5 fire behavior. 5 objectives (1 of 4) describe the chemistry of fire. define the three states of...
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5
Objectives (1 of 4)
• Describe the chemistry of fire.
• Define the three states of matter.
• Describe how energy and work are interrelated.
• Describe the conditions needed for a fire.
• Explain the chemistry of combustion.
• Describe the products of combustion.
5
Objectives (2 of 4)
• Explain how fires can spread by conduction, convection, and radiation.
• Describe the four methods of extinguishing fires.
• Define Class A, B, C, D, and K fires.
• Describe the characteristics of solid-fuel fires.
5
Objectives (3 of 4)
• Describe the ignition phase, growth phase, fully developed phase, and decay phase of a fire.
• Describe the characteristics of a room-and-contents fire.
• Explain the causes and characteristics of flameover, flashover, thermal layering, and backdraft.
5
Objectives (4 of 4)
• Describe the characteristics of liquid-fuel fires.
• Define the characteristics of gas-fuel fires.
• Describe the causes and effects of a boiling liquid expanding vapor explosion (BLEVE).
• Describe the process of reading smoke.
5
Introduction
• Fire has been around since the beginning of time.
• Destruction of lives and property by uncontrolled fires has occurred since just as long.
5
The Chemistry of Fire
• Understanding the conditions needed for a fire to ignite and grow will increase your effectiveness.
• Being well trained in fire behavior will allow the fire fighter to control a fire utilizing less water.
5
What Is Fire?
• Rapid chemical process that produces heat and usually light
• Fire is neither solid nor liquid.
• Wood is a solid, gasoline is a liquid, and propane is a gas—but they all burn.
5
Solids
• Definite shape
• Stokes most uncontrolled fires
• Expands when heated and contracts when cooled
5
Liquids
• Assume the shape of their containers
• Most will turn into gases when sufficiently heated
• Has a definite volume
5
Gases
• Have neither independent shape nor volume
• Expand indefinitely• Mixture of gases in air maintain a constant
composition– 21% Oxygen– 78% Nitrogen– 1% Other gases
5
Fuel
• Form of energy
• Energy released in the form of heat and light has been stored before it is burned
5
Chemical Energy
• Energy created by a chemical reaction.
• Some of these reactions produce heat and are referred to as exothermic reactions.
• Some of these reactions absorb heat and are referred to as endothermic reactions.
5
Mechanical Energy
• Converted to heat when two materials rub against each other and create friction
• Heat is also produced when mechanical energy is used to compress air in a compressor.
5
Electrical Energy
• Produces heat while flowing through a wire or another conductive material
• Other examples of electrical energy– Heating elements– Overloaded wires– Electrical arcs– Lightning
5
Light Energy
• Caused by electromagnetic waves packaged in discrete bundles called photons
• Examples of light energy– Candles– Light bulbs– Lasers
5
Nuclear Energy
• Created by nuclear fission or fusion– Controlled (nuclear power plant)– Uncontrolled (atomic bomb explosion)– Release radioactive material
5
Conservation of Energy
• Energy cannot be created or destroyed by ordinary means.
• Energy can be converted from one form to another.– Chemical energy in gasoline is converted to
mechanical energy when a car moves along a road.
5
Conditions Needed for Fire
• Three basic factors required for combustion:– Fuel– Oxygen– Heat
• Chemical chain reactions keep the fire burning.
5
Chemistry of Combustion (1 of 2)
• Compounds of atoms and molecules
• Almost all fuels are hydrocarbons– Consist of both hydrogen and carbon atoms– Wide variety of other molecules that release
toxic by-products
• Incomplete combustion produces large quantities of deadly gases
5
Products of Combustion
• Combustion produces smoke and heat.
• Specific products depend on:– Fuel– Temperature– Amount of oxygen available
• Few fires consume all available fuel.
5
Smoke
• Airborne products of combustion
• Consists of:– Ashes– Gases– Aerosols
• Inhalation of smoke can cause severe injuries.
5
Smoke Contents (1 of 2)
• Particles– Solid matter consisting of unburned, partially,
or completely burned substances
• Vapors– Small droplets of liquids suspended in air– Oils from the fuel or water from suppression
efforts
5
Smoke Contents (2 of 2)
• Gases– Most gases produced by fire are toxic.– Common gases include:
• Carbon monoxide• Hydrogen cyanide• Phosgene
5
Conduction
• Heat transferred from one molecule to another (direct contact)
• Good conductors absorb heat and transfer it throughout the object.
5
Convection
• Circulatory movement in areas of differing temperatures
• Creates convection currents
5
Convection Within a Room
• Hot gases rise, then travel along the ceiling.
• Convection may carry the fire outside the room of origin
5
Radiation
• Transfer of heat in the form of an invisible wave
• Travels in all directions
• Is not seen or felt until it strikes an object and heats its surface
5
Methods of Extinguishment
• Cool the burning material.
• Exclude oxygen.
• Remove fuel.
• Break the chemical reaction.
5
Class B
• Involve flammable or combustible liquids– Gasoline– Kerosene– Oils
• Shut off the fuel supply or use foam to exclude oxygen from the fuel
5
Class C
• Involve energized electrical equipment
• Attacking a Class C fire with an extinguishing agent that conducts electricity can result in injury or death.
5
Class D
• Involve combustible metals– Sodium– Magnesium– Titanium
• The application of water will result in violent explosions
• Must be attacked with special agents
5
Class K
• Involve combustible cooking oils and fats
• Special extinguishers are available to handle this type of fire.
5
Solid Fuels
• Most fires encountered involve solid fuels.
• Do not actually burn in the solid state– Must be heated or pyrolyzed to decompose
into vapor– May change directly from a solid to a gas
• Wood does not have a fixed ignition temperature
5
Ignition Phase
• Fuel, heat, and oxygen are present.
