Help with Building a Vacuum Chamber

[Bam654]

TL;DR

A client asked me to look into designing a vacuum chamber for drying out lumber. I am a decent fabricator but sadly not so great at the physics side of things.

A client asked me to look into designing a vacuum chamber for drying out lumber. I am a decent fabricator but sadly not so great at the physics side of things. I know the chamber I'm building will need pretty thick material and a lot of reinforcement. He wants it to be roughly 10'x3'x3' inside dimensions. I'm planning on building a shell out of steel tubing with supports every 10 inches but need to figure out what thickness of steel sheet metal. As well how to handle corners. Any advice is helpful

 

[Vanadium 50]

You realize there will be 150 tons of force on this device, right?

That much force, and I'd be calling an engineer and not trying to get by on "pretty thick".

 

[sophiecentaur]

If you want a strong enough structure to support a high vacuum then go for a cylinder with hemispherical ends and strong flanged joints. That's inherently self supporting and it's pretty universal (see submarine hulls!)
But, to the basics: what actual pressure do you reckon you need to speed up drying? (About 0.1 Bar, I have read).
Take a look at GOOGLE IMAGES with a search term "Vacuum Drying Lumber". I found that they use designs along the lines I described above. They look pretty expensive - which is why you want to make your own, I imagine. But they are expensive for a good reason. You would never get employer's insurance for equipment like this that isn't certificated.

 

[phinds]

He wants it to be roughly 10'x3'x3' inside dimensions.

AAAKKKK ! I think you will need to consult NASA

Is your customer filthy rich?

 

[sophiecentaur]

Is your customer filthy rich?

I was thinking of an offcut of Oil / Natural Gas Pipeline.
It would take a lot of dry timber to pay for itself, judging from those Google Images.

 

[berkeman]

Thread closed temporarily for a Google Images search...

 

[berkeman]

@Bam654 Welcome to the PF.

A client asked me to look into designing a vacuum chamber for drying out lumber.

If the overall design goal is to dry lumber, you may not need to use a high vacuum chamber for that (with the associated implosion dangers). It looks from my quick Google Images search that there are other options like kilns that do not involve pulling a vacuum.

I'll re-open this thread in case we can be of help with general questions about drying wood. @phinds is our local woodworking expert (and artist), so we should be able to help you with non-dangerous solutions to this technical challenge.

https://www.google.com/search?biw=1206&bih=564&tbm=isch&sxsrf=ACYBGNSEuwd9qzGG0zFw6n9wyXzdwwU7kw:1575422712909&sa=1&ei=-ArnXb-QN9O80PEP7vG24Ac&q=vacuum+chamber+for+drying+lumber&oq=vacuum+chamber+for+drying+lumber&gs_l=img.3...2824.4612..4677...1.0..0.65.477.8...0...1..gws-wiz-img...35i39j0i24.DViNx6OpzX8&ved=0ahUKEwi_7LH56prmAhVTHjQIHe64DXwQ4dUDCAY&uact=5

 

[phinds]

Yeah, I think a kiln is a much better solution. Likely to be an order of magnitude or two less expensive than a vacuum chamber. Lots of people use home made "solar kilns" which work very well, but do require more time than a vacuum chamber would.

 

[phinds]

@Bam654 if you do end up going w/ a kiln, couple of things. They are easy to build (for the size you're talking about), relatively cheap, and very easy to heat. The tricks you have to pay attention to are these: (1) you have to maintain a constant temperature, and sometimes it has to change at designated times, and (2) you have to have a good kiln schedule for each of the woods that are to be dried (it's not a "one size fits all" kind of thing) to know what temperature to use and to know when (if at all) the temperature needs to change.

 

[Bam654]

@Bam654 if you do end up going w/ a kiln, couple of things. They are easy to build (for the size you're talking about), relatively cheap, and very easy to heat. The tricks you have to pay attention to are these: (1) you have to maintain a constant temperature, and sometimes it has to changes at designated times, and (2) you have to have a good kiln schedule for each of the woods that are to be dried (it's not a "one size fits all" kind of thing) to know what temperature to use and to know when (if at all) the temperature needs to change.

