ASHRAE 4778-2005 On Atrium Smoke Management System Design《区域模型模拟的零售商店火灾》.pdf

1、477% On Atrium Smoke Management System Design W.K. Chow, PhD Member ASHRAE ABSTRACT Common design guides on smoke management in atria will be reviewed in this paper: Different approaches used in those guides are outlined. It is observed that the engineering principles behind those design guides are

2、basically the same, though the approaches might be different. Smoke ventilation appears to be a common approach for smoke management in an atrium. An acceptable smoke layer height can be kept, or at least the descending rate of the smoke layer could be reduced. Designs based on diflerentguides are c

3、ompared by refer- ring to some real cases in Hong Kong. There are deviations among different guides even under the same fire scenarios. At the moment, these guides only give general design principles, not yet covering all atria, especially the tall ones in the Far East. In other words, only simpliJi

4、ed design procedures for systems under some agreed fire scenarios in an atrium are outlined. Guidance on solving practical problems frequently encountered in atria apartfrom those cases is discussed, taking the huge development in the construction industry of China as an example. INTRODUCTION Many l

5、arge atria have been built in construction projects in the Far East. Fire safety in those big crowded spaces is a concern, as the number of fires appears to be increasing. In those big building fires (Chow 1998), smoke was identified to be a threat. In an atrium fire, smoke generated from the atrium

6、 floor itself, or in spaces adjacent to the atrium void, spreads rapidly. Though the smoke is quite “cool” due to the large atrium space, the consequences can be quite serious in expos- ing a large number of occupants to risk for atria located in J. Li crowded malls. Note that psychological effects

7、of occupants are key factors. The time for escape will be extended, and the performance of the fire protection systems and the fire-fight- ing activities will be affected. Smoke management systems, which are defined as engineered systems including all meth- ods that can be used singly or in combinat

8、ion to reduce smoke production or to modify smoke movement, are essential to provide a tenable environment for the safe evacuation of occu- pants (NFPA 1995,2000). Approaches to smoke management design in atria have been introduced in some codes and engineering guides. While basic approaches in thes

9、e guides might be different in many respects, the engineering principles behind are similar. For example, a stable smoke layer is assumed so that a zone model will work. By solving a set of equations describing smoke physics, smoke management systems can be designed. Common guidance to designers of

10、atrium smoke control systems within the UK is provided by the British standards BS 5588, Part 7, Code ofpractice for the Incorporation ofAtria in Buildings (BSI 1997); CIBSE Guide E, Fire Engineering (CIBSE 2003); BRE Report BR 258, “Design Approaches for Smoke Control in Atrium Buildings” (Hansel1

11、and Morgan 1994); and BRE Report BR 368, “Design Methodologies for Smoke and Heat Exhaust Ventilation” (Morgan et al. 1999). In the USA, guidance on calculation procedures for the design of smoke control systems in atria is described in NFPA 92B, Guide for Smoke Management Systems in Malls, Atria an

12、d Large spaces (NFPA 1995, 2000), and both versions of the ASHRAE smoke management design book (Klote and Milke 1992,2002). Approaches in the above design guides will be reviewed in this paper. This will give some information for engineers to W.K. Chow is professor and chair of architectural science

13、 and fire engineering and director of the Research Centre for Fire Engineering in the Department of Building Services Engineering, Hong Kong Polytechnic University, Hong Kong. J. Li is a lecturer in the College of Architecture and Civil Engineering, Beijing University of Technology, Beijing, China.

14、02005 ASHRAE. 395 select a workable guide by clarifjing some uncertainties. The UK guides and the US guides will also be compared. Approaches to be used for big atria in the Far East will be proposed. DESIGN OBJECTIVES Smoke management in an atrium normally includes management of smoke within the la

15、rge-volume space and any spaces that communicate with the large-volume space. The source of the smoke can be a fire within the large-volume space or within the communicating space. In the fire safety design of atrium buildings, smoke management can be utilized to satisSr one or more ofthe following

16、objectives (NFPA 1995, 2000; BSI 1997): To maintain a tenable environment in the means of egress from large-volume building spaces during the time required for evacuation To control and reduce the migration of smoke between the fire area and adjacent spaces To limit the rise of the smoke layer tempe

17、rature and toxic gas concentration and reduction of visibility To assist the firefighting personnel to approach, locate, and extinguish the fire In order to realize the objectives above, the following should be considered carefully (NFPA 1995,2000; BSI 1997; Klote and Milke 1992,2002; Milke 2002): G

