Air cycle machine
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An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized gas turbine-powered aircraft. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an air conditioning pack. The air cycle cooling process uses air instead of a phase changing material such as Freon in the 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.
Air cycle machines were first developed in the 19th century for providing chilling on ships. The technique is a reverse Brayton cycle (the thermodynamic cycle of a gas turbine engine) and is also known as a Bell Coleman cycle or "Air-Standard Refrigeration Cycle".
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), 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 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 describes a reduction from about 30 psi or 210 kPa to about 11 psi or 76 kPa). After the air has been cooled down, water vapor or moisture 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 type of water extraction system which uses coalescer bag is known as Low Pressure Water Extraction system. This system is incorporated in Boeing 737-200. One disadvantage is that the coalescer bag system dumps the extracted water directly on the tarmac. However today with improved technology, a High Pressure Water Extraction system has been introduced in Airbus aircraft and Boeing 737NG, Boeing 757 etc. In this system water is extracted in two stages . After air passes through secondary heat exchanger, it is sent through a split duct water extractor which extracts a certain amount . After this it passes through a reheater and condenser which both cools the air and finally to the primary water extractor which has swirl vanes and the motive flow of air makes the water to separate and flow further. The water extracted in these extractors are sprayed into inlet of the Ram Air Intake which draws in ambient air to cool the bleed air in the heat exchangers. This spraying of water helps in cooling the ambient air so as to increase heat transfer across heat exchanger.
The air can now be combined in a mixing chamber with a small amount of non-conditioned engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment.
Air cycle systems can be identified by the type of air cycle machine.
The types of air cycle machines may be identified as:
- Simple cycle consisting of a turbine and fan on a common shaft
- Two-wheel bootstrap consisting of a turbine and compressor on a common shaft
- Three-wheel consisting of a turbine, compressor, and fan on a common shaft
- Four-wheel/dual-spool consisting of a two turbines, a compressor, and a fan on a common shaft
The term air conditioning pack or A/C pack is used differently by different sources. Boeing, Airbus, Embraer, Bombardier and Lockheed use the term as an abbreviation of package, applied to both pneumatic and non-pneumatic systems. At least one business jet supplier uses the term PAC instead on their systems control panels, said to be an abbreviation for pneumatic air conditioning. Supposedly, PACK can also be an acronym for pneumatic air cycle kit, however, this abbreviation is not used by the major manufacturers above.
- Boeing. "Commercial Airliner Environmental Control System". Archived from the original on 2011-05-24.
- UK COT Secretariat, discussion paper on the cabin air environment
- Major Kong (6 June, 2013). "How Airliners Work - Pressurization". Daily Kos.
- What is air cycle?
- Legislation and guidance from the UK Government and the National Health Service (scroll to page 5 for schematic of ACM system)