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Chapter 03-Combustion Processes
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Chapter 03-Combustion Processes

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• 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
• 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

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  • 1. Combustion Processes Chapter 3
  • 2. 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
  • 3. 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
  • 4. 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
  • 5. 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
  • 6. What is Combustion (cont’d.) Figure 3-2 The new fire tetrahedron
  • 7. 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
  • 8. 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
  • 9. 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
  • 10. 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
  • 11. 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
  • 12. 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
  • 13. 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
  • 14. 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
  • 15. Figure 3-7 Temperatures associated with the stages of fire and the unique fire events
  • 16. Flame Over
    • Flames travel through or across unburned gases in upper portions of confined area during fire development
    Figure 3-8 Flame over/rollover
  • 17. 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
  • 18. 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
  • 19. 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
  • 20. 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
  • 21. 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
  • 22. Relative Humidity
    • Moisture in the form of water vapor
    • Always present
    • Affects amount of moisture in fuel
    • Impacts direction of fire gas movement
  • 23. 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
  • 24. Heat Measurement
    • Heat always flows from higher temperature materials to lower temperature materials
    • Four temperature scales
      • Kelvin
      • Rankin
      • Celsius
      • Fahrenheit
  • 25. Figure 3-11 Relationship among temperature scales
  • 26. 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
  • 27. 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
  • 28. Convection
    • Movement of heat energy by agitation of air molecules
      • Reduces density of molecules, making heated air lighter than cooler air
    Figure 3-13 Convection involves the transfer of heat by circulating currents
  • 29. 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
  • 30. Direct Flame Impingement Figure 3-15 Flames directly impinging upon the materials transfer the heat, raising their temperature to the point where combustion occurs
  • 31. 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