Smoke movement in buildings

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Smoke movement in buildings

McGuire, J. H.

Ser TE]-N2l_t2 no. 250 e . 2 BLDG

/-7 7 97

I NATIONAL RESEARCH COUNCIL OF CANADA CONSEIL NATIONAL DE RECHERCHES DU CANADA

Smoke Moaement in Buildings

by

J. H. McGuire

A N A L Y Z E D

Reprinted from Fire Technology Vol. 3, No. 3, August 1967

pp. 163-L74

Technical Paper No. 260 of the

Division of Building Research

OTTAWA November 1967

NRC 9867 Price 25 cents

t /

DEPLACEMENTS DE LA FUMEE/ DANS LES IMMEUBLES INCENDIES

SOMMAIRE

L'auteur d6crit les m6canismes d6terminant les d6place-ments de la fum6e dans un immeuble incendi6. Il explique I'im-portance relative des diff6rents facteurs, tels le vent, le tirage caus6 par le chauffage du bAtiment et celui qui est produit par I'incendie, et il 6tudie les ph6nombnes accompagnant un incendie imaginaire pour 6valuer les dur6es du d6placement de la fum6e d'une enceinte b I'autre. L'auteur d6crit ensuite les conditions qui pr6vaudraient dans un 6difice 6lev6.

REPRII{TED FROM FIRE TECHNOLOGY V o l . 3 N o . 3 A U G U S T 1 9 6 7

FT-25

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

Smoke is a major killer of building occupants in times of fire. How does it spread throughout a building and reach untenable accunulations before temperatures become dangerously high? In this, the first of two articles, the author discusses the move-ment of smoke. In the second, he will deal with methods for con-trolling the movement of smoke in buildings.

tfffn _{smoke generated by fires in buildings frequently creates greater} I problems for occupants attempting to escape, and for fire fighters, than does the heat. Eye irritation and reduced visibility caused by smoke can make it impossible to see, so that, in otherwise familiar circumstances, an occupant of a building may be unable to locate escape routes and exits. The toxic components of smoke, combined with reduced oxygen content of the atmosphere, can often prove fatal, even in regions where excessive temperatures have not developed. Statistics of fire deaths in Ontario indicatel that at least half the deaths result from asphyxia or carbon mon-oxide poisoning rather than from burns or other temperature effects. In fact, the proportion is probably higher. When individuals have been badly burned, it is not unlikely that the cause of death will be recorded as asso-ciated with burns, although in reality it might have been the result of asphyxia or some toxic component in the atmosphere.

The fire fighter can avoid toxic gas and irritation problems by wearing breathing apparatus. His efficiency is often impaired, however, by his inability to see once he is in a smoke-fiIled building; and he will often be unable, for example, to locate the origin of the fire. The magnitude of this problem is illustrated by the fact that firemen often make high-level open-ings in buildopen-ings to induce air flow through low-level inlets, thereby im-proving visibility near the inlets. The fire fighter's problems, however, are

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

Smoke Moaement in Buildings

Copyrishi 1967 NATIONAL FIRE PROTECTION ASSOCIATION 60 EAITERYMARCH ST.. BOSTON, MASS. O2IIO

Printed in U.S.A.

164 Fire TechnologY usually considered less important than the escape problems smoke creates for the occupants of a building.

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

So far as movement is concerned, smoky and normal atmospheres are virtually indistinguishable. The principal constituent of both is nitrogen, and although the oxygen and carbon dioxide contents may differ by about 10 per cent, this will not substantially affect the physical properties of the atmosphere. The properties of the particulate smoke component differ markedly from those of, say, nitrogen; but the concentration will not be sufficient to influence the over-all movement of the atmosphere even at levels that reduce visibility to virtually zero. The mechanisms to be dis-cussed are not, therefore, uniquely related to the movement of a smoky atmosphere as distinct from air.

