Man's physical universe

xanabras

THE ABSORPTION OR EVOLUTION OF HEAT 231

application of the physical changes which we have studied, but it

too involved to permit a thorough discussion in the text. In general, it

depends upon the driving off of ammonia from its water solution by

heat, condensing the ammonia by cool water or indirectly by relatively

cool air, cooling the refrigeration coil by expansion of the liquid ammonia,

and absorbing the ammonia in water again. The absorption of

the ammonia in water corresponds to the compressor in the mechanical

refrigeration unit.

Inasmuch as heat is used up in vaporization, the reverse process of

condensation evolves heat, which must be removed before the compressed

gas can be liquefied. In the gas refrigerator the vapor is cooled

by water or by air as in automobile engines. All automobile engines

are cooled by air.

Air-cooled airplane engines are so constructed that

their cylinders are cooled with a rapid stream of air produced by the

motion of the airplane. Most automobiles take advantage of the

specific heat of water, which permits it to transmit the heat to a radiator

specially designed to give efficient cooling when the air is kept in

circulation through it by a fan. The heat is thus transmitted from the

water to the metal which warms the air in contact with it. The fan

moves the heated air away from the radiator and thus maintains a

supply of relatively cool air.

In large refrigeration plants such as ice plants, the compressed gases

are cooled by water, which is cooled, in turn, by vaporization in evaporation

towers. Thus the heat used up by the vaporization of the water

in the evaporation towers is indirectly responsible for the freezing of

the ice in the ice plant.

One of the objections to electric refrigerators is that moisture is

removed from uncovered foods placed in the refrigerator. One method

of preventing this dehydration of foods is to increase the cooling area

and keep it about 10° F. above the freezing-point of water. Microorganisms

are more likely to multiply at this temperature than at the

lower temperatures usually required to preserve foods in refrigerators,

but this problem has been solved by the use of a germ-killing lamp to

destroy the bacteria in the air and in foods exposed to the ultraviolet

radiations given off.

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STUDY QUESTIONS

1. Why does liquid air boil on a piece of ice?

2. Give some examples of the change in prof)erties of matter at the temperature

of liquid air.

3. How is air liquefied?

4. Why can air not be licjuefied by compression alone?

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