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ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY
B.Tech (Food Technology)
Course No.: FDEN 123 Credit Hours: 3 (2+1)
Energy Generation and
Conservation
STUDY MATERIAL
Prepared by
Dr. S.Kaleemullah College of Food Science and Technology
Pulivendula - 516 390 and
Er.M.Sardar Baig College of Food Science and Technology
Bapatla – 522101
Department of Food Engineering
Course No : FDEN -123
Title : Energy Generation and Conservation
Credit hours : 3 (2+1)
Theory Lecture Outlines
L.No. Lecture outline P.No.
1 Thermodynamics-Definition - Thermodynamic system - Classification of
Thermodynamic systems - Properties of a system - Classification of properties
of a system - State of a system - Path of change of state – Thermodynamic
process - Thermodynamic cycle or Cyclic process.
5
2 Energy -Type of stored energy - Law of conservation of energy – Heat -
Specific heat -Thermal or heat capacity - water equivalent - Mechanical
equivalent of heat
9
3 Laws of Thermodynamics - Zeroth law of Thermodynamics - First law of
Thermodynamics - Second law of Thermodynamics - Refrigerator and heat
pump - Coefficient of performance
12
4 Ideal gas - Equation of state - General laws for expansion and compression -
Enthalpy of gas
19
5 Second law of Thermodynamic - Absolute thermodynamic scale - Carnot cycle
- Entropy - Reversibility and Reversibility
24
6 Renewable energy sources – Solar energy utilization – Biogas – Anaerobic
digestion of organic matter – Application of biogas
29
7 Types of biogas plants – Floating dome type – Fixed dome type – KVIC model,
Janata model – Deendandhu model – Running the plant – Maintenance of
biogas plant
33
8 Wind energy – Application of wind mill – Advantages and disadvantages of
wind energy – Types of wind mills – Savonius – Darrieus wind mill – Single
blade wind mill – Two blade wind mill – Multi blade wind mill – sail type wind
mill
38
9 Solar energy – Advantages and disadvantages – Flat plate collector -
Concentrating collector – Cylindrical parabolic concentrating collector –
Paraboloid concentrating collector
44
3
10 Natural Circulation water heating system – Forced circulation water system -
space heating – Space cooling and refrigeration – Drying – Cabinet dryer –
Convective dryer – Cooking – Solar distillation
48
11 Fuels – Introduction – Classification of fuels – Solids fuels – Liquids fuels -
Merits and demerits of liquid fuels over solid fuels
55
12 Gaseous fuel – Merits and demerits of gaseous fuels – Requirements of a good
fuel – Calorific value of fuels – Gross or higher calorific value – Net or lower
calorific value
58
13 Experimental Determination of Higher Calorific value - Bomb Calorimeter -
Boy’s Gas Calorimeter – problems on calorific values
62
14 Combustion of fuels - introduction - Combustion equations of solids fuels -
Combustion equation of gaseous fuel
68
15 Theoretical or minimum mass of air required for complete combustion -
Theoretical or minimum volume of air required for complete combustion –
Problems on theoretical mass or volume of air required for combustion
73
16 Conversion of volumetric analysis into mass analysis or gravimetric analysis -
Conversion of mass analysis into volumetric analysis - Mass of carbon in flue
gases – Mass of flue gases per kg of fuel burnt
76
17 Excess air supplied – Mass of excess air supplied – Flue gas analysis by Orsat
apparatus
78
18 Burners – Pulverized fuel burners – Long flame burners – Turbulent burners –
Tangential burners – Cyclone burners - Advantages
80
19 Oil burners – principles of oil firing – Classification of oil burners – Vaporising
oil burners – Atomizing fuel burners
85
20 Gas burners - Advantages 89
21 Formation and properties of steam – introduction – Temperature vs. total heat
graph during steam formation – wet steam – Dry saturated steam – Super
heated steam – Dryness fraction – Sensible heat of water – Latent heat of
vaporization – Enthalpy of stem – Specific volume of steam
90
22 Steam tables and their uses – Super heated steam – Problems on enthalpy of
steam – Advantage of super heating the steam
96
23 Steam Boiler – introduction – important term for steam boiler - Essential of a
good steam boiler – steam boiler – Selection of a steam boiler – Classification
99
4
of steam boilers
24 Simple vertical boiler – Cochran boiler – Scotch marine boiler – Lancashire
boiler – Cornish boiler
104
25 Locomotive boiler – Babcock and Wilcox boiler – comparison between water
tube and fire tube boilers
109
26 Boiler mounting viz., Water level indicator, Pressure gauge, Safety, Stop valve,
Blow off cock, Feed check valve, Fusible plug – their function. Boiler
accessories viz., feed pump, super heater, economizer, air – pre heater -
function
113
27 Performance of steam boilers – Equivalent evaporation – Boiler efficiency –
Problems on boiler efficiency
125
28 Boiler trial – Heat losses in a boiler – Heat balance sheet 128
29 Boiler draught – Classification of draughts – Types of draught – Advantages
and disadvantages of mechanical draught – Compression between forced
draught and induced draught – Height of chimney – Problems on draught of
chimney
130
30 Condenser – Advantages – Requirements of a steam condensing plant –
Classification of condensers
136
31 Indian Boiler’s Act – 1923 – insulation of steam piping - Pipe expansion bends 138
32 Compressors – Classification of air compressor – Working of single stage
reciprocating air compressor – Two stage reciprocating air compressor with
intercooler
141
5
Lecture No. 1
Thermodynamics:
The field of science, which deals with the energies possessed by gases and
vapor, is known as Thermodynamic.
Thermodynamics system:
Thermodynamic system may be broadly defined as a definite area or a space
where some thermodynamic process is taking place. The figure shows that a
thermodynamic process is taking place. The Fig.1 shows that a thermodynamic system
has its boundaries and any thing outside the boundaries is called its surrounding.
Fig. 1. Thermodynamic system.
Classification of Thermodynamic system:
The Thermodynamic system may be classified into three groups:
1. Closed system
2. Open system
3. Isolated system
Closed system:
This is a system of fixed mass and identity whose boundaries are determined by
the space of the matter (working system) occupied in it. Therefore, a closed system
(Fig. 2) does not permit any mass transfer across its boundary, but it permit transfer of
energy (work and heat).
Thermodynamics-Definition - Thermodynamic system - Classification of Thermodynamic
systems - Properties of a system - Classification of properties of a system - State of a system
- Path of change of state – Thermodynamic process - Thermodynamic cycle or Cyclic
process.
6
Fig. 2. Closed system.
Open system:
In open system (Fig.3), the mass of the working substance crosses the boundary
of the system. Thus an open system permits both mass and energy (heat and work)
transfer across the boundaries and the mass within the system may not be constant.
Fig. 3. Open system.
Isolated System:
A system which is completely influenced by surroundings is called an isolated
system. It is a system of fixed mass and no heat or work energy cross its boundary. In
other words, an isolated system does not have transfer of either mass or energy (heat or
work) with the surrounding. An open system with its surroundings (known as universe) is
an example of an isolated system.
Properties of a system:
The state of a system may be identified or described by certain observable
quantities such as volume, temperature, pressure and density etc. All the quantities,
which identify the state of a system, all called as properties of a system.
7
Classification of properties of a system:
The thermodynamic property of a system is divided into two general classes.
1. Extensive properties
2. Intensive properties
Extensive properties:
A quantity of a matter in a given system is divided, notionally into a numbers of
parts. The properties of the system, whose value for the entire system is equal to the
sum of their values for the individual parts of the system, are called extensive properties.
e.g total volume, total mass and total energy of a system or its extensive properties.
Intensive properties:
It may be noticed that the temperature of a system is not equal to the sum of
temperatures of its individual parts. It is also true for pressure and density of a system.
Thus properties like temperature, pressure and density are called Intensive properties.
The specific volume is an intensive property.
State of a system:
The state of a system (when the system is in thermodynamic equilibrium) is the
condition of the system at any particular moment which can be identified by the
statement of its properties, such as pressure, volume, temperature etc. The initial and
final states, on the pressure-volume diagram are shown in Fig.4.
Fig. 4. State of a system.
8
Path of change of state:
When a system passes through the continuous series of equilibrium state during
a change of state (from the initial state to final state), then it is known as path of change
of state. When the path is completely specified, it is known as path of the process.
Thermo dynamic process:
When the system changes its state from one equilibrium state to another
equilibrium state, then the path of successive state through which the system has
passed is known as thermodynamic process, In the Fig. 4, 1-2 represents a
thermodynamic process.
Thermodynamic cycle or cyclic process:
When the process or processes are performed on a system in such a way that
the final state is identical with the initial state, it is then known as a thermodynamic cycle
or cyclic process. In the Fig. 5, 1-A-2 and 2-B-1 are processes where 1-A-2-B-1 is a
thermodynamic cycle or cyclic process.
Fig. 5. Thermodynamic process or cyclic process.
9
Lecture No. 2
Energy:
The energy is defined as the capacity to do work. In others words a system is
said to posses energy when it is capable of doing work. The possessed by a system is of
the following two types:
1. Stored energy, and
2. Transit energy
Stored energy:
Stored energy is the energy possessed by a system within its boundaries. The
potential energy, kinetic energy and internal energy are the example of stored energy.
Transit energy:
Transit energy is the energy possessed by a system which is capable of crossing
its boundaries. The heat, work and electrical energy are the example of transit energy.
Types of stored energy
1. Potential energy:
It is the energy possessed by a body above the ground level. For example, a
body raised to some height above ground level possesses potential energy because it
can do some work by failing on the surface.
Let W = Weight of the body,
m = Mass of the body
g = Acceleration due to gravity = 9.81 m/s2
Therefore potential energy,
PE = Wz = m g z
It may be noted that potential energy will be in N-m.
Energy -Type of stored energy - Law of conservation of energy – Heat - Specific heat -
Thermal or heat capacity - water equivalent - Mechanical equivalent of heat
10
2. Kinetic energy:
It is the energy possessed by a body or a system for doing work by virtue of its
mass and velocity of motion.
Let m = Mass of the body and
V = Velocity of the body
Where m is in kg and v is in m/s, then kinetic energy will be in N-m
KE = 1/2 mv2
3. Internal energy:
It is the energy possessed by a body or a system due to its molecular
arrangement and motion of the molecules. It usually represented by U.
E (energy) = PE + KE + U
Law of conservation of energy:
It states, “The energy can neither be created nor destroyed, through it can be
transformed from one form to any other form, in which the energy can exist”.
Heat:
The heat is defined as the energy transferred, without transfer of mass, across
the boundary of a system because of a temperature difference between the system and
the surroundings. It is usually represented by Q and is expressed in joule (J) or
kilo-joule (kJ). The heat can be transferred in the three distinct ways, i.e. condition,
convection and radiation. All these modes occur across the surface area of a system
because of a temperature difference between the system and the surroundings.
Specific Heat:
The specific heat of a substance may be broadly defined as the amount of heat
required to raise the temperature of a unit mass of any substance through one degree. It
is generally denoted by c. The S.I unit of Specific heat (c) is kJ/kg K.
11
Thermal or heat capacity:
The thermal or heat capacity of a substance may be defined as the heat required
to raise the temperature of whole mass of a substance through one degree.
Mathematically.
Thermal or heat capacity = mc kJ
Where m = Mass of the substance in k
c = Specific heat of the substance in kJ/kg K
Water Equivalent:
The water equivalent of a substance may be defined as the quantity of water,
which requires the same quantity of heat as the substance to raise its temperature
through one degree. Mathematically,
Water equivalent of substance m= mc kg
m = Mass of the substance in kg, and
c = Specific heat of the substance in kJ/kg K
Mechanical equivalent of heat:
It was established by joule that heat and mechanical energies are mutually
convertible. He established, experimentally, that there is a numerical relation between
the unit of work. This relation is denoted by “J” and is known as Joule’s equivalent or
mechanical equivalent of heat.
12
Lecture No. 3
Laws of Thermodynamics:
The following three laws of thermodynamics are
1. Zeroth law of thermodynamics,
2. First law of thermodynamics, and
3. Second law of thermodynamics.
Zeroth Law of Thermodynamics:
This law states, "When two systems are each in thermal equilibrium with a third
system, then the two systems are also in thermal equilibrium with one another." This law
provides the basis of temperature measurement.
First Law of Thermodynamics:
This law may be stated as follows:
(a) "The heat and mechanical work are mutually convertible". According to
this law, when a closed system undergoes a thermodynamic cycle, the net heat transfer
is equal to the net work transfer. In other words, the cyclic integral of heat transfers is
equal to the cyclic integral of work transfers. Mathematically,
WQ∫ ∫= δδ
Where symbol ∫ stands for cyclic integral (integral around a complete cycle),
and δ Q and Wδ represent infinitesimal elements of heat and work transfers
respectively. It may be noted that δ Q and δ W are expressed in same units.
(b) “The energy can neither be created nor destroyed though it can be
transformed from one form to another”. According to this law, when a system
undergoes a change of state (or a thermodynamic process), then both heat transfer and
work transfer takes place. The net energy transfer is stored within the system and is
known as stored energy or total energy of the system. Mathematically
δ Q-δ W = dE
Laws of Thermodynamics - Zeroth law of Thermodynamics - First law of Thermodynamics -
Second law of Thermodynamics - Refrigerator and heat pump - Coefficient of performance
13
The symbolδ is used for a quantity which is inexact differential and symbol d is
used for a quantity which is an exact differential. The quantity E is an extensive property
and represents the total energy of the system at a particular state.
On integrating the above expression for a change of state from 1 to 2, we have
Q1-2 – W1-2 = E2 – E1 K (Q, W and E are in same units)
For a unit mass, this expression is written as
q1-2 – w1-2 = e2 - el
Q1-2 = Heat transferred to the system during the process from state 1 to state 2,
W1-2 = Work done by the system on the surroundings during the process, and
E1 = Total energy of the system at state 1
= PE1+ KE1 + U1 = m g z1+2
2
1Vm +U1
E2 = Total energy of the system at state 2
= PE2+KE2+U2 = m g z2+2
2
2Vm +U2
Thus the above expression may be written as
Q1-2 – W1-2 = E2 – E1 ... (1)
= (PE2 + KE2 + U2) - (PEl + KE1 + U1)
= (PE2 - PEl) + (KE2 – KE1) + (U2 – U1) ... (2)
= m (g z2,- g z1) + m
−
22
2
1
2
2 VV+(U2- U1)
For unit mass, this expression is written as
q1-2 – w1-2 = (g z2 – g z1) +
−22
2
1
2
2 VV+ (u2 – u1)
Notes:
1. When there is no change in potential energy of the system (i.e. when the
height of the system from the datum level is same), then PEl = PE2. Thus, the
above equation (2) is written as
Q1-2 – W1-2 = (KE2 – KE1) + (U2 - U1) ... (3)
14
2. When there is no change of PE and also there is no flow of the mass into or
out of the system, then PEl = PE2 and KE1 = KE2. Thus, the above equation
(2) is written as
Q1-2–W1-2 = U2 – U1 = dU ... (4)
In other words, in a closed or non-flow thermodynamic system,
PE = 0 and KE = 0
Thus the equation (iv) is known as Non-flow energy equation.
3. For an isolated system for which Q1-2 = W1-2 = 0, the above equation (1)
becomes
E2 = E1
This shows that the first law of thermodynamics is the law of conservation of energy.
Limitations of First Law of Thermodynamics
1. When a closed system undergoes a thermodynamic cycle, the net heat
transfer is equal to the net work transfer. This statement does not specify the
direction of flow of heat and work (i.e. whether the heat flows from a hot body to a cold
body or from a cold body to a hot body). It also does not give any condition under which
these transfers take place.
2. The heat energy and mechanical work are mutually convertible. Though
the mechanical work can be fully converted into heat energy, but only a part of heat
energy can be converted into mechanical work. This means that the heat energy and
mechanical work are not fully mutually convertible. In other words, there is a limitation on
the conversion of one form of energy into another form.
A machine which violates the first law of thermodynamics (i.e. energy can neither
be created nor destroyed, but can be trans- formed from one form to another) is known
as perpetual motion machine of the first kind (briefly written as PMM-I). It is defined as a
machine which produces work energy without consuming an equivalent of energy from
other source. Such a machine, as shown in Fig. 6, is impossible to obtain in actual
practice, because no machine can produce energy of its own without consuming any
other form of energy.
15
Fig. 6. Perpetual motion machine of the first kind.
Second Law of Thermodynamics: The second law of thermodynamics may be defined
in many ways, but the two common statements according to Kelvin - Planck and
Clausius are as follows:
Kelvin - Planck Statement:
According to Kelvin-Planck 'It is impossible to construct an engine working on a
cyclic process, whose sole purpose is to convert heat energy from a single thermal
reservoir into an equivalent amount of work'. In other words, no actual heat engine,
working on a cyclic process, can convert whole of the heat supplied to it, into mechanical
work. It means that there is a degradation of energy in the process of producing
mechanical work from the heat supplied.
(a) Perpetual motion machine of the (b) Heat engine second Kind Impossible
Fig. 7.
Thus Kelvin - Planck statement of the second law of thermodynamics is called as
law of degradation of energy. A heat engine which violates this statement of the second
law of thermodynamics (i.e. a heat engine which converts whole of the heat energy into
mechanical work) is known as perpetual motion machine of the second kind (PMM-II) or
100 percent efficient machine which is impossible to obtain in actual practice, because
no machine can convert whole of the heat energy supplied to it, into its equivalent
amount of work.
16
Thus for the satisfactory operation of a heat engine which is a device used for
converting heat energy into mechanical work, there should be at-least two reservoirs of
heat, one at a higher temperature and the other at a lower temperature, as shown in
Fig. 7 (b) In this case, consider that heat energy (Q1) from the high temperature reservoir
(or source) at temperature T1 is supplied to the engine. A part of this heat energy is
rejected to the low temperature reservoir (or sink) at temperature T2. If Q2 is the heat
rejected to the sink, then the remaining heat (i.e. Q1 - Q2) is converted into mechanical
work. The ratio of the maximum mechanical work obtained to the total heat supplied to
the engine is known as maximum thermal efficiency (η max) of the engine.
Mathematically,
1
2
1
2
1
21max 11
sup T
T
Q
Q
Q
pliedheatTotal
obtainedworkMaximum−=−=
−==η
Thus for the satisfactory operation of a heat engine which is a device used for
converting heat energy into mechanical work, there should be at-least two reservoirs of
heat, one at a higher temperature and the other at a lower temperature, as shown in
Fig. 7 (b) In this case, consider that heat energy (Q1) from the high temperature reservoir
(or source) at temperature T1 is supplied to the engine. A part of this heat energy is
rejected to the low temperature reservoir (or sink) at temperature T2. If Q2 is the heat
rejected to the sink, then the remaining heat (i.e. Q1 - Q2) is converted into mechanical
work. The ratio of the maximum mechanical work obtained to the total heat supplied to
the engine is known as maximum thermal efficiency (η max) of the engine.
Mathematically,
1
2
1
2
1
21max 11
sup T
T
Q
Q
Q
pliedheatTotal
obtainedworkMaximum−=−=
−==η
Note: For a reversible engine, Q1/T1 = Q2/T2.
1. A thermal reservoir is a body of infinite heat capacity which is capable of
absorbing or rejecting an unlimited quantity of heat without affecting its
temperature.
2. A perpetual motion machine of the second kind (PMM-II) does not violate the
first law of thermodynamics as such a machine would not create or destroy
energy.
17
3. In a heat engine, the reservoir (or body) at a higher temperature is known as a
source and the reservoir at a lower temperature is called a sink.
Clausius Statement:
According to Clausius statement "It is impossible for a self acting machine,
working in a cyclic process, to transfer heat from a body at a lower temperature to a
body at a higher temperature without the aid of an external agency." In other words, heat
cannot flow itself from a cold body to a hot body without the help of an external agency
(i.e. without the expenditure of mechanical work).
The device (such as a refrigerator or a heat pump), as shown in Fig. 8 (a)
violates the Clausius statement because no input work is supplied to the device to
transfer heat from a cold body to a hot body. Such a device is called perpetual motion
machine of the second kind. In order to achieve the object of transferring heat from a
cold body to a hot body, the refrigerator and a heat pump, while operating in a cyclic
process, require an input work, as shown in Fig. 8 (b) and (c) respectively.
(a) Perpetual motion machine (b) Refrigerator (c) Heat pump of the second Kind
Fig. 8.
Though there is no difference between the cycle of operations of the refrigerator
and a heat pump and achieve the same overall objective, but the basic purpose of each
is quite different. A refrigerator is a device which operating in a cyclic process, maintains
the temperature of a cold body (refrigerated space) at a temperature lower than the
temperature of the surroundings. On the other hand, a heat pump is a device which
operating in a cyclic process, maintains the temperature of a hot body at a temperature
18
higher than the temperature of surroundings. In other words, a refrigerator works
between the cold body temperature and the atmospheric temperature whereas a heat
pump operates between the hot body temperature and the atmospheric temperature.
The performance of refrigerator and heat pump is measured in terms of
coefficient of performance which is defined as the ratio of the maximum heat transferred
(i.e. heat taken from the cold body) to the amount of work required to produce the
desired effect. Mathematically, maximum coefficient of performance for a refrigerator
is
(C. O. P)R =21
2
21
22
TT
T
Q
W
Q
R −=
−=
and maximum coefficient of performance for heat pump is
(C. O. P)P = 121
2
21
1
21
11 +−
=−
=−
=TT
T
TT
T
Q
W
Q
P
= (C. O. P)R + 1
The C.O.P of a heat pump is greater than C.O.P of a refrigerator by unity.
1. In case of a refrigerator, the atmosphere acts as a hot body while in case of
a heat pump. The atmosphere acts as a cold body.
