RECIPROCATING COMPRESSORS

In these compressors, the gas volume decreases and pressure increases due to the action of one or more reciprocating piston moving axially in one or more cylinders. It may be single acting or double acting, single cylinder or multicylinder and single stage or multistage compressors.

Figure 10.2 shows the schematic of a reciprocating compressor. Reciprocating compressors consist of a piston moving back and forth in a cylinder, with suction and discharge valves to achieve suction and compression of the gas. Its construction and working are very similar to a two-stroke engine, as suction and compression of the gas are completed in one revolution of the crank. The suction (inlet) and the discharge (outlet) valves open and close due to pressure differences between the cylinder and inlet or outlet manifolds, respectively. The pressure in the inlet manifold is equal to or slightly less than the atmospheric pressure. Similarly the pressure in the outlet manifold is equal to pressure of the compressor at the end of compression. The purpose of the manifolds is to provide stable inlet and outlet pressures for the smooth operation of the valves and also provide a space for mounting the valves.

Figure 10.2 Schematic Diagram of Reciprocating Compressor

Working principle of ideal reciprocating compressors is diagrammatically shown in Figure 10.3. An ideal reciprocating compressor has the following assumptions:

Figure 10.3 Ideal Reciprocating Compressor

• There is no clearance volume in the cylinder.
• Working fluid behaves like a perfect gas.
• There is no friction loss.
• There is no loss during passing of fluid through valves.

Process 0−1: This is an isobaric suction process, during which the piston moves from the Top Dead Centre (TDC) to the Bottom Dead Centre (BDC). The suction valve remains open during this process and gas at a constant pressure P₁ flows into the cylinder.

Process 1−2: This is an isentropic compression process. During this process, the piston moves from BDC towards TDC. Both the suction and discharge valves remain closed during the process and the pressure of gas increases from P₁ to P₂. Various types of compression process in reciprocating compressors are shown in P−V and T−S diagrams in Figure 10.4.

Figure 10.4 ρ–V and T−S Diagrams for Isentropic, Polytropic, and Isothermal Compression

Isentropic compression	1−2′;	γ = 1.4 for air
Polytropic compression	1−2;	γ = 1.25 for air
Isothermal compression	1−2″;	Temperature remains constant.

 

Process 2−3: This is an isobaric discharge process. During this process, the suction valve remains closed and the discharge valve opens. Gas at a constant P₂ is expelled from the compressor as the piston moves to TDC.