ENGR 2213 Thermodynamics

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ENGR 2213 Thermodynamics

• F. C. Lai
• School of Aerospace and Mechanical
• Engineering
• University of Oklahoma

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Ideal Regenerative Rankine Cycles
Open Feedwater Heater
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Ideal Regenerative Rankine Cycles
Closed Feedwater Heater
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Ideal Regenerative Rankine Cycles
wp h2 h1 v1(p2 p1)
qin h4 h3
qout (1 y)(h6 h1) y(h8 h1)
yh8 (1 y)h6 h1
wt (1 y)(h5 h6) (h4 h5)
y(h5 h7) h3 h2
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Example 1

• Consider a steam power plant operating on the
  ideal
• regenerative Rankine cycle. The steam enters the
  
• turbine at 12 MPa and 520 ºC and is condensed in
  the
• condenser at a pressure of 6 kPa. Some steam
  leaves
• the turbine at a pressure of 1 MPa and enters the
  
• closed feedwater heater. While condensate exits
  the
• feedwater heater as saturated liquid at 1 MPa,
  the
• feedwater exits the heater at a temperature of
  170 ºC.
• Determine
• the fraction of steam extracted from the
  turbine,
• the thermal efficiency of this cycle.

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Example 1 (continued)
State 1 saturated liquid at p1 6 kPa
Table A-5 h1 hf 151.53 kJ/kg
v1 vf 0.001006 m3/kg
State 2 compressed liquid at p2 12 MPa
wp1 v(p2 p1) (0.001006)(12000-6)
12.07 kJ/kg
h2 h1 wp1 151.53 12.07
163.6 kJ/kg
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Example 1 (continued)
State 3 compressed liquid at p3 12 MPa
and T3 170 ºC
Table A-7 h3 725.86 kJ/kg
State 4 superheated vapor at p4 12 MPa
and T4 520 ºC
Table A-6 h4 3401.8 kJ/kg
s4 6.5555 kJ/kgK
State 5 p5 1 MPa and s5 s4 6.5555 kJ/kgK
Table A-5 sf 2.1387 kJ/kgK
sg 6.5865 kJ/kgK
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Example 1 (continued)
State 5 saturated mixture at p5 1 MPa
h5 hf x5hfg 762.81 0.993(2015.3)
2764.2 kJ/kg
State 6 saturated mixture at p6 6 kPa, s6 s4
h6 hf x6hfg 151.53 0.773(2415.9)
2018.3 kJ/kg
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Example 2 (continued)
State 7 saturated liquid at p7 1 MPa
Table A-5 h5 762.81 kJ/kg
State 8 p8 6 kPa and h8 h7
(a) y(h5 h7) h3 h2
(b) qin h4 h3 3401.8 725.86
2675.9 kJ/kg
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Example 1 (continued)
qout yh8 (1 y)h6 h1
0.281(762.81) (1 0.281)2018.3 151.53
1514.1 kJ/kg
wt (h4 h5) (1 y)(h5 h6) (3401.8
2764.2) (1 0.281)(2764.2 2018.3)
1174.0 kJ/kg
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Example 1 (continued)
wnet wt wp 1174.0 12.07 1161.9
kJ/kg
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Ideal Regenerative Rankine Cycles
Open Feedwater Heater Closed Feedwater Heaters
1. Simple Complex
2. Inexpensive to build Expensive
3. Good heat transfer Less effective in
heat characteristics transfer
4. Require a separate Do not require a
separate pump for each heater pump
for each heater
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Ideal Regenerative Rankine Cycles
Combined Feedwater Heaters
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Ideal Regenerative Rankine Cycles
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T
Combined Feedwater Heaters
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wp1 h2 h1 v1(p2 p1)
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3
y
2
1-y
wp2 h4 h3 v3(p4 p3)
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1
p4 p5 p9, T9 T3
S
h4 h3 v3(p4 p3)
h4 h5 h9
h9 h3 v3(p4 p3)
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Turbine
Boiler
y(h7 h3) (1-y)(h9 h2)
1-y
y
FWH
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7
2
Condenser
4
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FWH
1
P2
P1
3