Air-conditioning system, Simulation, Microchannel, Wet Air
A system model is developed in Engineering Equations Solver (EES) to simulate the performance of a residential air-conditioning system. The system model includes detailed component models of evaporator, compressor, condenser and expansion valve, while the components are linked through refrigerant mass flow rate, pressure and enthalpy. The evaporator and condenser 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. If the humidity ratio of air is high and dehumidification occurs, the air side is simulated based on the total enthalpy method. For round-tube heat exchanger, the tube circuiting is considered. For microchannel heat exchanger, it is assumed uniform distribution among parallel microchannel tube. The compressor model is based on the manufacturerâ€™s 10-coefficient map method. The expansion valve is simulated as an isenthalpic process. The inputs into the system model are air inlet conditions and refrigerant superheat and subcooling. The system model is 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 and condenser, while the round-tube evaporator is later replaced with a smaller microchannel evaporator. The system model shows good accuracy compared to the experimental results. The capacity and COP are within 2% while the saturation temperatures are within 2oC. The system model is applied further to investigate the benefits of replacing the baseline round-tube heat exchangers with microchannel heat exchangers. If the round tube evaporator and condenser are replaced with a respectively equal size microchannel evaporator and condenser, the capacity increases by 12% while the COP increases by 20%. To match the capacity, the compressor speed may be reduced or a smaller compressor may be used. Then, the COP may increase by 28%.