18 



CIRCULAR 74 0, U. S. DEPARTMENT OF AGRICULTURE 



cient heat to pass into the coils. This requires a low ammonia tem- 

 perature. If, because of resistance or insufficient surface in the cool- 

 ing coils, it is necessary to maintain a low ammonia temperature 

 (which means low suction pressure), the compressor is forced to boost 

 the temperature from a low point, and it cannot handle as much heat 

 as when the suction temperature is higher. 



The compressor must also discharge the ammonia at such temper- 

 ature that heat will flow from it to the cooling water in the condenser. 

 In general, a compressor can handle more heat if the temperature 

 in the cooling coils is kept as high as possible and the temperature 

 in the condenser as low as possible. The same conditions also reduce 

 the power necessary to remove a given quantity of heat. 



When the ammonia enters the condenser, heat passes from it into 

 the cooling water. As in the evaporating coils, the heat passing 



pressure 



Head pressure 



G It 



j i I [J.I.J J. I } ' Water 

 &:*»v:Wr?n^ inlet 



REFRIGERATOR 



LOW-PRESSURE SIDE 



HIGH-PRESSURE SIDE 



Figure 1.— Essential parts of a compression refrigeration system. 



from the ammonia to the cooling water depends upon (1) the tem- 

 perature difference between the ammonia and the water, (2) the 

 surface area exposed, and (3) the resistance to heat flow through 

 the condenser pipes. Here also, the resistance to the passage of heat 

 depends upon the water and the ammonia velocities and the cleanness 

 of the coil. Scale, which tends to collect on the pipes from the cooling 

 water, may increase the resistance markedly. If this scale is per- 

 mitted to build up or if there is not sufficient cooling surface, the 

 required quantity of heat can be transferred to the water only by hav- 

 ing a large temperature difference between ammonia and water. As 

 pointed out before, the high ammonia temperature in the condenser 

 means reduced compressor capacity and high power consumption. 

 An adequate supply of water as cold as possible will contribute toward 

 a low ammonia temperature in the condenser and therefore to low 

 power consumption. 



