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ME Thermodynamics II: Vapor Power Cycles

The document discusses ways to modify the basic Rankine vapor power cycle to increase thermal efficiency, including lowering the condenser pressure, superheating the steam to a higher temperature, and increasing the boiler pressure. Superheating the steam increases the thermal efficiency by raising the heat addition temperature while also increasing the steam quality during expansion. Raising the boiler pressure further increases efficiency but decreases steam quality during expansion.

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92 views11 pages

ME Thermodynamics II: Vapor Power Cycles

The document discusses ways to modify the basic Rankine vapor power cycle to increase thermal efficiency, including lowering the condenser pressure, superheating the steam to a higher temperature, and increasing the boiler pressure. Superheating the steam increases the thermal efficiency by raising the heat addition temperature while also increasing the steam quality during expansion. Raising the boiler pressure further increases efficiency but decreases steam quality during expansion.

Uploaded by

Jasmine_lai00
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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ME Thermodynamics II

Vapor Power Cycles

Dr Aklilu Tesfamichael
Lesson outcome
• Investigate ways to modify the basic Rankine
vapor power cycle to increase the cycle thermal
efficiency
Increasing Rankine Cycle Efficiency
The basic idea is to increase the average temperature
at which heat addition takes place, or to reduce the
temperature at which heat rejections takes place.

 Lowering the condenser pressure

 Superheating the steam to a higher temperature

 Increasing the boiler pressure


Lowering the Condenser Pressure
 Lower heat rejection
temperature
 Net work increase
 Heat input increase (very
small)
 Condenser pressure is
below the atmospheric
 Possible air leak into the
condenser
 Low moisture quality
during the turbine last
stages
Superheating the Steam

 Higher heat addition


temperature
 Net work increase
 Heat input increase
 High moisture quality
during the turbine last
stages
 Temperature is limited
according to material
Increasing the Boiler Pressure

 Higher heat addition


temperature
 Low moisture quality
during the turbine last
stages
EXAMPLE 10-3 Effect of Boiler Pressure and Temperature on
Efficiency
Consider a steam power plant operating on the ideal Rankine cycle.
Steam enters the turbine at 3 MPa and 350°C and is condensed in
the condenser at a pressure of 10 kPa. Determine
(a) the thermal efficiency of this power plant,
(b) the thermal efficiency if steam is superheated to 600°C instead of
350°C,
(c) the thermal efficiency if the boiler pressure is raised to 15 MPa
while the turbine inlet temperature is maintained at 600°C.
(a)
(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, = s,) are determined to be

Therefore, the thermal efficiency increases from 33.4 to 37.3


percent as a result of superheating the steam from 350 to 600°C.
At the same time, the quality of the steam increases from 81.3 to
91.5 percent (in other words, the moisture content decreases
from 18.7 to 8.5 percent).
(c) State 1 remains the same in this case, but the other states
change. The enthalpies at state 2 (15 MPa and s2, = s1), state 3 (15
MPa and 600°C), and state 4 (10 kPa and s4 = s3) are determined in
a similar manner to be

The thermal efficiency increases from 37.3 to 43.0 percent as a


result of raising the boiler pressure from 3 to 15 MPa while
maintaining the turbine inlet temperature at 600°C. At the same
time, however, the quality of the steam decreases from 91.5 to 80.4
percent (in other words, the moisture content increases from 8.5
to 19.6 percent).
2 minute paper
Consider a simple ideal Rankine cycle with fixed boiler and
condenser pressures. What is the effect of superheating the
steam to a higher temperature on

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