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590 CHAPTER 40 1959 Guide of the evaporative condenser have been started. In some cases, an automatic damper is used on the inlet of the evaporative copdfnapr to provide proportional control of bead pressure on the refrigeration system. See Chapter 43 for general information on controls. REFERENCES * W. D. Collins: Temperature of Water Available for Industrial Use in the United States (U. S. Geological Survey, Water Supply Paper No. 520F). *R. F. Throne: Cooling performance of an industrialised Kite surface (.Power, September 1951). * S. Hori. U. A. Patchett, and L. M. K. Boelter: Design of spray cooling ponds (ASHVE Journal Section, Heating, Piping and Air Conditioning, October 1942, p. 24). W. H. Walker, W. K. Lewis, W. H. McAdams, and E. R. Gilliland: Principles of Chemical Engineering (McGraw-Hill Co., New York, 1937, p. 480). D. Q. Kern: Process Heat Transfer (McGraw-Hill Co., New York, 1950, p. 563). * W. K. Lewis: The evaporation of a liquid into a gas (ASMS Transactions, Vol. 44, 1922, p. 325). F. Merkel: Verdustungs kuhlung (Forschungarbeiien, No. 275, 1925). * H. B. Nottage: Merkel's cooling diagram as a performance correlation for air-water evaporative cooling system (ASHVE Transactions, Vol. 47, 1941, p. 429). * W. H. McAdams: Heat Transmission (McGraw-Hill Co., New York, 1933, p. 157). 10 A. L. London, H. B. Nottage, and L. M. K. Boelter: De termination of unit conductance for heat and mass transfer by the transient method (Industrial and Engineering Chemistry, April 1941, p. 467). 11 Joseph Lichtenstein: Performance and selection of me chanical-draft cooling towers (ASMS Transactions, 1943, p. 779). D. R. Baker and L. T. Mart: Analysing cooling tower performance by the unit-volume coefficient (Chemical En gineering, December 1952, p. 196). J. C Albright and D. R. Baker: Summer Weather Data (The Marfey Co., 1944). 14 D. R. Baker: Studies relative to protection of cooling tower lumber (ASHE Oil, Gas and Power Division Proceedings, June 1954). CHAPTER 41 EVAPORATIVE AIR COOLING AND HUMIDIFICATION Apparatus for Humidification, Spray Generation, Spray Distribution, Unit Humidifiers, Air Washers, Humidification with Air Washers, Dehumtdtfication and Cooling with Air Washers, Evaporative Air Cooling, Types of Direct Evaporative-Cooling Equipment, Design Considerations, Controls, Maintenance EVAPORATIVE Air cooling and humidification increase induce air flow through a casing or duct for distribution of the moisture content of an air stream or the air in a the spray. Fan propulsion uses a power-driven fan to dis selected space, with or without a change in dry-bulb tempera tribute the spray. ture. Evaporative air cooling applies the well-known phenome Atomizing Humidifiers non of evaporating water at ambient temperature into an air stream. If no heat is added during the process, the dry-bulb temperature of the air stream is reduced along the thermo dynamic wet-bulb temperature line with a consequent in crease in latent heat and moisture content. Humidification, used industrially and in residential airconditioning and heating systems, may be accomplished by evaporative air-cooling apparatus, by the direct introduction of water spray into the air stream or space either by heated evaporating pans or steam jets. Some equipment can be used In several types of atomizing humidifiers nozzles are placed within the room. Compressed air effects complete atomiza tion of the water which is converted into vapor by the heat of the room sir. In some of these nozzles an aspirating effect draws the water into the nozzle and atomizes it. Others are operated on a combined air and water pressure. It is usual for nozzles of the water-pressure type to be controlled by a diaphragm valve actuated by the pressure of the atomizing air. for both cooling and hnmidifirAttnn High-Duty Humidifiers APPARATUS FOR HUMIDIFICATION Humidification may be direct or indirect, in., water in droplets or vapor form may be' introduced directly into the humidified space, or previously humidified air may be circu lated in or through the space. Evaporative air cooling is al ways indirect although the cooling may be accomplished by wetted surface or secondary cool surface. The classification of apparatus used for humidification and evaporative air cooling is shown-in Table 1. Water is supplied at about 150 psig to an impact-type spray-generating nozzle located in a cylindrical casing. A drainage pan provides for the collection of unevaporated wa ter which is returned to a filter tank. A powerful air current from a fan mounted above the unit, flows through the cyl inder and is charged with moisture. Air discharges from an opening below at a high velocity in a complete and nearly horizontal circle. The spray is evaporated and the resulting vapor is diffused and distributed. HUMIDIFICATION SPRAY GENERATION AND DISTRIBUTION Spray generation is obtained by (l) compressed air, (2) impact, (3) hydraulic separation, and (4) mftr.hftnir.ftl sepa ration. Atomisation can be accomplished by the use of a com pressed air jet to reduce the water particles to & fine spray. With the impact method, a jet of water under pressure im pinges directly on the end of a small round wire. Where hy draulic separation is employed, a jet of water enters a cy lindrical chamber and escapes through an axial port with a rapid rotation which causes it to separate in a fine coneshaped spray. In the mechanical separation process, water is thrown by centrifugal force from the surface of a rapidly revolving disc and separates into particles sufficiently gmall to be utilized in certain types of mechanical humidifiers. Spray distribution is obtained by (1) air jet, (2) induction, and (3) fan propulsion. Air-jet distribution utilizes the atomizing jet to distribute the spray through the space. Induction distribution utilizes the aspirating effect of an impact or centrifugal spray jet to Spray Humidifiers This type uses a spray nozzle in a cylindrical casing equipped with a drainage pan. The aspirating effect of the nozzle induces a moderate air current through the wising which distributes the entrained spray. The general method of circulating and returning the . water is Rrmilar to that em ployed for high-duty humidifiers. A suitable pump and cen trally-located filter *Anlr are required. Self-Contained Humidifiers In this type mechanical or centrifugal means are used to generate and distribute spray. They may be used other singly or in groups. In large installations, where suitable connections are provided to permit the cleaning and servicing of individual units without affecting the'room as. a whole, group control of the water and power may be employed. UNIT HUMIDIFIERS The term unit humidifier denotes an assembly of elements the principal function of which is to humidify. The essential