Enclosure fires

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Heat release rate required for a fl ashover to occur Data has been correlated from over 100 experimental fi res for varying room sizes. 21 These experiments have formed the basis for equation 18 below. This equation can be used to determine the amount of fuel burnt up when the fi re progresses to fl ashover. The heat release rate required for a fl ashover to occur can be described using the following formula: Q fo = 610(h kA TA W√H) 0.5 where Q fo – heat release rate required to trigger a fl ashover (kW) h k – heat transfer coeffi cient (kW/m 2 K), which specifi es the amount of heat absorbed by the enclosure surfaces A T – internal enclosure area in room (m 2 ) A W – ventilation opening’s area (m 2 ) H – ventilation opening’s height (m) Equation 18 has been produced for a normal-size room. But it has also proved to be valid for signifi cantly larger rooms. Many different types of fuel have been tested, including wood, plastic and various gases. Both windows and doors have been used as openings. The material which the walls and ceiling are made of have had a wide variety of properties. However, this equation does have certain limitations. It only applies to fuelcontrolled fi res and when the source of the fi re is not located close to any wall. If it was located close to a wall the coeffi cients in the equation would be different. The equation applies to temperatures between around 20 and 700 °C 7 . There is a sample calculation on page 181, which shows the scope of application for this equation. The equation is often used for scoping purposes. The fully developed fi re is ventilation controlled, which means that equation 18 cannot be applied at this stage. When the fi re is ventilation controlled the heat release rate, in particular, is limited in the room by the amount of air fl owing in through the opening. This amount is calculated using equation 19. When a fl ashover has defi nitely taken place we can use equation 19 to estimate the actual heat release rate. In actual fact, all the air which fl ows into the room is not used for combustion. Consequently, the amount of energy released is not as much as equation 19 gives, but the approximation must be regarded as being adequate for our purpose. m a = 0.5 A w √H Equation 18 Equation 19 where m a (kg/s) is the mass fl ow of the incoming air, A w (m 2 ) is the opening’s area and H (m) is the opening’s height. (cont. on next page P)
The amount of energy released when the oxygen in 1 kg of air fully reacts with fuel is about 3 MJ/kg. A standard door opening of 2 × 1 m 2 can produce a heat release rate of 4.2 MW, assuming that all the oxygen is fully combusted. Any extra fuel available will burn outside the room. Another important point to note is that some of it burns outside the room. Using the equation we can calculate the maximum amount of heat released inside the room. Figure 62. In this instance, the fi re is ventilation controlled. Heat is released partially inside the room and partially outside of it. 92
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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
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
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: 90 precisely the exact moment 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
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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