Atrium Smoke Control

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3 6 A S H R A E J o u r n a l a s h r a e . o r g J u n e 2 0 1 2

Smoke is recognized as the major killer in building res. Smoke controlin large-volume spaces is based on a long history of experienceand research going back to the 1881 Ring Theater re in Vienna that

killed 449 people. After that re, the Austrian Society of Engineers

conducted reduced-scale re tests that showed how roof vents over

the stage would have protected the audience from smoke. Thirty years

later, such smoke vents worked as intended in the Palace Theater re

in Edinburgh, Scotland.

In addition to such natural smokeventing, today there are a number ofdesign approaches to deal with smokein large-volume spaces. A large-vol-ume space is a space that is at least twostories high such as an atrium, a sportsarena, or an airplane hangar. In this ar-

ticle the term atrium is used in a ge-

neric sense to mean any large-volumespace.

This article is adapted from part ofChapter 15 of the new ASHRAE publi-cation, Handbook of Smoke Control En- gineering. 1 In this article, when a chap-ter number is mentioned, it is a chapter

in this new handbook.

Design ScenariosA design scenario is the outline of

events and conditions that are criticalto determining the outcome of alter-nate situations or designs. In additionto the re location and heat release rate(HRR), a design scenario may includemany other conditions such as the mate-rials being burned, the weather, the sta-tus of the HVAC system, and doors thatare opened and closed. A design analy-sis should include a number of design

scenarios to provide a level of assurancethat the smoke control system will oper-ate as intended.

Design res need to be realisticallyselected as discussed in Chapter 5. Ingeneral, a design analysis needs to in-clude design res located in the atriumand in communicating spaces. A com-municating space is one that has an open

About the AuthorJohn H. Klote, Ph.D., P.E., is is an expert in smoke

control technology in Leesburg, Va.

By John H. Klote, Ph.D., P.E., Fellow/Life Member ASHRAE

Basics ofAtrium Smoke Control

This article was published in ASHRAE Journal, June 2012. Copyright 2012 ASHRAE. Posted at www.ashrae.org. This article may not be copiedand/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.


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pathway to an atrium so that smoke from a re either in theatrium or the communicating space can move from one to theother without restriction. Figure 1A illustrates these spaces.

A separated space is one that is isolated from the atri-um by smoke barriers ( Figur e 1A ). For this handbook, asmoke barrier is a continuous membrane, either verticalor horizontal, that is designed and constructed to restrictthe movement of smoke in conjunction with a smoke con-trol system. Smoke movement at these smoke barriers canbe controlled by pressurization or by compartmentationalone.

Figure 1B shows a re in the atrium with smoke rising

above the re to form a smoke layer under the ceiling of theatrium. The most widely used approach to atrium smoke con-trol is smoke exhaust, but other approaches can also be used.Regardless of the smoke control approach, there is a distancearound the re where occupants cannot go because of the in-tensity of the re. To determine the minimum distance that aperson can be from a re for a few minutes without unbearablepain see Chapter 6.

For a scenario with the re in the atrium, the design redoes not normally take into account any benet of sprinklers.In spaces with high ceilings, the temperature of the smokeplume can drop so much that sprinklers may not activate or

activation may be so delayed that the spray may evaporate be-

fore it reaches the re. Information about the interaction ofsprinklers with the smoke layer is in the Handbook of SmokeControl Engineeri ng . For information about design res, seeChapter 5.

Smoke from a fire in a communicating space can flowinto the atrium and form a balcony spill plume as shownin Fi gure 1C . This figure shows smoke blocking of partsof balconies above the fire. It is beyond the capability ofsmoke control technology to prevent such smoke block-ing, but the balcony is not blocked away from the balconyspill plume ( Figure 2 ).The comments earlier regarding theminimum distance that a person can be from a fire also

apply here. For a scenario with the fire in a communicat-

Figure 1: Fire locations for atrium smoke control analysis.

Smoke exhaust through a plenum with a suspendedceiling not recommended. The pressures produced by theexhaust ow through a plenum with a suspended ceilingcan be high enough to lift ceiling tiles out of their frames.

Such relocation of ceiling tiles could have anadverse impact on the performance of thesmoke exhaust system. The effort involvedwith periodic testing of such a smoke exhaust

system can be signi cantly increased due to theneed for repair of suspended ceilings after testing.

