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StateoftheArt Atrium Smoke ControlHow engineering analysis ensures smokecontrol systems can handle significantdesignfire challenges

By JOHN H. KLOTE, DSc, PE, John H. Klote Inc., Leesburg, Va.

Unlike other building systems, it is virtually impossible to test an atrium smokecontrolsystem to design conditions. This primarily is because design conditions involve largedesign fires that can damage an atrium. Design conditions also can include wind, forwhich systems are nearly impossible to test. It is essential that an atrium smokecontrolsystem be designed properly and tested to verify it operates as intended and that systemcomponents be inspected to ensure they function as specified.

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This article discusses atria smokecontrol methods for a variety of large open spaces, suchas enclosed shopping malls, arcades, sports arenas, exhibition halls, and airplanehangers.

Smokecontrol technology has made significant advances in recent years. Design analysisof these systems commonly is accomplished via one or more techniques, such as algebraic

equations and zonefire and computationalfluiddynamics (CFD) modeling.1 A detailedmathematical treatment of these techniques is beyond the scope of this article; however,stateoftheart atrium smokecontrol technology is addressed.

BASIC CONCEPTS

When a fire occurs, smoke rises in a plume. As the plume rises, it pulls air from thesurrounding space, which causes the plume's mass flow to increase and its temperature todecrease. When the plume reaches the ceiling, it spreads out, forming a layer. An atriumsmokecontrol system exhausts smoke from that layer, providing a relatively smokefreeenvironment (figures 1 and 2).

Plume dynamics have been studied extensively, and algebraic equations have beendeveloped to calculate the mass flow and temperature of a plume based on plume heightand fire size. For steadystate conditions, exhausted smoke equals the mass flowing froma plume into a smoke layer. Thus, equations can be used to calculate the temperature andflow rate of smoke exhaust.

A book recently published by the International Code Council (ICC)2 focuses on the

requirements of the 2006 International Building Code (IBC),3 including the equationsneeded for system analysis. Intended for smokecontrol designers and code officials, thebook addresses all aspects of smoke control, including pressurization systems; atrium,stairwell, and elevator smoke control; design fires; smokecontrol equipment; andinspection and commissioning. The book also details the 2006 IBC's adoption of NationalFire Protection Association (NFPA) 92B: Standard for Smoke Management Systems in

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Malls, Atria, and Large Areas.4 A book on smoke control published by the AmericanSociety of Heating, Refrigerating and AirConditioning Engineers (ASHRAE)5 includesderivations of many of the equations used in smokecontrol design.

The algebraicequation approach is based on an idealization in which a smoke layer'stemperature is the same throughout and the bottom of the smoke layer is a horizontalplane, called the smokelayer interface. In this idealization, “smokefree” air is present0.001 in. below the interface. During a real fire, a transition zone actually exists betweenthe smoke layer and the air below. However, the algebraicequation approach is useful forsmokecontrol design.

Smoke filling, an alternative approach to smoke exhaust, requires that occupantsevacuate from or through the atrium as smoke fills the space. Smoke filling applies only toatria that have very large volumes above their highest walking surfaces, which createfilling times that are sufficient for evacuation, including the amount of time occupantsneed to become aware of a fire and prepare for movement to an exit. Smokecontrolequipment is not required for smoke filling.

Smokefilling time can be calculated via algebraic equations and CFD and zonefiremodels. Fillingtime algebraic equations can be found in the previously mentioned book

published by the ICC.2 Over the last three decades, many zonefire models have beendeveloped, the most sophisticated of which is Consolidated Model of Fire and SmokeTransport (CFAST), developed by the National Institute of Standards and Technology

(NIST).6 CFAST is available for free at http://fast.nist.gov. CFD modeling is addressedlater in this article.

Because few atria are large enough to rely on smoke filling, the remainder of this articledeals only with atrium smokeexhaust systems.

