Design principles for smoke ventilation in enclosed shopping centres

Transcription

1 Building Research Establishment Report Design principles for smoke ventilation in enclosed shopping centres H P Morgan, BSc, CPhys, MInstP, AIFireE, MSFSE and J P Gardner*, BSc *Colt International Ltd Fire Research Station Building Research Establishment Garston, Watford WD2 7JR

2 Prices for all available BRE publications can be obtained from: Construction Research Communications Ltd 151 Rosebery Avenue London, EC1R 4QX Telephone Facsimile BR 186 ISBN Crown copyright 1990 First published 1990 Reprinted with corrections 1991 Second reprint 1991 Republished on CD-ROM 1997 with permission of the Controller of HMSO and the Building Research Establishment, by Construction Research Communications Ltd Anyone wishing to use the information given in this publication should satisfy themselves that it is not out of date, for example with reference to the Building Regulations Applications to copy all or any part of this publication should be made to: Construction Research Communications Ltd, PO Box 202, Watford, Herts, WD2 7QG

3 Contents Page Foreword v Introduction 1 Chapter 1 General principles 3 Chapter 2 Design procedure for the mall smoke control system 6 Smoke reservoirs 6 Design fire size 6 Single-storey malls 7 Two-storey malls with small voids 8 Two-storey malls with large voids 10 Multi-storey malls 14 Calculating smoke temperature 15 Mall sprinklers 17 Flowing layer depth 18 Local deepening 18 Inlet air 19 Minimum number of extract points 20 Natural ventilation - area required per reservoir 21 Powered ventilation 22 Chapter 3 Large shop opening onto a mall 23 Chapter 4 Some practical design considerations 25 Factors influencing the design fire 25 The effect of wind on the efficiency of a smoke ventilation system 25 False ceilings in the mall 25 The use of a plenum chamber above a false ceiling in a shop 26 Stores with internal voids 27 Sloping malls 27 Assessment of effective layer depth 27 Smoke flow in low narrow malls 27 Basement service levels 27 Enclosed car parks 29 Smoke transfer ducts 29 Entrances within the smoke layer 30 Other situations 30 Chapter 5 Some operational factors 31 References 32 iii

4 Foreword This Report has been prepared by the Fire Research Station of the Building Research Establishment (BRE) and results from the latest scientific knowledge and practical experience in smoke movement and control. It is an update of Smoke control methods in enclosed shopping complexes of one or more storeys: a design summary published in 1979, and is based upon preliminary work carried out for BRE by Colt International Limited. The Report takes into account the comments of a Liaison Committee consisting of industrial and government representatives. The Fire Research Station is also grateful to the Society of Fire Safety Engineers, the Institute of Fire Engineers and the UK Chapter of the Society of Fire Protection Engineers for providing detailed comments which have as far as possible been included. The Fire Research Station acknowledges the assistance given by Colt International Limited in the preparation of the final Report. v

5 Introduction This Report is intended to assist designers of smoke ventilation systems in enclosed shopping complexes. Most of the methods advocated are the outcome of research into smoke control by smoke ventilation at the Fire Research Station, but also take into account the recommendations 1 of the Working Party on fire precautions in town centre redevelopment, as well as experience gained and ideas developed whilst the authors and their colleagues have discussed many proposed schemes with interested parties. The primary purpose of this Report is to summarise the design advice available from the Fire Research Station at the time of its preparation, in a readily usable form. As such, the Report is neither a detailed engineering manual nor is it a scientific review article. Perhaps most important of all, it is not a summary of the totality of approaches possible. New methods such as those based upon computational fluid dynamics, will be developed as time passes and there will always be special cases where existing alternative methods can be adopted. At peak times a shopping centre can be occupied by thousands of people, and some larger centres by more than a hundred thousand. A typical centre may comprise many individual shop units opening onto a common mall. Although the individual units may be separated from each other by a dividing wall of fireresisting construction, usually the shop is either open fronted or only separated from the mall by a glass shop front. This means that the public areas of the entire centre can be effectively undivided. Means of escape from within each shop unit will, in general, be specified (eg BS 5588 Part 2 2 ) in the same way as for shops which are not part of an enclosed complex. This means that escape from within the shops is specified as if the mall were as much a place of safety as the usual open-sky street. Unfortunately, the mall is a street with a roof, and so cannot be regarded as being as inherently safe as an open-topped street. People escaping from a shop into a mall will still need to travel along the mall before exiting to a true place of safety. It follows that the mall constitutes an additional stage to the escape route, which needs to be protected from the effects of fire and smoke. Ideally, it should approach the same level of safety as a street for as long as people need to escape through it even if smoke enters the mall from the shop on fire. Details of means of escape provisions for the malls can be found elsewhere 1. A shopping complex is a public building and the occupants will be a cross section of the community including the elderly, children and the disabled. They will not necessarily be familiar with the building, or perhaps more importantly, with all the escape routes that might be provided for them. In many types of building it is widely recognised that people will commonly try to escape by the same route they had used to enter the premises (see for example, Canter 3 ). It follows that escape via the malls must be assumed, even where other exits are provided 1. In this situation a long evacuation period can be expected. A detection and alarm system is required to give early warning of a fire, and sprinklers are needed to control its spread. Statistics of fire deaths show that the majority of fatalities are due to the effects of smoke. The ideal option would be to prevent any smoke from a shop fire entering the mall at all. In the majority of cases, it would either be very difficult or extremely expensive to fit a separate smoke extraction system to each and every shop, however small 1. Note that large shops are, however, an exception to this rule and the provision of smoke ventilation systems for such shops is discussed in Chapter 3. Occasionally circumstances dictate that smoke ventilation fitted to each shop unit, even small units, is the most viable option for protecting part or all of a mall. (This can apply, for example, when an old complex is being redeveloped, but the mall is too low and/or too narrow to allow the installation of a viable smoke ventilation system in the mall itself). There have been several examples of this. Nevertheless it remains generally true that this option is rarely found to be appropriate for most malls. Occasionally, one still hears the suggestion that the mall should be pressurised to prevent smoke moving from a shop into the mall. This is not usually a viable option by itself where the opening between shop and mall is large (eg an open-fronted shop, or a shop whose glazing has fallen away in whole or part). This is because the airspeed needed from mall into shop in order to prevent the movement of smoky gases the other way through the same opening, can vary between 1/2 and c. 2 ms -1 depending on fire size, gas temperature etc. All of this air must be continuously removed from within the shop unit in order to maintain the flow. The quantities of air-handling plant required will exceed the size of smoke ventilation systems for most typical shop-front openings. Where smoke from a fire in a single shop unit could spread rapidly via the malls through the entire centre, a smoke ventilation system in the malls is essential to ensure that escape is unhindered, by ensuring that any large quantities of thermally buoyant smoky gases can be kept separate from (ie above) people who may still be using escape routes through the malls. Therefore, the role of a smoke ventilation system is principally one of life safety. It should also be remembered, however, that firefighting becomes both difficult and dangerous in a smoke-logged mall. It follows that to assist the fire services, the smoke ventilation system should be capable of performing its design function for a period of time longer than that required for the 1

