Smoke ventilation can appear a 'black art' to architects, other designers, contractors and even some fire engineers struggle with it. The following is an attempt to demystify the subject.
Smoke ventilation not only requires openings to let the smoke from a fire out of a building but also similar openings to allow fresh air to flow into the building to replace it.
The amount of smoke generated by a fire is a function of the height through which it is able to rise and the size of the fire. The volume of smoke is not generally considered to be a function of the area of the building.
The size of a fire is usually expressed in terms of the area of burning and the amount of heat it is generating. Most smoke ventilation calculations are based on a ‘steady state' fire of a particular size. A common fire size used for smoke calculations is a fire, controlled by conventional sprinklers, having an area of 3 metres x 3 metres and a heat output of 5 megawatts.
A plume of smoke rising from a fire grows in volume the higher it rises above the source of the fire. This is because surrounding air becomes entrained within it at the boundary of the plume. As air is entrained within the plume it causes the smoke to cool and lose buoyancy.
If a fire is located in an area beneath a horizontal element such as a floor and flows out of an opening, research has indicated, that more air (approximately double the amount) is entrained in the plume after it changes direction and starts to rise vertically.
Smoke will cool when it is travelling horizontally. In order to limit the effect of this cooling, it is usual to restrict the distance it travels and spreads. This is normally achieved by limiting the overall size of any smoke reservoir and restricting the lateral spread of smoke travelling horizontally by channelling screens.
Hot smoke and the cold air around it, have differing densities and are like oil and water, they do not mix too easily. The exception is the air that is entrained at the boundary condition described above.
When rising smoke reaches a barrier such as the underside of a roof it starts to fill the space available to it from the top downwards. When there are ventilators or other openings in the roof the smoke will flow out of these openings. The rate at which the smoke will flow out is dependent on two factors, the size of the openings, and the depth of the smoke within the reservoir beneath it. A small opening with a deep smoke reservoir beneath it can discharge the same amount of smoke as a larger opening with a shallower smoke reservoir beneath it.
The amount of smoke that comes out of an opening can be increased by mechanical assistance in the form of a fire rated fan or fans. Fans do not however ‘pump' the smoke out of the building, they assist it to pass out more easily. It is the buoyancy of the smoke that does most of the work. The effect of fans is to reduce the aperture through which an identical amount of smoke can flow or to decrease the potential reservoir depth.
When the water level in a bath gets low, air is drawn into the plug hole with the water flowing out, which causes the familiar gurgling sound. If the depth of smoke in a reservoir being ventilated mechanically is reduced too much a fan can cause a similar effect, without the accompanying sound, which is referred to as ‘plug holing'. Instead of smoke being assisted out through the opening, clean air from below the smoke layer can be drawn through the smoke reservoir. The mixture of smoke and clean air flows out far less efficiently. The shallower the smoke reservoir the greater the number of smoke extract positions necessary to reduce the potential for plug holing. Plug holing does not occur to the same extent with a naturally ventilated smoke system.
A smoke ventilation system is usually designed so that the reservoir is above the heads of people on the highest level in the building or compartment of the building.