Help me finish my kiln/flue build, please.

[Chillucky]

I make furniture for a living, and for big tables our shop uses an aluminum honeycomb panel product. It is 2" thick, with the honeycombs running between the aluminum sheet skins.

I took a scrap of it and heated it in an oven to make sure it would hold up. The glue that holds the skins on starts to loosen up somewhere between 200-250°f. It never stank up the oven or anything, so I'll call it safe to try.

I was able to gaffle enough scraps of this stuff that I can make a box about 30" square and 48" tall inside dimensions for my kiln. I may just foil tape it together and have it be "knock down" for storage, I may go nuts and permanently assemble it.

From an abandoned home-brewing project, I have a Johnson Controls A419 unit to plug a crock pot into, which I will buy in the next couple of days.

My question is this: what am I missing? I think I need a way to observe relative humidity, right? What about fans or vents? Will convection do the work in that shape of vessel?

 

[deluxestogie]

Check the Index of Key Forum Threads for highly detailed instructions and discussions on building kilns.

• What are the thermal insulating properties of your metal honeycomb?
• The glue that holds it together sounds structurally stable at temp, but does it outgas anything toxic when it is heated?
• No need to directly measure humidity.
• Convection will sort of work, but not well, even for a kiln. I would expect significant cold spots. Convection will not work for flue-curing. You should plan on a humidity tolerant, circulating fan.
• Do you have a way to hang or hold the leaf inside the kiln?
• Is foil tape temp stable?

Bob

 

[Chillucky]

I've been going through the kiln building threads, but you guys have been very busy and it's taking me a minute. I apologise if I am cluttering up the boards with redundant topics.

I am hoping that the panels will be good insulators. Testing will tell.

I don't know if or what the glue offgasses. It didn't stink when I warmed it up, and the panel cut offs themselves are 3-5 years old. Hopefully they would be done off gassing.

In the flue schedule I have it mentions wet / dry bulb parameters. Do you suggest going by the feel of the leaf? Or is it just not as consequential as temperature to the process?

I will get a fan, thank you.

I will make a way to hang the leaf.

Good question about the tape. I will look into it. If not, I could bolt the box together with threaded rod, I suppose.

 

[deluxestogie]

wet / dry bulb

I have never used a wet bulb thermometer. They date back over a century and a half, to a time when there was no other simple way to measure humidity. You will not have a barn-size flue-cure at risk, and all your other conditions will be more closely controlled than in the bad old days. So you really don't need to measure anything for that RH curve you see on the graph. But you do need to understand what it is saying: keep the leaf viable an moist, until the lamina have yellowed adequately. Beyond that notion, just ignore the RH part of the graph, and figure out how to make your chamber follow the temp line after yellowing. If the leaf has yellowed successfully, then that RH curve is just a predictor of how the leaf will subsequently dry out.

For my own flue-curing, I add water to my Crockpot at the very start, when the green leaf goes into the chamber. Since I keep the door slightly ajar during yellowing, that water is usually gone by 1 to 2 days. I do not measure the humidity.

During the day (bottom leaf) or three (upper-stalk leaf) of yellowing, I peek at the leaf from time to time, to judge the coloration. Once I've decided to move on to the next phase (leaf kill), I latch the door, and never look inside again, until the entire, scheduled run is complete. It was either a good run or not so good. But peeking at any other time during the run screws up the temp, the humidity, the leaf.

Bob

 

[Chillucky]

Here are the bones of what I'm working with. The interior space is 21X27X42 High. I found a 300 watt crock pot with 5 power level settings and bought a 4" diameter "harsh environment" muffin fan.

First go-round, I will be loading from above with whatever's ripe from 7 Bright Virginia plans. Racks and a window still to be made.

 

[ChinaVoodoo]

I think this will work. It'll take some figuring out, how to balance the humidity and temperature for flue-curing at all the stages, while depending on only a temperature controller and a crockpot. The humidity will have to be done manually, (and differently, ) for every stage.

My recommendation is to put a thick piece of insulation loosely over the glass while you aren't actually looking in.

 

[deluxestogie]

I'm still skeptical about the thermal properties of that incredible, aluminum honeycomb. It looks like great stuff for engineering the skin of an aerospace vehicle. But it also looks like an effective design for a heat exchanger, with all of the cells allowing direct convection from the inner wall to the outer--and heat conductive aluminum. If the cells are empty (just a vacuum, no air) that would prevent any convective circulation within each one, but I can't clearly imaging how it might have been manufactured with a vacuum.

Conjecture aside, you'll find out as soon as you heat it. If the exterior feels warm or hot, then you'll need to cover the whole thing with insulation, in order for the Crockpot to provide sufficient heat flux.

Bob

 

[Chillucky]

I am also concerned about the material being a better conductor of heat than an insulator. If it turns out to be the case, it would be easy enough to skin it in sheet insulation and plywood. Testing will tell!

 

[deluxestogie]

XPS insulation walls (min 2") can be constructed by simply taping a box together with Tyvek tape. Stand the aluminum box onto a rectangle of the foam, then make a box of XPS foam that can slip over then entire thing, and can be lifted off to manage the kiln contents.

Bob

 

[Chillucky]

Yup. This is what will have to be done. Because, gentleman, what I have made here, is a radiator.

Ah, well. Tried with what I had on hand. I liked the idea of a metal interior for wash-down reasons.