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618 CHAPTER 41 1960 Guide K. P. Brace: Regenerative cooling for hot, dry climates (Heating and Ventilating, May 1932, p. 36). J. R. Watt and R. A. Bacon: Indirect evaporative air cooler (Heating, Piping and Air Conditioning, May 1956, p. 149). * R. S. Ash: Evaporative cooling for farm animals (Air Con ditioning, Heating and Ventilating, October 1956, p. 109). BIBLIOGRAPHY H. M. Hendrickson: How air washers perform when cooling (Heating, Piping and Air Conditioning, March 1954, p. 119). H. M. Hendrickson: How to calculate air washer performance when cooling (Heating, Piping and Air Conditioning, Septem ber 1954, p. 116). T. W. Reynolds: Evaporative cooling in air conditioning ap plications (Heating and Ventilating, March 1944, p. 53). F. W. Hutton: Useful tool--properly engineered evaporative foiling (Heating, Piping and Air Conditioning, July 1950, p. 96). R. S. Farr: Evaporative cooling for comfort (Refrigerating Engineering, May 1953, p. 527). M. H. Irons: Evaporative-cooling for textile management (Mechanical Engineering, February 1943, p. 115). G. T. Lang: Evaporative cooling for textile mills (Heating and Ventilating, December 1945, p. 89). W. L. Fleisher and J. H. Burges: Air conditioning the textile mill (Refrigerating Engineering, May 1954, p. 33). W. L. Fleisher: Evaporative cooling for comfort (Refrigerat ing Engineering, December 1936, p. 409). R. S. Ash: Cooling industrial hot spots with evaporative cool ers (Heating, Piping end Atr Conditioning, December 1955, p. 94). R. S. Ash: Summer cooling for greenhouses (Air Condition ing, Heating and Ventilating, August 1957, p. 67). Evaporative Cooling--A Symposium (Presented at ASHAE 1955 Semi-Annual Meeting). CHAPTER 42 DEHUMIDIFICATION BY SORBENT MATERIALS Types of Sorption DehumidHiers; Liquid Sorption Systems: Liquid Sorbents, Machine Operation and Design; Solid Adsorption Systems: Solid Desiccants, Machine Operation and Design; Sorption Dehumidifiers for Elevated Pressures; Applications for Sorption Dehumidifiers; Moisture Load Calculations; Vapor Transfer to Dehumidified Space EHUMIDIFICATION as used herein is the reduction 3. Stability. It should not break down over the range of use. D of the water-vapor content of a given volume of air or 4. Low viscosity and good heat-transfer characteristics. other gas. The term thus describes a special case of de 5. Widely available and economical to produce. hydration which covers the removal of moisture in any form 6. Capable of being regenerated at temperatures obtainable from matter. The degree of dehumidification required varies with low-pressure steam. greatly with different applications, and is one of the prime The equilibrium contact dew point is {dotted against con considerations influencing the choice of a method. tact temperature for various concentrations of a typical Dehumidification may be accomplished by chilling, as organic liquid absorbent in Fig. .1. The same data for a described in Chapters 19, 23, and 41, by the use of sorbents, typical inorganic absorbent is shown in Fig. 2. The concen or by compression in combination with chilling, sorbents, or . tration of the liquid absorbents used in commercial equip both chilling and sorbents. ment is dependent on the temperature of the cooling medium, Sorbents are substances which have the property of ex and on the latent and sensible heat-removal requirements. tracting and holding other substances (usually gases or The equilibrium contact dew points in Figs. 1 and 2 show vapors, eg;., water vapor) brought into contact with them. the dew point of air in equilibrium with the liquid absorbent. All materials are sorbents to a greater or lesser degree. The Reference should be made to the published performance weight of water held by a substance will increase or decrease, data for commercial apparatus for the solution of air-con depending upon whether the vapor pressure of the water ditioning and air dehumidification problems. Fig. 3 shows a held by the substance is less or greater, respectively, than rating curve for a liquid absorbent dehumidifier nong lithium the partial pressure of water vapor in the surrounding atmos chloride solution. phere. As generally used, however, the term sorbents refers to those materials having a capacity for moisture which is Liquid Absorption Operation large compared to their volume and weight. Such materials are divided into two general classifications: The operating cycle of a liquid absorption system with ex tended-surface contactor coils is shown by Fig. 4. For de 1. Absorbent--A sorbent which changes either physically, chemically, or both, during the sorption process. Calcium chloride is an example of a solid absorbent, while liquid absorbents include solutions of the halogen group (lithium chloride, lithium bromide), and the ethylene glycols. 2. Adsorbent--A sorbent which does not change physically or chemically during the sorption process. Certain solid ma terials, such as activated alumina, silica gel, activated bauxites, and activated charcoal have this property. The action of ad sorbents, most of which adsorb some gases and condensible vapors besides water vapor, is selective. Thus, in the case of a mixture containing both water and organic vapors, silica gel would remove the water vapor in preference to the organic va pors, while the reverse would be-true in the case of activated carbon. The selective property of adsorbents is made rise of in some instances for the removal of objectionable and contami nating vapors from an air or gas mixture. (See Chapter 7.) humidifying operation the strong absorbent solution is pumped from the sump of the unit and sprayed over the contactor coils. The hygroscopic solution at the required temperature and concentration is in intimate contact with the air which is flowing over the coil surface in the same direction as the liquid absorbent. Equipment is also available for flow counter current to the flow of liquid absorbent. Moisture is absorbed from the air by the solution due to the vapor-pressure dif ference between the air and the liquid absorbent. The mois ture content of the outlet air can be maintained at a constant condition by automatically regulating the flow of water through the cooler with a modulating water valve. The heat generated in absorbing moisture from the air is UQU1D ABSORPTION SYSTEMS called the heat of condensation of the water vapor in the solution. This consists of the latent heat of condensation of Liquid Absorbents water vapor and the heat of solution or the heat of muring. The heat of mixing varies with the liquid absorbent used A liquid absorbent has the property of absorbing moisture and with the concentration and temperature of the absorbent. from, or adding moisture to, the air, depending upon the The solution is maintained at the required temperature by vapor-pressure difference between the air and the solution. cooling with city, well, refrigerated, or cooling tower water, The equilibrium vapor pressure of the solution is dependent or refrigerant flowing inside the tubes of the contactor upon the temperature and concentration of the solution. coil. Hie quantity of water required is a function of the Absorbents for a liquid dehumidifying system should have water temperature and the total heat, either sensible, latent, the following characteristics: or both, removed from the air by the hygroscopic solution. 1. Suitable vapor-pressure characteristics. The absorbent should not crystallise at temperatures 10 to 15 deg below the temperature range used in the operating cycle. The dry-bulb temperature of the air leaving the liquid absorbent contactor is a function of the temperature of the liquid absorbent and the amount of contact surface between 2. Noncorrosive, odorless, nontoxic, and nonflammable. the air and the solution. In most commercial equipment the s' 6)9