drying 8/4 live edge slabs in a dh kiln

[John C.]

hi i have a nyle l53 kiln.i would like to dry some butternut live edge slabs in it.the slabs are 8/4 up to 25"wide 11ft long.they have been air drying for about a 30-40 days.looking for any tips from anyone who already has done this.did you use samples or  a meter? what mc content did you target?
thanks, john

[YellowHammer]

Sounds like some nice slabs.  This may be of no real help since I've never dried butternut, but if I was going to, I would find its max drying rate as 4/4 and then double it. I would also find out how prone it is to drying defects, and probably call up Nyle.  Also, although I have several moisture meters, I definitely have more confidence in weighing samples. 
YH

[John C.]

thanks, yellowhammer.hows your l53 working out?

[OneWithWood]

Look up the max drying rate for 4/4 and then divide it by 1.5.  You want to dry the 8.4 at a slower rate.  For example if the max drying rate for 4/4 is 5% per day the 8/4 rate would be 3%.

[Den Socling]

And you will want to use a Wagner moisture meter or something similar. You don't want to be cutting up your slabs to get MC samples. Also, I don't try to get slabs down to a uniform 7%. It's not necessary.

[YellowHammer]

The L53 has been running perfectly, nonstop, since the day it was installed with no mechanical issues.  Most of the time, it is loaded with either red or white oak, but has also has seen walnut, cherry, pecan, hickory, elm, maple, beech, poplar, and pine.  The only "problem," if you can call it that, encountered so far is that it has more apparent moisture removal capacity than advertised, so the kiln schedules provided with the manual end up being too agressive for the slower drying, less forgiving species, and call to leave the compressor on too long.  For example, with red oak greater than 50% MC, it can pull almost 6% per day at the 100% duty cycle specified in the manual, blowing the max safe drying rate curve and over drying if not caught fast.  Most times with red oak I start at 50% compressor duty cycle, and run the majority of time at 75% and only go to 100% for the last couple days.

It's nice to have the extra moisture removal capacity, especially with the variety of wood that I dry, it just means a little careful monitoring is required until I get the kiln schedules dialed in.

I have some 8/4 cherry slabs waiting to go in and don't want to screw them up, so am interested in learning how to dry thick slabs, safety and at a max safe rate.  When you say

don't try to get slabs down to a uniform 7%. It's not necessary.

Do you mean don't try to get all the slabs in the load to a uniform MC, or don't expect to get each slab, case and core, to a uniform MC?

YH

[Den Socling]

I mean both. Slabs go on tables and benches where they are expected to move. If you check a slab with a Wagoner, you might find MC varying from 12 down to 7%. I'd say that is fine. Same with variation from slab to slab. 5% variation is OK. I dry a lot of slabs and have never heard a complaint.

[John C.]

yellowhammer,i had the same results with my first load(730 bft 4/4 green from the mill quartersawn red oak).started at 60 percent compressor and 80-85 degrees vents closed,per your old post.had 6% loss daily(weighing samples).also had alot of water on the floor.called manufacturer, he said run it at 100 percent compressor like manual says to get rid of the water/extra moistuer in the chamber to avoid mold.i asked about the 6% and up daily moistuer loss,he said to deal with later in the schedule .any thoughts?i did not run 100% comp. until the wood was under 25% mc(the water came out very slow under 25%) just tried to target 3% loss a day. when wood was under 15% had trouble pulling 1% out a day.still ended up with some black mold on some boards on the bottom two rows.after alot of adjusting(compressor,temp,humidity) 44 days 7% average mc content.

[YellowHammer]

That's interesting, I've seen some of what you are experiencing but not at such a level.  I do see water occasionally on the floor, but its usually near the door seal where I'm getting condensation that pools up if I've messed up the seal.  On very cold days when the concrete is cold, water starts to condense on its surface but doesn't pool up, just sweats a little.  I'm wondering if a couple small drain holes drilled through the concrete floor into the gravel base would help drain pooling condensation if it becomes a problem.  My solar kiln has a drain in the floor for this.

I would be very interested in know how to recover from several days of sustained 6%+ daily loss with a modified schedule for red oak because about the only time I cussed the kiln (really was my fault) was one of the very first loads where I over dried red oak for a few couple days (compressor at 100%) because I hadn't gotten my oven dried sample weighed (lazy) and I saw some pretty serious and deep checking in the boards before I knew what was happening.  I slowed things down to finish the load, but those boards never recovered.  Red oak has a published 3.8% daily moisture loss, and I don't know how much margin that allows, so I try not to ever exceed it. Maybe somebody could comment on that. 

When I start to see lots of humidity building up in the kiln, I open the vents more to release the humidity and drop the temperature, which makes the wood slower to give up its moisture without adjusting the compressor and so drops the daily moisture loss.

I have noticed some of the lower lumber layers sometimes have a slightly higher moisture level, sometimes a few percent, but don't see any mold on the boards.  Due to the design of the kiln with its integrated blower/exhaust, I always felt that airflow near the floor of my kiln was weaker than it should be because the kiln inlet and return is set pretty high in my setup.  I also never liked having my wood so close to the damp concrete floor, whether the airflow was right or not.   To compensate, I put 8 inch concrete blocks on the floor of the kiln, so now I'm about 12 inches off the concrete floor with my first layer of lumber, which is stacked on 2x4 pallets.  I block most of the space with foam but leave decent gaps to let air under the stacks.  Seems I remember reading that pockets of mold, all things being equal, are due to insufficient localized airflow?

At the end of the drying cycle when moisture gets hard to pull, I'll crank the strips up to 115 F for the wood to release more water to finish the load, but its not unusual to only be able to get 1% to 1.5% removal rate for the last few days. 

I just pulled out a load of 1,000 bdft of red oak this morning, started at a very wet calculated 76 %MC  and ended at 6% in 35 days, including a day at 155F for sterilization.   I dug through the stacks, and found a few board with sticker stain, always a way to get my heart beat to speed up, but was able to plane it out on both sides with a 1/16 pass.

I've called the Nyle guys several times to fine tune my setup, have even emailed them a few of my measured drying curves for advice.  They have been very helpful. 

I'm alway trying to learn more about this setup, and would be glad for any advice from anyone.  My only other experience is with a solar kiln, and I'm always trying to learn as much as I can about this. 

YH

[GeneWengert-WoodDoc]

What would help when you see condensation on the floor is to insulate the floor.  As dry dirt is a good insulator, what we need to do is insulate the foundation...the outer perimeter of the kiln.  There should be some of that styrofoam blue board located vertically along the foundation and extending about 2-feet deep.  If there is concrete as part of the wall, the outside of the concrete should be insulated as well.  Concrete is not a good heat insulator.

[GeneWengert-WoodDoc]

The drying rates for 8/4 are in DRYING HARDWOOD LUMBER.  You take the 4/4 safe rate and divide by 2.5.  In other wrods, 8/4 takes 2.5 times longer to dry than 4/4.

On the other hand, the rates are for green lumber down to about 40% MC.  Drier than that, there is so little risk of developing new defects, that the rate is not necessary.

One caution:  If you air dry a light colored wood, you will get one color of the wood, based on the RH and temperature.  That is, the wood sugars and starches oxidize at a certain rate.  When you now put partly dried wood into the kiln(which means the core is still much wetter) and dry the core at a different temperature and RH, you will get a different color.  When you crosscut the lumber after drying, or rip it, you will see the two different colors---often dark on the outside and lighter on the inside.  So, it is always best, if air drying, to completely air dry to 20-25% MC before putting the lumber into the kiln in order to get uniform color.  (I have also seen this color pattern in oak and hickory.)