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Author Topic: Conditioning Phase of Drying Cycle  (Read 1196 times)

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Offline Downhill Cutter

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Conditioning Phase of Drying Cycle
« on: August 19, 2019, 08:26:08 PM »
I'm relatively new to the Forum.  I tried a few hours ago to float this subject, and I don't see my post (speculating operator error) , so if I'm redundant, I'm redundant. 

I'm just at the end of my first charge in my Nyle L53.  I have 1000-1200 bdf of 10-12/4 red oak slabs reaching 8-12% as measured surface and @3/4" deep with my Orion Moisture meter.  In the beginning of the cycle I successfully case-hardened the load per Seth at Nyle.  Thanks, Seth, for advising me to throw 5 gal of H2O on the floor, turn off the compressor, and heat it up to 140 for a couple of days.  Problem solved. . .  with any luck we killed bugs at the same time, if I understand correctly.  

Question:  Is it advisable for me to run a conditioning cycle on my material?  I don't read much about that, but is it customary to do so?  If I'm right, I'd dump about 1 gal of water on the floor, turn off the compressor, and run it at 120 dry-bulb for a day.  I'm just a little twitchy, as it's my first load and I'm pretty green at this process.

Thanks in advance

Downhill Cutter
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Offline WDH

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Re: Conditioning Phase of Drying Cycle
« Reply #1 on: August 19, 2019, 09:21:37 PM »
I do not purposely condition at the end of sterilization by adding water to the floor, but I do turn off all the fans and shut the vents (compressor is already off during the sterilization phase) and let the load sit and slowly cool down for a day or two to let everything come to equilibrium.   
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Offline YellowHammer

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Re: Conditioning Phase of Drying Cycle
« Reply #2 on: August 19, 2019, 10:28:17 PM »
The 8 % to 12 % spread on moisture for such thick wood concerns me, it should be much tighter at the end of the drying cycle, which may add considerably to the drying time, especially with oak.  

If thats measured from board to board, then I suggest getting them all down to 8% before removing them.  

If its measured internally on individual boards, then they have wet spots, and definately need to stay in the kiln until the moisture gradient in the board is very close.  This may take many days.  If it was white oak, it might take weeks.  

There is no need to dump a gallon if water in the kiln because there is enough that much and more in the lumber itself that will be driven out when reaching sterilization level temps.  I personally dont do a water dump unless its necessary.  

When all the wood is within a percent, or under 8%, jack up the temps to 145 F, close the vents, turn off the compressor, and heat the kiln up and when done let it sit and slowly cool down over a day or two.  The wood will relax and behave noticeable better.  

  


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Offline GeneWengert-WoodDoc

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Re: Conditioning Phase of Drying Cycle
« Reply #3 on: August 20, 2019, 08:10:30 AM »
Conditioning is the process of adding moisture quickly and at higher temperatures to the surface or shell of lumber to cause the surface to try and expand, which (because it cannot expand) creates stress that cancels or offsets the casehardening stress, which was caused by attempted shrinkage early in drying.  (Sometimes, conditioning is the term used for equalization...not a correct use.)

The prong test is used to measure casehardening, but it requires no moisture gradient within the piece of lumber in order to be accurate.  That is why putting it a microwave for 30 seconds or so after cutting, will help create uniform MC.  It is likely that varying moisture at the end of drying means that equalization has not been 
done, so the prong test will not work.

Note that when air drying, the high humidity every morning relieves the stress put in the previous day, so there is no casehardening stress, or need to condition, WITH WELL AIR DRIED LUMBER.  This is why some people report putting a small amount of water on the floor relieves stress...there was no casehardening stress in the first place.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Downhill Cutter

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Re: Conditioning Phase of Drying Cycle
« Reply #4 on: August 20, 2019, 08:56:12 AM »
Thanks, Doc.  You folks are really helping me think-through this process.

FF and its members make a dynamic community.

Thanks to all of y'all for being there.
If you do what you SHOULD do, WHEN you should do it, for Long enough . . . Pretty soon you'll be able to do what you WANT to do, WHEN you what to do it, For the rest of your life (within limits).

Offline scsmith42

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Re: Conditioning Phase of Drying Cycle
« Reply #5 on: August 20, 2019, 01:55:51 PM »
I've had good success with adding moisture via a high pressure misting system.  I dry a lot of oak and was never pleased with the "water on the floor" in my container kiln.  The misting system really works well and combined with a Delmhorst Kil-mo-trol system I can precisely match the core and shell MC%.
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Re: Conditioning Phase of Drying Cycle
« Reply #6 on: August 21, 2019, 11:10:26 AM »
Conditioning is the process of adding moisture quickly and at higher temperatures to the surface or shell of lumber to cause the surface to try and expand, which (because it cannot expand) creates stress that cancels or offsets the casehardening stress, which was caused by attempted shrinkage early in drying.  (Sometimes, conditioning is the term used for equalization...not a correct use.)

