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Tolley’s Basic
Science and Practice
of Gas Service
Gas Service Technology Volume 1
This page intentionally left blank
Tolley’s Basic
Science and Practice
of Gas Service
Gas Service Technology Volume 1
Fifth edition
John Hazlehurst
AMSTERDAM  BOSTON  HEIDELBERG  LONDON  NEW YORK  OXFORD
PARIS  SAN DIEGO  SAN FRANCISCO  SINGAPORE  SYDNEY  TOKYO
Newnes is an imprint of Elsevier
Newnes is an imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP, UK
30 Corporate Drive, Suite 400, Burlington, MA 01803, USA
First edition 1978
Second edition 1990
Third edition 1994
Fourth edition 2006
Fifth edition 2009
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or transmitted in any form or by any means electronic, mechanical, photocopying,
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email: Alternatively visit the Science & Technology website
at www.elsevierdirect.com/rights for further information

Notice
No responsibility is assumed by the publisher for any injury and/or damage to persons or
property as a matter of products liability, negligence or otherwise, or from any use or
operation of any methods, products, instructions or ideas contained in the
material herein
British Library Cataloguing in Publication Data
Hazlehurst, John.
Tolley’s basic science and practice of gas service. – 5th ed.
1. Gas appliances–Installation. 2. Gas appliances–Maintenance and repair.
I. Title II. Basic science and practice of gas service 683.8
0
8-dc22
Library of Congress Control Number: 2009925599
ISBN: 978-1-85617-671-2
For information on all Newnes publications visit
our website at elsevierdirect.com
Printed and bound in Great Britain
0910111213987654321
Table of Contents
Preface vi
1 Properties of Gases 1
2 Combustion 14
3 Liquefied Petroleum Gas 41
4 Burners 83
5 Energy 111
6 Pressure and Gas Flow 147
7 Control of Pressure 181
8 Measurement of Gas 207
9 Basic Electricity 240
10 Transfer of Heat 288

11 Gas Controls 311
12 Materials and Processes 361
13 Tools 387
14 Measuring Devices 439
Appendix 1 SI Units 461
Appendix 2 Conversion Factors 463
Index 466
v
Preface
Following comprehensive updates and revision of the two other volumes in this
series ‘Domestic Gas Installation Practice’ and ‘Industrial and Commercial Gas
Installation Practice’ (formerly Gas Service Technology 2 and 3), it was clearly
essential that this, the first volume in the series, be brought up to date. ‘Basic
Science and Practice of Gas Service’ leads the reader through the knowledge
and understanding required to put into practice the safe installation and
servicing procedures described in Volumes 2 and 3.
Changes to standards and legislation have been included, in particular the
European gas directive relating to the prevention of products of combustion
being released into a room in which an open-flued appliance is installed.
Chapter 8 covers the devices used to ensure that these types of appliances
conform to this directive. New types of combustion analysers and appliance
testers which take advantage of the new technology available have also been
included. Since the release of British Standards 7967 Parts 1, 2 & 3, on the
8thDecember2005and7967Part4on29thJune2007theindustryhasonce
again focused on Combustion Analysers. Compliance with the standards is of
no value without a full working understanding of using your chosen Elec-
tronic Combustion Analyser. Chapter 14 covers requirements of British
Standards 7967.
There have also been changes to the manner in which gas operatives are
required to prove their competence. It is now a legal requirement that all gas

operatives in the domestic field and most operatives working in industrial and
commercial sectors currently be registered with the Confederation of Registered
Gas Installers (CORGI). However, on 8th September 2008, HSE awarded
a 10-year contract to the Capita Group Plc to provide a new registration scheme
for gas installers from 1st April 2009. The current scheme has been in place for
more than 17 years. During this time the number of domestic gas related
fatalities has fallen significantly. However, a review in 2006 involving gas
industry stakeholders (including gas installers and their representatives) and
consumer groups identified no room for complacency and a strong case for
change. To achieve membership all operatives must be successful in a series of
initial gas safety assessments (Nationally Accredited Certification Scheme for
Individual Gas Operatives ACS) in the areas of work in which they operate. This
certification must be renewed every five years.
Scottish/National Vocational Qualifications are being amended to include
these ACS assessments as part of the qualification process. This volume and the
others in the series will prove invaluable to students studying for these
vi
qualifications and certificates, and for operatives wishing to improve their
knowledge and understanding of natural gas and Liquefied Petroleum Gas
(LPG) systems.
I would like to thank manufacturers for the use of photographs and
diagrams, in particular S.I.T. Gas Controls (ODS devices) and BW Technolo-
gies (flue gas analysers), and also Blackburn College for the use of their
facilities and resources.
Preface vii
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Chapter one
Properties of Gases
Chapter 1 is based on an original draft prepared by Mr E.W. Berry
INTRODUCTION

