cibse tm smoke

Technical MemorandaTM19:1995

Relationships for smoke control calculations

CIBSEThe Chartered Institution of Building Services Engineers

Delta House, 222 Balham High Road, London SW12 9BS

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No part of this publication may be reproduced, stored in a

retrieval system or transmitted in any form or by any meanswithout the prior permission of the Institution.

September 1995 The Chartered Institution of Building ServicesEngineers London

Registered charity number 278104

ISBN 0 900953 69 1

This document is based on the best knowledge available at thetime of publication. However no responsibility of any kind for anyinjury, death, loss, damage or delay however caused resulting

from the use of these recommendations can be accepted by theChartered Institution of Building Services Engineers, the authorsor others involved in its publication. In adopting these

recommendations for use each adopter by doing so agrees toaccept full responsibility for any personal injury, death, loss,

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therefore and agrees to defend, indemnify and hold harmlessthe Chartered Institution of Building Services Engineers, the

authors and others involved in their publication from any and allliability arising out of or in connection with such use as aforesaidand irrespective of any negligence on the part of thoseindemnified.

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Printed in Great Britain by Bourne Press Ltd. 311 Spring Road,Bournemouth, Dorset RH1 4QA

Foreword

The use of fire engineering concepts as an alternative to prescriptive codes and guides isincreasing and may be considered as essential for some complex projects. The purpose ofthese Technical Memoranda is to summarise the current state of knowledge of theengineering relationships for smoke movement which can be taken into account whendesigning smoke detection and control systems. The Memoranda can be used directly, buttheir chief purpose is to provide input to design guides and similar publications. Therelationships given are not rules and may be superseded or modified as more informationbecomes available.

The inspiration for these Memoranda arose from an approach by the CIBSE to the Instituteof Fire Safety (formerly the Society of Fire Safety Engineers) and it is hoped that suchcooperation will continue. The CIBSE gratefully acknowledges the contributions made byall members of the Task Group. The Task Group was particularly assisted by the SFPEHandbook of Fire Protection Engineering and the American National Standard ANSI/NFPA92B.

The Technical Memoranda are sponsored by CIBSE and the Institute of Fire Safety (IFS).

Professor Margaret LawTask Group Chairman

Task Group

Margaret Law MBE (Arup Fire) (Chairman)Gordon ButcherGeoff Cox (Fire Research Station)Graeme Hansell (Michael Slattery Associates; formerly Colt International Ltd.)Frank MillsAlan Porter (Warrington Fire Research)Philip ThomasChris Trott (Ove Arup & Partners) (Secretary)Peter Warren (Fire Research Station)

Publications Secretary

Ken Butcher

Editor

Ken Butcher

Note from the publisher

This publication is primarily intended to provide guidance to those responsible for thedesign, installation, commissioning, operation and maintenance of building services. It isnot intended to be exhaustive or definitive and it will be necessary for users of the guidancegiven to exercise their own professional judgement when deciding whether to abide by ordepart from it.

Contents

1 Introduction and context

2 Symbols and definitions

3 Essential requirements the hazards of smoke

3.1 The hazards of smoke

3.2 Critical conditions for toxicity design values

3.3 Critical conditions for temperature design values

3.4 Critical visibility design value

3.5 Firefighting

4 Design fire

4.1 Condition for flashover

4.2 Pre-flashover fires

4.3 Post-flashover fires

4.4 Fire products

4.5 Estimation of visibility

5 Plumes

5.1 Axisymmetric plume small diameter source

5.2 Axisymmetric plume larger diameter source

5.3 Line source

5.4 Flow from an opening

5.5 Ceiling flow

5.6 Plume temperature

5.7 Volume flow

5.8 Ceiling jet

5.9 Convective heat release

6 Accumulated ceiling layer

6.1 Zone models

6.2 Smoke filling a room with low level ventilation opening

6.3 Smoke filling an open room approaching flashover

6.4 Heat transfer to building surfaces

6.5 Heat transfer by radiation from smoke layer

6.6 Stratification

6.7 Number of extract points

7 Methods of smoke and heat management

7.1 General

7.2 Design principles of smoke management systems

7.3 Systems intended to protect the area in which the fire starts

7.4 Systems intended to protect parts of the building beyond thefire area

1

1

2

2

2

3

3

3

3

3

3

4

5

5

6

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6

7

7

8

8

8

8

8

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9

9

10

11

11

11

11

12

12

12

12

12

8 General design parameters for smoke managementsystems

8.1 Actuation

8.2 Design parameters for systems intended to maintain asmoke-free layer

8.3 Design parameters for systems intended to dilute the smoketo a tenable condition

12

12

13

8.4

8.5

Systems intended to protect other parts of the building enclosure to contain smoke

Systems intended to protect other parts of the building pressurisation

8.6

8.7

8.8

Systems intended to protect other parts of the building opposed air flow

Systems intended to protect other parts of the building depressurisation

Means of removal of smoke

References

Appendix 1 Dimensions of a room or compartment

Appendix 2 Computational fluid dynamics (CFD)

A2.1 Introduction

A2.2 Principles

A2.3 What the models predict

A2.4 What assuptions are necessary

A2.5 Numerical treatment

A2.6 Mode of working

Appendix 3 Background notes

Index

14

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Relationships for smoke control calculations

1 Introduction and context

These Technical Memoranda have been prepared in order toprovide engineering relationships which can be used as partof the overall fire safety design of buildings with atria andother spaces where large numbers of people may be exposedto smoke, toxic atmospheres and hot gases. The need forsmoke control depends on many aspects of the buildingdesign and use, including the combustibility of thecontents, mobility of occupants, and ease of escape. Thesmoke control measures needed, if any, may be simple perhaps exploiting the normal ventilation system or theymay require extra equipment and controls. These consider-ations are taken in context in fire safety engineering designand are dealt with elsewhere(1). This publication is intendedas a source document for design guidance.

The relationships are based on published, authoritativeinformation, where this is available, and the limits ofapplicability are suggested. In cases where the basis of arelationship is not firmly established, the relationship isgiven on the understanding that it may be superseded whenfurther information is available. This is made clear in thetext. It should normally be possible to use the information

given here without resort to computational fluid dynamics (CFD) or physical modelling, although these are valuable

hbhnhoIrKLlclsMMcMo

MoutmCOmsnvPQQaveQfQ pQp

Qpc

tools which can be used for unusual designs or to generatefuture design guidance. The basic principles involved inCFD and what is offered to the designer are described inAppendix 2.

Background notes and sources for these Memoranda aregiven in Appendix 3. However, information on smokegeneration and smoke control is increasing rapidly and newdata can be used to augment the guidance given here.

2 Symbols and definitions

Af floor area of room (m2)

Al leakage area (m2)

Anet internal surface area of room minus area ofopenings (i.e. (At Ao)) (m

2)Ao area of opening (window or doorway) of a room

(m2)As plan area of fire source (m

2)At internal surface area of room (walls, floor, ceiling)

(m 2 )Avi area of ventilation inlet (m

2)

Avob

CdcPcwdd1ds

gHch

Q*RrSTTcTm ToTsttctd

area of ventilation outlet (m2)depth of balcony (horizontal distance between edgeof balcony and plane of opening below) (m)discharge coefficient for vent ()specific heat capacity of air (kJ/kg K)specific heat capacity of wall material (kJ/kg K)depth of room behind opening (m)depth of smoke layer flowing under a ceiling (m)diameter of source or longer side of rectangularsource (m)acceleration due to gravity (m/s2)heat of combustion (kJ/kg)floor-to-ceiling height of room or height of ceilingabove base of fire (m)height of balcony above base of opening (m)height of neutral plane above base of opening (m)height of opening (window or doorway) (m)intensity of radiation (kW/m2)extinction coefficient (m1)fire load in equivalent weight of wood (kg)separation of channelling screens (m)length of line source (m)mass flow of entrained air (kg/s)mass flow at heig ht zc (kg/s)horizontal mass flow from opening of a roomcontaining a fire (kg/s)mass flow of vented (extracted) smoke (kg/s)mass rate of generation of carbon monoxide (kg/s)mass concentration of smoke aerosol (kg/m3)air changes per hour (h1)perimeter of plan area of fire source (m)rate of heat release (kW)average rate of heat input (kW)rate of heat release to cause flashover (kW)convective portion of heat release rate (kW)convective portion of heat release rate per unitlength of line source (i.e. (Qp/ls)) (kW/m)convective portion of heat release into corridor(kW)Qp / [ oTo cp (gh)

1/2h2] ()rate of burning (kg/s)horizontal distance from fire axis (m)visibility (m)(absolute) temperature of smoke (K)(absolute) axial temperature of Gaussian plume (K)average (absolute) temperature of plume (K)(absolute) ambient air temperature (K)average (absolute) temperature of smoke layer (K)time after effective ignition (s)characteristic burn time (s)time for smoke dilution (h)

1

r

TECHNICAL MEMORANDA

tgu

Vsvwwc

woYCO

YHCl

YHCN

Ysmoke

Z

z

characteristic growth time (s)velocity of smoke layer or jet under ceiling orairflow (m/s)volume of smoke (m3)volume flow of smoke or air (m3/s)width of wall containing an opening (m)width of channel under a ceiling or a corridor width

(m)width of an opening (window or doorway) (m)mass yield of carbon monoxide per mass of fueldecomposed (kg/kg)mass yield of hydrogen chloride per mass of fueldecomposed (kg/kg)

zbzczlzm

zo k

D P

s

w

o

s

w

s

3

3.1

mass yield of hydrogen cyanide per mass of fueldecomposed (kg/kg)mass yield of smoke particles per mass of fuel burnt

