Analysis of Steam Cycles

1. Analysis of Steam Cycles
Dr. Rohit Singh Lather, Ph.D.

2. Constant Pressure Steam Generation ProcessConstant Pressure Steam Generation ProcessConstant Pressure Steam Generation ProcessConstant Pressure Steam Generation Process
Theory of flowing Steam Generation
vdpdhq 
qHVmfuelSG  
Constant Pressure Steam Generation:
vdpdhq  = 0

3. Types of SteamTypes of SteamTypes of SteamTypes of Steam
Sensible HeatSensible Heat
It should be noted that the
points 2 and 3 are at the
same boiling point
temperature and pressure
and also that, at those
conditions, the liquid and the
steam (whether wet or dry)
are in equilibrium with each
other.
Sensible HeatSensible Heat
11/10/2014 Modern Power Plants 3
Sensible HeatSensible Heat
It should be noted that the
points 2 and 3 are at the
same boiling point
temperature and pressure
and also that, at those
conditions, the liquid and the
steam (whether wet or dry)
are in equilibrium with each
other.
Sensible HeatSensible Heat

4. • Wet steam: AA mixturemixture ofof waterwater plusplus steamsteam (liquid plus vapor) at the boiling
point temperature of water at a given pressure. Quality of steam refers to
the fraction or percentage of gaseous steam in a wet steam mixture.
• Dry steam: Steam, at the given pressure, that containscontains nono waterwater (also(also
referredreferred toto asas saturatedsaturated steam)steam).. TheThe steamsteam qualityquality == 100100%%.. At the top of
steam generator units for producing saturated steam, there are moisture
separators used to remove residual water droplets from outgoing steam.
• Superheated steam: Dry steam, at the given pressure, that has been
heatedheated toto aa temperaturetemperature higherhigher thanthan thethe boilingboiling pointpoint ofof waterwater atat thatthat
pressurepressure..
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• Wet steam: AA mixturemixture ofof waterwater plusplus steamsteam (liquid plus vapor) at the boiling
point temperature of water at a given pressure. Quality of steam refers to
the fraction or percentage of gaseous steam in a wet steam mixture.
• Dry steam: Steam, at the given pressure, that containscontains nono waterwater (also(also
referredreferred toto asas saturatedsaturated steam)steam).. TheThe steamsteam qualityquality == 100100%%.. At the top of
steam generator units for producing saturated steam, there are moisture
separators used to remove residual water droplets from outgoing steam.
• Superheated steam: Dry steam, at the given pressure, that has been
heatedheated toto aa temperaturetemperature higherhigher thanthan thethe boilingboiling pointpoint ofof waterwater atat thatthat
pressurepressure..

5. Temperature Entropy DiagramTemperature Entropy DiagramTemperature Entropy DiagramTemperature Entropy Diagram
Latent Heat of EvaporationLatent Heat of Evaporation

