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

520 | Thermodynamics

Then

Thus,

and

A comparison of these results with those obtained in Example 9–5 (single-

stage compression and expansion) reveals that multistage compression with

intercooling and multistage expansion with reheating improve the back work

ratio (it drops from 40.3 to 30.4 percent) but hurt the thermal efficiency (it

drops from 42.6 to 35.8 percent). Therefore, intercooling and reheating are

not recommended in gas-turbine power plants unless they are accompanied

by regeneration.

(b) The addition of an ideal regenerator (no pressure drops, 100 percent effec-

tiveness) does not affect the compressor work and the turbine work. There-

fore, the net work output and the back work ratio of an ideal gas-turbine

cycle are identical whether there is a regenerator or not. A regenerator, how-

ever, reduces the heat input requirements by preheating the air leaving the

compressor, using the hot exhaust gases. In an ideal regenerator, the com-

pressed air is heated to the turbine exit temperature T 9 before it enters the

combustion chamber. Thus, under the air-standard assumptions, h 5 
h 7 
h 9.

The heat input and the thermal efficiency in this case are

and

Discussion Note that the thermal efficiency almost doubles as a result of

regeneration compared to the no-regeneration case. The overall effect of two-

stage compression and expansion with intercooling, reheating, and regenera-

hth

wnet

qin

477.04 kJ>kg

685.28 kJ>kg

0.696 or 69.6%

 1 1395.971053.33 2  1 1395.971053.33 2 
685.28 kJ>kg

qin
qprimaryqreheat
 1 h 6 h 52  1 h 8 h 72

hth

wnet

qin

477.04 kJ>kg

1334.30 kJ>kg

0.358 or 35.8%

rbw

wcomp,in

wturb,out

208.24 kJ>kg

685.28 kJ>kg

0.304 or 30.4%

 1 1395.97404.31 2  1 1395.971053.33 2 
1334.30 kJ>kg

qin
qprimaryqreheat
 1 h 6 h 42  1 h 8 h 72

wnet
wturb,outwcomp,in
685.28208.24
477.04 kJ>kg

wturb,out
 21 wturb,out,I 2 
 21 h 6 h 72 
 21 1395.971053.33 2 
685.28 kJ>kg

wcomp,in
 21 wcomp,in,I 2 
 21 h 2 h 12 
 21 404.31300.19 2 
208.24 kJ>kg

h 7 
1053.33 kJ>kg

Pr 7 

P 7

P 6

Pr 6 

1

28

1 330.9 2 
117.0ST 7 
1006.4 K

Pr 6 
330.9

T 6 
1300 KSh 6 
1395.97 kJ>kg