Analysis of Air Side Cooling and Dehumidification Performance in Evaporator in Direct Expansion (DX) Variable Refrigerant Flow (VRF) Air-Conditioning (A/C) System

Direct Expansion Variable Refrigerant Flow Air-Conditioning (DX-VRF-A/C) units can be seen in buildings of all sizes. The evaporator in a DX-VRF-A/C unit (or DX air cooling coil), normally generates heat and mass transfer in the form of cooling and dehumidifying of hot and humid air flowing through the cooling coil. DX air-conditioning unit with single speed compressors and forced draft fans usually rely on switch control to maintain only the indoor dry bulb temperature which results in an uncontrolled balance of indoor humidity and reduces thermal comfort, and consequently causes poor indoor air quality (IAQ) as well as produces low energy efficiency. Thus, DX air-conditioning unit with variable-speed compressors and forced draft fans are increasingly adopted in pursuit of higher energy efficiency and thermal comfort indoor environment. Hence, study the simultaneous heat and mass transfer performance in DX air cooling coil of DX air-conditioning unit with variable-speed compressor and forced draft fan is essential, however, research work is still lacking in regards to this A/C system. A comprehensive analysis of the airside cooling and dehumidification process in DX cooling coil and its application are illustrated in this chapter. It commences with introducing the calculation of an important dimensionless parameter, namely steady state Equipment Sensible Heat Ratio (SHR) of DX air cooling coil, which is defined as the ratio of the output sensible cooling capacity of DX cooling coil to the total output cooling capacity. An experimental study on estimating on the dehumidification effect on the airside of the superheated region (SPR) in a DX cooling coil is given. The development of the modified Logarithmic Mean Enthalpy Difference (LMED) method for evaluating the total heat transfer rate in a wet air cooling coil operating under both unit and non-unit Lewis Factors (Le2/3) conditions is reported at the end of this chapter.