BS en 12101-5

Design approaches for smoke control in atrium buildings

G 0 Hansell*, BSc, PhD, CEng, MCIBSE, AlFireE H P Morgan, BSc, CPhys, MlnstP, AlFireE *Colt International Limited

Fire Research Station Building Research Establishment Borehamwood, Herts WD6 2BL

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Prices for all available BRE publications can be obtained from: BRE Bookshop Building Research Establishment Garston, Watford, WD2 7JR Telephone: 0923 664444 Fax: 0923 664400

BR 258 ISBN 0 85125 615 5 0 Crown copyright 1994 First published 1994

Applications to reproduce extracts from the text of this publication should be madc to the Publications Manager at the Building Research Establishment, Garston

Cover illustration by Bob Stoneman

Contents Page 1Foreword

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vi

Nomenclature Introduction

1

Chapter 1

General principles of smoke production, movement and control Fire growth and smoke production Pressurisation Depressurisation Throughflow ventilation

4 4 5 6 6

Chapter 2

Design fire size

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Chapter 3

Smoke control on the floor of fire origin Within the fire room Flow of hot gases out of the room of origin into the atrium Ventilation of the balcony space Smoke layer temperature Effects of sprinkler systems in smoke reservoirs Flowing layer depth Local deepening Inlet air Minimum number of extraction points Required ventilation rate (powered exhaust) Slit extract False ceilings The use of a plenum chamber above a false ceiling

9 9 10

Chapter 4

Smoke ventilation within the atrium Smoke movement in the atrium Channelling screens Entrainment into spill plumes rising through the atrium Fires on the atrium floor Throughflow ventilation - area of natural ventilation required Throughflow ventilation - remaining design procedures Limitations to the use of throughflow ventilation

21 21 22 23 33 33 35 35

Chapter 5

Design considerations other than throughflow ventilation Void filling Compartment separation DepressurisatiEn ventilation Principles Natural depressurisation Natural depressurisation and wind effects POwered depressurisation

37 37 37 37 37 38 40 41

Chapter 6

Depressurisatiodsmoke ventilation hybrid designs Principles Design procedures for hybrid systems Mass jZo w based systems Temperature based systems

42 42 42 42 43

13 14 14

16 17 17 18 19 19 20 20

(continued)

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Chapter 7

Atrium smoke layer temperature

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Chapter 8

Additional design factors Atrium roof-mounted sprinkler systems Smoke detection systems in the atrium Pressurisation of stairwells and lobbies Air-conditioned atria Channelling screens and hybrid systems Wind-sensing devices and natural depressurisation

46 46 46 46 46 46 47

Appendix A Case history

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Appendix B Users guide to BRE spill plume calculations Introduction Scenarios and assumptions Outline of procedure Detailed procedure

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Nomenclature used in Appendix B

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Acknowledgements

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References

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This Report is the culmination of a long-running collaborative project between the Fire Research Station of the Building Research Establishment and Colt International Limited on aspects of smoke movement and its control in atrium buildings. It is based on both the latest scientific knowledge and practical experience of smoke movement and control systems, and has been prepared under the overall supervision of the Fire Research Station. T h e present Report is intended to serve the designers of smoke control systems for atrium buildings in the same way that the earlier Building Research Establishment Report, Design principles f o r smoke ventilation in enclosed shopping centres, has served designers of smoke ventilation systems in shopping malls. As such, those graphs and tables it contains which are relevant to a particular design of building can be applied directly to that building; or the formulae cited can be used to apply the work to a broader range of circumstances. The Report does not exclude the options of using alternative methods where they are appropriate, or of using new techniques (such as computational fluid dynamics) as they are developed and validated.

A P S Ferguson Building Regulations Division Department of the Environment July 1993

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Nomenclature Note: The list of nomenclature used in Appendix B is given on page 54. Area of the fire (m2) Area of inlet (measured) (m2) Area of exhaust ventilator (measured) (m2) Area of opening into atrium from adjacent fire room (m2) Specific heat of air (kJkg-IK-') Coefficient of discharge for a vertical opening Entrainment coefficient Coefficient of discharge for an inlet Wind pressure coefficient acting on an inlet Wind pressure coefficient acting on the leeward side of building Wind pressure coefficient acting on an exhaust ventilator Coefficient of discharge for an exhaust ventilator Depth of smoke beneath an extraction point (m) Depth of a smoke layer under a balcony (m) Depth of a downstand fascia (m) Diameter of fire (m) Design depth of a smoke layer in a reservoir (m) Depth of a flowing smoke layer in a vertical opening (m) Maximum depth of smoke in an atrium (m) Acceleration due to gravity (ms-2) Height of a vertical opening (m) Height of a vertical opening with no upstand (m) Height of rise of a thermal line plume from an opening or balcony edge to the smoke layer (m) Height of the atrium (m) Height to the ceiling (m) Channelling screen separation (m) Mass flow rate (kgs-') Mass flow rate from the fire (kgs-') Mass flow rate under a balcony (kgs-') Mass flow rate entering (leaving) a smoke layer in a reservoir (kgs-I) Mass flow rate flowing through a vertical opening (kgs-') Critical exhaust rate (kgs-l) Number of exhaust points Perimeter of fire (m) Heat flux (kW) Convective heat flux of fire (kW) Convective heat flux passing through a vertical opening (or under a balcony) (kW) Convective heat flux per unit fire area (kWm-2) Absolute temperature of gases (K) Absolute temperature of gas layer under a balcony (K) Absolute temperature of gas layer in a reservoir (K) Absolute ambient temperature (K) vi

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Volumetric flow rate of gases (m3s-1) Volumetric flow rate of gases from a reservoir (m3s-I) Design wind velocity (ms-') Width of vertical opening (m) Width of balcony (distance from vertical opening to front edge of balcony) (m) Height from the base of the smoke layer to the neutral pressure plane (m) Height from the base of the fire to the smoke layer immediately above (m) Coefficient in critical exhaust rate equation (kgrnp3) Empirical height of virtual source below a balcony edge (m) Additional smoke depth due to local deepening (m) Temperature rise above ambient of smoky gases ("C) Temperature rise above ambient of smoky gases under a balcony ("C) Temperature rise above ambient of smoky gases in a reservoir ("C) Temperature rise above ambient of smoky gases in a vertical opening ("C) Density of gases (kgm-3) Density of ambient air (kgmW3)

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Introduction This Report is intended to assist designers of smoke ventilation systems in atrium buildings. Most of the methods advocated are the outcome of research into smoke movement and control at the Fire Research Station (FRS), but also take into account experience gained and ideas developed whilst the authors and their colleagues have discussed many proposed schemes with interested parties. The primary purpose of the Report is to summarise in a readily usable form the design advice available from FRS at the time of its preparation. As such, it does.not attempt to cover installation, detailed specification of hardware, or aspects of fire safety engineering other than smoke control. The predominant cause of death in fires in the UK has been attributed to the inhalation of smoke and toxic gases’, and the annual number of fire fatalities in the UK is approximately one thousand. However, the majority of these deaths occur in domestic premises. This implies that the life-safety measures required by legislation for most public and commercial buildings have been effective on the whole. Fire safety in buildings must in the UK conform to the relevant regulations (guidance for England and Wales is given in Approved Document B2). The principal objective of these regulations is to safeguard life by: @

reducing the potential for fire incidence,

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controlling fire propagation and spread, and

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the provision of adequate means of escape for the building’s occupants.

Means of escape in case of fire was first introduced to the Building Regulations for England and Wales in 1973. Prior to that date, the powers of control in England and Wales over means of escape had been contained in other legisIation~-‘‘,s. Historically, the prevention of fire growth within (or between) buildings has been achieved by the containment of the fire and its products by means of compartmentation and/or separation. The design of structural compartmentation and separation has been largely empirical, and the concepts gradually refined and enhanced in such a way that the Building Regulations now cover primarily life safety and the protection of means of escape. It is necessary to consider four major aspects of buildings - purpose, size, separation and resistance to fire - to promote safe design. Social and technical changes have led to changes in building environments which incorporate new (or revived) building forms and the use of innovative construction techniques and new synthetic materials. The buildings adopting these changes often have included within their design large spaces or voids, often integrated with many of the storeys. These large spaces

have been described as malls, atria, arcades and lightwells. The generic term for the building type tends to be ‘atrium’ and, by their very nature, atrium buildings often run contrary to the traditional Building Regulations’ approach in terms of horizontal compartmentation and vertical separation. The original atrium was an entrance hall in a Roman house and was one of the most important rooms in the building. The concept of this space has evolved architecturally over the past few hundred years and now applies to structures much larger than the typical Roman house. Atria today are designed as undivided volumes within a structure, intending to create visually and spacially an ideal external environment -indoors6.

