An air cycle machine (ACM) is the refrigeration unit of the environmental control system (ECS) used in pressurized

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

-powered

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an ''

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

pack''. The air cycle cooling process uses air instead of a phase changing material such as

Freon Freon ( ) is a registered trademark of the Chemours Company and generic descriptor for a number of halocarbon products. They are stable, nonflammable, low toxicity gases or liquids which have generally been used as refrigerants and as aerosol p ...

in the gas cycle. No

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.

History

Air cycle machines were first developed in the 19th century for providing chilling on ships. The technique is a reverse

Brayton cycle The Brayton cycle, also known as the Joule cycle, is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. It is characterized by isentropic process, isentropic compre ...

(the thermodynamic cycle of a gas turbine engine) and is also known as a Bell Coleman cycle or "Air-Standard Refrigeration Cycle".

Technical details

Air conditioning systems of a Sukhoi Superjet

The usual compression, cooling and expansion seen in any

refrigeration Refrigeration is any of various types of cooling of a space, substance, or system to lower and/or maintain its temperature below the ambient one (while the removed heat is ejected to a place of higher temperature).IIR International Dictionary of ...

cycle is accomplished in the ACM by a

centrifugal compressor Centrifugal compressors, sometimes called impeller compressors or radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. They achieve pressure rise by adding energy to the continuous flow of fluid through th ...

, two air-to-air

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

s and an expansion turbine.

Bleed air Bleed air in aerospace engineering is compressed air taken from the compressor stage of a gas turbine, upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPC) valves bleed air from low or high stage engine ...

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 , 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

. 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 heat 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 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 Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity is a ...

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 now 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 ACM are

Honeywell Aerospace Honeywell Aerospace Technologies is a manufacturer of aircraft engines and avionics, as well as a producer of auxiliary power units (APUs) and other aviation products. Headquartered in Phoenix, Arizona, it is a division of the Honeywell Intern ...

Liebherr Aerospace Liebherr-Aerospace is the aerospace equipment manufacturing division of Liebherr. The company is an original equipment manufacturer (OEM); its low visibility in the minds of end consumers can be attributed to the OEM nature of all of its oper ...

Collins Aerospace Collins Aerospace is an American technology company that is one of the world's largest suppliers of aerospace and defense products. It was formed in 2018 from the merger of Rockwell Collins and UTC Aerospace Systems. Headquartered in Charlotte, ...

, 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 The Boeing Company, or simply Boeing (), is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, and missiles worldwide. The company also provides leasing and product support s ...

Airbus Airbus SE ( ; ; ; ) is a Pan-European aerospace corporation. The company's primary business is the design and manufacturing of commercial aircraft but it also has separate Airbus Defence and Space, defence and space and Airbus Helicopters, he ...

Embraer Embraer S.A. () is a Brazilian multinational aerospace corporation. It develops and manufactures aircraft and aviation systems, and provides leasing, equipment, and technical support services. Embraer is the third largest producer of civil air ...

, 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''

References

External links

* {{Cite web , title=How Airliners Work - Pressurization , author=Major Kong , publisher=

Daily Kos Daily Kos ( ) is a group blog and internet forum focused on the U.S. Democratic Party (United States), Democratic Party and Modern liberalism in the United States, liberal American politics. The site publishes blog posts, polls, election and cam ...

, date=6 June 2013 , url=http://www.dailykos.com/story/2013/06/06/1213853/-How-Airliners-Work-Pressurization
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) Gas compressors Aircraft components