Analysis of A Vapor Power Plant

8/20/96
ME1361 T
hermo II

3.0 Abstract

The objective of this study is to construct a computer model of a water vapor
power plant. This model will be used to calculate the state properties at all
points within the cycle. Included is an analysis of the ideal extraction
pressures based on the calculated values of net work, energy input, thermal
efficiency, moisture content, and effectiveness.

4.0 Body 4.1 Introduction System to be Analyzed Steam enters the
first turbine stage at 120 bar, 520 °C and expands in three stages to the
condenser pressure of .06 bar. Between the first and second stage, some steam is
diverted to a closed feedwater heater at P1, with saturated liquid condensate
being pumped ahead into the boiler feedwater line. The Terminal Temperature
Difference of the feedwater heater is 5°C. The rest of the steam is reheated to
500°C, and then enters the second stage of expansion. Part of the steam is
extracted between the second and third stages at P2 and fed into an open
feedwater heater operating at that pressure. Saturated liquid at P2 leaves the
open feedwater heater. The efficiencies of all pumps are 80%, and the
efficiencies of all turbines are 85%.

Throughout this report the states will be referenced as depicted above with the
numbers 1-13. The analysis of the system will involve the use of the Energy Rate
Balance to isolate the specific enthalpies and associated values of temperature,
pressure, specific volume, and steam quality. The Entropy balance equation will
be used to calculate the specific entropy at all the above noted states. Energy
Rate Balance (assume KE&PE=0) dEcv/dt = Qcv-Wcv+Smi(hi) - Sme(he)

Entropy Rate Balance dScv/dt = SQj/Tj + Smi(si) - Sme(se) + scv
For simplicity, it is assumed in all calculations that kinetic and potential
energy have a negligible effect. It is also assumed that each component in the
cycle is analyzed as a control volume at steady state; and that each control
volume suffers from no stray heat transfer from any component to its
surroundings. The steam quality at the turbine exits will also be constrained
to values greater than or equal to 90% (Moran, 337). 4.2 Code Development
The C program "finalproject.c² was developed to calculate the state values given
the constraints listed in section 4.1. The program structure consists of three
parts: Header/variable declaration Calculation section Data Report section The
Header section includes all the variable declarations, functions to include and
system definitions. To obtain accurate data values, this program uses floating
point values. The Calculation section is the function that is used to calculate
all the state values. In essence this section consists of two nested while()
loops that are used to vary the extraction pressures from 12000 kPa to 300 kPa.
The while loops are set to terminate when the steam quality becomes less than
90% as defined in the constraints in 4.1. The Data Reporting section is found
within the nested while() loops and are used to report the values found in the
preceding Calculation section. 4.3 Results and Discussion T-s diagram

The T-s diagram above shows how specific entropy changes related to temperature.
At State 1 the water vapor has just left the boiler and is superheated. It then
undergoes an expansion through the turbine. Since the efficiency of the turbine
is not 100% the entropy increases and is denoted by the point labeled Œ2¹.
During the reheat the pressure remains constant but the entropy increases to
point Œ3¹. Then another two expansions occurs and the fluid reaches state 4 and
5 respectively. The fluid then condenses at constant pressure to a saturated
liquid at state 6. The working fluid then enters a pump of efficiency 80% to
state 7. The fluid is then heated in an open feedwater heater at constant
pressure until it is a saturated liquid at state 8. The fluid is then sent
through a pump to a pressure equal to that at state 1 at point Œ9¹. The closed
feedwater heater, heats the fluid at constant pressure to state 10; and then is
heated again by the mixing with the feedwater at state 13. The fluid is then
heated back to point Œ1¹ in the boiler. To find the optimum extraction pressure
it is necessary to analyze the net work, energy input, thermal efficiency and
moisture content at various extraction pressures. These results can be found in
the appendix and graphed on the following pages. In "Plot of Net Work as a
Function of Extraction Pressure P2,P4"" it is evident that net work is maximized
when extraction pressure, P2 is 300 kPa and pressure P4 is