Hvac Module 1

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1. THERMODYNAMICS

THERMODYNAMICS AND ENERGY

Thermodynamics can be defined as the science of energy. Although everybody has a

feeling of what energy is, It is difficult to give a precise definition for it. Energy can be viewed

as the ability to cause changes. The name thermodynamics stems from the Greek words therm

(heat) and dynanus (power), which is most descriptive of the early efforts to convert heat into

power. Today the same name is broadly interpreted to include all aspects of energy and energy

transformations, including power generation refrigeration, and relationships among the

properties of matter.

Thermodynamics may be defined as follows :

Thermodynamics is a science which deals with the relations among heat, work and

properties of system which are in equilibrium. It describes state and changes in state of physical

systems.

Or

Thermodynamics is the science of the regularities governing processes of energy

conversion.

Or

Thermodynamics is the science that deals with the interaction between energy and

material systems.

Thermodynamics, basically entails four laws or axioms known as Zeroth

, First, Second

and Third law of thermodynamics.

The Zeroth

law deals with thermal equilibrium and establishes a concept of temperature.

The First law throws light on concept of internal energy.

The Second law indicates the limit of converting heat into work and introduces the principle

of increase of entropy.

The Third law defines the absolute zero of entropy.

These laws are based on experimental observations and have no mathematical proof.

Like all physical laws, these laws are based on logical reasoning.

One of the most fundamental laws of nature is the conservation of energy principle. It

simply states that during an interaction, energy can change from one form to another but the

total amount of energy remains constant. That is, energy cannot be created or destroyed. A rock

falling off a cliff, for example, picks up speed as a result of its potential energy being converted

to kinetic energy. The conservation of energy principle also forms the back bone of the diet

industry. A person who has a greater energy input (food) than energy output (exercise) will gain

weight (store energy in the form of fat), and a person who has a smaller energy input than output

will lose weight. The change in the energy content of a body or any other system is equal to the

difference between the energy input and the energy output, and the energy balance is expressed

as Em - Em = ΔE

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The first law of thermodynamics is simply an expression of the conservation of energy

principle, and it asserts that energy is a thermodynamic property. The second law of

thermodynamics asserts that energy has quality as well as quantity, and actual processes occur in

the direction of decreasing quality of energy. For example, a cup of hot coffee left on a table

eventually cools, but a cup of cool coffee in the same room never gets hot by itself. The high-

temperature energy of the coffee is degraded (transformed into a less useful form at a lower

temperature) once it is transferred to the surrounding air.

It is well-known that a substance consists of a large number of particles called molecules.

The properties of the substance naturally depend on the behavior of these particles. For example,

the pressure of a gas in a container is the result of momentum transfer between the molecules and

the walls of the container. However, one does not need to know the behavior of the gas particles

to determine the pressure in the container. It would be sufficient to attach a pressure gauge to the

container. This macroscopic approach to the study of thermodynamics that does not require a

knowledge of the behavior of individual particles is called classical thermodynamics. It provides

a direct and easy way to the solution of engineering problems. A more elaborate approach, based

on the average behavior of large groups of individual particles, is called statistical

thermodynamics.

APPLICATION AREAS OF THERMODYNAMICS

All activities in nature involve some interaction between energy and mailer thus, it is

hard to imagine an area that does not relate to thermodynamics in some manner. Therefore,

developing a good understanding of bask principles of thermodynamics has long been an

essential part of engineering education.

Thermodynamics is commonly encountered in many engineering systems and other

aspects of life, and one does not need to go very far to see some application areas of it. In fact,

one does not need to go anywhere. The heart is constantly pumping blood to all parts of the

human body, various energy conversions occur in trillions of body cells, and the body heat

generated is constantly rejected to the environment. The human comfort is closely tied to the rate

of this metabolic heat rejection. We try to control this heat transfer rate by adjusting our clothing

to the environmental conditions.

Other applications of thermodynamics are right where one lives. An ordinary house is, in

some respects, an exhibition hall filled with wonders of thermodynamics. Many ordinary

household utensils and appliances are designed, in whole or in part by using the principles of

thermodynamics. Some examples include the electric or gas range, the heating and air-

conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the

shower, the iron, and even the computer and the TV. On a larger scale, thermodynamics plays a

major part in the design and analysts of automotive engines, rockets, jet engines, and

conventional or nuclear power plants, solar collectors, and the design of vehicles from ordinary

cars to airplanes. The energy-efficient home that you may be living in, for example, is designed

on the basis of minimizing heat loss in winter and beat gain in summer The size, location, and

the power input of the fan of your computer is also selected after an analysis that involves

thermodynamics.

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SYSTEM, BOUNDARY AND SURROUNDINGS

System- A system is a finite quantity of matter or a prescribed region of space Boundary.

The actual or hypothetical envelope enclosing the system is the boundary of the system. The

boundary may be fixed or it may move, as and when a system containing a gas is compressed or

expanded. The boundary may be real or imaginary. It is not difficult to envisage a real boundary

but an example of imaginary boundary would be one drawn around a system consisting of the

fresh mixture about to enter the cylinder of an I.C. engine together with the remnants of the last

cylinder charge after the exhaust process.

1. Closed System.

Refer above Fig. (a) If the boundary of the system is impervious to the flow of matter, it

is called a closed system. An example of this system is mass of gas or vapour contained in an

engine cylinder, the boundary of which is drawn by the cylinder walls, the cylinder head and

piston crown. Here the boundary is continuous and no matter may enter or leave.

2.Open System

Refer Fig. (c) An open system is one in which matter flows into or out of the system.

Molt of the engineering systems are open.

3.Isolated System

An isolated system is that system which exchanges neither energy nor matter with at

other system or with environment.

4. Adiabatic System

An adiabatic system is one which is thermally insulated from its surroundings. It can,

however, exchange work with its surroundings. If it does not, it becomes an isolated system.

Phase- A phase is a quantity of matter which is homogeneous throughout in chemical

composition and physical structure.

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5. Homogeneous System

A system which consists of a single phase is termed as homogeneous system. Examples :

Mixture of air and water vapour, water plus nitric acid and octane plus heptane.

6.Heterogeneous System

A system which consists of two or more phases is called a heterogeneous system.

Examples : Water plus steam, ice plus water and water plus oil.

MACROSCOPIC AND MICROSCOPIC POINTS OF VIEW

Macroscopic approach—(Macro mean big or total)

Microscopic approach—(Micro means small)

Macroscopic approach Microscopic approach

In this approach a certain quantity of matter is

considered without taking into account the

events occurring at molecular. This is known

as classical thermodynamic

In this molecules have different velocities and

energies. The values of these energies are

constantly changing with time. This approach

to thermodynamics which is concerned directly

with the structure of the matter is known as

statistical thermodynamics.

The analysis of macroscopic system requires

simple mathematical formulae.

The behaviour of the system is found by using

statistical methods as the number of molecules

is very large. So advanced statistical and

mathematical methods are needed to explain

the changes in the system.

The values of the properties of the system are

their average values. These properties like

pressure and temperature can be measured vary

easily. The changes in properties can be felt by

our sense.

The properties like velocity, momentum,

impulse, kinetic energy, force of impact etc

which describe the molecule cannot be easily

measured by instrument. Our senses cannot

feel them.

In order to describe a system only a few

properties are needed.

Large number of variables an needed to

describe a system. So the approach is

complicated.

PURE SUBSTANCE

A pure substance is one that has a homogeneous and invariable chemical composition

even though there is a change of phase. In other words, it is a system which is (a) homogeneous

in composition, (b) homogeneous in chemical aggregation. Examples: Liquid, water, mixture of

liquid water and steam, mixture of ice and water. The mixture of liquid air and gaseous air is not

a pure substance.

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THERMODYNAMIC EQUILIBRIUM

A system is in thermodynamic equilibrium if the temperature and pressure at all points

are same ; there should be no velocity gradient; the chemical equilibrium is also necessary.

Systems under temperature and pressure equilibrium but not under chemical equilibrium are

sometimes said to be in metastable equilibrium conditions. It is only under thermodynamic equi-

librium conditions that the properties of a system can be fixed.

Thus for attaining a state of thermodynamic equilibrium the following three types of

equilibrium states must be achieved :

1. Thermal equilibrium- The temperature of the system does not change with time has same

value at all points of the system.

2. Mechanical equilibrium- There are no unbalanced forces within the system or betwe the

surroundings. The pressure in the system is same at all points and does not change wit

respect to time.

3. Chemical equilibrium. No chemical reaction takes place in the system and the I cat

composition which is same throughout the system does not vary with time.

PROPERTIES OF SYSTEMS

A property of a system is a characteristic of the system which depends upon its state, bu

not upon how the state is reached. There are two sorts of property :

Intensive properties. These properties do not depend on the mass of the systen Examples :

Temperature and pressure.

Extensive properties. These properties depend on the mass of the system- Example ;

Volume. Extensive properties are often divided by mass associated with them to obtain the inten-

sive properties. For example, if the volume of a system of mass m is V, then the specific volume

of matter within the system is V/m =υ which is an intensive property.

STATE

State is the condition of the system at an instant of time as described or measured by

properties. Or each unique condition of a system is called a state. It follows from the definition

of state that each property has a single value at each stat Stated differently, all properties are state

or point functions. Therefore, all properties are identic for identical states.

On the basis of the above discussion, we can determine if a given variable is property or

by applying the following tests :

A variable is a property, if and only if, it has a single value at each equilibrium state.

A variable is a property, if and only if, the change in its value between any two pre-

scribed equilibrium states is single-valued.

Therefore, any variable whose change is fixed by the end states is a property.

PROCESS

A process occurs when the system undergoes a change in a state or an energy transfer at a

steady state. A process may be non-flow in which a fixed mass within the defined boundary is

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undergoing a change of state. Example : A substance which is being heated in a closed cylinder

undergoes a non-flaw process. Closed systems undergo non-flow processes. A process may be a

flow process in which mass is entering and leaving through the boundary of an open system. In a

steady flow process mass is crossing the boundary from surrounding at entry, and an equal mass

is crossing the boundary at the exit so that the total mass of the system remains constant. In an

open system it is necessary to take account of the work delivered from the surroundings to the

system at entry to cause the mass to enter, and also of the work delivered from the system at

surroundings to cause the mass to leave, as well as any heat or work crossing the boundary of the

system.

Quasi-static process. Quasi means 'almost'. This process is a succession of equilibrium

states and infinite slowness is its characteristic feature.

CONTINUUM

Matter is made up of atoms that are widely spaced in the gas phase. Yet. very convenient

to disregard the atomic nature of a substance and view ii a continuous, homogeneous matter with

no holes, that is, a continuum, continuum idealization allows us to treat properties as point

functions I assume the properties vary continually in space with no jump discontinuities. This

idealization is valid as long as the size of the system we deal is large relative to the space

between the molecules. This is the case in practically all problems, except some specialized ones.

The continuum idealisation is implicit in many statements we make, such as "the density of in a

glass is the same at any point.

CYCLE

Any process or series of processes whose end states are identical is termed a cycle. The

processes through which the system has passed can be shown on a state diagram, but a complete

section of the path requires in addition a statement of the heat and work crossing the boundary of

the system.

PATH FUNCTION AND POINT FUNCTION

With reference to Fig. it is possible to take a system from state 1 to state 2 along many

quasi-static paths, such as A, B or C.

Since the area under each curve

represents the work for each process, the

amount of work involved in each case is not a

function of the end states of the process, and it

depends on the path the system follows in

going from state 1 to state 2. For this reason,

work is called a path function, and đW is an

inexact or imperfect differential.

dW2

1≠ W2 − W1

Examples. Heat, work etc

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Thermodynamic properties are point functions, since for a given state, there is a definite

value for each property. The change in a thermodynamic property of a system in a change of

state is independent of the path the system follows during the change of state, and depends only

on the initial and final states of the system. The differentials of point functions are exact or

perfect differentials and the integration is simply dV2

1= V2 − V1 Examples. Pressure,

temperature, volume etc

The change in volume thus depends only on the end states of the system, irrespective of

the path the system follows. On the other hand, work done in a quasi-static process between two

given states depends on the path followed.

TEMPERATURE

The temperature is a thermal state of a body which distinguishes a hot body from a cold

body. The temperature of a body is proportional to the stored molecular energy, the average

molecular kinetic energy of the molecules in a system. (A particular molecule does not

behave a temperature, it has energy. The gas as a system has temperature).

Instruments for measuring ordinary temperatures are known as thermometers and those for

measuring high temperatures are known as pyrometers.

It has been found that a gas will not occupy any volume at a certain temperature. This

temperature is known as absolute zero temperature. The temperatures measured with

absolute zero as basis are called absolute temperatures. Absolute temperature is stated in

degrees centigrade. The point of absolute temperature is found to occur at 273. WC below

the freezing point of water.

Then : Absolute temperature (K) = Thermometer reading in oC + 273.15.

T(F)=1.8(ToC)+32

Absolute temperature is degree centigrade is known as degrees kelvin, denoted by K (SI

unit).

ZEROTH LAW OF THERMODYNAMICS

• "Zeroth law of thermodynamics' states that if two systems are each equal in tem-

perature to a third, they are equal in temperature to each other.

System '1' may consist of a mass of gas enclosed in a rigid vessel fitted with a pressure

gauge. If there is no change of pressure when this system is brought into contact with system 2 a

block of iron, then the two systems are equal in temperature (assuming that the systems 1 and 2

do not reacts each other chemically or electrically).

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Experiment reveals that if system T is brought into contact with a third system '8' again

with no change of properties then systems T and '3' will show no change in their properties when

brought into contact provided they do not react with each other chemically or electrically.

Therefore, 2 and '3' must be in equilibrium.

THE THERMOMETER AND THERMOMETRIC PROPERTY

The zeroth law of thermodynamics provides the basis for the measurement of

temperature. It enables us to compare temperatures of two bodies '1' and '2' with the help of a

third body '3‘ and say that the temperature of '1‘ is the same as the temperature of ‗2' without

actually bringing 'V and '2' in thermal contact. In practice, body '3' in the zeroth law it called the

thermometer. It is brought into thermal equilibrium with a set of standard temperature of a body

'2', and is thus calibrated. Later, when any other body '1‘ is brought in thermal communication

with the thermometer, we say that the body '1‘ has attained equality of temperature with the

thermometer, and hence with body It. This way, the body 'V has the temperature of body '2‘

given for example by, say the height of mercury column in the thermometer '3'.

The height of mercury column in a thermometer, therefore, becomes a thermometric

property.

Then are other methods of temperature measurement which utilize various other proper-

ties of materials, that an functions of temperature, as thermometric properties.

Six different kinds of thermometers, and the names of the corresponding thermometric

properties employed an given below:

Thermometer Thermometric property

1

2

3

4

5

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Constant volumes gas

Constant pressure gas

Alcohol or mercury-in-glass

Electric resistance

Thermocouple Electromotive

Radiation (pyrometer)

Pressure (p)

Volume (V)

Length (L)

Resistance (Ω)

force (F)

Intensity of radiation (I or J)

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DEFINITION OF PRESSURE

Pressure is defined as a force per unit area. Pressures are exerted by gases, vapours and

liquids. The instruments that we generally use, however, record pressure at the difference be-

tween two pressures. Thus, it is the difference between the pressure exerted by a fluid of interest

and the ambient atmospheric pressure. Such devices indicate the pressure either above or below

that of the atmosphere. When ft is above the atmospheric pressure, it is termed gauge pressure

and is positive. When it is below atmospheric, it is negative end is known as vacuum. Vacuum

readings are given in millimeters of mercury or millimeters of water below the atmosphere.

It is necessary to establish an absolute pressure scale which, is independent of the

changes in atmospheric pressure. A pressure of absolute zero can exist only in complete vacuum.

Any pressure measured above the absolute zero of pressure is termed an 'absolute pressure'. •

A schematic diagram showing the gauge pressure, vacuum pressure and the absolute

presssure is given in Fig.

Pgauge=Pabs-Patm; Pvac= Patm-Pabs

The fundamental SI unit of pressure is N/m2 (sometimes called pascal. Pa) or bar. 1 bar =

105N/m2. Standard atmospheric pressure = 1.01325 bar = 0.76 m for 760 mm) Hg.

Low pressures are often expressed in terms of nun of water or mm of mercury. This is an

abbreviated way of saying that the pressure is such that which will support a liquid column of

stated height.

Types of Pressure Measurement Devices

1. The manometer

An elevation change in a fluid corresponds to &P/ρg, which suggests that a fluid column

can be us measure pressure differences.

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2. The barometric and atmospheric pressure

Atmospheric pressure is measured by a device called a barometer; thus, the atmospheric pressure

is often referred to as the barometric pressure.

WORK

Work is one of the basic modes of energy transfer. In mechanics work is defined as the

product of force and distance moved in the direction of force! It is denoted by W and the unit of

work is N - m. 1 Nm = 1 J. In thermodynamics, the energy transfer across the boundary of a

system on account of reasons other than temperature difference is called work. Work is said to be

done by a system if the sole effect external to the system can be reduced to the lifting of a

weight.

Consider a storage electric battery as a system, which is connected to a resistor by means

of a switch as shown in fig. (a) When the switch is closed, current flows through the resistor and

the resistor becomes warmer. According to the definition of work in mechanics, no work is done.

The sole effect external to the system, ie., warming of resistor can be reduced to the lifting of

weight, if the resistor is replaced by a motor and a load as shown in fig. (b). When the switch is

closed, motor shaft rotates and the load is lifted. Hence when the switch is closed, the system

interacts with its surroundings and the sole effect could fig reduced to the lifting of a weight

Therefore the system ( battery ) does work when the switch is closed.

(a) (b)

A device based on this principle is called manometer, and

it is commonly used to measure small and moderate

pressure differences. A manometer mainly consists of a

glass or plastic U-t containing one or more fluids such as

mercury, water, alcohol, or oil keep the size of the

manometer to a manageable level, heavy fluids such

mercury are used if large pressure differences are

anticipated

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Like heat, work is also energy in transit. A system does not contain work, upon entering

the system it is converted into stored energy. Work is a path function and hence it is not a

property of the system.

(c) (d)

𝑑𝑊2

1≠ 𝑊2 −𝑊1 ; is the amount of work transferred during a process 1-2.

pdV-WORK OR DISPLACEMENT WORK

Let the gas in the cylinder (Fig. PdV

Work) be a system having initially the

pressure p1 and volume V1 The system is in

thermodynamic equilibrium, the state of

which is described by the coordinates p1, V1.

The piston is the only boundary which

moves due to gas pressure.

pdV work

Let the piston move out to a new final position 2, which is also a thermodynamic

equilibrium state specified by pressure p2 and volume V2. At any intermediate point in the travel

of the piston, let the pressure be p and the volume V. This must also be an equilibrium state,

since macroscopic properties p and V are significant only for equilibrium states. When the piston

moves an infinitesimal distance dl, and if V be the area of the piston, the force F acting on the

piston F = p.a. and the infinitesimal amount of work done by the gas on the piston.

dW = F dl = padl = pdV

where dV = adl = infinitesimal displacement volume. The differential sign in đW

When the piston moves out from position 1 to position 2 with the volume changing from

V1 to V2, the amount of work W done by the system will be

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𝑊1−2 = 𝑝𝑑𝑉𝑣2

𝑣1

Quasi-static pdV work

The magnitude of the work done is given by

the area under the path 1-2, as shown in Fig.

Since p is at all times a thermodynamic

coordinate, all the states passed through by

the system as the volume changes from V1

Jo V2 must be equilibrium states, and the

path 1-2 must be quasi-static. The piston

moves infinitely slowly so that every state

passed through is an equilibrium state. The

integration 𝑝𝑑𝑉 can be performed only on

a quasi-static path.

HEAT

The energy transfer across the boundary of a system on account of the temperature

difference between the system and surroundings is called heat. It is denoted by Q. Heat can be

identified only when it crosses the boundary of a system and hence it is a form of energy in

transit. A system does not contain heat because upon entering a system, heat is converted into

potential or kinetic energy of the molecules. When a system changes its state the amount of heat

transferred depends upon the path followed. Hence heat is a path function, and therefore it is not

a property of the system.

The integral of a differential change

in heat can be written as 𝑑𝑄2

1≠ 𝑄2 − 𝑄1

Q is the amount of heat transferred

during a process 1-2. Ref Fig (c)

Heat transferred to a system is

considered positive and heat transferred

from a system is considered negative. The

unit of heat; is Joule (J) or kilo Joule (kJ).

FIRST LAW OF THERMODYNAMICS

The study of thermodynamics is based on two general laws of nature, the first law of

thermodynamics and the second law of thermodynamics. These laws are based on physical

observations and hence cannot be proved mathematically. However no violation of these laws

has ever been noticed. Moreover various experiments have proved the validity of these laws.

The first law is a theorem of conservation of energy. It makes no distinction between the

various modes of energy and declares that all forms of energy are equivalent. The second law

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states that all forms of energy are not equivalent in their ability to do work. It declares that

certain processes are impossible to perform even though these processes do not violate the first

law.

The first law of thermodynamics can be stated as." For a system operating in a cycle, the

net heat transfer is equal to the net work transfer"

i.e. 𝑑𝑄 = 𝑑𝑊

since (𝑑𝑄 − 𝑑𝑊) = 0; ; where of stands for the summation over the cyclic process

or cyclic integral

PERPETUAL MOTION MACHINE OF THE FIRST KIND—PMM1

The first law states the general principle of the conservation of energy, Energy is neither

created nor destroyed, but only gets transformed from one form t another. There can be no

machine which would continuously supply mechanical work without some other form of energy

disappearing simultaneously (Fig.). Such a fictitious machine is called a perpetual motion

machine of the first kind, or in brief, PMM1. A PMM1 is thus impossible.

