Physics and Chemistry For Firefighters

Issued under the authority of the Home Office(Fire and Emergency Planning Directorate)

Fire Service Manual

Volume 1Fire Service Technology,Equipment and Media

Physics andChemistry forFirefighters

HM Fire Service Inspectorate

Publications Section

Issued under the authority of the Home Office(Fire and Emergency Planning Directorate)

Fire Service Manual

Volume 1Fire Service Technology,Equipment and Media

Physics and Chemistry forFirefighters

HM Fire Service Inspectorate Publications Section

London: The Stationery Office

© Crown Copyright 1998Published with the permission of the Home Officeon behalf of the Controller of Her Majesty's Stationery Office

Applications for reproduction should be made inwriting to The Copyright Unit. Her Majesty's Stationery Office,St. Clements House, 2-16 Colegate, Norwich, NR3 1BQ

ISBN 0 11 341182 0

Cover and half-title page photograph: Hampshire Fire and Rescue Service

Printed in the United Kingdom for The Stationery OfficeJ43472 4/98 C50

Physics and Chemistryfor Firefighters

Preface

In order to understand how fires behave and howthey can be extinguished, it is necessary to under-stand some physics and chemistry. This book isdivided into three parts which will introduce thereader to the relevant physical and chemicalprocesses and then show how these work togetherin the phenomenon that we call fire.

Extinguishing fires is a matter of interrupting oneor more of these processes so that burning cannotcontinue. Firefighters will appreciate that it is,therefore, important to acquire a good understand-ing of what happens in a fire, in order to be able tochoose the best method available to extinguish it,and to avoid making it worse.

In the first part of this book, some of the physicalproperties of matter will be discussed. Some mate-rials are heavier than others, bulk for bulk. Someheat up more easily than others. These and otherproperties greatly affect the way that materialsbehave when they are involved in a fire.

In the second part, the chemical processes relevantto fire will be discussed. Besides the burningprocess itself, the way that materials behave chem-ically in fire will also be discussed. It is hoped thatfirefighters will gain an understanding of the dan-gers that new materials present and the way theweapons they have to fight them work.

The third part of the book discusses fire extinction.

This book replaces: The Manual of Firemanship.Book 1 - Elements of combustion and extinction.

The Home Office is indebted to all those whoassisted in the production of this book, in particu-lar, Edinburgh University Department of Civil andEnvironmental Engineering, Dr. John Brenton andDr. Dougal Drysdale.

Physics and Chemistry for Firefighters iii


Contents

Preface iii

Chapter 1 Physical properties of matter 1

Physics and Chemistry for Firefighters V

Density1.1 Vapour density 21.2 Liquids of different density 21.3 Gases of different density 31.4 Matter and energy 51.5 Melting, boiling and evaporation 7

Chapter 2 Mechanics 9

2.1 Motion 92.2 Momentum and force 102.3 Work, energy and power 102.4 Friction 11

Chapter 3 Heat and Temperature 13

3.1 Measuring temperature 143.2 Thermometric scales 153.2.1 The Celsius or Centigrade scale 153.2.2 The Fahrenheit scale 153.3 Other methods of measuring temperature 153.3.1 The air or gas thermometer 153.3.2 Using solids to measure temperature 153.3.3 Thermocouples 153.3.4 Electrical resistance 153.3.5 Thermistors 163.3.6 Comparison by brightness 163.3.7 Infra-red 163.4 The Kelvin scale of temperature 163.5 Units of heat 173.5.1 The Joule 173.5.2 The calorie 183.53 The British thermal unit 183.6 Specific Heat 183.7 Change of state and latent heat 193.7.1 Latent heat of vaporisation 203.7.2 Effect of change of pressure on boiling point and latent heat 203.73 Latent heat of fusion 203.7.4 Cooling by evaporation 21

Chapter 4 Thermal Expansion 23

The Chemistry of Combustion 396.1 The basis of chemistry 396.2 Atoms and molecules 396.2.1 Compounds and mixtures 406.3 Symbols 416.3.1.Using symbols to write formulae 416.3.2. Radicals 416.4 Atomic mass 426.5 Molecular mass 426.6 Valency 436.6.1 Multiple valency 436.6.2 Nomenclature 436.7 Simple equations 446.8 Use of chemical equations 456.9 limitations of chemical equations 466.9.1 Reality 466.9.2 Physical state 466.9.3 Reaction conditions 466.9.4 Heat 466.9.5 Rate of reaction 46

vi Fire Service Manual

4.1 Thermal expansion of solids 234.1.1 Coefficient of linear expansion 234.1.2 Nickel-iron alloy (invar) 244.1.3 Allowing for expansion in large metal structures 244.1.4 Thermostats 254.1.5 Coefficients of superficial and cubical expansion of solids 264.2 Thermal expansion of liquids 264.2.1 Cubical expansion 264.2.2 The effect of expansion on density 264.3 The expansion of gases 274.3.1 Temperature, pressure, volume 274.3.2 The gas laws 274.3.2.1 Boyle's Law 274.3.2.2 Charles' Law 284.3.2.3 The Law of Pressures 284.3.2.4 The General Gas Law 294.4 The liquefaction of gases 304.4.1 Critical temperature and pressure 304.4.2 Liquefied gases in cylinders 304.5 Sublimation 31

Chapter 5 Heat transmission 335.1 Conduction 335.2 Convection 3553 Radiation 36

Chapter 6 The Basis of Chemistry 39

Chapter 7 Combustion 477.1 The Fire Triangle 477.2 Heat of reaction and calorific values 487.2.1 Oxidation 487.3 What makes a flame a flame? 487.4 Laminar flow and turbulent flow 497.5 Premixed and Diffusion Flames 507.6 Practical examples of premixed flames and diffusion flames 527.6.1 The Bunsen Burner 527.6.2 A candle flame 527.6.3 Flashpoint, firepoint and sustained fires 537.6.4 Fireball 537.6.5 Vapour cloud explosions 537.7 Ignition 547.7.1 Spontaneous ignition temperatures 547.7.2 Self heating and spontaneous combustion 547.7.3 Smouldering 557.8 Hazards of oxidising agents 557.8.1 Nitric acid and inorganic nitrates 557.8.2 Permanganates 567.8.3 Chlorates 567.8.4 Chromates and dichromates 567.8.5 Inorganic peroxides 567.8.6 Organic oxidising agents 567.8.7 Organic peroxides and hydroperoxides 57

Chapter 8 Simple organic stubstances 598.1 Aliphatic hydrocarbons (paraffins or alkanes) 598.2 Unsaturated aliphatic hydrocarbons 608.2.1 Olefines or alkenes 608.2.2 Acetylenes, or alkynes 618.3 Aromatic hydrocarbons 628.4 Liquefied petroleum gases (LPG) 638.5 Simple oxygen-containing compounds derived from hydrocarbons 648.5.1 Alcohols 648.5.2 Aldehydes 658.5.3 Ketones 658.5.4 Carboxylic acids 658.5.5 Esters 668.5.6 Ethers 66

Chapter 9 Polymers 699.1 Polymers 699.2 Fire hazards 709.2.1 Toxic and corrosive gases 709.2.2 Smoke 71923 Burning tars or droplets 71

Physics and Chemistry far Firefighters vii

9.2.4 Exotherms 719.2.5 Catalysts 719.2.6 Flammable solvents 719.2.7 Dusts 729.2.8 Self-extinguishing plastics 729.3 Monomer hazards 72

Acrylonitrile, Butadiene, Epichlorhydrin, Methyl Methacrylate, Styrene,Vinyl Acetate, Vinyl Chloride

9.3.1 Intermediates and hardeners 73Isocyanates, Chlorosilanes, Epoxides

Chapter 10 Other Combustible Solids 7510.1 Wood 7510.2 Coal 7510.3 Metals 7610.3.1 Properties of metals 7610.3.2 Reaction of metals with water or steam 7610.3.3 Reaction with oxygen 7710.4 Sulphur 7710.5 Phosporus 78

Chapter 11 Extiguishing Fires 7911.1 Classification of fires by type 7911.2 Classification of fires by size 8011.3 Extinguishing fire: Starvation, smothering, cooling 8011.3.1 Starvation 8011.3.2 Smothering 8211.3.3 Cooling 8311.4 Fire extinguishing media 8411.4.1 Water 8411.4.2 Inert gas 8411.43 Foam 8411.4.4 Vapourising liquids 8611.4.5 Carbon dioxide and inert gases 8611.4.6 Dry chemical powders 8611.4.7 Blanketing 8711.4.8 Beating out 87

Appendices 88A Metrication: Conversion tables 88B Material densities 90

Further reading 93

viii Fire Service Manual


Matter is the name given to all material things -anything that has mass and occupies space. Solids,liquids, gases and vapours are all matter. Theamount of matter is known as the mass, and ismeasured in kilograms. In everyday life, the massof a solid is measured in kilograms, although forliquids, gases and vapours, we are more accus-tomed to using volume, the amount of space occu-pied by a given substance, simply because it is eas-ier to measure. Thus, we talk about litres of petroland cubic metres of gas. However, gases, vapoursand liquids also have mass which can be expressedin kilograms.

The density of a material is defined as the mass ofone cubic metre of material. One cubic metre is thestandard "unit volume". A unit volume of iron hasa greater mass than a unit volume of wood and isthus more dense.

Calculating values for the densities of differentsubstances enables meaningful comparisons to bemade. The density of a substance is calculated bydividing the mass of a body by its volume.

Density

Understanding density is extremely important for afirefighter. For example, the density of a gas orvapour determines whether it will tend to rise orsink in air, and be found in the greatest concentra-tions at the upper or lower levels in a building. Thedensity of a burning liquid partly decides whetherit is possible to cover it with water to extinguishthe fire, or whether the firefighter will need to usefoam or other another extinguishing medium.However, another important factor is how well theburning liquid mixes with water, a property knownas miscibility.

Imagine two solid rods, both the same length andwidth, one made of wood and one from iron.Though they are the same size, the iron rod weighsmuch more than the wooden rod. The iron rod issaid to have a greater density than the wooden one.

If mass is measured in kilograms (kg) and the vol-ume in cubic metres (m3), the "units" of densitywill be kilograms per cubic metre (kg/m3). If massis in grams (g) and volume in cubic centimeters(cm3), density will be in grams per cubic centime-tre (g/cm3).

(Note: units should always be quoted, and caremust be taken not to mix, or confuse, units.)

Physics and Chemistry for Firefighters 1

Chapter 1 - Physical properties of matter

Mercury has the very high density of 13 600 kg/m3

or 13.6 g/cm3 and is, therefore, 13.6 times as denseas water.

If the density of a substance is lower than the den-sity of water, and does not mix with water, thenthat substance will float on water. To use our pre-vious example, the density of wood is lower thanthat of water and the density of iron is higher, sowood floats and iron sinks.

The term specific gravity or relative density issometimes used to give measure of density. Therelative density of a substance is the ratio of themass of any volume of it to the mass of an equalamount of water.

Relative density or specific gravity has no units asthe units on the top and bottom of the equation arethe same, so they cancel each other out when theone quantity is divided by the other.

1.1 Vapour density

Gases and vapours have very low densities com-pared with liquids and solids. At normal temper-atures and pressures (e.g., 20°C and 1 atmos-phere) a cubic metre of water has a mass of about1000 kg and a cubic metre of air has a mass ofaround 1.2 kg.

We have previously mentioned that specific grav-ity is a ratio of the density of the substance in

2 Fire Service Manual

question compared with the density of water, sospecific gravity is not a sensible thing to use forgases as their densities are so low: e.g., the spe-cific gravity of air is 0.0013. For this reason, thedensity of a gas or vapour (vapour density, usual-ly abbreviated to VD) is given in relation to thedensity of an equal volume of hydrogen, air oroxygen under the same conditions of temperatureand pressure.

Hydrogen is often used as a comparison for cal-culating vapour density because it is the lightestgas. The vapour density of air compared withhydrogen is 14.4, meaning that a given volumeof air is 14.4 times heavier than the same volumeof hydrogen at the same temperature and pres-sure. For carbon dioxide the vapour densitycompared with that of hydrogen is 22, so a givenvolume of carbon dioxide is about 1.5 (that is22/14.4) times as heavy as the same volume ofair at the same temperature and pressure. (If thetemperature and pressure are changed, the vol-ume of the gas will change. This will beexplained later.)

For fire service purposes it is much more conve-nient to compare the density of gases and vapourswith that of air. The reference gas should alwaysbe given to avoid confusion: for example, thevapour density of methane is 0.556 (air = 1), or 8(hydrogen =1).

1.2 Liquids of different density

As we have said, the density of a burning liquidpartly decides whether it is possible to cover itwith water to extinguish the fire, or whether thefirefighter will need to use foam or another extin-guishing medium.

Consider water poured into two tanks (A) and (B)(Figure 1.1) which are standing on a flat, horizon-tal surface and are connected by a horizontal pipeas shown. The water will assume equal levels ineach tank as, for the system to balance, there mustbe an equal "head" (or height) of water in eachtank above the lowest point of the pipe. The"head" determines the water pressure at any depth:with interconnected tanks as in Figure 1.1, the lev-els adjust to ensure that the pressures at the level ofthe pipe are equal.

Figure 1.1 Diagramsshowing the difference indensity in liquids.

Imagine now that petrol is poured into tank (A). Aspetrol has a specific gravity of about 0.75 and willnot mix with water, it will float on the water. Forthe system to balance, the pressures created by theheads of liquid in each tank must be equal asbefore, but because petrol is less dense (lighter)than water, a greater "head" of petrol is required toproduce the same pressure at depth. Consequently,the level of petrol is higher than the level of water.If water is added to tank (B), the water will raisethe petrol in (A) and eventually the petrol will spillover, well before tank (B) is full.

1.3 Gases of different density

Unlike many liquids, all gases and vapours arecompletely miscible. However, differences in den-sity will affect the way in which they mix. Thus,methane (the main component of natural gas) is alight gas with a vapour density of about 0.5 (air =1). If it is leaking into a room from a faulty gasappliance, it will rise to the ceiling, entraining


(mixing with) air as it rises to form a layer ofmethane and air mixture which will eventuallydescend to the level of the leak. (The concentrationof methane in the layer will increase as the layerdescends.) On the other hand, a leak of propanefrom a propane cylinder will produce a layer ofpropane/air mixture at low level in a similar fash-ion, as the vapour density of propane is roughly1.5 (air= 1).

All heavier-than air gases, like carbon dioxide (VD1.53, air = 1) and petrol vapour (VD 2.5, air = 1)will accumulate in low places such as wells andcellars, so creating dangers of asphyxia (suffoca-tion) as well as of fire or explosion in the case offlammable vapours.

Differences in density can also be created bychanges in temperature (see Chapter 4). Increase intemperature causes expansion, and a lowering ofdensity. Understanding the consequences of this isextremely important: it can be compared with ourexample of the behaviour of petrol and water ininterconnected tanks. As an example, we can con-sider the case of a chimney full of the hot productsof combustion from an open fire.

The chimney and the rest of the building (Figure1.2) act rather like our two tanks containing liquidsof different densities, in that the chimney is effec-tively a tank full of hot light gas joined at the baseto another tank full of cold, heavy gas, i.e. the sur-

rounding outside air. If we consider the hot, lessdense gases inside the chimney and compare themto a column of cold, more dense air outside thechimney we have two volumes of gases of equalheight but of different densities, and are thusunbalanced. The hot gases are said to be buoyantwith respect to the cold air.

In order to restore the balance, cold air from out-side flows into the base of the chimney and drivesthe hot gas out of the chimney. This would contin-ue until the chimney is full of cold air, but in prac-tice the fire at the base of the chimney continuous-ly replaces the hot chimney gas which is drivenfrom the top of the chimney.

If, however, the flow of hot gas from the chimney isprevented by a cover or damper, a pressure willdevelop at the top of the chimney. The weight of thecolumn of lighter hot gas in the chimney is notenough to balance the weight of the heavier cold out-side air. This unbalanced condition is responsible fora force which drives gas from the top of an openchimney. When the top is closed the force produces apressure (which can be calculated if the densities ofthe hot and cold gases are known), and combustionproducts may escape from the fireplace into the room.

The same thing happens in a burning building.The air inside is heated by the fire and sobecomes lighter. It rises and will escape throughany available opening provided it can be

Figure 1.2 Diagram of achimney showing the travelof convection currents.


1.4 Matter and energy

replaced by cold air entering at low level (com-pare with the fireplace). If there is no openingthrough which it can escape, a pressure willdevelop in the burning compartment and anyoneopening a door or window into this space willrelease the pressure, which may cause an outrushof hot gases and possibly flames which couldengulf them. (See Compartment Fires andTactical Ventilation. Fire Service ManualVolume 2.)

When there are low and high level openings (e.g.,broken windows and a hole in the roof), the build-ing acts as an open chimney, with flames and thehot gases escaping at high level with cold fresh airentering from below. Under these circumstances,the fire will be very intense.

Matter can exist in three states: solid, liquid orgas. Some substances are quite commonly foundin all three states - for example, water is found asice, liquid water and water vapour (steam) - butmost substances are, at normal temperatures,found only in one or two of the states. For exam-ple, steel is solid up to its melting point of around1400°C (the melting point varies according to thecomposition of the steel). Its boiling point, thepoint at which it turns into a vapour, is about30()0°C. Carbon dioxide is normally a gas, butunder pressure it can be liquefied and if it is cooledsufficiently, it solidifies. Oxygen is normally a gasbut it can be liquefied at very low temperatures(boiling point-183°C).

All matter is made up of extremely small particlescalled atoms. Atoms have a central core, or nucle-us which contains smaller particles called protonsand neutrons. Protons possess a positive electricalcharge. The nucleus is surrounded by a system ofelectrons, which each carry a negative electricalcharge. Atoms contain as many electrons as pro-tons. As the number of protons and electrons arematched, and each proton possesses an equal andopposite charge to each electron, atoms are electri-cally neutral. The number and arrangement ofelectrons around the nucleus determines the chem-ical behaviour of the atom, that is to say, it deter-mines which other atoms it will combine with.Chemical reactions take place when electrons

move between atoms. An atom which has lost orgained one or more electrons in a chemical inter-action will possess an electrical charge and iscalled either a positive or negative ion.

It now seems that it is not possible to get more thanabout 92 protons in a nucleus without it becomingso unstable that it falls apart. Otherwise, any num-ber of protons is possible.

An element is a substance which contains atomswhich are all of the same type: they all have thesame number of protons. As there can be anythingup to just over 92 protons in an atom, there are, justover 92 stable elements. Atoms of different ele-ments can combine to form molecules.

Some molecules consist of two or more atoms ofthe same kind: for example, an oxygen moleculeconsists of two oxygen atoms (O2). Other mole-cules consist of two or more atoms of differentkinds: carbon dioxide consists of two atoms ofoxygen and one of carbon (CO2), water consists oftwo atoms of hydrogen and one of oxygen (H2O).Carbon dioxide and water are chemical com-pounds and they can, by chemical means, be splitinto their component elements. This forms thebasis of the science of chemistry, which is dealtwith in Chapter 6.

Atom is a Greek word meaning 'indivisible': untilabout 70 years ago, it was believed that atomscould not be split into smaller particles. We knownow that this is wrong and that atoms of one ele-ment can be "split" or combined with other parti-cles to make new atoms of other elements. (Thesubject of atomic physics is discussed in theManual of Firemanship, Part 6c, Chapter 45,Section 11.)

Energy is expended in doing work and may be inone of a number of different forms. Heat, light andelectrical energy are well known from everyday


Figure 1.3 Diagram showing theconversion of potential intokinetic energy.fDiagram: North Hydro)

experience. There is also potential energy, whichis possessed by a body due to its position, forexample by water stored in a hydroelectic dam,and kinetic energy, which is energy possessed bya moving body, for example by the water from thedam flowing through the turbines in the turbinehall). The potential energy is converted into kinet-ic energy as the water flows under gravity to theturbine hall, where it is then converted into electri-cal energy (Figure 1.3).

sure of how hot something is, and is related to how"fast" the constituent molecules are moving.

The molecules which make up any substance, evena solid, are continually moving, although in a solidthey vibrate around a fixed position. They alsoexert a force of attraction to each other, whichbecomes greater the closer they are together. Themovement of molecules tends to spread them outwhile the attractive force, or force of cohesion,tends to bind them together.

If a solid is heated, heat energy is stored in the sub-stance as the vibrational energy of the molecules.As more energy is stored, they vibrate faster andtake up more space. At the same time, the temper-ature of the solid rises and thermal expansionoccurs.

A temperature is reached when the molecules arevibrating so much that they break free of the rigidframework in which they have been held by thecohesive forces and become free enough to slide

For the vast majority of purposes we can say thatenergy cannot be created or destroyed - it can onlybe converted into another form of energy. (Notethat when some radioactive processes occur,minute quantities of mass are "lost' and convertedinto large amounts of energy.)

The firefighter is mostly concerned with energy inthe form of heat. Heat may be produced by a chem-ical change, such as combustion, in which we saythat chemical energy is released as heat energy.Mechanical energy or kinetic energy can also beconverted into heat energy by friction (e.g., fric-tional heating of brake pads).

We are familiar with the concept of temperaturefrom everyday experience. Temperature is a mea-


past each other, although they do not have com-plete freedom. At this point the solid melts andbecomes a liquid.

Further heating causes the temperature to increase,and the energy is stored as kinetic energy of themolecules, which move with increasing rapidityuntil they are moving fast enough to overcome thecohesive forces completely. At this point, the liq-uid boils and turns into a gas (or more correctly, avapour). If heat is taken away from a substance,kinetic energy of the molecules decreases and thereverse processes occur.

1.5 Melting, boiling and evaporation

The temperature at which a solid turns into a liquidis called the melting point. If we are considering aliquid turning into a solid, the temperature is calledthe freezing point, though these two temperaturesare the same for the same substance under thesame conditions. The temperature at which a liquidboils and becomes a vapour is the boiling point.

Since energy is required to overcome the forces ofcohesion when a substance melts or boils, the heatwhich is supplied during these processes does notcause a rise in temperature of the substance.Conversely, when a vapour condenses or a liquidsolidifies, it gives up heat without any fall in tem-perature so long as the change is taking place. So,melting or freezing, for a given substance at agiven atmospheric pressure, take place at a certaintemperature: for the transition between water andice, at normal atmospheric pressure, this takesplace at ()°C. Boiling, for a given substance at agiven atmospheric pressure take place at anothercertain temperature: for the transition betweenwater and steam, at normal atmospheric pressure,this takes place at 100°C.

Even at temperatures below boiling point, somemolecules at the surface of the liquid may gainenough energy from colliding with other mole-cules for them to escape into the surrounding spaceas vapour. This process is evaporation.

Imagine a liquid in an enclosed space, where thereis already air, such as water in a saucepan. Even ifthe pan is not heated, evaporation will take place.

