Fire ventilation

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Ventilation-controlled fi reIn the case of a ventilation-controlled fire the fire scenariohas developed towards flashover, but then diminished andreduced in intensity as a result of the lack of oxygen. Thefire is, however, not fully extinguished and can sometimesmaintain a high smoke temperature in the magnitude of200–400°C. Large volumes of uncombusted gases can thereforebe produced. The concentration of these varies, butcan often be sufficient for them to start burning if air issupplied. If fire ventilation is implemented it is possible fora rapid flashover to take place, and the risk of backdraft canbe imminent. This requires special conditions, however.No doors or windows are open in this case, and theexchange of air can only take place through small leakageareas, for example through comfort ventilation. Large volumesof un combusted gases can also spread to adjoiningareas.The purpose of fire ventilation for poorly ventilatedrooms can be to force the fire scenario and to allow flashoverto take place under controlled conditions. It can also be toreduce the spread of fire gases by reducing the pressurebuild-up of hot gases in the fire room or adjoining rooms.The entire area is full offire gases. Air can flowin if the fire room isopened and a potentialignition could take placerapidly, causing a considerablepressure wave.111
Piercing nozzle(fog nail).Alternatives or supplements to fire ventilation, forexample the use of piercing nozzles, should be consideredwhen the fire is strongly ventilation controlled.If the fire gases can be cooled down in this way it will bepossible to implement fire ventilation or internal suppressionwith significantly less risks. Filling with foam can beanother alternative, which however assumes that the requisiteopening can be made. The size of such an opening isoften less than what would be needed for fire ventilation. Itcan often be appropriate to first secure the adjacent roomsor buildings before measures are taken for the fire room. Inthe case of ventilation-controlled fires it can be possible tosave lives in the fire room, since the impact of the heat ontrapped persons can be relatively moderate.Ventilation-controlled fires involve large risks, since thefire gases contains large volumes of uncombusted gases. Inthe event of rapid mixing of large volumes of air there is arisk of backdraft. Above all there is a risk of very rapid flashoverscenarios when fire ventilation is implemented.To take into consideration during ventilation-controlledfires:• Consider alternatives or supplementary methods tofire ventilation, for example cooling of the hotgases with piercing nozzles.• Observe great care during the implementation offire ventilation.• Observe great care during the choice of outlets,both in view of flames that can come out from thefire area and a possible backdraft.• Avoid turbulent mixing of fresh air in the fire gases.• Have a well-planned and coordinated ventilationand suppression operation, since the supply of airwill increase the intensity of the fire and the risk ofspreading the fire and fire gases.112
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To carry out fire and rescue operat
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The content of this book may not be
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Inlets 51Horizontal fire ventilatio
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ForewordIn this book fire ventilati
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chapter 1Fire ventilationThe fundam
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Openings to gain access to a fire t
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would be the case, for example, in
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esults in quicker and more simple e
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PyrolysisIrreversible chemicaldecom
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Flashover results in afully develop
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chapter 3Fire gasesFire gases is a
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Fire gases containsproducts such as
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The dynamic pressure differences ca
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Pressure differencesproducedby diff
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The hydrodynamic pressure differenc
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Direction of windnegative pressurep
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There can be verycomplex windcondit
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In buildings erectedduring the 1960
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Calculation of thermal expansionSta
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Calculation of thermal buoyancy for
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The different pressure differences
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For practical reasons it can someti
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Calculation of the size of outletsT
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almost always preferable to strive
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The mass flow through the inlet can
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Openings andventilation can beimple
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When a room or a firecell is consid
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Page 63 and 64: Negative pressure ventilation is no
Page 65 and 66: The pressurising of adjacent rooms
Page 67 and 68: Fans can during positivepressure ve
Page 69 and 70: A large outlet produces a larger vo
Page 71 and 72: The volume flow in the outlet is gi
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Page 76 and 77: chapter 6Creating openingsOpenings
Page 78 and 79: Electric motor powered toolsThe ele
Page 80 and 81: 12 3The circular saw is used to saw
Page 82: Water cutting.plastic explosive is
Page 85 and 86: Lowering deviceFall damperConnectin
Page 88 and 89: chapter 8Openings in differentroof
Page 90 and 91: its way through on its own, without
Page 92 and 93: drop down on personnel standing on
Page 94 and 95: It is a good idea if the outer open
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Page 98: In buildings that have systems for
Page 101 and 102: The measures that areinitiated, coo
Page 103 and 104: It is only through control that a s
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Page 107 and 108: elatively cold and diluted. Because
Page 109 and 110: Flashover has still nottaken place
Page 111: Flashover has takenplace and the fi
Page 115 and 116: To take into consideration at stair
Page 117 and 118: Further readingBengtsson, L. (1999)
Page 119 and 120: Särdqvist, S. (1994). Övertrycksv
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Fire ventilation

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