UNIVERSITY OF CALIFORNIA PUBLICATIONS * COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY. CALIFORNIA SOME FACTORS OF DEHYDRATER EFFICIENCY BY W. V. CRUESS and A. W. CHRISTIE BULLETIN No. 337 November, 1921 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1921 David P. Barrows, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Dean. Edward J. Wickson, Horticulture (Emeritus). , Director of Resident Instruction. C. M. Haring, Veterinary Science; Director of Agriculture Experiment Station. B. H. Crocheron, Director of Agricultural Extension. Hubert E. Van Norman, Dairy Management. James T. Barrett, Plant Pathology; Acting Director of Citrus Experiment Station. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. Ralph E. Smith, Plant Pathology. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture Frederic T. Bioletti, Viticulture and Fruit Products. Warren T. Clarke, Agricultural Extension. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. Walter Mulford, Forestry. Fritz W. Woll, Animal Nutrition. W. P. Kelley, Agricultural Chemistry H. J. Quayle, Entomology. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology L. D. Batchelor, Orchard Management. J. C. Whitten, Pomology, t Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. R. L. Adams, Farm Management. W. B. Herms, Entomology and Parasitology. F. L. Griffin, Agricultural Education. John E. Dougherty, Poultry Husbandry D. R. Hoagland, Plant Nutrition. G. H. Hart, Veterinary Science. L. J. Fletcher, Agricultural Engineering. Edwin C. Voorhies, Assistant to the Dean. DIVISION OF VITICULTURE AND FRUIT PRODUCTS F. T. Bioletti A. J. Winkler W V. Cruess G. Barovetto A. W. Christie J. H. Irish L. O. Bonnet t In cooperation with Bureau of Public Roads, U. S. Department of Agriculture. SOME FACTORS OF DEHYDRATER EFFICIENCY By W. V. CRUESS and A. W. CHRISTIE CONTENTS page Introduction 277 Plant Investment 277 Cost of Operation 279 Fuel Efficiency 279 Air Flow 285 The "Parallel Current" System 287 Static Pressure and Recirculation 288 Air Distribution 292 Fans 292 Trays 292 Control of Humidity 295 Dipping Equipment 296 Summary and Conclusions 298 Introduction. — During the past two years, more than 150 dehydra- ters have been built in California. There are also in existence not less than 150 driers of less modern design built before 1919. Some of these were erected merely as insurance against rain damage, but many have been used in place of sun-drying, as in prune and apple drying. Many different types are represented and several different systems of heat production and heat conveyance are employed. Obser- vations have been made upon man}^ of these plants. In several cases direct comparisons of important types were possible. Because of the improvements that are rapidty being made in the design, construc- tion and operation of dehydraters, this publication must be considered in the nature of a progress report. It is issued in the hope that the results, which in many instances are sufficiently conclusive, will be of value to operators and prospective purchasers or builders of dehydraters.* Plant Investment. — The cost of erecting dehydraters varies greatly according to the design, the materials of construction, and the acces- sory equipment. During the past two seasons, the cost of construction and the capacity in green tons per 24 hours of several distinct types were determined. Assuming uniform rates of interest, depreciation, insur- ance, and taxes, and a drying season of sixty days, calculations have * To avoid confusion of terms, "dehydrater" is used to designate the appa- ratus used for dehydration, and ' ' dehydrato?- ' ' fVi ° aerator of this apparatus. 278 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION been made of the proportion of the total cost of dehydration that may be assigned to the ' ' fixed charges ' ' of plant investment. For com- parative purposes, it has been assumed that the plants were operated on grapes and prunes thirty days each and that the drying ratios were 3.5 :1 and 2.5 :1 respectively, or an average of 3 :1. Most dehydraters were actually operated during the 1920 season for less than sixty days. The costs given in Table I therefore are, on the average, lower than the actual costs for the past season. How- ever, by operating on more kinds of fruit, the season can be prolonged and the costs thereby lowered. TABLE I Cost of Dehydration as Affected by Plant Investment Fix ed Charges per Green 1 'on Capac- Total Cost on basis of 60-day season * Type of ity Green First Cost of Plant Total Plant per No.f Plant Tons of per T , De- In- Dry per 24 Plant green inter- est precia- sur- Taxes Total Ton hours ton 24 tion ance hours A Air-blast tunnel, direct heat 25 $12,000 $480 $ .56 $ .80 $.10 $ .24 $1.70 $5.10 B University Farm type, Santa Clara County. 