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Does it matter where valve is placed in over-under?

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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Does it matter where valve is placed in over-under?

Unread postAuthor: Wizman » Sat Apr 23, 2011 5:30 pm

I'm getting ready to build a pneumatic cannon (actually, an "antenna/tennis ball" launcher using 4" PVC for the chamber and 2.5" PVC for the barrel.

My questions have to do with placement of the modified 1" sprinkler valve, the best size (diameter) of the PVC 90 degree elbows ("U"), and the overall effect they have on performance of an "over-under" pneumatic design:

I see some "over-under" pneumatic cannons with the chamber (located at the bottom) connected to the 1" valve (located at the top...directly attached to the barrel) using a "U" made up of two 90 degree PVC elbows between the chamber and the 1" valve.

On other over-under designs, I see the chamber directly connected to the 1" valve (both located at the bottom) connected to the barrel (on top) via the PVC "U" elbows.

(1) Does it matter, from a performance point of view, where the 1" valve is located (i.e. top or bottom)?

(2) If the 1" valve is mounted at the bottom with the chamber (i.e directly connected with a short piece of 1" PVC pipe), does the diameter of the 90 degree PVC elbows (i.e. 1" versus 1.25" versus 1.5") make much difference to overall performance??

(3) Does the diameter of the "U" make less of a difference if the 1" valve is mounted at the top along with the barrel?

I guess I'm confused by several designs where the 1" valve was mounted at the bottom along with the chamber and the designer felt it was important to machine out the chamber and barrel end-caps in order to directly glue in an over-sized "U"...made up of larger diameter elbows. This seems like a lot of extra work when larger "standard" 90 degree PVC elbows could be used instead...unless there is some obvious benefit to all of the extra machining.

I would sincerely appreciate your feedback so I can finalize my design and get it built. Thank you.


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Unread postAuthor: Hubb » Sat Apr 23, 2011 5:59 pm

The general consensus, from a performance perspective, is that the valve should be placed as close to the projectile as possible. This cuts back on dead space (the empty space the air must travel in order to reach the projectile). Essentially, the less dead space that is present, the higher the velocity is going to be. Honestly, though, you are probably not going to be able to tell a huge difference without a chronograph.

Other factors for valve placement in an over / under is due to space restraints or aesthetic purposes. Since you are going for performance, however, you should place it on the barrel side.

As far as the elbows go, the biggest issue is going to be the choke they put on the launcher. But your bigger concern with choked flow is going to come from the 1" sprinkler valve. My goal is to use the same sized fittings as the barrel. If that is an issue, just make them bigger than the valve.

As far as your build goes, I'm looking forward to it. And welcome to Spudfiles.
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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 8:38 pm

If you want performance, I would ditch the 1 inch sprinkler valve and use something with more flow for that 2.5 inch barrel.

For safety and a reduction in flow restriction through the bend, I would recommend keeping full chamber diameter through the bend, then reduce it for the valve. If it's within your budget, I recommend at least a 1.5 inch valve. 2 inch valves are a little expensive.

500 ml water bottles make excellent projectiles for towing a mason line over tall trees. Once the mason line is in a safe path, only then consider pulling a conductor.
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Unread postAuthor: jhalek90 » Sat Apr 23, 2011 8:44 pm

this http://www.spudfiles.com/forums/viewtopic.php?t=16190& would be a GREAT place to start. look at that gun, and build yours arounds the design there.

Good luck and welcome to the best website on the internet.
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Unread postAuthor: MrCrowley » Sat Apr 23, 2011 8:55 pm

Not on topic per se but awhile back I made an 'antenna launcher' only for the reason that it allowed me to build it at school as part of an assignment.

You can have a look at it here if you want.
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Unread postAuthor: mark.f » Sat Apr 23, 2011 8:55 pm

Technician1002 wrote:If you want performance, I would ditch the 1 inch sprinkler valve and use something with more flow for that 2.5 inch barrel.