The evaporating molecules build up a pressureknown as the vapour pressure. At the same time,some molecules will re-enter the liquid. For anygiven temperature below the boiling point there isa definite vapour pressure at which the number ofmolecules which escape is just balanced by thenumber which are recaptured by the liquid.

Boiling occurs when the vapour pressure hasbecome equal to the surrounding atmosphericpressure, the pressure of air. Vapour then forms notonly at the surface of the liquid, but also in thebody of the liquid, and we see bubbles.

If the external pressure is increased, the vapourpressure at which boiling will take place isincreased and so the temperature must increase. Ifthe external pressure falls, the reverse is true andthe temperature at which boiling occurs will belower.



Chapter 2 - Mechanics

2.1 Motion

Imagine a body moving from a starting point A toanother point B: for example a car movingbetween two cities.

The distance travelled by the car is the length ofthe line X. The average speed the car travelled atwill be the distance travelled by the car divided bythe time taken.

It has units of metres per second (m/s), or if dis-tance is measured in miles, and time in hours, theunits will be miles/hour (mph).

Although people tend to use speed and velocityinterchangeably, there is a difference betweenthem. Velocity has a direction associated with it: itis what we call a vector quantity.

The line Y shows the length of a straight line con-necting the starting and finishing point. The lengthand direction of this line together give the dis-placement of the car from the starting point.Displacement is also a vector quantity.

Imagine a car journey from London to Bristol.Bristol is 200 km west of London, so at the end ofthe journey the car has a displacement of 200 kmfrom London. However, the distance travelled willbe longer than this though, as the roads tend to takeconvenient routes through the countryside ratherthan following perfectly straight lines. There mayeven be times when the car is travelling North-South rather than East West as it follows the road.At those times, the displacement of the car fromLondon is not increasing, so although it has speed,its velocity is zero.

Figure 2.1 Car movingfrom A to B.

Previous pageis blank Physics and Chemistry for Firefighters 9

The units of speed and velocity are , whilethose for acceleration are m/s2.

2.2 Momentum and force

Momentum is the product of mass and velocity

23 Work, energy and power

So, a 2 kg object travelling at 10 m/s has a momen-tum of 20 kg.m/s

Force is the product of mass and acceleration

The units are kg.m/s2 which known as the Newton(N).

Imagine that our 2 kg object is initially at rest, andthen a force is applied which makes it accelerate at10m/s2. The applied force is equal to the body'smass times its acceleration.


The "force of gravity" provides an accelerationwhich acts on everything. At the earth's surface,anything which is dropped will accelerate, undergravity, at 9.81 m/s2 (this quantity is often referredto by the symbol g). It also determines the forcesthat are responsible for the movement of hot, buoy-ant gases in fires, and as described in Chapter 1.

The weight of a body is a measure of how strongthe force due to gravity is on an object. In every-day speech, we usually use mass and weight inter-changeably, but just as the words "speed" and"velocity" have different meanings, weight andmass also have different meanings.

Weight is the force due to gravity which acts on anobject. The acceleration due to gravity that anobject will experience is g = 9.81 m/s, so that amass of two kilograms experiences a force due togravity of

Giving definitions of weight and velocity whichare different from those used in every day speechmay seem like an unnecessary complication, butmuch science and engineering is only possible ifevery aspect of a problem is precisely defined, andevery scientist or engineer everywhere knowsexactly what is meant by each word.

Work is another every day term which has a rigiddefinition in science. If a body moves because aforce acts on it. we say that work is being done onthat object. Work is only being done if the objectmoves - if a force is being applied just to keep anobject stationary, no work is done. The mathemat-ical expression of work is:

2.4 Friction

where W is the work done (in Joules), F is the con-stant force applied (Newtons) and s is the distancemoved in the direction of the force (in metres).(The Joule (J) has units N.m, or (kg.m/s2).m, i.e.,kg.m2/s2: this illustrates the importance of ensuringthat a consistent set of units is used.)

Taking our 2 kg mass, if this is raised verticallythrough 2 m, the work done against gravity will be

Note that if the force acts in a different direction tothe direction of travel, less work will be done. Theinterested reader should refer to the various textbooks on this topic.

If something is capable of doing work, it possess-es energy. The energy that a body possesses byvirtue of the fact that it is moving is called kineticenergy. The kinetic energy that a body possesses isequal to the amount of work that must have beendone to it to increase its velocity from zero towhatever velocity it has. For a mass m movingwith velocity v:

The units are kg.m2/s2, i.e.. Joules.

Potential energy is the amount of energy that anobject possesses because of its position or thearrangement of its components.

A body held above the ground has potential ener-gy because it could do work while it is falling. Our2 kg mass raised vertically through 2 m has apotential energy equal to the work required to raiseit to that height, i.e., mass x acceleration x height(abbreviated as mgh), which as we have seen hasthe units of Joules. This is a consequence of theposition of the mass in the earth's gravitationalfield. Other forms of energy are encountered, e.g.,


When an object moves or tries to move over a sur-face, both the object and the surface experience africtional force along the common surface, each ina direction which opposes the relative motion ofthe surfaces.

Even on two very flat, well-polished surfacesthere are many microscopic imperfections whichmake the contact area much smaller than it wouldseem. These imperfections may interlock and, inthe case of metal, may even weld together undervery high local pressures.

We can see, then, that this locking and bondingwill inhibit motion and that energy will be neededto overcome the frictional force. This energyappears as heat: when energy is expended on over-coming friction, heating occurs. This is the princi-ple behind the rough strip on match boxes whichprovides the heat to start the chemical reaction in amatch, and is also why brake blocks get hot on acar or bicycle.

an electron in an electric field has electrical poten-tial energy, while a stretched rubber band has elas-tic potential energy.


Chapter 3 - Heat and Temperature

In Chapter 1 and 2 it was stated that:

Energy

• is the ability to do work;

• can neither be created nor destroyed; and

• can exist in a number of different forms.

Heat is one form of energy. It can be produced bychemical means, for example by burning coal oroil, or by mechanical means, by friction. Passing acurrent through an electrical resistance also pro-duces heat (e.g., an electric fire).

Heat can be converted into other forms of energy,for example into pressure energy in a steam boiler.

It is also possible to convert heat back into chemi-cal energy or electrical energy.

Heat always flows from high temperature to lowtemperature. If a hot body and a cold body areplaced in contact, the hot body (the one at the high-er temperature) loses heat and the cold body (theone at the lower temperature) gains heat.

The fact that heat and temperature are not the samething can be seen from a simple experiment.Imagine a piece of fine copper wire held in theflame of a match. After a couple of seconds, thewire will glow red hot, which tells us that the tem-perature of the wire has increased to 800 - 900°C.

Now imagine a similar match held under a kettlecontaining a litre of water. There would be nonoticeable change in the temperature of the water,yet the amount of heat supplied to the wire andsupplied to the water would be roughly the same.


The rise in temperature in a body to which heat issupplied is decided by three factors: the amount ofheat supplied (or "transferred") to the body, themass of the body and the specific heat capacity ofthe material from which the body is made.

The meaning of the term 'specific heat capacity'will be discussed later.

3.1 Measuring temperature

The human body cannot tell reliably whethersomething is hot or cold, it can only compare whatit is currently feeling with what it felt immediatelybeforehand. If you place one hand in a bowl ofcold water and the other in a bowl of hot water andthen, after an interval, both hands are placed in abowl of tepid water, the hand which was in thebowl of cold water will feel that the tepid water is'hot', while that from the hot water will feel thatthe tepid water is 'cold'.

Because it can only make comparisons, the humanbody cannot give a numerical value to temperature -people cannot step out into the street and reliablymeasure the air temperature just by the feel of the airon their skin.

Temperature can, though, be measured by makinguse of one of the effects of heat on materials. Thecommonest example is the use of the way that liq-uids expand as their temperature rises, the proper-ty of thermal expansion of a liquid. This is theprinciple behind the thermometer (Figure 3.1).

The thermometer consists of a narrow tube of finebore with a small bulb at one end, and sealed at theother, containing a suitable liquid. The liquid ismost commonly mercury, which has the advan-tages of a high boiling point (357°C), a uniformexpansion coefficient and a low heat capacity; it isalso opaque. However, its freezing point is about-39°C which makes it unsuitable for measuringtemperatures greatly below the freezing point ofwater. Alcohol has a lower freezing point (-112°C)but also a much lower boiling point (78°C) thanmercury. It can be is used for low temperatureswork. Coloured water is sometimes used for roughmeasurement of temperatures between the freezingpoint and boiling point of water.

Figure 3.1 Diagramshowing thermometer anda comparison between theCelsius and Fahrenheitthermometer scales.


3.2 Thermometric scales 3.3 Other methods of measuringtemperature

Two fixed points are required for the constructionof a thermometric scale. For the Celsius (orCentrigrade) scale of temperature, the meltingpoint of pure ice and the boiling point of purewater are taken as the fixed points at standardatmospheric pressure.

So, to fix the lower point of the scale, the bulb ofthe thermometer is placed in melting ice, while theupper fixed point of the thermometer is determinedby placing the bulb in the steam above the surfaceof boiling water (at standard atmospheric pres-sure). If the pressure is different from the standardatmospheric pressure, a correction has to beapplied to the upper fixed point. The level at whichthe liquid in the thermometer stands at each of thefixed points is marked on the stem of the ther-mometer.

Two thermometric scales are in common use:

3.2.1 The Celsius (or Centigrade) scale

On this scale the lower fixed point is marked 0.The upper fixed point is marked 100. The stembetween these two points is divided into 100 equaldivisions or degrees. These divisions are calledCelsius degrees.

3.2.2 The Fahrenheit scale

The inventor of this scale used a freezing mixtureto give him his lower fixed point, and the boilingpoint of water for the upper fixed point. The scalewas divided into 212 equal divisions, which gavethe freezing point of water as 32°F. There are 180Fahrenheit degrees between the freezing point andthe boiling point of water (at standard atmosphericpressure).

The 'liquid-in-glass' thermometer is not the onlymethod of measuring temperature. There are sev-eral other methods, including the following:

3-3.1 The air or gas thermometer

Instead of using a liquid, a bulb containing air orsome other gas can be used. In one such ther-mometer, the expansion of the gas causes a shortthread of mercury to move along a scale. Thesethermometers are very sensitive, but may requirecorrection to compensate for atmospheric pres-sure.

3.3.2 Using solids to measure temperature

The way that a solid expands when its temperaturerises can be used for temperature measurement.The expansion may be used directly, or the differ-ing expansion of two dissimilar metals may beused. This will be discussed further in the nextChapter.

3.3.3 Thermocouples

When the junction of wires of two different metals(for example iron and copper) is heated, an electri-cal potential (a voltage) appears at the junction. Acalibration can be made between the potential andtemperature, so that temperature can be measuredindirectly by measuring the potential with a sensi-tive voltmeter. There are various types of thermo-couple, some of which are capable of recordingextremely high temperatures. (See Figure 3.2)

Thermocouples junctions can be made very smalland so only take a short time to heat up or cooldown. They are very good for following very rapidchanges in temperature.

3.3.4 Electrical resistance

The electrical resistance of a wire increases with arise in temperature and the change of resistancemay be used to measure temperature. Platinum isnormally used as it has a high melting point and ahigh 'temperature coefficient of resistance", so thata small rise in temperature produces a (relatively)


Figure 3.2 Sketch of apyrometer of thethermo-couple type. Theheat sensitive element(below) is housed in acontainer (above) whichprotects it from the effectsof heat and mechanicalwear and tear.

large rise in resistance. Platinum resistance ther-mometers can measure between -2()0°C and1200ºC. Their disadvantage is that they are largecompared with thermocouples and so do not fol-low rapid changes in temperature very easily.

3.3.5 Thermistors

Thermistors are semiconductor devices, whichhave a negative temperature coefficient of resis-tance, so an increase in temperature produces adecrease in resistance. They are very robust andcan be made very small and so can follow rapidchanges in temperature. Their range is generallyfrom -70°C to 300°C, but they are less accuratethan resistance thermometers.

3.3.6 Comparison by brightness

At temperatures above about 750°C, objects startto glow, first a dull red, changing gradually to yel-low and brightening as the temperature is raised toabout 1250ºC. Temperature measurements can bemade by comparing the brightness of the objectwith the filament of an electric lamp whose bright-ness can be altered by varying the current flowingthrough it. If the current is too low, the filamentappears darker than the object, while if it is toolarge, the filament appears brighter. When the fila-ment "disappears" against the object, they have thesame brightness and temperature. The latter can befound indirectly by measuring the current throughthe filament.

33.7 Infra-red

Infra-red cameras and other sensors detect heat inthe same way that our eyes detect light. Light rep-resents only one portion of the electromagneticspectrum - a rainbow of different types of radia-tion which includes, in addition to visible light,radio waves, microwaves, ultra-violet radiationand X-rays.

Infra-red radiation is given off by bodies (andsome gases) when they are hot. Infra-red sensorsare sensitive to this type of radiation and can bedesigned to measure the temperature of an objectby analysing the strength and the wavelength ofthe radiation.

Infra-red radiation behaves in exactly the sameway as light, but it can pass through some thingsthat light can't and it is blocked by some thingsthat light can pass though. In particular, infra-redcan pass through smoke at concentrations whichblock visible light. This is why infra-red camerashave become so valuable in search and rescueoperations. The infra-red radiation emitted by anunconscious person lying in a cool environmentcan easily be detected.

3.4 The Kelvin scale of temperature

The Kelvin or "Absolute" scale of temperaturestarts at -273°C, which has been found (theoreti-cally and experimentally) to be the lowest temper-


ature that it is possible to achieve. We have dis-cussed before in the sections on melting and boil-ing, that the hotter a mass is, the faster the mole-cules that make up that body are moving. At-273°C, the molecules that make up a substancestop moving and it is not possible to cool the massany further.

The Kelvin or absolute scale of temperature has itszero at -273°C. Degrees on this scale are the samesize as Celsius degrees, and are denoted by thesymbol K, so

a gas. However, all real gases will condense to liq-uid or solid form at low temperatures.

Note that in equations, the symbol T is normallyused for temperature, but great care must be takenin remembering which temperature scale is beingused.

3.5 Units of heat

In the same way that length is measured in metresand temperature is measured in degrees, there areunits which are used to measure the amount ofenergy in a body - what is colloquially called"heat". The concept of energy was introduced inChapter 2. and discussed in the context of "workdone". Energy can neither be created or destroyed,but can be converted from one form into another,e.g., potential energy into kinetic energy, or chem-ical energy into electrical energy. The conversionprocess is never 100% efficient, and some of theenergy will appear in a different form, most com-monly as "heat". Thus, some of the energy expend-ed in bringing a moving car to rest is dissipated asheat generated by friction at the points of contactbetween the brake shoes and the brake drum.

3.5.1 The Joule (J)

The unit that scientists and engineers use to mea-sure heat is the same as that used for energy, i.e.,the Joule, which is named after a nineteenth cen-tury Manchester brewer who became interested inhow much energy was needed to heat water. It isdefined from mechanics; where energy is the abil-

Although the Celsius scale is the most widelyused, the Kelvin scale must be used in certain cir-cumstances - particularly when calculating howthe volume of a gas changes with temperature andpressure (see Chapter 4). This will be discussed inmore detail later, but Figure 3.3 shows how a givenmass of gas will occupy a smaller volume as itcools (assuming that it does not condense into aliquid). For most purposes, it is possible to assumethat the volume of this mass of gas is proportionalto its temperature in degrees Kelvin. Therefore, ifthe temperature is doubled at a given pressure, itsvolume will double. Conversely, if the temperatureis halved, the volume will halve. In principle, thevolume would become zero at 0 K if it remained as

Figure 3.3 Graph showingthe zero on the Kelvin orabsolute scale.


ity to perform work, so work and energy are mea-sured in the same units, as in Chapter 2. One Jouleof work is done when the point at which a 1Newton (1 N) force is applied moves through 1metre in the direction of the force.

For convenience, to save writing strings of zeroes,larger units based on the Joule are used, the kilo-joule and the megajoule:

In the same way that feet and inches were replacedby metres, there are older units which have nowbeen replaced by the Joule. These will now be dis-cussed briefly.

3.5.2 The calorie

This is defined as the quantity of heat required toraise the temperature of 1 gram of water through1°C. The energy content of food is often measuredin calories, though, confusingly the number of'calories' we talk about in a bar of chocolate ineveryday speech is actually the number of kilo-calories.

3.53 The British thermal unit (Btu)

This is the quantity of heat required to raise thetemperature of 1 lb of water through 1°F.

Another British unit is the therm, which is equalto 100 000 Btu (105 Btu).

3.6 Specific heat

As we have discussed, heat energy can only flowfrom a body at a higher temperature to one at alower temperature. Heat transfer continues either


until both bodies are at the same temperature (i.e.,the initially hot body has cooled and the initiallycold body has warmed), or until the two bodies areseparated. Whatever, a certain amount of heatenergy will have been transferred, which can bemeasured in Joules.

When heat is added to a body the temperaturerises. The rise in temperature of the body dependson three things:

• the amount of heat energy supplied to thebody;

• the mass of the body; and

· the specific heat capacity of the body.

The specific heat capacity of a material is the heatrequired to raise the temperature of one kilogramof the material by 1°C, and so is measured inJoules per kilogram per degree centigrade(J/kgºC). In equations, the letter c is used as asymbol for specific heat capacity.

Some texts may discuss the heat capacity of anobject. This is the specific heat capacity of the sub-stance it is made from multiplied by the mass.They are different quantities though easily con-fused. In general, the 'specific" is used when thevalue under discussion refers to a unit mass ofmaterial.

Imagine two containers, one containing water andthe other containing the same mass of oil. Nowimagine that a given amount of heat energy is sup-plied to each. This could be done by placing thecontainers on identical burners - ones that supplyheat at the same rate - for the same amount of time.

Both containers are equipped with thermometers,so that the temperature rise can be observed.

Now, after 3 minutes, it is found that the tempera-ture of the oil has risen by 10°C, but the tempera-ture of the water has only risen by 5°C in the sametime.

The masses of the oil and water are the same, andthe amount of heat energy supplied to the oil and

water was the same, so the difference in the tem-perature rise experienced by the two samples mustbe due to a difference in specific heat capacity.

The same amount of heat caused a greater rise intemperature in the oil than in the water. We knowthat specific heat capacity is a measure of howmuch heat it takes to bring about a one degree risein temperature in a given material. Therefore, thewater has a greater specific heat capacity than theoil as the water showed a smaller temperature risefor a given amount of heat supplied.

The other side to this statement is that materialswith a low specific heat capacity will heat up morerapidly in a fire situation than those of high specif-ic heat capacity.

Water has an unusually high specific heat capaci-ty: 4 200 J/kg per °C. There are very few sub-stances which have a higher value than this, themost notable being hydrogen at constant volume,and mixtures of certain alcohols with water.

low specific heat capacities are of capable of pro-moting fire risks.

[It should also be noted that the surfaces ofsolids of low density, such as polyurethanefoam, also heat up very rapidly when exposed toa heat transfer process (e.g., radiant heatingfrom an electric fire). This is a result of lowthermal conductivity. Instead of heat beingrapidly transferred into the body of the solid byconduction, it "accumulates" at the surface,resulting in a rapid temperature rise. As aconsequence low density combustible materialscan be ignited very much more easily thanmaterials of high density.]

3.7 Change of state and latent heatSome values of specific heat capacities are shownin Table 3.1.

Substances such as petrol, alcohol and the likehave low specific heat capacities. They are alsoreadily vaporised and may produce hazardousvapours. In general, combustible materials with

By 'change of state' we mean the changes betweenthe solid and liquid states (melting/freezing), liquidand gas states (boiling/condensation), and for a rel-atively few pure compounds, solid and gas states(sublimation). Freezing, melting, boiling, conden-sation and sublimation all cause a change of state.


3.7.1 Latent heat of vaporisation

When a kettle is put on to boil, heat enters thewater from the kettle element and the temperatureof the water rises until it reaches 100°C.

At this temperature the water boils, that is to saybubbles of vapour form at the bottom and rise tothe surface where they burst and escape as steam.Once the water has started to boil, the temperatureremains constant at 100°C - it doesn't get any hot-ter than 100°C. However, heat energy continuesflow from the element into the water. This energyis not increasing the temperature of the water, butis being used to allow the water molecules to pullthemselves apart from each other, converting thewater from the liquid state to the vapour state, i.e.,from liquid water into water vapour (steam).

Experiments show that 2 260 000 Joules (2.26MJ)are required to convert 1 kilogram of water at itsboiling point into steam at the same temperature.This is known as the specific latent heat ofvaporisation for water (latent means hidden - thelatent heat is hidden heat because it doesn't causea temperature rise). This extra heat goes into thevapour, but does not indicate its presence by pro-ducing a rise in temperature.

All liquids besides water absorb latent heat whenthey are turned into vapour. For example, 860 000 Jare required to convert lkg of alcohol into vapour atits boiling point.

Latent heat is measured in Joules per kilogram(J/kg), although it is more usually expressed inkilojoules per kilogram (kJ/kg).

3.7.2 Effect of change of pressure on boilingpoint and latent heat

Water 'normally' boils at 100°C. By 'normally' wemean that it boils at 100°C when the external airpressure is the standard atmospheric pressure of1.013 bars where 1 bar = 105 N/m2. This is the pres-sure of air which will support 760 mm of mercuryin a mercury barometer, and so is sometimes writ-ten as "760mm mercury" or "760mm Hg".

This effect is used in pressure cookers and in pres-surised cooling systems for car engines, where theincreased pressure raises the boiling point of theliquid in the system.

This behaviour is also used in the storage of "lique-fied gases" such as propane and butane. At increasedpressures, these gases liquefy at normal temperatures,and allow large amounts of "gas" to be stored in a rel-atively small volume (see Section 4.4).

Raising the boiling point increases the quantity ofheat needed to raise the temperature of the coldliquid to the new boiling point, but it decreases thelatent heat of vaporisation.

3.7.3 Latent heat of fusion

Just as latent heat is taken in when water changesto vapour at the same temperature, a similar thing

When steam condenses to form liquid water, thesame amount of latent heat is given out. This iswhy steam can cause serious burns.


happens when ice melts to form water. In this case,the latent heat is not so great. It requires 336 000 J(336 kJ) to convert 1 kg of ice at 0°C to water atthe same temperature. Likewise, when water at0°C freezes into ice, the same quantity of heat isgiven out for every 1 kg of ice formed. This iscalled the specific latent heat of fusion of ice. Thisis not confined to water alone; other substancesabsorb latent heat when they melt and converselythey give out latent heat on solidifying. This is thelatent heat of fusion.

The definition of the specific latent heat of fusionof a substance is the quantity of heat required toconvert unit mass of the substance from the solidto the liquid state without change in temperature.The same units (J/kg or kJ/kg, etc.) are used as forthe latent heat of vaporisation.

