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8/17/2019 Docfoc.com-Chapter 10 VAPOR AND COMBINED POWER CYCLES Mehmet Kanoglu University of Gaziantep Copyright © The McGraw-Hill Companies, Inc. Permission required fo…
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Chapter 10
VAPOR AND COMBINED POWERCYCLES
Mehmet Kanogl
!n"#er$"t% o& 'a("antep
Cop%r"ght ) *he M+'ra,-."ll Compan"e$/ In+ Perm"$$"on re"re2 &or repro2+t"on or 2"$pla%
*hermo2%nam"+$3 An Eng"neer"ng Approa+hSe#enth E2"t"on "n SI !n"t$
Yn$ A Cengel/ M"+hael A Bole$
M+'ra,-."ll/ 4011
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2
O56e+t"#e$
• Evaluate the performance of gas power cycles for
which the working fluid remains a gas throughout theentire cycle.
• Analyze vapor power cycles in which the working fluid
is alternately vaporized and condensed.
• Analyze power generation coupled with processheating called cogeneration.
• Investigate ways to modify the basic Rankine vapor
power cycle to increase the cycle thermal efficiency.
• Analyze the reheat and regenerative vapor powercycles.
• Analyze power cycles that consist of two separate
cycles known as combined cycles and binary cycles.
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*.E CARNO* VAPOR CYCLE
T-s diagram of two !arnot vapor cycles.
"he !arnot cycle is the most efficient cycle operating between two specified temperature
limits but it is not a suitable model for power cycles. #ecause$
%rocess &'2 (imiting the heat transfer processes to two'phase systems severely limits thema)imum temperature that can be used in the cycle *+,-! for water
%rocess 2' "he turbine cannot handle steam with a high moisture content because of the
impingement of li/uid droplets on the turbine blades causing erosion and wear.
%rocess ,'& It is not practical to design a compressor that handles two phases.
"he cycle in *b is not suitable since it re/uires isentropic compression to e)tremely high
pressures and isothermal heat transfer at variable pressures.
1-4 isothermal heat
addition in a boiler
4-7 isentropic e)pansion
in a turbine
7-8 isothermal heatre0ection in a condenser
8-1 isentropic
compression in a
compressor
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,
RANKINE CYCLE3 *.E IDEAL CYCLE
9OR VAPOR POWER CYCLES
1any of the impracticalities associated withthe !arnot cycle can be eliminated by
superheating the steam in the boiler and
condensing it completely in the condenser.
"he cycle that results is the Ran:"ne +%+le
which is the ideal cycle for vapor power plants.
"he ideal Rankine cycle does not involve anyinternal irreversibilities.
"he simple ideal
Rankine cycle.
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Energ% Anal%$"$ o& the I2eal Ran:"ne C%+le
"he efficiency of power plants in
the 4.5. is often e)pressed interms of heat rate which is the
amount of heat supplied in #tu6s
to generate & k7h of electricity."he thermal efficiency can be interpreted
as the ratio of the area enclosed by the
cycle on a T-s diagram to the area under
the heat'addition process.
5teady'flow energy e/uation
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DEVIA*ION O9 AC*!AL VAPOR POWER
CYCLES 9ROM IDEALI;ED ONES
*a 9eviation of actual vapor power cycle from the ideal Rankine cycle.
*b "he effect of pump and turbine irreversibilities on the ideal Rankine cycle.
"he actual vapor power cycle differs from the ideal Rankine cycle as a
result of irreversibilities in various components.
:luid friction and heat loss to the surroundings are the two common sources
of irreversibilities.
I$entrop"+ e&&"+"en+"e$
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.OW CAN WE INCREASE *.E
E99ICIENCY O9 *.E RANKINE CYCLE<
"he effect of lowering the
condenser pressure on the
ideal Rankine cycle.
