Modern Engineering Thermodynamics

480 CHAPTER 13: Vapor and Gas Power Cycles
EXAMPLE 13.7 (Continued )
b. Here, we remove the reheat loop by simply eliminating the pipes with monitoring stations 2 and 3 in Figure 13.27. The
power plant’s thermal efficiency becomes
ð
η T =
h 1 − h 4s Þðη s Þ pm
− ðh 6s − h 5 Þ/ ðη s
h 1 − h 6
Þ pm
where all the enthalpy values are the same as they were in part a except for h 4s . Here, s 4s = s 1 , and
x 4s = s 1 − s f 4
=
1:5874 − 0:1326
= 0:7883
s fg4 1:8455
so
h 4s = 69:7 + ð0:7883Þð1036:0Þ = 886:4 Btu/lbm
Then,
η T =
ð1350:6 − 886:4Þð0:82Þ − 2:93
1350:6 − 72:6
= 0:296 = 29:6%
Exercises
19. Determine the isentropic Rankine cycle thermal efficiency of the Crawford Avenue power plant without reheat discussed
in Example 13.7b. Assume all the variables except the turbine and boiler feed pump isentropic efficiencies remain
unchanged. Answer: (η T ) isentropic Rankine = 36.2%.
20. If the isentropic efficiency of the boiler feed pump in the Crawford Avenue power plant with reheat in Example 13.7(a)
is increased from 61.0% to 85.0%, determine the power plant’s new Rankine cycle thermal efficiency. Assume all the
other variables remain unchanged. Answer: (η T ) Rankine = 30.4% (there is no change in (η T ) Rankine to three significant
figures).
21. If the reheat pressure in the Crawford Avenue power plant with reheat in Example 13.7a is decreased from
100. psia to 80.0 psia, determine the power plant’s new Rankine cycle thermal efficiency. Assume all the
other variables remain unchanged. Answer: (η T ) Rankine = 30.4% (there is no change in (η T ) Rankine to three significant
figures).
Note that the interstage reheating used in Example 13.7 increases the Rankine cycle thermal efficiency by only
0.8%. However, it has the much more important effect of reducing the moisture content at the turbine exit. Wet
steam with a moisture content of more than 8 to 10% can produce serious blade erosion problems in the
low-pressure region of a turbine. The effect of reheating in this example keeps the exit moisture level within
this range, whereas without reheating, part b of the example shows that the exit moisture level would be
(1 – 0.7883)(100) = 21.2%, which is much too high.
13.10 MODERN STEAM POWER PLANTS
In the years since the 1930s, the advancements in boiler technology have been as dramatic as those in turbine
technology. Turbine inlet pressures and temperatures continued to increase over the years, mainly due to significant
improvements in high-temperature strength properties of various metal alloys. The simultaneous use of
superheat, reheat, and regeneration, along with improved turbine isentropic and mechanical efficiencies at
higher turbine inlet temperatures and pressures, allowed actual operating power station thermal efficiencies to
reach percentages in the low 40s by the 1980s (see Figure 13.9). In the 1930s, the turbine-generator unit output
reached 200 MW, and by the 2000s, it had surpassed 2000 MW. Figure 13.28 shows the 2800 MW combined
cycle gas turbine (CCGT) power plant built in Chiba, Japan.