This first volume of the manual deals with the elementary science or ‘tech-
nology’ which forms the foundation of all gas service work. It outlines the
principles involved and explains how they work in actual practice.
To do this it has to use scientific terms to describe the principles of things
like ‘force’, ‘pressure’, ‘energy’, ‘heat’ and ‘combustion’. Do not be put off by
these words – they are simply part of the language of the technology which you
have to learn. Every activity from sport to music has its own special words and
gas service is no exception. While the football fan talks of ‘strikers’, ‘sweepers’
and ‘back fours’ the gas service man deals with ‘calorific values’, ‘standing
pressures’ and ‘secondary aeratio n’.
It is necessary for him to know about these things so that he can be sure that
he has adjusted appliances correctly. He must also know what actions to take to
avoid danger to himself or customers or damage to customers’ property.
GAS: WHAT IT IS?
Every substance is made up of tiny particles called ‘molecules’ (see Chapter 2).
In solid substances like wood or metal, there is very little space between the
molecules and they cannot move about (Fig. 1.1).
In liquids, there is a little more space between the particles, so that a liquid
always moves to fit the shape of its container. The molecules cannot get very far
without bumping into each other, however, so they do not move very quickly
and only a few get up enough speed to break out of the surface and form
a vapour above the liquid (Fig. 1.2).
A gas has a lot more space between its molecules. So they are able to move
about much more freely and quickly. They are continually colliding with each
other and bouncing on to the side s of their container. It is this bombardment that
creates the ‘pressure’ inside a pipe ( Fig. 1.3).
Because the molecules are as likely to move in one direction as in any other,
the pressure on all of the walls of their container will be the same. Gases must, 1
therefore, be kept in completely sealed containers otherwise the particles would
fly out and mix, or ‘diffuse’, into the atmosphere (see section on Diffusion).

The word ‘gas’ is derived from a Greek word meaning ‘chaos’. This is
a good name for it, since the particles are indeed in a state of chaos, whizzing
about, colliding and rebounding with a great amount of energy.
KINETIC THEORY
‘Kinetic’ means movement or motion, so kinetic energy is energy possessed by
anything that is moving. A car has kinetic energy when it is moving along
a road. The effect of this can clearly be seen if it collides with another object!
Similarly gas molecules are in motion and possess kinetic energy at all normal
temperatures. The amount of energy increases as the temperature increases.
The Kinetic Theory states that:
1. The distance between the molecules of a gas is very great compared with
their size (about 400 times as great).
FIGURE 1.2 Molecules in a liquid.
FIGURE 1.1 Molecules in a solid.
FIGURE 1.3 Molecules in a gas.
2 Properties of Gases
2. The molecules are in continuous motion at all temperatures above
absolute zero, À273

C (see Chapter 5).
3. Although the molecules have an attraction for each other and tend to hold
together, in gases at low pressures the attraction is negligible compared
with their kinetic energy.
4. The amount of energy possessed by the molecules depends on their
temperature and is proportional to the absolute temperature (see Chapters
5 and 8).
5. The pressure exerted by a gas on the walls of the vessel containing it is due
to the perpetual bombardment by the molecules and is equal at all points.
DIFFUSION
If a small amount of gas is allowed to leak into the corner of an average-sized

