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 Environmental Control System of an airliner provides air supply, thermal control and pressurization for the passengers and crew. ... Cabin pressurization is the active pumping of air into the cabin of an aircraft to increase the air pressure within the cabin. ... A Siemens steam turbine with the case opened. ... Look up aircraft in Wiktionary, the free dictionary. ... Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. ... Water vapor condensing over a cup of hot tea Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapor) to a liquid. ... This article or section is in need of attention from an expert on the subject. ...

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. Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. ... A centrifugal compressor, also called a radial blower, squirrel cage, or squirrel wheel compressor, consists of an axle to which is mounted a cylindrical assembly of compressor blades. ... A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. ... A turboexpander (also referred to as turbo expander, expansion turbine or simply expander) is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work that is typically used to drive a compressor. ...

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. 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 after the expansion turbine is passed through a water seperator, which uses centrifugal force to throw the water particles into a coalescer bag that absorbs the moisture. 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. Bleed air in jet turbines is compressed air taken from within the engine, after the compressor stage(s) and before the fuel is injected in the burners. ...

References

1. ^ Discussion paper on the cabin air environment (published by UK COT Secretariat)

See also

• Refrigeration
• Brayton cycle

Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. ... The Brayton cycle is a cyclic process generally associated with the gas turbine. ...

External links

• What is air cycle?
• Commercial airliner environmental control system
• UK COT Secretariat paper (scroll to page 5 for schematic of ACM system)