6 public to escape: so an immediate attack on the fire can be made after the arrival of the fire brigade. Guidance on design principles for smoke ventilation systems was summarised in a report by Morgan 4 published in This was based on the knowledge available at that time. Since then a great deal of relevant research has been carried out, which for the most part has confirmed the guidance given in the original report, but has in some areas highlighted the need to modify that guidance. A great deal of practical experience has been gained in designing such systems. Also in the intervening years, shopping centres themselves have become larger and more intricate with many open levels, interconnecting voids, sloping floors and atrium features. These can result in a very complex path of smoke flow from the shop in which the fire starts to the eventual point of extraction. The purpose of this Report is to update the guidance available in the earlier design summary 4 to reflect these changes, to assist designers of smoke control systems in enclosed shopping centres. As was the case with the previous work, this Report only gives guidance in line with current knowledge and generally accepted practice. The guidance is based on results of research where possible, but also on the cumulative experience of design features, required for regulatory purposes, of many individual smoke ventilation proposals. Many of these design features have been evolved over a number of years by consensus between regulatory authorities, developers and fire scientists, rather than by specific research. Such advice has been included in this Report with the intention of giving the fullest picture possible. It is therefore likely that some of this guidance will need to be modified in the future, as the results of continued research become available. A Code of practice 5 for enclosed shopping centres is currently being prepared by the British Standards Institution (BSI). The aim of this Report is to provide guidance only on design principles for smoke ventilation and it is hoped to support the Code rather than pre-empt it. The Report cannot cover all the infinite variations of shopping centre design. Instead it gives general principles for the design of efficient systems, with simplified design procedures for an ideal model of a shopping mall and then further guidance on frequently encountered practical problems. Because the design procedures are of necessity simplified, the Report also gives their limitations so that, when necessary, a more detailed design by specialists can be carried out. A Code of practice for atrium buildings is also being prepared by the BSI. An atrium can be defined as any space penetrating more than one storey of a building where the space is fully or partially covered. Most atria within shopping centres may be considered as part of the shopping mall and treated accordingly. Where atria have mixed occupancies (eg shops and offices) then reference should be made to the above document, when available, or specialist advice sought. 2

