Air cycle machine

An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft.

Bleed air from the engines, an auxiliary power unit, or a ground source, which can be in excess of 150 °C and at a pressure of perhaps 32 psi (220 kPa),[1] is directed into a primary heat exchanger.

The pre-cooling through the first heat exchanger increases the efficiency of the ACM because it lowers the temperature of the air entering the compressor, so that less work is required to compress a given air mass (the energy required to compress a gas by a given ratio rises as the temperature of the incoming gas rises).

[2] The work extracted by the expansion turbine is transmitted by a shaft to spin the pack's centrifugal compressor and an inlet fan which draws in the external air for the heat exchangers during ground running; ram air is used in flight.

Cooling the air has caused any water vapor it contains to condense into fog, which can be removed using a cyclonic separator.