An Improved Design and Rating Analyses of Counter Flow Wet Cooling Towers

Abstract

Cooling towers are one of the largest heat and mass transfer devices that are in common use. In this paper, we present a detail model of counter flow wet cooling towers. The authenticity of the model is checked by experimental data reported in the literature. The values of number of transfer units (NTU) and tower effectiveness (ε) obtained from the model were compared with the commonly described models. Appreciable difference in NTU and ε values is found if the resistance to heat transfer in the water film and non-unity of Lewis number is considered in the calculations. The results demonstrate that the errors in calculating the tower effectiveness could be as much as 15 percent when considering the effect of air-water interface temperature. A procedure for the use of the model in designing and rating analyses of cooling towers is demonstrated through example problems. The limiting performance of the cooling towers; that is effectiveness equal to one, is explained in terms of air-approach temperature. The model is also used for obtaining the maximum possible mass-flow rate ratio of water-to-air, for different operating conditions.