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Heating Ventilating Air Conditioning Guide 1938 comer 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 growths, during warm weather, in cooling towers and spray ponds may be eliminated while the plant is in operation by the use of potassium permanganate. This chemical can be dissolved at the rate of 1 lb in 1J4 to \]/2 gal of hot water. About 10 parts of permanganate should be used per million parts of cooling water. The permanganate attacks the algae, forms a brown covering over it, and causes it to settle. Enough of the permanganate solution should be added periodically to cause the water to have a pink color for a period of from 15 to 20 min. Small additions of the permanganate daily do riot give concentrations which are effective. The best results are obtained when sufficient quantities are added periodically at intervals of several weeks, the time intervals being dependent upon local operating conditions. The chemical is non-poisorious and is non-corrosive when used as directed. 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 lower in this connection is somewhat of a misnomer as the apparatus is essentially a narrow spray pond with a high louver fence. As usually built, the nozzles spray down from the top of the structure and the distance from the center of the nozzle system to the fence on either side is not more than half the distance that the nozzles are elevated above the water basin. Heights range from 6 ft to 15 ft and the total width of a" structure is not usually . greater than its height. Spray cooling towers occupy less space on small jobs than spray ponds of equivalent, capacities because the towers have a capacity'of from 0.6 gpm to 1.5 gpm per square foot of tower area. The louvers are continually wet, and so add to the surface of water exposed to the cooling air. Natural Draft Deck' Type Towers In past years most of the atmospheric water cooling , on refrigeration work has been done with natural draft deck type towers, which are also referred to as wind or atmospheric towers. These towers consist of heavy wooden or steel framework from 15 to 80 ft high and from 6 to 30 ft wide, having open horizontal lattice-work platforms or decks at regular intervals from top to bottom, and a catch basin at the foot. The hot water is distributed over the upper part of the structure by means of troughs, splash heads, or nozzles, and it drips from deck to deck down to the basin. The object of the decks is to. arrest the fall of the water so as to present efficient cooling surfaces to the air, which passes through the tower parallel to the decks. The decks also add to the area of water surface exposed to the;air, but since they furnish a resistance to air flow, . too many decks, are a..detriment. 514 Chapter 25. Spray Equipment for Humidification & Dehumidification To prevent the loss of water on the leeward side of the tower, wide splash boards are attached at regular intervals from top to bottom. These boards or louvers extend outward and upward, and in most designs the top edge of each louver extends above the bottom edge of the brie above it. Efficiency of a deck tower is improved, within limits, by increased height, increased length, or increased width. The first'two increase the area of water exposed to the wind, and the latter increases the time of contact of the air with the water. Wind Velocities on Natural Draft Equipment Since natural air movement is the prime requirement for a deck type tower, spray cooling tower, or spray pond, the apparatus riiust be de signed to produce the desired cooling on days when the wind velocity is below average when the wet-bulb temperature is at the maximum chosen for design, and when the plant is operating at full load. The apparatus must also, for best results, be located with its longest axis at right angles to the'direction of the prevailing hot weather breeze. Table 1, Chapter 8, gives the average summer wind velocities arid directions in representative cities. Natural draft cooling equipment should be designed to operate properly with not more than one-half of the average wind velocity, and in no case for a wind velocity of more than 5 mph. It is obvious that natural draft towers and other natural draft equipment must be sp located that they are not obstructed by trees, buildings, or other wind deflectors. Mechanical Draft Towers Mechanical draft towers usually consist of vertical shells, constructed of wood, metal, or masonry, in which water is distributed uniformly at the' top and falls to a collecting basin at the bottom. The inside of the tower may be filled with wood checker-work over which the water drips, or the Water surface may be presented to the air by filling the.entire inside ofthe structure with spray from nozzles. Air is circulated through the tower from bottom to top by forced or induced draft fans;. Since the air flows counter to the water, the air is in contact with the hottest of the water just before leaving the top of the tower, and each unit of air picks up more heat than a similar unit would on natural draft equipment, so- the me chanical draft tower cools water by using less air than the other types of equipment need. As movement of the air through the towers is obtained by power-consuming fans, it is essential that the air used be reduced to a minimum so as to secure the lowest possible operating cost. The efficiency of a mechanical draft tower is increased by increasing height, area, or air quantity. Increasing the height increases the length of time the air is in contact with the water without affecting seriously the fan power required, but it increases the pumping power needed. In creasing the area while maintaining constant fan power increases the air quantity somewhat and because of lowered velocities it increases the time this air is in contact with the water. The surface area of water in contact with the air is increased in both cases. Increasing the air quantity decreases the time the air is in contact with the water, but, since a greater quantity is passing through, the average differential between the water temperature and the wet-bulb temperature of the air is increased, and 515