Chapter 4: Fire Dynamics Flashcards
Fire dynamics
describes the meeting point between fire science, material science, fluid dynamics of gases and heat transfer
Oxidizers
are not combustible but will support or enhance combustion
Using science to remain safe
o Identify fire dynamics
o Anticipate the next stages of the fire
o Anticipate how operations may impact fire behaviour
Physical science
is the study of matter and energy
Matter
physical materials that occupy space and have mass
A physical change
occurs when a substance remains chemically the same but changes in size shape or appearance
o ex: water freezing liquid to solid and boiling liquid to gas
Chemical reaction
when a substance changes from one type of matter into another such as two or more substances combining to form compounds
Oxidization
a chemical reaction involving the combination of an oxidizer such as oxygen in the air with other materials
o Can be slow such as the combination of oxygen with iron to form rust, or rapid, as the combustion of methane (natural gas) or an explosion
Energy :
Is the capacity to perform work
Work and In the case of heat
occurs when a force is applied to an object over distance or when a substance undergoes a chemical, biological, or physical change
in case of heat work means increasing a substances temperature
Potential energy
represents the amount of energy that an object can release at some point in the future
heat of combustion:
The potential energy available for release in the combustion process
o The rate at which fuel releases energy overtime depends on many variables including, chemical composition, arrangement, density of fuel, availability of oxygen for combustion
Kinetic energy
is the energy that a moving object possesses (releasing energy)
how does kinetic energy work
o When heat is introduced the molecules within the field begin to vibrate
o As the Heat (thermal energy) increases, these molecules vibrate more and more rapidly
There are many types of energy including
chemical, thermal, mechanical, electrical, light, nuclear, sound
All energy can
change from one type to another
Energy is measured in
joules
joule
international system of units (SI)
o The quality of heat required to change the temperature of 1 g of water by 1°C is 4.2 joules
British thermal unit (BTU):
customary system of measurement
o Is the amount of heat required to raise the temperature of 1 pound of water by 1°F
o 1055J= 1 BTU
Exothermic reactions
reactions that emit energy as they occur
o Fire is an exothermic chemical reaction that releases energy in the form of heat and sometimes light
Endothermic reactions
reactions that absorb energy as they occur
o Ex converting water to steam
Fire triangle and fire tetrahedron are used to
to explain the elements of fire and how fires can be extinguished
The fire triangle shows
three elements necessary for combustion to occur: fuel, oxygen and heat
o Remove any one of these elements in the fire will be extinguished
the fire tetrahedron model includes
the chemical chain reaction to explain flaming or gas phase combustion
o reducing agent (fuel)
o chemical chain reaction
o heat
o oxidizing agent
Fuels must be in a state to burn
Fuels must be in a gaseous state to burn
off gassing
When heat is transferred to a liquid or solid the substance as temperature increases in the substance starts to convert to a gaseous state
Pyrolysis
off gassing of solids
o Process/Chemical change of breaking down of a solid to a gas
o (solid fuel converted to gaseous fuel-lighter to wood off gases )
Vaporization:
physical change in liquids -off gassing of liquids (liquid converted to vapor)
Piloted ignition
occurs when a mixture of fuel and oxygen encounter an external heat source with sufficient heat or thermal energy to start the combustion reaction
o Most common form of ignition
Auto ignition
occurs without any external flame or spark to ignite the fuel gases are vapours
o The fuels surface is heated to the point at which the combustion reaction occurs (pyrolysis)
Autoignition temperature (AIT)
is the minimum temperature at which a fuel in the air must be heated in order to start self-sustained combustion
The autoignition temperature of a substance is always
The autoignition temperature of a substance is always higher than its piloted ignition temperature
Combustion
is a chemical reaction
There are two modes of combustion
non-flaming and flaming
Nonflaming combustion
occurs more slowly and at lower temperature producing a smouldering glow
o The burning may be localized on or near the fuel surface where it is in contact with oxygen
Flaming combustion and occurs when
produces a visible flame above the material surface
o occurs when a gaseous fuel mixes with oxygen in the correct ratio and heats to ignition temperature
Flaming combustion Requires
liquid or solid fuel to be converted to the gas phase
o When heated both liquid and solid fuels will emit vapours (pyrolysis) that mix with oxygen, gases ignite, producing flames above the material surface
The fire tetrahedron accurately reflects the conditions required for
required for flammable combustion
Each element of the tetrahedron must be in proper proportion and removing any element will interrupt the chemical chain reaction and stops flaming combustion
how does flaming combustion work
Ignition is where the combustion process begins. a heat source pyrolizes a fuel, creating fuel gases. Those glasses mix with oxygen and ignite creating fire
o As the heat transfers to the gasoues combustion products they expand and begin to rise and move away from the fire due to buoyancy
Products of combustion
as fuel burns its chemical composition changes which produces new substances
o Heat and smoke
Smoke
is the product of incomplete combustion
Incomplete combustion
