Chapter 1

Vapor Power
Systems
 Introduction

An important engineering goal is to devise systems that
accomplish desired types of energy conversion.

We are concerned with several types of power-generating

systems, each of which produces a net power output from a
fossil fuel, nuclear, or solar input.

Discussion on the practical arrangements employed for power

production and illustrate how such power plants can be
modeled thermodynamically.

The main objective of the present chapter is to study vapor

power plants in which the working fluid is alternately vaporized
and condensed.
 Photo Courtesy of GPU International

A view of the 80 MW natural gas fired plant

Component of a simple vapor power plant

The vast majority of electrical generating plants are

variations of vapor power plants in which water is the
working fluid.

The basic components of a simplified fossil-fuel vapor

power plant are shown schematically in Figure 1.1.

To facilitate thermodynamic analysis, the overall plant

can be broken down into the four major subsystems
identified by the letters A through D on the diagram.

The focus of our considerations in this chapter is

subsystem A, the important energy conversion from
heat to work occurs.
Components of a Vapor Power Plant
 Functions of the components
Subsystem B

To supply the energy required to vaporize
the water passing through the boiler.

In fossil-fuel plants, this is accomplished
by heat transfer to the working passing
through tubes and drums in the boiler
from the hot gases produced by the
combustion of a fossil fuel.

In nuclear plants, the origin of the energy
is a controlled nuclear reaction taking
place in an isolated reactor building.
 Functions of the components
Subsystem C and D

The shaft of the turbine is connected to an
electric generator at D.

The vapor leaving the turbine passes through the

condenser, where it condenses on the outside of
tubes carrying cooling water at C.

For the plant shown, the cooling water is sent to

a cooling tower, where energy taken up in the
condenser is rejected to the atmosphere. The
cooling water is then recirculated through the
condenser.
 Rankine Cycle – subsystem A

Turbine Pump
• •
Wt Wp
•
= h1 − h2 •
= h4 − h3
m m

Boiler Condenser (1 side):

• •
Q in Q out
•
= h1 − h4 •
= h2 − h3
m m
Overall Performance:
• • • •

η=
Wt / m−Wp/ m
=
( h1 − h2 ) − ( h4 − h3 )
• •
Qin/ m ( h1 − h4 )
 Principal Device Analysis

Turbine - Vapor from the boiler at state l, having an

elevated temperature and pressure, expands
through the turbine to produce work and then is discharged
to the condenser at state 2 with relatively low pressure

• •
Wt = m ( h1 − h2 )

ηt =
( h1 − h2 )
( h1 − h2 s )
Condenser
In the condenser there is heat transfer from the vapor to
cooling water flowing in a separate stream.
The vapor condenses and the temperature of the cooling
water increases.

• •
Q out = m ( h2 − h3 )
Pump
The liquid condensate leaving the condenser at 3 is
pumped from the condenser into the higher
pressure boiler.

ηp =
( h 4 s − h3 )
( h 4 − h3 )
• •
W p = m ( h4 − h3 )
Boiler

The working fluid completes a cycle as the liquid leaving the

pump at 4 called the boiler, feedwater, is heated to
saturation and evaporated in the boiler.

• •
Qin = m ( h1 − h4 )
Cycle Performance Parameters
• • • •
W t / m− W p / m
η= • •
Q in / m
• •
W p/m
bwr = • •
W t/m
Example 1.1
 Quiz 1

Draw the layout of a simple Rankine

Cycle that includes four major
component/devices as discussed today
and name them correctly.
Rankine Idealizations

Processes 1-2, 3-4: Isentropic

Processes 2-3, 4-1: Isobaric

Saturated liquid at State 3

Reversible Pump Work Equation:

 W• 
 • p  ≈ v3 ( p4 − p3 )
 m 
 int
rev

For Incompressible Fluids Only!

 Rankine Idealization
If there is no irreversibility in cycle, and no heat transfers to
the environment,
- there will be no frictional pressure drops in the boiler
- condenser and the working fluid would flow through these
components are at constant pressure.

