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Vacuum Kiln

S

Syd Sellers

Joined
Nov 26, 2011
Messages
138
Likes
8
Location
Rocky Mountain House, AB
Has anyone got plans for a shop made Vacuum Kiln. I have a heat source and a good old Vac Pump but I don't want to re invent the thing. I am getting tired of waiting several years for my wood to dry however. So it's time to get at it...

Any input would be great.

Thanks
 
There has been a fair amount of discussion on The Forestry Forum for vacuum kilns. A general one about radiant heat not moving through a vacuum. The commercial vacuum kilns use aluminum plate assemblies between layers of wood, that warm water is circulated through. Wood-Mizer sold one that used electrical resistance blankets between layers of wood. Some hobby ideas worked around the idea of putting the wet slab of wood in a vacuum bag, then submersing that under warm water. Discussions also head towards drawing all that moisture through a vacuum pump. The water from some woods is highly corrosive, and will eat up a pump in no time. Commercial units have chillers to pull the vapor through before seeing the inside of the pump. The benefits of the vacuum kiln is that water will boil in a vacuum around 90 degrees, depending on your elevation. The biggest down side, besides getting all the previous details worked out, is that density of the wood can vary how well each piece dries out. One straight grain blank, and a crotch blank will dry at very different rates. Sorry, no plans.
 
vaccum

Just a educated guess if you had a paint pressure pot and pulled a vaccum on it woud it not work the same? In hot as hell texas we never have too much moisture.
Ed
 
Just a small Unit.

ray hampton said:
are we talking about drying a whole tree 8 feet long or something about the size of a bowl
Click to expand...

If you are like me, I like to get a few pieces of wood from eBay every now and again, but as of late, most of this wood is storm recovered wood and it's green.

I don't want to wait several years for it to dry before I can use it. I'm not a green wood turner so I would like to help it along with the drying.

So if I could get a unit together to hold a Bowl blank I would be a happy guy.
Thanks
 
Technology of Vac Drying

I am slowly getting a feeling for all the problems associated with running a Vac dryer and the problems it creates for the pump. However, I have worked for years in the field of scientific research which uses freeze drying as a common way of drying tissue of all types. Plus I happen to live in the worlds largest freezer, it's called Alberta Canada so a small oil barrel sitting outside will provide all the protection I need for my pump by running the Vac line through the barrel that is sitting in -20 degree weather.

I am still wondering what the best way of providing heat and how much heat is needed to dry the blank. I do realize it has to be contact heating or maybe radiant heat of some kind. Maybe even infrared heat may do the job.

The reason I asked these questions is to try and reduce the amount of trial and error if I just dig in and make a unit. At this point in time, Temperature controllers and PCLs are pretty cheap, but do I need them???

Any input will be really good.
Thanks
 
Perfect timing; I've been working on a vacuum kiln for the same reasons. I have ordered Salesin's book on "Vacuum Kiln Drying For Woodworkers" on amazon. I should get it today or tomorrow. In the meantime, I have found two pieces of useful research which you may also like. The first is a short article by Lynn Diel, “Vacuum Kiln” which took some trouble to find on the internet (so much so I can’t find the working link now). The second is a more technical PhD dissertation by Chen “Primary Driving Force in Wood Vacuum Drying” (http://scholar.lib.vt.edu/theses/public/etd-02198-185538/materials/DISS.PDF). He has lots of data on red and white oak and most of it shows great results using “cyclic vacuum drying”. This entails heating the wood using conventional methods (or using the defrost setting in a microwave in my case) to 60 degrees celsius (about 140 degrees, hot to the touch). Then the vacuum is run at about 29.5” of mercury for 140 minutes. Each cycle seems to get about a 3-4% reduction in MC in his case, but the best reductions occur for high MC (above the fiber saturation point which is roughly 25-35% MC). I don’t think I did a very good job summarizing his findings, so take a look at it yourself.

