194 DAIRY TECHNOLOGY 



Ammonia leaks may be detected by holding a glass rod 

 dipped in hydrochloric acid to the places where the leaks 

 are suspected. When ammonia comes in contact with 

 hydrochloric acid, white fumes are formed. 



Principles of Producing Cold Artificially. — The chief 

 principle involved in producing artificial cold is that when 

 a substance passes from a liquid into a gaseous state, a 

 definite amount of 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 condi- 

 tions, 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 hquid state. We are 

 famihar with the coolness produced by rapid evaporation 

 of perspiration from the body. Mechanical refrigeration 

 is virtually a process of evaporation of the cooling medium 

 during which heat is absorbed; and then again liquefying 

 the cooHng medium by compression and cooling to remove 

 the absorbed heat. To increase the ability of the cooling 

 medium to absorb heat the cooling medium is compressed 

 and liquefied. So we might say that any compression 

 refrigerating system has three separate operations 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. 



I. 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 