6. • The horizontal axis is not enthalpy but instead is enthalpy divided by the mean
temperature at which the enthalpy is added or removed.
• By starting at the origin of the graph at a temperature of 0°C at atmospheric
pressure, and by adding enthalpy in small amounts, the graph can be built.
• As entropy is measured in terms of absolute temperature, the origin temperature of
0°C is taken as 273.15 K.
• The specific heat of saturated water at this temperature is 4.228 kJ/kg K. For the
purpose of constructing the diagram in the base temperature is taken as 273 K.
• Assuming 1kg of water at atmospheric pressure, and by adding 4.228 kJ of energy,
the water temperature would rise by 1 K from 273 K to 274 K. The mean temperature
during this operation is 273.5 K.
• The horizontal axis is not enthalpy but instead is enthalpy divided by the mean
temperature at which the enthalpy is added or removed.
• By starting at the origin of the graph at a temperature of 0°C at atmospheric
pressure, and by adding enthalpy in small amounts, the graph can be built.
• As entropy is measured in terms of absolute temperature, the origin temperature of
0°C is taken as 273.15 K.
• The specific heat of saturated water at this temperature is 4.228 kJ/kg K. For the
purpose of constructing the diagram in the base temperature is taken as 273 K.
• Assuming 1kg of water at atmospheric pressure, and by adding 4.228 kJ of energy,
the water temperature would rise by 1 K from 273 K to 274 K. The mean temperature
during this operation is 273.5 K.
• The horizontal axis is not enthalpy but instead is enthalpy divided by the mean
temperature at which the enthalpy is added or removed.
• By starting at the origin of the graph at a temperature of 0°C at atmospheric
pressure, and by adding enthalpy in small amounts, the graph can be built.
• As entropy is measured in terms of absolute temperature, the origin temperature of
0°C is taken as 273.15 K.
• The specific heat of saturated water at this temperature is 4.228 kJ/kg K. For the
purpose of constructing the diagram in the base temperature is taken as 273 K.
• Assuming 1kg of water at atmospheric pressure, and by adding 4.228 kJ of energy,
the water temperature would rise by 1 K from 273 K to 274 K. The mean temperature
during this operation is 273.5 K.
• The horizontal axis is not enthalpy but instead is enthalpy divided by the mean
temperature at which the enthalpy is added or removed.
• By starting at the origin of the graph at a temperature of 0°C at atmospheric
pressure, and by adding enthalpy in small amounts, the graph can be built.
• As entropy is measured in terms of absolute temperature, the origin temperature of
0°C is taken as 273.15 K.
• The specific heat of saturated water at this temperature is 4.228 kJ/kg K. For the
purpose of constructing the diagram in the base temperature is taken as 273 K.
• Assuming 1kg of water at atmospheric pressure, and by adding 4.228 kJ of energy,
the water temperature would rise by 1 K from 273 K to 274 K. The mean temperature
during this operation is 273.5 K.
This value represents the change in enthalpy per
degree of temperature rise for one kilogram of
water and is termed the change in specific
entropy. The metric units for specific entropy are
therefore kJ/kg K.
This value represents the change in enthalpy per
degree of temperature rise for one kilogram of
water and is termed the change in specific
entropy. The metric units for specific entropy are
therefore kJ/kg K.

7. • As the temperature increases, the change in entropy for each equal
increment of enthalpy reduces slightly.
• If this incremental process were continuously repeated by adding more
heat, it would be noticed that the change in entropy would continue to
decrease.
• This is due to each additional increment of heat raising the temperature
and so reducing the width of the elemental strip representing it. As more
heat is added, so the state point line, in this case the saturated water
line, curves gently upwards.
• As the temperature increases, the change in entropy for each equal
increment of enthalpy reduces slightly.
• If this incremental process were continuously repeated by adding more
heat, it would be noticed that the change in entropy would continue to
decrease.
• This is due to each additional increment of heat raising the temperature
and so reducing the width of the elemental strip representing it. As more
heat is added, so the state point line, in this case the saturated water
line, curves gently upwards.
• As the temperature increases, the change in entropy for each equal
increment of enthalpy reduces slightly.
• If this incremental process were continuously repeated by adding more
heat, it would be noticed that the change in entropy would continue to
decrease.
• This is due to each additional increment of heat raising the temperature
and so reducing the width of the elemental strip representing it. As more
heat is added, so the state point line, in this case the saturated water
line, curves gently upwards.
• As the temperature increases, the change in entropy for each equal
increment of enthalpy reduces slightly.
• If this incremental process were continuously repeated by adding more
heat, it would be noticed that the change in entropy would continue to
decrease.
• This is due to each additional increment of heat raising the temperature
and so reducing the width of the elemental strip representing it. As more
heat is added, so the state point line, in this case the saturated water
line, curves gently upwards.

8. • At 373.14 K (99.99°C), the boiling point of water is reached at atmospheric pressure, and further
additions of heat begin to boil off some of the water at this constant temperature.
• At this position, the state point starts to move horizontally across the diagram to the right, and is
represented by the horizontal evaporation line stretching from the saturated water line to the dry
saturated steam line.
• Because this is an evaporation process, this added heat is referred to as enthalpy of evaporation.
At atmospheric pressure, steam tables state that the
amount of heat added to evaporate 1 kg of water
into steam is 2256.71 kJ. As this takes place at a
constant temperature of 373.14 K, the mean
temperature of the evaporation line is also 373.14 K.
The change in specific entropy from the water
saturation line to the steam saturation line is
therefore:
At atmospheric pressure, steam tables state that the
amount of heat added to evaporate 1 kg of water
into steam is 2256.71 kJ. As this takes place at a
constant temperature of 373.14 K, the mean
temperature of the evaporation line is also 373.14 K.
The change in specific entropy from the water
saturation line to the steam saturation line is
therefore:
1: Saturated water line.
2 : Dry saturated steam line.
3 : Constant dryness fraction lines in the wet
steam region.
4: Constant pressure lines in the superheat
region.

