HEAT ENGINES STEAM, GAS, STEAM TURBINES AND THEIR AUXILIARIES BY JOHN R. ALLEN PROFESSOR MECHANICAL ENGINEERING UNIVERSITY OF MICHIGAN -AND- JOSEPH A. BURSLEY JUNIOR PROFESSOR MECHANICAL ENGINEERING UNIVERSITY OF MICHIGAN SECOND EDITION THOROUGHLY REVISED AND ENTIRELY RESET TOTAL ISSUE 6,000 McGRAW-HILL BOOK COMPANY, INC, 239 WEST 39TH STREET, NEW YORK 6 BOUVERIE STREET, LONDON, E. C. 1914 COPYRIGHT 1914, BY THE McGRAW-HiLL BOOK COMPANY, INC. COPYRIGHT 1910, BY THE McGRAW-HiLL BOOK COMPANY. THE. MAPLE. PBESS. YORK. PA PREFACE TO SECOND EDITION The advancement in heat engines during the past four years has been such as to make it desirable to publish a new edition of this text. Among the new subjects treated are the Stumpf Uniflow Engine, the Humphrey Gas Pump, and recent develop- ments in steam turbines and gas engines. It has been found also desirable to rewrite many of the chapters in order to clear up the points which experience has shown need more detailed explana- tion than was given in the first edition. At the same time an effort has been made not to increase materially the size of the book. The authors desire to thank those members of the faculty of the mechanical engineering department of the University of Michigan who have assisted in the preparation of this text by their timely suggestions, and the manufacturers who have kindly supplied many of the new cuts used. JOHN R. ALLEN. JOSEPH A. BURSLEY. ANN ARBOR, MICHIGAN Sept. 1, 1914. PREFACE TO FIRST EDITION In preparing this book, it has been the intention of the authors to present an elementary treatise upon the subject of Heat Engines, considering only those engines which are most com- monly used in practice. It is written primarily as a text-book, the subject-matter having been used in the classes at the University of Michigan for a number of years. The forms of heat engines discussed include the steam engine with its boiler plant and auxiliaries, the gas engine with its pro- ducer, oil engines, and the principal types of steam turbines. Under each division of the text, problems have been worked out in detail to show the application of the subject-matter just treated, and, in addition, a large number of problems have been introduced for class-room work. The use of calculus and higher mathematics has been largely avoided, the only place where it is used being in the chapter on thermo-dynamics, which subject has been treated in its elementary phases only. The matter of the design of engines has been left untouched, as it was felt that that subject did not properly come within the scope of this work. The authors wish to express their thanks to Messrs. H. C. Anderson, A. H, Knight, and J. A. Mover, for their assistance in compiling this work, to Mr. W. R. McKinnon who made a number of the drawings, and to the various manufacturers who have very kindly furnished illustrations and descriptions of their apparatus. JOHN R. ALLEN. JOSEPH A. BURSLEY. ANN ARBOR, MICHIGAN Sept. 1, 1910. VI CONTENTS PAGE PREFACE y-v LIST OF TABLES xii CHAPTER I HEAT THEORY AND MEASUREMENT OF HEAT 1 SPECIFIC HEAT 5 RADIATION, CONDUCTION, AND CONVECTION 7 DEFINITIONS OF ENERGY, WORK, AND POWER . 8 CHAPTER II ELEMENTARY THERMODYNAMICS FIRST AND SECOND LAWS OF THERMODYNAMICS 10 EQUATION AND LAWS OF PERFECT GASES 11 ABSORPTION OF HEAT 13 JOULE'S LAW 14 RELATION OF SPECIFIC HEATS . 15 EXPANSIONS IN GENERAL 17 WORK OF EXPANSION 18 GENERAL CASE OF HEAT ADDED 20 HEAT ADDED AT CONSTANT VOLUME OR CONSTANT PRESSURE 22 ADIABATIC EXPANSION 24 ISOTHERMAL EXPANSION 25 RELATION OF PRESSURE, VOLUME AND TEMPERATURE DURING EX- PANSION 27 THEORETICAL HEAT ENGINE " 29 CARNOT CYCLE 29 PROBLEMS 34 CHAPTER III PROPERTIES OF STEAM FORMATION OF STEAM ^ PROPERTIES OF STEAM . 40 STEAM TABLES . . ^ 42 CHAPTER IV CALORIMETERS AND MECHANICAL MIXTURES CALORIMETERS SEPARATING CALORIMETERS THROTTLING CALORIMETERS 50 vii viii CONTENTS PAGE QUALITY OF STEAM . 53 PROBLEMS 55 MECHANICAL MIXTURES 56 PROBLEMS 61 CHAPTER V COMBUSTION AND FUELS COAL ANALYSIS 64 HEATING VALUE OF FUELS 66 COAL CALORIMETERS 68 AIR REQUIRED FOR COMBUSTION 70 SMOKE 71 ANALYSIS OF FLUE GASES 71 THEORETICAL TEMPERATURE OF COMBUSTION 74 FUELS 75 PROBLEMS 79 CHAPTER VI BOILERS RETURN TUBULAR BOILERS 82 INTERNALLY FIRED BOILERS 87 WATER-TUBE BOILERS 91 HORSE-POWER OF BOILERS 97 HEATING SURFACE, GRATE SURFACE, AND BREECHING 98 BOILER ECONOMY 99 BOILER EFFICIENCY 100 BOILER LOSSES ! 101 BOILER ACCESSORIES 105 CHAPTER VII BOILER AUXILIARIES MECHANICAL STOKERS 109 INCLINED GRATE STOKERS 110 CHAIN GRATE STOKERS 113 UNDER-FEED STOKERS 115 BOILER FEEDERS '....... 119 FEED-WATER HEATERS 122 ECONOMIZERS 125 SUPERHEATERS 127 CHIMNEYS 128 MECHANICAL DRAFT 132 PROBLEMS 133 CONTENTS ix CHAPTER VIII STEAM ENGINES PAGE THE SIMPLE STEAM ENGINE 139 HORSE-POWER OF A STEAM ENGINE 142 LOSSES IN THE STEAM ENGINE 144 METHODS OF REDUCING INITIAL CONDENSATION 147 CLEARANCE AND COMPRESSION 149 PROBLEMS 150 CHAPTER IX TYPES AND DETAILS OF STEAM ENGINES CLASSIFICATION OF ENGINES 153 ENGINE DETAILS 160 CHAPTER X TESTING OF STEAM ENGINES STEAM ENGINE INDICATOR 168 INDICATED HORSE-POWER 172 DETERMINATION OF INITIAL CONDENSATION 174 ECONOMY OF VARIOUS FORMS OF ENGINES 177 BRAKE HORSE-POWER 178 MECHANICAL EFFICIENCY 179 ACTUAL HEAT EFFICIENCY 180 DUTY 180 PROBLEMS 184 CHAPTER XI VALVE GEARS PLAIN D-SLIDE VALVES 187 DEFINITIONS OF LAP, LEAD, ANGULAR ADVANCE, AND ECCENTRICITY . 188 RELATIVE POSITION OF VALVE AND PISTON 190 ZEUNER VALVE DIAGRAM 191 EFFECT OF CONNECTING ROD 196 VARIOUS TYPES OF VALVES 198 CORLISS VALVES 205 REVERSING GEARS 209 VALVE SETTING 212 INDICATOR DIAGRAMS 213 CHAPTER XII GOVERNORS TYPES 216 GOVERNOR MECHANISM . 217 x CONTENTS PAGE ISOCHRONISM 222 HUNTING . 222 FLY WHEEL 223 CHAPTER XIII COMPOUND ENGINES COMPOUND ENGINES 225 TANDEM COMPOUND ENGINES 226 CROSS COMPOUND ENGINES 227 RATIO OF CYLINDERS . . ... . . . 228 HORSE-POWER OF COMPOUND ENGINES 229 COMBINED INDICATOR CARDS 232 PROBLEMS 234 CHAPTER XIV CONDENSERS AND AIR PUMPS JET CONDENSERS . . ... . . . . . 237 SURFACE CONDENSERS 240 AIR PUMPS . . 240 COOLING WATER 240 PROBLEMS 242 CHAPTER XV . STEAM TURBINES HISTORY 244 CLASSIFICATION 246 ACTION OF STEAM IN TURBINE 247 TURBINE NOZZLES 248 SPEED OF TURBINES 249 DE LAVAL TURBINE 251 CURTIS TURBINE 255 RATEAU TURBINE 257 KERR TURBINE 259 STURTEVANT TURBINE 260 PARSONS TURBINE 262 DOUBLE-FLOW TURBINE 264 LOW-PRESSURE TURBINES 266 MIXED-FLOW TURBINES 267 CHAPTER XVI GAS ENGINES HISTORY 269 CLASSIFICATION 270 THEORETICAL EFFICIENCY 275 LOSSES 282 CONTENTS xi PAGE GAS-ENGINE FUELS GAS PRODUCERS . LIQUID FUELS FUEL MIXTURES . RATED HORSE-POWER . ACTUAL HORSE-POWER . CHAPTER XVII DETAILS OF GAS-ENGINE CONSTRUCTION DESCRIPTION OF PARTS . METHODS OF IGNITION . METHODS OF GOVERNING .... CARBURETORS OIL ENGINES FOR SHIPS HUMPHREY' GAS PUMP PROBLEMS .... CHAPTER XVIII ECONOMY OF HEAt ENGINES RELATIVE ECONOMY . . . COMMERCIAL ECONOMY 31 INDEX. 313 LIST OF TABLES PACK TABLE I. TEMPERATURE COLORS 4 TABLE II. SPECIFIC HEATS OF GASES 6 TABLE III. RADIATING POWER OF BODIES 7 TABLE IV. CONDUCTING POWER OF BODIES 8 TABLE V. HEAT AND TEMPERATURE CHANGES DEPENDENT UPON VALUE OF n DURING EXPANSION 18 TABLE VI. SPECIFIC HEATS OF SUPERHEATED STEAM 40 TABLE VII. PROPERTIES OF SATURATED STEAM 42 TABLE VIII. SPECIFIC HEATS OF LIQUIDS AND SOLIDS 57 TABLE IX. COMBUSTION PROPERTIES OF ELEMENTS 67 TABLE X. CALORIFIC VALUE OF WOODS 76 TABLE XI. CALORIFIC VALUE OF PEATS 77 TABLE XII. CALORIFIC VALUE OF LIGNITES 77 TABLE XIII. CALORIFIC VALUE OF BITUMINOUS COALS 78 TABLE XIV. CALORIFIC VALUE OF SEMI-BITUMINOUS COALS ... 78 TABLE XV. CALORIFIC VALUE OF ANTHRACITE COALS 79 TABLE XVI. DIAMETER OF BOILER TUBES - . 99 TABLE XVII. HEAT BALANCE IN BOILER PLANT 101 TABLE XVIII. CHIMNEY HEIGHTS 131 TABLE XIX. STEAM CONSUMPTION OF VARIOUS CLASSES OF ENGINES 178 TABLE XX. DUTY OF VARIOUS FORMS OF PUMPS 181 TABLE XXI. RELATIVE CHANGES IN VELOCITY, SPECIFIC VOLUME AND PRESSURE OF STEAM FLOWING THROUGH A NOZZLE 249 TABLE XXII. CALORIFIC VALUE OF GASEOUS AND LIQUID FUELS. . 287 TABLE XXIII. VOLUMETRIC EFFICIENCIES, v vj OF GAS ENGINES . . 289 TABLE XXIV. ECONOMIC EFFICIENCIES, v w , AND AIR CONSUMPTION, OF GAS ENGINES 290 TABLE XXV. THERMAL EFFICIENCIES OF PRIME MOVERS .... 309 TABLE XXVI. COMPARATIVE COSTS PER RATED HORSE-POWER . 311 xni HEAT ENGINES STEAM-GAS-STEAM TURBINES-AND THEIR AUXILIARIES CHAPTER I HEAT 1. Heat being the source of energy for the devices considered in this book, a short discussion of the nature and the more important properties of heat will assist the student to a better understanding of the subject-matter of this text. These phenomena will be con- sidered only as they affect perfect gases, steam, and water. 2. Theory of Heat. The accepted theory of heat at the pres- ent time is that it is a motion of the molecules of a body. Phys- ical experiments indicate this to be the fact. The intensity of the heat, or the temperature, is supposed to depend upon the velocity and amplitude of these vibrations. Most bodies when heated expand. This expansion is probably due to the increased velocity of the molecules which forces them farther apart and increases the actual size of the body. The vibration may become so violent that the attraction be- tween the molecules is partly overcome and the body can no longer retain its form. In this case the solid becomes a liquid. If still more heat is added, the attraction of the molecules may be entirely overcome by their violent motion, and the liquid then becomes a gas. The phenomena of heat is then a form of motion. This is often stated in another way, that is, heat is a form of kinetic energy. As heat is a form of motion, it must be possible to transform heat into mechanical motion. In the following pages, therefore, the most important methods of making this transformation will be discussed. 3. Temperature and Temperature Measurement. The ve- locity of the vibration of the molecules of a body determines the intensity of the heat, and this intensity is measured by 1 2 A : f '*: ; : ; i ff&AT ENGINES the temperature. If the molecules of a body move slowly it is at a low tempertaure; if they move rapidly it is at a high tem- perature. The temperature of a body is then determined by the rapidity of the motion of its molecules. Temperature is sometimes defined as the thermal state of a body considered with reference to its ability to transmit heat to other bodies. Two bodies are said to be at the same temperature when there is no transmission of heat between them. If there is transmission of heat between them, the one from which the heat is flowing is said to have the higher temperature. In mechanical engineering work, temperatures are usually measured on the Fahrenheit scale, and in this text, unless otherwise stated, the temperature will be taken on this scale. There is, however, an increasing use of the Centigrade scale among engineers, and certain quantities, such as the in- crease in temperature in a dynamo, are always expressed in Centigrade units. In the Fahrenheit scale the graduations are obtained by noting the position of the mercury column when the bulb of the ther- mometer is placed in melting ice, and again when it is placed in boiling water under an atmospheric pressure corresponding to sea level barometer. The distance between these two points is divided into 180 equal parts. The freezing point is taken as 32, making the boiling point 32 -f 180 = 212 above zero. In the Centigrade scale the distance between the freezing point and the boiling point is divided into 100 equal parts or degrees, and the freezing point on the scale is marked 0. The boiling point is then 100. Both the Fahrenheit and Centigrade scales assume an arbitrary point for the zero of the scale. Since in the Fahrenheit scale there are 180 divisions between the freezing and boiling points and on the Centigrade 100 divi- sions, it follows that 1 F. = f C., or 1 C. = f F. As, how- ever, the freezing point on the Fahrenheit scale is marked 32 and on the Centigrade scale 0, it is necessary to take account of this difference when converting from one scale to the other. If the temperature Fahrenheit be denoted by t F and the temperature Centigrade by t c , then the conversion from one scale to the other may be made by the following equations : *F=**c + 32; (1) *c = f (t F - 32). (2) HEAT 3 The measurement of temperature is not so simple a process as is generally supposed. The mercury of the ordinary glass thermometer does not expand equal amounts for equal incre- ments of heat, and the bore of the thermometer is not abso- lutely uniform throughout the whole length of the tube. These inaccuracies must be allowed for by accurate calibration. In measuring the temperatures of liquids, the depth to which the thermometer is immersed affects the reading, and it should be calibrated at the depth at which it is to be used. If a ther- mometer is used to measure the temperature of the air in a room in which there are objects at a higher temperature, its bulb must be protected from the radiant heat of those hot bodies. When accurate temperature measurements are desired, a careful study should be made of the errors of the instrument and the errors in its use. The ordinary form of mercury thermometer is used for tem- peratures ranging from 40 F. to 500 F. For measuring tem- peratures below 40 F, thermometers filled with alcohol are used. These are, however, not satisfactory for use at high temperatures. When a mercury thermometer is used for temperatures above 500 F., the space above the mercury is filled with some inert gas, usually nitrogen or carbon-dioxide, placed in the ther- mometer tube under pressure. As the mercury rises, the gas pressure is increased and the temperature of the boiling point of the mercury is raised, so that it is possible to use these ther- mometers for temperatures as high as 1000 F. This is the limit, however, as the melting point of glass is comparatively low. For temperatures exceeding 800 F., some form of pyrometer is generally used. The simplest of these is the metallic or mechan- ical pyrometer. This consists of two metals having different rates of expansion, such as iron and brass, attached to each other at one end and with the other ends free. By a system of levers and gears the expansion of the metals is made to move a hand over a dial graduated in degrees. This should not be used for tempera- tures over 1500 F. There are two types of electrical pyrometers in use to-day. In one, the thermo-electric couple is employed and the difference in temperature of the junctions of the two metals forming the couple produces an electric current which is proportional to this dif- ference, and which is measured on a galvanometer calibrated in 4 HEAT ENGINES degrees. By keeping one junction at a known temperature, the other may be computed. This may be used up to 2500 F. The second type, the electrical resistance pyrometer, depends upon the increase in electrical resistance of metals due to a rise in temperature. For still higher temperatures the optical pyrometer gives the most satisfactory results. This is based on the results of ex- periments made by Pouillet which show that incandescent bodies have for each temperature a definite and fixed color, as follows: TABLE I. TEMPERATURE COLORS Color Temp. C. Temp. F. Faint red 525 977 Dark red 700 1292 Faint ( herry f- 800 : . 1472 Cherry 900 1652 Bright cherry 1000 1832 Dark orange 1100 2012 Bright orange 1200 2192 White heat 1300 2372 Bright white 1400 2552 Dazzling white f 1500 \ 1600 (2732 \2912 4. Absolute Zero. In considering heat from a theoretical standpoint, it is necessary to have some absolute standard of comparison for the scale of temperature, so that the absolute scale is largely used. A perfect gas contracts TQI~~C of its volume at 32 F. for each degree that it is reduced in temperature. Hence if the tempera- ture be lowered to a point 491.6 below 32, its volume will be- come zero. This point is called the absolute zero and is mani- festly an imaginary one. (The lowest point so far actually reached by experiment is about 488.9 F.) For ordinary usage it is sufficiently accurate to consider absolute zero as 492 below the freezing point in the Fahrenheit scale. In other words, to convert to the absolute scale, add 460 to the temperature ex- pressed in degrees Fahrenheit. In this text absolute tempera- tures will be denoted by T and temperatures in degrees Fahren- heit by t. On the Centigrade scale the absolute zero is 273.1 below the HEAT 5 freezing point, and for all practical purposes, temperatures on the absolute scale may be found by adding 273 to the thermometer reading expressed in degrees Centigrade. 5. Unit of Heat. Heat is not a substance, and it cannot be measured as we would measure water, in pounds or cubic feet, but it must be measured by the effect which it produces. The unit of heat used in mechanical engineering is the heat required to raise a pound of water one degree Fahrenheit. The heat necessary to raise a pound of water one degree does not remain the same throughout any great range of temperature. For physical measurements where accuracy is required, it is neces- sary to specify at what point in the scale of temperatures this one degree is to be taken. The practice of different authors varies; the majority, however, specify that the heat unit is the amount of heat required to raise a pound of water from 39 to 40 Fahrenheit. The range from 39 F. to 40 F. is used because at this temperature water has its maximum density. This unit is called a British Thermal Unit, and is denoted by B.T.U. The heat unit used in Marks and Davis tables is the "mean B.T.U.," that is j-J-o of the heat required to raise one pound of water from 32 to 212 at . atmospheric pressure (14.7 pounds per square inch absolute). 6. Specific Heat. If the temperature of a body is raised or lowered a definite amount, a definite amount of heat must either be added to or given up by the body. Then dH = Cdt.- (3) where C is the heat necessary to change the temperature of the body one degree. Let a body of unit weight at a temperature T\ be heated to a temperature T 2) and at the same time let its heat content be increase from Hi to H z . Then the heat, H. added to cause this increase in temperature will be found by integrating equation (3) between the limits T\ and T 2 , or rTz H = H 2 HI = I Cdt. JTi If C is a constant and is equal to the heat necessary to raise the temperature of a unit weight one degree rT 2 H = C \ dt = C(T 2 - TO. (4) 6 HEAT ENGINES C in equation (4) represents the heat capacity of the body, or the heat required to raise the temperature of a unit weight of the body one degree. The heat capacity of any substance compared with that of an equal weight of water is called its specific heat. Expressed in English units, the heat capacity of one pound of water is one B.T.U., and specific heat may be defined as the heat necessary to raise the temperature of one pound of a substance one degree Fahrenheit expressed in British Thermal Units. Since a B.T.U. is the amount of heat required to raise a pound of water from 39 to 40 the specific heat will then represent the ratio of the heat necessary to raise the temperature of a unit weight one degree to the heat necessary to raise the temperature of the same weight of water from 39 to 40. TABLE II. SPECIFIC HEATS OF GASES Gas' Symbol Expressed in B.T.U Expressed in Ft. Lbs. i A fc< 05 *>' X Constant pressure Cp Constant volume Cv Constant pressure K P Constant volume Kv Air . - .2375 .1689 .453* .400 .5084 .350 .2450 .174 .2169; .167 .1569 .131 .4797 .450 3.40902.412 .2438 .1727 .2175 .1551 j VI j Page 40. 184.77 352.75 395.54 190.61 168.75 122.07 373.21 2652.20 189.68 169.22 131.40 311.20 272 . 30 135.37 129.93 101.92 350.10 1876.54 134.36 120.67 53.37 .406 41.55 .133 123.24 .452 55.24 .408 38.82 .299 20.15 .197 23.11 .066 775.66 .413 55.32 .412 48.55 .402 Alcohol C 2 H 6 O NH 3 CO C0 2 CS 2 C4HioO H ! See Tdbh Ammonia gas Carbonic oxide Carbonic acid Carbon disulphide. . Ether Hydrogen Nitrogen OxvKen Superheated steam. In solid and liquid substances it is necessary to consider but one specific heat, as the change in volume when a solid or a liquid substance is heated is so small that its effect may be neglected. In gases the change in volume when the gas is heated is large, and if it is heated under a constant pressure this change is directly proportional to the change in the abso- lute temperature. If there is a change in volume there must be external work done. On the other hand, when gas is con- HEAT 7 fined and is heated, it cannot expand. If it does not expand, there is no external work done. Therefore, in considering the specific heat of a gas, we must consider two cases: one in which the pressure remains constant and the gas expands when it is heated; and the other where the volume remains constant and the pressure increases when the gas is heated. Hence, in the case of a gas, there are two specific heats, the specific heat of constant pressure and the specific heat of constant volume. The specific heat of constant volume will be denoted by c v and the specific heat of constant pressure by c p , both being expressed in B.T.U. When expressed in foot-pounds they will be denoted by K v and K p respectively. 7. Radiation. The heat that passes from a body by radia- tion may be considered similar to the light that is radiated from a lamp. There is always a transfer of radiant heat from a body of a high temperature to a body of lower temperature. The amount of heat radiated will depend upon the difference in temperature between the bodies and upon the substances of which they are composed. The following table gives the radi- ating power of different bodies. TABLE III. RADIATING POWER OF BODIES Radiating power of bodies, expressed in heat units, given off per square foot per hour for a difference of one degree Fahrenheit. (PECLET.) B.T.U. Copper, polished 0327 Iron, sheet 0920 Glass 595 Cast iron, rusted 648 Building stone, plaster, wood, brick 7358 Woolen stuffs, any color 7522 Water 1 .085 8. Conduction. The heat transmitted by conduction is the heat transmitted through the body itself. The amount of heat conducted will depend upon the material of which the body is composed and the difference in temperature between the two sides of the body, and is inversely proportional to the thickness of the body. Heat may be conducted from one body to another when they are placed in contact with each other. The following table gives the conducting power of different bodies. 8 HEAT ENGINES TABLE IV. CONDUCTING POWER OP BODIES The conducting power of materials, expressed in the quantity of heat units transmitted per square foot per hour by a plate one inch thick, the surfaces on the two sides of the plate differing in temperature by one degree. (PECLET.) B.T.U. Copper 515 Iron 233 Lead 113 Stone 16.7 Glass 6.6 Brick work 4.8 Plaster 3.8 Pine wood .75 Sheep's wool 323 9. Convection. Loss by convection is sometimes called loss by contact of air. When air or other gas comes in contact with a hot body it is heated and rises, carrying away heat from the body. Heat carried off in this manner is said to be lost by convection. The loss by convection is independent of the nature of the surface wood, stone, or iron losing the same amount but it is affected by the form and position of the body. 10. Energy, Work, and Power. Work is the overcoming of resistance through space and is measured by the resistance multiplied by the space through which this resistance is over- come. The simplest form of work is the raising of a body against the force of gravity. Let M = the mass of the body. g = the force of gravity. w = the weight. I = the distance through which the weight is moved. W = work. Then Mg = w, and wl = W. If w is expressed in pounds and I in feet, then the unit of work will be the foot-pound (ft.-lb.). If we consider the work done by a fluid, let the volume be increased from v to v + d v, and the pressure against which the increase takes place be p, then the work done will be P [(v + 5 v) - v] = p d v = d W. HEAT 9 If a pressure p acts upon an area a through a distance I, then the work W = pla. Work may also be expressed as mass times acceleration times space. Energy is the capacity for doing work. Power is the time rate of doing work. The unit of power is the horse-power (H.P.). A horse-power is equivalent to raising 33,000 Ibs. one foot in one minute. This is the unit employed in determining the power of a steam engine. If r equals the resistance expressed in pounds, I the distance in feet through which the resistance r is overcome, and m the time in minutes in which the space is passed over, then the horse-power exerted is _J Xr 33,000 X m' Power is often expressed in electrical units. This is usually the case where an engine is used to drive a generator. An ampere is the unit of current strength or rate of flow. The volt is the unit of electromotive force or electrical pressure. The watt is the product of the amperes and the volts. One horse-power equals 746 watts, or one kilowatt equals 1.34 horse-powers. CHAPTER II ELEMENTARY THERMODYNAMICS 11. First Law of Thermodynamics. "'When mechanical energy is produced from heat, a definite quantity of heat goes out of existence for every unit of work done; and conversely, when heat is produced by the expenditure of mechanical energy, the same definite quantity of heat comes into existence for every unit of work spent." The relation between work and heat was first accurately determined by Joule in 1850. More recently Professor Rowland of John Hopkins University redetermined its equivalent with great accuracy. His results show that one British Thermal Unit is equivalent to 778 foot-pounds. This factor is often called the mechanical equivalent of heat, and is usually denoted by J. Heat and work are mutually convertible in the ratio of 778 foot-pounds equals one B.T.U. 12. Second Law of Thermodynamics. The second law of thermodynamics may be stated in different ways. Clausius states it as follows: "It is impossible for a self-acting machine, unaided by any external agency, to convey heat from one body to another of higher temperature.' 7 Rankine states the second law as follows: "If the total actual heat of a homogeneous and uniformly hot substance be conceived to be divided into a number of equal parts, the effects of those parts in causing work to be performed are equal." It follows from the second law that no heat engine can convert more than a small fraction of the heat given to it into work. From this law we derive the expression for the efficiency of a heat engine, i. e., _ heat added heat rejected heat added The second law is not capable of proof but is axiomatic. All our experiments with heat engines go to show that this law is true. 13. Laws of Perfect Gases. There are two laws expressing the relation of pressure, volume, and temperature in a perfect gas: the law of Boyle and the law of Charles. 10 ELEMENTARY THERMODYNAMICS 11 Boyle's Law. " The volume of a given mass of gas varies in- versely as the pressure, provided the temperature remains constant." If p = the pressure, and v = the volume of the initial condition of the gas, and p and v any other condition of the same gas, then P O V O = pv = a constant. Charles' Law. " Under constant pressure equal volumes of different gases increase equally for the same increment of tem- perature. Also if the gas be heated under constant pressure equal increments of its volume correspond very nearly to equal increments of temperature by the scale of a mercury ther- mometer." This law may also be stated as follows: When a gas receives heat at a constant volume the pressure varies directly as the absolute temperature, or when a gas receives heat at a constant pressure the volume varies directly as the absolute temperature. Letting a gas receive heat at a constant volume v , the pres- sure and absolute temperature varying from p , T to p, T', then If the gas now receives heat at this pressure p, the volume and temperature changing to v and T' ', then T 14. Equation of a Perfect Gas. Combining these two laws we have the equation of a perfect gas. Let one pound of a gas have a volume v, a pressure p, and be at an absolute tem- perature T. From Boyle's Law, if the pressure is changed to Pi and the volume to v', the temperature T remaining constant, then we have the following equation: pi v pv - = -,' or Pl = From the law of Charles, if the volume remains constant at v f and the temperature be changed to T r and the pressure to p' ', then Pl T p'T / f = ft' or P I = ~r' 12 HEAT ENGINES Combining equations (1) and (2), we have pv p'T v' ~~ T f ' Hence, p'v' p"v" f . = ~rT = v/ == a constant (3) Denoting this constant by R, then pv = RT, p'v' = RT r , and p"v" = RT". (4) The value of R given in this equation is for one pound of the gas. If we wish to state this law for more than one pound, let w equal the weight of the gas, then the law becomes % pv = wRT. (5) This equation is called the equation of the gas and holds true for any point on any expansion line of any perfect gas. These laws were first determined for air, which is almost a perfect gas, and they hold true for all perfect gases. A perfect gas is sometimes defined as a gas which fulfils the laws of Boyle and Charles. It is probably better to define it as a gas in which no internal work is done, or in other words, a gas in which there is no friction between the molecules under change of conditions. In the above expressions, p is the absolute pressure in pounds per square foot, is the volume in cubic feet, and T is the absolute temperature in degrees Fahrenheit. Absolute pressure must not be confused with gage pressure. The ordinary pressure gage reads the difference in pressure be- tween the atmospheric pressure outside the gage tube and the applied pressure inside the gage tube. The absolute pressure is equal to the gage pressure plus the barometric pressure. The value of R for any given substance may be determined, provided we know the volume of one pound for any given con- dition of pressure and temperature. For example, it has been found by experiment that for air under a pressure of 14.7 Ibs. per square inch absolute, and at a temperature of 32 F., the volume of 1 Ib. is 12.39 cu. ft. Substituting these values in equation (4) we have ELEMENTARY THERMODYNAMICS 13 R- V " K> rp 14.7 X 144 X 12.39 32 + 460 = 53.37 (compare with the value of R for air given in Table II). (6) Therefore for one pound of air with the units we have taken, pv = 53.37 T. (7) or for w pounds, pv = 53.37 wT. (8) This equation is always true for air at all times and under all conditions, as long as it remains a gas. Example. A tank*contains 5 Ibs. of air at 75 F., under a pressure of 100 Ibs. per square inch gage. Find the volume of the air. Solution. pv = wRT. p = (100+ 14.7)144 = 114.7 X 144 Ibs. per square foot, absolute. T = 75 + 460 = 535 absolute. Therefore, substituting in the equation of the gas, we have 114.7 X 144 X v = 5 X 53.37 X 535 142760 = 16520 v = 8 . 64 cu. ft. Example. Ten pounds of air under a pressure of 50 Ibs. per square inch gage occupy a volume of 10 cu. ft. Find the temperature. Solution. pv = wRT p = (50 + 14.7)144 = 64.7 X 144 Ibs. per square foot absolute. Therefore 64.7 X 144 X 10 = 10 X 53.37 X T 93200 533.7 T = 174.5 absolute T = 174.5 - 460 = - 285.5 F. 15. Absorption of Heat. When a gas receives heat this heat may be dissipated in one or all of three ways; by increasing its temperature, by doing internal work, or by doing external work. 14 HEAT ENGINES Let dH denote the heat absorbed, dS the heat used in increas- ing the temperature, dl the heat used in doing internal work, and dW the heat equivalent of the external work done. Then dH = dS + dl + dW ^ (9) The heat utilized in changing the internal energy of the sub- stance is represented by dS -f dl, and dl -f- dW represents the heat equivalent of the total work done. By ''internal work" is meant work done in overcoming changes in the physical state of the substance, and in overcoming the attraction of the molecules for each other, thus changing the po- tential energy of the body. An example of this is shown in the case of water at the boiling point being changed into steam. In this case dS in equation (9) becomes zero since the temperature remains constant. There- fore all the heat added goes to doing internal and external work. The external work will be equal to the change in volume from water to steam times the pressure under which the steam is being formed. This will be only a small part of the total heat added to accomplish the change, the balance being the heat going to in- ternal work or dl. Since no internal work is done in heating a perfect gas, the sec- ond term in equation (9) becomes zero and all the heat absorbed * goes either to increasing the temperature or doing external work. Therefore in the case of a perfect gas dH = dS + dW CH* rs 2 rv, .' . (dH = [dS + dW. JHi JSi Jvi Integrating # 2 - Hi = S 2 - Si + ] pdv ? rn Let H = # 2 - #1, S = S 2 - Si and W = I pdv Jvi Then H = S + W. (10) and a change in internal energy is indicated by a change in temperature alone. 16. Joules Law. When a perfect gas expands without doing external work and without taking in or giving out any heat, its temperature remains unchanged and there is no change in its ELEMENTARY THERMODYNAMICS 15 internal energy. This law was established by the following ex- periment performed by Joule. Two vessels a and b, Fig. 1, connected by a tube containing a stop-cock c were placed in a water-bath. One vessel contained air compressed to a pressure of 22 atmospheres, while a vacuum was maintained in the other. After the vessels had remained in the bath long enough so that the air and water were at the same temperature and there could therefore be no further flow of heat from one to the other, the stop-cock c was opened and the air allowed to flow from one vessel to the other until the pressure in each was 11 atmospheres. The temperatures of the air and FIG. 1. Joule's apparatus. water were then read again and found to be unchanged. From the conditions of the apparatus no work external to the two vessels could have been done. As the gas had done no work and had neither gained nor lost any heat, its internal energy must have remained unchanged. Although the pressure and volume of the gas had changed the temperature had not, thus proving that a change in internal energy depends upon a change in temperature only. 17. Relation of Specific Heats. If one pound of a perfect gas is heated at a constant pressure from a temperature Ti to a temperature T 2 , and the volume is changed from a volume Vi to a volume v 2 , the heat absorbed would equal K P (T, - ro (ii) and the work done, W = \ pdv. Integrating between limits w r /, = I pdv = p I dv = p(v 2 - Jvi Jv\ 16 HEAT ENGINES Since from the equation of a perfect gas pv 2 = RT 2 , and pv\ = substituting these values in the above expression for the work done, we have p(v 2 - vj = R(T, - 7\). (12) Since from equation (10), S = H ~ W, then the difference between equation (11) and equation (12) would be the heat which goes to increasing the temperature, which equals (K p - R) (T, - !Ti). (13) If the gas is heated at a constant volume from a tempera- ture Ti to a temperature T 2 , then the heat added would be K V (T, - T,), (14) and as no external work is done this heat all goes to increasing the temperature. But since equation (13) also represents the heat which goes to increasing the temperature, equations (13) and (14) are equal to each other, or (K p - R) (T 2 - TO =K 9 (T* - TO, therefore K v = K p - R, (15) or R = K P - K v . (16) The difference between the two specific heats, R, is the amount of work in foot-pounds done when one pound of a gas is heated one degree Fahrenheit at constant pressure. 7^ The ratio of the two specific heats, that is -^, is denoted /Vtf by 7. K Since K p K v = R, and J* = 7, A. v then Xp _ R K v K v ' or R ^"^^ and hence K, = -^ (17) Similarly K, = (18) ELEMENTARY THERMODYNAMICS 17 For air R = 184.77 -- 131.40 = 53.37 (compare equation 6) and K 184.77 131.40 1.406. (19) 18. Expansions in General. When air, steam, or any other gas is 'Used as the working substance in an engine, the gas is allowed to expand, doing work for a portion of the working stroke of the engine. The variation in pressure and volume during this expansion may be graphically represented by a mathematical curve on the pressure-volume plane. The same is true in the compression of these gases. On this plane the ordinates of any curve represent pressures and the abscissae represent volumes. FIG. 2. Paths of an expanding gas. Almost all the expansion or compression curves ordinarily occurring in steam, or gas engines, or the various forms of compressors, can be represented by the equation pv a constant. (20) During expansion, or compression, n in equation (20) may have any value between zero and infinity, but is constant for any given curve. Fig. 2 shows how the path of a gas will vary during expan- sion depending upon the value of n. The value of n will determine whether heat must be added, rejected, or remain constant, and whether the temperature will rise, fall, or remain constant during the expansion, or compres- sion, of a gas. These varying conditions are clearly shown in Table V. 18 HEAT ENGINES TABLE V. HEAT AND TEMPERATURE CHANGES DEPENDENT UPON VALUE OF n DURING EXPANSION Value of n Equation of path of gas Path as shown in Fig. 2 Heat Temperature n = . p = constant ab Added 1 Rises n > and < 1 n = 1 pv n = constant pv = constant ac ad Added Added Rises Constant n > 1 and < 7 pD* = constant ae Added Falls n = y pifY = constant af Constant Falls n > y and < n = oo pv n constant v = constant ag ah Rejected Rejected ' Falls I Falls For any path lying between ad and a/, heat is added and yet the .temperature falls. In other words the specific heat is nega- tive. In case the gas is being compressed instead of expanding, the changes in heat and temperature will be just the opposite of those shown in the table. 19. Work of Expansion. The curve ab in Fig. 3 represents graphically the relation between pressure and volume during expansion. Let the equation of this curve be pv n = a constant. In this figure pressures are represented by ordinates and volumes by abscissae. The gas expands from a point a, where the pres- sure is pi and the volume v\, to the point b where the pressure is pz and the volume v z . The area abed represents the work done during this expansion. Let W equal the work done during expansion. Then as W r2 pdv. (21) Since every point in the curve must fulfil the original conditions for the equation of the curve, , hence pv n = Substituting this expression in equation 21 , r I /Mi (22) (23) ELEMENTARY THERMODYNAMICS Integrating, W = piVi n - 1 n Multiplying out the parenthesis, we have w = PlPl * ^- l-n l-n FIG. 3. Pressure-volume diagram of an expanding gas. But = r, the ratio of expansion for the gas, therefore piVi (I r l ~ n ) W = ^j-y or substituting p 2 ^2 n for piv^ in equation (25), we have n-l 19 (24) (25) W = (26) (27) 20 HEAT ENGINES Substituting for pv its value in terms of R and T } equation (27) becomes ^ : = JiTi C 28 ) If w pounds of the gas is expanded, then equation (28) becomes = te#JTi_2\) t n 1 20. Heat Added General Case. In the case of any expan- sion, the heat added is equal to the algebraic sum of heat equivalent of the work done and the change in internal energy. As has been previously shown, the change in internal energy of a gas depends upon the change in temperature only and is equal to the heat necessary to change the temperature at constant volume. Therefore in the case of a perfect gas H = S + W K ,m m\ i /om wK v (1, fji _ rrt hence E = (compare equation 57) i i All experience in testing engines using either perfect or imperfect gases as their working medium goes to show that this law applies to all forms of engines no matter what the working medium may be. ELEMENTARY THERMODYNAMICS 33 27. Reversibility of Carnot Cycle. The Carnot cycle is a reversible one as the gas may be considered to first expand adiabatically along AD and then isothermally along DC, then to be compressed adiabatically along CB, and finally com- pressed isothermally along BA. It is thus possible to work around the cycle in the reverse direction. Having proved that the Carnot cycle is reversible and that rp _ np its efficiency is equal to ^r , it is now necessary to show that no cycle can be more efficient than a reversible one, and that no reversible cycle can have a greater efficiency than that of the Carnot cycle. Assume a non-reversible engine A and a Carnot engine B, both working between the same limits in temperature. Engine A takes Q A heat units from the hot body and rejects Q' A heat units to the cold body, while engine B takes Q B heat units from the hot body and rejects Q' B heat units to the cold body. If engine A is more efficient than engine B, it must take less heat from the hot body and reject less to the cold body, or in other words QA < QB and Q' A < Q' B . Now assume that B is to run in the reverse direction and that A is to drive 5, which acts as a heat pump. Since B is a reversible engine, it will reject to the hot body, when running in a reverse direction, the same amount of heat that it takes from that body when running direct. Therefore the combined unit of A B will, in each cycle, take from the hot body the quantity of heat Q A and reject to the hot body the quantity of heat Q B - But Q B > Q A which means that this " self-acting machine unaided by any external agency" is transferring heat from a body of lower to one of higher temperature. This is contrary to the Second Law of Thermodynamics. It is, therefore, impossible for engine A to be more efficient than engine B. As these represent any engines of these particular types, no non-reversible engine can be more efficient than a reversible one working in the Carnot cycle. Now assume engine A to be a reversible engine also. It can 3 34 HEAT ENGINES be similarly proven that it cannot be more efficient than the Carnot engine. The conclusion is therefore reached that no cycle can be more efficient than the Carnot cycle. It can also be proven that this cycle is the most efficient cycle that any engine can follow when working between any given temperature limits. This necessitates, however, a more thorough exposition of the principles of thermodynamics than it is deemed wise to include in this text, and will therefore be omitted. PERFECT GAS PROBLEMS 1. One pound of air under a pressure of 100 Ibs. per square inch absolute occupies .3 of a cubic foot in volume. What is its temperature in degrees F. ? 2. Ten pounds of air under a pressure of 10,000 Ibs. per square inch absolute have a temperature of 100 F. Find the volume occupied. 3. Five pounds of air at a temperature of 60 F. occupy a volume of 50 cu. ft. Find the gage pressure per square inch. 4. A tank containing air has a volume of 300 cu. ft. The pressure in the tank is 100 Ibs. per square inch absolute and the temperature is 70 F. Find the weight of air in the tank. 6. What is the weight of the quantity of air which occupies a volume of 10 cu. ft. at a temperature of 100 F. under a pressure of 50 Ibs. per square inch absolute? 6. What is the temperature of a pound of air when its volume is 5 cu. ft. and the pressure is 35 Ibs. per square foot absolute? 7. What is the weight of a cubic foot of air when the pressure is 50 Ibs. per square inch absolute and the temperature 160 F.? 8. A quantity of air at a temperature of 60 F. under a pressure of 14.7 Ibs. per square inch absolute has a volume of 5 cu. ft. What is the volume of the same air when its temperature is changed to 120 F. at constant pressure? '"9. The volume of a quantity of air at a temperature of 60 F. under a pressure of 14.7 Ibs. per square inch absolute is 10 cu. ft. What is the volume of the same air when the pressure is changed at constant temperature to 60 Ibs. per square inch absolute? 10. A tank contains 200 cu. ft. of air at a temperature of 60 F. and under a pressure of 200 Ibs. absolute, (a) What is the weight of the air? (b) How many cubic feet will the air occupy at atmospheric pressure? 11. A tank containing 1000 cu. ft. is half full of air and half full of water. The pressure in the tank is 60 Ibs. absolute and the temperature is 60 F. If half the water is withdrawn from the tank, what will be the resulting pressure, assuming the temperature to remain constant? 12. The volume of a quantity of air at 70 F. under a pressure of 16 Ibs. per square inch absolute is 20 cu. ft. What is the temperature of this air when the volume is 4 cu. ft. and the pressure is 70 Ibs. per square inch absolute? ELEMENTARY THERMODYNAMICS 35 13. A compressed air pipe transmission is 1 mile long. The pressure at entrance is 1000 Ibs. per square inch absolute; at exit, 500 Ibs. The velocity at entrance to pipe, which is 12 in. in diameter, is 100 ft. per second, (a) What must be the diameter of the pipe at the exit end to have the same velocity as at entrance, the temperature of the air in the pipe remaining con- stant? (&) What, if the velocity at exit is to be 90 ft. per' second? 