Man's physical universe



is merely a device to transfer heat from one place to another.

When a

compressor is used to cool a building, the heat is taken out of the building

and absorbed by the outside air (or by water as mentioned above),

which, though warm, is cooler than the compressed gas.

In the winter the heating coil is used to heat the inside air, while

the heat is taken from the outside air which is usually considerably

warmer than the cooling-coil.

In adding moisture to the air in the winter, heat is required for the

vaporization of the water.

The vaporization of water not only adds to


High pressure





^Expansion valve

Fig. 92. Vapor compression air conditioning system. (Courtesy of the General

Electric Air Conditioning Company.)

the amount of heat required for heating in the winter, but inasmuch as

moisture must be removed from the air in the summer and inasmuch as

this process of condensation gives off

heat, adding to the amount of

heat that must be removed, the control of moisture involves the expenditure

of energy in either case.

The Humidity Must Be Controlled.

Outdoor air in the winter contains little water because most of it is

removed as the air is cooled to low temperatures. If this outdoor air is

heated without adding moisture it becomes relatively even drier. We

have already pointed out the fact that low temperatures are more

comfortable if the relative humidity is not too low. It has been gen-

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