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

8–8 ■ EXERGY BALANCE: CONTROL VOLUMES

The exergy balance relations for control volumes differ from those for closed
systems in that they involve one more mechanism of exergy transfer:mass flow
across the boundaries.As mentioned earlier, mass possesses exergy as well as
energy and entropy, and the amounts of these three extensive properties are

Chapter 8 | 457

since KE 
PE 
0 and U
0 (the change in internal energy of an ideal

gas is zero during an isothermal process), and Ssys
Q/Tsysfor isothermal

processes in the absence of any irreversibilities. Then,

Therefore, the useful work output would be 2.65 kJ instead of 1.43 kJ if the

process were executed in a totally reversible manner.

Alternative Approach The reversible work could also be determined by apply-

ing the basics only, without resorting to exergy balance. This is done by

replacing the irreversible portions of the process by reversible ones that cre-

ate the same effect on the system. The useful work output of this idealized

process (between the actual end states) is the reversible work.

The only irreversibility the actual process involves is the heat transfer

between the system and the furnace through a finite temperature difference.

This irreversibility can be eliminated by operating a reversible heat engine

between the furnace at 1200 K and the surroundings at 300 K. When 2.43

kJ of heat is supplied to this heat engine, it produces a work output of

The 2.43 kJ of heat that was transferred to the system from the source is

now extracted from the surrounding air at 300 K by a reversible heat pump

that requires a work input of

Then the net work output of this reversible process (i.e., the reversible work)

becomes

which is practically identical to the result obtained before. Also, the exergy

destroyed is the difference between the reversible work and the useful work,

and is determined to be

which is identical to the result obtained before.

Xdestroyed
Wrev,outWu,out
2.651.43
1.22 kJ

Wrev
WuWHEWHP,in
1.431.820.61
2.64 kJ

WHP,in

QH

COPHP

c

QH

TH>1THTL 2

d

HP

2.43 kJ

1 400 K2>3 1 400  3002 K 4

0.61 kJ

WHE
hrevQH
a 1 

TL

TH

bQH
a 1 

300 K

1200 K

b12.43 kJ 2 
1.82 kJ

 2.65 kJ

 1 300 K 2

2.43 kJ

400 K

 1 1 kJ 2 a 1 

300 K

1200 K

b12.43 kJ 2

Wrev,out
T 0

Q

Tsys

Wsurra 1 

T 0

TR

bQ

SEE TUTORIAL CH. 8, SEC. 8 ON THE DVD.

INTERACTIVE

TUTORIAL