UNIVERSITY OF CALIFORNIA 
 
 COLLEGE OF AGRICULTURE 
 
 AGRICULTURAL EXPERIMENT STATION 
 
 BERKELEY, CALIFORNIA 
 
 The Smokiness of 
 Oil-Burning Orchard Heaters 
 
 WARREN R. SCHOONOVER AND F. A. BROOKS 
 
 BULLETIN 536 
 August, 1932 
 
 UNIVERSITY OF CALIFORNIA PRINTING OFFICE 
 BERKELEY, CALIFORNIA 
 
CONTENTS 
 
 PAGE 
 
 Summary 3 
 
 The smoke problem 4 
 
 The origin of the smoke nuisance 4 
 
 Heat, not smoke, effective for frost protec- 
 tion 5 
 
 Need of relief from the smoke nuisance 6 
 
 Economic importance of orchard heating 8 
 Initiation of the investigation of the smoke 
 
 problem 8 
 
 Outline of the problem 9 
 
 Methods used in measuring smoke 10 
 
 Methods available 10 
 
 Tests in the field at Pomona 11 
 
 Laboratory methods developed at Davis.. 12 
 
 Smoke units used for comparative results 13 
 
 Accuracy of the method 14 
 
 List of orchard heaters tested 15 
 
 Test results of standard makes of orchard 
 
 heaters 17 
 
 Effects of incomplete combustion 17 
 
 Smoke tests of open-container smudge 
 
 pots 18 
 
 Smoke tests of short-stack heaters 18 
 
 Influence of stack height on smokiness 22 
 
 Smoke tests of open-flame heaters 23 
 
 Smoke tests of heaters with straight lou- 
 
 vered stacks 25 
 
 Smoke tests of cone-combustion-chamber 
 
 heaters 29 
 
 Smoke tests of Hy-Lo 1929 Model orchard 
 
 heater 30 
 
 Smoke tests of "nondistilling"-type 
 
 orchard heaters 31 
 
 General discussion of results 33 
 
 Heater groups according to smokiness 34 
 
 Estimate of the number and smokiness of 
 
 heaters in use 36 
 
 Operation methods for reducing smoke 
 
 output 36 
 
 PAGE 
 
 Test results with different fuel oils 38 
 
 Analyses of oils used in tests of standard 
 
 heaters 41 
 
 Analyses of oils before aDd after burning 41 
 Analyses of oils used for testing influence 
 
 of oil character on smokiness 41 
 
 Method of testing influence of oil character 
 
 on smokiness 43 
 
 Variations of smokiness not consistent 
 
 with oil in different heaters 45 
 
 Tests on stacks of new design 46 
 
 Requirements of a good orchard heater 47 
 
 Interpretation of test results on new stacks 47 
 Smoke tests of annular-combustion-cham- 
 ber stacks 48 
 
 Smoke tests of enlarged-combustion- 
 chamber stacks 48 
 
 Smoke tests of straight tall stacks 51 
 
 Principles of combustion in open-flame 
 
 stacks 52 
 
 Smoke tests on new open-flame stacks 54 
 
 Field measurement of the smokiness of or- 
 chard heaters 54 
 
 Visible records of smokiness 55 
 
 Correlation with the light-interception 
 
 method 56 
 
 Use of felt method for smoke measure- 
 ments in the field 58 
 
 Acknowledgments 58 
 
 Appendix A : Description of apparatus and 
 
 test methods 59 
 
 Appendix B: Correlation between light 
 interception and weight of smoke par- 
 ticles 63 
 
 Additional studies on correlation of smoke 
 density and weight 66 
 
THE SMOKINESS OF 
 OIL-BURNING ORCHARD HEATERS 2 
 
 WARREN E. SCH00N0VER3 and F. A. BKOOKS* 
 
 SUMMARY 
 
 In southern California 70,000 acres of orange groves are protected 
 occasionally against frost damage by burning low-grade fuel oil in 
 simple sheet-iron orchard heaters and smudge pots. The great quantity 
 of smoke produced when burning millions of gallons of oil each frosty 
 night frequently results in a serious smoke nuisance. A study of the 
 smokiness of oil-burning orchard heaters was made to aid in the abate- 
 ment of this nuisance. 
 
 Visual methods ordinarily used for smoke estimation in industrial 
 regions are not applicable to a study of the smokiness of orchard heaters 
 because of operation at night and the prevalence of smoke throughout 
 the citrus districts. Four other methods were developed permitting an 
 accurate classification of orchard heaters as to smokiness. Two of these 
 methods classify according to smoke blackness, and two according to 
 smoke weight. 
 
 Tests were run to determine the smokiness of the heaters in general 
 use. The tests showed : 
 
 1. That the different heaters vary greatly in smokiness. 
 
 2. That it is possible to burn ordinary grades of fuel oil in simple, 
 inexpensive heaters without producing visible amounts of smoke at 
 normal burning rates. 
 
 3. That the smokiness of many types of heaters can be reduced by 
 proper regulation and frequent cleaning. 
 
 4. That the composition of fuel oils available commercially has no 
 consistent influence on the smokiness of different heaters. 
 
 i Keceived for publication July 12, 1932. 
 
 2 This project was initiated under the leadership of A. H. Hoffman. He died 
 soon after the completion of the field trials at Pomona. Mr. C. E. Barbee had 
 charge of the project for several weeks after the death of Mr. Hoffman and has 
 since been responsible for the construction and operation of all the testing' apparatus. 
 
 3 Extension Specialist in Citriculture, temporarily assigned by the Agricultural 
 Extension Service to the Experiment Station to continue the investigation. 
 
 4 Associate Agricultural Engineer in the Experiment Station, appointed August, 
 1931. 
 
4 University of California — Experiment Station 
 
 5. That laboratory tests run at summer temperatures are a reliable 
 indication of the relative smokiness of heaters *as operated in the field 
 during the winter. 
 
 Heater manufacturers have availed themselves of the smoke-measur- 
 ing 1 facilities afforded by the laboratory for a study of heater design in 
 relation to the smoke problem. As a result of these studies new stacks 
 for use on old heaters have been developed which reduce the smoke out- 
 put at normal burning rates to invisibility. 
 
 Portable apparatus was devised in order to measure the smokiness of 
 orchard heaters as operated in the field. Accurate and semipermanent 
 visible records of smokiness were obtained. A method of correlating 
 field records with laboratory determinations was found which permits 
 field records to be interpreted quantitatively. 
 
 THE SMOKE PROBLEM 
 
 The Origin of the Smoke Nuisance. — Experiments begun in Califor- 
 nia as early as 1896 gave some indication that smoke or steam was effect- 
 ive in protecting citrus orchards against frost. Smudges of smoldering 
 wet straw were lighted alongside orchards. Growers soon recognized 
 that for frost protection heat was at least as important as smoke. 
 
 Coal was used for some years but proved unsatisfactory in burners 
 then available. The first oil heaters employed were open pails which 
 produced both heat and smoke. The grades of cheap fuel available at 
 that time would not burn satisfactorily in open containers. However, 
 the advantages of oil — it is plentiful, cheap, and easily lighted and 
 extinguished — led to the initial efforts for improvement of orchard 
 heaters from the standpoint of better control of the burning rate and 
 ability to burn all of the oil contained in the heater. All of the heaters 
 then in use smoked badly, but the growers, while realizing the impor- 
 tance of heat, still believed the smoke to be of some benefit and no im- 
 provements for better combustion were demanded. The public was 
 doubtless annoyed by the smoke but recognized the importance of the 
 citrus industry to the economic welfare of southern California and so 
 little complaint was made. 
 
 The steady growth of orchard heating as a regular practice in cold 
 locations had established many thousands of smoky heaters in orchards 
 by 1922. The freeze of that year brought the first realization of the 
 seriousness of the smoke problem and of course of the value of heating. 
 At that time attempts to develop smokeless heaters began and encourag- 
 
Bul. 536] Smokiness of Oil-Burning Orchard Heaters 5 
 
 ing progress was made, but the rapid growth in number of heaters in 
 use increased the total smoke output so much that the smoke nuisance 
 continued to grow. 
 
 Heat, Not Smoke, Effective for Frost Protection. — The belief that 
 the smoke itself helped to conserve the heat in an orchard was widely 
 accepted because frosts rarely occur when low clouds are present. How- 
 ever, in 1920 Kimball and Young 5 found that the smoke had little if any 
 value. According to their measurements heavy smoke decreases the rate 
 of heat loss by radiation about 10 per cent but does not prevent tempera- 
 tures from reaching a minimum as low as that reached in similar smoke- 
 free locations. 
 
 The radiation frost has peculiar characteristics in that air, cooled 
 mainly by contact with the ground and other surfaces which radiate 
 rapidly on clear nights, tends to settle in low areas underpinning the 
 warmer air, which is relatively lighter. A wind would mix the warmer 
 air with the cold and decrease the probability of frost, but on calm, clear 
 nights the air next to the ground will cool rapidly even with no influx 
 from surrounding hills. This usually creates a "temperature inver- 
 sion, ' ,6 which fortunately acts as a virtual ceiling ; for the cold ground 
 air, if uniformly heated a few degrees, will rise only a short distance 
 before reaching the level of common density, thus limiting the volume 
 to be warmed. 
 
 That successful frost protection depends upon heat generated from 
 a relatively large number of small fires per acre is now well demon- 
 strated by ample field experience. Figure 1 shows thermograph records 7 
 obtained in an orchard equipped with open-pail oil heaters. It should be 
 noted that the temperature dropped rapidly in both the heated and 
 unheated orchards at 2 :45 a.m. and again at 4 :45 a.m., and that safe 
 temperatures were maintained by lighting more heaters until eventually 
 54 per acre were used. 
 
 Usually 50 heaters are required for fully protecting an acre of 
 orange orchard. Figure 2 8 shows thermograph records indicating that 
 on a severe night safe temperatures could not be maintained by burn- 
 
 5 Kimball, Herbert H., and Floyd D. Young. Smudging as a protection from 
 frost. U. S. Monthly Weather Eeview 48:461-462. 1920. 
 
 6 Young, Floyd D. Nocturnal temperature inversions in Oregon and California. 
 U. S. Monthly Weather Eeview 49:145. 1921. 
 
 7 Young, Floyd D., and C. C. Cate. Damaging temperatures and orchard heat- 
 ing in the Rogue River Valley, Oregon. U. S. Monthly Weather Review 51:617-631. 
 1923. 
 
 s Young, Floyd D. Notes on the 1922 freeze in southern California. U. S. 
 Monthly Weather Review 51:584. 1923. 
 
6 
 
 University of California — Experiment Station 
 
 ing 25 heaters per acre (even though a gallon or more of oil was burned 
 per heater per hour) , but that with 5,0 heaters burning, the temperature 
 was raised above the danger point. 
 
 Need of Belief from the Smoke Nuisance. — The protection now af- 
 forded to approximately 70,000 acres of citrus orchards is estimated to 
 consist of nearly 3,300,000 orchard heaters, of which about 2,900,000 are 
 oil burning. Sufficient oil for only one filling of these heaters totals 
 2,500 railway tank carloads. 
 
 11 p. m. 
 
 8 a.m. 
 
 Fig. 1. — Temperature records in a heated orchard and at an outside check 
 station on the same night. (From Ext. Cir. 40.) 
 
 If weather conditions should require general heating in all districts 
 as much as 15,000,000 gallons of oil might be burned during a single 
 night. No such conditions have occurred, but the burning of much 
 smaller quantities frequently has caused the smoke nuisance to become 
 acute. 
 
 Because of the nature of a radiation frost, there is little or no wind 
 to blow the smoke away. This smoke pall decreases the heat from the 
 sun and therefore necessitates longer hours of burning. Growers sustain 
 other losses from inefficient heaters that give off unburned fuel into 
 the atmosphere and from the added expense of washing smoky fruit. 
 In extreme cases there are price discounts when washing has been 
 unsuccessful. 
 
BUL. 536] SMOKINESS OF OlL-BURNING ORCHARD HEATERS 
 
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8 University of California — Experiment Station 
 
 The character of the soot is such that closed windows do not prevent 
 damage to furniture, draperies, clothes, and merchandise. Under these 
 conditions personal discomfort is, of course, intense. Highway traffic is 
 endangered and business is interfered with generally. 
 
 It is estimated that the oil consumption in orchard heaters for the 
 last two weeks of December, 1930, was 17,000,000 gallons. On the basis 
 of data now available it is evident that the carbon content of orchard- 
 heater smoke discharged into the air during this 14-day period is to be 
 reckoned in millions of pounds. It was generally acknowledged that the 
 smoke damage was considerable. 
 
 Economic Importance of Orchard Heating. — The citrus industry 
 brings into California about $100,000,000 a year, 9 and this income, 
 which supports many industries besides the actual growing of the fruit, 
 is in constant danger of reduction through various hazards. About one- 
 third of the citrus acreage is in locations cold enough to make orchard 
 heating an essential orchard practice, and there are few, if any, loca- 
 tions entirely free from damaging frosts. The efficient protection of the 
 colder orchards insures a regular supply of fruit so that markets can be 
 maintained, and also guarantees to consumers fruit of high quality 
 undamaged by frost. 
 
 The number of heaters lighted and their burning rates vary according 
 to weather conditions, but the average oil consumption will run about 15 
 to 20 gallons per acre per hour. The cost of heating at present prices of 
 fuel and labor, varies from about $0.75 to $1.00 per acre per hour 
 exclusive of interest and depreciation on equipment. 
 
 Despite general recognition of the fact that the economic welfare of 
 the entire state depends in large measure upon protecting the citrus 
 crop from frost damage, the public demands relief from the unnecessary 
 smoke produced by orchard heaters. 
 
 Initiation of the Investigation of the Smoke Problem. — The Agricul- 
 tural Experiment Station undertook to measure the smokiness of or- 
 chard heaters at the urgent request of representatives of the public, fruit 
 packers, and growers acting principally through the ' ' Orchard Heating 
 Improvement Committee, ' ' organized by the Los Angeles Chamber of 
 Commerce, January 9, 1931. L. D. Batchelor, Director of the Citrus 
 Experiment Station of the University of California at Riverside, empha- 
 sized the need and urgency of this investigation, which was assigned 
 priority over other engineering-research projects then under way at the 
 University Farm. 
 
 9 Approximate average of recent years, California Fruit Growers Exchange, 
 annual reports. 
 
BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS 9 
 
 The questions raised by the Committee 10 in regard to orchard heaters 
 were : 
 
 1. The value of the smoke emitted by such heaters as a frost pre- 
 
 ventive. 
 
 2. The relative value, for orchard heating, of a heater that does not 
 
 smoke and one that does. 
 
 3. If it shall be determined that all such heaters issue more or less 
 
 smoke, then the minimum amount of smoke, per heater, neces- 
 sary for operation. 
 
