Vacuum Drying in a Bag

[serg]

Den, hi! The good fellow, it is perfectly told. Sergey. 8) 8) 8) 8) 8)

[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.  :D 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