Power Machines N5

Power Machines N5

Module 1:

1.1 Introduction ..................................................................................................................................... 6
1.1.1 Absolute temperature............................................................................................................... 6
1.1.2 Standard temperature and pressure ( S.T.P. )..................................................................... 7
1.1.3 Normal temperature and pressure ( N.T.P. ) ....................................................................... 7
1.1.4 Standard atmosphere ( ICAO ) .............................................................................................. 7
1.1.5 Heating and expansion of gasses.......................................................................................... 7
1.1.6 Specific heat ................................................................................................................................ 7
1.2 Thermodynamic laws .................................................................................................................... 8
1.2.1 Joule’s law .................................................................................................................................... 8
1.2.2 Clausius’ theory ........................................................................................................................... 8
1.2.3 Boyle’s law .................................................................................................................................... 8
1.2.4 Charles’s law ................................................................................................................................ 9
1.2.5 Specific heat capacity of a gas ..........................................................................................10
1.3 Gas processes ...............................................................................................................................12
1.3.1 Isobaric process (constant pressure)...................................................................................12
1.3.2 Isometric process (constant volume)..................................................................................14
1.3.3 The polytropic process ............................................................................................................14
1.3.4 The isothermic process (constant temperature) .............................................................15
1.3.5 The adiabatic process.............................................................................................................17
1.4 Entropy ............................................................................................................................................18

Module 2:

2.1 Introduction ...................................................................................................................................29
2.2 Boilers ...............................................................................................................................................30
2.2.1 Fire tube boilers .........................................................................................................................30
2.2.2 Water tube boilers ....................................................................................................................34
2.3 Methods of firing boilers .............................................................................................................36
2.3.1 Solid coal firing...........................................................................................................................36
2.3.2 Pulverized coal firing ................................................................................................................37

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2.3.3 Oil firing ........................................................................................................................................39
2.4 Properties of steam......................................................................................................................39
2.4.1 Water............................................................................................................................................39
2.4.2 Steam ...........................................................................................................................................40
2.4.3 Saturated steam .......................................................................................................................40
2.4.4 Dry steam ....................................................................................................................................40
2.4.5 Superheated steam .................................................................................................................40
2.5 Enthalpy and steam generation..............................................................................................40
2.5.1 Triple point...................................................................................................................................40
2.5.2 Enthalpy of water......................................................................................................................41
2.5.3 Enthalpy of evaporation ( hfg )..............................................................................................41
2.5.4 Dryness fraction .........................................................................................................................42
2.5.5 The steam phase diagram.....................................................................................................44
2.5.6 Throttling calorimeter...............................................................................................................48

Module 3:

3.1 Introduction ...................................................................................................................................54
3.2 Types of condensers....................................................................................................................55
3.2.1 Surface condenser ...................................................................................................................55
3.2.3 Jet condenser............................................................................................................................57
3.3 Finding the amount of cooling water required ...................................................................59
3.4 Condenser pumps .......................................................................................................................61
3.4.1 Edwards air pump.....................................................................................................................62
3.4.2 Rotary air pump.........................................................................................................................63
3.4.3 Air ejector system......................................................................................................................63
3.4.4 Condenser efficiency ..............................................................................................................64
3.4.5 Vacuum efficiency...................................................................................................................64
3.4.6 Air leakage into condensers..................................................................................................64
3.4.7 Daltons law of partial pressures ............................................................................................65

Module 4:

4.1 Introduction ...................................................................................................................................69
4.1.1 Combustion ................................................................................................................................69

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4.2 Fuels..................................................................................................................................................69
4.2.1 Solid fuels.....................................................................................................................................69
4.2.2 Liquid fuels ..................................................................................................................................70
4.2.3 Gaseous fuels.............................................................................................................................70
4.3 Combustion ...................................................................................................................................71
4.3.1 Calorific value............................................................................................................................71
4.3.2 Measuring the calorific value................................................................................................71
4.3.3 higher heating value ...............................................................................................................77
4.3.4 Lower heating value ................................................................................................................77
4.3.5 Stoichiometry (chemically correct air fuel ratio) .............................................................78
4.4 Air fuel ratio....................................................................................................................................79

Module 5:

5.1 Introduction ...................................................................................................................................88
5.2 Construction ..................................................................................................................................89
5.2.1 Single-acting cylinder ..............................................................................................................89
5.2.2 Double-acting cylinder ...........................................................................................................91
5.3 Work done by a reciprocating compressor .........................................................................92
5.3.1 The clearance volume ............................................................................................................94

Module 6:

6.1 Introduction .................................................................................................................................101
6.2 Steam turbines ............................................................................................................................101
6.2.1 Impulse turbines.......................................................................................................................102
6.2.2 Reaction turbines....................................................................................................................103
6.2.3 Speed control ..........................................................................................................................103
6.3 Thermodynamics of a steam turbine ...................................................................................103
6.3.1 Velocities on blade inlet .......................................................................................................105
6.3.2 Velocities on blade exit ........................................................................................................106
6.3.3 Work done on the blades.....................................................................................................107
6.4 Governors .....................................................................................................................................110
6.4.1 Centrifugal governors............................................................................................................111
6.4.2 Watt governor..........................................................................................................................111

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6.4.3 Porter governor........................................................................................................................113
6.4.4 Spring-controlled governors ................................................................................................113
6.4.5 The Hartnell governor ............................................................................................................114
6.4.6 Sensitivity and friction ............................................................................................................115

Module 7:

7.1 Introduction .................................................................................................................................122
7.2 General requirements...............................................................................................................122
7.3 Duties of manufacturers...........................................................................................................123
7.4 Duties of users..............................................................................................................................123
7.5 Approval and duties of approved inspection authority.................................................124
7.6 Registration of a steam generator ........................................................................................125
7.7 Pressure equipment marking ..................................................................................................126
7.8 Pressure and safety accessories ............................................................................................127
7.9 Inspection and test....................................................................................................................128
7.10 Records.......................................................................................................................................130
7.11 Introduction to boiler and pressure vessels ......................................................................131

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Icons used in this book

We use different icons to help you work with this book; these are shown in the table
below.

