VALVE LIFTING MECHANISM.-The valve lifting mechanism consists of a sleeve and pushpin assembly mounted around each con-trolled cylinder. The valve lifting mechanism either holds the suction valve open or allows it to remain in a normal operating position. The position of the valve depends on the action of the valve lifting mechanism which, in turn, depends on actuation by the power element.

We will now explain the operation of the capacity control system and how the four major components work together to provide for changes in refrigerant load requirements of the plant. Follow along as we discuss how the system operates under both a loaded condition and an unloading condition.

LOADED OPERATION.-When a com-pressor is started with a warm load on the refrigeration system, the following events occur. A rise in the suction pressure increases the pressure against the capacity control valve bellows, com-pressing the range adjustment spring. The com-pression of the range adjustment spring allows the valve spring to move the pushpins and needle point of the valve toward the valve seat. The flow of control oil to the crankcase through the oil drain is throttled. Control oil pressure rises as oil enters the capacity control circuit through the orifice from the compressor oil pump circuit. Then increased control oil pressure advances the hydraulic relay piston (against the spring), which feeds oil at full pressure to one or more controlled cylinder power elements, depending upon the position of the control valve. The pump oil pressure in the unloader power elements forces the piston upward, pivoting the lifting fork(s) downward. The lifter pins then drop, allowing the suction valve(s) to seat and load the cylinder(s).

UNLOADING OPERATION.-A drop in suction pressure decreases the pressure against the control valve bellows. The range adjustment spring presses against the pushpins, compressing the valve spring. This moves the needle valve off the seat. Control oil bleeds from the hydraulic relay and control valve to the crankcase, relieving oil pressure on the hydraulic piston. The piston retracts, preventing transmission of pressurized oil to the controlled cylinder power element(s) and to the oil drains to the crankcase. As oil pump pressure to the power element drops, the piston moves downward. The lifting fork(s) is(are) pivoted upward, moving the lifting pins up-ward; the suction valves are raised from their seats; and the controlled cylinders are then unloaded.