12/14/2015 Practical Applications of SmokeControl Systems

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Practical Applications of SmokeControl SystemsFour basic design methods to help prevent smoke migration and maintain atenable environment for occupants

By ALLYN J. VAUGHN, PE, FSPE, LEED AP, and BRAD R. GEINZER, PE, LEED AP; jbaConsulting Engineers; Las Vegas, Nev. | HPAC Engineering

Design criteria for smokecontrol systems have changed over the years. Instead ofventilating an affected area, smokecontrol systems now must prevent the spread ofsmoke to nonaffected areas and/or provide a tenable environment in the area ofincident. How effective are current smokecontrol systems? Can buildingcoderequirements for smokecontrol systems be met reasonably? How can applications not

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described in building codes be addressed?

Smokecontrolsystem requirements have been in place for years, and smokecontrolsystem design has had to adapt to buildingcoderequirement changes. This article willaddress some practical design applications of these systems as well as how buildingcoderequirements can be applied. The article also will address ways to adapt a design to meetbuildingcode intent.

History of SmokeControl Requirements

Smokecontrol systems have been utilized in highrise buildings since the mid1970s. Inearly building codes, mechanical systems were required to provide a specific exhaust rateto ventilate a space; building codes later required specific airchange rates, typically sixair changes per hour (ACH).

The 1994 edition of the Uniform Building Code (UBC) required smokecontrol systems tobe designed using a performancebased approach according to one of four basic designmethods: pressurization, passive, airflow, and exhaust. These methods were tied toNational Fire Protection Association (NFPA) standards for smokecontrol systems. Designcriteria for the pressurization, passive, and airflow methods included maintaining smokein the zone of origin to prevent its spread throughout a building; the exhaust method'scriteria included maintaining the smoke layer above the highest walking surface in aneffort to maintain a tenable environment.

Since the adoption of the 1994 UBC, other model codes have incorporated these designrequirements. The International Building Code (IBC) and International Fire Code (IFC)include language similar to that in the 1994 UBC. Refinements have been made to theUBC to stay current with NFPA standards, but the criteria have remained much the sameas when the four design methods were introduced in 1994.

The IBC requires smokecontrol systems for atria, threestory covered malls, andunderground buildings, but offers them as an option in lieu of smoke venting. Although

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the IBC does not mandate smokecontrol systems for highrise buildings, certainjurisdictions have amended the code to require them, similar to what was required byprevious editions of the UBC. Since performancebased design criteria were introduced,many buildings have been outfitted with smokecontrol systems using this approach.

Pressurization Method

Smokecontrol systems using the pressurization method maintain a pressure difference

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between the zone in alarm and adjacent zones. To maintain a minimum pressuredifference of 0.05 in. w.c., a mechanical system typically is configured with 100percentexhaust, with no supply air introduced into the zone. Zone barriers are required to beintact and closed; doors and dampers must close and seal off the zone. This designmethod can be effective in preventing the spread of smoke to other areas of a building(Figure 1).

The pressurization method is most widely used for areas that have the ability to be sealed,such as individual floors and backofhouse spaces. However, barriers need to bemaintained to ensure the zone envelope is intact. Otherwise, the system cannot maintainthe required pressure difference. Maintaining smokezone barriers over long periods oftime can be difficult, in part because alterations to spaces adjacent to the barriers cancause leaks. Alterations also can result in work on smokebarrier walls by personnelunfamiliar with the walls' requirements. Without proper documentation of the location ofsmokebarrier walls, alterations can render these systems useless. A smokecontrol systemdesigned with the pressurization method can be effective if adjusted properly andmaintained.

Sizing exhaust fans for pressurization systems can be challenging. The exhaust requiredto achieve a desired pressure difference depends on construction leakage. Building codesprovide leakage rates that can be used in choosing an exhaust fan. However, a designmay need to accommodate construction that is tighter than building codes allow, whichcould result in overpressurization. Selecting fans to operate effectively under variousranges of operation can help minimize field changes or equipment replacement duringinitial commissioning.

To overcome construction issues and account for future renovations, variable exhaust from the space can

be utilized to build in some form of airflow adjustment. Many designers use variablefrequency drives

(VFDs) to control fan speed for HVAC design. Smokecontrol systems can be balanced by adjusting their

VFDs. If pressures need to be adjusted during initial setup and recommissioning, VFD settings can be

used to achieve the proper exhaust, eliminating the need to change motors or other equipment to adjust

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pressure differences. Using VFDs does pose some operational restrictions because they are part of the

smokecontrol systems and should not be adjusted by people unfamiliar with the systems' requirements.

A practical application of HVAC technology and the design of smokecontrol systems, thepressurization design method employs equipment to condition a space andsimultaneously serve as a smokecontrol system. Once a system is adjusted properly, it cancontinue to provide the protection intended by building codes. However, knowledge ofsystemdesign requirements, including the location of smokebarrier walls, is needed tomaintain the effectiveness of a smokecontrol system.