3.7.4 Cooling by evaporation

Some liquids have a low boiling point and thuschange from liquid to vapour quite easily at ordi-nary temperatures: these are called volatile liquids.Methylated spirit and ether are of this type. If youdrop a little methylated spirit or ether onto yourhand, it evaporates rapidly and your hand feelscold. Some local anaesthetics work in this way,'freezing' the pain.

The cooling is brought about because, to changefrom liquid to vapour, the liquid absorbs heat ener-gy from the hand to provide the latent heat ofvaporisation of the liquid. The hand therefore feelscold. Water would also cause the hand to becomecold but not so noticeably as methylated spirit. Thespirit has a lower boiling point than water and so itevaporates more quickly at the temperature of thehand.



Chapter 4 - Thermal Expansion

Within normal ranges of temperature, a solidwhich is "homogeneous" in structure, such as aniron bar, expands uniformly: the expansion of a barin each direction is proportional to the rise in tem-perature.

The expansion is also proportional to the length ofthe bar, but varies with the nature of the substanceof which the bar is made.

4.1.1 Coefficient of linear expansion

The amount by which unit length of a substanceexpands when its temperature is raised by onedegree is called the coefficient of linear expan-sion of the substance. The temperature scale mustbe stated. Thus we can say that the coefficient oflinear expansion of a solid is the fractionalincrease in length of unit length when its tempera-ture is raised by one degree Celsius.


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4.1 The thermal expansion of solids

As we have discussed in Chapter One and ChapterThree, a substance, whether solid, liquid or gas,will tend to expand when it is heated as long as itis not constrained by a container, a change of stateor a change in chemical composition.

When a solid is heated, it expands in all threedimensions and, therefore, increases in length,breadth and thickness. The increase in length isoften the most important, although the increase inarea and volume due to thermal expansion can bereadily calculated by considering the increase ineach dimension

For steel, the coefficient of linear expansion (denot-ed by the Greek letter α) is 0.000 012 per °C. Thus,a bar of steel 1 m long expands by 0.000 012 m foreach °C rise in temperature; a 1 km bar expands by0.000 012 km (12 mm) for each °C rise in tempera-ture, and so on.

Some other typical values of the linear expansioncoefficient, α, are:

Figure 4.1 Forth Road Bridge. (Photo: The Royal Commission on

the Ancient Historical Monuments of Scotland)

4.1.3 Allowing for expansion in large metalstructures

Large metal structures, such as bridges, often expe-rience large variations in temperature, so allowancemust be made for the linear expansion of the parts.

In large bridges, this expansion is quite large itself.For instance, the Forth Road Bridge is a steelstructure with a total length (1) of about 1960 m.The maximum temperature range between winterand summer is -30°C to +30°C, a range of ∆T =60°C. Using the formula above, the differencebetween the maximum and minimum lengths ofthe roadway would be:

Figure 4.2 Main expansion joint at one of the main

towers on the Forth Road Bridge.

(Photo: Forth Road Bridge Joint Board)


4.1.2 Nickel-iron alloy (invar)

Invar is an alloy of iron and nickel (64 % iron,36 % nickel) which has a coefficient of linearexpansion of 0.000 000 1 per °C i.e., less than1 per cent of that of steel. This is so small as tobe negligible in most cases.

It is used for making measuring rods and tapes,watch and clock parts and other components whichneed must remain the same over a range of tem-perature.

l × α × ∆T = 1960 × 0.000 012 × 60 = 1.41 m

Even a bridge with a span of 20 m could change by14 mm between the hottest and coldest tempera-tures. Allowance for this expansion is often madeby fixing one end of the bridge and resting the otheron rollers, or on a sliding bearing, so that the bridgemay expand and contract without exerting a sideload on its piers. Railway lines used to be laid in 45or 60 ft (13.7 or 18.3 m) lengths, with gaps to allowfor expansion and contraction, but modern methodsnow make it possible for the expansion to be takenup as a tension or compression in the rail, withexpansion joints at distances of about 800 m.

In buildings, the normal range of temperature isnot usually so great, since internal heating main-tains a reasonable temperature in winter and thebuilding fabric protects steelwork from excessiveexternal temperatures. Nevertheless, someallowance has to be made for expansion to preventthe steelwork from distorting the walls of thebuilding, even if the allowance is only made byleaving a small clearance between the steel frameand the brickwork. In a fire situation, however, theincrease in temperature may be very great and thesituation could arise in which a long beam couldexert sufficient side load on a wall and cause it tocollapse.

Problems of expansion are also encountered withmaterials which are poor thermal conductors. In afire situation, heating the inner face of a tall brickwall will cause that face to expand, while the outerface remains cool. This may cause the wall to leanoutwards at the top and can result in collapse of thestructure.

4.1.4 Thermostats

Imagine two strips of different metals with differ-ent coefficients of linear expansion, of the samelength, laid side by side. Imagine then that the tem-perature of the surroundings increase. As the stripswarm up, each will increase in length according toits own coefficient of linear expansion.

If the same two strips were fastened togetherthroughout their length, an increase in temperaturewould cause them to distort into a curve, formingthe arc of a circle. As the strip cooled, it wouldstraighten out again. Such a strip is known as a bi-metallic strip. (Figure 4.3).

If one end of a bi-metallic strip is fixed, a changein temperature will cause the free end to move.

Figure 4.3 Bi-metallicthermostat

LeftAbove a specified tempera-ture, the bi-metal strip willbend. This causes the elec-trical contact to be brokenwhich results in the currentbeing switched off.

RightA fire alarm using abi-metallic strip.


The movement of this free end can be made toopen or close an electric circuit, to cause an alarmto be operated, or to switch off a heater. Such adevice using a bi-metallic strip in this way is calleda thermostat. (The principle is also used in rate-of-rise heat detectors.)

4.1.5 Coefficients of superficial and cubicalexpansion of solids

It can be shown mathematically that the coefficientof superficial (or area) expansion of a solid is twicethe linear coefficient, and that of cubical expansionis three times the linear coefficient. Thus, theincrease in volume V of an object of volume Vwhen the temperature is increased by T is:

so that the new volume will be V + V (seeSection 4.1.1 for the equivalent equation for linearexpansion).

The expansion depends on the external dimensionsof the solid and is not affected by any voids. Thecubical expansion of a hollow metal box is thesame as that of a solid block of the same metal ofthe same (external) volume as the box.

4.2 Thermal expansion of liquids

4.2.1 Cubical expansion

Since liquids have no definite shape and, therefore,no fixed dimensions other than volume, the onlyexpansion which can be measured is that of cubi-cal expansion.

Since a liquid has to be contained in a vessel, theapparent expansion of the liquid is affected by theexpansion of the vessel, and the apparent expan-sion is, therefore, always less than the real expan-sion. However, the coefficient of cubical expan-sion of liquids is considerably greater than that ofsolids so (with the exception of water, which isdealt with below) the expansion of a liquid isalways greater than that of its container.

Consider this comparison of the cubical expansioncoefficients of glass, mercury and alcohol:


Thus, the thermal expansion of mercury is about 8times that of glass, while that of alcohol is nearly50 times that of glass. This is important in thedesign of thermometers.

The coefficient of cubical expansion of steel is0.000 036/°C, and that of many liquids is of theorder of 0 001/°C, i.e., about 30 times as much.Because of this, a sealed container (such as a stor-age tank) which is completely full of liquid may bea hazard in a fire situation (or even when exposedto strong sunlight) because of the internal pres-sures generated by expansion. If a pressure reliefvalve is fitted, this will allow the escape of liquid.The problem will be greatly reduced if the tank isnot completely full, and there is an air space.

The so-called "frangible bulbs" used in many con-ventional sprinkler heads are sealed glass bulbsfull of liquid. These break to operate and releasewater from the head when they are heated to aselected temperature, for example, when exposedto hot fire gases accumulating under the ceiling.

4.2.2 The effect of expansion on density

Since the density of a substance is the ratio of itsmass to its volume, an increase of temperatureresults in a decrease of density; or conversely, thevolume of a given mass of the substance increasesas its temperature rises.

Water behaves in a peculiar way. Its expansion isnot uniform: the expansion between 30°C and50°C is double that between 10°C and 30°C. Oncooling below 10°C, water contracts until its tem-perature reaches 4°C. On further cooling itexpands until its volume at 0°C is 1.000 120 timesgreater than its volume at 4°C. It also expands fur-ther when it freezes. This means that water inponds and lakes freezes from the top downwardand. once the temperature on the surface has fallento 4°C, further cooling of the lower level can only

43 The expansion of gases

occur by conduction. This conduction is slowbecause water is a poor conductor.

4.3.1 Temperature, pressure, volume

Since a gas expands to fill all the available space,the volume of a gas may be changed by altering thevolume of its container. If the volume is decreased,the pressure is increased.

This can be explained by saying that the samenumber of molecules of the gas occupy a smallerspace and, therefore, collide with each other andwith the container walls more frequently. The pres-sure is due to these collisions: more collisions,more pressure.

In a liquid, the molecules are much closer togetherto start with in than in a gas: the spacing of mole-cules in a liquid is comparable to that in a solid;though, because they are moving so quickly theydo not remain in the regular structure of a solid.Because molecules in a liquid are so close togeth-er to start with, they cannot be compressed further.This is why gases can be compressed but liquids,generally, cannot.

Heating a gas increases the kinetic energy of themolecules which, therefore, move faster, and col-lide more frequently. So, heating a gas increases itspressure - provided its volume is unchanged. Byincreasing its volume as it is heated, the pressurecan be kept constant.

We can see, then, that there are three variables whichchange with each other when dealing with a gas,namely temperature, pressure and volume. Whendealing with a solid or a liquid, temperature and vol-ume are important, but pressure is not so important.

4.3.2 The gas laws

There are three gas laws:

• Boyle's Law;

• Charles' Law; and

• The Law of Pressures.


These combine into the General Gas Law.

As we have seen, each solid or liquid will expandwith rise in temperature by an extent determinedby the coefficient of cubical expansion. However,all gases expand by the same amount for the sametemperature rise.

Changes in volume of a gas depend on changes intemperature and pressure. To study the interactionbetween temperature, pressure and volume, one ofthese quantities is kept constant and the depen-dence of the other two on each other can then bestudied. (Note: the mass of gas remains constant.)

This method of study provides the basis of thegas laws, the rules by which the behaviour ofgases can be determined.

4.3.2.1 Boyle's Law

The change in volume of a gas caused by changesin pressure alone is the subject of the first of thegas laws, known as Boyle's Law. This states that:

Experiments show that if the pressure applied to agiven volume of gas is doubled, the volume is halved.If the pressure is trebled, the volume is reduced toone-third, provided the temperature is constant.

If we have a cylinder whose capacity is lm3 it cancontain 1 m3 of a gas at 1 atmosphere, but if 120 m3

of gas at atmospheric pressure are compressed andpumped into the same cylinder, the pressure will be120 atmospheres (atm). If half of the gas is allowedto escape, then the pressure will fall to 60 atm.

This is why the pressure gauge on a breathingapparatus set is a measure of how much air there isin the cylinder.

43.2.2 Charles'Law

Experiments show that all gases expand by 1/273 oftheir volume at 0°C for each 1°C rise in tempera-ture, provided that they are maintained at constantpressure. Since the expansion for each 1°C rise intemperature is quite large, it is essential to take theinitial volume at 0°C. These experiments were car-ried out by a French scientist named Charles at thebeginning of the 19th century, and the law namedafter him states:

As gases expand, their density decreases and theybecome buoyant. This is why hot air rises, how hotair balloons work and why hot smoke and firegases collect at the top of rooms.

It will be seen from Figure 3.3 (page 17), whichshows the change of volume of a gas with temper-ature, that the Kelvin (or Absolute) scale of tem-perature must be used, and that the relationshipbetween volume and temperature is:

where V1 and T1 are the initial volume andabsolute temperature and V2 and T2 are the finalvolume and absolute temperature (the Kelvin tem-perature, NOT the Celsius temperature). In otherwords, the volume of a given mass of gas is direct-ly proportional to its absolute temperature, provid-ed that its pressure is kept constant.

4.3.2.3 The Law of Pressures

The previous two laws lead to a third law con-cerned with the relationship between the pressure


So,

Mathematically, if V1 and P1 are the initial volumeand pressure, and V2 and P2 are the final volumesand pressure, then

In practice, when a gas is compressed - for exam-ple, when a breathing apparatus cylinder ischarged, or a tyre in pumped up - heat is generat-ed and the temperature increases: the valve getswarm. This heat is only generated by the pumpingoperation. If the pressure of the gas is measuredbefore the temperature has returned to its originallevel, Boyle's Law does not hold, since the tem-perature of the gas is not the same as it was beforeit was pumped in: this is why the 'constant tem-perature' part of the law is important - the calcu-lations do not work if it is not fulfilled.

Figure 4.4 Breathing Apparatus Compressor.(Photo: Hamworthy Compressor Systems Limited)

Figure 4.5 Smoke layer at top of room.(Photo: The Fire Experimental Unit)

and temperature of a gas when the volume is keptconstant. This is the case when a cylinder of gas,whose valve is closed, is heated, as could happenif it were in a fire.

This is expressed mathematically as:

43.2.4 The General Gas Law

The three gas laws can be combined into a singlemathematical expression:

This general expression may be used for a givenmass of gas when pressure, temperature and vol-ume all change.

Remembering this law will allow you to rememberall three - you can just remove the quantity whichis being kept constant from both sides of the equa-tion. For example, if volume is being kept con-stant, remove V1 from the left-hand side and V2

from the right hand side (because they are equal,they cancel), and insert the pressure and tempera-ture values that you know.

It is important to remember that these gas laws areapplicable to all gases provided that they remain asgases over the temperature and pressure rangeinvolved. When the temperature and pressurereach levels at which the gas liquefies, the gas lawsno longer apply.


Figure 4.6 Fire involving liquefied petroleum gas tanks.(Photo: HM Fire Service Inspectorate)

4.4 The liquefaction of gases

As has been noted previously, an increase in pres-sure raises the boiling point of a liquid.

Many substances which are gases at normal tem-peratures and atmospheric pressure can be com-pressed to such an extent that their boiling point israised above atmospheric temperature and the gasliquefies (e.g., propane, ammonia).

Other gases cannot be liquefied at atmospherictemperature no matter how great a pressure isapplied.These are the so-called "permanent gases'.However, if the temperature is lowered sufficient-ly, it becomes possible to liquefy them by com-pression (e.g., methane, oxygen).

4.4.1 Critical temperature and pressure

For each gas, there is a critical temperatureabove which it cannot be liquefied by increasingthe pressure alone. For example, carbon dioxidecan be compressed to a liquid at 20°C, but at 40°Cit will remain a gas.

Its critical temperature is in fact 31.1°C. Belowthis temperature it can be liquefied by increasedpressure and it should properly be described as avapour. Above this temperature it cannot be lique-fied and is properly described as a gas, or, toemphasise the fact that it is above its critical tem-perature, a 'true gas'.

The pressure required to liquefy a vapour at itscritical temperature is called the critical pressure.

Some typical values of critical temperatures andcritical pressures are shown in Table 4.2.

4.4.2 Liquefied gases in cylinders

Many materials such as fuel gases are liquified underpressure and stored and transported in cylinders.

Liquefied gases in cylinders do not obey the gaslaws, since, below the critical temperature, anychange in temperature, pressure or volume willresult in either the liquefaction of gas or the evap-oration of liquid. Thus the pressure in a cylinder ofliquefied gas will remain constant as gas is drawn

off (provided the temperature remains constant)since more liquid will evaporate to make up for thegas drawn off until all the liquid is evaporated.

When liquefied gases are stored in cylinders,allowance must be made for expansion of the liq-uid in case the cylinder is heated beyond the criti-cal temperature and the liquid turns into a vapour.


This could lead to a substantial increase in pres-sure, with a risk of explosion. To minimise thisdanger, cylinders are never completely filled withliquid.

The amount of liquefied gas which may becharged into a cylinder is determined by its fillingratio, which varies from gas to gas and depends,among other things, on the density of the liquid.

The filling ratio for ammonia is 0.5, so that a cylin-der capable of holding 10 kg of water may only becharged with 5 kg of ammonia. A cylinder of thesame size could be charged with 12.5 kg of sulphurdioxide, for which the filling ratio is 1.25.

4.5 Sublimation

In the laboratory it is possible to produce such lowpressures that the boiling point of water can bereduced to 0°C and lower. When this happens, icedoes not melt to form water, but will vaporise com-pletely as the temperature rises.

This direct change from solid to vapour withoutforming an intermediate liquid is given the specialname of sublimation.

In order to achieve sublimation with water,extremely low pressures are required, but solidcarbon dioxide sublimes at atmospheric pressure.

At higher pressures, carbon dioxide shows the nor-mal sequence of melting followed, at a higher tem-perature, by boiling, so that under pressure - andonly under pressure - it is possible to have liquidcarbon dioxide.



Chapter 5 - Heat transmission

will only move a very small distance to either sideof their mean position; it is only the water whichpasses on. In conduction of heat, the moleculesvibrate about a mean position and pass on heatenergy by colliding with their neighbours.

Thermal conductivity, the ability to conduct heatvaries between materials. Most metals conductheat relatively easily and are, therefore, classed asgood conductors though the ability to conduct heatvaries between metals.

Heat energy always flows from regions of hightemperature to regions of lower temperature. Heatwill always flow when there is a temperature dif-ference, no matter how small that temperature dif-ference is.

There are three methods by which heat may betransmitted (Figure 5.1):

• conduction;

• convection; and

• radiation.

5.1 Conduction

Conduction may occur in solids, liquids or gases,although it is most clearly present in solids. In con-duction, heat energy is passed on from each mole-cule to its nearest neighbour, with heat flowingaway from the source of heat towards low temper-ature regions. The transfer of heat can be imaginedto take place in much the same way as water inbuckets being passed down a line of people in a'bucket chain". In the bucket chain each individual

Previous pageis blank

Figure 5.1 Diagram illustrating conduction, convectionand radiation.

EPhysics and Chemistry for Firefighters 33

The best conductors of heat are silver and copper.Aluminium has about half the thermal conductivi-ty of silver and iron about one-eighth. Non-metal-lic solids are poor conductors and, besides mer-cury, which is a metal, liquids and gases are verypoor conductors of heat.

In fact, some solids and also liquids and gases aresometimes referred to as heat insulators becausethey are such poor conductors. In general good con-ductors of electricity (e.g., metals) are good con-ductors of heat, while poor conductors of electricityare good thermal insulators (e.g., most plastics).

Thermal conductivity can be measured experimen-tally and is usually denoted by the symbol K.

The flow of heat is measured in Joules per second(J/s) and this unit is the Watt (W). So:

1 J/s = 1 W

Thermal conductivity in the SI system of units ismeasured in Watts per metre per degree Kelvin(W/m K).

Thermal conductivity is important at most stagesof a fire, but during the fully developed fire thereis the danger of fire spread. As steel conducts heatvery well, a steel girder passing through a fire wallmay conduct sufficient heat through to the neigh-bouring compartment (room) to start a fire there. Itis not necessary for flames to spread through thefire wall itself. (Figure 5.2).

Imagine a door built to separate rooms in case of afire. If a fire occurs, a plain steel door will conductheat rapidly to the other side, which could poten-tially cause the fire to spread outside the room.

Figure 5.2 Sketch showinghow fire may be spread ina building due to theconduction of heat alongan unprotected steel girder.


On the other hand, a wooden door, though it mayburn, is initially a better barrier to heat as it is sucha poor conductor. We can see then that the relativeconductivity of building materials may be animportant factor in the fire-resisting ability of astructure.

5.2 Convection

This occurs only in liquids and gases. It takes placefor example, when a pan of water is heated (seeFigure 5.3).

A pan full of water is heated from the bottom on agas ring. As the water warms up, it expands and,therefore, becomes less dense, and so a given vol-ume is lighter.

As the heated liquid is buoyant, it rises and colder,denser fluid takes its place at the bottom. This thenbecomes heated and so a circulation is set up. Heatenergy is carried throughout the fluid by the mole-cules as they move until the water is the same tem-perature throughout. Compare this to conduction,in which the molecules do not move from theirposition: in convection it is the movement of theliquid or gas molecules through the mass of fluidwhich spread the heat energy around.

Convection is used in domestic hot water systems(Figure 5.4) and in many heating systems using so-

called 'radiators'. Most of the heat from these radi-ators is in fact carried away by convection. It wasalso used in the 'thermo-syphon' system (nowlargely replaced by the pump-assisted system) ofcooling motor engines.

Convection also causes the updraft in chimneys(see Figure 1.2). When a fire occurs in a building,convection currents can convey hot gases pro-duced upwards through stairwells (Figure 5.5) andopen lift and service shafts, thereby spreading thefire to the upper parts of buildings.

If the hot gas products escape from the upper levels,cool air must enter at low level to replaces them.This will, in addition, help to maintain the burning.

Figure 5.3 Convection in healed water.

Figure 5.4 Small boreheating and hot watersystem.


Radiation Wavelength (m)

γ (gamma)-rays less than 10-13

Radio waves

(UHF to long wave) 10 - 1 4

Microwave 10-3 to 10 -1

Infrared 8 x 10-7 to 10-3

Visible light: from red 8 x 10-7 red

to violet 4 x 10-7 violet

Ultraviolet 4 x 10-7 to 10-8

X-rays 10-8 to 10-13

Figure 5.5 Sketch showing how fire on a lower floor can

spread to upper floors by convection.

[The term "convective heat transfer" is used todescribe the transfer of heat between a fluid (gas orliquid) and a solid. For example, a hot object in airloses heat partly by convection. The layer of airnext to the hot surface becomes heated and, there-fore, buoyant with respect to the surrounding coldair: it rises, carrying away the heat and is replacedby cold air. This in turn becomes heated, and aconvection current is set up which cools the solid.]

53 Radiation

Heat may also be transmitted by a means which isneither conduction nor convection, nor requires anintervening medium. Energy from the sun passesthrough empty space to warm the earth. A radiantheater placed at high level in a room can be felt atlower levels, where neither conduction nor con-vection can carry it. This method of heat transmis-sion is called radiation and does not involve anycontact between the bodies which are providingand accepting the heat. To all intents and purposes,it behaves in the same way as light ("visible radia-tion") in that it travels in straight lines, will castshadows, and will be transmitted through somematerials and not others.

Heat is radiated as infra-red radiation, which ispart of the spectrum of electromagnetic radiation.

Different parts of the electromagnetic spectrumhave been given different names simply for conve-nience. What we call "visible light" is so-calledbecause energy in the interval 8 x 10-7 to 4 x 10-7 mcan be detected by the eye. "Infra-red" radiationcannot be detected by the eye as it is beyond the"red" end of the visible spectrum. It also containsless energy than visible radiation, which in turn isless energetic that ultra-violet radiation. UV radia-tion causes damage to biological systems, but onlya small amount reaches the surface of the earthfrom the sun as it is absorbed by the ozone layer inthe upper atmosphere.

