dry kiln for drying bowls

[Greg_Kaldor]

Hi all,
My name  is Greg Kaldor and I live up in Northern Wi.  I want to build a dry kiln for drying my bowls as I am a woodturner.  I have everything I need except the de-humidifier.  Here is the problem.  I am interested in buying it used because the cost of a new one is so expensive.  Also my unit would be pretty small.  Anybody have any ideas?  Any input would be most helpful.

Thanks,
Greg

[Jeff]

Welcome Greg. You should probably pose this question in the drying portion of the forum. I can move this one over there if you like. I didnt want to move it without notifying you first since you are new to the Forestry forum.

[Greg_Kaldor]

Hi Jeff,
Thanks for the responce.  Yes I am new and yes go ahead and move me.

Greg

[Bill_B]

Welcome
 I have heard of other turners using an old frig or other old appliance for the box. Then you need a small fan to vent it. Also add a light bulb for heat.
Have you ever tried using the soap methed to help in drying?
Have fun

[Greg_Kaldor]

Hi Bill,
The idea with the fridge, this is to small and I do need a de-humidification unit.  Also I tried the soap method and God what a mess.

Greg

[beenthere]

At what stage of turning (I assume green) are you drying?  And are these bowls (species?) turned very thin, or thick in some places?  Do you have blanks for turning with (or without) the pith?  

There used to be a bowl turner in SW Wisconsin (Mineral Point) by the name of Harry Norr (sp?) and he turned large bowls (20" diam or so) to a very thin thickness (1/8 to 3/16") and worked in epoxy while the bowls were still on the lathe. He then had a clamp arrangement to hold the bowls while he dried them at (I think) 200 degree in the kitchen oven. When they came out (some were cracked, but most were not) he rubbed in more epoxy and polished them.  They were always mis-shapen due to drying, but were very nice looking and he sold them for some big bucks. I visited him in the 70's when he was doing his work. He was the retired PostMaster.  

Wondering how you are doing yours?  

[Greg_Kaldor]

Hi Beenthere,
I do most of my work in Birdseye, Cherry and Oak.  Alot of them are natural edge also.  All the ones that are not I turn to
a thickness of 10% of the dia.  A 20" bowl would be 2".  The
big ones have to sit 1 year... to long for me.  This is why I want the kiln.  I am also taking delivery next week of the VB36
it is a very large lathe that comes from England.  I seen a Demo of a 5 ft. dia. bowl out of Teak being turned.  I am told its capability of stock is 600lbs.  This is trully a BEAST.

Greg

[beenthere]

Sounds pretty impressive.

How do you manage the shrinkage of the wood when it dries?  Or do you tolerate the cracks in one years' drying, or not get any?  Have you been doing this for very long?  Any pictures?  2 inches of wood will be tough to air dry or kiln dry, IMO.

All the questions for now - I ran out ;D

[Jeff]

Greg I hope you can take pictures. I want to see that thing.

[Den Socling]

Since you sound serious, I suggest a vacuum kiln. Not the type we build but a 'discontinuous' vac kiln. These are fairly simple. They are slow compared to the type we build but a heck of a lot faster than a year.

With 'discontinuous', you heat the wood with warm air. If there is a chance of cracking, you might elevate the humidity. When the wood is up to temperature, pull a vacuum. When the wood has cooled off in the vacuum, repeat the process.

[Greg_Kaldor]

Hi Den,
I would like very much to hear more about the Vacume Kiln, this whole field is very greek to me.  Wood distorts most times in a predictable manner, but there can always be surprises.  About pictures I have to go thru my wife for the way to do it. This vacume kiln I would not Know where to go for info, please don't desert me in my time of need.  Ha Ha

Thanks,
Greg

[beenthere]

I found a site for the VB36. That looks like one beauty of a lathe for bowl turning. Congrats.

http://www.hegner.co.uk/system/index.html

(click on VB manufacturing)

[Den Socling]

Greg,
If you want to build a vac kiln. I'll lead you through it here. But it will take some money. do you weld? The expensive parts are the chamber and vacuum pump. The chamber can come from something like a propane tank but it needs to be cut. Then flanges, hinges and clamps need to be welded. Lots of metal work.
Den

[Bill_B]

Den
What would be a ballpark drying time say for 2 inch wall thickness?
Thanks

[Den Socling]

There's a lot of variables. I'd guesstimate around one week for 2" to get it to about 10%. It takes continuously heated vac kilns about 5 days to get 2" to 6%. The nice thing about vacuum for bowl turners is that you can dry from under the surface with vacuum. So very thick pieces can be dried.