18、eometric shape and dimensions of the atrium Building occupancy type and relative locations within the building Degree of separation between the atrium and the associ- ated floor area Egress routes from the large-volume space and any communicating space Relationship of the building to site boundaries

19、 Basically, as reviewed by Klote and Milke (1 992), smoke is controlled by imposing airflow opposite to the smoke flow, applying pressure differentials across a wall boundary, dilut- ing smoke, and extracting smoke. DESIGN FIRES The design fire is a key factor for fire hazard assessment. Specifying

20、an appropriate design fire is even more important for designing smoke management in large spaces. The calcu- lation of the quantity of smoke and heat produced by a fire requires knowledge of the fire, either its dimensions or its heat output. Design fire depends on the materials within an occu- panc

21、y. A database on combustibles should be worked out from full-scale burning tests. In addition, the possible size of a fire can also be deduced from analysis of fire statistics for different occupancies of interest. The design fires deduced from statis- tical analysis are based on implied acceptable

22、risk, which is different for various applications. Views from the general public should be surveyed to give an agreed design fire, though the Authority has specified some values (Morgan et al. 1999). The design fire can either be a steady burning fire with constant heat release rate or a transient g

23、rowing fire. The preferred choice of design fire would be a transient growing fire with heat release rate changes with time. The design of the means of escape and evacuation time for a particular building occupancy depends on that. This will allow the calculation of the increasing threat to occupant

24、s as time progresses, However, due to the lack of a database on fire growth rate in various occupancies and scenarios, steady burning fires have been used. Some design guides for smoke ventilation system are based on the principle of selecting a fixed fire size that would serve for almost all fire s

25、izes likely to be encountered in that class of occupancy. Adequate fire statistics would be helpful to determine the design fire. Based on UK statistics, a fire of 10 m2 area and 5 MW heat output is generally accepted as the design fire for retail premises with sprinklers. Other design fires commonl

26、y used for designing atrium smoke control systems in the UK (Hansel1 and Morgan 1994; Morgan et al. 1999) are: Sprinklered offices: 16 m2 area, 1 MW heat output Unsprinklered offices: 16 m2 area, 6 MW heat output Unsprinklered hotel bedrooms: the area of the design fire is taken to be the area of th

27、e largest bedroom with 1 MW heat output However, it is indicated in the NFPA 92B design guide that due to a lack of available data in North America, the design fire is only exceeded in a limited number of cases. A fixed design fire cannot be recommended in designing smoke management systems. A t2 pr

28、ofile was viewed as an appropri- ate approximation for a growing fire for design. The heat release rate Q (in kW) of the fire at the time (in s) after ignition is given by (1) 2 Q = lOOO(t/t,) . The constant tg is the growth time, which is defined by the time taken for the heat output to reach 1055

29、kW. The fires are conveniently classified as “slow,” “medium,” “fast,” and “ultra-fast, taking the values of growth time tg of 75 s, 150 s, 300 s, and 600 s, respectively. This kind of design fire is also recommended in CIBSE Guide E (CIBSE 2003) and BRFi report BR 368 (Morgan et al. 1999). The fire

30、 growth depends on the type of fuel and its arrangement. Note that using radiant energy calculations to determine whether adjacent combusti- bles are involved is described in NFPA 92B (NFPA 2000b). The fastest burning upholstered sofa and plastic goods stacked to a height of about 4.5 m give “ultra-

31、fast growth rates, while other upholstered furniture and lower piles of plastic goods give “fast” rates. Tightly rolled paper produces a “slow” growth rate. Experiments on burning computer workstations suggest “medium” to “fast” growth rates (NFPA 1995,2000). 396 ASHRAE Transactions: Research Based

32、on the experimental data, some suggested growth rates are (CIBSE 1997): Slow t2 fire: picture gallery, display area Medium t2 fire: dwelling, ofice, hotel reception, assem- bly hall seating, display area Fast? fire: shop, assembly hall seating, display area Ultra-fast t2 fire: warehouse DESIGN APPRO

33、ACHES The design method selected depends on the space where the smoke management system is required and the space where the smoke originates (NFPA 1995,2000; Hansell and Morgan 1994; Morgan et al. 1999; BST 1997). For fires occur- ring either on the atrium floor or in a compartment adjacent to the a