 4. Ways and means of elimination, by entrapment or otherwise, of 
 
 the smoke emitted from such heaters. . 
 
 On January 30, Mr. A. H. Hoffman, Director L. D. Batchelor, and 
 Mr. W. R. Schoonover of the College of Agriculture, met with Mr. R. L. 
 Willits and W. K. Beattie of the Orchard Heating Improvement Com- 
 mittee and Mr. Floyd D. Young of the United States Weather Bureau. 
 At this meeting it was agreed that the Agricultural Experiment Station 
 would undertake the investigation in cooperation with the Committee. 
 The claim that smoke was of value as a frost preventive was considered 
 adequately disproved by the experiments of Kimball and Young. 11 It 
 was agreed also that smoke from oil-burning orchard heaters constituted 
 the main problem because oil is the only fuel obtainable under present 
 conditions in adequate quantities at a reasonable price, and because 
 approximately 90 per cent of the heated acreage is equipped with oil- 
 burning heaters. 
 
 The project was restricted to an investigation of the smokiness of oil- 
 burning orchard heaters and does not include a study of the value of 
 orchard heating, or the efficiency of heaters. These two subjects have 
 been reported previously. 12 
 
 Outline of the Problem, — The problem then was to study the smoke 
 output of the oil-burning heaters in common use, using the grades of 
 fuel most readily obtainable, and also, if possible, to determine the 
 
 10 Anonymous. Orchard heating regulations considered at mass meeting. Cali- 
 fornia Citrograph 16:145, 180, 181. 1931. 
 
 ii Kimball, Herbert H., and Floyd D. Young. Smudging as a protection from 
 frost. Monthly Weather Review 48:461-462. 1920. 
 
 12 Webber, H. J., et al. A study of the effects of freezes on citrus in California. 
 California Agr. Exp. Sta. Bui. 304:243-321. 1919. (Out of print.) 
 
 Schoonover, Warren E., Robert W. Hodgson, and Floyd D. Young. Orchard 
 heating in California. California Agr. Exp. Sta. Bui. 398:1-69. 1925. (Out of 
 print.) 
 
 Hoffman, A. H. Laboratory tests of orchard heaters. California Agr. Exp. Sta. 
 Bui. 442:1-37. 1927. (Out of print.) 
 
 Schoonover, Warren R., Robert W. Hodgson, and Floyd D. Young. Frost pro- 
 tection in California orchards. California Agr. Ext. Cir. 40:1-73. 1930. 
 
10 University of California — Experiment Station 
 
 factors influencing smoke output. Large variations in smoke output 
 were expected because the heaters burn any kind of fuel oil whose pour 
 point is below 30° F, and, furthermore, at any bowl level, with large 
 variations of bowl and stack temperatures, with almost no control over 
 air-fuel ratio, and with secondary combustion sometimes above the 
 stack. Nevertheless, to control the smoke nuisance a practical method 
 of smoke measurement was required that would yield significant test 
 results with all heaters in question. 
 
 The subcommittee on research of the Orchard Heating Improvement 
 Committee agreed to choose the heaters to be tested, furnish used speci- 
 mens for the tests, supply the required amounts of the proper fuels, 
 secure a suitable location for field trials, supervise the regulation of 
 heaters, and formulate a decision as to what might be considered a 
 reasonable limit of smoke tolerance. The Agricultural Experiment Sta- 
 tion agreed to supply apparatus for making carbon determinations and 
 for determining fuel-consumption rates, make oil analyses for compari- 
 son of fuels used in tests with regular run of fuels used by the growers, 
 and supply personnel to conduct tests. 
 
 The purpose of the work was to. assist in eliminating the smoke 
 nuisance by : 
 
 1. Furnishing data to be used as the basis for legislation. 
 
 2. Furnishing information for the guidance of manufacturers in 
 
 improving their heaters. 
 
 3. Developing subject matter to be used by the Agricultural Exten- 
 
 sion Service for guiding growers in better heater operation. 
 
 METHODS USED IN MEASURING SMOKE 
 
 Methods Available. — Industrial smoke is usually measured by mak- 
 ing visual comparisons of its blackness with black and white color 
 standards such as the Ringelmann chart. 13 This method is not directly 
 applicable to orchard-heater tests because it cannot be used at night. 
 Furthermore, its grading in five steps of '20, 40, 60, 80, and 100 per cent 
 black is too coarse to register minor variations in orchard-heater smoke, 
 which are of considerable interest in determining the causes of smoki- 
 ness. Other methods considered for smoke determinations were : 
 
 1. Direct-weight methods such as the collection of a soot sample on 
 
 a weighable filter, or the electrical precipitation of smoke 
 
 particles. 
 
 13 Faust, H. M. Smoke and its prevention. Ohio Engin. Exp. Sta. Cir. 24:12. 
 1931. 
 
Bul. 536] Smokiness of Oil-Burning Orchard Heaters 11 
 
 2. Indirect-weight methods such as the collection of a sample on an 
 
 asbestos filter for making carbon determinations by chemical 
 methods. 
 
 3. Measurement of the opaqueness or apparent smoke density of the 
 
 products of combustion by a light-interception method. 
 
 4. Measurement of blackness by collecting a sample on a white filter 
 
 and determining the magnitude of the blackening effect by 
 photometric methods. 
 All of these methods were used and are discussed later. 
 
 Fig. 3. — Taking smoke sample by aspirator from tip of flame. 
 
 Tests in the Field at Pomona. — Mr. Young and the members of the 
 Committee were of the opinion that the tests should be conducted under 
 orchard conditions during cold weather. On the basis of experience 
 available at that time a direct-weight method using paper filters seemed 
 best adapted to a field study under orchard conditions. 
 
 Smoke determinations were made by holding a sampling tube in the 
 stream of gases discharged by a heater just above the tip of the flame 
 (see fig. 3) . The attempt to use this simple method to obtain weighable 
 samples on light paper proved unreliable and the method was aban- 
 doned for three reasons : (1) the smoke deposit could not be determined 
 because even when weighings were made in an air-controlled chamber 
 
12 University of California — Experiment Station 
 
 with complete reconditioning for moisture content, the final weights of 
 the filter papers were both greater and less than the original (owing to 
 change in composition of the paper itself because of exposure to the 
 flue gases, and to variable loss of oil processed in the paper) ; (2) the 
 samples were not dependable because it was impossible to draw them 
 from the exact center of the waving flame tip ; and (3) occasional breezes 
 disturbed the weighing of fuel consumption and affected stack combus- 
 tion. These difficulties forced the complete refraining of the project and 
 the development of new test methods. 
 
 Laboratory Methods Developed at Davis. — In order to get away 
 from the errors in the filter-paper method it was decided to estimate the 
 smoke on a light-interception basis, which is a standard method of smoke 
 determination. 14 The essential feature of the method is that a beam of 
 light from a constant source is passed through the smoke stream and the 
 light which is not intercepted falls upon a light-sensitive photo-electric 
 cell or a radiation pyrometer. Proper electrical instruments, indicating 
 the intensity of the light transmitted, measure the relative opaqueness 
 of the smoke. Then by measuring the total volume of the smoke stream 
 passing the light, per pound of fuel burned, the relative quantity of 
 smoke emitted by different heaters can be determined. 
 
 This method avoids the previous error in weighing filter papers, 
 eliminates the sampling error by dealing with the entire smoke stream 
 discharged from the heater, and avoids errors due to occasional breezes 
 because of its location indoors. The dilution of the smoke stream with 
 extra air simultaneously affects both the opaqueness and total volume 
 of air flow and thus does not influence the determination of relative 
 smokiness. The values of smokiness obtained with such an apparatus do 
 not represent actual weight or quantity of smoke no matter how accu- 
 rate the opaqueness and volume determinations may be. However, for 
 comparing different heaters the method is adequate — in fact goes one 
 step beyond the usual basis of smoke-abatement ordinances, which grade 
 by blackness without the volumetric determination. 
 
 If a reasonable correlation could be found between the opaqueness 
 of smoke and the density of its carbon particles, the light-interception 
 results might be interpreted in terms of weight within the limits shown 
 by the correlation. This, of course, was highly desirable, and the neces- 
 sary auxiliary apparatus for correlation data was added to the light- 
 interception equipment without sacrificing its advantages over all other 
 methods in being continuous, quick, and reliable. 
 
 14 American Society of Mechanical Engineers. Power test codes; instruments 
 and apparatus, Part 20, smoke-density determinations. Amer. Soc. Mech. Engin., 
 29 W. Thirty-ninth Street, New York City. 1930. 
 
Bul. 536] Smokiness of Oil-Burning Orchard Heaters 
 
 13 
 
 Figure 4 shows the apparatus used in making the smoke tests. The 
 orchard heater, at the extreme right, is placed on automatic self-balanc- 
 ing scales, which measure the loss of weight as the fuel is burned. The 
 centrifugal blower at the left creates a gentle suction, drawing the 
 smoke and some surrounding air into the hood, past the light, and 
 through the main orifice where the rate of flow is measured. Two small 
 orifices each with an area % o of the total orifice area divide the total 
 smoke stream to obtain 1 per cent samples for carbon weight deter- 
 minations. The sample from one orifice flows through an electric pre- 
 cipitator which collects the solid particles for direct weighing. The 
 
 Fig. 4. — Laboratory apparatus for measuring orchard-heater smoke. 
 
 other sample flows through an asbestos filter which retains the solid and 
 liquid particles for chemical analysis. A felt filter also can be used to 
 obtain a deposit for comparison of blackness. Detailed description of 
 this apparatus is given in Appendix A. 
 
 Smoke Units Used for Comparative Results. — The correlation be- 
 tween smoke opaqueness determined by light interception, and carbon 
 weight determined electrically and chemically is fully discussed in Ap- 
 pendix B. In early tests with three fuel oils in the standard heaters 
 there appeared to be a reasonable correlation between the opaqueness- 
 quantity unit and weight such that one ' ' pound-smoke ' ' unit as defined 
 below was approximately equivalent to 1 gram of carbon per pound of 
 fuel burned. However, tests with seven other orchard-heater fuel oils 
 indicate that the correlation between smoke opaqueness and density of 
 carbon particles is not generally satisfactory even at high burning rates, 
 and is unusable at low burning rates. 
 
14 University of California — Experiment Station 
 
 The correlation between opaqueness determinations and felt filter 
 blackness appears to be reasonably satisfactory as shown in figure 35. 
 Both methods are very fast. The light-interception method has the ad- 
 vantage of reading directly, continuously, and instantaneously while 
 the felts offer the advantages of simplicity and semipermanent record. 
 Each of these methods depends largely on the blackness of the smoke. 
 Either method gives accurate, comparative results which can be inter- 
 preted in terms of the other method. If the invisible vaporized carbon 
 compounds in the smoke are to be measured, another basis, such as the 
 determination of the weight of carbon particles would be required. 
 However, this characteristic of the smoke is not of general interest. 
 
 It therefore seems best to report the data on the basis of a quantity 
 unit representing the product of the average smoke-opaqueness units 
 (apparent density) times the total air flow (volume) during the period 
 of burning one pound of fuel. This unit has been designated as a 
 1 l pound-smoke ' ' unit. 
 
 Accuracy of the Method, — Since the weight-correlation problem is 
 avoided by comparing test results in pound-smoke units, the method as 
 a whole is subject only to the following small errors : 
 
 1. Error in weighing the loss of fuel as burning proceeds. For short 
 ^2-pound runs this might amount to 5 per cent, but the average error is 
 very small owing to the electric contact signals and the operating of 
 two stop watches by independent observers. The automatic balance 
 developed later reduced this error to a negligible amount, 
 
 2. Error in reading millivoltmeter because the smoke comes in puffs. 
 This error may amount to 20 per cent in the case of individual readings, 
 but the average error over the usual 10 readings is small. It is greatest 
 in percentage with the least-smoky heaters. 
 
 3. Error in determining air flow due to the assumption that dry air 
 is being measured, while what is measured is in fact a mixture of air 
 with products of combustion, including carbon dioxide, water vapor, 
 and unburned or partially cracked oil. This error has been carefully 
 estimated and is of a negligible magnitude. 
 
 4. Errors due to instruments and orifices. The instruments and ori- 
 fices have been calibrated, and the errors are known to be insignificant. 
 
 All of these errors are relatively unimportant. The determinations 
 are of such accuracy that all heaters show distinctive characteristics of 
 smokiness in spite of large momentary deviations in most heaters from 
 their average performance. 
 
 Figure 5 shows graphically typical results obtained with the present 
 apparatus when testing a heater of unusually smooth-burning character- 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 15 
 
 istics. The upper line shows that the burning rate (shown as a 3-minute 
 moving average) is rarely constant. This irregularity largely results 
 from the distillation cycle in the heater bowl. Some heaters even show 
 regularly spaced smoke peaks. The bottom line is the record of opaque- 
 ness, as observed each minute, corrected for temperature. The abrupt 
 
 ~o s /o 
 
 JO 40 SO 60 70 SO 
 
 T/ME AFTEe STARTING reST. Minutes. 
 
 Fig. 5. — Continuous smoke test of heater No. 32, Ily-Lo 1929 Model. 
 
 return to smokeless readings when the draft was cut down after a high 
 burning rate shows the sensitivity of the apparatus. The middle line of 
 smokiness (in pound-smoke units) is derived from the other two by 
 using the volume of total air flow during the period of burning one 
 pound of fuel. 
 
 LIST OF ORCHARD HEATERS TESTED 
 
 Table 1 lists the orchard heaters and stacks tested. Photographs of 
 many of these are given in the bulletin so that readers may identify 
 definitely the models used. The figure numbers in which these photo- 
 graphs appear are given in column 2. Graphs of the smoke-test records 
 and diagrammatic cross sections of the heaters and stacks will be found 
 in the figures listed in column 3. 
 
16 
 
 University of California — Experiment Station 
 
 table 1 
 List of Heaters. Tested and their Photograph and Ctjr.ve Numbers 
 
 No. 
 
 Photograph 
 
 Smoke test 
 
 
 
 fig. No. 
 
 fig. No. 
 
 
 1 
 
 2 
 
 3 
 
 4 
 
 1 
 
 18, 21,22 
 
 15,28 
 
 National Jumbo Cone 
 
 2 
 
 
 6 
 
 Garbage Pail 
 
 3 
 
 20 
 
 
 Hy-Lo, Double Stack, square bowl 
 
 4 
 
 20 
 
 12 
 
 Hy-Lo, single short stack, round bowl 
 
 5 
 
 
 17 
 
 Smith-Evans 
 
 6 
 
 18 
 
 17 
 
 Kittle 
 
 7 
 
 22 
 
 
 Citrus, Olsen Stack 
 
 8 
 
 19 
 
 15 
 
 National Baby Cone 
 
 9 
 
 20 
 
 8, 24, 25 
 
 Citrus Regular 
 
 10 
 
 
 
 Canco 5 gal. 
 