14. A street car has an air storage tank for its air brakes with a volume of 400 cu. ft. The pressure in the tank at starting is 200 Ibs. absolute and the temperature is 60 F. The air-brake cylinders take air at 40 Ibs. absolute and have a volume of 2 cu. ft. How many times can the brakes be operated on one tank of air, assuming the temperature of the air to remain constant? 15. To operate the air brakes on a car requires 1 cu. ft. of air at 40 Ibs. gage pressure. The car has a storage tank containing 100 cu. ft. of air at 250 Ibs. gage pressure. How many times will the tank operate the brakes? The compressed air tank on a street car has a volume of 250 cu. ft. The pressure in the tank is 250 Ibs. gage and the temperature is 60 F. There are two air cylinders each 8" X 10". The brakes take air aT*40 Ibs. gage pressure and 60 temperature. How many times will the tank operate the brakes? 17. How many B.T.U. will be required to double the volume of 1 Ib. of air at constant pressure from the temperature of melting ice? 18. A tank filled with 200 cu. ft. of air at atmospheric pressure, and at 60 F. is heated to 150. What will be the resulting air pressure in the tank and how many B.T.U. will be required to heat the air? AX^^ A tank contains 200 cu. ft. of air at 60 F. under a pressure of 40 Ibs. \ibk>lute. If the air has 1000 B.T.U. added to it, what will be the resulting temperature and pressure in the tank? 20. A tank contains 100 cu. ft. of air at 60 F. under a pressure of 50 Ibs. absolute. If the air in the tank receives 100 B.T.U. of heat, what will be the resulting temperature and pressure? \J 21. Ten pounds of air enclosed in a tank at 60 F. under a pressure of 100 Ibs. absolute are heated to 100 F. (a) What is the volume of the air? (&) What will be the final pressure? (c) How many B.T.U. will be required to heat it? 22. A tank contains 200 cu. ft. of air at 60 F. under a pressure of 50 Ibs. absolute, (a) Ho w many pounds of air in the tank ? (6) How many B.T.U. will be required to raise the temperature of the air in the tank to 100 F.? (c) What will be the pressure in the tank when the air has been heated to 100 F.? 23. A certain auditorium will seat 3000 people. If each person is supplied with 2000 cu. ft. of air per hour for ventilation, the outside temperature being F. and that in the hall being 70, how many pounds of air will be admitted per hour, and how many B.T.U. will be required to heat it? Weight of 1 cu. ft. of air at F. is .0863 Ibs.; at 70 is .075 Ibs. 24. A piece of iron weighing 5 Ibs. is heated to 212 F. and then dropped into a vessel containing 16.5 Ibs. of water at 60 F. If the temperature of the water is increased five degrees by the heat from the iron, what is the specific heat of the iron? \/ 25. How many foot-pounds of heat must be absorbed by 2 Ibs. of air in 36 HEAT ENGINES expanding to double its initial volume at constant temperature of 100 F.? 26. How many B.T.U. of work must be expended in compressing 3 Ibs. of air to one-fourth its initial volume at a constant temperature of 15 C.? 27. If 1 cu. ft. of air expands from a gage pressure of 4 atmospheres and a temperature of 60 F. to an absolute pressure of 1 atmosphere without the transmission of heat, find the final temperature. 28. An air compressor, the cross-section of which is 2 sq. ft., and stroke 3 ft., takes in air at 14 Ibs. absolute pressure and 60 F. and compresses it to 60 Ibs. gage pressure without the transmission of heat. Find the final temperature. 29. In problem 28, if the air at 60 Ibs. gage pressure and 70 F. expands adiabatically to a final pressure of 20 Ibs. gage, find the final temperature. 30. Two cubic feet of air at 60 F. and an initial pressure of 1 atmosphere absolute are compressed in a cylinder to 5 atmospheres gage pressure. If there be no transference of .heat, find the final temperature and volume. 31. A cylindrical vessel, the area of the base of which is 1 sq. ft., contains 2 cu. ft. of air at 60 F. when compressed by a frictionless piston weighing 2000 Ibs. resting upon it. Find the temperature and volume of the air if the vessel be inverted, there being no transmission of air or heat. 32. Given the quantity of air whose volume is 3 cu. ft. at 60 F. under a pressure of 45 Ibs. absolute, (a) Find the volume and temperature of this air after it has expanded adiabatically until its pressure is 15 Ibs. absolute. (b) What is the work done during the expansion? (c) What is the heat in B.T.U. converted into work? 33. Given a quantity of air whose volume is 2 cu. ft. at 60 F. under a pressure of 80 Ibs. absolute, (a) What is the weight of the air? (6) What will be the final temperature and pressure if the air be expanded adiabatically until its volume is 8 cu. ft.? (c) How much work will be done during this expansion? (d) How much work will be done if the air be expanded isother- mally until its volume is 8 cu. ft.? 34. Given a quantity of air whose volume is 2.2 cu. ft. at 80 F. under a pressure of 100 Ibs. absolute. It is made to pass through the following Carnot cycle: it is expanded isothermally until its volume is 4 cu. ft.; then expanded adiabatically until its temperature is 30 F.; then compressed isothermally; and finally it is compressed adiabatically until its volume, pres- sure, and absolute temperature are the same as at the beginning of the cycle. (a) Find the total heat added in B.T.U. (6) Find the total heat rejected in B.T.U. (c) Find the work done in foot-pounds during the cycle, (d) Find the efficiency of the cycle. 35. Given a quantity of air whose volume is 10 cu. ft. at 60 F. under a pressure of 20 Ibs. absolute. Heat is added at constant volume until its pres- sure is 200 Ibs. absolute; then added at constant pressure until its volume is 40 cu. ft.; then rejected at constant volume until its pressure is 20 Ibs. abso- lute; and then rejected at constant pressure until its volume is the same as at the beginning of the cycle, (a) Find temperature at end of first step. (6) Find temperature at end of second step, (c) Find temperature at end of third step, (d) Find total heat added in B.T.U. (e) Find total heat rejected in B.T.U. (/) Find work done in foot-pounds, (g) Find the efficiency of the cycle. ELEMENTARY THERMODYNAMICS 37 36. Given a quantity of air whose volume is 20 cu. ft. at 60 F. under a pressure of 20 Ibs. absolute. Heat is added at constant volume until its pressure is 200 Ibs. absolute; then the air is expanded adiabatically until its pressure is 20 Ibs. absolute; and then compressed at constant pressure until its volume is the same as at the beginning of the cycle, (a) Find tempera- ture at end of first step, (b) Find temperature at end of second step, (c) Find total heat added in B.T.U. (d) Find total heat rejected in B.T.U. 37. Given a quantity of air whose volume is 1 cu. ft. under a pressure of 100 Ibs. absolute. It is expanded under a constant pressure to 3 cu. ft. (a) What external work has been done during the expansion? (6) What heat has been added? 38. Two pounds of air occupying a volume of 6 cu. ft. under a pressure of 60 Ibs. absolute are expanded isothermally until the pressure is 20 Ibs. absolute, (a) What external work has been done during the expansion? (6) What heat has been added? 39. 1.3 cu. ft. of air under a pressure of 15 Ibs. absolute are heated at constant volume to 80 Ibs. absolute; then expanded adiabatically to a volume of 4.26 cu. ft. and a pressure of 15 Ibs. absolute; then compressed at a con- stant pressure to the original volume, (a) What is the total heat added in B.T.U.? (6) What is the Work done in foot-pounds? (c) What is the efficiency of the cycle? 40. Two cubic feet of air under a pressure of 15 Ibs. per square inch abso- lute are heated at constant volume to a pressure of 100 Ibs. per square inch absolute; then heated at constant pressure to a volume of 4 cu. ft.; then expanded to the original pressure; and finally compressed at constant pressure to the original volume. The expansion is pv 1 - 2 = a constant, (a) Find the heat added in B.T.U. (6) Find the heat rejected in B.T.U. (c) Find the work done in foot-pounds, (d) Find the efficiency of the cycle. ' (41, One pound of air is made to pass through the following cycle: it is expanded at constant pressure; then expanded isothermally; then com- pressed at constant pressure; and then compressed isothermally until the cycle is complete. Derive the expressions in terms of pressure and volume for, (a) the heat added in B.T.U.; (6) the heat rejected in B.T.U.; (c) the work done in foot-pounds; (d) the efficiency of the cycle. <* / / / yr WIA / /i/L/r ^ ^ ~T> /is -ri / f '* \ >^<&yvi/t tjv & // N 3-f fa ./ CHAPTER III PROPERTIES OF STEAM 28. Formation of Steam. In order to understand the opera- tion of a steam engine it is necessary to study the nature and properties of steam. Steam as produced in the ordinary boiler is a vapor, and often contains a certain amount of water in suspension, as does the atmosphere in foggy weather. Let us suppose that we have a boiler partly filled with cold water, and that heat is applied to the external shell of the boiler. As the water in the boiler is heated its temperature slowly rises. This increase of temperature continues from the initial tem- perature of the water until the temperature of the boiling point is reached, this latter temperature depending upon the pressure in the boiler. When the boiling point is reached small par- ticles of water are changed into steam. They rise* through the mass of water and escape to the surface. The water is then said "to boil." The temperature at which the water boils depends entirely on the pressure in the boiler. The steam produced from the boiling water is at the same temperature as the water, and under this condition the steam is said to be saturated. If we keep on applying heat to the water in the boiler, the pressure remaining the same, the temperature of the steam and the water will remain constant until all the water is evaporated. If more heat is added after all the water is converted into steam, the pressure still being kept unchanged, the temperature will rise. Steam under this condition is said to be superheated. In the formation of steam we divide the heat used into three different parts: (1) The heat which goes to raising the temperature of the water from its original temperature to the temperature of the boiling point, called "Heat of the Liquid." (2) The heat which goes to changing the water at the tem- perature of the boiling point into steam at the temperature of the boiling point, called "Latent Heat." (3) The heat which goes to changing the saturated steam at the temperature of the boiling point into steam at a higher 38 PROPERTIES OF STEAM 39 temperature but at the same pressure, called "Heat of Super- heat." 29. Dry Saturated Steam. Saturated steam always exists at the temperature of the boiling point corresponding to the pres- sure. If this saturated steam contains no moisture in the form of water, then it is said to be dry saturated steam, or, in other words, dry saturated steam is steam at the 'temperature of the boiling point and containing no water in suspension. Water so contained is often called entrained moisture. If heat is added to dry saturated steam, not in the presence of water it will become superheated. If heat is taken away from dry saturated steam it will become wet steam. Dry saturated steam is not a perfect gas, and the relation of pressure, volume, and temperature for such steam does not follow any simple law, but has been determined by experiment. The properties of dry saturated steam were originally deter- mined by Regnault between sixty and seventy years ago, and so carefully was his work done that no errors in his results were apparent until within very recent years, when the great diffi- culty in obtaining steam which is exactly dry and saturated became appreciated, and new experiments by various scientists proved that Regnault' s results were slightly high at some pres- sures and slightly low at others. The steam tables given in this book are based upon these recent experiments, and are probably correct to a fraction of 1 per cent. 30. Wet Steam. Wet steam is saturated steam which con- tains entrained moisture. When saturated steam is used in a steam engine, it almost always contains moisture in the form of water, so that the substance used by the engine as a working fluid is a mixture of steam and water. The steam and water in this case are at the same temperature. 31. Superheated Steam. Superheated steam is steam at a temperature higher than the temperature corresponding to the pressure of the boiling point at which it was formed. It is some- times called steam gas. If water were to be mixed with super- heated steam, this water would be evaporated as long as the steam remains superheated. Superheated steam at the same pressure as the boiling point at which it was produced can have any temperature higher than that of the boiling point. When raised to any considerable temperature above the temperature of the boiling point, it follows very closely the laws of a perfect 40 HEAT ENGINES gas, and may be treated as a perfect gas. The equation for superheated steam, considered as a perfect gas, is pv = 85.5 T, approximately. The specific heat of superheated steam is a variable and depends upon the pressure of the steam and the temperature to which the steam is superheated. For approximate calculations, the following values for the specific heat of superheated steam may be taken. TABLE VI. SPECIFIC HEATS OF SUPERHEATED STEAM Abs. press, in 14.7 25.0 50.0 75.0100.0125.0150.0175.0200.0225.0250.0275.0300.0 Ibs. per sq. in. Temp, of boil- ing point, F. 212.0 240.1 281.0 307.6 327.8 344.4 358.5 370.8381.9 391.9401.1 409.5417.5 Actual Temp. I of Steam. 250 47 48 275 .47 .48 I 300 47 a. 50 325 47 4.8 50 53 350 47 48 49 *> 9 55 58 375 .47 .47 .49 .51 .53 .56 .60 .66 400 .47 .47 .49 .50 .52 .54 .57 .60 .65 .72 425 .47 .47 .48 .50 .51 .52 .54 .56 .59 .63 .67 .74 .82 450 .47 .47 .48 .49 .50 .51 .52 .53 .55 .57 .60 .64 .67 475 .47 .47 .48 .49 .50 .50 .51 .52 .53 .54 .55 .56 .58 500 .47 .47 .48 .49 .49 .50 .50 .51 .52 .52 .53 .53 .54 525 .47 .47 .48 .48 .49 .49 .50 .50 .51 .51 .51 .52 .52 550 .47 .47 .48 .48 .49 .49 .50 .50 .50 .50 .51 .51 .52 600 .47 .47 .48 .48 .48 .49 .49 .49 .49 .50 .50 .50 .50 650 .47 .47 .48 .48 .48 .48 .49 .49. .49 .49 .50 .50 .50 700 .47 .47 .48 .48 .48 .48 .49 .49 .49 .49 .49 .50 .50 800 .48 .48 .48 .48 .48 .48 ! .48 .49 .49 .49 .49 .49 .49 When more accurate results are desired the value of specific heat should be taken from results given in Peabody's, or Marks and Davis's Steam Tables. The value of y for superheated steam is approximately 1.3. 32. Heat of the Liquid. The heat necessary to raise one pound of water from 32 to the temperature of the boiling point is called the heat of the liquid. This may be expressed numerically as follows: let c be the specific heat of the water, t the temperature of the boiling point, and h the heat of the liquid; then h = c (t - 32). (1) For approximate results c may be taken as 1, but where great accuracy is required the heat of the liquid should be taken from PROPERTIES OF STEAM 41 the steam tables as shown in Column 3. During this operation the change in the volume of the water is extremely small, and the amount of external work done may be neglected and all the heat of the liquid may be considered as going to increasing the heat energy of the water. 33. Latent Heat of Steam. When the water has reached the boiling point, more heat must be added to convert this water into steam. The heat necessary to convert one pound of water at the temperature of the boiling point into steam at the same tempera- lure is called the latent heat. We will denote the latent heat by L. Experiments show that the latent heat of steam diminishes as the pressure increases. When water is changed into steam, the volume is increased rapidly so that a considerable portion of the latent heat goes to external work. Let P equal the pressure at which the steam is formed; V equal the volume of the steam, and v equal the volume of the water: then the external work done equals P (V - v). (2) The volume of one pound of water under those conditions may be taken as approximately .017 cu. ft. At 212 the external work done in producing one pound of steam is equivalent to 73 heat units or about one-thirteenth of the latent heat. Experiments show that the latent heat of steam diminishes about .695 heat units for each degree the temperature of the boiling point is increased. If t be the temperature of the boiling point, then, approximately, L = 1072.6 - .695 (i - 32). (3) In condensing steam the same amount of heat is given up as was required to produce it. 34. Total Heat of Steam. The total heat of steam is the heat necessary to change one pound of water at 32 to one pound of steam at the temperature of the boiling point. The total heat of dry saturated steam will be designated by H. H = h + L. (4) The experimental results as given in the table for the value of the total heat may be approximated very closely by the formula H = 1072.6 + .305 (t - 32). (5) It is more accurate, however, to take the values of the total heat from the tables than it is to compute them from the formula given. 42 HEAT ENGINES If we let q represent the percentage of dry steam in a mixture of steam and water, then the latent heat in one pound of wet steam equals qL (6) and the total heat of one pound of wet steam equals h + qL. (7) 35. Steam Tables. The following table shows the properties of dry saturated steam. More complete tables will be found in Peabody's Steam Tables, Marks and Davis's Steam Tables, or in the Engineering Hand Books. Column 1 gives the ab- solute pressure of the steam in pounds per square inch. Column 2 gives the corresponding temperature of the steam in Fahren- heit degrees. Column 3 gives the heat of the liquid, or the heat necessary to raise one pound of water from 32 degrees to the boil- ing point corresponding to the pressure. Column 4 gives the latent heat, or the heat necessary to change a pound of water at the temperature of the boiling point into steam at the same temperature. Column 5 gives the total heat of the steam, and is the sum of the quantities in Column 3 and Column 4. Column 6 is the volume of one pound of steam at the different tempera- tures. Column 7 is the weight of one cubic foot of steam at the different temperatures. TABLE VII. PROPERTIES OF SATURATED STEAM ENGLISH UNITS ii. * *, M ii SS^ 1 5 in ^ d grc.s 2 si w Kg jc S _^ 5 fe VO II ?5 11^ l jyhi ils fift ^ o Q w In H OH <^ P * fc L /T i S P .0886 32 1072.6 1072.6 3301.0 .000303 .0886 .2562 60 28.1 1057.4 1085.5 1207.5 .000828 .2562 .5056 80 48.1 1046.6 1094.7 635.4 .001573 .5056 1 101.8 69.8 1034.6 1104.4 333.00 .00300 1 2 126.1 94.1 1021.4 1115.5 173.30 .00577 2 3 141.5 109.5 1012.3 1121.8 118.50 .00845 3 4 153.0 120.9 1005.6 1126.5 90.50 .01106 4 5 162.3 130.2 1000.2 1130.4 73.33 .01364 5 6 170.1 138.0 995.7 1133.7 61.89 .01616 6 7 173.8 144.8 991.7 1136.5 53.58 .01867 7 8 182.9 150.8 988.1 1138.9 47.27 .02115 8 PROPERTIES OF STEAM 43 PROPERTIES OF SATURATED STEAM Continued ENGLISH UNITS Abs. Pressure Pounds per Sq. In. 1 Temperature Degrees F. Heat of the Liquid Latent Heat of Evapora- tion S J-s * Density Pounds per Cu. Ft. Ahs. Pressure j Pounds per Sq. In. | P t h- I H V 3 P 9 188.3 156.3 984.8 1141.1 42.36 .023*61 9 10 193.2 161.2 981.8 1143.0 38.38 .02606 10 11 197.7 165.8 979.0 1144.8 35.10 .02849 11 12 202.0 170.0 976.4 1146.4 32.38 .03089 12 13 205.9 173.9 974.0 1147.9 30.04 .03329 13 14 209.6 177.6 971.7 1149.3 28.02 .03568 14 14.7 212.0 180.1 970.4 1150.4 26.79 .03733 14.7 15 213.0 181.1 969.5 1150.6 26.27 .03806 15 16 216.3 184.5 967.4 1151.9 24.77 .04042 16 17 219.4 187.7 965.4 1153.1 23.38 .04277 17 18 222.4 190.6 963.5 1154.1 22.16 .04512 18 19 225.2 193.5 961.6 1155.1 21.07 .04746 19 20 228.0 196.2 959.8 1156.0 20.08 .04980 20 21 230.6 198.9 958.0 1156.9 19.18 .05213 21 22 233.1 201.4 956.4 1157.8 18.37 .05445 22 23 235.5 203.9 954.8 1158.7 17.62 .05676 23 24 237.8 206.2 953.2 1159.4 16.93 .05907 24 25 240.1 208.5 951.7 1160.2 16.30 .0614 25 26 242.2 210.7 950.3 1161.0 15.71 .0636 26 27 244.4 212.8 948.9 1161.7 15.18 .0659 27 28 246.4 214.9 947.5 1162.4 14.67 .0682 28 29 248.4 217.0 946.1 1163.1 14.19 .0705 29 30 250.3 218.9 944.8 1163.7 13.74 .0728 30 31 252.2 220.8 943.5 1164.3 13.32 .0751 31 32 254.1 222.7 942.2 1164.9 12.93 .0773 32 33 255.8 224.5 941.0 1165.5 12.57 .0795 33 34 257.6 226.3 939.8 1166.1 12.22 .0818 34 35 259.3 228.0 938.6 1166.6 11.89 .0841 35 36 261.0 229.7 937.4 1167.1 11.58 .0863 36 37 262.6 231.4 936.3 1167.7 11.29 .0886 37 38 264.2 233.0 935.2 1168.2 11.01 .0908 38 39 265.8 234.6 934.1 1168.7 10.74 .0931 39 40 267.3 236.2 933.0 1169.2 10.49 .0953 40 41 268.7 237.7 931.9 1169.6 10.25 .0976 41 42 270.2 239.2 930.9 1170.1 10.02 .0998 42 43 271.7 240.6 929.9 1170.5 9.80 .1020 43 44 273.1 242.1 928.9 1171.0 9.59 .1043 44 45 274.5 243.5 927.9 1171.4 9.39 .1065 45 46 275.8 244.9 926.9 1171.8 9.20 .1087 46 44 HEAT ENGINES PROPERTIES OF SATURATED STEAM Continued ENGLISH 'UNITS Abs. Pressure Pounds per Sq. In. Temperature Degrees F. Heat of the Liquid Latent Heat of Evapora- tion Total Heat of Steam II^S cc g Density Pounds per Cu. Ft. ' Abs. Pressure Pounds per Sq. In. P t h L H V 1 I P 47 277.2 246.2 926.0 1172.2 9.02 .1109 47 48 278.5 247.6 925.0 1172.6 8.84 .1131 48 49 279.8 248.9 924.1 1173.0 8.67 .1153 49 50 281.0 250.2 923.2 1173.4 8.51 .1175 50 51 282.3 251.5 922.3 1173.8 8.35 .1197 51 52 283.5 252.8 921.4 1174.2 8.20 .1219 52 53 284.7 254.0 920.5 1174.5 8.05 .1241 53 54 285.9 255.2 919.6 1174.8 7.91 .1263 54 55 287.1 256.4 918.7 1175.1 7.78 .1285 55 56 288.2 257.6 917.9 1175.5 7.65 .1307 56 57 289.4 258.8 917.1 1175.9 7.52 .1329 57 58 290.5 259.9 916.2 1176.1 7.40 .1351 58 59 291.6 261.1 915.4 1176.5 7.28 .1373 59 60 292.7 262.2 914.6 1176.8 7.17 .1394 60 61 293.8 263.3 913.8 1177.1 7.06 .1416 61 62 294.9 264.4 913.0 1177.4 6.95 .1438 62 63 295.9 265.5 912.2 1177.7 6.85 .1460 ,63 64 297.0 266.5 911.5 1178.0 6.75 .1482 64 65 298.0 267.6 910.7 1178.3 6.65 .1503 65 66 299.0 268.6 910.0 1178.6 6.56 .1525 66 67 300.0 269.7 909.2 1178.9 6.47 .1547 67 68 301.0 270.7 908.4 1179.1 6.38 .1569 68 69 302.0 271.7 907.7 1179.4 6.29 .1591 69 70 302.9 272.7 906.9 1179.6 6.20 .1612 70 71 303.9 273.7 906.2 1179.9 6.12 .1634 71 72 304.8 274.6 905.5 1180.1 6.04 .1656 72 73 305.8 275.6 904.8 1180.4 5.96 .1678 73 74 306.7 276.6 904.1 1180.7 5.89 .1699 74 75 307.6 277.5 903.4 1180.9 5.81 .1721 75 76 308.5 278.5 902.7 1181.2 5.74 .1743 76 77 309.4 279.4 902.1 1181.5 5.67 .1764 77 78 310.3 280.3 901.4 1181.7 5.60 .1786 78 79 311.2 281.2 900.7 1181.9 5.54 .1808 79 80 312.0 282.1 900.1 1182.2 5.47 .1829 80 81 312.9 283.0 899.4 1182.4 5.41 .1851 81 82 313.8 283.8 898.8 1182.6 5.34 .1873 82 83 314.6 284.7 898.1 1182.8 5.28 .1894 83 84 315.4 285.6 897.5 1183.1 5.22 .1915 84 85 316.3 286.4 896.9 1183.3 5.16 ,1937 85 PROPERTIES OF STEAM 45 PROPERTIES OF SATURATED STEAM Continued ENGLISH UNITS Abs. Pressure Pounds per Sq. In. Temperature Degrees F. Heat of the Liquid Latent Heat of Evapora- tion "cS d l is s !i!h g.3g ftSa* Density Pounds per Cu. Ft. Abs. Pressure Pounds per Sq. In. P t h L H i V P 86 317.1 287.3 896.2 1183.5 5.10 .1959 86 87 317.9 288.1 895.6 1183.7 5.05 .1980 87 88 318.7 288.9 895.0 1183.9 5.00 .2002 88 89 319.5 289.8 894.3 1184.1 4.94 .2024 89 90 320.3 290.6 893.7 1184.3 4.89 .2045 90 91 321.1 291.4 893.1 1184.5 4.84 .2066 91 92 321.8 292.2 892.5 1184.7 4.79 .2088 92 93 322.6 293.0 891.9 1184.9 4.74 .2110 93 94 323.4 293.8 891.3 1185.1 4.69 .2131 94 95 324.1 294.5 890.7 1185.2 4.65 .2152 95 96 324.9 295.3 890.1 1185.4 4.60 .2173 96 97 325.6 296.1 889.5 1185.6 4.56 .2194 97 98 326.4 296.8 889.0 1185.8 4.51 .2215 98 99 327.1 297.6 888.4 1186.0 4.47 .2237 99 100 327.8 298.4 887.8 1186.2 4.430 .2257 100 101 328.6 299.1 887.2 1186.3 4.389 .2278 101 102 329.3 299.8 886.7 1186.5 4.349 .2299 102 103 330.0 300.6 886.1 1186.7 4.309 .2321 103 101 330.7 301.3 885.6 1186.9 4.270 .2342 104 105 331.4 302.0 885.0 1187.0 4.231 .2364 105 106 332.0 302.7 884.5 1187.2 4.193 .2385 106 107 332.7 303.4 883.9 1187.3 4.156 .2407 107 108 333.4 304.1 883.4 1187.5 4.119 .2428 108 109 334.1 304.8 882.8 1187.6 4.082 .2450 109 110 334.8 305.5 882.3 1187.8 4.047 .2472 110 111 335.4 306.2 881.8 1188.0 4.012 .2493 111 112 336.1 306.9 881.2 1188.1 3.977 .2514 112 113 336.8 307.6 880.7 1188.3 3.944 .2535 113 114 337.4 308.3 880.2 1188.5 3.911 .2557 114 114.7 337.9 308.8 879.8 1188.6 3.888 .2572 114.7 115 338.1 309.0 87.7 1188.7 3878 .2578 115 116 338.7 309.6 879.2 1188.8 3.846 .2600 116 117 339.4 310.3 878.7 1189.0 3.815 .2621 117 118 340.0 311.0 878.2 1189.2 3.784 .2642 118 119 340.6 311.7 877.6 1189.3 3.754 .2663 119 120 341.3 312.3 877.1 1189.4 3.725 .2684 120 121 341.9 313.0 876.6 1189.6 3.696 .2706 121 122 342.5 313.6 876.1 1189.7 3.667 .2727 122 123 343.2 314.3 875.6 1189.9 3.638 .2749 123 46 HEAT ENGINES PROPERTIES OF SATURATED STEAM Continued ENGLISH UNITS Abs. Pressure Pounds per Sq. In. Temperature Degrees F. 0> J3 ^"O n r Latent Heat of Evapora- tion Total Heat of Steam B o Density Pounds per Cu. Ft. Abs. Pressure Pounds per Sq. In. P t h L H V i V P 124 343.8 314.9 875.1 1190.0 3.610 .2770 124 125 344.4 315.5 874.6 1190.1 3.582 .2792 125 126 345.0 316.2 874.1 1190.3 3.555 .2813 126 127 345.6 316.8 873.7 1190.5 3.529 .2834 127 128 346.2 317.4 873.2 1190.6 3.5^3 .2855 128 129 346.8 318.0 872.7 1190.7 3.477 .2876 129 130 347.4 318.6 872.2 1190.8 3.452 .2897 130 131 348.0 319.3 871.7 1191.0 3.427 .2918 131 132 348.5 319.9 871.2 1191.1 3.402 .2939 132 133 349.1 320.5 870.8 1191.3 3.378 .2960 133 134 349.7 321.0 870.4 1191.4 3.354 .2981 134 135 350.3 321.6 869.9 1191.5 3.331 .3002 135 136 350.8 322.2 869.4 1191.6 3.308 .3023 136 137 351.4 322.8 868.9 1191.7 3.285 .3044 137 138 352.0 323.4 868.4 1191.8 3.263 .3065 138 139 352.5 324.0 868.0 1192.0 3.241 .3086 139 140 353.1 324.5 867.6 1192.1 3.219 .3107 140 141 353.6 325.1 867.1 1192.2 3.198 .3128 141 142 354.2 325.7 866.6 1192.3 3.176 .3149 142 143 354.7 326.3 866.2 1192.5 3.155 .3170 143 144 355.3 326.8 865.8 1192.6 3.134 .3191 144 145 355.8 327.4 865.3 1192.7 3.113 .3212 145 146 356.3 327.9 864.9 1192.8 3.093 .3233 146 147 356.9 328.5 864.4 1192.9 3.073 .3254 147 148 357.4 329.0 864.0 1193.0 3.053 .3275 148 149 357.9 329.6 863.5 1193.1 3.033 .3297 149 150 358.5 330.1 863.1 1193.2 3.013 .3319 150 152 359.5 331.2 862.3 1193.5 2.975 .3361 152 154 360.5 332.3 861.4 1193.7 2.939 .3403 154 156 361.6 333.4 860.5 1193.9 2.903 .3445 156 158 362.6 334.4 859.7 1194.1 2.868 .3487 158 160 363.6 335.5 858.8 1194.3 2.834 .3529 160 162 364.6 336.6 858.0 1194.6 2.801 .3570 162 164 365.6 337.6 857.2 1194.8 2.768 .3613 164 166 366.5 338.6 856.4 1195.0 2.736 .3655 166 168 367.5 339.6 855.5 1195.1 2.705 .3697 168 170 368.5 340.6 854.7 1195.3 2.674 .3739 170 172 369.4 341.6 853.9 1195.5 2.644 .3782 172 174 370.4 342.5 853.1 1195.6 2.615 .3824 174 176 371.3 343.5 852.3 1195.8 2.587 .3865 176 PROPERTIES OF STEAM 47 PROPERTIES OF SATURATED STEAM Concluded ENGLISH UNITS Abs. Pressure Pounds per Sq. In. Temperature Degrees F. Heat of the Liquid Latent Heat of Evapoi-a- tion Total Heat of Steam nh li*| >3*< o Density Pounds per Cu. Ft. Abs. Pressure Pounds per Sq. In. P t h L H V 1 . P 178 372.2 344.5 851.5 1196.0 2.560 .3907 178 180 373.1 345.4 850.8 1196.2 2.532 .3949 180 182 374.0 346.4 850.0 1196.4 2.506 .3990 182 184 374.9 347.4 849.3 1196.7 2.480 .4032 184 186 375.8 348.3 848.5 1196.8 2.455 .4074 186 188 376.7 349.2 847.7 1196.9 2.430 .4115 188 190 377.6 350.1 847.0 1197.1 2.406 .4157 190 192 378.5 351.0 846.2 1197.2 2.381 .4200 192 194 379.3 351.9 845.5 1197.4 2.358 .4242 194 196 380.2 352.8 844.8 1197.6 2.335 .4284 196 198 381.0 353.7 844.0 1197.7 2.312 .4326 198 200 381.9 354.6 843.3 1197.9 2.289 .4370 200 202 382.7 355.5 842.6 1198.1 2.268 .4411 202 204 383.5 356.4 841.9 1198.3 2.246 .4452 204 206 384.4 357.2 841.2 1198.4 2.226 .4493 206 208 385.2 358.1 840.5 1198.6 2.206 .4534 208 210 386.0 358.9 839.8 1198.7 2.186 .4575 210 212 386.8 359.8 839.1 1198.9 2.166 .4618 212 214 387.6 360.6 838.4 1199.0 2.147 .4660 214 216 388.4 361.4 8%7.7 1199.1 2.127 .4700 216 218 389.1 362.3 837.0 1199.3 2.108 .4744 218 220 389.9 363.1 836.4 1199.5 2.090 .4787 220 222 390.7 363.9 835.7 1199.6 2.072 .4829 222 224 391.5 364.7 835.0 1199.7 2.054 .4870 224 226 392.2 365.5 834.3 1199.8 2.037 .4910 226 228 393.0 366.3 833.7 1200.0 2.020 .4950 228 230 393.8 367.1 833.0 1200.1 2.003 .4992 230 232 394.5 367.9 832.3 1200.2 .987 .503 232 234 395.2 368.6 831.7 1200.3 .970 .507 234 236 396.0 369.4 831.0 1200.4 .954 .511 236 238 396.7 370.2 830.4 1200.6 .938 .516 238 240 397.4 371.0 829.8 1200.8 .923 .520 240 242 398.2 371.7 829.2 1200.9 .907 .524 242 244 398.9 372.5 828.5 1201.0 .892 .528 244 246 399.6 373.3 827.8 1201.1 .877 .532 246 248 400.3 374.0 827.2 1201.2 .862 .537 248 250 401.1 374.7 826.6 1201.3 1.848 .541 250 _ 275 409.6 383.7 819.0 1202.7 1.684 .594 275 ~ 300 417.5 392.0 811.8 1203.8 1.547 .647 300 350 431.9 407.4 798.5 1205.9 1.330 .750 350 CHAPTER IV CALORIMETERS AND MECHANICAL MIXTURES 36. Calorimeters. As we have already seen, steam may be either wet, dry and saturated, or superheated. By "quality" of steam is meant the per cent, of dry and saturated steam in the sample. The percent of moisture in the steam is found by subtracting the quality from 100 per cent. The quality is determined by means of a Calorimeter. There are two classes of these instruments in general use at the present time, the Separating Calorimeter and the Throttling Calor- imeter. In each of these classes there are several types or makes, but it will suffice to describe only one or two of each. As will be seen in Paragraph 39, the American Society of Mechanical Engineers recommend the use of a sampling nozzle, or calorimeter nipple, in connection with the calorimeter. This nipple is a piece of pipe extending nearly across the steam main, as shown in Fig. 9, with a cap on the end and a series of J-inch holes along and around its cylindrical surface. As the steam to be tested must enter the calorimeter through this nipple, a fair sample of the steam is insured. The sampling nozzle should be inserted in the steam main at a point where the entrained mois- ture is likely to be most thoroughly mixed. 37. Separating Calorimeters. The weight of the dry steam that will pass through a given size of orifice in a given time depends upon the pressure on the two sides of the orifice. If A is the area of the orifice in square inches, P the absolute pressure in pounds per square inch, and W the pounds of steam passing through the orifice into the atmosphere per second, then PA W=- 7 Q (Napier's Rule). (1) From Napier's Rule the weight of steam flowing through an orifice of known area is proportional to the absolute steam pres- sure. This law holds true until the lower pressure equals or exceeds .6 of the higher pressure. 48 CALORIMETERS AND MECHANICAL MIXTURES 49 The amount of steam flowing through any orifice may, there- fore, be determined. Professor R. C. Carpenter has a calorim- eter based upon this principle. Wet steam enters the calorim- eter, Fig. 7,through the pipe 6,,and is projected against the cup 14. The steam and water are then turned through an angle of 180, which causes the water to be thrown outward by cen- trifugal force through the meshes in the cup into the inner cham- ber 3. Causing the steam to strike the cup, instead of flowing directly into the chamber 3, prevents any moisture already thrown out being picked up again and carried on. The steam after leaving the cup passes upward and enters the top of the outer chamber 7. It then flows down around the inner chamber in the annular space 4, and is discharged through the ori- fice 8. The area of this orifice, which is known, is so small that there is no loss in pressure of the steam as it flows through the calorimeter. The pres- sure in the two chambers being the same, the temperature is the same, and there is no loss of heat from the inner chamber by radiation. The gage glass 12, connected with the in- ner chamber, is graduated in hun- dredths of pounds, so that the weight of moisture separated from the steam -can be read directty. The gage 9 is so calibrated as to read directly the FIG. 7. Carpenter's improved separating calorimeter. number of pounds flowing through the orifice 8 in a given time (generally ten minutes). These readings are not proportional to the pressure readings on the gage, which has two scales, for the latter readings are proportional to the pressures above the atmosphere and not to the absolute pressures. The accuracy of the results obtained by \ising the gage may be checked at any time by condensing and weighing the discharge from ori- fice 8 for a given period of time. If now we call w the weight of dry steam discharged from the orifice 8 in any given period of time, W the weight of mois- 50 HEAT ENGINES ture collected in 3 in the same period of time, and q the quality of the steam, then w may be obtained either from the reading of the gage 9, or by actually weighing the steam, and W is found by taking the difference between the readings on scale 12 at the beginning and end of the test. 38. Throttling Calorimeter. This form of calorimeter was invented by Prof. C. H. Peabody, and is the form recommended FIG. 8. Carpenter's throttling calorimeter. by the A.S.M.E. Committee on Standards (see paragraph 39 below). It is the most accurate form of calorimeter where it can be used, but is unsuitable for use in determining the quality of the steam if the steam contains over 3 or 4 per cent, of mois- ture, or if the temperature of the lower thermometer is below 225, which will be the case if the steam is at a very low pres- sure (below 5 or 6 Ibs. gage). The principle of its operation is as follows: a pound of saturated steam at a high pressure contains more heat than a pound of saturated steam at a lower pressure. If steam at a high pressure pass through an orifice into a space at a lower pressure without doing any external work, some of this heat must be given up, and as the only object that can absorb heat is the steam itself, it takes up this heat. If this steam contained some moisture at the higher pressure, part of the heat liberated when the pres- CALORIMETERS AND MECHANICAL MIXTURES 51 sure is lowered will go to evaporating this moisture, and the excess will go to superheating the steam. Let q = the quality of the steam. ti = the temperature of the wet steam before passing through trie orifice. Pi = the absolute pressure of the wet steam in the main. fa = the temperature corresponding to the absolute pressure on the low-pressure side of the orifice. i sup- = the temperature of the steam as shown by the thermom- eter on the low-pressure side of the orifice. hi and Z/i = heat of liquid and latent heat corresponding to the temperature fa, or the absolute pressure p\. h 2 and L 2 = heat of liquid and latent heat corresponding to the temperature fa. The heat contained in 1 Ib. of the mixture of steam and water at temperature fa, or pressure pi, would be hi + qLi. The heat contained in 1 Ib. of the steam on the low-pressure side of the orifice after expansion would be A 2 + L z + c p (t sup . - fa) = H 2 + c p (t sup . - fa), where c p is the specific heat of superheated steam. But since the heat in a pound of the substance must be the same on one side of the orifice as it is on the other, hi + qLi = H 2 + c p (t sup . - fa). (3) Solving for #, H 2 + c p (t sup . - fa) - hi q= Li The percentage of moisture equals 1 q. (5) Ordinarily fa is found from the tables by looking up the temperature corresponding to the absolute pressure in the calo- rimeter, i.e., the sum of the atmospheric pressure and the pres- sure shown by the manometer. This practice, however, is not permitted by the A.S.M.E. rules for finding the quality of steam, since t sup . is taken with a thermometer that has part of 52 HEAT ENGINES its stem exposed, and is thus subject to radiation,* nor does it take account of the radiation from the calorimeter itself, which may be considerable even though well covered. Therefore for accurate work it is necessary that we take a "normal reading" of the thermometer, as described in paragraph 39 to. correct for these errors. The calorimeter shown in Fig. 9 differs from- the one shown in Fig. 8 in that the temperature of the steam being admitted to the calorimeter is observed instead of the pressure. In other f FIG. 9. Barms' throttling calorimeter. words, hi and LI correspond to the temperature ti rather than to the absolute pressure p\. Another difference is that in the Barrus calorimeter, the exhaust is made very free and the pres- sure, pz, on the lower side of the orifice is assumed to be atmos- pheric. A long exhaust pipe will cause a back pressure in the calorimeter where we have assumed the pressure to be atmospheric. In case the atmospheric pressure is not known, it can be *When a considerable portion of the mercury column of a thermometer measuring high temperatures is exposed to the air, a correction K must be added to the readings to obtain the true temperature. Let t = the observed reading of the thermometer. t' = the temperature of the air surrounding the exposed stem of the thermometer. D = number of degrees on the scale from the surface of the liquid in the thermometer cup to the upper end of the mercury column. Then K = .000088 D (t t') , in Fahrenheit degrees. CALORIMETERS AND MECHANICAL MIXTURES 53 assumed as 14.7 Ibs. per square inch. If the barometer reading is given, however, it should always be used. This reading, as well as that of the manometer giving the pressure in the calo- rimeter, will be given in inches of mercury. To change this to pounds per square inch, multiply the inches of mercury by .491. 39. Quality of Steam. The following are the standard rules for finding the quality of steam as adopted by the A.S.M.E., and published in the Transactions of that society, Vol. 21, p. 43, and Vol. 24, p. 740: " The percentage of moisture in the steam should be determined by the use of either a throttling or a separating steam calorimeter. The sam- pling nozzle should be placed in the vertical steam pipe rising from the boiler. It should be made of |-in. pipe, and should extend across the diameter of the steam pipe to within half an inch of the opposite side, being closed at the end and perforated with not less than twenty f-in. holes equally distributed along and around its cylindrical surface, but none of these holes should be nearer than \ in. to the inner side of the steam pipe. The calorimeter and the pipe leading to it should be well covered with felting. Whenever the indications of the throttling or separating calorimeter show that the percentage of moisture is irregular, or occasionally in excess of 3 per cent., the results should be checked by a steam separator placed in the steam pipe as close to the boiler as convenient, with a calorimeter in the steam pipe just beyond the outlet from the separator. The drip from the separator should be caught and weighed and the percentage of moisture computed therefrom added to that shown by the calorimeter. "Superheating should be determined by means of a 'thermometer placed in a mercury-well inserted in the steam pipe. The degree of superheating should be taken as the difference between the reading of the thermometer for superheated, steam and the readings of the same thermometer for saturated steam at the same pressure, as determined by a special experiment, and not by reference to steam tables." "If it is necessary to attach the calorimeter to a horizontal section of pipe, and it is important to determine the quantity of moisture accu- rately, a sampling nozzle should be used which has no perforations, and which passes through a stuffing-box applied to the bottom of the pipe so that it can be adjusted up and down, and thereby draw a sample at different points ranging from the top to the bottom. "To determine the 'normal reading' of the calorimeter, the instrument should be attached to a horizontal steam pipe in such a way that the nozzle projects upward to near the top of the pipe, there being no per- forations and the steam entering through the open end. The test should be made when the steam in the pipe is in a quiescent state, and when the 54 HEAT ENGINES steam pressure is constant. If the steam pressure falls during the time when the observations are being made, the test should be continued long enough to obtain the effect of an equivalent rise of pressure. When the normal reading has been obtained, the constant to be used in determining the percentage of moisture is the latent heat of the steam at the observed pressure divided by the specific heat of superheated steam at atmospheric pressure, which is forty-six hundredths (.46). To ascertain this percentage, divide the number of degrees of cooling by the constant, and multiply by 100. "To determine the quantity of steam used by the calorimeter in an instrument where the steam is passed through an orifice under a given pressure, it is usually accurate enough to calculate the quantity from the area of the orifice and the absolute pressure. If it is desired to determine the quantity exactly, a steam hose may be attached to the outlet of the calorimeter, and carried to a barrel of water placed on a platform scale. The steam is condensed for a certain time, and its weight determined, and thereby the quantity discharged per hour." Example. Steam at 100 Ibs. pressure blows through a throttling calorimeter. The temperature of the lower thermometer is 275 and the manometer reading is 5.6 in. of mercury. Barometer reading 29 in. Find the quality of the steam. Solution. First find the atmospheric pressure and the pressure in the calorimeter. Atmospheric pressure = .491 X 29 = 14.25 Ibs. Pressure in calorimeter = .491 X 5.6 = 2.75 Ibs. Now from the steam tables find hi and LI corresponding to the pressure in the main, 114.25 Ibs. absolute, and also H z and 2 corresponding to the pressure in the calorimeter, 17 Ibs. absolute. Then from equation (4), H z + c p (t sup . - 2 ) - hi 1153.1 + .46 (275 - 219.4) - 308.5 RQ a = .988. 880.1 1153.1 + .46 X 55.6 - 308.8 869.6 880.1 880.1 Answer: 98.8 per cent. Example. (a) Find the quality of the steam in the preceding problem as shown by a separating calorimeter, if the data is as follows : weight of dry steam escaping through orifice, 4.5 Ibs.; weight of moisture col- lected, .05 Ibs. (6) Find the diameter of the orifice if the length of the run is 20 minutes. CALORIMETERS AND MECHANICAL MIXTURES 55 Solution. (a) From equation (2), w q ~ w + W = 4^+^05 = 4.55 = - 988 ' (6) Find weight of steam flowing through orifice per second, and call it w'. Then 45 45 w ' = 6n"<7n = ^ = -00375 Ibs . Zi\J X OU From equation (1), PA W = 70 70 X .OOS^ 1 -26250 P 1004- .49O<29 114.25 gV ( . *r* = .0023 %V^*> & * ^-r P r "--S"- 000732 * r = .027 d = .054 Answer: (a) 98.8 per cent (6) .054 in. PROBLEMS 1. Steam at 100 Ibs. pressure passes through a Barrus calorimeter. Tem- perature after passing through orifice is 246. What is the quality of the steam? Q^ni (2) Steam at 110 Ibalbliws through an orifice into the atmosphere. The temperature of the steam after passing through this orifice is 240. What per cent, of moisture is in the original steam? 3. One pound of a mixture of steam and water containing 2 per cent, moisture at 150 Ibs. absolute pressure expands through an orifice to 15 Ibs. absolute pressure. What will be the temperature at the lower pressure? ^ 4. Steam at a pressure of 100 Ibs. and a quality of 98 per cent, blows through an orifice to 15 Ibs. absolute. What will be its temperature? ^ vJ ^ eam a ^ 95 Ibs. pressure containing 2| per cent, moisture blows through an orifice into a chamber where the pressure is 8.2 in. of mercury above the atmosphere. What is the temperature of the steam after passing through the orifice? Barometer, 29.8 in. 6. Find the quality of the steam if, when tested with a separating calor- imeter, 4.5 Ibs. of dry steam blow through the orifice while 1.5 Ibs. of mois- 56 HEAT ENGINES ture are separated out. If the run is thirty minutes long and the steam pressure is 100 Ibs., determine the diameter of the orifice. 7. Steam at 10 Ibs. pressure blows through a separating calorimeter. The run is forty-five minutes long, 10.5 Ibs. of dry steam flow through the orifice and .5 Ibs. of moisture-are collected. Find the quality of the steam and the area of the orifice. 