room, the smell can be detected in all parts of the room after a few seconds. This
shows that the molecules of gas are in rapid motion and because of this, gases
mix or ‘diffuse’ into each other.
Graham’s Law of Diffusion
If two different gases at the same pressure were put into a container separated by
a wall down the centre and a small hole made in the wall as shown in Fig. 1.4
then, because the molecules are in continuous motion, some molecules of each
gas would pass through the hole into the gas on the other side. The faster and
lighter molecules would pass more quickly through the hole into the other gas.
After studying the rates at which gases diffuse into each other, Graham
discovered that the rates of diffusion varied inversely as the square root of the
densities of gases. Or,
Diffusion rate f
1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
density
p
Thus a light gas will diffuse twice as quickly as a gas of four times its density
(see section on Specific Gravity).
The effect can be demonstrated experimentally by filling a porous pot, made
from unglazed porcelain, fitted with a pressure gauge, with a dense gas such as
FIGURE 1.4 Diffusion of two gases.
Diffusion 3
carbon dioxide. Then place it in another vessel and fill that with a lighter gas
such as hydrogen (see Fig. 1.5). The pressure inside the inner pot will be seen to
rise, proving that the lighter gas is getting into the pot faster than the heavier gas
that is getting out.
CHEMICAL SYMBOLS
Symbols are often the initial letters of the name of the substance, like H for
hydrogen, O for oxygen. These symbols are not only a form of shorthand and

save a lot of writing, they also show the amount of the substance being
considered.
Each single symbol indicates one ‘atom’, which is the smallest chemical
particle of the substance (see Chapter 2). So H indicates one atom of hydrogen,
O is one atom of oxygen and so on.
It has previously been said that substances are made up of tiny particles
called ‘molecules’. This is true, but the molecules themselves consist of atoms.
Sometimes a molecule of a substance contains only one single atom, like
carbon, which is denoted by C. Often the molecules have more than one atom,
like those of hydrogen and oxygen which both have two atoms. So while an
atom is indicated by H, the smallest physical particle of hydrogen gas which can
exist is shown by H
2
. The ‘2’ in the subscript position indicates that the
molecule of hydrogen is made up of two atoms.
Some substanc es are made up of combinations of different kinds of atoms.
Water is an example. Water is composed of hydrogen and oxygen and its
formation is described in Chapter 2. The chemical formula for water is H
2
O.
This shows that a molecule of water has two atoms of hydrogen and one of
oxygen combined together. Similarly, methane gas, which forms the main part
of natural gas, is made up of one atoms of carbon and four atoms of hydrogen.
So its formula is CH
4
.
FIGURE 1.5 Experiment to demonstrate the effect of diffusion.
4 Properties of Gases
Table 1.1 shows the chemical symbols and formulae for some of the
substances met with in gas service work.

ODOUR
Gas can, of course, be dangerous. It can burn and it can explode. Some gases are
‘toxic’ or poisonous. But all fuels are potential killers if not treated properly.
Coke and oil both burn and can produce poisonous fumes. Electricity causes
more domestic fires than any other fuel and the first indication you get of its
presence could be your last!
Gas has the advantage of having a characteristic smell or ‘odour’ so it is
easily recognisable. Several of the combustible gases, including hydrogen,
carbon monoxide and methane, are colourless and odourless and could not
easily be detected without elaborate equipment. To make it possible for
customers to find out when they have a gas escape or have accidentally turned
on a tap and not lit the burner, a smell or odour, is added to the gas.
Gas manufactured from coal has its own smell, natural gas does not. But
suppliers of natural gas are required to add a smell to it before sending the gas
out to the customers. So an ‘odorant’ is used, originally a chemical called tet-
rahydrothiophene. Only a very small amount is added, something like 1/2 kg to
a million cubic feet of gas. Odorants now in use contain diethyl sulphide and
ethyl and butyl mercaptan.
Table 1.1 Chemical Symbols and Formulae for Substances Commonly
Used During Gas Service Work
Gases Metals
Substance
Chemical
Formulae Substance
Chemical
Formulae
Hydrogen H
2
Iron Fe
Oxygen O

2
Copper Cu
Nitrogen N
2
Lead Pb
Carbon monoxide CO Tin Sn
Carbon dioxide CO
2
Zinc Zn
Methane CH
4
Antimony Sb
Ethane C
2
H
6
Platinum Pt
Propane C
3
H
8
Nickel Ni
Butane C
4
H
10
Chromium Cr
Propylene C
3
H