As for my own progress, I have started with a decent gast vacuum pump (25" Hg) attached to a pressure cooker. It only holds two pieces about 5x5x3”, but enough to experiment for now. I can also set the whole thing on an electric glue pot and maintain the wood’s surface temperature at 115 degrees fahrenheit, even without any pre-heating in the microwave. My goal is to vacuum dry freshly cut holly before it loses its whiteness. Unlike oak, holly is not very permeable, so my results have not been as spectacular as I had hoped. Last night I did another cycle and managed to reduce 1414 grams of holly at about 27%MC to 1385 grams in 120 minutes. I don’t know how many more cycles it will take to finish the job. My moisture meter is not precise for MC above 20%; the 27% number came from someone else’s meter. Once I get it below 20 percent, I will have better data. I’ve also been experimenting with soaking the holly in DNA first, but this will prevent me from running the vacuum until the alcohol flashes off (or I will literally burn up my vacuum pump). I’ll post the more results if I find something that works for holly. In the meantime, I’ve got to return the wife’s pressure cooker and buy a larger one for my own use.
 
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Lars Stole said:
He has lots of data on red and white oak and most of it shows great results using “cyclic vacuum dryingâ€. This entails heating the wood using conventional methods (or using the defrost setting in a microwave in my case) to 60 degrees celsius (about 140 degrees, hot to the touch). Then the vacuum is run at about 29.5†of mercury for 140 minutes. Each cycle seems to get about a 3-4% reduction in MC in his case, but the best reductions occur for high MC (above the fiber saturation point which is roughly 25-35% MC).
Click to expand...

Strangely enough you get the same result with no vacuum using the microwave on roughs about a half inch thick around here. I'm around 700' above MSL, so standard day would be a barometer of 29.25 uncorrected. Wouldn't consider that a significant vacuum.
 
Good point. Chen's study used wood as thick as 2.5" so maybe his claim that conventional kiln heating doesn't dry wood nearly as fast as vacuum kilns should be understood in the context of thicker blanks. I haven't read the details carefully enough to be sure if this is the right way to consolidate the facts however.

Interestingly, Chen cites research that claims only high-temperature kilns (i.e., those with operating temperatures greater than 212 degrees fahrenheit) can dry wood as quickly as the vacuum kiln. Both processes get the internal water to a boil.
 
Too Much Heat...

This is very interesting data, but the whole idea of a Vacuum kiln is to keep the temp. low to prevent the wood fibers from cooking. My research so far points to a temp. max. of 66 degrees C. My guide point is that most of the wood blanks I get are coated with paraffin wax. It's melting point is in the lower 60's so all I'm going to do is heat till the wax starts to soften and then turn on the pump. May research to date also shows that the vacuum should be kept very low less than 1 PSI otherwise wood tissue damage could result due to cells collapsing. We don't want that. I am in the process of building a chamber at least 20" in Dia. with a Contact heater (Hot Plate) in the bottom to provide the heat. My biggest worry is that my pump is not Stainless steel, it's an old Cast Iron Vane pump oil filled, but the moisture that comes out of wood while under Vac. has a lot of acids in it.

The articles are fantastic, good luck to all. Keep trying!!!
 
Syd, why dont you just convert an old fridge into a bowl kiln using light bulbs as your heat source? the fridges get tossed every day and are free. A few 1/2 inch holes in the top and bottom and start with a 40 watt bulb in the bottom. If you are in no huge hurry the 40 watt will continue to dry enough blanks as you slowly move blanks from the top to the bottom. You will need to wax the outside of your blanks for this. Its cheap, its low tech and it works. A kiln full of wet blanks can be down to 6% in under 6 weeks.
 
1 psi?

Syd, I think we are both finding similar research regarding the ideal temperature for a vacuum kiln (I'm trying to hit 50-60 degrees celsius using my electric glue pot), but I'm curious about your findings that the vacuum should be kept very low "less than 1 PSI" otherwise wood tissue damage could result. I'm also confused about the conversion between inches of mercury and PSI. At sea level, there is about 15 PSI of pressure. To get about 1 PSI of pressure, wouldn't the vacuum have to be pull about 28" of mercury? Or does "1 PSI" mean a differential of 1 PSI between the inside and outside, in which case you would only be pulling only about 2" of mercury -- a very light vacuum? Thanks.
 
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Frige Kiln.

KellyDunn said:
Syd, why dont you just convert an old fridge into a bowl kiln using light bulbs as your heat source? the fridges get tossed every day and are free. A few 1/2 inch holes in the top and bottom and start with a 40 watt bulb in the bottom. If you are in no huge hurry the 40 watt will continue to dry enough blanks as you slowly move blanks from the top to the bottom. You will need to wax the outside of your blanks for this. Its cheap, its low tech and it works. A kiln full of wet blanks can be down to 6% in under 6 weeks.
Click to expand...

