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

A 1350-MW combined-cycle power plant built in Ambarli, Turkey, in
1988 by Siemens of Germany is the first commercially operating thermal
plant in the world to attain an efficiency level as high as 52.5 percent at
design operating conditions. This plant has six 150-MW gas turbines and
three 173-MW steam turbines. Some recent combined-cycle power plants
have achieved efficiencies above 60 percent.

EXAMPLE 10–9 A Combined Gas–Steam Power Cycle

Consider the combined gas–steam power cycle shown in Fig. 10–25. The top-

ping cycle is a gas-turbine cycle that has a pressure ratio of 8. Air enters the

compressor at 300 K and the turbine at 1300 K. The isentropic efficiency of

the compressor is 80 percent, and that of the gas turbine is 85 percent. The

bottoming cycle is a simple ideal Rankine cycle operating between the pressure

limits of 7 MPa and 5 kPa. Steam is heated in a heat exchanger by the exhaust

gases to a temperature of 500°C. The exhaust gases leave the heat exchanger

at 450 K. Determine (a) the ratio of the mass flow rates of the steam and the

combustion gases and (b) the thermal efficiency of the combined cycle.

Solution A combined gas–steam cycle is considered. The ratio of the mass

flow rates of the steam and the combustion gases and the thermal efficiency

are to be determined.

Analysis The T- sdiagrams of both cycles are given in Fig. 10–25. The gas-

turbine cycle alone was analyzed in Example 9–6, and the steam cycle in

Example 10–8b, with the following results:

Gas cycle:

h 5 ¿ 
h@ 450 K
451.80 kJ>kg

qin
790.58 kJ>kg¬wnet
210.41 kJ>kg¬hth
26.6%

h 4 ¿
880.36 kJ>kg¬¬ 1 T 4 ¿
853 K 2

Chapter 10 | 585

4

1'

4'

1

T, K

s

2'

3

5'

500 °C

7 MPa

5 kPa

7 MPa

450

300

3'

2

1300

FIGURE 10–25

T-sdiagram of the gas–steam

combined cycle described in

Example 10–9.