So, you've got lazy and settled on propane. Wimp... ;) On the other hand, welcome to the exciting new world of gaseous fuels. Here's some of what I know on the subject.
I am only familiar with Ron Reil's designs at the moment; mine are based on his work. Likewise, this article will concentrate on his style of naturally-aspirated burner.
The construction of these consists of: 1. a "jet tube" which connects to the propane supply and which shoots gas into the burner; 2. (optional) a venturi which helps to draw in more air; 3. a "burner tube" which the gas flows down for a certain length, giving it time to mix with the air and stabilize the flow (I guess); 4. a flare (usually optional inside the furnace) at the end; it provides a pressure 'step' which the flames rest in, 'holding' them.
Generally speaking, the jet tube is a small hole (pointing down the burner tube) which connects to the gas supply. The hole is typically less than 1/16" for most backyard foundry burners, but can get very large for low pressure (residential natural gas lines) or high output burners. It's either cross-drilled in a bit of steel or copper tubing, in the end of a capped or plugged tube, in an attachment (Tweeco tips are a good example), or any number of other possible ways.
The venturi, ideally, is a smooth bell curve as shown above, but usually you can settle for something a bit more, erm... crude. Namely, pipe reducers. They work perfectly, mind you! The tip of the jet should be placed a bit behind where it really starts to shrink in diameter, which would be with the tip at the cross in the picture above. You can even do without one, and I have a very capable burner (made from a .023" Tweco tip shooting down 6" of 3/4" o.d. EMT conduit tube) which proves it.
The burner tube should be at least 8 times longer than its diameter. Too short and it is unstable and will more easily start burning at the venturi and is sensitive to lean mixtures, too long and it's inconvienient. A smooth surface doesn't really matter because, one way or another, the high pressure gas is going to find the lower-pressure end and flow through. Besides, turbulence will just add to the mixing.
Lastly, the flare. This is needed to "hold" the flame in place. Without it, it would have to burn around the rim of the burner tube, which is no good as it only works at low velocities, and never even a neutral, let alone lean mixture is possible. It seems that, rather than a straight burner tube smoothly tapering out after a point, a step in the diameter (as seen by the gas flow) is preferable. The ideal slope for a flare (said taper) is 1:12, or 5°, but 0° seems to work fine with thicker burner tubes, such as those made using galvanized or black water pipe. The flare's length should be around 1 or 1-1/2 times the burner tube's diameter, too long (at least with little or no taper) and it invites instability (a fwoomp-fwoomp oscillation) at not just low pressure but higher levels as well; too short and it's hard to light and stay lit, as well as having some oscillatory troubles I think...
If the tuyere in the furnace is a close fit to the burner tube, it will suffice as a flare, thus one built on the burner may not be necessary in the furnace.
My Big Burner uses a 1" dia. 10" long black pipe nipple for the burner tube, a #57 size hole for the jet, uses a 1-1/2" to 1" reducer for venturi (normally about 1/4 to 1/2 choked off.. it's good to have some lean headroom with your mixture), and runs maybe 5-30PSI (I don't have a guage yet) with good stability. This (at 5-15PSI) provides enough heat to melt aluminum relatively quickly (compared to the wood and charcoal I had been using) in my Trashcan Furnace.
On the other hand, a .023" Tweco MIG welder tip soldered onto some tubing, shooting down a piece of EMT (as I mentioned above) works great for smaller work, especially brazing. I've brazed 1-1/4 x 1/4" steel bars in a tee joint with this thing, with plenty of heat to spare. (Of course, the real secret is piling a house of firebrick around it and filing the metal clean so it sticks first try, but that's beside the point. :-P )
If you've got a real big furnace lined up, say one made in a 55 gallon drum, then you'll want something in the 1-1/4 or 1-1/2" dia. burner tube range. (By the above ratio, that should be at least a foot long.) Something like a 1/16" dia. jet should do, but don't quote me on that. You may or may not need a venturi.
The burner should be matched to the furnace as well. For my Reverberatory Furnace I built a slightly smaller burner, using my first burner's spare plumbing... I haven't tried it yet but it should be a better match than the big guy.
This also shows my new torch-side plumbing, which includes a cutoff valve - very convienient. (The connectors go: (not shown) 20lbs. propane tank > tank to 1/4" pipe thread adapter > 0-35PSI regulator > 1/4" pipe thread to 1/4" hose barb > 6' of rubber fuel-rated hose > 1/4" hose barb to 1/4" pipe thread (this point screws into the valve shown) > 1/4" ball valve (pictured) > 1/4" pipe to 1/4" flare > flared 1/4" copper tube brazed onto .035" Tweco tip.)
On the bell (a 1 to 3/4" reducer) I brazed a flattened 3/16" dia. rod (it just happened to be flattened from my screwing around; it would stick on just as well if it were round), bent it so it comes out at a right angle to the surface, then bent it around as you can see, where the jet tube is attached. That's a square of 22ga. steel brazed on, which wouldn't be necessary if this end of the arm were forged flat too. The surface was filed clean and the jet tube silver soldered on.
At the other end, a few square inches of steel flashing is slipped on (it's been formed into a cylinder and held with a pop rivet) for the flare, completing the burner. Quite stable at low pressure, and despite the loud noise of what surely should be it blowing out, it withstands 30PSI over the full range of rich to lean.
One last note. All burners should have a choke if possible. My experience with these style venturis is they are more efficient at high pressure, that is, the mixture leans itself as pressure rises. (This might be a flare effect instead, the jury's out on that one.) On the above burner and the big burner, I drilled and tapped a hole in the end of the venturi and screwed on the choke plate. The EMT conduit burner I designed based on finding a burner tube matched to the jet orifice, so it doesn't need a choke to burn neutral. (After heavy use, its flare is turning to scale and crumbling, resulting in a shorter flare. I highly suspect the flare has a large effect on mixture, as with this shorter, unusually-shaped flare it is now burning lean and I have to hold my hand over the open end to choke it manually back to neutral. It also has tendencies towards instability, and as it heats up its characteristics change once again, but I think that's simply because the flare is short.)
There. Hope you managed to read through all that. (It took me long enough to type too, but that's my fault for typing at night when ze brain cells, dey verk less vell...) This should give you more than enough of an idea to mess around with your own burners. Just remember: once you get over the mostly groundless worries of high pressure potentially-explosive gas, and remember that brass brazing a Tweco tip tends to seal the hole, ;) you can have a lot of fun wasting gas.
Copyright 12-28-2003 by Tim Williams. All rights reserved.