Refrigeration . 155 



than this is easily obtained for cooling the compressed gas. 

 With the reduction of the temperature of the gas below its 

 boiling point, the gas condenses or liquefies, giving up its heat 

 to the colder water surrounding the pipe. 



Control of boiling point by pressure. The boiling point of 

 liquid ammonia at ordinary pressure is about 29 F. Such 

 a low temperature is unnecessary in refrigeration. The liquid 

 is therefore usually liberated at a somewhat higher pressure. 

 Suppose it is released in the evaporating coil under an absolute 

 pressure of 22 pounds. At this pressure its boiling point is 

 about 12T.2 F. The difference between the pressure above, 

 and that below, the regulating valve is still very great. The 

 temperature of the liquid in the collecting coil under 156 pounds 

 pressure is much below its boiling point. The liquid liberated 

 into a pressure of 22 pounds begins immediately to vaporize 

 and to expand rapidly. It takes heat largely from itself as 

 it vaporizes and expands, and thereby lowers its temperature 

 to about the point at which it boils under this comparatively 

 low pressure. 



At this temperature the gas moves through the pipes, ab- 

 sorbing heat from them and from the brine, expanding con- 

 tinuously until it is drawn into the cylinder of the compressor. 

 The brine gives up heat but does not freeze, because its freez- 

 ing point is far below that of fresh water, perhaps as low as 

 4 F. The brine takes heat from the water in the molds, 

 and by reducing its temperature to the freezing point, eventu- 

 ally causes it to become solid. 



Ammonia is of common occurrence. It may be detected 

 by its odor wherever putrefying organic matter, animal and 

 vegetable, is found, as about stables, cesspools and manure 

 piles. It may be found by chemical tests in water where pollu- 

 tion has occurred from decaying organic matter or sewage. Its 

 source for commercial purposes is the ammonia liquor produced 

 in the destructive distillation of soft coal (page 163) in making 

 illuminating gas. In a former age it may have been prepared 



