Mathematical Modelling of a Low Approach Evaporative Cooling Process for Space Cooling in Buildings

This paper describes a mathematical model of a low approach open evaporative cooling tower for the production of high temperature indirect cooling water (14-16°C) for use in building radiant cooling and displacement ventilation systems. There are several potential approaches to model evaporative cooling, including: the Poppe method, the Merkel method and the effectiveness-NTU (ε-NTU) method. A common assumption, applied to the Merkel and ε-NTU methods, is that the effect of change in tower water mass flow rate due to evaporation is ignored, which results in a simpler model with reduced computational requirements, but with somewhat decreasedaccuracy. In this paper, a new improved method, called the corrected ε-NTU approach is proposed, where the water loss due to evaporation is taken into account. It is expected by this correction the results of improved ε-NTU in the category of heat transfer will be more close to the results ofmore rigorous Poppe method.The current mathematical model is evaluated against experimental data reported for anumber of open tower configurations, subject to different water temperature and ambient boundary conditions. It is shown that the discrepancies between the calculated and experimental tower outlet temperatures are to within ±0.35°Cfor a low temperature cooling water process (14-16°C), subject to temperate climate ambient conditions and ±0.85°C for a high temperature cooling water process (29-36°C),subject to continental climate ambient conditions.Considering the associated tower cooling loads, predicted results were found to be within a 6% root-mean-square differencecompared to experimental data.