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Bleed air
Bleed air in aerospace engineering is compressed air taken from the compressor stage of a gas turbine engine, upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPC) valves bleed air from low- or high-pressure engine compressor sections; as the pressure varies with engine operation, low-stage air is used during high-power operation, and high-stage air is used during descent and other low-power operations. Bleed air from that system can be utilized for internal cooling of the engine, cross-starting another engine, engine and airframe anti-icing, cabin pressurization, pneumatic actuators, air-driven motors, pressurizing the hydraulic reservoir, and waste and water storage tanks. Some engine maintenance manuals refer to such systems as "customer bleed air".
Bleed air is valuable in an aircraft for two properties: high temperature and high pressure (typical values are 200–250 °C (400–500 °F) and 275 kPa (40 psi), for regulated bleed air exiting the engine pylon for use throughout the aircraft).
In civil aircraft, bleed air's primary use is to provide pressure for the aircraft cabin by supplying air to the environmental control system. Additionally, bleed air is used to prevent the formation of ice on parts of the aircraft on which it would endanger the aircraft (such as wing leading edges).
Bleed air is used to power aircraft systems because it is readily available from the engines. For example, bleed air from one engine can be used to start the other engines. Lavatory water storage tanks are pressurized by bleed air that is fed through a pressure regulator.
When used for cabin pressurization, bleed air from the engine must first be cooled because it leaves the compressor at temperatures up to 250 °C (500 °F). It is passed through an air-to-air heat exchanger cooled by the cold outside air. It is then fed to an air cycle machine that regulates the temperature and flow of air into the cabin, keeping the environment comfortable. New bleed air is continuously flowing into the cabin and stale air leaving. Part of it is reused after passing through filters. This process is contrary to the common misconception that cabin air in airplanes is the same air being recycled.
Bleed air is also used to heat the engine intakes. This prevents ice from forming, accumulating, breaking loose, and being ingested by the engine, which could damage it.
On aircraft powered by jet engines, a similar system is used for wing anti-icing by the 'hot-wing' method. In icing conditions, water droplets condensing on a wing's leading edge can freeze. If that happens, the ice build-up 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, hot bleed air is pumped through the inside of the wing's leading edge, heating it to a temperature above freezing, which prevents the formation of ice. The air then exits through small holes in the wing edge.
On propeller-driven aircraft, it is common to use bleed air to inflate a rubber boot on the leading edge, breaking the ice loose after it has already formed.
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Bleed air
Bleed air in aerospace engineering is compressed air taken from the compressor stage of a gas turbine engine, upstream of its fuel-burning sections. Automatic air supply and cabin pressure controller (ASCPC) valves bleed air from low- or high-pressure engine compressor sections; as the pressure varies with engine operation, low-stage air is used during high-power operation, and high-stage air is used during descent and other low-power operations. Bleed air from that system can be utilized for internal cooling of the engine, cross-starting another engine, engine and airframe anti-icing, cabin pressurization, pneumatic actuators, air-driven motors, pressurizing the hydraulic reservoir, and waste and water storage tanks. Some engine maintenance manuals refer to such systems as "customer bleed air".
Bleed air is valuable in an aircraft for two properties: high temperature and high pressure (typical values are 200–250 °C (400–500 °F) and 275 kPa (40 psi), for regulated bleed air exiting the engine pylon for use throughout the aircraft).
In civil aircraft, bleed air's primary use is to provide pressure for the aircraft cabin by supplying air to the environmental control system. Additionally, bleed air is used to prevent the formation of ice on parts of the aircraft on which it would endanger the aircraft (such as wing leading edges).
Bleed air is used to power aircraft systems because it is readily available from the engines. For example, bleed air from one engine can be used to start the other engines. Lavatory water storage tanks are pressurized by bleed air that is fed through a pressure regulator.
When used for cabin pressurization, bleed air from the engine must first be cooled because it leaves the compressor at temperatures up to 250 °C (500 °F). It is passed through an air-to-air heat exchanger cooled by the cold outside air. It is then fed to an air cycle machine that regulates the temperature and flow of air into the cabin, keeping the environment comfortable. New bleed air is continuously flowing into the cabin and stale air leaving. Part of it is reused after passing through filters. This process is contrary to the common misconception that cabin air in airplanes is the same air being recycled.
Bleed air is also used to heat the engine intakes. This prevents ice from forming, accumulating, breaking loose, and being ingested by the engine, which could damage it.
On aircraft powered by jet engines, a similar system is used for wing anti-icing by the 'hot-wing' method. In icing conditions, water droplets condensing on a wing's leading edge can freeze. If that happens, the ice build-up 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, hot bleed air is pumped through the inside of the wing's leading edge, heating it to a temperature above freezing, which prevents the formation of ice. The air then exits through small holes in the wing edge.
On propeller-driven aircraft, it is common to use bleed air to inflate a rubber boot on the leading edge, breaking the ice loose after it has already formed.