Physics of air pressure [pictures]

[maldotcom2]

I'm planning to make a simple contraption capable of shooting a wad of paper at high velocities by using compressed air from an air compressor (for another experiment). I'm considering different ways of attaining a sudden gush of air to the barrel to launch the paper. Below, ive drawn my three ideas crudely in paint to depict what i'm talking about. Red represents a quick release valve and green represents the wad of paper. The curly thing is the air hose. The air hose and barrel assembly are roughly to scale.



Simplest possible solution, just fit the air compressor outlet to the end of the pipe.



The idea behind this design is there would be more air volume to gush through the pipe when i open the valve. But am i really achieving more pressure by having a secondary air reservoir at the same pressure as the source?



I don't know why i thought of this design.


So, do these designs differ from each other in terms of supplied kinetic energy to the projectile? If so, which is the best design for maximum power? Feel free to suggest your own design if you think it would work well, but don't forget to supply a crude paint drawing.

Thanks guys.

[jkmatt]

No 2

Calculate the volume of your barrel, then make the plenum behind the barrel roughly twice the volume.

Depending on the size of the barrel the air pressure could start to drop the further the wad goes down the barrel, becuase of low flow through the air hose/fittings. the plenum would supply extra

No specific knowledge behind that, just sounds good to me

[R3Dl2ULZ]

My basic understanding would suggest no 2 aswell, for the same reasons suggested by jk_matt. i dont think the plenum being bigger than twice the size would be detrimental, but you might need to taper down from the plenum to the valve to reduce turbulence, although im not very familiar with compressed gases, it was mainly to do with liquids..
If you could get a bigger valve and put it at the barrel end of the plenum it would probably help with flow.

[TERRA Operative]

You want number 2. It's called an accumulator and allows a high volume to flow straight into the barrel, rather than through a long restrictive tube.

Make sure you have a very fast acting valve too, to make sure the air rushes out in a big hit, rather than 'leaking' past as the valve is opened.

[underskore]

R3Dl2ULZ +1


Click to view full-sized image!
Hosted by UGBox Image Store


instead of the picture i drew i would aim for a plenum of X diameter, then have it stepped down to about X/2 for the valve, and stepped down after the valve to the barrel size.

i imagine that a suitable electronic solenoid valve wouldnt be to pricey. (and i believe most are a membrane instead of gate type, they should give the effect that TERRA Operative describes. )

[maldotcom2]

Thanks for the helpful responses guys! I was thinking No. 2 aswell, for the same reason TERRA Operative mentioned.

Originally Posted by TERRA Operative

Make sure you have a very fast acting valve too, to make sure the air rushes out in a big hit, rather than 'leaking' past as the valve is opened.

Originally Posted by underskore

i imagine that a suitable electronic solenoid valve wouldnt be to pricey. (and i believe most are a membrane instead of gate type, they should give the effect that TERRA Operative describes. )

Hmm, i've been thinking a lot about the valve but i havent seen too many options. Only valves designed for water, like ball valves.

This Solenoid valve sounds interesting but may prove hard to find and/or expensive. But it would be electrical, which would make it super easy to rig a trigger system rather than a clunky mechanical trigger. It also has to be the right size to fit/screw into the other components which will be either Pressure PVC or brass. I shall research. Does anyone know a local supplier?

[Bastard Child]

Ahhh... ye ol' days of spun guns. Most of my bookmarks are no longer working though in this age of [s]Islamophobia[/s] 'terrism'

http://www.spudtech.com/
http://www.instructables.com/tag/?so...3Atype%3Aid=on
http://www.instructables.com/id/Air-...Potato-Cannon/
http://www.spudfiles.com/spudtech_message.html
http://www.engineering.uiowa.edu/~ha...1/potato.shtml
http://www.geocities.com/ResearchTri...spudindex.html



EDIT: As for a valve you may want some kind of electric sprinkler valve
EDIT2: Strike tags don't work on this forum???

[maldotcom2]

Thanks for the links. I'm focusing on pneumatic rather than combustion and please note i'm not making a weapon. An interesting read all the same.

Originally Posted by Bastard Child

EDIT: As for a valve you may want some kind of electric sprinkler valve

Cheers, i'll look into them.

[underskore]

i was thinking of a standard 25mm irrigation type solenoid valve, available at most hardware stores, expected price < $30
edit: most are 24V but i rekon you could get away 2 9V batteries

[maldotcom2]

Thanks mate, legend. I'll ring around today and make a few trips. Good thing you specified irrigation, i was looking at jaycar and such.

[maldotcom2]

Ok i bought all the parts i need today, including a 24V solenoid valve. The only trouble is, it requires 24 VAC.

a) Where the hell am i gonna get that?

b) Does anyone know what pressure "pressure pvc fittings" are rated to? I was thinking of going to 130PSI which would be the maximum of my hardware.

c) When using PVC primer and cement, do i have to let the primer evaporate before using the cement or do i slap on the cement while it's still wet?

[stewpot]

Originally Posted by maldotcom2

Ok i bought all the parts i need today, including a 24V solenoid valve. The only trouble is, it requires 24 VAC. Where the hell am i gonna get that?

The solenoid valve will still work with DC. If you run it at 24VDC it will draw much more current than with 24VAC. It'll overheat if you run it long term from 24VDC. I've had success with running an AC solenoid on reduced voltage.

For this application, the solenoid will only be on for a very short time, so too much current won't be a disaster. Just pick a DC voltage that operates the solenoid quickly.

[aXis]

The primer evaporates very quickly, as far as I am aware dry is fine. If in doubt read the instructions on the container.

[maldotcom2]

Thankyou stewpot, thats what i wanted to hear. The valve only cost me $20 so if i kill it using DC atleast i didnt have to pay big bucks for a 24VAC solution. I think that logic works...? Hmm, anyway.

If anyone knows the answer to question B that would be awesome!

EDIT: Cheers axis. Yes the primer is MEK which evaporates damn fast. The container gave no specifics so i thought it best to ask.

[maldotcom2]

I tried the solenoid valve using 2 * 9V batteries and there is a distinct click as though something movies, but when it's supposedly open i cant blow air through it very freely, it only leaks through very slowly and my cheeks get sore.

I'll pick up a 24V door bell battery tommorrow and try that.