Capacity Control on the Carrier/Carlyle 06D and 06E compressors is accomplished in one of two ways, hot gas by-pass (HGBP) or suction cut-off unloading (SCOU).
Both types can be electrically controlled with a solenoid or operated with a pressure operated capacity control valve. The hot gas by-pass shown here consists of the unloader head, the unloader control, and a matching HGBP valve plate. The unloader head has two chambers, one connected to the suction side of the compressor, the other connected to the discharge side of the compressor.
These two chambers are inter-connected with a passage between the two. This passage can be open or closed depending on the position of the bypass piston. When open, the refrigerant pumped by the cylinders in the unloader bank is by-passed back to the suction side of the compressor. When closed, the refrigerant is forced into the discharge manifold.
The discharge sides of all cylinder banks are connected internally via a discharge manifold. Since this type of unloading opens a port between both the discharge and suction sides of the compressor a check valve is necessary to prevent an unloader from bypassing the gas from all cylinders back into the suction. This check valve is part of the HGBP valve plate assembly and must be used with this type capacity control.
These compressors are shipped with a bypass plug and spring in all unloader heads. This plug and spring manually closes the unloader bypass port and loads the compressor. If an capacity control valve is installed the plug and spring must be removed.
Proper operation of the unloader control valve should be checked when installed. This includes the control set point and differential settings on the pressure operated control valve. An improperly installed or defective control valve can result in compressor failure.