Cooling Towers Design
Classified by the air-circulation method used, there are two types of cooling towers. They are either natural-draft or mechanical-draft towers. Figure 8-15 shows the operation of the natural-draft cooling tower. Figure 8-16 shows the operation of the mechanical-draft cooling tower. The forced-draft cooling tower shown in Fig. 8-17 is just one example of the mechanical-draft designs available today.
Cooling tower ratings are given in tons. This is based on heat-transfer capacity of 250 Btu/(min·ton). The normal wind velocity taken into consideration for tower design is 3 mi/h. The wet bulb temperature is usually 80°F (27°C) for design purposes. The usual flow of water over the tower is 4 gal/min for each ton of cooling desired. Several charts are available with current design technology. Manufacturers supply the specifications for their towers. However, there are some important points to remember when use of a tower is being considered:
1. In tons of cooling, the tower should be rated at the same capacity as the condenser.
2. The wet-bulb temperature must be known.
3. The temperature of the water leaving the tower should be known. This would be the temperature of the water entering the condenser.
Towers present some maintenance problems. These stem primarily from the water used in the cooling system. Chemicals are employed to control the growth of bacteria and other substances. Scale in the pipes and on parts of the tower also must be controlled. Chemicals are used for each of these controls.