MINIMUM DESIGN SMOKELAYER DEPTH

It is important that sufficient space be available for a smoke layer to form. When a smoke

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plume reaches the ceiling, smoke flows away from the point of impact in a radialdirection, forming a “ceiling jet.” When the ceiling jet reaches a wall, smoke flows aroundand under the ceiling jet. The ceiling jet and the smoke flow under the ceiling jet eachhave a depth of about 10 percent of the floortoceiling height, meaning that the normalminimum smokelayer depth is about 20 percent of the floortoceiling height. The smokelayer needs to be at least this deep, unless an engineering analysis shows otherwise(figures 1 and 2). Such an analysis can be based on fullscale or scalemodel fire tests orCFD modeling.

SMOKE PLUMES

There are many kinds of smoke plumes, but axisymmetric plumes and balcony spillplumes are most commonly considered during smokecontrol design.

FIGURE 1. Atrium fire with an axisymmetric plume.

An axisymmetric plume is expected to accompany a fire that originates on the floor of anatrium away from walls (Figure 1). In this case, air is entrained into the plume from allsides and along the entire height of the plume. The mass rate of an axisymmetric plumedepends on the size of the fire and the distance from the base of the fuel to the smoke

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layer interface.

A balcony spill plume flows under and around a balcony before rising. Air is entrainedinto the plume as the plume flows under and around the balcony and smoke rises abovethe balcony (Figure 2). The mass rate of a balcony spill plume depends on the size of thefire, the dimensions of the balcony, and the distance from the balcony edge to the smokelayer interface.

FIGURE 2. Atrium fire with a balcony spill plume.

A window plume comes from a room that has a fully developed fire. Because a fullydeveloped fire is not expected with a properly functioning sprinkler system, windowplumes are considered only for very unusual applications.

MAKEUP AIR

Makeup air can be provided by fans or openings to the outside. Makeup air must beprovided for exhaust fans to operate at design flow rates. Makeupair supply pointsshould be below the smokelayer interface. When makeup air is provided via openings tothe outside, the effect of wind on the makeupair openings should be analyzed.

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Large openings and small paths are used to provide makeup air via openings to theoutside. Large openings include open doors, windows, vents, etc. Small paths includeconstruction cracks and gaps around closed doors and windows.

Makeup air for fanpowered/mechanical smokeexhaust systems should be less than themass flow rate — approximately 85 to 95 percent — of exhaust. The remaining 5 to 15percent of the air will enter the largevolume space via small paths. Supplying less than100percent makeup air avoids positively pressurizing a largevolume space.

FIGURE 3. System failure caused by makeupair velocity greaterthan 200 fpm.

Unless a higher velocity is supported by engineering analysis, makeup air should notexceed 200 fpm in areas in which it could come into contact with a smoke plume. Thislimit prevents significant deflection of the plume and disruption of the smoke layer(Figure 3). This type of engineering analysis could be based on comparisons developedwith fullscale, scale, or CFD modeling.

SMOKE STRATIFICATION

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The result of rooftop solar radiation, a hot layer of air often forms under an atrium'sceiling. The temperature of this layer often exceeds 120°F. As previously mentioned,plume temperature decreases as plume height increases. For a 1,800Btupersecond firein an atrium with a floortoceiling height of 60 ft, the ceilingsmoke temperature wouldbe only about 95°F.

When a plume's average temperature is less than a hotair layer's, smoke will stratifyunder the hotair layer, preventing smoke from reaching ceilingmounted smokedetectors. Projectedbeam smoke detectors should be used for applications in whichsmoke stratification is possible. Three commonly accepted arrangements of projectedbeam smoke detectors can be used for atrium smoke control: upward beams that detectthe smoke layer, horizontal beams that detect the smoke layer at various levels, andhorizontal beams that detect the smoke plume. For these arrangements, the spacing ofdetectors is critical. Spacing recommendations can be found in the previously mentionedbook published by the ICC.°

DESIGN FIRES

Design fires, which can be deemed steady or unsteady, have a major impact on an atriumsmokecontrol system. Designfire size is expressed in terms of heatrelease rate. Typicallyranging from 1,800 to 8,000 Btu per second, design fires should be evaluated as part of asmokecontrol system's engineering analysis. For a discussion of the concepts behind

design fires, see the HPAC Engineering article “Design Fires: What You Need to Know”7

and the conference paper “Determining Design Fires for DesignLevel and Extreme

Events.”8 A detailed treatment of design fires can be found in the previously mentioned

book published by the ICC.2

Designers should not make the blunder of thinking that an atrium with almost nomaterials should have a very small design fire. This kind of thinking does not account forchanges in space use or transient fuels. Transient fuels are materials that reside in a spacetemporarily, such as holiday decorations, paint and solvents used for redecorating,

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cardboard boxes awaiting removal, and upholstered furniture. Transient fuels must notbe overlooked when analyzing designfire size.

