chapter 4 fire-related chemistry and physics
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Chapter 4
Fire-Related Chemistry and Physics
Introduction
• To appreciate how fire is controlled, we must first understand the chemical and physical properties of fire itself
• Information can be used to predict what the fire will do with the available fuel and where it is headed
• With this knowledge, we are able to choose the proper extinguishing agent and its method of application
Learning Objective 1Fire Triangle and Fire Tetrahedron
FIRE DEFINED
• Rapid, self-sustaining oxidation process accompanied by the evolution of heat and light in varying intensity
• Combustion described as a chemical reaction that releases energy as heat and usually as light
RUSTING OF IRON• Not considered combustion• Proceeds at a slow rate
FIRE TRIANGLE
• Originally based on three elements Fuel Air Heat
FIRE TETRAHEDRON• Fourth component called chemical chain reaction• Produces free radicals
Learning Objective 1Fire Triangle and Fire Tetrahedron
Learning Objective 2Describe What Constitutes an Oxidizer
CHEMISTRY OF FIRE
• Two basic necessary components Oxidizer Fuel
OXIDIZER• Evolves or generates oxygen• Oxygen is most common oxidizer• Most fires are burning at 21% oxygen
FLOURINE AND CHLORINE
• Listed under halogen family on periodic table
• Both naturally occur as gases
• Flourine is a much stronger oxidizer than oxygen
• Fire in a flourine atmosphere will burn more rapidly than one occurring in air
Learning Objective 2Describe What Constitutes an Oxidizer
Learning Objective 3Describe What Constitutes a Fuel
FUEL
• Described as anything that will burn Carbon and hydrogen are two most common
• Hydrocarbons Main element in organic fuels, fuels that were at
one time living things• Other elements as fuels, including metals
Sodium, aluminum, magnesium
FIRES INVOLVING CARBON, HYDROGEN, AND OXYGEN
• Two byproducts of complete combustion Water vapor Carbon dioxide
• Byproducts of incomplete combustion Smoke Carbon monoxide (CO) Carbon dioxide (CO²) Other fire gases
Learning Objective 3Describe What Constitutes a Fuel
Learning Objective 4Illustrate the States of Matter
FUEL MAY OCCUR IN THREE STATES OF MATTER
• Solid• Liquid• Gas
MOLECULES AND TEMPERATURE• Molecules in liquids vibrate faster than those in solids,
and gas molecules vibrate the fastest of the three• As this vibration increases, solids become liquids and
liquids become gases
Learning Objective 5Explain the Process of Pyrolysis
PYROLYSIS
• Chemical decomposition of matter through heat
• Combustion occurs when fuel has been converted to vapor or gas
• Oxidizer and fuel must be gaseous for recombination
• Solid and liquid fuels are converted to gaseous state by the application of energy
IGNITION TEMPERATURE
• Combustion continues without
external input of heat
FREE RADICALS
• Longer molecules break into shorter molecules
• Byproduct of the fuel that directly combines with the oxidizer
Learning Objective 5Explain the Process of Pyrolysis
Learning Objective 6Properties Affecting Solid Fuels
FACTORS AFFECTING SOLID FUEL PYROLIZATION
• Mass
• Arrangement
• Continuity
• Moisture content
MEASURING FLAME SPREAD
• Steiner tunnel
Test consists of a 25’ vented tunnel
A fan draws the flame across the surface of the material being tested
Flame spread is determined visually through windows built into the tunnel
Learning Objective 6Properties Affecting Solid Fuels
Learning Objective 7Properties Affecting Liquid Fuels
LIQUID FUELS
• Molecules flow freely but do not readily separate
• Will assume the shape of their container
• Physical properties make them difficult to extinguish
• They increase the hazards to persons and property
• If spilled, will flow and increase in size
OTHER IMPORTANT PROPERTIES OF LIQUIDS
• Specific gravity• Volatility• Vapor pressure• Boiling point• Vapor density• Flash point• Miscibility
Learning Objective 7Properties Affecting Liquid Fuels
Learning Objective 8Properties Affecting Gas Fuels
GAS AND VAPOR FUEL CHARACTERISTICS
• Defined as a fluid that has no shape or volume and tends to expand indefinitely
• Always fill the container in which they are stored
FLAMMABLE RANGES• Upper flammable limit• Lower flammable limit • Flammable range
Learning Objective 9Differentiate Heat and Temperature
SOURCES OF HEAT (FORM OF ENERGY)
• Chemical• Mechanical• Electrical• Nuclear
MEASUREMENTS OF HEAT
• British thermal unit (BTU)• Calorie (metric term)• Joule (international system of units)
MEASUREMENTS OF TEMPERATURE
• The measure of the hotness or coldness of an object Fahrenheit Celsius Kelvin Rankine
Learning Objective 9Differentiate Heat and Temperature
Learning Objective 10Illustrate the Four Methods of Heat Transfer
ENERGY TRANSFERS FROM HEAT SOURCE TO FUEL
• Conduction
• Convection
• Radiation
• Direct flame impingementor autoexposure
STRUCTURE FIRE AND HEAT TRANSFER METHODS
• Fire starts in a stack of boxes in a warehouse, convection catches the ceiling on fire
• Fire spreads up the stack due to direct flame impingement
• Radiated heat spreads the fire to piled stock several feet away
• A pipe running through the wall heats to the point that stock on the other side of the wall ignites
Learning Objective 10Illustrate the Four Methods of Heat Transfer
Learning Objective 11Illustrate the Five Classifications of Fire
FIRE HAS BEEN DIVIDED INTO BASIC CLASSIFICATIONS
• Class A: Ordinary combustibles• Class B: Flammable liquids• Class C: Energized electrical• Class D: Combustible metals• Class K: Cooking materials
FOUR STAGES OF FIRE DEVELOPMENT
• Ignition Fire ignites and reaches a point where it no
longer needs input heat from outside sources to keep burning
• Growth Fire releases heat, bringing more fuel to its
ignition temperature Heat is transferred to nearby surfaces though
radiationCont.
Learning Objective 12Describe the Four Stages of Fire
FOUR STAGES OF FIRE DEVELOPMENT
• Fully developed All available fuels in the fire’s perimeter are
burning
• Decay Occurs when the fire has run out of available
fuel Suppression action has reduced the fire to
smoldering embers
Learning Objective 12Describe the Four Stages of Fire
FLASHOVER
• The contents in the room are brought to their ignition temperature and if sufficient oxygen is present, flashover can occur
• If a flashover were to take place, temperatures, even at floor level, would rise dramatically
• Firefighters cannot survive in a room with a flashover, even wearing full PPE and SCBA
Learning Objective 12Describe the Four Stages of Fire
BACK DRAFT
• Flame may die out and glowing combustion takes place
• Pyrolysis continues to occur, with amounts of combustible gases produced
• Room is now superheated and charged with smoke and combustible fire gases
• Fire gases and smoke are alternately forced out and sucked back into the structure
Learning Objective 12Describe the Four Stages of Fire
Summary
• This chapter introduces the chemical and physical properties of the elements found in fires and what actually occurs during the combustion reaction
• A solid understanding of the combustion process and the stages of fire must be in place before we discuss the selection, application, and tactics of extinguishing agents
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