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

Chapter 10 | 563

(a) This is the steam power plant discussed in Example 10–1, except that
the condenser pressure is lowered to 10 kPa. The thermal efficiency is
determined in a similar manner:

State 1:

State 2:

State 3:

State 4:

Thus,

and

Therefore, the thermal efficiency increases from 26.0 to 33.4 percent as a
result of lowering the condenser pressure from 75 to 10 kPa. At the same
time, however, the quality of the steam decreases from 88.6 to 81.3 percent
(in other words, the moisture content increases from 11.4 to 18.7 percent).

(b) States 1 and 2 remain the same in this case, and the enthalpies at
state 3 (3 MPa and 600°C) and state 4 (10 kPa and s 4
s 3 ) are deter-
mined to be

Thus,

and

hth
 1 

qout

qin

 1 

2188.5 kJ>kg

3488.0 kJ>kg

0.373 or 37.3%

qout
h 4 h 1 
2380.3191.81
2188.5 kJ>kg

qin
h 3 h 2 
3682.8194.83
3488.0 kJ>kg

h 4
2380.3 kJ>kg¬ 1 x 4
0.915 2

h 3 
3682.8 kJ>kg

hth
 1 

qout

qin

 1 

1944.3 kJ>kg

2921.3 kJ>kg

0.334 or 33.4%

qout
h 4 h 1 
 1 2136.1191.81 2 kJ>kg
1944.3 kJ>kg

qin
h 3 h 2 
 1 3116.1194.83 2 kJ>kg
2921.3 kJ>kg

h 4 
hfx 4 hfg
191.810.8128 1 2392.1 2 
2136.1 kJ>kg

x 4 

s 4 sf

sfg

6.74500.6492

7.4996

0.8128

s 4 
s 3

P 4
10 kPa¬ 1 sat. mixture 2

P 3 
3 MPa

T 3 
350°C

f¬

h 3 
3116.1 kJ>kg

s 3 
6.7450 kJ>kg#K

h 2 
h 1 wpump,in
 1 191.813.02 2 kJ>kg
194.83 kJ>kg

3.02 kJ>kg

wpump,in
v 11 P 2 P 12 
 1 0.00101 m^3 >kg 2313000  102 kPa4a

1 kJ

1 kPa#m^3

b

s 2 
s 1

P 2 
3 MPa

P 1 
10 kPa

Sat. liquid

f¬

h 1
hf (^) @ (^) 10 kPa
191.81 kJ>kg
v 1
vf (^) @ (^) 10 kPa
0.00101 m^3 >kg