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

Chapter 8 | 463

The maximum power output (reversible power) is determined from the rate
form of the exergy balance applied on the extended system(system + imme-
diate surroundings), whose boundary is at the environment temperature of
T 0 , and by setting the exergy destruction term equal to zero,

Rate of net exergy transfer Rate of exergy Rate of change

by heat, work, and mass destruction in exergy

Note that exergy transfer by heat is zero when the temperature at the point
of transfer is the environment temperature T 0 , and the kinetic and potential
energies are negligible. Therefore,

That is, we could have produced work at a rate of 4588 Btu/min if we ran a
heat engine between the hot and the cold fluid streams instead of allowing
them to mix directly.
The exergy destroyed is determined from

Thus,

since there is no actual work produced during the process (Fig. 8–47).
Discussion The entropy generation rate for this process was determined in
Example 7–20 to be S

.
gen
8.65 Btu/min · R. Thus the exergy destroyed
could also be determined from the second part of the above equation:

The slight difference between the two results is due to roundoff error.

X

#

destroyed
T 0 S

#

gen
^1 530 R^21 8.65 Btu>min#R^2 
4585 Btu>min

X

#

destroyed
W

#

rev,out
4588 Btu/min

X

#

destroyed
W

#

rev,outW

#

u^ 
T 0 S

#

gen

4588 Btu/min

¬ 1 322.7 lbm>min 23 97.99 Btu>lbm 1 530 R 21 0.18174 Btu>lbm#R 24

¬ 1 22.7 lbm>min 23 1162.3 Btu>lbm 1 530 R 21 1.7406 Btu>lbm#R 24

1 300 lbm>min 23 18.07 Btu>lbm 1 530 R 21 0.03609 Btu>lbm#R 24

W

#

rev,out
m

#

11 h 1 T 0 s 12 m

#

21 h 2 T 0 s 22 m

#

31 h 3 T 0 s 32

W

#

rev,out
m

#

1 c 1 m

#

2 c 2 m

#

3 c 3

m

#

1 c 1 m

#

2 c 2 
W

#

rev,outX

#

heat

Q^0

m

#

3 c 3

X

#

in
X

#

out

X

#

inX

#

out¬ ^ X

#

destroyed^ 
dXsystem>dt^ 
^0

EXAMPLE 8–17 Charging a Compressed Air Storage System

A 200-m^3 rigid tank initially contains atmospheric air at 100 kPa and 300 K
and is to be used as a storage vessel for compressed air at 1 MPa and 300 K
(Fig. 8–48). Compressed air is to be supplied by a compressor that takes in
atmospheric air at P 0
100 kPa and T 0
300 K. Determine the minimum
work requirement for this process.

Solution Air is to be compressed and stored at high pressure in a large
tank. The minimum work required is to be determined.

FIGURE 8–47

For systems that involve no actual

work, the reversible work and

irreversibility are identical.

© Reprinted with special permission of King

Features Syndicate.

AIR

V = 200 m^3

100 kPa → 1 MPa

300 K

100 kPa

300 K

Compressor

FIGURE 8–48

Schematic for Example 8–17.

S^0 (reversible) S^0 (steady)

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