Stairwell smoke control by ventilation.
Stair pressurization can be very difficult in tall buildings due to stack effect and floor-to-floor variations in flow resistance. The impact of stack effect on stair pressurization is well known, but the impact of variations in floor-to-floor flow resistance is less well known. These variations can be illustrated by the 80 story building shown in Figure 1. This building consists of the following kinds of floors: underground parking, general hotel spaces with rental spaces, hotel guest floors, condominium floors, and a penthouse. At each floor, pressurization air from the stairs needs to flow through the building to the outside, and the difference in flow resistance of the different kinds of floors makes successful stair pressurization difficult. For further information about stack effect and pressurized stairwells, readers are referred to the ASHRAE smoke control manual (Klote and Milke 2002).
[FIGURE 1 OMITTED]
Smoke control by means of ventilation has the advantage of maintaining a tenable environment in the stairwells of tall buildings without the difficulties mentioned above. A tenable environment is one in which the smoke is not life threatening. Ventilation smoke control maintains a tenable environment by using ventilation air to dilute smoke.
In the early days of smoke control, smoke control by ventilation received limited attention. In those days, there was no way to evaluate the performance of such systems. What is significant about this paper is that it presents a new concept of ventilation smoke control based on modern methods of analysis, and the feasibility of this new concept is demonstrated. Ventilation smoke control has the potential to be used for numerous smoke protection applications, but the focus of this paper is on stairwell smoke protection. This paper includes a discussion of modern analytical methods. Further, the results of simulations are discussed that demonstrate the feasibility of stairwell smoke control by ventilation.
The analytical tools used for ventilation smoke control are tenability analysis, computational fluid dynamics (CFD), and network modeling. These tools are discussed below.
Toxic gases, heat and thermal radiation are the direct threats to human life from flames and smoke. In thick smoke, people see poorly and walk slowly or become disorientated which prolongs exposure to smoke. In many applications the primary threat results from reduced visibility, but the other threats still need to be considered.
Exposure to Toxic Gases: The models that can be used to predict the results of gas exposures are (1) the fractional effective dose (FED) model, (2) the N-gas model (Levin 1996), and (3) the Purser model (Purser 2008). The FED model is the oldest and simplest, and it is sufficient for most smoke control applications. The FED is
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [m.sub.f] is the mass concentration of fuel burned, t is the exposure time, and [LCt.sub.50] is the lethal exposure dose from animal test data. An FED greater than or equal to one indicates fatality. For values of [LCt.sub.50] readers are referred to the ASHRAE smoke control manual.
Exposure to Heat: Heat exposure happens when a person comes into contact with hot gases. Figure 2 is a graph of the heat tolerance of naked humans at rest with low air movement (Blockley 1973). This figure shows 250[degrees]F (121[degrees]C) as a rule of thumb demarcation between skin burns and heat stroke (hyperthermia). The figure is for naked people, but clothing tends to protect people from thermal exposures. Thus the figure is conservative for people with clothing. Exposures to temperatures above 250[degrees]F (121[degrees]C) can result in skin pain and burns, and exposures to temperatures below this temperature can result in heat stroke. Because of the water vapor in smoke, the curve for humid air should be used for smoke control applications. From Figure 2, it can be seen that a person can tolerate an exposure to 120[degrees]F (49[degrees]C) for about one hour.
[FIGURE 2 OMITTED]
Exposure to Thermal Radiation: This exposure happens when a person is subjected to thermal radiation from nearby flames or hot gases. A method of evaluating the effect of exposure to thermal radiation has been developed by Stoll and Chianta (1969). Exposure to thermal radiation is not relevant to the design of most smoke control systems. This can be illustrated by considering both heat and thermal radiation exposures to a gas for the same period of time. If the temperature of the gas is such that heat exposure to that gas can be tolerated, then the exposure to the thermal radiation of the gas can also be tolerated.