8.6 5,000 581 .67 .97 .12 .26 1.92 5.76 C Air-blast tunnel, direct heat 24 14,000 583' .68 .98 .12 .26 1.94 5.82 D Air-blast tunnel, direct heat 52 36,000 596 .69 .99 .12 .27 2.07 6.21 E University Farm type, Davis 6 4,000 667 .78 1.11 .14 .29 2.32 6.96 F Air-blast, tunnel direct heat 35 25,000 715 .83 1.19 .15 .32 2.49 7.47 G Air-blast, tunnel direct heat 5 5,500 1,100 1.28 1.83 .23 .48 3.82 11.46 H Air-blast cabinet 12 15,000 1,250 1.46 2.09 .26 .55 4.36 13.08 I Small Oregon tunnel type .75 1,000 1,330 1.55 2.20 .28 .59 4.62 13.86 J Air-blast tunnel 20 25,000 1,250 1.46 2.08 .26 .67 4.47 13.41 K Small stack type .75 1,500 2,000 2.33 3.55 .42 .89 7.19 21.57 L Air-blast stack type 1.50 4,000 2,666 3.11 4.44 .55 1.48 9.28 27.84 M Stack type, large size 9 25,000 2,778 3.24 4.63 .58 1.23 9.68 28.04 N Ceramic oven 6 25,000 4,167 4.17 6.94 .87 1.39 13.37 40.01 *Interest at 7%; depreciation at 10%; insurance at 2^% of }/% value; taxes at 4% of 2 /3 value. tThese letters are used to designate the same dehydraters in subsequent tables. Bulletin 337] some factors of dehydrater efficiency 279 As stated above, the eosts reported in Table I are based upon the capacity for a sixty-day season and represent fixed charges for each green ton and each dry ton of fruit, respectively, handled during the sixty-day period. A depreciation of 10 per cent is assumed for all plants, although it varies greatly with the type of construction. It is evident from Table I that most of the dehydraters listed represent too large an investment for the tonnage of fruit dried. If the plants were operated throughout the year, the relative fixed charges would be much less. Under such conditions, a high initial investment might be economically sound. For the average fruit grower who would operate during not more than two to three months of the year, the initial investment should be kept as low as is compatible with efficiency. This is necessary par- ticularly in cases where the dehydrater is to be used for only one variety of fruit. If the plant is to be used only as insurance against rain damage, as low cost of construction as is compatible with efficiency is essential. It has been demonstrated that a thoroughly satisfactory fruit dehydrater can be built at a cost not exceeding (at present prices of labor and materials) $500 per green ton capacity per 24 hours. This includes cost of trays, cars, and all equipment used in the dehydrater proper, but not dipping and packing equipment. A less expensive, but also less efficient dehydrater is often sufficient for insurance against rain damage. Cost of Operation. — The amount of labor, fuel, power, and mater- ials used in the operation of the University Farm dehydrater was carefully determined and similar data were obtained for several com- mercial plants of various designs and capacities. From these data, the cost was calculated by assuming prices of 6 cents per gallon for fuel oil, 2!/2 cents per kilowatt hour for power, and 50 cents per hour for labor. Table II summarizes the results of these calculations. These data and calculations, which are supported by less complete data obtained for several plants not included in Table II, lead to the conclusion that the type of dehydrater consisting of a tunnel through which the fruit is progressively moved on trucks and dried in a blast of recirculated air is the most efficient of those now in use in California. Fuel Efficiency. — We may define fuel efficiency as the proportion of the total heating value of the fuel that is actually utilized in evaporating moisture from the fruit. For example, if an amount of fuel is burned sufficient to evaporate 1000 pounds of water and meas- urement shows that only 500 pounds of water is evaporated from the fruit, the fuel efficiency is ^-r- or 50 per cent. 280 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE II Comparative Costs of Dehydration* Type — j Fruit Cost per Green Ton Plant No. Labor Fuel Power and light Total operat- ing charges Fixed charges from Table I Total cost of produc- tion D Direct Heat Air-blast tunnel Prunes Prunes Grapes Prunes Prunes Grapes Prunes $3.19 4.75 4.16 4.80 5.70 4.56 9.75 $ .94 1.28 2.05 1.63 1.70 1.44 3.26 $ .48 .56 .45 .55 1.18 .59 .20 $4.61 6.59 6.66 6.98 8.58 6.59 13.21 $2.07 1.92 2.32 4.36 4.47 13.37 9.68 $6.68 B E H J University Farm type Santa Clara Co. University Farm type Davis Air-blast Cabinet Air-blast tunnel 8.51 8.98 11.34 13.05 N Ceramic Oven 19.96 M Stack -type Gravity Air Flow 22.89 Approximately 1000 B. T. U. (British Thermal Units) are required to evaporate one pound of water. Fuel oil furnishes on complete combustion approximately 135,000 B. T. U. per gallon. Therefore one gallon of oil will evaporate at 100 per cent efficiency about 135 pounds of water. Since the amount of water evaporated in a given dehydrater is obtained by subtracting the weight of dry fruit from that of green fruit, we have by application of the above facts the following simple formula for the calculation of the approximate fuel efficiency : Pounds green fruit — Pounds dry fruit ^ , ffl . Gallons of oil consumed X 135 The exactness of this formula will vary slightly with the tempera- ture of the air used in drying and with the heat value of the oil. It is sufficient for comparative purposes. Application of this formula to data secured at several dehydraters during the past season gives the results presented in Table III. The air-blast tunnel type of dehydrater is shown by the data in Table III to be much more efficient than the gravity air-flow type. These results are confirmed by similar observations made in 1919 and by data of the 1920 season not reported in Table III. ♦Figures for labor, fuel, power, and light represent actual observations, while fixed charges are based upon cost of plant and an operating season of sixty days. BULLETIN 337] SOME FACTORS OF DEHYDRATER EFFICIENCY 281 14 Direct heat" dehydraters in all instances gave high fuel efficiency. This was to be expected because in this type of dehydrater, stack losses are eliminated. Fig. 1. — Students of fruit products operating University Farm dehydrater. In general, it was possible to obtain higher fuel efficiency in the dehydration of free drying fruits, such as apples and apricots, than of fruits which case-harden, such as pears and prunes. By "case- hardening" is meant the dessication or over-drying of the surface of fruits, which makes them almost impervious to the escape of moisture from within and thereby greatly lengthens the drying time. If the humidity and the recirculation of the air were under perfect control, differences in fuel efficiency for different fruits would be lessened. 