ch.3-Fire Behavior

Exothermic

Reactions that give OFF energy as they occur

Endothermic

Reactions that ABSORB energy as they occur

Combustion

An exothermic chemical reaction that is a self-sustaining process of rapid oxidation of a fuel, that produces heat and light.

Fire Triangle

Oxygen, Heat, and Fuel

Fire Tetrahedron

Oxygen, Fuel, Heat, and a self-sustained chemical reaction. - Best concept that describes the flaming combustion.

Potential Energy

Stored energy possessed by an object that can be released in the future to perform work.

Kinetic Energy

The energy possessed by a moving object.

Joules

Measure of heat energy in the metric system. A Joule is equal to 1 newton over a distance of 1 meter.

Btu (British thermal unit)

Measure of heat in the customary system. Amount of heat required to raise the temperature of a pound of water 1 degree F.

Pyrolysis

The chemical decomposition of a substance through the action of heat.

Autoignition

Same as ignition temperature except that no external ignition source is required for ignition because the material itself has been heated to ignition temperature. The fuel surface is chemically heated to the point at which the combustion reaction occurs.

Piloted Ignition

when a mixture of fuel and oxygen encounter an external heat source with sufficient heat energy to start the combustion reaction.

Sources of Heat Energy (6 things):

Chemical, mechanical, electrical, light, nuclear, and sound energy can cause a substance to heat up by increasing the speed with which molecules are moving. (Chemical heat energy is the most common in combustion reactions).

Electrical heat energy (4things)

1. Resistance Heating
2. Overcurrent or overload
3. Arching
4. Sparking

Mechanical heat energy is generated by?

Friction or compression

Heat of friction is generated by?

the movement of 2 surfaces against each other.

Heat of compression is generated when?

A gas is compressed. (diesel engines use this principle to ignite fuel vapor without a spark plug)

Conduction

The transfer of heat within body or to another body by direct contact. Conduction is the heat flow through and between solids.

Convection

The transfer of heat energy from a fluid (liquid or gas) to a solid surface. In the fire environment, this usually involves transfer of heat through the movement of hot smoke and fire gases.

Radiation

The transmission of energy as an electromagnetic wave (such as light waves, radio waves, or x-rays) without a intervening medium.

Passive Agents

Materials that absorb heat but do not participate actively in the combustion reaction.

Reducing Agent

The fuel that is being oxidized or burned during combustion.

Oxidation

When a combustable comes in contact with oxygen.

What is the most common source of heat in combustion reactions?

Chemical heat energy.

2 key factors influecing the combustion process?

-Physical state of the fuel
-and its distribution or orientation(horizontal or vertical)

Vapor density

describes the density of gasses in relation to air

Specific gravity

Weight of a substance compared to the weight of a equal volume of water at a given temp.

Vaporization

Transformation of a liquid to vapor or gaseous state.

Vapor pressure

The pressure produced or exerted by vapors released by a liquid.

Flash point

The temp. at which a liquid gives off sufficient vapors to ignite, but not sustain, combustion.

Fire point

The temp. at which sufficient vapors are being generated to sustain the combustion reaction.

Solubility

Describes the extent to which a substance will mix with water.

Miscible

Materials that are capable of being mixed.

Pyrolysis

The chemical decomposition of a substance through the action of heat.

Suface-to-mass ratio

The surface area of the fuel in relation to its mass.
Objects with a greater suface area than mass generate combustible pyrolisis products more quickly.

Heat Release Rate (HRR)

The energy released per unit of time as a given fuel burns and is usually expressed in kilowatts (kW).
Dependent on the type, quantity, and orientation of the fuel.

Oxidizers are not combustible but like oxygen they will...

...support combustion.

Flammable range

The range between the upper flammable limit and lower flammable limit in which a substance can be ignited.

Lower flammable limit

The minimum concentration of fuel vapor and air that supports combustion.

Upper flammable limit

The concentration above which combustion can not take place.

Carbon Monoxide (CO)

Byproduct of the incomplete combustion of organic materials.
Acts as a chemical asphyxiant by binding with hemoglobin in the blood that transports oxygen throughout the body.

Hydrogen Cyanide (HCN)

Produced in the combustion of materials containing nitrogen. Commonly encountered in smoke at lower concentrations than CO.
Acts as a chemical asphyxiant.

