Piston compressors

What is piston compressor?

The piston compressors are the oldest and most common industrial compressors. In principle, the piston or reciprocal compressors consist of a crank case with crank shaft and connecting rod, piston and cylinder, as well as intake and pressure valve.

The rotary motion of the crank shaft results in the reciprocating motion of the piston. When the piston strokes backward, the air is drawn from the atmosphere via the open intake valve into the cylinder space. When, piston strokes upward, the piston compresses the drawn in ambient air inside the cylinder chamber, the intake valve is closed and the air in the cylinder undergoes compression.

The air is therefore compressed to a fraction volume of its original volume while, at the same time, the pressure in the cylinder is increased. The relationship of volume and pressure changes can be explained with Gas Laws.  Once the pressure enclosed by the cylinder exceeds the spring force of the pressure valve, this valve opens and the compressed air is expelled out from the cylinder.

Piston compressor structures.

Compression types for piston compressors can be separated as single-stage or multi-stage. With single-stage compression, the compressed air is immediately pushed into the compressed air system directly. With multi-stage compression, the air will pass through further stages of compression follow the first stage, then is pushed in to the compressed air system.

A two-stage compressor generally consists of two single-stage compressors mounted in series upon a common crank shaft casing. With two stage or multi-stage compression, the mechanical strain on the pressure bearing parts is thus diminished, achieving a longer service life of the piston compressor.

An intercooler is installed between the first and the second stage of the compressor. Without an intercooler, the work of compression would, in the course of the second stage, continue to heat and expand the air volume and have an effect upon the compressor output temperature. Then the compression effect is same as single-stage compression. Cooling by means of the intercooler permits compression towards the final pressure in the face of reduced air volume. It is a further advantage of two- stage compression that the final temperature of the air after compression is lower as a result of cooling after compression.

Two-stage compression to pressure at 9 bar causes a delivery temperature of 110°C whereas, if the same pressure had been reached by single-stage compression, the delivery pressure would be 240°C. Such a high temperature resulting from single-stage compression would naturally cause problems, regarding the lubrication of the compressor.

Piston compressor working theory

Piston compressors have various forms. The standard form is the complete compressor assembly, with an electric motor as a drive unit and the fundamental accessories such as intake filter, inter and after-cooler, automatic water drain and safety release valves. 

An integrated control system is very important, because it must be able to optimize the loading, idle running and off-loading of the compressor. Control depends on a signal provided by monitoring the pressure transmitter set for the operating with a lower and upper pressure limit value.

If the lower limit value is reached, the compressor automatically and continuously displaces compressed air into the system until the upper switching point has been reached, thus causing the switching off of the compressor. Off-loading control switches off the compressor when the upper switching limit has been reached. This type of control is applied if the compressed air use is subject to great variations and is irregular. This type of control is the most economical, but the highest permitted switching frequency of the electric motor must not be exceeded in its application.

Idle running control is used when frequent short off-loading times leading to high frequency of switching of the compressor. When the upper switching frequency limit is reached, the throughput is set to “zero”. The compressor then continues running idly, using little current.

At present, standards provide the combination of off-loading and idle running control, with the additional refinement, the duration of the idle running can be set. This makes it possible to have an automatic adaptation to operating conditions when these involve high air consumption with a correspondingly high frequency of switching.

Now, nearly all manufacturers of compressors provide custom designed integrated control systems for different operating conditions.

Piston compressor products

Single-stage compressors are usually for the lower pressure ranges and small outputs. For higher pressures, two stage and multi-stage compressors are usually installed.

Up to medium range output quantities, piston compressors are normally supplied solely in the air cooled types, except this, manufacturers provide the options of air and also water cooled compressors.

Piston compressor effective shaft power

Diagram below shows the shaft output power of piston compressors as a function of the throughput and the operational gauge pressure (delivery pressure). This diagram is based on typical piston compressors with a rotary speed of 1450 r.p.m.

The shaft power levels given in the diagram do not refer to a particular type but are mean values based on a uniform design series.