In Roman times the control of any smoke and hot gases that may have issued from a fire in a room adjacent to the atrium was likely to have been a simple matter. Providing there were no adverse wind conditions (due to local topography of adjacent structures), then the smoke and heat would undoubtedly vent through the open portion of the atrium roof known as the ‘compluvium’ (generally used for lighting purposes). Modern atrium buildings tend to contain large quantities of combustible material and often have open-plan layouts which increase the risk of the spread of fire. The populations within such buildings are also greater; hence there has been a substantial increase in the number of people to be protected and evacuated in an emergency. Modern atrium buildings are usually designed with the atrium as a feature which can be appreciated from within the adjacent rooms. The room/atrium boundary is usually either glazed or completely open. Thus when compared to ‘conventional’ buildings, this architecturaVaesthetic requirement imposes additional problems of life safety during a fire, since smoke, hot gases and even flames may travel from one (or more) rooms into the atrium and thence affect areas which would have remained unaffected in the absence of an atrium. In conventional multi-storey structures there is always the possibility of fire-spread up the outside of the building, with flames issuing from one room and affecting the floors above. Recent examples of this mode of fire-spread have been an office block in Siio Paulo7 and the Villiers building in London. If the escape facilities from the various rooms are of a suitable standard and segregated from other compartments (as required in the UK), there should not (in theory) be any serious hazard to life safety in this fire condition. It is only when the means of escape are inadequate or the parameters dictating their design are violated, that the loss of life may occur.

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If a building has an atrium then this fire condition can also occur internally, since there is generally a maximisation of the window area and/or open boundary between the rooms and the atrium. Hence there is an increased risk to other levels of the entry of smoke, toxic gases and possibljl flames from a fire. Recent experience of fires in atrium buildings in the has shown the problem of flame travel internally through the atrium to be minor in comparison to that of hot and toxic gases accumulating and building down in the atrium - spreading throughout the building and affecting escape routes. Thus there appears to be a need for a properly designed smoke control system in atrium buildings. The ideal option would be to prevent any smoke from a room fire entering the atrium at all. An easily understood way of achieving this is to ensure that the boundary between the room and the atrium is both imperforate and fire-resisting, and that the atrium base has only a very restricted use. This option has frequently been used, but is widely regarded as being architecturally restrictive. Consequently it is not often favoured by designers. The concept has been labelled the ‘Sterile tube’6. Where the boundary between the room and the atrium is open, it is sometimes feasible to provide a smoke ventilation system within the room, to maintain smoky fire gases above the opening to the atrium. Unfortunately it is often very difficult, impracticable, or extremely expensive to fit a separate smoke extraction system to each and every room, however small. Occasionally circumstances dictate that smoke control dedicated to each room in this way is the most viable option for protecting the atrium (this can occur, for example, when the room layout is of a large area, is predominantly open-plan and open-fronted). There have been several examples of this. Nevertheless it remains generally true that this option is rarely found to be appropriate for most atrium buildings. Another possibility is that the atrium should be pressurised to prevent smoke moving into it from a room. This is not usually a viable option where the opening between the room and the atrium is large (for example, an open-fronted room or a room whose glazing has fallen away in whole or in large part). This is because the air speed needed from the atrium into the room in order to prevent the movement of smoky gases the other way through the same opening, can vary between about 0.5 ms-l and approximately 4 ms-1 depending on gas temperature, etc. All of this air must be continuously removed from within the fire room in order to maintain the flow. The quantities of airhandling plant required will exceed the size of smoke ventilation systems for many typical atrium room openings. Note however, that pressurising the atrium may be a viable option where the atrium faqade has only relatively small leakage paths.

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Where smoke from a fire in a room can spread into the atrium, with the possibility of rapid further spread affecting other parts of the building, there will be an extreme threat to safe evacuation of occupants from those parts. Similar threats will occur if there is a serious fire in the atrium space itself. In either case, the threat to means of escape which are either within the atrium or in spaces open 1.0 the atrium, can develop rapidly unless some form of smoke control is used in the atrium in order to protect those means of escape. In other words a smoke control system in the atrium is essential to make certain that escape is unhindered, by ensuring that any large quantities of thermally buoyant smoky gases can be kept separate from people who may still be using escape routes, or awaiting their turn for evacuation. Therefore the role of a smoke control system is principally one of life safety. It should however also be remembered that firefighting becomes both difficult and dangerous in a smoke-logged building. It follows that to assist the fire services, the smoke control system should be capable of performing its design function for a period of time longer than that required for the public to escape, allowing a speedier attack on the fire to be made after the arrival of the fire service. There has been no readily usable guidance available to designers of atrium smoke control systems within the UK. There have been a number of purely qualitative papers, as well as papers on work using relatively simple models of smoke movement within atria (see for example References 10-12). The National Fire Protection Association of the USA has recently been developing a Code13which sets out a fire engineering approach to the design of smoke control for atria (termed ‘smoke management’ in the USA). While this code is in many ways very comprehensive and broader in purpose than the present Report, some of the approaches used differ from alternatives with which UK designers are more familiar, or are more approximate than methods currently used by the Fire Research Station. This particularly applies to smoke entering the atrium from adjacent rooms. The purpose of this Report is to provide guidance to assist designers of smoke control systems in atrium buildings in line with current knowledge. The guidance is based on results of research where possible, including as yet unpublislied results of experiments, but also on the cumulative experience of design features required for regulatory purposes of many individual smoke control 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.

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A Code of practiceI4 for atrium buildings is currently being prepared by the British Standards Institution (BSI). The aim of this present Report is to provide guidance only on design principles of smoke control and it is hoped to support the code rather than to preempt it. The Report cannot cover all the infinite variations of atrium design. Instead it gives general principles for the design of efficient systems, with simplified design procedures for an ideal model of an atrium, and then further guidance on frequently encountered practical problems. As the design procedures are of necessity simplified, it also gives their limitations so that, when necessary, a more detailed design by specialists can be carried out.

Figure 1 ‘Stcrile tube‘ atrium

Such an approach may employ field modelling, which exploits the new techniques of computational fluid dynamics (CFD) to deduce how, and at what rate, smoke would fill an enclosure. It does this by avoiding resort (as far as is currently possible) to experimental correlations, and returning to first principles to solve the basic laws of physics for the fluid flow. As a consequence, with adequate validation, this type of modelling should have a wide applicability. The use of a computer is necessary since the technique involves the solution of tens of thousands of mathematical equations for every step forward the simulation makes. 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. A BSI Code of practiceIs specifically for shopping complexes has been published, and also a BRE ReportI6 giving advice on smoke ventilation of enclosed shopping centres. Where atria have mixed occupancies including shops then reference should be made to these documents, or specialist advice sought.

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Figure 2 Closcd atrium

In order for a design to be achieved, it is necessary to identify the various ‘types’ of atrium that are built. These can be simply defined as follows: (a) The ‘sterile tube’ atrium The atrium is separated from the remainder of the building by fire-resisting glazing (FRG). The atrium space generally has no functional use other than as a circulation area (Figure 1).

Figure 3 Partially open atrium

(b) The closed atrium The atrium is separated from the remainder of the building by ordinary (non fire-resisting) glass. The atrium space may well be functional (cafeterias, restaurants, recreation, etc) (Figure 2). (c) The partially open atrium Here some lower levels are open to the atrium and the remaining levels closed off by glazing (Figure 3). (d) The fully open atrium Some of the upper levels or all of the building levels are open to the atrium (Figure 4).

Figure 4 Fully open atrium

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Fire growth and smoke production In most instances, a room (compartment) fire may be assumed to burn in either of two ways: (a) Fuel-bed control When the rate of combustion, heat output and fire growth are dependent upon the fuel being burned. (The 'normal' fire condition found in most singlestorey buildings whilst the fire is still small enough for successful smoke control.) (b) Ventilation control Where the rate of combustion, etc is dependent upon the quantity of air available to the fire compartment (assuming any mechanical ventilation systems are inactive).

temperature. The smoke spreads out radially underneath the ceiling and forms a layer which deepens as the compartment begins to fill. If the compartment is open to the atrium, then the gases flow out immediately they reach the opening. If the compartment is glazed or the opening is below a deep downstand then the smoke steadily deepens. As the layer gets deeper there is less height for the plume of smoke to rise before it reaches the smoke layer, hence less air is being entrained, wit'h the result that the temperature of the smoke layer increases with layer depth, even for a steady fire. Most fires will continue to grow larger as the layer deepens, reinforcing this effect.

