COMPRESSOR CLASSIFICATION AND SELECTION

COMPRESSOR PERFORMANCE

COMPRESSOR INSTALLATION

MOBILE COMPRESSORS

ACCEPTANCE TESTS

COMPRESSOR NOISE REDUCTION

COMPRESSOR CONTROLS

AIR RECEIVERS

COMPRESSOR LUBRICATION

HEAT EXCHANGERS AND COOLERS

FIGURE 1 – Basic compressor types.

FIGURE 2

FIGURE 3

FIGURE 4

• Maximum, minimum and mean demand. If there is an intermittent requirement for air, but a large compressor set is needed to cater for peak requirements, the installation can be very uneconomic to operate unless the control system is well designed. There may be a maximum placed on the power (electricity or gas) consumption. Depending upon the characteristics of the peak demand (ie whether it is long or short term), it may be appropriate to consider a large air receiver as an economic alternative to a larger compressor.

• Ambient conditions – temperature, altitude and humidity. At high altitude, because of the reduced air density, the compressor efficiency and capacity is reduced. High humidity can result in large quantities of water which have to be disposed of.

• Methods of cooling available – availability of cooling water or ambient temperature for air cooling. The possibility of using the waste heat for space heating or for process heating should be considered.

• Environmental factors – noise and vibration. Special foundations may be required, particularly with reciprocating compressors.

• Requirements for skilled maintenance personnel.

FIGURE 5 – Generalised performance envelopes for different types of compressors.

Reciprocating compressors

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 (see the chapter on Compressor Performance). 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 multi-cylinder 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:

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

Reciprocating compressors for oil-free applications

For oil-free applications, special precautions have to be taken. Figure 7 shows how particular designs produce a high degree of cleanliness in the air. No lubrication is necessary in the upper cylinder because the pistons are fitted with PTFE rings; the combination of these rings, austenitic liners and a crosshead guide to minimise piston side loads extends operating life without complicating maintenance. To prevent oil entering the cylinder, a distance-piece incorporating rubber seals is fitted between each cylinder and crankcase. Pure air is produced straight from the...