2. The performance of a heat engine is measured in terms of thermal
efficiency.
19
Lecture No. 4
Ideal gas (Perfect gas):
“Ideal gas is defined as a state of a substance, whose evaporation from its liquid
state is complete, and strictly obeys all the gas laws under all conditions of temperature
and pressure”. In actual practice there is no real or actual gas which strictly obeys the
gas law over the entire range of temperature and pressure.
Laws of Perfect Gases:
The physical properties of a gas are controlled by the following three variables:
1. Pressure exerted by the gas
2. Volume occupied by the gas and
3. Temperature of gas
The behavior of a perfect gas, undergoing any change in the above mentioned
variable, is governed by the following laws.
Boyle’s Law
Charles’ Law
Gay-Lassac Law
Boyle’s law:
This law was formulated by Robert Boyle in 1662. It states,
“The absolute pressure of a given mass of a perfect gas varies inversely as its
volume, when the temperature remains constant”. Mathematically
vp
1α
or pv tcons tan=
p1v1 = p2v2 = p3v3 = 4. = constant
Charles’ law:
This law was formulated by a Frenchman Jacques A.C. in 1787. It states,
(1) “The volume of a given mass of a perfect gas varies directly as its absolute
temperature, when the absolute pressure remains constant”. Mathematically
Ideal gas - Equation of state - General laws for expansion and compression - Enthalpy of gas
20
Tvα tconsT
vor tan=
tconsT
v
T
v
T
vor tan.....
3
3
2
2
1
1 ====
(2) “All perfect gases change in volume by of its original volume at 0oC for
every 1oC change in temperature, when the pressure remains constant”.
Let vo = volume of a given mass at 0oC, and
vt = Volume of the same mass of gas at toC
Then, according to the above statement,
tconsT
Tv
tvtvvvt tan.....]
273
1[
273
1
0
0000 ==×=+
=+=
0
0
T
v
T
vor t =
Where T = Absolute temperature corresponding to toC
T0 = Absolute temperature corresponding to 0oC
Gay-Lassac law:
“The absolute pressure of a given mass of a perfect gas varies directly as its
absolute temperature, when the volume remains constant”. Mathematically.
tconsT
pTp tan==α
tconsT
p
T
p
T
ptan....
3
3
2
2
1
1 =====
General Gas Equation:
In actual practice, all the three variable i.e., pressure, volume and temperature,
change simultaneously. In order to deal with all practical cases, the Boyles’s law and
Charles’ law are combined together, which gives us a general gas equation.
According to Boyle’s law
21
vp
1α
pv
1α= (T constant)
and according to Charles’ law
Tvα (P constant)
pv
1α and T both or
pv
1α
Tpvα= or TCpv =
where C is a constant, whose value depends upon the mass and properties of
the gas concerned.
===3
33
2
22
1
11
T
vp
T
vp
T
vpK = (constant)
Characteristics Equation of a gas:
It is a modified form of general equation. If the volume (v) in the general gas
equation is taken as that of 1 kg gas (known as its specific volume, and denoted by vs)
then the constant C (in the general gas equation is represented by another constant R
(in the characteristic equation of gas).
Thus the general gas equation may be rewritten as
pvs = RT
where R is known as characteristics gas constant or simply gas constant.
For any mass m kg of a gas, the characteristics gas equation becomes:
m p vS = m R T
pv = m R T )...( vmvs =Q
Note:
1. The unit of gas constant (R) may be obtained as
KkgJKkgmNKkg
mN
Kkg
mmN
mT
pvR //
/ 32
=−=×−
=××
==
)11...( JmN =−
2. Its value for atmospheric air taken 287 J/kg K or 0.287 kJ/kg K
22
3. The equation pv = m R T may also be expressed in another form
i.e RTRTv
mp ρ==
)...( ρ=
v
mQ
Where ρ (rho) is density of the given gas.
Enthalpy of a gas:
In the thermodynamic, one of the basic quantities most frequently recurring is the
sum of the internal energy (U) and the product of pressure and volume (pv). The sum
(U+pv) is termed as enthalpy and is written as H. Mathematically
Enthalpy, H = U + pv
Since (U + pv) is made up entirely of properties, therefore enthalpy is also a
property.
For a unit mass, specific enthalpy,
h = u + pvS
Where u = Specific internal energy, and
vS = specific volume
Note: Q1-2 = dU + W1-2 = dU+ pdv
When gas is heated at constant pressure from an initial condition 1 to a final
condition 2, then change in internal energy.
dU = U2 - U1
and workdone by the gas,
W1-2 = p dv = p (V2 - V1)
Q1-2 = (U2 - U1) + p (V2 - V1)
= (U2 + pv2) - (U1 + pv1) = H2 - H1
and for a unit mass, q1-2= h1 - h2
Thus, for a constant pressure process, the heat supplied to the gas is equal to
the change of enthalpy.
General laws for expansion and compression:
The general law of expansion or compression of a perfect gas is PVn = Constant.
The general law for the expansion and compression of gases, is also known as
23
polytropic process, It gives the relationship between pressure and volume of a given
quantity of gas. Where n is a polytropic index. The value of n depends upon the nature
of gas, and condition under which the changes (i.e. expansion or compression) take
place. The value of n may be between Zero and infinity. But the following values of n are
important:
1. When n=0. The mean PV0 = Constant, i.e P = constant. In other words, for the
expansion or compression of a perfect gas at constant pressure, n=0.
2. When n=1; the PV = constant, i.e., the expansion or compression is isothermal
or hyperbolic.
3. When n lies between 1 and n, the expansion or compression is polytropic, i.e
PVn = constant.
4. When n =γ , the expression or compression is adiabatic, i.e PVγ = Constant.
5. When n=∞, the expression or compression is at constant volume i.e
V = constant.
Fig.9. Curves for various values of n
Fig.9 shows the curve of expansion of a perfect gas for different values of n. It is
obvious that greater the value of n, steeper the curve of expansion
24
Lecture No. 5
Second law of Thermodynamic:
The second law of thermodynamics may be defined in many ways, but the two
common statements according to Kelvin - Planck and Clausius are as follows:
Kelvin - Planck Statement:
According to Kelvin-Planck ‘‘it is impossible to construct an engine working on a
cyclic process, whose sole purpose is to convert heat energy from a single thermal
reservoir into an equivalent amount of work'’.
Clausius Statement:
According to Clausius statement "It is impossible for a self acting machine,
working in a cyclic process, to transfer heat from a body at a lower temperature to a
body at a higher temperature without the aid of an external agency".
Reversibility and irreversibility:
If the process is assumed to take place sufficiently slowly so that the deviation of
the properties at the intermediate states is infinitesimally small, then every state passed
through by the system will be in equilibrium. Such a process is called quasi-static or
reversible process and it is represented by a continuous curve on the property diagram
(i.e. Pressure-volume diagram) as shown in Fig.10 (a).
(a) (b)
Fig. 10. Reversibility and irreversibility
Second law of Thermodynamic - Absolute thermodynamic scale - Carnot cycle - Entropy -
Reversibility and Reversibility
25
If the process take place in such a manner that the properties at the intermediate
state are not in equilibrium state (except the initial and final state), then the process is
said to be non-equilibrium or irreversible process. This process is represented by the
broken lines on the property diagram as shown in Fig. 10 (b).
Absolute thermodynamic temperature scale:
The temperature, below which the temperature of any substance can not fall, is
known as absolute zero temperature. The absolute zero temperature, for all sorts of
calculation, is taken as -273oC in case of Celsius scale and -460oF in case of Fahrenheit
scale. The temperature in Celsius scale is called degree Kelvin (K); Such that
K = oC+273. Similarly, absolute temperature in Fahrenheit scale is called degree
Rankine (oR); such that oR = oF+460.
Entropy:
The term ‘entropy’ which literally means transformation, was first introduce by
Clausius. It is an important thermodynamic property of a working substance, which
increases with the addition of heat, and decrease with its removal.
Fig. 11. Temperature entropy diagram
The entropy is usually denoted by S (Fig. 11) and the unit of entropy is kJ/K. But
change in entropy of the working substance defined as, in a reversible process, over a
small range of temperature, the increase or decrease of entropy, when multiplied by
absolute temperature, gives the heat absorbed or rejected by the working substance.
Mathematically, heat absorbed by the working substance,
Q = TdS
where T= absolute temperature, and
dS = Increase in entropy
26
Types of Thermodynamic Cycles:
Though there are many types of thermodynamic cycles, yet the following are
important from the subject point of view:
1. Carnot cycle, 2. Stirling cycle, 3. Ericsson cycle, 4. Joule cycle, 5.Otto cycle, 6. Diesel
cycle, and 7. Dual combustion cycle.
Carnot Cycle:
This cycle was devised by Carnot, who was the first scientist to analyse the
problem of the efficiency of a heat engine, disregarding its mechanical details. He
focussed his attention on the basic features of a heat engine. In a Carnot cycle, the
working substance is subjected to a cyclic operation consisting of two isothermal and
two reversible adiabatic or isentropic operations. The p-v and T-S diagrams of this cycle
are shown in Fig.12 (a) and (b).
Fig. 12. Carnot cycle.
The engine imagined by Carnot has air (which is supposed to behave like a
perfect gas) as its working substance enclosed in a cylinder, in which a frictionless
piston A moves. The walls of the cylinder and piston are perfect non-conductor of heat.
However, the bottom B of the cylinder can be covered, at will, by an insulating cap (I.C.).
The engine is assumed to work between two sources of infinite heat capacity, one at a
higher temperature and the other at a lower temperature.
27
Now, let us consider the four stages of the Carnot's cycle. Let the engine cylinder
contain m kg of air at its original condition represented by point 1 on the p-v and T-S
diagrams. At this point, let P1, T1, and v1 be the pressure, temperature and volume of the
air, respectively.
First stage (Isothermal expansion). The source (hot body, H.B.) at a higher
temperature is brought in contact with the bottom B of the cylinder. The air expands,
practically at constant temperature T1, from v1 to v2. It means that the temperature T2 at
point 2 is equal to the temperature T1. This isothermal expansion is represented by curve
1-2 on p-v and T-S diagrams in Fig.12 (a) and (b). It may be noted that the heat supplied
by the hot body is fully absorbed by the air, and is utilized in doing external work.
Heat supplied = *Work done by the air during isothermal expansion
Q1-2 = p1,v1, loge.
1
2
v
v= m R T1, loge,
1
2
v
v
K (QP1v1 = mRT1)
= 2.3 m R T1 log r
r = Expansion ratio = v2/v1.
Second stage (Reversible adiabatic or isentropic expansion). The hot body is
removed from the bottom of the cylinder B and the insulating cap I.C. is brought in
contact. The air is now allowed to expand reversibly and adiabatically. Thus the
reversible adiabatic expansion is represented by the curve 2-3 on p-v and T-S diagrams.
The temperature of the air falls from T2 to T3. Since no heat is absorbed or rejected by
the air, therefore
Decrease in internal energy = Work done by the air during adiabatic expansion
11
323322
−−
=−−
=γγ
mRTmRTvpvp ... (Qpv = mRT)
( )
1
31
−−
=γ
TTmR ... (QT1 = T2)
Third stage (Isothermal compression. Now remove the insulating cap I.C. from the
bottom of the cylinder and bring the cold body C.B. in its contact. The air is compressed
practically at a constant temperature T3 from v3 to v4. It means that the temperature T4
(at point 4) is equal .to the temperature T3. This isothermal compression is represented
28
by the curve 3-4 on p-v and T-S diagrams. It would be seen that during this process, the
heat is rejected to the cold body and is equal to the work done on the air.
∴Heat rejected = Work done on the air during isothermal compression
Q3-4 = P3 V3 loge
4
3
v
v = m R T3 loge
4
3
v
v
... (Qpv = mRT)
= 2.3 m R T3 log r
r = *Compression ratio = v3/v4
Fourth stage (Reversible adiabatic or isentropic compression). Now again the
insulated cap I.C. is brought in contact with the bottom of the cylinder B, and the air is
allowed to be compressed reversibly and adiabatically. The reversible adiabatic
compression is represented by the curve 4-1 on p-v and T-S diagrams. The temperature
of the air increases from T4 to T1. Since no heat is absorbed or rejected by the air,
therefore
Increase in internal energy = Work done on the air during adiabatic compression
11
414411
−−
=−−
=γγmRTmRTvpvp
... (Qpv = mRT)
= ( )
1
31
−−
γTTmR
... (QT3 = T4)
We see from the above discussion that the decrease in internal energy during
reversible adiabatic expansion 2-3 is equal to the increase in internal energy during
reversible adiabatic compression 4-1. Hence their net effect during the whole cycle is
zero. We know that
Work done, W = Heat supplied - Heat rejected
= 2.3 m R T1 log r - 2.3 m R T3 log r = 2.3 m R log r (T1 - T3)
And efficiency **( )
rmRT
TTrmR
pliedHeat
doneWork
log3.2
log3.2
sup 1
31 −==η
= 1
3
1
31 1T
T
T
TT−=
−
29
Lecture No. 6
Renewable source of energy:
The source which can be renewed by reproduction or by physical, mechanical or
chemical processes are known as inexhaustible or renewable source of energy.
Renewable sources of energies are those which are continuously restore by nature. Ex.
Biomass, solar energy and wind energy.
Solar energy utilization:
Solar energy is a very large, inexhaustible source of energy. The power from the
sun intercepted by the earth is approximately 1.8×1011 MW which is many thousands of
times larger than the present consumption rate on the earth of all commercial energy
sources. Unlike fossil fuels, the solar energy is free and available in adequate quantities
in almost all parts of the world where people live.
The main problem is that it is a dilute source of energy. Even in the hottest region
on earth, the solar radiation flux available rarely exceeds 1 kW/m2. Consequently large
collecting areas are required in many applications and these results in excessive costs.
A second problem with the use of solar energy is that its availability varies widely with
times. Classification of the various methods of solar energy utilization is shown Fig.13.
Fig.13. Classification of solar energy utilization.
Solar
energy
utilization
Direct
method
Indirect
method
Thermal
Photovoltaic
Biomass
Wind
Water
power
Ocean
temperature
differences
Renewable energy sources – Solar energy utilization – Biogas – Anaerobic digestion of
organic matter – Application of biogas
30
Biomass:
Biomass refers to all organic matter except fossil fuels. Biomass energy refers to
the energy released when organic matter is eaten, decayed or burnt. Biomass is created
by photosynthesis process. Biomass include all plant life i.e. plants, grasses, trees etc.
Classification of the various methods of biomass utilization is shown in Fig. 14.
6H2O + 6CO2 → C6H12O6 + 6O2
Fig.14. Classification of biomass utilization.
Biogas:
Biogas is the term used to describe the mixture of gases produced during
anaerobic digestion. It is also known as gobar gas or methane gas. It mainly consists of
CH4 (methane) of 55 - 65%, CO2 (carbon dioxide) 30 - 40% and other gases such as H2,
H2S, N2 etc. The calorific value of biogas is 5,000 to 5,500 kcal/m3 of gas. The gas
production may be 0.036 m3/kg of wet dung.
Conditions for gas production
1. Gas production is at optimum at temperature between 30-35°C and the
process retard very much below 15°C.
2. pH of the slurry for gas production is between 7 to 8. Bacteria die when the
pH is above 8.
Biomass
Directly
Ex. Burning the wood for
Indirectly
Ex. Converting biomass in
the form charcoal and
Producer gas
Gas by pyrolysis
Gas by gasification
31
Uses of biogas:
1. Used for Cooking
2. Used for Heating
3. Used for Lighting
4. Used for running dual fuel engines (80% biogas + 20% diesel)
Application of biogas:
One cu. m of biogas can do the following operations.
1. It can illuminate a mantle lamp (60 W) for a period of 7 hours.
2. It can cook three meals for a family of five.
3. It can run 2 HP engine for one hour.
4. It can run 100 lit capacity refrigerators for 9 hours.
5. It can generate electricity of 1.25 kwh.
Anaerobic digestion of organic matter:
The digestion takes place in the absence of oxygen in a digester, where three
principal reactions take place as shown in Fig.15. They are
Fig. 15. Anaerobic digestion of organic matter.
32
Solubilization:
In this process insoluble or partially soluble of the organic matter take place to
form a soluble complex organic.
Acidification:
In this process the organic substance contained in the waste is acted upon by
certain kind of bacteria called acid former. The material is broken up into small chain of
acid.
Methanogenesis:
In this process, these acids are acted upon by methanogenic bacteria which
produce methane and carbon dioxide.
33
Lecture No. 7
Types of biogas plants:
Biogas plants are basically of two types.
Floating dome type:
In this type the gas holder floats on the top of the digester.
Ex. KVIC model (Khadi and village industries commission)
Fixed dome type:
In this type the gas holder is fixed on the top of the digester
Ex. Janata model (chinese model) and Deenabandhu model
KVIC model:
In KVIC model, there is a masonary structure which is the digester. It has a depth
of 3.5 to 6 m as shown in Fig. 16. Above the digester, there is a gas holder and will be
made of mild steel sheet. It is cylindrical in shape with concave top surface. There is a
guide frame made up of angular iron which holds the gas holder. A central a central
guide pipe is provided to prevent holder from tilting. The gas holder sinks into the slurry
due to its own weight and rests up on the ring constructed for this purpose. When gas is
produced, the holder rises and floats freely on the surface of slurry.
A partition wall is provided in the centre which divides the digester vertically and
submerges in the slurry when it is full. The inlet and outlet tanks are connected by AC
pipes to the digester. While construction, care should be taken that the opening of the
inlet tank should be at least 15 cm above than the opening of the outlet tank. Otherwise,
the digested slurry may come out through inlet tank if kept at the same level. The cost of
the gas holder is about 40% of the total cost of the plant. The slurry will be prepared
thoroughly by mixing cow dung & water in the ratio of 4:5.
Janata model:
It is also called as Chinese drumless model as shown in Fig. 17. It consists of a
underground well sort of digester made up of bricks and cement having a dome shaped
roof which remains below the ground level. Dome shaped roof is fitted with pipe at its top
Types of biogas plants – Floating dome type – Fixed dome type – KVIC model, Janata model
– Deendandhu model – Running the plant – Maintenance of biogas plant
34
which is the gas outlet of plant. The other parts of the plant are outlet chamber and inlet
chamber. When the gas is produced, it ascends towards the top of the dome and pushes
the slurry down. No steel is used for the construction of the plant.
Fig.16. KVIC model.
Fig.17. Janata model.
35
Deenbandhu model:
It consists of two segments of spheres of different diameters as shown in
Fig. 18. The diameter of the digester is more than the diameter of the gas holder. The
digester is made of concrete (1:2:4). The structure is connected to the inlet tank by AC
pipe. Outlet tank is also a masonry structure. At the centre of the dome is the gas outlet.
This structure is also constructed below the ground level. As no steel is used, the cost is
less than the other two models.
Fig.18. Deenbandhu model.
Running the plant:
Initially the digester is filled with uniformly premixed dung and water in 1:1 ratio
removing all trapped air by opening gas outlet. The digester may be filled in a week or
10 days time depending upon the availability of dung. In order to facilitate good
production, addition of 5 - 10% inoculum taken from a running gas plant hastens up the
process. If the gas holder level or slurry level in the outlet tank increases, it indicates the
production of gas. First, second or third installments of gas will not burn because of
excess CO2. It takes 4-8 weeks for correct population of bacteria to develop. After
digestion has stabilized, feeding can be stabilized according to recommended dose.
Points to be considered during construction and maintenance of biogas plant:
1. Biogas plant should be located nearer to the kitchen to reduce the cost of
pipe line.
2. The excavated soil should be kept atleast one meter away from the pit.
3. Provide slight upward slope towards the kitchen while laying the pipeline.
36
4. To get more gas during winter, lay compost paddy straw all round the digester
to warm up the digester.
5. The gas drum should be painted outside every alternate year with good quality
paint.
6. The surface of water supply should be atleast 15 m away from the plant.
7. Avoid construction of well or digester under water.
8. Do not allow the sand particles to enter into digester.
9. If possible use PVC pipes instead of A.C. pipes for inlet and outlet.
10. Only the recommended quality of the cow dung should be supplied to the
digester.
11. As for as possible, the plant should be nearer to the cattle shed.
Problems:
Example 1. A former wants to construct biogas plant for the following requirements. He
wants to utilize the biogas to cook food for 6 members, to light lamps for 2 h, to run dual
fuel engine for 3 h. Calculate the capacity of biogas plant that the farmer should have.
Also calculated the number of cows required to meets the demand if each cow gives
dung daily?
Solution.
Cooking = 0.2 m3 gas/member
= 0.2 × 6 = 1.2 m3 K (1)
Light mantle lamps = 0.15 m3/h
= 0.15 × 2 = 0.3 m3 K (2)
Dual fuel engine = 0.5 m3/h
= 0.5× 3 = 1.5 m3 K (3)
Adding equation (1)+(2)+(3)
Gives total gas required
= 3.0 m3
∴ Capacity of gas = 0.04 m3/kg dung
10 kg dung gives = 0.4 m3 = 1 Cow dung
To produce 0.04 m3 gas, no. of
cows required
=1
To produce 3.0 m3 gas, no. of cows
required
= 04.0
3 = 75 kg = 8 Cows
37
Example 2. A farmer wants to construct biogas plant for the following requirements. He
wants to utilise the biogas to cook food for 5 members, to light lamps for 3h, to run dual
fuel engine for 4h. Calculate the capacity of biogas plant required and the number of
cows meet the above demand if each cow gives 10 kg dung daily?
Solution.
