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

The value of entropy at a specified state is determined just like any other

property. In the compressed liquid and superheated vapor regions, it can be

obtained directly from the tables at the specified state. In the saturated mix-

ture region, it is determined from

where xis the quality and sfand sfgvalues are listed in the saturation tables.

In the absence of compressed liquid data, the entropy of the compressed liq-

uid can be approximated by the entropy of the saturated liquid at the given

temperature:

The entropy change of a specified mass m(a closed system) during a

process is simply

(7–12)

which is the difference between the entropy values at the final and initial

states.

When studying the second-law aspects of processes, entropy is commonly

used as a coordinate on diagrams such as the T-sand h-sdiagrams. The

general characteristics of the T-sdiagram of pure substances are shown in

Fig. 7–11 using data for water. Notice from this diagram that the constant-

volume lines are steeper than the constant-pressure lines and the constant-

pressure lines are parallel to the constant-temperature lines in the saturated

liquid–vapor mixture region. Also, the constant-pressure lines almost coin-

cide with the saturated liquid line in the compressed liquid region.

¢Sm¢sm 1 s 2 s 12 ¬¬ 1 kJ>K 2

s@ T,Psf @ T¬¬ 1 kJ>kg#K 2

ssf
xsfg¬¬ 1 kJ>kg#K 2

340 | Thermodynamics

T, °C

081 2 3 4 5 6 7

100

200

300

400

500

Saturated

liquid line

Critical

state

Saturated

vapor line

P

= 10 MPa

P

= 1 MPa

s, kJ/kg • K

v = 0.1 m

(^3) /kg
v^ = 0.5 m
(^3) /kg
FIGURE 7–11
Schematic of the T-sdiagram for
water.
Superheated
vapor
T
s
13
2
Saturated
liquid–vapor mixture
TP (^3) s 3
3 }
PT (^1) s 1 ≅ sƒ@T 1
1 }
Compressed
liquid
xT^2 s^2 = sƒ + x^2 sƒg
2 }
FIGURE 7–10
The entropy of a pure substance is
determined from the tables (like other
properties).