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 Amazon.com

You can also get more information by visiting: http://vacuumkilndrying.com/

It 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 !!!!!!!!!!!!!!!!!!!!!