282 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION TABLE III Comparative Fuel Efficiencies of Several Types of Dehydraters Plant No. Type of Dehydrater Fruit Dried Fuel Efficiency per cent E N F N E E B J J E H M M University Farm type, Davis Ceramic Oven Air-blast Tunnel Direct Heat Ceramic Oven University Farm type, Davis University Farm type, Davis University Farm type, Santa Clara Co. . . . Air-blast Tunnel Air-blast Tunnel (same design as pre ceding but in another location) University Farm type, Davis Air-blast Cabinet Stack type Gravity Air Flow Small Stack type Gravity Air Flow Apricots Apples Grapes Grapes Peaches Pears Prunes Prunes Prunes Grapes Prunes Prunes Apricots 58 50 48 44 43 43 42 39 38 38 30 24 14 A well-designed dehydrater should have a fuel efficiency of at least 40 per cent in drying prunes or grapes. An efficiency above 50 per cent is very difficult to attain under usual conditions. With freely drying fruit during hot weather, 50 per cent efficiency may be exceeded, as in the first test, Table III. Where the products of combustion are passed directly over the fruit, it has been found that over 90 per cent of the heat generated is taken up by the air. Where a furnace and an efficient system of radiating flues are used, it is possible to transmit as much, as 85 per cent of the heat generated in the furnace to the air used in drying. This was demonstrated at the University Farm dehydrater during 1919 and 1920. # The "direct heat" system requires the use of fuel which burns free of soot, smoke, or objectionable odor. For this reason, more expensive fuel is neces- sary than for the radiation system. This fact tends to counterbalance the greater efficiency of the direct heat system. There has been no opportunity to compare the fuel efficiencies of steam-heated dehydra- ters with those of dehydraters heated by furnaces or direct heat. It is known, however, that only 55 to 65 per cent of the heat value of fuel is transmitted to steam, though as much as 95 per cent of this 55 to 65 per cent may be transferred to the air used in drying. Crude oil and stove oil are the fuels in most common use in dehydraters in California. The latter burns practically without soot, if a forced draft furnace is used. Crude oil even in the best burners BULLETIN 337J SOME FACTORS OF DEHYDRATER EFFICIENCY 283 to < o GO r+-' P H -s P u p < to O p a - o CO CO x P Q p o P r+ O d 5. o '-i 05 P o p 284 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION forms an appreciable amount of soot. This makes it necessary to seal all flue connections tight, so that no soot is carried through the drying tunnel to contaminate the fruit. Observation has demonstrated that this is a real difficulty. Functions of the Air. — Air serves two purposes in the dehydration of fruits. It conveys to the fruit the heat necessary to evaporate the surplus moisture and it carries away the water vapor after evapora- tion has taken place. Much more air is required for the former than for the latter function. TABLE IV Examples of Air-Flow Measurements and Drying Times Plant F C A O E G B M M N N Type of Dehydrater Air-blast direct heat. Air-blast direct heat. Air-blast direct heat. Air-blast tunnel batch type University Farm type Air-blast direct heat University Farm type Stack type Gravity Air Flow Stack type Gravity Air Flow Ceramic Oven Ceramic Oven Fruit Velocity of air across trays. Linear feet per minute Total volume of air. Cubic feet per minute Volume of air per 100 sq. ft. of tray surface Cu. i\. per minute Grapes 424 20,800 290 Grapes 485 15,800 275 Prunes 510 44,000 255 Grapes 450 17,500 250 Grapes 265 8,600 250 Grapes 197 7,486 235 Prunes 357 Less than 11,390 200 Prunes 20 Less than 4,500 130 Prunes 20 Less than 4,800 100 Grapes 20 6,017 110 Apples 20 6,660 105 Approx- imate drying time. Hours 24 24 24 18-24 18-30 22 30 30 36 60 18 To evaporate one pound of water at the average temperature used in dehydrating fruit requires the heat furnished by 1750 cubic feet of air dropping 40° F., but one pound of water vapor will saturate only 235 cubic feet of air at 110° F. For example, if dry air enters the drying chamber at 150° F. and leaves it saturated with water at 110° F., 235 cubic feet will carry away one pound of water. But under these conditions with a 40° temperature drop, 1750 cubic feet is necessary to evaporate one pound of water, or more than 7 times as much as that required to. carry away the evaporated moisture. The ratio will be less than 7 :1 unless the entering air is perfectly dry, or the escaping air completely saturated with moisture. For example, Bulletin 337] SOME FACTORS OF DEHYDRATER EFFICIENCY 285 if the entering air is 10 per cent saturated with moisture vapor at 150° F., and the exhaust air is at 110° F. and saturated with moisture vapor. 