Slow Drying at the End of Dehumidification Kiln Run

If drying slows or stalls at close to the target moisture content, a small amount of added heat is appropriate. December 8, 2012

Question
I was wondering for those of you running the Nyle 200 or if you find that when running 4/4 oak lumber that the drying rate really slows down at the end? I usually air dry my lumber to 15%MC or so, then put in the kiln. Running 100% compressor and 120 degrees I can remove about 2% MC per day until I get to around 9%MC, then it usually slows to a crawl, almost stopping. It might be 1/2% MC per day? This usually requires me to turn off the compressor and raise the temps to 140-150 as suggested by someone at Nyle and slowly heat the last bit out over three-four days.

Is this typical of drying oak or do I have a weak link in my system somewhere? I've got a fairly old used Nyle unit, maybe 1995 model. I'm wondering if it’s just not pulling the moisture it should? I just had it recharged with virgin R12 digitally weighed in. (Last service guy fixed a leaky valve cap and refilled with wrong freon). I could still have a leak though.

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
This is fairly common with well air dried lumber. It is common with steam kilns, DH, etc. The common expression is that heat is used to "break loose the water.” Here is a quote from DRYING HARDWOOD LUMBER, p. 102: “What can be done when drying seems to stop in the middle of the kiln run? Drying will never actually stop if the EMC of the kiln atmosphere is lower than the core MC of the lumber, but the drying rate is extremely slow. To correct this problem of “slow drying,” the dry-bulb temperature can be slightly increased (perhaps 5°F) during the intermediate stages of drying. Higher increases are not recommended because of the risk of degrade. If a kiln operator decides to increase temperature above 30% MC, the operator should make sure that the kiln controller is properly calibrated.”
In your case, I would try 130 or 140 F for a few hours.

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From the original questioner:
Thanks Gene. If it’s common I won't worry too much. Curious you mentioned this in connection with air dried lumber. Does green lumber allow the moisture to exit more easily then? You won't find a slow down at the end? One last question: You mentioned trying a few hours at 140. I've been needing to keep the kiln at 140 degrees for three-four days with the compressor off to get oak down to 6% MC in my kiln, could this system possibly be part of the trouble I seem to have with stress in my finished product? It feels like case hardening, but having been air dried first, I didn't see how I could be getting case hardening so frequently. It’s possible that the lumber would have dried to 6%MC at 120 if I gave it more time. I seem to have the RH in the kiln at 25% so it should be dry enough to dry. It just quits making water and seemingly stays at the same MC day after day at the end.

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From Gene Wengert, forum technical advisor:
Yes, green lumber seldom seems to stop. Well air-dried. Lumber is the most common. Heating to 140 F and then after a few hours going to a lower temperature seems to work. You do not put casehardening stress into lumber in the kiln at this low mc level.