Monday, May 14, 2007
HW #8, P1 - Back-Work Ratio of a Steam Power Cycle - 4 pts
Consider a steam power plant that operates between the pressure limits of 10 MPa and 20 kPa. Steam enters the pump as a saturated liquid and leaves the turbine as a saturated vapor. Determine the back work ratio (BWR is the ratio of the work delivered by the turbine to the work consumed by the pump). Assume the entire cycle to be reversible and the heat losses from the pump and the turbine to be negligible.
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7 comments:
I can find the properties of states 1,3, and 4 easily, but when I attempt to look up state two in the superheated steam tables using the fact that S2=S3, I find that S3 is, according to my definition, off the charts. I am also using the fact that S1=S4 to find the properties of the steam at state 1. Am I doing something wrong? or is the temperature at state 2 really so high that it is off the tables?
Giro, I had the same problem. You can use NIST webbook. Keep it the same conditions and pick from a really large range and look at the S values. Then you can narrow down until you find ~approximately the same value.
But doing this...I get a BWR of ~206-207
Giro & Parwiz:
Make yourself a TS Diagram and it will be very clear to you that stream 2 is a subcooled liquid ! You can readily interpolate on the subcooled liquid table for 10 MPa using S2 = S1. Or, of course you can use the webbook. But it will be easiest if you use the TFT Excel plug-in ! Try it.
My BWR is about 30 off from the posted answer.
I used all NIST values, single interpolation for the superheated enthalpy, and for the subcooled state enthalpy, isobaric properties at 8MPa w/ temperature ranging in very small increments from 335.5-333.6 K (after narrowing it down from a larger T range using the entropy for that state).
Is the answer that is posted using different tables/reference states or is there something wrong in my solution?
can you explain how to start this problem am lost. and i seem for some reason not being able to install the FTF Excel plug-in
Maria M:
30 off is a problem when it is out of 242 or 252.
I used the TFT plug-in and got BWR = 252 and I used the tables in the back of the book and got 242.
I doubt NIST could yield results that differ from the book by 30 since I got the data in the book from NIST !
It is hard for me to tell for sure, because small differences in the enthalpy of the subcooled liquid can make a big difference in the BWR. But I just calculate Wpump using NIST and it was less than 0.5% different from the value I got using interpolation on the values in the book. So, I think you did something wrong. Maybe Ben can help you in office hours later today. Or you can try blogging again.
Help:
You do not need the plug-in. Use NIST or the tables in the book.
Apply the 1st Law to each process in the cycle. Use the tables/NIST/TFT to get properties, especially H & S. The pump and turbine are isentropic. This lets you determine H for the streams coming out of them. Calculate W for each and the ratio is the BWR.
Come to class.
Best of luck !
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