Pressure differentials associated with winds, blowers, fans, and me-chanical ventilation systems will contribute to smoke movement; however, temperature differentials and variations are usually even more important factors. In the immediate area of a developing fire, the continuous increase in temperature causes the familiar expansion process that can displace the greater proportion of the volume of the gases involved.

Expansion only occurs as temperatures rise, but another effect, known colloquially as "chimney effect," proceeds continuously whenever there is any temperature differential between interior and exterior. During a fire, chimney effect, rather than expansion, is responsible for the greater proportion of smoke migration. Because some level of temperature differ-ential usually exists between a building and the exterior atmosphere, chimney effect is also responsible for much of the normal air movement in buildings. As the temperature differential of smoke-Iaden gas can fall off rapidly at any distance from the immediate fire area, it is this normal move-ment of air within a building that is largely responsible for the widespread distribution of smoke.

T H E R M A L L Y I N D U C E D M O V E M E N T

The displacement of gases by the expansion mechanism can be con-veniently discussed on the basis of the universal gas law, which states that for any given mass of gas the product of pressure, P, artd volume, V, is proportional to absolute temperatwe, T. The zero of the absolute scale of temperature is approximately -273" C or -460" F. In other words, PV : R? where ,E is a constant (usually taken to relate specifically to 1 g mole of a gas).

The pressure differentials required to establish substantial flow ve-Iocities ar.e very small compared with absolute atmospheric pressure (see Table 1). It may be said, therefore, that the volume of a given mass of gases is proportional to its absolute temperature since, during all the rele-vant processes short ofexplosions, P in the above expression hardly varies'

Srnoke Movernent 165 Teer.n L. Magnitude of Thermal and Wind Effects

Height of heatud cornpartment (ft) 800" C aboue

Pressu.re Head ambient (i.e.. on (in. of water) fi.ri)

9O" F aboue arnbient Windspeed(mph) Flnw beneath a door (cu ft/min) 0 . 1 o.2 0 . 5 1 2 9.5 1 9 . 1 47.7 95.5 191_ 33.9 67.7 169.3 339 677 14.4 20.4 32.2 45.6 64.5 103 L46 230 326 467

Prior to a fire, a compartment temperature might be 20o C, or approxi-mately 300' K (absolute), and during the course of the fire it will probably rise to at least 900' K (627" C). The volume occupied by the gases originally in the compartment will multiply by three, and two-thirds will migrate to other parts of the building.

The process known as chimney effect is illustrated in Figure 1, and in-volves the entry of air into the compartment at a low level and the outflow of gases at a high level. In other words, exterior pressure exceeds interior pressure at a low level, and the reverse is true at a high level. At an inter-mediate level, labeled "neutral pressure plane" in Figure 1, the interior and exterior pressures are the same. The prevailing conditions are analyzed by equating the pressure drop created by the flow through the openings to that resulting from the differences in density of the internal and external atmospheres (using the neutral pressure plane as a reference datum). The location of the neutral plane proves to be such that

hz hr

A f T Ar'To where the symbols are defined by Figure 1.

The chimney effect and the concept of a neutral pressure plane are so vital to gas movement in a building that other phenomena (e.g. wind) may be discussed in terms of the manner in which they modify the chimney effect. To adopt this approach, the right-hand portion of Figure 1 will be used, where the pressure difference across the vertical compartment boundaries is depicted graphically, following the example set by Tamura and Wilson.z The horizontal scale, with a zero as marked, represents pres-sure difference across the exterior boundary.

A N C I L L A R Y E F F E C T S

To discuss the effects of wind in terms of theirinfluenceonthechimney effect is complicated by the fact that wind effects are themselves a function of the shape and size of a building. The intermediate step of discussing

166 Fire Technology G a s d e n s i 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 et e m p T o

Figure 7. Chimney efect.

the exterior pressure differentials created by wind will, therefore, be intro-duced and followed by a discussion of the influence of exterior pressure differentials on chimney effect. The subject of the exterior pressure differ-entials created by a wind is complex. Textbooks on the aerod5mamics of bluff bodies give detailed coverage, and a popular description is available.3

The first approximation one might make in evaluating the pressure differential created by a wind of velocity, u, is to equate it to the stagnation pressure, Dp. The latter is related to the kinetic energy of the gas, being given by

where p is the density of the gas.

