Control of smoke in building fires

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Control of smoke in building fires

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Reprinted from Fire Technology

Vol. 3, No. 4, November 1967 pp. 281-290

OTTAWA February 1968

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Technical Paper No. 268

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L'auteur propose que les responsables s'occupent de pr6voir des moyens de ventilation m6canique pour lib6rer les escaliers et autres issues de secours des 6difices de la fum6e d'un in-cendie. Un certain nombre de mesures auxiliaires seraient n6cessaires h cette fin. L'auteur 6tudie l'utilisation de I'effet de tirage pour lutter contre la fum6e dans les alentours imm6diats du feu. D'autres ph6nombnes, tels le vent et I'effet de tirage caus6 par le chauffage du bAtiment, pourraient annihiler I'effet d6sir6; l'article d6crit les moyens de surmonter ces difficult6s.

REPRINTED FROM

FIRE TECHNOLOGY Vol. 3 No.4 NOVET\^BER 1967

FT-27

Control of Smoke in Building Fires

J. H. McGUIRE, SFPE Diuision of Building Research National Research Council (Canada)

In the August issue, the author discussed the movement of smoke in buildings. In this, a companion article, he examines an ap-proach to controlling the movement of smoke to permit safe evacu-ation of a building and to lessen its hampering effect on fire fighting operations.

THE _^ migration of smoke throughout a building during a fire is usually a greater hazard, to life and a greater impediment to conventional fire fighting than the heat itself. A calculation or measurement of the rates at which gases can flow through the cracks around a closed doorr readily indicates the magnitude of the problem.

M E C H A N I S M S O F S M O K E M O V E M E N T

The movement of smoke throughout a building may result from pressure differentials created by winds, fans, blowers, and ventilating systems in general, and from the chimney effect caused by gas density differentials originating from temperature differentials.

Chimney effect is illustrated in Figure 1 in which the temperature, ?, within the enclosure is assumed to be higher than the e.xterior temper-ature, 7.. Air enters such an enclosure at a low level, and gases leave it at a high level; in other words, exterior pressure exceeds interior pressure at a low level, whereas the reverse is the case at a high level. At some inter-mediate level known as the neutral pressure plane, interior and exterior pressures are the same. The concept of the neutral pressure plane is sig-nificant in a discussion of smoke control in buildings.

One further mechanism is instrumental in moving smoke in the im-mediate vicinity of a developing fire. During the early stage, the at-mospheric temperature will rise and the contained gases will expand and migrate. The absolute scale of temperature (with a zero of about -273" C or -460o F) should be noted here for, unless constrained, the volume of a body of gas is proportional to its absolute temperature. The maximum gas temperature attained during a fire might exceed the original by a factor

Nore: This paper is a contribution from the Division of Building Research, Na-tional Research-Council, Canada, and is published with the approval-of the Director of the Division.

Copyright 1967 NATIONAL FIRE PROTECTTON ASSOCIATION 60 BATTERYMARCH ST., 8OsTON, MASS. O2IIO

28t

282 Fire Technology of three or even five, in terms of the absolute scale, which means that two-thirds to four-fifths of the gases originally in the compartment will be displaced. A r e a A , V e l o c l l y v z D e n s l t y p " G a s D e n s l t y p o N e u t r a l - P r e s s u r e P l a n e A b s o l u t e T e m p T o A r e a A l

Figure 1. Chimney effut.

O V E R A L L B U I L D I N G P R O T E C T I O N

Confinement of smoke to the immediate area of a fire is feasible, but there are so many potential sources of fire in a building that it is often more practicable to control smoke initirally by maintaining certain areas free of smoke.

MncnaNrc^ol, VnNrrr,mtoN

A mechanical ventilation system with some degree of recirculation will probably distribute smoke throughout much of the building unless its characteristics are modified. In areas adjacent to the fire, exhaust systems should be turned off and fresh air injected. In the fue region, air injection should be terminated and exhausting continued only if it can be done safely. The complicated arrangements required to make a ventilating system meet such requirements are probably not justified, however, when merely switching off all mechanical ventilation provides an acceptable alternative.

