Vacuum Drying in a Bag

[Glen Evans]

It seems like the major inhibiting factor for the home hobbyist is the vacuum chamber.  Am I crazy or could a person place wet lumber in vacuum bags place the vacuum bag in a heated enclosure and then draw a vacuum on the bag?  A simple electrical moisture sensor could tell you when you've reached your desired MC.  Seems to me you could actually have several bags attached to one pump, stacked with a generous air space to allow heat to move around the bags in the heated enclosure.  I know air isn't a great conductor but still.  My question is how large a vacuum pump would the home hobbyist who wanted to do maybe 300 bd ft at a time need?  Comments anyone?

[Den Socling]

What you do is put the wood in a bag and then put the bag in a tub of hot water. It can work for one piece at a time.

[Glen Evans]

Thanks for the reply Den.  I've read a ton of your posts and learned alot from them.  Back to the bag idea for a minute--any reason you couldn't place multiple boards side by side in the bag?  If yes, any reason why you couldn't place multiple bags in the water fed off of the same pump?  If yes what other mediums other than air could be considered--how about sand with heating cables imbedded??  Why not hot air as a medium--too inefficient.

I guess I'm just trying to dumb down the whole thing by eliminating the pressure vessel and I'd rather not replace it with a watertight tub if a knock-down insulated box could do the job.

[Ianab]

Hi Glen

Interesting questions
I think the one piece at a time is because the heat has to be conducted thru the bag from the water to the wood. If several pieces are in the bag the middle ones would not be heated properly. Side by side could work I guess so long as they can be kept flat. Several bags from one pump should not be a problem, just need a bigger pump I guess.

I'm not sure about pump size, if the pump is too small it wont be able to pull enough vacuum and drying will take longer, but it should work.

I think the water is probably the most efficient way of getting the heat into the wood, sand and air are reasonably good insulators. The commercial kilns seem to use metal plates to transfer heat to the wood so that may be an option too.
 
If you do some experiments let us know what happens. Lots of interest in this sort of technology.

Cheers

Ian

[Larry]

Ok you guys...I have been sperminting with this vacuum drying stuff on the sly with another guy.  Few successes but mostly dead ends and nothing to write home about.  Were trying to come up with a way to dry walnut crotches with minimum degrade.  About ready to think it is not possible due to the convoluted grain...but were going to chase it a while more.

Been trying a electric blanket for our heat source...think we need a electric blanket running on 220 though. 

The links are all geared towards veneering...but should be the same process for drying.

For parts and simple tutorial go here  http://www.joewoodworker.com/veneering/welcome.htm

For more parts and stuff go here  http://www.vacupress.com/

[Den Socling]

Ian is right about the heat transfer through multiple layers. There are ways to move heat in a low pressure atmosphere (vacuum) but they involve complex and expensive methods.

Water would be the best medium for heat transfer (for the hobbyist). I helped a guy do this a couple years ago and, if I remember correctly, he built a wood box and lined it with plastic. He wanted to dry a big flitch for a bar top, if I remember correctly.

A compressor from a refrigerator could pull the vacuum but it wouldn't last long. It wouldn't take much of a pump.

[Buzz-sawyer]

I use refrigerator compressors for both vac and compressed air.......I use the vac to produce refined vegitable oil under vacuum and have a friend that has used one several times a week several hours at a time, for 2 years.....it really depends on how the thing lubricates itself, and how ya care for it

[Den Socling]

The problem when drying wood is that it will pull water vapor.

[GaS]

To clarify :

The compressor will suck water vapor into its innards...typically water contaminated with junk from the wood you are drying (as if regular water wasn't bad enough for its inner workings).

[Buzz-sawyer]

We use them to remove water and methanol. For biodiesel production.