At the center of a plane face normal to a wind, the resulting pressure differential is the stagnation pressure. The differential falls off as one ap-proaches the boundaries, however, often becoming negative at the other faces. The leeward face, in particular, will usually be subjected to a nega-tive pressure differential between -0.5 and -0.7 times the stagnation pressure.

A further complicating feature is the fact that wind velocities are them-selves a function of height above the ground, being nearly zeto at ground level and increasing with height. Variation is usually substantial over the first hundred feet above ground level.

The hypothetical problem of Figure 2 illustrates the way in which exterior pressure differentials modify chimney effect; at any height the exterior pressure on the wall on the left is greater than that on the right by an amount Ap. It will be noted that the inlet and outlet in Figure 2

6p:+

(2) A r e a A , T E o o

Srnoke Movement d - d q t l e i € € o t - @ -c* 5 -

.c-r-'r---f

---.1

l t r l W i n d ( g i v i n g p r e s s u r e a P I _ J - - - + - - - - + - t - | r . i i I

= *ii

^ : l € |

i

- - \ - - r - ; - - 1

i l . = i

A=-- r N P N P N P l = 2 = ) -N e u t r a l p r e s s u r e N e u t r a l p r e s s u r e N e u t r a l D r e s s u r e p l a n e ( l e f t h a n d w a l l ) p l a n e ( r i g h t h a n d w a l l ) p l a n e i n t h e a b s e n c e o f w i n d ( b o t h w a l l s )

Figure 2. Effect of wind.

are on different sides of the compartment. If both were on the same side, the pressure differential would have no effect on the level of the neutral plane.

The pressure differential will result in a separation of the neutral pres-sure planes of the walls on the left and right, as illustrated. The exterior pressure differential, Ap, will be equal to the "head" given by the difference in level between the two neutral planes. No zero has been marked on the horizontal pressure differential scale on the right side of Figure 2. The location of the zero is dependent on whether the reference point is taken as the left- or the right-hand exterior pressure. The two appropriate refer-ence il(es are marked "L. H. 'Wall" and "R. H. Wall."

Analysis of the conditions shows that, using the symbols as defined in Figure 2, Equation 1 continues to apply. Furthermore, the displacement of the neutral planes from the level NP3, prevailing in the absence of an exterior pressure differential, will also be in the proportion given by Equation 1.

The magnitude of a wind's effect can be considerable. With the idealized conditions illustrated in Figure 2, attributing a value of 10 ft to H, the flow pattern could be reversed for wind speeds of more than about 15 mph. As the wind speed approaches 15 mph the values of hz and ftr would tend to approach zero, and for higher speeds, no neutral pressure planes would exist.

168 Fire Technology Before attempting to extend the concepts illustrated in Figures L and,2 to describe the flow of gases in a building, the effect of flow constrictions within a compartment must be discussed. In Figure 3, part of the over-all pressure head appears across the interior constrictions to establish the appropriate flow through them. The lower of the two constrictions illus-trated produces only the effect just mentioned, but the upper one creates an additional neutral plane. The horizontal scale in the diagram, referenced to the vertical axis, O, represents pressure difference between interior and exterior. Thus, the fact that the vertical axis (i.e., thle zero of the scale) is crossed at two levels means that there will be two neutral planes.