40. Mechanical Mixtures. Problems involving the resulting temperature and final condition when various substances are mixed mechanically are often met with. They are best treated by first determining the heat in B.T.U. that would be available for use if the temperature of all the substances were brought to 32 F., and then using this heat (positive or negative) to raise (or lower) the total weight of mixture to its final temperature and condition. Another method of solving mixture problems is by equating the heat absorbed to the heat rejected and letting z. represent the resulting temperature. It is often difficult to decide upon which side of the equation a material should be placed. In such a case a trial calculation should be made, and the tempera- ture determined by this trial will settle this question. In the mixture of substances which pass through a change of state during the mixture process, it is almost necessary to make a trial calculation. Take, for example, the mixing of steam with other substances. The steam may all be condensed and the re- sulting water cooled also; the steam may be condensed only; or the steam may be only partially condensed. The equations in each case would be different. If one pound of dry saturated steam at a temperature ti is condensed and then the temperature of the condensed steam is lowered to a temperature 2 , the amount of heat H r given off would be H' = Li + c(t, - t z ). (6) where LI is the latent heat corresponding to the temperature t\. If the steam was condensed only, the heat given off would be H' = Li (7) and the temperature of the mixture is the temperature corre- sponding to the pressure. If the steam is only partly condensed, let q equal the per cent, of steam condensed. Then H' = qLi (8) and the temperature of the mixture is the temperature corre- sponding to the pressure. CALORIMETERS AND MECHANICAL MIXTURES 57 The general laws of thermodynamics do not apply in the case of mixtures as the equations become discontinuous. The general expression for heat absorbed in passing from a solid to a gaseous state may be stated as follows: Let Ci be the specific heat in the solid, c 2 in the liquid and c 3 in the gaseous state, w the weight of the substance, t the initial temperature, t\ the temperature of the melting point, t 2 the temperature of the boiling point, t 3 the final temperature, H f heat of liquefaction, and L heat of vaporization. H r = w[d (t, - t) + H f + c 2 (t z - O + L + c 3 (h - fe)] (9) TABLE VIII. SPECIFIC HEATS OF LIQUIDS AND SOLIDS Substances Specific heat, c. Mercury .0333 Alcohol .615 Turpentine Wrought iron .462 .114 Cast iron .129 Copper Ice .095 504 Spermaceti Sulphur .320 .177 Glass Graphite .187 .200 Latent heat of fusion of ice = 144 B.T.U. Example. Find the final temperature and condition of the mixture after mixing 10 Ibs. of ice at 20; 20 Ibs. of water at 50, and 2 Ibs. of steam at atmospheric pressure. Mixture takes place at the pressure of the steam. Solution. First Method Heat to raise ice to 32 = 10 X .5 (32 - 20) 60 Heat to melt ice = 10 X 144 = 1440 Total heat necessary to change the ice to water at 32 = 1500 B.T.U. Heat given up by water when temperature is lowered to 32 = 20 X (50 - 32) = 360 Heat in steam above 32 (from tables) = 2X1150.4 =2300.8 Total heat given up in lowering water and steam to 32 = 2660.8 B.T.U. Heat available for use = 2660.8 - 1500 = 1160.8 B.T.U. 58 HEAT ENGINES 1160 8 Degrees this heat will raise the mixture = 36.3. oZ .'. final temperature of mixture = 36.3 -f 32 = 68.3 F. Ans. 32 Ibs. water at 68.3 F. Second Method Assume that the steam is all condensed and that the temperature of the mixture is t. Then the heat necessary to raise the ice to the melting point equals 10 X .5(32 - 20). The heat necessary to melt the ice equals 10 X 144; the heat necessary to raise he melted ice to the temperature of the mixture equals 10( 32); the heat necessary to raise the water to the temperature of the mixture equals 20 (t 50) ; the heat given up by the steam in chang- ing to water at the temperature of the boiling point equals 2 X 970.4, and the heat given up by the condensed steam when its temperature is lowered to the temperature of the mixture equals 2 (212 t). Combining the preceding parts into one equation, we have 10 X .5 (32 - 20) + 10 X 144 + 10 (t - 32) + 20 (t - 50) = 2 X 970.4 + 2 (212 - t) 60 '+ 1440 + Wt - 320 + 20* - 1000 = 1940.8 + 424 - 2t 32 = 2184.8 t = 68.3 Since t is less than the temperature of the boiling point corresponding to the pressure at which the mixture takes place, all the steam is con- densed. Ans. 32 Ibs. water at 68.3 F. Example. Find the resulting temperature and condition after mixing 10 Ibs. of ice at 20, 20 Ibs. of water at 50, 40 Ibs. of air at 82, and 20 Ibs. of steam at 100 Ibs. pressure and containing 2 per cent, moisture. Mixture takes place at the pressure of the steam. Solution. First Method 10 X .5(32 - 20) = 60 10 X 144 = 1440 1500 B.T.U. = heat to raise ice to water at 32. CALORIMETERS AND MECHANICAL MIXTURES 59 20 X (50 - 32) 360 40 X .2375(82 - 32) 475 20(308.8 +.98 X 879.8) = 23420 24255 B.T.U. = heat given up by air, 1500 water, and steam. 22755 B.T.U. = heat available. 40 X .2375(337.9 - 32) = 2905 B.T.U. = heat to raise air to,337.9. 19850 B.T.U. = heat available to raise the water. 50 X 308.8 = 15440 B.T.U. = heat to raise water to 337.9. 4410 B.T.U. = heat available to evapo- rate water. 4410 oTft o = 5.01 Iks. steam, o/y . o Ans. 40 Ibs. air 44 .99 Ibs. water > at 337 .9. 5.01 Ibs dry saturated steam J Second Method Assume the steam to be all condensed and let the temperature of the mixture be t. Equating the heat gained by the ice, water and air, and the heat lost by the steam, we have 10 X .5(32 - 20) + 10 X 144 + 10( - 32) + 20(* - 50) + 40 X .2375(1 - 82) = 20 X .98 X 879.8 + 20(337.9 - 60 + 1440 + IQt - 320 + 20* - 1000 + 9.5Z - 779 = 17250 + 6758 - 20t 59.5* = 24670 t = 413. 6 F. This result is of course absurd, as the temperature of the mixture cannot be higher than the temperature of the boiling point corresponding to the pressure at which the mixture takes place. Therefore our assumption that all the steam is condensed must be wrong, and we know that part of it remains in the form of steam, and hence the temperature of the mix- ture is equal to the temperature of the boiling point corresponding to the pressure at which the substances are mixed. Then substituting for t its value, and letting x represent the number of pounds of steam condensed, we have 10 X .5(32 - 20) + 10 X 144 + 10(337.9 - 32) + 20(337.9 - 50) + 40 X .2375(337.9 - 82) = 879. 8* 60 HEAT ENGINES 60 + 1440 + 3059 + 5758 + 2431 = 879. 8x 879. 8 x = 12748 x = 14.49 Ibs. condensed. 20 X .98 = 19.6 Ibs. = original weight of dry steam. Ans. 40 Ibs. air ) 10 + 20 + (20 - 19.6) + 14.49 = 44.89 Ibs. water [ at 337.9. 19.6 - 14.49 = 5.11 Ibs. dry saturated steam The difference between the results obtained in these two methods of working this problem is due to the fact that in the first method we took account of the variation in the specific heat of water by using the heat of the liquid, h, from the tables, in place of (t 32) wherever possible, while in the second method we assumed this specific heat to be constant and equal to 1. Example. Find the resulting temperature and condition after mixing 10 Ibs. of ice at 20, 20 Ibs. of water at 50, and 30 Ibs. of steam at 100 Ibs. pressure and 400 temperature. Mixture takes place at 25 Ibs. pressure. Solution. First Method 10 X .5(32 - 20) = 60 10 X 144 = 1440 1500 B.T.U. = heat to raise ice to water at 32. 20 X (50 - 32) = 360 30 X .57(400 - 337.9) = 1062 30 X 1188.6 35658 37080 B.T.U. = heat given up by water and steam. 1500 35580 B.T.U. = heat available. 60 X 235.7 = 14142 B.T.U. = heat to raise water to 266.8. 21438 B.T.U. = heat available to evaporate water. 21438 22 . 97 Ibs. steam ' at 266. 8 F. 933.3 Ans. 37 . 03 Ibs. water 22 . 97 Ibs. dry saturated steam Second Method Assume the steam to be all condensed and let the temperature of the mixture be t. Then CALORIMETERS AND MECHANICAL MIXTURES 61 10 X .5(32 - 20) + 10 X 144 + 10(* - 32) + 20(J - 50) = 30 X .57(400 - 337.9) + 30 X 879.8 + 30(337.9 - 60 + 1440 + 10* - 320 + 20 - 1000 = 1062 + 26394 + 10137 - 60 t = 37413 t = 623.6 This result is, of course, impossible and we see at once that only part of the steam is condensed, and that the temperature of the mixture must be that of the boiling point corresponding to the pressure at which the mixture takes place. This problem differs from the previous ones in that the pressure of the mixture is different from the original steam pressure, and we must pro- ceed in a slightly different manner. Assume for the moment that the steam has all been condensed and that we have 60 Ibs. of water at 623 . 6 F. Then assume that the temperature of the water is dropped to the temperature of the boiling point (266.8) corresponding to the pressure (25 Ibs.) at which the mixture is made. Each pound will give up, approximately (623 .6 - 266 . 8) B.T.U. This heat can then be used to re-evaporate part of the water. Therefore since the latent heat corresponding to 25 Ibs. is 933 . 3, we have 60(623,6-266.8) 60X356.8 21408 : *)33.3 933.3 = 933.3 = 22. 94 Ibs. re-evaporated. VY ^ An*. 37.06 Ibs. water \ 1 ^2.94 Ibs. dry saturated steam / PROBLEMS 1. Required the temperature after mixing 3 Ibs. of water at 100 F., 10 ll>s. of alcohol at 40 F., and 20 Ibs. of mercury at 60 F. 2. Required the temperature and condition after mixing 5 Ibs. of ice at 10 F. with 12 Ibs. of water at 60 F. 3. Required the temperature and condition after mixing 10 Ibs. of ice at 15 F. with 1 Ib. of steam at 212 F. 4. Required the temperature and condition of the mixture after mixing 5 Ibs. of steam at 212 F. with 20 Ibs. of water at 60 F. One pound of ice at 32 is mixed with 10 Ibs. of water at 50 and 20 Ibs. of steam at 212. What is the temperature and condition of the resulting mixture? 6. Ten pounds of steam at 212 are mixed with 50 Ibs. of water at 60 and 2 Ibs. of ice at 32. What will be the resulting temperature and condi- tion of the mixture? 7. Ten pounds of steam at atmospheric pressure, 5 Ibs. of water at 50 and 10 Ibs. of ice at 32 are mixed together, (a) What will be the resulting JV *\temperature of the mixture? (6) What will the condition of the mixture V be? (c) If the steam is not all condensed, determine what per cent, of the steam will be condensed. : 62 HEAT ENGINES -7- 8. Five pounds of steam at atmospheric pressure, 10 Ibs. of water at 60, and 2 Ibs. of ice at 20 are mixed at atmospheric pressure. What will be the resulting temperature? 9. Ten pounds of ice at 10, 20 Ibs. of water at 60 and 5 Ibs. of steam at atmospheric pressure are mixed at atmospheric pressure. Find the resulting temperature and condition of the mixture. Y- 10. Twenty pounds of steam at atmospheric pressure, 10 Ibs. of water at 60 and 50 Ibs. of air at 100 are mixed together at the pressure of the steam, (a) What will be the resulting temperature? (6) If the steam is not all condensed, determine what per cent, of the steam will be condensed. 11. A mixture is made of 10 Ibs. of steam at atmospheric pressure, 5 Ibs. of ice at 20, 10 Ibs. of water at 50, 30 Ibs. of air at 60. (a) What will be the temperature of the resulting mixture? (6) What will be the percentage by weight of air, steam, and water in the mixture? 12. What would be the resulting temperature and condition of a mixture of 10 Ibs. of water at 40, 20 Ibs. of water at 60, and 8 Ibs. of steam at 5 Ibs. pressure? Mixture takes place at 5 Ibs. pressure. 13. Ten pounds of steam at 5 Ibs. pressure, 1 Ib. of ice at 32, and 20 Ibs. of water at 60 are mixed at 5 Ibs. pressure. What will be the temperature and condition of the resulting mixture? ,4 14. Five pounds of ice at 5, 10 Ibs. of water at 50,20 Ibs. of air at80, and 5 Ibs. of steam at 20 Ibs. pressure are mixed at the pressure of the steam. Find the resulting temperature and condition of the mixture. 15. Required the temperature and condition of the mixture after mixing 10 Ibs. of steam at a pressure of 30 Ibs. absolute and a temperature of 250.3 F., 2 Ibs. of ice at 10 F., and 20 Ibs. of water at 40 F. Mixture takes place at the pressure of the steam. 16. Fifty pounds of air at 100, 10 Ibs. of steam at atmospheric pressure, and 10 Ibs. of water at 60 are mixed at atmospheric pressure. What is the temperature of the mixture and how much steam is condensed? 17. Required the final temperature and condition after mixing at the pressure of the air 100 Ibs. of air at a temperature of 500 and a pressure of 100 Ibs. absolute, and 2 Ibs. of steam at 100 Ibs. absolute having a quality of 98 per cent. )~^is. Five pounds of steam at 5 Ibs. gage pressure are mixed at atmospheric pressure with 10 Ibs. of water at 60. What is the temperature and condition of the resulting mixture? 19. Thirty pounds of water at 60, 10 Ibs. of steam at 115 Ibs. absolute and a temperature of 400 F., and 10 Ibs. of ice at 20 are mixed at atmos- pheric pressure. What will the resulting temperature be? What is the condition of the mixture? 20. Ten pounds of ice at 20 F., 18 Ibs. of water at 80, and 10 Ibs. steam at 75 Ibs. pressure and 90 per cent, quality, are mixed at atmospheric pres- sure. What is the resulting temperature and condition of the mixture? 21. Two pounds of steam at 150 Ibs. absolute and a temperature of 400, 5 Ibs. of ice at 22, and 10 Ibs. of water at 60 are mixed at atmospheric pres- sure. Find the final temperature and condition of mixture. 22. Required the final temperature and condition after mixing at atmos- CALORIMETERS AND MECHANICAL MIXTURES 63 pheric pressure 3 Ibs. of ice at 22 and 3 Ibs. of steam at 100 Ibs. pressure and containing 2 per cent, moisture. 23. Find the resulting temperature and condition of a mixture of 10 Ibs. of steam at 150 Ibs. absolute and a temperature of 400 F., 10 Ibs. of water at 60 F., and 50 Ibs. of air at 112 F. Mixture takes place at atmospheric pressure. 24. Five pounds of ice at 0, 20 Ibs. of water at 75, and 15 Ibs. of steam at 50 Ibs. absolute and 95 per cent, quality are mixed at 20 Ibs. absolute. What is the resulting temperature and condition of the mixture? 25. How many pounds of water will 10 Ibs. of dry steam heat from 50 to 150 if the steam pressure is 100 Ibs. gage? 26. If 10 Ibs. of steam at 100 Ibs. gage raises 93 Ibs. of water from 50 to 140, what per cent, of moisture is in the steam, radiation being zero? 27. A pound of steam and water occupies 3 cu. ft. at 110 Ibs. absolute pressure. What is the quality of the steam? CHAPTER V COMBUSTION AND FUELS 41. Coal Analysis. The source of heat which is used to pro- duce steam in a boiler is the fuel. The principal ingredients of all fuels are carbon and hydrogen. For the purpose of making comparison between the product of various mines, and to determine the relative value of these fuels for different uses, coal is subjected to an ultimate and a proximate analysis, and it is tested in a coal calorimeter to ascertain its calorific or heating value. In the ultimate analysis the proportions of carbon, hydrogen, oxygen, nitrogen and sulphur are determined. In the proximate analysis determinations are made of the amounts of moisture, volatile matter, fixed carbon and ash. The volatile gases are hydrocarbons such as marsh or olefiant gas, pitch, tar and naphtha. All of these must be distilled from the fuel before being burned. The fixed carbon and ash are left after all the volatile gases have been driven off. The ash con- sists of the incombustible material which remains after the fuel has been completely burned. It should be noted that the term " proximate" does not mean that the analysis is only " approximate," the facts being actually to the contrary. The analyses are made of air-dried coal. Therefore, the various percentages of carbon, hydrogen, etc., in the ultimate analysis, and of fixed carbon, ash, etc., in the proximate analysis of "coal as received," "moisture free" or "dry coal," and "coal, moisture and ash free," as given, for example, in the U. S. Bureau of Mines Bulletin No. 22 on the "Analyses of Coals in the United States," "were not obtained directly but were calculated from the values obtained by the analyses of air-dried coal." 64 COMBUSTION AND FUELS 65 "Calculations from 'Air Dried' to 'Moisture Free' Condition "* 'Air dried' condition 'Moisture free' condition 100 Volatile matter < 100 _ moisture = volatile matter 100 Fixed carbon X 100 _ molstur^ = fixed carbon Ash X 100 -Toisture = ash 100 Sul P hur < 100 - moisture = sulphur 100 Hydrogen (-1/9 moisture) X 100 _ mo i s ture = h y dr g en 100 Carbon < 100 - moisture = carbon Nitrogen X 100 -Toisture = nitrogen 100 Oxygen (- 8/9) moisture) < ^ = oxy ^ en 100 Calorific value < 100 _ mo i sture = calorific value "The analyses are calculated to the 'moisture and ash free' basis by taking 100 (moisture + ash) as a divisor and proceeding otherwise exactly as in the calculation to the 'dry coal' or 'moisture free' basis. "The air-drying loss of a mine sample indicates to some degree the loss in weight after mining from the evaporation of loosely retained moisture. The analysis of the coal 'as received' shows the actual composition of the coal in the mine. After the coal has left the mine its moisture content lies between the limits of coal 'as received. and coal 'air dried.' "The analysis on a 'moisture free' basis represents the composition of the coal after drying at 221 F. (105 C.). "The analysis stated on a 'moisture and ash free' basis represents approximately the heating value and composition of the dry organic matter. This relation seems to be fairly constant for the same coal bed in certain districts, especially in the Appalachian region. Com- parison of numerous analyses shows that the 'moisture and ash free' calorific values of different samples from the same mine and bed usually agree closely, provided the proportion and the character of the ash and the sulphur do not vary greatly. "For the commercial valuation of coals a proximate analysis and a calorific value determination are usually sufficient. Moisture and ash are of importance; they not only displace their own weights of com- bustible matter, but the evaporation of the moisture wastes heat. * U. S. Bureau of Mines Bulletin No. 22. 5 66 HEAT ENGINES A high percentage of ash increases the cost of handling coal in a power plant and decreases the efficiency of the furnace. "The ratio of the volatile matter to the fixed carbon indicates in a way the type of furnace best adapted for burning a coal with maximum efficiency. "The smokeless combustion of coal containing a low percentage of volatile matter is not difficult in furnaces of ordinary types, but to burn a high volatile coal without smoke requires a suitably designed furnace. A high percentage of sulphur is undesirable in coal used for the manufac- ture of coke and gas. For ordinary steaming purposes sulphur is not a serious drawback unless associated with elements, such as iron or lime, that promote clink ering." 42. Heat of Combustion. The term combustion as applied here refers to the union of oxygen with some other substance pro- ducing heat. The perfect combustion of ordinary fuel should result in carbon dioxide, nitrogen, water vapor, and a trace of sulphur dioxide. "The calorific power, or heating value, of a fuel is the total amount of heat developed by the complete combustion of a unit weight of fuel." The calorific power as determined by a calo- rimeter is the higher heating value. When a fuel is burned water vapor is formed, and this will be condensed only when the temperature falls below the boiling point. So long as this water remains in the form of vapor, the heat necessary to main- tain it as such, i.e., the latent heat of steam at atmospheric pressure times the weight of vapor, is unavailable for use. The difference between the higher heating value and this latent heat is called the lower heating value. This is the " available calorific value" in nearly all cases. For example, in a boiler plant the temperature of the stack gases, and in a gas engine the temperature of the exhaust, are both above the temperature of the boiling point of water, and therefore the heat actually avail- able for use in either case is the lower heating value of the fuel. The heat given off per pound by the elements ordinarily met with in fuels, together with the air required for combustion and the combining volumes and weights, are shown in Table IX. COMBUSTION AND FUELS 67 l|-i - ~ 10 a '3 3 00 CO^ 00 IO CO CO CO a> ?|i >s " IN ** t>- CO i-l i-l s III? 1 )8 1 .^ CO (N "si ill 1 .2> * H ro" TjT O~ . ^ p? ^ * B -d 1 I S - 1 1 ^ -i CO I p B B + + 4 00 8 O O ~ ~ 5 5 55 555 %~ ^-v ' CN i-H S~ g SCO i-l " y ' CO PH v ^ v* c^ 03 M \^_ y PH 6 o 8 8 i 02 P B "3 "o p "> 1 PQ 2 e c CN M CM (N 3 II || II 2 "3 M a '3 1 O "3 O o 1 1 h- 1 IP IM M CN H _L 1 I -(- 3 6 ^ w 8 - H t 1 1 1 ^ (N M "" CN d o O B 8 CN o o 8" i io '. C^ ^ ** M g 11 o II 6 6 6 ' ++ tt + + 6- s CN o 8 J- 8 P c^ rt X o g 6 - .. . Combustioi H to H 2 O (Water) 81 o a -^ 'S'o -fi 5 "2 =3 "2 O O 3 "5 * -^ 8 te ii - ^ C 2 H 4 (Ethylene 68 HEAT ENGINES When a coal is analyzed the percentage of hydrogen shown includes not only the free hydrogen in the sample but also that which existed in combination with the oxygen in the form of water (for all the oxygen in the coal will be united with hy- drogen). As 16 parts by weight of oxygen unite with 2 parts of hydrogen, the weight of hydrogen which was in combination with the oxygen will be equal to one-eighth the total weight of oxygen. The balance of the hydrogen is available for producing heat, and in determining the heat value of a fuel, the number of B.T.U. in the coal may be found from the ultimate analysis by the following formula: Heat value of fuel in B.T.U. per pound = 14,650 C + 62,100 (H -- g + 4000 S, (1) where the symbols C, H, and S represent the weights of car- bon, hydrogen, oxygen and sulphur in 1 Ib. of the fuel. This is called Du Long's formula. The heat value obtained from equation (1) is only an approxi- mate result, and where greater accuracy is desired it is necessary actually to test the coal experimentally in a coal calorimeter. 43. Coal Calorimeters. One form of calorimeter very com- monly used for determining the heating value of solid fuels is the Mahler Bomb Calorimeter. This consists of a strong steel vessel into which a known weight (usually 1 gram) of finely powdered air-dried coal is introduced. This coal is placed in a platinum cup or dish suspended from the cover of the bomb by a wire electrode. Another wire passes through the cover, although well insulated from it, and extends down into the coal. The cover is then screwed down tight and the bomb charged with oxygen to a pressure of from 150 to 250 Ibs. This allows a considerable excess of oxygen over that theoretically required for the combustion of the coal. After the bomb has been charged it is placed in a vessel containing a known weight of water and an electric current is passed through the wire electrodes, igniting the coal. While the combustion is going on, the water in the containing vessel is kept thoroughly stirred by the ap- paratus. The rise in temperature of the water is carefully noted, and after making allowances for radiation, the heat generated by the electric current, etc., the heating value of the coal can be com- COMBUSTION AND FUELS 69 puted, since the heat gained by the water must equal the heat given up by the coal (after the allowances just mentioned have been made). Another type which has found considerable use in cases where it is not convenient to secure a supply of oxygen under pressure, N FIG. 10. Parr coal calorimeter. FIG. 11. Cartridge in Parr calorimeter. is the Parr Calorimeter shown in Fig. 10. This is simpler to use than a bomb calorimeter, but the results obtained are not as accurate. The charge consists of .004 of a pound of finely powdered coal and eighteen times as much by weight of sodium peroxide to supply the oxygen for combustion. After the charge has been placed in the cartridge, Fig. 11, and A, Fig. 10, and the cover has been tightly screwed down, it should be thoroughly 70 HEAT ENGINES mixed by shaking. The calorimeter is then immersed in a vessel containing water and a short piece of white hot wire is dropped in the top of the long neck and a blow on the upper end opens the valve at M, Fig. 10, and allows the wire to drop into the charge igniting it. The water is stirred by fins attached to the sides of the cartridge which is turned on a pivot bearing at the . bottom by a belt run by an electric motor. The rise in tempera- ture of the water due to the combustion of the coal is carefully noted. After making allowances for the heat radiated, the heat given up by the combustion of the sodium peroxide and by the wire used for ignition, the heating value of the coal is found just as in the case of the bomb calorimeter. 44. Air Required for Combustion. The oxygen furnished to the fuel in order to burn it is obtained from the air. Air is a mechanical mixture containing by weight 23 per cent, oxygen and 77 per cent, nitrogen, and by volume 21 per cent, oxygen and 79 per cent, nitrogen. The oxygen only is used in the combustion of the fuel, the nitrogen being an inert gas and having no chemical effect upon the combustion. For the complete combustion of 1 Ib. of hydrogen there is required 8 Ibs. of oxygen, and for the complete combustion of 1 Ib. of carbon to carbon dioxide there is required 32 -f- 12 = 2.66 Ibs. of oxygen. For each pound of hydrogen there will Q be required ^ = 34.8 Ibs. of air, and for each pound of carbon . Zo f\ C*C* 00 = 11.6 Ibs. of air to produce combustion. . Zo As has already been stated, the oxygen in the fuel unites with its equivalent of hydrogen to form water and in determining the weight of air theoretically required for combustion this hy- drogen should be disregarded. The air required for the com- plete combustion of any fuel may then be found from its analysis by the following expression: Weight of air per pound of fuel = 11.6 C + 34.8 (H -- |) + 4.35 S (2) In equations (1) and (2) it has been assumed that each atom of hydrogen and carbon comes in contact with a proper pro- portion of oxygen. In actual practice this condition does not exist and an excess of air is furnished in order to insure com- COMBUSTION AND FUELS 71 plete combustion. Theoretically most coals require for com- plete combustion approximately 12 Ibs. of air. In actually burning coal under a boiler with natural draft we find that the coal requires about 24 Ibs. of air per pound of coal. For forced draft there is usually required about 18 Ibs. per pound of coal. If insufficient air is admitted to the fire, only a por- tion of the carbon will unite with the oxygen to form C02, the balance forming CO. In the actual operation of a boiler plant, one of the most important considerations is the admission of a proper quantity of air to the fire. As will be seen later, the less the quantity of air given to the fire the better the efficiency of combustion, provided enough air enters so that all the carbon is burned to C0 2 . 45. Smoke. Smoke is unburned carbon in a finely divided state. The amount of carbon carried away by the smoke is usually small, not exceeding 1 per cent, of the total carbon in the coal. Its presence, however, often indicates improper handling of the boiler, which may result in a much larger waste of fuel. Smoke is produced in a boiler when the incandescent particles of carbon are cooled before coming into contact with sufficient oxygen to unite with them. It is necessary that the carbon be in an incandescent condition before it will unite with the oxygen. Any condition of the furnace which results in carbon being cooled below the point of incandescence before sufficient oxygen has been furnished to unite with it, will result in smoke. Smoke once formed is very difficult to ignite, and the boiler furnace must be handled so as not to produce smoke. Fuels very rich in hydrocarbons are most apt to produce smoke. When the carbon gas liberated from the coal, is kept above the temperature of ignition and sufficient oxygen for its combus- tion added, it burns with a red, yellow, or white flame. The slower the combustion the larger the flame. When the flame is chilled by the cold heating surfaces near it taking away heat by radiation, combustion may be incomplete, and part of the gas and smoke pass off unburned. If the boiler is raised high enough above the grate so as to give room for the volatile matter to burn and not strike the tubes at once, the amount of smoke given off and of coal used will both be reduced. 46. Analysis of Flue Gases. In all large power houses and carefully conducted power plants the flue gases leaving the 72 HEAT ENGINES boilers are analyzed from time to time. In some cases records are kept, by an automatic device, of the percentage of carbon dioxide in the flue gases. In analyzing the flue gases it is cus- tomary to use some modification of the Orsat apparatus. This consists of three pipettes, a measuring tube, and a wash bottle, as shown in Fig. 12. The first pipette D contains a saturated solution of potassium hydrate and absorbs CO 2 , the second pipette E contains potassium pyrogallate and absorbs O, and the third pipette F contains cuprous chloride and absorbs CO. The gas is passed through the pipettes in the order named, and FIG. 12. Orsat apparatus. the remainder is assumed to be nitrogen. The readings obtained from this apparatus give the per cent, composition of the gases by volume. The following directions will show how the reagents used in the Orsat apparatus are prepared. Potassium Hydrate. (1) For the determination of C02, dissolve 500 grams of the commercial hydrate in 1 liter of water. 1 c.c. of this solution will absorb 40 c.c. of C02. (2) For the preparation of potassium pyrogallate for use in case the per cont. of oxygen is high, dissolve 120 grams of the commercial hydrate in 100 c.c of water. COMBUSTION AND FUELS 73 Potassium Pyrogallate. Put 5 grams of the solid pyrogailic acid in a funnel placed in the neck of the pipette E, and pour over this 100 c.c. of potassium hydrate, solution (1) or (2). So- lution (1) may be used in case there is not more than 25 per cent, of in the gas. Otherwise solution (2) must be used or CO may be given up. 1 c.c of this solution absorbs 2 c.c. of 0. Cuprous Chloride. Pour from J to \ an inch of copper scale into a 2-liter bottle and also place in the bottle a number of long pieces of copper wire. Then fill the bottle with hydro- chloric acid of 1.10 sp. gr. (1 part muriatic acid to 1 part water). Let the bottle stand, shaking it occasionally until the solution becomes colorless. Then pour the liquid into the pipette F, which is filled with copper wires. 1 c.c of this solution will absorb from 1 to 2 c.c of CO. Example. A stack gas shows the following analysis: C02, 12 per cent. ; CO, 1 per cent. ; 0, 7 per cent. ; N, 80 per cent. Find the air used in burning a pound of coal, if the coal contains C, 80 per cent.; H, 4 per cent.; 0, 2 per cent. Solution. Vol. in 100 Density Weight cu. ft. Carbonic acid, C0 2 12 X .12341 = 1.481 Carbonic oxide, CO 1 X .07806 = .078 Oxygen, 7 X .08928 = .625 One pound of carbon dioxide contains i\ of a pound of oxygen, and 1 Ib. of carbonic oxide contains y of a pound. The weight of the oxgyen in 100 cu. ft. of the flue gases would therefore be: In carbonic acid A X 1.481 = 1.077 In carbonic oxide * X .078 = .045 Free oxygen = .625 Total weight of oxygen = 1.747* pounds and the weight of the carbon would be: In carbonic acid A X 1.481 = .404 In carbonic oxide 2 i X .078 = X)33 Total weight of carbon 437 pounds Air contains 23 per cent, of oxygen by weight; hence the pounds of air required to burn .437 Ibs. of carbon would be 1.747 ^ .23 = 7.6, 74 HEAT ENGINES and the pounds of air to burn 1 Ib. of carbon under the conditions of the flue gases would be 7.6 -f- .437 = 17.4 The pounds of air used to burn a pound of coal of the given analysis would be 17.4 C + 34 = 17.4 X .80 + 34.8 (0.4 - ~\ = 13.92 + 1.31 = 15.23 Ibs. It should be noted here that in this solution the weight of air theoretic- ally required to burn the hydrogen has been added to the weight actually required to burn the carbon as shown by the stack gas analysis. While this is, of course, not exactly correct, it is approximately so, and the error is slight, as the amount of air used to burn the hydrogen is small as com- pared with the total amount required. The above results are such as might be expected in a boiler plant using induced draft. 47. Theoretical Temperature of Combustion. If the total and specific heats of the materials of a given coal are known, the temperature that might result from their combustion may be approximately calculated. The calculated temperatures are often very much higher than can be obtained in practice, this being probably due to the fact that the specific heat of the products of combustion is very much larger at the high temperatures, and also to the fact that carbon and oxygen will no longer unite above a given temperature, probably about 3500 Fahrenheit. . Example. Assume the following composition of coal: Carbon, 75 per cent.; hydrogen, 5 per cent.; oxygen, 3 per cent.; nitrogen, 2 per, cent.; the ash and sulphur may be disregarded. Find the theoretical and actual rise in temperature of the products of combustion. Solution. A coal of the above composition has a heat value of 13,860 B.T.U. The theoretical amount of air required to burn 1 Ib. of it is 10.62 Ibs. 10.62 Ibs. of air contain 10.62 X .77 = 8.18 Ibs. nitrogen, to which must be added the .02 Ibs. of nitrogen in the coal, giving us a total of 8.2 Ibs. nitrogen. Total C0 2 formed = .75 X 3.66 = 2.745 Ibs. } (See Total H 2 formed = .05 X 9 = .45 Ibs. J Table IX) The thermal units required to raise the products of combustion through 1 would be COMBUSTION AND FUELS 75 Sp. ht. B.T.U. Carbonic acid 2.75 X .217 = .596 Water vapor 45 X .460 = .207 Nitrogen 8.2 X .244 = 2.000 Total 2.803 The theoretical rise in temperature of the products of combustion would be 13,860 -h 2.8 = 4950 In the actual operation of a boiler it is found necessary to add 50 to 100 per cent, more air than is required for combustion. This addi- tional air, as the following calculation shows, materially reduces the theoretical temperature of combustion. Assuming 100 per cent. more to be required, there would then be added 10.62 additional pounds of air. The heat to raise this 1 degree would be 10.62 X .2375 = 2.522 Add for undiluted products 2.803 Total B.T.U. per degree 5.325 The theoretical rise in temperature would be, then, 13,860 -^ 5.325 = 2600 This is more nearly the temperature obtained in a boiler plant with hand firing. If the temperature of the boiler room is given, the final temperature of the products of combustion may be found by adding to this temperature the rise in temperature as found above, the assumption being made that the temperature of the coal is the same as that of the boiler room. In boilers operated by automatic stokers, temperatures in the fire of over 3000 F. have been observed. Such temperatures are usually obtained when the boilers are being crowded to their full capacity and their operation is being given careful atten- tion, especially with reference to the amount of air admitted to the furnace. 48. Fuels. Fuels may be divided into three general classes, solid, liquid, and gaseous. The larger proportion of the fuels used are in solid form. The principal solid fuels are wood, peat, lignite, and coal. Coal may be divided into three principal kinds, anthracite, semi- bituminous and bituminous coal. 76 HEAT ENGINES The liquid fuels are usually some of the mineral oils, generally unrefined petroleum. In some gas plants liquid tar is used. The most commonly used gaseous fuel is natural gas, but there are a good many plants using gas which is a waste product from a manufacturing operation. In the steel mills the "down comer" gases from the blast furnaces are often used as a fuel for the steam boilers. Coke-oven gases are similarly used. In some cases the coal is distilled in a gas producer, and this producer gas used as a fuel. 49. Woods. Woods may be divided into two general classes, soft and hard. The commonest hard woods are oak, hickory, maple, beech, and walnut. The commonest soft woods are pine, elm, birch, poplar, and willow. When first cut, wood contains about 50 per cent, of moisture, but after being dried this is reduced from 10 to 20 per cent. The following table gives the chemical composition and heat value of some of the more common woods. (From Poole's Calorific Value of Fuels.) TABLE X. CALORIFIC VALUE OF WOODS Name C H O N Ash B.T.U per Ib. combustible Ash 49 2 6 3 43.9 .07 .57 8480 Beech 49.0 6.1 44.2 .09 .57 8590 Birch 48 9 6 44.7 10 29 8590 Elm . ..... 48.9 6.2 44.3 .06 50 8510 Oak 50.2 6.0 43.4 .09 .37 8320 Pine 50 3 6.2 43.1 .04 37 9150 In boiler tests a pound of wood is usually assumed as equal to .4 of a pound of coal. 50. Peat. Peat is an intermediate between wood and coal. It is formed from the immense quantity of rushes, sedges, and mosses that grow in the swampy regions of the temperate zone. These in the presence of heat and moisture are subject to a chemical change which leaves behind the hydrocarbons, fixed carbon, and 70 to 80 per cent, of moisture. It is usually cut in blocks and air dried. Good air-dried peat contains about 60 per cent, of carbon, 6 per cent, of hydrogen, 31 per cent, of oxygen an,d nitrogen, and 3 per cent, of ash. The following table gives the heat value of some of the different peats: COMBUSTION AND FUELS 77 TABLE XI. CALORIFIC VALUE OP PEATS Location Fixed carbon Volatile matter Ash B.T.U. per Ib. combustible Northern Michigan 4 4 11 000 Southern Michigan 33 3 61 2 5 5 8 900 Southern Michigan 29.0 68 5 2 3 9 500 New York 29 2 65 6 8 25 10200 Wisconsin 27.6 60 5 11 8 8 250 51. Lignite Coal. Lignite is coal of very recent formation, and its analysis is similar to peat. It usually resembles wood in appearance, and is of brownish color. It is uneven of fracture and of a dull luster. It is found quite generally west of the Mississippi River. The composition is given in the following table: TABLE XII. CALORIFIC VALUE OF LIGNITES Location Fixed carbon Volatile matter Ash B.T.U. per Ib. combustible California 9063 Colorado 46 32.7 2 74 11,360 52. Bituminous Coal. Coals that contain over 20 per cent, volatile matter are usually classed as bituminous coals. Bitu- minous coals are divided into coking, non-coking, and cannel coals. " Coking coal" is a term used in reference to coals that fuse together on being heated and become pasty These coals are used in gas manufacture, and are very rich in hydrocarbons. Non-coking coals are free burning and the lumps do not fuse together on being heated. " Jackson Hill" is an example of this kind of coal. Cannel coal is very rich in carbon, ignites readily, and burns with a bright flame. It is very homogeneous, breaks without any definite line of fracture, and has a dull, resinous luster. It is very valuable as a gas coal so that it is little used for steaming purposes. The principal bituminous coals used are mined in Ohio, West Virginia, Pennsylvania, and Illinois. The following table gives the properties of the commonest varieties of the bituminous coals used for steaming purposes: 78 HEAT ENGINES TABLE XIII. CALORIFIC VALUE OF BITUMINOUS COALS Location Fixed carbon Volatile matter Ash B.T.U. per Ib. combustible Water Illinois: Big Muddy . . 53 7 30 1 9 2 13,610 Streator 44.0 39.2 12.3 13,690 4.5 Wilmington 44 9 36 8 13 3 14,050 13 3 Michigan: Saginaw 6 1 13,470 Ohio: Brier Hill... 59.1 36.4 4 5 14,200 Hocking Valley 49.1 36.1 8.5 13,980 6.4 Jackson .... ... 54 6 34.3 7 13,955 4 1 Pennsylvania: Pittsburg No. 8 54 6 35.5 9 9 14,200 Turtle Creek 56.6 34.4 8.0 15,080 1.0 Youghiogheny . . 54 7 32 6 12 7 15,000 West Virginia-' Clover Hill 56 8 31 7 10 1 14,265 Thacker 56.2 35.5 6.8 15,240 53. Semi-Bituminous. This is a softer coal than anthracite, but in appearance it looks like the latter. It is lighter than anthracite and burns more rapidly, and is a valuable coal where it is necessary to keep a very intense heat. Its composition is given in the following table : TABLE XIV. CALORIFIC VALUE OF SEMI-BITUMINOUS COALS Location Fixed , carbon Volatile matter Ash B.T.U. per Ib. combustible Blassburg, Pa 73 15 11.0 13,500 Cumberland, Md 80.8 13.0 5.0 16,320 Pocahontas, W. Va . . . 74 5 18 1 6 6 15,740 A semi-bituminous coal should not contain, usually, more than 20 per cent, volatile matter as compared with the fixed carbon. 54. Anthracite. This coal ignites very slowly and burns at a high temperature. Its principal component is fixed carbon. Consequently it gives off almost no smoke and the flame is very short. Owing to its smokeless burning, it is almost all consumed for domestic purposes. Nearly all anthracite used in this country comes from Pennsylvania. An anthracite coal should contain not COMBUSTION AND FUELS 79 less than 92 per cent, of fixed carbon as compared with the volatile matter. The following is a table of the composition of various anthracite coals: TABLE XV. CALORIFIC VALUE OF ANTHRACITE COALS Location Fixed carbon Volatile matter Ash B.T.U. per Ib. combustible Lackawam Lykens Va Scran ton la 84.0 81.0 84.4 5.0 5.0 6.5 11.0 14.0 9.0 13,900 13,650 13,800 Hey 55. Efficiency of Fuels. The commercial value of a fuel is determined by the number of pounds of water it will evaporate into steam per hour from and at 212. This, however, involves the efficiency of the boiler, so that to compare fuels in actual use, they should be burned in the same boiler. In practice the value of a fuel in any given plant is affected by the form and character of the furnace, the amount of air supplied, and the intensity of the draft. There are, in fact, so many variables entering into the problem that it is difficult to make an accurate comparison of the value of the different coals. It is easy to burn either anthracite or semi-bituminous coal in almost any boiler. For bituminous coals containing less than 40 per cent, volatile matter, plain grate bars with a fire- brick arch over the fire give very good results. With coals containing over 40 per cent, volatile matter, it is desirable to use some form of furnace arranged so that the gases are mixed with warm air, and with these a large combustion chamber should be provided. The commercial results obtained from a given coal are usually determined by the cost to evaporate 1000 Ibs. of water into steam from and at 212. This cost varies from 10 cents to 18 cents. Where the principal cost of the coal is in the freight rate, it is usually more economical to burn a good grade of coal than a cheap grade. PROBLEMS 1. An anthracite has the following composition: C, 90 per cent.; H, 2 per cent.; O, 2 per cent. Find the heating value of the coal. 2. A semi-bituminous coal has the following composition: C, 80 per cent.; H, 5 per cent.; O, 3 per cent. Find the heat units in the coal. 80 HEAT ENGINES v 3. A Pennsylvania bituminous coal contains: C, 75 per cent.; H, 5 per cent.; O, 12 per cent. Find the heat value of the coal and the air required to burn 1 Ib. 4. An Illinois bituminous coal has the following composition: C, 62 per cent.; H, 5 per cent.; O, 15 per cent. Find the heat units in the coal and the air required to burn 1 Ib. 5. A coking coal has the following composition: C, 85 per cent.; H, 5 per cent. ; O, 4 per cent. Find the heat value of the coal and the air required to burn 1 Ib. ^ 6. A coal contains C, 80 per cent.; H, 2 per cent.; O, 6 per cent. What is its heat value and how many pounds of air will be required to burn 1 Ib. of it? 7. A coal contains C, 70 per cent.; H, 5 per cent.; O, 8 per cent. What is its heat value and how much air will be required to burn 1 Ib. of it? 8. A coal has the following composition: C, 80 per cent.; H, 3 per cent.; O, 4 per cent. How much heat will be lost if one-half of the carbon is burned to CO and the balance to CO 2 , and what is the weight of air required to burn 1 Ib. of the coal under these conditions? ~" 9. A coal contains C, 90 per cent.; H, 1 per cent.; O, 2 per cent. If three- quarters of the carbon is burnt to CO 2 and the balance to CO, what will be the B.T.U. given off per pound, and what will be the air required to burn 1 Ib. under the above conditions? 10. A flue -gas shows the following composition: CO 2 , 8 per cent.; CO, per cent. ; O, 14 per cent. ; N, 78 per cent. Find the pounds of air used per pound of coal if the coal contains C, 80 per cent.; H, 5 per cent.; O, 3 per cent.; and N, 1 per cent. ^/. 5 < 11. A flue gas shows the following composition: CO 2 , 8.1 per cent.; CO, per cent.; O, flrtper cent.; Nj^fc&pjer cent. Find the pounds of air us^d per pound of coal, n the coal contains ^u; 75 per cent.; H, Skper cent.; O, '8 per cent. * 12. A flue gas shows the following composition: CO 2 , 5 per cent.; CO, per cent.; O, 15 per cent.; N, 80 per cent. Find the pounds of air used per pound of coal if the coal contains C, 75 per cent.; H, 5 per cent.; O, 8 per cent. 13. A flue gas shows the following composition: CO 2 , 4.1 per cent.; CO, per cent.; O, 16 per cent.; N, 79.9 per cent. Find the pounds of air used per pound of coal if the coal contains C, 75 per cent.; H, 5 per cent.; O, 8 per cent. 14. A flue gas shows the following composition: CO 2 , 4.3 per cent.; CO, pelf cent. ; O, 12.7 per cent. ; N, 83 per cent. Find the pounds of air required per pound of coal if the coal contains C, 75 per cent.; H, 5 per cent.; O, 8 per cent. 16. A flue gas shows the following composition: CO 2 , 8.3 per cent.; O, 10.8 per cent.; N, 80.9 per cent. How much air is burned per pound of coal if the coal Contains C, 75 per cent.; H, 6 per cent.; O, 4 per cent.? 16. A coal contains C, 80 per cent.; H, 5 per cent.; O, 3 per cent.; N, 1 per cent. Find the theoretical temperature of combustion if 30 per cent, more air is used in the combustion than is necessary. Temperature of boiler room, 70. COMBUSTION AND FUELS 81 17. A coal has C, 80 per cent.; H, 5 per cent.; O, 3 per cent.; and N, 1 per cent. Find the theoretical temperature of combustion if 50 per cent, more air is used than is necessary for the combustion. Temperature of boiler rjaom, 80. 