Enclosure fires

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Flashover Sample calculation: Heat release rate required for a fl ashover Let us take a room measuring 3.6 m × 2.4 m × 2.4 m. The opening is in the form of a door 0.8 m wide and 2 m high. The enclosure material is lightweight concrete. Calculate the heat release rate required for the fi re in the room to progress to fl ashover. To start with, h k is calculated. For fairly short periods of time h k is determined by h k = (k3c/t) 0.5 , where k3c represents the material’s properties and t is the time from when the fi re started. k3c for lightweight concrete is roughly 75000 W 2 s/ m 4 K 2 . The time is assumed to be 10 minutes. We can assume that the fi re service has arrived by that point. h k is calculated using the expression √k3c/t and will equal 0.0112 kW/m 2 K. The values are entered in the equation: Q fo = 610(h k A T A W √H ) 0.5 This gives: Q fo = 610 (0.0112 × 44.48 × 1.6 × 2 0.5 ) 0.5 = 650 kW This can be compared to a sofa, which releases between 1 and 2 MW. The heat release required is therefore much less than this. If we had calculated instead the heat release rate required to cause a fl ashover after 2–3 minutes, it would be much higher. You can even try by varying the time (t) in the expression for h k. 181
Backdraught Sample calculation: The speed of the incoming air current The room has the dimensions 2.4 × 2.4 × 6 m 3 and the opening is wide, roughly a third of the room’s height, i.e. 0.8 metres. The opening is about 2.2 metres wide. The smoke gas temperature is about 150°C. This gives a rough smoke gas density of 0.84 kg/ m 3 . The air density is about 1.2 kg/m 3 , representing b. b = (1.2 – 0.84)/0.84 u 0.42 v* is obtained from reference 32. This value is connected to the size of the opening in relation to its geometry, in this case, the wide opening. v* = v/(g × h × b) 0.5 , which gives: v = 0.35 × (9.81 × 2.4 × 0.42) 0.5 u 1 m/s Based on the equation above, this gives a speed of about 1 m/s, which visually seems to be a good approximation. It is important not to use this result as an exact value, but just as an approximation. You should also note on this subject that it can take a number of seconds for a combustible mixture to reach the ignition source, if it is placed far inside the room. The above equation is also useful for applying to the outgoing air current and it is important to mention that it can take many seconds for the air current, which has refl ected against the back wall, to reach the door opening again. In this case, the mixture in the room may be very well mixed and ignition can result in a very rapid heat release rate with fatal consequences. 182
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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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110
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112
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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
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 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: You should bear in mind that this c
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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