Vacuum Kiln or Atomic Bomb -- You Decide

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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 Amazon.com

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

I
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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Participated in the
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1 Person Made This Project!

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30 Comments

0
BillWWT
BillWWT

Question 4 days ago on Step 7

What glue did you use to glue two pieces of Corian together?

0
KenDiPietro
KenDiPietro

7 years ago on Introduction

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.

0
bucklipe
bucklipe

Reply 2 years ago

Easier to fill an intermediate jar with an indicator dessicant.
Air to the vacuum pump goes through the dessicant media and gets absorbed.
When the dessicant turns pink, bake it, dry it and reuse it.
No moisture in pump oil...

0
tuijtubby
tuijtubby

Reply 3 months ago

The same low temperature and vacuum that's evaporating the moisture from the wood would also evaporate the moisture from a dessicant. Its like when you dry a dessicant in the oven.

So the idea of lowering the temperature in the water trap is valid, if the use of dessicants is the route you're taking.

0
CWKr
CWKr

Reply 7 years ago on Introduction

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.

0
kethevanecellard
kethevanecellard

Question 8 months ago on Step 2

Thanks for the article! Great to know you can make this kind of kiln with paying full price. I'm looking into vacuum kilns to get rid of the woodworms that might attack my wood. I know you can kill the bugs even at the larvae state using vacuum, leaving it in for a month or so. But I'm not sure if it works irl… anyone uses it for this purpose?

0
CWKr
CWKr

Answer 8 months ago

You are welcome. I am sorry -- but I don't know about entomology. I would suggest that you contact the agriculture department of a state "Ag school" and ask a professor.

0
Ronald Tumuhairwe
Ronald Tumuhairwe

9 months ago

Good day! Thanks for this project. I am working on a prototype for this same vacuum kiln. I have a couple of questions.
1. What are the minimal requirements for the vacuum pump to carryout this project? Can I use a pump of less than 17cfm?
2. Can you replace the heating pad with a salvaged water heating coils to boil the water at 70c in the kiln? What is the alternative to a heating pad?
3. What was the black hose pipe taking in/out of the kiln?

0
CWKr
CWKr

Reply 8 months ago

Hi -- Sorry it took so long for me to respond. You are welcome. This is the reference that I used to build the kiln: https://www.amazon.com/Vacuum-Kiln-Drying-Woodwork...

You must put heat into kiln. This book talks about the fact that since the wood is in a vacuum, you need to use heat that transfers via contact.

The black pipe is a 6" threaded 3/4" black pipe from Home Depot. You need to tap the 12" diameter PVC. I purchased the taps at Harbor Freight. You need to use a PID and make sure that you keep the heat input as low as possible or the wood will crack.

The issue with the pump is that you need to get as close to 29.5 (adjusted for altitude) as possible -- You need a pump that will work for A/C freon systems.

If you have more questions -- I will answer sooner.

0
okuzster
okuzster

2 years ago

a question: i keep hearing that fast drying will cause cracks on wood, did you (or anyone who tried) experience anything like that with vacuum drying?

0
workislove
workislove

3 years ago

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!

0
Cotherman
Cotherman

5 years ago

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.

0
JoW9
JoW9

5 years ago

Kilnbrick

0
drobertson123

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.

0
eranox
eranox

6 years ago

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!

0
ronald44181000
ronald44181000

6 years ago on Step 8

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?

0
eranox
eranox

Reply 6 years ago

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).

0
Bill WW
Bill WW

7 years ago on Introduction

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.

0
CWKr
CWKr

Reply 7 years ago on Introduction

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.