Basics of Atrium Smoke Control

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1 This article was published in ASHRAE Journal, June Copyright 2012 ASHRAE. Posted at This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www. ashrae.org. Basics of Atrium Smoke Control By John H. Klote, Ph.D., P.E., Fellow/Life Member ASHRAE Smoke is recognized as the major killer in building fires. Smoke control in large-volume spaces is based on a long history of experience and research going back to the 1881 Ring Theater fire in Vienna that killed 449 people. After that fire, the Austrian Society of Engineers conducted reduced-scale fire 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 fire in Edinburgh, Scotland. In addition to such natural smoke venting, today there are a number of design approaches to deal with smoke in large-volume spaces. A large-volume space is a space that is at least two stories high such as an atrium, a sports arena, or an airplane hangar. In this article the term atrium is used in a generic sense to mean any large-volume space. This article is adapted from part of Chapter 15 of the new ASHRAE publication, Handbook of Smoke Control Engineering. 1 In this article, when a chapter number is mentioned, it is a chapter in this new handbook. Design Scenarios A design scenario is the outline of events and conditions that are critical to determining the outcome of alternate situations or designs. In addition to the fire location and heat release rate (HRR), a design scenario may include many other conditions such as the materials being burned, the weather, the status of the HVAC system, and doors that are opened and closed. A design analysis should include a number of design scenarios to provide a level of assurance that the smoke control system will operate as intended. Design fires need to be realistically selected as discussed in Chapter 5. In general, a design analysis needs to include design fires located in the atrium and in communicating spaces. A communicating space is one that has an open About the Author John H. Klote, Ph.D., P.E., is is an expert in smoke control technology in Leesburg, Va. 36 ASHRAE Journal ashrae.org June 2012

2 A B Smoke Layer Separated Space Separated Space Communicating Space Atrium (Large-Volume Space) Smoke Exhaust Is Not Shown Plume Communicating Space C Spaces Related to Atriums Smoke Layer D Fire in the Atrium Smoke Layer Balcony Spill Plume Smoke Exhaust Is Not Shown Smoke Exhaust Is Not Shown Window Plume This Room is Fully Involved in Fire Fire in a Communicating Space Figure 1: Fire locations for atrium smoke control analysis. Fully Developed Fire in a Room Open to the Atrium pathway to an atrium so that smoke from a fire either in the atrium or the communicating space can move from one to the other without restriction. Figure 1A illustrates these spaces. A separated space is one that is isolated from the atrium by smoke barriers (Figure 1A). For this handbook, a smoke barrier is a continuous membrane, either vertical or horizontal, that is designed and constructed to restrict the movement of smoke in conjunction with a smoke control system. Smoke movement at these smoke barriers can be controlled by pressurization or by compartmentation alone. Figure 1B shows a fire in the atrium with smoke rising above the fire to form a smoke layer under the ceiling of the atrium. The most widely used approach to atrium smoke control is smoke exhaust, but other approaches can also be used. Regardless of the smoke control approach, there is a distance around the fire where occupants cannot go because of the intensity of the fire. To determine the minimum distance that a person can be from a fire for a few minutes without unbearable pain see Chapter 6. For a scenario with the fire in the atrium, the design fire does not normally take into account any benefit of sprinklers. In spaces with high ceilings, the temperature of the smoke plume can drop so much that sprinklers may not activate or activation may be so delayed that the spray may evaporate before it reaches the fire. Information about the interaction of sprinklers with the smoke layer is in the Handbook of Smoke Control Engineering. For information about design fires, see Chapter 5. Smoke from a fire in a communicating space can flow into the atrium and form a balcony spill plume as shown in Figure 1C. This figure shows smoke blocking of parts of balconies above the fire. It is beyond the capability of smoke control technology to prevent such smoke blocking, but the balcony is not blocked away from the balcony spill plume (Figure 2).The comments earlier regarding the minimum distance that a person can be from a fire also apply here. For a scenario with the fire in a communicat- Smoke exhaust through a plenum with a suspended ceiling not recommended. The pressures produced by the exhaust flow through a plenum with a suspended ceiling can be high enough to lift ceiling tiles out of their frames. Such relocation of ceiling tiles could have an adverse impact on the performance of the smoke exhaust system. The effort involved with periodic testing of such a smoke exhaust system can be significantly increased due to the need for repair of suspended ceilings after testing. June 2012 ASHRAE Journal 37

