Sonntag, Borgnakke and Wylen
9.138E A steam turbine inlet is at 200 psia, 900 F. The exit is at 40 psia. What is the lowest possible exit temperature? Which efficiency does that correspond to? We would expect the lowest possible exit temperature when the maximum amount of work is taken out. This happens in a reversible process so if we assume it is adiabatic this becomes an isentropic process. Exit: 40 psia, s = sin = 1.8055 Btu/lbm R ⇒ T = 483.7 F The efficiency from Eq.9.27 measures the turbine relative to an isentropic turbine, so the efficiency will be 100%. 9.139E A steam turbine inlet is at 200 psia, 900 F. The exit is at 40 psia. What is the highest possible exit temperature? Which efficiency does that correspond to? The highest possible exit temperature would be if we did not get any work out, i.e. the turbine broke down. Now we have a throttle process with constant h assuming we do not have a significant exit velocity. Exit: 40 psia, h = hin = 1477.04 Btu/lbm ⇒ T = 889 F w η=w =0
Efficiency:
s
T
P
i
i
h=C e
e v
s
Remark: Since process is irreversible there is no area under curve in T-s diagram that correspond to a q, nor is there any area in the P-v diagram corresponding to a shaft work.