"he basic idea behind all the modifications to increase the thermal efficiencyof a power cycle is the same$ Increase the average temperature at which heat is
transferred to the working fluid in the boiler, or decrease the average
temperature at which heat is rejected from the working fluid in the condenser.
Lo,er"ng the Con2en$er Pre$$re =Lowers T lo,/a#g>
"o take advantage of the increasedefficiencies at low pressures the condensers
of steam power plants usually operate well
below the atmospheric pressure. "here is a
lower limit to this pressure depending on the
temperature of the cooling medium
S"2e e&&e+t3 (owering the condenserpressure increases the moisture content of
the steam at the final stages of the turbine.
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"he effect of superheating the
steam to higher temperatures
on the ideal Rankine cycle.
Sperheat"ng the Steam to ."gh *emperatre$
=Increases T h"gh/a#g>
#oth the net work and heat input
increase as a result ofsuperheating the steam to a higher
temperature. "he overall effect is
an increase in thermal efficiency
since the average temperature at
which heat is added increases.5uperheating to higher
temperatures decreases the
moisture content of the steam at
the turbine e)it which is desirable.
"he temperature is limited bymetallurgical considerations.
%resently the highest steam
temperature allowed at the turbine
inlet is about 82
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In+rea$"ng the Bo"ler Pre$$re =Increases T h"gh/a#g>
"he effect of increasing the boiler
pressure on the ideal Rankine cycle.
:or a fi)ed turbine inlet temperature
the cycle shifts to the left and the
moisture content of steam at the
turbine e)it increases. "his sideeffect can be corrected by reheating
the steam.
A supercritical Rankine cycle.
"oday many modern steam power
plants operate at supercritical
pressures *P > 22.
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*.E IDEAL RE.EA* RANKINE CYCLEHow can we take advantage of the increased efficiencies at higher boiler pressures
without facing the problem of ecessive moisture at the final stages of the turbine!
&. 5uperheat the steam to very high temperatures. It is limited metallurgically.2. E)pand the steam in the turbine in two stages and reheat it in between *reheat
"he ideal reheat Rankine cycle.
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&&
"he average temperature at whichheat is transferred during reheating
increases as the number of reheat
stages is increased.
"he single reheat in a modern power
plant improves the cycle efficiency by , to
3> by increasing the average
temperature at which heat is transferred
to the steam."he average temperature during the
reheat process can be increased by
increasing the number of e)pansion and
reheat stages. As the number of stages is
increased the e)pansion and reheat
processes approach an isothermalprocess at the ma)imum temperature.
"he use of more than two reheat stages
is not practical. "he theoretical
improvement in efficiency from the
second reheat is about half of that which
results from a single reheat.
"he reheat temperatures are very close
or e/ual to the turbine inlet temperature.
"he optimum reheat pressure is about
one'fourth of the ma)imum cycle
pressure.
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&2
*.E IDEAL RE'ENERA*IVE RANKINE CYCLE
"he first part of the heat'addition
process in the boiler takes place at
relatively low temperatures.
?eat is transferred to the working fluid
during process 2'2 at a relatively lowtemperature. "his lowers the average
heat'addition temperature and thus the
cycle efficiency.
In steam power plants steam is e)tracted
from the turbine at various points. "his
steam which could have produced morework by e)panding further in the turbine is
used to heat the feedwater instead. "he
device where the feedwater is heated by
regeneration is called a regenerator or a
&ee2,ater heater =9W.>.
A feedwater heater is basically a heat
e)changer where heat is transferred from
the steam to the feedwater either by
mi)ing the two fluid streams *open
feedwater heaters or without mi)ing them
*closed feedwater heaters.
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&
Open 9ee2,ater .eater$
An open *or 2"re+t-+onta+t &ee2,ater
heater is basically a miing chamber,
where the steam e)tracted from the
turbine mi)es with the feedwater e)itingthe pump. Ideally the mi)ture leaves
the heater as a saturated li/uid at the
heater pressure.
