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Saturday, April 01, 2006
HW #2 - P #6 - An Application of Equations of State
A 0.016773 m3 tank contains 1 kg of R-134a at 110oC. Determine the pressure of the refrigerant using: a.) the ideal gas EOS b.) the generalized compressibility chart c.) the refrigerant tables d.) van der Waals EOS e.) Soave-Redlich-Kwong EOS
8 comments:
Anonymous
said...
I am trying to read the compressibility chart given in class. Are the dashed lines the ideal reduced molar volume or something else? I can tell the Tr line and I assume our Z and Pr values are where the two lines meet. How accurate do we need to be? (I got 0.4 for a Pr value and that set off my answer pretty far...) Thank you for your time!
Lisa 3:54 & 4:05 AM Wow, doing thermo at 4 AM ? Sleep ! Yes, the dashed lines are lines of constant ideal reduced molar volume.
Correct again. To summarize for other readers... Pr = 0.4 is not so bad. Yes, this gives an answer MUCH different than the IG EOS. But it is the IG EOS that is not accurate ! That is really the whole point of using better EOS's such as the compressibility charts.
On part d) of this problem, I'm getting an answer that is about 370 kPa higher than the answer given on the homework. I've double- and triple-checked my calculations and units as well as the correctness of the Van der Waal's equation P = (RT/V-b) - a/V^2 (V being V wiggle that worked for all other parts). Is there some common mistake here that I could be overlooking?
Carina 10:22 PM The most common mistakes when using the more sophisticated EOS eqns involve units. I suggest ou stick with P [=] Pa, V~ [=] m^3/mole and T [=] Kelvin. Can you tell me what values you got for a & b ? Maybe your values for T & V~ and their units would help too.
8 comments:
I am trying to read the compressibility chart given in class. Are the dashed lines the ideal reduced molar volume or something else? I can tell the Tr line and I assume our Z and Pr values are where the two lines meet. How accurate do we need to be? (I got 0.4 for a Pr value and that set off my answer pretty far...) Thank you for your time!
Never mind, I got it (the difference surprised me, then I realized it's supposed to be!) Thanks anyway!
Lisa 3:54 & 4:05 AM
Wow, doing thermo at 4 AM ? Sleep !
Yes, the dashed lines are lines of constant ideal reduced molar volume.
Correct again. To summarize for other readers...
Pr = 0.4 is not so bad. Yes, this gives an answer MUCH different than the IG EOS. But it is the IG EOS that is not accurate ! That is really the whole point of using better EOS's such as the compressibility charts.
Good work, especially for 4 AM !
I must have missed something in class but what are the refrigerant tables?
Anon @ 6:58
R-134a is a refrigerant. In this case, the refrigerant tables are the R-134a tables.
On part d) of this problem, I'm getting an answer that is about 370 kPa higher than the answer given on the homework. I've double- and triple-checked my calculations and units as well as the correctness of the Van der Waal's equation P = (RT/V-b) - a/V^2 (V being V wiggle that worked for all other parts). Is there some common mistake here that I could be overlooking?
Carina 10:22 PM
The most common mistakes when using the more sophisticated EOS eqns involve units. I suggest ou stick with P [=] Pa, V~ [=] m^3/mole and T [=] Kelvin. Can you tell me what values you got for a & b ? Maybe your values for T & V~ and their units would help too.
Carina 10:22 PM
The other possibility is R. With the units I mentioned in my last comment, you should use R = 8.314 J/mol-K = 8.314 Pa-m^3/mol-K.
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