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7/27/2019 Advanced thermodyamic cycles for Power Plants.pps
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7/27/2019 Advanced thermodyamic cycles for Power Plants.pps
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All conventional and most non-conventional power plants alwayshave a generator which is drivenby a prime mover.
The prime mover may be adevice which traps any form ofenergy found in nature and
converts into work or it may be aheat engine.
There can again be several kindsof heat engines in a power plant
viz. steam turbine or gas turbine.However, here we consider themost common type: a steamturbine power plant.
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Rankine cycle Coal fired thermal power plants
Oil fired boiler power plants Nuclear power plants
Brayton cycle Gas turbine power plant
Stirling cycle Solar power generation units.
(where the temperature difference is
low)
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540
Turbine
Condensor
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540
I.PTurbine
Condensor
L.P
Turbine
H.P
Turb
ine
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Reduces maximum temperatureto safe limits.
Decreases heat consumption ofthe boiler.
The same mount of heating at highertemperature would be less efficient,
because the higher heat dissipation.Yet reheating is a byproduct ofheating.
Moves closer to the Carnot cyclethus increases efficiency.
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540
I.PTurbine
Condensor
L.P
Turbine
H.P
Turb
ine
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ARE SO MANY REHEATS USEFUL?
Putting many turbines with less
work output (through low
pressure drops) will not beuseful.
It may give rise to more entropy
generation only
It is desired that we go to
regeneration after we reach
saturated pressure.
That will help in regeneration
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0
100
200
300
400
500
600
0 2 4 6 8 10s
T
Region boundary
Main steam/water cycle
540
LP REGENERATION
CONDENSATION
HP
TURBINE
IP
TURBINE
LP
TURB
INE
BOILER DRUM
CEP
BFP
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Main advantages ofregeneration
Less heat required by boiler.
Less size of turbine (since some steam isbled).
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Where and when can we useregeneration?
It is better to use regeneration withsaturated steam as hot fluid ratherthan superheated steam.
So we use regeneration as soon as the steambecomes saturated.
Sat
urationpoint
Distance
Temperature
Distance
Temperature
Regeneration withsuperheated steam
Regeneration withsaturated steam
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0
100
200
300
400
500
600
0 2 4 6 8 10s
T
Region boundary
Main steam/water cycle
Regeneration
540
HP REGENERATION
LP REGENERATION
CONDENSATION
HP
TURBINE
IP
TURBINE
LP
TURB
INE
BOILER DRUM
CEP
BFP
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Supercritical
Rankine CycleFrom the principle of the Carnot cycle we see by raising the
boiler temperature, the efficiency can be increased. In the
Rankine cycle, which only is a modification of the Carnot
cycle, higher boiler temperature means higher boiler
pressure. In the T-S diagram of water density difference of
water and steam decreases at higher pressure and finally
becomes zero at critical pressure. Most older boilers
required this density difference to produce flow in the boiler.
But modern forced convection boilers can handle water at
supercritical pressure.
Turbine
Condensor
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Cogeneration PlantsThe temperature which is usually the boilertemperature (heat addition temperature) of a Rankine
cycle using steam is generally same or below theexhaust (heat rejection temperature) of an ordinarygas turbine. So, the flue gas from the gas turbine isused to heat the water (in the boiler) of the Steamturbine.
Thermodynamic cycles for cogent plants naturallyconsist of two separate cycles having different workingfluids. Only the exhaust temperature of the gas turbineis a little higher than the inlet temperature of thesteam turbine and the inlet temperature of the gas
should be a little higher the exit temperature of theBFP of the steam turbine.
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0
100
200
300
400
500
600
0 2 4 6 8 10
T
sBoiler Feed Pump
Turbine
Condensor
Schematic block diagram of combined cycle power plant
Thecombinedcycle
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Acknowledgements
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