Fire ventilation

Calculation of the size of outletsThe maximum outlet is determined among other things by the Froudenumber, which gives the square root from the relationship between either theinertial force and gravitational force, or the kinetic energy and potentialenergy. From critical values of the Froude number it is possible to derive asimple rule of thumb for the maximum opening area A v (separate openings):A v < 2 × d 2where d is the thickness of the smoke layer.This means, for example, that if in an industrial building completely filledwith fire gases and with a ceiling height of 3 m, it is wished to raise the layer offire gases to half the ceiling height, then each separate opening (or group ofopenings in direct connection with each other) should not be made largerthan approximately:3A v, max ≈ 2 × −− = 4,5 m 222In total, however, a larger opening area than this can be required. This can thenbe achieved by creating several openings with the maximum opening area.Large outlets can causefresh air to flow in andcool the smoke. Thisreduces the effect of thefire ventilation.The single most important parameter that influences thesize of outlets is the temperature. The temperature is directlyrelated to the intensity of the fire, and also to the pressuredifference over the layer of fire gases. A high temperaturecorresponds to a high intensity and means that more firegases are produced, but also that there is larger thermalbuoyancy force. The size of the opening or openings neededfor fire ventilation increases in relation to the developmentof the intensity of the fire.To achieve the best effect with fire ventilation it is importantnot to make the openings too large in relation to thefire and the thickness of the layer of fire gases. If the openingis too large fresh air from the inlets can flow directlyout with the fire gases, which naturally reduces the effect ofthe opening. This is, however, usually only a minor problemin the context of fire and rescue operations.50