is combustion that is incomplete when any of the fuel is left after combustion has occurred
Combustion is incomplete in a structure fire meaning that some of the fuel does not burn but instead gets entrained with hot gases and rises aloft
unburned fuel is smoke and has a potential to
burn
When the air supply is limited the level of incomplete combustion is
higher which produces more smoke
Most structure fires involve multiple types of fuel including
o Carbon-based fuels: wood, cotton
o Hydrocarbon fuels: plastics, synthetic fibers
- gases
Carbon monoxide
is a toxic product of the incomplete combustion of organic materials
Colorless, odourless gas present at almost every fire
CO combines with haemoglobin about 200 times more effectively than oxygen does
Hydrogen cyanide
toxic and formable substance produced in the combustion and materials containing nitrogen is also commonly found and smoke
Natural fibre such as wool, cotton, silk
Resin such as carbon fibre or fibreglass
Synthetic polymer such as nylon or polyester
Synthetic rubber such as neoprene silicone and latex
HCN is 35 times more toxic than CO
Prevents the body from using oxygen at the cellular level
Can be inhaled ingested or absorbed into the body where it then targets the heart and brain
Enters the bloodstream and prevents the blood cells from using oxygen properly killing the cells
Carbon dioxide
it’s a product of complete combustion of organic materials
Not toxic in the same way as CO or HCN displaces existing O2 creating an oxygen deficient atmosphere
Formaldehyde
- Colourless gas with a pungent irritating odour that is highly irritating to the nose
- 50-100 ppm can cause severe irritation to respiratory tract a serious injury
- Exposure to high concentrations can cause injury to the skin
- Formaldehyde is a suspected carcinogens
Nitrogen dioxide
- Reddish brown gas or yellowish brown liquid which is highly toxic and corrosive
Irritants in smoke
are substances that cause breathing discomfort and inflammation of the eyes, respiratory tract and skin
- Smoke contains significant amounts of unburned fuels in the form of solid and liquid particulates
Pressure
Is the force per unit of area applied perpendicular to a Surface
Standard temperature 68F 20c and atmospheric pressure gases…
remain calm and don’t move
Differences and pressure above or below standard atmosphere pressure,it creates
movement in gases
Gas always moved from areas of —-pressure to areas of —— pressure
Gas is always moved from areas of high-pressure to areas of low pressure
Heated gasses will
rise remain aloft and generally travel up and out
cooler fresh air will
generally travel inward toward the fire this exchange of air creates a convective flow
Heat release rate
total amount of heat released per unit time
The heat release rate is typically measured in kilowatts or megawatts
Heat
is the thermal kinetic energy needed to release the potential chemical energy in a fuel
how does heat work at the molecular level
o Heat vibrates fuel molecules
o Fuel changes from solid or liquid to gas
o The fuel emits flammable vapours which can ignite and release thermal energy
o Thermal energy heats uninvolved fuels
Temperature
is the measurement of heat
o Temperature is the measurement of average kinetic energy in the particles of a sample of matter
o Measured using several different scales
A dangerous misconception is that temperature…
is an accurate predictor for measurement of heat transfer
Heat flux
is a measure of heat energy transfer rate
o Measured in kilowatts per square meter
Heat transfer and PPE
temperature tells wether its safe to go in environment
o Heat transfer rate determines how long PPE will protect the firefighter in environment
Sources of thermal energy
- Chemical, electrical, and mechanical energy are common sources of heat that resukt in the ignition of a fuel
Chemical energy
oxidation
- Chemical energy: the most common source of heat in combustion reactions
The potential for oxidization exists when
any combustible fuel is in contact with oxygen
Self heating
a form of oxidation, is a chemical reaction that increases the temperature of a material without the additional external heat
Spontaneous ignition
which is ignition without the addition of external heat
For self heating to progress to spontaneous ignition, following factors are required:
o The insulation properties of a material immediately surrounding the fuel must be such that heat cannot dissipate as fast as it is generated
o The rate of heat production must be great to raise the temperature of the material to its autoignition temperature
o The available air supply in and around
rags soaked in linseed oil have a potential for spontaneous ignition because
the natural oxidization of this vegetable oil and the cloth will generate heat if some method of heat transfer such as air movement around the rags does not dissipate heat
o also charcoal and hay/manure
The rate of most chemical reactions increases as
temperature of the reacting materials increases
Electrical energy can generate temperatures
high enough to ignite any combustible materials near the heated area
Resistance heating
electric current flowing through a conductor produces heat
Overcurrent or overload
is unattended resistance heating
o when the current flowing through a conductor exceeds its design limits
arcing
the arc is a high temperature luminous electric discharge across a gap or through a medium such as charred insulation
Arcs may be generated when there is
a gap in the conductor such as a cut or frayed wire or when there is a high voltage, static electricity or lightning
Sparking:
when an electrical arc occurs, luminous (glowing) particles can form and splatter away from the point of arcing
Mechanical energy
friction and compression.
