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

384 | Thermodynamics

Analysis We take the throttling valve as the system(Fig. 7–66). This is a

control volumesince mass crosses the system boundary during the process.

We note that there is only one inlet and one exit and thus m

.

1 m

.

2 m

.

.

Also, the enthalpy of a fluid remains nearly constant during a throttling

process and thus h 2 h 1.

The entropy of the steam at the inlet and the exit states is determined from

the steam tables to be

State 1:

State 2:

Then the entropy generation per unit mass of the steam is determined from

the entropy balance applied to the throttling valve,

Dividing by mass flow rate and substituting gives

This is the amount of entropy generated per unit mass of steam as it is

throttled from the inlet state to the final pressure, and it is caused by

unrestrained expansion. The increase of entropy principle is obviously

satisfied during this process since the entropy generation is positive.

sgens 2 s 1 7.00466.63530.3693 kJ/kg#K

S

#

genm

# 1 s

2 s 12

m#s 1 m#s 2 
S

#

gen^0

S

#

inS

#

out¬
¬S

#

gen¬¬dSsystem>dt

P 2 3 MPa

h 2 h 1

f¬s 2 7.0046 kJ>kg#K

P 1 7 MPa

T 1 450°C

f¬

h 1 3288.3 kJ>kg

s 1 6.6353 kJ>kg#K

T,°C

s

1

2

s 1

h = const.

450

s 2

Throttling

process

P 1 = 7 MPa

T 1 = 450°C

FIGURE 7–66 P 2 = 3 MPa

Schematic and T-sdiagram for
Example 7–18.

123

Rate of net entropy

transfer by heat

and mass

123

Rate of entropy

generation

123

Rate of change

in entropy

0 (steady)

¡