Assuming mechanical ventilation to be nonexistent, the migration of slnoke throughout a building, except in the immediate vicinity of a fire, will be primarily due to the normal air movements associated with chimney effect, wind, and other natural forces. Figures 2 and.3 illustrate typical conditions established in both the absence and presence of a wind. In the absence of a high wind, a high-level fire in the building wil not usually foul the stairwell, whereas a low-level fire will probably introduce smoke into it.

Stairwells are probably the most desirable areas of a building to keep srnoke-free. This may be achieved by injecting air into them at such a rate that all openings communicating with them will constitute outlets instead of inlets. In terms of fan specifications, the pressure differential involved

Control of Srnoke 283 will be quite small, probably less than 0.1 in. of water for a ten-story build-ing. More detail about the pressure differential, which will involve such features as building height, temperature differential, and exterior wind velocity, may be found elsewhere.r'2

FiCqfe. 2. .Chimnc-y- effect with idealized Figure 3. Chimney efect in a building

conditions in a building Qn wind). unA.er windy anditions.

The most significant of the required fan characteristics will be the low-pressure differential capacity, for a flow rate of 100 cfm (cubic feet per minute) can be established beneath a door with a rh-in. cleatance by a differential of 0.1 in. water gage. While the air in the stairwell remains warm, the pressure differential acro$s the doors communicating with the stairwell will range from near zero at the bottom of the well to a maximum at the top. A fan with a capacity of only 1,000 cfm might well satisfy the requirements of a stairwell in a ten-story building with ten doors opening into it and a miscellaneous leakage equal to that around the doors.

Shortly after the sounding of an alarm, one or more of the doors to the stairwell will probably be opened, and the planned effect of the fan on the level of the neutral presswe plane will virtually disappear. As often as not, smoke will then flow into the stairwell around the edges of the door leading from the region involved in fire. To dissipate the smoke fairfu rapidly, the fan should be able to deliver about one enclosure volume every 5 or 10 min when all doors are closed.

The suggestion to subdivide stairwells at, for example, every fifth storys has two advantages. First, the required fan capacity will be reduced ac-cording to approximately the square of the number of subdivisions. The reduction follows a square, rather than a linear, law because the subdivi-sions reduce both the number of doorways and miscellaneous openings per compartment and the average pressure difference across these openings. Second, owing to pressure considerations, smoke flows will be lower while

N e u l r a l P r e s s u t e P l a n e s

284 Fire TechnologY doors are open. In the event of a fan failure, however, a philosophical dis-advantage might arise, in that one portion of alstairwell might be impassable without its being obvious at other levels. With no subdivisions, the whole stairwell would probably become impassable and that would be obvious at all levels.

Elevator shafts and certain corridors will also require air injection in the event of a fire. Great care must be exercised, however, to ensure that the regions chosen do not, themselves, becorrle involved in fire while injec-tion is continued. A region could be considefed unlikely to be involved in fire if it were a fire resistant compartment with arrangements to complete its enclosure (e.g., closed doors and dampers at its boundaries) at the out-break of a fire, and if its wall and ceiling linings and contents were sub-stantially nonflammable. Air injection into a region not meeting these re-quirements should only be permitted where it can be terminated immedi-ately if the area becomes involved in fire.

Extending the concept to corridors generally introduces a problem that makes the proposition almost worthless in the present context. It has been implicitly assumed, so far, that the areas adjoining pressurized regions have a greater opening area to the exterior or to other areas than to the pressurized area. When this is true, as it is in the average stairwell, the pressure differential is established as planned across the partition separat-ing the two areas. This will not always be the case, however, in corridors such as those serving hotel bedrooms where the leakage area between a bedroom and the exterior may be much less than that between the bedroom and the corridor. In the event of a fire in a bedroom, smoke migration into the corridor will probably not be noticeably reduced by injecting air into the corridor. If, on the other hand, the room is vented upon the outbreak of a fire, air injection into the corridor can be a valuable adjunct. ANcrr,r,env Mnasunns

The use of mechanical ventilation to maintain areas smoke-free has been suggested on the premise that the building design meets a number of requirements. It has been assumed that the building is subdivided into fire resistant compartments, each stairwell and each story constituting at least one compartment and, in the latter case, preferably at least two. The partitions between compartments will be perforated by numerous openings, such as doorways and ventilating ducts, and arrangements must be made to close these in the event of a fire.

