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Technical Note (National Research Council of Canada. Division of Building Research), 1965-04-01
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The Principles Underlying the Movement of Smoke in Buildings
McGuire, J. H.
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DIVISION OF BUILDING RESEARCH
No.
NATIONAL RESEARCH COUNCIL OF CANADA
436 c
NOTE
'IfE
C
1HIN ][ CAlL
PREPARED BY J. H. McGuire CHECKED BY GWS APPROVED BY NBH
DATE
April
1965
PREPARED FOR General Information
SUBJECT The Principles Underlying the Movement of Smoke in Buildings
The purpose of this Note is to try to provide, for easy reference by fire prevention officers especially, a description of the mechanism responsible for the movement of smoke and gases in a burning building.
THE NATURE OF A GAS
Any gas is composed of individual molecule s which are extremely numerous' even if the total quantity of the gas involved is minute. These molecules move about at random and their average energy of movement (kinetic energy) is directly proportional to the absolute temperature of the gas (having a zero at about min11S 4600
F). The absolute temperature of a gas (i.e. its temperature measured on a scale with a zero near minus 460°F) can, in fact, be conveniently regarded as the measure of the average energy of movement of the molecules.
The concept of molecular movement also provides an explanation of how it is that gase s exert a pre s sure. If a quantity of gas is put in a box, then gas molecules keep striking the sides of the box and rebounding. It is this mechanisrn that results in a pressure being exerted on the box sides. The rate at which these collisions occur is so astoundingly great that, so far as an observer is concerned, the resultant pressure is quite steady.
-2-THE GAS LAW
Development of the concept of molecular movement will readily illustrate why it is that gases obey the law PV=RT, where
V is the volume of a definite weight or mas s of gas P is its pressure
T its absolute temperature and R a constant.
If, in the above expression, temperature is considered constant, then the relation states that pressure should vary inversely as volume. If a quantity of gas in one box is transferred to a second box of only half the volume, then the number of molecules per unit volume in the second box will be twice the previous value. The collision rate at the box sides will thus be double and hence the pressure in the second box will be twice that which prevailed in the first. It is therefore to be expected that, at constant temperature, the law PV= constant ｷｩＱｾ be obeyed.
If the case of constant volume/ variable temperature is con-sidered..the molecular movement concept will again be compatible with the universal gas law. Regardless of the temperature of the gas in a box the number of molecules per unit volume will, of course, be constant. Irence an increase in pressure can only result if individual molecules hit the side of the box more frequently and/or if each collision involves greater momentum reversal. An increase in the velocity of a molecule gives a proportional increase in both these quantities so that, for constant volume, pressure will vary as {molecular velocity)2. The temperature or energy of molecular movement of the gas, however, is also dependent on {molecular velocity)2. Thus pressure will vary directly as temper-ature, in compliance with the universal gas law.
DIFFUSION
Molecular movement in gases is responsible for the fact tnat gases of different densities are usually found uniformly mixed in an
atmosphere. Thus if (gaseous) propane, which is more dense (i. e. heavier) than air, is allowed to escape from a cylinder into a room, it will initially flow across the floor and behave in the same sort of manner as a liquid. If the cock of the cylinder is closed and the room is left undisturbed, it will be found, some time later, that the propane is uniformly distributed throughout the room.
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-3-PRACTI CAL SIGNIFICANCE AND TYPI CAL VALUES OF PRESSURE When bulk movement of a gas take s place, i. e. a flow from one region to another, it is pressure difference which is responsible. The gas flow is from the region at the higher pressure to the one at lower pressure. A complicating feature concerning gravity and "fluid head" is also involved, but it is hoped that the part it plays will become apparent as the general theme is developed.
Before proceeding further, it is convenient to list values of pressure and pressure difference commonly encountered. These values are such that certain simplifying assumptions can be made.
The reason atmospheric air, at ground level, does not expand indefinitely until its volume becomes infinite and its pressure zero is that it is supporting a mass of air above it. Above every square inch of surface on the earth there is a weight of approximately 14.7 lb of air. Following the law pressure x volume equals a constant, (at a fixed
temperature) the air at ground level is compressed until its pressure rise s to this same value, which is approximately 14. 7 lb/ sq inch.
A pressure is often expressed in terms of the head of water it can hold up. A 34-ft column of water of area one square inch weigh's
14.7 lb and hence atmospheric or ambient pressure may be described as being 34 ft water gauge.
Now a pressure difference of only O. 1 in. water gauge either side of an opening can give rise to a flow velocity of about 20 ft/ sec. (14 mph). From this relationship alone a fire-fighter will appreciate that it is most unlikely that he will ever encounter steady pressures higher than this during the course of a fire.
More substantial transient pressures can result from explosions, or rapid flame propagation through an ideal fuel-air mixture, but such events will be rare and obviously the high pressure will not be maintained for periods longer than the odd few seconds,
It can also be shown theoretically that pressure differences associated with fires will not usually exceed O. 1 in. water gauge and it is interesting to note that this level is lower than, for example, the value of 0.3 in. water gauge which can be developed by a 25-mph wind.
