Bleed air
From Wikipedia, the free encyclopedia
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.
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.
[edit] 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.
When used for cabin pressurization, the air from the engine must first be cooled (as it exits the compressor stage (converging) 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 conditioning unit which regulates the temperature and flow of air into the cabin, keeping the environment comfortable.
A similar system is used for wing de-icing. 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 fatal loss of 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. Alternatively, the bleed air may be used to inflate a rubber boot glued to the leading edge, breaking the ice loose.
[edit] 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. Additionally, eliminating bleed air may reduce the aircraft's mass by removing a whole series of pumps, heat exchangers and other heavy equipment. Lastly, advocates of the design say it improves safety as heated air is confined to the engine core, as opposed to being pumped through pipes and heat exchangers in the wing and near the cabin, where a leak could damage surrounding systems.
Skeptics point out that eliminating bleed air creates a requirement for another source of energy for cabin heating, anti-ice/de-ice systems, and other functions previously covered by bleed air. The other source can be a "dummy engine" - which can require electrical energy from the main engine(s). Therefore, from a complete system point of view, this approach can be less efficient than might initially be thought. The dummy engine has an inlet, a compressor, a turbine, an exhaust, an electric motor. Instead of a combustion chamber it has much like a turbocharger connections to deliver and take back air. A reverse flow heat exchanger can be used to lower energy requirements for the cabin ventilation.
Airbus does not currently (as of November 2004) 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 [1] and The Observer [2] 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 [3] 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.
