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

Chapter 7 | 387

Under the stated assumptions and observations, the mass and energy

balances for this steady-flow system can be expressed in the rate form as

follows:

Mass balance:

Energy balance:

Rate of net energy transfer Rate of change in internal, kinetic,

by heat, work, and mass potential, etc., energies

Combining the mass and energy balances gives

The desired properties at the specified states are determined from the steam

tables to be

State 1:

State 2:

State 3:

Substituting,

which gives

The rate of entropy generation during this process can be determined by

applying the rate form of the entropy balance on an extended systemthat

includes the mixing chamber and its immediate surroundings so that the

boundary temperature of the extended system is 70F 530 R:

m

#

1 s 1 m

#

2 s 2 
m

#

3 s 3 

Q

#

out

Tb

S

#

gen^0

S

#

in
S

#

out¬¬S

#

gen¬¬dSsystem>dt

m

#

2 22.7 lbm>min

180 Btu>min 3300 18.07m

#

2 1162.3
^1300 m

#

22 97.99^4 Btu>min

P 3 20 psia

T 3 130°F

f¬

h 3 hf @ 130°F97.99 Btu>lbm

s 3 sf @ 130°F0.18174 Btu>lbm#R

P 2 20 psia

T 2 240°F

f¬

h 2 1162.3 Btu>lbm

s 2 1.7406 Btu>lbm#R

P 1 20 psia

T 1 50°F

f¬

h 1 hf @ 50°F18.07 Btu>lbm

s 1 sf @ 50°F0.03609 Btu>lbm#R

Q

#

outm

#

1 h 1 m

#

2 h 2 
^1 m

#

1 m

#

22 h 3

m# 1 h 1 m# 2 h 2 m# 3 h 3 Q

#

out^1 since W

#

0, ke
pe
 02

E

#

inE

#

out^

E

#

in
E

#

out^ ^ dEsystem/dt^0

m

#

in
m

#

out dmsystem>dt^0 S^ m

#

1 m

#

2 m

#

3

⎭⎪⎪⎬⎪⎪⎫ ⎭⎪⎪⎬⎪⎪⎫

0 (steady)

¡

0 (steady)

¡

123

Rate of net entropy

transfer by heat

and mass

123

Rate of entropy

generation

123

Rate of change

in entropy

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