to flare or not to flare

T

trogdorr

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Jan 3, 2015
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I am making a horizontal propane forge and I have a quick question about the construction of the burners. These are venturi burners with 3/4 inch tubes. I have seen a couple different burners online, and my newbie questions are, is it even necessary to have a flare? What benefits would a flare have over just a straight pipe into the forge? If I were to make a flare would it be better to have a section of pipe that fits over the tube or could I just flare the tube its self? what would be the advantages/disadvantages of no flare vs. a flare that fits over the tube vs. flaring the tube its self? I've also seen burners with bell reducers for flares, would this be a good way to go? Any input would be appreciated. This is my first time building a forge and I'm trying to everything as correctly as I can the first time, so I thought I would tap into the plethora of knowledge contained within this forum.

Thanks for your time,
Trogdorr
 
A flare is only needed if you want the burner to stay lit while clamped into a vise. The entry into the chamber of the forge will perform the same function as the flare in practical use.

Venturi burners are super easy to make and don't require the complexity of the side-arm to function. Mine have 3 parts, but I have the parts custom made. If you want to build it from hardware store parts, it will require 5: Tube, 3/8" flare to 1/4" NPT(depending on your regulator hose fitting), 1/4" to 1/8" steel bushing, 1/8" nipple, and 1/8" cap. The largest size orifice I've got to work with 3/4" is .052" with a 1.25" schedule 40 port. It maxxed out at around 18 PSI, I think you can go higher if you had a bigger port.
 
A large flare is certainly not needed. I have always preferred a slight flare. Often it is just chamfering the inside of the last inch of the burner tube. In other cases, I have forged the tube against a tapered mandrel to flare about .25" or so. Even a small flare drops the speed and pressure as the gas exits the burner. This cuts down on turbulence and blow-out. Probably of more use at full pressure than at the low end.

I haven't though of it in the terms Charles addressed, but once running, the forge will take care of the burn and some backpressure, so the flare probably doesn't do all that much once the chamber is hot.
 
I have nothing to add, I just wanted to remind you not to use the forge in a thatched roof cottage and try not to burninate the country side or the peasants. [emoji12]
 
I've gotten dozens of e-mails from people complaining that their burners won't light. Every time it turns out that they were trying to light it while clamped into a vise, or in one case, a 55 gallons steel drum rigged up as a smoker. Yes, during the initial stages a flareless burner sputters more, but once red the flare hasn't made a difference in any of my tests. My reasoning has been that as the burner enters at an angle, the angled cutaway of the forge wall acts as a latitudinal flare, rather than a longitudinal flare. Probably not as efficient, but still effective. Plus, it has the added benefit of not allowing someone to burn their garage down because they thought a 100K BTU blow torch would be neat.
 
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