Author Topic: Vacuum Drying in a Bag (Read 19648 times)

Ironwood · « **Reply #20 on:** February 11, 2005, 10:23:39 PM »

All I can say is that we all have our specialties. If and when I need the vaccum chamber rest assured I will call the boy's at PC SPECALTIES. I..you could spend thousands of dollars and hundreds of hours of time not finding a solution. If it's for entertainment well that's another matter.

Reid

GaS · « **Reply #21 on:** February 12, 2005, 12:55:41 AM »

Tyler! Glad to see you join the conversation! I still feel that the core temperature is high...was your wood pre-dried at all? I am overly cautious when it comes to schedules (especially considering this is so 'by the seat of the pants'). Also, I was thinking that perhaps you might want to increase the down time to four 1/2 (or even full one) hour segments rather than two 1 hour segments. Its really hard to tell what might be happening, but my gut keeps saying 'over drying the shell'. Once the wood MC is lower, you can 'beat' the wood, but you should go slow at the beginning.

Thanks for the vote of confidence, Reid. It has taken Den thousands of hours of research to learn all that he knows today. I only have a couple hundred of pondering so far, but I am able to pick his brain daily!

Larry · « **Reply #22 on:** February 12, 2005, 08:56:10 AM »

T,
Are you using a platen?

Glen Evans · « **Reply #23 on:** February 14, 2005, 08:57:48 AM »

Tyler,

Exciting to hear about your prototyping work! A couple of quesions. Are you drying a single piece of wood, or do you have multiple boards in the bag? Do you circulate the air at all in the box? How did you decide on those operating temperatures? I'm really interested in hearing how this works out for you Tyler.

It's great to hear that air can work as a heating medium--I'm sure the efficiency is no where near what water could be but I figure that just means you need to have a greater difference in temperature between the air and the surface of the wood.

One thing I'm still wondering about with the bag method versus the chamber method is whether the vinyl bag collapsing on the surface of the wood actually prevents the water vapour from exiting over large areas of the board. I wonder if we don't need to allow channels, (1/2" pvc pipe cut in half lying on top of the board extending all the way to ends of the board, with vacuum connections at both ends of the bag), to allow the vacuum to be applied to surface and the ends of the board and to allow water vapour an easy path to exit. At the end of the day, the bag that we are compressing around the board, (especially if it is poly) is a vapour barrier!

Anyone have any comments?

Glen

Den Socling · « **Reply #24 on:** February 14, 2005, 12:47:30 PM »

I'm a control freak.

I like it when I control all the parameters and the 'process' happens exactly as expected. When drying with a vacuum, the vapor pressure of the water in the wood is equally as important as the chamber pressure (vacuum). If water is used as the medium, not only will you benefit in heat transfer but the temperature of the water, with it's mass, would be easy to maintain as a stable parameter. If you use water, I think you could control from the temperature of the water bath. If you use air, you probably need to be more concerned with wood temperature.

The wood temperature can be confusing. If the wood is wet, the maximum temperature is set by the chamber pressure (vacuum). There is a table at our website. It shows the relationship between reduced pressure and the boiling point.
http://www.pcspecialties.com/torroconvHC.html
If you pull to 30 Torr which is 1.18" Hg absolute which is 28.8" Hg vacuum, water boils at 84'F. It can't get hotter unless the pressure goes up.

serg · « **Reply #25 on:** February 14, 2005, 01:32:00 PM »

Den, hi! The good fellow, it is perfectly told. Sergey.

Buzz-sawyer · « **Reply #26 on:** February 14, 2005, 01:53:59 PM »

Glen
I have not tried this except on small scale experiments,
Put your bag around a solid object that can withstand atmospheric pressure...like a strong barrel or tank then your issue is solved

Glen Evans · « **Reply #27 on:** February 14, 2005, 02:19:44 PM »

Den and Buzz Sawyer,

Den I agree with you point about the water being easier to control particularly with respect to the temperature of the wood. I still wonder about the effectiveness of having the vacuum bag sucked tight to 90% plus of the wood. In theory at least, slapping a poly bag tight to surface is going to prevent vapour transmission isn't it?

The problem with you solution Buzz is heating.

One variation on the classic platten system came to me this morning. Piping hot water through the condensor coils of old refridgerators layed flat between layers of wood in a large vacuum bag. This space between layers would also facilitate the evacuation of water vapour from the wood.

The only real difference I suppose between this approach and a more conventional approach is that the vacuum chamber is a vinyl bag! I'd still use a condenser to avoid pumping water through the pump.

What am I missing Guys? I really appreciate the feedback

Hey Tyler how's it working today?

