Transcript Fire_Behavior - Evfd

Fire Behavior
Module 2
TS 2–5
DEFINITION OF POWER
• Amount of energy delivered over a given
period of time
• Units of power
– English or Customary System —
Horsepower
– International System of Units (SI) — Watts
HEAT & TEMPERATURE
TERMS
TS 2–6a
• Heat — Energy transferred from one body to another
when the temperatures of the bodies are different
• Temperature — Measure of warmth or coldness of an
object based on some standard (usually freezing and
boiling points)
• Degrees Celsius — SI unit of temperature measurement
– 0C = freezing point of water
– 100C = boiling point of water
HEAT & TEMPERATURE
TERMS (cont.)
TS 2–6b
• Degrees Fahrenheit — Customary unit of temperature
measurement
– 32F = freezing point of water
– 212F = boiling point of water
• Joule — Approved SI unit of all forms of energy, including
heat
• Calorie — Amount of heat required to raise the
temperature of 1 gram of water 1 degree Celsius
HEAT & TEMPERATURE
TERMS (cont.)
• British thermal unit — Amount of heat
required to raise the temperature of 1
pound of water 1 degree Fahrenheit
• Mechanical equivalent of heat
– 1 calorie = 4.187 joules
– 1 Btu = 1,055 joules
TS 2–6c
Thermal Balance
• A fire seeks some sort of equilibrium between
products flowing towards and away from the fire
itself.
– The energy coming into a fire must equal the energy
being released.
• The degree of thermal balance existing in a
closed room during a fire’s development is
dependent upon fuel supply and air availability
as well as other factors.
• The hot area over the fire (often termed the fire
plume or thermal column) causes the circulation
that feeds to the fire.
Thermal Balance cont.
• However, when the ceiling and upper parts
of the wall linings become super-heated,
circulation slows down until the entire
room develops a kind of thermal balance
with temperatures distributed uniformly
horizontally throughout the compartment.
Thermal Imbalance
• Occurs through turbulent circulation of
steam and smoke in the fire area and this
leads to decreased visibility and
uncomfortable conditions.
Chemical Byproducts of
Combusiton
• Gases
– Most gases produced by fire are toxic
– Common gases include:
• Carbon monoxide
• Hydrogen cyanide
• Phosgene
IDLH Table
Toxic Atmosphere
Sensibility
IDLH PPM
Caused By
Miscellaneous
Carbon Dioxide
Colorless /
Odorless
40,000
Free burning
End products of
complete combustion of
carboniferous materials
Carbon Monoxide
Colorless /
Odorless
1,200
Incomplete Combustion
Cause of most fire related
deaths
Hydrogen
Chloride
Colorless to
slightly yellow
with a pungent
odor
50
Burning plastics such as PVC
Irritates eyes and
respiratory tract
Hydrogen
Cyanide
Colorless with a
bitter almond odor
50
Burning wood, polyurethane,
nylon, foam, rubber and paper
Chemical asphyxiate,
hampers respiration at
the cellular tissue level
Nitrogen Dioxide
Red Brown with
an acid odor
20
Given off around grain bins
also liberated when pyroxylin
plastics decompose
Irritates nose and throat
Phosgene
Colorless with an
odor of musty hay
and tasteless
2
Produced when refrigerants
such as Freon contact a flame
Forms hydrochloric acid
in the lungs due to
moisture
Carbon Monoxide
• Because carbon monoxide causes more fire
related deaths than any other toxic gas, a
greater explanation is needed.
• This colorless, odorless gas is present with
every fire. The poorer the ventilation and the
more inefficient the burning, the greater the
quantity of carbon monoxide is formed.
• A rule of thumb, though subject to much
variation, is that the darker the smoke, the
higher the carbon monoxide levels. Black smoke
is high in particulate carbon and carbon
monoxide because of incomplete burning.
CO (Continued)
• Carbon monoxide concentrations in air above fivehundredths of one percent (0.05 percent) or 500 parts
per million (ppm) can be dangerous.
• When the level is more than 1 percent, unconsciousness
and death can occur without symptoms.
• Therefore, firefighters should not use signs, and
symptoms as safety factors. Dizziness, headaches,
nausea, vomiting, and cherry-red skin can occur at many
concentrations, depending on the firefighter’s dose and
exposure.
CO (Continued)
• The higher the carbon monoxide concentrations the
quicker carboxyhemoglobin occurs
• A firefighter’s general physical condition, age, degree of
physical activity, and length of exposure all affect the
actual carboxyhemoglobin level in the blood.
• Firefighters frequently exposed to carbon monoxide
develop a tolerance to it, and they can function without
symptoms with residual levels of serum
carboxyhemoglobin that would produce significant
symptoms in an average adult.
More CO
• A 1-percent concentration of carbon monoxide will cause a
50-percent level of carboxyhemoglobin in the blood in 2½ to 7
minutes.
• A 5-percent concentration will cause a 50-percent level of
carboxyhemoglobin in the blood in 30 to 90 seconds
• A firefighter previously exposed to high levels of carbon
monoxide may react later in a safer atmosphere because the
newly formed carboxyhemoglobin may be traveling through
his body.
• These firefighters should not be allowed to resume firefighting
activities until the danger of toxic reaction has passed. Even
with protection, a toxic condition could be endangering
consciousness.
Diffusion Flame
• Consists of (3) basic elements:
– Fuel
– Oxygen
– Heat
Impact of Changing Conditions
• Structure fires can be dynamic
• Factors influencing fire development can
change as fire extends from one
compartment to another
• Changes in ventilation likely most
significant factors in changing behavior
Temperature Reduction
• One of the most common methods of fire
control/extinguishment
• Depends on reducing temperature of fuel
to point of insufficient vapor to burn
• Solid fuels, liquid fuels with high flash
points can be extinguished by cooling
Temperature Reduction
• Use of water is most effective method for
extinguishment of smoldering fires.
• Enough water must be applied to absorb
heat generated by combustion.
• Cooling with water cannot reduce vapor
production enough to extinguish fires in
low flash point flammable liquids/gases.
Temperature Reduction
• Water can be used to control burning
gases/reduce temperature of products of
combustion above neutral plane.
• Water absorbs significant heat as
temperature raised, but has greatest effect
when vaporized into steam.
Fuel Removal
• Effectively extinguishes any fire
• Simplest method is to allow a fire to burn
until all fuel consumed.
TS 2–26
FUEL REMOVAL
• Is used on solid, liquid, or gas fuels
• Stops flow of liquid or gaseous fuel
• Moves solid fuel out of fire path
• Allows fire to consume all fuel
Oxygen Exclusion
• Reduces fire’s growth and may totally
extinguish over time
• Limiting fire’s air supply can be highly
effective fire control action.
TS 2–27
OXYGEN EXCLUSION OR
DILUTION
• Is used on solid, liquid, or gas fuels
• Prevents air from reaching fuel
(smothering)
• Dilutes or displacing oxygen with an inert
gas
Chemical Flame Inhibition
• Extinguishing agents interrupt combustion
reaction, stop flame production
• Effective on gas, liquid fuels because they
must flame to burn
• Does not easily extinguish surface mode
fires
INHIBITION OF CHAIN
REACTION
TS 2–28
• Is used on gas and liquid fuels
• Uses dry chemicals and halogenated
hydrocarbons
• Interrupts chemical chain reaction (stops
flaming)
Velocity & Pressure
• The speed at which smoke leaves a
building is referred to as velocity.
• Only two things can cause smoke to
pressurize within a structure:
– Heat
– Volume