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Noob dual-valve question

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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Noob dual-valve question

Unread postAuthor: torquey » Thu Jan 07, 2010 9:12 pm

I did look and search - I promise!

OK, I think dual QEV's is a fairly accepted model, and most designs I see have included some way of equalizing the pressure across two pressure chambers.

My question is this: Is there any advantage to two chambers with a small connection for equalization? In other words, all else being equal, will two 3" diameter chambers with a 1/2" tube connecting them and a 3/4" QEV on each perform better than a single 3" diameter chamber of the same volume with the same valves at either end?

Thanks in advance, and holy crap, some of you have some insane toys!
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Attachments
ex3.JPG
straight chamber, two valves
ex3.JPG (36.3 KiB) Viewed 290 times
ex2.JPG
single u-shaped chamber, two valves
ex2.JPG (27.33 KiB) Viewed 290 times
ex1.JPG
two chambers, 1/2" equalization connection, two valves
ex1.JPG (28.5 KiB) Viewed 290 times

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Unread postAuthor: MrCrowley » Thu Jan 07, 2010 10:22 pm

ex3.jpg and ex2.jpg will probably have similar performance depending on chamber size and barrel length.

Download GGDT and play around with chamber size, amount of valves, barrel length etc to see which configuration is best for you. Any questions you have about GGDT can be answered by looking through his website:
http://www.thehalls-in-bfe.com/GGDT/main_menu.html

The thing is that a lot of people seem to think giant chamber = better performance than one half its size. This is generally true, but depending on many factors, sometimes the performance increase is so small, there's little point having a giant chamber.

The only time you need a huge chamber is for when you have a very long barrel. But anyway give us more specs and we'll help you with your design. Start with these:

Barrel diameter and length
Chamber diameter and length
Projectile
Pressure
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Unread postAuthor: torquey » Fri Jan 08, 2010 1:01 am

MrCrowley wrote:ex3.jpg and ex2.jpg will probably have similar performance depending on chamber size and barrel length.

Download GGDT and play around with chamber size, amount of valves, barrel length etc to see which configuration is best for you. Any questions you have about GGDT can be answered by looking through his website:
http://www.thehalls-in-bfe.com/GGDT/main_menu.html

The thing is that a lot of people seem to think giant chamber = better performance than one half its size. This is generally true, but depending on many factors, sometimes the performance increase is so small, there's little point having a giant chamber.

The only time you need a huge chamber is for when you have a very long barrel. But anyway give us more specs and we'll help you with your design. Start with these:

Barrel diameter and length
Chamber diameter and length
Projectile
Pressure


Hey, thanks a bunch, Mr. C!

I'm not really concerned with the actual size of the chamber, though... I'm wondering if two chambers connected by pressure hose to equalize with one QEV each (as in ex1) would show a performance advantage over a single chamber (of the same volume) with a valve at each end.

I'm obviously not expressing this clearly...

I'll give the specifics:

Barrel: The plan is for interchangeable barrels, GB, TB, maybe BB... but lets say for the purpose here that it's a 3-4' GB barrel, 1 1/2" SDR 21 inside 2" Sched 80.

Chambers: That's the question. The valves are Humphrey QE-5 (3/4" QEVs rated to 150 psi) I'm thinking around 100 cubic inches total chamber volume, but planned to research a proven ratio. Many examples I've seen online seem to have two 3-4" diameter chambers with a QEV (or sprinkler valve, for that matter) at one end, and a small equalizing connection at the other end. My thought is that however I create that connection between the chambers is either going to be a potential point of failure (like 1/4" NPT fittings into tapped end cap/pipe overlap) or expensive (like reducing couples/bushings and 1" pipe and elbows).

I was thinking of just doing a single chamber of the same volume, either U-shaped like ex2 or straight like ex3, with the same QEVs at each end. I haven't seen this done, and figured there was a reason for that, some advantage to the separate chambers model that wasn't occurring to me.

Projectiles: Eventually it will vary, but again, let's just assume golf balls for the question.

Pressure: I planned on a 100 PSI max. I will run the gun off my compressor or a 20 lb. CO2 tank regulated to 100 PSI.

Thanks again for any and all help. I know the info is probably here somewhere, but I couldn't find the right search parameters.
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Unread postAuthor: psycix » Fri Jan 08, 2010 2:04 pm

I would go for design one or two.

In any way, the most important thing of all is that the T the 3/4" QEV's connect to is larger than 3/4"
If you would take a 3/4" T, it would constrict flow and the dual valves are near useless.
The T should be (almost) the same same size as the barrel.
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