1 The fire starts for whatever reason, its rate of growth depending upon the materials involved. In most practical compartments there is sufficient oxygen to support combustion in the first few minutes, and the fire growth and smoke production are controlled by the fuel, ie, fuel-bed control.

3 6 mm plate glass may shatter when exposed to gases as little as 100 K warmer than ambient. Thus once this temperature is passed, there is an increasing likelihood that the glass will fracture. If the compartment is sprinklered and the water spray hits the glass, the localised heating of the glass by radiation from the fire and by the gas layer, combined with sudden cooling due to the water spray will increase the likelihood of the glass breaking. The smoke and hot gases will then flow externally to atmosphere, or enter the atrium, or both, depending upon the nature of the compartment and its relative position in the building, the size and position of the fire in the compartment, and the strength of differing glazing systems. If the fire can be accidentally or deliberately vented externally then the threat to other levels via the atrium is greatly reduced.

2 Smoke from the fire rises in a plume to the ceiling. As the plume rises, air is entrained into it, increasing the volume of smoke and reducing its

There will, however, be instances when a fire will vent all its effluent gas into the atrium, and this is generally the worst design scenario (Figure 5).

The quantity of smoky gases produced (ie the mass flow rate of gases) in and from the compartment, and the energy (heat flux) contained therein are different for both regimes. It is therefore important to identify the regime which applies. Hence the mass flow and heat flux within the smoky gases may be determined. It is important to understand the basic mechanisms which control the fire condition. A step-by-step history of a growing fire may be as follows:

Area A, Perimeter P

Figure 5 Smoke entering an atrium from a fuel-bed-controlled fire in an adjacent room

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There is so much mixing of ambient air into the plume that, except close to the fire itself, the hot smoky gases can be regarded as consisting of warmed air when calculating the quantity (mass flow rate) being produced in the compartment. Initially this mass flow rate of smoke will be controlled by the fuel-bed, as mentioned above. However, the geometry of the opening on to the atrium has a crucial effect. As the fire grows large in comparison to the area of the opening, the air supply to the fire is 'throttled', causing it to burn inefficiently. This leads to the situation where the inability of the compartment to vent the gases effectively due to the restricted area available causes the layer to deepen further which, combined with the increasing fire area, causes the layer temperature to rise. Once the layer temperature reaches approximately 600 "C, then in most compartments the downward radiation from the gas layer is sufficient to cause auto-ignition of the remaining combustible materials in the compartment (Figure 6). Where there is sufficient fuel within the compartment for the entire compartment to become involved, the layer temperature will rapidly rise to flame temperature, very approximately 1200 K (930 "C). The rate of burning, heat output and mass Qpw leaving the compartment are now strongly dependent upon the geometry of the opening, ie ventilation control (Figure 7).

The transition from the fuel-bed-controlled fire with a layer at 600 "C to the ventilation-controlled condition is very rapid, and may take only seconds. This condition is often known as 'flashover'. There may be an intermediate situation where the compartment has flashed over or the fire simply grown to encompass the entire width of the compartment, but where the quantity of air now rq9uired to maintain combustion is adequate, even

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though the only surface available for air entrainment is the width of the opening (as opposed to the fire perimeter for a fuel-bedcontrolled fire). This condition is known as the 'fully-involved, large-opening fire'".

8 There are many factors which determine the prevailing condition, including the type and disposition of the fuel, the dimension of the enclosure and the dimensions of the ventilation opening. They can however be reduced to two principal parameters for most compartments: A,VH

This is the area of the opening into the atrium A,", multiplied by the square root of its height H.

Ar

The area of the fire.

Note: Both the fully-involved large-opening fire and ventilation-controlled fire conditions will almost certainly produce flames from the opening into the atrium.

9 The presence of sprinklers will usually serve to prevent fire growth proceeding to full involvement, and will usually maintain the fire in a fuel-bedcontrolled state where extinguishment is not achieved.

Air is introduced into an escape route (usually a stairway) at a rate sufficient to hold back any smoke trying to pass on to that route. The pressure difference across any small opening on to the route must be large enough to offset adverse pressures caused by wind, building stack effect and fire buoyancy. It must also be low enough to allow the escape doors to be opened with relative ease. The air supply must also be large enough to produce a velocity sufficient to hold back smoke at any large opening on to the pressurised space. Experience of pressurisation designs suggests that the technique is well-suited to the protection of

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Heat radiation

Figure 6 T h c onset of flashover

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stairways used as escape routes in tall buildings, though it can also be useful in other circumstances.

Depressurisation This is a special case of pressurisation, where gases are removed from the smoke-affected space in a way that maintains the desired pressure differences and/or air speeds across leakage openings between that space and adjacent spaces. Note that depressurisation does not protect the smoke-affected space in any way; instead it protects the adjacent spaces. In the circumstances of an atrium it is sometimes possible to use the buoyancy of the smoky gases themselves to create the desired depressurisation effects. This is explained in more detail in Chapter 5.

Air mixes into the fire plume as it rises, giving a larger volume of smoky gases. These flow outwards below the ceiling until they reach a barrier (eg the walls, or a downstand). The gases then form a deepening layer, whose buoyancy can drive them through natural ventilators (or alternatively smoky gases can be removed using fans). For any given size of fire, an equilibrium can be reached where the quantity of gases being removed equals the quantity entering the layer in the fire plume - no significant mixing of air occurs upwards into the base of the buoyant smoke layer. Sufficient air must enter the space below the layer to replace the gases being removed from the layer, otherwise the smoke ventilation system will not work.

Throughflow ventilation Smoke ventilation (throughflow ventilation) is used when the fire is in the same space as the people, contents or escape routes being protected, without it filling that space. The intention is to keep the smoke in the upper reaches of the building, leaving clean air near the floor to allow people to move freely. This stratification or layering of the smoke is made possible by the buoyancy of hot smoky gases produced by the fire, and it follows that to be most successful the highlevel smoke layer must remain warm. Smoke ventilation is therefore only suitable for atria where fires can cause smoke to enter the atrium space. Such fires can either be fuel-bed-controlled fires at the base of the atrium, or fires in adjacent spaces (rooms) which allow smoky gases to enter the atrium. Much of this Report is concerned with the calculation of design parameters for smoke ventilation systems tailored to the circumstances found in various types of atria. First though, it is worth reviewing the underlying principles of smoke ventilation and the general approach needed for successful design.

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Figure 7 A fully-involved ventilation-controlled fire

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Chapter 2 Design fire size The calculation of the quantity of smoke and heat produced by a fire requires a knowledge of its size in terms of area, perimeter and heat flux developed per unit area or from the fire as a whole. When designing smoke ventilation or depressurisation systems, the mass flow rate and heat flux developed in the room are major parameters in the calculation of the system requirements, changes in which can substantially affect all of the subsequent smoke flow conditions. The preferred choice of design fire would be a timedependent growing fire, to which the means of escape and evacuation time for the particular building occupancy could be related, allowing the increasing threat to occupants to be calculated as time progresses. 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 areas typically associated with atria. 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. Such assessments can sometimes be based upon available statistics on fire damaged areas, but may have to depend upon experienced judgement based on the anticipated fire load where a more rigorous approach is not feasible. ‘Work on design guidance for smoke ventilation systems in shopping centres used the principle of selecting a fixed size of fire that would cater for almost all of the fire sizes likely to be found in that class of occupancy, and then deducing a pessimistic heat output from that fire16,18. This procedure has been adopted for occupancies other than retail which are also commonly associated with atrium buildings offices and hotel bedrooms1y*20. The’procedure has no time dependency and does not reveal any information regarding the growth characteristics of the fire. It is therefore usual to assume that the fire is at ‘steady-state’. This assumption allows the smoke control system to cater for all fires within the accepted design fire size, and by not considering the growth phase of the fire, introduces a significant margin of safety to the system design. It follows from the foregoing that there is a strongly subjective element in assessing what fire size is acceptably infrequent for safety design purposes. Various design fires have been suggested for occupancies associated with atria. A wide range of

fires may potentially be adopted. In the present work the following, in terms of fire area and convective heat flux, are used to illustrate the calculation procedures adopted:

(a) RetailI6 (sprinklered shops) 10 m2, 12 m perimeter. (b) Offices” (sprinklered) 16 m2, 14 m perimeter.