7 Chapter 1 General principles Smoke from a fire in a shop rises in a plume to the ceiling. As the plume rises, air is entrained into it, increasing the volume of smoke and reducing its temperature. The smoke spreads out underneath the ceiling and forms a layer which deepens as the shop begins to fill with smoke. As the layer deepens, there is less height for the plume of smoke to rise before it reaches the smoke layer. Less air is being entrained, with the result that the temperature is higher on reaching the smoke layer. As this continues, the increasing smoke layer temperature at some point will operate the sprinklers. The fire may shatter the shop front glazing (if present) unless that glazing is fireresistant. The smoke will then flow out of the shop into the mall (Figure 1). The change of direction as the smoke flows out of the shop front results in increased mixing with the surrounding air. There is so much mixing that, except close to the fire itself, the hot smoky gases can be regarded as consisting entirely of warmed air, when calculating flow. The quantity of smoke particles (and hence the optical density and visibility through the smoky gases) produced in a fire, depends strongly on the nature of the burning material 6. The quantity of hot gases carrying these particles is, however, mainly dependent on the size and rate of burning (related to the heat output per second) of the fire. Increasing the visibility through these gases requires their dilution with clean air, but improving visibility (and reducing toxicity) by dilution alone in a mall to safe levels after smoke has entered, is not a practical proposition. One suggested safe level of visibility 7 (about 8 metres rather a short distance for a mall) would require the hot smoky gases to be diluted by a mass ratio possibly larger than 300, even after the gases leave the shop of origin. It follows then that hot smoky gases should always be kept spatially separated from escaping people. One can state as a general principle that air will mix into a rising stream of hot smoky gases in large quantities, but will not mix appreciably into a horizontally flowing stream of hot smoky gases except under special (and usually local) conditions. Smoke flowing from the shop onto the mall will rise to the mall ceiling (Figures 1 and 2). Air will mix into the smoke as it rises. If no smoke control measures are present, the gases will then flow along the mall as a ceiling layer (Figure 2a) at a speed 8 typically between 1 and 2 m/s. This is faster than the probable escape speed of pedestrians in a crowded mall 9. When the smoke reaches the end of the closed mall it will dip down to a low level and be drawn back towards the fire 10 (Figure 2b). If the end of the mall is open, even light winds blowing into or across the opening will cause severe local disturbance and mixing, and once again smoke at low level will be drawn back towards the fire 10 (Figure 2c). Hence a single-storey mall could become smoke logged within minutes. An unsprinklered fire in a single-storey shopping centre in Wolverhampton 11, is thought to have caused a 100-metre-long mall to become untenable within one minute. Similarly short times can be expected to apply to the upper floors of a multi-storey mall. Since it is not usually practical to prevent smoke entering the mall, except for larger shops, a mall smoke control system is necessary to control and remove heat and smoke. The Fire Research Station has extended the ideas developed during its earlier work on the fire venting of factories 12 to apply to malls 8 in order to use the buoyancy inherent in fire gases to keep those gases safely above the heads of people in the malls (Figure 3). There are three essential features of a smoke ventilation system, without any one of which it will not function effectively: 1 There must be some means of forming a smoke reservoir to prevent the lateral spread of smoke, Figure 1 Smoke spread and main entrainment sites in single and two-storey malls 3

8 Figure 2a Creation of a moving smoke layer beneath the ceiling of an unventilated mall, showing movement of the displaced air Figure 2b Recirculation of smoke in an unventilated and closed mall Figure 2c Mixing of smoke into the air being drawn into an open-ended mall, caused by wind Figure 3 Principles of system needed to contain smoke in a well-defined layer (section along mall) which would result in excessive loss of buoyancy. This reservoir must be designed to contain the smoke layer base well above head height. Generally speaking, malls with high ceilings or rooflights will allow a deeper smoke reservoir and hence a more effective smoke ventilation system than in low, narrow malls. 2 There must be extraction of smoke within the reservoir, to prevent the smoke layer building down below the design depth. This can be natural, 4 buoyancy driven ventilation or mechanical extraction, depending on the circumstances. The rate of the exhaust must equal the rate at which the smoke enters the reservoir from below. 3 Since the gases being extracted consist almost entirely of air that has mixed with the original fire gases, fresh air must enter the mall to take its place. It must enter at a rate equal to the rate of extraction of smoke and at a low enough height not to mix prematurely with the smoke.

9 In a fire most of the important factors are time dependent: the time for a fire to grow from ignition to the design fire size, time for the mall to smokelog and time needed for evacuation. Currently there are no reliable data on fire growth rates in retail premises. Smoke filling times are known to be rapid but are not always easily quantified. The time needed for evacuation is unknown but it could be considerably longer than the 2.5 minutes escape time used to size exit widths to cope with the peak flow rates during this evacuation period. There is considerable anecdotal evidence within the UK fire services supporting this view. These uncertainties lead to many problems in designing a time dependent smoke ventilation system. The design principles given in this Report are based instead on steady state conditions. A design fire is used which has a low probability of being exceeded, and the smoke ventilation system is designed to remove the mass flow rate of smoke necessary to maintain clear conditions in the mall for escape. Sprinklers in shops are an essential part of the smoke ventilation design in order to prevent the fire growing beyond the design fire size. There are other essential factors that should be borne in mind when designing a smoke control system. When the mall has more than one level, it is also necessary to restrict or channel smoke on the lower level in order to restrict entrainment into the plume rising through the upper level. Once the smoke has entered a ceiling reservoir it will flow towards the extraction points. Since this horizontal flow is driven by the gases own buoyancy, there will be a minimum depth for any particular combination of smoke temperature and mass flow rate. This is discussed in greater detail in Chapter 2, but this minimum depth will set a limit to the design for the smoke reservoir, which cannot be shallower. The capacity of the extraction system depends mainly on the height the gases have risen to the layer base, and not at all on the area of the floor or the volume of the space. Therefore a simple percentage rule for natural venting, such as 3% of the floor area, or an air change rate, such as six air changes per hour for powered ventilation, are misleading. 5

