What's new in vacuum drying bowl blanks?

[Phil Light]

I'm surprised that the latest posts regarding DIY vacuum kilns for drying bowl blanks are from 2012 - assuming I was searching correctly. So, I thought I'd start a new thread and discuss my experiences so far. I bought a 15-gallon vacuum chamber from Amazon, along with a 16" Vacuum chamber digital heating pad some valves and other plumbing parts, and I'm using the Frugal Oil-FreeVacuum Pump that I already had for my vacuum chucks.


https://www.amazon.com/gp/product/B08PSP45TB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1


https://www.amazon.com/gp/product/B01L0M494Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1


https://www.frugalvacuumchuck.com/home.html

I'm not affiliated - just posting links if anyone is interested. I've only dried one load of two large walnut blanks so far, and I didn't really get them dry enough - I had to finish in the microwave. I have a second small load in the chamber now (two 12" and one 14" blanks 3 and 4 inches thick, respectively, very wet, green, and waxed from eBay), and I have a few questions that I hoped someone with more experience could answer. I get a lot of condensation on the glass lid. Big droplets precariously cling to and cover the entire inside surface of the heavy glass lid. The walls and floor of the pot also get pretty damp. Should I stop the vacuum now and then, remove the lid, and dry most of the liquid water out? Just letting air flow back into the chamber causes many droplets to dislodge and fall onto the blanks below. Or is it better to just keep the vacuum and heat running without interruption? I have the heating mat set to 140 degrees, but I don't have anything to measure the temperature of the wood in the chamber. In other words, should the drying/heating process be continuous or cyclic for best results? I hope to get some other pointers and spur a more up-to-date conversation.

 

[Mark Jundanian]

Though I am less experienced (read that as none) I am very interested in vacuum drying, too. Have you read Joshua Salesin's little book on vacuum drying?

Amazon.com

I recommend it.

Also, I think this video is worth watching:

View: https://www.youtube.com/watch?v=-I9DygHrLCo

This is a transcript, sort of:

https://edmontonwoodturners.ca/wp-content/uploads/2020/08/Vacuum-Kiln-Wood-Drying-PDF.pdf

I was under the impression that the "Frugal's" oil-less pumps do not pull enough of a vacuum for this application, so that may be a factor in not completely drying your blank. Also I think ideally you'd want to drain the condensate water. Though, I don't know if interrupting the dry cycle would be a good idea or not. Perhaps some absorbent material in the chamber would help if a drain can't be plumbed? Have you tried extending the dry cycle time?

Are you able to get a 14 x 14 x 4 inch blank into your chamber? If not how large will it accommodate?

 

[Richard Coers]

It takes more to vacuum dry that what you linked to. You need a controllable heat source at 90 degrees F contacting the wood that works in a vacuum.

 

[Kevin Jenness]

IDry kilns

 

[Phil Light]

Though I am less experienced (read that as none) I am very interested in vacuum drying, too. Have you read Joshua Salesin's little book on vacuum drying?

Amazon.com

I recommend it.

Also, I think this video is worth watching:

View: https://www.youtube.com/watch?v=-I9DygHrLCo

This is a transcript, sort of:

https://edmontonwoodturners.ca/wp-content/uploads/2020/08/Vacuum-Kiln-Wood-Drying-PDF.pdf

I was under the impression that the "Frugal's" oil-less pumps do not pull enough of a vacuum for this application, so that may be a factor in not completely drying your blank. Also I think ideally you'd want to drain the condensate water. Though, I don't know if interrupting the dry cycle would be a good idea or not. Perhaps some absorbent material in the chamber would help if a drain can't be plumbed? Have you tried extending the dry cycle time?

Are you able to get a 14 x 14 x 4 inch blank into your chamber? If not how large will it accommodate?

Yes, a 14" blank will fit easily in that pot. It's right around 16" ID (I don't want to take the lid off right not to get a precise measurement.)

The pump I'm using pulls just shy of 27" of mercury. I have two gauges on the system—one read 27", and the other read 26.8 just now. The bottom of the container's temperature reads 138 degrees F. I do have some cotton surgical towels in the bottom to collect what drips down from the lid.

And yes, I had seen that video, but I have not read the book.

 

[Phil Light]

It takes more to vacuum dry that what you linked to. You need a controllable heat source at 90 degrees F contacting the wood that works in a vacuum.

The heater is easy to control. It comes with a digital controller. I have it set at 140° F, which I read somewhere was an optimal temperature. The bottom of the chamber is that temperature, but I don't have it in direct contact with the lowest blank because of all the condensation. As I tried to dry the first batch, it seemed counterintuitive to have the bottom blank in direct contact with hot water that collected in the bottom. So, I have some cotton surgical towels in the bottom to soak up liquid water that drips down from the lid and sides and the bowl blanks elevated on 3/4" triangular stickers. My strategy this time was to leave the vacuum off for 24 hours so the wood would heat through to 140°, then switch on the vacuum. I stopped it once so far, yesterday, to dry the water from the lid and the little collected in the bottom. Less than 1/4 cup, I'd say.