Offices”(unsprink1ered) 47 m2, 24 m perimeter.

(c) Hotel bedrooms2” Floor area of the largest bedroom.

It should be noted that the design fire size for (a) was originally chosen by the Home Office and Scottish Home and Health Department, and that for those of (b) and (c) there is no official ‘approved’ choice. After considering the heat losses to the structure of the room, and any losses to sprinkler sprays, etc, the commonly used heat outputs are approximatelyl’: Sprinklered office

1 MW

Unsprinklered office

6 MW 1 MW

Hotel bedroom*

Gases flowing into the atrium from a fire deep inside a large-area office with operating sprinklers may be cooler than is assumed in the ‘sprinklered office’ design fire above. The mass flow rate of gases entering the final reservoir will be less than would be calculated using the value given above. Even for this scenario therefore, the above value should err on the side of safety. Designers wishing to take sprinkler cooling in the fire compartment more rigorously into account should adopt a fully fire-engineered approach appropriate to their specific circumstances, for example by using the methods described in the section ‘Effects of sprinkler systems in smoke reservoirs’ (page 14) to assess the effect of sprinkler cooling on the outflowing gases. The hotel bedroom fire represents a fully-involved unsprinklered fire2”. Where sprinklers are present this will be clearly unrealistic and a value of 500 kW (for a 6 m perimeter fire) may be more appropriate for designers wishing to adopt a fire-engineering approach to a design. This Report will however assume the fully-involved value for design purposes as this will introduce a large safety margin to the design, in particular to the calculation of the mass flow rate leaving the room through the opening.

* Assuming a floor area of approximatcly 20 m2 7

The use of the bedroom floor for the hotel bedroom design fire reflects the situation where there are no sprinklers present. Unpublished research on sprinklered bed fires (P G Smith and J V Murrell, Fire Research Station; private communication, 1986), where the low heat output per unit area was comparable to values for hotel bedrooms, suggests that the much lower fuel load (compared to an office) expected in a hotel bedroom utilising conventional sprinklers should make it possible for the smoky gases to be cooled sufficiently to be retained within the room of origin (assuming the window is not open). The operation of sprinklers is likely to cool any smoke from a fire and suppress that fire to such an extent that the glazing to the bedroom will probably remain intact. This is particularly true for double-glazed windows. The same research indicates that the use of conventional sprinklers in a residential environment may not however allow conditions within the room to remain tenable, and it may be inferred that the presence of an open window to the room could produce hazardous conditions in the atrium, at least above the floor of fire origin. Since there is no statistical data available on fires in sprinklered hotel bedrooms in the UK, any choice of design fire size will be subjective. Should a designer wish to examine the effect of a plume emanating from an open window in a sprinklered hotel bedroom, it would not seem unreasonable to use a value of 6 m perimeter (equivalent to a single bed) with a convective heat output of around 500 kW as the design fire. Research into the use of fast-response sprinklers in a residential environment21,22has clearly shown that at the time of operation of these sprinklers the conditions inside the rooms were still tenable, ie there was no lifesafety risk from the smoke, even with excessive ceiling level temperatures. This indicates that for any gases flowing into the atrium (eg through an open window) the further entrainment induced by the rising smoke plume will ensure that conditions within the atrium must be tenable, regardless of the smoke temperature or smoke production rate in the room. While it is possible that this may also be true for cellular offices employing fast-response sprinklers, there is no evidence (experimental or empirical) to validate this, and so, to err on the side of safety, this Report will regard sprinklered offices employing fast-response sprinklers in the same way as offices using conventional sprinklers. Further research and statistical data are desirable in this area. As mentioned in the Introduction the retail atrium is considered separately in the Report on covered shopping complexesI6, and will not be considered further in this Report. The fire sizes on the previous page (excluding (a)) will be those used throughout. Furthermore, when considering an unsprinklered office occupancy, there exists the potential for flashover to occur and for the entire floor to become involved in fire. Even if the building geometry can

8

accommodate this fire condition, the destructive power of a fully-involved office room fire is such that smoke control systems cannot usually be designed to satisfactorily protect means of escape in this situation, except for fires in small rooms. An accurate assessment of the mass flow rate and heat flux from a room fire will allow the potential for flashover to be estimated, and thence whether additional precautionary measures are required, eg sprinklers. This Report will only provide guidance for the design of smoke control systems for a fuel-bed-controlled fire in an office, and a fully-involved fire in a hotel bedroom. Should a different design fire be considered for whatever reason, the equations, figures, etc given here may no longer apply, and advice should be sought from experts.

>, '

.

i'

.

,

. *

_.

"

_.

.

.

,,

. , . h

-...,-.-

. ..; LC.

~.

... .

Chapter 3 Smoke control on the floor of fire origin Within the fire room

Exhaust fromf ComDartment

In any situation involving the potential movement of smoke into escape routes, it is always preferable to control the smoke in the fire room and hence prevent its passage to otherwise unaffected areas. Ventilation of the fire room may be achieved by either a dedicated smoke exhaust system or by adapting and boosting an air-conditioning or ventilating system. If the compartment is open to the atrium, then it must have either a downstand barrier to create a reservoir within the compartment, or a high-powered exhaust slot at the boundary edge to achieve a similar effect (Figure 8). The minimum height of the smoke layer base in the room must be compatible with the openings on to the atrium, with the layer depth being no lower than the soffit of the opening (Figure 9). Where no downstand exists and an exhaust slot is used instead, the exhaust capacity provided will need to be compatible with the layer depth (Figure 10). See the section on exhaust slots ('Slit extract') on page 19.

(a) Use of a downstand to create a smoke reservoir Exhaust from compartment

*Volumetric flow rate sufficiently great to prevent smoke spillage beneath downstand for height of rise Y

Figure 9 Plume height and layer depth with a downstand

Having established the clear layer height in the room, the mass flow rate of smoke can then be calculated. Recent work by Hansel123drawing on work by Zukowski et a124and Quintiere et a125has shown that the rate of air entrainment into a plume of smoke rising above a fire ( M , ) may be obtained by using the equation: Mr = C, P Y 3'2

kgs-'

...(1)

where C, = 0.188 for large-space rooms such as auditoria, stadia, large open-plan offices, atrium floors, etc where the ceiling is well above the fire. C, = 0.210 for large-space rooms, such as open-plan offices, where the ceiling is close to the fire.

(b) Use of a 'slot exhaust' to prevent smoke entering the atrium Exhaust from compartment

Boundary edge exhaust slot

Note: As the two values are approximately similar and the demarcation between them uncertain, then the value for all large-space rooms is taken to be 0.188 for the purposes of design. C, = 0.337 for small-space rooms such as unit

shops, cellular offices, hotel bedrooms*, etc with ventilation openings predominantly to one side of the fire (eg from an office window in one wall only). Most small rooms will therefore take this value. P = Perimeter of the fire (m). Y = Height from the base of the fire to t h e smoke layer (m) Figure 8 Smoke ventilation within a compartmcnt

* Prior to flashover or full involvement

9

Exhaust from* compartment

Boundary e d g g exhaust slot

Flow of hot gases out of the room of origin into the atrium The mass flow rate of smoky gases passing through a vertical opening ( M , ) may be found from23:

L

L

U *

Total volumetric flow rate sufficiently great t o prevent smoke spillage beyond the exhaust slot for height of rise Y

where

h = Height of the opening above the floor (m)

Figure 10 Plume height and layer depth with a slot exhaust

Equation 1has been validated experimentally26for values of Y up to 10 times U A f ,for fires in large spaces, for values of the heat release rate between 200 and 750 kWm-*. 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. The quantity of smoke entering a ceiling reservoir or flowing layer given by Equation 1 is shown graphically in Figures 11 and 12 for both cellular and open-plan offices (C, = 0.337 and C, = 0.188 respectively) and for sprinklered and unsprinklered offices ( P = 14 m and P = 24 m respectively). For further discussion on the criteria for selecting a value of C,, see the section ‘Flow of hot gases out of the room of origin into the atrium’ that follows. Whilst Figures 11 and 12 show the mass flow production curves for cellular offices, many such configurations will not in practice have a fixed wall construction with a good enough fire resistance, or have a large enough opening to sustain the replacement air supply needed for such large fires (see the section ‘Inlet air’ on page 17).

