Tweet
The suppressive effects water may have on Class 'A' fires are - (Grant & Drysdale FRDG 1/97)
Fuel Cooling - Cooling of the combustible solid fuel surface, which reduces the rate of Pyrolysis and thus the supply rate of fuel to the flame zone. This reduces the rate of heat release by the fire; consequently the thermal feedback from the flame is also reduced and this augments the primary cooling effect of the suppression agent. The application of a water spray to the fuel bed is typical of this method;
Flame Cooling - Cooling of the flame zone directly; this reduces the concentration of free radicals (in particular the chain-branching initiators of the combustion reaction). Some proportion of the heat of reaction is taken up by heating an inert substance (such as water) and therefore less thermal energy is available to continue the chemical break-up of compounds in the vicinity of the reaction zone. One function of the new water mist technology is to act in this manner, the fine droplets providing a very large surface area per unit mass of spray in order to increase the rate of heat transfer;
Flame Inerting - Inerting the air feeding the flame by reducing the oxygen partial pressure by the addition of an inert gas (eg N2, CO2, H2O vapour). This is equivalent to the removal of the oxidiser supply to the flame by the production of water vapour. This is the dominant mechanism by which water mists can suppress large confined fires.
In a discussion of water-mist fire extinction mechanisms Mawhinney added to the above the possibilities of thermal radiation attenuation, dilution of the flammable vapour/air mixture and chemical inhibition
The use of water-fog, or fine sprays, as opposed to the solid straight stream attack in structural fire situations is nothing new. In fact, there has been an ongoing debate for the past fifty years as to which is the best form of attack. Even quite recently, there have been research projects that lasted several years to measure the effectiveness of water-fog patterns in cooling fire gases and extinguishing fires under various ventilation parameters, in comparison to the benefits and extinguishing capabilities of straight streams. In real terms, either form of attack has advantages over the other, depending on varying conditions.
The term 'three-dimensional water-fog' is not to be confused with that of 'indirect water-fog' and the associated applications of either style are completely different. Quite simply, the 'indirect' approach is where water is applied in a fine spray form onto the heated surfaces of a fire involved compartment, or room, to create steam. This massive expansion of steam creates a positive pressure within the room and smothers the fire to extinction within seconds. However, the steam will create a sudden rise in compartmental temperature and cause a major problem for firefighters occupying the compartment. The 'three-dimensional' approach, when used as a firefighting tool, places the water-fog directly into the heated fire gases and avoids excessive contact with hot surfaces. This, in effect, causes a negative pressure within the fire compartment and firefighters are not burned by steam expansion. This effect is achieved by resorting to specific 'pulsing' techniques at the nozzle and by selecting ideal fog 'cone' angles and pattern diameters. The application is precise and requires both equipment and nozzles that are able to function with optimum effect and firefighters that fully understand and are trained in the application techniques.
------------------
www.firetactics.com
Fuel Cooling - Cooling of the combustible solid fuel surface, which reduces the rate of Pyrolysis and thus the supply rate of fuel to the flame zone. This reduces the rate of heat release by the fire; consequently the thermal feedback from the flame is also reduced and this augments the primary cooling effect of the suppression agent. The application of a water spray to the fuel bed is typical of this method;
Flame Cooling - Cooling of the flame zone directly; this reduces the concentration of free radicals (in particular the chain-branching initiators of the combustion reaction). Some proportion of the heat of reaction is taken up by heating an inert substance (such as water) and therefore less thermal energy is available to continue the chemical break-up of compounds in the vicinity of the reaction zone. One function of the new water mist technology is to act in this manner, the fine droplets providing a very large surface area per unit mass of spray in order to increase the rate of heat transfer;
Flame Inerting - Inerting the air feeding the flame by reducing the oxygen partial pressure by the addition of an inert gas (eg N2, CO2, H2O vapour). This is equivalent to the removal of the oxidiser supply to the flame by the production of water vapour. This is the dominant mechanism by which water mists can suppress large confined fires.
In a discussion of water-mist fire extinction mechanisms Mawhinney added to the above the possibilities of thermal radiation attenuation, dilution of the flammable vapour/air mixture and chemical inhibition
The use of water-fog, or fine sprays, as opposed to the solid straight stream attack in structural fire situations is nothing new. In fact, there has been an ongoing debate for the past fifty years as to which is the best form of attack. Even quite recently, there have been research projects that lasted several years to measure the effectiveness of water-fog patterns in cooling fire gases and extinguishing fires under various ventilation parameters, in comparison to the benefits and extinguishing capabilities of straight streams. In real terms, either form of attack has advantages over the other, depending on varying conditions.
The term 'three-dimensional water-fog' is not to be confused with that of 'indirect water-fog' and the associated applications of either style are completely different. Quite simply, the 'indirect' approach is where water is applied in a fine spray form onto the heated surfaces of a fire involved compartment, or room, to create steam. This massive expansion of steam creates a positive pressure within the room and smothers the fire to extinction within seconds. However, the steam will create a sudden rise in compartmental temperature and cause a major problem for firefighters occupying the compartment. The 'three-dimensional' approach, when used as a firefighting tool, places the water-fog directly into the heated fire gases and avoids excessive contact with hot surfaces. This, in effect, causes a negative pressure within the fire compartment and firefighters are not burned by steam expansion. This effect is achieved by resorting to specific 'pulsing' techniques at the nozzle and by selecting ideal fog 'cone' angles and pattern diameters. The application is precise and requires both equipment and nozzles that are able to function with optimum effect and firefighters that fully understand and are trained in the application techniques.
------------------
www.firetactics.com
Comment