 11 
 
 
 7 
 
 Dunn 
 
 12 
 
 19 
 
 13 
 
 National, Exchange model, 5j-inch stack 
 
 13 
 
 
 6 
 
 Hamilton Bread Pan with stack 
 
 14 
 
 
 13 
 
 Wheeling 
 
 15 
 
 
 
 Canco 3 gal. 
 
 16 
 
 
 
 Chinn 
 
 17 
 
 
 6 
 
 Hamilton Bread Pan 
 
 18 
 
 
 
 Hamilton, oblong with stack and down draft 
 
 19 
 
 
 7 
 
 Hamilton, square bowl with down draft 
 
 20 
 
 19 
 
 10 
 
 National Double Stack 
 
 21 
 
 
 17 
 
 Bothwell 
 
 22 
 
 20,21 
 
 8 
 
 Citrus, 15-inch stack 
 
 23 
 
 20 
 
 12 
 
 Hy-Lo Double Stack, round bowl 
 
 25 
 
 20,22 
 
 
 Hy-Lo, single short stack, square bowl 
 
 26 
 
 19,22 
 
 9,29 
 
 Citrus, high stack 
 
 27 
 
 20 
 
 
 Citrus Gas Flame 
 
 29 
 
 18 
 
 17 
 
 Fugit 
 
 30 
 
 19 
 
 11 
 
 National Junior Louver, 15 inch 
 
 31 
 
 18 
 
 14 
 
 National, Exchange model, 7-inch stack 
 
 32 
 
 18,21 
 
 5,16,24,25 
 
 Hy-Lo 1929 Model 
 
 34 
 
 19 
 
 14, 29, 30 
 
 National, Exchange model, 6-inch stack 
 
 New stocks 
 
 52 
 
 26 
 
 27 
 
 Lamco Gyradiant 
 
 60 
 
 26 
 
 27 
 
 Hy-Lo Giant 
 
 61 
 
 
 28 
 
 Hy-Lo Giant Junior 
 
 62 
 
 26 
 
 28 
 
 Hy-Lo Drum Stack 
 
 63 
 
 
 32 
 
 Hy-Lo, straight stacks 
 
 75 
 
 
 32 
 
 Hy-Lo, tapered stacks 
 
 76 
 
 
 32 
 
 National Junior Louver, 18 inch, holes spaced spirally 
 
 80 
 
 26 
 
 29 
 
 Hinchcliff, 36-inch 
 
 81 
 
 
 29 
 
 Hinchcliff , 30-inch 
 
 90 
 
 26 
 
 32 
 
 National Junior Louver, 18 inch, holes spaced in rows 
 
 91 
 
 26 
 
 32 
 
 O'Keefe and Merritt, 6-inch straight stack 
 
 91A 
 
 
 31 
 
 O'Keefe and Merritt, 7-inch straight stack 
 
 100 
 
 
 27 
 
 O'Keefe and Merritt Corrugated 
 
 148 
 
 26 
 
 32 
 
 Hy-Lo, tapered stack 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 17 
 
 TEST RESULTS OF STANDARD MAKES OF ORCHARD HEATERS 
 
 The heaters were tested in the regular runs over the entire range of 
 burning rates permitted by the draft regulators. The average burning 
 rate in the field is from 4 to 5 pounds of fuel an hour. The normal operat- 
 ing range is from 2 1 /2 to 7 pounds an hour. 
 
 Figures 6 to 17 show the detailed results obtained with the various 
 heaters. The marked points indicated by crosses, triangles, circles, etc., 
 show the estimated smokiness in pound-smoke units as calculated from 
 four to ten light-interception readings taken at 40-second intervals 
 while burning % pound of fuel at the rate indicated. Development of 
 the automatic balance permitted a continuous determination of burning 
 rates, so that smokiness could be determined for each minute as shown 
 in figures 5 and 30. Where such continuous observations were made, they 
 are shown on the smoke-test graphs by single checks ( V )• Each curve 
 indicates the line of trend of smoke production as the burning rate is 
 varied. The curve shown does not indicate the exact smokiness but 
 merely the probable center of a band of variable width. It is to be 
 expected that any single reading might deviate from the line of trend 
 by as much as 25 per cent, Occasionally a reading may deviate to a 
 much greater extent from the line of trend because of unstable burning 
 conditions. 
 
 Effects of Incomplete Combustion. — The field trials at Pomona, sup- 
 plemented by orchard observations and laboratory tests, indicate that 
 many factors influence the smokiness of an oil burner, but that the only 
 factor of great importance is the degree to which complete combustion 
 is approached. If oil is completely burned the products will be carbon 
 dioxide, water, and very small amounts of ash and sulfur dioxide. Sub- 
 stantially complete combustion occurs under the following conditions : 
 (1) oil thoroughly atomized and mixed with the proper amount of air 
 for combustion; (2) high combustion-chamber temperature to sustain 
 combustion ; and (3) combustion completed before the gases are chilled. 
 The best conditions result in a blue smokeless flame. If there is a less 
 favorable admixture of air with the combustible gases arising from the 
 bowl of a heater the flame may be yellow and either smokeless or smoky. 
 A flame is yellow because it contains particles of incandescent carbon 
 which did not come into contact with enough oxygen for complete com- 
 bustion at the instant of reaching the ignition temperature. Under 
 favorable temperature conditions these particles may be consumed be- 
 fore leaving the flame and there will be practically no smoke. If such a 
 flame is cooled many unburned carbon particles will escape to form 
 
18 University of California — Experiment Station 
 
 smoke. For example, the flame from a kerosene lamp or a candle may be 
 smokeless, though yellow, but if a cold object is held in the flame it will 
 become coated with soot or if a puff of cold air disturbs the flame it will 
 become smoky. 
 
 Smoke may be caused also by the cracking of the complex hydro- 
 carbons at such a distance from the region most favorable to combustion 
 that carbon atoms can agglomerate to form particles too large to be 
 burned readily. ' ' Cracking ' ' is the name given to a process extensively 
 used today for the purpose of breaking up the more complex hydro- 
 carbons into the simpler ones, either in vapor or liquid phase. Fuel oil 
 produces simpler hydrocarbons when heated between 525° and 650° F. 
 A representative type of reaction for such a mixture might well be 
 C 14 H 30 + heat -^C 7 H 10 + C 6 H 14 + C. During the cracking process, 
 which occurs when oil vapors are superheated, the larger molecules are 
 broken down, producing lighter hydrocarbons and leaving a residue of 
 carbon. In the refinery the process is carried on in a still and the carbon 
 is left in the still as a hard coke. In a heater partial cracking may take 
 place if the oil vapors are sufficiently heated to break the molecules while 
 the air supply is insufficient for complete combustion. Carbon resulting 
 from cracking appears as soot accumulations in the bowl and stack or 
 as smoke. 
 
 Smoke Tests of Open-Container Smudge Pots. — Figure 6 shows the 
 performance of what are essentially open containers with no regulation 
 except control of the size of the fire by sliding covers on or off. The oil 
 is vaporized and burns at the surface, producing a flickering, smoky 
 flame. Air is deficient in the mass of flaming gases and temperatures are 
 favorable for some cracking. No. 13 has a short stack, which, however, 
 has little effect, because the sliding cover permits free burning from the 
 surface. 
 
 Smoke Tests of Short-Stack Heaters. — The next stage in the develop- 
 ment of heaters was the addition of drafts for regulating burning rates. 
 Heaters 10, 15, and 16 fall in this class. In addition they have short 
 stacks, which also aid in controlling the burning rate. The stacks were 
 intended to improve the combustion, but these heaters are little if any 
 less smoky than open containers. The smokiness varied between 16 and 
 28 and averaged about 24 pound-smoke units. 
 
 Figure 7 shows the performance of short-stack heaters of larger 
 capacity having down-draft tubes designed to concentrate a generating 
 fire in a hot spot on the oil so as to facilitate burning the entire charge 
 even though low-grade oil is used. Hamilton down-draft heaters of other 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 19 
 
 models but similar to No. 19 were tested with results nearly like those 
 shown on the diagram. The points determining the curves were obtained 
 at air temperatures of about 90° F, and those indicated by checks at 
 
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 Fig. 6. — Smoke tests of open-container smudge pots, heaters 2, 13, and 17. 
 
 46° to 50° F. The determinations are too scattered to permit an accu- 
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 fication of the heater is not changed. 
 
20 
 
 University of California — Experiment Station 
 
 These heaters are more smoky than open containers. Apparently the 
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 cracked and there is nothing in the construction to cause effective mix- 
 
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 Fig. 7. — Smoke tests of short-stack orchard heaters, Nos. 11 and 19. 
 
 ing of air with the burning gases. In fact the air is admitted in such a 
 manner that it probably chills the flame to below the most favorable 
 combustion temperatures. 
 
BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 21 
 
 Figure 8 shows the performance of two models of Citrus heaters — 
 the short-stub stack or Kegular, and the perforated 15-inch stack. 
 Warm and cold-weather tests are shown. From a smoke standpoint these 
 
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 Fig. 8. — Smoke tests of Citrus Kegular and Citrus 15-inch heaters, Nos. 9 and 22. 
 
 heaters are little better than ordinary smudge pots. The 15-inch stack 
 increases the smokiness at low burning rates, probably because of stack 
 temperatures more likely to cause cracking. It is to be noted that the use 
 of kerosene reduced the smoke output to less than half the usual amount. 
 
22 
 
 University of California — Experiment Station 
 
 Influence of Stack Height on Smokiness. — In order to meet the de- 
 mand for a tall stack and less smoky heater the manufacturers of the 
 Citrus heater sold a top stack to be put on above the 15-inch stack. This 
 
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 Fig. 9. — Influence of stack height on smokiness of Citrus high-stack heater, No. 26. 
 
 stack has been sold in 30, 24, and 18-inch lengths. Figure 9 shows the 
 effect of stack height on this type of heater tested with various lengths of 
 top stack varying from 12 to 36 inches. 
 
BUL. 536] SMOKINESS OF OlL-BURNING ORCHARD HEATERS 23 
 
 The 12 and 15-inch top stacks do not produce enough draft to in- 
 crease the air intake into the lower stack sufficiently to give improved 
 combustion. On the contrary they promote cracking. The 18-inch top 
 stack provides slightly better combustion than the 15-inch bottom stack 
 alone. The 24-inch top stack improves combustion noticeably at low 
 burning rates and gives reasonably good results at high rates. However, 
 there seems to be some instability in the lower range, where a slight 
 change in conditions causes cracking to take place and the smokiness 
 may rise to as high as 35 pound-smoke units. 
 
 Many other heaters show unstable combustion at certain burning 
 rates, the most pronounced effect being found in heater No. 7 (Citrus 
 heater with Olsen Stack) in which the smokiness varied between 6 and 
 36 pound-smoke units with little change in burning rate and rose to 51 
 units during one test, Similar performance may be expected from the 
 Apollo heater, which varies from scarcely visible smoke at 4 pounds an 
 hour to 75 pound-smoke units at 5 pounds an hour. 
 
 The Citrus heater can be made fairly satisfactory by use of a 30-inch 
 or 36-inch top stack above the 15-inch section. The stronger draft pull 
 of the high stack draws in sufficient air for practically complete combus- 
 tion over the burning range to 3 to 6 pounds of fuel per hour. These tall 
 stacks can be used only on models of recent construction which have 
 tight-fitting covers. The draft is so strong that burning rates cannot be 
 controlled with heaters having loose-fitting covers. It is also difficult to 
 extinguish the fires in these tall-stack modifications of the Citrus heater. 
 
 Smoke Tests of Open-Flame Heaters. — The open-flame stack (some- 
 times called "lazy-flame ' ' stack) is very popular because of ease of light- 
 ing and regulation, the release of the hot products of combustion near 
 the ground, and the low cost and depreciation. No stack parts are heated 
 excessively. In this type the mixing of air and gases takes place in the 
 stack and most of the combustion occurs at the top of the stack. The 
 proportion of total heat in radiant-energy form is greater with the open- 
 flame than with tall-stack heaters. Figure 10 shows the performance of 
 the National Double Stack heater (No. 20) belonging to the open flame 
 type. This stack is not very satisfactory in regard to smoke but it offers 
 interesting possibilities, some of which are shown in the lower curves, 
 which indicate the effect of introducing extra air at various points. 
 These experiments with extra air were continued on newer types of 
 open-flame stacks and will be discussed in another section. 
 
 Figure 11 shows the smokiness of a heater (No. 30) with a Junior 
 Louver stack, an open-flame type, on both round and square bowls. 
 Tests were made with clean and dirty stacks during warm and cold 
 
24 
 
 University of California — Experiment Station 
 
 weather. The performances of these stacks on the two types of bowl are 
 similar and there is little difference between the 15-inch and 18-inch 
 heights. This type of stack, as well as the one shown in figure 10, admits 
 just enough air along the stack to burn fairly well at a low rate, but at 
 average rates and higher it tends to cause cracking, and consequently 
 smokes. 
 
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 Fig. 10. — Smoke tests of National Double Stack open-flame orchard heater, No. 20. 
 
 Heater No. 27, the Citrus Gas Flame, was tested clean and dirty and 
 with and*without the baffle usually supplied with the bowl for use with 
 this stack. With the baffle this heater will not operate practically in the 
 field. Without the baffle its smokiness was above 20 pound-smoke units 
 in tests at all burning rates, 
 
 Figure 12 shows the performance of the low-stack or open-flame 
 models, heaters 4 and 23, manufactured by the Scheu Products Com- 
 pany. The stacks of these heaters have a great many small holes uni- 
 formly distributed from the top nearly to the bottom. As combustible 
 gases rise in the stack, air is admitted through each hole and a small 
 
BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 25 
 
 tongue of flame shoots from the hole into the stack. The remaining un- 
 burned gases burn with a smoky flame at the top of the stack. Some of 
 the smoke is probably caused by cracking within the stack. The per- 
 formance of this heater is greatly improved by a new bottom collar pro- 
 viding for air intake near the base of the stack. 
 
 Similar results were obtained with square-bowl heaters, No. 3 and 
 No. 25, using the same stacks. The heaters manufactured by this com- 
 
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 Fig. 11. — Smoke tests of National Junior Louver stack heater, No. 30. 
 
 pany have had numerous changes in attachments for improving com- 
 bustion and gas generation within the bowl. Most of these have only a 
 minor influence on smoke output. Lack of time has prevented a study 
 of all the possible combinations. 
 
