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My First Pressure gun ever. Over/Under Design, need help.

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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Unread postAuthor: Technician1002 » Mon Nov 22, 2010 9:03 pm

Diameter of 2 inches. 4 feet long
is equal to,
Diameter of 4 inches, 1 foot long


They are equal. Do the math. The volume of a cylinder is the area of one end times the length. Area of a circle is Pi times radius squared.

The first cylinder is Pi times 1 squared (one) times 48 inches. This is 3.14 X 48.

The second cylinder is Pi times 2 inches squared (2 squared = 4) times 12.

First cylinder = 150.72 cubic inches.
Second cylinder = 150.72 cubic inches.

This diameter vs area is why a 2 inch valve will outflow a 1 inch valve by about 4X.

Due to one 2 inch valve having less friction surface than four 1 inch valves in parallel, the 2 inch valve will flow more than 4 times better than four 1 inch valves in parallel.


I love larger valves. :D
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Unread postAuthor: Yukondano2 » Mon Nov 22, 2010 9:31 pm

saefroch wrote:The rotation in Google Sketchup can be rather irritating, agreed.

A modded sprinkler valve will be better than your current ball valve. However, I can tell you that even a ball valve cannon can be quite impressive at 150psi to a complete newbie (which was me not too long ago).

Don't wish to be rude, but that little "Preview" button next to "Submit" is wonderful. I use it all the time. :P


What did i do wrong?:shock::shock::shock::shock:
But is there anything wrong with my Cannon that i should know about? Again, besides the valve?
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Unread postAuthor: Yukondano2 » Mon Nov 22, 2010 9:32 pm

Technician1002 wrote:
Diameter of 2 inches. 4 feet long
is equal to,
Diameter of 4 inches, 1 foot long


They are equal. Do the math. The volume of a cylinder is the area of one end times the length. Area of a circle is Pi times radius squared.

The first cylinder is Pi times 1 squared (one) times 48 inches. This is 3.14 X 48.

The second cylinder is Pi times 2 inches squared (2 squared = 4) times 12.

First cylinder = 150.72 cubic inches.
Second cylinder = 150.72 cubic inches.

This diameter vs area is why a 2 inch valve will outflow a 1 inch valve by about 4X.

Due to one 2 inch valve having less friction surface than four 1 inch valves in parallel, the 2 inch valve will flow more than 4 times better than four 1 inch valves in parallel.


I love larger valves. :D


Fascinating. 8)
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Unread postAuthor: saefroch » Mon Nov 22, 2010 10:07 pm

Technician1002 wrote:Due to one 2 inch valve having less friction surface than four 1 inch valves in parallel, the 2 inch valve will flow more than 4 times better than four 1 inch valves in parallel.
Are you asserting that friction is a function of area? So far as I know (check if you want), friction is equal to the coefficient of friction times the normal force. Nothing about area in there...

Youkondano2 wrote:What did i do wrong?
Just your quote tags, the quote was closed by the sample quote tag I gave you, and not by the one you tried to use. Just being a nit-picker.

Also keep in mind, as Ragnarok has said many times and Technician1002 never seems to wrap his mind around, valve opening speed has some serious diminishing returns. My current piston valve opens in 2.5milliseconds at 1,000psi. I can halve that and not see any muzzle velocity change. But that is a rather extreme case. Most times you'd see at least some difference, though rather small once you get to such fast opening speeds.
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Unread postAuthor: Yukondano2 » Mon Nov 22, 2010 10:23 pm

Ok, last time. Not caring about any of that other stuff for now. DO YOU GUYS THINK, MY GUN IS OK TO BUILD? No issues i'm going to run into after its made?
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Unread postAuthor: mlz3000 » Mon Nov 22, 2010 10:33 pm

It looks fine to build. Make sure to prime and glue it properly, and you should be A-OK. :)
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Unread postAuthor: Yukondano2 » Mon Nov 22, 2010 10:41 pm

mlz3000 wrote:It looks fine to build. Make sure to prime and glue it properly, and you should be A-OK. :)


Kthx
finally an answer :P
I'ma build tomorrow, its late and i got hw. Peace
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Unread postAuthor: Technician1002 » Tue Nov 23, 2010 2:14 am

Just a quick note on speed, There is a point of diminishing returns when valves get faster than a certain speed, much like there is diminishing returns on really large chambers.

Also keep in mind, as Ragnarok has said many times and Technician1002 never seems to wrap his mind around, valve opening speed has some serious diminishing returns.


This gets brought up because I designed valves to be fully open before the projectile in the barrel has moved more than it's diameter. Much slower valves produce very similar muzzle velocities even if the projectile moves servial times it's diameter in the barrel as the valve opens.

