Reciprocating Compressors – Compression Cycle

In a reciprocating compressor, compression takes place as a result of the back-andforth travel or stroke of a piston in a cylinder equipped with inlet and discharge valves. The cycle is usually described by a Pressure-Volume (PV) diagram, similar to the one shown in Figure 300-2.

The “ideal indicator diagram” is followed by a series of cylinder illustrations depicting piston movement and valve position. The figure shows, in diagram form, one complete crankshaft revolution representing a complete compression cycle.

To begin the cycle, refer to the figure at:

(a) where the piston is at the lower end of the stroke (bottom dead center) and is at path point 1 on the indicator diagram. At this point the cylinder has filled with gas at intake pressure P1. Note that the valves are both closed. (b) the piston has started to move to the left. This is the compression portion of the cycle and is illustrated by Path 1-2.

When the piston reaches point 2 on the indicator diagram, the exhaust valve starts to open. The discharge portion of the cycle is shown at (c). This is shown on the indicator diagram Path 2-3. Note that the discharge valve is open during this period while the intake valve is closed. The gas is discharged at the discharge line pressure P2.

(c) When the piston reaches point 3 it has traveled to the upper end of its stroke (top dead center). Physically, at this point in the stroke, there is a space between the piston face and the head. The space results in a trapped volume and is called the clearance volume.

(d) Next in the cycle, the piston reverses direction and starts the expansion portion of the cycle, as illustrated at (d) in the figure. Path 3-4 shows this portion of the cycle. Here the gas trapped in the clearance volume is re-expanded to the intake pressure. Note that the discharge valve has closed and the intake valve is still closed.

(e) At point 4 the expansion is complete and the intake valve opens. The intake portion of the cycle is shown at (e). This is indicated by Path 4-1 on the indicator diagram. The cylinder fills with gas at intake line pressure P1. When the piston reaches point 1 the cycle is complete and starts to repeat.