Fire ventilation

Stairwells with hot gasesDuring apartment fires relatively hot fire gases can in certaincases flow out into the stairwell. The stratification inthe stairwell becomes apparent, with an upper area filledwith fire gases and a lower area more or less free from firegases. Evacuation from the upper apartments can be difficultto achieve.Without some form of ventilation measure, and withclosed doors, there will be a negative pressure in the bottomparts of the stairwell and a positive pressure in the upperparts. Fire gases can flow into the upper apartments, as aresult among other things of the type of comfort ventilationand the temperature of the fire gases.Since the fire gases has a relatively high temperature it ispossible to use natural ventilation with satisfactory results.This assumes, however, that there is an openable hatch orwindow in the upper part of the stairwell, which is often thecase. If positive pressure ventilation is used and the fan isplaced at ground level outside the stairwell, the entire stairwellcan be pressurised (see figure below) with a greater riskof the fire gases being forced into the upper apartments. Ifpositive pressure ventilation is properly coordinated withthe extinguishing operation for the apartment fire thereare, however, the prerequisitesfor a successful operation.The upper part of thestairwell is full of hotfire gases.113