redcordes
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787 Electric-Compressor Pressurization

Thu Jun 08, 2006 1:48 am

I've read that by not using bleed air from the engines for pessurization, the 787 will be more efficient and have more engine power available for thrust. If electric compressors are used for pressurization, don't they put an additional electrical load on the generators and thus the engines themselves? Can there really be a gain in efficiency when there is ultimately the same amount of work to be done by the engines?
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Dougloid
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 5:07 am

I've asked this question before, and apparently the engineers have decided that using generator power to run the cabin environmental systems represents a net savings in energy that's worth the doing.

One of the appealing things about turbine engines has always been there was a lot of bleed air to do stuff like run air conditioners and environmental systems.

I'm not convinced either....perhaps some well placed people could fill in the blanks?
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scbriml
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 5:23 am

Quoting Dougloid (Reply 1):
I'm not convinced either....perhaps some well placed people could fill in the blanks?

After the initial hype around bleedless engines (mostly from Boeing), haven't the engine manufacturers admitted that there's little engine efficiency gain in going bleedless? That's not to say that a totally bleedless plane isn't more efficient overall, or that bleedless engines aren't easier to design/build/maintain, but purely on engine fuel consumption, I believe the difference between bleed and bleedless is negligible.

Then again, I might be talking out of my sphincter. scratchchin 
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KELPkid
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 5:44 am

A little OT here, but didn't the original 707 use piston engine superchargers, driven directly by the jet engines, for cabin pressurization?
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RichardPrice
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 5:50 am

Quoting Scbriml (Reply 2):

I recall reading an article some time ago (6 months or so) which had the various manufacturers bleedless offerings against the same engine with bleed, and there was only a percentage point in the efficiency between bleedless and bleeding.

The 787/A350 arguement does seem to be all about weight, not whether the engines are bleed or not, so I guess it doesnt make that much of a difference to the airlines. I think the main arguement is that electricity cables weigh less than compressed air ducts.

Also for cabin air heating means, deicing etc, the heater on bleedless configurations has to work with ambient air rather than preheated engine air, so theres a larger temperature rise to force.
 
texfly101
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 6:03 am

Quoting Redcordes (Thread starter):
I've read that by not using bleed air from the engines for pessurization, the 787 will be more efficient and have more engine power available for thrust. If electric compressors are used for pressurization, don't they put an additional electrical load on the generators and thus the engines themselves? Can there really be a gain in efficiency when there is ultimately the same amount of work to be done by the engines

Yes, by doing this, they eliminate the mechanical systems that are necessary for bleed air distribution and the associated weight, redundacy and control systems. The electrical systems, being driven off a power feed from the turbine is basically simpler in both operation and mechanical design and the associated distribution system is basically an electrical wire distribution system rather than a pnuematic system. While if you look at just one aspect of the system, like does it really help engine efficiency, you might not get a gain in efficiency, but if you look at the combined systems as a whole, then the net gain is positive due to the losses being less in the combined system. Add in that you can get by with an electrical system alone but you can't get by with just a pneumatic system and the overall control systems can be integrated for redundancy, control panels, etc....so its the overall bits and pieces that add up to the overall net efficiency.
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redcordes
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 6:28 am

Texfly101, thanks for the explanation--makes sense. Would this system also allow for the reduction of outside air introduced into the cabin while the aircraft is on the ground and thereby reduce the smell of exhaust when operating near other aircraft?
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Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 7:07 am

Quoting Texfly101 (Reply 5):
they eliminate the mechanical systems that are necessary for bleed air distribution and the associated weight, redundacy and control systems.

And introduce enormous generators, and big air compressors.
I read on Anet a while ago that with the present electrical generators the effect is tiny but positive, but Boeing hopes that in 5 years time someone will come up with a better generator. Noone will ever come up with a better bleed system. This is like the piston engine in 1956, each new development is just a tiny improvement, while electric generator technology is leaping ahead.

Quoting Redcordes (Reply 6):
Would this system also allow for the reduction of outside air introduced into the cabin while the aircraft is on the ground and thereby reduce the smell of exhaust when operating near other aircraft

NO. By law you must introduce fresh air into the cabin from outside. This air will be better than before because it doesnt pass through an engine, but I dont expect you would notice.
 