You are right Den as a rule they dont last long and take a while to draw down

These pumps only last a few months (@ 200 litres per week of biodiesel production)But if you arent drawing vapor/chemicals over them they last a bit

[Glen Evans]

Thanks for all the great comments guys.  I hadn't thought of making the tub with a liner.  That would allow me to stick with the knock down idea.  With respect to the pump and water degrade is it a stupid question to ask if you can't filter out the the water vapour, in-line, a head of the pump?  I know the volume of water is much greater but I'm thinking along compressed air line terminology...

[Den Socling]

You should take out the water before it gets to the pump. Set up a condenser. We use shell & tube, single pass heat exchangers mounted vertically with the pump connected to the top and the kiln (or bag) connected to the bottom.

[Glen Evans]

Thanks Den, I'm doing some digging on the web now to see how to plumb that.  As an alternative would I be able to pull enough of a vacuum with a venturi valve connected to my air compressor, (5 hp 6 cfm @ 90 psi) and effectively blow the water out with the evacuated air??

[Glen Evans]

Den,

Can you elaborate on the set up for condensor and heat exchanger you recommend please.  I'm not familiar with "shell and tube" and so on.

Glen

[Ironwood]

Well, as am sure Den will tell you it is a rather complex process. I have a 9000lb wood oven, vaccum, and steam chamber. It was constructed with the thought of using it to dry round stock with the bark on. Envision Old Hickory funiture, the problem in the rustic furniture industry is how to dry the wood before the bugs get to it. I have never been a volume rustic builder but the idea was berthed many years ago before my busness evolved into custom one off creations. The "pig" as we affectionately refer to the beast was constructed by a friend with several 12' "drop" pieces or heavy walled 36" and 30" pipe from huge cantilievered outdoor advertising signs. I have yet to hook up the water jacket to a boiler, which is the entire outside of the tube.
I simply use the water jacket as a flue to heat the chamber as a wood oven.
from a wood burner below.

   I researched quite a bit and the authorities seem to be PC Specialties and several other custom vaccum kiln guys around the country, including Fred Lamb down at Virginia Tech. Fred tends to know all the ins and outs an actually encouraged me to seek Gov. grants avalible at the time. Sounds like some great projects for you retired  guys. Anyway the devil is in the heat tranfer issue and "cycling" the thing with vaccum. The wood web drying archive has some fairly invovled discussions on the science.  Reid

[Den Socling]

Glen,

I don't think a venturi will move the volume unless it's big and uses a large volume of air. I use little venturis on suction cups to lift our heating plates. They use only 0.74CFM but, even at that low volume, .74 x 60 minutes x 24 hours = a lot of air.

Shell and tubes have a bundle of tubes surrounded by a shell. The shell side, in my case, has cold water circulating through it. The vapor is pulled into the tube side where it is condensed and drops into a tank. One of the guys I've helped with a DIY sloped a pair of long copper pipes up the side of his chamber. One pipe is inside the other. Cold water is pumped through the outer pipe and warm vapor is pulled into the inner pipe.

Den

[Larry]

Glen,
We are working with the venturi valve from joewoodworker...5 hp compressor.  Not ideal but does work for a bag.

Reid has the right idea when he said

"Anyway the devil is in the heat tranfer issue and "cycling" the thing with vaccum."

[Glen Evans]

Thanks Guys,

The lights are starting to go on!  Given that I live in Ottawa, Canada keeping that chilling water cold shouldn't be a problem for at least half of the year!  I'm assuming that the condensate exits through some kind of valve after the heat exchanger/ before the vacuum pump.  Do you have to worry about any kind of additional filter infront of the pump?

[GaS]

You can do some manual condensate dump with two three valves and a tank.  Put the tank low so that water drains down.  When it fills, close the valve which leads back to the vacuum.  Open two other valves, one on top to bleed vacuum and on one bottom to drain the water.

