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What is a Rotary Compressor?

Lea Miller
Lea Miller

A rotary compressor is a machine that compresses ambient air using a rotary motion. The air at the intake is gradually compressed using a screw configuration and positive displacement. The unit may be cooled using oil, air, or water. Rotary compressors may be single-stage or used in series to compress air in two stages.

The function of an oil-cooled rotary screw compressor begins with air being introduced to a central chamber that is filled with oil. Most often, two interlocking rotary screws, wider at one end and narrower at the other, turn in opposite directions to gradually force air toward the narrow end. As the rotation continues, it creates a vacuum at the intake point, drawing more air into the system. Some units may be configured with a single screw.


The compressed air exits into a separator that segregates the oil so it can be cooled and directs the cleaned air to an outlet pipe. The compressed air may continue into service lines connected to the industrial process or into an air receiver that stores it for future need. The oil is cooled with either air or water before being returned to the compression chamber. This type of unit is also called a wet rotary screw compressor.

For applications where true oil-free air is required, a rotary compressor using no oil in the compression chamber is preferred. Oil-free air is commonly specified in industries such as food processing and pharmaceutical manufacturing. These units may be air or water cooled, and they may also be called dry rotary screw compressors.

When the pressure required is relatively low, it is possible to reach the desired compression level in a single step. Otherwise, the partially compressed air exiting the first rotary compressor chamber may be directed into another unit connected in series. The air is compressed further in the second unit to reach the desired pressure level.

The positive displacement aspect of the rotary compressor refers to its production of a fixed volume of air at a specific pressure. Rotary screw compressors are designed to operate continuously. The limited number of moving parts allows them to be reliable and easily maintained.

The motor powering a rotary compressor can be connected directly to the compression chamber or it may be belt-driven. Stationary units are most often powered by an electric motor, but portable units used for road construction or repairs can also be powered by gasoline or diesel fuel. Rotary screw compressors are available in sizes ranging from a few horsepower to several hundred horsepower.

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