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

ambient in a liquid-to-air heat exchanger. This heat rejectedusually amounts

to 60 to 90 percent of the power input, and thus it represents a huge amount

of energy that can be used for a useful purpose such as space heatingin win-

ter,preheatingthe air or water in a furnace, or other process-related purposes

(Fig. 7–80). For example, assuming 80 percent of the power input is con-

verted to heat, a 150-hp compressor can reject as much heat as a 90-kW

electric resistance heater or a 400,000-Btu/h natural gas heater when operat-

ing at full load. Thus, the proper utilization of the waste heat from a com-

pressor can result in significant energy and cost savings.

400 | Thermodynamics

SUMMARY

The second law of thermodynamics leads to the definition of

a new property called entropy, which is a quantitative mea-

sure of microscopic disorder for a system. Any quantity

whose cyclic integral is zero is a property, and entropy is

defined as

For the special case of an internally reversible, isothermal

process, it gives

The inequality part of the Clausius inequality combined with

the definition of entropy yields an inequality known as the

increase of entropy principle, expressed as

where Sgen is the entropy generated during the process.

Entropy change is caused by heat transfer, mass flow, and

irreversibilities. Heat transfer to a system increases the

entropy, and heat transfer from a system decreases it. The

effect of irreversibilities is always to increase the entropy.

The entropy-changeand isentropic relationsfor a process

can be summarized as follows:

1.Pure substances:

Any process:

Isentropic process: s 2 s 1

¢ss 2 s 1

Sgen 0

¢S

Q

T 0

dSa

dQ

T

b

int rev

2.Incompressible substances:

Any process:

Isentropic process:

3.Ideal gases:

a.Constant specific heats (approximate treatment):

Any process:

Isentropic process:

b.Variable specific heats (exact treatment):

Any process:

s 2 s 1 s° 2 s° 1 R ln ¬

P 2

P 1

a

P 2

P 1

b

sconst.

a

v 1

v 2

b

k

a

T 2

T 1

b

sconst.

a

P 2

P 1

b

1 k 1 2>k

a

T 2

T 1

b

sconst.

a

v 1

v 2

b

k 1

s 2 s 1 cp,avg ln¬

T 2

T 1

R ln¬

P 2

P 1

s 2 s 1 cv,avg ln ¬

T 2

T 1

R ln ¬

v 2

v 1

T 2 T 1

s 2 s 1 cavg ln ¬

T 2

T 1

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