6
Tungsten W
Mercury Hg
Odour 5
TOXICITY
A number of gases are ‘toxic’ or poisonous and inhaling them can resu lt in
death. Newspaper reports of people being ‘gassed’ are usually referring to
carbon monoxide poisoning.
Carbon monoxide, CO, is the ingredient which causes the problem. By
replacing oxygen in the bloodstream it prevents the blood from maintaining life
and so the organs of the body become poisoned.
CO is one of the constituents of gas made from coal or oil, and inhaling
the unburnt gas can prove fatal. Natural gas does not contain CO and so it is
‘non-toxic’.
This means, of course, that people can no longer commit suicide by gassing
themselves. There is another hazard, however. All fuels which contain carbon
can produce carbon monoxide in their flue gases if the carbon is not completely
burned. So people can still be gassed if the appliances are not flued or ventilated
correctly (see Chapter 2). There is always a risk of suffocation if the presence of
the gas reduces the amount of oxygen in the air.
CALORIFIC VALUE
All gases which burn give off heat (energy) and the ‘calorific value’, or CV,
indicates the heating power. It is the number of heat (energy) units which can be
obtained from a measured volume of the gas.
To measure CV in SI units, megajoules per cubic metre are used, written as
MJ/m
3
. The CV of natural gas in the UK is about 39.3 MJ/m
3
but does vary

slightly from district to district. Because customers pay for the gas they use
measured in heat units, The Gas supplier has to declare the calorific value of its
supply and this is printed on every gas bill. The actual CV is monitored at
official testing stations by gas examiners, appointed by the Dep artment of Trade
and Indus try.
Meters presently used by The Gas supplier measure gas in cubic feet
(100 ft
3
¼ 2.83 m
3
) and until April 1992 custom ers were charged, based on the
number of ‘therms’ used (1 therm ¼ 105.506 MJ).
EC directive 80/181 required Britain to change their method of billing from
imperial to metric units and The Gas supplier implemented this change from
April 1992 using the metric kilowatt hour (kWh) as a basis for charge. It is the
common unit used in Europe and is of course the basis of charge for electricity.
The total amount of heat obtained from gas is, in fact, the Gross CV. If
however, the water vapour in the products of combustion is not allowed to
condense into water, the amount of heat obtained is the Net CV.
SPECIFIC GRAVITY (RELATIVE DENSITY)
Every substance has weight or ‘mass’, including gas. Some complicated scien-
tific equipment would be needed to do the weighing, but it can be weighed. It is
necessary, for various reasons, to compare weights of gases and to do this
6 Properties of Gases
a comparison is made of their ‘densities’. The density of a substance is the weight
of a given volume. In Imperial units it is the number of pounds per cubic foot
(lb/ft
3
), and in SI units it is the number of kilograms per cubic metre (kg/m
3

).
Densities of substances vary very considerably. Lead is heavier than wood
and wood is lighter than water. In order to compare densities they are related to
a standard substance. For solids and liquid s the standard is water. For gases the
standard is air.
This relationship between the density of a substance and the density of the
standard is known as the ‘relative density’ or ‘specific gravity’. Let us take the
example of mercury. The specific gravity (or SG) of liquid mercury is 13.57. So
it is about 13 V2 times as heavy as the same bulk of water, or 1 l would weigh
13.57 kg.
The specific gravity of natural gas is in the region of 0.5. So it is about half
the weight of the same volume of air. The specific gravity of liquefied petroleum
gas (LPG) is greater than that of air.
WOBBE NUMBER
The Wobbe number of Wobbe ‘index’ gives an indication of the heat output
from a burner when using a particular gas (the terms Wobbe number and Wobbe
index are used interchangeably in this book).
The amount of heat which a burner will give depends on the following factors:
1. The amount of heat in the gas as given by its calorific value.
2. The rate at which the gas is being burned. This rate depends on the
following.
2.1 The size of the ‘jet’ or ‘injector’.
2.2 The pressure in the gas, pushing it out of the injector.
2.3 The relative weight of the gas. This affects how easily the pressure
can push it out and is indicated by the specific gravity.
Looking at these factors it can be seen that they divide up into two groups.
1. Factors depending on the gas:
– calorific value (1),
– specific gravity (2.3).
2. Factors depending on the appliance:

