Search

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.

Meteorological enthalpy analysis of temperate and maritime climates above 45˚N suggests that the water-side evaporative cooling technique has considerable unrealised potential with contemporary "high temperature" building cooling systems - such as chilled ceilings. As low approach conditions are the key to exploiting the cooling potential of the ambient air, thermal performance at such conditions needs to be investigated. To address the research issues a test rig, based on an open cooling tower and plate heat exchanger and designed to maximise evaporative cooling potential, has been constructed at DIT. A combination of experimental measurement and analysis is used in the investigations. The performance of open cooling towers, resulting from experimental research, is usually correlated, as a function of the water and air flow rate, in terms of the cooling tower coefficient, or number of transfer units (NTU) achieved. A new correlation has been developed for the experimental tower, which shows a significant increase in the NTU level, at the lower water to air flow rate ratios of interest. As the cooling tower in this application is predominantly a mass transfer device, the evaluation of the total volumetric heat and mass transfer coefficient (kg/sm3) is of particular interest. This coefficient has been determined for the experimental tower and provides a key parameter for the design of this form of heat dissipation in buildings.

Recent developments in enhancing heat transfer in cooling towers, together with the success of chilled ceilings, have prompted a review of the evaporative cooling technique. 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. This paper presents the results of experimental research into the thermal efficiency of a water-side open indirect evaporative cooling test rig designed to achieve low (1-4 K) approach conditions. Secondary efficiencies in the range 0.24-0.76 have been achieved.

Evaporative cooling has had limited application in maritime, temperate, climates due to the low levels of cooling water availability which result, when low temperature (5 to 8˚C), convection based, building cooling systems are used. However, the success of "high temperature" radiant cooling, in the form of chilled ceilings, has prompted a review of evaporative cooling in maritime, temperate, conditions. In order to maximize evaporative cooling availability, however, in this application, it is necessary to achieve low wet bulb temperature approach conditions, at viable levels of primary energy consumption. This paper presents the results of experimental research into the energy performance of an evaporative cooling test rig, designed to maximize cooling water availability at the temperatures required for chilled ceilings (14 to 18°C). Results are compared with typical energy efficiencies of conventional, refrigeration based, building cooling systems. A significant potential for improved energy performance, is shown.

Recent developments have prompted a review of the use of cooling tower based evaporative cooling technology as an effective means of cooling modern buildings. Prominent among these developments is the success of high temperature cooling systems such as radiant ceiling panels and chilled beams. At present, however, there is little published literature which gives a quantitative, in depth analysis of the performance or energy efficiency of cooling towers, used in maritime climates, in conjunction with heat exchangers and run at low approach and low wet bulb temperatures throughout the free cooling season. This lack of knowledge has meant that many current opportunities to benefit from the technology are not availed of by building design teams. To address this issue an automated laboratory test rig has been specifically developed with the aim of optimising the performance and demonstrating the potential of this form of cooling in maritime conditions. This paper, which reports on work in progress, describes the design and development of the rig and presents and analyses the preliminary test results.