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Flamethrower question.

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Flamethrower question.

Unread postAuthor: PVC Arsenal 17 » Thu May 24, 2007 3:59 pm

I'm looking to build a portable flame thrower to use for effects in a movie I'm making with my friends.

I pretty much have the design down, but I have one question.

Because this thing must be portable and I won't be near an air source, can I pressurize the fuel tank with co2? I'm not sure if it will choke the flame or not.
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Unread postAuthor: Freefall » Thu May 24, 2007 4:02 pm

Yes, you can pressurize the fuel tank with CO2. It won't choke the flame, as the flame has no contact with the CO2 in the fuel tank.

Whatever you do, do NOT pressurize the tank with air. if the fuel runs out, there is the small possibility that the pressurized air-fuel mix in the line will transmit the flame back up into the tank, resulting in an injured performer.
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Unread postAuthor: Pete Zaria » Thu May 24, 2007 4:03 pm

CO2 would probably work (see notes below), but most people use propane. As long as there's more than about 10% propane in the chamber (and you should have 80-90%...) there's no way it can ignite; not enough oxygen (too high a fuel/air ratio).
Propane canisters are $3, put out at least 90psi, and will last for longer than it seems like they should.

I see no reason why CO2 wouldn't work except that it could make your tank very cold and brittle.

I'd really suggest a backflash valve (a special check valve made to prevent a flame or pressure spike from traveling up a tube), just in case.

A home made flamethrower is really just an industrial-grade super-soaker with a metal nozzle and pressurized by a non-oxidizing gas.
Safety needs to come first. A pressure gauge, a safety valve behind the main trigger valve, preferably a check/backflash valve, and lots of common sense are all requirements.

What are you going to use for fuel? Denatured alcohol is expensive but works great. Don't use gasoline or diesel in a PVC chamber.

What's going to be your pilot? One advantage of using propane to pressurize the tank is that you use it for the pilot, also.

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Unread postAuthor: PVC Arsenal 17 » Thu May 24, 2007 4:38 pm

Propane as a pilot as well? How does that work out? Are there two separate lines- one for the pilot light and one to pressurize the chamber?

And I'll definitely look into a backflash valve. Last thing I need is to blow my hand off...
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Unread postAuthor: hi » Thu May 24, 2007 4:54 pm

im not sure if im right or not, but i would think that you would want to use co2 because as freefall said, it might ignite with air, but with co2 that posibility would be basicly zero.

i dont know if im right or not, but thats what i think.
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Unread postAuthor: sandman » Thu May 24, 2007 5:00 pm

if that was not true then global warming would not be a problem, lol

i would say that CO2 would be your best bet
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Unread postAuthor: Pete Zaria » Thu May 24, 2007 5:26 pm

Guys, it doesn't really matter what you use to pressurize the tank with, given two things:

It's not an oxidizer.
There's very little oxygen inside the tank.

To use CO2, you'd need a tank and regulator ($100+)

To use propane, you'd a tank ($2.99), a torch head ($6.99), a pressure gauge ($10), some tubing, some hose clamps, and a hose barb or two.

I know it seems counter-intuitive to pressurize the tank on a flamethrower with a flammable gas, but you have to realize, the upper explosive limit of propane is 10.1% ( http://www.engineeringtoolbox.com/explo ... d_423.html ). If there's more than that in the chamber, there's not enough oxygen (or too high a fuel/air ratio) to ignite. If you were to pressurize the chamber to, say, 60psi with propane, you'd be well above 50% propane; zero chance of ignition. I'm too lazy to do the math to figure out how much it would be, but there's no way a spark inside the tank could light it off with that much fuel to that little air.

The only way it could ignite is if the chamber pressure got VERY low, and as the very last bit of propane was leaving the chamber, a flame was able to travel back up the tube. By this point, there should be very little fuel left in the chamber anyway. But to prevent this, you can use a backflash/check valve, and stop using the flamethrower when the pressure gets very low.

My conclusion is that propane is a cheaper and easier solution for pressurizing a flamethrower tank. It's also more convenient in that you can use it for the pilot also, and when the tank runs out, you can just grab a new one ($3) instead of having to get it refilled at a paintball shop. This is just my opinion, though.

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Unread postAuthor: paaiyan » Thu May 24, 2007 5:32 pm

I've got three words. Napalm.
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Unread postAuthor: PVC Arsenal 17 » Thu May 24, 2007 5:36 pm

Thanks Pete! You seem to know your stuff. I'm going to use propane, but I'm going to modify the concept a bit.

