58 



CIRCULAR 7 4 0. U. S. DEPARTMENT OF AGRICULTURE 



creases as the head pressures increase and the suction pressures de- 

 crease with ammonia compressors. Suction pressures as high as 35 

 to 40 pounds and head pressures as low as 100 to 120 pounds can 

 be obtained under favorable conditions. Pressure gages should be 

 checked occasionally for accuracy, since they may get out of adjust- 

 ment after long use. The temperature of liquid ammonia at various 

 gage pressures is given in table 10. 



Table 10. — Temperatures of liquid ammonia at various gage pressures 



R:i:_ 



Gage Temper- 

 pressure ature 



Ranee 



- 



Suction pressure (usual 

 range) 



Pounds ; 



! 



10 

 15 

 20 

 25 

 30 

 35 



■F. 

 -28 



-17 



-8 



— 1 



5 



11 



17 



21 



(i) 



Head pressure (usual range) 





Gage 



Temper- 



pressure 



ature 



Pounds 



°F. 





40 



26 



• 



50 



34 





75 



50 





r ioo 



63 





125 



o 





150 



84 





175 



93 





{ 200 



101 



i Suction pressures seldom occur below 10 or above 35 pounds; head pressure seldom below 100 or above 

 200 pounds. 



TROSTED COILS 



Accumulation of heavy layers of frost on cooling coils retards the 

 passage of heat (fig. 25). Pipes or finned coils need to be defrosted 



frequently to get the most from a cooling system. Disposal of the ice 



3 



Figttre 25.— The accumulation of frost on the coils lowers the efficiency of the plant. 