[Greg_Kaldor]

Hi Den,
I do not weld but enough people owe me favors.  I would just have to know ahead of time just how big a project this is going to be.  Also how long would it take to dry bowl 2" thick.
maybe you can send me some more info. I am most
grateful.  one more thing how much money would you think.
Greg

[Jeff]

I would like to see this discussion continued here and not off forum. Valuable information for others.

[ronwood]

Den,

Does a 'discontinuous' vac kiln  dry a load of wood quicker than dehumidification kiln say from Nyle or Ebac. Would you get less degrade?

Thanks
Ron

[Fla._Deadheader]

  I too hope this thread goes the distance. We are very fortunate to have Den advise us. He has so much 'sperence to help those of us that are seriously looking at small kilns.

  Being that we ARE welders, this info will be most valuable. Thanks, Den ;) ;)

[Greg_Kaldor]

Hi Den,
There seems to be a great deal of interest regarding this subject.  I know that woodturners such as myself have always searched for a quick safe way to dry our rough turned pieces.  I agree that this would be best searved by keeping this thread open so that everybody now interested may still
benefit.  My only intention for asking you direct was for clarity.  It is quite difficult to gage this project on the whole by
this method though.  I see that the vacume kiln might be the Holy Grail for me if it is not to expensive.

Greg

[Den Socling]

Discontinuous vac kilns used to be built by an Italian company and VacuTherm tried to sell them. With thick stock, they are probably 5 or 10 times faster than conventional kilns. But they are slow compared to continuously heated vac kilns. I've worked on them and the wiring, relays and timers needed for automation in the 'old days' are a nightmare. The patent has expired and one might be built by an enthusiast.

The basics are as follows: You heat the wood by blowing warm air over it. Lumber gets stickers. Blocks would just take time. When you heat the wood, you increase a property of the water in the wood. This property is called vapor pressure. If you heated until the vapor pressure was (for discussion) 50 and then you pulled the chamber pressure down to 50, all of the water would instantly evaporate. This doesn't happen in reality because, when water evaporates, it takes heat with it. The remaining water cools and it's vapor pressure is lower - say it's 40. Then too, the evaporating water fills space and eliminates some of the vacuum so the chamber pressure might become 60. Instead of all of the water evaporating, only some can and then the 50/50 doesn't exist anymore.

With discontinuous vac drying, you heat with air and then pull vac. Repeatedly. You could do it by hand. You could do some automation. You could make it completely automated. The cost, for the most part, depends on how much you will scrounge for parts and how much you want automated.

[shopteacher]

Ya got us all setting on the edge of our seat Den. Please continue.

[Greg_Kaldor]

I have a good friend where I work who is an instrument man and I am sure will give me tech. advice.  Most of this is all Greek to me.  But I will try very hard this is very important to me.  I will be watching for more.  Thankyou.

Greg

[Den Socling]

It's been a long day with a DanG 50 HP freq drive blowing up in my face, it's dinner time and I'm ready for a beer.

But you need a chamber. I'm always wondering about propane tanks. They come in all sizes. They are stronger than needed. If you could just cut the end off and turn it into a door.

[etat]

careful cutting into a propane tank.  first remove the valve, then either fill full of water and cut, or hook up a hose to the exhaust of an engine and cut while co2 is pumping into the cylinder.  Otherwise you might find you head going into orbit, along with the tank

[Bill_B]

The chamber that is used needs to be totaly sealed?
What kind of vacumn unit would be needed?
What temp should  be reached?
Rough figures are only needed right now.
This sounds like a interesting project.

[Den Socling]

If I planned on using a propane tank, I'd take it to professionals to be cut. There are shops that repair gas tanks. I'd use one.

