Guide to Smoke Extraction in Buildings

WFRC No. C49378 Page 2 of 76

C O N T E N T S

PAGE

1. INTRODUCTION 3

2. WWEN IS SMOKE EXTRACTION NECESSARY? 3

3. MECHANICS OF SMOKE PRODUCTION AND MOVEMENT 4

4. FUNDAMENTALS OF DESIGN 5

5. DESIGN GUIDE FOR DIFFERENT BUILDING TYPES 18

6. NATIONAL DIFFERENCES AND REQUIREMENTS 32

7. REFERENCES 34

8. TABLES 38-40

9. APPENDICES

Al. Duct Sizing 41

Al. Construction Of Fire-resisting Smoke Control Ducts

A3. Worked Examples

10. FIGURES 50-76


WARRINGTON FIRE RESEARCH CONSULTANTS Fire Safety & Consultancy

Holrnesfield Road, Warrington WA1 2DS Telephone Warrington 551 16

GUIDE TO SMOKE EXTRACTION IN BUILDINGS

A report prepared for Promat GmbH

1. INTRODUCTION

1.1 Smoke kids more people in fires than heat, flames orstructural collapse. It is therefore being increasingly realisedthat occupant safety in a fire can be greatly improved by providing an efficient smoke extraction system. Moreover, such systems can limit property damage, both directly by reducing the spread of smoke, and indirectly by providing better visibility and thus easier access to the seat of the fire for fire fighters.

1.2 The purpose of this document is to provide basic guidance and information to the designers of smoke extraction systems. It should be noted, however, that as buildings can differ widely it is not possible to cover all eventualities. The design of any smoke extraction system involves experienced engineering judgement to tailor the system to the building and specific design objectives.

2. WHEN IS SMOKE EXTRACTION NECESSARY

Smoke extraction is one of the tools which the fire safety engineer may use to ensure adequate fire safety within a building. As such it should not be considered in isolation, but as an integral part of the total package of f i e safety measures designed for the building. Thus, the need for smoke extraction in any building must be decided in context with the means of escape, compartmentation and active supression systems in the particular circumstances of that building. In general terms, however, smoke extraction should be considered and may be found particularly useful in the following circumstances:

a) Smoke extraction for life safety

Smoke extraction for life safety purposes is of benefit in buildings where means of escape to the open air cannot be achieved within a short period of time and in which the means of escape could be severely contaminated with smoke and become impassable. Examples include shopping malls, atrium buildings, and high-rise buildings with phased evacuation (i.e. when aproportion of the occupants are expected to stay in the building throughout the duration of a fire).


b) Smoke extraction for fire-fighter access

Buildings where either

i. fire brigade access is difficult, e.g. basements, high-rise buildings

or ii. rapid attack on a fire is desirable to reduce fire spread and property damage, e.g high value warehouses,

will benefit from the provision of an appropriate smoke extraction system.

Buildings where smoke clearance by natural means may be difficult (e.g. basements, windowless buildings, and high-rise buildings without openable windows) may require a mechanical smoke purging system. .- (" .% Kt- ,,.,.. / *. *c ~~ C . 3"' okp. 3)<- i~<,>~%<~. ji\? ~l.:.!?~.~

I

3. MECHANICS O F SMOKE PRODUCTION AND MOVEMENT

Smoke is a hot bouyant gas - basically hot air plus contamination. As such it obeys the fundamental laws of fluid mechanics. Several basic principles should be understood by all designers of smoke extract systems.

i . A smoke extract system does not "push" or "suck" the smoke from an area being protected. Instead, it merely exhausts smoke which has migrated to the area of the extract opening under the influence of its own bouyancy.

ii. The amount of smoke produced by a fire is a function of its size (and heat output) and the path through which the smoke flows. In particular, it is related to the size of the rising smoke plume: i.e. its perimeter and height. This is because the turbulence around the perimeter of the rising plume entrains the surrounding air as it rises. This air is then incorporated into the plume increasing the total volume of smoke but reducing its temperature and concentration.

iii. Conversely, a non-turbulent smoke layer, such as may be formed beneath a horizontal ceiling, does not entrain significant quantities of air unless excessive horizontal travel occurs.

iv. As smoke production is largely related to fire size, it is obviously not directly related to floor areanor to compartment volumeexcept as far as this affects f i e size and height of smoke rise. Simple approaches relating extract requirements to apercentage of floor area or number of air changes per hour cannot therefore generally be justified. (But see 4.2.1.)


v. Smoke removed by a smoke extraction system must be replaced by an equivalent volume of inlet or make-up air. This air must enter at a sufficiently low velocity where it encounters a smoke layer. If it enters at high velocity, it can induce turbulence and mix air into an otherwise stable smoke layer and cause downward mixing of the layer.

vi. To maintain a smoke layer at a given height, the mass flow rate of smoke entering the layer must equal the mass flow rate of smoke being extracted from that layer.

vii. As certain types of extraction systems rely on the bouyancy of the smoke, there is a limit to the size of a "reservoir" from which smoke can be extracted as smoke in an overlarge reservoir may cool and lose its bouyancy. Large areas may therefore require to be divided into separate smoke extraction zones of limited area.

4. FUNDAMENTALS OF DESIGN

4.1 Why do it?

The f i s t question to be answered before attempting any smoke extraction design is "What is the purpose of this system?". There are three basic possibilities.

a) Life Safety

The system is to bedesigned to maintain tenable conditions on escape routes and in other areasthroughout theperiod they arelikely to bein use by occupants ofthe building.

b) Fire fighting accesslproperty protection

The system is to be designed to increase visibility for, and reduce heat exposure to, trained Fie fighters thus allowing earlier and less hazardous attack on the Fie. Such systems will help to reduce property damage by increasing fire brigade effectiveness.

C) Smoke purging

The system is to be designed to enable smoke to be cleared from a building after the fire has been brought under control.

It is necessary to decide which of these or combination of the three objectives (if any) is to be achieved before commencing a design. However, naturally a system designed for life safety will usually provide a secondary benefit in terms of fire-fighting and property protection functions.

WFRC No. C49378 Page. 6 of 76

4.2 Determine design fire size

It is at~uism that all fires start small and grow larger. As pointed out in 2 (ii), the amount of smoke ~roduced de~ends to a large extent on the size of the fue. However, it is simulv not to desi& a smoke extract system to cope with any size of fue. ~a re f ; l consideration should therefore be given to determining a fire size for design purposes. This cannot simply be the largest possible fire as this will invariably be a post-flashover, fully-developed fire involving a whole fire compartment. Smoke extraction systems cannot generally be designed to cope with such fires.

4.2.1 One case where fire size is not relevant is a smoke purging system as described in 4.l.(c). In such systems where the rate of smoke extraction is not critical a "rule- of-thumb" approach can be adopted.

Vent maequal to 2'/,% of floor area will usually be considered sufficient if natural cross-ventilation can be achieved. If not, powered ventilation may be desirable. A design based on 6 air changes per hour is often used, but if this is impractical the choice of a lower figure will not be critical.

4.2.2 In life safety and fire fighting systems, however, choice of an appropriate design fire is essential. The most satisfactory way of arriving at this size is by direct consideration of the building contents. If, for example, the largest possible fire source in a fire compartment is an oil bath which is separated from other combustibles, design for a fire in the oil bath. If the building or compartment is a car park, tests have shown that fire is likely to be confined to a single car. Design for a fire in one car.

4.2.3 More often than not, however, no obvious single fire source and therefore fue size presents itself. A frequent strategy then is to relate design fire size to sprinkler operation. Experience has shown that fires in "normal" combustiblesrarely exceed aheat output of 0.5MWM. In sprinklered premises it is also frequently assumed that the maximum fire area will be that area contained within the sprinkler array, and that maximum fue perimeter will be that of the array (e.g. with a 3 x 3m spinkler array, maximum fire perimeter= 3+3+3+3 = 12m.) Following work at the British Fire Research Station (Ref. 1)this approach has been shown to bestatistically justified for shopping malls where a 12m perimeter is generally used for design purposes.


