mini propane burner

[MattBower]

I wanted a propane burner to fire a very small casting furnace or forge -- something with more "oomph" potential than the handheld propane torches, but smaller than the typical 3/4" Reil/Porter/Price style induction burners. So I scaled one down. I started with a version of something called the mako burner from another forum, but ended up with a scaled down version of one of the burners from Michael Porter's book. Although "measure twice, cut once" is always excellent advice, I didn't heed it in this case and ended up paying for it in the end. It's running OK now, but it still needs tweaking.


So far, it's gone like so.

1/8" brass pipe nipple. Length of 1/4" OD copper tube. 0.030" MIG tip. The MIG tip was brazed shut. The tube was chucked up in a hand drill and polished with 150 grit abrasive. Unfortunately the chuck crushed the end of the tube, which required cutting it shorter than I originally planned (seen later). I should've started over with a longer piece of tube, but I got in a hurry -- to my later regret.



Tube soft soldered into the pipe nipple (then pickled with ferric chloride, which is why the solder looks dark).



Tube trimmed. And since the pipe nipple was a leftover from an earlier burner, the small hole in the side was soldered shut.



Tube ID reamed to 7/32", then tapped 1/4-28. MIG tip and tube end pickled in ferric, fluxed, threaded together and soft soldered.



Checking out how far the orifice will protrude into the burner tube.



Starting to come together. The burner tube is 1/4" NPT, joined to the orifice assembly with a 1/8"x1/4" coupler.



Orifice drilled with a #75 drill in a pin vise. (My drill press routinely breaks tiny drills.) That took a little while.



I don't have any more WIP photos, but I will describe the rest of the process.

The next step was to cut "gill slits" (air intakes) into the burner tube. Since I wanted to keep the slits as close as possible to the tip of the propane orifice, and the orifice didn't initially end up protruding as far into the burner tube as I had wanted, I cut the slits quite close to one another, as near to the orifice end as I could get them. The consequence was that I could only make two rows of slits, diametrically opposed to one another. Since my burner was going to be much smaller than the original, I tried to get away with cutting the slits with a hacksaw, which proved inadequate: the burner couldn't draw enough air and ran very rich. I later enlarged the slits with the edge of a big mill file, and the discrete slits turned into two larger, rectangular openings, as seen in the video. I also drilled two 3/16" holes near the front of the burner for supplementary air, similar to the original. I now suspect that was unnecessary.

During my initial test firing the burner ran stable but very rich. tI wasn't drawing enough air. One of the solutions was to enlarge the air intakes, as described above. But I also suspected that part of the problem was that the orifice wasn't seated far enough into the burner tube, for reasons already mentioned. To fix that, I ended up reaming out the pipe threads on the side of the reducer coupling that the injector assembly (MIG tip, plus copper tube and brass pipe nipple) had formerly threaded into. That allowed me to insert the injector assembly much deeper into the burner, at the cost of simplicity: I could no longer just thread it into the coupling and be done with it. So I ended up inserting the injector assembly into the burner tube as deeply as I could (the non-working end of the burner tube ended up butting up against the end of the 1/8" pipe nipple, which kept the injector from moving beyond a certain depth). That gave me enough extra depth to fix the problem, but I had to resort to JB-welding the injector into the coupler, which cost me adjustability. There's a better way that I could've done that.

Prior to the second test run I also chucked up the entire injector assembly in a drill and used files and abrasives to put a nice, rounded taper on my MIG tip. That should let the burner breathe more easily. If you use the TWECO tips that are usually recommended in these sorts of tutorials, they're pre-tapered.

The next test firing didn't work very well because I had not quite gotten the orifice axially aligned with the burner tube when I epoxied it in place, and I didn't yet have even a temporary flare; I used an unattached piece of scrap square tubing as a flare, which only kinda worked. Fortunately the copper tube that the orifice is mounted in is reasonably bendable, so a little judicious pressure through the air intake slots got the alignment problem fixed. I also attached a 1/4"x1/2" reducer coupling as a temporary flare for R&D purposes, and I inserted a piece of metal window screen across the opening of the burner tube where it enters that coupling, to serve as a flame holder and hopefully help a little with mixing the propane with intake air. The screen shouldn't be necessary, but I figured it couldn't hurt.

Here's the result so far:

My apologies for the bad videography, All I have to take video with is a phone, and I shot that at night because it was my first opportunity. I'll get something better when I have a chance.

[Bigred1o1]

i look forward to seeing how this works out as i am hoping to make a long forge with several small burners and this looks like a nice burner plan