 Smoke Tests of Heaters with Straight Louver ed Stacks. — Figures 13 
 and 14 show results with heaters having tall stacks with louvers in the 
 lower section. Stack diameters range from 5 inches in No. 14 to 7 inches 
 in No. 31. The curves represent the characteristics of each stack when 
 properly cleaned. When the air intakes become partially clogged by 
 
26 
 
 University of California — Experiment Station 
 
 soot the smokiness increases as shown by the upper curve for heater 
 No. 34 (fig. 14) . When this record was taken the soot accumulation had 
 decreased the stack diameter to approximately that of heater No. 14. 
 
 A study of the curves and of the heaters in operation indicates cer- 
 tain causes for the performance as illustrated. In the case of heater 
 No. 14 the smallest possible gas-generating fire in the bowl results in the 
 
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 12 
 
 Fig. 12. — Smoke tests of open -flame heaters, Nos. 4 and 23, manufactured by the 
 Scheu Products Company. 
 
 production of a hydrocarbon-air mixture too rich to burn in the small- 
 diameter stack except at the topmost of the rows of louvers. The 
 primary combustion at this point seems to crack part of the gases and 
 cause a heavy smoke output. As the supply of combustible gases is in- 
 creased a smaller percentage of the combustion occurs in the stack and 
 less cracking takes place. There is also an increase in the secondary com- 
 bustion above the top of the stack. As the heat is increased at this point 
 some of the elementary carbon formed during the cracking is consumed 
 and the smoke output decreases as the burning rate is increased. Stacks 
 
BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS 
 
 27 
 
 of diameter larger than 5 inches admit enough air to permit practically 
 complete combustion in the louvered section at low burning rates. At 
 the points where the air enters, the air-fuel mixture is lean enough to 
 
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 Fig. 13. — Smoke tests of 5 and S^-inch louvered-stack heaters, Nos. 14 and 12. 
 
 burn explosively ; growers call this ' ' louvering. ' ' These small explosions 
 increase turbulence and cause better mixing of air and gas, which im- 
 proves the combustion. For each size of stack there is a burning rate at 
 
28 
 
 University of California — Experiment Station 
 
 which the air-fuel ratio is satisfactory and combustion is apparently 
 completed in the lower part of the stack. Under these specific conditions 
 the smoke output is small. The range of burning rates over which air- 
 
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 Fig. 14. — Smoke tests of 6 and 7-inch louvered-stack heaters, Nos. 34 and 31. 
 
 fuel ratios are reasonably satisfactory increases as the stack diameter is 
 increased. With all stack diameters the smoke output increases rapidly 
 as the generation of oil vapor in the bowl is pushed beyond the capacity 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 29 
 
 of the combustion chamber, reaches a maximum, and then decreases 
 coincidentally with the development of a secondary blaze at the top of 
 the stack. 
 
 It is unfortunate that heaters of this type cannot be lighted without 
 opening the drafts far beyond the amount required for normal burning. 
 This results in excessive smokiness during the warming-up period and 
 also perhaps later because of soot accumulation while burning at too 
 high a rate. This same difficulty of lighting is experienced to a greater 
 or less degree with all heaters of the lean mixture, explosive-fire type. 
 Possibly this difficulty can be eliminated by the use of wicks to permit 
 lighting with less draft opening. 
 
 It is apparent that heaters with straight louvered stacks are very 
 erratic. They need careful draft regulation and frequent cleaning of 
 stacks to give satisfactory performance. The best results are obtained 
 when "louvering" takes place throughout the entire length of the 
 louvered stack section. If No. 34 is burned without a top stack, * ' louver- 
 ing" occurs at burning rates of 1 or 2 pounds an hour and the smokiness 
 is about 4 pound-smoke units, but at a 3V2-p°und rate it is 36 units. 
 With the top section on, however, "louvering" occurs at all rates be- 
 tween 2 and 6 or 7 pounds and good results are obtained as long as the 
 heater is clean. 
 
 The worst over-all performance shown in any of the tests was that of 
 heater No. 31 without a top stack. Apparently this type of stack per- 
 mitted the greatest amount of cracking to take place. The smokiness at a 
 burning rate of 3^2 pounds an hour was as high as 65 pound-smoke units. 
 
 Smoke Tests of Cone-Combustion-Chamber Heaters. — Figure 15 
 shows the smokiness of heaters with louvered cone-shaped combustion 
 chambers. It is similar to that with the louvered straight stacks. The 
 National Baby Cone, heater No. 8, has a very narrow range of satisfac- 
 tory air-fuel ratio. When the air supply becomes too limited there is a 
 very sudden and steep rise in smoke output ; this occurs at a 3-pound 
 burning rate. Closing the top three rows of louvers, which can be done 
 with a hammer, lowers the fire into the larger part of the cone and 
 considerably improves the performance at usual burning rates. If the 
 cone is altered in this manner a hot area develops at the top, which may 
 result in rapid oxidation of the stack at high burning rates. The National 
 Jumbo Cone, heater No. 1, with larger combustion chamber, is satisfac- 
 tory over the entire normal operating range as long as it is clean. The 
 upper curves show the smokiness of the same heater when using a dirty 
 stack. The latter records were taken after burning about 14 gallons of 
 fuel without cleaning the stack. 
 
30 
 
 University of California — Experiment Station 
 
 Smoke Tests of Hy-Lo 1929 Model Orchard Heater. — Figure 16 
 shows the performance of the Hy-Lo 1929 Model, heater No. 32. This 
 heater usually gives satisfactory results over a wide range of burning 
 
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 Fig. 15. — Smoke tests of cone-combustion-chamber heaters, Nos. 1 and 8. 
 
 rates as shown by the line of trend. At times it becomes very smoky, 
 owing usually to having the fire jump back and burn at the holes in the 
 base of the burner. When burning normally, combustion is of the explo- 
 sive, lean-mixture type. This heater is the only one showing significant 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 31 
 
 differences between warm and cold-weather tests. It was less smoky on 
 
 the cold run. Figures 5, 24, and 25 give further test results on this heater. 
 
 Smoke Tests of "Nondist Ming "-Type Orchard Heaters.— figure 17 
 
 shows the performance of the separate-container or so-called "non- 
 
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 dt/PMNG &ATE - Pounds per r/oar 
 
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 Fig. 16.— Smoke tests of Hy-Lo 1929 Model heater, No. 32. 
 
 distilling ' ' type of heater. This type differs from the distilling types in 
 burning fresh oil of constant composition. Kittle, heater No. 6 (gravity 
 feed), and Fugit, No. 29 (pressure feed) require an oil of higher grade 
 
32 
 
 University of California — Experiment Station 
 
 than that commonly supplied for other orchard heaters. In both of these 
 heaters, gas is generated from the oil in a hot burner and combustion is 
 completed at the point of gas generation without any opportunity for 
 
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 Fig. 17. — Smoke tests of "nondistilling ''-type heaters, Nos. 5, 6, 21, and 29. 
 
 cracking to take place. Results are very satisfactory under proper ope- 
 rating conditions. No. 6 becomes smoky if the oil is fed too rapidly for 
 the capacity of the burner. 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 33 
 
 Smith-Evans, heater No. 5, and Bothwell, heater No. 21, are similar 
 in general design. They both drip oil onto a hot plate in a burner which 
 admits insufficient air. A type of combustion takes place which results 
 in considerable cracking. It is apparent that in addition to smoke these 
 heaters discharge some unburned gases, but the quantities are not great 
 enough to cause more than intermittent blazes at the top of the stack. 
 It would appear that burner and stack design are more important in 
 preventing smokiness than the method of supplying oil to the burner. 
 
 GENERAL DISCUSSON OF RESULTS 
 
 The tests show that there is great variation in the smokiness of or- 
 chard heaters, the range being from less than 1 pound-smoke unit to 
 more than 60 at a burning rate of 5 pounds of fuel an hour. An indi- 
 vidual heater varied from as low as 4 pound-smoke units to as high as 
 46 at this burning rate, the increase being due to sooting up of the air 
 passages. Heaters vary considerably in smoke output as the burning 
 rate is changed, usually producing more smoke as the burning rate is 
 increased until the secondary combustion at the top of the stack becomes 
 effective. Also when the burning rate is reduced below the critical point 
 the smokiness usually increases, partly because the longer time con- 
 sumed in burning a pound of fuel increases the accumulation of soot 
 measured. 
 
 The tests so far conducted probably represent the best the heaters 
 can do with the fuel used. The heaters were always cleaned before start- 
 ing the regular test, filled to an average level with clean oil and placed 
 level ; covers were made tight, and drafts properly set. Furthermore, 
 they were under constant observation while burning and were shielded 
 from breezes. No extended studies have been made of any one heater. 
 Repeated tests while changing only one variable at a time, such as the 
 oil level, would probably aid in determining the exact causes of smoke 
 production in each case. 
 
 However, a study of the results indicates the validity of certain con- 
 clusions as follows : 
 
 1. Smokiness is governed to a large extent by the design of the stack. 
 In several tests the same stack on different bowls with different draft 
 devices showed substantially the same characteristic. 
 
 2. The influence of air temperature on smokiness is slight and it may 
 be expected that laboratory or summer field tests usually will be com- 
 parable with winter field tests. 
 
34 
 
 University of California — Experiment Station 
 
 3. Accumulation of soot in the heaters has no consistent influence on 
 smokiness except on heaters having tall stacks either with or without 
 combustion chambers. Soot accumulations in such stacks and combus- 
 tion chambers greatly increases the smokiness. The smokiness of low- 
 stack smudge pots and of open-flame heaters is not greatly influenced 
 by soot. The principal effect is a decrease in burning rate, which may 
 continue to the point of practically extinguishing the heater. This 
 effect is particularly pronounced in the Citrus heaters. 
 
 4. It is possible to burn a relatively crude fuel in a very simple, 
 inexpensive orchard heater and keep the smokiness below the level of 
 ordinary visibility. 
 
 Fig. 18. — Standard heaters reasonably free from smoke: A, heater No. 1, Na- 
 tional Jumbo Cone ; B, No. 6 Kittle ; C, No. 29, Fugit ; D, No. 31, National, Exchang( 
 model, 7-inch stack; E, No. 32, Hy-Lo 1929 model. 
 
 5. Unburned carbon and hydrocarbons given off in the smoke amount 
 to a direct fuel loss of 16 per cent in some cases and probably would 
 average 5 per cent for all the heaters now in use. It is not correct to 
 judge the efficiency of combustion by the blackness of the smoke because 
 of the presence of invisible combustible carbonaceous matter. However, 
 with the usual orchard heater the loss of unburned fuel can be con- 
 sidered below 1 per cent when the smoke output is not visible. 
 
 Heater Groups According to Smokiness. — It appears logical to divide 
 the standard heaters tested into the following four groups : 
 
 1. Heaters 1, 6, 29, 31, and 32 (fig. 18), which are reasonably free 
 from smoke at all burning rates under good operating conditions. 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 35 
 
 2. Heaters 8, 12, 20, 26, 30, and 34 (fig. 19), which can be operated 
 with little smoke up to burning rates used in moderately cold weather, 
 but which may produce excessive smoke under certain conditions. 
 
 
 J 
 
 B 
 
 E 
 
 Fig. 19. — Photographs of standard heaters which can be nearly smokeless at cer- 
 tain low burning rates : A, heater No. 8, National Baby Cone ; B, No. 12, National 
 Exchange model, 5%-inch stack; C, No. 20, National Double Stack; D, No. 26, Cit- 
 rus, high stack; E, No. 30, National Junior Louver, 15-inch; F, No. 34, National 
 Exchange model, 6-inch stack. 
 
 3. Heaters 3, 4, 9, 22, 23, 25, and 27 (fig. 20), which are smoky but 
 commercially important. Nos. 3, 4, 23, 25, and 27 can be operated so as 
 to give results similar to those in group 2, but they are erratic and as 
 burned in the field usually would be much worse than the group 2 
 heaters. 
 
 3L. 11 & 
 
 ■ 
 
 A 
 
 B 
 
 E 
 
 G 
 
 Fig. 20. — Photographs of standard heaters which are smoky but commercially 
 important: A, heater No. 3, Hy-Lo Double Stack, square bowl; B, No. 4, Hy-Lo, 
 single short stack, round bowl; C, No. 9, Citrus Eegular; D, No. 22, Citrus, 15-inch 
 stack; E, No. 23, Hy-Lo Double Stack, round bowl; F, No. 25, Hy-Lo, single short 
 stack, square bowl ; G, No. 27, Citrus Gas Flame. 
 
 4. Heaters 2, 5, 7, 10, 11, 13, 14, 15, 16, 17, 18, 19, and 21 which are 
 very smoky but which are mostly of obsolete types. 
 
36 University of California — Experiment Station 
 
 Estimate of the Number and Smokiness of Heaters in Use. — The 
 Fruit Frost Service of the United States Weather Bureau completed in 
 June, 1932, a survey of the numbers and types of orchard heaters now 
 being used by citrus growers. 15 
 
 According to these estimates the 2,900,000 oil-burning heaters may 
 be classified into distinctive groups and the smoke output estimated 
 from a study of the test data as follows : about 1,350,000 heaters believed 
 to be of such a t; T pe that the smokiness will average around 12 pound- 
 smoke units at a burning rate of 5 pounds an hour; 50,000 with an 
 average smokiness of 4 pound-smoke units at the 5-pound burning rate ; 
 and 1,500,000 with an average smokiness of 28 pound-smoke units. 
 
 Of the 1,500,000 heaters averaging 28 pound-smoke units, 500,000 
 are of such a type that it will be difficult to reduce the smoke output. 
 Most of these are obsolete heaters of very little value. This figure 
 includes approximately 55,000 garbage pails, which can be used for 
 orchard storage of oil. These heaters should be replaced by others hav- 
 ing a smoke output of 8 pound-smoke units or less over the normal 
 operating range. The remainder of the smoky heaters consist mainly of 
 about 200,000 Dunn heaters and about 800,000 Citrus heaters with short 
 or 15-inch stacks. It is believed that stacks can be put on all of these at 
 a relatively small expense and the smoke output can be brought below 
 8 pound-smoke units. 
 
 If changes as indicated were made and if the 1,350,000 heaters men- 
 tioned above as averaging 12 pound-smoke units were cleaned regularly 
 and adjusted so as to cut the smoke output down to an average of 8, the 
 total smoke output of the community might be cut to less than half its 
 present amount. 
 
 It should be pointed out that the smoke nuisance will not be elimi- 
 nated until it is feasible to keep the smoke production of all heaters 
 below the limit of visibility. This limit is from 3 to 5 pound-smoke units 
 at a burning rate of 5 pounds an hour. If a pound-smoke unit be con- 
 sidered equal to 1 gram of smoke carbon, such a limit would still allow 
 smoke particles to be discharged from an orchard heater at the rate of 
 0.4 grams a minute at the above average burning rate. 
 
