Piston compressors are the most common type of compressor in use and a basic single cylinder piston compressor form is shown in below figure 1. As the piston descends during the inlet stroke (shown in figure 1 a), the inlet valve opens and air is drawn into the cylinder. As the piston passes the bottom of the stroke, the inlet valve closes and the exhaust valve opens allowing air to be expelled as the piston rises (figure 1 b).
Figure 1 shows the valves are similar to valves in an internal combustion engine. In practice, spring-loaded valves are used, which open and close under the action of air pressure across them. One common type uses a feather of spring steel which moves above the inlet or output port, as shown in figure 1 c.
A single cylinder compressor gives significant pressure pulses at the outlet port. This can be overcome to some extent by the use of a large receiver, but more often a multi cylinder piston compressor is used. Multi cylinder piston compressor are usually classified as vertical or horizontal in-line arrangements and the more compact, V, Y or W constructions.
A piston compressor which produces one pulse of air per piston stoke (of which the example of figure 1 is typical) is called a single acting compressor. A more even air supply can be obtained by the double acting action of the piston compressor as shown in figure 2, which uses two sets of valves and a cross head to keep the piston rod square at all times.
Piston compressors described so far go direct from atmospheric to required pressure in a single operation. This is known as single stage piston compressor. The general gas law shows the compression of a gas to be accompanied by a magnificent rise in gas temperature. Of the exit pressure is about 5 bar in a single acting compressor, the compressed air temperature can rise to over 200°C and the motor power needed to drive the compressor rises accordingly.
For pressure over a few bar it is far more economical to use a multistage piston compressor with cooling between stages. Figure 3 shows an example of an two-stage piston compressor. As cooling is undertaken by inter-cooler device, reduces the volume of the gas to be compressed at the second stage there is a large energy saving. Normally two stages are used for pneumatic pressures of 10 to 15 bar, but multistage compressors are available for pressures up to around 50 bar.
Multistage compressors can be manufactured with multi-cylinders as shown in figure 3 or, more compactly, with a single cylinder and a double diameter piston as shown in figure 4.
There is contact between pistons and air, in standard piston compressors, which may introduce small amounts of lubrication oil from the piston walls into the air. This very small contamination may be undesirable in food and chemical industries. Figure 5 shows a common way of giving a totally clean supply by incorporating a flexible diaphragm between piston and air.