Lotronex

Q: For a certain experiment, R-134a vapor is contained in a sealed glass tube at 20C. It is desired to know the pressure in this condition, but there is no means of measuring it, since the tube is sealed. However, if the tube is cooled to -20C small droplets of liquid are observed on the galss walls. What is the initial pressure?

We got this question on a HW assignment, but we never did any type of problem solving like this, it was mostly just the professor in front drawing P-T diagrams and the like.
We think that -20C is the Tsat at that pressure, but we have know idea where to go. If anyone with experience in these matters could point me in the right direction to get this solved, it would be much appreciated.

Amano

What information do you have about the properties of R-134a? I'd say use the equations of state to determine the vapor pressure at -20 and work your way back up to +20.

Bossnass

*it has only been one year since I took my thermo course. I did poorly and I forget most of it. Nonetheless, I would proceed as follows*...

Just past the water tables in your text appendix, you should find some r134a tables.

So you need to find the pressure that corresponds to -20. Make sure youre dealing on the right hand side of your saturation curve because youre nearly a saturated vapor, not nearly saturated liquid.

Then use your ideal gas equation (with an R for r134a) and back calc to your initial temp. Depending on where you are in the course, you might need to make a general compressibility adjustment.

again, take that advice with a grain of salt.

kutulu

At -20, the vapor is saturated. As Bossnass said, you should be able to look up the corresponding Psat for the R134a in a table. After that you can use the idea gas law to get the original pressure. Since the volume and number of moles are constant, they (along with the gas constant) drop out of the equation. It reduces to:

P1/Psat = T1/Tsat (of course make sure you use absolute temp)

Bossnass - Are you an ME? I remember hearing that ME's like to combine the ideal gas constant, R with the molecular weight.

Amano

Right, i'd agree with everythhing that kutulu and bossnass have said, except to watch out for one thing. By "equation of state" in my original post, i was referring to equations that relate P,T,V, etc. If you're making the ideal gas assumption, then PV=nRT holds, but depending on how you're going to approximate a "real" gas, you may need to use the van der Waals equation or a virial expansion. Depending on what you've been taught in class.

Bossnass

I'm currently in a 'requalifying year'. I went back as an older student (a 23 year old freshman), but found it pretty tough to handle 6 courses of engineering while working a full time job. I didn't do bad enough to get kicked out, but I didn't do well enough to be accepted into one of the streams. I'm still torn between a MecE and a ChemE. Hell, CivE still appeals for that matter. All I know is that I don't want to go EE, CompE, PetE, or MinE. I have till April to decide.

pig

i could be wrong here - i'd have to think about it some more. i would avoid igeos and other equations of state and try the following:

1. at -20 you know it's saturated vapor. therefore, you also know the specific volume, I believe. you're in a confined volume, and the r134 is a gas. thus, as long as you're strictly in vapor phase, specific volume is constant.

2. i *think* that you can therefore perform an interpolation using information in the table, at 20 deg. C and the given specific volume, and back out the pressure. I believe this will eliminate any theoretical assumption of equation of state representation, and give an answer that is empirically based.

/could be wrong

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kutulu

6 courses is a tad ambitious (even for someone who doesn't work). I worked full time and never took more than 4. Took me a while to get out though...