Dougloid
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 10:11 pm

Quoting KELPkid (Reply 3):
A little OT here, but didn't the original 707 use piston engine superchargers, driven directly by the jet engines, for cabin pressurization?



I'd be surprised, with all that lovely bleed air.
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Corsair2
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 10:25 pm

Swapping between bleed air and electric compressor air sounds like it does not significantly improve efficiency. Novel concept though. Sounds a lot like how I clean up at home. Instead of throwing stuff out, I end up moving it from one room into the other room!
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Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 10:33 pm

Quoting KELPkid (Reply 3):
A little OT here, but didn't the original 707 use piston engine superchargers, driven directly by the jet engines, for cabin pressurization?

Don't know, but the VC10 did. It had Rootes blowers to supply cabin air. Superchargers in another name.
 
Tod
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 10:44 pm

Quoting TristarSteve (Reply 7):
By law you must introduce fresh air into the cabin from outside

Per 14CFR25.831, .55 pounds of fresh air per pax per minute.

Tod
 
vc10
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RE: 787 Electric-Compressor Pressurization

Thu Jun 08, 2006 10:56 pm

Quoting TristarSteve (Reply 10):
It had Rootes blowers to supply cabin air

We always called the VC-10 blowers "Godfrey Blowers" as Godfrey was the name of the manufacturer. They were Helical screws meshed together and in the early days constantly had oil leaks which was the main cause of air conditioning smoke on the flight deck.

The B707 had air driven compressors for the air conditioning

Like texfly101 says , if you have the compressor electrically driven it can be positioned almost anywhere and would save a lot of big diameter pipe runs

vc10
 
Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Fri Jun 09, 2006 1:07 am

Quoting Vc10 (Reply 12):
We always called the VC-10 blowers "Godfrey Blowers

Thanks, didn't know that. I never had a licence on a VC10, just did odd jobs on the GF ones out in BAH
 
katekebo
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RE: 787 Electric-Compressor Pressurization

Fri Jun 09, 2006 1:31 am

In the overall picture of complete engine fuel consumption, the difference is very small. However, on the individual basis, there are several reasons why electrical system is more efficient. I'll try to explain:

- Jet engine compressors are designed to compress the air to the optimum pressure from the thermodynamic standpoint for the engine operations (usually limited by turbine exhaust temperature). The thermodynamics requirements are very different from the optimum pressure required for the pressurization of the cabin. Also, the bleed system is designed to provide enough pressurization and air flow to the cabin at all engine operating conditions, from idle to full throttle. That mean that under average cruise conditions the system is terribly oversized, operating far from its optimal efficiency point and therefore very wastefull from the energy standpoint. I don't know the exact number, but I estimate the overall efficiency of the pressurization system using bleed air to be somewhere between 20 and 30%, maybe even less than that.

- A modern generator and electrical motor can easily achieve 90%+ efficiency. A centrifugal or axial compressor running at its optimal efficiency point can also deliver 80%+ effciency. So, by designing a system that will operate most of the time at its optimum efficiency point, an electrical pressurization system can achieve somewhere between 60 and 70% effciency.

- Additional benefits of a bleedless system are lower weight and relatively simplicity. Bleed pressurization system requires a lot of ducts. Electrical wires are lighter and easier to route through the airplane.

As with any new technology, the initial benefits vs. a mature, highly optimized technology are modest. However, electrical systems is the way of the future.
 
rsbj
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RE: 787 Electric-Compressor Pressurization

Fri Jun 09, 2006 11:01 am

"A little OT here, but didn't the original 707 use piston engine superchargers, driven directly by the jet engines, for cabin pressurization?"

The engines used "turbo-compressors" because the FAA at the time didn't think the bleed air off a turbine would be clean. I don't know what they thought would contaminate it; perhaps oil past bearings. These turbo compressors used bleed air to turn a compressor, which pressurized the cabin.

Later, but I don't know when, they were removed.