 Operation Methods for Reducing Smoke Output. — It is evident that 
 considerable improvement in smoke production may be obtained by 
 grower cooperation without the necessity of turning to other fuels or the 
 purchase of large quantities of new and expensive equipment. 
 
 15 Unpublished data furnished to the Orchard Heating Improvement Com- 
 mittee by Floyd D. Young, Senior Meteorologist, United States Weather Bureau. 
 
Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 37 
 
 The most important suggestions for growers are : 
 
 1. Not to burn oil in open pails and other obsolete heaters such as 
 most of those manufactured prior to 1915. Figure 6 shows the perform- 
 ance of heaters in this class. 
 
 2. Clean soot from stack and drafts, particularly of heaters with 
 combustion chambers or louvered stacks. Figure 21 shows how drafts 
 and stacks become clogged. Frequently stacks become sooted up during 
 the warming-up period. 
 
 Fig. 21. — Photographs of soot collection on covers and stacks of orchard heaters: 
 A, on underside of cover of heater No. 1, National Jumbo Cone; B, in throat of 
 heater No. 1, National Jumbo Cone; C, in throat of heater No. 22, Citrus 15-inch 
 stack; D, around thimble in heater No. 32, Hy-Lo 1929 model. 
 
 3. Regulate heaters systematically so as to maintain the best com- 
 bustion rate for each type of heater. Figure 22 shows how smokiness 
 varies as burning rate is changed. Most heaters smoke more at high 
 rates. 
 
38 
 
 University of California — Experiment Station 
 
 4. Study the recommendations of the Fruit Frost Service of the 
 United States Weather Bureau with regard to temperatures at which 
 to begin to light heaters. Have an adequate supply of tested and 
 properly sheltered thermometers. Be careful not to burn more oil than 
 is necessary to maintain safe temperatures. 
 
 Fig. 22. — Photographs of smokiness at different burning rates: A, heater No. 1, 
 National Jumbo Cone; B, No. 7, Citrus, Olsen Stack; C, No. 26, Citrus, high stack; 
 D, No. 25, Hy-Lo, single short stack, square bowl. 
 
 TEST RESULTS WITH DIFFERENT FUEL OILS 
 
 The Fruit Growers Supply Company has found that it is not feasible 
 to place rigid specifications on the oil used for orchard heating. Growers 
 have storage facilities for 35,000,000 gallons, which is only a little more 
 than enough for one filling of the heaters. Therefore they have to buy 
 on short notice grades of oil normally carried in storage at the refineries. 
 
BUL. 536] SMOKINESS OF OlL-BlTRNING ORCHARD HEATERS 
 
 39 
 
 The oil formerly used was a straight-run distillate of from 32° to 
 34° Baume gravity. At present no regular cut is made between Diesel 
 engine fuel oil of 27°+ and kerosene base of 38° to 40° Baume gravity. 
 Most growers use the Diesel engine fuel. During rush periods it is the 
 only satisfactory fuel oil available in large quantities because kerosene 
 base is not normally carried in storage by the refineries. However, there 
 is some opportunity to choose between various lots of Diesel fuel. The 
 specifications ordinarily used are : 
 
 1. Sulfur content less than 0.75 per cent (some samples have shown 
 
 as high as 3 per cent but the normal content is about 0.25 per 
 cent). 
 
 2. Carbon residue less than 0.50 per cent. 
 
 3. Pour point below 15° F (some samples have run from 30° to 
 
 40° F pour point) . 
 
 TABLE 2 
 Source of Oils Used in Standard Tests 
 
 Oil No. 
 
 Description and source 
 
 Amount used 
 
 1 
 2 
 3 
 4 
 
 5 
 
 Kittle fuel as used in Los Angeles County (Pomona) 
 
 Orchard heater fuel used in Los Angeles County (Pomona) 
 
 Orchard heater fuel— 30 gravity— Woodland 
 
 Orchard heater fuel — 30 gravity — Cooks Oil Co., Emeryville. Delivered 
 
 from Suisun, California 
 
 Orchard heater fuel — 35 gravity — Cooks Oil Co., Emeryville. Delivered 
 
 Sample 
 Sample 
 100 gals. 
 
 100 gals. 
 
 35 gals. 
 
 6 
 
 7 
 
 Orchard heater fuel — 30 gravity — Cooks Oil Co., Emeryville. Delivered by 
 Sheldon Oil Co., Suisun 
 
 Orchard heater fuel — 30 gravity — Cooks Oil Co. Shipped from Emeryville, 
 9-9-31 
 
 100 gals. 
 200 gals. 
 
 
 
 
 TABLE 3 
 
 Analyses of Heater. Oils* 
 
 
 Flash 
 
 point 
 
 (Cleveland 
 
 Open 
 
 tester) 
 
 Viscosity 
 (Say bolt 
 
 Universal 
 100° F) 
 
 Distillation test 
 
 Pour 
 point 
 
 Carbon 
 
 residue 
 
 (Conradson 
 
 method) 
 
 Sulfurt 
 
 Oil 
 No. 
 
 10 per cent 
 over at 
 
 90 per cent 
 over at 
 
 End-point 
 
 (Parr 
 Sulfur 
 Bomb) 
 
 
 °F 
 
 sec. 
 
 °F 
 
 °F 
 
 op 
 
 °F 
 
 per cent 
 
 per cent 
 
 1 
 
 149 
 
 36 
 
 395 
 
 495 
 
 510 
 
 Below 12 
 
 047 
 
 0.62 
 
 2 
 
 230 
 
 43 
 
 435 
 
 615 
 
 627 
 
 Below 12 
 
 .301 
 
 57 
 
 3 
 
 158 
 
 33 
 
 360 
 
 490 
 
 515 
 
 Below 12 
 
 .020 
 
 .26 
 
 4 
 
 195 
 
 39 
 
 405 
 
 620 
 
 632 
 
 Below 12 
 
 .160 
 
 .43 
 
 5 
 
 222 
 
 40 
 
 444 
 
 616 
 
 630 
 
 Below 12 
 
 .075 
 
 .47 
 
 6 
 
 190 
 
 39 
 
 400 
 
 600 
 
 622 
 
 Below 12 
 
 .283 
 
 .49 
 
 7 
 
 221 
 
 43 
 
 438 
 
 640 
 
 652 
 
 Below 12 
 
 0.028 
 
 53 
 
 * Analyses reported by H. W. Allinger, Division of Chemistry, 
 t Corrected for NaCl occlusion. 
 
40 
 
 University of California — Experiment Station 
 
 -ujn 
 
 a^i Sui 
 q aSBjaAy 
 
 e» 
 
 lbs. 
 per 
 hour 
 
 5.00 
 3.98 
 3.65 
 5.47 
 
 
 4.95 
 4.28 
 3 95 
 5.81 
 
 a> hf 
 
 -rt CD-- 
 
 a> c« h 
 
 S3 a 
 Ph g 
 
 
 per 
 cent 
 
 29.4 
 38.4 
 44.2 
 25.3 
 
 
 23 5 
 25 2 
 
 29.8 
 21.8 
 
 < 
 
 OS 
 
 lbs. 
 
 25.25 
 33.00 
 38.00 
 21.75 
 
 
 o © o © 
 to o o o 
 
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 CM CM CM -H 
 
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 00 
 
 lbs. 
 
 86.00 
 86.00 
 86.00 
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 87.25 
 87.25 
 87.25 
 87.25 
 
 1 
 
 C 
 _o 
 "+3 
 
 "-J3 
 co 
 
 6 
 
 AaaAooa^j 
 
 - 
 
 per 
 cent 
 
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 47 
 
 
 
 
 
 
 
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 ^ cj O O O O O 
 
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 d © © © © 
 
 xu M 
 
 - 
 
 . *» i^ o co cm r- 
 
 co S O) O ■* N tN 
 
 D « 
 
 " O O to ^ -* Tt< 
 
 49 
 3.49 
 3 41 
 3.38 
 4.02 
 
 U'eqdsv 
 
 - 
 
 fe s s a 2 s § 
 
 Q_ CO 
 
 ^ co O O O O O 
 
 j_i. f- ■<*< CO >— 1 
 
 T* © o o -h 
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 JUIOd 8JIJ 
 
 GO 
 
 o CM 
 
 
 
 
 
 
 
 ■}uiod qfiBj^j 
 
 ©J 
 
 O 
 
 
 
 
 
 
 
 J o 09 
 
 - 
 
 "API 
 
 33.7 
 
 
 
 
 CO 
 
 o 
 
 CO 
 
 
 .S 6 
 
 0) |H 
 
 mi 
 
 
 Before burning 
 9 
 
 22 
 
 20 
 
 1 
 
 Before burning 
 9 
 
 22 
 
 20 
 
 1 
 
 
 
 
 6 
 
 z; 
 
 
 - 
 
 
 
 | 
 
 
 CM 
 
 
 Q _ 
 %4 
 
 a 
 
 
 hS 
 
 SEH ^ CD 
 
 < 
 o 
 
 
 02*0 
 
 ^ S -2 
 
 o 
 O 
 
 § 
 
 B 
 
 J* 
 
 1 
 
 02 
 
 
 minations: 
 y-Martens Closed C 
 and Open Cup A.S. 
 
 test. 
 
 s City Testing Labo 
 dson test, A.S.T.M. 
 test A.S.T.M. D129 
 Saybolt Universal V 
 .M. D97-28. 
 .M. D95-27. 
 traction; after burni 
 S.T.M. D158-28. 
 
 XI 
 OJ 
 T3 
 
 S3 
 
 a 
 
 
 d in detei 
 2: Pensk 
 3:Cleve 
 4: Holde 
 5: Kansa 
 6: Conra 
 7: Bomb 
 8 and 9: 
 10: A.S.T 
 11: A.S.T 
 12: By ex 
 13-17: A. 
 
 Is 
 
 cl II 
 
 V 
 
 <n _' ^-.' _ 
 
 3 o o o 
 
 . . . to ... SO 
 
 'o'o'o'o'o'o'o'o 
 
 S3 
 
 «OOOOOOOOOOU 
 
 «*EhO b^ 
 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 41 
 
 Analysis of Oils Used in Tests of Standard Heaters. — In testing the 
 smokiness of standard heaters an average grade of 30° Baume gravity 
 orchard-heater oil was used. It was not possible to reorder the fuel oil 
 as required and receive exactly the same kind used in the previous tests. 
 Table 2 shows the source of the oils used in the standard tests and table 3 
 shows the analyses. Oils 3, 4, and 6 have been used in most of the tests 
 except for the Kittle and Fugit heaters in which oil No. 5 was used. 
 Kerosene was burned in a few tests (see fig. 8) and was shown to produce 
 considerably less smoke than ordinary orchard-heater oil. 
 
 It was noted in connection with the tests of standard heaters that 
 when return was made to a given burning rate after operating several 
 hours at various other rates, the smokiness reading was approximately 
 the same as earlier in the run, indicating that smokiness was not changed 
 appreciably as a result of changes taking place in the oil as burning 
 progressed. 
 
 Analyses of Oils Before and After Burning. — Specific data on the 
 changes in two oils after burning are shown in table 4. Four heaters 
 were used and the oils analyzed before and after burning down to resi- 
 dues of approximately 0.2 to 0.4 of the original weight. The composition 
 changes in the oil as a result of burning were marked but were less than 
 the natural differences occurring between the various fuel oils available 
 to growers. It is to be noted that the original pour points (-10° F and 
 +15° F) of the two oils both rose to 35-40° F, and the initial boiling 
 points rose on the average 36° F and 51° F, respectively. Furthermore, 
 the percentages of wax and Conradson carbon residue were increased to 
 a greater value than can be accounted for by the reduction in oil volume 
 due to burning. 
 
 TABLE 5 
 Source of Oils Used for Testing Influence of Oil Character on Smokiness 
 
 Lot 
 
 Gravity, 
 
 
 No. 
 
 ° Baume 
 
 Source 
 
 20 
 
 30 
 
 Blended from Standard Oil Co. 27°+ and 32°+, Cooks Oil Co., Emeryville 
 
 21 
 
 32+ 
 
 Standard Oil Co., El Segundo 
 
 22 
 
 27+ 
 
 Standard Oil Co., El Segundo 
 
 23 
 
 27+ 
 
 General Petroleum Corporation 
 
 24 
 
 27+ 
 
 Shell Oil Co. 
 
 25 
 
 30+ 
 
 St. Helen's Petroleum Corporation 
 
 26 
 
 27+ 
 
 Union Oil Co. 
 
 Analyses of Oils Used for Testing Influence of Oil Character on 
 Smokiness. — Six lots of oil were furnished by the Fruit Growers Supply 
 Company for testing the influence of the chemical and physical char- 
 acteristics of the fuel oil on smokiness. These were compared with oil 
 
42 
 
 University of California — Experiment Station . 
 
 TABLE 6 
 Analyses of Oils Used for Testing- Influence of Oil Character on Smokiness 
 
 
 Flash 
 point 
 (Cleveland 
 Open 
 tester) 
 
 Viscosity 
 
 (Say bolt 
 
 Universal 
 
 100° F) 
 
 Engler distillation test* 
 
 Pour 
 
 point 
 
 Carbon 
 residue 
 
 (Conrad- 
 son 
 
 method) 
 
 
 Oil 
 
 No. 
 
 10 per cent 
 
 at 
 
 90 per cent 
 at 
 
 End 
 
 point 
 
 Sulfurf 
 
 20t 
 
 °F 
 181 
 
 sec. 
 39 
 
 °F 
 382 
 
 °F 
 580 
 
 op 
 620 
 
 Below 12 
 
 per cent 
 305 
 
 per cent 
 209 
 
 21 
 
 180 
 
 36 
 
 380 
 
 560 
 
 600 
 
 Below 12 
 
 .027^ 
 
 .125 
 
 22 
 
 228 
 
 44 
 
 450 
 
 590 
 
 600 
 
 Below 12 
 
 .0341 
 
 .188 
 
 23 
 
 243 
 
 49 
 
 485 
 
 650 
 
 680 
 
 30 
 
 .375 
 
 .151 
 
 24 
 
 206 
 
 37 
 
 420 
 
 540 
 
 550 
 
 Below 12 
 
 .081 
 
 .433 
 
 25 
 
 245 
 
 44 
 
 475 
 
 630 
 
 645 
 
 12 
 
 .028 
 
 .298 
 
 26 
 
 190 
 
 37 
 
 410 
 
 560 
 
 600 
 
 Below 12 
 
 0.213 
 
 0.302f 
 
 * On distilling No. 23 the condensed distillate congealed between 80 and 90 per cent, and on No. 25 
 between 86 and 90 per cent. 
 
 t Sulfur determined from oxygen bomb rinsings following calorific valve determinations at Berkeley. 
 The sulfur on No. 26 is an estimate made from part of the rinsings. 
 
 t Analyses made by H. W. Allinger. 
 
