112 



ELEMENTS OF GENERAL SCIENCE 



of the air by increase of pressure (see sect 118). Thus, while 

 the boiling point is 28 F. at atmospheric pressure, its boil- 

 ing point is raised to 80 F. by a pressure of 155 pounds to 

 the square inch. 



Figure 60 represents the essential features of an ice-manu- 

 facturing plant The condensing pump forces the gaseous 

 ammonia into the pipes to the right with a pressure of at least 

 155 pounds to the square inch, and 

 the gas is cooled to the necessary 

 degree by the water which flows over 

 the cooling pipes. The gas therefore 

 condenses into a liquid. This liquid 

 ammonia is allowed to escape through 



Cooling pipes 



Cold-water 



Expansion 

 valve Ammonia pipe 



FIG. 60. An artificial-ice machine 

 Note names of parts and the use of these parts as described in section 125 



the valve into the pipes at the left only as rapidly as the ex- 

 cess is removed by the pump, and the pressure in these pipes 

 therefore never rises much above 30 pounds to the square inch. 

 The liquid which has escaped into this place of lower pressure 

 immediately begins to change into a gas, and in so doing it 

 absorbs heat from its surroundings. The pipes into which the 

 liquid ammonia expands are usually surrounded with brine, 

 and this is cooled by the evaporating ammonia several degrees 

 below the freezing point of water. The brine, in turn, cools 

 the water which has been placed in the freezing chambers. 

 The gaseous ammonia passes on to the pump and is used again. 