18. A coal gives the following analysis: C, 75 per cent.; H, 6 per cent.; O, 4 per cent.; and N, 2 per cent. Seventy-five per cent, excess of air is used in burning it. What is the ideal rise in temperature of the gases? CHAPTER VI BOILERS 56. Boilers may be divided, from the path taken by the fire, into fire-tube or tubular boilers and water-tube or tubulous boilers. In the fire-tube boiler the hot gases from the fire pass through the tubes, while in the water-tube boiler these gases pass around the tubes. Boilers are also divided into two classes depending on the position of the fire; these are known as externally fired and inter- nally fired boilers. In the externally fired boiler, the fire is entirely external to the boiler and is usually confined in a brick chamber. These boilers are largely used for stationary plants. The internally fired boiler is most commonly used for loco- motive and marine boilers. The fire is entirely enclosed in the steel shell of the boiler and no brick setting is necessary. These boilers are more expensive per horse-power than the ordinary forms of stationary boilers. The various forms of boilers under proper operating condi- tions give essentially the same economical results. 57. Return Tubular Boilers. Fig. 13 shows the plan and elevation of the setting of a fire-tube boiler of the return type. The coal burns upon the grates, which rest upon the front of the boiler setting and upon the bridge wall. The flames pass under and along the boiler shell, then turn in the back combustion chamber D and pass through the tubes of the boiler, then out through the smoke nozzle N and through the breeching to the chimney. The smoke nozzle is shown at the front of the boiler setting. There are usually two man-holes in the boiler, one in front under the tubes and one in the top of the boiler. These open- ings are reenforced with flanged steel reinforcements. The shells are made of boiler steel having a tensile strength of 55,000 to 66,000 Ibs. The shell of the boiler is rolled to form and riveted together. The heads of the boiler which form the tube sheet 82 BOILERS 83 and into which the tubes are fastened are made of flanged steel of about 55,000 Ibs. tensile strength. The tubes are made of steel, usually lap welded. Charcoal iron tubes are the best, but are difficult to get, so that most manufacturers use a hot-rolled, lap-welded steel tube. Feed Pipe M Manhole 1 1 i J i F u I _-i :; 1 U > '5 ! ;; J> - \ 3 J 1 1 '^ 3} ' i; pi i ; 1= =^ N r J These boilers are set in brick settings, and in all brick-set boilers great care should be taken in building the setting. Air leaks in the brick work should be carefully avoided as they cause serious loss in economy. All brick should be set with full flush 84 HEAT ENGINES mortar joints so as to make the setting strong and avoid leakage. Fig. 13 shows the return flue boiler with the boiler resting upon the brick work. FIG. 14. Steel frame boiler support. FIG. 15. Return tubular boiler with loops for suspension setting. Boilers of this type are often supported by a steel frame- work as shown in Figs.. 14 and 15. This method is preferable as it leaves the boiler independent of the setting. The brick setting of BOILERS 85 a boiler has very little strength and this arrangement leaves the boiler setting free from all strain due to the weight of the boiler. In earlier boiler construction it was customary to place a steam dome on all boilers. The object of doing this was to FIG. 16. Dry pipe. provide dry steam. Most engineers have discarded the use of steam domes on high-pressure boilers as they weaken the boiler shell and add to the expense of the boiler construction. To avoid' getting wet steam from the boiler a dry-pipe is provided as shown in Fig. 16. FIG. 17. Brick setting for fire-tube boiler with overhanging shell. Fig. 17 shows a return flue boiler and solid brick setting. Some engineers prefer a setting having a 2-in. air space in the center of the wall. The brick walls enclosing a fire-tube boiler are made very heavy so as to give good heat insulation, preventing anexcessive loss of heat from the boiler, and also to prevent the 86 HEAT ENGINES BOILERS 87 filtration of air through the setting and the consequent cooling of the hot gases passing away from the fire. 58. Internally Fired Boilers. Another large class of return tubular boilers are the internally fired boilers. These boilers have been extensively used for marine purposes. Fig. 18 shows an internally fired Scotch marine boiler. The cut shows two internal furnaces. In the larger sizes these boilers are often FIG. 19. Scotch marine boiler. made with three or even four furnaces (see Fig. 19). These can be built in large sizes, and are very compact, making them par- ticularly suitable for marine work. Fig. 20 shows one of these boilers built for stationary pur- poses. The steel back combustion chamber used in marine work, shown in Fig. 18, is replaced by brick construction in Fig. 20. In very large boilers of this type, furnaces are provided at each end, opening into a common combustion chamber in the middle of the boiler. 88 HEAT ENGINES BOILERS 89 59. Locomotive Type of Boiler. A special type of fire-tube boiler is used on locomotives. In this boiler the combustion space, including the grates, and the sides of the ash pit are sur- rounded by a water space. The gases pass directly from the fire through the tubes and up the stack. As in the internally fired boiler, the hot gases do not come in contact with the shell of the boiler. This permits of the use of higher pressures in these boilers, often as high as 225 Ibs. Modifications of this type of boiler are used for threshing and other types of portable boilers. They are sometimes used for stationary purposes, particularly FIG. 21. Locomotive type of boiler. for heating where a compact form of boiler is desirable. Fig. 21 shows the side elevation of a boiler of this class designed for stationary use. 60. Use of Tubular Boilers. The fire-tube boiler, as shown in Fig. 13, has certain limitations in use. Its construction is such that hot gases pass outside the shell, with cold water on the inside of the shell. This produces a large difference of temperature on the two sides of the shell, and a strain is produced in the metal of the shell, owing to this difference of temperature. The thicker the shell the greater is the difference in temperature between the two sides of the shell. In practice it is found that the thickness of the shell should not exceed \ in. This limitation in the thickness of the shell limits the diameter of the boiler and the pressure that the boiler can carry. It is customary to use this 90 HEAT ENGINES class of boilers for pressures not to exceed 125 Ibs. per square inch and in sizes not larger than 125 boiler horse-power. A majority of the more recent plants are being operated at over 125 Ibs. pressure and therefore a fire-tube boiler cannot be used. In addition the horse-power of each boiler unit is so small that a very large number of boiler units would be necessary. In a power plant of say 50,000 horse-power, such as exists in the larger cities, if this type of boiler were used, there would be required 400 boilers and the space required for this number of units would make it almost impossible to install such a plant. Safety Valve FIG. 22. Babcock and Wilcox boiler. The internally fired boiler is not as limited in the pressure that it can carry as is the return fire-tube type, since the fire does not come in contact with the boiler shell and the shell can be made thicker. The increased thickness of shell permits the building of larger boilers of this type than of the return fire tube, and they have been built in units of 500 horse-power carrying 200 Ibs. pressure. They have not been much used for stationary purposes owing to their first cost and the cost of repairs where conditions are not favorable to their use. BOILERS 91 Hand Hole / Plates N 61. Water-tube Boilers. The demand for increased pressure and for larger sized boiler units has led to the introduction of water-tube boilers, and all the larger power stations to-day are using water-tube boilers almost exclusively. The principal reasons for using the water-tube boilers in large power stations are: adaptability to high pressure, reduced space taken by the boiler, and greater safety in operation. There are a great many different makes of water-tube boilers on the market of various types, both vertical and horizontal. Fig. 22 shows a Babcock and Wilcox boiler in longitudinal cross- section. Gases from the fire pass up through the tubes, being de- flected vertically by a baffle wall located between the tubes and directly above the bridge wall. They then pass down around the tubes to the space back of the bridge wall, being deflected by another baffle, then up between the tubes and out through the smoke opening which is in the rear of the boiler setting and above the tubes. As it is heated, the water in the tubes tends to rise toward their up- per, or front end, then rises through the front header and connection into the steam and water drum, where the steam separates from the water, and the latter flows back in the drum and down through the rear header. The feed water enters the boiler through a pipe passing through the front end of the drum and extending back about one third its length. Fig. 52 shows a Babcock and Wilcox boiler with a superheater attached. Fig. 23 shows the front and side views of a header in a Babcock and Wilcox boiler and indicates clearly the way the tubes are "staggered." This class of boiler gives very satisfactory service for high- pressure work, having large disengaging surfaces for the steam to leave the water, and ample ste'am space. Side view of vertical header Front view of vertical header FIG. 23. Tube header in Bab- cock and Wilcox boiler. 92 HEAT ENGINES Fig. 24 shows a sectional side elevation of a Stirling boiler. "This consists of three transverse steam and water drums set parallel and connected to one mud drum by water tubes so curved that their ends enter the tube sheets at right angles to the surface. This curvature of the tubes gives ample and effi- cient provision for expansion and contraction. The front and middle steam drums are connected by curved ^equalizing tubes FIG. 24. Stirling boiler. above the water line and curved circulating tubes below the water line, while the rear and middle drums are connected by curved equalizing tubes above the water line only. The steam generated in the three banks of tubes passes into the middle drum, which is set higher than the other two to give additional steam space, thence it passes through the main steam outlet, which may be located anywhere along the top of the drum. BOILERS 93 The safety valves are located on the top of the middle steam drum. The feed pipe connection passes through the top of the rear drum into a trough by which the water is dis- tributed along the whole length of the drum. The blow- off connection is attached to the bottom of the mud drum at the center and passes out through a sleeve in the rear wall, just outside of which the blow-off valve is located. The water column, located at one side of the front of the boiler, is con- nected to one head of the center steam and water drum. The feed water enters the upper rear drum and passes downward through the rear bank of tubes to the lower drum, thence up- ward through the front bank to the forward steam and water drum. The steam formed during the passage upward through the front bank of tubes becomes separated from the water in the front drum, and passes through the upper row of cross tubes into the middle drum, from which point it enters the steam main. The water from the front drum passes through the lower cross tubes into the middle drum, and thence down- ward through the middle bank of tubes to the lower drum, from which it is again drawn up the front bank to retrace its former course. The steam generated in the rear bank of tubes passes through the cross tubes to the center drum. In its pas- sage down the rear bank of tubes the feed water is heated so that much of the scale-forming matter is precipitated and gathers in the rear bank of tubes and in the mud drum, where it is protected from high temperatures and can be washed and blown out as frequently as tire case demands." The hot gases circulate in the reverse direction. On leaving the fire they are deflected by baffle walls so as to pass up between the tubes to the first drum, then down around the tubes from the second drum, and again up between the tubes to the rear drum. The burned gases leave the boiler at the rear near the upper end of the last bank of tubes. This boiler represents the ideal cir- culation as far as the paths of the water and gases are concerned; that is, the coldest gases come in contact with the coldest water in the boiler, and the hottest gases come in contact with the hottest water. The drums with their connecting tubes are supported by a steel frame built into the brick work of the boiler. The brick setting only serves to enclose the gases and is under no strain due to the weight of the boiler. 94 HEAT ENGINES There is a man-hole in one end of each of the four drums and by the removal of the man-hole plates the drums may be entered. Fig. 25 shows a cross-section of a Heine water-tube boiler. In this boiler the gases of combustion pass over the bridge wall into the combustion chamber, where they are completely burned. They then pass upward back of the lower baffle wall (which consists of a row of tiling) and then forward around the tubes, and parallel to them, to the front of the boiler, where they turn up in front of the forward end of the upper baffle wall and then FIG. 25. Heine boiler. pass back around the shell to the opening to the breeching. The feed water enters the boiler through the front head, pass- ing into the mud drum where the dirt and sediment are de- posited, then flows back along the bottom of the drum and then forward along the top and out of the drum at the front end. From here the circulation is toward the back of the boiler, down the rear water-leg, forward through the tubes, and up the front water-leg into the boiler again. The steam, which is formed very largely in the tubes, is carried along with the water and discharged into the boiler from the front water-leg. Fig. 26 shows a side elevation of the water-legs, shell and tubes in the Heine boiler. BOILERS 95 Where a plant is very limited in the floor space available, it is often desirable to use a vertical water-tube boiler. Fig. 27 shows a cross-section of the Wickes vertical boiler. The grates are located in a " Dutch Oven" front built out from the main boiler setting. The gases pass up around the tubes in the for- ward half of the boiler and down around them in the rear half, leaving the boiler in the rear near the lower drum. The water inside the tubes flows in the same direction as the gases, in both the front and rear compartments. These boilers are quick steamers and occupy relatively small floor space. Fig. 28 shows a vertical boiler of the Rust type. The furnace and combustion chamber project from the front of the boiler as a " Dutch Oven" front. FIG. 26. Heine boiler showing water-legs, shell and tubes. " The products of combustion travel up the first pass, down the second pass and out to the stack. "The water, which is fed into the center of the rear water-and- mud drum, passes across through the circulating tubes to the front water-and-mud drum, up the vertical tubes in the front pass to the front steam-and-water drum where the steam generated in the front pass is separated from the water. The water then passes over through the circulating tubes to the rear steam-and-water drum and down the rear bank of tubes to the starting place. "The steam liberated in the front steam-and-water drum passes through the steam tubes into the steam space of the rear steam- and-water drum, the entrained moisture dropping into the 96 HEAT ENGINES Steam Outlet Manhole Safety Valve Water Column Feed Inlet - Downcomers 1 FIG. 27. Wickes boiler. BOILERS 97 water space, while the steam passes along the top of the drum through the dry pipe to the steam outlet." 62. Horse-power Rating of Boilers. The term " horse- power," as applied to boilers, has no definite value and is only used as a matter of convenience. The ability of a boiler to , Steam and Water Drums -, Steam \ \ Steam ; Tubes ! \ Outlet Water Circulating Tubes-- Water Column ^-Blow-off ' ' * Water and Mud Drums-'' FIG. 28. Rust boilef. make steam depends on the amount of heating surface in it. Experience has determined that for the best results in the ordinary form of boiler, a square foot of heating surface should not evaporate more than 3 Ibs. of water per hour (if economy 7 98 HEAT ENGINES is highly desired). In writing specifications for boilers, it is customary to state the number of square feet of heating surface the boiler is to contain and the pounds of water it is to evaporate per hour under the given conditions, rather than the boiler horse-power. In order to give the term " boiler horse- power" a definite meaning, the American Society of Mechanical Engineers has adopted the following rating for boilers: A "boiler horse-power" is 34.5 Ibs. of water evaporated per hour from and at 212 into dry and saturated steam. Most boilers will produce from 25 to 50 per cent, more steam than their rating, depending upon the amount of heat generated in the furnace and the amount of heat that is given to the water in the boiler. The amount of heat given off by the fuel will depend upon the kind of fuel used, the area of the grate, the amount of draft, and the skill of the fireman. A very rapid rate of combustion usually results in a large escape of heat to the stack and reduced economy. There is no relation between a boiler horse-power and an engine horse-power. The number of boiler horse-power required to supply steam for a given engine horse-power will be determined by the number of pounds of steam the engine requires to develop a horse-power. The steam required per horse-power hour varies through a wide range in the different types of engines. 63. Heating Surface, Grate Surface, and Breeching. The water heating surface in a boiler is that part of the boiler which has water on one side and hot gases on the other. Superheating surface has steam on one side and hot (jases on the other. In both cases the side in contact with the hot gases is the one to be measured. The proportion of grate surface to heating surface depends upon the kind of fuel and the intensity of the draft. In small boilers such as are used for heating purposes, with light draft and hard coal it is usual to allow 1 sq. ft. of grate to from 20 to 30* sq. ft. of heating surface. In large power boilers the ratio of grate surface to heating surface varies from 1 to 50, to from 1 to 70. In locomotive boilers with forced draft the ratio is from 1 to 50, to 1 to 100. The rate of combustion varies with the kind of coal and with the draft. With anthracite coal and moderate draft, not ex- ceeding five-tenths of an inch of water, it is from 12 to 15 Ibs. per square foot of grate surface per hour, and with bituminous coal from 15 to 20 Ibs. The air opening in the grate depends BOILERS 99 upon the kind of coal and usually does not exceed 50 per cent. of the grate area. Anthracite and the better grades of bitu- minous coal require less air opening than the poorer grades of coal. The following rule is used for determining the heating surface of a horizontal return flue fire-tube boiler: the heating surface is equal to two-thirds the cylindrical surface of the shell, plus the internal area of all the tubes, plus two-thirds the area of both tube sheets, minus twice the combined external cross-sectional area of all the tubes, all expressed in square feet. In water-tube boilers it is customary to allow 10 s_q. ft. of heating surface per bQi]grrsp-pnwr,nrl in fire-tube boilers 12 sq. ft. The connection for carrying the hot gases from the boiler to the chimney is called the breeching. The area of the breech- ing is from J to | of the area of the grates, depending on the strength of the draft. The breeching is usually made of sheet steel well braced, and should be provided with a door for cleaning and inspection. TABLE XVI. DIAMETER OF BOILER TUBES Outside Inside Inches Feet Inches Feet 2 .167 1.80 .150 2 .208 2.28 .190 3 .250 2.78 .232 3 .292 3.26 .272 4 .333 3.74 .312 4 . .375 4.24 .353 5 .417 4.72 .393 64. Boiler Economy. The economy of a boiler is usually ex- pressed as the number of pounds of water fed to the boiler per pound of coal fired. The water evaporated by a boiler is equal to the weight of water fed to the boiler (corrected for leakage), provided that the steam formed is dry and saturated. If the steam is wet, it is necessary to make a correction in order to determine how much dry and saturated steam might have been formed. When the percentage of moisture is less than 2 per cent, this correction may be made by simply subtracting the moisture from the total weight of water fed. If the percentage is more than 2 per cent. 100 HEAT ENGINES or if great accuracy is desired, the weight of water fed must be multiplied by a " factor of correction" which is equal to / h - (t - 32) \ ff + (1 - ff) { H^T- ~d) } where q is the quality of the steam, h is the heat of the liquid and H the total heat of the steam at the given pressure, and t the temperature of the feedwater. In order to compare boilers working under different conditions of feed temperature and steam pressure and with different coals, it is better to reduce them all to the same conditions, and the economy may be expressed as the number of pounds of equivalent evaporation from and at 212 per pound of combustible burned. By "equivalent evaporation from and at 212 " is meant the number of pounds of water that would be evaporated from a feed temperature of 212 into dry and saturated steam at 212 by the expenditure of the same amount of heat as is actuall^ased in evaporating the water under the given conditions. The "factor of evaporation" is that factor by which the water evaporated, corrected for moisture in the steam, must be multiplied in order to get the equivalent evaporation. It is equal to the heat necessary to make 1 Ib. of dry and satu- rated steam under the given conditions divided by the heat necessary to make 1 Ib. from and at 212. With a good boiler and high-grade bituminous coal, a boiler will evaporate from 9 to 12 Ibs. of water per pound of coal. The average performance under usual working conditions is from 8 to 10 Ibs. of water per pound of coal. The economy of boiler operation depends not only upon the construction of the boiler, but also upon the skill of the fireman. This is par- ticularly true with hand firing, and a careful record of the fire- man should be kept, in order to prevent a waste of coal due to improper handling of the fires. 65. Efficiency of Steam Boilers. The efficiency of boiler, furnace and grate is the ratio of the heat absorbed per pound of dry coal fired, to the heating value of a pound of dry coal. The efficiency of boiler and furnace is the ratio of the heat absorbed per pound of combustible burned to the heating value of a pound of combustible. The "heat absorbed" per pound of dry coal or combustible is found by multiplying the equivalent evaporation from and at 212 F. per pound of dry coal, or combustible, by 970.4. BOILERS 101 The "dry coal fired" is found by deducting the moisture in the coal from the total weight of coal supplied to the grates. The "combustible burned" is determined by deducting from the weight of coal fired, the weight of moisture in the coal plus the weight of ash and refuse taken from the ash pit plus "the weight of dust and soot, if any, withdrawn fron the tubes, flues and combustion chamber, including ash carried away in the gases, if any, determined from the analysis of coal and ash." The "heating value of a pound of combustible" is equal to the heating value of a pound of dry coal divided by 1 minus the percentage of ash in the dry coal as shown by analysis. Actual tests of various boilers show that the efficiency under ordinary working conditions varies from 60 to 80 per cent. Seventy per cent, might be considered as a good average efficiency. 66. Losses in Boiler. The principal losses in a boiler are the heat that is carried away by the flue gases, the loss due to hydrogen in the coal, the loss through the grates, and the loss by radiation. Of these, the largest is the heat carried up the chimney by the stack gases. The following table shows the relative proportions of these losses in a well-operated boiler plant, and is termed the heat balance. The total heat in 1 Ib. of combustible in the coal was 15,070 B.T.U. TABLE XVII. HEAT BALANCE IN BOILER PLANT Distribution of heat of dry coal B.T.U. Per cent. 1 Heat absorbed by the boiler 10982 77 82 2. Loss due to evaporation of moisture in the coal . . 3. Loss due to heat carried away by steam formed by the burning of hydrogen in the coal 4. Loss due to heat carried away in dry chimney gases. 5. Loss due to carbon monoxide 27 628 1,635 96 .19 4.45 11.57 68 6 Loss due to combustible in ash and refuse 319 2 26 7. Loss due to heating moisture in the air 40 .28 8. Loss due to unconsumed hydrogen and hydro- carbons, to radiation, and unaccounted for. 388 2.75 Total 14,115 100 The heat carried away by the chimney gases depends upon the amount of air admitted to the fire and upon the tempera- ture at which the gases leave the boiler. In a properly operated plant, the gross loss of heat up the chimney should not exceed 102 HEAT ENGINES 20 per cent. It is often much more than this owing to the fact that the fireman admits too much air to the coal; more than is necessary for its complete combustion. This excess of air is heated from the temperature of the boiler room to the tem- perature of the stack gases, and all the heat used for this pur- pose passes up the chimney and is wasted. It is, therefore, very important that the amount of air admitted to the fire should not be more than is absolutely necessary. This is determined by the amount of carbon dioxide in the stack gas analysis which has been previously described. In a well-operated plant, the CO2 as shown by the analysis varies from 9 to 10 per cent., and under exceptional conditions an analysis showing 16.8 per cent, of C0 2 has been obtained. It is usually undesirable to have more than 12 to 13 per cent, of CO2 in the stack gases. Larger per- centages generally indicate the presence of CO. Example. A 48 in.' X 12 ft. return flue fire-tube boiler has thirty 4-in. tubes. It evaporates 1400 Ibs. of water per hour from a feed temperature of 120 into steam at 100 Ibs. What per cent, of its rating is the boiler developing? Solution. First find heating surface from the rule in paragraph 63. H.S. of cylindrical portion of shell ^-* , = ?~X-3.l4i6 X 4 X 12. =100. 6 sq.ft. 3^74 H.S. of tubes = 30 X 3.1416 X -^r* X 12 = 352.5 sq. ft. -. \.i - - ' H.S. of tube sheets = 2[f(3.1416 XJ2 X 2) -(30 X 3.1416 X i X = 2(8 . 38 - 2 . 62) =11 . j> sq. ft. Total heating surface = 464 . 5 sq. ft. From paragraph 63, the rate4 horse-power = ' = 38 . 7 Now find actual horse-power developed. The heat actually used in evaporating a pound of water is equal to the total heat in a pound of steam at thfc given pressure minus the heat already in the feed water. The heat used in evaporating water under actual conditions = 1400 [1188. 6 - (120 - 32)] = 1,541,000 B.T.U. From paragraph 64, the equivalent evaporation from and at 212 1,541,000 = -Q 7 Q A = 1588 Ibs. per hour, and from paragraph 62, the boiler horse-power BOILERS 103 1588 34.5 46. 46 5^ = 1.19 = 119 per cent. 00. I Ans. Boiler is developing 19 per cent, overload. Example. A boiler evaporates 8.23 Ibs. of water per pound of coal fired. Feed temperature, 120; steam pressure, 100 Ibs. Coal as fired contains 2 per cent, moisture. Dry coal contains 5 per cent, ash and has a heating value of 12,800 B.T.U. per pound. Twelve per cent, of coal fired is taken from ash pit in form of ash and refuse, (a) Find the efficiency of the boiler, furnace and grates combined. (6) Find the efficiency of the boiler and furnace. Solution. (a) Heat necessary to evaporate 1 Ib. of water = 1188.6 - (120 - 32) = 1100.6 B.T.U. Water evaporated per pound of dry coal fired 8.23 8.23 = 1. 00^02 = T98 = Heat utilized per pound of dry coal fired = 8.4 X 1100.6 = 9245 B.T.U. Efficiency of boiler, furnace and grates combined _ Heat utilized per pound of dry coal fired Heating value of 1 Ib. of dry coal 0045 = = .7223 = 72.23 per cent. (6) Heating value of 1 Ib. of combustible 12800 12800 , = EOO"- .05 = ~96~ " 13 ' 474 RT - U ' Water evaporated per pound of combustible burned 8 '1 3 = 9.57 Ibs. 1.00 -(.02+ .12) .86 Heat utilized per pound of combustible burned = 9.57 X 1100.6 = 10,533 B.T.U. Efficiency of boiler and furnace _ Heat utilized per pound of combustible burned , Heating value of 1 Ib. of combustible 104 HEAT ENGINES 10533 13474 ~ - 7817 " = 78 - 17 Percent.* . f (a) 72.23 per cent. ?< \(6) 78. 17 per cent. Example. If 26 Ibs. of air are used to burn a pound of coal containing 13,500 B.T.U., and the temperature of the stack gases is 550, what per cent, of heat is lost up the stack, if the temperature of the boiler room is 70? Solution. If there were no ash in the coal, each pound burned would give off a pound of gas and the total weight of stack gas per pound of coal fired would be 26 + 1 = 27 Ibs. This, however, is never the case as there is always some ash and unburned coal, and hence the actual weight of stack gas per pound of coal is something a little less than 27 Ibs. The average of the specific heats of the various components of the stack gase? is a little higher than that of air, .2375. To be absolutely correct, then, it would be necessary to multiply the weight of each of the various gases in the stack gas by its particular specific heat, and then add these prod- ucts together to get the B.T.U. necessary to raise the products of com- bustion one degree. This, however, is never done, the method used being to assume the specific heat of the stack gases to be the same as that of air, .2375, although really it is slightly higher, and to assume that 1 Ib. of gas is given off from 1 Ib. of coal, although in reality it is a little less. Thus one assumption practically offsets the other, and the result is approximately correct. Hence, the heat necessary to raise the products of combustion one degree = .2375(26 + 1) = 6.41 B.T.U. Rise in temperature of the stack gases = 550 - 70 = 480. * This answer may be checked as follows: Efficiency of grate alone _ Combustible burned per pound of coal fired Combustible fired per pound of coal fired = = - 9237 = 92 - 37 per cent - Efficiency of boiler and furnace _ Efficiency of boiler, furnace and grate Efficiency of grate alone 722S - = 1)237 = - 7819 =78 - 19 P er cent - BOILERS 105 Heat necessary to raise the stack gases 480 = 480 X 6.41 = 3080B.T.U. Per cent, of heat lost up the stack 3080 = 1Q =.2281 = 22.81 per cent. loOUU 67. Boiler Accessories. In order to determine the physical condition of the steam and water in a boiler, all boilers are provided with a steam gage showing the pressure per square inch in the boiler, a gage glass to indicate the water level in the boiler, and a safety valve which automatically relieves the pressure in the boiler should it exceed the safety point. The feed-water pump, or other feeding device, supplies the boiler Elevation. Interior mechanism FIG. 29. Pressure gage. with water to take the place of water which has been made into steam. The blow-off cock is attached to the lowest point of the- boiler and drains the water from the boiler. This is usually opened from time to time to blow the mud and settlings out of the boiler. The ordinary form of pressure gage is shown in Fig. 29. Pressure gages should be placed at a convenient point for easy observation, and the piping should be as short as possible. The gage should always be provided with a siphon containing water so that the hot steam cannot enter the gage. If hot steam enters the gage it changes the length of the copper gage-tube, which changes the calibration of the instrument. It should also have a gage cock and union so that it may be easily removed. The operat- ing portion of the gage consists of a flattened copper tube bent in a circle and closed at the end. One end is fixed, or, as shown in Fig. 29, there are two such tubes. When fluid pressure is 106 HEAT ENGINES applied to the inside of the tube, its cross-section tends to assume a circular form and the tube tends to straighten. The greater the pressure the more the straightening of the tube. By proper mechanism this change of form due to pressure is registered on a dial, which when properly calibrated shows the pressure in the boiler. Fig. 30 shows the elevation and cross-section of a water column with its gage glass. The section shows the float so arranged that it will blow a whistle when the water in the boiler is too high or too low. This is called a "high and low water alarm." Elevation Cross-section. FIG. 30. Water column. The water gage and water column to which it is attached are important accessories in boiler operation. The length of the water gage on the boiler should be such as to cover the ordi- nary fluctuations of water in the boiler. It should always be attached to a water column. On this water column are placed tri-cocks, or gage cocks, used as a check upon the water column, as the water column is sometimes clogged with dirt. The lowest point in the gage glass should be set about 3 in. above BOILERS 107 the highest point of the tubes in tubular boilers. The position of the gage glass in water-tube boilers is usually determined by the manufacturers. The top of the water column should be attached to the steam space so that it will get dry steam, and the bottom of the water column to the water space at a point in the boiler. There should be blow-off valves on both the water column and the water gage. The water columns and gage cocks should be blown off frequently. Fig. 30 shows the ordinary arrangement of water column, water gage, and tri-cocks. Safety valves are constructed in a great many different forms, but in general they consist of a valve opening outward and held in place by a spring, and in the old forms by an arm and weight. Fig. 31 shows the construction of the ordi- nary safety valve. The size of the safety valve is usually determined by the grate surface and the steam pressure carried. The following rule may be used : Let G = the grate surface in square feet; P = the pressure in pounds per square inch gage; A = the total area of safety valve, or valves, in square inches. 22. 5G FIG. 31. Safety valve. Then, AJL ~ T * P + 8.62 Some authorities allow in spring-loaded safety valves 1 sq. in. of safety valve for every 3 sq. ft. of grate surface. Formerly the lever safety valve was the type most used, but it was easily tampered with. At the present time the pop safety valve is almost universally used. Safety valves are adjusted so as to blow at one pressure, and seat at a pressure usually 2 Ibs. less than that at which they open. The safety valve on the boiler should be tried once a day at least, to see if it is in working condition. In an article presented to the A.S.M.E., the following expres- sions have been developed for determining the size of safety valves to be used on boilers: 108 HEAT ENGINES For 45 valve seats Z) = - 0095 ifp'- For locomotives For fire-tube and water-tube stationary boilers H J LXP' For marine boilers E = Pounds of steam discharged, or boiler-evaporation, per hour. L = Vertical lift of the valve in inches. P = Steam pressure (absolute) in pounds per square inch. D = Nominal diameter of valve (inlet) in inches. H = Total boiler heating surface in square feet. The average lift for a safety valve is about . 1 of an inch. More exact results may be obtained by reference to a paper on this subject by P. G. Darling in the A.S.M.E. Proceedings for 1909. The feed pipe to the boiler is always provided with a valve and check valve. In case of accident to the feed valve the check valve will close and prevent the water from leaving the boiler. It sometimes happens that a boiler shell may become over- heated and a boiler explosion results from this accident. Such accidents are avoided by having screwed into the boiler a plug consisting of a brass bushing filled with a metal, which melts before any damage can be done to the boiler. These plugs are called fusible plugs and are often used. CHAPTER VII BOILER AUXILIARIES 68. Mechanical Stokers. In firing a boiler the best results are obtained by firing the coal in small quantities, or by pro- gressive burning of the coal. With hand firing these results are difficult to accomplish. Most firemen prefer to shovel the Transverse Section FIG. 32. Murphy stoker cross-section. coal into the furnace in relatively large amounts and then rest. It is difficult to get them to give the proper attention to the handling of their fires. With mechanical stokers it is possible to introduce small quantities of coal frequently, or so arrange the stoker that there may be progressive burning of the coal. The first stoker was introduced into England by Brunton in 1822, and at nearly the same time by Stanley. These were 109 110 HEAT ENGINES both of the sprinkling type. The first chain grate was brought out by John Juckes. The first American stoker was invented FIG. 33. Murphy stoker view from rear of furnace. by Thomas Murphy of Detroit, Mich, in 1878, and it was prod- bly the first to have a sloping grate. Stokers may be divided into .three principal classes: the inclined grate, the chain grate, and the under-feed. FIG. 34. Elevation of Detroit stoker. 69. Inclined Grates. The Murphy stoker, shown in Figs. 32 and 33, is an example of the inclined grate known as the side overfeed or opposed type. BOILER AUXILIARIES 111 "At either side of the furnace, extending from front to rear, is a coal magazine into which the coal may be introduced either mechanically from conveyors, or by hand. At the bottom of this magazine is the coking plate, against which the inclined grates rest at their upper ends. The stoker boxes, operated by seg- ment gear shaft and racks, push the coal out over the coking plate and on to the grates. The grates are made in pairs one fixed, the other movable. The movable grates, pinioned at their upper ends, are moved by a rocker bar at their lower ends, alternately above and below the surface of the stationary grates. The stationary grates rest upon the grate bearer, which also contains the clinker or ash grinder. This grate bearer is cast hollow and receives the FIG. 35. Detroit stoker view from rear of furnace. exhaust steam from the stoker engine. This steam escapes through small openings at regular intervals on either side of the clinker grinder and lower ends of the grates, to soften the clinker and so assist the cleaning process." Figs. 34 and 35 show the elevation and rear view of a Detroit stoker. This is very similar to the Murphy. In small plants a worm conveyor type of feed is used (see Fig. 35). These conveyors revolve through the hoppers and feed the coal in through the magazines. This is a convenient arrangement where the coal is shoveled into the hoppers from the floor by hand. Both the Murphy and Detroit stokers are adaptable to all 112 HEAT ENGINES grades of bituminous coal, but are not suited to the use of lignite or anthracite. The Dutch oven, or extension settings, are generally re- garded as the most effective both as to efficiency and as to the elimination of smoke. Still another form of inclined grate stoker is the Roney, shown in Fig. 36, in whichTh!Tc7)aT7s~T^ of the boiler, and is pushed upon an inclined grate from the front. The feeding mechanism is shown in Fig. 37. To the FIG. 36. Roney stoker side view. main operating shaft which runs horizontally just beneath the hopper, is keyed an eccentric which gives a pendulum motion to the agitator. This agitator is connected with the grates, giving to them a rocking movement. It also, through the rock-shaft, imparts a reciprocating motion to the pusher, the length of this motion being regulated by a hand- wheel. As the pusher recedes, the fuel in the hopper settles down in front of it, and as it ad- vances the fuel is pushed into the furnace. As the main operating shaft runs at constant speed, the quantity of fuel fed is propor- tional to the travel of the pusher. The reciprocating motion of the rocker bar is imparted through BOILER AUXILIARIES 113 a connecting rod, the free end of which works freely through a sliding bearing in the lower end of the agitator an adjusting nut on the connecting rod makes it possible to regulate the re- ciprocating motion of the rocker bar, and consequently the amplitude of the rocking motion of the grate bars. The inclined grate stoker has given excellent satisfaction, particularly in using diversified coals. In conditions of excessive loads this is probably not so smokeless as some other forms of stoker, but when carefully operated is one of the most satis- factory forms. hopper Plate Feed Wheel-., Agitator-Sector Sheath-Nut Hock- Shaft Agitator.. Lock- Nut - Main Operating Shaft--- Connecting Rod- Rocker Bar- FIG. 37. Roney stoker feed mechanism. 70. Chain Grates. Fig. 38 shows the elevation of a chain grate. The coal is fed into a hopper, the bottom of which is open to the chain grate, composed of a series of flexible links rotating upon two cylinders, one at each end of the grate. The grate is driven by a small engine the speed of which can be adjusted to the particular form of fuel burned and the load on the boiler. This speed should be regulated so that the fuel is completely burned just as it reaches the back of the grate. If 114 HEAT ENGINES the speed is too fast, unburned coal will be carried over the back of the grate into the ash pit. If it is too slow, there will be holes FIG. 38. Green chain grate. FIG. 39. Green chain grate stoker applied to Stirling boiler. in the fire toward the rear, allowing an excess of air to pass through the grate. The coal drops from the hopper upon this slowly moving grate, the thickness of the bed of coal being ad- BOILER AUXILIARIES 115 justed by an apron at the front of the boiler. This form of grate gives excellent satisfaction with non-coking coals and uniform loads on the boiler, and will be almost smokeless under proper conditions of operation. It is not adapted to the use of semi- bituminous, or anthracite, coal. The greatest difficulty is im- proper installation, which will permit of the passing of an excess of air through the grate. This, however, may be avoided by careful setting. In installing these grates, provision should be made for the easy removal of the ashes. Fig. 39 shows the FIG. 40. Jones under-feed stoker. cross-section of a chain grate installed under a Stirling boiler. It also shows the ash pit and sub-basement for easy removal of the ashes. .This is a desirable arrangement with most forms of stokers. 71. Under-feed Stokers. One of the commonest forms of under-feed stokers is the Jones, shown in Fig. 40 applied to a boiler plant, and in Fig. 41 in cross-section. In this form, coal is dropped down from hoppers in front of a piston at regular intervals depending upon the load. This piston moves forward 116 HEAT ENGINES and pushes the coal in under the burning fuel. In this way coal is always introduced under the fire, and all the gases are FIG. 41. Section of Jones under-feed stoker. passed through the incandescent fuel. This is the most smokeless form of stoker. Fig. 42 shows the American type of stoker, which is similar FIG. 42. American under-feed stoker. in operation, but in this stoker the piston of the Jones is replaced by a worm which continuously feeds the coal underneath the fire. BOILER AUXILIARIES 117 The under-feed form of stoker produces a very intense heat directly above the fire. The ash accumulates on the top of the fire and falls over to the sides of the furnace, from where it is taken out. Owing to the ashes being raised to a high tempera- ture, coal containing ash which is high in sulphur and iron should not be used in a stoker of this type, as it will produce very large, hard clinkers. In under-feed stokers, the resistance of the fuel bed to the passage of air is so great that it is necessary to use a blower FIG. 43. Taylor stoker cross-section. to force the air through the fuel bed. This blower is usually driven by a steam engine, and the excessive amount of steam used is one of the objections offered to the use of these stokers. The Taylor, Figs. 43 and 44, is an inclined under-feed stoker. "Coal from the hopper is fed into the retort, from which two cylindrical rams, assisted by gravity, introduce it into the fur- nace at an angle to the fire surface. Movement of the upper 118 HEAT ENGINES ram pushes the green coal outward and upward, properly dis- tributing it in the coking zone. The action of the lower ram is similar, but instead of bringing in fresh coal it pushes the fuel bed and refuse toward the dump plates at the rear. The retort or fuel magazine is formed by two tuyere boxes. Air for combustion enters the tuyere boxes from the wind box, and escaping from the tuyere openings mingles with the gases distilled from the coal and with the coked fuel pushed outward and upward by the rams. Both rams are actuated by connecting rods and links from a crank shaft which is driven from the speed shaft. The speed shaft in turn is driven by the fan engine. FIG. 44. Taylor stoker perspective, showing dumping plates down. "The dump plates, which are combination dump plates and fire guards, are hung on the rear of the wind box; these plates receive the burned out refuse and are dumped periodically, as the conditions of service may require. The dump plates are operated from the front of the stoker, raised, latched in position and released by a hand lever." 