4.2.4 If there are no sprinklers and no obvious f i e cenaes, choosing a fixed design fire size becomes unrealistic. It must be expected that, barring fire brigade intervention, the fire will continue to grow until flashover occurs. But as already pointed out, a smoke extract system cannot generally be expected to cope with post-flashover conditions.

4.2.5 A simple approach sometimes used is to assume that unsprinklered fires can be expected to be double the size of sprinMered fires. Thus an unsprinklered shop can be expected to suffer a 17m perimeter, 10 MW fire. It should be noted, however, that there is no sound theoretical basis for choosing fire sizes in this way. This approach is therefore not recommended.

4.2.6 A more realistic and theoretically sound approach is to assume a growing fire. The calculations necessary can, however, be simplified by either picking a fire size expected after a set time interval - (for example related to expected escape times) or choosing the f i e to be expected just before flashover.

The latter approach is adopted in NFPA 204M (Ref. 2). The designer should refer to the tables in NFPA 204M to determine the area of natural ventilators or the mechanical exhaust capacity required.

If adopting the former approach it is recommended that fire sizes be based on what is commonly known as the T squared fire. Work by the National Bureau of Standards in the USA based upon a series of f i e tests and analysis of real fires (Ref. 29) has provided a basis for evaluating the growth of various types of fire which can be approximated by a simple equation of the form.

Where Q = heat output of the fire (kW)

a = f i e growth constant

t = time since ignition

The f i e growth constant varies with the type of materials present and the configuration of the fire room. Whilst no two fires are ever identical, four basic categories of fire growth have been defined which are considered to form a good basis for design purposes:


T Squared Fire Scenarios

Notional Typical Equivalent Fire Time for Fire Materials Growth fire to

Description Constant grow to (a) 1055kw

(approx. 1 MW)

Slow 0.0029 600s

Moderate Cotton/polyester 0.012 300s sprung mattress

Fast Full mail bags, 0.047 150s plastic foam, stacked timber pallets, 4.5m high stacked cartons

Ultra-fast Methyl alcohol 0.19 75s pool fire, fastest burning upholstered furniture

If there is doubt about what rate of fire growth to assume, err towards a faster (i.e. "worst case") fire or seek expert advice.

Assuming such a fire is circular and has a heat output of 500kW/mz, the area of the fire is thus Q/500 and its perimeter =

If life safety is the only major consideration, it is recommended that t is taken as the time necessary to ensure safe escape. Time should be allowed for delayed response to an alarm and for queuing at doorways.

If the system is to be designed for fire fighting access rather than means of escape, it is recommendedthat t is taken as fire brigade response time. This should be the slowest expected response following raising of the alarm.


In both the abovecasesit is assumedthat the alarm will be automatically given following operation of an automatic smoke detection system (see 4.8.2). For the purpose of calculation, operation of the system can usually be taken as occuring at t = 0.

4.2.7 In Table 4.2.7 some suggested fire sizes are given. Use these &if none of the approaches given above is more suitable. In particular it should be noted that except forthat given forcar parks, the fire sizes suggested for unsprinklered buildings have insufficient justification to form a sound basis of a smoke extraction design.

4.2.8 A similar approach may be adopted in sprinklered buildings where the maximum fire size is assumed to be at the time at which sprinklers activate.

4.3 Determine acceptable smoke layer depth

4.3.1 Havingdecided upon adesign fire, it is then necessary to determine an acceptable smoke layer depth. As pointed out in 3(ii), the amount of smoke produced depends not only on the fue but also on the height of rise of the smoke plume. This is the height to the smoke layer base (see Fig. 1).

4.3.2 The higher the smoke layer base, the greater the plume rise and the greater the quantity of smoke which needs to be extracted. However, obviously the smoke layer base should be above the heads of people trying to escape beneath it. Suggested minimum heights for this purpose are

Single-storey compartments - 2.5m

Upper storey of two-storey compartments (e.g. shopping malls) - 3.0m

N.B. A greater smoke layer height is required on the upper of two storeys because such smoke tends to be cool and the base of the smoke layer less well-defined.

4.3.3 If thesmokecontrolsystemis designed for fire fighteraccessonly, alowersmoke base than thatrecommended above may be acceptable. Recommended smoke layer levels for this purpose are 2m (lower or single-storey), 2.5m (upper storey).


4.3.4 In other designs, particularly thoseprimarily related to property protection, smoke layer depth cannot be determined in relation to clear layer height beneath the smoke. In suchdesigns, the usualcriterionis toestablish the smoke layer depth at alevelthatprevents smoke flowing into an adjoining area of the building (see for example Fig. 2).

4.4 Identify smoke reservoirs

If smoke is held in too large an area(reservoir) it will tend to cool losing its bouyancy. This may resultin mixingofthe smokelayerinto the clean air below. Maximum suggested reservoir sizes (appropriate to shopping mall design) are:-

2000m2 - natural extraction

2600m2 - powered extraction.

Smoke curtains used to limit the horizontal spread of smoke can either be permanent features of abuilding orretractable dropping into position on operation of the alm system. In either case they must extend below the base of the smoke layer. Often the maximum practical depth of such screens determines smoke layer depth rather than vice versa

4.5 Calculate smoke volume and temperature

4.5.1 Single storey compartments

An equation which can be used to relate smoke production to fue size and plume rise is:

Where M = mass flow of smoke (kg/s)

P =perimeter of fire (m)

y =height to base of smoke layer (m)

The above equation is not theoretically justifiable but is simple to use and generally provides conservative results, other more accurate equations are available but generally involve more complex calculations.

The temperature4 of this smoke is given by Q/M°C above ambient where Q = design fne size in kW.


The volume of the smoke (V) is then given by

V = M(To+Q)

Where To = ambient temperature in degrees Kelvin

Assume ambient temperature = 20°C (293K) unless local circumstances dictate otherwise.

Table 4.5.1 gives values for smoke production, smoke temperature and smoke volume for different smoke layer depths and fire sizes using the above formulae.

With large design fires and small plume heights (y) calculated values of smoke temperatux (0) using the above formulacan be excessive. With high smoke temperatures (say above 300°C) it is reasonable to assume that some of the energy of the f i e is lost to the building's structure. A common rule of thumb is to assume that '1, of the energy of the f i e is lost in this way.

At this stage the possibility of dangerously high smoke temperatures should be considered. If people are expected to escape beneath a smoke layer, values of 0 above 200°C should not be tolerated. However, in a sprinkler controlled f i e such temperatures would not be anticipated. Either redesign the smoke extract system or use sprinklers to reduce the heat output of the f i e and smoke temperatures.

4.5.2 Two and multi-storey compartments

Where a smoke layer resulting from a fire on one level flows from that level and then rises vertically to f o m a smoke layer at a higher level it entrains more air and the total volume of smoke can be greatly increased. (See Fig. 3.) This problem can be particularly significant in atrium buildings makiig it difficult to maintain a smoke free layer more than three storeys above the f i e floor. However, the dilution of smoke which occurs in these circumstances can significantly reduce the hazard.

4.6 Calculate minimum number of extract points

4.6.1 If an attempt is made to extract too much smoke through one vent the result can be that a "ho1e"is punched through theunderlying smoke layer and fresh air and areduced quantity of smoke is extracted. This occurs at a critical extract rate given by:


Vcrit = 2 ( g x d s x O x T 0 ) ~ . ~

Tc

Where Vcrit = maximum extract rate m3/s

d = distance between centre line of extract point and base of smoke layer

To = ambient temperature K

Tc = temperature of smoke K

8 = temperature of smoke above ambient OC

4.6.2 The minimum number of extract points is therefore the number which ensures

i. Vcrit is not exceeded,

and ii. that no smoke needs to flow more than 30m to an extract point (i.e. no part of the smoke control zone is more than 30m from an extract point - greater distances than this can cause excessive cooling of the smoke and therefore mixing between the smoke layer and the underlying air). These extract points should be spaced evenly over the smoke reservoir.