The converse of the above statement is also true, i.e., there can be no machine which

would continuously consume work without some other form of energy appearing simultaneously

(Fig. ).

A PMM 1 Converse of PMM1

INTERNAL ENERGY

Energy storage in a system is neither heat nor work, and is given the name internal

energy or simply, the energy of the system. Internal energy of a substance may be defined as the

algebraic sum of internal kinetic energy and internal potential energy of its molecules and is

denoted by U. It is very difficult to determine the absolute value of internal energy possessed by

a substance.

In most of the thermodynamic applications we are mainly interested only in the changes

in the internal energy of a system. The total energy of a system is the sum of potential energy,

kinetic energy, internal energy and other energies due to electricity, magnetism etc. In

engineering thermodynamics the concern is with the first three types of energies and electrical

energy, magnetic energy etc, can be neglected.

ΔE = Q - W =PE + KE + U

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Change in energy, ΔE = ΔPE+ ΔKE + ΔU

For a stationary closed system undergoing

CYCLIC HEAT ENGINE

A heat engine cycle is a thermodynamic cycle in which there is a net heat transfer to the

system and a net work transfer from the system. The system which executes a heat engine cycle

is called a heat engine.

A heat engine may be in the form of a mass of gas confined in a cylinder and piston

machine Fig. (a) or a mass of water moving in a steady flow through a steam power plant Fig.

(b).

(a) I.C engine (b) Steam power plant Block Diagram

In the cyclic heat engine, as represented in Fig. (a), heat Q1 is transferred to the system,

work WE is done by the system, work Wc is done upon the system, and then heat Q2 is rejected

from the system. The system is brought back to the initial state through all these four successive

processes which constitute a heat engine cycle. In Fig. (b) heat Q1 is transferred from the furnace

to the water in the boiler to form steam which then works on the turbine rotor to produce work,

then the steam is condensed to water in the condenser in which an amount Q2 is rejected from the

system, and finally work Wp is done on the system (water) to pump it to the boiler. The system

repeats the cycle.

The net heat transfer in a cycle to either of the heat engines

𝑄𝑛𝑒𝑡 = 𝑄1 − 𝑄2

Net work transfer in the cycle,

𝑊𝑛𝑒𝑡 = 𝑊𝑇 −𝑊𝐸

By first law of thermodynamics Wnet = Qnet

𝑖𝑒.𝑄1 − 𝑄2 = 𝑊𝑇 −𝑊𝐸

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The block diagram indicating the various energy interactions during a cycle. Boiler (B),

turbine (T), condenser (C), and pump (P), all four together constitute a heat engine. A heat

engine is here a certain quantity of water undergoing the energy interactions, as shown, in cyclic

operations to produce net work from a certain heat input

The function of a heat engine cycle is to produce work continuously at the expense of

heat input to the system. So the net work and heat input Q1 referred to the cycle are of primary

interest. The efficiency of a heat engine or a heat engine cycle is defined as,

η = 𝑁𝑒𝑡 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 𝑜𝑓 𝑡𝑒 𝑐𝑦𝑐𝑙𝑒

𝑁𝑒𝑡 𝑒𝑎𝑡 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡𝑒 𝑐𝑦𝑐𝑙𝑒

= 𝑊𝑛𝑒𝑡

𝑄1=

𝑊𝑇−𝑊𝐸

𝑄1=

𝑄1−𝑄2

𝑄1 = 1 −

𝑄2

𝑄1

This is also known as the thermal efficiency of a heat engine cycle. A heat engine is very

often called upon to extract as much work (net) as possible from a certain heat input, i.e., to

maximize the cycle efficiency

ENERGY RESERVOIRS

A thermal energy reservoir (TER) is defined as a large body of infinite heat capacity,

which is capable of absorbing or rejecting an unlimited quantity of heat without suffering

appreciable changes in its thermodynamic coordinates. The changes that do take place in the

large body as heat enters or leaves are so very slow and so very minute that all processes within

it are quasi-static.

The thermal energy reservoir TERH from which heat Q1 is transferred to the system

operating in a heat engine cycle is called the source. Eg:- Sun. The thermal energy reservoir

TERL to which heat Q2 is rejected from the system during a cycle is the sink. Eg. Sea. A typical

source is a constant temperature source where fuel is continuously burnt, and a typical sink is a

river or sea or the atmosphere itself.

A mechanical energy reservoir (MER)

is a large body enclosed by an adiabatic

impermeable wall capable of storing work as

potential energy (such as a raised weight or

wound spring ) or kinetic energy (such as a

rotating flywheel). All processes of interest

within an MER are essentially quasi-static. An

MER receives and delivers mechanical energy

quasi-statically.

Cyclic heat engine (CHE) with source and sink

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REFRIGERATOR AND HEAT PUMP

A refrigerator is a device which, operating in a cycle, maintains a body at a temperature lower

than the temperature of the surroundings. Let the body A (Fig) be maintained at which is lower

than the ambient temperature t1. Even though A is insulated, there will always be heat leakage Q2

into the body from the surroundings by virtue of the temperature difference.

A cyclic refrigeration plant

In order to maintain, body A at the constant temperature t2, heat has to be removed from

the body at the same rate at which heat is leaking into the body. This heat (Q2) is absorbed by a

working fluid, called the refrigerant, which evaporates in the evaporator E1 at a temperature

lower than t2 absorbing the latent heat of vaporization from the body A which is cooled or

refrigerated (Process 4-1). The vapour is first compressed in the compressor C1 driven by a

motor which absorbs work Wc (Process 1-2), and is then condensed in the condenser C2 rejecting

the latent heat of condensation Q1 at a temperature higher than that of the atmosphere (at t1]) for

heat transfer to take place (Process 2-3). The condensate then expands adiabatically through an

expander (an engine or turbine) producing work WE, when the temperature drops to a value

lower than t2 such that heat Q2 flows from the body A to make the refrigerant evaporate (Process

3-4). Such a cyclic device of flow through E1-C1-C2-E2 is called a refrigerator. In a refrigerator

cycle, attention is concentrated on the body A. Q2 and Ware of primary interest. Just like

17

efficiency in a heat engine cycle, there is a performance parameter in a refrigerator cycle, called

the coefficient of performance, abbreviated to COP, which is defined as,

[COP]Ref = 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑒𝑓𝑓𝑒𝑐𝑡

𝑊𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡=

𝑄2

𝑊=

𝑄2

𝑄1−𝑄2

A heat pump is a device which, operating in a cycle, maintains a body, say B (Fig.), at a

temperature higher than the temperature of the surroundings. By virtue of the temperature

difference, there will be heat leakage Q1 from the body to the surroundings. The body will be

maintained at the constant temperature t1, if heat is discharged into the body at the same rate at

which heat leaks out of the body. The heat is extracted from the low temperature reservoir, which

is nothing but the atmosphere, and discharged into the high temperature body B, with the

expenditure of work W in a cyclic device called a heat pump. The working fluid operates in a

cycle flowing through the evaporator E1, compressor C1 condenser C2 and expander E2, similar

to a refrigerator, but the attention is here focused on the high temperature body B. Here Q1and W

are of primary interest, and the COP is defined as,

[COP]HP = 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑒𝑓𝑓𝑒𝑐𝑡


𝑄1

𝑊

= 𝑄1

𝑄1−𝑄2= 1 +

𝑄2

𝑄1−𝑄2

Therefore,

[COP]HP = [COP]Ref + 1

Cyclic Heat Pump

KELVIN-PLANCK STATEMENT OF SECOND LAW

The efficiency of a heat engine is given by,

η = 𝑊𝑛𝑒𝑡

𝑄1 = 1 −

𝑄2

𝑄1

Experience shows that Wnet < Q1 since heat Q1 transferred to a system cannot be

completely converted to work in a cycle. Therefore, η is less than unity. A heat engine can never

be 100% efficient. Therefore, Q2 > 0, i.e., there has always to be a heat rejection. To produce net

work in a thermodynamic cycle, a heat engine has thus to exchange heat with two reservoirs, the

source and the sink.

The Kelvin-Planck statement of the second law states: “It is impossible for a heat engine to

produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed

temperature”.

18

If Q2 = 0 (i.e., Wnet = Q1 or η = 1.00), the heat engine will produce net work in a complete

cycle by exchanging heat with only one reservoir, thus violating the Kelvin-Planck statement

Such a heat engine is called a perpetual motion machine of the second kind, abbreviated to

PMM2. A PMM2 is impossible.

PMM2

CLAUSIUS' STATEMENT OF THE SECOND LAW

Heat always flows from a body at a higher temperature to a body at a lower temperature.

The reverse process never occurs spontaneously.

Clausius' statement of the second law gives: “It is impossible to construct a device which,

operating in a cycle, will produce no effect other than the transfer of heat from a cooler to a

hotter body”.

Heat cannot flow of itself from a body at a lower temperature to a body at a higher

temperature. Some work must be expended to achieve this.

EQUIVALENCE OF KELVIN-PLANCK AND CLAUSIUS STATEMENTS

At first sight, Kelvin-Planck's and Clausius' statements may appear to be unconnected,

but it can easily be shown that they are virtually two parallel statements of the second law and

are equivalent in all respects. The equivalence of the two statements will be proved if it can be

shown that the violation of one statement implies the violation of the second, and vice versa.

(a) Let us first consider a cyclic heat pump P which transfers heat from a low temperature

reservoir (t2) to a high temperature reservoir (t1) with no other effect, i.e., with no expenditure of

work, violating Clausius statement,

Heat Engine producing net work in

a cycle by exchanging heat at two

different temperatures

19

Let us assume a cyclic heat engine E operating between the same thermal energy

reservoirs, producing in one cycle. The rate of working of the heat engine is such that it draws an

amount of heat Q1 from the hot reservoir equal to that discharged by the heat pump. Then the hot

reservoir may be eliminated and the heat Q1 discharged by the heat pump is fed to the heat

engine. So we see that the heat pump P and the heat engine E acting together constitute a heat

engine operating in cycles and producing net work while exchanging heat only with one body at

a single fixed temperature. This violates the Kelvin-Planck statement.

(b) Let us now consider a perpetual motion machine of the second kind (E) which

produces net work in a cycle by exchanging heat with only one thermal energy reservoir (at t1)

and thus, violates the Kelvin-Planck statement (Fig).

Let us assume a cyclic heat pump (P)

extracting heat Q2 from a low temperature

reservoir at t2 and discharging heat to the

high temperature reservoir at t1 with the

expenditure of work W equal to what the

PMM2 delivers in a complete cycle. So E

and P together constitute a heat pump

working in cycles and producing the sole

effect of transferring heat from a lower to a

higher temperature body, thus violating the

Clausius statement.

20

REVERSIBILITY AND IRREVERSIBILITY

A process is said to be reversible if, when the process is carried out in the reverse;

direction, using the same amount of work and heat transferred during the forward process, the

system passes through the same states as it does in the forward direction. In other words a process

is said to be reversible, if both the system and surroundings can be restored to the original state

after the process is completely reversed. A process which cannot be completely reversed without

leaving a change either in the system or surroundings is called irreversible process. All actual

process is irreversible. The conception of reversibility is purely hypothetical and hence

irreversibility is a natural tendency. Work done during an irreversible process will be less than the

work done during the same process if the process is assumed to be reversible.

ie. W irr. < W rev.

The difference between the work done during reversible process and irreversible process is

called irreversibility and is denoted by I

Irreversibility, I = Wrev - Wirr.

Irreversibility is also known as lost work or degradation

AVAILABILITY

According to second law of thermodynamics, complete conversion of heat in to work is

not possible. Any heat engine must reject a portion of heat supplied to it to a low temperature

reservoir. Only the remaining portion of heat can be converted into work. Heat converted into

work will be more when the temperature of the low temperature reservoir is less. The lowest

temperature of a reservoir corresponds to the temperature of atmosphere, which is not at absolute

zero. Therefore the entire heat supplied to an engine is not available for conversion into work. The

portion of heat, which is available for conversion into work, is called available energy (exergy)

and the portion of heat, which is rejected to the low temperature reservoir, is called unavailable

energy(anergy).

The potential energy of a system can be considered as completely available form of

energy, because the elevation to calculate potential energy is measured from the surface of earth.

If the centre of earth is selected as the datum of zero potential energy, then a large part of

potential energy of the system is unavailable.

If a system which is in a state of equilibrium is in equilibrium with its surroundings, then

there can be no interaction between the system and surroundings. This is called the dead state of a

system. The work done by a system during a reversible process will be maximum, if the process is

continued until the system reaches the dead state. The availability of a system at a given state is

defined as the maximum useful work that can be obtained during a process in which the system

21

comes to equilibrium with its surroundings (dead state). Availability is thus a composite property

depending on the state of both the system and surroundings.

EQUATIONS OF STATE

A perfect or an ideal gas is the gas which strictly obeys all the gas laws under all

conditions of pressure and temperature. In fact no actual or real gas which exists in nature is

perfect. In engineering applications gases such as air, nitrogen, hydrogen etc, are considered as

perfect gases. Boyle's law and Charles law govern the behavior of a perfect gas.

Boyle's Law

This law was formulated by Robert Boyle in 1662, on the basis of his experimental results,

It states that, if the temperature remains constant, the volume of a given mass of gas is inversely

proportional to its absolute pressure.

i.e, υ ∝ 1

𝑇, if T is constant P

pυ = constant, if T is constant.

From the above expression, it is clear

that at constant temperature, the product of

absolute pressure and volume of a given mass

of a perfect gas is constant. The equation pυ =

constant represents a rectangular hyperbola as

shown in fig.

Charles' Law

This law was formulated by A.C Charles in 1787. It states that, the volume of a given

mass of a perfect gas varies directly as its absolute temperature, if the pressure remains constant.

V α T, if p is constant

The above expression shows that at constant pressure, the volume increases with absolute

temperature. It is represented by a horizontal line in the p-V diagram as shown in figure At the

state l. 𝑣1

𝑇1= constant.

At the state 2. 𝑣2

𝑇2= constant.

Experiments prove that at constant

pressure any perfect gas changes its volume

by 1

273 of its volume at 0°C for every 1° C

change in its temperature.

22

Avogadro’s Law

A mole of a substance has a mass numerically equal to the molecular weight of the

substance.

One g mol of oxygen has a mass of 32 gm, 1 kg mol of oxygen has a mass of 32 kg, 1 kg mol

of nitrogen has a mass of 28 kg, and so on. It is denoted by the letter ‗m‘

Avogadro's law states that the volume of a g mol of all gases at the pressure of 760 mm

Hg and temperature of 0°C is the same, and is equal to 22.4 litres. Therefore, 1 g mol of a gas has

a volume of 22.4 x 103 cm3 and 1 kg mol of a gas has a volume of 22.4 m3 at normal temperature

and pressure (N.T.P.).

For a certain gas, if m is its mass in kg, and μ its molecular weight, then the number of kg

moles of the gas, n, would be given by

n = 𝑚

𝑘𝑔

𝑘𝑔

𝜇𝑘𝑔

𝑘𝑔 𝑚𝑜𝑙

= 𝑚

𝜇 kg moles

The molar volume, v, is given by

v = 𝑉

𝑁 𝑚

3

𝑘𝑔 𝑚𝑜𝑙 where V is the total volume of the gas in m3.

Characteristic Gas Equation

A relationship between the three properties pressure, volume and temperature of a perfect

gas is obtained by combining Boyle's law and Charles law.

Let a given mass m of a perfect gas be expanded from state 1 to state 2. Let first part of

expansion, ie., from 1 to 1' be at constant temperature and the second part of expansion ie, from 1'

to 2 be at constant pressure. For the first part of expansion applying Boyle's law,

p1V1 = p'1V'1

p1 V1 = p2V'1 [Since p'1 = p2]

For the second part of expansion at constant pressure, applying Charles law,

𝑉1′

𝑇1′=

𝑉2

𝑇2

𝑉1′

𝑇1=

𝑉2

𝑇2 [Since T1' = T1]

𝑉1′ =𝑉2

𝑇2x T1

Substituting this value of V1' in the expression, p1 V1 = p2V'1

p1 V1 = p2 x 𝑉2

𝑇2 x T1

23

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2=

𝑝 𝑉

𝑇= 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡

pV= constant x T; This constant depends upon the mass of gas, properties of gas and

temperature scale.

pV =mRT where R is a constant, the value of which depends upon the properties of gas

and the temperature scale. This constant R is called characteristic gas constant its unit is kJ/kg K

it is depend on molar mass and ‗m‘ is molar mass. The equation pV=mRT is called characteristic

gas equation or equation of state of a perfect gas. R for air is 287J/kg K, O2=260 J/kg K

N2=296J/kg K etc

pV=nȒT Where ‗n‘ is the number of moles of substance and Ȓ is the universal gas

constant and its value is 8.314kJ/kg mole K. It is given by formula Ȓ = 𝑅

𝑚

SPECIFIC HEAT

Specific heat of a substance is defined as the amount of heat required to raise the

temperature of unit mass of the substance by unit degree. Since a gas can be heated under constant

pressure and under constant volume, it has two specific heats, specific heat at constant pressure

and specific heat at constant volume.

Specific Heat at Constant Pressure

It is defined as the amount of heat required to raise the temperature of unit mass of the

substance by unit degree, when it is heated at constant pressure. It is denoted by letter Cp.

Let Q be the amount of heat supplied to a gas at constant pressure in J, m is the mass of

the gas in kg, T1 and T2 the initial and final temperature of the gas in K, then,

𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑎𝑡,𝐶𝑝 =𝑄

𝑚(𝑇2−𝑇1) 𝑘𝐽/𝑘𝑔.𝐾

Specific Heat at Constant Volume

It is defined as the amount of heat required to raise the temperature of unit mass of the

substance by unit degree, when it is heated at constant volume. It is denoted by letter Cv.

Let Q be the amount of heat supplied to a gas at constant pressure in J, m is the mass of

the gas in kg, T1 and T2 the initial and final temperature of the gas in K, then,

𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑒𝑎𝑡,𝐶𝑣 =𝑄

𝑚(𝑇2−𝑇1) 𝑘𝐽/𝑘𝑔.𝐾

For air, Cp = 1.005 kJ/ kg. K and Cv = 0.718 kJ/kg.K

RATIO OF SPECIFIC HEATS

The ratio of specific heat at constant pressure to specific heat at constant volume of gas is

denoted by Greek letter gamma (γ)

24

When a gas is heated at constant pressure it expands. The heat supplied to it is used partly

in raising its temperature and partly in doing work against the external pressure. When the gas is

heated at constant volume, no work is done and the heat supplied is used only for raising the

temperature. Hence the amount of heat required to be supplied to 1 kg of gas to raise its

temperature by 1°C at constant pressure will be more than that at constant volume. Therefore the

specific heat at constant pressure Cp is greater than the specific heat at constant volume Cv. Hence

the value of γ will be always greater than 1. For air the value on is 1.4.

Relationship between the specific heats and the characteristic gas constant.

According to Joule's law the change of internal energy is proportional to the change of

temperature.

Change in internal energy, ΔU α ΔT

ΔU = mass x a constant x ΔT

Irrespective of the type of process, or manner of heating or cooling, the constant in the

above expression is always equal to the specific heat at constant volume Cv .

ΔU = m x Cv x ΔT

Consider 'm' kg of a gas being heated at constant pressure from state 1 to 2. For stationary

closed system undergoing a process 1 - 2,

ΔU= 1Q2 − 1W2

Change in internal energy,

ΔU = m Cv (T2 –T1)

Heat supplied,

1Q2 =m Cp(T2 –T1)

Work done,

1W2 = p (V2 –V1)

= mR(T2 –T1) [since PV= RT]

Substituting these values of, Q and W in the expression, ΔU= 1Q2 − 1W2

m Cv (T2 –T1) = m Cp(T2 –T1) − mR(T2 –T1)

Cv = Cp – R

From the results Cp – Cv =R and Cp/Cv= γ the following useful results can be obtained.

𝐶𝑣 𝐶𝑝

𝑐𝑣− 1 = 𝑅

Cv (γ-1) = R

Cv = 𝑹

𝛄−𝟏 Cp = γ Cv =

𝛄𝑹

𝛄−𝟏

25

ENTHALPY

The sum of internal energy and pressure volume product (flow work) appears so

frequently in thermodynamic calculations. It is very convenient to use a single letter to denote the

sum of these two energies. Thus enthalpy is defined as the sum of internal energy and; pressure

volume product (flow work). It is denoted by H.

H = U + pV

Since U, p and V are properties of the system, H is also a property of the system. The

change of enthalpy during a process 1 - 2 is given by

dH = dU + d(pV)

H2 – H1=(U2 + p2 V2 ) − (U1 + p1 V1)

= (U2 −U1 ) + (p2 V2 −p1 V1)

= mCv(T2−T1 )+ (mRT2 − mRT1)

= mCv(T2−T1) + mR(T2−T1)

= m(Cv+R)( T2−T1)

H2 – H1= m Cp (T2−T1)

ΔH= m Cp ΔT

ENTROPY

The first law of thermodynamics is a theorem of conservation of energy. It makes no

distinction between various forms of energy and declares that all forms of energy are equivalent.

According to second law of thermodynamics all forms of energy are not equivalent For example

work can be completely converted into heat but, according to Kelvin - Planck statement, heat

cannot be completely converted into work. Work is considered as a high grade energy and heat as

a low grade energy. The complete conversion of a low grade energy into high grade energy is

impossible.

