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Electrical Packs

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Electrical Packs

Old 11th Feb 2019, 01:59
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Electrical Packs

Hello all, does anyone know why airbus opted for engine bled packs on the 350 rather than the electrical packs on the 787?
MD83FO is offline  
Old 11th Feb 2019, 08:57
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Many reasons. If you ditch bleed air from your engines, you need cabin air compressors; huge generators; increased electrical redundancy; big inverters + cooling; alternative engine starting; alternative wing anti-icing technology; bigger APU (potentially), and a few other things.

All these have a weight penalty.

Bleed air cabin pressurisation is a proven technology, although it has the risk that an engine issue in the compressor section can contaminate the cabin air supply.
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Old 11th Feb 2019, 17:34
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Originally Posted by Fursty Ferret View Post
Many reasons. If you ditch bleed air from your engines, you need cabin air compressors; huge generators; increased electrical redundancy; big inverters + cooling; alternative engine starting; alternative wing anti-icing technology; bigger APU (potentially), and a few other things.

All these have a weight penalty.

Bleed air cabin pressurisation is a proven technology, although it has the risk that an engine issue in the compressor section can contaminate the cabin air supply.
The question was only about packs, not about ditching the whole bleed system. All the other systems except pressurization would be conventional, you would just replace the packs with a system that would pressurize and cool outside air as opposed to bleed air. I think getting rid of all the fume events might make it worth looking into.
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Old 11th Feb 2019, 17:40
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Originally Posted by hans brinker View Post
The question was only about packs, not about ditching the whole bleed system. All the other systems except pressurization would be conventional, you would just replace the packs with a system that would pressurize and cool outside air as opposed to bleed air. I think getting rid of all the fume events might make it worth looking into.
Pressurise and COOL!
err........
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Old 11th Feb 2019, 19:10
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Originally Posted by FE Hoppy View Post
Pressurise and COOL!
err........

Yes, because when you pressurize the temp will go up, so you might need to cool it before it goes into the cabin. I know it is -56deg C outside, but the temperature increase will be bigly...

From NASA (who are a lot smarter than me):
T2 / T1 = (p2 / p1) ^ [(gamma - 1)/gamma]
During the compression process, as the pressure is increased from p1 to p2, the temperature increases from T1 to T2 according to this exponential equation. "Gamma" is just a number that depends on the gas. For air, at standard conditions, it is 1.4. The value of (1 - 1/gamma) is about .286.

So:
Altitude FL400, so p1 = 19KPa, T2 = -56deg C = 220K
Cabin 6,000ft so p2 = 81KPa
T2/220 = (81/19)^0.286
T2 = 220x(81/19)^0.286
T2 = 333 or 60deg C so a little warmer than I prefer my cabin....
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Old 11th Feb 2019, 23:44
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The original drive for going 'bleedless' on the 787 was for fuel efficiency - some people at Boeing became convinced that if you got rid of service bleed, the engine design could be optimized much better - enough to easily compensate for the massive weight of the needed power generation and electrically driven air cycle machines. Addressing engine related fume events did not play a significant role in the decision.
The improved fuel efficiency didn't play out - turns out that any improvements in engine cycle efficiency was minimal - to the point where the high pressure compressor is common between the bleedless GEnx-1B (787) and the conventional bleed GEnx-2B (747-8). The GEnx-1B simply incorporates blanking plates over the service bleed ports.
That's why you don't see bleedless engines on the 777X or the 737 MAX - it simply is not worth the trouble.

BTW, there has been a least one 'oil fume' event on the 787 - caused by a malfunction of the air cycle unit (which uses the same lube oil as the engines). So getting rid of engine bleed air is not some magic cure for oil fume events.
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Old 12th Feb 2019, 16:49
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Originally Posted by hans brinker View Post
Yes, because when you pressurize the temp will go up, so you might need to cool it before it goes into the cabin. I know it is -56deg C outside, but the temperature increase will be bigly...

From NASA (who are a lot smarter than me):
T2 / T1 = (p2 / p1) ^ [(gamma - 1)/gamma]
During the compression process, as the pressure is increased from p1 to p2, the temperature increases from T1 to T2 according to this exponential equation. "Gamma" is just a number that depends on the gas. For air, at standard conditions, it is 1.4. The value of (1 - 1/gamma) is about .286.

So:
Altitude FL400, so p1 = 19KPa, T2 = -56deg C = 220K
Cabin 6,000ft so p2 = 81KPa
T2/220 = (81/19)^0.286
T2 = 220x(81/19)^0.286
T2 = 333 or 60deg C so a little warmer than I prefer my cabin....
Correct but you haven't cooled the outside air. it was -56c and now it's considerably warmer!

One of the wonders of the traditional air cycle machine is that it is self powering. Take your bleed air at 40psi and 200C and use it's own energy to cool itself well below ambient.
The electrical power requirements of bleed less systems demonstrate just how much energy you need to pump the volume of air required to not only pressurise but adequately ventilate an aircraft. That energy in the end all comes from fuel. If you are using bleed for some services you may as well use it for all. i don't see splitting pneumatic supplies between bleed and electric driven compressors as being an optimised solution. Neither do any of the airframe manufacturers.
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