4.7 Calculate vent or fan sizes. Determine fan temperature rating , ? !4 :.I i"P t'"; -"

4.7.1 Powered extract

Reauued fan sizes are those necessan, to remove the volume of smoke calculated in 4.5 assuming, as necessary, pressure losses as calculated in %&' In most cases the fans should berated to overate at the temveratureTo +8 alsocalculatedin4.5. (NB when smoke is being extracted from two or more interconnected storeys (eg. shopping malls, see 5.5 )the highest smoke temperatures will occur with fire on the highest storey.) However, if sprinklers are installed it can be assumed that they will cool the smoke. In such cases it is generally safe to assume a maximum smoke temperature of 250°C.

It should be noted that fan performance is related to air density which is reduced at elevated temperatures. Seek advice from fan suppliers.


If the fans are connected to the exaact points by ductwork then they should be rated to take account of frictional pressure losses in the ductwork as discussed in Appendix 1.

4.7.2 Natural ventilation

Sizing n a h d ventilation is more complex than sizing fans.

Either:- a. Follow in its entirety the method of NFPA 204M.

or b. Use Nomogram 3 of Ref. 3.

or c. Use the following iterative procedure taken from Ref. 4.

First calculate vent area Av from

Where M = mass flow kg s -l (See 4.5)

0 = smoke temperature above ambient OC (See 4.5)

d = distance between centre lime of vent and base of smoke layer

This equation assumes that inlet area (Ai) = vent area (Av)

If this is not the case, guess a different value of Av (call it av) and recalculate Av from

Continue until a value is found such that Av = av. This is the required vent area in m2. (NB. The above simplified procedure assumes that both the coefficient of discharge of the ventilators and the coefficient of entry of the inlets = 0.6.)

WFRC No. ~ 4 9 3 7 8 Page 14 of 76

4.8 Other design factors

4.8.1 Inlet air

For a smoke extraction system to operate effectively make-up airmust beprovided to replace the mass of smokey gases removed.

Inlet air should enter at low level (generally at least 1.5m) beneath the designed smoke layer. It should be ensured as far as possible that:

a. If natural inlet is provided (i.e. doors, windows, louvred openings) these open automatically on operation of the extract system.

b. Inlets are sited such that no stagnant areas are formed and clean air "flushes through" all areas. Use ducted (mechanical) air inlets if there are insufficient natural inlets.

c. Inlets are sited away from extract points.

d. Inlet velocities are eitherkept below 3 d s (natural inlets) or fan power is increased to compensate for the increased resistance of air flow through the inlets. (In any case inlet velocities through doorways or on escape routes should not exceed 3 d s as greater air speeds than this can hinder escape.)

Inlet velocities are less critical with mechanical (powered) inlet.

Where inlets (for example doors) cannot be sited at least 1.5m below the smoke layer, either a smoke curtain or a horizontal shelf should be used to prevent inlet air distorting the smoke layer - see Fig. 4.

4.8.2 System operation

A powered smoke extract system for means of escape should operate automatically on operation of:-

a. a smoke detection system to recognized standards (e.g. BS5839 - Ref. 5).

and, if fitted

b. sprinkler system. (Via a water flow switch.)

W F W No. C49378 Page 15 of 76

F i e fighting and smoke purging systems may be operated as above or manually by the f i e brigade.

If false alarms are likely to be a nuisance and the system is not required for life safety, operation of the system by a smoke detection system can be on the cross-zoned ("double-knock") principle (i.e. two detectors in different detector zones need to operate before system operation).

If required for life safety, natural ventilators should also be operated by a smoke detector system. Otherwise heat detectors (often integral with the ventilator) are satisfactory.

If smoke curtains are used they should automatically activate (i.e. lower) on operation of the smokedetection system. They should"fai1 safe" i.e. powerfailure should result in them failing into their active (i.e. down) position.

Smoke extract fans should have a duplicate power supply. Power and instrument cabling supplying the fans should be of atype resistant to f i e e.g. mineral insulated metal sheathed cable to BS 6207: Part 1 (Ref. 6) or category CWZ cable to BS 6387 (Ref. 7), alternatively, cabling can be run via a f i e protected route.

In all cases, in addition to automatic operation there should be provided afire fighters over-ride switch or mechanism.

4.8.3 Ductwork

Ductwork used for smoke extract purposes must be of a standard at least capable of withstanding the anticipated smoke temperatures as calculated in Section 4.5. (Although as with fans it is generally safe to assume amaximum temperature of 250°C if the building is fully sprinklered.) But if the ductwork passes through a fire-resisting barrier it must also be capable of satisfying the fire resistance requirements.

Experience has shown that standard galvanised steel ductwork (for example to HVCA specification DW142 [Ref. 81) can resist relatively high temperatures up to 400°C without failure,provided that no aluminium rivets, combustible seals or low temperature connectors are used. Such ductwork is therefore generally suitable for use as smoke extract ductwork in sprinklered buildings and where high smoke temperatures are not anticipated.


If smoke temperatures above 400°C are anticipated fire-resisting ductwork will be required to be used (see 4.8.3.1).

Fire-resisting ductwork is also required to maintain fire-resisting compartmentation. A general requirement exists worldwide to ensure that a building is provided with a level of structural fire protection and compartmentation such that the building is capable of surviving a full bum out even if a sprinkler system is installed. This concept allows for the possibility of the sprinklers either failing to operate effectively due to poor maintenance, equipment failure or the inability to control an unexpectedly rapidly growing fire.

Therefore, if a smoke extract (or any ventilation ductwork) passes through afire- resisting compartment bamer it is necessary to ensure that both fire and smoke cannot readily spread from one area of the building to another via the ductwork. For normal ventilation ductwork oneof the followingthree methods may beused to limit firespread:

i. Fire protection of steel ductwork

ii. Construct duct from proprietry fire-resisting material

iii. Install smoke or temperature operated f i e dampers in ductwork where compartment boundary is crossed. (Not suitable for smoke extract ductwork.)

In the case of smoke extract or make-up air supply ductwork the use of fire dampers is clearly inappropriate and the only solutions are either the construction of ductwork from fi-resisting materials or the fire protection of steel ductwork.

4.8.3.1 Fie-resisting ductwork

The fire resistance of the duct is required for three reasons:-

1. To prevent fire inside the duct breaking out into another fire compartment (see Fig. 6).

2. To prevent fire outside the duct breaking into the duct and hence entering another fire compartment [see Fig. 61 (e.g. via an inlet grille).


3. To prevent fire outside the duct breaking into the duct and then breaking out into another fire compartment (see Fig. 6).

In most cases the duct will be required to provide fire resistance to both an external and an internal (i.e. inside the duct) fire.

There are occasions, however, when resistance will only be required to fire inside the duct (e.g. see Fig. 7). Where such occasions are identified then ductwork resistant only to fire inside the duct can be used.

The standard of fire resistance required of a duct may also vary. The duct should normally have a resistance to an external fire equal to the compartment through which it passes; a resistance to an internal fire equal to the compartment which it serves (see Fig. 8).

Insulation

In some cases even when aduct needs to be fire-resisting there is no need for it to fully comply with the insulation criterion of the fire resistance test. However, it is necessary to satisfy the insulation criterion for afire inside the duct in the following circumstances:-

i. The duct passes through a circulation or means of escape area where it can hazard the escape of people from the building (see Fig.%).

9 and/or ii. The duct is or could come into contact with combustible materials (see

Fig. 9).

