Document R2BNqBxGYwb3y5RNyD2exYomX

752 CHAPTER 46 1965 Guide And Data Book Cooling Towers and Spray Ponds 753 WET.BULB TEMPERATURE - F Rg. 14 .... Rating Chart for 7 Deg Range0 WET-BULB TEMPERATURE -F Rg. 15.... Rating Chart for 10 Deg Range19 ' distribution system have a significant effect on the charac teristic. Air channeling is caused by the cell structure and this b most significant 'in a counterflow tower, due to the restricted air inlet and the changes in the direct of air flow. A portion of the cooling occurs in the spray chamber above the filling and also in the open space below the filling All of the cooling b credited to the filled volume. The indicated available coefficient reflects the overall performance of the entire1 assembly. The characteristic of a fill pattern varies -widely, and the variations are greater in counterflow towers. A true tower characteristic must be developed from full waI* tests of the actual assembly. DESIGN CONDITIONS A cooling tower b selected to operate at a given set of design conditions, but these conditions must first be selected by considering the function of' the tower within - the overall operation. The cooling tower b one component of the system, all parts of which must operate in equilibrium. As drown schematically in Fig. 13, the heat load may be transferred from the process at a constant or variable tem perature. The heat flows from the process to the circulating water in the beat exchanger, and the heat b then rejected to the atmosphere by the cooling tower. A vapor condenses at a constant temperature corresponding to the saturation pressure. A temperature rise occurs where the heat b transferred to the circulating water. The condensing temperature must be above the hot-water temperature to provide a potential difference. The temperature rise, for a given heat load, b determined by the circulating water rate. The cooling range of the tower will equal tins temperature rise at equilibrium conditions. The degree of elevation of the process temperature above the ambient conditions b composed of the sum of the approach of the cold-water to the wet-bulb temperature in the tower, the cooling range which equals the temperature rise in the heat exchanger, and the terminal difference in the exchanger. A reduction in operating temperature, always desirable for eco nomic reasons, may be obtained by increasing the capability of either the cooling tower or heat exchanger. An increase in the circulating water rate, at constant heat load, reduces the temperature rise and cooling range. The characteristics of a cooling tower are such that thb results in an increase in approach, but the increase in cold-water tem perature b less than the decrease in hot-water temperature. The heat exchanger operates at a constant log-mean tempera- ture difference so the effect of the change in water tempera tures b to reduce the condensing temperature. The overall effect of an increase in water rate b to reduce the <wrHnning temperature, but at the expense of higher pumping costs. Similar conditions exist when a fluid b cooled although the heat b removed from the process at a varying temperature. A temperature differential must exist throughout the heat exchanger, therefore the cooled fluid-must be above the coldwater temperature and the hot fluid must be above the hot' water temperature. The designer selects the water rate and the optimum combi nation of cooling tower and condenser. The temperature levels throughout the system are dependent on the entering wet-bulb temperature, so the design of the system b based 4>n a variable weather pattern. The wet-bulb temperature fol- WET-BULB TEMPERATURE - f Rg. 16 .... Rating Chart for 15 Deg Range0 Rg. 17 .... Rating Chart for 25 Deg Range0 lows daily cycles that peak around noon. The daily cycles vary with the and the average conditions vary from year to year, so the design must be based on 'an average weather pattern for the locality. A refrigerating system has a maximum condensing tem perature, above which it either ceases to function, or must be shut down because of high head pressure. Similar conditions prevail in all processes. A system must be capable of func tioning at aQ tirnpfl when needed; therefore, the minimum rise cooling tower must be capable of maintaining the operation within acceptable limits at the peak wet-bulb temperature. Operating economy b of no importance during-peak condi tions because such periods are rare and of short duration. Operating economy improve as the wet-bulb temperature decreases so the optimum cooling tower b selected on the basis of an evaluation that considers the local average weather pattern which b obtainable from the Weather Bureau or from various other sources.*-* The analysis considers the tower characteristics which relate water temperatures to the wefrr bulb temperature, and the weather pattern which considers the frequency that various wet-bulb temperatures prevail. Thb means that the cooling tower b selected on the basis of a performance curve that covers a wide range of operating conditions. A single set of conditions must be selected as the design point, and any point on the performance curves may be used for thb purpose. It b seldom possible to conduct an acceptance test at the design point, and guarantees are usually restricted to a narrow range of deviations from the dwdgn point. A common practioe b to select a darign wet-bulb temperature .that will prevail or be exceeded 5 percent of the time during the four summer months, or about 150 hours per year.. A low design point b desirable because it increases the number of hours per year that the tower will operate within the guarantee zone. It should be noted that the size or capability of the tower b determined on the basis of an economic evaluation. Once thb b done, any point on the performance curve may be selected for the design point, and the design wet-bulb temperature b chosen on the basis of convenience of testing. PERFORMANCE CURVES Cooling tower performance b specified in terms of range, approach, wet-bulb temperature, and water rate, or in some variation of these terms. The rating of a cooling tower b estjAlinhniH by developing a series of charts that relate these variables. Relative capabilities are compared on the basb of the required sq ft/gpm. The ratio varies slightly with oper ating conditions, therefore comparisons should be made at some standard set of conditions. A convenient standard b 90 F hot-water, $0 F cold-water, and 70 F wet-bulb temperature, referred to hereafter as the 90-80-70 point Variations in tower design are reflected as variations in the area required to meet a given set of conditions. The variation between rating charts of different towers may be n-nnimiepri by introducing the term Tower Unit, which b defined as the area required to cool 1 gpm at 90-80-70. The sq ft/gpm at 90-80*70 becomes unity and all rating charts coincide at that point The family of curves b quite simitar for all towers. The deviations become greater as the conditions become more remote from the 90-80-70 point The rating charts10 shown in Figs. 14, 15, 10, and 17 are constructed in thb manner and represent the average of a