Process 1-2: Isentropic expansion of the working fluid

through the turbine from saturated vapor at state 1 to the
condenser pressure.

Process 2-3: Heat transfer from the working fluid as it

flows at constant pressure through the condenser with
saturated liquid at state 3.

Process 3-4: Isentropic compression in the pump to

state 4 in the compressed liquid region.

Process 4-1: Heat transfer to the working fluid as it

flows at constant pressure through the boiler to complete
the cycle.
 Rankine Idealization

The ideal Rankine cycle also includes the possibility of

superheating the vapor, as in cycle 1'2'341'. The
importance of superheating is discussed in Section 1.3.

Since the ideal Rankine cycle consists of internally

reversible processes, areas under the process lines of
Figure 1.3 can be interpreted as heat transfers per unit of
mass flowing.

Here, the area 1bc4a1 represents the heat

transfer to the working fluid passing through the boiler and
area 2bc32, is the heat transfer from the working
fluid passing through the condenser, each per unit of mass
flowing.

The enclosed area 1234a1 can be interpreted as

the net heat input or, equivalently, the net work output,
each per unit of mass flowing.
 Fully reversible pump

Because the pump is idealized as operating without
irreversibilities, the pump work can be evaluated as,

 W& P  4
  = ∫ vdp
 m&  int
rev
3

The subscript "int rev" has been retained as a reminder that

this expression is restricted to an internally reversible
process through the pump.

A plausible approximation to the value of the integral can be

had by taking the specific volume at the pump inlet, v3 as
constant for the process.
 W& P 
  ≈ v3 ( p 4 − p3 )
 m&  int
rev
Principal Irreversibilities and Losses
Turbine

The principal irreversibility experienced by the working fluid is

associated with the expansion through the turbine.
Heat transfer from the turbine to the surroundings represents a
loss, but since it is usually of secondary importance, this loss
is ignored in subsequent discussions.
As illustrated by Process 1
2 of Figure 1.6, an actual adiabatic
expansion through the turbine is accompanied by an increase
in entropy.
The work developed per unit of mass in this process is less
than for the corresponding isentropic expansion 1
2s.
The isentropic turbine efficiency allows the effect of
irreversibilities within the turbine to be accounted for in terms
of the actual and isentropic work amounts.

ηt =
(W& m& )
t
=
h1 − h2
(W& m& )
t S
h1 − h2 S
Adiabatic and Isentropic expansion
Principal Irreversibilities and Losses
Pump

The work input to the pump required to overcome frictional effects

also reduces the net power output of the plant.
In the absence of heat transfer to the surroundings, there
would be an increase in entropy across the pump.
Process 3
4 of Figure 1.6 illustrates the actual pumping
process.
The work input for this process is greater than for the
corresponding isentropic process 3 4s.
The isentropic pump efficiency allows the effect of
irreversibilities within the pump to be accounted for in terms of
the actual and isentropic work amounts.

ηp =
(W& m& )
P S
=
h4 S − h3
(W& m& )
P h4 − h3
Example 1.2
JANAMANJUNG PLANT LAYOUT
 Improving Cycle Performance
One key is in the pressures:

Each method increases cycle thermal efficiency!

 Improving Performance
An increase in the boiler pressure or a decrease in the condenser
pressure may result in a reduction of the steam quality at the exit of
the turbine.

This can be seen by comparing states 2' and 2" of Figures 1.4a and
1.4b to the corresponding state 2 of each diagram.
 Improving Performance
If the quality of the mixture passing through the turbine
becomes too low, more droplet will exist in the vapor.

The impact of liquid droplets in the flowing liquid-vapor

mixture can erode the turbine blades, causing a decrease in
the turbine efficiency and an increased need for maintenance.

Accordingly, common practice is to maintain at least 90%,

quality (x) at the turbine exit.

The cycle modifications known as superheat and reheat

permit advantageous operating pressures in the boiler and
condenser and yet offset the problem of low quality of the
turbine exhaust.
 Improving Cycle Performance

Superheat and Reheat protect the turbine, and increase the thermal efficiency
 Superheat
The fact : not all the liquid (water) can be changed into vapor

Therefore there is no assurance to having saturated vapor at

the turbine inlet

Hence, further energy can be added by heat transfer to the

steam, bringing it to a superheated vapor condition at the
turbine inlet.

