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Chamber diameter vs barrel diameter?

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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Chamber diameter vs barrel diameter?

Unread postAuthor: boyntonstu » Wed Jul 01, 2009 1:59 pm

Chamber diameter vs barrel diameter?

Is there a technical reason why the chamber diameter should be any larger than the barrel diameter?

It seems to me that smaller diameters can take higher pressures.

If the same size diameter can supply all the compressed air required for the barrel, wouldn't that be best?

The only thing that I would consider make larger is the size of the ball valve.

A 3/4" ball valve opens much more area per arc angle faster than a 1/2" valve.

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Re: Chamber diameter vs barrel diameter?

Unread postAuthor: Ragnarok » Wed Jul 01, 2009 3:06 pm

boyntonstu wrote:Is there a technical reason why the chamber diameter should be any larger than the barrel diameter?

Greater volume for the same length.

Alternatively, in very serious launchers, there are more reasons for it involving relative gas flow speeds, and the characteristics of decompression, but I'm not going to go into that much detail here.

Basically, higher diameter is about the volume, but if there's enough volume and as long as your chamber diameter is at least the size of your barrel or valve, then it's not essential for most purposes.

The only thing that I would consider make larger is the size of the ball valve

I'd just go with a more powerful valve full stop. Ball valves aren't good.
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Re: Chamber diameter vs barrel diameter?

Unread postAuthor: Technician1002 » Wed Jul 01, 2009 6:18 pm

Ragnarok wrote:Basically, higher diameter is about the volume, but if there's enough volume and as long as your chamber diameter is at least the size of your barrel or valve, then it's not essential for most purposes.


I'll cover it.. It has to do with flow in high volume high pressure cannons. For this the barrel diameter is ignored and only the chamber is considered.

For a long chamber with small diameter, the high speed discharge has a maximum flow simply from the size of the pipe and friction on the pipe. If we consider the flow at 1/2 the pressure drop in the chamber, at 1/2 pressure (for simplicity the temperature change is ignored but is a factor) the volume of the remaining gas has doubled, so the flow near the barrel is high speed. Air traveling the length of a pipe has a pressure loss related to the speed. To go twice as fast, it requires 4X the pressure just to get to the barrel.

A chamber twice the diameter, would therefore have the flow velocity at this pressure and same volume of 1/4 that of before (area of a circle twice the diameter has 4 times the cross section) so it is traveling 1/4 the speed to get to the barrel. Therefore there is 1/2 the pressure loss in the pipe getting to the barrel.

The orifice leaving the chamber to the barrel can therefore have higher pressure during the shot because there is less pressure loss to friction in the chamber.

In a combustion, there are two more very important factors. 1 the speed of the flame front. A short length is a faster burn time to reach the other end. Long chambers benefit from multiple gap spark strips for this very reason. It lights the gas in many places at the same time.

The second factor is cooling. In a combustion, hot gas cools quickly (with resulting drop in pressure) as there is more surface area close to the burnt gass in a long chamber. The shape with the least surface to contain it is a sphere. As a result, that shape is most efficient at keeping the burnt gas hot for the shot.

Old timer experts, please feel free to correct errors. I have never built a combustion yet, so others may know more on the subject that I have missed.

General use for spudding the chamber at twice the diameter of the barrel gives very good results as chamber flow would be 1/4 the maximum you can get into the barrel.
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Unread postAuthor: Hotwired » Wed Jul 01, 2009 6:38 pm

It is regarding pneumatics but no matter.

I would also add that the above makes the assumption that power is the factor you hold above all else.

There are other reasons such as function or aesthetics which can mean you don't need or desire to have potential muzzle energy leading the design.

Not many seem to be big on losing out on their cannons "potential power" mind you. However multishot capability is all about that and flow comes secondary to the mechanism to allow it.
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Re: Chamber diameter vs barrel diameter?

Unread postAuthor: boyntonstu » Wed Jul 01, 2009 6:40 pm

Technician1002 wrote:
Ragnarok wrote:Basically, higher diameter is about the volume, but if there's enough volume and as long as your chamber diameter is at least the size of your barrel or valve, then it's not essential for most purposes.


I'll cover it.. It has to do with flow in high volume high pressure cannons. For this the barrel diameter is ignored and only the chamber is considered.

For a long chamber with small diameter, the high speed discharge has a maximum flow simply from the size of the pipe and friction on the pipe. If we consider the flow at 1/2 the pressure drop in the chamber, at 1/2 pressure (for simplicity the temperature change is ignored but is a factor) the volume of the remaining gas has doubled, so the flow near the barrel is high speed. Air traveling the length of a pipe has a pressure loss related to the speed. To go twice as fast, it requires 4X the pressure just to get to the barrel.

A chamber twice the diameter, would therefore have the flow velocity at this pressure and same volume of 1/4 that of before (area of a circle twice the diameter has 4 times the cross section) so it is traveling 1/4 the speed to get to the barrel. Therefore there is 1/2 the pressure loss in the pipe getting to the barrel.

The orifice leaving the chamber to the barrel can therefore have higher pressure during the shot because there is less pressure loss to friction in the chamber.

