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.

4.0Body 4.1Introduction 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). 4.2Code 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. 4.3Results 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 2¹. During the reheat the pressure remains constant but the entropy increases to point 3¹. 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...