MECHANICAL REPRIGERATION 207 



When a liquid evaporates or changes into the gaseous 

 state it absorbs a definite amount of heat called heat of 

 vaporization or ''latent" heat. Thus to change water from 

 212° F. to steam at 212° F. requires a considerable 

 amount of heat which is apparently lost, hence the term 

 latent (hidden) heat. 



Ether changes into its gas at a much lower temperature 

 than water which is illustrated by its instant evaporation 

 when poured upon the hand. The heat of the hand in this 

 case is sufficient to cause vaporization and the sensation 

 of cold indicates that a certain amount of heat has been 

 abstracted from the hand in the process. 



Manifestly for refrigerating purposes a liquid must be 

 used that can be evaporated at a very low temperature; 

 for the cold in mechanical refrigeration is produced by 

 the evaporation of the liquid in iron pipes, the heat for 

 the purpose being absorbed from the room in which the 

 pipes are laid. Anhydrous ammonia has thus far proven 

 to be the best refrigerant for ordinary refrigeration. 



Anhydrous Ammonia (Refrigerant). This substance 

 is a gas at ordinary temperatures but liquifies at 30° F. 

 under one atmospheric pressure. In practical refrigera- 

 tion the ammonia is liquified at rather high temperatures 

 by subjecting it to pressure. The ammonia is alternately 

 evaporated and liquified so that it may be used over and 

 over again almost indefinitely. 



Circulation of Ammonia. The cycle of operations in 

 mechanical refrigeration is as follows : The liquid am- 

 monia starts on its course from a liquid receiver, and 

 enters the refrigerating coils in which it evaporates, ab- 

 sorbing a large amount of heat in the process. By means 

 of a compression pump, operated by an engine, the am- 

 monia vapors are forced in the condenser coils where the 



