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Faro
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Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 9:09 am

I understand that the function of the air conditioning packs is to lower the temperature of the air coming off the high pressure compressor and reduce its pressure in order to transform it into cabin air.

Why in that case not bleed that air off the low pressure compressor, thereby taking lower-temperature and lower-pressure air to work with? That would conceivably make the size and weight of the air conditioning packs lower, wouldn't it?


Faro
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Starlionblue
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 9:43 am

The air to have high enough pressure and temperature to actually power the air cycle machines in the packs. My guess is you wouldn't get what you need from the low-pressure compressor (fan).

I can only speak for the Rolls-Royce Trent 772 and Trent XBW. They take bleed air from the intermediate pressure compressor. In case bleed demand exceeds supply, additional bleed air is taken from the high-pressure compressor. Air from the low-pressure compressor (fan) is used to regulate the temperature of the bleed air.
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Faro
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 9:53 am

Starlionblue wrote:
The air to have high enough pressure and temperature to actually power the air cycle machines in the packs. My guess is you wouldn't get what you need from the low-pressure compressor (fan).

I can only speak for the Rolls-Royce Trent 772 and Trent XBW. They take bleed air from the intermediate pressure compressor. In case bleed demand exceeds supply, additional bleed air is taken from the high-pressure compressor. Air from the low-pressure compressor (fan) is used to regulate the temperature of the bleed air.



I guess the question is then why would one need power input to the packs? What work would have to be injected into the HP bleed air if what you are looking to do is reduce its energy per unit volume? That is, reduce both the temperature and pressure. Overall, the packs are taking energy out of the HP bleed air, not adding it. In what way do they need even more energy to do that?


Faro
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Starlionblue
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 10:15 am

I'm not an air cycle machine expert, but my guess there is you need a lot of air to condition and pressurise the entire aircraft and it is simply inefficient to use low-pressure spool air.
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SAAFNAV
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 10:16 am

Faro wrote:

I guess the question is then why would one need power input to the packs? What work would have to be injected into the HP bleed air if what you are looking to do is reduce its energy per unit volume? That is, reduce both the temperature and pressure. Overall, the packs are taking energy out of the HP bleed air, not adding it. In what way do they need even more energy to do that?


Faro


The pack is a turbine spinning to compress air, in order to use the expansion thereafter for cooling.
You need energy to do work. The energy in this case is the temperature difference between the hot and the cold air.

For the same reason you need electricity to drive your aircon, or hot exhaust gasses to power your car's turbo charger, nothing is for free.
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dragon6172
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 1:19 pm

Smaller jet I worked on powered the packs with LP air supplemented with HP air. Anything above 80-85% N1 the LP bleed was enough to power the pack. From idle up to 80-85% N1, HP bleed air was mixed in to provide enough pressure.
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Lpbri
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 5:56 pm

Do away with engine bleed air entirely, like a 787. Bleed air is wasteful no matter what Airbus may say. All electric is the only way to go.
 
chimborazo
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 6:45 pm

Lpbri wrote:
Do away with engine bleed air entirely, like a 787. Bleed air is wasteful no matter what Airbus may say. All electric is the only way to go.


But it’s still not free energy. It’s taking taking energy from the engine via the gearbox driving the generator. There are associated losses in the energy conversions. I don’t know which is a more efficient energy solution butbit may be that the electrically driven packs are simpler.

You can see that when the a/c kicks in on a car at idle... the revs momentarily drop as the a/c compressor engages and then the engine controls increase the throttle to maintain idle speed. Not as noticeable on modern cars- I assume there is some pre-emotive control that throttles the engine up slightly as the compressor clutch is engaged to smooth it all out.
 
strfyr51
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Re: Bleed AIr Off the Low Pressure Compressor?

Thu Jun 06, 2019 8:20 pm

Faro wrote:
Starlionblue wrote:
The air to have high enough pressure and temperature to actually power the air cycle machines in the packs. My guess is you wouldn't get what you need from the low-pressure compressor (fan).

I can only speak for the Rolls-Royce Trent 772 and Trent XBW. They take bleed air from the intermediate pressure compressor. In case bleed demand exceeds supply, additional bleed air is taken from the high-pressure compressor. Air from the low-pressure compressor (fan) is used to regulate the temperature of the bleed air.



I guess the question is then why would one need power input to the packs? What work would have to be injected into the HP bleed air if what you are looking to do is reduce its energy per unit volume? That is, reduce both the temperature and pressure. Overall, the packs are taking energy out of the HP bleed air, not adding it. In what way do they need even more energy to do that?


