A method and apparatus for maintaining a relatively constant temperature of a working fluid in an evaporator of a refrigeration system by providing a constant volumetric displacement compressor and a heat exchanger for exchanging heat between the high pressure and low pressure portions of a refrigeration circuit to superheat, and hold substantially constant, the temperature of the refrigerant entering the compressor. In doing this, the pressure of the refrigerant in the low pressure portion of the circuit, including the evaporator, and the mass flow rate of the refrigerant remain substantially constant. As a result, the temperature of the saturated refrigerant in the evaporator remains substantially constant.