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

external to the system. A second-law analysis of these cycles reveals where

the largest irreversibilities occur and where to start improvements.

Relations for exergyand exergy destructionfor both closed and steady-

flow systems are developed in Chap. 8. The exergy destruction for a closed

system can be expressed as

(9–30)

where Tb,inand Tb,outare the temperatures of the system boundary where

heat is transferred into and out of the system, respectively. A similar relation

for steady-flow systems can be expressed, in rate form, as

(9–31)

or, on a unit–mass basis for a one-inlet, one-exit steady-flow device, as

(9–32)

where subscripts iand edenote the inlet and exit states, respectively.

The exergy destruction of a cycleis the sum of the exergy destructions of

the processes that compose that cycle. The exergy destruction of a cycle can

also be determined without tracing the individual processes by considering

the entire cycle as a single process and using one of the relations above.

Entropy is a property, and its value depends on the state only. For a cycle,

reversible or actual, the initial and the final states are identical; thus se
si.

Therefore, the exergy destruction of a cycle depends on the magnitude of

the heat transfer with the high- and low-temperature reservoirs involved and

on their temperatures. It can be expressed on a unit–mass basis as

(9–33)

For a cycle that involves heat transfer only with a source at THand a sink at

TL, the exergy destruction becomes

(9–34)

The exergies of a closed system fand a fluid stream cat any state can be

determined from

(9–35)

and

(9–36)

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

c
 1 hh 02 T 01 ss 02 

V^2

2

gz¬¬ 1 kJ>kg 2

f
 1 uu 02 T 01 ss 02 P 01 vv 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

Xdest
T 0 sgen
T 0 asesi

qin

Tb,in



qout

Tb,out

b¬¬ 1 kJ>kg 2

X

#

dest
T 0 S

#

gen
T 01 S

#

outS

#

in^2 
T 0 aa

out

m#sa

in

m#s

Q

#

in

Tb,in



Q

#

out

Tb,out

b¬¬ 1 kW 2

T 0 c1S 2 S 12 sys

Qin

Tb,in



Qout

Tb,out

d¬¬ 1 kJ 2

Xdest
T 0 Sgen
T 01 ¢SsysSinSout 2

528 | Thermodynamics