Principles of smoke control: For smoke to get out, air must come in

Posted by Conor Logan on 30/09/11 10:42

A vacuum in a water bottleBuildings are becoming increasingly airtight to meet energy efficiency and low carbon emission regulations, such as the Approved Document L (ADL).  This means that, when designing the building’s smoke control system, it is essential to include not only means of extracting the smoke, but also ways to let the air in.

Why is that so much more important with airtight buildings?

You’re probably familiar with the phrase about nature abhorring vacuums. You can see it when you try to empty a bottle by upending it over the sink - it takes a while for the liquid to come out. It ‘glugs’ as the force of the fluid trying to leave the bottle battles with the air trying to get back in to fill the vacuum. To empty the bottle as efficiently as possible, you need to tilt the bottle so that the incoming air takes a different path to the outgoing fluid.

Let the air in

The same thing happens when you try to extract smoke, heat or air from an enclosed space. If no air comes in to replace what you are trying to extract, nothing will move. So it’s not enough to install a smoke and heat extraction system, you also need a path for air inlet.
There is more. When hot smoke is extracted through a ventilator, cold air is sucked in to replace it. If this has to come in through the same ventilator, it reduces the flow of smoke in exactly the same way as our fluid glugged out of our bottle. The ventilator’s efficiency is drastically reduced, since part of its surface is being used to let air into the building, instead of it being entirely dedicated to extraction.
In addition, as the hot exhaust air mixes with the colder air coming through the ventilator, the smoke layer is disturbed and cooled, increasing the volume of smoke and lowering its buoyancy.

Separate outgoing smoke from incoming air

As with the bottle, for a smoke control system to exhaust smoke efficiently, the outgoing smoke must be separated from the incoming air: you need to provide separate inlet ventilation to ensure enough replacement air is coming in.

Don’t disturb the smoke layer

The air inlet must be provided at low level or remote from the smoke layer so that the incoming air doesn’t disturb the smoke layer. This can be achieved with ventilators in other zones, doors or windows that open automatically or, if they are not sufficient, air inlet ventilators.

The Computational Fluid Dynamics models in Figures 1 and 2 clearly show the benefits of inlet ventilators as part of a correctly designed smoke control system.

Figure 1: CFD model of a fire in a single storey building with roof vents but no inlet.

CFD model of a fire in a single storey building with roof vents but no inlet

Figure 2: CFD model of the same scenario, but with an equal area of roof vents and low-level inlet vents. The smoke ventilators are clearly discharging much more smoke and the clear layer above floor level, through which the occupants can escape and fire fighters can search and attack the fire, is much higher.

CFD model of a fire in a single storey building but with an equal area of roof vents and low-level inlet vents

Use certified ventilators

Ventilators should comply with EN12101-2 to ensure a robust and reliable system, which will operate in all conditions.

Use available tools for effective design

There are calculation methods and tools, such as BS 7346-4 or Computational Fluid Dynamics (CFD) analysis, to assess the required areas of smoke exhaust and air inlet, which need not be identical. If both are not fully provided the system will not work as intended and will not properly protect you, your workforce or your customers.

Image: Paul/

Conor LoganConor Logan is a Technical Manager at Colt UK Smoke and Climate Control Division. Designing innovative smoke control and HVAC systems, Conor is also Chairman of the Smoke Control Association.

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Topics: Smoke Control, CFD, Regulations, Smoke ventilation