Reduced Visibility: Based on research at the Fire Research Station (FRI) in Japan (Jin 1975), the relation between visibility and smoke obscuration is
s = [K/[alpha]] (2)
where S is visibility, K is a proportionality constant, and [alpha] is the extinction coefficient. K is 3 for reflecting signs and 8 for illuminated signs. A value of K of 3 is often used for building components seen with reflected light.
With a CFD model, the properties of the smoke vary from point to point, and visibility is often thought of as visibility at a point. The visibility at a point is the distance that a person could see if he or she were in a space with smoke that had the same extinction coefficient as at the point. The shortcoming of visibility at a point is it does not account for the small spaces of relatively dense smoke that often form in fire situations. To account for such "pockets" of smoke, visibility along a path can be used. Visibility along a path can be calculated from smoke obscuration as
S = -[Kx/[[log.sub.e](1 - [lambda]100)]] (3)
where [lambda] is the percent obscuration along the path and x is the path length. When the visibility is greater than or equal to the path length, a person can see to the end of the path. Often when smoke is diluted such that the visibility criterion for a project is met, the smoke is so diluted that the threats of toxic gases, heat and thermal radiation are not an issue.
CFD modeling was developed in the 1970s, and in recent years CFD modeling has become commonly used largely due to advances in computer hardware and numerical methods. The idea of CFD modeling is to divide a space into a large number of small cells, and use a computer to solve the governing equations for the flows, pressures and temperatures throughout the space. The governing equations consist at least of the conservation equations for mass, momentum and energy.
Fire dynamic simulator (FDS) is a CFD model that was developed at the National Institute for Standards and Technology (NIST) specifically for fire applications (McGrattan et al. 2008a, 2008b). FDS has been extensively validated (McGrattan et al. 2008c, 2008d). Because FDS was developed at NIST, it is available from NIST at no cost. FDS is extensively used around the world for fire applications, and it was used for the smoke flow simulations described in of this paper.
Many network models have been developed (Butcher Fardell, and Jackman 1969; Barrett and Locklin 1969; Sander 1974; Wakamatsu 1977; Evers and Waterhouse 1978; Yoshida et al. 1979; Klote 1982). The CONTAM model (Walton and Dols, 2005) has superior numeric routines and graphic input. Even though CONTAM was developed for indoor air quality applications, it is extensively used throughout the world for smoke applications. Because CONTAM was developed at NIST, it is available from NIST at no cost.
In a network model, a building is represented as a network of spaces or nodes, each at a specific pressure and temperature. The stairwells and other shafts are modeled by a vertical series of spaces, one for each floor. Air flows through leakage paths from regions of high pressure to regions of low pressure. These leakage paths are doors and windows that may be opened and though gaps around closed doors. Leakage can also occur through cracks in partitions, floors, and exterior walls and roofs. The airflow through a leakage path is a function of the pressure difference across the leakage path.
Smoke flow throughout the building can be simulated by modeling the flow of contaminants. In network models, the temperature and concentration of contaminants are considered to be uniform throughout each space. The pressures throughout the building and steady flow rates through all the flow paths are obtained by solving the airflow network, including the driving forces such as wind, forced ventilation, and inside-to-outside temperature difference.
STAIRWELL VENTILATION APPROACH
The following is a discussion of an approach that could be used to analyze a ventilation smoke control system for stairwells in a tall building like that of Figure 1. In such a system, a minimum flow past stair doors needs to be determined, and this can be done with a CFD model. Such a CFD analysis is described later. Once this minimum flow has been established, CONTAM or another network model can be used to design a system of stair supply and venting that will result in the minimum flow past all stair doors under conditions of all stair doors closed and a design number of stair doors open. CONTAM has been extensively used for analysis of pressurized stairs, and anyone experienced with this application would be capable of using it to analyze it for a stairwell ventilation system. For this reason, the use of network models is not discussed further.