Cooking = 0.2 m3 gas/member
= 0.2 × 5 = 1.0 m3 K(1)
Light mantle lamps = 0.15 m3/h
= 0.15 ×3 × 2 = 0.9 m3 K(2)
Dual fuel engine = 0.5 m3/h
= 0.5× 4 = 2.0 m3 K(3)
Adding equation (1)+(2)+(3)
Gives total gas required
= 3.9 m3 = 4 m3
Gas production = 0.04 m3/kg dung
10 kg dung gives = 0.4 m3 of gas
4 m3 gas required =
4.0
4 = 10 cows
38
Lecture No. 8
Wind energy:
The wind derives its energy from the solar radiation that reaches the earth’s
surface. Uneven heating of the earth from the equator to the poles and over the oceans
and the continents derives the motions of the atmosphere. Also the rotation of the earth
about its axis and its movement around the sun causes the flow of wind. The wind
energy on the earth surface is estimated to be 1.6 × 107 MW. A wind velocity of over
6.5 km/h is necessary for the satisfactory operation of wind mills.
Applications of wind mills:
1. Pumping water for drinking and irrigation purpose.
2. Grinding grain
3. Generation of power etc.
Factors affecting the wind:
1. Latitude of the place
2. Attitude of the place
3. Topography of the place
4. Scale of the hour, month or year
Suitable places for the erection of wind mills:
1. Off shore and on the sea coast: wind energy availability – 2400 kWH/m2
2. Mountains: 1600 kWH/m2/year
3. Plains: - 750 kWH/m2/year
Places unsuitable for wind mills:
1. Humid equatorial region: Wind velocity is minimum.
2. Warm, windy region: Frequency of cyclones is more.
Advantages of wind energy:
1. It is a renewable source of energy
2. Non polluting and no adverse influence on the environment.
3. Cost of electricity under low production is comparatively low.
Disadvantages of wind energy:
1. It is dilute and fluctuating in nature
Wind energy – Application of wind mill – Advantages and disadvantages of wind energy –
Types of wind mills – Savonius – Darrieus wind mill – Single blade wind mill – Two blade wind
mill – Multi blade wind mill – sail type wind mill
39
2. Storage facility should be there.
3. Wind energy machines are noisy in operation
4. Large area in required for wind mill.
5. Present wind mills are not maintenance free.
Types of wind mills:
1. Vertical axis wind mills – Axis of rotation is vertical
Ex: a) Savonius wind mill (Low wind velocity)
b) Darrieus wind mill (High wind velocity)
2. Horizontal axis wind mills – Axis of rotation is horizontal
Ex: a) Single blade wind mill
b) Double blade wind mill
c) Sail type wind mill.
Vertical axis wind mills:
Vertical axis wind mill (or wind axis) is a unit whose axis of rotation is
perpendicular to the direction of the wind.
a) Savonius wind mill:
It consists of a hollow circular cylinder sliced in half as shown in Fig. 19. The two
halves being fixed to a vertical axis (forming ‘S’ shape) with a gap in between.
Irrespective of direction of wind it rotates. Torque is produced by the pressure difference
between the two sides of the half facing the wind. This type was inverted by
S.J. Savonius in 1920. It has become popular since it requires relatively low velocity
winds for operation.
Direction of wind
Fig.19. Savonious wind mill.
40
The force of the wind is more on the cupped face than on rounded face. A low
pressure is created on the convex sides of half cylinders. The rotating rotor can be used
for working such as water pumping, battery charging, grain winnowing etc. It works on
the same principle as cup anemometer, with the addition that wind can pass between
the bent sheets. It is useful and economical for small power requirements.
b) Darrieus rotor:
Georges Darrieus of paris filed a us patent in 1926 for a vertical axis rotor.
Darrieus rotor as shown in Fig. 20 has long, thin blades (3 m) in the shape of loops
connected at top and bottom of the axle; often called “egg beater” wind mill because of
its appearance. The force in the blade due to rotation is torsion. The aerofoil blades
provide a torque about the central shaft in response to the wind direction. The shaft
torque is transmitted to a generator at the base of the central shaft for power generation.
Fig. 20. Darrieus rotor.
Horizontal axis wind mills:
Horizontal axis wind mill (or wind axis) is a unit whose axis of rotation is parallel
to the direction of the wind. The power loss in contrast to the trapezoidal form and it is
mainly used for slow rotors.
41
A) Single blade wind mill:
In this type of wind mill, a long blade is mounted on a rigid hub. Induction
generator is arranged as shown in Fig. 21. If extremely long blades (> 60m) are mounted
on the rigid hub, blades may be bent due to gravity and sudden shifts in the wind
direction. A counter weight is recommended for balancing long blade centrifugally
Fig. 21. Single blade wind mill.
B) Two blade wind mill:
In this type of wind mill (Fig. 22), rotor drives a generator through a step up gear
box. When the mill is in operation, the rotor blades are continuously flexed (bent) by
unsteady aerodynamics, gravitational and inertial loads. If the blades are made of metal,
flexing (bending) reduces their life due to fatigue loading.
The gear box contains a spring mechanism that allows the tail to fold parallel to
the wheel in a high wind. This effectively shuts down the mill and prevents its self
destruction. When the gale subsides, the tail is released so it may turn the wheel back
into the wind.
42
Fig. 22. Two blade wind mill.
C) Multiblade wind mill:
This type of wind mill will have 12 to 20 blades as shown in Fig. 23. The diameter
of the wheel ranges from 6-16 ft. The blades will be made by aluminum sheets. The rotor
is strong enough to with stand a wind speed of 60 km/h. it has good power coefficient,
high torque, simple and is low in cost. This will be commonly used for pumping water.
Fig. 23. Multiblade wind mill.
D) Sail type wind mill:
It is of recent origin, the sail type has three blades made by stretching out
triangular pieces of cloth, nylon, plastics, canvas as shown in Fig 24. It runs at low
speeds 60 to 80 revolutions per minute.
43
Fig. 24. Sail type wind mill
44
Lecture No. 9
Solar energy:
Solar energy is a very large, inexhaustible source of energy. The power from the
sun intercepted by the earth is approximately 1.8 × 1011 MW, which is may thousands of
times larger than the present consumption rate on the earth of all commercial energy
sources. Solar energy is one of the most promising of the non-commercial energy
sources.
Advantages of solar energy:
1. Available in a very large quantity.
2. It is an environmentally clean source of energy unlike fossil fuels and nuclear
power.
3. It is free and available in adequate quantities in almost all parts of the world
where people live.
Disadvantages of solar energy:
1. Solar energy is a dilute source of energy. Even in the hottest regions on earth
the solar radiations flux available rarely exceeds 11 kw/m2, which is a law
value for technology utilization consequently, large collecting areas are
required in many applications and these result in excessive costs.
2. The available of solar energy varies widely with time the energy collected
when the sun is shinning must be stored for use during periods when it is not
available the need for storage adds significantly to the cost of any system.
Thermal conversion – collection and storage:
In any collection device, the principle usually followed is to expose a dark
surface to solar radiation so that the radiation is absorbed. A part of the absorbed
radiation is then transferred to a fluid like air or water. When no optical concentration is
done, the device in which the collection is achieved is called a flat plate collector. If
optical concentration is done, the device in which collection is achieved is called a
concentration collector.
Solar energy – Advantages and disadvantages – Flat plate collector - Concentrating collector
– Cylinderical parabolic concentrating collector – Paraboloid concentrating collector
45
1. Flat plate collector:
Flat plate collector can be used for a variety of applications in which
temperatures ranging from 40°C to about 100°C are required for the purpose.
a) Liquid flat plate collector:
The basic elements required for making up a conventional liquid flat plate
collector as shown in Fig. 25: (1) The absorber plate (2) The tubes fixed to the absorber
plate through which the liquid to be heated flows (3) The transparent cores and (4) The
insulated container. The solar radiation falls on the absorber plate after coming through
one or more transferred to a liquid flowing through tubes which are fixed to the absorber
plate or integral with it. This energy transfer is the useful gain.
Fig. 25. Liquid flat plate collector
The remaining part of the radiation absorbed in the absorber plate is lost by
convection and re-radiation to the surroundings from the top surface, and by conduction
through the back side and the edges. The transparent covers help in reducing losses by
convection and re-radiation, while thermal insulation on the back side and the edges
helps in reducing the conduction heat loss. The liquid most commonly used is water. A
liquid flat plate collector is usually held titled in a fixed position on a supporting structure;
facing south is located in the northern hemisphere.
b) Solar air heater (Flat plate collector for heating air):
A schematic cross – section of a conventional flat plate collector for heating air
(Commonly required to as a solar air heater) is shown in Fig. 26. The construction of
such a collector is essentially similar to that of a liquid flat-plate collector except for the
46
passages through which the air flows. These passages have to be made larger in order
to keep the pressure drop across the collector within manageable limits. In the diagram
shown, the air passage is simply a parallel plate duct.
Fig. 26. Solar air heater.
2) Concentrating collector:
It is used when temperature higher than 100oC are required.
a) Cylindrical parabolic concentrating collector:
When temperatures higher than 100°C are required, it becomes necessary to
concentrate the radiation. This is achieved using focusing or concentrating collectors. A
schematic diagram of a typical concentrating collector is shown in Fig. 27.
Fig. 27. Cylindrical parabolic concentrating collector.
The concentration shown is a mirror reflector having the shape of a cylindrical
parabola. It focuses the sunlight onto its axis where it is absorbed on the surface of the
47
absorber tube and transferred to the fluid flowing through it. A concentric glass cover
around the absorber tube helps in reducing the convective ad radiative losses to the
surroundings. In order that the suns rays should always be focused onto the absorber
tube, the concentrator has to be rotated. This movement is called tracking. In the case of
cylindrical parabolic concentrators, rotation about a single axis is generally required.
Fluid temperatures up to around 300°C can be achieved in cylindrical parabolic focusing
collector systems.
The absorber tube is usually made of mild steel or copper. It is coated with a heat
resistant black paint. Water is used for temperatures below 100°C, while organic heat
transfer liquids (sometimes referred to as thermic fluids) are used for high temperature.
The reflecting surface is often made from anodized aluminum sheet. Long strips of glass
are also sometimes used as an approximation to the parabolic shape.
b) Paroboloid concentrating collector:
Higher working temperature (> 300°C) can be achieved by using paraboloid
reflectors (Fig. 28), which have a point focus. These require two-axis tracking (Vertical
and horizontal axis) so that the sun is in line with the focus and the vertex of the
paroboloid.
Fig. 28. parabolic concentrating collector.
48
Lecture No. 10
Thermal applications:
1. Water heating 2. Space heating 3. Space cooling or refrigeration 4. Drying
5. Cooking 6. Distillation.
1. Water heating:
A. Natural circulation water heating system:
A natural circulation water heating system (Fig.29) consists of two main
components viz., the liquid flat plate collector and the storage tank. The tank is located
above the level of the collector. As the water in the collector is heated by solar energy it
flows automatically to the top of the water tank, and its place is taken by cold water from
the bottom of the tank. Hot water for use is withdrawn from top of the tank. Whenever
this is done, cold water automatically enters at the bottom. The cold water gets heated
up in the collector by absorbing solar energy and hot water will flow to the storage tank
from upwards by thermo- sphere system circulation.
Fig. 29. Natural circulation water heating system.
Natural Circulation water heating system – Forced circulation water system - space heating
– Space cooling and refrigeration – Drying – Cabinet dryer – Convective dryer – Cooking –
Solar distillation
49
These systems have capacities ranging from 100 to 200 litres and adequately
supply the needs of a family of four or five persons. The temperature of the hot water
delivered ranges from 50 to 70°C.
B. Forced circulation water heating system:
When large amounts of hot water are required, a natural circulation system is not
suitable. Large arrays of flat plate collectors are then used and forced circulation is
maintained with a water pump (Fig. 30). The restriction that the storage tank should be
at a higher level is thus removed. whenever hot water is withdrawn for use, cold water
takes its place because of the ball-float control shown.
Fig. 30. Forced circulation water heating system.
The pump is operated by an on-off controller which senses the difference
between the temperature of the water at the exit of the collectors and a suitable location
inside the storage tank. The pump is switched on whenever this difference exceeds a
certain value and off when it falls below a certain value. Provision is also usually made
for auxiliary heater. The systems of this type are well suited for places like hospitals,
hotels, offices, hostels and factories.
3. Space heating:
A space heating system is illustrated in Fig. 31. Water is heated in the solar
collectors (A) and stored in the tank (B). The air is blown (or drawn) across a pipe coil
50
(heat exchanger) through which the hot water flows. Energy is transferred to the air
circulating in the house by means of the water to air heat exchanger (E). Two pumps (C)
provide forced circulation between the collectors and the tank, and between the tank and
the heat exchanger. Provision is also made for adding auxiliary heat.
Fig. 31. Space heating
3. Space cooling and refrigeration:
Space cooling may be done with the objective of providing comfortable lining
conditions or of keeping a food product cold. A diagram of a simple solar operated
absorption refrigeration system is shown in Fig. 32. Water heated in a flat plate collector
array is passed through a heat exchanger called the generator, where it transfers heat to
a solution mixture of the absorbent and refrigerant, which is rich in the refrigerant.
Refrigeration vapour is boiled off at a high pressure and goes to the condenser
where it is condensed into high pressure liquid. The high pressure liquid throttled to a
low pressure and temperature in an expansion valve and passes through the evaporator
coil. Here, the refrigerant vapour absorbs heat and cooling is therefore obtained in the
space surrounding this coil. The refrigerant vapour is now absorbed back into a solution
mixture withdrawn from the generator, which is weak in refrigerant concentration, and
then pumped back to the generator, thereby completing the cycle.
Some of the common refrigerant absorbent combinations used are ammonia-
water (NH3-H2O) and water–lithium bromide, the latter being used essentially for air
conditioning purposes. Values for the coefficient of performance (the ratio of the
refrigerating effect to the heat supplied in the generator) range between 0.5 and 0.8. By
using the above technique, a solar cold storage unit can be constructed.
51
Fig. 32. Solar absorption refrigeration system
3. Drying:
One of the traditional uses of solar energy has been for drying of agricultural
products. The drying process removes moisture and helps in the preservation of the
product. Traditionally, drying is done on open ground. The disadvantages associated
with this are that the process is slow and that insects and dust get mixed with the
product. The use of dryers helps to eliminate these disadvantages. Drying can then be
done faster and in a controlled fashion. In addition, a better quality product is obtained.
a) Cabinet type solar dryer:
A cabinet type solar dryer, suitable for small scale use, is shown in Fig. 33. The
dryer consists of an enclosure with a transparent cover. The material to be dried is
placed on perforated trays. Solar radiation entering the enclosure is absorbed in the
product itself and the surrounding internal surfaces of the enclosure. As a result moisture
is removed from the product and the air inside is heated. Suitable openings at the
bottom and top ensure a natural circulation. Temperature ranging 50 to 80°C are usually
attained and the drying time ranges from 2 to 4 days. Typical products which can be
dried in such devices are dates (fruits of date palm), apricots, chilies, grapes etc.
b) Connective dryer:
For large scale drying, a connective dryer is used (Fig. 34). In this dryer, the solar
radiation does not fall on the product to be dried. Instead, air is heated separately in a
52
solar air heater and then ducted to the chamber in which the product to be dried is
stored. Generally forced circulation is used. Such dryers are suitable for food grains and
many other products like tea & tobacco.
Fig. 33. Cabinet type solar dryer.
Fig. 34. Connective dryer dryer.
6. Cooking: An importnant domestic thermal application is that of cooking.
a) Box type cooker:
It consists of a rectangular enclosure insulated on the bottom and sides and
having two glass covers on the top as shown in Fig. 35. Solar radiation enters through
the top and heats up the enclosure in which the food to be cooked is placed in shallow
vessels. A typical size available has an enclosure about 50 cm square and 12 cm deep.
The solar radiations entering the box are of short wave length. The higher wave length
53
radiation is not able to pass through the glass cover i.e. re-radiation from absorber plate
to out side the box is minimized by providing glass cover.
Fig. 35. Box type cooker.
Temperatures around 100°C can be obtained in these cookers on sunny days
and pulses, rice, vegetables etc. can be readily cooked. The time for cooking depends
upon the solar radiation and varies from 0.5 to 2.5 hours. The best time of the day for
cooking is between 11 am and 2 pm.
b) Box type cooker with reflector:
A single glass reflector whose inclination can be varied is sometimes attached to
the box type cooker. A sketch of such a cooker the addition of the mirror helps in
achieving enclosure temperatures which are higher by about 15 to 20°C. As a result the
cooking time is reduced. In winter when sun rays are much inclined to horizontal surface,
reflector is most useful.
The item to be cooked has only to be placed inside and taken out, so that with
some experience, the operator does not have to spend much time in the sun. The
disadvantage of box type cooker is that it can not be used for making items like chapatti
or frying items like puris which require higher temperatures in box type cooker.
Merits:
1. No orientation to sun is needed.
2. No attention is needed during cooking.
3. No fuel, maintenance and recurring cost.
4. Simple to use and fabricate.
54
5. No pollution.
6. No loss of vitamin in Food. It is nutritive and delicious with natural taste.
Demerits:
1. Cooking can be done only when there is sunshine.
2. Takes more time to cook food.
3. All types of food can not be cooked.
7. Distillation:
In many small communities, the natural supply of fresh water is inadequate, but
brackish (salt) or saline water is available. Solar distillation can prove to be an effective
way of supplying drinking water to such communities.The principle of solar distillation is
simple and can be explained with reference to Fig. 36 in which a conventional basin type
solar still is shown. The still consists of a shallow basin lined with a black, impervious
material which contains the saline water. The solar radiation is transmitted through the
cover and is absorbed in the black lining. It thus heats up the water and causes it to
evaporate. The resulting vapour rises, condenses as pure water on the underside of the
cover and flows into condensate collection channels on the sides. An output of about 3
litres/m2 can be obtained in a well designed still on a good sunny day.
Fig. 36. Solar still.
55
Lecture No. 11
Fuel:
A fuel, in general terms, may be defined as a substance (containing mostly
carbon and hydrogen) which, on burning with oxygen in the atmospheric air, produces a
large amount of heat. The amount of heat generated is known as calorific value of the
fuel. As the principal constituents of a fuel are carbon and hydrogen, it is also known as
hydrocarbon fuel. Sometimes, a few traces of sulphur are also present in it.
Classification of Fuels:
The fuels may be classified into the following three general forms:
1. Solid fuels,
2. Liquid fuels, and
3. Gaseous fuels.
Each of these fuels may be further subdivided into the following two types:
(a) Natural fuels, and
(b) Prepared fuels.
Solid Fuels:
The natural solid fuels are wood, peat, lignite or brown coal, bituminous coal and
anthracite coal. The prepared solid fuels are wood charcoal, coke, briquetted coal and
pulverised coal. The following solid fuels are important from the subject point of view:
Natural solid fuels:
1. Wood: It consists of mainly carbon and hydrogen. The wood is converted into
coal when burnt in the absence of air. The average calorific value of the wood is
19,700 kJ/kg.
2. Peat: It is a spongy humid substance found in boggy land. It may be regarded
as the first stage in the formation of coal. It has a large amount of water contents (up to
30%) and therefore has to be dried before use. It has a characteristic odour at the time
of burning, and has a smoky flame. Its average calorific value is 23,000 kJ/kg.
3. Lignite or brown coal: It represents the next stage of peat in the coal
formation, and is an intermediate variety between bituminous coal and peat. It contains
nearly 40% moisture and 60% of carbon. When dried, it crumbles and hence does not
Fuels – Introduction – Classification of fuels – Solids fuels – Liquids fuels - Merits and demerits
of liquid fuels over solid fuels
56
store well. Due to its brittleness, it is converted into briquettes, which can be handled
easily. Its average calorific value is 25,000 kJ/kg.
4. Bituminous coal: It represents the next stage of lignite in the coal formation
and contains very little moisture (4 to 6%) and 75 to 90% of carbon. It is weather-
resistant and burns with a yellow flame. The average calorific value of bituminous coal is
33,500 kJ/kg. The bituminous coal is of the following two types:
5. Anthracite coal: It represents the final stage in the coal formation, and
contains 90% or more carbon with a very little volatile matter. It is thus obvious, that the
anthracite coal is comparatively smokeless, and has very little flame. It possesses a high
calorific value of about 36,000.
6. Wood charcoal: It is made by heating wood with a limited supply of air to a
temperature not less than 280°C. It is a good prepared solid fuel, and is used for various
metallurgical processes.
7. Coke: It is produced when coal is strongly heated continuously for 42 to 48
hours in the absence of air in a closed vessel. This process is known as carbonization of
coal. Coke is dull black in colour, porous and smokeless. It has high carbon content
(85 to 90%) and has a higher calorific value than coal.
If the carbonisation of coal is carried out at 500 to 700°C, the resulting coke is
called lower temperature coke or soft coke. It is used as a domestic fuel. The coke
produced by carbonisation of coal at 900 to 1,100 °C, is known as hard coke. The hard
coke is mostly used as a blast furnace fuel for extracting pig iron from iron ores, and to
some extent as a fuel in cupola furnace for producing cast iron.
8. Briquetted coal: It is produced from the finely ground coal by moulding under
pressure with or without a binding material. The binding materials usually used are pitch,
coal tar, crude oil and clay etc. The briquetted coal has the advantage of having,
practically, no loss of fuel through grate openings and thus it increases the heating value
of the fuel.
9. Pulverised coal: The low grade coal with high ash content, is powdered to
produce pulverised coal. The coal is first dried and then crushed into a fine powder by
pulverising machines. The pulverised coal is widely used in the cement industry and also
in metallurgical processes.
57
Note: Out of all the above mentioned types of solid fuels, anthracite coal is commonly
used in all types of heat engines.
