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Whats the best method for drying large slabs on a budget

Started by urbanlumberinc, August 22, 2006, 11:52:45 PM

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urbanlumberinc

A vacuum kiln would be the obvious choice, but due to current financial constraints, how would I best go about quickly drying large slabs for a minimal investment?

Ga_Boy

Your constraints are exclusive of one another......You need to choose quickly or inexpensive.

Have you checked around your area for Vacuum Kilns in your are that accept custom jobs? 

If there are no Vacuum Kilns in your area; for air drying allow one year per inch. 

You can build a conventional kiln, I have about $20K in mine.  The $20K is for the material, this does not include my time.

I fire my kiln from an out door wood fired furnance (burn my wood waste), my only power requirement is for running the fans and sensors.

I can not offer insights on DH units.  But I would think that the power requirements for drying thick slabs would make it too expensive.



10 Acers in the Blue Ridge Mountains

DanG

I'm thinking that Den once said something about using a vacuum bag for drying a single piece.  Maybe he'll jump in here and enlighten us further. ;D
"I don't feel like an old man.  I feel like a young man who has something wrong with him."  Dick Cavett
"Beat not thy sword into a plowshare, rather beat the sword of thine enemy into a plowshare."

metalspinner

I do what the little voices in my wife's head tell me to do.

Den Socling

Yes, I would suggest a bag as a cheap substitute to a vacuum chamber. It has been discussed here before and has been done successfully. I would build a tank and use water as the heating medium. Each slab would need to be in it's own bag and submerged with the heating medium against both sides. Then pull vac.

jimF

Zhangjing Chen has a patent on such a setup.  Check out the patent office for information.  P# 6,634,118

beenthere

Found the following about the bag vacuum drying in heated water by Chen

Chen Zhangjing of Blacksburg, a research specialist in the T.M. Brooks Forest Products Center in the College of Natural Resources at Virginia Tech, and Fred M. Lamb of Christiansburg, professor emeritus of wood science in the college, have received patent 6,634,118 for a "Method and apparatus for vacuum drying wood in a collapsible container in a heated bath."

The system can be constructed to dry small or large dimension wood--from cabinet pieces to logs for homes, for instance. The vacuum system dries wood with little stress or variation in moisture content in individual boards. Zhangjing demonstrated that the vacuum system can dry red oak nine times faster than conventional drying methods. "It is simple to operate and, because only the area surrounding the wood is heated, this system will reduce equipment and energy costs," he said.

The patented technology transfers heat efficiently and effectively at lower temperature, so wood dries without changing color. It also can be modified to dry wood of mixed species, different thicknesses, and different initial moisture contents. The patent has been licensed to American Moistening Company (AMCO) of Pineville, N.C., which specializes in humidification technology. It is in use by a wood turning and carving company in Hickory, N.C.

Zhangjing received his Ph.D. from Virginia Tech in 1998.The technology was the subject of his dissertation research. Lamb, who was an Extension specialist in wood products and processing, was Zhangjing's committee chair.

south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

DanG

"I don't feel like an old man.  I feel like a young man who has something wrong with him."  Dick Cavett
"Beat not thy sword into a plowshare, rather beat the sword of thine enemy into a plowshare."

urbanlumberinc

So, if I understand this right, you place the lumber in a vacuum bag, imerse it in heated water, apply a vacuum, and the lumber dries.  Is the moisture just sucked out the vacuum bag?

DanG

That's about it in a nutshell, Urban.  Actually though, vacuum drying is a fairly complex process.  Like the other methods, there's a lot more to it than meets the eye.  The key is the fact that water boils at very low temperatures when the atmospheric pressure is greatly reduced.  If the internal temp of the wood is above the boiling point, the water in it evaporates and exits more easily.  Drying times are severely reduced.  The thicker the stock, the more dramatic the "vacuum advantage" becomes.  You could, of course, dry lumber very quickly by using very high temperatures, but the lumber would be damaged beyond use.