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Assessment / Activity Multimedia

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Demonstration/ observation Presentation/ Lecture

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Example Safety

Experiment Site visit
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play, etc. Take note of
In the workplace Theoretical – questions,
reports, case studies, etc.
Keywords Think about it

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Learning Outcomes

On the completion of this module the student must be able to:
 Describe the laws of thermodynamics
 Describe the gas processes
 Calculate the heat transferred on a gas
 Calculate the work done on or to a gas
 Calculate the internal energy of a gas

1.1 Introduction

Since temperature and air pressure vary from place to place it is
necessary with a standard reference condition to compare testing
and documentation of chemical and physical processes.
Note:
Thermodynamics is a relation between heat and work and the
properties of a gas like mass, specific heat capacity etc.
The following are some terms used for taking accurate measurements:
1.1.1 Absolute temperature
Absolute or thermodynamic temperature is defined by the third law of
thermodynamics in which the theoretically lowest temperature is the zero
point. At this point, absolute zero, the particle constituents of matter, has
minimal motion and can become no colder.
Absolute temperature is measured in Kelvin ( K ), where:

0

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1.1.2 Standard temperature and pressure (S.T.P.)
S.T.P. is commonly used to define standard conditions for temperature and
pressure which is important for the measurements and documentation of
chemical and physical processes where:

1.1.3 Normal temperature and pressure (N.T.P.)
N.T.P. is commonly used as a standard condition for testing and
documentation of fan capacities where:

1.1.4 Standard atmosphere (ICAO)
Standard model of the atmosphere adopted by the International Civil Aviation
Organization (ICAO) where:

1.1.5 Heating and expansion of gasses
Thermal expansion is the tendency of a gas to change in volume in response
to a change in temperature. This is through heat transfer.
Temperature is a function of the average molecular kinetic energy of a gas.
When a gas is heated, the kinetic energy of its molecules is increased. Thus, the
molecules begin moving more and usually maintain a greater average
separation.
The degree of expansion divided by the change in temperature is called the
gasses coefficient of thermal expansion and generally varies with temperature.
1.1.6 Specific heat

Definition: Specific heat
The amount of heat transferred (energy) required to raise a unit
mass of gas by 1 degree raise in temperature.

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1.2 Thermodynamic laws

1.2.1 Joule’s law
The first law of thermodynamics is a version of the law of conservation of
energy, adapted for thermodynamic systems. The law of conservation of
energy states that the total energy of an isolated system is constant. Energy
can be transformed from one form to another, but cannot be created or
destroyed.

The first law is often formulated by stating that the change in the internal
energy of a closed system is equal to the amount of heat supplied to the
system, minus the amount of work done by the system on its surroundings.

And

Where:
Uf
U1
Also
Q
W

Looking at the formula above:
 When the system gains heat, Q is positive and when it loses heat, Q in

negative.
 When work is done by the system, W is positive and when work is done on

the system, W is negative.

1.2.2 Clausius’ theory
The Clausius theorem (1855) states that for a system exchanging heat with
external reservoirs and undergoing a cyclic process (i.e. a process which
ultimately returns a system to its original state) heat will always flow from a
reservoir with a higher temperature to a reservoir with a lower temperature.

1.2.3 Boyle’s law
Boyle's law is an experimental gas law which describes how the pressure of
a gas tends to increase as the volume of a gas decreases.

Note: A modern statement of Boyle's law is:
The absolute pressure exerted by a given mass of an ideal gas is
inversely proportional to the volume it occupies if
the temperature and amount of gas remain unchanged within
a closed system.

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Boyle's law can be stated as:

Where:
P1
P2
V1
V2
T1 =
T2 =

1.2.4 Charles’ law
Charles' law (also known as the law of volumes) is an experimental gas
law which describes how gasses tend to expand when heated. Figure 1.1

A modern statement of Charles' law is:
When the pressure on a sample of a dry gas is held constant, the Kelvin
temperature and the volume will be directly related.

This directly proportional relationship can be written as:

Figure 1.1 Boyle’s law and Charles’ law on a PV diagram

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Equating (1) and (2):

It follows then that for any contained mass of gas, the change of state is
governed by the equation:

But for a specific gas under specific conditions, R = characteristic gas constant.

When the mass of the gas is considered:

This is the characteristic equation for a perfect gas.
1.2.5 Specific heat capacity of a gas
Equal to the ratio of the heat added to (or removed from) a gas to the
resulting temperature change.
Specific heat capacity at constant volume ( Cv )
The amount of heat transfer when the volume remains constant and the
temperature changes by one degree.
Specific heat capacity at constant pressure ( Cp )
The amount of heat transfer when the pressure remains constant and the
temperature changes by one degree.

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Worked Example 1.1

A gas, whose original pressure, volume and temperature was 140 kN/m2, 0.1

m3 and respectively, is compressed to a new pressure of 700 kN/m2, and

a new temperature .

Find the new volume of the gas.

Solution:

Worked Example 1.2

A quantity of gas has a pressure of 350 kN/m2 when its volume is 0.03 m3 and

its temperature is . If R = 0.29 kJ / kg.K, find the mass of gas present.

Solution:

Worked Example 1.3

A gas in its original state has the following:

Pressure = 275 kPa
Volume = 0.09 m3
Temperature =

This gas has its state changed at constant pressure until its temperature

becomes . Find how much heat is transferred from the gas.

