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1. Power Plant Vapor Power Cycles
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Chapter Two
Vapor Power Cycles
2-1 Introduction :
A power plant is assembly of systems or subsystems to generate electricity,
i.e., power with economy and requirements. The power plant itself must be
useful economically and environmental friendly to the society.
A power plant may be defined as a machine or assembly of equipment that
generates and delivers a flow of mechanical or electrical energy.
The major power plants are:
1. Steam power plant
2. Diesel power plant
3. Gas turbine power plant
4. Nuclear power plant
5. Hydro electric power plant
The Steam Power Plant, Diesel Power Plant, Gas Turbine Power Plant and
Nuclear Power Plants are called THERMAL POWER PLANT, because
these convert heat into electric energy.
Steam Power Plant (SPP) is a power plant in which the prime mover is
steam driven. Water is heated, turns into steam and spins a steam turbine
which drives an electrical generator. After it passes through the turbine, the
steam is condensed in a condenser. Figure 2.1shown the block diagram of
steam power plant.

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Figure 2.1: Steam Power Plant
2-2 Power plant cycles
Thermodynamic cycles which are used in devices producing power are
called power cycles. Power production can be had by using working fluid
either in vapour form or in gaseous form. When vapour is the working fluid
then they are called vapour power cycles, whereas in case of working fluid
being gas these are called gas power cycles.
Power plants cycle generally divided in to the following groups,
Vapor Power Cycle
Vapor power cycles can be further classified as,
1. Carnot vapour power cycle
2. Rankine cycle
3. Reheat cycle
4. Regenerative cycle.

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Gas Power Cycles
Gas Power Cycles can be further classified as,
1. Otto cycle 2. Dual combustion cycle
3. Gas turbine cycle 4. Diesel cycle
2-2-1 Carnot vapor power cycle
Carnot cycle has already been defined earlier as an ideal cycle having
highest thermodynamic efficiency. Carnot cycle uses to get positive work
with steam as working fluid. As shown in figure 2.2 arrangements proposed
for using Carnot vapour power cycle is as follows:
Figure 2.2: Arrangement of Carnot cycle

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Pump:
Water enters the pump at state 1 as saturated liquid and is compressed
isentropically to the operating pressure of the boiler. The water temperature
increases somewhat during this isentropic compression process due to a
slight decrease in the specific volume of water.
Boiler
Water enters the boiler as a compressed liquid at state 2 and leaves as a
superheated vapor at state 3. The boiler is basically a large heat exchanger
where the heat originating from combustion gases, nuclear reactors, or other
sources is transferred to the water essentially at constant pressure. The
boiler, together with the section where the steam is superheated (the
superheater), is often called the steam generator.
Turbine
The superheated vapor at state 3 enters the turbine, where it expands
isentropically and produces work by rotating the shaft connected to an
electric generator.
Condenser
Condenser is a closed vessel in which steam is condensed by abstracting the
heat and where the pressure is maintained below atmospheric pressure.
The pressure and the temperature of steam drop during this process to the
values at state 4, where steam enters the condenser. At this state, steam is
usually a saturated liquid vapor mixture with a high quality. Steam leaves
the condenser as saturated liquid and enters the pump, completing the cycle.

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Thermal analysis for Carnot cycle:
The network for unit mass flow:
Wnet = WT - Wp
1
2
4
3 h
h
h
h
Wnet
For Boiler:
2
3 h
h
Qadd
For Condenser:
1
4 h
h
Qrejected
The thermal efficiency:
add
net
carnot
Q
W
2
3
1
2
4
3
h
h
h
h
h
h
carnot
The heat added and rejected can be given as function of temperature and
entropy as follows:

6. Power Plant Vapor Power Cycles
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Since : s
T
Q
2
3
2 s
s
T
Qadd
1
4
1 s
s
T
Qrejected
Also : s1 = s2 and s3 = s4
H
L
carnot
T
T
1
2
1
1
T
T
EXAMPLE 1
A Carnot cycle works on steam between the pressure limits of 7 MPa and
7 kPa. Determine thermal efficiency, turbine work and compression work
per kg of steam.
SOLUTION:

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2-2-2 Ideal Rankine cycle:
Many of the impracticalities associated with the Carnot cycle can be
eliminated by superheating the steam in the boiler and condensing it
completely in the condenser. As shown in figure 2.3 the cycle called
Rankine cycle, which is the ideal cycle for vapour power plants.
Figure 2.3: Schematic Layout of ideal Rankine cycle.