Chapter 2 Chemistry of Fire Flashcards
Flammable/Explosive Limit
% of a substance (vapor) in air that will burn once it is ignited
Most have upper (too rich) / lower (too lean) flammable limits
Flash Point
Minimum temperature at which a liquid fuel gives off sufficient vapors to form an ignitable mixture w/air near the surface
At this temperature, the ignited vapors will flash but will not continue to burn
Heat
The form of energy that raises temperature
Ignition Temperature
The minimum temperature to which a fuel in air must be heated to start self-sustained combustion w/o a separate ignition source
Fire is a byproduct of
combustion
Combustion-
a self-sustaining process of rapid oxidation of a fuel which produces heat and light
Conduction:
Direct contact of the two bodies or by an intervening heat conducting minimum; Aluminum copper and iron are good conductors while fibrous materials are not
Convection:
Transfer of heat by the movement of air or liquid; Upward travel of hot gases within an open stairwell; Mushrooming effect is convection; Direct flame contact is a form of convection
Radiation:
Heat can travel where matter doesn’t exist;
Example: Heat from the sun reaches the earth; Radiated heat will travel though a space until it reaches an opaque object; Major source of fire spread to an exposure
Law of heat flow:
Heat tends to flow from a hot substance to a cold substance
Fire Tetrahedron: 4 Components:
Oxygen (oxidizing agent), Fuel, Heat, Self-Sustaining chemical reaction
Fuel: Can be found in 3 states of matter:
Solid, Liquid or Gas
- Only gases burn;
- Solid fuels are evolved to gases by pyrolysis;
- Liquids are evolved to gases by vaporization;
- Gaseous fires are the most dangerous b/c they are already in their natural state required for ignition
Oxygen: Need approximately ___% for fire;
Need approximately 16%; Some fuels contain their own oxygen source
Phases of Fire:
3 main progressive phases: Incipient, Steady state burning, and hot smoldering
All Phases in order: Incipient Phase Rollover Steady State Burning Phase Flashover Hot Smoldering Backdraft
Incipient Phase:
Fire is limited to original materials of ignition. Temperature in room is only slightly increased
Rollover:
Last warning before flashover. Unburned combustible gases released during the incipient or early steady state accumulate at the ceiling level. When the flammable range is reached they ignite and a fire develops at the ceiling level
Steady state burning phase:
Sometimes referred to as free burning phase. Sufficient oxygen and fuel are available for fire growth and open burning to a point where total involvement is possible. Temperature at upper regions of the room exceed 1300 degrees. The fire will continue to grow until there is an insufficient amount of oxygen to react with the fuel, then enter the smoldering phase
Flashover:
Occurs when flames flash over the entire surface of a room or area. Caused by the buildup of heat. Can be avoided by directing water at the ceiling level and room contents to cool materials below their ignition points. Today’s bunker gear lasts about 2 seconds in a flashover
Hot smoldering phase:
Fire is reduced to glowing embers. Room completely filled with dense smoke and gases. Can force smoke through small cracks in building. Room temp in excess of 1000 degrees
Backdraft:
Can occur in the late steady state burning phase or hot smoldering; Introducing oxygen into an oxygen deficient atmosphere
Warning signs of a backdraft:
Dense black (carbon-filled) smoke Pressurized smoke exiting small openings Black smoke becoming dense gray yellow Confinement and excessive heat Little or no visible flame Smoke leaving the building in puffs or at intervals Smoke stained windows Muffled sounds Sudden rapid movement of air inward as an opening is made
Thermal Layering of gases:
Also known as heat stratification and thermal balance; Smoke is a heated mixture of air gases and particles as it rises
Products of combustion:
When a fuel burns it undergoes a chemical change; 4 products of combustion: Light, Heat, Smoke, Fire Gases
Flame:
is the visible luminous body of a burning gas; More complete combustion make the fire hotter and less luminous;
Smoke:
consists of a mixture of oxygen, nitrogen, carbon dioxide, CO, carbon particles among others; CO is 210X more attractive to red blood cells than oxygen; The hotter the smoke is the faster it’s going to move; Most fire related death occur from CO
Fire extinguishment theory:
Can be achieved by Temperature reduction, Fuel Removal, Oxygen Dilution, Chemical flame inhibition
Classes of fire:
CLASS A: Wood, Paper, Rubber, Plastic
CLASS B: Liquids, Greases, Gases
CLASS C: Electrical (Equipment)
CLASS D: Combustible Metals
