178 THE AMERICAN APPLE ORCHARD 



"The refrigerating- gases generally used are anhy- 

 drous ammonia, sulphuric acid, and carbonic acid 

 (also known as carbon anhydrid) and carbon dioxide. 

 The cold temperature in the warehouses is usually 

 produced by either of two methods, commonly known 

 as the compression and the absorption systems. 



"The compression system takes its name from the 

 fact that the refrigerating gas — whether ammonia, 

 carbonic acid, or sulphuric acid — is first compressed 

 in a machine called a compressor. Heat is generated 

 by the compression; the gas is then cooled and con- 

 densed in pipes or coils called the condenser, either 

 immersed in water or having water running over 

 them, and this converts the gas into a liquid. The 

 liquefied gas then passes an expansion valve to pipes 

 or coils called the refrigerator cooling coils or cooler, 

 where it is evaporated by the heat which is with- 

 drawn from the surroundings. The gas formed by 

 the evaporation of the liquid returns to the compress- 

 or, is again condensed, then re-evaporated, and the 

 cycle of refrigeration is repeated over and over. 



"In the absorption system the gas is obtained by 

 heating strong aqua ammonia in a still, thereby driv- 

 ing off the ammonia gas. The gas is then reduced in 

 a condenser to a liquid in a manner similar to the 

 compression system. The liquefied ammonia pro- 

 duces refrigeration by evaporating in the cooling 

 coils, and the gas is then absorbed by weak aqua 

 ammonia in coils called an absorber. The resulting 

 strong liquor is then pumped back to the still. The 

 cycle of refrigeration is repeated continuously, and 

 consists, first, in the generation of a gas by heating 

 strong aqua ammonia in a still ; second, in condensing 

 the gas which is deposited from the water to a liquid 