– size of injector (2.1),
– pressure of the gas (2.2).
Since the factors in group 2 are fixed by the design or the adjustment of the
appliance, the only alterations in heat output would be brought about by
changes in the group 1 factors. That is, changes in the characteristics of the gas,
the CV and the SG. The Wobbe number links these two characteristics and is
obtained by dividing the CV by the square root of the SG, thus:
Wobbe number ¼
CV
ffiffiffiffiffiffiffi
SG
p
Wobbe number 7
It is essential to ensure that the heat outputs of appliances are kept reasonably
constant. To do this the Wobbe number of the gas must be maintained within
fairly close limits.
Natural gas with a CVof 39.33 MJ/m
3
and a SG of 0.58 would have a Wobbe
number of 51.64.
FAMILIES OF GASES
To ensure that appliances operate correctly the gas quality must be maintained
within close limits. In practice, it is kept to a quality range indicated by Wobbe
numbers.
There are three ranges or ‘families’ which have been agreed internationally
(Table 1.2). Family 1 covers manufactured gases, family 2 covers natural gases
and family 3 covers liquefied petroleum gas (LPG).
Manufactured gases are generally made from coal, oil feed-stocks or
naphthas and also include LPG/air mixtures.
The demand for natural gas has historically been supplied from the North

Sea reserves, with excess gas being supplied to Continental Europe. However,
more recently the UK have become a net importer of gas. It is estimated that
additional supplies will be required by 2010; the UK is projected to import up to
half of its gas demand and that by 2020 imports may be as high as 90%.
LIQUEFIED NATURAL GAS (LNG)
The reasoning behind storing and transporting LNG as opposed to gaseous
natural gas is the physical property of a reduction in volume of 600 times.
Although there may be some modifications of the make up of the revapourised
gas which change its combustion characteristics.
Liquefied petroleum gases include propane, butane and mixtures. They will
be dealt with in more detail in Chapter 3.
Table 1.2 Families of Gases
Family Approximate Wobbe Number Range Type of Gas
1 22.5–30 Manufactured
(inc. LPG/air)
2

L 39.1–45 Natural
H 45.5–55
3 73.5–87.5 LPG
Appliances are designed to operate on gas of a particular family.
8 Properties of Gases
AIR REQUIREMENTS
Everything that burns must have oxygen in order to do so. Fuel gases contain
carbon and hydrogen compounds which burn when they are lit and allowed to
combine with oxygen (see Chapter 2).
Fortunately the atmosphere consists of about 21% oxygen and 79% nitrogen
(with a tiny amount of other gases). This means that if a gas flame is allowed to
burn freely in the open, it can get the oxygen it needs from the surrounding air.
For each cubic metre (m

3
) of gas burned, the amount of air required is:
 4.89 m
3
for butane/air,
 9.75 m
3
for natural gas,
 23.8 m
3
for commercial propa ne.
AERATION
Gas can be burned straight out of a jet, getting the air it requires from the
atmosphere around the flame. This happens with a ‘neat’ or ‘non-aerated’
burner.
For a number of uses a different kind of flame is necessary and to get this
a proportion of the air is allowed to mix with the gas before it is burned. This
happens in the ‘aerated burner’ and the air added first is called ‘primary air’, as
distinct from ‘secondary air’ which is the remaining air required and obtained
from around the flame.
The proportion of air mixed before ‘combustion’ or burning is called the
‘primary aeration’. It is usually expressed as a percentage. For example,
a burner with 50% primary aeratio n would have half the total air requirement
mixed with the gas before it was burned.
GAS MODULUS
The ‘gas modulus’ is a numerical expression which relates the heat output from
a burner with the pressure required to provide a satisfacto ry amount of aeration.
It gives a figure which indicates how aeration and heat loading conditions may
be maintained when changing from one gas to another.
The modulus is obtained by dividing the square root of the pressure by the

