A component in a high-pressure pneumatic system that removes most of the water vapor from the compressed air. When the compressed air is used, its pressure is dropped, and this pressure drop causes a drop in temperature. If any moisture were allowed to remain in the air, it would freeze and block the system.

The separation is normally done by centrifugal action of the air that slings the moisture out into an area where it can be collected and drained when the system is shut down.