APPLICATION OF REFRIGERATION TO HANDLING OF MILK. 29 
the transfer of heat can be made to take place between it and the 
cooling water at ordinary temperatures, a compressor or a suitable 
gas pump must be used with a compression system of refrigeration. 
There must also be furnished a suitable cooling chamber or con- 
denser in which the cooling water may be brought sufficiently near 
the refrigerating medium to allow the necessary heat to flow from the 
hot refrigerant to the cool water. Therefore the essential parts of 
a compression refrigerating system and their functions are : 
A compressor, which is nothing more than a specially designed 
pump, which takes the gas from the evaporator coils and com- 
presses it into the coils of the condenser, reducing the volume of the 
gas and increasing its temperature by changing work into heat. 
A condenser, which consists of coils of pipe over which water is 
allowed to flow, or in some constructions the cooling water passes 
through an inner tube and the gas is discharged by the compressor 
into the annular space between the inner and the outer pipes. The 
heat imparted to the gas is given up to the cooling water, thereby 
liquefying the gas. 
An evaporator, in which the liquid is allowed to boil and absorb 
heat from the surrounding media, again changing into the gaseous 
state. In other words, the process of refrigeration by means of the 
compression system is divided into three distinct stages, namely, 
compression, condensation, and expansion. 1 
Compression. — The gaseous refrigerant is drawn into the com- 
pressor or pump and there compressed to a pressure dependent upon 
the refrigerant and the temperature and quantity of cooling water 
used in the condenser. The latent heat of the vapor, that is, the 
quantity of heat imparted to it to effect its vaporization from a liquid 
to a vapor is converted into active or sensible heat during this 
compression. 
Condensation.' — The vapor is forced into the condenser coils under 
high pressure by the compressor, where the nonactive and sensible 
heat developed during compression is absorbed by the cooling water, 
thus removing from the vapor the heat required to keep it in the 
gaseous state, and thereby reconverting it into a liquid at the tem- 
perature and pressure existing in the condenser. 
Expansion. — The refrigerant, after being liquefied in the coils of the 
condenser, is passed first to a liquid receiver and then into pipe coils 
through a throttling or expansion valve which is capable of being 
adjusted to allow the liquid to pass in small quantities. As there is 
a materially lower pressure maintained in these pipe coils by the com- 
1 In order to make the compression system perfectly reversible there is, in addition to the three stages 
mentioned above, a fourth, which consists in the reduction of the temperature of the liquid from the con- 
denser temperature to that of the refrigerator, by the vaporization of a part of the liquid and by doing work 
by moving a piston. 