The movement of two surfaces against each other creates
heat of friction that generates heat and or Sparks
Heat is generated when a gas is…
because…
compressed
o When a compressed gas expands, the gas absorbs the heat
Heat transfer:
transfer of heat from one point or object to another is a part of the study of thermodynamics
Heat transfer warmer objects to cooler objects because
heated materials will naturally return to a state of thermal equilibrium
The greater the temperature differences between the bodies the greater the transfer rate
Thermal equilibrium
in which all areas of an object are a uniform temperature
- Objects at the same temperature do not transfer heat
Heat transfers from one body to another by three mechanisms
conduction, convection, and radiation
Conduction:
and occurs when:
is the transfer of heat through and between solids
- Occurs when the material is heated as a result of direct contact with the heat source
conduction Results from
increased molecular motion and collisions between a substances molecules, resulting in the transfer of energy through the substance
The more closely packed the molecules of a substance are…
the more readily it will conduct heat
Heat transfer due to conduction is dependent upon three factors:
o Area being heated
o Temperature difference between the heat source and the material being heated
o Thermal conductivity of the heated material
Insulating material
Good insulators are materials that
The best insulators are those made of
slow the conduction of heat from one solid to another
Good insulators are materials that do not conduct heat well because their physical make up disrupts the point-to-point transfer of heat or thermal energy
The best insulators are those made of particles or fibers will void spaces between them filled with a gas such as air
Convection
the transfer of thermal energy by the circulation or movement of a fluid (liquid or gas)
convection In the Fire environment
In the Fire environment, convection usually involves transfer of heat through the movement of hot smoke and fire gases
The heat flows from the hot fire gases to the cooler structural surfaces, building contents and air
convection occurs in what direction?
- Convection may occur in any direction
- Vertical movement is due to the buoyancy of smoke and fire gases
- Lateral movement is usually the result of pressure difference
Radiation
is the transmission of energy as electromagnetic waves,
Radiant heat
can become the dominant mode of heat transfer as the fire grows in size
- can have a significant effect on the ignition of objects located some distance from the fire
- Significant factor in the fire development and spread in compartments
- Numerous factors influence radiant heat transfers including:
o Nature of the exposed surfaces: dark coloured materials in it and absorb heat more effectively than light coloured materials; smoother highly polished surfaces reflect more radiant heat the rough surfaces
o Distance between the heat source in the exposed surface: increasing distance reduces the effect of radiant heat
o Temperature of the heat source: unlike other methods of heat transfer that depends on the temperature of both the heat source and exposed surface, radiant heat transfer primarily depends on the temperature of the heat source
radiated heat energy travels
radiated heat energy travels in a straight line at the speed of light
Radiation is a common cause of
Radiated heat travels through
Materials that reflect, absorb, or scatter radiated energy will
- Radiation is a common cause of exposure fires
- Radiated heat travels through vacuums and air spaces that would normally disrupt conduction or convection
- disrupt the heat transmission
Interaction among the methods of heat transfer
- The fire radiates heat, causes convection heat through hot fuel gases, and conducts heat through burning materials or metals that are involved in the fire
- Convected heat and radiated heat that reaches walls and ceilings heat still surfaces which begin to conduct heat to whatever extent possible based upon the materials thermal conductivity
- But heat us surface well then begin to radiate heat which could lead to ignition, combustion, convection and so on
Fuel
the oxidized or burn material or substance in the combustion process
o gas, liquid or solid
reducing agent
the fuel in a combustion reaction
o fuels may be inorganic or organic
inorganic:
organic
- inorganic: do not contain carbon
- organic: contains carbon
o most common fuels are organic
organic fuels can be divided into hydrocarbon based fuels such as:
o gasoline
o plastics
o fuel oil
o cellulose based materials (wood and paper)
the fuels heat of combustion is:
the total amount of thermal energy released when a specific amount of that fuel burns
synthesized petroleum products
o have higher heats of combustion and may generate higher heat release rates than wood on a per mass basis
power
and in terms of fire
is the rate at which energy transfers- the rate at which energy converts
in terms of fire behaviour, power is the heat release rate during combustion
watt
the standard international (SI) unit for power
o 1 watt = 1 joule per second (j/s)
Heat release rate
is the energy released per unit of time as a fuel burns and is usually expressed in kilowatts (kW) or megawatts (MW)
Heat release rate depends on
the type, quantity and orientation of the fuel
Heat release rate directly relates to
oxygen consumption because the combustion process requires a continuous supply of oxygen to continue
The more oxygen available the ——- the heat release rate
the higher the heat release rate
The heat release rate decreases if
all available oxygen is consumed and not replenished
For flaming combustion to occur,
fuels must be in gaseous state
Thermal energy is required to? in regards to gases
change solids and liquids to gaseous state
Vapour:
the gaseous state of fuel that would normally exist as a liquid or a solid at standard temperature and pressure
most dangerous of all gas fuels
Gaseous fuels such as methane (natural gas), hydrogen, and acetylen
Vapour density
describes the density of gases in relation to air
o Air has a vapour density of 1
o Gases with a vapour density of less than 1 such as methane will rise
o Vapour density greater than 1 such as propane will sink
