MEM 310 Design Project Assignment
Prepared by
Bradley R. Schaffer
Drexel University
Philadelphia, PA 19104
Submitted to:
Dr. William J. Danley of MEM 310 - Thermodynamic Analysis I
on
May 28, 2004
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Study Guide - Thermodynamic Analysis I | MEM 310, Papers of Mechanical Engineering
Material Type: Paper; Class: Thermodynamic Analysis I; Subject: Mechanical Engr & Mechanics; University: Drexel University; Term: Spring 2004;
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MEM 310 Design Project Assignment
Prepared by
Bradley R. Schaffer
Drexel University
Philadelphia, PA 19104
Submitted to:
Dr. William J. Danley of MEM 310 - Thermodynamic Analysis I
on
May 28, 2004
Abstract
With todayís soaring energy demands and continually increasing fuel costs, it is detrimental to a company to overlook opportunities that would increase their power plantís efficiency. Companies that have high efficiencies in their plants are given an upper hand in the market. Companies that donít utilize the latest advancements in power generation technology are jeopardizing their economic stability. Weíve studied and simulated your system using our proprietary software that utilizes the renowned Danley transfer functions and have concluded that the highest efficiency obtainable is 41.06%. This was the optimal efficiency value that met all of your companyís specifications. In addition to testing the system that you specified, we also did further research and analysis on improving this efficiency. These improvement options range from simple additions to your current cycle to scaling up projects to meet future demands greater than 550 megawatts. The end result of these improvements has the potential to raise the overall efficiency of your plant above 50%.
- AbstractÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ
- System Problem StatementÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- SolutionÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ
- Discussion and ConclusionÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- Recommendations for Future AnalysisÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- ReferencesÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- Appendix A: Steam Cycle DevelopmentÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- Appendix B: Rankin Cycle Flow DiagramÖ.ÖÖ.ÖÖÖÖÖÖÖÖÖÖÖÖÖÖ.
- Appendix C: Additional Feedwater Heater Flow DiagramÖÖÖÖÖÖÖÖÖÖÖ..
- Appendix D: Combined Cycle Flow DiagramÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- Appendix E: Thermal Efficiency Increases vs. YearÖÖÖÖÖÖÖÖÖÖÖÖÖÖ
- Appendix F: Transfer Function Links and RelationsÖÖÖÖÖÖÖÖÖÖÖÖÖÖ
- Appendix G: Detailed Hand CalculationsÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ.
- Appendix H: Temperature ñ Entropy DiagramÖ..ÖÖÖÖÖÖÖÖÖÖÖÖÖÖ..
- Table 1: Design SpecificationsÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ......................... Table of Figures
- Table 2: Optimized Design SpecificationsÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ...
- Table 3: Transfer Function SpecificationsÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ
- Table 4: Rankine Cycle Component Mass Flow RatesÖÖÖÖÖÖÖÖÖÖÖÖÖ
- Table 5: Rankine Cycle State Temperatures & PressuresÖÖÖÖÖÖ...ÖÖ...ÖÖ..
- Table 6: Transfer Function ResultsÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ...
- Table 7: Tabulated Additional Feedwater Heater CalculationsÖÖÖ.ÖÖÖÖÖÖ...
System Problem Statement
The objective of this project was to produce a 550 megawatt vapor power plant that meets certain criteria yet maintains a high efficiency. To begin this problem, a complete analysis of the Rankine cycle must be completed. The simplest form of this cycle consists of an isentropic pump, an isobaric boiler, an isentropic turbine, and an isobaric condenser. The cycle analysis starts with the condenser. The condenser is a heat exchanger that acquires exhaust steam from the turbine and then removes heat from the exhaust until it becomes a saturated liquid. This saturated liquid is then sent to the pump. The pump then pressurizes the saturated liquid up to the turbine inlet pressure. After the liquid leaves the pump, it is routed through a boiler which adds heat to the liquid, converting it into a superheated vapor. At the final stage, the superheated vapor is sent through the turbine which internally expands the steam and in return, the output shaft of the turbine rotates. This mechanical energy is then used to turn the input shaft of a generator, thereby producing electricity. This is the simplest form of the Rankine cycle. This cycle will meet the output requirement. However, it doesnít meet the desired efficiency. The first attempt at increasing efficiency was seen in the early 1920ís by implementing regeneration by the use of feedwater heaters (See Appendix A). Feedwater heaters are heat exchanges that use superheated steam bled from the turbine to heat the feedwater before it enters the boiler. This increases the average temperature of heat addition which increases the overall efficiency of the cycle (Cegel, 522). Feedwater heaters come in two types: open and closed. The open feedwater heater mixes the superheated steam directly with the
Solution
Once the transfer functions were properly connected, any changes made to the variables instantly showed changes in both efficiency and the quality of the low pressure turbine exhaust (See Appendix F). Knowing that the minimum quality was 98.5%, variables could be changed one at a time until the desired quality was obtained. The first variable that was experimented with was the maximum temperature. When the maximum temperature was decreased, the efficiency dropped substantially but no effect on quality was seen. The next variable that was modified was the condenser temperature. The condenser temperature had to be increased to 94 oC in order to obtain acceptable quality levels. This reduced efficiency by 5.01%. After the condenser temperature was tested, changes in maximum pressure were explored. This produced the same results that were seen when the maximum temperature was decreased. Efficiency was decreased but no effect on quality was seen. The maximum reheat temperature was the next variable that was modified. As this temperature increased so did the quality of the steam. The final variable that was manipulated was the temperature of the open feed water heater exit. When temperature was decreased, quality increased sharply yet efficiency only fell slightly. After experimenting with modifying individual variables, combinations of variables were manipulated. The only combination that had the desired outcome of higher quality was the minimum temperature and the open feedwater heater exit temperature. The best combination that met all specifications was a lowered open feedwater heater temperature of 165 o^ C and an increased condenser temperature of 60.5oC (See Table 2).
