Gas cooling.pdf - Industrial Fire Journal

GAS COOLING IN COMPARTMENTS
Gas cooling to control the
compartments
In today’s built environment, a compartment fire
develops quickly. Synthetic materials used in
furnishings and interior finish have a much higher heat
of combustion and heat release rate than the natural
materials found in the built environment of the 1950s
and 1960s. When a compartment fire develops,
buoyant, hot smoke and entrained air form a layer at
the ceiling and begin to fill the compartment.
As the depth of the layer increases, the hot smoke and
air begins to flow out through the opening into
adjacent compartments, writes Chief Edward E Hartin,
(MS, EFO, MIFireE, CFO) of Central Whidbey Island Fire
& Rescue in the State of Washington, US, and owner of
CFBT-US, LLC, an international fire behaviour training
Figure 1: energy
and mass transfer
in a compartment
fire.
and related consulting service.
The upper layer presents several hazards: 1) toxicity, 2)
thermal energy, and 3) flammability. The hot upper layer
transfers energy to fuel packages and compartment linings
and begins to pyrolize other fuel packages into the gas phase fuel
necessary for flaming combustion. The most unrecognised hazard
is that smoke is fuel.
When ventilation is limited, the oxygen concentration becomes
insufficient for continued fire growth. In this ventilation controlled
burning regime, combustion efficiency decreases and
concentration of unburned flammable products in the smoke
increases. Heat release rate is reduced, but the upper layer
remains hot, resulting in ongoing pyrolysis. Failure of a window or
creation of an opening as a firefighter enters provides additional
oxygen and may result in rapid fire progression and depending on
conditions may result in ventilation induced flashover or backdraft.
How can these hazards be mitigated to provide a safer working
environment for firefighters?
Thermodynamics and firefighting
Energy is transferred from materials of higher temperature to
those of lower temperature. In a compartment fire, flames and
the hot upper layer transfer energy to everything else in the
compartment (including firefighters). Mass transfer not only
involves movement of hot smoke out of the compartment, but
also inward movement of air providing the oxygen necessary for
continued combustion and fire growth.
Firefighting tactical operations influence this thermodynamic
system in a number of ways, principally by applying and
changing the ventilation profile. All fire control and ventilation
tactics influence the compartment fire as a thermodynamic
system.
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fire environment in
appropriate under all circumstances! Firefighters who work to
become masters of their craft recognise that strategies, tactics,
and techniques must fit the problems presented by the incident.
Gas Cooling is the application of an appropriate amount of
water fog into the upper layer to reduce the temperature of the
gases.
Vapourisation of water in the upper layer reduces the
temperature of the hot gases, thermal load on the firefighters
working below, and potential for ignition. While this sounds simple
and fairly intuitive, this basic technique to control upper layer
hazards is frequently misunderstood.
Many firefighters in the United States believe that application of
water into the upper layer in a fog or spray pattern will result in
production of a large amount of steam which will fill the
compartment and make conditions untenable. In many cases this
is consistent with firefighters’ fireground experience. These
firefighters are sceptical of cooling the upper layer with the
application of water fog to improve conditions. The answer can be
found in a statement made by Floyd Nelson (1989, “In principle,
Ed Hartin, MS, EFO,
MIFireE, CFO
serves as Fire Chief
with Central
Whidbey Island Fire
& Rescue in
Washington (USA),
and is the owner of
CFBT-US, LLC.
Ed was co-author
of 3D Firefighting:
Techniques, Tips,
and Tactics, and he
delivers practical
fire dynamics
training
internationally and
across the US.
Figure 2:
short pulse.
Fire control tactics
It is essential to understand the interrelationship between
ventilation strategies such as tactical ventilation (removal of
smoke and introduction of air) and tactical anti-ventilation
(confinement and exclusion of air) and fire control.
It is fair to say that amongst fire services in the US offensive
firefighting involves aggressive use of tactical ventilation and direct
attack with high flow hoselines. In other parts of the world such as
Europe and Australia, use of tactical ventilation is limited or highly
controlled, and flow rates from hoselines are lower than those
typically used in the US.
While tactics vary, firefighters have often been taught not to put
water on smoke. These cautions were based on the need to put
water onto burning fuel for an effective direct attack and to limit
production of steam which could add to the upper layer and
worsen conditions for firefighters working at floor level. However,
what happens when the fire is shielded from direct attack?
Firefighters must advance their hoseline to the seat of the fire,
while working underneath the flammable upper layer. In a
ventilation controlled regime, the air introduced when the
firefighters made entry for fire attack can result in increased heat
release rate and rapid fire progression. How can this hazard be
mitigated?
Gas cooling as a fire control technique
First a qualifier, there are no universal solutions to control the
hazards presented by a compartment fire. Firefighters are often
presented with ideas such as positive pressure ventilation (PPV),
compressed air foam systems (CAFS), high pressure or ultra high
pressure water fog, and gas cooling. and see these technologies
or tactics as the solution. No one tactic or technique is
Figure 3: heating
and cooling
curves of smoke
and water.
