Drying Schedule for Red Oak

      Advice for a kiln operator on drying rates and humidity and heat settings for drying Red Oak in a dehumidifier kiln. October 3, 2009

I am drying red oak (I don’t know the exact species) and would like information on how fast/slow it should be dried to avoid drying defects.

I made a dehumidification drying kiln. The kiln dimensions are 133” x 39” x 36” with a heat/dehumidification area of 24” x 39” x 36” and a drying box of 109” x 39” x 36”. There is a 100 cfm fan that moves the air from the dehumidification chamber to the drying box and a 300 cfm fan that circulates the air within the drying box. The air is then returned via 3-3.25” holes in the bottom section opposite the 100 cfm fan (which is at the top). The air circulates from the top left of the drying chamber to the top right, then to the bottom right and back to the dehumidification chamber via the bottom left.

I am drying approximately 300bf of 5/4 red oak lumber. The lumber was frozen going into the kiln. It was also fresh cut and very green. The current RH in the kiln is 42% and the temp is 94F and I am removing 2.5-3 gallons of water per day (20-24 lbs per day). The moisture content of the wood is above 22% (above the highest my meter can read) after 2 weeks in the kiln.

At what rate should I continue to dry? 3 gallons a day at 40% rh until the wood reads X% mc, then Y gallons per day at Z% rh?

I am guessing my kiln may hit a max temp of 95F-100F.

Forum Responses
From contributor B:
For 1000' there is 4 gallons loss per 1%. I don't have my notes at hand but seems like you can dry oak at about 2.5-3% per day for 4/4.

From contributor S:
Seems to me like your 2.5 - 3 gallons a day is about right. Any faster and you'll have degrade. Your rate of water production may drop as your MC goes down. With my D/H kiln (an ebac) it seemed like everything went well until the MC went below 20% or so, then the rate of drying really slowed down.

From Professor Gene Wengert, forum technical advisor:
The conditions for drying of oak are based on the MC of the wood. This is covered in detail in "Drying Hardwood Lumber." You can download the book from WOODWEB and also from the US Forest Products Lab's site. It is no longer available in hard copy from the US Forest Service.

Drying Hardwood Lumber

Note that initially, oak should be dried at about 87% RH, so you are well below the suggested RH. As the wood dries, the RH is slowly lowered to under 30% RH. I suspect your RH measurement might be in error. Also, the suggested air flow velocity is around 200 to 300 fpm. This means with an 8' long load and about 20 sticker spaces that are 3/4" high, you need about 3000 cfm.

Note that at the 100 F maximum, you will not kill powder post beetles or other insects in wood. Also, without more heat, which might ruin your compressor, drying will really go slow under 30% MC. And under 25% MC, air flow is no longer a big factor.

As 1000 BF of oak weighs about 5400 pounds green and has an oven-dry weight of 3000 pounds (round numbers), each 30 pounds of water is 1% MC loss. For 300 BF, this would be about 10 pounds per % MC. The safe drying rates are given in "Drying Hardwood Lumber." For 5/4, we might target 2-1/4% MC loss per day average, so 25 pounds is about right.

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