It is particularly important to ensure that sound or thermal insulation to ductwork willnotberaisedtoignition temperature as a result of heatconductionthrough - - the ductwork.

WFRC No. C49378 ' Page 18 of 76

Fire Stopping

In all cases where smoke control ductwork passes through a fire-resisting barrier fire stopping should be provided around the 'duct to maintain the full fire-resistance of the barrier. As steel ductwork expands when heated, f i e stopping around uninsulated steel ductwork needs to be capable of accommodating this expansion .

Particular care should be taken where fie-resisting ductwork is to be used on one side of a firecompartmentboundary, non fire-resisting steel ductwork on the other. Buckling or collapse of the non fie-resisting ductwork adjacent to the fire-resisting wall or floor can result in gaps around the wall penetration, thus allowing fire spread.

Details on the construction of fire-resisting ductwork are given in Appendix 2.

5. DESIGN GUIDE FOR DIFFERENT BUILDING TYPES

The general guidelines given in Section 4 above are applicable to all buildings types. However, more detailed design guidance is given below on the following building types.

a Multi-storey offices and similar buildings.

b. Warehouses.

c. Underground car parks.

d. Atrium buildings.

e. Shopping malls.

If preparing adesignfor one of these building types,follow the procedures in Section 4 but modify it as appropriate as indicated below.

5.1 Multi-storey office and similar buildings

5.1.1. In a multi-storey building it is frequently impractical to construct.staircases capable of evacuating all floors of the building immediately in the event of fire. Instead it is usual to cany out a "phased evacuation" whereby the fire floor and the one above are evacuated immediately and other floors are evacuated later on the advice of f i e marshals. In high-rise buildings the total evacuation time before all persons are able to leave the building can be in excess of 60 minutes.


5.1.2 It is therefore necessary to ensure that smoke does not suread from the fire floor to other still-occupied areas and that staircases which provide m k s of escape and access for fire fi~htersremain free of smoke. The fust of these obiectives can be achieved using a smoke &tract system, the second using a staircase pressurization system. ~ o ~ e t h e r they form the smoke control system for the building.

5.1.3 Pressurization is beyond the scope of this guide, but the smoke extract system can be designed according to the basic principles described here. Either a dedicated smoke extract system can be provided, or, as more usually the building's normal HVAC system can be modified to act also as a smoke control system when required.

(Guidance on the design of pressurization systems is given in Refs. 10 and 12.)

5.1.4 The basics of such a "zoned" smoke extract system are shown in Fig. 10. Assumingeach storey is a fire compartment, smoke is extractedfromthe fire storey into acommon extract duct or shaft and from there to the exterior. At the same time motorised smoke and fire dampers operate at each floor level other than the fire floor. If the normal HVAC systemrecirculates air, the return (recirculating) air damper (and fan iffitted) is shut preventing smoke re-entering the supply ductwork. The supply system therefore runs on fresh air only and this is fed to all floors except the fire floor where a motorised damper prevents air entering.

5.1.5 The result is that the fire storey will tend to depressurize in relation to other storeys so restricting the spread of smoke to other storeys. Instead, air will tend to leak into the fire storey from other storeys, from pressurized and unpressurized lift shafts, and in particular from pressurized staircases.

5.1.6 The following design points should benoted:-

i. The system is not designed to provide a smoke free layer on the fire storey. It is assumed that means of escape is adequate to enable escape from a storey on fire to a protected staircase without the need for smoke control.

ii. Both the supply andextract ducts or shafts should be constructed with fire resistance equal to the fire resistance of thecompartment floors (see 4.8.3) to prevent fire spread f'rom level to level.


iii. The exhaust fan should be rated to run at the highest expected smoke temperature, but do not need to operate under f i e resistance test conditions.

iv. The plant room containing the smoke extraction equipment should be constructed as a f i e compartment and should preferably contain no other safety equipment (such as staircase pressurization plant).

v. The power supply to the HVAC fans should be duplicated. Power and instrumentation cablimg to the extract fans and motorised dampers should be of a type resistant to f i e or be run in a protected route (see 4.8.2).

vi. The smoke and fie-rated dampers should be to a suitable specification. In the absence of suitable local standards, a Class I damper to UL 555s (Ref. 11) is recommended for the return air damper, where leakage is critical. Class 11, III, or IV dampers are recommended for all other dampers. TheUtemperature rating of the dampers will depend on expectedsmoke temperatures and fire resistance requirements (see below). (NB. The use of high leakage dampers leading to smoke extract ductwork, i.e. Class 111 or IV. meansthat more air willleak into the smoke extract ductworkthrourrhthesedarn~ers.

A .

and therefore larger ductwork and/or fans will be required to main& the required extraction rate on the fire floor.)

vii. In tall multi-storey buildings where extemal wind pressures can be very high, smoke control systems will be ineffective if there is appreciable breakage of extemal windows. Such asystem is thereforeonly of benefit where thereisamechanismto reduce the fire size and hence the degree of window breakage in a f i e i.e. the building is sprinklered.

viii. In a multi-storey building, operation of the system by smoke detectors can result in the system operating on a level other than the fire storey. It is therefore suggested that system operation is by heat detectors or following the operation of a sprinkler system. There should also be fire brigade override control as described in 4.8.2.


5.1.7 Calculating extract required

In Section 4it was suggested that smoke extract volumes and hence smoke layer depths may be estimated using the following equation:

Where M = mass of smoke kgls

P = perimeter of design fire m

y =height from floor to smoke layer base m

However, there are difficulties with such an approach when designing the smoke extract rate required in a multi-storey zoned smoke control system because it is not necessary or practical to specify a design smoke layer base. Indeed, the f i e floor can become completely smoke loggedprovided smoke does not leak into surrounding areas. The purpose of the system is to prevent leakage by creating pressure differences across leaky boundaries such as doors and floors.

The calculation of required flows and pressure differences is beyond the scope of this guide. It is most easily camed out using a computer model such as ASCOS (Ref. 12). However, for most purposes an extract rate of 6 air changes per hour from the f i e floor will provide an acceptable level of depressurization.

5.1.8 Smoke temperature

Having calculated the smoke extract volume it is possible to estimate expected smoke temperature once a design f i e has been chosen (see 4.2). For sprinklered office buildings adesign fire of heat output 1.5MWissuggested. Determining fire size for anunsprinklered office is difficult, however. If the building is subdivided into small rooms, then the heat output of a fire fully involving one (the largest) room could be chosen. A reasonable approximation to the maximum heat release rate Q max is given by:

Q max = L where L = total fire load of the - room expressed in MJ 1200

But it must be accepted that a smoke control system of the type discussed is unlikely to uerform satisfactorilv or be worthwhile uns~rinklered building if asevere f i resul ts in appreciable breakage of external windows.


5.2 Warehouses

5.2.1 Purpose of system

The main use of a smoke extract system in a warehouse would generally be to assist in fire fighting and to provide a means of reducing property damage. Means of escape would normally be adequate to ensure evacuation without a smoke extraction system.

5.2.2 Single-storey warehouses

The design of a natural or powered smoke venting system for a single-storey warehouse or similar building, as shown for example in Fig. 12, is covered fully in Section 4. The main difficulty in designing such systems is in determining fire size. This is discussed in 4.2.

5.2.3 Multi-storey warehouses

For the purposes of smoke control, multi-storey warehouses can be treated as a stack of single-storey warehouses provided that there is sufficient headroom on each storey to allow the establishment of a smoke layer and that each storey forms a single fire compartment.

Fig. 13 shows the recommended system. A single plant room can be used except in very tall buildings where friction losses in common ductwork become excessive.

Other design points are:-

a. Ductwork should be fire-resisting to maintain compartmentation as discussed in 4.8.3.

b. Fans should be provided with duplicate power supply and all cabling associated with them should be of fie-resisting type (see 4.8.2). They should be rated to run at the highest expected smoke temperature (which will occur with fire on the highest floor).

c. Other design points as 5.1.6 iv - viii.