A design fire needs to be determined, and this fire would be outside of the stairs either near a stair door or in an adjacent space. An example of an adjacent space is a hotel guest room with the guest room door open to the corridor. The fire could be as small as a shielded fire or as large as a fully developed fire. A shielded fire is a sprinklered fire where the spray cannot directly fall on the burning material because of the presence of a shielding surface such as a table top. A fully developed fire is one where every object that can burn in a room is burning. Of these design fires, the least stringent is a shielded fire in an adjacent space, and the most stringent is a fully developed fire near a stair door. For a specific application, an engineering analysis should be conducted to determine an appropriate design fire.
For the CFD analysis discussed below, the most stringent design fire above was used. The burning material was upholstered furniture filled with polyurethane foam which is relatively common and produces large quantities of dense black smoke. The ventilation airflow can be upward or downward in the stairs. An upward airflow results in nearly smoke free conditions in the stair below the fire, and this upward flow was used for the CFD analysis.
The possibility of door warping needs to be considered. For a shielded fire door warping is probably minimal. When a door is subject to hot smoke or flames from a fully developed fire, warping can be significant. The extent of door warping depends on (1) the temperature of the gases near the door, (2) door materials and (3) door fabrication methods. However, there is limited data on this subject (Fire International 1968; Van Geyn 1994). For the CFD simulations, the warped door opening was arbitrarily chosen as 1 inch (25 mm) at the top side away from the hinges. The opening area consisted of two isosceles triangles which have bases of 1 inch (25 mm) and sides of 3 ft (0.91 m) and 7 ft (2.1 m).
There are no generally accepted visibility criteria for smoke control analyses, but visibility criteria for smoke control applications are usually chosen for visibility at a point. However, it seems that for stairwell ventilation analysis, visibility criteria should be along a path. The visibility criterion for the CFD analysis is that a person on the landing directly above the fire floor is able to see down to the fire floor landing. This is equivalent to saying that a person on the fire floor landing is able to see the landing directly above. The idea behind this criterion is that if it is met, visibility would also be maintained from landing to landing throughout the stairs. The CFD simulations show that this idea was true for the stairwell of these simulations.
One way to deal with the threat of toxic gases is to choose the worst location and to calculate the FED for an exposure time. This approach only works when it is obvious that there is a location with the densest smoke. The results of the CFD analysis will show that the worst situation for tenability would be a person waiting in a wheelchair inside the stairs on the fire floor landing. While the likelihood of a person with a mobility limitation seeking refuge in this stair on the fire floor is relatively low, it is possible. People on the fire floor would see the fire near the stairs, and they would use another stair rather than move past the fire. However, it is possible that during the early stages of the fire a person might be between the fire and the stairs, and that person could use the stairs. For the CFD analysis, it was arbitrarily chosen that the FED was determined for a person waiting on the fire floor landing in a wheelchair for one hour.
The above discussion does not include the relatively undiluted smoke within about 1.5 ft (0.46 m) of the door gaps. This smoke near the door gaps is not a concern because it is human nature to avoid such dense smoke.
To evaluate the minimum flow needed past stair doors, CFD simulations including tenability analysis were done for a four story section of stairwell. The cell size was about 3.7 inches (0.094 m) which was selected based on a sensitivity analysis. Smoke leakage into the stairs was approximated by smoke generation at the gaps at the top and side of the stair door.
Figure 3(a) shows simulated smoke flow in the stairwell without forced ventilation. The stairs were filled with dense smoke resulting in visibility of only 2 to 4 ft (0.6 to 1.2 m). Such smoke would make it impossible for a people to see down to their feet. This visibility is definitely unsatisfactory.
[FIGURE 3 OMITTED]
Simulations were made with ventilation rates of 10,000; 15,000 and 20,000 cfm (4.72, 7.08 and 9.44 [m3/s). It will be shown that with 20,000 cfm (4.72, 7.08 and 9.44 m3/s), a tenable environment was maintained in the stairs. The following discussion focuses on simulations with a ventilation rate of 20,000 cfm (4.72, 7.08 and 9.44 m3/s). Figures 3(b) and 3(c) show smoke at this ventilation rate with and without people. In Figure 3(c) a "pocket" of smoke forms in front of the door on the fire floor. It will be explained later that a person on the landing above the fire floor can see down to the fire floor door with the exception of occasional puffs of smoke.