[isimitibiti]

hi
special thanks to denis and garret at pc specialties.
your help is unexpected and appreciated.
i have been experimenting with drying in bag.
im am half way through the first real life and size prototype of kiln.

ive gone through the heated tank idea and while this is probabbly the best heat transfer,
it is also tough to keep an even and constant temperature in a giant tank.
ive done the electric blanket with some results, but it was kind of ugly and just seemed llike a fire
hazard. im right now using a simple heated 2" insulation board box with an electric heater inside.
i have a thermo coupler in the core, on the surface and in the box. heated is controlled by the
core temp. also ive a probe for monitoring MC. im pulling about 29"hg.
im getting water!!!!! so this is the start. i was worried the heated air chamber would not transfer well.
it really is working fine. i dont think there is need for the water tank, ive also keeping some buckets of water
in the box to keep it humid to help tranfer heat to the vinyl. all three temps will fluctuate, but there seems to be no problem keeping the core and surface of the slab at a reasonablely stable temp. im running at 105-110F for the core, 100-115 for the surface and 100-140F for the chamber. temperatures vary as the thermostat is controlled
by the core temp whic has a lag time to reheat (its set for 110) the other temps stay within these ranges as the system runs itself. this seems reasonble, seem to be working. yesterday it seemed to stop working as well.
i shut it off and let it acclimate, then it seemed to spew more water. im installing a timer to run discontinuous.
this shoudl help to even the drying. im drying 12/4 lumber. walnut. biggest problem im haveing is the water through the pump. 1/2 horse kinney kc8, ive been changing the oil daily, its a mess but of course i have no collection chamber. tomorrow im running the pre pump out-pipe through a chest freezer in a flexibel copper coil then putting
a 2" copper pipe "tank" that i can install a motorized valve to empty when the pump stops for the down cycle.
this should work, itll hold more than ill produce in a seveeral hour pump cycle. and the chest freezer should be an adequate cool down through 50' of copper. then ill run the out put of the pump into the freeqer to capture any
other vapors i migh miss for measurments sake of exactly how much water im removing.
im convinced this is a reasonble DIY method. of course itll take some experimenting. but i believe
i have enough control over the variables that seem to be crucial. next is developing some sort of schedule.
but im most impresed that the heated box seems to keep the wood temp where its needed to work.
(by the chart i have water vaporizes in 29"hg at 80 degrees, if so this system should work.) i havent tried it that low, maybe im jusst excited to see the water come out. ill try that next, its in an overnight down cycle right now for 100F. well see what it does in the am.

T

[Ironwood]

All I can say is that we all have our specialties. If and when I need the vaccum chamber rest assured I will call the boy's at PC SPECALTIES. I..you could spend thousands of dollars and hundreds of hours of time not finding a solution. If it's for entertainment well that's another matter.

                                       Reid

[GaS]

Tyler!  Glad to see you join the conversation!  I still feel that the core temperature is high...was your wood pre-dried at all?  I am overly cautious when it comes to schedules (especially considering this is so 'by the seat of the pants').  Also, I was thinking that perhaps you might want to increase the down time to four 1/2 (or even full one) hour segments rather than two 1 hour segments.  Its really hard to tell what might be happening, but my gut keeps saying 'over drying the shell'.   Once the wood MC is lower, you can 'beat' the wood, but you should go slow at the beginning.

Thanks for the vote of confidence, Reid.  It has taken Den  thousands of hours of research to learn all that he knows today.  I only have a couple hundred of pondering so far, but I am able to pick his brain daily!

[Larry]

T,
Are you using a platen? 

[Glen Evans]

Tyler,

Exciting to hear about your prototyping work!  A couple of quesions.  Are you drying a single piece of wood, or do you have multiple boards in the bag?  Do you circulate the air at all in the box?  How did you decide on those operating temperatures?  I'm really interested in hearing how this works out for you Tyler. 

It's great to hear that air can work as a heating medium--I'm sure the efficiency is no where near what water could be but I figure that just means you need to have a greater difference in temperature between the air and the surface of the wood.

