Chart for estimating cooling tower blowdown

If the air entering the tower is saturated, as much as one-third of the heat removed from the water may go into heating the air, while the balance will go into evaporating the water. Thus the water consumption will be only about two-thirds of what would be required if the entire heat load went into evaporation of the water. On the other hand, under unusual conditions at light loads, with a low temperature range and very dry air, evaporation of the water may actually reduce the air dry-bulb temperature so that heat is removed from, rather than added to, the air, and the amount of heat going to evaporate the water actually exceeds the heat load on the tower. 

Under some wind conditions, a portion of the warm moist air leaving the tower may recirculate back through tire tower inlet and thus degrade performance. Forced-draft towers have recirculation rates that are about double those of induced-draft towers. Both water loading and tower height play the dominant role in- recirculation. Correlations exist in the literature for defining the effects of these parameters, and corrections can be applied to the wet-bulb temperature [2,3], Cooling tower fabricators can supply data to estimate the severity of the problem. 

Our discussion up to now has concerned the cooling of hot process waters exclusively. However, we insisted back in Chapter 1 that a cooling tower is nothing more than a device that transfers heat from one mass to another. Therefore, gas coolers are governed by the same theory of operation and design principles as are water cooling towers. 

This correction becomes important in gas cooling operations. Unfortunately, we cannot evaluate Equation 6.19 in a straightforward manner. 

For the gas cooling case in which Le = 1, we must redefine our basic equations for humidification and dehumidification developed back in Chapter 5. 

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Figure 6.15 Chart for estimating cooling tower blowdown.

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