This is accomplished in a separate heat exchanger called a

superheater.
The combination of boiler and superheater is referred to as a
steam generator.
 Superheat

Figure 1.3 shows an ideal Rankine cycle with superheated

vapor at the turbine inlet: cycle 1'
2'
3
4
1'.
 Superheat

The cycle with superheat has a higher

average temperature of heat addition
than the cycle without superheating
(cycle 1
2 3 4
1), so the thermal
efficiency is higher.

Moreover, the quality at turbine exhaust state 2' is greater than at

state 2, which would be the turbine exhaust state without
superheating.

Accordingly superheating also tends to alleviate the problem of

low steam quality at the turbine exhaust.

With sufficient superheating, the turbine exhaust state may even

fall in the superheated vapor region.
 Reheat
A further modification normally
employed in vapor power plants.

With reheat, a power plant can

take advantage of the increased
efficiency that results with higher
boiler pressures and yet avoid
low-quality steam at the turbine
exhaust.

In the ideal reheat cycle shown in Figure 1.7, the steam does not
expand to the condenser pressure in a single stage.

The steam expands through a first-stage turbine (Process 1

2) to
some pressure between the steam generator and condenser
pressures.

The steam is then reheated in the steam generator (Process 23).

Ideally, there would be no pressure drop as the steam is reheated.

After reheating, the steam expands in a second-stage turbine to the

condenser pressure (Process 3 4).
 Reheat

The principal advantage of reheat is to increase the quality of the

steam at turbine exhaust.
 Reheat

This can be seen from the T-s diagram of Figure 1.7 by

comparing state 4 with state 4', the turbine exhaust state
without reheating.
 Reheat
Thermal efficiency of a reheat
cycle,
it is necessary to account for
the work output of both turbine
stages

the total heat addition

occurring in the vaporization or
superheating and reheating
processes.

This calculation is illustrated in

Example 1.3.
 Supercritical Cycle
The temperature of the steam entering
the turbine is restricted by metallurgical
limitations imposed by the materials used
to fabricate the superheater, reheater, and
turbine.

High pressure in the steam generator

also requires piping that can withstand
great stresses at elevated temperatures.

Although these factors limit the gains that can be realized through
superheating and reheating, improved materials and methods of
fabrication have permitted significant increases over the years in the
maximum allowed cycle temperatures and steam generator
pressures, with corresponding increases in thermal efficiency.
 Supercritical Cycle
This has progressed to the extent that
vapor power plants can be designed to
operate with steam generator pressures
exceeding the critical pressure of water
(22.1 MPa, 3203.6 lbf/in.2) and turbine
inlet temperatures exceeding 600°C
(1100°F).

Figure 1.8 shows an ideal reheat cycle with a supercritical steam

generator pressure.

Observe that no phase change occurs during the heat addition

process from 6 to 1.

In the next example, the ideal Rankine cycle of Example 1.1 is

modified to include superheat and reheat.
 Example 1.3

In this example, the ideal Rankine cycle of Example 1.1 is

modified to include superheat and reheat.
 Example 1.4
This example illustrate the effect of turbine irreversibilities on
the ideal reheat cycle of Example 1.3.

Look at it carefully, it is one good example for a vapor plant.

 Quiz 1

Draw the layout of a simple Rankine

Cycle that includes four major
component/devices as discussed today
and name them correctly.
 Regenerative vapor power cycle
Another commonly used method for increasing the thermal
efficiency of vapor power plants regeneration is
regenerative feedwater heating, or simply regeneration

To introduce the principle underlying regenerative feedwater

heating, consider Figure 1.3 once again.
Regenerative vapor power cycle
In cycle

1
2
3
4
a
1,

the working fluid would

enter the boiler as a
compressed liquid at
state 4 and be heated
while in the liquid
phase to state a

With regenerative feedwater heating, the working fluid would enter

the boiler at a state between 4 and a.