(kg/kg)height above base of fire (inside fire room) or heightabove top of opening (outside fire room) divided byfloor-to-ceiling height of room or height ofceiling above base of fire (i.e. (z / h)) ()height above base of fire (inside fire room) or heightabove top of opening (outside fire room) (m)height above a balcony (m)critical height of clear layer (m)limiting height (intermittent flames) (m)maximum height of smoke rise above base of fire(stratification) (m)height of virtual source above base of fire (m)effective heat transfer coefficient (kW/m2 K)pressure differential across the barrier (Pa)emissivity of smoke layer ()thermal conductivity of wall material (kW/m K)density of smoke at temperature T (kg/m3)density of ambient air (kg/m3)average density of smoke layer (kg/m3)density of wall material (kg/m3)StefanBoltzmann constant (kW/m2 K4)t (g / h)1/2(h2 / Af) ()

Essential requirements the hazards of smoke

The hazards of smoke

The toxic products of fires consist of irritant and narcoticcomponents, which can cause disorientation, incapacit-ation or death, the effect depending upon the concentrationand length of exposure. The predominant irritant com-

Table 3.1 Exposure to common toxic products of combustion

ponents are organic smoke products and acid gases such ashydrogen chloride. The immediate effects of irritants areconcentration related, consisting of pain to the eyes andlungs, accompanied by difficulties in breathing. Thepredominant narcotic component, in pre-flashover fires iscarbon monoxide, hydrogen cyanide also being importantin pre-flashover fires. Narcotic effects, disorientation andcollapse, occur only when a certain dose (i.e. the product ofexposure concentration and exposure time) has beeninhaled over a period.

The temperature of smoke is significant since it can causeburns by convection, to exposed skin and lungs, and byradiation. With long exposure times there is risk ofhyperthermia.

Smoke particles and irritant products can reduce visibility.While loss of visibility is not directly life-threatening, it canprevent or delay escape and thus expose people to the risk ofbeing overtaken by fire.

In general, people who can see at least 8 m in front of themare prepared to walk through smoke. If a tenability limit forvisibility for escape purposes is set at 8 m (for lightreflecting surfaces) then, for most fire types, it is likely thatsmoke at this concentration would cause some eye irritationbut it is unlikely to contain irritants at high enough con-centrations seriously to inhibit escape or cause collapse.Neither well ventilated pre-flashover fires, nor post-flashover fires at smoke densities up to this limit are likelyto contain sufficient narcotic gases to cause disorientationor collapse within a 5 minute exposure period, althoughsuch effects may become a hazard over longer time scales. Itmay therefore be convenient to define critical conditionsfor short exposures in terms of visibility only. However, hotsmoke layers above head height can still pose a radiationhazard and the concentration of toxic smoke to whichpeople may be exposed can increase very rapidly.

For the purposes of these Technical Memoranda, it isassumed that the fires are flaming and well ventilated, sincethese are the types of fire which are of interest to thedesigners of smoke control systems (see section 4: Designfire).

3.2 Critical conditions for toxicity

design values

Table 3.1 is provided to enable the designer to estimatewhether there is likely to be a toxic threat to the occupantsof a building. For that purpose, the effects of these common

Chemical product 5 minutes 30 minutes

Incapacity Death Incapacity Death

Carbon monoxide CO (ppm) 6000 12000 1000 2500

Hydrogen cyanide HCN (ppm) 150 250 90 170

Hydrogen chloride HCl (ppm) 500 16 000 200

toxic products may be considered as approximatelyadditive: for example, there would be incapacitation after5 minutes for a concentration of 4000 ppm CO plus 50 ppmHCN (two thirds of CO dose plus one third of HCN dose). Amore detailed study of toxic effects, which is outside thescope of this publication, would consider other aspects suchas oxygen deficiency and increase in carbon dioxide.

For well ventilated pre-flashover fires the contributionfrom low oxygen and HCN may be considered relativelyminor.

The estimation of toxic concentrations in smoke isdescribed in section 4.4, and that of smoke extinctioncoefficient (K) in section 4.5.

3.3 Critical conditions for

temperature design values

The critical temperatures for various conditions ofexposure are given in Table 3.2. The temperature attainedby smoke is described in section 5.

Table 3.2 Critical temperature for different exposure conditions

Type and period Effect Temperature

of exposure (C)

Radiation Severe skin pain 185

Conduction (metal)

(1 second) Skin burns 60

Convection (30 minutes) Hyperthermia 100

Convection (< 5 minutes) Skin/lungs burns

by hot gases 120

Convection (< 1 minute) Skin/lungs burns

by hot gases 190

black-body: 2.5 kW/m2

Critical visibility designvalue

For the purposes of escape, visibility should be at least 8 m.The estimation of visibility in smoke is described in section4.5.

3.5 Firefighting

Firefighters wearing breathing apparatus can feel their waythrough thick smoke, provided that the smoke temperaturedoes not exceed the values given in Table 3.2.

4 Design fire

The design fire is characterised here by the variation of heatoutput with time. In the initial stages of fire growth it isassumed here that the fire is well ventilated, its rate ofburning being characterised by the type, amount and

RELATIONSHIPS FOR SMOKE CONTROL CALCULATIONS

configuration of the fuel. The fire is assumed to be confinedinitially to a single object or group of objects.

Some measured rates of heat output for various objects aregiven in Table 4.2.

If unchecked, the fire may spread to adjacent objects andonce flames reach the ceiling then flashover may occur,when the whole room or compartment becomes involved ina fully developed fire. After flashover, the rate of smokeproduction can be so great that smoke control becomesimpractical. However, if there is a post-flashover fire in asmall room, it may be possible to design a smoke controlsystem which protects an adjacent large-volume space, suchas an atrium, when smoke emerges from a window ordoorway of the room in which the fire has occurred.

4.1 Condition for flashover

For design purposes it may be assumed that flashover doesnot occur if the smoke layer at ceiling level is at a temp-erature of less than 600C. Methods of calculating thistemperature are given in section 5.

Note that in the plume above a fire the temperature at the tip ofintermittent flames is about 350C and at the tip of sustainedflames is about 550C.

If sprinklers operate it may be assumed that flashover willnot occur since sprinklers are designed to operate while thesmoke layer is at a temperature much lower than 600C.

4.2 Pre-flashover fires

4.2.1 Fire classes

The variation of heat output with time may be based onmeasurements. Alternatively design fires may be used; theclass of fire may be selected by referring to Table 4.1 whichgives values obtained by fitting curves to observed data.

A t-squared fire is given as follows:

Q = 1000 (t / tg)2

(4.1)

where Q is the rate of heat release (kW), t is the time aftereffective ignition (s) and tg is the characteristic growthtime (s).

Table 4.1 Characteristic growth

times for different classes of fires

Class tg (s)

Slow 600

Medium 300

Fast 150

Ultra-fast 75

3

3.4

TECHNICAL MEMORANDA

Illustrations of different fire classes are given in Table 4.2.The complete burn-out of single items or packages of fuel,from growth to decay, can be characterised by assumingthat the variation of rate of heat release (RHR) with timefollows a triangular curve. Thus, in using Table 4.2, it maybe assumed that the latex foam pillow, with a fire durationof 240 s, has a maximum rate of heat release of 117 kW at120 s after ignition and is burnt out after a further 120 s.

Table 4.2 Maximum rates of heat release and approximate fire duration

Localised fire Fire class Maximum rate

of heat release

(kW)

Duration (s)

Waste basketpaper Slow 18 400

Television set Medium 290 640

Latex foam pillow Medium 117 240

Christmas tree Ultra-fast 650 70

Wardrobe:

3.2 mm plywood Ultra-fast 6400 100

12.7 mm plywood Ultra-fast 3100 350

with clothes

4.2.2 Steady-state fires not sprinklered

Once fire has spread from item to item until all the availablefuel is burning, the heat output will reach a steady valuebefore eventually declining as the fuel decays. The estim-ation of the steady value is given in section 4.3.

4.2.3 Steady state fires sprinklered

For design purposes the rate of heat release (Q) may beassumed as steady after operation of the first sprinkler, i.e.no further items of fuel ignite, and the value of mass flow inthe plume is calculated accordingly. After operation it maybe assumed that the sprinklers cool most of the smoke layerto below the operating temperature of the sprinklers. Withconventional heads, it is considered that an average smokelayer temperature of 100C may be assumed in calculations,while the sprinklers are operating, giving a constant valueof 373 K for Ts.

4.2.4 Transient fires

To simplify calculations of smoke-filling during the trans-ient phase (see section 6) an average rate of heat release(Qave) may be used:

tQ dt

Qave = 0

t(4.2)

For t-squared fires:

Qave = 333 (t / tg)2 (4.3)

4.3 Post-flashover fires

The heat release rate is given by:

Q=Hc R (4.4)

where Hc is the heat of combustion (kJ/kg) and R is themass rate of burning (kg/s).

The heat of combustion (Hc) is discussed in section 4.4 andvalues are given in Table 4.4; the mass rate of burning (R) isdescribed in section 4.3.1.

4.3.1 Ventilation controlled fires

The mass rate of burning R (kg/s) is given by:

R = 0.02 [Aoho1/2 (At Ao) (w / d)]

1/2 (4.5)

where R is the mass rate of burning (kg/s), Ao is the area ofthe ventilation opening (window or doorway) of a room(m2), ho is the height of the ventilation opening (m), At isthe internal surface area of the room (walls, floor, ceiling)(m2), w is the width of wall containing the opening (m) andd is the depth of room behind the opening (m).

Effective values of the above parameters can be derived for aroom with more than one opening by using the proceduresin Appendix 1.

