If you have ever used an electric drill in a piece of wood or metal, you may have noticed that the chips or spirals of metal follow the contour of the flutes up and out of the hole (most of them, at least).
A similar phenomenon occurs inside the rotary-screw compressor housing.
At the wide end of the screw (sometimes there is one screw operating against a housing, sometimes more than one), an inlet valve allows free air into the screw housing when there is a demand. Free air flows into the housing from the outside as there is a partial vacuum formed inside the rotary-screw housing as the screw(s) rotate.
Inside the screw housing are the screws in a bath of oil. The oil is there to provide a viscous, flowing, sealing method to help trap the air in the rotary-screw flutes.
The air–oil mixture in the screw housing moves along the flutes from the wide end of the screw toward the narrow end, pulling a vacuum behind, thus drawing more air into the screw housing.
As the air–oil blend is pulled along the flutes of the screw, the space in which the air is contained gets smaller and smaller. The diameter of the screw is larger at the inlet end and smaller at the discharge end, thus compressing the air. The amount of air trapped in the screw flutes does not change as the air is moved along the narrowing path, but the volume that air is in gets steadily smaller, thus compressing the air.
Manufacturers of rotary-screw compressors have their own ideas of what constitutes the ideal geometry of the screw within their air compressor.
Rotary-screw compressors may have just one screw (also sometimes known as augers) or maybe two or more. Single-screw compressors function the same way as multiple-screw units, with the air being compressed between the housing of the screw compartment and the screw itself, rather than between two or more screws.
The following drawing will give you an idea of how the rotary-screw concept works with two screws. The actual guts of the rotary-screw compressor will vary depending on the designs of the company that manufactured that particular compressor. The drawing shows two screws. They would be housed inside the screw compartment of the compressor, in a bath of oil.
At the narrow end there would be an outlet valve, which feeds the compressed air–oil mixture from the screw compartment and into a separator.
The separator has the job of removing as much oil from the compressed air as possible, and then to release that compressed air into the compressor receiver or into the plant main air lines.