Enclosure fires

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3.4 Summary In many situations the smoke gas layer will contain unburnt products. The poorer the ventilation, the larger the amount of unburnt gases. As a result of this and in conjunction with a large number of pyrolysis products from material not in proximity to the actual seat of the fi re, the smoke gases may ignite. The energy in the smoke gas layer can be released if fuel and air are in the right proportions, and there is also an ignition source close by. If the energy in the smoke gas layer is released, i.e. if fl ames occur, the radiation level in the room will rise dramatically. When the smoke gas layer ignites two types of fl ames can occur: Diffusion fl ames, where the oxygen diffuses into the fuel. Premixed fl ames, where the fuel and air have mixed prior to ignition. Diffusion fl ames occur in the boundary layer between the fuel and air. In most enclosure fi res, what we see are diffusion fl ames. During a compartment fi re a variety of pressure conditions will control the fl ow of smoke gases. It is important to be able to recognise pressure conditions as this will effect the type of operation carried out. The pressure generated by fi res can be classifi ed by two types: – Thermal expansion – Thermal buoyancy. The pressure difference resulting from inhibited thermal expansion is most often small in normal buildings, as there is always a certain level of leakage. If the fi re room has a large opening the fl ow will be controlled by thermal buoyancy. The pressure is mostly in the order of 20–30 Pascal, but even this can infl uence the type of operation carried out. If a smoke gas layer ignites inside a building a high pressure build-up can occur. If the whole gaseous mass or large parts of it are mixed prior to ignition there will be a considerable in- 81
82 crease in pressure, unless the room has openings which are suffi ciently large to reduce the pressure inside the room. Only a small amount of the room’s total volume is suffi cient for the pressure to rise dramatically. The larger the openings, the less the build-up of pressure will be and the more energy the gases contain, the higher the burning velocity. Scenarios involving premixed gases often cause extremely dangerous situations and there is a very high risk of burns and injuries caused by increased pressure. If diffusion processes are dominant in the smoke gas layer the level of gas expansion will not be so high and the process is easier to control, which means the increase in pressure in the room will not be as dramatic. When talking about fi res, occasionally the terms lean and rich are used to describe the smoke gas layer’s composition. These concepts assume that the gaseous mass is premixed, which is not the case in most compartment fi res. Test your knowledge! 1. When can we use the terms “lean” and “rich” when talking about a fi re? 2. What level of pressure can theoretically build up in a fi re room if a premixed gaseous mass ignites? 3. Describe the different categories of pressure difference there are. Which are caused by the fi re? 4. Explain the difference between the various types of fl ames. 5. What speed does a premixed fl ame front move at? 6. What is a defl agration? 7. What is a detonation? 8. The concepts “fuel control” and “ventilation control” are often used when talking about fi res. What is the difference between these concepts? Why is it important to know whether the fi re is fuel controlled or ventilation controlled? 9 . Which products are formed when a fi re burns? 10. Which substances can you fi nd in a smoke gas layer? 11. What factor controls the speed of diffusion fl ames? 12. Describe the pressure conditions in an open room. 13. Describe the pressure conditions in a hermetically sealed
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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
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
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 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
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
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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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