10 Chapter 2 Design procedures for the mall smoke control system The following sections outline a general procedure which can be followed when designing the mall smoke control system. First plan the positions of the smoke reservoirs, calculate the mass flow rate of smoke entering the reservoir, and either the ventilation area of a natural ventilation system or the extract rate of a powered system to remove the same amount of smoke. These procedures have been developed from idealised models of shopping malls studied at the Fire Research Station. Commonly encountered variations are discussed further in Chapter 4. Smoke reservoirs No smoke layer has a perfectly defined interface with the colder, clearer air below; there is always a small amount of cross mixing. This cross mixing has no significant effect on the quantity of smoke in a horizontal ceiling layer but can cause progressive loss of visibility in the air beneath. This loss of visibility occurs more rapidly when: a ceiling layer is cool (indeed very cool gases will not persist as a layer) the hot smoke and the air beneath have a high relative velocity turbulence disturbs the interface Heat is lost from the smoke layer by radiation downwards and by radiation and conduction to the surfaces of the reservoir. To prevent excessive heat loss the size of an individual smoke reservoir within the mall is usually taken to be limited to 1000 m 2 (or 1300 m 2 if mechanical extraction is used as its efficiency is less susceptible to heat loss). This recommendation evolved during the early 1970s and is perhaps best understood by noting that in many industrial simple undivided compartment applications of smoke ventilation, it has long been the practice to limit reservoir areas to between 2000 m 2 and 3000 m 2. The smoke reservoir can often be formed by the downstand fascia of the shops, combined with screens at intervals along the mall (Figure 4). The fascias are then necessary, not to prevent the smoke flowing into the mall from a fire in the shop, but to prevent smoke contained in the mall reservoir flowing into the shops adjoining the mall. If there are raised rooflights above the malls, these can often be utilised as smoke reservoirs. Although the height through which the smoke may have to rise to a ventilator installed at the top of a high rooflight may be greater than for lower malls, provided that the smoke is allowed to build down to the same level, the mass flow rate of smoke entering the reservoir is no greater, but the deeper reservoir of smoke will produce a greater buoyancy pressure at the ceiling. The screens which form the smoke reservoir should be arranged so that smoke from a fire in any shop can only flow into one reservoir. Even with a smoke reservoir limited to 1000 m 2, excessive cooling and/or downward mixing can occur in stagnant regions of the smoke layer. To prevent this the extraction outlets, be they natural or mechanical, should be distributed over the reservoir so as to prevent stagnant regions being formed. Potentially stagnant regions of the smoke reservoir can sometimes be avoided by using smoke transfer ducts (see Chapter 4). Design fire size Before any smoke ventilation system can be designed it is essential to determine a suitable size of fire, for design purposes. This fire size then forms the basis of a smoke ventilation system design. In a mall, smoke from shops of up to 1000 m 2 area (if the mall is naturally ventilated) or up to 1300 m 2 (if the mall has extract fans) can enter the mall reservoir, giving a total area affected by smoke similar to the long-standing practice stated above. The maximum distance between screens forming the boundary of the reservoir should be 60 m. This distance recommendation follows earlier considerations 1 (confirmed in this form by the Liaison Panel consulted in preparation of the present document), and derives from concern over the distance people should be expected to walk below a smoke layer while escaping. For complex geometries of smoke reservoirs specialist advice should be sought. Figure 4 Smoke reservoir in mall with a ceiling height greater than that in shops (section across mall) 6

11 Ideally the design fire should show the physical size and heat output of the fire increasing with time, allowing the growing threat to occupants to be calculated as time increases. Unfortunately there is no available research, at the time of preparing this Report, which allows assessment of the probability distribution describing the variation of fire growth curves for retail areas. Clearly, one does not want an average fire for safety design, since typically half of all fires would grow faster. It is much simpler to assess the maximum size a fire can reasonably be expected to reach during the escape period, and to design the system to cope with that. Note that even here, the statistical evidence is not strong (see for example Morgan and Chandler 13 ) for shopping malls. Further research is currently in hand to improve this statistical basis. It may be necessary to raise the smoke layer base above this for practical reasons. This will result in an increased mass flow rate of smoke due to the additional entrainment, but as the smoke layer base is higher, a greater degree of safety will have been achieved. Shop fascias should extend below this height otherwise the layer base would be low enough to enter unaffected shops. Indeed, fascias serve no other useful smoke controlling function, except as part of a separate smoke extraction system within a shop, or to contain smoke from a very small fire. Having established the clear layer height in the mall, the mass flow rate of smoke can then be calculated. Recent work by Hinkley 15 has confirmed the rate of entrainment of air into a plume of smoke rising above a fire as: It follows from the foregoing that there is a strongly subjective element in assessing what fire size is acceptably infrequent for safety design purposes. A 12 m perimeter (3 m 3 m) 5 MW sprinkler controlled fire has become the accepted basis in the UK for a smoke ventilation system in a sprinklered shopping centre 1,4,8. where M = 0.19 P Y 3/2 (1) M = mass flow rate of smoke entering the smoke layer within the shop (kg/s) P = the perimeter of the fire (m) Y = the height from the base of the fire to the smoke layer (m) Some factors affecting the design fire size are discussed in Chapter 4. The design principles outlined in this Report are based upon a 12 m perimeter 5 MW fire. Should a different design fire be considered for whatever reasons, the equations, figures etc given in this Report may no longer apply and advice should be sought from experts. Other fire sizes have occasionally been specified by designers, for both sprinklered and unsprinklered shops. The problem of unsprinklered shops has been discussed in more detail by Gardner 14, who has shown the importance of considering flashover in such units and the consequent need to consider potentially very large fires. Single-storey malls The minimum height of the smoke layer base must be 2.5 m from the mall floor to ensure safety, and preferably at least 3 m in a single-storey mall (Figure 5). There is no information available to show how Equation 1 (or any current alternatives) should be modified to allow for the effects of sprinkler-spray interactions. Consequently it is used here unmodified. Experiments have shown 16 that the smoke flowing from the shop onto the mall becomes turbulent with increasing mixing of air. The mass flow rate of smoke entering the reservoir is approximately double the amount given by Equation 1, where Y is now the height from the fire to the base of the mall smoke layer (Figure 5). Figure 6 shows the mass flow rate of smoky gases entering the layer at different layer heights in a single-storey mall. This can be calculated from: M = 0.38 P Y 3/2 (2) The Fire Research Station is currently studying entrainment into smoke flow from compartments. The purpose of this work is to determine more accurately the influence of such factors as compartment opening geometry, the presence of a downstand fascia and balcony/downstand combinations. It follows that Equation 2 may be superseded in due course. Figure 5 Smoke ventilation in a single-storey mall If the clear layer in the mall is much higher than the shopfront opening, then the plume of smoke rising to the smoke layer will be similar to a two-storey shopping centre (Figure 7). Where this height difference (h r ) between the shopfront opening and the smoke layer base is more than 2 m (Figure 7) the mass flow rate should be obtained using the procedure for two-storey shopping centres given later in this chapter on page 10. 7