Hey thanks for the help! Yeah the customer really wanted to do a vacuum because he uses a fair amount of oak and the dry time for oak is sped up when using a vacuum vs kiln. I'm just going off what he has told me. But he was open to the idea of a regular kiln sounds like that's his best option.

 

[berkeman]

It sounds like such a system doesn't use high vacuum, right? Does anybody know what level of vacuum is pulled on typical systems with the large vacuum lumber plank drying tanks? How do you heat a pile of wood planks in a vacuum?

 

[phinds]

It sounds like such a system doesn't use high vacuum, right? Does anybody know what level of vacuum is pulled on typical systems with the large vacuum lumber plank drying tanks? How do you heat a pile of wood planks in a vacuum?

I don't really know that stuff but I do seem to recall* that you first apply the vacuum and then after a while you re-pressurize the chamber and use heat.

*but remember, I'm old and my memory is not what it used to be **

** Actually, my memory never was what it used to be.

 

[sophiecentaur]

The ‘problem’ with vacua is that even moderately low pressure involves large Forces. A guy who supplies me with logs told me that it’s common to use old shipping containers with a fire of burning logs beneath. (A ready fuel source).
Cycles of heating and rapid air extraction gives good drying, BUT it is not good if you want stress free timber for making things with.
Edit: I don't use kiln dried but have a lot of regular logs, stored for ages. Cheaper.

 

[jrmichler]

How much faster does a vacuum kiln dry wood than a typical dry kiln? If twice as fast (don't know, just as an example), then two dry kilns would dry as much lumber as one equal sized vacuum kiln. And the two dry kilns would cost less to to buy and operate. The Dry Kiln Operator's Manual, USDA Agriculture Handbook No. 188, is a good source for estimating drying time of a dry kiln: https://www.esf.edu/wus/documents/DryKilnOperatorsManual.pdf. That book is a good read on how to operate a dry kiln to get good quality dried wood.

My dad once built a solar lumber kiln. It worked well because he gave it regular attention to control the drying rate.

 

[phinds]

I have it anecdotally that rapid drying promotes drying defects. That's why kiln schedules involve long times (weeks not hours or days) for proper drying. Of course it depends on the thickness of the planks but in general rapid drying is not a good idea for anything but small pieces. Artisans working with small pieces will sometimes dry them quickly in a microwave but in general that's not a good idea.

 

[sophiecentaur]

Kiln dried wood is not as good as naturally seasoned wood. I remember seeing images of a tree trunk, sawn into planks and then re- assembled with small air spaces left between the parts. Natural drying out that way is the best way to avoid stresses and consequent warping.
But, if it's firewood you want then heat gets the job done quickly. What quality of lumber is the client after?

 

[phinds]

Kiln dried wood is not as good as naturally seasoned wood.

That is true but kiln drying is how almost all wood gets dried to use for furniture, both fancy and plain, and it is quite adequate. You lose a bit to drying defects but that is acceptable for commercial operations which can't wait for a couple of years while wood dries.

 

[Keith_McClary]

1.What is vacuum pressure impregnated wood?

Vacuum pressure impregnated wood is wood that, through the application of a preservative, is protected for many years against fungi and insects. To achieve this effect, wood is put in a closed horizontal cylinder Ð the autoclave Ð and then the air is drawn out of the cylinder and out of the wood cells. Then the preservative is admitted and the content of the treatment cylinder is put under pressure. By doing this, the preservative enters deep into the wood cells. After this, the vacuum is sucked again to make sure that any excess preservative is completely removed. The process is brought to an end by a fixation treatment to ensure that the applied preservative will remain in the wood by entering into a chemical reaction with the wood.

https://www.showood.gr/en/about/woodimpregnation
Image of chamber:
https://www.showood.gr/pictures/original/b_23831_or_b_22095_or_vaccuum_(1).jpg

This one
http://www.isve.com/en/vacuum-impregnation-plants-imp-vac
says:

vacuum of -730 mmHg

 

[phinds]

https://www.showood.gr/en/about/woodimpregnation
Image of chamber:
https://www.showood.gr/pictures/original/b_23831_or_b_22095_or_vaccuum_(1).jpg

Why are you bringing in something that no one asked about and that has nothing to do with this thread? Vacuum impregnation is an entirely separate topic than vacuum drying.