335 instead of 235 cubic feet will be required to absorb one pound of water vapor from the fruit. Again, if the entering air at 150° F. is 10 per cent saturated and the exhaust air at 110 F. is only 75 per cent saturated with moisture, 522 cubic feet of air will be required to remove one pound of water vapor from the fruit ; the ratio between the amount of air required to furnish heat and that necessary to remove the water vapor becoming 3.35 :1. For simplicity the above calculations disregard the slight differences in the volume of air caused by changes in temperature. Knowing the tons of green fruit per charge, the drying ratio of the fruit and the estimated drying time, it is possible to calculate the minimum air-flow requirement for a dehydrater. For example, if a dehydrater is to hold ten tons of prunes with a drying ratio of 2y 2 :1, and is to dry the fruit in 24 hours, it will be necessary to remove 12,000 pounds of water per 24 hours or 8.3 pounds per minute. If the temperature drop is 40° F., there will be required 8.3 X 1750, or 14,525 cubic feet of air per minute. If various heat losses are included, the designer should allow at least 20,000 cubic feet of air per minute for a dehydrater of this size. The importance of adequate air flow cannot be over-emphasized. More dehydraters have failed because of insufficient air supply than for all other reasons combined. It must be realized that at least five times as much air is usually necessary to furnish heat for drying as is required for the removal of the evaporated moisture. With these principles in mind, the air flow in a number of dehydraters was studied during the past two seasons. The results obtained are given in Table 4. The air flow in all instances was determined by means of an anemometer. This instrument was placed in various positions in the drying tunnels or air ducts and the results represent in each case the average of several determinations. The anemometer is in common use by heating and ventilating engineers. It is an inexpensive and useful instrument for the operator of a dehydrater. Some typical air-flow measurements are given in Table IV. Unfortunately, an exact comparison of these data on air-flow and drying times is impossible because of the effect of many other variables, such as variety of fruit, treatment before drying, load per square foot, temperature, humidity, moisture content of finished product, etc., each of which directly affects the drying time. Experiments are now in progress in which it is hoped to establish more exactly the inter- 286 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION relations of all the factors. However, the writers feel from their observations of the past two seasons that air-blast dehydraters, to be reasonably efficient, should be supplied with a velocity of air over the trays of not less than 500 feet per minute and a volume of at least 250 cubic feet per minute for each 100 square feet of tray surface. Most of the dehydraters tested were operated at temperatures of 165° F. to 185° F. at the "finishing point." The best quality with Fig. 3. -Anemometer used to determine the velocity of air in dehydraters. most fruits is most readily attained by finishing at temperatures not in excess of 150° F., but with the same volume of air per unit of drying surface drying is less rapid than at 165° F. Within certain limits, the same rate of drjdng at lower temperatures can be main- tained by increasing the volume of air. Owing to the facts that horsepower increases very rapidly with increase in air velocity and that a certain minimum time is required for any fruit to give up its moisture to the surrounding air, increase of air velocit}^ beyond a certain maximum becomes uneconomical. Although this maximum will vary greatly with the variety of fruit and its preliminary treat- ment, the maximum efficient velocity for most products would prob- ably not exceed 1100 feet per minute. The horse power necessary to operate a fan increases proportionately to the cube of the revolutions BULLETIN 337] SOME FACTORS OF DEHYDRATER EFFICIENCY 287 per minute, and roughly in proportion to the volume of air delivered and to the velocity across the trays. Taking all factors into con- sideration, economical drying can best be obtained by a velocity of not less than 500 feet per minute for fruits which case-harden, and a velocity of at least 750 feet per minute for freely drying fruits. In the drying of grapes, sliced apples, and other freely drying sub- stances, it is probable that velocities of 800 to 1000 feet per minute would sufficiently accelerate drying to compensate for the increased cost of power for the fan. Dehydraters depending to a considerable extent upon direct radiation of heat require less air than air-blast dehydraters to accomplish the same amount of drying within the same time. Such dehydraters are, however, limited in their rate of drying by the amount of heat reaching the fruit by direct radiation and by the velocity of air flow which it is possible to obtain by natural draft. Attempts to equip stack driers with fans have not proved satisfactory because their construction does not permit uniform air distribution. Our observations on the air flow in natural draft dehydraters are insufficient to base recommendation on regarding minimum air-flow requirements. However, natural-draft dehydraters even of the most approved design give a slower rate of drying, a less uniformly dried product, and a lower fuel efficiency than the best air-blast dehydraters. These advantages of the latter are obtained for a smaller initial plant investment, when the drj-ing capacity is considered. The "Parallel Current" System of Dehydration. — In most tunnel dehydraters the fresh fruit enters at the air exhaust end and the dried fruit leaves at the air intake end of the drying compartment. During drying, the