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Energy stored in compressed air

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: jimmy101 » Wed Feb 25, 2009 8:03 pm

P*V is clearly wrong since it ignores heat loss and the like.

Seems to me you have to model the compression as an isothermal process since heat loss is probably fairly fast compared to the rate of compression.

Unless you are pressurizing from a tank, then you have cold gas expanding into the chamber which then warms up (and increases it's internal energy).

During firing it's an adiabatic process (the temperature of the gases changes as fast as the gas expands). There would be some heat gain from the chamber walls but that is probably insignificant since the temperature gradient is so small.

I would think GasEq would handle this type of calculation.

1. Isothermal compression, record Gibbs energy for the ending state.

2. Adiabatic expansion, record final Gibbs energy for the ending state.

3. Energy change is just the difference in energy of #1 and #2.

4. Efficiency is KE of your projectile over the number in #3.
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Unread postAuthor: btrettel » Wed Feb 25, 2009 11:09 pm

I've been examining the discrepancy you mentioned jimmy as I've designed my next pneumatic gun. Time scale seems to be the main difference between gas entering the chamber and firing. I'm tempted to try a polytropic coefficient like (k + 1)/2 to get something between isothermal and adiabatic for the gas entering the chamber. Whether that's reasonable or not for the time scale I'm looking at (10 seconds to a few minutes between shots), I don't know, but it's a start.
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Unread postAuthor: jimmy101 » Thu Feb 26, 2009 11:46 am

btrettel

I would think you don't need anything that complicated. Heat loss (or gain) from a gas is pretty darn quick. In addition, the heat capacity of the compressed gas is small compared to the heat capacity of the chamber. That means the gas cools off (or heats up depending on how you pressurize the chamber) but the chamber itself really doesn't change temperature very much.

Heck, you should be able to measure the temperature changes just by monitoring the chamber pressure for a minute or two after filling. If the pressure has stabilized within a minute or so then you've lost (or gained) all the heat needed to get the gases to ambient temperature.

My shop compressor cycles off at 120 PSIG. It cycles back on at about 100 PSIG. If I fire it up when the tank is "empty" it'll run for a couple minutes then shut down. A couple minutes later it'll cycle on again as the fairly hot compressed air cools and the pressure in the tank drops from ~120 to ~100. Compressors generate very hot air, on my compressor the line between the compressor cyclinder and the storage tank gets too hot to touch.

Of course, now you've got the problem of how are you going to calculate efficience? Heat loss (or gain) means the efficience of the gun depends on how long it sat between the time it was pressurized and the time it was fired. Furthermore, depending on how you pressurize the chamber the efficiency could increase or decease with the delay time.
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Unread postAuthor: btrettel » Thu Feb 26, 2009 3:08 pm

Interesting thoughts.

The heat loss is partly why I only look at the gas once everything has stabilized when evaluating efficiency. This is the most straightforward way to look at it as far as I can tell.

Of course, it's less efficient than it seems due to the heat transfer, but there's not much I can do about that, especially given that my air reservoir's made from aluminum and has a rather large surface area. I figure at least looking at efficiency will put me ahead of the curve so I don't worry too much about this.

Speaking of this gun... I'll be completing it next week given I'm not too busy so I should have something interesting to post soon.
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Unread postAuthor: Biopyro » Thu Feb 26, 2009 4:54 pm

you know when a concept just suddenly clicks and you don't know how you didn't understand it before? Just got that! Hopefully I'll really be able to pick up some good marks on this now! Thanks so much for all your help.
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Unread postAuthor: Solar » Fri Feb 27, 2009 3:10 am

ok you nerds... I can't keep up with all the equation stuff, but I can answer one of the first questions in the authors post. "Where does the energy come from" --- It is SOLAR energy, since all energy derived from compressing air comes from the sun. :-)

Here is a site that has some great info on the subject and might be useful for you. Check out the whole site, very cool info about air engines.

www.aircaraccess.com/solar.htm

I have noticed with the Eclipse launcher that if I use a slower regulator, the launcher tends to heat up on the opposite end of the ballast chamber. Subsequent rounds will enter the aluminum chamber, come to regulated pressure, then the pressure will increase about 15% since it is recliming residual energy in the form of extracted heat from the previous cycle. If I use a faster regulator it appears that the chamber doesn't have time to heat up significantly and/or the air rushing in is fast enough to actually cool the volume.
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