R E L A T I V E M A G N I T U D E S A N D L E A K A G E A R E A S To analyze the probable conditions in a building, a concept of the rela-tive magnitudes of various effects is required. Table 1 illustrates how par-ticular values of pressure head may be established by a fire, the heating of a building, and a wind. To calculate the values in the second column, it was assumed that a fire had raised the temperature in a compartment by 800' C; to calculate the values in the third column, that building heating had produced a 90'F differential between interior and exterior. In typical compartments of the heights listed in Columns 2 and 3, the respective pressnre differentials would, in practice, tarely be found. The values given in Table 1 relate to the sum of the pressure differentials across the top and bottom of the compartment. It would be the exception rather than the rule for the whole of the pressure differential to occur at one level

(the pressure differential at the other approaching zero).

Column 5 lists the flow rates that would be established by the pressure heads given in Column 1 beneath a 3-ft 3-in. wide door with th-in.

clear-ance from the floor.

Information is also required on the leakage characteristics of buildings, and this is a field in which not many measurements have been made. In Canada, however, the characteristics of four buildings ranging in height

e u t r a l p r e s s u r e p l a n e s F l o o r F l o o r l e '_{t ' :}

\

--Et=

A b s o l u t e t e m p T

smoke Movernent 169 from 9 to 44 stories and in floor plan area from 12,000 to 20,000 sq ft have been studied. OnIy the findings of the first study - a 9-story, brick-faced, reinforced concrete structure of plan dimensions approximately 75 by 265 tt - have so far been published.'

In all fout cases, the leakage between adjacent stories exceeded that from any one story to the exterior. The over-all equivalent leakage area to the exterior in the 9-story building was about 50 sq ft (excluding ventila-tion duct openings, and with windows and exterior doors closed). It was estimated that less than a quarter of this total was associated with windows. For the other three buildings, over-all leakage (normalized to cater for different building dimensions) was appreciably less, in one case by a factor of three.

The levels of the principal neutral planes in the buildings ranged be-tween 0.35 and 0,72 of the total heights, the lower values being associated with the higher buildings.

T Y P I C A L B U I L D I N G C O N D I T I O N S

Before discussing more complex problems, it is convenient to consider the simple case of any building containing a mechanical ventilation system thabis largely recirculating and has only one central plenum chamber. Smoke from a fire anywhere in the building will obviously be distributed throughout. Furthermore, although the primary smoke from the fire area will be greatly diluted in the plenum chamber, it will usually be of unde-sirably high density in the delivery duct; and even a moderate fire would be capable of smoke-logging a whole building via the ventilation system.

The level of smoke distributed throughout a building will depend on whether the outlets from the fire region open to the exterior or to other parts of the building. It has been showna that this feature permits the con-trol of smoke in buildings. In most current designs normal flow patterns discharge much of the smoke into the building so that it reaches remote areas, even though the fire itself may be localized. Only in the immediate

area of the fire is the flow of gases substantially affected by the thermal effects created by the fire. An analysis of smoke migration is, therefore, equivalent to an analysis of the normal movement of air in a building.

Figure 4 represents the conditions that could prevail in a building with no automatic ventilation system, no exterior wind, and an internal temper-ature higher than external, as in winter. The arrows shown in Figure 4 indicate direction of flow but not magnitude. It should be noted that the further an opening is, vertically, from the neutral pressure plahe, the greater will be the pressure differences between interior and exterior, and hence, the greater the flow. Ttre pressure differences also increase in almost direct proportion to the temperature differential between interior and exterior.

If a fre were to originate under winter conditions on the second floor, it might be inferred that no noticeable level of smoke would develop on

170 Fire Technology the ground floor. Smoke will be found on floors lower than that on which the fire originates only as a result of mechanical ventilation, anomalous wind conditions, or an exterior temperature exceeding the interior temper-ature, thus producing a reversed chimney effect. In the first stages of the fire, gases displaced from an unvented fire region by an expansion mechan-ism might also be forced into a lower story; this process, however, would not continue for long.