fruit is moved from air of moderate temperature (100° F. to 120° F.) at the start of drying to temperatures of 150° F. to 190° F. near the end of the drying period. This is termed the "counter current" system. During the first stages, very little drying occurs because of the moist condition and relatively low temperature of the air. The drying process is completed in air of high temperature and low relative humidity, conditions that favor case-hardening and scorching of the fruit. In the so-called "parallel current system," the fruit enters at the air intake end of the drying compartment and is taken from the dehydrater at the air exhaust end. In other words, the drying process is started in hot, dry air and is completed in warm, moist air. For some fruits this system possesses the following advantages : 1. Evaporation of the surplus moisture is very rapid during the initial stages of the drying period when the fruit is moist and in the best condition to give up its water. 288 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION 2. The wet fruit is more nearly at the temperature of the wet- bulb thermometer because the fruit contains sufficient moisture to maintain a rapid rate of evaporation which reduces its temperature proportionately. This permits higher drying temperatures than are now used, thus still further increasing the rate of drying. In the ' ' counter current ' ' system the fruit near the end of the drying process, because of its low moisture content and slow rate of drying, is very apt to approach the temperature of the hot, dry air and become scorched and carmelized. The "parallel current" system takes fuller advantage of the great drying power of air direct from the heating chamber. 3. The fruit gradually progresses during drying toward a region of lower temperature and higher humidity so that scorching and over- drying are avoided. 4. The fruit emerges after drying at a relatively low temperature so that much less heat is carried to the outside atmosphere by heated cars, trays, and fruit than is the case with the "counter current" system. The "parallel current" system therefore conserves heat, A preliminary test of this method was made in a large commercial dehydrater. Two carloads of grapes which received the high initial temperature, dried so rapidly that it was necessary to remove them from the tunnel several hours before cars which had received a low initial temperature. Further tests on the "parallel current" system, as applied to apples and cherries have been conducted in the Fruit Products Laboratory and in commercial plants with favorable results. Static Pressure and Recirculation. — Recirculation of a large pro- portion of the air used in drying conserves fuel and makes possible the regulation of the humidity of the air, which is a factor of great importance in the drying of fruits which tend to case-harden. Under average conditions, from five to eight times as much air is required for heat transfer as for moisture removal. Therefore, it is often possible to return 75 per cent or more of the air to the heating chamber. This fact has been successfully utilized in many of the dehydraters built during 1919 and 1920. Return of the air to the heating chamber doubles the distance traveled and consequently increases the load upon the fan and motor. The return air duct, therefore, must be of large cross-section in order that the static pressure (air friction) therein shall not be excessive. If this return duct is too small, it will greatly reduce the volume of air handled by the fan and tend to counterbalance the advantages of recirculation. Bulletin 337] some factors OF dehydrater efficiency 289 For the same reason, the air delivery duct between the fan and the drying tunnel must be as short and direct as possible and large enough to avoid serious static pressure. The velocity of the air in ducts leading to and from the fan should not exceed the velocity of the air across the trays, i.e. 500 to 1100 feet per minute. Velocities of over 3500 feet per minute have been observed in the small air passages of several dehydraters. This causes very high static pressure and low air flow in the drying compartment. Fig. 4. — Recording thermometer, showing a 24-hour temperature record during dehydration of grapes in the University Farm dehydrater. Static pressure may be considered as the pressure necessary to overcome the frictional resistance offered to the flow of air. It is measured by a Pitot tube and is expressed as "inches of water pres- sure.' There are three pressures to consider in Pitot tube measure- ments : first, static or frictional resistance pressure ; second, velocity pressure due to the velocity of the air ; and third, the total or impact pressure, which is the sum of the static and velocity pressures. Since velocity pressure is the difference between the total pressure and static pressure, any increase in static pressure will result in a decrease in 290 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION velocity pressure and consequently a decrease in volume of air de- livered. The effect of increased static pressure can be overcome to a certain extent by increasing the speed of the fan, but this in turn involves increased expenditure of power. TABLE V Comparative Static Pressures in Various Dehydraters Type of Dehydrater Static Pressure in Inches of Water Plant No. At Fan Intake (Suction) At Fan Discharge (Pressure) Total E University Farm, Davis, with partial recirculation - .65 - .93 - .81 -1.63 -1.76 - .85 - .16 .51 .86 .43 .07 .13 .67 .74 .52 E N University Farm, Davis, with total recirculation Ceramic Oven .56 1.16 G G Air-blast tunnel, direct heat Air-blast tunnel, direct heat, no recirculation 1.79 1.24 G Air-blast tunnel, direct heat, comolete recirculation 2 63 Air-suction tunnel, no recircula- tion 1.70 Air-suction tunnel, partial re- circulation 1.89 Air-suction tunnel, total recir- culation 2.19 P Air-suction tunnel, tray and slide type .70 B University Farm type No recirculation 1.52 Total recirculation .90 P No recirculation. Smaller model than above dehy- drater P 1.28 The excessive static pressures found to exist in several dehydraters ar^ due to the use of very narrow and crooked air ducts. Air ducts should contain as few bends as possible ; where bends are necessary, these should be wide, to reduce air friction to a minimum. Taking into account the precautions necessary in such measure- ments, determinations were made of the static pressure in several dehydraters, with the results shown in Table V. It is evident from this table that the assumption frequently made that the static pressure in tunnel dehydraters would be y 2 to 1 inch is too low. Estimates based on this assumption have led to disappointment in the perform- ance of certain dehydraters. Since the air velocities in the different BULLETIN 337] SOME FACTORS OF DEHYDRATER EFFICIENCY 291 plants tested were not the same, no exact comparisons of static pressures can be made, because static pressure varies directly as the square of the air velocity. The effect of high air velocity on static pressure is shown in Plant 0, in which high air velocity is used. The volume of air delivered by a fan decreases as static pressure increases. Thus, a Number 7 Sirocco fan revolving at 350 r.p.m. has a rated capacity of 33,000 cubic feet per minute at one inch static pressure, while at the same speed and two inches static pressure the capacity is reduced Fig. 5. — Two types of speed indicators for determining the speeds of motors and fans. to 17,000 cubic feet. A direct test was made at the University Farm dehydrater to determine the effect of the size of the return flue on the volume of air delivered. With a return flue of two square feet in area and complete recirculation, 5150 cubic feet of air per minute passed through the return flue. When the return flue was increased to four square feet in area, the volume of air increased to 6250 cubic feet per minute or an increase of over 20 per cent. In one type of dehydrater manufactured in California the cross-section area of the air duct connecting the fan and the drying tunnel was about 1.7 per cent of the cross-section area of the drying compartment. Static pressure in this case so reduced air flow that the drying time of one variety of fruit was about four times as long as in well designed plants. 292 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Air Distribution. — Some dehydraters, though equipped with fans and motors of sufficient capacity, have not dried the fruit so rapidly as they should have done with the volume of air furnished. Investi- gation proved that an excessive proportion of the air was flowing above or below the cars, between the cars, or between the cars and the walls. Such air, of course, accomplishes no drying. A typical instance will indicate the importance of this factor. A certain dehydrater loaded with about 25 tons of prunes gave air-flow readings behind the last car as shown in Table VI, Column 2. Baffles were then placed below the car frames and on the ceiling of the dehydrater, and the air-flow readings given in Column 3 were then obtained. Before baffles were installed, over 75 per cent of the total volume of air passing through the dehydrater was flowing beneath or above the cars and accomplishing little or no drying. The installation of baffles increased the velocity of air flow across the trays about 42 per cent and shortened the drying time materially. In other dehy- draters the velocity of the air was much higher near the top of the cars of trays than in the center of the load or near the bottom. In a few instances the greatest velocity was found near the bottom of the stack of trays. Such irregularity of air flow causes uneven rates of drying. Uneven distribution can be overcome by means of suitable dampers at one or both ends of the tunnel. Also if the intake or discharge of the fan be placed below the center of the tunnel, it will counterbalance the tendency of heated air to rise. By the use of an anemometer the velocity of the air in different parts of the dehydrater can be deter- mined and it is then usually possible to correct faulty air distribution. Fans. — Previous conclusions given in our Bullettin 322, The Evaporation of Grapes, in regard to the relative efficiency of different types of fans were confirmed by observations during the past season. Disc fans are not satisfactory for use in long tunnels or against high static pressure because they cannot force air against high resistance. They can be used for short tunnels in which there is wide clearance (at least 3 inches) between trays. Such fans are inexpensive and therefore suitable for "rain damage insurance" driers. However, for the greatest efficiency a multivane or steel plate fan is necessary, despite the greater cost. These fans are capable of overcoming high static pressure and make possible the use of a much higher air velocity than can be obtained with the disc fan. Trays. — The size and design of trays in use vary almost as much as the dehydraters themselves. Although