As a result, the average temperature of heat addition would be

increased, thereby tending to increase the thermal efficiency.
Improving Cycle Performance
Adding an Open Feedwater Heater (with pump).

h6 − h5
y=
h2 − h5
Regenerative Cycle : Open Feedwater Heater
Let us consider how
regeneration can be
accomplished using an
open feedwater

It is - a direct contact-
type heat exchanger in
which streams at
different temperatures
mix to form a stream at
an intermediate
temperature.

Shown in Figure 1.9 are the schematic diagram and the

associated T-s diagram for a regenerative vapor power cycle
having one open feedwater heater.
Regenerative Cycle : Open Feedwater Heater
For this cycle,

 the working fluid passes

isentropically through the
turbine stages and pumps

the flow through the steam

generator, condenser, and
feedwater heater takes place
with no pressure drop in any
of these components.

Steam enters the first-stage turbine at state 1 and expands to

state 2, where a fraction of the total flow is extracted, or bled,
into an open feedwater heater operating at the extraction
pressure, p2.
Regenerative Cycle : Open Feedwater Heater

The rest of the steam expands

through the second-stage
turbine to state 3.

This portion of the total flow is

condensed to saturated liquid,
state 4

It is pumped to the extraction

pressure and introduced into
the feedwater heater at state 5.

A single mixed stream exits the feedwater heater at state 6.

Regenerative Cycle : Open Feedwater Heater
For the case shown in Figure
1.9,

the mass flow rates of the

streams entering the
feedwater heater are chosen
(controlled)

Therefore, the stream exiting

the feedwater heater is a
saturated liquid at the
extraction pressure.

The liquid at state 6 is then pumped to the steam generator pressure

and enters the steam generator at state 7.

Finally, the working fluid is heated from state 7 to state 1 in the steam
generator.
 Open Feedwater -
Discussions
Referring to the T-s diagram of the
cycle, note that the heat addition would
take place from state 7 to state 1, rather
than from state a to state 1, as would
be the case without regeneration.

Accordingly, the amount of energy that must be supplied from the

combustion of a fossil fuel, or another source, to vaporize and superheat
the steam would be reduced. This is the desired outcome.

Only a portion of the total flow expands through the second stage
turbine (Process 23), however, so less work would be developed as
well.

In practice, operating conditions are chosen so that the reduction in heat

added more than offsets the decrease in net work developed, resulting
in an increased thermal efficiency in regenerative power plants.
 Open Feedwater – Cycle Analysis
Consider next the thermodynamic
analysis of the regenerative cycle
illustrated in Figure 1.9.

An important initial step in analyzing

any regenerative vapor cycle is the
evaluation of the mass flow rates
through each of the components.

Taking a single control volume

enclosing both turbine stages, the
mass rate balance reduces at steady
state to
m& 2 + m& 3 = m& 1
where,
m1 : the rate at which mass enters the first-stage turbine at state 1,
m2 : the rate at which mass is extracted and exits at state 2,
m3 : the rate at which mass exits the second-stage turbine at state 3.
 Open Feedwater – Cycle Analysis
Dividing by m1, places this on the
basis of a unit of mass passing
through the first-stage turbine

m& 2 m& 3
+ =1
m& 1 m& 1
Denoting the fraction of the total
flow extracted at state 2 by y
(y=m2/m1), the fraction of the total
flow passing through the second-
stage turbine is

m& 3
= 1− y
m& 1
 Open Feedwater – Cycle Analysis
The fraction y can be determined by
applying the conservation of mass
and conservation of energy
principles to a control volume
around the feedwater heater.

Assuming no heat transfer between

the feedwater heater and its
surroundings and ignoring kinetic
and potential energy effects, the
mass and energy rate balances
reduce at steady state to give

0 = yh2 + (1 − y )h5 − h6
And solving for y h6 − h5
y=
h2 − h5
 Open Feedwater – Cycle Analysis
Expressions for the principal work and heat transfers of the
regenerative cycle can be determined by applying mass and
energy rate balances to control volumes around the individual
components.

