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Suppressing fire by throwing water onto ithas been used since ancient times. To pro-vide an automatic spray of water to controla fire, sprinkler systems were developed inthe late 19th century. Since then, automaticsprinklers have become the most commonfixed fire suppression system for providingfire safety in buildings.

Sprinklers control fire development bywetting and cooling the fuel surface. Theyare effective for fires involving solid materi-als (referred to as solid fuels) but are not

effective for flammable liquids (calledliquid fuels) such as gasoline, diesel andjet fuels. The old adage “oil and water donot mix” must be kept in mind. Fire sup-pression systems for liquid fuels typicallyuse foam or dry chemicals, which cover the

fuel surface, hence limiting thermal feed- back to the liquid fuel surface and fuelvapourization.

Fires in electrical and electronic facilitiescall for special solutions for suppression.Carbon dioxide (CO2) systems were usedearly on. They deliver large amounts of CO2 to the fire area to reduce the oxygenconcentration to below the level requiredfor combustion. For total flooding systems,in which pressurized cylinders are con-nected to a fixed piping and nozzle system

to discharge the gas into an enclosed space,the oxygen concentration can be reducedto below the level required by occupants.As a result, CO2 systems are typicallylimited to unoccupied spaces.

In the 1940s, there was a major effort tocome up with a more effective fire suppres-sion agent than the ones being used at thattime. This led to the development of halonchemicals, including Halon 1301, whichwas typically used for total flooding appli-cations. Halons are excellent fire suppres-sants, but they contribute significantly tostratospheric ozone depletion. As a result,fire protection halons were phased out indeveloped countries around 1990 when aninternational consensus (the MontrealProtocol) was reached to regulate the useof ozone-depleting substances.

By Andrew Kim

Fire suppression systems for the protection of buildings have evolved inresponse to new requirements, environmental pressures and advances intechnology. This Update provides an overview of four recently developedsystems, reviews research on the performance of each, and provides

guidance on system selection, design and use.

Advances in FireSuppression Systems

Construction Technology Update No. 75

There are two basic methods for applying an extinguishingagent: total flooding and local application. Total flooding meansapplying an extinguishing agent to a three-dimensionalenclosed space in order to achieve a concentration of theagent adequate to extinguish the fire. These types of systemsmay be operated automatically or manually. Local applica-

tion means applying an extinguishing agent directly onto afire (usually a two-dimensional area), or into the three-dimensional region immediately surrounding the substanceor object on fire. The main difference between local appli-cation and total flooding design is the absence of physical

barriers enclosing the fire space.

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The phase-out led to extensive researchto find halon substitutes that would workwell for various applications without harm-ing the environment and human health.Several new fire suppression systems have

been developed and some old approacheshave gained renewed interest. This Updatereviews four such technologies: inert andhalocarbon gaseous agents, water mist sys-tems, compressed-air-foam systems, andsolid gas generators. A subsequent Updatewill focus solely on compressed-air foamtechnology, drawing on extensive researchcarried out by the NRC Institute forResearch in Construction (NRC-IRC).

Gaseous Systems Two types of gaseous agents are

available for use in total flood-ing systems: halocarbon agentsand inert gases. A generalrequirement for such systems isthat the enclosure must becapable of holding the gas and

be able to withstand the highpressures produced duringdischarge.

To address limitations andproper use of systems involvinggaseous agents, the NationalFire Protection Associationpublished NFPA 2001[1]

“Standard on Clean Agent FireExtinguishing Systems.” NFPA2001 is not a design handbook

but rather a guide for thosedealing with clean agent extin-

guishing systems, to help ensure that suchequipment will function as intended. Thestandard contains information on the useand limitations of clean agents, such as thephysical properties of halocarbon and inertagents, maximum concentrations allowed,and toxicity of the agents. It also addressessystem components and hardware, systemdesign, inspection, maintenance, testingand training.Halocarbon AgentsHalocarbon agents are chemicals similar tohalon except that their molecular structurehas been modified to reduce the number of,or to eliminate completely, the chlorine and

bromine atoms, which are responsible forozone depletion. These agents extinguishfires primarily by cooling.

Acceptance of a halocarbon agent byregulatory authorities hinges on the agent’stoxicity. Two toxicological aspects must beconsidered. One is the toxicity of the agentitself, and the other is the toxicity of com-

bustion by-products of the agent producedunder fire conditions.

