Enclosure fires

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If there is a risk of a backdraught occurring it is best to ventilate the room directly out to the open air, if possible. Ventilation must be done as high up as possible to force out any unburnt smoke gases. As there is a build-up of positive pressure, it is important that there is a “clear path” out for any backdraught which occurs. Otherwise, there can be a very large increase in pressure. A backdraught is a very short event. This means that fi refi ghters wearing the appropriate protective clothing have a good chance of coping, even if they are caught unawares by an ignition. You can also try to cool down the smoke gases. The best way to do this is without opening the room, for instance, by using piercing nozzle or a cutting extinguisher. The piercing nozzle option comes in two versions: piercing nozzle attack and piercing nozzle containment. You proceed with this option as follows. First of all, you make a hole in the structure. You then insert the piercing nozzle. Given that, with this exting uishing method, you are not opening any doors and windows or piercing a large hole, this prevents any additional excess air reaching the fi re, which could make the fi re more intense. The piercing nozzle is dispensed at around 70 l/min. at a pressure of 8 bar. Piercing nozzle is a good method to use in the case of attic fi res, for instance, when the fi re still has not yet broken through the ceiling (see Figure 108). The cutting extinguisher is a device which is under development in Sweden. Cutting extinguishers have the unique ability to penetrate walls and outer layers, along with effi ciently extinguishing fi res. These are features which are considerably valuable during a fi refi ghting operation. The water droplets coming through the jet reduce the temperature more effectively than a standard mist nozzle. 39 Figure 108. Piercing nozzle. Figure 109. Hand-held lance version of cutting extinguisher. 141
Figure 110. The fi xed lance version of the cutting fi re extinguisher. 142 There are two types of cutting extinguisher available. The fi rst type which has been produced is fi xed to the outside of the hydraulic platform cage. With this version, a pressure of 200 bar is applied with a fl ow of about 40–50 litre/min. The second version is a hand-held lance and operates at a pressure of 300 bar and fl ow of around 30 litre/min. In some cases a rapid internal operation may need to be carried out from inside, e.g. in a life-saving situation. Sometimes you need to open a room. In these scenarios there are two alternative approaches: 1. Open the door quickly. Cool down. Close the door quickly. Repeat this procedure until the temperature and pressure in the room have dropped. You should note that if the door is opened just for a couple of seconds this is enough for too much air to get in, which could cause a backdraught. 2. The BA fi refi ghters crawl in and close the door behind themselves as quickly as possible and cool down the smoke gases. This method can entail major risks for the fi refi ghters. It is important not to get caught in the open doorway. Apart from the risks being greater, it also helps to generate more turbulence.
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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
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
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: Figure 107. The smoke gases are str
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 and 182: You should bear in mind that this c
Page 183 and 184: Backdraught Sample calculation: The
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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