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GAS COOLING IN COMPARTMENTS
Figure 4: gas
versus surface
cooling.
Figure 5: volume
changes during
gas cooling.
Note: adapted
from Water and
other
extinguishing
agents (p. 155),
by Stefan
Särdqvist,
2002, Karlstad,
Sweden (copyright
Räddnings
Verket).
firefighting is very simple. All one needs to do is put the right
amount of water in the right place and the fire is controlled.”
As demonstrated by Superintendent Rama Krisana
Subramaniam, Bomba dan Penelamat (Fire & Rescue Malaysia)
[see figure 2] a short pulse can place a large number of small
water droplets in the upper layer that develops during a
compartment fire. When a pulse (brief application) of water fog is
applied into a layer of hot smoke and gases with a temperature of
500 o C (932 o F) what happens? As the small droplets of water
pass through the hot gas layer, energy is transferred from the hot
smoke and gases to the water. If done skillfully, the upper layer
will not only be cooler and less likely to ignite, but it will contract
(or at least stay the same volume) providing a safer working
environment below. When water is turned to steam, it expands. At
its boiling point, water vapourised into steam will expand 1700
times. However, due to the tremendous amount of energy
required to vapourise water (and resulting reduction in gas
temperature), the final volume of the mixture of hot gases and
steam is less than the original volume of hot gases within the
compartment. Heating the water to 100 o C and production of
steam transfers a tremendous amount of energy from the hot
smoke and gases to the water, reducing the temperature of the
hot gases. At constant pressure, the volume of a gas is directly
proportional to its absolute temperature (if absolute temperature
is reduced by 50%, volume will also be reduced 50%).
The key
The temperature of the gases is lowered much more than the
temperature of the water is increased. As illustrated in Figure 3,
the specific heat of smoke is approximately 1.0 kJ/kg (Särdqvist,
2002; Yuen & Cheung, 1999) while the specific heat of water is
4.2 kJ/kg and even more importantly the latent heat of
vapourisation of water is 2,260 kJ/kg. What this means is that it
requires over four times the energy to raise the temperature of a
kilogram of water by 1 o C than it does to lower the temperature of
smoke by the same amount. In addition, it requires 2,260 times
the energy to turn 1kg of water to steam at 100 o C than it does to
lower the temperature of 1kg of water by 1 o C.
While water expands as it turns to steam, the hot gas layer
contracts as its temperature drops. If the initial temperature of the
hot gas layer is 500 o C (773 Kelvin) and its temperature is
lowered to 100 o C (373 Kelvin) the absolute temperature is
reduced by approximately half (773 K-373 K=400 K).
Correspondingly the volume of the hot gases will also be reduced
by half.
Gas versus surface cooling
When vapourised in the upper layer, energy is transferred from
the hot gases in the upper layer to; 1) raise the temperature of
the water to its boiling point of 373.15 K (100 o C); 2) to change
its state from liquid phase to gas phase; and 3) to raise the
temperature of the steam until reaching equilibrium (hot gases
and steam are at the same temperature).
When water is vapourised on contact with a hot surface, it does
not absorb significant energy while travelling through the hot
gasses of the upper layer. The energy necessary to raise the
temperature of the water to its boiling point and vapourise it is
absorbed from the surface. Steam produced in this manner will
also absorb energy from the hot gases of the upper layer (but
increasing the temperature of the water in liquid form and
vapourisation on contact with hot surfaces does not take
significant energy from the hot gasses of the upper layer).
As illustrated in Figure 5, the relative volume (expansion or
contraction) of the upper layer during gas cooling is dependent
on the percentage of water vapourising as water passes through
the hot gases of the upper layer and the percentage of water
vapourising on contact with hot surfaces such as compartment
linings.
Different parts of the elephant
Firefighters’ perspectives on the use of water fog for interior
structural firefighting can be compared to the Indian fable of The
six blind men and the elephant (Saxe, 1963). In this fable, the six
men tried to determine what an elephant was. As none of the
men could see, they used their sense of touch. However, each
grasped a different part of the elephant. One touched the side
and thought an elephant was like a wall, another the trunk and
thought an elephant was like a snake, and so forth. What you
believe may be limited by your point of observation and your
understanding of what you are looking at.
References
Shaw, E. (1876). Fire protection. London: Charles and Edwin Layton.
Nelson, F. (1991). Qualitative fire behavior. Ashland, MA: International Society of Fire
Service Instructors.
Särdqvist, S. (2002). Water and other extinguishing agents. Karlstad, Sweden:
Räddnings Verket.
Yuen, K. & Cheung, T. (1999). Calculation of smoke filling time in a fire room - a
simplified approach. Journal of Building Surveying, 1(1), p. 33-37
Saxe, J. (1963). The blind men and the elephant. New York: McGraw-Hill
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