When mechanical work is converted into heat, the energy is degraded. Entropy is defined

as a measure of energy that cannot converted into useful work or the measure of degradation

which energy experiences as a result of energy conversion. It is also defined as the measure of

irreversibility associated with any process. The more the irreversibility, the more will be the

change in entropy. Entropy is a property, which cannot be measured directly, but the change of

entropy during any process can be calculated.

Eg:- Melting of ice,

26

Small change in entropy, dS is defined as the ratio of small amount of heat transfer, dQ, to

the absolute temperature, T, at which heat is transferred.

ie. 𝑑𝑠 =𝑑𝑄

𝑇

During a process, 1-2

𝑑𝑠2

1=

𝑑𝑄

𝑇

2

1

S2-S1 = 𝑑𝑄

𝑇

2

1

𝑑𝑄

𝑇 is independent of path and hence

entropy is not a path function. It is a point

function and hence entropy is a property.

Calculation of entropy change

From first law of thermodynamics, dQ = dW + dU

= pdV + mCv dT

𝑑𝑄

𝑇 =

pdV + mC v dT

𝑇

ds = 𝑝

𝑇𝑑𝑣 + 𝑚𝐶𝑣

𝑑𝑇

𝑇

= mR 𝑑𝑣

𝑣+ 𝑚𝐶𝑣

𝑑𝑇

𝑇

For unit mass, ds = R 𝑑𝑣

𝑣+ 𝐶𝑣

𝑑𝑇

𝑇

For process 1-2

𝑑𝑠2

1= (R

𝑑𝑣

𝑣+ 𝐶𝑣

𝑑𝑇

𝑇

2

1)

s2-s1 = 𝑹 𝒍𝒏 𝒗𝟐

𝒗𝟏 + 𝑪𝒗 𝐥𝐧

𝑻𝟐

𝑻𝟏 ……………….. (1)

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2

𝑠𝑖𝑛𝑐𝑒,𝑉2

𝑉1=

𝑝1𝑇2

𝑝2𝑇1

s2-s1 = 𝑅 𝑙𝑛 𝑝1𝑇2

𝑝2𝑇1 + 𝐶𝑣 ln

𝑇2

𝑇1

= 𝑅 𝑙𝑛 𝑝1

𝑝2 + 𝑅 𝑙𝑛

𝑇2

𝑇1 + 𝐶𝑣 ln

𝑇2

𝑇1

= 𝑅 𝑙𝑛 𝑝1

𝑝2 + (𝑅 + 𝐶𝑣 )𝑙𝑛

𝑇2

𝑇1

27

= 𝑅 + 𝐶𝑣 𝑙𝑛 𝑇2

𝑇1 − 𝑅 𝑙𝑛

𝑝2

𝑝1

− 𝑹 𝒍𝒏 𝒑𝟐

𝒑𝟏 …………….(2) s2-s1 = 𝑪𝒑 𝒍𝒏

𝑻𝟐

𝑻𝟏

𝑇2

𝑇1=

𝑝2𝑉2

𝑝1𝑉1

s2-s1 = 𝐶𝑝 𝑙𝑛 𝑝2𝑉2

𝑝 1𝑉1 − 𝑅 𝑙𝑛

𝑝2

𝑝1

= 𝐶𝑝 𝑙𝑛 𝑝2

𝑝1 − 𝑅 𝑙𝑛

𝑝2

𝑝1 + 𝐶𝑝 𝑙𝑛

𝑉2

𝑉1

= (𝐶𝑝 − 𝑅)𝑙𝑛 𝑝2


𝑉2

𝑉1

s2-s1 = 𝑪𝒗 𝒍𝒏 𝒑𝟐

𝒑𝟏 + 𝑪𝒑 𝒍𝒏

𝑽𝟐

𝑽𝟏 …………..(3)

Temperature - Entropy diagram

In temperature - Entropy diagram, temperature, T is plotted along the Y - axis and

entropy, S is plotted along the X - axis as shown in fig. The curve 1 -2 shows a reversion process.

It can be shown that the area under the curve 1-2 represents the heat transferred during the process

1-2.

The infinitesimal change in entropy, dS, due to reversible heat transfer dQ, at temperature,

T, is given by, 𝑑𝑠 =𝑑𝑄

𝑇 ; 𝑑𝑄 = 𝑇𝑑𝑠 area of the element of width dS and height T.

For a reversible process 1-2 𝑑𝑄2

1= 𝑇𝑑𝑠

2

1

1Q2 = 𝑇𝑑𝑠2

1 = area under curve 1-2

28

2. THERMODYNAMIC PROCESSES

When a system changes its state from one equilibrium condition to another it is said to

have undergone a process. When a gas undergoes a thermodynamic process, the various

properties of the gas such as pressure, volume, temperature, energy, entropy, etc. may change.

The thermodynamic process may be performed in different ways. Some of them are:

a. Constant volume (isochoric) process

b. Constant pressure (isobaric) process

c. Constant temperature (isothermal) process

d. Adiabatic process

e. Polytropic process

f. Free expansion

g. Throttling process

Using the laws of thermodynamics some useful relations applicable to the above said

processes can be developed.

a) Constant volume (isochoric) process

Consider 'm' kg of a gas being heated in a cylinder at constant volume from an initial

temperature T1 to final temperature T2. This process is represented on a p-V diagram, shown in

Fig. that shows the process represented on a T-S diagram. The path of the process is represented

by the vertical line 1 -2 in the p-V diagram and a curve 1-2 in the T-S diagram. Since there is no

change in volume, no external work is done by the gas. The entire heat supplied will be stored in

the form of internal energy.

(i) p - V - T relationship

For a perfect gas,

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2

Since V1=V2

Then, 𝑝1

𝑇1=

𝑝2

𝑇2

(ii) Work done

1W2 = 𝑝𝑑𝑣2

1

Since, v= constant, then dV=0

Therefore, 1W2 = 0

29

(iii) Change in internal energy

Since there is a rise in temperature from

T1 to T2,

ΔU = m Cv(T2−T1)

(iv) Heat supplied

From first law of thermodynamics,

1Q2 = ΔU + 1W2

But, 1W2 = 0

1Q2 = ΔU = m Cv(T2−T1)

(v) Change in entropy

Change in entropy during process 1 -2

s2-s1 = 𝑅 𝑙𝑛 𝑣2

𝑣1 + 𝐶𝑣 ln

𝑇2

𝑇1 = 𝐶𝑣 𝑙𝑛

𝑝2


𝑉2

𝑉1

Since V1 =V2

Then, s2-s1 = 𝐶𝑣 ln 𝑇2

𝑇1 = 𝐶𝑣 𝑙𝑛

𝑝2

𝑝1

( b) Constant pressure ( isobaric ) process

Consider ‗m‘ kg of gas being heated at constant pressure from state 1 to 2. The heating of

the gas under constant pressure causes an increase in the volume and temperature. There will be

some external work done due to the increase in the volume. This process, represented on a p-V

diagram is as shown in Fig. (a) and (b) shows the process represented on a T - S diagram. The

horizontal line 1 -2 in Fig. (a) represents the process in pV diagram. The curve 1-2 in Fig. (b)

represents the process in T-S diagram. A part of heat supplied during the process is utilised to

increase the internal energy and the remaining part is utilised to do external work.


For a perfect gas,

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2

Since p1=p2

Then, 𝑉1

𝑇1=

𝑉2

𝑇2

30

(ii) Work done

1W2 = 𝑝𝑑𝑣2

1

= p{𝑉}12

Therefore, 1W2 = 𝑝(𝑉2 − 𝑉1)


Since there is a rise in temperature from T1 to T2, ΔU = m Cv(T2−T1)

(iv) Heat supplied


1Q2 = ΔU + 1W2

But, 1W2 = 𝑝(𝑉2 − 𝑉1)

1Q2 = m Cv (T2−T1) + 𝑝 𝑉2 − 𝑉1

For constant pressure process P1=P2

1Q2 = m Cv (T2−T1) + (P2V2 – P1V1)

Since PV = mRT

= m Cv(T2−T) + mRT2 − mRT1

= m Cv(T2−T1) + 𝑚𝑅 𝑇2 − 𝑇1

= m (Cv+𝑅) (T2−T1)

1Q2 = m Cp (T2−T1) = Enthalpy (H)

Fig (b)



s2-s1 = 𝐶𝑝 𝑙𝑛 𝑇2


𝑝2

𝑝1 = 𝐶𝑣 𝑙𝑛

𝑝2


𝑉2

𝑉1

Since P1 =P2

Then, s2-s1 = 𝐶𝑝 𝑙𝑛 𝑇2

𝑇1 = 𝐶𝑝 𝑙𝑛

𝑉2

𝑉1

(c) Constant temperature (isothermal) process

A process in which a gas receives or rejects heat in such a way that its temperature

remains constant is called isothermal process. It can be represented on p-V diagram as shown in

31

Fig (a). Fig. (b) shows the process represented on T-S diagram. The line 1-2 in the figures

represent isothermal heat addition process. In this case, the entire heat supplied to the gas is used

up in doing external work.


For a perfect gas,

Since T1=T2

Then, 𝑝1𝑉1 = 𝑝2𝑉2

(ii) Work done

1W2 = 𝑝𝑑𝑣2

1

For isothermal process 𝑝1𝑉1 = 𝑝2𝑉2 = 𝑝𝑉 = C

Or 𝑝 = 𝑝1𝑉1

𝑉

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2

Then, 1W2 = 𝑝1𝑉12

𝑉𝑑𝑣

1

1W2 = 𝑝1𝑉1 𝑑𝑣

𝑉

2

1 = 𝑝1𝑉1 ln

𝑉2

𝑉1

1W2 = 𝑝1𝑉1 ln 𝑉2

𝑉1

For isothermal process 𝑝1𝑉1 = 𝑝2𝑉2

ie. 𝑉2

𝑉1=

𝑝1

𝑝2

will get, 1W2 = 𝑝1𝑉1 ln 𝑝1

𝑝2


Since T1 = T2, ΔU = 0

(iv) Heat supplied


1Q2 = ΔU + 1W2

But, 1W2 = 𝑝1𝑉1 ln 𝑉2

𝑉1

and ΔU = 0

1Q2 = 𝑝1𝑉1 ln 𝑉2

𝑉1

32




𝑣1 + 𝐶𝑣 ln

𝑇2

𝑇1 = 𝐶𝑝 𝑙𝑛

𝑇2


𝑝2

𝑝1

Since T1 =T2 Then, s2-s1 = 𝑅 𝑙𝑛 𝑣2

𝑣1 = 𝑅 𝑙𝑛

𝑝1

𝑝2

(d) Adiabatic process

In an adiabatic process, the gas neither receives nor rejects heat. In this process the heat

exchange Q =0. ie. Here there is only work transfer but no heat transfer. Work is done by the gas

at the expense of internal energy.

From the first law,

Q2= ΔU + 1W2 But for an adiabatic process,

0 = ΔU + 1W2 or 1W2 = −ΔU ……….. (1)

Change in internal energy, ΔU = m Cv(T2−T1)

Substituting in Equation 1,

𝑝𝑑𝑉 =2

1−m Cv(T2−T1)

Write in differential form, 𝑝𝑑𝑉 = −CvmdT..(2)

Considering the general equation, PV= mRT

Differentiating, pdV + Vdp = mRdT

(a)

ie. 𝑝𝑑𝑉 + 𝑉𝑑𝑝

R = mdT Sub this value in in Equ (2)

Will get, 𝑝𝑑𝑉 = − Cv 𝑝𝑑𝑉 + 𝑉𝑑𝑝

R

𝑝𝑑𝑉 = − (𝑝𝑑𝑉 + 𝑉𝑑𝑝) 𝐶𝑣

𝑅

=>𝑅 𝑝𝑑𝑉 = − (𝑝𝑑𝑉 + 𝑉𝑑𝑝)𝐶𝑣

=>𝑅 𝑝𝑑𝑉 + 𝐶𝑣𝑝𝑑𝑉 = − 𝐶𝑣𝑉𝑑𝑝

=>𝑝𝑑𝑉(𝑅 + 𝐶𝑣)= − 𝐶𝑣𝑉𝑑p

Since, Cp = R + Cv and Cp/Cv= γ

=> 𝐶𝑝 𝑝𝑑𝑉 = − 𝐶𝑣𝑉𝑑p

(b)

33

=> 𝐶𝑝

𝐶𝑣 𝑝𝑑𝑉 = − 𝑉𝑑p

=> γ𝑑𝑉

𝑉+

𝑑p

𝑝

After integration it will become, γ ln V + ln p = Constant

=> ln 𝑝𝑉𝛾 = 𝐶1 where C1 is the constant of integration

Or 𝒑𝑽𝜸 = 𝑪 Where C is another constant.

Therefore for an adiabatic process,

𝑝1𝑉1𝛾 = 𝑝2𝑉2

𝛾 = 𝐶

An adiabatic process can be represented an a p-V diagram as shown in Fig. (a) Fig. (b)

shows the process represented on a T-S diagram. The path of the process is represented by curve

1- 2 in the p-V diagram and by a vertical line 1 - 2 in the T - S diagram.


Relation between p and V for an adiabatic process is,


𝛾 = 𝐶

Since, 𝒑𝟏

𝒑𝟐=

𝑽𝟐

𝑽𝟏 𝜸

……… . (1)

Relation between V and T for an adiabatic process is,

From the general gas equation,

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2or

𝑝1

𝑝2=

𝑉2

𝑉1 𝑇1

𝑇2 ………..(2)

Sub Equ (2) in (1) will get,

𝑉2

𝑉1 𝑇1

𝑇2=

𝑉2

𝑉1 𝛾

=> 𝑇1

𝑇2=

𝑉2

𝑉1 𝛾−1

…………(3)

Relation between p and T for an adiabatic process is,

For an adiabatic process,


𝛾 = 𝐶

=> 𝑉2

𝑉1=

𝑝1

𝑝2

1

𝛾……………..(4)


𝑝1

𝑝2=

𝑉2

𝑉1 𝛾

𝑇1

𝑇2=

𝑉2

𝑉1 𝛾−1

𝑇1

𝑇2=

𝑝1

𝑝2

𝛾−1𝛾

34

𝑇1

𝑇2=

𝑝1

𝑝2

1

𝛾

𝛾−1

𝑻𝟏

𝑻𝟐=

𝒑𝟏

𝒑𝟐

𝜸−𝟏

𝜸…………(5)

(ii) Work done

1W2 = 𝑝𝑑𝑉2

1



𝛾 = 𝑝𝑉𝛾 = 𝐶

Then, p = 𝐶

𝑉𝛾

=> 1W2 = 𝐶2

𝑉𝛾𝑑𝑉

1

= 𝐶 𝑑𝑉

𝑉𝛾

2

1

After integration it becomes, 1W2 = 𝐶 𝑉2−𝛾+1

−𝑉1−𝛾+1

−𝛾+1

Substitute value of C,

1W2 = 1

−𝛾+1 𝑝2𝑉2

𝛾 𝑉2−𝛾+1

− 𝑉1𝛾 𝑉1

−𝛾+1

=> 1W2 = 𝑝2 𝑉2−𝑝1𝑉1

−𝛾+1

1W2 = 𝒑𝟏𝑽𝟏− 𝒑𝟐 𝑽𝟐

𝜸−𝟏


Since there is a rise in temperature from T1 to T2, ΔU = m Cv (T2−T1)

For adiabatic process 1Q2= 0, ie work done at the expense of internal energy.

ΔU = − 1W2

ie. => ΔU = 𝑝1𝑉1− 𝑝2 𝑉2

𝛾−1

(iv) Heat supplied

For adiabatic process 1Q2= 0


Change in entropy during a process, s2-s1 = 𝑑𝑄

𝑇

2

1

Hear dQ = 0 then, s2-s1 = 0

35

(e) Polytropic process

In this process both volume and pressure changes in a certain specified manner.

(a) (b)

The curve of expansion or compression follows the law pVn = constant, where 'n' is a

constant called polytropic index of expansion or compression. This process represented on a p-V

diagram is shown in Fig. (a) and on T - S diagram is shown in Fig. (b).

(i) p- V-T relationship

Relation between p and V for a polytropic process is,

𝑝1𝑉1𝑛 = 𝑝2𝑉2

𝑛 = 𝐶

Since, 𝒑𝟏

𝒑𝟐=

𝑽𝟐

𝑽𝟏 𝑛

……… . (1)

Relation between V and T for a polytropic process is,

From the general gas equation,

𝑝1𝑉1

𝑇1=

𝑝2𝑉2

𝑇2or

𝑝1

𝑝2=

𝑉2

𝑉1 𝑇1

𝑇2 ………..(2)


𝑉2

𝑉1 𝑇1

𝑇2=

𝑉2

𝑉1 𝑛

=> 𝑇1

𝑇2=

𝑉2

𝑉1 𝑛−1

…………(3)

Relation between p and T for a polytropic process is,



𝑛 = 𝐶

36

=> 𝑉2

𝑉1=

𝑝1

𝑝2

1

𝑛……………..(4)


𝑇1

𝑇2=

𝑝1

𝑝2

1

𝑛

𝑛−1

𝑻𝟏

𝑻𝟐=

𝒑𝟏

𝒑𝟐

𝒏−𝟏

𝒏…………(5)

(ii) Work done

1W2 = 𝑝𝑑𝑉2

1

For a polytropic process,


𝑛 = 𝑝𝑉𝑛 = 𝐶

Then, p = 𝐶

𝑉𝑛

=> 1W2 = 𝐶2

𝑉𝑛𝑑𝑉

1

= 𝐶 𝑑𝑉

𝑉𝑛

2

1

After integration it becomes, 1W2 = 𝐶 𝑉2−𝑛+1−𝑉1

−𝑛+1

−𝑛+1

Substitute value of C,

1W2 = 1

−𝑛+1 𝑝2𝑉2

𝑛 𝑉2−𝑛+1 − 𝑉1

𝑛 𝑉1−𝑛+1

=> 1W2 = 𝑝2 𝑉2−𝑝1𝑉1

−𝑛+1

1W2 = 𝒑𝟏𝑽𝟏− 𝒑𝟐 𝑽𝟐

𝒏−𝟏=

𝒎𝑹(𝑻𝟏−𝑻𝟐)

𝒏−𝟏

Since pV= mRT


Since there is a rise in temperature from T1 to T2, ΔU = m Cv (T2−T1)

(iv) Heat supplied


1Q2 = ΔU + 1W2

= m Cv (T2−T1) + 𝑚𝑅 (𝑇1−𝑇2)

𝑛−1= m Cv (T2−T1) −

𝑚𝑅 (𝑇2−𝑇1)

𝑛−1

1Q2 = m (T2−T1) Cv − 𝑅

𝑛−1 ….(6)

Since, Cp – Cv =R and Cp/Cv= γ the following useful results can be obtained.

𝑝1

𝑝2=

𝑉2

𝑉1 𝑛

𝑇1

𝑇2=

𝑉2

𝑉1 𝑛−1

𝑇1

𝑇2=

𝑝1

𝑝2

𝑛−1𝑛

37

𝐶𝑣 𝐶𝑝

𝑐𝑣− 1 = 𝑅

Cv (γ-1) = R

Cv = 𝑅

γ−1

Therefore equation (6) becomes

1Q2 = m (T2−T1) 𝑅

γ−1−

𝑅

𝑛−1

= m 𝑅 (T2−T1) 1

γ−1−

1

𝑛−1 = m 𝑅 (T2−T1)

𝑛−1 − γ−1

γ−1 (𝑛−1)

= m 𝑅 (T2−T1) 𝑛−𝛾

γ−1 (𝑛−1) = m 𝑅 (T1−T2)

𝛾−𝑛

γ−1 (𝑛−1)

1Q2 = 𝛾−𝑛

γ−1

m 𝑅 (T1−T2)

γ−1

1Q2 = 𝛾−𝑛

γ−1 𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒


Change in entropy during a process, s2-s1 = 𝑅 𝑙𝑛 𝑣2

𝑣1 + 𝐶𝑣 ln

𝑇2

𝑇1

Since, 𝑉2

𝑉1=

𝑇1

𝑇2

1

𝑛−1

Change in entropy become, s2-s1 = 𝑅 𝑙𝑛 𝑇1

𝑇2

1

𝑛−1 + 𝐶𝑣 𝑙𝑛

𝑇2

𝑇1

= 𝐶𝑣 𝑙𝑛 𝑇2


𝑇2

𝑇1

1

𝑛−1

s2-s1 = 𝐶𝑣 𝑙𝑛 𝑇2

𝑇1 −

𝑅

𝑛−1 𝑙𝑛

𝑇2

𝑇1 = 𝑙𝑛

𝑇2

𝑇1 𝐶𝑣 −

𝑅

𝑛−1

Since, 𝐶𝑣 = 𝑅

γ−1

s2-s1 = 𝑙𝑛 𝑇2

𝑇1

𝑅

𝛾−1−

𝑅

𝑛−1 = 𝑅 𝑙𝑛

𝑇2

𝑇1

1

𝛾−1−

1

𝑛−1

= 𝑅 𝑙𝑛 𝑇2

𝑇1

𝑛−1 − 𝛾−1

𝛾−1 𝑛−1 = 𝑅 𝑙𝑛

𝑇2

𝑇1

𝛾−𝑛

𝛾−1 (𝑛−1)

= 𝑅

𝛾−1

𝛾−𝑛

(𝑛−1) 𝑙𝑛

𝑇2

𝑇1 = 𝐶𝑣

𝛾−𝑛

(𝑛−1) 𝑙𝑛

𝑇2

𝑇1

s2-s1 = 𝑪𝒗 𝜸−𝒏

(𝒏−𝟏) 𝒍𝒏

𝑻𝟐

𝑻𝟏

(vi). Expression for poly tropic index ‗n‘

For polytropic process, 𝑝1𝑉1𝑛 = 𝑝2𝑉2

𝑛

38

ie. 𝑝1

𝑝2=

𝑉2

𝑉1 𝑛

ln 𝑝1

𝑝2 = 𝑛 𝑙𝑛

𝑉2

𝑉1 ; n =

𝒍𝒏 𝒑𝟏𝒑𝟐

𝒍𝒏 𝑽𝟐𝑽𝟏

POLYTROPIC PROCESSES FOR VARIOUS VALUES OF n.

