An air cycle machine (ACM) is the refrigeration unit of the
environmental control system
In aeronautics, an environmental control system (ECS) of an aircraft is an essential component which provides air supply, Air conditioning, thermal control and cabin pressurization for the Aircrew, crew and passengers. Additional functions incl ...
(ECS) used in
pressurized
{{Wiktionary
Pressurization or pressurisation is the application of pressure in a given situation or environment.
Industrial
Industrial equipment is often maintained at pressures above or below atmospheric.
Atmospheric
This is the process by ...
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...
-powered
aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
. 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 or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
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 prop ...
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 to ...
or
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
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 is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas tur ...
(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](https://upload.wikimedia.org/wikipedia/commons/b/b5/Air_conditioning_systems_of_a_Sukhoi_Superjet.jpg)
The usual compression, cooling and expansion seen in any
refrigeration
The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...
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 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 (ASCPCs) valves bleed air from high or low stage engine compressor sections. Lo ...
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
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 ...
. 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 i ...
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 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 International conglome ...
,
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 its operatio ...
,
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
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and product ...
,
Airbus
Airbus SE (; ; ; ) is a European Multinational corporation, multinational aerospace corporation. Airbus designs, manufactures and sells civil and military aerospace manufacturer, aerospace products worldwide and manufactures aircraft througho ...
,
Embraer
Embraer S.A. () is a Brazilian multinational aerospace manufacturer that produces commercial, military, executive and agricultural aircraft, and provides aeronautical services. It was founded in 1969 in São José dos Campos, São Paulo, where i ...
,
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
*
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas tur ...
*
Refrigeration
The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...
*
Aerotoxic syndrome
Aerotoxic syndrome relates to ill-health effects that are claimed to be caused by breathing contaminated airliner cabin air. This condition is not an established medical diagnosis.
Potential sources of contamination
Modern jetliners have envi ...
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 and liberal American politics. The site includes glossaries and other content. It is sometimes considered an example of "netroots" activism.
Daily Kos was f ...
, 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