Heat pump refrigerant charge control system

A microcomputer controlled charge control system for modulating charge in a heat pump refrigerant circuit. A charge receiver has its interior in thermal communication with the compressor suction line and has a single charge flow line connected via a charge control valve to the refrigerant circuit at a point intermediate two controllable expansion valves in the high side of the circuit. Super-heating and subcooling strategies are employed to maintain the charge level in the circuit at optimum performance levels, automatically adapting to changes in environmental load conditions on the heat pump heat exchange coils. Additional control strategy is employed to shift refrigerant charge into and out of the circuit during transient operation, such as during start-up, stop and reversal into defrost, to optimize operating efficiency during these transient conditions and also to assure correct placement of the refrigerant charge, for example, to avoid compressor slugging during start-up following an off cycle or a reversal in refrigerant flow.