Beginning with the turbine, the total work is the sum of the
work developed by each turbine stage.

Neglecting kinetic and potential energy effects and assuming

no heat transfer with the surroundings, we can express the
total turbine work on the basis of a unit ~ mass passing
through the first-stage turbine as

W& t
= (h1 − h2 ) + (1 − y )(h2 − h3 )
m& 1
 Open Feedwater – Cycle Analysis

The total pump work is the sum of the work required to

operate each pump individually.

On the basis of a unit of mass passing through the first-

stage turbine, the total pump work is

W& P
= (h7 − h6 ) + (1 − y )(h5 − h4 )
m& 1
 Open Feedwater – Cycle Analysis
The energy added by heat transfer to the working fluid passing through
the steam generator, per unit mass expanding through the first stage
turbine, is
Q& in
= h1 − h7
m& 1
And the energy rejected by heat transfer to the cooling water is,

Q& out
= (1 − y )(h3 − h4 )
m& 1
Example 1.5 illustrates the analysis of the regenerative cycle with one
open feedwater heater, including the evaluation of properties at state
points around the cycle and the determination of the fractions of the
total flow at various locations.
 Example 1.5
Example 1.5 illustrates the analysis of the regenerative
cycle with one open feedwater heater, including the

 evaluation of properties
at state points around
the cycle, and

the determination of
the fractions of the total
flow at various locations.
 Quiz 2

What is the main advantage of the

regeneration system?
Improving Cycle Performance
Adding a Closed Feedwater Heater.

h6 − h5
y=
h2 − h7
 Closed Feedwater Heaters
Regenerative feedwater heating also can be accomplished with
closed feedwater heaters are shell-and-tube-type recuperators in
which the feedwater temperature increases as the extracted steam
condenses on the outside of the tubes carrying the feedwater.

Since the two streams do not mix, they can be at different pressures.
There are two different schemes for removing the condensate from
closed feedwater heaters.

(a) It is accomplished by means of a pump whose function is to pump

the condensate forward to a higher-pressure point in the cycle.

(b) The condensate is allowed to pass through a trap into a feedwater

heater operating at a lower pressure or into the condenser. A trap is a
type of valve that permits only liquid to pass through to a region of
lower pressure.
 Closed Feedwater Heaters

(a) The condensate is moved forward to a higher-pressure point in

the cycle by a pump.

(b) The condensate is allowed to pass through a trap into a

feedwater heater operating at a lower pressure or into the
condenser. A trap is a type of valve that permits only liquid to pass
through to a region of lower pressure.
 Regenerative vapor power cycle with one
close feedwater heater

For this cycle, the working fluid passes isentropically through the
turbine stages and pumps, and there are no pressure drops
accompanying the flow through the other components.
 Regenerative vapor power cycle with one
close feedwater heater

For this cycle, the working fluid passes isentropically through the
turbine stages and pumps, and there are no pressure drops
accompanying the flow through the other components.
 Closed Feedwater Heaters
The T-s diagram shows the principal states
of the cycle.

The total steam flow expands through the

first-stage turbine from state 1 to state 2.

At this location, a fraction of the flow is

bled into the closed feedwater heater,
where it condenses.

Saturated liquid at the extraction pressure

exits the feedwater heater at state 7.

The condensate is then trapped into the condenser, where it is reunited

with the portion of the total flow passing through the second-stage
turbine.

The expansion from state 7 to state 8 through the trap is irreversible, so

it is shown by a dashed line on the T-s diagram.
 Closed Feedwater Heaters
The total flow exiting the condenser as
saturated liquid at state 4 is pumped to the
steam generator pressure and enters the
feedwater heater at state 5.

The temperature of the feedwater is

increased in passing through the
feedwater heater.

The feedwater then exits at state 6.

The cycle is completed as the working fluid is heated in the steam

generator at constant pressure from state 6 to state 1.

Although the closed heater shown on the figure operates with no

pressure drop in either stream, there is a source of irreversibility
due to the stream-to-stream temperature differences.
 Closed Feedwater Heaters
Cycles analysis
The fraction of the total flow extracted, y,
can be determined by applying the
conservation of mass and conservation of
energy principles to a control volume
around the closed heater.