All types of electromagnetic radiation produce aheating effect when they are absorbed by a body.This will depend on the amount of energyabsorbed. A proportion of the energy radiated fromthe sun is radiated as visible light. If a body is heat-ed above ambient temperature, it will radiate heatin the infra-red region of the spectrum. These"energy waves" have wavelengths longer thanthose of visible light.

All forms of electromagnetic radiation travel instraigh

o waves, microwaves, visible light and X-rays

makes one form of radiation different from anoth-er is the wavelength of the radiation. The follow-ing table shows how the different forms of radia-tion occupy the electromagnetic spectrum.


to l0

araree al alll par partt o off thi thiss spectrum spectrum;; th thee onl onlyy thing which

Figure 5.6 Diagram show-ing the inverse square lawas it applies to radiation.

Radiation travels at this speed in a vacuum, but moreslowly through matter such as air, water and glass.

The intensity of radiation - that is, how much ener-gy reaches a surface of a given size - falls offinversely as the square of the distance from thesource of radiation. This means that at twice the dis-tance the intensity is one quarter; at three times thedistance, the intensity is one-ninth, and so on. Thisinverse-square law is demonstrated in Figure 5.6.

The square with 1 metre sides placed at, say,2 metres from the source will throw a shadow with2 metre sides on a second sheet placed 4 metresfrom the source. Thus the energy falling on 1 m2 isthe same as that which would have fallen on an areaof 2 m x 2 m = 4 m2 at a distance of 4 m. So theenergy per square metre at 4 m is one quarter thatat 2 m, i.e. one quarter at twice the distance. This isimportant when considering the effect of radiationfrom a heat source such as a fire: a body of a givensize and composition will heat up more slowly thefurther it is away from the radiation source.

When radiant energy (which, of course, includesinfra-red radiation) falls on a body, there are threepossibilities:

Transmission. If energy passes through thebody without warming it, it has been trans-mitted through the body. For example, 'trans-parent' materials transmit light.

Absorption. The energy is absorbed by thebody, whose temperature is raised; and

• Reflection. The energy may be reflected backfrom the surface in the way that light is froma shiny surface. Reflected energy does notenter the body, it just "bounces off" the sur-face.

Some substances absorb selectively: they allowsome forms of radiation to pass, but not others.Glass, for example, allows light to pass butabsorbs infra-red radiation - so glass may be usedas a fire screen: the heat is stopped but the fire maybe seen through it. However, for other reasons,such as its tendency to break under relatively lowpressures and its behaviour at high temperatures,ordinary glass is not a good fire barrier.

Carbon dioxide and water vapour also exhibit thisproperty. The sun's radiant energy, falling on theearth passes through the atmosphere and warmsthe ground, while the resulting infra-red radiationfrom the ground is absorbed by the atmosphereand so does not readily escape back into space.This is the cause of the 'Greenhouse Effect' - asindustry, homes and transport have released morecarbon dioxide into the atmosphere, the tendencyto retain heat at the earth's surface has increased. Itis believed that this may be producing a noticeableeffect on the climate.


•

•

The ability of carbon dioxide and water vapour toabsorb infra-red radiation in very narrow regionsof the electromagnetic spectrum means that theyalso emit radiation in the same regions when theyare hot. Radiation from small flames (e.g., at theearly stage of a fire) is dominated by radiationfrom these gases, specifically at wavelengths of2.8 x 10-7 and 4.4 x 10-7 m. Many infra-red detec-tors make use of this fact to distinguish betweenradiation from a flame and a hot solid object (suchas a heating element) which emits over a widerange of wavelengths.

Some substances, such as pitch, transmit infra-redradiation, but absorb light.

The condition of the surface of a body affects itsability to absorb or reflect radiation. White or pol-ished metal surfaces are the best reflectors andpoor absorbers, while matt black surfaces are badreflectors and good absorbers.

This is why white clothes, white-painted houses andcars, etc., are often used in hot climates. Snow and iceare poor absorbers of heat and reflect radiation verywell. Because of this they melt slowly in direct sun-light. Melting will occur if the air temperature is raised,and heat is transferred by convection (Section 5.2).

Experiments have been carried out in which coaldust or other black powders have been spread onsnow in order to accelerate melting. The sun's heatis more readily absorbed by the black powder, soits temperature rises and heat is transferred by con-duction from the powder to the snow beneath.

Good reflectors of heat are also poor radiators. Apolished silver teapot retains its heat better than ablackened teapot, in spite of silver being a goodconductor. For the so-called radiators of a hotwater system to radiate effectively they should bepainted black, not a light colour as is usually thecase. However, the principal way by which theywarm a room is by heating the air immediatelyaround them, which then rises, producing convec-tion currents which spread heat throughout theroom. The other side of the coin is that the heateris being cooled by convection.

Many fires have been caused by radiation - one ofthe most common is clothing being ignited by

being placed too close to a source of radiation(Figure 5.7), as sometimes happens when peopleair clothes on a clothes horse placed too near a fire.Radiant heat from the sun passing through a glasswindow has sometimes been concentrated by anobject inside the house which acts as a lens, suchas a magnifying glass or a shaving mirror. The old-fashioned "bottle glass" used in windows will alsoconcentrate the sun's rays and could, in principle,cause a fire, but a bottle, or pieces of broken bottlecannot.

Figure 5.7 Clothing max be ignited by radiation whenplaced too close to a source of radiated heat.



Chapter 6 - The Basis of Chemistry

The Chemistry of Combustion 6.2 Atoms and molecules

Up to now. we have considered the physical prop-erties of matter and heat, the properties that decidehow bodies will behave when energy is suppliedto them.

In the rest of this volume we will deal with thechemistry of combustion - the reactions by whichenergy is released in fires. Before discussing com-bustion in detail, it is necessary to talk about under-stand some of the basic concepts of chemistry.

Chemistry is a complicated subject bristling withlong and difficult names to pronounce, and withintricate formulae used by the chemist. There are,of course, many text books available to the studenton chemistry and, in presenting an opening to thestudy of fire-fighting techniques, it is difficult todecide exactly how much should be included.Many new processes and materials have becomeavailable in recent years. Firefighters are facedwith so many new substances, particularly newbuilding materials, during the course of their work,that they must have some idea how they will reactwhen involved in fire. The particular hazards ofmany flammable materials and chemicals are dealtwith separately in Parts 6b and 6c of the Manual ofFiremanship. However, in this Chapter it is pro-posed to deal with those aspects of chemistry whichapply to the study of fire techniques, and to lead onto discuss some of the more hazardous chemicalsubstances from a purely chemical point of view.

6.1 The basis of chemistry

Chemistry is the science of the composition ofsubstances, their properties and reactions witheach other. Substances may be solids, liquids orgases, in living or non-living systems, but all haveone common factor - they consist of chemicals.


Chemists recognise two distinct classes of sub-stances; those which consist of a single chemical(elements and compounds) and those which aremixtures. A mixture may be separated into its con-stituents by some physical or mechanical means;for example, a mixture of salt and sand can be sep-arated by dissolving the salt in water, leaving thesand behind. But to separate or change a singlechemical substance, a chemical reaction isrequired.

Whether the substance is single or a mixture, it ismade up from many millions of very tiny particleswhich the chemist calls molecules. (See Section1.5) A mixture will contain more than one type ofmolecule, whereas a chemical compound containsonly one type of molecule. Molecules of the samesubstance are all exactly alike in their propertiesand behaviour.

A molecule can be said to be the smallest particleof a compound capable of existing independently.The common substance chalk occurs in largequantities and in many different forms. For exam-ple, it is found in cliffs as lumps, or as a powder; itis, nevertheless, always recognisable as the samematerial, known chemically as calcium carbonate.This material is formed from innumerable calciumcarbonate molecules. Each molecule is composedof even smaller particles called atoms. Every cal-cium carbonate molecule is exactly the same; eachcontains five atoms.

Molecules are formed from atoms. The number ofdifferent atoms comprising their molecules is rela-tively small. The molecules of all substances com-prise various combinations of atoms, from approx-imately 90 different types of atom.

Atoms are the 'building blocks' of all substances.Unlike molecules, which can be broken down orchanged during chemical reactions, atoms cannotbe split chemically into anything smaller1. Duringchemical reactions the atoms rearrange to formdifferent molecules, but the atoms themselvesremain the same. They are the smallest particles totake part in chemical changes. Atoms are extreme-ly small, their diameters being

Substances formed entirely from one type of atomare called elements. There is an element corre-sponding to each different type of atom. Thus car-bon, being formed entirely from carbon atoms, isan element. Similarly iron, containing only ironatoms, is another element. Elements may be com-posed of molecules made up from identical atomsjoined together, or they may be composed of sin-gle atoms. The element oxygen consists of oxygenmolecules, each molecule being two oxygen atomsjoined together, whereas the element magnesiumconsists of single magnesium atoms. When weagain consider the molecule of calcium carbonate(chalk) we find that it is composed of one atom ofthe element calcium, one atom of the element car-bon, and three atoms of the element oxygen, allchemically bound together. A list of the names ofsome elements is given in Table 6.1; a full list isgiven in Appendix B.

6.2.1 Compounds and mixtures

When two or more atoms of different elements arechemically bound together to form molecules, allexactly the same, the substance formed is called achemical compound. For example, each molecule ofthe compound calcium carbonate contains five atomschemically bound together (one of calcium, one ofcarbon and three of oxygen). The compound formedfrom identical molecules can only be broken down orchanged by a rearrangement of the atoms, known as achemical reaction. A mixture (formed from two ormore different sorts of molecules) can be separated byphysical or mechanical means into the substanceswhich make up the mixture.

1 They can be split by nuclear processes, which will bediscussed elsewhere.


since they are based on the old Latin or Greeknames. For example, the element sodium has thesymbol Na which is derived from the Latin natri-um, and lead has the symbol Pb, derived fromplumbum (hence plumber, and plumb line).

63.1 Using symbols to write formulae

When a symbol is written it represents one atom ofthe element. Thus: H represents one atom ofhydrogen; O represents one atom of oxygen.

A formula always represents one molecule of thesubstance and shows which atoms are present inthe molecule and how many of them there are.Thus H2O represents one molecule of water, con-taining two atoms of hydrogen and one atom ofoxygen, bound together chemically. Similarly car-bon dioxide has the formula CO2, representing themolecule which contains one atom of carbon andtwo atoms of oxygen. If a molecule contains morethan one atom of the same type, the number ofsimilar atoms is written at the bottom right of theappropriate symbol:

Calcium carbonateCaCO3 1 atom of calcium, one atom of carbon

and 3 atoms of oxygen

Phosphorous pentoxidePO5 1 atom of phosphorus and 5 atoms of

oxygen.

To represent more than one molecule we write anumber in front of the formula: thus three mole-cules of water are represented by 3H2O. Thisgroup of three water molecules contain six hydro-gen atoms and three oxygen atoms.

2MgO represents two molecules of magnesiumoxide (and, therefore, a total of two magnesiumatoms and two oxygen atoms).

63.2 Radicals

Certain groups of atoms, common to families ofrelated compounds, are known as radicals. A rad-ical can be defined as: 'a group of atoms presentin a series of compounds which maintains itsidentity regardless of chemical changes whichaffect the rest of the molecule'.


63 Symbols

Chemical symbols are used as a way of describingchemicals in terms of formulae, which are com-plete descriptions of molecules in terms of the con-stituent atoms. Symbols give as much informationas possible, whilst still being simple and quick touse. Formulae may also be used to describe theway that atoms in a molecule are grouped togeth-er. This information may give clues to how onechemical compound may react with another.

Every element is assigned a symbol (see Table6.1), which is different from that of all the otherelements. A symbol may be one letter or two; inthe latter case the convention is to write the secondletter as a small letter. Thus the symbol for nickelis written Ni and not NI. NI would be interpretedas a molecule containing one nitrogen atom (N)and one iodine atom (I) (such a compound doesnot exist). In many cases the symbols are the firstletter of the name of the element, often followedby a second letter taken from that name. However,there are several common elements whose sym-bols bear no relationship to their modern names.

To show these radicals, formulae are often writtenwith brackets enclosing the radical and with anumber beyond the bracket to indicate how manyof these radicals are in the formula.

Radicals are not complete molecules and have noindependent existence. For example, the formulaof one molecule of calcium hydroxide (slakedlime) is Ca(OH)2, indicating that it contains onecalcium atom and two hydroxyl (OH) radicals. Themolecule contains two oxygen atoms and twohydrogen atoms, but they are always paired, asOH. Another common radical is NO3, the nitrateradical. The formula for aluminium nitrate is writ-ten Al (NO3)3, indicating that the trivalent alumini-um atom is combined with three monovalentnitrate radicals. Schematically:

The meaning of valency is discussed in Section 6.6.A list of common radicals is given in Table 6.2.

6.4 Atomic mass

The mass of one atom or one molecule is extreme-ly small - of the order of 10-22 grams. It is of littlepractical value to quote the actual masses of atoms,but because atoms of different elements containdifferent numbers of protons and neutrons, know-ing the mass is a big step towards identifyingwhich element the atom belongs to.

It is, therefore, important to know how heavy oneatom is in comparison with any other. The chemist,therefore, uses a relative atomic mass scale and notthe actual masses of the atoms. Various scales havebeen proposed and, for technical reasons, the onemost generally used is based on oxygen, which isgiven the atomic mass of 16.000. On this scalehydrogen has an atomic mass of 1.008. However,for normal purposes, the atomic masses can berounded off, making hydrogen equal to 1.

We can then compare other atoms with hydrogento see how many times heavier they are, so that wehave the definition:


For example, the atomic mass of sodium is 23(written Na = 23), meaning that an atom of sodiumis 23 times heavier than an atom of hydrogen.

6.5 Molecular mass

In the same way, molecular mass is the mass ofone molecule of the substance compared to themass of one atom of hydrogen. For example, themolecular mass of water is 18 which means thatone molecule of water is 18 times as heavy asone atom of hydrogen. Since a molecule consistsof atoms joined together, the mass of the mole-cule is the sum of the masses of its componentatoms. The molecular mass is found by addingtogether the atomic masses of those atoms pre-sent, thus: The molecular mass of sulphur diox-ide (SO2) is 64.

Molecular Mass = atomic mass of sulphur +2 x atomic mass of oxygen

= 32 + (2 x 16)= 64

Similarly: nitric acid HNO3, molecular mass 63;

Mol = atomic + atomic + 3 x atomicmass mass H mass N mass O

1 + 1 4 + (3 x 16)

63

6.6 Valency

6.6.1 Multiple valency

Several elements show more than one valency,including iron (Fe), copper (Cu) and nickel (Ni). Thevalency that the element shows depends on the par-ticular circumstances - the other elements with whichthe element is combined, as well as on the conditionsunder which the reaction in which the compound isformed is carried out. More detailed knowledge ofthe chemistry of the element is required to predictwhich valency will be shown in any particular reac-tion. However, the names of the compounds formedare often adapted to help in deciding which valencystate the element is in, in that particular compound,and hence to determine the correct formula.

6.6.2 Nomenclature

(1 ) -OUS and-IC-OUS and -IC are used where an element showstwo valencies.-OUS always indicates the lower and -IC the high-er valency. For example:

IronFerrous: valency 2, e.g., FeCl2 ferrous chloride.Ferric: valency 3, e.g., FeCl3 ferric chloride.TinStannous: valency 2, e.g., SnBr2 stannous bro-mide.Stannic: valency 4, e.g., SnBr4 stannic bromide.

(2) Use of Roman numeralsA modern approach to the problem of multiplevalency is to indicate which valency is being usedby inserting the appropriate Roman numeral afterthe name or symbol of the element concerned, e.g.,Iron (II) chloride Fe(II)Cl2,Iron (III) chloride Fe(III)Cl3,Tin (II) bromide Sn(II)Br2,Tin (IV) bromide Sn(lV)Br4

(3) -IDE-IDE is used to indicate that a compound is madeup of two elements only. By convention, metalsare written before non-metals in names and formu-lae, thus:

Magnesium oxide MgO(Mg valency 2; O valency 2).

Potassium sulphide K2S(K valency 1; S valency 2)


When atoms combine to form molecules they doso in definite fixed ratios. For example, one sodi-um (Na) atom always combines with one chlorine(Cl) atom to give NaCl (common salt), but onemagnesium (Mg) atom combines with two chlo-rine (Cl) atoms to give MgCl2 (magnesium chlo-ride). The 'combining power' of an atom dependson the arrangement and number of its electrons,but the mechanism of this is too complicated todiscuss easily here.

The valency of an atom tells us how many chemi-cal bonds the particular atom, or group of atoms(radicals) will form. Valencies are given in Table 3.When molecules are formed, the atoms or radicalsgenerally combine in ratios in which the valenciesare balanced. This property enables us to work outthe correct formulae of many chemical com-pounds. For example, in magnesium oxide, Mghas a valancy of 2, and O has a valency of 2. Tobalance the valencies we need one Mg atom andone O atom: hence the formula MgO.

In potassium carbonate, potassium (K) has a valen-cy of 1, while the carbonate radical has a valencyof 2. Potassium carbonate requires two K atoms tocombine with one carbonate radical, thus the for-mula is K2CO3.

In aluminium sulphate Al has a valency of 3, thesulphate radical has a valency of 2. To form thecompound, two Al atoms are required to balancethree SO4 radicals (total number of "bonds" = 6),so that the formula for aluminium sulphate isA12(SO4)3.

=

=

-IDE is also used exceptionally for a few radicals,e.g., -OH hydroxide. Thus:

Calcium hydroxide Ca(OH)2

(Ca valency 2; OH valency 1).

(4) -ITE and-ATE-ITE and -ATE are used where a compound con-tains more than two elements, one of which is oxy-gen. For two related compounds, that named -ITEalways contains less oxygen than that named-ATE.

Sodium sulphite Na2SO3

(Na valency 1; SO3 valency 2).

Sodium sulphate Na2SO4

(Na valency 1; SO4 valency 2).

Potassium nitrite KNO2

(K valency 1; NO2 valency 1).

Potassium nitrate KNO,(K valency 1; NO, valency 1).

The ending -ITE and -ATE are related to the end-ing -OUS and -IC where the latter are used in thenames of acids. -OUS leads to -ITE and -IC to-ATE. For example:

Sulphurous acid H2SO3 gives sulphites -SO3

Sulphuric acid H2SO4 gives sulphates -SO4

Nitrous acid HNO2 gives nitrites -NO2

Nitric acid HNO, gives nitrates -NO3

(5) Mono-, Di-, Tri-, Tetra-, PentaMono-, di-, tri-, tetra- and penta- are used in namesto tell how many of a particular atom or radical arepresent.

Mono- 1 e.g., carbon monoxide CO,Di- 2 e.g., carbon dioxide CO2,Tri- 3 e.g., sulphur trioxide SO3,Tetra- 4 e.g., carbon tetrachloride CC14,Penta- 5 e.g., phosphorus pentachloride PC15.

(6) Per- always denotes that there is moreoxygen present in the compound thanwould normally be the case:Hydrogen oxide (water) H2O,Hydrogen peroxide H2O2,Sodium chlorate NaClO3,Sodium perchlorate NaC104.

6.7 Simple equations

Consider a simple chemical reaction. When sul-phur (a yellow solid element) burns in air, it com-bines with oxygen from the air, producing acolourless gas with a pungent choking smell. Thisgas is called sulphur dioxide (formula SO2)

This can be stated simply as: "sulphur reacts withoxygen to form sulphur dioxide". A further simpli-fication can be made by replacing 'reacts with' by"+" and 'to form' by an equals sign, "=". We thenhave:

Sulphur + oxygen = sulphur dioxidereacts with to form

This statement can be simplified even further byreplacing the names of the chemicals by symbolsand formulae. (The molecules of oxygen, likethose of most common elements that are gases,contain two atoms, but sulphur, like other solidelements, is assumed to consist of single atoms.)This gives:

S + O2 = SO2

This final statement represents the chemical equa-tion for this reaction. It tells us that every sulphuratom involved reacts with one oxygen molecule toform one sulphur dioxide molecule.


Consider this other example. Magnesium (a metal)burns in oxygen to form magnesium oxide (a whitepowder). Magnesium reacts with oxygen to formmagnesium oxide:

Magnesium + oxygen = magnesium oxide

Mg + O2 = MgO

In this case, although there is the same number ofmagnesium atoms on each side of the equation,this is not true in the case of the oxygen atoms,where there are two on the left hand side and onlyone on the right hand side. This implies that oxy-gen atoms disappear during the reaction. The equa-tion must be balanced before the equation is of anypractical use - before it can tell us how the ele-ments combine.

The equation above can be balanced by placingtwo molecules of magnesium oxide on the righthand side, thus:

Mg + O2 = 2MgO

We now have equal numbers of oxygen atoms oneach side of the equation, but the magnesium is'out of balance'. By having two atoms of magne-sium on the left hand side (instead of one):

2Mg + O2 = 2MgO

the equation is now correctly balanced. Each sidenow contains equal numbers of each type of atominvolved. It is of course possible to balance theequation by cheating. The equation:

Mg + O2 = MgO

"balances" if we change the formula of magne-sium oxide, thus:

Mg + O2 = MgO2

but MgO2 does not exist. A chemical equation canonly be balanced by changing the number of mol-ecules present, not their formulae.

6.8 Use of chemical equations

The balanced equation: 2Mg + O2 = 2MgO tells us


that two atoms of magnesium react with one mol-ecule of oxygen to produce two molecules of mag-nesium oxide.

These atoms and molecules have masses which areexpressed in terms of their atomic and molecularmasses (as described in Section 6.3). The atomicmass of magnesium is 24, that of oxygen is 16, andif we use this information together with the equa-tion, we obtain:

2 Mg +O2 = 2MgO2 x24 2 x 16 2(24+ 16)48 units 32 units 80 unitsTwo Two Two MgOmagnesium oxygen molecules eachatoms atoms containing 1

magnesium and1 oxygen atom

The 'units' are mass units, where one unit repre-sents the mass of one hydrogen atom. Therefore,according to the equation, 48 units of magnesiumwill react with 32 units of oxygen to form 80 unitsof magnesium oxide. In other words the ratio:

An actual reaction between magnesium and oxy-gen will obviously involve millions of moleculesof each substance. Suppose we 'scale up' this reac-tion until we have two million magnesium atomsinstead of two. Then this number of magnesiumatoms will react with one million oxygen atoms.

Two atoms of magnesium weigh 48 units, there-fore, two million atoms of magnesium weigh48 000 000 units.

Similarly, one molecule of oxygen weighs 32units; therefore, one million molecules of oxygenweigh 32 000 000 units, so that the ratio:

6.9 Limitations of chemicalequations

No matter to what extent the amounts of magne-sium and oxygen are scaled up, the ratio willalways be 48/32 Therefore:

if 48 grams of magnesium are used, 32 grams ofoxygen are needed. If 48 kg of magnesium areused, 32 kg of oxygen are needed, and 80g or 80kg of magnesium oxide will be produced. For anyother mass of magnesium, the masses of oxygenneeded and magnesium oxide produced can befound by simple proportion.

tion releases more heat then the first, by an amountequal to the latent heat of evaporation of water.

