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StateoftheArtAtriumSmokeControl

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HOME > FIRE / SMOKE > STATE-OF-THE-ART ATRIUM SMOKE CONTROL

StateoftheArtAtriumSmokeControl
Howengineeringanalysisensuressmokecontrolsystemscanhandlesignificant
designfirechallenges
ByJOHNH.KLOTE,DSc,PE,JohnH.KloteInc.,Leesburg,Va.
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Unlikeotherbuildingsystems,itisvirtuallyimpossibletotestanatriumsmokecontrol
systemtodesignconditions.Thisprimarilyisbecausedesignconditionsinvolvelarge
designfiresthatcandamageanatrium.Designconditionsalsocanincludewind,for
whichsystemsarenearlyimpossibletotest.Itisessentialthatanatriumsmokecontrol
systembedesignedproperlyandtestedtoverifyitoperatesasintendedandthatsystem
componentsbeinspectedtoensuretheyfunctionasspecified.
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Thisarticlediscussesatriasmokecontrolmethodsforavarietyoflargeopenspaces,such
asenclosedshoppingmalls,arcades,sportsarenas,exhibitionhalls,andairplane
hangers.
Smokecontroltechnologyhasmadesignificantadvancesinrecentyears.Designanalysis
ofthesesystemscommonlyisaccomplishedviaoneormoretechniques,suchasalgebraic
equationsandzonefireandcomputationalfluiddynamics(CFD)modeling.1Adetailed
mathematicaltreatmentofthesetechniquesisbeyondthescopeofthisarticlehowever,
stateoftheartatriumsmokecontroltechnologyisaddressed.

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BASICCONCEPTS

Whenafireoccurs,smokerisesinaplume.Astheplumerises,itpullsairfromthe
surroundingspace,whichcausestheplume'smassflowtoincreaseanditstemperatureto
decrease.Whentheplumereachestheceiling,itspreadsout,formingalayer.Anatrium
smokecontrolsystemexhaustssmokefromthatlayer,providingarelativelysmokefree
environment(figures1and2).

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Plumedynamicshavebeenstudiedextensively,andalgebraicequationshavebeen
developedtocalculatethemassflowandtemperatureofaplumebasedonplumeheight
andfiresize.Forsteadystateconditions,exhaustedsmokeequalsthemassflowingfrom
aplumeintoasmokelayer.Thus,equationscanbeusedtocalculatethetemperatureand
flowrateofsmokeexhaust.
AbookrecentlypublishedbytheInternationalCodeCouncil(ICC)2focusesonthe
requirementsofthe2006InternationalBuildingCode(IBC),3includingtheequations
neededforsystemanalysis.Intendedforsmokecontroldesignersandcodeofficials,the
bookaddressesallaspectsofsmokecontrol,includingpressurizationsystemsatrium,
stairwell,andelevatorsmokecontroldesignfiressmokecontrolequipmentand
inspectionandcommissioning.Thebookalsodetailsthe2006IBC'sadoptionofNational
FireProtectionAssociation(NFPA)92B:StandardforSmokeManagementSystemsin

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WEBINAR

Malls,Atria,andLargeAreas.4AbookonsmokecontrolpublishedbytheAmerican
SocietyofHeating,RefrigeratingandAirConditioningEngineers(ASHRAE)5includes
derivationsofmanyoftheequationsusedinsmokecontroldesign.
Thealgebraicequationapproachisbasedonanidealizationinwhichasmokelayer's
temperatureisthesamethroughoutandthebottomofthesmokelayerisahorizontal
plane,calledthesmokelayerinterface.Inthisidealization,smokefreeairispresent
0.001in.belowtheinterface.Duringarealfire,atransitionzoneactuallyexistsbetween
thesmokelayerandtheairbelow.However,thealgebraicequationapproachisusefulfor
smokecontroldesign.
Smokefilling,analternativeapproachtosmokeexhaust,requiresthatoccupants
evacuatefromorthroughtheatriumassmokefillsthespace.Smokefillingappliesonlyto
atriathathaveverylargevolumesabovetheirhighestwalkingsurfaces,whichcreate
fillingtimesthataresufficientforevacuation,includingtheamountoftimeoccupants
needtobecomeawareofafireandprepareformovementtoanexit.Smokecontrol
equipmentisnotrequiredforsmokefilling.
SmokefillingtimecanbecalculatedviaalgebraicequationsandCFDandzonefire
models.Fillingtimealgebraicequationscanbefoundinthepreviouslymentionedbook
publishedbytheICC.2Overthelastthreedecades,manyzonefiremodelshavebeen
developed,themostsophisticatedofwhichisConsolidatedModelofFireandSmoke
Transport(CFAST),developedbytheNationalInstituteofStandardsandTechnology

