APPLICATION OP REFRIGERATION TO HANDLING OF MILK. 25 



tion, and a small specific volume. A low boiling point is desirable, 

 because it makes operation possible with comparatively low pressure 

 in all parts of the system; therefore, the machines and accessories 

 may be of lighter construction, with smaller loss of gas by leakage. 

 As the latent heat of vaporization is, to a certain extent, a direct 

 measure of the cooling effect, it is obvious that the greater the heat 

 of vaporization the better the refrigerant. The specific volume of the 

 refrigerating agent determines the volume of the cylinders of the 

 compressor, consequently the size and weight of the machine. 



In comparing the three refrigerating agents which are considered 

 applicable to the dairying industry, viz, ammonia, carbon dioxid, 

 and sulphur dioxid, it will be noted by referring to tables giving 

 the main characteristics of the agents that, assuming the limits of 

 operation are between 5° F. and 85° F., the absolute pressures are: 

 Ammonia from 27 to 175 pounds, carbon dioxid from 290 to 1,000 

 pounds, and sulphur dioxid from 9 to 65 pounds. Taking the boil- 

 ing points of the liquids at the temperature at which the liquid boils 

 under atmospheric pressure, it will be noted that there is a wide 

 difference in their boiling points as well as their latent heats of 

 vaporization. Ammonia boils at 28.5° F. below zero and has a latent 

 heat of vaporization of 572.8 B. T. U. Carbon dioxid boils at 110° F. 

 below zero and has a latent heat of vaporization of 140 B. T. U. at 

 a pressure of 182 pounds per square inch absolute. The latent heat 

 at atmospheric pressure is not definitely known. Sulphur dioxid 

 boils at a temperature of 14° F. and has a latent heat of vaporization 

 of 162.2 B. T. U. 



For practical purposes the value of a refrigerant depends upon its 

 boiling point, its latent heat of vaporization, and upon the pressure 

 at which it can be used. 



To maintain a zero temperature with ammonia as the refrigerant 

 an absolute pressure of 30 pounds per square inch is required in the 

 evaporating coils; with carbon dioxid, 310 pounds absolute; and 

 for sulphur dioxid, 10 pounds. 



Ammonia has a much greater latent heat of vaporization and the 

 working pressures are not excessive, but it has the disadvantage 

 that it corrodes brass or any other copper alloy; consequently only 

 iron or steel can be used in the construction of those parts of the 

 machine with which the agent comes in contact. The pressures of 

 carbon dioxid are so high as to cause trouble in keeping the stuffing 

 box and joints tight. A relief valve is often placed in the high- 

 pressure side of the system in order to protect it from excessive 

 high pressures. It is noncorrosive, nonexplosive, and is not dan- 

 gerous to life when diluted with air. The high pressures necessary, 

 combined with the small specific volume of the gas, make it suitable 

 for use with a very compact machine. As the lower pressure of 



