A comprehensive analysis of a laboratory scale counter flow wet cooling tower using the first and the second laws of thermodynamics

Abstract

Cooling towers are an integrated part of most of the energy systems. Energy-saving strategies focus on enhancing the overall performance of energy systems. However, to enhance the performance of an existing cooling tower, it is necessary to obtain the tower characteristic equation. When this equation is known, the tower performance could be predicted. In this paper, thermodynamic properties of air and water corresponding to different inlet conditions are determined along the tower by a numerical procedure. Then, by using the calculated properties and the second law of thermodynamics, variations in water and air exergy along the height of the cooling tower are investigated. Entropy generation at each section is calculated to analyze the efficiency of the cooling tower. Results obtained are validated against the experiments performed on an experimental cooling tower. Moreover, mass transfer coefficient variations with water-to-air mass flow rate ratio is obtained. As the water temperature decreases along the tower from top to bottom, both the air wet- and dry-bulb temperatures increase linearly with the height of the cooling tower from bottom to top. Air wet-bulb temperature approaches its dry-bulb temperature at the tower exit, which is an indication of water evaporation. Most of the air exergy exchange is due to the chemical exergy exchange in the tower. Calculating irreversibility at different sections of the cooling tower helps to choose packing of high quality, as based on the importance of heat and mass transfer in entropy generation, higher entropy generation is an indication of the higher efficiency of the cooling tower packing. Total irreversibility along the tower is calculated for each experiment to determine the optimum working conditions for the tower.