One thing I'm still wondering about with the bag method versus the chamber method is whether the vinyl bag collapsing on the surface of the wood actually prevents the water vapour from exiting over large areas of the board.  I wonder if we don't need to allow channels, (1/2" pvc pipe cut in half lying on top of the board extending all the way to ends of the board, with vacuum connections at both ends of the bag), to allow the vacuum to be applied to surface and the ends of the board and to allow water vapour an easy path to exit.  At the end of the day, the bag that we are compressing around the board, (especially if it is poly) is a vapour barrier!

Anyone have any comments?

Glen

[Den Socling]

I'm a control freak.  I like it when I control all the parameters and the 'process' happens exactly as expected. When drying with a vacuum, the vapor pressure of the water in the wood is equally as important as the chamber pressure (vacuum). If water is used as the medium, not only will you benefit in heat transfer but the temperature of the water, with it's mass, would be easy to maintain as a stable parameter. If you use water, I think you could control from the temperature of the water bath. If you use air, you probably need to be more concerned with wood temperature.

The wood temperature can be confusing. If the wood is wet, the maximum temperature is set by the chamber pressure (vacuum). There is a table at our website. It shows the relationship between reduced pressure and the boiling point.
http://www.pcspecialties.com/torroconvHC.html
If you pull to 30 Torr which is 1.18" Hg absolute which is 28.8" Hg vacuum, water boils at 84'F. It can't get hotter unless the pressure goes up.

[serg]

Den, hi! The good fellow, it is perfectly told. Sergey.

[Buzz-sawyer]

Glen
I have not tried this except on small scale experiments,
Put your bag around a solid object that can withstand atmospheric pressure...like a strong barrel or tank then your issue is solved

[Glen Evans]

Den and Buzz Sawyer,

Den I agree with you point about the water being easier to control particularly with respect to the temperature of the wood.  I still wonder about the effectiveness of having the vacuum bag sucked tight to 90% plus of the wood.  In theory at least, slapping a poly bag tight to surface is going to prevent vapour transmission isn't it?

The problem with you solution Buzz is heating.

One variation on the classic platten system came to me this morning.  Piping hot water through the condensor coils of old refridgerators layed flat between layers of wood in a large vacuum bag.  This space between layers would also facilitate the evacuation of water vapour from the wood. 

The only real difference I suppose between this approach and a more conventional approach is that the vacuum chamber is a vinyl bag!  I'd still use a condenser to avoid pumping water through the pump.

What am I missing Guys?  I really appreciate the feedback

Hey Tyler how's it working today?

[Den Socling]

Without looking up the numbers, I'd guess that the topography at the surface of wood would look like the Grand Canyon to a water molecule. Think about those tiny pores and vessels that it went through on the way out of the wood. I don't think that plastic that is vacuum packed against the wood is going to stop the vapor.

[Glen Evans]

Touchee.

Good to perspective to look at from.

Thanks

Glen

[Glen Evans]

Let me try that again,

Good perspective to look at things from!

Glen

PS  How nuts is the heating idea Den??

[Dan_Shade]

i can see the heating coil as only heating up specific areas of the board, since wood is a good insulator...

[Den Socling]

Glen,

Depending on the species of wood, the refrigerator evaporator coils would be eaten up by acids in no time.

Sandwiching the heat source between two layers of aluminum would do much better at spreading the heat and avoiding the problem that Dan mentions.

If you wanted to make one or two custom platens, you could buy a few feet of our extrusion. A do-it-yourselfer recently bought $45,000 worth.    You probably don't need that much.

Den

[Dan_Shade]

I've been thinking about this a bit...

when you vacuum bag composites, you use a "breather cloth" so that you can get a vacuum pulled equally over the entire surface, problem I can see is if you use a breather cloth, it may not transfer heat that well...  but I think I've seen fiberglass guys heat the stuff with heat lamps so it must transfer heat sorta ok.

[Glen Evans]

Den,

Yeah, I think $45,000 might be a shade high for me!! I was puzzling over this yesterday and got to thinking about water bladders for heating.  Think 4' x 8' by 1" thick vinyl water bladders with hot water entering one corner and exiting the diagonally opposite corner.  Obviously I wouldn't be able to stack more than maybe 4 lifts at a time for fear of rupturing a bladder. 

