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Dehumidifier Kiln "Hanging Up"

Dehumidifier kilns sometimes stall as moisture content readings get closer to the target. An extra shot of heat can help a lot. July 3, 2008

I have a Nyle L200 loaded with 2300 board feet of mixed hardwoods - ash, cherry and walnut. All were shed dried to 10-12% MC and needed to be dried to 6%. Temp in kiln is 110 degrees. Day one, water flows nicely from discharge tube. Day two, water drips quickly. Day 3, occasional water drips. Day 4, nothing.

Now, I know you shouldn't dry lumber by meter readings, but I dry mostly AD lumber and I haven't seen the need yet to set up equipment to dry and weigh samples. Meters are what I must depend on. Right now, a Delmhorst J2000 with hammer in insulated pins and a Wagner pin-less meter are both telling me the lumber is at 8-9% and has been for days. I have a cheap hygrometer telling me I'm at 20% RH in the chamber and a dry-bulb/wet-bulb depression of 34 degrees corroborates that reading. The charts are telling me my lumber should be equalizing at 4% EMC. So why isn't it? Are both meters that inaccurate in a kiln?

Forum Responses
(Sawing and Drying Forum)
From contributor S:
I have an EBAC dehumidification kiln and I have the same problem... getting that last bit of moisture out. It seems like it takes an eternity. I put an additional electric heater in the kiln to push the temperature higher and added more fans, but that will add to your operating costs. It took nature a long time to grow the trees, so I just try to relax and not be in such a rush to dry the lumber.

From contributor F:
I have had some loads "pause" for 2 or 3 days before the last 3 or 4 percent of moisture will come out.

From contributor B:
I have noticed the same thing with my Nyle l-200. Usually more with the walnut hanging up the #2 category hardwoods or red oak hanging up the hard dryers. What I found when I started using oven dried samples was that the pin meters aren't that great at low MCs when measuring a hot piece of wood.

I have been bashed on this forum for saying this before, but I take my Wal-Mart digital (grams) food scale, take a cut sample of about 150 grams, record the weight, dry it in the microwave for 2 minutes, record the weight, dry for 30 second bursts until the weight stops changing, then calculate the MC (take the initial weight divide by dry weight, subtract one, multiply by one hundred). What you see is that the drying is just slowing down, but you still should be seeing a steady half to full percent per day. I know that this method easily discerns this difference.

Funny thing: I just finished putting about 1200 bd/ft of air dried poplar in the kiln one day last September, when my buddy who was helping me asked what the moisture content was. I got my meter and checked it for laughs. It measured 6%! Both on the meter and oven sample. It was so hot and dry here over the summer it took poplar from green to 6% in 40 days. Good luck with your drying. It takes the Nyle about 17 days from green!

From contributor D:
110 is too low at this point. Turn it to 120. If you want, you can push it to 125 or 130 if there is no water coming out at 120 and you are stuck. Don't go beyond 120 unless you are clearly stuck. Temperature is the most important factor affecting drying time. Humidity is much less important.

From contributor B:
Forgot to say what contributor D said. I definitely don't run below 120 degrees when I have a MC that is hanging up. I have picked up other good pointers by calling Nyle with problems.

From Professor Gene Wengert, forum technical advisor:
I also agree with contributor D about the temperature. In fact, if you can isolate the DH unit, and if you have an auxiliary heat source, give the lumber a shot of hot air for a few hours and then go back to 120 F. In case you are concerned about this information, this subject is discussed in DRYING HARDWOOD LUMBER, so what you are being told is indeed well known and is the correct answer.

From contributor D:
Gene's suggestion can be done with an L200. You set the compressor timer to zero and then you can run the heat to a higher temperature. They are made to be able to go to 160F (72C). Wood-Mizer and Bailey versions can do the same.