12 Figure 6 Figure 7 Rate of production of hot smoky gases in a single-storey mall from a 5MW fire Additional entrainment with the smoke base well above the shop front opening Two-storey malls with small voids When a fire occurs on the lower level of a two-storey mall, a plume of smoke has further to rise before entering the smoke reservoir. This results in greater entrainment of air, hence a larger quantity of smoke. The smoke layer on the upper level will then be cooler and less well defined. In this case the smoke layer should be not less than 3 m above the upper floor level and preferably more than 3.5 m. Again, shop fascias should extend below this height otherwise the layer base would be low enough to enter unaffected stores. In a shopping centre which has small voids connecting levels, smoke from a fire in a lower level shop will flow out of the shop and spread in a complicated horizontal circulation pattern beneath the ceiling (ie beneath the upper deck) (Figure 8). Where smoke reaches the edge of a void linking the two levels, some will flow over the edge producing an extensive plume above each void, rising through the upper level. Air mixes into these plumes, resulting in extremely large quantities of very cool gas collecting in the upper level ceiling reservoir. This in turn reduces the efficiency of buoyancy-driven venting as well as increasing downward mixing from the ceiling layer. To minimise this mixing of air into the plume, smoke screens of at least 1.5 m depth 17 (actuated by smoke detectors or as permanent features) should be hung below the lower level ceiling (ie below the upper deck) in order to restrict the lateral spread of smoke and ensure that all the smoke from a fire passes through only one void (Figures 9 and 10). The approach outlined in this section will only apply when the void is small enough in relation to the mall, and the screens are arranged so that smoke can flow freely into the void around its perimeter in a swirling pattern (Figure 9). The perimeter of the rising plume, which strongly affects the rate of the mixing of air into the plume, then depends only on the size of the void. Experiments with scale models 17 suggest that the largest void consistent with this type of smoke control system would have a perimeter of between 35 and 45m. There are no adequate theories to relate the quantities of smoke entering the reservoirs, to the height from the upper floor to the smoke base. Figure 11 shows the values of the mass flow rate (M) entering the reservoir for different heights of ceiling reservoir smoke base above the upper floor, and for three different sizes of void (indicated by their respective perimeters), based on the voids used in the scale-model experiments. Other void perimeters must be inferred by interpolation. Figure 11 applies to a 5 MW fire occurring in a shop on the lower levels, and is derived from experiments on a one-tenth scale model 17 representing a mall of 5 m floor-to-floor height. Note that extrapolating too far 8

13 Figure 8 Plan view of smoke circulation pattern below upper deck. Without smoke-restraining screens Figure 9 Plan view of smoke circulation pattern below upper deck. With smoke-restraining screens Figure 10 Smoke and air movements in two-storey mall with small connecting voids 9

14 Figure 11 Mass of smoky gas entering ceiling reservoir per second from a 5 MW fire small voids will risk an increasing (but unknown) error, since these curves are purely empirical. The fire on the upper level can be treated for design purposes as a fire in a single-storey mall. If the smoke ventilation system will cope with the smoke from a fire on the lower level, it should usually be able to cope with fires on the upper level. Note, however, that upper level fires can result in higher temperature smoke in the reservoir. Two-storey malls with large voids When a shopping centre has large voids through the mall connecting levels, the smoke flow is somewhat different from that given earlier for small voids. An idealised model of a two-storey mall with large voids is shown in Figure 12, where the upper pedestrian walkways take the form of a balcony on either side of a large void. Again, the worst case is when a fire occurs in a shop on the lower level. Smoke will flow from the shopfront to the underside of the balcony. It will then flow forward to the balcony edge, as well as sideways beneath the balcony (Figure 12). Because of this sideways flow beneath the balcony, there may be an extensive line plume flowing upwards from the balcony edge like an inverted waterfall. Unless the length of this line plume flowing over the edge of the balcony is limited, the extensive 10 entrainment of air that takes place may result in very large quantities of cool smoke. Screens installed beneath the balcony to channel the smoke from the shopfront to the edge of the balcony will restrict lateral spread of smoke (Figure 13), thereby producing a more compact line plume. This results in less entrainment of air and therefore a more manageable quantity of hotter smoke. These screens can be permanent structures or, alternatively, screens activated automatically on smoke detection. Recent, as yet unpublished, research 18 suggests that channelling screens may be unnecessary if the balcony projects no more than 1.5 m beyond the shop front. The minimum depth required for a pair of these screens to channel all the smoke is dependent on their separation at the void edge (L). Some values for a 5 MW fire in a typical mall are given in Table 1a for a balcony with a downstand at the void edge, which is deep in relation to the approaching layer, and Table 1b for a balcony without a downstand at the void edge (based on Morgan 19 ). If sideways spillage occurs in quantity, the visibility on the upper level can be significantly worse. The mass flow rate entering the reservoir can, in principle, be related to the height from the balcony to the reservoir layer base and to the plume length (ie to the screen separation) by the theory of Morgan and Marshall 20. Unfortunately, this theory applies to a plume rising through free space, where the air outside the plume is uniformly at ambient temperature.

