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 Reheat Rankine Cycles
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Ideal Reheat Rankine Cycles
wp h2 h1 v(p2 p1)
qin (h3 h2) (h5 h4)
wt (h3 h4) (h5 h6)
qout h6 h1
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Ideal Reheat Rankine Cycles
The reheat process in general does not
significantly change the cycle efficiency.
The sole purpose of the reheat cycle is to
reduce the moisture content of the steam at the
final stages of the expansion process.
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Example 1

• Consider a steam power plant operating on the
• ideal reheat Rankine cycle. The steam enters
• the turbine at 15 MPa and 600 ºC and is condensed
• in the condenser at a pressure of 10 kPa. If the
  
• moisture content of the steam at the exit of the
  low-
• pressure turbine is not to exceed 10.4,
  determine
• the pressure at which the steam should be
• reheated.
• (b) the thermal efficiency of this cycle.

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Example 1 (continued)
State 6 saturated mixture at p1 10 kPa,
x6 0.896
h6 hf x6hfg 191.83 0.896(2392.8)
2335.8 kJ/kg
s6 sf x6sfg 0.6493 0.896(7.5009)
7.370 kJ/kgK
State 5 superheated vapor at T5 600 ºC
s5 s6 7.370 kJ/kgK
Table A-6, p5 4 MPa
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Example 1 (continued)
State 1 saturated liquid at p1 10 kPa
Table A-5 h1 hf 191.83 kJ/kg
v1 vf 0.001008 m3/kg
State 2 compressed liquid at p2 15 MPa
wp v(p2 p1) (0.001008)(15000-10)
15.11 kJ/kg
h2 h1 wp 191.83 15.11
206.94 kJ/kg
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Example 1 (continued)
State 3 superheated vapor at p3
15 MPa and T3 600 ºC
Table A-6 h3 3582.3 kJ/kg
s3 6.6776 kJ/kgK
State 4 superheated vapor at p4 4 MPa
s4 s3 6.6776 kJ/kgK
Table A-6 h4 3154.3 kJ/kg
T4 375.5 ºC
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Example 1 (continued)
qin (h3 h2) (h5 h4) (3582.3
206.94) (3674.4 3154.3) 3895.46 kJ/kg
qout h6 h1 2335.8 191.83
2143.97 kJ/kg
0.43 (without reheat)
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Ideal Regenerative Rankine Cycles
Open Feedwater Heater
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Ideal Regenerative Rankine Cycles
wp1 h2 h1 v1(p2 p1)
wp2 h4 h3 v3(p4 p3)
wp (1 y)wp1 wp2
qin h5 h4
qout (1 y)(h7 h1)
wt (h5 h6) (1 y)(h6 h7)
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Ideal Regenerative Rankine Cycles
yh6 (1-y)h2 h3
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Example 2

• Consider a steam power plant operating on the
• ideal regenerative Rankine cycle using open
• feedwater heater. The steam enters the turbine
• at 15 MPa and 600 ºC and is condensed in the
• condenser at a pressure of 10 kPa. Some steam
• leaves the turbine at a pressure of 1.2 MPa and
• enters the feedwater heater. Determine
• the fraction of steam extracted from the
• turbine.
• (b) the thermal efficiency of this cycle.

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Example 2 (continued)
State 1 saturated liquid at p1 10 kPa
Table A-5 h1 hf 191.83 kJ/kg
v1 vf 0.001008 m3/kg
State 2 compressed liquid at p2 1.2 MPa
wp1 v(p2 p1) (0.001008)(1200-10)
1.20 kJ/kg
h2 h1 wp1 191.83 1.2
193.03 kJ/kg
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Example 2 (continued)
State 3 saturated liquid at p3 1.2 MPa
Table A-5 h3 798.65 kJ/kg
v3 0.001139 m3/kg
State 4 compressed liquid at p4 15 MPa
wp2 v3(p4 p3) (0.001139)(15000-1200)
15.72 kJ/kg
h4 h3 wp2 798.65 15.72
814.37 kJ/kg
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Example 2 (continued)
State 5 superheated vapor at p5 15 MPa and
T5 600 ºC
Table A-6 h5 3582.3 kJ/kg
s5 6.6776 kJ/kgK
State 6 p6 1.2 MPa
s6 s5 6.6776 kJ/kgK
Table A-6, h6 2859.5 kJ/kg
State 7 p7 10 kPa
s7 s6 s5 6.6776 kJ/kgK
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Example 2 (continued)
State 7 saturated mixture at p4 10 kPa
h7 hf x7hfg 191.83 0.804(2392.8)
2115.6 kJ/kg
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Example 2 (continued)
qin h5 h4 3582.3 814.37
2767.93 kJ/kg
qout (1 y)(h7 h1) (1
0.227)(2115.6 191.83) 1487.1 kJ/kg
0.43 (without reheat)