Rankine cycle at various steam pressure and temperature
with reheating
this note calculates plot of η vs steam plant pressure with fixed condenser pressure (Temperature) and max
temperature with reheating to saturation 460 deg C and 560 deg C. steam is extracted at 400kPa from hp turbine for
reheating to boiler outlet temperature.
1 - vacuum; saturated liquid T=40 C
3
hp
Turbine
Boiler
2 - sub cooled liquid at boiler
pressure P=3, 6, 9, 12 MPa
Wt1
4
Q_dotH
Wt2
lp
Turbine
3 - saturated vapor, superheated
vapor T=460 C, T=560 C, p = p 2
5
6
2
Feed
Pump
4 - extraction steam; superheated
vapor @ 400 kPa or ... vapor + liquid
@ saturation temperature and
pressure tbd
Condenser
Q_dotL
1
5 - superheated vapor @ saturation
temperasature for p 2 , T=460 C,
T=560 C, p = p4 = 400 kPa
Wfp
6 - vapor + liquid @ saturation
temperature (40 C) and pressure or
...superheated vapor @ saturation
pressure for 40 C; tbd
the results are shown below
in the details of calculation for state 4 - extraction steam; superheated vapor @ 400 kPa or ... vapor + liquid @
saturation temperature and pressure tbd - it can be seen that combinations of
p=3 MPa, T = 460 C
p=3 MPa, T = 560 C and ....
p=6 MPa, T = 460 C
result in superheated vapor extraction (x 6 calculated >1) while the other combinations result in x 6 < 1
also ... state 6 - lp turbine exhaust - is superheated vapor @ saturation pressure for 40 C when boiler temperature is
560 C
The approach will be as the regenerative example; calculate x; if > 1 use super heat interpolation for results.
calculations
10/26/2005
1
0.45
0.44
0.42
thermal efficiency
0.41
0.39
0.38
0.36
0.35
0.33
0.32
0.3
30
39
48
57
66
75
84
pressure bars
93
102
111
120
with reheat - saturated
with reheat - 460 deg C
with reheat - 560 deg C
no reheat - saturated
no reheat - 460 deg C
no reheat - 560 deg C
N.B. efficiency decreases at saturation reheat, and at 460 C reheat at 12 MPa. This is observable when looking at
the T - s plot below for saturation temperature. The effect is noticeable at all pressures, but most significant at 12
MPa. Recall the entropy average temperature concept and observe that the temperature where heat is being added
is lowered.
The benefit from reheat is the exhaust moisture in the turbine output (in the latter stages of the single turbine)
quality out of hp turbine
 0.8615

0.8034
x4 = 
 0.7619
 0.7258

1.0427 1.0989 
0.972 1.0315 
0.9267 0.9899 
0.8913 0.9586 
quality out of lp turbine
 0.8773

0.8985
x6 = 
 0.9117
 0.9214

0.9767 1.0124 
0.9767 1.0124 
0.9767 1.0124 
0.9767 1.0124 
quality out of pressure temperature file
 0.731

0.692
x=
 0.664
 0.64

N.B. values of x > 1 are bogus and actual x = 1, i.e. steam is superheated vapor
data for saturation curve
10/26/2005
2
0.851 0.889 
0.804 0.844 
0.774 0.816 
0.75
0.795 
select_pressure :=
this plot for
ip := select_pressure − 1
p2
select_temperature :=
30
60
90
120
ip
= 12 MPa
saturated
superheat to 460 deg C
superheat to 560 deg C
iT := select_temperature − 1
TT3
data for T s and H s plots
T-s Plot
400
T degrees C
300
200
100
0
0
1
2
3
4
5
6
s entropy kJ/(kg*K)
7
8
9
10
h-s Plot
4000
h enthalpy kJ/kg
3000
2000
1000
0
10/26/2005
0
2
4
6
s entropy kJ/(kg*K)
3
8
10
ip , iT
= 324.75