MECHANICAL REFRIGERATION 69 



"Anhydrous ammonia" or as "Aqua ammonia." The fonner 

 is free from water and exists in both gaseous and liquid form; 

 the latter is in the form of a solution of which about 29 1/2 per 

 cent is anhydrous ammonia. The anhydrous ammonia is used 

 in refrigerating machines of the compression type; aqua am- 

 monia is used in refrigerating machines of the absorption type. 



Pure anhydrous ammonia, under ordinary temperature and 

 atmospheric pressure, exists in the form of gas or vapor; but 

 under atmospheric pressure at a temperature of — 30° F., it be- 

 comes liquid. It will also liquefy at higher temperatures if the 

 pressure is suflSciently increased. In its hquid form it is color- 

 less, has a strong alkaline reaction, boils at — 29° F. under 

 atmospheric pressure, and its latent heat of vaporization is 

 555 B. T. U.i Its specific gravity at 32° F. is 0. 6364. One 

 pound of this liquid, after being vaporized and the vapor raised 

 to 32° F., will occupy a space of 21.017 cubic feet under atmos- 

 pheric pressure. 



Ammonia attacks copper and brass but not iron or steel, and 

 this fact determines the metal from which compression ma- 

 chinery must be made. Ammonia decomposes at 900° F. if in 

 pure condition; but if it contains impurities, it may decompose 

 at comparatively low temperatures. Ammonia may contain 

 impurities which will not influence its stability; but if mixed 

 with another brand of ammonia containing a different kind of 

 impurities which alone would not have any influence on its 

 stability, such ammonia may deteriorate much more readily 

 owing to the fact that the impurities of one supplement the 

 impurities of the other and cause decomposition. This explains 

 the advisability of not mixing diS^erent brands of ammonia. 



COMPRESSION SYSTEM 



In this type of refrigeration, the ammonia operates in a 

 cycle: First, the liquid or inactive stage; second, the vaporizing 

 or actively refrigerating stage, taking place in the expansion 



' The latent heat of vaporization for any liquid is the total amount of heat, ex- 

 pressed in B. T. U., which is required to change i pound of that liquid at the boiling 

 point into vapor. 