Since I'm still going for a portable flamethrower, I'm going to buy one of Gort's adapters: http://www.launchpotatoes.com/index.php ... oductId=37 and connect a tire chuck to it.

That way, I put a shrader valve on the chamber and fill up when I need to rather than carrying a propane tank around all the time.

How much should I fill the chamber with denatured alcohol? I'm not sure if the air left inside will causes the propane to ignite. And can I use any old brass check or do I specifically need a backflash valve?
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Unread postAuthor: Panzerfaust » Thu May 24, 2007 8:06 pm

Propane definitely sounds like a good idea for this type of thing, if i remeber correctly the military pressurized thier flamethrowers with nitrogen for the same reason. It might help people out if you posted where backflash valves are sold, or if they are really any different from a standard check-valve. I posted this in the last flamethrower thread, but its kinda dead so i dont think any one noticed. Filling your chamber with non-flammable gases (or gass that will prevent flame by lowering oxygen content) is a great idea, but i definitely think making a chamber out of PVC is not. I would recommend a water-type fire extinguisher, they have built in pressure gauges to 100 psi, are aluminum that would not be affected by your fuel (and very light), and have built in schrader valves for filling and a built in valve on top for firing. Heres a you-tube video showing just how well they work.
http://www.youtube.com/watch?v=PuPu-q8dDvY&mode=related&search=
that thing has some serious range, and filled with propane i would think it would be about as safe as you can get a flamethrower. only thing id change is to put a valve on the end of the hose and use that for shooting.
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Unread postAuthor: Pete Zaria » Thu May 24, 2007 8:21 pm

Using a fire extinguisher tank sounds like an excellent idea.

Some info on flash arrest valves:

http://www.airgas.com/browse/product_li ... d=Category

http://www.praxair.com/praxair.nsf/7a11 ... enDocument

http://www.scottecatalog.com/equipspc.n ... enDocument

I've seen people use two regular brass check valves in series on a home made flame thrower. I'm not sure what the differences are between a check valve and a flash arrest valve.

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Pete Zaria.
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Unread postAuthor: Fnord » Thu May 24, 2007 8:32 pm

I would think that the only way a flame thrower could explode is if the flame front travels faster than the air coming out, otherwise it wouldnt be able to enter the chamber.
Of course you would need to shut off the valve before the pressure gets too low inside.

Does anyone know what speed a gasoline vapor/air mixture flame front can travel at?.
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Unread postAuthor: turbohacker » Thu May 24, 2007 8:33 pm

DONT USE PROPANE!!
Use only non-flamible and non-oxidizers
I do not remember where i saw it but i know propane used in aflame thrower is extremly dangerous. Becides, CO2 is cheaper, can be compressed more (therfore being smaller) and if somehow a flame trys to make it back into your tank you will have no problom because a flame will be no longer be because their is no air. These propertys of CO2 will simplify your design because you will no longer need a backflash valve (which are ussually small anyways, restricting performance)

P.S. use only liquid fuels because they will only light when exposed to air ( some gases have ther own oxidizers)
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Unread postAuthor: Pete Zaria » Thu May 24, 2007 8:38 pm

turbohacker wrote:DONT USE PROPANE!!
Use only non-flamible and non-oxidizers
I do not remember where i saw it but i know propane used in aflame thrower is extremly dangerous. Becides, CO2 is cheaper, can be compressed more (therfore being smaller) and if somehow a flame trys to make it back into your tank you will have no problom because a flame will be no longer be because their is no air. These propertys of CO2 will simplify your design because you will no longer need a backflash valve (which are ussually small anyways, restricting performance)

P.S. use only liquid fuels because they will only light when exposed to air ( some gases have ther own oxidizers)


Research traditional military flamethrowers. I explained above exactly why it's safe to use propane for the propellant - not for the fuel, of course. The propane is only used to pressurize the fuel (denatured alcohol, etc....). In this design scenario, it's perfectly safe.

CO2 is more expensive and more complicated to use for a simple design like this, and if it IS safer, it's only by a teeny tiny fraction of a percentile.

Peace,
Pete Zaria.
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Unread postAuthor: turbohacker » Thu May 24, 2007 8:44 pm

I misunderstood, I thaught you were going to use propane as the fuel (which would be pretty lame effect anyways.) I still think that CO2 would be easyer to use i.e. remote paintball setup
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