9. H – S Diagram
Isenthalpic expansion of steam through a
control valve is simply represented by a
straight horizontal line from the initial
state to the final lower pressure
The isentropic expansion of steam through a
nozzle is simply a line from the initial state
falling vertically to the lower final pressure.

10. • As the expansion through a control valve orifice is an isenthalpic process
Accelerate at high Speed
Borrowing energy from its enthalpy and converting it to
kinetic energy.
Heat
Drop
This part of the process is
isentropic
This part of the process is
isentropic
SteamSteam DeacceleratesDeaccelerates
Fall in velocity requires a reduction in kinetic
energy which is mostly re-converted back into
heat and re-absorbed by the steam.
The heat drop that caused the initial increase
in kinetic energy is reclaimed (except for a
small portion lost due to the effects of
friction), and on the H - S chart, the state point
moves up the constant pressure line until it
arrives at the same enthalpy value as the initial
condition.

11. Steam Generation : Expenditure Vs WastageSteam Generation : Expenditure Vs Wastage
Liquid +Vapour
Vapour
h
s
Liquid

12. Variable Pressure Steam GenerationVariable Pressure Steam GenerationVariable Pressure Steam GenerationVariable Pressure Steam Generation
s
h

13. Pressure, MPa Enthalpy, kJ/Kg Entropy, kJ/Kg/K Temp, C Volume, m³ /kg
1 1 3500 7.79 509.9 0.3588
2 5 3500 7.06 528.4 0.07149
3 10 3500 6.755 549.6 0.03562
4 15 3500 6.582 569 0.02369
Analysis of Steam Generation at Various PressuresAnalysis of Steam Generation at Various PressuresAnalysis of Steam Generation at Various PressuresAnalysis of Steam Generation at Various Pressures
4 15 3500 6.582 569 0.02369
5 20 3500 6.461 586.7 0.01776
6 25 3500 6.37 602.9 0.01422
7 30 3500 6.297 617.7 0.01187
8 35 3500 6.235 631.3 0.0102

14. More Availability of EnergyMore Availability of EnergyMore Availability of EnergyMore Availability of Energy
Temp, C Pressure, MPa Volume m³/kg Enthalpy, kJ/kg Entropy, kJ/kg/K
575 5 0.0762 3608 7.191
575 10 0.03701 3563 6.831
575 12.5 0.02917 3540 6.707
575 15 0.02393 3516 6.601
575 17.5 0.02019 3492 6.507
575 20 0.01738 3467 6.422
575 22.5 0.0152 3441 6.344
575 25 0.01345 3415 6.271
575 30 0.01083 3362 6.138
575 35 0.008957 3307 6.015

15. 6.6
6.8
7
7.2
7.4
Creation/Reduction of WastageCreation/Reduction of WastageCreation/Reduction of WastageCreation/Reduction of Wastage
5.8
6
6.2
6.4
6.6
0 10 20 30 40
s
MPap,

16. Less Fuel for Creation of Same TemperatureLess Fuel for Creation of Same TemperatureLess Fuel for Creation of Same TemperatureLess Fuel for Creation of Same Temperature
3500
3550
3600
3650
kg
kJh,
3250
3300
3350
3400
3450
0 10 20 30 40
MPap,
kg
kJh,

17. Carnot Cycle (NoCarnot Cycle (No IrreversablitiesIrreversablities))Carnot Cycle (NoCarnot Cycle (No IrreversablitiesIrreversablities))
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1. No heat device can generate work withoutwithout netnet rejectionrejection ofof heatheat toto aa lowlow
temperaturetemperature reservoirreservoir..
2. It is impossible for any device that operates in a cycle toto receivereceive heatheat fromfrom aa singlesingle
highhigh temperaturetemperature reservoirreservoir andand produceproduce aa netnet amountamount ofof workwork and no other effects.

