Chapter 5: Fire Behavior1

Pre-Lecture

I. You Are the Fire Fighter

Time: 5 Minutes

Small Group Activity/Discussion

Use this activity to motivate students to learn the knowledge and skills needed to understand fire behavior.

Purpose

To allow students an opportunity to explore the significance and concerns associated with fire behavior.

Instructor Directions

  1. Direct students to read the “You Are the Fire Fighter” scenario found in the beginning of Chapter 5.
  2. You may assign students to a partner or a group. Direct them to review the discussion questions at the end of the scenario and prepare a response to each question. Facilitate a class dialogue centered on the discussion questions.
  3. You may also assign this as an individual activity and ask students to turn in their comments on a separate piece of paper.

Lecture

I. Introduction

Time: 5 Minutes

Slides: 1–6

Level: Fire Fighter I

Lecture/Discussion

  1. An understanding of fire behavior is the basis for all firefighting operations.
  2. Knowing how fire behaves helps fire fighters to determine what tactics to employ.
  3. Knowing what goes on physically and chemically to make a fire occur helps fire fighters to learn proper methods of extinguishments.

II. The Chemistry of Fire

Time: 25 Minutes

Slides: 7–20

Level: Fire Fighter I

Lecture/Discussion

  1. Introduction to the chemistry of fire
  2. Understanding how fire ignites and grows will assist in the fire fighter’s ability to extinguish fire situations.
  3. Being well-trained in fire behavior will allow the fire fighter to control a fire utilizing less water.
  4. What is fire?
  5. Rapid chemical process that produces heat and usually light
  6. Fire is neither solid nor liquid.

3. Wood is a solid, gasoline is a liquid, and propane is a gas—but they all burn.

  1. States of matter
  2. Solids
  3. Maintain a definite shape
  4. Most fuels are solids, but only gases burn.
  5. Solids must be converted to gases before they burn.
  6. As the solid heats up, pyrolysis releases individual molecules into the atmosphere where they mix with oxygen and combust.
  7. Surface to mass ratio (STMR)
  8. Fuels with a higher STMR combust more easily and rapidly.
  9. The larger the surface area, the faster the rate of combustion.
  10. Liquids
  11. Do not have a definite shape; assume the shape of their container
  12. Vaporization occurs as a liquid fuel is heated, releasing molecules of fuel from the surface into the gaseous state where they mix with oxygen and combust.
  13. Liquids with a high surface to volume ratio combust more easily and rapidly.
  14. Gases
  15. Have neither shape nor volume and expand indefinitely
  16. Oxygen molecules and fuel molecules mix freely and come into direct contact with each other
  17. Need very little energy to ignite
  18. The ratio of fuel to air must be within a certain range for combustion to occur.
  19. Fuels
  20. Fuels are a form of energy.
  21. The energy released in the form of heat and light has been stored in the fuel before it is burned.
  22. Types of energy
  23. Chemical energy
  24. Energy created by a chemical reaction
  25. Some of these reactions produce heat and are referred to as exothermic reactions.
  26. Some of these reactions absorb heat and are referred to as endothermic reactions.
  27. Mechanical energy
  28. Converted to heat when two materials rub against each other and create friction
  29. Heat is also produced when mechanical energy is used to compress air in a compressor.
  30. Electrical energy
  31. Produces heat while flowing through a wire or another conductive material
  32. Examples of electrical energy
  33. Heating elements
  34. Overloaded wires
  35. Electrical arcs
  36. Lightning
  37. Light energy
  38. Caused by electromagnetic waves packaged in discrete bundles called photons
  39. Examples of light energy
  40. Candles
  41. Light bulbs
  42. Lasers
  43. Nuclear energy
  44. Created by nuclear fission or fusion
  45. Conservation of energy
  46. Energy cannot be created or destroyed by ordinary means.
  47. Energy can be converted from one form to another.
  48. Chemical energy in gasoline is converted to mechanical energy when a car moves up and down the road.
  49. Conditions needed for fire
  50. The three basic ingredients required to create a fire form the fire triangle.
  51. A combustible fuel
  52. Oxygen in sufficient quantities
  53. A source of heat
  54. Adding a fourth factor, chemical chain reactions, forms the fire tetrahedron.
  55. A self-sustaining series of chemical chain reactions must occur to keep a fire burning.

