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April-May 2006 |
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Ventilation Principles, Part I
By John Mittendorf
Similar to the term firefighting, the word ventilation has several definitions and
may be successfully performed using a variety of operations and techniques.
In spite of its potential complexity, the word ventilation is easily defined
and applied to firefighting operations as follows: "procedures necessary to effect the planned and systematic
re-direction and removal of fire, smoke, heat, and fire gases from a
structure". However, this definition is too simplistic when we
consider that safe, timely, and effective ventilation operations are often
performed in hazardous atmospheres and under dangerous conditions.
In addition
to its definition, ventilation can also be divided into two categories:
theoretical and experiential. Theory provides a foundation to build an
understanding of ventilation, and experience provides the practical opportunity
to develop necessary expertise. So, let's review basic ventilation theory.
Ventilation does not put out fires. It never has and probably never will.
However, safe, timely and effective ventilation is integral in the attack,
control and extinguishment of structure fires. Do all structure fires need
ventilation? Obviously not. However, in many structure
fires, ventilation should be a primary concern.
To understand how fireground conditions can dictate why ventilation is
important, consider a simple example. An unconfined fire will draw cold air
into the bottom of the fire as heated smoke, gases and air rise vertically.
Because the fire in our example is unconfined, it is able to draw as much cold
air as necessary to sustain combustion. Any fire must have heat, fuel and
oxygen. As our fire is able to use the correct amount of oxygen, the
result is a free-burning fire. Now transfer and confine our fire to a simple
old-fashioned wood burning stove. With the ash-box door and damper open, the
free-burning fire will produce heat, some smoke, and fire gases comprised of
carbon monoxide, carbon dioxide, sulfur dioxide and other gases that
depend on the materials burning in the stove. The heated smoke and fire gases
are lighter than the cooler air at the base of the fire so they will rise
vertically and exhaust out of the open damper. However, if the ash-box door and
damper are closed and the fuel-box door is opened, cool air will be drawn into
the bottom of the fire. The rising volume of hot smoke and fire gases will
reach the highest level of the stove and begin to bank downward (mushrooming),
forcing the cooler air to the bottom of the stove and the base of the fire. As
this condition continues, a natural air circulation from the bottom of the
stove to the top and back down to the base of the fire is developed. Depending
on the material that is burning, the temperature inside the stove will increase
and can easily exceed l,000-degrees F. Additionally,
the level of carbon monoxide is increasing at a rapid rate and the level of
oxygen has been reduced. If the fuel-box door is closed, resulting in an
airtight stove, the fire will be reduced to a smoldering phase due to a further
reduction of oxygen, and the stove will completely fill with hot smoke and fire
gases, with temperatures possibly exceeding l,300-degrees
F. At this stage, the stove is pressurized, flammable carbon monoxide is
abundant, temperatures are high and oxygen levels are low. In this example,
restoring the stove to a proper operating condition would require opening the
damper first to exhaust the heated and flammable contents. However, if the ash-box door were opened first, fresh oxygen would suddenly be introduced into the
flammable and oxygen-deprived environment with a predictable result — an
explosion.
Now, let's apply the preceding example to a multi-story building
that is not compartmentalized. As our fire initially burns, heated gases and
smoke will also rise to the highest point within the structure and will then
begin to bank down and fill other portions of the structure. If this process continues, the
structure will become filled with heated smoke and fire gases, offer poor
visibility, and have developed a level of heat within the structure that can be
intolerable for occupants and suppression personnel. At this point, suppression
personnel are faced with a choice:
-
Enter the structure to suppress the fire without any type of ventilation and
encounter a hot, flammable environment that is dangerous to suppression
personnel.
-
Use horizontal and/or vertical ventilation to improve visibility, reduce the
dangerous concentrations of heat, smoke, and fire gases, and allow suppression personnel to effect a timely and safe extinguishment.
This simple ventilation example points to a
common problem. Whether a fire is small or large, fire gases and other products
of combustion are a byproduct of any fire and can be
expected to be encountered by building occupants and/or suppression personnel. This condition will present three
primary dangers:
-
Fire
Gases. The term fire gases refers to the gases or
vaporized products of combustion. Common combustible materials contain carbon
which, when burned with an ample supply of oxygen, produces carbon dioxide
which has little toxic effect on the human body. However, carbon dioxide
displaces air (oxygen) and can lead to asphyxiation. It also contributes to the
accelerated inhalation of other toxic fire gases. When the supply of oxygen is
reduced below 21 per cent, the production of other fire gases is accelerated.