18. Introduction to Rankine cycleIntroduction to Rankine cycleIntroduction to Rankine cycleIntroduction to Rankine cycle
• It is difficult if not impossible, to maintainmaintain perfectperfect constantconstant temperaturetemperature
heatheat additionaddition andand heatheat rejectionrejection..
• By usingusing waterwater asas thethe workingworking fluid,fluid, andand consideringconsidering thethe latentlatent heatheat
conceptconcept can closely resemble the theoreticaltheoretical CarnotCarnot cyclecycle.
•• Rankine cycle is a waterRankine cycle is a water--vapor cycle that describe the operation of steamvapor cycle that describe the operation of steam
power generation system in it’s thermodynamic aspects.power generation system in it’s thermodynamic aspects.
• Rankine cycle is also referred to as a “practical Carnot cycle”“practical Carnot cycle” due to :
1. The TThe T--s diagram resembless diagram resembles the Carnot cycle.
2. Heat addition in the boiler and heat rejection in the condenser takes
place :
i– Isothermally in Carnot[ ΔT = 0 ]
ii –Isobarically in Rankine[ ΔP = 0 ]
3. Steam is converted in the condenser to saturated liquidsaturated liquid
• It is difficult if not impossible, to maintainmaintain perfectperfect constantconstant temperaturetemperature
heatheat additionaddition andand heatheat rejectionrejection..
• By usingusing waterwater asas thethe workingworking fluid,fluid, andand consideringconsidering thethe latentlatent heatheat
conceptconcept can closely resemble the theoreticaltheoretical CarnotCarnot cyclecycle.
•• Rankine cycle is a waterRankine cycle is a water--vapor cycle that describe the operation of steamvapor cycle that describe the operation of steam
power generation system in it’s thermodynamic aspects.power generation system in it’s thermodynamic aspects.
• Rankine cycle is also referred to as a “practical Carnot cycle”“practical Carnot cycle” due to :
1. The TThe T--s diagram resembless diagram resembles the Carnot cycle.
2. Heat addition in the boiler and heat rejection in the condenser takes
place :
i– Isothermally in Carnot[ ΔT = 0 ]
ii –Isobarically in Rankine[ ΔP = 0 ]
3. Steam is converted in the condenser to saturated liquidsaturated liquid
11/10/2014 Modern Power Plants 18

19. Rankine CycleRankine CycleRankine CycleRankine Cycle
A Rankine cycle consists of the following processes:-
• Process 4 - 1 Isentropic Compression: The working
fluid is pumped from low to high pressure.
• Process 1 - 2 Isobaric Heat Supply: The high pressure
liquid enters a boiler where it is heated at constant
pressure by an external heat source to become a dry
saturated vapor.
• Process 2 - 3 Isentropic Expansion: The dry saturated
vapor expands through a turbine, generating power.
This decreases the temperature and pressure of the
vapor, and some condensation may occur.
• Process 3 - 4 Isobaric Heat Rejection: The wet vapor
then enters a condenser where it is condensed at a
constant pressure to become a saturated liquid.
A Rankine cycle consists of the following processes:-
• Process 4 - 1 Isentropic Compression: The working
fluid is pumped from low to high pressure.
• Process 1 - 2 Isobaric Heat Supply: The high pressure
liquid enters a boiler where it is heated at constant
pressure by an external heat source to become a dry
saturated vapor.
• Process 2 - 3 Isentropic Expansion: The dry saturated
vapor expands through a turbine, generating power.
This decreases the temperature and pressure of the
vapor, and some condensation may occur.
• Process 3 - 4 Isobaric Heat Rejection: The wet vapor
then enters a condenser where it is condensed at a
constant pressure to become a saturated liquid.
11/10/2014 Modern Power Plants 19

