trouble-shooting the mini sidearm burner

[jcornell]

I built a little coffee-can forge as my old firebrick forge was falling apart after a couple of years of delightful use. I built a mini-sidearm burner. When I had the burner in the vise I was able to get a nice blue flame with a quiet roar (rumble?) but once it was in the forge I couldn't get the roar, and it seemed like the flame was weaker, with some stuttering. Turning up the gas pressure yields yellow flames out of the mouth of the forge and the back pass-through hole.

Is this too much back pressure, or another problem?

Any suggestions?

[timgunn1962]

There are lots of different things happening in an atmospheric burner and they all affect each other.

Although they are not any more efficient or any hotter when set correctly, blown burners can be easier to set up, because the gas and air feeds are independent.

With your atmospheric burner (I'm being pedantic and not calling it a Venturi because not all atmospheric burners use a Venturi), the gas leaving the jet generates a low-pressure zone, into which air is drawn. The air mixes with the gas as it travels along the burner tube. The mixture really needs to be travelling along the burner tube faster than the flame-front can move through the mixture. If the flame-front is moving through the mixture faster than the mixture is moving in the opposite direction, the flame will burn back down the tube. Sometimes this will appear as stuttering and can usually be overcome by turning up the gas pressure.

The gas speed out of the jet has more of an effect on the amount of air entrained than does the volume of gas. It sounds, from your description of the yellow flame, like you either have a restriction at the exit from the forge (the volume of hot gases you need to get out of the forge will be at least 5 times the volume of cold gas and air going in, simply because of thermal expansion), or you have too large a gas jet.

When you had your nice blue flame out of the forge, what did it look like? I'm guessing you had a tight blue cone (the primary flame), surrounded by a bushy outer flame (the secondary flame). The primary flame is where the gas and the (primary) air, drawn in through the mixer, burn. The secondary flame is where more air mixes with the partially-burnt gases from the primary flame and finishes the burn.

If you stick the burner in a forge and don't let air in between the burner and the hole it fits through, the secondary air cannot get to the partially-burnt gases from the primary flame until after they leave the forge. The burner that looked great with both primary and secondary air, now only has primary air and it ain't enough.

It sounds to me like you need a smaller gas jet.

[jcornell]

I'm fairly certain that I'm using an .023 size tip - is there anything smaller?

[Frosty]

Well said Timgunn, you've played with these enough to understand them and better yet explain what's going on and why.

As Tim said a smaller jet may do the job. OR you can trim it shorter, the farther the jet's tip is from the throat (the transition point from the "T" to the tube) the more air it will entrain and the leaner it'll burn.

Judging by your description there's another possible cause I find more common and that's back pressure in the forge itself. A number of things can cause too much back pressure, too small a forge volume, restricted exhaust porting (doors) and improper burner alignment.

Without knowing the specifics of your burner and forge I'll have to let you figure out the cause and solutions, yes solutionS, there are often more than one way to fix a problem, these are only simple devices in principle.

Okay, how large is your burner? If you're using a 0.023" jet it should be a 1/2" ID tube with a length of 6" give or take, longer is better than shorter within reason, an inch more is just fine.

What's the volume and shape of your forge chamber? A 1/2" burner will bring about 175 cu/in to welding heat depending on shape. A 3/4" burner is good for abut 300-350 cu/in. Heat output is a direct factor of how much fuel it can burn efficiently and this is an effect of the cross section area of the tube. Another really important factor is exhaust, all that burned gas and hot nitrogen HAS to go somewhere after it's done it's thing. you need to have reasonably clear exhaust ports so the exhaust can escape or it'll develop enough back pressure to kill the flame. Exhaust porting is in the forge chamber size and shape because the ports are an integral part of the forge chamber. Inadequate exhaust porting is my first impression when I hear the "complaints" you describe. Open the door and or the pass through to reduce back pressure. Position of the burner is occasionally the problem, if it's too close to an opposing surface or is aimed into a pocket it can be reflected back into the burner and will develop back pressure. This IS rare but does happen. It's also why I recommend aiming the burner so it's at an angle to the far wall so the flame can't be reflected directly back. AND that's it for burner alignment as a problem.

How you direct the flame is a matter of personal preference which is usually a matter of purpose. Aiming the flame so it impacts the far parallel wall at an angle makes a vortex which is good for more even heat in the chamber. Aiming the flame so it impacts the opposing wall perpendicularly makes areas in the forge of different heats. Both have advantages for instance someone wanting to heat treat blades wants as even heating as possible so a vortex may be best. A smith who wants to work smaller sections, say, for working finials where having say the 8" next to it makes for more distortion would find it hard to heat just one part of the project in a vortex, everything in the forge will get hot. Perpendicular alignment and the differential chamber heat may be his/er preference.

Well, ain't that enough rambling for now. Let us know some particulars and maybe post a pic or two and we can be more help than my theoretical windyness.

Frosty The Lucky.

[jcornell]

I did some troubleshooting this weekend and got success.

 

1) I took a layer of insulation out - I had 2" and went to 1" - the chamber was just to small and I had too much back pressure.

 

2) My mig tip was not quite tight with the pipe nipple - gas was escaping around the tip, rather than just through the .023 tip - so I was getting too much propane.

 

Carefully sealing the joint gave me a wonderful burner.

 

Now I need to reseal the insulation with a home-brew mix of kaolin and zircopax to up the efficiency.