Document jgnJ87bMdnJwgD5NpkzwLRE62

582 CHAPTER 55 1962 Guide And Data Book as in the previous example. The heat absorbing capacity of the meat in wanning to 40 F during the trip would then be 30,000 X 0.77 X 5/24 - 4810 Btuh In this case the unit would only have to supply a cooling capacity of 6000--4810=1190 Btu per hr as compared to 15,620 Btu per hr. This is a very significant factor, and the only difference between the two cases is a 15 F difference in the temperature of the product at time of loading. Sometimes a different situation occurs. The unit operates continuously during the haul and the product climbs slowly but steadily in temperature. -When this happens it is a clear indication that the unit is not adequate, not operating prop erly or not applied properly to provide the required refrigerat ing capacity. Improper air circulation within the vehicle is a major cause of inadequate refrigeration.*'11'14 The top of the load may actually be undercooled several degrees but, be cause the bottom or sides of the load are not cooled, the aver age product temperature climbs, with actual spoilage occur ring at hot spots in the load. In order to protect the product in these potential hot spots, the rest of the vehicle is some times purposely undereooled. Under these conditions, the return air temperature at the unit is lowered, and the capacity of the unit decreases. If it is necessary to operate with --20 F return air to keep a hot spot in the load at satisfactory tem perature, the capacity of the unit may be reduced as much as 50 percent as compared to normal operation at 0 F return air. At the same time the heat load is probably greater because of tiie undercooling of part of the vehicle and load. Many times, because of improper air distribution, the thermostat will cycle the unit at 0 F return air temperature and hot spots in the load are not detected until the vehicle is unloaded. Example 1 illustrates the load calculation and selection of the eutectic plates for a retail delivery truck for frosen foods. Example l: Determine the total heat gain, net heat load, area of eutectic plate required, and minimum condensing unit ca pacity for refreeziiig the plates, for a frozen food delivery truck. The inside dimensions of the truck body are 160 in. long, 76 in. wide, and 80 in. high. The mean surface areas are: roof and floor - 99.2 sq ft each; side walla, 94.5 X 2 = 189 eq ft; front and rear, 49 X 2 = 98 sq ft. The body is insulated with 6 in. of a material having a thermal conductivity k of 0.27 Btu per (hr) (sq ft) (F deg per in.). The maximim ambient temperature is 100 F, with the marimum roof temperature asmimed to be 120 F, and the maxi mum floor temperature assumed to be 110 F. The temperature inside the body is 0 F. The truck makes 60 stops (a heavy service load) during a 10 hr delivery schedule. The average product load is 5900 lb of frozen food having a specific beat of 0.35 Btu per U>, loaded daily at --15 F. The eutectic plate temperature (as recommended by the manufacturer) is --8.5 F. and the plate has ` a thermal conductivity k of 2.0 Btu per (hr) (sq ft) (F deg). Allowance* to be used are:framing and other heat gam* = 20 per cent of the heat transmission through the insulation; service load =* 100 percent of total heat gain: heat absorption of the product -- H product load wanning to 0 F. Solution: The heat transmission is calculated through the insulation, since the additional film resistances are negligibleThe coefficient of transmission is, therefore, 0.27/6 = 0.045 Btu per.(hr) (sq ft) (F deg). Heat Transmission Through Insulation Roof = 99.2 X 0.045 (1204)) Floor = 99.2 X 0.045 (1104)) Side Walls = 189 X 0.045 (1000) Front and Rear = 98 X 0.045 (1000) Btuh - 537 - 491 = 851 = 441 Total Framing and other heat gains (20%) - 2,320 . = 464 Service load (100%) 2,784 - 2,784 Total heat gain = 5,568 Total heat load for 10 hr trip Heat absorption by product (10 hr) (0.5) 5900 X 0.35 (0 - ( - 1S)J -- 55 g~. - IS.Sqq Net heat load for 10 hr trip Net heat load . -* 40 m*. -- 4joig The plate area required is 4018/(2.0 X 8.5) = 236 sq ft. Plates are selected to provide this area. Using 2 plates mounted on the ceiling and 2 plates mounted on the walls, each plate could be 36 X 120 X 1 in. thick. Such plates would have an approxi mate weight of 1360 lb. The total holdover capacity of the plates at - 8.5 F (from manufacturer's data) would be 48,500 Btuh. If 12 hr a day are available to refreese the plates, the minimum densing unit capacity for refreezing is 10 br/day load + 14 hr/oight load 12 40,180 +35,750 12 6330 Btuh It should be noted that the above capacity is required at the prevailing ambient temperature and at --23 F suction. All figures and steps given here are general Manufacturers' recommendations for selection of plates *r.H condensing unit should be obtained for particular applications. If outside truck surface area had been used for determining insulation hn>t gain instead of mean surface area, the