Chapter 04

transcript

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

Fuel Heat Oxygen

FIRE TETRAHEDRON

Fuel Heat Oxidizer Chain Reaction

Fuel Oxygen

ELEMENTS OF FIRE

FUEL - Something that will Burn. HEAT - Ignition Source, Starts the FIRE OXYGEN - Supports Combustion Chemical Chain Reaction - Must have

all three elements.

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

• 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

PYROLYSIS• The chemical decomposition of matter

through the action of heat

• Solid and liquid fuels are converted to the gaseous state by the application of energy thru this process i.e. the flame of a candle floating a small distance above the wick

FIRE TERMS

FIRE POINT

• The minimum temperature to which a material must be heated in order to sustain combustion after ignition by an external source.

• Also called Ignition Temperature.

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, the lower the Flash point in degrees the more DANGEROUS the fuel.

• 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

CLASS A FIRES

• Ordinary Combustible Materials

• Wood, Paper, Cloth, Plastic, etc.

CLASS B FIRES

• Flammable & Combustible Liquids

• Flammable & Combustible Gases

• Gasoline, Kerosene, Grease, Oil

CLASS C FIRES

• Energized Electrical Equipment

• Wires, Motors, Switches, Lights, Cables, Receptacles, Computers

CLASS D FIRES

• Combustible Metals

• Magnesium, Titanium, Sodium.

• Identified by the Suffix “ IUM “

• React Violently with WATER

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

STAGES OF FIRE

INCIPIENT PHASE

• Just Started.

• Free Burning Fire.

• Normal Oxygen 21%.

• Don’t Have High Temperatures.

• Thermal Balance is not Yet Disturbed.

• Breathing is barely affected.

FIRE BEHAVIOR

FREE BURNING PHASE OR

GROWTH STAGE

• Fire is expanding.

• Oxygen Supply less than 21%.

• Breathing Difficult, SCBA must be used

• Carbon monoxide levels are high.

• Temperatures between 1200° - 1600° F.

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

FIRE BEHAVIOR

SMOLDERING PHASE

• Free burning has ended.

• Insufficient oxygen. Less than 16%.

• Carbon monoxide, fatal range.

• Backdraft explosion possible.

• Most Dangerous stage to the Firefighter.

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

EXTINGUISHMENT of FIRE• Removal of one of the

three elements, or by breaking the chemical chain reaction.

• Removal of Heat -Water is used to cool the fuel below its ignition temperature.

• Removal of Oxygen- Co2 is used to sweep away the oxygen.

EXTINGUISHMENT of FIRE

• Removal of Fuel- Shut off valve, deenergize electrical equipment.

Summary

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

– Understand the combustion process

– Learn about heat transfer

– Know the classifications and stages of fire

Chapter 04

Education

Chapter 04

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