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

where Tb,inand Tb,outare the temperatures of the system boundary where
heat is transferred into and out of the system, respectively.
The exergy destruction associated with a cycledepends on the magnitude
of the heat transfer with the high- and low-temperature reservoirs involved,
and their temperatures. It can be expressed on a unit mass basis as

(10–20)

For a cycle that involves heat transfer only with a source at THand a sink at
TL,the exergy destruction becomes

(10–21)

The exergy of a fluid stream cat any state can be determined from

(10–22)

where the subscript “0” denotes the state of the surroundings.

EXAMPLE 10–7 Second-Law Analysis of an Ideal Rankine Cycle

Determine the exergy destruction associated with the Rankine cycle (all four

processes as well as the cycle) discussed in Example 10–1, assuming that

heat is transferred to the steam in a furnace at 1600 K and heat is rejected

to a cooling medium at 290 K and 100 kPa. Also, determine the exergy of

the steam leaving the turbine.

Solution The Rankine cycle analyzed in Example 10–1 is reconsidered. For

specified source and sink temperatures, the exergy destruction associated

with the cycle and exergy of the steam at turbine exit are to be determined.

Analysis In Example 10–1, the heat input was determined to be 2728.6 kJ/kg,

and the heat rejected to be 2018.6 kJ/kg.

Processes 1-2 and 3-4 are isentropic (s 1 
s 2 , s 3 
s 4 ) and therefore do

not involve any internal or external irreversibilities, that is,

Processes 2-3 and 4-1 are constant-pressure heat-addition and heat-

rejection processes, respectively, and they are internally reversible. But the

heat transfer between the working fluid and the source or the sink takes

place through a finite temperature difference, rendering both processes irre-

versible. The irreversibility associated with each process is determined from

Eq. 10–19. The entropy of the steam at each state is determined from the

steam tables:

Thus,

1110 kJ/kg

1 290 K2c16.74501.2132 2 kJ>kg#K

2728.6 kJ>kg

1600 K

d

xdest,23
T 0 as 3 s 2 

qin,23

Tsource

b

s 4
s 3
6.7450 kJ>kg#K¬¬ 1 at 3 MPa, 350°C 2

s 2
s 1
sf (^) @ (^) 75 kPa
1.2132 kJ>kg#K
xdest,12
0 ¬and¬xdest,34
0
c
1 hh 02 T 01 ss 02 
V^2
2
gz¬¬ 1 kJ>kg 2
xdest
T 0 a
qout
TL

qin
TH
b¬¬ 1 kJ>kg 2
xdest
T 0 aa
qout
Tb,out
a
qin
Tb,in
b¬¬ 1 kJ>kg 2
Chapter 10 | 577