Fire Behaviour

Ventilation

Aim

To provide students with information

to give them an understanding of the behaviour of fire.

Learning OutcomesAt the end of the session students will be able to:

• Describe the effects of forced and natural ventilation on the development of a range of fires.

Ventilation

Defined as;

The removal of heated air, smoke and other airborne contaminants from a structure, and their replacement with a supply of fresher air.

Ventilation• Self ventilation

• Automatic ventilation

• Tactical ventilation

Before fire control

After control but before fire extinction

After fire extinction.

Smoke• Mixture of fine particles, water droplets,

other liquids and gases given off by the burning material

• Usually toxic

• Can be hot

• Can burn.

Value of ventilation

• Assist escape

• Aid rescue operations

• Improve firefighters’ safety

• Earlier entry

• Reduce damage

• Restrict fire spread.

The effect

of a broken

window.

When to use ventilation

• A tactic to be used by the Officer in charge

• Vertical ventilation

• Horizontal ventilation

• Defensively

• Offensively.

Smoke layer

in an atrium.

Vertical ventilation

Advantages;

• Minimize risk of backdraught

• Minimize fire spread

• Provide rapid smoke clearance

Disadvantages;

• Need to work to make the vent from above the fire.

The stack

effect.

Vertical ventilation – Offensive.

Vertical ventilation – Defensive.

Making vents

• Use any inbuilt systems

• Use roof lights / windows

• Lift slates or tiles

• Work from ladder or aerial appliance

• Work from below the vent.

Vertical ventilation(1)

Making a

roof vent.

Vertical ventilation(2)

Clearing the escape route and improving access for firefighters.

Horizontal ventilation

• Most used type of ventilation

• Use where vertical not suitable

• Open exhaust vent on downwind side

• Inlet vent on upwind side

• Only open any vent as part of the ventilation plan.

Horizontal ventilation – Offensive.

Offensive ventilation.

Defensive ventilation.

Making vents

• Open windows where possible

• Cover exit vents with jets

• Ensure no-one working above vent

• Fresh air may result in backdraught, withdraw all personnel from danger area

• Glass may travel long distances.

Inlet and outlet vents - Same window

Viewed

from

inside

the

room.

Inlet and

outlet vents

Different

windows.

Making a vent

Working from below and to the side.

Forced ventilation - Advantages

• Smoke removed rapidly

• Makes horizontal ventilation more effective

• Reduces the need for vertical ventilation

• Less affected by wind conditions

• More controllable.

Negative pressure ventilation.

PPV

Wide opening Extra air pressure needed.

Forced ventilation - Disadvantages

• It requires mechanical device

• Can increase the intensity of the fire

• If large compartment - large fan or several smaller ones needed

• Time to set up

• When used defensively it means a room by room approach.

Main techniques

• Positive pressure ventilation (PPV)

• Negative pressure ventilation (NPV)

• Heating, ventilation and air conditioning systems

• Powered exhaust systems.

Defensive PPV - Advantages

• Rapid removal of gases

• Cooler and easier working conditions

• Hot spots seen easier - have hose line to deal with.

PPV sequence of operations• Isolate unaffected areas

• Position the fan

• Crew members to open exhaust vent

• Start the fan

• Check smoke taking expected route

• Monitor until fan stopped.

Offensive PPV

• Used before the fire is out

• Part of firefighting tactics

• If not used effectively can cause fire spread.

Summary• Note and use prevailing wind

• Consider and use correct tactic

• Do firefighters need to withdraw

• Outlet first - high and downwind

• Cover outlet vents with charged hoselines

• Start inlet vent

• Communications

• Monitor.

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

Learning Outcomes• Describe the effects of forced and

natural ventilation on the development of a range of fires.

THE END