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

or

(8–35)

This relation is referred to as the exergy balanceand can be stated as the
exergy change of a system during a process is equal to the difference
between the net exergy transfer through the system boundary and the exergy
destroyed within the system boundaries as a result of irreversibilities.
We mentioned earlier that exergy can be transferred to or from a system
by heat, work, and mass transfer. Then the exergy balance for any system
undergoing any processcan be expressed more explicitly as

General: (8–36)

Net exergy transfer Exergy Change

by heat, work, and mass destruction in exergy

or, in the rate form,as

General, rate form: (8–37)

Rate of net exergy transfer Rate of exergy Rate of change

by heat, work, and mass destruction in exergy

where the rates of exergy transfer by heat, work, and mass are expressed
asX

.

heat
(1 T 0 /T)Q

.

,X

.

work
W

.

useful, and X

.

mass
m

.
c, respectively. The
exergy balance can also be expressed per unit mass as

General, unit-mass basis: (8–38)

where all the quantities are expressed per unit mass of the system. Note that
for a reversible process,the exergy destruction term Xdestroyeddrops out from
all of the relations above. Also, it is usually more convenient to find the
entropy generation Sgenfirst, and then to evaluate the exergy destroyed
directly from Eq. 8–33. That is,

(8–39)

When the environment conditions P 0 and T 0 and the end states of the system
are specified, the exergy change of the system Xsystem
X 2 X 1 can be
determined directly from Eq. 8–17 regardless of how the process is exe-
cuted. However, the determination of the exergy transfers by heat, work, and
mass requires a knowledge of these interactions.
A closed systemdoes not involve any mass flow and thus any exergy
transfer associated with mass flow. Taking the positive direction of heat
transfer to be to the system and the positive direction of work transfer to be
from the system, the exergy balance for a closed system can be expressed
more explicitly as (Fig. 8–33)

Closed system: (8–40)

or

Closed system:aa 1  (8–41)

T 0

Tk

bQk 3 WP 01 V 2 V 124 T 0 Sgen
X 2 X 1

XheatXworkXdestroyed
¢Xsystem

Xdestroyed
T 0 Sgen¬or¬X

#

destroyed
T 0 S

#

gen

1 xinxout 2 xdestroyed
¢xsystem¬¬ 1 kJ>kg 2

X

#

inX

#

out¬ X

#

destroyed
dXsystem>dt^1 kW^2

XinXout¬ Xdestroyed
¢Xsystem 1 kJ 2

XinXoutXdestroyed
¢Xsystem

Chapter 8 | 445

System

∆Xsystem

Xdestroyed

Xin Xout

Mass

Heat

Wo r k

Mass

Heat

Wo r k

FIGURE 8–32

Mechanisms of exergy transfer.

∆Xsystem

Xdestroyed

Q

Xheat

Xheat – Xwork – Xdestroyed = ∆Xsystem

W

Xwork

FIGURE 8–33

Exergy balance for a closed system

when the direction of heat transfer is

taken to be to the system and the

direction of work from the system.

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