Monday, January 01, 2007

HW #1, 6.49 - Enthalpy and Entropy Changes of Vaporization - 12 pts

From data in the steam tables:

a.) Determine values for Gliq and Gvap for saturated liquid and vapor at 150 psia. Should these be the same ?
b.) Determine values for DHvap / T and DSvap at 150 psia. Should these be the same ?
c.) Find values for VR, HR, and SR for saturated vapor at 150 psia.
d.) Estimate a value for dP*/dT at 150 psia and apply the Clapeyron Equation to evaluate DSvap at 150 psia. Does this result agree with the steam table value ?
e.) Use the SRK EOS to evaluate VR, HR, and SR for saturated vapor at 150 psia. Do these results agree with the values you found in part [c] ?

8 comments:

Anonymous said...

To find the residual values for V, H, and S, we have to find the ideal gas values for each. For enthalpy would you like us to integrate the heat capacity equation in table C1 from the given lowest "ideal" limit (298K) to (358.43+459.67)/1.8 K (our ideal in kelvin) and then convert to Btu/lbm H2O?

Anonymous said...

I second that question, and if we do use the Cp, what units does the integration from the table in the back of the textbook yield? I'm tempted to think kj/kg but I'd like to be sure.

Anonymous said...

ahayles 3:44 PM
Do not use the heat capacity in this problem. Use the steam tables instead. This makes it easy to lookup the actual values of V, H & S at 150 psia and Tsat. Use the IG EOS to calculate V{IG} at 150 psia and Tsat. For H, we can estimate H{IG,Tsat,150 psia} = H{IG,Tsat, 1 psia} because H is not a fxn of P for an IG. The cool part is that at 1 psia and Tsat steam is very nearly an idea gas AND we can lookup the value of H{Tsat, 1 psia} in the steam tables.

Be careful with S ! Entropy depends on P even for an ideal gas ! Lookup S{IG,Tsat, 1 psia} ~ S{Tsat, 1 psia} in the superheated steam tables. Then, apply Eqn 5.14 to an IG undergoing a process from {Tsat, 1 psia} to {Tsat, 150 psia} to determine S{IG,Tsat, 150 psia}. Use this result to calculate S{R,150 psia, Tsat}.

Anonymous said...

spoonman 7:26 PM
1- Don't use Cp in this problem.
2- The equations for Cp in the back of the book take the form: Cp/R = polynomial. Cp/R is dimensionless. So, the polynomial can be applied to any system of units you like by choosing an appropriate value of R. For example, using R = 8.314 J/mole-K gives you Cp in J/mole-K. This is a kind of cool way to make the Cp polynomial equations a bit more flexible.

Anonymous said...

so i got parts a,b,c,d but i have a question on part e. in the hints it said to use SRK instead of the generalized corralations. But i seem to remember that you said in lecture that SRK wasn't good for water. I could be remembering it wouldn't be the first time. But if SRK isn't a very good approx for water then won't this give us a big error when calc the Vr at 150 psi. Not really a question about how to do the calculations i can do that i was just wondering if it was very accurate or not?

Dr. B said...

Brendan 9:38 PM,
You are correct. Use SRK like you might have done in class on Tuesday. You are also correct that it will not work well for water. That is kind of the point of this problem.

I am glad you are thinking when you are working these problems. You will learn more that way.

Anonymous said...

I am working through the SRK equation to find Z from the equation that is on page 3-1 on the lecture notes. I have found a Z that is the same as the one you listed in the homework problem ~.949. I'm running into problems when I get to solving for S(r) and H(r) from page 3-5,6 of the notes. When I solved the S equation using A,B,(alpha), m, and z, I get -Z back for S. When I solve the next equation for H(r) it gives me back the same H I found in part c for H(IG).

I've checked my equations and can't figure out where I'm going wrong. Please help.

Anonymous said...

confused 1:35 PM
I cannot tell what the problem is from what you have given me. Give me numbers and units. Once you have Z correct, calculating Hr and Sr is plug-and-chug using the eqns in the class notes. This is just like in the problem you almost did in class on Tuesday. Blog again and perhaps I can help. Otherwise, come to office hours tomorrow and we will figure it out together.