Heated gases expand and become —- dense
When cooled they ———–
less
When cooled they contract and become more dense
Liquids
have mass and volume but no definite shape except for a flat surface or the shape of their container
Unlike gasses, liquids will not expand to fill all of the container
Liquid density is compared to
water
Specific gravity
is the ratio of the mass of a given volume of a liquid compared to the mass of an equal volume of water at the same temperature
o Like vapour density but with liqid
Liquids with a specific gravity less than 1 such as gasoline and most other flammable liquids are
lighter than water and will float on its surface
Liquids with a specific gravity greater than 1, such as corn syrup are
heavier than wastere and will sink
To burn, liquids must
vapourize
Vaporization
is the transformation of a liquid to a vapour or a gaseous state
For vaporization to occur the escaping vapours must
be a greater pressure than atmospheric pressure
Vapour pressure
the pressure that vapours escaping from a liquid exert
o Indicate how easily a substance will evaporate into air
o Flammable liquids with a high vapour pressure present a special hazard to firefighters
Rate of vaporization
determined by the vapour pressure of the substance and the amount of thermal energy applied
Water converts to steam at 1701 degrees
Water converts to steam at 1701 degrees
Flash point:
is the minimum temperature at which a liquid gives off sufficient vapours to ignite, but not sustain combustion, in the presence of a piloted ignition source (flash but go out immedietly because it cant produce enough heat to make vapours)
Flash point is Commonly used to indicate
the flammability hazard of liquid fuels
Liquid fuels that vaporize sufficiently at temperatures under ——-F present a significant flammability hazard
o Liquid fuels that vaporize sufficiently at temperatures under 100F (38c) present a significant flammability hazard
Fire point
the temperature at which a piloted ignition of sufficient vapors will begin a sustained combustion reaction (flame is sustained because it can produce enough heat to make vapours)
Solubility
describes the extent to which a substance will mix with water
o May be expressed in qualitive terms (slightly or completely) or as a percentage
Miscible:
in water will mix in any proportion
Some liquids are lighter than water and wont mix
Polar solvents
flammable liquids such as alcohols (methane,ethol) will mix readily with water
o Use alcohol resistant fire fighting foams specifically designed for polar solvents
Hydrocarbon fuels: lighter and don’t mix with
water
Liquids less dense than water are more
difficult to extinguish using water as the sole extinguishing agent- may disperse the buring liquid instead of extinguishing it
o Foam or chemical agent it extinguish liquid fuels that are not water soluble
Water soluble liquids will mix with
- Water soluble liquids will mix with some water based extinguishing agents such as many types of firefighting foam
o Extinguishing agent will mix with water and become less effective
o Use alcohol resistant fire fighting foams specifically designed for polar solvents
Solids
have definite size and shape
solids React differently when exposed to heat
- Wax and metals will change their state and melt
o Wood and plastics will not
When solid fuels are heated
they begin to pyrolize (off gass) and release fuel gases and vapour
o The solid will begin to decompose and emit combustible vapours
Solid fuels have a definite shape and size which significantly affects
how easly they ignite
Surface to mass ratio:
the surface area of fuel in proportion to its mass
o Higher surface to mass ratio, less energy needed to ignite
o Low surface to mass ratio, more energy needed to ignite
- As this ratio increases the fuel particles become more finely divided, like shavings or sawdust
The proximity and orientation of a sloid fuel relative to the source of heat also affects
the way fuel burns
o Ex if you ignite one corner of a sheet of plywood lying horizontally (flat) the fire will consume the fuel at a relatively slow rate. The same type of ply wood in a vertical position (standing up) burns more rapidly because the heated vapours rise over more surface area
Normal air consist of about
Normal air consist of about 21% oxygen
The energy released in fire is directly proportional t
the amount of oxygen available for combustion
At normal ambient temperatures materials can ignite and burn at oxygen concentrations as low as
15%
When oxygen concentration is limited the flaming combustion will
diminish causing non flaming combustion and can continue at extremely low oxygen
High ambient temperature, flaming combustion may
continue at considerably lower oxygen concentrations
Many materials that do not burn at normal oxygen levels will burn in
oxygen enriched atmospheres
Flammable (explosive) range
the range of concentrations of the fuel vapour and air
o The fuels flammable range is reported using the percent by volume of gas or vapour in air for the lower explosive (flammable) limit and for the upper explosive (flammable) limit
o There is an ideal concentration at which there is exactly the correct amount of fuel and oxygen required for combustion
lower explosive (flammable) limit (LEL)
the minimum concentration of fuel vapour and air that supports combustion
o concentrations below the LEL are said to be too lean to burn
upper explosive (flammable) limit (UEL):
the maximum concentration of fuel vapour and air that supports combustion
o concentrations below the UEL are said to be too rich to burn
free radicals
formed by molecules of a fuel gas and oxygen that break apart during flaming combustion
o combine with oxygen or with the elemnts released from the fuel gas to form new substances (molecules) and even more free radicals
o the process increases the speed of the oxidation reaction
chemical flame inhibition:
occurs when an extinguishing agent such as dry chemical or Halon-replacement agent interferes with this chemical reaction forms a stable product and terminates the combustion reaction
compart fire development depends on
whether the fire is fuel limited or ventilation limited
fuel limited
when sufficient oxygen is available for flaming combustion
