Air cycle machine
Encyclopedia
An air cycle machine 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.
(the thermodynamic cycle of a gas turbine engine) and is also known as a Bell Coleman cycle or "Air-Standard Refrigeration Cycle".
cycle is accomplished in the ACM by a centrifugal compressor
, two air-to-air heat exchanger
s and an expansion turbine.
Bleed air
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 32psi, 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. 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 work 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 is obtained by the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system describes a reduction from about 30psi to about 11psi). After the air has been cooled down, water vapor in the air condenses, forming fog or high humidity. To get rid of this, the air exiting the expansion turbine is passed through a water separator, which uses centrifugal force to throw the water particles into a coalescer bag (Not Airbus) that absorbs the moisture (this condensate can be injected into the outside air entering the secondary heat exchanger to improve its performance).
The air can now be combined in a mixing chamber with a small amount of uncooled engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment.
The types of Air Cycle Machines may be identified as:
The term Air Conditioning "Pack" or A/C "Pack" is used differently by different sources. Boeing
, Airbus
, Embraer
, Bombardier and Lockheed
use the term as an abbreviation of "package", applied to both pneumatic and non-pneumatic systems. At least one business jet supplier uses the term "PAC" instead on their systems control panels, said to be an abbreviation for "Pneumatic Air Conditioning." Supposedly, "PACK" (P.A.C.K.) can also be an acronym for "Pneumatic Air Cycle Kit", however, this abbreviation is not used by the major manufacturers above.
Environmental Control System
The environmental control system of an aircraft provides air supply, thermal control and cabin pressurization for the crew and passengers...
(ECS) used in pressurized gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
-powered aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft 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.Although...
. Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred as an Air Conditioning Pack
Pack (aircraft)
PACK, Pneumatic Air Cycle Kit. The Kit usually consists of an Air cycle machine , a set of heat exchangers, water separators and temperature and pressure control valves and sensors. Additional components may be included depending on the design....
.
The air cycle cooling process uses air instead of a phase changing material such as Freon in the gas cycle. No condensation
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
or evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
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 cycleBrayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...
(the thermodynamic cycle of a gas turbine engine) and is also known as a Bell Coleman cycle or "Air-Standard Refrigeration Cycle".
Technical details
The usual compression, cooling and expansion seen in any refrigerationRefrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
cycle is accomplished in the ACM by a centrifugal compressor
Centrifugal compressor
Centrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller...
, two air-to-air heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s and an expansion turbine.
Bleed air
Bleed air
Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage and before the fuel is injected in the burners. While in theory bleed air could be drawn in any gas turbine engine, its usage is generally restricted to jet engines used in aircraft...
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 32psi, 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. 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 work 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 is obtained by the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system describes a reduction from about 30psi to about 11psi). After the air has been cooled down, water vapor in the air condenses, forming fog or high humidity. To get rid of this, the air exiting the expansion turbine is passed through a water separator, which uses centrifugal force to throw the water particles into a coalescer bag (Not Airbus) that absorbs the moisture (this condensate can be injected into the outside air entering the secondary heat exchanger to improve its performance).
The air can now be combined in a mixing chamber with a small amount of uncooled engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment.
Nomenclature
Air Cycle systems can be identified by the type of Air Cycle Machine.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 a Turbine 1, Turbine 2, Compressor and Fan on a common shaft
The term Air Conditioning "Pack" or A/C "Pack" is used differently by different sources. Boeing
Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
, Airbus
Airbus
Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....
, Embraer
Embraer
Embraer S.A. is a Brazilian aerospace conglomerate that produces commercial, military, and executive aircraft and provides aeronautical services....
, Bombardier and Lockheed
Lockheed Martin
Lockheed Martin is an American global aerospace, defense, security, and advanced technology company with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is headquartered in Bethesda, Maryland, in the Washington Metropolitan Area....
use the term as an abbreviation of "package", applied to both pneumatic and non-pneumatic systems. At least one business jet supplier uses the term "PAC" instead on their systems control panels, said to be an abbreviation for "Pneumatic Air Conditioning." Supposedly, "PACK" (P.A.C.K.) can also be an acronym for "Pneumatic Air Cycle Kit", however, this abbreviation is not used by the major manufacturers above.
External links
- What is air cycle?
- Commercial airliner environmental control system
- Legislation and guidance from the UK Government and the National Health Service (scroll to page 5 for schematic of ACM system)