Doors could rely for closure on a simple spring or on the sophisticated hydraulically damped, spring-actuated devices now widely used. In many cases, traffic will require that the door normally be held open; this can be done by using an electromagnet from which the power will be disconnected when fire occurs. Interconnection with a fire alarm system and, if it exists, an automatic fire detection or sprinkler system must be relied on to remove power from all the door holders in the building.

Control of Srnoke 285 In some circumstances, such as in the evacuation of bedridden patients, it would be desirable to hold doors open after the sounding of an alarm. A highly sophisticated system might provide a timer and an additional power supply at designated doors so that power could be reapplied to the electro-magnet for an appropriate period (e.g., 2 min).

Dampers in ventilation ducts might also be spring loaded and held open by electromagnets. Indicator lamps would be required so that, fol-lowing power failures, the task of manually resetting the dampers could be simply carried out without neglecting any. In many cases it might be prac-tical to avoid the resetting problem by using solenoids instead of electro-magnets, provided a trouble-free design could be found. Smoke migration might render temperature sensitive catches unsatisfactory, partly because diluted smoke is not necessarily very hot, and partly because high normal air delivery temperatures in supply air ducts might necessitate the specffi-cation of too high a temperature for the catch release.

Great emphasis has been placed on the use of mechanical ventilation in stairwells, elevators, and certain corridors. Some care must be exercised to ensure that the fans operate successfully during a fire. They can be switched on by interconnection with the fire alarm and fire detection systems, but a supervised or latched circuit technique will be essential so that the fans will not be disconnected after the fre destroys the systems. The electrical supply to a fan must remain intact for as long as fre is excluded from the region it serves. This, in most cases, should be indefinitely, and could be achieved by appropriate fire resistant protection or, in stairwells, by bring-ing the supply into the buildbring-ing via the stairwell.

In general, the highest level is the most appropriate level at which air should be injected into a stairwell or elevator shaft. Unheated air will probably be used, and will then ensure dilution and subsequent dissipation of smoke at all levels.

A S I M P L I F I E D C A S E O F S M O K E C O N T R O L I N T H E F I R E A R E A

The chimney effect created by the heat in the immediate fire area can often be utilized to direct smoke to the exterior and to ensure that it does not migrate to other parts of the building. In general, the pressure differ-entials created by the local chimney effect will have the effects of wind and the chimney action in the whole building resulting from building heat-ing superimposed upon them. For the moment, however, it will be as-sumed that the parts of the building not involved in fire are at the exterior ambient temperature and that no wind is involved.

TIre most important feature in utilizing chimney effect created by the fire is to control the level of the neutral plane, because all openings above it will constitute outlets and those below it, inlets. For the simple case

286 Fire Technology illustrated in Figure 1, the level of the neutral plane is grven by the ex-pression

h2 _ AfT

hr Az'7.

Where there are multiple inlets and outlets the expression becomes

{TY (or*,",'/hr^-,1

: \/T"f1or,,,,,

,rr^,y)

In general, the ratio T/T,rn expressions 1 and 2 might be taken as 3 on

the assumption that ? : 900" K (627" C) and ? : 300o K (27" C). To ensure that smoke flows to the exterior and not to other parts of the building, all openings above the level of the neutral plane must com-municate with the exterior and not with the interior. Unless an attempt is made to seal some openings that are essentially at a high level (for example, a crack at the top of a closed door), the neutral plane will have to be raised to at least this level. The controllable variables are the inlet and outlet area€r. Considering expression 1, Ar must be minimized and then a high-level opening to the exterior, Az, creaffi to give a suitably low value of the ratio Ar/Az.