o as long as the fire can reach more ignitable fuel it will continue to burn
o usually progresses through the stages of fire development in order
ventilation limited
have access to all of the fuel needed to maintain combustion but the fire does not have access to enough o2 to continue to burn or spread to all the available fuels
o tend to enter an early stage of decay at the end of the growth stage because there is no longer enough o2 for the fire to become fully developed
all compartment fires begin in the —– stage
once it reaches the —- stage
the fire will either remain —– ——
or the fire will consume all the oxygen and become —– ——
all compartment fires begin in the incipient stage, once it reaches the growth stage the fire will either remain fuel limited or the fire will consume all the oxygen and become ventilation limited
stages of fire development
- 4 stages
incipient,growth,fully developed and decay
3 key factors that control how the fire develops:
o The fuel properties
o The ventilation available
o Heat conservation
Incipient stage:
starts with ignition when the three elements of the fire triangle come together and the combustion process begins
o At this point the fire is small and confied to a small portion of the fuel first ignited
Growth stage
as the fire transitions from incipient to growth stage more of the initial fuel package becomes involved and the production of heat and smoke increases
o The fire may continue to grow to become full developed or may enter an early state of decay depending upon available o2
Fully developed stage
when all the combustible materials in the compartment are burning at their peak heat release based on the available oxygen
o The fire is consuming the maximum amount of oxygen that it can
Decay stage
: as the fire consumes the available fuel or oxygen and the heat release rate begins to decline, the fire enters the decay stage
o Fuel limited fires may self extinguish in this phase or reduce to smoldering
o Ventilation limited fires may also self extinguish however if oxygen becomes available before complete extinguishment these fires are likely to re-enter the growth stage and rapidly become fully developed
Open buring or free burn
condition provides the most basic fire growth curve
o Representative of a fuel limited fire such as a campfire, or a sofa in a large empty warehouse
o Fuel controlled
a single item burning either outside or in a large well ventilated space means ther is sufficient oxygen available to burn the fuel until it can no longer sustain combustion
- Development in the incipient stage depends largely upo
the characteritics and configuration of the fuel involved
The following describe what occurs when a compartment fire enters the incipient stage:
o Radiant heat warms the adjacent fuel and co tinues the process of pyrolysis. A thin plume of hot gases and flame rises from the fire and mixes with cooler air in the compartment
o Ceiling jet: the hot gases in the plume rise until they encounter the ceiling and then begin to spread horizontally
o Hot gases in contact with the surface of the compartment and its contents transfer heat to other material
incipient stage temperature
- The temperature is only slightly above ambient
- Occupants can safely escape and an extinguisher can extinguish
A visual indicator that a fire is leaving the incipient stage is
flame height
o When flames reach 2.5F high, radiated heat begins to transfer more heat than convection and will then enter growth stage
Growth stage
- A variety of fire behaviours can occur
- The fire may consume all the oxygen and enter ventilation limited state of decay
or - Ventilation may provide enough oxygen for rapid development and or growth to full development
Rapid fire development usually occurs during
the growth stage
Entrainment
drawing in air
Unconfined fires draw air from
all sides
the entrainment of air reduces what
cools the plume of hot gases reducing flame length and extension
The following describes entrainment based on the positioning of fuel packages:
o Fires in fuel packages in the middle of the room can entrain air from all sides
o Fires in fuel packages near walls can only entrain air from 3 sides
o Fires in fuel packages in corners can only entrain air from 2 sides
Combustion zone
area where sufficient air is available to feed the fire
When the fuel package is not in the middle of the room the combustion zone…
expands vertically and a higher plume results
A higher plume increases
the temperature in the developing hot gas layer at the ceiling level and increase the speed of development
A fire is in the growth stage unitl
the fire’s heat release rate has reached its peak either because of lack of fuel or lack of oxygen
2 common routes to full development are as follows:
o Fires that consume all available oxygen and transition to a ventilation limited decay
o Fire that have enough oxygen and move through the growth phase and possibly into rapid fire development
Thermal layering
is the tendency of gases to form into layers according to temperature, gas density and pressure
o The hottest gases will form the highest layer while cooler gases will form lower layers
——— from the hot gas layer acts to heat the interior surfaces of the compartment and its content
radiation
Changes in ventilation and flow path can
significantly alter thermal layering
- As the mass and energy of the hot gas layer increases so does
the pressure
Higher pressure causes the hot gas layer to spread
downward within the compartment and laterally through any openings such as doors and windows
- If there are ni openings for lateral movement, the higher pressure gases have no lateral path to follow to an area of lower pressure
Isolated or intermittent flames
move through the hot gas layer up by roof
o Combustion of these hot gases indicates that portions of the hot gas layer are within their flammable range, and that there is sufficient heat to cause ignition
o The appearance of isolated flames is sometimes an immediate indicator of flashover
Neutral plane
the interface between the hot gas layers and cooler layer of air
o The net pressure is zero, or neutral where the layers meet
o Neutral plane exists at openings where hot gases exit and cooler air enters the compartment
o Hot gases at higher than ambient pressure exit through…..
