A cylinder is a pressure vessel that holds the gas during the compression cycle. There are two basic types:
1. Single-acting cylinders are those where compression occurs only once per crankshaft revolution, 2. Double-acting cylinders are those where compression occurs twice per crankshaft revolution.
Figure 11-4 is a cut-away drawing of a compressor with single-acting cylinders. True single-acting cylinders are typical of low horsepower air compressors. Single-acting process compressors are typically double-acting cylinders with the outer end suction valves removed. Figure 11-5 is a cut-away of a double-acting cylinder.
Cylinders are made of different kinds of materials. Generally, cast iron is used for cylinder operating pressures up to 1,000 to 1,200 psig, nodular iron or cast steel for operating pressure in the 1,000 to 2,500-psig range, and forged steel for pressures greater than 2,500 psig.
Like all pressure vessels, the cylinder has a maximum allowable working pressure (MAWP). The maximum allowable working pressure of the cylinder determines the setting of the relief valve that is downstream of the cylinder. The MAWP of the cylinder should be a minimum of 10% or 50 psi greater than its operating pressure.
A cylinder liner such as that shown in Figure 11-6 may be used to help prolong the life of the cylinder and improve operating flexibility. Any damage caused by the action of the piston or heat generated by compression will affect the cylinder liner, which may be removed and replaced. As the surface of the liner wears, it is much easier and quicker to repair it than to repair the cylinder itself. In addition, liners enable the diameter of the piston to be varied without changing the cylinder and thus provide flexibility to respond to different conditions of pressure and flow rate.
The disadvantages of liners are that they increase the clearance (discussed in more detail below) by increasing the distance between the piston and the valve, and they decrease the bore of the cylinder. Therefore, the cylinder will have less capacity and lower efficiency (at high ratios) than if there were no liner.