Modified Airflow/Exhaust Method for Parking Garages

Parking garages requiring smoke control present unique challenges. It generally has beenaccepted that a parking garage meeting buildingcode requirements for an “open garage”does not need mechanical smoke control because the number of openings along thebuilding perimeter do not allow smoke to collect within the garage. Mechanicallyventilated garages pose design challenges when smokecontrol systems are required, butdesign methods in the IBC do not readily lend themselves to practical solutions. Designersoften use hybrid designs to provide effective smoke control in parking garages.

Enclosed garages typically are required to have mechanical ventilation for vehicleexhaust(carbonmonoxide [CO]) removal. These systems can be effectively used for smoke controlif certain design parameters are established. Until recently, the exhaust method was notpossible in spaces in which a 10ft smoke layer needed to be maintained because of thelimited clearance height of a typical garage. The 2006 IBC lowered the smokelayerheight to 6 ft, making this design method slightly more practical, but the limited smokelayer depth leaves exhaust inlets susceptible to plugholing. Also, CO exhaust inlets are notlocated solely at the ceiling, meaning an inlet could be located below the smoke layer ifthe exhaust method is used.

The pressurization method also can be used to fulfill buildingcode performancerequirements for a garage. Because the pressurization method requires a sealed area, the

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garage would require rolldown doors for any openings, such as drive lanes. However,this can present a scenario in which doors close off traffic lanes, restricting emergencyresponse vehicles. While the smokecontrol system will perform as intended by code, it isnot an effective solution because it can restrict emergencyresponse access.

A hybrid design combining the exhaust and airflow methods also can be used for smokecontrol in enclosed parking garages. This concept provides an exhaust rate for the garagewhile maintaining velocities of less than 200 fpm across an opening, as required by theairflow method. The hybrid design recognizes that garages cannot effectively have asmokelayer height of even 6 ft above the floor because of limited clearances and theamount of ductwork routing required to extract smoke at the ceiling level. While thisapproach is not one of the prescribed buildingcode design methods, it can be effective inexhausting smoke from an enclosed garage using equipment already in place.Additionally, the use of VFDs can allow the adjustment of fans in the field to achieve thedesired airflow.

Exhaust Method for Large Spaces

Using the exhaust method for very large areas, such as an assembly space or casino, is apractical choice for smoke control. Relatively high ceilings and common returnairplenums make exhausting large quantities of air straightforward. Makeup air can comefrom adjoining areas. Draft curtains can separate a large space from adjoining retailpromenades, meeting rooms, restaurants, etc.

However, various issues need to be addressed when the exhaust method is beingconsidered for a large space. Smokezone areas may become too large, requiring multiplesmokeexhaust fans. Exhaust fans should be within 200 ft of a fire for smoke to beextracted. For large open areas, multiple fans are necessary to limit the travel of smokefrom the fire to the exhaust inlet. When multiple fans are used, the exhaust quantity isincreased, which impacts the ability to provide adequate makeup air. A single extractionpoint can provide 50,000 cfm of exhaust easily when makeup air is equal to the exhaustrate. However, if a large zone requires four extraction points, the increased total exhaust

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and makeupair rate of 200,000 cfm is more difficult to accommodate.

To manage large areas using the exhaust method, smaller smoke zones are needed. Utilizing smaller

zones reduces the amount of exhaust and helps manage makeup air. However, creating smaller zones

within a larger area requires additional smoke dampers, as well as draft curtains below ceilings. These

items will increase construction costs and could have negative effects on the aesthetics of a space. Draft

curtains are difficult to create in highceiling areas.

A more practical solution is to modify the exhaust approach, utilizing the entire area as asingle smoke zone and creating smaller “activation zones” within the common smoke zone(Figure 2). This works well when a common returnair plenum is used. An overall smokezone surrounded by appropriate boundary walls contains multiple activation zones thatare served individually by smokeexhaust fans chosen based on the designfiresize/smokelayer height and corresponding smokeexhaust rate in the activation zone.When a smokecontrol system is activated, all of the smokeexhaust fans within theactivation zone are energized. This provides the required exhaust rate for the activationzone and obtains makeup air from adjoining zones, keeping velocities across the openingsfor the overall smoke zone within buildingcoderequired limits.

Smoke migration to other activation zones is not considered an issue because the smoke is

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within the same smoke zone. However, because exhaust inlets are located within theactivation zone, smoke has a tendency to be drawn toward the exhaust fans in operation.This limits the potential for smoke damage remote from the area of incidence whilekeeping initial construction expenses down. Draft curtains are not required betweenactivation zones because they are treated as subzones to the overall smoke zone. This canbe an effective way to provide required smoke control while minimizing the impact on theconstruction costs and aesthetics of a space.

