I know from my hour(s) of piddling around with the little propane torch (and not with the usual pencil/bushy flame tips, but with the teeny jet of propane alone, blowing down various bits of tubing as in one of these style burners) what a rich, neutral and lean flame are. And that first burner was too rich (just by a little, but still too rich), even with no choke plate. So either I need a smaller jet or a larger venturi. The former increases the velocity of the gas jet - that is, for the same volume (and thus power output) you need to increase the pressure behind it. As a result of the higher velocity, more air is drawn in, and you'll get a leaner mixture. For a larger venturi, there is more area to draw in that air.
Now, I only have this 0-35PSI regulator, so I'd rather keep pressure down. (I've already had the first burner up to full throttle and it seems like it can still take a lot more gas.) So a smaller jet hole is out of the question, given a certain power output. (Again, I don't want to lose power.) That leaves a larger venturi.
So let's go shopping! Ace doesn't sell 1 1/2" to 3/4" reducers, so I'm stuck with something bigger, say 1" burner tube. So I'll grab a 1 1/2" to 1" reducer and a 10" long 1" black pipe nipple huh huh, he said nipple.. <-- How'd that get in there? :-p Anyway, that should work for the base. I'll need to make a new flare, since the old one's too small.. no problem.. that leaves the gas delivery itself... I don't want to do it the same as the first because I had to add that strap to keep the jet tube from moving, that's a cover-your-ass move. I don't want to do that on a new design! So humm... ah! Two tight bends, three holes and a little brazing later and we got ourselves a new burner...
...Sorry for the crappy pictures but again, this camera sucks, especially in low light...
Overall and closeup view. You can see the gas tube comes from a flare connection - I also picked up some flare connectors at Ace so I can swap between burners while using the same hose, without all the wear and bad sealing that happens when I use the same compression ring over several times... Anyway, construction, the tube goes from a 1" nipple of 1/4" copper, flared on the end, brazed to a piece of smaller steel tubing I had laying around, which makes one bend to get it away from the burner, attaches to a steel clip (actually silver soldered), screwed to the reducer, then another bend to get it to a cross-hole. As it goes across, a #57 hole shoots propane down the tube. The far end is sealed off. And yes, there is no other connection holding the tube in besides that one clip - works pretty well!
Side-by-side comparison of the two burners. This one is almost twice as long! :)
That reminds me, another detail of burner contruction is burner tube dimensions - too short and the gas won't be mixed enough to burn, too long and it's impractical. (Among my experiments with the little torch (see above), I lit up the end of 5' of 3/8" vinyl tubing - don't do this yourself, the burning plastic releases chlorine-based nasties! - but anyway, I did it, it worked, it looked funny.) A safe minimum length is 8 times the diameter. Diameter typically depends on the power level usually: 1/2 to 3/4" is good for a 6 to 8" i.d. furnace for aluminum or brass melting, 1" for cast iron or larger furnaces, etc. (Note that you can get away with very little power in a relatively large furnace; the price is maximum temperature and melt time.) The other price is flame velocity: for a given volume of air, speed will be higher with a smaller pipe. You might exceed the permissible flame speed for your design and it'll never stay lit! (This is one thing a flare (the sheetmetal cylinder on the end of the pipe) helps do.) With a very large tube, velocity will be low and the flame might start somewhere back from the intended point (which is inside the flare). Or it'll oscillate and sound like a pulsejet! As-is, I have just this sort of instability (burning too far up the tube), though not the same cause. I imagine you'll have a similar problem if you try building a burner like these - so I'll tell you how to get around that. Start with gas flow off, then turn it on slowly while holding a match/lighter near either end of the burner, to get it burning right away. Turn up the pressure. If it's like these, then it'll do that oscillation pulsejet-ish thing. Turn up the pressure a bit more. If it's still farting away, put your hand over the intake end to reduce air into the burner: the idea is now to choke off the air, so where it's burning up front, it can't anymore. You'll get a big sooty orange fireball out the other end when doing this, careful! :-) Slowly remove your hand to restore airflow, and it should start burning like a clean torch or jet (without the high-pitched whine) sound, and the flame should be burning in the flare as shown below:
Neutral flame. Unfortunately the camera can't capture the full spectrum of colors going on here, but it's a smooth blue, with a light blue inner cone (the bright part coming out of the flare). For that matter, the flare is *supposed* to be glowing orange. ;-p
Here the choke has been advanced a little, cutting off more air. The result is a more greenish inner cone (and the flame overall), the flame becomes generally lazy and starts rising as convection takes over. The flame is also cooler, and you can see the flare glowing less. For very rich flames, the inner cone becomes elongated and is very close to the outer cone. Any richer and carbon starts seperating, forming a yellow peak, or worse still, a sooty flame (see above about choking to start it up).
Here the choke is wide open. As is typical with lean fires, the flame temperature is much higher, as evidenced by the brighter flare. (DON'T take this to mean you can get faster melt times from a lean flame). The inner cone is more blue, starting to turn purple in fact. It also gets longer, and for very lean flames is indistinguishable from the outer cone. At this point it almost looks like, "what's keeping that flame burning!?" Indeed, any more lean and it *will* blow out.
Hope that's enough information in succinct enough form for you to get by... some day I guess I should condense that into a propane burner design handbook.. oh well.. I gots homework to do....