Enclosure fires

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Neutral plane The differences in pressure generated by the fi re can be classifi ed by two types: • Differences in pressure resulting from thermal expansion being inhibited • Differences in pressure resulting from the smoke gases’ buoyancy. The relative size of these factors varies according to a building’s design and position, external infl uences, etc. Circumstances can vary enormously both within buildings and between different buildings, as well as over time. But it is natural for the differences in pressure generated by the fi re itself to be dominant in proximity to the fi re. When the distance from the fi re increases and the smoke gases cool down, normal pressure differences are much more dominant. Differences in pressure cause smoke gases to fl ow, resulting in them spreading quickly to adjacent parts of the building. This spreading process can occur via vents or openings in the ceiling, fl oor or walls, along corridors, up (and in some cases down) vertical shafts for staircases, lifts or ventilation as well as via ventilation system ducts. In this book we are focusing on the differences in pressure generated by the fi re. We will discuss these differences in detail and then talk about what the pressure conditions are like in three separate scenarios which we are often faced with when we are at the scene of a fi re. Figure 43. Example of the pressure conditions in a fi re room. The room is open. 67
Usually there is a certain amount of leakage in the fi re room, which means that the pressure will be seldom more than twenty or thirty Pascal. 68 The three scenarios are: 1. Room is closed, or almost completely closed. 2. Room is open, with a door or window open, for instance. 3. Pressure build-up in a room when a gaseous mass ignites. But we must remember that the other differences in pressure which can arise, for example, caused by the wind, can be of the same magnitude as those generated by the fi re. You are recommended to read a more detailed description of the different types of pressure difference. 7,8,15 3.3.1 Inhibited thermal expansion When fi re breaks out in a compartment which is entirely closed, the pressure will build up. This is a result of the smoke gases being heated, but being prevented from expanding. Pressure build-up in a closed room The heat release rate Q from a waste paper basket is constant at around 100 kW. Let us make the volume of the room V 60 m 3 . The starting point for the calculation is that this pressure is the mass and energy balance for a limited control volume. You should note that the release rate Q is expressed in kW. We assume an air temperature of 293 K (20 °C), 3 a is the air density and is set to 1.2. C v is the specifi c heat capacity at a constant volume, which is set to 0.7. We use the equation: (p–p a) = Qt p a V3 ac vT a (p–p a) = Qt = (100 × 1) P approx. 700 Pa p a V3 ac vT a 60 ×1.2 × 0.7 × 293 Equation 12 The rise in pressure will then be around 700 Pa/s. After 10 seconds a 1 m 2 window pane is subjected to a pressure of around 7 kN, which, in real conditions, is more than suffi cient for the pane of glass to crack.
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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
Page 18 and 19: chapter 2 How a fi re starts People
Page 20 and 21: smoke gases in the compartment, but
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 70 and 71: Where the differences in temperatur
Page 72 and 73: Positive pressure Negative pressure
Page 74 and 75: Dp = 353(1/Ta - 1/Tg)gh Let us take
Page 76 and 77: Figure 51 (top). The pressure condi
Page 78 and 79: Vent just during the early stage of
Page 80 and 81: In Figure 57 calculations have not
Page 82 and 83: 3.4 Summary In many situations the
Page 84: 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 113 and 114: 112
Page 115 and 116: 114
Page 117 and 118: 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
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Sample calculations Flammability li
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You should bear in mind that this c
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Backdraught Sample calculation: The
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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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