Exhaust Method for Tall Atria

Since the 1980s, atria have been required to have smokecontrol systems. Design criteriain early building codes were based on the airchange method, in which the volume of thespace dictated the required exhaustair quality. (A larger atria may require only fourACH, while a smaller atria may require six.) Morerecent building codes haverecommended the exhaust design method, which maintains the smoke layer at a certainheight.

NFPA exhaustmethod standards include formulas that depend on two basic variables:the height of the smoke layer above the fire and the fire size. The height of the smokelayer is dictated by building design, and the fire size is dictated by fuel loads andsuppressionsystem design. While the density of the air at the smoke layer has someimpact, smokelayer height and fire size are more important issues for the equations.

Building design dictates smokelayer height because a smoke layer must be a minimum of6 ft above the highest walking surface. Within tall atria, this can be very high. Unless thespace is redesigned, this variable cannot be changed. Fire size can be adapted to thespace by limiting fuel or controlling the fire in its early inception stage with quickresponse sprinklers or detection systems operating deluge systems.

The equations found in NFPA standards can produce large exhaust requirements for tall atria. Building

codes also require an equal amount of makeup air produced by mechanical or natural means or a

combination of the two. Building codes limit makeup air to no more than 200 fpm of airflow toward the

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fire. While large quantities of exhaust air can be difficult to manage, tackling large quantities of makeup

air is even more difficult. Buildings often have insufficient space to allow vents and louvers to introduce

makeup air under the prescribed maximum velocity.

Makeup air must be introduced below the smoke layer. In tall spaces, multiple levels maybe available to introduce makeup air. In smaller areas, wall space may be insufficient. Inthese cases, operable panels or doors in an exterior wall can be used to introduce makeupair by natural means. Makeup air also can be introduced by adjacent zones whenmultiple zones are present (Figure 3).

Makeup air can be difficult to provide in atria because of the physical constraints of the

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building design. Tall atria with walking surfaces located near the top can require exhaustrates exceeding 300,000 to 400,000 cfm. When makeup air is equal to the exhaust rateand limited to no more than 200 fpm, smokecontrol systems can require up to 1,500 to2,000 sq ft of free vent area. If grilles are used, the free area often is much smaller thanthe actual size of the grille. If panels or doors are used, a large quantity may be needed tomeet the freearea requirements.

There are ways to limit the impact of smoke control in tall atria. The best method is toreduce the height of the highest walking surface in the atrium or provide a way toseparate the walking surface from the atrium under a fire condition, such as with fireshutters. If the building design cannot accommodate either option, the fire size needs tobe limited with sprinkler control and smaller fuel loads. Another approach is to use acomputational flow analysis to determine actual exhaust quantities based on performanceobjectives. These types of analyses can help reduce NFPAstandard exhaust rates forexhaustmethod systems or allow higher velocities in makeupairflow rates.

If makeup air can be managed, benefits of the exhaust design method include the abilityto open adjacent zones to one another. Exhaustmethod smokecontrol systems also areeasier to commission because they do not rely on the balancing of pressure differences.

Conclusion

Smokecontrol systems have been required by building codes for decades. For much ofthis time, design criteria were straightforward. For the last 16 years, smokecontrol designhas focused on performance criteria based on the physics of fire. Applying the designmethods found in current codes has posed some unique challenges. Often, the prescribedapproach does not fit within a building's constraints. In these situations, designers arerequired to adapt or modify the design so the systems can prevent smoke migrationeffectively. During the last 16 years, HVAC professionals have learned how to meetbuildingcode requirements effectively and adapt a design to meet a building code'sintent.

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Current smokecontrol systems can be very effective in meeting buildingcode designcriteria. Using pressurization or passive design methods, smokecontrol systems can beeffective in preventing the spread of smoke throughout a building. The exhaust methodcan help smokecontrol systems be effective in creating a tenable environment. How thesystems are adapted to a respective building depends on whether a practical approachhas been used. The more elaborate the system, the more chances for failure. Withpractical application and design, code requirements — both prescribed and intended —can be achieved in today's buildings.

Vice president of growth markets for jba Consulting Engineers, Allyn J. Vaughn, PE,FSPE, LEED AP, has more than 28 years of experience in fireprotection and smokecontrolsystem design and commissioning. He has been responsible for thirdpartytesting of smokecontrol systems in Las Vegas for more than 13 years. Vice president ofengineering for jba Consulting Engineers, Brad R. Geinzer, PE, LEED AP, has morethan 20 years of experience in smokecontrolsystem design and commissioning. He wasinvolved in the design and commissioning of the first Las Vegas building to use theperformancebased criteria of the 1994 edition of the Uniform Building Code.

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