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2A1. General. Refrigeration is the process
of removing heat from matter. The matter
may be a solid, a liquid, or a gas. Removing
heat from the matter cools it, or lowers its
temperature. There are a number of ways of
lowering temperatures, some of which are of
historical interest only.
2A2. Some older methods. Lowering of temperature may be accomplished by the rapid
expansion of gases under reduced pressures.
Thus, cooling may be brought about by compressing air, removing the excess heat produced in compressing it, and then permitting
it to expand.
Evaporation also has a cooling effect. The
canvas-covered water bottle, or canteen, is
familiar to all. The canvas cover is drenched
with water and placed in a draughty shaded
place, where the moisture begins to evaporate.
Heat is required to change this water from
liquid to vapor; some of this heat that is absorbed comes from water in the bottle. The
water vapor carries away heat from the bottle,
in turn cooling the water within.
A lowering of temperatures is produced by
adding certain salts, such as sodium nitrate,
sodium thiosulfate (hypo), and sodium sulfite
to water. The same effect is produced, but to
a lesser extent, by dissolving common salt or
calcium chloride in water.
The mixing of common table salt (sodium
chloride) with cracked ice lowers the temperature of the mixture several degrees.
2A3. Two common methods of refrigeration.
The methods just discussed are limited to
small-scale operation. Two common methods
by which large-scale refrigeration is obtained
include: 1) the use of ice, and 2) the use of
mechanical devices.
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2A4. Ice. Ice has been used in refrigeration
since ancient times and it is still widely used.
Refrigeration by means of ice depends upon
either natural or forced circulation of air.
The circulating air passes around blocks of
ice. Some of the heat of the circulating air is
transferred to the ice, thus cooling the air.
The heat absorbed by the ice melts it. The
chilled air then passes around other articles,
absorbing some of their heat, and thus cooling
them. A continuous cycle of refrigeration is
maintained, but as a result of this process, the
ice is melted and more ice must be supplied
at regular intervals. On board a submarine,
ice would last only a few days.
2A5. Mechanical refrigeration. Under certain conditions, some liquids boil at temperatures actually lower than the freezing point
of water. In a suitable mechanical system,
such liquids can draw heat from surrounding
substances and thus cool them. Cooling by
this method is known as mechanical refrigeration.
2A6. Refrigerant defined. In mechanical refrigeration, a refrigerant is a substance capable of transferring heat that it absorbs at low
temperatures and pressures to a condensing
medium; in the region of transfer, the refrigerant is at higher temperatures and pressures.
By means of expansion, compression, and a
cooling medium, such as air or water, the refrigerant removes heat from a substance and
transfers it to the cooling medium.
The process of mechanical refrigeration
offers so many important advantages over ice
that it has taken first place. On shipboard it
is invaluable.
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