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Potential energy question???

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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Potential energy question???

Unread postAuthor: boyntonstu » Sat May 01, 2010 12:17 pm

Is there a formula that calculate the potential energy of compressed air?

From Wiki http://en.wikipedia.org/wiki/Compressed ... gy_storage:

Thus if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bars (20 MPa), the potential energy stored is 530 kilojoules (0.15 kW·h).

Calculate the PE of 10 cu-in at 600 psi.
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Re: Potential energy question???

Unread postAuthor: jackssmirkingrevenge » Sat May 01, 2010 1:46 pm

boyntonstu wrote:Calculate the PE of 10 cu-in at 600 psi.

10 cubic inches at 600 psi is 0.00016387064 cubic metres at 41.36854368 bar, and the total amount of ambient air required would be 0.006779 cubic metres... can you take it from there?
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Unread postAuthor: btrettel » Sat May 01, 2010 1:47 pm

You can calculate how much energy is required to go from one state (atmospheric gas) to another (gas in a pressure chamber at set volume) in an idealized case. I detailed the equations here: http://www.spudfiles.com/forums/viewtop ... tml#223368

Use the adiabatic relationship.

Note that this gives you how much energy is required to go from one state to another. How you go from one state to the other matters. This isn't some sort of "absolute" energy. I'm not completely sure how to quantify that, though, after I finish my stat. thermo. class next semester, I should know.
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Re: Potential energy question???

Unread postAuthor: Ragnarok » Sat May 01, 2010 6:32 pm

boyntonstu wrote:Thus if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bars (20 MPa)

I'm going to be pedantic and say that's not quite right.

Don't confuse bar and atmospheres. While roughly equivalent, they are not identical.
At the same time, don't forget to use absolute pressure, not gauge pressure.

If I'm being really pedantic, then I also need to mention compressibility factor.
Boyle's law is only valid for ideal gases. Real gases do not compress according to the perfect P<sub>1</sub>V<sub>1</sub> = P<sub>2</sub>V<sub>2</sub> behaviour. They compress according to P<sub>1</sub>V<sub>1</sub>Z<sub>2</sub> = P<sub>2</sub>V<sub>2</sub>Z<sub>1</sub>, where Z is the compressibility factor for the given temperature and pressure. While it is normally approximately 1, sometimes you can't make that assumption.

It can make quite some difference when high pressure or temperature changes are involved.
I spent ages wrestling around trying to incorporate it into Apocalypse, where it proved to be a bigger factor than I was expecting. It shifted around the projected muzzle velocities for 3vo by several m/s.

It can be fudge factored out by messing with the valve coefficient, but that's not an ideal solution.
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