Equation 4.5 is derived from experiments with wood cribsand can be used for most types of fire load found in houses,offices and shops. Conventionally fire load may be expres-sed in equivalent weight of wood. Fire loads expressed inMJ or MJ/m2 may be converted to kg or kg/m2 of wood bydividing by 18 MJ/kg, e.g. 360 MJ/m2 = 20 kg/m2 woodequivalent.

4.3.2 Fuel bed controlled fires

For low values of fire load, equation 4.5 will lead tooverestimation of R.

With typical furnishings found in houses, offices andshops, an effective fire duration of 20 minutes may beassumed and R is then given by:

R = L / 1200

where L is the total fire load (kg), or:

(4.6)

L = (L / Af ) Af

where Af is the floor area (m2).

(4.7)

Values of (L / Af) (kg/m2) are derived from surveys or design

data.

For design purposes, R should be calculated usingequations 4.5 and 4.6 and the lower value adopted. Sometypical values of rate of heat release per unit floor area forfuel-bed controlled fires are given in Table 4.3.

4

Table 4.3 Typical values of rate of heat

release per unit floor area for fuel bed

controlled fires

Fuel Maximum rate

of heat release

per unit floor

area (kW/m2)

Display 100

Offices 250

Domestic 250

Shops 500

4.4 Fire products

The combustion of material vapours in fires is not completeand the heat of combustion (Hc) is always less than the netheat of complete combustion. Suggested values of Hc forflaming fires are given in Table 4.4 in units of heatproduced per mass burnt (kJ/kg). The convective portion isabout 65% for wood fires and 5060% for commonly foundplastics. Some values of the yield of carbon monoxide (YCO)for flaming fires are given in Table 4.4 in units of mass ofCO per unit mass burnt (kg/kg). Values of the yields ofhydrogen cyanide (YHCN) and hydrogen chloride (YHCI) arealso given. Table 4.4 also gives some values for the yield ofsmoke particles (Ysmoke) for flaming fires in units of mass ofsmoke per unit mass burnt (kg/kg).

For individual materials, the yields of smoke particulatesand toxic products vary considerably with the decom-position conditions, being at a minimum for the earlystages of well ventilated fires. The figures shown in Table4.4 are therefore very much best case values for toxicproduct yields and can be applied only to pre-flashoversituations where there is no restriction on ventilation forthe fire and no significant reduction in compartmentoxygen concentration. Under vitiated post-flashover con-ditions the yields of CO are much higher at approximately0.25 kg/kg for most fuels, and HCN yields from flexible andrigid polyurethanes are also much higher at approximately0.010.05 kg/kg. Smoke yields are also higher undervitiated combustion conditions (by a factor of up to 10).

4.5 Estimation of visibility

Visibility in smoke is defined by S, the furthest distance atwhich an object can be perceived. Light-emitting objects

Table 4.4 Fire products with flaming combustion


such as electric lights are more easily perceived than objectsreceiving ambient illumination.

For light-emitting signs:

KS=8 (4.8)

For light-reflecting signs:

KS=3 (4.9)

where K is the extinction coefficient (m-l).

For flaming combustion of wood or plastics:

K 7.6 103 ms (4.10)

where ms is the mass concentration of smoke aerosol(kg/m3).

For a fire burning at a rate R (kg/s) for a duration t (s):

ms = Y smoke R t / Vs(4.11)

where Vs is the volume of smoke (m3).

For light-emitting signs:

S = 8 Vs / ( 7.6 103 Y smoke R t )


(4.12)

S = 3 Vs / ( 7.6 103 Y smoke R t ) (4.13)

For fires of predominantly wood-based fuel (e.g. timber,paper, cotton etc.) the following can be derived by sub-stituting Q = 13 103 R and Y smoke = 0.025 (conservativeassumption).

Hence for light-emitting signs:

S = 545 Vs / Qt


S = 205 Vs / Qt

(4.14)

(4.15)

Material Hc Y CO YHCN or YHCI Ysmoke(kJ/kg) (kg/kg) (kg/kg) (kg/kg)

Timber 13.0 103 0.020 0 < 0.010.025

Polyvinyl chloride 5.7 103 0.063 0.250.5 0.120.17

Polyurethane (flexible) 19.0 103 0.042 0.001 < 0.010.23

Polyurethane (rigid) 17.9 103 0.180 0.011 0.090.11

Polystyrene 27.0 l03 0.060 0 0.150.17

Polypropylene 38.6 l03 0.050 0 0.0160.10

HCI yield depending upon formulation, i.e. plasticised or rigid

5

TECHNICAL MEMORANDA

5 Plumes

The following section gives relationships for the mass flowand temperature of the ambient air when it is entrained intofire plumes. They are based on theory and experimentaldata. Except where stated, the mass flow of fuel is negligibleand not taken into account.

At a given height, entrainment depends on the heat outputand, at small plume heights, on the geometry of the source.The geometry is characterised here as a point, circle,rectangle or line. At large plume heights, entrainment isequivalent to that above a point source. The plume itselfmay be in the room of fire origin (directly above the source)or it may be outside the room, having emerged from anopen door or window. For the purposes of smoke controldesign, the zone of interest is above the luminous part of theplume.

5.1 Axisymmetric plume small

diameter source

The axisymmetric plume is expected for a fire originatingon the floor away from the walls; see Figure 5.1. It has avirtual point source. Air is entrained from all sides andalong the entire height of the plume until the plumebecomes submerged in the smoke layer beneath the ceiling.

The height of luminous zone above the base of the fire isgiven by:

z1 = 0.20 Qp2/5

(5.1)

where Qp2/5 > 14.0 ds and where z1 is the limiting height

(intermittent flames), Qp is the convective portion of theheat release rate (kW) and ds is the diameter or largerdimension of the source (m).

The value of z1 denotes the limiting height for use of theentrainment equation 5.2, i.e:

For z > z1:

M = 0.071 Qp1/3 ( z - zo )

5/3 (5.2)

where M is the mass flow by entrainment (kg/s), z is theheight above base of fire (m) and zo is the height of thevirtual source above base of fire ( 0 ) (m)

Luminous

zone

6

Figure 5.1 Plume above a fire on

the floor away from the walls of

the room

Notes :

(a) Estimates, of the value of the coefficient in equation5.1 vary from 0.17 to 0.23.

(b) The location of the virtual source has been deter-mined for pool-type fires only. For most solid fuelsfound in buildings the value of zo is likely to besmall and for design purposes may be taken as zero(i.e. the source is at the base of the fire).

Entrainment in the far field of a fire against a wall can beconsidered approximately as that for half an axisymmetricpoint source plume and, using equation 5.2:

M = 0.044 Qp1/3 z5/3 (5.3)

Entrainment for a point-source fire in a corner can beconsidered approximately as that for one quarter anaxisymmetric point source plume and, using equation 5.2:

M = 0.028 Qp1/3 z5/3 (5.4)

5.2 Axisymmetrlc plume larger

diameter source

For a circular source of diameter ds or a square source ofside ds, originating on the floor away from the walls, theheight of the luminous zone is given by:

z1 = 0.035 Qp2/3 / ( ds + 0.074 Qp

2/5 )2/3 (5.5)

where Qp2/5 < 14.0 ds.

For z > z1:

M = 0.071 Qp1/3 z5/3 (5.6)

Equations 5.5 and 5.6 may also be used for rectangularsources where the longer side, does not exceed three timesthe shorter side. In such cases dimension ds is taken as thediameter of a circular source of the same area.

An alternative equation for M may be used for circular orsquare sources as follows.

For z < 2.5 p and 200 < Qp / As < 750:

M = 0.188 p z 3/2 (5.7)

where p is the perimeter of the source (m) and As is the planarea of the source (m2).

Notes:

(a) The coefficient in equation 5.5 is believed torepresent the upper limit.

(b) When ds is small in relation to z1, equation 5.5reduces to equation 5.1 for the axi-symmetricplume. When ds is large, equation 5.5 gives the sameanswer as early work using square-based fires ofwood cribs.

(c) Equation 5.6 is the same as equation 5.2.

Section Front view

Section Front view

(d) Equation 5.7 is justified theoretically for z z1. Forlarger values of z it is justified only empirically,within the limits stated.

5.3 Line source

A line source is defined here as a rectangular source whereds is the longer side and is greater than three times theshorter side. For a line source originating on the floor awayfrom the walls the height of the luminous zone is given by:

z1 = 0.035 Qp 2/3 / ( ds+ 0.074 Qp

2/5)2/3 (5.8)

For z1 < z < 5 ds:

M = 0.21 Qp1/3 ds

2/3 z (5.9)

For z > 5 ds:

M = 0.071 Qp1/3 z5/3 (5.10)

Notes:

Equation 5.8 is the same as equation 5.5.

The coefficient in equation 5.9 is believed to be anupper limit.

Equation 5.10 is the same as equation 5.2 for theaxisymmetric plume.


5.4 Flow from an opening

The horizontal mass flow from an opening of a roomcontaining a fire, see Figure 5.2, is given by:

Mo = 0.09 (Qp wo2) l/3 ho (5.11)

Entrainment in the vertical plume above the opening isgiven by:

M = 0.23 Qp1/3 wo

2/3 (z + ho) (5.12)

where wo is the width of the opening (m), ho is the height ofthe opening (m) and z is the plume height above the top ofthe opening (m).

Where there is a balcony above the opening, see Figure 5.3,the entrainment in the vertical plume above the balcony isgiven by:

M = 0.36 Qp1/3 lc

2/3 (zb + 0.25 hb) (5.13)

where lc is the separation of the channelling screens (m), hbis the height of the balcony above the base of the opening(m) and zb is the plume height above the balcony (m).