difference, 13 percent, would have accounted for a part of the 20 percent allowed in the example for framing and other heat gains. Another method used to allow for these extra heat gains is to use out ride area and also increase all k factors by 10 percent If this had been done in the example, the total allowed would have been 23 percent instead of 20 percent. If the truck was to be used for a 15 hr trip instead of the 10 hr trip figured, and the hourly load was about the same, the additional capacity could be obtained by increasing the thickness of the plates. For example, plates V/i in. thick, instead of the 1 in. in the example, would provide holdovercapacity of about 83,400 Btu as compared to the 48,500 Btu capacity of the 1 in. plates. If credit had not been allowed for product warmup from --15 F to 0 F, the hourly refrigerating load would have been 5100 Btu per hr instead of 4020 Btu per hr. Both plate area and holdover capacity would have to be increased. The plate sur face could be reduced if a eutectic temperature of --13 F was used instead of a eutectic temperature of -- 8.5 F as figured. Empirical values of service load have been used in many ways to determine how much refrigerating capacity is needed because of door openings during a delivery run. The assump tion that this load is related to the heat transfer of the body is not realistic, but, up to now, no other general method is used. Careful study of the table on page 12 of the report* by Guilfoy, in addition to showing that a curtain at the door of a retail delivery truck reduces the entry of warm air into the vehicle, shows that the average air temperature in the vehicle during the entire delivery period of 8V hr was higher than the product temperature at the end of the day. This suggests three conclusions: (1) the refrigeration capacity of the trucks was inadequate; (2) the product was cooling the air surround ing it; and (3) the service load had no functional relationship to the heat transfer of the refrigerated body. The need for further study of methods for determining the service load of a delivery vehicle is clearly indicated. In the example shown, the service load was assumed to be 2780 Btu per hr, equal to 100 percent of the baric heat transfer plus an estimated 20 percent for framing, air leakage and other heat gains. If it is. assumed that an air change of only one-third of the vehicle in* tenor volume, or 186 cu ft, occurs at each door opening the heat gain per door opening would be 495 Btu if the ambient Trucks and Trailers 583 j^npyature was 80 F at 50 percent rh, or 750 Btu, if H was lOO F and 50 percent rh. With 60 door openings in 10 hr, not including loading and unloading at the beginning and end of the day the service load heat gain, then, would be 2970 Btu ,,er hr at. 80 F and 50 percent rh, and 4500 Btu per hr at 100 F aod 50 percent rh. It is possible that more than one air change occurs at each door opening. In Example t, the calculations for mechanical refrigerating equipment for a long-haul trailer for frozen foods are illus trated. Example t: Determine the total heat gain and minimum set MLnaritv of the cooling unit for a long-haul frozen food trailer ^hinside body dimanainnn of 39 ft length, 7 ft width, and 7 ft foieht. The r"Mri surface areas are: roof and floor = 296 sq ft fX: ride walls - 296 X 2 - 592; front and rear = 56.2 X 2 ,, 112.4 sq ft The trailer is insulated with 6 in. of a material having a thermal conductivity k -- 0.27 Btu per (hr) (sq ft) (F dec per in.). The trailer body temperature is 0 F, with a nuuri- nJumambient temperature of 100 F. Maximum roof and floor- temperaturra are assumed to be 105 and 110 F, respectively. The average length of trip is in excess of 48 hr, with an average product load of approximately 40,000 lb of frozen food having a specific heat of 0.35 Btu per lb, loaded at -- 5F. Allowances to be made axe: framing and other heal gains -- 35 percent of heat transmission through insulation; increased heat gain due to mtjp* 25 percent of total heat gain; and refrigeration effect rfproduct wanning to OP. ...... Solution: As in Example 1, the beat transmission ts calculated through the insulation only, neglecting the effects of film re- Beat Transmission Through Insulation Roof = 296 X 0 045 (105-0) Floor = 296 X 0.045 (110-0) Side Walla - 592 X 0.045 (10<M)) Front and Rear = 112.4 X 0.045 I KM = 1,400 - 1,465 - .2,660 = 506 Framing and other heat gains (35%) - 6,031 - 2,110 Increased heat gain due to motion (air leakage, etc.) 8,141 - 2,040 Total heat gain = 10,181 The minimum set capacity of the cooling unit required, without allowance for product warm-up is.10,181 Btuh. (The unit prob ably should be selected for thia capacity because subsequent loads may not be undercooled.) B allowance is made for the product warm-up, the minimum capacity can be reduced. The refrigerating effect of product warm up ia - M.000 X 0.3510 -(-- j-_ 1460Btub48 The net capacity would be 10,181 -- 1460 = 8721 