3 ing space, the growth of the design fire generally stops upon sprinkler activation. Figure 1D shows a fully developed fire and smoke forming a window plume. A fully developed fire would not happen when a sprinkler system is operating properly. Because most new commercial buildings in the United States are fully sprinklered, design fire scenarios that include a fully developed fire are uncommon in the United States. In countries where fully sprinklered buildings are uncommon, design fire scenarios may include fully developed fires. It is also possible that some building owners or building managers may want the very high level of protection associated with a smoke control system that can handle even a fully developed fire. Design Approaches Design approaches that have been used for atrium smoke control are (1) natural smoke filling, (2) steady mechanical smoke exhaust, (3) unsteady mechanical smoke exhaust, (4) steady natural smoke venting, and (5) unsteady natural smoke venting. These approaches are discussed later. Airflow can also be used to control smoke flow in conjunction with these approaches, but care must be exercised because airflow has the potential to provide combustion air to the fire. Many design approaches are intended to prevent occupants from coming into contact with smoke. The idea is to control smoke so that it descends only to a predetermined height during the operation of the smoke control system. In many locations, there are code requirements for the predetermined height. This height is often in the range from 6 to 10 ft (1.83 to 3.05 m) above the highest walking surface that forms a portion of a required egress in the atrium. Other design approaches are intended to maintain a tenable environment when people come into contact with smoke. When the products of combustion are sufficiently diluted, the resulting diluted smoke can be tenable, and tenability analyses routinely deal with reduced visibility and exposure to toxic gases, heat and thermal radiation. See Chapter 6 for more information about tenability. The following discussion of design approaches address systems that are intended to prevent occupant contact with smoke, but these systems can be modified to ones that address tenability. Natural Smoke Filling This approach consists of allowing smoke to fill the atrium without any smoke exhaust or other smoke removal. For some spaces the smoke filling time with the design fire is more than sufficient for evacuation. The smoke filling time is the time from ignition until the smoke descends to the predetermined height. Applications that are appropriate for natural smoke filling are not common, because there needs to a very large space above the highest occupied level of the atrium. Any of the methods of analysis discussed below can be used for this system. It is essential that calculations of evacuation time Smoke exhaust is not shown. Smoke Layer Balcony Spill Plume Figure 2: Front view of balcony spill plume. include the times needed for recognition, validation and premovement as discussed in Chapter 4. Steady Mechanical Smoke Exhaust This is the most commonly used approach in North America. This system consists of mechanical smoke exhaust sized to keep the bottom of the smoke layer at the predetermined height for the design fire. Unsteady Mechanical Smoke Exhaust This approach also uses mechanical smoke exhaust, but the flow rate of the exhaust is less than steady mechanical exhaust such that the exhaust only slows the rate of smoke layer descent for a time that allows occupants to safely egress from the space. This method needs to maintain at least the predetermined height mentioned previously for the time it takes the occupants to safely evacuate. The considerations about calculation evacuation time for natural smoke filling systems also apply here. Steady Natural Venting As previously mentioned, this kind of venting has a history going back to the Ring Theater fire of This approach is not 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 top of the atrium. Often this kind of venting is called gravity venting because the smoke is vented due to buoyancy. The flow rate of the smoke through the vents needs to be such that the bottom of the smoke layer is kept at the predetermined height for an indefinite time. The previous comments regarding the predetermined height also apply here. An equation for the steady mass flow rate through a natural vent is discussed later. It is recommended that steady natural venting systems be analyzed with the aid of a computational fluid 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 smoke layer descent for a time that allows occupants to safely egress from the space. This method needs to maintain at least the 38 ASHRAE Journal ashrae.org June 2012