III. Chemistry of Combustion

Time: 5 Minutes

Slides: 21, 22

Level: Fire Fighter I

Lecture/Discussion

  1. Combustion is a very rapid, self-sustaining process that combines oxygen with another substance and results in the release of heat and light.
  2. Oxidation is the process of chemically combining oxygen with another substance to create a new compound.
  3. Rusting is an example of a slow oxidation process.
  4. Combustion produces changes in the physical state of matter of the materials involved (the fuel).
  5. Rapid, self-sustaining process that combines oxygen with another substance and results in the release of heat and light.
  6. Pyrolysis is the decomposition of a material caused by external heating.
  7. Pyrolysis can eventually produce enough heat to cause combustion.

IV. Products of Combustion

Time: 20 Minutes

Slides: 23–26

Level: Fire Fighter I

Lecture/Discussion

  1. Specific products of combustion released will depend on:
  2. Fuel being burned
  3. Temperature of the fire
  4. Amount of oxygen available
  5. Very few fires consume all of the available fuel.
  6. This results in incomplete combustion and a variety of by-products.
  7. Smoke
  8. The airborne products of combustion are referred to as smoke and include three major components:
  9. Particles
  10. Vapors
  11. Gases
  12. Inhalation of smoke can cause severe injuries.
  13. Toxic droplets can be dangerous if inhaled, and some can be poisonous if absorbed through the skin.
  14. Gases in smoke vary depending on the fuel being burned.
  15. Many of the gases liberated during combustion are toxic.
  16. Common gases in smoke include:
  17. Carbon monoxide
  18. Hydrogen cyanide
  19. Phosgene

V. Fire Spread

Time: 30 Minutes

Slides: 27–39

Level: Fire Fighter I

Lecture/Discussion

  1. Heat is required to ignite a fuel.
  2. Once ignited, the combustion process gives off heat, which can ignite other nearby fuels.
  3. Heat energy is transferred from a hotter mass to a colder mass.
  4. There are three mechanisms of heat transfer:
  5. Conduction
  6. Convection
  7. Radiation
  8. Conduction
  9. The process of transferring heat from one molecule to another
  10. Objects that transfer heat well are called conductors.
  11. Metals generally conduct heat very well.
  12. Objects that transfer heat poorly are called insulators.
  13. Fiberglass is a type of insulating material.
  14. Convection
  15. The movement of heat through a fluid medium such as air or a liquid
  16. Convection currents:
  17. Molecules rise as they are heated and then fall as they cool off.
  18. A plume of smoke is actually a convection current.
  19. Winds can push convection currents in different directions, causing the fire to spread.
  20. Convection within a room
  21. Hot gases rise to the ceiling.
  22. Gases travel horizontally along the ceiling until they hit a wall.
  23. If there are no openings for gases to escape:
  24. Gases will bank back down the walls, creating a downwardly expanding layer of heated air.
  25. If there are openings for gases to escape:
  26. Gases will move outside the room of origin to other parts of the building.
  27. Gases will flow toward the highest level of the building through any openings.
  28. Gases will collect under the roof.
  29. Superheated gases can be hot enough to ignite other materials.
  30. Radiation
  31. The transfer of heat in the form of an invisible wave
  32. Heat radiated to a nearby structure (an exposure) can ignite it.
  33. The effect of radiation is not seen or felt until the radiation strikes an object and heats the surface of the object.
  34. Radiated heat can pass through glass windows and ignite objects in other buildings.
  35. Methods of extinguishment
  36. Can be broken down into four methods:
  37. Cooling the burning material
  38. Excluding oxygen from the fire
  39. Removing fuel from the fire
  40. Interrupting the chemical reaction with a flame inhibitor
  41. Classes of fire
  42. Fires are classified according to the type of fuel that is burning.
  43. A fire can fit into more than one class.
  44. The five classes of fires are:
  45. Class A
  46. Class B
  47. Class C
  48. Class D
  49. Class K
  50. Extinguishing agents are classified to match the type(s) of fires they extinguish.
  51. Class A fires:
  52. Involve ordinary solid combustible materials such as:
  53. Wood
  54. Paper
  55. Cloth
  56. Extinguishing agents:
  57. Water is most frequently used because it is plentiful and efficient in absorbing large quantities of heat.
  58. Class B fires:
  59. Involve flammable or combustible liquids such as:
  60. Gasoline
  61. Kerosene
  62. Oils
  63. Paints
  64. Tar
  65. Extinguishing agents:
  66. Foam creates a barrier to separate the fuel and the oxygen.
  67. Carbon dioxide is applied as a smothering agent to exclude oxygen.
  68. Dry chemicals interrupt the fire’s chain reactions.
  69. Water mist is sometimes used.
  70. Class C fires:
  71. Involve energized electrical equipment
  72. The underlying fuel is often Class A or Class B.
  73. Special classification is required due to the hazards of electricity.
  74. Extinguishing agents:
  75. Carbon dioxide and dry chemical agents are common extinguishing agents because they do not conduct electricity.
  76. The use of water is not advised.
  77. Be sure to shut off power to equipment before applying water.
  78. Class D fires:
  79. Involve burning metals such as:
  80. Sodium
  81. Potassium
  82. Lithium
  83. Zirconium
  84. Magnesium
  85. Aluminum
  86. Extinguishing agents:
  87. Special salt-based extinguishing powders are available.
  88. Dry sand is another option.
  89. Do not use water.
  90. Many metals are water reactive!
  91. Class K Fires:
  92. Involving combustible cooking media found in commercial kitchens such as:
  93. Cooking oils
  94. Grease
  95. Class K designation is new and coincides with a new classification of Class K extinguishing agents.