15 Figure 12 Smoke spread beneath a balcony producing a long line plume The smoke layer in a mall ceiling reservoir does not have a well defined base (especially in a two-storey mall where there might be a deep layer of cool smoke). Even below the nominal layer base (ie d 1 below the ceiling, Figure 13, which corresponds closely to the visible layer base 17 ), there is a temperature excess relative to the temperature of the incoming air which increases with height (Figure 14). To apply the theory 20 to a calculation of the mass flow rates of smoke entering the reservoir, one must introduce a correction factor for the smoke layer depth in a reservoir. Experiments with flat roofed models 21 have shown that for calculating plume entrainment, the effective layer depth (d 2 ) is 1.26 times the nominal and visible layer depth (d 1 ) which has been chosen for reasons of visibility and safety. In practice many shopping malls have roofs which are not flat, and it is necessary to assess the result of this on the effective layer depth. This is examined more closely in Chapter 4. Results from such calculations for a number of values of channelling screen separation, are shown graphically in Figure 15. These results include an allowance for entrainment of air into the ends of the plume. This correction method follows that of Morgan and Marshall 21, which supersedes their earlier approach 20. Once the height of the nominal layer base (h-d 1 ) has been chosen on safety grounds, and the channelling screens separation L (and hence also channelling screen depth using Tables 1a or 1b) has been chosen on practicability grounds, (eg such that the screens contact the walls separating the shop) then Figure 15 can be used to find the mass flow rate of smoke entering the reservoir. It should be noted that once the height of the layer base (h-d 1 ) has been selected, d 1 can be used instead of d 2 for greater simplicity in interpreting Figure 15 and in designing the consequent smoke extraction. This results in an overdesign of the extract capacity, which 11

16 Figure 13 The use of smoke-channelling screens to produce a compact rising plume thus errs on the side of safety. In experiments values of (h-d 2 ) as low as a full-scale equivalent 0.75 m were obtained. For a very deep layer, one sometimes finds that (h-d 2 ) can sometimes be negative. This corresponds to a plume moving downwards which is impossible in this context and shows that the method breaks down under these conditions. It follows that wherever (h-d 2 ) is less than 0.75 m, it would be safe to use (h-d 1 ) instead for estimating entrainment. The results given in Figure 15 are representative of typical shop/balcony geometries. In practice the shopfront geometry, presence or absence of a deep downstand fascia and a balcony will affect the mass flow rate of smoke. For example, many malls will have the upper walkway set back above the shop units on the storey below with no balcony projecting beyond the lower shop front. In such designs the shop walls themselves act as channelling screens. Where such a shop has a downstand fascia, the plume s rise (h-d 2 ) should be measured from the bottom of the downstand. Figure 15 can again be used to estimate the entrainment into the plume, but a more precise calculation for this case is feasible 22. Similarly, results given in Figure 15 are for a line plume rising through an upper level where air is entrained on both sides. If the plume is rising against a vertical surface (such as a wall or shopfront on the level above), then air will only be entrained into one side. Recent research work 22,23 has enabled a more detailed analysis of the fire compartment conditions and subsequent plume entrainment to be carried out taking into account these factors. This fire engineering approach is of necessity more complicated and needs individual consideration. An alternative method of calculating the entrainment into the line plume is due to Thomas 24. This treats the plume in a 'far plume' approximation apparently rising 12

