Monitoring Progress in a Dehumidifier Kiln

      Collecting and measuring the removed water is not a reliable way to gauge drying progress, especially when drying mixed thicknesses of wood. June 4, 2012

I've been running a solar kiln for years, but added a Nyle for drying thick stock faster. And of course, my first batch is a mixture of 6/4 to 12/4, both red and white oak. I set the unit up so I can also collect water in a barrel to monitor water loss. Obviously even in my tight kiln I can also expect some moisture is lost via air and I'll need to reconfirm my numbers on that, but it should be a fairly small number when the chamber is closed. So here's the question: based on gallons removed, I should be able to run the compressor much harder and still maintain a very slow initial drying rate, but I'm also watching wetbulb depression and based on that, I'm pushing the limit for thick oak, with a 5-6 degree drop. Do I just watch wetbulb, or is my notion of watching the water barrel valid enough to crank the compressor farther despite wetbulb depression?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
After a short while, the thinner lumber will not lose moisture very fast, as it is getting dry. So the overall or total moisture loss rate is not a good idea when drying several thicknesses together.

Stated another way, as the thinner material gets drier, you can indeed lower the RH, but the thick stock will not tolerate such a dry MC as the thick stock must be dried slower (higher RH) and at the same time will also be higher in MC than the thin stock. Therefore, your drying must be based on the 12/4 only at this point.

Note that oak lumber thicker than 8/4 cannot usually be kiln dried without excessive checking and honeycomb in a kiln. Drying times will be 90 days or so at best. The schedule is T3-C1. Therefore, very slow air drying is suggested prior to going into the kiln. Hopefully you are using a 3 F depression; 5-6 F is not appropriate until the MC is 30% MC. Note that if your measuring equipment is not quite accurate and you actually have a 4 F depression, that will dry about 33% faster and that can be damaging to the oak. You need to hold this 3.0 F depression until the average MC of the 12/4 reaches 40% MC. (This is what C1 means.) Do you have a copy of Drying Hardwood Lumber?

How are you measuring WB depression? Do you have good air flow (600 fpm) across the wick?

In short, you need to use 12/4 kiln samples until the drier, thin lumber reaches its final MC. Then you need to equalize. I am curious on how you are determining how much water loss (%MC per day) you can have safely. I thought that loss rates (% MC per day) were only for 4/4 through 8/4, so if you have 12/4 rates, I would like to see them. How do you convert them to pounds or gallons?

From the original questioner:
I went into this batch assuming a 100 day minimum cycle and all my calculations are assuming I'd keep the initial period at maximum 1% per day wood moisture loss. I'm measuring air with a wetbulb hygrometer located between the DH unit and fan, so good air movement and measuring the driest air. I've also been assuming that for the first several weeks I'd be seeing more movement off the thinner stock and put the thick material on the inner pile of a two pile deep stack.

Actually running this kiln is still new, but as a fair disclosure, before quitting the day job I was an engineer with just enough background in psychrometrics and grain drying principals to be dangerous. So I've already researched all I can find. The desire to measure "drippings" was based on my desire to minimize the number of samples taken knowing I'm in a 100 day cycle and limited in how much of this client's wood can be sacrificed.

From Professor Gene Wengert, forum technical advisor:
Drying time for green oak in the kiln is 90 -100 days for 8/4, so for 12/4 you would be looking at about 1.7 times longer or 150 - 170 days. Because you have mixed thicknesses, you may have to slow the drying at the end (equalization) because the thinner material will be reaching the target MC first while the thicker stock still needs more time. This can extend time for a few more days.

As mentioned, the book says to use a 3 F depression and so your 5 to 6 F depression is going to dry the lumber twice as fast. For 4/4 oak, we use a 4 F depression initially. Finally, samples are used to prevent sacrificing or damaging the wood. So, we do cut a few pieces, but the overall results will be better as we can now protect the entire load.

From the original questioner:
I understand the need for actual samples, but given the very long cycle, I was opting for less frequent sampling (I was thinking weekly) and then watching both wetbulb depression and dehumidifier removal rates daily. Based on both the initial couple weekly samples and barrel drippings I'm probably more in line with your 150 day estimate, in that barrel drippings account for less than 0.5% MC drop per day and I'm not venting because I wanted fairly accurate measurement of loss, so it's just drippings and leakage. Also for clarification, the $ slabs are actually only 2.5 to 2.75, so I have one test board at that thickness, one that is a shade over 3 thick, but not as wide as the primary slabs. I wasn't sure how relevant a prong test would be at this early stage of drying or if that is just an after the fact confirmation of whether you already blew it.

I take it from your message, however, that you would pay no attention to the barrel in the corner, and just watch wetbulb depression, even if test samples continue confirming an actual rate on the thick stock under 0.5%?

From Professor Gene Wengert, forum technical advisor:
Yep, especially with your mixed thicknesses.

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