Air cycle machine
Refrigeration unit of aircraft environmental control systems.
An air cycle machine (ACM) is the cooling component within an aircraft’s environmental control system (ECS). Most pressurized planes powered by gas turbines carry two or three of these units. Each ACM, together with its connected pipes and parts, is commonly called an air conditioning pack. Instead of using a refrigerant that changes phase, like freon, the ACM cools air through a gas cycle. There is no condensation or evaporation of a refrigerant; the chilled air it produces goes straight to the cabin for ventilation or to cool electronic gear.
The concept dates back to the 1800s, when it was first used to cool ships. Technically, it operates on a reverse Brayton cycle—the same thermodynamic cycle found in a gas turbine engine—and is also known as the Bell Coleman cycle or air-standard refrigeration cycle.
In operation, the ACM uses a centrifugal compressor, two air-to-air heat exchangers, and an expansion turbine to achieve the standard steps of compression, cooling, and expansion. Hot bleed air—coming from the engines, an auxiliary power unit, or a ground source—can exceed 150 °C at around 32 psi (220 kPa). This air first enters a primary heat exchanger, where outside ambient air cools it. After that, the air is compressed by the centrifugal compressor, which raises its temperature to about 250 °C. It then moves to a secondary heat exchanger, again cooled by outside air. The initial cooling in the first heat exchanger improves efficiency because compressing cooler air requires less work.
The compressed, cooled air now sits at a temperature slightly above the outside air. It flows through the expansion turbine, which extracts heat as the air expands, dropping its temperature below ambient—down to −20 °C or −30 °C. This means the ACM can produce air below freezing even when the plane is on the ground in hot weather. The turbine’s work drives a shaft that spins the pack’s compressor and an inlet fan; the fan pulls in external air for the heat exchangers while on the ground, and ram air takes over in flight. The system gets its power from the pressure drop between the incoming bleed air and the exiting cooled air—typically from about 30 psi (210 kPa) down to 11 psi (76 kPa).
Next, the air is dehumidified. Cooling causes water vapor to condense into fog, which a cyclonic separator removes.
- Typical number per aircraft
- two or three
- Bleed air inlet temperature
- in excess of 150 °C
- Bleed air inlet pressure
- approximately 32 psi (220 kPa)
- Maximum temperature after compression
- about 250 °C
- Typical output temperature
- down to −20 °C or −30 °C
- Typical pressure differential
- from about 30 psi (210 kPa) to about 11 psi (76 kPa)
Lore & Background
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'.
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 is directed into a primary heat exchanger, where outside air at ambient temperature and pressure is used as the coolant. After cooling, the air is compressed by the centrifugal compressor, which increases its temperature, then sent to a secondary heat exchanger. The pre-cooling through the first heat exchanger increases efficiency because it lowers the temperature of the air entering the compressor. The compressed, cooled air then travels through the expansion turbine, which extracts heat as the air expands, cooling it to below ambient temperature. 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 external air for the heat exchangers during ground running; ram air is used in flight.
Reader's Guide
The air cycle machine is notable as the core refrigeration unit of the environmental control system in pressurized gas-turbine–powered aircraft, typically with two or three units per aircraft. Its significance lies in using air as the refrigerant rather than a phase-changing material like freon, avoiding condensation or evaporation of a refrigerant. The process follows a reverse Brayton cycle, also known as the Bell Coleman cycle, which was first developed in the 19th century for ship chilling. The ACM's design includes a centrifugal compressor, two air-to-air heat exchangers, and an expansion turbine, with the power for the pack coming from the reduction of pressure of incoming bleed air relative to exiting cooled air. The system can produce air cooled to below 0 °C even in hot ground conditions. Dehumidification is achieved via a cyclonic separator, and newer ACMs spray extracted water into outside-air intakes to improve efficiency. Major manufacturers include Honeywell Aerospace, Liebherr Aerospace, Collins Aerospace, and PBS Velka Bites. The equipment is referred to variously as PAC, air conditioning pack, or A/C pack, with a lack of consistency and agreement as to the derivations and meanings of these abbreviations.
Did You Know?
- The air cycle cooling process uses air instead of a phase changing material such as freon.
- The ACM can produce air cooled to less than 0 °C even when outside air temperature is high.
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