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Okay: I have steam leaving a boiler at T = 873 K and P = 15 MPa and entering a turbine which it leaves at a pressure 2.1 MPa (these data are given), and I want to determine the other thermodynamic properties at this second point. Finding the enthalpy should be straightforward because I know that the isentropic efficiency of the turbine is 0.91. I want to start of course with s2 isentropic = s1 and work back from there.

Now my problem is this: I can't cross-tabulate the isentropic entropy with this pressure in the steam tables--there's no point in the steam tables where this entropy exists at that pressure--so I went instead to the tables for saturated water. Starting at P2 = 2.1 MPa, I then found the "quality" (or "vapour fraction") by applying the lever rule. When applying the lever rule, I can find results which are fairly close (like about 0.5% away from the printed result) from those the lecturer provided, but I am a bit skeptical because maybe it's some sort of coincidence?

My question is: when can I move to using the saturated water tables instead of the steam tables? Is there some kind of rule I can use to do this? It seems almost arbitrary but it has some important repercussions.

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  • $\begingroup$ Don't your steam tables also have tables for saturated water/steam, including liquid and vapor enthalpies, internal energies, and entropies? $\endgroup$ – Chet Miller Nov 23 '18 at 3:12
  • $\begingroup$ Yes, they do, I'm just wondering if there is a rule of thumb to know when to go to steam tables and when to go to saturated water tables. $\endgroup$ – Abed Nov 23 '18 at 9:02
  • $\begingroup$ I don't think so. I think you have just got to check to see whether the entropy at the final pressure is going to be between that of the saturated liquid and the saturated vapor. The values in the supersaturated stream tables should tell you that, for the final entropy needed, you are going to be below that of the saturated vapor. $\endgroup$ – Chet Miller Nov 23 '18 at 12:41
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..there's no point in the steam tables where this entropy exists at that pressure-

You need a complete superheat steam table (many stop at pressures well below 15 MPa). Check Ohio.edu. You will find the entropy for 600 C, 15 MPa, is 6.680.

You will also find that the steam exiting the turbine also appears to be superheat. You can find the entropy there as well (you will need to extrapolate). From there, knowing the isentropic efficiency, you should be able to estimate the other properties.

Hope this helps.

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  • $\begingroup$ You shouldn't assume that saturated steam exits the turbine. Water droplets on the turbine blades lead to erosion, so turbines are designed to avoid saturated conditions. $\endgroup$ – David White Nov 23 '18 at 17:06
  • $\begingroup$ @DavidWhite I said superheat leaves the turbine, not saturated steam. Please re-read. I looked it up in tables. $\endgroup$ – Bob D Nov 23 '18 at 17:36
  • $\begingroup$ My comment was info for the OP. I probably should have posted higher on the page. $\endgroup$ – David White Nov 23 '18 at 19:14
  • $\begingroup$ @DavidWhite Sorry David, I didn't look at his entire post. I see now he is referring to quality. If he looks at the saturated steam table for 1.2 MPa he will see that the entropy for sat vapor is less that 6.50 and realize he should be looking at the superheat steam table. $\endgroup$ – Bob D Nov 23 '18 at 21:34

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