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5 A Ceiling Jet B Exhaust Uniform Smoke Layer Transition Zone Plume Smoke Layer Interface Plume Smoke Exhaust Not Shown Fire Fire Atrium Fire Zone Model Idealization of an Atrium Fire Figure 3: Sketch of an idealized zone model representation of an atrium fire. predetermined height mentioned previously for the time it takes the occupants to safely evacuate. It also is recommended that unsteady natural venting systems be analyzed with the aid of a CFD model. The considerations about calculation evacuation time for natural smoke filling systems also apply here. Methods of Analysis The methods that can be used for analysis of atrium smoke control systems are algebraic equations, zone fire modeling, CFD modeling and scale modeling. Algebraic Equations Atrium smoke control makes use of many algebraic equations. Some of these are based on the fundamental principles of engineering, and others are empirical correlations based on experimental data. Equations for smoke filling, natural venting and the airflow velocity to prevent smoke backflow are discussed later in the chapter. Chapter 16 addresses the algebraic equations for steady mechanical smoke exhaust, and these equations are based on the zone fire model concepts discussed in the next section. In the following section on zone fire modeling, the discussion about smoke exposure in the transition zone also applies to systems designed with the algebraic equations of Chapter 16. When another method of analysis is used, algebraic equations are often used to determine starting points for the analysis. Zone Fire Modeling In an atrium fire, smoke flows upward in a plume that entrains air as it rises. When the plume reaches the ceiling, it turns and becomes a ceiling jet that flows under the ceiling (Figure 3A). Figure 3B shows an idealized zone model representation of an atrium fire. Zone fire models are simple models that consider a fire compartment 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 flowing into and out of it. In a real fire, the temperature and concentration of contaminants vary throughout the smoke layer with the highest values tending to be near top of the smoke layer. In real fires, there is also a gradual transition zone between the smoke layer and the lower layer as shown in Figure 3A. In a zone fire model, the smoke layer has a uniform temperature and uniform concentrations. This means that the temperature at any place in the smoke layer is the same as everywhere else in the smoke layer, and the same can be said about the concentration of each contaminant. Zone fire models do not simulate the transition zone, but the bottom of the smoke layer is simulated as a horizontal plane called the smoke layer interface as shown in Figure 3B. The zone model considers the air a fraction of an inch (or centimeter) below the smoke layer interface to be as free of smoke as the rest of the lower layer. Occupants in the lower layer near the smoke layer interface will actually be in the transition zone exposed to some smoke. Unfortunately, neither zone fire models nor the algebraic equations of Chapter 16 can be used to evaluate this smoke exposure. It is believed that in many situations, conditions in the transition zone may be tenable. CFD modeling can be used to evaluate tenability at this location. Zone fire models do not simulate the time it takes for the plume to reach the ceiling, which is small in a normal size room but larger in an atrium. Empirical equations for this lag time are discussed later in this Chapter. Zone fire models do not simulate plume flow, but they use empirical equations to calculate plume temperature and the mass flow. Even with the previous limitations, zone fire models have proven to be very useful tools for many applications, but they must be used with care. Chapter 18 has more detailed information about zone fire models. CFD Modeling CFD consists of dividing a space of interest such as an atrium into a large number of cells, and using a computer program to solve the governing equations for each cell. CFD is 40 ASHRAE Journal ashrae.org June 2012

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7 capable of highly realistic simulations. The plume, ceiling jet, smoke layer and the transition zone are all simulated by the CFD model. CFD models are capable of simulating plugholing, and they can simulate any adverse effects of makeup air velocity on plume formation. Plugholing is discussed later. CFD modeling requires a level of knowledge and experience beyond that of zone fire modeling, and CFD simulations typically require hours and sometimes days of computer time. For more information about this kind of modeling, see Chapter 20. Scale Modeling Scale modeling is capable of highly realistic simulations. This kind of modeling consists of conducting fire tests in a small model of the atrium or other facility, and converting the data from those tests to the full scale facility. Scale modeling is addressed in Chapter 21. Ceiling Jet Plume Figure 4: Minimum smoke layer depth. Minimum Smoke Layer Depth is 20% of Floor-to-Ceiling Height Flow Under Ceiling Jet Atrium Temperature For systems that rely on mechanical smoke exhaust, the