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Mechanical smoke ventilation Mechanical smoke ventilation may be used as an alternative to natural ventilation systems. Some of the benefits of mechanical systems are specified extraction rates, low wind sensitivity, known capability to overcome system resistances and reduced shaft cross sections. Requirements of mechanical systems include a maintained power supply, fire resisting wiring, temperature classified equipment and a standby fan. Suitable air inlet and exhaust is needed to
prevent damage to the system and to ensure that excessive pressurisation or depressurisation of the ventilated area does not occur. This ensures that excess smoke is not drawn from the apartment of fire origin, and that escape doors are neither rendered inoperable nor pulled open.
Design should be based on a single floor
level being affected by the fire, and therefore only the smoke vents on the floor of fire origin and any other design critical vents (such as the head of the smoke shafts and staircase) are required to open. System designers should avoid opening ventilators on multiple floor levels, especially where connected by a smoke shaft, to avoid smoke spread to otherwise unaffected parts of the building, and/or reduction of extract rate from the floor of fire origin.
Pressure differential systems It is generally recognised that pressure differential systems can provide a high level of protection
42 SEPTEMBER 2018
www.frmjournal.com Installation and testing
To ensure that the smoke control system operates correctly when called upon, it is essential that the processes of planning, design, installation, commissioning and maintenance are undertaken by competent parties, and that clear responsibilities are established. A smoke control system is usually made up of a number of components, interconnected and controlled in a way which meets the performance requirements set out in the system design. The products will generally meet European or international product standards.
to stairs and lobbies. The aim of a pressure differential system is to establish a pressure gradient (and thus an airflow pattern), with the protected escape stair at the highest pressure and the pressure progressively decreasing through lobbies and corridors. With the correct level of pressure differential, it is possible to be certain that smoke from a typical apartment fire will not enter the stair under normal conditions. Air will naturally try to move from an area of higher pressure to an area of lower pressure. By increasing the pressure in the protected areas (ie the escape routes) to above that in the areas where the fire is likely to occur (in this case the apartments), it is possible to prevent smoke spread into these escape routes. This is usually achieved by pressurising the parts of the escape route to be protected.
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