Application of Water Mist Pre-cooling on the Air-cooled Chillers

The operation of air-cooled chillers accounts for significant energy consumption in local commercial buildings, and low operational efficiency especially under partial load conditions and poor control are part of reasons for such huge energy consumption. Chiller efficiency can be improved by enhancing heat transfer rate in condensers. This paper considers how the chiller performance can be improved by using water mist to pre-cool ambient air entering the condensers to decrease compressor power. A simulation analysis on an air-cooled chiller equipped with a water mist pre-cooling system under head pressure control shows that applying water mist precooling enables the coefficient of performance (COP) to increase. Variable condensing temperature control is superior to head pressure control, where condenser effectiveness is enhanced by staging all condenser fans to decrease the condensing temperature. When the chiller with a water mist pre-cooling system operates under condensing temperature control, the analysis indicates that the COP improvements would be more considerable. The cooling energy saving potential for a representative office building in Hong Kong will be assessed when water mist pre-cooling and variable condenser temperature control are applied to the air-cooled chiller plant. INTRODUCTION Air-cooled chillers are widely used to provide cooling energy in the form of chilled water in airconditioned buildings in the subtropical regions. The operation of chillers usually takes up the highest proportion of the total electricity consumption of buildings (Yik and Burnett, 2001; Yu and Chan, 2005a; Yu and Chan, 2005b). The operating efficiency of air-cooled chillers is low compared to water-cooled chillers. However, air-cooled chiller systems gain popularity due to its flexibility, especially for the cities that have water shortage problem. Low operational efficiency especially under partial load conditions and poor control are part of reasons for such huge energy consumption. To improve energy efficiency, chiller systems should be properly operated to meet the cooling load under external and internal conditions with least energy consumption. The operating efficiency of air-cooled chillers is low compared with water-cooled chillers. The deficient performance of air-cooled chillers is mainly due to head pressure control (HPC) under which the condensing temperature floats around a high set point of 50 ̊C based on a design outdoor temperature of 35 ̊C, irrespective of different chiller loads and weather conditions. Some researchers have stated the opportunity to lower the condensing temperature to save compressor power to improve the operating efficiency of air-cooled chillers. Variable condensing temperature control (CTC) is proposed as an alternative to HPC to lower the condensing temperature in response to changes of the ambient and load conditions (Yu and Chan, 2005b; Yu and Chan, 2006a). As air-cooled chillers apply condenser fans to force outdoor air to condense and slightly subcool the refrigerant. Under such conditions, the extent of the condensing temperature decrease is constrained by the dry-bulb temperature of outdoor air. As evaporative cooling could decrease the temperature of the outdoor air, it is possible to use evaporative pre-coolers to cool outdoor air before entering aircooled condensers. Evaporative pre-coolers could be installed in front of air-cooled condensers, which can pre-cool outdoor air before entering the condensers while consuming less than 15% of the cooling water required by cooling towers and evaporative condensers (Yu and Chan, 2006b). With these coolers, air-cooled chillers can operate more efficiently because the condensing temperature drops followed by any reduced outdoor temperature. However, the condensing fans have to increase their suction power to maintain the heat rejection rate while the pressure of the air entering the condenser is reduced after passing through the evaporative pre-cooler. The pressure drop across the pre-coolers incurs additional fan power, but the compressor power dropped following the decrease in condensing temperature. The use of the pre-cooler resulted in the beneficial tradeoff (Zhang et al. 2000). Zhang (2000) have indicated that the use of evaporative pre-coolers can bring about a 14.7% increase in the COP of air-cooled chillers working in a hot and dry environment. According to a simulation study (Yu and Chan, 2005c), a 1.4–14.4% decrease in chiller power and a 1.3–4.6% increase in the Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009