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(NIST).6CFASTisavailableforfreeathttp://fast.nist.gov.CFDmodelingisaddressed
laterinthisarticle.

FundingHVACREnergyEfficiency
ResearchandDevelopment

Becausefewatriaarelargeenoughtorelyonsmokefilling,theremainderofthisarticle
dealsonlywithatriumsmokeexhaustsystems.

byLawrence(Larry)Clark,
QCxP,GGP,LEEDAP+
Posted6daysago

MINIMUMDESIGNSMOKELAYERDEPTH

inClark'sRemarks

Itisimportantthatsufficientspacebeavailableforasmokelayertoform.Whenasmoke

GivingThanks

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plumereachestheceiling,smokeflowsawayfromthepointofimpactinaradial
direction,formingaceilingjet.Whentheceilingjetreachesawall,smokeflowsaround
andundertheceilingjet.Theceilingjetandthesmokeflowundertheceilingjeteach
haveadepthofabout10percentofthefloortoceilingheight,meaningthatthenormal
minimumsmokelayerdepthisabout20percentofthefloortoceilingheight.Thesmoke
layerneedstobeatleastthisdeep,unlessanengineeringanalysisshowsotherwise
(figures1and2).Suchananalysiscanbebasedonfullscaleorscalemodelfiretestsor
CFDmodeling.
SMOKEPLUMES

byLawrence(Larry)Clark,
QCxP,GGP,LEEDAP+
Posted2weeksago
inClark'sRemarks

AcceleratingEnergyProductivity
byLawrence(Larry)Clark,
QCxP,GGP,LEEDAP+
Posted4weeksago
inClark'sRemarks

Therearemanykindsofsmokeplumes,butaxisymmetricplumesandbalconyspill
plumesaremostcommonlyconsideredduringsmokecontroldesign.

FIGURE1.Atriumfirewithanaxisymmetricplume.

Anaxisymmetricplumeisexpectedtoaccompanyafirethatoriginatesonthefloorofan
atriumawayfromwalls(Figure1).Inthiscase,airisentrainedintotheplumefromall
sidesandalongtheentireheightoftheplume.Themassrateofanaxisymmetricplume
dependsonthesizeofthefireandthedistancefromthebaseofthefueltothesmoke
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layerinterface.
Abalconyspillplumeflowsunderandaroundabalconybeforerising.Airisentrained
intotheplumeastheplumeflowsunderandaroundthebalconyandsmokerisesabove
thebalcony(Figure2).Themassrateofabalconyspillplumedependsonthesizeofthe
fire,thedimensionsofthebalcony,andthedistancefromthebalconyedgetothesmoke
layerinterface.

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FIGURE2.Atriumfirewithabalconyspillplume.

Awindowplumecomesfromaroomthathasafullydevelopedfire.Becauseafully
developedfireisnotexpectedwithaproperlyfunctioningsprinklersystem,window
plumesareconsideredonlyforveryunusualapplications.

