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2
REFRIGERATION
 
A. METHODS OF REMOVING HEAT
 
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.

  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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