Condensed aerosol fire suppression is a particle-based method of fire extinction. It is uses pyrogenic, condensed aerosol fire suppressants to extinguish fires. It is a fire suppression method for class A, B, C, E and F. Similar to other fire-extinguishing agents, it is not applicable to metal fires (class D). Some aerosol-generating compounds (e.g., potassium nitrate-based) produce a corrosive by-product that may damage electronic equipment, although later generations have lowered the effect.
Condensed aerosol fire suppression systems employ a fire-extinguishing agent consisting of very finely divided solid particles, suspended in an inert gas. Those superfine aerosol particles are pyrotechnically generated via the combustion of an aerosol-forming agent (AFA) which is stable at room temperature and does not need to be stored in a pressurized container.
Benefits include high performance (3 times more effective than banned Halon 1301, with the aerosol leveraging both cooling, dilution and chemical inhibition), general availability (from plant-size systems to compact and lightweight portable tooling), low toxicity, environmentally friendliness (e.g., 0% ozone depleting potential), non-pressurized systems, and overall cost-effectiveness.
Aerosol chemistry That fire-extinguishing medium is not stored "as is" but rather produced "on demand": the aerosol microparticles and effluent gases are generated by an exothermic reaction initiated within a condensed, solid, aerosol-forming compound. Typically, a fuse ignites the aerosol-forming compound to generate hot aerosol colloids.
The hot aerosol fire suppression technology went through several generations: gen I (oil tank suppression system) and gen II (potassium nitrate-based, efficient but with corrosive byproducts) denoted as K-type systems, then gen III (boosted strontium salt-based, less deliquescent and generating less corrosive byproducts) denoted as S-type systems. All generations leverage alkaline or alkaline earth metal nitrates (which act as oxidants in the hot aerosol generating process), and different kinds of reductants.
Typically, in K-type systems, fire-extinguishing aerosol particulates consist of potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3) that are produced from the thermal decomposition of potassium nitrate (KNO3) mixed with various reducers, binders (e.g., organic resin or melamine resin) and additives:
KNO3 (solid) + CnHmNpOq (solid) = KHCO3 (solid) + K2CO3 (solid) + CO2 (gas) + N2 (gas) + H2O (gas)
with CnHmNpOq being a gas-generating oxidizer
The exothermic reaction produces an enormous amount of micron-sized dry particles (typically in the form of potassium or strontium carbonates such as alkaline or alkaline earth oxides, hydroxides or chlorides), as well as a gaseous mixture (carbon dioxide, nitrogen and water vapor). This very hot aerosol mixture, akin to a vapor, is cooled within the dispersion device itself by passing through a cooling solid agent (a thermally absorbent grid which vaporizes, inducing its endothermic chemical decomposition). It eventually is discharged as solid microparticles within a gaseous fluid. Final microparticles dispersed in solution can be sodium, potassium or magnesium chlorides, lithium iodide, potassium acetate, sodium hydroxide, potassium ferrocyanide and PCCs (phosphorus containing compounds), depending on the exact aerosol-generating compound used.
Comparison with other fire suppression methods
Compared to gaseous suppressants (which emit only gas) and dry chemical suppression agents (which are powder-like particles of a large size – 25–150 micrometers), the National Fire Protection Association defines condensed aerosols as those that release finely divided solids of less than 10 micrometers in diameter.
The solid particulates have a considerably smaller mass median aerodynamic diameter (MMAD) than those of dry chemical suppression agents. The particulates are subject to Brownian motion: the colloid's high diffusive ability and long suspension time mean the microparticles remain airborne significantly longer and leave much less residue within the protected area than alternate (dry or gas) agents. Their large combined surface area efficiently attracts free radicals through surface adsorption.
Condensed aerosols are flooding agents. In closed premises, they are effective regardless of the location and height of the fire. This can be contrasted with dry chemical systems, which must be directly aimed at the flame. In open spaces, they are reasonably effective when targeting the top of the fire, unlike gaseous and dry agents which must be directed at the base of the flames.
The condensed aerosol agent can be delivered by means of mechanical operation, electric operation, or combined electro-mechanical operation.
Wet chemical systems generally found in foam extinguishers must, similarly to dry chemical systems, be sprayed directionally onto the fire.
Methods of fire extinction
Hot, condensed aerosol fire-extinguishing agents act on both physical and chemical levels. They leverage four methods to extinguish fires, for they act on the four elements of what is known as the fire tetrahedron. These four means of fire extinction are:
Reduction or isolation of fuel Reduction or isolation of oxygen Reduction of heat Inhibiting the chain reaction of the above components
Condensed aerosols' primary extinguishing mechanism involves the fourth element of the fire tetrahedron by means of chemical reactions with the free radicals of the flame, therefore interfering with the combustion process of the fire.
