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flow through the tower. Where intermittent operation of, a system is ], employed, water in outside basins may cause considerable damage due to freezing. To prevent this, sucih basins are drained when out of service . and on some small roof installations a tank large enough to hold , all the water in the system may be installed inside the building. In such cases the pump should be protected from the freezing weather. REFERENCES Temperature of Water Available for Industrial Use in the United States, by W. D.Collins (U. $. Geological Survey, Water Supply Paper No. 520 F). . p. 2--Cooling 615). Tower Performance Studies, by L. M. K. Boelter (A.S.H.V.E! Transactions, Vol. 45,1939, * --A.S.H.V.E. Research Paper--Design of Spray Cooling Ponds, by S. Hori, U. A, Patchett and L. M. K. Boelter (A-S.H.V.E. Journal Section, Beating, Piping and Air Conditioning, October, 1942, p. 624). 4--Principles of Chemical Engineering, by W. H. Walker, W. K. Lewis, W. H. McAdams and E. R. Gilliland.(McGraw-Hill Co., 1937, p. 480); --Absorption and Extraction, by T..K. Sherwood (McGraw-Hill Co., 1937, p. 91). ' ^ --Performance Characteristics of a Mechanically Induced Draft, Counterflow. Packed Cooling Tower, by A.'L. London, W. E. Mason and L. M. K. Boelter (A Transactions, January, 1940, Vol. 62, p. 41). 7_Determination of Unit Conductances for Heat and Mass Transfer by the'Transient Method, by AV.olL. .33L.opn.d4o6n7.).H. B. Nottage and L. :M. KU Boelter.{Industrial and Engineering .Chemistry, 'April, .1941, .-^Graphical Method of Determining Number Transfer Units, by T. Baker (Industrial and Engineering Chemistry, August. 1935, Vol. 27, p. 977).- --Loc. Cit. Note 4. p. 79. 10-Loc. at. Note 4, p. 142. * 11--Performance and Selection of Mechanical-Draft Cooling Towers, by Joseph Lichtenstein (A .5.34.. Transactions,,October, 1943, Vol..65,.No..7, p. 779). . t. .1 ;Heat .Transmission, by W. H. McAdams (McGraw-Hill Co., 1933, ;p. 157). 1 *--A.S.H.V.E. Research Report No. 1189^-Performance Characteristics of a Forced Draft,'Counter->` .flow Spray Cooling Tower, by H. H. Niederman. E. D. Howe, J..P,. Longwell, R, A. Seban and L. M. K. Bodter (A.S.H.VX: Transactions, VoL 47,'1941. p.413). ' * >--A.S.H.V.E. Research Report No. 1240---Spray Nozzle Performance In a Cooling Tower, by L. M. . K. Boelter and S. Hori (A.S.H.V.E. Transactions, Vol. 49, 1943, p. 309); Chapter 38 . 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 Barriers 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 moisture1: 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 accomplished by latent heat removal together with: sensible'heat removal, as described in Chapters 7,37, and.43, or by . the use of sorbents. Sorbents are, substances which have the property-of extracting , and holding 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 substance is less or greater respectively: than the partial pressure , of water vapor in the surrounding atmosphere. As generally used, how- ever, 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:- , 1. . 1. Adsorbent--A sorbent which does not change physically or chemically during the - sorption process: Certain solid materials, such as activate! alumina, silica gel; acti, vated. bauxites, and activated charcoal have this property. The: action of adsorbents, most of, which adsorb some gases and condensible, vapors besides water vapor, is selec tive! 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 reverse . 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. contaminating vapors from an air or gas mixture. (See Chapter 10.) 2. 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 lithium chloride, calcium chloride, lithium bromide, and the ethylene glycols. ADSORBENTS i. The ability of an adsorbent to remove water- vapor from , a gas is explained by the fact that the vapor pressure of the water in the adsorbent 1 (when in the reactivated condition) is less than the partial pressure of `the water vapor in the surrounding atmosphere: ' For instance;.when an active adsorbent 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 equilibrium with the partial pressure of the water in the sur rounding gas, with the result that water is extracted by the adsorbent and its weight increased, while the moisture content of the gas'is correspond ingly reduced. (The adsorbent is said to be saturated for a given con dition when equilibrium is attained.) The weight of water a given adsorbent will extract is dependent upon the relative humidity (ratio of the partial pressure in the gas to the saturation pressure at a' given temperature) and the temperature of the adsorbent. ' The process is - 673