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Author Topic: Vacuum Drying in a Bag  (Read 18010 times)

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Offline logboy

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Re: Vacuum Drying in a Bag
« Reply #60 on: October 25, 2011, 01:58:52 PM »
Do you know what isimitibiti was using?
I like Lucas Mills and big wood.  www.logboy.com

Offline Den Socling

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Re: Vacuum Drying in a Bag
« Reply #61 on: October 26, 2011, 11:21:02 AM »
No I don't know what he used but a "chamber" might be made from plastic sheathing with duct taped seems. The tricky part would be the connection for the pump.

Offline logboy

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Re: Vacuum Drying in a Bag
« Reply #62 on: November 07, 2011, 12:26:20 AM »
I dont think the bags or the connection would be an issue as long as one of these would work. The down side would be the "softening point" of 150 degrees. The hot box would need to be kept below that temp.  What kind of vacuum

http://www.veneersupplies.com/categories/Vacuum__Press__Items/Vacuum__Bags__%26__More/Dura-Max__Extreme%E2%84%A2__Vacuum__Bags/
I like Lucas Mills and big wood.  www.logboy.com

Offline Den Socling

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Re: Vacuum Drying in a Bag
« Reply #63 on: November 07, 2011, 09:32:47 AM »
Those bags look like they might fit the bill. But odd sizes. Too big.

Many species will reach 7% at 140'F. A couple, like Black Walnut burl, needs 180'F.

I usually run the vacuum around 55 Torr (55mm Hg). A chamber pressure of 65 will work. If you go above 65, the boiling point (and wood temperature) goes above 110'F.

Offline logboy

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Re: Vacuum Drying in a Bag
« Reply #64 on: November 07, 2011, 10:20:57 AM »
I was checking over your chart and 65 Torr correlates  to 109 degrees F. So theoretically with a "chamber" temp of 125 the wood would be at the required temp and yet still not soften the bags.

What kind of vacuum pump would be necessary to correctly run this setup? I kept reading about oil/water mixture problems. Is a different pump the solution, or a condenser of some sort?
I like Lucas Mills and big wood.  www.logboy.com

Offline Den Socling

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Re: Vacuum Drying in a Bag
« Reply #65 on: November 07, 2011, 10:57:30 AM »
If your wood is wet, evaporation keeps the temperature at a level set by chamber pressure. So, if you have 65 Torr, the wood can't go above 110. If you were heating in a bath of 125, the extra energy would speed up drying.

I use shell and tube heat exchangers as condensers. I use liquid ring vacuum pumps because they can take some water if any gets past the condenser. There are dry, screw pumps but they cost two or three times that of liquid ring. The thing about drying in a bag is that a tiny pump would do.

Offline logboy

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Re: Vacuum Drying in a Bag
« Reply #66 on: November 07, 2011, 03:33:41 PM »
If I am reading right from the other posters the pump would not need to be run continuously either. Some sort of small pump hooked up to a basic timer in a thermostat controlled hot box might do the trick.
I like Lucas Mills and big wood.  www.logboy.com

Offline Den Socling

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Re: Vacuum Drying in a Bag
« Reply #67 on: November 07, 2011, 03:59:35 PM »
That's correct.

Offline JasonRerras

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Re: Vacuum Drying in a Bag
« Reply #68 on: September 14, 2018, 11:51:01 AM »
Can anyone recommend a liquid ring vacuum pump?  Can these be had for a couple hundred dollars or is it going to be several thousand?

Alternatively, would an cheap automotive A/C vacuum pump (ala harbour freight) work as a more temporary solution assuming you could effectively condense the water before the pump??

 I have a number of slabs that are air dried, and it would be nice to be able to kiln dry them one at a time as needed. 

I've searched all over this forum and google, and this thread seems to be the only info I can find.



MS880 60" CSM and stuff

Offline btulloh

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Re: Vacuum Drying in a Bag
« Reply #69 on: September 14, 2018, 05:33:58 PM »
I use a Robinair pump that was intended for HVAC service work.  It works fine and I got it used for a couple hundred dollars.  With any of these, you probably need to build a vacuum reservoir and put a vacuum switch and unloader valve in the plumbing.  Those pumps are not designed to run continuously.  I found a general design somewhere on the web - maybe one of the woodworking magazines.  It was probably 15 years ago.