R = 0.29 kJ / kg.K and kJ / kg.K

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Solution:
Find the mass of gas:

Heat transferred:

1.3 Gas processes

Note:
A gas is said to have undergone a process when the state of a gas
has changed due to an operation that has been carried out on the
gas.

There are 5 different processes that we will consider in this course. The other
three processes will be dealt with at a later time.

1.3.1 Isobaric process (constant pressure)

Figure 1.2 Work done by a gas on a cylinder
Consider Figure 1.2 The pressure of a gas in the cylinder is kept constant while
the piston moves a distance. The force on the piston will remain constant. The
pressure = force on the piston x the inside area of the cylinder.
Work done by the gas:

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But A x L = swept volume, so we get:
Figure 1.3 shows the P V graph of the process above:

Figure 1.3 Work done by a gas
It is obvious that with the pressure remaining constant, the gas has to receive
heat and the piston has to move. Thus:

H2 = final enthalpy =
H1 = original enthalpy =

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1.3.2 Isometric process (constant volume)
The constant volume process is also referred to as the isochoric process.
If there is no change in volume in the cylinder in Figure 1.2, then the distance
moved by the piston = 0. Therefore:

The change in internal energy:
So the change of the total heat:

1.3.3 The polytropic process
A polytropic process is a thermodynamic process that obeys the relation:

where P is the pressure, V is specific volume, n is the polytropic index and C is a
constant. Figure 1.4
All processes that can be expressed as a pressure and volume product are
polytropic processes. Some of those processes are unique. This equation can
accurately characterize a very wide range of thermodynamic processes, that
range from n=0 to n= which covers, n=0 ( isobaric ), n=1 ( isothermal ), n=γ (
isentropic ), n= ( isochoric ) processes and all values of n in between.

Figure 1.4 PV diagram of a polytropic process

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The polytropic process equation is particularly useful for characterizing
expansion and compression processes which include heat transfer. The one
restriction is that the process should display a constant energy transfer ratio K
during that process:

Work done:

Combining the polytropic law and the characteristic gas equation
of a perfect gas:

From the polytropic law:

()
From the characteristic equation:

Combining equation (1) and (2):

()

Also: ()
And

() ()

1.3.4 The isothermic process (constant temperature)
An isothermal process is a change of a system, in which the
temperature remains constant:

ΔT = 0.

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This typically occurs when a system is in contact with an outside thermal
reservoir, and the change occurs slowly enough to allow the system to
continually adjust to the temperature of the reservoir through heat exchange.
In an adiabatic process is where a system exchanges no heat with its
surroundings (Q = 0). In other words, in an isothermal process, the value ΔT = 0
and therefore ΔU = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic
process, ΔT ≠ 0 but Q = 0.

Note:
The law of the isothermal process is:
PV = C
Figure 1.5 shows an isothermal process plotted on a PV diagram. The change
of state from 1 to 2 is given by:

Figure 1.5 PV diagram of an isothermal process where the temperature
at 1 equals the temperature at 2
Work done:

Introducing:

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1.3.5 The adiabatic process
An adiabatic process is one that occurs without transfer of heat or matter
between a thermodynamic system and its surroundings.
In an adiabatic process, energy is transferred only as work. The adiabatic
process provides a rigorous conceptual basis for the theory used to expound
the first law of thermodynamics, and as such it is a key concept in
thermodynamics.

Did you know?
Some chemical and physical processes occur so rapidly that they
may be conveniently described by the "adiabatic approximation",
meaning that there is not enough time for the transfer of energy as
heat to take place to or from the system.
No heat is transferred and so the adiabatic process is a specific part of the
polytropic process where:

The adiabatic index ( ) satisfies the particular case of this process. Work done
then becomes:

The non-flow energy equation:

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1.4 Entropy

Definition: Entropy
The idea of entropy comes from a principle of thermodynamics
dealing with energy. It usually refers to the idea that everything in
the universe eventually moves from order to disorder, and entropy is
the measurement of that change.

The word entropy means "a turning toward" or "transformation." The word was
used to describe the measurement of disorder by the German physicist
Rudolph Clausius and appeared in English in 1868. A common example
of entropy is that of ice melting in water. The resulting change from formed to
free, from ordered to disordered increases the entropy.

Think about it!
Entropy is very similar to energy.
Energy measures the capability of an object or system to do work.
Entropy, on the other hand, is a measure of the "disorder" of a
system.

What "disorder refers to is really the number of different microscopic states a
system can be in, given that the system has a particular fixed composition,
volume, energy, pressure, and temperature. By "microscopic states", we mean
the exact states of all the molecules making up the system.

The idea here is that just knowing the composition, volume, energy, pressure,
and temperature doesn't tell you very much about the exact state of each
molecule making up the system.

For even a very small piece of matter, there can be trillions of different
microscopic states, all of which correspond to the sample having the same
composition, volume, energy, pressure, and temperature.

Why should it be important, after all, if you know the bulk properties? Isn't that
all one usually needs? It turns out that no, in fact if you want to, say, exact
energy from say steam and convert it to useful work, those details turn out to
be crucial.

Entropy is denoted by (S) and the change in entropy can be shown by:

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Worked Example 1.4
A quantity of gas has an initial pressure, volume and temperature of 140 kPa,
0.14 m3 and 25 C respectively. It is compressed to a pressure of 1.4 MPa
according to the law PV1.25 = constant.
Find the change of entropy. Cp = 1.041 kJ/kg.K and Cv = 0.743 kJ/kg.K
Solution:

For 1 kg gas:
Also:

() ()

For 0.221 kg of gas:

Figure 1.6 and Figure 1.7 show a summary of the expansion and compression
laws of gasses

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Figure 1.6 Expansion curves. Work done is the area under the graph in each
case

Figure 1.7 Compression curves. Work done is the area under the graph in
each case

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Worked Example 1.5

A gas whose original pressure and temperature were 300 kN/m3 and 25
degrees Celsius respectively, is compressed according to the law PV1.4 = C
until its temperature is 180 degrees Celsius. Find the new pressure of the gas.