Figure 4 indicates that smoke from a fire on the second story would migrate directly to the floor above and, via the elevator or stair shafts, to

S t a i r w e l l o r e l e v a t o r s h a t t

e u t r a l

p r e s s u r e p l a n e s

Figure 4. Idealized conditions in a building.

the floors above the neutral pressure plane. Thus it is probable that smoke density on the top floor will be greater than that on the fourth and inter-vening floors.

The flow pattern illustrated in Figure 4 results from a temperature differential between interior and exterior. Variation in this differential, from winter to summer, for example, will affect the magnitude of all the flows. If the interior building temperature is lower than ambient, as can occasionally happen at the height of summer, the directions of flow can be reversed, high-level openings constituting inlets and low-level openings, outlets. Such negative temperature differentials are the exception rather than the rule, however, and as they are bound to be of smaller magnitude

Srnoke Movernent f?l than the positive differentials prevailing in the depth of winter, the flow velocities are correspondingly lower.

The principles already discussed make it obvious that a wind of any magnitude (e.g., 20 mph) will have a substantial influence on a flow pat-tern, such as that of Figure 4. Velocities of the order of those given in Table l- will make one side of the building an inlet and the other an outlet. There will still be an upward flow within the building, inlet velocities on the windward facing wall being minimal at the top and outlet velocities on the lee wall minimal at the bottom. These conditions develop out of the more elementary case illustrated in Figure. 2. The critical velocities may actually be less than those listed in Table 1, because of the effect of internal constrictions illustrated in Figure 3, and because the total pressure differential established by a wind across a building can exceed the stagna-tion pressure.

It is common practice for fire fighters to establish large vents at high levels during a building fire. This induces air at low-level inlets and permits fire fighters to enter, attacking the fire as they proceed. Before adopting this approach, wind conditions, in particular, should be noted. The most favorable vent is one on the lee side of a building (as well as at a high level). Similarly, the most satisfactory inlet and entry point will be to windward. If the intended flow pattern is achieved, the venting of the fire will prob-ably increase the rate of burning greatly and, hence, destroy the building much more rapidly unless fire fighting is successftrl.

Another cornmon practice, incorporated in the design of many build-ings, is to vent the top of elevator and even stair shafts with the intention of maintaining them tenable for as long as possible. The merits of this technique are questionable so far as smoke control is concerned; they can be discussed with reference to Figure 4. A highJevel opening, large in comparison with the sum of all the other openings, will raise the neutral pressure planes associated with the shaft almost to roof level. AII inlet flows will thus be increased and several openings that were previously out-lets will become inout-lets. Fires on the floors associated with these latter openings would, of course, now introduce smoke into the shaft where they might not have done so had the vent not existed. Fires at lower levels will introduce more smoke than previously, but the resulting smoke level will be lower because air flow will have risen by a greater factor. An alternative technique that will eliminate the smoke problem in elevator and stair shafts is discussed in Reference 4.

The flows shown in Figure 4 relateto a fairly symmetrical distribution of openings, and the effect of having some doors open and others closed has not been discussed in detail. An example will illustrate possible dis-astrous effects. Suppose that a fire is in progress on the second story of the idealized building illustrated in Figure 4, and. that the exterior windows on that floor have been destroyed by heat. Leaving the lobby and stair shaft doors open would not only greatly increase the flow of smoke into the stair

172 Fire Technology shaft, it would also lower the neutral pre$sure planes and hence increase the number of stories into which smoke would flow from the stair shaft. If the area of the open doors were large in comparison with the sum of the other openings to the stair shaft, the neutral planes would be near the level of the second story and smoke would flow into all the higher stories.

T I M E S C A L E

To give an indication of the time scale of the movement of smoke in a building, a hypothetical problem has been solved analytically. The hypo-thetical building (Figure 5) consists of several compartments, each of vol-ume, V, one on top of the other. The number of compartments (or stories)

F l o w r a t e f r o m c o m p a r t m e n t t o c o m D a r t m e n t = 2 / ( v o l u m e / u n i t t i m e ) . V o l u m e o f e a c h c o m D a r t m e n t = V

Figure 5. Hypothetical probl,em.

proves to be unimportant and may vary from two to infinity. Each com-partment is linked to the next by an opening, and it is assumed that gases flow from one compartment to the next at a rate, u. A temperature differ-ential between the interior and exterior would establish such conditions. It is also assumed that mixing takes place in each compartment.

