How Reciprocating Compressors Work in Air Conditioning Systems

The reciprocating compressors (piston compressors or recip compressors) work exactly like a bicycle pump. The principle difference is an electrical motor drives the compressor.

Their function is identical: to draw in gas and discharge it.

Let’s remind ourselves how the recip compressor operates.

The piston is at the bottom dead centre (BDC) at A. The cylinder is full of refrigerant vapour at low pressure.

At B, the piston rises in the cylinder and compresses the vapour. As a result, the pressure and temperature in the cylinder continually increase, and the valve reads remain closed and gas-tight.

At C, the pressure of the vapour in the cylinder becomes just greater than the HP that exists above the discharge valve, and the valve opens.

Because of the difference in pressure between one side of the LP valve and the other, the valve remains closed and gas-tight. As a result, as the piston rises, the HP vapour discharges into the high-pressure pipework.

This pressure difference causes the HP valve’s opening (or closure).

At D, the piston has reached the piston top dead centre (TDC), and it starts its descent.

At E, the piston continues its descent. When the pressure in the cylinder becomes less than the LP pressure, the suction valve opens. At this point, LP vapour sweeps into the cylinder as the piston continues to descend.

At F, the piston moves downwards in the cylinder until it reaches the bottom dead centre (BDC). At this point, the cylinder is full of LP vapour, and the suction valve shuts.

The compressor shaft has now made one complete revolution, and the piston has made one complete rise and fall in the cylinder: we have returned to our starting point. The cylinder is full of refrigerant vapour at low pressure, and the piston is about to start rising in the cylinder once again.

The pressure difference alone causes the opening (and closure) of the LP valve.

The opening of the LP and HP valves is entirely due to the pressure difference between the gas trapped in the cylinder and the LP and HP pressures. You must clearly understand that the compressor does not control the value of the LP and HP pressures. It simply functions as a lift that allows the vapour to rise from the LP pressure level to the HP pressure level.