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CABA Home & Building Automation Quarterly, Spring, pp. 24-25, 1999-03-01
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Research viewpoints by IRC : water-mist fire-suppression challenges
controls industry
Kim, A. K.
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Research viewpoints by IRC : water-mist
fire-suppression challenges controls industry
Kim, A.K.
A version of this paper is published in / Une version de ce document se trouve dans : CABA Home & Building Automation Quarterly, Spring 1999, pp. 24-25
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Water-mist fire-suppression technology poses challenges for controls industry By Andrew K. Kim
An international ban invoked 15 years ago on Halon-1301 has resulted in the development of water-mist fire-suppression systems. The challenge now is to develop the sophisticated fire-detection system necessary to make the new suppression technology commercially viable.
Halon-1301 was one of the most effective chemical fire suppressants ever developed, but was banned in 1994 by the Montreal Protocol because of its adverse effects on the earth's ozone layer. Since then, water mist has received considerable attention as an environmentally friendly, non-toxic, economical and effective halon replacement.
At the moment, though, there remains a need for detection systems that are sensitive enough to provide early detection to trigger the
fire-suppression system without false alarms, and that can pin-point the precise location of the fire.
Unfortunately, highly sensitive early detection still goes hand in hand with false alarms; water mist could be released when it is not needed.
The concept of using water mist to extinguish liquid- and solid-fuel fires is not new, but its development only began to seem practical as a result of the halon ban. As a result, there has been a number of studies of the capabilities of water-mist systems for various applications. Some have been done at the National Research Council of Canada (NRC).
Among the applications considered was use in shipboard machinery and turbine engine rooms, aircraft cabins, engine and dry bays. One area of considerable potential is shipboard machinery spaces.
A concern here was the effectiveness of water mist in a large space with a high ceiling and a many obstructions. Full-scale tests conducted by NRC and other research agencies showed, however, that water-mist systems can effectively extinguish large fires in unventilated machinery spaces using a small amount of water. The performance was comparable to that of halon.
Based on the results of the studies, water-mist systems were identified by both U.S. and Canadian navies as a possible replacement for Halon-1301 in shipboard machinery spaces.
Another study, the NRC's IntelMist project, examined the feasibility of using water mist to protect facilities with a large array of electronic equipment.
It has been found that such water-mist characteristics as drop-size distribution, flux density and spray momentum relate directly to the effectiveness of fire suppression. To be effective, a water-mist system must generate and deliver to the fire optimum-sized droplets in adequate concentration. The selection of the optimum size of droplets is dependent on the potential size of the fire, the properties of the combustibles, and the degree of obstruction and ventilation in the compartment. No one drop-size distribution fits all fire scenarios.
Spray momentum (determined by the spray mass and velocity and its direction relative to the fire plume) is also an important parameter in
determining the success or failure of water-mist fire suppression.
Water-mist systems will be most efficient when the maximum amount of the spray is directly at the fire plume. This is particularly important for
The objective of the IntelMist project was to use state-of-the-art fire-detection technology to control a zoned system, so that water could be applied only to the smallest possible area directly associated with the fire.
As part of the project, a series of full-scale fire-suppression tests was conducted using water mist in electronic cabinets, under-floor cable plenums and overhead cable trays-areas where the telecommunications and utilities industries have traditionally been reluctant to use water
because of potential damage.
The experimental investigations demonstrated that the traditional total-flooding approach (used for Halon-1301), whether in a single cabinet, an under-floor plenum or in a large space with many cable trays, was unreliable when applied to water mist. On the other hand, reliable fire suppression was achieved with water through careful control of spray direction. This was accomplished by laying out the nozzles to suit the physical arrangement of the obstructions or structural elements.
The tests showed that a water mist system can be used to suppress fires in electrical and electronic equipment without causing short circuits or damage to electrical and electronic components. They also showed that with suitable spray characteristics and an intelligent detection system with appropriate signal-processing logic, water mist can replace halon in electronic and telecommunication equipment rooms.
That brings us back to the task facing the controls industry. Determining the precise location of the fire is of paramount importance. One approach might be to use many sensors, then determining which was the one that triggered the alarm. This may be somewhat crude, however, and there could be a more advanced way of discovering the location of the fire-perhaps by using some sort of rotating detector that pinpoints the fire by "pointing" to it. Infrared detection is also a possibility.
Sensitivity is the other paramount factor, in order to avoid having the water-mist system activated by a false alarm.
The answer here probably lies in a form of multi-sensing. The system might detect smoke first, for example, then heat. It could detect the presence of carbon monoxide or carbon dioxide. There could be many sensors, but the fire-suppression system would not be triggered until specific, well-defined criteria have been met.
Ultimately, it is for the industry to decide which variables to address and to develop designs that will allow water-mist fire-suppression systems to fulfil the promise they show.
In the meantime, NRC will continue its work since there is much yet to be understood. Evaluation of water-mist systems has, until now, been based on costly full-scale tests, which slows development of the technology.
However, a computational fluid dynamics (CFD) model can be used to study fluid dynamics and heat transfer as well as the integration between water mist and the flame. Such software tools greatly enhance researchers' understanding of the subject and at a cost much lower than that incurred by full-scale testing.
For example, to study liquid pool fire extinguishment by water mist, NRC recently developed a CFD model, which showed that the fire created a strong upward plume directly over the fire, and did not allow a low-momentum water spray to penetrate that plume. However, water mist cooled the fire plume rapidly and introduced a flow of cool gases over pan fires.
NRC is planning further development of computer models in order to develop an understanding of the extinguishment processes and to assist in
the design of water-mist systems. ______________________________
Andrew K. Kim, Ph.D., P.Eng., is a researcher in the Fire Risk
Management Program of the National Research Council's Institute for Research in Construction. He can be reached via e-mail at: [email protected]
