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Performance study of an HVAC counterflow cooling tower in a humid climate.

Article Excerpt
INTRODUCTION

The United Arab Emirates (UAE) is one of the most wetted regions in the world. The weather in UAE is hot and humid, and conventional air-conditioning systems are used to cool and dehumidify the air. Air-cooled chillers are normally selected at a dry-bulb temperature of 46.6[degrees]C (115.9[degrees]F) and installed in high-rise buildings. It is rare to install water-cooled chillers because of the high humidity in the summer season. Water-cooled condensers can be used to reject heat from the refrigeration system in moderate and dry climates. Accordingly, water cooling towers occupy a considerable area of the roof space and, hence, limit the architectural designs.

Recently, district cooling was used in Abu Dhabi and Dubai for high-rise buildings. The Plaza is one of the new buildings under construction in Abu Dhabi (see Figure 1). It consists of five towers designed with a conical shape on the top. This makes the use of air-cooled chillers difficult because of the large quantity of chillers needed to condition the five towers. Moreover, there is no district cooling station close to the towers. It was suggested to use water-cooled chillers, specifically five water-cooled chillers chosen compared to 22 air-cooled. The use of cooling towers is mandatory in such a case. Cooling towers are widely used in dry and moderate climates to remove heat from industrial processes and from refrigeration and air-conditioning systems. Simultaneous heat-and mass-transfer processes at every section of the cooling tower give rise to complicated design equations.

[FIGURE 1 OMITTED]

Experimental investigations of cooling towers should lead to better designs and efficiency. In counterflow cooling towers, hot water is sprayed into an airstream, heat and mass are transferred, and the water enthalpy decreases while that of air increases. In order to increase the cooling rate, the interface area between air and water is increased by providing packed and fluidized beds. However, the UAE has a humid climate that affects the performance of cooling towers. The present study is conducted to investigate the possibility of mixing exhaust air from conditioned spaces in various proportions with fresh air so as to arrive at an optimum mixing ratio to suit efficient cooling tower operation to reduce the entering air wet-bulb temperature to the cooling tower.

Balanced mechanical ventilation with heat recovery is normally used in high-rise buildings in Abu Dhabi. Constant treated fresh air is supplied by using roof-mounted air-handling units and distributed by insulated ducts to the building floors. Also, air will be centrally extracted from kitchens and toilets by central roof-mounted extract fans. Therefore, there is no added horsepower to get conditioned air to the cooling tower.

There has been a rising trend over recent...

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