I've seen this idea before, and it is a real good alternative to a Vac Kiln. My floor space is one of the problems I hit right away. If I had a place to put it when it wasn't in use I would be set.

But as you say, it's a very cheap set up.
 
Low Vac Pressure.

Lars Stole said:
Syd, I think we are both finding similar research regarding the ideal temperature for a vacuum kiln (I'm trying to hit 50-60 degrees celsius using my electric glue pot), but I'm curious about your findings that the vacuum should be kept very low "less than 1 PSI" otherwise wood tissue damage could result. I'm also confused about the conversion between inches of mercury and PSI. At sea level, there is about 15 PSI of pressure. To get about 1 PSI of pressure, wouldn't the vacuum have to be pull about 28" of mercury? Or does "1 PSI" mean a differential of 1 PSI between the inside and outside, in which case you would only be pulling only about 2" of mercury -- a very light vacuum? Thanks.
Click to expand...

The data I found was from a commercial manufacturer of Vac. kilns and they stated the pressure in Torr. They maintain a Vac of 40 Torr.

This is where the fun comes in. I found a conversion table that stated 1 PSI is equal to 51 Torr. Now was this corrected to Sea Level or what???

I need to dig into this a bit more. As you say, this is a very small Vac., but if the wood tissue is green, it is very soft and mushy. A heavy Vac would suck it flat so maybe you start off really low and as the wood hardens up you increase the vac. Toward the end of the dry cycle, you should be able to put all the Vac on that the pump will pull.

We agree on the Temp. but lets see what this Vac is all about and is there a fancy ramp that one is to follow. You know the controller that is on some of these Vac. kilns is well into the thousands of Dollars so something has to be going on... Plus, it all changes as you change the wood type.
 
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Vacuum Kiln update.

Just to keep those interested abreast of what progress I have made.

First off, I found a 32 Gal. pressure tank that had a broken bladder so I was able to modify it to make my kiln tank. I had a flange welded on the tank and lid and I made a good silicone gasket to seal it. So I now have a nice tight tank that I can connect to may Vacuum pump and I can get it down to 26" of HG.
I then got an IR heater (600 Watt) to install in the lid so I can heat the wood.

On my real first test of drying some green Black Walnut, I put in approx. 3 cubic ft. of wood and set the heater to low and turned on my good old Welch 1402 vane pump. Within seconds I had a vacuum pump full of water!!!

So, my idea is good, but the use of an oil filled Vane pump isn't so good.
I am now looking for a Liquid Ring pump made to create a vacuum with the use of water for the seal. These pumps don't care how much water comes in. I had never heard of such a think, but they do exist. There are several companies making them.

Update to follow. So far total investment $200.00 Quantity of dry wood ZERO...
But I am making progress... I think.
 
Since you have $200 bucks in this project so far you probably will keep spending money. I dont know how much space your 32 gal thing takes up. My smallest fridge kiln is 30 in wide and 26 in deep. I also use the space on top to stack stuff. Not on one thats used for really wet stuff. But I have 4 of these. Two in the studio for finish drying. i have wood items on the top that also catch the heat. Total cost per kiln about 10 bucks. Burned out light bulbs are a recuring cost. 1st one made in 89. Here in the tropics and under my house it does not look so good. Plenty of rust and I have to use a stick to keep the door shut. Even if you had to have the thing outdoors a hundred watt bulb going 24-7 produces a very warm convection current of air. When I teach a class on making this cheap kiln I am astounded at folks who think they can build a plywood version or go hitec cheaper than a free fridge. Made by folks who KNOW how to keep the cold in and the heat out. We are just going backwards. I am glad you gave an update. So I am glad I gave you one also. Good luck.
 
I wet turn my bowls to about 20mm place it 50-50 dishwash & clean water for 2 weeks remove to a drying rack away form to much light and they dry in about 2 weeks without the cost of power or pumps ,The other way is to use a sealed container size depending on how much you are drying and use a dehumidifyer but this is slower
 
Josh Salesin wrote a fine book on making your own vacuum kiln from scratch. This is for small stuff (not for large timber). For details, see http://vacuumkilndrying.com/

I built one from an old pressure cooker after reading his book. If you use the pressure cooker idea, just be sure to use one of the higher pressure type (some are only rated for low pressure). Also, use one with a gasket fit lid -- not a taper-fit metal-to-metal fit (or you'll never get it open after pulling a vacuum).
 
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