critical pressure

[mongo blongo]

I thought i would post this here because it's not e&w related.
I have something which is confusing the fuck out of me. I have obtained a refrigerant which i will be using for an improvised freezing apparatus. It is 1,1,1,2-tetrafluoroethane, AKA HFC-134a, Freon 134a, SUVA-134a Genetron-134a, Forane-134a KLEA-134a etc.
It's critical pressure is known to be 588 psi which is a shit load of pressure. Well, i have obtained it in the form of an aerosol spray called "Freez-It" which is used for freezing electrical components. The contents is a liquid and it is very pure! The data sheet can be found at http://www.farnell.com/datasheets/8594.pdf
I thought no way could an aerosol can stand up to 588 psi!! The can is a little bit thicker than normal aerosols but still..
I thought I would check the pressure myself and I found it to be 115 psi !
I was defiantly measuring the pressure of the liquid.
So does anyone know how it could possibly be a liquid at 115 psi which is well below the known critical pressure??

[nbk2000]

If I remember correctly, critical pressure is the highest pressure that will keep a gas liquified at its critical temperature, above which no amount of pressure will liquify the gas.

So, if the critical temp of the chemical was 100C, then at 100C the pressure would have to be 588PSI to keep the gas liquified, but at 101C, the pressure could be 2,000PSI and it wouldn't be able to keep it liquid.

[mongo blongo]

yea it works both ways. If the pressure was anything below 588 psi then it is impossible to keep it a liquid no matter what the temp is. it could be -500 deg c at 587 psi and it couldn't become a liquid. this is compleatly against my findings as it was liquid at room temp at 115 psi. i think i'm going crazy.

[Sparky]

This is just a shot in the dark, but if you're sure of what it is and of the critical pressure, maybe it is like acetylene. That is to say dissolved in a solvent and possibly absorbed in something. I suppose you're sure it's a liquid and not just a compressed gas? Easy enough to tell by shaking the can I guess.

[rikkitikkitavi]

mongo blongo , you have missunderstood it slightly.

a gas can exist as a liquid below its critical pressure, water is one exampel (374 C, 227 bar)

what critical point (temperature, pressure) defines is at what point the gas and liquid has identical enthalpy. it also means that the liquid and gas has similar densities at that point, hence there is no differnce between them.

F e x , if boiling water is boiling under pressure, you can see a phase boundary between steam and water. If temperature and pressure is increased , this phase boundary disappears @ 374 C, 227 bar, it will turn into a homogenous mix.

The pressure in your aerosol can is the vapour pressure if the freon at ambient temperature,which appears to be 115 psi (this can be found in tables fex on the internet) ,care taken for measurement errors.

/rickard

[mongo blongo]

rikkitikkitavi- thanks for clearing that up for me dude! it seems that my sources of info were far from incomplete. I knew there was something wrong with that. :) well on with my experimentation. This new info has given rise to a new problem for me though!
now I don't know what pressure to fill up my refrigerant system with! :D (see post called "freezing at low temps")