Fire Behaviour

Chemistry of Combustion

Aim

To provide students with information to give them an understanding of the behaviour of fire.

Learning Outcomes At the end of the session students will be able to:

• Describe the characteristic development and behaviour of a range of fires

• Describe the combustion process within a range of fires and the contribution made by different fuels to fire growth and development

• Recognize and interpret the characteristic development and behaviour of a compartment fire.

The fire triangle.

The triangle

of combustion.

• Conduction

• Convection

• Radiation.

3 methods of heat transfer

Heat transfer

• Metals conduct - some better than others, silver & copper are best

• Electrical insulators do not conduct heat well (plastics)

• Liquids and gases do not conduct heat well.

This shows

how a fire

may be

spread in a

building due

to the

conduction

of heat

along an

unprotected

steel girder.

Convection

• Only occurs in liquids and gases

• Central heating systems

• Chimney stacks

• Shafts in buildings• In fire situations in a room -mushrooming.

Convection in heated water.

Central heating system using convection.

Convection causes the updraft in chimneys.

This shows how fire can spread by convection.

Radiation

• Heat is transmitted by infra-red radiation

• Travels in straight lines

• Casts shadows

• Will transmit through some materials and not others

• Absorption

• Reflection.

The inverse square law as applied to radiation

Double the distance - quarter the radiation.

How fire can start from radiated heat.

Chemistry of combustion

• Atom - smallest particle• Molecule - atoms chemically bound

together• Elements - contain one sort of atom• Compounds - formed from one type of

molecule which contains more than one kind of atom

• Mixtures - contain more than one kind of molecule.

Combustion• Flashpoint

• Firepoint

• Fireball

• Spontaneous ignition temperature

• Spontaneous combustion

• Smouldering.

Development of compartment fire• Fire starts• Gases rise - form plume• Convection takes place - mushrooming• Plume radiates heat outwards• Other fuels start to generate flammable

gases• Conditions ripe for flashover.

The fire plume.

Smoke layer

at top of room.

Products of combustion

• Heat

• Smoke

• Light

• Gases that are toxic/noxious/irritant

• Carbon monoxide

• Carbon dioxide

• Water vapour.

Safety

• Wear full firefighting kit (inc. gloves)

• Wear BA with balaclava

• Use recognised BA procedures

• Limit exposure to high levels of heat and humidity.

Confirmation Assessments will be based on this lesson and the corresponding study note

Learning Outcomes• Describe the characteristic development and

behaviour of a range of fires• Describe the combustion process within a

range of fires and the contribution made by different fuels to fire growth and development

• Recognise and interpret the characteristic development and behaviour of a compartment fire.

THE END