Refrigeration Manual - HVAC and Refrigeration Information Links

multiple evaporators are used. On large installations,large numbers of solenoid valves may be necessary forsatisfactory automatic control.similarly to the crankcase pressure regulator, except thatit is responsive to inlet pressure. It should be located inthe suction line at the evaporator outlet.CRANKCASE PRESSURE REGULATING VALVESThis type of valve, commonly called a CPR valve ora holdback valve, limits the suction pressure at thecompressor below a preset limit to prevent overloadingof the compressor motor. The valve setting is determinedby a pressure spring, and the valve modulates fromfully open to fully closed in response to outlet pressure,closing on a rise in outlet pressure.The crankcase pressure regulating valve should belocated in the suction line between the evaporator andthe compressor. Since the power requirement of thecompressor declines with a fall in suction pressure, theCPR valve is normally used to prevent motor overloadingon low temperature units during pulldown or defrostcycles. Use of the valve permits the application of aAn EPR valve modulates from fully open to fully closed,closing on a fall in inlet pressure, and its sole function isto prevent the evaporator pressure from falling below apredetermined value for which the regulator has beenset.HOT GAS BYPASS VALVESHot gas bypass valves are used where it is desirableto modulate the compressor capacity and at the sametime prevent the suction pressure from falling toobjectionable low levels. These valves operate in thelarger displacement compressor without overloading agiven size motor, but pressure drop through the valvemay result in an unacceptable loss of system capacityunless the valve is adequately sized.EVAPORATOR PRESSURE REGULATING VALVEOn systems with multiple evaporators operating atdifferent temperatures, or on systems where theevaporating temperature cannot be allowed to fall belowa given temperature, an evaporator pressure regulatorvalve is frequently used to control the evaporatingtemperature. This valve, often called an EPR valve, acts7-9© 1967 Emerson Climate Technologies, Inc.All rights reserved.