iDRY Vacuum Kilns

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dry kiln for drying bowls

Started by Greg_Kaldor, February 12, 2004, 02:46:13 AM

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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.
Bill B

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
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

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) :)
"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."

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.  :o ;)  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
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

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
"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."

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
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

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.
Bill B

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.  
Proud owner of a LT40HDSE25, Corley Circle mill, JD 450C, JD 8875, MF 1240E
Tilt Bed Truck  and well equipted wood shop.

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
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

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
All truth passes through three stages:
   First, it is ridiculed;
   Second, it is violently opposed; and
   Third, it is accepted as self-evident.

-- Arthur Schopenhauer (1788-1860)

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
"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."

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.
Bill B

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. :)
"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."

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.
Proud owner of a LT40HDSE25, Corley Circle mill, JD 450C, JD 8875, MF 1240E
Tilt Bed Truck  and well equipted wood shop.

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.

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