Document LJ9RjY888VGNMK9o1o2Mxwr67

814 CHAPTER 35 1955 Guide type of atmospheric water cooling apparatus to be used. This effective ness is expressed as the percentage ratio of the actual cooling effect to the maximum possible cooling effect. Since the wet-bulb temperature of the entering air is the equilibrium temperature to wjiich the water could be cooled, the effectiveness of water cooling apparatus can be indicated thus: ^ (hot water temperature -- cold water temperature) X 100 Ei = ............ . hot water temperature -- wet-bulb temperature of entering air (2) where Et = water cooling effectiveness, percent. Magnitudes of this effectiveness ratio will vary through wide limits in accordance with construction and conditions of operation. Values indica tive of the commercial range of the effectiveness ratio are given in Table 3, although unusual designs may operate outside these ranges. Example 1: A mechanical refrigeration installation requires 3 gpm of cooling water, per ton of refrigeration, with the hot-water temperature at 95 F and the coldwater at 84 F, with a design wet-bulb temperature of 78 F. Find the water-cooling effectiveness of a mechanical draft tower for the above conditions. Solution: Substituting known conditions in Equation 2, 95 _ g4 Water-cooling effectiveness = ------- -- X 100 = 64.7 percent (typical). 95 -- 78 From a consideration of the factors which include the water-cooling range and the design wet-bulb temperature of the ambient air, the quan tity of water required can be calculated from the amount of heat to be rejected. The average quantities of heat to be removed from various types of mechanical equipment that require cooling are listed in Table 4. WATER-COOLING TOWER DESIGN Because of the many variables5 in water-cooling tower calculations and performance, it is difficult to provide simple handbook equations and tables whereby an engineer can readily select the type and size of unit for a definite requirement. Each manufacturer has a semi-confidential method of sizing a tower, based largely upon research and actual performance correlated with definite requirements; selection of water-cooling equipment for any specified service must ultimately depend upon overall considera tions established from reliable design and performance data. Some of the variables encountered in water-cooling tower work are: continuously changing air and water temperatures throughout the struc ture; varying moisture, content, pressure, and volume of the moving air; caprice of the weather, ambient air changes in temperature, humidity, wind velocity and direction, and the amount of sunshine. Other less important physical properties of the air and water affecting tower per formance are: density, specific heat, conductivity, viscosity, vapor pressure, surface tension, latent heat, coefficient of expansion, vapor diffusivity, emissivity, and molecular weight. The air velocity, overall and in dif ferent parts of the tower, and also the type of air movement provided by natural draft, forced draft, induced draft, counterflow, or crossflow design have an important effect on heat transfer. Different features of construc tion will produce dissimilar velocities of water, and will affect its distribu tion and diffusion as well as the size of drops, jets, sprays, and sheets. The pressure and elevation of the water supply system, as well as the adsorption Spray Apparatus 815 Table 3. Effectiveness of Water Cooling Equipment * Cooling Equipment Spray Ponds.................... *................ Spray Filled Atmospheric Towers.. Atmospheric Deck Towers............... Mechanical Draft Towers................. Wateb Cooling Effectiveness--Pebcent Minimum 30 40 50 50 Typical 40 to 50 55 to 75 Maximum 60 60 90 93 and interfacial surface tension of the wetted tower areas also affect dis tribution. Dissolved gases and other impurities in the water influence the watercooling process. Additional cooling tower variables affecting its perform ance include: location (ground, roof, nearby obstructions, wind orienta tion), dimensions, relative proportions (contour) of tower structure, ma terials, type and arrangement of interior surfaces; the louver and drifteliminator designs as they facilitate the air flow to and from the tower; temperature of the structure at different points as influenced by the external and internal conditions. Other cooling tower factors to be considered are: loss of water by entrainment (drift loss), design and location of water collecting basin, and surface evaporation therefrom; also the noise generated by the air, water, fan, and structure vibration. Basically, a water-cooling tower is a heat exchanger in which heat flows from the water to the air (1) by a flow of sensible heat from the warm water to the cooler air, and (2) by an exchange of latent heat resulting from the evaporation of a small part of the circulating water to increase the humidity ratio of the air by a corresponding amount. The general principles involved are similar to those encountered in the processes of diffusion in absorption and extraction equipment.6'6 ance have wie applic_a_t_io_n__U__i _m_e_piuueoa m water-cooung tower pertormbeen published by various authorities,7'8'9'10'n'12 and those interested in the derivation of the various equations should refer to these references, as listed at the end of this chapter. The approach in each case is based on a heat balance in which the total heat given up by the water equals the total heat absorbed by the air. These derivations are also based on. certain assumptions, viz: that the specific heat of water is unity at the temperatures encountered; that there is no loss in weight of the Table 4. Heat Absohbed bt Cooling Wateb Mechanical Equipment Btu fbb Min pee Ton Refrigeration Compressor....................... Refrigeration, Absorption System........ Steam Turbine Condenser....................... Steam Jet Refrigerating Condenser.... Diesel Engine Jacket & Lube Oil: Four-cycle, Supercharged.................... Four-cycle, Non-supcrcharged........... Two-cycle, Crank-case Compressor........ Two-cycle, Pump Scavenging, Large Unit. Two-cycle, Pump Scavenging, High Speed, Natural Gas Engine: I Four-cycle............................................ Two-cycle............................................. 250 550 -- 550 -- -- -- -- -- -- Btu peb Lb op Steam _ -- 1000 1100 -- -- -- -- -- -- Btu peb BHF-HR ___ ___ -- 2600 3000 2000 2500 2200 4500 4000