Steam Power Plant Reheat Efficiency Calculation

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Discussion Overview

The discussion revolves around the calculation of reheat efficiency in a steam power plant, focusing on the application of thermodynamic principles and equations related to heat transfer and work output. Participants are attempting to solve a homework problem involving enthalpy values and efficiency calculations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • The initial post presents a calculation attempt for the net work and efficiency of a steam power plant, including specific enthalpy values derived from a T-S diagram.
  • One participant suggests that the enthalpy value for h3 may be incorrect, proposing it should be equal to h2 plus the heat given up by the high-pressure stream.
  • Another participant questions the method for calculating the heat given up, proposing the use of the formula Q=mcT.
  • A response confirms that the heat can also be obtained from steam tables as a change in enthalpy (Δh), suggesting both methods should yield similar results.
  • A later reply indicates that a correction was made to the schematic diagram, leading to successful recalculations of the answers.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of specific enthalpy values and the methods for calculating heat transfer. There is no consensus on the initial calculations, but later adjustments seem to lead to agreement on the revised answers.

Contextual Notes

Participants noted potential errors in the schematic diagram and the need for adjustments in their calculations. The discussion includes unresolved assumptions regarding the accuracy of enthalpy values and the formulas used for calculations.

Who May Find This Useful

Students and practitioners interested in thermodynamics, particularly in the context of steam power plants and efficiency calculations.

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Homework Statement


vuKJEna.png


Homework Equations


Heat added, Qa (This I'm not sure how since the steam passed through a reheater before it enters the HPT. That is what I understand based on the problem above)
Work Net, Wnet
Efficiency, Wnet/Qa
Heat Rate, WP/EP (Not sure if that is the correct formula)

The Attempt at a Solution


First I made a T-S Diagram
w4NX6Ax.jpg


Based on the diagram above, I got all these enthalpy values.
OZzSVlg.png

h1 = h @ 3 MPa & 370C
h2 = h @ 0.7 MPa & s1
h3 = h @ 0.7 MPa & 500C
h4 = hf4 +x hfg4 (All values, 0.007 MPa)
where x = (s4 - sf4)/sfg4
h5 = hf @ 0.007 MPa
h6 = vf5 (P6-P5)
WP1 = h6-h5
ha = h @ 3 MPa and 500C

The WNet that I got is, 1485.491
For the Qa = (ha-h6) + (h3-h2) = 3959.022

Eff = .3752

Was not able to solve Heat Rate.

-------------
I can't seem to understand how the answers shown in the problem was got. Please enlighten me. Thanks in advance!
 
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It looks to me like h3 is wrong. It should be equal to h2 plus the heat given up by the high pressure stream between 500 C and 370 C. So, it should be about 3100 J/kg.
 
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Thank you for your reply.
Should I get the heat given up by using this formula?
Q=mcT
 
Sent said:
Thank you for your reply.
Should I get the heat given up by using this formula?
Q=mcT
Sure. Or you can get it from the steam tables as the ##\Delta h##. Both should give about the same answer.
 
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Chestermiller said:
Sure. Or you can get it from the steam tables as the ##\Delta h##. Both should give about the same answer.
Hello,

We was informed that the label in the schematic diagram that was given to us was wrong. I adjusted my TS diagram based on the correction and tried to solve it again. Thankfully I got the answers now.

Thank you very much for your replies.
 

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