Using Outdoor Cold for Drying Dehumidification

Unlike a powered dehumidifier, a cold kiln wall dumps heat to the outdoors, and so expends energy just as venting does. Here's a long discussion of alternatives to heat for kiln drying, and why they're mostly not practical. April 10, 2007

Question
I have an old refrigeration trailer. If I run 40-50 degree water through the refrigerator coil and have a fan blowing over it constantly, will it dehumidify okay? I will also be blowing hot air from my wood boiler at the same time. The wood is ash, cherry, and a little birch and maple that has been air drying about 1.5 years.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
Yes, it can work, but the coil will corrode quickly. Why not just use the wood stove and run a conventional heat and vent kiln? You also need a lot of water flow. For a 4000 BF chamber you would need to bring and waste about 10-20 gallons a minute.


From the original questioner:
I was going to circulate water in a closed loop in a 400 ft coil and submerse it in a large vat of water outdoors, and it's cold out now, so... I have a lot of wood to burn, but to just leave the trailer open while heating seems wasteful.

From contributor B:
It is nice to be able to measure the liquid coming out of your load in DH drying.

From Professor Gene Wengert, forum technical advisor:
Here is an example. If you dried 1000 BF of red oak from green to 7% MC, you would have to evaporate 2500 pounds of water or more, which would require 2.5 million BTU just for evaporation. If you have hot water that is at 50 F and you have 1000 gallons, then the water would reach 150 F by adding 100 BTU per pound or 800 BTU per gallon. So, this huge amount of water would not be enough cooling (800,000 BTU). What you propose will be hard to do indeed. Note that air-dried would require closer to 800,000 BTU per 1000 BF. In this example, I did not include the difficulty of transferring heat as the water heats (which means high water flow). In other words, you need a constant supply of cold water, which means you cannot recycle the water. (This example assumes 100% efficiency, which is not really possible in the real world. So, increase everything about 50% or more.)

From contributor B:
This is one of my favorite topics even if I do have a hard time understanding it well. It would seem possible to engineer some "free" DH cooling when the temps outside the kiln drop into the freezing range.

From Professor Gene Wengert, forum technical advisor:
You are correct, but it is hard to talk about a system that only works 6 months a year. However, you also have to remember that if you dump the heat outside, it is the same as venting, and so there is no energy savings. What makes the electric DH so attractive is that the heat of evaporation is recovered and put back into the kiln.

From contributor W:
I would think you would freeze up your system if the power went out. I don't quite understand what you're doing, but every system I've been around needs a pump to circulate the water and if water doesn't move, it freezes. Am I wrong?

From contributor S:
I thought about using a shipping container as a kiln. These containers are air tight and the walls would serve as a condenser in the winter time. Evaporation is taking place inside the kiln, so no heat loss there. As Gene mentioned, it wouldn't work well in the summer time unless you have a cold water spring close by. It would also be difficult to control humidity. The walls would be condensing water very fast and the air would become dry unless you were pouring the heat to it. It would also be difficult to achieve the temps necessary to get down to 6% in an uninsulated chamber. It never hurts to think outside the box (or shipping container) every now and then.

From Professor Gene Wengert, forum technical advisor:
Regarding the idea of using a container... Let's assume that the outside temperature is 30 F. Let's also assume that the wet-bulb in the kiln will be 35 F; if it tries to get any higher, the walls will condense moisture. So, if we want a dry condition, but not too dry, inside the kiln, we will run it about 55 F. Of course, at this cold temperature, wood dries very slowly. If we raise the temperature, then the RH in the kiln will be too low. Of course, a lot of the energy that we recapture from the vapor condensing will actually be used for heat losses through the uninsulated wall. So, the bottom line is that it will not be practical if made in the manner you describe.

From contributor S:
If we raise the temperature in this chamber, wouldn't we also be raising the temperature of the kiln walls? If so, wouldn't this slow down condensation and allow vapour to circulate through the stack? Also, if we add more heat, aren't we creating more water vapour, thus raising the RH? This may not be relevant, but when the water turns to a liquid, there is a considerable lowering of the volume of the atmosphere, due to the vapour turning to a liquid. One could actually control the RH by using a pressure relief valve on the chamber since it's air tight. The rate at which we add heat will effect the rate of condensation.

Why would anyone start a drying schedule at 55F? A lot of kiln operators air dry oak on the yard when the outside temps are as high as 90F with no problems. The outside temp of 30F would be an advantage since it makes for a perfect condenser. I initially thought you wouldn't be able to get the temperature up inside the kiln, but I'm not so sure of that now. It is a closed system and we would be recapturing the heat given off by evaporation. You mentioned that any heat gained by evaporation would be lost through the uninsulated wall. Wouldn't this only hold true if the kiln walls stayed the same temperature as the outside air? When the water condenses, it gives out a lot of heat - this risks warming the surface so the water will stop or slow its condensation. One water molecule is multiplying 1700 times when it turns into vapour. I've heard of some conventional kiln operators speeding their drying times substantially by leaving the kiln doors partially open during the early stages of drying. In light of that, is condensing too fast that much of a concern?


From the original questioner:
Thanks for your replies. Let me explain my situation. I have approximately four thousand board feet of mostly ash, some cherry, and a little birch and hard maple, about 1", stickered in a 40' insulated trailer for a year and a half. This is mostly country/rustic boards of widths from 3-10" that my friend and I are going to plane and t&g for my house flooring that I'm ready for. My house has radiant heat and I want wide planks, so I need this wood dry. I have teed off my outdoor boiler that heats my home with a supply and return of 3/4 pipe that enters the trailer 70' away through two 1" holes in the aluminum floors located at the corners in the rear of the trailer to a large radiator. I was going to simply circulate 180 f water and have a fan blowing constantly. The refrigerant coil is on top, so if I scratch that idea of cold water circulation to dh and can't buy a commercial dh, I can leave a little flap door approximately 1'x1' located in the rear top corner wall of trailer open. Is this my best way to go?
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