I was microwaving a piece dry this AM and got to wondering. Since several woodturners have vacuum pumps, has anyone tried vacuum drying wood? Should work, just need a decent pump and a good bell jar. At a low enough pressure the water would boil out, I would think similar to microwaving. Ought to be faster as you don't have to worry about scorching the wood. Pull a vacuum and leave it be for a bit.
Maybe another reason to give SWMBO as to why I need a vacuum pump.
You run into problems with vacuum drying. You have to keep bleeding the vacuum and the wet air out of the system. Also, since you're not providing heat to the process, the bonds binding the water to the cellulose are not going to let loose as fast, because they are at lower energy.
Since wood dries by losing moisture, the rate of which is determined by the relative humidity, it seems the smart money would go for direct control of that, rather than the indirect methods.
"Bill B" wrote: (clip) Pull a vacuum and leave it be for a bit. (clip) ^^^^^^^^^^^^^^^^ Here's what will happen: You pull a vacuum, which removes air and water vapor from the chamber. Water evaporates from the wood and comes to equillibrium with the wood at the same partial pressure as though the air were present. Even though your gauge shows you still have a good vacuum, there's noting going "dryingwise." So you would have to keep the pump running, or starting and stopping it at intervals. If the humidity where you live is low, you could probably dry the wood just as fast by letting it sit on a shelf or in a pile.
I'd do it slowly - don't want a large bubble to explode. I'd think of doing it in a light finishing oil or light cooking oil maybe. It will keep the water out and will replace the water once the vac is reduced.
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder
Bill B wrote:
When you work on an air conditioning system you "vacuum it down" after you close it back up. The moisture is discharged through the vacuum pump outlet in vapor form. If I recall you have to exceed 27" or even 29" (30" perfect?) before any "boiling/vaporizing" begins. You will go far below RH or EMC. Automotive repair equipment works just fine on auto sized systems.
A problem would be constructing the perfect enclosure since the tiniest vacuum leak defeats the process preventing you from achieving the 27" or so. I'm sure there must be industrial pumps and systems.
TomNie
There are threads in our archives re drying wood by vacuum drying and freeze drying, also by osmotic pressure. As far as I know these methods have never been of any use for woodturners due to physical and economic problems.
I guess if there are many solutions to the drying problem, it probably means that there is no one best method ...or just maybe the problem hasn't been solved.
Turn to Safety, Arch Fortiter
Have you all ready forgotten that LDD eliminates those complex forms of methodism?
Leif
Sorry Soapy, my abject apology. Of course LLD is the one best solution. "'In Detergent, Veritas" or to be PC, "E Pluribus Unum" :)
Turn to Safety, Arch Fortiter
oops, I meant LDD! Let's keep the lawyers out of this. :)
Turn to Safety, Arch Fortiter
Arch,
OK - let me add something I saw at the library at Arrowmont. The Woodturning Masters Class, I think, by Tony Boase showed a couple times a huge rough bowl being dried with a heat gun (hair dryer?) from the outside. These were the famous "masters" at work.
Makes me wonder if one of those infra-red heaters next to the bowl while turning would help. Get a woodturner's tan if nothing else.
Another thing would excite George and start some ..... Turn the bowl up to over 3000rpm and just let it fling the water out for awhile. Sorta like LDD without the soap.
TomNie
With the right jack pulley, you could get 10,000 rpm, not even the bugs could hang in there then :-)
Compressed air from the inside will eject unbound water with much greater safety. Won't work well on the small spots of face grain, of course. What it will do is cut down on the growth of mildew, especially if you use an occlusive coating on wet wood. I don't use the coating, but the air helps get a light wood to dry with no mildew spots.
As to drying, if you think of it, you're not gaining more than a week of dry time. It's the bound water that counts, and since end grain dries ten times faster than face anyway, and application of mechanical energy won't speed it up loss of bound water....
it's just that all those soap bubbles clog the vacuum pump...
mac
Please remove splinters before emailing
Tom.. "spin drying" is fairly common.. I learned it here...
Last spring, when I was turning a lot of very wet pine that had been under the snow during the winter, it helped a lot..
I'd rough turn it at normal speed, which would already have the shop wall and ceiling soaked, then turn it up to the highest rpm I thought was safe and let it fling any left over water out... and LOTS of additional water got flung... (some early attempts got my wife's wood burning stuff wet and led to re-aiming be pivoting the lathe)
mac
Please remove splinters before emailing
The problem with vacuum drying is that you have to remove the moisture from the enclosure without letting the vacuum escape. Normally this is done by having a very cold pipe run thru the enclosure, with a drain gutter below it. Captured moisture then drains (by gravity) into a chamber with valves above and below. To release the moisture, the top valve is closed, the bottom valve then releases the trapped moisture. Then the bottom valve is closed and the top valve is opened to start the process again. Commercial vacuum kilns do this automatically.
So not only do you have to have a vacuum-proof container, a vacuum pump, but you also have to run a refrigeration system and associated piping, etc. Not so simple a deal.
Regards, James Johnson
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