Document ppMxmk6QDmqvVqa7qbMekwKVa

c American Society of Heating and Ventilating Engineers Guide, 1934. pheric water cooling apparatus is expressed as the percentage ratio of the actual cooling range to the possible cooling range. Since the wet-bulb temperature of the entering air is the lowest temperature to which the water could possibly be cooled this is: Percentage cooling efficiency of atmospheric water cooling equipment = (hot water temperature -- cold water temperature ) X 100 hot water temperature -- wet-bulb temperature of entering air Efficiencies of various types of atmospheric water cooling apparatus vary through wide limits, depending upon air velocity, concentration of water per square foot of area, and the type of equipment. The commercial range of efficiencies is given in Table 3 although unusual designs may operate outside these ranges. Table 3. Efficiency of Atmospheric Water Cooling Equipment Equipment Natural Draft Deck or Atmospheric Mechanical Draft................................... Cooling Efficiency--Peb Cent Minimum Usual Maximum 30 60 40 60 35 50 to 70 90 35 55 to 75 90 From consideration of the factors which include the cooling range and design wet-bulb temperature, the quantity of water required can be calculated from the amount of heat to be dissipated. The normal amounts of heat to be removed from various parts of the cooling equipment are: Compressor refrigeration................................ 220 to 270 Btu per minute per ton Condenser turbine........................................... 950 to 980 Btu per pound of steam Steam jet refrigerating appartus.................. 1030 to 1150 Btu per pound of steam Diesel engine................................................... ..2800 to 4500 Btu per horsepower Cooling Ponds A natural pond is often used as a source of condensing water. The' hot water should be discharged close to the surface at the shore line, as natural air movement over the surface of the water will cause evaporation and carry away heat. Because increased density due to the loss of heat causes the cooled water to sink to the bottom of the pond, the suction connection for intake water should be placed as far below the surface as possible, and at as great a distance from the discharge as practicable. Spray Cooling Ponds The spray pond consists of a basin, above which nozzles are located to spray water up into the air. Properly designed spray nozzles break up the water into small drops, but not into a mist because the individual drops must be heavy enough to fall back into the basin and not drift off. The water surface exposed to the air for cooling is the combined area of all the small drops. Since the rate of heat removal by atmospheric water cooling is a function of the area of water exposed to the air, the difference in 154 Chapter 11--Humidifying and Dehumidifying Equipment temperature between the water and the wet-bulb temperature of the air, the relative velocity of air and water, and the duration of contact of the ajr with the water, a much larger quantity of heat may be dissipated in a given area with the spray pond than with the cooling pond, because of (1) the speed with which the drops travel as they are propelled into the air and fall back into the water basin, (2) the increased wind velocity at a point above the surrounding structures or terrain, (3) the increased volume of air used, and (4) the vastly increased area of contact between air and water. Spray pond efficiencies are increased by (1) elevating the nozzles to a higher point above the surface of the water in the basin, (2) increasing the spacing between nozzles of any one capacity, (3) using smaller capacity nozzles, to decrease the concentration of water per unit area, and (4) using smaller nozzles and increasing the pressure to maintain the same concentration of water per unit area. Usual practice is to locate the nozzles from 3 ft to 6 ft above the edge of the basin, to supply from 5 lb to 12 lb pressure at the nozzles, using nozzles spraying from 20 gpm to 60 gpm each and spacing them so the average water delivered to the surface of the pond is from 0.1 gpm per square foot per minute in a small pond to 0.8 gpm per square foot per minute in a large pond. Increasing the pressure, spacing the nozzles farther apart, or increasing the elevation of the nozzles will increase the cross section of spray cloud exposed to the air, and therefore increase the quantity of air coming in contact with the water. Best results are obtained by placing the nozzles in a long relatively narrow area located broadside to the wind. Spray ponds may be located on the ground if they have an earthen or a concrete basin, or they may be placed on roofs having special waterproof roofing. To prevent excessive drift loss, or the carrying of entrained water beyond the edge of the pond by the air on the leeward side, louver fences are required for roof locations and for those ground locations where space is so restricted that the outer nozzles cannot be located at least 20 ft'to 25 ft from the edge of the basin. Such fences usually are con structed of horizontal louvers overlapping so the air is forced to turn a corner in passing through the fence, and the heavier drops of water are thrown back, owing to their inertia. The louvers also restrict the flow of air, particularly at the higher wind velocities, and thus further reduce the possibility of water being carried off. The height of an effective fence should be equal to the height of the spray cloud. Louver boards are preferably of red gulf cypress or California redwood supported on castiron, steel or wood posts. Where building ordinances forbid the use of combustible materials, sheet metal is customarily used. Algae formations may be a considerable nuisance in a spray pond. Such growths are killed by the periodic addition of potassium permanga nate to the pond water. Addition of the dissolved chemical should be made until the water holds a faint pink color' for at least 15 min. Spray Cooling Towers Where not more than 30,000 Btu per minute are to be dissipated, the spray cooling tower-is a satisfactory apparatus. The word tower in this connection is somewhat of a misnomer as the apparatus is essentially a 155