Efficiency of evaporative water cooling towers

Summary

This paper describes a new method for calculating the interdependence of temperatures of the air and water streams which exists in an ideal, 100% efficient evaporative cooling tower and which pertains to various counter current air/water flow ratios. The interrelation is presented in the form of a graph, which facilitates the establishment of the average air/water flow ratio, prevailing in a real cooling tower, by means of the recorded initial and final enthalpy levels (temperatures) of the two streams. The graph enables also the determination of the minimum air/water flow ratio which is required for producing an equal water cooling effect in an ideal operation at the given initial enthalpy levels. The quotient of the two flow ratios (ideal/real) represents a new measure of cooling tower efficiency which is expressed in fractions of the ideal maximum heat absorption by the air stream. Conversely, the reciprocal value of this quotient, minus one (real/ideal — 1) represents the fractional amount of the prevailing excess air flow. Based on the reviewed principles of the evaporative cooling process, a series of conclusions are drawn with regard to the influence of some operational variables on tower performance. The computations presented in this paper refer to normal near atmospheric pressures. They have to be recalculated in accordance with the indicated method for application to cooling towers which operate at a different pressure level.