Thread: Drying timber with vacuum and heat?

Only one slight problem, most of the vacuum pumps that we use do not like moisture and fail rather quickly.

Yes Moby, one of the videos mentioned that. He spoke of using a piston driven vac. pump instead of the usual "oil-less" types. I am thinking that there would be some sort of moisture trap available, either ready made or DIY?
Come to think about it, the rotary vane vac. pumps they used in milking plants exhausted heaps of water and a small amount of Alfa Laval vacuum pump oil as a mist. More research is needed.

Forced drying plus vac can becomes expensive in terms of electrical power.

To run say a large fridge size kiln most vac pumps don't move enough air internally to generate evenly heated air so at least say 100W of fan moving air inside such a kiln i required.
Lets say you used a 500w vac pump and 150w of heating (say a heat lamp) for a total of 750W is needed. - running 24hrs for a week @ say 25c/kWhr =$32 a week and I reckon for say 4x2 timbers at least 2 weeks will be needed so that's $64!. In other words the timber better be worth drying. Its overkill and problematic if just a few boards are put in such a kiln - read on

In Australia you won't need a heater or a vac, especially in the summer if direct solar heating is employed so the heat lamp can be ditched but not heating at night means it takes a bit longer. In fact it has been demonstrated that if australian timber is allowed to cool off at night (still keep fans running) and equilibrate the MC across the drying direction in the wood. Although it takes longer there are usually less cracks and twists.

Larger kilns are usually more efficient especially if direct solar heating can be employed. The CSIRO have tested and developed a standard seatainer size kiln and recommend at least 1500w of internal fan power and 150w of internal/external fan power be used - on such a setup, timber slabs take about 3 weeks to dry BUT that's ~$208!

You can't really set and forget these things without risking turning the wood into "pretzels", so some form of monitoring and control is needed especially if just one or two pieces are put into such kilns. I've done this with seatainer on more than one occasion.

These kilns work better if they are near full because a larger mass of timber buffers rapid changes in MC.

Well Bob, I was thinking of something the size of a 45Kg gas bottle suitably modified with a closing door and a heating pad sitting on a shelf that the timber would sit on. The problem is that there is moisture in the air that is withdrawn from the chamber so that needs to be filtered out somehow. If the chamber is well constructed and sealed effectively then the vacuum pump only needs to run intermittently to maintain the vacuum and the heating pad would be the only thing that runs continuously. The problem is the filter for the inlet of the vacuum pump and there is probably more than one design for a moisture trap out there on the internet somewhere.
It is just a theoretical exercise really. Can I dry pen turning blanks more efficiently if I place them in a warm vacuum compared to putting them in a microwave oven set on "low" for a while? How many pen blanks can I fit in a 45Kg gas bottle compared to a large microwave oven? It's a theoretical exercise at the moment. It may never happen. A cheap "used" large microwave oven may come my way and that will be the end of the matter.
As for the solar kiln, well the Paulownia plantation and our sawmill will have to work out if that is viable. At the moment it isn't. That too may change. Who knows what the future holds?

Originally Posted by Old Hilly

Well Bob, I was thinking of something the size of a 45Kg gas bottle suitably modified with a closing door and a heating pad sitting on a shelf that the timber would sit on. The problem is that there is moisture in the air that is withdrawn from the chamber so that needs to be filtered out somehow. If the chamber is well constructed and sealed effectively then the vacuum pump only needs to run intermittently to maintain the vacuum and the heating pad would be the only thing that runs continuously.

Sounds like good way to make pretzels.The wood near the heat pad will get too hot compared to the rest of the wood and dry out too quickly. This will set up strain between the hot and cooler rods and twist and crack the wood. This will be far less of a problem for very short pieces.

You could be right about the pretzels Bob but I doubt that a heat pad designed to keep pets warm on cold winter nights will really overheat the wood to any great extent. I'll do some more research and hopefully find someone in Australia that has something working that I can pinch a few ideas from.

There is a book precisely about vacuum kiln drying wood by Joshua Salesin that you should read. Not a very big book but the information is good. It is geared for small scale work like drying a few roughed bowls from green to dry in a few days.
https://www.amazon.ca/Vacuum-Kiln-Dr...0575828&sr=8-1

Pete

Thank you QCInspector, I was getting all excited there until Amazon decided they won't ship to Australia. Perhaps there is a local source. I will consult Prof. Goggle and see what he says.
Edit...I still have to sort out some sort of moisture trap to go between the cylinder and the vacuum pump. Perhaps a suitable container with an inlet and outlet suspended low down in a bucket of cold water?
Or perhaps I go back to the microwave oven idea?

Interesting this is mentioned.

I'm aware that libraries that preserve books do exactly this.

I'm a little foggy, but a while back a flood swamped a large number of rare expensive books. USA Florida? Italy?... its jsut out of my reach....

Aannyy way, it was decided to keep them wet. The were frozen first, then put into a large kiln-sized vacuum chamber. They dried out quick smart and no ruined by the process.

Normal drying would have left a clagged together mess.

Edit - we are! Stanford Update on flood damage to books: 4/5/98


I don't see why this wouldn't work with timber/logs. A book is but veneer....

edit 2 - 3.12 Freezing and Drying Wet Books and Records — NEDCC

Lets do some science!

-- Freeze those pen blanks
-- Pop them into a vacuum chamber
-- Give it full blast for a day
-- Pretzels?

Remember kiddos - knowledge matters - I before E except within science

Actually woodPixel, I would heat the timber first. How about we put the timber into a vacuum chamber fitted with a waveguide and feed microwaves down the waveguide and into the chamber? Easier than building a vacuum chamber around a microwave oven and working out how to cool the magnetron cool in a vacuum. Then we need to catch the moisture vapour before it gets to the pump.