Results from both small-scale and full-scale tests have shown that the halocarbonreplacement systems extinguish fires well,though not as effectively as halons. To pro-vide the same level of fire protection ashalons, larger amounts of halocarbon agentsare needed. This means larger and heaviercylinders are required, which may createweight and space problems (Figure 1).

The test results also show that halocarbonagents produce five to ten times more toxicgases than Halon 1301 during fire suppres-sion. These gases include hydrogen fluoride(HF) and carbonyl difluoride (COF2), withlevels produced in test fires significantlyexceeding all human exposure limits. Thelevels of HF and COF2 likely to be producedin actual applications will depend on manyfactors such as agent type and concentration,fire type and size, and discharge and extin-guishment times. Halocarbon agents alsoproduce carbon monoxide (CO) during fireextinguishment, which could be anothersafety issue, depending on the concentrations

and the occupancy of the space.Some halocarbon agents have a longAtmospheric Life Time (ALT) and couldcontribute to global warming. This may

become a determining factor in selectingsuitable suppression agents in the future.

Inert Gas AgentsInert gas agents are applied as total floodingagents. They extinguish fire by displacingthe oxygen in the enclosed space andeventually reducing its concentration belowthe level required for combustion. Inertgases, such as nitrogen, argon and helium,are clean and naturally occurring, have zero

ozone depletion potential and no globalwarming potential. They are not subjectto thermal decomposition when used inextinguishing fires, and hence form nocombustion by-products.

One of the disadvantages of using inertgas systems is that a large volume of agentis required to extinguish a fire. As well,inert gases cannot be liquefied and must bestored in cylinders as high pressure gases,

2

Figure 1. Weight and space issues can arise with the use of cylinders

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which has implications for space and weight.Inert gases also require a discharge systemsufficiently robust to withstand the highpressures involved. The rapid displacementof oxygen, high noise levels and rapidcooling are also a concern if the agent is to

be discharged into an occupied space.

Water MistThe term "water mist" refers to fine watersprays in which 99% of the volume of thespray is in droplets smaller than 1000 micronsin diameter.

For water mist systems, a standardNFPA 750[2], “Standard on Water Mist FireProtection Systems,” was developed byNFPA. The standard specifies minimumrequirements for the design, installation,

maintenance, and testing of water mist fireprotection systems. It does not providedefinitive fire performance criteria nor doesit offer guidance on how to design a systemto suppress a fire.

Fire suppression by water mist is mainly by physical mechanisms. No significantchemical effects are involved. While earlystudies identified flame cooling and oxygendisplacement as the dominant mechanisms,recent investigations suggest that there areadditional ones. The main one is radiationattenuation, which can stop the fire fromspreading to an un-ignited fuel surface and

reduces the vapourization at the fuel sur-face. Tests conducted at NRC-IRC showedthat the radiant heat transfer to the walls of the test compartment was reduced by morethan 70%. Other secondary extinguishingmechanisms include dilution of flammablevapours released by the burning objects,and wetting and cooling of these objects bydirect impingement.

Water mist does not behave like a “true”gaseous agent. The NRC-IRC compartmenttests showed that its effectiveness in firesuppression is substantially affected by thefire size, the degree of obstruction, ceilingheight, and the ventilation conditions.Water mist characteristics, such as varietyof drop sizes and spray momentum, have adirect influence on effectiveness. To effec-tively suppress a fire, a water mist systemmust generate and deliver optimum-sizeddroplets with an adequate concentration.The selection of the optimum size of dropletsfor the design of the system is dependent

on the potential size of the fire, propertiesof the combustibles, and the degree of obstruction and ventilation in the compart-ment. There is no one drop size distributionto fit all fire scenarios.

There are several water mist systemsavailable commercially. Some employ highor intermediate pressures of water throughsmall orifices in a nozzle to produce themist, while others use twin fluid nozzles(water and air).

Water mist systems have demonstrated anumber of advantages, such as good firesuppression capability, no environmentalimpact and no toxicity. As a result, theyhave been considered for numerous appli-cations. One potential application is ship-

board machinery spaces. Water mistsystems are able to extinguish a wide varietyof fires when natural ventilation, such asopen doors and hatches are allowed, whereasgaseous agents were not effective undersuch conditions. Water mist systems alsorapidly reduced the compartment tempera-ture and significantly improved visibility.These advantages allow accessibility to thecompartment during fire suppression.