(i) When n = 0 , pVn = C becomes pV° = C or

p = C, which represents a constant

pressure process.

(ii) When n = 1, pVn = C becomes pV

1 = C or

pV = constant, which represents

isothermal process.

(iii) When n=γ , pV n

= C becomes pV γ = C,

which represents an adiabatic process

(iv) pVn = C , taking n

th root of both sides p

1/n.

V = C1 (where C1 is another

constant.

When n = α

p1/n

. V = C1; becomes, p0. V = C1

ie. V = C which represents constant volume

process. These processes are shown in fig.

(e) Free expansion

When a gas expands without a restraining force being exerted by the surroundings the

process is called free expansion. Free expansion occurs when a gas is allowed to expand suddenly

into a vacuum chamber. Work does not cross (he boundary of the system and hence no external

work is done. There is no heat interaction during the free expansion process since Q and W are

zero, the change in internal energy is zero and hence temperature before and after free expansion

will be the same. Though this process involves a change in volume the work is zero. This is

because the free expansion process is irreversible, and hence the work is not equal to pdV

Consider two vessels A and B which

are connected to each other by a pipe and a

valve. Vessel A is initially filled with a fluid

at a certain pressure and B is completely

evacuated. By opening the valve, the fluid in

the vessel A will expand until it fills both

vessels. This process is known as free or

unresisted expansion.

(g) Throttling process

A throttling process is defined as a process in which there is no change in enthalpy from

state one to state two, h1 = h2; no work is done, W = 0; and the process is adiabatic, Q = 0

39

When a fluid is allowed to expand from a high pressure to a low pressure, by passing

through a narrow throat, slightly opened valve, small orifice etc, without any work interaction and

heat transfer or change in kinetic energy and potential energy, the type of process is called

throttling. Even though the velocity is high in the region of restriction, change in kinetic energy

across the restriction will be very small.

An example of a throttling process is an ideal gas flowing through a valve in mid

position.

Process P,V,T relationship 1W2 1Q2 s2-s1

Const. Volume

V=C

𝑝1

𝑇1

= 𝑝2

𝑇20 m Cv(T2−T1) 𝐶𝑣 𝑙𝑛

𝑇2

𝑇1 𝑜𝑟 𝐶𝑣 𝑙𝑛

𝑝2

𝑝1

Const. Pressure

p = C

𝑉1

𝑇1

= 𝑉2

𝑇2 𝑝(𝑉2 − 𝑉1) m Cp (T2−T1) 𝐶𝑝 𝑙𝑛 𝑇2

𝑇1 𝑜𝑟 𝐶𝑝 𝑙𝑛

𝑉2

𝑉1

Const. Temp

T = C 𝑝1𝑉1 = 𝑝2𝑉2 𝑝1𝑉1 𝑙𝑛

𝑉2

𝑉1

𝑝1𝑉1 𝑙𝑛 𝑉2

𝑉1

𝑅 𝑙𝑛 𝑣2

𝑣1 or

𝑅 𝑙𝑛 𝑝1

𝑝2

Adiabatic

pVγ = C


𝛾 1W2 = 𝑝1𝑉1− 𝑝2 𝑉2

𝛾−10 0

Polytropic

pVn = C


𝑛 𝑝1𝑉1 − 𝑝2 𝑉2

𝑛 − 11Q2 =

𝛾−𝑛

𝛾−1 1W2 𝐶𝑣

𝛾 − 𝑛

(𝑛 − 1) 𝑙𝑛

𝑇2

𝑇1

𝑝1

𝑝2=

𝑉2

𝑉1 𝛾

𝑇1

𝑇2=

𝑉2

𝑉1 𝛾−1

𝑇1

𝑇2=

𝑝1

𝑝2

𝛾−1𝛾

Cp – Cv =R Cp/Cv= γ

Cv = 𝑅

γ−1 Cp =

γ𝑅

γ−1


𝑣1 + 𝐶𝑣 ln

𝑇2

𝑇1

s2-s1 = 𝐶𝑝 𝑙𝑛 𝑇2


𝑝2

𝑝1

s2-s1 = 𝐶𝑣 𝑙𝑛 𝑝2


𝑉2

𝑉1

pV =mRT; pV=nȒT

𝑝𝑣 = 𝑅𝑇; Ȓ = 𝑅

𝑚 𝑣 =

𝑉

𝑚

Ȓ =8.314kJ/kg mole K ΔH= m Cp ΔT; ΔU= m CV ΔT

𝑑𝑠2

1

= 𝑑𝑄

𝑇

2

1

Q = U + W

1W2 = 𝑝𝑑𝑉2

1

[COP]Ref = 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑒𝑓𝑓𝑒𝑐𝑡


𝑄2

𝑊

[COP]HP = 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑒𝑓𝑓𝑒𝑐𝑡


𝑄1

𝑊

[COP]HP = [COP]Ref + 1

40

3. AIR STANDARD CYCLES

INTRODUCTION

A cycle is defined as a repeated series of operations occurring in a certain order. It maybe

repeated by repeating the processes in the same order. The cycle may be of imaginary perfect

engine or actual engine. The former is called ideal cycle and the latter actual cycle. In ideal cycle

all accidental heat losses are prevented and the working substance is assumed to behave like a

perfect working substance.

Many of the power producing device use gas as the working fluid. The working fluid in

an internal combustion engine does not operate on a cycle. For the sake of simplification, the

analysis of internal combustion engine is carried out in terms of an air standard cycle. An air

standard cycle is an idealized cycle in which air is taken as the working fluid. The actual

combustion process is replaced by a heat transfer process. The exhaust process is replaced by a

heat rejection process. All the processes are assumed to be reversible.

A part of heat transferred to the air is converted into useful work and the remainder is

rejected . Therefore the work done by the air is equal to the difference between the heat supplied

and heat rejected, if there is no mechanical loss, then,

Work done during a cycle = Heat supplied - Heat rejected.

Thermal efficiency of a cycle may be defined as the ratio of the work done to the heat

supplied during the cycle. The thermal efficiency obtained with air as the working fluid is known

as air standard efficiency.

Work done Air standard efficiency = 𝑊𝑜𝑟𝑘 𝐷𝑜𝑛𝑒

𝐻𝑒𝑎𝑡 𝑆𝑢𝑝𝑝𝑙𝑖𝑒𝑑

= Heat supplied − Heat Rejected


=1 −Heat Rejected


I. THE CARNOT CYCLE

This cycle has the highest possible efficiency and consists of four simple operations namely,

(a) Isothermal expansion

(b) Adiabatic expansion

(c)Isothermal compression

(d)Adiabatic compression.

41

Following are the four stages of the Carnot cycle. Refer figure

Stage 1. Line 1-2 in the figure represents the isothermal expansion which takes place at

temperature T1 when source of heat is applied to the end of cylinder.

Stage 2. Line 2-3 represents the application of non-conducting cover to the end of the cylinder.

This is followed by the adiabatic expansion and the temperature falls from T1 to T2

Stage 3. Line 3-4 represents the isothermal compression which takes place. Heat is rejected

during this operation.

Stage 4. Line 4-1 represents repeated application of non-conducting cover and adiabatic

compression due to which temperature increases from T2 to T1

It may be noted that ratio of expansion during isotherm 1-2 and ratio of compression during

isotherm 3-4 must be equal to get a closed cycle.

Work done Air standard efficiency =1 −𝑄2

𝑄1 = 1 −

𝑇2

𝑇1

42

From this equation, it is quite obvious that if temperature T2 decreases efficiency

increases

and it becomes 100% if T2 becomes absolute zero which, of course is impossible to attain.

Further

more it is not possible to produce an engine that should work on Carnot's cycle as it would

necessitate the piston to travel very slowly during first portion of the forward stroke (isothermal

expansion) and to travel more quickly during the remainder of the stroke (adiabatic expansion)

which however is not practicable.

II. CONSTANT VOLUME OR OTTO CYCLE

This cycle is so named as it was conceived by 'Otto'. On this cycle, petrol, gas and many

types of oil engines work. It is the standard of comparison for internal combustion engines. Fig.

21.6 shows the theoretical p-V diagram and T-s diagrams of this cycle respectively.

The point 1 represents that cylinder is full of air with volume V1, pressure p1 and absolute

temperature T1.

Line 1-2 represents the adiabatic compression of air due to which p1,V1 and T1 change to

p2,V2 and T2 respectively.

Line 2-3 shows the supply of heat to the air at constant volume so that p2 and T2 changes to

p3 and T3 (V3 being the same as V2)

Line 3-4 represents the adiabatic expansion of the air. During expansion p3, V3 and T3

change to a final value of p4, V4 or Vl and T4 respectively.

43

Line 4-1 shows the rejection of heat by air at constant volume till original state (point 1)

reaches.

𝜂𝑜𝑡𝑡𝑜 = 1 − 1

𝑟𝛾−1

Where r is the compression ratio = V1/V2 and γ= adiabatic index

This expression is known as the air standard efficiency of the Otto cycle. It is clear from

the above expression that efficiency increases with the increase in the value of r, which means

we can have maximum efficiency by increasing r to a considerable extent, but due to practical

difficulties its value is limited to about 8.

III. CONSTANT PRESSURE OR DIESEL CYCLE

This cycle was introduced by Dr. R. Diesel in 1897. It differs from Otto cycle in that heat

is supplied at constant pressure instead of at constant volume. Fig. shows the p-v and T-s

diagrams of this cycle respectively.

This cycle comprises of the following operations :

i. 1-2 Adiabatic compression.

ii. 2-3 Addition of heat at constant pressure.

iii. 3-4 Adiabatic expansion.

iv. 4-1 Rejection of heat at constant volume.

Point 1 represents that the cylinder is full of air. Let p1, Vl and T1 be the corresponding

pressure, volume and absolute temperature. The piston then compresses the air adiabatically (i.e

44

pVγ= constant) till the values become V2 and T2 respectively (at the end of the stroke; at point

2 Heat is then added from a hot body at a constant pressure. During this addition of heat volume

increases from V2 to V3 and temperature T2 to T3, corresponding to point 3. This is called the

point of cut off. The air then expands adiabatically to the conditions p4, V4 and T4 respectively

corresponding to point 4. Finally, the air rejects the heat to the cold body at constant volume till

the point 1 where it returns to its original state

𝜂𝐷𝑖𝑒𝑠𝑒𝑙 = 1 − 1

𝛾𝑟 𝛾−1 𝜌𝛾−1

𝜌−1

Where 𝜌 is the cut off ratio = V3/V2

It may be observed that from the above equation for efficiency of diesel cycle is different

from that of the Otto cycle only in bracketed factor. This factor is always greater than unity,

because ρ>1. Hence for a given compression ratio, the Otto cycle is more efficient.

IV BRAYTON CYCLE

Brayton cycle is a constant pressure cycle for a perfect gas. It is also called Joule cycle.

The heat transfers are achieved in reversible constant pressure heat exchangers. An ideal gas

turbine plant would perform the processes that make up a Brayton cycle. The cycle is shown in

the Fig. and it is represented on p-v and T-s diagrams as shown in Fig.

45

The various operations are as follows :

Operation 1-8. The air is compressed isentropically from the lower pressure pl to the upper

pressure p2, the temperature rising from T1 to T2. No heat flow occurs.

Operation 2-3. Heat flows into the system increasing the volume from V2 to V3 and tem-

perature from T2 to T3 whilst the pressure remains constant at p2.

Operation 3-4. The air is expanded isentropically from p2 to p1 the temperature falling from

T3 to T4. No heat flow occurs.

Operation 4-1. Heat is rejected from the system as the volume decreases from V4 to V1 and

the temperature from T4 to T1 whilst the pressure remains constant at p1

𝜂𝐴𝑖𝑟 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 = 1 − 𝐻𝑒𝑎𝑡 𝑅𝑒𝑗𝑒𝑐𝑡𝑒𝑑

𝐶𝑦𝑐𝑙𝑒𝐻𝑒𝑎𝑡 𝑆𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝐶𝑦𝑐𝑙𝑒

𝜂𝐵𝑟𝑎𝑦𝑡𝑜𝑛 = 1 − 𝑚𝑐𝑝 𝑇4−𝑇1

𝑚𝑐𝑝 𝑇3−𝑇2

𝜂𝐵𝑟𝑎𝑦𝑡𝑜𝑛 = 1 − 𝑇4−𝑇1

𝑇3−𝑇2

Since, 𝑇2

𝑇1=

𝑝2

𝑝1

𝛾−1

𝛾 Then, T2 = T1

𝑝2

𝑝1

𝛾−1

𝛾= T1 𝑟𝑝

𝛾−1

𝛾

Where rp = Pressure ratio that is equal to p2/p1

𝑇3

𝑇4=

𝑝3

𝑝4

𝛾−1

𝛾 Then, T3 = T4

𝑝3

𝑝4

𝛾−1

𝛾= T4

𝑝2

𝑝1

𝛾−1

𝛾= T4 𝑟𝑝

𝛾−1

𝛾

46

Then, 𝜂𝐵𝑟𝑎𝑦𝑡𝑜𝑛 = 1 − 𝑇4−𝑇1

T4 𝑟𝑝 𝛾−1𝛾 −T1 𝑟𝑝

𝛾−1𝛾

=1 − 𝑇4−𝑇1

T4−T1 𝑟𝑝 𝛾−1𝛾

= 1 − 1

𝑟𝑝 𝛾−1𝛾

The above equation shows that the efficiency of the ideal joule cycle increases with the

pressure ratio. The absolute limit of upper pressure is determined by the limiting temperature of

the material of the turbine at the point at which this temperature is reached by the compression

process alone, no further heating of the gas in the combustion chamber would be permissible and

the work of expansion would ideally just balance the work of compression so that no excess

work would be available for external use.

47

4. REFRIGERATION

INTRODUCTION

Refrigeration is the science of producing and maintaining temperatures below that of the

surrounding atmosphere. This means the removing of heat from a substance to be cooled. Heat

always passes downhill, from a warm body to a cooler one, until both bodies are at the same

temperature. Maintaining perishables at their required temperatures is done by refrigeration. Not

only perishables but to-day many human work spaces in offices and factory buildings are air-

conditioned and a refrigeration unit is the heart of the system.

Before the advent of mechanical refrigeration water was kept cool by storing it in semi-

porous jugs so that the water could seep through and evaporate. The evaporation carried away

heat and cooled the water. This system was used by the Egyptians and by Indians in the South-

west. Natural ice from lakes and rivers was often cut during winter and stored in caves, straw-

lined pits, and later in sawdust-insulated buildings to be used as required. The Romans carried

pack trains of snow from Alps to Rome for cooling the Emperor's drinks. Though these methods

of cooling all make use of natural phenomena, they were used to maintain a lower temperature in

a space or product and may properly be called refrigeration.

In simple, refrigeration means the cooling of or removal of heat from a system. The equip-

ment employed to maintain the system at a low temperature is termed as refrigerating system and

the system which-is kept at lower temperature is called refrigerated system. Refrigeration is

generally produced in one of the following three ways :

1. By melting of a solid.

2. By sublimation of a solid.

3. By evaporation of a liquid.

Most of the commercial refrigeration is produced by the evaporation of a liquid called

refrigerant. Mechanical refrigeration depends upon the evaporation of liquid refrigerant and its

circuit includes the equipments naming evaporator, compressor, condenser and expansion valve.

It is used for preservation of food, manufacture of ice, solid carbon dioxide and control of air

tempera-tars and humidity in the air-conditioning system.

Important refrigeration applications:

1.Ice making

2.Transportation of foods above and below freezing

3.Industrial air-conditioning

4.Comfort air-conditioning

5.Chemical and related industries

48

6.Medical and surgical aids

7.Processing food products and beverages

8.Oil refining and synthetic rubber manufacturing

9.Manufacturing and treatment of metals

10.Freezing food products

11.Miscellaneous applications :

a) Extremely low temperatures

b) Plumbing

c) Building construction etc.

ELEMENTS OF REFRIGERATION SYSTEMS

All refrigeration systems must include at least four basic units as given below:

i. A low temperature thermal "sink" to which heat will flow from the space to be

cooled.

ii. Means of extracting energy from the sink, raising the temperature level of this

energy, and delivering it to a heat receiver.

iii. A receiver to which heat will be transferred from the high temperature high-

pressure refrigerant.

iv. Means of reducing of pressure and temperature of the refrigerant as it returns

from the receiver to the "sink".

REFRIGERATION SYSTEMS

The various refrigeration systems may be enumerated as below:

1. Ice refrigeration

2. Air refrigeration system

3. Vapour compression refrigeration system

4. Vapour absorption refrigeration system

5. Special refrigeration systems

a. Adsorption refrigeration system

b.Cascade refrigeration system

c. Mixed refrigeration system

d.Vortex tube refrigeration system

e. Thermoelectric refrigeration

f. Steam jet refrigeration system

49

CO-EFFICIENT OF PERFORMANCE (C.O.P.)

The performance of a refrigeration system is expressed by a term known as the "co-

efficient of performance", which is defined as the ratio of heat absorbed by the refrigerant

while passing through the evaporator to the work input required to compress the refrigerant in

the compressor; in short it is the ratio between heat extracted and work done (in heat units).

If, Rn - Net refrigerating effect, W = Work expanded in by the machine during the same

interval of time,

Then, C.O.P= 𝑹𝒏

𝑾

STANDARD RATING OF A REFRIGERATION MACHINE

The rating of a refrigeration machine is obtained by refrigerating effect or amount of heat

tracted in a given time from a body. The rating of the refrigeration machine is given by a unit of

refrigeration known as ―standard commercial tonne of refrigeration‖ which is defined as the

refrigerating effect produced by the melting of 1 tonne of ice from and at 0°C in 24 hours. Since

e latent heat of fusion of ice is 336 J/kg, the refrigerating effect of 336 x 1000 kJ in 24 hours is

rated as one tonne, i.e.

1 tonne of refrigeration (TR) =𝟑𝟑𝟔 ×𝟏𝟎𝟎𝟎

𝟐𝟒= 14000 kJ/h.

Note: Ton of refrigeration (TR). A ton of refrigeration is basically an American unit of

refrigerating effect (R.E.). It originated from the rate at which heat is required to be removed to

freeze one ton of water from and at 0oC. Using American units this is equal to removal of200

BTU of heat per minute, and MKS unit it is adopted as kcal/min or 3000 kcal/hour. In S.I. units

its conversion is rounded of to 3.6 kJ/s (kW) or 210 kJ/min.

AIR REFRIGERATION SYSTEM / REVERSED BRAYTON CYCLE

Air cycle refrigeration is one of the earliest methods of cooling developed. It became

obsolete several years because of its low co-efficient of performance (C.O.P.) and high operating

costs. It , however, been applied to aircraft refrigeration systems, where with low equipment

weight, it utilise a portion of the cabin air according to the supercharger capacity. The main

character-: feature of air refrigeration system, is that throughout the cycle the refrigerant remains

in gaseous state.

The air refrigeration system can be divided in two systems:

1. Closed system 2. Open system.

In closed (or dense air) system the air refrigerant is contained within the piping or

components parts of the system at all times and refrigerator with usually pressures above

atmospheric pressure.

50

In the open system the refrigerator is replaced by the actual space to be cooled with the

air expanded to atmospheric pressure, circulated through the cold room and then compressed to

the cooler pressure. The pressure of operation in this system is inherently limited to operation at

atmospheric pressure in the refrigerator.

A closed system claims the following advantages over open system :

(i) In a closed system suction to compressor may be at high pressure. The sizes of

expander and compressor can kept within reasonable limits by using dense air

(ii) In open air system, the sir picks up moisture n the products kept in the refrigerated

chamber ; the moisture may freeze during expansion and is likely to choke the

valves whereas it does not happen in closed system

(iii)In open tern, the expansion of the refrigerant can be carried only upto atmospheric

pressure prevailing the cold chamber but for a closed system there is no such

restriction.

practice it may or may not be done eg:- in some aircraft refrigeration systems which employ air

refrigeration cycle the expansion work may be used for driving other devices.

This system uses reversed Brayton cycle which is described below:

Fig shows a schematic diagram of an air refrigeration

system working on reversed brayton cycle. Elements of this

systems are :

1. Compressor

2. Cooler (Heat exchanger)

3. Expander

4. Refrigerator.

In this system, work gained from expander is

employed for compression of air, consequently less

external work is needed for operation of the system. In

pr

51

Fig. a and b shows p-V and T-s diagrams for a reversed Brayton cycle. Here it is assumed

that

(i) Absorption and rejection of heat are constant pressure processes.

(ii) Compression and expansion are isentropic processes.

C.O.P= 𝑇3−𝑇2

𝑛

𝑛−1

𝛾−1

𝛾 𝑇4−𝑇3+𝑇2−𝑇1

Merits and Demerits of Air refrigeration System.