Liquid Fuels:
Almost all the commercial liquid fuels are derived from natural petroleum (or
crude oil). The liquid fuels consist of hydrocarbons. The following liquid fuels are
important from the subject point of view:
1. Petrol or gasoline: It is the lightest and most volatile liquid fuel, mainly used
for light petrol engines. It is distilled at a temperature from 65 to 220°C.
2. Kerosene or paraffin oil: It is heavier and less volatile fuel than the petrol,
and is used as heating and lighting fuel. It is distilled at a temperature from 220 to
345°C.
3. Heavy fuel oils: The liquid fuels distilled after petrol and kerosene are known
as heavy fuel oils. These oils are used in diesel engines and in oil-fired boilers. These
are distilled at a temperature from 345 to 470°C.
Merits and demerits of liquid fuels over solid fuels:
Merits
1. Higher calorific value.
2. Lower storage capacity required.
3. Better economy in handling.
4. Better control of consumption by using valves.
5. Better cleanliness and freedom from dust.
6. Practically no ashes.
7. Non-deterioration in storage.
8. Non-corrosion of boiler plates.
9. Higher efficiency.
Demerits
1. Higher cost.
2. Greater risk of fire.
3. Costly containers are required for storage and transport.
58
Lecture No. 12
Gaseous Fuels:
The natural gas is usually found in or near the petroleum fields, under the earth's
surface. It essentially consists of marsh gas or methane (CH4) together with small
amounts of other gases such as ethane (C2H6), carbon dioxide (CO2) and carbon
monoxide (CO). The following prepared gases can be used as fuels.
1. Coal gas: It is also known as a town gas. It is obtained by the carbonisation of
coal and consists mainly of hydrogen, carbon monoxide and various hydrocarbons. It is
very rich among combustible gases, and is largely used in towns for street and domestic
lighting and heating. It is also used in furnaces and for running gas engines. Its calorific
value is about 21,000 to 25,000 kJ/m3.
2. Producer gas: It is obtained by the partial combustion of coal, coke, anthracite
coal or charcoal in a mixed air-steam blast. It is mostly used for furnaces particularly for
glass melting and also for power generation. Its manufacturing cost is low, and has a
calorific value of about 5,000 to 6,700 kJ/m3.
3. Water gas: It is a mixture of hydrogen and carbon monoxide and is made by
passing steam over incandescent coke. As it burns with a blue flame, it is also known as
blue water gas. The water gas is used in furnaces and for welding.
4. Mond gas: It is produced by passing air and a large amount of steam over
waste coal at about 650°C. It is used for power generation and heating. It is also suitable
for use in gas engines. Its calorific value is about 5,850 kJ/m3.
5. Blast furnace gas: It is a by-product in the production of pig iron in the blast
furnace. This gas serves as a fuel in steel works, for power generation in gas engines,
for steam raising in boilers and for preheating the blast for furnace. It is extensively used
as fuel for metallurgical furnaces. It has a low heating value of about 3,750 kJ/m3.
6. Coke oven gas: It is a by-product from coke oven, and is obtained by the
carbonisation of bituminous coal. Its calorific value varies from 14,500 to 18,500 kJ/m3. It
is used for industrial heating and power generation.
Gaseous fuel – Merits and demerits of gaseous fuels – Requirements of a good fuel –
Calorific value of fuels – Gross or higher calorific value – Net or lower calorific value
59
Merits and demerits of gaseous fuels:
Merits
1. The supply of fuel gas, and hence the temperature of furnace is easily and
accurately controlled.
2. The high temperature is obtained at a moderate cost by pre-heating gas and
air with heat of waste gases of combustion.
3. They are directly used in internal combustion engines.
4. They are free from solid and liquid impurities.
5. They do not produce ash or smoke.
6. They undergo complete combustion with minimum air supply.
Demerits
1. They are readily inflammable.
2. They require large storage capacity.
Requirements of a Good Fuel:
Though there are many requirements of a good fuel; yet the following are
important from the subject point of view:
1. A good fuel should have a low ignition point.
2. It should have a high calorific value.
3. It should freely burn with a high efficiency, once it is ignited.
4. It should not produce harmful gases.
5. It should produce least quantity of smoke and gases.
6. It should be economical, easy to store and convenient for transportation.
Calorific Value of Fuels:
The calorific value (briefly written as C.V.) or heat value of a solid or liquid fuel
may be defined as the amount of heat given out by the complete combustion of 1 kg of
fuel. It is expressed in terms of kJ/kg of fuel. The calorific value of gaseous fuels is,
however, expressed in terms of kJ/m3 at a specified temperature and pressure.
Following are the two types of the calorific value of fuels:
1. Gross or higher calorific value, and
2. Net or lower calorific value.
Gross or Higher Calorific Value:
All fuels, usually, contain some percentage of hydrogen. When a given quantity
of a fuel is burnt, some heat is produced. Moreover, some hot flue gases are also
produced. The water, which takes up some of the heat evolved, is converted into steam.
60
If the heat, taken away by the hot flue gases and the steam is taken into consideration,
i.e. if the heat is recovered from flue gases and steam is condensed back to water at
room temperature (15°C), then the amount of total heat produced per kg is known as
gross or higher calorific value of fuel. In other words, the amount of heat obtained by the
complete combustion of 1kg of a fuel, when the products of its combustion are cooled
down to the temperature of supplied air (usually taken as 15°C), is called the gross or
higher calorific value of fuel. It is briefly written as H.C.V. If the chemical analysis of a
fuel is available, then the higher calorific value of the fuel is determined by the following
formula, known as Dulong's formula:
H.C.V. = 33,800 C + 1,44,000 H2 + 9,270 S kJ/kg .... (i)
where C, H2 and S represent the mass of carbon, hydrogen and sulphur in 1 kg
of fuel, and the numerical values indicate their respective calorific values.
If the fuel contains oxygen (O2), then it is assumed that the whole amount is
combined with hydrogen having mass equal to 1/8th of that of oxygen. Therefore, while
finding the calorific value of fuel, this amount of hydrogen should be subtracted.
H.C.V. = 33,800 C + 1,44,000
−8
22
OH + 9,270 S kJ/kg K. (ii)
Net or Lower Calorific Value:
When the heat absorbed or carried away by the products of combustion is not
recovered (which is the case in actual practice), and the steam formed during
combustion is not condensed, then the amount of heat obtained per kg of the fuel is
known as net or lower calorific value. It is briefly written as L.C.V. If the higher calorific
value is known, then the lower calorific value may be obtained by subtracting the amount
of heat carried away by products of combustion (especially steam) from H.C.V.
L.C.V = H.C.V. - Heat of steam formed during combustion
Let ms = Mass of steam formed in kg per kg of fuel = 9 H2
Since the amount of heat per kg of steam is the latent heat of vaporisation of
water corresponding to a standard temperature of 15°C, is 2,466 kJ/kg, therefore
L.CV� = H.C.V. – ms X 2466
= H.C.V. – 9 H2 x 2,466 kJ/kg (Qms = 9 H2)
61
Problems on calorific value of fuels:
Example 1. A fuel consists of 80% carbon; 17% hydrogen; 3% residual matter by mass.
Find the higher and lower calorific values per kg of the fuel?
Solution. Given: C = 80% = 0.80 kg; H2 = 17% = 0.17 kg; Residual matter = 3% =
0.03 kg
Higher calorific value per kg of fuel
H.C.V. = 33,800 C + 1,44,000 H2 + 9,270 S
= (33,800 x 0.8) + (1,44,000 x 0.17) + (9,270 x 0) = 51,520 kJ/kg
Lower calorific value per kg of fuel
L.C.V. = H.C.V. - (9 H2 x 2,466)
= 51520 - (9 x 0.17 x 2,466) = 47,747.02 kJ/kg
Example 2. A sample of coal has the following composition by mass: Carbon 72%;
hydrogen 9%; oxygen 8%; nitrogen 2.5%; sulphur 1.5%; and ash 7%. Calculate its
higher and lower calorific values per kg of coal?
Solution. Given, C = 72% = 0.72 kg; H2 = 9% = 0.09 kg; O2 = 8% = 0.08 kg; N2 = 2.5% =
0.025 kg; S = 1.5 % = 0.015 kg; Ash = 7% = 0.07 kg
Higher calorific value per kg of coal
H.C.V. = 33,800 C+ 1,44,000 [ H2 - 8
2O ] + 9,270 S
= (33,800 x 0.72) + (1,44,000 (0.09 - 8
08.0) )+ (9,270 x 0.015)
= 24,336 + 11,520 + 139.05 = 35,995.05 kJ/kg
Lower calorific value per kg of coal
L.C.V. = H.C.V. - (9 H2 x 2466)
= 35,995.05 – (9 x 0.09 x 2,466)
= 33,997.59 kJ/kg
62
Lecture No. 13
Experimental determination of higher calorific value:
The most satisfactory method of obtaining the calorific value of a fuel is by actual
experiment. In all these experimental methods, a known mass of fuel is burnt in a
suitable calorimeter, and the heat so evolved is found by measuring the rise in
temperature of the surrounding water. The calorimeters used for finding the calorific
value of fuels are known as fuel calorimeters. Bomb calorimeter and Boy’s gas
calorimeter are used to determine the calorific values of the fuels.
Bomb Calorimeter:
It is used for finding the higher calorific value of solid and liquid fuels. In this
calorimeter (Fig. 37), the fuel is burnt at a constant volume and under a high pressure in
a closed vessel called bomb.
Fig. 37. Bomb calorimeter.
The bomb is made mainly of acid-resisting stainless steel, machined from the
solid metal, which is capable of withstanding high pressure (upto 100 bar), heat and
corrosion. The cover or head of the bomb carries the oxygen valve for admitting oxygen
and a release valve for exhaust gases. A cradle or carrier ring, carried by the ignition
Experimental Determination of Higher Calorific value - Bomb Calorimeter - Boy’s Gas
Calorimeter – problems on calorific values
63
rods, supports the silica crucible, which in turn holds the sample of fuel under test. There
is an ignition wire of platinum or nichrome which dips into the crucible. It is connected to
a battery, kept outside, and can be sufficiently heated by passing current through it so as
to ignite the fuel.
The bomb is completely immersed in a measured quantity of water. The heat,
liberated by the combustion of fuel, is absorbed by this water, the bomb and copper
vessel. The rise in the temperature of water is measured by a precise thermometer,
known as Beckmann thermometer which reads upto 0.01 °C.
Procedure:
A carefully weighed sample of the fuel (usually one gram or so) is placed in the
crucible. Pure oxygen is then admitted through the oxygen valve, till pressure inside the
bomb rises to 30 atmosphere. The bomb is then completely submerged in a known
quantity of water contained in a large copper vessel. This vessel is placed within a large
insulated copper vessel (not shown in the figure) to reduce loss of heat by radiation.
When the bomb and its contents have reached steady temperature (this temperature
being noted), fuse wire is heated up electrically. The fuel ignites, and continues to burn
till whole of it is burnt. The heat released during combustion is absorbed by the
surrounding water and the apparatus itself. The rise in temperature of water is noted.
Let mf = Mass of fuel sample bunt in the bomb in kg
H.C.V. = Higher calorific value of the fuel sample in kJ/kg
mw = Mass of water filled in the calorimeter in kg
me = Water equivalent of apparatus in kg
Cw = Specific heat of water, 4.2 KJ/kg. K
t1 = Initial temperature of water and apparatus in °C, and
t2 = Final temperature of water and apparatus in °C
We know that heat liberated by fuel
= mf × H.C.V. .... (1)
and heat absorbed by water and apparatus.
= (mw + me) Cw (t2 – t1) ... (2)
Since the heat liberated is equal to the heat absorbed (neglecting losses),
therefore equating equations (1) and (2),
mf × H.C.V. = (mw + me) Cw (t2 – t1)
64
H.C.V. = ( ) ( )
kgkJm
ttCmm
f
wew /12 −+
Notes:
1. To compensate for the loss of heat by radiation (which cannot be totally
eliminated), a cooling correction is added to the observed temperature rise. This
corrected temperature rise is used in the above expression.
∴H.C.V. = ( ) ( )
f
cwew
m
tttCmm ][ 12 +−+
where tc = Cooling correction.
2. This calorimeter gives H.C.V. of the fuel because any steam formed is
condensed (since it cannot escape) and hence heat is recovered from it.
Problems:
Example 1. Calculate the higher calorific value of a coal specimen from the following
data:
Mass of coal burnt = 1.5 g
Quantity of water in calorimeter = 3 kg
Increase in temperature of water = 2.8°C
Water equivalent of apparatus = 400 g
If the fuel used contains 7% of hydrogen, calculate its lower calorific value.
Solution. Given : mf = 1.5 g = 0.0015 kg; mw = 3 kg; t2 – t1 = 2.8°C; me = 400 g = 0.4 kg;
H2 = 7% = 0.07
Higher calorific value
We know that higher calorific value,
H C V = ( ) ( ) ( )
kgkJm
ttcmm
f
wew /0015.0
8.22.44.0312 ×+=
−+
= 26,656 kJ/kg
Lower calorific value
We know that lower calorific value
= H.C.V. - (9 H2 X 2,466)
= 26,656 – (9 x 0.07 x 2,466) = 25,102.42 kJ/kg
65
Boy's Gas Calorimeter:
Boy’s gas calorimeter (Fig. 38) is primarily used for gaseous fuels, though it can
be modified for liquid fuels also. It gives the higher calorific value only. But the lower
calorific value may be calculated, since the amount of water produced can be collected
and measured.
Fig. 38. Boy's Gas Calorimeter.
It consists of a suitable gas burner at B, in which a known volume of gas at a
known pressure is burnt. The hot gases, produced by combustion, rise up in the copper
chimney or combustion chamber, which is surrounded by a double metal tubing through
which a continuous flow of water under a constant head is maintained. From the top of
inner chamber, hot gases are deflected downwards through the space containing the
inner water tubes M. From here the gases are deflected upwards through the space
containing the outer water tubes N. Then the gases escape into the atmosphere from the
top and their temperature is recorded just before their exit. During this process of playing
up and down the water tubes, the gases give out, practically, whole of their heat so that
any steam formed during combustion is condensed back into water.
The temperature of the circulating water is measured at inlet and outlet by
thermometers T1 and T2 respectively. After an initial warming up period during which
conditions are established, simultaneous readings are taken of:
66
1. The volume of gas burnt in a certain time.
2. The quantity of water passing through the tube during the same time, and
3. The rise in temperature of water.
The volume of the gas consumed is reduced to some standard conditions of
pressure and temperature (15°C and 760 mm of Hg), the normal temperature and
pressure being 0°C and 760 mm of Hg.
Let v = Volume of gas burnt at standard temperature and pressure (S.T.P.) in m3,
mw = Mass of cooling water used in kg,
H.C.V. = Higher calorific value of the fuel in kJ/m3,
t1 = Temperature of water at inlet, in °C, and
t2 = Temperature of water at outlet in °C.
We know that heat produced by the combustion of fuel = v × H.C.V. K (1)
and heat absorbed by circulating water = mw cw (t2 – t1) K. (2)
Since the heat produced by the combustion of fuel is equal to the heat absorbed
by the circulating water (neglecting losses), therefore equating equations (1) and (2),
v x H.C.V. = mw cw (t2 - tl)
H.C.V. = ( )v
ttcm ww 12 −
Notes:
1. The lower calorific value may be calculated if the water produced during the
combustion is drained off from the bottom of the calorimeter, collected and
weighed. If this mass is m, then
L.C.V. = H.C.V. – (m x 2,466) kJ/kg
2. The modification of this gas calorimeter is known as Junker's calorimeter.
Problem:
Example 2. The following data refers to a calorific value test of a fuel by means of a gas
calorimeter.
Volume of gas used = 0.8 m3 (reckoned at S.T.P.); mass of water heated = 30 kg;
rise in temperature of water at inlet and outlet = 15°C; mass of steam condensed
67
= 0.03 kg. Find the higher and lower calorific value per m3 at S.T.P. Take the heat
liberated in condensing water vapour cooling the condensate as 2,470 kJ/kg.
Solution. Given: v = 0.8 m3; mw = 30 kg; t2 – t1 = 15 °C; ms = 0.03 kg ; Heat liberated =
2,470 kJ/kg
Higher calorific value
H.C.V. = ( ) 312 /5.362,2
8.0
152.430mkJ
v
ttcm ww =××
=−
Lower calorific value
L.C.V. = H.C.V. - Mass of steam condensed in kg/m3 x Heat liberated
= 2,362.5 – ( 24708.0
03.0× ) = 2,269.875 kJ/m3
68
Lecture No. 14
Introduction:
The combustion of fuels may be defined as a chemical combination of oxygen in
the atmospheric air and hydro-carbons. It is usually expressed both qualitatively and
quantitatively by equations known as chemical equations. A chemical equation shows in
a concise form the complete nature of the chemical action or reaction taking place.
Elements and Compounds:
The elements are those substances which have so far not been resolved by any
means into other substances of simpler form. About 109 such elements are known so
far. The examples are hydrogen, oxygen, nitrogen, helium, iron, carbon, etc.
The compounds are formed by the combination of different elements in simple
proportion. The number of compounds, that can be formed, is almost infinite. The
examples are water (combination of hydrogen and oxygen), carbon dioxide (combination
of carbon and oxygen), methane gas (combination of carbon and hydrogen), etc.
Atoms and Molecules:
The elements are made up of minute and chemically indivisible particles known
as atoms. The smallest quantity of a substance, which can exist by itself, in a chemically
recognizable form, is known as molecule. A molecule may consist of one atom, two
atoms, three atoms or even more. Such a molecule is known as monoatomic, diatomic,
triatomic or polyatomic molecule respectively.
Atomic Mass:
Hydrogen is the lightest known substance. By taking the mass of hydrogen atom
as unity, it is possible to obtain relative masses of other atoms and molecules. It may be
noted that actual masses of these atoms are extremely small. The atomic mass of an
element is the number of times, the atom of that element is heavier than the hydrogen
atom. For example, the atomic mass of oxygen is 16. It means that the oxygen atom is
16 times heavier than the hydrogen atom.
Combustion of fuels - introduction - Combustion equations of solids fuels - Combustion
equation of gaseous fuel
69
Molecular mass:
The molecular mass of a substance is the number of times a molecule of that
substance is heavier than the hydrogen atom. For example, one molecule of oxygen
consists of two atoms of oxygen, each of which is 16 times heavier than hydrogen atom.
It is thus obvious, that a molecule of oxygen is 2 x 16 = 32 times heavier than hydrogen
atom. In other words, the molecular mass of oxygen is 32.
Symbols for Elements and Compounds:
The elements and compounds are, generally, represented by symbols. The
symbol of an atom is, usually, the initial letter of the element in capital. The symbol for
carbon atom, for example, is C; for hydrogen atom H; for oxygen atom O; and so on. A
molecule is represented by a single expression. For example, molecule of oxygen is
written as O2, where the suffix 2 represents the number of atoms in one molecule of
oxygen.
Similarly CO2 stands for one molecule of carbon dioxide gas, which consists of
one atom of carbon and two atoms of oxygen. The coefficients or prefix of an expression
indicates the number of molecules of a substance, e.g. 7 CO2 means 7 molecules of
carbon dioxide, each of which consists of one atom of carbon and two atoms of oxygen.
The symbols, atomic mass and molecular mass of the following substances are
important from the subject point of view:
Substance Symbol Atomic mass
Molecular mass
Hydrogen H2 1 2
Oxygen O2 16 32
Nitrogen N2 14 28
Carbon C 12 -
Sulphur S 32 -
Carbon monoxide CO - 28
Methane gas CH4 - 16
Acetylene C2H2 - 26
Ethylene C2H4 - 28
Ethane C2H6 - 30
Carbon dioxide CO2 - 44
Sulphur dioxide SO2 - 64
Steam or Water H2O - 18
70
Note: If the atomic mass of a substance is known, then its molecular mass may be
easily obtained. For example, molecular mass of CO2 = 12 + (2 x 16) = 44
Combustion Equations of Solid Fuels:
The combustion of a fuel is the chemical combination of oxygen. The following
equations are important from the subject point of view:
1. When carbon burns in sufficient quantity of oxygen, carbon dioxide is
produced along with a release of large amount of heat. This is represented by the
following chemical equation:
C+O2 = CO2
1 mol. + 1 mol. = 1 molecule. .. (By volume)
12 kg + 32 kg = 44 kg .. (By mass)
1 kg + 3
8 kg =
3
11 kg
It means that 1 kg of carbon requires 8/3 kg of oxygen for its complete
combustion, and produces 11/3 kg of carbon dioxide gas.
2. If sufficient oxygen is not available, then combustion of carbon is incomplete. It
then produces carbon monoxide instead of carbon dioxide. It is represented by the
following chemical equation:
2C+O2 = 2CO
2 mol. + I mol. = 2 mol. .. (By volume)
2 x 12 kg + 2 x 16 kg = 2 x 28 kg .. (By mass)
1 kg + 3
4 kg =
3
7 kg
It means that 1 kg of carbon requires 4/3 kg of oxygen, and produces 7/3 kg of
carbon monoxide.
3. If carbon monoxide is burnt further, it is converted into carbon dioxide. Thus
2CO + O2 = 2CO2
2 mol. + 1 mol. = 2 mol. .. (By Volume)
2 x 28 kg + 2 x 16 kg = 2 x 44 kg .. (By mass)
1 kg + 7
4 kg =
7
11 kg
71
It means that 1 kg of carbon monoxide requires 4/7 kg of oxygen, and produces
11/7 kg of on dioxide.
4. When sulphur burns with oxygen, it produces sulphur dioxide. This is
represented by the following chemical equation:
S+O2 = SO2
1 mol. + 1 mol. = 1 mol.