• Flame produces a small amount of radiated energy.
• Convection and radiation heat the fuel.
5
Growth Phase
• Kindling starts to burn, increasing convection of hot gases upward.
• Energy radiates in all directions.
• Major growth in an upward direction
5
Fully Developed Phase
• Produces the maximum rate of burning
• Fire will burn as long as fuel and oxygen remain.
5
Key Principles of Solid-Fuel Fire Development (1 of 2)
• Hot gases and flame tend to rise.• Convection is the primary factor in
spreading the fire upward.• Downward spread occurs primarily from
radiation and falling chunks of flaming material.
• If there is no remaining fuel, the fire will go out.
5
Key Principles of Solid-Fuel Fire Development (2 of 2)
• Variations in the direction of fire spread occur if air currents deflect the flame.
• The total material burned reflects the intensity of the heat and the duration of the exposure to the heat.
• An adequate supply of oxygen must be available to fuel a free-burning fire.
5
Room Contents
• Synthetic products prevalent today made from petroleum products.– These produce dense smoke that can be
highly toxic.
• Newer paints
• Carpets
• Furniture
5
Ignition Phase
• Flame begins small and localized
• Convection of hot gases is the primary means of fire growth
• Fire could probably be extinguished with a portable fire extinguisher
5
Growth Phase
• Additional fuel is drawn into the fire.
• Convection current carries hot gases to the ceiling
• Flames spread upward and outward
• Radiation starts to play a greater role
• Growth is limited by the fuel and oxygen available
5
Fully Developed Phase
• Flammable materials are pyrolyzed.
• Volatile gases are being released.
• Flashover– All combustible materials in a room ignite at
once.– Temperatures can reach 1000 °F.– Fire fighters cannot survive for more than a
few seconds in a flashover
5
Decay Phase
• Burning decreases to the point of smoldering fuel
• May continue to produce a large volume of toxic gases
5
Flameover (Rollover)
• Flaming ignition of hot gases layered in a developing room or compartment fire
• Flames can extend throughout the room at the ceiling level
5
Thermal Layering
• Gases rise and form layers
• Thermal balance – Water applied to a fire creating steam– Steam displaces hot gases at the top of the
room
• Ventilate while attacking the fire
• Avoid directing water at the ceiling
5
Backdraft (1 of 3)
• Requires a “closed box”
• Explosion that occurs when oxygen is suddenly admitted to a confined area that is very hot and filled with combustible vapors
5
Backdraft (2 of 3)
• Signs of an impending backdraft:– Confined fire with a large heat build-up– Little visible flame from the exterior– “Living fire”– Pressurized smoke– Smoke-stained windows– Turbulent smoke– Ugly yellowish smoke
5
Backdraft (3 of 3)
• Prevention of backdrafts:– Ventilate at a high level to allow superheated
gases to escape before or just as additional oxygen is introduced.
– Well-coordinated fire attack
5
Liquid-Fuel Fires (1 of 2)
• A liquid must be converted to a gaseous state before it will burn.
• Conditions required for ignition:– Fuel–air mixture within flammable limits– An ignition source with sufficient energy– Sustained contact between ignition source
and fuel–air mixture
5
Liquid-Fuel Fires (2 of 2)
• Flammability is determined by the compound with the lowest ignition temperature
• Flash point is the lowest temperature at which vapor is produced
• Flame point (or fire point) is the lowest temperature at which sufficient vapors are produced
5
Gas-Fuel Fires (1 of 2)
• Vapor Density– Weight of a gas fuel– Gas with vapor density less than 1 will rise.– Gas with vapor density greater than 1 will
settle.– Knowing vapor density helps predict where
the danger of ignition will be.
5
Gas-Fuel Fires (2 of 2)
• Flammability limits– Below the lower flammability limit
• Too little fuel = too lean
– Above the upper flammability limit• Too much fuel = too rich
5
BLEVE (1 of 2)
• Boiling liquid, expanding vapor explosion
• Occurs when a vessel storing liquid fuel under pressure is heated excessively
5
BLEVE (2 of 2)
• Vessel is heated.
• Internal pressure rises past ability to vent.
• Temperature exceeds the boiling point of the liquid causing the vessel to fail.
• Liquid immediately turns into a rapidly expanding cloud of vapor.
• Vapor ignites into a huge fireball.
5
Smoke Reading (1 of 4)
• Enables the fire fighter to learn where the fire is, how big it is, and where it is moving
• Fires are dynamic events.
• Smoke is the fuel all around you at a fire.
• The best place to observe patterns of smoke is outside of the fire building.
5
Smoke Reading (2 of 4)
• Determining the key attributes of smoke– Smoke volume– Smoke velocity– Smoke density– Smoke color
• Black fire
5
Smoke Reading (3 of 4)
• Determine the influences on the key attributes– Size of the structure– Wind conditions– Thermal balance– Fire streams– Ventilation openings– Sprinkler systems
5
Smoke Reading (4 of 4)
• Determine the rate of change– Changes in the four key attributes indicate
changes in the fire
• Predict the event– Consider the key attributes, what influences
them, and their rate of change– Communicate key parts to the company
officer
5
Smoke Reading Through a Door
• If smoke exits through the top half and clean air enters through the bottom half
• If smoke rises and the opening clears
• If smoke thins, but still fills the door
5
Summary (1 of 3)
• Characteristics of solids, liquids, and gases are different.
• Fire triangle and fire tetrahedron represent conditions necessary for combustion.
• Five classes of fire require specific extinguishing methods.
5
Summary (2 of 3)
• Knowledge of fire spread
• Typical fires pass through four distinct phases.
• Liquid-fuel fires, gas- fuel fires, and interior fires have unique characteristics.
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