English Oak - high moisture content - advice please!

[Sgian Dubh]

so ... would it be worth experimenting with - if you had a vacuum pump for veneering for instance ?

Probably not. You need some method to warm the water in the wood and the vacuum effectively reduces the pressure in the container, and therefore effectively reduces the boiling point of water. In your case the proposed container is your vacuum bag.

To warm the water in the wood you can commonly do one of three things:

• 1. use an external heat source, eg, wrap you vac bag in a heated blanket or similar;
  2. put heated platens in between your layers of wood within the vac bag;
  3. Use radio frequency to excite the water molecules in the wood, ie, use a microwave device around your vac bag.

You might be able to rig something up on a small scale, but as far as I know the technology isn't yet efficient enough for commercial kiln operators to make money with.

Nearly all the kiln operators I know use either conventional high temperature kilns or dehumidification kilns. There are also some that operate solar kilns and others operating huge progressive kilns. Many wood kilning operations use a mixture of one form or another of air drying in conjunction with one of the most common forms of kiln. Slainte.

 

[Digit]

I improvised an unused greenhouse as a solar kiln for drying timber. With the wood covered to keep it out of direct sunlight and the warm air circulated around it with a fan the normal air drying time is shortened considerably.

Roy.

 

[big soft moose]

nice - i built a big insulated box and stuck a dehumidifier inside it with the wood. it works okay for part drying turning blanks but not to the kind of drying point i imagine you would need for furniture

 

[Tusses]

so ... would it be worth experimenting with - if you had a vacuum pump for veneering for instance ?

Probably not. You need some method to warm the water in the wood and the vacuum effectively reduces the pressure in the container, and therefore effectively reduces the boiling point of water. In your case the proposed container is your vacuum bag.

To warm the water in the wood you can commonly do one of three things:

• 1. use an external heat source, eg, wrap you vac bag in a heated blanket or similar;
  2. put heated platens in between your layers of wood within the vac bag;
  3. Use radio frequency to excite the water molecules in the wood, ie, use a microwave device around your vac bag.

You might be able to rig something up on a small scale, but as far as I know the technology isn't yet efficient enough for commercial kiln operators to make money with.

Nearly all the kiln operators I know use either conventional high temperature kilns or dehumidification kilns. There are also some that operate solar kilns and others operating huge progressive kilns. Many wood kilning operations use a mixture of one form or another of air drying in conjunction with one of the most common forms of kiln. Slainte.

my understanding of vacuum drying was the the water was 'sucked' out of the wood, not boiled / warmed out . so no heat required ?

 

[Sgian Dubh]

my understanding of vacuum drying was the the water was 'sucked' out of the wood, not boiled / warmed out . so no heat required ?

You could quite easily suck the free liquid water out of a block of wood I suppose with a vacuum. Once the free water is removed and the wood has reached FSP, ie, 30% MC the rest of the water in the fibres is bound water. You have to persuade that bound water to unbind itself from the cellular structure. This means creating lower vapour pressure outside the wood cells than the vapour pressure within the cells. Water moves from higher vapour pressure areas to lower vapour pressure areas.

There are different ways of creating lower vapour pressure areas: you can increase air movement, apply heat, reduce the pressure, or any combination of the three. In theory I suppose it might be possible to create enough vacuum to reduce the boiling point of water to about room temperature, ie ~20ºC -- I'm not sure if this actually possible. If it is possible this would cause the cells to release their water as vapour which you could theoretically suck out.

If you've got a vac bag why not try an experiment using a plank of wet wood? Some cheap pine or badly knotted hardwood you can't use for anything would do. Cut out a section as I described earlier and oven dry it in the microwave to establish the MC prior to your experiment. Stick the rest of your board in the vac bag for a week or two and then repeat the oven drying test to find out the moisture content at the end of the experiment. You would then be in a position to analyse and evaluate the worthiness of the vacuum drying method using the equipment you have.

Personally, I'm not sure it's an experiment that I would conduct because it is only of academic interest to me and has little bearing on any practical woodworking I do. Slainte.

 

[Tusses]

ok - what is all this about boiling points and vapor pressures ???

a puddle on the pavement dries at room temperature in a few hours

the kitchen work top dries at room temperature in a few minutes

I thought the theory was that the negative vacuum pressure on the wood caused the wood to expel water by capillary action through the wood fibers ? nothing about latent heat of vaporisasion an all thet (I think thats what we called it at school LOL )

the advantages are obvious, that with the same vacuum pump you can bag a test cube, or a full board, or a whole stack of timber. Which wouldn't fit in the microwave

mined you, the only vacuum dryer I have seen was like a giant torpedo tube with a whole stack of wood loaded in !

but theory is... theory does ... n all that

 

[Digit]

Water cannot exist in a vacuum as a fluid, it passes from liquid to vapour independent of temperature and so passes through the timber into its surroundings.
The reverse of how it absorbed moisture from its surrounding in the first place.

Roy.

 

[Sgian Dubh]

Water cannot exist in a vacuum as a fluid, it passes from liquid to vapour independent of temperature and so passes through the timber into its surroundings.
The reverse of how it absorbed moisture from its surrounding in the first place.

Roy.