PLUGHOLING

FIGURE 4. Plugholing can result in system failure.

Plugholing occurs when air below is pulled through a smoke layer and into smokeexhaust (Figure 4). Plugholing lowers the smokelayer interface and can expose people tosmoke. Lowering the interface can result in system failure; however, plugholing can beprevented by keeping flow relatively low at each smokeexhaust inlet. To avoidplugholing, the maximum flow rate at smokeexhaust inlets must be calculated correctlyand the number of inlets chosen carefully.

CFD MODELING

CFD modeling divides a space into a large number of cells and solves the governingequations for each. (Governing equations are nonlinear, partial differential equations forconservation of mass, momentum, and energy.) Atrium applications can have 100,000 to1 million cells. Obstructions, such as walls, balconies, and stairs, should be considered.

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Boundary conditions, including smoke exhaust and makeup air, should be defined.

Atrium smokecontrol designs based on the conventional algebraicequation approachtend to be conservative and have exhaust flow rates that are somewhat high. Conversely,CFD modeling can be the basis for exceptions for the requirements to smokelayer depth,the 200fpm limitation on makeup air, and plugholing.

CFD modeling provides a high degree of confidence that a tenable environment will bepreserved. CFD modeling's strength is that it can simulate fireinduced smoke flows,which algebraic equations cannot.

There are some good generalpurpose CFD models, but the NIST's Fire Dynamics

Simulator (FDS)9 is for fire applications. While the annual fee to use a commercial CFDmodel can be tens of thousands of dollars, the FDS and its associated documents can bedownloaded for free at www.fire.nist.gov/fds/downloads.html. For a nonmathematicaldiscussion of CFD modeling, see the previously mentioned HPAC Engineering article. Thepreviously mentioned book published by the ICC includes a detailed introduction to CFD

modeling.3

REFERENCES

1. Klote, J.H. (2006, June). CFD: A new way to design atrium smoke control. HPACEngineering, pp. 1927.

2. Klote, J.H., & Evans, D.H. (2007). A guide to smoke control in the 2006 IBC.Country Club Hills, IL: International Code Council.

3. International Code Council. (2006). 2006 International Building Code. CountryClub Hills, IL: International Code Council.

4. NFPA. (2005). NFPA 92B: Standard for smoke management systems in malls,atria, and large areas. Quincy, MA: National Fire Protection Association.

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Known worldwide as a smokecontrol expert, John H. Klote, DSc, PE, is a consultingengineer based in Leesburg, Va. Formerly, he conducted fire research for the NationalInstitute of Standards and Technology.

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5. Klote, J.H., & Milke, J.A. (2002). Principles of smoke management. Atlanta:American Society of Heating, Refrigerating and AirConditioning Engineers.

6. Peacock, R.D., Jones, W.W., Reneke, P.A., & Forney, G.P. (2005). NIST specialpublication 1041: CFAST — Consolidated model of fire growth and smoketransport (version 6): User's guide. Gaithersburg, MD: National Institute ofStandards and Technology.

7. Klote, J.H. (2002, September). Design fires: What you need to know. HPACEngineering, pp. 4351.

8. Bukowski, R.W. (2006, June). Determining design fires for designlevel andextreme events. Paper presented at the 6th International Conference onPerformanceBased Codes and Fire Safety Design Methods, Tokyo, Japan.

9. McGrattan, K. (2004). NIST special publication 1018: Fire dynamics simulator(version 4): Technical reference guide. Gaithersburg, MD: National Institute ofStandards and Technology.

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