Cd = Effective coefficient of discharge for the opening Where the smoke flow directly approaches a ‘spill edge’ with no downstand (eg where the ceiling is flush with the top of the opening), Cd = 1.0. For other scenarios the following procedure may be adopted: Where the smoke flows beyond a downstand or lower ceiling level in the form of a plume of height Dd (Figure 13(a) arid 13(b)), it has been shown23that the height of rise of the plume has an effect on the rate of flow of smoke leaving the opening. This effect can be expressed as a modification to the coefficient of discharge as follows: Cd = 0.65

I

32

30 I 28VI

26VI

f

24-

0

)pen -plan offices :,=0.188

22-

;

20-

r o

18-

en

6

12

Mass flow rates should be above this shaded zone for the smoke control systems to operate safely.

Height of smoke base (m)

Figure 11 Rate of production of hot smoky gases sprinklered offices

10

...(a

--0 4

h

Figure 12 also has a ‘cut-off‘ below which the temperature of the gas layer will exceed 600 “C and flashover of the room will almost certainly have occurred. The mechanism of flashover may well start to occur prior to this critical point, and gas temperatures in excess of 500 “C may be considered a conservative lower limit for flashover potential27.The ‘danger-zone’ is shown shaded on Figure 12.

W = Width of opening (m)

:ellular offices

where D , = Flowing layer depth in the plane of the opening (m).

:,=0,337

Dd = Depth of downstand or height of rise of plume beyond the opening (m). Where Dd 2 1.0 then Dd may be taken as 1.0 for most openings of practical interest. For a plain opening with no downstand obstruction (Figure 14), Dd can be considered as the rise of the plume beyond the balcony edge. The flowing layer depth (0,)may be found from:

$ >

Y

401

)pen-pla Jffices :e = 0.188

35

E

.*.(4)

6 30r r

A simple procedure for calculating the mass flow rate, etc is as follows: (a) Set C, to 0.65. (b) Calculate mass flow rate from: 15t

C, P W h 312 Mw=

[

g3 ]

//

Increasing potential for flashover

I

3/2

w213+;[

Height of smoke base (m)

Figure 12 Rate of production of hot smoky gases -

unsprinklcred offices

(c) Calculate buoyant layer depth from: 213

Figure 13 Flow out of an opening

c,

D w = ~ 2w [ " ]

m

(d) Calculate discharge coefficient from: Cd=0.65

[y] Dd DW+

113

(e) Use the new value of Cdand repeat from step (b), until the difference between the currently calculated value of Mw(Mw(n))and the previously calculated value of Mw(Mw(n-l)) is less than 0.1 Y0. ie: Mw(n) - Mw(n - 1)

4-

U

(a) With downstand and projecting balcony

x 100 < 0.1

MW(n) This procedure usually converges after about five iterations and will therefore quickly yield M,, C, and D,. Figures 15 and 16 give t h e mass flow values in graphical form for various opening heights and widths, using t h e above procedure. A ceiling and projecting balcony 4 m above the floor have been assumed. I t should be noted that the shaded areas on the graphs represent t h e onset of flashover (calculated using M, and Q, appropriate to the example illustrated and a layer temperature of approximately 550 "C), and values of mass flow lying within this band should be regarded with caution.

I

7 (b) With high balcony

11

---

I

.-

---- -

--

-

-

i i.

i

lDi'

Figure 14 Plain opening Cd= 0.8

2

2 h= 4 ' 0 m h = 3.5m

5

h

i

.20

h = 3.0m

W

Y

i

15

h= 2.5m

10

h= 2.0m h=4,0m h= 3.5m h= 3.0m

10

h= 2.5m h= 2.0m I

0

5

I 10

I 15

I

1

1

1

20

25

30

35

40

W (m)

Figure 15 Mass flow rate through a vertical opening -open-plan

offices

3

35t

5

7

7 h= 4'0m

301 25

-I

W v)

Y

2

0

0

5t

5

15 W (m) Sprinklered 10

20

2

-0

5

Figure 16 Mass flow rate through a vertical opening - cellular offices

12

10

15 W (m) Unsprinklered

20

25

The demarcation between a cellular room and an open-plan layout is determined by the ability of the incoming air to flow into the rising plume from all sides. The narrower the room becomes, the less easily' the air can flow behind the plume. In this Report cellular offices are considered to be those in which the maximum room dimension is less than or equal to five times the diameter of the design fire size, and the incoming air can only enter from one direction (Figure 17). This demarcation dimension was chosen arbitrarily and has no theoretical derivation. Research in this area is highly desirable. Width<

5 x Df

Restricted air flow to fire and plume

'

positioned across the balcony to limit the size of this reservoir to ensure that the smoke retains its buoyancy. Each reservoir should be limited to an area not exceeding 1300 m2, with a maximum length of 60 m by analogy with shopping maIlsI6.The screens around the balconies will, in general, be fairly close to potential fire compartments (eg offices). 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 (see page 17). 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 atrium (Figure 20). The mass flow rate of smoke to be exhausted from the atrium roof will then be that calculated for the under-balcony condition28.

Cellular room

Early experiments with smoke flow in shopping malls29 and unpublished workI7 at FRS (also N R Marshall, Fire Research Station; private communication, 1984) have shown that the smoke flowing from a room with a deep downstand and then under a balcony beyond the opening becomes turbulent with increasing mixing of air. This subsequent evidence suggests that for the purpose of engineering design the mass flow rate of smoke entering the balcony reservoir (MB) can be taken to be approximately double the amount given by Equation 2, ie: ..

Opening

...(5 )

4

Figure 17 Limiting size of a cellular room

Ventilation of the balcony space 1

If the smoke cannot be contained within the room of origin because: 0

the rooms have demountable partitions,

0

insufficient replacement air can be provided, or

0

the engineering implications are too costly or difficult to apply,

Common balcony space

I Atrium I

w Common balcony space

then the smoke and hot gases will be able to travel from the room of origin into the space beyond, including the atrium. Figure 18 Schematic section of an atrium with balconies

Some atria are designed with balconies around the perimeter of the void, serving all the rooms at that level (Figure 18). Figure 19 illustrates in schematic form an atrium with floors (two levels only are shown in both the figures) which have balconies that leave a considerable area for pedestrians. On each level there is a large area situated below each balcony. If screens (activated by smoke detectors or as permanent features) are hung down from the balcony edges, the region below each balcony can be turned into a ceiling reservoir. This is similar to the procedure used in multi-storey shopping complexesI6. This balcony reservoir can then be provided with its own extraction system. Other screens can be

U Exhaust from balcony reservoir

Figure 19 An under-balcony smoke reservoir

13

0

alternative escape routes,

0

shorter escape paths along the balcony, and the installation of sprinklers to cool the gases further.

Effects of sprinkler systems in smoke reservoirs Offices, shops, assembly, industrial and storage or other non-residential purpose groups are now expected to have sprinklers if they have a floor more than 30 metres above ground level. Multi-storey buildings in the assembly, shop, industrial or storage purpose groups will also be fitted with sprinklers if individual uncompartmented floors exceed a given size. Sprinklers may also be required in other circumstances for insurance purposes.

I Figure 20 Under-balcony smoke reservoir venting into an atrium smoke reservoir

Entrainment into smoke flows from compartments is being studied23.The purpose of this is to determine more accurately the influence of such factors as compartment opening geometry, the presence of a downstand fascia and balconyldownstand combinations. It follows that Equation 5 may be superseded in due course.

