MECHANICAL REFRIGERATION 377 



latent heat is absorbed. When water in a kettle on the stove 

 begins to boil and passes off into steam, no higher temperature 

 can be reached. No matter how much heat is applied under 

 those same conditions, the temperature remains the same. This 

 extra heat is used in transforming the water into steam. If this 

 steam were confined, and that heat removed, by cooling, the 

 steam would again pass into a liquid state. We are familiar 

 with the coolness produced by rapid evaporation of perspiration 

 from the body. Mechanical refrigeration is virtually a process 

 of evaporation of the cooling media, during which heat is absorbed, 

 and liquefaction of the cooling medium by compression and cool- 

 ing to remove that absorbed heat. To increase the ability of the 

 cooling medium to absorb heat it is compressed and liquefied. 

 So it may be said that in any compression refrigerating system 

 three separate operations are necessary to form the complete 

 cycle of mechanical refrigeration, viz. : 



1. Compression of the ammonia gas. 



2. Condensation of the ammonia gas. 



3. Expansion of the ammonia gas. 



1 . The machine which causes the compression of the ammonia 

 gas is called the compressor. In construction it is much like a 

 steam-engine. Small machines are single, but large machines are 

 double acting. Gas is drawn in, on the suction stroke, com- 

 pressed and discharged on the return stroke. The pressure gen- 

 erated varies between 120 and 175 pounds per square inch. 

 During the compression heat is developed in proportion to pres- 

 sure exerted. The greater the pressure the higher the tempera- 

 ture of the gas. Part of the heat of compression is carried off by 

 means of a continuous stream of water running through a jacket 

 around the cylinder. 



2. From the compressor the gas is forced through the pipes 

 into the condensing coils, in which the warm compressed gas is 

 cooled still more. When sufficient heat has been removed from 

 this gas, it assumes a liquid condition and is ready to expand into 

 a gaseous form for the purpose of absorbing heat and producing 

 cold. During the cooling and condensing processes each pound 

 of ammonia parts with about 560 units of heat, which amount 



