Air cycle machine

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". ==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 , 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 comes from the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system; typical differentials are from about to about . The next step is to dehumidify the air. Cooling the air has caused any water vapor it contains to condense into fog, which can be removed using a cyclonic separator. Historically, the water extracted by the separator was simply dumped overboard, but newer ACMs spray the water into the outside-air intakes for each heat exchanger, which gives the coolant a greater heat capacity and improves efficiency. (It also means that running the ACM on an airplane parked on the tarmac does not leave a puddle.) The air can then be combined in a mixing chamber with a small amount of non-conditioned engine bleed air. This warms the air to the desired temperature, and then the air is vented into the cabin or to electronic equipment. == Manufacturers ==

Major manufacturers of ACMs are Honeywell Aerospace, Liebherr Aerospace, Collins Aerospace, and PBS Velka Bites. == Nomenclature ==

Types 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 two turbines, a compressor, and a fan on a common shaft Abbreviations The equipment is referred to variously as PAC, air conditioning pack, or A/C pack, but there is a lack of consistency and agreement as to the derivations and meanings: • Pack. as an abbreviation of package, applied to both pneumatic and non-pneumatic systems (Boeing, Airbus, Embraer, Bombardier and Lockheed) • PAC as an acronym meaning either passenger air conditioning or pneumatic air conditioning (the latter being found on systems control panels of at least one business jet supplier) • PACK as an acronym for pneumatic air cycle kit or pressurization & air conditioning kit ==See also==