and causing the refrigerant liquid to boil into a vapor according to certain embodiments the fan may be replaced by a pump that draws fluid through the evaporator the evaporator may be a shell and tube heat exchanger brazed plate heat exchanger or other suitable heat exchanger the refrigerant then flows to compressor 48 as a low pressure and temperature vapor compressor 48 reduces the volume available for the refrigerant vapor consequently increasing the pressure and temperature of the vapor refrigerant the compressor may be any suitable compressor such as a screw compressor reciprocating compressor rotary compressor swing link compressor scroll compressor or turbine compressor compressor 48 is driven by a motor 66 that receives power from a variable speed drive vsd or a direct ac or dc power source according to an exemplary embodiment motor 66 receives fixed line voltage and frequency from an ac power source although in certain applications the motor may be driven by a variable voltage or frequency drive the motor may be a switched reluctance sr motor an induction motor an electronically commutated permanent magnet motor ecm or any other suitable motor type the refrigerant exits compressor 48 as a high temperature and pressure vapor that is ready to enter the condenser and begin the refrigeration cycle again the control devices 44 which include control circuitry 68 an input device 70 and a temperature sensor 72 govern the operation of the refrigeration cycle control circuitry 68 is coupled to the motors 56 62 and 66 that drive condenser fan 54 evaporator fan 60 and compressor 48.