Even where building heating and winds are absent, applying the above principles in a building involves the complication illustrated in Figure 4, ,which represents an a.rea adjacent to a fire intended to be kept smoke-free. The single neutral pressure plane of Figure t has divided into the two shown in Figure 4; the one on the right being distinctly lower, and the one on the left a little higher, than would have prevailed had the srnoke-free area been completely open to the exterior. In Figure 1, the two vertical walls separated the same pair of atmospheres, yfu., the interior and ex-terior atmospheres. In Figure 4, there is a pressure differential between the smoke-free area and the exterior, which produces a flow of frestr air into that space. The resulting depression of the neutral plane can be minimized by ensuring that the area of the ftesh air entry to the snoke-free area is large in comparison with the total area of the openings between the smoke-free and fire areas.

Achieving the appropriately low ratio of. At/A, when the confinement of smoke to a potential fue area is considered mlght itself involve problems. 'It is worth dividing these into two categories associated, respectively, with minimizing inlet areas and with creating an appropriate highJevel vent to the exterior. To minimize inlet areas, doors and any low-level openings, such as registers, must be closed in the manner discussed earlier.

If lowJevel windows fall out drring the fire, they will constitute inlet

(1)

Control ofSrnoke 287 areas and hence lower the level of the neutral plane. This condition may be alleviated by adopting one or more of the following procedures:

o There should be no windows below the desired level of the neutral pre$[rre plane.

o All windows below the level of the neutral plane should be balanced by highJevel windows, expression t or 2 being the balance criterion. The validity of this recommendation depends on the assumption that high-level windows will fall out before low-high-level ones.

r Windows below the level of the neutral plane should be of wired glass or glass block. It is popularly held that wired glass and many glass blocks will remain in place about 45 min and t hour, respectively, following out-break of a substantial fire.

A reasonably priced and effective technique to provide a highJevel opening to the exterior would be to close the required area (probably about 9 inches square) by a spring-loaded cover and secure it by a metal pin with a low.melting temperature. If the potential fire area were monitored by a mroke or fire detector, the detector would open the high-level vent, which could then be spring-loaded and normally held closed by an electromagnet.

P l a n e s

F i r e A r e a

S m a l l 0 p e n i n g

a - t o E x t e r i o r

( a t a n y l e v e l )

Figure 4. Utilizatian of chimnqr effed. L O C A L C O N T R O L

U N D E R D I F F I C U L T C I R C U M S T A N C E S

Smoke control as just described can be made ineffective either as a result of adverse wind conditions or if the fire region is several stories lower than the level of the neutral plane in relation to a communicating interior shaft (i.e., one of the lower stories in a tall building). Under either circumstance, gases will flow from the fire area to the vertical shaft at all levels, regardless of any openings made in the exterior wall.

Both adverse conditions can be overcome by using a flue to achieve the high-level vent area. A cross-sectional area of about % qft would usually be sufficient, and such a flue could often serve as many as four roor$ on every floor it traversed. In a tall building, therefore, it might well serve a hundred rooms.

2BB Fire Technology In the event of a fire in one particular room, it is important that only that room be connected to the flue. A damper should separate each room from its flue, and it should open only upon the outbreak of fire in the room it serves. Under these circumstances mains-operated electrical control of the necessary damper might not be the wisest approach. A spring-loaded damper held closed by a temperature-sensitive metal would prob-ably be as satisfactory an arrangement as any.

No sophisticated arrangement is necessary at the top of the flue. Wind blowing across it will create a negative pressure that is an appreciable fraction of the stagnation pressure. A special cowl would be necessary only where a flue had been improperly located in relation to an adverse roof contour.

The height of the flue between the level of the room involved and the top of the flue could have a substantial effect on the level of the relevant neutral planes. The variation of the effect with height would be of the precise nature that is desirable. In the absence of a flue, fire in a low-level region would usually result in the flow of smoke to stairwells and the like, as stated previously and illustrated in Figure 2. Because of its substantial height, a flue would have a marked effect, and all openings to the fire area would become inlets, thus preventing smoke migration to other parts of the building.

