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noob question by spudfarm

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: jimmy101 » Sat Mar 22, 2008 10:57 am

So, for a 20X mix using stoich. propane + air and assuming "20X" means the final pressure in the chamber is 20ATM.

Originally, the air in the chamber was 79.05% N<sub>2</sub> and 20.95% O<sub>2</sub>. (Ignoring the trace gases like water, etc.) But, we always add propane until the final percentage is 4.02%, so the N<sub>2</sub> and O<sub>2</sub> have to be decreased by 4.02%. This is the same as mutliplying by (1-0.0402)=0.9598.

So, we calculate the partial pressures of the three main gases;

P<sub>N<sub>2</sub></sub> = (0.7905)(20 ATM)(0.9598) = 15.174 ATM
P<sub>O<sub>2</sub></sub> = (0.2095)(20 ATM)(0.9598) = 4.022 ATM

The amount of propane doesn't get decreased by the 0.9598 factor since 4.02% is the final precentage we are aiming for.
P<sub>propane</sub> = (0.0402)(20 ATM) = 0.8040 ATM

Double check that the total pressure comes out to 20 ATM;
P<sub>total</sub> = P<sub>N<sub>2</sub></sub> + P<sub>O<sub>2</sub></sub> + P<sub>propane</sub> = 15.174 ATM + 4.022 ATM + 0.8040 ATM = 20.00 ATM

Double check that the final propane concentration is indeed the 4.02% we were aiming for;
P<sub>propane</sub>/(P<sub>N<sub>2</sub></sub> + P<sub>O<sub>2</sub></sub> + P<sub>propane</sub>) = 0.8040 ATM / (15.174 ATM + 4.022 ATM + 0.8040 ATM) = 4.02% propane

Now, the pressure that controls whether or not the porpane liquifies is the propane partial pressure, P<sub>propane</sub>, which is only 0.804 ATM. We can look at the graph Spudfarm posted and see that the boiling point at that propane pressure is well below 0°C. So the propane will not liquify in this 20X mixture at normal spudgun temperatures.
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Unread postAuthor: SpudBlaster15 » Sat Mar 22, 2008 11:51 pm

DYI wrote:So you're saying I was right all along? :wink:


No, (*looks back at DYI's first posts on the matter, notices 6.44psi was his original assumption*) I am saying that you were correct at the beginning, but failed to entirely understand why, and were thus mislead by my slight assumption that turned out to thwart the calculation by a substantial amount.
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Unread postAuthor: DYI » Sun Mar 23, 2008 6:54 pm

So I was right :lol:
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Unread postAuthor: SpudBlaster15 » Sun Mar 23, 2008 7:00 pm

If it makes you feel better, sure. :D
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Unread postAuthor: DYI » Sun Mar 23, 2008 7:06 pm

2) Order KNO3 and KMnO4.
3) Purchase enough brass media to fill my 6" x 9" PVC ball mill jar.


Getting ready for summer already I see :D
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