Where there are no channelling screens beneath thebalcony the entrainment in the vertical plume is given by:

M = 0.36 Qp1/3 (wo + b)

2/3 (zb + 0.25 hb) (5.14)

Figure 5.2 Vertical plume from

the opening of fire room

Figure 5.3 Vertical plume from

the opening of fire room, flowing

round a balcony

7

(a)

(b)

(c)

TECHNICAL MEMORANDA

M M Tm M Q pv = = = + o To o o T o c p

where b is the depth of the balcony (m).

Notes:

Equation 5.12 is for a free-standing plume. If theplume attaches to a wall above, the entrainmentmay be reduced by about one-third.

Equations 5.11, 5.12, 5.13 and 5.14 are empiricaland being so do not necessarily coincide at theextremes.

At large heights the plume can be considered asaxisymmetric and it is suggested that for z > 5 ho or

zb > 5 hb, the entrainment is calculated usingequation 5.2 with zo taken as zero if this gives amore conservative solution. If the critical conditionis smoke temperature or smoke concentration, thenthe lower value of M gives a conservative solution.If the critical condition is smoke volume, then thehigher value of M gives a conservative solution.

5.5 Ceiling flow

The velocity of a flowing layer beneath a ceiling along achannel of width wc is given by:

u = 0.7 (g Qp T / po cp To2 wc)

1/3 (5.15)

where u is the velocity of the layer (m/s), T is the (absolute)smoke temperature (K), To is the (absolute) ambient airtemperature ( 290) (K).

Assuming conservation of heat:

d1 = {M T / [38 wc ( T To )1/2 ]}2/3 (5.16)

where M is the mass flow entering the layer (kg/s) and d1 isthe depth of the layer (m).

Note: T may be calculated as the average temperature of thesmoke plume as it enters the layer.

5.6 Plume temperature

The average temperature of the plume is given by:

Tm To = Qp / M c p (5.17)

where Tm is the (absolute) average plume temperature (K),cP is the specific heat capacity of air ( 1) (kJ/kg K).

For an assumed Gaussian temperature distribution acrossthe plume, the axial temperature is given by:

Tc - To = 2 ( Tm To ) (5.18)

where Tc is the (absolute) axial temperature (K).

8

5.7 Volume flow

The volume flow is given by:

(5.19)

where v is the volume flow (m3/s), is the smoke density(kg/m3), o is the ambient air density ( 1.2) (kg/m

3); ( oTo= 352 kg K/m3).

5.8 Ceiling jet

When the plume above a fire reaches a ceiling, a horizontaljet is formed. It is necessary to know the characteristics ofthis jet in order to predict the performance of ceiling-mounted detectors. Such a prediction is outside the scope ofthese Memoranda but the following relationships are givenhere for completeness.

For an axisymmetric plume below an unconfined ceiling(no accumulated warm upper layer) the ceiling jet has thefollowing properties when the source is at least 1.8 times theceiling height from the walls.

For r / h 0.18:

T To = 16.9 Q2/3 / h5/3

For r / h > 0.18:

= 5.38 (Q / r)2/3/ h

For r / h 0.15:

u = 0.96 (Q / h)1/3

For r / h > 0.15:

(5.20)

(5.21)

(5.22)

u = 0.195 Q1/3 h1/2/ r5/6 (5.23)

where T is the temperature of the jet (K), h is the height ofthe ceiling above base of fire (m), r is the horizontal distancefrom the fire axis (m), Q is the total rate of heat release (kW)and u is the velocity of the jet (m/s)

Note: the above equations are for a steady heat release rate.They can be modified to take into account t-squared fires(2).

5.9 Convective heat release

The proportion of the total heat release rate which is in theplume varies with the type of combustible material and thecharacteristics of the compartment (for flow out of anopening). For the purposes of design the following may beassumed:

Qp = Q / 1.5 (5.24)

(a)

(b)

(c)

r r r r

r rr

T To

6 Accumulated ceilinglayer

6.1 Zone models

The simplest zone model postulates that smoke rises toform a smoke layer of uniform depth and temperature witha substantially smoke free layer below it. As described insection 7, smoke control systems are frequently designed tomaintain a minimum height of the smoke-free layer for aspecified time.

Figure 6.1 Smoke filling a room with a low level opening

1.0

6.2 Smoke filling a room with a

low level ventilation opening

In rooms of this type there is no smoke flow out of the lowlevel opening in the wall; see Figure 6.1. Heat loss to theroom surfaces is neglected, which is conservative.

Z 0.5

6.2.1 Axisymmetric plume

The elapsed time at which the smoke free layer is at a heightz is obtained by solving the differential equation:

0.00 5 10 15

(Q*)1/3

Figure 6.2 Solution of equation 6.2 for an axi-symmetric plumedz

o Af Q p+ M + = 0

dt To cp(6.1)

where Af is the floor area (m2).

The variation of M with z is described in section 5.

Equation 6.1 may be written in dimensionless form asfollows:

dZ M+

d o (g h)1/2 h2

+ Q* = 0 (6.2)

The average density of smoke layer is given by:

s / o = 1 Q* (1 Z) (6.4)

The average temperature of the smoke layer Ts is given by:

(Ts To)/ To = 1 / [1 Q* /(1 Z )] (6.5)

Where an impurity such as carbon monoxide can be relatedto Q as follows:

where: mCO = C (Q / cp To) (6.6)

Z = z / h The mass fraction in the ceiling layer is given by:

Q* = Qp / [ o To cp (g h)1/2 h2] = Q / (1100 h5/2)

= t (g / h)1/2 (h2 / Af) = (3.13 t h3/2) / Af

This equation is solved in Figure 6.2 for an axisymmetricplume (M from equation 5.2) with constant QP from timezero.

fm = C Q* /(l z)

where fm is the mass fraction.

Note:

Figure 6.2 solves the following integral:

dZ=

Z

1

0.195 (Q*)1/3 Z5/3 + Q* (6.3)

mCO = YCO R = YCO Q / Hc

C = YCO cp To / Hc

where Hc is the heat of combustion (kJ/kg).


(6.7)

9

r

t r

t

r

t

t

t

tr r

t

h

z

z

Q* = 0.05 0.01 0.002

TECHNICAL MEMORANDA

6.2.2 Line plume

For a plume from a line source, such as that given byequation 5.9, 5.12, 5.13 or 5.14, i.e:

M = Q 1/3 ls2/3 z (6.8)

where ls is the length of the line source (m)

From equation 6.1, the differential equation is:

r o Afdz Qp

+ Qp1/3 ls

2/3 z + = 0To cp

(6.9)dt

The solution to equation 6.9 with constant Qp from timezero is:

The temperature of the vented smoke, under steady-stateconditions will be given, assuming conservation of heat, by:

Ts To = Qp / (Mout cp) (6.12)

and the volume flow by:

V = (Mout / o ) + Qp / ( o To cp ) (6.13)

With natural ventilation, the mass flow of the vented smokeis given by:

Cd Avo o [ 2g (h z)(Ts To)To ]1/2

M out =Ts1/2 [Ts+ (Avo/Avi)

2To]1/2

(6.14)

where Avo is the outlet ventilation area (m2), Avi is the inlet

ventilation area (m2) and Cd is the discharge coefficient( 0.7).

(Q*)1/3 =11n

2

2 + (Q*)2/3

2 Z + (Q*)2/3

(6.10)

Smoke filling an open room

approaching flashover

where:

Q* = Qp / [ ro To cp (gh)1/2 (h ls)] = Q / (1100h

3/2ls )

= t (g/h)1/2(hls /Af) = 3.13 t h1/2ls /Af

2 = (Tocp / ro 2 ) 1/3= 2.72

Z = z / h

(Q*)1/3 t = 0.303 (Qls2)1/3 t / Af

Equations 6.5, 6.6 and 6.7 can be used to calculate averagetemperature, density and mass fraction by inserting theabove values.

Note: this solution can be used where smoke flows from acommunicating space into a large-volume space such as amall or atrium, by entering equation 5.12, 5.13 or 5.14 andthe dimensions Af and h of the large volume.

6.2.3 Room filling with smoke extract fromlayer

A critical height of the smoke layer may be dictated by theneed to keep it above eye level, inside a reservoir or, ifotherwise too hot, well above head level.

If the critical clear layer height zc would be reached beforethe occupants have escaped, then extract from the smokelayer can be provided as follows, under steady-stateconditions:

Mout = Mc (6.11)

where Mc is mass flow in the plume at height z = zc.

Note: values for discharge coefficients are provided by thevent manufacturer.

6.3

The calculations in section 6.2 are not suitable whereflames are approaching ceiling height or where smoke flowsout of the wall opening. In such circumstances thefollowing equation may be used:

Ts To = 9.15 [ (Qp2/ (Aoho

1/2 k At) ]1/3 (6.15)

where Ao is the area of the ventilation opening (window ordoorway) of a room (m2), ho is the height of the ventilationopening (m), k is an effective heat transfer coefficient(kW/m2 K) and At is the internal surface area of the room(walls, floor, ceiling) (m2).

Note: equation 6.15 was derived for At / (Aoho1/2) = 16 to 530

m1/2.

By substituting (Ts To) = 580 K in equation 6.15, the valueof Qp at flashover, Qf, iS given by:

Qf = 505 (Aoho1/2 k At) 1/2 (6.16)

recommended:For Anet/(Aoho

1/2) < 10 m1/2, the following value for Qf is

Qf = 5.2 Anet + 252 Aoho 1/2 (6.17)

where Anet is the internal surface area of the room minus thearea of openings (i.e. (At Ao)) (m

2).

