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Environmentally safe fire

The International Montreal Protocol on Substances That Deplete the Ozone Layer, which entered into force on 1 January, 1989, set the timescale for the cessation of production of specific ozone depleting chemical compounds for the end of 1993 in the developed world and 2010 in developing countries. The NFPA 12A Halon 1301 (also known as bromotrifluoro-methane CF3Br), an effective and widely used chloro-fluorocarbon (CFC) fire extinguishing gas agent has the highest ozone depletion potential of any man-made ozone depleting substances, consequently ending production of this agent in the United States in 1994 in compliance with the US Clean Air Act of 1990.

Manufacturers of fire suppression technologies quickly recognised the impact the phase-out of the production of halon 1301 will have on the fire protection market, resulting in the entry of halon 1301 replacement gaseous and water-based products in the early 1990s such as the NFPA 2001 halocarbon (HFC) agents and NFPA 750 water-mist technologies. However, despite the rapid development of these technologies to provide similar fire protection as halon 1301, none of the technologies could replace halon 1301 on a one-to-one basis by stored agent weight and volume. Clearly the need for true halon 1301 replacement technology was not completely satisfied.

The U.S. EPA first formally recognised condensed aerosol technology (also referred to as inert gas/powdered aerosols) as an acceptable halon 1301 substitute as early as 1995 (Federal Register Vol. 60, Number 113, 13 June, 1995).

The early condensed aerosol fire suppression technologies became commercially available in the international fire protection market during the mid-1990s, essentially derived from the development of the technology by the former Soviet Union for its military and space programmes around the 1970s.

Condensed aerosol systems have been commercially sold and installed in the international market for over a decade. In the late 1990s the European and Australian marine classification societies and approval agencies have begun to issue approval certificates for aerosol units for the protection of the engine rooms of small recreational and commercial vessels. However, with the exception of research by a few US government defence laboratories, the technology remained little known in the industrial fire protection market. That is, until recently.

In 2006, NFPA recognised aerosol fire suppression agents as a distinct technology from existing gaseous total flooding agents such as the NFPA 2001 halocarbon, fluoroketone, and inert gas clean agents and NFPA 12 carbon dioxide gas and voted to release NFPA 2010, the Standard for Fixed Aerosol Fire-Extinguishing Systems.

Condensed aerosol technology

The flame quenching capability of the potassium ion (or radical) is well known and potassium-based compounds used as flame-halting agents have been proven in highly effective fire suppression agents such as Purple-K dry chemicals (potassium bicarbonate KHCO₃-based mixed with other chemicals) and Karbaloy wet agents (potassium carbonate K₂CO₃-based mixed with water).

Fire extinguishing systems producing ions that directly inhibit the chemical chain reaction which forms one of the four sides of the fire tetrahedron (heat – oxygen – fuel – chemical reaction) is not new. For example, the Halon 1301 and Halon 1211 agents (producing bromine ions) have been widely used for the fire protection of special hazards against Class A surface, B, and C fires since the 1960s.

The invention of condensed aerosol fire extinguishing technology has discovered a method to disperse, as a total flooding agent, potassium-based micro-particles propelled by inert gases. Condensed aerosol, as defined in NFPA 2010, is an extinguishing medium consisting of finely divided solid particles, generally less than 10 microns in diameter, and gaseous matter, generated by a combustion process of a solid aerosol-forming compound.

Based on Stokes Law, the settling velocities for fine solid spherical particles in air over a retention period of 10 minutes can be determined. Particles with mass median aerodynamic diameter (MMAD) < 10 microns, the vast majority of particles will remain suspended during the retention period. For particles with MMAD > 10 microns (micrometer) will agglomerate and fall out of suspension.

To illustrate, the Stat-X aerosol microparticles will have a MMAD of 1-2 microns with a settling velocity of 0,0035 to 0,013 cm/s. Generally Purple-K dry chemical particles will have a MMAD greater than 20 microns with a settling velocity of 1,2 cm/s, 90-340 times that of the Stat-X aerosol particles. Clearly, once dispersed, aerosol fire extinguishing agents will tend to remain suspended in air over a long period of time, compared with the much larger dry chemical particles which will quickly settle within seconds forming significant surface residue. Once fires are extinguished, the suspended aerosol agents remain buoyant in air and can be naturally or forced vented from the protected area before the micro-particles can settle. Consequently, when properly vented during post-fire operations, aerosol systems will leave little residue.

Fire suppression mechanics

During combustion, the flame propagation radicals including hydroxyl OH, hydrogen H and oxygen O sustain the flame chemical chain reaction. Condensed aerosol microparticles generally consist of a large population of solid potassium carbonate K₂CO₃ and potassium bi-carbonate KHCO₃ compounds generated from a proprietary aerosol forming solid pellet stored inside a fire extinguishing unit. The pellet also generates inert nitrogen gas which entrains and propels the potassium microparticle compounds into the protected space in the form of an aerosol cloud.

The cloud, composed of a large population of dispersed microparticles, collectively offers a very large surface area for the absorption of heat. Thermal cooling of the flame temperature will occur as the potassium solid compounds absorb heat and vaporise to gas. Dilution of the fire combustion zone with aerosol microparticles and inert gases will slow down the velocity of the flame front propagation and inhibit additional fuel molecules from participating in the combustion process.

On the surface of the particles recombination of the flame propagation radicals OH, H, and O will take place to form OH and water molecules.

Endothermic chemical reactions take place where potassium radicals K are generated and will combine with the flame radicals OH and O to form potassium hydroxide KOH and potassium oxygen KO. The KOH compound will further combine with the H radical, to release more potassium radicals K. The population of potassium radicals is therefore propagated since they are both consumed and produced by the reaction with the fire free radicals. Generally the common by-product from these chemical reactions is the water molecule H₂O.

Condensed aerosol fire extinguishing systems consequently attack all four sides of the fire tetrahedron. The enhanced fire suppression efficiency of condensed aerosol systems is such that when designed as total flooding systems to extinguish Class B flammable liquid fires, the mass flooding rate per volume for condensed aerosol systems is five times less than an equivalent NFPA 12A halon 1301 system, 10 times less than an equivalent NFPA 2001 halocarbon clean agent system, and 14 times less than an equivalent NFPA 12 carbon dioxide system.

STAT-X aerosol fire suppression products

Fireaway, a Minneapolis-based company founded in 2005 has developed a family of products under the brand name of Stat-X, that is compliant with NFPA 2010 and the first manufacturer listed by UL under category FWSA. The Stat-X products are UL listed for Class A and B fires, and Class C (with Class A and B involvement). Stat-X fire detection and extinguishing systems are approved on the EPA SNAP List for normally unoccupied and unoccupied spaces.

Stat-X 30T generator extinguishing a flammable liquid fire in one second

The Stat-X product lines include three families of fixed and portable fire suppression units of similar design but with different methods of application, including electric release with a fire detection system, automatic/mechanical release as a standalone fire extinguishing unit with integral fixed-temperature heat detector, portable hand tossed unit for local area flame suppression and control. All units supplied for fixed fire extinguishing applications are hermetically sealed generators of stainless steel construction.

The patented Stat-X electric operated aerosol generators are available in units with integral aerosol forming compounds supplying 30, 60, 100, 250, 500, 1000, 1500, and 2500 grams of aerosol fire suppressant agent. The electrically activated generators, when coupled with a fire detection and releasing panel and appropriate detection devices, provide a fire suppression system that is significantly more cost effective than any other available technology.

The second part of this article will be published in the next issue of Hi-Tech Security Solutions.

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