17 Table 1a Minimum depth of channelling screens downstand at void edge Screen separation at edge L (m) Minimum screen depth (m) Note: The minimum depth is that below the lowest transverse obstacle Table 1 b Minimum depth of channelling screens no downstand at void edge Screen separation at edge L (m) Minimum screen depth (m) Note: The minimum depth is that below the lowest transverse obstacle from a line source of zero thickness some distance below the void edge. The relevant formula is: gql 2 1/3 M = 0.58ρ (z + ) (3) ρc p T (z + 2 ) 2/3 1 + L where M = mass flow of smoky gases passing height z (kg s -1 ) ρ = density of warm gases at height z (kg m -3 ) Q = heat flux in gases (kw) L = length of void edge past which gases spill (m) C p = specific heat of air (kj kg -1 K -1 ) T 1 = absolute ambient temperature (K) = empirical height of virtual source below void edge (m) z = height above void edge (m) Morgan s re-analysis 25 of Law s earlier paper 26, concluding that the effective depth d 2 of the reservoir layer should be used when the plume rises within a closed space such as a mall, should apply equally to Thomas s work. This means that z should be taken to be (h-d 2 ), as earlier in this section. Again from Morgan 25, one can take = 0.3 times the height of the compartment (ie shop unit) opening, although comparisons between Equation 3 and the modified form 22 of Morgan and Marshall s method 20 suggest that the value of may be sensitive to the parameters of the horizontal flow approaching the void. In this context it is noteworthy that Law 26 derived a value of = 0.67 times the height of the compartment based on different experimental data. It should be realised that the derivation of Equation 3 limits its application to scenarios where smoky gases issue directly from the compartment on fire, with a balcony projecting beyond. For two-storey malls Equation 3 and Figure 15 should give broadly similar 13

18 Figure 14 A typical temperature profile for a reservoir layer results, but under some circumstances significant discrepancies can occur because of the apparent variability of. Nevertheless, Equation 3 can be a very useful way of estimating entrainment for geometries departing significantly from Figure 15. Multi-storey malls From the previous section it can be seen that when a fire occurs in a ground floor shop and rises through an upper level, a very large quantity of smoke is produced. If this is extended to a three-storey mall the result is an impracticably large quantity of very cool smoke. Therefore it is not usually possible to design a practical smoke ventilation system which allows smoke from more than the top two levels of a multi-storey mall to rise up through the mall and maintain a clear layer for escape on the upper level. The limiting factor here is not the height to the top of the smoke reservoir, but the height of rise to the smoke layer base. A multi-storey mall can instead be treated as a stack of single-storey malls, with each level having a separate smoke ventilation system. Clearly this technique can also be used in a two-storey mall if so desired. Figures 16 to 18 illustrate in schematic form a mall whose upper floor (two levels only are shown in the figures) is penetrated by voids which leave a considerable area for pedestrians. On the lower level there is a large area situated below this upper deck. If screens (activated by smoke detectors or as permanent features) are hung 14 down from the void edges, the region below the upper deck can be turned into a ceiling reservoir similar to that of a single-storey mall, albeit a more complicated geometry. This reservoir can then be provided with its own smoke extraction system. Other screens can be positioned across the mall to limit the size of this reservoir, as for a single-storey mall. The screens around the voids will, in general, be fairly close to potential fire compartments (ie shops). Being close, smoke issuing from such a compartment will deepen locally on meeting a transverse barrier. The depth of these screens should take into account local deepening 27 see page 18. Smoke removed from these lower level reservoirs should usually be ducted to outside the building but can be ducted into the ceiling reservoir of the top floor ( Figure 18). The mass flow rate of smoke exhaust for the top floor can be calculated as if it were a single-storey mall. There will often be some small smoke spillages under the void screens, which will contribute to a fogging of the upper levels. This can be controlled by permitting some ventilation of these upper levels to operate, to flush out this stray smoke. To minimise such spillages and limit the amount of smoke below the ceiling layer on the affected level, it is desirable to simplify the geometry of the ceiling reservoir where possible.the

19 Figure 15 Mass of smoke entering ceiling reservoir per second from a 5MW fire large voids void screens tend to split the smoke flow into separate streams within the reservoir (Figure 16). These streams can meet further along the reservoir, as shown. Such opposed smoke flows produce turbulence and downward mixing of smoke into the air below. It follows that it is an advantage to keep a simple reservoir geometry. It is also important to provide the full flushing clean air inflow below the ceiling reservoir at the affected level. A lesser amount of flushing air flow is desirable on the top level when lower reservoirs are vented to a common top level reservoir (Figure 18). This can easily be provided by increasing the smoke extraction from the top reservoir by 10% above that needed to remove all the smoke arriving from below. Calculating smoke temperature The temperature rise of the smoke layer, above ambient, is given in Table 2, for a 5 MW fire (ignoring any further cooling) and can be calculated from: θ = Q MC p where Q = heat flow rate (kw) θ = temperature rise of the smoke layer above ambient ( C) C p = specific heat capacity of the gases (kj/kgk) M = mass flow rate of smoky gases (kg/s) (4) High smoke layer temperature will result in intense heat radiation causing difficulties for people escaping 15

20 Figure 16 Schematic plan of multi-storey mall with a smoke reservoir on each level Figure 17 Schematic section of a two-storey mall with a smoke reservoir on the lower level 16 Figure 18 Schematic section of a two-storey mall with lower smoke reservoir venting into the upper reservoir

