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© 2009 Delmar, Cengage Learning

Combustion Processes

Chapter 3


Objectives

• Explain the theories underlying combustion processes

• Describe how fire researchers have identified combustion processes using a variety of different classifications

• Provide a description of the stages and events of fire as it progresses from the initial stage to its final stage


Objectives (cont’d.)

• Explain the causes of flame over, flashover, and backdraft and review the procedures to prevent and protect against such events

• Describe the various methods by which heat and unburned gases move in a confined environment

• Define the five classes of fires and explain how they are classified


Introduction

• This chapter:– Considers physical and chemical process involved in

fire combustion and relates them to procedures of fire services to confine, control, and extinguish uncontrolled fires

– Emphasizes combustion processes as fires progress– Reviews fire classification methods, fire extinguishing

agents, and their advantages and disadvantages


What is Combustion?

• Planned and controlled, self-sustaining chemical reaction between fuel and oxygen with evolution of heat and light

• Differs from fire• Represented by fire tetrahedron

– Heat– Fuel– Oxygen– Chemical reaction


What is Combustion (cont’d.)

Figure 3-2 The new fire tetrahedron


Spontaneous Combustion

• Does not require independent ignition source• Material heats to piloted ignition temperature • After ignition, flames spread• Coal is an example of a porous solid material

that when heated, eventually reaches ignition temperature and combustion begins


Methods of Fire Classification

• Type of combustion• Rate of fire growth• Available ventilation• Type of materials that are burning• Stages or phases of a fire


Types of Combustion

• Three stages– Pre-combustion

• Fuel heated to ignition point

• Particulates released

• Entrainment gathers additional oxygen

• Heat energy radiated back into fuel

– Smoldering combustion– Flaming combustion


Smoldering Combustion

• Absence of flame• Presence of hot materials on surface where

oxygen diffuses into fuel• Two phases

– Solid– Gas

• Incompleteness creates very high levels of carbon monoxide


Flaming Combustion

• Encountered in most emergency incidents• Presence of flames• Gas or vapor has to be burning• Two categories

– Gaseous fuel premixed with air before ignition– Diffusive flaming

• Flames are generally yellow due to incomplete burning process

• Light and heat also emitted


Fire Classification by Type of Substance Burning

• Class A– Fires involving combustion of ordinary cellulosic

materials

• Class B– Fires involving flammable liquids

• Class C– Fires involving energized electrical equipment or

wires


Fire Classification by Type of Substance Burning (cont’d.)

• Class D– Fires involving combustible metals

• Class K– Fires involving cooking oils

• Saponification: process of chemically converting the fatty acid contained in a cooking medium (oil or grease) to soap or foam


Fire Classification by Stages and Events

• Fire stages:– Ignition stage– Growth stage– Fully developed stage– Decay stage

• Fire events:– Flameover or rollover– Flash over– Backdraft


Figure 3-7 Temperatures associated with the stages of fire and the unique fire events


Flame Over

• Flames travel through or across unburned gases in upper portions of confined area during fire development

Figure 3-8 Flame over/rollover


Flashover

• When heating is enough to bring other materials in room to ignition temperature, igniting all fuel materials in the room into flaming combustion

Figure 3-9 Flashover


Backdraft

• Additional oxygen entering the compartment is heated and expands

• Increased pressure inside room• Windows, walls, and weak points in the building

suddenly pushed outward• Firefighters caught in the sudden, explosive rush

of fire can be killed instantly


Building Construction and Fire Spread

• Efficiency declines if fires move vertically through buildings or bypass horizontal construction barriers– Pre-WWII concrete construction inhibited vertical

movement

• Post-WWII drywall spreads fires to other areas of building quickly

• Compartmentation is safe areas in high-rises


Fire Rating of Materials

• Building’s ability to withstand a fire differ because of:– Variations in workmanship– Methods of installation– Different sets of test methods– Sizes of test specimens

• Rated fire resistance of construction has some but not a substantial impact on the spread of fire


Weather Conditions

• Impact the burning characteristics of inside building fires and outside fires

• Stack effect: temperature difference between the outside temperature of building and temperature inside the building

• Windy conditions outside can impact horizontal ventilation activities


Relative Humidity

• Moisture in the form of water vapor• Always present• Affects amount of moisture in fuel• Impacts direction of fire gas movement


Mass/Drying Time

• Impacts how long it will take source of ignition to raise material to ignition temperature

• Thicker or heavier mass will take longer to raise the temperature of the material

• Law of latent heat of vaporization: heat absorbed when 1 gram of liquid transformed into vapor at boiling point under 1 atmosphere of pressure– Result in BTUs per pound or calories per gram


Heat Measurement

• Heat always flows from higher temperature materials to lower temperature materials

• Four temperature scales– Kelvin– Rankin– Celsius– Fahrenheit


Figure 3-11 Relationship among temperature scales


Heat Transfer

• Important in all aspects of combustion process• Responsible for continuance of combustion

process• Four methods of transfer:

– Conduction– Convection– Radiation– Direct flame impingement


Conduction

• Transfer of heat energy from hot to cold side of medium by means of energy transfer from molecule to adjacent molecule or atom to atom

Figure 3-12 Conduction is the transfer of heat energy from a material by direct contact between the movements of molecules of another higher energy material


Convection

• Movement of heat energy by agitation of air molecules– Reduces density of

molecules, making heated air lighter than cooler air

Figure 3-13Convection involvesthe transfer of heat by circulating currents


Radiation

Figure 3-14 Radiation is energy that travels across a space and does not need an intervening medium, such as a solid or a fluid


Direct Flame Impingement

Figure 3-15 Flamesdirectly impinging upon the materials transfer the heat, raising theirtemperature to thepoint where combustion occurs


Summary

• Combustion process defined by type, rate of fire growth, amount of ventilation, and type of substance that burns

• Classifications of fires: Class A, B, C, D, and K • Physical and chemical properties of fuels

feeding fires affect how a fire will burn, spread, and quickness of burning rate