78 MANAGEMENT OF DAIRY PLANTS 



denser in liquid form, thus completing the second and final step 

 in liquefaction. 



On its journey through the system, the ammonia has risen in 

 temperature owing to heat acquired in two places — the latent 

 heat of vaporization taken up in the expansion coils, and the 

 heat of compression that represents work done on the ammonia 

 by the compressor. Some of this total acquired heat is absorbed 

 by the water jacket of the compressor; the remainder passes 

 into the condenser and is there removed by the cold water. 



The temperature of the gas is, therefore, higher in the com- 

 pressor and condenser coils than in the expansion coils. But 

 there is a definite relation between the pressure of the gas and 

 its temperature. At a pressure of 15 pounds, the ammonia gas 

 has a temperature of 0° F., while at a pressure of 150 pounds it 

 has a temperature of 80° F. Consequently, a great disparity 

 between the temperature in the condenser coils and that in 

 the expansion coils means a corresponding disparity between 

 the high pressure in the condenser coils and the low pressure 

 (or back pressure) in the expansion coils. The less this dis- 

 parity, the more efficient is the working of the machine. Con- 

 sequently, the low or back pressure should be reasonably high, 

 since the capacity of the machine is governed by the density of 

 the gas. The high or condenser pressure should be as low as 

 possible, since it requires less power to pump against a low 

 pressure, and since the temperature of compression is decreased 

 with decreasing pressure. As the temperature of the wate'- used 

 for cooling determines the pressure of the gas, the water used 

 for that purpose on compressor and condenser coils should be as 

 cold as it can be had. 



The most common styles-of condenser are the atmospheric 



ammonia condenser and the double-pipe ammonia condenser. 



former is moi;e economical and is usually placed on the roof. 



:t '. used mostly wher& the condenser is placed inside. 



Expansion Valve. — The expansion or regulating valve is 

 the valve between the receiver and the expansion coil. When 

 this valve is opened the liquid escapes into the expansion coil, 

 and thus the cycle is completed. 



