Common Features

Rotary compressors as a group make up the balance of the positive displacement machines. This group of compressors has several features in common despite differences in construction. Probably the most important feature is the lack of valves as used on the reciprocating compressor. The rotary is lighter in weight than the reciprocator and does not exhibit the shaking forces of the reciprocating compressor, making the foundation requirements less rigorous. Even though rotary compressors are relatively simple in construction, the physical design can vary widely. Both multiple- and single-rotor construction is found. Rotor design is one of the main items that distinguishes the different types. Size and operating range is another area unique to each type of rotary. The following sections cover some of the more common rotary compressors in detail.

Arrangements and Drivers

Rotary compressors are frequently arranged as single units with a driver. Occasionally the compressors are also used in series arrangements, with or without an intercooier. The series configurations may use a form of tandem drive or multiple pinion gears to permit the use of a common driver.

For most of the rotary compressors in process service, the driver is an electric motor. Compressors in portable service, however, particularly the helical-lobe compressor, use internal combustion engines. Many of the rotary compressors require the high speed that can be obtained from a direct-connected motor. The dry type helical-lobe compressor is probably the main exception as the smaller units operate above motor speed and require a speed increasing gear which may be either internal or external (see Figure 4-1). The helical-lobe compressor is the most likely candidate for a driver other than the electric motor. Aside from the portables already mentioned, engines are used extensively as drivers for rotaries located in the field in gas-gathering service. Steam turbines, while not common, probably comprise most of process service alternate drive applications.

Figure 4-1. A skid-mounted oil-free helical-lobe compressor. (Courtesy of A-C Compressor Corporation)
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