Approach vs. Wet Bulb Temperature

After the heat load is calculated and the necessary cold water temperature has been determined, the economies of carrying the gpm circulated vs. range must be determined. Table 3-8 shows that a shorter range vnth additional water circulation reduces the cost per gpm cooled but increases the cost per million Btu/hr. dissipated. 

Design wet bulb temperature is determined by geographical location. Usually the wet bulb temperature selected is not exceeded more than 5% of the time in any area. Wet bulb temperature is a factor in cooling tower selection. The higher the wet bulb temperature the smaller the tower required to give a specified approach to the wet bulb at a constant range and gpm. 

A cooling tower selected to cool 28,500 gpm of water from 118 to 88° F at 70° F wet bulb would be smaller than a cooling tower selected to cool 28,500 gpm from 118 to 88°F at an 80°F wet bulb. In spite of the fact that the wet bulb temperature has increased, the reduction of approach from 18 to 8°F dictates that the tower must be larger. Table 3-9 indicates the effect of wet bulb temperature in sizing a tower when the gpm and range remain constant. 

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Table 3-9. Approach vs. Wet-Bulb Temperature on a Given Tower...

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