Round-Tube and Microchannel Heat Exchanger Modeling at Wet Air Condition

This paper discusses the modeling of round tube plate fin heat exchanger (RTPF) and microchannel heat exchanger (MCHX) under wet air conditions. Â The heat exchanger models are based on finite volume method. In each control volume, the empirical heat transfer and pressure drop correlations for refrigerant and air are adopted and the effectiveness-NTU method is applied for heat transfer calculation. For the round tube heat exchanger, the tube circuiting is considered. For microchannel heat exchanger, both uniform distribution and maldistribution among parallel microchannel tubes are investigated and compared. When the air dew point temperature is higher than the tube wall temperature, dehumidification occurs. Two methods are compared to simulate the wet air condition: (1) the air side is simulated based on the total enthalpy method; (2) the air side is simulated based on simultaneous heat and mass transfer method. The heat exchanger models are validated against the experimental results of a 2.5 ton residential air-conditioning system in Air-Conditioning and Refrigeration Center at University of Illinois. The experiment was conducted based on AHSI/AHRI standard 210/240 at Conditions A, B (wet coil) and C (dry coil). The baseline system contains round-tube evaporator, while the round-tube evaporator is later replaced with a microchannel evaporator. The microchannel heat exchanger is tested at both direct expansion and flash gas bypass conditions. The models show good accuracy compared to the experimental results. The capacities are within 2% while the saturation temperatures are within 2oC