5.3 Underground car parks

5.3.1 Experimental work has shown that even in unsprinklered car parks fire originating in one vehicle does not spread to adjacent vehicles. The design fice size for a smoke extract system for an underground carpark is therefore one car- usually taken to have a heat output of 1.5MW for sprinklered car parks and 3.OMW for unsprinklered car parks (Ref 13).


5.3.2 It has also been shown by experience that in sprinklered car parks, the action of sprinklers is such as to bring smoke to floor level even if a smoke extract system is installed. Thus, the systemisunlikely toprovide a clear smoke-freelayer for f i e fighting purposes and the system willtherefore be essentially a smoke purging system which will assist fire fighters by reducing the temperature and enabling clearance of smoke once the fire has been brought under control.

5.3.3 In unsprinklered carparks, the design of the smoke extract may be carried out on the basis of the guidelines in Section 3 using a 2m design smoke layer for f i e fighting purposes.

5.3.4 Generally, both inlet and extract to anundergroundcarparkneedto be ducted. The ducts themselves should be designed in accordance with 4.8.3, inlets should be at low level, exuact should be at high level and should be distributed evenly overthe whole area of the car park.

5.3.5 The number and therefore size of individual fans is up to the designer provided sufficient volume of smoke is extracted and maximum inlet velocities (see 4.8.1) are not exceeded. Fans should berated to withstand expected smoke temperatures and powered as described in 4.8.2.

5.4 Atrium buildings

An atrium which unites several storeys within a building provides aroute by which smoke and f i e can potentially spread from level to level much more rapidly than in a similar building which iscompartmented at eachlevel by means of solid fie-resisting floors. Such spread of smoke can have a substantial effect upon the time available for escape of the occupants, the activities of fire fighters and the damage to the building structure and contents. (See Fig. 15)

5.4.1 The open nature of atrium buildings often means that it is necessary to rely heavily on active systems such as smoke extraction, sophisticated detectiodalarm systems and sprinkler production to compensate for the lack of compartmentation.

The risk associated with fire in an atrium building will depend upon such factors as the type and usage of a building, the number of people present, the type of fire protection systems and the means of escape urovided. Hospitals. offices, shops. leisure centres - . eic. may all be designed to incorporate one or more Aaeach presentingbifferent problems for means of escaue. Offices traditionally have proved to oresent a very low risk to life but entertainmenis and shopping complexes containing k g e numbeiof people who are unfamiliar with the building represent a significantly different case.


5.4.2 The only current UK document which deals with the design of smoke control systems in buildmgs containing atria is "Fire Safety in Atrium Buildings", Fire Safety Guide No. 2 (Ref. 16) which has been produced by the London District

Surveyors Association (LDSA). Whilst this guide has no statutory standing outside centralLondon it is widely usedin the design and approval of atrium buildings. The LDSA guide puts particular emphasis on meansofescape and draws a clear distinction between the fire protection measuresrequired in buildings where total evacuation can be achieved and buildings where "phased evacuation is utilised" requiring a proportion of the occupants toremain in the building for an extended period. Essentially the LDSA document requires that all storeys which are not separated from the atrium space are capable of being evacuated simultaneously.

5.4.3 The guide states that the provision of a smoke control system is necessary in order to:

i. reduce heat building up on the fire floor and thereby limit the risk of failure of the atrium enclosure (if one exists).

ii. inhibit smoke spread into the atrium in the event of a local failure of the enclosure.

iii. remove heat from the atrium to reduce the risk of failure of the atrium enclosure on the levels above the fire floor.

In addition, the smoke control system for the individual floors is required to be designed to prevent the early spread of smoke into the atrium space. The requirements of the LDSA document can lead to the specification of very wide staircases if a large number of open storeys are planned: which according to the guides provisions must be capable of being evacuated simultaneously.

However in practice, by the provision of a well designed smoke control system, it is often possible to maintain tenable conditions within the open floor areas and increase the number of storeys open to the atrium which may be served by a given staircase specification.

Absolute protection against the consequences of fire can never be achieved in building design. However, a building containing an atrium should be designed in such a manner that it presents no greater risk to life than would be associated with a more traditional building of similar type and occupancy but which does not contain an atrium.


5.4.4 A wide range of building designs have been constructed ranging from fully enclosed atria glazed with fire-resisting materials and containing no fire load to totally open with no separation between the atrium void and the associated floor areas.

If fire occurs on a floor connected to an unenclosed atrium, hot smoke will rise to ceiling level of that storey and spread in all possible directions to form a layer beneath the ceiling. Smoke will thence flow from the ceiling laver into the atrium void where it will tend;o rise upwards due to its natural buoyancy. Xs &e smoke rises through the atrium itwillentrainlargeauantities of cool air from within the atrium reducing the temperature - and increasing 6 e mass and volume of smokey gases involved.

As the smoke plume rises and cools its density increases such that at some height its temperature may fall to that of the surrounding air and will cease to rise by its own buoyancy.

Having risen to an upper limit the smoke will then build downwards to produce alayer of increasing depth which will spread horizontally into any open storeys within the depth of the layer.

5.4.5 Because a substantial proportion of the smoke and toxic fumes arising from a fire on an open floor level will spread directly into the atrium volume the rate of smoke layer development on the fire floor will be significantly reduced. This can provide a substantial increase in the time available for escape on the level at which the fire started.

If the upper levels of the atrium are enclosed with glazing which will remain intact the atrium may, depending upon its size and form, provide alarge smoke reservoir which will contain the smoke and substantially prolong the time available for escape from the lower levels. Also the dilution of smoke which arises from the mixing of air with the rising plume in large atriacansignificantly reduce the hazard presented by the smoke. For most design purposes it is possible that if the smoke from a 1MW fire is mixed in a volume of 75m3s the visibility and toxicity of the smoke will not be sufficient to prevent escape if exposure is for a limited period only.


5.4.6 Buildings containing aaia can be considered in terms of the following main categories for the purposes of f i e safety design:

A. FULLY ENCLOSED - Atrium separated from all floor levels except at the atrium base level.

i. Fire-resisting enclosure (sterile tube). [Fig. 161

ii. Enclosure of limited combustibility but not fire-resisting.

B. PARTIALLY OPEN ATRIUM -Floors open directly to atrium at low level with enclosure above.

i. Fire-resisting enclosure.

ii. Enclosure of limited combustibility but not fie-resisting.

C. FULLY OPEN ATRIUM - No enclosure between atrium and floor areas.

Asnoted above,nonationally agreed guidance iscurrently available in the UKregarding the design for fire safety in amum buildings, however, considerable work is being canicd out at the present time to develop a new British Standard for f i e safety in atrium buildings (BS 5588: Part 7). However, in the interimthe following general principles aresuggested - - & a basis for design of high-rise atrium buildings:

5.4.7 A Fully enclosed atrium

F.x-resisting enclosure to all levels except at atrium base.

If the rit resisting enclosure provides both integrity and insulation inaccordance w:th BS 47f at22 or otherrecognised fire resistance standard for the appropriate period no other action is necessary (other than to comply with statutory requirements fornon- atrium buildings).

ii. If the fire-resisting enclosure provides integrity only, the fire protection systems should be designed to maintain the temperature within the atrium below 300°C so as to reduce tht. ;otential for heat radiation onto the other floors.


This may be achieved by either the installation of sprinklers or by means of smoke/ heat extract systemdesigned tomaintain temperatures within theatriumat orbelow 300°C.

No other action is necessary than to comply with statutoryrequirements fornon atrium buildings.

iii. Atrium enclosed with non-combustible but non-fire resisting glazing.

a. Building should be sprinklered throughout, sprinklers on normal grid.

b. Smokeheat extract system designed to maintain glazing at two levels above fire floor below 300°C or failure temperature of glass whichever is the lower.

aKeA c. If atrium facade contains perforation of limited and phased evacuation is utilised k an extract system designed to maintain the atrium pressure negative relative to the

non-fire floors is recommended.