27" of vacuum is the best I'm getting with that pump.

 

[Phil Light]

Here are some shots of my current setup. You can see the condensation clinging to the underside of that inch-thick glass lid.

 

[Phil Light]

IDry kilns

Those are way bigger than I would want for my little hobby shop.

 

[Kevin Jenness]

Those are way bigger than I would want for my little hobby shop.

I posted the link just as a source of information on commercial alternatives. You might want to contact them for advice on kiln schedules. Andrew Pierce has one of their units in his bowl turning operation here in VT.

Why don't you plumb a drain with a ball valve into the bottom of your vacuum vessel? If you tilt the vessel out of level and locate the drain near the wall the big drops might not build up so on the lid and you could minimize the water buildup in the bottom with regular draining. You could have an automatic drain if desired..

 

[Richard Coers]

The heater is easy to control. It comes with a digital controller. I have it set at 140° F, which I read somewhere was an optimal temperature. The bottom of the chamber is that temperature, but I don't have it in direct contact with the lowest blank because of all the condensation. As I tried to dry the first batch, it seemed counterintuitive to have the bottom blank in direct contact with hot water that collected in the bottom. So, I have some cotton surgical towels in the bottom to soak up liquid water that drips down from the lid and sides and the bowl blanks elevated on 3/4" triangular stickers. My strategy this time was to leave the vacuum off for 24 hours so the wood would heat through to 140°, then switch on the vacuum. I stopped it once so far, yesterday, to dry the water from the lid and the little collected in the bottom. Less than 1/4 cup, I'd say.

27" of vacuum is what I'm getting with that pump.

In a hard vacuum, water will boil at 90 degrees F. 140 is used in a conventional kiln to sterilize the wood, it is way too high to start green wood in any type of kiln.

 

[Phil Light]

I posted the link just as a source of information on commercial alternatives. Andrew Pierce has one of their units in his bowl turning operation here in VT.

Why don't you plumb a drain with a ball valve into the bottom of your vacuum vessel? If you tilt the vessel out of level and locate the drain near the wall the big drops might not build up so on the lid and you could minimize the water buildup in the bottom with regular draining. You could have an automatic drain if desired..

I have thought about that, but I would have to make sure I don't introduce a leak.

 

[robo hippy]

I had some lumber that was dried in a vacuum kiln. Different woods, as is always the case, require different optimum temperatures to cure and dry. For madrone, which really moves, they would strap it down to the floor of the kiln. First efforts had a lot of cracks in it. Turning down the temperatures really helped, but I have no idea what temps he used. What I liked about the wood was that if you ripped a board lengthwise, you got no warping, twisting, or cupping. It worked like air dried lumber. If I was going to dry bowls in a vacuum kiln, I would rough turn them first. No matter what you do, drying thick blanks is difficult to do with no cracking. For heat source, they ran extruded sheets of aluminum, I think, that were filled with antifreeze, in between layers of wood.

robo hippy

 

[Phil Light]

Richard Coers said:
In a hard vacuum, water will boil at 90 degrees F. 140 is used in a conventional kiln to sterilize the wood, it is way too high to start green wood in any type of kiln.

Well, it looks like I've done everything wrong, then. I thought that since I couldn't create a hard vacuum, that 90° boiling temperature that I had also read on the internet was irrelevant to my situation. With my dusty setup, I felt I could reliably generate and maintain a vacuum of 25 to 26 inches of mercury. I consulted some engineering tables that told me the boiling point of water at those pressures is 124.77° F to 133.22° F. I'm measuring my temperatures at the externally applied heating pad, not inside the chamber or the wood itself. Since we know that vacuum is an excellent insulator and my heated surface was not directly touching the wood, I figured the middle of a 4" thick bow blank would not likely approach that externally measured temperature. I based my 140° figure on those engineering tables, and an OSU article I read said that the vacuum kiln temperature should be maintained at 120°F to 190°F.

 

[Phil Light]

This is an experiment in progress, and I'm just sharing it because I figured others might be interested in my experience, mistakes and all. I also figured I might get some advice from others with more experience in small-batch vacuum drying than me. And this is a very small batch. We're talking a capacity of maybe five bowl blanks at a time. So far, three is the most I've tried. Today is day three for this experiment. Tomorrow I'll open the kiln, dry out the chamber, scrape off the remaining wax coating on these bowl blanks, let their surfaces dry, and measure their water content. I think both bowl blanks in the pot right now are Claro walnut.

