Enclosure fires

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5. The main protection against a fl ashover is to be able to predict when a fl ashover is imminent. You need to have a good knowledge of a fi re’s development to do this. It is very important to know what the signs are. At a more practical level, it involves reducing the smoke gases’ temperature so that the reradiation level and heat release rate will not be too high. 6. The reference literature mentions some of the following signs: change in the smoke gas layer’s position, the speed at which the smoke gases leave the room, pyrolysis from the various fuel packages and fl ames in the smoke gas layer. The smoke gases’ colour or a change in colour may also be used, but with a certain degree of caution. 7. In many cases, the fi re does not spread from the initial object and as a result, the heat release from the fi re is not suffi cient to cause a fl ashover. In some situations the fi re is ventilation controlled and the heat release rate is limited before the progression to fl ashover is reached. 8. Just a few percent. 9. In the premises where concrete is used in the enclosure structure. This is because more energy disappears through conduction. Chapter 6 1. Both premixed and diffusion fl ames are involved in a backdraught. 2. Unburnt smoke gases accumulate, air fl ows in, the gases and air mix, they ignite with some kind of ignition source, a turbulent defl agration occurs and a fi re ball appears outside the room. 3. The conditions required for a backdraught to occur are very particular. The chances of an ignition source being present at the very moment the premixed area forms in the compartment are very low. 4. Pulsations from small openings around doorframes, for instance, hot doors and windows, no visible sign of a fi re and a whistling sound around openings. 5. A backdraught occurs when the fi re has been ventilation controlled for a while. 175
176 6. By cooling down the smoke gases before they ignite or by ventilating the smoke gases out before you enter the room. There are many options for doing this. Wearing suitable protective clothing is vital in these situations. 7. The best tactic is to try to cool down the smoke gases before you draw too much air in. 8. No. It depends on the radiation levels and the types of material which are close by. 9. There must be a high concentration of unburnt smoke gases. These can accumulate if the building is well insulated, there is a great deal of combustible material at the top of the room, if there are fewer leakage areas right down at the bottom of the room and if an ignition source can appear. 10. It is common in sauna fi res. 11. When combustion takes place in a fairly enclosed room positive pressure will build up, which causes smoke gases to be pushed out through the openings. The oxygen content also drops and the fl ame goes out. After a while the temperature falls, which creates negative pressure and fresh air can then by drawn into the room. Ignition is possible again after this. It is very diffi cult to predict whether a fi re will pulsate or not. 12. Think about at which stage the fi re is at, the length of time it has been burning, then weigh up the risks and compare with the possible outcome of the operation. 13. A backdraught has occurred, the fi re resumes its development, the fi re may have spontaneously gone out or the smoke gases auto-ignite in the door opening. 14. The fi re has spontaneously gone out or the fi re resumes its development and the fi re becomes more intense. 15. This is not common. This phenomenon occurs as the smoke gases’ temperature is higher than the auto-ignition temperature. The gases ignite on direct contact with the air. 16. If the fi re is ventilation controlled, when we open the door, the fi re receives new impetus and its development can then resume.
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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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Figure 78. Fire pulsating. Figure 7
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Figure 81 (above). The fi re is abo
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Figure 85. The fi re resumes its de
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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 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: 174 13. In a closed room there will
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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