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Modern Engineering Thermodynamics

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480 CHAPTER 13: Vapor and Gas Power Cycles<br />

EXAMPLE 13.7 (Continued )<br />

b. Here, we remove the reheat loop by simply eliminating the pipes with monitoring stations 2 and 3 in Figure 13.27. The<br />

power plant’s thermal efficiency becomes<br />

ð<br />

η T =<br />

h 1 − h 4s Þðη s Þ pm<br />

− ðh 6s − h 5 Þ/ ðη s<br />

h 1 − h 6<br />

Þ pm<br />

where all the enthalpy values are the same as they were in part a except for h 4s . Here, s 4s = s 1 , and<br />

x 4s = s 1 − s f 4<br />

=<br />

1:5874 − 0:1326<br />

= 0:7883<br />

s fg4 1:8455<br />

so<br />

h 4s = 69:7 + ð0:7883Þð1036:0Þ = 886:4 Btu/lbm<br />

Then,<br />

η T =<br />

ð1350:6 − 886:4Þð0:82Þ − 2:93<br />

1350:6 − 72:6<br />

= 0:296 = 29:6%<br />

Exercises<br />

19. Determine the isentropic Rankine cycle thermal efficiency of the Crawford Avenue power plant without reheat discussed<br />

in Example 13.7b. Assume all the variables except the turbine and boiler feed pump isentropic efficiencies remain<br />

unchanged. Answer: (η T ) isentropic Rankine = 36.2%.<br />

20. If the isentropic efficiency of the boiler feed pump in the Crawford Avenue power plant with reheat in Example 13.7(a)<br />

is increased from 61.0% to 85.0%, determine the power plant’s new Rankine cycle thermal efficiency. Assume all the<br />

other variables remain unchanged. Answer: (η T ) Rankine = 30.4% (there is no change in (η T ) Rankine to three significant<br />

figures).<br />

21. If the reheat pressure in the Crawford Avenue power plant with reheat in Example 13.7a is decreased from<br />

100. psia to 80.0 psia, determine the power plant’s new Rankine cycle thermal efficiency. Assume all the<br />

other variables remain unchanged. Answer: (η T ) Rankine = 30.4% (there is no change in (η T ) Rankine to three significant<br />

figures).<br />

Note that the interstage reheating used in Example 13.7 increases the Rankine cycle thermal efficiency by only<br />

0.8%. However, it has the much more important effect of reducing the moisture content at the turbine exit. Wet<br />

steam with a moisture content of more than 8 to 10% can produce serious blade erosion problems in the<br />

low-pressure region of a turbine. The effect of reheating in this example keeps the exit moisture level within<br />

this range, whereas without reheating, part b of the example shows that the exit moisture level would be<br />

(1 – 0.7883)(100) = 21.2%, which is much too high.<br />

13.10 MODERN STEAM POWER PLANTS<br />

In the years since the 1930s, the advancements in boiler technology have been as dramatic as those in turbine<br />

technology. Turbine inlet pressures and temperatures continued to increase over the years, mainly due to significant<br />

improvements in high-temperature strength properties of various metal alloys. The simultaneous use of<br />

superheat, reheat, and regeneration, along with improved turbine isentropic and mechanical efficiencies at<br />

higher turbine inlet temperatures and pressures, allowed actual operating power station thermal efficiencies to<br />

reach percentages in the low 40s by the 1980s (see Figure 13.9). In the 1930s, the turbine-generator unit output<br />

reached 200 MW, and by the 2000s, it had surpassed 2000 MW. Figure 13.28 shows the 2800 MW combined<br />

cycle gas turbine (CCGT) power plant built in Chiba, Japan.

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