Atrium(Large-Volume Space)

SeparatedSpace

SeparatedSpace

CommunicatingSpace

CommunicatingSpace

Smoke ExhaustIs Not Shown

Smoke Layer

Plume

A B

C DSmoke Layer Smoke Layer

Window PlumeSmoke ExhaustIs Not Shown

BalconySpill Plume

This Room is Fully Involved in Fire

Spaces Related to Atriums Fire in the Atrium

Fire in a Communicating Space Fully Developed Fire in a Room Open to the Atrium

Smoke ExhaustIs Not Shown


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ing space, the growth of the design fire generally stopsupon sprinkler activation.

Figure 1D shows a fully developed re and smoke form-ing a window plume. A fully developed re would not happenwhen a sprinkler system is operating properly. Because most

new commercial buildings in the United States are fully sprin-klered, design re scenarios that include a fully developed reare uncommon in the United States. In countries where fullysprinklered buildings are uncommon, design re scenariosmay include fully developed res. It is also possible that somebuilding owners or building managers may want the very highlevel of protection associated with a smoke control systemthat can handle even a fully developed re.

Design ApproachesDesign approaches that have been used for atrium smoke

control are (1) natural smoke lling, (2) steady mechanicalsmoke exhaust, (3) unsteady mechanical smoke exhaust,(4) steady natural smoke venting, and (5) unsteady naturalsmoke venting. These approaches are discussed later. Air-ow can also be used to control smoke ow in conjunctionwith these approaches, but care must be exercised becauseairow has the potential to provide combustion air to there.

Many design approaches are intended to prevent occupantsfrom coming into contact with smoke. The idea is to controlsmoke so that it descends only to a predetermined height dur-ing the operation of the smoke control system. In many lo-cations, there are code requirements for the predeterminedheight. This height is often in the range from 6 to 10 ft (1.83 to3.05 m) above the highest walking surface that forms a portionof a required egress in the atrium.

Other design approaches are intended to maintain a ten-able environment when people come into contact with smoke.When the products of combustion are sufciently diluted, theresulting diluted smoke can be tenable, and tenability analysesroutinely deal with reduced visibility and exposure to toxicgases, heat and thermal radiation. See Chapter 6 for more in-formation about tenability.

The following discussion of design approaches addresssystems that are intended to prevent occupant contact withsmoke, but these systems can be modied to ones that address

tenability.

Natural Smoke Filling This approach consists of allowing smoke to ll the atrium

without any smoke exhaust or other smoke removal. For somespaces the smoke lling time with the design re is more thansufcient for evacuation. The smoke lling time is the timefrom ignition until the smoke descends to the predeterminedheight. Applications that are appropriate for natural smokelling are not common, because there needs to a very largespace above the highest occupied level of the atrium. Anyof the methods of analysis discussed below can be used for

this system. It is essential that calculations of evacuation time

include the times needed for recognition, validation and pre-movement as discussed in Chapter 4.

Steady Mechanical Smoke Exhaust This is the most commonly used approach in North Amer-

ica. This system consists of mechanical smoke exhaust sizedto keep the bottom of the smoke layer at the predeterminedheight for the design re.

Unsteady Mechanical Smoke Exhaust This approach also uses mechanical smoke exhaust, but the

ow rate of the exhaust is less than steady mechanical exhaustsuch that the exhaust only slows the rate of smoke layer de-scent for a time that allows occupants to safely egress from thespace. This method needs to maintain at least the predeterminedheight mentioned previously for the time it takes the occupantsto safely evacuate. The considerations about calculation evacua-tion time for natural smoke lling systems also apply here.

Steady Natural VentingAs previously mentioned, this kind of venting has a history

going back to the Ring Theater re of 1881. This approach isnot common in the United States, but it is common in Europe,Australia, New Zealand and Japan. Rather than exhaust fans,this approach uses non-powered smoke vents at or near the topof the atrium. Often this kind of venting is called gravity venting because the smoke is vented due to buoyancy.

The ow rate of the smoke through the vents needs to be such

that the bottom of the smoke layer is kept at the predeterminedheight for an indenite time. The previous comments regard-ing the predetermined height also apply here. An equation forthe steady mass ow rate through a natural vent is discussedlater. It is recommended that steady natural venting systems beanalyzed with the aid of a computational uid dynamics (CFD)model, discussed in Chapter 20.

Unsteady Natural Venting This approach is like steady natural venting except the

smoke venting rate is such that it only slows the rate of smokelayer descent for a time that allows occupants to safely egress

from the space. This method needs to maintain at least the

Figure 2: Front view of balcony spill plume.

Smoke Layer

BalconySpill Plume

Smoke exhaustis not shown.


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predetermined height mentioned previously for the time ittakes the occupants to safely evacuate. It also is recommendedthat unsteady natural venting systems be analyzed with the aidof a CFD model. The considerations about calculation evacu-ation time for natural smoke lling systems also apply here.

