A couple thoughts on hybrid design...

[D_Hall]

To get around the explosion hazard you just have to show that there is no ignition source.

LOL.... Ah, how naive we are (no offense).

It's not a matter of showing something as simple as "no ignition source." That's easy.

You have to get explosive certification. You have to sit down before a formal review board and explain how there's no way that the wires could be damaged and lead to arcing in "other" areas of the system. In short, you're probably talking about spending $30k just to prove your point. I'm not talking hardward; those are just the labor costs of the meetings and such you're gonna be sitting in on.

Oh, and certified explosion proof motors are EXPENSIVE right off the bat. Are they any different? Probably not, but the certification process ain't cheap on the vendor's end either.

Another possiblity would be an air driven fan system. A lot more complex that electric fans but zero chance of sparking in the chamber.

That's a real possibility! I'll have to think on that some.

Any idea how long it would take for fuel to diffuse the length of the chamber? My WAG is that it'll take a lot more than an hour, indeed it might take weeks.

Pure diffusion? No idea. But realistically convection currents and such would be doing more work than diffusion.

For what it's worth, right now I'm thinking of a two point eductor system in the U. One pointing down each leg of the chamber with the inlet to both being at the "center" of the system.

I do like the air motor idea though. I'll definately have to give that some thought!

[jimmy101]

Good point about certification, I tend to forget the difference between doing something safely and proving to managment that it is safe.

So, why not a commercial explosion proof fan? Perhaps the fan used in spray-painting booths? Hmm, they appear to be a bit pricey, $900 for one that move ~2500CFM. (be pretty cool in a toroidal chamber though, mix, vent, and give a turbulent flame front all at the same time!)

Other explostion proof fans;
http://www.bowlinent.com/vent/fans1.htm?kc=NaFU1
still ~$500 but a 24"D

There must be a source of more reasonably sized (and priced) explosion proof fans. Chemical reactors must occasionally include gas circulating fans.

[SpudMonster]

Would something along the lines of an external shaft drive be too complex or risky in the eyes of the big nabobs?

[D_Hall]

Good point about certification, I tend to forget the difference between doing something safely and proving to managment that it is safe.

So, why not a commercial explosion proof fan?

Because explosion proof means "won't cause an explosion." It doesn't mean "guarenteed to survive an explosion."

My point being... OK, it's safe to use the first time. I fire the gun. Is it safe to use again? It didn't CAUSE the explosion, but did all it's seals and such survive? Is it STILL "explosion proof"? Can you GUARENTEE that?

[D_Hall]

Would something along the lines of an external shaft drive be too complex or risky in the eyes of the big nabobs?

No, they'd be totally cool with it from a safety perspective, but now I've got a potential maintenance headache with the seals and such.

[jimmy101]

My point being... OK, it's safe to use the first time. I fire the gun. Is it safe to use again? It didn't CAUSE the explosion, but did all it's seals and such survive? Is it STILL "explosion proof"? Can you GUARENTEE that?

OK, then don't call it a "mixing fan". Call it the ignition system.

And, just in case the "fan based ignition" doesn't work you have a high voltage spark backup.