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Author Topic: Jim Fuller's ideas  (Read 7744 times)

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Offline old3dogg

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Re: Jim Fuller's ideas
« Reply #20 on: March 30, 2004, 11:30:10 AM »
In a vacuum the water is moved from the center of wood to the surface.How fast you evaporate from the surface dictates how fast you can move water from the surface.
So in vac drying:Slow at the begining,a little faster toward the middle and after the MC is under 20% it really doesnt matter.

Offline old3dogg

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Re: Jim Fuller's ideas
« Reply #21 on: March 30, 2004, 11:32:27 AM »
Thanks Tom.
I ill ceeck it ouy! :D

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #22 on: March 30, 2004, 04:20:02 PM »
I'm home again.

In vacuum drying, evaporation is not limited to the surface. The better the heat transfer technique, the deeper into the wood the drying goes. If you use too much heat in an effort to force heat transfer, you create the stress that Mike is talking about.

Mike,

If you apply what I taught you years ago (I know it's hard with the control systems you have), your old kilns could dry as almost as fast as my new kilns. The answer is pressure control.

Jim,

The pressure control consist of water vapor pressure as set by water temperature and the gradient to chamber pressure. With the method we use (patent pending), we use the gradient to minimize moisture gradient. It's very effective.

Den

Offline jimF

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Re: Jim Fuller's ideas
« Reply #23 on: March 31, 2004, 09:30:48 AM »
Den,
whats your patent application number?

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #24 on: March 31, 2004, 09:55:32 AM »
Jim,
Our Do-all girl (Kelley) is doing the paper work from where I left off some time ago. I'll let you know when I know. There's been an exchange with the patent office but I'm not involved.  :D
Den

Offline jimF

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Re: Jim Fuller's ideas
« Reply #25 on: April 02, 2004, 01:08:39 PM »
Den,
Are you  saying there is a pressure gradient in the void of the chamber? or pressure gradient  in the wood?  How are you measuring it?
If you are applying heat continuously by platens,you are only evaporating water from the liqiud boundary  surface, not from the surface of the wood nor throughout the whole peice of wood.  It is just like at atmospheric pressure just at a different pressure.  Heat transfer is the same not matter what the pressure is..

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #26 on: April 02, 2004, 01:35:58 PM »
Jim,

The gradient is between the vapor pressure of the water (that is set by the temperature of the water) and the chamber pressure.

We're applying heat continuously but there's more than one way to skin the cat.

I can set the boiling point with the chamber pressure. I can put an RTD in the core and verify that the change in boiling point occurs at the core.

Den

Offline old3dogg

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Re: Jim Fuller's ideas
« Reply #27 on: April 02, 2004, 02:36:08 PM »
I would have to agree with Den on this one.I have seen the core temp of squares dried in a vacuum the same as the shell temp.I feel that if you can get the core and shell temp even during drying then you can dry without honeycomb and stress.
Heck!I have even seen this done in an old RF/V kiln!

Offline jimF

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Re: Jim Fuller's ideas
« Reply #28 on: April 02, 2004, 05:34:28 PM »
Den,
It sounds like what you are calling pressure gradient is the partial vapor pressure in the chamber compared to the total pressure of the chamber.  Is that what you are talking about?  This would tell you the percent vapor in the total vapor/gas composite.  This is a measure of humidity, though not relative humidity.
By applying heat to the surface the heat has no other way to get into the wood other than going through it.  For this to happen there must be a temperature gradient thrugh the wood.  Heat energy only travels from high temperature to low temperature.  No matter how small of a temperture difference there is, it still has to be there for heat to be transfered.

