Fire and Smoke Understanding the Hazards (1986) / Chapter Skim
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2 A Primer on Fire and Fire Hazard
2 A Primer on Fire and Fire Hazard
Pages 23-44
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From page 23... ...
Except in smoldering fires, the combustion reaction itself occurs in the vapor phase, where fuel vapor and oxygen (O2)
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From page 24... ...
A sizable fraction of the heat produced by combustion appears as radiant energy, some of which is absorbed by the fuel surface beneath, so the evolution of fuel vapors continues. Adjacent surfaces are also heated until they are hot enough to evolve combustible amounts of vapors; this is how the flame spreads.
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From page 25... ...
The hot combustion products rise from the fire, entraining additional air and forming a distinct, hot, smoky upper layer just below the ceiling, which will deepen as the fire continues to burn. When the hot layer extends down to the top of a doorway, open window, or other vent, smoke will begin to spill out of the room, some of it into the rest of the building.
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From page 26... ...
As the upper part of a room becomes filled with very hot combustion products, this hot layer, like the flame itself, radiates energy to the fuel bed. The extra radiant heat makes the fuel burn faster than it would otherwise.
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From page 27... ...
This phenomenon, called flashover, is the typical result of an unchecked fire in a residence or a commercial occupancy that contains an abundance of combustible materials. At flashover, more combustible fuel vaDor is being Produced than can be consumed by the ~ ~ _ _ _ _ ~ ~ : An ~~~ ~ he ~~~w=~~ air coming in, so not vapors are carrlea cur Alla UWLW= where they burn as they encounter more air.
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From page 28... ...
The temperature of the hot gases coming out of the room as the fire approaches flashover typically exceeds 700°C; fuel is consumed at rates of around 0.5 kg/s; CO content of the smoke might be 5% -- high enough for a few breaths to be disabling or lethal. Such a fire produces hot gases at several cubic meters per second, so an entire floor of a building can be filled with smoke within a few minutes.
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From page 29... ...
Assessing smoke hazard in this scenario involves, for most purposes, only elementary calculations; required inputs are the mass of combustible material involved, the volume of the room, and the lethal concentration of the combustion products, assuming a substantial exposure time. The simple refinement of considering the ventilation rate through the compartment would provide a dilution factor.
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From page 30... ...
In either case, the temperature will approach a limiting value, governed by the relative size of the fire and the rate at which hot gases escape from the doorway. If Figure 2-3 were drawn for a longer period, the temperature would eventually decrease as the item of fuel burned itself out.
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From page 31... ...
The situation is different when one wishes to evaluate hazards associated with the smoke. Figure 2-5 shows the same fire growth curve as Figure 2-3, with a dotted curve added to show smoke production.
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From page 32... ...
of toxic conditions, one can then estimate when, in a developing fire, the smoke will be dense enough to block sight-directed escape." 89 In reality, however, smoke might effectively impede vision at concentrations below those at which it blocks light transmission, if it irritates the eyes; no adequate biologic model is available for assessing such properties. The time available for escape or rescue begins when the fire is detected.
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From page 33... ...
o <~ Toxic Dose (1 o in / / ~~/ / / 71 t1 Time t2 FIGURE 2-6 Comparison of smoke production and development of toxic smoke dose for two different materials.
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From page 34... ...
The five components labeled N1-N4 and As' permit computation of the time needed (N) for escape; the seven labeled A1-A7 permit computation of the time available (A)
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From page 35... ...
- - ll l l i l 1 1 1 Iz 1 ~ 35 -1 o ._ A Q E E Woo l .
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From page 36... ...
T IME AVAILABLE FOR ESCAPE As discussed above, the sublethal effects of smoke from various materials are not understood in sufficient detail for their influence on TNE to be predicted with confidence, and it has been suggested that those effects and reduced visibility be approximated by applying an appropriate multiplier, a 'safety factor," to a computed TNE. If it is assumed that TNE is a constant for a given scenario, usually independent of the burning material, and that, in contrast, the fire and smoke properties of a given material influence TAE, then, of a series of materials postulated to be burning in a given scenario, that with the largest TAE offers the greatest opportunity of escape.
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From page 37... ...
The details of fire hazard depend on the scenario. For example, a rapidly developing flaming fire whose products accumulate in a relatively confined space, such as a small apartment, produces a well-defined hot gas layer that descends rapidly.
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From page 38... ...
50, the concentration-time product required for death to occur in 50% of animals exposed to the smoke. In a smoke toxicity test, this product is obtained by continuously monitoring the smoke concentration to which the animals are exposed and reporting the time integral of this quantity when the animals die.
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From page 39... ...
When the burning rate of the fire and the associated mass loss rate are known, it is simple to compute the average smoke concentration in the hot layer. Assuming that the occupants have been exposed to the smoke from the time when the hot layer was at the 1-m level, the time to receive a lethal dose of smoke, TAE, is given by the integral over time, aft: MAE L(ct)
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From page 40... ...
For a fire to produce so little smoke in the foregoing scenario, it would have to have a mass optical density of 20 m2/kg, which is about one-tenth the smoke-producing potential of a typical furnishing material, such as polyurethane. If, instead of a flaming fire, the fire on the furniture is smoldering, too little heat will probably be generated to maintain a stable upper layer, and the smoke will disperse generally uniformly through the compartment volume.
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From page 41... ...
In other cases, however, the sequence is likely to be predictable -- combustible materials, such as plastic pipe or wiring, behind a wall are much more likely to be exposed to heat from a fire in the room than to be ignited directly by a small ignition source. Figure 2-9 shows the buildup of temperature in a room as the result of a known fire, the standard time-temperature curve for the ASTM E119-83 fire endurance test.)
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From page 42... ...
. [ It mln # FIGURE 2-9 Time-temperature profile of fire simulating ASTM E119 fire endurance test and of cavity behind gypsum wallboard.
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From page 43... ...
The smoke concentration depends both on the mass loss rate of the fuel and on the point in the building at which the hazard is being assessed. Hence, knowledge of the building layout (A5')
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From page 44... ...
In comparing two alternative materials for the same use behind the wall, it is possible to compute a difference in TAE associated with the change from one material to another. How this is accomplished is the subject of Chapter 6.
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