Mastershake

Once again, thank you all for your input. This is an awesome and informative forum. I started forging with my little charcoal burner after retiring from the Air Force after 24 years and I was quickly hooked. I beat the crap out of that little forge just practicing hammering and drawing out techniques. I am ready to upgrade to propane and thank you all for your help!

I don't want to seem overly negative, but the .pdf in the original post scares the wotsits out of me. I've only had dial-up speeds this week, so one of the links I've not followed may have mentioned this, but just in case:

If I'm reading it correctly, the gas/air mixing is done prior to the plenum (big pipe) top right. The plenum and individual burner feeds are therefore, by design, filled with a flammable gas/air mixture during normal operation, with an ignition source (flame in the forge) at the end of the burner feeds. The only thing that can stop the mixture flashing back is the speed of the mixture down the burner feeds. As long as this is higher than the flame speed through the mixture, it should be safe, but as soon as the mixture speed drops, there will be a flashback into the plenum. The flame will start to move down the burner feed pipe, radiating heat into the mixture in front of it and generating pressure. As the pressure and heat build, the flame will accelerate, generating more pressure and radiating more heat, accelerating the flame and so on.This may or may not cause a major safety issue (eg, operator fatality).

The pressure rise/flame front acceleration will stop either when the mixture runs out, or when the pressure has risen enough to overcome the forces constraining it (Translating from my native geek; the plenum explodes).

It is certainly possible to engineer the plenum, burner feed pipe sizes, and so on to keep it safe, but the fact that you've asked the question on here tends to suggest that you aren't that familiar with the issues and that your time would be better spent on a design which eliminates the risk.

It is the gas/air mixture that is the dangerous bit. Having it confined in the plenum is what gives rise to the explosion risk. If you just run air to the plenum and pipe gas to a separate nozzle near the outlet of each burner, there is only a gas/air mixture for the last bit of the burner feed pipe. Any flashback can only get as far as the gas nozzle, so it can't build the speed and pressure that would make it dangerous.

I'll admit the chances of actually killing yourself with the original design are not particularly high, but assuming the plenum and pipework are built strong enough to contain things, the only route for the hot, high-pressure, gases to leave the plenum is via the burners. The risk of damage to the forge lining when this happens is quite high.

Regards

Tim

^{Thanks for all the great input. I will try to start messing with some pipes this week.}

Awesome work and very artistic design!

I have only used a homemade charcoal forge up until now. It's beat up and not that efficient so I decided to upgrade to a forced air propane forge instead of spending another $18.00 for a bag of charcoal. I have scrounged a blower and the sheet metal to construct the forge already.

I have an idea to construct a 4 jet forced air propane forge. The blower would hook into a 1" pipe with the propane input centered a few inches down from there. The 1" pipe would spilt into 4 - 1/2" line and I would place them at angles into the forge clreating a fire swirl. Is this efficient? Please give any inputs. I am attaching a PDF file illustrating the idea.

FORCED AIR FORGE.pdf

It holds up pretty good unless your stabbing it with metal and moving it all over the place. Its actually pretty hard once its fired a couple of times. I like to have that with a brick bottom to sat your work on. I think thats the best of both worlds for my situation.

Thanks for the information, So the coated wool would be the longer lasting?

I have been using a coal mini forge I built myself and want to upgrade to using propane. I want to build a forced air propane forge and had a few quick questions.

Which is better and lasts longer in this type of forge, fire brick or that wool stuff with the protectant painted on it?

Where does the combustion take place in a forced air propane forge, actually after the nozzle inside the forge of where the propane is delivered farther up in the assembly?

Thanks to whoever replies!