 

[Mark Jundanian]

Keep up the experimental work, Phil. You are definitely pulling water out of the blanks. Maybe there's nothing wrong with the idea of periodically breaking the vacuum to remove the bulk of the water, then reapplying the vacuum.

Clarify for me, do you have the heating pad placed on the outside of the vac chamber, or the inside? And if inside is it in contact with one side of the blank?

 

[Richard Coers]

Richard Coers said:
In a hard vacuum, water will boil at 90 degrees F. 140 is used in a conventional kiln to sterilize the wood, it is way too high to start green wood in any type of kiln.

Well, it looks like I've done everything wrong, then. I thought that since I couldn't create a hard vacuum, that 90° boiling temperature that I had also read on the internet was irrelevant to my situation. With my dusty setup, I felt I could reliably generate and maintain a vacuum of 25 to 26 inches of mercury. I consulted some engineering tables that told me the boiling point of water at those pressures is 124.77° F to 133.22° F. I'm measuring my temperatures at the externally applied heating pad, not inside the chamber or the wood itself. Since we know that vacuum is an excellent insulator and my heated surface was not directly touching the wood, I figured the middle of a 4" thick bow blank would not likely approach that externally measured temperature. I based my 140° figure on those engineering tables, and an OSU article I read said that the vacuum kiln temperature should be maintained at 120°F to 190°F.

Are you talking dry bulb temperatures or wet bulb? I just used the boiling point as an example. The idea of kiln drying is not to boil the water. Here's a link to kiln schedules on conventional kilns, and your vacuum kiln should be considerable less. https://www.fpl.fs.usda.gov/documnt...s-for-commercial-woods-temperate-and-tropical

 

[Gary Beasley]

The problem with heating something under vacuum is heat will not transfer through a vacuum. Think of a thermos. Best heat transfer I think would be an infrared heat lamp directed through the lid. You would have to figure a way to keep the glass from overheating. Another factor is the pump. An oil type pump will accumulate water in the oil. Better to use an oilless vane pump like a Gast. Either way the water vapor should get pumped out. Another small fact is the water vapor presents itself as a small amount of pressure keeping the gauge from showing full vacuum.

 

[Phil Light]

The problem with heating something under vacuum is heat will not transfer through a vacuum. Think of a thermos. Best heat transfer I think would be an infrared heat lamp directed through the lid. You would have to figure a way to keep the glass from overheating. Another factor is the pump. An oil type pump will accumulate water in the oil. Better to use an oilless vane pump like a Gast. Either way the water vapor should get pumped out. Another small fact is the water vapor presents itself as a small amount of pressure keeping the gauge from showing full vacuum.

Well, the piston pump I'm using is oilless so water contamination of oil oil is not an issue. I agree an infrared lamp shining through the glass would heat the top bowl blank, but, as you say, it would be hard to control to prevent overheating of various components, including the glass lid. If I heat the pot intermittently - with the wood blanks stacked in an enclosed oven full of air, it seems like it should heat through just like baking a turkey.

 

[Phil Light]

Are you talking dry bulb temperatures or wet bulb? I just used the boiling point as an example. The idea of kiln drying is not to boil the water. Here's a link to kiln schedules on conventional kilns, and your vacuum kiln should be considerable less. https://www.fpl.fs.usda.gov/documnt...s-for-commercial-woods-temperate-and-tropical

If I recall from science class, a hygrometer measures humidity in air. Dry and wet bulbs are not applicable to measuring the temperature of liquid water. We used a sling psychrometer in high school science class.

 

[Tim Hunter]

Heat does travel through vacuum…by radiation rather than conduction.

Tim

 

[Mark Jundanian]

Heat does travel through vacuum…by radiation rather than conduction.

Tim

Yes, but I believe that process is very inefficient.

Joshua Salesin has the heating pad inside the vacuum chamber in direct physical contact with the wood being dried.

If I recall, he also uses a water trap on the vacuum line.

 

[Leo Van Der Loo]

The problem with heating something under vacuum is heat will not transfer through a vacuum. Think of a thermos. Best heat transfer I think would be an infrared heat lamp directed through the lid. You would have to figure a way to keep the glass from overheating. Another factor is the pump. An oil type pump will accumulate water in the oil. Better to use an oilless vane pump like a Gast. Either way the water vapor should get pumped out. Another small fact is the water vapor presents itself as a small amount of pressure keeping the gauge from showing full vacuum.

Wonder why we can feel the Sun if "heat" will not transfer through a vacuum, as you say ??

 

[Gary Beasley]

Wonder why we can feel the Sun if "heat" will not transfer through a vacuum, as you say ??

