Construction Concerns: Fuels and Fire Behavior
Article and photos by Gregory Havel
June 29, 2015
Firefighters are familiar with many of the terms used in describing fire behavior. Photo 1
shows an acquired structure used for live fire training compliant with NFPA 1403,
Standard on Live Fire Training Evolutions, at the point where the remaining structure
was allowed to burn down.
(1)
June 29, 2015
During the past few years, the laboratory and acquired structure studies of fire behavior
by the National Institute for Standards and Technology (NIST) and Underwriters
Laboratories (UL) have exposed us to a large volume of scientific data and reports which
express measurements in the International System of Units (SI, or metric system, which
is not yet in daily use in the United States). The data also includes some terms and data
which may not have been included in our firefighter training.
To understand the scientific data that is available to us today, we need to understand the
relationship between SI and the system of measurement that is common in the United
States.
The definitions below in quotation marks are taken from NFPA 921, Guide for Fire and
Explosion Investigations, 2014 edition, Chapter 3.
Temperature. “The degree of sensible heat of a body as measured by a thermometer or
similar instrument”. [NFPA 921:3.3.172]
It is measured by a thermometer, or by a thermocouple (Photo 2) for more extreme
temperatures. Do an Internet search for “thermocouple”, “thermocouple types”, and
“thermocouple wire” for an explanation of the theory and function of theromocouples.
(2)
The three common temperature scales are the Fahrenheit, Celsius, and Kelvin. The
United States and a few other countries use Fahrenheit. The rest of the world uses
Celsius. The scientific community uses Celsius and Kelvin.
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The Fahrenheit thermometer dates to 1724. It has zero as the temperature at
which brine freezes, and 98.6 as the human body’s core temperature. On this
June 29, 2015
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scale, pure water freezes at 32°F and boils at 212°F at sea level and
atmospheric pressure.
The Celsius thermometer dates to 1742. It uses a 100° interval between pure
water freezing at 0oC and boiling at 100°C. This scale has also been called
“Centigrade”.
The Kelvin scale dates to 1848. It uses the unit of the °C with 0K as absolute
zero (the point at which all thermal motion ceases at the molecular level; -273.15
o
C; -459.67 oF); pure water freezes at 273.16K and boils at 373.13 K.
To convert temperatures between these scales, use these formulas:
 oF = 9/5 oC + 32
 oC = 5/9 oF – 32
 K = oC + 273.2
 K = oF + 459.7
Flash Point. “The lowest temperature of a liquid, as determined by specific laboratory
tests, at which the liquid gives off vapors at a sufficient rate to support a momentary
flame across its surface.” [NFPA 921:3.3.82]
Ignition temperature (piloted ignition temperature). “The minimum temperature a
substance should attain in order to ignite under specific test conditions.” [NFPA
921:3.3.106] This is usually higher than the flash point for a liquid.
Auto-ignition temperature. “The lowest temperature at which a combustible material
ignites in air without a spark or flame.” [NFPA 921:3.3.15] This is usually higher than
either the ignition temperature or flash point.
The temperature of a fire varies depending on the type of fuel; the geometry (dimensions
and arrangement of the fuel); the position of the thermocouple in the flame, thermal
column, or flow path; and on the amount of oxygen available for combustion.
Pressure is traditionally measured in pounds per square inch (PSI). In the SI or metric
system, the following applies:
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1 PSI = 6.895 Kilopascals (kPa).
1 kPa = 0.145 PSI.
1 bar = 100 kPa, or 14.5 PSI (atmospheric pressure at sea level).
These are the units used when discussing partial vacuum; atmospheric pressure;
compressed gases in containers; fuels in pipelines, tanks, and pressure vessels; and the
expansion of combustion products because of heating.
Energy is usually measured as kinetic energy. From this measurement, it is possible to
calculate the potential energy of a fuel as British Thermal Units (Btus) per pound or as
kilowatts per kilogram in the SI or metric system.
BTUs are “The quantity of heat required to raise the temperature of one pound of water
1°F at the pressure of one atmosphere and temperature of 60°F; it is equal to 1,055
joules, 1.055 kilojoules, and 252.15 calories.” [NFPA 921:3.3.21]
In the SI or metric system:
June 29, 2015
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A joule is “The …heat produced when one ampere is passed through a
resistance of one ohm for one second, or it is the work required to move a
distance of one meter against a force of one newton. There are 4.184 joules in a
calorie, and 1055 joules in a British thermal unit (Btu). A watt is a joule/second.”
[NFPA 921:3.3.114] (A Newton is a unit of force required to impart an
acceleration of one meter per second per second to a mass of one kilogram).
A watt (W) is a “Unit of power, or rate of work, equal to one joule per second, or
the rate of work represented by a current of one ampere under the potential of
one volt.” [NFPA 921:3.3.191] (A watt = 0.00134 horsepower (HP) = 3.42 BTU
per hour).
A kilowatt (kW) is energy released at the rate of 1000 joules per second. A kW =
1.34 HP = 3,415 BTU per hour.
In the science of fire and combustion, some additional measurements are needed.
These are usually expressed in the SI or metric system.
Heat release rate (HRR) (or rate of heat release) is “The rate at which heat energy is
generated by burning.” [NFPA 921:3.3.99] It is usually measured in kW. The kW is 1,000
energy units (joules) released per second. The rate of heat release can be affected by
several factors including moisture content of the fuel (fuels with higher moisture content
burn more slowly) and available oxygen (compare a smoldering fire in an oxygendeficient atmosphere to a free-burning fire, a wind-driven structure fire, a fire in a
blacksmith’s forge, or a fire in a blast furnace).
Heat flux is “The measure of the rate of heat transfer to a surface, expressed in
kilowatts/m2; kilojoules/m2 · second; or Btu/ft2 · second.” [NFPA 921:3.3.97] This can be
affected by a change in the rate of heat release due to the availability of oxygen.
From the scientific studies performed at UL and NIST using these units and
measurements, we can make the following conclusions about fire behavior:
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A fire that contains fuel with a high potential and kinetic energy will require more
cooling before extinguishment than a fire that contains fuel with a lower potential
and kinetic energy. Firefighters or an automatic sprinkler system will need to
apply a larger volume of water on these fires.
Fuels with high kinetic energy and high rates of heat release will result in fires
with rapid growth, high levels of radiant and convection heat, and rapidlyexpanding thermal columns and higher-velocity flow-paths.
Adding a ventilation opening to a burning structure by forcible entry for fire attack
will change the ventilation profile of the structure as well as the fire, adding
oxygen for more rapid combustion and flashover and the potential for firefighters
to be trapped in the ventilation flow path.
A structure fire with a high rate of heat release that is combined with the oxygen
available from an added ventilation opening can produce a heat flux that will
exceed the level of protection provided by firefighters’ personal protective
equipment, resulting in serious injury or fatality.
June 29, 2015
View the results of NIST studies at
http://www.youtube.com/watch?v=07c4Tu_QrHc&list=PLeDTEhgchmb0gyMh3Of8Q6u
EVQnsvoZof.
The results of the UL and UL Firefighter Safety Research Institute are available online
at www.uluniversity.us and http://ulfirefightersafety.com.
Gregory Havel is a member of the Town of Burlington (WI) Fire
Department; retired deputy chief and training officer; and a 30-year
veteran of the fire service. He is a Wisconsin-certified fire instructor
II, fire officer II, and fire inspector; an adjunct instructor in fire
service programs at Gateway Technical College; and safety director
for Scherrer Construction Co., Inc. Havel has a bachelor's degree
from St. Norbert College; has more than 30 years of experience in
facilities management and building construction; and has presented
classes at FDIC.
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