The combination of lowered feedwater heater temperature and increased condenser temperature produced a quality of 98.53%, and an efficiency of 41.06%. This efficiency could have been increased further if the open feedwater heater temperature had been lowered below 165 o^ C and the condenser temperature kept at 55 oC. However, this wasnít possible because the transfer function specified a minimum mid-temperature of 165 oC (See Table 3). Therefore, the condenser temperature had to be increased to account for the final increase in quality.
decreased because the average low temperature increased. However, the quality increased due to the fact that as the condenser temperature rises, the decrease in s (^) fg is greater than the increase in s (^) f. This produces an overall increase in the denominator that is greater than the decrease in the numerator of the quality equation, thereby producing a higher quality. The third variable that was modified was the reheat temperature. As this temperature decreased, both the efficiency and the quality also decreased. The reheat cycle allows higher boiler pressures without causing moisture problems in the low pressure turbine. The reheat cycle reheats the high pressure turbine exhaust before it enters the low pressure turbine. This increase in temperature increased the entropy of the superheated steam. The efficiency also increases due to the fact that as the maximum pressure increases, the temperature of liquid entering the boiler increases which means a higher average temperature of heat addition. The final variable that was modified was the open feedwater heater exit temperature. As this temperature decreased, efficiency also dropped, but quality went up. The efficiency declined due to the fact that as this temperature decreases, the feedwater entering the boiler decreases as well which produces a lower average temperature of heat addition in the boiler. However, the quality of the steam rises because as the temperature of the feedwater heater decreases, the pressure of the steam entering the reheat cycle decreases. When steam is heated to a specified temperature, the entropy of the superheated vapor after the reheat process is inversely proportional to its pressure. In conclusion, there are only three variables that affect the quality of the low pressure turbine exhaust. The first variable-- reheat temperature-- affects quality and
efficiency in an adverse way. The only two variables that will increase quality are condenser temperature and open feedwater heater temperature. Although modifying these two variables lowered efficiency, they were modified in such a way that limited this decrease. This systemís efficiency was only reduced by 1.91%. This decrease in efficiency can be recouped along with a gain in overall efficiency through the use of additional components in the cycle, namely multiple feedwater heaters.
continually decreases. A cost-benefit analysis must be done to decide how many feedwater heaters will optimize the cycle. Some of todayís larger power generation plants use up to eight feedwater heaters (Cengel, 531). A second improvement to the cycle would be a larger scaling-up operation. If the demand exceeds the wattage that the original cycle was designed to supply, a second generation system may need to be implemented to increase the overall net output of the power plant. A combined cycle, which is a combination of the Rankine cycle and the Brayton cycle is a valuable cycle to consider (See Appendix D). This cycle can produce efficiencies in the 50 percent range (See Appendix E). This is a greater efficiency than either cycle could obtain individually (Siemens).
References
- Cengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. New York: Jack P. Holman, 2002.
- Kutz, Myer. Mechanical Engineers' Handbook (2nd Edition). New York: 1998
- ìCombined Cycle Plant Ratings,î [Internet]. Siemens. (2004 [cited 20 May 2004]); available from
Table 2
Optimized Design Specifications
Optimized Specifications
Maximum feedwater heater temperature 165 oC Maximum pressure entering the High pressure turbine 20 MPa Minimum condenser temperature 60.5 oC High pressure turbine adiabatic efficiency 89 % Low pressure turbine adiabatic efficiency 93 % Low pressure pump adiabatic efficiency 87 % High pressure pump adiabatic efficiency 89 % Minimum steam quality entering the condenser 98.5 % Maximum steam temperature 600 oC Maximum steam temperature exiting reheater 460 oC
Table 3
Transfer Function Specifications
Transfer Function Specifications
Low 0-100 oC Mid 165-260 oC Reheat 440-500 oC High 575-625 oC
Table 5
Rankine Cycle State Temperatures & Pressures
State Pressure & Temperature State Pressure (Mpa) Temperature ( oC) 1 .02041 60. 2 .70029 60. 3 .70029 165 4 20 177. 5 20 600 6 .70029 165 7 .70029 460 8 .02041 60.
Table 6
Transfer Function Results
State qin Pump win Turbine wout 4 to 56 to 7 2816.45 KJ/Kg620.64 KJ/Kg HPLP 0.7922 KJ/Kg24.03 KJ/Kg HPLP 762.56 KJ/Kg762.53 KJ/Kg
qin 3327. qnthout 1961.6260. wy net 1366.3370. x 8 0.
State 1 20.4068P (kPa) T (C)60.5 253.22hf hfg (^) 0.837423sf sfg (^) 0.001018v (^) f x 0 hs (^) 253.22ha (^) 0.837423sa 23 700.29700.29 (^165) 0.001108 0 253.91 254.0122697.28 0. 45 2000020000 600 718.6669 721.30963537.755 6. 67 700.29700.29 460 2680.95 2775.1993395.836 6.5047877. 8 20.4068 60.5 253.22 2357.272 0.837423 7.064151 0.985 2575.914 2633.309 7.