Why do I get condenstation

[MikeG.]

Which makes my point for me, in response to Steve "Grainoftruth". An insulated surface will attract less condensation, even in an unheated buiding.

Mike

 

[JohnBrown]

I can't disagree with that!

John

 

[aesmith]

Moving air dries stuff.

I think I'm 100% sure that this is not quite true. For it to be true the moving air needs to be dry enough to do the job. If the air's not moving then a wet object will be surrounded by air at near to 100% RH. Moving air clears that away and replaces it with new air.

However if the air is wet enough to cause condensation on a cold surface, then more air will cause more condensation. We used to see that first hand in the steading at our old place.

 

[devonwoody]

I'll come in with a comment, why do crystals soak up damp. I'm thinking of those gimmicy things I used to put on window cills in a plastic container years ago before double glazing.

What is happening there?

 

[Tom K]

I'll come in with a comment, why do crystals soak up damp. I'm thinking of those gimmicy things I used to put on window cills in a plastic container years ago before double glazing.

What is happening there?

Dunno John its like they are absorbent or something :?

 

[Steve Maskery]

They are hygroscopic crystals. The molecular structure can exist in 2 forms, dehydrated and hydrated. They prefer hydrated. So if you put them in a damp environment when they are dehydrated, they will absorb water. Not like a sponge which soaks up free water, but it actually bonds the water to the crystalline structure. Once the crystals are saturated (ie there are no more bonds available to, er, bond) the crystals stop working, but can be reactivated by heating, which drives off the water and they can be used again.

HTH
Regards,
Mr. S. Alec, BSc (Chemistry).

 

[devonwoody]

And how many tons of the stuff would you need for 50 cu. mtrs of workshop?

 

[Digit]

About 50 cubic mtrs? 8)

Roy.

 

[Steve Maskery]

That is exactly the problem.
S

 

[MikeG.]

Moving air dries stuff.

I think I'm 100% sure that this is not quite true. For it to be true the moving air needs to be dry enough to do the job. If the air's not moving then a wet object will be surrounded by air at near to 100% RH. Moving air clears that away and replaces it with new air.

However if the air is wet enough to cause condensation on a cold surface, then more air will cause more condensation. We used to see that first hand in the steading at our old place.

I don't quite follow, so I'll try my own example.

If you have 2 identical saucers of water in identical rooms...the only difference being that in one of the rooms there is a fan blowing over the top of the saucer........which saucer of water will evaporate the quickest?

I suggest that this will apply whatever the temperature or humidity of the air.

I did the experiment at school 40 years ago: the moving air one dries quickest........whereas if I read your reply correctly you seem to think that they will both evaporate at the same rate? Or am I not following?

Mike

 

[Digit]

Having a blotting paper mind I love science debates, (apart from global warming) but I see the views here as moot as it's bloody cold both inside and outside my workshop. Therefore, using fresh air circulation to control condensation carries the burden of heat losses if I raise the temp in the shop, or the cost of a heat exchanger system!
By the way of an aside I red on the Net 'tother day that the government subsidy for photo voltaic installations is to be scrapped this coming year, seems like a strange 'Green' idea to me.

Roy.

 

[aesmith]

Not quite, what I'm saying is that air movement speeds up the process, so moving wet air could create more condensation than still air, whereas moving dry air will dry quicker than still dry air.

Moving air speeds up the process, but doesn't determine which way the exchange will go.

I know you hate them, but consider that a dehumidifier wouldn't work at all if moving air didn't deposit water onto the cold part.

 

[JohnBrown]

I'd have to agree with you on that. If you blow 100% relative humidity air over your saucer of water, or your washing for that matter, I don't see how it can aid evaporation.

 

[aesmith]

By the way of an aside I red on the Net 'tother day that the government subsidy for photo voltaic installations is to be scrapped this coming year, seems like a strange 'Green' idea to me.

I think the idea behind the new "Feed In Tariff" is that you get rewarded for producing energy, rather then subsidised for the upfront costs.

 

[jack55]

A dehumidifyer is out of the question as there is no power in this shed its just used for storage. There should be plenty of air movement as the eaves are not closed and the first foot or so of the roof from the eaves is bone dry.

So it could be the point Grainotruth made about the roof being the coldest point. Putting the fire risk aside would polystyrene stuck to the roofing boards do the trick. i'm not trying to insulate as I don't work in this shed. I'm planning on rebuilding my workshop next year, once I work out how to get the drawings done for planning permission. So I will insulate that building. For that i'm considering Planwell roofing www.planwell.co.uk, which is a steel sheet with insulation. They also do an anti condensation sheeting system Permadri 1000 has anyone any experience of this?

 

[MikeG.]

I'd have to agree with you on that. If you blow 100% relative humidity air over your saucer of water, or your washing for that matter, I don't see how it can aid evaporation.

Granted, although the chances are that 100% humidity air that is then moved will warm up (as the particles bash into each other), and will thus not be at 100% humidity anymore. Furthermore, how often is air 100%? If it was 99% it would still dry a bit, until it became saturated again.

Bingo............here's the other way that moving air can dry: Moving air is at a lower pressure than still air, and should thus attract moisture out of the higher pressure air around it. 8)

We are splitting hairs here, chaps.........basically, moving air dries stuff.........with occasional exceptions.

Mike

 

[jlawrence]

which means that you have to stump up all the initial instalation costs.
Erm, we're in a recesion - or I thought we were supposed to be. I can't see many people having the funds available to actual put these things in.
My usage is an average 350kWh per month (very roughly) so to have an excess I'd need to be putting in at least a 5KW system - I'd imagine that the cost of that is going to be well in excess of 20K. So much as I'd love to be less reliant on main electricity it simply ain't financially viable as far as I can tell. If I knew I was going to be staying in this house for 20+ years, or it would considerably increase the value of house then it might be different - I might then be able to justify another 20K on the mortgage.
But that's only looking at the (admitidly very rough) installation costs, not considering maintenance.

 

[aesmith]

I'm sure your correct, but the renewable energy industry's been campaigning hard for feed-in tariffs for at least a few years.

 

[Digit]

From what I red Tony the two are currently running together with the one to cease next March. The pay back time is lengthy, which is a handicap as apparently the Mr Average moves house every seven years!

Roy.

 

[MikeG.]

Time for a new thread, methinks? This 'ere deep greenie isn't convinced at all by photovotaics in the cloudy UK. Does anyone have the enthusiasm to re-start this conversation in a new thread? Its OK, I think..........I haven't seen Rich about for a while.....we might be safe!

Mike