direct comparisons were not made of all styles of trays, the relative efficiency of the more common types were determined. Bulletin 337] some factors of dehydrater efficiency 293 TABLE VI Effect on Air Distribution of Proper Placing of Baffles Location of Test Velocity of air between trays near top of car Velocity of air between trays near bottom of car Velocity of air below cars Velocity of air above top tray Velocity Before Installing Baffles Ft. per Min. 320 400 1500 2800 Velocity After Installing Baffles Ft. per Min. 600 420 500 500 It was found that trays 3' X 6' or 3' X 8' were not satisfactory where the air flows across the greater length of the tray, because the fruit on the end of the tray nearest the heating chamber dried much more rapidly than that on the opposite end of the tray. Air during its passage across such long trays loses a great deal of its drying power because of increased humidity and decreased temperature. Reversal of the air current during drying does not entirely overcome the defect because there is then a tendency for the fruit in the center of the tray to dry more slowly than that at the ends. It is believed that forty inches should be the maximum length for trays, unless the 3' X 6' or 3' X 8' trays are placed crosswise to the direction of the air flow. Very large trays are necessarily ' ' two man ' ' size and inconvenient for stacking. Trays 36" X 36" or 36" X 40" have proved very satis- factory for tunnel dehydraters and are more durable than larger trays. Where it becomes necessary to use field trays, as in "rain damage'' dehydraters, blocks must be placed between them so that there is at least two inches of free space between their edges. Where this is not done, air flow is so much impeded that drying becomes excessively slow. One-inch blocks are usually sufficient for 2'X3 r raisin trays because the sides of these trays are only about % inch in height and the ends 1% inches. A small stack evaporator was used experimentally at the Univer- sity Farm during 1920. The fruit was dried on 2' X 3' wooden trays with 3-inch sides and slat bottoms. The trays were satisfactory for this type. Slat bottom trays are preferrable to galvanized iron screen trays for all fruits that require sulfuring, such as white grapes, apricots, peaches, pears, and apples. Galvanized screen trays corrode rapidly in sulfur fumes and impart a disagreeable metallic flavor to the fruit. Peeled peaches and very juicy fruits stick badly to wooden trays, 294 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION although sticking is slightly reduced by coating the trays with a neutral mineral oil each time they are used. Many different kinds of paints and protective coatings for screen trays were tested. So far, no satisfactory material has been discovered. All the paints tested either soften and stick to the fruit because of the high temperature or become brittle and scale off the screen during the removal of the dried fruit. Fig. 6. — Angle stem thermometers. Convenient for insertion through walls of drying compartment. It has been found that dry fruit may be most readily removed from the trays as soon as it has cooled after removal from the dehydrater. If an attempt is made to scrape the fruit from the trays while still warm and soft, the fruit is broken and syrup is forced from it, causing it to stick to the trays; but, if left several hours, it often tends to sweat or soften by diffusion of moisture toward the surface and is then also difficult to remove. Often fruit which will "rattle" on the trays when first taken from the dehydrater will become softened and pliable after several hours standing. This effect is due to sweating after case-hardening. BULLETIN 337] SOME FACTOTS OF DEHYDRATER EFFICIENCY 295 Control of Humidity. — For the rapid and uniform drying of certain fruits, especially halved pears, peaches, and large prunes, it is necessary to use air of relatively high humidity. Very dry air causes such fruits to case-harden, a condition which results in very slow drying. Moist air permits diffusion of water outward to keep pace with evaporation from the surface and thus prevents case- hardening. The desired humidity at the air-intake end of the tunnel cannot always be maintained by recirculation of the air, as the fol- Fig. 7. — A convenient and accurate form of hygrometer for the determination of the relative humidity of air in dehydraters. lowing consideration demonstrates. Air at the exhaust end of the tunnel, if at 110° F. and 80 per cent relative humidity, when returned to the furnace room and reheated to 160° F. and mixed with about 25 per cent of its volume of outside air, is reduced below 20 per cent relative humidity, whereas it is essential in some instances that the humidity be increased to 35 or 40 per cent. It was found that steam admitted to the air return duct did not bring about the desired result because though the relatively cool air was saturated with steam it became greatly reduced in relative humidity as it passed through the 296 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION air heating system. It is necessary, therefore, to introduce the steam into the reheated air as it leaves the air heating system on its way to the drying chamber. Allowing water to drip upon the furnace and radiating pipes gave fair results. Much better results were obtained by the use of two cyclone spray nozzles set at such an angle in the furnace room as to play a fine spray of water against the furnace and pipes and into the air stream. Humidities