Comments??

Glen

[Den Socling]

Glen,

If you put a bladder full of water in a bag and then pulled a vacuum, that bladder is going to expand to fill every bit of space.

Think about this. I use a bladder to press stuff like Holly flat while drying. If the bladder is 288" long and 40" wide and I let 14 PSI (one atmosphere) into the bladder while I pull pressure down to 2 PSI, that bladder is pushing out with a total force of 138,240 lbs. You better have a tough bag to hold that. 

Den

[Glen Evans]

Den,

Yeah I wondered about that--you have a wonderful way of illustrating concepts with numbers that makes me wince!!  Its almost like there's a good reason for the way you make your plattens!!!!!

I'm fast running out of ideas--but not done yet.  Next up is an idea retrofitted from the heated floor industry.  (I can almost see everyone rolling their eyes!)  1/2" thick cement board with electric radiant floor heating cable laid onto the surface on 2" centers and parged over.  The entire board could be laid between lifts, additional boards could be connected in series and controlled by a thermostat.

I hesitate to ask for comments??

Glen

[Glen Evans]

Something I said??

Hey I hope I didn't offend anyone, maybe you're just tired of my ramblings--

Thought  I'd send out another SOS to see if anyone thinks the idea of using radiant electric floor heating wires embedded in 1/2" cement board to conduct heat and act as warming plattens between lifts of wood in the vacuum bag is viable?

Any feedback would be appreciated.

Glen

[GaS]

well, I'll say that it would be a lot less expensive than aluminum plates!

hard to say what type of heat transfer you'd get.   concrete, like wood, isn't a great insulator...but it isn't a great conductor either.  hmm...

depending upon how deep the heating elements were embedded, you'd probably get reasonably even heating...

wouldn't last long in an acidic environment...

heavy...but would help hold down boards that would want to cup and might improve your heat transfer...

 

[Glen Evans]

Thanks for the input.  Think I might just give it a try

Glen

[Larry]

Confused as normal...somebody set me straight.

Pulling 30 torr so I have to get the temperature to 84 degrees for vapor.

Pulling 100 torr so now I have to get the temperature to 124 degrees for vapor.

So....quality wise which way is best and why?

[Den Socling]

Conventional wisdom says that the wood fiber is softened and more likely to degrade if it's above 110 to 115'F. I'm not an expert on cellulose. Maybe that's a myth from conventional kilns but I keep wood cold in our kilns.

[isimitibiti]

hi all, sorry for the abscense
ive been immersed in this.

first, i looked into getting a real system a year or so ago from pcs
but i needed a 25' long chamber and that was way too expensive for the 8 boards im drying.
albeit, long single slabs, worth an investment but not that big of one.
if i were doing 10' pieces, id be ahead had i gotten a real system from pcs.
im convinced there isnt a productive way to do diy vac drying unless you have the need for giant pieces.
ive got about 15k invested so far, not including time.
dring regular lumber this way would be a joke.

in the end the bags wear out, little leaks etc,after some handling as well.

i believe the water tank would be best, but hard to control temp at 25' without building a giant
jacuzzi, it was one of my plans but if the "hot box" would work, its easiest.
my other idea was to build a giant heat press using silicone heating plates.
but again a big monster for for my limited needs and could only do one at a time.

the hot box worked well, and given that my schedules and temeratures are novice.
i took one piece from green to 12-18% in about 2 weeks, with some degrade but tolerable for my needs. piece was air dried for maybe six months.
still above 40% (maximum on my meter) it is definately uneven, but i believe tolerable for my applications. so with this, i dont think a complicated heat transfer is my biggest issue.
it is a big kiln box with fans etc. ive buried thermo couplers in the wood and it seems to get hot into the core fine and i can keep the temp there no problem as its "drying".
at the end of the day

the bag sucking to the wood seemed to not be a problem for the moisture, (the grand canyon effect) but seems to cause a drop in pressure, im pulling 28" at the inlet, and am reading 25" on the other end 25' away. this is definately a problem.

