Enclosure fires

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smoke gases in the compartment, but the actual fi re is very easy to extinguish. This situation is very common with house fi res in Sweden. In the case of scenario 2 (the fi re grows – see Figure 9), we need to give a bit more thought. Also, as it is interesting to see what happens when the fi re spreads further, in the next section we will look at how and in what way the fi re will be able to spread during the initial phase. The fuel’s arrangement in the compartment is crucial to the fi re’s continuing behaviour. Porous and wood-based materials in furnishings contribute to the fi re’s rapid development. Plastics sometimes cause fi res to spread very quickly due to the fact that they drip to form pools of fi re on the fl oor. We will now look at how the material ignites and fl ames spread with objects. It is important to understand these processes in order to be able to learn how a fi re’s intensity increases. Figure 8. The fi re does not spread. Figure 7. The initial cause of a fi re can be, for instance, a desk lamp or a cigarette which ignites a sofa. Figure 9. The initial fi re in the sofa grows bigger. The fi re’s area increases. 19
Ignition Figure 10. Combustion process. Infl ammability in solid materials is estimated using the time it takes for ignition to occur. 20 + O2 Energy + H2O + CO2+ CO + carbon particles, etc. Exothermic process 2.2 Ignition of solid material Combustion is a chemical reaction. It involves, to be more precise, a whole series of chemical reactions when the fuel is oxidised. Fuel and oxidising agents react with each other. This releases heat and light. As a result, the chemical process is accompanied by physical effects. Heat is the physical energy which is released during the chemical process. Light is the physical consequence of the fact that there is energy stored in the soot particles, for instance. Ignition is the fi rst visible sign of combustion. The combustible material may auto-ignite due to the high temperature or it can be ignited by an external source such as a match or spark. In the case of solid materials, there is a critical temperature at which ignition takes place. But this generally varies according to the material which is burning and can therefore not be used as a measure of infl ammability. With solid materials, the surface must be heated up to 300–400°C for ignition to occur with a pilot fl ame. If there is no fl ame nearby the surface temperature must be higher. Wood needs to reach a surface temperature of 500–600°C before it auto-ignites. 5 Infl ammability in solid materials is estimated using the time it takes for ignition to occur. Ignition takes place when suffi cient combustible gases have built up on the solid material’s surface for them to be ignited by a small fl ame. Materials such as wood or paper (organic polymers) need to emit 2 g/m 2 s (grams per square metre and second) of combustible gases to be able to ignite. As for plastics (synthetic polymers), which have a high energy content, they need about 1 g/m 2 s of combustible gases to be able to ignite. Figure 11 shows what happens on the fuel surface when the
Page 1 and 2: Lars-Göran Bengtsson Enclosure fi
Page 3 and 4: The content of this book may not be
Page 5 and 6: 4.4 Risk assessment 99 Smoke gases
Page 8 and 9: Foreword The aim of this book is to
Page 10: Overview This book assumes that its
Page 13 and 14: Figure 1. Fire growth curve featuri
Page 15 and 16: Fire extinguished/burnt out Just sm
Page 18 and 19: chapter 2 How a fi re starts People
Page 22 and 23: Pyrolysis gases material is subject
Page 24 and 25: Ignition time Ignition time can als
Page 26 and 27: Smouldering fi res can therefore re
Page 28 and 29: 2.4.1 Thermal inertia kρc The fl a
Page 30 and 31: grow to 50 cm, then a 1 m high fl a
Page 32 and 33: Flammability in solid materials is
Page 34: 6. The fl ame spread rate varies wi
Page 37 and 38: Figure 22. Smoke gases start to acc
Page 39 and 40: Figure 25. The different sections i
Page 41: The production of unburnt gases is
Page 44 and 45: Heat release rate from a fuel surfa
Page 46 and 47: Carbon monoxide (CO) is the next mo
Page 48 and 49: ment to accelerate, but this is obv
Page 50 and 51: is far too high for a premixed fl a
Page 52 and 53: is fi lled with fuel molecules, con
Page 54 and 55: Figure 33. Flames on the under side
Page 56 and 57: Flammability limits Vent For a prem
Page 58 and 59: CH4 + 2 + 2 + + 2O2 = CO2 + 2H2O +
Page 60 and 61: Temperature impact on fl ammability
Page 62 and 63: energy is required, which can be ex
Page 64 and 65: ustion takes place in under-ventila
Page 66 and 67: If turbulence affects the system th
Page 68 and 69: 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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110
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112
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114
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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
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Figure 89. A backdraught entails a
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Figure 90. The picture shows a vent
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128 The current which results in sm
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130 fans should not be used. You sh
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Figure 97. A larger part is premixe
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Figure 101. There are still combust
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Figure 103. Sauna. A common scenari
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Fires in enclosed spaced with minim
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Figure 107. The smoke gases are str
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Figure 110. The fi xed lance versio
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Four fi re scenarios: 1. The fi re
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146 to occur. If a backdraught occu
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148 Fire at 62 Watts Street The New
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chapter 7 Smoke gas explosion Throu
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ignition source required to ignite
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ity is also affected by turbulence
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7.3.2 Course of action If smoke gas
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Test your knowledge! 1. Why do smok
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Corridor Space in between Sauna app
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164 gases the fl ames would come ou
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Figure 119. Plan drawing of the bas
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168 Glossary Adiabatic fl ame If al
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Heat of combustion, This measures t
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172 Suggested solutions to test que
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174 13. In a closed room there will
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176 6. By cooling down the smoke ga
Page 179 and 180:
Sample calculations Flammability li
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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
Page 193 and 194:
192 List of fi gures Drawn illustra
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