S P E C I A L D E S I G N S

Where extreme precautions against smoke haza:rd, are desirable, con-sideration should be given to using several spatially separated buildings to house one occupancy, instead of one building containing one or more occupancies. This approach has already been adopted on several occa-sions; escape stairwells have been constructed as separate units linked to the main building at each story by ventilated or open bridges.

An interesting and rewarding application of this technique would be in the design of hospitals, where evacuation is normally a problem. Clear-ing a conventional hospital of bedridden patients in the event of a fire is time-consuming because descent to the ground floor means using either elevators or long shallow rnmps. If a hospital were to consist not of one building but two, joined at each level by ventilated bridges, no patient would have to change levels to be transferred to the second building.

Figure 5 illustrates a design in which the fact that a hospital consists of more than one building is not necessarily apparent to the patients and need cause no inconvenience. The pairs of doors at each end of the con-necting bridge perfomr a dual function. Normally they constitute the sides of the corridor, closing off the emergency vents to the exterior. Upon the operation of any fire detector, fre alarm, automatic sprinkler, or other suitable device or circuit, the doors should spring to the other position,

Control of Srnoke

closing off the two buildings and leaving the connecting bridge freely ventilated. A timer and overriding arrangement would be necessary as it would be desirable to have the doors held open while beds were transferred from one building to the other.

E F F E C T O F F U E L

Most building fires create large quantities of smoke regardless of the nature of the fuels involved; this is not to say, however, that the fuel has no effect on smoke generation. There ar'e two properties of fuel that have considerable bearing on the rate of smoke production. The first is its flammability, as determined by various flame spread or fire hazard tests; this, in conjunction with many other factors (e.g., distribution, ventilation), will govern the likelihood and time scale of the development of a fire.

The second property is less well defined than flammability, and per-tains to the rate at which a specified amount of the materiral will generate smoke under conditions intended to represent those prevailing in practice. The property is assessed in terms of a smoke index.a'5 The smoke generation rate of a burning material is highly dependent on the completeness of com-bustion. To some degree, combustion is always incomplete in building fires, and some materials generate much more srroke than others. For this reafion, some restriction on the use of materials by regulating permissible smoke indexes is usefuI in combatting smoke problems.

Figure 5. A sqgested lnspitat building design for controlling smohc'

Unfortunately, the present level of knowledge on the subject does not allow truly sound recommendations. One interesting suggestion is that the snoke index of any material to be used in a compartment should not be higher than the highest index of materials whose use in quantity is unavoidable. In many cases, the index that will result from such a con-sideration win be that of some species of wood.

290 Fire Technology

C O N C L U S I O N S

The building designer's initial approach to the control of smoke move-ment during fires should be to maintain critical areas (e.g., stairwells) free of smoke. Mechanical ventilation is necessary, together with essential ancillary measures such as door closing arrangements and locating blower wiring in a fire-safe region.

If necessary, smoke may be confined to the immediate area of a fire's origrn by a combination of venting and limiting inlet areas. Such an ap-proach involves adopting the necessar5l measures in every enclosure in which a fire could take place and hence could prove expensive. To ensure confinement even in the presence of an adverse wind or where the bottom story of a very high building is involved, the further complication of venting flues is necessarJr.

R E F E R E N C E S

1 McGuire, J. H., "Smoke Movement in Buildings," Fhe Technalngy,YoL S, No. 3, August 1967, pp. 163-t74.

I Tamura, G. T. and A. G. Wilson, "Pressure Differences for a Nine-Story Build-lng as a_Result of Chimney Etrect and Ventilation System Operation," ASHRAE, YoL72, Paft I, 1966, pp. 180-9.

3 Tamura, G. T., Private communication.

a'Standard _{Method of Test for Surface Burning Characteristics of Building} Materiale" ASTM E 84-61, 1964 Book of ASTM Standards, Part 14, p. 331-337.

6 Gross D. and Loftus, J. J. "A Laboratory Tbst for Measuring Smoke from Burn-ing Materials," presented to Sirty-Ninth Arinual Meeting of thi American Society for Teeting and Materials, Atlantic City,27 June to I July 1966.