Chapter 03-Combustion Processes
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Transcript of Chapter 03-Combustion Processes
© 2009 Delmar, Cengage Learning
Combustion Processes
Chapter 3
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
What is Combustion (cont’d.)
Figure 3-2 The new fire tetrahedron
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Figure 3-7 Temperatures associated with the stages of fire and the unique fire events
© 2009 Delmar, Cengage Learning
Flame Over
• Flames travel through or across unburned gases in upper portions of confined area during fire development
Figure 3-8 Flame over/rollover
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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
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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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
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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
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Relative Humidity
• Moisture in the form of water vapor• Always present• Affects amount of moisture in fuel• Impacts direction of fire gas movement
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Heat Measurement
• Heat always flows from higher temperature materials to lower temperature materials
• Four temperature scales– Kelvin– Rankin– Celsius– Fahrenheit
© 2009 Delmar, Cengage Learning
Figure 3-11 Relationship among temperature scales
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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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
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
© 2009 Delmar, Cengage Learning
Direct Flame Impingement
Figure 3-15 Flamesdirectly impinging upon the materials transfer the heat, raising theirtemperature to thepoint where combustion occurs
© 2009 Delmar, Cengage Learning
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