Propane tanks aren't the only possibility. Any cylinder designed to hold pressure might work. A total vacuum puts only 14 psi on the tank (from the outside). This doesn't sound like much but it can flatten a 'box' design real quick. That's why I suggest a cylinder. Something from an air compressor?

After you cut off the end, you would need to weld pipe flanges on both pieces. Door movement (hinge, boom, truck or whatever) would depend on size. The chamber needs to be totally sealed but you would not need all the bolts that a pipe flange might have. As soon as you start pulling vacuum, atmospheric pressure will push it in tight.

During the heating cycle, I would pump warm water through an automobile radiator inside the chamber. Nipples for water connection in and out are easy. I might try low temperature, maybe 110F for wet wood. Put a fan behind the radiator to circulate. If there is a chance of cracking, elevate RH by spraying water on the radiator.

Now the necessary process control. You would need a temperature sensor (an RTD) in a sample of wood. You would run the heating cycle only until the wood was 110F.

When the wood is up to temperature, I'd start a little, two-stage, liquid ring vacuum pump. To keep things cheap and simple, I would use a partial recovery system and dump some water to keep it cool and clean. After starting the pump, I'd watch the wood temperature. The wood is going to start to cool. When the temperature stops dropping, there is no use in running the vac any longer. Open a valve, let air in and heat it again.

As the the MC of the wood comes down, I'd raise the temperature of the water in the radiator. 120, 140 and maybe 160 if you wanted really dry.

[Fla._Deadheader]

  Oh man, this is too simple.  ;) ;)

  Den, on our soaking wet wood, using the system you just described, how long would it take to get the wood to 12%?? I know exact days are tough, just ballpark. 4 weeks, 6 weeks?? I would use a 500 gallon tank.  Maybe there is a formula of MC divided by volume or something????????

  Once you pull the vacuum, can you shut the pump off and hold the vacuum with a valve of some sort, or is that not neccessary???   Great info. I got it built already, in my mind. ;D ;D

[Den Socling]

Drying time would depend a lot on the porosity of the species that you are drying. If you didn't waste any time on cycles, I would expect 12% in about a week or two. Even a log that is soaked is going to give up a majority of that water quickly and easily. Naturally, a very dense wood is going to take longer.

The size of the vac pump would also influence drying time. A gallon of water makes about 227 cubic feet of vapor. If you wanted to remove a gallon per hour and your heating cycle took one half hour, then you would need a pump that could move about 8 CFM (227 divided by 30 minutes).

If everything was sized right, the volume of wood would not affect drying time. Since we are talking about a car radiator as a heat exchanger, volume could have some influence.

Now you can begin to see where automation is extremely useful. If you ran it only a couple hours a day while you could watch it, it could take 4 to 6 weeks. If a controller was watching the wood temperature and controlled the cycling, the kiln could run 24/7. The Italians made the mistake of trying to use timers.

Once you get to the end of the vac cycle, I'd turn the pump off. If the system is automated, you need a solenoid valve to turn the water off. A check valve will prevent the vacuum in the chamber from pulling water out of the pump. Then open a valve to bleed air into the chamber.

[DanG]

Well I'll be DanGed!  I had thought of using a propane tank for a vac kiln chamber, but I figgered the idea was too harebrained to even mention. ??? I, too, already have this thing planned in my head. I have a heater out of an old bus that would make a perfect heat exchanger for such a small space.

Den, would one of your controllers adapt to this application? What would a ballpark price be on such a controller? Do you think it would be practical to weld 2 tanks end to end, to get more length for lumber?

This is a great  thread! 8) 8) 8)

[Den Socling]

Our controllers aren't designed to run something like this but I could design one. Jason of WI might be able to come up with something cheaper.

If you can weld, there would be nothing wrong with putting two chambers end to end. The old VacuTherms were more than 30 feet long. The problem is air flow. The Italians had about a dozen fans along the side. And they made the chamber with a double wall so that they could heat the air with hot water circulated through the wall.

[Fla._Deadheader]

  Too many brains thinkin alike, here.   ;)  I already got the 2 tank system figgered AND cutting out both ends and loading from each end, with roller tables and racks

  I just gotta study up on vac pumps. Only kind I'm familiar with, are Dairy milker pumps, Vane types???