Another application where water misthas potential as an effective halon alterna-tive is the protection of electronic equip-ment. The telecommunications industry

and utilities have generally been reluctantto use water as a fire suppressant becauseof concerns about damage to electrical andelectronic equipment. A preliminary study

by NRC-IRC on the feasibility of using awater mist system to suppress in-cabinetelectronic fires showed that the system waseffective without causing short circuits ordamage to the equipment.

Recent NRC-IRC research[3] in a testcompartment has shown that the fire sup-pression performance of water mist can befurther improved by using a cycling dis-charge. This feature involves a continuous

alternation of an ON and OFF cycle of themist discharge. When a fire was minor, theextinguishment was easy and the contribu-tion made by the cycling discharge wassmall. For more challenging fire scenarios,such as ventilated conditions, the cyclingdischarge significantly reduced extinguish-ment times and water requirements. Morewater vapour and combustion productswere produced, which increased the rate of

3

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oxygen depletion. In addition, the recurrentdynamic mixing created by a cycling dis-charge effectively dilutes the oxygen andfuel vapour available for the fire.

Compressed-Air FoamFor decades, foam systems have been usedto provide fire protection in the chemicaland petroleum industries and in militaryinstallations. The overall effectiveness of current fixed-pipe foam systems, whichincorporate aspirating-type nozzles and

blower-type foam generators, is limitedsince they are unable to provide foams withhigh injection velocities. As well, the foamproduced using traditional systems is not asstable and consistent, and expansion ratiosare not as high as desired for some applica-

tions, because the air to generate foam atthe nozzle, which comes from the fire envi-ronment, may be contaminated. However,if compressed air is used for foam generation,the foam possesses superior quality andsubstantial injection velocity, as well asrequiring a much smaller quantity of waterand foam concentrates.

Compressed-air foam (CAF) is generated by injecting air under pressure into a foamsolution stream (Figure 2). The process of moving the solution and air mixture throughthe hose or piping, if done correctly, formsa foam. The energy for the CAF comes

from the combined momentum of the foamsolution and air. One significant advantageof such systems is the increased momentumof the foam, enabling it to penetrate flamesand reach the fuel surface. Another advan-tage of CAF is that it possesses greater sta-

bility with respect to drainage (foam does

not collapse easily) than air-aspiratedfoams, since it is characterized by a narrowdistribution of bubble sizes.

Early attempts to adapt CAF to fixedinstallations failed, owing to two fundamen-tal technical difficulties: first, traditionalsprinkler-type nozzles cannot distributecompressed-air foam without collapsing it,and second, the foam itself degenerates infixed piping. Recently, NRC-IRC overcamethese difficulties and developed a means of producing CAF using Class A and Class Bfoam concentrates in a fixed-pipe system,using a new and innovative foam distribution

nozzle. Foam break-up, which preventedthe development of this technology in thepast, was avoided by careful engineeringdesign of the nozzle and the piping system.

NRC-IRC conducted full-scale fire teststo evaluate the performance of a prototypeCAF system (Figure 3). The tests demon-strated the superior performance of thesystem in extinguishing both liquid fuel and

4

Figure 3. CAF extinguishment versus water (deluge) extinguishment

Figure 2. Schematic of a CAF system

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wood crib fires with a small amount of water.Also, CAF requires a smaller amount of foam concentrate to provide effectivesuppression, compared to systems based onair-aspirated nozzles. In the NRC-IRC tests,less than one half of the normally recom-mended (for air-aspirated systems) Class Aand Class B foam solutions were used with-out compromising the extinguishment effi-ciency of compressed-air foams [4].

Recently, NRC-IRC developed a prototypeCAF system for practical application in

providing fire protection to very largestructures, such as aircraft hangars andpower transformers. Full-scale experimentscarried out with the prototype proved thesuperior performance of CAF in extinguishingsimulated fuel spill and transformer fires(Figure 4).

Currently pre-engineered CAF systemsare being developed for a variety of com-mercial applications.

Gas Generators Based on automotive airbag technology, gasgenerators have been developed for fire

suppression applications. Gas generatorscan produce a large quantity of gases (mainlyN2, CO2 and water vapour) by combustionof solid propellants. Solid propellantsconsist of oxidizers and fuel ingredients,and are able to burn without ambient air.Gas generators can be very compact andcan provide very fast discharge (in a few

milliseconds). Currently, there are twotypes of gas generators available: conven-tional and hybrid.