The prong test is used to measure casehardening, but it requires no moisture gradient within the piece of lumber in order to be accurate.  That is why putting it a microwave for 30 seconds or so after cutting, will help create uniform MC.  It is likely that varying moisture at the end of drying means that equalization has not been
done, so the prong test will not work.

Note that when air drying, the high humidity every morning relieves the stress put in the previous day, so there is no casehardening stress, or need to condition, WITH WELL AIR DRIED LUMBER.  This is why some people report putting a small amount of water on the floor relieves stress...there was no casehardening stress in the first place.
Okay, I'm trying to wrap my mind around this and my thoughts are not clear or organized. It sounds like conditioning is something to avoid  through proper, "well air dried lumber." I assume that some degree of case hardening occurs in all air drying, but that it becomes detrimental at some point where the case dries too much faster than the the rest of the board. I have a thousand questions about avoiding detrimental case hardening, but I'll start with one;

Is there a way to measure/monitor case hardening? Let's assume I measure the moisture at approximate pin depth of 3/8" and again at 1/3 or 1/2 the depth of the board with nails or hammer pins. Is there a percentage difference that can warn me that detrimental case hardening is a threat?
Thank you for sharing your expertise with noobs like me.

Offline GeneWengert-WoodDoc

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Re: Conditioning Phase of Drying Cycle
« Reply #7 on: August 21, 2019, 11:20:52 AM »
Nope.  No easy way, as there is a MC gradient that makes the prong test invalid.  BasicallY, air dried lumber gets a small amount during the day and relieves it at night.   Casehardening is created the first five to ten days of initial drying, when the shell is trying to shrink but the core is still too wet to shrink.  The details are discussed in DRYING HARDWOOD LUMBER.
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Offline YellowHammer

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Re: Conditioning Phase of Drying Cycle
« Reply #8 on: August 21, 2019, 06:22:13 PM »
The moisture gradient in wood and its effects on the drying rate and for that matter, the final behavior of the piece is a very complex relationship.  To make matters worse, each species as it own characteristic moisture loss "signature" and becomes very apparent when air drying or using low temperature dehumidification kilns.  So tests like the prong test, although very useful (I run them routinely) are invalid until there is virtually no moisture gradient, but getting to a no moisture gradient is very difficult in some species and thicknesses, which is what prompts the desire to run a prong test in the first place, to try to understand what is happening to specific pieces, or kiln loads of wood.  

The properties of each species moisture releases rate and depth of release rate, which is a function of the actual cell walls and how they collapse, is something that you will develop a "feel" for with experience, but is definitely not as simple as throw a bucket of water in the kiln.

For example, some species, such as cherry, will absolutely drop water out, with no restriction, virtually independent of thickness of for that matter temperature.  It will have a vey steady release rate, from core to shell and develops virtually no gradient, even on thick wood, whether it was air dried initially, put in the kiln, or whatever.  Interestingly enough, cherry doesn't even have a very high allowable moisture release rate.

Walnut, on the other hand, has a very high allowable moisture release rate, leading one to believe that removing moisture from it will be easy, like poplar.  However, due to many reasons, walnut is one of the most ill behaved wood in a low temp DH kiln, and frequently "locks up" during the drying process, sometimes having huge moisture gradients in boards.  I've had it as high as 18% MC in one board and the board next to it in the pack be at 7%.  Add the fact that walnut is usually is very internally stressed wood to begin with, then thats why it is such a pain to dry correctly, get a low moisture gradient, get it done in a reasonable time, and have straight boards.  Walnut is a pain.

Other species can be difficult also, just when you think to have them on the ropes.  Maple, for example, will trick you.  It has to be dried fast to keep from sticker staining, but sometimes the high delta in the wet bulb/dry bulb will cause all kinds of moisture gradients, that need to be addressed later in the cycle, once past the sticker stain stage.

In your case, thick oak is a real challenge, red oak having a tendency to lock up about halfway or near the end of the cycle, when air dried initially, making it very difficult to achieve any kiln condition that will allow it to dry close to its max drying rate.  It really has to be beat into submission at the end of its cycle.  Interestingly enough, it has a completely different drying characteristic from green, especially in the beginning of the cycle, when it wants to seriously overdry and blow through its max drying rate curve and surface check before you know whats happening.  White oak is red oak on steroids, and thick white oak is even worse still.

