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B787 More Humid Air Conditioning

Wed Jul 11, 2007 4:14 am

Does anyone understand how this is?
The normal jet aircraft air conditioning pack takes all the water out of the air.
The old B737-200/300 type had a bag type water separator, which just about worked when it was new, and then a 35degF sensor that kept the air above freezing to stop the remaining water freezing in the heat exchangers.
Later packs from B757/ B734/A320 onwards had really efficient water extractors which guaranteed that all the water was removed. The packs could then run down into Minus temps, and M15degC is not unusual. This greatly increased the efficiency of the pack. On the ground in the humid Middle East this was greatly appreciated.
But the air in the cabin was very dry as a result. Hunidifiers were tried on some long haul aircraft for the flight deck, but not very successfully.
Now long ago I remember flying on the Gulf Air VC10s, and when we descended into BAH the roof vents rained on us as we descended into the humid air of the Gulf. Not very nice! The VC10 had rootes blowers instead of engine bleed air to produce the air supply. The B787 has something similar, but electric powered instead of driven off the engine gearbox. Perhaps this is how they get more humidity in the cabin (but hopefully not too much!)

All this is OK below say 10000ft.
But 90pc of the B787 flying will be at 37000ft where the air is dry outside. Where will the more humid air come from? Will they carry water on board? Or is there something I don't understand?

Another point from the roll out show was the claim that the B787 can be more humid because carbon fibre does not corrode so much and aluminium aircraft must be kept dry!
Well I meet B777 that arrive from the Far East where it rains a lot. As they taxy in there is water draining out of all the fuselage drains. Water that was on the freight as it was loaded, and had drained into the bilges, and is now draining out as the aircraft has depressurised. If aircraft had to be kept dry, everyone would fit these Swedish Dryers that are advertised in Flight, but very few do.

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RE: B787 More Humid Air Conditioning

Wed Jul 11, 2007 1:29 pm

Quoting TristarSteve (Thread starter):
Where will the more humid air come from?

Evaporated perspiration and moisture in exhaled breath. The resulting humidity of the cabin is controlled by how much of this moisture you remove; in practice there is never a need to humidify the cabin.

I've also wondered how the higher humidity level (and corresponding condensation on the inner surface of the fuselage skin) would be feasible. I once lived in a top-floor apartment with poor roof insulation, and the ceiling was constantly moist in the winter. Sure enough, after two years I had a massive bloom of mold. How will the growth of various hardy organisms behind the cabin walls (and potential cabin air contamination) be prevented?
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RE: B787 More Humid Air Conditioning

Wed Jul 11, 2007 3:09 pm

Just today I was training some guys, and was telling them about a Dash8 that had proximity sensor problems, that in the end were due to condensation ( a big Dash problem) dripping on to the computer, producing faults that grounded the aircraft. The Dash8 often drips condensation onto the pax from the overhead. There's an aircraft that certainly doesn't need more humidity!
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RE: B787 More Humid Air Conditioning

Wed Jul 11, 2007 4:43 pm

The specs I have seen for the 787 say the cockpit humidifies are an optional item, I know for example Air Mauritius also has the Airbus optional humidifiers on their A343s.

BTW I dont see the 787 spending most of its time at 39000ft, I expect it initial cruise to be low 30s on long haul operations, much the same as a 773ER.
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RE: B787 More Humid Air Conditioning

Wed Jul 11, 2007 4:46 pm

We also often have GPS, ATC and TCAS problems on the 737NG due to moisture from condensation entering the antenna plugs.

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RE: B787 More Humid Air Conditioning

Wed Jul 11, 2007 5:13 pm

Quoting MD11Engineer (Reply 4):
BTW I dont see the 787 spending most of its time at 39000ft, I expect it initial cruise to be low 30s on long haul operations, much the same as a 773ER.

This might be the case. Wasn't part of the 787 marketing that it would fly higher than other current commercial aircraft and that this would also result in a smoother ride (less turbulence)?

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