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Gas Forge Furnace: Chimney and Damper?


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Researching gas forge furnace designs as I plan/build my first unit.

As of now, I am collecting parts to build a forge similar to Larry Zoeller's, but am considering ways to potentially improve efficiency:

I have been reviewing designs of commercial furnaces with regard to efficiency, and, (roughly speaking,) it seems they seal the inside of the furnace, then provide a Flue/Chimney(s) with a damper(s) to regulate the expulsion of Flue/exhaust-Gases.

This helps to reduce heat losses caused by hot air/gasses escaping before heat is transferred. Sealing the furnace also helps to retain and focus I.R. radiant heat, which is the largest component responsible for heating materials placed in the furnace. Coating the interior with high emissivity coatings (like ITC-100) helps with this as well.

Also, flue-gases are sometimes used to pre-heat combustion-air, prior to mixing/burning it in the furnace. I have seen burner-tubes, jacketed by exhaust-stacks, for this purpose.

Has anyone successfully incorporated these concepts into their small forge furnaces? I was particularly interested in the idea of a chimney/damper, so that the furnace could be sealed and not explode.

Maybe I'm over-thinking this, (considering the small size of the furnace,) but a more efficient furnace can be made to run hotter and use less fuel.

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The Sandia recuperative forge uses the exhaust to preheat the air.

You have to have adequate exhaust or you forge will pressurize to the point back pressure will prevent the burner from working.

There are a lot of very good techniques for keeping the heat inside the chamber as long as possible. One of my favorite (untested by me) is the recuperative wall furnace. This is a double wall furnace design where the exhaust gasses are circulated in the annulus between the inner refractory wall and an outer refractory wall.

In a number of cases the outer wall also serves to preheat the intake air and gas.

The burners in these stop in the outer liner and fire across the annulus. This causes negative pressure in the annulus and encourages strong circulation of the exhaust gasses. It also consumes all the gas and oxy.

Regardless you have to let it out sometime. Kind of like the kids you know.


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Generally in blacksmithing we have pieces that will stick out of the forge and so there has to be a gap somewhere!

I think you are over thinking it. Until you know what you like doing with it it's kind of hard to build the "one true forge". I tell folks to expect to build a number of forges over time.

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Appreciate the replies/thoughts. Since it is my first forge furnace, I agree that I'll likely modify it and/or build more than one furnace over time.

Frosty, that sounds like similar elements as used in open-hearth steel-making furnaces.

I have some experience relining heat-treating furnaces, as I was a Supervisor for a Heat-Treat department many years ago.

Just trying to think through some possibilities. I read over my ASM International "Metals Handbook," with particular attention to furnace type/design and specifics about Thermocouples.

When I used to heat-treat professionally, we made-up thermocouples from ceramic-insulators (strung like beads on a string,) with Ni-Chrome and Ni-Aluminum wire, by twisting the wire-ends together and gas-welding to draw a bead on the tip. Then we would attach the T/C to the load at center and edge.

Found a Furnace Supply company (Industrial Furnace and Insulation, Inc., a company that builds/relines furnaces/kilns, etc.) that said he'll sell me hard-fire-brick ($3 /ea.), rigidizer ($18/gal.) to coat the Kaowool and Thermocouple Wire setup ($35.)

Edited by DerekC
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Unless you're thinking about getting into high precision heat treating you just don't need anything too fancy in a forging furnace. This is a situation where brute force counts for a lot, especially when you're first getting started.

A box or pipe lined with an adequate refractory and a burner with enough beans to get to forging heat is what you really need right now. By the time you have a handle on the basics you'll have a much better idea what you need in a forge.

Were I to go out on a limb and actually say out loud what type gas forge I think is the most versatile. I'd have to say a chip bed forge/hearth but never having used one I could be off by the width of the pond.

And yes, recuperative wall furnaces are pretty common in the iron/steel industry. I spent many happy hours drawing plans for one on a scale I could use but that's all I did. Construction of the thing was complicated and the overall size was about double what the usable chamber would've been. Maybe if I were melting Buicks.


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