Troubleshooting Drying Stress

      Some general instruction about the causes of drying stress for the owner of a dehumidifier kiln. April 2, 2013

Question
I'm still getting really stressed lumber out of my kiln and recently I've made it worse. I bought a simple core lumber system from Lignomat and discovered that my lumber cores, especially the thicker stock, were much wetter than I realized. Now I'm running the lumber a lot longer in the kiln to get the core down to 7%MC, however this is making the jacket a lot dryer. My pin meter doesn't even go low enough, but it's reading 4.2MC on the surface.

I've tried a number of suggestions from the folks at Nyle (who have spent a lot of time talking with me and are helpful with ideas), but still haven't found a way to condition or even out the MC in the lumber. I'm afraid I've lost a lot of customers as my wood has always been very stressed and terrible to work with, especially fresh out of the kiln. It makes real curls coming off the tablesaw and is pretty much junk.

A little recap... I have a Nyle 200 with 1000-2000 bd ft that I run different types of species, but all have the same result. Usually we air dry for a few months, hardly ever green, but it doesn't seem to matter as the wood turns out the same, which makes sense as the moisture leaves the wood from the surface so it would always be dryer. The recommended procedure of dumping a bucket of water on the kiln floor doesn't work for me. So I added a 4 gallon commercial rice cooker which kept the RH around 50% for 6-8 hours at 140 degrees with no permanent change in the lumber. I added another 4 gallon rice cooker and recently boiled 18 gallons of water through them, keeping the RH at 70-98% for 2 days, with no change in the lumber surface.

What I think is happening is that the lumber actually reads an increase on the surface when I open the kiln, but after it cools for a day, seeing as the lumber is 140 degrees inside the core, it simply dries off again as it cools. Back to 4.2MC on my Lignomat pin meter.

Gene mentioned somewhere that the lumber needs to be steam heated quickly, and I seem to recall it needs to be over 165 degree for the lumber cells to open up?

I don't understand why I'm the only small kiln operator that seems to not be able to condition their lumber, so something is not happening properly in my operation. I've been doing this for 5 years and am about ready to pull the plug and take it to a commercial operator as I can't see any way to make this kiln produce even moisture lumber without steam injection.

Gene has mentioned a steam generator for cleaning engine blocks. Is this still the best idea for conditioning? I couldn't really find anything that wasn't diesel powered and less than a few thousand dollars.

I'm sure there is something I'm not understanding. My last conversation with Nyle they thought I should condition the air dried lumber first to even things out, but I don't see how that works as the surface would dry first in any event. I'm willing to try anything, though.

How about just spraying all the lumber with a deck sprayer? I don't know how wet that will leave it, though. Seems like a bad system...

Forum Responses
(Sawing and Drying Forum)
From contributor T:
I don't claim to know anything about the process, I only own a homemade D/H kiln. From my very little knowledge, stresses can be created with too high beginning temp or moisture loss causing an extreme drying condition on the outside of the lumber and not relieving the MC on a more balanced level throughout the wood. Yes, the outside will always be dryer than internal, but if outside is extremely more, inside is not drying properly/releasing its moisture to balance out. Basically in redneck terms, drying too fast, causing stress.

Has this been going on from day one, or just recently? If from day one, maybe something got hooked up wrong. Nyle has a good reputation and I haven't heard this problem as a common one. Something may be giving you false readings or if hooked up wrong, incorrect readings.

Gene, could a person start kilning for a day or two without the heat to balance an MC draw, then add the heat to increase the drawing of moisture, kinda like air drying in the beginning but with a controlled boost? Seems to me the heat may be sealing moisture exchange too quickly, causing a stress?



From the original questioner:
Thanks. I've had this problem all along. I'm sure it's something incorrect that I'm doing without realizing it, and not something Nyle's kiln does. It's just more complicated than one realizes. You don't just turn it on for such and such amount of time.

I turned the rice cookers on again this evening now that the kiln and lumber is all down to 60 degrees or less. I'm just using the fans and the rice cooker to see if doing it cool allows the lumber to wick up the moisture and keep it. I've had 88% RH for most of the day. So I'll check the surface tomorrow and the next day to see if that makes a difference.

One thing I should add is that the lumber has gained some moisture on the surface, averaging around 4.5-5mc on my Lignomat, and I have done oven dry samples and compared this meter to my other Wagner L606 meter and everything seems to agree.

Another component might be that though my material is air dried I'm sure the avgerage MCs are different by over 5%MC. If this difference is maintained as the lumber dries, some might be overly dried. Maybe these are taking up most of the moisture. Very hard to figure out what is happening.