6.93 Reaction conditions

Equations say nothing about the reaction condi-tions; whether heat must be used or pressureapplied.

6.9.4 Heat

Equations do not tell us whether heat is given outor absorbed during a chemical reaction.

6.9.5 Rate of reaction

Equations say nothing about the rate of the reac-tion; whether it is slow, fast or inherently violent;or whether or not a catalyst: is necessary to makethe reaction occur at a reasonable rate.

6.9.1 Reality

A chemical equation must be a summary of aknown chemical reaction. For instance it is per-fectly possible to write down the equation:

Cu + 2HNO3 = Cu(NO3)2 + H2

copper + nitric acid = copper nitrate + hydrogen

but such an equation is useless because it is foundthat when copper is placed in nitric acid, hydrogenis never produced. Therefore, the equation is not'telling the truth', even though the sides balance.

6.9.2 Physical state

The equations we have been considering containno information about the physical state of thechemicals, whether they are solids, liquids orgases, whether they are pure substances or are dis-solved in water or some other solvent, or whetherthe solutions are dilute or concentrated.Sometimes it is important to specify the physicalstate. For example, the reaction of hydrogen andoxygen to form water vapour is associated with therelease of a certain amount of heat. If the watervapour is allowed to condense, the latent heat ofvaporisation is released. Thus, if we are quotingthe amount of heat released by the reaction ofhydrogen and oxygen, the physical form of thewater must be stated. This can be done as follows:

2H2 + O2 = 2H2O(g)2H2 + O2 = 2H2O(1)

where g and 1 refer to the gaseous (vapour) and liq-uid states respectively. As written, the second reac-


2 A catalyst is a substance that alters the rate of a chemicalreaction, but does not itself undergo a chemical change.


Chapter 7 - Combustion

(from the air) and the surface of the fuel: this is acomplex process and in general only occurs withsolid fuels which char on heating.

A flame is a region in which a sustained, heat-releasing reaction between a fuel in the vapourstate and oxygen takes place. This region alsoemits light, usually with a strong yellow colour,though there are substances such as methanolwhich burn with a weak blue flame which cannotbe seen in strong light.

One way of discussing burning is in terms of thetriangle of combustion (Figure 7.1). For combus-tion to occur three things are necessary: heat, oxy-gen and fuel. Combustion will continue as long asthese three factors are present. Removing one ofthem leads to the collapse of the triangle and com-bustion stops.

Figure 7.1 The triangle of combustion.


7.1 The Fire Triangle

For combustion or burning to occur, oxygen, usu-ally from the air, must combine with a fuel. A fuelmay be in any one of the three states (gas, liquid orsolid) initially, but for flaming combustion tooccur, a solid or liquid fuel must be converted intoa vapour, which then mixes with air and reactswith oxygen. Smouldering combustion, on theother hand, involves a reaction between oxygen

7.2 Heat of reaction and calorific value

All combustion reactions release heat energy and are, therefore, called exothermic reactions. The quantity of heat produced per unit weight of fuel can be calculated and is known as the calorific value of the fuel. For example, when 12 grams of carbon (the "gram atomic mass") are burned to carbon dioxide 392 920 Joules of heat are pro-duced. This is the "heat of combustion", as tabu-lated in textbooks and handbooks: it refers to a standard amount of the fuel (the "mole") and has units kJ/mol. The calorific value for carbon is then:

(ii) The combustion may take place using oxygen which is contained within the burning material, the combustible material and the supporter of combus-tion being together in the same compound:

12CO2 + 10H2O + 6N2 + O2

(iii) Oxygen may be provided by one of the materials in a mixture of compounds. The 'thermite reaction' illustrates this principle:

A12O3 + 2Fe + heat

= 32 743 Joules per gram (J/g)

as one "mole" of carbon contains 12 g.

Besides calorific value, the rate of heat release is also important. For example, burning magnesium produces less heat than the burning of carbon, but when rates of reaction are considered, we find magnesium has a much higher rate of combustion than carbon so that the heat is released much more rapidly. Heat release rate is now considered to be a major factor in whether a fire will spread over materials, and a device called a Cone Calorimeter has been developed to measure this quantity for wall linings and building materials in general as part of the assessment of their flamma-bility and suitability for their intended use.

7.2.1 Oxidation

An oxidation reaction is a reaction which involves combination with oxygen or other oxidising agents. The following reactions are typical exam-ples of combustion:

(i) The oxygen may be supplied by the air.

2C + O2 2CO (carbon monoxide) 2CO +O2 2CO2 2H2 + O2 2H2O

(note that the oxidation of C to CO is not a flam-ing reaction: the others are).

(iv) Elements other than oxygen may be consid-ered as oxidising agents; examples of these are chlorine and fluorine. "Combustion" may occur with these substances; for example, hydrogen will burn explosively with chlorine:

H2 + Cl2 2HC1

Many organic materials (i.e., those based on car-bon) will burn readily in halogen gases:

C10H16 + 8C12 16HC1 + 10C turpentine

Nitrogen is not usually thought of as an oxidising agent or even a reactive element, but some metals will burn vigorously in this gas. Magnesium, alu-minium and their alloys form nitrides in combus-tion reactions:

3Mg + N2 Mg3N2 magnesium nitride

7.3 What makes a flame a flame?

If a pool of paraffin is heated, its temperature will rise and combustible vapours will evaporate from the surface. When the temperature of the liquid


4C3H5(NO3)3 nitroglycerine

Fe2O3 + 2A1 thermite mixture

392920 12

7.4 Laminar flow andturbulent flow

surface reaches about 50 - 55°C (the firepoint, seeSection 7.6.3), the rate of evaporation is highenough for the vapours to be ignited by a smallflame, or spark, and support continuous flamingabove the surface. After the paraffin has beenburning for some time, the surface of the fuel willbe close to its boiling point, supplying flammablevapours to the flame.

Once a flame has been established and flammablevapours are rising from the fuel surface, heat andwhat are called chain carriers are produced wherethe flame reactions are occurring. A proportion ofthese will pass into the next layer of gas and startthe oxidation and heat release processes there,rather as in a relay race. Chain carriers are believedto be atoms or fragments of molecules known asfree radicals which are extremely reactive. Thetype of chemical reaction which occurs in theflame is known as a chain reaction.

In fact, there are two distinct types of flame: thepremixed flame and the diffusion flame. Theyhave different properties, though both are familiarfrom everyday experience. Understanding thateach behaves differently is important: under differ-ent circumstances fuel and air can combine in dif-ferent ways to produce very different results.

Before we discuss flames, it is useful to define twotypes of gas flow: laminar flow and turbulent flow.

Laminar flow [Figure 7.2] is steady flow in whichtwo particles starting at any given point follow thesame path. Particles never cross each other's paths,so the particle paths are bunched together likeuncooked spaghetti in a packet. At any given time,the velocities of all particles on one path are thesame, but the velocities of particles in differentpaths might be different. Laminar flow is associat-ed with slow flow over smooth surfaces.

In turbulent flow, |Figure 7.2] there are randomchanges in velocity and direction of the flow,although the flow as a whole is moving in a defi-nite direction. If we consider wind blowing downa street on a windy day, leaves and litter may beblown up, down, across and around, revealinglocal changes in the flow, but the general directionof the wind is still down the street. Turbulent flowtends to occur in fast flows over rough surfacesand around obstacles.

Figure 7.2 Laminar andturbulent flow.


7.5 Premixed and diffusionflames

As we have seen, a flame is the region in which thechemical reactions take place which turn unburntfuel vapours into burnt gases — the combustionproducts: for example, methane and oxygen reactto give carbon dioxide and water vapour.

A certain amount of heat energy is required to startthis reaction but more heat is produced by the reac-tion than it takes to initially start it, so the burningprocess is self-sustaining.

Premixed flames occur when a fuel is well-mixedwith an oxidant, e.g., 10% methane mixed with air.For ignition to occur, energy must be supplied tothe system in the form of a spark or small flame. Aself-sustaining flame will then be establishedaround the ignition source and propagate outwardsin all directions.

The flame consists of a zone where cold unburntgas (reactants) is transformed into hot burnt gas(products). The flame zone of a premixed flamemay be less than 1 mm thick. As the volume of thehot burnt gas is greater than that of the same massof cold unburnt gas, the flame front is pushed out-wards from the ignition point, like the skin of aninflating balloon.

Not every mixture of air and fuel will burn.Depending on the type of fuel and oxidant involved(air or pure oxygen, for example), a mixture initial-ly at room temperature and pressure will only burn


if the concentration of fuel lies between certainwell-defined limits, called flammability limits.For example, mixtures of methane and air will onlyburn if the concentration of methane in air liesbetween 5% and 15%, whereas hydrogen will burnin air at concentrations between 4% and 75%.

The figures quoted for limits of flammability mayvary as there are a number of factors which mayslightly alter the value: pressure, temperature,dimensions of the test apparatus, direction offlame propagation and moisture content of themixture all have some effect. (In general, the lim-its widen with rise in temperature.)

Within these ranges, there is an optimum mixturein which there is just sufficient fuel to use up allthe oxygen. This is the stoichiometric mixture.Mixtures containing more fuel than the stoichio-metric mixture are known as rich mixtures, andones containing less fuel are lean mixtures. Thestoichiometric mixture for methane is 9.4%. A 7%mixture is lean, while a 12% mixture is rich.

For each mixture of fuel and air between the flam-mability limits, there is a characteristic burningvelocity at which a premixed flame will propagatethrough a stationary gas.

Burning velocities usually lie between 0.1 and 1.0m/s. They tend to peak at the stoichiometric com-position and fall away towards the flammabilitylimits [Figure 7.3]. Burning velocity is dictated bythe chemical processes involved - how quickly thefuel reacts with the oxygen. The methane and oxy-gen molecules do not simply combine instanta-neously to form carbon dioxide and water vapour,but form free radicals and intermediates such asformaldehyde and carbon monoxide along the wayto completing the reaction.

If the premixture flows into a flame with a laminarflow whose velocity is equal to the burning veloc-ity of the mixture, the flame can be held stationary.This is how premixed flames on Bunsen burners,domestic gas rings etc., are held steady.

Local air currents and turbulence caused by obsta-cles can cause a flame to move at speeds muchhigher than the burning velocity. The speed atwhich a flame moves relative to an observer is the

Figure 7.3 Flammabilitylimits.

flame speed, which is different to the burningvelocity. For example, the burning velocity of astoichiometric methane-air flame is 0.45 m/s. Ifthe unburnt gases are no longer stationary, theflame propagates at the local flow speed plus theburning velocity. As the flame gets faster, theflame front wrinkles as turbulence is produced inthe unburnt gas, increasing the surface area of theflame front. This increases the reaction rate,increasing the rate at which burnt gas is produced,so pushing the flame front forward faster. In explo-sion situations, flame speeds of hundreds of metresper second can be achieved in gas-air mixtures,though the burning velocity of the mixture will bemuch lower than this. It is worthwhile making thedistinction between these terms, as some oldertexts use flame speed to mean burning velocity. Itis possible to achieve supersonic flame speeds, inwhich the combustion region is strongly coupledto a shock wave: this phenomenon is called deto-nation.

Diffusion flames occur at the interface where fuelvapour and air meet. Unlike premixed flames, thefuel vapour and the oxidant are separate prior toburning. The dominant process in the diffusion

flame is the mixing process. The fuel vapour andoxygen mix with each other by molecular diffu-sion, which is a relatively slow process, though thehigh temperatures associated with flames increasethe rate at which diffusion occurs.

Diffusion flames themselves fall into two broadtypes. In slow-burning diffusion flames, such ascandle flames, the fuel vapour rises slowly fromthe wick in a laminar flow and molecular diffusiondominates. This type of flame is a laminar diffu-sion flame.


In industrial burners, fuel is injected at high veloc-ity into the air as a spray or jet. Turbulence isinduced at the interface where mixing takes place.This gives the flame an extremely large surfacearea in comparison to the relatively small surfacearea of the smooth fuel/air interface of the candleflame. In this turbulent case, it is the large interfacearea, rather than the rate of molecular diffusion,which determines the rate of mixing. This type offlame is a turbulent diffusion flame.

In a large fire (e.g., more than 1 m in diameter), theflames are turbulent diffusion flames, the turbu-lence generated by the strong buoyancy of theflames themselves. Inside the flame, there areregions of high temperature and low oxygen con-centration where the fuel vapour is subjected to amixture of pyrolysis (chemical decomposition inthe absence of oxygen) and partial oxidation, lead-ing to the formation of soot particles and productsof incomplete combustion, in particular carbonmonoxide (CO). These are the source of smoke,and of the gaseous species that render the fireproducts toxic.

7.6 Practical examples of premixedflames and diffusion flames

7.6.1 The Bunsen Burner

The Bunsen burner (Figure 7.4) should be familiarfrom school laboratories. It can produces both typesof flame - premixed and diffusion. Gas is forced outof a jet at the base of the burner. If the air inlet col-lar at the bottom is open, air is entrained into thefuel flow and mixing occurs in the burner column.A pale blue conical flame is visible just above thetop of the burner. This is a laminar premixed flame.When the air inlet to a Bunsen burner is closed, ayellow diffusion flame results.

7.6.2 A candle flame

When a match is held close to the wick of an unlitcandle, the wax melts and rises up the wick by cap-illary action. There it evaporates, and a flame isestablished at the interface between the evaporat-ing fuel and the surrounding air. The fuel and airare not mixed before burning, so this is a diffusion

Figure 7.4 (a) Premixedflame on a Bunsen burnerwith full aeration;(b) diffusion flame.


flame. Once the flame is established, the processof melting, evaporation and burning is self-sus-taining because heat is transferred from the flameback to the wick to sustain the melting and evapo-ration processes.

7.63 Flashpoint, firepoint and sustained fires

Imagine a dish of flammable liquid, such as paraf-fin. A region will exist above the liquid surface inwhich the evaporating fuel vapour is well mixedwith air. If the paraffin is heated above about 40°C,this well-mixed region will become flammable -that is, the vapour concentration in air is above itslower flammability limit. The lowest tempera-ture at which this occurs is called the 'FLASH-POINT', the liquid temperature at which appli-cation of an ignition source will cause a flame toflash across the surface of the liquid. This is apremixed flame moving through the vapour/airmixture but, just above the flashpoint, it burns out,or self-extinguishes, because it has consumed allthe vapour. If heating is continued, a temperaturewill be reached at which ignition of the vapourswill lead to a "flash", followed by the developmentof a sustained diffusion flame at the surface flame.This temperature is known as the 'FIRE-POINT', the lowest temperature at which therate of supply of fuel vapours (by evaporation)can sustain the flame.

There are several types of apparatus for determin-ing the flash point of a liquid. The most commonis the Pensky-Martens "closed cup" test in whichthe vapours cannot diffuse away from the surface,but can achieve a uniform concentration in thehead space above the liquid surface. The Abelapparatus is also a closed cup test. The Cleveland"open cup" test gives a slightly higher flashpointthan the Pensky-Martens, but can be used to deter-mine the firepoint. Clearly, it is always necessaryto quote the method and the type of apparatusused, and whether the result is an "open cup" or"closed cup" flashpoint. Note that the flashpoint isaffected slightly by pressure: values quoted inhandbooks, etc., are adjusted to normal atmospher-ic pressure. Corrections should be considered forhigh altitude applications.

The term "flash fire" is used to describe what hap-pens if the temperature of the fuel is much greater

than the firepoint and a flammable vapour/air zoneexists at some distance from the liquid surface.This may happen if there is a spillage of petrol(firepoint approx. -40°C) which forms a large pool.If an ignition source is introduced into the flam-mable zone, a premixed flame will flash back,igniting the fuel in the fuel-rich mixture above theliquid surface and giving rise to a large fire (turbu-lent diffusion flame).

In a sustained fire of this type, flames burn contin-uously above the surface until the fuel is consumed(or the fire is extinguished). In principle, com-bustible solids burn in the same way, although theformation of fuel vapours involves chemicaldecomposition of the solid which requires moreenergy than simple evaporation. For this reason,solids tend to burn much more slowly than com-bustible liquids.

7.6.4 Fireball

A fireball can occur when a mixture of vapour andmist droplets forms a cloud containing very littleair, for example when a vessel containing pres-surised liquid fuels, such as LPG, ruptures. Theoxygen concentration within this cloud is far toolow for premixed combustion to take place, but, ifthere is a source of ignition at the boundarybetween the fuel and the surrounding air, a pre-mixed flame will flash through the flammablezone at the boundary, leading to the establishmentof a diffusion flame. The fireball has then beenestablished and will rise as it burns.

As burning progresses, instabilities are introducedin the surface of the flame increasing the surfacearea available for the reaction to take place. Thefireball will increase in size until the fuel has beenused up. Then it will shrink and extinguish.

7.6.5 Vapour cloud explosions

If a gas or vapour escapes under pressure from arupture in a storage tank or pipeline, there is likelyto be rapid mixing of the fuel with air, producing acloud of fuel-air mixture, some of which will be ofconcentrations which fall between the flammabili-ty limits. If an ignition source is present in thecloud, a premixed flame will move outwards in alldirections from the source. The flame will propa-


gate through that region of the cloud which liesbetween the flammability limits.

After ignition, obstacles such as process vesselsand pipework will introduce turbulence in thevapour/air mixture ahead of the flame, and causewrinkling of the flame. This will increase theeffective surface area of the flame, increasing therate of burning, and the flame speed. This can gen-erate overpressures which under severe conditionscan cause blast damage. It is believed that thismechanism was responsible for the extensive dam-age caused at Flixborough in June 1974.

7.7 Ignition

For ignition to occur, sufficient heat energy is sup-plied to gaseous fuel and oxidant, either mixed towithin flammability limits or at the interfacebetween the fuel and the oxidant, to start a self-sustaining chemical reaction. This energy is gener-ally supplied by a flame or a spark or a hot surface.

In this short section we will look at less obviousways in which ignition can occur.

7.7.1 Spontaneous ignition temperature

This is the lowest temperature at which the sub-stance will ignite spontaneously, that is the sub-stance will burn without the application of a flameor other ignition source. This is sometimes referredto as the auto-ignition temperature.*

For some materials, the ignition temperature maybe so low that there is a danger of them ignitingunder normal conditions, or in the range of tem-peratures that the material would experience dur-ing day-to-day use. Such materials are normallywell documented and information availableregarding their safe handling.

7.7.2 Self heating and spontaneous combustion

Certain materials may react with oxygen at roomtemperature. Compounds such as linseed oil whichcontain carbon-carbon double bands are very

prone to this reaction, but any organic materialstored in bulk quantities may be suspect, especial-ly if it has been stored at an elevated temperature.

Imagine a pile of cloths soaked in linseed oil,which have been discarded after, for example aroom has been decorated with paint or varnishcontaining a large proportion of linseed oil.

As the cloth is porous, oxygen in the air will beable to reach the centre of the pile. The linseed oilwill oxidise slowly, even at room temperature,releasing heat. Because the centre of the pile iswell insulated by the surrounding cloths, heat willbuild up and the temperature will rise. As the tem-perature rises, the reaction rate increases - rough-ly for every 10°C rise in temperature, the reactionrate doubles - so even more heat is given out andthe temperature rises more quickly.

If heat is being produced more rapidly than it canescape, the temperature will continue to rise to astage at which active combustion begins, usuallywell within the mass of material. Combustion willbegin as a smouldering process, burning throughto the surface of the pile where naming combus-tion will be initiated.

Spontaneous combustion should be considered asa possible cause of a fire for which there is noobvious ignition source, but it is necessary to showthat the material involved has the propensity to selfheat, and that a sufficient quantity has been storedin such a way as to provide the necessary thermalinsulation for the inside of the pile. A useful rule-of-thumb is that if the material does not produce arigid char when it is heated, it is very unlikely toself-heat to ignition.

Sometimes the action of bacteria on certain organ-ic materials can cause a rise in temperature even-tually leading to active combustion (haystackswere prone to this). Powdered material, such aspowdered coal used in power stations and somemetals, can give rise to spontaneous combustion.Stocks of coal at power stations, if incorrectlystored in very large piles, can self-heat to ignition.In the manufacture of some plastics (e.g.,polyurethane foams), the cross-linking processwhich creates the final molecular structure of thematerial is exothermic, and can lead to sponta-


* The auto-ignition temperature is not a true property of amaterial. It depends on how it is measured.

neous combustion if slabs of foam are storedbefore the process is complete (see Chapter 10).

7.7.3 Smouldering

Smouldering only occurs in porous materialswhich form a solid carbonaceous char when heat-ed. Paper, sawdust, fibreboard and latex rubber canall undergo smouldering.

Smouldering is the combustion of a solid in an oxi-dising gas such as air, without the appearance of aflame. The process is very slow, but smoulderingfires can go undiscovered for a very long time andcan produce a large amount of smoke. This isflammable, but it must accumulate and reach itslower flammablity limit before it can be ignited.This has been known to happen on a few, fortu-nately rare, occasions (e.g., the Chatham Dockyardmattress fire in November 1974).

Smouldering will undergo a transition to flamingunder favourable conditions. The best documentedexamples involve cigarette ignition of upholstered fur-niture. The mechanism is not fully understood, and itis impossible to predict how long after the commence-ment of smouldering that the transition will occur.

7.8 Hazards of oxidising agents

Nearly all combustion reactions involve oxidationwhich in its most simple form is combination withoxygen, such as the combustion of hydrogen:

2H2 + O2 = 2H2O

The oxygen in this case may be called an oxidisingagent. The word oxidation also has a broadermeaning where elements other than oxygen maybe considered as oxidising agents. For example,most metals will react with chlorine, and otherhalogens: this is also a type of oxidation.

Mg + Cl2 = MgCl2

Here chlorine is the oxidising agent.

There are certain compounds which do not neces-sarily burn themselves but, on decomposition,release oxygen which can greatly assist a combus-tion reaction. Some of these compounds may be

relatively stable at room temperature but at hightemperatures they could be extremely hazardous.

Some of the more common oxidising agents areconsidered below.

7.8.1 Nitric acid and inorganic nitrates

Concentrated nitric acid is a very powerful oxidis-ing agent and reacts vigorously with many organiccompounds. Carbon itself reacts with the hot acid,in the following way:

C + 4HNO3 = CO2 + 4NO2 + 2H2O

When the concentrated acid mixes with carbona-ceous (carbon-containing) material there is a vio-lent reaction giving off a great deal of heat andnitrogen dioxide (nitrous fumes). Clearly, carbona-ceous materials like sawdust and wood chippingsmust never be used to soak up a spillage of thisacid. The nitrates (salts of nitric acid) are also goodoxidising agents. They are used in large quantitiesin industry and agriculture. An example is the useof molten nitrate salt baths for the heat treatmentof metals.

If they are strongly heated the nitrates of sodiumand potassium give off oxygen and the metal nitrite:

2KNO2 = 2KNO2 + O2

Most other metal nitrates form the metal oxide,giving off nitrogen dioxide ('nitrous fumes') andoxygen.

