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Risks to firefighters

TECHNICAL

The 'smoke shaft' is retained in a reissued bs as a means of clearing smoke from firefighting lobbies. The authors believe it doesn't work

For more than 30 years the uk has led the world in recognising the risk of smoke and hot gases in places of safety, means-of-escape routes and lobbies for the protection of firefighters. The greatest development, going back to before the London Building Act Section 20, was to understand the contribution made by positive pressure. Introduced into the staircase or lobby, it ensures that no smoke or hot gases from fire on adjacent floors can penetrate that protected zone.

Research has shown that using mechanical air-handling systems is effective for pressurisation. It also shows that natural systems with automatic opening windows/vents into the lobby or staircase can provide the same facility. However, in some cases using these openings is less successful due to the effect of external air pressures.

The exception to this understanding is found in the reference of Building Regulations (England and Wales) Approved Document B to firefighting shafts. In this case it is stated that smoke ventilation should be in accordance with bs 5588, Part 5. The bs describes three types of smoke control as acceptable - pressurisation and automatic openable windows, as noted above, but also a 'smoke shaft'.

Are all these three smoke ventilation measures effective? As noted above, pressurisation is known to be completely effective, both by experimental test and from fire experience. Openable windows on an external wall are satisfactory because fresh air is readily available, particularly if cross- ventilation can be arranged. But the natural ventilation 'smoke shaft' method (see illustration) is of very questionable status.

As far as we know it has never been shown to work, either in experimental test or in fire incident experience. And there have been cases where it has definitely not worked, both in experimental tests and in fire situations.

Against first principles

The 'smoke shaft' method as described in bs 5588: Part 5 consists of a vertical shaft with a cross-sectional area of at least 3m2, connected to each lobby by a dampered opening operated by smoke detector. The idea is that any smoke entering the firefighting lobby from the space where the fire is seated will move out of the lobby into the smoke shaft and so keep the lobby clear. This smoke movement is unlikely to take place.

In most conditions, the air in the unheated smoke shaft will be colder than the air in the building. Consequently, when a damper is opened between a lobby and the smoke shaft, the denser cold air in the shaft will move into the lobby to replace the less dense warm air in the building.

Any smoke which has entered the lobby will be relatively cool, since the fire is not in the lobby but in the space separated from the lobby by a door (which may or may not be closed). The initial movement of air out of the smoke shaft into the building will prevent the cool smoke in the lobby from entering the shaft.

Even when the smoke in the lobby becomes hot enough to move into the shaft against the cold air flow, it will not remain as a discrete hot bubble of smoke rising through the seas of cold air in the shaft. Rather it will immediately mix with the large quantity of cold air in the shaft, so there will be no upward movement of air and smoke.

This will continue until the smoke in the lobby has become hot enough to move into the shaft and heat up the large volume of cold air in it. By this time conditions in the lobby will be quite untenable and it is likely that more smoke will be entering the lobby than can be removed by the smoke stack. (Bear in mind that the smoke is cooling all the time while it is moving from the fire space through the lobby and into the shaft, and also that cold air is entering the shaft via the bottom opening. Any upward movement in the shaft will not be particularly vigorous.) Hence the firefighting lobby will continue to be completely smoke-logged.

A flawed design

The only way to make a smoke shaft work effectively is to provide mechanical extraction at its head, have the bottom of the shaft closed, open the lobby to the shaft on the fire floor only, and provide replacement air to each lobby. The volumetric flow of replacement air should equal the volumetric flow extracted by the fan at the shaft head.

It would not be satisfactory to draw replacement air from spaces around the lobby via door cracks, since this would create pressure differentials which would encourage smoke to enter the lobby. Replacement air should be drawn into the lobby directly from open air, if necessary via a duct. The introduction of replacement air directly into the vertical shaft by means of a bottom opening or by the U-tube arrangement, as in the diagram, is technically incorrect and should not be used.

It is clear that the Building Regulations (England & Wales) are correct in not directly suggesting the use of smoke shafts for firefighting lobbies. However reference is made to bs 5588, Part 5. The system of smoke shafts continues in its newly revised version. Its guidance on smoke shafts should be regarded with extreme reservation.

Gordon Butcher and Alan Parnell are directors of Fire Check Consultants

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