Yes, true, but does a vac bag used for bending wood and veneering jobs in a furniture workshop create a vacuum? Or does the pump do no more than just about hold its own against incoming replacement air? Slainte.

 

[Digit]

Either way it's a lower pressure than the water vapour was when the timber was absorbed do the timber must give up the moisture.
It's the same with heat in a kiln, it's not the temp that dries the wood, it's the fact that warm air has a lower density/pressure so the water moves to equilibrium.

Roy.

 

[Mr T]

Hi

I am surprised no one has picked up on Big sft Moose's suggestion of using an enclosure round a dehumidifier.

I have done this before by building and enclosure in viscreen stacking the wood sticked in the enclosure and running the dehumidifier. I ran a pipe from the dehumidfier reservour to the outside so that I did not need to worry about emptying the tank

Chris

 

[big soft moose]

thats pretty much what i did mr T - my experience is that its great for removing the free water but less effective at removing the bound water in the cellular structure.

this means that it was fine for reducing water content in turning blanks , which would then be partially turned and then allowed to dry in the workshop - speeding up the drying process after part turning to about 1 month. however it would be less effective at drying wood for furniture where movement would be more of a problem.

(although that said , thinner stock dries faster than 3" turning blanks anyway)

 

[Digit]

When I used a greenhouse for a period of bulk drying I soon found that, in effect, any moisture that was lost during the day was soon replaced, at least in part, during the night when the interior of the glass became covered in condensation.
I picked up a de-humidifier and ensured that the timber was covered in and sealed as much as possible at night with the de-humidifier inside. Even on cool days the de-humidifier extracted a surprising amount of moisture, some of which was as a result of an imperfect seal I'll grant, but overall it worked well enough.

Roy.

 

[bjm]

Water cannot exist in a vacuum as a fluid..

This is not true. A common, but inefficient, method of impregnating is to immerse a sample into a liquid and pull a vacuum. It used to be used as a method of preservative-treating timber (Vac-vac).

Brian

 

[Sgian Dubh]

ok - what is all this about boiling points and vapor pressures ???

Well, as I said, if you are curious about the viability of vacuum drying why not do the experiment I suggested?

Experimental vacuum drying kilns use heat. The reason for adding heat is, as far as I'm aware, because practical vacuum kilns don't create a vacuum, but they do reduce pressure. Presumably they don't reduce pressure enough to force the water out of the wood fast enough to make the technique economically viable.

Here is some reading material for you that has information on wood drying techniques, including vacuum drying, see Pratt in particular. It's a limited selection from the bibliography of a manuscript on timber technology I'm working on. Reading them all in one sitting might keep you occupied for half an hour or so, ha, ha-- ha, ha, ha. Slainte.

• Bousquet, D, (2000) Lumber Drying: An Overview of Current Processes, University of Vermont Extension. [Online] Available at: http://www.uvm.edu/extension/publicatio ... drying.pdf
  Brown, WH, (1995) The Conversion and Seasoning of Wood: a guide to principles and practice, Stobart Davies, Hertford, Great Britain
  Denig, J; Wengert, E and Simpson, W T, (2000) Drying Hardwood Lumber, United States Department of Agriculture, Forest Products Laboratory. [Online] Available at: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf
  Forest Products Laboratory, (1999) Wood Handbook: Wood as an Engineering Material, United States Department of Agriculture, Forest Products Laboratory, USA. [online] Available at http://www.fpl.fs.fed.us/documnts/fplgt ... gtr113.htm
  Garrahan, P and Kidlark, J, (2001) Drying Hardwoods, Northern Hardwood Initiative. [Online] Available at http://www.cfquesnel.com/nhi/TableOfContents/TOC_5.htm
  Hoadley, R B, (2000) Understanding Wood: a craftsman’s guide to wood technology, revised edition, The Taunton Press, CT, USA.
  Pratt, GH, (2003) Timber Drying Manual, 3rd Edition, Building Research Establishment, London.
  Wengert, EM, (2006) Principles and Practices of Drying Lumber, Lignomat USA Ltd., Brooks Forest Products Center, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA.

 

[Digit]

Brian, that would have to be in a condition where the water was unable to escape in some way or where the loss of the water to a vapour was not a problem.
Liquid water cannot exist in a vacuum it simply boils away. The boiling point of any liquid is determined by the air pressure surrounding it. Water can exist in a vacuum as ice but then converts straight to vapour with no liquid stage

http://www.philosophychatforum.com/bull ... c93ef16dcf

Roy.

 

[dickm]

A common, but inefficient, method of impregnating is to immerse a sample into a liquid and pull a vacuum. It used to be used as a method of preservative-treating timber (Vac-vac).

Presumably what happened there is that the reduced pressure removed <air> from the timber, and the preservative moved into the space left?

 

[Digit]

That is how they used to impregnate motor windings with varnish ****.

Roy.

 

[bjm]

Liquid water cannot exist in a vacuum it simply boils away.

Only to the extent of reaching saturation-not a problem in the vastness of space.

 

[Tusses]

That is how they used to impregnate motor windings with varnish ****.

Roy.

ah ... the good old days !

(and I'm only 40 :lol: )

 

[Digit]

Yeah! Provided you didn't mind arriving home every night smelling of burnt out motors 'cos the varnish was heated as well. Horrible job! And I was only the foreman!

Roy.