32 kg+2x 16 kg = 64 kg
1 kg + 1 kg = 2 kg
It means that 1 kg of sulphur requires 1 kg of oxygen for complete combustion
and produces of 2 kg of sulphur dioxide.
Combustion Equations of Gaseous Fuels:
The gaseous fuels are, generally measured by volume (in m3) than by mass (in
kg). It is thus obvious, that their combustion equations are usually stated quantitatively in
volume form, though we may use masses also. Following are the important equations for
the chemical combination of oxygen (representing combustion) from the subject point of
view:
1. 2CO+O2 = 2CO2
=+
=333 21m2
Volumes2 Volume 1 + Volumes 2
mm... (By volume)
=+
×=×+×
kgkgkg
kgkgkg
7
11
7
41
442321282
K (By mass)
It means that, 2 volumes of carbons monoxide require 1 volume of oxygen and
produces 2 volumes of carbon dioxide.
Or in other words, 1 kg of carbon monoxide requires 4/7 kg of oxygen and produce
11/7 kg of carbon dioxide.
2. 2H2 +O2 = 2H2O
=+
=+333 212
.2.1.2
mm
VolVolVol .... (By Volume)
72
=+
×=×+×
kgkgkg
kgkgkg
981
18232122 K (By mass)
It means that, 2 volumes of hydrogen require 1 volume of oxygen and produce 2
volumes of water or steam.
(Or) in other words, 1 kg of hydrogen requires 8 kg of oxygen and produces 9 kg of
water or steam.
3. CH4 + 2O2 = CO2 + 2H2O
+=+
+=+3333 2121
.2.1.2.1
mmmm
VolVolVolVol .... (By volume)
+=+
×+=×+
kgkgkgkg
kgkgkgkg
4
9
4
1141
1824432216
K (By mass)
It means that, 1 volume of methane requires 2 volumes of oxygen and produces
I volume of carbon dioxide and 2 volumes of water or steam.
Or in other words, 1 kg of methane requires 4 kg of oxygen and produces 11/4 kg
of carbon dioxide and 9/4 kg of water or steam.
4. C2H4+3O2 = 2CO2+2H2 O
+=+
+=+3333 2231
.2.2.3.1
mmmm
VolVolVolVol ... (By volume)
+=+
×+×=×+
kgkgkgkg
kgkgkgkg
7
9
7
22
7
241
18244232328
K (By mass)
It means that, 1 volume of ethylene requires 3 volumes of oxygen and produces 2
volumes of carbon dioxide and 2 volumes of water or steam.
Or in other words, 1 kg of ethylene requires 24/7 kg of oxygen and produces 22/7
kg of carbon dioxide and 9/7 kg of water or steam.
73
Lecture No. 15
Theoretical or minimum mass of air required for complete combustion:
The theoretical or minimum mass (or volume) of oxygen required for complete
combustion of 1 kg of fuel may be calculated from the chemical analysis of the fuel. The
mass of oxygen, required by each of the constituents of the fuel, may be calculated as
follows.
Now consider 1 kg of a fuel.
Mass of carbon = C kg
Mass of hydrogen = H2 kg
Mass of sulphur = S kg
We know that 1 kg of carbon requires 8/3 kg of oxygen for its complete
combustion. Similarly, 1 kg of hydrogen requires 8 kg of oxygen and 1 kg of sulphur
requires 1 kg of oxygen for its complete combustion.
.:. Total oxygen required for complete combustion of 1 kg of fuel
kgSHC ++= 283
8 K (1)
If some oxygen (say O2 kg) is already present in the fuel, then total oxygen for
the complete combustion of 1 kg of fuel
= [ C3
8 + 8 H2 + S] – O2 kg ... (2)
It may be noted that the oxygen has to be obtained from atmospheric air, which
mainly consists of nitrogen and oxygen along with a small amount of carbon dioxide and
negligible amounts of rare gases like argon, neon, and krypton etc. But for all
calculations, the composition of air is taken as:
Nitrogen (N2) = 77%; Oxygen (O2) = 23% K (By mass)
Nitrogen (N2) = 79%; Oxygen (O2) = 21 % K (By volume)
It is thus obvious, that for obtaining 1 kg of oxygen, amount of air required
Theoretical or minimum mass of air required for complete combustion - Theoretical or
minimum volume of air required for complete combustion – Problems on theoretical mass or
volume of air required for combustion
74
= kg35.423
100=
.:. Theoretical or minimum air required for complete combustion of 1 kg of fuel
=
−
++ 2283
8
23
100OSHC kg
Problem:
Example 1. A fuel has the following composition by mass: Carbon 86%, Hydrogen
11.75%, Oxygen 2.25%. Calculate the theoretical air supply per Kg of fuel, and the mass
of products of combustion per Kg of fuel?
Solution. Given: C = 85% = 0.85 kg; H2 = 12.75% = 0.1275 kg; O2 = 2.25 % = 0.0225 kg
Theoretical air supply per kg of fuel
We know that theoretical air supply per kg of fuel
−
++= 2283
8
23
100OSHC kg
=
−
+×+× 0225.001275.0885.03
8
23
100kg
= 14.19
Mass of products of combustion
The chemical equations of carbon and hydrogen with oxygen are
C+O2 = CO2
2 H2 + O2 = 2 H2O
We know that 1 kg of carbon produces 11/3 kg of carbon dioxide and 1 kg of
hydrogen produces 9 kg of water.
.:. Total mass of the products of combustion
= ( C3
11) + (9 H2) kg
= ( ×3
110.85) + (9×0.1275) = 4.26 kg
Theoretical or minimum volume of air required for complete combustion:
The volume of oxygen required for complete combustion of 1 m3 of gaseous fuel
may be calculated from the chemical analysis of the fuel. The volume of oxygen,
75
required by each of the constituents of the fuel, may be calculated as follows. Now
consider 1 m3 of a gaseous fuel.
Let Volume of carbon monoxide = CO m3
Volume of hydrogen = H2 m3
Volume of methane = CH4 m3
Volume of ethylene = C2H4 m3
We know that 2 volumes of carbon monoxide require 1 volume of oxygen. Or in
other words, 1 volume of carbon monoxide requires 0.5 volume of oxygen for its
complete combustion. Similarly, 2 volumes of hydrogen require 1 volume of oxygen. Or
in other words, 1 volume of hydrogen requires 0.5 volume of oxygen for its complete
combustion. Similarly, 1 volume of methane requires 2 volumes of oxygen and 1 volume
of ethylene requires 3 volumes of oxygen for its complete combustion.
.:. Total oxygen required for complete combustion of 1 m3 of fuel
= 0.5 CO + 0.5 H2 + 2 CH4 + 3 C2H4 m3
If some oxygen (say O2 m3) is already present in the fuel, then total oxygen
required for complete combustion of 1 m3 of fuel
= [0.5 CO + 0.5H2 + 2CH4 + 3C2H2] - O2 m3
Since the oxygen present in the air is 21 % by volume, therefore theoretical or
minimum volume of air required for complete combustion of 1 m3 of fuel
= 21
100[(0.5 CO+0.5 H2+2 CH4+3 C2H4) - O2) m3
Problem:
Example 2. A producer gas, used as a fuel, has the following volumetric composition:
H2 28%; CO 11%; CH4 2%; CO2 17% and N2 43% Find the volume of air required for
complete combustion of 1 m3 of this gas. Air contains 21% by volume of oxygen?
Solution. Given: H2 = 26% = 0.26 m3; CO = 11% = 0.11m3; CH4 = 2% = 0.02 m3;
CO2 = 17% = 0.17 m3; N2 = 43% = 0.43 m3
We know that theoretical air required
= 21
100[(0.5 CO + 0.5 H2 + 2 CH4 + 3C2H4) - O2] m
3
= 21
100 [(0.5 × 0.11) + (0.5 × 0.26) + (2 × 0.02)+0) - 0] m3
= 1.071 m3
76
Lecture No. 16
Conversion of volumetric analysis into mass analysis or gravimetric analysis:
When the volumetric composition of any fuel gas is known, it can be converted
to gravimetric composition by applying Avogadro's Law, Thus
1. Multiply the volume of each constituent by its own molecular mass. This gives
the proportional mass of the constituents.
2. Add up these masses and divide each mass by this total mass, and express it
as a percentage.
3. This gives percentage analysis by mass.
Conversion of mass analysis into volumetric analysis:
The conversion of mass analysis of a fuel gas into the volumetric analysis may
be done by the following steps:
1. Divide the percentage mass of each constituent by its own molecular mass.
This gives the proportional volumes of the constituents. .
2. Add these volumes and divide each volume by this total volume, and express
it as a percentage.
3. This gives percentage analysis by volume.
Mass of Carbon in Flue Gases:
The mass of carbon, contained in 1 kg of flue or exhaust gases, may be
calculated from the mass of carbon dioxide and carbon monoxide present in them.
We know that 1 kg of carbon produces 11/3 kg of carbon dioxide. Hence I kg of
carbon dioxide will contain 3/11 kg of carbon. Also 1 kg of carbon produces 7/3 kg of
carbon monoxide, hence 1 kg of carbon monoxide will contain 3/7 kg of carbon.
.:. Mass of carbon per kg of flue gas
Conversion of volumetric analysis into mass analysis or gravimetric analysis - Conversion of
mass analysis into volumetric analysis - Mass of carbon in flue gases – Mass of flue gases
per kg of fuel burnt
77
COCO7
3
11
32 +=
where CO2 and CO represent the quantities of carbon dioxide and carbon
monoxide present in 1 kg of flue gases.
Mass of Flue Gases per kg of Fuel Burnt:
The mass of dry flue gases may be obtained by comparing the mass of carbon
present in the flue gases with the mass of carbon in the fuel, since there is no loss of
carbon during combustion. Mathematically, mass of flue gas per kg of fuel
=gasflueofkgincarbonofMass
fuelofkgincarbonofMass
1
1
78
Lecture No. 17
Excess Air Supplied:
To ensure complete and rapid combustion of a fuel, some quantity of air, in
excess of the theoretical or minimum air, is supplied. It is due to the fact that the excess
air does not come in contact with the fuel particles. If just minimum amount of air is
supplied, a part of the fuel may not burn properly. The amount of excess air supplied
varies with the type of fuel and firing conditions. It may approach to a value of 100 per
cent, but the modem tendency is to use 25 to 50 per cent excess air.
Mass of Excess Air Supplied:
The mass of excess air supplied may be determined by the mass of unused
oxygen, found in the flue gases. We know that in order to supply one kg of oxygen, we
need 100/23 kg of air.
Similarly, mass of excess air supplied
= ×23
100 Mass of excess oxygen
∴ Total mass of air supplied = Mass of necessary air + Mass of excess air
Flue Gas Analysis by Orsat Apparatus:
To check the combustion efficiency of boilers, it is essential to determine the
constituents of the flue gases. Such an analysis is carried out with the help of Orsat
apparatus as shown in Fig. 39. It consists of a graduated measuring glass tube (known
as eudiometer tube) and three flasks A, Band C, each containing different chemicals for
absorbing carbon dioxide, carbon monoxide and oxygen. An aspirator bottle containing
water is connected to the bottom of the eudiometer tube by means of a rubber tube. It
can be moved up and down, at will, for producing a suction or pressure effect on the
sample of the flue gas.
Excess air supplied – Mass of excess air supplied – Flue gas analysis by Orsat apparatus
79
Fig. 39. Orsat apparatus.
The flak-A contains caustic soda (Na OH) and is used for absorbing carbon
dioxide in the sample of the flue gas. The flask-B contains caustic soda (Na OH) and
pyrogallic acid, which absorbs oxygen from the sample of the flue gas. The flak-C
contains a solution of cuprous chloride (Cu2 Cl2) in hydrochloric acid (HCl). It absorbs
carbon monoxide (CO) from the sample of the flue gas. Each of the three flasks has stop
cock 'a', 'b' and 'c' respectively and a three-way cock 'd'
The sample of the flue gas to be analysed is first sucked in the eudiometer tube,
and its volume is noted. It can take usually 100 cm3 of gas. By manipulating the level of
aspirator bottle, the flue gas can, in turn, be forced into either of the flasks A, Band C by
opening the respective cocks a, b and c. The flue gas is left there for sometime and then
sucked back into eudiometer tube. The chemicals, in the three flasks absorb carbon
dioxide, and carbon monoxide and the resulting contraction in volume enables the
percentage of each gas present in the sample to be read on the eudiometer tube.
Since the flue is collected over water in the tube, any steam present in it will be
condensed. Similarly, the sulphur dioxide present in it will also be absorbed. Hence the
percentages of dry flue gases are only obtained by Orsat apparatus.
80
Lecture No. 18
Burners:
Primary air that carries the powdered coal from the mill to the furnace is only
about 20% of the total air needed for combustion. Before the coal enters the furnace, it
must be mixed with additional air, known as secondary air, in burners mounted in the
furnace wall.
In addition to the prime function of mixing, burners must also maintain stable
ignition of fuel-air mix and control the flame shape and travel in the furnace. Ignition
depends on the rate of flame propagation. To prevent flash, back into the burner, the
coal-air mixture must move away from the burner at a rate equal to flame front travel.
Too much secondary air can cool the mixture and prevent its heating to ignition
temperature.
The requirements of a burner can be summarised as follows:
1. The coal and air should be so handled that there is stability of ignition.
2. The combustion is complete.
3. In the flame the heat is uniformly developed avoiding any superheat spots.
4. Adequate protection against overheating, internal fires and excessive abrasive
wear.
Pulverised fuel burners:
Pulverised fuel burners may be classified as follows:
1. Long flame burners
2. Turbulent burners
3. Tangential burners
4. Cyclone burners.
Long flame burners:
These are also, called U-flame or steamlined burners. In this type of burner coal
is floated on a portion of air supply (primary air) and supplied to the burner in one
Burners – Pulverized fuel burners – Long flame burners – Turbulent burners – Tangential
burners – Cyclone burners - Advantages
81
stream. Secondary and tertiary air supplies are maintained as shown in Fig. 40. The
length of flame is increased in the combustion chamber by downward initial flow of the
flame. The flame produced is stable, long and intense but it can be made short and
intense by adding much secondary air. Tertiary air enters through the burner and forms
an envelope around the primary air and fuel and provides better mixing. Furnaces for
low volatile coal are equipped with such burners to give a long flame path.
Fig. 40. Long flame burners.
Turbulent burners:
A turbulent burner is also called a short flame burner. As shown in Fig.41. These
burners can fire horizontally or at some inclinations by adjustment. The fuel-air mixture
and secondary hot air are arranged to pass through the burner in such a way that there
is good mixing and the mixture is projected in highly turbulent form in the furnace. Due to
high turbulence created before entering the furnace, the mixture burns intensely and
combustion is completed in short distance.
This burner gives high rate of combustion compared with other types. This is
generally preferred for high volatile coals. All modern plants use this type of burner.
82
Fig. 41. Turbulent burners.
Tangential burners:
These burners are set as shown in Fig. 42. In this case four burner are located in
the four corners of the furnace and are fired in such a way that the four flames are
tangential to an imaginary circle formed at the centre. The swirling action produces
adequate turbulence in the furnace to complete the combustion in a short period and
avoids the necessity of producing high turbulence at the burner itself.
Fig. 42. Tangential burners.
83
The tangential burners have the following advantages:
1. The parts of the burner are well-protected by furnace wall tubes.
2. The operation of these burners is very simple.
3. High heat release with complete and effective utilisation of furnace volume is
possible.
4. The completeness of combustion is exceptionally good and a maximum
degree of turbulence exists throughout the furnace.
5. Liquid, gaseous and pulverised fuels can be readily burned either separately
or in combination.
4. Cyclone burners:
Two main disadvantages associated with pulverised coal firing are:
(1) High cost.
(2) 70% of the ash escapes as fly-ash which requires expensive dust collectors in
the flue gas path.
These disadvantages are offset by a cyclone burner. In a cyclone burner coal is
crushed to a Maximum size of 6 mm and blows into a cylindrical cyclone furnace. The
fuel is quickly consumed and liberated ash forms a molten film flowing over the inner
wall of the cylinder. Owing to inclination of the furnace, the molten ash flows to an
appropriate disposal system.
The description of a cyclone burner (developed by M/s Babcock and Wilcox) is
given below: refer Fig. 43.
Fig. 43. Cyclone burners.
It consists of a horizontal cylindrical drum having a diameter varying from 2 to 4
meter, depending upon the capacity of the boiler. Depending upon the capacity of the
84
burner the number cyclone burners used may be one or more. If the number of cyclone
burners used is more than one, the diameter of each burner is less. These burners are
attached to the side of the furnace wall and have vents for primary air, crushed coal (6
mm diameter maximum size) and secondary air. It is water-cooled. The horizontal axis of
the burner is slightly deflected towards the boiler.
The cyclone burner receives crushed coal carried in primary air (at 80 cm water
pressure) at the left end. Tangential entry of the coal throws it to the surface of the
cylindrical furnace. Secondary air enters the furnace through tangential ports at the
upper edge at high speed and creates a strong and highly turbulent vortex. Extremely
high heat liberation rate and use of preheated air cause high temperatures to the tune of
2000°C in the cyclone. The fuel supplied is quickly consumed and liberated ash forms a
molten film flowing over the inner wall of the cylinder. Due to horizontal
Axis of the burner being tilted the molten ash flows to an appropriate disposal
system. The cyclone furnace gives best results with low grade fuels.
Advantages:
1. High furnace temperatures are obtained.
2. Simplified coal existing equipments can be used instead of costly pulverised
mills.
3. The cyclone burners reduce the percentage of excess air used.
4. It can bum poorer and cheaper grades of coal.
5. As the swirling effect and consequently the mixing of air and crushed coal is
better, it provides for a higher furnace capacity and efficiency.
6. Boiler efficiency is increased.
7. The cost of milling in cyclone fire is less as the finer particles are not required
in this case.
8. Combustion rates can be controlled by simultaneous manual adjustment of
fuel feed and air flow and response in firing rate changes is comparable to that
of pulverised coal firing.
85
Lecture No.19
Oil burners:
Principle of oil firing. The functions of an oil burner are to mix the fuel and air
in proper proportion and to prepare the fuel for combustion. Fig. 44 shows the principle
of oil firing.
Fig. 44. Principle of oil firing.
Classification of oil burners:
The oil burners may be classified as:
1. Vapourising oil burners:
1) Atmospheric pressure atomising burner
2) Rotating cup burner
3) Recirculation burner
4) Wick type burner
2. Atomising fuel burners:
1) Mechanical or oil pressure atomising burner
2) Steam or high pressure air atomising burner
3) Low pressure air atomising burner.
Oil burners – principles of oil firing – Classification of oil burners – Vaporising oil burners –
Atomizing fuel burners
86
1. Vapourising oil burners:
Following are the requirements of a vapourising evaporation oil burner:
1) To vapourise the fuel before ignition.
2) To mix the vapourised fuel thoroughly with the air.
3) To minimise the soot formation.
4) To give high heat release by burning large quantity of oil per hour.
5) To allow for efficient combustion of fuel at part load operation.
1) Atmospheric pressure atomising burner.
This burner makes use of highly volatile liquid fuels such as neptha, volatile
gasoline etc. Here the fuel at low pressure is passed through a tube adjacent to the
flame before being released through an orifice. While passing through the hot tube, most
of the fuel is vapourised so that the fluid ejected from the orifice is more or less a vapour.
The required quantity of primary air is supplied to burn the vapour stream in a cylindrical
tube.
2) Rotating cup burner.
In this type of burner, the fuel oil flows through a tube in the hollow shaft of the
burner and into the cup at the furnace end. An electric motor or an air turbine runs the
shaft and the cup at high, speeds (3000 to 10000 r.p.m.). As a result of centrifugal force
fuel is split into small droplets. About 10 to 15 percent of air is supplied, as primary air.
This air is supplied from a blower surrounding the cup. The shape of the flame is
governed by the sharp edge of the cup and the position of air nozzle. These burners are
used on low as well as medium capacity boilers.
3) Recirculating burner.
The part of the combustion products may be recirculated in order to heat up the
incoming stream of fuel and air. Low ratio of the mass of recirculated combustion
products to the mass of un burnt fuel-air mixture results in less temperature rise of the
mixture, whereas, high ratio may extinguish the flame due to increased proportions of
circulated products. An optimum ratio may be determined for different fuels
experimentally. In recirculation burner (utilising the above principle) circulation system is
separated from the combustion by a solid wall.
87
4) Wick burners.
In this type of a burner a cotton or asbestos wick is used which raises the liquid
fuel by, capillary action. The fuel from the uppermost part of the wick is evaporated due
to radiant heat from the flame and the nearby heated surfaces. Air is admitted through
holes in the surrounding walls. A wick burner is suitable for models or domestic
appliances.
2. Atomising fuel burners.
Following are the requirements of an automising fuel burner:
1. To automise the fuel into fine particles of equal size.
2. To supply air in required quantity at proper places in the combustion chamber.
3. To give high combustion intensity.
4. To give high thermal efficiency.
5. To operate without difficulty at varying loads.
6. To create necessary turbulence inside the combustion chamber for proper
combustion of fuel.
7. To minimise soot formation and carbon deposit, particularly on the burner
nozzle.
1) Mechanical atomising burners.
A mechanical atomising oil burner consists of the following four principal parts:
(1) Atomiser (2) Air register (3) Diffuser (4) Burner throat opening.
Atomiser breaks up the oil mechanically into a fine uniform spray that will burn
with minimum of excess air when projected into the furnace. The spray is produced by
using relatively high pressure to force oil at high velocity through small tangential
passages of sprayer plate.
2) Steam atomising burners.
This method may, however, absorb some 4 to 5% of the total amount of steam
generated. These burners may be divided into two categories:
(1) The outside mix
(2) The inside mix.