[Edited 2006-06-09 04:03:04]
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PolymerPlane
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RE: 787 Electric-Compressor Pressurization

Fri Jun 09, 2006 11:27 am

This is an opinion from a layman, without any expertise in aircraft engine and aerodynamic.

I think one of the reason that bleedless engine might have a better efficiency compared to bleed engine because the energy from the engine core is too high to be wasted on the bleed air.

One of the most efficient way to extract this energy is by converting the pressure and thermal energy to shaft work on the air expansion.

Just look how efficient a high bypass engine compared to a turbojet. High bypass engine coverts the gas expansion to shaftwork and to the fan, thus extra thurst. It might work the same way with pressurization system. By decreasing thrust waste through bleed air, there should be an efficiency gain by doing so.

Cheers,
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Dougloid
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RE: 787 Electric-Compressor Pressurization

Sat Jun 10, 2006 11:12 am

Quoting Rsbj (Reply 15):
The engines used "turbo-compressors" because the FAA at the time didn't think the bleed air off a turbine would be clean. I don't know what they thought would contaminate it; perhaps oil past bearings. These turbo compressors used bleed air to turn a compressor, which pressurized the cabin.

That's the definition of an air pack. They don't use direct engine bleed air to pressurize the cabin.

You can pretty much figure out how hot the bleed air is going to be by its pressure. The Garrett bleed air off the second stage of compression (two stage radial compressor) was about 600 degrees

[Edited 2006-06-10 04:14:56]
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Buzz
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RE: 787 Electric-Compressor Pressurization

Mon Jun 12, 2006 2:33 pm

Hi Dougloid, Buzz here. One thing I won't miss is troubleshooting bleed air leak faults on an A320. Maybe there will be a significant weight / efficiency savings with fewer ducts and bleed valves.
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JayinKitsap
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RE: 787 Electric-Compressor Pressurization

Mon Jun 12, 2006 2:58 pm

Quoting Dougloid (Reply 17):
That's the definition of an air pack. They don't use direct engine bleed air to pressurize the cabin.

Are you saying that the airpack is a small turbine/compressor with the turbine run with the bleed air. I've had the impression that bleed air is used for direct cabin pressurization but it is higher pressure and temp than desired. It does make sense to run a compressed air (bleed air) powered blower and then blend the two streams in a proportion that is the right temp and pressure.

I see the potential to be as efficient with less weight, also as bleed air is pretty hot, are there issues with high temp bleed air in close proximity to CRFP for extended periods.

With variable speed motors, a known pax load, and a given altitude the compressor would be able to be near optimum for a wide range of conditions. Yes, supplemental heating would be needed, but there also could be the ability to do energy recovery from the cabin air (preheat the compressor feed). It is often a lot of little things to finally develope noticable energy improvements.
 
buckfifty
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RE: 787 Electric-Compressor Pressurization

Mon Jun 12, 2006 5:58 pm

Quoting JayinKitsap (Reply 19):
Are you saying that the airpack is a small turbine/compressor with the turbine run with the bleed air. I've had the impression that bleed air is used for direct cabin pressurization but it is higher pressure and temp than desired. It does make sense to run a compressed air (bleed air) powered blower and then blend the two streams in a proportion that is the right temp and pressure.

Bleed air does not directly pressurize the cabin. It is used to power the compressor in the pack itself, which actually draws in outside air to provide pressurization at altitude. However, the bleed air is also piped into the cabin to provide heating as a separate source from the pack air. So if the engines have gone through a wash recently, you will smell it in the cabin.

The piping involved in this is quite extensive. And it provides not only extra weight, but lots of maintenance issues. I can see why bleedless would be an advantage here.

But I have not read of any solutions to the de-icing issue. Any news on that?
 
rsbj
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RE: 787 Electric-Compressor Pressurization

Mon Jun 12, 2006 9:16 pm

Every modern commercial plane I am familiar with now uses bleed air to directly pressurize the cabin. A "pack" is a series of compressors, radiators, and turbines to compress the air, therefore heating it, running it through the radiator cooling it, then running the air through a turbine cooling it even further. The picture below is the Pack Synopitic on a Boeing 737-300/500.