temperature of the air below the smoke layer quickly approaches the outdoor temperature. This is because of the very large amounts of makeup air that enter the atrium. For design analysis of systems using mechanical smoke exhaust, the outside design temperature should be used for the ambient temperature of the atrium. As the gas temperatures increase, the density of the gas decreases, and the volumetric flow rate needed to maintain a constant mass flow increases. Atrium exhaust fans need to be sized for the maximum volumetric flow needed to control smoke for the design conditions. This maximum volumetric flow will happen when the summer outside design temperature is used for the ambient temperature of the atrium. For this reason, smoke exhaust fans need to be sized with an ambient temperature of the atrium equal to the summer outside design temperature. Minimum Smoke Layer Depth The minimum smoke layer depth needs to be 20% of the floor-to-ceiling height except when an engineering analysis using full scale data, scale modeling, or CFD modeling indicates otherwise. The formation of the minimum smoke layer depth is shown in Figure 4. When a smoke plume reaches the ceiling, the smoke flows away from the point of impact in a radial direction forming a ceiling jet. When the ceiling jet reaches a wall the smoke flow turns down and flows back under the ceiling jet. The ceiling jet has a depth of about 10% of the floor-toceiling height, and the smoke flow under the ceiling jet is also about 10% of the floor-to-ceiling height. This means that the smoke layer depth is about 20% of the floor-to-ceiling height. Makeup Air Makeup 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 air has to be provided so that the exhaust fans can remove the design quantities of smoke and that the door opening force requirements are not exceeded. Makeup air must be supplied far enough below the smoke layer interface so that it does not disrupt the smoke layer. When providing makeup air through openings to the outside, some air also flows by way of leakage paths. The large openings (such as vents, doors and windows) need to open automatically on system activation. The leakage paths consist of construction cracks, gaps around closed doors, gaps around closed windows, and other similar small paths. The large openings should be sized to provide about 85% to 95% of the makeup air with the rest coming through the leakage paths. When makeup air is provided by mechanical fans, the makeup air should be less than the mass flow rate of the mechanical smoke exhaust. It is recommended that makeup air for fan powered smoke exhaust systems be designed at 85% to 95% of the exhaust. The idea is that the remaining air (5% to 15%) will enter the large-volume space through leakage paths preventing positive pressurization of the atrium. The makeup air must not exceed 200 fpm (1.02 m/s) where the makeup air could come into contact with the plume unless a higher makeup air velocity is supported by an engineering analysis. The primary reason for this 200 fpm (1.02 m/s) limit is to prevent significant deflection of the plume and disruption of the smoke layer. 2 Deflection of the plume results in increased air entrainment that can cause smoke control system failure. A secondary reason for this velocity restriction is that it reduces the potential for fire growth and spread due to airflow. The 200 fpm (1.02 m/s) limitation is not relevant in communicating spaces one story high that are sprinklered. At these locations, successfully sprinklered fires do not form plumes as they would in the atrium, and successfully sprinklered fires limit fire growth. However, air introduced in a communicat- 42 ASHRAE Journal ashrae.org June 2012

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9 A Exhaust Fan B Exhaust Fan Plugholing of Air Into Smoke Exhaust It Cannot Be Seen, But The Fan is Still Pulling Air Into the Smoke Exhaust Smoke Layer Height Falling Due to Plugholing Figure 5: Plugholing causing the smoke layer to fall below the intended height. Smoke Below the Intended Height Due to Plugholing ing space needs to slow down to meet the 200 fpm (1.02 m/s) limitation when it reaches the atrium. For example, consider makeup air supplied to the communicating space on the ground floor of Figure 1A. The makeup air enters the communicating space at a velocity above the limitation. A jet of supply air forms as it would from an HVAC diffuser. The velocity of this jet needs to drop to 200 fpm (1.02 m/s) or less when it reaches the atrium space. The design calculations need to include velocity calculations of this makeup air jet at the point where it reaches the atrium. When makeup air is provided by openings to the outside, the design analysis of the system needs to address wind effects as discussed below. Wind Atrium smoke control systems need to be designed to minimize the potential for wind to result in: (1) velocities greater than 200 fpm (1.02 m/s) where the makeup air could come into contact with the plume, and (2) smoke feedback from the smoke 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 (1.02 m/s) inside the atrium. The wind can blow into openings facing one direction and out the other openings. A simple approach for minimizing wind effects inside an atrium is to have all the makeup air openings face in the same direction. Another simple approach is using mechanical fans for both smoke exhaust and makeup air such that the impact of the wind is minimized. When such simple approaches are not feasible, a detailed analysis is needed that takes into account the prevailing wind directions. Such an analysis can be done with a network model or a CFD model. Smoke can be carried by the wind from the smoke exhaust 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 openings (or inlets): (1) far away from each other, and (2) such that the prevailing wind directions carry the smoke away from makeup air openings (or inlets). When this simple approach is not feasible, CFD analysis or wind tunnel analysis is needed to evaluate the potential for smoke feedback into the atrium. Plugholing Plugholing is a phenomenon where air from below the smoke layer is pulled through the smoke layer into the smoke exhaust. Plugholing can cause system failure, but it can be easily prevented. Plugholing reduces the exhaust from the smoke layer, which tends to lower the smoke layer and expose occupants to smoke. The following discussion of plugholing applies to steady mechanical smoke exhaust systems. Figure 5A shows a smoke layer that is at the intended design height, but the layer is still descending due to plugholing. As the smoke layer depth increases, the buoyancy forces of the smoke layer increase, and the amount of plugholing decreases. Eventually, the smoke layer becomes deep enough that a state of equilibrium is achieved with a constant smoke layer height as shown in Figure 5B. Plugholing has resulted in a smoke layer below what was intended. The important forces for plugholing are the kinetic forces of the smoke exhaust and the buoyancy forces of the smoke layer. When kinetic forces dominate, there will be plugholing. When the buoyancy forces dominate, there will be no plugholing. The kinetic forces depend on the flow rate of the smoke exhaust, and the buoyancy forces depend on the temperature and depth of the smoke layer. When these forces are balanced at an exhaust inlet, the flow at that inlet is the maximum that can be achieved without plugholing. Plugholing can be prevented by using a number of smoke exhaust inlets such that the flow rate at each inlet is at or below this maximum value. 44 ASHRAE Journal ashrae.org June 2012

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11 There is an empirical equation in Chapter 16 for the maximum volumetric flow rate that can happen at an exhaust inlet without plugholing. This equation and the earlier discussion also apply to systems that use natural venting. Scale modeling and CFD modeling can simulate plugholing without the use for the empirical maximum flow rate equation of Chapter 16. This empirical equation can be conservative, and it is possible that an analysis using scale modeling or CFD modeling would result in a lower number of exhaust inlets than an analysis using the empirical equation. Stratification A hot layer of air can form under the ceiling of an atrium due to solar radiation on the atrium roof. The temperature of such a layer can be 120 F (50 C) or more. When the aver- age temperature of the plume is less than that of the hot air layer, a stratified smoke layer can form under the hot air layer preventing smoke from reaching ceiling-mounted smoke detectors. If smoke stratification can occur, projected beam smoke detectors should be used, and three arrangements of these detectors are discussed in the handbook. Control and Operation Atrium smoke control systems must be activated automatically to quickly provide smoke protection for the occupants. For atria where smoke stratification can happen, projected beam smoke detectors should be used as mentioned previously. Some other methods of system activation are ceiling mounted smoke detectors, heat detectors and sprinkler water flow. The smoke control system needs to reach full operation before conditions in the atrium reach the design conditions. Determination of the time for the system to become operational needs to take into account (1) the time for detection of the fire and (2) the HVAC system activation time including shut-down and start-up of air-handling equipment, opening and closing dampers, and opening and closing natural ventilation devices. A means of manually starting and stopping the smoke control system needs to be provided at a location acceptable to the fire department. These manual controls need to be able to override the automatic controls. For general information about controls of smoke control systems see Chapter 8. References 1. Klote, J.H Handbook of Smoke Control Engineering. Atlanta: ASHRAE. 2. Hadjisophocleous, G., J. Zhou Evaluation of atrium smoke exhaust make-up air velocity. ASHRAE Transactions, Part ASHRAE Journal June 2012

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