Bleed air

Bleed air produced by gas turbine engines is compressed air that is taken from the compressor stage of those engines, which is upstream of the fuel-burning sections. In modern airliner engines, two regulator valves (high stage and low stage) turn on and off automatically and are controlled by at least "...two air supply and cabin pressure controllers (ASCPCs) which open and close appropriate valves. Engine Bleed Air comes from the high stage or low stage engine compressor section. Low stage air is used during high power setting operation, and high stage air is used during descent and other low power setting 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, 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 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 keep critical parts of the plane (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, bleed air from an airplane engine is used to start the remaining engines. Lavatory water storage tanks are pressurized by bleed air that is fed through a pressure regulator.

When used for cabin pressurization, the bleed air from the engine must first be cooled (as it exits the compressor stage at temperatures as high as 250 °C) by passing it 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.

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Wikipedia