air circuloation via ducting in large unheated room

Chris Hogg · 2022-10-25T08:31:41+00:00

As I may have mentioned before, I volunteer at my local Archive. Wehave a large room where we do our research etc. for enquirers, withrows of shelves stacked with ring-folders, books, files etc. It iscold in winter, and the humidity is much higher than that recommendedfor document storage. The dimensions of the room are 12.2 x 8.35 x4.84 metres (l x w x h), giving a volume of 493 cu. metres; call it500 cu. metres. The cost of heating this room is getting exorbitant,may be beyond our means, and we are looking into ways of making itcheaper.When I was gainfully employed, we had a large aircraft-hanger-like labthat housed big items for chemical engineering research and pilotscale development, probably several times the size of the researchroom at the Archive. It was heated by industrial space heaters whichwere expensive to run, although I can't now remember if they weregas-fired or electric. Then someone pointed out that most of the warmair went straight up to the roof, and they had the bright idea ofdrawing it back down through ducting, and recirculating it at floorlevel. This was very simply and cheaply done using a few lengths ofdrainage pipe stacked vertically and a duct fan at the bottom to drawthe air down. Apparently it was a great success, but I have nodetails.I have been wondering if we might use something similar at theArchive. Does anyone have any experience of this sort of thing?

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Theo 3 years ago

Sounds like a good experiment.

One thing worth doing is to weigh the water tank (assuming there is a tank) and record the weight and the date each time it is emptied.

For example, if the tank is full but the humidity isn't going down, it suggests there's source of humidity somewhere (either water ingress, or a large collection of damp paper that needs drying out). You may find the buffering effect of the paper means it doesn't reduce the room RH as much as you might expect, until all the paper has dried out. It doesn't mean it isn't working, though.

I'm not sure how efficiency compares, a small unit running flat out compared with a larger unit. However once you have the data for what you have, maybe it's possible to check them against efficiency numbers of a larger unit.

Theo

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Chris Hogg 3 years ago

Yep - me!

Or will you mount it higher and use the water outlet to go to a

That too. Emptying twice a week.

60%RH, which is the upper limit of what's recommended for document storage, to see how things go.

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Chris Hogg 3 years ago

The last time a short experiment was tried, it took out much more water than could be simply held by the volume of air in the room. Since then, doors have been draught-proofed and windows (frames plus glass) covered in bubble-wrap partly to reduce heat loss and partly to reduce draughts. Any excess moisture being removed will be coming from the structure and the documents as they slowly lose moisture.

Hopefully it will 'bottom out' and reach equilibrium at some stage, at which point we can do no more with the one dehumidifier. If it's not at an acceptable humidity, get a second to run alongside it.

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Andrew 3 years ago

But how are you measuring %RH ?.

I have a trusty old Bannen? wet and dry mercury thermometer and it is in my living room at the moment, where it measures DRY 19C, WET 17C.

This means an RH of about 80% which seems too high. My Ebac dehumidifier extracted about 600ml of water over a 6 hour period and the wet bulb reading didn't budge. Even during the heatwave the Wet bulb reading was never greater than 4C less than the dry reading.

I read somewhere that wet bulb thermometers will only give an accurate reading if the airflow is 1 m/sec which seems a lot. There is no airflow at all in my lounge, unless it is a windy day and the patio door is opened.

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Chris Hogg 3 years ago

I'm using a little TZ-TempU03 data logger

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I've checked it against humidities in a sealed container over saturated salt solutions. A saturated solution of ammonium sulphate gives a relative humidity of 81-82%, fairly independent of temperature, and a saturated solution of magnesium chloride gives a relative humidity of 33-34%, also fairly independent of temperature.

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I'm assuming the humidity reading on the dehumidifier is roughly OK, but it isn't critical, as long as the %RH gets to below 60% on the data-logger.

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Animal 3 years ago

0.2kW for a variable percentage of the time. You won't notice any temp change.

no it won't, unless it's already at 100% & there's water waiting to evaporate. Dehumidifying it makes it hold less.

cobblers.

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Animal 3 years ago

17 & 19c plus or minus how much? I'm inclined to think it an inaccurate method unless very precise thermometers with a well stretched scale are used, not something one would expect at home.

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Thomas Prufer 3 years ago

Yes -- it needs the airflow. "sling psychrometer" is one where you swing the lot around, some have a small fan, used to be clockwork, sometimes.

They are simple, but require care in use: clean "sock", distilled water, no radiant heat, etc.

Thomas Prufer

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Chris Hogg 3 years ago

A bit like a football rattle!

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Jeff Layman 3 years ago

From

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at at fixed relative humidity of 60%, air at 5°C will hold 4.1g/m^3 of water, and at 10°C will hold 5.6g/m^3 of water - ie the absolute humidity is higher. As I said, /slightly/ more water at a higher temperature.

? By using a dehumidifier you're trying to cut down on the amount of water in the air; any additional water in the air, no matter how small, will be in opposition to the activity of the dehumidifier.

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Animal 3 years ago

You fail to state what you're comparing, and it's far from obvious given your odd comparison.

No-one's putting extra water into the air by dehumidifying. If you think they are, I'm already thinking this discussion is another pointless thread of lunacy.

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air circuloation via ducting in large unheated room

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