Smaller units are usually electric-motor driven (direct or belt); for medium to large units, a wide choice of drives are available. These include motors (synchronous, induction, low or high speed); steam turbines (back-pressure, condensing or controlled extraction); internal-combustion engines (integral or direct connected); gas turbines (single or double shaft); and expanders.
The selection of a drive depends to some extent on the compressor service, but more important are the overall energy balance, energy utilization and availability, and heat-rejection methods. Within the limits imposed by these criteria, the selection should stress a drive system that is simple, dependable, and straightforward. The compressor is the reason for the drive, not the other way around.
The drives of internal-combustion engines and steam turbines can ordinarily be operated over a fairly large range of speeds. This may not be the case, however, if gas-turbine or electric-motor drives are used.
Let us first consider gas turbines, almost all of which have axial-type air compressors for pressure–air supply to the compressors. For single-shaft units (air compressor, gas turbine, and driven unit on one shaft), the speed range is most often determined by the steep performance curve of the axial compressor rather than by the much flatter curve of the centrifugal process compressor. Double-shaft machines permit constant speed for the axial air compressor and variable speed for the process compressor. The selection of sizes, speeds, and horsepower outputs of commercially available gas turbines is limited. Very often, a wide freedom of choice is not available as to single- or double-shaft units.
Motor drives usually have a constant speed: In a limited number of cases, variable-speed couplings, wound-rotor or multipole (PAM) motors may be used. Large motors may be of the synchronous or induction kind. For a unit driven at above-synchronous speeds (3,600 rpm for 60 Hz), the choice should be based on the total cost of the motor and the speed increaser. Thus, a 1,800-rpm induction motor and its speed increaser may cost less (including operating costs) than a 1,200-rpm synchronous motor with its speed increaser.
Constant-speed centrifugals in process plants tend to operate at a high enough average load so that the economic rewards of power-factor correction obtained by the use of a synchronous motor are minor.
Fossil-fuel drives are used when initial and operating costs are more attractive than those of steam or motor drives, sufficient electric power is not available, and electric or steam sources are not reliable. In this last case, the entire system must be carefully specified to ensure that minor items such as cooling-water pumps, pressure switches, control air, etc. are independent of any source of power, not as dependable as the compressor drive.
Internal-combustion engines are usually turbo-supercharged and may be two or four cycle, integral with or separate from the compressor. The type of engine can normally be selected on the basis of drive features, including accessories and costs (purchase order, installation, fuel consumption, spare parts, maintenance) independent of the compressor. It is best, however, to include the drive as part of the compressor system.
The gear mechanical rating, including the American Gear Manufacturers Association (AGMA) service factor, should be selected so that the gear rating does not become the limiting factor in the compressor and drive train. Steam-turbine drives combined with a gear (with the turbine at a lower speed than the compressor) are sometimes lower in cost than higher-speed turbines. The policy of the user and his insurance carrier on warehouse spares for gears affects the choice, since the gears and additional couplings increase the probability of outage.