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Author Topic: Vacuum Drying in a Bag  (Read 19177 times)

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Offline JasonRerras

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Re: Vacuum Drying in a Bag
« Reply #80 on: September 19, 2018, 08:54:49 AM »
From what I've read, a heating pad can work, but the hot water bath is superior.

There's a local turner here in Virginia Beach that I've met that did his Masters Thesis on drying in a microwave. 

Thinking the easiest thing for me to do right now is going to be build a small DH kiln. 

Thanks for all the ideas.
MS880 60" CSM and stuff

Offline GeneWengert-WoodDoc

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Re: Vacuum Drying in a Bag
« Reply #81 on: September 20, 2018, 03:35:27 AM »
The rf-vac kilns use a lower frequency Rf in order to avoid somewhat heating the wood too hot in one spot and causing a fire or rapid charring.  Also, the issue with high Rf frequencies is trying to get them to hit the load uniformly.  Why does a home microwave have a rotating trey...because we know the microwave is not even in the oven. Even so, part of a item in a MW will be very hot and another, cool.

Therefore, generating the Rf including the microwave is one thing and then conveying the "waves" into the kiln structure is another.  The uneven distribution of Rf has led to several issues including uneven final MC, hot spots with charring, wet pockets, and high equipment costs to get the Rf somewhat uniform throughout the chamber.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Don P

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Re: Vacuum Drying in a Bag
« Reply #82 on: September 20, 2018, 07:40:13 AM »
Gene, I'm wondering if the vac bag being sucked tight to the surface of a slab wouldn't force all moisture to exit through the wood from the small area right in front of the exit tube. That's where I was wondering if some form of "mesh" wrapping the wood is needed to let the released vapor travel to the exit freely. I'm thinking something like one of the foundation drainage mats. but, in doing so the wood is then insulated from conduction from outside.

Even using FM radio waves in glue presses depending on energy input/ moisture/ conductivity/ cycle time, you can char or start a fire. When we first got one in a component shop the learning curve was kind of steep as we learned just how many variables there were. It was interesting to open up a glueline and see a multi fingered lightening strike through it, whoops! 
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Online Crusarius

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Re: Vacuum Drying in a Bag
« Reply #83 on: September 20, 2018, 07:48:40 AM »
What about placing MDF between the bag and board? Probably wouldn't last long but would definitely allow the water to escape.

Online Crusarius

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Re: Vacuum Drying in a Bag
« Reply #84 on: September 20, 2018, 07:50:45 AM »
The real question is "Are we over thinking this"? When I did my little experiment with a vacuum chamber the center of the board dried nicely while the ends stayed moist. my thought was the vacuum was pulling the moisture out through the end grain.

Offline GeneWengert-WoodDoc

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Re: Vacuum Drying in a Bag
« Reply #85 on: September 20, 2018, 08:14:42 AM »
When vacuum drying without RF, the vacuum lowers the boiling point of the liquid water (moisture above 30% MC).  For example 103F is the boiling point.  When the water in the wood boils, it creates pressure and so the vapor is forced out of the wood through the many pathways, mainly lengthwise, (technical term is mass flow), compared to much slower vapor diffusion in "normal" drying.  The pressure difference between core and outside the wood is the one key to the speed as well as the permeability of the wood.  Red oak has an open structure so mass flow is easy and fast; black locust, Osage orange,and white oak are quite impermeable. So mass flow is slow and the build up of pressure can actually damage the wood.

The early RF vacuum systems had such variability of final MC due to RF variability, incoming MC variability,  and wood permeability variability, that they only worked reasonably well in production situations if the wood was air-dried first to a uniform MC.  Obviously, this approach made them impractical.

RF gluers do indeed have issues especially when the incoming wood MC is variable.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Spacegrey

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Re: Vacuum Drying in a Bag
« Reply #86 on: January 22, 2019, 10:05:55 AM »
Hi All, I'm new to the forum.
I'll be a next guy trying Vacuum bag method
Wish me luck :)

Offline boardmaker

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Re: Vacuum Drying in a Bag
« Reply #87 on: January 22, 2019, 10:13:12 AM »
Welcome Spacegrey,

Let us know what you find out and what process you plan on using.  You may want to start your own thread.  Be great if you gave us feedback as you're doing it, as some of us may be able to help. 

Good luck
Lucas

Offline Spacegrey

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Re: Vacuum Drying in a Bag
« Reply #88 on: January 22, 2019, 01:09:14 PM »
Thanks, Boardmaker.
I'll start a new thread if/when I got some results worth sharing.
For now I got a vacuum pump, build vacuum reservoir, finally got figured out how to make my DIY vacuum bag completely sealed and how to make air tight lines to connect it all.
Next is to get it all assembled and start testing.

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Re: Vacuum Drying in a Bag
« Reply #89 on: January 22, 2019, 01:26:16 PM »
Don't forget gauges and temperature sensors (thermal couples)

All that information would be very helpful. I have been playing with this a little in the vacuum chambers at work.

Offline boardmaker

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Re: Vacuum Drying in a Bag
« Reply #90 on: January 23, 2019, 09:27:36 AM »
I wonder how cookies would dry in a bag.  If they would still crack as bad or if drying in the vacuum would help with the stresses.

Offline YellowHammer

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Re: Vacuum Drying in a Bag
« Reply #91 on: January 23, 2019, 09:43:39 AM »
I’m very interested in this also, it occurs to me that if a simple vacuum bag or other approach can reduce the green moisture in thick boards,  bring the moisture down to the fiber saturation point without the addition of auxiliary heat, then the boards can then be safely finished in one of our DH kilns. It could save months of initial air drying time, especially for one of a kind high value slabs I want to cycle and sell quickly.  
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Offline Spacegrey

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Re: Vacuum Drying in a Bag
« Reply #92 on: January 24, 2019, 01:33:34 PM »
I wonder how cookies would dry in a bag.  If they would still crack as bad or if drying in the vacuum would help with the stresses.
Can be done with no cracks if frozen and then vacuumed to sublimation (aka Freeze-drying)


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