 I don't use it for drying, I use it for vacuum bag glue ups.
HM126

Offline JasonRerras

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Re: Vacuum Drying in a Bag
« Reply #70 on: September 16, 2018, 08:51:55 AM »
Thanks, the unloader takes the load off the pump after the bag had reached vacuum, right?
MS880 60" CSM and stuff

Offline btulloh

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Re: Vacuum Drying in a Bag
« Reply #71 on: September 16, 2018, 07:44:22 PM »
The unloader just allows the pump to be able to start again.  Same as with a compressor.  Maybe it's called a MAC valve in a vacuum situation.  It was quite a few years ago when I built the thing.

Here's a source of supplies:  

Vacuum Press Parts - Build Your Own and Save

This is similar to the design I used:

Do-It-Yourself Vacuum Veneer Press

Mine was for a vacuum press, so I don't really know how that applies for your task, but pulling a vacuum for an extended time is all about the same.  You may have to do something about the moisture that is coming out, but I think the HVAC vacuum pump handles that ok.  I'm not an expert, I just learned enough to get the vacuum press working, and it's been fine for 15 years.
HM126

Offline JasonRerras

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Re: Vacuum Drying in a Bag
« Reply #72 on: September 16, 2018, 08:03:25 PM »
Thanks for the info.  The moisture is the big issue as there will be gallons coming out, and you definitely need a condenser to capture the water vapor before the pump.  Looks like one can get a used liquid ring pump for a little more than an conventional pump, and that will tolerate the moisture much better while still using a condenser.
MS880 60" CSM and stuff

Offline Don P

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Re: Vacuum Drying in a Bag
« Reply #73 on: September 16, 2018, 09:04:06 PM »
Far from knowing but I think moisture is why the HVAC guys are always changing the oil in those.

Offline btulloh

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Re: Vacuum Drying in a Bag
« Reply #74 on: September 16, 2018, 09:44:59 PM »
Sounds about right.  Oil doesn't improve much  when you mix it with water . I guess that's where the condenser comes into play.  I haven't needed to chainge the oil, but I'm not dealing with much moisture in a press.  

I'll be curious to see how this turns out.  
HM126

Offline barbender

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Re: Vacuum Drying in a Bag
« Reply #75 on: September 16, 2018, 11:20:58 PM »
I don't know if this would help in any way, but Dodge Cummins pickups have a vaccum pump (at least my 97 did) it just pulls atmospheric air all the time in a harsh under the hood environment.  I don't know how many hg's they're pulling but for a small system to dry one slab at a time one of those and some ingenuity might do the trick?
Too many irons in the fire

Offline JasonRerras

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Re: Vacuum Drying in a Bag
« Reply #76 on: September 17, 2018, 01:25:00 PM »
Interesting idea. Random comment in another forum said it should be pulling 28-29", but I doubt it would hold up to the corrosive liquid coming out of the wood. 

Currently looking at something like this:  https://www.ebay.com/itm/Travaini-TRMB-32-50-GH-KW-2-2-Single-Stage-Liquid-Ring-Vacuum-Pump-Used/273444427632?hash=item3faa8ed770:g:kj0AAOSwVzdbj9Fv

Just trying to wrap my head around on the ancillary stuff that it will need to make it work. 
MS880 60" CSM and stuff

Offline boardmaker

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Re: Vacuum Drying in a Bag
« Reply #77 on: September 18, 2018, 08:53:12 AM »
Wish I could help you more Jason.  I had researched this a few years ago.  If I remember correctly, Den always said you needed a liquid ring pump so your ebay pump should work. 

I also wondered if instead of using a vat of water to heat the slab bag if you could wrap the bag with an electric blanket?  I know they tried something like that in previous models but I thought if you then wrapped the blanket with insulation to hopefully equalize the temps.

Offline shenandoahsawmill

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Re: Vacuum Drying in a Bag
« Reply #78 on: September 18, 2018, 07:38:03 PM »
Anyone try microwaves as a heat source for a vacuum kiln? You can buy used microwave ovens for next to nothing and pull the magnetron out. Microwaves excite water molecules and provide heat proportionally depending on moisture content. It also kills living bugs and larvae as a side benefit. I would imagine that setting up an array of magnetrons in a lead lined chamber would do the trick. The microwave would definitely penetrate the vacuum bag and wood. Probably many slabs as well. I use my microwave oven to dry wood turning stock and kill bugs quite often. Thoughts? 

Offline Don P

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Re: Vacuum Drying in a Bag
« Reply #79 on: September 18, 2018, 09:04:32 PM »
That is basically RF-V, the heating pad or water bath is using conduction. Either ought to work in my mind. I think you might need a mesh or something to let the vapor get to the vac outlet easily?


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