Methods of Analysis The methods that can be used for analysis of atrium smoke

control systems are algebraic equations, zone re modeling,CFD modeling and scale modeling.

Algebraic EquationsAtrium smoke control makes use of many algebraic equa-

tions. Some of these are based on the fundamental principlesof engineering, and others are empirical correlations based onexperimental data. Equations for smoke lling, natural vent-ing and the airow velocity to prevent smoke backow arediscussed later in the chapter.

Chapter 16 addresses the algebraic equations for steady me-chanical smoke exhaust, and these equations are based on thezone re model concepts discussed in the next section. In thefollowing section on zone re modeling, the discussion aboutsmoke exposure in the transition zone also applies to systemsdesigned with the algebraic equations of Chapter 16.

When another method of analysis is used, algebraic equationsare often used to determine starting points for the analysis.

Zone Fire ModelingIn an atrium re, smoke ows upward in a plume that en-

trains air as it rises. When the plume reaches the ceiling, itturns and becomes a ceiling jet that ows under the ceiling(Figure 3A ). Figure 3B shows an idealized zone model repre-sentation of an atrium re.

Zone re models are simple models that consider a re com-partment to be divided into two zones: (1) a smoke layer and (2)a lower layer that is free or nearly free of combustion products.

The smoke layer can change in size based on the mass owing

into and out of it. In a real re, the temperature and concentra-tion of contaminants vary throughout the smoke layer with thehighest values tending to be near top of the smoke layer. In realres, there is also a gradual transition zone between the smokelayer and the lower layer as shown in Figure 3A.

In a zone re model, the smoke layer has a uniform tem-perature and uniform concentrations. This means that the tem-perature at any place in the smoke layer is the same as every-where else in the smoke layer, and the same can be said aboutthe concentration of each contaminant.

Zone re models do not simulate the transition zone, but thebottom of the smoke layer is simulated as a horizontal planecalled the smoke layer interface as shown in Figure 3B . Thezone model considers the air a fraction of an inch (or centime-ter) below the smoke layer interface to be as free of smoke asthe rest of the lower layer. Occupants in the lower layer nearthe smoke layer interface will actually be in the transition zoneexposed to some smoke. Unfortunately, neither zone re mod-els nor the algebraic equations of Chapter 16 can be used toevaluate this smoke exposure. I t is believed that in many situ-ations, conditions in the transition zone may be tenable. CFDmodeling can be used to evaluate tenability at this location.

Zone re models do not simulate the time it takes for theplume to reach the ceiling, which is small in a normal size

room but larger in an atrium. Empirical equations for this lagtime are discussed later in this Chapter. Zone re models donot simulate plume ow, but they use empirical equations tocalculate plume temperature and the mass ow.

Even with the previous limitations, zone re models haveproven to be very useful tools for many applications, but theymust be used with care. Chapter 18 has more detailed infor-mation about zone re models.

CFD ModelingCFD consists of dividing a space of interest such as an atri-

um into a large number of cells, and using a computer pro-

gram to solve the governing equations for each cell. CFD is

Figure 3: Sketch of an idealized zone model representation of an atrium re.

A BUniform Smoke Layer

PlumePlume

Ceiling Jet

Transition Zone

Fire Fire

Smoke LayerInterface

Exhaust

Smoke ExhaustNot Shown

Atrium Fire Zone Model Idealization of an Atrium Fire


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capable of highly realistic simulations. The plume, ceiling jet,smoke layer and the transition zone are all simulated by theCFD model. CFD models are capable of simulating plughol-ing, and they can simulate any adverse effects of makeup airvelocity on plume formation. Plugholing is discussed later.

CFD modeling requires a level of knowledge and experi-ence beyond that of zone re modeling, and CFD simulationstypically require hours and sometimes days of computer time.For more information about this kind of modeling, see Chap-ter 20.

Scale ModelingScale modeling is capable of highly realistic simulations.

This kind of modeling consists of conducting re tests in asmall model of the atrium or other facility, and converting thedata from those tests to the full scale facility. Scale modelingis addressed in Chapter 21.

Atrium TemperatureFor systems that rely on mechanical smoke exhaust, the

temperature of the air below the smoke layer quickly ap-proaches the outdoor temperature. This is because of the verylarge amounts of makeup air that enter the atrium. For designanalysis of systems using mechanical smoke exhaust, the out-side design temperature should be used for the ambient tem-perature of the atrium.