Solution:

For a polytropic compression, the relationship between pressure and
temperature is:

()

And from this:

() ()
()

Worked Example 1.6

0.25 kg of air at a pressure of 140 kPa occupies 0.15 m3 and from this
condition it is compressed to 1.4 MPa according to the law PV1.25 = C.

Find the change of internal energy of the air if Cp = 1.005 kJ / kg.K and Cv =
0.718 kJ / kg.K Also find the work done on or by the air.

Solution:
Change in internal energy:

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Figure 1.8

() ()
Change in internal energy: ./

This is a positive change in energy hence it is a gain of internal energy to the
air.

Work done:

This is negative hence work is done on the air.

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Worked Example 1.7

A certain gas, which has a mass of 2,268 kg, is compressed polytropically
from a pressure of 103,4 kPa and a volume of 2,125 m3 to a pressure of 3 102
kPa and a volume of 0,125 m3. The gas constants R is 0,288 kJ/kg.K and Cv is
0,67 kJ/kg.K.

Calculate the following:

1. The index of compression
2. The initial temperature in Kelvin
3. The temperature after compression in Kelvin
4. The work done in kilojoules
5. The change in internal energy

Solution: =
1. =

()

( )=
= 30
=
= 1,2

2. =
=
=
= 336,39 K

3. =
=
=
= 593,63 K

4. =

=
= -840,125 kJ

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5. =
= 2m268 X 0,67 X (593,63 – 336,39)
= 390,89 kJ

Worked Example 1.8
0,625 kg gas at a pressure of 103 kPa and a temperature of 288 kelvin is
compressed isothermally to a pressure of 310,2 kPa. Its volume is now
reduced at constant pressure to 0,085 m3 and finally, its pressure is raised at
constant volume to 413,6 kPa. R= 0,286 kJ/kg.K.
Calculate the following:
The initial volume of the gas
The volume of the gas after the isothermal compression
Solution:

1. =
=
=
= 0,4998 m3

2. =
=
=
= 0,6596 m3

3. =
=
=
= 147,508 K

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4. =
=

=

= 196,677 K
4. =

=
= -25,11379 kJ

Activity 1.1

Draw up a table for the summary of the gas laws. Use 6 columns with the
headings: Process, Law symbol, Heat added or rejected (Q), Change in
internal energy, and Other formula. Use 6 rows for the five processes.

[find all formula for each process]

Activity 1.2

0,6 kg of gas is at a pressure of 1,5 MPa and a temperature of 300ºC. This gas
is expanded reversibly and politropically to a pressure of 150 kPa. The
following data is applicable to this gas:

 Index of expansion = 1,24
 Gas constant = 0,287 kJ/kg.K
 Specific heat capacity at constant pressure = 1,005 kJ/kg.K

Calculate the following:
1. The temperature of the gas after expansion
2. The work done
3. The heat flow
4. The change in entropy during the expansion for 1 kg of steam

[367; 0.06578; 0.42126; 147.8375; 0.718; -88.7448; 59.0927; 0.21309]

Activity 1.3

The density of a certain gas at 0ºC and 100 kPa is 1,28 kg/m3. 5 kg of this gas
is compressed adiabatically from a temperature of 0ºC and a pressure of 100
kPa to a pressure of 5 200 kPa. The gas constant Cv is 0,754 kJ/kg.K.

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Calculate the following:
1. The gas constant R
2. The adiabatic index of compression, gamma
3. The initial volume at 0ºC and 100 kPa
4. The final volume at 5 200 kPa
5. The work done during compression

[0.286; 1.38; 3.9; 0.22263; - 2020.2]

Activity 1.4

3,5 kg of air at 410 kPa and 130ºC expands politropically to 101,3 kPa and 18
ºC. The specific heat capacity at constant pressure is 1,005 kJ/kg.K and the
gas constant R is 0,287 kJ/kg.K for air.

Calculate the following:

1. The original volume
2. The final volume
3. The value of the index of expansion
4. The work done in kilojoules
5. The change in entropy

[0.98735; 2.88558; 1.3; 375; 0.259]

Activity 1.5

A certain gas has a density of 1,5 kg/m3 at free air conditions with a pressure
of 101,3 kPa and a temperature of 18 ºC. It takes 389 kJ to heat 1,67 kg of this
gas at constant pressure from 18 ºC to 250 ºC.

Calculate the following:

1. The characteristic gas constant R of the gas
2. The specific heat capacity of the gas at constant pressure
3. The specific heat capacity of the gas at constant volume
4. The change in internal energy
5. The work done

[0.232; 1.004; 0.772; 299.104; 89.896]

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Activity 1.6

0,5 kg of gas is expanded from 2 068 kPa and a volume of 0,0566 m3 to 103,4

kPa according to the law . The gas constant ‘R’ is 0,288 kJ/kg.K and

Cv is 0,72 kJ/kg.K.

Calculate the following:

1. The final volume
2. The original absolute temperature
3. The final absolute temperature
4. The work done in kilojoules
5. The heat received or rejected in kilojoules

[0.481; 812.84; 345.39; 168.3; 0]

Activity 1.7

0,125 m3 of a gas is heated at constant volume from a temperature of 350 ºC

and a pressure of 930 kPa to a pressure of 3 930 kPa. This gas then undergoes

an adiabatic expansion to a pressure of 230 kPa when . The gas

constant for this gas is 0,288 kJ/kg.K and the specific heat capacity of this gas

at constant volume is 0,702 kJ/kg/K.

