iDRY Vacuum Kilns

Sponsors:

Vac Kiln - Can It Be Used to Alter Board Shape?

Started by YellowHammer, January 27, 2020, 09:55:13 AM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

YellowHammer

Can a Vac Kiln be used to flatten, bowed wood off the sawmill?  Does the wood that goes into a vac kiln come out the same shape as it went it?  For example, if a bowed and stressed board goes into the vac kiln, will it come out equally bowed and stressed, less, or more? A major selling point of Vac Kilns is that they will not induce drying stress, but can they be used to alter stress? 

I have a continuous interest in producing exceptionally flat wood, no bow, and have had some success using special techniques with my air drying, DH kilns, lumber sticking, etc to relax and flatten bowed boards, and have been asking questions about steaming, also.  However, I haven't researched the issue with Vac Kilns.

Any thoughts?  

YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

boonesyard

With my limited experience (2 + months of use) with the vac kiln, it does not induce stress and the wood does not change shape. If it goes in flat, it comes out flat. If it goes in bowed, it comes out bowed. 
LT50 wide
Riehl Steel Edger
iDRY Standard kiln
BMS 250/BMT 250
JD 4520 w/FEL
Cat TH255 Telehandler
lots of support equipment and not enough time

"I ain't here for a long time, I'm here for a good time"

doc henderson

@YellowHammer I have done a little steam bending.  my understanding is both moisture and the heat are needed to soften the fibers and loosen the lignin glue.  in a vacuum water boils at a lower temp, so no extra moisture and no extra heat.  one issue with steam bending is memory so you usually bend tighter than needed, i.e. over bend so it ends up where you want.  interesting to make a big steam press that could heat boards and inject steam and apply pressure like a hydraulic press to straiten high value boards.  it would be an energy sump trying to heat the wood, make the steam and generate the force.  could use steam or air pressure instead of traditional hydraulic pressure.  or make a heating steam chamber then place in a press to cool and dry.  this may help permanently alter the position of fibers relative to each other.  like a micro laminated beam that hold its shape as it is fastened to the neighboring fibers.
 could use clamps/all-thread in a jig, or weight on top of the wood on a flat slab, instead of a press.  i can see it now, people dropping off their wood to be pressed.  "I will pick it up on Monday!"   :)
@GeneWengert-WoodDoc ,  @Southside  , @WDH .  lets get this party started!!!   :D
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

GeneWengert-WoodDoc

Just like taking flat boards and bending them curved, we can do the same with warped lumber.  However, bending is done above 22% MC for severe bends and above 16% MC for gentle bends.  Also, when bending, you bend a little further than needed, as there is always spring back when the pressure is relieved.  So, all these requirements make bending of warped dry lumber difficult.  However, lumber thaT is warped right when dawn, you can bend it flat, but some pieces will need more bending pressure than others, so you would have to do each piece separately and the amount of bend would be somewhat guess-work.  You also may notice more movement than normal once the 15% MC or higher is dried to the 7% MC in use.  also, there is still stress within the board, so you cannot rip and crosscut or heavily machine the piece without seeing a lot more warp.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

doc henderson

@GeneWengert-WoodDoc is there a temp above which combined with moisture, that the spring back would not occur.  You have spoken about stress relief by letting the temp drop and humidity come up in a solar kiln (case hardening)  although that is due to the differential MC in the center and the outside.  Like in a hot box with steam.  then flattened?
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

YellowHammer

As an example, we mill a lot of cherry, and it is one of the more bow prone woods.  So even after careful processing, there still is a certain percentage of bowed boards in every load.  As we deadstack each bundle, we take all the noticeably bowed boards, and rwsticker them in specific patterns.  After several kiln runs will eventually amass a full pallet, maybe 800 bdft of bowed cherry.  Then, we do our thing with our Nyle kilns to them, and about a week later, we unsticker them and about 50% to 70% are now flat.  Thats a huge percentage.

YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

GeneWengert-WoodDoc

YH...
MAYBE...I do believe that much of the bow you notice is due to a difference in MC from one side to the other, likely due to differences in drying rate.  Have you noticed that they tend to bow toward the bark?  So, by dead stacking, you actually are allowing the moisture gradient to equalize, which allows the shrinkage on both faces to equalize and the piece becomes flat.  This bow I often seen when we dry at cooler temperatures, compared to 160 F in a steam kiln...the heat seems to provide more equal MC within the lumber.

