Vacuum Kiln or Atomic Bomb -- You Decide





Introduction: Vacuum Kiln or Atomic Bomb -- You Decide

I wanted to build a vacuum kiln for drying wood at low temperatures. I was able to make it at Tech Shop.

Wood dries in a Kiln because the water is flashed to vapor. This normally occurs when you heat the wood to a temperature of greater than 212 degrees. When you boil water, you get water vapor.

Everyone knows that water boils at 212 right? If you live in Denver -- you know that water boils at 201 degrees. This is because the air pressure is less since Denver is 1 mile above sea level. At a lower air pressure than sea level, water boils at a lower temperature.

If you can significantly reduce the air pressure -- using an industrial vacuum pump -- you can get water to boil at 76 degrees Fahrenheit if you pull a 29" vacuum (at sea level). Note: A near perfect vacuum is 29.5" at sea level -- this is the equivalent of being at 100,000 feet above sea level. For every 1000' of increase of altitude, vacuum pumps lose the ability to pull 1" of mercury. For example -- in Denver -- the mile high city, the best you can pull is 24.5", because you are 5280' above sea level. See this vacuum chart.

See the Boiling Point of Water at Various Vacuum Levels PDF attached.

This is how you build a Vacuum Kiln -- yes -- it's da bomb -- just not an atomic one. This thing would never ship on a plane -- it just has too many wires and tubes coming out of it -- but it does look cool ;-)

The reason why I wanted to build this is that I want to dry "found wood" and burls to turn on a lathe. A vacuum kiln can dry wood at near room temps in 2 - 3 days without inducing stress. You can dry the wood at about 85 degrees. Slightly above room temperature.

Step 1: Not an Original Thought

This is not my design.  I read a book called "Vacuum Kiln Drying For Woodworkers: How To Build And Use A Vacuum Kiln For Drying Wood" on

You can also get more information by visiting:

t is worthwhile to purchase the book.  I'm not going to go into great detail since don't want to take sales from the author.

You can put this together for about $500 if you get a great deal on a vacuum pump -- see this instructable if you want to refurb a pump.  You can also get these on eBay or local auctions.  On average you can pick one of these up for about $300 - 500 worst case.  A Welch 1400 will work just as well and weigh 1/3 of the Welch 1397

Step 2: Prepare the Vacuum Chamber

Start with a 4 foot section of 12" diameter schedule 40 PVC Pipe.  These are available from your locale hardware store -- NOT!  I ended up buying mine on eBay.  It runs about $35 a foot.

Cut it to size and then sand the ends smooth.  You will need to tap holes for the vacuum port, a drain in the bottom , electrical cables for heating on the inside, control wires to measure the temperature, and a vacuum release port.  All told, I put 7 holes in this pipe.

If you want to see how to tap a large pipe, click on this instructable

Step 3: Make the Ends

I used two 1/2 pieces of Corian that were glued together with special cement.  You can get this from a counter top fabricator.

Click on this instructable to see how to make the ends and also how to make an O-Ring.

You can also use 1/4" metal plates and bell gaskets as an alternative.

Step 4: Wrap the Chamber With an Insulating Blanket

The pipe is going to contain a light bulb and also a heated plate to heat the wood up to 85 degrees to heat the wood so that the water flashes to vapor.

You will need to insulate the pipe.  I bought a water heater blanket at Home Depot and cut it down.

I made sure that I had marked the location for the holes with a sharpie marker and cut "X"s in the right places.

The blanket comes with a silver tape to connect the foil.  I ended up buying more from Home Depot.

Step 5: Make a Stand

I milled some 2 x 4s to make a stand and allow for the drain line.

I used a Kreg Jig to drill pocket holes for the screws.

Step 6: Connect the Gauges and Test the Vacuum

You need a thermometer, a vacuum gauge, a big pipe for the vacuum line, a ball valve to break the vacuum, and  waterproof connections to get 110V AC into the pipe as well as control wires.

Instead of putting the wires in, I just used plugs for the holes for the wires so that I could troubleshoot any leaks.

Make sure that you use teflon tape.  I coated each of the ends of the threads with clear silicone to make a seal.

After I did this, I put the ends on and fired up the vacuum pump -- It works!!