Den Socling · « **Reply #28 on:** February 14, 2005, 04:02:21 PM »

Without looking up the numbers, I'd guess that the topography at the surface of wood would look like the Grand Canyon to a water molecule. Think about those tiny pores and vessels that it went through on the way out of the wood. I don't think that plastic that is vacuum packed against the wood is going to stop the vapor.

Glen Evans · « **Reply #29 on:** February 14, 2005, 04:05:25 PM »

Touchee.

Good to perspective to look at from.

Thanks

Glen

Glen Evans · « **Reply #30 on:** February 14, 2005, 04:08:38 PM »

Let me try that again,

Good perspective to look at things from!

Glen

PS How nuts is the heating idea Den??

Dan_Shade · « **Reply #31 on:** February 14, 2005, 04:14:32 PM »

i can see the heating coil as only heating up specific areas of the board, since wood is a good insulator...

Den Socling · « **Reply #32 on:** February 14, 2005, 05:26:14 PM »

Glen,

Depending on the species of wood, the refrigerator evaporator coils would be eaten up by acids in no time.

Sandwiching the heat source between two layers of aluminum would do much better at spreading the heat and avoiding the problem that Dan mentions.

If you wanted to make one or two custom platens, you could buy a few feet of our extrusion. A do-it-yourselfer recently bought $45,000 worth.

You probably don't need that much.

Den

Dan_Shade · « **Reply #33 on:** February 15, 2005, 12:51:53 PM »

I've been thinking about this a bit...

when you vacuum bag composites, you use a "breather cloth" so that you can get a vacuum pulled equally over the entire surface, problem I can see is if you use a breather cloth, it may not transfer heat that well... but I think I've seen fiberglass guys heat the stuff with heat lamps so it must transfer heat sorta ok.

Glen Evans · « **Reply #34 on:** February 16, 2005, 08:36:05 AM »

Den,

Yeah, I think $45,000 might be a shade high for me!! I was puzzling over this yesterday and got to thinking about water bladders for heating. Think 4' x 8' by 1" thick vinyl water bladders with hot water entering one corner and exiting the diagonally opposite corner. Obviously I wouldn't be able to stack more than maybe 4 lifts at a time for fear of rupturing a bladder.

Comments??

Glen

Den Socling · « **Reply #35 on:** February 16, 2005, 09:35:48 AM »

Glen,

If you put a bladder full of water in a bag and then pulled a vacuum, that bladder is going to expand to fill every bit of space.

Think about this. I use a bladder to press stuff like Holly flat while drying. If the bladder is 288" long and 40" wide and I let 14 PSI (one atmosphere) into the bladder while I pull pressure down to 2 PSI, that bladder is pushing out with a total force of 138,240 lbs. You better have a tough bag to hold that.

Den

Glen Evans · « **Reply #36 on:** February 16, 2005, 09:55:59 AM »

Den,

Yeah I wondered about that--you have a wonderful way of illustrating concepts with numbers that makes me wince!! Its almost like there's a good reason for the way you make your plattens!!!!!

I'm fast running out of ideas--but not done yet. Next up is an idea retrofitted from the heated floor industry. (I can almost see everyone rolling their eyes!) 1/2" thick cement board with electric radiant floor heating cable laid onto the surface on 2" centers and parged over. The entire board could be laid between lifts, additional boards could be connected in series and controlled by a thermostat.

I hesitate to ask for comments??

Glen

Glen Evans · « **Reply #37 on:** February 21, 2005, 03:56:41 PM »

Something I said??

Hey I hope I didn't offend anyone, maybe you're just tired of my ramblings--

Thought I'd send out another SOS to see if anyone thinks the idea of using radiant electric floor heating wires embedded in 1/2" cement board to conduct heat and act as warming plattens between lifts of wood in the vacuum bag is viable?

Any feedback would be appreciated.

Glen

GaS · « **Reply #38 on:** February 21, 2005, 06:15:09 PM »

well, I'll say that it would be a lot less expensive than aluminum plates!

hard to say what type of heat transfer you'd get. concrete, like wood, isn't a great insulator...but it isn't a great conductor either. hmm...

depending upon how deep the heating elements were embedded, you'd probably get reasonably even heating...

wouldn't last long in an acidic environment...

heavy...but would help hold down boards that would want to cup and might improve your heat transfer...

Glen Evans · « **Reply #39 on:** February 22, 2005, 01:49:44 PM »

Thanks for the input. Think I might just give it a try

Glen

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Author Topic: Vacuum Drying in a Bag (Read 19648 times)

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