Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 10 | 583

or

where

Thus

Discussion Note that 26.2 MW of the heat transferred will be utilized in the

process heater. We could also show that 11.0 MW of power is produced in

this case, and the rate of heat input in the boiler is 43.0 MW. Thus the uti-

lization factor is 86.5 percent.

10–9 ■ COMBINED GAS–VAPOR POWER CYCLES

The continued quest for higher thermal efficiencies has resulted in rather
innovative modifications to conventional power plants. The binary vapor
cyclediscussed later is one such modification. A more popular modification
involves a gas power cycle topping a vapor power cycle, which is called the
combined gas–vapor cycle,or just the combined cycle.The combined
cycle of greatest interest is the gas-turbine (Brayton) cycle topping a steam-
turbine (Rankine) cycle, which has a higher thermal efficiency than either of
the cycles executed individually.
Gas-turbine cycles typically operate at considerably higher temperatures
than steam cycles. The maximum fluid temperature at the turbine inlet is
about 620°C (1150°F) for modern steam power plants, but over 1425°C
(2600°F) for gas-turbine power plants. It is over 1500°C at the burner exit
of turbojet engines. The use of higher temperatures in gas turbines is made
possible by recent developments in cooling the turbine blades and coating
the blades with high-temperature-resistant materials such as ceramics. Because
of the higher average temperature at which heat is supplied, gas-turbine
cycles have a greater potential for higher thermal efficiencies. However, the
gas-turbine cycles have one inherent disadvantage: The gas leaves the gas
turbine at very high temperatures (usually above 500°C), which erases any
potential gains in the thermal efficiency. The situation can be improved
somewhat by using regeneration, but the improvement is limited.
It makes engineering sense to take advantage of the very desirable charac-
teristics of the gas-turbine cycle at high temperatures andto use the high-
temperature exhaust gases as the energy source for the bottoming cycle such
as a steam power cycle. The result is a combined gas–steam cycle, as shown

26.2 MW

¬ 1 12 kg>s 21 640.09 kJ>kg 2

Q

#

p,out
^1 1.5 kg>s^21 3411.4 kJ>kg^2 ^1 10.5 kg>s^21 2739.3 kJ>kg^2

m

#

7 
m

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4 m

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5 
1.510.5
12 kg>s

m

#

5 
^1 0.7^21 15 kg>s^2 
10.5 kg>s

m

#

4 
^1 0.1^21 15 kg>s^2 
1.5 kg>s

Q

#

p,out
m

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4 h 4 m

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5 h 5 m

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7 h 7

m

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4 h 4 m

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5 h 5 
Q

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7 h 7

E

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in
E

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out