上海交通大学：《热力学 Thermodynamics（I）》课程教学资源（课件讲义）Lecture 43_superheat and reaheat

上游充通大学 SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 43 Chapter 10 Vapor and Combined Power Systems Spring,2017 强 Prof.,Dr.Yonghua HUANG AMLMAM http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G

Engineering Thermodynamics I Lecture 43 Spring, 2017 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: IDEAL CYCLE Turbine Irreversibility WWWM 3 in the pump Pressure drop in the boiler 3 Boiler 2 out Irreversibility Cooling water in the turbine Pump Condenser ACTUAL CYCLE 2 >4 Pressure drop/ in the condenser 上游充通大 Wednesday,May 03,2017 2 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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 TB Question: Why not increase Te 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 上降文通大学 Wednesday,May 03,2017 3 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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 T3 is limited by TH and materials Question: Why not increase TB 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 TB superheat

Continue Rankine Cycle Performance Trends Continue effect of superheat on performance: Consequence of superheating: △Tsup〔C) 7 X4 0 0.186 0.858 54 0.190 0.913 278 0.217 -(superheated) Note: Increasing ATsup does not have big effect on thermal efficiency,but is needed to increase x4 toa“practical"value！ 上游充通大学 Wednesday,May 03,2017 4 SHANGHAI JLAO TONG UNIVERSITY

Wednesday, May 03, 2017 4 Continue Rankine Cycle Performance Trends Continue effect of superheat on performance: • Consequence of superheating: DTsup ( oC) h x4 0 0.186 0.858 54 0.190 0.913 278 0.217 - (superheated) Note: • Increasing DTsup does not have big effect on thermal efficiency, but is needed to increase x4 to a “practical” value!

Continue Rankine Cycle Performance Trends Effect of boiler pressure on performance: T Increasing boiler pressure Increase 3 3 decreases heat input at in Wnet max relatively constant work output Decrease >Increase in efficiency in Wnet Upper limit depends on heat supply temperature and materials \Possible? Also decreases quality S 上游充通大 Wednesday,May 03,2017 5 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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 TB 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) Pg (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 t to maintain X4 maximum pressure dictated by temperature of heat source and materials optimal pressure:trade-off between 1st costs and operating costs 上游充通大学 Wednesday,May 03,2017 6 HANGHAI JLAO TONG UNIVERSITY

Wednesday, May 03, 2017 6 Continue Rankine Cycle Performance Trends Continue effect of boiler pressure on performance: • Assume: TC = 66oC [pC = psat(TC ) = 0.255 bar] x4 = 0.95 [specifies T3 ] TB ( oC) pB (bar) T3 ( oC) h 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

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<P4 Lowering pressure Increase in Wnet decreases quality →Must increase T3to keep quality constant 上游充通大 Wednesday,May 03,2017 7 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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:TR 172.3C and x=0.95 Tc(C) Pc(bar) T3 (C) 7 93.31 0.8 552 0.289 65.61 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 T! Need superheat to get xa>0.9 Would rather increase T than T3 上游充通大 Wednesday,May 03,2017 8 SHANGHAI JLAO TONG UNIVERSITY

Wednesday, May 03, 2017 8 Continue Rankine Cycle Performance Trends Effect of condensation temperature, TC , on performance: • Assume: TB = 172.3oC and x4 = 0.95 TC ( oC) pC (bar) T3 ( oC) h 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 • Tc is limited by sink temperature Summary: • Want high Tb and low Tc ! • Need superheat to get x4 > 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 To without reducing quality High- pressure turbine Steam generator Condenser 6 Pump 4'4 5 上降文通大学 Wednesday,May 03,2017 9 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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→2 3→4 5→6 W net Wn+Wr2-Wp QB+QR 6→1 2→3 T --5-1 --Tavg.rcheat Multi-staging with Reheat: Idea:Would rather raise TB than Ts Consider limit of infinite-stages: 上游充通大学 Wednesday,May 03,2017 10 SHANGHAI JIAO TONG UNIVERSITY

Wednesday, May 03, 2017 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 T2 P net th in B R W W W W Q Q Q   h    12 34 56 61 23 TH TL 5 TB