APPLICATION OF 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 
