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

Steam cycle:

(a) The ratio of mass flow rates is determined from an energy balance on the

heat exchanger:

Thus,

That is, 1 kg of exhaust gases can heat only 0.131 kg of steam from 33 to

500°C as they are cooled from 853 to 450 K. Then the total net work out-

put per kilogram of combustion gases becomes

Therefore, for each kg of combustion gases produced, the combined plant

will deliver 384.8 kJ of work. The net power output of the plant is deter-

mined by multiplying this value by the mass flow rate of the working fluid in

the gas-turbine cycle.

(b) The thermal efficiency of the combined cycle is determined from

Discussion Note that this combined cycle converts to useful work 48.7 per-

cent of the energy supplied to the gas in the combustion chamber. This value

is considerably higher than the thermal efficiency of the gas-turbine cycle

(26.6 percent) or the steam-turbine cycle (40.8 percent) operating alone.

hth

wnet

qin

384.8 kJ>kg gas

790.6 kJ>kg gas

0.487 or 48.7%

384.8 kJ>kg gas

 1 210.41 kJ>kg gas 2  1 0.131 kg steam>kg gas 21 1331.4 kJ>kg steam 2

wnet
wnet,gasywnet,steam

m#s

m#g

y
0.131

m#s 1 3411.4144.78 2 
m#g 1 880.36451.80 2

m

#

s^1 h 3 h 22 
m

#

g^1 h¿ 4 h¿ 52

m

#

g h 5 ¿ m

#

s^ h 3 
m

#

g^ h 4 ¿ m

#

s^ h 2

E

#

in
E

#

out

wnet
1331.4 kJ>kg¬¬hth
40.8%

h 3
3411.4 kJ>kg¬¬ 1 T 3
500°C 2

h 2
144.78 kJ>kg¬¬ 1 T 2
33°C 2

586 | Thermodynamics

With the exception of a few specialized applications, the working fluid pre-

dominantly used in vapor power cycles is water. Water is the bestworking

fluid presently available, but it is far from being the idealone. The binary

cycle is an attempt to overcome some of the shortcomings of water and to

approach the idealworking fluid by using two fluids. Before we discuss the

binary cycle, let us list the characteristics of a working fluid most suitable

for vapor power cycles:

TOPIC OF SPECIAL INTEREST* Binary Vapor Cycles

*This section can be skipped without a loss in continuity.