modesto2
Posts: 2681
Joined: Sun Jul 16, 2000 3:44 am

Engine Bleed Air

Fri Jul 09, 2004 12:30 am

Where is the bleed air valve located for a typical airliner? If it varies greatly for each aircraft, then let's use the A320 as an example. Is the air taken from bypass air or from the core? If from the core, is it off a compressor or turbine stage? And if it's from a compressor/turbine stage, is it a high pressure or low pressure stage? Thanks!
 
phollingsworth
Posts: 635
Joined: Sat Mar 06, 2004 6:05 am

RE: Engine Bleed Air

Fri Jul 09, 2004 12:36 am

Where is the bleed air valve located for a typical airliner? If it varies greatly for each aircraft, then let's use the A320 as an example. Is the air taken from bypass air or from the core? If from the core, is it off a compressor or turbine stage? And if it's from a compressor/turbine stage, is it a high pressure or low pressure stage? Thanks!

Always from the Core, as you want a greater than ~2 pressure ratio, and always from the compressor, the turbine is a bad place because the flow will be significantly hotter. On most aircraft there is a high and low pressure bleed, which obviously dictates where it is extracted. On GE engines all of the bleed is off of the compressor (HPC), not the booster (LPC).
 
SlamClick
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Joined: Sun Nov 23, 2003 7:09 am

RE: Engine Bleed Air

Fri Jul 09, 2004 12:45 am

Actually lots of CFM users do extract fan discharge air. It is not used by the pneumatic system, but blown across the heat exchangers to cool the core air for a/c use.

I know more about the JT-8D than about the CFM, even though I have not flown one in many years. On that engine there were utility airbleeds at the 6th, 8th and 13th stages of the compressor section.

BTW the turbine section is not used for airbleed largely because it contains carbon monoxide, being exhaust from the burner section.

The only plane I recall using 6th stage was the 727. My ignorance there. But all users tapped primarily the 8th stage for a/c and pressurization. If needed, the augmentation valves would open and add 13th stage air. This had to be pre-cooled (with fan discharge air) because what we needed was more pressure or more mass flow, but not higher temperature.

Anti-icing and deicing require hotter air and may favor the 13th stage some.

On the CFM-powered Airbus the arrangement will be similar. Most air of an intermediate stage of compression, augmented as needed by highest stage air.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
A/c train
Posts: 674
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RE: Engine Bleed Air

Fri Jul 09, 2004 12:50 am

The Hp compressor, because its hot, its easier to cool air down than to heat it back up again, you've also as phollingsworth states got a good pressure.
The RB211 for example has an HP 2 and an HP 6 bleed located at the 12 o clock position, bloody good engine the RB211  Smile/happy/getting dizzy
 
SlamClick
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RE: Engine Bleed Air

Fri Jul 09, 2004 1:08 am

Just a couple more items about airbleed.

Jet engines take in more air than they need, most of the time. In a typical turbine engine there may be bleed valves or bleed bands at most of the compression stages. Excess air may be bled off and discharged through the fan duct any time less than full thrust is called for.

Turbine engines use bleed air internally for gas path flow control and other services. Some discharge bleed air radially, out through some of the turbine blades or stators for cooling purposes.

Rarely is so much power called for from an engine that it cannot deliver pneumatic air for air conditioning etc. When such a "bleeds off" takeoff is made, the APU can be used to power the a/c packs, or the takeoff must be made "packs off" until reaching the first power reduction.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
cdfmxtech
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RE: Engine Bleed Air

Fri Jul 09, 2004 1:21 am

Actually lots of CFM users do extract fan discharge air. It is not used by the pneumatic system, but blown across the heat exchangers to cool the core air for a/c use.

Slamclick,
Are you saying that fan discharge air is used for the Air Conditioning Pack Heat Exchangers? Or are you just talking about Precooling (Precooler and Control valves, Fan Air, etc)?
 
SlamClick
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Joined: Sun Nov 23, 2003 7:09 am

RE: Engine Bleed Air

Fri Jul 09, 2004 1:26 am

Just for precooling.

At least that is my understanding from a dumbed-down pilot groundschool fourteen years ago.
Like I said, I know lots more about the JT-8D than I do the other jet engines that carry me around.

I guess I was just arguing with the statement that it is "not used" and being pedantic about the words.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
darkblue
Posts: 227
Joined: Thu Sep 11, 2003 10:27 pm

RE: Engine Bleed Air

Fri Jul 09, 2004 1:54 am

The driving force behind where you take your bleed is air pressure, not necessarily bleed air temperature. To meet aircraft pneumatic requirements, a bleed pressure level is defined by airframer.

Ideally you want to take the "cheapest" bleed air from the engine. As you go deeper and deeper into the compressor, the bleed air becomes more and more "expensive". This is because the compressor has done more work on this air to reach that level of pressure.

At high power, the cheapest bleed that meets the aircraft pressure requirements usually comes from the middle stages of the compressor. However, at idle power, the middle stages do not provide enough bleed pressure, therefore bleed must come from the more expensive compressor discharge air.

Before going into the aircraft, the bleed air must be cooled. Fan bleed air is used to cool the compressor bleed using the precooler.

