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Rapid recompression of PVC guns

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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Rapid recompression of PVC guns

Unread postAuthor: skyjive » Sun Oct 26, 2008 7:55 pm

So if I have a rapid-firing pneumatic made of sch40 pvc, is there any significant danger from the rapid filling and emptying of the gun causing the PVC to become cold and brittle and shatter? I am trying to use a compressed gas tank rather than a compressor as a power source for portability, so what kind of gas would minimize this danger? I've heard that CO2 can be dangerous, so would Nitrogen or something be less so?
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Unread postAuthor: frankrede » Sun Oct 26, 2008 8:00 pm

During rapid recompression of a air cannon the chamber will actually get warm using normal air.
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Unread postAuthor: Lentamentalisk » Sun Oct 26, 2008 8:05 pm

right, but that is just when you are using a compressor straight. If the gas ever gets stored (read HPA, CO2, N2, or propane tank) then when it boils (or to a lesser extent, expands, in the case of HPA and N2 tanks,) it will chill considerably, and possibly cause failure.

Under no circumstances should unregulated CO2 ever be used with PVC for so many reasons, and continual use with even regulated CO2 can be dangerous.
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Unread postAuthor: jimmy101 » Mon Oct 27, 2008 10:37 am

Depends on exactly how rapidly the gun is cycled and how many times it is cycled. A couple shots in a couple seconds probably won't be a problem. Easy enough to check though, does it get cold to the touch after a couple shots?

If you are good enough to design and build a gun that'll fire many times for many seconds then you can ameliorate the cooling affects by using a radiator in the supply line. A coil of copper pipe might be enough. For more cooling add a fan blowing across the loops or submerge the loops in a liquid like water or oil.
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