72. Grate Surface in Stokers. The grate surface in a stoker with an inclined grate is taken as the area of the horizontal projection of the grates, and is termed projected area. The ratio of projected grate area in the stoker to the heating surface in the boiler varies from 1 to 55, to 1 to 65. BOILER AUXILIARIES 119 73. Advantages and Disadvantages. The principal advan- tages of mechanical stokers are: smokeless operation of the furnace, adaptability to the burning of cheaper grades of coal, uniformity of furnace conditions and steam pressure, which adds to the economy of the plant, and in larger plants, a saving in the labor charge for plant operation. Their disadvantages are: high initial cost, large repair bills, cost of operating stoker mechanism, which in most stokers is from J to 3 per cent, of the steam generated, and, if fan blast is used, from 3 to 5 per cent, of the steam generated. FIG. 45. General arrangement of a modern boiler room. In small plants where coal-handling machinery is not pro- vided, stokers will not reduce the labor charge. In large plants where the coal is delivered mechanically to the stoker hoppers, stokers will materially reduce this charge. Fig. 45 shows a plant with stokers fed from overhead hoppers. In such plants the ash is usually removed from a basement under the boiler room floor. 74. Boiler Feed Pumps. The feed water is forced into a boiler either by a feed pump or an injector. There are wot general types of feed pumps: belted feed pumps which may 120 HEAT ENGINES be driven from the machinery, or by independent motor; and independent pumps driven by their own steam cylinders. The independent feed pump is most commonly used as it has the advantage of being independent of the operation of the main engine, and in addition its speed can be adjusted so as to give uniform feeding. Its principal disadvantage is in the large steam consumption of pumps of this type. Small feed pumps use from 150 to 300 Ibs. of steam per indicated horse-power per hour; large steam pumps, from 80 to 150 Ibs.; compound con- densing feed pumps of the direct-acting type, from 60 to 75 Ibs. The mechanical efficiency of these pumps is about 80 per cent. FIG. 46. Worthington boiler feed pump. Fig. 46 shows a modern form of feed pump having four single- acting water cylinders. This pump has -two plungers working in these cylinders. The plungers are in the center of the pump and have the packing glands outside the cylinder. This type of pump is called an outside center packed pump. The belt-driven pump is often used to overcome the steam wasted when using the independent direct-acting pump. These pumps may be driven from the shaft of the main engine or from the line shafting. In some cases they are driven by an electric motor. This arrangement has its disadvantages. The speed of the pump being constant, it is necessary to regulate the amount of water pumped by a by-pass allowing part of the water pumped to go back from the pressure to the suction side of the pump. BOILER AUXILIARIES 121 If the feed is suddenly shut off from all the boilers, provision must be made for the discharge from the pump being turned back to the suction automatically. It is not possible to use a belted feed pump except when the engine is running, and there must be an auxiliary feeding device provided that can be operated when the main engine is shut down. In very large plants steam turbine driven, turbine pumps are being used. These pumps being of the centrifugal type, it is not necessary to change the speed of the pump for changes of load. The speed of a turbine pump determines the pressure Overflow Water FIG. 47. Steam injector cross section. only and the amount of water pumped depends upon the demand. This is, therefore, automatic and requires very little attention. 75. Steam Injectors. Boilers are often fed by an injector, a device invented by M. Giffard, a French engineer. Fig. 47 shows the cross-section of an injector, the operation of which is as follows: The handle, 137, is pulled back slightly, thus raising valve 130 from its seat and admitting steam through valve 126 to the lifter nozzel 101. "The discharge of steam from this nozzle into the lifter combining tube, 102, entrains the air in the suction pipe finally producing sufficient vacuum to lift the water. The flow of water passes through both the intermediate overflow, 121, and the forcer combining tube, 104, and out of the final overflow, 117. A further movement of the lever opens the forcer steam valve, 126, and admits 122 HEAT ENGINES steam to the forcer steam nozzle, 103, while at the same time the final overflow valve is approaching its seat, producing a consevuent increase of pressure in the delivery chamber. This pressure closes the intermediate overflow valve, 121, and opens the intermediate or line check valve, 111, and when the final overflow valve, 117, is brought to its seat the injector will be in full operation. The intermediate overflow valve, 117, operates automatically, its only function .being to give direct relief to the lifter steam nozzle when lifting or priming, and comes to its seat when the forcer steam is applied and is held there by the pressure exerted by the forcer." There are many different forms of injectors made for different conditions. The injector, however, is a very inefficient pump for general pump purposes. It is installed, however, as an auxil- iary method of feeding the boiler in case of accident to the regular feed pump. As a boiler feeder it has a thermal efficiency of al- most 100 per cent, since all the heat of the steam used by the in- jector, except that lost by radiation, goes into the feed water. In locomotives, injectors only are used for feeding the boiler, as they take very little space and warm the feed water. Each locomotive is provided with two injectors. 76. Pump Connection. When a pump or injector is han- dling cold water, the lift on the suction side of it should not exceed 25 ft. Most engineers try to install pumping apparatus with a head on the suction not more than 15 ft. When hot water is to be handled, the pump should be below the level of the water on the suction side. By hot water is meant water exceeding 120. Injectors are seldom used to handle hot water as they are very difficult to start with water exceeding 100. Where pumps are installed handling hot water from a feed-water heater, the level of water in the heater should be 5 ft. above the center line of the pump cylinders if pos- sible. Hot water cannot be raised by a pump, as the lowering of the pressure in the suction pipe lowers the temperature of the boiling point of the water in the suction pipe, the water in the suction boils and all the pump draws from the suction is steam. 77. Feed-water Heaters. It is very important that a boiler be fed with warm water, usually at a temperature over 180. This saves part of the heat necessary to make steam, and in addition prevents strains in the boiler due to a difference in BOILER AUXILIARIES 123 temperature of different parts of the boiler shell. Feeding a boiler with cold water often causes a leak. In all modern power plants some means is provided for heating the feed water before entering the boilers. This is usually accomplished in one of two ways; by heating the water with the exhaust steam from the engine, which is by far the commonest method used, or with waste gases from the boilers. Devices for using the exhaust steam for heating the water are called feed-water heaters, and the device for using the gases from the boiler for heating the feed water is termed an economizer. The principal advantages of the feed-water heater are the saving in B.T.U. due to the increase 'in the temperature of the feed, and the saving in wear and tear on the boiler due to in- troducing hot instead of cold water, thereby reducing the strain on the boiler. A heater which increases the temperature of the feed water from 70 to 200 will save about 12 per cent, of the fuel, and the installation of a heater will usually pay for itself in a few months. 78. Types of Feed-water Heaters. There are two general types of heaters: the open and the dosed. The open feed- water heater, Fig. 48, consists of a cast- or wrought-iron shell into which the exhaust steam is led. The cold water is admitted at the top of the heater, and is allowed to pass through the exhaust steam in streams or sheets of water. In this type of heater the feed water and exhaust steam come into direct con- tact with each other. The water usually passes over pans, or trays, upon which any scale-producing matter can be deposited. When it is desired to clean the heater, it is only necessary to take out these pans and clean them. Before entering the heater the exhaust steam should be passed through an oil separator. The hot feed water is usually passed through some form of filter before going to the feed pumps. The feed-water heater should be located at a sufficient height above the feed pump so that the water will enter at a pressure. This distance should be 5 ft. or more. The heater may also be used as a receptacle for the hot water which is drained from the steam mains, and for other hot condensed steam which does not contain oil. A uniform water level is maintained in the heater by a float valve which automatically allows water to enter the heater when the level gets below a certain point. The closed heater shown in Fig. 49 consists of a cylindrical 124 HEAT ENGINES shell of cast iron, or steel, containing tubes extending from the header at one end of the heater to the header at the other end, FIG. 48. Open feed-water heater. or tubes in the form of coils of pipe. The exhaust steam is admitted on one side of the tubes and the feed water on the other. In a closed heater the feed water and the steam used Blow Feed Steam Outlet or Inlet FIG. 49. Closed feed -water heater. do not come in contact with each other. The closed heaters are usually used where it is desired to pass the water through BOILER AUXILIARIES 125 the heaters under pressure. They are more expensive than the open heaters and are more difficult to clean. Where possible it is better to use an open heater. 79. Installation of Heaters. Open heaters are placed on the suction side of the feed pump, and the feed water must be brought to the heater. The level of the water in an open heater should be at least 5 ft. above- the center of the feed-pump cylin- der as a feed pump cannot lift hot water. Injectors are never used with an open heater as they cannot be used with hot water. Closed heaters are placed on the discharge side of the pump and the feed pump may lift its supply directly from the source of water. An injector may be used with a closed heater. Heaters cost from $2 to $4 per boiler horse-power served by them. 80. Economizers. Any device which heats the feed water by means of the heat in the gases which leave the boiler is termed End elevation. Side elevation. FIG. 50. Economizer. an economizer. Fig. 50 shows the elevations of an economizer. The cold water is pumped into the lower pipe header, and after being heated, passes out from the upper header to the boiler. The flue gases from the boiler pass around the pipes and headers containing the feed water. The tubes as shown in the cut are provided with scrapers operated from time to time to remove the soot from the pipes. The general arrangement of an econ- omizer is shown in Fig. 51. An economizer is always provided 126 HEAT ENGINES with a duct, or by-pass, passing around it, so that it can be cleaned without shutting down the plant. The economizer is placed in a brick or sheet metal flue which carries the gases from the boiler to the chimney. Economizers are installed so as to make use of the heat in the gases leaving a boiler and thus reduce the waste in heat going up the stack. Economizers may be installed also to increase the capacity of a boiler plant which is too small for its services. They deliver the water to the FIG. 51. Economizer, showing location in breeching. boiler at a high temperature, reducing the strain and the leakage caused by the admission of cold water. Their particular dis- advantage is in reducing the strength of the draft owing to the fact that the economizer causes additional friction. Econo- mizers should never be used except with chimneys having a strong draft or with mechanical draft. The first cost of the economizer is very high, varying from $5 to $6 per horse-power for plants of 1000 horse-power or over. A number of tests have been made of the economizer where a BOILER A UXILI ARIES 127 net saving of 10 per cent, was shown, allowing for cost of econo- mizer, cost of operation, interest, depreciation, and repairs. From 4 to 5 sq. ft. of economizer surface should be allowed per boiler horse-power. 81. Superheaters. In the past few years the use of super- heated steam with both reciprocating engines and turbines has become very general. The benefits derived are many. The steam remains in a dry condition until all the superheat is lost. The heat lost by the steam while passing through the piping from the superheater to the place where it is to be used, does not B FIG. 52. Superheating coil in Babcock and Wilcox boiler. cause condensation as it is simply superheat which is given up. The initial condensation loss in reciprocating engines is greatly reduced, or entirely eliminated, depending upon the amount of superheat in the steam. In turbines the absence of moisture is particularly desirable, as the water coming in contact with the blading at a high velocity has an eroding effect, thus in- creasing the clearance between the blades and the casing and consequently increasing the steam consumption. Recent experiments have shown that when steam is super- heated from to 100 F. there is a saving of 1 per cent, in 128 HEAT ENGINES steam consumption for every 10 degrees of superheat, and when superheated from 100 to 200 F. there is a saving of 1 per cent, for every 12 degrees of superheat. These results are based on a comparison between superheated and dry saturated steam. If the steam is wet. the saving will, of course, be much larger. The degree of superheat to be used will depend largely upon the conditions. In the majority of cases it has been found that the highest commercial efficiency is secured by the use of from 125 to 150 of superheat in turbine plants and slightly less in the case of reciprocating engine plants. A superheating coil placed in a Babcock & Wilcox boiler is shown m Figure 52. It has been frequently stated that cast-iron fittings and valves should not be used with superheated steam, as the iron deterio- rated at the high temperatures. Recent developments have shown that the trouble has been caused by fluctuating rather than high temperatures. In the transactions of the A.S.M.E., Vol. 31, page 1037, Professor Hollis states: "When the temperature is constant, even though as high as 600 or 700 F., the change in cast iron is not serious enough to prohibit us from its use, but where the temperature varies considerably, the metal is certain to develop cracks and distortion that render it unsuitable for steam pipes and other parts under steam pressure. "The use of cast-iron fittings for superheated steam is inadvisable where the temperature is likely to fluctuate, but it can be safely used where the temperature is to be constant." 82. Chimneys. The chimney is a very important part of a steam-power plant, and the operation of the plant depends upon the draft and capacity of the chimney. 83. Draft. The draft in a chimney is produced by the dif- ference in weight between the column of hot gases inside the chimney and a column of gases of the same dimensions outside the chimney. The hot gases, being light, are forced up the chimney by the cold gases coming through the grates. The height of the chimney then determines the intensity of the draft. The draft is always measured in inches of water, and for a given height of stack may be determined as follows: BOILER AUXILIARIES 129 Let H = the height of the chimney. T = the absolute temperature of the gases outside the chimney. T' = the absolute temperature of the gases inside the chimney. w = the weight of a cubic foot of air at a temperature /T7O w' '= the weight of a cubic foot air at a temperature T'. Then assuming the chimney to have an area of 1 sq. ft., the weight of the hot gases equals Hw' = Hw ~r (1) The weight of the cold gases equals Hw = Hw e ~ (2) Hence the force of the draft, rpo F' = Hw - Hw' = Hw - Hw 7^ Therefore F' = Hw(l - J)- (3) This is in pounds per square foot. To reduce to inches of water this must be multiplied by .192. Hence the force of the draft in inches of water, F = .W2Hwl - ~- (4) The intensity of the draft as shown in equation (4) is determined by the height of the chimney and the temperature inside and outside the chimney. 84. Chimney Capacity. -The capacity of a chimney is the quantity of gases that it will pass per hour, and upon the capacity of a chimney depends the number of pounds of coal that the plant will burn. The theoretical quantity of coal that a chimney will burn may be found as follows: Let h = the head producing velocity. Then the weight of the gases producing the head equals hw', and hw' = Hw - Hw' = Hw' ~ - Hw'. (5) 130 HEAT ENGINES Therefore IT' \ h = H (^ - 1.) (6) Let u = the velocity of the entering gases and u' = the velocity of the leaving gases in feet per second. Then the veloc- ity of the leaving gases (7) Let W = the total weight of the gases passing up the chimney per second, then W = wu = w'u' -- or For an outside temperature of 70 F., w = .075 and T = 530. Assume the temperature in the chimney to be 500 F. Then T' equals 960. Substituting these values in equation (8), = .602 # X .247. (9) If A = area of the chimney in square feet, then W = .30 A V# in pounds per second, (10) or in pounds per hour TF! = 3600 X .3 A Vff. (11) This assumes the efficiency of a chimney to be 1, but experi- ence shows the average efficiency of a chimney to be about 35 per cent., so that the actual weight of air passed per hour is W a = 3600 X .35 X .3A V# = 3784 V#. (12) Each pound of coal requires 24 Ibs. of air to burn it, and as each boiler horse-power requires about 5 Ibs. of coal, the boiler horse-power of a chimney is B.H.P. = ^ VH = 3.15A V#- (13) BOILER AUXILIARIES 131 Various authors give values of the constant in this expression varying from 3.5 to 3.0. 85. Height of a Chimney. The height of a chimney is always measured from the level of the grate and, in any given case, depends upon the kind of fuel that is to be burned under the boiler. The following table gives the minimum height of chimney for various kinds of fuels : TABLE XVIII. CHIMNEY HEIGHTS For straw or wood 35 feet. bituminous lump, free burning 100 " ordinary slack 100 ordinary bituminous coal 115 *' small slack or anthracite 125 " anthracite pea coal 150 " The height of the chimney should not be too short for its diameter. A very large diameter of chimney in proportion to the height may show reduced capacity. As an example, a chim- ney 100 ft. high should not exceed 6.5 ft. in diameter. In general the inside diameter of a chimney should not exceed 8 per cent, of its height. 86. Materials Used. Brick or hollow tile is more extensively used in building chimneys than any other material where per- manent chimneys are desired. The life of a brick chimney is probably forty or fifty years. These materials are used in plants where few changes are expected. In most plants the station is not expected to remain without extensive changes more than twenty or twenty-five years, and the expense of a brick chimney is not warranted. Many of the recent power houses are using self-sustaining steel chimneys. For temporary use the unlined sheet steel chimney is very commonly used. It is necessary to brace these chimneys with steel guy wires. The life of these chimneys is short, at the best not more than ten years, and where the coal contains much sulphur not more than five years. 87. Brick Chimneys. Brick chimneys, as shown in Fig. 53, are built in two parts, an outer shell and an inner shell, usually lined with fire-brick which forms a flue for the burned gases. There should be an air space between the outer and the inner shells so that the inner shell is free to expand. Brick chimneys are expensive to erect, but very permanent in character. Care should be taken in investigating the ground which is to support 132 HEAT ENGINES a chimney, as unequal or excessive settlement may endanger the chimney. The radial brick chimney is constructed of hollow tile and has no lining. These chim- neys are much lighter than the solid brick chimney. They are much less expensive than the brick and cost but little more than a self- sustaining steel chimney. 88. Steel Chimneys. Steel chimneys of the self-sustaining type are built of boiler plates riveted together. They are supported on am- ple foundations to which they are bolted by very heavy anchor bolts. The pressure of the wind - against the chimney is carried to the foundation by these bolts, and the foun- dation must be of sufficient size and weight to prevent overturning. Chimneys of this type are lined with fire-brick usually for their full length. 89. Mechanical Draft. In some cases con- ditions will not permit of the construction of a tall chimney, and in other cases the draft required is more than the ordinary chimney will give. It is then necessary to resort to some form of forced or mechanical draft. Mechanical draft is entirely independent of the temperature inside or outside of the chim- ney. Where economizers are used, the tem- perature in the chimney may be so low and the resistance of the economizer such as to require mechanical draft. 90. Systems of Mechanical Draft. There are three systems that may be used to produce mechanical draft. (1) A steam jet may be used to force air into the ash pit. (2) A fan may be used to force air into the FIG. 53. Brick ^ ., chimney. ash P 1 ^ Both of the above systems require a closed ash pit and are termed forced draft, as the air is forced through the fire. CC BOILER AUXILIARIES 133 (3) The third system, or induced draft, is more commonly used. With this system a fan is placed in the smoke connection to the chimney, or, as in the case of locomotives, the cylinders exhaust directly into the stack, and air is drawn through the fire. The action in this case is analogous to the action of the chimney. Under ordinary conditions the rate of combustion may be taken as from 15 to 30 Ibs. of coal per square foot of grate surface per hour with mechanical draft. With mechanical draft the air required to burn a pound of coal may be reduced to 18 Ibs. With induced draft the pressure of the draft usually varies from 1.5 to 2 in. of water. The operation of an induced draft plant may be made partially automatic. This is done by driv- ing the fan with an engine and having the speed of the engine controlled by the steam pressure in the boilers. PROBLEMS 1. Calculate the factor of evaporation for a gage pressure of 75 Ibs. and an initial temperature of the feed water of 135. 2. A boiler evaporates 500 Ibs. of water per hour from a feed temperature- of 145 into steam at 80 Ibs. pressure. What is the equivalent water evapo- rated per hour from and at 212? V3. A boiler evaporates 85 Ibs. of water per pound of coal. Pressure in % boiler, 125 Ibs.; feed temperature, 150. f What will it evaporate from and at 212? 4. A boiler evaporates 8 Ibs. of water per pound of coal. Pressure, 100 Ibs.; feed temperature, 100. What will it evaporate if the pressure is 80 Ibs. and feed 200; and what will it evaporate from and at 212? 5. A boiler evaporates 8 Ibs. of water per pound of coal. Steam pressure, 120 Ibs. ; feed temperature, 150. What will it evaporate with a steam pres- sure of 5 Ibs. and a feed temperature of 200? / 6. A boiler evaporates 9 Ibs. of water per pound of coal. Steam pres- r^/sure, 100 Ibs.; feed temperature, 50. What will it evaporate if the steam / \pressure is 200 Ibs. and the feed temperature 150? 7. A boiler evaporates 8000 Ibs. of water per hour. Steam pressure, 120 Ibs.; feed temperature, 180. What would it evaporate if the steam pressure were 60 Ibs. and the feed temperature 60? 8. A boiler plant evaporates 6 Ibs. of water per pound of coal. Steam - pressure, 150 Ibs.; feed temperature, 120. What will it evaporate if an economizer is added increasing the feed temperature to 230? 9. A boiler evaporates 5000 Ibs. of water per hour from a feed-water- temperature of 70 into steam at 120 Ibs. pressure. What is the evaporation from and at 212? If the efficiency of the boiler, furnace and grate is 70 per cent, and coal that contains 13,500 B.T.U. per pound is used, how many pounds of water will be evaporated from and at 212 per pound of coal? 10. A coal contains 14,000 B.T.U. per pound dry. If all the heat in this 134 HEAT ENGINES coal should be utilized, how many pounds of water would be evaporated per pound of dry coal? Steam pressure, 200 Ibs.; feed temperature, 250. *" 11. Efficiency of a boiler, furnace and grate is 65 per cent. Coal burned contains 12,000 B.T.U. per pound. Steam pressure, 200 Ibs.; feed tempera- ture, 180. How many pounds of water will be evaporated per pound of coal? "* 12. A boiler burns coal containing 13,000 B.T.U. per pound. Steam pres- sure, 200 Ibs.; feed temperature, 200; efficiency of the boiler, furnace and grate, 75 per cent. What would be evaporated from and at 212? 13. One hundred pounds of coal containing 13,000 B.T.U. per pound will evaporate how many pounds of water at 200 into steam at 100 Ibs. pressure? What will it evaporate from and at 212? Efficiency of the boiler, furnace and grate, 70 per cent. -* 14. How many pounds of water can be evaporated from and at 212 by the heat evolved by the complete combustion of 1 Ib. of dry coal contain- ing C, 65.2 per cent.; H, 4.92 per cent.; O, 8.64 per cent.? 16. A coal contains C, 75 per cent.; H, 5 per cent.; O, 4 per cent. Effi- ciency of the boiler, furnace and grate, 70 per cent.; feed temperature, 180; steam pressure, 150 Ibs absolute. Steam contains 2 per cent, moisture, (a) What is the actual evaporation per pound of coal? (6) What is the equiva- lent evaporation from and at 212 per pound of coal? * 16. A coal contains C, 80 per cent. ; O, 7 per cent. ; H, 3 per cent. ; and ash, 10 per cent. It is used in a boiler carrying 100 Ibs. pressure with a feed tem- perature of 180. The efficiency of the boiler, furnace and grate is 70 per cent. What will be the evaporation per pound of coal? m 17. If 40 per cent, of the heat of combustion of coal containing 12,750 B.T.U. per pound is lost, how many pounds of coal will be required .to evaporate 5650 pounds of water from an initial temperature of 130 and under a pressure of 80 Ibs.? 18. A coal contains 12,500 B.T.U. and requires 24 Ibs. of air per pound to burn it. Temperature of boiler room, 70; temperature of stack gases, 500. What per cent, of the heat of the coal goes up the stack? '** 19. If the temperature of the boiler room is 70 and the temperature of the stack gases is 500 and 30 Ibs. of air are used per pound of coal, what per cent, of heat is lost up the stack, if the coal contains 14,500 B.T.U. per pound? - 20. A boiler evaporates 3500 Ibs. of water per hour from an initial tem- perature of 120 and under a pressure of 80 Ibs. A second boiler evaporates 4000 Ibs. of water from an initial temperature of 110 and under a pressure of 60 Ibs. Which of the two boilers utilizes the greater amount of heat per hour? 21. A boiler evaporates 6000 Ibs. of water per hour. Coal contains 13,000 B.T.U. Steam pressure, 100 Ibs.; feed temperature, 180; efficiency of boiler and grate, 70 per, cent. How many pounds of coal will the boiler burn per hour? /\k "" 22. An engine uses 30 Ibs. of steam per I.H.P. per hour. Feed tem- . perature, 120; steam pressure, 120 Ibs. The boiler evaporates 9 Ibs. of water per pound of coal. How many pounds of coal are required per I.H.P. per hour? 23. A boiler evaporates 7.5 Ibs. of water per pound of coal. Steam BOILER AUXILIARIES 135 pressure, 150 Ibs. ; feed temperature, 200. Coal costs $2.50 per ton. What' is the cost to evaporate 1000 Ibs. of water from and at 212? 24. A 72-in. return tubular boiler 18 ft. long has seventy 4-in. tubes Find the heating surface and rated B.H.P. (Boiler Horse-power). 25. A 66-in. boiler 16 ft. long has ninety-eight 3-in. tubes. Find the heating surface and rated B.H.P. 26. A 60-in. boiler 16 ft. long has forty-four 4-in. tubes. Find the heating - surface and rated B.H.P. 27. A 60-in. boiler 16 ft. long has fifty-six 3^-in. tubes. Find the heating surface and rated B.H.P. 28. A 48-in. boiler 12 ft. long has twenty-six 4-in. tubes. Find the heat- ing surface and rated B.H.P. 29. A 36-in. boiler 12 ft. long has twenty-six 3-in. tubes. Find the heating surface and rated B.H.P. 30. A boiler evaporates 4000 Ibs. of water per hour from a feed tempera- ture of 60 into steam at 150 Ibs. pressure and 100 of superheat. What is the factor of evaporationT^oiler H.P., and number of pounds of coal used per< hour, if the boiler, furnace and grates combined have an efficiency of 70 per cent, and the coal contains 14,000 B.T.U. per pound dry. 31. A boiler evaporates 9000 Ibs. of water per hour. Steam pressure, 150 Ibs.; feed temperature, 120. How many boiler horse-power is it developing? 32. What is the H.P. of a boiler which evaporates 3080 Ibs. of water per hour from an initial temperature of 135, ari under a pressure of 100 Ibs.? 33. A 1000-H.P. engine uses 15 Ibs. ef steam per H.P. per hour. Steam pressure at boiler, 180 Ibs.; feed water temperature, 120. What boiler H.P. should we have to supply steam for the engine? 34. A boiler evaporates 4000 Ibs. of water per hour at 100 Ibs. pressure from a feed temperature of 120. Quality of steam, 98 per cent. What is the boiler H.P.? 35. A fire-tube boiler is 60 in. X 16 ft. and has fifty-four 4-in. tubes. If it evaporates 3000 Ibs. of water per hour, is it working over or under its rated H.P. and how much? Steam pressure, 100 Ibs.; feed temperature 200 36. A return fire-tube boiler is 60 in. in diameter, 16 ft. long, and has fifty-two 4-in. tubes. It evaporates 4000 Ibs. of water per hour. Steam pressure, 100 Ibs.; feed temperature, 150. Is it working above or below its rated H.P., and how much? 37. A boiler is reported to evaporate 12.5 Ibs. of water per pound of coal. Coal contains 13,000 B.T.U. and uses 24. Ibs. of air per pound to burn it. Temperature of the boiler room, 70, and of the stack, 550. Steam pressure, 100 Ibs.; feed temperature, 70. Would this result be possible? If not, how many pounds of water could the boiler evaporate per pound of coal? - 38. A plant burns 1500 Ib. of coal per hour. The height of the stack is 130 ft. Temperature- of boiler room is 70 and of the stack gases, 500, and 24 Ibs. of air are used to burn 1 Ib. of coal. Coal contains 12,000 B.T.U. per pound. What should be the area of the stack? What per cent, of heat is lost up the stack? What is the pressure of the draft in tenths of inches of water? * "* , r. 136 HEAT ENGINES /5. H* K/*rt ~ 39. A boiler is to evaporate 12,000 Ibs. of water per hour. Steam pressure, 100 Ibs. ; feed temperature, 200. (a) What should be the horse-power of the boiler? (6) How many square feet of heating surface should the boiler con- tain? (c) How many square feet of grate surface should it have? (d) What should be the area of the breeching? 40. In a 100 H.P. boiler plant what should be the area of the grates, and the diameter of the stack, if the stack is 125 ft. high? If the plant carries 130 Ibs. gage pressure, would a water or a fire-tube boiler be used, and why? 41. A 400 H.P. Corliss engine uses 26 Ibs. of steam per H.P. per hour. The auxiliaries use 25 per cent, as much as the engine. The boilers to supply the plant should contain how many square feet of heating surface and grate surface, and about what should be the area of the fluet Pressure, 150 Ibs.; feed temperature, 200. How many pounds of coal will the plant burn per hour if the coal contains 13,500 B.T.U. per pound and the efficiency of the boiler, furnace and grate is 70 per cent. ? - 42. A boiler evaporates 7 Ibs. of water per pound of coal. Steam pressure, 100 Ibs. ; feed temperature, 50. A feed-water heater is added increasing the feed-water temperature to 200. Heater costs $400. Allowing 5 per cenfo interest and 5 per cent, for depreciation and repairs, would it pay to install the heater if the plant burns 750 tons of coal per year, coal costing $2.50 per ton? 43. A boiler evaporates 1Q Ibs. of water per pound of dry coal from and at 212. Dry coal contains 13,000 B.T.U. pet pound. What is the combined efficiency of the boiler, furnace and grate? - 44. A boiler evaporates 7.5 Ibs. of water per pound of coal. Coal con- tains: 13,000 B.T.U. Steam pressure, 100 Ibs.; feed temperature, 150. What is the combined efficiency of the boiler, furnace and grate? 45. What is the combined efficiency of a boiler, furnace and grate that evaporates 8 Ihs. of water per Ib. of coal from a feed temperature of 150 into steam at 150 Ibs. pressure? Coal contains 13,000 B.T.U. per pound. - 46. A boiler evaporates 9 Ibs. of water per pound of dry coal containing 13,500 B.T.U. per pound. Steam pressure, 100 Ibs. ; feed temperature, 200. What is the combined efficiency of the boiler, furnace and the grate? 47. A coal contains C, 80 per cent.; H, 4 per cent.; O, 2 per cent. What is the heat value of the coal? If this coal is used in a boiler carrying 100 Ibs. pressure with a feed temperature of 190 and evaporates 8 Ibs. of water per pound of coal, what is the combined efficiency of the boiler, furnace and grate? 48. A boiler evaporates 15,000 Ibs. of water per hour into steam at 100 Ibs. pressure; temperature of feed, 200. Nine pounds of water are evaporated per pound of dry coal containing 13,000 B.T.U. per pound, (a) What is the H.P. developed by the boiler? (6) What is the combined efficiency of the boiler, furnace and grate? ^ 49. A boiler evaporates 11 Ibs. of water from and at 212 F. per pound of dry coal containing 14,000 B.T.U. per pound. What is the combined efficiency of the boiler, furnace and grate? At the same efficiency, what will it evaporate with a steam pressure of 200 Ibs. and feed temperature at 200? >-J "50. A boiler uses 1 Ib. of dry coal containing 13,000 B.T.U. to evaporate 6 Ibs. of water. Steam pressure, 100 Ibs.; feed temperature, 100. (a) BOILER AUXILIARIES 137 What is the efficiency of the boiler plant? (&) What will be the efficiency of the plant if a heater is added which heats the feed to 200 F.? (c) What will be the evaporation per pound of coal after the feed-water heater is installed? 51. A boiler evaporates 9 Ibs. of water per pound of coal fired. Feed temperature, 70; steam pressure, 150 Ibs. Coal as fired contains 3 per cent, moisture. Dry coal contains 14,000 B.T.U. per pound and has 6 per cent, ash by analysis. Twelve per cent, of coal fired is taken from the ash pit in form of ash and refuse, (a) What is the efficiency of the boiler and furnace? (6) What is the efficiency of the boiler, furnace and grates combined? 52. A boiler plant burns coal which contains C, 75 per cent.; H, 6 per cent.; and O, 8 per cent. Two-thirds of the carbon is burned to CO 2 and the balance to CO. The evaporation is 8 Ibs. of water per pound of coal. Steam pressure, 100 Ibs.; feed temperature, 170. (a) What is the efficiency of the boiler and furnace? (6) What is the efficiency of the boiler, furnace and grates combined? 53. A boiler evaporates 20,000 Ibs. of water per hour from a feed tem- perature of 180 into dry saturated steam at 115 Ibs. pressure absolute. Dry coal contains 4 per cent, ash by analysis and 13,000 B.T.U. per pound. Ten per cent, ash and refuse are taken from the ash pit. The actual evapora- tion per pound of dry coal is 9 Ibs. (a) What H.P. is being developed by the boiler? (6) What is the efficiency of the boiler, furnace and grates Combined? (c) What is the efficiency of the boiler and furnace alone? if 54. Given the following data from a boiler test : Duration of test, 24 /hours; total amount of water fed to boilers, 240,000 Ibs.; total weight of dry coal fired, -30,000 Ibs.; total weight of ash and refuse, 3000 Ibs.; tem- perature of feed water, 180 F.; steam pressure, 150 Ibs. absolute; quality of steam, 98.5 per cent.; dry coal contains 13,000 B.T.U. per pound and 3 per cent, ash by analysis, (a) What H.P. is the boiler developing? (6) What is the evaporation from and at 212 per pound of dry coal? (c) What is the combined efficiency of the boiler, furnace and grates? (d) What is the efficiency of the boiler and furnace alone? (e) What should be the heating and grate surfaces in this boiler? 55. A boiler received 10,000 Ibs. of water per hour at 100 F. Steam pressure, 150 Ibs. absolute; quality of steam, 98| per cent. Dry coal burned per hour, 1250 Ibs., each pound containing 13,000 B.T.U. Per cent, of ash by analysis, 3 per cent.; ash and refuse taken from ash pit per hour, 125 Ibs. Coal costs $3 per ton. Plant runs 10 hours a day, 300 days in the year, (a) What H.P. is the boiler developing? (6) What is the efficiency of the boiler, furnace and grates combined? (c) What is the efficiency of the boiler and furnace alone? (d) If the interest and depreciation is 10 per cent., how much could you pay for a heater that would increase the tem- perature of the feed water to 212? 56. A boiler evaporated 9000 Ibs. of water per hour from a feed tempera- ture of 80 into steam at 145.8 Ibs. absolute. Coal contains 13,500 B.T.U. and costs $2.50 per ton. Efficiency of the boiler, furnace and grate, 70 per cent. If we add a feed-water heater that will increase the temperature to 138 HEAT ENGINES 212, what will be the saving in coal cost per year, if the plant operates 10 hours a day, 300 days in the year? 67. A boiler plant evaporates 30,000 Ibs. of water per hour. Feed tem- perature, 70; steam pressure, 150 Ibs. The evaporation is 8 Ibs. of water per pound of coal, and coal costs $2.50 a ton. If a feed- water heater is in- stalled that will increase the temperture of the feed water to 180, how much money will be saved per year and how much can be paid for the heater if the interest and depreciation are 10 percent.? Plant runs 10 hours per day, 300 days in the year. 68. A feed-water heater increases the temperature of the feed from 100 to 200. Steam pressure, 100 Ibs. The plant evaporates 10,000,000 Ibs. of steam per year. The cost to evaporate 1000 pounds of steam without the heater is 15 cents. What can we afford to pay for a heater allowing 5 per cent, interest and 8 per cent, depreciation and repairs? 59. A boiler plant develops 500 B.H.P. and uses 4 Ibs. coal per H.P. per hour. Coal contains 13,000 B.T.U. per Ib. Steam pressure, 150 Ibs. Feed temperature, 120. A feed-water heater is added raising the temperature of water from 120 to 200. Heater costs $500. The plant operates 10 hours a day for 300 days a year. The cost of coal is $3 per tori, (a) Allowing 7 per cent, depreciation, what interest will the owner make on the investment? (6) If later an economizer is added which raises the feed water from 200 to 300, allowing 5 per cent, interest and 7 per cent, deprecia- tion, how much can the owner pay for the economizer? (c) What would be the efficiency of the plant under this last condition? 60. A boiler plant runs 24 hours per day for 300 days in the year. It burns 30 tons of coal per day costing $3 per ton. The analysis of the stack gases is CC>2, 5 per cent.; O, 15 per cent.; N, 75 per cent. The coal contains C, 80 per cent. ; H, 6 per cent. ; and O, 4 per cent. The plant is changed so that the stack gas analysis is CO 2 , 14 per cent.; O, 6 per cent.; N, 75 per cent. What will be the saving in dollars per year? Stack gas temperature, 600 F. Boiler room temperature, 70. Boiler radiation loss, 4 per cent. After this change is made, an economizer is installed which reduces the temperature of the stack gases from 600 to 400. The evaporation is 9 Ibs. of water per pound of coal. Feed-water temperature is 120 P . What will be the final temperature of the feed water? What will be the saving in dollars per year after this second change is made? CHAPTER VIII STEAM ENGINES 91. The Simple Steam Engine. A simple form of stationary steam engine and one in general use is shown in Fig. 54. It is a small direct double-acting engine with a balanced slide valve and a cast-iron cylinder closed at its ends by cylinder heads bolted on. The engine has no steam jacket and is surrounded on the outside by non-conducting material and cast-iron lag- ging. Fig. 55 shows the steam chest containing the valves and the ports leading from the steam chest to the cylinder. The FIG. 54. Vertical section of Skinner engine. steam is admitted and exhausted through these ports. The piston is made a loose fit in the cylinder. The spring rings shown in the piston serve to prevent leakage from one side of the piston to the other. The piston rod is usually fastened into the piston head by means of a taper-ended rod and nut, and is then carried through the cylinder head, the gland and packing serving to make a steam-tight joint. The other end of the piston rod is connected with the cross-head. The power is 139 140 HEAT ENGINES communicated from the connecting rod to the crank, which is attached to the main shaft. To this main shaft the eccentric is fastened by means of set-nuts. The valve of the engine is driven by the eccentric through the eccentric rod and the valve stem, The valve stem passes through the steam chest, being made tight by the glands and packing, as in the case of the piston rod, and is fastened by lock nuts to the valve. The function of this valve is to admit the steam surrounding the valve to each end of the cylinder alternately. On the opposite stroke, Valve .Balance Plate Steam Port / Steam Chf-st Cover Packing Packing Gland / Packing Picking Gland Lagging FIG. 55. Section through steam engine cylinder and valve. Counter Bore Piston 'Ring the valve opens up the ends of the cylinder to the exhaust space in the center of the valve, this space being connected to the ex- haust pipe of the engine, and the space outside of the valve being connected to the steam pipe admitting the steam to the engine. Fig. 55 shows the slide valve in a position admitting steam to the head end of the cylinder. On the crank end, the cylinder is open to exhaust. As the steam enters behind the piston, the steam in the space on the opposite side of the piston is forced out through the space under the valve and out of the exhaust port. When the piston reaches the opposite end of the stroke, the valve STEAM ENGINES 141 will have been moved to a similar position at the opposite end. Steam will then be admitted at that end, and the end previously receiving steam will be open to exhaust. 92. Action of the Steam in the Steam Engine. In the simplest form of steam engine, the steam is admitted for the full stroke of the piston and, when the valve opens the cylinder to exhaust, the steam is exhausted at nearly full boiler pressure. This action of the engine is, of course, very uneconomical, and early in the development of the engine it was found desirable to allow the steam to expand in the cylinder. This is accomplished by having the valve close the entrance port before the piston has reached the end of its stroke, then, for the balance of the stroke, as the piston is forced out, the steam pressure in the cylinder is greatly reduced, due to the increased volume of the cylinder. FIG. 56. Indicator diagram. Fig. 56 shows graphically the action which goes on in the cylinder. The ordinates of the diagram represent the steam pressure, and the abscissas represent cylinder volumes as the piston moves out. The steam enters along the line CDE, the pressure at D being a little below boiler pressure. At the point E, known as the point of cut-off, the valve closes. The steam expands from the point E to F, along the expansion line EF. At the point F, called the point of release, the valve opens, and from the point F to the point H the exhaust occurs. At the point H the valve closes the exhaust port and compression of the steam left in the cylinder begins, continuing along the line HC to the 142 HEAT ENGINES point C. At this point steam is again admitted to the cylinder. A similar action occurs on the opposite end of the cylinder, so while the steam is being admitted at one side, at the opposite side of the piston we have exhaust pressure. Such a diagram is termed an indicator diagram and may be graphically produced by an instrument known as the indicator. 