Orion Kindel

Questions about propane tank forge insulation

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Hey folks, long time lurker first time poster. I'm about halfway into a propane tank forge build with a pair of 3/4" Frosty T burners and have been perusing IFI mindlessly for the last couple weeks absorbing all the information I can.

I'm planning on following Wayne build guide, which is essentially a 20# propane tank forge with the flat bottomed chamber. The entire tank is cut lengthwise following a line drawn across the opening of the collar, which is roughly 1/3 from the bottom of the 7" dia collar circle.

For the bottom "half," he advises to use 2 x 1" layer of ceramic wool then (from what I can tell) filled and screed with castable refractory which from rough guesstimations would be about 1-2" of solid refractory. Wouldn't the forge get to heat faster if I filled the void with more wool, rather than an inch of solid refractory, and instead shoot for the surface be around 1/4" of castable?

And for the top "half" he builds his forges with 1 layer of wool topped with 2 full 5lb bags of castable, which to my uninitiated mind seems like a lot more than 1/4", which seems to be the target thickness of castable from other threads here on IFI.

Should I shoot for a thinner castable layer on both halves for efficiency's sake?

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The reason the floor is thicker is because of the abuse it takes. You're going to be "stabbing" it with stock as you put it in, regardless of how gentle you are. You don't want to chip or gouge that refractory to the point you're having to reline it. Also, depending on the refractory that you're using, it'll hold up to flux to a certain extent. Thinner refractory will get eaten through faster, and then once you get flux on wool it's game over, like water on cotton candy. So you're giving up some thermal efficiency in exchange for durability.

As for the ceiling/walls, they're going to heat/cool faster than the floor, and you're going to experience thermal cycle shock to the refractory and cracking will be catastrophic if you're running a thin coat. Just like the floor, thicker is designed for durability. And keep in mind too that the castable doesn't go as far as you think it will. 10lbs of castable seems like a lot, but it's really not.

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Gotcha, thank you Maxwell!
Couple more questions, I'm going to line the burner openings in the chamber with refractory as well, does the thickness there matter much?
And would it behoove me to shape the castable there into a ~12° flare for the burners?

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If you rigidize the ceramic blanket there's no need for 1-2" to make a good solid floor. Fill the curve with rigidized blanket and lay 1/2" or so water setting hard refractory over it. A lot of folk use a thinner coat for the walls and roof but I've found they all get beaten up, especially be beginners. 

Mine are 1/2" of Kastolite 30 all the way around and hold up nicely.

Yes, line the burner ports as you propose and kiln wash them. The burner ports will be the hottest part of your forge and need the protection.

Frosty The Lucky.

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