Due to the "application" he specified, you really don't need too much performance. If you take a look at some of these launchers, you won't see a valve with more than about 1" porting.
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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 8:58 pm

With the low performance valve the restriction and dead space become less of a factor. Anyone want to play with the numbers of both designs in GGDT?
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Unread postAuthor: Lockednloaded » Sat Apr 23, 2011 8:59 pm

Tennis balls can get pretty far, even from a 1" valve with a tight barrel. I'd use 4" elbows to make the U, then you can bring it down to 1"
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Unread postAuthor: Technician1002 » Sat Apr 23, 2011 9:13 pm

Due to the volume, there is no need for elbows at all with a 1 inch valve, a 2.5 inch barrel and 4 inch chamber.
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At 7 bar, this little launcher can launch a rolled up t shirt 200 feet.
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Unread postAuthor: Hubb » Sun Apr 24, 2011 2:23 am

Tech, now, to be fair, you should not compare your valve with a sprinkler valve. :wink:
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Unread postAuthor: Technician1002 » Sun Apr 24, 2011 6:04 am

The high loss in a sprinkler valve is why I recommend a larger valve to achieve a similar CV to my 1 inch valve. I've directly compared performance to a sprinkler valve.

The advantage in the field is less pumping. If this is to be a backpack launcher, low efficiency results in higher effort to pump it up. Higher efficiency means a smaller chamber can be used at lower pressure for much less work. The launcher can be smaller overall for less pack weight. The chamber does not need to be larger than a couple 4 inch elbows or straight pipe volume equivalent.

A larger chamber running at higher pressure means a lot more air is required to do the job.
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Unread postAuthor: Wizman » Wed Apr 27, 2011 11:09 pm

I sincerely appreciate all of your excellent comments and suggestions.

Based on all of your feedback and after some additional research, I've decided to use two 4" elbows plus an additional short piece of 4" PVC pipe (~14") for the chamber and will use a 2" valve instead of a 1" valve (modded, of course). :P

I have also decided to use several different size barrels (each capable of threading into the 2" valve) depending on the type and size of my projectiles.

Question: Can PVC "Quick Disconnect Unions" be used effectively to easily change barrels? I'm thinking of threading one end of a Quick Disconnect Union into the 2" valve and have the other end attached to a specific barrel. Seems like using multiple (identical) unions with different barrels would make changing the barrel very quick and convenient. Any down sides to this idea??

I'm going to try two different sizes of 2.5" PVC barrels for tennis balls...schedule 40 and SDR 21 (class 200). My initial application is to launch a tennis ball with and without fishing line attached...for erecting antennas.

I will also try a 1.5" barrel (golf balls) and a 1.25" barrel (PVC projectiles). Obviously, my design allows me to experiment in various ways, to learn by doing, and to have fun doing it.

Question: regarding painting. I assume it's best to assemble everything, test it, and then dissemble for painting? I'm pretty good with PVC pipe, but know how difficult it is keeping the purple primer from staining the outside of the PVC. I will try masking everything as I assemble it, but still believe some painting will likely be necessary after I'm done.

I'll post some photos after I get the "cannon" built. Thanks again for all the help!
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Unread postAuthor: Technician1002 » Wed Apr 27, 2011 11:36 pm

In regards to the 14 inch length of 4 inch pipe; Are you planning on using a compressor or hand pumping it? If hand pumping it, you may find the effort to be tiring. My small launcher used a small chamber for a reason. It was designed with a hand pump in mind.

To see if your proposed chamber is too big for hand pumping, find the volume of your proposed barrels. At 3-7 bar of operating pressure a chamber 1/2 the volume of the barrel works well. A chamber 10X the volume requires 10X the effort to pump up. It makes a difference of a 30 second pump up or a 5 minute workout.

Use GGDT to see if the larger chamber is worth the extra effort.

If you are using a compressor, then this is not important.

The small chamber was used with hand pumping in mind.
[youtube]http://www.youtube.com/watch?v=FusGJKJ4EL0&[/youtube]

In regards to primer, take care at the angle you hold the parts as you apply primer. Primer runs downhill. Use that to your advantage. Hole a socket opening up and prime the inside. Keep it off the outside. When priming pipe ends, hang it down and prime the bottom. Drips will run to the end, not along the length.

When done properly the primer can fully coat just the area of the joint and not stain the pipe outside of the joint. Here is an example I did.
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Unread postAuthor: Labtecpower » Thu Apr 28, 2011 1:42 am

My 6 liter chamber is a 5 minute workout :D
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