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Example 62
One of the most efficient types of power plant in service
today is a Combined Cycle Power Plant, which includes
both a high temperature gas turbine and a low
temperature steam turbine. In one such plant, 480 MW of
net power is produced. The cooling water that removes
heat from the cycle enters at 45°F, leaves at 65°F, and
flows at a rate of 1.52 x10
4
lbm/s. The firing temperature
in the gas turbine is 2600°F. Compare the efficiency of
this cycle to the efficiency of a theoretical maximum power
cycle operating between these temperature limits. Also
compute the efficiency of the Carnot cycle for the same
temperature limits.
Solution:
Heat is rejected from the cycle in the cooling water at the
rate:
Lp
Qm
c
T
=∆
±
±
Btu
cp(water,
55,
14.7)
1.00
lbm R
p
cT
P
==
=
=
⋅
()
4o
lbm
Btu
1.52 10
1.00
65
45
F
sl
b
m
R
L
Q
⎛⎞
⎛
⎞
=×
−
⎜⎟
⎜
⎟
⋅
⎝⎠
⎝
⎠
±
5
Btu
3.04 10
s
L
Q
±
The first law for a cycle is
H
net
L
QW
Q
=+
±±
±
5
1000 kw
Btu
1.055 kw
480MW
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 Spring '08
 Kaminski

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