 

[Spinnor]

Vacuum driers for small to medium sized carpenter’s workshops
The ISVE EM2V vacuum drier has been designed and manufactured especially for small to medium sized carpenter’s workshops, with small quantities of wood to be dried, but requiring high quality and short drying times.

From, http://www.isve.com/en/vacuum-dry-kilns-model-em2v

Description

• High frequency heating.A new technology totally different from the traditional conductive heating.Heating wood from inside to outside because HF wave can penetrate thick wood.Where there is moisture,there is high frequency.
• Vacuum condition.It can create a minus pressure,the moisture will be sucked out fastly.The water will vapor in a more lower temperature.It realize low temperature drying,more suitable to wood easy to spark.
• PLC control system. The PLC comes with common hardwood drying program ready for use. The dryer can run with set program without person guiding or control, gives you perfect drying result without cracking,bending or color change.
• Application.It is suitable for all kind of wood,from normal solid wood,like pine, walnut,merbau, cypress and so on,to veneer laminate,avoiding shrinking and bending

From, https://www.ecvv.com/product/4763751.html

Both from, https://www.google.com/search?q=Vac...biJD6sQ_AUoAnoECA8QBA&biw=931&bih=604#imgrc=_

 

[Baluncore]

Why are you bringing in something that no one asked about and that has nothing to do with this thread? Vacuum impregnation is an entirely separate topic than vacuum drying.

No. They both employ the same vacuum pump and vessel, and they both operate at similar pressures. I built a 6.5 m long drier that did both jobs. The only difference is that at the end of the process the wood is filled with preservative instead of air.

The product of positive pressure and volume is a good indicator of risk. Stored energy is the liability, but a vacuum vessel actually has negative energy. Only when something might be ejected, such as glass, or an electron gun from a CRT, is there a real danger. The danger of implosion is often exaggerated. When a 20 litre drum collapses, as steam condenses in a classroom demonstration of air pressure, you must be careful not to pinch your fingers.

The construction of a vacuum chamber for drying should employ a circular section vessel, but it does not require a massively heavy steel wall. If it has a thin wall, then there must be hoop rings to prevent out-of-round buckling and collapse. A round concrete pipe with an external coat of paint would probably meet the required specifications.

Efficient loading and unloading are important. Rails inside the vessel allow carriages of wood to be quickly wheeled in/out or through.

 

[sophiecentaur]

I built a 6.5 m long drier that did both jobs.

Good job! What pressure did it operate at? What kind of pump did you use?

 

[Baluncore]

I tried various pumps from industrial scrap. Best was a vane pump from an old vacuum truck. Time and pressure depend on temperature, species and weight reduction requirement. Aim to pull the chamber down to 1 psi absolute.

If I needed to do it again I would build two chambers, next to each other, only one pumping system is needed. You must consider economy of operation, throughput, loading and unloading. A portable system would use steel tubes and be built as a module, to fit on a semitrailer or slide tray truck. A static system would use larger diameter concrete pipes.

 

[sophiecentaur]

Aim to pull the chamber down to 1 psi absolute.

I've been looking at temperature / partial pressure tables and I can't come to a conclusion about what would be best value. Where did your 1 psi figure come from? Experiments? The latent heat could perhaps be involved?

 

[Baluncore]

I believe there are too many variables to provide a single answer. Wood dries faster or slower depending on species, thickness, temperature and pressure. I think these temperatures for the boiling point of water at different pressures are appropriate to this discussion.
psi = °C
14.6 = 100.
5.00 = 72.1
1.00 = 38.1
0.70 = 32.0
0.50 = 26.7
0.35 = 20.5
0.25 = 15.5
0.20 = 11.6
0.15 = 7.3
0.10 = 2.2

I air dry the timber I mill, so I do not need a vacuum chamber/kiln for special species projects with critical delivery times. I got the prototype system working for a bush engineer friend and his mate. I later adapted it for vacuum impregnation of preservative. It seemed happy to dry timber at 1 psi, maybe the vacuum gauge was unreliable and it was running at slightly lower pressures (0.2 psi) when colder.

