<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (August 2011) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (August 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> 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". </P>

How does air conditioning work on an airplane
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