COMPRESSORS SELECTION CONSIDERATIONS

Not all types of machines are made in all pressure–volume ranges. Figure 1.1 indicates, in a very general manner, the capacities of reciprocating, centrifugal, rotary-screw, and axial compressors available. The more common usage is indicated by the deeper shading. Although this figure does not indicate the theoretical or engineering limits of any design (the limits are continually being expanded), it may be used as a guide to current technology.

Since sealing systems for axial compressors are not as versatile as for other types, normally only those gases whose leakage to the atmosphere can be tolerated should be handled by this type of machine.

Rotary-lobe, sliding-vane, liquid-piston, and diaphragm compressors have relatively small capacities and, typically, atmospheric-pressure suction. Of these four types, the rotary-lobe compressor can deliver the most gas, as its maximum suction volume is about 30,000 actual cfm (acfm). A maximum discharge pressure of about 40 psia can be attained. However, rotary-lobe compressors are most competitive at capacities of 17,500 acfm or less and discharge pressures of about 22 psia.

Maximum inlet capacities of sliding-vane units are about 3,000 acfm, or double this amount if a duplex compressor is used. The latter consists of two compressors attached to a single drive. Maximum discharge pressures of standard machines are about 65 psia in a single stage and 140 psia in two stages.

The liquid-piston compressor has a maximum capacity of about 10,000 acfm and can deliver this amount of air (or gas) at about 30 psia. Volumes of 300 acfm or less can be compressed to about 115 psia. The three foregoing compressor types can produce moderate to high vacuum, particularly in multiple stages.

Diaphragm compressors have much smaller volumetric capacities, with maximum flows ranging from 40 to perhaps 200 acfm. These machines, however, can develop pressures up to 40,000 psi. Before selecting a compressor type, one must decide how many machines will be needed to handle the process load. In former years, reciprocating machines were used for almost all process applications. Since the compressor capacity was low, large plants would require trains of machines. As machine reliability and capacity increased, the tendency to install two machines started, each with 55 percent or 60 percent capacity, perhaps with a third unit as a spare.

The spare unit ensured operation at full capacity, but at an increased compressor cost of about 50 percent. If the spare compressor were omitted, but two half-size machines installed, one could still be reasonably sure of continued operation at all times. This was particularly important when the process included equipment that could not be shut down frequently, such as furnaces.

Later, to take advantage of larger machine capability, several services were placed on the same frame.

Today, the situation is somewhat different, as more centrifugal compressors are being used (Figure 1.3). For one thing, the downtime of a rotating equipment generally is appreciably less than that of a reciprocating equipment. Therefore, in many instances a single centrifugal compressor may be satisfactory. However, it must be recognized that when a compressor is down, it will usually take longer to repair or overhaul a centrifugal unit than a reciprocating one—unless a complete spare rotor is available.

Also, the pricing structure of centrifugal compressors is quite different from that of reciprocating ones. As a first very rough approximation, one may assume that halving the size of a reciprocating compressor will halve its cost. Yet, halving the size of a small centrifugal compressor may only decrease its cost 20 percent, and halving the size of a large machine may only reduce its cost 30 percent.

Furthermore, because of their flat operating characteristics, the running of centrifugal compressors in parallel may result in surging unless very careful attention is given to avoiding unstable operation. Therefore, in many process applications for which one centrifugal compressor will have adequate capacity, an installed spare is not provided. In these instances, a complete spare rotor may be bought.

The choice between reciprocating and centrifugal compressors is not always simple, particularly for high-head, medium-capacity service such as gasfield repressuring. If several reciprocating compressors are used, each can be multiple staged to develop the desired head. The shutting down of one machine would merely cause a decrease in plant output. But if several centrifugal compressors were used in series, the failure of one would stop the entire operation.

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