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MAKEUPAIR

Makeupaircanbeprovidedbyfansoropeningstotheoutside.Makeupairmustbe
providedforexhaustfanstooperateatdesignflowrates.Makeupairsupplypoints
shouldbebelowthesmokelayerinterface.Whenmakeupairisprovidedviaopeningsto
theoutside,theeffectofwindonthemakeupairopeningsshouldbeanalyzed.
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Largeopeningsandsmallpathsareusedtoprovidemakeupairviaopeningstothe
outside.Largeopeningsincludeopendoors,windows,vents,etc.Smallpathsinclude
constructioncracksandgapsaroundcloseddoorsandwindows.
Makeupairforfanpowered/mechanicalsmokeexhaustsystemsshouldbelessthanthe
massflowrateapproximately85to95percentofexhaust.Theremaining5to15
percentoftheairwillenterthelargevolumespaceviasmallpaths.Supplyinglessthan
100percentmakeupairavoidspositivelypressurizingalargevolumespace.

FIGURE3.Systemfailurecausedbymakeupairvelocitygreater
than200fpm.

Unlessahighervelocityissupportedbyengineeringanalysis,makeupairshouldnot
exceed200fpminareasinwhichitcouldcomeintocontactwithasmokeplume.This
limitpreventssignificantdeflectionoftheplumeanddisruptionofthesmokelayer
(Figure3).Thistypeofengineeringanalysiscouldbebasedoncomparisonsdeveloped
withfullscale,scale,orCFDmodeling.
SMOKESTRATIFICATION
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Theresultofrooftopsolarradiation,ahotlayerofairoftenformsunderanatrium's
ceiling.Thetemperatureofthislayeroftenexceeds120F.Aspreviouslymentioned,
plumetemperaturedecreasesasplumeheightincreases.Fora1,800Btupersecondfire
inanatriumwithafloortoceilingheightof60ft,theceilingsmoketemperaturewould
beonlyabout95F.
Whenaplume'saveragetemperatureislessthanahotairlayer's,smokewillstratify
underthehotairlayer,preventingsmokefromreachingceilingmountedsmoke
detectors.Projectedbeamsmokedetectorsshouldbeusedforapplicationsinwhich
smokestratificationispossible.Threecommonlyacceptedarrangementsofprojected
beamsmokedetectorscanbeusedforatriumsmokecontrol:upwardbeamsthatdetect
thesmokelayer,horizontalbeamsthatdetectthesmokelayeratvariouslevels,and
horizontalbeamsthatdetectthesmokeplume.Forthesearrangements,thespacingof
detectorsiscritical.Spacingrecommendationscanbefoundinthepreviouslymentioned
bookpublishedbytheICC.
DESIGNFIRES

Designfires,whichcanbedeemedsteadyorunsteady,haveamajorimpactonanatrium
smokecontrolsystem.Designfiresizeisexpressedintermsofheatreleaserate.Typically
rangingfrom1,800to8,000Btupersecond,designfiresshouldbeevaluatedaspartofa
smokecontrolsystem'sengineeringanalysis.Foradiscussionoftheconceptsbehind
designfires,seetheHPACEngineeringarticleDesignFires:WhatYouNeedtoKnow7
andtheconferencepaperDeterminingDesignFiresforDesignLevelandExtreme
Events.8Adetailedtreatmentofdesignfirescanbefoundinthepreviouslymentioned
bookpublishedbytheICC.2
Designersshouldnotmaketheblunderofthinkingthatanatriumwithalmostno
materialsshouldhaveaverysmalldesignfire.Thiskindofthinkingdoesnotaccountfor
changesinspaceuseortransientfuels.Transientfuelsarematerialsthatresideinaspace
temporarily,suchasholidaydecorations,paintandsolventsusedforredecorating,
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cardboardboxesawaitingremoval,andupholsteredfurniture.Transientfuelsmustnot
beoverlookedwhenanalyzingdesignfiresize.
PLUGHOLING

FIGURE4.Plugholingcanresultinsystemfailure.

