Bleed air in gas turbine engines is compressed air taken from within the engine, after the compressor stage(s) 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. This compressed air can be used within the aircraft in many different ways, from de-icing, to pressurizing the cabin, to pneumatic actuators. However, bleed air is quite hot and when being used in the cabin or other low temperature areas, it must be cooled or even refrigerated. Bleed air is valuable in an aircraft for two properties: its high temperature and its high pressure [typical values are 200-250°C and 275 kPa (40 PSI), for regulated bleed air exiting the engine pylon for use throughout the aircraft].
Newer aircraft rely more on electricity, reducing the need for compressed air. Since most gas turbine engines use multiple compressor stages, some newer engines have the bleed air inlet between compressor stages to reduce the temperature of the compressed air.
Merits of bleed air
In civil aircraft, its primary use is to provide pressure for the aircraft cabin by supplying air to the Environmental Control System. Additionally, bleed air is used to keep critical parts of the aircraft (such as the wing leading edges) ice-free.
Bleed air is used on many aircraft systems because it is easily available, reliable, and a potent source of power. For example, air turbine starters used to start large jet engines are much smaller and lighter than an electric motor of equivalent power output. Bleed air for starting is provided by an on board Auxiliary Power Unit (APU) or an external huffer if the APU is inoperative. Once a single engine is started, its bleed air can be used to drive the starter on the remaining engines. Lavatory water storage tanks are pressurized by bleed air that is fed through a pressure regulator. Even the outside air probe on some aircraft utilize bleed air to drive a venturi pump to draw outside air in to a temperature sensor chamber. Early jet aircraft even used bleed air to drive the gyroscopes in their cockpit artificial horizons.
When used for cabin pressurization, the air from the engine must first be cooled (as it exits the compressor stage at temperatures as high as 300°C) by passing the bleed air through an air-to-air heat exchanger cooled by cold outside air. It is then fed to an air cycle machine unit which regulates the temperature and flow of air into the cabin, keeping the environment comfortable.
Bleed air is also used to heat the engine intakes. A small amount of bleed air is removed from the core of the engine, and is piped to the engine pod shroud where it heats the back side of the fan case. This prevents any ice from accumulating then breaking loose, where it would be immediately ingested by the engine, possibly damaging it.
A similar system is used for wing de-icing by the 'hot-wing' method. In icing conditions, water droplets condensing on a wing's leading edge can freeze at the ambient temperatures experienced during flight. This build-up of ice adds weight and changes the shape of the wing, causing a degradation in performance, and possibly a critical loss of control or lift. To prevent this, warm bleed air is pumped through the inside of the wing's leading edge. This heats up the metal, preventing the formation of ice. The air then exits through small holes in the wing edge. Alternatively, the bleed air may be used to inflate a rubber boot glued to the leading edge, breaking the ice loose.
Recent developments in civil aircraft
Bleed air systems have been in use for several decades in passenger jets. Recently, Boeing announced that its new aircraft, the 787 would operate without use of bleed air (and the two engines proposed for the aircraft, the General Electric GEnx and the Rolls-Royce Trent 1000, are designed with this in mind). This represents a departure from traditional winged aircraft design, and proponents state that eliminating bleed air improves engine efficiency, as there is no loss of mass airflow and therefore energy from the engine, leading to lower fuel consumption. In a bleed air system, air is compressed to several atmospheres by the engines, only to then be cooled and expanded (energy "lost") to a narrow pressure and temperature margin (requires complex systems to regulate temperature and pressure) to be piped around the plane, only then to be cooled or expanded again (energy "lost") to roughly one atmosphere of pressure. Additionally, eliminating bleed air reduces the aircraft's mass by removing a whole series of ducts, valves, heat exchangers and other heavy, maintenance intensive equipment and systems. The APU (auxiliary power unit) no longer has to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. To pressurize the cabin, electric air compressors are used. By only spinning cabin air supply compressors at the absolute minimum required speed, no energy wasting modulating valves are required. High temperature, high-pressure air cycle machine (ACM) packs can be replaced with low temperature, low pressure packs to increase efficiency. At cruise altitude, where most aircraft spend the majority of their time, and burn the majority of their fuel, and where the outside air is typically very cold, the ACM packs can be bypassed entirely, saving even more energy. Since no bleed air is taken from the engines for the cabin, engine oil contamination of the cabin air supply is eliminated. Lastly, advocates of the design say it improves safety as heated air is confined to the engine pod, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak could damage surrounding systems.
Because the aircraft has no bleed air powered engine starters, and no requirement for bleed air for cabin pressurization during flight, there is no need to have the APU drive a load compressor to supply bleed air to the aircraft during a period of engine loss, or on the ground at the start of each flight. This simplifies aircraft design, eliminates another maintenance requirement, and saves additional weight.
Skeptics point out that eliminating bleed air creates a requirement for another source of energy for cabin pressurization, anti-ice/de-ice systems, and other functions previously covered by bleed air. The other source is electricity from large generators fitted to the main engines and APU. Therefore, from a complete system point of view, this approach can be less efficient than might initially be thought. Rather than high pressure air from the engine (pneumatic energy) being used directly to pressurize the cabin, the pneumatic energy is converted to mechanical energy by the engine itself, and this shaft horsepower (mechanical energy) is taken from the engine to drive a generator (electrical energy) that is then used to drive a motor (mechanical energy) that is used to drive a compressor (pneumatic energy). Energy is lost at each conversion step, but despite all of these conversions, Boeing still expects a net energy savings. In the 787, the compressor motor is driven by the same power converters that are used to drive the electric engine starters for major weight savings.
While the 787 is considered a "bleedless" aircraft, a minor amount of bleed air is still used for engine pod intake de-ice. The amount of bleed air requirement for engine pod de-ice is so small, and since engine pod deice is not used for the entire flight, the engines are designed as if there were basically no typical bleed air requirements.
Airbus does not currently (as of November 2007) have any plans to eliminate bleed air from its 787 competitor, the A350, while Boeing is actively pursuing this technology, touting it as one of the main advantages of its design.
There have been claims by Dagbladet and The Observer that hazardous cabin gases can be caused by leakage of toxic gases produced by modern synthetic engine oils under high temperatures in the gas turbines, which subsequently gets distributed through the bleed air.
A non profit group called AOPIS was set up by crew members in 2001 to tackle the health and flight safety issues of bleed air being contaminated by synthetic jet engine oils and hydraulic fluids. A UK group, the Aerotoxic Association has been recently launched with similar aims to AOPIS.
Ill health effects
Certain neurological and respiratory ill health effects have been linked to exposure to contaminated bleed air on commercial and military aircraft. The associated long-term illness is known as "Aerotoxic Syndrome". One potential contaminant is Tricresyl phosphate.
Psychologist Sarah Mackenzie Ross and colleagues have recently studied the possible health implications for pilots breathing contaminated aircraft cabin air.
Published - July 2009
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