Transient Characteristics of Split Air-Conditioning Systems Using R-22 and R-410A as Refrigerants

All air-conditioning systems experience transient processes during start-up and shutdown and varying boundary conditions during operation due to changes in space loads and outdoor air loads, control loop operation, etc. Transient characteristics of split air conditioners that use R-22 and R-410A as refrigerants are presented in this paper. The model used for the overall system simulation comprises compressor, capillary, condenser, and evaporator submodels. The condenser and evaporator are modelled using continuity, momentum, and energy equations over the space and time span, using the implicit finite difference approach. Both of the heat exchangers are assumed to be made up of microfin tubes, although a comparison of transient pressure response with smooth tubes is presented in the paper. Appropriate correlations are used for calculating the two-phase heat transfer coefficients, the liquid vapor slip, and the friction factors. A simulation program is developed in the MATLAB (Mathworks 2004) environment linked with REFPROP for refrigerant properties. The program is validated with measured and published experimental data. The transient simulation results including the pressure, the temperature, and the mass flow rate response for a start-up transient are compared for R-22 and its alternative R-410A. The transient loss during start-up due to the reconditioning of thermal mass of the system and redistribution of the refrigerant among the system components is also compared for the two refrigerants.

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