Performance evaluation of hot-gas by-pass capacity control schemes for refrigeration and air-conditioning systems

The capacity control of a vapor-compression refrigeration system is investigated by injecting hot gas and liquid refrigerant into the suction side of the compressor. Three different possibilities for the by-pass schemes are investigated for HFC-134a by considering finite size of the components that are used in the refrigeration systems. The model considers the finite-temperature difference in the heat exchangers, thus allowing the variations in the condenser and evaporator temperatures with respect to capacity and external-fluid inlet temperatures. It is demonstrated that the compressor discharge temperatures increase significantly when the hot-gas from the compressor discharge is extracted and injected (without any liquid injection) directly into the suction side of the compressor. A comparative study is also performed of these schemes in terms of the system coefficient of performance (COP), operating temperatures and the refrigerant by-pass fraction as a function of the percentage capacity reduction.COP Coefficient of performanceh Enthalpy (kJ.kg^-1)ṁ Mass-flow rate of refrigerant (kg.s^-1)Q̇ Capacity of the evaporator or condenser (kJ.s^-1)R Liquid to total by-pass ratio of the refrigerantT or t Temperature (°C)y By-pass fraction of refrigerant(εĊ) Product of heat-exchanger effectiveness and fluid-capacitance rate (kW.K^-1)η Compressor efficiencySubscripts1 to 6 state pointscond condenseref,c external-fluid condenseref,e external-fluid evaporatorevap evaporatorref refrigerantSuperscriptin inlet