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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Chapter 03-Combustion Processes

  1. 1. Combustion Processes Chapter 3
  2. 2. Objectives <ul><li>Explain the theories underlying combustion processes </li></ul><ul><li>Describe how fire researchers have identified combustion processes using a variety of different classifications </li></ul><ul><li>Provide a description of the stages and events of fire as it progresses from the initial stage to its final stage </li></ul>
  3. 3. Objectives (cont’d.) <ul><li>Explain the causes of flame over, flashover, and backdraft and review the procedures to prevent and protect against such events </li></ul><ul><li>Describe the various methods by which heat and unburned gases move in a confined environment </li></ul><ul><li>Define the five classes of fires and explain how they are classified </li></ul>
  4. 4. Introduction <ul><li>This chapter: </li></ul><ul><ul><li>Considers physical and chemical process involved in fire combustion and relates them to procedures of fire services to confine, control, and extinguish uncontrolled fires </li></ul></ul><ul><ul><li>Emphasizes combustion processes as fires progress </li></ul></ul><ul><ul><li>Reviews fire classification methods, fire extinguishing agents, and their advantages and disadvantages </li></ul></ul>
  5. 5. What is Combustion? <ul><li>Planned and controlled, self-sustaining chemical reaction between fuel and oxygen with evolution of heat and light </li></ul><ul><li>Differs from fire </li></ul><ul><li>Represented by fire tetrahedron </li></ul><ul><ul><li>Heat </li></ul></ul><ul><ul><li>Fuel </li></ul></ul><ul><ul><li>Oxygen </li></ul></ul><ul><ul><li>Chemical reaction </li></ul></ul>
  6. 6. What is Combustion (cont’d.) Figure 3-2 The new fire tetrahedron
  7. 7. Spontaneous Combustion <ul><li>Does not require independent ignition source </li></ul><ul><li>Material heats to piloted ignition temperature </li></ul><ul><li>After ignition, flames spread </li></ul><ul><li>Coal is an example of a porous solid material that when heated, eventually reaches ignition temperature and combustion begins </li></ul>
  8. 8. Methods of Fire Classification <ul><li>Type of combustion </li></ul><ul><li>Rate of fire growth </li></ul><ul><li>Available ventilation </li></ul><ul><li>Type of materials that are burning </li></ul><ul><li>Stages or phases of a fire </li></ul>
  9. 9. Types of Combustion <ul><li>Three stages </li></ul><ul><ul><li>Pre-combustion </li></ul></ul><ul><ul><ul><li>Fuel heated to ignition point </li></ul></ul></ul><ul><ul><ul><li>Particulates released </li></ul></ul></ul><ul><ul><ul><li>Entrainment gathers additional oxygen </li></ul></ul></ul><ul><ul><ul><li>Heat energy radiated back into fuel </li></ul></ul></ul><ul><ul><li>Smoldering combustion </li></ul></ul><ul><ul><li>Flaming combustion </li></ul></ul>
  10. 10. Smoldering Combustion <ul><li>Absence of flame </li></ul><ul><li>Presence of hot materials on surface where oxygen diffuses into fuel </li></ul><ul><li>Two phases </li></ul><ul><ul><li>Solid </li></ul></ul><ul><ul><li>Gas </li></ul></ul><ul><li>Incompleteness creates very high levels of carbon monoxide </li></ul>
  11. 11. Flaming Combustion <ul><li>Encountered in most emergency incidents </li></ul><ul><li>Presence of flames </li></ul><ul><li>Gas or vapor has to be burning </li></ul><ul><li>Two categories </li></ul><ul><ul><li>Gaseous fuel premixed with air before ignition </li></ul></ul><ul><ul><li>Diffusive flaming </li></ul></ul><ul><ul><ul><li>Flames are generally yellow due to incomplete burning process </li></ul></ul></ul><ul><ul><ul><li>Light and heat also emitted </li></ul></ul></ul>
  12. 12. Fire Classification by Type of Substance Burning <ul><li>Class A </li></ul><ul><ul><li>Fires involving combustion of ordinary cellulosic materials </li></ul></ul><ul><li>Class B </li></ul><ul><ul><li>Fires involving flammable liquids </li></ul></ul><ul><li>Class C </li></ul><ul><ul><li>Fires involving energized electrical equipment or wires </li></ul></ul>
  13. 13. Fire Classification by Type of Substance Burning (cont’d.) <ul><li>Class D </li></ul><ul><ul><li>Fires involving combustible metals </li></ul></ul><ul><li>Class K </li></ul><ul><ul><li>Fires involving cooking oils </li></ul></ul><ul><ul><ul><li>Saponification: process of chemically converting the fatty acid contained in a cooking medium (oil or grease) to soap or foam </li></ul></ul></ul>
