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THERMODYNAMIC CYCLE OF STEAM FLOW
RANKINE CYCLE (IDEAL , ACTUAL ,REHEAT)
LAYOUT OF STEAM POWER PLANT
MAJOR COMPONENTS AND THEIR
ADVANTAGES AND DISADVANTAGES
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Use of steam power started when it was first used in
locomotive invented by James watt.
Steam power is used to rotate the prime mover of
electric generator and it is known as steam power
In this process heat energy is converted into
mechanical energy and then to electrical energy
through turbine generator system heat energy may be
obtained by proper combustion of commercial fuel
such as coal ,gas ,oil etc..
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steam power plants are also called thermal power
plants. Prime movers of steam plant may be operated
either in non-condensing or condensing mode
In non-condensing operation the steam is exhausted
from the prime movers and is discharged at
atmospheric pressure or greater than that
Whereas in condensing the steam is exhausted into a
condenser in which pressure is less and steam is
converted to water
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thermodynamic cycle of
From laws of thermodynamics when heat is transferred
to water its enthalpy and physical state change as
heating takes place its temperature rises and density
decreases the vapour formed in this process is called
steam which is gaseous state but does not entirely
follow the laws of perfect gas
The Carnot cycle cannot be applied to steam turbine
as compression phase does not exist hence a steam
power plant works on rankine cycle
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The impracticalities associated with Carnot
cycle can be eliminated by superheating the
steam in the boiler and condensing it
completely in the condenser. This cycle results
as the Rankine cycle, which is the ideal cycle for
vapour power plants
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The Rankine cycle consists of the following
1-2 : Isentropic compression in pump
2-3 : Constant pressure heat addition in boiler
3-4 : Isentropic expansion in turbine
4-1 : Constant pressure heat rejection in a
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rankine cycle: actUal cycle
The cycles encountered in actual devices are difficult
to analyze because of the presence of complicating
effects, such as friction and the absence of sufficient
time for establishment of the equilibrium conditions
during the cycle.
The actual vapour power cycle differs from the ideal
Rankine cycle, as a result of irreversibility's in various
components. Fluid friction and heat loss to the
surroundings are the two common sources of
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• Fluid friction causes pressure drop in the boiler, the
condenser and the piping between various
components. Also the pressure at the turbine inlet is
somewhat lower than that at the boiler exit due to the
pressure drop in the connecting pipes. To compensate
for these pressure drops, the water must be pumped to
a sufficiently higher pressure than the ideal cycle. This
requires a large pump and larger work input to the
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Reheat Rankine CyCle
The efficiency of the Rankine cycle can
increase by expanding the steam in the turbine
in two stages, and reheating it in between.
Reheating is a practical solution to the excessive
moisture problem in turbines, and it is
commonly used in modern steam power plants.
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The ideal reheat Rankine cycle differs from the simple
ideal Rankine cycle in that the expansion process take
place in two stages.
In first stage (the high-pressure turbine), steam is
expanded isentropically to an intermediate pressure
and sent back to the boiler where it is reheated at
constant pressure, usually to the inlet temperature of
the first turbine stage.
Steam then expands isentropically in the second stage
(low-pressure turbine) to the condenser pressure.
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The different types of systems and components used
in steam power plant are as follows:
1.Coal handling system
2. High pressure boiler
3. Air preheater, economizer, super heater ,feed
4. Pumping system
5. Feed water purification plant
6. Condensers and cooling towers
7. Draught system
8. Prime mover
10. Ash and dust handling system
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The flow circuit of a thermal power plant consists of the
following four main circuits:
(a)Feed water and steam flow circuit.
(b) Coal and ash circuit.
(c)Air and gas circuit.
(d) Cooling water circuit.
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Boiler has the function to convert water into steam.
The process of change of water to vapour done by heating
the water in the pipes with heat from burning fuel.
Combustion processes carried out continuously in the
combustion chamber with fuel and air flow from the
majoR Components and
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. Steam production quantities dependent on the surface
area of heat transfer, flow rate, and the heat of combustion
is given. Boiler construction consisting of water-filled pipes
called a water tube boiler
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Boiler feed pump
function is to feed the water to be steamed in the boiler of
thermal power plant.
Condensate water to be coming out from condenser and
make up water coming from water treatment plant ad together
forming total feed water.
In modern power plant pulverized coal is used.
Once pulverized, coal mills are employed for
crushing the coal into a powder form. This increases
the efficiency of boiler hence the plant.
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The economizer is a feed water heater,
deriving heat from the flue gases.
The justifiable cost of the economizer depends
on the total gain in efficiency. In turn this
depends on the flue gas temperature leaving the
boiler and the feed water inlet temperature.
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Steam turbine working to change the heat energy
contained in the steam into rotary motion.
Steam with high pressure and temperature were directed
to push turbine blades mounted on the shaft, so the shaft
Due to perform work on the turbine, the pressure and
temperature of steam coming into the turbine down to
saturated vapour. This steam then flows to the condenser,
while the rotary power is used to turn a generator.
Today almost all of the steam turbine is a type of
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The flue gases coming out of the economizer is
used to preheat the air before supplying it to the
An increase in air temperature of 20 degrees
can be achieved by this method.
The pre heated air is used for combustion and
also to dry the crushed coal before pulverizing.
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The fuel used in thermal power plants causes soot and
this is deposited on the boiler tubes, economizer tubes,
air pre heaters, etc. This drastically reduces the
amount of heat transfer of the heat exchangers.