Smoke layer temperature The mean temperature rise of the smoke layer above ambient (0)can be calculated from: QW

__

"C

...(6)

Mc where Qw= Heat output at window or exit point (kW)

M = The mass flow of smoke (eg M , or MB)(kgs- ')

The action of a sprinkler system in an office on the cooling of gases flowing from that office to the atrium is accounted for in the derivation of the 1 MW heat output17.Where the smoke layer is contained wholly within the room of origin by a smoke control system and has a large area, the sprinklers will cool the smoke layer further. Similarly, where smoke is collected within a balcony reservoir adjacent to sprinklered offices, operation of sprinklers under the balconies 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 balcony ceiling, and will not be pulled out of the layer by the sprinkler sprays. When the fire occurs in an office, the operation of sprinklers under the balcony will not assist in controlling it. If too many sprinklers operated under the balcony, sprinklers in the office could become less effective as the available water supply approached its limits. It follows therefore that sprinklers need only be installed in a smoke reservoir i f

= Speciticheat capacity of the gases (kJkg-'K-')

0

Tables 1 and 2 give the temperature rise (0)for a 1 MW and 6 MW fire, taking into account the cooling processes mentioned in Chapter 2.

the smoke layer temperature is likely to exceed 200 "C and thus produce sufficient radiation to impede escape, or

0

if there is the likelihood of sufficient combustibles being present to pose a significant threat of excessive fire-spread.

c

In unsprinklered fire situations a high smoke layer temperature will result in intense heat radiation which may cause difficulties for people escaping along a balcony beneath the smoke layer, especially if the balconies form a major escape route. The maximum smoke layer temperature which will allow safe evacuation without undue stress is in the order of 200 "C. If this gas temperature (or lower) cannot be achieved then consideration should be given to: 14

A powered extract system, to a reasonable approximation. removes a fixed volume of smoke irrespective of temperature. Therefore if the extent of sprinkler coolirig is overestimated, the system could be underdesigned. A system using natural ventilators depends on the buoyancy of the hot gases to expel smoke through the

Table 1 Volume flow rate and temperature of gases from a 1 MW fire (including cooling within room of origin) Mass flow rate (Mass rate of extraction) (kgs-9

Temperature of gases above ambient . ("C)

Volume rate of extraction (at maximum temperature) (m's-')

4 6 8 10 12

250 I67 125 83

6.0 8.0 9.5 11.0 12.5

15 20 25 30 35

67 50 42 33 28

15.0 19.5 22.5 27.5 32.0

40 50 60

25 20 17

36.0 44.5 53.0

100

temperature beyond the radius of operation of the sprinklers. This radius is generally not known.

ventilators. In this case the system would be underdesigned if the sprinkler cooling were underestimated.

In the absence of better information, it may be reasonable to assume that no more sprinklers will operate than are assumed when calculating the design of sprinkler systems and their water supply (eg 18 heads for Ordinary Hazard Group 3).

The heat loss from smoky gases to sprinklers is currently the subject of research, although data suitable for design application are not yet available. Nevertheless, an approximate estimate can be obtained as follows: If the smoke passing a sprinkler is hotter than the sprinkler operating temperature, that sprinkler will eventually be set off and its spray will 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 be assumed to be approximately equal to the sprinkler operating

For powered extract systems the cooling effect of sprinklers can be ignored in determining the volume extract rate required. This will err on the side of safety. Alternatively, this further cooling and the consequent contraction of smoky gases can be approximately estimated on the basis of an average value between the sprinkler operating temperature and the calculated . initial smoke temperature. Where the fan exhaust openings are sufficiently well separated it can be assumed that one opening may be close to the fire, and

Table 2 Volume flow rate and temperature of gases from a 6 MW fire (including cooling within room of origin) Mass flow rate (Mass rate of extraction) (kgs-9

Temperature of gases above ambient ("C)

Volume rate of extraction (at maximum temperature) (1113s- ')

10 12 15 20 25

600 500 400 333 240

25.5 27.0 29.5 32.0 38.0

30 35 40 50 60

200 171 150 120 100

41 .5 46.5 50.5 59.0 67.5

75 I10 130 150

80 67 54 46 40

80.0 92.5 107.0 123.5 140.0

200 300 400

30 20 15

181.0 263.0 345.0

90

15

,

.

will extract gases at the full initial temperature given by Equation 6. The other openings in these circumstances can be assumed to be outside the zone of operating sprinklers, and will extract gases at the sprinklers’ effective operating temperature. The number of potential ‘hot’ and ‘cool’ intakes must be assessed when calculating the average temperature of extracted gases. If the sprinkler operating temperature is above about 140 “C, or above the calculated smoke layer temperature, then sprinkler cooling can be ignored for natural ventilators. 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 0 has been estimated, the new heat flux can be found using Equation 6.

Flowing layer depth Smoke entering a ceiling reservoir will flow from the point of entry towards the exhaust points. This flow is driven by the buoyancy of the smoke. Even if there is a very large ventilation area downstream (eg if the ceiling downstream were to be removed), this flowing layer would still have a depth related to the width available under the remaining ceiling (which can now be considered a balcony), the temperature of the smoke and the mass flow rate of smoke. Work by Morgan30 has shown that this depth can be calculated for unidirectional flow as follows:

...(7)

where D B= Flowing smoke layer depth under the balcony (m)

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 3(b) rather than 3(a).

Table 3 Minimum reservoir depths or minimum channelling screen depths required under balconies for both 1 MW and 6 MW convective heat output (a) Unimpeded Itlow Mass flow rate entering reservoir (kgs-’1

4

10 15 20 25 30

Width of reservoir W , or channelling screen width L (m)* 6

8

10

12

15

1.1 1.4 1.7 2.0 2.3

0.8 1.1 1.3 1.5 1.7

0.7 0.9 1.1 1.2 1.4

0.6 0.8 0.9 1.1 1.2

0.5 0.7 0.8 1.0 1.1

0.5 0.6 0.7 0.8 0.9

40 50 70 90 110

2.8 3.4 4.5 5.6 6.7

2.2 2.6 3.4 4.3 5.1

1.8 2.1 2.8 3.5 4.2

1.5 1.8 2.4 3.1 3.6

1.4 1.6 2.2 2.7 3.3

1.2 1.4 1.9 2.3 2.8

130 150

7.8 9.0

6.0 6.8

4.9 5.6

4.2 4.9

3.8 4.3

3.2 3.7

(b) Impeded flow - deep downstand

WB = Balcony channel width (m)

Mass flow rate entering reservoir

= Coefficient of discharge

Note: Values of Cd will vary for differing flow geometries. However, for the purpose of engineering design c d can be taken to be 0.6 if a deep downstand is present at right angles to the flow, or 1.0 in the absence of a downstand. At the time of writing, values of Cd for intermediate depth downstands cannot be stated with confidence for the wide range of geometries to be found in practice. It is suggested that either of the extreme values should be adopted in seeking a conservative design approach. The resulting values of layer depth for different balcony reservoir widths and mass flow rates of smoke are

. .-

shown in Table 3 for layer temperatures 0B2 65 OCl7. This ignores the effects of cooling (See the section ‘Smoke layer temperature’ on page 14). 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.

(kgs-’)

16

~.,_..

MB= Mass flow rate under the balcony (kgs-I)

c d

I

-... .

4

Width of reservoir W , or channelling screen width L (m)*

6

8

10

12

15

10 15 20 25 30

1.8 2.3 2.8 3.3 3.8

1.4 1.8 2.2 2.5 2.9

1.2 1.5 1.8 2.1 2.4

1.0 1.3 1.5 1.8 2.1

0.9 1.1 1.4 1.6 1.8

0.8 1.0 1.2 1.4 1.6

40 50 70 90 110

4.7 5.7 7.5 9.4 11.2

3.6 4.3 5.8 7.2 8.6

3.0 3.6 4.8 6.0 7.1

2.6 3.1 4.1 5.1 6.1

2.3 2.7 3.6 4.5 5.4

2.0 2.4 3.1 3.9 4.7

130 150

13.1 15.0

10.0 11.5

8.2 9.5

7.1 8.2

6.3 7.2

5.4 6.2

* For bi-directional

flow of smoky gases this should be twice the actual reservoir width

.(

.

..

- ... ...

.. -

1,

Local deepening Where a buoyant layer of hot smoke flows along beneath a ceiling and meets a transverse barrier, it deepens locally against that barrier3' and, as the gases, are brought to a halt, the kinetic energy of the approaching layer is converted to buoyant potential energy against the barrier.