The effective heat transfer coefficient is derived from:

k = ( l w w cw / tc )1/2 (6.18)

where l w is the thermal conductivity of the wall material(kW/m K), w is the density of the wall material (kg/m

3), cwis the specific heat capacity of the wall material (kJ/kg K)and tc is the characteristic burn time (s)

10

a

a

t a

a

a

t

a a a

r r

r

a

a r

r

a

a

Values of k are given in Table 6.1 for a characteristic burntime of 900 s.

Table 6.1 Effective heat transfer coef-

ficient to surfaces of room or compartment

Material of surface k (kW/m2 K)

Concrete 55 10-3

Brick 36 10-3

Plaster 21 10-3

Plasterboard 13 10-3

Fibre insulating board 5.2 10-3

Flashover is not expected until there are sustained flames atceiling level. For axisymmetric sources of base dimensionless than the ceiling height, the minimum condition forflashover is given by

h < 0.094 Qp 2/5 (6.19)

where h is the height of ceiling above base of fire (m).

For extended area sources, the minimum condition forflashover is:

h < 0.035 Qp2/3 / ( ds + 0.074 Qp

2/5)2/3 (6.20)

where ds is the the longer dimension of the source (m).

Note: equation 6.20 is conservative, because it uses equation5.8 for intermittent flames.

6.4 Heat transfer to building

surfaces

In the simple room tilling model of section 6.2, heattransfer to the ceiling and wall surfaces is neglected. This isa conservative assumption, in that the volume of smokeis overestimated. However, if low-temperature smoke isfilling a large reservoir, then cooling may lead to loss ofbuoyancy and should be taken into account. In the absenceof experimental data, it is suggested that cooling effectsshould be allowed for, using computational fluid dynamics(CFD), where the area of the reservoir is greater than 2000m2, and/or the average layer temperature is less than 10 Kabove ambient when calculated by neglecting cooling.

6.5 Heat transfer by radiation

from smoke layer

The radiation emitted from a hot smoke layer is given by

Ir = e s s Ts4 (6.21)

where Ir is the intensity of emitted radiation (kW/m2), e s is

the emissivity of smoke layer, s is the StefanBoltzmannconstant (= 5.7 10-11) (kW/m2 K4) and Ts is the (absolute)smoke layer temperature (K).


As a conservative assumption es may be taken as unity.Alternatively it may be estimated for a ceiling layer from:

e s = 1 exp[ (0.33 + 470 ms ) (h z) ] (6.22)

where h is the height of ceiling above base of fire (m), z isthe height of layer interface above base of fire (m) and msis the mass concentration of smoke aerosol (kg/m3) (seesection 4.5).

6.6 Stratification

When the ambient temperature at ceiling level issignificantly higher than at the level where the fire startsthen the upward movement of the smoke plume may cease,due to lack of buoyancy, and stratification may occur.

The maximum height of rise of an axisymmetric plume isgiven by

zm = 5.54 Qp1/4 ( d T / d z )3/8 (6.23)

where zm is the maximum height of smoke rise above baseof fire (m), Qp is the convective heat release rate (kW) and(dT / dz) is the rate of change of ambient temperature withrespect to height (assumed to be linear) (K/m).

The maximum height of rise of a line plume of length ls atsource is given by:

zm = 4.81 (Qp')1/3 (dT / dz)1/2 (6.24)

where:

Qp' = Qp / ls

6.7 Number of extract points

When the smoke layer is relatively shallow, a high extractrate at any point may lead to plug-holing, whereby someair is extracted along with the smoke. Accordingly morethan one extract point may be needed, with an extract ratefrom one point not exceeding M (kg/s) given by:

M = b [g (h z)5 (TsTo) To ]0.5 / Ts (6.25)

where g is the acceleration due to gravity (= 9.81) (m/s2)and b is a numerical factor which takes the value 2.0 wherethe extract point is near a wall and 2.8 where the extractpoint is distant from a wall.

Note: the estimates of b are based on approximatemeasurements but are thought to be reasonable.

11

a

a

TECHNICAL MEMORANDA

7 Methods of smoke andheat management

management is, however, often used for less critical applic-ations such as the removal of cold smoke from a space after afire has been extinguished or cross ventilation of a space asan aid to fire fighting.

7.1 General

There is a wide range of smoke management methodsavailable for life safety or property protection purposes.

Smoke management methods can vary from systemsdesigned with a high degree of sophistication to achievespecific design objectives to the use of, for example,openable windows to remove cold smoke after a fire hasbeen extinguished.

Smoke management systems are often installed as an aid tofirefighting. In such cases it may be important to identifywhether the firefighting is in support of life safety orproperty protection. In many cases these systems are moreconcerned with the removal of heat rather than smoke, forexample the use of pavement lights for venting basementfires, as the former is often more critical in preventingeffective firefighting operations.

7.2 Design principles of smoke

management systems

Smoke management systems have one or both of thefollowing objectives:

to protect the area of the building where the firestarts

to protect areas of the building beyond where thefire has started.

Where both systems are installed, they must be compatible.

7.3 Systems intended to protect

the area in which the fire starts

Systems of this type can be based on:

(a) preventing a smoke layer from reaching a criticallevel


parts of the building beyond

the fire area

Systems of this type can be based on:

(a) enclosing a space with barriers to contain smoke

(b) establishing a favourable pressure difference acrossleaks in a barrier to prevent smoke flow (pres-surisation).

Control of smoke movement by enclosing the space withwalls and doors having a specified standard of fireresistance is historically the most widely used method ofsmoke management. In practice this form of smokemanagement is usually only partially successful in pre-venting smoke movement because of leakage through theenclosing walls e.g. at doors, through ductwork etc.Enclosure of a space need not be by fire resistingconstruction but can be by any form of screen, or curtain,which will contain the smoke for the required time period.

Pressurisation of a space to prevent smoke entering thespace is a widely used form of smoke management.Conventionally it is used to prevent the passage of smokeinto stairways or other protected shafts. This method ofsmoke management has been fully codified in the UK andUSA.

The use of induced air movement to prevent smokeentering unaffected parts of the building has been widelyused in the USA but less so in the UK although it is inprinciple similar in concept to pressurisation. Systems canbe of a variety of different types such as in office buildingswhere air is extracted from a fire affected space such as anopen floor, while air is being supplied to the other openspaces. Systems can be designed to prevent smoke spreadthrough large openings but the velocities required general-ly make this method impractical for buildings unless asprinkler system is relied on to keep the fire small. There is

(b) diluting the smoke within the space with cool clearextensive guidance available on this method of smoke

air, to reduce its temperature and improvemanagement in USA publications.

visibility.

Systems designed in accordance with (a) are the mostcommonly encountered form of designed smoke manage-

8 General designment system and are usually based on maintaining the base parameters for smokeof the smoke layer above peoples heads for a predeterminedperiod or for a steady state condition. Smoke collects in a

management systemsreservoir, which may or may not be vented; sprinklers maylimit the fire size. Systems designed in this way are oftenused for life safety purposes such as in shopping malls. The

8.1 Actuation

relevant relationships are given in section 6.All smoke management systems, other than those based on

Systems designed in accordance with (b) are more rarelypassive measures, require a means of actuation. Actuation

used for life safety purposes because of the very highof a smoke management system may be:

dilution rates required. Dilution as a means of smoke by detection of smoke

12

on operation of a sprinkler flow switch

by manual operation.

The choice should take into account the time for operationand reliability.

8.2 Design parameters for systems

intended to maintain a smoke-

free layer

These systems are usually designed on the basis ofmaintaining the base of the smoke layer at a predeterminedheight above the floor level for a specified time. If it isnecessary to maintain the base of the smoke layer at thepredetermined height indefinitely the rate of smoke extractmust equal the rate at which smoke flows into the smokelayer (see section 6).

In practice some contamination of the air below the smokelayer must be expected because of mixing at the horizontalinterface between the smoke layer and the clear air below,turbulence caused by sprinklers etc. However, if thefollowing parameters are considered in the design processthen any contamination should not affect the designobjectives being achieved although, in the case of propertyprotection, some smoke damage may still occur.

(a) Area of reservoir: smoke contained with a reservoirloses heat by convection and radiation. The heatloss is dependant on the area of the reservoir. If thereservoir size is excessive then the temperature ofthe smoke will reduce such that downward mixingwill occur with the clear air below. Conventionallyan area of 2000-3000 m2 has been adopted as themaximum reservoir size. This area limit wasoriginally adopted for fires in warehouse typebuildings and more recently for shops and retailcomplexes.

The maximum area of reservoir to preventexcessive cooling and downward mixing isdependent on a number of factors including:

the temperature of the smoke entering thesmoke layer

the nature of the construction forming theboundary to the reservoir, particularly thefloor or roof construction forming the topsurface of the reservoir

the presence of sprinklers in the reservoir

turbulence in the layer caused by obstruc-tions, e.g. beams, ducts whose depth is largein relation to the depth of the smoke layer

mixing at the horizontal interface with theclear air layer below the smoke layer

(b)

depth-to-width ratio of the reservoir.

At present there is no analytical model whichincorporates all of these parameters.

Reservoir screens and curtains: the screens or curtainsenclosing the edges of a reservoir must be

(c)

(d)

(e)

(f)


constructed from materials which can withstandthe calculated smoke temperature for the requiredperiod. These screens should be impermeable butsome leakage at for example the junction of screensis not likely to be critical for most applications. Thedepth or drop of the screens or curtains should bethe same as the calculated depth of the smoke layer.It is not considered to be necessary to add a marginof safety into the depth of the screen, i.e. thecalculated depth can be the actual depth.