With the exception of the "pocket" of smoke, the number and arrangement of people on the stairs in Figure 3(c) is that used by Achakji and Tamura (1988) for full people loading of stairs. The CFD simulations were made without people in this "pocket" of smoke, because it is human nature to avoid such a smoky space. Even avoiding the "pocket" of smoke, people would have sufficient room to walk past the fire floor landing. Achakji and Tamura also experimentally determined that they could use cylinders in place of people in the stairs when measuring the flow resistance. Because of the requirements of FDS, it was not feasible to model people by cylinders, but people were modeled in FDS by rectangular solids. One rectangular solid was used for the body and limbs and another smaller one was used for the head.
Figure 4 shows the visibility on a path from the landing above the fire floor to the stairwell door on the fire floor. The ends of the path were 5 ft (1.52 m) above floor. The path length was 14.8 ft (5.4 m), and this length is shown on Figure 4 as a dashed line. For visibilities greater than the dashed line, a person can see the door on the fire floor. For all of the simulations without people, visibility is above the dashed line. This means that without people, the visibility criterion is met.
[FIGURE 4 OMITTED]
With the stairs full of people, it can be seen from Figure 4 that most of the time the visibility is above the dashed line. This means that most of the time a person on the landing can see the fire floor door. Visibility is below this line only for short periods of time. What is happening is that occasional puffs of smoke in front of the door prevent a person from seeing the door along the path length. However, a person on the landing would be able to see the door on the sides of the puffs of smoke. This means that with the stairs full of people, the visibility criterion is also met. Also a person on the fire floor landing would be able to see to the landing above and to the landing below.
For a person in a wheelchair waiting an hour in the stairs on the fire floor, the FED was 0.02. Remembering that an FED greater than or equal to one indicates fatality, this means that toxic exposure would not be an issue for this person. However, such a person might experience some anxiety and possibly nausea. The temperature in the stairwell on the fire floor ranged from 74[degrees]F to 98[degrees]F (23[degrees]C to 37[degrees]C). This temperature does not result in any threat concerning either heat exposure or thermal radiation exposure. Thus a tenable environment was maintained in the stairs on the fire floor landing. The results of the CFD simulations showed that the conditions were worst in the stairs on the fire floor landing, and a tenable environment was maintained throughout the stairs.
While the CFD analysis shows that the ventilation approach is feasible, it should be verified by full scale fire experiments. Door warping is another topic of needed research. At present, a stairwell ventilation design should include a CFD analysis and a tenability analysis. A research project could develop a design approach that would eliminate the need for CFD analysis and a tenability analysis for most stairwells.
SUMMARY & CONCLUSIONS
Stair pressurization can be very difficult in tall buildings due to stack effect and the floor-to-floor variation of building flow resistance. For tall buildings, stairwell smoke control by ventilation can minimize these difficulties. The CFD analysis discussed above demonstrates that stairwell smoke control by ventilation is feasible. This analysis was based on stringent conditions of design fire and door warping, and it showed that 20,000 cfm (9.44 m3/s) of ventilation air was needed to maintain a tenable environment. A design based on less stringent conditions would be expected to need less ventilation air.
FED = fractional effective dose (dimensionless)
K = proportionality constant (dimensionless)
[LCt.sub.50] = lethal exposure dose from test data, lb ft-3 min (g m-3 min)
[m.sub.f] = mass concentration of burned fuel, lb/ft3 (g/m3)
S = visibility, ft (m)
t = exposure time, sec (sec)
x = path length, ft (m)
[alpha] = extinction coefficient [ft.sup.-1] ([m.sup.-1])
[lambda] = percent obscuration along the path
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John H. Klote, P.E., D.Sc.
John H. Klote is a consultant in Lansdowne Virginia.
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|Author:||Klote, John H.|
|Date:||Jan 1, 2011|
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