Soot blowers control the formation of soot and reduce
its corrosive effects.
The types of soot blowers are fixed type, which may be
further classified into lane type and mass type
depending upon the type of spray and nozzle used.
The other type of soot blower is the retractable soot
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The use of a condenser in a power plant is to improve
the efficiency of the power plant by decreasing the
exhaust pressure of the steam below atmosphere.
Another advantage of the condenser is that the steam
condensed may be recovered to provide a source of good
pure feed water to the boiler and reduce the water
softening capacity to a considerable extent.
A condenser is one of the essential components of a
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The importance of the cooling tower is felt when the
cooling water from the condenser has to be cooled.
The cooling water after condensing the steam
becomes hot and it has to be cooled as it belongs to a
The Cooling towers do the job of decreasing the
temperature of the cooling water after condensing the
steam in the condenser.
The type of cooling tower used in the Columbia
Power Plant was an Inline Induced Draft Cross Flow
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This tower provides a horizontal air flow as the water falls
down the tower in the form of small droplets.
The fan centered at the top of units draws air through two
cells that are paired to a suction chamber partitioned beneath
The outstanding feature of this tower is lower air static
pressure loss as there is less resistance to air flow.
The evaporation and effective cooling of air is greater when
the air outside is warmer and dryer than when it is cold and
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The superheater consists of a superheater header and
Steam from the main steam pipe arrives at the saturated
steam chamber of the superheater header and is fed into the
Superheated steam arrives back at the superheated steam
chamber of the superheater header and is fed into the steam
pipe to the cylinders.
Superheated steam is more expansive.
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The reheater functions similar to the superheater in that
it serves to elevate the steam temperature.
Primary steam is supplied to the high pressure turbine.
After passing through the high pressure turbine, the steam
is returned to the steam generator for reheating (in a
reheater) after which it is sent to the low pressure turbine.
A second reheat cycle may also be provided.
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to overcome the resistance offered by the pipelines, air
ducts, fuel beds, dampers, chimney, etc. for the flow of
air, flue gases draught system is required in thermal power
Normally FD (forced draft) fan, ID (induced draft) fan
and chimney facilitate this function.
FD fan provides air from atmosphere to the boiler with
the positive pressure, where as ID fan along with chimney
sucks an air and waste gas from boiler to the atmosphere.
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In thermal power plant, turbo alternator is usually
coupled unit of steam turbine and electrical generator.
Steam turbine rotates with the help of superheated
steam expanded from high pressure and high
As the turbine rotates, alternator also rotates gives rise to
generation of electricity.generator is the main unit of any power plant.
In this unit the generation of electricity is done. It is
mechanically coupled with the turbine, so that when turbine
rotates, it also rotates. The speed of the generator is given by,
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Alternator cooling system:
In thermal power plant, large amount of heat is generated
in the windings due to copper and Eddy current losses.
This heat should be removed continuously so as to keep
the insulating material from damaging.
There are two methods of cooling systems, open-circuit
and closed-circuit method.
For low capacity generators open-circuit cooling system is
employed in which atmospheric air is passed through one
side and sucks out from the another side
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For medium capacity generators closed circuit cooling
system is employed in which same air passed again and
again after cooling it.
In case of high capacity generators same closed circuit
cooling system is employed only with difference of air is
replaced with oxygen.
Finally the overall efficiency of the thermal power plant is
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protection of turbo
Being one of the expensive part in the power plant
alternator must be protected from the faults which may
be either in stator or rotor and other abnormal
Some of the protections provided are as follows
Faults in the windings protection
Over load protection
Over heating of windings or bearings
Over speed protection
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Loss of Excitation protection
Motoring operation protection
single phase or unbalanced current protection,
out of step operation protection
sub-synchronous oscillations protection
earth fault protection
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Alternator control both power and voltage with two
different control loops. The control of input are called
M-W frequency control loop . It is also known as load
frequency control or automatic load frequency control
or automatic generation control second control loop is
MVAR control loop or excitation control loop
Excitation system control the terminal voltage of
generator and reactive power generation, it also
responds to system disturbances
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For satisfactory operation of the system it must fulfil
Meeting specified response criteria , providing limiting
and protective functions ,flexible operation , reliability
The excitation system can be categorized based on the
supply given such as dc excitation , ac excitation and
Ac excitation is more commonly used now
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Governing is the procedure of monitoring and
controlling the flow rate of steam into the turbine with
the objective of maintaining its speed of rotation as
The flow rate of steam is monitored and controlled by
interposing valves between the boiler and the turbine.
Depending upon the particular method adopted for
control of steam flow rate, different types of governing
methods are being practiced
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Some of the governing methods are
By pass governing
Flyball speed governing
The main parts of governing system are speed
governor, amplifier , speed changer and linkage
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The fuel used is quite cheap.
Less initial cost as compared to other generating
It can be installed at any place irrespective of the
existence of coal.
The coal can be transported to the site of the plant by
rail or road.
It require less space as compared to Hydro power
Cost of generation is less than that of diesel power
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It pollutes the atmosphere due to production of
large amount of smoke and fumes.
It is costlier in running cost as compared to Hydro
Transportation of fuel is one of the major
difficulties for the plants located away from coal fields.
Life of the plant is hardly 3 to 4 decades compared
with hydro-plant (1 to 2 centuries).
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