ASHRAE LV-11-C059-2011 Stairwell Smoke Control by Ventilation.pdf

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1、John H. Klote is a consultant in Lansdowne Virginia. Stairwell Smoke Control by Ventilation John H. Klote, P.E., D.Sc. ASHRAE Fellow ABSTRACT The idea of using ventilation to keep spaces tenable during building fires goes back to the early days of smoke control. In those days, analysis of the perfor

2、mance of ventilation smoke control was not possible. Because of recent advances in analytical tools, these systems can be analyzed today. This paper presents the idea of using ventilation for stairwell smoke control in tall buildings including a description of the design approach. The analytical too

3、ls used are: (1) tenability analysis, (2) computational fluid dynamics (CFD), and (3) network modeling. The results of a CFD analysis including tenability analysis are described, and this analysis demonstrates the feasibility of stairwell smoke control by ventilation. Stairwell ventilation has the a

4、dvantage over stair pressurization of mitigating the adverse impacts of stack effect and floor-to-floor variations in flow resistance. INTRODUCTION 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 e

5、ffect 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

6、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 diffic

7、ult. For further information about stack effect and pressurized stairwells, readers are referred to the ASHRAE smoke control manual (Klote and Milke 2002). Smoke control by means of ventilation has the advantage of maintaining a tenable environment in the stairwells of tall buildings without the dif

8、ficulties 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 thos

9、e 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 po

10、tential 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

11、 ventilation. ANALYTICAL TOOLS The analytical tools used for ventilation smoke control are tenability analysis, computational fluid dynamics (CFD), and network modeling. These tools are discussed below. LV-11-C059478 ASHRAE Transactions2011. American Society of Heating, Refrigerating and Air-Conditi

12、oning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.Tenability Analysis Toxic gases, heat

13、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 b

14、e considered. Figure 1. Example Building. Figure 2. Heat tolerance for humans at rest, naked, with low airflow 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

15、 Purser model (Purser 2008). The FED model is the oldest and simplest, and it is sufficient for most smoke control applications. The FED is (1) where mfis the mass concentration of fuel burned, t is the exposure time, and LCt50is the lethal exposure dose from animal test data. An FED greater than or

16、 equal to one indicates fatality. For values of LCt50readers 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 197

17、3). This figure shows 250F (121C) 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 2

18、50F (121C) 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 exposu

19、re to 120F (49C) for about one hour. 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 2011 ASHRAE 479Chianta (1969).

20、 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, th

21、en 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 (2) where S is visibility, K is a proportionality constant, and is the ext

22、inction 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 vi

23、sibility 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 situati

24、ons. 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 (3) where 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 perso

25、n 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 CFD modeling was developed in the 1970s, and in recent years CFD modeling h

26、as 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 govern

27、ing 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 ex

28、tensively 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. Network Modeling Many network model

29、s 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

30、 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

31、. 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 al

32、so occur through cracks in partitions, floors, and exterior 480 ASHRAE Transactionswalls 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 networ

33、k 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 ventil

34、ation, 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

35、 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 un

36、der 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

37、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 s

38、mall 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 ro

39、om 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

40、 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 airf

41、low 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 develo

42、ped 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 warpe

43、d 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 an

44、alyses, 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 a

45、bove 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 througho

46、ut 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 w

47、ith 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 i

48、s 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 us

49、e the stairs. For the CFD analysis, it was arbitrarily chosen that the FED was determined 2011 ASHRAE 481for 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. Figure 3. CFD simulations showed that ventilation can provide tenable conditions. CFD ANALYSIS To evaluate the minimum flow needed past stair doors, CFD simulations incl