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1/2" valve seat and 3/4" barrel? Please explain

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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1/2" valve seat and 3/4" barrel? Please explain

Unread postAuthor: velocity3x » Mon Jul 06, 2009 4:06 pm

The model below is for a 1/2" valve seat. It shows projectile velocity increases using a 3/4" barrel against the 1/2" valve seat as opposed to using a 1/2 barrel / 1/2 valve seat. I've always thought gas passing from a smaller orifice into a larger barrel would reduce gas velocity and volume. Apparently it does not. Can someone please explain why using a large barrel dia with a smaller valve actually increases performance

<img src="http://i145.photobucket.com/albums/r240/velocity3x/44mm%20HUNGUS/05invalveseat.jpg">?
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Re: 1/2" valve seat and 3/4" barrel? Please explai

Unread postAuthor: Ragnarok » Mon Jul 06, 2009 4:42 pm

velocity3x wrote:I've always thought gas passing from a smaller orifice into a larger barrel would reduce gas velocity and volume. Apparently it does not.

It does!

The fact is, although the gas is depressurizing further (and is thus at a lower pressure), there's a larger area for the gasses to work on, so the actual forces are higher.
That is to say, increases in barrel volume - either via lengthening it or increasing diameter) will increase muzzle energy (to a finite limit).

If you haven't scaled the projectile mass, this extra energy will exhibit as greater velocity.
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Unread postAuthor: velocity3x » Mon Jul 06, 2009 6:52 pm

Ragnarok,
Thanks for explaining. I never considered the additional force generated by the larger barrel area. I was born dumb and often experience a relapse.
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Unread postAuthor: Technician1002 » Mon Jul 06, 2009 8:22 pm

A big barrel on a big valve is the best combination. This was part of the testing we did for the t shirt competition. Which is better on a 2 inch valve.. A 2.5 inch barrel or a 3 inch barrel. The 3 inch barrel won by a small margin.

Leaving the barrel size fixed and just increasing the valve size will also make improvements. This is the big reason for my current project. It's not for a bigger barrel (which is possible) but better performance with a golf ball barrel.

A 4 inch barrel on a 3 inch valve will have serious power.

A six inch barrel on a 6 inch valve can do this at 60 - 100 PSI.
Not my video..

[youtube]http://www.youtube.com/watch?v=YoIy4GkaIZQ[/youtube]
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