Two air start compressors are normally supplied which must be capable of charging the air receivers from empty to full in one hour.

They are usually two stage reciprocating with inter and afterstage cooling. Relief valves will be fitted to each stage which will limit the pressure rise to 10% of design pressure, and a high temperature cut out or fusible plug to limit the HP discharge to 121°C.

Intercoolers are also fitted with bursting disks or relief valves on the water side.