HEAT AND MASS TRANSFER ANALYSIS OF MECHANICAL DRAUGHT COUNTER FLOW WET COOLING TOWER

Abstract

Counter flow wet cooling towers are widely used in power plant and air conditioning application. The function of the cooling tower is to reduce the temperature of circulating water so that it may be reused in condensers and other heat transfer equipments. The hot water in the cooling tower is cooled by evaporation cooling mechanism. In the present work, heat and mass transfer equations used in Merkel, e-NTU and Poppe methods are discussed. A computer programs based on these methods are developed to solve sizing and performance analysis problem of cooling towers. The model is capable for calculating air temperature, humidity ratio, cooling tower outlet temperature, and evaporation loss and tower volume. This model also considers variation in Lewis number and evaporation loss. The differences in the heat and mass transfer analyses and solution techniques of the Merkel, e- NTU and Poppe methods are discussed for different input atmospheric conditions with the help of enthalpy diagrams, percentage error variation graphs for Merkel number and for tower outlet temperature. The comparison of Merkel and e-NTU shows almost similar results because these methods are based on similar simplifying assumptions. The comparison of Merkel, e-NTU and Poppe methods for different initial conditions shows that variation in Merkel number calculated by Poppe method varies from 0.2 % to 1.8 %. Percentage average error variation in tower volume for Poppe and Merkel methods is 16.3 %. For Poppe method percentage errors in outlet water temperature varies 2.53 % to 5.87 % and percentage evaporation varies from 1.26 % to 4.62 %. Outlet water temperature remains smaller for Merkel Method than calculated by Poppe method. Key words: Cooling tower, Merkel number, Tower volume, Evaporation loss, Lewis number, outlet water temperature, Percentage error in tower volume.