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.
(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.