20. Losses in Rankine CycleLosses in Rankine CycleLosses in Rankine CycleLosses in Rankine Cycle
• At the end of condensationcondensation thethe liquidliquid pressurepressure mustmust bebe raisedraised toto boilerboiler pressurepressure by the action of
the feed water pump, and this is bring about the first kind of power losses.
• The pumped saturated liquid inside the boiler is out of the liquid saturation line, and therefore heatheat
mustmust bebe addedadded atat constantconstant pressurepressure toto returnreturn toto saturationsaturation pointpoint, such heat added is another kind
of power losses in the cycle.
• The efficiency of Rankine cycle is limited by the working fluid possible temperature range to avoid
construction material failure, and on the other side to avoid possible condenser leakage and
inability of heat removal. Such limits (Th=565 °C) and (TL = 30 °C) will results in Carnot thermal
efficiency of (63%) compared to (42%) in modern coal fired power station.
• Work required by the pump consumes only ( 1 to 3 %)of the turbine power o/p, since all vapor is
converted to liquid in the condenser. However heat must be added in the boiler to raise pumped
liquid to saturation temperature. i.e
Pump work is improved →→→ thermal eff. Is increased
Overall heat added →→→ thermal eff. Is reduced
Therefore, the resultant effects → is better thermal eff.
• At the end of condensationcondensation thethe liquidliquid pressurepressure mustmust bebe raisedraised toto boilerboiler pressurepressure by the action of
the feed water pump, and this is bring about the first kind of power losses.
• The pumped saturated liquid inside the boiler is out of the liquid saturation line, and therefore heatheat
mustmust bebe addedadded atat constantconstant pressurepressure toto returnreturn toto saturationsaturation pointpoint, such heat added is another kind
of power losses in the cycle.
• The efficiency of Rankine cycle is limited by the working fluid possible temperature range to avoid
construction material failure, and on the other side to avoid possible condenser leakage and
inability of heat removal. Such limits (Th=565 °C) and (TL = 30 °C) will results in Carnot thermal
efficiency of (63%) compared to (42%) in modern coal fired power station.
• Work required by the pump consumes only ( 1 to 3 %)of the turbine power o/p, since all vapor is
converted to liquid in the condenser. However heat must be added in the boiler to raise pumped
liquid to saturation temperature. i.e
Pump work is improved →→→ thermal eff. Is increased
Overall heat added →→→ thermal eff. Is reduced
Therefore, the resultant effects → is better thermal eff.
11/10/2014 Modern Power Plants 20

21. Rankine CycleRankine CycleRankine CycleRankine Cycle
• SFEE for boiler
Q₁ = h₁ - h₄
• SFEE for turbine
Q₂ = h₂ – h₃
• SFEE for pump
Wp = h₄ – h₃
• The efficiency of Rankine cycle:
Ƞ = Wnet / Q₁
= (Wt – Wp) / Q₁
= (h₁ – h₂) - (h₄ – h₃) / (h₁ - h₄)
• SFEE for boiler
Q₁ = h₁ - h₄
• SFEE for turbine
Q₂ = h₂ – h₃
• SFEE for pump
Wp = h₄ – h₃
• The efficiency of Rankine cycle:
Ƞ = Wnet / Q₁
= (Wt – Wp) / Q₁
= (h₁ – h₂) - (h₄ – h₃) / (h₁ - h₄)

22.  TheThe capacitycapacity ofof aa steamsteam powerpower plantplant isis oftenoften expressedexpressed inin termsterms ofof SteamSteam
RateRate oror SpecificSpecific SteamSteam ConsumptionConsumption (S(S..SS..C)C)..
 DefinedDefined asas thethe raterate ofof steamsteam flowflow (kg/s)(kg/s) requiredrequired toto produceproduce unitunit shaftshaft
outputoutput ((11 kW)kW)..
SteamSteam RateRate (S(S..RR..)) == 11//WnetWnet ;;kg/kWkg/kW ss
 TheThe cyclecycle efficiencyefficiency isis sometimessometimes expressedexpressed alternativelyalternatively asas HeatHeat RateRate
whichwhich isis thethe raterate ofof heatheat inputinput (kJ/s)(kJ/s) requiredrequired toto produceproduce unitunit shaftshaft outputoutput..
HeatHeat RateRate (H(H..RR..)) == Q₁Q₁ // WtWt –– WpWp == 11//ȠȠ;; kJ/kWkJ/kW ss
 TheThe capacitycapacity ofof aa steamsteam powerpower plantplant isis oftenoften expressedexpressed inin termsterms ofof SteamSteam
RateRate oror SpecificSpecific SteamSteam ConsumptionConsumption (S(S..SS..C)C)..
 DefinedDefined asas thethe raterate ofof steamsteam flowflow (kg/s)(kg/s) requiredrequired toto produceproduce unitunit shaftshaft
outputoutput ((11 kW)kW)..
SteamSteam RateRate (S(S..RR..)) == 11//WnetWnet ;;kg/kWkg/kW ss
 TheThe cyclecycle efficiencyefficiency isis sometimessometimes expressedexpressed alternativelyalternatively asas HeatHeat RateRate
whichwhich isis thethe raterate ofof heatheat inputinput (kJ/s)(kJ/s) requiredrequired toto produceproduce unitunit shaftshaft outputoutput..
HeatHeat RateRate (H(H..RR..)) == Q₁Q₁ // WtWt –– WpWp == 11//ȠȠ;; kJ/kWkJ/kW ss

23. Latent heat of
Vaporization 5 - 6
Const. Pr. Heating
Heated 4 – 5 till it becomes
saturated liquid
Latent heat of
Vaporization 5 - 6
Sensible Heating