The KC-135 goes so far as to have an alternate pressurization system that pumps raw bleed air directly into the cabin. This is used primialy because bleed air used to be drawn off the 5th stage compressor on the old engines, and it couldn't keep the cabin warm enough. In fact even with this system, I was flying a 135 that had a Coke explode back there flying high over Alaska.
It also served as a nice pressurization backup in case our one pack failed.

Just to clarify, the 707's Turbocompressors were mounted forward in the engine strut. Look at KC-135E's to maybe get a glimpse of one. Some Air Guard planes had them taken off, some didn't.
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Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Mon Jun 12, 2006 11:19 pm

Quoting Dougloid (Reply 17):
They don't use direct engine bleed air to pressurize the cabin



Quoting Buckfifty (Reply 20):
Bleed air does not directly pressurize the cabin



Quoting Rsbj (Reply 21):
Every modern commercial plane I am familiar with now uses bleed air to directly pressurize the cabin

RSBJ is right. Bleed air is taken from the engine compressore, HP at idle and LP (IP on RR) at higher powers. This bleed air can be very hot and high pressure. It is fed through a precooler to coll it down before it leaves the engine and then through a Pressure Reducing Valve which brings it down typically to 40 psi. This air then pressurises the pneumatic system. Air from the pneumatic system is fed to the packs, the wing anticing, air driven hyd pumps, water tank pressurisation, hyd tank pressurisation, and to a jet pump on the TAT gauge.(this is the hissing noise you hear when boarding the aircraft if the APU is running.
When this air goes into the packs, some is directed directly to the mixing chamber, but most goes into the pack where it is compressed, dried, cooled and sent to the mixing chamber and into the cabin. This is all taken care of by a spinning assy of a compressor and a turbine and sometimes a fan. Air is drawn in from outside the aircraft and directed through two interstage heat exchangers to cool the air. At altitude, this cooling air is sufficient and the turbine assy is stopped, but on the ground it is running. All the air from the pneumatic system that enters the pack ends up in the cabin. The cooling air through the heat exchangers all goes back overboard. The temp of the air in the pack can go as cold as Minus 20degC.
This system applies to nearly every large pax airliner.
Older designs like B737-200 and B727 and L1011 the temp was controlled to minimum Plus 2degC to prevent ice, but from B757 onwards the water separators could take out all the water and Minus temps were normal.

[Edited 2006-06-12 16:23:45]
 
Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Tue Jun 13, 2006 1:21 am

By the way the "plus temp" pack system with the 35degF control stat that is fitted to B737-200 is also on B737-300 and 500. But the B737-400 has a negative temps type Pack which is very similar to the B757/B767 pack.
 
JayinKitsap
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RE: 787 Electric-Compressor Pressurization

Sat Jun 17, 2006 1:06 am

Quoting Rsbj (Reply 21):

Thanks for the photo of the diagram, its now very clear. It appears to me that doing cabin pressurization in a bleedless would be far simpler. It would require a heat exchanger to take the heat from the cabin exhaust air and preheat the incoming air, a 10 psi blower, and an electric heater. There is a lot of energy used in the bleed air that doesn't provide useful work.
 
LMP737
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RE: 787 Electric-Compressor Pressurization

Sat Jun 17, 2006 2:35 am

Quoting Scbriml (Reply 2):
After the initial hype around bleedless engines (mostly from Boeing), haven't the engine manufacturers admitted that there's little engine efficiency gain in going bleedless? That's not to say that a totally bleedless plane isn't more efficient overall, or that bleedless engines aren't easier to design/build/maintain, but purely on engine fuel consumption, I believe the difference between bleed and bleedless is negligible.

From a maintenance standpoint whether the aircraft be an Airbus, Boeing, Douglas or Fokker the pneumatic system tends to be the biggest headache. By going bleedless that headache will go away. Hopefully another one is not created.
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Fly2HMO
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RE: 787 Electric-Compressor Pressurization

Sat Jun 17, 2006 5:20 am

So what kind of electric compressor would be used then? Reciprocating? (like your shop compressor from Home-Depot Big grin) Centrifugal? Rotary?
 
