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Heat release rate from a fuel surface The heat release rate can be calculated using the following formula: Q = m"A f DH Cx where Q = heat release rate in W m" = mass loss rate in kg/m 2 s A f = area of fuel surface in m 2 DH C = heat of combustion in the case of complete combustion in MJ/kg x = combustion effi ciency, which measures how effi ciently the fuel is used up. 1.0 equals complete combustion, i.e. all the energy is extracted. You now perform a simple sample calculation. Calculate the heat released from the fi re when a heptane pool with a diameter of 1.2 metres is burning. The equation can be used for both solid and liquid fuels. We need to have the following values (taken from another reference 5 ) in the calculations: DHC = 44.6 MJ/kg x = Assumed to be 0.7 (standard value for hydrocarbons) m" = 0.075 kg/m2s Af = p × D2 /4=3.14 × 1.2 × 1.2/4 = 1.13 m2 Equation 3 This gives a total heat release rate of 0.075 × 1.13 × 0.7 × 44.6 × 10 6 = 2.6 MW Equation 3 shows indirectly how much energy can be stored in a smoke gas layer. 1 – x is the part of the energy accompanied by the fi re plume and contained in the smoke gases, which is, in this case, about 1.1 MW. Material k (W/mK) c (J/kgK) 3 (kg/m 3 ) k3c (W 2 s/m 4 K 2 ) Chipboard 0.14 1,400 600 120,000 Wood fi bre board 0.05 2,090 300 32,000 Polyurethane 0.034 1,400 30 1,400 Table 2. Different k3c values for different materials. 43
Both material properties and ventilation system are crucial factors in the production of unburnt gases, as well as in the extent to which a material represents a fi re hazard in a particular situation. Certain plastics have a high energy content. If the ventilation is poor you can end up with a large number of incomplete products among the smoke gases. When plastics combust, this often produces the conditions which can lead to a fl ashover occurring. 44 DH L is known as the combustion value and represents the ratio between the heat released and the heat consumed in vaporising the burning material. The fi gure corresponding to DH C and DH L indicates how many times more heat is generated than used up during combustion. Solid materials generate around 3–30 times more heat than they use up. Comparing different materials by comparing their individual properties does not give any indication at all of how they will behave in a real fi re. But it is still worthwhile familiarising yourself with the different materials and with the differences between them. For instance, certain plastics, known as thermoplastics, soften and melt when the temperature is raised. In this situation, these materials behave like a liquid. The radiation heat emitted from the fl ames in a fi re can even cause plastic objects a fairly large distance away from the fi re to soften or melt, without them even being directly involved in the fi re. The melted material can then ignite through radiation from the fi re, burning drips or as a result of a burning object falling on top of the melted material. The heat of combustion is the amount of heat which can be released from burning material. The amount of heat energy released is expressed in J/kg. The amount of heat which can be released by wood in the case of complete combustion is between 17 and 20 MJ/kg. There are large differences in the heat of combustion values for plastics. Some generate almost no energy at all. Other plastics, such as thermoplastics, can generate a heat of combustion which is comparable to that of heating oil, at around 40–50 MJ/kg. 3.1.2 Content of unburnt smoke gases As we have previously mentioned, combustion produces a large number of products. If combustion takes place with a good oxygen supply present this produces large amounts of carbon dioxide and water, which are in themselves not combustible. Numerous other products can also be formed, depending on the accessible oxygen supply and the material’s constituents. Some of them are described below. 5
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 and 14: Figure 1. Fire growth curve featuri
Page 15 and 16: Fire extinguished/burnt out Just sm
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 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 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
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106 clear that we need to reach the
Page 109 and 110:
This example illustrates how crucia
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110
Page 113 and 114:
112
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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
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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
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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Enclosure fires

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