VI. Characteristics of Solid-Fuel Fires

Time: 15 Minutes

Slides: 40–57

Level: Fire Fighter I

Lecture/Discussion

  1. As a typical fire progresses, it will pass through four distinct phases, unless the process is interrupted.
  2. The four phases of fire are:
  3. Ignition
  4. Growth
  5. Fully developed
  6. Decay
  7. Ignition phase
  8. Fuel, heat, and oxygen are present.
  9. Fuel is heated to its ignition temperature.
  10. Growth phase
  11. Additional fuel becomes involved in the fire.
  12. As more fuel is ignited, the fire grows larger.
  13. Convection currents draw more air to the fire.
  14. Hot gases collect at the ceiling and begin to bank downward.
  15. The fire will continue to grow as long as oxygen and fuel are present.
  16. Flashover
  17. All combustible materials in a room ignite at once.
  18. Temperatures can reach 1000 ºF in a matter of seconds.
  19. Flashovers are deadly to fire fighters and victims.
  20. Fully developed phase
  21. All combustibles have ignited.
  22. Heat is being produced at the maximum rate.
  23. Oxygen is consumed very rapidly.
  24. Fire will burn openly as long as fuel and oxygen are available.
  25. Decay phase
  26. The fire is running out of fuel.
  27. Intensity will reduce as fuel is consumed.
  28. When all fuel is consumed, the fire will go out.
  29. Key principles of solid-fuel fire development

1. Hot gases and flame are lighter and tend to rise.

2. Convection is the primary factor in spreading the fire upward.

3. Downward spread of the fire occurs primarily from radiation and falling chunks of flaming material.

4. If there is not more fuel above or beside the initial flame that can be ignited by convection or radiated heat, the fire will go out.

5. Variations in the direction of upward fire spread will occur if (and when) air currents deflect the flame.

6. The total material burned reflects the intensity of the heat and the duration of the exposure to the heat.

7. An adequate supply of oxygen must be available to fuel a free-burning fire, although some parts of the flame may have a limited supply of oxygen.

I. Room contents

  1. Many fires in buildings burn the contents of the structure, but do not involve the structure itself.
  2. Most modern rooms are heavily loaded with materials made of plastics and synthetic materials.
  3. These produce dense smoke that can be highly toxic.
  4. Newer upholstered furniture is more resistant to ignition from glowing sources, such as cigarettes, but it has little resistance to ignition from flaming sources.
  5. Finishes used on walls and ceilings can burn readily.
  6. This can increase the intensity and spread of the fire.
  1. Special considerations
  2. There are three special conditions that must be understood to extinguish fires safely: flameover, the thermal layering of gases, and backdraft.
  3. Flameover (Rollover)
  4. A warning sign that the fire may soon flashover
  5. Licks of flame ignite momentarily in the upper layers of smoke.
  6. Aggressive cooling of the atmosphere, immediate exit, or immediate ventilation is required at this point.
  7. Thermal layering and thermal balance
  8. Gases in thermal balance are allowed to seek their own level.
  9. Superheated gases collect near the ceiling.
  10. Temperatures are lowest near the floor.
  11. The thermal balance can be upset by fire fighters.
  12. Water applied to a fire creates steam, which expands and rises.
  13. The steam can displace the hot gases at the top of the room, forcing them down on fire fighters (and victims).
  14. Avoid this problem by coordinating fire attack with ventilation and using a straight stream to minimize steam formation.
  15. Backdraft
  16. An explosion that occurs when oxygen is suddenly admitted to a confined area that is very hot and contains large amounts of combustible vapors and smoke
  17. Usually occurs when the fire is smoldering because it has consumed most of the available oxygen in the room
  18. The room is filled with carbon monoxide and other products of combustion.
  19. Sudden introduction of air will explosively feed the fire.
  20. Signs of an impending backdraft:
  21. Little or no flame is visible from the exterior (the fire is smoldering, not flaming).
  22. Smoke is emanating under pressure from cracks around doors, windows, and eaves.
  23. No large openings in the building such as open doors or windows
  24. “Living fire” is visible—smoke is puffing from the building and being drawn back in so that it looks like it is breathing.
  25. Unexplained change in the color of the smoke
  26. Glass is smoke stained and blackened due to heavy carbon deposits from the smoke.
  27. Signs of extreme heat conditions are present.
  28. To prevent backdrafts:
  29. Ventilate at a high level to allow superheated gases to escape.
  30. Conduct a well-coordinated fire attack.