Merits

1. Since air is non-flammable, therefore there is no risk of fire as in the machine using

NH3 as the refrigerant.

2. It is cheaper as air is easily available as compared to the other refrigerants.

3. As compared to the other refrigeration systems the weight of air refrigeration system

per tonne of refrigeration is quite low, because of this reason this system is employed

in aircrafts.

Demerits

1. The C.O.P. of this system is very low in comparison to other systems.

2. The weight of air required to be circulated is more compared with refrigerants used in

other systems. This is due to the fact that heat is carried by air in the form of sensible

heat.

SIMPLE VAPOUR COMPRESSION SYSTEM

Out of all refrigeration systems, the vapour compression system is the most important

system from the view point of commercial and domestic utility. It is the most practical form of

refrigeration. In this system the working fluid is a vapour. It readily evaporates and condenses or

changes alternately between the vapour and liquid phases without leaving the refrigerating plant.

During evaporation, it absorbs heat from the cold body. This heat is used as its latent heat to

converting it from the liquid to vapour. In condensing or cooling or liquefying, it rejects heat to

external body, thus creating a cooling effect in the working fluid. This refrigeration system thus

acts as a latent heat pump since it pumps its latent heat from the cold body or brine and rejects it

or delivers it to the external hot body or cooling medium.

In a simple vapour compression system fundamental processes are completed in one

cycle

These are:

1. Compression 2. Condensation 3. Expansion 4. Vapourisation.

52

The vapour at low temperature and pressure (state '2') enters the "compressor" where it is

compressed isentropically and subsequently its temperature and pressure increase considerably

(state '3'). This vapour after leaving the compressor enters the "condenser" where it is condensed

into high pressure liquid (state '4') and is collected in a "receiver tank'. From receiver tank it

passes through the "expansion valve", here it is throttled down to a lower pressure and has a low

temperature (state '1')- After finding its way through expansion "valve" it finally passes on to

"evaporator" where it extracts heat from the surroundings or circulating fluid being refrigerated

and vapourises to low pressure vapour (state ‗2‘)

53

Merits and demerits of vapour compression system over Air refrigeration system

Merits :

1. C.O.P. is quite high as the working of the cycle is very near to that of reversed

Carnot cycle.

2. When used on ground level the running cost of vapour-compression refrigeration

system is only 1/5th of air refrigeration system.

3. For the same refrigerating effect the size of the evaporator is smaller.

4. The required temperature of the evaporator can be achieved simply by adjusting the

throttle valve of the same unit.

Demerits :

1. Initial cost is high.

2. The major disadvantages are inflammability, leakage of vapours and toxity. These

have been overcome to a great extent by improvement in design.

Functions of Parts of a Simple Vapour Compression System

Here follows the brief description of various parts of a simple vapour compression

system shown in Fig

1. Compressor. The function of a compressor is to remove the vapour from the evaporator,

and to raise its temperature and pressure to a point suck that it (vapour) can be

condensed with available condensing media.

2. Discharge line (or hot gas line). A hot gas or discharge line delivers the high-pressure,

high-temperature vapour from the discharge of the compressor to the condenser.

3. Condenser. The function of a condenser is to provide a heat transfer surface through

which heat passes from the hot refrigerant vapour to the condensing medium.

4. Receiver tank. A receiver tank is used to provide storage for a condensed liquid so that a

constant supply of liquid is available to the evaporator as required.

5. Liquid line. A liquid line carries the liquid refrigerant from the receiver tank to the

refrigerant flow control.

6. Expansion valve (refrigerant flow control). Its function is to meter the proper amount of

refrigerant to the evaporator and to reduce the pressure of liquid entering the evaporator

so that liquid will vapourize in the evaporator at the desired low temperature and take

out sufficient amount of heat.

7. Evaporator. An evaporator provides a heat transfer surface through which heat can pass

from the refrigerated space into the vapourizing refrigerant.

8. Suction line. The suction line conveys the low pressure vapour from the evaporator to

the suction inlet of the compressor.

54

5. HEAT ENGINES

A heat engine is a device which transforms the chemical energy of a fuel into thermal

energy and uses this energy to produce mechanical work. Heat engines are classified into two

broad types:

(a) External combustion engines, and

(b) Internal combustion engines.

External Combustion Engines Internal Combustion(IC) Engines

1 In this products of combustion of air and

fuel transfer heat to a second fluid which

is the working fluid of the cycle

In this products of combustion are directly the

motive fluid.

2 Advantages: The use of cheaper fuels

including solid fuels, and high starting

torque (internal combustion engines are

not self-starting).

Advantages: It has greater mechanical

simplicity, lower ratio of weight and bulk to

output due to absence of auxiliary apparatus like

boiler and condenser and, hence lower first cost

(except in the case of very large units), higher

overall efficiency, and lesser requirement of

water for dissipation of energy through cooling

system.

3 It is mainly used for large electric power

generation

These are mainly used for transport vehicles —

automobiles, locomotives, aircrafts, etc. External

combustion engines are less suitable for

transport vehicles because of bulk and weight,

and difficulty of transporting the working fluid

55

Classification of Heat Engine

Classific

ation

Name of Engines Reciprocating

or Rotary

Maximum

size inkW

Principal Use

(a)Internal

combustion

engines

1. Gasoline or petrol

engine (SI)

Reciprocating 4000 Road vehicles, small,

industrial, small marine,

(propulsion of ships), small

aircrafts

2. Gas engine (SI) Reciprocating 4000 Industrial, electric power

3. Diesel engine (CI) Reciprocating 40,000 Road vehicles, industrial,

locomotives

electric power, marine

4.Wankel engine (SI,

CI)

Rotary 400 Road vehicles, small aircrafts

5.Open cycle gas

turbine

Rotary 15,000 Electric power, aircraft

6. Jet engine Rotary 8000 Aircraft

7. Rocket No mechanism Very big Missiles, space travel

(b)External

combustion

engines

1. Steam engine Reciprocating 4000 Locomotives, ships

2. Steam turbine Rotary 5,00,000 Electric power, large marine

3. Stirling or hot air

engine

Reciprocating 800 Experimental, power in space,

vehicles

4. Closed cycle gas

turbine

Rotary 80,000 Electric power, marine

SI = Spark Ignition CI = compression Ignition

56

IC ENGINE COMPONENTS

Figure shows the cross-section of a single cylinder spark-ignition internal combustion

engine. The cylinder is supported in position by the cylinder block at the top end is covered by

cylinder head. In the cylinder a piston travels in reciprocating motion. The space enclosed

between the upper part of the cylinder and the top of the piston during the combustion process is

called the combustion chamber.

A mixture of air and fuel enters the cylinder through the carburattor in spark ignition

engine via the inlet manifold i.e the pipe which connects the inlet port of the engine to the air

intake. In carburettor a throttle is provided to control the mass of mixture entering the

combustion chamber. In the cylinder head are inlet valves for taking the charge in the cylinder

and exhaust valves for discharging the products of combustion.

A spark plug near the top of the cylinder initiates the combustion. The energy of the

expanding gas is transmitted by the piston (having piston rings to prevent leakage) through the

gudgeon pin to the connecting rod. The connecting rod and the crank arm of the crankshaft

translate the reciprocating motion of piston into rotational motion of the crankshaft.

57

The crankshaft is supported in

bearings attached to the crankcase. The

crankcase is the main body of the engine to

which the cylinder is attached. The products of

combustion leave through exhaust port and

exhaust manifold. Both the intake and exhaust

valves are operated by the valve mechanism. A

camshaft is driven by the crankshaft through

timing gears. Lobed cams on the camshaft

actuate the push rods and rocker arms for

opening the valves against the force of valve.

Since the power stroke exists for only a part of

the total time, a flywheel is used to smooth out

the power pulses and thus obtain a uniform

rotation of the crankshaft. For control of speed

under varying load conditions a governor is

provided.

Fig. Cross-section of spark-ignition engine

NOMENCLATURE

Fig. Four stroke SI Engine

58

1. Cylinder bore (D). The nominal inner diameter of the working cylinder.

2. Piston area (A). The area of a circle of diameter equal to the cylinder bore.

3. Stroke (L). The nominal distance through which a working pistons between two

successive reversals of its direction of motion.

4. Dead centre. The position of the working piston and the moving

parts which are mechanically connected to it at the moment when the

action of the piston motion is reversed (at either end point of the stroke).

a. Bottom dead centre (BDC) .Dead centre when the piston is nearest to the

crankshaft. In horizontal engines it is also called outer dead centre (ODC).

b. Top dead centre (TDC). Dead centre when the position is farthest from the

crankshaft. In horizontal engines it is also called inner dead centre (IDC).

5. Displacement volume or piston swept volume (Vs). The nominal

volume generated by the working piston when travelling from one dead

centre to next one, calculated as the product of piston area and stroke.

Vs = A X L

6. Clearance volume (Vc). The nominal volume of the space on the combustion side of the

piston at top dead centre.

7. Cylinder volume (V). The sum of piston swept volume and clearance volume

V = Vs + Vc (1.2)

8. Compression ratio (CR or r). The numerical value of the cylinder volume divided by the

numerical value of the combustion space volume or clearance volume

Compression ratio r = 𝑉

𝑉𝑐

FOUR STROKE SPARK IGNITION (SI) ENGINES

Working Strokes

59

1. Suction stroke. Suction stroke starts when

the piston is at top dead centre and about to

move downwards. The inlet valve is open at

this time and the exhaust valve is closed. Due

to the suction created by the motion of the

piston towards bottom dead centre, the charge

consisting of fresh air mixed with the fuel is

drawn into the cylinder. At the end of the

Suction stroke the inlet valve closes.

Stroke Valve position

Suction stroke. Suction valve open.

Exhaust valve closed.

Comp. stroke. Both valves closed.

Expansion stroke. Both valves closed.

Exhaust stroke. Exhaust valve open.

Suction valve closed.

2. Compression stroke. The fresh charge taken into the cylinder during suction stroke is

compressed by the return stroke of the piston. During this stroke both inlet and exhaust valves

remain closed. The air which occupied the whole cylinder volume is now compressed into

clearance volume. Just before the end of the compression stroke the mixture is ignited with the

help of an electric spark between the electrodes of the spark plug located in combustion chamber

wall. Burning takes place when the piston is almost at top dead centre. During the burning

process the chemical energy of the fuel is converted into sensible energy, producing a

temperature rise of about 2000°C, and the pressure is also considerably increased.

3. Expansion or power stroke. Due to high pressure the burnt gases force the piston towards

bottom dead centre, both the inlet and exhaust valves remaining closed. Thus power is obtained

during this stroke. Both pressure and temperature decrease during expansion.

4. Exhaust stroke. At the end of the expansion stroke the exhaust valve opens, the inlet valve

remaining closed, and the piston is moving from bottom dead centre to top dead centre sweeps

out the burnt gases from the cylinder.

FOUR-STROKE COMPRESSION IGNITION (CI) ENGINES

The four-stroke CI engine is similar to four-stroke SI engine except that high

compression ratio is used in the former, and during the suction stroke air alone, instead of a fuel-

air mixture, is inducted. Due to high compression ratio, the temperature at the end of

compression stroke is sufficient to ignite the fuel which is injected into the combustion chamber.

In the CI engine a high pressure fuel pump and an injector is provided to inject fuel into

combustion chamber. The carburettor and ignition system, necessary in the SI engine, are not

required in the CI engine.

60

The ideal sequence of operation for the four-stroke CI engine is as follows:

1. Suction stroke. Only air is inducted during the suction stroke. During this stroke intake

valve is open and exhaust valve is closed.

2. Compression stroke. Both valves remain closed during compression stroke.

3. Expansion or power stroke. Fuel is injected in the beginning of the expansion stroke.

The rate of injection is such that the combustion maintains the pressure constant. After

the injection of fuel is over (i.e. after fuel cut off) the products of combustion expand.

Both valves remain closed during expansion stroke.

4. Exhaust stroke. The exhaust valve is open and the intake valve remains closed in the

exhaust stroke.

TWO-STROKE ENGINE

In two-stroke engines the cycle is

completed in two stroke, i.e., one resolution

of the crankshaft as against two revolutions

of four-stroke cycle. The difference between

two-stroke and four-stroke engines is in the

method of filling the cylinder with the fresh

charge and removing the burned gases from

the cylinder. In a four-stroke engine the

operations are performed by the engine

piston during the suction and exhaust

strokes, respectively. In a two stroke engine

suction is accomplished by air compressed

in crankcase or by a blower. The induction

of compressed air removes the products of

combustion through exhaust ports.

Therefore no piston strokes are required for suction and exhaust operations. Only two

piston strokes are required to complete the cycle, one for compressing the fresh charge and the

other for expansion or power stroke. Figure shows the simplest type of two-stroke engine. The

air or charge is sucked through spring-loaded inlet valve when the pressure in the crankcase

reduces due to upward motion of the piston during compression stroke. After the compression,

ignition and expansion takes place in the usual way. During the expansion stroke the air in the

crankcase is compressed. Near the end of expansion stroke piston uncovers the exhaust ports,

and the cylinder pressure drops to atmospheric as the combustion products leave the cylinder.

Further motion of the piston uncovers transfer ports, permitting the slightly compressed air or

mixture in the crankcase to enter the engine cylinder. The top of the piston sometimes has a

61

projection to deflect the fresh air to sweep up to the top of the cylinder before flowing to the

exhaust ports. This serves the double purpose of scavenging the upper part of the cylinder of

combustion products and preventing the fresh charge from flowing directly to the exhaust ports.

The same objective can be achieved without piston deflector by proper shaping of the transfer

port. During the Upward motion of the piston from bottom dead centre, the transfer ports and

then the exhaust port close and compression of the charge begins and the cycle is repeated.

COMPARISON OF SI AND CI ENGINES

Description SI Engine CI Engine

1. Basic cycle Based on Otto cycle. Based on Diesel cycle.

2. Fuel Petrol (Gasoline). High self

ignition temperature

desirable.

Diesel oil. Low self-ignition temperature

desirable.

3. Introduction of fuel Fuel and air introduced as a

gaseous mixture in the suc-

tion stroke. Carburetor

necessary to provide the

mixture Throttle controls the

quantity of mixture

introduced.

Fuel is injected directly into combustion

chamber at high pressure at the end of

compression stroke. Carburetor is eliminated but

a high pressure fuel pump and injector

necessary. Quantity of fuel regulated in pump.

4. Ignition Requires an ignition system

with spark plug in the com-

bustion chamber.

Self ignition due to high temperature, caused by

high compression of air, when fuel is injected.

Ignition system and spark plug is eliminated.

5. Compression ratio

range

6 to 10.5. Upper limit of C.R.

fixed by antiknock quality of

fuel.

14 to 22. Upper limit of C.R. is limited by the

rapidly increasing weight of the engine structure

as the compression ratio is further increased.

6. Speed Higher maximum revolution

per minute due to lighter

weight.

Maximum r.p.m. lower.

7 Efficiency Maximum efficiency lower

due to low compression ratio.

Higher maximum efficiency due to higher

compression ratio.

8. Weight Lighter. Heavier due to higher pressures.

62

COMPARISON OF FOUR-STROKE AND TWO-STROKE CYCLE ENGINES

Four-stroke cycle Two-stroke cycle

1 The cycle is completed in four strokes of

the piston or in two revolutions of the

crankshaft. Thus one power stroke is

obtained in every two revolutions of the

crankshaft

The cycle is completed in two-strokes of the

piston or in one revolution of the crankshaft.

Thus one power stroke is obtained in each

revolution of the crankshaft.

2 Because of the above, turning movement

is not so uniform and hence heavier

flywheel is needed

More uniform turning movement and hence

lighter flywheel is needed.

3 Again, because of one power stroke for

two revolutions, power produced for

same size of engine is small, or for the

same power the engine is heavy and

bulky.

Because of one power stroke for one revolution,

power produced for same size of engine is more

(theoretically twice, actually about 1.3 times),

or for the same power the engine is light and

compact.

4 Because of one power stroke in two

revolutions lesser cooling and

lubrication requirements. Lesser rate of

wear and tear

Because of one power stroke in one revolution

greater cooling and lubrication requirement.

Greater rate of wear and tear.

5 The four-stroke engine contains valves

and valve mechanism.

Two-stroke engines have no valves but only

ports (some two-stroke engines are fitted with

conventional exhaust valve or reed valve).

6 Because of the heavy weight and

complication of valve mechanism,

higher in initial cost.

Because of light weight and simplicity due to

the absence of valve mechanism, cheaper in

initial cost.

7 Volumetric efficiency more due to

greater time of induction.

Volumetric efficiency less due to lesser time for

induction.

8 Thermal efficiency higher, part load

efficiency better than two-stroke cycle

engine.

Thermal efficiency lower, part load efficiency

lesser than four-stroke cycle engine.

In two-stroke petrol engines some fuel is

exhausted during scavenging.

63

9 Used where efficiency is important, in

cars, buses, trucks, tractors, industrial

engines, aeroplanes, power generation,

etc

Used were (a) low cost, and (b) compactness

and light weight important. Two-stroke (air-

cooled) petrol engines used in very small sizes

only: lawn mowers, scooters, motor cycles,

mopeds etc. (Lubricating oil mixed with petrol).

Two-stroke diesel engines used in very large

sizes, more than 60 cm bore, for ship

propulsion because of low weight and

compactness.

MULTI POINT FUEL INJECTION (MPFI) ENGINES

M.P.F.I. means Multi Point Fuel Injection system. In this system each cylinder has

number of injectors to supply/spray fuel in the cylinders as compared to one injector located

centrally to supply/spray fuel in case of single point injection system. This is used in petrol

powered vehicle as well as in diesel powered vehicles.

Advantage of M. P. F. I.

More uniform A/F mixture will be supplied to each cylinder, hence the difference in

power developed in each cylinder is minimum. Vibration from the engine equipped with

this system is less, due to this the life of engine components is improved.

No need to crank the engine twice or thrice in case of cold starting as happens in the

carburetor system.

Immediate response, in case of sudden acceleration / deceleration.

64

Since the engine is controlled by ECM (Engine Control Module), more accurate amount

of A/F mixture will be supplied and as a result complete combustion will take place. This

leads to effective utilization of fuel supplied and hence low emission level.

The mileage of the vehicle will be improved.

ECM ( Engine Control Module) and its function.

The function of ECM is to receive signal from various sensors, manipulate the signals

and send control signals to the actuators.

Sensors; Sensing different parameters (Temperature, Pressure, Engine Speed etc.) of the

engine and send signal to ECM.

Actuators; Receives control signal from ECM and does function accordingly (ISCA,

PCSV, Injectors, Power Transistor etc.)

Case I: If ECM fails to send control signal to all actuators then the engine won't get started.

Case II: If ECM fails to service from all sensors then also the engine won't get started

COMMON RAIL DIRECT INJECTION (CRDI) ENGINES

CRDI has also provided a tremendous

boost in diesel-engine performance. The

improvement is mainly due to the common-rail

design, which has tubes that connect all the

injectors. These injectors are based on the

direct-injection concept, as was the case in the

past. But the common-rail design was quite a

step forward.

Fuel in the common tube or ―rail‖ is

under a set amount of pressure which causes

the fuel to be ―atomized‖ or broken down to its

smallest particles. This allows the fuel to

combine with the air much more efficiently.

With proper direct injection, fuel use is highly

efficient, with much less waste fuel escaping

the system unused.

The newest electronic technology has

also allowed CRDI engines to better control the

amount of fuel used, the pressure within the

system and the timing of both the injection of

fuel and the electronic charge applied to make

the fuel burn.

65

Injectors in the common rail direct injection engine have controls on the injector heads

that allow slight variances in the amount of fuel put into the cylinders

Disadvantages

Like all good things in life, these engines also come at a price; they are at least 25% more

costly than the conventional engines. They also require a higher degree of maintenance and

spares are not cheap also.

Advantages

CRDI method greatly reduces engine and vehicle vibration, allows the engine and vehicle to

run more quietly and reduces the cost of operation significantly.

CRDI engine fitted cars offer 25% more power than the normal direct injection engine with a

superior pickup and torque offering sometimes up to 70% more power than the conventional

diesel engines.

They are smooth less noisy and immensely fuel efficient giving around 24 kilometers to a

liter of Diesel. The fact that Diesel is cheaper than petrol in India further attributes greatness

to the engine.

In a CRDI engine, a tube or a common rail connects all the injectors and contains fuel at a

constant high pressure. This high pressure in the common rail ensures that when injected, the

fuel breaks up into small particles and mixes evenly with the air, thereby leaving little un-

burnt fuel thus reducing pollution.

CARBURETOR

Carburetor is a device that blends air and fuel for petrol engine. The carburetor works

on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its

dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid

fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the

engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn

into the airstream.

Under all engine operating conditions, the carburetor must:

Measure the airflow of the engine

Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range

Mix the two finely and evenly

This job would be simple if air and gasoline (petrol) were ideal fluids; in practice, however,

their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a great deal

of complexity to compensate for exceptionally high or low engine speeds. A carburetor must

provide the proper fuel/air mixture across a wide range of ambient temperatures, atmospheric

pressures, engine speeds and loads, and centrifugal forces:

http://en.wikipedia.org/wiki/Earth%27s_atmosphere

http://en.wikipedia.org/wiki/Fuel

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

http://en.wikipedia.org/wiki/Static_pressure

http://en.wikipedia.org/wiki/Dynamic_pressure

http://en.wikipedia.org/wiki/Throttle

http://en.wikipedia.org/wiki/Gasoline

http://en.wikipedia.org/wiki/Centrifugal_force

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In addition, modern carburetors are required to do:

Hot start

Idling or slow-running

Acceleration

High speed / high power at full throttle

Cruising at part throttle (light load)

To function correctly under all these conditions, most carburetors contain a complex set

of mechanisms to support several different operating modes, called circuits.