Replacement air: for any smoke removal system towork effectively it is necessary to have a source ofreplacement air. The replacement air can besupplied by natural means or mechanically, al-though the former is to be preferred. Replacementair must enter below the depth of the smoke layer.The velocity of the incoming air should becontrolled, particularly if it is entering through fireexit doors, so that the escape of the occupants is notadversely affected. A velocity of 3 m/s has tradition-ally been adopted as reasonable for incoming airbut this is not critical and recent tests indicate that5 m/s can be tolerated. However it should be notedthat in practice replacement air will enter via theflow paths of least resistance which may not be theinlets assumed in the design or the balance of inletsas summed in the design. For example, externalwind pressures may affect the balance of supply, i.e.the rate of inflow through all inlets may not beequal. Therefore the calculated inlet velocityshould allow some margin for variations which mayoccur in practice and the design inlet velocityshould not normally exceed 3 m/s.

In some instances, such as shopping malls wherethere are several roof level smoke control zones,adjacent zones may be used for the supply ofreplacement air. In such instances considerationshould be given to avoiding the possibility of smokere-entering the building through roof vents beingused to provide replacement air.

When designing mechanical smoke removal sys-tems the replacement air requirement should bebased on a volume balance and not a mass balance.

Number of extract points: for a smoke removal systemto work effectively there must be sufficient extractpoints to prevent air below the smoke layer beingdrawn up into the smoke layer (see section 6.7). Therequired number of extract points should beuniformly distributed across the reservoir.

Plenum extract systems: in some cases smoke may beextracted via a plenum. Smoke extract plenumsshould contain no cavity barriers or combustiblematerials. Sprinklers should be provided over thefire risk, not in the plenum.

Suspended ceilings: where suspended ceilings areprovided below the level of smoke extract pointsthey should be at least 25% open uniformlydistributed.

13

TECHNICAL MEMORANDA

8.3 Design parameters for systems

intended to dilute the smoke to

a tenable condition

Smoke management can be based on diluting the smokewithin a space such that the design criteria within thatspace, e.g. tenability limits, are not exceeded. This can bebased on simple dilution either with or without the removalof air from the space.

Except in relatively large spaces with relatively small fires itis unlikely that sufficient dilution can be obtained tomaintain tenable conditions for any substantial timeperiod. In general smoke would need to be diluted approx-imately 100 times to increase the visibility to a tolerablelevel.

As stated earlier, dilution as a means of smoke managementis often used for removal of cold smoke from a large spaceafter the fire has been extinguished. An extract rate of sixair changes per hour has been widely adopted for thispurpose. The time to improve the visibility within a spaceto a predetermined level can be calculated from thefollowing equation:

ms /mso = exp ( nv td ) (8.1)

where td is the dilution time (h), ms is the concentration attime td, mso is the initial concentration and nv is the numberof air changes per hour (h1)

Removal of cold smoke would normally be by mechanicalmeans but can be by natural venting utilising the stackeffect. This arrangement is less predictable than mech-anical venting because of the dependency of stack effect onexternal conditions and the temperature within the internalspace, which could quickly approximate to the external airtemperature as replacement air is drawn into the space. Thenumber of air changes produced can be calculated usingconventional principles.

DP = 34601 1

hnTo Ts

For dilution to work effectively where it involves theremoval of smoke, a source of replacement air must beprovided. In principle this should meet similar criteria tothose given in section 8.2 except that, where the objective isremoval of smoke after a fire has been extinguished (or inother non-critical applications), the replacement air can beprovided by manually opening doors or windows etc. asrequired.

Cross ventilation has been widely used as a means of smokedilution and/or dispersal, particularly for firefightingoperations. This has traditionally been based on providingvent areas of 2% or 5% of the floor area of the buildingequally distributed on two sides of a space. No theoreticaljustification has been given for this arrangement althoughit is reasonable to suppose that the relatively large ventareas which result in large spaces are such that they would,in most instances, provide an effective means of removal ofheat and smoke.

In applications below ground where vents are provided aspavement lights or where the vents are provided at roof

14

level, the rate of smoke (and heat) removal can be calculatedusing the principles given in section 8.2 except that sucharrangements have not traditionally been provided with adesigned source of replacement air. This would need to beprovided by, for example, holding doors open.


other parts of the building

enclosure to contain smoke

The use of passive fire protection measures to containsmoke is traditionally the most widely used method ofcontrolling smoke movement and has been based onrequiring the surrounding construction to a space to meet aspecified standard of fire resistance as defined in BS 476:Part 20 (3). This relatively simple arrangement has provedvery effective at controlling smoke movement. However,leakage of smoke through the enclosing construction canreduce the effectiveness of the arrangements and additionalmeasures are described below.

8.5 Systems inteded to protect


pressurisation

Pressurisation of stairways and other protected shafts is anestablished method of smoke control and has beenextensively documented and codified. Detailed recom-mendations are given in a British Standard code ofpractice(4) and in NFPA publications(5) . The relationshipsadopted are as follows:

(8.2)

v = 0.827 A1 (D P)1/N

(8.3)

hn = 2ho /3 (8.4)

where DP is the pressure differential across the barrier (Pa),hn is the height above the neutral plane (m), To is the(absolute) temperature of ambient air (K), Ts is the(absolute) temperature of the smoke (K), V is the air flowrate (m3/s), A1 is the leakage area (m

2), N is a numericalfactor (see Note below) and ho is the height of the opening

(m).

Note: N takes values between 1 and 2. For large leaksaround doors, N = 2; for small leaks around windows,N = 1.6.

Design values of A1 are given in references 4 and 5 .

It can be important to differentiate between the use ofpressurisation to protect stairways for means of escapepurposes and for firefighting purposes. In the former casethe doors, particularly to the floor affected by the fire,would be open for only a relatively short time whereas inthe latter case the doors may be open for prolonged periods.


8.8 Means of removal of smoke

Removal of smoke can be by natural or mechanical means.Each method has its advantages and disadvantages.

Natural venting relies on the buoyancy pressure in thesmoke layer below the vents to produce flow through thevents. The relationship between the mass flow of smokethrough the vents and the temperature and depth of thesmoke layer is given in section 6.2. The principal advantageof natural venting systems is simplicity of operation.

Mechanical venting systems are more likely to be effectivewith relatively cold smoke than are natural ventingsystems. However, as the extract rate is fixed it cannotcompensate should the fire size, be larger than that forwhich the system was originally designed.

With both systems, vents should be located to avoid theeffect of positive wind pressures. However, additionaloverpressure can be allowed for in the performance ofmechanical systems. Consideration should be given to thepossibility of subsequent developments on adjacent sitesadversely affecting wind pressures, particularly where thesystems play a critical role in life safety.

With mechanical extract systems, apart from their greaterdependency on maintenance and need for standby powersupplies, very large extract rates should be avoided becauseof the possibility of high air velocities being induced andtheir possible effect on the rate of fire growth, the pattern ofsmoke movement and their effect on occupants trying toescape. In general, it is worth questioning a mechanicalsmoke extract rate much in excess of 100 m3/s, to confirmthat it is appropriate to the design goal.

References

3

Fire engineering CIBSE Guide (London: Chartered Institutionof Building Services Engineers (to be published)

SFPE Handbook of Fire Protection Engineering (Boston MA:Society of Fire Protection Engineers/Quincy MA: National Fire

Protection Association) (1988)

BS 476: Fire tests on building materials and structures: Part 20: 1987Method for determination of the fire resistance of elements ofconstruction (general principles) (London: British StandardsInstitution) (1987)

BS 5588: Fire precautions in the design, construction and use ofbuildings: Part 4: 1978 Code of practice for smoke control in protectedescape routes using pressurization (London: British StandardsInstitution) (1978)

NFPA 92A: Recommended Practice for Smoke Control Systems(Quincy MA: National Fire Protection Association) (1988)

NFPA 92B: Smoke management systems in malls, atria and largeareas (Quincy MA: National Fire Protection Association) (1991)

15

1

2

4

5

6



opposed air flow

This method of smoke control has been widely used in theUSA and detailed recommendations have been publishedin NFPA standards(5,6). It is based on inducing an air flowtowards the area of the building containing the fire suchthat the air velocity is sufficient to prevent the outflow ofsmoke. The required airflow rate for an opening can becalculated from the following equation:

u = 0.64 [g ho (Ts To) / Ts]1/2 (8.5)

For a corridor:

u = 0.292 (Qpc / wc)1/3 (8.6)

where u is the air velocity (m/s), Qpc is the energy releaserate into a corridor (kW) and wc is the corridor width (m).

For large openings the volumes of air required aresubstantial except where the smoke temperature isrelatively low. This makes the method of smoke controlsuitable only for buildings protected by a sprinkler system.For sprinkler-controlled fires the smoke temperature canreasonably be assumed to be close to the operatingtemperature of the sprinkler (see section 4.2) and a value of373 K for Ts will give conservative results.



depressurisation

This method of smoke control is based on the extraction ofair/smoke from the part of the building affected by fire toreduce the pressure in the space to less than that in theadjacent parts of the building. The induced pressuredifferential then inhibits the spread of smoke to adjacentparts of the building.

The method is fully described in NFPA standards forsituations where, for example, the fire affected part of thebuilding is an occupied floor adjacent to a large space andextraction can be effectively carried out using the normalair handling systems provided in building. In general theair handling systems operate in extract mode on the fireaffected floor and supply in the adjacent floors. For thismethod to prevent the spread of smoke the areas availablefor leakage must be relatively small and the method wouldnot be effective if, for example, a large area of enclosureseparating two spaces were to fail. Therefore this arrange-ment is only suitable for buildings which are protected bysprinklers.

This method of smoke management can also be used toprevent the spread of smoke from a large volume space to aconnecting space through an opening by depressurisationof the larger space. This method has not been widely usedbut has been extensively described in the literature.