93. Theoretical Horse-power of a Steam Engine. In deter- mining the theoretical horse-power of a steam engine it is as- sumed that there is no clearance, that the full pressure of steam is maintained during admission, that the cut-off and release occur instantly, and that the engine acts without compression. Then the indicator card of the engine would be as shown in Fig. 57. o e FIG. 57. Theoretical indicator card. The curve of expansion is assumed to be a rectangular hyperbola, the equation of which is pv = a constant, as this is the curve which coincides most nearly with the actual expansion curve in a simple non-condensing engine. Let the pressure at the point of cut-off b be pi, and the volume Vi] and let the pressure at the point d be pz, and the volume, v z . The area of work is represented by the area abode = oabg + gbcf oedf. Area oabg = Area, gbcf /v 2 Vi pdv. Area oedf = Substituting these values in the previous equation, the area of work, /l>2 pdv p 2 v 2 . (1) i STEAM ENGINES 143 t As vi and ^ are the volumes before and after expansion, fhe ratio of expansion, Since the expansion curve be is a rectangular hyperbola, pv = piVi. Hence substituting for p its value in terms of pi and v i} the equation for work becomes /v 2 dv / ("v 2 dv\ - -P&z = piVi I 1 + I ) - P& 2 . Vi V J Vi V / Integrating, and substituting r for --, we have abode = piVi(l + log e r) p 2 v 2 . (3) The average pressure on the card, which is termed the mean effective pressure, is found by dividing this by the length of the card v 2 , or M.E.P. = Pl - : ^- -p* (4) In actual practice, however, the assumptions made are not fulfilled, and the actual mean effective pressure is less than the theoretical mean effective pressure. The proportion borne by the actual M.E.P. to the theoretical M.E.P. is termed the dia- gram factor, e. (Trans. A.S.M.E., Vol. 24, p. 751.) The actual mean effective pressure is (5) This diagram factor is found by experiment and varies from 70 to 90 per cent. To determine the indicated horse-power of a steam engine, it is necessary to find the work done in the engine cylinder; / Assume the engine to have a cylinder a square inches in cross- section and 1 ft. long, that it is double-acting and makes n revolutions per minute (r.p.m.), and that the mean effective pressure determined from equation (5) acting on the piston is p pounds per square inch. Then the total pressure against the piston will be pa pounds and the space traveled per minute by the piston will be 2 In', hence the foot-pounds of work done per 144 HEAT ENGINES minute is 2 plan. Since 1 horse-power equals 33,000 ft.-lbs. per minute, the indicated horse-power of the engine is 2plan : 33,000' Example. A 12" X 15" double-acting engine runs 200 r.p.m. Cut' oft 7 , | stroke; steam pressure, 100 Jbs.; back pressure, 2 Ibs. absolute- Card factor, 80 per cent. Find the rated horse-power of the engine. Solution. From equation (2), the ratio of expansion, v 2 1 ^ = l = 4 ' \ and from equation (5) the M.E.P. = e< ^(1 + %-r) = .80 [ i^p(l + log A) ~ 2 j> = .80(28.7(1 + 1.39) - 2) = .80J68.5 - 2} = .80 X 66.5 = 53 . 2 Ibs. The cross-sectional area of the cylinder, a = ^2 = 3.U16 x 6 2 = 113.3 sq. in. From equation (6), the I.H.P. = ?" 33,000 2 X 53.2 X 1.25 X 113.3' X 200 33,000 #**- 91.4. Ans. 91.4 rated horse-power. 94. Losses in a Steam Engine. The action of the steam in the steam engine is different from that which has been assumed as the ideal action. The action of the ideal engine is useful, however, as a basis of comparison for the action of the steam in actual engines. In the actual engine the steam is never expanded completely, and has at the end of the expansion a higher pressure than the back pressure in the exhaust pipe. It is not advisable to give the steam complete expansion, as there will be no added work due to the complete expansion of this steam, the pressure being insufficient to overcome the friction of the engine. Qwing STEAM ENGINES 145 to the fact that we do not have complete expansion, it is necessary to open the exhaust valve before the end of the stroke so as to bring the pressure at the end of the stroke down to the back pressure. Comparing the ideal diagram, Fig. 57, with the actual diagram, Fig. 56, it will be noticed that the steam during admis- sion in the actual diagram does not remain at full boiler pressure, but that there is a reduction of the pressure due to wire drawing of the steam through the ports of the valve. In the ideal engine there is no transmission of heat to the steam except in the boiler, but in the actual engine there is a transfer of heat from the steam to the cylinder walls during a portion of the stroke, and during other portions of the stroke from the cylinder walls to the steam. In an actual engine the back pressure in the cylinder is always greater than the vacuum in the condenser owing to the resistance of exhaust valve and passage. In the ideal engine the whole volume of the cylinder is swept through by the piston, and in the actual engine there must be a space at the end of the cylinder to prevent the piston striking the head. The principal losses of heat from an engine are given as follows, as nearly as possible in the order of their importance. 1. Heat lost in the exhaust. This loss is usually 70 per cent, or more of the entire heat admitted to the engine. 2. Initial condensation. 3. Wire drawing at admission and in exhaust valve. 4. Condensation in the clearance space during compression. 5. Radiation and conduction from the cylinder. 6. Leakage past the piston and valves. 95. Heat Lost in the Exhaust. Most of the heat brought to the engine by the steam is rejected by the engine in the exhaust. This loss varies from 70 per cent, of the heat of the steam in the best engines to over 90 per cent, in the poorer types. In many steam plants this heat is partly recovered by using the exhaust for heating or manufacturing purposes. The steam leaving the exhaust of an engine usually contains from 10_to 20 per cent, of water. 96. Initial Condensation and Re -evaporation. Early ex- perimenters in steam-engine economy found that the surfaces of the cylinder wall and steam ports played a very important part in the economy of the steam engine. The inner surfaces exposed to the action of the steam in the engine naturally have a tempera- ture very close to that of the steam itself. When the steam 10 146 HEAT ENGINES enters the cylinder, it comes in contact with the walls of the cylinder which have just been exposed to exhaust steam and are necessarily at a lower temperature. A part of this steam will, therefore, be condensed in warming the walls, and as the piston moves out more, more of the walls will be exposed, so that con- densation increases to a point even beyond the point of cut-off. After the point of cut-off the steam expands, the pressure falls, and the temperature drops until a point is reached where the temperature of the cylinder walls is about the same as the tem- perature of the steam in the cylinder. Condensation ceases at this point and the cylinder walls are by this time covered with a film of moisture. If the expansion of the steam is still further increased, the temperature in the cylinder corresponding to the steam pressure will be less than the temperature of the cylinder walls, and this film of moisture on the surface will begin to re- evaporate. Usually the amount of re-evaporation is very much smaller than the initial condensation and the cylinder walls are still wet when the exhaust valves open. This re-evaporation also continues during the exhaust stroke. It is very desirable that all the moisture on the surface of the cylinder be evaporated before the end of the exhaust. If it is not evaporated, the cylin- der walls will be wet when steam is again admitted to the cylinder and the initial condensation will be greatly increased. The trans- fer of heat from the steam to the walls of the cylinder is always increased by the presence of moisture. In the average non-condensing engine, initial condensation is from 15 to 20 per cent., in small reciprocating steam pumps an initial condensation as high as 75 per cent, sometimes occurs, and in the most perfect engines it is from 10 to 12 per cent. 97. Factors Affecting Initial Condensation. Initial conden- sation is always increased by increasing the range of temperature in the cylinders. It also increases as the proportion of the area of the cylinder walls to the volume of the cylinder increases. The greater this ratio, the less the economy, as the more wall that is exposed the more heat the wall will take up. This accounts for the large consump- tion of steam shown by most rotary engines. Time is also important, and other conditions being the same, the slower the speed of the engine, the greater the initial con- densation, as the whole action depends upon the time during which the heat has an opportunity to be taken up or given off by STEAM ENGINES 147 the cylinder walls. As the element of time during which the steam is in contact with the walls of the cylinder increases, the initial condensation increases. The changes of temperature only affect the inner surfaces of the cylinder, and the greater the time, the greater the depth of cylinder walls that will be affected. Initial condensation increases as the ratio of expansion is increased, that is, as the cut-off becomes shorter. This is easily explained; as the cut-off is shortened, the weight of steam admitted to the cylinder becomes less and the amount of heat taken up by the cylinder walls remains sub- stantially the same, so that the proportion of steam condensed increases. With very short cut-offs this initial condensation becomes very large. When the cut-off is reduced below a certain point, the increased initial condensation offsets the increase in economy due to longer expansion. If the cut-off is shortened to less than this point, the steam consumption of the engine will be increased. The point of greatest economy in most single-cylinder engines is from one-quarter to one-fifth stroke. In an engine having a short cut-off and using a high steam pressure, the econ- omy may often be increased by reducing the steam pressure, thereby increasing the cut-off. 98. Steam Jacket. The action of initial condensation is increased by the loss of heat through the cylinder wall by con- duction. This may be reduced by surrounding the cylinder with steam at boiler pressure. Such an arrangement is called a steam jacket. The effect of the steam jacket is to reduce initial condensation and to increase the re-evaporation. The steam used by the steam jacket is always charged to the engine as though it had been used in the cylinder. Engines with jackets show increased economy, particularly when operated at slow speed. The higher the speed of the engine, the less is the element of time during which the jacket can affect the steam in the cylinder and the less effective the jacket becomes. In cases of slow-speed engines with large ratios of expansion, the use of the jacket will show a saving of from 10 to 20 per cent. 99. Superheating. Superheating the steam previous to its admission to the engine is used as a means of reducing initial condensation. A sufficient amount of superheat should be given to the steam so that on admission of steam to the cylinder, the cylinder walls take up this superheat instead of condensing the 148 HEAT ENGINES steam. The effect of this is to leave the cylinder walls entirely dry, reducing the amount of heat which would be conducted to the walls, as dry gas is one of the best non-conductors of heat. The experiments of Professor Gutermuth show that with sufficient superheat the economy of a simple non-condensing engine may be made to equal that of a compound condensing engine. 100. Compound Expansion. By increasing the steam pressure and using a longer range of expansion, the range of temperatures in the cylinder of a steam engine is much increased, thereby increasing the initial condensation. In order to reduce the range of temperatures in the cylinder, it has been found more economical partially to expand the steam in one cylinder and then exhaust the steam into a second cylinder in which the expansion is completed. By this means the range of temperature in each cylinder is reduced and initial condensation reduced. Compound cylinders are only used when the steam pressure is sufficiently high so that the initial condensation would be excessive if the steam were expanded in one cylinder. With steam pressures less than 100 Ibs., compound engines are seldom used. It is not necessary to use compound engines for less than 125 Ibs. pressure unless the ratio of expansion is very large. 101. Wire Drawing. The resistance offered by the valves, #orts, and passages lowers the pressure of the steam in the cylinder during admission and raises the pressure during exhaust. As the valves do not close instantly when the valve nears the point of closing, or cut-off, the pressure is reduced owing to the small port opening. This is shown by the rounded corners at cut-off and release. This resistance is often called "throttling" or "wire drawing." The effect of this throttling of the steam is to slightly dry the steam and, if it were absolutely dry to start with, there would be a small amount of superheating. It will be noticed in the indicator diagram, Fig. 56, that the initial line DE is not abso- lutely horizontal, but that there is a gradual reduction of pres- sure from D to E. The initial pressure line is always lower than the boiler pressure, owing to the resistance of the passages between the boiler and the cylinder. The steam in passing through the piping leading to the engine loses a certain quantity of heat, with a corresponding condensa- tion. It is customary to place a separator in the main just before it reaches the engine so that this water of condensation can be removed from the steam. STEAM ENGINES 149 102. Clearance and Compression. In order that the piston may not strike the end of the cylinder, it is necessary to leave a small space between the piston and the cylinder head. In addition there is always some space in the steam ports between the valve and the cylinder. The volume of the ports between the valves and the cylinder, together with the space between the pis- ton at the end of its stroke and the cylinder head, is called the clearance. It is usually determined by placing the piston at the extreme end of its stroke and filling the clearance space with water. Knowing the weight and temperature of the water put into the clearance space, the volume of the water may be deter- mined. Dividing the volume of the clearance by the volume of O G FIG. 58. Theoretical indicator card showing clearance. the piston displacement gives the per cent, of clearance. The clearance in engines varies from 1 to 10 per cent. The steam in the clearance affects the expansion curve of the engine. In Fig. 58, ED represents the piston displacement, and AB represents the volume of the steam admitted to the cylinder. The apparent ratio of expansion is ED. AB Actually, however, the steam expanding includes not only the steam admitted from the .boiler, but also the steam left in the clearance, so that the real ratio of expansion is ED+AF AB+AF (8) 150 HEAT ENGINES The clearance of the engine alters the amount of steam con- sumed per stroke of the engine. In order to reduce the amount of live steam to fill the clearance at each stroke, the exhaust valves of the engine are closed before the end of the stroke, and for the balance of the stroke the steam is compressed. This compression of the steam serves to fill the clearance space with steam at a higher pressure than the exhaust pressure. In addition, compres- sion of steam in the clearance space serves to retard the reciprocat- ing masses in the engine and bring them to rest at the end of the stroke. If an engine is operated with^too little compression, it will be found to pound at the end of the stroke. The effect of compression, or the cushioning of the piston, is materially increased by the lead of the engine. The lead is the amount the valve is open when the piston reaches the end of its stroke. In order to have lead it is necessary to open the valves before the end of the stroke, and this steam admitted before the end of the stroke serves to assist in cushioning the piston and reciprocating parts. We may consider the steam occupying the cylinder as composed of two parts: the part that has been left in the clearance, which is called cushion steam; and the part that has been admitted from the boiler, which is called cylinder feed. If it is desired to determine the amount of steam that is expanding in an engine, it is necessary to add to the weight of the steam fed from the boiler the weight of the steam left in the clearance space. The sum will be the total steam expanding in the engine. The compression of the steam in the clearance space always involves a loss. Just previous to compression, the cylinder walls have been exposed to the exhaust steam. During compression the steam compressed has its temperature increased, and when tlie temperature of the compressed steam exceeds the temperature of the walls, condensation begins to occur. The action is similar to initial condensation. PROBLEMS 1. An electrical plant runs a factory having five 10 H.P. motors, two 20 H.P. motors, four 30 H.P. motors. Efficiency of the motors, 80 per cent. ; transmission, 80 per cent. ; of the engine and dynamo combined, 80 per cent. What should be the H.P. of the engine plant and kilowatts of the generator? 2. A street car plant uses ten cars each requiring an average horse-power of 75 at the wheels. Efficiency of car is 60 per cent.; of transmission, 75 per cent.; of sub-stations, 75 per cent.; and of main engines and dynamo, 75 per cent. M.E.P. of engine, 40 Ibs.; r.p.m., 150. Plant has two engines STEAM ENGINES 151 of equal size. What are the dimensions of their cylinders? Assume 600 ft. per minute piston speed. 3. Assume the mean effective pressure to be 40 Ibs., the number of revo- lutions to be 75 per minute, and the length of the stroke to be 42 in., and determine the diameter of the cylinder of a Double-acting engine which will develop 200 H.P. 4. An engine is 18" X 36" and runs 100 r.p.m. Initial pressure, 100 Ibs.; back pressure, atmospheric; cut-off, i stroke. What H.P. will be developed? Assume card factor of 85 per cent. 6. An engine is 8" X 12"; initial steam pressure, 100 Ibs. gage; back pressure, 3 Ibs. gage; cut-off, ; the expansion curve is an isothermal of a perfect gas; r.p.m., 250. What is the horse-power of the engine? Card factor, 85 per cent. 6. Determine the horse-power of a 13" X 18" double-acting engine when making 220 r.p.m. while taking steam at 80 Ibs. gage and cutting off at 1 stroke. Neglect the clearance and assume that the mean back pressure is 20.5 Ibs. absolute, and that the card factor is 80 per cent. 7. An engine is 18" X 30"; cut-off, | stroke. It runs 100 r.p.m. Initial steam pressure, 80 Ibs. Exhaust, atmospheric. What would be the increase of horse-power if the cut-off was increased to \ stroke and initial pressure to 150 Ibs.? Card factor, 80 per cent. ^8. An engine is 8" X !." and makes 300 r.p.m.; cut-off, i stroke; ex- haust, atmosphere. What would be the horse-power of the engine at the following gage pressures: 60, 80, 100, and 120 Ibs.? Card factor, 75 per cent. 9. What would be the horse-power developed under the different condi- tions stated in Problem 8, if a condenser were added and the back pressure reduced to 2 Ibs. absolute? 10. An engine is 18" X 30"; runs 100 r.p.m., and. initial pressure is 100 Ibs. Atmospheric exhaust. A condenser is added bringing the exhaust down to 2 Ibs. absolute. In both cases cut-off occurs at \ stroke. Card factor, 80 per cent, (a) How much is the horse-power of the engine in- creased? (6) If the power is sold for $60 per horse-power per year, how mifch could be paid for a condenser, allowing 5 per cent, for interest and 6 per cent for depreciation? 11. An engine is to develop 600 H.P. at a piston speed of 600 ft. per minute. Initial steam pressure, 100 Ibs. Exhaust pressure, 1 Ib. gage. Cut-off at \ stroke. Speed, 100 r.p.m. Card factor, 85 per cent, (a) What should be the stroke and diameter of the cylinder? (6) What should be the diameter if the back pressure is 2 Ibs. absolute? J2. An engine is to develop 1000 H.P. at \ cut-off and 120 r.p.m. Initial steam pressure, 125 Ibs; back pressure, atmospheric; piston speed not to exceed 720 ft. per minute. What should be the dimensions of the cylinder? Card factor, 70 per cent. 13. The cylinders of a locomotive are 19 in. in diameter and have a 24-in stroke. The driving wheels are 7 ft. in diameter, and the mean back pres- sure against which the piston works is 19 Ibs. absolute. Determine the horse- power developed by the locomotive when taking steam at 150 Ibs. gage 152 HEAT ENGINES and cutting off at f stroke, while traveling at a speed of 40 miles per hour Card factor, 75 per cent. 1,4. An engine has a clearance volume which is 0.08 of the volume swept through by the piston per stroke. If the steam be cut off at I stroke, what will be the number of times it is expanded? 16. A 12" X 14" double-acting engine develops 97 H.P. when running 260 r.p.m. and at f cut-off. Pressure, 70 Ibs. What is the weight of steam actually used per I. H.P. per hour, assuming that one-quarter of that theo- retically required is lost through condensation, radiation, etc. 16. A tank contains 1000 cu. ft. of air at a pressure of 1000 Ibs. per square inch absolute and a temperature of 60 F. This tank is used to run an 8" X 12" double acting air engine; \ cut-off; 200 r.p.m. The initial pressure of air entering the engine is 60 Ibs. per square inch absolute. How long will the tank run the engine? 17. A tank contains 200 cu. ft. of air at 200 Ibs. absolute and a temperature of 60 F. How long will it operate a 4" X 6" double-acting air engine run- ning 100 r.p.m.? Cut-off | stroke. Engine takes air at 60 Ibs. absolute. Temperature constant. 18. A double-acting compressed air locomotive has two air tanks each 3' X 12'. These tanks supply two 8" X 12" cylinders. The cylinders take their air through a pressure reducing valve at 100 Ibs. per square inch abso- lute, the original pressure in the tanks being 1000 Ibs. per square inch abso- lute, (a) If the air acts at a constant temperature of 60 F. and the expan- sion in the engine is isothermal, how long will the tanks run the engine at | cut-off in the cylinder? (6) How many horse-power will be developed when the engine runs 150 r.p.m., assuming a card factor of 90 per .cent.? CHAPTER IX TYPES AND DETAILS OF STEAM ENGINES 103. Classification. Engines may be classified according to whether they exhaust into the atmosphere or into a condenser, into: 1. Non-condensing engines. 2. Condensing engines. In the non-condensing engine the exhaust passes directly to the atmosphere. In condensing engines the exhaust steam passes into a cold chamber where, by means of a cooling medium, the steam is changed to water. This produces a vacuum so that the exhaust occurs at a pressure lower than that of the atmosphere. The condensed steam is removed and the vacuum is sustained by means of an air pump. Another classification may be made according to the way in which their speed is governed, as: 1. Throttling engines. 2. Automatic engines. In the throttling engines the speed of the engine is controlled by means of a valve in the steam pipe which regulates the pressure of the steam entering the engine. In the automatic engine the pressure of the entering steam remains constant and the governor controls the amount of steam admitted to the cylinder. Engines may also be classified according to the number of cylinders in which the steam is allowed to expand successively as: 1. Simple engines. 2. Compound engines. 3. Triple expansion engines. 4. Quadruple expansion engines. In a simple engine the steam expands in but one cylinder, and is then allowed to exhaust. In a compound engine a portion of the expansion occurs in the high-pressure cylinder, and from there the steam passes to the low-pressure cylinder, where it is further expanded to a pressure approximating the exhaust pressure. In the triple-expansion engine the steam expands successively in three cylinders, and in the quadruple in four. 153 154 HEAT ENGINES TYPES AND DETAILS OF STEAM ENGINES 155 A fourth classification depends upon the position of the cyl- inder, as: 1. Vertical engines. 2. Horizontal engines. 104. Plain Slide Valve Engine. The simplest form of engine is the plain D-slide valve engine, as shown in Fig. 59. The valve is shown in its normal position in the steam chest. A cross-section of a valve of this type showing the steam ports is shown in Fig. 90. FIG. 60. Portable engine and boiler. This type of engine is used where high economy is not neces- sary. It requires little attention, and is easily repaired and adjusted. Fig. 60 shows a boiler and engine of this type arranged so as to be portable. These engines are governed by a throttling governor of the fly-ball type, as shown in Fig. 60, which controls the speed of the engine by changing the pressure of the steam in the steam chest. 156 HEAT ENGINES 105. Automatic High-speed Engine. This class of engines has developed rapidly since the introduction of electrical lighting machinery, and is designed primarily for the direct driving of FIG. 61. Governor, eccentric rod, rocker shaft, valve and valve stem. electric generators. These engines have balanced slide valves such as are shown in Fig. 55. The governors in this class of engines control the valve directly, and it is necessary that the FIG. 62. Bed of high-speed, center-crank engine. valve be balanced so that it may be moved easily by the governor. Fig. 61 shows the governor, eccentric rod, rocker shaft, valve stem, and valve. TYPES AND DETAILS OF STEAM ENGINES 157 158 HEAT ENGINES TYPES AND DETAILS OF STEAM ENGINES 159 Engines of this class are well adapted to a high rotative speed. The stroke of these engines is usually short, so that the average piston speed may exceed 600 ft. per minute when the engine runs at a large number of revolutions per minute. Most engines of this class are of the center crank type so that all parts of the engine are supported on one casting. Fig. 62 shows the bed of a center-crank high-speed engine. This bed is so designed that all parts are accessible and may be removed. It may be machined at one setting. This insures perfect alignment of the various parts of the engine. This bed casting is bolted to a suitable brick or cement foundation. 106. Corliss Engine. These engines are described and the action of their valves explained in paragraph 136. Figs. 63 and 64 show a plan and side elevation of a Corliss engine. 107. The Stumpf Uniflow Engine. In 1910 Professor Stumpf of Charlottenburg, Germany, brought out an engine which he called a uniflow engine and which promises to give materially better economy than the ordinary reciprocating engine. This engine was not new in principle as the patent had been taken out in 1886 but had not been used. It obtains its economy largely through reducing the initial condensation losses. It is of the four-valve type. Fig. 65* shows a section through the cylinder. This cylinder has no exhaust valves but in the middle of the cylinder there is a ring of ports which are uncovered by the piston at the end of each stroke so that the piston is the exhaust valve. There are two steam valves A in the cylinder heads, and the steam spaces over the valves have the clearance pockets B which completely steam jacket the heads. In the uniflow engine, the piston faces and cylinder heads are exposed to exhaust temperature only during the short time that the piston uncovers and covers the exhaust ports. On the return stroke the steam remaining in the cylinder is compressed in the clearance spaces up to the admission pressure. The temperature also increases in the compression space, not only due to compression but also to the absorption of heat from the jacketed head. The cylinder Fig. 65 is a simple cylindrical casting with a belt cast in the middle for the exhaust passage. The steam chest is integral with the cylinder and provided with two drums C to take up expansion without distorting the cylinder. Each cylinder_has * Taken from Power, June 11, 1912, vol 35, No. 24, p. 830. 160 HEAT ENGINES a large valve D open into a pocket D in the cylinder head. This valve opens automatically to serve as a relief valve to let out entrained water. It also serves as extra clearance to prevent exces- sive pressure if the vacuum should be lost when the engine is operating non-condensing. In the particular form of inflow engine described the valves are Corliss valves and operated by the usual Corliss valve mechanism. These engines have shown very low steam consumption particularly with superheated steam. In a recent test a simple single-cylinder engine, non- condensing, developed a horse-power with 11J Ibs. of steam. In addition they have a flat economy curve and are capable of taking very heavy overloads. FIG. 65-. Section of Stumpf engine. 108. Engine Details. Fig. 66 shows the piston and piston rod. The piston is turned a little smaller than the cylinder, and is made tight in the cylinder by means of spring rings. These rings are shown in the figure leaning against the piston rod. They are made of cast iron and are so constructed that they have to be compressed in order to get them into the cylinder, and when the piston is in place, the rings bear firmly against the cylinder walls. The piston with rings in place is shown in Fig. 67. In Fig. 68 is shown a piston, piston rod, and cross-head. The piston is attached to the piston rod by a taper pin and lock- nut, and the other end of the piston rod is screwed into the cross- head and fastened by a lock-nut. The cross-head pin is also shown in the cross-head. TYPES AND DETAILS OF STEAM ENGINES 161 Fig. 69 shows a solid-ended connecting rod. These rods are usually made of forged steel. The bearings that enclose the pin are made of brass and fitted into the ends of the rods. These FIG. 66. Piston and piston rings. FIG. 67. Piston with rings in place. FIG. 68. Piston, piston rod and crosshead. bearings, or brasses, are taken up when they wear by means of wedges held by lock-nuts as shown in the cut. Fig. 70 shows a strap-ended connecting rod. In this form of rod the brasses are held in place by steel straps that encircle them. These straps are fastened to the body of the connecting 11 162 HEAT ENGINES rod by means of a taper key and a cotter. The brasses in this rod are shown lined with babbitt metal which is much softer than the steel pins themselves. FIG. 69. Solid end connecting rod. Fig. 71 shows the crank-shaft and its counterbalance weights which are bolted to the crank. The crank-shaft is a solid forging of open-hearth steel. The counterweights are made of cast iron. '*!' FIG. 70. Strap end connecting rod. The crank-shaft shown in the figure is designed for a center-crank engine. Fig. 72 shows one of the main bearings for the crank-shaft. The figure shows what is called a four-part bearing. The FIG. 71. Counter-balanced crank. bearing proper is made up of four pieces. The two side pieces, or brasses, take up most of the wear in the bearing and are TYPES AND DETAILS OF STEAM ENGINES 163 adjusted by means of set screws fastened with lock-nuts. The upper part of the brasses is adjusted by a screw in the top of the bearing. The brasses are supported by the main frame of FIG. 72. Main bearing, four part. the engine and held down by a main bearing cap bolted to the main frame of the engine. Fig. 73 shows the eccentric strap and eccentric rod. The eccentric strap is driven by an eccentric sheave the position of FIG. 73. Eccentric strap and eccentric rod. which is determined by the governor. Fig. 74 shows the eccen- tric sheave. In Fig. 75 is shown the eccentric strap more in detail. The strap is split in two parts and bolted together so that it can be placed over the sheave. In Fig. 76 is shown the main frame for a side-crank engine. 164 HEAT ENGINES This cut shows a main bearing with a three-part box. The side brasses in this box are adjusted by wedges moved by set-nuts on the top of the bearing. FIG. 74. Eccentric sheave for shaft governor. Figs. 77 and 78 show two views of a main engine-bearing having an oil cellar. The lower part of the cellar is filled with oil which is carried up onto the bearing by means of a chain FIG. 75.' Eccentric strap. which hangs over the shaft and dips into the cellar. The chain is moved by the rotation of the shaft, bringing the oil up on to the shaft, TYPES AND DETAILS OF $TEAM ENGINES 165 109. Lubricators. Although not a part of the engine itself, the lubricator is so intimately associated with it that it seems desirable to describe its action at this point. FIG. 76. Frame for side crank engine. FIG. 77. Main engine bearing with oil cellar cross-section. A cross-section of a sight feed lubricator is shown in Fig. 79. The lubricator is connected to the steam main just before the main enters the steam chest, and its purpose is to supply oil for 166 HEAT ENGINES lubricating the engine cylinder. This oil is carried into the cylin- der by the entering steam. FIG. 78. Main bearing with oil cellar transverse-section. FIG. 79. Sight-feed lubricator. " Steam being admitted into pipe 'B' and condenser 'F' con- denses, thus forming a column of water which exerts a pressure TYPES AND DETAILS OF STEAM ENGINES 167 equal to its head plus the difference in specific gravity between oil and water, through the tube 'P' on the oil in reservoir 'A.' By this excess pressure the oil is forced from reservoir ' A' through the tube 'S' and sight feed nozzle 'N' into the sight feed chamber 1 H.' The sight feed chamber being filled with water, the drop of oil floats to the top and passes to the point to be lubricated through the passage "I" and support arm 'K.'" CHAPTER X TESTING OF STEAM ENGINES 110. The Indicator. The indicator is a device by which the pressure of the steam for each point in the stroke of the engine is graphically recorded. It was first invented by James Watt and has since reached a high state of perfection. There are three principal things determined by an indicator: FIG. 80. Crosby indicator. First, the average pressure of the steam acting against the piston, which is called the mean effective pressure (M.E.P.). Second, the distribution of the steam in the engine; that is, the point at which the valves of the engine are opened and closed. By the use of the indicator we are able to determine whether or not the engine has a proper distribution of steam. 168 TESTING OF STEAM ENGINES 169 Third, from the indicator we may determine the actual weight of steam which is being worked in the engine cylinder. The indicator makes possible a complete analysis of the action of the steam engine. Fig. 80 shows a cross-section of a Crosby steam-engine indi- cator. This instrument is attached to the engine cylinder, and the space under the piston 8 is in direct communication with the FIG. 81. Crosby indicator with outside spring. engine cylinder. The pressure of the steam acts agianst the piston 8, compressing a spring above it. The pressure of the steam raises an arm 16, and the attached pencil at 23. The drum 24 is covered with a sheet of paper; a cord passing over a pulley 34 is attached to the engine cross-head through a reducing motion, so that with each stroke of the engine the drum makes almost a complete revolution. The movement of the drum corresponds 170 HEAT ENGINES to the movement of the piston, and the upward movement of the pencil corresponds to the pressure in the cylinder. We have a diagram, therefore, of the pressure in the cylinder for each point in the stroke of the engine. The springs used above the piston are of various strengths. What is termed a 40-lb. spring would be one of such strength that a pressure of 40 Ibs. per square inch under the piston would move the pencil one inch. These springs are carefully calibrated so that certain movements of the piston give a corresponding movement of the pencil on the paper. A brass stylus is sometimes used in place of a pencil. This has the advantage of always keeping a sharp point. In this case the indicator cards are taken on a specially prepared paper with a metallic surface, as no mark would be made on ordinary paper. Elevation. Cross-section. FIG. 82. Thompson indicator. One disadvantage of the use of the stylus and metallic surfaced paper is that the outline traced by the brass point is not permanent, but will fade out in a comparatively short time. Fig. 81 shows a similar indicator with the spring external to the indicator piston. The temperature of the spring in this indicator is independent of the steam pressure. The spring in this indicator may easily be changed without removing the indicator piston. This form is particularly adapted for indicator work where great accuracy is desired. Fig. 82 shows the elevation and cross-section of the Thomp- son indicator. This form of indicator is particularly well adapted to hard service. 111. Use of Indicator. The accuracy of an indicator depends upon the accuracy with which the pressure in the cylinder is TESTING OF STEAM ENGINES 171 recorded on the indicator drum, and also upon the accuracy with which the motion of the piston is conveyed to the indicator drum. In order to have the pressure recorded properly, the following conditions should be observed: the piping leading to the indicator should not be more than 18 in. long, and should be \ in. in diameter; the indicator should never be connected to a pipe through which a current of steam is passing; the holes connecting the indicator with the cylinder should be drilled into the clearance space so that the piston will not cover the opening ; the indicator should, if possible, be placed in a vertical position. Where great accuracy is desired the indicator spring_should be calibrated before and after the test. FIG. 83. Reducing motion, showing method of attachment. The motion of the drum may be taken from any part of the engine which has the same relative motion as the engine piston. The movement of the drum, which is usually taken from the cross- head, must be reduced to the length of the indicator diagram by some form of mechanism which makes the reduced motion an exact ratio to the movement of the engine piston. The indicator drum is then connected with this reduced motion of the piston by means of a cord. A reducing lever and segment is one of the commonest means used to accomplish this reduction. There are also on the market various forms of reducing wheels which make the reduction by means of gearing and pulleys. These reducing motions are more satisfactory when they are provided with a clutch so that the drum may be disengaged without 172 HEAT ENGINES- removing the cord connection from the reducing motion to the engine. Fig. 83 shows a simple form of reducing motion made of hard wood splines and a brass segment. It is better to use a segment of a circle at the point 6, so that db is the same distance for every point of the stroke. Fig. 84 shows a reducing wheel having a clutch, so that it is not necessary to disconnect the motion from the cross-head when the paper on the drum is replaced. Cord that has been stretched should be used on the indicator and reducing motion, so that the give of the cord will not reduce the length of the card. Wherever very long cords are found necessary, it is better to replace the cord with piano wire. FIG. 84.-Reducin g wheel. 112 - Taking an Indicator Card.- Before attaching the indicator, oil the parts of the mechanism with watch oil and the piston with cylinder oil. Be sure the piston is working freely in the cyl- inder. The piston should drop by gravity in the cylinder when the spring is removed. The pencil should have a smooth, fine point. Be sure there is no lost motion in the instrument. The reducing motion should be adjusted so that the length of the card is from 2^ to 3 in. The higher the speed, the shorter should be the card. The tension of the indicator drum spring should be just sufficient to prevent slackness in the cord. Before taking a card, try the indicator and see that it does not strike the stops at either end of the stroke. The cord should run to the indicator over the center of the guide pulleys. Steam should be turned on the indicator a few moments before taking the card so as to warm up the instrument. 113. To Find the Power of the Engine. The piston area is the cross-section of the cylinder. The diameter of the cylinder should be obtained with a caliper and the corresponding area is the piston area a. The piston area is not the same at bojh ends of the stroke, as on the crank end the area of the piston rod must be subtracted. The travel of the piston in feet per minute for each end of the stroke is found by multiplying the length of the stroke by the revolutions of the crank-shaft per minute. TESTING OF STEAM ENGINES 173 The mean effective pressure is obtained from the indicator card. The usual method is to measure the area of the card with an instrument called a plartimeter. In Fig. 85 is shown a standard form of planimeter. In using a planimeter the point B is placed on a point on the indicator card to be measured and the vernier E set at zero. The point B is then made to trace the card in a clockwise direction, going all around the card and returning to the starting-point. The reading of the scale on the rotating wheel C will then show the number of square inches enclosed by the diagram. Dividing the area of the FIG. 85. Polar planimeter. card by the length of the diagram will give the average height of the card in inches, and this multiplied by the value of the spring gives the mean effective pressure, M.E.P. The M.E.P. should be determined for each end of the cylinder separately. The mean ordinate from the card may also be obtained by dividing the card into ten spaces by vertical lines drawn equi- distant apart. Then measure the distance from the back pres- sure line to the forward pressure line at the center of each space. The average of these lengths will be approximately the mean ordinate. 174 HEAT ENGINES Let ph be the mean effective pressure for the head end, and p c , for the crank end; a h , the cross-sectional area of the piston in square inches for the head end, and a c , for the crank end; I, the length of the stroke in feet; and n, the number of revolutions per minute. Then the indicated horse-power will be IHP (2) The total I.H.P. of the engine is the sum of the I.H.P. for the head end and the crank end'. 114. Indicator Diagrams. The indicator is very often used to determine the setting of the valve and the distribution of steam o o FIG. 86. Indicator card from non-condensing Corliss engine. in the cylinder. Fig. 86 shows a typical indicator card from a Corliss engine running non-condensing. AB is the atmospheric line, and 00' , the line of absolute vacuum, or zero pressure abso- lute. OY is the line of no volume for the head end, and O'Y' for the crank end of the cylinder. The horizontal distance between the lines OY and CD represents the clearance volume for the head end of the cylinder. The clearance on the crank end is similarly shown. 