Document Gm29KaGVZqyD0vjN7wVJxMjQv

786 CHAPTER 35 1954 Guide tion in performance. This reduction could be as much as 20 percent under certain conditions. During cold weather, recirculation hnay cause ice formation on adjacent equipment and buildings, as well as in the tower fan ring, with possible resultant fan breakage. Fan sizes are limited to 12 ft or less, and therefore more fans, motors, starters, and wiring are needed than for induced draft towers. Induced draft towers, since fans and motors are not visible, are therefore somewhat more adaptable to architectural treatment. In the spray-filled mechanical draft tower, the area presented to the air is the combined surface area of the small drops present in the tower at any one time. The net free cross-sectional area of the air spaces in a spray-filled tower is greater than that of the wood-filled tower for the same plan area. Before discharging to the atmosphere, the water-laden exhaust air passes through a drift eliminator to remove entrained moisture. This Spray Apparatus 787 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 de creases the time the air is in contact with the water, but since a greater quantity of air is passing through, the average differential between the water temperature and wet-bulb temperature of the air is increased, and this speeds up the heat transfer rate. Increased air quantities are obtained only at the expense of increased fan power, which, for fans of the disc type, increases approximately as the cube of the air handled. The performance of mechanical draft towers is independent of wind velocity; hence, it is possible to design them for more exacting performance. Fig. 7. Forced Draft Cooling Tower type of tower is particularly applicable for installations in restricted areas where city ordinances require fire-proof construction. In the wood-filled tower, lumber of various cross sections is laid hori zontally across the space on as close centers, horizontally and vertically, as required, without introducing too great a resistance to air flow. The water is distributed over the top layer by means of spray nozzles, troughs, splash heads, or through evenly spaced nozzles located in the floor of an overhead open-type water distribution basin, and drops from piece to piece of the wood filling as it progresses downward. As the air moves upward or across the wood filling, the latter presents a large wetted surface, repeatedly breaks up the falling drops of water, and continuously provides new drop surfaces whose integrated areas are several times that of the wood-fill area. The efficiency of a mechanical draft tower is improved by increasing the amount of filling, 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 increases the pumping power. Increasing the area while maintaining constant fan power increases the air quantity somewhat and, because of lowered velocity, increases the time Fig. 8. Counterflow Induced Draft Cooling Tower They require less space and less piping than atmospheric deck towers, and the pumping head varies from 11 to 26 feet, depending upon the design. Overall plant economy, due to colder water temperature, usually more than offsets the additional operating expense and initial cost as compared with those of atmospheric towers. The counterflow (conventional) type of induced draft tower has the fan located at the top, Fig. 8, to provide vertical air movement across the nlling. Air is discharged upward at a high velocity to prevent recirculation. Another type, for small requirements, has the induced draft fan 10 one end (see Fig. 9) to provide horizontal flow. Another induced draft tower, developed for the purpose of obtaining compactness, larger capacity, increased flexibility and improved perormance, is the crossflow type. This type of tower employs multiple ans centered along the top, each fan drawing air through two cells paired fitwf sPc^'on chamber which is partitioned midway beneath the fans and ted with drift eliminators that turn the air upward toward the fan outlet, ms tower obtains a horizontal air movement as water falls in a cascade