Now, this only covers aircraft bleed air. The engine itself requires all sorts of parasitic flows. Cooling air is taken off the fan and compressor to cool various parts of the engine. The key to where you take this bleed air is once again what pressure do you need. To cool the stage 1 HPT nozzle, you need cooling air that is at a higher pressure level, otherwise the hot turbine air will flow back into the compressor... not a good thing. Therefore HPT cooling air has to come from the compressor discharge, while LPT cooling can come from mid stage compressor and the core compartment cooling and bore cooling can come from the fan and booster bleed air.
 
Mr Spaceman
Posts: 2723
Joined: Thu Mar 15, 2001 5:09 am

RE: Engine Bleed Air

Fri Jul 09, 2004 2:59 am

Hi guys.

>> Modesto2, here's a link that has excellent info about how the engine bleed air is utilized on a Boeing 767-300ER with P&W 4000 engines.

http://www.boeing.com/commercial/cabinair/ecs.pdf


I hope this info helps. Big grin


Chris  Smile

[Edited 2004-07-08 20:00:53]
"Just a minute while I re-invent myself"
 
G4Doc2004
Posts: 120
Joined: Thu Feb 19, 2004 7:49 am

RE: Engine Bleed Air

Fri Jul 09, 2004 10:47 am

The Tay 611-8 on a Gulfstream G-IV uses 7th stage and 12th stage air. At idle, it uses 12th stage, and as power is increased, a valve senses the pressure rise and shifts bleed air to the 7th stage. Fan air is tapped off at the aft end of the bypass duct and is used to cool the hot bleed air via an intercooler in the pylon. The actual bleed air control valves are in the tail compartment.

[Edited 2004-07-09 03:52:05]
"Failure to prepare is preparing to fail"--Benjamin Franklin
 
MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

RE: Engine Bleed Air

Fri Jul 09, 2004 11:38 pm

The bleed valve is usually sited either directly on the engine or in the pylon. It is also often refered to as the PRSOV (Pressure regulating and shut-off valve), because this are the jobs it does.
Airframe bleed air for airconditiomning and wing anti-ice is normally taken from one of two different compressor stages, one at for low engine RPMs (idle) and one for high RPMs. Normaly a compressor stage close to the middle of the compressor is used, however at low RPM, the pressure will not be enough, so at engine idle a high compressor stage is used. This selection is done be a high-stage valve and a check valve in the low pressure duct. from there the air normally goes to a precooler either in the pylon or attached to the engine. Fan air, regulated by a fan air valve, is blown over this cooler to cool the bleed air down to about 190- 200°C. From there it passes the PRSOV. In case the pressure or temperature becomes too high, the PRSOV closes. Downstream of the PRSOV is often a branch pipe going to the starter valve and the starter. Afterwards the air joins the crossover duct and goes to the aircraft systems (A/C, anit-ice)
As has been said, the engines use internal bleed air for pressurising and sealing bearing chambers as well as muscle pressure for operating valves. Also there aare several tap offs at various points of the engine used to measure pressures for regulation and stall protection of the engine.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
Bungle
Posts: 93
Joined: Fri Nov 03, 2006 6:26 am

RE: Engine Bleed Air

Sat Jul 10, 2004 5:35 am

The cfm 56-2a ( E3D) Takes its bleed air from the 5th and 9th stages of the compresser section.
 
320tech
Posts: 489
Joined: Mon May 17, 2004 11:38 am

RE: Engine Bleed Air

Sat Jul 10, 2004 2:28 pm

Bungle is correct. The A320 / CFM-56 has two sources, the 5th and 9th stages. The 9th stage ("high pressure") is used primarily when the engine is at low power. When the engine is running at higher power, the 5th stage ("intermediate pressure") provides the air. The 9th stage has a valve (HPV), the 5th stage has only a check valve.

Both sources provide air at about 36 - 44 psi.

From there, the air travels through a Pressure Regulating Valve, an Overpressure Valve (shuts off air at 85 psi, IIRC), through a precooler, and to the various services. A Fan Air Valve opens to cool the air flowing through the precooler if the temperature exceeds 200 degrees C. The Fan Air Valve uses bypass air. The PRV will shut off bleed air if the system exceeds 240 C.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
delta-flyer
Posts: 2631
Joined: Mon Jul 30, 2001 9:47 am

RE: Engine Bleed Air

Sat Jul 10, 2004 11:55 pm

Someone mentioned "expensive" bleed air and "Jet engines take in more air than they need". Indeed, both are true - engines are designed to take in more air than is used strictly for propulsion in order to service the peripheral pneumatic system needs.

This is where one of the 7E7 innovations will occur - the pneumatic system's big users - such as the ECS and air turbine driven pumps, will be electric. It is cheaper (and more efficient, in the long-run) to extract mechanical power from the engine core to drive a generator than to extract the equivalent power as bleed air. So, with all else being equal, a slightly smaller compression stage will be able to deliver all system requirements using electrical systems instead of pneumatic. This, in turn, reduces power demand from the engine, reducing fuel burn.

Pete
"In God we trust, everyone else bring data"

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