As the gas temperatures increase, the density of the gasdecreases, and the volumetric ow rate needed to maintaina constant mass ow increases. Atrium exhaust fans need tobe sized for the maximum volumetric ow needed to controlsmoke for the design conditions. This maximum volumetricow will happen when the summer outside design tempera-ture is used for the ambient temperature of the atrium. For thisreason, smoke exhaust fans need to be sized with an ambienttemperature of the atrium equal to the summer outside designtemperature.

Minimum Smoke Layer Depth The minimum smoke layer depth needs to be 20% of the

oor-to-ceiling height except when an engineering analysis us-ing full scale data, scale modeling, or CFD modeling indicatesotherwise. The formation of the minimum smoke layer depth

is shown in Figure 4. When a smoke plume reaches the ceil-ing, the smoke ows away from the point of impact in a radialdirection forming a ceiling jet. When the ceiling jet reaches awall the smoke ow turns down and ows back under the ceil-ing jet. The ceiling jet has a depth of about 10% of the oor-to-ceiling height, and the smoke ow under the ceiling jet is alsoabout 10% of the oor-to-ceiling height. This means that thesmoke layer depth is about 20% of the oor-to-ceiling height.

Makeup AirMakeup air is outdoor air either supplied by openings to

the outside or by mechanical fans. For systems that have fan

powered smoke exhaust, makeup air needs to be provided by

mechanical fans or by openings to the outside. Makeup airhas to be provided so that the exhaust fans can remove thedesign quantities of smoke and that the door opening forcerequirements are not exceeded. Makeup air must be suppliedfar enough below the smoke layer interface so that it does notdisrupt the smoke layer.

When providing makeup air through openings to the out-side, some air also ows by way of leakage paths. The largeopenings (such as vents, doors and windows) need to openautomatically on system activation. The leakage paths con-sist of construction cracks, gaps around closed doors, gapsaround closed windows, and other similar small paths. Thelarge openings should be sized to provide about 85% to 95%of the makeup air with the rest coming through the leakagepaths.

When makeup air is provided by mechanical fans, themakeup air should be less than the mass ow rate of the me-chanical smoke exhaust. It is recommended that makeup airfor fan powered smoke exhaust systems be designed at 85% to95% of the exhaust. The idea is that the remaining air (5% to15%) will enter the large-volume space through leakage pathspreventing positive pressurization of the atrium.

The makeup air must not exceed 200 fpm (1.02 m/s) wherethe makeup air could come into contact with the plume unless

a higher makeup air velocity is supported by an engineeringanalysis. The primary reason for this 200 fpm (1.02 m/s) limitis to prevent signicant deection of the plume and disrup-tion of the smoke layer. 2 Deection of the plume results inincreased air entrainment that can cause smoke control sys-tem failure. A secondary reason for this velocity restriction isthat it reduces the potential for re growth and spread due toairow.

The 200 fpm (1.02 m/s) limitation is not relevant in commu-nicating spaces one story high that are sprinklered. At theselocations, successfully sprinklered res do not form plumesas they would in the atrium, and successfully sprinklered res

limit re growth. However, air introduced in a communicat-

Figure 4: Minimum smoke layer depth.

Plume

Minimum SmokeLayer Depth is 20% ofFloor-to-Ceiling Height

Ceiling Jet

Flow UnderCeiling Jet


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ing space needs to slow down to meet the 200 fpm (1.02 m/s)limitation when it reaches the atrium. For example, consid-er makeup air supplied to the communicating space on theground oor of Figure 1A . The makeup air enters the commu-nicating space at a velocity above the limitation. A jet of sup-ply air forms as it would from an HVAC diffuser. The velocityof this jet needs to drop to 200 fpm (1.02 m/s) or less whenit reaches the atrium space. The design calculations need toinclude velocity calculations of this makeup air jet at the pointwhere it reaches the atrium.

When makeup air is provided by openings to the outside, thedesign analysis of the system needs to address wind effects asdiscussed below.

WindAtrium smoke control systems need to be designed to mini-

mize the potential for wind to result in: (1) velocities greaterthan 200 fpm (1.02 m/s) where the makeup air could comeinto contact with the plume, and (2) smoke feedback from thesmoke exhaust (or smoke vents) into the makeup air.

When makeup air openings face in different directions,wind forces can result in velocities exceeding 200 fpm (1.02m/s) inside the atrium. The wind can blow into openings

facing one direction and out the other openings. A simpleapproach for minimizing wind effects inside an atrium is tohave all the makeup air openings face in the same direction.Another simple approach is using mechanical fans for bothsmoke exhaust and makeup air such that the impact of thewind is minimized. When such simple approaches are not fea-sible, a detailed analysis is needed that takes into account theprevailing wind directions. Such an analysis can be done witha network model or a CFD model.