  14. 14. Fire Classification by Stages and Events <ul><li>Fire stages: </li></ul><ul><ul><li>Ignition stage </li></ul></ul><ul><ul><li>Growth stage </li></ul></ul><ul><ul><li>Fully developed stage </li></ul></ul><ul><ul><li>Decay stage </li></ul></ul><ul><li>Fire events: </li></ul><ul><ul><li>Flameover or rollover </li></ul></ul><ul><ul><li>Flash over </li></ul></ul><ul><ul><li>Backdraft </li></ul></ul>
  15. 15. Figure 3-7 Temperatures associated with the stages of fire and the unique fire events
  16. 16. Flame Over <ul><li>Flames travel through or across unburned gases in upper portions of confined area during fire development </li></ul>Figure 3-8 Flame over/rollover
  17. 17. Flashover <ul><li>When heating is enough to bring other materials in room to ignition temperature, igniting all fuel materials in the room into flaming combustion </li></ul>Figure 3-9 Flashover
  18. 18. Backdraft <ul><li>Additional oxygen entering the compartment is heated and expands </li></ul><ul><li>Increased pressure inside room </li></ul><ul><li>Windows, walls, and weak points in the building suddenly pushed outward </li></ul><ul><li>Firefighters caught in the sudden, explosive rush of fire can be killed instantly </li></ul>
  19. 19. Building Construction and Fire Spread <ul><li>Efficiency declines if fires move vertically through buildings or bypass horizontal construction barriers </li></ul><ul><ul><li>Pre-WWII concrete construction inhibited vertical movement </li></ul></ul><ul><li>Post-WWII drywall spreads fires to other areas of building quickly </li></ul><ul><li>Compartmentation is safe areas in high-rises </li></ul>
  20. 20. Fire Rating of Materials <ul><li>Building’s ability to withstand a fire differ because of: </li></ul><ul><ul><li>Variations in workmanship </li></ul></ul><ul><ul><li>Methods of installation </li></ul></ul><ul><ul><li>Different sets of test methods </li></ul></ul><ul><ul><li>Sizes of test specimens </li></ul></ul><ul><li>Rated fire resistance of construction has some but not a substantial impact on the spread of fire </li></ul>
  21. 21. Weather Conditions <ul><li>Impact the burning characteristics of inside building fires and outside fires </li></ul><ul><li>Stack effect: temperature difference between the outside temperature of building and temperature inside the building </li></ul><ul><li>Windy conditions outside can impact horizontal ventilation activities </li></ul>
  22. 22. Relative Humidity <ul><li>Moisture in the form of water vapor </li></ul><ul><li>Always present </li></ul><ul><li>Affects amount of moisture in fuel </li></ul><ul><li>Impacts direction of fire gas movement </li></ul>
  23. 23. Mass/Drying Time <ul><li>Impacts how long it will take source of ignition to raise material to ignition temperature </li></ul><ul><li>Thicker or heavier mass will take longer to raise the temperature of the material </li></ul><ul><li>Law of latent heat of vaporization: heat absorbed when 1 gram of liquid transformed into vapor at boiling point under 1 atmosphere of pressure </li></ul><ul><ul><li>Result in BTUs per pound or calories per gram </li></ul></ul>
  24. 24. Heat Measurement <ul><li>Heat always flows from higher temperature materials to lower temperature materials </li></ul><ul><li>Four temperature scales </li></ul><ul><ul><li>Kelvin </li></ul></ul><ul><ul><li>Rankin </li></ul></ul><ul><ul><li>Celsius </li></ul></ul><ul><ul><li>Fahrenheit </li></ul></ul>
  25. 25. Figure 3-11 Relationship among temperature scales
  26. 26. Heat Transfer <ul><li>Important in all aspects of combustion process </li></ul><ul><li>Responsible for continuance of combustion process </li></ul><ul><li>Four methods of transfer: </li></ul><ul><ul><li>Conduction </li></ul></ul><ul><ul><li>Convection </li></ul></ul><ul><ul><li>Radiation </li></ul></ul><ul><ul><li>Direct flame impingement </li></ul></ul>
  27. 27. Conduction <ul><li>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 </li></ul>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. 28. Convection <ul><li>Movement of heat energy by agitation of air molecules </li></ul><ul><ul><li>Reduces density of molecules, making heated air lighter than cooler air </li></ul></ul>Figure 3-13 Convection involves the transfer of heat by circulating currents
  29. 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. 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. 31. Summary <ul><li>Combustion process defined by type, rate of fire growth, amount of ventilation, and type of substance that burns </li></ul><ul><li>Classifications of fires: Class A, B, C, D, and K </li></ul><ul><li>Physical and chemical properties of fuels feeding fires affect how a fire will burn, spread, and quickness of burning rate </li></ul>

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