| The use of Direct-Expansion (DX) air conditioning (A/C) systems offers many advantages. However, most DX A/C systems are equipped with single-speed compressor and supply fan, relying on On-Off cycling compressor as a low-cost approach to maintaining only indoor air dry-bulb temperature, whereas the indoor air humidity is not controlled directly. In a hot and humid climate like Hong Kong, indoor humidity may remain at a high level in a space served by an On-Off controlled DX A/C System. A poor indoor humidity control can cause discomfort to occupants and degrade indoor air quality, and decrease the energy efficiency of an air conditioning system.;This thesis addresses the indoor humidity control using DX A/C systems. It firstly presents a study on condensate retention on a louver-fin-and-tube air cooling and dehumidification coil, which is commonly used in DX A/C systems. Compared to previously related work focusing on the influence of condensate retention on the heat and mass transfer between air and a cooling coil, the study emphasizes the impacts of operating parameters on condensate retention on a cooling coil. Secondly, a new model for evaluating the wet fin efficiency of cooling and dehumidification coils has been developed and is reported. The condensate film moving on fin surfaces and its impacts on heat and mass transfer have been taken into account. Thirdly, an experimental study on the inherent correlations between the total output cooling capacity and the Equipment Sensible Heat Ratio (SHR) of a DX A/C system is reported. Finally, a new control algorithm, named H-L control, to replace the traditional On-Off control algorithm widely used in DX A/C systems is developed. Extensive experiments suggest that the use of the H-L control would result in a better control performance, in terms of an improved indoor humidity level and a higher energy-efficiency for DX A/C systems, when compared to the use of traditional On-Off control. |