of 30 to 45 per cent at temperatures of 145° F. to 155° F. were easily maintained by this means in the University Farm dehydrater. It is recommended that all air blast tunnel dehydraters be equipped with an air humidifying device. Steam heated dehydraters can easily be equipped with open steam jets for humidity control. Dipping Eqmpmemt. — Wine grapes and prunes are usually dipped in a dilute boiling lye solution before dehydration in order to check the skins and thereby increase the rate of drying. Several types of dippers are in use for this purpose. One of these is the common hand-power dipper found in many prune dry yards in California. This machine is fairly satisfactory but the heating equipment has often proved inadequate. "Where forced draft oil burners are installed instead of wood burning grates or gravity distillate burners, hand- power dippers have been fairly satisfactory. Hand-power dippers may be attached to a cam shaft and operated by a small gasoline engine. The revolving drum type of dipper used with success for prunes has not been satisfactory for grapes because of the difficulty of regula- tion and the shattering of grapes from the bunches. A continuous draper scalder for grapes was manufactured during the past season. It was of large capacity and it was found impossible to furnish sufficient heat by means of a furnace and hot water circu- lation to maintain the lye solution at the boiling point. However, by connecting the dipper to a 20 h.p. boiler, good results were obtained. One of the most successful dipping machines used during the last season was a lye-spray type of peach peeling machine. In this machine the fruit is carried on a broad, metal cloth conveyor, beneath sprays of hot water which heat the fruit, then through sprays of boiling lye and finally through sprays of rinsing water. By regulating the concentration of lye and the speed of the conveyor, dipping may be ac- curately controlled. A 25 h.p. motor and a 25 h.p. steam boiler are re- quired. A royalty must be paid upon all fruit dipped in this machine. Regardless of the type of machine used, it is essential that the lye solution be maintained at proper strength and at or very near the BULLETIN 337] SOME FACTOTS OF DEHYDRATER EFFICIENCY 297 Dry 3ulb / % / / / . . / / /. ,/ s\ RtLATivc Humidity Chart paromew 2S>92fof Ha Ml -3a« FrjncJsc^, CjI Degrees fchrefthefj Dry Bulb Fig. 8. — Chart showing the humidity of air from the wet and dry bulb ther- mometer readings. (Drawn by G. B. Kidley.) boiling point. The proper strength of lye solution can be readily determined by an experienced operator from the appearance of the dipped fruit, although it is feasible to use a simple method of deter- mining it by means of titration with a standard acid solution. Grapes should be rinsed in water after dipping, in order to remove adhering lye solution, which tends to darken the flesh, injure the flavor, and form a white deposit on the dry fruit. Sprays are used in continuous dippers and a second vat supplied with fresh water should be used with hand-dipping outfits. 298 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION ACKNOWLEDGMENTS The investigations reported in this bulletin were made possible by funds from the appropriation for investigations in Deciduous Fruits made by the state legislature of 1919. The writers are indebted to the many manufacturers, owners, and operators of dehydraters who made possible the securing of much of the data reported herein. They also wish to express their appreciation to Professor F. T. Bioletti for helpful revision of the manuscript. SUMMARY AND CONCLUSIONS 1. The cost of a dehydrater erected by the average fruit grower for operation during a season of only one or two months must be as low as is compatible with reasonable efficiency if it is to be profitable. 2. A completely equipped and satisfactory dehydrater can be built for $500 or less per green ton capacity per 24 hours. 3. The air-blast tunnel type of dehydrater is the most economical to operate in regard to both fixed charges and operating costs. 4. For efficiency, the velocity of air across trays should not fall below 500 feet per minute, while the total volume of air per 100 square feet of tray surface should not be less than 250 cubic feet per minute. 5. In order to reduce static pressure and secure maximum fan capacity, all air passages should be as large in area, as short in length, and as direct as possible. 6. Inefficiency will result unless all the heated air flows between the trays of drying fruit and is equally distributed among the several trays. This can be accomplished readily by proper relative dimensions of the drying chamber and trays, supplemented by the intelligent use of baffles and dampers. 7. Multivane or steel-plate fans, although more costly, more than repay their extra cost by their greater efficiency, especially in large dehydraters, where high static pressures must be overcome. 8. Fruits which are sulfured should be dried on wooden trays, preferably with slat bottoms. Unsulfured fruits are most rapidly dried on screen trays. 9. Air of 20 to 50 per cent relative humidity is advantageous in the dehydration of fruits which case-harden readily. Such moist air permits the steady evaporation of moisture from the fruit at relatively high temperatures. 10. Prunes and grapes are most rapidly dried if previously dipped in a boiling lye solution. The first requisite of any dipper is a source of heat sufficient to maintain the lye solution boiling constantly during operation.