24. • As the pressure increases, the latent heat decreases and so
the heat absorbed in the evaporator decreases and the
fraction of the total heat absorbed in the superheater
increases.
• In high pressure boilers, more than 40% of the total heat is
absorbed in the superheaters.
• There is a physical limit to the amount of heat that water
can absorb in any state. As the amount of heat that is used
to raise the temperature of water before vaporization can
take place increases, the amount of latent heat used to
accomplish that vaporization decrease.
Pressure
• As the pressure increases, the latent heat decreases and so
the heat absorbed in the evaporator decreases and the
fraction of the total heat absorbed in the superheater
increases.
• In high pressure boilers, more than 40% of the total heat is
absorbed in the superheaters.
• There is a physical limit to the amount of heat that water
can absorb in any state. As the amount of heat that is used
to raise the temperature of water before vaporization can
take place increases, the amount of latent heat used to
accomplish that vaporization decrease.
Latent Heat

27. • The total heat of the steam increases with the system pressure up to
approximately 30 bar.
• Thereafter, the total heat decreases with increasing pressure.
• As the steam system approaches the critical pressure, little expansion can
take place.
• The specific volume of steam decreases as the system pressure increases.
• At atmospheric pressure a pound (0.4535 kg) of steam occupies about
1,600 times the volume of a pound of condensate.
• As pressure increases in the system, the specific volume of the steam
decreases.
• Because of this principle, a greater quantity of steam is available in a
smaller space (pipe, heat exchanger) at higher pressures.
• This is why steam is often distributed at higher pressure but is reduced to
a lower pressure to perform specific heat-transfer functions.
• The total heat of the steam increases with the system pressure up to
approximately 30 bar.
• Thereafter, the total heat decreases with increasing pressure.
• As the steam system approaches the critical pressure, little expansion can
take place.
• The specific volume of steam decreases as the system pressure increases.
• At atmospheric pressure a pound (0.4535 kg) of steam occupies about
1,600 times the volume of a pound of condensate.
• As pressure increases in the system, the specific volume of the steam
decreases.
• Because of this principle, a greater quantity of steam is available in a
smaller space (pipe, heat exchanger) at higher pressures.
• This is why steam is often distributed at higher pressure but is reduced to
a lower pressure to perform specific heat-transfer functions.

28. • Rising pressure constrains the molecules from moving apart when water vaporizes.
At the critical pressure of 221 bar, the specific volumes of liquid and gas are equal.
• Differences in density are generally referred to by the measurement of specific
volume. As pressure increases in the system, the specific volume of the liquid rises
marginally.
Water is incompressible, and therefore is not affected by steam pressure. As
pressure increases, however, so does the steam temperature. This temperature
increase results in greater motion in the molecules of the water and a slight
increase in its specific volume.
• Rising pressure constrains the molecules from moving apart when water vaporizes.
At the critical pressure of 221 bar, the specific volumes of liquid and gas are equal.
• Differences in density are generally referred to by the measurement of specific
volume. As pressure increases in the system, the specific volume of the liquid rises
marginally.
Water is incompressible, and therefore is not affected by steam pressure. As
pressure increases, however, so does the steam temperature. This temperature
increase results in greater motion in the molecules of the water and a slight
increase in its specific volume.

29. Improving Rankine CycleImproving Rankine CycleImproving Rankine CycleImproving Rankine Cycle
 The efficiency of the steam turbine will be limited by water droplet
formation. As water condenses, water droplets hit the turbine blades at
high speed, causing ‘’pitting & erosion’’, and so, gradually decreasing the
life and efficiency of the turbine.
 In order to avoid above disadvantages and improve cycle efficiency, the
followings are to be used wherever possible :
 LoweringLowering thethe CondenserCondenser PressurePressure
 IncreasingIncreasing thethe BoilerBoiler PressurePressure
 CycleCycle withwith SuperheatSuperheat
 CycleCycle withwith ReheatReheat
 CycleCycle withwith RegenerationRegeneration
 The efficiency of the steam turbine will be limited by water droplet
formation. As water condenses, water droplets hit the turbine blades at
high speed, causing ‘’pitting & erosion’’, and so, gradually decreasing the
life and efficiency of the turbine.
 In order to avoid above disadvantages and improve cycle efficiency, the
followings are to be used wherever possible :
 LoweringLowering thethe CondenserCondenser PressurePressure
 IncreasingIncreasing thethe BoilerBoiler PressurePressure
 CycleCycle withwith SuperheatSuperheat
 CycleCycle withwith ReheatReheat
 CycleCycle withwith RegenerationRegeneration
11/10/2014 Modern Power Plants 29