Step 7: Install the Drain, 110V AC, and Low Voltage Control Wires

The drain at the bottom of the pipe will allow you to drain the condensate from the steam   You can use 1/2" schedule 40 PVC and a PVC valve.  It will withstand the vacuum.

The waterproof electrical connections allow SOOW 110v wire (get it at Home Depot) to enter the chamber.  This will allow you to provide power the the silicone rubber heating pad.  This pad draws 4 watts of power.  Search the web for "silicone rubber heating pads"

I also put in a light bulb.  Since you are heating wood in a vacuum, you don't get convection.  There is only radiant energy from the light or conduction from the heating pad.

I affixed the pad to an aluminum plate that rests in the chamber so that you can have something to hold the wood.

After I installed the drain and the power and control wires, I tested the vacuum again.

Step 8: Build the Control Box and Test

You need to use PIDs to control the temperature.  A PID is a proportional-integral-derivative controller.  It is the black box in the picture.  Search eBay and you can buy them in the US for about $35.  They come with a relay that allows you to control the set point temperature.  They also come with the thermo-couple so that the PID knows the temperature.

If you don't use a PID, you can't control the temperature.  The heating pad can quickly got to 400 degrees.  Just in case, I installed a thermostatic switch that closes at 140 degrees.  I also have a relay in the control box that will cut the power to the system if the thermostatic switch closes.  I have no desire to burn my garage down.

I put the PIDs and a few control switches in an acrylic box that I made.  See this instructable to learn how to make the box.

I put two PIDs in the controller since I had both a light and a heating pad.  It is easy to wire the PIDs.  Just make sure that you ground everything since there is going to be a lot of moisture in the pipe,

This shows the first test that I did -- I put a pint of COLD water in a plastic cup and set it on the heated plate.  I waited about 45 minutes and pumped the atmosphere out of the pipe for 20 minutes.  Note that the temperature on thermometer that measures only 76 degrees.  The water turned to vapor in the vacuum.

My vacuum pump really sucks !!!

I'm now ready to quickly dry "found wood"

I MADE THIS AT TECH SHOP !!!!!!!!!!!!!!!!!!!!!



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Thank you SOOOOO much. I was looking into vacuum systems for stabilizing wood and stumbled onto your 'ible. I teach woodworking at Techshop San Francisco and this is just the most logical thing in the world that I never thought of - thank you for opening my eyes to this possibility!

There is no need for a pump to run 24hrs a day with this. Get the ends of the tube to seal well and use desiccant beads inside. Hold the negative pressure with a valve and the beads will take care of the moisture.



Great 'ible. You just opened my eyes to a totally new approach.

I have a question about your comment that this is less stressful on the wood while drying.

I have a number of Sea Grape logs I would like to dry for woodworking. The wood is some of the most amazing I have ever seen. It is very dense, fine figured grain with subtle purple and yellow tones through the wood. You can polish it to a shine without any finishes at all.

The problem is that seasoning the wood without it cracking is almost impossible. The whole log is full of stress due to the way the trees grow sideways to the ground. The only approach I have had success with is covering the whole log with primer to slow the drying and let it sit in the dark back end of the garage for about 1.5 years.

I am extremely curious about your thoughts on how this would work for my problem.

Thanks for this 'ible! I had planned on building a vacuum chamber for stabilizing and staining wood using liquid resins. It never occurred to me that I could use it to dry green wood! I think I'll use the heating pad externally and forego the light bulb. Great project!

Wouldn't a fan used in a PC be of any value to circulate the condensate and to aid in distributing the radiant heat required?

1 reply

Normally yes, but in this case there is no air for the fan to circulate since the system is under vacuum.

Awesome project….
Since the temperatures required are so low, could it be warmed from the outside? Some heating pads under the exterior insulation could warm up the chamber walls, which would either conduct the heat directly to any wood in contact with the wall, or through radiant heating across the vacuum. It might be slower than using an interior heat pad & light bulb (because the bulb is hotter), but it certainly would be more uniform (and perhaps faster because it's more uniform).
Also, for more uniform heating: Instead of a single light bulb at one spot, perhaps a string of incandescent christmas lights could be strung over the length of the chamber. (It might be safer to spread out the heat too).
Do you preheat the wood before putting it into the chamber? That should speed up the process initially and put less demand on the heating elements (since everything would be at working temperatures) . Perhaps load the chamber and preheat everything with a hair drier blowing through it. (NB: the vacuum only reduces the boiling temperature of water, but not the heat of vaporization. So most of the heat required would still have to be supplied during the process, and preheating might not speed things up that much).

