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MD83FO
11th Feb 2019, 01:59
Hello all, does anyone know why airbus opted for engine bled packs on the 350 rather than the electrical packs on the 787?

Fursty Ferret
11th Feb 2019, 08:57
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.

hans brinker
11th Feb 2019, 17:34
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.

FE Hoppy
11th Feb 2019, 17:40
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........

hans brinker
11th Feb 2019, 19:10
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....

tdracer
11th Feb 2019, 23:44
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.

FE Hoppy
12th Feb 2019, 16:49
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.

AeroTech
16th Feb 2019, 20:12
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.

If the 777X and 737 MAX are fitted with bleedless engines, this will require a lot of changes at least:
The ECS (environmental control system) will change which may cause the change of AC bay----> this may cause the change the wing-body fairings
The APU will change: no bleed air for aircraft/engines
The wings anti-ice will change ----->change in the slats
The electrical system may change: VFSG's instead of IDG's
All these changes mean extra costs in addition the cost of certification of these new systems.

I don't know if it is worth to fit bleedless engine in a new plane versus bleed air engine in new plane such B787 vs A350. But I don't think it is worth for derivative
planes such 737 MAX or 777X.

A summary of expected benefits of bleedless engines B787 according to Boeing:
A key benefit expected from the Boeing 787's no-bleed architecture is improved fuel consumption as a result of more efficient engine cycle and more efficient secondary power extraction, power transfer, and energy usage.Eliminating the maintenance-intensive bleed system is also expected to reduce airplane maintenance needs and improve airplane reliability because there are fewer components on the engine installation; there are no IDGs, pneumatic ducts, pre-coolers, valves, duct burst protection, and over-temperature protection; and there is no compressed air from the APU, resulting in a simplified and more reliable APU.The 787 no-bleed architecture also features modern power electronics and motors that will provide increased overall reliability, decreased costs, and improved performance. This is an extract from this article: https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_1.html (https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_1.html)

tdracer
16th Feb 2019, 22:00
If the 777X and 737 MAX are fitted with bleedless engines, this will require a lot of changes at least:
The ECS (environmental control system) will change which may cause the change of AC bay----> this may cause the change the wing-body fairings
The APU will change: no bleed air for aircraft/engines
The wings anti-ice will change ----->change in the slats
The electrical system may change: VFSG's instead of IDG's
All these changes mean extra costs in addition the cost of certification of these new systems.

I don't know if it is worth to fit bleedless engine in a new plane versus bleed air engine in new plane such B787 vs A350. But I don't think it is worth for derivative
planes such 737 MAX or 777X.

I agree for the 737 MAX - aside from the engines not a lot changed on the MAX relative to the NG. However the 777X barely qualifies as a derivative and more of an all new aircraft - new engines, new wings, new flight deck, nearly all new avionics, heavily revised fuselage - aside from the outside fuselage diameter there is precious little commonality between the 777 and the 777X. If going bleedless gave anywhere near the 3% fuel burn improvements that they thought they'd get for the 787, it would have made sense to go bleedless on the 777X. But it didn't, and they didn't.
As for the quoted 'advantages', note that was written over a year before the 787 actually flew - and most of that stuff didn't pan out. I already addressed the "more efficient engine cycle" and APU reliability is a laugh (the 787 APU has far and away the worst reliability of any Boeing installation). Sure, you get rid of bleed hardware, but you add lots of very heavy electrical hardware.
BTW, not positive, but memory says the 777X is getting rid of IDGs and going with VSCF architecture for electrical power generation.