30. Mean Temperature of Heat AdditionMean Temperature of Heat Addition
ηrankine = 1 – T₂/ Tm
Lowering the condenser pressure, Higher will be the
efficiency of Rankine cycle.
Higher the mean temperature higher will be the cycle efficiencyHigher the mean temperature higher will be the cycle efficiency
We can improve the efficiency of a Rankine
power cycle by increasing the pressure of the
boiler and by decreasing the pressure of the
condenser.
Lowering the operating pressure of the
condenser lowers the temperature at which
heat is rejected. The overall effect of lowering
the condenser pressure is an increase in the
thermal efficiency of the cycle.
We can improve the efficiency of a Rankine
power cycle by increasing the pressure of the
boiler and by decreasing the pressure of the
condenser.
Lowering the operating pressure of the
condenser lowers the temperature at which
heat is rejected. The overall effect of lowering
the condenser pressure is an increase in the
thermal efficiency of the cycle.
We can improve the efficiency of a Rankine
power cycle by increasing the pressure of the
boiler and by decreasing the pressure of the
condenser.
Lowering the operating pressure of the
condenser lowers the temperature at which
heat is rejected. The overall effect of lowering
the condenser pressure is an increase in the
thermal efficiency of the cycle.
We can improve the efficiency of a Rankine
power cycle by increasing the pressure of the
boiler and by decreasing the pressure of the
condenser.
Lowering the operating pressure of the
condenser lowers the temperature at which
heat is rejected. The overall effect of lowering
the condenser pressure is an increase in the
thermal efficiency of the cycle.

31. Increasing the Boiler PressureIncreasing the Boiler Pressure
Increasing the operating pressure of the
boiler, automatically raises the
temperature at which boiling takes
place.
This raises the average temperature at
which heat is added to the steam and
thus raises the thermal efficiency of the
cycle.
Increasing the operating pressure of the
boiler, automatically raises the
temperature at which boiling takes
place.
This raises the average temperature at
which heat is added to the steam and
thus raises the thermal efficiency of the
cycle.
Increasing the operating pressure of the
boiler, automatically raises the
temperature at which boiling takes
place.
This raises the average temperature at
which heat is added to the steam and
thus raises the thermal efficiency of the
cycle.
Increasing the operating pressure of the
boiler, automatically raises the
temperature at which boiling takes
place.
This raises the average temperature at
which heat is added to the steam and
thus raises the thermal efficiency of the
cycle.

32. Effect of SuperheatingEffect of Superheating
The average temperature at which heat is
added to the steam can be increased without
increasing the boiler pressure by superheating
the steam to high temperatures.
Superheating the steam to higher
temperatures has another very desirable
effect: It decreases the moisture content of the
steam at the turbine exit.
The average temperature at which heat is
added to the steam can be increased without
increasing the boiler pressure by superheating
the steam to high temperatures.
Superheating the steam to higher
temperatures has another very desirable
effect: It decreases the moisture content of the
steam at the turbine exit.
The average temperature at which heat is
added to the steam can be increased without
increasing the boiler pressure by superheating
the steam to high temperatures.
Superheating the steam to higher
temperatures has another very desirable
effect: It decreases the moisture content of the
steam at the turbine exit.
The average temperature at which heat is
added to the steam can be increased without
increasing the boiler pressure by superheating
the steam to high temperatures.
Superheating the steam to higher
temperatures has another very desirable
effect: It decreases the moisture content of the
steam at the turbine exit.