o Lower pressure air from outside the compartment…..
o Hot gases at higher than ambient pressure exit through the top of the opening above the neutral plane
o Lower pressure air from outside the compartment entrains in the opening below the neutral plane
Most residential fires that develop beyond the incipient stage become
ventilation limited
- As the interface height of the hot gas layer descends toward the floor, the greater volume of smoke begins to interrupt the entrainment of fresh air and oxygen to the seat of the fire and into the plume
o This interruption causes the fire
o This interruption causes the fire within the compartment to burn less efficiently
o As the efficiency of combustion decreases (incomplete combustion) the heat release rate decreases and the amount of unburned fuel within the hot gas layer increases
o As the efficiency of combustion decreases (incomplete combustion) the heat release rate decreases and the amount of unburned fuel within the hot gas layer increases
- The fire is now in a state of ventilation limited decay because:
o There is not enough oxygen to maintain combustion
o The heat release rate has decreased to the point that fuel gases will not ignite
- Although the heat release rate decreases in ventilation limited, the temperature
- Although the heat release rate decreases in ventilation limited, the temperature in the room may remain high
o Even if temperatures decrease, pyrolysis can continue
- If there are no other source of oxygen exists the compartment will
fill with black smoke and slowly cooling fuel gases
o If there is oxygen introduced it creates a rapid increase in the heat release rate and growth of the fire
Rapid fire development
refers to the rapid transition from the growth stage or early decay stage to a ventilation limited, fully developed stage
o Among these events are flashover and backdraft
To protect yourself and your crew, you must be able to:
o Recognize the indicators of rapid fire development
o Know the conditions created by each of these situations
o Determine the best action to take before they occur
Flashover:
rapid transition from the growth stage to the fully developed stage is known as flashover
o When flashover occurs the combustible materials and fuel gases in the compartment ignite almost simultaneously; resulting in full room fire involvement
flash over Typically occurs during
o Typically occurs during fires growth stage but may occur during fully developed stage as a result of ventilation
o During a flshover the environment of the room changes from
o During a flshover the environment of the room changes a two layer (hot on top, cool on bottom) to a single well mixed hot gas condition from floor to ceiling
o The environment is untenable
o As flashover occurs the gas temperatures in the room reach
1100 F (593c) or higher
radiations part in flashover
o Radiation from the upper layer heats the compartments contents until they reach their ignition temperature simuletaneously
o When the upper layer ignites the amount of radiation increases to levels which rapidly ignite contents in the room
o During flashover the volume of burning gases can increase from
approximately ¼ to ½ of the rooms upper volume to fill the rooms entire volume and extend out of any openings
o A significant indicator of flashover is
rollover
rollover
a condition where the unburned fire gases that have accumulated at the top of the compartment ignite and flames propagate through the hot gas layer or across the ceiling
o Rollover may occur during the
the growth stage as the hot gas layer forms at the ceiling of the compartment
o Flames may appear in the layer when the combustible gases reach their ignition temperature
Rollover will generally precede
flashover but it may not always result in flashover
Rollover contributes to flashover conditions because
the buring gas athe the upper levels of the room generate tremendous amounts of radiant heat which transfers to other fuels in the room
- There are 4 common element of flashover:
o Transition in fire development: represents transition from growth to fully developed
o Rapidity: happens rapidly often in a matter of seconds
o Compartment: there must be an enclosed space such as a single room or enclosure
o Pyrolysis of all exposed fuel surfaces: fire gases from all the combustible surfaces ignite
Two interrelated factors determine whether a fire within a compartment will progress to flashover
- there must be sufficient fuel and the heat release rate must be sufficient for flashover conditions
- ventilation. A developing fire must have sufficient oxygen to reach flashover
SCBA face pieces fail after 5 minutes of exposure to a heat flux
15kW/m2
You must be aware of the following flashover indicators to protect yourself:
o Building indictors: interior configuration, fuel load, thermal properties and ventilation
o Smoke indicators: rapidly increasing volume, turbulence, darkening color, optical density and lowering of the hot gas layer and or neutral plane
o Heat indicators: rapidly increasing temperature in the compartment, pyrolysis of contents or fuel packages located some distance away from the fire or hot surfaces
o Flame indicators: isolated flames or rollover in the hot gas layers or near the ceiling
- Level of neutral plane observed from the exterior of the structure are also good indicators of fire behaviour within the structure as follows:
o High neutral plane: fire in early stages of development
Note high ceilings can hide a fires stage
A high neutral plane can also indicate fire above your level
o Mid level neutral plane: could indicate compartment has not yet ventilated or that flashover is approaching
o Very low neutral plane: may indicate that fire is reaching backdraft conditions
Could also mean fire is below you
- When fire is in ventilation limited decay, the introduction of new oxygen can trigger
flashover quickly
- Flashover may not occur in every compartment fire, such as
large compartments
- The fire may not progress to flashover but instead become ventilation limited and enter decay stage while continuing pyrolysis and increasing fuel conent of the smoke
Backdraft
an expolosivley rapid combustion of the flammable gases
what kind of fire can produce backdraft
o a ventilation limited compartment fire that can produce a large volume of flammable smoke and other gaes due to incomplete combustion. Fuel sources are unburned
back drafts occur in spaces containing
containing high concentration of heated flammable gaes that lack sufficient oxygen for flaming combustion
backdraft indictors include:
o Building indicators - Interior configuration, fuel load, thermal properties, amount of trapped fuel gases, and ventilation
o Smoke indicators - Pulsing smoke movement around small openings in the building; smoke-stained windows
o Air flow indicators - High velocity air intake
o Heat indicators - High heat, crackling or breaking sounds
o Flame indicators - Little or no visible flame
The effects of a backdraft can vary considerably depending on a number of factors, including:
o Volume of smoke
o Degree of confinement
o Temperature of the environment
o Pressure
o Speed with which fuel and air mix
2 main types of fully developed fires
ventilation limited and fuel limited
“Fully developed” means
- Means fire has grown as much as it can but can continure to grow if fuel/oxygen is reintroduced
- Fuel limited conditions
: the available fuel limits the peak heat release in a fuwl limited fully developed fire
Ventilation limited conditions
lacks the oxygen available to grow because the number and size of openings in the compartment limit the entainment of air
Ventilation liited fully developed fires present a hazardous situation for firefighter because
the potential for a window failure to provide fresh oxygen and increase peak heat release rate can endanger firefighters and civilians
o Firefighters must transition the fire from ventilation limited to fuel limited
Fuel limited decay
after a fuel limited fire reaches the fully developed the stage the fire will decay as the fuel is consumed
o heat release rate begins to decrease but temperature of objects stay high
o compartment fires rarely enter state of fuel limited decay unless compartment burns all the way to the ground
ventilation limited decay
not necessarily the last stage of the fires development
o to ensure that the decay stage of a ventilation limited fire is the fires final stage, a controlled transition from ventilation limited to fuel limited must take place
o firefighters must ensure no ventilation until heat release is complete
structures:
composed of individual compartments connected by hallways, stairways or openings such as doorways
flow path
- the method by which the fire receives the needed oxygen to sustain the combustion reaction occurs through one or many flow paths
the flow path is composed of 2 regions:
o the ambient air flow in
o hot exhaust flow out
flow path direction
- unidirectional due to pressure differences where ambient air flows toward the seat of the fire and reacts with the fuel
- the products of combustion flow
away from the fire toward the low pressure outlet
- in a structure fire the floor plan and openings within the structure determine available flow path
- a flow paths effectiveness to transport ambient air to the seat of the fire is based on the following:
o size of the ventilation opening
o length of the path traveled
o number of obstructions
o elec=vation differences between the base of the fire and the opening
fireifghters working in the exhaust portion of the flow path will feel
the increase of temperature as the velocity and/or turbulence increases
unplanned ventilation
occurs when a structural member fails usually because exposure to heat and intorduces a new source of oxygen to the fire
o could result from a failure of a: window, roof, doorway, wall
o can also originate from inside the building if the floors collapse
unplanned ventilation is often a result of:
o occupant action
o fire effects on the building
o actions other than planned, systematic and coordinated tactical ventilation
wind conditions
the wind can increase pressure inside and drive smoke and flames to unburned portions and onto firefighters
o must be aware of wind direction and velocity
wind speeds as low as –kmph can create wind driven fire conditions
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smoke explosion:
occurs when a mixture of unburned fuel gases and oxygen comes in contact with an ignition source
o the result will be explosive, rapid combustion
o violent because they involve premixed fuel and oxygen
firefighters can influence fire dynamics in a number of ways:
o temperature reduction: usimng water or foam agent
