RACA Journal I March 2026 www. refrigerationandaircon. co. za 35
SOLUTIONS voices
The compression cycle of a scroll compressor. SCROLL TYPE This compressor uses a unique design with two scrolls, one fixed and one movable, both inter-leaved. The movable spiral is able to orbit or oscillate without actually fully rotating. The movable scroll is connected to the input shaft via a concentric bearing. As the movable spiral oscillates within the fixed spiral, a number of pockets are formed between the spirals. As these pockets decrease in volume the refrigerant is compressed, the pressure and temperature increase, and it is finally discharged through a reed valve at the discharge port in the rear section of the compressor.
The compression cycle is illustrated below. From left to right the low-pressure, low-temperature refrigerant is drawn into the compressor( in blue); and as the compressor scroll orbits the inlet port is closed and the volume of the space containing the refrigerant reduces, compressing the refrigerant( in red).
Finally, the high-pressure refrigerant reaches the discharge port where the refrigerant enters the discharge line to the condenser. Simultaneously, the inlet port opens, allowing lowpressure refrigerant to enter the compressor, repeating the cycle.
VARIABLE STROKE The variable stroke compressor is a variable displacement compressor.
The compressor varies displacement to control capacity to meet A / C system demand at all operating conditions. The compressor features a variable angle wobble plate in five-cylinder axial piston design.
Displacement is controlled by a bellows-actuated control valve located in the rear cylinder head. This control valve senses and responds to the system suction pressure or air-conditioning system demand. Through regulation of the compressor crankcase pressure the wobble plate angle, which determines the length of the piston stroke and therefore compressor displacement, is variable.
To facilitate understanding, when the wobble plate is at a theoretical 90 ° to the pistons stroke, the actual stroke will be virtually zero and at this point the compressor displacement will be zero. Then the wobble plate is at the maximum angle the displacement is at the compressor’ s maximum.
ROTARY VANE Rotary vane compressors consist of a rotor with three or four vanes and carefully shaped rotor housing. As the compressor shaft rotates, the vanes and housing form chambers. The refrigerant is drawn through the suction port into these chambers, which become smaller as the rotor turns. The discharge port is located at the point where the gas is fully compressed.
Variable stroke compressor.
RACA Journal I March 2026 www. refrigerationandaircon. co. za 35