Ammonium nitrate is widely used as an agricultur-al fertiliser under various trade names. It is a whitecrystalline solid, very soluble in water (all nitratesare soluble in water). It does not burn by itself, butmixing it with a fuel (e.g., sugar) produces a pow-erful explosive. It decomposes violently whenheated giving nitrous oxide and water:

NH4NO3 = N2O + 2H2O

Brown nitrous fumes (NO2) are also given off onheating; decomposition is complex. These fumesof nitrogen dioxide will support combustion in asimilar manner to oxygen.


Ammonium nitrate can detonate, but only largequantities, and under extreme conditions.

7.8.2 Permanganates

Of the permanganates, sodium (NaMnO4) andpotassium (KMnO4) permanganates are the mostcommon. They are powerful oxidising agents andmay react violently with oxidisable organic mate-rials. When mixed with glycerol (glycerine), spon-taneous ignition occurs. With concentratedhydrochloric acid, permanganates produce thehighly toxic chlorine gas as a result of oxidation.

7.8.3 Chlorates

Chlorates are often used as their sodium or potas-sium compounds. On heating, oxygen is released:

2KC1O3 2KC1 + 3O2

Very violent reactions occur on contact with oxi-disable materials and may occur merely by fric-tion.

Potassium perchlorate (KC1O4) has a similarchemical formula, but is, in fact, stable. Anhydrousperchloric acid (HC104) is a powerful oxidisingagent and will explode on heating. Sodium chlo-rate is used as a weed killer and has also been usedin home-made explosives.

7.8.4 Chromates and dichromates

The most common compounds of this type arepotassium chromate (K2CrO4) and potassiumdichromate (K2Cr2O7); these materials are yellowand orange respectively and are oxidising agents.They are soluble in water and will produce a high-ly combustible mixture with oxidisable sub-stances.

7.8.5 Inorganic peroxides

Peroxides are a group of compounds which con-tain a higher proportion of oxygen than the 'nor-mal' oxide. This extra oxygen is easily liberated,making these compounds good oxidising agents.Inorganic peroxides may be considered to derivefrom hydrogen peroxide (H2O2). Pure hydrogenperoxide is a clear viscous liquid with a specific

gravity of 1.46 (at ()°C). It is soluble in water andis used at various concentrations. Above 70 percent concentration in water it is a powerful oxidis-ing agent and decomposes explosively:

2H2O2 O2 + 2H2O

This decomposition may occur on heating, but canalso occur in by the presence of a catalyst: smalltraces of metallic dust, charcoal or even stronglight may be sufficient. Concentrated solutions ofhydrogen peroxide are often known as 'high testperoxide' (HTP).

Common metal peroxides, derived from hydrogenperoxide, are those of sodium (Na2O2) and barium(BaO2). Sodium peroxide is a pale yellow solidwhich reacts vigorously with water, releasingoxygen:

2Na2O2 + 2H2O O2 + 4NaOH

A great deal of heat is released in this reaction andthis could cause a fire in any nearby combustiblematerial. The fire would be made worse by theoxygen evolved.

Sodium peroxide can absorb carbon dioxide,releasing oxygen as a product:

2Na2O2 + 2CO2 2Na2CO3 + O2

7.8.6 Organic oxidising agents

When nitric acid reacts with organic compounds,two important types of substance are formed: organ-ic nitrates (-NO,) and nitro-compounds (-NO2).

These compounds are oxidising agents and fur-thermore they carry oxidisable carbon-containingmaterial within their own molecules.Consequently, both the organic nitrates and thenitro-compounds are highly flammable. Some thatcontain several nitrate or nitro groups in the mole-cule are explosive. Typical examples are glyceryltrinitrate (used in dynamite) and trinitrotoluene(TNT) - an important military explosive.

Most organic nitrates and nitro- compounds aretoxic and many of them, including glyceryl trini-trate, may be absorbed through the skin.


7.8.7 Organic peroxides and hydroperoxides

These compounds have a structure similar to that ofhydrogen peroxide (H2O2, arranged as H-O-O-H),with both hydrogen atoms replaced by organicgroups, thus forming an organic peroxide. If only onehydrogen is replaced, a hydroperoxide is formed.

As would be expected peroxides and hydroperoxidesare powerful oxidising agents and, because there is acarbon-containing part of the molecule which can beoxidised, they are highly flammable. Some are explo-sive and sensitive to heat and mechanical shock.Because of this they are often diluted or 'dampeddown' with either water or stable esters.

Peroxides are extensively used as catalysts, espe-cially in the plastics industry. They are toxic andare especially irritating to the skin, eyes andmucous membranes. Skin contact and breathing ofvapours should be avoided. In all respects, organicperoxides and hydroperoxides should be treatedwith extreme caution.



Chapter 8 - Simple organic substances

simplest member of paraffins, or alkanes, ismethane, the main constituent of natural gas. It hasthe formula CH4 and the structure of the moleculeis conveniently represented as:

The carbon atom uses each of its four valencies tojoin it to four hydrogen atoms which each have avalency of one. The CH4 molecule can also beregarded as a combination of the group:

called a methyl group

with a hydrogen atom. Methane has well-definedchemical and physical properties. It is a relativelyunreactive gas, although it is flammable and formsexplosive mixtures with air at concentrationsbetween 5% and 15% by volume. In common withother hydrocarbons, it burns completely to pro-duce carbon dioxide and water:

CH4 + 2O2 = CO2 + 2H2O.

Larger molecules are built up by linking the car-bon atoms together in chains. Hydrogen atoms areattached to the carbon atoms in accordance withthe valency rules. For example:

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Carbon forms a very large number of compounds,especially with hydrogen, oxygen, nitrogen andthe halogens. It forms so many compounds thatchemistry is divided into two branches:

• organic chemistry which deals with thechemistry of the carbon compounds; and

• inorganic chemistry which deals with thechemistry of all the other elements.

There are believed to be over a million stable car-bon compounds, which explains why a separatebranch of chemistry is necessary to study them.

Carbon atoms differ from almost every other typeof atom in that they are able to link up with othercarbon atoms and form chains and rings. Mostother atoms only join with others of the same kindin twos or threes. In all these organic compoundsthe valency of carbon is always four.

Organic chemicals are divided into two classes:

• aliphatic compounds, which contain chains ofcarbon atoms; and

• aromatic compounds which contain a specialkind of ring of six carbon atoms, known as abenzene ring.

Most organic chemicals are capable of burning.Our most important fuels, such as natural gas,petrol, paraffin and diesel oil are mixtures oforganic compounds which contain carbon andhydrogen - the hydrocarbons.

8.1 Aliphatic hydrocarbons(paraffins or alkanes)

The aliphatic hydrocarbons are a series of com-pounds containing only carbon and hydrogen. The

Ethane (C2H6) is another constituent of natural gas.Because the molecule is larger, the physical prop-erties are different from methane. The boilingpoint, melting point and vapour density of ethaneare higher than those of methane, whereas its spon-taneous ignition temperature is lower.

Another increment in the chain length of the mol-ecule results in propane (C,H8), a constituent ofliquefied petroleum gas.

• their physical properties vary in a regularway.

As the number of carbon atoms increases:

• melting point, vapour density, boiling pointand flash point increase; and

• solubility in water and spontaneous ignitiontemperature decrease.

8.2 Unsaturated aliphatichydrocarbons

8.2.1 Olefines or alkenes

There is another series of aliphatic compoundsknown as the olefins or alkenes. The first memberof the series is ethylene (C2H4), the formula forwhich is represented as:

where there is a double bond between the two car-bon atoms. The carbon still has its valency of 4 andhydrogen that of 1, but each carbon atom uses twoof its valency bonds to link to the other carbonatom. Compounds containing double bonds (andthose with triple bonds) are known as unsaturat-ed compounds.

Unsaturated compounds are much more reactivethan the paraffins. Not only do they burn, but theyreact readily with chlorine, hydrogen chloride,bromine, and other reagents. For example:

The reactivity of ethylene makes it an importantstarting material in the production of plastics andother synthetic materials (see Chapter 9).

Propane is chemically similar to ethane andmethane, but once again, the physical propertiesdiffer (see Table 8.1).

In principle, the carbon chain can be extendedindefinitely, until the chain consists of many thou-sands of carbon atoms, as in polyethylene (seeChapter 9). The longer the chain, the higher theboiling and melting points of the substance.Methane, ethane, propane and butane are all gasesat room temperature and pressure, but heavier mol-ecules, starting at pentane, C5H12, are liquids, andfrom hexadecane, C16H34 they are solids.Compounds near C8H18 (octane) are found inpetrol, those near C10H22, in paraffin, those nearC14H30 in diesel oil, those nearC18H38 in petroleumjelly (Vaseline) and those near C25H52 in paraffinwax. The following points may be noted concern-ing these compounds:

• they form a series in which each differs fromthe next by one -CH2 unit;

• they have similar chemical properties; and


Other olefins can be obtained by progressivelyincreasing the length of the carbon chain. As withthe paraffins, the physical properties alter in a reg-ular way as the size of the molecule increases (seeTable 8.2).

8.2.2 Acetylenes, or alkynes

Acetylenes contain a carbon-carbon triple bond.The only important member is the gas acetylene(C2H2). It is an unsaturated compound and the only

way of arranging the normal valency bonds of thecarbon and hydrogen is:

H-C C-HAcetylene

Here, each carbon atom uses three of the availablevalencies to make three bonds with the other. Thistriple bond makes acetylene very reactive: it canexplode on exposure to heat or mechanical shock,even when air or oxygen are absent. Acetylene is


flammable and forms mixtures in air with wideflammability limits (2.5% - 80%). Some of itsphysical properties are given in Table 8.3.Acetylene is used in the manufacture of plastics(e.g., PVC), certain chemicals and in oxyacetylenewelding. It is stored by dissolving it in acetone,which is absorbed on an inert porous material con-tained in cylinders.

8.3 Aromatic hydrocarbons

The simplest member of the aromatic hydrocar-bons is benzene, C6H6. It has a unique structure,consisting of six carbon atoms arranged in a ring,apparently with alternating single and doublebonds:


However, the six carbon atoms are linked in a spe-cial way, and benzene does not behave like anolefin. In fact, the benzene ring is remarkably sta-ble, so that aromatic compounds - those contain-ing the benzene ring, such as toluene - are lessreactive than olefins.

Benzene is a flammable liquid, but when it burns,the aromatic ring proves difficult to oxidisebecause of its stability: the high proportion of car-bon in the molecule leads to the formation of athick black smoke. It is no coincidence that theprecursors of smoke in flames from any fuel havebeen found to have an aromatic structure. Theseare formed in the flame in regions where tempera-tures are high, but the oxygen concentration is low.If the fuel already contains an aromatic structure, itcan be said quite categorically that it will burnwith a very smoky flame.

Other aromatic compounds are formed by replacingthe hydrogen atoms of benzene by other atoms orgroups of atoms, such as methyl radicals. For example:

Again, the physical properties of the members of thisseries vary in a regular way as the molecular weightincreases, and the chemical properties remain simi-lar. Some aromatic compounds, especially tolueneand the xylenes are important solvents.

It is worth noting that hydrocarbons do not dis-solve to any extent in water, but will float as theirspecific gravity is less than 1. Some aromatic com-pounds are toxic; benzene for example is highlytoxic, both as a vapour and by skin absorption ofthe liquid. Many, more complex, aromatic com-pounds have been identified as carcinogens.Similar compounds are to be found in smoke.

8.4 Liquefied petroleum gases(LPG)

Propane (C3H8) and butane (C4H10) are gases atroom temperature and pressure, but are easily liq-uefied using pressure alone. A very small amountof liquid will produce a great volume of gas and soby liquefying the gas a large amount can be storedin a small volume. As both gases are highly flam-mable and are widely used as fuel gases, installa-tions containing the liquid gases are very wide-spread.

The important property for LPG is the critical tem-perature - the temperature above which it isimpossible to liquefy a gas by pressure alone (seeSection 4.4). For propane, the critical temperatureis 96.7°C and for butane, 152°C. When in the rightkind of container they will evaporate, increasingthe pressure until there is no further net evapora-tion. Thus, inside each liquid gas container there isa liquid with pressurised vapour above it. As thegas is let out for use as fuel, more liquid evaporatesto keep the pressure of the vapour at its originalvalue.

As discussed, both propane and butane are highlyflammable. If propane liquid escapes, it will quick-ly boil into a large amount of flammable vapour: 1litre of liquid will produce 270 litres of vapour.

The vapours of both propane and butane are heav-ier than air and will seek lower ground; they areodourless and colourless, though very frequently, astenching agent (mercaptan) is added.


When propane and butane evaporate they takeheat from their surroundings. Propane has a boil-ing point of -42°C at atmospheric pressure, so atnormal ambient temperatures and pressures, theliquid boils easily. This applies to vast majority ofpractical situations. However, butane has a boil-ing point of around -1 °C. so that in winter condi-tions, the vapour pressure may be too low to pro-vide a flow of fuel vapour. For this reason, "LPG"generally consists of a mixture of propane andbutane.

The whole series of alcohols is formed by adding-CH2 groups to methanol:

• ethyl alcohol or ethanol (C2H5OH);

• n-propyl alcohol or n-propanol (C3H7OH);and

• n-butyl alcohol or n-butanol (C4H9OH).

As the molecular weight increases, there is a gen-eral increase in the:

• melting point;

• boiling point; and

• flash point.

This is accompanied by a decrease in the:

• solubility in water; and

• spontaneous ignition temperature.

The first few members of the series dissolve com-pletely in water but members higher than butylalcohol are only slightly soluble. All alcohols areless dense than water: the insoluble ones float.

Chemically the alcohols resemble each other. Thefirst members of the series are highly flammableliquids. Ethyl alcohol is sometimes used as a fueland is also used in rocket propulsion systems.Alcohols are also intermediaries in various chemi-cal processes. Methanol and ethanol are widelyused as solvents in industry and ethanol is the mostimportant ingredient of beer, wines and spirits.Propanol and butanol are also used as solvents andsome of the higher alcohols are used to makedetergents. Nearly all alcohols are to a greater orlesser extent toxic according to type.

Full details of the storage and fire-fighting tech-niques associated with liquefied petroleum gaseswill be found in the Manual of Firemanship,Part 6c.

8.5 Simple oxygen-containingcompounds derived fromhydrocarbons

There are many different types of organic com-pound which contain oxygen in addition to carbonand hydrogen. Some are reactive, and may beencountered in industrial processes (e.g., aldehy-des) whereas others are relatively unreactive andare used as solvents (e.g., ketones). The moreimportant types are discussed briefly.

8.5.1 Alcohols

The structure of the commonest alcohols is similarto that of paraffins, but with one of the hydrogenatoms replaced by a hydroxyl group O —H:


8.5.2 Aldehydes

These compounds all contain the group:

The higher aldehydes (i.e. containing more thantwo carbon atoms) are rather less important.The physical properties of these materials varyin the usual way as the molecular weightincreases.

8.5.3 Ketones

The simplest ketone is acetone [(CH3)2CO] whichhas the structure:

attached to such organic groups as methyl (CH3-),ethyl (C2H5-) and so on. The simplest member ofthe group is formaldehyde:

Acetone is by far the most commercially importantmember of the series. It is a colourless, highlyflammable liquid which is readily soluble in waterand has a minty smell. It is toxic to the extent thathigh concentrations have an anaesthetic effect andthe liquid dissolves the fats out of the skin, and socan give rise to dermatitis and skin irritations.Acetone is a very important industrial solvent formaterials such as paint removers, cellulose acetate,fats, waxes and acetylene (see Section 8.2).

The next member of the series is methyl ethylketone (MEK):

It is another important industrial solvent and close-ly resembles acetone, but with a higher flashpoint,etc.

8.5.4 Carboxylic acids

The carboxylic acids or 'fatty acids' are a group ofweak acids, related to the aliphatic hydrocarbonswith a similar chain structure. They all contain thegroup:


with the formula CH2O. Formaldehyde is a colour-less, flammable gas with a pungent suffocatingsmell, though it is more usual to find it as a 4%solution in water, which also contains a littlemethanol. This solution is called formalin andgives off flammable vapour if heated above itsflash point. This varies according to the formalde-hyde and methanol concentration. The vapour istoxic. Formaldehyde is used in the manufacture ofseveral plastics, as an antiseptic and as a preserva-tive of anatomical specimens.

The next member of the series, acetaldehyde(CH,CHO) has the structure:

and differs from formaldehyde by one CH2

group. It is a colourless liquid with a strongfruity smell. The compound dissolves readily inwater, is flammable and toxic and the vapourforms explosive mixtures with air. It is used asan intermediate in the manufacture of otherchemicals and plastics.

which may be found attached to various organicgroups such as methyl (-CH3) and ethyl (-C2H6).The first member of the carboxylic acid series isformic acid, HCOOH:

Formic acid is a colourless liquid with a pungentsmell. It is toxic and can cause burns on the skin.It is used in the textile industry, in electroplatingand in the leather and rubber industries.

The next member of the series is acetic acid(CH3COOH), which is present as a dilute solutionin vinegar. It is flammable, dissolves readily inwater and can burn the skin if concentrated. Thevapour and the concentrated acid are toxic. It isused as a solvent in chemical manufacture.

The properties of some of these acids are given inTable 8.4.

8.5.5 Esters

The esters can be thought of as being derived fromthe carboxylic acids by the replacement of the hydro-gen atom in the COOH group by a methyl or otherradical. For example, "ethyl acetate" (see opposite).

Esters are flammable, colourless liquids or solidsand are usually only slightly soluble in water, onwhich they float. They have fruity smells and areoften found in fruit and in scents. The higher solidmembers of the series are found in beeswax. Theesters are used as solvents, and in pharmaceuticals,perfumery and foodstuffs. Some properties of afew esters are given in Table 8.5.

8.5.6 Ethers

These all contain an oxygen atom -O- which joinstwo organic groups such as methyl or ethyl groups.The only one of commercial significance is diethylether (C2H5-O-C2H3). This substance is oftenreferred to merely as 'ether'. It is a colourless, high-ly flammable, volatile liquid with a characteristicsmell. It is less dense than water and immiscible init. It is toxic in high concentrations and at lowerconcentrations has an anaesthetic effect.


Diethyl ether has a boiling point of 34.5°C, a flashpoint of -4.8°C, flammable limits of 1.85 to 36.5%in air and a self-ignition temperature of 180°C. Itmay contain a substance known as ether peroxidewhich, if the ether is evaporated to dryness, cancause an explosion.



Chapter 9 - Polymers

9.1 Polymers or polystyrene:

Many organic solids, including wood, plastics andrubbers, are polymers. This means that the mole-cules of which they are composed consist of verylong chains of carbon atoms which can consist ofmany thousands of atoms.

For many years now, chemists have been able tocreate or synthesise polymer molecules in the lab-oratory. Many of these have passed into commer-cial use as plastics and synthetic rubbers. Polymersare formed by taking small molecules with two ormore reactive groups and arranging for these tolink up end to end and form long chains. For exam-ple in ethylene H2C=CH2, the double bond can be"opened" or broken to give:

stands for the benzene ring, which is the simplestaromatic structure (Section 8.3).

Polymers such as polyethylene, polypropylene andpolystyrene soften and eventually melt at tempera-tures in excess of 100-150°C Such materials arecalled thermoplastics.

Thermosetting plastics do not melt but breakdown and char on heating. In these plastics, thelong chains are also linked together sideways bycarbon-carbon bonds: the material is said to becross-linked. Figure 9.1 illustrates this point, thelines representing polymer molecules and theircross-links.

Here the symbol:

which will very rapidly combine with other mole-cules of the same type to produce polyethylene:

which consists of a long chain of -CH2- groupsjoined to each other. In a case such as this we callethylene the monomer, polyethylene the resultingpolymer and the process polymerisation.

Simple straight-chain polymers of this type arewell known; many of them have small side groupsattached to the chain, as in polypropylene:


Fig. 9.1 The cross-links of polymer molecules of ther-mosetting plastics.

Industry uses the fact that thermoplastics soften ormelt when they are processed. The same tech-niques can not be used in thermosetting plastics.These have to be processed as short chain mole-cules and then heated or a catalyst added to makethe molecules cross-link.

Plastics often have other materials mixed intothem to improve their properties or to make themcheaper:

• Inert fillers such as china clay, wood flourand carbon black, or, in laminated plastics,sheets of paper or glass-cloth;

• Fire retardants, to make the polymer moredifficult to ignite;

• Plasticisers, mixed with some thermoplasticsto make them more pliable (e.g., PVC insula-tion);

• Stabilisers to inhibit degradation due toatmospheric oxidation, attack by sunlight ordecomposition under conditions of mildheating; and

• Colouring materials.

9.2 Fire hazards


However, many synthetic polymers produce muchmore smoke than "traditional" materials such aswood, and the rate of fire growth may be muchgreater, particularly if the material melts and drips,spreading fire as a burning liquid.

9.2.1 Toxic and corrosive gases

If plastics only contain the elements carbon andhydrogen, or carbon, hydrogen and oxygen, themain toxic gas to be expected is carbon monoxide(CO) which is formed when all organic materialsare burned in quantity. The amount of CO pro-duced increases if there is a relative shortage ofoxygen. This gas is a well-known hazard to firefighters. As it is odourless, colourless and pro-duced in large quantities in fires in buildings, it isresponsible for most of the fire fatalities. It canalso cause the deaths of people who are confinedto unventilated rooms with faulty gas heaters.

Many other toxic products are produced from plas-tics containing only carbon, hydrogen and oxygen.A whole range of toxic and corrosive species areproduced under conditions of poor ventilation.Their nature depends on the structure of the poly-mer, and can include aldehydes and many otherpartially oxidised products.

Many plastics contain nitrogen in addition to car-bon, hydrogen and oxygen. Plastics in this catego-ry include cellulose nitrate, nylon, polyurethanefoams, melamine-formaldehyde plastics, urea-formaldehyde plastics, ABS (acrylonitrile-butadi-ene-styrene), some epoxy resins and nitrile rub-bers. (Note that certain "natural polymers" such aswool and silk also contain nitrogen.) The fire prod-ucts from these will contain nitrogen-containingspecies such as organic nitriles, hydrogen cyanideand NO2. All of these are toxic. Intense, well ven-tilated burning will convert most of the originalnitrogen into NO2.

The class of material known as polyurethanefoams (PUF) includes the "standard PUF" intro-duced in the 1970's as well as the newer "combus-tion modified foams" which began to appear in the1980's. The standard foams have been extensivelyinvestigated and are known to produce appreciableamounts of CO and HCN in the fire gases. Other,highly toxic nitrogen-containing products may be

produced, depending on the type of polyurethanefoam and the conditions of burning.