Dougloid
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RE: 787 Electric-Compressor Pressurization

Sun Jun 18, 2006 12:53 pm

Well, whatever it is it is going to be a monster. When I worked at Douglas we'd do ground pressurization checks on the MD11 and run them up to max differential cabin pressure.

The air sources we used were the huffer truck which had a giant Roots blower powered by two 8V71 Detroit Diesel's back to back/ The other was a single Roots powered by a twin turbo Detroit Diesel 12V92. What that means is power and plenty of it is needed-about 1,200 hp for this rig. And a mean Roots blower taking up space somewhere-a lot of space.
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n8076u
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RE: 787 Electric-Compressor Pressurization

Fri Jun 23, 2006 7:48 am

Just to clarify a common misconception, the engine/APU bleed air IS what is put into the cabin on modern aircraft. The outside air that goes in the pack inlet on the fuselage is only used to cool the bleed air through a heat exchanger and doesn't physically intermix with the bleed air in any way.

Some older jet aircraft had turbocompressors (707, DC-8) which took outside air and pressurized it to supply the air to the cabin. The DC-8 ones were powered by pneumatic bleed from the engines (seems redundant, no?), as they were in the nose of the aircraft (and fresh air would be taken in by the nose scoops), rather than in the pylon like the 707 (which I believe were shaft-driven directly from the engine). I know the DC-8 had centrifugal compressors, and I would assume on the 707 as well. They also had freon refrigeration systems to control the air temperature, as the "packs" as we know them today would come later, or would be an option later on as in the case of the 707. Packs were also an optional upgrade when the 60-series DC-8s were getting converted into 70-series, with the CFM-56 engines.

Any oil seeping from the engine bearings forward of where the bleed was taken would introduce an oily odor to the cabin, which was probably another reason for taking outside air on older aircraft. Bearing technology has come a long way since then, and that is now only an occasional issue, but it does happen, and when it does, it is time for an engine change.  Wink

Keep in mind the older jets (especially the turbojets) were also very sensitive to any amount of bleed air being removed from the core of the engine. Even abrupt throttle movements could cause problems, where that's not the case with modern aircraft. Better fuel control and adjustable guide vanes in later engines helped the situation, among other things like better engineering, tighter construction tolerances and better materials, and has allowed for bleed air to be used in many ways all at the same time, including pressurization/running the packs, anti-icing the engines/wings/tail leading edges, running air driven hydraulic pumps and actuating leading edge flaps (747).

Chris
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Jetlagged
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RE: 787 Electric-Compressor Pressurization

Sat Jun 24, 2006 1:01 am

Quoting N8076U (Reply 28):
The DC-8 ones were powered by pneumatic bleed from the engines (seems redundant, no?), as they were in the nose of the aircraft (and fresh air would be taken in by the nose scoops), rather than in the pylon like the 707 (which I believe were shaft-driven directly from the engine). I know the DC-8 had centrifugal compressors, and I would assume on the 707 as well.

As has been stated previously, the 707 used turbo-compressors. Air bled from the engine drove a turbine, which in turn drove a compressor which pressurised ram air. They were not shaft driven. The turbine and compressor used would certainly have been the centrifugal type.

Apart from doubts about the cleanliness of bleed air, early jets used such compressors because that was the kind of technology used by their piston engined predecessors.
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roseflyer
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RE: 787 Electric-Compressor Pressurization

Sat Jun 24, 2006 8:43 am

I work for the company producing the generators and environmental control system for the 787. I have a fair bit of experience working with the generators on the 787.

Quoting Scbriml (Reply 2):
After the initial hype around bleedless engines (mostly from Boeing), haven't the engine manufacturers admitted that there's little engine efficiency gain in going bleedless?

Going bleedless alone isn't what makes the engine more efficient. However without bleed air, the engine can have a higher compression ratio gain efficiencies in other ways. It also saves weight.

Quoting TristarSteve (Reply 7):
And introduce enormous generators, and big air compressors.
I read on Anet a while ago that with the present electrical generators the effect is tiny but positive, but Boeing hopes that in 5 years time someone will come up with a better generator. Noone will ever come up with a better bleed system. This is like the piston engine in 1956, each new development is just a tiny improvement, while electric generator technology is leaping ahead.

