Truck/Trailer Refrigeration Unit - Sunbelt Transport Refrigeration

1.10 COMPRESSOR PRESSURE REGULATINGVALVE (CPR)This adjustable regulating valve is installed on thesuction line of the compressor to regulate the amount ofsuction pressure entering the compressor. The CPR valveis adjusted to maintain a maximum suction pressure. ForCPR settings, refer to Section 1.5.The suction pressure is controlled to avoidoverloading the electric motor or engine during high boxtemperature operation. To adjust the CPR valve, refer toSection 4.20.1.11 ACCUMULATORThe accumulator is a refrigerant holding tanklocated in the suction line between the evaporator andcompressor. The purpose of the accumulator is toprevent or minimize entry of any liquid refrigerant (thatmay be entrained in the suction line) into the compressor,causing internal damage.This is accomplished by the compressor drawing therefrigerant vapor through the outlet pipe of theaccumulator, which is equipped with an orifice. This orificecontrols the oil return to the compressor and prevents theaccumulation of oil within the accumulator tank.1.12 COMPRESSOR UNLOADERSa. Temperature Control1. Cool light (CL) or heat light (HL) illuminated(depending on mode of operation).2. In low speed cooling or heating, unloader relays(UFR, or UFR & URR) may energize to unloadcompressor banks. Refer to Table 2-2 or Table 2-31.12.2 Suction Pressure OperationSuction pressure is used for controlling thecompressor unloaders to reduce capacity in thecompressor at low suction pressures.If suction pressure falls to 0 psig or below and nounloaders are energized, then UFR will energize.If suction pressure falls to 0 psig or below, and UFR isenergized, URR shall also be energized.If suction pressure rises to 6 psig when URR andUFR are energized, then URR is de-energized.If suction pressure rises to 6 psig when UFR isenergized, then UFR shall be de-energized.There is a five minute time delay set between any ofthe load/unload steps due to suction pressure that arelisted above.This algorithm is turned off during defrost.1.12.1 Unloading in Temperature ModeThe compressor is equipped with unloaders(electronically controlled by the microprocessor) forcapacity control.The capacity controlled cylinders are easilyidentified by the solenoid which extends from the side ofthe cylinder head. When the solenoid is energized thecylinders unload. The unloaded cylinders operate withlittle or no pressure differential, consuming very littlepower. A de-energized solenoid reloads the cylinders.NOTES1. The unloader relay is locked in for aminimum of five minutes once it isenergized due to suction pressure.2. There is a delay of 30 seconds betweende-energizing one set of unloaders tode-energizing the other set of unloaders.There are two modes of unloader operation;temperature control and suction pressure control.1-25 62-02884