Axial Piston Variable Capacity Compressor

By 2 August 2014

The axial piston type (Figure 2.5) is one of the most common types of compressor and can be fixed or variable capacity. The pumping cylinders are circumferentially situated around the outside of the drive shaft (4) and parallel to its axis. Each cylinder has a double ended piston with a separate pumping chamber at each end. Each pumping chamber has a set of inlet and discharge reed valves.The inlet reed valves are connected to the inlet port of the compressor via internal drillings and the discharge valves are connected to the discharge port of the compressor via internal drillings.

Pumping operation
A swash plate (3) is attached to the drive shaft.The swash plate is located at an angle.The pistons inside the pumping chamber are connected to the swash plate via swivelling ball joints. The rotation of the swash plate causes the pistons to reciprocate inside their pumping chambers. When the volume above a piston increases the refrigerant flows into the chamber via the inlet reed valves – induction. When the volume above a piston is reducing the pressurised refrigerant exits the pumping chamber via the pumping chamber discharge reed valves. The position of the swash plate will determine the length of travel by the piston which allows the compressor to vary its output.

Control of the compressor swept volume is regulated by the control valve ((5) in Fig. 2.5), located in the compressor rear end plate. The control valve attempts to keep the compressor low pressure side at a constant pressure, also known as the control point, which is determined by factory settings and cannot be adjusted in service.

The operation of the variable capacity swash plate compressor is based on the creation of three distinctive pressures which are sensed and controlled by a control valve and adjusted to create a mechanical equilibrium within the compressor (this can be seen on Figs 2.7 and 2.8):

A. High pressure (discharge pressure).
B. Low pressure (suction pressure from the evaporator (TX system or accumulator FOV).
C. Compressor internal pressure (generated by the high pressure but only a fraction of it).

The displacement is the difference between the low pressure and the internal pressure, this is called ‘delta P ’. The change in delta P varies the angle of the swash plate causing the capacity of the compressor to vary.