Rotary compressors are positive-displacement machines. Figure 10-7 shows a typical vane compressor. The operation is similar to that of a vane pump. A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors.
![Vane-type rotary compressor. [Courtesy of Dresser-Rand Company,]](https://www.oilngasprocess.com/wp-content/uploads/2009/09/vane-type-rotary-compressor.JPG "vane-type-rotary-compressor")
A relatively large quantity of oil is injected into the flow stream to lubricate the vanes. This is normally captured by a discharge cooler and after-scrubber and recycled to the inlet.
Vane compressors tend to be limited to low pressure service, generally less than 100 to 200 psi discharge. They are used extensively as vapor recovery compressors and vacuum pumps. Single-stage vane compressors can develop 27 in. Hg vacuums, two-stage compressors can develop 29,9 in. Hg, and three-stage compressors can develop even higher vacuums.
The major characteristics of vane compressors are:
Size
• Common sizes up to 250 bhp, but mostly used for applications under 125bhp.
• Available in sizes to 500 bhp.
• Discharge pressures to 400 psig.
• Single- or two-stage in tandem on same shaft.
Advantages
• Good in vacuum service.
• No pulsating flow.
• Less space.
• Inexpensive for low hp vapor recovery or vacuum service.
Disadvantages
• Must have clean air or gas.
• Takes 5 to 20% more horsepower than reciprocating.
• Uses ten times the oil of a reciprocating. Usually install after-cooler and separator to recycle oil.