i got ambitious and build a big steel flat shelving system that can be dismantled.
iv got it loaded with six giant slabs in bags.
origianlly i used some rubber screen/mesh to help air flow, but put a few test pieces in without, the test pieces dried as well, so i thought the mesh would only act as insulation so it was removed.
no i am having trouble with even pressure.

with one board (36" w 120" long 10/4) i was getting aobut 1/2 gallon a day, now with about 6 time that much wood im getting only slightly more. something is wrong and i cant yet figure it out.
i assuming its pressure,capcity,pipe size/length related. but im not sure.

regading temp and schedule, im going by the seat of my pants. i have a basic steam table that give the pressure vs temp at which water boils. i figure as long as im hotter than that, water will come out.
dennis if you could explain why getting the wood to hot is a problem i would be all ears. its my intuition that tells me its ok because with a bag sucked down tight theres no where for the water to unless i revove it by evaporation with the pressure. if i control the pressure, with disicontinous operation, isnt it the more heat (energy) applied force the water to move through the wood and equalize itself faster/easier. again im not a scientist, and regrettably, my curiosities would get me into a lot less trouble if i were.

one more question for you dennis.
how improtant is exact even heating?
with the plates there must be some variance in temperature as well.
on my bags i use one of those laser temp shooter things and get about 5 degrees
difference throughout, another reason i keep the temp higher to make sure no spots drop below
the steam temp.
any help from anyone appreciated.

thanks all

tyler

[Den Socling]

Tyler,

With a bagged system 25' long, I would expect a difference in pressure from end to end. Maybe a piece of perforated pipe would fix that. The pipe that is used for radiant heat in floors could be drilled and still be tough enough for the job.

Scaling up often causes unexpected problems.  Going from one to six boards just overloaded your system. Possibly, you aren't getting heat evenly to all six or, probably, the vacuum pump capacity isn't sufficient. If you are reading inches of mercury with a mechanical gage, you can't see small changes that make a big difference.

Look at the temperature of your wood and you will know the approximate vapor pressure of water in the wood. Pull the chamber (bag) pressure below the vapor pressure and the gradient will cause water to come out. Too much heat will cause too much vapor pressure and you will never get the pressure down. Too much heat will cause too much drying in the outer portions of the wood. Discontinuous vacuum helps you out but, for nonporous wood, the vacuum doesn't have a deep effect. Excessive moisture gradient causes cracks. Finally, as the easy water is removed, you need higher temperatures to get the wood to 6%.

Even heating is very important in vacuum drying. Years ago, I considered many methods of heating and decided plates were best. I designed my aluminum extrusion to conduct heat from the water evenly. I designed the heating water system to carry the heat to the aluminum evenly. A difference of five degrees is enough to make one spot dry very rapidly while another spot barely dries at all.

Den

[isimitibiti]

hi all
update on the progress.
dennis your certainly right about the pressure drop across the plank.
i rebagged every thing, (six slabs) and wrapped them with a single layer of plastic mesh.
i was getting about a 1/2 gallon a day from the six slabs before, after putting the mesh
over to allow the air some room to travel there was ten gallons of water the next day.

den im using a kinney kc8 with a 3/4 hp motor, as far as over loading the system i think its working
fine. its destroying the pump as i cant get all the water to condense before the pump and after 24 hrs or so its mostly water in the pump. it still pulls 27.5" and after going through 5 gallons of oil i just stopped replacing it daily, liquid ring is the way to go, but ive got a few kc8s to burn through.
so ill see how they last running like this.
all in all im really satisfied with the luck im having. its consistently pumping out 5 gallons a day
(the first 10 gallons was because the boards sat in the hot box under vacuum but not evaporating, they must have been sweating a ton so there was alot of surface water) on my thumbnail calculations after 60 or so gallons (a week or so) ill have the bulk of the pieces dry to 10-12% and the degrade is pretty nominal. and if i were to cycle it with more moderate temps im convinced ill be able to control
this. i threw in a test piece of true green maple, dripping wet, through the bag it looks to be drying
well, the knots have tons of little checks, and the piece itself has only minimal case hardening.
im unloading some of it this am, ill see how even its dried.

