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


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Buildings are becoming increasingly airtight to meet energy efficiency and low carbon emission regulations, such as the Approved Document L (ADL). Read more!

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

  1. 1. Principles of smoke control: For smoke to get out, air must come inBuildings are becoming increasingly airtight to meet energyefficiency and low carbon emission regulations, such as theApproved Document L (ADL). This means that, when designingthe building’s smoke control system, it is essential to include notonly means of extracting the smoke, but also ways to let the airin.Why is that so much more important with airtight buildings?You’re probably familiar with the phrase about nature abhorringvacuums. You can see it when you try to empty a bottle byupending it over the sink - it takes a while for the liquid to comeout. It ‘glugs’ as the force of the fluid trying to leave the bottlebattles with the air trying to get back in to fill the vacuum. Toempty the bottle as efficiently as possible, you need to tilt thebottle so that the incoming air takes a different path to theoutgoing fluid.Let the air inThe same thing happens when you try to extract smoke, heat or air from an enclosed space. If no aircomes in to replace what you are trying to extract, nothing will move. So it’s not enough to install asmoke 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. Ifthis has to come in through the same ventilator, it reduces the flow of smoke in exactly the same wayas our fluid glugged out of our bottle. The ventilator’s efficiency is drastically reduced, since part of itssurface 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 smokelayer is disturbed and cooled, increasing the volume of smoke and lowering its buoyancy.Separate outgoing smoke from incoming airAs with the bottle, for a smoke control system to exhaust smoke efficiently, the outgoing smoke mustbe separated from the incoming air: you need to provide separate inlet ventilation to ensure enoughreplacement air is coming in. © 2012 Colt UK
  2. 2. Don’t disturb the smoke layerThe air inlet must be provided at low level or remote from the smoke layer so that the incoming airdoesn’t disturb the smoke layer. This can be achieved with ventilators in other zones, doors orwindows 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 inletventilators 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.Figure 2: CFD model of the same scenario, but with an equal area of roof vents and low-level inletvents. The smoke ventilators are clearly discharging much more smoke and the clear layer above floorlevel, through which the occupants can escape and fire fighters can search and attack the fire, is muchhigher. © 2012 Colt UK
  3. 3. Use certified ventilatorsVentilators should comply with EN12101-2 to ensure a robust and reliable system, which will operatein all conditions.Use available tools for effective designThere 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. Ifboth are not fully provided the system will not work as intended and will not properly protect you, yourworkforce or your customers.Image: Paul/ Conor 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. © 2012 Colt UK
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