Chapter 5 Fire Behaviour Flashcards
Combustion
Chemical process of oxidation that occurs fast enough to produce heat and usually light
Fire definition
Rapid oxidation, which is a chemical process resulting in heat and light
Heat
Form of energy characterized by vibration of molecules and capable of initiating and supporting chemical change
Temperature
Measure of a materials ability to transfer heat energy to other objects.
Measure of average kinetic energy of particles in a sample of matter
Fuel
A material that will maintain combustion
Oxidizer
Any material that readily yields oxygen or other oxidizing gas or that readily reacts to promote or initiate combustion
Matter
Anything that takes up space and has mass
All fire
Involves a heat producing chemical reaction between some type of fuel and oxidizer
Oxidation
Can be rapid like methane combusting or slow like rust.
Chem process when a substance combines with oxygen in air
Energy
Capacity to perform work
Joules
A force of one newton through a distance of one meter.
For ffers, the energy required to increase temp. 4.2joules to raise one gram of water one degree.
1055 J = 1 btu
Pyrolysis
Chemical decomposition of a solid material by heating. Pyrolysis often precedes combustion
Vaporization
Liquid into a gaseous state. Rate depends on heat, substances, pressure and surface area
Ignition
The process of initiating self sustained combustion
Piloted ignition
Moment when mix of fuel and oxygen encounter external heat sufficient to start combustion reaction
Auto ignition
Initiation of combustion by heat without spark or flame. NFPA 921
Always a higher temp for a given substance than its piloted temp
Fire triangle
Heat oxygen fuel
Fire tetrahedron
Fuel heat oxygen and self sustaining chemical reaction
Passive agent
Material that absorbs heat but does not participate actively in combustion process. E g drywall.
Nonflaming combustion
Burning is localized on or near the fuels surface where it is in contact with oxygent
Eg burning charcoal or smouldering wood or fabric
Flaming is when substance is vaporized and the vapours are ignited forming the flame
CO
Colourless odourless
0.2 percent unconscious in 30 minute. High concentration immediate collapse and unconscious
Formaldehyde
Colourless gas. Pungent odor. 50-100ppm can cause severe irritation to respiratory track and serious injury. Can injure skin in high concentrations. Suspected carcinogen
Nitrogen dioxide
Reddish brown gas or yellowish brown liquid which is highly toxic and corrosive
Sulfur dioxide
Colourless gas with a choking or suffocating odor. Toxic and corrosive. Irritates mucus membranes
Hydrogen cyanide
Colourless toxic and flammable liquid below 26C. Produced by combustion of nitrogen containing products. Lower smoke concentrations than CO. Common from polyurethane furniture and bedding
Carbon dioxide
Colourless odourless heavier than air. Neither burns nor supports combustion. Used to extinguish class B or C fires by displacing oxygen
Irritants in smoke
Hydrogen chloride
Formaldehyde
Acrolein
Flame brightness
Luminosity will decrease with more complete combustion
Thermal energy
Heat. Kinetic energy transferred from high to low temp substance
Heat is the thermal kinetic energy needed to release the potential chemical energy in a fuel
Molecules vibrate. Release vapours which can ignite and release thermal energy
Temperature
Measurement of heat, more specifically measure of average kinetic energy in particles in a sample of matter
Sources of thermal energy
Chemical mechanical electrical (most common the result in ignition of fuel) but also light nuclear and sound
Chemical energy
Most common source of heat
Self heating is a form of oxidation, normally lost to surroundings nearly as fast as generated. More heat means faster reaction means more heat
Common self heating materials
Charcoal linseed oil soaked rags straw and manure
4 types of electrical heating
Resistance heating
Overcurrent (a type of resistance heating; too much electricity for design)
Arcing
Sparking
Mechanical heating
Friction or compression
Heat flux
Measure of rate of heat transfer to a surface expressed in kilowatts/m2, kJ/m2 x sec or btu/ft2 x sec
* squared not doubled
Heat transfer is based on
Temp differential and thermal conductivity
Conduction
More dense molecules will more readily it will conduct heat Based on 3 factors: Area being heated Temp difference Thermal conductivity
Thermal conductivity of common objects in W/mK
Copper 386
Steel 36-54
Concrete 0.8-1.2
Wood 0.13
Radiation transfer factors
Nature of exposed surface - dark emits and absorbs heat better. Smooth or polished reflect more than rough surfaces
Distance
Temp difference - as temp increases radiant energy increases by a factor to the fourth power.
Radiant heat travels in straight line at speed of light
Reducing agent
Fuel that is being oxidized or burned during combustion
Heat of combustion
Total amount of thermal energy by combustion reaction if a fuel were completely burned. Measured in btu per lb or mJ per kg
Heat release rate
Total amount of heat released per unit time
Power
Measured in watts. 1 watt = one J per second
Defined as energy transferred over time or rate at which energy is converted from one form to another.
In terms of fire power is described as hrr
Common flammable gases
Methane - vapour density 0.55, ignition temp 540C
Propane 1.52 450C
CO 0.96, 609C
Vapour density
1.0 is air. Under one rises
Specific gravity
Density compared to water. Less than one floats
Flammable liquid
Flash point below 37.8C (100F) and vapour pressure less than 40 psi absolute
Vapour pressure
Measure of the tendency of a substance to evaporate. The pressure at which a vapour is in equilibrium with its liquid phase for a given temp.
High VP is more likely to evaporate
Flash and fire point
Flash is minimum temp for a liquid to ignite but not sustain
Fire point is enough for a liquid to support combustion (for 5 seconds). Usually a few degrees higher for fire point
Flash point is used to indicate flammability
Solubility
Degree to which a substance dissolves in a solvent (usually water)
May be qualitative like slightly, completely or a percentage
Miscible
Will mix with water in any proportion