There has been a lot of discussion about it on these pages, and there is more info on Den Socling's website.  Read and enjoy. ;)
"I don't feel like an old man.  I feel like a young man who has something wrong with him."  Dick Cavett
"Beat not thy sword into a plowshare, rather beat the sword of thine enemy into a plowshare."

urbanlumberinc

Ok, here's my last (and probably dumbest) question - How is the moisture removed from the vacuum system, do you need to pull suction through some type of resivoir to collect the moisture?

footer

Quote from: urbanlumberinc on August 25, 2006, 11:35:19 AM
Ok, here's my last (and probably dumbest) question - How is the moisture removed from the vacuum system, do you need to pull suction through some type of resivoir to collect the moisture?
I'm no expert, but I assume it is expelled through the vacuum pump.

LeeB

I thought it was a pretty good question myself as I was a wondering the same thing. LeeB
'98 LT40HDD/Lombardini, Case 580L, Cat D4C, JD 3032 tractor, JD 5410 tractor, Husky 346, 372 and 562XP's. Stihl MS180 and MS361, 1998 and 2006 3/4 Ton 5.9 Cummins 4x4's, 1989 Dodge D100 w/ 318, and a 1966 Chevy C60 w/ dump bed.

Den Socling

Some vacuum pumps can handle the vapor but I never recommend that method for a long-lived kiln. We pull the vapor through a heat exchanger that condenses the water to keep it out of the pump. But, if you only want to dry a slab now and then, use a liquid ring pump and feed it fresh water. The vapor will be condensed in the pump.

urbanlumberinc

My final question then is:  Could I realistically expect to employ a DIY vacuum kiln to dry oversize slabs on a regular basis reliably and acuratley?  Based upon the information you guys have provided (thanks a ton by the way), I think I've got a pretty good picture in my head of how I'd go about building this contraption.  What are the possible pitfalls of this type approach?  If I've priced the right stuff I think I'll be able to construct a servicable vacuum bag drying system for around $5-600.  Would I be wasting my time and money to try and pull it off for that ammount?

Ga_Boy

To me the thermodynamics is the biggest problem to takle.  You need a constant supply of hot water to keep the drying process going.  This means you need a vessel large enough for your material yet insulated sufficently to allow the heat to transfer to the slabs you are drying. 

To dry more than one piece of material you will need a bag and hot water container for each or a way to layer multipule bags allowing enough room for water to circulate.   Lots of technical questions to tackle.   Vacuum drying is all about thermodynamics.....

Do not forget you will also need a way to weight or restrain the material to keep it flat.
10 Acers in the Blue Ridge Mountains

firecord

First off I know nothing about drying wood!!!!!!
But that's why I'm Here!!

Vacuum Kiln   The word drew my attention.   I googled it because that's what I do when I want to learn something.  Some info out there but no smoking gun that describes it.

Ok I get the premmes - Water boil at 212 at sea level - this temp will boil the water out of the wood - but damages it.  Reduce the atmosphere and water boils at lower temps.   So in theroy - place the wood in a pressure chamber - pull a vacuum - apply a low temp - wood dries faster.   Am I close

QuoteOk, here's my last (and probably dumbest) question - How is the moisture removed from the vacuum system, do you need to pull suction through some type of resivoir to collect the moisture?

This question really sparked my intrest  then this responce came.

QuoteSome vacuum pumps can handle the vapor but I never recommend that method for a long-lived kiln. We pull the vapor through a heat exchanger that condenses the water to keep it out of the pump. But, if you only want to dry a slab now and then, use a liquid ring pump and feed it fresh water. The vapor will be condensed in the pump

Lets say a fella  has a 48 in. dia.  schedule 40 steel pipe that happens to be 22 foot long.  OK it's me.   I weld  a cap on one end and build a pressure holding sealed door on the other.   I then put a series of rollers across the inside of the pipe (to roll 7x11 cant's inside)  I then weld some couplings onto said pipe and hook to the suction side of a lagre compressor I may have lying around, with gate (valves)of course.   Then add some heater elements - some controlls - etc.  Wrap the whole thing in insulation.  Put the cant's in - pull a vacuum - close valves - turn the heat on.  (The heaters are regulated and go on and off to maintain the desired temp.)   