How are you measureing the temperature?  How do you put the RTD in the wood?  How do you seal it up?  If you drill a hole in the wood, the bottom of the hole is the outside of the wood.  No matter how deep the hole is.  It has direct exposure to the chamber pressure.  The hole would have to be absolutely completely sealed up with the RTD in it for the RTD to accurately record temperature. That is almost impossible to do.  Wax is too brittle or is not able to adhere to wood well enough.  Most polymeric material will not adhere to wet wood well enough to seal.
It's like when my wife checks her baking with a thermometer, continues to bake it and then rechecks it in the same hole.  That hole had become the surface of the cake becuase it is exposed to the oven air.  It always "seems " to cook faster than when she rechecks in a different spot.  Because that new spot was not exposed to the oven air between the two times.  Or consider ...the inside of a balloon becomes the outside of the balloon when you pop it.  The internal pressure becomes the same as the external pressure and collpases.
Old3doff,
If the frequency of the RF in a RF?VAC kiln is just right, the energy seems not to totally absorbed at the water boundary layer, but some of it is able to penetrate deeper.  This enables some RF/VAC kilns to heat more evenly.  However, the energy is still going from a high energy state to a low energy state,  RF tube to the interior of the wood.  Since the dry outside portion of the wood does not have water in it to absorb the energy, it does not heat up.
Boy, this discussion gets the old noodle going. jim

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #29 on: April 02, 2004, 07:17:32 PM »
Yep. if you obey the gas laws, it partial vapor pressure.

If you press in an RTD with a plastic sleeve, you get a tight fit. Especially when the wood begins to shrink. And vapor is the conveyance.

The frequency of an RF oscillator is rarely 'just right' because the kiln charge is part of the circuit and it changes as MC goes down.

Offline jimF

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Re: Jim Fuller's ideas
« Reply #30 on: April 03, 2004, 06:27:26 AM »
I agree with your assertion that the wood is part of the circuit and rarely is the frequency right.  That is why I part "seems" in my post.  This is what a campany claims and I have no way of proving or disporving it.  It is common to get burned araes just below the surface.  This is where and when the liquid boundary layer is slightly below the surface and all the energy is absorbed there.  The heat can not be transfered quick enough to the center or surface, even in a vacuum system that is conntrolled by vapor is the conveyance.
Have you tested your plastic sleeve with a pressure probe inserted?  I have a hard time getting a consistant seal with compression fittings on tubes let alone in wood.

Offline old3dogg

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Re: Jim Fuller's ideas
« Reply #31 on: April 03, 2004, 10:56:53 AM »
jimF.
Lets say I put a load of wood in a vac kiln.I set the heat at 100 and the WB at 100 and then I pull the CP.I get down to lets say 40torr.The water at the surface of the wood cant go anywhere because of the WB being at the same temp as the temp of the wood wont let it evaporate.
This I feel will bring the whole load to the same temp.Core and shell.
Just a thought.

Offline jimF

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Re: Jim Fuller's ideas
« Reply #32 on: April 05, 2004, 07:15:07 AM »
At 40 torr and 100/100 how much water are you producing /BDFT/hr?

Offline jimF

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Re: Jim Fuller's ideas
« Reply #33 on: April 06, 2004, 06:12:04 AM »
old3dogg,
Where do you measure the wetbulb temp and what temperature are the heating platens?

Offline jimF

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Re: Jim Fuller's ideas
« Reply #34 on: April 09, 2004, 12:58:17 PM »
I never did get to the conclusion I was pointing to.  Being drying stresses are the limiting factor in drying, which causes surface checks and honeycomb, and moisture content nor gradient is a good measure of drying stresses, why measure moisture content?  (Stain and mold all dictate to dry very quickly)  What should be monitored is something that indicates drying stress - strain fits the bill.  Strain is directly related to stress.  But what stress are you looking for?  Often what is called stress reversal is mentioned when discussing drying stresses and is one reason in standard schedules a change in settings is dictated.   But what you really are looking for is a reduction in stress, not a complete reversal.  This point I call peak stress and the idea was first mentioned by a person called Reitz.  He said it occurs approximately when an 1/8 of the original MC is loss but an axact figure can not be given because it is not directly related to moisture content.  This is the point in the standard schedules that the first change in settings is dictated.  If one measures strain you can determine when peak stress occurs and increase the drying rate at that point and safely avoid and degrade.