 1 Carbon residues analyzed by W. B. Dye. 
 
 TABLE 7 
 
 Vacuum Distillation Range and Heat Values of Orchard-Heater Oils 
 
 Nos. 20 to 26 
 
 Per cent 
 
 No. 20 
 
 No. 21 
 
 No. 22 
 
 No. 23 
 
 No. 24 
 
 No. 25 
 
 No. 26 
 
 over 
 
 
 Vapor temperature, °F 
 
 (pressure 10 
 
 mm mercur: 
 
 / absolute) 
 
 
 Start 
 
 108 
 
 125 
 
 173 
 
 185 
 
 160 
 
 170 
 
 156 
 
 5 
 
 160 
 
 160 
 
 228 
 
 234 
 
 194 
 
 232 
 
 184 
 
 10 
 
 180 
 
 176 
 
 244 
 
 254 
 
 204 
 
 250 
 
 194 
 
 20 
 
 212 
 
 192 
 
 260 
 
 284 
 
 216 
 
 268 
 
 206 
 
 30 
 
 234 
 
 208 
 
 270 
 
 304 
 
 226 
 
 278 
 
 216 
 
 40 
 
 250 
 
 220 
 
 279 
 
 320 
 
 235 
 
 290 
 
 224 
 
 50 
 
 258 
 
 230 
 
 290 
 
 338 
 
 248 
 
 300 
 
 234 
 
 60 
 
 264 
 
 246 
 
 308 
 
 * 
 
 257 
 
 316 
 
 242 
 
 70 
 
 275 
 
 254 
 
 328 
 
 * 
 
 268 
 
 336 
 
 254 
 
 80 
 
 312 
 
 280 
 
 * 
 
 424 
 
 280 
 
 362 
 
 276 
 
 90 
 
 376 
 
 322 
 
 406 
 
 470 
 
 302 
 
 398 
 
 324 
 
 95 
 
 440 
 
 * 
 
 444 
 
 518 
 
 326 
 
 434 
 
 352 
 
 End point 
 
 476 
 
 416 
 
 468 
 
 535 
 
 365 
 
 462 
 
 418 
 
 Per cent recovery 
 
 98 
 
 98 
 
 Heat values as B. t. u. per lb. (to an accuracy of ±25 B. t. u. per lb.) 
 
 000 19,200 18 
 
 19,050 
 
 19,300 
 
 ,700 
 
 19,100 
 
 * Determination uncompleted. 
 
Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 43 
 
 No. 20, which was used for most of the tests run during the winter of 
 1931-32. Table 5 shows the source of these oils. 
 
 These oils were carefully analyzed by the Division of Chemistry, 
 according to the standard methods of the American Society for Testing 
 Materials. The vacuum distillation range and heat values were deter- 
 mined by the College of Engineering. Table 6 shows the results of the 
 oil analyses. Table 7 (plotted in fig. 23) gives the vacuum-distillation 
 data and heat values. 
 
 1 
 
 | 
 Is 
 
 I 
 
 
 eel// S' 
 
 <?£*&* 
 
 
 £2 
 
 /o 
 
 JPO JO 40 
 0/ST/LL/IT/Off, 
 
 50 60 70 
 
 Per Cenr Over. 
 
 SO 90 /OO 
 
 Fig. 23. — Vacuum distillation range of orchard-heater oils 20 to 26 inclusive 
 
 (see table 7). 
 
 Method of Testing Influence of Oil Character on Smokiness. — Tests 
 were run using heater No. 9, Citrus Regular, as typical of a smoky heater 
 and No. 32, Hy-Lo 1929 Model, as typical of a relatively smokeless 
 heater. These heaters were chosen because in the standard tests their 
 smokiness had been exceptionally steady, which is of considerable ad- 
 
44 
 
 University of California — Experiment Station 
 
 vantage in determining small differences between fuels. The procedure 
 in running the tests was as follows : Heaters were cleaned thoroughly 
 between tests. Heater bowls were filled uniformly to a level 3 inches 
 
 « 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Si 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 -T*%* 
 
 £§S_ 
 
 B*^J 
 
 li% 
 
 <Vl 
 
 — £/ 
 
 ~25 
 
 ■tW- 
 
 h: 
 
 
 
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 //f/l7fff /Yo. S£>, //y-io /S£9 MO0£L 
 
 
 
 
 
 
 
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 TE-P 
 
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 Fig. 24. — Influence of oil character on smokiness of heaters No. 9 and No. 32. 
 
 from the top at the start of each test. Each test included four burning 
 rates. At the beginning of a test the heater was warmed up by burning 
 at a high rate and then reduced to a rate of 3 pounds an hour or less. 
 When the burning rate had become steady twenty-five or more readings 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 45 
 
 of smokiness and weight change were taken at one-minute intervals. 
 Then the drafts were opened, and after conditions became steady the 
 tests were repeated for each of the three other burning rates. 
 
 § 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 •J? 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ID 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 S 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 12- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 7.C// 
 
 ^<75 G&1/LA& 
 
 
 
 
 
 
 
 
 
 
 
 / /»// 
 
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 t-H 
 
 
 
 
 
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 4M-* 
 
 
 
 
 
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 irfG 
 
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 r / 
 
 
 
 
 
 ^ 
 
 
 
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 wfe //! 
 
 tervot. 
 
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 J f J" <5 
 
 BL//2M/H6 £/)T£ -Pounds per /Your 
 
 Fig. 25. — Smoke-test observations of heaters No. 9 and No. 32, burning oil No. 21 
 
 Variation of Smokiness Not Consistent with Oil in Different Heaters. 
 — Figure 24 shows the results of the oil studies. The curves are based on 
 averages of ten consecutive smokiness determinations for each burning 
 rate. Figure 25 indicates the spread of the individual smokiness deter- 
 
46 University of California — Experiment Station 
 
 urinations for oil No. 21. This figure shows results typical of those ob- 
 tained with the other oils. The scale of burning rate used in figures 24 
 and 25 is twice that used in reporting the standard tests. The oil test 
 results indicate some difference between fuels, especially with heater 
 No. 32, in which the smokiness from the best oil is about half that from 
 the poorest. This degree of variability is present over the full range of 
 burning rates but the individual oil curves cross so that there is no con- 
 sistent variation between any two oils. This difference has little real 
 value as the worst smoke was scarcely visible from this heater. When 
 the normal spread of individual determinations is taken into considera- 
 tion it can scarcely be said that the character of the oil, within the range 
 shown by the analyses, had a significant influence upon the smokiness of 
 either of the heaters used in these tests. This is especially true if cross 
 comparisons are made between the two heaters. Oil No. 20, which gave 
 the least smoke in heater No. 9 at low burning rates and the most smoke 
 at high rates, was about average throughout the whole range in heater 
 No. 32. Oil No. 25, which was the best in heater No. 32, was about 
 average in No. 9. 
 
 Conceivably the divergence of the curves for heater No. 9 at low 
 burning rates and the departure of several oils from the general per- 
 formance curve for this heater as shown in figure 8 are partially caused 
 by different rates of soot accumulation. Observations in the field and 
 laboratory indicate that new heaters or ones which have been very 
 thoroughly cleaned do not display their normal smokiness immediately 
 after lighting. Probably, then, the warming-up period before the low- 
 burning-rate tests were started caused different degrees of soot forma- 
 tion with different oils. The only general conclusion which seems jus- 
 tified is, that the smokiness of a great variety of heaters cannot be mate- 
 rially reduced by placing more rigid specifications on oil than those now 
 in use. 
 
 TESTS ON STACKS OF NEW DESIGN 
 
 As soon as it became apparent that smoke-abatement ordinances 
 would be adopted in some counties, the manufacturers of heaters began 
 intensive study of stack design. The tests on standard heaters showed 
 the need for improvements and in some instances suggested lines of 
 procedure. The availability of test apparatus enabled manufacturers 
 to make a systematic study of stack design in relation to smokiness. 
 
 No new heaters were studied in the laboratory, for it was felt that 
 the citrus industry could best be served by aiding the development of 
 stacks designed primarily for the improvement of heaters already in 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 47 
 
 use. Manufacturers have made frequent visits to the laboratory to study 
 stacks. More than sixty different stacks have been tested over the 
 normal range of burning rates to determine their smokiness, and many 
 others have received single-point tests to indicate whether or not further 
 study might be desirable. 
 
 Requirements of a Good Orchard Heater. — The problem of develop- 
 ing simple new heater stacks is not easily solved. Smokelessness is only 
 one of many requirements all of which a heater must meet as closely as 
 possible in order to be useful. According to the generally accepted ideas 
 as to the more important specifications of a heater, it should : 16 
 
 1. Hold sufficient fuel to burn all night without refueling, even 
 
 though 7 pounds or more of oil an hour be burned at times. 
 
 2. Be capable of sufficient regulation to give its greatest heat just 
 
 before sunrise even though the fuel in the reservoirs, is low by 
 this time (ordinary burning rates are from 3 to 5 pounds an 
 hour) . 
 
 3. Be able to burn any of the ordinary grades of heating fuels on the 
 
 market without smoking and without leaving a heavy residue. 
 
 4. Be rain-tight. 
 
 5. Deliver the heat and products of combustion near the ground, but 
 
 without heating the ground unnecessarily. 
 
 6. Be easy to light and regulate by inexperienced labor under all 
 
 weather conditions. 
 
 7. Be capable of being lighted with the draft opening set as required 
 
 for normal burning. 
 
 8. Be readily extinguished by merely closing the regulator and cap- 
 
 ping the stack. 
 
 9. Be arranged for filling and for soot removal without taking off 
 
 the stack or cover. 
 
 10. Be so designed that if it burns dry the bottom of the heater will 
 
 not be damaged. 
 
 11. Avoid oil condensation on the stack or cover. 
 
 12. Be of reasonable cost. 
 
 13. Be made of good material and show small annual depreciation. 
 
 14. Be easy to take apart, clean, and store. 
 
 Interpretation of Test Results on New Stacks. — All of the standard 
 heaters tested were known to have reliable burning characteristics in the 
 field. An observer could also notice differences from usual operation if 
 for instance a cover happened to be loose. However, in testing a new 
 
 16 Schoonover, Warren R., Robert W. Hodgson, and Floyd D. Young. Frost pro- 
 tection in California orchards. California Agr. Ext. Cir. 40:1-73. 1930. 
 
48 
 
 University of California — Experiment Station 
 
 design one cannot be familiar with its normal characteristics, and of 
 course a new stack has not been subject to the ordinary orchard operat- 
 ing practice. The new stacks have been tested for smokiness only. A 
 season's use in an orchard might increase the smoke output. Field trials 
 alone can prove their practicability for general orchard use. However, 
 the great improvement in test results of most of the new designs in com- 
 parison with the standard heaters is so encouraging that it appears 
 advisable to report the results in spite of the limitations. 
 
 Not all of the new experimental results can be reported, but the 
 smokiness of the better stacks developed in each group is shown in figures 
 27, 28, 29, 31, and 32. These are so much better than most of the standard 
 heaters that smokiness and burning rate are shown on scales twice as 
 
 A 
 
 B 
 
 T) 
 
 G 
 
 Fig. 26. — Photographs of new smokeless stacks for orchard heaters: A, No. 52, 
 Lamco Gyradiant; B, No. 60, Hy-Lo Giant; C, No. 62, Hy-Lo Drum; D, No. 80, 
 Hinchcliff 36-inch; E, No. 90, National Junior Louver, 18-inch; F, No. 91, O'Keefe 
 and Merritt, 6-inch straight stack ; G, No. 148, Hy-Lo tapered stack. 
 
 large as before. The photographs in figure 26 show a number of the new 
 stacks. All types have been studied including simple, slip-on stacks for 
 use with the Citrus heater to stacks much larger than any heretofore in 
 use and giving complete combustion within the stack over a wide range 
 of burning rates. 
 
 Smoke Tests of Annular-Combustion-Chamber Stacks. — Figure 27 
 shows the smokiness of stacks which, have large annular or ring-shaped 
 combustion chambers. In these stacks the gases rising from the bowl 
 come into contact with excess air at the base of the stack. Conditions in 
 the burner at the bottom of the stack are favorable for complete and 
 practically smokeless combustion. Because this involves high tempera- 
 tures the stack must be made of high-grade material. A large surface 
 for radiation serves to deliver considerable heat close to the ground. 
 
 Smoke Tests of Enlarged-Combustion-Chamber Stacks. — Figure 28 
 shows the smokiness of three models of Hy-Lo stacks with medium-sized 
 
BlTL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 49 
 
 s: 
 
 I 
 
 No.se 
 
 oc lU No. 60 
 
 
 
 3 4 S 6 
 
 8Vm//VG P/ITE- Pounds per Hour 
 
 Fig. 27. — Smoke tests of annular-combustion-chamber stacks, Nos. 52, 60, and 100. 
 
 peuM 
 
 3 4 S 6 7 
 
 3£/GN/ffG P/)7£ - Poarc/s per ftour 
 
 Fig. 28. — Smoke tests of enlarged-combustion-chamber stacks, Nos. 1, 61, and 62. 
 
50 
 
 University of California — Experiment Station 
 
 combustion chambers. Their prototype, the National Jumbo Cone, is 
 also shown for comparison. These heaters are similar in burning char- 
 acteristics to the heaters with larger ring-shaped combustion chambers 
 
 J 4 S 6 7 9 
 
 BU&MMG GATE - Po ana's per rfoar. 
 
 Fig. 29. — Smoke tests of straight tall stacks on Citrus bowls : stacks 26D, 45, 
 
 80, and 81. 
 
 and are excellent at burning rates permitting completion of combustion 
 in the burner or lower part of the stack. The smokiness increases greatly 
 when the capacity of the burner is exceeded. Stack No. 61A had a 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 51 
 
 smaller stack diameter at the top than No. 61 and although the burner 
 was the same the performance was not as good, and this experimental 
 design was dropped. 
 
 These stacks are all of the lean-mixture type requiring warming up 
 before being adjusted to normal burning rates. 
 
 Smoke Tests of Straight Tall Stacks. — A number of attempts have 
 been made to improve the Citrus heater by equipping it with some sort 
 of a tall stack. Figure 29 shows the results of such attempts. The best 
 
 jo 40 so 60 ro 
 
 T/MF AFTES 5T/1£T//VG TEST, Af/nufes. 
 