  Did I ever post the pic of the 500 gallon STILL we built in Arkyland??? IT had a compressor radiator in it to heat the mash to distill "spirits".  ;D ;D ;D ;D  Had a water heater outside and plumbed into the tank to the radiator ;D ;D ;D ;D

  This is gonna be SLICK. Thanks again, Den. ;) ;D ;D

[Den Socling]

Do a Google for liquid ring vacuum pumps. SIHI and Nash are good brands. Used pumps are available. Don't buy bigger than you need as HP goes up quickly. You want 2-stage. Anything under 20 CFM is considered very small. A 20 CFM pump will use about 2 HP. The pump on my 1000 bf kiln cost $1324 new and bare.

Pressure control is not an issue here since drying is not going to be continuous. However, you want the pump to pull low enough to boil the water out at low temperature. If you can pull to 55 Torr (55 mm Hg), the wood temperature will drop rapidly to under 100F.

I'm glad you appreciate this. Vacuum drying is suppose to make a living for me!  :D Actually, I just can't figure out how to make a really small kiln cheap. Welders cost a lot of money. Sure helps if you can do it yourself. Carbon steel will rust if you dry Red Oak. I have to use stainless. (Use coal tar epoxy in your recycled tanks.) I can't scrounge for used parts. They all have to be available off a shelf. I have to have multiple sensors and safeties and tight control for guaranteed results, etc., etc.

[DanG]

I was thinking of mounting the radiator and fan at one end and running a duct with multiple outlets along one side, then channeling the return air along the other side to the input side of the rad. Sound feasible?

Another possibility for a chamber might be an old pressure treating vat. They might be available if one were to look in the right places. They already have the track and trolley system for loading lumber, and the seals to run the pressure up and down. :P

[Den Socling]

The duct work sounds interesting. Last summer, I considered blowing over the top and into the end of a baffled kiln charge. The heat exchanger and fan would be above the load. Periodically, you could change fan direction to give more even drying.

Another source of a chamber is tire vulcanizing chambers. They look just like small vac kilns. More likely, cylindrical vac kilns look just like big tire vulcanizing tanks.

[Fla._Deadheader]

  Ya know, I believe ALL the info like this is really appreciated on this Forum. Sometimes the info is taken and no thanks is offered. That makes the person with the smarts feel UN-appreciated. I hope folks here get a little more "Thankful" and express it more. I KNOW it helps.  Thanks AGAIN, Den

  Too bad we don't live closer. Stainless is our "other" business. ;D ;D

[Bill_B]

Thanks Den. You have presented a lot of good ideas.
I was on a woodturning site and they were talking about boiling wood bowls for 1/2 hour to help dry and to prevent cracking.
Your idea is much better.

[Greg_Kaldor]

HI Den,
You are the one with the smarts.  All of us are very grateful to
you for you valuable input.
Thanks again,
Greg

[shopteacher]

Den have you ever thought about  putting together a kit for the do-it-yourselfer?  Something like  a set of written plans, heat exchanger, selnoid valve and a simple controller, etc.  Let the builder come up with a suitable chamber and pump.  Rather than add expense with redundant controls as part of the instruction pack tell what could be done with  multiple sensors and safeties and tight control for guaranteed results.
   I realize it not prudent from a selling point to give out the information you've worked and researched to gain. I do assure you I for one and probably everyone else reading these threads are very appreciative to have you and your expertise on this forum.  

[Ianab]

What about plastic pipe for the chamber?
It's gonna limit your size of course, but for bowls and carving blocks you should be able to get a good amount into a long pipe.
I found this web site with some info on a DIY vacumn dryer, no doubt it could be improved with some of Den's ideas?

http://carverscompanion.com/Ezine/Vol2Issue4/LynnDiel/LynnDiel.html

ian

[Den Socling]

 :D Thank you for the thanks. This is pretty funny. We ate Chinese tonight and my fortune cookie said that I will be on the list of "The Excellent People" this year.   :D ;D 8)

I talked to people at WoodWeb about Diel's design. One problem is no heat. One guy had an idea of wrapping the wood in a heating blanket.