These gases include formaldehyde, carbon monoxide, acrolein,
sulfur dioxide, ammonia, hydrogen cyanide/chloride, chlorine, oxides of
nitrogen, phosgene, isocyanates
and so on. Fire fatalities from the inhalation of fire gases exceed fire deaths
from all other fireground causes combined.
Investigations examining fire fatalities as a result of exposure to toxic
atmospheres (fire gases) has confirmed that carbon monoxide is the primary
toxicant. Other studies have suggested that other fire gases are just as
deadly.
-
Occupants
and Firefighters. Inhalation of fire gases is a primary danger to building
occupants who obviously are not protected by appropriate protective equipment.
However, suppression personnel are equipped with protective equipment and SCBA;
and yet, suppression personnel still die each year from the inhalation of toxic
fire gases. Interestingly, deaths to firefighters per year is
slowly increasing to active and
retired firefighters. A portion of a three-part study on the relationship
between cancer and firefighting by the Institute for Cancer and Blood Research
for a major fire department indicated the death rate from cancer among
firefighters was doubled from 17 per cent to 34 per cent between 1950 and 1980,
and increased exposure to fire gases of petrochemical fires could be the cause.
Additionally, the incidence of brain cancer as a cause of death was 129 percent
higher among firefighters than the general population, and a 30-year
firefighter faces a one-in-three chance of dying from cancer, compared to one in
five among the general population. During the 1930's and 1940's, firefighters
were primarily exposed to gases from wood or paper fires, but as the use of
petrochemical plastics and other synthetic materials in furniture, wall
coverings, building materials and a myriad of other items has increased, so has
the danger to fireground personnel. Additionally, as
the flammability properties of various materials are increased, the toxic level
of the resultant fire gases from common materials (which often contain chlorinated
compounds) is also increased. With these thoughts in mind, ventilation should
be considered as a primary tactic to reduce a deadly hazard to building
occupants and fireground personnel who are often
subjected to the presence of fire gases and other products of combustion.
-
Thermal
Layer. For a moment, let's briefly consider three scenarios that are based on
the following definition of thermal layer: "a
combination of smoke, fire gases, and heat that is capable of burning, often
with a significant intensity and rise in temperature":
-
#1. The first arriving engine company to a reported
structure fire finds a fire in a two-story hotel. A quick size-up indicates a
working fire in one of the apartments on the first floor has charged the
apartment and first-floor hallway with heat and smoke, resulting in minimal
visibility and heat conditions that are steadily increasing. Two firefighters
quickly develop an attack line into the hallway. As it is 1960, the firefighters do not have the protection of SCBA and hoods.
However, as they slowly and cautiously advance down the hallway towards the
seat of the fire, they stay as low as possible and monitor the heat levels with
their ears. Reaching the open door to the involved apartment, they stop to the
side of the door and deftly direct a spray stream towards the upper portion of
the apartment and quickly close the door. After a short period of time, the
firefighters open the door and observe the fire has virtually been
extinguished. Final extinguishment quickly follows.
-
#2. With the same conditions as in scenario #1, the two
firefighters quickly develop an attack line into the hallway. As it is 1980,
the firefighters do not have the protection of hoods. However, as they
cautiously advance down the hallway, their SCBA enhances their advance towards
the seat of the fire while they also monitor the heat levels in the hallway environment
with their ears. Approaching the open doorway to the involved apartment, the
advancing firefighters notice that conditions in the hallway are rapidly
deteriorating and several tongues of flame are starting to appear at the
ceiling. Reaching the open doorway, the firefighters quickly direct a spray
stream with a rotating movement into the involved apartment. Although the fire
is knocked down, the disturbed thermal layer and expanding steam quickly
envelop the firefighters, causing extreme discomfort and some steam burns.
-
#3. With the same conditions as in scenarios #1
& 2, the two firefighters quickly develop an attack line into the hallway.
As it is 1997, the
firefighters have state-of-the-art protective clothing, including SCBA and
hoods. As they cautiously advance the attack line down the hallway, their rate
of deployment is enhanced by their protective equipment. However, in their
desire to reach the seat of the fire in a timely manner, their protective
equipment has masked the worsening
conditions in the hallway. As the firefighters approach the open doorway to the
involved apartment, the apartment and
hallway suddenly flashover, enveloping the advancing firefighters. Depending on
your fire service seniority, do any or all of the
preceding scenarios sound familiar? Interestingly, scenarios #1, 2, and 3 all
share a common thread of commonalty with the fireground
of yesterday and today.
This discussion will be continued in Parts 2 and
3 coming up in the June and August issues.
Chief Mittendorf is the author of Truck Company
Operations and Facing the Promotional Interview. To
purchase, return to the Main Page and scroll down to Fire Engineering Books.
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