Calculate the following:

1. the temperature of the gas at the commencement of the adiabatic
expansion

2. the temperature of the gas at the completion of the adiabatic expansion
3. the quantity of heat absorbed by the gas during the isochoric process
4. the work done during the adiabatic process

[0.08334; 1759.64; 1.3; 0.01488; - 7.39054]

Activity 1.8

A certain gas has a density of 1,28 kg/m3 at 200 kPa and 267 ºC. The

law, , Is used to expand 0,7 kg of this gas to 2,5 times Its original

volume, from 200 kPa and 267 ºC.

Calculate the following:

1. The characteristic gas constant

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2. The original and final volume of the gas
3. The final pressure of the gas
4. The final temperature of the gas
5. The work done during the expansion

[0.289; 1.3655; 61.332; 413.991; 87.9]

Activity 1.9

A constant volume process was used to heat 3,3 kg of gas from a
temperature of 21ºC and 0,87 m3 to a temperature of 137 ºC. The specific
heat capacity of the gas at constant volume Is 0,718 kJ/kg.K and the gas
constant Is 0,269 kJ/kg.K.

Calculate the following:

1. The quantity of heat transferred in kJ
2. The final pressure of the gas after heating

[274.85; 322.285; 449.445]

Self-Check

I am able to: Yes No

 Describe the laws of thermodynamics

 Describe the gas processes

 Calculate the heat transferred on a gas

 Calculate the work done on or to a gas

 Calculate the internal energy of a gas

If you have answered ‘no’ to any of the outcomes listed above, then speak to

your facilitator for guidance and further development.

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Learning Outcomes

On the completion of this module the student must be able to:
 Describe the main types of boilers and their specific application
 Describe the different methods of firing a boiler
 Calculate the enthalpy, volume and internal energy of wet steam
 Calculate the enthalpy, volume and internal energy of dry steam
 Calculate the enthalpy, volume and internal energy of superheated

steam
 Calculate the dryness fraction using the calorimeter

2.1 Introduction

Many call the water vapor that comes out of a boiling kettle, steam.
But this vapor that is seen as a light cloud has too little energy to be
used to generate power in industry.

Figure 2.1 Simplified steam generation plant
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Referring to Figure 2.1, some of the steam generation equipment that is going
to be covered is the boiler, condenser and air pump.

2.2 Boilers

2.2.1 Fire tube boilers
A fire-tube boiler is a type of boiler in which hot gases from a fire pass through
one or (many) more tubes running through a sealed container of water.
The heat of the gases is transferred through the walls of the tubes by thermal
conduction, heating the water and ultimately creating steam.

Did you know?
This type of boiler was used on virtually all steam locomotives in the
horizontal "locomotive" form.
This has a cylindrical barrel containing the fire tubes, but also has an extension
at one end to house the "firebox". This firebox has an open base to provide a
large grate area and often extends beyond the cylindrical barrel to form a
rectangular or tapered enclosure.
The horizontal fire-tube boiler is also typical of marine applications, using
the Scotch boiler. Vertical boilers have also been built of the multiple fire-tube
type, although these are comparatively rare; most vertical boilers were either
flued, or with cross water-tubes.

Figure 2.2 A three pass fire-tube boiler

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2.2.1.1 The Cornish boiler:
The earliest form of fire-tube boiler was the "high-pressure" Cornish boiler. This is
a long horizontal cylinder with a single large flue containing the fire. The fire
itself was on an iron grating placed across this flue, with a shallow ash-pan
beneath to collect the non-combustible residue.

Did you know?
Although considered as low-pressure (perhaps 25 psi) today, the
use of a cylindrical boiler shell permitted a higher pressure than the
earlier "haystack" boilers.
As the furnace relied on natural draught (air flow), a tall chimney was required
at the far end of the flue to encourage a good supply of air (oxygen) to the
fire. Figure 2.3.
For efficiency, the boiler was commonly encased beneath by a brick-built
chamber. Flue gases were routed through this, outside the iron boiler shell,
after passing through the fire-tube and so to a chimney that was now placed
at the front face of the boiler.

Figure 2.3 A Cornish fire-tube boiler with one fire tube
2.2.1.2 The Scotch marine boiler
The Scotch marine boiler differs dramatically from its predecessors in using a
large number of small-diameter tubes. This gives a far greater heating surface
area for the volume and weight. The furnace remains a single large-diameter
tube with the many small tubes arranged above it.
They are connected together through a combustion chamber – an enclosed
volume contained entirely within the boiler shell – so that the flow of flue gas

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through the fire tubes is from back to front. An enclosed smokebox covering
the front of these tubes leads upwards to the chimney or funnel.

Note:
Typical Scotch boilers had a pair of furnaces, larger ones had three.
Above this size, such as for large steam ships, it was more usual to
install multiple boilers.
2.2.1.3 The locomotive boiler
A locomotive boiler has three main components: a double-walled fire box; a
horizontal, cylindrical "boiler barrel" containing a large number of small flue-
tubes; and a smoke box with chimney, for the exhaust gases.
The boiler barrel contains larger flue-tubes to carry the superheater elements,
where present. Forced draught is provided in the locomotive boiler by injecting
exhausted steam back into the exhaust via a blast pipe in the smokebox.
Locomotive-type boilers, Figure 2.4 are also used in traction engines, steam
rollers and portable engines and some other steam road vehicles. The inherent
strength of the boiler means it is used as the basis for the vehicle: all the other
components, including the wheels, are mounted on brackets attached to the
boiler.