Smoke can be carried by the wind from the smoke ex-haust or from smoke vents to makeup air openings or inlets.

The simple approach to minimize the potential for this is to

locate the smoke exhaust (or vents) and makeup air open-

ings (or inlets): (1) far away from each other, and (2) suchthat the prevailing wind directions carry the smoke awayfrom makeup air openings (or inlets). When this simple ap-proach is not feasible, CFD analysis or wind tunnel analysisis needed to evaluate the potential for smoke feedback intothe atrium.

PlugholingPlugholing is a phenomenon where air from below the

smoke layer is pulled through the smoke layer into the smokeexhaust. Plugholing can cause system failure, but it can beeasily prevented. Plugholing reduces the exhaust from thesmoke layer, which tends to lower the smoke layer and exposeoccupants to smoke.

The following discussion of plugholing applies to steadymechanical smoke exhaust systems. Figure 5A shows a smokelayer that is at the intended design height, but the layer is stilldescending due to plugholing. As the smoke layer depth in-creases, the buoyancy forces of the smoke layer increase, andthe amount of plugholing decreases. Eventually, the smokelayer becomes deep enough that a state of equilibrium isachieved with a constant smoke layer height as shown in Fig- ure 5B . Plugholing has resulted in a smoke layer below what

was intended. The important forces for plugholing are the kinetic forcesof the smoke exhaust and the buoyancy forces of the smokelayer. When kinetic forces dominate, there will be plughol-ing. When the buoyancy forces dominate, there will be noplugholing. The kinetic forces depend on the ow rate of thesmoke exhaust, and the buoyancy forces depend on the tem-perature and depth of the smoke layer. When these forcesare balanced at an exhaust inlet, the ow at that inlet is themaximum that can be achieved without plugholing. Plughol-ing can be prevented by using a number of smoke exhaustinlets such that the ow rate at each inlet is at or below this

maximum value.

Figure 5: Plugholing causing the smoke layer to fall below the intended height.

Plugholing of Air IntoSmoke Exhaust

Exhaust Fan A B

Exhaust Fan

It Cannot Be Seen, ButThe Fan is Still Pulling AirInto the Smoke Exhaust

Smoke Layer Height Falling Due to Plugholing Smoke Below the Intended Height Due to Plugholing


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There is an empirical equation in Chapter 16 for the maxi-mum volumetric ow rate that can happen at an exhaust inletwithout plugholing. This equation and the earlier discussionalso apply to systems that use natural venting.

Scale modeling and CFD modeling can simulate plughol-

ing without the use for the empirical maximum flow rateequation of Chapter 16. This empirical equation can beconservative, and it is possible that an analysis using scale

age temperature of the plume is lessthan that of the hot air layer, a stratiedsmoke layer can form under the hot airlayer preventing smoke from reachingceiling-mounted smoke detectors. Ifsmoke stratication can occur, projectedbeam smoke detectors should be used,and three arrangements of these detec-tors are discussed in the handbook.

Control and OperationAtrium smoke control systems must

be activated automatically to quicklyprovide smoke protection for the occu-pants. For atria where smoke stratica-tion can happen, projected beam smokedetectors should be used as mentionedpreviously. Some other methods ofsystem activation are ceiling mountedsmoke detectors, heat detectors andsprinkler water ow. The smoke controlsystem needs to reach full operation be-fore conditions in the atrium reach thedesign conditions. Determination of thetime for the system to become opera-tional needs to take into account (1) thetime for detection of the re and (2) theHVAC system activation time includingshut-down and start-up of air-handlingequipment, opening and closing damp-ers, and opening and closing naturalventilation devices.

A means of manually starting andstopping the smoke control system

needs to be provided at a location ac-ceptable to the re department. Thesemanual controls need to be able to over-ride the automatic controls. For generalinformation about controls of smokecontrol systems see Chapter 8.

References1. Klote, J .H. 2012. Handbook of Smoke

Control Engineeri ng . Atlanta: ASHRAE.2. Hadjisophocleous, G., J. Zhou. 2008.

Evaluation of atrium smoke exhaust make-upair velocity. ASHRAE Transactions , Part 1.

modeling or CFD modeling would result in a lower num-ber of exhaust inlets than an analysis using the empiricalequation.

Stratifcation

A hot layer of air can form under the ceiling of an atriumdue to solar radiation on the atrium roof. The temperature ofsuch a layer can be 120F (50C) or more. When the aver-


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