My theory
The wood would dry  - the water would condinsate onto the pipe - and when I open the door the condinsate would roll out.  Because I built it at a slight tilt.

Am I way off base or would this work????

Den Socling

First of all, use PVC pipe or your wood could be ruined with stain. Next problem is heat transfer. Look up discontinuous vac discussed here. Then, with the PVC pipe comes condensation or lack of condensation. And the the amount of water - maybe a quart per board foot. Then pressure control. If the pressure isn't kept down where it belongs, the boiling point and wood temperature will rise. Water increases in volume approximately 1700x when it evaporates.

Vacuum kilns are the way of the future for large scale and difficult to dry species and thicknesses but it isn't simple. Otherwise, we would have competition.  :D

serg

I agree with Den.
Vacuum drying can dry all breeds of wood. A log diameter 220 õ6000ìì, a bar 230 õ 300 õ6000 mm.
Oak thick 80 mm, 90 mm a nut. All African trees. The vacuum lowers temperature of boiling of water. Constant temperature 75C, vacuum 0.5 humidity of a tree 65 - 25 % water not êåïèò. From 25 % - 5 % of humidity of a board to create vacuum 0.9, water boils on all section of wood. Result of humidity on thickness of 0.2 %, a board thickness of 50 mm. The faltering vacuum can be not applied.
Sergey.

urbanlumberinc

Okay then, about how much does a small sized comercially produced vacuum kiln cost?

Ga_Boy

$150K  for a 1MBF unit

This will get you the kiln, a heat source, condinsing unit and a trolly to handle the heating plates.
10 Acers in the Blue Ridge Mountains

Den Socling


firecord

Den

I went back and read the material you suggested.  Great stuff!  Although I won't try to say I understand it, I think I get the picture.  Thanks

Ga_Boy

Den,

When I priced your unit about 6 months ago, the kiln was $90K.

Then I had to buy a condinsing unit a heat source and a ganty crain to handle the heat plates and have all this stuff hooked up and powered up.

When the unit was installed and operational I had spent almost $150K


10 Acers in the Blue Ridge Mountains

Larry

Interesting and thought provoking article in the last edition of Sawmill & Woodlot.  Vacutherm 500 bf kiln...$50,000 with a purported payback of less than a year.  I thought the energy use was quite interesting also.

The bag thing will work...cause I did it as a small experiment.  Don't know how practical it would be in most cases.
Larry, making useful and beautiful things out of the most environmental friendly material on the planet.

We need to insure our customers understand the importance of our craft.

Den Socling

Hi Mark,

The numbers don't sound right. The cooling water system isn't included because many people are planning more kilns and it is economical to put a bigger system up front. A cooling system for a 1.25 cost less than $3000.

A vacuum grab to lift the plates is recommended for big kilns or multiple kiln installations. We have never sold one for the short plates of the 1.25. Two guys can easily build the kiln charge.

The last 1.25 that we sold included a propane boiler. I don't remember the cost but it was around $2000.

Installation cost anywhere from $400 plus expenses to $2000 plus expenses depending on how much you want to do yourself. Some customers have done everything themselves but we now say we do start up and that cost $400 plus travel expenses.

Den

Ga_Boy

Den,

My pricing allowed the use of an outdoor wood furnance.  The cost of the furnance and parts to bring it online is around $10K.  I also allowed for the cost of the gantry crain, I work alone most of the time.  Dependable help is hard to come by in my area, I have a bum back, (thanks to a US Navy Surgeon).  I do not remember the cost info on the condenser and chiller unit. 

The number that stuck in my head was to buy the kiln, the needed suport systems and bring the system online was kicking $150K.....
10 Acers in the Blue Ridge Mountains

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