DOC...
Springback occurs because wood is elastic before the stress becomes a. permanent shape deformer (sometimes called the pastiche range).  Kinda like a rubber band...you can stretch it a little, then release the stress, and it will return to its original shape.  But stretch is really far and hold it there for a while, and it will be larger forever.  Temperature changes the point at which elasticity stops and plastic flow begins, called the proportional limit, but not all that much to make it an important part of a processing procedure.  

You see the same effect with a wooden tongue depressor...it is elastic if you do not push too hard and so it springsback to flatness when you take it out of a patient's mouth. 
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

YellowHammer

Part of my specific flattening cycle is to spend a good deal of time, several days to sometime over a week, at 160° to 170°F with several thousand pounds of weight on the stack of bowed lumber.  I have not seen a moisture gradient in the boards, but there is always that possibility with some of them.  I check the bowed kilns loads routinely, and when I can lift the weights off the stacks with a forklift and they don't expand like an accordion, then I know its working.  If not, I replace the weights on top and give it more time and heat.  Some of the boards have sapwood, some do not.  At some point, I run out of patience, time, or whatever, and unstick the stacks and put the now flat boards into out normal process and the refusing to flatten boards into another, much higher effort to process, stream.  So every board I can successfully flatten is a significant labor savings to me. 

Most of the boards are identified as being stressed right off the mill when sticking.  I always segregate the green and stressed (bowed) boards from the dead flat boards when we initially sticker them, so will have two distinct piles off the mill from the same species, even the same logs. We even label them differently in our database to be aware of their characteristics.  The green flat boards are handled differently from the green bowed boards, and we use a different drying and finishing process for them because getting to the higher temps in DH style kilns is very hard on the kilns, even if they are not operating at that temperature, and its one of the reasons I keep my L53, among other things, as it is used and abused as a flattening kiln.  I have tried doing this process at lower temps, but the hotter I can get, the noticeably flatter I can make the wood.  

I have no experience with steam cycle kilns, so still am not sure exactly what the mechanism I'm doing really is, but the best I can describe it as "elevated temperature wilting" or putting a "set" in the wood.  Some of the boards from our routine drying also are culled, and added to these stacks, to make a total stack of problem boards.  Those particular boards may have moisture gradients, but the rest of the boards in those non stressed low temp kiln dried stacks turn out well and very flat and go on the sales racks.

I do not inject humidity, but do close all the vents, turn up the heat, turn off the DH unit, and give the wood a little quality time to relax and get squashed using the combination of heat, tonnage and time.   

If I could get to even higher temperatures, I would do it, much the way @doc henderson describes, I would like to high temperature straighten the wood.  Interesting comments by @Wood Doctor as well.  That sheds soem light on the process.    

Our techniques works reasonably well, and overcomes some of the limitations of a low temperature DH kiln, but still leaves some to be desired.  I'd still like to improve it.  There are just some boards I can't flatten, although I am tempted to try to build a chamber (hence my questions on a steam chamber in another post) that will get to even higher temps to try it out.  Since what I'm doing is hard on my DH equipment, I'd like to build a specific "flattening chamber" if I knew exactly what I wanted to build.

Some of the boards, especially walnut, if there is sapwood, will have lot of internal stress off the log, sometimes several inches of bow that we could never get to flatten and would be useless wood for us.  However, high temp steam kilns that operate at higher temps routinely are able to use this type of wood and it comes out flatter than I can do with a conventional low temp DH cycle.  I have enough good results with this technique where we can now specifically process and sell our highly sapwood stressed walnut boards as a relatively flat and profitable product.

Trouble is, I'm stuck in a gray area where my results seem to have hit a wall, there is certain percentage of wood I just can't flatten, and I'm looking for alternatives and knowledge.

Perhaps the process I am using is utilizing the heat to drop the wood out of the elastic into the plastic range to an accessible force level where my weights have an effect on the wood.  Using Gene's analogy of the UTS curve then I am able to permanently deform the wood.  It may also be a combination of high temperature equalization.  Or both.  Either way, I'd like to permanently deform the wood in a much shorter time frame.