Clarifying, thermal conductivity by way of molecular motion.
Arent commercial vacuum kilns set up with low level microwave heating? Seems a better way to avoid stress cracking. Not that a homemade kiln can be done that way. An infrared source is easier to handle and can be used through a vacuum and monitored with an off contact thermometer.
The polished sides of the vacuum pot should reflect and redistribute the energy around the wood if room is allowed for it. I wouldnt think very high wattage would be needed, just enough to maintain a warm environmant in the pot.
You are right Leo, though its different from the way a heating pad transfers heat via contact.

 

[Richard Coers]

If I recall from science class, a hygrometer measures humidity in air. Dry and wet bulbs are not applicable to measuring the temperature of liquid water. We used a sling psychrometer in high school science class.

Wait, you set the temperature of your drying process by measuring the temperature of the water? I don't understand. I find it difficult to talk kiln drying here. Mixed terms and lots of confusion. How much wood drying experience do you have? Maybe you are way ahead of me and I'm barking up the wrong tree.

 

[Phil Light]

Richard Coers wrote:​

"Wait, you set the temperature of your drying process by measuring the temperature of the water? "
LOL! Yes, there is definitely some communication breakdown! I can't directly measure the temperature of the water inside the wood, which I ideally want to be a little greater than 124°F. With my current equipment, I can only monitor and adjust the temperature of the aluminum plate that comprises the bottom wall of the vacuum chamber, where the bowl blanks are resting. While the side walls and lid of the vacuum chamber feel quite warm, I don't currently possess a tool to measure their temperature accurately. I have not yet drilled holes in my vacuum chamber to run wires for power or data, but that may change. I've just started playing with vacuum drying.

When you declared that 140°F was 'way too high,' I tried to explain why I chose that temperature for my experiment. Did you understand this is the external temperature of the vacuum chamber's bottom (floor) plate? My rationale was based on the same understanding of the exchange of thermal energy between physical objects that any well-educated old guy has and the data from the engineering tables I consulted, which indicate that water boils at 124.77°F to 133.22°F in a partial vacuum of 25 to 26 inches of mercury - my experimental range.

You then asked if I was referring to dry bulb or wet bulb temperatures, which just didn't make sense to me—and still doesn't. When measuring solids or liquids like the aluminum plate that the kiln floor and walls are made from (or the wet wood inside), there isn't such a thing as a 'wet bulb/dry bulb' temperature measurement. That's a method used to calculate relative humidity and dew point in the air. I used this info years ago to calculate cooling loads. As far as I know, wet bulb/dry bulb thermometers are not used to measure or describe the temperature of materials. I learned to use an old-fashioned sling psychrometer in high school earth science class in the 1960s when calculating dew point and relative humidity, but I'm not interested in the relative humidity of the small air volume remaining in my partial vacuum chamber.

I've been a woodworker for a little over fifty years. Although I occasionally use kiln-dried lumber, most of my work has been made from air-dried wood. These days, I mostly just turn bowls. I live in rural Kentucky, which is filled with native hardwoods. I've dried wood stacked in my barn, in wood and electric heated kilns that can hold thousands of board feet (friends' kilns, not my own), primitive solar kilns, and in my microwave oven. At 71 years old, I realize that much of my wood will STILL be too wet to use after I die - especially since I favor the larger bowl blanks. I seldom turn anything smaller than 16"X4-5."

 

[Richard Coers]

Just one more comment, then I'm done. I sent you kiln schedules so you would see that no one starts drying wood at 140 degrees like you said in your original post. That's the temperature at the end.

 

[Phil Light]

Just one more comment, then I'm done. I sent you kiln schedules so you would see that no one starts drying wood at 140 degrees like you said in your original post. That's the temperature at the end.

Where can I find these vacuum kiln schedules? Or what is the usual starting temperature for the heat pad underneath an externally heated vacuum kiln?
Edit: Oops. Never mind - I found the link you provided above. Thanks. But that article is aimed at using a conventional warm-air drying kiln. Vacuum kilns work differently.

 

[Phil Light]

Yes, but I believe that process is very inefficient.

Joshua Salesin has the heating pad inside the vacuum chamber in direct physical contact with the wood being dried.

If I recall, he also uses a water trap on the vacuum line.

He does, but I'm not yet ready to source and buy more heat pads or drill into my vacuum pot. That may come, and it would clearly improve efficiency and control. I am planning on making a moisture trap using his suggestions.

 

[Kevin Jenness]

Where can I find these vacuum kiln schedules?

I would recommend contacting IDry. IDry kilns I know they have done some kiln runs to develop schedules for thick hardwoods and could at least give guidelines for bowl blanks. Andrew Pearce would be another potential source of info. andrewpearcebowls

I would not hesitate to drill into the vacuum chamber as needed for monitoring, interior heating or drainage. Adequately sealing penetrations should not be onerous at the moderate vacuum level you are using.