88
In case of outside mixing [Fig.45 (b)] type burners, oil is rejected through one
side of the holes and is blasted by a high velocity jet of steam issuing from other holes.
Mixing, however, occurs outside the burner.
Fig. 45. (a) Inside mixing (b) Outside mixing.
In case of inside mixing type burners steam and oil are mixed inside the burner
before the mixture is projected in the furnace. These burners provide high efficiency at
the high firing rates and flexible flame shape.
3) Low pressure air atomising burners.
In this case air pressure required ranges from 0.015 bar to 0.15 bar.
89
Lecture No. 20
Advantages of Gas burners:
1. It is much simpler as the fuel is ready for combustion and requires no
preparation.
2. Furnace temperature can be easily controlled.
3. A long slow burning flame with uniform and gradual heat liberation can be
produced.
4. Cleanliness.
5. High chimney is not required.
6. No ash removal is required.
For generation of steam, natural gas is invariably used in the following cases:
1. Gas producing areas.
2. Areas served by gas transmission lines.
3. Where coal is costlier.
Typical gas burners:
Typical gas burners used are shown in Fig. 46.
In this burner [Fig. 46 (a)], the mixing is poor and a fairly long flame results.
This is a ring type burner [Fig. 46 (b)] in which a short flame is obtained.
This arrangement [Fig. 46 (C)] is used when both gas and air are under pressure.
In order to prevent the flame from turning back, the velocity of the gas should be
more than the "rate of flame propagation".
(a) (b) (C)
Fig. 46. Typical gas burners.
Gas burners - Advantages
90
Lecture No. 21
Introduction:
Steam is a vapour of water, and is invisible when pure and dry. It is used as the
working substance in the operation of steam engines and steam turbines. Steam does
not obey laws of perfect gases, until it is perfectly dry. It has already been discussed that
when the dry vapour is heated further, it becomes superheated vapour which behaves,
more or less, like a perfect gas.
Formation of steam at a constant pressure from water:
Consider 1 kg of water at 0°C contained in the piston-cylinder arrangement as
shown in Fig. 47 (a). The piston and weights maintain a constant pressure in the
cylinder. If we heat the water contained in the cylinder, it will be converted into steam as
discussed below:
(a) (b) (c) (d) (e)
Fig. 47. Formation of steam at a constant pressure.
1. The volume of water will increase slightly with the increase in temperature as
shown in Fig. 47 (b). It will cause the piston to move slightly upwards and hence work is
obtained. This increase in volume of water (or work) is generally, neglected for all types
of calculations.
2. On further heating, temperature reaches boiling point. The boiling point of
water, at normal atmospheric pressure of 1.013 bar is 100°C, but it increases with the
increase in pressure. When the boiling point is reached, the temperature remains
Formation and properties of steam – introduction – Temperature vs. total heat graph during
steam formation – wet steam – Dry saturated steam – Super heated steam – Dryness fraction
– Sensible heat of water – Latent heat of vaporization – Enthalpy of stem – Specific volume of
steam
91
constant and the water evaporates, thus pushing the piston up against the constant
pressure. Consequently, the specific volume of steam increases as shown in Fig. 47 (c).
At this stage, the steam will have some particles of water in suspension, and is termed
as wet steam. This process will continue till the whole water is converted into wet steam.
3. On further heating, the water particles in suspension will be converted into
steam. The entire steam, in such a state, is termed as dry steam or saturated steam as
shown in Fig. 47 (d) Practically, the dry steam behaves like a perfect gas.
4. On further heating, the temperature of the steam starts rising. The steam, in
such a state, is termed as superheated steam as shown in Fig. 47 (e).
Temperature vs. Total Heat Graph during Steam Formation:
The process of steam formation may also be represented on a graph, whose
abscissa represents the total heat and the vertical ordinate represents the temperature.
The point A represents the initial condition of water at 0°C and pressure p (in bar) as
shown in Fig. 48. Line ABCD shows the relation between temperature and heat at a
specific pressure of p (in bar).
Fig. 48. Temperature-total heat graph during steam formation.
During the formation of the superheated steam, from water at freezing point, the
heat is absorbed in the following three stages:
92
1. The heating of water up to boiling temperature or saturation temperature (t) is
shown by AB in Fig. 48. The heat absorbed by the water is AP, known as sensible heat
or liquid heat or total heat of water.
2. The change of state from liquid to steam is shown by BC. The heat absorbed
during this stage is PQ, known as latent heat of vaporisation.
3. The superheating process is shown by CD. The heat absorbed during this
stage is QR, known as heat of superheat. Line AR represents the total heat of the
superheated steam.
If the pressure is increased, the boiling temperature also increases.
The line passing through the points A, B, E, K is known as saturated liquid line
which forms boundary line between water and steam. Similarly, a line passing through
dry steam points L, F, C is known as dry saturated steam line which forms boundary line
between wet and superheated steam. Sometimes, these terms are briefly written as
liquid line and dry steam line, but the word "saturated" is always understood.
It may also be noted from the figure, that when the pressure and saturation
temperature increases, the latent heat of vaporisation decreases. It becomes zero at a
point (N) where liquid and dry steam lines meet. This point N is known as the critical
point and at this point, the liquid and vapour phases merge, and become identical in
every respect. The temperature corresponding to critical point N is known as critical
temperature and the pressure is known as critical pressure. For steam, the critical
temperature is 374.15°C and critical pressure is 221.2 bar.
Important Terms for Steam:
1. Wet steam. When the steam contains moisture or particles of water in
suspension, it is said to be wet steam. It means that the evaporation of water is not
complete, and the whole of the latent heat has not been absorbed.
2. Dry saturated steam. When the wet steam is further heated, and it does not
contain any suspended particles of water, it is known as dry saturated steam. The dry
saturated steam has absorbed its full latent heat and behaves practically, in the same
way as a perfect gas.
93
3. Superheated steam. When the dry steam is further heated at a constant
pressure, thus raising its temperature, it is said to be superheated steam. Since the
pressure is constant, therefore the volume of superheated steam increases. It may be
noted that the volume of one kg of superheated steam is considerably greater than the
volume of one kg of dry saturated steam at the same pressure.
The superheated steam is produced in a separate apparatus known as
superheater, so that it is out of contact with water from which it was formed.
4. Dryness fraction or quality of wet steam. It is the ratio of the mass of actual
dry steam, to the mass of same quantity of wet steam, and is generally denoted by 'x'.
Mathematically,
x = m
m
mm
m g
fg
g =+
where mg = Mass of actual dry steam,
mf = Mass of water in suspension, and
m = Mass of wet steam = mg + mf
Note: The value of dryness fraction, in case of dry steam, is unity.
5. Sensible heat of water. It is the amount of heat absorbed by 1 kg of water,
when heated at a constant pressure, from the freezing point (0 °C) to the temperature of
formation of steam, i.e. saturation temperature (t). The sensible heat is also known as
liquid heat.
The specific heat of water at constant pressure is usually taken as 4.2 kJ/kg K.
Therefore heat absorbed by 1 kg of water from 0°C to t °C or sensible heat
= Mass x Sp. heat x Rise in temperature
= 1 x 4.2 [(t + 273) - (0 + 273)] = 4.2 t kJ/kg
Thus the sensible heat of water in kJ/kg may be-obtained directly by multiplying
the specific heat of water and the saturation temperature (t) in °C.
6. Latent heat of vaporisation. It is the amount of heat absorbed to evaporate
1 kg of water, at its boiling point or saturation temperature without change of
94
temperature. It is denoted by hfg and its value depends upon the pressure. The heat of
vaporisation of water or latent heat of steam is 2257 kJ/kg at atmospheric pressure.
The value of hfg decreases as the pressure increases and it is zero at critical
pressure. If the steam is wet with a dryness fraction x, then the heat absorbed by it
during evaporation is x hfg.
7. Enthalpy or total heat of steam. It is amount of heat absorbed by water from
freezing point to saturation temperature plus the heat absorbed during evaporation.
∴ Enthalpy or total heat of steam
= Sensible heat + Latent heat
It is denoted by hg and its value for the dry saturated steam may be read directly
from the steam tables. The expressions for the enthalpy of wet steam, dry steam and
superheated steam are as follows:
(1) Wet steam. The enthalpy of wet steam is given by:
h = hf + x hfg K (1)
where x is the dryness fraction of steam.
(2) Dry steam. We know that in case of dry steam, x = 1.
h = hg = hf+ hfg K (2)
(3) Superheated steam. If we further add heat to the dry steam, its temperature
increases while pressure remaining constant. This increase in temperature shows the
superheat stage of the steam. Thus, the total heat required for the steam to be
superheated is:
hsup = Total heat for dry steam + Heat for superheated steam
= hf+ hfg + cp (tsup - t) = hg + cp (tsup - t) **
where
cp = Mean specific heat at constant pressure for superheated steam,
tsup = Temperature of the superheated steam, and
t = Saturation temperature at the given constant pressure.
Notes: 1. The difference (tsup - t) is known as degree of superheat.
95
2. The value of cp for steam lies between 1.67 kJ/kg K to 2.5 kJ/kg K.
8. Specific volume of steam. It is the volume occupied by the steam per unit
mass at a given temperature and pressure, and is expressed in m3/kg. The value of
specific volume decreases with the increase in pressure.
96
Lecture No. 22
Steam Tables and their Uses:
The properties of dry saturated steam like its temperature of formation (saturation
temperature), sensible heat, latent heat of vaporisation, enthalpy or total heat, specific
volume, entropy etc., vary with pressure, and can be found by experiments only. These
properties have been carefully determined, and made available in a tabular form known
as steam tables.
There are two important steam tables, one in terms of absolute pressure and
other in terms of temperature.
Table 1 (Pressure)
Absolute pressure
(p) in bar
Temperature (t) in °C
Specific volume In m3/ kg
Specific enthalpy In kJ/kg
Specific entropy In kJ/kg K
Water (vf)
Steam (vg)
Water (hf)
Evapora tion (hfg)
Steam (hg)
Water (sf)
Evaporation (sfg)
Steam (sg)
0.010
0.015
0.20
6.983
13.04
60.09
0.001000
0.001001
0.001017
129.21
87.982
7.649
29.3
54.7
251.5
2485.1
2470.8
2358.4
2514.4
2525.5
2609.9
0.106
0.196
0.832
8.871
8.634
7.077
8.977
8.830
7.909
Table 2 (Temperature)
Temperature (t) In °C
Absolute pressure
(p) in bar
Specific volume In m3/ kg
Specific enthalpy In kJ/kg
Specific entropy In kg K
Water (vf)
Steam (vg)
Water (hf)
Evapora tion (hfg)
Steam (hg)
Water (sf)
Evaporation ns (sfg)
Steam (sg)
0
5
10
0.00611
0.00872
0.01227
0.00100
0
0.00100
0
0.00100
0
206.31
147.16
106.43
0.0
21.0
42.0
2501.6
2489.7
2477.9
2501.6
2510.7
2519.9
0.000
0.076
0.151
9.158
8.951
8.751
9.158
9.027
8.902
1 bar = 1 × 105 N/m2
Steam tables and their uses – Super heated steam – Problems on enthalpy of steam –
Advantage of super heating the steam
97
Problem:
Example 1. Calculate the enthalpy of 1 kg of steam at a pressure of 8 bar and dryness
fraction of 0.8. How much heat would be required to raise 2 kg of this seam from water
at 20°C.
Solution. Given: p = 8 bar, x = 0.8 = dryness fraction
Enthalpy of 1 kg of steam
From steam tables, corresponding to a pressure of 8 bar, we find that
hf = 720.9 kJ/kg and hfg = 2046.5 kJ/kg
Enthalpy of 1 kg of wet steam,
h = hf+ x hfg = 720.9 + (0.8 x 2,046.5) = 2358.1 kJ
Heat required to raise 2 kg of this steam from water at 20°C
We have calculated above the enthalpy or total heat required to raise 1 kg of
steam from water at 0°C. Since the water, in this case, is already at 20°C,
Heat already in water = 4.2 t = 4.2 x 20 = 84 kJ
∴Heat required per kg of steam
= 2358.1 - 84 = 2,274.1 kJ
and heat required for 2 kg of steam
= 2 x 2274.1 = 4,548.2 kJ
Superheated steam:
Whenever dry steam or saturated steam is further heated, then the steam is
termed as superheated steam. The process of superheating is assumed to follow
constant pressure process (or Charles' law).
The values of saturation temperature, specific volume, specific enthalpy and
specific entropy at a given pressure of the superheated steam are also given in the
tabular form known as steam tables of superheated steam.
98
Advantages of Superheating the Steam:
1. The superheated steam contains more heat contents, and hence its capacity
to do work is also increased without increasing its pressure.
2. The superheating is done in a superheater, which obtains heat from waste
furnace gases. These gases would have otherwise passed, uselessly, through
the chimney.
3. The high temperature of the superheated steam results in an increase of
thermal efficiency.
4. Since the superheated steam is at a higher temperature than that
corresponding to its pressure, therefore it can be considerably cooled during
expansion in an engine cylinder. This is done before the temperature of
superheated steam falls below that at which it condenses and, thereby,
becomes wet. It is thus obvious, that heat losses due to condensation steam
on cylinder walls, etc., are avoided to a great extent.
99
Lecture No. 23
Introduction:
A steam generator or boiler is, usually, a closed vessel made of steel. Its function
is to transfer the heat produced by the combustion of fuel (solid, liquid or gaseous) to
water, and ultimately to generate steam. The steam produced may be supplied:
1. To an external combustion engine, i.e. steam engines and turbines,
2. At low pressures for industrial process work in cotton mills, sugar factories,
breweries, etc., and
3. For producing hot water, this can be used for heating installations at much
lower pressures.
Important terms for steam boilers:
1. Boiler shell. It is made up of steel plates bent into cylindrical form and riveted
or welded together. The ends of the shell are closed by means of end plates. A boiler
shell should have sufficient capacity to contain water and steam.
2. Combustion chamber. It is the space, generally below the boiler shell, meant
for burning fuel in order to produce steam from the water contained in the shell.
3. Grate. It is a platform, in the combustion chamber, upon which fuel (coal or
wood) is burnt. The grate, generally, consists of cast iron bars which are spaced apart so
that air (required for combustion) can pass through them. The surface area of the grate,
over which the fire takes place, is called grate surface.
4. Furnace. It is the space, above the grate and below the boiler shell, in which
the fuel is actually burnt. The furnace is also called fire box.
5. Heating surface. It is that part of boiler surface, which is exposed to the fire
(or hot gases from the fire).
6. Mountings. These are the fittings which are mounted on the boiler for its
proper functioning. They include water level indicator, pressure gauge, safety valve etc.
It may be noted that a boiler cannot function safely with out the mountings.
Steam Boiler – introduction – important term for steam boiler - Essential of a good steam
boiler – steam boiler – Selection of a steam boiler – Classification of steam boilers
100
7. Accessories. These are the devices, which form an integral part of a boiler,
but are not mounted on it. They include super heater, economiser, feed pump etc. It may
be noted that the accessories help in controlling and running the boiler efficiently.
Essentials of a Good Steam Boiler:
Following are the important essentials of a good steam boiler:
1. It should produce maximum quantity of steam with the minimum fuel
consumption.
2. It should be economical to instal, and should require little attention during
operation.
3. It should rapidly meet the fluctuation of load.
4. It should be capable of quick starting.
5. It should be light in weight.
6. It should occupy a small space.
7. The joints should be few and accessible for inspection.
8. The mud and other deposits should not collect on the heating plates.
9. The refractory material should be reduced to a minimum. But it should be
sufficient to secure easy ignition, and smokeless combustion of the fuel on
reduced load.
10. The tubes should not accumulate soot or water deposits, and should have a
reasonable margin of strength to allow for wear or corrosion.
11. The water and flue gas circuits should be designed to allow a maximum fluid
velocity without incurring heavy frictional losses.
12. It should comply with safety regulations as laid down in the Boilers Act.
Selection of a Steam Boiler:
The selection of type and size of a steam boiler depends on the following factors:
1. The power required and the working pressure.
2. The rate at which steam is to be generated.
3. The geographical position of the power house.
4. The fuel and water available.
5. The type of fuel to be used.
6. The probable permanency of the station.
7. The probable load factor.
101
Classifications of Steam Boilers:
Though there are many classifications of steam boilers, yet the following are
important from the subject point of view:
1. According to the contents in the tube.
The steam boilers, according to the contents in the tube may be classified as:
(1) Fire tube or smoke tube boiler, and
(2) Water tube boiler.
In fire tube steam boilers, the flames and hot gases, produced by the combustion
of fuel, pass through the tubes (called multi-tubes) which are surrounded by water. The
heat is conducted through the walls of the tubes from the hot gases to the surrounding
water. Examples of fire tube boilers are: Simple vertical boiler, Cochran boiler,
Lancashire boiler, Cornish boiler, Scotch marine boiler, Locomotive boiler, and Velcon
boiler.
In water tube steam boilers, the water is contained inside the tubes (called water
tubes) which are surrounded by flames and hot gases from outside. Examples of water
tube boilers are: Babcock and Wilcox boiler, Stirling boiler, La-Mont boiler, Benson
boiler, Yarrow boiler and Loeffler boiler.
2. According to the position of the furnace.
The steam boilers, according to the position of the furnace are classified as:
(1) Internally fired boilers, and (2) Externally fired boilers.
In internally fired steam boilers, the furnace is located inside the boiler shell. Most
of the fire tube steam boilers are internally fired.
In externally fired steam boilers, the furnace is arranged underneath in a brick-
work setting.
Water tube steam boilers are always externally fired.
102
3. According to the axis of the shell.
The steam boilers, according to the axis of the shell, may be classified as:
(1) Vertical boilers, and (2) Horizontal boilers.
In vertical steam boilers, the axis of the shell is vertical. Simple vertical boiler and
Cochran boiler are vertical boilers.
In horizontal steam boilers the axis of the shell is horizontal. Lancashire boiler,
Locomotive boiler and Babcock and Wilcox boiler are horizontal boilers.
4. According to the number of tubes.
The steam boilers, according to the number of tubes, may be classified as:
(1) Single tube boilers, and (2) Multi tubular boilers.
In single tube steam boilers, there is only one fire tube or water tube. Simple
vertical boiler and Cornish boiler are single tube boilers.
In multi tubular steam boilers, there are two or more fire tubes or water tubes.
Lancashire boiler, Locomotive boiler, Cochran boiler, Babcock and Wilcox boiler are
multi tubular boilers.
5. According to the method of circulation of water and steam.
The steam boilers, according to the method of circulation of water and steam,
may be classified as:.
(1) Natural circulation boilers, and (2) Forced circulation boilers.
In natural circulation steam boilers, the circulation of water is by natural
convection currents, which are set up during the heating of water. In most of the steam
boilers, there is a natural circulation of water.
In forced circulation steam boilers, there is a forced circulation of water by a
centrifugal pump driven by some external power. Use of forced circulation is made in
high pressure boilers such as La-Mont boiler, Benson boiler, Loeffler boiler and Velcon
boiler.
103
6. According to the use.
The steam boilers, according to their use, may be classified as
(1) Stationary boilers and (2) Mobile boilers.
The stationary steam boilers are used in power plants, and in industrial process
work. These are called stationary because they do not move from one place to another.
The mobile steam boilers are those which move from one place to another.
These boilers are locomotive and marine boilers.
7. According to the source of heat.
The steam boilers may also be classified according to the source of heat
supplied for producing steam. These sources may be the combustion of solid, liquid or
gaseous fuel, hot waste gases as by-products of other chemical processes, electrical
energy or nuclear energy, etc.
104
Lecture No. 24
Simple Vertical Boiler:
A simple vertical boiler produces steam at a low pressure and in small quantities.
It is used for low power generation or at places where the space is limited. The
construction of this type of boiler is shown in Fig. 49.
Fig. 49. Simple vertical boiler.
It consists of a cylindrical shell surrounding a nearly cylindrical fire box. The fire
box is slightly tapered towards the top to allow the ready passage of the steam to the
surface. At the bottom of the fire box is a grate. The fire box is fitted with two or more
inclined cross tubes F, F. The inclination is provided to increase the heating surface as
well as to improve the circulation of water. An uptake tube passes from the top of the fire
box to the chimney. The handholes are provided opposite to the end of each water tube
for cleaning deposits. A manhole is provided at the top for a man to enter and clean the
boiler. A mudhole is provided at the bottom of the shell to remove the mud that settles
down. The space between the boiler shell and fire box is filled with water to be heated.
Cochran Boiler or Vertical Multitubular Boiler:
There are various designs of vertical multitubular boilers. A Cochran boiler is
considered to be one of the most efficient types of such boilers. It is an improved type of
simple vertical boiler.
Simple vertical boiler – Cochran boiler – Scotch marine boiler – Lancashire boiler – Cornish
boiler
105
This boiler consists of an external cylindrical shell and a fire box as shown in
Fig. 50. The shell and fire box are both hemispherical, the hemispherical crown of the
boiler shell gives maximum space and strength to withstand the pressure of steam inside
the boiler. The hemispherical crown of the fire box is also advantageous for resisting
intense heat. The fire box and the combustion chamber is connected through a short
pipe. The flue gases from the combustion chamber flow to the smoke box through a
number of smoke tubes. These tubes c generally have 62.5 mm external diameter and
are 165 in number. The gases from the smoke box pass to the atmosphere through a
chimney. The combustion chamber is lined with fire bricks on the shell side. A manhole
near the top of the crown on the shell is provided for cleaning.
Fig. 50. Cochran boiler.
At the bottom of the fire box, there is a grate (in case of coal firing) and the coal is
fed through the fire hole. If the boiler is used for oil firing, no grate is provided, but the
bottom of the fire box is lined with firebricks. The oil burner is fitted at the fire hole.