Assuming no heat transfer between the feedwater heater and its

surroundings and neglecting kinetic and potential energy effects, the
mass and energy rate balances reduce at steady state to give

0 = y (h2 − h7 ) + (h5 − h6 )
h6 − h5
y=
And solving for y, h2 − h7
 Multiple Feedwater Heaters

The thermal
efficiency of the
regenerative cycle
can be increased by
incorporating
several feedwater
heaters at suitably
chosen pressures.

The number of feedwater heaters used is based on economic

considerations, since incremental increases in thermal efficiency
achieved with plant designers use computer programs to simulate
the thermodynamic and economic performance of different
designs to help them decide on the number of heaters to use, the
types of heaters, and the pressures at which they should operate.
 Example 1.6
This example shows the function of multi stages turbines and
the effect of multi feedwater (closed and open) in a vapor
plant.

In ordinary plant, this might be the closest example.

 Quiz

Give three ways to increase

the cycle performance or
thermal efficiency in a
vapor plant.
Other Vapor Cycle Aspects

In this section we consider aspects of vapor

cycles related to working fluid characteristics,
binary vapor cycles, and cogeneration systems.
 Working fluid characteristics
Water is used as the working fluid in the vast majority of
vapor power systems.

This is because it is plentiful and low in cost, nontoxic,

chemically stable, and relatively noncorrosive.

Shortcoming

Water has a relatively large change in specific enthalpy

when it vaporizes at ordinary steam generator
pressures

This tends to limit the mass flow rate for a desired

power plant output.
 Working fluid characteristics
The properties of liquid water and water vapor are also
such that the back work ratios achieved are
characteristically quite low

Therefore, the techniques of superheat, reheat, and

regeneration can be effective for increasing power plant
efficiencies.

As a conclusion, water is less satisfactory in sofar as

some other desirable working fluid characteristics are
concerned.

For example, the critical temperature of water is only

374.14°C (705.4°F, which is about 225°C (440°F) below
the maximum allowable turbine inlet temperatures.
 Working fluid characteristics
Accordingly, to achieve a high average temperature of
heat addition and realize the attendant higher thermal
efficiency, it may be necessary for the steam generator to
operate at supercritical pressures.

This requires costly piping and heat exchanger tubes

capable of withstanding great stresses.

Another undesirable characteristic of water is that its

saturation pressure at ordinary condenser temperatures
is well below atmospheric pressure.

As a result, air can leak into the system, necessitating

the use of special ejector pumps attached to the
condenser or deaerating feedwater heaters to remove the
air.
 Working fluid characteristics

Although water has some shortcomings as a working

fluid, no other single working fluid has been found that
is more satisfactory overall for large electrical
generating plants.

Still, vapor power cycles intended for special uses

may employ working fluids that are better matched to
the application at hand than water.
 Working fluid characteristics
Cycles that operate at relatively low temperatures may
perform best with a refrigerant such as ammonia as the
working fluid.

Power systems for high-temperature applications may

employ substances having desirable performance
characteristics at these temperatures.

Moreover, water may be used together with some other

substance in a binary vapor cycle to achieve better
overall performance than could be realized with water
alone.
 Binary Vapor Cycle
In a binary power cycle two working fluids are used, one with
good high-temperature characteristics and another with good
characteristics at the lowertemperature end of the operating
range.

Example
Binary vapor cycle
using water and a
suitable liquid metal.

Each substance in
both the liquid and
vapor phases.
 Binary Vapor Cycle
In this arrangement, two ideal Rankine cycles are combined, with the
heat rejection from the high-temperature cycle (the topping cycle)
being used as the energy input for the low-temperature cycle.

This energy transfer

is accomplished in
an interconnecting
heat exchanger.

It serves as the
condenser for the
metal cycle and the
boiler for the water
cycle.
 Binary Vapor Cycle
Since the increase in the specific enthalpy of the water as it
passes through the heat exchanger is typically several times
the magnitude of the specific enthalpy decrease of the metal,
several units of mass of metal must circulate in the topping
cycle for each unit of mass of water in the other cycle.

