A&P Mech From , joined Dec 1969, posts, RR:
Reply 1, posted (15 years 10 months 1 week 2 days 12 hours ago) and read 6320 times:
On commercial aircraft turbine engines, bleed air valves on the engine control surging/stability AND supply air to the pneumatic system plenum/manifold, which can then supply air to these systems:
-Potable Water Systems
-Cargo Loading Systems
Example: The PW 4460 Engine-
There are bleed air valves at stages 2.5, 2.9, 4, 8, 9, 12, and 15. The ones at stages 2.5, 2.9, 4 (fan section), and 9 (HP compressor section) are related to controlling engine surging and stability.
The 2.5 bleed valve and actuator controls the quantity of air that goes into the high pressure compressor (HPC). At low engine RPM, the low pressure compressor (LPC) supplies more air than is necessary for operation. The 2.5 bleed valve bleeds this unwanted 4th stage air into the fan discharge secondary airflow. The valve closes when the engine RPM is sufficient to let the full flow of fan air go into the N2 compressor. This makes the compressor more stable at start, during transient operation, and during reverse thrust operation.
The two 9th stage compressor bleeds, (HPC 9th stage start/stability (2.9) bleed system) gives increased compressor stability during engine start, decelerations, and shutdown. The valves bleed 9th stage air from the engine compressor. This gives smoother accelerations to idle during start, and prevents possible compressor surges or stalls during operation. The bleeds are controlled as a function of high rotor speed (N2) and engine inlet total temperature (T2). One is for Engine starting, the other is for stability. Valves bleed air into the bypass airstream.
The bleed air valve locations and purposes are:
2.5 Stage = Compressor surge/stability control (controls venting of 4th stage air).
2.9 Stage = Compressor surge/stability control.
4 Stage = Vented into the bypass airstream to control surge/stability. Driven by 2.5 stage air.
8 Stage = Low pressure pneumatics plenum (Air Conditioning/Pressurization, etc).
9 Stage = Vented into the bypass airstream to control surge/stability. Driven by 2.9 stage air.
12 Stage = Ported into the turbine section for cooling blades, vanes, and #3 bearing.
15 Stage = High pressure pneumatics plenum (Air Conditioning/Pressurization, etc).