...(8)

When designing a smoke ventilation system for atria, in which the balconies are acting as reservoirs, it is often necessary to control the path of smoke flow using downstand smoke curtains. These are typically installed around the edge of the voids to prevent smoke flowing up through the voids. If the void edge is close to the room this local deepening could cause smoke to underspill the smoke curtain and flow up through the void, possibly affecting escape from other storeys. Clearly, the void edge screens must be deep enough to contain not only the established layer, but also the additional local deepening outside the room on fire. The extent of local deepening can be found from Figure 21. The depth of the established layer (DB in Figure 21) under the balcony immediately downstream of the local deepening must first be found using the design procedure given in the preceding sections. Usually this means in the channel formed between the void edge screen and the room faqade. The additional depth ADBcan then be found by inspection of Figure 21, allowing the necessary minimum overall depth (DB + ADB) of the void edge screen to be found. It can be shown that the following scale-independent formula can be used to approximate Figure 21:

I

I

I

1.0

1.5

2.0

where ADB= the additional deepening at the transverse barrier (m)

H , = the floor-to-ceiling height (m)

D B= the established flowing layer depth (m) I

WB = distance between the opening and the transverse barrier (ie balcony depth) (m)

Inlet air There must be adequate replacement air for the efficient operation of a smoke ventilation system. When ventilating compartments directly, if the faiade is normally sealed then facilities should be provided for the necessary quantity of replacement air to be supplied to the fire room automatically. This requirement often makes the provision of smoke ventilation to the room of origin prohibitive or . undesirable. The provision of replacement air to a system employing balcony reservoirs is far easier, provided the balconies are open to the atrium. If the area available for inlet becomes too restricted, incoming airflow through escape doors may be at too high a velocity for easy escape. Such air inflows through doors in public buildings could hinder

Dg fm)

Figure 21 Local deepening at a transverse barrier

17

.

...

-

escapees. Recent research32into the ability of people to move through an exit against an opposing airflow has shown that movement is not impeded for airspeeds below 5 ms-', and is not seriously impeded below 10 ms-' (although some discomfort was reported at these higher airspeeds). This suggests that inflow airspeeds should not usually exceed 5 ms-I. Other values may be appropriate for other circumstances. For example, in buildings where the population is largely familiar with the escape routes; where the incoming air is entering the fire room directly, or where (in the instance of the inlet air being supplied via the atrium) the major escape routes are away from the atrium; then a less onerous parameter can be applied. Current advice regarding pressurisation system design33recommends a maximum pressure drop across a door of 60 Pa. This accords to a face velocity across a rectangular inlet opening of about 6 ms-I. The pressure drop criterion may be increased if the population of the building is adult and physically fit, to perhaps 100 Pa (8 ms-I). A fan-driven inlet air supply may be employed, but can give problems when mechanical extraction is used (the building will usually be fairly well sealed in such circumstances). This is because the warmed air taken out will have a greater volume than the inlet air. As the fire grows and declines, the mismatch in volume between the inlet air and the extracted fire-warmed air will also change. This can result in significant pressure differences appearing across any doors on the escape routes. For this reason simple 'push-pull' systems should be avoided.

Minimum number of extraction points The number of extraction points within the reservoir is important since, for any specified layer depth, there is a maximum rate at which smoky gases can enter any individual extraction point. Any further attempt to increase the rate of extraction through that point merely serves to draw air into the orifice from below the smoke layer. This is sometimes known as 'plugholing'. It follows that, for efficient extraction, the number of extraction points must be chosen to ensure that no air is drawn up in this way. Table 4, which is subsequently modified based on experimental ( A J M Heselden, Fire Research Station; private communication, 1976), lists the minimum numbers needed for different reservoir conditions and for a variety of mass flow rates being extracted from the ventilators in the reservoir. Table 4 strictly applies to ventilators which are small compared to the layer depth below the ventilators (eg where the diameter is much less than the depth of the layer). Where sprinklers are installed and additional cooling of the smoke layer needs to be accounted for, the number of extraction points required will differ from those shown in Table 4. The number can be determined by calculating the critical exhaust rate for 18

an opening (A J M Heselden, Fire Research Station; private communication, 1976), beyond which air will be drawn through the layer. This critical exhaust rate (MCRIT)may be found from:

where

p=

1.3 for a vent near a wall ( k g ~ n - ~ )

or

p=

1.8 for a vent distant from a wall (kgm-')

g = Acceleration due to gravity (ms-2)

D = Depth of smoke layer below the extraction point (m) To = Absolute ambient temperature (K)

0 = Excess temperature of smoke layer ("C) T = To + 0 (K)

The required number of extract vents (N) is then given by: M

N2--

...( 10)

MCRIT where M = The mass flow rate entering the layer (ie Mf o r M s ) (kgs-I)

Table 4 Minimlim number of extraction points needed in a smoke reservoir (a) 1 MW heat output Total mass rate of extraction (kgs-') 0.5

Depth of layer below extraction point (m) 0.75

1.0

1.25

1.5

1.75

2.0

9 12 15 18

21-28 2940 39-54 50-68

8-11 11-15 14-20 18-25

4-5 6-8 7-10 9-13

3-3 3-5 4-6 5-7

2-2 2-3 3-4 4-5

1-2 2-2 2-3 3-3

1-2 2-2 2-3

20 25 30 40

57-79 77-1 07 99-137 149-205

21-29 28-39 36-50 54-75

10-14 14-19 18-25 27-37

6-8 8-11 10-14 15-21

4-6 5-7 7-9 10-14

3-4 4-5 5-6 7-9

2-3 3 4 4-5 5-7

Total mass Depth of layer below extraction point (m) rate of extraction (kgs-I) 0.5 0.75 1.0 1.25 1.5 1.75

2.0

1-1

(b) 6 MW heat output

12 15 18

26-35 31-43 37-51

10-13 12-16 14-19

5-7 6-8 7-9

3-4 4-5 4-6

2-3 2-3 3-4

2-2 2-2 2-3

1-2 1-2 2-2

20 2.5 30 40

41-56 51-70 62-85 85-118

15-21 19-26 23-31 31-43

8-10 9-13 11-15 15-21

5-6 6-8 7-9 . 9-12

3 4 4-5 4-6 6-8

2-3 3-4 3-4 4-6

2-2 2-3 2-3 3-4

Note: In both tables the first number of each pair denotes extraction points wcll away from the walls, and thc second is for those close to the walls.

Where very large or physically extensive ventilators are used (eg a long intake grill in the side of a horizontal duct) an alternative method is possible. For this case, Table 3(a) or 3(b) can be used with the ‘width of reservoir’ being taken as the total horizontal accessible perimeter of all the ventilators within the reservoir (eg the total length of intake grilles in the example above) and the ‘minimum reservoir depth’ corresponds to the depth of the smoke layer beneath the top edge of the intake orifice. In practice, for a given mass flow rate and layer depth, Table 3(a), 3(b) or Equation 7 can be used to find the minimum value of accessible perimeter.

Such a system is likely to work best with further extraction distributed within the fire room, which for a sprinklered room may possibly be provided by the normal ventilation extraction system (the normal ventilation input and recirculation of air being stopped) or, for an unsprinklered room, by a partial smoke exhaust system. Whilst this system is designed to prevent smoke entering the atrium void, it will not necessarily maintain a clear layer within the room itself, and any balcony may become ‘fogged’. The extraction should be provided very close to the opening from a continuous slit which may be situated in the plane of the false ceiling.

Intermediate size intakes (ie where the ventilator size is comparable to layer depth) cannot be treated so simply and it is recommended that Table 4 be used since it errs on the side of safety.

It has been shown that powered exhaust from a slit at right angles to a layer flow could completely prevent smoke passing that slit, provided that the extraction rate at the slit was at least 5/3 times the flow in the horizontal layer flowing towards the slit (H G H Wraight, Fire Research Station; private communication, 1984). This allows a useful general method for sizing such an extract:

Required ventilation rate (powered exhaust) A powered smoke exhaust system consists of fans and associated ductwork designed to remove the mass flow rate ,of smoke entering the smoke reservoir, and to be capable of withstanding the anticipated smoke temperatures.

(a) First calculate the flow rate of gases approaching the opening (or gap in the balcony edge screens) using as appropriate the following sections:

The controls and wiring should of course be protected, to maintain the electrical supply to the fans during a fire.

I

I

I

The mass flow rate of smoke determined from the previous sections can be converted to the corresponding volumetric flow rate and temperature, using Tables 1 or 2 or the following equation for selection of the appropriate fans:

M T

v,=

PO

where

U

...(11)

To

Drop screen

I ni I1 I Slit

1313s-1

~

Balcony reservoir screen

,

Atrium void

I /I

Fire room

VI = Volumetric exhaust rate required in the reservoir (m3s-I)

Drop screen

M = M for M B determined from the previous section (kgs-I)

Balcony reservoir screen

T = e + T, (K)

Plan

po = Density of ambient air (kgmP3)

Slit extract When removing the smoke from a common balcony reservoir, and there is no possibility of using downstand screens to prevent the passage of smoke into the atrium, a slit extract system may be employed over the length of the flow path to supplement the underbalcony exhaust system and replace the screens (Figure 22). Similarly, a slit extract system can be used across a room’s openings to prevent any outflow of smoke.

