Controlling smoke in buildings by pressurisation requires careful fan selection. Help is at hand from Woods Air Movement.
There are basically two main methods for controlling smoke in buildings to prevent it contaminating escape routes: by ventilation and by pressurisation. Powered ventilators are often used for exhausting hot smoke from fires; the other option is to select fans for use in pressurisation in accordance with BS 5588 Part 4: 1998.

The objective of any smoke control system is to keep the smoke and toxic gases out of the escape route long enough to allow the occupants to escape or seek a safe refuge. In addition an adequate smoke control system will help the fire fighters deal with the fire and smoke.

In large open-spaced buildings, car parks, shopping centres, exhibition halls, etc, the usual method of smoke control is by ventilation – extracting the smoke from the area.

In high rise, multi-room buildings, where the staircases, lift lobbies and corridors provide the escape route, smoke extraction may only serve to worsen the situation. A smoke extraction system (figure 1) will provide negative pressure in the escape routes which will tend to draw the smoke in to the very spaces requiring protection.

However, it is possible to hold back smoke from a fire by simply supplying clean air into the escape routes, thereby developing excess, or positive pressure in the spaces requiring protection. Figure 2 illustrates the method which is known as pressurisation.

In a pressurisation system, the airflow must always be away from the escape routes to ensure that they will be at a higher pressure than the surrounding area.

A pressurisation system has two main components. First, there is a supply air system designed to blow in to the protected spaces a sufficient quantity of air to maintain the required pressure level or air velocity. This will always be fan powered.

Second, there is an exhaust air system to enable the pressurising air to escape from the unpressurised areas of the building via the fire floor. This can either be a natural or fan-powered method. The fans used would be required to handle the hot fire smoke and comply with BS 7346 part 2.

The supply side of the pressurisation system has two and occcasionally three modes of operation:

  • mode 1, the detection phase – to raise a pressure differential in the protection space (staircase, corridor etc) by the required amount (50 Pa in the UK) when all doors are closed;
  • mode 2, the escape phase – to maintain a specified air velocity (0·75 m/s) through the open doors onto the fire floor with various other doors open, or a pressure difference of 10+ Pa with the fire floor doors closed and various other doors open;
  • mode 3, fire fighting phase – to maintain a specified air velocity (2 m/s) through the open doors onto the fire floor with various other doors open.

    The exhaust air system must be designed to provide a low resistance route for the supply air to leave the building via the fire floor. This can be achieved by: natural leakage (in practice this is unrealistic); through automatically opened windows or vents around the perimeter of the building (in practice almost 0·5 m2 of vent area on every floor would be required for each pressurised staircase); the provision of a vertical duct through the building with a damper arranged to open automatically on the fire floor (often the best solution, although the size of the duct can cause problems); mechanical, powered extract from the unpressurised space. The latter can be combined with the vertical duct option, thus reducing the size of the duct, or any existing exhaust system from the unpressurised space can be utilised.

    Smoke control using pressure differential can be seen as simple in concept but difficult in practice. However, fan engineering technology is well able to provide the requirements of both supply and exhaust air fans of a pressurisation system for smoke.

      

    Pillars of wisdom

    The smoke extract system at a mainline station is one of the most vital public safety elements. At London’s Cannon Street Station the extract system is located in ductwork above the tracks and is capable of dealing with a fully-developed train fire. Merlin Gerin is supplying NS feeder pillars to provide dedicated lv power to the smoke extract system. M&E contractor MJN Colston is installing the feeder pillars which provide the specified form of separation, control and monitoring circuits all in a fully-assembled outdoor switchboard solution. The upgrade at Cannon Street is part of a huge Railtrack programme to upgrade the smoke containment arrangements in 14 London mainline stations. Merlin Gerin’s NS feeder pillars incorporate cassette-mounted circuit breakers that allow circuits to be added or removed to meet any future changes in power demand. The outgoing circuits are capable of powering seven fans, rated from 50 kW to 90 kW. This power requirement was well beyond the old supply.

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    Electrical and Mechanical Contractor

    Postscript

    For more information on the selection of fans for fire safety, circle the enquiry number for your copy of the Woods Air Movement technical paper WTP41– Smoke control by pressurisation.