Btuh. Note that the roof temperature was assumed to be 105 F for this trailer when the ambient air temperature was figured at 100 F, while the roof temperature for the truck was assumed to be 120 F. This is because experience has shown that the effect of incident solar radiation is significantly reduced when the vehicle is in motion as compared to being stationary. In asmuch as the trailer is presumed to remain essentially in mo tion the lower figure was used. Similarly, a 25 percent increase over total transmission heat grins was figured to allow for the probable increase in heat gain due to increased air infiltration caused by the pressures created by the forward motion of the vehicle. When the vehicle is stationary in bright sunlight, the tolar heat gain will increase and the air infiltration heat grin rill decrease, tending to offset each other. The allowance for heat gain due to framing members and *** leakage can be reduced as trailer construction improves and the amount of conductive framing is lessened, the amount of air infiltration is reduced, and the air passages through the sections of insulation are more nearly eliminated. If air leakage could be eliminated entirely, tiie increase in heat gain due to motion would be less than the increase in heat gain due to solar radiation on the roof when the vehicle is stationary. As mentioned previously, the effect of solar heat grin is not more than 15 percent over the baric heat gain in an ambient tem perature of 100 F, if the vehicle is stationary during tiie hours of sunlight. Careful selection of materials may reduce this amount. REFERENCES I W. EL Petersen: Refrigeration of delivery vehicles (Refriger ating Engineering, April 1951, p. 351). * M. B. Green: How to truck frozen foods (Refrigerating Engineering, November 1956, p. 44). EL W. Krotzer: Truck refrigeration (Refrigerating Engineering, October 1945, p. 324). 4 P. B. Reed: Refrigerated transport (Refrigerating Engi neering, August 1946, p. 115). * M. V. Stagg: Low temperatures on the highway (Refuger ating Engineering, March 1946, p. 231). * State Size and Weight Limits for Trucks and Truck-Trailers (Truck-Trailer Manufacturers Association, Inc., 710 Albee Building, Washington 5, D. C.). * D. W. Kuenzli: Effect of New Trailer Designs on Transit Tem peratures of Settled Frozen Foods--Frozen Pies and TV Dinners (AMS Report No. 446, Agricultural Marketing Service, Trans portation and Facilities Research Division, USDA, June 1961). * M. V. Genity and H. D. Johnson: Motortruck Transportation of Freddy Killed Beef (Marketing Research Report No. 119, Transportation and Facilities Branch, USDA, June 1956). * W. H. Redit ci al: Transportation of Frozen Citrus Concentrate by Railroad and Motortruck from Florida to Northern Markets (Agricultural Information Bulletin No. 62, Bureau of Plant Indus try, Soils, and Agricultural Engineering, USDA, June 1951). t* H. D. Johnson and C. E. Garver: Test of a Mechanical Re frigerating Unit Designed to Maintain Low Temperatures in Motortruck Transportation (Marketing and Facilities Branch, USDA, March 1953). u H. D. Johnson and M. V. Gerrity: Report of Tests on Trans portation of Frozen Poultry with Mechanically Refrigerated Trucks (An Interim Report) (AMS Report No. 144, Marketing and Facilities Branch, USDA, May 1952). ** D. W. Kuenzli: The Cold Wall Trailer Maintaining Frozen Pood Below Zero (AMS Report, Agricultural Marketing Service, Transportation and Facilities Division, USDA, 1962). u Zero hauling comes of age (Fleet Owner, June 1961, p. 69, 11page reprint available). 14 P. R. Achenbach: Chilled-air distribution in refrigerated trailers (Proceedings of Commission 7 Meeting m Padua, Italy, June 1961, International Institute of Refrigeration, 177, Boule vard Malesherbes, Paris (17e) France). " C. P. Lentz, E. A. Rooke, and M. A. Foley: Performance Tests with a Refrigerated Trailer (NRC Report No. 6302, Division of Applied Biology, National Research Council, Ottawa, Canada, 1961). >* H. D. Johnson and P. L. Breakiron: Protecting Perishable Foods during Transportation by Truck (Agriculture Handbook No. 105, Transportation and Facilities Branch, USDA, December 1956). II Zero hauling problems (Fleet Owner, February 1962, p. 71). * H. D. Johnson, R. F. Guilfoy, and R. W. Penney: Transpor tation of Hanging Beef by Refrigerated Rad Cars and "Piggyback" Traders (Marketing Research Report No. 485, Agricultural Mar keting Service, USDA June 1961). J. L. Mischou: Liquid nitrogen boomed as intransit re frigerant (Food Engineering, August 1951, p. 67). ** Will foamed insulation help? (Cornmercial Car Journal, March 1961, p. 100). * George Chieger: Design and testing of refrigerated van bodies (Trader/Body Budders, April 1961, p. 24). * S. W. Eby ana R. L. Collister: Insulation in refrigerated transportation body design (Refrigerating Engineering, July 1955, p. 51). ** C. W. Phillips et al: A Rating Method for Refrigerated Trader Bodies Hauling Perishable Foods (Marketing Research Report