What a great idea, thanks! Never thought of drying wood this way, most of us (me included) are too impatient to let our wood dry naturally - sometime a year.

I have a couple of vacuum pumps (don't ask why!) so have a start on equipment. I believe Harbor Freight sells them for cheap.

Thanks for the good pressure/altitude chart. The discussions about elevation and vacuum are correct, but elevation will not affect your vacuum kiln. You are really concerned about the absolute pressure the wood is subjected to. If you could pull a full vacuum anywhere, regardless of the vacuum gauge reading, the absolute pressure will be zero.

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Thanks Bill. Make sure that you get a pump that will run for 3 days and still work. You're right about the altitude / pressure thing. More or less -- all things equal -- 29" at sea level = 24" at 5000' As you go up, the air gets thinner.

I would suggest that you're probably going to run into problems as the water you're removing from the wood is going to end up in the vacuum pump.

A reasonably inexpensive way of dealing with this is to create an intermediate chamber which is chilled by cooled water or ice will suffice. The water will condense in the trap and not make it into the pump.

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The Welch pumps have a gas ballast valve to avoid that problem. It is easier and less expensive to change the oil in the pump, than to get into chilling water.

How did you attach the ends or doors? Why cut the pipe off? If I paid $350 for a piece of pipe, I think I would use the whole thing. How much current does it draw with everything turned on? Do you actually get any condensate out of your drain?

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Take a close look at the ends. They have a 1/2" groove. If you don't use corian, then you just need to use a bungee cord just to help them from falling off when there is no vacuum. The vacuum holds the ends on while the pump is running. The vacuum pump has to be a continuous duty rated pump, so the motor will run 24 hours a day. Electrical usage is based on the size of the motor. The water condenses on the walls of the chamber depending on the temperature outside. You only need a 2' - 4' length of pipe.

This is amazing, I've never thought of such a thing, but it makes sense. I intend to buy a vacuum pump for making veneer and bent laminations whenever I can afford it, this just gives me even more incentive. Thanks!

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Most of the ones I have seen are made of metal tanks, think air tank with the end cut of and flanges welded on, with hinging end caps that clamp shut on a rubber seal. Drying wood is just one use. Taxidermists use them to dehydrate dead animals, people use them to dry food, cool dry delicate fabrics, as a vacuum "storage" tank for vacuum forming, and many other things. Can be very useful to a tinkerer.

Is anyone else having trouble typing certain characters or cutting text out of this textbox?

This is an excellent project to use for wood that you're planning to bend. I'm building a wood-strip kayak and I'm finding that wood dried in a standard kiln is much more difficult to successfully bend with a heat gun, probably because the high heat of a kiln alters the lignin. I wonder if this project would prevent that problem, or if getting the wood drier than air drying does would alter the lignin anyhow.

Odd, I used to haul lumber into Wilderness Pallet near Paso Robles, Ca. They were surrounded by cabbage fields. They would put pallet loads of cabbage into a large vacuum chamber to cool it for shipping. When they would unseal and open the doors there would be slush around the bottom of them. The operator said it was like the ol' airplane altitude effect. - Dunno'.? : - }

Well you have just given me another project to build and another book to buy. Great ible. I saw a dehumidifier kiln in Fine Woodworking magazine years ago and they used a fan to circulate the air for even heating. would this improve the efficiency or would the low air volume take care of that?

This 'ible contains the common assumption that the reader lives at sea level. If you live at altitude, the vacuum gauge readings need to be adjusted downward by about 1"Hg per 1000' MSL. So in Denver, don't expect to pull 29" of vacuum...24" is about as low as possible there. Note that Barometric pressure values are adjusted for altitude, so the Barometric pressure the Denver weatherman states is NOT the ambient pressure on the ground, but an estimate of what it would be in a mile deep