Working

A carburetor basically consists of an open pipe through which the air passes into the inlet

manifold of the engine. The pipe is in the form of a venturi: it narrows in section and then widens

again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a

butterfly valve called the throttle valve a rotating disc that can be turned end-on to the airflow, so

as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the

flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity

of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The

throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely

http://en.wikipedia.org/wiki/Inlet_manifold



http://en.wikipedia.org/wiki/Butterfly_valve

http://en.wikipedia.org/wiki/Bowden_cable

67

by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or

equipment.Fuel is introduced into the air stream through small holes at the narrowest part of the

venturi and at other places where pressure will be lowered when not running on full throttle. Fuel

flow is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.

http://en.wikipedia.org/wiki/Hillman_Imp

http://en.wikipedia.org/wiki/Automobile_pedal

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6.PROPERTIES OF FUELS

INTRODUCTION TO FUELS

The various types of fuels like liquid, solid and gaseous fuels are available for firing in

boilers, furnaces and other combustion equipments. The selection of right type of fuel depends

on various factors such as availability, storage, handling, pollution and landed cost of fuel.

The knowledge of the fuel properties helps in selecting the right fuel for the right purpose

and efficient use of the fuel. The following characteristics, determined by laboratory tests, are

generally used for assessing the nature and quality of fuels.

PROPERTIES OF LIQUID FUELS

Liquid fuels like furnace oil and LSHS are predominantly used in industrial application. The

various properties of liquid fuels are given below.

1. Density

This is defined as the ratio of the mass of the fuel to the volume of the fuel at a reference

temperature of 15°C. Density is measured by an instrument called hydrometer. The knowledge

of density is useful for quantity calculations and assessing ignition quality. The unit of density is

kg/m3.

2. Specific gravity

This is defined as the ratio of the weight of a given volume of oil to the weight of the

same volume of water at a given temperature. The density of fuel, relative to water, is called

specific gravity. The specific gravity of water is defined as 1. Since specific gravity is a ratio, it

has no units. The measurement of specific gravity is generally made by a hydrometer.

3. Viscosity

The viscosity of a fluid is a measure of its internal resistance to flow. Viscosity depends

on temperature and decreases as the temperature increases. Any numerical value for viscosity has

no meaning unless the temperature is also specified. The measurement of viscosity is made with

an instrument called Viscometer. Viscosity is the most important characteristic in the storage and

use of fuel oil. It influences the degree of pre-heat required for handling, storage and satisfactory

atomization. If the oil is too viscous, it may become difficult to pump, hard to light the burner,

and tough to operate. Poor atomization may result in the formation of carbon deposits on the

burner tips or on the walls. Therefore pre-heating is necessary for proper atomization.

4. Flash Point

The flash point of a fuel is the lowest temperature at which the fuel can be heated so that

the vapour gives off flashes momentarily when an open flame is passed over it. Flash point for

furnace oil is 66oC.

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5. Pour Point

The pour point of a fuel is the lowest temperature at which it will pour or flow when

cooled under prescribed conditions. It is a very rough indication of the lowest temperature at

which fuel oil is readily pumpable

6. Specific Heat

Specific heat is the amount of kcals needed to raise the temperature of 1 kg of oil by 1oC.

The unit of specific heat is kcal/kgoC. It varies from 0.22 to 0.28 depending on the oil specific

gravity. The specific heat determines how much steam or electrical energy it takes to heat oil to a

desired temperature. Light oils have a low specific heat, whereas heavier oils have a higher

specific heat.

7. Calorific Value

The calorific value is the measurement of heat or energy produced, and is measured

either as gross calorific value or net calorific value. The difference being the latent heat of

condensation of the water vapour produced during the combustion process. Gross calorific value

(GCV) assumes all vapour produced during the combustion process is fully condensed. Net

calorific value (NCV) assumes the water leaves with the combustion products without fully

being condensed. Fuels should be compared based on the net calorific value. The calorific value

of coal varies considerably depending on the ash, moisture content and the type of coal while

calorific value of fuel oils are much more consistent.

8. Sulphur

The amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to

a lesser extent on the refining process. The normal sulfur content for the residual fuel oil (furnace

oil) is in the order of 2-4 %. The main disadvantage of sulphur is the risk of corrosion by

sulphuric acid formed during and after combustion, and condensing in cool parts of the chimney

or stack, air pre heater and economiser.

9. Ash Content

The ash value is related to the inorganic material in the fuel oil. The ash levels of

distillate fuels are negligible. Residual fuels have more of the ash-forming constituents. These

salts may be compounds of sodium, vanadium, calcium, magnesium, silicon, iron, aluminum,

nickel, etc. Typically, the ash value is in the range 0.03-0.07 %. Excessive ash in liquid fuels can

cause fouling deposits in the combustion equipment. Ash has erosive effect on the burner tips,

causes damage to the refractories at high temperatures and gives rise to high temperature

corrosion and fouling of equipments.

10. Carbon Residue

Carbon residue indicates the tendency of oil to deposit a carbonaceous solid residue on a

hot surface, such as a burner or injection nozzle, when its vaporisable constituents evaporate.

Residual oil contains carbon residue ranging from 1 percent or more.

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11. Water Content

Water content of furnace oil when supplied is normally very low as the product at

refinery site is handled hot and maximum limit of 1% is specified in the standard. Water may be

present in free or emulsified form and can cause damage to the inside furnace surfaces during

combustion especially if it contains dissolved salts. It can also cause spluttering of the flame at

the burner tip, possibly extinguishing the flame and reducing the flame temperature or

lengthening the flame.

PROPERTIES OF COAL

CLASSIFICATION

Coal is classified into three major types namely anthracite, bituminous, and lignite.

However there is no clear demarcation between them and coal is also further classified as semi-

anthracite, semi-bituminous, and sub-bituminous. Anthracite is the oldest coal from geological

perspective. It is a hard coal composed mainly of carbon with little volatile content and

practically no moisture. Lignite is the youngest coal from geological perspective. It is a soft coal

composed mainly of volatile matter and moisture content with low fixed carbon. Fixed carbon

refers to carbon in its free state, not combined with other elements. Volatile matter refers to those

combustible constituents of coal that vaporize when coal is heated.

The common coals used in Indian industry are bituminous and sub-bituminous coal.

Normally D,E and F coal grades are available to Indian Industry.

PHYSICAL PROPERTIES

a) Fixed Carbon

Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off. It consists

mostly of carbon but also contains some hydrogen, oxygen, sulphur and nitrogen not driven off

with the gases. Fixed carbon gives a rough estimate of heating value of coal

b) Volatile Matter:

Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and

incombustible gases like carbon dioxide and nitrogen found in coal. Thus the volatile matter is an

index of the gaseous fuels present. Typical range of volatile matter is 20 to 35%.

Volatile Matter

• Proportionately increases flame length, and helps in easier ignition of coal.

• Sets minimum limit on the furnace height and volume.

• Influences secondary air requirement and distribution aspects.

• Influences secondary oil support

c) Ash Content:

Ash is an impurity that will not burn. Typical range is 5 to 40%

71

Ash

• Reduces handling and burning capacity.

• Increases handling costs.

• Affects combustion efficiency and boiler efficiency

• Causes clinkering and slagging.

d) Moisture Content:

Moisture in coal must be transported, handled and stored. Since it replaces combustible matter, it

decreases the heat content per kg of coal. Typical range is 0.5 to 10%

Moisture

• Increases heat loss, due to evaporation and superheating of vapour

• Helps, to a limit, in binding fines.

• Aids radiation heat transfer.

e) Sulphur Content:

Typical range is 0.5 to 0.8% normally.

Sulphur

• Affects clinkering and slagging tendencies

• Corrodes chimney and other equipment such as air heaters and economisers

• Limits exit flue gas temperature.

Preparation of Coal

Preparation of coal prior to feeding into the boiler is an important step for achieving good

combustion. Large and irregular lumps of coal may cause the following problems:

1. Poor combustion conditions and inadequate furnace temperature.

2. Higher excess air resulting in higher stack loss.

3. Increase of unburnts in the ash.

4. Low thermal efficiency.

(a) Sizing of Coal

Proper coal sizing is one of the key measures to ensure efficient combustion. Proper coal sizing,

with specific relevance to the type of firing system, helps towards even burning, reduced ash

losses and better combustion efficiency.

Coal is reduced in size by crushing and pulverizing. Pre-crushed coal can be economical for

smaller units, especially those which are stoker fired. In a coal handling system, crushing is

limited to a top size of 6 or 4mm. The devices most commonly used for crushing are the rotary

breaker, the roll crusher and the hammer mill.

72

It is necessary to screen the coal before crushing, so that only oversized coal is fed to the

crusher. This helps to reduce power consumption in the crusher. Recommended practices in coal

crushing are:

1. Incorporation of a screen to separate fines and small particles to avoid extra fine

generation in crushing.

2. Incorporation of a magnetic separator to separate iron pieces in coal, which may damage

the crusher.

PROPERTIES OF GASEOUS FUELS

Gaseous fuels in common use are liquefied petroleum gases (LPG), Natural gas, producer

gas, blast furnace gas, coke oven gas etc. The calorific value of gaseous fuel is expressed in

Kilocalories per normal cubic meter (kCal/Nm3) i.e. at normal temperature (20

oC) and pressure

(760 mm Hg)

Calorific Value

Since most gas combustion appliances cannot utlilize the heat content of the water

vapour, gross calorific value is of little interest. Fuel should be compared based on the net

calorific value. This is especially true for natural gas, since increased hydrogen content results in

high water formation during combustion.

LPG

LPG is a predominant mixture of propane and Butane with a small percentage of

unsaturates (Propylene and Butylene) and some lighter C2 as well as heavier C5 fractions.

Included in the LPG range are propane (C3H8), Propylene(C3H6), normal and iso-butane

(C4H10) and Butylene(C4H8).

LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure,

but may be condensed to the liquid state at normal temperature, by the application of moderate

pressures. Although they are normally used as gases, they are stored and transported as liquids

under pressure for convenience and ease of handling. Liquid LPG evaporates to produce about

250 times volume of gas.

LPG vapour is denser than air: butane is about twice as heavy as air and propane about

one and a half times as heavy as air. Consequently, the vapour may flow along the ground and

into drains sinking to the lowest level of the surroundings and be ignited at a considerable

distance from the source of leakage. In still air vapour will disperse slowly. Escape of even small

quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus

cause considerable hazard. To aid in the detection of atmospheric leaks, all LPG's are required to

be odorized. There should be adequate ground level ventilation where LPG is stored. For this

very reason LPG cylinders should not be stored in cellars or basements, which have no

ventilation at ground level.

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NATURAL GAS

Methane is the main constituent of Natural gas and accounting for about 95% of the total

volume. Other components are: Ethane, Propane, Butane, Pentane, Nitrogen, Carbon Dioxide,

and traces of other gases. Very small amounts of sulphur compounds are also present. Since

methane is the largest component of natural gas, generally properties of methane are used when

comparing the properties of natural gas to other fuels.

Natural gas is a high calorific value fuel requiring no storage facilities. It mixes with air

readily and does not produce smoke or soot. It has no sulphur content. It is lighter than air and

disperses into air easily in case of leak.

TYPES OF LIQUID FUELS

1. FURNACE OIL / FUEL OIL (FO)

Internationally Furnace oil is known as Fuel oil and is traded in many varieties based on its

specifications of viscosity and sulfur percentage. Fuel Oil is used as an industrial fuel. It is a dark

viscous residual fuel obtained by blending mainly heavier components from crude distillation

unit, short residue and clarified oil from fluidized catalytic cracker unit.

2. LOW SULPHUR HEAVY STOCK (LSHS)

It is a residual fuel processed from indigenous crude. This fuel is used in the same

applications where furnace oil is suitable.

Its key features include the following:

Has higher pour point than that of fo and hence to be maintained at 75oC at all times.

Low sulphur content is its main advantage.

Emits lesser quantity of sulphur dioxide.

Gross calorific value of LSHS is more than that of furnace oil hence consumption of

fuel oil will be reduced.

It is a low viscosity fuel oil.

Chemically stable and incompatible with strong oxidizers.

Ingestion may cause spontaneous vomiting, irritation of mouth throat and gastro

intestinal tract.

3. LIQUEFIED NATURAL GAS (LNG)

Liquefied natural gas, or LNG, is natural gas in a liquid form that is clear, colorless,

odorless, non-corrosive, and non-toxic. LNG is produced when natural gas is cooled to minus

259 degrees Fahrenheit through a process known as liquefaction. During this process, the natural

gas, which is primarily methane, is cooled below its boiling point, whereby certain

concentrations of hydrocarbons, water, carbon dioxide, oxygen, and some sulfur compounds are

74

either reduced or removed. LNG is also less than half the weight of water, so it will float if

spilled on water.

4. DISEL FUELS(HSD/LDO)

A diesel fuel is any fuel suitable for burning in diesel or compression ignition engines.

Petroleum diesel fuels may be distillates or blends of distillates and residual fuels. In a

compression ignition engine, air alone is drawn into cylinder and compressed until it is very hot

(about 500 deg C). At this stage, finely atomized fuel is injected at a very high pressure, which is

ignited by the heat of compression and hence the term compression ignition (C.I.). A spark

ignition engine on the other hand, relies upon a carburetor to supply into the cylinder a mixture

of gasoline vapour and air, which after compression, is ignited by a spark.

The average compression ratio of a diesel engine is much higher (about 15:1) than that of

a gasoline engine (about 8:1) and this is the reason for the higher thermal efficiency of the diesel

engine (about 33% as compared to about 25% of the gasoline engine) which makes for economy

in operation.

Two main grades of diesel fuel are marketed in India, High Speed Diesel (HSD) and

Light diesel oil (LDO). The former is a 100% distillate fuel while the latter is a blend of

distillate fuel with a small proportion of residual fuel.

HSD is normally used as a fuel for high speed diesel engines operating above 750 rpm

i.e. buses, lorries, generating sets, locomotives, pumping sets etc. Gas turbine requiring distillate

fuels normally make use of HSD as fuel. LDO is used for diesel engines, generally of the

stationery type operating below 750 rpm

When fuel is injected into the combustion chamber of a diesel engine, ignition does not

occur immediately. The interval between the commencement of fuel injection and the

commencement of combustion is known as the " ignition delay" and is a measure of the ignition

quality of the fuel. This delay period depends on the nature of the fuel, the engine design, and on

the operating conditions. If the delay is too long, the engine may be hard to start and when the

accumulated fuel does ignite, the rate of pressure rise may be so great that it causes roughness or

diesel knock. The effects of diesel knock are similar to the effects of knocking in gasoline

engines, viz. loss of efficiency and power output and a possibility of mechanical damage to the

engine if the knocking is prolonged.

CETANE NUMBER

The most accurate method of assessing the ignition quality of a diesel fuel is by

measuring its cetane number in a test engine, the higher the cetane number the higher the ignition

quality. The cetane number of a fuel is defined as the percentage of cetane, arbitrarily given a

cetane number of 100, in a blend with alphamethyl-naphthaline (cetane number -0 ), which is

equivalent in ignition quality to that of the test fuel.

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OCTANE NUMBER

Measure of the ignition quality of gas (gasoline or petrol). Higher this number, the less

susceptible is the gas to 'knocking' (explosion caused by its premature burning in the combustion

chamber) when burnt in a standard (spark-ignition internal combustion) engine. Octane number

denotes the percentage of volume of iso-octane in a combustible mixture (containing iso-octane

and normal-heptane) whose 'anti-knocking' characteristics match those of the gas being tested. In

the older vehicles, high octane numbers were achieved by adding lead tetraethyl to the gas (the

'leaded gas'), a pollutant that contributes to lead poisoning. In the newer vehicles, the same result

is achieved by the engine design that increases turbulence in the combustion chamber or by

adding aromatic hydrocarbons such as xylenes and oxygenates (oxygen-containing compounds

such as alcohols) to the gas. Also called Octane rating.

BIO GAS

Biogas is a clean and efficient fuel, generated from cow-dung, human waste or any kind

of biological materials derived through anaerobic fermentation process. The biogas consists of

60% methane with rest mainly carbon-di-oxide. Biogas is a safe fuel for cooking and lighting.

By-product is usable as high-grade manure.

A typical biogas plant has the

following components: A digester in

which the slurry (dung mixed with water)

is fermented, an inlet tank - for mixing

the feed and letting it into the digester,

gas holder/dome in which the generated

gas is collected, outlet tank to remove the

spent slurry, distribution pipeline(s) to

transport the gas into the kitchen, and a

manure pit, where the spent slurry is

stored.

Advantages of Bio Gas technology

It provides a better and cheaper fuel cooking, lighting and for power generation.

It produces good quality, enriched manure to improve soil fertility.

It proves an effective and convenient way for sanitary disposal of human excreta,

improving the hygienic conditions.

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76

It generates social benefits such as reducing burden on forest for meeting cooking

fuel by cutting of tree for fuel wood, reduction in the drudgery of women and

children etc.

As a smokeless domestic fuel, it reduces the incidence of eye and lung diseases.

It also helps in generation of productive employment

BIOFUEL

Biofuels provided 1.8% of the world's transport fuel in 2008. Liquid biofuel is usually

either bioalcohol such as bioethanol or an oil such as biodiesel. Bioethanol is an alcohol made

by fermenting the sugar components of plant materials and it is made mostly from sugar and

starch crops. With advanced technology being developed, cellulosic biomass, such as trees and

grasses, are also used as feedstocks for ethanol production. Ethanol can be used as a fuel for

vehicles in its pure form, but it is usually used as a gasoline additive to increase octane and

improve vehicle emissions. Bioethanol is widely used in the USA and in Brazil.

Biodiesel is made from vegetable oils, animal fats or recycled greases. Biodiesel can be

used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce

levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.

Biodiesel is produced from oils or fats using transesterification and is the most common biofuel

in Europe.

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http://en.wikipedia.org/wiki/Vegetable_oil

http://en.wikipedia.org/wiki/Animal_fat

http://en.wikipedia.org/wiki/Oil

http://en.wikipedia.org/wiki/Transesterification

77

7. LAYOUT OF POWER PLANT

7.1 INTRODUCTION

Power plays a greater role wherever man lives and works. Electricity is the only form of

energy which is easy to produce, transport and easy to control. The types of power plants which

is used to produce the electricity are thermal, nuclear, hydraulic, gas turbine and geothermal.

In the above said power plants, the thermal power plants generate more than 80% of the

total electricity produced in the world. The availability of electrical energy and its per capita

consumption is regarded as an index of national standard of living in the present day civilization.

Fossil fuel like coal, fuel oil and natural gas are the energy sources and steam is the

working fluid. Thermal power production cost in India is more when compared to nuclear power

due to rise in oil prizes. Hydraulic power plants are essentially multi purpose. It is used for

power generation as well as irrigation.

Nuclear energy has enlarged the world's power resources. The amount of energy released

by burning one kilogram of uranium is equivalent to the energy obtained by burning 4500tonnes

of high-grade coal.

The renewable energy source like solar energy in India has ideal1 geographical

situations. If the sunshine is bright for an average for 8hours/day. then the amount of heat

equivalent is more than 200MW per square kilometer. This numerical figure shows that

enormous potential' is available for developing solar thermal power plants.

In this unit, the various layouts of power plant are discussed. They are

1.Steam or thermal power plant.

2.Hyde power plant.

3.Diesel power plant.

4.Nuclear power plants.

5.Gas power plants.

6.Tidal Power Plants

7. Wind Power Plants

8.Geothermal Power Plants

9.OTEC Power Plants.

7.2 STEAM POWER PLANT

Steam is an important medium for producing mechanical energy. A steam power plant

continuously converts the energy stored in fossil fuels (coal, oil and natural gas). Steam has the

advantage that it can be raised from water which is available in abundance. The steam power;

stations are very much suitable where coal is available in abundance. The pressure range is from

I0kg/cm2 to super critical pressure and temperature varies from 250°C to 650°C.

Working principle:

Steam or thermal power plant is using steam as working fluid. Steam is produced in a

boiler using coal as fuel and used to drive the prime mover (Steam turbine).

78

The heat energy is converted into mechanical energy by die steam turbine and that

mechanical energy is used for generating power with the help of generator.

The layout of the steam power plant consists of four main circuits. These are

1. Coal and ash circuit. 2. Air and flue gas circuit

3. Water and steam circuit. 4. Cooling water circuit.

1. Coal and Ash circuit:

This circuit consists of coal storage, ash storage, coal handling and ash handling systems.

The handling system consists of belt conveyor screw conveyors etc. coal from the storage yard is

transferred to the boiler furnace by means of coal handling equipment. Ash resulting from the

combustion of coal in the boiler furnace is removed to ash storage through ash handling.

The Indian coal contains 30 to 40% of ash and a power plant 100MW produces normally

20 to 25 tones of hot ash per hour.

2. Air and Flue gas circuit.

This circuit consists of air filter, air preheater, dust collector chimney. Air is taken from

the atmosphere to the air preheater, the dust from the air is removed by means of using air filter.

By using the waste heat of the flue gas which is passing to the chimney, the air is preheated in

the preheater.

79

After combustion in the furnace, the flue gas which has sufficient quantity of heat is

passed around the boiler tubes, dust collector, economiser and preheater before being exhausted

to the atmosphere through the chimney. By passing the flue gas around the economiser and air

preheater, the water and air are preheated before going to the boiler,

3.Feed water and steam circuit.