Conventional gas generators contain apropellant and an electrical initiator. Whena signal is received from a detector/controller,the electrical initiator ignites a charge tostart a combustion process in the propellant.Rapid combustion of the solid propellantgenerates large amounts of N2, CO2 andwater vapour, which rapidly increases theinternal pressure. A hermetic seal isruptured and the gas products are dis-charged within milliseconds into the pro-tected space. Suppression is by oxygendisplacement and gas discharge dynamics(blowing effect).

A hybrid gas generator consists of anelectrical initiator, a solid propellantchamber and a suppression agent chamber(Figure 5). The heat and pressure generated

by the combustion of the propellant are usedto heat and expel the liquefied suppressant.

Gas generators are limited for use inunoccupied spaces only, because of theirhigh temperature and high momentumdischarge.

5

Figure 5. Hybrid gas generator

Figure 4. Extinguishment of a transformer fire using CAF

Initiator

Solidpropellant

Liquidsuppression

agent

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Guidance for Users

Each of the fire suppression systems dis-cussed here work well and have certainadvantages and disadvantages dependingon the specific application, summarizedas follows.

For gaseous systems, the fire hazard must be in an enclosed space. If the room hasmany openings, a gaseous system may not

be able to maintain its design concentration,and may fail to extinguish the fire. Also,

because halocarbon gaseous agents producetoxic gases, steps must be taken to minimizegas production and exposure to it.

Water mist systems work well with a largefire in a reasonably enclosed space, suchas a room with doors and windows open.If the fire is hidden or shielded from thewater mist spray, the fire may not be extin-guished. Fire suppression effectivenessdepends on many factors, such as roomgeometry, location of the fire with respectto the nozzle, and obstruction. A watermist system is used to replace a sprinklersystem where the need for reducing waterdamage is great, such as in museums andart galleries.

A CAF system works best in extinguishingliquid fuel fires. It can suppress fires in anopen area and does not require an enclosure.

It also requires far less water than conven-tional water-based systems. However, a CAFsystem will have difficulty extinguishing3-D fires, such as a spray fire.

Gas generators work well in relativelysmall and airtight compartments. They aretypically used where installation of pipingfor conventional fire suppression systemsis difficult, or where there is a need forquick fire extinguishment. They can only

be used in a location where no person ispresent, such as an engine compartmentor storage space.

Conclusions

Since the phase-out of halon, a major thrustto develop new advanced fire suppression

systems has produced several effectiveoptions. These include halocarbon andinert gas systems, water mist, compressed-air-foam, and gas generators.

All of these systems extinguish fires attheir design conditions. However, no onesystem can be chosen as the best system forall applications. Some perform better thanothers in a specific application. All havelimitations and raise concerns that have to

be dealt with. It is important that all theadvantages and disadvantages of each firesuppression system be considered in rela-tion to the specific requirements to select

the best system for the application.References

[1] NFPA 2001 (2000), “Standard on CleanAgent Fire Extinguishing Systems,”National Fire Protection Association,Quincy, MA, U.S.A., 2000 Edition,pp. 1-104.

[2] NFPA 750 (2010), “Standard on WaterMist Fire Protection Systems,” NationalFire Protection Association, Quincy,MA, U.S.A., 2010 Edition, pp. 1-69.

[3] Kim, A.K., Liu, Z. and Su, J.Z. (1999),“Water Mist Fire Suppression usingCycling Discharges,” Proceedings of Interflam ’99, Edinburgh, UK, p. 1349.

[4] Kim, A.K. and Dlugogorski, B.Z. (1997),“Multipurpose Overhead Compressed AirFoam System and its Fire SuppressionPerformance,” Journal of Fire ProtectionEngineering , Vol. 8, No. 3, p. 133.

Dr. A.K. Kim is a senior research officer in theFire Research program of the National ResearchCouncil Institute for Research in Construction.

“Construction Technology Updates” is a series of technical articles containing practical information distilled from recent construction research.

For more information, contact Institute for Research in Construction,National Research Council of Canada, Ottawa K1A 0R6.

© 2011National Research Council of CanadaMarch 2011ISSN 1206-1220

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