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.