Plugholingoccurswhenairbelowispulledthroughasmokelayerandintosmoke
exhaust(Figure4).Plugholinglowersthesmokelayerinterfaceandcanexposepeopleto
smoke.Loweringtheinterfacecanresultinsystemfailurehowever,plugholingcanbe
preventedbykeepingflowrelativelylowateachsmokeexhaustinlet.Toavoid
plugholing,themaximumflowrateatsmokeexhaustinletsmustbecalculatedcorrectly
andthenumberofinletschosencarefully.
CFDMODELING

CFDmodelingdividesaspaceintoalargenumberofcellsandsolvesthegoverning
equationsforeach.(Governingequationsarenonlinear,partialdifferentialequationsfor
conservationofmass,momentum,andenergy.)Atriumapplicationscanhave100,000to
1millioncells.Obstructions,suchaswalls,balconies,andstairs,shouldbeconsidered.
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Boundaryconditions,includingsmokeexhaustandmakeupair,shouldbedefined.
Atriumsmokecontroldesignsbasedontheconventionalalgebraicequationapproach
tendtobeconservativeandhaveexhaustflowratesthataresomewhathigh.Conversely,
CFDmodelingcanbethebasisforexceptionsfortherequirementstosmokelayerdepth,
the200fpmlimitationonmakeupair,andplugholing.
CFDmodelingprovidesahighdegreeofconfidencethatatenableenvironmentwillbe
preserved.CFDmodeling'sstrengthisthatitcansimulatefireinducedsmokeflows,
whichalgebraicequationscannot.
TherearesomegoodgeneralpurposeCFDmodels,buttheNIST'sFireDynamics
Simulator(FDS)9isforfireapplications.WhiletheannualfeetouseacommercialCFD
modelcanbetensofthousandsofdollars,theFDSanditsassociateddocumentscanbe
downloadedforfreeatwww.fire.nist.gov/fds/downloads.html.Foranonmathematical
discussionofCFDmodeling,seethepreviouslymentionedHPACEngineeringarticle.The
previouslymentionedbookpublishedbytheICCincludesadetailedintroductiontoCFD
modeling.3
REFERENCES
1. Klote,J.H.(2006,June).CFD:Anewwaytodesignatriumsmokecontrol.HPAC

Engineering,pp.1927.
2. Klote,J.H.,&Evans,D.H.(2007).Aguidetosmokecontrolinthe2006IBC.

CountryClubHills,IL:InternationalCodeCouncil.
3. InternationalCodeCouncil.(2006).2006InternationalBuildingCode.Country

ClubHills,IL:InternationalCodeCouncil.
4. NFPA.(2005).NFPA92B:Standardforsmokemanagementsystemsinmalls,

atria,andlargeareas.Quincy,MA:NationalFireProtectionAssociation.
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5. Klote,J.H.,&Milke,J.A.(2002).Principlesofsmokemanagement.Atlanta:

AmericanSocietyofHeating,RefrigeratingandAirConditioningEngineers.
6. Peacock,R.D.,Jones,W.W.,Reneke,P.A.,&Forney,G.P.(2005).NISTspecial

publication1041:CFASTConsolidatedmodeloffiregrowthandsmoke
transport(version6):User'sguide.Gaithersburg,MD:NationalInstituteof
StandardsandTechnology.
7. Klote,J.H.(2002,September).Designfires:Whatyouneedtoknow.HPAC

Engineering,pp.4351.
8. Bukowski,R.W.(2006,June).Determiningdesignfiresfordesignleveland

extremeevents.Paperpresentedatthe6thInternationalConferenceon
PerformanceBasedCodesandFireSafetyDesignMethods,Tokyo,Japan.
9. McGrattan,K.(2004).NISTspecialpublication1018:Firedynamicssimulator

(version4):Technicalreferenceguide.Gaithersburg,MD:NationalInstituteof
StandardsandTechnology.

Knownworldwideasasmokecontrolexpert,JohnH.Klote,DSc,PE,isaconsulting
engineerbasedinLeesburg,Va.Formerly,heconductedfireresearchfortheNational
InstituteofStandardsandTechnology.

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