Document 82x2xDnn4vn9Jbm6yZkRkOZK

842 CHAPTER 76 1 W. P. Chapman: Design of snow melting systems (Beating and Ventilating, April 1052, p. 95, and November 1952, p. 88). * W. P. Chapman: Calculating the beat requirements of a snow melting system (Air Conditioning, Heating and Ven tilating, September 1956 through August 1957).. *W. P. Chapman: Snow melting system hydraulics (Air Conditioning, Heating and Ventilating, November 1955). P. B. Gordon: Antifreeze protection for snow melting syo- 1962 Guide And Data Book terns (Heating, Piping and Air Conditioning Contractors Htui tional Association Official Bulletin, February-1950, p. 21). / C. S. Cragoe: Properties of Ethylene Glycol and Its Agueoul Solution (National Bureau of Standards, Society of Auto, motive Engineers, CKC Report No. 9). ' W. P. Chapman: Are thermal stresses a problem in soo melting systems? (Heating, Piping and Air Conditioning June 1955, p. 104, and August 1955, p. 92). ' CHAPTER 77 EVAPORATIVE AIR COOLING System Design} Applications for Residential, Commercial, Industrial, Equipment, and Process Coofrng; Form Building, Produce, and Greenhouse Cooling VAPORATIVE cooling is the oldest method used in takes place. The water in the cooler MnmM the wet-bulb E man's attempt to produce comfort in hot climates. In temperature of the air, and cooling proceeds with the enthalpy biblical times,1-'4 wetted grass mats and porous jars utilised or total heat content of the air remaining essentially constant. the evaporative process to cool air or water. The early settlers Humidification occurs as a result of the vapor pressure exerted of the Southwestern United States found the Indians using the by the water which is higher than that corresponding to the gjimA devices. Since then evaporative cooling has been mecha entering air dew point. In the process the dew-point tempera nized. Various devices have been developed and manufactured ture rises and the dry-bulb temperature falls with the wet- to utilise evaporation directly in the air stream or indirectly to bulb temperature remaining constant. The matimum possible cool the air in a heat exchange process. Today direct evapora dry-bulb temperature reduction is the difference between the tive cooling represents the vast majority of installations, as entering air dry-bulb and wet-bulb temperatures, which is indirect systems are usually too costly to justify their use. usually referred to as the wet-bulb depression. If it were pos Evaporative cooling equipment, including unitary equip sible to cool the air to the wet-bulb temperature, the air would ment and air washers, is discussed in Chapter 38 of the 1961 be completely saturated. Since this is not the case in actual Goins And Data Boos. practice, evaporative cooling systems operate at less than 100 SYSTEM DESIGN percent effectiveness. The temperature of the air resulting from the evaporative cooling process should never' be con Various methods for designing evaporative cooling systems are in common use. Rtmplifipd rule-of-thumb methods use an assumed air change rate as a basis. Good engineering practice however, requires an evaluation of the building heat gain, in ternal loads, and the cooling season wet-bulb temperature pat tern. The suitability of evaporative air cooling for a particular application is in the first instance, determined by the climatic sidered as the room temperature, since the latter will be sev eral degrees higher. The temperature difference is usually called the diffusion temperature and depends upon the heat load and the amount of cool air discharged. In calculating the heat load, no attention need be paid to the latent load as the system operates on the baas of 100 percent outdoor air and any moisture pick up is exhausted. Fig. 1 is a skeleton psychro- conditions. Since the lowering of the dry-bulb temperature of the air during the evaporative process is accompanied by a corresponding increase in moisture content, it follows that the greatest application of evaporative cooling for conditions of human occupancy is in areas in which the periods of high temperature are accompanied by low outdoor relative humid ities. The areas in which the climate is suitable for the reliable production of optimum comfort conditions by evaporative cooling are therefore limited. Despite this, evaporative air cooling is extensively used over areas with less favorable climates as a means of improving conditions for human oc cupancy. It finds good acceptance *!> in industrial applica tions, particularly those in which an improvement of dry- bulb temperature can be used to advantage. Successful appli cations of this type are spot and area cooling in factories such as steel mills, metal casting plants and foundries. Laundries and dry cleaning plants, with large exhaust air loads may also take advantage of evaporative cooling. Electric motor cooling and turbine cooling are other examples of industrial demands for cooling where evaporative cooling is practical and eco nomical. In agriculture, evaporative cooling has been used for cattle and poultry barn cooling with excellent results even in climates which give poor results with human occupancy. Greenhouses and common storages for fruits and vegetables use evaporative cooling to reduce temperature and raise humidity. The tobacco industry, especially cigar plants, is Another example of this use for evaporative cooling. New ap plications continue to arise as the control of summer condi tions is looked upon more as a necessity than a luxury. The evaporative cooling process involves what is known as nn adiabatic exchange of heat. The sensible heat of the air is reduced proportionally to the amount of evaporation that Rg. 1 .... Solution of Example 1 on Pydirmetric Chart 843