Enclosure fires

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Temp 4 nising these signs can be completely crucial to the outcome of the rescue operation. If there are only leakage paths in a compartment, which is otherwise closed, the fi re does not progress to a fl ashover due to the lack of oxygen. The fi re’s intensity diminishes before a fl ashover can occur. This can happen in many different ways, as shown in Figure 2 and is discussed in detail below. Ventilation control means that the extent of the fi re is determined by limiting the amount of oxygen. In many cases the fi re will be ventilation controlled when the fi re service arrives on the scene. The time it takes for a fi re to reach ventilation control varies. Let us take the example of a TV set catching fi re. A burning TV set can generate between around 200 and 500 kW. This means that the oxygen in a normal-size room will already be used up 3–6 minutes after the fi re has started. After that, the intensity of the fi re will diminish and it then usually just smoulders or spontaneously goes out. A large number of the house fi res which occur in Sweden are ventilation controlled when the fi re service arrives on the scene. But the problems remain as the fi refi ghters open the door to the premises. Air will then rush into the room where the fi re is, which may result in the smoke gases in the room igniting. However, this does not happen very often, only in a few percent of all fi res (represented by line 3 in Figure 2). If fi refi ghters wearing breathing apparatus take the appropriate action and immediately cool down the smoke gases, the risk of the smoke gases igniting is kept to a minimum. 1 2 3 Time Figure 2. Typical fi re behaviour when ventilation control occurs. 13
Fire extinguished/burnt out Just smoke development 14 Fire at initial object Fire in one room Fire in several rooms Fire in several fire cells Extent not specified Figure 3. Percentage distribution indicating the extent of a building fi re by the time the fi re service arrives at the scene. Note that the fi re has only spread in a few cases to other rooms when the fi re service arrives. It is important to be able to understand a fi re’s behaviour in order to be able to carry out a safe, effective operation. 0 5 10 1520 2530 35 40 % In a few cases, the smoke gases may ignite very quickly and the fl ames shoot out of the room at a speed of a couple of metres per second. This phenomenon is known as a backdraught and is dealt with at great length in Chapter 6. Backdraughts entail major risks, which can even, in some instances, result in fi refi ghters getting killed. This is why it is very important to learn to recognise the signals warning of an imminent backdraught. A backdraught is illustrated by line 1 in Figure 2. In many apartment fi res the impact of the fi re is limited to a few objects and there is only slight smoke damage. The fi re is often still burning the fi rst object when the fi re service arrives. There are mainly two common scenarios in this case. In the fi rst scenario, the fi re spontaneously goes out due to a lack of oxygen. It has hardly spread from the initial object. This is indicated by line 2 in Figure 2. The temperature in this case is fairly low, but there can be a lot of smoke in the apartment. In the other common scenario, the fi re is still fuel controlled when the fi re service arrives. In this case, air is freely accessible and the fi re is controlled by the amount of fuel. Fuel control may be due to the total fi re load being small, which means that the fi re does not release suffi cient heat to cause a fl ashover. It may also be due to the fact that the fuel arrangement in the compartment, i.e. the combustible objects’ locations in relation to each other, means that the fi re cannot spread from the initial object.
Page 1 and 2: Lars-Göran Bengtsson Enclosure fi
Page 3 and 4: The content of this book may not be
Page 5 and 6: 4.4 Risk assessment 99 Smoke gases
Page 8 and 9: Foreword The aim of this book is to
Page 10: Overview This book assumes that its
Page 13: Figure 1. Fire growth curve featuri
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
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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
Page 84:
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
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90 precisely the exact moment when
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The amount of energy released when
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Figure 63. Thermal balance on a fue
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Figure 64. Flame spread under the c
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Access to fuel Figure 67. Fire’s
Page 101 and 102:
The signs indicating that a fl asho
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Figure 69. Different signs indicati
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Smoke gas temperature 700 600 500 4
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106 clear that we need to reach the
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This example illustrates how crucia
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110
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112
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114
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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
Page 169 and 170:
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
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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Enclosure fires

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