Faro

the PACK Valves are electrically Controlled, Pneumatically operated Valves on most airplanes. other than the 787, the electrics power solenoids on or OFF to open or close the valves. There are other Air Tap offs besides the Air conditioning for engine Bleed air. Engine Inlet anti icing and wing anti-icing on many jets is Pneumatic as well.
 
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Starlionblue
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Re: Bleed AIr Off the Low Pressure Compressor?

Fri Jun 07, 2019 2:53 am

Lpbri wrote:
Do away with engine bleed air entirely, like a 787. Bleed air is wasteful no matter what Airbus may say. All electric is the only way to go.


Airbus didn't go all electric on the A350, indicating that with today's tech going all electric is by no means the obviously more efficient solution.

As chimborazo says, it's not free energy. You have to get the energy from the engines anyway. How you get that extract that energy can be electric or pneumatic. Both methods have pros and cons, and with time as technology evolves the scale may tip more one way or the other.
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Okcflyer
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Re: Bleed AIr Off the Low Pressure Compressor?

Fri Jun 07, 2019 3:15 am

Most expert analysis I’ve read indicate their isn’t a fuel advantage to the all electric typology but rather a large maintenance advantage.

Several of the 787 electrical system are overly complex / over engineered compared to what’s common in non-aviation sectors for the same basic processes today. Still a few patents on critical components. Some are quite novel but the automation industry has since advanced past it.

I suspect if Boeing’s next clean sheet will further develop and optimize many of these subsystems for aviation duty, and these will ultimately port back to the 787.

But this more about reduced maintenance and higher reliability than any potential fuel efficiency savings.
 
strfyr51
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Re: Bleed AIr Off the Low Pressure Compressor?

Fri Jul 26, 2019 2:24 am

Bleed Air from the engine comes off of whatever /stage of the compressor that the aircraft needs it to. the 737 Had it one place on the -200 another on the -300,-500 and NG there IS NO set place. It all depenfs on the model and the Engine combination.
 
JayinKitsap
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Re: Bleed AIr Off the Low Pressure Compressor?

Sat Jul 27, 2019 2:48 am

Electric packs are more efficient. Bleed air is compressed, causing heat. Only a portion of the heat is needed, and there is often excess when air temps are above -50F. So cooling of the air is needed, causing more energy used. Electric allows for the compression and temperature to not exceed the requirement, saving the excess. Not a huge number on the packs for energy, but huge on maintenance and initial cost. Bleed air ducting is expensive and heavy. compared to power cables.
 
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Re: Bleed AIr Off the Low Pressure Compressor?

Sat Jul 27, 2019 3:07 am

Starlionblue wrote:
Lpbri wrote:
Do away with engine bleed air entirely, like a 787. Bleed air is wasteful no matter what Airbus may say. All electric is the only way to go.


Airbus didn't go all electric on the A350, indicating that with today's tech going all electric is by no means the obviously more efficient solution.


Leahy said that his engineers told him that they didn't want to go all-electric on the A350 because they were concerned that offering an entirely new architecture like that would adversely affect reliability. And it certainly did for the 787, didn't it? But Boeing seems to have nailed those issues down by now. I suspect Airbus will eventually take the plunge.

My understanding is that the efficiency gain from all-electric architecture comes mostly during the climb phase, but also in weight savings (no need for all that extra ducting). I would expect that all new Boeing types from now on will feature all-electric architecture.
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LDRA
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Re: Bleed AIr Off the Low Pressure Compressor?

Sun Jul 28, 2019 9:35 pm

LP stage air density too low, so air duct size need to increase to avoid excessive pressure loss in air duct
 
strfyr51
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Re: Bleed AIr Off the Low Pressure Compressor?

Mon Jul 29, 2019 11:28 pm

I think you're speaking of the Fan air valve. The Fan air is used while sitting stationary while on the ground. The Fan Air is no longer needed after takeoff thus the valve closes and Ram air takes over.
 
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fr8mech
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Re: Bleed AIr Off the Low Pressure Compressor?

Tue Jul 30, 2019 12:30 am

strfyr51 wrote:
I think you're speaking of the Fan air valve. The Fan air is used while sitting stationary while on the ground. The Fan Air is no longer needed after takeoff thus the valve closes and Ram air takes over.


No, the fan air valve opens or closes in response to bleed air temperature. It allows fan air to flow across the heat exchanger.
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strfyr51
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Re: Bleed AIr Off the Low Pressure Compressor?

Tue Jul 30, 2019 6:41 pm

Lpbri wrote:
Do aw with engine bleed air entirely, like a 787. Bleed air is wasteful no matter what Airbus may say. All electric is the only way to go.

the 787 produces massive amounts of electrical power from the engines. Another engine if not already set up to do that would need extensive modification to carry that same power and the airplane would need to be set up for Electrical only and not bleed air. Even Airbus couldn't do it.

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