chapter 04

Copyright © 2006 Thomson Delmar Learning

Introduction to Fire Protection 3rd EditionRobert Klinoff

Introduction to Fire Protection3rd Edition



Chapter 4

Chemistry and Physics of Fire



Objectives

• Define the difference between the fire triangle and the fire tetrahedron

• Describe what constitutes an oxidizer• Describe what constitutes a fuel• Illustrate the states of matter



Objectives (con’t.)

• Explain the process of pyrolysis• Describe the properties affecting solid fuels• Describe the properties affecting liquid fuels• Describe the properties affecting gas fuels



Objectives (con’t.)

• Differentiate heat and temperature• Illustrate the four methods of heat transfer• Illustrate the five classifications of fire• Describe the four stages of fire



Introduction

• Fire can be better controlled by understanding its chemical and physical properties– Ability to predict what fire will do with available

fuel and where it is headed

– Ability to choose appropriate extinguishing agent and method of application



Fire Defined

• Fire: Rapid self-sustaining oxidation process accompanied by the evolution of heat and light in varying intensities

• Combustion: A chemical reaction that releases energy as heat and, usually, light



Fire Triangle

• Three sides represent elements (see Figure 4-1)– Originally: fuel, heat, and air

– More accurate: fuel, energy, and oxidizer



Fire Tetrahedron

• Four sides represent elements (see Figure 4-2)– Fuel

– Energy

– Oxidizer

– Chemical chain reaction



Oxidizer

• Oxygen is the most common– Air contains 21% oxygen

• Increasing amount of oxidizer will increase intensity of fire

• Other oxidizers include fluorine and chlorine



Fuel

• Described as anything that will burn• Most common fuels contain carbon and

hydrogen• Complete combustion yields carbon dioxide

and water vapor• Most combustion is incomplete due to several

factors– Size, arrangement, contaminants, lack of

sufficient oxidizer– Yields smoke and other fire gases



Fuel (con’t.)

• Occurs in three states of matter• State is often temperature dependent• Both fuel and oxidizer must be in gaseous state

to combine• Pyrolysis: fuel is vaporized by input heat• Ignition temperature: when fuel is hot enough

to self-sustain combustion



Solid Fuels

• Factors affecting rate of pyrolysis– Size

– Arrangement

– Continuity

– Moisture content



Flame Spread

• Steiner Tunnel tests how rapidly a fire spreads on interior finishes – Measures flame spread, temperature, and

smoke density

– May measure and analyze combustion gases



Liquid Fuels

• Liquids flow like water but do not readily separate

• Specific gravity: weight of a liquid compared to the weight of an equal volume of water

• Volatility: ease with which a fuel gives off vapors



Liquid Fuels (con’t.)

• Vapor pressure: pressure exerted by vapor molecules on the sides of a container

• Boiling point: when the vapor pressure equals atmospheric pressure

• Vapor density: relative density of a vapor or gas as compared to air



Liquid Fuels (con’t.)

• Flash point: minimum temperature of a liquid at which it gives off vapors sufficient to form an ignitable mixture with air

• Miscibility: ability of a substance to mix with water



Gas/Vapor Fuels

• Gas/vapor fuels: fluid that has neither independent shape nor volume but tends to expand indefinitely

• Upper flammable limit: maximum concentration of gas or vapor in air above which it is not possible to ignite the vapors (too rich to burn)



Gas/Vapor Fuels (con’t.)

• Lower flammable limit: lower concentration of gas or vapor in air below which it is not possible to ignite vapors (too lean)

• Flammable range: proportion of gas or vapor in air between the upper and lower flammable limits



Gas/Vapor Fuels (con’t.)

• Classification of gases– Flammable and nonflammable

• Some nonflammable support combustion (oxygen)

• Caution: flammable vapors are not always visible



Heat and Temperature

• Heat is a form of energy• Sources of heat

– Chemical: breaking down and recombination of molecules

– Mechanical: friction, friction sparks

– Electrical: arcs and sparks

– Nuclear: fission and fusion



Heat and Temperature (con’t.)

• British thermal unit (BTU): amount of heat required to raise the temperature of one pound of water one degree Fahrenheit

• Calorie: amount of heat required to raise the temperature of one gram of water one degree Celsius



Temperature

• Measure of the hotness or coldness of something expressed in degrees

• Fahrenheit– Freezing temperature: 32o

– Boiling temperature: 212o

• Celsius– Freezing temperature: 0o

– Boiling temperature: 100o



Heat Transfer

• Conduction: transfer of heat through a medium without visible motion

• Convection: transfer of heat through a circulating medium (see Figure 4-16)



Heat Transfer (con’t.)

• Radiation: transfer of heat through wavelengths of energy

• Direct flame impingement (auto exposure) combines all three (see Figure 4-18)



Classification of Fires

• Class A: ordinary combustibles • Class B: flammable liquids• Class C: energized electrical• Class D: combustible metals• Class K: cooking materials

Note: Many fires involve more than one

classification



Stages of Fire

• Incipient– Smoke and heat produced

• Free burning– Heat production increases

– Fire spreads to other fuels

• Smoldering– Flames die out, glowing combustion

• Phases of fire have evolved into four stages



Stages of Fire (con’t.)

• Ignition• Growth• Fully developed• Decay



Ignition Stage

• Oxygen in surrounding air approximately 21%• Fire ignited and burns on its own• Combustion reaction begins to accelerate



Growth Stage

• More fuel reaches ignition temperature• Heat transferred to other combustibles• Fire builds in intensity• In a confined area, room temperature

increases– Ceiling temperature may easily reach 10000 F

• Rollover starts to occur



Growth Stage (con’t.)

• Water applied to ceiling to reduce temperatures

• Flashover may occur if not cooled– Tongues of flame roll across ceiling

– Radiant heat affects materials in room, raising them to ignition temperature

– Materials in room ignite all at once

• Firefighters can not survive flashover• PPE and breathing apparatus may fail



Fully Developed Stage

• All fuels burning• Structure

– Room and contents or entire structure

• Wildland– Fire is moving across the countryside



Decay Stage

• Fire has run out of fuel or oxygen• In a sealed environment

– When oxygen is below 15%, combustion is slowed

– Pyrolysis continues to occur

– Room is superheated and charged with smoke and combustible gases

– If oxygen is introduced, backdraft can occur



Backdraft

• Combustible fire gases are prevalent in atmosphere

• Gases are at or above their ignition temperature• Oxygen content is too low for ignition (too rich to

burn)• Oxygen is introduced• Gases ignite with explosive force



Summary

• In order to choose and apply the proper extinguishing agents to fires, you must:– Study the physical and chemical properties of

fire

– Understand the combustion process

– Learn about heat transfer

– Know the classifications and stages of fire

chapter 04

Technology

temperature heat

transfer of heat

evolution of heat

gas fuels

heat transfer conduction

methods of heat transfer

gasvapor fuels gasvapor

temperature measure