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already present. But if this plant is not present, then the additional cost associated with a system installed only for smoke control may be excessive.

Natural smoke venting

Dilution and smoke clearance through external wall vents, introduced above, can be classed also as a natural smoke venting scheme. However, the term is more generally applied to schemes where there is an attempt to exhaust smoke at high level and provide replacement fresh air at lower level. By placing vents at high level in the corridor/lobby, there may in the right conditions be a degree of smoke clearance through the vent(s). However, these schemes, as for the dilution methods discussed above, are prone to adverse wind effects.

Smoke shafts have been proposed, and used (in the UK in particular), throughout the world. Some of the early work, particularly in North America, was orientated towards the venting of smoke from the fire compartment itself. However, smoke shafts have also been used to vent smoke from corridors, lobbies and stairwells. While a naturally- ventilated smoke shaft can be expected to work (i.e. draw smoke into the shaft and vent it out of the top) with sufficiently buoyant smoke, they may be prone to adverse wind and building stack effects when the smoke is not sufficiently buoyant [see, for example, Tamura & Shaw, 1973]. Given the condition of cool smoke, for various weather conditions (influencing the building stack pressures in particular) the passage of smoke in a smoke shaft with bottom and top openings could be either upwards or downwards [Tamura & Wilson, 1970].

Where stairwell pressurisation is not employed, there is generally provision for natural venting to 'clear' smoke that enters the stairwell. This will often take the form of a vent at the top of the stairwell, sometimes augmented by vents at ground level and at intermediate storeys. The behaviour of the smoke inside the stairwell, for a given arrangement of ventilation openings, will be influenced strongly by wind building stack effects. It has been suggested [e.g. Poreh & Trebukov, 2000] that there is a benefit in having open vents at the top and bottom of the stairwell, as this gives better overall smoke clearance performance for a wide range of conditions. It is argued that the addition of a bottom vent will in general reduce the amount of smoke passing into the stairwell on the fire floor, and that in ‘adverse’ conditions where the controlling forces dictate a downwards movement of smoke within the stairwell it is the better to have a bottom vent.

The location of vents at rooftop level requires special care [Saathoff et. al., 2002], and the design of the opening may then also be important [Ghosh, 1993]. Given a complex roof top arrangement, the assumption of suction in all weather conditions may be incorrect.

There is no worldwide consensus on the most appropriate way to naturally ventilate stairwells. For example, the Canadian code [National Research Council Canada, 1995] specifies that for a naturally ventilated stairwell the vent be located at ground level so that in winter conditions, where 'reverse stack effects' are significant, conditions are

© Building Research Establishment Ltd 2005

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