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Propane as a compressed air source

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Propane as a compressed air source

Unread postAuthor: vex » Tue Apr 27, 2010 1:16 pm

I have two questions:

1. Would propane make an ok compressed air source for a Pneumatic cannon? Not talking about a combustion cannon. I have a big tank of propane and it would be easy to take off the grid compared to a compressor and easier to use than a pump.

2. What would happen if the exhaust from the cannon were ignited by a source as it exited the chamber? Perhaps a sparkler or flare or some source of ignition. Is there a danger of it traveling back into the tank and blowing everything up? Melting the barrel?

I am building a cannon for a few special effects and was hoping to get some experienced answers before I actually went through. I am currently using 3" PW PVC (2:1 ratio) and a modified 1" rainbird valve with the solenoid and power supply.

Thanks!
Chris
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Re: Propane as a compressed air source

Unread postAuthor: Hubb » Tue Apr 27, 2010 1:36 pm

vex wrote:1. Would propane make an ok compressed air source for a Pneumatic cannon?
I've done it plenty of times before. In fact, that is pretty much what "green gas" is. It has a vapor pressure of approximately 90 psi, and I use it for a constant air source when I have a PVC chamber.

vex wrote:2. What would happen if the exhaust from the cannon were ignited by a source as it exited the chamber? Perhaps a sparkler or flare or some source of ignition. Is there a danger of it traveling back into the tank and blowing everything up? Melting the barrel?
I would assume that it would produce a fireball, if it don't blow out the flame due to the mixture being too rich. But I've never tried it so I'm not sure.

vex wrote:I am building a cannon for a few special effects and was hoping to get some experienced answers before I actually went through. I am currently using 3" PW PVC (2:1 ratio) and a modified 1" rainbird valve with the solenoid and power supply.
Although I've used sprinkler valves in propane pneumatics before without issue, it may need to be tested to make sure the seals hold up properly.
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Unread postAuthor: clemsonguy1125 » Tue Apr 27, 2010 3:36 pm

Yeah its been done before but it will take longer to fill than regulated HPA, heres a topic from a while ago of a propane tank with an expansion chamber, it was originally going to be used for a BBMG but with a quick connect it could easily be converted to a fill station.
http://www.spudfiles.com/forums/propane ... 20242.html
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Unread postAuthor: irisher » Tue Apr 27, 2010 3:36 pm

I have ignited the exhaust on my propane pneumatic before. It makes a pretty spectacular fireball that can be adjusted to do some crazy things. Be sure to have a bucket of sand or water nearby though. The muzzle will occasionally catch on fire.
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Unread postAuthor: clemsonguy1125 » Tue Apr 27, 2010 3:40 pm

If it does catch fire the flame could travel up the barrel and mess up the seal in the sprinkler valve
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Unread postAuthor: irisher » Tue Apr 27, 2010 3:56 pm

For me the barrel was long enough and the mix was off enough that it would not light. However propane in the barrel would leak out and accelerate the fire at the muzzle.
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Unread postAuthor: clemsonguy1125 » Tue Apr 27, 2010 4:03 pm

Depends how long after he shot it got ignited, eventually the mix would be right, I've had gun ignite 15 minutes after I fueled them
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Unread postAuthor: Hubb » Tue Apr 27, 2010 4:09 pm

In fact, I have recently updated my Portable Tank Calculator. I'm updating it again to include the use of propane as a pneumatic. It should be done within the next few hours or so.
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Unread postAuthor: vex » Tue Apr 27, 2010 6:18 pm

Thanks for the responses. I guess question 1 has been answered in the affirmative and question 2 looks promising. I have made several special effect "cremora" fireballs using black powder and a mortar but I was looking into launching the non-dairy creamer in a quieter way and thought pneumatics might be the way to go. Then I started thinking about propane. Perhaps the 2 together? Anyway, great forum, I'll let you know how it goes.
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Unread postAuthor: Lentamentalisk » Tue Apr 27, 2010 7:18 pm

Here is a video of a fireball cannon I made a while back. It is basically a 2"x12" chamber with a 1/2" ball valve, hooked directly up to a 20lb propane tank. I just left the tank valve open so I could get repeated shots. You will notice the flame does not go out between fireballs. That is probably partly due to the fact that the ballvalve seal got a little messed up when I sweated it on to the pipe, but also, once you fire the cannon, the barrel is still full of propane not yet mixed with air, so it burns as it wafts out. My suggestion is to shy away from a plastic barrel, or at least have a metal/ceramic end to the barrel, to avoid serious meltage.

[youtube]http://www.youtube.com/watch?v=5PPhPT5XO78[/youtube]

Also, like mentioned above, it may take a long time to fill, especially if you are using a small 1lb tank.
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Unread postAuthor: Marffy » Tue Apr 27, 2010 9:43 pm

i'd agree with hubb and try to test your valve for propane. (quick note) propane doesn't expand as fast as air (its slower) so filling time will be effected and i'm not sure if velocity will be affected.
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Unread postAuthor: Lentamentalisk » Tue Apr 27, 2010 11:43 pm

Propane's SoS is lower, because it is heavier. That also means it has more mass, which if you are not pushing the SoS, will give you better results. However, the gas comes out cold, as it has been boiling off the liquid, which can lower pressure.
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