21 Table 2 Volume flow rate and temperature of gases from a 5 MW fire (ignoring cooling) Mass flow rate Temperature of gases Volume rate of extraction (Mass rate of extraction) above ambient (at maximum temperature) kg/s C m 3 /s beneath the smoke layer in the mall. To reduce the intensity of heat radiation the smoke layer temperature in the malls should be less than 200 C. In general a higher clear layer beneath the smoke layer will lead to more air being entrained into the rising smoke plume and therefore lower smoke temperatures. If the height of the mall is restricted, then it may not be practical to increase the clear layer height in order to reduce the smoke layer temperature, in which case consideration may be given to installing sprinklers into the malls specifically to reduce the smoke layer temperature by sprinkler cooling. This is discussed further in the following section. Mall sprinklers Sprinklers in shops are an essential part of the smoke ventilation design in order to prevent a fire growing beyond the design fire size. Sprinkler operation in the malls will lead to increased heat loss reducing the buoyancy of smoke, which in turn can contribute to a progressive loss of visibility under the smoky layer. However, gases sufficiently hot enough to set off sprinklers will remain initially as a thermally buoyant layer under the ceiling. When the fire occurs in a shop, operation of sprinklers in the mall will not assist in controlling it. If too many sprinklers operated in the malls sprinklers in the shops could become less effective as the available water supply approaches its limits. Malls should be sprinklered if they contain sufficient combustibles to support a fire larger than the design fire size of 5 MW, 12 m perimeter, during their operational lifetime. Note however that sprinklers installed at high level in a multi-storey mall are unlikely to operate unless the fire size reached is much larger than this. Sprinkler cooling can be used in the malls to reduce the smoke layer temperature to below 200 C, above which heat radiation from the layer is likely to impede escape beneath. A natural ventilation system relies on the buoyancy of the smoke for extraction, therefore if sprinkler cooling is underestimated, the use of unrealistically high smoke temperatures could lead to the system being underdesigned. Conversely a powered extract system, to a reasonable approximation, removes a fixed volume of smoke irrespective of temperature. Therefore if the extent of sprinkler cooling is overestimated the system could be underdesigned. The heat lost from smoky gases to sprinklers in the mall is currently the subject of research although data suitable for design application is not yet available. An approximate calculation approach can be used, as follows: If the smoke passing a sprinkler is hotter than the sprinkler operating temperature that sprinkler will eventually be set off. The sprinkler spray will then cool the smoke. If the smoke is still hot enough, the next sprinkler will operate, cooling the smoke further. A stage will be reached when the smoke temperature is insufficient to set off further sprinklers. The smoke layer temperature can thereafter simply be assumed to be approximately equal to the sprinkler operating temperature if natural vents are used. The cooling effect of sprinklers in the malls can be ignored in determining the volume extract rate required for fans. This will err on the side of safety. Alternatively this cooling and the consequent contraction of the smoky gases can be approximately estimated on the basis of an average value between the sprinkler operating temperature and the calculated 17

22 initial smoke temperature, since a fire may occur close to at least one of the extract openings, whereas the other openings may be well outside any probable zone of operating sprinklers. The number of potential hot and cool intakes must be assessed separately for each shopping complex when calculating the average temperature of extracted gases. If the sprinkler operating temperature is set high enough and is above the calculated smoke layer temperature, then sprinkler cooling in the malls can be ignored. Note that the effect of sprinkler cooling is to reduce the heat flux Q without significantly changing the mass flux. It follows that once a new value of θ has been estimated, the new heat flux can be found using Equation 4. Note that sprinklers must not be installed close to ventilator extract positions. Flowing layer depth Smoke entering the ceiling reservoir will flow from the point of entry towards the vent or fans. This flow is driven by the buoyancy of the smoke. Even if there is a very large ventilation area downstream (eg if the downstream roof were to be removed) this flowing layer would still have a depth related to the width of the mall, the temperature of the smoke and the mass flow rate of smoke. Work by Morgan 19 has shown that this depth can be calculated for unidirectional flow under a flat ceiling, as follows: where MT c 2/3 (5) d 1 = γ θ 1/2 W d 1 = flowing smoke layer depth (m) T c = absolute temperature of the smoke layer (K) θ = temperature rise of the smoke layer above ambient ( C) W = channel width (m) γ = downstand factor 36 if deep downstand is present at right angles to the flow; downstand factor 78 if no downstand is present at right angles to the flow M = mass flow rate of smoky gases (kg/s) The resulting values of layer depth for different reservoir widths and mass flow rates of smoke are shown in Table 3a. This ignores the effects of cooling in the layer, therefore if sprinklers are installed in the mall Equation 5 should be used after estimating the effects of such cooling (see previous section). Each depth shown in this table is the minimum possible regardless of the smoke extraction method employed downstream: consequently it represents the minimum depth for that reservoir. The depth must be measured below the lowest transverse downstand obstacle to the flow (eg structural beams or ductwork) rather than the true ceiling. Where such structures exist and are an appreciable fraction of the overall layer depth, the depth below the obstacle should be found using Table 3b rather than 3a. It is also necessary in some malls to determine the flowing layer depth between channelling screens beneath balconies. This flowing layer depth will be altered by the presence or absence of a downstand which is deep in comparison to the approaching layer. If there is a deep downstand at the balcony edge use Table 1a. Local deepening When a buoyant layer of hot smoke flows along beneath the ceiling and meets a transverse barrier it deepens locally against that barrier 27 ; the kinetic energy of the approaching layer is converted to buoyant potential energy against the barrier as the gases are brought to a halt. Table 3a Minimum reservoir depth (m) Mass flow rate Width of reservoir (m)* entering reservoir (kg/s) * For bi-directional flow of smoky gases this should be twice the actual reservoir width Table 3b Minimum reservoir depth deep downstand across the flow Mass flow rate Width of reservoir (m)* entering reservoir (kg/s) * For bi-directional flow of smoky gases this should be twice the actual reservoir width 18