Appendix 1: Dimensions of a roomor compartment

TECHNICAL MEMORANDA

The following dimensions and areas should be calculated:

Af Floor area (m2)

Ao Area of opening (window or doorway) of room (m2)

Anet Internal surface area of room minus area ofopenings (m2)

cd

Core dimension (m)Depth of opening (m)

h Floor-to-ceiling height of room or height abovebase of fire (m)

ho Height of opening (window or doorway) (m)w Width of wall containing an opening (m)wo Width of an opening or doorway(m)

(a) Simple case

Wall 3 Wall 4

Af = w1 w2 Ao = wo ho

Anet = 2Af + 2h (w1 + w2) Ao

d / w = w2 / w1

(b) More than one window

Ao1 = wo1 ho1 Ao2 = wo2 ho2 etc.

Ao = A1 + A2 + etc. wo = wo1 + wo2 + etc.

(c) Windows in more than one wall

(Wall 1 contains the greatest window area)

Aow1 = window area on wall 1

Aow2 = window area on wall 2 etc.

Ao = Aow1 + Aow2 + etc.

d / w =w2 Aow1

w1 Ao

(d) Compartment with core

(Wall 1 contains the greatest window area)

Af = w1 w2 c1 c2

Anet = 2Af + 2h (w1 + w2 + c1 + c2) Ao

(w2 c2) Aow1d / w =

(w1 c1) A o

h =Aol + Ao2 + etc.

Ao

16

Appendix 2: Computational fluiddynamics (CFD)

(r f) ( r ui f ) f+ G = S f

A2.1 Introduction t xi xi xi

Since its emergence in the mid-1970s as a practical designand analysis tool, computational fluid dynamics has madean increasingly significant contribution to the solution of Time rate

fluid flow problems in many branches of engineering. of change

Examples of its application can be found in many areas ofendeavour ranging from airframe, ships hull and car bodydesign through to analyses of the efficiency of gas turbines,cement kilns and glass furnaces. It is also beginning toemerge as a useful tool for the examination of indoor airmovements and in the design of clean rooms.

By solving numerically the partial differential equation setdescribing the principles of local conservation of mass,momentum, energy and species, subject to the particularboundary conditions of the problem, the models canprovide calculations of hazard from a developing firewithout making the assumptions often necessary in moretraditional smoke movement analysis. For example, it isnot necessary to assume, as do the zone models described insection 6, that the smoke from a fire will instantaneouslycover the ceiling of a compartment in the form of ahomogeneous layer and then fill it from the top downwards.The influences of pre-fire ambient stratification which may r uidistort this simple picture, at least in the early stages, can be ui =

taken into account. Similarly the transit of the ceiling jet r

over a large ceiling can be examined, as can the possibleenhancement of plume air entrainment caused by fire-induced winds. These effects are all incorporated naturallyby the more fundamental approach of CFD. It is not neces-sary to assume that plume entrainment is described by, forexample, equations 5.2 or 5.7; instead it will be determinedduring the solution of the underlying equations. Further-more the influences of external wind forces, difficult toaccount for in the zonal treatment, are taken account ofthrough choice of appropriate boundary conditions.

r .ui f = G f f

xiA2.2 Principles

Only a brief summary of the methodology is offered here.

CFD models start with the exact instantaneous partialdifferential equation set describing the local conservationprinciples. For smoke movement problems these are thensolved subject to the following critical decisions:

(a) how to treat the problem of turbulent mixing

(b) which algorithm is to be used to calculate thenumerical solution of the resulting equations atinterior points of the flow domain

(c) how properly to approximate boundary conditionsalong the domain boundaries

(d) how to treat combustion and thermal radiation.

The basic equation set for the simulation of fires in en-closures comprises time-averaged conservation equations


for mass, momentum, energy and chemical species of thegeneral form:

(A2.1)

i.e:

+ Convection Diffusion = Source/sink

where f is a generic variable which may represent, forexample, the three Cartesian velocity components ui, theenthalpy h or the mass fraction of a particular species mj.(The mass continuity equation is represented by the casef = 1.) S f is a source term appropriate to f which in-corporates, for example, the effects of chemical productionand radiative heat loss.

A Cartesian grid is not essential but is assumed here forsimplicity. All dependent variables in equation A2.1 aretime-averaged quantities and, since density fluctuations areusually neglected, may be viewed as implicitly density-weighted, for example:

(A2.2)

The diffusion term incorporates the effects of bothturbulent and molecular diffusion through the exchangecoefficient G f . In most field modelling studies of fire it isassumed that the Reynolds stresses and scalar fluxes, whichinvolve the correlations of fluctuating properties, can bemodelled by use of the gradient transport hypothesis,which for scalars is:

(A2.3)

To determine the local value of G f, two further transportequations are solved for k, the turbulence kinetic energyand its rate of dissipation, e. The effects of buoyancy onextra turbulence production (in rising plumes) andinhibition (in stratified layers) require special attention.

The modelled conservation equations are then discretisedand solved iteratively on a numerical grid of probably tensor hundreds of thousands of elementary control volumesfilling the computational domain. This is achieved usingguess-and-correct operations.

Solution of these equations alone, together with theappropriate initial and boundary conditions to incorporatethe effects of heat and momentum loss to the envelopingstructure, conditions of vents and any important pre-fireinfluences is sufficient to capture the major features of thesmoke movement problem for a known fire size.

17

f

TECHNICAL MEMORANDA

Where it is necessary to estimate hazard to human life dueto inhalation of toxic gases or to radiant and convective heatexposure the further sub-models of combustion andthermal radiation are also needed.

Incorporating a combustion model can be very importantin modelling extended release of heat over a volumedetermined by local mixing conditions.

A2.2.1 Combustion

The treatment of the effects of turbulent transport hasalready been mentioned briefly. Unfortunately the turbu-lent mixing process also has a significant influence on themean rate of chemical reaction. The hydrodynamic mixingof fuel with air is much slower in fires than is the rate ofreaction and so it is the former which controls the rate offuel disappearance, Rfu, and of product yield.

A simple method for dealing with this difficulty is to allowthe combustion process to be controlled only by the rate ofsmall-scale turbulent mixing between the reactants and forthat rate to be further controlled by the concentration ofdeficient reactant (either air or fuel). In air-rich locations,the reaction is controlled by lack of fuel and vice-versa infuel-rich locations, thus:

Rfu = C r e moxmfu;k s

min

(A2.4)

where mfu, mox are the local mass fractions of fuel and air, s isthe stoichiometric ratio and C is a numerical constant.

A transport equation for mfu, incorporating the abovesource term, is solved in addition to one for a normalisedmixture fraction, f, where:

mox mox, mfu +s

f = s

mox, mfu, 0 +

s

(A2.5)

Since f is conserved its transport equation does not involvea source term. (The subscripts (0, ) denote conditions inthe fuel supply and ambient air respectively). This methodcan be used to predict the major features of a wide range ofbuilding fire problems including the stable species of CO2and H2O. Models containing this simplistic treatment ofcombustion chemistry are now available for use asapplication tools to assess a variety of smoke movementproblems.

A2.2.2

Two quite distinct difficulties need to be addressed for therealistic modelling of radiant heat transfer. The firstconcerns geometrical problems associated in particular

with the exchange of radiant energy between remoteemitters and receivers, be they solid surfaces such ascompartment walls or particulate/gas phase mixtures suchas smoke and flames. The second difficulty concerns thecalculation of local emissive power. The relativecontributions from broadband soot and banded gaseousemissions will vary substantially between flame and smokeproducts. In addition, as with transport processes andcombustion chemistry, the effect of turbulent fluctuationsin temperature and gas composition may influence radiantheat transfer. In smoke movement assessment this latterinfluence is generally ignored. Many smoke movementanalyses assume a grey gas of fixed absorptivity andcalculate radiant heat transfer only in the six Cartesiancoordinate directions based on timemean predictions oflocal gas temperature.

A2.3 What the models predict

The primary output of this kind of model is a series of timehistories for each of the variables solved in the equation setA2.1 (i.e. gas velocities, gas temperatures, fuel, oxidant andcombustion product concentrations together with pres-sures) at each elementary control volume throughout thecalculation domain. Mass fluxes through ventilation open-ings as well as convective and radiative heat fluxes acrossthe face of solid boundaries are also provided.

Secondary variables can be deduced from the primary onesby the use of further assumptions. Smoke obscuration ineach control volume, for example, can be deduced fromlocal combustion product concentration, as can concen-trations of detailed chemical species. However, deductionsof these secondary variables are heavily dependent on theassumptions made and are in the spirit of the zonalmodelling approach. The validity of these assumptionsmust always be kept under review.

A2.4 What assumptions are

necessary?

As has been mentioned, models of this type avoid many ofthe assumptions implicit in the construction of a zonalmodel. However, as with models of all types, the CFDmodels also rely on assumptions regarding the choice ofdesign fire. The state of development of fire science is, asyet, unable to make accurate a priori predictions of firegrowth for practical situations. For design purposes it isnecessary to resort to assumed fire growth curves of the typedescribed in section 4.

A2.5 Numerical treatment

It is not the intention of this appendix to consider thedetails of the numerical treatment, however the implic-ations of using such a method need to be appreciated.

The computer solutions are discretised approximations tothe continuous partial differential equation A2.1, calc-ulated at the grid points of a numerical mesh spanning the

18

Thermal radiation

domain of interest. The variation of property between gridpoints has to be assumed.

The discrete nature of the solution means that it cannotaccurately capture all the physical features of the truesolution at length or time scales less than those associatedwith the numerical mesh and time step. Cell dimensions aretypically quite large, usually greater than 0.1 m, and, par-ticularly in large compartments, often substantially larger,i.e. perhaps several metres. Phenomena at smaller lengthscale are described by approximate sub-grid scaleturbulence modelling.

