How is cabin humidity managed?
 

The 787 claims to have higher humidity in the cabin. It is said that the composite construction enables this to be achieved without creating corrosion problems. Also the cabin altitude is around 6000 feet compared with a more typical 8000 feet in other aircraft, so the cabin pressure is higher and the partial pressure of water vapour is correspondingly higher.

But I was under the impression that the amount of water vapour in the atmosphere was very low at high altitude. This could be because the partial pressure of water vapour must be lower unless there are high altitude clouds, and I think that with extreme low temperatures, say minus 40 or lower, the air must necessarily be dryer. So even if you heat and compress this air, I would expect it to contain little water vapour.

So if the 787 has more humidity in the cabin, where does it come from?

Not necessarily. If that was the case, clouds wouldn't exist. :O

What you are probably thinking of is how conventional air conditioning packs/air cycle machines extract most of the humidity from the ambient air, which the 787 doesn't.

See above.

ah, well ask a daft question and get....
 

Thank you. I have heard of, and mentioned, clouds. :-) And Wikipedia, which tells me

The percentage water vapor in surface air varies from 0.01% at -42 °C (-44 °F)[16] to 4.24% when the dew point is 30 °C (86 °F).[17] Approximately 99.13% of it is contained in the troposphere.

I had supposed the low amount of water in the atmosphere at altitude was mainly a function of temperature. The Wikipedia entry does not make it quite clear if pressure plays a part. Obviously in the extreme case water boils above 100C down a mine, and at near 0C in a vacuum.

I also found this: http://www.boeing.com/commercial/aer...ly/m01txt.html , which contains this: Air-conditioning system design The amount of outside air per occupant supplied to the airplane affects the in-flight humidity level. Increasing the outside air per occupant decreases the cabin humidity, which decreases the condensation rates. and this: Seating density More people produce more moisture, causing higher cabin humidity levels and increased condensation rates.

Unless I am told different I will assume that much of the water vapour in an aircraft comes from the respiration of the passengers themselves, and that air conditioning removes much of it, but less so in the 787. I also assume that while the cabin pressure might be a bit higher in the 787 than other airliners, the rate of change of air is not necessarily higher, indeed it might even be lower unless there is a means of capturing and recirculating water vapour separately.

I should have asked when I was at Everett.

I believe that BA has or had humidifiers on certain aircraft presumably operating off the potable water supply - perhaps someone could confirm this. This would be the obvious way of increasing humidity but would require additional kit and lugging water around.

Here are a couple of results from googling.

How It Works - CTT

https://www.liebherr.com/shared/medi...ion-system.pdf

Yes, BA's 747-100s and 747-200s had humidifiers in the roof space fed by potable water. I even overhauled them when I worked in one of their the workshops for a while around 1985. :O

I believe that flight distance/time determines target humidity.

On a short flight, no need to moisten it up.