The 4 1 inch valves in comparison to a single 2 inch valve is not related to valve speed but a factor called CV, which is related to the pressure drop on a a valve for a given flow. This is often listed for the flow for a 1 PSI drop over the valve for water and 15 PSI for air. A single 2 inch valve has more flow than the total of 4 1 inch valves in parallel.


The flow rate through a valve is determined by the nature of the design and by the type of flow. The size of valve required for a particular application is generally established by the Cv rating. This figure is evolved for standardized units and conditions, i.e. flowrate in GPM and using water at a temperature of between 40°F and 86°F at a pressure drop of 1 PSI. Cv ratings for each valve are quoted. A standardized system of flowrate values is also used for pneumatics. In this case the air flow in SCFM upstream and a pressure drop of 15 PSI at a temperature of 68°F.

Quote from here; http://www.omega.com/TechRef/techprinc.html

It is a simple matter of looking up some CV values for some common valves online for gate, globe, ball, butterfly and other valves to compare the CV value for various size valves.

Edit;

Located a table of ball valves here; http://www.jomarautomation.com/products/pdf/A1000-flowCoefficients.pdf

A 1 inch ball valve has a CV of 87 and the 2 inch 396.

The launcher is fine to build. Enjoy it.
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Unread postAuthor: Yukondano2 » Tue Nov 23, 2010 1:09 pm

Technician1002 wrote:Just a quick note on speed, There is a point of diminishing returns when valves get faster than a certain speed, much like there is diminishing returns on really large chambers.

Also keep in mind, as Ragnarok has said many times and Technician1002 never seems to wrap his mind around, valve opening speed has some serious diminishing returns.


This gets brought up because I designed valves to be fully open before the projectile in the barrel has moved more than it's diameter. Much slower valves produce very similar muzzle velocities even if the projectile moves servial times it's diameter in the barrel as the valve opens.

The 4 1 inch valves in comparison to a single 2 inch valve is not related to valve speed but a factor called CV, which is related to the pressure drop on a a valve for a given flow. This is often listed for the flow for a 1 PSI drop over the valve for water and 15 PSI for air. A single 2 inch valve has more flow than the total of 4 1 inch valves in parallel.


The flow rate through a valve is determined by the nature of the design and by the type of flow. The size of valve required for a particular application is generally established by the Cv rating. This figure is evolved for standardized units and conditions, i.e. flowrate in GPM and using water at a temperature of between 40°F and 86°F at a pressure drop of 1 PSI. Cv ratings for each valve are quoted. A standardized system of flowrate values is also used for pneumatics. In this case the air flow in SCFM upstream and a pressure drop of 15 PSI at a temperature of 68°F.

Quote from here; http://www.omega.com/TechRef/techprinc.html

It is a simple matter of looking up some CV values for some common valves online for gate, globe, ball, butterfly and other valves to compare the CV value for various size valves.

Edit;

Located a table of ball valves here; http://www.jomarautomation.com/products/pdf/A1000-flowCoefficients.pdf

A 1 inch ball valve has a CV of 87 and the 2 inch 396.

The launcher is fine to build. Enjoy it.


Thanks for the info. I'll build it after i get a chance to cut the brace out. It snowed today strangely, though just BARELY. If you stick your finger in the snow on top of a table that was outside, it goes up to your nail. Not very deep at all.

But yeah thanks for the help.


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I noticed i got specialist. I wonder why, I haven't even built a gun. Perhaps i just post too much :l. Crazy
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Unread postAuthor: Technician1002 » Tue Nov 23, 2010 1:48 pm

saefroch wrote: Are you asserting that friction is a function of area? So far as I know (check if you want), friction is equal to the coefficient of friction times the normal force. Nothing about area in there...


I have not responded to this yet. This is simply a function of how close the majority of the flow is next to a non moving wall. Running 4 flows through 4 1 inch pipes means that all of the flow is within 1/2 inch of a non moving wall, rubbing against it. With the same area on a single 2 inch opening means the center of the flow is now 1 inch away from the friction surface. 1/2 inch away from the center of the flow is flow traveling with the flow.. so the center sees less slowdown do to friction against the walls.

If you want to play further with the math, the area is enclosed by the wall. The circumference of the circle the water flows through is simply Pi X Diameter.

A two inch pipe has all the flow exposed to the circle of pipe 6.28 inches of wall.

4 1 inch pipes has more friction area of drag. This would be 4 each of Pi times 1 inch. This turns out to be a grand total of Pi X 1 and 4 of them for a grand drag area to the flow of 12.34 inches of wall surrounding the flow. That is twice the drag area.

I hope this makes sense. It isn't the area of the flow, it is the area of the wall surrounding the flow that provides drag on the flow.