A2.5.1

Before any engineering judgement is made on a numericalsolution it is important that an acceptable level ofconvergence be demonstrated. Convergence is the term thatdescribes whether or not the solutions of the discretisedform of the equations approach the true solution of thepartial differential equations having the same initial andboundary conditions, as the numerical mesh is refined andas the number of numerical iterations increases. Since thetrue solution is not known, this is determined for a givenmesh from inspection of the behaviour of the residualerrors in each of the conservation equations as iterativesolutions proceed.

These residual errors are the magnitudes of imbalancebetween right- and left-hand sides of the discretisedequations using the latest solution of the guess-and-correct operation for each variable. It is not enough simplyto demonstrate that a solution does not change as iterationproceeds. This could occur if considerable under-relaxationhas been used in an attempt to procure convergence and thesolution then becomes frozen but not converged. Further-more it is important to examine the sensitivity of thesolution to grid refinement. This can be an expensive task;refining a grid by a factor of two in each co-ordinatedirection will increase computational cost roughly by afactor of eight so it is often necessary to strike a compromisebetween cost and accuracy. Most general purpose CFDpackages provide diagnostic information on the progress ofresidual errors for each of the equations solved. However, itis important to be satisfied that overall mass and energybalances for the whole domain are within acceptablebounds. Compartment mass outflows must balance massinflows and heat lost into the structure taken together withheat lost from the compartment through its openings mustbalance that generated by the fire.

There will be occasions where the model may suggestunexpected behaviour. If a physical simulation were toproduce something unexpected, the engineer would useintuition to explain what has been seen or what has beenmeasured and to relate it to the practical problem in hand.However, with a numerical simulation such an eventualityis more disturbing since it can have two explanations;either it is genuine and would have been observed in aphysical simulation or alternatively it is some sort ofmisleading numerical artefact.

The possibility of the latter cannot, be completelydiscounted with such complex numerical simulations as


those involved in CFD. It is therefore essential to shadowthe numerical simulation, where possible, with knownsimple calculation methods.

A2.6 Mode of working

Since computational fluid dynamics is a particularlycomplex undertaking its practitioners tend to be specialistswith little experience in fire science and fire engineering.Therefore, when applying CFD to fire problems, it isparticularly important that the project be supervised by amulti-disciplinary team to ensure that the problem is prop-erly posed and that the results produced are sensible.

The responsibilities of the fire engineer and CFDpractitioner can be summarised as follows.

The fire engineer must:

simplify the problem to its essentials; the presenceof design features may influence some aspects of aproblem but not others, e.g. structural beams whichmay significantly affect detection times but notsmoke filling times

specify the way that the fire source is to be treated(constant or growing fire) and decide in discussionwith the CFD practitioner whether the fire is to betreated simply as a heat source of known volumeor area or whether a combustion sub-model isrequired

shadow the CFD simulations with simplecalculations to determine whether the results ob-tained from the CFD are sensible.

The CFD practitioner must:

decide on the placement and refinement of the gridmesh in discussion with fire safety engineer (todetermine where steep property gradients areexpected etc.) and the time step size

demonstrate convergence, energy and mass balanceinformation

provide a statement on the degree to which gridinsensitive solutions have been obtained

set down the assumptions on which any secondaryvariables are based.

Bibliography (Appendix 2)

Cox G Combustion fundamentals of fire (London: Academic Press) (1995)

Patankar S V Numerical heat transfer and fluid flow (Washington DC:Hemisphere) (1980)

Fire modelling BRE Digest 367 (Garston: Building Research Establish-ment) (1991)

19

Acceptance of the solution

TECHNICAL MEMORANDA

Appendix 3: Background notes

Not e: the publications cited as references are listed at theend of this appendix.

Section:

3.2

3.3

4.1

4.2.1

4.2.4

4.3

4.4

4.5

5.1

5.2

5.3

20

Critical conditions for toxicity: from SFPEHandbook(1) (Chapter 1-14) and comments by FireResearch Station (Committee Paper No. 82).

Critical conditions for temperature: from SFPEHandbook(1) (Chapter 1-14).

Condition for flashover: based on work byMcCaffrey et. al.(2) and discussion in Drysdale(3).

Pre-flashover fires: equation 4.1 and Table 4.1 fromNFPA 92B (4); Table 4.2 from SFPE Handbook(1)

(Chapter 2-1).

Transient fire calculation: use of average valuefrom Zukoski (5).

Post flashover fires: equation 4.5 from Thomas(6);equation 4.6 from Law(7); Table 4.3 based onequation 4.6 and fire load data from Fire ResearchNote No. 808/1970, CIB Design Guide Structural FireSafety(8).

Fire products: Table 4.4 from SFPE Handbook(1)

(Chapters 1-13 and 1-25) and comments by FireResearch Station (Committee Paper No. 82).

Estimation of visibility: equations 4.8 to 4.12 fromSFPE Handbook(1) (Chapter 1-25).

Axi-symmetric plume small diameter source:

equation 5.1: the coefficient for Q2/5 is given as0.166 by NFPA 92B(4), as 0.2 by McCaffrey(9) andCox and Chitty(10), and as 0.23 by Heskestad(11).A mid value of 0.2 has been chosen for theseMemoranda

equation 5.2: estimates of the coefficient 0.056 byRouse, Yih and Humphreys (12), 0.063 by Yokoi (13),0.077 by Zukoski et. al.(14), 0.082 by Cox andChitty(10), 0.071 by NFPA 92B (4); for theseMemoranda it is considered there is no reason tochange from NFPA 92B value of 0.071 (CommitteePaper No. 40)

equation 5.3: from Zukoski et. al.(14);

M = 0.5 0.071 (2 Qp)1/3 z5/3 = 0.044 Qp

1/3 z5/3

equation 5.4: see explanation for equation 5.3.

Axi-symmetric plume larger diameter source:

equation 5.5 uses data from Hasemi andNishihata(15) as interpreted by Thomas(16)

equation 5.7: from Thomas et. al .(17); limits forapplicability from Hinkley(18).

Line source: equation 5.9 from Thomas(19), Lee andEmmons(20); range of applicability from Hasemiand Nishihata(15).

5.4

5.5

5.8

5.9

6.2

6.3

6.5

6.6

6.7

8.3

8.5

8.6

Flow from opening:

equation 5.11: based on various data collected byLaw(21) and recent data from Hansell et. al(22); seeCommittee Paper 59b and review by Thomas(23)

equation 5.12: based on data from Hansell et. al(22)

and Port e(24) ; see Committee Paper 59b. It isattached plume(22) increased by 50%

equation 5.13 based on Law(25) and data fromHansell et. al.(22); see Committee Paper 59b

equation 5.14: based on the recent data fromHansell et. al.(22) as interpreted by Law(26); seeCommittee Paper 66.

Ceiling flow: equation 5.15 from Hinkley (27).

Ceiling jet: equations 5.20 to 5.23 from Alpert(28),SFPE Handbook(1) (Chapter 1-9).

Convective heat release: equation 5.24 based onSFPE Handbook(1) (Chapter 1-13)

Smoke filling a room, with smoke extract: equation6.14 from Thomas et. al.(17).

Smoke filling a room approaching flashover:

equation 6.15 from McCaffrey et. a.l(2). (Note: theirequation has been modified, using Qp (where Qp =

Q/1.5)

equation 6.17 from Thomas(29) with Qp = Q/1.5

equation 6.18 and Table 1 from Drysdale(3)

equation 6.19 uses measurements of Cox andChitty(10) for the continuous region.

Heat transfer by radiation from ceiling layer:equation 6.21 from SFPE Handbook(1) (Chapter 2-

2).

Stratification:

equation 6.23 from NFPA 92 B (4), based on Morton,Taylor and Turner(30)

equation 6.24 derived by Thomas (CommitteePaper 81) using Morton, Taylor and Turnerapproach. A paper has been prepared forpublication(31).

Number of extract points: equation 6.25 beenderived from Fire Research Note 1001(32) (asamended 1976) and Fire Research Note 954(33). Ittakes into account data not considered in BR186(34).

Design parameters for systems intended to dilutethe smoke to a tenable condition: equation 8.1 fromSFPE Handbook(1) (Chapter 3-9).

Systems intended to protect other parts of thebuilding pressurization: equations 8.2 and 8.3from NFPA 92A(35) (Committee Paper 70).

Systems intended to protect other parts of thebuilding opposed air flow:

equation 8.5 from NFPA 92B(4) after Heskestad(36)

equation 8.6 from SFPE Handbook(1) (Chapter3-9).

.

.

.

13

4

5

6

7

References (Appendix 3)

2

8

9

10

11

12

13

14

15

16

17

18

SFPE Handbook of Fire Protection Engineering (Boston MA:

Society of Fire Protection Engineers/Quincy MA: National Fire

Protection Association) (1988)

McCaffrey B J, Quintire J G and Harkleroad M F Estimating

room temperatures and the likelihood of flashover using fire test

data correlations Fire Technology 17 98119 and 18 122

Drysdale D D An introduction to fire dynamics (Chicester:John

Wiley and Sons) (1985)

NFPA 92B: Smoke management systems in malls, atria and large

areas (Quincy MA: National Fire Protection Association) (1991)

Zukoski E E Development of stratified ceiling layer in early

stages of a closed-room fire Fire and Materials 2(2) 5462 (1978)

Thomas P H Behaviour of fires in enclosures some recent

progress Proc. 14th Inter. Symp. on Combustion (Pittsburgh PA:

Combustion Institute)(l973)

Law Margaret Fire safety of external building elements th

cibse tm smoke

Documents

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smoke movement