Apparatus and method for mass flow control of a working fluid

In combination with a heat transfer system of the type having a fluid circuit comprising a compressor for pressurizing a working fluid received from an evaporator, a condenser and condenser fan for cooling working fluid received from the compressor, and an electrically controlled expansion valve for controlling flow of the working fluid between the condenser and the evaporator:

• a quality sensor operative in response to a control signal and disposed in the fluid circuit on the outlet side of the evaporator, a high pressure sensor disposed in the fluid circuit on the high pressure side of the expansion valve, and control means for providing said control signal to said quality sensor;
• said quality sensor detecting quality of the working fluid in the heat transfer system;
• said control means producing output signals for activating and deactivating the compressor and condenser fan and regulating flow of the working fluid through the expansion valve in response to said high pressure sensor output and said quality sensor output as a function of control setpoints;
• said control means operating said valve in a flow reduction mode to reduce high side pressure before deactivating the compressor.