Figure 2.4 A locomotive fire-tube boiler

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Did you know?
It is rare to find superheaters designed into this type of boiler, and
they are generally much smaller (and simpler) than railway
locomotive types.
2.2.1.4 The Lancashire boiler:
The Lancashire boiler is similar to the Cornish, but has two large flues containing
the fires. Figure 2.5. It was the invention of William Fairbairn in 1844, from a
theoretical consideration of the thermodynamics of more efficient boilers that
led him to increase the furnace grate area relative to the volume of water.
Later developments added Galloway tubes (after their inventor, patented in
1848), crosswise water tubes across the flue, thus increasing the heated surface
area.
As these are short tubes of large diameter and the boiler continues to use a
relatively low pressure, this is still not considered to be a water-tube boiler. The
tubes are tapered, simply to make their installation through the flue easier.

Figure 2.5 A Lancashire fire-tube boiler with two fire tubes
2.2.1.5 The vertical fire-tube boiler
A vertical fire-tube boiler is a vertical boiler where the heating surface is
composed of multiple small fire tubes, arranged vertically.
These boilers were not common, owing to drawbacks with excessive wear in
service. The more common form of vertical boiler, which was very similar in
external appearance, instead used a single flue and water-filled cross tubes.

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Another form used horizontal fire-tubes, even where this added complexity,
such as the Cochran boiler.
2.2.2 Water tube boilers
Fuel is burned inside the furnace, creating hot gas which heats water in the
steam-generating tubes. In smaller boilers, additional generating tubes are
separate in the furnace, while larger utility boilers rely on the water-filled tubes
that make up the walls of the furnace to generate steam. Figure 2.6.
The heated water then rises into the steam drum. Here, saturated steam is
drawn off the top of the drum. In some services, the steam will re-enter the
furnace through a superheater to become superheated.

Definition: Superheated steam
Superheated steam is defined as steam that is heated above the
boiling point at a given pressure.
Superheated steam is a dry gas and therefore used to drive turbines, since
water droplets can severely damage turbine blades.

Figure 2.6 A typical water tube boiler arrangement
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2.2.2.1 Thornycroft boiler
The Thornycroft type features a single steam drum with two sets of water tubes
either side of the furnace. These tubes, especially the central set, have sharp
curves.
Apart from obvious difficulties in cleaning them, this may also give rise to
bending forces as the tubes warm up, tending to pull them loose from the
tubeplate and creating a leak. There are two furnaces, venting into a
common exhaust, giving the boiler a wide base tapering profile.
Figure 2.7 shows this boiler with its tube configuration along with three mud
drums and one central steam drum at the top.

Figure 2.7 The Thornycroft type boiler
2.2.2.2 Yarrow boiler
Named after its designers, the then Poplar-based Yarrow ship builders, this type
of three drum boiler has three drums in a delta formation connected by water
tubes. The drums are linked by straight water tubes, allowing easy tube-
cleaning.

Figure 2.8 The Yarrow type boiler

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This does, however, mean that the tubes enter the drums at varying angles, a
more difficult joint to caulk. Outside the firebox, a pair of 'cold-leg' pipes
between each drum act as 'downcommers'.

Did you know?
Due to its three drums, the Yarrow boiler has a greater water
capacity. Hence, this type is usually used in older marine boiler
applications. Its compact size made it attractive for use in
transportable power generation units during World war 2.

Figure 2.8 shows a Yarrow boiler tube arrangement with two mud drums and a
single steam drum at the top.

2.3 Methods of firing boilers

2.3.1 Solid coal firing
The combustion process with solid fuel can be performed by mechanical
spreader stoker or hand stoker firing. Solid fuel firing in steam boiler can be
classified as underfeed stoker firing and overfeed stoker firing.

2.3.1.1 Underfeed stoker firing
Underfeed stoker firing is the process of combustion in which the new coal is
heated by radiation in the presence of air and located under ignited fuel bed.

The heating of coal is running less rapidly and release volatile matter combine
with air, so generating low smoke.

2.3.1.2 Overfeed stoker firing
Overfeed stoker firing is the process of combustion in which the unignited fuel
or incoming coal is located above ignited fuel bed. The ignited fuel transfer
heat to the incoming coal by radiation.

Moreover coal is heated by convection from hot gases that have been
through the combustion.

Secondary air is added to perform complete combustion unless steam boiler
will produce more smoke because the hot gases contain little oxygen.

Figure 2.9 shows a mechanical boiler feed for solid coal.

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Figure 2.9 Solid coal firing of boiler
2.3.1.3 Travel grate stoker solid coal firing
Here, the coal is burnt on a chain grate which continuously travels forwards.
Combustion takes place during the journey of coal from first end to last end of
the furnace.
At the end of combustion heavier ash content falls into ash pit by gravitational
force as the grate chain moves like conveyor belt. The lighter ash particles and
combustion gases fly away with primary air.
2.3.2 Pulverized coal firing
A pulverized coal-fired boiler is an industrial or utility boiler that generates
thermal energy by burning pulverized coal (also known as powdered coal or
coal dust since it is as fine as face powder in cosmetic makeup) that is blown
into the firebox.
The basic idea of a firing system using pulverized fuel is to use the whole
volume of the furnace for the combustion of solid fuels. Coal is ground to the
size of a fine grain, mixed with air and burned in the flue gas flow. Biomass and
other materials can also be added to the mixture.

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Coal contains mineral matter which is converted to ash during combustion.
The ash is removed as bottom ash and fly ash. The bottom ash is removed at
the furnace bottom.

Did you know?
This type of boiler dominates the electric power industry, providing
steam to drive large turbines.

Pulverized coal provides the thermal energy which produces about 50% of the
world's electric supply.

For getting most calorific value of coal, the coal is pulverized in fine powder
and then mixed with sufficient air. The mixture of coal powder and air is fired in
the steam boiler furnace to achieve most efficient combustion process. This is
most modern and efficient method of boiler firing.

Due to pulverization, the surface area of coal becomes much larger, and in
this method air required for combustion is much less. As the quantity of
required air and fuel both are less, loss of heat in this method of boiler firing is
much less, hence temperature can easily be reached to the designated level.