5.4.8 B. Partially open atrium

As per enclosed atrium design at enclosed levels but following provisions apply to open areas:

i. Occupancy - Awake, mobile and familiar with building (e.g. offices).

If single stage evacuation of open storeys with standard exit widths and travel distances no special exit provisions required provided that it can be shown that untenable conditions do not arise on any open floor within the expected evacuation period.

ii. Occupancy - awake, mobile but unfamiliar with building (e.g. shops).

Single stage evacuation is recommended together with a smoke control system designed to maintain the smoke layer base above highest open storey assuming a steady state design fire. Alternatively, aspecific fire growthhime evacuation study may be carried out and a smoke control system designed to ensure that the occupied levels will remain tenable throughout the evacuation period.


5.4.9 C. Fully open atrium

Occupancy - awake, mobile and familiar with building.

For all high-rise buildings a sprinkler system should be provided throughout.

The smoke control system should be designed to ensure that untenable conditions do not arise (other than on the fire floor) within a period of at least equal to the anticipated evacuation time.

An alternative strategy would be to provide "waiting areas" in the form of separate fire compartments at each level which are capable of holding the total occupancy of the floor whilst awaiting evacuation via the staircase. Under non-emergency conditions such an areacould serve as an office or fulfil some other purpose provided that sufficient space is available for waiting space. If this strategy is adopted the smokecontrol system should be designed to maintain the waiting area at a positivepressure relative to the atrium space to ensure that smoke does not hazard the refuge areas.

5.4.10 Provisions for fire fighting

To assist fire fighting operations and smoke clearance it is common practice to provide a smoke extract system designed to provide an hourly air change rate of 6 (although certain U.S. codes accept lower air changerates) basedupon the totalvolume of the atrium space and the associated open floor area.

5.5 Shopping malls

5.5.1 The smokecontrolrequirement for shopping malls of more than one storey arise essentially as the malls must be treated as escape routes. Smoke escaping from a shop on the ground storey will entrain air as it rises and the large volume of smoke produced can fill the mall area within a relatively short period unless provision for sprinkler protection and smoke control is made.

5.5.2 There are two common approaches to this problem:-

a. extract smoke from each individual shop unit so that smoke is prevented from entering the mall.

b. extract smoke from the mall itself to maintain pedestrian levels clear.


5.5.3 Extract from individual shop units

This approachisillustrated inFig. 17. There are two main advantages of such amethod

i. smoke is kept off the mall and there is no need for shoppers to escape through or beneath a hot smoke layer.

ii. very large fan sizes as required for mall extraction are not needed.

Set against this is the high additional cost complexity that these systems pose over mall extract systems.

Design features to note are:-

i. Shops must be sprinklered. In the U.K. design is usually based upon a fire of 5MW, with a 12m perimeter sprinkler controlled fire.

ii. To reduce cost use combined ductwork and fans serving many shops. Fans and ductwork should be sized to cope with the "worst case"demand - usually extract from the shop unit most remote from the fans. Power and cabling to fans should be as described in 4.8.2.

iii. Common ductwork which passes through compartment walls should be inherently fire-resisting or encased with fire protection material as described in 4.8.3.

iv. Smoke and fire rated dampers are needed where ductwork within a shop enters common ductwork. They should be to UL 555s Class I, II, IJI or IV. (NB. More leaky dampers, i.e. Class EIor IV, means that more air will be drawn into the smoke extract ductwork through these dampers, therefore requiring larger ductwork and/or fans.)

v. Because the primary purpose of the smoke extract system is not to maintain a high smoke layer within the shop to facilitate escape within the shop (it is assumed that means of escape within the shop is such to allow escape before potentially dangerous conditions arise) the design smoke layer can be lower than the 2.5 - 3.0m recommended in 4.3.2. The main constraint on smoke layer depth is the need for people to pass beneath reservoir smoke screens at the junction between shop and mall, suggesting a minimum design smoke base height of 2m.


5.5.4 Extract from mall

This approach is illustrated in Figs. 18 and 19. Detailed guidance on one method of design is given in Ref.l., although the use of an engineering design based upon the principles outlined in Section 4.2.6 may often provide a more effective and economic design.

i. The amount of smoke produced by the rising line plume increases markedly with height. It is not therefore practical to use this method for malls of more than 2 storeys and the approach described for atria in Section 5.4 will be more appropriate.

ii. To limit the amount of smoke entering the rising plume it is usually necessary to channel smoke so that the perimeter of the plume is reduced. This is done using channelling screens as shown in Fig. 20.

iii. The volume of smoke in the plume can be estimated either by using the figures in Ref.1, or by using the following formula (simplified from Ref. 17.).

where M = mass flow of smoke kgls

Q =heat output of fire kW

L = width of plume (i.e. distances between chanelling screens) m

Z = height of rise of plume from base of upper floor slab to smoke layer base m

A = half height of lower storey m (Ref. 18.).

(NB. This formula assumes an ambient temperature of 20°C.)

WFRC No. C49378 Page 3 1 of 76

iv. The closer chanelling screens are placed together, the smaller the mass of smoke that needs to be extracted, but the greater the required depth of the screens or curtains themselves. A design compromise has to be made. The required depth of screen can either be taken from tables in Ref. 1. or can be calculated using the following equation:

Where D = depth of flowing smoke layer (i.e. minimum depth of chanelliig screenlsmoke curtains) m

and W = width between chanelling screens or smoke curtains.

v. The width between channelling screens should not be less than the shop front they serve. See Fig. 20.

vi. A fireinashoplargerthan 1000m2may result in cool smoke enteringthe mall which cannot always be successfully extracted by this method. In such cases consideration should begiven to providing such large shopunits with their own independent extraction system.

*NB. This is a simplified formula assuming that ambient temperature is 2PC, fire size is 5MW with a 12mperimeter and that the height ofrise to the base of the smoke layer within abuming shop is 2.5m. The mass of smoke flowing out of a shop (which involves additional entrainment of air) is assumed to be double that produced by the smoke plume in the shop itself i.e. M = 2 x 0.19 PY'.~.


6. NATIONAL DIFFERENCES AND REQUIREMENTS

6.1 Hong Kong

The Hong Kong Fire Services Department have published a code of practice for smoke control systems (Ref. 19).

In particular, the following points should be noted:-

6.1.1 Powered ("Dynamic") Systems

i. The FSD code refers to systems designed to assist fire fighters. Accordingly a smoke layer depth of 2m is considered acceptable.

ii. The FSD limit individual smoke reservoirs to a maximum of 500mZ (compared to 2000mZ suggested in this guide).

iii. A minimum extract rate of 8 air changes per hour is required (or 10 air changes of the volume of means of escape comdors in hotels).

iv. No one system can serve more than 10 fue compartments.

v. As well as being activated by an automatic fire detection system and by operation of a sprinkler if fitted the system should also be operated by any other fire detection or fire protection system installed in the area.

vi. Ductwork should resist mechanical damage as tested by BS 5669 (Ref. 20) and be pressure tested to HVCA DW 143 (Ref. 28).

vii. Duplicate fans and ancillary equipment are required if a system protects a sleeping risk. Duplicate plant are also required for basements, but in thiscase eachplantneed only provide half the-required extract or inlet capacity.

viii. There are strict requirements on completion procedures, testing and maintenance. F L M frjwcr.t G

6.1.2 I3qmmd"static" systems

i. Smoke curtains ("smoke barriers") are required to have 1 hour's fire resistance.


ii. Systems can be based on an aerodynamic vent area* of 2% of floor area of which half must be permanently open or automatically operated (i.e. half may be by openings breakable by the fire service e.g. pavement lights).

