Okay... a little bit of fire mechanics for you...
A flame will not flash back into a combustible mixture in an enclosed space that is moving faster than the flame front speed. For propane that is about 12ft/sec. What that means is that if you lit a fire at your blower as it is now, it would flash, pop, and be blown out into the forge and not continue to burn. You are at a relatively low pressure, so no explosive (supersonic detonation) effects should ever be seen. However, you have a real chance for deflagration (subsonic burning) IF you get an ignition source back into the blower/manifold area. The problem is this: if that deflagration happens in a large enough volume of propane/air mixture, your blower assembly may come apart from overpressure.
I'm a chemical engineer, so I like new ideas like this, and instead of dissuading you from doing it I'll try to help you take as much risk out of it as possible. This has the potential to be more efficient and consistent than a post mix burner.
Here's what you are going to have to do to do this right:
You need to have a blower with a known capacity (preferrably one with a published speed vs. flow curve) and size your outlet manifold to always stay above 12 ft/sec even at the lowest fan setting. You also need to make the outlet manifold out of pipe that can withstand the high pressure that may encountered when things go haywire. That's pretty much any steel pipe. That manifold needs to be as short as possible to minimize the amount of gas that would be involved in a flashback. There can be ZERO leaks. That means welded or caulked joints and daily leak testing with soapy water WITHOUT gas in the blower. The blower needs sealed bearings (which they propbably are). Also, you need to make sure you always shut the gas off before you shut the motor down.
Now, in reality you aren't going to find that blower. It's okay. You can use the one you have now if you do an easy test. Using a suitable outlet pipe (2" should work since it's the blower outlet size) without your flare on the end, set up the blower to blow toward something that won't catch on fire (outside). Set up a propane torch to the side of the outlet with the flame pointing toward the outlet at an angle. Turn your blower on full blast, then add gas until you get a flame out of the end that will just sustain itself. Now, cut back on the gas until the flame goes out and dial the blower back until it relights. Do it again and again until finally you get a flashback. These are your unsafe minimum gas/blower settings. To get a safe minimum, add 20% to both to account for the pressure drop you get in your burner and a safety factor and DO NOT GO BELOW EITHER OF THESE SETTINGS!!!!! Here's what you just did: the gas and air combine to make some given velocity in the tube. By setting the blower and adding gas until it will just sustain a flame, you are finding the lower flamability limit. Then by cutting the gas back and chasing it with the burner you are doing the same over and over again. The LFL is the leanest mixture at which a gas will burn, and also happen to be the point at which you are putting the least gas with the most air. This makes the lowest manifold velocity possible and also keeps you away from the safety issues associated with a richer mixture, since what is coming out of the end of the manifold will NEVER catch on fire if it is too lean. When you find the point you have a flashback, the combined air and gas velocity was less than 12 ft/sec, and is unsafe. You set your minimum above this because putting the flare on the end will slow things down a bit because of pressure drop, but you would not get a reliable flashback with it on there, and that's why you test without it, then you add a little for safety.
So, now you've found an operating envelope and you can adjust gas and blower anywhere above your lower limits and have a combined velocity above 12 ft/sec and will not get a spontaneous flashback. Now, if an ember gets into the blower inlet it will flash, but since you have a small manifold it will be over quickly and with little gas involved. DO NOT add an expansion chamber with this setup. Slow is bad, and an expansion chamber slows things down. That could become a combustion chamber and get real hot real fast.
This is a good idea; it just has to be done safely. This is what I would do if it were me doing it, but you are free to do however much or little of it as you see fit.
Taylor
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and that was it. but because the blades where spinning the flame lasted a mil a sec and was gone back into the forge.
Be safe, serioulsly though. Blow your air into the gas stream, not gas into your supply
