上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 41-42 Chapter 10 Vapor and Combined Power Systems Spring,5/9/2018 通 Prof.,Dr.Yonghua HUANG VALLAMAAA http://cc.sjtu.edu.cn/G2S/site/thermo.html SHANG 1日 ERSITY
Engineering Thermodynamics I Lecture 41‐42 Spring, 5/9/2018 Prof., Dr. Yonghua HUANG Chapter 10 Vapor and Combined Power Systems http://cc.sjtu.edu.cn/G2S/site/thermo.html
Rankine Cycle Improvements Basic Rankine Cycle: T IDEAL CYCLE Turbine 3 Irreversibility in the pump Pressure drop in the boiler 3 Boiler O out Irreversibility Cooling water in the turbine Pump- 2 Condenser ACTUAL CYCLE )4 Pressure drop in the condenser 上游充通大率 May9,2018 2 SHANGHA BAO TONG LINIERSITY
May 9, 2018 2 Rankine Cycle Improvements Basic Rankine Cycle: 1 2 3 4
Rankine Cycle Performance Trends Effect of superheat on performance: ·Purpose: Increase average temperature for heat addition at a given boiler pressure T increase in performance Increase in Wnet superheat T3 is limited by TH and materials Question: Why not increase Tg instead? Answer: Need high steam quality at turbine outlet to avoid corrosion (want x4 0.9,better >0.95) -Would require higher boiler pressure materials,safety 上游充通大￥ May9,2018 3 SHANGHA BAO TONG LINIERSITY
May 9, 2018 3 Rankine Cycle Performance Trends Effect of superheat on performance: • Purpose: Increase average temperature for heat addition at a given boiler pressure increase in performance T 3 is limited by T H and materials Question: Why not increase T B instead? Answer: ‐ Need high steam quality at turbine outlet to avoid corrosion (want x 4 > 0.9, better > 0.95) ‐ Would require higher boiler pressure materials, safety T B superheat
Continue Rankine Cycle Performance Trends Continue effect of superheat on performance: Consequence of superheating: △Tsup〔C) n X4 0 0.186 0.858 54 0.190 0.913 278 0.217 -(superheated) Note: ·Increasing△T'sup does not have big effect on thermal efficiency,but is needed to increase x4 toa“practical'"value！ 上游充通大粤 May9,2018 4 HANGHAI HAO TONG LINIVERSITY
May 9, 2018 4 Continue Rankine Cycle Performance Trends Continue effect of superheat on performance: • Consequence of superheating: Tsup ( oC) x 4 0 0.186 0.858 54 0.190 0.913 278 0.217 ‐ (superheated) Note: • Increasing Tsup does not have big effect on thermal efficiency, but is needed to increase x 4 to a “practical” value!
Continue Rankine Cycle Performance Trends Effect of boiler pressure on performance: T Increasing boiler pressure Increase decreases heat input at in Wnet relatively constant work output Decrease >Increase in efficiency in Wnet Upper limit depends on heat supply temperature and materials 4' Possible? Also decreases quality S 上游充通大粤 May9,2018 5 SHANGHA BAO TONG LINIERSITY
May 9, 2018 5 Continue Rankine Cycle Performance Trends Effect of boiler pressure on performance: ‐ Increasing boiler pressure decreases heat input at relatively constant work output Increase in efficiency ‐Upper limit depends on heat supply temperature and materials ‐ Also decreases quality T B Possible?
Continue Rankine Cycle Performance Trends Continue effect of boiler pressure on performance: .Assume:TC=66C [pc=Psat(Tc)=0.255 bar] x4=0.95 [specifies T3] TB(C) PB(bar) T3(C) 7 232 29 400 0.246 288 72 506 0.290 343 153 604 0.324 Notes: T3 increases faster than TBoiling to maintain X4 maximum pressure dictated by temperature of heat source and materials . optimal pressure:trade-off between 1st costs and operating costs 上游充通大 May9,2018 6
May 9, 2018 6 Continue Rankine Cycle Performance Trends Continue effect of boiler pressure on performance: • Assume: TC = 66 oC [ p C = psat ( TC) = 0.255 bar] x 4 = 0.95 [specifies T3] TB ( oC) p B (bar) T3 ( oC) 232 29 400 0.246 288 72 506 0.290 343 153 604 0.324 Notes: • T3 increases faster than TBoiling to maintain x 4 • maximum pressure dictated by temperature of heat source and materials • optimal pressure: trade‐off between 1st costs and operating costs
Continue Rankine Cycle Performance Trends Effect of condensing pressure on performance: Lower condenser pressure increase work output at the same heat input >Increase in efficiency -Lower limit depends on environmental temperature P4<P Lowering pressure Increase in Wnet decreases quality →Must increase T3to keep quality constant S 上游充通大率 May9,2018 7 SHANGHA BAO TONG LINIERSITY
May 9, 2018 7 Continue Rankine Cycle Performance Trends Effect of condensing pressure on performance: ‐ Lower condenser pressure increase work output at the same heat input Increase in efficiency ‐Lower limit depends on environmental temperature ‐ Lowering pressure decreases quality Must increase T3 to keep quality constant
Continue Rankine Cycle Performance Trends Effect of condensation temperature,Tc,on performance: ·Assume:TB=172.3℃andx4=0.95 Tc(C) Pc(bar) T3(C) n 93.3 0.8 552 0.289 65.6 0.255 604 0.324 37.8 0.066 670 0.359 Notes: T3 must increase to maintain x4 T.is limited by sink temperature Summary: Want high T and low Te! Need superheat to get x4>0.9 Would rather increase Tp than T3 上游充通大率 May9,2018 8 HANGHAI HAO TONG LINIVERSITY
May 9, 2018 8 Continue Rankine Cycle Performance Trends Effect of condensation temperature, TC, on performance: • Assume: TB = 172.3 oC and x 4 = 0.95 TC ( oC) p C (bar) T3 ( oC) 93.3 0.8 552 0.289 65.6 0.255 604 0.324 37.8 0.066 670 0.359 Notes: • T3 must increase to maintain x 4 • Tc is limited by sink temperature Summary: • Want high Tb and low Tc! • Need superheat to get x 4 > 0.9 • Would rather increase Tb than T3
Continue Rankine Cycle Improvements -----Two-stage Cycle with Reheat Reheat section Advantages: .Higher quality (or superheated vapor)existing the turbine without Low-pressure turbine large superheat .For a given TH can increase T without reducing quality High- pressure turbine Steam generator Condenser 6 Pump 上游充通大￥ May9,2018 9 SHANGHA BAO TONG LINIERSITY
May 9, 2018 9 Continue Rankine Cycle Improvements ‐‐‐‐‐Two ‐stage Cycle with Reheat Advantages: •Higher quality (or superheated vapor) existing the turbine without large superheat •For a given TH can increase Tb without reducing quality
Continue Rankine Cycle Improvements Overall thermal efficiency: 1→23→4 5→6 W WrI WT2-Wp h et. QB+Qg 6→12→3 TA © Multi-staging with Reheat: Idea:Would rather raise Ts than Ts Consider limit of infinite-stages: 上游充通大率 May9,2018 10 SHANGHAI BAO TONG LINIERSITY
May 9, 2018 10 Continue Rankine Cycle Improvements Overall thermal efficiency: Multi‐staging with Reheat: • Idea: Would rather raise TB than T5 • Consider limit of infinite‐stages: T1 T 2 P net th in B R W WW W Q QQ 1 2 3 4 5 6 6 1 2 3 T H T L 5 T B