’//////,///////////////////////////,

Fire room

Section

Figure 22 Slit extraction

19

.- ‘Flow of hot gases out of the room of origin into the atrium’ (page 10) ‘Ventilation of the balcony space’ (page 13), and ‘Smoke layer temperature’ (page 14). (b) Multiply this mass flow rate by 5/3. (c) Using the known layer convective heat flux (and allowing for sprinkler cooling, referring if appropriate to ‘Effects of sprinkler systems in smoke reservoirs’ on page 14), calculate the volumetric exhaust rate required from the slit, using Equation 6 to calculate the mean gas temperature drawn through the fan, and Equation 11 to calculate the required fan capacity.

False ceilings Where there is an unbroken false ceiling in the fire room or balcony it must be treated as the top of the smoke layer. If the false ceiling is porous to smoke, ie if it has an appreciable free area, any smoke screens forming the smoke reservoir must be continued above the ceiling. If the proportion of free area is large enough, the reservoir and its screens may even be totally above the false ceiling. The permeable ceiling ought not to interfere appreciably with the flow of smoke from the fire to the smoke ventilation openings above the false ceiling. It has been shown e ~ p e r i m e n t a l l ythat ~ ~ a minimum free area of 25% can be used as a rule of thumb value for allowing safe escape. For balcony reservoirs cool smoke can be expected to affect some nearby rooms under some circumstances, but would not significantly hinder safe escape. Free areas of less than 25% are possible in some circumstances, but expert advice should be sought where this possibility is felt desirable.

The use of a plenum chamber above a false ceiling Some designs have been seen in which the space above the mainly solid false ceiling in a roof or above a balcony is used for the extraction of air for normal ventilation purposes. A fan extracting air from this space (effectively a plenum chamber) reduces its pressure and so draws air from the space below through a number of openings in the false ceiling. In the event of a fire, a fan of suitably larger capacity starts up and draws smoky gases into the chamber in a similar way.

.

A potentially valuable bonus of such a system in a sprinklered building is that the sprinklers which are normally required in the space above the false ceiling will cool the smoky gases before they reach the fan. Furthermore, it can be desirable to leave the false ceiling below the extraction points ‘solid’ (ie not able to pass smoke) to prevent air being drawn up through the smoke layer. A sufficiently extensive area of solid ’

20

L J

.

false ceiling will ensure that the smoke passes through at least one sprirtkler spray en route to the extract. The plenum chamber should not be larger in area than its associated smoke reservoir. Larger chambers should be subdivided by smoke screens extending the full height of the chamber and below the false ceiling to form a complete smoke reservoir below. The minimum number of openings through the false ceiling required within a single subdivision can be found from Table 4. The total area of such openings per reservoir should be decided by consideration of the design pressure differences between chamber and smoke layer, and of the flow impedance of the openings concerned. A system of reasonably wide (perhaps oneor two-metre) slots surrounding a region of false ceiling could perhaps be used instead of screens below the false ceiling.

Chapter 4 Smoke ventilation within the atrium Smoke movement in the atrium When the smoke and heat cannot, for various reasons, be confined and removed from the room of origin or associated balcony space, the use of 'throughflow' or steady-state ventilation from the atrium itself is usually considered. This form of smoke control is that most readily understood by most as 'smoke ventilation' and is based upon a defined buoyant smoke layer being established at some point within the structure, with a 'clean' layer of air beneath. The mass flow of gases entering this layer is equivalent to that flowing out through the exhaust system (Figure 23). The base of such a layer is usually at a height chosen for safety reasons or to avoid breaching the practical 'cut-off' limits outlined in the section 'Limitations to the use of throughflow ventilation' on page 35. Once the height of this layer base is chosen for a lowest-level fire, the height above the top of the opening (or above the edge of the projecting canopy or balcony over the opening where relevant) must be established where the fire is in an adjacent room. Note that when the fire is on the floor of the atrium and is directly below the smoke layer that forms under the atrium ceiling, entrainment into the rising plume is different to entrainment into spill plumes. This special case of fires on the atrium floor is discussed later on page 33. In general however, the worst condition to be catered for is a fire in an adjacent room on the lowest level, as

4

-

Inlet

......................... ........................ ................

...................... . ..........................

___Y.....

'r

I

this results in the most entrainment in the rising smoke plume and lience the largest quantity of smoky gas entering the buoyant layer. The fire condition in the compartment (the design fire) should be specified, and the mass flux leaving through the compartment opening and any entrainment under the projecting balcony or canopy can be calculated as described in the first three sections of Chapter 3.

As the smoke flows through the room opening into the atrium space it will either: 0

rotate upwards around the top edge of the opening and pass directly into the atrium space as a plume, or

0

flow under a horizontal projection such as a balcony beyond the opening, pass to the edge of the projection and rise upwards into the atrium space as a plume.

Such plumes are often referred to as 'spill' plumes, or as 'thermal' line plumes. The term 'line' denotes that the base of the plume immediately following rotation is long and relatively narrow. Line plumes may take one of two forms:

1 Adhered plumes, where the smoky gases project directly from a compartment opening, and the plume attaches to the vertical surface above the opening whilst rising upwards. This will also occur when there is a vertical surface immediately above any rotation point into the void. The surface of the plume in contact with the ambient atmosphere in the atrium will cause additional air to be entrained

,

I

Figure 23 Throughflow ventilation of the atrium

'21

I

into it (Figure 24(a)). This type of plume is also known variously as a single-sided, attached or wall plume. 2 Free plumes, where the smoky gases project into space beyond a horizontal projection, eg a balcony, thus allowing the forming plume to rise upwards unhindered. This creates a large surface area for entrainment on both sides of the plume along its spill width (Figure 24(b)), for which reason they are also known as double-sided plumes.

The degree of entrainment into the rising plume, and hence the total quantity of gases entering the smoke layer forming under the ceiling of the atrium space, is

Reductions in the mass flow rate of smoke entering the smoke layer can usually be effected by changes to (c) and (d). In practice the height of rise of the plume is usually chosen to permit safe evacuation, leaving only a dependency on the length of the line plume.

Channelling screens When the atrium has a plane f a p d e with no horizontal projections, the length of the plume is determined by the width of the opening through which the smoke is passing. When, however, smoke is able to flow unrestricted under a horizontal projection, eg a balcony, it will flow forwards towards the balcony edge, and laterally sideways. It will continue to flow sideways until it meets an obstruction or loses

(a) Adhered plume

Figure 24 Line plumes within the atrium

22

(b)

Free plume

sufficient energy to become stagnant, and will then rise into the atrium space as a very long line plume (Figure 25(a)). This results in large quantities of air being entrained and hence a very large mass flow rate of smoke entering the layer in the atrium roof. This excessive entrainment can be reduced by restricting the sideways travel of the smoke under the balcony and hence reducing the length of the line plume. The devices used to achieve this are commonly known as channelling screens, and literally ‘channel’ the smoke from the exit of the room to the balcony edge (Figure 25(b)). This concept is used in smoke control systems in multi-storey shopping centresI6. 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 1 MW and 6 MW fires in offices are given in Table 3(a) for a balcony with no downstand obstruction at the void edge (unimpeded flow), and Table 3(b) for a balcony with a downstand at the void edge, which is deep relative to the approaching layer (impeded flow). Alternatively the minimum channelling screen depth may be calculated using Equation 7, where WB will be the channelling screen separation width L. Whichever procedure is chosen, the resulting depth is the maximum flowing smoke layer depth and hence minimum screen depth. Good practice suggests that a safety margin should be considered. An additional depth of 0.25 m would not seem unreasonable. Screens may be fixed or may descend upon smoke detection. As described above, the final mass flow entering the layer is a function of four initial parameters, one of which is the plume width at the balcony edge. The narrower the plume at its base, the less the mass flow entering the layer. Thus the closer the screens may be installed to each other, the more the smoke base may be allowed to rise for the same heat flux.

Figure 25(a) Smoke spreading sideways beneath a projecting canopy or balcony

Figure 25(b) Smoke confined to a compact spill plume by channelling screens

These screens must, of course, meet the wall of a compartment where it meets the balcony. Any screen fixed midway across a compartment opening will serve no purpose, since smoke will flow on both sides simultaneously.

Entrainment into spill plumes rising through the atrium

Recent research37suggests that channelling screens may be unnecessary if the balcony projects no more than 1.5 m beyond the fire room. This research has also shown that balconies which are shallow (