USES OF WATER 



109 



ammonia gas cannot be liquefied is, 130 Centigrade (the 

 Critical Temperature). At temperatures below this 

 point it may be liquefied by subjecting it to great pres- 

 sure. At 80 Fahrenheit a pressure of 155 pounds per 

 square inch will produce its liquefaction. 



Figure 99 is a diagram of the essentials of a modern ice 

 plant. 



The condensing pump forces the ammonia gas into the 

 condensing pipes at a pressure of 155 pounds to the 



FIG. 99. A Modern Ice Plant. 



square inch of surface. The heat of the condensation of 

 ammonia is carried off by the cold water which is kept in 

 contact with the condensing coils. From the condenser 

 the liquid ammonia is allowed to pass slowly through the 

 expansion valve at D into the coils of the evaporator C 

 from which the evaporated ammonia is pumped, so that 

 the pressure in the evaporating pipes is maintained at 

 about the pressure of two atmospheres. The pump 

 shown in the diagram, acts both as a compressor for the 

 ammonia in the condensing coils and an exhaust for the 



