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I Forge Iron

Concept: horizontal front-mount burner

Dr. Rusty

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I've been processing the info on here as best as I can, and I've been struck by the concept of keeping the flame in the forge as long as possible so that ALL THE FUEL CAN COMBUST before exiting. Velocity. Volume. Thermodynamics. Aerodynamics. Mixing. Reradition. Flow. Flow path.The brain tries to model it, but inevitably builds itself a universe of unique physics and it's hard to separate reality from fantasy.

So I doodled one of these thoughts and I'm curious if there is any merit, or off it has already been tried (and failed). I had a go at the forum search but came up dry.

(I'm making some simplifications to hone in on the big concepts, in a similar way to how a physicist might simplify a cow into a ball with udders for the purposes of calculations)

So. Regularly, a forge has burners mounted on side walls, top, or bottom. Never facing an open door. From the perspective of the flame, it is thrust into a box that has an open exit hole that is always 90 degrees from its entry point. Pressure in the forge from the burner ultimately turns the bulk flow to at least one direction perpendicular to the entry point. I emphasize bulk, because the local flame flow at any particular position in the forge is influence by various -dynamics.

Anyways. Lots of mixing going on for some of the flame, but for other portions, there is quick ejection out the forge door.

And I thought, what if the forge exit forced a flow path that was 180 degrees from the point of entry. What if the burning gasses couldn't just hang a left so easily to escape into the environment. What if it needed to do u-turn or hairpin turn to exit? I THINK it would decrease the proportion of flame getting shot right out the door before combusting all the fuel. Increased residence time, Slower velocity, Hotter forge?

According to this doodle I found on my phone: would there be value in horizontally front-mounting a burner on the same forge wall as the door?

For this theoretical exercise, same forge, burners, psi, etc. Only difference is burner entry point. The metrics being graded/considered are completeness of fuel burn, relative maximum forge temperature, and heating eveness vs hotspot vs convenience. Safety is ALWAYS a concern for all forges ;)

I'd love to hear thoughts or opinions. Don't be afraid to tear this down with cold hard facts and logic.


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I have a Don Fogg style propane forge; this is essentially a burner in front; firing off-center in a round vertical chamber; with openings higher. This makes the flame swirl below the openings; and exit the forge quite evenly. It works reasonably well; but is especially effective for making damascus as you don't have a floor for the borax to eat :D ; and you could insert a stainless bowl below the burner to capture the borax drops which you could empty easily.

You can move the burner to the front if you wish; but I like the distance between exit and burner to be larger.

picture found on google for illustration : 





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I believe it was  Don Fogg that talked with me at the 2004 Kentucky conference about his idea to blow an oil flame into the forge, under its floor (think kiln shelf). up an opening at the rear, and back forward to exit out the front, in a similar fashion to what you present. Your idea is a sound concept, BUT the devil is in the details...

Sometimes those details are too nasty to be worth the effort. Meantime, a swirling exhaust path is not the only way to extract heat from combustion gases. A baffle wall just outside of the exit forces a final turn in the flame path, while bouncing radiant energy back into the forge. Just a thought; the one doesn't prevent you from doing he other :rolleyes:

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I've given this a lot of thought too and don't have many "good" ideas. My intuition keeps looking at your basic concept and whispering " no sale, back pressure."  The second down check is the exhaust flame is directed straight at the operator. How are you going to lay long material without a couple feet being in the super heated exhaust? That is assuming complete combustion.

Don't knife forge does what he claims and is a darned good design, a little awkward for general smithing but very good for blades.

Increasing hang time for the reasons you describe is highly desirable but it's not so easy. As Mike says some of the solutions are so involved as to make them unacceptable, even to experiment with. 

One of the limiting factors is back pressure, unless you use gun burners back pressure is the deal killer for many concepts. #1 flame and heat retention "tricks" is a complete deal killer for a naturally aspirated burner, close the doors and limit exhaust ports. 

My thing is NA (Naturally Aspirated) burners and they're pretty susceptible to back pressure, so I have to make "free air" forges. The exhaust has to vent as much as the burners input. Call it a GIGO forge, (Gas in Gas out).  The two main methods I like are: #1 low velocity flame and #2 prolonged flame path (hang time). Lower velocity increases hang time so it should probably be #1 but I'm not swapping things around. 

