When smoke is present on any corridor, the relevant Smoke Detector (D) sends a signal to the Control Panel (A).
The panel then sends a signal to the respective lobby-to-shaft vent (F) on the 'fire' floor, and this opens into the shaft. Simultaneously, the lobby vents on the other floors are electronically locked to prevent the spread of smoke, fumes and combustible gases into other parts of the building.
These lobby vents can be proprietary ventilators with either a aluminium grille screen or plasterboard cover to match the corridor wall, or the vents can be replaced with fire doors fitted with special actuators to open the door into the shaft on receipt of the appropriate signal.
Where Fire doors are used as lobby-to-shaft vents, there must be either a physical barrier 1100mm high fitted in front of the door, or more usually a floor grille on each floor inside the shaft, to eliminate the possible fall hazard into the shaft when the fire door is open.
At the same time that the shaft vent is opened, the Control Panel sends a signal to the OpenSky head-of-shaft vent (L) to open, thereby allowing the smoke to rise up through the shaft and out of the building by the natural buoyancy effect of the hotter air.
The System can also be configured to open the head-of-stair vent (S) whenever one of the lobby vents is open, in order to allow the ingress of replacement cooler air which aids the flow of smoke out through the shaft.
The top of the smoke shaft should be at least 2.5m above the highest vent into the shaft. Where this is not possible or aesthetically desirable, the top floor corridor will often not vent into the smoke shaft and will have its own external vent out through the roof.
Airsun Systems would not recommend the use of naturally vented shafts on buildings with more than 5-6 storeys. For taller buildings, mechanically assisted smoke shafts should be provided.