Chapter Five, Fire Behavior

Knowledge objectives

  • Describe the chemistry of fire

  • List the three states of matter

  • list the five forms of energy

  • Explain the concept of the fire triangle

  • Explain the concept of the fire tetrahedron

  • Combustion

  • Flow path

  • Four methods of extinguishing fires

  • Class A,B,C,D, K fires

  • Describe the importance of the following characteristics in solid-fuel fires: composition of fuel, amount of fuel, and configuration of fuel

  • Describe the four stages of fire development incipient stage, growth stage, fully developed stage

  • TermsL thermal layering, neutral plane, rollover, flashover, backdraft, fuel-limited fires, ventilation-limited fires, and smoke explosion

  • Describe conditions that cause thermal layering

  • Describe the conditions that lead to rollover, flashover, backdraft.

Chemistry of fire

  • understanding conditions needed for a fire to ignite and grow will increase your effectiveness

  • Being well trained in fire behavior will allow the fire fighter to control a fire with less water

What is fire

  • Rapid chemical process that produces heat and usually light

  • Fire requires fuel in the form of combustible vapors

    States of matter

    Three states: solid liquid gas

    Solids have a definite size and shape.

Fuels

  • fuels are materials that store energy

  • Energy released in the form of heat and light has been stored before it burns

Exothermic (fire) versus endothermic (ice)

Conditions needed for fire

  • A combustible fuel

  • Oxygen in sufficient quantities

  • A source of heat

A fourth factor must be added to maintain a self sustaining fire

  • Chemical Chain reactions

If you remove any of these elements, the fire will go out

Chem of combustion

  • Pyrolysis is the process that liberates gaseous fuel vapors due to the heating of a solid fuel

  • Almost all fuel consists of hydrocarbons

  • Incomplete combustion results in large quantities of deadly gasses and compounds

  • Toxic by product of combustion

  • Composed of

  • - particles , vapors, gasses

Temp of smoke will vary depending on the conditions of the fire and the distance the smoke travels

Inhalation of hot gasses in smoke may cause severe injuries in the form of severe burns of the skin and respiratory tract.

Heat transfer

  • measured as energy flow per unit of time

  • Occurs when there is a difference in temperature between two objects

  • Heat will flow from a hotter object to a cooler temperature until they reach equal temperature

  • when two objects have the same temp heat transfer will not occur

  • The rate of heat transfer is dependent on two factors

  • difference in temp

Conduction

  • Process of transferring heat to and through one solid to another

  • Objects that have more tightly packed molecules are more efficient in conducting heat

  • Dependent on thermal conductivity, area, and difference in temp

Convection

  • the transfer of heat by the flow of gases or fluid from hotter areas to cooler areas

  • Involves primarily smoke and hot gasses generated by the fire

  • Transfer is from a hotter gas to a cooler surface

Radiation

  • Transfer of heat in the form of an invisible wave

  • travels in all directions

  • Not seen or felt until it strikes an object

Methods of extinguishment

  • Cool the burning material

  • exclude oxygen

  • remove fuel

  • interrupt the chemical reaction

Class Afire

  • Ordinary solid combustibles (wood)

  • Cool it below ignition temperature

  • Remove oxygen

Class B fire

  • Liquid fire

  • Put out with foam (removes oxygen component)

  • Break the chemical reaction with foam

Class C fire

  • Electrical fire

  • Energized fire

  • Remove the energy then it becomes B or A

Class D fires

  • Combustible metals

  • Magnesium Potassium Lithium

  • Must be attacked with special agents

Class K

  • Combustible cooking oils and fats

  • Special K extinguishers

Solid fuels

  • A variety of solid fuels are found in most buildings. wood and wood based products, fabrics, paper, carpeting.

  • Building materials and the building contents will influence how a fire burns.

  • Pyrolysis: when fuels are heated, they begin to change chemically. Pyrolysis is a process by which a solid (or a liquid) undergoes thermal degradation into smaller volatile molecules, without interacting with oxygen or any other oxidant

Solid fuels

  • Fuel limited fire, the fire has sufficient oxygen but limited fuel

  • All else being equal, when more fuel is available there is a higher heat release rate. (HRR)

Solid fuel fire development

  • Progresses through four classic stages

  • - Incipient stage

  • Growth stage

  • Fully developed stage

  • Decay stage

Signs of a backdraft

  • Large heat build up

  • little of no visible flame from exterior

  • Living fire

  • smoke stained windows

  • no smoke showing

  • turbulent smoke

Rapid fire growth: introduced air into a ventilation limited fire can result in explosive growth.

Decay stage

  • Can occur because of a decreasing fuel supply

    Oxygen limited

  • - Rate of combustion slows

  • visible flames decrease or disappear

  • Mels will continue to pyrolyze and create additional flammable vapors and

Bleve

Occurs when a vessel storing liquid fuel under pressure is heated excessively

The vessel can fail releasing all of the heated fuel in a massive explosion