Hamilton Sundstrand has been making generators for years and they keep getting more powerful while also getting smaller. The current generators are a third the weight of the ones found on the first jets and yet produce more power. The 787 generators are the latest in technology. Things are constantly being improved with newer designs that reduce weight. The 787 generators can produce 500KVA, which is a lot of power. That is more than an 18 wheeler truck. Also the 787 generators are also the starters for the engines, which reduces a significant amount of weight.

Hamilton Sundstrand recently broke ground on the company's new Airplane Power System Integration Facility in Rockford, Illinois. The facility will play a pivotal role in developing and testing multiple systems for the Boeing 787 Dreamliner.

Through high-speed data links with Boeing and with other Hamilton Sundstrand facilities in Connecticut and California, the facility will operate as a virtual workspace. Engineers thousands of miles apart will have the ability to test and verify systems in a real-time, collaborative environment, simplifying the aircraft development process.

According to David Hess, Hamilton Sundstrand president, "This unique facility further enhances our systems integration capabilities and our more-electric technology, enabling us to deliver a total integrated airplane power system to the customer."

Hamilton Sundstrand was chosen by Boeing to provide the 787's environmental control system, electric power generation and start system, remote power distribution system, auxiliary power unit, primary power distribution system and high-voltage DC equipment racks, emergency power system, nitrogen generation system and electric pump subsystem.
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KELPkid
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RE: 787 Electric-Compressor Pressurization

Sat Jun 24, 2006 9:34 am

Quoting RoseFlyer (Reply 30):
nitrogen generation system

What on earth would the dreamliner need a nitrogen generation system for? Keeping the tires inflated? Or is it the 300+ airbags built into the seat backs and bulkheads  Wink
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n8076u
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RE: 787 Electric-Compressor Pressurization

Sat Jun 24, 2006 9:40 am

Quoting RoseFlyer (Reply 30):
Hamilton Sundstrand has been making generators for years and they keep getting more powerful while also getting smaller.

I agree with you there. The 777 engine driven generator with integrated constant speed drive is about the same size as just the generator on a 747 "classic", and has higher output as well.

Chris
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roseflyer
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RE: 787 Electric-Compressor Pressurization

Sat Jun 24, 2006 11:11 pm

Quoting KELPkid (Reply 31):
What on earth would the dreamliner need a nitrogen generation system for?

Nitrogen is pumped into the fuel tanks as fuel is consumed to ensure that there is an inert gas to reduce the risk of possible fire or explosion. It is used to ensure an inert atmosphere in the fuel tanks, which is something that has been made even more important since the TWA 800 incident.
If you have never designed an airplane part before, let the real designers do the work!
 
Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Sun Jun 25, 2006 2:50 am

Quoting RoseFlyer (Reply 30):
The 787 generators can produce 500KVA,

If the generators are 500KVA, how much ground power does the aircraft need if the APU is u/s, or as we have in ARN, not allowed to be run on the ground. Nowadays on the B777 we need two 90KVA supplies, and there is nothing on the airport bigger than 90KVA, so to replace one generator we will need six GPUs? Our handling agent doesn't own six, and there is only one FEP on each stand.
Are all airports in the world putting in new FEP systems?
 
roseflyer
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RE: 787 Electric-Compressor Pressurization

Sun Jun 25, 2006 11:46 pm

Quoting TristarSteve (Reply 34):
If the generators are 500KVA, how much ground power does the aircraft need if the APU is u/s

I'm not sure how I see you relating the generators to the APU. The generators can produce 500KVA in an overspeed condition. This would only happen under extreme conditions for brief periods of time. I'm not that knowledgeable on the inner workings of IDGs, CSDs, APUs or other generators.
If you have never designed an airplane part before, let the real designers do the work!
 
Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Mon Jun 26, 2006 3:32 am

Quoting RoseFlyer (Reply 35):
I'm not sure how I see you relating the generators to the APU

When the aircraft is on the ground, the APU when running provides enough electrical power for the aircraft. The B777 has one 120KVA generator on the APU and this is sufficient.
Normal airport ground power is 90KVA. This is sufficient on a B777 unless you start turning on high load pumps like Hyd pumps when 2 x 90KVA GPUs are required.
Now if the B777 engine generator is 120KVA and needs two GPUs, what I am asking is if the B787 generator is 500KVA how many GPUs are required when the aircraft is on the ground? Say you want to run the packs and the hyd pumps. How much KVA is required?
 
PolymerPlane
Posts: 832
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RE: 787 Electric-Compressor Pressurization

Mon Jun 26, 2006 3:44 am

Quoting Tristarsteve (Reply 36):
Now if the B777 engine generator is 120KVA and needs two GPUs, what I am asking is if the B787 generator is 500KVA how many GPUs are required when the aircraft is on the ground? Say you want to run the packs and the hyd pumps. How much KVA is required?

On the ground the cabin pressurization is not needed right? so I would guess, the electrical requirement is not going to be significantly different than B777

Cheers,
PP
One day there will be 100% polymer plane
 
n8076u
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RE: 787 Electric-Compressor Pressurization

Mon Jun 26, 2006 3:54 am

Quoting PolymerPlane (Reply 37):
On the ground the cabin pressurization is not needed right? so I would guess, the electrical requirement is not going to be significantly different than B777

Although "pressurization" is not needed, there may not be air conditioning hookups at the gate to cool the aircraft, and the aircraft's air conditioning system may need to be used. Since the 787 will be bleedless, I can only assume that it won't have the traditional "packs" as those are run by pneumatics, but will rather have some sort of electrically driven air conditioning system as well as the pressurization pumps.

Chris
Don't blame me, I don't work here...
 
roseflyer
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RE: 787 Electric-Compressor Pressurization

Mon Jun 26, 2006 5:24 am

Quoting Tristarsteve (Reply 36):
Now if the B777 engine generator is 120KVA and needs two GPUs, what I am asking is if the B787 generator is 500KVA how many GPUs are required when the aircraft is on the ground? Say you want to run the packs and the hyd pumps. How much KVA is required?

I can't remember what the 787 generators are rated at. It has been a while since I have seen the power curve for them. I do know that 500KVA is a rough estimate and is an overspeed condition. I'll see if I can find out.
If you have never designed an airplane part before, let the real designers do the work!
 
mrocktor
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 5:04 am

Quoting N8076U (Reply 38):
I can only assume that it won't have the traditional "packs" as those are run by pneumatics, but will rather have some sort of electrically driven air conditioning system as well as the pressurization pumps.

As far as I know the air from the compressors does go through packs that operate based on the same principle as bleed air packs for temperature and pressure control. Of course since the input tempreature and pressure ranges are likely very different, the particulars (heat exchangers, dimensions etc.) of these packs are likely to be different as well.

One possible explanation for the small gains in engine efficiency from the bleedless concept is that the 787 engines are in fact bleedless versions of engines that actually can be bled. Perhaps the "provisions for" bleed are enough of a compromise in the design of the core to corrode some of the gains from a truly bleedless engine.

mrocktor
 
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Jetlagged
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 5:21 am

There's no such thing yet as a truly bleedless engine. Some air bleed will likely be necessary to stabilise the engine at off-design conditions (such as surge bleed valves). In other words when running at idle or part power.

Service bleeds provided additional anti surge margins. If service bleed is no longer used then it's possible additional surge bleed offtakes may be necessary.

Regarding running the electric pressurisation on ground, ground air supply would be the best answer, rather than run the compressors on ground (could be noisy). The total power required by the aircraft for ground servicing should be less than current designs if it's so efficient  Smile The APU generator has to be able to replace a failed engine generator in flight. All that power should not be needed on ground, so the GPU could have a lower rating.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
mrocktor
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 6:02 am

Quoting Jetlagged (Reply 41):
There's no such thing yet as a truly bleedless engine. Some air bleed will likely be necessary to stabilise the engine at off-design conditions (such as surge bleed valves). In other words when running at idle or part power.

Such bleeding would not significantly affect the engine's maximum thrust rating (because it does not need stabilization at that thrust setting) or its fuel burn during cruise (since the engine will be designed for this setting). Or am I missing something?