i think the only hurdle at this point for the bag system (beyond contolling the temps and schedule which ill figure out through time is the even pressure, i will also run some of the pex tubing with holes in the next run.

tyler

[Glen Evans]

Tyler,

Sounds like you're having some success!  I don't know if the pump issue is a concern or not but I looked further into the idea of using venturi valves with an air compressor and found one that combines excellent vacuum with fast cfm evacuation rates by 2L @ http://www.2linc.com/venturi_vacuum.htm.  The straight through design seems better than most that I've seen.  This approach coupled with a vacuum switch might help avoid eating up your pumps.

Thanks for keeping us updated on your efforts.  Can you brief us on your drying schedule?

Thanks

Glen

[Den Socling]

I've been wanting to test venturis with a lot of condensed water but here it is Sunday night, again. What I'm guessing is that a venturi with a lot of moisture is going to ice-up.

[isimitibiti]

hi every one

ill be honest, my drying schedule is very laxed.
and even so, ive had tremendous results.

ive run the heat, due ot my laxed attention anywhere from 110 - 150F.
and even forgot to open the vacum valve for 5 days.
when the pieces are sucked down in the bag the water just migrates to the surface
but doesnt leave so there isnt so much degrade.

dont get me me wrong, im not getting perfect lumber, main problem is some minor case hardening
but with 12 and 16/4 lumber not much worse then ive seen from regular drying.
and for my needs pretty acceptable.
as well with some paitience and attention  and experince with drying in general
im pretty sure i could get perfect lumber.

one test piece was dripping wet 16/4 maple, i got minimal case hardening and and the piece was too dry to read on my meter in about 12 days. i also ran six bags in this run off one pump and it nailed all of them. pretty exiting to dump about 5 gallons a day out of my collection chamber.

dennis, i dont think the venturi would freeze as the air coming out of a chamber is 120 degrees or so.
im running mine through a 5/8" 50' copper coil that sits in a small chest freezer full of water
i cant get the freezer to keep up with the heat coming out. the water gets to 65F at best when the pump is running, turn of the pump and it goes down to 33 and starts freezing in an hour or so.

i do like the venturi idea, where was this one availabe that someone mentioned, and is it possible to pull far enough with them, i tried this before but could only get it to 27" which means i didnt have as much flexibility with the kiln temp.

tyler

[isimitibiti]

oops, thanks for the link glen.
these venturi pumps would only pull 26" max, i dont have my chart in front of me but i think youd need to get like 125F or more to get steam, my feeling is its better to have the flexibility to keep it cool.
when i get my rebuilt pumps back, theyre supposed to pull 29.5 and i could get water at 80 degrees or something like that. im still experimenting.
but for as little as i know, and the results im getting, this system is pretty amazing.
tyler

[Glen Evans]

Tyler,

Good point about the vacuum, and temperature, take alook here http://www.vaccon.com/PDFs/pg10.pdf  this puppy has 29.5" just slower on the vacuum cfm numbers.  This same outfit has a unit which combines high vacuum cfm with 28".  In the bag system, I just don't seen time to evacuate the air as being a huge problem--it's not as though you're emptying a chamber the size of a house.

Glen

[Den Socling]

isimitibiti (Tyler) was here this afternoon. He wants to build a discontinuous vac now. He said the bag was working but it's time to move up.

I asked him a half dozen times to get back in here and pass 'bag-vac technology' along to you wood turners and guys after one dry slab. Hope he does. Also would be nice if he shares his discontinuous vac experience.

