Fire ventilation

chapter 4The spread offire gasesThe flow of fire gases always takes place from higher to lowerpressure. The magnitude of the difference between the higherand the lower pressure determines the size of the flow, and howquickly this flow takes place. The magnitude of the pressuredifference is in turn determined by the size of the openingsbetween rooms, the wind conditions, the size of the fire andhow it develops, and the ventilation system etc. Differences inpressure can cause fire gases and the fire to spread long distances,and in directions that cannot always easily be predicted.With a knowledge of the different types of pressure differencesin buildings and how they arise, the spread of fire gases can toa certain degree be predicted, and in certain cases also prevented.Sometimes it is in fact possible to change the direction ofthe fire gases, and to steer it through and out from a building.It can, however, be very difficult to produce an overall view ofwhat the pressure differences look like inside buildings.Pressure differences in buildingsIt is relatively well known how the build up of pressure takesplace and how fire gases spreads in a room that is burning,but when a complete building is affected by the fire the problembecomes somewhat more complex. When a fire is inprogress it is seldom, or never possible to make a more extensiveanalysis of the pressure differences and their causes, butit is important to have a certain understanding of what it isthat influences the spread of fire gases in a building and outfrom the building.27