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748 CHAPTER 34 1952 Guide .velocity- in a complete.-and nearly horizontal circle. -The spray is evapo rated and the resulting vapor diffused. . This-distribution.of fine spray overthe maximum possible area promotes complete and rapid vaporization.- 'Spray Humidifier's. ;This type-consists of an impact spray nozzle,in a cylindrical casing , with a drainage pan below it. . The aspirating effect of the nozzle induces a moderate air current through the casing which dis tributes the entrained spray. The general method of circulating and returning the water is similar, to that employed for high-duty humidifiers; . A suitable pump-and .centrally-located filter tank-are-required.- . Self-Contained Humidifiers. The self-contained or centrifugal humidi-' fier has the ability to generate and distribute spray without the use of air compressors, pumps, or other auxiliaries. These may be used either singly or in groups. In large, installations, where suitable connections are pro vided to permit the cleaning and'semcing of individual units without, af fecting the room as a whole, group control of the water and power may be employed. : - ' ' - UNIT HUMIDIFIERS ; The term unit hurnidifier denotes an assembly of elements the principal" function of which is to'humidify. The essential element of a unit humidi fier is an atomizer or evaporator. To this may. be added a fan, a heater, outlet varies'or diffusere, and a housing to enclose the various parts. Unit humidifiers faff into four general classificatioris, depending on the method of causing evaporation. These are as follows: (1)' Nozzle Type, (2) Rotary Type, (3) Cascade Type, and (4) Heater Type. ; : In the nozzle type of humidifier water is sprayed into the air and evapo ration is -effected by -adiabatic exchange -of energy. Units of this "type in simplest form, spray a fine mist of water directly into the air in a space. They are, used to a great extent in the, textile industry. " , Iri the rotary type of humidifier the spray is created by rotating vanes or discs which throw the water by centrifugal force, and in- so doing-break itup into a fine mist; In all other respects, this type of humidifier is similar to the nozzle type. It has the advantage over the nozzle type of being-less liable to become clogged. In the cascade type the humidification takes place by water falling in sheets over a series of baffles or trays:; This type -is usually furnished with a fan, air-heater, and air filter all enclosed in a housing. ' ' '. In the heater type of humidifier the water is heated either to .the boiling point or to a temperature at which the water vapor readily passes into the air stream; : There are many variations of this type of humidifier. - In the - simplest form the heating element using steam, hot water, gas, oil, or elec tricity is placed in a pan or vessel of water, and the vapor passes from the surface of the water to the stream of air. A modification of this type of humidifier is the combination of spray nozzle and heater type in which the water is sprayed over a hot surface and evaporated. It has the disadvantage of accumulating scale on the surfaces of the vessel or heating surface. WATER-COOLING TOWERS The removal and . dissipation of heat from a compressed refrigerant or from exhaust steam are important factors in the efficient operation of a refrigerating plant or an electric steam-generating station. This heat removal is generally accomplished by first transferring the heat of the gas Spray Apparatus 749 to coolirig water in a heat exchanger' The water, if cheap or plentiful, may be wasted to the nearest sewer or open waterway, such as a river or lake. Where-water usage is.restricted or expensive, or where the available water contains dissolved salts which would form scale on the heat-exchange apparatus,- it is necessary to recirculate the water, and to cool it, after each passage through the heat-exchanger, by contact with moving air in- some type of water-cooling apparatus.- . Water Use and Conservation r ,: . . Many communities have found thiat. present water systems are not sufficiently large to satisfy the increasing demands of domestic and indus trial users. The reasons for such shortages are primarily: (a) inadequate purification and water-distribution systems; .(b) inadequate sanitary arid storm-sewer disposal -systems; or (c) inadequate sources of water. Even when an adequate supply of water is available from the water mains or private"wells, many cities do not have sufficient sanitary'or storm sewer facilities to handle increasing demands. The sanitary systems are usually limited because of the capacity of the filtration plants, and therefore many cities restrict the use of the sanitary system to sewage. Rivers and Lakes " Until the year 1920, large generating stations were usually located on the banks of rivers, lakes, or artificial ponds. The removal and dissipation of the heat from the Diesel cylinder or the exhaust steam of a turbine was accomplished by. taking in .the circulating water at a considerable distance from the discharge, thus preventing mixing of the heated discharge with the inlet water. The use of water from streams for this purpose has the following disadvantages: the site may be far removed from the fuel source or from power consumers; water supply may limit plant expansion; munici pal restrictions on use of water may hamper operation; costly intake struc tures with screens and sediment basins may be required; drastic flood or drouth conditions, the vagaries of most rivers. upstream pollution, scale-' forming constituents, debris, sand, algae, and formation of troublesome ice may cause operating difficulties. When lakes and cooling ponds have been used as a source of circulating water, the hot water is discharged close to the surface at the shore fine. Natural air movement over the surface of the water causes evaporation over that area, thus carrying the heat away at a rate of about 4 Btu per (hr) (sq ft) .(F-deg temp difference between air and water). Increased density of the water due to loss of heat, causes the cooled water to sink to the bottom, of the pond. The suction connection is therefore located as. far below the, surface, as possible, and at as great a distarice from the dis charge as practicable. The area required by such cooling ponds is about 50 times that of a spray pond, or. about 1000 times that of .a.water-cooling tower to. dissipate the same quantity of heat and achieve-equal operating costs. If the surfaces of such ponds were below the level of surrounding terrain, and ..the shore were wind-sheltered by trees , or other vegetation, so that natural air movement across the surface of the water would be re tarded, the use of a spray pond or water-cooling tower would be indicated. SPRAY COOLING PONDS The spray pond consists of a water, collecting basin, above which spray nozzles ato located in an ari-imgemerit such as shown in !Fig:'4 to Spray