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keithl
27th Aug 2004, 13:27
I've checked the Tech Log Philosophy page and this does have a (very) "tenuous connection with aviation". It's about condensation on my windows and I'm asking here because I expect to find experts on Saturation, Dew Point, and Atmosphere.

My (original, Victorian) sash windows have been showing a bit of condensation on recent mornings. I understand why (analogy of cold beer glass). What I don't understand is why that condensation appears at the bottom of each sash (sliding window section). I would have expected the whole glass area to be equally affected. Not gravity - the droplets have not moved.

I'm not looking for DIY solutions - just a technical explanation.

Genghis the Engineer
27th Aug 2004, 13:44
This is guesswork

Glass is not, technically, a solid - it is an incredibly viscous liquid. If you measure the thickness of glass in very old windows, it is invariably thicker near the base than the top. So, the bottom will have greater thermal inertia than the top, and is likely to hold it's night time (cold) temperature longer.

So, not only is it colder, than the top of the window and more likely to attract condensation, but the gradient over the surface is likely to cause the air to circulate downwards along the surface of the window, until it reaches it's dewpoint which, if conditions are marginal for condensation, will be part way down the window.

I could, of-course, be completely wrong.

G

keithl
27th Aug 2004, 13:50
That's convincing, Gengis, - thanks. The glass is indeed of the old and wrinkly sort. (As, indeed, is its owner).

Mr Levitator
28th Aug 2004, 19:26
The above is a good suggestion. To add to it, what about the mounting frame of the window? And the circulation of air within the cabin. Also, because most passenger windows have a perspex inner and a glass outer, what about the air in the trapped space?

Mr L

SyllogismCheck
29th Aug 2004, 11:08
I'd say Genghis is spot on with his suggestion of the cooling air flowing downwards and cooling further until dew point is reached, it was the exact same one which popped into my head before I scrolled down and read his reply.

The glass thickening adding to this effect I admit did not occur to me. As a point of interest though one or two panes are likely to have been replaced and be identifiable by their superior optical correctness, are these less prone to this pattern of condensation build up?
Have to say although the reasoning is spot on I would imagine that the glass would react to changing external air temperature through its thickness so quickly as to rule out a significant pre condensation temperature difference, the thickness difference being miniscule in relation to the surface area of the pane.

However once the condensation has formed due to the downward air flow on hitting the (relative to room air temperature) cold glass it would have the effect of cooling that area of the glass still further by the fact that some of that moisture would be evaporating which as we all know has a cooling effect.
Oh dear! Is that possible evaporation AND condensation happening in the same place at the same time? Sounds like a contradiction now I've written it!

I think it can, even if only locally, say at the top of the condensation band where the glass is in its trasition between wet and dry. This in turn could provide an additional reason why the condensation 'grows' up the glass from the bottom as the top of the band is actually more vigorously 'self cooling' than the rest, couldnt it?

Ouch, brain fade! Its always the simple thing that do it!
Is any of that plausible, or complete nonsense?
Any thermodynamics experts care to quell my self inflicted confusion?

Notso Fantastic
29th Aug 2004, 14:26
Genghis, I can't go along with the glass thickening at the bottom theory. It does so at such a slow rate that I can't imagine it being even 1% thicker after a century, which will make negligible difference to thermal resistance. How about air in contact with glass rapidly cools, flows down window, starts depositing condensation?

An even bigger mystery in life to me is why car airconditioning cold air is better at demisting than warm air from the heater. Now that one doesn't make sense to me!

CJ Driver
29th Aug 2004, 15:36
The demisting effect of air conditioning is easy to explain - when the air blows over the evaporator it is chilled and condenses out much of the moisture present. When you blow this very dry air over the windows, it picks up the moisture, even though it is cold. Depending on how your car controls work, if you have a simple car (a/c button and simple heat control, versus "climate control") then the best demisting of all is to turn on the a/c and turn the heater temperature control up.

Which, to bring this back to aviation, is actually how moisture is removed from the cabin of some light jets. For example, the CE525's don't have an air cycle machine, they have separate warm bleed air and refrigerant based air conditioning. In normal operation, the air conditioner only cycles on when the cabin is too warm - the "climate control" model. But, when you switch on the defog system in the descent, it forces the a/c compressor and forward evaporator to run, thus removing moisture from the air that it then blows on the inside of the windshield. Because this air is cold, the cockpit also gets extra (warm) bleed air flow from the flow divider.

Sadly, the two don't quite meet, and the result is that 525 crew suffer hypothermia on every descent... :(

SyllogismCheck
29th Aug 2004, 17:21
Notso, quite agree with you that the 'down flow' and subsequent further cooling of the air causes the condensation to build from the bottom of the glass.
Genghis did also state this reason in the second part of his post. I agree with you that it is a secondary and most likely barely perceptible cause.
As I said the theory is sound, but I feel too tiny to be influential in practice.