If it is mled that, at time t : 0, compartment No. 1 becomes smoke-logged, the smoke density being 100 units per unit volume, then the smoke density in compartment,l/ is given by

D* : ' =1oo , Io't' ,(*-ile-rd,,

(N -2)lJ "

: 1 o o { t -

" [ t

, r r r r P , T , T n - r r -r 2 r - gl -[

+#%ll

Srnoke Movernent where

173

T : ut/V.

It is interesting to note that at the second story the solution reduces to the familiar expression

D : 100 (l - e-,ttv1

These expressions are illustrated in Figure 6, and it may be seen that a smoke density of one-tenth that prevailing on the first story will occur at a time ? : ut/V : 0.L on the second story and at time T : ut/V : 5.4 on the tenth story.

A typical example has been evaluated to illustrate the actual times that these values are likely to represent, and a few sample results are second story, 5.6 min (3 min); third story, 29 min; and tenth story, 4 hours 50 min. It has been assumed that each compartment has a volume of 3,000 cu ft and is linked to the next by the crack referred to earlier as typical of that beneath a single door (i.e., 3 ft 3 in. by 0.5 in.). A 90' F temperature differential between interior and exterior has been assumed, except for the bracketed value (viz., 3 min) where fire conditions giving L6 times the differ-ential (i.e., 800" C) have been assumed. In passing, it should be noted that the assumption of a temperature differential corresponding to a fire instead of to building heat gives less than a twofold increase in gas flow rates (and hence the reduction from 5.6 to 3 min).

The evaluation indicates that even the crack beneath a door might, within a few minutes, create untenable conditions in a compartment ad-jacent to one involved in a fire. As the source of the fire becomes further

t 0 0 rF - 6 0 z v 4 0 = . 0 0 5 . 0 1 . 0 ? . 0 t . 1 . 2 . 5 t l t M E , T - u l l V Figure 6. Smake, dilution.

t74 Fire Technology removd, in terms of the number of compartments through which the smoke must pass, the time scale increases substantially.

C O N C L U S I O N S

Recent analysis of the leakage characteristics of several large buildings indicates that the leakage between adjacent stories appreciably exceeds that from any one story to the exterior. Gas movements between a local-ized arca of a building and the exterior cannot, therefore, be discussed individually but only as a part of the flow complex throughout the building.

Most gas movements in a building may be discussed in terms of their influence on chimney effect. It is interesting that gas flow from a compart-ment on fire to an adjacent one is not likely to be more than double the value resulting from building heating under severe winter conditions in the absence of fire. Even though the temperature differential created by the fre rnay be sixteen times that resulting from normal building heating, it will establish less than double a typical winter pressure differential. Prior to breaking windows or establishing other conditions uniquely associated with a fre, the smoke movement may be discussed in terms of the normal gas movements within the building.

R E F E R E N C E S

1 Williams-Leir, G., private communication.

2 Tamura, G. T., and Wilson, A. G., "Pressure Differences for a Nine-story Build-ing as a Result of Chimney Efrect and Ventilation System Operation," Transactions of the American Society of Heating, Refrigerating, and Air--onditioning Engineers, Yol. 72, Part I, 1966, pp. 180-189.

3 Dalgligsh, W. A., and Schriever, W. R., "Wind Pressures on Buildings," National Research Council of Canada, Division of Building Research, CBD 34, Oct. L962.

a McGuire, J. H., "Control of Smoke in Buildings," scheduled for publication in Fire Technolngy, Vol. 3, No. 4, Nov. 1967.