What exactly is the Bleed air?

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What exactly is the Bleed air?

Post by captainalex »

Hi all
I need a detailed explain please :oops: :oops: :?: :?:
747-400.... What can i say...

Ron D.
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Post by Ron D. »

Bleed Air in gas turbine engines is compressed air taken from the engines, after the compressor stage and before fuel is injected in the burners. This compressed air is used for many things but mainly for pressurising the cabin, air conditioning and de-icing systems. This air is very hot and is pumped to wing leading edges etc. for de-icing or to inflate rubber boots to remove ice. When used for pressurisation and air conditioning it must be cooled or refrigerated first.
Boeing have announced that the new 787 may not have bleed air systems as it removes air from the engine and the new engines ie. GEnx and Rolls-Royce Trent 1000 can deliver more power and fuel efficiency this way. Airbus however, claim that the added weight and complex components of an alternative system outweigh the claimed fuel saving and currently have no plans to eliminate bleed air from it's aircraft.



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Post by jackpilot »

To add a bit to this, and with some variations in type, compressed air is needed to start spinning the engines (like wind on a windmill) in this case it is supplied by the APU ( which is a small turbine engine but electrically started which supplies electrical power AND compressed air) or by a ground compressor on an airport truck.
This is why the aircraft has a duct system with valves that the pilot opens or closes with overhead switches to manage the bleed air supply coming from different sources, Ground/APU/Engine1/2/3/4...and used for different purposes...Check http://en.wikipedia.org/wiki/Bleed_air

Air Isolation Valve
The air isolation valve serves two purposes. First, it is used to cross start engines. After starting one engine on the auxiliary power unit (APU) or ground air, with the APU switch in the off position, the engine crank switch will automatically open the air isolation valve when starting the other engine. Bleed air from the engine running is routed through the air isolation valve to the engine starter control valve (fig. 3-1) of the engine to be started. As the engine accelerates to a self-sustaining speed, the switch automatically returns to the off position. The air isolation valve is then closed by spring pressure. The air isolation valve can also be used to route APU air to augment the bleed-air supplv to the air-conditioning system at times when engine output is low. This could be when waiting to launch, with engines at idle power and air temperatures high and humid.

This is a quote from an excellent and complete article on the subject;..see
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Post by Stoney3K »

Does the cabin air get cooled forcibly (via refrigeration units) or does it get cooled by mixing it with ram air from the outside (which is about -40C or colder depending on flight level)... would be a terrible waste of power to use refrigeration units when you have a source of cold air running outside. Maybe this also depends on the aircraft. I always thought the "packs" used for air conditioning had ram air inlets (I know they have on the 737) which are used to get the compressor air to a more livable temperature.

Fabio Miguez
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Post by Fabio Miguez »

Hey Stoney,

WAIVER: The system described below is related to a Falcon 900.

The cold air that cools the cabin is air straight from the bleed system, that has been cooled via the packs. On the ground, most bleed systems have an electric or pneumatic fan that blows air over a radiator, inside which hot or warm air gets cooled.

In the air, ram air is used in lieu of the fan, to run through the radiator and cool the bleed air. But outside air is never used directly in the cabin. At first, this may seem a waste, due to the extremely cold temperatures outside, but the packs do an exceedingly good job at capturing that temperature in the radiator, or, more technically speaking, transfering heat from the bleed air to the outside air.

There are a lot of inctricacies related to the bleed system, including several details I ommited for clarity, but should anyone express interest, I can discuss this further, including diagrams.

Best regards,
Fabio Miguez

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Post by captainalex »

Thaaaaaaaanks guys
I understand now :D :D :D :D
747-400.... What can i say...

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