There are ways to overcome these obstacles, but different approaches for different problems.  From a practical standpoint, there are only so many knobs you can turn once the wood is in the kiln, such as heat with compressor, heat with no compressor, very high heat with no compressor, time, frustration and patience when the wrong method is chosen.  Oh yeah, not to mention the electricity bill.

There are things to do when air drying, also, and in that case, Mother Nature can help or hurt, and I try to pick the wood that I'm sawing based on all of that, if possible.  For example, I just milled up about $80,000 retail bucks of 9/4 walnut because it is now a perfect time to air dry it and get it down in a hurry without wasting time in my kilns.  Near 100F temps every day outside in Alabama, very high humidities at night, going into the end of summer when it gets dryer and dryer every week.  Perfect for thick walnut.  A natural kiln schedule.

However, these are just about the worst conditions for maple.

Anyway, there are many times that a load of wood seems to fit the description of case hardening, but in reality, in my experience, it there are many more variables involved that manifest themselves as issues with drying wood, and case hardening is just a good catch all term that encompasses many of them.  Some times drying difficulties and the inability for a piece of wood to come down in moisture gradient is as simple as it has a cold 😁.  Or rather it has a bacterial infection and won't release its moisture.




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Re: Conditioning Phase of Drying Cycle
« Reply #9 on: August 21, 2019, 06:56:44 PM »
There are ways to overcome these obstacles, but different approaches for different problems


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Offline WDH

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Re: Conditioning Phase of Drying Cycle
« Reply #10 on: August 21, 2019, 08:19:12 PM »
That is the last resort.
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Re: Conditioning Phase of Drying Cycle
« Reply #11 on: August 21, 2019, 10:42:28 PM »
Yep, when its time to implement the magic and Vodoo techniques, its pretty much a last resort.  
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Re: Conditioning Phase of Drying Cycle
« Reply #12 on: August 22, 2019, 07:12:12 AM »
That part is an art, not a science as it is not included in Drying Hardwood Lumber. 
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Offline scsmith42

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Re: Conditioning Phase of Drying Cycle
« Reply #13 on: August 22, 2019, 07:21:42 AM »
Robert, incredibly well written post above.  I salute you!
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Offline boardmaker

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Re: Conditioning Phase of Drying Cycle
« Reply #14 on: August 22, 2019, 10:53:51 AM »
I agree with Scott, great post Robert.

Scott, do you have any way of tracking how much water your are adding with your system.  I'm curious?

Offline scsmith42

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Re: Conditioning Phase of Drying Cycle
« Reply #15 on: August 22, 2019, 01:18:18 PM »
I agree with Scott, great post Robert.

Scott, do you have any way of tracking how much water your are adding with your system.  I'm curious?
I dont have anything in place, but it would not be that hard to put a meter on the incoming water line.
Mine is controlled by a separate RH sensor.  When conditioning, I can enter whatever RH% that Id like and the system will maintain it.
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Re: Conditioning Phase of Drying Cycle
« Reply #16 on: August 22, 2019, 01:53:15 PM »
What kiln system are you running Scott?
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Re: Conditioning Phase of Drying Cycle
« Reply #17 on: August 23, 2019, 04:47:44 PM »
What kiln system are you running Scott?
I have four solar kilns and two Nyle 200 series (one not installed).  The fogging system is in the Nyle.
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Offline Downhill Cutter

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Re: Conditioning Phase of Drying Cycle
« Reply #18 on: August 31, 2019, 06:20:57 AM »
I feel like I've just been sat down and talked to by a bunch of Uncles . . . who KNOW stuff.  

Beautiful post, YH.  Thanks so much!

Thanks Dr Gene!

And Thanks, Scott!

Jerry Cox

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Re: Conditioning Phase of Drying Cycle
« Reply #19 on: September 12, 2019, 08:54:16 AM »
Walnut, on the other hand, has a very high allowable moisture release rate, leading one to believe that removing moisture from it will be easy, like poplar.  However, due to many reasons, walnut is one of the most ill behaved wood in a low temp DH kiln, and frequently "locks up" during the drying process, sometimes having huge moisture gradients in boards.  I've had it as high as 18% MC in one board and the board next to it in the pack be at 7%.  Add the fact that walnut is usually is very internally stressed wood to begin with, then thats why it is such a pain to dry correctly, get a low moisture gradient, get it done in a reasonable time, and have straight boards.  Walnut is a pain.


Tru'dat! I just took some 5/4 walnut out that was measuring 6-8%.  Unstickered it and did some spot checks with the pinless meter: 10-12%. "Naw, this can't be right..." Got out the trust Delmhorst pin meter and yep...was right. :-X Back in the kiln with it. :-\
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