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Airflow (critical velocities)

By generating a sufficient horizontal flow of air at the location of the fire it is possible to force all the smoke in one given direction, thus creating safe conditions in the other direction(s). This technique is employed widely in tunnels, where forcing the smoke in a given direction often forms the core of the smoke management strategy, and where other systems such as ceiling level mechanical smoke extraction may present difficulties in respect to installation and maintenance. Furthermore, the geometry of tunnels and the evacuation and fire-fighting strategy will often make this approach attractive, particularly in uni-directional traffic bores.

However, applying an airflow at the source of the fire is not generally considered for buildings, where one problem that has been highlighted is that a high volume of fresh air may ‘fan the fire’, increasing the likelihood of fire growth. Furthermore, this is not a practical option within residential dwellings. While it could be considered within common access areas such as corridors, it is not appropriate if they are considered as ‘sterile’, i.e. the fire source is not located here.

Note that achieving a ‘critical airflow’ remote from the fire source can be considered as part of a successful pressure differential system, where for example the smoke is kept back from protected spaces at open doorways. Here, by maintaining the flow of air above some critical velocity the smoke is ‘held back’ at the door. However, this method of smoke control is generally considered as part of a pressure differentials scheme and not an airflow control method.

Pressurisation (pressure differentials)

The term pressurisation (or pressure differentials) generally includes the range of schemes whereby designated ‘safety critical’ regions of the building are protected from the ingress of smoke by maintaining them at a higher pressure than neighbouring spaces.

Pressurisation is widely used worldwide to protect stairwells [e.g. ASHRAE, 1999, BSI, 1998, CEN, 2000, Standards Australia & Standards New Zealand,1998], in particular when they are considered the most important part of the egress route to protect. To eliminate, or at least reduce to acceptable levels, the passage of smoke from the adjacent spaces, the stairwell is maintained at a sufficiently high pressure by means of forced fresh air supply, either at the bottom of the stairwell, the top of the stairwell, or at distributed locations within the height of the stairwell.

Pressurisation may be extended to vestibules and corridors, as specified for example in the UK [BSI, 1998]. However, careful design is require in these cases to ensure that appropriate pressure differentials are maintained between the stairwell, the lobby/vestibule and the corridor. Provision for pressure relief is required at each stage. Unless this is correctly accounted for, the pressures differentials across the various compartmentation boundaries will be reduced or eliminated, rendering the pressurisation system ineffective. Another problem that has been identified is that pressure relief is not properly engineered the pressure differentials may be too great, so that persons (especially the young and weak) cannot open doors on the escape route [NFPA, 2000b].

© Building Research Establishment Ltd 2005

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