Air cycle machine

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An Air Cycle Machine (ACM) is the refrigeration unit of the environmental control system used in pressurized turbine-powered aircraft. Normally an aircraft has two to three of these machines arranged in a system called a "pack". The cooling process uses air instead of freon in a gas cycle. No condensation or evaporation is involved, and the cooled air output from the process is used directly for cabin ventilation.

The usual compression cooling and expansion seen in any refrigeration cycle is accomplished in the ACM by a centrifugal compressor, two heat exchangers and an expansion turbine.

Engine bleed air, which can be in excess of 150°C, is directed into a primary heat exchanger before going through the compressor. Once the hot air is cooled, it is then compressed which heats the air back up and it is then sent to the secondary heat exchanger. The pre-cooling through the first heat exchanger increases the efficiency of the ACM because of the heating from compression. The air is again cooled in the secondary heat exchanger and travels through the turbine which expands the air and cools it even further. It is possible for the ACM to to produce air cooled to less than 0°C.^[1] The work extracted by the expansion turbine is transmitted by a shaft to spin the compressor which allows this cooling process to take place.