VII. Characteristics of Liquid-Fuel Fires

Time: 20 Minutes

Slides: 58–60

Level: Fire Fighter I

Lecture/Discussion

  1. Liquids must be converted to a gaseous state before they will burn.
  2. Three conditions must be present for a vapor and air mixture to ignite:

1.The fuel and air must be present at a concentration within a flammable range.

2.There must be an ignition source with enough energy to start ignition.

3.The ignition source and the fuel mixture must make contact for long enough to transfer the energy to the fuel–air mixture.

  1. Terms used to describe the flammability of liquids:

1.Flash point

  1. The lowest temperature at which a liquid produces a flammable vapor
  1. Flame point (fire point)
  2. The lowest temperature at which a liquid produces enough vapor to sustain a continuous fire

VIII. Characteristics of Gas-Fuel Fires

Time: 20 Minutes

Slides: 61–65

Level: Fire Fighter I

Lecture/Discussion

  1. Vapor density and flammability limits describe the characteristics of flammable gases and vapors:
  2. Vapor density
  3. The weight of a gaseous fuel
  4. A gas with a vapor density of less than 1.0 will rise to the top of a confined space or rise into the open atmosphere.
  5. A gas with a vapor density greater than 1.0 is heavier than air and will settle close to the ground.
  6. Knowing the vapor density allows fire fighters to predict whether the danger of ignition is near the floor or ceiling of a room or space.
  7. Flammability limits
  8. Mixtures of flammable gases and air will burn only when they are mixed in certain concentrations.
  9. Too little fuel (vapor) in the mixture:
  10. Not enough fuel to support combustion
  11. Too lean
  12. Too much fuel (vapor) in the mixture:
  13. Not enough oxygen present to support combustion
  14. Too rich
  15. These lower and upper flammable (or explosive) limits set the range where combustion will occur.
  16. Boiling liquid expanding vapor explosion (BLEVE)
  17. BLEVEs can occur when a tank storing liquid fuel under pressure is exposed to fire or excessive heat.
  18. Fire heats up the liquid in the tank, causing it to generate more vapors.
  19. Internal pressure rises.
  20. The tank ruptures catastrophically.
  21. Liquid fuel is immediately released.
  22. Liquid is at or above its boiling point and immediately turns into a rapidly expanding cloud of vapor that ignites into a huge fireball.
  23. BLEVEs can injure and even kill fire fighters and civilians.
  24. Fireballs are created when the fire ignites the rapidly expanding vapors.
  25. Large pieces of the tank are propelled great distances.

IX. Smoke Reading

Time: 20 Minutes

Slides: 66–69

Level: Fire Fighter I

Lecture/Discussion

A. Understanding how to read smoke will assist the fire fighter in knowing three distinct things:

1. Where the fire is

2. How big the fire is

3. Where the fire is going

B. Determining the key attributes of smoke

1. Four key attributes

a. Smoke volume

i. The volume of smoke gives an idea of how much fuel is being heated to the point that it gives off gas.

b. Smoke velocity (speed)

i. Suggests how much pressure is accumulating in the building

ii. Laminar flow or turbulent flow

c. Smoke density

i. Suggests how much fuel is contained in the smoke

d. Smoke color

i. Gives some indication as to what stage the fire is in

ii. Can give evidence as to what substances are burning

C. Determine what is influencing the key attributes

1. The following considerations must be made:

a. Size of the structure

b. Wind conditions

c. Thermal balance

d. Fire streams

e. Ventilation openings