A Theoretical Study on Optimum Charging rate of Refrigerant at the Vapor-Compression Air-Conditioner.
A theoretical model for the transient performance of vapor-compression, air-conditioning system has been developed to evaluate the influence of the refrigerant charge on the system performance. The model is based on a system which has an indoor and an outdoor unit and is rated at 3,500 kcal/h cooling capacity. The major components of the system are an evaporator and a condenser, capillary tube, and a reciprocating compressor.
A set of mass and energy equations for the heat exchangers and the capillary tube and an appropriate model for the compressor are solved numerically based on the finite volume integral method. The momentum equation is not considered in the present model because the pressure drop is typically small compared to the pressure drop across the expansion devise in vapor compression refrigeration.
For a base-case system charged with 750 gram of R-22 refrigerant, the present model successfully predicts the transient behavior of the vapor-compression air-conditioner from the startup. For indoor air of 27oC and outdoor air of 35oC, the evaporating pressure is lowering, the condensing pressure is rising and reaches a steady-state value after about 30 seconds. The refrigerant flow in the compressor is high at the beginning but it gradually decreases, and becomes equal to the capillary flow rate at the steady-state condition. At the steady-state, about 90% of the refrigerant mass is distributed in the condenser and the liquid line.
An estimation of the optimum refrigerant charge is obtained after conducting a set of calculations with different refrigerant charge from 500 grams to 1000 grams. As the refrigerant charge is increased, both the evaporating and condensing pressures increase gradually, but the cooling rate and the COP show a maximum in the range of 750-800 grams of refrigerant charge. This amount of refrigerant mass is determined to be the optimum charge of the system.
The differences between condensing and evaporating pressure are about the same throughout the variation of the refrigerant charge. It implies that it is misleading to use these pressures in seeking the optimum charge. The superheat of refrigerant vapor at the evaporator exit is 1∼6℃ at the range of the optimum charge. The results of the present work suggests that the optimum refrigerant charge in a refrigeration system be determined by examining the variations of cooling rate, COP, and suction vapor superheat, which may vary depending upon the system capacity and the indoor and outdoor operational conditions. Also, the effect of outdoor air temperature on the optimum refrigerant charge is discussed.