Chlorine is present in polyvinyl chloride (PVC)and certain related co-polymers, in neoprene and incertain types of self-extinguishing fibre-glass poly-ester resin. In PVC fires, almost all the chlorinegoes to form hydrogen chloride (HC1) gas in thefire gases. HC1 is both toxic and corrosive, havinga very sharp smell and forming a corrosive solutionwith water (hydrochloric acid). Apart from corrod-ing many metals, the acid may cause long termchanges in alkaline mortar. Ferroconcrete may bemuch less affected; nevertheless, copious washingafter incidents involving PVC is desirable. Otherchlorine containing polymers may also givehydrochloric acid gas and possibly other chlorinecontaining toxic compounds as well.

PTFE (poly-tetrafluoroethylene - 'Teflon') andsome related materials, such as 'Kel-F' and somesynthetic rubbers, sometimes known as "vitons'.contain fluorine.

9.2 3 Burning tars or droplets

Thermoplastics melt on heating and so in a fire mayform burning droplets which could help the fire tospread. Although polyurethane foams are not tech-nically thermoplastics, they do give burning dropsof tar in a fire. On the other hand PVC, which is athermoplastic, does not give burning droplets, butmerely forms a tarry coke-like product.

9.2.4 Exotherms

The polymerisation process may well produceheat. This is a problem for those manufacturersproducing the raw plastics (normally in pelletform for thermoplastics), but there is a highdegree of control of the processes involved, andit rarely causes a problem. However, the cross-linking or curing process involved in the manu-facture of thermosetting materials may also beexothermic; for example, if blocks ofpolyurethane foam are stored before the exother-mic curing process is complete, self-heating mayoccur, leading to spontaneous combustion(Section 7.7.2).

9.2.5 Catalysts

Various types of catalyst are used in polymerisationprocesses including acids, alkalis, complex organo-metallic compounds and organic peroxides.

Acids and alkalis present well known hazards. Anorgano-metallic catalyst may be in the form of aslurry in flammable solvents: some of these com-pounds, such as aluminium triethyl, react violentlywith water. Organic peroxides are oxidising agentsand, therefore, present a considerable fire risk.Under some conditions such materials can beexplosive.

9.2.6 Flammable solvents

Flammable liquids, such as acetone, methyl ethylketone, toluene, industrial alcohol and methylalcohol are widely used as solvents in variousprocesses and also as cleaning fluids, so may bepresent in fires on industrial premises. See alsoChapter 8.

9.2.2 Smoke

Fires involving materials which contain the aro-matic (benzene) ring structure (Section 8.3) willtend to produce large quantities of smoke. Theseinclude polyurethanes, phenol-formaldehyderesins, polystyrene (Section 9.1), polyesters, epoxyresins and polycarbonates.

Although PVC does not contain the aromaticstructure, it produces large amounts of smoke infires, because it decomposes in the solid phase toproduce aromatic structures.


9.2.7 Dusts

Some processes produce fine plastic dusts. Thesemay present an explosion hazard if dispersed asa suspension in air.

9.2.8 Self-extinguishing plastics

Many plastics are described as "self-extinguish-ing". While PVC and phenol-formaldehyde resinsare naturally so, others may be made self-extin-guishing by chemical changes in the polymer mol-ecule or by the use of special additives.

The term 'self-extinguishing' means that, while aflame may (or may not) cause the material to burn,it will not continue to burn if the applied flame isremoved. However, the term refers to the perfor-mance of plastics in a specific small scale test. In afire situation, it will burn if surrounded by otherburning materials, or perhaps on its own if a largeenough area of the plastic has been ignited to pro-duce flames which are self-sustaining.

this problem, monomers are flammable, andsome are toxic. It is also possible that somemonomers in bulk quantities could start to poly-merise in a fire with the ensuing added hazard ofthe heat given out by the polymerisation reaction.

Some of the more notable monomers used in theplastics industry are detailed below, but the num-ber of monomers is very large and ever increasing,so the list cannot be considered as exhaustive.

Acrylonitrile

• colourless, partially water-soluble, flamma-ble liquid with faint, pungent smell;

• polymerises explosively with some organicperoxides or concentrated caustic alkalis;

highly toxic: can be absorbed through theskin and also through leather; and

• used as a starting material in the manufactureof ABS (acrylonitrile-butadiene-styrene)plastics and certain synthetic rubbers.

Butadiene

vapour at room temperature and pressures,but easily liquefiable at room temperature;

• polymerises readily, especially in the pres-ence of peroxide catalysts or air;

• flammable; and

• slightly toxic, narcotic in high concentration.

Epichlorhydrin

• used in the manufacture of epoxy resins;

colourless, slightly water-soluble liquid withan irritating odour;

• polymerises exothermally with acids, basesand some salts; and

highly toxic material and in fires may pro-duce toxic gases including phosgene.

93 Monomer hazards

Monomers by definition are reactive compoundscapable of polymerisation. Some, like ethylene,do not polymerise very easily and need exactlythe right conditions of temperature and pressure,perhaps with a catalyst. Others, like styrene, maypolymerise by accident, due to the presence ofimpurities, water, heat or other causes, and whenthis happens a great deal of heat may be givenout. Some monomers have to be transported witha polymerisation inhibitor added to prevent theprocess occurring spontaneously.

As these monomers are mostly poor conductorsof heat, the heat cannot get away easily, tempera-tures may rise and a fire may result. In addition to

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Methyl methacrylate

clear liquid with acrid odour, used in themanufacture of acrylics (poly-(methyl-methacrylate));

flammable;

toxic;

polymerises exothermally with peroxide cat-alysts; and

normally stabilised, but heat accelerates thepolymerisation.

Styrene

used in the manufacture of polystyrene plas-tics and fibre-glass polyester resins;

slightly yellow liquid, strong smell;

normally stable, polymerisation greatlyaccelerated by heat or added peroxides;

exothermic polymerisation: risk of fire andeven explosion; and

moderately toxic, vapour is an irritant to theeyes.

Vinyl acetate

colourless, slightly water-soluble, flammableliquid, faint odour;

polymerises with organic peroxides or whenheated; and

low toxicity, may act as an eye irritant.

Vinyl chloride

sweet-smelling vapour at room temperatureand pressure, easily liquefied;

severe explosion hazard when exposed toheat or flame;

intermediates in silicone plastic manufacture;

mostly fuming clear liquids;

highly toxic;

Chlorosilanes

used as intermediates in polyurethane foammanufacture;

mostly brown liquids, slightly water-soluble,characteristic odour;

skin irritants, may cause dermatitis, toxic byskin absorption;

flammable - emit toxic gases when on fire;

isocyanate vapours cause bronchial spasmrepeated exposure may bring sensitisation;

great caution should be exercised in dealingwith them; and

made harmless using special solutions ofammonia in water, to which an emulsifyingagent has been added.

Isocyanates

93.1 Intermediates and hardeners

moderately toxic acts as an anaesthetic inhigh concentrations;

liquid may cause freeze burns due to rapidevaporation;

flammable; and

combustion gases contain hydrogen chloridewhich is both toxic and corrosive.


flammable; and

react with water to produce hydrogen chlo-ride gas - reaction is strongly exothermic inmany cases.

Epoxides

Amine hardeners - generally toxic. Somemay cause dermatitis.



Chapter 10 - Other Combustible Solids

10.1 Wood Many methods are available to reduce the com-bustibility of wood. The most successful involveimpregnating the wood with chemicals (e.g.,ammonium phosphates, etc.) which catalyse thechar-forming reaction at the expense of thedecomposition process which produces flamma-ble vapours. The resulting vapours are of verylow flammability (mainly a mixture of CO, CO2

and H2O) and will not support flame, or con-tribute significantly to a fire when other materi-als are burning.

10.2 Coal

Coal is a very complex mixture of carbon and avariety of resinous organic compounds. There aremany varieties, with harder coal containing morecarbon. Plates of inorganic, noncombustible mate-rials are also found in coal: these consist of lime-stone and compounds of iron, magnesium andmanganese.

In large heaps, such as those used for storage ofcoal at power stations, self-heating can occur,which may lead to spontaneous combustion (seesection 7.7). The smaller the coal particles, thethe greater the danger. It is promoted by mois-ture, and the greater the oxygen content of thecoal, the greater the danger, so that coal, espe-cially pulverised coal containing more than 10per cent of oxygen, may be dangerous in storage.In addition, coal dust can form an explosivemixture in air.

Storage heaps must be kept as free as possiblefrom excess of air and protected from externalsources of heat. Coal is sometimes sprayed witha high flash-point mineral oil that reduces dusti-ness and protects coal surfaces against oxidation.Fires in coal stacks are dealt with in the Manualof Firemanship, Part 6c.


Wood is a complex polymeric material of naturalorigin. In spite of the widespread use of syntheticmaterials, wood still accounts for a high propor-tion of the combustible material which is used inbuildings, not only as fittings and furniture, butalso as structural members. The principal con-stituent of wood is cellulose, a polymer of D-glu-cose, which occurs in all higher plants.

There is a high water content in wood and the dif-ference in moisture content between green andwell-dried wood is significant in regard to fire risk.Considerable quantities of heat are required to drytimber, due to the high latent heat of vaporisationof water.

When wood is heated, decomposition starts attemperatures of around 170°C, forming char,with the evolution of carbon dioxide, carbonmonoxide and water. The proportion of flamma-ble vapours released at this stage is low. Above300°C, the decomposition process which pro-duces flammable vapours becomes the dominantpyrolysis reaction, but they will be mixed withsome CO2 and H2O vapour from the char-formingprocess which still occurs, but no longer domi-nates. This mixture of gases and vapours is lessflammable than the decomposition productsfrom, for example, polyethylene which will be100% hydrocarbon, undiluted by non-flammablegases. This, and the fact that there is always a sig-nificant amount of char produced which providesprotection to the wood underneath, accounts forthe remarkable fire properties of wood. Forexample, (i) thick sections of wood cannot burnin isolation and (ii) thick timber beams can sur-vive longer in a fire than unprotected steel beams,because the char forms a protective layer aroundthe sound timber below.

103 Metals

Three-quarters of all the elements are metals. To achemist a metal is a substance which can lose elec-trons and form positive ions (ions are chargedatoms or groups of atoms - see Chapter 1). In addi-tion, metals tend to have a group of propertiesassociated with them; if an element possesses mostof them, we describe it as a metal.

103.1 Properties of metals

Metals will show most of the properties listed here,although there are exceptions to most of these. Forexample, most metals are malleable and ductile -can be hammered into shape and can be drawn outinto wire - but antimony is very brittle and willshatter if hammered!

(i) all, except mercury, are solids at room tem-perature, though they have a wide range ofmelting and boiling points;

(ii) they form positive ions;

(iii) they are malleable and ductile;

(iv) they are good conductors of heat and electric-ity;

(v) they can form alloys;

(vi) the oxides and hydroxides are basic (and socan be alkali solutions in water); and

(vii) most dissolve in mineral acids, normallyreleasing hydrogen.

Metals show a wide range of chemical properties,and range from dangerously reactive metals suchas sodium and potassium, to inert metals such asplatinum and gold. Metals can be arranged in an'Activity Series'(Table 10.1).

In this Table, the most reactive metals are at the topand the least reactive at the bottom. Whateverchemical property is considered, those metals atthe top of the series react most vigorously, indeedoften violently, and those at the bottom react slow-ly or not at all.

Although hydrogen is not a metal, it is included inthe Table as it also forms a positive ion. Manyimportant metal reactions involve displacement ofhydrogen either from water or from acids.

10.3.2 Reaction of metals with water or steam

It is obviously important for the firefighter tounderstand how these heated metals will react withwater as, usually, water will be the most readily


available firefighting medium. It will be shownthat for some metals, though, the addition of watercould be dangerous or disastrous.

Potassium to calciumThese metals react immediately with water torelease flammable hydrogen gas and leave a metalhydroxide. In some cases the hydroxide so formedis itself a corrosive alkali. In the case of potassiumthe reaction is so vigorous that the metal seems toignite immediately on contact with water. A smallpiece of sodium will move rapidly over the surfaceof water and if prevented from doing so, willignite. Larger pieces of these metals are in dangerof explosion on contact with water. Calcium reactssteadily with cold water but vigorously with hot.

Magnesium to ironThese metals react little with cold water, evenwhen powdered. At higher temperatures, the reac-tion rate increases and a steady flow of hydrogenis produced by reaction with steam. If the metalsare already burning the reaction with cold waterbecomes very fast, producing a lot of hydrogenwhich may lead to an explosion. Going down theseries, the rate of reaction decreases until, withiron, there is little reaction unless the red-hot metalis exposed to steam.

Cobalt to leadHere the white-hot metals must be treated withsteam before reaction will take place.

Bismuth to goldThese metals do not react with water or steam asthey are below hydrogen in the Activity Series.

10.3.3 Reaction with oxygen

Metals at the top of the Activity Series react mostreadily in air*. Sodium and potassium are so reac-tive that they are stored in paraffin oil to preventoxygen reaching them. Many other metals willburn in air or oxygen with increasing difficultygoing down the series. Even metals like tin andlead will burn at very high temperatures.

When a metal is powdered, it presents a very largesurface area compared with a block of the samemass. Combustion is made much easier as thepowder or dust particles are so small and can beheated extremely rapidly to the temperature atwhich they will burn. Some metallic dusts canburn or explode spontaneously when dispersedin air. When this occurs at ordinary temperaturesthe material is said to be pyrophoric (e.g., "RaneyNickel", which is a finely divided form of Nickel,used as a catalyst).

Many flammable metal powders and dusts arepyrophoric, especially magnesium, calcium, sodi-um, potassium, zirconium, hafnium. Some metalpowders will burn in carbon dioxide and nitrogen(e.g., magnesium) or under water. Metal powderswhen damp may also cause fires and explosions,even in the absence of air and often without warn-ing, and in the absence of heat.

10.4 Sulphur

This is usually found either as a yellow powder(known as 'flowers of sulphur') or as yellow crys-tals, but it is sometimes produced as blocks orsticks. It burns with a blue flame to give sulphurdioxide:

S + O2 = SO2

Sulphur is used in the manufacture of rubber, insulphur compounds, such as sulphur dioxide andsulphuric acid and in certain drugs. It has a lowtoxicity, but the dust presents an explosion haz-ard. Sulphur dioxide, however, is a highly toxicgas with a sharp pungent odour which can be eas-ily liquefied under pressure at ambient tempera-tures. It has many uses, especially as a bleachingagent and as a food preservative.

Hydrogen sulphide (H2S), which is also known assulphuretted hydrogen, is formed as a by-productfrom many chemical processes, including thedecomposition of organic sulphur compounds; forthis reason it is frequently found in sewer gases.Hydrogen sulphide has a characteristic odour ofrotten eggs and is highly toxic. It is flammableand under certain conditions can produce anexplosion risk.


* Note that reaction in pure oxygen is always much morevigorous than in air.

10.5 Phosphorus

The element phosphorus is extremely reactive andis found in nature combined with other elements,mostly as phosphates (compounds containing thePO4 group). It is also present in all living matter.The pure element exists in two different forms: redphosphorus and white (or yellow) phosphorus.Their properties are itemised in the Manual ofFiremanship, Part 6C, Section 16. White phospho-rus is extremely dangerous as it will ignite in air attemperatures as low as 30°C giving dense whiteclouds of toxic fumes of phosphorus pentoxide:

4P + 5O2 2P2O5

White phosphorus should never be touchedwith the bare hands as their warmth may causeignition; moreover, phosphorus burns heal veryslowly.

Red phosphorus is relatively safe if handled withcare, and is used in making safety matches. Thewhite form, because of its toxicity, is converted tophosphorus sulphide (P4S3) for use in non-safetymatches.

Inorganic phosphates are crystalline solids whichare normally safe unless one of the toxic metals isinvolved. Some are used as fertilisers.



Chapter 11 - Extinguishing Fires

ed into two groups: those that mix (are miscible)with water and those that do not (are immiscible).

Extinguishing agents are chosen according towhether the liquid fuel will mix with water or not.Agents which may be used include water spray,foam, light water, vaporising liquids, carbon diox-ide and dry chemical powders.

Class CThese are fires involving gases or liquefied gasesin the form of a liquid spillage, or a liquid or gasleak, and these include methane, propane, butane,etc. Foam or dry chemical powder can be used tocontrol fires involving shallow liquid spills,though water in the form of spray is generally usedto cool the containers.

Class DThese are fires involving metals. Extinguishingagents containing water are ineffective, and evendangerous. Carbon dioxide or dry chemical pow-ders containing bicarbonate will also be hazardousif applied to most metal fires. Powdered graphite,powdered talc, soda ash, limestone and dry sand arenormally suitable for Class D fires. Special fusiblepowders have been developed for fires involvingsome metals, especially the radioactive ones.

Electrical firesElectrical fires are not treated as a class of theirown, since any fire involving, or started by, elec-trical equipment must, in fact, fall into one of theother categories.

The normal procedure for dealing with an electri-cal fire is to cut off the electricity and use an extin-guishing method appropriate to what is burning.

If this cannot be done with certainty, special extin-guishing agents will be required which are non-


11.1 Classification of fires by type

The current British/European Standard BS EN 2:1992 Classification of fires defines four categoriesof fire, according to the type of material burning.

Class AThese are fires on solid materials, usually organic,leaving glowing embers. Class 'A' fires are the mostcommon and the most effective extinguishing agentis generally water in the form of a jet or spray.

Class BThese are fires involving liquids or liquefiablesolids. For the purpose of choosing effective extin-guishing agents, flammable liquids may be divid-

conductors of electricity and non-damaging toequipment. These include vaporising liquids, drypowders and carbon dioxide. Very fine water mistshave proven to be very effective at extinguishingfires using very little water, and their developmenthas been hastened recently as they are seen to bean environmentally friendly replacement forhalons. The rapid cooling that can be broughtabout by carbon dioxide extinguishers may affectsensitive electronic equipment - though it is thesmothering effect of the gas, rather than the cool-ing which extinguishes the fire.

11.2 Classification of fires by size

To describe the size of a fire, the Central FireBrigades Advisory Council has made the follow-ing recommendation:

Major fire 20-jets (or more)

Large fire 8-19 jets

Medium fire 3-7 jets

Small fire 1-2 jets, or 3 + hose reels

Minor fire 1-2 hose reels, or handextinguishers.

113 Extinguishing fire: Starvation,smothering, cooling

We have seen from the triangle of combustion (seeFigure 7.1), that three things are needed to allowburning to take place:


Fire extinction is largely a matter of depriving thefire of one or more of these factors, so methods ofextinguishing fire can be classified in terms ofremoving these factors:

In practice, fire extinguishing methods often usemore than one of these principles, but it will beconvenient to group them according to the mainprinciple involved.

11.3.1 Starvation

Fires can be starved of fuel (Figure 11.1, top) inthree ways:

1. By removing potential fuel from the neigh-bourhood of the fire. For example, by:

draining fuel from burning oil tanks;

working out cargo at a ship fire;

cutting trenches or creating fire breaks in, forexample, peat, heath and forest fires,demolishing buildings to create a fire stop;and

counter-burning in forest fires.

2. By removing the fire from the mass of com-bustible material - for instance, pulling apart aburning haystack or thatched roof.

3. By dividing the burning material into smallerfires which may be left to burn out or which can beextinguished more easily by other means. Thebeating out of a heath fire owes much of its effec-tiveness to this.

Figure 11 .1 The triangle of combustion.Top: starvation - or the limitation of the combustible material. Centre: smothering - or thelimitation of oxygen. Bottom: cooling - or the limitation of temperature.


11.3.2 Smothering

If the oxygen supply to the burning material can besufficiently reduced, burning will cease (Figure 11.1,centre).

The general procedure in methods of this type is totry to prevent fresh air from reaching the to the seatof the fire and so to allow the combustion to reducethe oxygen content in the confined atmosphereuntil it extinguishes itself. This is less effectivewhere, as in the case of celluloid, the burningmaterial contains within itself in a chemicallycombined form the oxygen it requires for combus-tion.

Smothering is the principle behind snuffing outcandles and capping oil well fires. The batteningdown of a ship's hold when a fire breaks out belowdecks will sometimes hold the flames in checkuntil port is reached. Small fires, such as thoseinvolving a person's clothing, can be smotheredwith a rug, blanket, etc., while the use of sand orearth on a small metal fire is a further instance ofthe same principle.

Foam is an important practical smothering agent.Foams form a "blanket" over the burning surfaceand so separate the fuel from the air, thus prevent-ing fuel vapours from mixing with air while at thesame time shielding the surface from direct heattransfer from the flames.

Fires can be smothered with a cloud of fine drypowder, usually sodium bicarbonate, shot from apressurised extinguisher, though research suggeststhat chemical interaction (inhibition) by the pow-der may be as important as the smothering action.

Another technique using the smothering principleis the use of ternary eutectic chloride powder foruse on metal fires. This is applied using a gas car-tridge pressurised extinguisher. The fusing temper-ature of the powder is in the region of 580°C, andit is applied to form a crust over the burning metaldepriving it of oxygen from the air.

Inert gases such as nitrogen and carbon dioxidecan be used to smother a flame temporarily. Ifthese gases are vigorously discharged in the imme-diate vicinity of the fire, the oxygen content of the

atmosphere may be reduced to such an extent thatburning cannot be supported. Patented mixtures ofinerting gases are now extensively used instead ofhalons in computer installations. "Total floodingsystems" are used to protect special risks such ascomputer installations and rare book collections inlibraries. This requires that the inerting gas isreleased into a closed space, as the appropriateinerting concentration must be reached followingthe discharge, and then maintained.

For larger fires, however, inerting agents aren't souseful, as the convection currents set up are suffi-ciently powerful to dilute the inert blanket beforethe extinguishing action can take effect. Strongwinds may have the same effect. Applying inertingagents in a liquid form, which is then vaporised bythe fire likely to be more effective, particularly asthe burning region is also cooled by this. However,inerting gases can be used to great effect inenclosed environments such as electrical cabinets.

Very fine water mists have been shown to be ableto extinguish fires using very small amounts ofwater. These have shown their worth in situationswhere halons would previously have been usedand on offshore installations. They act mainly byinerting: a great deal of water vapour is createdwhen water mist is discharged into a confinedspace with hot surfaces, and this smothers the fire.As the droplets are so small, they evaporate veryquickly, and can rapidly smother the flames. Theirsmall size which makes them such a good extin-guishing agent also means, unfortunately, that theyare easily swept away from the fire by opposing airmovement and so are not suitable for fighting larg-er fires in unconfined spaces.

In the 1970's and 80's, halogenated hydrocarbonsor halons were developed and used extensively asextinguishants. The first and probably the simplestof these was carbon tetrachloride, but it is toxicand its use was soon discontinued. A number ofothers of lesser toxicity were developed and foundfavour.

Many halons have been considered. They are iden-tified by numbers which denote how many carbonand halogen atoms are in the molecule. The firstdigit gives the number of carbon atoms, the secondgives the number of fluorine atoms, the third gives


the number of chlorine atoms and the fourth givesthe number of bromine atoms. A fifth digit may, ormay not, be present which gives the number ofiodine atoms. For example, bromochlorodifluoro-methane has a formula CF2ClBr, and so is knownas Halon 1211.

