Fire ventilation

The relationship between the size of inlets and outletsduring positive pressure ventilationv FA F• 2A Th• 1D T= Hv T0 •v 0A 0V .0D 0Consider a room with an inlet with an area of A T , an air speed of V T and aheight of H (=D T , height/diameter of air cone), and an outlet with an area ofA F and a gas speed of v F . The fan has an area of A 0 and a diameter of D 0 , andthe flow through the fan is V 0 . We assume in other words that the air conefrom the fan completely covers the inlet.We disregard the actual fire. In those cases where positive pressure ventilationis primarily applicable this is a reasonable assumption, since in thesecases the fan creates a much greater pressure than the fire. We also disregardthe effect of the wind.The law on the conservation of momentum gives:v 0 × D 0V T × D T = v 0 × D 0 dvs. v T = −−−−−−−−−− Equation 1D TThe dynamic pressure exerted by the air cone (from the fan) to the inletis given by:ρ 0 × V T2P dyn = −−−−−−−− Equation 22The flow from the fan is given by:π × D 02V˙ 0 = v 0 × A 0 = v 0 × −−−−−−−− Equation 3469