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- PublicationSensitivity Studies of a Low Temperature Low Approach Direct Cooling Tower for Building Radiant Cooling SystemsRecent interest in cooling towers as a mechanism for producing chilled water, together with the evolutionof radiant cooling, have prompted a review of evaporative cooling in temperate maritime climates. The thermal efficiency of such systems is a key parameter, as a measure of the degree to which the system has succeeded in exploiting the cooling potential of the ambient air. The feasibility of this concept depends largely however, on achieving low approach water temperatures within an appropriate cooling tower, at acceptable levels of energy performance. Previous experimental work for a full scale evaporative cooling system has shown that it is possible to produce cooling water at low process approach conditions (1-3 K), at the higher temperatures required in radiant and displacement systems (14-18°C), with varying levels of annual availability in different temperate climate locations. For such conditions, evaporative cooling has the potential to offer an alternative approach for producing chilled water, particularly in temperate climates, where conventional mechanical air-conditioning systems can, for certain buildings, be considered to be an over engineered solution but where passive cooling is insufficient to offset cooling loads. The current paper describes the development of a mathematical model which analysesthe behavior of a low approach open evaporative cooling tower. The model is used to carry out a series of sensitivity studies assessing the performance of the cooling tower subject to various weather and climatic boundary conditions.
- PublicationMathematical Modelling of a Low Approach Evaporative Cooling Process for Space Cooling in BuildingsThis 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.