  

 
YellowHammerisms:

Take steps to save steps.

If it won't roll, its not a log; it's still a tree.  Sawmills cut logs, not trees.

Kiln drying wood: When the cookies are burned, they're burned, and you can't fix them.

Sawing is fun for the first couple million boards.

Be smarter than the sawdust

doc henderson

I think you need heat and humidity, then a force to flatten.  the trick (that is too tedious on a large scale) would be trying to over bend a board to make it permanent.  I do this on a small scale with my engravings.  heat to 160 and then flex by hand and get good results.  these are thin pieces 1/4 to 1/2 inch, that I can hold in my hand.  you should build a small steam chamber and play with steam bending.  assuming you have not already done this.  as you know, some wood is more amenable to bending than others.  I have bent walnut and oak.  I am sure there will be changes in the wood as well.  I remember using a flat iron and a wet paper towel to raise grain in a dented board.  permanent swelling of the fibers.
Timber king 2000, 277c track loader, PJ 32 foot gooseneck, 1976 F700 state dump truck, JD 850 tractor.  2007 Chevy 3500HD dually, home built log splitter 18 horse 28 gpm with 5 inch cylinder and 32 inch split range with conveyor powered by a 12 volt tarp motor

Stephen1

Quote from: YellowHammer on January 27, 2020, 09:55:13 AM
Can a Vac Kiln be used to flatten, bowed wood off the sawmill?  Does the wood that goes into a vac kiln come out the same shape as it went it?  For example, if a bowed and stressed board goes into the vac kiln, will it come out equally bowed and stressed, less, or more? A major selling point of Vac Kilns is that they will not induce drying stress, but can they be used to alter stress?

I have a continuous interest in producing exceptionally flat wood, no bow, and have had some success using special techniques with my air drying, DH kilns, lumber sticking, etc to relax and flatten bowed boards, and have been asking questions about steaming, also.  However, I haven't researched the issue with Vac Kilns.

Any thoughts?  


I Have flattened wood from the mill in my VK Put it at the bottom of the pile. Tonnage! I watch the slabs comming off the log and it seems the outer pieces with the sap wood move more. they go at the bottom. 
I have straightened some warped boards out of the kiln by doiing the same thing, cup down, back in the kiln at the bottom of the pile, Tonnage. Because I custom dry mostly thicker wood, slabs, 6/4 8/4 and such, and only been at it for a while, I haven't gathered enough experience to know if it really works. 
My thoughts are by adding green wood and dry or almost dry wood into the kiln at the same time , it increases the length of time before that dry wood is down to the 8%. The green wood introduces a lot of moisture into the kiln which is then absorbed by the the dryer wood, especially the outer shell. I think it also allows the wood to bend and flatten. 
The temp in VK runs in the 130-150 range, when it finnaly gets "hot" 160 that is the upper temp. 

My thoughts. 
IDRY Vacum Kiln, LT40HDWide, BMS250 sharpener/setter 742b Bobcat, TCM forklift, Sthil 026,038, 461. 1952 TEA Fergusan Tractor

GeneWengert-WoodDoc

Warp in green lumber is due to stress in the tree, called growth stress.  Indeed, heat does relieve some of this stress, but is not 100%.  

Growth stress, the cause of most bow, has been an issue and has had tons of research, including initial steaming and heating, with limited success.  YH has the most reasonable approach indeed...we cannot afford much more.

There was one company in Australia that had a warped frame for stacking and they stacked the warped lumber so it was bent a little more than flat.  It worked but not 100% and was labor intensive.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

WDH

It is aggravating, but I cut the bowed boards into shorter lengths to take out some of the bow, then run them over the jointer to flatten one face, then plane flat on both sides.  Adds one more step to planning, but is fairly efficient without having to have separate bowed stacks and extra time and heat in the kiln.  The reason this works for me is that I sell a lot of boards less than 8' long, and on all these shorter, jointed one face before planing, boards are sold at full price.  Most of them turn out nice.  But it does take time and that time takes away from other things that take time too.
Woodmizer LT40HDD35, John Deere 2155, Kubota M5-111, Kubota L2501, Nyle L53 Dehumidification Kiln, and a passion for all things with leafs, twigs, and bark.  hamsleyhardwood.com

Thank You Sponsors!