Enclosure fires

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This is one of the reasons why we do not come across backdraughts very often. 6.5.5 Conditions resulting in a backdraught It is diffi cult to specify exactly the conditions which create a backdraught. We can, however, indicate some of the factors which are a prerequisite to a backdraught occurring. The fact that backdraughts do not occur very often is because we are good at cooling down the smoke gases with water and there is seldom a large enough amount of combustible gases and an ignition source present at the same time. Generally speaking, it is diffi cult to achieve the large quantities of fuel which are required. The following factors have an infl uence on whether a backdraught will occur: • Presence of an ignition source. There needs to be an ignition source within the combustible area. Usually the most combustible area is the boundary layer between the fuelrich smoke gas layer and the incoming air. In many cases ignition sources can be very low down in the room. This is perhaps the main reason why backdraughts are not a very common occurrence. • Fuel’s arrangement (and type of fuel). The higher up in the room the fuel is located, the more combustible pyrolysis products will accumulate there. 35, 36 The obvious requirement, of course, is that there is suffi cient fuel in the room so that the gas concentration required to cause a backdraught can be achieved. • Openings’ location/size (original openings). The lower down the opening is, the smaller the proportion of pyrolysis pro ducts which disappear through the opening. If the opening is too small it is likely that the fi re will spontaneously go out. If it is too large the fi re will progress immediately to fl ashover. The opening needs to be suffi ciently large. You should note that this applies to the opening in the room at the time the fi re starts, in other words, not the opening made as a result of entry during a rescue operation. Fires in saunas can cause a backdraught. 135
Figure 103. Sauna. A common scenario which can cause a backdraught. 136 • Insulation in the room. The better insulated the room is, the higher the temperature rises in it. The temperature can also be maintained for a long time, even if the fi re has almost spontaneously gone out. A large quantity of unburnt products accumulate, especially if the fuel is located high up in the room. The lower the fuel’s pyrolysis temperature, the easier it is to reach the concentration required for a backdraught to be able to occur. The fuel concentration needs to be very high. 37 In the case of fi res in saunas, most of these factors occur. The fuel is certainly located high up in the room. The room is well insulated, which means that the temperature is maintained for a long time. An opening at the lower edge of the door can often allow a small amount of oxygen into the fi re room. There is obviously a suffi cient content of unburnt gases in sauna fi res.
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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
Page 87 and 88: Figure 59. The fi re has spread to
Page 89 and 90: Figure 60. Flashover occurrence ove
Page 91 and 92: 90 precisely the exact moment when
Page 93 and 94: The amount of energy released when
Page 95 and 96: Figure 63. Thermal balance on a fue
Page 97 and 98: Figure 64. Flame spread under the c
Page 99 and 100: Access to fuel Figure 67. Fire’s
Page 101 and 102: The signs indicating that a fl asho
Page 103 and 104: Figure 69. Different signs indicati
Page 105 and 106: Smoke gas temperature 700 600 500 4
Page 107 and 108: 106 clear that we need to reach the
Page 109 and 110: This example illustrates how crucia
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Page 115 and 116: 114
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: Figure 101. There are still combust
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 and 166: 164 gases the fl ames would come ou
Page 167 and 168: Figure 119. Plan drawing of the bas
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
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v* = v/(g × h × b) 0.5 , which gi
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188 gram), Institutionen för brand
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Index 62 Watts street 148 Adiabatic
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192 List of fi gures Drawn illustra
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Enclosure fires

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