Scotch Marine Boiler:
The marine steam boilers of the scotch or tank type are used for marine works,
particularly, due to their compactness, efficiency in operation and their ability to use any
type of water. It does not require brick work setting and external flues.
It has a drum of diameter from 2.5 to 3.5 metres placed horizontally. These steam
boilers may be single ended or double ended. The length of a single ended steam boiler
may be upto 3.5 meters while for double ended upto 6.5 meters. A single ended boiler
106
has one to four furnaces which enter from front end of the boiler. A double ended boiler
has furnaces on both of its ends, and may have furnaces from two to four in each end.
Fig. 51. Scotch Marine Boiler.
A single ended scotch marine steam boiler is fired by four furnaces, as shown in
Fig. 51.The furnaces are generally corrugated for strength. Each furnace has its own
combustion chamber. There are fine flat plates in the combustion chamber, which
require staying, i.e. the top plate, back plate, two side plates and the tube plate. There
are a number of smoke tubes placed horizontally and connect the combustion chamber
to chimney. The front and back plates of the shell are strengthened by longitudinal stays.
The combustion chamber walls form the best heating surface. The furnace tubes,
smoke tubes and the combustion chamber, all being surrounded by water, give a very
large heating surface area in proportion to the cubical size of boiler. The water circulates
around the smoke tubes. The level of water is maintained a little above the combustion
chamber. The flue gases, from the combustion chamber, are forwarded by draught
through the smoke tubes, and finally up the chimney. The smoke box is provided with a
door for cleaning the tubes and smoke box.
107
Lancashire Boiler:
It is a stationary, fire tube, internally fired, horizontal and natural circulation boiler
It is used where working pressure and power required are moderate. These boilers have
a cylindrical shell of 1.75 m to 2.75 m diameter. Its length varies from 7.25 m to 9 m. It
has two internal flue tubes having diameter about 0.4 times that of shell. This type of
boiler is set in brick work forming external flue so that part of the heating surface is on
the external shell.
A Lancashire boiler with brick work setting is shown in Fig. 52. This boiler consists
of a long cylindrical external shell (I) built of steel plates, in sections riveted together. It
has two large internal flue tubes (2). These are reduced in diameter at the back end to
provide access� to the lower part of the boiler. A fire grate (3) also called furnace, is
provided at one end of the flue tubes on which solid fuel is burnt. At the end of the fire
grate, there is a brick arch (5) to deflect the flue gases upwards. The hot flue gases,
after leaving the internal flue tubes pass down to the bottom tube (6). These flue gases
move to the front of the boiler where they divide and flow into the side flue (7). The flue
gases then enter the main flue (9), which leads them to chimney.
Fig. 52. Elevation, side and plan of Lancashire boiler.
108
The damper (8) is fitted at the end of side flues to control the draught (i.e. rate of
flow of air) and thus regulate the rate of generation of steam. These dampers are
operated by chain passing over a pulley on the front of the boiler.
A spring loaded safety valve (10) and a stop valve (11) is mounted as shown in
Fig. 52. The stop valve supplies steam to the engine as required. A high steam and low
water safety valve (12) is also provided.
A perforated feed pipe (14) controlled by a feed valve is used for feeding water
uniformly. When the boiler is strongly heated, the steam generated carries a large
quantity of water in the steam space, known as priming. An antipriming pipe (15) is
provided to separate out water as far as possible. The stop valve thus receives dry
steam.
A blow-off cock (16) removes mud, etc., that settles down at the bottom of the
boiler, by forcing out some of the water. It is also used to empty water in the boiler,
whenever required for inspection. Manholes are provided at the top and bottom of the
boiler for cleaning and repair purposes.
Cornish Boiler:
It is similar to a Lancashire boiler in all respects, except there is only one flue tube
in Cornish boiler instead of two in Lancashire boiler, as shown in Fig. 53. The diameter
of Cornish boiler is generally 1 m to 2 m and its length varies from 5 m to 7.5 m. The
diameter of flue tube may be about 0.6 times that of shell. The capacity and working
pressure of a Cornish boiler is low as compared to Lancashire boiler.
Fig. 53. Cornish boiler.
109
Lecture No. 25
Locomotive Boiler:
It is a multi-tubular, horizontal, internally fired and mobile boiler. The principal
feature of this boiler is to produce steam at a very high rate. A modem type of a
locomotive boiler is shown in Fig. 54. It consists of a shell or barrel having 1.5 metres
diameter and 4 metres in length. The coal is fed into the fire box through the fire door
and burns on grate. The flue gases from the grate are deflected by a brick arch, and thus
whole of the fire box is properly heated. There are about 157 thin tubes or fire tubes F
(47.5 mm diameter) and 24 thick or superheated tubes G (130 mm diameter). The flue
gases after passing through these tubes enter a smoke box. The gases are then lead to
atmosphere through a chimney. The barrel contains water around the tubes, which is
heated up by the flue gases and gets converted into steam.
Fig. 54. Locomotive Boiler.
A stop valve as regulator is provided inside a cylindrical steam dome. This is
operated by a regulator shaft from the engine room by a driver. The header is divided
into two portions, one is the superheated steam chamber and the other is the saturated
steam chamber. The steam pipe leads the steam from the regulator to the saturated
steam chamber. It then leads the steam to the superheated tubes, and after passing
Locomotive boiler – Babcock and Wilcox boiler – comparison between water tube and fire
tube boilers
110
through these tubes, the steam returns back to the superheated steam chamber. The
superheated steam now flows through the steam pipe to the cylinder, one on each side.
The draught is due to the exhaust steam from the cylinders, which is discharged through
the exhaust pipe. The front door can be opened for cleaning or repairing the smoke box.
The safety valves and a steam whistle are provided as shown in Fig. 54.The ash
from the grate is collected in ash pan and is discharged out from time to time by opening
it with the help of dampers operated by rods and levers.
Babcock and Wilcox Boiler:
It is a straight tube, stationary type water tube boiler, as shown in Fig. 55. It
consists of a steam and water drum (1). It is connected by a short tube with uptake
header or riser (2) at the back end.
Fig. 55. Babcock and Wilcox Boiler.
The water tubes (5) (100 mm diameter) are inclined to the horizontal and
connects the uptake header to the down take header. Each row of the tubes is
connected with two headers, and there are plenty of such rows. The headers are curved
when viewed in the direction of tubes so that one tube is not in the space of other, and
hot gases can pass properly after heating all the tubes. The headers are provided with
hand holes in the front of the tubes and are covered with caps (18).
111
A mud box (6) is provided with each down take header and the mud, that settles
down is removed. There is a slow moving automatic chain grate on which the coal is fed
from the hopper (21). A fire bricks baffle causes hot gases to move upwards and
downwards and again upwards before leaving the chimney. The dampers (17) are
operated by a chain (22) which passes over a pulley to the front of a boiler to regulate
the draught.
The boiler is suspended on steel girders, and surrounded on all the four sides by
fire brick walls. The doors (4) are provided for a man to enter the boiler for repairing and
cleaning. Water circulates from the drum (I) into the header (2) and through the tubes (5)
to header (3) and again to the drum. Water continues to circulate like this till it is
evaporated. A steam super heater consists of a large number of steel tubes (10) and
contains two boxes; one is superheated steam box (II) and other is saturated steam box
(12).
The steam generated above the water level in the drum flows in the dry pipe (13)
and through the inlet tubes into the superheated steam box (11). It then passes through
the tubes (10) into the saturated steam box (12). The steam, during its passage through
tubes (10), gets further heated and becomes saturated steam box the steam is now
taken through the outlet pipe (14) to the stop valve (15). The boiler is fitted with usual
mountings, such as safety valve (19), feed valve (20), water level indicator (8) and
pressure gauge (9).
Comparison Between Water Tube and Fire Tube Boilers:
Following are the few points of comparison between a water tube and a fire
tube boiler.
S.No.
Water tube boiler
Fire tube boiler
1.
The water circulates inside the tubes which are surrounded by hot gases from the furnace.
The hot gases from the furnace pass through the tubes which are surrounded by water.
2.
It generates steam at a higher pressure upto 165 bar.
It can generate steam only upto 24.5 bar.
3.
The rate of generation of steam is high, i.e. upto 450 tonnes per hour.
The rate of generation of steam is low, i.e. upto 9 tonnes per hour.
112
4.
For a given power, the floor area required for the generation of steam is less, i.e. about 5 m2 per tonne
The floor area required is more, i.e. about 8 m2 per tonne per hour of steam generation.
5.
per hour of steam generation. Overall efficiency with economiser is upto 90%.
Its overall efficiency is only 75%.
6.
It can be transported and erected easily as its various parts can be separated.
The transportation and erection is difficult.
7.
It is preferred for widely fluctuating loads.
It can also cope reasonably with sudden increase in load but for a shorter period.
8.
The direction of water circulation is well defined.
The water does not circulate in a definite direction.
9. The operating cost is high. The operating cost is less.
10. The bursting chances are more. The bursting chances are less.
11.
The bursting does not produce any destruction to the whole boiler.
The bursting produces greater risk to the damage of the property.
12. It is used for large power plants. It is not suitable for large plants.
113
Lecture No. 26
Introduction:
The boiler mountings and accessories are required for the proper and
satisfactory functioning of the steam boilers.
Boiler Mountings:
These are the fittings, which are mounted on the boiler for its proper and safe
functioning. The following are important from the subject point of view:
1. Water level indicator; 2. Pressure gauge; 3. Safety valves; 4. Stop valve; 5. Blow off
cock; 6. Feed check valve; and 7. Fusible plug.
Water Level Indicator:
It is an important fitting, which indicates the water level inside the boiler to an
observer. It is a safety device, upon which the correct working of the boiler depends.
This fitting may be seen in front of the boiler, and are generally two in number. A water
level indicator, mostly employed in the steam boiler is shown in Fig 56.
Fig. 56. Water level indicator.
Boiler mounting viz., Water level indicator, Pressure gauge, Safety, Stop valve, Blow off
cock, Feed check valve, Fusible plug – their function. Boiler accessories viz., feed pump,
super heater, economizer, air – pre heater - function
114
It consists of three cocks and a glass tube. Steam cock C1 keeps the glass tube
in connection with the steam space. Water cock C2 puts the glass tube in connection
with the water in the boiler. Drain cock C3 is used at frequent intervals to ascertain that
the steam and water cocks are clear.
In the working of a steam boiler and for the proper functioning of the water level
indicator, the steam and water cocks are opened and the drain cock is closed. In this
case, the handles are placed in a vertical position as shown in Fig 56. The rectangular
passage at the ends of the glass tube contains two balls. In case the glass tube is
broken, the two balls are carried along its passages to the ends of the glass tube. It is
thus obvious, that water and steam will not escape out, The glass tube can be easily
placed by closing the steam and water cocks and opening the drain cock. When the
steam boiler is not working, the bolts may be removed for cleaning. The glass tube is
kept free from leaking by means of conical ring and the gland nut.
Pressure Gauge:
A pressure gauge is used to measure the pressure of the steam inside the steam
boiler. It is fixed in front of the steam boiler. The pressure gauges generally used are of
Bourden type. A Bourden pressure gauge, in its simplest form, consists of an elliptical
elastic tube ABC bent into an arc of a circle, as shown in Fig 57. This bent up tube is
called Bourden’s tube.
Fig. 57 Bourdan type pressure gauge.
115
One end of the tube gauge is fixed and connected to the steam space in the
boiler. The other end is connected to a sector through a link. The steam, under pressure,
flows into the tube. As a result of this increased pressure, the Bourden’s tube tends to
straighten itself. Since the tube is encased in a circular curve, therefore it tends to
become circular instead of straight. With the help of a simple pinion and sector
arrangement, the elastic deformation of the Bourden’s tube rotates the pointer. This
pointer moves over a calibrated scale, which directly gives the gauge pressure.
Safety Valves:
These are the devices attached to the steam chest for preventing explosions
due to excessive internal pressure of steam. A steam boiler is, usually, provided with two
safety valves. These are directly placed on the boiler. The function of a safety valve is to
blow off the steam when the pressure of steam inside the boiler exceeds the working
pressure. The following are the four types of safety valves:
1. Lever safety valve, 2. Dead weight safety valve, 3. High steam and low water
safety valve, and 4. Spring loaded safety valve.
Lever Safety Valve:
A lever safety valve used on steam boilers is shown in Fig. 58. It serves the
purpose of maintaining constant safe pressure inside the steam boiler. If the pressure
inside the boiler exceeds the designed limit, the valve lifts from its seat and blows off the
steam pressure automatically.
A lever safety valve consists of a valve body with a flange fixed to the steam
boiler. The bronze valve seat is screwed to the body, and the valve is also made of
bronze. It may be noted that by using the valve and seat of the same material, rusting is
considerably reduced. The thrust on the valve is transmitted by the strut. The guide
keeps the lever in a vertical plane. The load is properly adjusted at the other end of the
lever.
When the pressure of steam exceeds the safe limit, the upward thrust of steam
raises the valve from its seat. This allows the steam to escape till the pressure falls back
to its normal value. The valve then returns back to its original position.
116
Fig. 58 Lever safety valve.
Dead Weight Safety Valve:
A dead weight safety valve, used for stationary boilers, is shown in Fig. 59. The
valve is made of gun metal, and rests on its gun metal seat. It is fixed to the top of a
steel pipe. This pipe is bolted to the mountings block, riveted to the top of the shell. Both
the valve and the pipe are covered by a case which contains weights.
Fig. 59. Dead weight safety valve.
These weights keep the valve on its seat under normal working pressure. The
case hangs freely over the valve to which it is secured by means of a nut. When the
pressure of steam exceeds the normal pressure, the valve as well as the case are lifted
up from its seat. This enables the steam to escape through the discharge pipe, which
117
carries the steam outside the boiler house. The centre of gravity of the dead weight
safety valve is considerably below the valve which ensures that the load hangs vertically.
High Steam Low Water Safety Valve:
This valve is placed at the top of Cornish and Lancashire boilers only. It is a
combination of two valves, one of which is the lever safety valve which blows off steam
when the working pressure of steam exceeds. The second valve operates by blowing off
the steam when the water level becomes too low.
It consists of a main valve (known as lever safety valve) is shown in Fig. 60. In
the centre of the main valve, a seat for a hemispherical valve is formed for low water
operation. When the water level falls, the weight E comes out of water and the weight F
will not be sufficient to balance weight E. Therefore weight E comes down. There are
two projections on the lever to the left of the fulcrum which comes in contact with a collar
attached to the rod. When weight E comes down, the hemispherical valve is lifted up and
the steam escapes with a loud noise, which warns the operator.
Fig. 60. High steam low water safety valve.
Spring Loaded Safety Valve:
A spring loaded safety valve (Fig. 61) is mainly used for locomotives and marine
boilers. It is loaded with spring instead of weights.
118
Fig. 61. Spring Loaded Safety Valve.
Steam Stop Valve:
It is the largest valve on the steam boiler. It is, usually, fitted to the highest part of
the shell by means of a flange as shown in Fig. 62. The principal functions of a stop
valve are:
1. To control the flow of steam from the boiler to the main steam pipe.
2. To shut off the steam completely when not required.
Blow off Cock:
The principal functions of a blow-off cock are :
1. To empty the boiler whenever required
2. To discharge the mud, scale or sediments which are accumulated at the
bottom of the boiler.
The blow-off cock, as shown in Fig. 63 is fitted to the bottom of a boiler drum and
consists of a conical plug fitted to the body or casing.
119
Fig. 62. Steam stop valve.
Fig. 63. Blow off cock.
Feed Check Valve:
It is a non-return valve, fitted to a screwed spindle to regulate the lift. Its function
is to regulate the supply of water, which is pumped into the boiler, by the feed pump. A
feed check valve for marine boilers is shown in Fig. 64. The spindle is made of muntz
metal.
120
Fig. 64. Feed Check Valve.
Fusible Plug:
Its object is to put off the fire in the furnace of the boiler when the level of water in
the boiler falls to an unsafe limit, and thus avoids the explosion which may take place
due to overheating of the furnace plate. A fusible plug (Fig. 65) must not be refilled with
anything except fusible metal.
Fig. 65. Fusible Plug.
121
Boiler Accessories:
These are the devices which are used as integral parts of a boiler, and help in
running efficiently. Though there are many types of boiler accessories, yet the following
are important from the subject point of view:
1. Feed pump 2. Super-heater 3. Economiser and 4. Air preheater.
Fig. 66 shows the schematic diagram of a boiler plnat with the above mentioned
accessories.
Fig. 66. Schematic diagram of a boiler plant.
Feed Pump:
The water in a boiler, is continuously converted into steam, which is used by the
engine. Thus a feed pump is needed to deliver water to the boiler. The pressure of
steam inside a boiler is high. So the pressure of feed water has to be increased
proportionately before it is made to enter the boiler. Generally, the pressure of feed
water is 20% more than that in the boiler.Double acting reciprocating pump is commonly
used as a feed pump these days. These pumps may be classified as simplex, duplex
and triplex pumps according to the number of pump cylinders. The common type of
pump used is a duplex feed pump (Fig. 67).
Superheater:
Its purpose is to increase the temperature of saturated steam without raising its
pressure. It is generally an integral part of a boiler, and is placed in the path of hot flue
gases from the furnace. The heat, given up by these flue gases, is used in superheating
the steam. Such superheaters, which are installed within the boiler, are known as
integral superheaters. A Sudgen's superheater commonly employed with Lancashire
boiler is shown in Fig. 68.
122
Fig.67. Duplex feed pump.
Fig. 68. Superheater.
123
Economiser:
An economiser is a device used to heat feed water by utilising the heat in the
exhaust flue gases before leaving through the chimney. A well known type of
economiser is Greens economizer. It is extensively used for stationary boilers, especially
those of Lancashire type. It consists of a large number of vertical pipes or tubes placed
in an enlargement of the flue gases between the boiler and chimney as shown in Fig. 69.
These tubes are 2.75 m long, 114 mm in external diameter and 11.5 mm thick and are
made of cast iron.
The economizer is built-up of transverse section. Each section consists of
generally six or eight vertical tubes (1). These tubes are joined to horizontal pipes or
boxes (2) and (3) at the top and bottom respectively. The top boxes (2) of the different
sections are connected to the pipe (4), while the bottom boxes are connected to pipe (5).
The pipes (4) and (5) are on opposite sides, which are outside the brickwork enclosing
the economizer. The feed water is pumped into the economizer at (6) and enters the
pipe (5). It then passes into the bottom boxes (3) and then into the top boxes (2) through
the tubes (1). It is now led by the pipe (4) to the pipe (7) and then to the boiler. There is a
blow-off cock at the end of the pipe (5) opposite to the feed inlet (6). The purpose of this
valve is to remove mud or sediments deposited in the bottom boxes. At the end of pipe
(4) (opposite to the feed outlet) there is a safety valve.
It is essential that the vertical tubes may be kept free from deposits of soot, which
greatly reduce the efficiency of the economiser. Each tube is provided with scraper for
this purpose. The scrapers of two adjoining sections of tubes are grouped together and
coupled by rods and chains to the adjacent group. The pulley (9) of each chain
connected to a worm wheel (10) which is driven by a worm on a longitudinal shaft. The
scrapers automatically reverse when they reach the top or bottom end of the tubes. The
speed of scraper is about 46 m/h.
The temperature of feed should not be less than about 35°C, otherwise there is a
danger of corrosion due to the moisture in the flue gases being deposited in cold tubes.
Following are the advantages of using an economiser:
1. There is about 15 to 20% of coal saving.
2. It increases the steam raising capacity.
3. It prevents formation of scale in boiler water tubes.
4. Strains due to unequal expansion are minimised.
124
Fig. 69. Economiser.
Air Preheater:
An air preheater is used to recover heat from the exhaust flue gases. It is
installed between the economiser and the chimney. The air is passed through the tubes
of the heater internally while the hot flue gases are passed over the outside of the tubes.
The following advantages are obtained by using an air preheater
1. The preheated air gives higher furnace temperature which results in more heat
transfer to the water.
2. Increase of about 2% in the boiler efficiency.
3. Better combustion with less soot, smoke and ash.
4. A low grade fuel to be burnt with less excess air.
125
Lecture No. 27
Introduction:
The performance of a steam boiler is measured in terms of its evaporative
capacity. The evaporative capacity or power of a boiler is the amount of water
evaporated or steam produced in kg/h. It may also be expressed in kg/kg of fuel burnt or
kg/h/m2 of heating surface. The feed temperature usually adopted is 100°C and the
working pressure as normal atmospheric pressure, i.e. 1.013 bar.
Equivalent Evaporation:
It is the amount of water evaporated from feed water at 100°C and formed into
dry and saturated steam at 100°C at normal atmospheric pressure. It is, usually, written
as "from and at 100°C”.
Equivalent evaporation “from and at 100°C”,
E = 2257
waterfeedevapiratetorequiredheatTotal
E = ( )2257
1fe hhm −
Where,
me = mass of water actually evaporated or steam produced in kg/h or kg/kg
h = enthalpy or total heat of steam in kJ/kg of steam corresponding to a given
working pressure (from steam tables)
hf1 = enthalpy or sensible heat of feed water in kJ/kg of steam corresponding to
t1°C (from steam tables)
Note: The factor 2257
1fhh − is known as factor of evaporation, and is usually denoted by Fe.
Its value is always greater than unity for all boilers.
Performance of steam boilers – Equivalent evaporation – Boiler efficiency – Problems on
boiler efficiency
126
Boiler Efficiency:
It is the ratio of heat actually used in producing the steam to the heat liberated in
the furnace. It is also known as thermal efficiency of the boiler.
