COLD STORAGE FOR APPLES AND PEARS 17 



but no manipulation of air movement or special stacking of boxes 

 or other adjustment can prevent the accumulation of heat if it is 

 being introduced or produced faster than it is being removed. 



THREE STEPS IN THE REFRIGERATING PROCESS 



Heat, like air, is handled in definite quantities, but unlike air it 

 cannot be moved bodily from one point to another. By its nature 

 heat moves from a place of high temperature to one of low tem- 

 perature. A refrigerating system, or heat pump, takes advantage of 

 this tendency. 



Heat from the storage room moves through the walls of the evap- 

 orator cooling coils to the ammonia or other refrigerant inside, which 

 is at a lower temperature. The compressor then takes the vaporized 

 ammonia with the heat it has picked up in the evaporator and, by 

 compressing the gas, raises its temperature. The heat from the hot 

 ammonia finally moves into the condenser water because the water is 

 at a lower temperature. Thus the heat from the storage is now in 

 the condenser cooling water, which may either be wasted or cooled 

 by aeration for recirculation. These three steps in heat removal are 

 accomplished by the three essential parts of the refrigerating sys- 

 tem — the evaporator, the compressor, and the condenser (fig. I). 2 



In the evaporator, or cooling coils, the quantity of heat picked up 

 depends upon (1) the temperature difference between the refrigerant 

 (ammonia) in the coils and the air outside, (2) the area of coil sur- 

 face exposed, and (3) the resistance to heat flow through the walls 

 of the pipes. The resistance to passage of heat into the coil in turn 

 depends not only upon the cleanness of the coil but also upon the 

 velocity of air (or brine if a brine cooling system is used) passing 

 the coil and the velocity of the refrigerant (whether liquid or vapor). 

 The resistance is increased by an accumulation of frost, or if not 

 enough piping surface is exposed a large temperature difference will 

 be necessary between the inside and outside of the coil to permit suffi- 



2 Bowen (4, pp. 2-3) describes the operation of the refrigerator shown in figure 1 as 

 follows : 



To utilize its latent heat of vaporization for refrigeration and to conserve the refrigerant, 

 application is made of the physical law that the temperature at which a fluid boils or 

 condenses is raised or lowered, respectively, by increasing or reducing the pressure. To 

 cause the refrigerant to boil at a low temperature in the evaporating coils and hence absorb 

 heat on a low-temperature plane, the pressure in the coils is lowered by the suction of the 

 compressor. ... To free the fluid of the heat absorbed in the refrigerator and return 

 it to liquid form, the cold refrigerating gas coming from the evaporating coils is compressed 

 until its temperature is raised above that of the water flowing through the condenser so 

 that the contained heat can pass from the gas to the water. (In very small machines, 

 air may be used instead of water.) 



The essential parts of a compression-refrigerating system are an evaporator, a compressor, 

 and a condenser. 



In the evaporator (the coils in the refrigerator) the liquid boils and in the process absorbs 

 heat from the surrounding medium. The compressor is a specially designed pump that takes 

 the gas from the evaporator coils and compresses it into the condenser coils, reducing its 

 volume and increasing its temperature. The condenser consists of coils of pipe over or 

 through which water or air flows to absorb the heat from the gas, which is thereby liquefied. 

 In some systems the cooling water passes through an inner tube, and the gas from the 

 compressor through the annular space between the inner and the outer pipes. From the 

 condenser the refrigerant passes first to a liquid receiver, and then through a throttling 

 or expansion valve into the evaporator coils, to repeat the process of transferring heat 

 from the refrigerator to the water flowing through the condenser. The temperature of the 

 liquid ammonia is reduced from the temperature of the receiver to that of the refrigerator 

 by vaporizing a part of the liquid. 



The expansion valve is of a special design and is capable of very fine adjustment. Its 

 function is to so regulate the flow of the liquid refrigerant that suitable pressure and tem- 

 perature conditions will be maintained. It is largely responsible for the control of tem- 

 perature in the evaporating or cooling coils. 



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