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esult in ignition. A smoke gas explosion, on the other hand, occurs most often in an area adjacent to the fi re room. Generally speaking, a backdraught causes less pressure to build up than a smoke gas explosion. A backdraught requires the ventilation conditions to change, which, in itself, helps to ease the pressure. In the chapter on smoke gas explosions, we also discussed the fact that a smoke gas explosion can occur in the fi re room, but that this is a rare occurrence. This is because there is very often an ignition source, which consumes the combust ible gaseous mass as soon as it is formed. As a rule, the size of the combustible gaseous mass formed is not suffi ciently large. We need to try and get some idea of the extent to which ignition is dependent on whether the ventilation opening has changed or not. If the situation has arisen due to a change in ventilation we are dealing with a backdraught. 8.5 Differentiating between a fl ashover and a smoke gas explosion Both these scenarios are the simplest to differentiate between, as a fl ashover involves diffusion fl ames, whereas premixed fl ames are involved in a smoke gas explosion. 8.6 Summary To sum up, we can say that it is important to understand the processes which control how fi re-related phenomena occur so that we can realise what has actually taken place. The terms used, of course, also are fundamentally important, as in situations requiring quick, appropriate action, there is no room for any misunderstanding. Generally speaking, it is diffi cult to differentiate between the various phenomena which have the same type of fl ame involved. For instance, diffusion fl ames are involved in a fl ashover, whereas both premixed and diffusion fl ames are involved in a backdraught. This can make it diffi cult to distinguish between them. On the other hand, if we compare a fl ashover with a smoke gas explosion, where premixed fl ames are involved, it is much easier to differentiate between these. 165
Figure 119. Plan drawing of the basement level. Note that the stairwell is open and leads up to entrance level. 166 Sauna fi re in Kiruna This incident took place on 5 October 1999. 43 The Kiruna fi re service received an emergency call to attend a sauna fi re at the Malmfälten high school. One minute after this, the fi re engine left the station, there was also the automatic alarm from the high school itself. The fi refi ghters noted at the scene that the stairwell by the sauna was fi lled with white, light smoke. Firefi ghters with breathing apparatus entered the building to go down to the basement via the stairwell and extinguish the fi re. They fi rst did a search from the pool room to the sauna to make sure there was nobody left in the building and then proceeded towards the sauna. They did not notice the temperature at the door being any higher, however, they could hear a crackling noise coming from the sauna. When the BA fi refi ghters opened the sauna door they felt a very strong inward pull through the door opening. A moment later, a bluish fl ame shot out from the sauna. The smoke gases outside the sauna ignited and a bang was heard, all within a fraction of a second. The offi cer in charge was paying out the hose when he heard Ancillary room Ancillary room Sauna Path of action Pool room
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Lars-Göran Bengtsson Enclosure fi
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The content of this book may not be
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4.4 Risk assessment 99 Smoke gases
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Foreword The aim of this book is to
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Overview This book assumes that its
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Figure 1. Fire growth curve featuri
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Fire extinguished/burnt out Just sm
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chapter 2 How a fi re starts People
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smoke gases in the compartment, but
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Pyrolysis gases material is subject
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Ignition time Ignition time can als
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Smouldering fi res can therefore re
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2.4.1 Thermal inertia kρc The fl a
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grow to 50 cm, then a 1 m high fl a
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Flammability in solid materials is
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6. The fl ame spread rate varies wi
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Figure 22. Smoke gases start to acc
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Figure 25. The different sections i
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The production of unburnt gases is
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Heat release rate from a fuel surfa
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Carbon monoxide (CO) is the next mo
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ment to accelerate, but this is obv
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is far too high for a premixed fl a
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is fi lled with fuel molecules, con
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Figure 33. Flames on the under side
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Flammability limits Vent For a prem
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CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
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Temperature impact on fl ammability
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energy is required, which can be ex
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ustion takes place in under-ventila
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If turbulence affects the system th
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Neutral plane The differences in pr
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Where the differences in temperatur
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Positive pressure Negative pressure
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Dp = 353(1/Ta - 1/Tg)gh Let us take
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Figure 51 (top). The pressure condi
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Vent just during the early stage of
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In Figure 57 calculations have not
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3.4 Summary In many situations the
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oom and in a fairly closed room (on
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Figure 59. The fi re has spread to
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Figure 60. Flashover occurrence ove
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90 precisely the exact moment when
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The amount of energy released when
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Figure 63. Thermal balance on a fue
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Figure 64. Flame spread under the c
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Access to fuel Figure 67. Fire’s
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The signs indicating that a fl asho
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Figure 69. Different signs indicati
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Smoke gas temperature 700 600 500 4
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106 clear that we need to reach the
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This example illustrates how crucia
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Page 117 and 118: Figure 78. Fire pulsating. Figure 7
Page 119 and 120: Figure 81 (above). The fi re is abo
Page 121 and 122: Figure 85. The fi re resumes its de
Page 123 and 124: 122 is suffi ciently big. A fi re
Page 125 and 126: Figure 89. A backdraught entails a
Page 127 and 128: Figure 90. The picture shows a vent
Page 129 and 130: 128 The current which results in sm
Page 131 and 132: 130 fans should not be used. You sh
Page 133 and 134: Figure 97. A larger part is premixe
Page 135 and 136: Figure 101. There are still combust
Page 137 and 138: Figure 103. Sauna. A common scenari
Page 139 and 140: Fires in enclosed spaced with minim
Page 141 and 142: Figure 107. The smoke gases are str
Page 143 and 144: Figure 110. The fi xed lance versio
Page 145 and 146: Four fi re scenarios: 1. The fi re
Page 147 and 148: 146 to occur. If a backdraught occu
Page 149 and 150: 148 Fire at 62 Watts Street The New
Page 152 and 153: chapter 7 Smoke gas explosion Throu
Page 154 and 155: ignition source required to ignite
Page 156 and 157: ity is also affected by turbulence
Page 158 and 159: 7.3.2 Course of action If smoke gas
Page 160 and 161: Test your knowledge! 1. Why do smok
Page 162: Corridor Space in between Sauna app
Page 165: 164 gases the fl ames would come ou
Page 169 and 170: 168 Glossary Adiabatic fl ame If al
Page 171 and 172: Heat of combustion, This measures t
Page 173 and 174: 172 Suggested solutions to test que
Page 175 and 176: 174 13. In a closed room there will
Page 177 and 178: 176 6. By cooling down the smoke ga
Page 179 and 180: Sample calculations Flammability li
Page 181 and 182: You should bear in mind that this c
Page 183 and 184: Backdraught Sample calculation: The
Page 185 and 186: E = (T f/T i)( N b/N u) Tf Ti Nb Nu
Page 187 and 188: v* = v/(g × h × b) 0.5 , which gi
Page 189 and 190: 188 gram), Institutionen för brand
Page 191 and 192: Index 62 Watts street 148 Adiabatic
Page 193 and 194: 192 List of fi gures Drawn illustra
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Enclosure fires

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