Swirl is a biggy and your concept falls in this category. Sort of. One of the ideas I haven't experimented with much is directing the swirl to travel as long a path as possible in the chamber. For an exaggerated example, the burner is placed at the doorway end, aligned to impinge the liner tangentially to cause a strong swirl. It is further aligned to encourage the swirl to move towards the far end in a vortex. Envision the flame following a path like threads in a nut.  See what I'm envisioning? 

Where does the exhaust go though? Hopefully it'll follow the low pressure zone in the vortex, back the other direction in the center of the chamber and out the doorway. If you search "Vortex Tube" you'll see that a vortex will reflect and travel back up the center of the parent vortex. IF you can get it to reverse direction, clockwise to counter clockwise then you have a heat pump of MAJOR effectiveness. 

You mechanics will recognize vortex tubes as choke testers, air hose connection off center and it blows 300f air out one end and -50f out the other. I've enjoyed decades of fantasies about vortex tube appliances.

I haven't built a cylindrical forge since my first, I've been building rectangular chambers so directly playing with swirl and vortices is pretty moot for me.

 So, how do I prolong hang time in a rectangular chamber? If we shift terminology a little there's a way. For the purposes of conversation let's call, flame "hang" time, flame "contact" time. The longer flame contacts the liner the more energy it will transfer. We may not be able to do much to prolong the flaming 4th dimension so how about increasing the contact part? More surface, more contact more energy transfer, say the cooling fins on an air cooled engine. 

I've collected the stuff to make a corrugated floor for my next forge to increase contact time and as a side bonus no flame shadow so the flame stays in contact with the forge floor the work is laying on. Hmmm?

There's another I've been fantasizing about for decades, the "recuperative WALL" forge. It's in common use in commercial furnaces and is so elegantly simple it just screams out to be part of my forge. They're simple a furnace with a double wall, the burner fires into the chamber and heats the wall like ours do. The the exhaust ports are a series of vents to a open space (Annulus) on the outside of the thin inner wall. The flame travels between the inner and outer furnace walls before being allowed to vent outside. This heats the inner liner from both sides and does some impressive things to efficiency and effectiveness of energy transfer.

However no matter what you do outside of my fantasy flaming vortex tube, the forge can NOT be hotter than the flame. Sir Isaac  Newton explained it so I'll skip that part.

And there are my current thoughts for your consideration. :)

Frosty The Lucky.

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I think the "coffin lid" on this design is that, if all of its problems are figured out, it is already superseded by ribbon burner forges, which do the best possible job of getting the most heat transference from gas flames, and can be combined with any and all new burner bodies that anyone figures out.

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Yeah, multiple nozzle burners reduce velocity for a treat but there's still a lot of energy going out the doorways that could maybe be recupped by circulating it through a second space. The problem I envision is a ribbon might not generate enough velocity to power a second circulation loop. 

With a recuperative wall the final exhaust shouldn't be the doorways so you can close the chamber completely without inhibiting flow. 

Knowing how you like experimenting I don't need to say "kiln shelf" do I. The only available to me is stupid expensive or I wouldn've been messing with the idea for years. I believe you have much better access than I. 

Trying to cast a hard liner with an annulus is one of those stupid too hard to pull off things and I gave it a LOT of thought, even modeled it a couple times. PITA in spades is an understatement. 

I'm not familiar with the "coffin lid" by that name anyway. Pictures? :)

Frosty The Lucky.


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I think you'd like to suck me down the ribbon burners rabbit hole, and of course, that is only a matter of time. But right know I'm waist deep in a book on small burners made by repurposing $20 air/propane torch-heads, to reduce the work and expense of making Mikey burners, and hybrid designs,  from scratch. But, yes, ribbon burner ideas keep creeping into the back of my head; they won't stay quiet indefinitely. I will probably drill my first kiln shelf this summer. You know what a stickler I am for clean flames :rolleyes:

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  • 2 weeks later...

Frosty, Mikey, I can never get enough of your input, even if it is burying an idea. You both share so much in return that I win every time. Yet more ideas sparking from what you've both said, but at least now I'm getting better at identifying the limitations and which potential design forks aren't worth the energy. Thanks!

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Your thinking is pretty much cutting edge, BUT something I did not see mentioned was deceleration. This creates a huge hole in your mental picture. As soon as combustion gases leave the flame envelope (good), or envelopes (not desirable). they start decelerating. Small flames slow down much faster than large flames. So, distance from flame to exhaust opening is only half of the oicture...can you say ribbon burner? :rolleyes:

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