Wobbe index. Thus,
Gas modulus ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pressure
p
Wobbe index
Using the modulus shows that to change from a manufactured gas with a Wobbe
index of 27.2 supplied at a pressure of 6.23 mbar to a natural gas with a Wobbe
index of 49.6 required the pressure to be increased to 20.62 mbar to maintain the
same operating conditions (see Chapter 4, section on Modifying Appliances to
Burn Other Gases).
Gas modulus 9
FLAMMABILITY LIMITS
Mixtures of gas and air will burn, but only within limits. If there is either too
much gas or too much air, the mixture will not burn. The ‘flammability limits’
are those air and gas mixtures at each end of the range which will just burn. For
natural gas the range is from 5% up to 15% of gas in the mixture. So the limits
are 5% gas for the lower explosive limit (LEL) and 15% gas for the higher
explosive limit (HEL). For commercial propane the limits are 2.0–10.0% and
for butane/air the limits are narrower being from 1.6 to 7.75% gas.
FLAME SPEED
All flames burn at particular rates. You can watch a flame burning its way along
a match or taper. Similarly a gas flame is burning along the mixture ( Table 1.3)
as it comes out of the jet or burner at a particular speed. The speed at which the
mixture is coming out has to be adjusted so that the flame will stay on the tip of
the burner. If it came out too fast it would blow the flame off and if it was too
slow the flame could burn its way back inside the tube!
The flame speed of gas is measured in metres per second. Typical flame
speeds are:
 natural gas: 0.36 m/s,

 butane/air: 0.38 m/s,
 commercial propane: 0.46 m/s.
Table 1.3 Constituents of Gases
Constituent Formulae
Typical Percentage by Volume
Natural
Gas
Commercial
Propane Butane/Air
Oxygen O
2
17.01
Nitrogen N
2
2.7 63.99
Carbon dioxide CO
2
0.6
Methane CH
4
90.0
Ethane C
2
H
6
5.3 1.5
Propylene C
3
H
6

12.0
Propane C
3
H
8
1.0 85.9 2.5
Butane C
4
H
10
0.4 0.6 16.5
100 100 100
10 Properties of Gases
FIGURE 1.6 Substitute natural gas (SNG) plant. A catalytic rich gas producer with a double methanation process.
Flame speed 11
IGNITION TEMPERATURES
Gases need to be lit or ‘ignited’ before they will burn. This is done by heating
the gas until it reaches a sufficiently high temperature to burst into flame and
keep burning.
Heating the gas to the required temperature may be done with a match,
a small gas flame or ‘pilot’, an electric spark, or a coil of wire or ‘filament’ made
red hot by an electric current. Ignition temperatures of the common gases are:
 natural gas: 704

C,
 butane/air: 500

C,
 commercial propane: 530


C.
SUBSTITUTE NATURAL GAS
Substitute natural gas (SNG) is manufactured either as a direct substitute for
natural gas or as a means of providing additional gas to meet peak loads. It can
be made from a range of feed-stocks in a number of different types of plant. The
feed-stocks commonly used are LPG or naphthas, which are light petroleum
distillates. The feed-stock is mixed with high pressure steam and passed over
a catalyst to produce a Catalytic Rich Gas (CRG). After this it may pass through
additional processes to increase its percentage of methane and to remove carbon
dioxide. An example of an SNG plant is shown in Fig. 1.6.
For combustion to be useable, it must be controlled; uncontrolled it will be
dangerous and inefficient. To control combustion and achieve fuel efficiency,
and complete combustion process, it is necessary to understand the character-
istics of the fuel and the way it burns. Table 1.4 below provides the comparison
of prope rties of typical gases.
Table 1.4 Comparison of Properties of Typical Gases
Property Units
Natural
Gas
Commercial
Propane Butane/Air
Caloric value MJ/m
3
39.3 97.3 23.75
Specic gravity air ¼ 1 0.58 1.5 1.19
Wobbe number MJ/m
3
51.64 79.4 21.79
Air required vol/vol 9.75 23.8 4.89
Flammability

limits
% gas
in air
5–15 2–10 1.6–7.75
Flame speed m/s 0.36 0.46 0.38
Ignition
temperature

C 704 530 500
12 Properties of Gases
The characteristics of SNG from the ‘double methanation process’ are
shown in Table 1.5. The gas produced is a non-toxic, high-methane, low-inert
gas interchangeable with natural gas, and it can be supplied directly into
pipelines.
Table 1.5 Characteristics of Typical SNG
Constituent Formulae
Percentage by
Volume
Methane CH
4
98.5
Hydrogen H
2
0.9
Carbon monoxide CO 0.1
Caloric value 38 MJ/m
3
Specic gravity 0.555 (air ¼ 1.0)
Wobbe number 51 MJ/m
3

