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

Entropy Change of a System, Ssystem

Despite the reputation of entropy as being vague and abstract and the intim-

idation associated with it, entropy balance is actually easier to deal with

than energy balance since, unlike energy, entropy does not exist in various

forms. Therefore, the determination of entropy change of a system during a

process involves evaluating entropy of the system at the beginning and at

the end of the process and taking their difference. That is,

or

(7–69)

Note that entropy is a property, and the value of a property does not change

unless the state of the system changes. Therefore, the entropy change of a

system is zero if the state of the system does not change during the process.

For example, the entropy change of steady-flow devices such as nozzles,

compressors, turbines, pumps, and heat exchangers is zero during steady

operation.

When the properties of the system are not uniform, the entropy of the sys-

tem can be determined by integration from

(7–70)

where Vis the volume of the system and ris density.

Mechanisms of Entropy Transfer, Sinand Sout

Entropy can be transferred to or from a system by two mechanisms:heat

transferand mass flow(in contrast, energy is transferred by work also).

Entropy transfer is recognized at the system boundary as it crosses the

boundary, and it represents the entropy gained or lost by a system during a

process. The only form of entropy interaction associated with a fixed mass

or closed system is heat transfer, and thus the entropy transfer for an adia-

batic closed system is zero.

1 Heat Transfer

Heat is, in essence, a form of disorganized energy, and some disorganization

(entropy) will flow with heat. Heat transfer to a system increases the

entropy of that system and thus the level of molecular disorder or random-

ness, and heat transfer from a system decreases it. In fact, heat rejection is

the only way the entropy of a fixed mass can be decreased. The ratio of the

heat transfer Qat a location to the absolute temperature Tat that location is

called the entropy flowor entropy transferand is expressed as (Fig. 7–58)

Entropy transfer by heat transfer: (7–71)

The quantity Q/Trepresents the entropy transfer accompanied by heat trans-

fer, and the direction of entropy transfer is the same as the direction of heat

transfer since thermodynamic temperature Tis always a positive quantity.

Sheat

Q

T

¬¬ 1 Tconstant 2

Ssystem s dm

V

sr¬dV

¢SsystemSfinalSinitialS 2 S 1

Entropy changeEntropy at final stateEntropy at initial state

378 | Thermodynamics

Surroundings

SYSTEM

Sheat =

Q = 500 kJ

Q

Tb

Tb = 400 K

= 1.25 kJ/K

FIGURE 7–58

Heat transfer is always accompanied
by entropy transfer in the amount of
Q/ T, where Tis the boundary
temperature.