Boiler efficiency or thermal efficiency,
( )C
hhm
furnacetheinliberatedHeat
steamproducinginusedactuallyHeat fe 1−==η
Where
me = mass of water actually evaporated or actual evaporation in kg/kg of fuel
C= calorific value of fuel in kJ/kg of fuel
me = fuelofkgkgm
m
f
s /
Where
ms = mass of water evaporated into steam in kg
mf = mass of fuel used in kg
∴( )
Cm
hhm
f
fs
×
−= 1η
If a boiler consisting of an economiser and superheater is considered to be a
single unit, then the efficiency is termed as overall efficiency of the boiler.
Problems on boiler efficiency:
Example 1. A boiler evaporates 3.6 kg of water per kg of coal into dry saturated steam
at 10 bar. The temperature of feed water is 32°C. Find the equivalent evaporation “from
and at
100°C” as well as the factor of evaporation.
Solution. Given: me = 3.6 kg/kg of coal; p = 10 bar; t1 = 32°C
From steam tables, corresponding to a feed water temperature of 32°C, we find that
hfl = 134 kJ/kg
and corresponding to a steam pressure of 10 bar, we find that
h = hg = 2776.2 kJ/kg K. (For dry saturated steam)
Equivalent evaporation 'from and at 100°C',
127
E = ( )2257
1fe hhm −=
( )2257
1342.27766.3 −= 4.2 kg / kg of coal
Factor of evaporation
= 2257
1fhh − =
2257
1342.2776 − = 1.17
Example 2. The following observations were made in a boiler trial:
Coal used 250 kg of calorific value 29,800 kJ/kg; water evaporated 2,000 kg; steam
pressure 11.5 bar; dryness fraction of steam 0.95 and feed water temperature 34°C.
Calculate the equivalent eyaporation ''from and at 100°C" per kg of coal and the
efficiency of the boiler.
Solution. Given: mf = 250 kg; C = 29,800 kJ/kg; ms = 2,000 kg; p = 11.5 bar; x = 0.95;
tl = 34°C
From steam tables, corresponding to a feed water temperature of 34 °C, we find that
hf1 = 142.4 kJ/kg
and corresponding to a steam pressure of 11.5 bar, we find that
hf = 790 kJ/kg; hfg = 1991.4 kJ/kg
Enthalpy or total heat of steam,
h = hf + x hfg = 790 + (0.95 x 1991.4) = 2681.8 kJ/kg
and mass of water evaporated per kg of coal
me = ms/mf = 2000/250 = 8 kg / kg of coal
∴ Equivalent evaporation 'from and at 100°C'
E = ( )2257
1fe hhm − =
( )2257
4.1428.26818 − = 0.682 Kg/kg of coal
Efficiency of the boiler,
( )C
hhm fe 1−=η =
( )2257
4.1428.26818 − = 0.682 or 68.2%
128
Lecture No. 28
Boiler Trial:
The main objects of a boiler trial are:
1. To determine the generating capacity of the boiler.
2. To determine the thermal efficiency of the boiler when working at a definite
pressure.
3. To prepare heat balance sheet for the boiler.
Heat Losses in a Boiler:
The efficiency of a boiler is the ratio of heat utilised in producing steam to the
heat liberated in the furnace. The heat utilised is always less than the heat liberated in
the furnace. The difference of heat liberated in the furnace and heat utilised in producing
steam is known as heat lost in the boiler.
1. Heat lost in dry flue gases
2. Heat lost in moisture present in the fuel
3. Heat lost to steam formed by combustion of hydrogen per kg of fuel:
4. Heat lost due to unburnt carbon in ash pit
5. Heat lost due to incomplete combustion of carbon to carbon monoxide (CO)
6. Heat lost due to radiation
Heat Balance Sheet:
A heat balance sheet shows the complete account of heat supplied by 1 kg of dry
fuel (equal to the calorific value of the fuel) and heat consumed. The heat supplied is
mainly utilised for raising the steam and the remaining heat is lost.
Heat utilised in raising steam per kg of fuel
= me (h - hfl)
The heat balance sheet for a boiler trial per kg of fuel is drawn as below:
Boiler trial – Heat losses in a boiler – Heat balance sheet
129
Heat supplied kJ Heat consumed kJ %
Heat supplied
by 1 kg of dry
fuel
X 1. Heat utilised in raising steam
2. Heat lost in dry flue gases
3. heat lost to steam by
combustion of hydrogen
4. Heat lost to steam by
combustion of hydrogen
5. Heat lost due to unburnt
carbon in ash pit
6. Heat lost due to incomplete
combustion
7. Heat lost due to radiation,
etc. (by difference)
x1
x2
x3
x4
x5
x6
X-(x1 + x2 + x3 + x4
+ x5 + x6)
K
K
K
K
K
K
K
Total X Total X 100%
130
Lecture No. 29
Introduction:
The rate of steam generation, in a boiler, depends upon the rate at which the fuel
is burnt. The rate of fuel burning depends upon the availability of oxygen or in other
words availability of fresh air. The fresh air will enter the fuel bed, if the gases of
combustion are exhausted from the combustion chamber of the boiler. This is possible
only if a difference of pressure is maintained above and below the fire grate. This
difference of pressure is known as draught.
The main objects of producing draught in a boiler are:
1. To provide an adequate supply of air for the fuel combustion.
2. To exhaust the gases of combustion from the combustion chamber.
3. To discharge these gases to the atmosphere through the chimney.
Classification of Draughts:
In general, the draughts may be classified into the following two types:
1. Natural draught. It is the draught produced by a chimney due to the
difference of densities between the hot gases inside the chimney and cold atmospheric
air outside it.
2. Artificial draught. The artificial draught may be a mechanical draught or a
steam jet draught. The draught produced by a fan or blower is known as mechanical or
fan draught where as the draught produced by a steam jet is called steam jet draught.
The artificial draught is provided, when natural draught is not sufficient. It may be
induced or forced.
Types of Draughts:
In general, the draughts are of the following three types:
1. Chimney draught. The draught produced by means of a chimney alone is
known as chimney draught. It is a natural draught and has induced effect. Since the
atmospheric air (outside the chimney) is heavier than the hot gases (inside the chimney),
Boiler draught – Classification of draughts – Types of draught – Advantages and
disadvantages of mechanical draught – Compression between forced draught and induced
draught – Height of chimney – Problems on draught of chimney
131
the outside air will flow through the furnace into the chimney. It will push the hot gases to
pass through the chimney. The chimney draught varies with climatic conditions,
temperature of furnace gases and height of chimney.
2. Mechanical or fan draught. The draught, produced by means of a fan or
blower, is known as mechanical draught or fan draught. The fan used is, generally, of
centrifugal type and is driven by an electric motor.
In an induced fan draught, a centrifugal fan is placed in the path of the flue gases
before they enter the chimney. It draws the flue gases from the surface and forces them
up through the chimney. The action of this type of draught is similar to that of the natural
draught.
In case of forced fan draught, the fan is placed before the grate, and air is forced
into the grate through the closed ash pit.
3. Steam jet draught. It is a simple and cheap method of producing artificial
draught. In a steam jet draught, the exhaust steam, from a non-condensing steam
engine, is used for producing draught. It is mostly used in locomotive boilers, where the
exhaust steam from the engine cylinder is discharged through a blast pipe placed at the
smoke box and below the chimney.
In an induced steam jet draught, the steam jet issuing from a nozzle, is placed in
the chimney. But in a forced steam jet draught, the steam jet issuing from a nozzle is
placed in the ash pit under the fire grate of the furnace.
Advantages and Disadvantages of Mechanical Draught:
These days the mechanical draught is widely used in large boiler plants.
Advantages:
1. It is more economical.
2. It is better in control.
3. The flow of air through the grate and furnace is uniform.
4. It produces more draught.
5. Its rate of combustion is very high.
6. Low grade fuel can be used.
7. The air flow can be regulated according to the changing requirements.
8. It is not affected by the atmospheric temperature.
132
9. It reduces the amount of smoke.
10. It reduces height of chimney.
11. It increases efficiency of the plant.
12. It reduces the fuel consumption. About 15% of fuel is saved for the same
amount of work.
Disadvantages:
1. Its initial cost is high.
2. Its running cost is also high.
3. It has increased maintenance cost.
Comparison between Forced Draught and induced draught
The following table gives the comparison between forced draught and induced
draught.
S.No. Forced draught Induced draught
1. The fan in placed the fire grate. The fan is placed after the fire grate.
2.
The pressure inside the furnace is
above the atmospheric pressure.
The pressure inside the furnace is
below the atmospheric pressure.
3.
It forces fresh air into the combustion
chamber.
It sucks hot gases from the
combustion chamber, and forced
them into the chimney.
4.
It requires less power as the fan has
to handle cold air only. Moreover,
volume of air handled is less
because of low temperature of the
cold air.
It requires more power as the fan has
to handle hot air and flue gases.
Moreover, volume of air gases is more
because of high temperature of the air
and gases.
5.
The flow of air through grate and
furnace is more uniform.
The flow of air through grate and
furnace is less uniform.
6.
As the leakages are outward,
therefore there is a serious danger of
blow out when the fire doors are
opened and the fan is working.
As the leakages are inward, therefore
there is no danger of blow out. But if
the fire doors are opened and the fan
is working there will be a heavy air
infiltration.
133
Balanced Draught:
It is a combination of induced and forced draught. It is produced by running both
induced and forced draught fans simultaneously.
Height of Chimney:
Natural draught is produced by means of a chimney. Since the amount of
draught depends upon the height of chimney, its height should be such that it can
produce a sufficient draught.
Let H = Height of chimney above the fire grate in metres.
h = Draught required in terms of mm of water.
T1 = Absolute temperature of air outside the chimney in K.
T2 = Absolute temperature of the flue gas inside the chimney in K.
v1 = Volume of outside air at temperature T1 in m3/kg of fuel.
v2 = Volume of flue gases inside the chimney at temperature T2 in m3/kg of
fuel.
m = Mass of air actually used in kg/kg of fuel.
m + 1 = Mass of flue gases in kg per kg of fuel.
Draught pressure,
P = 3463 H
+−
21
11
mT
m
T N/m2 K.. (1)
In actual practice the draught pressure (h) is expressed in mm of water as
indicated by a manometer. Since 1 N/m2 = 0.101937 mm of water,
h = 353 H
+−
21
11
mT
m
T mm of water ... (2)
Notes:
1. The equations (i) and (ii) give only the theoretical value of the draught and is
known as static draught. The actual value of the draught is less than the
theoretical value.
2. The draught may also be expressed in terms of column of hot gases. If H’ is
the height in metres of the hot gas column,
134
H’ = H
−
×
+1
1 1
2
T
T
m
m meters
3. The velocity of flue gases through the chimney under a static draught of H’
metres is given by
V = '2gH = 4.43 'H KK.. (Neglecting friction)
The efficiency of chimney is less than 1 percent.
Problems on draught of chimney:
Example 1. A chimney is 28 m high and the temperature of the hot gases in the chimney
is 320°C. The temperature of outside air is 23°C and the furnace is supplied with 15 kg
of air per kg of coal burnt. Calculate draught in mm of water.
Solution. Given: H = 28 m; T2 = 320°C = 320 + 273 = 593 K; T1 = 23 °C = 23 + 273 =
296 K; m = 15 kg /kg of coal
Draught,
h = 353 H
+−
21
11
mT
m
T = 353 x 28
×+
−59315
115
296
1
= 15.6 mm of water
Example 2. A boiler uses 18 kg of fuel. Determine the minimum height of chimney
required to produce a draught of 25 mm of water. The mean temperature of chimney
gases is 315°C and that of outside air 27°C.
Solution. Given: m = 18 kg/kg of fuel; h = 25 mm of water; T2 = 315°C = 315 + 273 =
588 K; T1 = 27°C = 27 + 273 = 300 K
Let H = Minimum height of chimney required.
Draught (h),
25 = 353 H
+−
21
11
mT
m
T = 353 H
×+
−58818
118
300
1 = 0.53 H
H = 47.2 m
135
Example 3. The following data pertain to a steam power plant:
Height of chimney = 30 m; Draught produced = 16.5 mm of water gauge; Temperature of
flue gas = 360°C and that of outside air 27°C.
Solution. Given: m = 30 m; h = 16.5 mm of water; T2 = 360°C = 360 + 273 = 633 K;
T1 = 28°C = 28 + 273 = 301 K; P0 = 1.013 bar
Let m = Quantity of air used per kg of fuel.
Draught (h),
16.5 = 353 H
+−
21
11
mT
m
T = 353 × 30
×+
−633
1
301
1
m
m
30353
5.16
301
1
633
1
×−=
×+
m
m= 0.001 76
m + 1 = 0.00176 (m × 633) = 1.114 m
m = 8.772 kg /kg of fuel
136
Lecture No. 30
Introduction:
A steam condenser is a closed vessel into which the steam is exhausted, and
condensed after doing work in an engine cylinder or turbine. The condensed steam is
called condensate.
Advantages of a Condenser in a Steam Power Plant:
1. It increases expansion ratio of steam, and thus increases efficiency of the
plant.
2. It reduces back pressure of the steam, and thus more work can be obtained.
3. It reduces temperature of the exhaust steam, and thus more work can be
obtained.
4. The reuse of condensate (i.e. condensed steam) as feed water for boilers
reduces the cost of power generation.
5. The temperature of condensate is higher than that of fresh water. Therefore
the amount of heat supplied per kg of steam is reduced.
Requirements of a Steam Condensing Plant:
The principle requirements of a condensing plant, as shown in Fig. 70 are as
follows:
1. Condenser. It is a closed vessel is which steam is condensed. The steam
gives up heat energy to coolant (which is water) during the process of condensation.
2. Condensate pump. It is a pump, which removes condensate (i.e. condensed
steam) from the condenser to the hot well.
3. Hot well. It is a sump between the condenser and boiler, which receives
condensate pumped by the condensate pump.
4. Boiler feed pump. It is a pump, which pumps the condensate from the hot
well to the boiler. This is done by increasing the pressure of condensate above the boiler
pressure.
5. Air extraction pump. It is a pump which extracts (i.e. removes) air from the
condenser.
Condenser – Advantages – Requirements of a steam condensing plant – Classification of
condensers
137
Fig.70. Steam condensing plant
6. Cooling tower. It is a tower used for cooling the water which is discharged
from the condenser.
7. Cooling water pump. It is a pump, which circulates the cooling water through
the condenser.
Classification of Condensers:
The steam condensers may be broadly classified into the following two types,
depending upon the way in which the steam is condensed:
1. Jet condensers or mixing type condensers, and
2. Surface condensers or non-mixing type condensers.
138
Lecture No. 31
THE INDIAN BOILERS ACT, 1923
(Act No.5 of 1923)*
The Indian boilers act is to consolidate and amend the law relating to steam-
boilers. it is hereby enacted as follows:
1. Short title, extent and commencement
2. Definitions
2A. Application of Act to feed-pipes
2B. Application of Act to economisers
3. Limitation of application
4. Power to limit extent
5. Chief Inspector, Deputy Chief Inspectors and Inspectors
6. Prohibition of use of unregistered of uncertificate boiler
7. Registration
8. Renewal of certificate
9. Provisional orders
10. Use of boiler pending grant of certificate
11. Revocation of certificate or provisional order
12. Alterations and renewals to boilers
14. Duty of owner at examination
15. Production of certificates, etc.
16. Transfer of certificate etc.
17. Powers of entry
18. Report of accident
19. Appeals to chief Inspector
20. Appeals to appellate authority
20A. Power of central Government to revise order of appellate authority
21. Finality of orders
22. Minor penalties
23. Penalties for illegal use of boiler
24. Other penalties
Indian Boiler’s Act – 1923 – insulation of steam piping - Pipe expansion bends
139
25. Penalty for tampering with register mark
26. Limitation and previous sanction for prosecutions
27. Trial of offences
27A. Central Boilers Board
28. Power to make regulations
28A. Power of central Government to make rules
29. Power to make rules
30. Penalty for breach of rules
31. Publication of regulations and rules
31 A. Power of Central Government to give directions
32. Recovery of fees etc.
33. Applicability to the Government
34. Exemptions
35. Repeal of enactments
Piping system:
Many fluids such as steam, water, oil, gas, air etc. are used in power plants.
These fluids can be completely controlled-co-ordinated. The piping system can be
divided into following categories
1. Steam piping: For main auxiliary, reheat, bleed, exhaust and process steam.
2. Water piping: For condensate, feed-water, raw water etc.
3. Blow off piping: For boiler, evaporator and feed treatment.
4.Others: Including service water, fuel, lubricating oil, compressed air, soot
blowing, fire acting, chemical feed etc.
The pipes employed for different purposes have different specifications. Mostly,
the pipes are for steam and water. Standards for different classification of piping have
been laid down for pipes working under different temperatures and pressures.
The pipes may be broadly divided into various categories as low pressure pipes,
medium pressure, pipes and high pressure pipes at high temperature. The temperature
of the fluid in the pipes, are an important role specially at higher pressures.
Materials used for pipes:
1. Wrought iron
2. Cast iron
140
3. Steel
1. Wrought iron. Wrought iron pipe is used where corrosive conditions exist
especially in hot water lines, underground piping and special plumbing
installation.
2. Cast iron. Cast iron pipes are also use for low pressure. It is used for water
services upon a pressure of 15 kg/cm2 or for gas, drain piping etc. Steam pipes
do not use cast iron.
3. Steel. Steel pipes are being widely used having different percentage of metals
in the alloy of steel depending upon the purpose for which they are used.
Insulation of Steam Piping:
Insulating hot pipes not only conserves heat but also avoids an uncomfortable
overheat, atmosphere in the vicinity of pipe. The insulating material used for steam pipes
should possess following properties:
1. Should possess high insulating efficiency.
2. Should not be too costly.
3. Should be easily moulded and applied.
4. Should not be affected by moisture.
5. Should have high mechanical strength.
6. Should be able to withstand high temperature.
7. Should not cause corrosion of pipes if chemically decomposed.
8. Should not overload the pipe by its dead weight.
Pipe Expansion Bends:
As the forces produced by expansion are practically irresitible, the pipe is
invariably allowe expand and its movement is prevented from unduly stressing the
fittings and connections b, providing (i) suitable bends, (ii) expansion joints .
Expansion joints provide flexibility to the pipiIig system by, permittg
relativemotion of given pipe sections. They compensate for changes in dimensions due
to temperature variation joints being flexible, can isolate vibration and can allow the
ground setting if used, under ground. Most of the joints permit considerable axial
movement.
141
Lecture No. 32
Introduction:
An air compressor is a machine to compress the air and to raise its pressure.
The air compressor sucks air from the atmosphere, compresses it and then delivers the
same under a high pressure to a storage vessel. From the storage vessel, it may be
conveyed by the pipeline to a place where the supply of compressed air is required.
Since the compression of air requires some work to be done on it, therefore a
compressor must be driven by some prime mover.
The compressed air is used for many purposes such as for operating pneumatic
drills, riveters, road drills, paint spraying, in starting and supercharging of internal
combustion engines, in gas turbine plants, jet engines and air motors etc. It is also
utilised in the operation of lifts, rams, pumps etc.
Classification of Air Compressors:
The air compressors may be classified in many ways, but the following are
important from the subject point of view:
1. According to working
(a) Reciprocating compressors, and
(b) Rotary compressors.
2. According to action
(a) Single acting compressors, and
(b) Double acting compressors.
3. According to number of stages
(a) Single stage compressors, and
(b) Multi-stage compressors.
Compressors – Classification of air compressor – Working of single stage reciprocating air
compressor – Two stage reciprocating air compressor with intercooler
142
Working of Single Stage Reciprocating Air Compressor:
It consists of a cylinder, piston, inlet and discharge valves, as shown in Fig. 71,
from the geometry of the compressor. When the piston moves downwards (suction
stroke), the pressure inside the cylinder falls below the atmospheric pressure. Due to
this pressure difference, the inlet valve (I.V.) gets opened and air is sucked into the
cylinder. When the piston moves upwards (delivery stroke), the pressure inside the
cylinder goes on increasing till it reaches the discharge pressure. At this stage, the
discharge valve (D.V.) gets opened and air is delivered to the container. At the end of
delivery stroke, a small quantity of air, at high pressure, is left in the clearance space. As
the piston starts its suction stroke, the air contained in the clearance space expands till
its pressure falls below the atmospheric pressure. At this stage, the inlet valve gets
opened as a, result of which fresh air is sucked into the cylinder, and the cycle is
repeated.
a) Suction stroke b) Delivery stroke
Fig. 71. Working of single stage reciprocating air compressor
It may be noted that in a single acting reciprocating air compressor, the suction,
compression and delivery of air takes place in two strokes of the piston or one revolution
of the crankshaft.
Note: In a double acting reciprocating compressor, the suction, compression and
delivery of air takes place on both sides of the piston. It is thus obvious, that such a
compressor will supply double the volume of air than a single acting reciprocating
compressor (neglecting volume of piston rod).
143
Two-stage Reciprocating Air Compressor with Intercooler:
A schematic arrangement for a two-stage reciprocating air compressor with water
cooled intercooler is shown in Fig. 72.
Fig. 72. Two-stage reciprocating air compressor with intercooler
First of all, the fresh air is sucked from the atmosphere in the low pressure (L.P.)
cylinder during its suction stroke at intake pressure P1 and temperature Tl. The air, after
compression in the L.P. cylinder (i.e. first stage) from 1 to 2, is delivered to the
intercooler at pressure P2 and temperature T2. Now the air is cooled in the intercooler
from 2 to 3 at constant pressure P2 and from temperature. T2 to T3. After that, the air is
sucked in the high pressure (H.P.) cylinder during its suction stroke. Finally, the air, after
further compression in the H.P. cylinder (i.e. second stage) from 3 to 4, is delivered by
the compressor at pressure P3 and temperature T4.
