What do bleed valves do on Jets?

Question

What do bleed valves do on Jets?

Answer 1

Compressor bleed valves on jet engines allow excess compressor air to bleed off. Besides the safety aspect of preventing over pressure in the compressor section of the engine they also work in conjunction with such devices as inlet guide vanes to prevent compressor surge or what we call compressor stalls. Bleed valves will normally be open at idle RPM since there is little need for much compressed air with so little fuel being delivered to the combustor. At take of you can hear the bleed valves close and trap all available compressable air to mix with the fuel required for high power.

Mark the valves you refer to are normally called HP regulator valves and have nothing to do with the bleed function of bleed valves. HP valves pressurize a bleed air manifiold from which bleed air can be withdrawn for the uses that you state but these are not bleed valves.

Answer 2

Bleed valves may be installed on the LP, IP, or HP compressor section to provide air flow as required from each stage. Bleed air is a loss to the duty cycle of the engine (i.e. you are compressing the air but you are not using it for thrust, so you are wasting fuel in terms of engine specific fuel consumption). HP bleed is "more costly" in this regard than IP or LP air, since you have already done more work to compress it. However, it is also hotter and higher in pressure, which means you can use less of it as a source for de-icing systems or even cabin air - once it is passed through a heat exchanger. The proper source of air for these systems really depends on the design of the system and the design of the engine.

Bleed air valves are occasionally opened briefly during start or rapid acceleration transients to increase compressor stall margin. When this is done, the air is normally not used for cabin pressure or de-icing, it is just dumped into the bypass duct or overboard. Therefore, this is only done when necessary to prevent compressor surge (which can be disconcerting but is rarely dangerous) during such transients. If an engine is running with bleed valves dumping air at idle to prevent surge, it is wasting a lot of fuel.

Answer 3

There are two main type of Bleed Valves used in transport category aircrafts engines. One like John said is used like a flapper valve to release extra pressure buildup so that the compressor blades do not stall and the other is the engine bleed valve usually in the intermediate compressor (intermediate is used mainly because the high pressure compressor air would be too hot) which is used for a tonne of stuff like Air conditioning, pressurization and reverse thrusters.

Answer 4

They open in order to bleed air from engine compressor sections.  Bleed air is used for starting, air conditioning, pressurization, moving thrust reverser clamshell doors, and  heating engine inlets and ducts,  leading edges,  some antennas,   fuel, etc.

Answer 5

On a Boeing 727

AC motor driven bleed valves admit or shut off bleed air from their respective engines to the pneumatic manifold.  On engines 1 and 3 the valve is upstream of the bleed air precooler.

The bleed air valves are normally controlled by their respective switches on the engineer's upper panel.  Pulling the respective engine fire pull handle overrides the normal control switch for the bleed valve on the effected engine and closes the valve.

Opening bleed air switches for engines 1 or 3 will open the respective bleed valve and admit starting air to the start valve or bleed air to the pneumatic manifold

Only intermediate stage bleed air can be made available from engine 2 to the pneumatic manifold.  Air supplied from the (#2) engine is not cooled by a precooler