6.2 Singapore

The Singapore code - Fire Precautions for Buildings 1982 (Ref. 21) calls for smoke extract in basements. The requirement is for a vent area of 0. lmZ for every 150m3 of the compartment volume arranged to ensure cross-ventilation. The vents can be stallboards or pavement lights.

If smoke extract shafts are used to feed the vents they are required to have the full period of fire resistance of the floors through which they pass.

As an alternative to natural vents a mechanical extract system can be used. Designers are recommended to follow thisguide, and also Singapore Standard CP13:1980 (Ref. 22). CP 13 makes two specific requirements for smoke extract systemsfor basement carparks:-

a. heavy guage (1.2mm) steel should be used for horizontal ductwork.

b. the exhaust fan should be rated to operate at 250°C and have a secondary power supply.

6.3 Taiwan, Malaysia, Philippines, Indonesia

There are as yet no specific requirements in these countries or codes for smoke ventilation systems.

* Footnote: Aerodynamic vent area = physical area of vent (Av) x coefficient of discharge (C) (C normally = 0.6)


8. REFERENCES

I . H.P. Morgan.

Smoke control methods in enclosed shopping complexes of one or more storeys: a design summary. (1979).

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

2. National Fire Protection Association.

Guide for smoke and heat venting.

NFPA 204M. (1985). National Fire Protection Association, Batterymarch Park, Quincy, MA 02269, USA.

3 . Thomas PSI. and Hinkley P.L.

Design of roof-venting systems for single-storey buildings.

Fire Research Technical Paper No.10, Joint Fire ResearchOrganisation. UK. (1971). Available from Colt a eat in^ andventilation ~imited, ~ o w L u n e , ~ a v a n t , ~ a n t s , PO9 2LY, U.K.

4. Morgan H.P.

A simplified approach to smoke-ventilation calculatiom.

Building Research Establishment Information Paper IP 19/85. (1985). BRE, Fire Research Station, Borehamwood, Hertfordshire WD6 2BL, UK.

5 . BS 5839: Fire detection and alarm systems in buildings: Part I : 1988: Code of Practice for installation and servicing.

6. BS 6207: Part 1: 1985: Copper sheathed cables with copper conductors.

7. BS 6387: 1983: Spec ification for performance requirements for cables required to maintain circuit integrity under fire conditions.

References5.6 and 7from British Standardslnstitution, Linford Wood, Milton Keynes, MK14 6LE, UK.

8. HVCA DW142: Specification for sheet metal ductwork (1982).

HVCA Publications, Old Mansion House, Eamont Bridge, Penrith, Cumbria, CAI0 2BX, U.K.


9. BS 476: Part 24: 1987: Methodofdetermination of the fire resistanceofventilation ducts.

10. BS 5588: Part 4: 1978: Code of Practice for smoke control in protected escape routes using pressurization.

Refs 9 and 10 available from BSI (See Ref. 7).

11. Underwriters Laboratories.

Leakage rated dampers for use in smoke control systems.

UL 555S, (1983). Underwriters' Laboratories.

12. Klote J, Fothergill J.W.

Design of smoke control systems for buildings.

National Bureau of Standards Handbook 141. (1983). Available from Superintendent of Documents, US Government Printing Office, Washington DC 20402. (USA).

13. Guide to fire safety in Section 20 buildings.

London District Surveyor's Association.

14. Hansell G.O.

Smoke control in atrium buildings.

Building Control (UK) MaylJune 1987. Institute of Building Control Oflcers.

15. Fothergill J.W.

The atrium as a fresh air channel - a diSferent concept in smoke control.

ASHRAE Transactions, 86(1) 1980 pp 624 - 635 American Society of Heating, Refrigeration and Air Conditioning Engineers Inc., Atlanta, GA 30329. (USA).

16. London District Surveyor Association.

Fire Safety in atrium buildings.

Fire Safety Guide No. 2 (1989) LDSA Publications.


17. Pa. Thomas.

On the upward movement of smoke and related shopping mall problems Fire Safety Journal 12 (1987),pp 191-203.

18. A. Porter.

Large scale tests to evaluate mass flow of smoke in fire plumes.

Paper given to Symposium at Fire Research Station, Borehamwood, Herts, UK, 27th June 1989.

19. Hong Kong Fire Services Department.

Code of practice for minimumfire service installations and equipment. Part V specification and testing. S.26 Smoke extraction systems.

20. BS 5669: 1979: Specifcation for wood chipboardand methods of testfor particle board.

Available from BSI - See Ref. 7.

21. Fire Precautions for buildings 1982.

Development and Building Control Division (PWD) Singapore.

22. Code ofpractice for mechanical ventilation and air-conditioning in buildings.

Singapore Standard CP13: 1980. Singapore Institute of Standards and Industrial Research.

23. Chartered Institution of Building Services.

Design notes for ductwork (1983).

CIBSE, 222 Balham High Road, London SW12 9BS, UK.

24. HEVACOMP, 212-218 West Street, Sheffield, SI 4EU, UK

25. R.G. Gewain.

Fire Research for steel HVAC systems.

Fire Journal, November 1984.


26. American Iron and Steel Institute.

Summary report. Fire resistance of major duct materials.

American Iron and Steel Institute, 1000 16th Street N.W., Washington D.C. 20036.

27. National Fire Protection Association.

Standard for the installation of air conditioning and ventilating sytems.

NFPA 90A. NFPA, Batterymarch Park, Quincy, MA 02269, USA.

28. HVCA. DW142.

Guide to ductwork leakage testing.

Obtainable from HVCA. See Ref. 8

29. H.E. Nelson.

Engineering analysis of the early stages of fire development. The fire at the Du Pont Plaza Hotel and Casino - December 31,1986.

NBSlR 87-3560. National Bureau of Standards, USA.


TABLE 4.5.1

SMOKE PRODUCTION AND SMOKE TEMPERATURE FOR VARYING FIRE SIZES

Scenario Fire Perimeter Fire Output Height To Smoke Smoke Smoke Smoke Base Production Temperature Volume

P Q Y M 0 V m MW m kgls OCabove m3/s

ambient

Sprinklered office 12 15 2.0 6.5 233 9.6 2.5 9.0 166 11.7 3.0 9.9 152 12.4

Sprinklered shop 12 5 2.0 6.5 775 19.4 2.5 9.0 555 21.6 3.0 9.9 506 22.3

Sprinklered warehouse 18 10 2.0 9.7 1034 36.2 2.5 13.5 740 39.4

Sprinklered hotel 12 2.5 2.0 6.5 388 12.4 (public area) 2.5 9.0 277 14.5

3.0 11.9 211 16.8

Sprinklered basement 15 7 2.0 8.1 868 26.5 service area

( low park)

Unsprinklered car park 13.5 3.0 2.0 7.3 414 14.6


APPENDIX 1

DUCT SIZING

Ducts for smoke extract and inlet should be sized using a standard method such as the Equal Friction Loss Method as described in Ref. 23. Alternatively any standard computer duct-sizing method (eg. Ref. 24) suitably modified can be used. The main differences between sizing ductwork for smoke extract rather than standard HVAC purposes are:

a. Noise and running costs (within limits) are not a problem. Thus velocities can be increased beyond that assessed in HVAC systems.

A recommended "working" velocity for the design of smoke extract ductworkis 20m/s. At much greater velocities than this friction losses become excessive.

b. Smoke temperatures are frequently appreciably greater than ambient and hence air density is less than 1.2kg/m3 as assumed for example in Ref. 23. Corrections therefore have to be made for air density in values of pressure loss obtained from standard tables. The values should be multiplied by a correction factor E obtained from:

Where /3 = air density at smoke temperature.

Table A1 gives values of E for smoke temperatures between 343K (70°C) and 793K (52OOC).

