Reciprocating compressors are suitable for operating over a very wide range of speeds, with a practical maximum of about 10 bar delivery pressure from a single-stage unit, and up to 70 bar for a two-stage machine. Multi-stage reciprocating compressors may be built for special purposes capable of supplying delivery pressures up to and in excess of 700 bar. By carefully selecting the number of stages, the designer can also produce a machine which approaches the ideal or isothermal compression curve more closely than with any other type, with the possible exception of very large volume axial flow compressors. In the double-acting compressor the space on the other side of the cylinder is enclosed and so both sides are used for compression, giving two compression strokes for each revolution of the crank shaft. Alternatively, one side of the piston may be used for one stage and the other for the second stage in a two stage compressor. Individual cylinders may be also used for multi-stage compression, disposed in a number of arrangements.
Most reciprocating compressors are single-stage or two-stage, ranging from fractional horsepower units to very large machines with input requirements of the order of 2250 kW. The smaller compressors are usually single stage of a single or V-twin layout with air cooled cylinders and are powered by electric motors.
Intermediate sizes comprise a variety of different configurations. Vertical compressors may comprise one or more cylinders in line, ‘V’, ‘W’ and ‘H’ arrangements for multicylinder units, and also ‘L’ configuration which has both vertical and horizontal cylinders disposed about a common crankshaft. The angled arrangement of cylinders offers certain advantages, notably reduced bulk and weight and superior machine balance (since with careful design the primary forces can be accurately balanced). The ‘L’ configuration with a vertical low pressure cylinder and a horizontal high pressure cylinder is advantageous for larger machines, facilitating installation, assembly, maintenance and dismantling. Larger reciprocating compressors are commonly horizontal double acting, tandem or duplex. Basic two-stage units are shown in Figure 2. Variations include transferring the side thrust by means of a crosshead. When the space under the piston is used, it is essential to use a crosshead to convert the rotary action of the crankshaft into reciprocating motion in order to obtain a satisfactory seal of the piston rod where it passes through the compression chamber. The crosshead ensures that all the side thrust component from the crank shaft is taken by the crosshead guide and not transferred to the piston and cylinder. Single-acting reciprocating compressors are normally of the trunk piston type, whilst differential-piston double-acting compressors can be of either type – see Figure 3. Differential or stepped pistons may also be used as shown in Figure 4. Here one stage of compression takes place in the annular space between the shoulder of the piston and the corresponding shoulder in the cylinders.
Figures 2 to 4 also show designs where a compressor piston is connected to the same crank as the reciprocating engine which drives it. A common arrangement uses three cylinders of a four-cylinder block to form the engine, whilst the fourth cylinder forms the compressor cylinder. This is a convenient way of balancing the forces with a single cylinder compressor.
Figure 6 shows the pattern of reciprocating air compressors in industrial service, according to a recent survey. This represents most of the industrial units in service. For special purposes, as mentioned above, compressors are found well outside the range indicated. The versatility of reciprocating compressors means that they are the most common of all types. The disadvantages of reciprocating compressors are:
• They require special foundations to cater for the unbalanced inertial forces of the reciprocating pistons and connecting rods.
• The maintenance they need has to be done by skilled personnel.
• The inlet and delivery valves are prone to failure.
• The discontinuous flow of the compressed medium can cause vibrational resonance in the delivery passages and the distribution system.
In mobile compressors and in the medium range of stationary compressors, reciprocating compressors tend to have been superseded in recent years by screw and sliding vane designs in oil flooded and dry versions. They still maintain their dominance for large stationary (factory) applications.