115. Graphical Determination of Initial Condensation. Initial condensation may be determined graphically from the TESTING OF STEAM /ENGINES 175 indicator card. In determining the amount of steam working in the engine cylinder, the amount supplied to the engine per stroke is determined by either weighing the water entering the boiler, which passes over as steam into the engine, or by weighing the steam condensed in a condenser attached to the exhaust of the engine. This total quantity of steam used by the engine is then reduced to the amount of steam used per stroke, and this is called the cylinder feed. To this must be added the cushion steam. To determine the amount of cushion steam, an average indicator card is selected, and at a point after compression has begun and it is certain that the valve is closed, the pressure is measured and the volume determined. This volume must include the volume of the clearance. From this pressure and volume, by reference FIG. 87. Indicator card and saturation curve, showing effect of initial condensation. , to the steam tables, the weight of the cushion steam may then be calculated, assuming the steam to be saturated. The total steam in the cylinder during expansion is then found by adding this cushion steam to the cylinder feed. A curve of saturation for this total quantity of steam can then be drawn upon the indi- cator diagram, and this curve will represent at each point of the stroke the volume of steam if no initial condensation has occurred. Fig. 87 shows a saturation curve constructed on an indicator card. YR represents the volume of the steam as supplied to the engine per stroke, or in other words, it represents the volume of the total steam in the cylinder at boiler pressure if all the steam entering remained steam. The curve RS represents the volume 176 HEAT ENGINES of this same weight of steam for the varying pressures of expan- sion. The difference in the volume between this theoretical expansion line and the actual expansion line represents the loss in volume due to condensation. The percentage of initial con- C 1 ! densation at the point of cut-off would be 77 J and at any other kl point, such as k, would be .,- Example. An 8" X 12" engine runs 230 r.p.m. and uses 700 Ibs. steam per hour. Steam pressure, 100 Ibs.; exhaust, atmospheric; clear- ance, 10 per cent.; scale of indicator spring, 60 Ibs. Find the total weight of steam in the cylinder durmg expansion. Solution. First find the cylinder feed, or amount of steam supplied by the boiler to the engine per stroke. Strokes per hour = 230 X 2 X 60 = 27,600. Cylinder feed = 700 -f- 27,600 = .02536 Ibs. To find the amount of cushion steam, first lay off from u, Fig. 87, the distance uO equal to 10 per cent, of uv, since the clearance is 10 per cent, and -uv represents the volume of the cylinder. If the length uv of the card is 2.9 in., the total length Ov is 3.2 in. The volume swept through by the piston is3.1416X4X4X12 = 602.4 cu. in. The clearance volume is then 60.2 cu. in., and the total volume 662.6 cu. in. In other words, each inch of length of the line Ov represents 662.6 -f- 3.2 = 207 cu. in. Now take a point on the compression curve after the exhaust valve has closed, such as N. The ordinates of this point measured from the axes OY and OX are, p = 34.8 Ibs. absolute, and v = 124.2 cu. in. = .07187 cu. ft. From the steam tables we find that 1 cu. ft. of dry saturated steam at 34.8 Ibs. absolute weighs .0836 Ibs. The weight of .07187 cu. ft., or the cushion steam, will then equal .07187 X .0836 = .006 Ibs. The total weight of steam in the cylinder during expansion is therefore .0254 + .006 = .0314 Ibs. Finally plot the curve of saturation for .0314 Ibs. of steam. To do this, take any pressure such as 80 Ibs. absolute and from the steam tables find the volume of 1 Ib. of steam at that pressure. This equals 5.47 cu. ft. The volume of .0314 Ibs. would then be .0314 X 5.47 = .1718 cu. ft. = 297 cu. in. \ TESTING OF STEAM ENGINES 177 Hence the ordinates of this point will be en P = 60 = L33 in. 297 and v = 2^7 = 1.43 in. This point is then plotted, and others are found and plotted in the same way. A curve drawn through these points will be the saturation curve. 116. Determination of Steam Consumption. When the engine is used with a surface condenser, the steam consumption may be determined by weighing the steam condensed. It is seldom, however, that this can be done, and usually it is necessary to measure the amount of feed water going to the boiler which supplies steam to the engine to be tested. When this is done, great care should be taken to see that all the steam produced from this feed water goes to the engine. If all the steam does not go to the engine, the amount going to other purposes should be measured and deducted from the total feed, the difference being the engine feed. Tests of this character should be at least 10 hours in length, and still better 24 hours, so as to allow for the effect of varying conditions such as level of water in the boiler. The engine should be credited with the moisture in the steam. The engine should be operated for some time before the test be- gins so that the heat conditions may be uniform. During the test the engine should be run as nearly as possible at a uniform load. Indicator cards are usually taken every 10 or 15 minutes, and the average horse-power shown by the cards is taken as the average horse-power developed during the test. As has already been stated, to determine the number of pounds of steam used by a steam engine per horse-power per hour, the water entering the boiler is weighed and all the water that actually goes to the engine is charged to the engine. This weight of water reduced to pounds per hour is divided by the average horse-power devel- oped by the engine; the result is the number of pounds of steam used by the engine per horse-power per hour. The American Society of Mechanical Engineers has adopted a standard method of testing steam engines, which will be found in Volume XXIV of their Proceedings. The number of pounds of steam used by the various forms of 12 178 HEAT ENGINES engines are summarized in the following table. These results are very general for the various classes of engines. TABLE XIX. STEAM CONSUMPTION OF VARIOUS CLASSES OF ENGINES Pounds Simple throttling engine, non-condensing 44 to 45 Simple automatic engine, non-condensing 30 to 35 Simple Corliss engine, non-condensing 26 to 28 Simple automatic engine, condensing 22 to 26 Simple Corliss engine, condensing 22 to 24 Compound automatic engine, non-condensing 25 to 30 Compound automatic engine, condensing 18 to 20 Compound Corliss engine, condensing 14 to 16 Triple Corliss engine, condensing 12.25 to 13 Uniflow engine, simple condensing, superheat 11.25 to 12 117. Brake Horse-power. All of the economies given in Table XIX are based on the indicated horse-power of the engines. But this does not represent the actual useful work that can be FIG. 88. Prony brake. obtained from the engine, as part of this power must be used in overcoming the friction of the engine itself. The actual power of the engine delivered upon the fly-wheel is usually measured by a Prony brake or some similar device. The horse-power obtained at the brake is termed the "brake," or "effective" horse-power. The brake used to determine the brake horse-power usually consists of an adjustable strap which encircles the rim of the brake wheel which is fastened to the crank-shaft of the engine. The brake wheel should be provided with interior flanges for holding water used for keeping the rim cooled. To the strap encircling the brake wheel is rigidly fastened an arm which rests on a platform scales. The friction of the strap DE, Fig. 88, TESTING OF STEAM ENGINES 179 tends to carry the arm FK in the direction of rotation of the wheel. The force tending to depress the arm^-K is measured on the scales. The net force on the scales times the distance AC is the moment of friction, and this multiplied by the angular velocity equals the rate of doing useful work. The weight of the lever on the scales must either be counterbalanced, or else found by suspending the lever on a knife-edge vertically over A and noting the scale reading. This weight plus the weight of the standard C is called the tare, and is then subtracted from the weight shown on the scales to determine the net weight due to the force of friction. The standard C must be of such a length that when the engine is running the arm FK is held in a horizontal position. Let w = the net weight on the scales, n the revolutions of the shaft per minute, I the horizontal distance AC in feet, or the brake arm, and B.H.P. the brake horse-power. Then RHP 2irlwn - m : 33000 118. Mechanical Efficiency. The brake horse-power divided by the indicated horse-power is the mechanical efficiency of the en- gine, and the indicated horse-power minus the brake horse-power is called the friction horse-power. The mechanical efficiency of an engine is usually about 85 per cent., and in well-built engines may be as high as 90 per cent, and over. In large engines it is not possible to obtain the brake horse- power, as such an engine would require a very elaborate brake. In such cases it is customary to obtain the horse-power lost in friction, approximately, by what is termed a friction card. A friction card is obtained by removing all the load from the engine so I/hat the only load acting upon the engine is the friction of the engine itself. An indicator card is taken from the engine under these conditions, and the horse-power shown by this card is called the friction horse-power. A card so taken does not give the actual friction of the engine, as the friction increases with an increase of load. After finding the friction horse-power, the actual output of the engine may be determined by subtracting this friction horse-power from the indicated horse-power. If the power taken by the friction card is more than 10 per cent, of the full-load capacity of the engine, the friction of the engine is considered to be excessive. Where an engine is used to drive a dynamo, the mechanical efficiency of the engine may be deter- 180 HEAT ENGINES mined from the electrical output of the generator, if the electrical efficiency of the generator is known. 119. Actual Heat Efficiency. The actual thermal efficiency of an engine is the heat equivalent of one horse-power per hour divided by the number of heat units consumed by the engine per H.P.-hour, either indicated or brake. Since a horse-power is 33,000 foot-pounds per minute, then the heat equivalent of one horse-power per hour is Let S equal the steam consumption of an engine per horse- power per hour, q the quality of the steam, L the latent heat, h the heat of the liquid above 32, and t the temperature of the feed-water. (The British practice assumes this temperature to be the temperature corresponding to the exhaust, or back, pressure.) Then the actual thermal efficiency would be : ____ 2 s45 M S[h+qL- (J-32)}' 120. Duty. The economy of pumping engines is usually expressed not as the number of pounds of steam per I.H.P. per hour, but in terms of "duty." In the earlier history of pumping engines, the definition of duty was the number of foot-pounds of work done in the pump cylin- der per 100 Ibs. of coal burned in the boiler. The objection to this method of determining duty is that it includes both boiler and engine economy. In purchasing a pumping engine it was necessary to allow the contractor to furnish the boilers also. To obviate this difficulty it is better to express duty as the number of foot-pounds of work obtained in the pump cylinders per 1000 pounds of steam furnished to the engine. The speci- fications state at what pressure this steam must be furnished. Duty may also be expressed as the number of foot-p'ounds of work done in the pump cylinders per 1,000,000 B.T.U. consumed by the engine. This is the best way of expressing duty, as it eliminates all considerations of the steam pressure. Engines working under widely different conditions may be compared when their duty is based on foot-pounds developed in the pump cylinder per 1,000,000 B.T.U. furnished to the engine. The amount of "work done" is equal to the weight of water pumped times the "head" pumped against. The total head is made up of the pressure shown by the gage on the discharge line plus that on the suction fine, both reduced to feet, plus the TESTING OF STEAM ENGINES 181 vertical distance between the center of the pressure gage and the point of attachment of the suction gage to the main. The duty that may be obtained in the various forms of pumping engines is given in the following table: TABLE XX. DUTY OF VARIOUS FORMS OF PUMPS Ft. Ibs. Small duplex non-condensing pumps 10,000,000 Large duplex non-condensing pumps 25,000,000 Small simple fly-wheel pumps, condensing 50,000,000 Large simple fly-wheel pumps, condensing 65,000,000 Small compound fly-wheel pumps, condensing 85,000,000 Large compound fly-wheel pumps, condensing 120,000,000 Large triple-expansion fly-wheel pumps, condensing 165,000,000 Large triple-expansion pumps, condensing, of exceptional economy 180,000,000 The capacity of a pump is the number of gallons pumped in 24 hours. 100 I w 3 80 P< W 70 a 30 I M 20 10 10 20 30 50 60 70 80 90 100 110 120 130 Indicated Horse Power FIG. 89. Curves showing steam consumption. 121. Variation of Steam Consumption. Most engines work at a varying load, so that it is important to know the steam con- sumption of the engine at the different loads. Fig. 89 shows the variation of steam consumption in a 100 horse-power engine at 182 HEAT ENGINES various loads. The upper curve shows the steam consumption when the engine was running non-condensing, and the lower curve when it was running condensing. In these curves the ordinates represent the steam consumption per horse-power per hour, and the abscissae represent the indi- cated horse-power. Example. The area of the indicator card from the head end of an 8" X 12" double-acting steam engine running 227 r.p.m. is 1.34 sq. in., and from the crank end 1.16 sq. in. The length of both cards is 2.19 in., and the scale of the spring used was 60 Ibs. The diameter of the piston rod is 1| in. A Prony brake was attached to the engine and the gross weight on it was 103.5 Ibs. The length of the brake arm is 54 in., and the tare 28.5 Ibs. Find the (a) I.H.P., (6) B.H.P., (c) F.H.P., and (d) mechanical efficiency. Solution. (a) The average height, or mean ordinate, of the card is equal to the area divided by the length, and this multiplied by the scale of the spring used will give the mean effective pressure. Hence, 1 34 Head end = ' X 60 = 27.7 Ibs. M.E.P.I J;JJ Crank end = X 60 = 23.95 Ibs. Area Head end = 3.1416 X 4 X 4 = 50.26 sq. in. Crank end = (3.1416 X 4 X 4) - (3.1416 X .75 X .75) = 48.50 sq. in. Y) / fl 7? The indicated horse-power for each end equals oonnn* Hence, , , 27. 7 X 1 X 50. 26X227 Head end = - H P / c - 1 , 23.95X1X48.5X227 Crank end = ^3000 Total I.H.P. = 9.58 + 8.02 = 17.6. (6) Net weight on brake = 103.5 - 28.5 = 75 Ibs. 54 Length of brake arm = ~ = 4.5 ft. 2V*3 1 This equation shows that theoretically the action of the gover- nor is independent of the weight of the balls. Practically, there is considerable friction in the mechanism of the governor, and the balls must have considerable weight in order easily to over- come the friction of the governor. If the number of revolutions' of the governor balls be n per minute, then > Substituting in equation (4), the value of F 2 as found from (5), and solving for n n = svs Substituting equation (5) for F 2 in equation (3), and letting k = 2, then W ' w 2936 (l +. ) - n*h. (8) This expression gives the relation of the principal items of the governor design. 220 HEAT ENGINES 148. Shaft Governor. There are two forces that may be util- ized to control the speed of an engine by means of a shaft gover- nor. In the earlier form of governors, the principal force was centrifugal force. FIG. 126. Elementary centrifugal governor. FIG. 127. Actual construction of centrifugal governor. In Fig. 126, the governor weight is so suspended that it moves approximately in a radial direction due to the action of centrifugal force. In the actual construction of the governor, the centrifugal GOVERNORS 221 force acts against the resistance of a spring. In this figure, as the speed of the wheel increases, the centrifugal force increases FIG. 128. Elementary inertia governor. and the weight M will move out against the resistance of the spring. Fig. 127 shows the actual construction of a governor which is FIG. 129. Actual inertia governor. actuated by centrifugal force. The governor in this case regu- lates the position of the eccentric, as is shown by the dotted lines. 222 HEAT ENGINES The angular advance and eccentricity are changed at the same time, leaving the lead almost constant for all positions of the governor. In Fig. 128 the weight M is fastened so that centrifugal force has no effect upon the movement of the weight, but only produces a stress in the arm SM. But, if the wheel were suddenly stopped, the weight would continue to move, due to the inertia, and exert a force upon a spring (not shown) against the resistance of which the governor ball acts. The motion of this weight is arranged to change the position of the valve. Inertia alone is not used as the actuating force, but a combination of centrifugal force and inertia is used. Fig. 129 shows a form of governor combining these two forces. The two governor weights are fastened to a single arm which rotates around a pin (shown shaded). One weight has a longer arm than the other, arid is the dominating weight. As the engine revolves, this weight tends to take a radial position. This action gives the governor its initial position and determines the position of the valve. The governor weights are suspended so that if the speed of the engine changes, the inertia of the weights moves the governor against one or the other of the stops shown. The governor weights act against the resistance of a spring. The speed at which the engine is to run may be changed by changing the tension of this spring. The valve is driven by a pin fastened to the governor arm. 149. Isochronism. For a given governor, w and W are fixed quantities, and if the governor is so constructed that h is constant, then n must be constant, and the governor becomes isochronous. An isochronous governor is one in which the balls are in equilib- rium at one speed and only at one, except for friction, and any variation from this speed will send them to the limit of their travel in one direction or the other. The friction of the governor makes it impossible for a governor to be perfectly isochronous. This result is approximately obtained by using crossed arms so that the governor balls have a parabolic path, and the height h will remain approximately constant. In some forms of governors the balls are guided in a parabolic guide so that their motion is an exact parabola and give h a uniform value. 150. Hunting. Over-sensitive governors often exhibit the phenomena known as "hunting." No matter how quickly a governor may change its position in response to a demand for more or less steam, the engine does not respond instantly. This GOVERNORS 223 is in consequence of the energy stored in the moving parts of the engine, and in the element of time that must elapse between the moment when the steam is admitted by the governor and the time that it acts on the piston. Therefore when a sudden demand for power is made on an engine in which the governor is too sensi- tive, or too nearly isochronous, the drop in speed will be sufficient to force the governor into a position of over-control, so that too much steam is admitted. This causes the revolutions tojncrease beyond the desired point and the same over-control is exercised in the opposite direction. In other words, the governor balls fly first in one direction and then the other, " hunting" for the position of equilibrium: The effect is to make the speed of engine change rapidly, first having an excess of speed, and then a speed below the normal. This trouble may be overcome by adding a small weight to one of the governor balls, and changing the tension of the governor spring. 151. Practical Considerations. When a properly designed engine does not govern properly, the trouble is often due to undue friction in the vajve mechanism, which may be caused by a tight- ening of the glands or the journals, or by friction in the dash pot and springs. It may also be due to excessive leakage in the valve, unbalancing it, or by the valve being too tight. The governor should also be examined to see that the weights have not been changed. The tension of the springs should be uniform, if more than one spring is used. If the engine operates at a lower speed than that desired, the tension of the governor spring should be increased. If this ten- sion has been increased to the limit of the spring, then additional weight should be placed in the governor balls. In all forms of governors it is necessary that the friction of the valve mechanism be made as small as possible, and it should, if possible, be a constant quantity. It is better to have balanced valves, where they are directly operated by the governor, and the valves should have a small travel. In the D-slide type of valve, small travel is obtained by using a double-ported valve. In direct connected engines, 2 per cent, variation in speed is the maximum allowable, and most specifications require the va- riation to be less than 1 per cent. In mill engines a variation of 5 per cent, is sometimes allowed. 152. Fly-wheel. The governor of an engine controls the speed within certain limits by controlling the action of the valve. It 224 HEAT ENGINES takes a few revolutions, however, to bring the governor into action. The steam engine, however, has fluctuations of speed that occur in the fraction of a revolution, and these fluctuations must be controlled by the fly-wheel. These fluctuations of speed are due to three principal causes: First. The pressure of steam is not the same at all points of the stroke. Second. The motion of the piston is carried to the shaft by means of the connecting rod and crank. This means of changing reciprocating into rotary motion causes a turning effort which varies from zero to a maximum. Third. The reciprocating motion of the engine piston and other parts necessitates these parts being brought to rest and started again twice each revolution. The overcoming of the inertia effect, caused by the action described, causes a variable force to be transmitted to the crank. A fly-wheel is fastened to the main shaft of the engine to reduce the variation of speed of the engine in the fraction of a revolution. The inertia of the fly-wheel serves to carry the engine at those portions of the stroke where the piston is not giving sufficient power to the shaft to carry the load. The effectiveness of the fly-wheel depends upon the energy stored in it. As most of the weight of the wheel is in the rim, we may consider, for an approximation, the action of the rim as giving the fly-wheel effect. If W is the weight of the fly-wheel rim in pounds, and R is the average radius in feet, and the wheel makes n revolutions per minute, then the energy of the rim W 2g V 60 4rr 2 2 ft.lb, (9) The expression shows that the effectiveness of a fly-wheel de- pends upon the weight of the rim, the square of the radius of the wheel, and the square of the number of revolutions that it makes. CHAPTER XIII COMPOUND ENGINES 153. Compound Engines. Any engine in which the expansion of steam is begun in one cylinder and continued in another is called a compound engine] although this term as commonly used refers to an engine in which the expansion takes place in two cylinders successively. A triple-expansion engine is one in which the steam is expanded successively in three cylinders. When steam is expanded in two or more cylinders successively, the number of expansions per cylinder is less than when only one is used, and therefore the range of temperature in each cylinder is less. Reducing the range of temperature in the cylinder reduces the condensation losses. -The principal object of compounding is to reduce the amount of steam used per horse-power per hour, and, under proper conditions, compounding accomplishes this, owing to the reduction of initial condensation. The radiation losses from a compound* engine are usually larger than from a simple engine, and very often the mechanical losses are increased by compounding. The tendency, then, in a compound* engine, is to increase the radiation loss and to increase the mechanical losses. On the other hand, compounding decreases the thermodynamic losses by decreasing the range of temperature in each cylinder. With low pressure and a small number of expansions, a single-cylinder engine is more economical than a compound* engine, but with high-pressure steam and a larger number of expansions, the reverse is the case. The higher the pressure and the larger the number of expansions the greater the economy of the compound* engine. For pressures under 100 Ibs., the single-cylinder condensing engine is more economical than the compound engine. But for pressures above 100 Ibs. the compound engine is usually more economical. In the case of the non-condensing engine, the com- *The term "compound" as here used includes triple-expansion, quad- ruple-expansion, etc. 15 225 226 HEAT ENGINES pound engine does not show any economical advantage until the pressure reaches 150 Ibs. The compound condensing engine becomes less economical than the triple-expansion engine for pressures greater than 150 Ibs. The single-cylinder engine, Fig. 112, is more economical than the compound engine when the number of expansions of the steam is less than four. When there are from four to six ex- pansions there is very little difference in the economy. With from six to fifteen expansions the compound engine is more economical. When the number of expansions exceeds fifteen it is usual to use a triple-expansion engine. FIG. 130. Tandem arrangement of cylinders. . 154. Tandem Compound Engines. A tandem compound engine, Fig. 130, is one in which the two cylinerds are placed one in front of the other. The pistons of the two cylinders are attached to the same piston rod, and there is but one connecting rod and crank. The steam flows directly from the high-pressure cylinder into the low-pressure cylinder, and the connecting pipes are relatively small, there being no receiver except the piping between the cylinders. The tandem compound engine occupies less space than the cross compound. The principal objection to this form of engine is the difficulty of getting at COMPOUND ENGINES 227 the cylinder which is nearest the crank-shaft. This is the earliest form of compound engine used. 155. Cross-compound Engine. In the cross-compound engine, Fig. 131, the two cylinders are placed side by side, and each 228 HEAT ENGINES cylinder has its separate piston rod, connecting rod, and crank. The steam, after leaving the high-pressure cylinder, usually enters a steam reservoir called a receiver, and from this receiver the low-pressure cylinder takes its steam. The cranks in a cross-compound engine are usually set 90 apart, so that when the high-pressure cylinder is at the beginning of its stroke the low-pressure cylinder is at mid-stroke. A cross-compound engine with cranks at 90 must always be provided with a receiver, as the low-pressure cylinder may be taking steam when the high-pressure cylinder is not exhausting. The cross-compound engine occupies a much larger space than the tandem engine, but the parts are lighter. Each piston, cross-head, connecting rod, and crank does only approximately one-half the work that they would do in a tandem engine. The turning effort on the crank-shaft is made more uniform by placing the crank at 90. This reduces the size of the fly-wheel necessary to overcome the fluctuation of the speed of the engine, and also assists the governing. A vertical cross-compound engine is often termed a "fore and aft" compound. 156. Ratio of Cylinders in the Compound Engine. In the compound engine the strokes of the two cylinders are usually the same. If we represent the ratio of the volumes of the two cylinders by L, and the diameter of the high-pressure cylinder by d, and that of the low-pressure cylinder by D, then * - f The value of L should be such as to avoid a fall in pressure, termed "drop," between the exhaust pressure in the high-pres- sure cylinder and the admission pressure in the low-pressure cylinder. The value of L varies from 2J to 4 for automatic high-speed engines, and from 3 to 4| for engines of the Corliss type. L is equal to the quotient of the number of times the steam is expanded in the engine divided by the number of expansions in the high-pressure cylinder. The ratio of expansion, r, in a compound* engine is equal to the ratio of the total volume of the low-pressure cylinder, or cylinders, to that of the high up to the point of cut-off. That is, it is, as in * See note at bottom of page 225. COMPOUND ENGINES 229 the case of a single-cylinder engine, the ratio of the final to the initial volume occupied by the steam while in the engine. This ratio, r, may be varied in an engine by varying the point of cut-off in the high-pressure cylinder. It is customary to proportion an engine and so set the valves that each cylinder does an equal amount of work. This, however, is not always the case, some, engines being designed to give equal ranges of temperatures in the cylinders. Theoretically this gives the best economy. The proportion of work that is done by each cylinder may be adjusted by changing the low-pressure cut-off. The shorter the cut-off in the low-pressure cylinder, the less the steam taken from the receiver and the higher the pressure in the receiver. Increasing the pressure in the receiver causes a higher back pressure for the high-pressure cylinder, and consequently less work done by that cylinder. Increasing the low-pressure cut-off will decrease the work done by the low-pressure cylinder. Theo- retically, changing the cut-off in the low-pressure cylinder does not change the gross horse-power developed by the engine, but in actual practice this does not hold absolutely true, although the change is very slight. The equalization of the work in the two cylinders cannot be accomplished in most engines, as in equalizing the work at different loads an excessive drop may be produced be- tween the cylinders. 157. Horse-power of a Compound* Engine. In determining the horse-power of a compound* engine from the indicator cards, the card from each end of each cylinder is worked up and the horse-power calculated for each, and the sum of the horse-powers determined from each card will be the horse-power of the engine. In determining the horse-power that a compound* engine ought to develop it is necessary to know the absolute initial steam pressure, the total number of expansions of steam, the number of strokes per minute, the length of the stroke, and the diameter of the high- and low-pressure cylinders. The horse-power is then determined as though there were but one cylinder, and that one the size of the low-pressure cylinder, and the total expansion of steam took place in that cylinder. The reason for this is apparent when we consider that the power of any engine per stroke depends on the weight of steam admitted and its * See note at bottom <5T page 225. 230 HEAT ENGINES ratio of expansion, and that all the power of the compound* engine could be developed in its low-pressure cylinder if we ad- mitted into that cylinder the same weight of steam as was ad- mitted to the high-pressure cylinder, expanded the steam in this cylinder the same number of times as it was expanded in the whole engine, and exhausted against the same back pressure. If the horse-power obtained by assuming all the work done in the low-pressure cylinder be multiplied by a card factor, the result will be equal to the horse-power of the engine. This may be expressed mathematically as follows: Let D = the diameter of the low-pressure cylinder. d = the diameter of the high-pressure cylinder. A = the area of the low-pressure cylinder in square inches. / = the length of stroke of the engine in feet. p = the mean effective pressure for the whole engine. n = number of revolutions per minute. x = the per cent, of the stroke to the point of cut-off in the high-pressure cylinder. T = ratio of expansion for the whole engine. e = the card factor. Pi = initial pressure steam entering the engine. pz = pressure of the exhaust. D 2 Then r - ~ 2 = ( 2 ) Pi (1 + logs) . and p = e - p 2 (3) 2plAn Horse-power . The value of the factor e depends upon the type of the engine, and varies from .70 to .80 for automatic high-speed engines, and from .75 to .85 for a Corliss engine. ^ ^ ^ t> Example. A 15" X 24" X 36" X 30" engine runs 100 r.p.m. Cut-off in the H.P. cylinder, f stroke; in the intermediate cylinder, f stroke; in the L.P. cylinder, \ stroke. Steam pressure, 225 Ibs. * See note at bottom of page 225. COMPOUND ENGINES 231 Engine exhausts into a condenser having a vacuum of 26 in. Ba- rometer reading, 28.65 in. Assume a card factor of .80. Indicator cards were taken from the engine with the following areas: H.P. cylinder, head end, 1.32 sq. in., crank end, 1.35 sq. in. ; intermediate cylinder, head end, 1.8 sq. in., crank end, 1.71 sq. in.; L.P. cylinder, head end, 2.01 sq. in., crank end, 2.04 sq. in. Length of all cards, 3 in. A 160 Ib. spring was used on the H.P. cylinder, a 50 Ib. spring on the intermediate, and a 20 Ib. spring on the L.P. The diameters of the piston rods were as follows: H.P. cylinder 2 in.; intermediate cylinder, 2^ in.; L.P. cylinder, 3 in. (a) What is the rated H.P. of the engine? (6) What per cent, of the rated H.P. is being developed? Solution. (a) Atmospheric pressure = 28.65 X .491 = 14 Ibs. Exhaust pressure, p 2 , = (28.65 - 26) X .491 = 1.3 Ibs. D 2 36 X 36 8 X 36 X 36 r = xd* f X 15 X 15 3 X 15 X 15 = 15.35. M.E.P = e + log2. Producers using anthracite coal have been in successful use for a number of years, and bituminous producers are now coming into use. The prin- cipal difficulty in using bituminous coal as a fuel for producers is in removing, or preventing, the formation of tar. The future success of the bituminous producer depends upon the thorough removal of the tar. There are two types of producers: (a) pressure, and (b) suction producers. In the pressure type, the air and steam are furnished FIG. 173. Cross-section of suction gas producer. to the producer by a fan. The rate of production is independent of the engine's demand and the gas must be stored. The gas is furnished to the engine at the pressure produced by the fan, usu- ally equivalent to a pressure of a 2- or 3-in. column of water. In the suction type, the air is drawn through the producer by the suction formed in the engine cylinder, so that the rate of pro- duction of gas in the producer depends upon the demand of the engine. The producer then automatically furnishes the necessary amount of gas for the operation of the engine, so that no storage tank is required. The suction producer is becoming very popular for use with the gas engine, particularly in the smaller sizes. The pressure pro- THE INTERNAL COMBUSTION ENGINE 285 ducer is more expensive in installation than the suction type, as it involves a gas holder, but it can be used with inferior grades of fuel. The suction producer occupies less space and costs less than the pressure type. It is best adapted to the use of high-grade fuels. The most successful suction producers use anthracite coal. Fig. 173 shows a cross-section of a suction producer. A is a blower which is used to furnish draft during the starting of the fires. B is the generator with a double-valved hopper for admit- ting the coal to the fuel bed of the producer. C is a vaporizer in which steam is formed, the steam being mixed with the air enter- ing the producer. D is the scrubber, consisting of a coke tower with a spray of water for washing the gas. E is the cleaner con- taining trays filled with wood shaving, through which the gas passes to remove dust and dirt. F is the cleaning pot which col- lects the heaviest dust and dirt coming over with the gas. By the admission of water to the cleaning pot on shutting down, the rest of the apparatuses water sealed and the gas remaining in it is kept for use in starting up again. G is a damper which is closed while the blower is running. After the blower has been shut down, the damper G is opened and the air enters the producer at H, passes over the surface of the water in the vaporizer C and down the pipe 7, entering the generator B at the bottom. The pipe J leads to the engine. To operate the plant, a fire is lighted just as in an ordinary coal stove, and the blower is run until a good fire is burning, with the relief valve R open. After fifteen or twenty minutes, the fire is sufficiently hot to give off gas. The relief valve is then closed and the gas allowed to pass through the apparatus to the engine, the blower being kept running until the proper quality of gas is obtained at the test cock near the engine. The engine is then started, the blower stopped and the formation of gas becomes automatic, the suction of the engine furnishing the draft through the fire. The efficiency of the gas producer should be a little higher than that of a steam boiler. Actual tests show efficiencies as high as 85 per cent., but efficiencies ordinarily do not exceed 80 per cent. The consumption of fuel in a gas engine operating with gas pro- ducers does not usually exceed 1 Ib. per horse-power per hour, and in large installations is less than one pound. The heat value of producer gas varies from 100 to 150 B.T.U. per cubic foot. 286 HEAT ENGINES 192. Vaporization of Oil. The lighter oils, such as gasoline, are easily vaporized by either spraying the oil into a current of air, or allowing a current of air to pass over the surface of the oil. This vaporization may be increased or assisted in four ways: (a) By the application of heat; (6) By increasing the surface of the oil exposed to the air; (c) By reduction of pressure or increase in vacuum; (d) By keeping the air which is in contact with the gasoline as fresh or as far from the saturation point as possible. With the heavier oils, such as distillates and crude oil, it is necessary to provide some other means of vaporizing the oils. There are two general methods to accomplish this purpose. In engines such as the Hornsby-Akroyd, the oil is injected into a cylinder against hot plates, or a hot ball, and is almost instantly vaporized by the contact with the red-hot surface. In other engines the oil is vaporized in a heated chamber external to the engine. Initial vaporization is often produced by artificially heating the chamber, and after the engine is in operation, the oil is heated by means of the exhaust passing through pipes located in this chamber. Engines have been placed on the market which used crude oil just as it comes from the wells, and have given fair satisfaction. The difficulty in using crude oils is in taking care of the heavier ingredients, such as paraffine and asphalt, that occur in them. The hot surface must be at a sufficient tempera- ture so that in vaporizing these heavier oils they will be broken up into lighter compounds which are more easily vaporized. Asphalts cannot be broken up and must be removed. 193. Alcohol. Alcohol is similar in its nature to kerosene, except that it will stand a very much higher compression, so that, while alcohol does not contain the heat value of the petroleum oils, it will, nevertheless, give almost as much power per pound, owing to the fact of the higher efficiency which may be obtained by its higher compression. In this country, alcohol has not yet been extensively used, but it has been largely used in Europe and Central America. In using alcohol in connection with the engine, it is usually necessary to provide some means of heating it so as to produce more rapid vaporization. Commercial alcohol usually contains not less than 5 per cent, water, and the percentage may be much higher. For satisfactory operation in a gas engine, it should not contain more than 10 per cent, water. THE INTERNAL COMBUSTION ENGINE 287 194. Heating Value of Fuels. The heating values of the vari- ous fuels are given in the following table: TABLE XXII. CALORIFIC VALUE OF GASEOUS FUELS Lower heating value per cu. ft., B.T.U. Least air required for combustion per cu. ft., cu. ft. Oil gas (Pintsch) 1000 D 5 Natural gas 950 9 1 Illuminating gas 565 5 25 Coke-oven gas 545 5 Producer gas (from soft coal) 145 1 25 Producer gas (from anthracite) .... Producer gas (from coke) Blast-furnace gas 145 135 100 1.15 1.0 7 CALORIFIC VALUE OF LIQUID FUELS Lower heating value per cu. ft. of oil gas, B.T.U. Least air required for combustion per cu. ft., cu. ft. Heating value B.T.U. per pound Heavy crude oil (West Virginia). Light crude (West Virginia) Heavy crude (Pennsylvania).. . . Kerosene Gasoline 94.6 95.0 99.2 95.8" 97 7 IS.Olbs. 15.0 ' 15.0 ' 15.0 ' 15 ' 18,320 18,400 19,210 18,520 19 000 Benzol, C 8 H 6 99 3 13 4 ' 17 190 Alcohol, 100 per cent. . . 103 8 6 ' 11 664 Alcohol 90 per cent 104 7 8 ' 10 080 195. Fuel Mixtures. The mixture of air and gas in internal combustion engines is very important. The possible power derived from an engine depends upon obtaining the proper mixture of air and gas. Under ordinary conditions of pressure and temperature, a mixture of CO and air will be explosive when the range is from 16 to 74 per cent., by volume, of CO. With illuminating gas, the range of mixture is from 8 to 19 per cent.; with gasoline, from 2J to 5 per cent. It will be noticed that the possible range of mixtures varies very widely with the nature of the gas used. Experiments show that the best results are obtained when the air in the cylinder is slightly in excess of the theoretical mixture. 196. Flame Propagation. A very important point in gas en- gine operation is the rate of flame propagation through the mass of the gas. If this rate is slow, the pressure will not be obtained 288 HEAT ENGINES quickly enough for the engine to give its maximum horse-power. The rate of flame propagation depends upon the mixture of the gas and upon the method of ignition. In large engines it is becoming a custom to put more than one igniter upon an engine so as to produce more rapid flame propagation. High compression has a tendency to reduce the rate of flame propagation. On tfhe other hand, however, compression of the gases increases the ease with which they may be ignited, and the range of the explosive mixture. Owing to slow flame propagation, ignition takes place before the beginning of the working stroke. 197. Rated Horse -power. The determination of the power of a gas engine from its dimensions is much more difficult than of a steam engine. The theoretical diagram, although quite definitely defined is not of much value in determining the horse-power. The actual diagram is influenced by so many conditions such as the quality and purity of gas, temperature of the mixture, condi- tions of combustion, heat losses, location and kind of ignition, form of combustion chamber and other items, that it is possible to obtain almost any result. The card factor as applied to the steam engine is of little value as it shows variations under different conditions as high as 100 per cent. It is not surprising therefore that numerous methods exist for determining the.principal dimen- sions of internal combustion engines, all of these based on assump- tions giving only approximate results. One of the best methods is based on the amount of air necessary for combustion and on the thermal and volumetric efficiencies when the engine is operating with the quantity of air assumed. This method was developed by Hugo Giildner and for many years this has been used with success for all sizes and types of engines and for various fuels. Let N n = normal or rated horse-power. n = r.p.m. H = the lower heating value of the fuel in B.T.U. per cubic foot for gas per pound for liquids. Ch = fuel consumption per hour at normal output in cubic feet for gas in pounds for liquids. y w = economic or thermal efficiency at the brake. N n X 33000 X 60 2545 X N n ,. rhen ' * = ^ ~ 2545 X N, whence G& = ^rw~ rj w A /i THE INTERNAL COMBUSTION ENGINE 289 Let C st = fuel consumption per suction stroke. L st = air comsumption per suction stroke. L = proper amount of air in cubic feet required per cubic foot of gas or per pound of liquid fuel. D = diameter of the cylinder. S = stroke of the engine. t] v = volumetric efficiency. Then, for a single acting four-cycle engine C h ^.SXN^ C h XL 84.8XJV n XL = 30 Xn~ nXHXrj w For two-cycle engines equations (27) and (28) must be divided by 2 . During one suction stroke the volume of the actual charge drawn in is _ r T _ D 2 X ?r X S _ actual piston displacement C. + L st - 4 x ^- volumetric efficiency 84.8XAT n X(l+L) . .... n - m Cublc feet Solving for D, S, and n, we get for engines using gaseous fuels: = Vsx SfS^-A () n f X^X^ (1+L) Xr, v ^' m ' (32) For liquid fuel engines the term (1-f- L) may be put equal toL, as the volume of fuel is very small compared with the volume of air. In view of the amount of experimental data available a selec- tion of the efficiencies rj w and t] v , and the air consumption for different types and sizes of engines can be easily made. For this purpose Tables XXIII and XXIV are inserted. TABLE XXIII. VOLUMETRIC EFFICIENCY rjv, OF GAS ENGINE rj v = .88 .93 For slow speed engines with mechanically operated inlet valve. ?7v = .80 .87 For slow speed engines with automatically operated inlet valves. rj v = .78 -85 For high speed- engines with mechanically operated inlet valve. ?7v = .65 .75 For high speed engines with automatic inlet valve. r) v = .50 .65 For very high speed automobile engines with automatic inlet valves and air cooling. 19 290 HEAT ENGINES w PLJ i SwcN^ C^ SwMM CO . 3 '.'.'.'. '. '.'.'.'. . . CO CO 00 O O tC oa : : : : 1 : :! : : :| 1 1 1 - ' i H 1 4J . . . . ' eC ; o(> .. .oo"oo"'o5cr ^.81 S OCDrHbt -^1(N- (NO O- ' I '. ICU5COCO - rH.rH- rHrH U5- 1 J jj i ^ * "^ 1 s ? * 1 S -8 ? ^ 1 il.s-.2t5 1 ^ s O 73 S3^ o. _, 203 | i to o 1 5 S3 5 r i a o THE INTERNAL COMBUSTION ENGINE 291 Diameter D, stroke S and speed n have a certain relation to each o other. Present-day engines can safely be built with ^ = 1 to 2.5, and with piston speeds up to 800 ft. per minute for large engines and 1200 for smaller ones. Example. Determine the cylinder diameter, stroke and revolutions per minute of a four-cycle, single-acting, one-cylinder anthracite pro- ducer gas engine of 170 H.P., having a piston speed of 800 ft. per minute and a stroke-diameter ratio -w = 1.35 Solution. From table XXIII use rj v = .90 T From table XXIV use 17. =.26 H = 140 B.T.U. and since n = ^ = ^= Zo Z.IL) solving for D I W8XN n (l+L) = I \S XnXHXirvX 77, V 1 - 3 108 X 170 X 2.5 X 2.7 .35 X 800 X 140 X .26 X .9 = 1.877' feet = 22.5 inches S = 1.35 X 22.5 = 30 inches 800 77 = 2~X30 = ; r ' P ' m> Example. Determine the diameter and stroke of a two-cycle, single-acting, 4-cylinder Diesel Oil Engine of 150 H.P., having 350 r.p.m. and a piston speed of 700 ft. per minute. Solution. From table XXIII rj v = -80 From table XXIV i, w = .32 L ~ 18 ' Each cylinder must develop 2 . = 18.75 H.P. per cycle. 