Has anyone ever tried vacuum drying wood ?

[KeithO]

Seeing references to kiln dried cord wood popping up, makes me wonder why one would not use a vacuum to dry wood instead ? If one lowers the ambient pressure, the water in the wood will boil off without requiring heat. One needs a chamber in which to load the wood and draw the vacuum and of course a vacuum pump (one can use salvaged refrigerator motors which do not draw to much power to begin with). Boiling off water at low presure requires far less energy than at normal pressures, so it seems like it would be more efficient ?

Is the reason behind using heat that a certain amount of the yield (dried wood) is used as a ready energy source in the kiln drying process ?

 

[Harley]

Interesting concept, Keith

I haven't seen ads for kiln dried firewood before - maybe that comes from some woodworking shops that have some space in their drying rooms to store some cord wood, and let it dry in the heat with the blowers. There has to have an easy way to get the stuff in and out of the room. I don't think that would be easy to do if you had to keep it under negative pressure though.

Doing it on a small scale would be an interesting experiment, but I really don't think it would be too much better than some good old sun and wind exposure in the long run.

 

[KeithO]

I'm not suggesting that this substitute normal summer outside sun drying. In some of the colder latitudes it woud be useful to be able to season wood any time of the year. Here in MI we do have a humidity problem in summer that slows down the rate that wood cures. I guess building a more dedicated drying rack that makes effective use of solar gain and keeps rain off would accelerate the process, but I still have wood that is far from dry after it was cut spilt and stacked before last winter. In fact, some of it is now covered with fungus, which got established prior to the (dead) tree being felled. Maybe a small scale experiment would be in order to understand just what is involved.

 

[Gooserider]

I would suspect that it is something that would work as an experiment on say one split, but wouldn't scale to the woodpile sized application. To get a significant decrease in the energy needed to dry the wood you'd need to pull some pretty heavy vacuum, and it gets difficult (spelled E-X-P-E-N-$-I-V-E) to build a structure that is 1. air tight, 2. big enough to hold a useful volume of wood, and 3. Strong enough to withstand the atmospheric crushing force.

If I were wanting to optimize drying, I'd look at trying to build some form of solar kiln (which I've seen many plans for) or other way of utilizing waste heat to increase evaporation through temperature rather than looking at vacuum technology.

Gooserider

 

[elkimmeg]

Why not build a solar Kilm powered by our sun?

If you need plans I have some here I could did up

 

[pgmr]

Why not build a solar Kilm powered by our sun?

If you need plans I have some here I could did up

Yes, a solar kiln powered by our sun would work much better than one powered by, say, Alpha Centauri....

 

[mtarbert]

I think......the kilns for drying wood are designed to make sure the lumber does not Check and Warp so much as to render the stock useless. However when drying firewood ....who cares if it splits and warps? Until the price of storage containers went thru the roof I was thinking about buying a 20footer painting it black and installing a de-humidifier with the hose from the condensate running out the side. Temps can reach 160f in the summer inside the sea containers.
Mike

 

[mayhem]

So lets get some solr kiln plans up here and try it out. I'm one of those guys who is always going to be dropping trees in August/September and trying to split them and burn them this winter...I have three trees lying in the yeard right now that I dropped 2 weeks ago that I'll be needing this winter actually. A solar kiln would be most useful.

 

[Eric Johnson]

They tried vacuum drying for lumber back in the late '80s, but it never caught on. You've the physics right, KeithO, but apparently it either wasn't practical or it wasn't cost effective. Or both.

And if it didn't work out for drying valuable lumber, it sure as shootin' wouldn't pay off for drying firewood.

 

[backpack09]

Why not put your splits in the refrigerator? The fridge works as a dehumidifier..

 

[wahoowad]

How about a big microwave?

 

[Gooserider]

I've seen plans for solar wood kilns on the net, Google is your friend... The designs looked pretty simple though, essentially a woodshed with insulated walls and a leanto style roof made from solar appropriate materials. They designed it to open up one end to get the wood in and out, and had a few fans inside to circulate the air and cause it to change periodically in order to get rid of the humidity.

While they were effective, I suspect that you might still be pushing it to try and get newly cut trees ready to burn in time for this winter, especially since this time of year is less than the best for solar gain.


Gooserider

 

[Eric Johnson]

The business end of most solar dry kilns I've seen consists of the plastic covering, upon which moisture condenses and then drips down into the ground or into a drain. The sun comes up, heats the wood and vaporizes some moisture, which condenses out onto the plastic sheeting and drains away. No fans or venting needed. This works well for lumber, since it's a far gentler process than a conventional steam or dehumidification kiln, and obviously less energy intensive.

With firewood, I would think that vents at the top of the kiln would speed the drying process. As has been noted, nobody cares about splits and checks with firewood.

You can dry lumber with a microwave, Warren, but you don't want to see the electric bill. You can also squeeze it out, put it in a room with a dehumidifier, etc. Doing all this in a cost-effective fashion that doesn't degrade the product is a lot more difficult.

 

[karl]

Eric that makes we wish I had gotten the microwave assist option on my Summit

 

[DiscoInferno]

The microwave idea came up in the past for measuring moisture content. I tried it on a lark with a smallish piece of fresh pine. 5 minutes at a time on high (600-750W), then stop, weigh, and rotate (it was too long for the carousel). After about 20 minutes of that, a dry hotspot must have formed (rest of split was still wet), it made a deep "woof" sound, and briefly burst into flames. The smoke smothered the fire pretty quickly. Took days to get the smell out of the house, and months to get the smell out of the microwave. The wife was not pleased.

 

[Eric Johnson]

They never are, man. They never are.

 

[KeithO]

Dan: The refrigerator dehumidifies AIR, but it will have a harder time drying wood if the moisture is captured internally by freezing. Some rounds I cut last winter were certainly frozen and were still damp at the start of this past summer when I split them. How do you like the Morso 7110 ? I'm strongly leaning towards that model right now for my living room. I like the close install clearances and the fact that the door is hinged on the left (almost everything else is hinged on the right). Any coments on burn time and the size of room you heat with it ?

Generally, I think most forced drying processes have certainly been set up for high value lumber, ordinarily with the longest possible continuous length. On the other hand, I think that split cordwood has a very favorable surface area for drying in this fashion with lots of exposed surace area on the ends and on the split faces. I wouldn't try it on solid rounds.

Why not put your splits in the refrigerator? The fridge works as a dehumidifier..