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View Full Version : How low should the m/c % be before putting into a dehumidification kiln



David Mealey
06-20-2018, 7:08 PM
I've been asked to kiln dry and large black walnut slab, approx. 12' x 30-48" (90bf) that's currently 27-30%. I've been told that it's not a good idea to put wet wood (>25%) into the kiln as it's prone to excessive cracking, warping and even case hardening. What's been others experience with black walnut. Is it stable enough to chance it? I've explained all the risks to the guy but he still wants to have it kiln dried.

Jim Andrew
06-20-2018, 9:07 PM
It is best to air dry first, then kiln dry the lumber. Air drying will allow the mc to get down to about 12%.

Keith Outten
06-21-2018, 8:58 AM
If your operating a De-humidification kiln you can put lumber in your kiln right from the sawmill. I operated an EBAC kiln for ten years and although the drying schedule was much longer with wet wood the quality was much better.

Scott T Smith
06-21-2018, 9:40 PM
David, you did not mention the thickness of the slab but I would presume that it is around 3". What type of kiln are you using?

Typically you can dry thick black walnut as long as you follow the appropriate kiln schedule. However, what else is going to be in the kiln with the slab and what schedule were you planning to follow?

Daily MC% reduction for 12/4 BW is around 1% per day - which is a very slow rate.

David Mealey
06-27-2018, 5:20 PM
Hi Scott,

Yes, the slab in question is 10-12/4. I'm going to include a stack of similar large, thick 10-12/4 walnut and some thinner boards 5-6/4 walnut, maple and maybe some cherry. I'm using an EBAC 3000 and the kiln measures 16'w x 12'h x 12' deep. I recently changed the front doors from swing to sliders and now I've got some significant leaks I need to plug. I can get a to a comfortable 115-120 F inside the kiln but can't seem to get it over that temp.

Scott T Smith
07-02-2018, 8:30 PM
ok, so you're using a DH kiln. Typically you can mix anything in a DH kiln once it dries to 25% or below. With thick BW you really have to be careful about case hardening.

At 30% in my nyle I would be setting the kiln for 100F and 60% RH. Once it dries below 25% I would turn the temp up to 120F and the TH% down to 40%.

You should be ok going into the kiln at that MC% as long as you don't turn the heat up or the RH% down.

David Mealey
07-25-2018, 6:34 PM
On the Ebac, I can control temp and on/off time. There's no direct control of RH%. I typically start out at low temp 30 degrees C (86F) and wait until it stops putting out water, gradually increasing on time and temp to get to Constant and 120F. It's been a 3-4 weeks and I'm up to 45C(114F) and 75% on time. Does that sound reasonable?

Bill Dufour
07-27-2018, 11:21 AM
Wonder what all this means for my climate. I like to leave pressure treated wood outside for a week or two before i build a fence etc. I save the receipts and return 10-20% as it warps too much while drying in the sun.
This summer has been hotter then most about 100 degrees the last two months.
Today will be 104 and the humidity is high at 33%
Bil lD.

Tom Hogard
07-27-2018, 2:08 PM
You can cut down on the waste if, when you bring PT wood home, you stack it just like you would freshly milled lumber, until you are ready to use it.