Piston compressors
In principle, the piston or reciprocal compressor consists of a crank case with crank shaft and connecting rod, piston and cylinder, as well as intake and pressure valve.
Through the rotary motion of the crank shaft and the reciprocating motion of the piston, the air is drawn from the atmosphere via the open intake valve into the cylinder space. When, in the course of its upward stroke movement, 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 take up only a fraction of its original volume while, at the same time, the pressure in the cylinder is increased to a multiple of the original. Once the pressure enclosed by the cylinder exceeds the spring force of the pressure valve, this valve opens and the compressed air is expelled from the cylinder.
There are different types of compression. Either the compressed air is immediately pushed into the compressed air system directly, thus representing single-stage compression, or further stages of compression follow the first stage and we then have a multi-stage compression of air.
A two-stage compressor in principle consists of two single-stage compressors mounted in series upon a common crank shaft casing. In the first one, the pressure of the ambient atmosphere is compressed to a pressure level corresponding to the square root of the absolute ultimate pressure. 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 fitted 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 as though it was 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 p = 9 bar causes a delivery temperature of T2 = 110°C whereas, if the same pressure had been reached by single-stage compression, T1 would be 240°C. Such a high temperature resulting from single-stage compression would naturally cause problems, regarding the lubrication of the compressor.
Piston compressors exist in various forms. The basis is the complete compressor assembly, fitted with an electric motor as a drive unit and the fundamental accessories such as intake filter, inter and aftercooler, automatic water drain and safety valves. An integrated control system forms an important factor, because this must be suitable for the cut-out and idle running of the compressor. Control depends on a signal provided by a pressure monitor set for the operating situation and 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.
Cut-off 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 short cut-out periods leading to high frequency of switching of the compressor is called for. When the upper switching limit is reached, the throughput is set to "zero". The compressor then continues running idly, using little current.
Present day standards provide the combination of cut-out and idle running control, with the additional refinement that the duration of the idle running period can be determined. This makes it possible to have an automatic adaptation to operating conditions when these involve elevated air consumption with a correspondingly high frequency of switching.
Today, nearly all manufacturers of compressors offer custom designed integrated control systems for particular operating conditions.
One usually comes across single stage compressors 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 offered solely in the air cooled types, beyond this manufacturers offer the choice of air and also water cooled compressors.
The below diagram shows the shaft output power of piston compressors as a function of the throughput and the operational gauge (output) 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.
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