Document 5bqkoD3432MnN2g3DBJRjo46z

,M 934 CHAPTER 36 1957 Guide;-- series of curves, each curve being drawn for a given condensing pressure: The performance of a direct expansion coil at two different air velocities is plotted on the same graph. The operating point will be, of course, where the two curves cross. Data given in Table 10 illustrate two types of conditioned enclosures having the same total load of 148,000 Btu per hour, but with two different ratios of sensible to total heat. In the case of the office with a ratio of 82 percent sensible to total heat, the operating point A in Fig. 17 is found to be 42.2 F evaporating temperature, with a face velocity of 500 fpm. In the case of the restaurant, with a ratio of 69.5 percent sensible to total heat, the air velocity is lowered to 300 fpm, and the evaporating tempera- Table 10. Typical Operating Conditions foe Two Types of Load Ttfb or Enclosure Load, Btu peb Houb Sensible Latent Total Ratio Sen* BIBLE TO Total Aib Entering Operating Balance Point Con. F Deg Per Cent Evapo rator Temp F Deg Con denser Per Cent Pressure Sensible Lb per Heat Sq in. Restaurant.... 103,000 45,000 148,000 0.695 82 45 34.4 123 69.9 121,000 27,000 148,000 0.820 82 45 42.2 100 82.T ture is lowered to 34.4 F as shown in point B of Fig. 17. In order to obtain the same capacity, a larger condensing unit is used. This illustration assumes zero pressure drop through the suction line. The pressure drop can be taken into account by shifting the compressor performance curves by the amount of pressure drop expressed in Fahrenheit degrees. REFERENCES 1 Air Cycle Refrigeration, by Paul C. Scofield (.Refrigerating Engineering, Vol. 57, No. 6, June 1949, p. 558). * Application and Economy of Steam Jet Refrigeration to Air Conditioning, by A. R. Muinford and A. A. Markson (A.S.H.V.E. Transactions, Vol. 44,1938, p. 33). A New Development in Absorption Refrigeration, by A. A. Berestneff (Re frigerating Engineering, Vol. 57, No. 6, June 1949, p. 553). BIBLIOGRAPHY Refrigerating Data Book, Vol. 1 (American Society of Refrigerating Engineers). Refrigeration Engineering, by H. J. Macintire (John Wiley & Sons). Theory of Mechanical Refrigeration, by N. R. Sparks (McGraw-Hill Book Co.). Refrigeration and Air Conditioning Engineering, by B. F. Raber and F. W. Hutchinson (John Wiley & Sons, 1945). Refrigeration, by J. A. Moyer and R. U. Fittz (McGraw-Hill Book Co.). Refrigerants and Absorbents, by W. R. Hainsworth (Refrigerating Engineering, August and September, 1944). Air Conditioning and Refrigeration, by B. H. Jennings and S. R. Lewis (Inter national Textbook Company, 1944). Refrigeration and Air Conditioning, by Jordan and Priester (Prentice-Hall, Inc., 1948). Heat Pumps, by P. Sporn, E. R. Ambrose and T. Baumeister (John Wiley and Sons, 1947). Heat Pump Applications, by E. N. Kemler and S. Oglesby, Jr. (McGraw Hill Book Co., 1950). CHAPTER 37 dehumidification by sorbent materials Definitions and Principles, Adsorbents, Dehumidification by Solid Adsorbents, .Dehumidification Equipment Using Solid Adsorbents, Absorbents, Dehumidification by Liquid Absorbents, Dehumidification Equipment Using Liquid Absorbents, Calculation of Moisture Load, Vapor Transfer to Dehumidified Space DEHUMIDIFICATION as used herein is the reduction of the water vapor content of a given volume of air or other gas. The term thus describes a special case of dehydration which covers the removal of moisture in any form from matter. The degree of dehumidification required varies greatly with different applications, and is one of the prime considerations influencing the choice of a method. Dehumidification may be accom plished by latent heat removal, together with sensible heat removal, as described in Chapters 29, 34, and 35, or by the use of sorbents. Sorbents are substances which have the property of extracting and hold ing other substances (usually gases or vapors, e.g., water vapor), brought into contact with them. All materials are sorbents to a greater or lesser degree. The weight of water held by a substance will increase or decrease, depending upon whether the vapor pressure of the water held by the sub stance is less or greater, respectively, than the partial pressure of water vapor in the surrounding atmosphere. As generally used, however, the term sorbents refers to those materials having a capacity for moisture which is large compared to their volume and weight. Such materials are divided into two general classifications: L Adsorbent--A sorbent which does not change physically or chemically during the sorption process. Certain solid materials, such as activated alumina, silica gel, activated bauxites^ and activated charcoal have this property. The action of adsorb ents, most of whicn 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 vapors, while the rverse would be true in the case of activated carbon. The selective property of adsorbents is made use of in some instances for the removal of objectionable and con taminating vapors from an air or gas mixture. (See Chapter 8.) 2. Absorbent--A sorbent which changes either physically, chemically, or both, aurmg the sorption process. Calcium chloride is an example of a solid absorbent, ynile liquid absorbents include solutions of lithium chloride, calcium chloride, lith ium bromide, and the ethylene glycols. The ability of an adsorbent to remove water vapor from a gas is explained 6y the fact that the vapor pressure of the water in the adsorbent (when in the reactivated condition) is less than the partial pressure of the water vapor in the surrounding atmosphere. For instance, when an active ad sorbent is brought into contact with a gas of high humidity, there is a tendency for the vapor pressure of the water in the adsorbent to reach equihonum with the partial pressure of the water in the surrounding gas, with 935