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Is a short one as good as a long one ? (honestly)

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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Is a short one as good as a long one ? (honestly)

Unread postAuthor: LeMaudit » Wed May 13, 2009 1:53 pm

:-D Couldn't resist...

I need again your advises, to sort out all the ideas that cross my mind :roll:

I am considering using long chambers with small diameters. However, I have no idea if it is better to have a compact chamber than a long one, in term of air pressure efficiency.

To pick an example with simple numbers, considering 3 chambers all with an identical 1 inch seat:

- 4 inch diameter, 4 inch long
- 2 inch diameter, 16 inch long
- 1 inch diameter , 64 inch long.

All have the same cubic inch volume, about 50 inch3. Would it make a difference in term of flow rate or pressure force when the seat opens? GGDT says it’s the same but I’d just like to be sure.

The idea behind that is that I could use high pressure flexible tubing and use them as chambers, so I could have several chambers that could fit in a tinier space. But again, would I loose some efficiency?

Thanks again!
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Unread postAuthor: jeepkahn » Wed May 13, 2009 2:03 pm

http://www.spudfiles.com/forums/viewtop ... tml#244387

read page 2 of the linked thread...

in short, larger diameter=more efficient...
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Unread postAuthor: LeMaudit » Wed May 13, 2009 2:27 pm

Not sure to understand, sorry. It seems to me this thread (that I was reading BTW) refers to different chamber and pilot volume, and pilot design. I was just wondering about the shape of the chamber, with identical compressed air volumes. Whatever the release mechanism is (say its a burst disk).

Maybe I need another expresso :-D And I'll read the thread again.
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Unread postAuthor: SpudFarm » Wed May 13, 2009 2:33 pm

lool i started to think once i saw the title! good job :P
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Unread postAuthor: jeepkahn » Wed May 13, 2009 2:46 pm

LeMaudit wrote:Not sure to understand, sorry. It seems to me this thread (that I was reading BTW) refers to different chamber and pilot volume, and pilot design. I was just wondering about the shape of the chamber, with identical compressed air volumes. Whatever the release mechanism is (say its a burst disk).

Maybe I need another expresso :-D And I'll read the thread again.


I got off onto a tangent about chamber sizes and flow rates toward the end of page 2, drink your espresso and read my posts in particular...
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Unread postAuthor: MountainousDew » Wed May 13, 2009 3:36 pm

I don't know how much of a difference the shape of the chamber would actually effect the performance of the gun as long as the volume is the same, but what you could do is make three or so different chambers to attach to your gun... The sizes you stated above would be great variables in the experiment, or you could do some small scale testing first because of the price of buying the pipe. And afterwards, you could share your results with us.
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Unread postAuthor: LeMaudit » Wed May 13, 2009 3:51 pm

afterwards, you could share your results with us.


:-D As soon as I can I will !

it is just that I was designing a gun more sleek than my first idea as a barreled gun (to please the wife, go figure..) and if at the end I have a sensible loss of power because of the design it makes no sense. The idea to use flexible tubing could gives me a way to have a multi-shot PCP gun, with the equivalent of multiple not-so-small chambers, one per fire, with no lost of pressure between the shots like with a single PCP chamber and make the chambers conform the "classical" shape of a flat semi-automatic.
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Unread postAuthor: Lentamentalisk » Wed May 13, 2009 4:02 pm

basically as long as your chamber is the same diameter or larger than the porting on your valve or the diameter of your barrel, you should be good.

In theory, how ever, a short fat chamber will give the best results, but it will most likely be fairly minimal.
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Unread postAuthor: D_Hall » Thu May 14, 2009 4:52 pm

My wife assures me that short is just as good as long. HOWEVER, skinny is NOT as good as fat.
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Unread postAuthor: LeMaudit » Thu May 14, 2009 5:05 pm

Wife opinion doesn't count. It is biased.
:-D
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Unread postAuthor: Technician1002 » Thu May 14, 2009 5:21 pm

MountainousDew wrote:I don't know how much of a difference the shape of the chamber would actually effect the performance of the gun as long as the volume is the same, but what you could do is make three or so different chambers to attach to your gun... The sizes you stated above would be great variables in the experiment, or you could do some small scale testing first because of the price of buying the pipe. And afterwards, you could share your results with us.


My theory is the longer the chamber the faster the flow has to be before it reaches the valve. It takes energy to accelerate the gas before the valve. The more the gas has expanded before it reaches the valve, the less that will fit through the valve.

A short fat chamber provides more gas to the valve at higher pressure (more gas) for expansion after the restriction of the valve.

High speed air doesn't like a turbulent path through a valve and will quickly lose energy. Lower speed air that has less distance to travel to the valve goes through at higher pressure and slower speed (MPH) for less loss to turbulence. It may be part of the reason I am seeing good performance with the QDV.

Wife opinion doesn't count. It is biased.

And even may be wrong. :)
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Unread postAuthor: LeMaudit » Thu May 14, 2009 6:23 pm

My theory is the longer the chamber the faster the flow


Makes sense. The additional question would be: Will the loss be significant for the size I seek? I am thinking of something like a 3/8" dia. chamber, 12 inch long (that's about 1.33 inch3).


And even may be wrong


To quote an universal law: Anything that can be wrong will be wrong.
Now the wife may be wrong is a pleonasm
:oops:
hum.. Hi honey...

:violent1:
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Unread postAuthor: Technician1002 » Thu May 14, 2009 6:38 pm

LeMaudit wrote:
My theory is the longer the chamber the faster the flow


Makes sense. The additional question would be: Will the loss be significant for the size I seek? I am thinking of something like a 3/8" dia. chamber, 12 inch long (that's about 1.33 inch3).


And even may be wrong


To quote an universal law: Anything that can be wrong will be wrong.
Now the wife may be wrong is a pleonasm
:oops:
hum.. Hi honey...

:violent1:


I'm not sure you understood. Faster before the valve is bad. To get air fast before the valve, there was expansion. Expansion means lower pressure to the valve. Higher speed in the valve means more energy loss to turbulence.

Lots of air through the valve at a slower speed = less loss. Higher pressure through the valve = more pressure behind the projectile.

More pressure behind the projectile = more force on the projectile.

It is best to get the gas to expand after the valve as much as possible.
Expanded gas before the valve is less that can go through it in any given time.

For the sizes considered;
- 4 inch diameter, 4 inch long
- 2 inch diameter, 16 inch long
- 1 inch diameter , 64 inch long.


Try using GGDT and using these as barrels matched to the respective chamber for a 1:1 burst disk. Use a super light projectile the same mass for each and compare results.

Which ejected the projectile the first? The discharge time for each is related to the flow limitations of each. The time to accelerate the mass of a long column of air is considerable. The friction in small diameters is high.
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Unread postAuthor: LeMaudit » Thu May 14, 2009 6:50 pm

Yes, I think I understood. But thanks for the clarification.

I was wondering if the loss you describe would be significant for a relatively low volume or air if I use a tubing as a chamber. Your chamber is a monster compare to what I have in mind.

Reading your explanations I understand it will...

Some food for the evening thoughts :-D
And more fun with GGDT!
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