The smoke temperature values used should be 293 + 0 with 0 calculated according to 3.5 unless the building is sprinklered. In sprinklered buildings use 293 + 8 or 250°C whichever is the lesser.

Inlet ductwork should be designed using standard pressure loss values i.e. assuming ambient air conditions.


TABLE A1 - Values of of E for different smoke temperatures

Smoke Temperature (deg. K)


APPENDIX 2

Construction of the Fire-Resisting Smoke Control Ducts

F i e resistance of smoke control ductwork can be achieved in two ways:

a. Fire-resisting enclosure

The steel ductwork may be enclosed by a fire-resisting, non-combustible enclosure (for example constructed from fire-resisting boards) which provides the necessary degree of fire resistance. In multi-storey buildings the enclosureeffectively forms a fire-resisting protected shaft - see Fig.5.

No essential safety services should be included in smoke extract ductwork enclosures if they will be adversely affected by duct temperatures.

b. Fire-resisting ductwork

The ductwork itself provides thenecessary fire resistance (either inherently orby virtue of a protective cladding).

In cases of doubt the fire resistance of ductwork should be demonstrated by testing to an appropriate test such as BS 476: Part 24 (Ref. 9).

In practice fie-resisting ductwork can be constructed in three ways:

a galvanised steel with additional protection

b. fire-resisting boards

c. proprietary fire-resisting ductwork.

c. Galvanised steel with additional protection

This is probably the most common form of fire-resisting duct construction. Fire resistanceupto4hours both to internal andexternal fire can be achievedprovided sufficient thickness of fire-~esisting insulating material (e.g. boards or sprayed materia1)isapplied and f i ings are adequate.

Designers are advised to follow the recommendations of fire protection manufacturers. A typical method of fixing fire-resisting boards to a large duct is shown in Fig. 21.


APPENDIX 2 (Continued)

b. Fire-resisting board

If it is necessary to protect ductwork with fire resisting board then it is often more cost- effective to construct the ductwork entirely from this material. Designers are advised to contact board manufacturers or independent organisations for details of tested designs. One such design which can provide 4 hours fire resistance is shown in Fig. 22.

c. Proprietary systems

There are several proprietory ductwork systems specifically designed foruse as smoke extract ducts. The suppliers of these systems should be contacted for further details.

In all the above cases the designer should ensure that auurouriate test data and/or - A

assessments are available for an independent laboratory to justify the particular form of construction, itssuitability for theparticular location and the required fire resistanceperiod.

Hangers

To maintain its fie-resisting stability and integrity, it is necessary that fire-resisting ductwork should not fail. To ensure this, hangers should be designed such that they can support the weight of the duct and any additional applied fire protection under f i e conditions. This can be achieved with unprotected steel hangers if the stress limits shown in Table A2.1 are not exceeded.

If higher loads are applied it will become necessary to protect the hangers with an appropriate thickness of fie-resisting material to ensure that the hangers do not reach the critical temperature at which they are unable to support the weight of the duct plus fire- resisting cladding.

Table A.2.1 Maximum stress in unprotected hangers

Required Fire Resistance Maximum Stress Minutes N/mmz


APPENDIX 3 Worked Examples

A.3.1 Example 1 Multi storey warehouse

See Fig. 23.

The system is for amulti-storey warehouse. Each storey is a 2 hour fire compartment measuring 20 x 15m. Two staircases are provided, door to staircases are 1.8m high.

Floor to ceiling height is 3.2m. The building is not sprinklered, but there is an automatic fire detection system l iked directly to the f i e brigade which expects to respond within five minutes. Following Section 4.

1. Why do it? The system is required to aid f i e fighting operations.

2. Determine fire size Study of contents of warehouse (motor parts in boxes stored on pallets) suggests afast

fire growth rate.

Thus Q = 0.047t2 0 . ~ q 1

:. Q = W x 3002 = 4230kW

Perimeter = 2 QlJ - 2 4230 IT =@ J5, - J500

?I2 Using M = 0.19py

= 0 . 1 9 ~ 10.3 x 2

Assume '1, of energy of fire absorbed by building structure, i.e. effective heat output of f i e = 4230 x 2/3 = 2820kW.

Thus 8 = a = 2820 = 'S~%'L M 5.5

: , , 0 , ."b. 0 3 .: ,S :: ass 2: ,.,

V = M (To + 01 354.5 r: fi + f . a . l l ( 3 % ~ .~. ~. i,< ~. .. ..

2 0


EXAMPLE 3 (Continued)

3. Calculate minimum number of extract points

With d = 3.2 (ceiling height) - 2 = 1.2m

Vent = 2 (e x 1.25 x 513 TO)O.~ Tc

:. No. of extracts required = 12.5 = 3 4.8

Consider run of extract ductwork. Suggestedrun as shown. No part of reservoir greater than 30m from extract.

4. Consider inlet

Extract = 12.5m3/s

Suggest natural inlet using automatically opening low level vents as shown.

Using maximum inlet velocity 3m3/s

:. Minimum vent area = 12.5 = 4.2m2 3

5. Design ductwork and fans

Calculated smoke tempelture 513OC above ambient

:. Fi-resisting ductwork required.

If smoke extract riser serves several storeys, fire and smoke-sealed dampers required at entry to riser.

Ductwork and fan sizes should be calculated following 4.7 and 4.8. Fan should be designed to withstand smoke temperature in excess of 513OC and therefore suggestedrating 6WC.



A.3.2 Example 2 - 3-Storey shopping mall

See Fig. 24.

The system is for a 3-storey shopping mall. Floor to ceiling height 4.5m. Rooflight over mall (3m to eaves). Shops are sprinklered.

Concept

As 3-storey mall, impractical to extract smoke from ground storey fire via extract in rooflight. Thus extract upper 2 storeys from rooflight and ground storey extract from individual shops.

Following Section 4.

1. Why do it?

The systemis required for life safety, i.e. to maintain asmoke free layer inmall andupper walkways.

2. Determine fire size

From Table 4.2.7. Choose design fire of 12m, 5MW.

3. Determine acceptable smoke layer base

Design for 3m at first storey, 3.5m at upper storey (see 4.3.2). On ground storey, no necessity to provide smoke free layer in shop itself. Smoke layer depth determined by shop fascia/downstand. Sugest set at 2.5m above floor level (i.e. Fascialdownstand = 4.5 - 2.5 = 2m).

4. Identify smoke reservoirs

Ground storey - each individual shop.

First and second storeys - mall rooflight. Area = 80 x 100 = 800m2 = acceptable.

5. Calculate smoke volume and temperature

Ground storey.

From Table 3.5.1 choosing P = 12m, Q = 5000kW, y = 2.5m

M = 9.0lkgIs



Mall extract

From Table 4.6.1

8 = 34OC Tc = 327K

d = depth from centreline of fan intake to smoke layer base = 4m.

:. Vent = 2 ix 9.81 x 4 5 ~ 327 x 2931 = 62047 = 190m3/s - 327 327

Required extract = 136m3/s

:. Minimum number extract = 1

However, if only 1 outlet smoke must travel >30m to reach extract. Therefore, suggest 2 No. fans in end wall of rooflight as indicated in Fig. 29A.

7. Design ductwork and fans Ground storey.

Calculated smoke temperature in shop = 550°C.

However, shop sprinklered. :. Assume maximum250°C :. ductworkneed not be fire- resisting. However, ductwork passes through fire compartment boundaries between shops and between floors. :. F i e resisting ductwork to be provided as shown in Fig. 29.

Fans to be. rated at 250°C.

Fire and smoke rated dampers as shown in Fig. 25 necessary to isolate shops not on fire.

Size ductwork and fans according to 4.7 and 4.8.

Mall extract

Fans to be sized at 37.1 = 19 = 20m3/s at 180°C. 2

5th March 1990 WP Ref: M E W B

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Page 54 o f 76

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Fire-resisting construction

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Fire and smoke-rated damper

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