They are defined as a variable displacement devices incorporating a number of sliding vanes inserted into a rotor inside an outer housing.
As the eccentrically mounted rotor rotates, the vanes slide out of the housing through centrifugal motion, encountering and sealing against the inside of the outer housing.
Since the rotor is installed off-center, the vanes on one side extend further than they do on the other, since the distance from the axis of the chamber varies as well.
Air is introduced through the housing via an inlet valve. The inlet valve is located closest to the area of the vanes that offers the largest possible volume. In other words, where air is ingested is the area that can hold the largest volume of free air, and the vanes are extended the farthest from the rotor.
As the rotor rotates, the vane length is shortened, this because the rotor containing the vanes is installed off-center in housing. This reduction in vane length means the chamber that the free air is in is now becoming smaller, thus compressing the air in that chamber.
At the location of highest compression, where the vanes have been shortened as much as possible, and the air chamber is as small as it is going to get, the now-compressed air is vented from the inner housing.
As the air exits the chamber it flows through an oil separation device, and then via an outlet valve, to the compressor receiver or the air mains.