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alcohol question

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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alcohol question

Unread postAuthor: pat123 » Sat Jun 12, 2010 10:20 am

Will liquid isopropyl alcohol evaporate to a combustible mix if poured into a closed container? also if more air was pumped in to the chamber to try to create a higher atmospheric mix, wouldn't less alcohol evaporate? does anyone know what the highest mix you can do before the alcohol can't combust?
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Unread postAuthor: Technician1002 » Sat Jun 12, 2010 2:22 pm

It depends on the temperature. At the flashpoint, it will. Below the flashpoint it won't.

Be aware most over the counter Isopropyl, is denatured and only 70% alcohol.

http://en.wikipedia.org/wiki/Isopropyl_alcohol
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Unread postAuthor: jimmy101 » Sat Jun 12, 2010 4:39 pm

Yes, it'll evaporate. The flash point for IPA is 12C to 13C, which is about 55F.

"Pour" is not an accurate description of how you would measure the fuel. Try an eye dropper. For a 1L chamber (61ci) the amount of IPA would be about 0.24g or (very) roughly 3 drops. The stoichiometric mixture for IPA in air at 1 ATM is 4.7% IPA. IPA would be expected to have about the same energy as propane. (This is for pure IPA, if you have 70% IPA, the rest is water, then you need to adjust things accordingly.)

Yes, you could add additional fuel and air as in a hybrid. The additional air pressure won't make much difference to the evaporation of the IPA but the IPA will only vaporize to whatever it's partial pressure is at the particular temperature. See http://en.wikipedia.org/wiki/File:LogIo ... essure.png for a graph of IPA vapor pressure vs. Temperature. At 25C it looks like the vapor pressure is about 30 Torr. 30/760 = 4% so at 25C (77F) you almost get to the stoichiometric mixture. Higher temperatures would be better. Looks like you'll need a temperature of 86F to get to 1.5X and ~93F to get to 2X.


http://www.inpharmix.com/jps/Liquid_Com ... fuels.html
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Unread postAuthor: pat123 » Sun Jun 13, 2010 4:03 pm

Ok thanks. so if I heat the alcohol I also have to add more air to keep the correct mix? I think this will be too complicated for my current design since the alcohol would have to stay heated for a long time before the shot. If i heat it and it cools off, the alcohol that evaporated will just condense right? I think it will be easier to get a good mix of propane to air than of alcohol to air. measuring in fractions of a drop seams difficult.
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Unread postAuthor: starman » Sun Jun 13, 2010 6:28 pm

I've used isopropyl alcohol before...spray and pray style. Spray it out of a hand spray bottle like hair stylists and barbers use...one small mist does the trick. It is fairly temperamental though with air temperature and humidity. An average dry summer afternoon it works fine. A summer evening after a rain shower, high humidy and it won't fire at all.

Sticking with propane is a far superior choice.
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Unread postAuthor: jimmy101 » Tue Jun 15, 2010 6:17 pm

pat123 wrote:If i heat it and it cools off, the alcohol that evaporated will just condense right?

Maybe, though it is possible to get a super-saturated mixture.
pat123 wrote:I think it will be easier to get a good mix of propane to air than of alcohol to air. measuring in fractions of a drop seams difficult.

Drops are hard to measure, especially with low boiling liquids. (There is a trick to it if anyone cares.) Usually though, the liquid just piddles right out'a the dropper before you get to doing the actual measurement.

Propane (or butane) is cheaper. Just get a $0.99 disposable butane lighter and a $2 syringe and you're good to go.
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Unread postAuthor: jimmy101 » Tue Jun 15, 2010 6:19 pm

double post 12345
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