Humidity Control in a Kiln

      A kiln operator gets advice on airflow and humidity settings. April 27, 2007

I'm currently finishing drying approximately 4k of mostly ash 1" random width planks for flooring in a container. They were air dried to 12-14% and have been in my kiln for 8 days with temps of 95-139f, averaging 110. The wood is stickered in 5 piles with 9 box fans blowing through. The humidistat is in the middle of the container on top of the center wood pile approximately 5 feet up hooked up to a shutter fan vented outdoors in the front of the trailer where my heat source is. The humidistat has settings of 10 20 30 40 50 60 70, stop, and a switch for auto and on. I have it on auto and above 70... just before stop. I have only heard it come on once, but I'm by the trailer often. When I go into the trailer, there is no condensation on the walls, though it feels humid. What should I have the humidistat set on and how often should it come on?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
First off, get yourself some sort of hygrometer so you know the actual relative humidity in the container. This is very important info to have. Temp fluctuation seems a little much too... I'd try to regulate that a little better. I'm guessing that your settings on the humidistat are for RH, so you are set to turn on only over RH of 70%. At 100 degrees F, you probably want your RH to be around 40-45% (giving your lumber an equilibrium MC of about 8%). I'm not sure about your setup, but play with your controls of heat and temp to get the RH around 40%. I use pretty cheap digital thermometer/hygrometers from Wal-Mart that are only about $10.

From Professor Gene Wengert, forum technical advisor:
To achieve 7% MC final, you will have to run about 30% RH or slightly lower.

From the original questioner:
Thanks. Does that mean I have been wasting time these last 8 days? That's a 45 difference! Will a $10 hygrometer from Wal-Mart be useful to see if it calibrates with my humidistat and set it all for 30? Also, are the fans necessary at this point?

From contributor R:
Air flow (even air flow throughout the lumber) is always important.

From contributor D:
At your current MC, you need only minimal air flow, something like 3 mph. Kiln operators talk about feet/min or seconds; you can do the math. With a smoking device, any movement of smoke on the opposite side of the lumber pile should be enough air flow.

From the original questioner:
So for future reference... If I'm looking for rustic/country wood flooring (not worried about perfect color), I can air dry to below 20% and then turn the heat to 130 and the humidity down to >30% with little airflow and finish wood to final mc of 6%? Approximately how long should that take? And can you overdry, and if so, what happens?

From contributor R:
Get a table showing EMC for various temps and RH. I think 130 degrees 30% RH is a little harsh... Would give you an EMC of around 5%. There is a good table in "Wood Handbook" put out by the Forest Service from Madison, WI. It takes me about 10-12 days to get air dry oak (12-15%) down to my target of 8%. It you overdry, your flooring will swell in use.

From contributor P:
Contributor R, what kind of kiln are you running? I have about 300 bdft in a dehumidifier kiln for a week. It was at 16% MC; now it's gone to 20%. Not sure why. I took 4 pints of water off in the last 12 hours and I have the heat at 105 to 95. Any suggestions?

From contributor R:
I'm running a Nyle set up in a container. Your problem doesn't make much sense to me. Only thing I can figure is your initial MC reading of 16% was wrong, or at one odd dry location in your lumber. Also, MC readings should be adjusted for temp. The lumber is hot now in the kiln, but I don't think that would have shown that much variation... check your meter and take an average of multiple readings in the kiln.

From Professor Gene Wengert, forum technical advisor:
Are you using a pin meter with insulated needles? Are you driving the needles to the core? You need to use a setting of under 30% RH to achieve 7% MC in the core of the wood. An RH of 30% RH will achieve 6% MC on the surface. (Under 30% RH is written as a listing of EMC and RH values is provided in "Drying Hardwood Lumber," which is in the archives here. Look on p.15 of the document.)

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