Because blowers are expensive, I opted to go with a Venturi style burner. These tend to be a little more fickle, and a little more complicated to get right, but when you do they work great. Ron Reil is where every one gets their first inspiration for building one of these. I would suggest you look through his web page, because he's done it all.
Venturi burners work on the Bernoulli's principal, where a high velocity stream of gas creates a low pressure in the burner tube. This low pressure will draw in air that mixes with the propane and ignites in the forge chamber. The stream of gas/air mixture is moving faster than the propane is igniting. Until it reaches the chamber where it slows down enough. Since we're relying on a physical phenomena to pull air into the system, it can be temperamental, and sensitive to certain details.
The diameter of the nozzle, and the gas pressure will have an effect on the velocity of the gas, which in turn will dictate the air/fuel mixture. The geometry of the bell, and any irregularities in side of it or the nozzle can cause turbulence, or reduce the negative pressure. All of this can cause drastically different results in the performance of your forge.
Another property of significant importance is the amount of back pressure caused by the forge itself. Ideally you'll have zero back pressure, resulting in free flowing mixture of air and propane. Unfortunately this is never the case. Too much backpressure can cause a number of issues, one in particular is back burning. My paint can forge has suffered from "back burning" since day one. I've never understood what the issue is until recently.
Here is my theory: The forge chamber has a diameter of 2", and a burner tube diameter of 3/4". The rule of thumb for chamber diameters is eight times the burner tube diameter. My 2" is way too small. The forge behaves normally for 15 minutes, until it reaches a certain temperature where the back pressure becomes too great, the gas velocity slows down to a point were the flame burns back up into the bell. I'm going to upgrade to a larger forge and see if that solves the problem.
Many designs include a flare at the flame end of the burner tube. I can't say that I completely understand what advantage is provides, but I have a few ideas. In order for the flame to stay lit, the gas velocity has to be slow enough to continue the burn, but not slow in the burner tube (otherwise it would cause back burning). Without a flare, the gas would continue out the burner at full speed faster than the flame can keep up, and the gas concentration decreases below the propane flammability limit. A flare gradually increases the pressure and slows the velocity to a point where the flame stays lit at the flare. But here's the thing, the forge chamber can act as the flare. It won't be the "perfect" shape to match the gas expansion, but it will be close enough for a functioning forge. It's possible having the perfect flare improves the efficiency of the forge, but I can't say for sure.
A forced air forge doesn't have many of these issues. The air/fuel mixture depends directly on the speed of the blower and the pressure of the gas. And the burner will perform as well as the blower is able to supply enough air.
Now that we've got some of my musings out of the way, let's get to building. Naturally the best way to start is to draw it out.
For the body of the forge, I used a 1 gallon paint can. 2 inches of Kaowool and satanite to seal the fibers. It's important to seal the fibers, because you don't want little pieces of ceramic being blown out of your forge where you can breath them in. Kaowool is also not very strong, and will deteriorate rapidly if it's left exposed to the open flame, hot steel, etc. Satanite gives the inside of the forge a nice rigidity, and helps keep the forge at a stable temperature.
Here is a video of it running.
At the end of the project, I was able successfully heat treat some knives. Despite the back burning problem.
The forge is now used as a proof of concept for some blacksmithing. But the back burning is too much of a problem for this to be viable at all. Guess that means I get to rebuilt it :D I'll write up a whole post about that. But for now, here is my testing setup, complete with the original forge, and my anvil.
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