Design Optimization of Chilled and Hot Water Coil for an Air Handling Unit

Abstract

An Air Handling Unit (AHU) is one of the important components of an air conditioning system. The primary function of the AHU is to maintain the required indoor air conditions. The type and rating may vary based on application in various sectors such as pharmaceuticals, hospitality, hospitals, commercial building, auditoriums, research labs, etc. The AHU along with supporting sensor and controllers can maintain the required indoor air conditions mainly temperature, relative humidity (RH) and air quality (cleanliness). The heat exchangers are the back bone of an AHU’s performance. There are several types of cooling or heating coils which are used for cooling, dehumidification and heating. For cooling and dehumidification process, chilled water (CHW) or direct expansion (DX) coils are used while for heating hot water (HW) / steam coils are deployed. The performance of CHW coil and HW coil govern the temperature and moisture contents of air at outlet based on coil selection data at inlet of coil. There are several parameters which affect the performance of heat exchanger. Each parameter must be accurately worked out in order to ensure the optimum performance and size of the coil. The coil performance parameters can be broadly classified as designing parameters and operating parameters. In the present study, Air Conditioning Heating and Refrigeration Institute (AHRI) Standard 410-2001 for Forced Circulation Air Cooling and Air Heating Coils is used for the determination of coil performance and optimization of coil design. The study shows that any coil design with staggered arrangement is preferable than the inline arrangement. The present study concludes that half circuiting arrangement gives higher heat transfer than the full circuiting connection and the double circuiting connection. The heat transfer observed in half circuiting is almost 25% higher than it is in full circuiting and the air temperature at coil off condition is observed to be 2 °C lower than in half circuiting. The only drawback is water side pressure drop which is 4 times higher in half circuiting connection than that of in full circuiting but the air side pressure in half circuiting remains unchanged as compared with the full circuiting. The present study recommends half circuiting of coil design for the Air Handling Unit (AHU).