To further confirm CJ Driver on his reasoning behind the air conditioning question, have you ever been stationary in your car with the air con running then reversed? If so you may have noticed a considerable damp spot on the ground where the front 1/3 of the car was. This is indeed where the intake air is passed over the cooling element and in doing so is stripped of its moisture as it hits dew point, the moisture condenses onto, then drips from the element into a collector tray and is discharged though a drain pipe to waste (the ground) under the vehicle. Result; dehumidified air with a large moisture carrying capacity passing over the screen stripping away any moisture present there.

When you turn the a/c off the opposite tends to happen for a short while, the air begins to strip the moisture from the element and deposit it on the windscreen as the air is now carrying ambient moisture plus an additional load making it keen to give this excess moisture load up, very annoying that little quirk I find. Freeze or fog the screen for a few minutes.

Correction to answer the question as actually asked:
The difference between the warm and cold 'conditioned' air is most likely that the warm air is added in after the cooling process, therefore putting moisture carrying air back into the mix so although the air is warm it carries more moisture than the 'pure' conditioned and dehumidified air. There will obviously be a transitional phase and change over point where the heat of the 'wet warm' air will have a superior 'drying' effect than the 'stripping' effect of the 'cold but dry' air.

Bomber Harris
29th Aug 2004, 22:44
the answer is not too difficullt. Warm air holds more moisture than cold air. Also warm air rises above cold air.

The air at the top of the room is the warm air. At a specific point (in height above ground) the air will be cold enough that it will release its moistue onto the cold window. Above that point the air is warm enough to hold all it's moisture thence no condensation

keithl
31st Aug 2004, 12:22
Bomber - can't accept your explanation, as if it were as simple as that then the condensation would all be at and below a particular level on the whole window. The situation described is that it appears at the lower half of each sash . In other words: Bottom half of lower sash misty, top half clear.
Bottom half of upper sash misty, top half clear.

That effect also rules out the simple downflow theory, and at the moment the uneven pane thickness is still convincing. There are no new panes in the windows in question which are actually 160 or so years old.

Thanks all for some interesting ideas, didn't think the question would run like this...

Captain Stable
31st Aug 2004, 14:43
But doesn't the divider between each sash section restrict the airflow from circulation around the lower part?

Paul Wilson
31st Aug 2004, 19:40
It seems that both Notso Fantastic and Genghis may both be right. Following a bit of googling, it seems that the majority of glass for windows produced up until the mid nineteenth century was made using the Crown Glass (http://en.wikipedia.org/wiki/Crown_glass_process) technique. Basically a big bubble of glass flattened out. This led to the glass being thinner nearer the edge. The common practice at the time was to put the thick side down.

So Genghis take a bow for the "thicker at the bottom theory"

and Notso Fantastic for the "Glass isn't liquid" statement

other URL's

The London Crown Glass Company (http://www.londoncrownglass.co.uk/History.html)

A Usenet Physics FAQ (http://math.ucr.edu/home/baez/physics/General/Glass/glass.html)

keithl
1st Sep 2004, 15:00
Excellent! Thanks, chaps.

Capn.S: That would be reasonable, but the dividers between the panes in each sash are just as much of a barrier to circulation, therefore I would have expected the pattern to be:
Lower half of each Pane misty, upper half clear if that were the case.

Romeo Tango Alpha
1st Sep 2004, 23:52
I believe Geghis is more right than you may believe.

Glass IS a HIGHLY VISCOUS liquid. That was eve taught to me way back when at school. It takes a LONG time for the glass to deform in one way or another due to gravity, but it DOES happen. SLOWLY....

His circulation points and those of others make sense, including Bombers: "The air at the top of the room is the warm air. At a specific point (in height above ground) the air will be cold enough that it will release its moistue onto the cold window. Above that point the air is warm enough to hold all it's moisture thence no condensation"

What people fail to realise, is that the windows, naturally, are attached to walls. Now, the walls are going to retain heat LONGER than any glass, so even if the wall is below the level of dew point, the wall will be hotter than the glass above it, and therefore NOT condense the air around it. The glass, being relatively thin, will be significantly colder, and cause the water vapour to condense on it.

Now, also, having said that, the glass will warm up MUCH quicker than the walls, so that at a point as the day progresses, the glass will be warmer than the walls....

It's all fairly straighforward thermodynmaics.... an IR camera would undoubtedly show a warmer spot on the underlaying walls, and the glass a bluer spot.

Again, take it as you will...

keithl
2nd Sep 2004, 10:56
RTA - thanks for the input, but I don't see (I may be missing something) how that explains the "banded" effect that my original Q was all about.