 Fig. 30. — Continuous smoke test of 6-inch Exchange stack on Citrus bowl. 
 
 results were obtained with a 36-inch top section placed on the 15-inch 
 stack of heater No. 26 and with the Hinchcliff stack of 36-inch length. 
 With these stacks combustion is practically completed in a limited por- 
 tion of the lower stack much as in the enlarged-combustion-chamber 
 stacks. The straight stacks lack the large radiating surface of the latter 
 and develop local hot spots. Field experience will be required to deter- 
 mine if the stack life is long enough to make them practical. 
 
 The 6-inch exchange stack is even more erratic on the Citrus bowl 
 than on the round bowl with a down-draft tube. Curves show clean and 
 
52 
 
 University of California — Experiment Station 
 
 dirty runs and checks indicate cold-run smokiness. The peculiar per- 
 formance of this stack is well illustrated in figure 30, which shows a con- 
 tinuous record of the cold-weather run with smokiness plotted at minute 
 intervals. A periodicity in smokiness peaks is indicated at all burning 
 rates. 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
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 r° 
 
 
 
 
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 o 
 
 
 
 
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 <V| 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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 r 
 
 
 
 
 
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 $ 
 
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 7 
 
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 9 
 
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 X 
 
 */ 
 
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 f X 
 
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 f ,*> 
 
 
 
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 f 
 
 
 
 
 
 
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 U 
 
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 r 
 
 
 
 
 
 1 /y<?. i?//4 
 
 1 ■ 
 
 
 o 
 
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 / 
 
 
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 *+' 
 
 i 
 
 
 
 
 \H 
 
 
 s$H 
 
 : 
 
 
 
 
 
 
 
 
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 ^ 
 
 1 
 
 
 J 4 S 6 7 
 
 &L/eWrtG /2/irf - Povoc/s per //our. 
 
 Fig. 31. — Influence of bottom air openings on smokiness of open-flame stack, 
 
 No. 91A. 
 
 Principles of Combustion in Open-Flame Stacks. — Combustion in a 
 good open-flame stack should be similar to that in an ordinary Bunsen 
 burner. That is, sufficient air for a good air-fuel ratio should be mixed 
 thoroughly with the combustible gases at the base of the stack and the 
 flame should be at the top. If anything causes the fire to "strike back" 
 
Bul. 536] Smokiness of Oil-Burning Orchard Heaters 
 
 53 
 
 and burn at the air-mixing holes the gases are cracked and the smokiness 
 is greatly increased. In a Bunsen burner using gas under pressure the 
 normal blue flame changes when "striking-back" occurs to a yellow 
 
 3 4 S 6 7 
 
 BUPN/NG PATE- Pounds per Hoar. 
 
 Fig. 32. — Smoke tests of new open-flame stacks on Citrus bowl. 
 
 somewhat smoky flame. In a heater burning oil vapors under natural 
 draft the change is from a yellow smokeless flame to a reddish-colored 
 very smoky flame. 
 
54 University of California — Experiment Station 
 
 Apparently it is relatively easy to design an open-flame stack which 
 will burn without smoke at a single fixed burning rate, but very difficult 
 to design one which will be satisfactory over the whole range of burning 
 rates desired by citrus growers. The problem of air mixing is not simple. 
 If adequate air is admitted for complete combustion at high burning 
 rates it will be too much for low burning rates. The mixture then will 
 be very lean and there will be a tendency for the fire to drop below the 
 air-intake zone and burn the rich oil vapors in the collar, thus producing 
 a smokiness like that of the old smudge pots. A lean mixture, further- 
 more, tends to burn explosively, which may cause the generating fire in 
 the bowl to blow out. If the air intake is correct for low burning rates 
 it will be insufficient for higher burning rates and the stack will show 
 characteristics similar to early models of lazy-flame heaters such as Nos. 
 4, 20, 23, and 30. Figure 31 shows the effect on smokiness of changing 
 the bottom air openings, 
 
 Smoke Tests on New Open-Flame Stacks. — The manufacturers have 
 worked constantly to develop successful stacks of this general type, with 
 results which have sometimes been exceptionally promising. Figure 32 
 gives data from tests on various experimental models. The repeated 
 efforts made in the development of stacks of this general type are indi- 
 cated by the progressive lowering of the smoke curve. The data from all 
 the tests on new open-flame stacks are now being used by manufacturers 
 as the basis for developing the models they expect to sell. 
 
 The new production models have not been tested, but tests on experi- 
 mental stacks indicate the possibility of making open-flame stacks the 
 smoke of which is scarcely visible over burning rates of from 2.5 to 6 
 pounds of oil an hour. It also seems probable that such stacks can be 
 both practical and inexpensive. 
 
 FIELD MEASUREMENT OF THE SMOKINESS OF ORCHARD HEATERS 
 
 During the early winter of 1931 field measurements were made with 
 the electrical precipitator in connection with a portable smoke-collecting 
 device mounted oh a truck. The portable apparatus was similar to 
 the laboratory apparatus in all basic principles with the omission of 
 the light-interception parts. One per cent of the total smoke discharged 
 was passed through the precipitator while burning % pound of fuel. 
 Although dependable results were obtained, the method is not well 
 adapted to field studies, for it is cumbersome and expensive. Technically 
 trained operators are required. 
 
BUL. 536] SMOKINESS OP OlL-BuRNING ORCHARD HEATERS 
 
 55 
 
 Visible Records of Smokiness. — In the course of field, demonstrations 
 conducted by the Agricultural Extension Service, visible records oi 
 relative smokiness were made by pulling 1 per cent of the total smoke 
 stream for 1 minute through a 5-inch square of white filter paper. This 
 method was very effective in demonstrating smokiness and it appeared 
 to offer possibilities of development into a simple, accurate, and portable 
 means of measuring orchard heater smoke. 
 
 After experimenting in the laboratory, it was found that satisfac- 
 tory records could be made on a white filtering felt, such as is used in 
 some automobile air cleaners. The smoke sample was collected on a felt 
 
 Fig. 33. — Felt sampling apparatus. The felt is held in the square frame between 
 the two rectangular funnels (to the right of the manometers). 
 
 9 inches square with %-inch margin, giving an exposure of 56.25 sq. in. 
 This size was chosen because preliminary studies indicated that this area 
 was sufficient to permit taking the sample while using the suction 
 normally developed by the centrifugal fan. (See fig. 33.) A large num- 
 ber of tests were run to determine the sensitivity of the method and also 
 the amount of smoke which could be collected on the felt in order to give 
 good comparisons between all of the heaters to be tested. The whiteness 
 of the felt, the depth of penetration of the soot, the degree of dispersion 
 of the carbon, the presence of filterable vapors, etc., influence the black- 
 ness of the deposit. It finally appeared that a satisfactory record would 
 be obtained by collecting on this white felt the soot from 0.1 per cent of 
 the total smoke produced by burning 1 pound of fuel. Such a record 
 shows a light gray with the good heaters and still is not too black with 
 
56 University of California — Experiment Station 
 
 the bad heaters for accurate calibration of the smokiness. For the very 
 best heaters it is advisable to collect 0.5 per cent and for the worst 0.05 
 per cent in order to obtain measurable records. 
 
 The grayness of these felt samples can be measured electrically by 
 reflected light with the same equipment used for the light-interception 
 method in measuring the opaqueness of smoke. However, the standard 
 color wheel is more commonly used because of its simplicity. The nature 
 of the felt smoke records and the method of grading them by the color 
 wheel are illustrated in figure 34. The color wheel used has one disk 
 covered with black velvet and the other with a piece of the same white 
 felt material used in making the tests. The areas of black and white 
 exposed can be varied, and when the wheel is spun to blend the black 
 
 Fig. 34. — Photographs of color wheel (stationary and running) used to 
 measure blackness of felt sample. 
 
 and white into a gray, an exact match can be obtained from any smoked 
 felt sample. The relative smokiness is expressed in the terms of the per- 
 centage of black required. 
 
 Correlation with the Light-Interception Method. — This felt method 
 provides another quantitative means of evaluating the relative smoki- 
 ness of orchard heaters. Furthermore, one can compare the results with 
 tests made with the light-interception method and thus interpret them 
 in pound-smoke units. The correlation between percentage of blackness 
 and pound-smoke units is shown in figure 35. The curve is based on 
 simultaneous determinations of smoke opaqueness and sample collection 
 by the felt. Opaqueness readings were taken at 5-second intervals while 
 the smoke was being drawn through the felt and burning rates were 
 determined on the automatic balance. The curve indicates a very high 
 degree of correlation between the smokiness as determined from per- 
 centage of blackness on the color wheel and the smokiness as determined 
 in pound-smoke units by the light-interception method. This is to 
 be expected, for both methods depend upon the quantity and black- 
 
Bul. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 57 
 
 ness of the soot. If the accuracy of the color-wheel determinations is 
 not required, quick and dependable comparisons of the grayness of 
 the felt smoke records can be made with properly prepared gray color 
 standards. 
 
 According to the correlation shown in figure 35, the very best heaters 
 would produce felt smoke records less than 30 per cent black, or a smoki- 
 ness under 3 pound-smoke units. A good heater, that is, one which would 
 give off smoke scarcely visible to the eye at a 5-pound burning rate, 
 
 s 
 
 & 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 / 
 
 
 J* 1 
 
 
 
 
 
 ° 
 
 1 
 
 
 Very Best tteoterj 
 
 Good tteoters 
 
 Far tieoters 
 
 Smoky fieoters 
 
 8od 1 tieoters 
 
 ^ c ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 /o 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 
 
 
 >» ^ 
 
 
 
 
 
 y~ 
 
 
 
 
 
 1* 
 
 
 
 
 
 / 
 
 
 
 
 
 
 t* 
 
 
 
 
 
 
 S 
 
 
 
 
 
 
 
 & ^ 
 
 
 
 
 
 ntf 
 
 
 
 
 
 
 
 1 * 
 
 
 
 o 
 
 !z 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Jfi* 
 
 >^< 
 
 
 
 
 
 
 
 
 
 
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 ""-^ 
 
 £o 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 o 
 
 """^T 
 
 
 
 
 
 
 
 
 
 
 
 
 
 *s 
 
 ^j^-K4 — 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 JO 40 JO 60 
 
 0LACf<rtE55 Of r~£ir t Per Cent 3 toe A. 
 
 Fig. 35. 
 
 -Correlation between blackness of felt sample and quantity of soot 
 collected. 
 
 would produce records from 30 to 45 per cent black or a smokiness of 
 from 3 to 5 pound-smoke units. The fair heaters burning under favor- 
 able conditions would produce records from 45 to 60 per cent black, or 
 a smokiness of from 5 to 10 pound-smoke units. A smoky heater would 
 produce records from 60 to 75 per cent black, or a smokiness of from 10 
 to 17 pound-smoke units. The bad heaters would produce records 
 blacker than 75 per cent, or above 17 pound-smoke units. 
 
 The above grades of blackness are only slightly different from the 
 Ringelmann chart, which has been adopted by many cities as the of- 
 ficial standard of comparison for the enforcement of smoke abatement 
 ordinances. 17 
 
 !7 Anonymous. A digest of smoke ordinances in American cities and Canada. 
 Power 74:447-482. 1931. 
 
58 
 
 University of California — Experiment Station 
 
 Use of Felt Method for Smoke Measurements in the Field. — The felt 
 method appears to meet the requirements for use in the field ; it is simple, 
 inexpensive, and accurate. All the apparatus required can be mounted 
 readily on a light truck capable of being driven into orchards while 
 frost-protection operations are in progress. (See fig. 36.) Inexperi- 
 
 Fig. 36. — Field apparatus for measuring orchard-heater smoke. 
 
 enced operators can easily be trained to collect dependable records of 
 the smokiness of orchard heaters. This type of apparatus is also suit- 
 able for manufacturers who wish to carry on studies leading to heater 
 improvement. 
 
 ACKNOWLEDGMENTS 
 
 This project received very effective cooperation from many persons 
 outside the Experiment Station. Thanks are due particularly to the 
 Orchard Heating Improvement Committee, under the chairmanship of 
 Mr. B. R. Holloway, for its personal attention and the equipment 
 furnished. The California Fruit Growers Exchange has also been gen- 
 erous in supplying technical information and several lots of fuel oil. 
 Mr. Floyd D. Young of the United States Weather Bureau rendered 
 considerable assistance in starting the project and Mr. F. T. Seabern of 
 Pomona very kindly arranged for the experimental field work to be done 
 in his orchard. 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 59 
 
 APPENDIX A: DESCRIPTION OF APPARATUS AND TEST METHODS 
 
 Equipment for making smoke-density determinations on the light- 
 interception basis as developed and built in the laboratories of the Agri- 
 cultural Experiment Station is shown in figure 4. The work of testing 
 standard heaters was completed before the apparatus was remodeled to 
 increase its sensitivity in determining both opacity and burning rate. 
 
 Fig. 37. — Automatic self-balancing scales used for determining burning rate. 
 
 The details of the original equipment need not be given ; for all of the 
 results reported were developed on apparatus similar in all essential 
 details to that described below. 
 
 The heater being tested is placed on scales under the hood at the ex- 
 treme right hand. The scales (fig. 37) are self -balancing ; the beam car- 
 ries one end of a chain (like a ''Chainomatic" balance) and a mercury 
 contact which operates an auxiliary device while in the low position to 
 raise the other end of the chain, thus lightening the beam load. To in- 
 crease the sensitivity of the scales an electro-magnet in series with an 
 
60 
 
 University of California — Experiment Station 
 
 ordinary light flasher causes the beam to oscillate regularly. Since the 
 frequency of the contact period is constant, its duration is what regu- 
 lates the winding-up device. This duration of the contact period is 
 governed in turn by the average position of the beam, thus keeping the 
 scales in perfect balance. A vernier slide attached to the winding-up 
 mechanism indicates accurately the loss of fuel weight and hence the 
 burning rate. 
 
 Vrho/es*. 
 
 tbncentrofea n/a 
 
 \-S/Je /vt>es-/ 
 
 to mi Ih vo/t me/er 
 
 Jjus/ol/e //gf>f socket 
 'Sec/ion of smoke s/ocA 
 
 Fig. 38. — Diagram of light-interception apparatus for measuring opaqueness 
 
 of smoke. 
 
 A centrifugal fan at the extreme left draws the entire smoke stream 
 with some surrounding air into the hood and up through a 6-inch stack 
 into a 10-inch horizontal pipe. The sudden change of section and the 
 right angle turn serve to mix thoroughly the smoke particles and air, 
 giving a stream of uniform opaqueness under steady burning condi- 
 tions. The smoke stream is then flattened to 4 1 / 4 inches and spread 
 horizontally to 24 inches where it crosses the path of the light beam. 
 Small air holes in the 4-inch pipes enclosing the light beam, drilled near 
 the channel wall allow just enough additional air to seep in to prevent 
 
 Fig. 39. — Felt strip used to check uniformity of smoke distribution and 
 length of light path through the smoke. 
 
 the smoke from swirling out into the light tubes. (Glass screens could 
 not be used to confine the smoke stream because they would become 
 coated with soot, ) The plan of construction of the light tube is shown in 
 figure 38. The sharpness of boundary and uniformity of smoke spread 
 was tested by a felt strip left in the line of light just long enough to 
 become gray with soot. (See fig. 39.) 
 
