vapour power cycles
TRANSCRIPT
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Vapour Power Cycles
Introduction
Carnot Cycle
Rankine Cycle
Reheat Cycle
Regenerative Cycle
Steam Cycles for Nuclear Power
Plants
Plant Efficiency
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Introduction
Transfer of heat from a reservoir to a
working fluid taken through athermodynamic cycle
Working fluid is a condensable vapour
Cycle consists of a succession ofsteady flow processes
Each process carried out in a separatecomponent designed for the purpose
Each component is an open system
All components are connected inseries and the working fluid passes
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through a cycle of mechanical andthermodynamic states
To simplify analysis change in kineticand potential energy of the fluid areassumed to be negligible
Operating cost related to overall
efficiency of plant
SourceFuel and Air Products ofCombustion
Sink
(atmosphere)
Q1
Q2
W
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Overall Efficiency = Cycle Efficiency xCombustion Efficiency
Cycle efficiency expresses theproportion of the heat available that isconverted into useful mechanical work
Efficiency of a vapour power cycle isknown as the Ideal Cycle Efficiency ()
when all the processes are assumedto be reversible.
Ratio of actual cycle efficiency to ideal
cycle efficiency is called the efficiency
ratio.
Ratio of the network output (|W|) to the
gross work output is defined as the
Work Ratio (rw)
A direct indication of the size of vapour
power plant is provided by the Specific
Steam Consumption (SSC) usually
expressed in kg/kWh
SSC is the mass flow of steam
required per unit power output
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It can be shown that
||
3600
WSSC =
Carnot Vapour Power Cycle
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An ideal cycle in which the heat is
taken in at a constant upper
temperature and rejected at a constantlower temperature suggested by Sadi
Carnot.
Consists of two reversible isothermal
processes connected by two isentropicprocesses.
Saturated water in state 1 is
evapourated in the boiler at constant
pressure to form saturated steam in
state 2.
Heat added to the boiler
1212 hhQ =
Steam is expanded isentropically to
state 3, while doing work in a turbine.
Turbine work
3223 hhW =
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After the expansion, steam is partially
condensed at constant pressure until
state 4 is reached.
Heat rejected in the condenser
4334 hhQ =
Finally, steam is subjected to
compression isentropically, in thecompressor to state 1.
Compressor work
4141 hhW =
Ideal Cycle Efficiency
( ) ( ) ( )[ ]( )12
4132
12
4123
12
||||||
hh
hhhh
Q
WW
Q
W
=
==
or
1
31
T
TT =
Work Ratio
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( ) ( ) ( )[ ]( )32
4132
23
4123
23
||||||
hh
hhhh
W
WW
W
Wrw
=
==
Specific Steam Consumption
( ) ( ) ( )[ ]41324123
3600
||||
3600
||
3600
hhhhWWWSSC
=
==
Drawbacks ofCarnot VapourPower Cycle
Possesses a low Work Ratio
Having to run a bulky compressor
with higher power consumption
Practical difficulties associated with
the compression
Difficult to control the condensation
process so that it stops at state 4.
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Compression of a very wet vapour
in the compressor. i.e. Liquid tends
to separate out from the vapourbecause of the non-homogeneous
nature of the mixture.
Hence Carnot cycle is not used inpractice.
Simple Rankine Cycle
Vapour is completely condensed in the
condenser and a small feed pump
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compresses the liquid to the boiler
pressure.
Efficiency of the cycle is less than that
of the Carnot cycle operating between
the same temperatures, since all the
heat supplied is not transferred at the
upper temperature High Work Ratio
Less Specific Steam Consumption
Smaller plant size
The processes of Rankine cycle are as
follows
Saturated water in state 5 is
evapourated in the boiler at constant
pressure to form saturated steam in
state 2.
Heat added to the boiler
5212 hhQ =
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Steam is expanded isentropically to
state 3, while doing work in a turbine.
Turbine work
3223 hhW =
After the expansion in the turbine,
steam is completely condensed at
constant pressure.
Heat rejected in the condenser
4334 hhQ =
Finally, liquid is compressed to boiler
pressure isentropically, in the feedpump.