34、trium, preventing smoke spreading from the fire in the associated floor area to the atrium, and, subsequently, into other storys via the atrium, might be the main design objective. An easy way of achieving this is to provide an imperforate boundary between the atrium and the associated floor area wi

35、th adequate fire resistance. This method is frequently used in smoke management system design for an atrium. However, the atrium cannot be utilized as a functional space, and so this approach is not too welcomed by architects and building users. The entire atrium might be functioning as a fire-resis

36、ting light- well (Hansell and Morgan 1990, 1994; Morgan et al. 1999). Smoke venting might be a common approach for smoke control in an atrium. Removing smoke from the atrium can limit the accumulation of heat and smoke within the atrium or can arrest the descent of the smoke layer. When the corridor

37、s, stairways, or other means of egress are located within the atrium, or when the communicating spaces are not separated from the atrium, using smoke venting to maintain the smoke layer at an acceptable height above the highest walking level, or at a level higher than that of the highest opening to

38、the communicating space for a specified period, would give a safe egress (Klote and Milke 2002; NFPA 1995, 2000). Provided that a high enough clear layer can be maintained during the evacuation, other objectives for smoke control would become not so important for most atria with a high ceiling. Such

39、 a smoke management system can be effective, as demonstrated by full-scale burning tests reported by Yamana and Tanaka (1985). Two kinds of ventilation systems-mechanical and natu- ral ventilation-are used. Mechanical venting is known as “dynamic smoke exhaust” in Hong Kong (Fire Services Department

40、 1998) and is most commonly used in the US as specified by NFPA 92B. Natural ventilation, which is known as “static smoke exhaust” in Hong Kong (Fire Services Department 1998) is used in the UK and Australia (Klote A natural smoke venting system might be the most viable and cost-effective solution w

41、hen the quantity of smoke is large, the smoke is hot enough to give adequate buoyancy, and when there are no adverse wind effects (Hansell and Morgan 2000). 1994; Morgan et al. 1999, Klote 2000). More importantly, most atria in the Far East are located in the central core of a building complex and o

42、nly roof space is available for smoke exhaust. There might be problems in allocating places for installing fans and ducts in a mechanical smoke exhaust system. A high enough smoke layer interface height can be kept by opening a smoke vent if the exhaust capacity is equal to the rate at which smoke f

43、lows into the smoke layer. The base ofthe smoke layer is usually at a height chosen for safety reasons and agreed to by the local authority. “Cold” smoke is usually found in an atrium fire, and the thermal radiation effect is not signif- icant. For example, the smoke layer interface height is recom-

44、 mended to be not less than 3 m for public buildings, and 2.5 m for nonpublic buildings in BS 5588, Part 7 (BSI 1997) and BRE report BR 368 (Morgan et al. 1999), but the value can be down to 1.8 mas pointed out by Klote and Milke (1 992,2002). If the smoke layer interface height cannot be kept at a

45、high enough level, smoke exhaust can be designed to slow down the rate of descending to give a longer time for occupants to evac- uate. For designing smoke venting, the minimum design depth of the smoke layer is determined by both the thickness of the ceiling jet and the depth necessary to prevent “

46、plugholing.” “Plugholing” is the condition where air from below the inter- face is pulled through a relatively shallow smoke layer due to a high exhaust rate at that point (CIBSE 1997; NFPA 2000). The thickness of the ceiling jet has been reported by Beyler (1 986) to be in the range of 10% to 20% o

47、f the distance from the fire source to the top of the space (NFPA 2000). Because of the large volume of space in an atrium, a large quantity of entrained air would require a very large exhaust rate. Further, a cooler smoke layer would be formed. For economic reasons and in order to keep the smoke la

48、yer stable, limitations proposed by BRE guides for using the ventilation systems (applied to both natural and mechanical ventilation systems) through the atrium are: maximum mass flow rate of 150 to 200 kg/s and/or minimum smoke layer temperature of 20C above the ambient (Hansell and Morgan 1994; Mo

49、rgan et al. 1999). One of the other limits is usually reached when the height of rise above the fire opening exceeds 8 to 12 m. If the atrium is too high, other means of smoke control, such as a pressuriza- tion or depressurization system in different areas, should be considered. Smoke filling, known as “void filling” in BRE design guides (Hansell and Morgan 1994), is a passive approach for smoke management in atrium space when the fire size is rela- tively small compared to the size of the space or when the time taken to fill the space with smoke is relatively long compared to the time