Air cycle machine

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 or 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 of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.

Technical details

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

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 32psi , is directed into a primary heat exchanger. Outside air at ambient temperature and pressure is used as the coolant in this air-to-air heat exchanger. Once the hot air has been cooled, it is then compressed by the centrifugal compressor. This compression heats the air (the maximum air temperature at this point is about 250°C) and it is sent to the secondary heat exchanger, which again uses outside air as the coolant. 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).

At this point, the temperature of the compressed cooled air is somewhat greater than the ambient temperature of the outside air. The compressed, cooled air then travels through the expansion turbine which extracts work from the air as it expands, cooling it to below ambient temperature (down to -20°C or -30°C). It is possible for the ACM to produce air cooled to less than 0°C even when outside air temperature is high (as might be experienced with the aircraft stationary on the ground in a hot climate).

The work extracted by the expansion turbine is transmitted by a shaft to spin the air 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. The power for the air conditioning pack is obtained by the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system (Boeing describes a reduction from about 30psi to about 11psi). After the air has been cooled down, water vapor in the air condenses, forming fog or high humidity. To get rid of this, the air exiting the expansion turbine is passed through a water separator, which uses centrifugal force to throw the water particles into a coalescer bag that absorbs the moisture (this condensate can be injected into the outside air entering the secondary heat exchanger to improve its performance).

The air can now be combined in a mixing chamber with a small amount of uncooled engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment.

See also

• Refrigeration
• Brayton cycle

External links

• This is ECS is an Aerospace Environmental Control Systems Consulting Firm
• What is air cycle?
• Commercial airliner environmental control system
• UK COT Secretariat paper (scroll to page 5 for schematic of ACM system)