I am using XSteam IF97 to evaluate steam power plant cycles.

However, I miss some important input options such as pressure as a function of entropy and temperature ("p_st()") - when I assume isobaric heat addition (neglecting pressure any pressure losses) while reheating the steam. I end up looking at i-s diagram which is highly inefficient while optimizing cycle for different inputs.

Is there any way around or any more sophisticated software (in the best scenario, available for MS Excel). Or how do you work with steam power plant cycles? (I intend to avoid any commercial software)

  • $\begingroup$ What is an i-s diagram? Anyway, try using a p-H diagram. $\endgroup$ – Chet Miller Nov 13 '20 at 12:43
  • $\begingroup$ @ChetMiller Enthalpy–entropy diagram (rather h-s, in my language it's also called i-s). However, my point is not to use any diagrams at all, I would like to obtain the pressure with XSteam or another software, because there is no point computing the whole cycle in Excel for fast parameters switching, while I would have to get one or two values manually. $\endgroup$ – Josh E. Nov 13 '20 at 20:42
  • $\begingroup$ Aren't there any other functions in if97 that are more suited for looking at this than temperature-entropy., like one where you input temperature and pressure and output enthalpy? $\endgroup$ – Chet Miller Nov 13 '20 at 21:31
  • $\begingroup$ @ChetMiller The situation is: At a certain point, the pressure is unknown to me (for this point I also don't know enthalphy, I know only entropy). However, I can get to this point by following isobare from the point "above" (with a higher temperature), hence I need to obtain isobare based on entropy and temperature. $\endgroup$ – Josh E. Nov 14 '20 at 9:36
  • $\begingroup$ I'm confused. If you know the pressure at the point "above," and it an isobar, then you know the pressure at the point in question. What am I missing? $\endgroup$ – Chet Miller Nov 14 '20 at 12:01

If you have an initial estimate for the pressure, $p_0$, then you can quickly solve iteritively for the pressure that matches the desired entropy $s=s^*$ using modified Newton's method: $$p_{n+1}=p_n-\frac{s(p_n,T)-s*}{\left(\frac{\partial s}{\partial p}\right)_T}$$The partial derivative of s with respect to p can be obtained by finite difference at the initial guess for p.


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