ch08 - Copy
Transcript of ch08 - Copy
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Introducing Vapor Power Plants
In fossil-fueled plants, the energy required for
vaporization originates in combustion of the fuel.
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Power Cycle Review
in
cycle
Q
W
The first law of thermodynamicsrequires the net work developedby a
system undergoing a power cycle to
equalthe net energy added by heat
transferto the system:
The thermal efficiencyof a power
cycle is
Wcycle= QinQout
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Power Cycle Review
The second law of thermodynamics requires the thermal
efficiency to be less than 100%. Most of todays vapor power
plants have thermal efficiencies ranging up to about 40%.
Thermal efficiencytends to increase as the average
temperature at which energy is added by heat transfer
increasesand/or the average temperature at which energy is
rejected by heat transfer decreases.
Improved thermodynamic performanceof power cycles, as
measured by increased thermal efficiency, for example, also
accompanies the reduction of irreversibilities and losses.
The extent of improved power cycle performance is limited,
however, by constraints imposed by thermodynamicsand
economics.
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The Rankine CycleEach unit of mass of water circulating through the
interconnected components of Subsystem B ofFig. 8.1(a)undergoes a thermodynamic cycle known as the Rankine cycle.
All energy transfersby work and heat are taken as positive in
the directions of the arrowson the schematic and energy
balances are written accordingly.
There are four principal
control volumes involving
these components:
Turbine
Condenser
Pump
Boiler
http://www.wiley.com/college/moran/0470495901/ig/Ch8/pages/fig_08_03.htm -
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Turbine
Condenser
Pump
Boiler
The Rankine Cycle
34
p
hhm
W
21t hhm
W
41in hhm
Q
(Eq. 8.3)
(Eq. 8.4)
(Eq. 8.2)
(Eq. 8.1)
32out hhm
Q
Applying mass and energy rate balances
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Thermal Efficiency
The Rankine Cycle
)(
)(
/
/bwr
21
34
t
p
hh
hh
mW
mW
(Eq. 8.6)
(Eq. 8.5a)
Performance parameters
)(
)()(
/
//
41
3421
in
pt
in
cycle
hh
hhhh
mQ
mWmW
Q
W
Back Work Ratio
Back work ratio is characteristically low for vapor
power plants. For instance, in Example 8.1, the
power required by the pump is less than 1% of the
power developed by the turbine.
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Ideal Rankine Cycle
Since the ideal Rankine cycle involvesinternally reversible processes, results from
Sec. 6.13apply.
Applying Eq. 6.51c, the pump work input perunit of mass flowing is evaluated as follows
)( 343
s
ppp
m
W
v
(Eq. 8.7b)
wherev3is the specific volume at the pump
inletand the subscript ssignals the isentropic
processof the liquid flowing through the pump.
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)()(
/
)/(
s21
21
st
t
t hh
hh
mW
mW
Principal Irreversibilities
Isentropic turbine efficiency, introduced in Sec. 6.12.1,
accounts for the effects of irreversibilities within the turbinein
terms of actual and isentropic turbine work, each per unit of
mass flowing through the turbine.
(Eq. 8.9)
work developed in the actual
expansion from turbine inlet state
to the turbine exit pressure
work developed in an isentropic
expansion from turbine inlet
state to exit pressure
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)()(
/
)/(
34
3s4
p
sp
phh
hh
mW
mW
Principal Irreversibilities
While pumpwork input is much less than turbine work
output, irreversibilities in the pump affect net power outputofthe vapor plant.
Isentropic pump efficiency, introduced in Sec. 6.12.3,
accounts for the effects of irreversibilities within the pumpin
terms of actual and isentropic pump work input, each per unit
of mass flowing through the pump.
(Eq. 8.10a)
work input for the actual process from pump
inlet state to the pump exit pressure
work input for an isentropic process
from pump inlet state to exit pressure
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With fraction yknown, mass and energy rate
balancesapplied to control volumes around theother componentsyield the following expressions,
each on the basis of a unit of mass entering the
first turbine stage.
Regenerative Vapor Power Cycle Using
an Open Feedwater Heater
Applying steady-state mass and energy ratebalancesto a control volume enclosing the
feedwater heater, the fraction of the total flow yis
(Eq. 8.12)
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Regenerative Vapor Power Cycle Using
an Open Feedwater Heater
For the pumps
(Eq. 8.14)
For the steam generator
(Eq. 8.15) (Eq. 8.16)
For the condenser
For the turbine stages
(Eq. 8.13)