Document 9kXLyYNqOnxxYJpO7zqg13Le

744 CHAPTER 46 1965 Guide And Data Book Table 1.... Spray Pond Design Data Canwnfranal Up-Sproy $|nhd IMtt . Standard. Mm . Max Water capacity per nozzle.. gpm Nozzles per 12 ft length of pipe..;...:.................. Height of nozzles ` above water level............................ ft Nozzle pressure.:.:. ~ prig Size of nozzles and nozzle 35 to 50 25 60 6 4' 6- 6 5 12 6 ~ -5 - - 7 Distance between spray lateral piping............................ Distance nozzles from pond aide, unfenced........................ Distance nozzles from pond side, fenced............................ Height of louver fence.......... Depth pond basin.................. Friction loss allowed per 10Q ft pipe.................................... Design mod velocity............ ft . 25. ft. : 25 to 35 ft 15 to 20 ft 12. ft 4 to 5 ft 1 to 3 S 13 , 20. 15 12 2 3 38 50 25 : i2 -- __ -- .Table 2 .... Degree Adjustments to be Applied to Leaving Air Wet-Bulb Temperature to find Cooled Water Temperatures of Spray Ponds* Coating Rang* FOg - Oitaifag . .Wet-fefb*. i Adjustment k> F Oigiwi teegtfa of Air fraW 100 ` 50 25 10 80 WB -3 +2 " +4 70 WB -2 +3 +5 60 WB -1.5 +3:5 +5.5 V 15 80 WB -5.0 +1 +5 70 WB -4 +2 +6 , 60 WB -3.5 +2:5 +5.5 20 80 WB -7 0 +6 70 WB -6 +1 +7 60 WB -6.5 +1.5 +7.5 * Coate+mter tempemtsr* -- vet-bulb tempexauu* of leTin* sir phis or minus tbe nluea shown. b-Wet-bulb temperature of iir catenas rpray-fllUd Miuiast 8m text length of air tiarel through iprvg-filti Mfwae. The broadside air flow is 1270 X 440 + 14.2 ~ 39,296 -lb. The end air flow is 720 X 440 + 14.2 - 22,310 lb. ' Tbe beat rise of the air is 500,000 + 39,296 - 12.72 Btu/lb for broadside wind, or 500,000 + 22,310 -- 22.41 Btu/lb Tor end wind. The entering enthalpy of the air is 35.83 Btu/lb, so-the out going air is: Broadside: 35.83 + 12.72 ' '48.55 Btu/lb, corresponding to 84 F wet-bulb. End: 35.83 + 22.41 -- 58^4 Btu/lb, corresponding to 92 F wet-bulb. By mnng Table 2 and interpolating, the cold-water tempera ture for end wind operation will be 2 F below the outgoing wet- bulb temperature, or 90 F. Tbe cold-water temperature for a broadside wind will be 3 F above the outgoing wet-bulb, or 87-F. Wind pairing through a pond carries away entrained water as drift. This creates a nuisance in the area on the leeward side of the pond. This must be considered when selecting the location of the pond. The drift loss can be minimigpH by en closing the spray pond within a louver fence 10 to 12 ft high. Fogging, which can occur during cold weather, presents another hazard to buildings or roadways. A pond should be located with due regard for seasonal variations in wind direc tions, to provide maxiimmi cooling during the hot summer months and minimum hazard to roadways during the winter. Lower water temperatures can be obtained from a spray pond than from a cooling lake, and a much smaller area is needed. This reduces the initial cost when a natural lake is not available. This saving in cost is offset by the cost of pip ing, supports, and spray nozzles. Pumping costs may also be higher, due to the back pressure imposed by the spray nozzles. Atmospheric Towers An atmospheric spray-filled tower is essentially a spray pond with louvered walls and an elevated spray system which usually sprays downward.-While the spray pond uses large nozzles handling 25 to SO gpm each, the spray tower will have | in. to } in. nozzles handling 2 to 5 gpm each. Water loadings vary from 0.6 to 3.0 gpm per sq ft of plan area, and the heights vary from 6 to 15 ft. The design conditions, are generally based on .a 3 mph wind,, but .the down-spray noz zles have an aspirating effect that can'induce a downward air movement of up to 400 fpm. However, wind is needed;to carry the hot vapors away and prevent recirculation... Atmospheric spray towers seldom exceed 50 ft in length. They frequently serve small refrigerating systems, or cool jacket water of internal combustion engines. The performance can be estimated by calculating the outgoing wet-bulb tem perature, as with the spray pond. The cold-water temperature will be approximately equal to the outgoing wet-bulb tem perature, and performance c*n be estimated on that basis. An inadequate spray system will increase the water temperature several degrees. Spray towers require less basin area, less piping, and no more mechanical equipment than spray ponds, but savings may.be offset by the extra cost of the structure. The atmospheric deck tower contains wooden latticework Cooling Towers and Spray Ponds 745 tWirn, regularly spaced from top to bottom. This filling inter rupts the falling water, greatly increases the exposed water surface, and prolongs the time of contact with the air. Towers, such as shown in Fig. 2, are 20 to 50 ft high and 8 to 16 ft wide. The hot water is distributed at the top by a spray sys tem or, more frequently, by open troughs. Except for the vagaries of the wind, the performance is quite predictable.'' This is done by the method described later for cross+low, mechanical-draft towers. The cooling range of spray ponds or spray towers,,which is the temperature difference between the water entering and leaving the pond or tower, seldom exceeds 15 -F deg. [t is difficult to obtain less than a 10 F deg approach, which is a temperature difference of les than 10 F deg between the leaving water and the air wet-bulb, unless the cooling range is lower. These performance .limitations do not apply to the atmospheric .deck tower. The designer can control perform ance by variations in tower height,'water loadings, and density of the filling. The drift loss of a deck tower is considerable, and the nui sance is similar to that caused by the spray pond or spray towers. Both spray and deck towers are frequently located on building roofs, but the drift nuisance is seldom tolerated in congested areas. Water.cooling systems that depend on the wind for air' movement are hot suitable for many services where tiie temperature requirements are more exacting. Mechanical-Draft Towers The mechanical-draft tower is equipped with fans to pro vide a positive and constant air flow. Since performance does not depend on the wind, it is possible to design mechanicaldraft towers for exacting conditions. The fans may operate to provide forced or induced draft, depending on their loca tion at the inlet or outlet of the tower. The tower may be cross-flow, as shown in Fig. 3, or counter-flow, as shown in Fig. 4. The addition of the fan makes it passible to design wider towers that are more compact than the long, narrow atmospheric towers. Early mechanical-draft towers were frequently of the spray-filled, forced-draft type. These sprayfilled towers are rarely used at present, except in congested areas where fire codes prohibit' the use of wood filling. ' The mechanical-draft tower is subject to recirculation of the exhaust vapors. These vapors will usually rise vertically on a still day and cause no trouble. Wind blowing across a tower creates a low-pressure area on the leeward side. The exhaust vapors may then move downward and flow back into the tower.'This increases the entering wet-bulb temperature' and affects-the performance accordingly.' Recirculation is worse with forced-draft towers because of their low exit velocities. As a result, most towers built since 1940 are of induced-draft type. The forced-draft towers, however, have fig. 4 .... Counter-flow Induced-Draft Cooling.Tqwer