[Fla._Deadheader]

  Guess ole "Confucious" was pretty smart, EH ??? :D :D

[DanG]

Confucious cheated! You made the list before he baked the DanG cookie. ;D  I, too, appreciate the info you have shared, both here and on the forum on your own website.  Things move a mite slow over there, but I've learned a few things by checking in from time to time.

What I was thinking on the duct work was to put an oscillating vane in front of each opening. All the vanes could be operated by a pittman arm or bellcrank on a small gearmotor, with all the vanes connected to a common rod.  With the radiator and circulating fan located at the end, you wouldn't waste any of the precious height in an already tight space. Simple baffles along the top and bottom of the stack would isolate the duct side from the return side to keep the air flowing correctly. The lower baffle could be a permanent part of the trolley, and the upper would be a drop-curtain affair that you could raise and lower from the open end of the chamber. :P

[Bill_B]

DanG
What do you think of a duct on both top and bottom of the tank. Have a fan in each. Let the air return through the lumber.
Have holes in the duct for venting.

[Greg_Kaldor]

Hi all,
That freind that I have that is an instrument man in the paper mill where I work said that they have a type of heater that they use to heat vats of oil when needed.  You see that one set of plans that I had for that homemade kiln said to use a light bulb ina a tin can for heat.  Very inefficient.  With this other type of heater he said with some sensor you could controll the heat to with-in a degree.

Greg

[Den Socling]

Some ideas for heat: If your kiln is tiny, a little immersion heater in a tank might be used. The water is circulated in a closed loop. I put a tee in the water line going to the heat exchanger. Then I use a compression fitting to put a RTD in the water. Connect the RTD to a controller that runs a contacter that energizes the heater when needed.

Don't forget that heating circuits need a couple safeties. Pressure relief valves are cheap. An expansion tank is a good idea. Connect it to 'city' water through a pressure regulator to keep it from going dry from drips. Include an air release valve to bleed air.

For my vac kiln, I bought a steam boiler from a salvage yard for $300. People don't want steam for heat in houses and they are sitting around here in the north. I take steam from the boiler and feed it to a brazed plate heat exchanger proportionately. The water circulates constantly through the other side of the exchanger. An RTD in the water line tells a controller how to set a ball valve that controls the steam. This type of system can control water within a tenth of a degree. The boiler makes a lot more steam than the kiln needs so I use some for radiant heat in the shop floor.

You can use an outdoor boiler for heat. Use a water to water heat exchanger with two closed loops circulating. Use a diverter valve to send more or less to the heat exchanger as the kiln needs.

A couple details: Water will condense on the walls inside the chamber.You will need a floor drain.

Liquid ring vacuum pumps can take some vapor. But they work better if you pull the vapor through a condenser to keep it out of the pump.

Placement of the sample that you are watching for control is critical. It would be better to have RTD's in a couple locations and have the capability to switch control.

There are times that you might want to dry something fast and easy like soft maple. A condenser inside the chamber can assist the vacuum pump.

[DanG]

Bill_B, my thought on placement of the duct was to get a good airflow horizontally across the stack. Vertically, the airflow would be blocked by the solid layers of lumber. Longitudinally, it would be blocked by the stickers. Also, the bottom would likely interfere with the cart you would need to load the kiln, and at the top, I would probably put a couple of narrow layers of boards, pyramid style, to squeeze in a few extra boards. Since the sides of the stack have to be square, and it is in a round space, you are left with this handy little opening for the duct.

[shopteacher]

Den: Can you elaborate a little on condensers? I'm imagining a container between the chamber and the pump where the vapor can condense and be drained off before getting to the pump. I can't figure how it would work though without loosing vacuum or drawing outside atmosphere into the pump.

[Den Socling]

A shell and tube heat exchanger can be a condenser. They lay around in scrap yards but you need to find a single-pass unit. In these, the tubes go in one end, straight through and out the other. To make it a vac pump condenser, stand it on end, connect the vac pump to the top, the chamber to the bottom and cooling water to the shell.

I've got one customer who built two of his own by welding a pipe inside a larger pipe. It's long and tilts. Water is circulated through the outer pipe. The pump is connected to the high end and the bottom drains back into the kiln.

Inside the kiln, the fin tube that is used in baseboard heating works well.