Did you know?
As the combustion is most efficient pulverized coal firing increases
the overall efficiency of steam boiler.

As handling of lighter coal dust is much easier than handling of heavier coal
chips, it is quite easy to control the output of the boiler by controlling supply of
fuel to the furnace.

Hence fluctuation of system load can smoothly be met. In addition, these
advantages, pulverized coal firing system has many disadvantages. Such as:

 The initial cost of installing this plant is very high.
 Not only initial cost, running cost of this plant is quite high as
 Separate pulverization plant to installed and run additionally.
 High temperature causes high thermal loss through flue gas.
 This type of method of boiler firing has always a risk of explosion.
 This is also difficult and expensive to filter fine ash particles from
 Fine gas. Moreover, the quantity of ash particles in the flue gas is
 More in pulverized system.

Process of pulverization is discussed here in brief.

First the coal is crushed by preliminary crasher. The coal is crushed to 2.5 cm.
or less.

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Then this crushed coal is passed through magnetic separator to separate any
iron content in the coal. Iron must be removed; otherwise during pulverizing
iron particles will cause spark which results unwanted fire hazard.

After that, crushed coal is dried properly before pulverization. The moisture
content must be less than 2% after drying operation.

Then the coal is crushed again in fine particles in ball mill. This process is
referred as pulverization.

This pulverized coal is then puffed with air and put into furnace as fluid.

2.3.3 Oil firing
An oil burner or oil furnace is a heating device which burns heating oil, diesel
fuel or other similar fuels. The fuel is atomized into a fine spray usually by forcing
it under pressure through a nozzle. This spray is usually ignited by an electric
spark with the air being forced through by an electric fan.

Note:
Fuel is injected into the combustion chamber by spray nozzles.

The nozzles are usually supplied with high pressure oil. Because of problems
with erosion, and blockage due to lumps in the oil, they need frequent
replacement, typically every year. Fuel nozzles are usually rated in fuel volume
flow per unit time.

2.4 Properties of steam

2.4.1 Water
Water exists between 0 degrees Celsius and Superheated water.

Superheated water is liquid water under pressure at temperatures between the
usual boiling point, 100 °C and the critical temperature, 374 °C. It is also known
as "subcritical water" or "pressurized hot water." Superheated water is stable
because of overpressure that raises the boiling point.

Water vapour, or aqueous vapour, is the gaseous phase of water. Water
vapour can be produced from the evaporation or boiling of liquid water or
from the sublimation of ice. Unlike other forms of water, water vapour is
invisible.

Under typical atmospheric conditions, water vapour is continuously generated
by evaporation and removed by condensation. It is lighter than air and triggers
convection currents that can lead to clouds.

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2.4.2 Steam
The enthalpy of evaporation is the energy required to turn water into the
gaseous form when it increases in volume by 1,700 times at standard
temperature and pressure; this change in volume can be converted
into mechanical work by steam engine such as reciprocating piston type
engines and steam turbines.

2.4.3 Saturated steam
If saturated steam is reduced in temperature (whilst retaining its pressure) it will
condense to produce water droplets, even if it is still considerably above the
boiling point of 100 °C at standard pressure. These condensation droplets are a
cause of damage to steam turbines blades, the reason why such turbines rely
on a supply of dry, superheated steam.

2.4.4 Dry steam
Dry steam is saturated steam that has been very slightly superheated. This is not
sufficient to change its energy appreciably, but is a sufficient rise in
temperature to avoid condensation problems, given the average loss in
temperature across the steam supply circuit.

Did you know?
Towards the end of the 19th century, when superheating was still a
less-than-certain technology, such steam-drying gave the
condensation-avoiding benefits of superheating without requiring
the sophisticated boiler or lubrication techniques of full
superheating.

2.4.5 Superheated steam
Superheated steam is a steam at a temperature higher than its vaporization
(boiling) point at the absolute pressure where the temperature is measured.

The steam can therefore cool (lose internal energy) by some amount, resulting
in a lowering of its temperature without changing state (i.e., condensing) from
a gas, to a mixture of saturated vapour and liquid.

If saturated steam (a mixture of both gas and saturated vapour) is heated at
constant pressure, its temperature will also remain constant as the vapour
quality (think dryness, or percent saturated vapour) increases towards 100%,
and becomes dry (i.e., no saturated liquid) saturated steam.

Continued heat input will then "super" heat the dry saturated steam. This will
occur if saturated steam contacts a surface with a higher temperature.

2.5 Enthalpy and steam generation

2.5.1 Triple point
All the three phases of a particular substance can only coexist in equilibrium at
a certain temperature and pressure, and this is known as its triple point.

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The triple point of water, where the three phases of ice, water and steam are
in equilibrium, occurs at a temperature of 273.16 K and an absolute pressure of
0.006 112 bar. This pressure is very close to a perfect vacuum.
If the pressure is reduced further at this temperature, the ice, instead of
melting, sublimates directly into steam.
2.5.2 Enthalpy of water
This is the heat energy required to raise the temperature of water from a
datum point of 0°C to its current temperature.
At this reference state of 0°C, the enthalpy of water has been arbitrarily set to
zero. The enthalpy of all other states can then be identified, relative to this
easily accessible reference state.

Did you know?
Sensible heat was the term once used, because the heat added to
the water produced a change in temperature. However, the
accepted terms these days are liquid enthalpy or enthalpy of
water.
At atmospheric pressure (0 bar), water boils at 100°C, and 419 kJ of energy are
required to heat 1 kg of water from 0°C to its boiling temperature of 100°C.
It is from these figures that the value for the specific heat capacity of water (
Cp ) of 4.19 kJ/kg °C is derived for most calculations between 0°C and 100°C.
2.5.3 Enthalpy of evaporation ( hfg )
This is the amount of heat required to change the state of water at its boiling
temperature, into steam.
It involves no change in the temperature of the steam/water mixture, and all
the energy is used to change the state from liquid (water) to vapor (saturated
steam).