These vaporising liquids act partly as inertingblankets similar to those mentioned in the preced-ing section, but mainly by chemical interference(inhibition) with the chain reactions in the flame,"mopping up" free radicals.

Although extremely effective, halons are no longerused except under some exceptional circum-stances, as they are known to be very harmful tothe earth's protective ozone layer. No more will bemanufactured, though such as are already in stockwill continue to be used in fixed installationswhere there is perceived to be a special risk, suchas in certain military situations.

Fires can also be extinguished by separating thefuel from the flame by blasting it away. This iswhat happens when a candle is blown out and, ona larger scale oil well fires can be extinguished bythe blast from exploding dynamite. This methoddoes not work simply by depriving the flame offuel, but also by making the flame unstable whenair is supplied at high velocity in the vicinity of thefuel surface.

11.3.3 Cooling

If the rate at which heat is generated by combus-tion is less than the rate at which it is lost from theburning material, burning will not continue.(Figure 11.1. bottom).

So. to extinguish a fire by cooling, the rate atwhich heat energy is lost from the burning materi-al must be increased by removing some of the heatenergy. This reduces the temperature of the burn-ing mass, reducing the heat release rate.Eventually, the rate at which heat is lost from thefire may be greater than the rate of heat productionand the fire will die away.

Cooling the fuel is the main way in which water isused to extinguish fires. There are many varia-tions: for example, a tank fire involving a high

flashpoint oil (boiling point » 100"C) can beextinguished by a high velocity sprinkler spraywhich apparently produces a water-in-oil emulsionat the surface, thus causing rapid cooling. Thisneatly avoids the problem of water sinking to thebottom of the tank before it has had much effect onthe temperature of the surface layer.

When it is applied to a fire, the extinguishingmedium - water for example - itself undergoeschanges as it absorbs heat from the fire:

(a) its temperature will rise;(b) it may evaporate (boil);(c) it may chemically decompose (not water);

and(d) it may react chemically with the burning

material.

For the extinguishing medium to achieve maxi-mum effect, it is clear that the quantity of heatenergy absorbed when these changes occur shouldbe as high as possible. That is to say that, referringto the points above in order, in a good coolant, thefollowing properties should be as high as possible:

the specific heat capacity;

the latent heat of vaporisation; and

the heat of decomposition.

The action of water depends predominantly on (a)and (b), the latter being far more important: it takesabout six times as much heat to convert a givenmass of water at its boiling point into steam as isrequired to raise the temperature of the sameamount of water from the usual atmospheric valueto its boiling point. Water is most efficiently usedif it is applied to a fire in liquid form and in such away that as much as possible is converted to steam.The smothering effect of the steam produced at theseat of the fire is thought to play a part in assistingin the extinguishing process.


When the heat of a fire is considerable, as in itsearly stages, the steam formed will not be visible,but as the temperature falls the steam will con-dense above the fire. This is widely recognised byexperienced fire-fighters as a sign that a fire isbeing brought under control.

On the basis of thermal capacity and latentheat of vaporisation, water is an excellent fireextinguishant, since both figures are high. Thisfact, combined with its availability in largequantities, makes it by far the most useful fireextinguishant for general purposes. The role ofdecomposition is insignificant in the case ofwater, but certain substances, for example carbon-ates, absorb heat in this way (see the reference todry powder extinguishers under Section 2,'Smothering').

Water does not react with ordinary materials, butmay prove dangerous with some fuels, evolvingheat rather than absorbing it. Moreover, the reac-tion may result in the formation of a flammableproduct, thus adding fuel to the fire. The action ofwater on burning magnesium exemplifies boththese effects, since it reacts with the metal exother-mically (i.e., producing heat) with the formation ofhydrogen, which is readily ignited. In the case ofother media the reaction products may be undesir-able in other senses, as in the case of the halonswhich can produce toxic gases which can be haz-ardous in enclosed spaces.

11.4 Fire extinguishing media

11.4.1 Water

Water is the most efficient, cheapest and mostreadily available medium for extinguishing fires ofa general nature. It is used by fire brigades for themajority of fires, although the methods of applica-tion have undergone a number of improvements.

If more water is applied than is actually required tocontain and extinguish the fire, the surplus willdrain off and may seep through floors and perhapscause more damage to goods and property thanthat caused by the fire itself. Accordingly, themethod of applying water to a fire varies accordingto the size of the fire.


If only small quantities are required, portablewater extinguishers or hand pumps may be suffi-cient. Hose reels are used for larger fires. These arefed from a tank on the appliance and water ispumped through the tubing on the reels by meansof a built-in pump. For major fires, greater quanti-ties of water are necessary: the built-in pumps dri-ven by the vehicles' engines are often capable ofpumping up to 4500 litres per minute, giving thewater the necessary energy to provide adequatethrow to penetrate deep into a building.

A variation in the application of water can be madeusing nozzles that produce jets or sprays rangingfrom large size droplets down to "atomised" fog.Judicious use of this type of application can notonly cut down the amount of water used, minimis-ing water damage, but also ensure that it is used togreatest effect. Atomised spray (log) nozzles havebecome standard equipment on fire brigade appli-ances in the UK. They are quite effective whenused in the correct situations, but their range islimited. Special pumps and ancillary equipmentare used with high pressure fog, giving a greaterrange of application.

11.4.2 "Inert gas"

On cargo ships, a fire in a hold may be containedby "inerting" the space using the exhaust gasesfrom the ship's engines to displace air. These gaseshave low oxygen and high carbon dioxide concen-trations. (In the past, steam has been tried as a firesuppression agent to control fires in the petro-chemical industry, but it is very expensive, requir-ing a fixed installation and an available source ofhigh pressure steam.)

11.4.3 Foam

Firefighting foams have been developed primarilyto deal with the hazards posed by liquid fuel fires.

Although water is used for most firefighting inci-dents, it is generally ineffective against firesinvolving liquid fuels. This is because water has adensity that is greater than most flammable liquidsso, when applied, it quickly sinks below their sur-faces, often without having any significant effecton the fire.

Finished firefighting foams, on the other hand,consist of bubbles that are produced from a com-bination of a solution of firefighting foam concen-trate and water that has then been mixed with air.These air filled bubbles form a blanket that floatson the surface of flammable liquids. In so doing,the foam blankets help to knock down and extin-guish these fires in the following ways:

by excluding air (oxygen) from the fuel sur-face;

by separating the flames from the fuel sur-face;

by restricting the release of flammablevapour fro m the surface of the fuel;

by forming a radiant heat barrier which canhelp to reduce heat feedback from flames tothe fuel and, hence, reduce the production offlammable vapour; and

by cooling the fuel surface and any metal sur-faces as the foam solution drains out of thefoam blanket. This process also produces steamwhich dilutes the oxygen around the fire;

The main properties of firefighting foams include:

Expansion: the amount of finished foam pro-duced from a foam solution when it is passedthrough foam-making equipment;

Stability: the ability of the finished foam toretain its liquid content and to maintain thenumber, size and shape of its bubbles. Inother words, its ability to remain intact;

Fluidity: the ability of the finished foam to beprojected on to, and to flow across, the liquidto be extinguished and/or protected;

Contamination resistence: the ability of thefinished foam to resist contamination by theliquid to which it is applied;

Sealing and resealing: the ability of the foamblanket to reseal should breaks occur, and itsability to seal against hot and irregular shapedobjects;

Knockdown and extinction: the ability of thefinished foam to control and extinguish fires;and

Burn-back resistance: the ability of the fin-ished foam, once formed on the fuel, to stayintact when subjected to heat and/or flame.

The amount of air added to the foam solutiondepends on the type of equipment used. Hand-heldfoam-making branches generally only mix rela-tively small amounts of air into the foam solution.Consequently, these produce finished foam withlow expansion (LX) ratios, that is to say, the ratioof the volume of the finished foam produced bythe nozzle, to the volume of the foam solution usedto produce it, is 20:1 or less. Other equipment isavailable which can produce medium expansionfoam (MX) with expansion ratios of more than20:1 but less than 200:1, and high expansion foam(HX) with expansion ratios of more than 200:1 andpossibly in excess of 2000:1.

There are a number of different types of foam con-centrate available. Each type normally falls intoone of the two main foam concentrate groups, thatis to say, they are either protein based or syntheticbased, depending on the chemicals used to pro-duce them.

Protein based foam concentrates include:

Protein (P);Fluoroprotein (FP);Film-forming fluoroprotein (FFFP); andAlcohol resistant FFFP (FFFP-AR).

Synthetic based foam concentrates include:

Synthetic detergent (SYNDET);Aqueous film-forming foam (AFFF); andAlcohol resistant AFFF (AFFF-AR).

The characteristics of each of these foam concen-trates, and the finished foams produced from them,vary. As a result, each of them has particular prop-erties that makes them suitable for some applica-tions and unsuitable for others.

Various types of surface active agents (or surfac-tants) are added to many firefighting foam concen-


trates. These are used to reduce the amount of fuelpicked up by the finished foam on impact with fueland to increase the fluidity of the finished foam.Surface active agents are also used as foamingagents because they readily produce foam bubbleswhen mixed with water.

In film-forming foam concentrates, surface activeagents form an aqueous film of foam solutionwhich, in certain conditions, can rapidly spreadover the surface of some burning hydrocarbons toaid knockdown and extinction. This ability canmake them ideal for use in certain firefighting sit-uations such as aircraft crash rescue. However, theassociated foam blanket tends to collapse quickly,so providing very poor security and resistance toburnback.

Water-miscible liquids, such as some polar sol-vents, mix freely with water and can quickly attackfinished foams by extracting the water they con-tain. This rapidly leads to the complete destructionof the foam blanket. Fires involving these liquidscan be extinguished by diluting them with largequantities of water. However, containment of theresulting mixture can cause problems and theapplication of sufficient quantities of water toachieve extinction can take a long time.Consequently, 'alcohol resistant' foam concen-trates have been developed to deal with these par-ticular types of liquid.

Further technical detail regarding foam will befound in the Fire Service Manual - Volume 1 -'Fire Service Technology, Equipment andExtinguishing Media - Firefighting Foam' anddetails of operational use - in Volume 2 - FireService Operations - 'Firefighting Foam'.

11.4.4 Vapourising liquids

This category consists mainly of halons as dis-cussed in Section 11.3.2. Halons have the propertyof vapourising rapidly when released from theirpressurised container. The vapours are heavierthan air, but when entrained into the flames, theyinhibit the chain reactions and suppress flaming.

Due to environmental concerns, halons have large-ly been replaced with inerting gases (see Section11.4.5) and fine water mists.

11.4.5 Carbon dioxide and inert gases

At normal temperatures, carbon dioxide is a gas1.5 times as dense as air. It is easily liquefied andbottled in a cylinder, where it is contained under apressure of approximately 51 bars at normal tem-peratures. When discharged from the cylinder,cold CO2 vapour and some solid CO2 are expelledfrom the "horn", which rapidly cools in theprocess. The solid quickly sublimes, and some ofthe liquid CO2 evaporates to maintain the pressurein the cylinder. The gas, however, extinguishes bysmothering, effectively reducing the oxygen con-tent of the air. About 20 to 30 per cent is necessaryto cause complete extinction, depending on thenature of the burning material. In fact, materialswhich have their own oxygen "supply" will con-tinue to burn, as will any material that tends toreact with the carbon dioxide, such as burningmagnesium. Apart from these considerations, car-bon dioxide is quick and clean, electrically non-conducting, non-toxic and non-corrosive. Mostfabrics are unharmed by it.

For special risk situations, such as in transformerrooms and rare book collections in libraries, totalflooding of the compartment may be required. Forthis, fixed carbon dioxide installations may bebuilt in. However, although it is non-toxic, it is anasphyxiant at the concentrations necessary toextinguish a fire. The operation of total floodingCO2 systems requires prior evacuation of all per-sonnel.

Carbon dioxide is also available in bulk to fireauthorities, by special arrangement with certainmanufacturers who have agreed to supply tankerscontaining 10 tonnes of liquid to any fire onrequest.

11.4.6 Dry chemical powders

New problems have been produced for the fire-fighter by the use in industry of an ever wideningrange of risks and materials.

The rise of new plastics is one example of this, andthe fabrication of reactive metals such as titanium,zirconium and beryllium is another. Sometimes,water cannot be used; on most fires involvingburning metals, the result of applying water can be


disastrous, often leading to an explosion. Newmethods of extinction have had to be evolved.

Chief among these are the dry chemical powderswhich are stored in cylinders under pressure, orwhich can be ejected by the release of gas underpressure.

The basis of most of these is sodium bicarbonate,which, with the addition of a metallic stearate as awaterproofing agent, is widely used as an extin-guishant both in portable extinguishers and forgeneral application in large quantities. Apart fromstearates, other additives are sometimes used todecrease the bulk density and to reduce packing inthe cylinder. Dry powder is very effective at extin-guishing flame ("rapid knock-down"), and is par-ticularly valuable in tackling a fire involving anincident in which someone's clothes have beensoaked in flammable liquid, and ignited.

Dry chemical is expelled from containers by gaspressure and directed at the fire in a concentratedcloud by means of specially designed nozzles. Thiscloud also screens the operator from the flamesand enables a relatively close attack to be made.Dry chemical powder can also be supplied in poly-thene bags for metal fires, as it is more effective tobury the fire under a pile of bags which melt andallow the contents to smother the fire.

Dry chemical powders are also tested for theircompatibility with foam, as it was discovered thatthe early powders tended to break down foam. Thetwo can complement each other on fires wherefoam is the standard extinguishant.

Ternary eutectic chloride powders have beendeveloped for some metal fires, especially for theradioactive metals such as uranium and plutonium.These contain an ingredient which melts, thenflows a little and forms a crust over the burningmetal, effectively sealing it from the surroundingatmosphere and isolating the fire.

Some burning materials, such as metals, whichcannot be extinguished by the use of water, may bedealt with by means of dry earth, dry sand, pow-dered graphite, powdered talc, soda ash or lime-stone, all of which act as a smothering agent.

Dry sand may also be used to prevent burning liq-uids, including paints and oils, from flowing downdrains, basement lights, etc., and for confiningshallow layers of such liquids, thus permitting theuse of foam or spray branches. On no accountshould sand be used for extinguishing fires inmachinery, such as electric motors, since its usemay well necessitate dismantling the entiremachine for cleaning, even though the fire damageis negligible.

11.4.7 Blanketing

Another fire extinguishing method is blanketing,which deprives the fire of oxygen. This is espe-cially useful if someone's clothes are burning. Theperson should be laid down and covered or rolledin a rug, coat, jacket, woollen blanket, etc.

For dealing with fires in small utensils, such asthose containing cooking fats, the best method is tosmother the fire with a fire resisting blanket, or acloth or doormat which has been wetted first.

11.4.8 Beating out

Small fires in materials, such as textiles, etc., mayoften be extinguished by beating them out, or byrolling and screwing up the burning material tight-ly to exclude the air. Beating is also the methodnormally employed to extinguish heath, crop andother similar fires in rural areas when water is notreadily available.


APPENDIX A

MetricationList of SI units for use in the fire service.

Quantity and basicor derived SI unitand symbol

Lengthmetre (m)

Areasquare metre (m2)

Volumecubic metre (m3)

Volume, flowcubic metre per second(m3/s)

Masskilogram (kg)

Velocitymetre per second (m/s)

Accelerationmetre per second2 (m/s2)

ForceNewton (N)

Approved unit ofmeasurement

kilometre (km)metre (m)millimetre (mm)

square kilometre (km2)square metre (m2)square millimetre (mm2)

cubic metre (m3)litre (1) (= 103m3)

cubic metre per second(m3/s)litres per minute(l/min= 10-3m3/min)

kilogram (kg)tonne (t)(1 tonne = 103kg)

metre/second (m/s)International knot (kn)

kilometre/hour (km/h)

metre/second2

kiloNewton (kN)Newton (N)

Conversion factor

1 mile = 1.609 km1 yard = 0.914m1 foot = 0.305m1 inch = 25.4 mm

1 mile2 = 2.590 km2

1 yard2 = 0.836 m2

1 foot2 = 0.093m2

1 inch2 = 645.2 mm2

1 cubic foot = 0.028 mJ1 gallon = 4.546 litres

1 foot-Vs = 0.028 m3/s1 gall/min = 4.546 1/min

1 lb = 454 kg1 ton= 1.016 t

1 foot/second = 0.305 m/s1 Int. knot = 1.852 km/h1 UK knot = 1.853 km/h1 mile/hour =1.61 km/h

1 foot/second2 = 0.305 m/s2

'g' = 9.81 m/s2

1 ton force = 9.964 kN1 lb force = 4.448 N


Quantity and basicor derived SI unitand symbol

Energy, workJoule (J)(= 1 Nm)

Powerwatt (W)(= 1 J/s = 1 Nm/s)

Pressurenewton/metre2 (N/m2)

Heat, quantity of heatJoule (J)

Heat flow ratewatt (W)

Specific energy, calorificvalue, specific latent heatjoule/kilogram (J/kg)

Temperaturedegree Celsius (°C)

Approved unit ofmeasurement

joule (J)kilojoule (kJ)kilowatt-hour (kWh)

kilowatt (kW)watt (W)

bar= 105 N/m2

millibar (m bar)(= 102 N/m2)metrehead

joule (J)kilojoule (kJ)

watt (W)kilowatt (kW)

kilojoule/kilogram (kJ/kg)kilojoule/m3 (kJ/m3)joule/m3 (J/m3)megajoule/m3 (MJ/m3)

degree Celsius (°C)

Conversion factor

1 British thermal unit= 1.055 kJ1 foot lb force = 1.356 J

1 horsepower = 0.746 kW1 foot lb force/second= 1.356W

1 atmosphere =101.325 kN/m2 =1.013 bar1 lb force/in3 =6894 76 N/m2 = 0.069 bar1 inch Hg = 33.86 m bar1 metrehead = 0.0981 bar1 foothead = 0.305 metrehead

1 British thermal unit= 1.055 kJ

1 British thermal unit/ hour= 0.293 W1 British thermal unit/ second= 1.055 kW

1 British thermal unit/ lb= 2.326 kJ/kg1 British thermal unit/ft3

= 37.26 kJ/m3

1 degree centigrade =1 degree Celsius


APPENDIX B

List of the elements with atomic number atomic weight and valency

Name ofelement

ActiniumAluminiumAmericiumAntimonyArgon

ArsenicAstatineBariumBerkeliumBerylliumBismuthBoron

BromineCadmiumCalciumCaliforniumCarbonCeriumCaesiumChlorineChromiumCobalt

CopperCuriumDysprosiumEinsteiniumErbiumEuropiumFermiumFluorineFranciumGadoliniumGalliumGermaniumGoldHafniumHelium

HolmiumHydrogenIndium

AcAlAmSb

ArAsAt

BaBkBeBiBBrCd

Ca

CfCCeCsClCrCo

CuCmDyEsEr

EuFmFFrGdGaGeAu

HfHeHo

HIn

891395511833855697

483

535482098

65855172427299666996863

100

9876431327972

267

149

227.027.0

243.0122.040.0

75.0210.0137.0249.0

9.0209.0

11.080.0

112.040.0

251.012.0

140.0133.035.552.059.063.5

247.0162.5254.0167.0152.0257.0

19.0223.0157.070.073.0

197.0178.5

4.0165.0

1.0115.0

33

3,4 ,5 ,63,50

3,51,3,5,723,423,531

22

23,4112,3,6

2,31,233

3

2,3

1132,34

1,340313

Symbol Atomicnumber

Atomicweight

Valency


IodineIridiumIronKryptonLanthanumLawrencium

LeadLithiumLuteciumMagnesiumManganeseMendeleviumMercuryMolybdenumNeon

NeptuniumNickelNiobiumNitrogenNobeliumOsmiumOxygenPalladiumPhosphorusPlatinumPlutoniumPoloniumPotassiumPraseodymiumPromethiumProtactiniumRadiumRadonRheniumRhodiumRubidiumRutheniumSamariumScandiumSeleniumSiliconSilver

I

IrFeKrLaLwPbLiLuMgMnMd

HgMoNeNpNiNbNNoOs

OPdPPtPuPoKPrPmPaRaRnReRhRbRuSmScSeSi

Ag

5377263657

10382

3711225

101804210932841

7

10276

84615789484195961918886754537446221341447

127.0192.056.084.0

139.0257.0207.0

7.0175.024.055.0

256.0201.0

96.020.0

237.059.093.014.0

253.0190.0

16.0106.031.0

195.0242.0210.0

39.0141.0145.0231.0226.0222.0186.0

103.085.5

101.0150.045.079.028.0

108.0

1

3,42,303

2,4

1322 ,3 ,4 ,6 ,7

1,23.4,604,5 ,62,33,53,5

2,3 ,4 ,82

2,4,63,52,43,4,5,62,3,41

3

35202 ,3 ,4 ,6 ,7

313,4,6 ,82,332,4,641

SymbolName ofelement

Atomicnumber

Atomicweight

Valency


APPENDIX B continued

Name ofelement

Sodium

StrontiumSulphurTantalumTechnetiumTelluriumTerbiumThalliumThoriumThuliumTinTitaniumTungstenUraniumVanadiumXenon

YtterbiumYttrium

ZincZirconium

Na

SrSTa

TcTeTbTlThTmSnTiW

UVXeYbYZn

Zr

11

3816

7343526581906950227492235470393040

23.088.0

32.0181.099.0

128.0159.0204.0232.0169.0119.048.0

184.0238.0

51.0131.0173.0

89.065.091.0

122,4,65

6,72,4,6

31,343

2,43,4

64,63,502324

Symbol Atomicnumber

Atomicweight

Valency


Suggestions for further reading

Books about Combustion, Flame and FireJ.F. Griffiths and J.A. Barnard "Flame andCombustion". Blackie Academic andProfessional.

"Thermal Radiation Monograph" The Institutionof Chemical Engineers.

ReferencesD.D. Drysdale "An Introduction to FireDynamics". John Wiley 1985.

J.F. Griffiths, J.A. Barnard "Flame andCombustion" Blackie Academic and Professional1995.

G.B. Grant D.D. Drysdale "A Review of theExtinction Mechanisms of Diffusion FlameFires". Fire Research and Development GroupPublication Home Office 6/96.

Books about PhysicsMuncaster "A-Level Physics". Stanley Thornes(Publishers) Ltd.Nelkon and Parker "Advanced Level Physics".Heinemann Educational Books

Books about ChemistryOpen University Foundation Course in Science

GeneralI. Asimov, "Asimov's New Guide to Science".(Penguin Books, 1985).

"The SFPE Handbook of Fire ProtectionEngineering" (second edition). Society of FireProtection Engineers, Boston, Massachusetts,USA.

AcknowledgementThe author would like to thank the Editor of theFire Engineers Journal for allowing portions ofthe article "Flames in Fires and Explosions" byJ. R. Brenton and D.D. Drysdale to be reproducedherein.


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