230 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME pressures in boilers as does water at the high temperatures necessary for high efficiency in power generation, and thus replaces the boiler water in some power plants. A liquid suitable for use in refrigeration must be readily liquefied by pressure at ordinary temperatures; i.e., its critical temperature must be above room temperature. Water should meet the requirements of a good refrigeration liquid because it is easily condensed and has a high heat of vaporization, but inasmuch as its boiling-point is 80" C. above ordinary room temperatures, it does not vaporize rapidly enough at ordinary pressures to be effective. Air is often cooled by the vaporization of water, however. A simple house air-cooler, shown in the Fig. 72. A very simple type of air-conditioning unit. the long wet wicks. The fan blows air over illustration, depends upon increasing the. cooling by evaporation by directing a current of air against a wet cloth by a fan. Desert water bags are kept cool by the evaporation of the moisture which seeps through the canvas bags. Refrigerators covered with moistened cloth are sometimes used for camping trips. Nearly everyone has experienced the cooling of the body when standing in a wind when clothed with wet garments. Inasmuch as the liquid in a refrigeration system must be condensed as rapidly as it vaporizes, some provision must be made for this process. In the majority of refrigeration systems, small or large, the vapors are condensed by power-driven compressors. The gas-operated refrigerator originated as an invention of two Swedish students, Carl Munters and Baltzar van Platen, who made their brilliant discovery while still undergraduates at the Royal Institute of Technology at Stockholm. The process represents an ingenious
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. is 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?