For an unheated vacuum drying chamber, the operating pressure will depend on the temperature of the charge. The temperature will fall as the vacuum is pulled. There will then be a problem with supplying the energy needed to evaporate the water.

Wood holds onto water so the boiling point has little effect on tightly bound water. But once the water is free, if it condenses onto surfaces, it will be in equilibrium and prevent further drying. At a fixed temperature, removing a volume of steam will boil more water and stabilise the pressure again. The chamber will then have a steam atmosphere. That steam must be continuously removed economically to keep the drying progressing.

The liquid water in the timber gradually evaporates to a much greater volume of steam. That volume is increased again 14 times at 1 psi abs. The pump used to pull the initial vacuum needs a high capacity. The pump that removes the water vapour towards the end of the process may be a different pump, it needs to operate efficiently and economically for several days.

The low-density steam being extracted would normally condense in the exhaust port of the extractor pump. But if it could be condensed in the chamber it would help to maintain the low operating chamber pressure. One experiment I have contemplated would be to have a cooler and drip tray in the chamber, (insulated from below). Steam at chamber temperature would condense into the tray, then flow as liquid to a cool external port where the water could be extracted more efficiently. The volume pumped multiplied by the pressure difference would effect a huge reduction in pumping costs compared with the low-density steam extraction. Let me know if anyone tries it.

I believe there are also chemical changes that take place over time while air drying timber. I expect the vacuum is helping to dry the timber, while only the heat of a kiln can accelerate the chemical changes. Near room temperature, for each increase by 10°C, the rate of a chemical reaction approximately doubles. A kiln increases the temperature, so it increases the rate of chemical changes. Going from 20°C to 90°C is a 70°C increase, 2^7 = 128. So a one year reaction would take 3 days. To boil at 1 psi requires warming to 38.1°C, which is easy to do from 20°C. But the chemical reactions will only speed up by a factor of 4. So a one year reaction would take 3 months. It would be interesting to know what chemistry takes place, and when it happens if the material was vacuum dried quickly.

 

[phinds]

It would be interesting to know what chemistry takes place, and when it happens if the material was vacuum dried quickly.

I'm not really sure about this but I think that unless you are using pretty high heat and the wood has something like resin (especially resin pockets) in it, there really isn't much chemical action going on. What DOES happen is that there are structural changes since some of the cells / cell walls collapse when the water goes out and there is an overall shrinkage of the wood.

 

[sophiecentaur]

I air dry the timber I mill,

Your posts on this thread score some of the lowest BS factors that I have seen on PF. Congratulations!.

(I love the Condensing Clothes Dryer technology that you snuck in there.)

 

[Baluncore]

(I love the Condensing Clothes Dryer technology that you snuck in there.)

I was thinking of a split cooling system, but now I'm contemplating an extractor tray with fins above, cooled by peltier elements. The heat is usefully released within the chamber. Maybe the extractor can then be a simple peristaltic pump activated by a float switch.

 

[Baluncore]

In 2018, CRC Press released a new book in the series; Advances in Drying Technology.
Title; Industrial Heat Pump-Assisted Wood Drying. Edited by; Vasile Minea.
ISBN -13: 9781138041257 (Hardback)

This is essential reading for those building vacuum drying systems for wood.
Here are a couple of short extracts to encourage readers.

Section 6.6.2 Radio Frequency And Microwave Drying.
“In practice, the use of vacuum in combination with radio-frequency and microwave power inputs is very convenient, especially to avoid generating high internal pressures in the lumber that may develop on drying at atmospheric pressure (Keey et al. 2000). The radio-frequency drying of Australian hardwood and Mountain ash at absolute pressures of 1 kPa compared with 10 kPa increases the initial drying rate twofold at the lower pressure, and there is a rapid rise in the core temperatures at the higher pressure (10 kPa) even at low power inputs (Rozsa 1994). However, for power inputs of 1,250 W and, vacuum pressures of up to 10 kPa may need expensive pressure vessels as drying chambers, thus increasing the capital cost (Antti 1992a, b).”

Section 6.6.4 Vacuum Drying.
“In order to compensate for the loss of thermal capacity of the air, a slightly higher pressure level (e.g., more than 100 mbar), together with a air velocity (100 m/s m or more), may be very effective.”