Document jmDYppZq4XpqK1yKq1mzdRz89

.72`i CHAPTER 37 1949' Guide or'7 gpm per sq ft because of blanketing effect while the capacity of'the most efficient types range up to 9 or 10 gpm. ' , When an inside cooling tower' is required some adaptation of a spray filled or wood filled induced draft tower is often used and occasionally an air washer is converted to this service. In this type of application pre cautions must be taken to prevent the discharged air'from short circuiting to. the intake: MECHANICS OF ATMOSPHERIC WATER-COOLING The heat exchange in atmospheric water-cooling equipment is accom plished partially by a transfer of sensible heat which raises the wet-bulb temperature of the moving air, but.most of the cooling is due to ah exchange of latent'heat resulting from the evaporation of a small part of the water. Fig. 9: .Small, Horizontal Induced Draft Cooling Tower for 3 to. 50,-ton . Refrigerating Units If all of the water were cooled by evaporation, the rate of evaporation would be approximately one per cent for each 10 deg of cooling. In prac tice, .the loss of . circulating water by evaporation will approximate 1 per cent-: for 12 to 14 deg of actual cooling due to the additional amount of cooling by sensible heat transfer,,and the rate of evaporation will vary from about 0.64 per. cent of the water circulated in the winter to 0.88 per cent in the.summer for a.water-cooling range of 10 deg. -The lowest temperature to which.water, may be cooled in atmospheric cooling equipment is to the temperature of adiabatic saturation, which is at. the wet-bulb temperature of the air. Performance is measured in terms of approach (5 to 10 F deg, with 7 F deg ayerage) of the cooled water to, the wet-bulb temperature of the ambient air when cooling the water through some desired range. The water -cooling range in some;installations will vary from 10 to .12 F deg when a spray pond is used, and from 5. to 17 F;deg (with i0 F deg average) for a mechanical draft cooling tower. Heat absorption: by the moving air in .an'atmospheric water-cooling tower- continues as long as the wet-bulb temperature of the air- is'-lower than the temperature of the water. The rate of heat transfer depends .upon : (1) the area of water in contact with. the.air, (2) the relative velocity of It Spray Apparatus . . 725: the air and water during contact, (3) the difference between the wet-bulb temperature of the air and the initial temperature of the water, and (4), ; the time of contact of the air with the water. The rate of heat dissipation ' is also influenced by many other lesser factors* which further complicate -' the cooling tower design. Ultimate selection of water-cooling equipment for any specified service depends on over-all economic considerations estab lished from correlated performance data. As the enthalpy of the moving air increases, its wet-bulb temperature rises (see Chapter 3). Since it is impracticable to allow the air to be in contact with the water for a long enough time to permit the wet-bulb temperature of the moving air and the temperature of the water to reach equilibrium, atmospheric water cooling equipment aims to circulate only enough air to cool the water to the desired,temperature with least,expenditure of power. DESIGN CONDITIONS FOR WATER-COOLING; The maximum wet-bulb (design) temperature at which the total quan tity of circulating water must be cooled through a specified range by water- Table 3. Effectiveness of Water Cooling Equipment Cooling Equipment Spray Ponds.................. :........................ Spray Filled Atmospheric Towers.. Atmospheric Deck Towers................. Mechanical Draft Towers.................. Water Cooling Eppectiyeness--Per Cent Minimum Typical Maximum 30 40 to 50 60 40 45 to 55 60 50 SO to 60 90 50 55 to 75 93 cooling equipment is never selected as the highest wet-bulb temperature: ever known to have occurred for some locality nor the average wet-biilb temperature over any period of time. The maximum basis would require cooling equipment several times larger than normal capacity, and the average basis would result, for a large part of the time, in higher condenser temperatures than those for which the plant was designed. Accepted design practice for water-cooling towers, evaporative con-, densers, and spray,ponds, is to use the maximum hourly outdoor dry-bull), temperature .which will be exceeded no more than 2) per cent, of-the time' for the months of June to September, alk> to use the maximum hourly wet-bulb temperature which will be exceeded no more than 5 per cent of the total hours for the same period. Tabulation of these data has not been completed. The limited portion of such data as are available is given in Table 3, Chapter 15 for airport weather stations; for other locali ties design dry-bulb and wet-bulb temperatures in use locally are tabulated as a guide to design temperatures. More complete summer weather data,- statistics, charts, maps, and technical analysis have been prepared by' Albright.4 > -^ Equipment for steam turbine condensers and internal combustion en-' gines is usually based upon somewhat lower design temperatures if peak loads occur at night or during winter months when outdoor temperatures are lower. Knowing the hot water temperature and the wet-bulb temperature, for which, the equipment must be designed, the-cold water temperature must be chosen to place the requirement within the effectiveness range , of the