700 X 12 * - 2 X 350 = 108 X 18.75 X 312 _ 7 -- = / .o X 350 X 18,000 X .32 X 0.80 The American Association of Automobile Manufacturers determines the normal output of four-cycle automobile engines by the formula B.H.P. = ^|y^ (33) where d = the diameter of the cylinder in inches, and N the num- ber of cylinders. This rule is based on a piston speed of 1000 ft. 292 HEAT ENGINES per minute and has, of course, an arbitrary and conventional value only. The rated horse-power of a gas engine to drive a given size electric generator is quite different from that of a steam engine to drive the same machine. This is due to the fact that a gas engine as rated has very little overload capacity, while a steam engine can carry a 25 per cent, overload continuously and a 50 per cent, overload for a short period of time. In order to allow for the overload capacity of the generator, the gas engine must be sufficiently large to drive the generator under that condition. As an example, to drive a 2000 k.w. generator, a 4500 H.P. gas engine is used, while to drive the same generator with a steam engine, a 3000 H.P. engine is used. It should be noted that, at present, gas engines are rated on their output, or brake horse-power, while steam engines are rated on their indicated horse-power, and that, as stated above, gas engines are rated at practically their maximum capacity, while steam engines are rated at the I. H.P. at which they give the best economy. 198. Actual Horse -power. The actual indicated horse- power (I. H.P.) of a gas engine already built and in operation may be determined in almost exactly the same way as was done in the case of the steam engine, the only difference being that in the formula, plan i.tt.r. - 33000' n = explosions per minute, when finding the horse-power of the gas engine, while when finding the power of the steam engine, it was equal to the revolutions per minute. In both cases, I = the length of stroke in feet, and a = the cross-sectional area of the piston in square inches. The mean effective pressure p is found by taking indicator cards from the engine and then multiplying, by the scale of the spring used, the quotient found by dividing the area of the card by its length. The cross-sectional area of the piston in the gas engine indi- cator is usually one-fourth of a square inch, while that of the THE INTERNAL COMBUSTION ENGINE 293 steam-engine indicator is one-half a square inch, the difference being due to the fact that the initial pressure in the gas-engine cylinder is so much greater. The brake horse-power (B.H.P.) of a- gas engine is found in exactly the same manner as the B.H.P. of a steam engine, the expression being (35 > where I = the length of the brake arm in feet, w = the net weight on the brake, and n = the number of revolutions per minute. It is thus seen that in making a test of a gas engine to obtain the I.H.P. and B.H.P., both the explosions per minute and the revolutions per minute must be noted. Example. A 10|" X 16|" single-acting gas engine runs 200 r.p.m. and makes 96 explosions per minute. The gross weight on the brake was 140 Ibs., the tare 20 Ibs., and the length of the brake arm, 60 in. The area of the indicator card was 1.07 sq. in. and the length 3 in., and the scale of the spring used was 219 Ibs. Find the (a) I.H.P.; (6) B.H.P.; (c) F.H.P.; and (d) mechanical efficiency. Solution. (a) M.E.P. = ~'^ X 219 = 78.1 Ibs. o a = TT X 5 T 3 F X 5f\ = 84.5 sq. in. I = 16J -5- 12 = 1.406 ft. plan _ 78.1 X 1.406 X 84.5 X 96 _ 885000 I.M.I . - 330()() - = - 27. (6) Net weight = 140 - 20 = 120 Ibs. Length of brake arm = 60 4- 12 = 5 ft. 2irlnw 2 X 3.1416 X 5 X 200 X 120 B.H.P. = 33000 33000 _ 755000 "33000 (c) F.H.P. = I.H.P. - B.H.P. = 27 - 22.85 = 4.15. (d) Mech. Eff. = '- = ^" = - 846 = 84 - 6 P er cent - CHAPTER XVII DETAILS OF GAS-ENGINE CONSTRUCTION 199. In general, the frame and working parts of the gas engine are heavier in construction than the corresponding parts of a steam engine. This is largely due to the fact that the number of impulses given the gas engine for the same power is less than those given the steam engine, and hence each impulse in the gas engine must exert more force. FRAME. Fig. 174 shows the frame of a modern gas engine of medium size. The barrel of the cylinder is cast with the frame. The main bearing supports are cast in the same frame. FIG. 174. Gas engine frame. CYLINDER AND PISTON. The inner lining of the cylinder is inserted in the frame as a separate piece, except in the smaller engines. Fig. 175 shows the piston and piston rings. Three rings, at least, and often six or seven, are used in a gas engine. It is very important that the piston fit the cylinder as closely as possible so as to hold the compression. The piston shown is for a single- 294 DETAILS OF GAS-ENGINE CONSTRUCTION 295 acting engine, and serves both as piston and cross-head. The cross-head pin is shown at the top of the figure, and is placed in the hole shown in the side of the piston. This is the most com- monly used construction for small and medium size engines. CONNECTING RODS. The connecting rods used in gas engines are similar to those in steam-engine practice. VALVE MECHANISM. The valves used have been almost the same for all types of gas engines, and are of the poppet type. The exhaust valves are always mechanically operated, but the inlet valves may be either automatic or mechanically operated. Fig. 176 shows the cross-section of a four-cycle gas engine, and shows both inlet and exhaust valves. These valves are operated from a cam shaft at the side of the engine by means of roller cams. In some engines these cams are replaced by eccentrics. FIG. 175. Piston and rings for gas engine. WATER-JACKET. -In all except small air-cooled engines, the cylinder and cylinder head are cooled by being surrounded by a water-jacket, and in the best engines the valves are also water- jacketed. The water-jackets are shown in Fig. 176, surrounding the valves and reaching between the valves. 200. Ignition. One of the most important details of gas- engine construction has been the development of a suitable means of ignition. The first successful form of ignition was by means of an open flame which was drawn into the cylinder at the proper time. Flame ignition, however, is uncertain and difficult of application, and is not economical and so has been abandoned in recent engines. The next form of ignition was the hot tube, in which a closed tube connected with the engine cylinder was kept at red heat 296 HEAT ENGINES by means of an external flame. The compression of the gases into the hot tube ignites them at the proper time in the stroke. The time of ignition is more or less regulated by the tempera- ture of the tube. In some cases the admission of the gas into the hot tube was controlled by a valve. This form of ignition is satisfactory in small engines, but is hardly sufficient to ignite a large volume of gas such as is admitted to a large engine, and does not admit of a change in the time of sparking. One of the simplest forms of igniters is that used by the Deisel Engine Company. In this engine the air is compressed to a very DETAILS OF GAS-ENGINE CONSTRUCTION 297 high pressure and the temperature is then sufficient to ignite the entering charge of oil, or gas, which is delivered to the cylinder at a pressure slightly higher than the compression pressure. This then requires no special igniting apparatus, and the time of ignition is controlled by the time of admission to the cylinder. In the Hornsby-Akroyd oil engine, a hot bulb, used for vaporizing the entering oil, serves also as an igniter. At the present time, the most used and the most successful form of ignition is by electric spark. This has proven to be the FIG. 177. Magneto. most satisfactory in the large majority of internal combustion engines and in automobiles it is used exclusively. It is by far the most reliable and flexible method in use. There are various means of generating the current used in electrical ignition, such as a battery, dynamo, or magneto, the one most commonly employed at present being the magneto, Fig. 177. This is a piece of apparatus consisting of a permanent steel magnet bent in the form of a letter U, with a coil of wire revolving in the opening between the poles or ends of the magnet. As the wire revolves it cuts the magnetic lines of force and an electric current is set flowing through the wire. When the armature is in a position such that the magnetic lines flow through the core, this core becomes magnetized. If the armature is then turned so that the lines no longer flow through 298 HEAT ENGINES the core, the core loses its magnetism and it is this dying away of the magnetism of the core that produces an electric current in the winding. A magneto thus gives a current only at certain points in the FIG. 178. Bottom of make- and-break jignitor block, show- ing contact points. FIG. 179. Top of make-and-break ignitor block. revolution of the armature and it must be so driven that it will give this current at the time the engine needs a spark. The more suddenly the magnetism changes strength the more intense the current will be. If the armature is turned slowly, the current may not be strong enough to form a spark. FIG. 180. Section of cylinder head showing make-and-break ignition system. A battery of dry cells is frequently used to furnish current for ignition of small engines. There are two forms of electric ignition; viz. the make-and- break or low tension, and the jump spark or high tension systems. Of these, the make-and-break is the simpler. In the make-and-break system there are two contact points, as shown in Figs. 178 and 180, located inside of the cylinder, and DETAILS OF GAS-ENGINE CONSTRUCTION 299 in addition, in series with the circuit is placed what is called a spark coil. This consists of a number of turns of comparatively heavy wire wrapped around a core composed of iron wires. This coil acts as an inductive resistance, and when the circuit is broken it serves to cause a hot spark at the point of the break. The circuit of the make-and-break igniter, then, consists of a battery, or magneto, and a spark coil, both of which are placed in series with two contact points in the engine. Just before the point of sparking, the two contact points A and B, Fig. 178, are brought together, and at the point of sparking the mechanism is so constructed (see Fig. 179) that the two FIG. 181. Diagram of jump-spark ignition. points are quickly separated, producing a sufficient spark to ignite the charge. The advantages of the make-and-break system are: (a) hot spark ensuring ignition; (6) little trouble with insulation. The disadvantages are : (a) moving mechanism required in the cylinder; (6) points of contact become foul and wear away. The make-and-break igniter is used in a great many engines, and is advocated by many, owing to the low tension at which it is operated. It is the most common form of ignition on station- ary engines. In jump-spark ignition, Figs. 181 and 182, the current is taken from a battery B, Fig. 181, or generator at a low voltage and passed through an induction coil C, having an interrupter. The induction coil has a primary and secondary coil. The interrupted current passing through the primary coil induces a high-tension current in the secondary coil. This current at a high voltage is carried to what is known as a spark plug E, located in the engine cylinder. 300 HEAT ENGINES This spark plug contains two points about V m - apart, across which a high-tension current is made to j ump at the time of igni- tion. The time of ignition is controlled by a timer D, fastened to the engine shaft, and, at the proper time of the stroke, this timer closes the battery circuit, the high-tension current is generated in the induction coil, and the spark jumps across the air gap causing ignition in the cylinder. There are a great many detailed modifi- cations of this device, but the above description covers the general construction of them all. In some cases the current is furnished by an alternating-current magneto. With an alternating current, no interrupter is necessary. This system is almost universally used on automobiles. The advantages of the jump-spark system are: (a) absence of moving parts in the cylinder; (b) easy adjustment of the time of FIG. 182. Section of cylinder head showing jump-spark ignition system. ignition. The disadvantages are: (a) high insulation required; (b) liability of spark plug becoming fouled with oil or dirt; (c) intensity of spark varies with pressure in cylinder. In all forms of gas-engine igniters, some means should be pro- vided for changing the time of ignition, so that the pressure may reach a maximum at the proper time in the stroke. In the jump- spark igniter this is done by moving the position of the commuta- tor relative to the piston position. The proper time for ignition depends upon the mixture and the speed of the engine. Ignition is not instantaneous and in order to have the greatest pressure against the piston when it begins the power stroke, the mixture must be set on fire before the completion of the compres- sion stroke. This is called advancing the spark. 201. Governing. The aim of all governors is to obtain the maximum thermal efficiency at all loads. The governing of a gas engine is different from that of a steam engine. In a steam engine under a constant load, each cycle of the engine is DETAILS OF GAS-ENGINE CONSTRUCTION 301 practically the same, while in the gas engine, even with a constant load, there is always some change in the cycle of the engine. This is due to changes of mixture and time of ignition. This makes the problem of governing in the gas engine more diffi- cult than in the steam engine. The following general methods of governing are used in gas engines: I. The "hit and miss" system. II. Variation in the quantity of charge entering the cylinder, the mixture of gas and air being constant. III. Variation of the mixture of gas and air, the load determin- ing the quality of the mixture. IV. Governing by changing the time of ignition. V. Combinations of the above methods. 1. Hit and Miss. The most common of all these systems of governing is the "hit and miss." In this form of governing, when the speed exceeds the normal, the supply of gas is cut off and the engine gets no explosion, causing the engine to "miss." The loss of the explosion causes the speed to slacken, the governor opens the inlet valve and the engine again receives an impulse, or a "hit." This is most economical and simplest method of governing, but does not provide the closest regulation in speed. In this system the "miss" may be occasioned by (a) holding the exhaust valve open and thus allowing no suction, or (b) by failing to open the gas valve. There may be considerable variation in speed. This method of governing is not desirable for large engines because of the high pressures after a "miss." 2. Quantity governing may be accomplished by varying the weight, or quantity, of the mixture of gas and air entering the cylinder. This result may be obtained in two ways. 1. By cutting off the charge before the piston reaches the end of the suction stroke. 2. By throttling the charge during the suction stroke. The disadvantage of this system is that the compression varies with the size of the charge. Reducing the compression reduces the efficiency, and hence this form of governing is not as univer- sally efficient as the "hit and miss." 3. Quality Governing .In this system the weight of the charge remains the same, but the proportion of gas to air is varied the governor usually controlling the supply of gas. 302 HEAT ENGINES As the load decreases, the amount of gas is reduced for the same total charge. This system has the advantage over Method No. 2, that the pressure of the compression always remains the same. On light loads, however, it is not so economical as Method No. 2, for when the load is very light the mixture may be so weak that the charge will not ignite. Method No. 4. Controlling the speed by changing the time of ignition is used on automobile engines. As the load diminishes, the time of sparking is brought nearer to the working stroke, that is, it is advanced, and it may even occur after the dead center (just previous to the working stroke). As the spark is advanced, the engine develops less and less power. The quan- tity and quality of the charge, however, remains the same. This system of speed control is very uneconomical at light loads. Method No. 5. A great many different combinations of the above systems have been used. Often engines having "quan- tity" and "quality" governors for the heavy and medium loads change the governing system to " hit and miss " for light loads. A combination largely used in electric lighting work, on account of the close regulation obtained, is quality governing at high loads and quantity governing at low loads. The governing of an automobile is a combination of quality governor by the throttle, and governing by spark advance with the ignition device. Kerosene and fuel oil engines are commonly governed by by- passing the fuel so that a greater or less amount of it is injected into the cylinder. Gas-engine governing is at present almost as perfect as govern- ing in the steam engine. There is no difficulty in obtaining suffi- ciently accurate governing so that alternators driven by gas engines may be operated in parallel. 202. Carburetors. A carburetor is a device used for vaporizing oil, particularly gasoline. It is largely used in connection with automobile or small launch engines. In a carburetor the air may be passed over or through the gasoline, or the gasoline may be mechanically sprayed into the current of incoming air. Fig. 183 shows a cross-section of one type of Stromberg carbu- retor. A float M operates a pair of levers and through them the needle valve K, thus controlling the supply of gasoline to the spray nozzle C. The gasoline enters the carburetor from the source of supply through and the dirt in it is removed by the DETAILS OF GAS-ENGINE CONSTRUCTION 303 strainer N. The hot water in the jacket J keeps the carburetor warm and assists in vaporizing the gasoline. At each charging stroke of the engine, air is drawn in through the fixed air inlet S and passes at a high velocity up through the venturi tube D and around the nozzle C. The gasoline is sucked in a jet from this nozzle and is mixed with the air in the mixing chamber 1. The throttle valve H regulates the supply of the mixture to the engine. As the speed of the engine increases, the proportion of air to gasoline must be increased. This is taken care of by the auxiliary \ EXPLANATION A- Low .speed adjusting nut K- Needle valve B-High speed adjusting nut L- Glass float chamber C - Spray nozzle D- Venturi tube E- Auxiliary air valve F- Low speed spring G-High speed spring H- Throttle valve I - Mixing chamber J- Water jacket M- Metal float N- Gasoline strainer O - Gasoline line coupling P - Drain cock Q- Hot air horn R- A.ir shut-off for starting S - Fixed air inlet T- Season adjustment FIG. 183. Cross-section of Stromberg gasoline carburetor. air valve E, which is opened or closed a greater or less amount as the speed of the engine increases or decreases. 203. Vertical Versus Horizontal Engines. The advantages of the vertical engine are: higher rotative speed, better balancing, occupy less floor space, less wear on the cylinders and pistons. The advantages are obtained because multi-cylinder engines are more easily built of the vertical type. Therefore more cylinders, each of smaller size, may be used for the same power than would 304 HEAT ENGINES be the case with a horizontal engine. This means a shorter stroke and hence higher rotative speed for the same power. An increase in the number of cylinders means better balancing and less vibra- tion. The reciprocating masses of the horizontal engine tend to cause the engine to move on its foundation and heavier founda- tions are necessary than in case of vertical engines. The disadvantages of the vertical engine are: increased first cost, and, in the enclosed type, too much oil may get in the cylin- der, causing trouble. The open end of the cylinder on the hori- zontal engine assists in cooling the piston. In the larger size FIG. 184. Koerting two-cycle gas engine. engines, the cylinders are generally horizontal, while most auto- mobile and small launch engines have vertical cylinders. 204. Large Gas Engines. In the large sizes, the single-acting engine has been replaced by the double-acting engine, similar in its arrangement to the steam engine. Fig. 184 shows a block plan of a modern two-cycle gas engine of the double-acting type. In this figure, the device for cooling the piston and piston rod is not shown. In most large engines, however, of the double- acting type, the piston and piston rod are cooled by allowing a circulation of water through them. Usually the water enters through a flexible pipe connected to the cross-head, and is re- moved by a tail rod projecting through the cylinder head. DETAILS OF GAS-ENGINE CONSTRUCTION 305 205. Oil Engines for Ships. For use in marine work certain conditions are required for successful operation of the engine. 1. It should be able to be started quickly from any position without having to be barred round. 2. It should be capable of rapid reversal. 3. It should be able to run continuously for long periods with- out a stop. 4. It should work economically at various speeds. 5. It should start under a load. 6. It should admit of easy inspection and adjustment. 7. It should work smoothly in a rough sea when the propeller is sometimes partly out of the water. The relative advantages and disadvantages of Diesel engines as compared with steam engines and boilers for use on large ships are as follows: Advantages : (a) Have much higher thermal efficiency; (6) Weigh about half as much and occupy about two-thirds the space for the same power; (c) Make possible cleaner, quicker and easier "coaling;" (d) Require less attendants; (e) Eliminate funnels and dirt; (f) Start quicker; (g) Eliminate stand-by losses. Disadvantages. (a) Are not so easily reversed or maneuvered in harbors; (b) Fuel is more expensive in most places and not so readily available ; (c) There is an absence of steam for working the auxiliary devices. 206. Humphrey Gas Pump During the Brussels Exposition in 1910 there was exhibited a new type of pumping engine known as the Humphrey Gas Pump. Since that time this gas pump has gained a world-wide reputation. It has been successfully intro- duced into this country and has been greatly improved by Ameri- can designers. The largest pump in the world, pumping water for the City of London, is of this type. Its operation can best be understood by reference to Fig. 185. The pump consists of a vertical gas cylinder A with inlet and outlet valves B and C. These valves interlock with each other. On the water side of the pump there is a suction pipe D, 20 306 HEAT ENGINES a suction valve S, and a pressure pipe E connecting the cylinder with the pressure tank F. The water column G forms a gas-tight piston. The operation of the pump is as follows: We will assume at the beginning that the gas cylinder is filled with a mixture of gas and air. This charge of gas and air is ignited and the pressure is suddenly increased. While this takes place the volume will scarcely change so that combustion practically takes place at- constant volume. The water column FIG. 185. Diagram of Humphrey gas pump. owing to the increased pressure on its surface is rapidly acceler- ated by the pressure in the gas cylinder and the gases undergo adiabatic expansion. When the gas has reached a predetermined pressure the exhaust valves on the top of the cylinder and the suc- tion valves on the water inlet begin to open automatically. The inflowing water follows the moving water column and fills the gae cylinder, replacing the burned gases. The hydrostatic pressurs from the water tank reverses the water column closing the water inlet valves and forcing out the bur-ned gases through the exhaust valve. When the water level reaches the position V 3 , the exhaust valve closes and the water column compresses the remaining burned gases to the volume V*. Now the water column reverses DETAILS OF GAS-ENGINE CONSTRUCTION 307 again. Reexpansion of the compressed gases takes place and the pressure falls below the atmospheric pressure. The mixing inlet valve opens and the new charge is taken in until the volume Vi is filled. The water column again reverses and compresses the charge to the volume Vz- Ignition lakes place and the whole cycle is started over again. The engine works on the four-cycle principle, the expansion and contraction occurs adiabatically and the cycle is carried on by the oscillation of the water column due to the changes of pressure. The action of the pump is not altered if instead of delivering into the elevated tank it is discharged into an open air vessel or into an open tower. The pump has the advantage of being capable of handling enormous quantities of water. In the large pump installed in the City of London, 15 tons of water are discharged at each discharge of the pump, the pump having a capacity of 150 million gallons. PROBLEMS 1. A gasoline engine uses 1 Ib. of gasoline per I.H.P. per hour. If the gasoline contains 19,500 B.T.U. per pound, what is the actual heat emciency of the engine? 2. A gas engine uses 20 cu. ft. of gas per horse-power per hour. Each cubic foot of gas contains 600 B.T.U. Initial temperature in the engine is 2000 and the final temperature 800. What is the actual and theoretical thermal efficiency of the engine? 3. What is the mechanical efficiency of an 8\" X 14" single-acting gas engine if it runs 225 r.p.m., makes 106 explosions per minute, has a net weight of 50 Ibs. on the brake, and the M.E.P. is 76.8 Ibs. ? The length of the brake arm is 62.75 in. and the tare of the brake is 19 Ibs. 4. A card from an 8j"X 14", single-acting gas engine has an area of .9 sq. in. and its length is 3 in. Scale of spring, 240 Ibs.; r.p.m., 225. Explosions per minute, 100. There is a Prony brake on the engine, the length of the brake arm being 63 in. and the net weight on the brake 42 Ibs. Find the I.H.P. ; B.H.P.; F.H.P.; and the mechanical efficiency. 6. An 8"X 10", single-acting steam engine running 250 r.p.m. and having an average M.E.P. of 35 Ibs. uses 20 Ibs. of steam per I.H.P. per hour. Steam pressure, 100 Ibs.; feed temperature, 200; coal costs $2.50 a ton and contains 13,500 B.T.U. per Ib. Efficiency of the boiler plant, 70 per cent. A gas engine is being considered for the place. The engine is 8^" X 14", single acting, running 223 r.p.m. and making 75 explosions per minute. It uses 2 1 Ibs. coal per I.H.P. per hour. The area of the average indicator card is 1.04 sq. in. and the length 3.33 in. Scale of spring, 240 Ibs. The engines are to run ten hours a day, three hundred days in the year. Gas producer uses the same coal as the boiler plant. Which would be the cheaper to run and how much per year? If a Prony brake is placed on each engine, that on the steam engine having a length of 4 ft. and carrying a net weight of 50 Ibs., 308 HEAT ENGINES and that on the gas engine having a length of .63 in. and carrying a gross weight of 58 Ibs., the tare being 19 Ibs., which engine will develop the larger output and how much? Which has the greater mechanical efficiency and how much? 6. A steam engine uses 20 Ibs. of steam per I.H.P. per hour and develops 200 H.P. A 20" X 24" single acting gas engine running 220 r.p.m. is being considered for the place. It uses 10,000 B.T.U. per I.H.P. per hour when making 105 explosions per minute and developing an average M.E.P. of 100 Ibs. Efficiency of the boiler plant, 70 per cent.; efficiency of gas pro- ducer, 80 per cent. Steam engine plant costs $20,000. Gas engine and gas producer plant costs $30,000. Cost of labor is the same for both plants. Coal costs $3 a ton and contains 13,000 B.T.U. perlb. The steam pressure in the boiler plant is 100 Ibs., and the temperature of the feed water, 180. If the interest charges are 5 per cent., and the repairs and the depreciation, 10 per cent., which would be the cheaper plant, and how much, to run ten hours a day for three hundred days a year? CHAPTER XVIII ECONOMY OF HEAT ENGINES 207. Relative Economy of Heat Engines. Primarily the efficiency, and in most cases, the economy, of heat engines depends upon the range of temperature of the working medium in the engine. As has been shown, the thermal efficiency of an engine theoretically equals r, where T\ is the initial absolute temperature of the working medium and T z is its final absolute temperature. In practice it is found that the best heat engines are able to realize actually only about 60 per cent, of the theoretical efficiency. An examination of the range of temperatures in the various forms of heat engines will give some clue to their probable actual efficiency. The following table gives a general idea of the possible efficiency of some of the more important prime movers. TABLE XXV. THERMAL EFFICIENCIES OF PRIME MOVERS Range of tempera- ture in cylinders Theore- tical efficiency Probable actual efficiency Average non-condensing steam engine 116 14 5 8.7 Average condensing steam engine 226 27 8 16 7 High-pressure non-condensing steam engine. . . High-pressure condensing steam engine 194 279 22.4 32 2 13.4 19.3 High-pressure steam engine, superheated steam Average condensing steam turbine, saturated steam 381 381 39.6 39 6 23.8 23 8 High-pressure condensing steam turbine, super- heated steam 429 43.3 25.7 Small gas engine 900 39.5 19.5 Large gas engine 1300 47.0 28.0 Large gas engine high compression 1400 52.2 31.6 Diesel motor very high compression .... 1900 60.0 36.0 This table gives some idea of the development and future 309 310 HEAT ENGINES possibilities of the various prime movers considering them from a standpoint of heat efficiency. The internal combustion engine is theoretically approximately twice as efficient as the steam engine. 208. Commercial Economy. Heat efficiency, however, is not the only consideration. In actual operation, the important thing is the cost to produce a horse-power for a given period of time. A convenient unit of time is one year. This cost of production involves a great many considerations. In determining this cost the following items should be considered : (1) Interest on the capital invested; (2) Depreciation of machinery and building structures; (3) Insurance and taxes; (4) Fuel cost; (5) Labor of attendance; (6) Maintenance and repairs; (7) Oil, waste, water, and other supplies. The first three of these items are called the "fixed charges," and remain the same no matter what the load on the plant may be. The last four items are the ''operating expense," and vary with the conditions of operation. The sum of the fixed charges and operating expense is the total operating cost. In most plants the cost of coal is from 25 to 30 per cent, of the total operating expense. A saving in the coal cost of operat- ing is not always a saving in the total cost of operating. This saving may involve so much increased cost of installation that the additional fixed charges on the new capital invested will more than offset the saving in coal. This is well illustrated by the condition which exists in localities having very cheap coal. A careful comparison of plant-operating costs for a condensing and a non-condensing plant often shows that the cost of operating the non-condensing is less than that of the condensing plant, due to the fact that the increased cost of the condensing plant adds more to the interest and depreciation charges than is saved on the cost of coal used, which is less than in a non-condensing plant. The following table gives the comparative itemized costs of operating for a compound condensing engine, a gas engine with gas producer, and a steam turbine. These are assumed to be operating an electric generating unit. ECONOMY OF HEAT ENGINES 311 Comparison of a 1000 B.H.P. compound condensing engine, a 1000 B.H.P. bituminous gas producer and gas engine plant, and a 1000 B.H.P. steam turbine. Bituminous coal assumed to cost $3 per ton, with lower heat value of 12,000 B.T.U. per pound TABLE XXVI. COMPARATIVE COSTS PER RATED HORSE POWER Reciprocating engine Gas engine Steam turbine Installation. Engine $18.00 8.00 3.50 29.50 27.00 18.00 10.00 $40.00 3.50 43.50 20.00 18.00 10.00 $15.00 6.00 5.00 26.00 23.50 14.00 7.50 Piping Condensers and pumps Engine plant Producer Boiler 12.00 10.00 5.00 20.00 10.00 9.00 4.50 Chimney, breeching and pumps Stokers Boiler, or producer, plant. . . Generator, switchboard and connections 4.25 5.95 1.70 4.58 6.40 1.83 3.55 4.79 1.42 Building Total cost of plant Operation. Interest, 5 per cent Depreciation, 7 per cent Insurance and taxes. . . $84.50 11.90 43.05 $91.50 12.81 31.85 $71.00 9.76 36.73 Fixed charges 16.20 2.75 12.90 21.60 2.13 13.00 Coal per brake horse-power per year . . 27.00 2.55 13.50 Repairs 3 per cent. Attendance, oil, waste and supplies Operating expense Total cost of operation $54.95 $44.66 $46.49 The above table assumes the plant to operate 24 hours per day and 300 days per year, and the average load to be one-half of the full rated load. 312 HEAT ENGINES As the cost of coal increases, the gas engine and gas producer will make a more favorable showing. If full load could be carried for the 24 hours, the showing will be more favorable to the recip- rocating engine. With smaller units the cost of operation is less for the gas engine, as small gas engines are more economical than small reciprocating steam engines, or steam turbines. With large gas engines the first cost is high and the upkeep expensive. INDEX Adiabatic expansion, definition of, 24 change of temperature during, 28 Advance angle, 188, 190 Advancing the spark, 300 Air, composition of, 70 pump, 240 Alcohol, use of in gas engines, 286 Ampere, 9 Angle of advance, 188, 190 Anthracite coal, 78 A. S. M. E. rule for finding, horse-power rating of boilers, 98 quality of steam, 53 Automobile engines, rated horse- power of, 291 Babcock and Wilcox boiler, 91 Barker's mill, 244 Barrus, 52 Barsanti, 271 Bearing, 162 Bituminous coal, 77 Blow-off cock, 105 Boiler accessories, 105 Boiler, A. S. M. E. rule for horsepower rating of, 98 Babcock and Wilcox water- tube, 91 classification of, 82 dry-pipe, 85 economy, 99 efficiency, 100 feed pump (see Feed pump) fire-tube, 82 heating surface of return fire-tube, 99 Heine water-tube, 94 horse-power, actual, 98 rated, 97 internally fired, 87 locomotive, 89 losses, 101 problems, 133 Boiler, Rust water-tube, 95 Scotch marine, 87 setting, 83, 85 Stirling water-tube, 92 tubes, diameter of, 99 tubular, 82 tubulous, 82 use of tubular, 89 vertical, 95 water- tube, 91 when use fire-tube and when water-tube, 89 Wickes water-tube, 95 Boyle's law, 11 Brake horse-power (see Horse-power, brake). Branca's, Giovanni, turbine, 245 Brasses, 163 Breeching, definition of, 99 ratio of to grate surface, 99 British Thermal Unit (B.T.U.), 5 Brunton, 109 Buckeye riding cut-off valve, 205 Calorific power of fuel, 66, 76, 77, 78, 79 Calorimeter, 48 Barrus throttling, 52 Carpenter separating, 49 throttling, 50 coal, 68 nipple, 48 "normal reading" of, 48 Peabody throttling, 50 problems, 55 separating, 48 throttling, 48, 50 Capacity of pump, 181 Carburetor, 302 Carnot cycle, description of, 29 most efficient cycle, 34 reversibility of, 33 Carpenter, 49 313 314 INDEX Charles' law, 11 Chimneys, boiler horse-power of, 130 brick, 131 capacity of, 129 dpaft of, 128 efficiency of, 130 height of, 131 materials used in, 131 steel, self-sustaining, 132 unlined, 132 Clausius, 10 Clearance, 149 per cent, of, 149 Clerk, Dougal, 269, 278 Coal analysis, 64 proximate, 64 ultimate, 64 Coal, anthracite, 78 bituminous, 77 calorimeter, 68 dry, 64 semi-bituminous, 78 Cock, blow-off, 105 gage, 106 three-way, reversing by means of, 212 tri-, 107 Combustion, air required for, 70 heat of, 66 problems, 79 rate of, 98 theoretical temperature of, 74 Compound engines, combined dia- gram from, 232 cross, 227 cut-off in low-pressure cylinder of, 229 "fore and aft," 228 horse-power of, 229 number of cylinders in, 225 problems, 234 ratio of cylinders in, 228 tandem, 226 Compound expansion, effect upon initial condensation, 148 Compounding, gains due to, 225 losses due to. 225 principal object of, 225 Compression, 149 Condensation, initial (see Initial condensation). Condensers, barometric, 238 cooling surface required in, 242 cooling water used by, 240 for steam turbine use, 242 increase in power due to, 242 jet, 237 location of hot-well for use with, 238 surface, 240 types of, 236 Conduction, 7 Connecting rod, effect of on Zeuner diagram, 196 solid-ended, 161 strap-ended, 161 Convection, 8 Cover plate, 201 Corliss engine card, 174 trip gear, 206 valve, 205 effect of using, 208 Counter-balance, 162 Crank-shaft, 162 Crosby indicator, 169 Cross-compound engine, 227 Cross-head, 160, 161 pin, 161 Curtis turbine, 255 "Cushion steam," 150 Cut-off valve (see Valve, cut-off). Cycle, Beau de Rochas, 271 Carnot, 29 four-, 272 Otto, 271 two-, 272 "Cylinder feed," 150 Dash-pot, 206 Davis, 5, 40, 42 Dead center, method of placing engine on, 212 De Laval turbine, 251 Diagram factor, 143 Diesel, 269 motor, 273, 296 INDEX 315 Draft, chimney, 128 forced, 132 induced, 133 mechanical, 132 systems of, 132 "Dutch Oven," 95, 112 Duty, 180 Eccentric rod, 163 sheave, 163 strap, 163 throw of, 190 Eccentricity, 188, 190 Economizers, cost of, 126 description of, 125 size of, 127 Economy, engine, commercial, 310 relative, 309 governor, relative, 216 Efficiency, boiler, 100 boiler and grates combined, 100 chimney, 130 fuel, 79 gas engine, 275 producer, 285 heat engine, 10, 29, 32 actual, 180 mechanical, 179 turbine, best, 249 Energy, 8 change in internal, due to change in temperature, 15 Engine, automobile (see Automobile engine). commercial economy of, 310 Corliss (see Corliss engine). gas (see Gas engine). heat (see Heat engine). steam (see Steam engine). Equivalent evaporation, 100 Evaporation, equivalent, 100 factor of, 100 per pound of coal, 100 Exhaust, heat lost in, 145 lap, 189 'effect of on Zeuner diagram, 195 Expansion, adiabatic, 24 change of temperature during, 27 Expansion, compound, effect of upon initial condensation, 148 general case, 17 isothermal, 25 ratio of, 26, 147 work of, 18 Factor, diagram, 143 of evaporation, 100 Feed pump, location of, 122 use of, 119 Worthington boiler, 120 Feed-water heaters, advantages of, 123 closed, 123 cost of, 125 location of, 125 open, 123 types of, 123 use of, 123 "Fixed charges," 310 Flame propagation, 287 Flue gas, analysis of, 71 Fly-wheel, 223 Forces of impulse and reaction, 245 Frame, 164 "Free-piston" gas engine, 270 "Fore and aft" compound engine, 228 Fuel, air required for combustion of, 70 classification of, 75 composition of, 64 efficiency of, 79 gas engine, 283 heating value of gas and oil, 287 value of, theoretical, 68 mixtures, proper, 287 Fusible plug, 108 Gage cocks, 106 glass, 105 Gas engine, Barsanti and Matteucci's, 271 classification, 270 construction, details of, 294 Diesel's, 273, 280, 296 efficiency of, 275 316 INDEX Gas engine, four-cycle, 272 "free-piston," 270 fuels, 283 governors, types of, 300 history of the, 269 horizontal vs. vertical, 303 horse-power, actual, 292 rated, 288 ignition, kinds of, 295 Langen's, Otto and, 270 Lenoir's, 270 losses in a, 282 Otto (and Langen's), 270 problems, 307 two-cycle, 272 use of alcohol in, 286 vertical vs. horizontal, 303 Gas producers, 283 efficiency of, 285 pressure, 284 suction, 284 Gas pump, Humphrey, 305 Gear, Corliss trip. 206 Joy, 212 radial, 211 reversing, 209, 210 Walschaert, 211 Giffard, M., 121 Governor, automatic, 216 centrifugal, 220 design, relation of items in, 219 fly-ball, 218 "hit and miss," 301 inertia, 221 isochronous, 222 mechanism, 217 quality, 301 quantity, 301 shaft, 218, 220 throttling, 216 used with double-ported valve, 203 variable cut-off, 216 Governors, gas-engine, types of, 300 practical considerations in con- nection with, 223 relative economy of, 216 variation in speed allowable, 223 Grate surface in stokers, 118 ratio of, to breeching, 99 to heating surface, 98 Gutermuth, 148 Giildner, 269 Heat, absorption of, 13 added at constant pressure, 22 at constant volume, 22 general case, 20 balance in boiler plant, 101 capacity, 6 lost in exhaust, 145 up stack, 101 of fusion of ice, latent, 57 of liquid, 38, 40 of steam, latent, 38, 41 total, 41 of superheat, 38 relation between, and work, 10 between specific, of constant pres- sure and of constant volume, 15 specific (see Specific heat), theory of, 1 unit of, 5 Heat engines, efficiency of (see Effi- ciency, heat engine), ideal, 29 relative economy of, 309 Heater, feed-water (see Feed-water heaters). Heating surface, definition of, 98 of fire-tube boilers, rule for finding, 99 ratio of, to grate surface, 98 to rated boiler horse-power, 99 Heating value of fuel, 66, 287 higher, 66 lower, 66 of combustible, 101 Heine boiler, 94 Hero's turbine, 244 "Hit and miss" governor, 301 Hollis, 128 Horse-power, boiler, actual, 98 heating surface per, 99 rated, 97 INDEX 317 Horse-power, brake, gas engine, 293 steam engine, 178 chimney, 130 comparison of rated, for steam and gas engines, 292 engine, automobile, rated, 291 compound, 229 definition of, 9 gas, actual, 292 rated, 288 steam, indicated, 143, 173 theoretical, 142 friction, 179 of gas and steam engines, how rated, 292 Hornsby-Akroyd oil engine, 286, 297 Hot-well, 238 Humphrey gas pump, 305 "Hunting," 222 Ice, latent heat of fusion of, 57 Ignition, flame, 295 hot tube, 295 jump-spark, 299 kinds of gas engine, 295 "make and break," 298 Impulse, 245 Indicator, accuracy of, 170 Indicator cards, 174 combined, 232 method of taking, 172 various forms of, 213 Indicator, Crosby, 169 difference between steam and gas engine, 292 external spring, 170 manner of connecting to engine, 171 reducing motions, 171 setting valve by, 213 things determined by, 168 Thompson, 170 use of, 170 Initial condensation, action of, 145 amount of, 146 factors affecting, 146 graphical determination of, 174 Injectors, description of, 121 location of, 122 Isochronous governor, 222 Isothermal expansion, 25 Joule, 10, 14 Joy gear, 212 Juckes, John, 110 Jump-spark ignition, 299 Kerr turbine, 259 Knock-off cam, 206 Langen, 270 Lap, exhaust, 189 effect of on Zeuner diagram, 195 Lap steam, 189 effect of on Zeuner diagram, 192 Lead, 150, 188, 190 Lenoir, 270 Lignite, 77 Losses, boiler, 101 due to compounding, 225 gas-engine, 282 steam-engine, 144 Lubricators, 165 Magneto, 297 Mahler bomb coal calorimeter, 68 "Make and break" igniter, 298 Marks, 40, 42 Matteucci, 271 Mean effective pressure, 143, 173 Mechanical draft, 132 systems of, 132 efficiency, 179 equivalent of heat, 10 mixtures, 56 stokers (see Stokers, mechanical). Meyer riding cut-off valve, 204 Mines, U. S. Bureau of, 64, 65 Mixture problems, 61 Mixtures, fuel, 287 mechanical, 56 Moisture in steam, 48 Murphy, Thomas, 110 Napier's rule, 48 318 INDEX Nozzle, turbine, 248 "Normal reading" of calorimeter, 53 Oil, vaporization of, 286 engine, Hornsby-Ackroyd, 297 for ships, 205 "Operating expense," 310 Orsat apparatus, 72 Otto, 269 Parr coal calorimeter, 68 Parsons turbine, single-flow, 262 Peabody, 40, 42, 50 Peat, 76 Peclet, 7, 8 Perfect gases, definition of, 12 equation of, 12 laws of, 11 problems in, 34 Piston, 160, 161 position of relative to valve, 190 rod, 160, 161 valve, 198 Planimeter, 173 Port opening, 193 Power, 9 Pressure, absolute, 12 gage, 12 gas producer, 284 heat added at constant, 22 mean effective, 143, 173 relation between volume, tem- perature, and, 12, 27 specific heat of constant, 7 Problems, boiler, 133 calorimeter, 55 combustion, 79 economic, 242 engine, gas, 307 steam, actual, 184 compound, 234 theoretical, 150 mixture, 61 perfect gas, 34 Producers, gas (see Gas producers). Pumps, capacity of, 181 circulating, 240 Pumps, dry air, 240 feed (see Feed pumps). Pyrometers, 3 286, Quality of steam, 48, 53 Radial gears, 211 Radiation, 7 Rankine, 10 Rateau turbine, 257 Ratio of expansion, 147, 228 Re-action, 245 Reducing motion, indicator, 171 Re-evaporation, 145 Regnault, 39 Reversing gear, 209, 210 by means of three-way cock, 212 Rochas, 271 Rotation, changing direction of, 209 Rowland, 10 "Run over," 209 "Run under," 209 Rust boiler, 95 Safety valve, size of, 107 Sampling nozzle, 48, 53 Saturation curve, 175 Semi-bituminous coal, 78 Smoke, 71 Southwark-Rateau turbine, 259 Specific heat, definition of, 6 of constant pressure, 7 and of constant volume, rela- tion between, 15 of superheated steam, 40 of constant volume, 7 theory of, 5 Stanley, 109 Steam, action of, in turbine, 247 A. S. M. E. rules for finding qual- ity of, 53 boiling point of, 38 consumption, determination of, 177 variation of at different loads, 181 cushion, 150 dry saturated, 39 U.C.BERKELEY LIBRARIES - UNIVERSITY OF CALIFORNIA LIBRARY 1