Geothermal dehumidification kiln

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Can water piped from below ground help with dehumidification? July 24, 2001

I just happen to have 1000' of plastic pipe 9' deep near my proposed new solar kiln. It would be cheap to place a radiator (or just serpentine copper tubing) in my return air path and pump this 60 F water through. Would this provide much dehumidification?

Forum Responses
To condense 1 pint (or one pound) of water requires 970 Btu. When the vapor condenses, the cold water heats. To supply this energy, you would have to (for example) have 45 pints of water going from 60 F to 80 F. Note that to change the MC of 100 BF of lumber by 1%, MC requires about 30 pints of water to be evaporated and then condensed, so you would need 1,350 pints of water coming in at 60 F and leaving at 80 F.

Gene Wengert, forum technical advisor

From contributor G:
You should be able to lower the wet bulb temperature close to 60 F with a good water to air heat exchanger. However, there is no need to do that in a solar kiln, since you can easily lower the wet bulb temperature by venting and you do not have to reheat the humid air.

You must remember the purpose of drying is to remove moisture from the wood. Condensing the moisture from the air has no benefit unless you need the water. In a dehumidification kiln, condensing is only done because the energy required to condense is used as the heat source and thus condensing saves the energy you would lose in venting.

From the original questioner:
I'm confused. You say I can lower the wet bulb temperature (I assume this is the same as RH) by venting. The RH here (Lex, KY) can be 90% in summer. And you say "re-heat the humid air". Wouldn't I be re-heating "dry(er)" air? My geothermal DH is almost free but maybe it would only make sense in the summer...or maybe not.

From contributor G:
You should sign up for a kiln drying course. I would highly recommend Harlan Petersen's at the U of Minnesota this summer.

In a solar kiln, you work very hard to capture as much heat energy as you can. The dehumidification is provided free every evening as the sun goes down and it is this cycle that produces relatively stress-free wood. To add a cooling coil to dehumidify would just suck up some of your heat with no benefit. The outside air at, say, 90 F and 90% RH does not stop your drying because when the sun heats it to, say, 120 F it will be only 38% RH. That is why it would be more efficient to vent excess moisture (if that is ever required) and heat outside air to replace it.

I could be wrong, but I just can't see a need in a solar kiln to dehumidify. In a dehumidification kiln, it is only done because it is a free by-product of the vapor-compression heat pump cycle and it allows you to run a closed cycle system with the only energy losses through the walls, floor and ceiling.

More comments: The RH in KY is not 90% except for a few hours in the early morning or during a rain storm--check with a local weather station for details. Our bodies do not accurately measure the RH (yesterday here it was "very humid", everyone said, but in truth the RH was 54%!).

The reheat the humid air comment--when you would condense the moisture, the cold air leaving the condenser will be 100% RH. You need to reheat this air. If you use conventional venting, the outside air is already warm (except on a few cold days).

Gene Wengert, forum technical advisor

I think (geothermal) DH is beneficial in a few special cases:
1. On humid, hot summer days, to keep your kiln running when you are using low temperatures.
2. On cold winter days, to save energy.

It is a matter of calculation to estimate the economic benefit against the costs of a DH installation.