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liquid propane calculations

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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liquid propane calculations

Unread postAuthor: singularity » Fri Jul 20, 2007 3:29 pm

i recently acquired a green gas chamber complete with green gas valve, heres some pics

<img src="http://i78.photobucket.com/albums/j85/irwinner_2006/DSCN2674.jpg">

<img src="http://i78.photobucket.com/albums/j85/irwinner_2006/DSCN2675.jpg">

<img src="http://i78.photobucket.com/albums/j85/irwinner_2006/DSCN2672.jpg">

the actual chamber is about 2" long and has a diameter of 3/4" and intend on using it to store the fuel for a airsoft propane mortar i am in the process of making. i have already made a propane meter with a reg on it and the chamber has been built, it is a 1' long and 3" in diameter. now what im wondering is how many shots can i get out of the cannon before i have to refill the green gas chamber? that is assuming the chamber will be filled with liquid propane. it doesn't have to be exact
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Unread postAuthor: singularity » Fri Jul 20, 2007 7:41 pm

come on someone answer just a rough estimate is all i need
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Unread postAuthor: Pete Zaria » Fri Jul 20, 2007 7:50 pm

I wish I could just write you up a reply, but I have no idea how to give you an accurate figure.

Here's how I'd figure it out, though:

First, measure the volume of the propane chamber in question. The easiest way to do this is to fill it with water, then pour the water into a measuring cup or etc... and check the volume.

When you fill it with liquid propane gas (LPG), you'll get it pretty full but not 100% full, because there will be a pocket of propane gas / the residual air that was in the chamber before filling. You might find some numbers on this, but assume you'll get it 90% full of LPG, so subtract 10% from the total volume.

Then, find out (via Googling or asking BLB, he seems to be able to find all these figures) how much gaseous propane 1 cu or oz of LPG will produce, calculating in that it will be under pressure. Then you should be able to calculate out how many shots you can get from that volume of LPG (figure out how much it takes to fill your meter pipe at X pressure, etc...).

I'll see if I can find you some charts on liquid propane.

For a *really* rough estimate... I get about 150 shots from a benzomatic tank in my big combustion gun, 250ci chamber.

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Unread postAuthor: singularity » Fri Jul 20, 2007 8:02 pm

well if your chamber is 250ci and you get 150 shots off then you could about 7 times as many shots off with a 36ci chamber (250/36=6.94). so with the same tank i could get about 1050 shots (150*7=1050) off the same tank. so all thats left is finding out how much liquid propane is stored in a benzomatic tank then i can get my estimate. you have been very helpful thank you
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Unread postAuthor: Pete Zaria » Fri Jul 20, 2007 8:59 pm

14.1 Oz / 400 grams of LPG in a benzomatic tank, according to the label.

That's not a very accurate estimate, but whatever :-p

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Unread postAuthor: boilingleadbath » Sun Jul 22, 2007 1:29 pm

We have one way of calculating it:
1) Find the mass of the propane in the storage tank, and the mass required to fuel your launcher, and do the obvious calculation.

Every other method is a simple derivative of this.

1) Find the VOLUME of liquid propane in storage
2) Find the MASS of the liquid propane in storage
3) Find the number of MOLES of liquid propane...

and...

a) Find the MASS of propane required
b) Find the VOLUME of propane, at a known pressure&temperature, required
c) Find the number of MOLES of propane required

Pick any number and any letter; the conversion is simple enough it doesn't matter.

Personally, I'd find #2 by weighing the tank... the actual metal part is probably a fairly small fraction of that weight, so it gives you a reasonably accurate number, and find "B" through the standard maths required to find the volume of propane required.

However, if #2 isn't an option, you could find the volume of the exterior of the tank (perhaps by water displacement). The walls are fairly thin, and the air cavity is fairly small, so it allows you to get a good estimate of the volume of liquid propane inside.
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Unread postAuthor: SpudBlaster15 » Sun Jul 22, 2007 3:25 pm

Pete Zaria wrote:I get about 150 shots from a benzomatic tank in my big combustion gun, 250ci chamber.


You must have a severe leak in your meter, because:

400 grams of propane = 400/45 = ~8.9 moles of propane.

8.9 moles = 8.9*24 = 213.6 liters of propane.

250 cubic inches = 4,090 milliliters.

4,090 milliliters of chamber volume = 4,090*0.042 = 171.8 milliliters per shot.

213,600/171.8 = 1,243 shots per tank, which is approximately what I was getting with my large combustion gun.
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Unread postAuthor: Pete Zaria » Sun Jul 22, 2007 7:38 pm

SpudBlaster15 wrote:You must have a severe leak in your meter, because:

400 grams of propane = 400/45 = ~8.9 moles of propane.

8.9 moles = 8.9*24 = 213.6 liters of propane.

250 cubic inches = 4,090 milliliters.

4,090 milliliters of chamber volume = 4,090*0.042 = 171.8 milliliters per shot.

213,600/171.8 = 1,243 shots per tank, which is approximately what I was getting with my large combustion gun.


Wow, thanks a bunch for that math.

I know I have a small leak in my meter, on a hose clamp joint, I know this because if I leave the meter pressurized (to 74psi, my normal meter pressure) and let it sit, it will leak down to about 20 psi over a few hours. Also, if you hold a flame to the outside of that joint, it will produce a very small flame. However, I didn't think the leak was that significant. Now I have some motivation to fix it. I'll find some new hose and some epoxy...

I doubt I've fired my new combustion gun 300 times yet, and I've gone through a tank and a half. I stopped using the tank when it took forever to get upto my meter pressure, though there was probably a few dozen shots left in it.

Thanks.

Peace,
Pete Zaria.
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Unread postAuthor: VH_man » Sun Jul 22, 2007 7:58 pm

GHAHH whered u get that?

im not kidding, if you have any more, i will buy the green gas fill valve off ya........

i finally figured out how a GBB works, and im gonna make a GBB electro-pneumatic airsoft gun.......... thats right, electronic actuation, electronic feed, electronic trigger............., pneumatic propulsion/blowback.
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Unread postAuthor: singularity » Sun Jul 22, 2007 11:26 pm

i got that from a NNB gas gun magazine more specifically <a href="http://www.airsplat.com/Items/GP-UG161-MAG.htm">THIS</a> gas gun magazine. the magazine casing is made from two pieces of plastic held together with two screws once the screws are removed you will see that sliding bb loader a plastic hammer valve and the gas chamber pictured above (unless they changed it)

and are you serious over 1000 shots from a tank
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Unread postAuthor: VH_man » Mon Jul 23, 2007 8:19 pm

hmm. im getting one of those along with my M-11 magazine (1 is just not enogh with this thing!!!!!)

and 1000 shots seems reasonable.

i may just be completely full of crap, but seeing how small meter chambers are, and knowing you can get about 3000 shots with a GBB from a coleman propane cylinder, it seems reasonable.

1000 shots is alot for a combustion.............. my combustion only lasted 50 shots.....
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