Binary vapor power cycles can

operate with higher average
temperatures of heat addition
than conventional cycles using
water only and thus can attain
higher thermal efficiencies.

However, the higher

efficiencies achieved in this
manner must justify the
increased costs related to the
construction and operation of
the more complex cycle
arrangement.
 Cogeneration
The binary cycle considered so far is just
one example of how systems can be linked
to obtain overall systems that utilize fuel
more effectively.

Other examples including the multi-use

strategy known as co-generation.
Cogeneration Concept

Direct combustion
heating and the
generation of process
steam together account
for a substantial portion
of industrial energy
resource use.

Since the heat and steam are often required at a

relatively low temperature, good use is not made of
the relatively hightemperature products of
combustion obtained by burning fuel.
Cogeneration Concept

This source of inefficiency

can be reduced with a
cogeneration
arrangement.

Here, the fuel is consumed to produce both electricity and

steam (or heat), but with a total cost less than would be
required to produce them individually.

The figure is labeled with the fractions of the total flow

entering the steam turbine that remain at various locations.
Cogeneration Concept

On the basis of a unit of mass

entering the turbine at state 1, a
fraction y is extracted at an
intermediate point 2

It is diverted to some process that

requires steam at this condition.

The remaining fraction 1-y, expands to the turbine exit at stage 3,

producing power addition to that developed by the first turbine.

Eventually, this fraction rejoins the amount extracted and the combined
steam returned to the steam generator.

When no process steam is required, all of the steam generated by the

steam generator is allowed to expand through the turbine.
Cogeneration Concept
Industries such as pulp and paper
production and food processing
are examples of cogeneration.

They require steam for various

processes in addition to electricity
for operating machines, lighting,
etc.,

Another cogeneration arrangement being used increasingly involves

district heating which is commonly used cold climate countries.

In this application a power plant is integrated into a community.

It provides electricity for industrial, commercial, and domestic use

together with steam for process needs, space heating, and domestic
water heating.
Exergy Accounting of a Vapor
Power Plant
The discussions to this point show that a useful
picture of power plant performance can be obtained
with the conservation of mass and conservation of
energy principles.

However, these principles provide only the

quantities of energy transferred to and from the plant
and do not consider the utility of the different types
of energy transfer.
Exergy Accounting of a Vapor
Power Plant
With the conservation principles alone, a unit of energy exiting
as generated electricity is regarded as equivalent to a unit of
energy exiting in relatively low-temperature cooling water, even
though the electricity has greater utility and economic value.

Also, nothing can be learned with the conservation principles

alone about the relative significance of the irreversibilities
present in the various plant components and the losses
associated with those components.

The method of exergy analysis introduced so far allows issues

such as these to be dealt with quantitatively.
 Exergy
Exergy of a system is the maximum work possible
during a process that brings the system into equilibrium
with a heat reservoir.

When the surroundings are the reservoir, exergy is the

potential of a system to cause a change as it achieves
equilibrium with its environment.

Exergy is then the energy that is available to be used.

After the system and surroundings reach equilibrium,

the exergy is zero.
 Exergy Accounting
In this section we account for the exergy entering a power plant with
the fuel.

A portion of the fuel exergy is ultimately returned to the plant

surroundings as the net work developed.

However, the largest part is either destroyed by irreversibilities within

the various plant components or carried from the plant by the cooling
water, stack gases, and unavoidable heat transfers with the
surroundings.

These considerations are illustrated in the present section by three

solved examples, treating respectively the boiler, turbine and pump,
and condenser of a simple vapor power plant.

The irreversibilities present in each power plant component exact a

tariff on the exergy supplied to the plant, as measured by the exergy
destroyed in that component.

The component levying the greatest tariff is the boiler, for a significant
portion of the exergy entering the plant with the fuel is destroyed by
irreversibilities within it.
 Exergy Accounting
There are two main sources of
irreversibility in the boiler:

(1) the irreversible heat transfer

occurring between the hot
combustion gases and the
working fluid of the vapor power
cycle flowing through the boiler
tubes

(2) the combustion process itself.