Removal of free radicals As the aerosol particles (K2CO3, KHCO3) surround and come into contact with the hot flame, they absorb the flame heat energy, breaking down and releasing large concentrations of potassium radicals (K+, noted K• below – ie. ions with an unpaired electron). Those potassium radicals bond with the hydroxide (OH+), hydrogen (H+) and oxygen (O+) free radicals that typically sustain combustion, producing harmless by-product molecules like potassium hydroxide (KOH) and water (H2O), and breaking the chain reaction required to keep combustion active.
Free radicals are thus removed from the chain reaction by way of the following chemical reactions:
K• + OH• = KOH
KOH + H• = K• + H2O
Aerosol suppressants are highly effective since potassium radicals are "propagated": they are both consumed and produced by reaction with the fire’s free radicals. Disrupting the reactions necessary to sustain the flame's combustion, the cycle continues until the combustion's chain reactions are terminated and the flame is extinguished. The whole process does not deplete oxygen content, merely displaces it.
Other fire-extinguishing effects Condensed aerosol agents also have secondary extinguishing mechanisms implicating the other three elements of the fire tetrahedron: it isolates the fire's fuel, displaces oxygen and reduces heat.
Aerosol particles and gases mixing with the gaseous components of the flame. The aerosol cools the flame by engulfing it within a cloud with large concentrations of microparticles, each with a mass median aerodynamic diameter (MMAD) as small as 1 to 2 micrometers. Though the surface area of each microparticle is extremely small, the large quantity of particles surrounding and penetrating the flame offers a sufficiently large combined surface area to absorb the flame's heat. That large surface area also allows using smaller quantities of extinguishing agent when compared with other methods.
Oxygen displacement, homogenous distribution Aerosol-based fire suppression systems are usually designed to generate a "knock-down" effect by discharging the aerosol at a fairly high rate and for a fairly long time. The aerosol is said to be "condensed" for it consists of a dense fog of microparticles "gaseous vector" which exhibits gas-like, three-dimensional properties, including a rapid, homogeneous distribution even in hard-to-reach, confined spaces. Re-ignition is prevented by both the "knock-down" effect and the sustained flooding of the fire area which displaces oxygen.
Recombination of free radicals The microparticles provide a tremendously greater reactive surface than typically found in a combustion process, boosting the natural free radical's recombination rate, which leads to a net fire's energy loss:
O• + H• = OH•
H• + OH• = H2O
Heat sinks Thermal cooling is achieved through a couple of reactions:
Heat absorption via endothermic phase changes of the aerosol particles: K2CO3 (solid) -> K2CO3 (liquid) -> K2CO3 (gas) Heat absorption via endothermic decomposition reaction: 2KHCO3 (solid) -> K2CO3 (solid) + CO2 (gas) + H2O (gas)
Performance
Attacking all the elements of the fire tetrahedron, condensed aerosol fire suppression agents are among the more effective flame-extinguishing agents. For example, some condensed aerosol fire suppressants can extinguish a Class B flammable liquid pool fire with 1/5 the amount of Halon 1301 agent or 1/10 the amount of a hydrofluorocarbon or fluoroketone based clean agent gaseous fire suppression system, in terms of kilogram mass of agent per cubic meter.
The extinguishing performance of condensed aerosol fire suppressants is dependent on the density of aerosol particulates in the immediate vicinity of the flame. As with gaseous fire suppression systems, the faster the agent can build around the flame, the more efficient the extinguishing agent is at stopping combustion. The extinguishing and design densities of aerosol fire suppression agents are generally expressed in kilograms per cubic meter (kg/m3). Thus, the efficiency of aerosol extinguishing agents varies depending on a number of factors, such as the location of the aerosol relative to the flame, the proximity of other combustible flammable materials, the type of fuel involved, etc.
Condensed aerosol devices are designed to provide a controlled discharge. The aerosol-forming compound is installed inside of the device, which is then fitted with an electric or mechanical initiator. The electric initiator is interfaced with a fire-detection control unit or panel, which can be remotely operated by physical means such as by cable, operated by hand with a fuse mechanism such as those used in smoke dispensing grenades, or automatic and self-triggering when outfitted with an integral heat-sensing device.
Uses and applications Automated fire suppression systems are widely used within the industry to protect large-scale, enclosed facilities and rooms. They can leverage hot aerosol agents, with the added benefit of low installation and maintenance costs (long shelf storage, non-pressurized storage and distribution). Whether static or portable, hot aerosol extinguishers consist of a thin metal or plastic casing (neither the aerosol-generating compound nor the resulting aerosol needs to be pressurized) and sometimes piping (although usually unnecessary), an initiation device (actuator for the exothermic reaction, either electrical or thermal), some aerosol-forming compound mixed with a reaction component and additives, a solid cooling component and some discharge port(s).
Condensed aerosol-based fire extinguishing systems (CAFES) are widespread among industries and public facilities. There are two uses for applying fire extinguishing agents: as a total flooding fire protection system or as a local application fire suppression system.