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Figure 2.10 Enthalpy temperature curve
The old term latent heat is based on the fact that although heat was added,
there was no change in temperature. However, the accepted term is now
enthalpy of evaporation.
Like the phase change from ice to water, the process of evaporation is also
reversible. The same amount of heat that produced the steam is released
back to its surroundings during condensation, when steam meets any surface
at a lower temperature. See Figure 2.10.
This may be considered as the useful portion of heat in the steam for heating
purposes, as it is that portion of the total heat in the steam that is extracted
when the steam condenses back to water.
2.5.4 Dryness fraction
Steam with a temperature equal to the boiling point at that pressure is known
as dry saturated steam. However, to produce 100% dry steam in an industrial
boiler designed to produce saturated steam is rarely possible, and the steam
will usually contain droplets of water.

Note:
In practice, because of turbulence and splashing, as bubbles of
steam break through the water surface, the steam space contains
a mixture of water droplets and steam.

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Steam produced in any shell-type boiler, where the heat is supplied only to the
water and where the steam remains in contact with the water surface, may
typically contain around 5% water by mass.

If the water content of the steam is 5‰ by mass, then the steam is said to be
95% dry and has a dryness fraction of 0.95.

The actual enthalpy of evaporation of wet steam is the product of the dryness
fraction ( ) and the specific enthalpy ( ) from the steam tables. Wet steam
will have lower usable heat energy than dry saturated steam.

Actual enthalpy of evaporation

Therefore:

Actual total enthalpy

Because the specific volume of water is several orders of magnitude lower
than that of steam, the droplets of water in wet steam will occupy negligible
space. Therefore the specific volume of wet steam will be less than dry steam:

Actual specific volume

Where is the specific volume of dry saturated steam.

Worked Example 2.1

4 637,5 kJ of heat is dissipated from 1,75 kilograms of wet steam which is at a
pressure of 2 000 kPa. The supply water which is used to form the steam enters
the evaporator at 26,5 ºC. The wet steam Is then superheated to 350 ºC. The
specific heat capacity of the water is 4 187 J/kg.K.

Calculate the following:
1. The dryness fraction of the wet steam
2. The change in enthalpy from wet steam to superheated steam for 1 kg of

steam
3. The increase in the specific volume of the steam for 1 kg of steam

Solution: =
1. = 2 650 kJ/kg
=
= 4,187 X 26,5

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= 111 kJ/kg
=
= 2650 -111
= 2539 kJ/kg
But =
=
=
= 0,863
2. =
=
= 3 138 – 2 539
= 599 kJ/kg

2.5.5 The steam phase diagram
The data provided in the steam tables can also be expressed in a graphical
form. Figure 2.11 illustrates the relationship between the enthalpy and
temperature of the various states of water and steam; this is known as a phase
diagram. From the diagram, note the following:

A to B:
Heat is added to water which will reach a specified pressure and, depending
on that pressure, the saturation temperature ( ts ) will be reached. This is the
sensible phase of the process of generating steam. The enthalpy change is hf.

B to C:
At point B, vapour starts forming and as heat continues to be added the
enthalpy of evaporation is hfg, changes at constant temperature,

C to D:
The vapour is no longer in contact with water and the superheat phase starts.
The temperature starts to rise again.

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Figure 2.11 Enthalpy temperature diagram

Enthalpy values as they occur in the formation of steam are set out in tabular
form.

The first phase in the generation of steam is warming up the water from an
original temperature to saturation temperature. The energy added to the
water during this phase if liquid enthalpy ( )

Enthalpy of sensible heat:

[ ]

The second phase is when energy continues to be added, creates an enthalpy
of evaporation ( ). This occurs at constant saturation temperature. The liquid
is transformed into dry saturated vapour.

Enthalpy of latent heat:

The third phase as energy continues to be added is when the temperature
starts to raise again at constant pressure. This is the enthalpy of dry saturated
vapour ( ).

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Find in steam tables
Enthalpy of superheated steam:

()

Enthalpy total:

()

Worked Example 2.2

10 kg of water, at a pressure of 920 kPa and a temperature of 60 ºC, is
heated to boiling point, then to dry saturated steam and then to
superheated steam, with a temperature of 210 ºC. The specific heat
capacity of superheated steam is 2,1 kg/kg. ºC and that of water is 4,187
kJ/kg. ºC.

Calculate the following with the aid of steam tables:

1. The enthalpy of the sensible heat in MJ
2. The enthalpy of the latent heat In MJ
3. The enthalpy of the superheat in kJ
4. The enthalpy of the total heat required for superheating the water In MJ
5. The saturation temperature of the steam

Solution: ={ }
1. ={ }
= 4 957,8 kJ
= 4,9578 MJ

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2. = (from steam tables)

=

= 20 260 kJ

= 20,26 MJ

3. = }
={

= 707,7 kJ

4. =

= 4 958 + 20 260 + 707,7

= 25 925,7 kJ

= 25,9257 MJ

5. = 176,3 ºC

Worked Example 2.3

Steam which contains 4,2% moisture enters a superheater at a pressure of 4
MPa. The steam is then superheated to a temperature of 400 ºC at constant
pressure.

Calculate the following with the aid of steam tables:
1. The heat transferred in the superheater in kJ/kg
2. The volume of steam entering the superheater in m3/kg
3. The volume of the superheated steam leaving the superheater in m3/kg

Solution: = }
1. ={
={ }
2.
3. = 3 214 – 2 728,054

= 485,946 kJ/kg

=

=

= 0,04766 m3/kg

= 0,0773 m3/kg (@4 000 kPa and 400 ºC)

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