33. Reheat CycleReheat Cycle
1. Increases thermal efficiency
2.Increases dryness fraction of the steam at
turbine exhaust, this will reduce blade erosion.
3.Increase work done per unit mass of
steam, thus reducing boiler size.
4. Increases plant cost due to re-heater
requirement and it’s long piping system.
5.Increases condenser capacity due to the
increase in steam dryness fraction.
1. Increases thermal efficiency
2.Increases dryness fraction of the steam at
turbine exhaust, this will reduce blade erosion.
3.Increase work done per unit mass of
steam, thus reducing boiler size.
4. Increases plant cost due to re-heater
requirement and it’s long piping system.
5.Increases condenser capacity due to the
increase in steam dryness fraction.
1. Increases thermal efficiency
2.Increases dryness fraction of the steam at
turbine exhaust, this will reduce blade erosion.
3.Increase work done per unit mass of
steam, thus reducing boiler size.
4. Increases plant cost due to re-heater
requirement and it’s long piping system.
5.Increases condenser capacity due to the
increase in steam dryness fraction.
1. Increases thermal efficiency
2.Increases dryness fraction of the steam at
turbine exhaust, this will reduce blade erosion.
3.Increase work done per unit mass of
steam, thus reducing boiler size.
4. Increases plant cost due to re-heater
requirement and it’s long piping system.
5.Increases condenser capacity due to the
increase in steam dryness fraction.

34. Regenerative CycleRegenerative Cycle
The regenerative Rankine cycle is so named because
after emerging from the condenser, water is heated
by steam tapped from the hot portion of the cycle.
Direct contact heating: The fluid at 2 is mixed with the
fluid at 6 (both at the same pressure) to end up with the
saturated liquid at 3
Bleed steam from turbine between stages and sent
to feedwater heaters to preheat the water on its way
from the condenser to the boiler. These heaters do not
mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed
feedwater heaters".
The regenerative Rankine cycle is so named because
after emerging from the condenser, water is heated
by steam tapped from the hot portion of the cycle.
Direct contact heating: The fluid at 2 is mixed with the
fluid at 6 (both at the same pressure) to end up with the
saturated liquid at 3
Bleed steam from turbine between stages and sent
to feedwater heaters to preheat the water on its way
from the condenser to the boiler. These heaters do not
mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed
feedwater heaters".
The regenerative Rankine cycle is so named because
after emerging from the condenser, water is heated
by steam tapped from the hot portion of the cycle.
Direct contact heating: The fluid at 2 is mixed with the
fluid at 6 (both at the same pressure) to end up with the
saturated liquid at 3
Bleed steam from turbine between stages and sent
to feedwater heaters to preheat the water on its way
from the condenser to the boiler. These heaters do not
mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed
feedwater heaters".
The regenerative Rankine cycle is so named because
after emerging from the condenser, water is heated
by steam tapped from the hot portion of the cycle.
Direct contact heating: The fluid at 2 is mixed with the
fluid at 6 (both at the same pressure) to end up with the
saturated liquid at 3
Bleed steam from turbine between stages and sent
to feedwater heaters to preheat the water on its way
from the condenser to the boiler. These heaters do not
mix the input steam and condensate, function as an
ordinary tubular heat exchanger, and are named "closed
feedwater heaters".

35. Effect of Inlet PressureEffect of Inlet PressureEffect of Inlet PressureEffect of Inlet Pressure
Superheaters, Valves, Pipelines, inlet stages of the turbine are subjected to high
pressure and temperatures due to metallurgical limit.
The maximum temperature is fixed by this limit , as the operating steam pressure at
which the heat is added to the boiler increases from p1 to p2, the mean temperature
of heat addition increases.
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36. Effect of Variation of Steam Condition on Thermal Efficiency ofEffect of Variation of Steam Condition on Thermal Efficiency of
Steam Power PlantSteam Power Plant
Effect of Variation of Steam Condition on Thermal Efficiency ofEffect of Variation of Steam Condition on Thermal Efficiency of
Steam Power PlantSteam Power Plant
For inlet steam pressure above 100 bar, there is continuous but decreasing rate of
improvement of cycle efficiency.
Increase in the steam pressure is limited by considerations of mechanical stresses and
higher cost of equipment.
Considerable improvement in the cycle efficiency with decrease of condenser
pressure. Depends on the temperature of cooling water.
Less in warm region and more in cold region.
11/10/2014 Modern Power Plants 36
For inlet steam pressure above 100 bar, there is continuous but decreasing rate of
improvement of cycle efficiency.
Increase in the steam pressure is limited by considerations of mechanical stresses and
higher cost of equipment.
Considerable improvement in the cycle efficiency with decrease of condenser
pressure. Depends on the temperature of cooling water.
Less in warm region and more in cold region.

37. Rankine Cycle in a Power PlantRankine Cycle in a Power PlantRankine Cycle in a Power PlantRankine Cycle in a Power Plant
11/10/2014 Modern Power Plants 37

38. Power Plant CyclePower Plant Cycle

39. Combined CycleCombined Cycle