A thermodynamic analysis of the counter flow wet cooling tower (CWCT) in the Steelmaking unit of the Khuzestan Steel Company (KSC) is performed in this paper. We evaluated both energy and exergy formulations for analyzing the heat and mass transfer, exergy and second-law efficiency in this cooling tower. The Lewis factor Lef is an indication of the relative rates of heat and mass transfer in an evaporative process. In many performance analyses of the cooling tower, Lef is assumed to be 1 in order to simplify the heat and mass transfer equations between air and water. If so, evaporative loss is negligible. But in this paper, although heat and mass transfer equations and their numerical solutions are more complicated, Lef is calculated for all parts of the tower in order to achieve more accurate results in predicting air and water conditions into the cooling tower in the analysis on the performance of this cooling tower.