To simplify the present discussion, the boiler is considered to consist

of a combustor unit in which fuel and air are burned to produce hot
combustion gases, followed by a heat exchanger unit where the cycle
working fluid is vaporized as the hot gases cool.
 Table 8.1 : Vapor Power Plant
Exergy Accounting
Outputs
Net power out (b) 30%
Losses
Condenser cooling water (c) 1%
Stack gasses 1%
Exergy destruction
Boiler
Combustion unit (assumed) 30%
Heat exchanger unit (d) 30%
Turbine (e) 5%
Pump (f) -
Condenser (g) 3%

Total 100%
 Exergy Accounting

For the purposes of illustration, let

us assume that 30% of the exergy
entering the combustion unit with
the fuel is destroyed by the
combustion irreversibility and 1%
of the fuel exergy exits the heat
exchanger unit with the stack
gases.

The corresponding values for an

actual power plant might differ
from these nominal values.

However, they provide characteristic values for discussion.

This is for evaluating the combustion exergy destruction and the

exergy accompanying the exiting stack gases are introduced in
discussion for Combustion in Chapter 4.
 Exergy Accounting

Using the foregoing values for the

combustion exergy destruction
and stack gas loss, it follows that
a maximum of 69% of the fuel
exergy remains for transfer from
the hot combustion gases to the
cycle working fluid.

It is from this portion of the fuel

exergy that the net work
developed by the plant is obtained.

In Examples 1.7 through 1.9, we account for the exergy supplied by the
hot combustion gases passing through the heat exchanger unit. The
principal results of this series of examples are reported in Table 1.1.
 Table 8.1 : Vapor Power Plant
Exergy Accounting
Outputs
Net power out (b) 30%
Losses
Condenser cooling water (c) 1%
Stack gasses 1%
Exergy destruction
Boiler
Combustion unit (assumed) 30%
Heat exchanger unit (d) 30%
Turbine (e) 5%
Pump (f) -
Condenser (g) 3%

Total 100%
 Exergy Conclusion
The entries of Table 1.1 suggest some general observations about
vapor power plant performance.

First, the table shows that the exergy destructions are more
significant than the plant losses.

The largest portion of the exergy entering the plant with the fuel is
destroyed, with exergy destruction in the boiler overshadowing all
others.

By contrast, the loss associated with heat transfer to the cooling

water is relatively unimportant.

The cycle thermal efficiency (calculated in the solution to Ex. 1.2) is

31.4%, so over two-thirds (68.6%) of the energy supplied to the cycle
working fluid is subsequently carried out by the condenser cooling
water.
 Exergy Conclusion
By comparison, the amount of exergy carried out is virtually
negligible because the temperature of the cooling water is raised
only a few degrees over that of the surroundings and thus has
limited utility.

The loss amounts to only 1% of the exergy entering the plant with
the fuel.

Similarly, losses accompanying unavoidable heat transfer with

the surroundings.

The exiting stack gases typically amount only to a few percent of

the exergy entering the plant with the fuel

It is generally considered from the perspective of conservation of

energy
 Exergy Analysis

It allows the sites where destructions or losses occur to

be identified and rank ordered for significance.

This knowledge is useful in directing attention to

aspects of plant performance that offer the greatest
opportunities for improvement through the application
of practical engineering measures.

However, the decision to adopt any particular

modification is governed by economic considerations
that take into account both economies in fuel use and
the costs incurred to achieve those economies.
 Exergy Analysis

The calculations presented in the examples 1.7 ~ 1.9 illustrate the

application of exergy principles through the analysis of a simple
vapor power plant.

There are no fundamental difficulties, however, in applying the

methodology to actual power plants, including consideration of the
combustion process.

The same procedures also can be used for exergy accounting of

the gas turbine power plants considered in the following chapter
and other types of thermal systems.
 Quiz

Suggest two methods that

can be applied in the
vapor plant to maximize
its application.
 END
OF
CHAPTER 1