An exact analytic method for evaluating heat and mass transfer in closed circuit cooling towers, previously developed by the author, has been expanded and revised to provide a computerized means to predict the thermal performance and determine the associated energy requirements for specified tower design. The validity of the model has been verified and fine-tuned by extensive laboratory testing. After a brief overview of the analytical model, it is demonstrated how this model can be effectively applied to both counterflow and parallel flow spray water-air flow arrangements in actual tower designs. The main computer program options are then discussed, with emphasis on the iterative nature of the solution. Finally, the temperature profiles of the process fluid, spray water, and air wet-bulb temperature are presented and discussed for both flow arrangements. An overview of the laboratory test program is also presented. In summary, this paper reports on the successful combination of the latest state-of-the-art analytical procedures with advanced programming techniques and extensive laboratory testing to create a workable and powerful engineering software package for evaluating the thermal performance of large closed-circuit cooling towers.