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Example 92
Saturated water at 15 kPa enters the pump in an ideal
reheat Rankine cycle. It is pumped to 25 MPa and heated
in a boiler to 700°C. After expansion through the first
stage turbine, the steam pressure is reduced to 6MPa.
The steam is reheated to 700°C and expanded in the
secondstage turbine. If the power output desired is 400
MW, find the mass flow rate. Estimate the cycle efficiency
assuming pump work is negligible.
Solution:
Define a control volume around the boiler and reheater. The
first law for this control volume is
ee
ii
QW
m
h
m
h
−=
−
∑
∑
±
±
±±
ei
mmm
==
±±±
Since there is no work and the total heat added to the cycle
is the sum of the heat in the boiler and reheater
()
3524
H
Qm
h
h
h
h
=+
−
−
±
±
To find
h
2
, we will have to work our way across the pump. At
the pump inlet
1
kJ
enthalpy(steam,
0,
15)
226
kg
hx
P
=
=
( )
21
P
Wm
h
h
m
v
P
P
=−
=
−
±
3
m
volume(steam,
0,
15)
0.001014
kg
vx
P
=
=
hh
v
P
P
−
3
2
kJ
m
226
0.001014
25000 15 kPa
kg
kg
h
⎛⎞
−
⎜⎟
⎝⎠
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kJ
251.2
kg
h
=
3
kJ
enthalpy(steam,
700,
25000)
3773
kg
hT
P
==
=
=
To find
h
4
, consider the second law for the first stage turbine
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 Spring '08
 Kaminski

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