Conditioning DH-kiln-dried white oak

      July 18, 2000

We are thinking about adding a small, 4,000-board-foot (BF) dehumidification (DH) kiln to our operation, for drying our 4/4 and 5/4 quartersawn white oak.

We plan on air drying (AD) to below 30 percent before going to the kiln. Our question concerns conditioning in a DH kiln.

Is there a small steam generator on the market that could be used, or is the air drying and DH drying going to produce satisfactory lumber?

What about 80 percent to 6 percent in a DH? Stress?

You trust a salesman only so far; they tend to tell you everything will be fine.

Ask the salesman for a few references of people that are doing what you want. I have heard of a source for small boilers -- I once used a Chromolox brand electric boiler.

Will AD and DH produce stress-free lumber? Maybe -- it dpeends on our definition of stress-free. One hundred percent stress-free? No!

I encourage you to run the numbers on AD and DH, versus 100 percent DH. The cost of inventory, extra handling, and the extra air drying degrade you'll experience with oak (and maybe a few other species) can easily make green-from-the-saw DH a better deal.

I am not in favor of air drying valuable woods, especially oak and hard maple, if quality and cost are the main criteria for success.
Gene Wengert, technical advisor

From the original questioner:
It is your feeling that green-to-dry in a DH is better for the lumber than AD/DH?

Cost of drying is not a factor in our decision to air dry first; quality 5/4 quartersawn white oak is.

Do you think a steam generator to condition the lumber at the end of the cycle is necessary for DH? If so, why don't DH manufacturers include a steam system?

"It is your feeling that green-to-dry in a DH is better for the lumber than AD/DH?"

Yes, this is for certain.

"Do you think a steam generator to condition the lumber at the end of the cycle is necessary for DH. If so, why don't DH manufacturers include a steam system?"

A steam system would be a good idea for many operations, but it raises the price, hence, many manufacturers do not even discuss it. In some cases, the conditioning process is not needed.

We've been running two, 10,000 BF Driline DH kilns for the last eight years, drying primarily 4/4 red oak and secondarily 4/4 white oak. We quickly found the that manufacturer's idea of conditioning, with moisture left in the kiln when lumber was around 20 percent, didn't work well. Since we plumbed steam for back-up heat, adding steam spray was easy and we could give our lumber a real conditioning.

I would suggest if you were looking for a steam source for conditioning that you also look at back up heat, to help electric/resistant heat. This might get you looking at a small, wood-waste-fired boiler. We added some relays and let kiln controllers actuate motor-driven ball valves. Results were shortened schedules, particularly during winter, while not jeopardizing the quality of the product. It would take a week sometimes to get up to set points with electric heat alone in winter.

As far as manufacturers of DH equipment, I have a hard time recommending Driline because of our experience. When we replace any of these units we'll be looking at a Nyle.

One last bit of advice: Don't undersize your DH units (compressors) for kiln size. Our five-horsepower (HP) units run constantly, resulting in frequent repairs. We figure a 10- or 15-HP unit in each kiln would have given us better service by virtue of not running constantly.

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  • KnowledgeBase: Woodworking Miscellaneous: Accessories

  • KnowledgeBase: Primary Processing: Kiln Operation

  • KnowledgeBase: Primary Processing: Lumber Grading

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  • KnowledgeBase: Wood Engineering: Wood Properties

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