Enclosure fires

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Relationship between heat release rate and mass loss rate Q = mDH cx where DH c – heat of combustion (J/kg) x – combustion effi ciency m – mass loss rate (kg/s) been previously mentioned, is fuel controlled. When there is limited access to oxygen and the heat release rate is affected by this, this type of fi re is described as ventilation controlled. The heat release rate is related to the available equipment and what you manage to extinguish. Just to give you a comparison, fi refi ghters can more or less manage to tackle a fi re with a heat release rate of 15–20 MW. This is a higher release rate than in a normal house fi re. 22 The ability to extinguish the fi re is obviously dependent on accessibility and the individual ability of the fi refi ghters. The heat release rate required for a fl ashover to occur can be converted to a mass loss rate (the amount of pyrolysis from the fuel), using equation 20. 11 4.2.2 Increasing the mass loss rate In this section we will discuss the processes which contribute to an increase in the mass loss rate and indirectly, in the heat release rate, allowing a fl ashover to occur. Figure 63 illustrates the thermal balance on a fuel surface. In this instance, we are assuming that we have an object on the fl oor. This is obviously not completely indicative of a standard compartment fi re, but we will use this fi gure to make it easier to understand the mechanisms involved. There is a big difference between, for instance, a piece of furniture burning outside in the open air and inside a room, where there is an impact from radiation emitted by the fl ames and walls, as well as from smoke gases. Reradiation increases and then the fuel emits more pyrolysis gases and the heat released by the fi re increases. We have therefore noticed that a particular heat release rate Equation 20 93
Figure 63. Thermal balance on a fuel surface. 94 is required in the form of a mass loss rate. The mass loss rate is controlled by: 1. Flame spread and ignition (the area of the initial fi re increases) 2. Higher intensity of combustion on a particular fuel surface. These two mechanisms are each then dependent on three processes closely linked to one another, specifi cally: q" loss (the energy produced by the fuel, but not used for vaporisation) q" f (heat from the fl ame) q" ext (radiation from the surrounding area) q" loss When a fi re is at the start of its development, a large amount of the energy transferred to the fuel is used to raise the temperature of the fuel rather than for generating fumes from the surface. Burning thermoplastic provides an example of this: it will not reach its maximum mass loss rate until a large amount of the material has melted to form a pool fi re. This process uses up a lot of energy. After a while, however, the fuel and its underlying surfaces will have excess heat and q" loss will drop. According to equation 21, the mass loss rate will then increase. External radiation Heat from flames Heat transferred to material
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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
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
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: The amount of energy released when
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
Page 111 and 112: 110
Page 113 and 114: 112
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 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
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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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