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #35 on: April 09, 2004, 03:14:17 PM »
Jim,

I see what you're saying but I think the stress is caused by the moisture gradient. Maybe I'm oversimplyfying but, with a change in water volume, cells shrink. If all cells don't change volume at about the same rate, some shrink more than others. If too many shrink too much, stress is created. Too much stress and you end up with kiln degrade.

In conventional drying, the surface has to be dryer than the core to get the water out of the core. Therefore, you have to create some stress. So, as you say, you can measure the stress to control drying. But isn't the stress caused by MC gradient?

Den

Offline jimF

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Re: Jim Fuller's ideas
« Reply #36 on: April 10, 2004, 01:06:40 PM »
Yes, the stress is caused by any moisture gradient and with TOO much stress you end up with kiln degrade (surface checks, honeycomb and collpase).  But but, drying wood does not act as a simple mildly stretched steel spring where it returns to the original position when the force is released.  That is called elastic strain, think rubberband. Similar to a steel spring when it is stretched  too far, the spring remains stretched, as does wood during drying.  Other types of strain occur during drying.  Viscous strain and visco-elastic strain occur over time during drying - drying takes time.  Another strain called mechano-sorptive strain occurs when a material loses moisture and becomes stretched.  I know these are big technical terms but the point is that wood does not behave simply like a new car spring during drying.  It is because of these different types of strains that occur during drying that you can not use moisture gradient.  We  do not know the values of these strains to be able to predict stress level when a so and so moisture gradient occurs.  More than a few people have performed many experiments to try to get these values and have not succeeded yet.  A few people have tried to use moisture gradient for control and have not gotten very far. Neither moisture content nor moisture gradient tells you enough to accurately control drying.  Current drying practices are just a guesstimate of what is going on.
It is because of these various types of strains that you end up with less stress after conditioning if you push the wood to the limit but not damage it by producing surface checks.  Many people say that kiln dried wood is cooked and cuts differently than air-dried wood.  Don't confuse this concern with pushing the wood to the limit.  Realize a commercial kiln is very large. They can be 20 feet high.  The top portions of the kiln receive air that is hotter than the wood on the bottom and where the temperature sensors are.  The difference can be as much as 8F.  When the EMC at the bottom is 4%, an increase in temperature at the top by 8F can reduce the EMC down to 1-1.5%  This is one of the sources of kiln dried wood being of possible lower quality than air-dried wood.
jimF



Offline jimF

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Re: Jim Fuller's ideas
« Reply #37 on: April 10, 2004, 01:18:39 PM »
Den,
You say "In conventional drying..." and that "you can skin a cat more than one way" to get heat into the wood.in vacuum kilns.  I'm still not convinced vacuum drying is totally different than conventional.  Yes, it occurs faster but.  All energy and mass transfer requires a temperature or pressure difference.  It's been stated here that the chamber temperature, wood surface temperture, and center of the wood are all the same temperature.  That the pressure in the chamber and in the center of the wood are all the same pressure.  Can you explain how you can skin the cat differently? ie getting energy into the center of the wood.
jimF
PS Is everyone that visits this board vacuum driers or is this just a good drying board?

Offline Den Socling

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Re: Jim Fuller's ideas
« Reply #38 on: April 10, 2004, 02:25:48 PM »
Jim,

I don't think that anybody who participates in this forum uses a vac kiln except Mike and I.

I don't know if my explanation of other ways to skin the ol' cat would have any effect on me trying to patent cat skinnin' or not but I think I'll keep it private, for now. It's the same stuff I've been expounding for years.

Den

Offline stevareno

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Re: Jim Fuller's ideas
« Reply #39 on: April 11, 2004, 10:12:56 AM »
Jim,
I've been reading this thread with interest.  We just ordered a vacuum kiln from Dennis so I'm trying to absorb everything I can get my eyes on.   Thanks for sharing some good information!

Steve


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