"he ideal regenerativeRankine cycle with an open
feedwater heater.
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&,
Clo$e2 9ee2,ater .eater$
"he ideal regenerative Rankine cycle with a closed feedwater heater.
Another type of feedwater heater fre/uently used in steam power plants is
the +lo$e2 &ee2,ater heater in which heat is transferred from the
e)tracted steam to the feedwater without any mi)ing taking place. "he twostreams now can be at different pressures since they do not mi).
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&3
"he closed feedwater heaters are more comple) because of the internal tubing
network and thus they are more e)pensive. ?eat transfer in closed feedwater
heaters is less effective since the two streams are not allowed to be in direct contact.
?owever closed feedwater heaters do not re/uire a separate pump for each heater
since the e)tracted steam and the feedwater can be at different pressures.
@pen feedwater
heaters are simple
and ine)pensive and
have good heat
transfer
characteristics. :or
each heater however
a pump is re/uired to
handle the feedwater.
1ost steam powerplants use a
combination of open
and closed feedwater
heaters.
A steam power plant with one open and three closed feedwater heaters.
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&8
SECOND-LAW ANALYSIS O9 VAPOR
POWER CYCLES
E)ergy destruction for a steady'flow system
5teady'flow one'
inlet one'e)it
E)ergy destruction of a cycle
:or a cycle with heat transfer
only with a source and a sink
5tream e)ergy
A second'law analysis of vapor power cycles reveals where the
largest irreversibilities occur and where to start improvements.
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&+
CO'ENERA*ION
A simple process'heating plant.
1any industries re/uire energy input in the form of heat called process
heat . %rocess heat in these industries is usually supplied by steam at 3 to
+ atm and &3< to 2
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&; An ideal cogeneration plant.
4tilization
factor
• "he utilization factor of the
ideal steam'turbine
cogeneration plant is
&
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&=
A cogeneration plant with
ad0ustable loads.
At times of high demand for process heat all
the steam is routed to the process'heating units
and none to the condenser *m+
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2<
COMBINED 'AS?VAPOR POWER CYCLES
• "he continued /uest for higher thermal efficiencies has resulted in rather
innovative modifications to conventional power plants.
• A popular modification involves a gas power cycle topping a vapor power cycle
which is called the +om5"ne2 ga$?#apor +%+le or 0ust the +om5"ne2 +%+le.
• "he combined cycle of greatest interest is the gas'turbine *#rayton cycle topping
a steam'turbine *Rankine cycle which has a higher thermal efficiency than
either of the cycles e)ecuted individually.
• It makes engineering sense to take advantage of the very desirablecharacteristics of the gas'turbine cycle at high temperatures and to use the high'
temperature e)haust gases as the energy source for the bottoming cycle such as
a steam power cycle. "he result is a combined gasDsteam cycle.
• Recent developments in gas'turbine technology have made the combined gasD
steam cycle economically very attractive.
• "he combined cycle increases the efficiency without increasing the initial cost
greatly. !onse/uently many new power plants operate on combined cycles and
many more e)isting steam' or gas'turbine plants are being converted to
combined'cycle power plants.
• "hermal efficiencies over 3 are reported.
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2&!ombined gasDsteam power plant.
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Smmar%
• "he !arnot vapor cycle
• Rankine cycle$ "he ideal cycle for vapor power cycles Energy analysis of the ideal Rankine cycle
• 9eviation of actual vapor power cycles from idealized ones
• ?ow can we increase the efficiency of the Rankine cycle
(owering the condenser pressure *"owers T lowavg
5uperheating the steam to high temperatures *Increases T highavg
Increasing the boiler pressure *Increases T highavg
• "he ideal reheat Rankine cycle
• "he ideal regenerative Rankine cycle
@pen feedwater heaters
!losed feedwater heaters
• 5econd'law analysis of vapor power cycles
• !ogeneration
• !ombined gasDvapor power cycles