The most interesting conclusion, from the theoretical point of view, is that pressure differences are invariably small compared with the absolute value of pressure (excepting, possibly, during the course of an explosion). This means (invoking the law PV =: RT) that, to a first
.
ｾ-4-proportional to its temperature.
With the basic statements so far developed, it is easy to derive an explanation of the mechanism responsible for the movement of smoke in building s .
FIRST MECHANISM: EXPANSION
The first (but not the more important) mechanism responsible for the movement of smoke in a building is, quite simply, that the atmos-phere in the fire region expands and therefore much of it must leave that region.
As stated in the previous section the volume of a gas will be proportional to its (absolute) temperature. During the course of a fire the temperature in a compartment might rise from, say, 70°F to 900°F or more (2000°F on occasions). On the absolute scale these temperatures will be 5300
R (70°F) and l3600R (900°F). The latter temperature is well over twice the former so that at least half the original atmosphere in the compartment will be forced out, to the exterior or to other regions of the building.
SECOND MECHANISM: THE "STEADY STATE" MECHANISM
Anyone familiar with fire ill a furnace, stove, or building will be aware that hot gases (smoke, etc.) leave the fire area at high level openings and are replaced by gase s entering at low level openings.
It is this mechanism that is primarily responsible for the movement of smoke in buildings and it is therefore desirable to have a thorough understanding of it. It is hoped that this can be readily achieved by reference to Figure 1.
Figure 1 represents a room or other enclosure in which the temperature T is appreciably higher than that outside (T0)' as the result, say, of a fire in the room. Now as air is flowing in at the lower opening and out at the higher opening it follows that either side of the lower
opening the pressure is greater outside, whereas either side of the higher opening the pressure is greater inside. Continuing to compare internal and external pressures at different levels it can also be seen that there will probably be a particular level at which the interior and exterior pre s sure s are the same. This is, in fact, so and such a level has been indicated by a dotted line in Figure 1 and marked "neutral pressure plane".
The mechanism responsible for the various pressure differences depends on the result established earlier in this Note that the volume of a
-5-mass of gas is directly proportional to its absolute temperature. As the ｴ･ｭｰ･ｲ｡ｴｵｦｾ viithin the room is higher than outside, the "amount, II i.e. the weight of gas needed to occupy a particular
volume, will be less than the amount that would occupy the same volume outside. Now the pressure both inside and outside at the level of the lower opening equals the pressure at the neutral plane plus the weight of the column of gas between the neutral plane and the level of the opening. As the weight of the exterior column is greater than the weight of the interior column it follows that the exterior pressure will be greater than the interior pressure at the level of the lower opening.
A corre sponding argument will demonstrate why the pressure at the level of the upper opening is greater within the room than it is outside. It will also be seen that the pressure difference between exterior and interior or vice versa at any particular level varies as its height above or below the neutral pressure plane.
The concept of the neutral pressure plane has a tremendous significance. Gases will flow out of any opening above the neutral pressure plane whereas any opening at a lower level will constitute an inlet.
THE LEVEL OF THE NEUTRAL PRESSURE PLANE
The fact that gases will usually flow out of an opening only when it is above the level of the neutral pressure plane is of tremendous importance in the fire field for it provides a means of ensuring that smoke and toxic gases will not flow from one area to another. It is obviously interesting to know the factors governing the level of the neutral plane to see if this level can in fact be adjusted to
suit any particular requirements that might arise in buildings.
In terms of the symbols given in Figure I, the level of the neutral plane is given by the expression
.
Z
Z
hZ/h, = A T/A Z To'
A typical temperature to be expected in a burning compartment will be 1, OOO°F (1,4600
R) so that the ratio T/To will often have a value of about 3. It is unlikely ever to exceed 5. The" expression can therefore
usually be approximated to h /h = 3A Z/A Z
Z ' Z •
If some action is called for, it will always be to raise the neutral plane and not to lower it. In other words it will be desirable to reduce the value of hZ/hI' The action necessary therefore will be to reduce the area of low level openings (A) or to increase the area of the high level openings (A
-
-
-6-One important feature that has not been emphasized so far is that a still atmosphere has been assumed to surround the burning compartment. If this is not the case and the two openings are in different sides of the compartment then the
position of the neutral plane can be substantially in:fluenced. The pressure associated with a 25-mph wind can be 0.3 in. water gauge whereas the pressure differences generated by the fire will
probably not exceed 0.1 in. water gauge. It is therefore possible to establish openings so that wind conditions completely over -ride density head considerations and air enters at high level openings on the windward side of the building and gases leave at low level openings on the leeward side.
CONCLUSION
The principles outlined in this Note can be utilized to design buildings that will not become smoke laden in the event of a fire. Some complications are involved, however, the most significant of which is the wind problem just mentioned. A
discussion of these features, indicating, for example, the measures necessary to take advantage of wind pressures, form the subject matter of a report which, it is hoped, will shortly be published.