Service bleed is completely different from stabilization bleed, it takes high energy air precisely when you need it (take off with anti-ice on, pressurization during cruise).

Since service bleed may or may not be extracted at any given time, it should make your stabilization problem worse since instead of designing a core to run at various thrust settings you have to design it to run at various settings within the full range of possible service bleed extraction.

mrocktor
 
pygmalion
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 8:32 am

My recollection is that the 787 will have 2 generators on each engine and 2 more on the APU. My understanding is that they will be nominally rated for 250 kva on the engines and 225kva each on the APU. There will be 4 90kva ground power connections, 1 pair forward and 1 pair just aft of the main gear well. Only the forward pair are needed for normal ground ops.

link:
http://www.boeing.com/commercial/787family/programfacts.html

Edited to show correct ratings and provide link...

[Edited 2006-06-29 01:50:19]
 
roseflyer
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 10:18 am

Quoting Pygmalion (Reply 43):
My recollection is that the 787 will have 2 generators on each engine

Yes there is a Main and Aux generator on each engine. They are staying as separate systems to ensure redundancy. I believe that is where the number 500kVA that I have heard comes from. Of course ratings aren't really strict as in they don't mean the absolute power. The ratings usually reflect the maximum continuous operation. They can overspeed to 150%-200% for shorter periods of time without overheating.

For reference the 777 also has two generators on each engine, but there is the main and then back up generator (affectionally known as the BUG).
If you have never designed an airplane part before, let the real designers do the work!
 
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Jetlagged
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 10:13 pm

Quoting Mrocktor (Reply 42):
Such bleeding would not significantly affect the engine's maximum thrust rating (because it does not need stabilization at that thrust setting) or its fuel burn during cruise (since the engine will be designed for this setting). Or am I missing something?

No, you're not missing anything, I did say it only affects "off design" conditions. I was only pointing out that the term bleedless is a bit misleading. Also that since current engines are designed with (fairly constant) pressurisation service bleeds in mind, simply removing such bleed will require some airflow redesign, or possibly even additional surge bleed.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
Dougloid
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RE: 787 Electric-Compressor Pressurization

Thu Jun 29, 2006 10:36 pm

Quoting Mrocktor (Reply 42):
Service bleed is completely different from stabilization bleed, it takes high energy air precisely when you need it (take off with anti-ice on, pressurization during cruise).

Good observation. Where's the anti ice going to come from on the 787? Is that going to be 'total electric living' as well?

The key here will be the level of reliability of the electric production and distribution systems that can be attained under field conditions, and the cost to repair and downtime when it breaks-not if it breaks.

There's a lot riding on the reliability of this approach, which is yet to be proven "in Peoria" as Donald Douglas used to say.

BTW mrocktor, where ya been? Haven't seen your handle around in a while.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn
 
Tristarsteve
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RE: 787 Electric-Compressor Pressurization

Fri Jun 30, 2006 4:34 am

Quoting Dougloid (Reply 46):
Where's the anti ice going to come from on the 787

Engine antice will be an air offtake. The only air offtake.
Wing antice will be electric.
 
mrocktor
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RE: 787 Electric-Compressor Pressurization

Fri Jun 30, 2006 4:59 am

Quoting Tristarsteve (Reply 47):
Wing antice will be electric.

Anyone know if it will be EMEDS, heating or a mixture of both?

Quoting Dougloid (Reply 46):
mrocktor, where ya been? Haven't seen your handle around in a while.

Just taking a breather, nothing special. Too much work I guess
 Big grin

mrocktor
 
Dougloid
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RE: 787 Electric-Compressor Pressurization

Fri Jun 30, 2006 10:48 am

Quoting Tristarsteve (Reply 47):
Wing antice will be electric.

You're gonna need a whole lot more amps then. Problem with that is, the IDGs will have to be big enough to handle the entire calculated load with a safety factor. Unless i miss my guess, you'll have to have one IDG capable of handling the entire essential load and electric anti ice is essential load....

This bird's gonna be schlepping some monster generators.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

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