In a combustion, there are two more very important factors. 1 the speed of the flame front. A short length is a faster burn time to reach the other end. Long chambers benefit from multiple gap spark strips for this very reason. It lights the gas in many places at the same time.

The second factor is cooling. In a combustion, hot gas cools quickly (with resulting drop in pressure) as there is more surface area close to the burnt gass in a long chamber. The shape with the least surface to contain it is a sphere. As a result, that shape is most efficient at keeping the burnt gas hot for the shot.

Old timer experts, please feel free to correct errors. I have never built a combustion yet, so others may know more on the subject that I have missed.

General use for spudding the chamber at twice the diameter of the barrel gives very good results as chamber flow would be 1/4 the maximum you can get into the barrel.


Very interesting.

BTW A 3/4" ball valve has 2.25 times the open area than a 1/2" valve.

Therefore a rotation of only 40 degrees of a 3/4" valve will equal the area of a 1/2" valve at its full 90 degree position.

One may say, that a 3/4" ball valve can open 2.25 times faster than a 1/2" valve with the same friction.

Does this have any impact on whether the chamber feeding a 1/2" barrel is 3/4 or 1/2?
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Re: Chamber diameter vs barrel diameter?

Unread postAuthor: Technician1002 » Wed Jul 01, 2009 7:02 pm

boyntonstu wrote:
Very interesting.

BTW A 3/4" ball valve has 2.25 times the open area than a 1/2" valve.

Therefore a rotation of only 40 degrees of a 3/4" valve will equal the area of a 1/2" valve at its full 90 degree position.

One may say, that a 3/4" ball valve can open 2.25 times faster than a 1/2" valve with the same friction.

Does this have any impact on whether the chamber feeding a 1/2" barrel is 3/4 or 1/2?


Other than a larger dead space, you are right on. It is the reason I am building a 3 inch valve as my next project, even though the goal is supersonic marshmallows and golf balls. The dead space volume is larger, but the opening speed factor and flow restriction when open is what it's all about. Well done. I get fantastic speeds on jawbreakers using a 3/4 inch barrel on my 1 inch valve cannon.
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2X4 split with jawbreakers
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Unread postAuthor: ralphd » Wed Jul 01, 2009 8:52 pm

I was thinking about this very subject. I have a 2 1/2" barrel and a 2 inch chamber with a 2" piston. The tube that my piston seals against is 2". It launches tennisballs almost as far as my combustion launcher. I was thinking if I make everything with 2 1/2" parts(except for the "T" - 3 or 4") it should launch much farther with the same pressure due to the improved flow.
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Unread postAuthor: Hotwired » Wed Jul 01, 2009 9:25 pm

ralphd wrote:I have a 2 1/2" barrel and a 2 inch chamber with a 2" piston. If I make everything with 2 1/2" parts it should launch much farther with the same pressure due to the improved flow.


...and the 60% increase in chamber volume by going from 2" to 2.5" ^^
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Unread postAuthor: Technician1002 » Wed Jul 01, 2009 11:21 pm

ralphd wrote:I was thinking about this very subject. I have a 2 1/2" barrel and a 2 inch chamber with a 2" piston. The tube that my piston seals against is 2". It launches tennisballs almost as far as my combustion launcher. I was thinking if I make everything with 2 1/2" parts(except for the "T" - 3 or 4") it should launch much farther with the same pressure due to the improved flow.


The flow rate into the valve is why my 2 inch uses a large diameter tank instead of a long pipe. We were allowed to use up to 100 PIS, but we were hitting our targets (260 feet away an up 81 feet) in the back of the cheap seats on 25 PSI. Due to this we were able to go to a shorter than the most efficient length for a barrel and use 35-45 PSI at the game. They still tagged the retired numbers above the 300 section.

This was done with a 2 inch valve and 3 inch barrel. It works very well. The ball valves seldom made it past the box seats between the 200 and 300 sections. They also put some shirts against the back wall behind the 300 section, so the back row got shirts.

In testing we got this shot of an orange we launched. How fast does an orange have to go to have a shock wave? :D
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Attachments
orange.jpg
High speed orange.
orange.jpg (34.79 KiB) Viewed 313 times
Shots.JPG
Retired numbers, Back wall, Scoreboard, all hit with t shirts.
Launch.JPG
The launch team in action.
Last edited by Technician1002 on Thu Jul 02, 2009 11:12 am, edited 2 times in total.
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Unread postAuthor: ralphd » Thu Jul 02, 2009 5:32 am

I was planning to keep everything volumetrically simular so it wont get heavier than what it is now but I'll try the shorter barrel first.
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Unread postAuthor: Technician1002 » Thu Jul 02, 2009 10:57 am

ralphd wrote:I was planning to keep everything volumetrically simular so it wont get heavier than what it is now but I'll try the shorter barrel first.


We lost range with a shorter barrel. We just didn't need the long barrel efficiency in the arena to hit the back row of the 300 section. :D We used a shorter barrel and raised the pressure from 25 to about 40-ish for the same range.
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