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Modern Engineering Thermodynamics

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472 CHAPTER 13: Vapor and Gas Power Cycles<br />

EXAMPLE 13.6 (Continued )<br />

Solution<br />

a. First, draw a sketch of the configuration of the isentropic Rankine cycle system before boiler feedwater regeneration is<br />

added (Figure 13.21).<br />

Q B<br />

1 Prime mover<br />

Boiler Stage 1 Stage 2<br />

W E<br />

4s<br />

Pump<br />

3 2s<br />

Condenser<br />

W P<br />

Q C<br />

FIGURE 13.21<br />

Example 13.6, part a.<br />

Its thermal efficiency is<br />

ðη T Þ isentropic<br />

Rankine<br />

ð<br />

= h 1 − h 2s Þ− ðh 4s − h 3 Þ<br />

ðh 1 − h 4s Þ<br />

where, assuming an incompressible liquid condensate, Eq. (13.6) gives h 4s = h 3 + v 3 (p 4 – p 3 )andv 3 = v 4 (1.00 psia) =<br />

0.01614 ft 3 /lbm. The monitoring station data are as follows:<br />

Station 1<br />

p 1 = 200: psia<br />

x 1 = 1:00<br />

h 1 = 1199:3 Btu/lbm<br />

s 1 = 1:5466 Btu/ðlbm . RÞ<br />

Station 3<br />

p 3 = 1:00 psia<br />

x 3 = 0:00<br />

h 3 = 69:7 Btu/lbm<br />

s 3 = 0:1326 Btu/ðlbm . RÞ<br />

Station 2s<br />

p 2s = p 2 = 1:00 psia<br />

s 2s = s 1 = 1:5466 Btu/ðlbm . RÞ<br />

h 2s = 863:5 Btu/lbm<br />

Station 4s<br />

p 4s = p 4 = 200: psia<br />

s 4s = s 3 = 0:1326 Btu/ðlbm . RÞ<br />

h 4s = h 3 + v 3 ðp 4 – p 3 Þ<br />

= 69:70 + ð0:01614Þð200: – 1:00Þð144/778:16Þ<br />

= 69:70 + 0:594 = 70:3 Btu=lbm<br />

where, at station 2s, we use<br />

x 2s = s 2s − s f 2<br />

= s 1 − s f 2 1:5466 − 0:1326<br />

= = 0:7662<br />

s fg2 s fg2 18455<br />

then,<br />

h 2s = 69:70 + ð0:7662Þð1036:0Þ = 863:5 Btu=lbm<br />

The thermal efficiency is now<br />

ðη T Þ isentropic<br />

Rankine<br />

=<br />

1199:3 − 863:5 − 0:594<br />

1199:3 − 70:3<br />

= 0:297 = 29:7%<br />

b. Now, draw a sketch of the configuration of the<br />

isentropic Rankine cycle system with one open loop<br />

boiler feedwater regenerator (Figure 13.22).<br />

6<br />

7s<br />

Q B<br />

W P2<br />

Boiler<br />

Regenerator<br />

5s<br />

1<br />

4s<br />

Prime mover<br />

Stage 1 Stage 2<br />

2s<br />

3<br />

Condenser<br />

W P1<br />

W E<br />

Q L<br />

The properties at monitoring stations 1, 2, and 3 are the<br />

same as they were in part a, but pump 1 brings the condensate<br />

pressure up to only 80.0 psia (to match the vapor inlet<br />

FIGURE 13.22<br />

Example 13.6, part b.

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