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Compressing propane...

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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Compressing propane...

Unread postAuthor: saefroch » Sun Oct 17, 2010 9:53 pm

Has anyone ever fed propane out of a tank into a pump or compressor? I'm thinking of building an attachment to feed propane into the pump I'm working on while I wait for parts, and am wondering if pressure will build up in chamber more quickly than a normal pneumatic pump/compressor because of the lower compressibility of a liquid.
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Last edited by saefroch on Mon Oct 18, 2010 2:36 pm, edited 1 time in total.
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Unread postAuthor: c11man » Sun Oct 17, 2010 9:55 pm

i dont quite understand what your trying to do. care to explain it more?
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Unread postAuthor: saefroch » Sun Oct 17, 2010 10:04 pm

I'm trying to compress propane beyond whatever pressure it is in the tank, thus filling a chamber with compressed liquid propane.
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Unread postAuthor: ilovefire » Sun Oct 17, 2010 10:15 pm

for some reason that dose not sound very safe to me!
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Unread postAuthor: saefroch » Sun Oct 17, 2010 10:21 pm

Well considering everything I plan to use is rated to 1,000psi and I'll be watching the pressure, I think it'll be pretty safe, unless you can offer some significant reason why it wouldn't be.
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Unread postAuthor: ilovefire » Sun Oct 17, 2010 11:05 pm

i am still not really following what you are trying to do if you are only going to have just propane in the system then i think it would be fine but if any oxygen were to get in then there is a possibility of hybrid like ignition, lets just say i still wouldn't do it but if done properly it should be relatively safe
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Unread postAuthor: Technician1002 » Sun Oct 17, 2010 11:58 pm

You will want to study the gas phase model and vapor pressures vs temperatures before proceeding. Liquid propane is about as compressable as liquid steam (water).

If you get a saturated vapor of propane in the chamber, it will condense into a liquid. The gas (air bubble) will contain the total pressure of all the vapor pressures combined such as gasses in air and propane. The pressure of the condensing saturated vapor varies with temperature.

Please study the subject and understand it before proceeding.

To get higher pressure than is in the tank, either raise the temprature of the chamber or mix propane vapor with other gasses so the combined vapor pressures are higher. This is how a 20X mix hybrid works without condensing the propane to a liquid. Most of the vapor pressure is other gasses from air. The vapor pressure in the mix is below the saturated vapor pressure.

Other subjects to study include fractional distallation, and refrigeration basics.
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Last edited by Technician1002 on Mon Oct 18, 2010 12:02 am, edited 1 time in total.
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Unread postAuthor: lozz08 » Mon Oct 18, 2010 12:01 am

Get a fridge or an AC compressor. Trying to compress liquid propane will break your compressor because it will expand into a gas in it's chamber and freeze the rubber seals and cause damage. If you want high pressure gas as a liquid get a CO2 tank.
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Unread postAuthor: Gippeto » Mon Oct 18, 2010 12:01 am

saefroch wrote:I'm trying to compress propane beyond whatever pressure it is in the tank, thus filling a chamber with compressed liquid propane.


Propane is usually stored as a liquid, so I am missing the point.

Vapour pressure chart for propane...

http://www.engineeringtoolbox.com/propa ... _1020.html

You can fill with liquid propane by simply inverting the full bottle, and chilling the empty bottle.

Filling to 100% capacity can lead to massive pressure spikes if the temperature increases. (...be cautious.)
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Unread postAuthor: saefroch » Mon Oct 18, 2010 2:26 pm

The purpose of this question, which I probably should have stated at the beginning, was to see if I could squeeze a bit more power out of a propane-fueled pneumatic (which should not be at all damaged if an incredibly rich mix combusts).

Technician1002 wrote:Liquid propane is about as compressable as liquid steam (water).

Wikipedia wrote:The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.

Is this correct? If so, does that mean that increasing the pressure of a chamber of propane would not displace any more propane into the chamber, and therefore the effect of this extra pressure on a piston valve opening speed would be negligible?

Gippeto wrote:Filling to 100% capacity can lead to massive pressure spikes if the temperature increases. (...be cautious.)

Thank you for the warning, but I believe that in a few days, I will be able to accomplish this safely and sanely. Aka: cautiously

Technician1002 wrote:Other subjects to study include fractional distallation, and refrigeration basics.

Subjects suitably reviewed, thank you.

Does anyone know where I can find a complete phase diagram for propane?
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Unread postAuthor: r00kie » Mon Oct 18, 2010 8:02 pm

saefroch wrote:I'm trying to compress propane beyond whatever pressure it is in the tank, thus filling a chamber with compressed liquid propane.


Gippeto wrote:Propane is usually stored as a liquid, so I am missing the point.


I have to agree with Gippeto here, though I don't exactly understand what you're trying to do. I can't think of a reason you'd have to compress propane past it's liquid form for a combustion launcher. If you're trying to use the natural expansion of propane to power a pneumatic launcher, I strongly advise against it. You would get a relatively low pressure (around 120-150 psi), your lines would tend to freeze up depending on usage, and with propane being heavier than air you would have a combustible mix floating at your feet after each shot.

I am fairly new to the forum (and pneumatic launchers in general), but being a gasfitter, I've worked with LPG for quite a few years.

Also, you will find it rather hard to compress further once it becomes a liquid.

Being rather new to launchers, my opinion doesn't weigh as much as the veterans, but that's my AU $0.02.
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Unread postAuthor: saefroch » Tue Oct 19, 2010 6:50 am

r00kie wrote:with propane being heavier than air you would have a combustible mix floating at your feet after each shot.

I'd actually forgotten about that, thanks.

r00kie wrote:I don't exactly understand what you're trying to do.

That's because you're not suppoused to. :wink:

I think my questions have now been sufficiently answered, thank you all.
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