Feed pump work
( )4545 ppvW f =
or
From steady flow energy equation
4545 hhW =
Ideal Cycle Efficiency
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( ) ( ) ( )[ ]( )52
4532
52
4523
52
||||||
hh
hhhh
Q
WW
Q
W
=
==
Work Ratio
( ) ( ) ( )[ ]( )32
4532
23
4523
23
||||||
hh
hhhh
W
WW
W
Wrw
=
==
Specific Steam Consumption
( ) ( ) ( )[ ]45324523
3600
||||
3600
||
3600
hhhhWWWSSC
=
==
Rankine Cycle with superheat
It is possible to raise the steam
temperature without raising the boiler
pressure by sending the saturated
steam away from the boiler to a
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separate bank of tubes placed in the
combustion gases which is known as
the super-heater.
Average temperature at which heat is
supplied is increased by superheating
and hence the ideal cycle efficiency is
increased. Considerable reduction in SSC
Ideal Cycle Efficiency
( ) ( ) ( )[ ]( )25
1265
25
1256
25
||||||
hh
hhhh
Q
WW
Q
W
=
==
Work Ratio
( ) ( ) ( )[ ]( )65
1265
56
1256
56
||||||
hh
hhhh
W
WW
W
Wrw
=
==
Specific Steam Consumption
( ) ( ) ( )[ ]12651256
3600
||||
3600
||
3600
hhhhWWWSSC
=
==
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Although superheating increases the
dryness fraction of steam at the turbine
outlet, with the present metallurgicallimit it is not always sufficient to
maintain the dryness fraction above
the specified value (88%)
Reheat Cycle
Expansion takes place in two turbines
Steam expands in the high pressure
turbine to some intermediate pressure
Sent through another bank of tubes in
the boiler where it is reheated at
constant pressure, generally to the
original superheat temperature
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Expands in the low pressure turbine to
the condenser pressure
Ideal Cycle Efficiency( )
6725
127856
6725
||||||||
QQ
WWW
QQ
W
+
+=
+
=
( ) ( ) ( )[ ]( ) ( )[ ]6725
128765
hhhh
hhhhhh
+
+=
Work Ratio
( )
7856
127856
7856
||||||||
WW
WWW
WW
Wrw
+
+=
+
=
( ) ( ) ( )[ ]
( ) ( )[ ]8765
128765
hhhh
hhhhhhrw
+
+=
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Specific Steam Consumption
( )|||||| 3600||3600 127856 WWWWSSC
+
==
( ) ( ) ( )[ ]128765
3600
hhhhhhSSC
+
=
Regenerative Cycle
Raises the ideal cycle efficiency by
increasing the average temperature at
which heat is added from an external
source to the working fluid.
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Steam is expanded to an intermediate
state 3, at which a certain quantity is
bled off and taken to a feed waterheater.
Remaining quantity is expanded to
condenser pressure and leaves the
turbine in state 4. After condensationwater is compressed in the first feed
pump to the bleeding pressure.
It is mixed in the feed water heater
with the bled steam in state 3 and total
mixture leaves the heater in state 7.
A second feed pump compresses the
water to boiler pressure at state 1.
It can be shown that the cycle
efficiency is a maximum when the
temperature
2
423
TTT
+=
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Consider the feed water heater as an
adiabatic open system (Q=0) for
calculating the appropriate mass y kg to
be bled off. No work is done and the
energy equation reduces to( ) 637 110 hyyhh = Also h6=h5
53
57
hh
hhy
=
Ideal Cycle Efficiency( )
12
71563423
12
||||||||||
Q
WWWW
Q
W +==
( ) ( ) ( ) ( ) ( ) ( )[ ]( )12
71564332 11
hh
hhhhyhhyhhy
+=
Work Ratio( )
3423
71563423
3423
||||||||||
WW
WWWW
WW
Wrw
+
+=
+
=
( ) ( ) ( ) ( ) ( ) ( )[ ]( ) ( ) ( )4332
71564332
1
11
hhyhhy
hhhhyhhyhhyrw
+
+=
Specific Steam Consumption
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( )||||||||3600
||
3600
71563423 WWWWWSSC
+
==
( ) ( ) ( ) ( ) ( ) ( )[ ]71564332 113600
hhhhyhhyhhySSC
+
=
Economiser and Air preheater