Thanks for any ideas out there. I do have 7 separate Lignomat probes spaced over the stack, but these tell me the MC at about 3/4" deep, hence I found my cores were wetter than I thought on my old schedules, now I'm drying a lot longer and harder to achieve a 7% core MC.



From Professor Gene Wengert, forum technical advisor:
Equalizing is the process of getting the moisture equal, from shell to core. If there is a moisture difference or gradient, shell to core, then it is impossible to measure the drying stress (or casehardening). So, that is why you are unable to get rid of the stress... you are not measuring it correctly, so you probably have no stress at all.

Air dried lumber has very little stress in it. The reason that stress (also called casehardening) develops is the shell fibers try to shrink, but are restrained by the wet core that is not yet shrinking. So the shell dries in a stretched-out condition. It is not related to drying speed. To get rid of this stress, we add water to the dry shell and so it attempts to swell, but cannot due to the dry core. So, the attempted swelling here offsets the attempted shrinkage at the beginning of drying, leaving us at a stress-free condition.

The hotter the lumber is, the faster the stresses are removed and, if fast, we do not add much water to the dry lumber.

Never steam the lumber at a mid-point or when done with air drying. That will create oodles of problems.



From contributor T:
"The hotter the lumber is, the faster the stresses are removed and, if fast, we do not add much water to the dry lumber."

Gene, my simple mind doesn't agree, but I haven't seen the studies either. Air drying relieves the stress itself due to its slower more equal M/C loss and the day and night temps expanding and contracting the wood. Has this ever been mimicked in studies of kilning? The high heat would case harden it quicker unless the moisture pull can be increased from within.

Now I did notice the steam injection is done in the early stages, which would somewhat mimic the contracting then steam expanding of wood fibers relieving stresses.



From Professor Gene Wengert, forum technical advisor:
More info is in the archives here:
Equalizing and Conditioning Report

Casehardening is a poor word, as nothing is harder than anything else. For that reason, we usually prefer using "drying stress."

Air drying relieves the stress at night when the RH reaches 100%. This is also what happens in a solar dryer. The amount of stress does vary by the speed of drying, so slow air drying develops less stress than drying in a kiln, but still develops more stress than we can tolerate (except for the automatic relief at night). Incidentally, temperature has no huge effect in the amount of stress developed; it is relative humidity mainly, as well as the shrinkage properties of wood.

The stress that develops results because the outside fibers try and shrink, but cannot. This happens very early in drying. Stress relief in kiln drying always occurs at the end of drying, and as I stated, the way it occurs by adding moisture to the outer fibers and they try to swell. Heat makes the moisture move faster, which is why we get faster relief. Heat also helps relax the fibers.



From contributor K:
I've had some good success with steaming the lumber while it was cold. The lumber appears to be keeping the moisture that I've added in this cycle. It seems to me that the problem was that adding water to lumber that is 140 degrees, simply evaporates as the lumber cools unless the environment could be kept at high humidity until the lumber was cold. I'll keep checking over the next couple days to see how it maintains the latest moisture content.


From Professor Gene Wengert, forum technical advisor:
Cooling before steaming is discussed in Drying Hardwood Lumber. It works, but it can be over-done and that creates new problems.

Incidentally, after conditioning, the lumber's surface is a bit wetter than desired. A typical conditioning EMC is 4% EMC wetter than the target MC. So, after conditioning, we let the lumber sit for a day or two and let the surface moisture evaporate. With steam on cold lumber, the water addition to the surface can be somewhat larger than with hotter lumber and so we need to allow a longer time for the surface water to evaporate.

In general, we add moisture only to the surface when conditioning and not to the core. We start conditioning when the core is at the correct MC level. Overall, we might add 1% MC to the average MC of the entire piece, as the surface would typically be a bit too dry after equalization. But with the EMC 4% higher than the target (maybe 6% higher than the surface MC), the surface can safely gain 2% MC. If more than 2% MC, we need to let this excess evaporate. When steaming cold lumber, the surface will be at 28% MC or so, so we do need to evaporate the surface MC after conditioning. We do not want this excess moisture to go into the core, as the core was already at the correct MC when we started conditioning. So, heating or hot lumber will help avoid getting moisture into the core and will accelerate the loss of moisture after conditioning.



From Professor Gene Wengert, forum technical advisor:
I should have mentioned that the prong test, used to evaluate across the grain drying stress (casehardening), only works if there is no drying gradient, shell to core.


From contributor R:
I have designed and installed hot water misting heads along kiln ceiling for this purpose. To release stress during conditioning.

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