Smoke and Heat vent opening calculations - NL519

The dimensions of smoke and heat exhaust outlets should be such that smoke and combustion gases can be discharged to the atmosphere in sufficient quantities to ensure that an adequate smoke free layer is maintained inside a building in the event of a fire. This can only be achieved by ensuring that the smoke ventilation system has been correctly designed following all current design practices and guidlines. Once the design has established a vent area requirement, it is necessary to select size and site the smoke vents.

When air and hot gases pass through an opening, a permanently open vent, open window, smoke flap or louver, there is a restriction to the flow imposed by the resistance of the perimeter edges of the opening including the resistance of the ventilator opening mechanisms, louver vanes or or tilting sashes.

This resistance to flow is defined in the from of a co-efficient that must be applied to the ‘geometric area’ of the vent opening to arrive at it’s ‘aero-dynamic free area’ it is this figure that must be applied in selecting the number of ventilators to meet specific design needs.

Some ventilators, rooflights and windows will have flow coefficents established by test but in the absence of any specific test figures a factor must always be applied to the geometric area of the ventilator to arrive at an aerodynamic free area.

Typical flow coefficients for an outward opening tilting sash without considersation of side winds is illustrated in the table above.

Where it is necessary to provide designated air inlet vents to compensate any shortfall in make up air provided by infiltration through doorways, entrances and other smoke outlet vents in parts of the building not affected by the fire. A flow coefficient must be applied to these vents. Inlet vents may take the form of top hung outward opening: centre pivot or bottom hung inward opening windows.


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