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

Chapter 10 | 573

State 3:

State 4:

State 5:

State 6:

State 7:

The energy analysis of open feedwater heaters is identical to the energy
analysis of mixing chambers. The feedwater heaters are generally well insu-
lated (Q

.


0), and they do not involve any work interactions (W

.

0). By
neglecting the kinetic and potential energies of the streams, the energy bal-
ance reduces for a feedwater heater to

or

where yis the fraction of steam extracted from the turbine (
m. 6 /m. 5 ). Solv-
ing for yand substituting the enthalpy values, we find

Thus,

and

hth
 1 

qout

qin

 1 

1486.9 kJ>kg

2769.1 kJ>kg

0.463 or 46.3%

1486.9 kJ>kg

qout
 11 y 21 h 7 h 12 
 11 0.2270 21 2115.3191.81 2 kJ>kg

qin
h 5 h 4 
 1 3583.1814.03 2 kJ>kg
2769.1 kJ>kg

y

h 3 h 2

h 6 h 2

798.33193.01

2860.2193.01

0.2270

yh 6  11 y 2 h 2 
 11 h 32

E

#

in
E

#

out^ S^ a

in

m

#

h
a

out

m

#

h

h 7 
hfx 7 hfg
191.810.8041 1 2392.1 2 
2115.3 kJ>kg

s 7 
s 5 x 7 

s 7 sf

sfg

6.67960.6492

7.4996

0.8041

P 7 
10 kPa

P 6 
1.2 MPa

s 6 
s 5

f¬

h 6 
2860.2 kJ>kg

1 T 6 
218.4°C 2

P 5 
15 MPa

T 5 
600°C

f¬

h 5 
3583.1 kJ>kg

s 5 
6.6796 kJ>kg#K

h 4 
h 3 wpump II,in
 1 798.3315.70 2 kJ>kg
814.03 kJ>kg

15.70 kJ>kg

 1 0.001138 m^3 >kg 231 15,000 12002 kPa4a

1 kJ

1 kPa#m^3

b

wpump II,in
v 31 P 4 P 32

s 4 
s 3

P 4 
15 MPa

P 3 
1.2 MPa

Sat. liquid

f¬

v 3
vf (^) @ (^) 1.2 MPa
0.001138 m^3 >kg
h 3
hf (^) @ (^) 1.2 MPa
798.33 kJ>kg