 For a light source, a standard Balopticon 1,000 watt lamp and its 
 concave mirror is used. On the far side of the smoke stream a condens- 
 ing lens focuses the parallel rays on a radiation pyrometer (fig. 40). 
 The voltage impressed on the source lamp is held constant at the low 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 61 
 
 value of approximately 65 volts, which produces a pyrometer reading 
 of 15 millivolts with clear air. This setting is checked before and after 
 test. The opaqueness of the smoke is indicated by a decrease in the 
 electromotive force (e.m.f.) of the pyrometer and is measured in con- 
 centration units. 18 Each successive concentration unit represents a re- 
 duction of 10 per cent of the original light intensity. That is, one con- 
 centration unit reduces the millivolt reading to 13.500, two units to 
 
 Fig. 40. — End view of laboratory apparatus for measuring smoke, showing 
 radiation pyrometer at the end of the light path (see arrow at the right). 
 
 12.150, three units to 10.935, etc. The smoke-stream temperature at the 
 light beam is noted and the value in concentration units is then increased 
 by the correction for temperature to a standard-density basis. 
 
 From the light, the smoke stream passes into a large drum 18 inches 
 in diameter through a honeycomb of %-inch tubes 3 inches long and 
 slowly approaches the orifice plate. Three orifices are provided : one 
 large central orifice (3-inch diameter) for measuring total flow and two 
 fractional orifices to obtain 1 per cent samples for carbon collection, the 
 
 is Simon, A. W., L. C. Kron, C. H. Watson, and H. Eaymond. A recording dust 
 concentration meter Eev. Sci. Instruments 2:67-83. 1931. 
 
62 University of California — Experiment Station 
 
 first by electric precipitator, and the second by a Buchner funnel for a 
 chemical analysis or by a felt for color-wheel comparison. The pressure 
 drop across each small orifice is balanced exactly with the main orifice 
 by butterfly valves some distance downstream. A definite fraction of 
 the total flow can be obtained unaffected by the temperature or pressure, 
 which apply alike to the three orifices. 19 Calibration of the main orifice 20 
 was accomplished by using a special elliptically rounded approach 
 nozzle with a Venturi expanding cone. 21 The flow through the frac- 
 tional orifices was measured in a large displacement tank with a stop 
 watch by noting the rate of discharge of water, which was regulated to 
 balance the manometer between the downstream taps of the large and 
 small orifices. Both the pressure differential and temperature are ob- 
 served at the orifice for the calculation of air flow. 
 
 The radiation pyrometer e.m.f. readings are taken at definite and 
 frequent intervals so as to give a practically continuous record of rela- 
 tive smoke density at the point of observation. The average value of 
 opaqueness, suitably corrected for temperature (expressed as smoke- 
 concentration units), multiplied by the number of thousand cubic feet 
 of air (standard conditions) pulled through the stack while burning a 
 pound of fuel gives a convenient smokiness unit. This quantity unit, 
 which has been designated heretofore as a "pound-smoke" unit, al- 
 though not actually weight, is comparable with a mass unit per pound 
 of fuel burned. 
 
 A typical calculation is as follows : 
 
 Opaqueness (average of 6 readings) = 13.55 millivolts 
 
 = 0.965 concentration units 
 
 Thermocouple (average reading at light _ 218 millivolts 
 path) 
 
 = 134° F (calibrated) 
 
 Temperature correction to standard air _ 458 + 134 
 
 conditions (32° F) ~ 490 
 
 Therefore, opaqueness (corrected for 
 
 temperature) 
 
 = 1.165 concentration units 
 
 /458 + 134\ 
 0.965 ( j^ ) concentration units 
 
 lo Hodgson, John L. The laws of similarity for orifice and nozzle flows. Amer. 
 Soc. Mech. Engin. Trans. 51 (FSP) :303-332. 1929. 
 
 20 Bean, H. S., E. Buckingham, and P. S. Murphy. Discharge coefficients of 
 square-edged orifices for measuring the flow of air. Bur. Standards Jour. Research 
 2:561-658. 1929. 
 
 2i Schiller, Ludwig. Hydro- und Aerodynamik. In: Wien, W., and F. Harms. 
 Handbuch der Experimentalphysik 4(1):581. Akademische Verlagsgesellschaft. 
 M.B.H., Leipzig, Germany. 1931. 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 63 
 
 Differential pressure (drop across = ^ incheg water 
 
 orifice) 
 Thermocouple (at orifice) = 2.04 millivolts 
 
 Therefore, differential (corrected for 
 temperature as above) 
 
 = 128°F 
 
 = 3.95 inches 
 
 And the air flow? 2 = 127 V 3.95 
 
 = 252 cubic feet per minute (standard 
 conditions) 
 Change of fuel weight = 0.35 pounds in 6 minutes 
 
 Therefore the burning rate = 3.5 pounds per hour 
 
 — 17.15 minutes per pound 
 
 The total air flow per pound of fuel i^-ir v O ro i • -p + 
 
 1 l 17.15 X 252 cubic feet 
 
 burned is therefore 
 
 = 4.32 thousand cubic feet 
 
 Therefore the smokiness = 1.165X 4.32 pound-smoke units 
 
 == 5.02 pound-smoke units. 
 
 In this way it is possible to calculate, on the basis of fuel consumed, 
 the relative smokiness of different heaters at different burning rates 
 without error due to sampling or to admixture of outside air with the 
 products of combustion. 
 
 APPENDIX B: CORRELATION BETWEEN LIGHT INTERCEPTION AND 
 WEIGHT OF SMOKE PARTICLES 
 
 In order to secure data for determining the weight of carbon per 
 pound of fuel burned corresponding to one pound-smoke unit, a series 
 of chemical determinations was run. Smoke-density readings on the 
 millivolt meter and the necessary data for calculating air flow were 
 taken simultaneously. In making the chemical determinations during 
 the first part of the investigation gas samples were withdrawn from the 
 smoke chamber through an M-shaped sampling tube introduced just 
 downstream from the point of making the opaqueness determinations. 
 Small holes approximately y iG inch in diameter were drilled in the 
 sampling tube at right angles to the direction of gas flow. The suction 
 for removing the gas samples was created by allowing water to flow 
 from a steel oil drum mounted on scales, The rate at which the water 
 discharged from the barrel was controlled to collect the gas sample con- 
 
 22 127 is the numerical value of the coefficient of discharge and dimensional 
 factors of the appropriate air flow formulas given in the Bureau of Standard 
 Journal of Research. (Bean, H. S., E. Buckingham, and P. S. Murphy. Discharge 
 coefficients of square-edged orifices for measuring the flow of air. Bur. Standards 
 Jour. Research 2:561-658, 1929.) 
 
64 University of California — Experiment Station 
 
 tinuously while burning a known weight of fuel. The volume of gases 
 (approximately 6 cubic feet) withdrawn from the smoke chamber was 
 calculated from the weight of the water which ran from the barrel. The 
 gas volumes were reduced to standard conditions except that the correc- 
 tion for water vapor was not applied. 
 
 Later chemical determinations were based on the 1 per cent sample 
 taken through one of the small orifices described above. 
 
 For making a carbon determination of a sample, the flue gases were 
 passed through a filter mat of shredded asbestos supported in a 130-mm 
 Buchner funnel. The weight of smoke absorbed on the filter was not 
 determined directly ; instead, its carbon content was found by combus- 
 tion and weighing the carbon dioxide. The Division of Chemistry co- 
 operated effectively in establishing test procedure, in making all chemi- 
 cal determinations, and in interpreting the data. Standard analytical 
 methods were used with additional refinements which made certain that 
 the variations observed resulted entirely from the test heater and not 
 from the method of determination. This method gives the weight of 
 carbon in the absorbed smoke, from which can be calculated the meaning 
 of 1 pound-smoke unit in terms of weight of carbon per pound of fuel 
 burned. 
 
 The smoke analyses show a mixture of carbon and unburned car- 
 bonaceous matter. Some of the smoke particles are practically pure 
 carbon and some are rather oily. This difference in character of the 
 smoke was determined by taking a second sample on another filter while 
 maintaining as nearly as possible the same burning conditions. The oily 
 matter was extracted by ether, the ether driven off, and the remaining 
 carbon determined as before by combustion. 
 
 For the fuel oils used in the tests of standard heaters, figure 41 gives 
 the results of the chemical determinations compared with pound-smoke 
 units. It will be seen that the value per unit is not constant but varies 
 between 0.81 grams and 1.58 grams per unit with an average value of 
 1.00 grams of carbon in the smoke for each pound-smoke unit. The 
 greatest dependable value is 24 per cent above the average and the 
 smallest 19 per cent below. Forty-two determinations were made, repre- 
 sentative of three different oils and 21 different heaters operated over 
 a range of burning rates varying between 3.5 and 15.0 pounds of fuel 
 per hour. The dotted line drawn on figure 41 shows how the value varied 
 with one oil in one heater over a range of burning rates. The average 
 correlation value of 1.0 gram per pound-smoke unit applies to a light 
 path 24 inches long through the smoke stream. For any different length 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 
 
 65 
 
 the value would change in inverse proportion. 23 The above data indicate 
 that the weight of smoke for the three fuel oils used in testing the 
 standard heaters might be estimated from light-interception measure- 
 ments if the accuracy of 25 per cent is satisfactory, but further tests 
 with seven other fuel oils showed much wider variation, especially at 
 low burning rates. 
 
 ////o- 
 
 >660 
 
 Wf «63G 
 
 _| 64/). 
 
 o 73 C 
 
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 o 26 C 
 
 v °2jc 
 
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 *>63/> 
 
 '///is 7'> 
 7 °/C 
 
 66A0 69 *™ 6 * 3 
 
 \j /¥ ( t " 
 
 6/E 
 24/) 
 
 *>™F 
 
 ?&**& 
 
 >SSA 
 
 
 f/6t/e£S €£P££5£/YT T£5T £(/// /Yl/Af0££5 
 
 S 6 7 9 /O 
 3(/g/Y///6 £/)T£ -Pounds per //our. 
 
 /£ 
 
 /4 
 
 /6 
 
 Fig. 41. — Correlation between chemical determinations of total carbon 
 and pound-smoke units. 
 
 This variation may be explained as follows : 
 
 1. Weight of soot particles of given density varies substantially as 
 the cube of the linear dimension of the particles, while the area for light 
 interception varies as the square. Thus the degree of dispersion of the 
 smoke particles influences the result. 
 
 23 Simon, A. W., L. C. Kron, C. H. Watson, and H. Eaymond. A recording dust 
 concentration meter Ee v. Sci. Instruments 2:67-83. 1931. 
 
66 
 
 University of California — Experiment Station 
 
 2. The smoke is not pure carbon. Adsorbed oily matter may increase 
 the weight of individual particles without increasing their size. 
 
 3. The smoke comes from some heaters in distinct puffs. This makes 
 it difficult to get a correct figure for the average number of millivolts 
 even when reading at intervals of 40 seconds. The filter sample is taken 
 continuously and represents an average. 
 
 4. Relatively transparent smokes seem to contain colorless carbon 
 compounds, capable of being collected by the asbestos filters. 
 
 In the last three cases the felt method should show slightly better 
 correlation with weight than would the light-interception method. 
 
 Fig. 42. — Electrical precipitator and its connections with sampling orifice 
 and high-tension transformer. 
 
 Additional Studies on Correlation of Smoke Density and Weight. — 
 The additional data mentioned above as bearing on the correlation 
 between opaqueness and weight were developed in connection with 
 studies of the influence of oil composition on the smoke output of a 
 smoky heater and a typically good heater. Seven different fuel oils of 
 variable characteristics were used. These data were obtained by the 
 same chemical methods as before, by use of asbestos filters for collecting 
 samples, and also by use of an electric precipitator to collect smoke 
 samples which could be weighed directly and later analyzed for carbon, 
 hydrogen, and ash content. The precipitator was developed with the 
 cooperation of the Western Precipitation Company of Los Angeles. It 
 
BUL. 536] SMOKINESS OF OlL-BuRNING ORCHARD HEATERS 67 
 
 is a laboratory model, alternating current, Cottrell precipitator 24 capa- 
 ble of handling 2 cu. ft. of smoky gases per minute (see fig. 42). The 
 precipitating potential is 30,000 volts developed by a small transformer 
 operating on a 110-volt circuit. Smoke is precipitated both on the cen- 
 tral high voltage electrode and on the glass container, which is sur- 
 rounded by a wire-mesh grounded electrode. 
 
 Precipitation of the smoke particles carried in the hot gases is visu- 
 ally complete up to a certain load. With very smoky gases some smoke 
 may be carried over, especially if too long a run is made. The chemical 
 studies seem to indicate that light smokes contain heavy molecules, prob- 
 ably compounds of carbon, hydrogen, and oxygen, which intercept very 
 little light and are not precipitated electrically but which are adsorbed 
 onto the asbestos filters. However, for average conditions, the direct 
 smoke weight data correlate fairly well with weights determined chemic- 
 ally. The precipitated smoke contains about 10 per cent ash, which 
 would tend to give higher results with this method because ash was not 
 determined on the filtered smoke. For extremely light or heavy smokes 
 the correlation between the chemical and electrical methods is not good. 
 
 For the average conditions applying to all the tests of the old heaters 
 supplied by the committee 1 pound-smoke unit may be considered ap- 
 proximately equal to 1 gram of carbon in the smoke from 1 pound of 
 fuel. Examination of the weight data obtained during the oil studies 
 indicates that the correlation between smoke opaqueness and weight is 
 not satisfactory at average or high burning rates, only a general trend 
 being evident. At low burning rates, no consistent correlation was 
 found. It varies with different oils and with different heaters as well 
 as with the burning rate, as is pointed out in figure 41. One pound- 
 smoke unit may represent as little as 0.75 grams or as much as 3 grams 
 of carbon in the smoke per pound of fuel burned. This indicates that 
 the pound-smoke unit results, if interpreted in terms of grams weight 
 per pound of fuel burned, should be considered as minimum values. 
 
 24 [Anderson, E.] Cottrell processes of electrical precipitation for removing 
 suspended particles from gases. Leaflet issued by the Western Precipitation Com- 
 pany, 1016 W. Ninth Street, Los Angeles. 
 
 Simon, A. W., and L. C. Kron. Electrical precipitation. Amer. Inst. Elec. 
 Engin. Paper 32-32. Eev. in: Elec. Engin. 51:93-95. 1932. 
 
 27m-9,'32