Document e5Rpz4rLneYwOZ1Bd4RBXOxY4

562 CHAPTER 53 The refrigeration loads and unit selections may be deter mined by the foregoing methods. bibliography D. E. Brady: Curing important in Southern lockers (Qutefc Frozen Poodt, July 1944, p. 53). R. D. Haynes and W. E. Garner: Locker room usage heat loads (Repbioesaung Engineering, April 1951, p. 359). D. C. McCoy: Comments on borne freezers (luraiGBaanNO Engineesino, March 1945, p. 194). Operation of Cooperative Frozen Pood Lockers in Illinois (Farm Credit Administration, USDA Special Report No. 77, January 1941). 1962 Guide And Data f A. Todoroff: Roto to Build and Operate a Locker Plant (v. Hoffman Press, St. Louis, Missouri, 1946). * D. K_ Tressler and C. F. Evers: Into the Freezer and Out /A-Publishing Co., Westport, Conn., 1953). D. K. Tresaer and C. F. Evers: The Freezing Prcsmcium Foods (Avi Publishing Co., Westport, Conn., VoL t and 2,1957? D. K. Treater: Home frewere--presentand future (RerBinT-i' ATINQ Engineering, February 1945, p. 97). H; P. White, W. M. Hurst, and W. E. Gamer: Frozen Locker Plants in Georgia, (College of Agriculture, University of Georgia, 1947). P. C. Wilkins, L. B. Mann, and B. D. Miner: 1955 Suren m Frozen Food Locker Plants (Utilisation Research Report L Fanner Cooperative Service, USDA, May 1957). H CHAPTER 54 PRECOOLING of Precoofihg, Cooling with Air, Cooling with Contact Ice, Cooling with Top Ice, Cooling with Water, Cooling with Vacuum, Effects of Containers and Stocking Patterns THE term precooling as used in the produce industry refers cooling will be governed by the volume and velocity of the re to the rapid removal of heat from a commodity prior to frigerating medium. The rate of heat transfer is directly re tfaipment or storage. Since the term rapid is relative it may, lated to the velocity of the cooling air or water, and adequate to the method used and the needs of the commodity, volume is essential for thorough contact and the maintenance involve a period of from H to 24 hr or more. Precooling is of velocity. The sweeping action of the coolant reduces the in ^yntiyl to the proper handling of the more perishable fruits termediate temperature zone around the product. As the tsd vegetables. Ripening, deterioration, and decay develop- velocity of the air or water increases, this intermediate zone ojent occur much more rapidly at high than at low tempera- decreases and the surface temperature of the product ap tores. The more quickly field heat is removed after harvest, proaches that of the cooling medium. There is, of course, an the longer the product can be maintained in good marketable optimum velocity for each situation, beyond which the small condition. increase in cooling rate does not justify the added cost. Storage rooms designed for holding produce under refrigera Accessibility of the product to the cooling medium refers to tion normally do not have either the refrigeration capacity or the thoroughness with which the air or the water can come in the sir movement needed for rapid cooling. In most cases contact with all surfaces of the product to be cooled. An in ffwvling in transit is also unsatisfactory. In conventional ice- dividual ear of corn will cool more rapidly than a packed crate rririgersted rail cars, with fans operated from the car wheels, of corn, and an individual peach will cool faster than a basket air movement during the first 24 hr in transit is too variable of peaches, when both go through the same hydrocooler. In a for rapid cooling. Mechanically refrigerated rail and truck tightly packed crate or basket, the water may not reach any equipment generally does not have enough capacity for transit surface of some products. Pads or liners in the container that precooling. However, there is some evidence that newly de restrict the flow of the refrigerant may also hinder heat veloped equipment which provides continuous, high-speed fan transfer. When cooling with air in a room or car, spaced stacks operation in ice-refrigerated cars will be satisfactory for the will cool much more rapidly than a solid block, unless the con rapid removal of field heat during the first 18 to 24 hr after tainers in the solid block are ventilated and stacked so that completion of loading. Nevertheless, precooling for either air can be forced through instead of around the containers. storage or transport is generally a distinct and separate opera When a package is cooled by movement of air over its outer tion, requiring special facilities and equipment, which may be surfaces, cooling is more rapid if all surfaces, not just the ends rooms with additional refrigerating surface and air volume, or or sides, of the package are exposed. The chimney load of supplementary refrigeration and- blower capacity for cooling carton-packed produce is difficult to cool by conventional beds in rati cars or highway vans. methods because only a small portion of the total package Precooling can be accomplished by a number of methods, surface is exposed. This factor of accessibility will be discussed. but in the simplest terms it is transfer of heat from the com in more detail in a later section. modity to a cooling medium, such as air, water, or ice. The best exchange system must cany beat from the product to the METHODS OF PRECOOUNG source of refrigeration. Such a source can be ice, ammonia or baloc&rbon refrigerant evaporator coils, or cold briae. The effectiveness of any of the conventional methods of precooling is controlled by four factors: (1) the nature of the cool ing medium, (2) the temperature.differential between the product to be cooled and the cooling medium, (3) the volume &nd velocity of the cooling medium, and (4) the accessibility As previously noted, many methods of precooling are avail able to the handler of fresh produce. Selection of method de pends on the needs and adaptability of the commodity and the availability of equipment. Results with any of the methods will be satisfactory only when the available knowledge is applied to conduct the process at optimum effectiveness. ofthe product to the cooling medium. Because of heat transfer factors, some cooling media are more effective than others. Be Cooling with Air cause cooling be accomplished much more rapidly with With moving air as the refrigerating medium, produce can vster than it can with air, hydrocoolers are effective for those be precooled satisfactorily in refrigerated rooms adapted to commodities which will withstand exposure to water and for that purpose, in refrigerated rail care or highway vans, on which very rapid cooling is desirable. continuous conveyors in tunnels, or by the forced-air method Cooling rate is substantially affected by the difference in of passing air through the containers by pressure differential.- temperature between the product and the cooling medium Each of these methods is used commercially, and each is suit Since commodity temperatures at the beginning of precooling able for certain commodities when properly applied. ue not generally subject to control, the use of a large tempera Produce can be satisfactorily precooled in rooms, cars, or ture differential to promote rapid cooling requires a coolant vans when the following conditions are met: (1) an ade faniperature as low as practicable without freezing. quate source of refrigeration, (2) an air volume of 6000 to With a given differential between commodity and re 8000 cfm per cariot equivalent of product, with air directed in frigerant temperatures and a constant exposure, the rate of such a way that all containers in the load are exposed to 563