This circuit consists of boiler feed pump, boiler, turbine, and feed heaters. The steam

generated in die boiler passes through super heater and is supplied to the steam turbine. The

steam is expanded in the steam turbine then passed to the condenser where it is condensed.

The condensate is heated in the HP and LP heaters using the steam tapped from different

points of the turbine. The feed water is passing through the economiser, where it is further heated

by means of flue gases. Using the economizer, the feed water is heated by the feed water heaters

and then it is fed into the boiler.

Part of the steam and water are lost while passing through different components of the

system. So, feed water is supplied from the external source to compensate losses.

4.Cooling water circuit:

This circuit consists of circulating water pump, condenser, cooling water pumps and

cooling tower. Abundant quantity of water is required for condensing the steam in the condenser.

Adequate water supply is available from various sources like river or lake. If adequate quantity

of water is not available at plant sites, the warm water coming out from the condenser is cooled

in cooling tower and it is recirculated again and again.

Characteristics of steam power plant:

1. Low cost compared with hydro power plant.

2. High efficiency.

3. Reduced water requirement.

4. Higher reliability and availability.

5. Reduced environmental impact in terms of air pollution.

Advantages of steam power plant:

1. The power production does not depend on nature mercy.

2. Initial investment is low.

3. The power plant can be located near load center, so the transmission cost and losses

are considerably reduced.

4. The time requirement for construction and commissioning of thermal power plant

require less period of time. -

Disadvantages of steam power plant:

1. As compared to hydro powerplant, life and efficiency are less.

2. Transportation of fuel is a major problem in this type of power plant.

80

3. Power generation cost is considerably high when compared to hydro plant.

4. Air pollution is the major problem calling for additional investment.

5. It cannot be used as peak load plant.

6. The coal (fuel) needed may be exhausted by gradual use.

7.3. HYDEL POWER PLANT

Water is the cheapest source of power. A hydroelectric power plant is aimed at

harnessing from water flowing under pressure. In hydroelectric power plants, the energy of water

is utilized to drive the turbine hydro or waterpower is important only next to the thermal: power.

Hydroelectric power was initiated in India in 1987 near Darjeeling. The arrangement of different

components used in hydraulic power plants is discussed below.

COMPONENTS OF HYDROELECTRIC POWER PLANT:

I . Water reservoir:

Continuously availability of water is the basic necessity for a hydroelectric plant. The

main purpose of the reservoir is to store the water during rainy season and supplied the same to

the turbine continuously throughout the year. Water surface in the storage reservoir is known as

headrace.

2. Dam:

The dam is used in hydro power plants to increase the height of water level and thereby it

increases the capacity of reservoir. The dam also helps to increase the working head of the power

plant.

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3. Spillway:

Water after certain level in the reservoir overflows through spillway without allowing the

increase in water level in the reservoir during rainy conditions. Spillway is acting as a safety

valve for the dam. It must have the capacity to discharge major floods without damage to the

dam.

4. Trash rack:

The water is taken from the dam or from the forebay is provided with trash rack.

5. Pressure tunnel:

Pressure tunnel carries water from the reservoir to surge tank.

6. Penstock:

A pipeline fixed between the surge tank and prime mover is known I as penstock. It is

commonly made of reinforced concrete or steel.

7. Forebay:

It serves as a regulating reservoir. It stores the water temporarily I when the load on the

plant reduces and provides water for initial I increment of an increasing load.

8. Surge Tank:

There is sudden increase in pressure of the penstock due to sudden backflow of water as

load on the turbine is reduced. This sudden arise pressure in the penstock above normal due to

reduce in load the generator is known as water hammer. The surge tank is introduced between

the dam and powerhouse to keep in reducing the sudden rise the penstock.

9. Water turbine:

Water through the penstock enters into the turbine through the inlet valve. The water

turbine converts kinetic energy of water into mechanical energy to produce electrical energy.

Prime mover which is in common use such as Pelton turbine, Francis turbine and Kaplan turbine.

The mechanical energy available at the turbine shaft is used to run the electric generator.

10.Draft Tube

The draft tube is connected at the outlet of the water turbine. It allows the turbine to be

placed over tailrace level. It is used to tap the remaining kinetic energy of water coming out of

the turbine.

11. Tailrace:

It is a waterway to lead the water discharged from the turbine to the river.

12. Transformer:

The transformer is to raise the voltage generated at the generator terminal before

transmitting the power to consumers and workstation.

■

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WORKING PRINCIPLE:

In the hydroelectric power plants, the potential energy of water is converted into kinetic

energy. The potential energy of water is used to run water turbine to which the electric generator

is coupled. The mechanical available at the shaft of the turbine is converted into electrical energy

through a generator or alternator. The water is first passed through the penstock to the turbine

from the dam.

CLASSIFICATION OF HYDROPOWER PLANTS:

The hydropower plants are classified according to the head of water. The operating head

of water exceeds 70meters, the plat is known | as "high head power plant". Pelton turbine is used

as prime mover in this type of power plants.

The operating head of water ranges from 15 to 70meters then the power plant is known as

medium head power plant. The operating head of water is less than 15meters that power plant is

known as low head power plant.

Advantages of hydroelectric power plant:

1. Water is the cheapest source of energy. The fuels needed for die thermal, diesel and nuclear

plants are exhaustive and expansive.

2. Water is the renewable source of energy. It is neither consumed nor converted into

something else.

3. The fuel cost is totally absent.

4. There is no problem of handling the fuel and ash. No nuisance of smoker exhaust gases and

soot's and no health hazards due to air pollution.

5. The running cost of hydropower installation is very low as compared to thermal or nuclear

power stations.

6. The hydraulic power plant is relatively simple in concept and self-contained in operation.

7. Variable load does not affect the efficiency in the case of a hydro-plant.

8. Modem hydropower equipment has greater life expectancy and can easily last 50 years or

more.

9. Hydro-plants provide auxiliary benefits like irrigation, flood control.

10. The efficiency does not change with age.

11. Maintenance cost is low.

12. It requires less supervising staff for the operation of the plant.

Disadvantages of hydroelectric power plant:

1. Hydropower projects are capital-intensive with a low rate of return.

2. Power generation is dependent on die quantity of water available, which may vary season-

to-season and year-to-year.

3. Initial cost of the plant is high.

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4. The hydel power plants are often far away from the load center and require long

transmission lines to deliver power.

5. Large hydro-plants disturb the ecology of the area, by way of deforestation, destroying

vegetarian and uprooting people.

6. It takes considerably longer time for its installation, compared with thermal power plants.

7.4. DIESEL POWER PLANT

Diesel power plant is suitable for small and medium outputs. Diesel electric plants in the

range of 2 to 50MW capacity are used as central stations. The diesel power plants are commonly

used where fuel prices or reliability of supply favour oil over coal, where water supply is limited

where loads are relatively small.

COMPONENTS OF DIESEL POWER PLANT:

1. Diesel engine:

This is the main component of a diesel power plant. The engines are classified as two-

stroke engine and four stroke engines. Engine is generally directly coupled to the generator for

developing power. In diesel engine, air admitted into the cylinder is compressed, the

compression ratio being 12 to 20. At the end of compression stroke, fuel is injected. The fuel is

burned and the burning gases expand and do work on the piston. The shaft of the engine is

directly coupled to the generator. After the combustion, the burned gases are exhausted to the

atmosphere.

2. Air fiber and super charger:

The air filter is used to remove the dust from the air which is taken by the engine.

The function of the supercharger is to increase the pressure of the air supplied to the

engine and thereby the power of the engine is increased.

3. Engine starting system:

This includes air compressor and starting air tank. This is used to start the engine in cold

conditions by supplying the air.

5. Fuel system:

It includes the storage tank, fuel pump, fuel transfer pump, strainers and heaters. The fuel

is supplied to the engine according to the load variation.

6. Lubrication system:

It includes oil pumps, oil tanks, filters, coolers and pipes. It is used to reduce the friction

of moving parts and reduce wear and tear of the engine.

7. Cooling system:

The temperature of burning fuel inside the combustion chamber is 1500°C to 2000°C. To

maintain the temperature as reasonable level, water is circulated around the engine in water

jackets which is passed through the cylinder, piston, combustion chamber. Hot water leaving the

jacket is sent to heat exchanger.

84

8. Governing system:

It is used to regulate the speed of the engine. This is done by varying the fuel supply

according to the engine load.

9. Exhaust system:

It includes silencers and connecting ducts. The exhaust gas has high temperature and so it

is used to preheat the oil and air.

WORKING PRINCIPLE:

The air and fuel mixture are working medium in diesel engine power plant. The

atmosphere air is coming inside the combustion chamber during the suction stroke and die fuel is

injected through the injection pump. The air and fuel is mixed inside the engine and the charge is

ignited due to high compression inside the engine cylinder. The basic principle in diesel engine is

the thermal energy which is converted into mechanical energy and this mechanical energy is

converted into electrical energy to produce the power by using generator or alternator.

Applications of diesel power plant

1. Quite suitable for mobile power generation.

2. Used as peak load plants in combined with thermal or hydro plants.

3. Used as stand by plants for emergency service.

85

Advantages:

1 Diesel power plants are cheaper.

2. Plant layout is simple.

3. Location of the plant is near the load center.

4. Quick starting and easy pick-up of loads.

5. Skilled manpower is not required.

6. Time schedule for manufacturing are shorter.

7. Diesel plants operate at high overall efficiency than steam plants.

8. Fuel handling is easier and no problem of ash disposed.

9. Efficiency does not fall so much as that of a steam plant during pans loads

10.It has no stand by losses

Disadvantages:

1. The repair and maintenance cost are high.

2. Plant capacity is limited to about 5QMW of power.

3. Life of the diesel plants is low when compared to thermal plants.

4. Diesel fuel is much more expansive.

5. The efficiency of the diesel engine is about 33% only.

7.5. NUCLEAR POWER PLANT

The nuclear power plants are now comparable to or even lower than the unit cost in coal

fired power plants. Nuclear power utilization can help to save a considerable amount of fossil

fuels which can be used in other areas. The heat produced due to fission of U and Pu is used to

heat water to generate steam which is used for running turbo generator.

One kilogram of U can produce as much energy as possible by burning 4500tonnes of

high-grade coal.

Nuclear fission:

Uranium exists in the isotopic form of U which is unstable. The nuclear energy is derived

from splitting or by the fission of the nuclear of fissionable atom. When a neutron enters the

nuclear of U235

, the nuclear is splitted into two equal fragments and also releases 2.5 fast moving

neutrons with a velocity of 1.5 > 107 m/sec and producing energy at 200million-electron volts.

The isotopes of Uranium are U"", If", IT" are the most unstable topic, which is easily

fissionable.

Chain reaction:

The neurons released during the fission can be made to fission and the other nuclei of U23

causing a "Chain Reaction". During the fission process, when a neutron is captured by the

nuclear of an atom U235

, the atom is splitted into two fragments of more or less of equal mass and

about 2.5 neutrons are released.

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The atomic explosion is caused by the under controlled chain reaction.

We know in fission process, about 2.5 neutrons are released out of which one neutron is

used to sustain the chain reaction, 0.9 neutron is converted into fissionable material pU239

and 0.6

neutron is absorbed by control rod and moderator.

The moderator function is to reduce the energy of neutrons and to maintain the chain reaction.

MAIN COMPONENTS OF NUCLEAR POWERPLANTS

1. Fuel:

The fuel which is used in the nuclear reactors are U235

, pU239

and U233

2.Nuclear reactor:

It consists of reactor cone, reflector, shield etc. It may be regarded as a substitute for the

boiler fire box of a steam power plant. During the fission the large amount of heat is liberated by

U235

. This large amount of heat is absorbed by the coolant and it is circulated through the core.

The various types of reactors used in nuclear power plant is

1. Boiling water reactor

2. Pressurised water reactor

3. Heavy Water-cooled reactor.

3. Steam generator:

It is fed with feed water and the feed water is converted into steam by the heat of the hot

coolant. The coolant is used to transfer the heat from the reactor core to the steam generator.

4. Moderator:

It is used to the Kinetic energy of fast neutrons into slow neutrons and to increase the

probability of chain reaction.

87

5 . Reflector:

It is used in the reactor to conscience the neutrons in order to reduce the consumption of

fissile material.

6.Turbine:

The steam produced by the steam generator is passed to the turbine and it is connected to

the generator.

7. Control Rods

It is used to control the nuclear chain reaction and functions of the nuclear reactor.

8. Coolant pump and feed pump:

It is used to maintain the flow of coolant and feed water in the power plant. The ordinary

water or heavy water is a common coolant.

9. Shielding:

The reactor is a source of intense radioactivity. These radiations are very harmful,

shielding is provided to absorb the radioactive rays. A thick concrete shielding and a pressure

vessel are provided to prevent the radiations escaped to atmosphere.

WORKING PRINCIPLE OF NUCLEAR POWER PLANT:

The fission reaction of Uranium fuel takes places in the reactor and large amount of heat

is released. This reaction is controlled by moderators.

88

The large amount of heat produced in the reactor is absorbed by the coolant which is

circulated through the reactor core.

The hot coolant goes to the steam generator where the feed water is absorbed the heat

from the coolant and converted into steam.

This steam is used to run the steam turbine and it is connected to the steam generator. The

used steam is condensed and it is reused in the boiler. The mechanical energy of the steam

turbine is converted into electrical energy by an electric generator.

Advantages of nuclear power plant:

1. Space requirement of a nuclear power plant is less.

2. It is easily adopted where water and coal resources are not available.

3. There is increased reliability of operation.

4. It is not affected by adverse whether conditions.

5. It does not require large quantity of water.

6. Nuclear power plant consumes very small quantity of fuel.

7. Nuclear power plants are well suited to meet large power demands.

8. Space for fuel storage is not needed.

9. No ash handling.

Disadvantages:

1. It is hot suitable for variable load conditions.

2. Radioactive wastes may affect the health of workers and other population.

3. It requires high initial cost.

4. It requires well-trained personnel.

7.6. GAS TURBINE POWER PLANT

The gas turbine power plant has relatively low cost and can be quickly put into

commission. Gas turbine installations require only a fraction of water used by their steam turbine

counter-parts. The size of gas turbine plants used varies from 10MW to 50MW and the thermal

efficiency of about 22% to 25%. It is very much useful in peak load. The gas turbine plant

requires less space only. The classification of gas turbine plants are

1. According to type of load

a. Peak load

b. Stand by

c. Base load

3. According to fuel

a. Liquid

b. Gas

c. Solid

2. According to the application

a. Aircraft

b. Marine

c. Locomotive

d. Transport

89

4. According to cycle of operation

a. Open cycle

b. Closed cycle

5. According to number of shafts

a. Single shaft

b. Multi shaft

ELEMENTS OF A GAS TURBINE PLANT

1. Compressor

2. Intercoolers

3. Regenerator

4. Combustion chambers

5. Gas turbine

6. Reheating unit

1. Compressor:

In gas turbine plant, the axial and centrifugal flow compressors are used. In most of the

gas turbine power plant, two compressors are used. One is low-pressure compressor and the

other is high-pressure compressor.

In low-pressure compressor, the atmospheric air is drawn into the compressor through the

filter. The major part of the power developed by the turbine (about 66%) is used to run the

compressor.

This low-pressure air goes to the high-pressure compressor through the intercooler. Then

the high-pressure air goes into the regenerator.

2. Intercooler:

The intercooler is used to reduce the work of the compressor and it is placed in between

the high pressure and low-pressure compressor. Intercoolers are generally used when the

pressure ratio is very high. The energy required to compress the air is proportional to the air

temperature at inlet. The cooling of compressed air in intercooler is generally done by water.

3.Regenerator:

Regenerators are used to preheat the air which is entering into the combustion chamber to

reduce the fuel consumption and to increase the efficiency. This is done by the heat of the hot

exhaust gases coming out of the turbine.

4. Combustion chambers:

Hot air from regenerator flows to the combustion chambers and the fuel like coal, natural

gas or kerosene are injected into the combustion chamber. After the fuel injection, the

combustion takes place. These high-pressure, high- temperature products of combustion are

passed through the turbine.

5. Gas turbine:

Two types of gas turbines are used in gas turbine plant 1. High pressure turbine. 2. Low

pressure turbine. The combustion products from the combustion chamber are first expanded in

high-pressure turbine and then it expands in low-pressure turbine. Due to the expansion taking

place in the gas turbine, the heat is converted into mechanical work. The gas turbine

classification is

1. Open cycle gas turbine: 2. Closed cycle gas turbine

90

In open cycle gas turbine plant, the air (or) gases coming out from the gas turbine are exhausted

to the atmosphere.

In closed cycle gas turbine plant, the air (or) gases coming out from the gas turbine plant

are cooled in the cooler and it is again recirculated. The working fluid is continuously used in the

system without change of phase.

6. Reheating unit:

In this unit, the additional fuel is added to the exhaust gases coming out from the high

pressure turbine, and the reheated combustion products goes into the low pressure turbine.

WORKING OF GAS TURBINE PLANT:

The working of gas turbine plant is shown in fig. The atmosphere air is drawn into the

low-pressure compressor through the air filter and it is compressed.

The compressed low-pressure air goes into the high pressure compressor through the inter

cooler. Here, the heat of the compressed air is removed.

Then the high-pressure compressed air is goes into the combustion chamber through the

regenerator.

In the combustion chamber, the fuel is added to the compressed air and the combustion of

the fuel takes place. The product of the combustion goes into the high-pressure turbine. The

exhaust of the high pressure turbine goes to the another combustion chamber and the

additional fuel is added the exhaust and it goes to the low pressure turbine.

After the expansion in the low-pressure turbine, the exhaust is used to heat the high-

pressure air coming to the combustion chamber through the regenerator. After that, the exhaust

goes to the atmosphere.

Advantages of gas turbine plant:

1. Smaller in size and weight as compared to an equivalent steam power plant.

2. Natural gas is a very suitable fuel.

3. The gas turbine plants are subjected to less vibration.

91

4. The initial cost is lower than an equivalent steam plant.

5. The installation and maintenance costs are less than thermal plants.

6. There are no standby losses in gas turbine plants.

7. It requires less water as compared to a steam plant.

8. Any quantity of fuels can be used in gas turbine plants.

9. It can be started quickly.

10. The exhaust of the gas turbine is free from smoke.

11. Gas turbines can be built relatively quicker and requires less space.

Disadvantages:

1. Part load efficiency is poor.

2. The unit is operated at high temperature and pressure, so special metals are required

to maintain the unit.

3. Major past of the work, (about 66%) developed in the turbine is used to drive the

compressor.

4. The devices that are operated at high temperature are complicated.

7.7. COMPARISON AND SELECTION

1 . Thermal power plant Vs Hydro-plant:

No. Thermal power plant Hydro power plant

1. Initial cost is low. Initial cost is high.

2. Located near to load center. Not like that.

3. Transmission losses are less. Transmission losses are high.

4. Power production is not dependent on

nature's mercy.

It is Only dependent on nature's mercy.

5. Construction time is less. Initial construction requires long time.

6. Power generation cost is high. Power generation cost is less.

7. Air pollution is more. No air pollution.

8. Fuel transportation is difficult. No fuel transportable

9. Life of the plant is less. Life of the plant is high.

10. Efficiency of the plant is less. Efficiency of the plant is high.

11. Not suitable for peak load plant. It is suitable.

92

2.Diesel power plant Vs Gas turbine power

No. Diesel power plant Gas turbine power plant

1. The efficiency of the diesel power

plant is about 35 to42%.

The efficiency of the simple gas turbine power plant

= 20to 25%.

2. Particular fuel should be used in this

power plant.

The varying fuel quality also used in this plant

3. The work output is high. The network output is less.

4 It is not require special metals. The unit is operated at high temperature and pressure

so special metals are required.

5. Cost of the plant is less. Gas turbine cost is high.

6. Limited plant capacity. Capacity of the plant is higher than diesel power plant.

7. Not suitable for continuous over

loads.

It can be work on over loads.

8. Lubrication cost is high. No lubrication cost.

9. No ash-handling problem. Ash handling problem is there.

10. Life of the plant is less. Life of the plant is high compared with diesel power

plant.

3. Steam power plant Vs Nuclear power

No. Steam power plant Nuclear power plant

1. It is not suitable wherever water and

coal resources are not available.

It is suitable for (hat.

2. Fuel storage space is required. No fuel storage space.

3. Workmen required is very high. Very less number of workmen is

required.

4. Capital cost is high. Capital cost is less when size of plant is

increased.

5. No radioactive material. Radioactive wastes.

6. Space requirement is high Space requirement is less.

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7. Steam power plants are affected by

whether conditions.

Not affected by adverse weather

conditions.

8. It requires large quantity of water. It does not require large quantity of

water.

9. Large quantity of fuel is required. Less quantity of fuel required.

10. Maintenance cost is less. Maintenance cost is high.

11. Operating cost is high. Operating cost is less.

12. Steam power plant efficiency = 20 to

30%.

Nuclear power plant efficiency = 30 to

32%.

7.8 TIDAL POWER PLANTS

Tide is periodic rise and fall of the water level of the sea. Tides occur due to the attraction

of seawater by the moon. These tides can be used to produce electrical power which is known as

tidal power.

When the water is above the mean sea level, it is called flood tide and when the level is

below the mean level, it is called ebb tide. A dam is constructed in such a away that a basin gets

separated from the sea and a difference in the water level is obtained between the basin and sea.