If you wish to point out my mistakes, please start here;
http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm

Find the friction of a 2 inch pipe and then find the friction of the same length of 1 inch pipe. Is the friction on the 1 inch pipe exactly 4 times the 2 inch pipe? Will 4 1 inch pipes in parallel have the same friction of 1 2 inch pipe?
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Unread postAuthor: Selador » Tue Nov 23, 2010 2:03 pm

Once that is understood, it is easy to see how the turbulence caused by bends in the pipe, affects the flow rate.
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Unread postAuthor: Technician1002 » Tue Nov 23, 2010 2:16 pm

This is why any coaxial piston valve tends to knock the socks off the same size sprinkler valve in a competition. When the valve is built into the chamber as a coaxial, the plumbing leading into the valve is non existent. The flow out of the valve is a single bend from the sides into the outlet.

Oh, and large valves rock.
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Unread postAuthor: Yukondano2 » Tue Nov 23, 2010 4:48 pm

Technician1002 wrote:
saefroch wrote: Are you asserting that friction is a function of area? So far as I know (check if you want), friction is equal to the coefficient of friction times the normal force. Nothing about area in there...


I have not responded to this yet. This is simply a function of how close the majority of the flow is next to a non moving wall. Running 4 flows through 4 1 inch pipes means that all of the flow is within 1/2 inch of a non moving wall, rubbing against it. With the same area on a single 2 inch opening means the center of the flow is now 1 inch away from the friction surface. 1/2 inch away from the center of the flow is flow traveling with the flow.. so the center sees less slowdown do to friction against the walls.

If you want to play further with the math, the area is enclosed by the wall. The circumference of the circle the water flows through is simply Pi X Diameter.

A two inch pipe has all the flow exposed to the circle of pipe 6.28 inches of wall.

4 1 inch pipes has more friction area of drag. This would be 4 each of Pi times 1 inch. This turns out to be a grand total of Pi X 1 and 4 of them for a grand drag area to the flow of 12.34 inches of wall surrounding the flow. That is twice the drag area.

I hope this makes sense. It isn't the area of the flow, it is the area of the wall surrounding the flow that provides drag on the flow.

If you wish to point out my mistakes, please start here;
http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm

Find the friction of a 2 inch pipe and then find the friction of the same length of 1 inch pipe. Is the friction on the 1 inch pipe exactly 4 times the 2 inch pipe? Will 4 1 inch pipes in parallel have the same friction of 1 2 inch pipe?


What you said made perfect Sense. I didn't do the math you suggested at the bottom myself, But the concept makes PERFECT sense. However, I didn't think that air could be affected by friction :l. I learned something new today :D
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Unread postAuthor: Yukondano2 » Tue Nov 23, 2010 4:50 pm

Technician1002 wrote:This is why any coaxial piston valve tends to knock the socks off the same size sprinkler valve in a competition. When the valve is built into the chamber as a coaxial, the plumbing leading into the valve is non existent. The flow out of the valve is a single bend from the sides into the outlet.

Oh, and large valves rock.


Fascinating.
Well i have absolutely not idea what a coaxial piston is. I understand the Axial part, probably referring to an axis. Not sure about the co. Where can I thread on pistons?

-EDIT-
http://www.spudfiles.com/forums/piston- ... t8157.html

Woots for visuals :D

Hotwired wrote:Just a flap of rubber over the equalisation hole on the chamber side of the piston should work as a check valve. The rubber will be forced against the hole with the pressure difference as the pilot vents.


Quite helpful of him :D. I was planning on using a one way check valve for my next gun.

Woots for figuring out how to use quote tags :P.
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Unread postAuthor: saefroch » Tue Nov 23, 2010 5:44 pm

So I tried out that calculator to try to verify Technician1002's assertation about diameter affecting drag. What I found is incredibly bizzarre. Plugging in 750psi absolute pressure, 150m/s for a flow rate, 2cm pipe diameter, 0.0000015 pipe roughness(the value for brass), .5m pipe length, 0 elevation change, 1.2kg/m^3 for fluid density (the density of air), and 0.00089 Pa-s for fluid viscosity (again, of air), I get a flow of 47.1 l/s. Changing that diameter to 1cm, it gives me 11.8, which multiplied by 4 pipes gives me 47.2l/s flow rate. So if the calculator is to be believed, Technician1002 has it backwards, and the four pipes with half the diameter have better flow than the one pipe with double the diameter of the other pipes.

Which makes no sense, and therefore I would attribute that .1l/s difference to some sort of rounding-based error. Which brings me to the conclusion that (at least on this scale) drag down a reasonably smooth flow path is insignificant. And therefore 4 pipes with a 1in diameter have the same flow as one pipe with a 2in diameter.

It is however correct that the friction factor is much greater with the 4 pipes than the one pipe (4 smaller pipes: 0.1264, 1 larger pipe: 0.0386), but with some realistic (or at least to me) numbers, it's insignificant.
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