Carbon Dioxide (CO2)

Product of complete combustion of organic materials.
Acts as a simple asphyxiant by displacing oxygen.

Class A Fires

Involve ordinary combustible materials such as wood, cloth, paper, rubber, grass, and plastics.
Primary method of extinguishment: cooling to reduce the temp. of the fuel to slow or stop the release of pyrolysis products.

Class B Fires

Involve flammable and combustable liquids and gasses such as gasoline, oil, lacquer, paint, mineral sports, and alcohol.
Primary method of extinguishment: shutting off gas supply. Appropriately applied foam and/or dry chemical agents.

Class C Fires

Involve energized equipment. Household appliances, computers, transformers, electric motors, overhead transmission lines.

Class D Fires

Combustible metals such as aluminum, magnesium, potassium, sodium, titanium, and zirconium.
Primary method extinguishment: No single agent effectively controls fires.

Class K Fires

Involves oils and greases normally found in commercial kitchens and food preparation facilities using deep friers.
Primary method of extinguishment: extinguishment agent specifically formulated for the materials.

2 modes of combustion

1. Flaming Combustion - When heated, liquid fuel gives off vapors (it is being vaporized) that then mix with oxygen, burn and produce flames.
2. Smoldering Combustion - Solid fuels that undergo oxidation at the surface. This oxidation is nonflaming.

Modes of combustion

Differentiated based on where the reaction is occuring.

Temperature

Measurement of kinetic energy

Resistance heating

occurs when electric current flows through a conductor at a normal design limit.

4 stages of fire development

1. Incipient stage
2. Growth stage
3. Fully developed stage
4. Decay stage

Fuel controlled fire

When sufficient oxygen is available, fire development is controlled by the characteristics and configuration of the fuel.

Ventilation controlled fire

when fire development is limited by the air supply.

Incipent stage

starts with ignition and is the point when the three elements of the fire triangle come together and combustion occurs.

Mushrooming

As the smoke hits the ceiling, hot gases begin to spread horizontally across the ceiling
Scientific or engineering refer to it as a CEILING JET.

plume

hot gases and flames rise from the fire and mix with the cooler air within the room.

Growth stage

Early stage of fire where fuel and oxygen is unlimited. Characterized by a rapidly increasing release of heat.

Thermal layering

gases form into layers according to temperature..Hottest gases tend to be in the top layer.

Isolated flames (Ghosting)

pockets of flames may be observed moving through the hot gas layer above the neutral plane.

Rollover/ Flameover

fire gases accumulate at the top of a compartment ignite and flames propagate through the hot gas layer or across the ceiling.

flashover

When the temp. in a compartment results in the simultaneous ignition of all of the combustible contents in the space.
Conditions in the compartment change very rapidly from partial to full involvement of the compartment.

3 things that happen just before flashover?

1. Temps rapidly rise
2. Additional fuel becomes involved
3. Fuel in the compartment gives off combustible gases

Fully developed stage

stage of burning when all combustible material in the compartment are burning.

Decay stage

Stage of fire development where the fuel becomes consumed or the oxygen concentration fall to the point where flaming combustion can no longer be supported.

Backdraft

An increase in ventilation in a fire that is in the decay stage that can result in deflagration.

Signs of backdraft

- little or no visible flames
- smoke leaving building in puffs
- inwardly drawn smoke( sucking phenomenon)
- black smoke becoming dense gray- yellow
- smoke stained windows with heat induced cracks
- pressurized smoke exiting small openings.

Factors that affect fire development

- fuel type
- availability and location of more fuel
- comartment geometry
- ventilation and changes in ventilation
- ambient conditions

Temperature reduction

Reducing the temp. of a fuel to a point where it does not produce sufficient vapor to burn.
most common method of fire control and extinguishment by cooling with water

Fuel removal

Removing the fuel source.

Oxygen exclusion

Reducing the oxygen available to the combustion process. Reduces fire growth and may totally extinguish it over time.

Chemical flame inhibition

Interrupts the combustion reaction and stops flame production.

How can fire be controlled and extinguished?

-Temp. reduction
-Fuel removal
-Oxygen exclusion
-Chemical flame inhibition