The constructed basin is filled during high tide and emptied during low tide passing through

sluices and turbine respectively. The Potential energy of the water stored in the basin is used to

drive the turbine which in turn generates electricity as it is directly coupled to an alternator.

Factors affecting the suitability of the site for tidal power plant

The feasibility and economic vulnerability of a tidal power depends upon the following

factors.

1. The power produced by a tidal plant depends mainly on the range of tide and the cubature of

the tidal flow occurring in the estuary during a tidal cycle which can be stored and utilized for

power generation. The cubature of the tidal flow not only depends on the tidal range but on

the width of estuary mouth.

2. The minimum average tide range required for economical power production is more.

3. The site should be such that with a minimum cost of barrage it should be possible to create

maximum storage volume. In addition to this, the site selected should be well protected from

waves action.

4. The site should not create interruption to the shipping traffic running through the estuary

other wise the cost of the plant will increase as locks are to be provided.

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5. Silt index of the water of the estuary should be as small as possible to avoid the siltation

troubles. The siltation leads to reduction of the range of tides and reduces the power potential

of the plant.

6. The fresh water prism that falls into the reservoir of the tidal plant (due to the surface flows in

the streams having out fall in the estuary) eats away the valuable storage created for storing

the tidal prism. Therefore, the ratio of fresh water prism to tidal water prism becomes an

important index in determining the economic feasibility of a tidal scheme. The effective and

cheaper will be the power production with decreasing the ratio mentioned above.

CLASSIFICATION OF TIDAL POWER PLANTS

The tidal power plants are generally classified on the basis of the number of basins used

for the power generation. They are further subdivided as one-way or two-way system as per the

cycle of operation for power generation.

The classification is represented with the help of a line diagram as given below

COMPONENTS OF TIDAL POWER PLANTS

There are three main Components of a tidal Power plant. i.e,

(i) The Power house

(ii) The dam or barrage

(iii) Sluice-ways from the basins to the sea and vice versa.

The turbines, electric generators and other auxiliary equipment's are the main equipments

of a power house. The function of dam to form a barrier between the sea and the basin or

between one basin and the other in case of multiple basins.

The sluice ways are used either to fill the basin during the high tide or empty the basin

during the low tide, as per operational requirement. These are gate controlled devices.

It is generally convenient to have the power house as well as the sluice-ways in alignment

with the dam.

The design cycle may also provide for pumping between the basin and the sea in either

direction. If reversible pump turbines are provided, the pumping operation can be taken over at

any time by the same machine. The modern tubular turbines are so versatile that they can be used

either as turbines or as pumps in either direction of flow. In addition, the tubular passages can

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also be used as sluice-ways by locking the machine in to a stand still. As compared to

conventional plants, this, however, imposes a great number of operations in tidal power plants.

For instance, the periodic opening and closing of the sluice-way of a tidal plant are about 730

times in a year.

1. Dam (Barrage)

Dam and barrage are synonymous terms. Barrage has been suggested as a more accurate

term for tidal power scheme, because it has only to with stand heads a fraction of the structure's

height, and stability problems are far more modest. However, the literature does not always make

the distinction, even though heads are small with tidal power cutoffs.

Tidal power barrages have to resist waves whose shock can be severe and where pressure

changes sides continuously.

The barrage needs to provide channels for the turbines in reinforced concrete. To build

these channels a temporary coffer dam in necessary, but it is now possible to built them on land,

float them to the site, and sink them into place.

Tidal barrages require sites where there is a sufficiently high tidal range to give a good

head of water - the minimum useful range is around three meters. The best sites are bays and

estuaries, but water, can also be impounded behind bounded reservoir built between two points

on the same shore line.

The location of the barrage is important, because the energy available is related to the size

of trapped basin and to the square of the tidal range. The nearer it is built to the mouth of bay, the

larger the basin, but the smaller the tidal range. A balance must also be struck between increased

out put and increased material requirements and construction costs.

2. Gates and Locks

Tidal power basins have to be filled and emptied. Gates are opened regularly and

frequently but heads very in height and on the side where they occur, which is not the case with

conventional river projects. The gates must be opened and closed rapidly and this operation

should use a minimum of power. Leakage, is tolerable for gates and barrages. Since we are

dealing with seawater, corrosion problems are actuate, they have been very successfully solved

by the catholic protection and where not possible by paint. Gate structures can be floated as

modular units into place.

Though, in existing plants, vertical lift gates have been used. The technology is about

ready to substitute a series of flap gates that operates by water pressure. Flap gates are gates that

are positioned so as to allow water in to the holding basin and require no mechanical means of

operation. The flap gates allow only in the direction of the sea to basin. Hence, the basin level

rises well above to sea level as ebb flow area is far less than flood flow area.

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3. Power house

Because small heads only are available, large size turbines are needed; hence, the power

house is also a large structure. Both the French and Soviet operating plants use the bulb type of

turbine of the propeller type, with revisable blades, bulbs have horizontal shafts coupled to a

single generator. The cost per installed kilowatt drops with turbine size, and perhaps larger

turbines might be installed in a future major tidal power plant.

WORKING

1. Single basin-One-way cycle

This is the simplest form of tidal power plant. In this system, a basin is allowed to get filled

during flood tide and during the ebb tide. The water flows from the basin to the sea passing

through the turbine and generates power. The power is available for a short duration during ebb

tide.

Single basin Tidal Power Plant

Fig shows a single tide basin before the construction of dam, at the mouth of the basin and

power generation during the falling tide.

2. Single-basin two-way cycle

In this arrangement power is generated both during flood tide as well as ebb tide also. The

power generation is also intermittent but generation period is increased compared with one-way

cycle. However the peak power obtained is less than the one-way cycle. The arrangement of the

basin and the power cycle is shown in fig

1

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The main difficulty with this arrangement, the same turbine must be used as Prime mover

as ebb and tide flows pass through the turbine in opposite directions. Variable pitch turbine and

dual rotation generator are used for such schemes.

3. Single-basin two-way cycle with pump storage

The Range tidal power plant in France uses this type of arrangement. In this system,

power is generated both during flood and ebb tides. Complex machines capable of generation

Power and Pumping the water in either directions are used.

A part of the energy produced is used for introducing the difference in the water levels

between the basin and the sea at any time of the tide and this is done by pumping water into the

basin up or down. The period of power production with this system is much longer than the other

two described earlier. The cycle of operation is shown in Fig

4. Double basin type

In this arrangement, the turbine is set up between the two basins as shown in Fig 5.5. one

basin is intermittently filled by the flood tide and other is intermittently drained by the ebb tide.

Therefore a small capacity but continuos power is made available with this system as shown in

Fig5.5. The main disadvantage of this system is that 50% of the Potential energy is sacrificed in

introducing the variation in the water levels of the two basins.

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5. Double basin with Pumping

In this case, off peak power from the base load plant in a interconnected transmission

system is used either to pump the water up the high basin. Net energy gain is possible with such

a system if the pumping head is lower than the basin-to-basin turbine generating head.

General Layout of Tidal Powerplant

Advantages

1. Exploitation of tidal energy will in no case make demand for large area of valuable land

because they are on bays.

2. It is free from pollution as it does not use any fuel.

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3. It is much superior to hydro-power plant as it is totally independent of rain which always

fluctuates year to year. Therefore, there is certainty of power supply a the tide cycle is very

definite.

4. As in every form of water power, this will also not produce any unhealthy waste like gases,

ash, atomic refuse which entails heavy removal costs.

5. Tidal Power is superior to conventional hydro power as the hydro plants are know for their

large seasonal and yearly fluctuations in the output of energy because they are entirely

dependent upon the nature's cycle of rainfall, which is not the case with tidal as monthly

certain power is assured. The tides are totally independent on nature's cycle of rainfall.

6. Another notable advantage of tidal power is that it has a unique capacity to meet the peak

power demand effectively when it works in combinatiion with thermal or hydroelectric

system.

7. It can provide better recreational facilities to visitors and holiday makers, in addition to the

possibility of fish forming in the tidal basins.

Disadvantages

1. These Power plants can be developed only if natural sites are available.

2. As the sites are available on the bay which will be always far away from the load centers. The

power generated must be transported to long distances. This increases the transportation cost.

3. The supply of power is not continuous as it depends upon the timing of tides. Therefore some

arrangements (double basin or double basin with pump storage) must be made to supply the

continuous power. This also further increases the capital cost of the plant.

4. The capital cost of the plant (Rs.5000/kw) is considerably large compared with conventional-

power plants (hydro, thermal)

5. Sedimentation and siltration of the basins are some of the added problems with tidal power

plants.

6. The navigation is obstructed.

7. It is interesting to note that the output of power from tidal power plant varies with lunar

cycle, because the moon largely influences the tidal rhythm, where as our daily power

requirement is directly related to solar cycle.

7.9 WIND POWER PLANTS

Winds are caused because of two factors.

(1) The absorption of solar energy on the earth's surface and in the atmosphere.

(2) The rotation of the earth about its axis and its motion around the sun.

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Because of these factors, alternate heating and cooling cycles occur, differences in

pressure are obtained, and the air is caused to move. The potential of Wind energy as a source of

power is large. This can be judged from the fact that energy available in the wind over the earth's

surface is estimated to be 1.6x107 K.W Besides the energy available is free and clean.

The problems associated with Utilizing wind energy are that:

(i) The energy is available in dilute form, because of this conversion

machines have to be necessarily large.

(ii) The availability of the energy varies considerably over a day and with the seasons.

For this reason some Means of storage have to be devised if a continuous supply of power

is required.

A wind mill converts the kinetic energy of moving air into mechanical energy that can be

either used directly to run the machine or to run the generator to Produce electricity.

CLASSIFICATION OF WIND MILLS

(a) Horizontal Wind mills

i. Horizontal Axis single blade Wind mills

If extremely long blades are mounted

on rigid hub. Large blade root bending

moments can occur due to tower shadow,

gravity and sudden shifts in wind directions

on a 200ft long blade. Fatigue load may be

enough to cause blade root failure.

To reduce rotor cost, use of single long

blade centrifugally balanced by a low cost

counter Weight as shown in fig. The

relatively simple rotor hub consists of a

Universal Joint between the rotor shaft and

blade allowing for blade. This type of hub

design contains fewer parts and costs less. Single blade wind mill

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https://sites.google.com/site/mubarakmes

(ii) Horizontal axis - two bladed wind mills

In this arrangement rotor drives

generator through a step-up gear box. The

components are mounted on a bed plate

which is mounted on a pinttle at the Top of

the tower. The two blade rotor is Usually

designed to be oriented down wind of the

tower. The arrangement of all the

Components used in horizontal axis wind

mill is shown in fig

When the machine is operating its

rotor blades are continuously flexed by

Unsteady aerodynamic, gravitational and

inertial loads.

Double blade wind mill

If the blades are metal, flexing reduces their fatigue life. The tower is also subjected to

unsteady load and dynamic interactions between the components of the machine-tower system

can cause serious damage.

Horizontal Axis - Multi bladed Wind Mills

This type of wheel have narrow rims

and Wire spokes. The wire spokes support light

weight aluminum blades. The rotors of this

design have high strength to weight ratios and

have been known to survive hours of free

wheeling operation in 100kmph winds. They

Fig Multi bladed wind mill have good power

Co-efficient, high starting torque ad added

advantage of simplicity and low cost.

(b) Vertical Wind Mills

Wind turbines mounted with the axis

of rotation in a vertical Position have

advantage that they are omni-directional that

is, they need not to be turned to Force the

wind. The Vertical mounted Wind Machines

eliminates the need for some of the complex

mechanical devices and control systems

necessary for horizontal mounted wind

Machines.

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Two types of vertical axis Wind Machines have received attention.

The Darrievs rotor consists of two or

three convex metal blades with an air foil

cross section, mounted on a Central shaft

which is supported by bearings at the top

and bottom. The rotor assembly is held in

position by guy wires running from the top

of the rotor to the ground.The savonius rotor

consists of a long solid's- shaped surface

mounted to turn at the center of 's' the

savonius rotor is self starting and has an

efficiency of about 31% while the Darrievs

rotor has a slightly higher efficiency of 35%

but is not self starting.

Fig (a) Savonius rotor

Advantages of Wind energy

(1) The wind energy is free, inexhaustible and does not need transportation.

(2) Wind mills will be highly desirable and economical to the rural areas which are far from

existing grids.

(3) Wind power can be used in combination with hydroelectric plants. Such that the water level

in the reservoir can be maintained for longer periods.

Disadvantage of Wind energy

(1) Wind power is not consistent and steady, which makes the complications in designing the

whole plant.

(2) The wind is a very hazard one. Special and costly designs and controls are always required.

(3) The cost factor, which has restricted the development of wind power in large scale for

feeding to the existing grid .

(4) It has low power coefficient.

(5) Careful survey is necessary for plant location.

7.10 GEOTHERMAL POWER

It has been estimated that the growing need for energy will exhaust all fossil fuels within

a few decades. More over the pollution hazards arising out of fossil fuel burning has become

quite significant in recent years. Also, the nuclear fuels have posed a number of problem Thus,

there a need for tapping new unconventional, sources of energy such as geothermal ocean

thermal, ocean tide, wind and sun. It is also hoped that these alternative energy sources will be

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able to meet considerable part of the energy sources will be able to meet considerable part of the

energy demand in coming future. Out of all these sources, geothermal energy has a great

potential and is already being commercially utilised in some of the developed countries.

According to various theories, 95 % of the earth is molten. Only a thin outer layer (rang-

ing from 15 to 150 km) is in the solid form. Under the earths crust lies the mantle, which is 2900

km thick . It is estimated that the temperature at the centre of the earth is around 3000°C while

the temperature towards the outer layer approaches to about 1200°C. As t crust of the earth is an

excellent insulator, it allows only very little heat to enter the surface But at some locations, there

is transfer of heat from the mantle to the shallower levels of the earth crust by some geothermal

fluid. From these heat sources geothermal systems developed.

The geothermal fluid is nothing but water containing dissolved minerals and salt, water

gets heated from the magmas and becomes less dense which creates a convection cell or system

under some covering rocks. This part of the system constitutes the reservoir. Leaks from the

reservoir to the surface are manifested by steam vents or hot springs.

The geothermal system utilising the geothermal fluid are, divided into two general classes:

1. Vapour dominated system 2. Liquid dominated system.

A vapour dominated system (also called dry steam system) liberates saturated to slightly

superheated steam, with temperatures around 250° C and pressures of 30 to 35 bars, from the

wells drilled in the geothermal reservoir. The reservoir is generally located in highly fractured

rocks, and the well flows may range from a few thousand to cover 250,000 kg of Italy, USA and

Japan. The first electric power generating station using the natural dry steam was built in Italy.

The arrangement of the components of the system used is shown in fig. a.

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The dry steam coming out of the

reservoir through the drilled wells is directly

fed to the turbines. These turbines are

coupled with electric generators for

generating electricity. The steam, after

expansion in the turbine are send to the

condenser. The condensate coming out of

the condenser is fed back to the hot reservoir

with the help of a pump.

A high enthalpy system is employed

in the geothermal zone where water with

highly dissolved solids (Brine solution) is

available. The temperature of the solution

available in such fields ranges from200 to

350°C. Wells drilled into mis type of

reservoir liberate the hot brine solution.

A typical arrangement used in Japan for power generation using a high enthalpy system is

shown in fig. b

Brine solution lat high temperature

coming. out from the hot well is flashed in a

flash chamber to produce steam. The steam

produced is then passed through the turbine

and then passed to the condenser. The

condensate from the condenser along with

the brine separated in the flash chamber is

again pumped back to the geothermal field.

Low enthalpy system is used in

places where the temperature of the

geothermal fluid is not sufficient to produce

the flash steam. Under this situation, the

heat in the geothermal fluid (hot brine) is

utilised in a closed system as shown in fig.

c. In this system a low boiling organic fluid

is used as the working fluid which is

continuously recirculated.

(a)

(b)

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The fluid gets heat from the geothermal fluid and vapourises. This vapour is passed

through the turbine for generating power. It is then condensed in a condenser and then

recirculated. The geothermal fluid, after giving up its heat to the working fluid, is fed back to the

geothermal field.

In addition to the geothermal reservoirs,

geothermal energy can also be found in the

form of the heat content of hot dry rock. In

order to utilise this form of energy heat will

have to be transferred to the surface by

means of artificially injected water through

specially constructed pathways in the rock.

The capital costs of discovery,

development and installation of geothermal

system have tendency to discourage their

use for power generation. The exceptions

are the areas where the geothermal resource

was found by chance. For the production of

electric power from geothermal source,

however, the costs are lower than those of

all other sources except hydel power.

7.11 OCEAN THERMAL ENERGY CONVERSION (OTEC OR OTE)

Ocean thermal energy conversion uses the difference between cooler deep and warmer

shallow waters to run a heat engine. As with any heat engine, greater efficiency and power

comes from larger temperature differences. This temperature difference generally increases with

decreasing latitude, i.e. near the equator, in the tropics. Historically, the main technical challenge

of OTEC was to generate significant amounts of power efficiently from small temperature ratios.

Modern designs allow performance approaching the theoretical maximum Carnot efficiency.

OTEC offers total available energy that is one or two orders of magnitude higher than

other ocean energy options such as wave power;[ but the small temperature difference makes

energy extraction comparatively difficult and expensive, due to low thermal efficiency. Earlier

OTEC systems were 1 to 3% efficiency, well below the theoretical maximum of between 6 and

7%.Current designs are expected closer to the maximum. The energy carrier, seawater. Expense

comes from the pumps and pump energy costs. OTEC plants can operate continuously as a base

load power generation system. Accurate cost-benefit analyses include these factors to assess

performance, efficiency, operational, construction costs, and returns on investment.

(c)

http://en.wikipedia.org/wiki/Heat_engine

http://en.wikipedia.org/wiki/Temperature

http://en.wikipedia.org/wiki/Equator

http://en.wikipedia.org/wiki/Tropics

http://en.wikipedia.org/wiki/Carnot_efficiency

http://en.wikipedia.org/wiki/Orders_of_magnitude

http://en.wikipedia.org/wiki/Wave_power

http://en.wikipedia.org/wiki/Thermal_efficiency

http://en.wikipedia.org/wiki/Seawater

http://en.wikipedia.org/wiki/Base_load



http://en.wikipedia.org/wiki/Cost-benefit_analysis

http://en.wikipedia.org/wiki/Returns_on_investment

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A heat engine is a thermodynamic device placed between a high temperature reservoir

and a low temperature reservoir. As heat flows from one to the other, the engine converts some

of the heat energy to work energy. This principle is used in steam turbines and internal

combustion engines, while refrigerators reverse the direction of flow of both the heat and work

energy. Rather than using heat energy from the burning of fuel, OTEC power draws on

temperature differences caused by the sun's warming of the ocean surface. Much of the energy

used by humans passes through a heat engine.

The only heat cycle suitable for OTEC is the Rankine cycle using a low-pressure turbine.

Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that

are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use the

water heat source as the working fluid.

Cycle types

Cold seawater is an integral part of each of the three types of OTEC systems: closed-

cycle, open-cycle, and hybrid. To operate, the cold seawater must be brought to the surface. The

primary approaches are active pumping and desalination. Desalinating seawater near the sea

floor lowers its density, which causes it to rise to the surface.

The alternative to costly pipes to bring condensing cold water to the surface is to pump

vaporized low boiling point fluid into the depths to be condensed, thus reducing pumping

volumes and reducing technical and environmental problems and lowering costs.[

1. Closed

Diagram of a closed cycle OTEC plant

http://en.wikipedia.org/wiki/Thermodynamic

http://en.wikipedia.org/wiki/Work_%28thermodynamics%29

http://en.wikipedia.org/wiki/Steam_turbine

http://en.wikipedia.org/wiki/Internal_combustion_engine



http://en.wikipedia.org/wiki/Refrigerator

http://en.wikipedia.org/wiki/Rankine_cycle

http://en.wikipedia.org/wiki/Ammonia

http://en.wikipedia.org/wiki/R-134a

http://en.wikipedia.org/wiki/File:OTEC_diagram.gif

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Closed-cycle systems use fluid with a low boiling point, such as ammonia, to power a

turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger to

vaporize the fluid. The expanding vapor turns the turbo-generator. Cold water, pumped through a

second heat exchanger, condenses the vapor into a liquid, which is then recycled through the

system.

2. Open

Open-cycle OTEC uses warm surface water to make electricity. Placing warm seawater

in a low-pressure container causes it to boil. The expanding steam drives a low-pressure turbine

attached to an electrical generator. The steam, which left its salt and other contaminants in the

low-pressure container, is pure fresh water. It is condensed into a liquid by exposure to cold

temperatures from deep-ocean water. This method produces desalinized fresh water, suitable for

drinking water or irrigation.

3. Hybrid

A hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid,

warm seawater enters a vacuum chamber and is flash-evaporated, similar to the open-cycle

evaporation process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on

the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to produce

electricity. The steam condenses within the heat exchanger and provides desalinated water.


http://en.wikipedia.org/wiki/Turbine

http://en.wikipedia.org/wiki/Seawater

http://en.wikipedia.org/wiki/Heat_exchanger

http://en.wikipedia.org/wiki/Steam

http://en.wikipedia.org/wiki/Electrical_generator

http://en.wikipedia.org/wiki/Salt

http://en.wikipedia.org/wiki/Fresh_water

http://en.wikipedia.org/wiki/Desalinization

http://en.wikipedia.org/wiki/Drinking_water

http://en.wikipedia.org/wiki/Irrigation



http://en.wikipedia.org/wiki/Desalination

Hvac Module 1

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