Substitute natural gas 13
Chapter two
Combustion
Chapter 2 is based on an original draft prepared by Mr E.W. Berry
COMBUSTION
Fuel gases burn when they are ignited and allowed to combine with oxygen,
usually taken from the air. This burning or ‘combustion’ is in fact a chemical
reaction taking place, a reaction which produces heat and changes the gas and
air into other gases. In order to understand what exactly is taking place and what
new substances are being produced by the combustion, it is necessary to study
a little very simple chemistry. Start by looking at atoms and molecules in a little
more detail.
ATOMS
The atom was introduced in Chapter 1 as the smallest chemical particle into
which a substance may be divided by chemical means. Atoms are, however,
made up of three components. There are over 100 different kinds of atoms, each
containing different numbers of the three basic components.
All atoms consist of a relatively heavy central core or ‘nucleus’ with very
light ‘electrons’ revolving or orbiting round it at a little distance. The nucleus
has a positive (or þ) electrical charge and the electrons have negative (or À)
electrical charges. The positively charged particles in the nucleus are called
‘protons’. Usually the number of electrons and protons in an atom is the same so
that the negative and positive charges balance out and the atom is electrically
neutral. The simplest of all atoms is the common hydrogen atom (Fig. 2.1). It
has a single proton round which revolves a single electron.
Some atoms have additional particles in the nucleus which are similar to
protons but are electrically neutral (i.e. without charge). These particles are
called ‘neutrons’. A few hydrogen atoms have neutrons as well as protons, and
the nucleus of ‘heavy hydrogen’ is thought to have one proton and one neutron
(Fig. 2.2).

The weight or mass of an atom depends on the number of protons and
neutrons in the nucleus. The electrons are so tiny, by comparison, that they do14
not affect the overall mass of the atom. So the three components of an atom
are:
1. electrons,
2. protons,
3. neutrons.
Electrons move around the nucleus of an atom in fixed orbits. There can be up to
seven orbits of electrons in the more complex atoms. The inner orbit or ‘shell’
can contain up to two electrons. The next holds up to eight electrons. Each shell
holds a fixed number of electrons.
The chemical behaviour of an atom depends largely on the number of
electrons in its outer shell. This particularly affects the ability of the atom to
combine with others to form molecules. Stable substances are those whose outer
shell contains its full complement of electrons. Other substances, with outer
shells which are not completely full, are less stable and so combine more readily
together to form more stable substances. Carbon, for example, has two electron
shells. With two electrons in its inner shell, but only four in its outer shell, it
combines readily with other substances.
MOLECULES
A molecule is the smallest particle of a substance which can exist independently
and still retain the properties of that substance.
A molecule of methane, CH
4
, consists of one atom of carbon and four atoms
of hydrogen (Fig. 2.3). You can see that, by sharing electrons with the hydrogen
atoms, the carbon atom can fill up its outer shell to the full eight electrons and so
form a stable substance.
FIGURE 2.1 Structure of the hydrogen atom.
FIGURE 2.2 Structure of the deuterium (or ‘heavy hydrogen’) atom.

Molecules 15
ELEMENTS
An element is a substance whose molecules contain only one kind of atom.
Examples of elements are hydrogen, H
2
, oxygen, O
2
, nitrogen, N
2
, iron, Fe,
copper, Cu.
COMPOUNDS
Compounds are substances whose molecules contain more than one kind of
atom. For example, water, H
2
O, contains the atoms of hydrogen and oxygen.
Similarly methane, CH
4
, is a compound since it contains both carbon and
hydrogen atoms.
MIXTURES AND COMPOUNDS
It is possible for substances to be brought together either as compounds or as
mixtures and there are important differences between the two. Table 2.1 shows
the main differences.
FIGURE 2.3 Structure of a molecule of methane.
Table 2.1 Differences Between M ixtures and Compounds
Compounds Mixtures
There are fixed proportions of
constituents
May have variable proportions

Produced by a chemical reaction usually
associated with heat
Made by adding constituents
together in some container
Cannot be separated by physical means Can be separated by physical means
Has properties often very different from
those of its constituents
Has properties related directly to its
constituents, each contributing its
own particular property to the whole
16 Combustion