[Ironwood]

Someone out there has a patent on a bag vaccum drying process.(I won't say anything else) It is possible. Reid

[Den Socling]

Reid,

That's Zhangjing Chen with the patent. He's a friend of mine. Just don't try to market a system using a bag without his involvement.

Den

PS There are patents for steam vacs, also.

[Ironwood]

Den,

  Unless he bought it from the guy I know (several offers were tendered but not accepted) it is a differing patent. It is an international patent. Reid

[Den Socling]

Hi Reid,
I almost have an world patent. Its a long, long process. They investigate the dickens out of them. Mine has been going on for a year or more and is suppose to be final in the 'first half' of this year. If a similar patent exist in a country that signed the WIPO treaty, a world patent doesn't happen. I've read Chen's patent and it is current. Chen is a researcher at Virginia Tech @ Blacksburg with Fred Lamb.
Den

[Ironwood]

Den,

 Fred is a nice guy. I spoke with him for what seemed like hours a few years back about my project. He was very helpful and at the time informed me of some federal grant monies available for my application, it was just too much regulation for me and I didn't see it benefiting me that much. Great guy, definitely one of the vac. gurus.

                                Reid

[iain]

I talked today Ian Holmes of Becker pumps UK,

his pumps will run 24-7 for years doing this job, his advice to anyone (when i splaned what we wanted to do and the problems)
use pumps that will run at 60c or above and that stops the moisture condensing into the oil it gets passed straight out as exhaust

sounds like a plan to me for when i get to the new work shop,


here's the link for north Americadia

 http://www.beckerpumps.com/


 iain

[Den Socling]

In the systems that I have seen running oil, some oil goes out with the pump discharge and that makes a real mess. One company in PA had a 'dead zone' outside the factory where they were discharging. Another company in NY was told by their 'EPA' to change to water or shut down. Oil might work if you used some way of catching the oil mist.

[logboy]

I've been looking into the bag technique for drying hardwood slabs for a while.  Did anyone ever follow up on it? Where are the bags from? What is the rough setup?

[Den Socling]

I believe the idea can work. Vacuum storage bags may be suitable but I don't know how big they go.

[logboy]

Do you know what isimitibiti was using?

[Den Socling]

No I don't know what he used but a "chamber" might be made from plastic sheathing with duct taped seems. The tricky part would be the connection for the pump.

[logboy]

I dont think the bags or the connection would be an issue as long as one of these would work. The down side would be the "softening point" of 150 degrees. The hot box would need to be kept below that temp.  What kind of vacuum

http://www.veneersupplies.com/categories/Vacuum__Press__Items/Vacuum__Bags__%26__More/Dura-Max__Extreme%E2%84%A2__Vacuum__Bags/

[Den Socling]

Those bags look like they might fit the bill. But odd sizes. Too big.

Many species will reach 7% at 140'F. A couple, like Black Walnut burl, needs 180'F.

I usually run the vacuum around 55 Torr (55mm Hg). A chamber pressure of 65 will work. If you go above 65, the boiling point (and wood temperature) goes above 110'F.

[logboy]

I was checking over your chart and 65 Torr correlates  to 109 degrees F. So theoretically with a "chamber" temp of 125 the wood would be at the required temp and yet still not soften the bags.

What kind of vacuum pump would be necessary to correctly run this setup? I kept reading about oil/water mixture problems. Is a different pump the solution, or a condenser of some sort?

[Den Socling]

If your wood is wet, evaporation keeps the temperature at a level set by chamber pressure. So, if you have 65 Torr, the wood can't go above 110. If you were heating in a bath of 125, the extra energy would speed up drying.

I use shell and tube heat exchangers as condensers. I use liquid ring vacuum pumps because they can take some water if any gets past the condenser. There are dry, screw pumps but they cost two or three times that of liquid ring. The thing about drying in a bag is that a tiny pump would do.

[logboy]

If I am reading right from the other posters the pump would not need to be run continuously either. Some sort of small pump hooked up to a basic timer in a thermostat controlled hot box might do the trick.

[Den Socling]

That's correct.