o fuel removal: eliminating sources of fuel in the path of fire
o oxygen exclusion/flow path control: using door control tactical ventilation techniques
o chemical flame inhibition: using extinguishing agents to inhibit or interrupt combustion reaction
2 primary types of dangerous building conditions are:
o Conditons that contribute to the spread and intensity of the fire
o Conditons that make the building susceptible to collapse
There may be many unkown factors that are not relected in fire resistance estimates such as:
o The duration of the fire up to the time of arrival
o The buildings contents
o Ways the building contents affect the heat release rate
o The heat release rate and intensity of the fire
o Renovations to the interior that may have compromised fire resistance
Fuel load/fire load
the total quanity of combustible contents of a building, space or fire area
- At scene youwil onlybe able to estimate fuel load base dupon ypur knowledge and experience
The orientation of the fuels as well as their surface to mass ratio will influence rate and intensity
- Fires originating on upper levels generally extend downward much more slowly following the fuel path or as a result of collapse
Assuming that there is available oxygen, the higher the fuel load, the more likely the fire will behave in the following ways:
o If structural members are part of the fuel load, structural integrity of the building will deteriorate faster.
o The longer the fire burns, the more fire spread accelerates.
o the fire may have a higher heat release rate.
o The structure may self-ventilate, introducing even more oxygen to the fuel-limited fire and accelerating fire development and involvement of combustible structural members.
- If fires are ventilation-limited, higher fuel loads indicate
a greater amount of unburned fuel that could reignite with the introduction of a new oxygen source.
- There may also be a greater amount of unburned fuel gases in the air because fuel packages pyrolized but did not begin combustion before the building became oxygen-limited.
- Such buildings are subject to backdrafts and flashovers if firefighters do not coordinate ventilation.
Building compartmentation
is the layout of the various open spaces in a structure and includes
o Stories above or below
o Floor plan
o Openings between floors
o Continuous voids or concealed spaces
o Barriers to fire spread
- Each of these elements may contribute to fire spread or containment
Any space with no complete fire barrier dividing it is considered a
compartment
- Two rooms that a door way connects are considered two compartments only if the door is closed between them
Legacy construction/older structures prior to approximately 1990 had the following features:
o Smaller compartments
o More compartments within the same square footage as modern homes
o Windows that could be opened for ventilation
o Air pockets in empty wall cavities; this construction technique used the air as insulation
Modern single family structures may feature:
o Open floor plans
o High ceilings
o Atriums
o Lightweight manufactured structural components
o Sealed windows
o Wall cavities
o Synthetic insulation
Building characteristics to consider when ventilating include:
o Occupancy classification
o Construction type
o Square footage and compartmentation
o Ceiling height
o Number of stories above and below ground level
o Number and size of exterior windows, doors, and other wall openings
o Number and location of staircases, elevator shafts, dumbwaiters, ducts, and roof openings
o External exposures
o Extent to which a building connects to adjoining structures
o Type and design of roof construction
o Type and location of fire protection systems
o Contents
o Heating, ventilation, and air conditioning (HVAC) system
a fire in a large compartment will normally develop
slower due to greater volume of air and increased distance radiant heat must travel
tall ceilings can make determining the extent of the fire development more difficult as
smoke can accumulate in the ceiling while conditions at floor level remain unchanged
- thermal properties of a building include:
o insulation: contains heat within the building which causes localized increase in the temperature and fire growth
o heat reflectivity: increases fire spread through the transfer of radiant heat
o retention: maintains temperature by slowly absorbing and releasing large amounts of heat
failure of lightweight trusses and joists
- unprotected engeneered steel and wooden trusses can fail after 5-10mins of exposure to fire
- traditional wood joist roof tend to loose their strength when exposed to fire and beome soft and spongy before collapse
- until they fall there may be noindicatio of danger
- construction, renovation and demolition hazards:
o additional fuel loads and ignition sources
o inoperative standpipes and sprinkler systems
o subject to rapid fire spread when they are partially completed
breached walls, open stairwells, missing doors
o occupants may still be inside
o accumulation of debris can block exits
o contractors and owners/occupants don’t always follow local building codes
What is the measurement of the average kinetic energy in the particles of a sample of matter?
Temperature
Only mechanism to transition ventilation limited to fuel limited is to
Extinguish some of the burning fuel
-it is not possible to make enough openings
