It's that part of the machine where the actual compression takes place. It is the heart of the rotary screw air compressor. Often, the compressor screw element is also called air-end or airend. They are constant flow (volume) with variable pressure compressors. Which mean that at a given speed (rpm), they always supply the same amount of air (in liters per second for example), but can do so at different pressures.

Why are they so popular? Since it's a continuous process (as opposed to reciprocating piston compressors) they supply a steady, non-pulsating air flow, with minimum vibrations and maintenance and maximum lifetime. They can operate 24/7, 365 days a year. The normal lifespan of a screw element is about 24,000 40.000 running hours depending on usage, brand and service history.