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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
The chalice not my son
 
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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.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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.

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