Q&A from 'The general principles of smoke control' webinar

Posted by Paul Compton on 19/01/16 12:00

--united.local-dfs-usr-uk-ukevajon-my_documents-my_pictures-img_smoke_control.jpegI received some excellent questions during the Q&A section during the recent webinar that I presented. Here you can see my answers to these questions, slightly edited for clarity.

There is also a recording of the webinar available.

Does the installation of attenuators on mechanical systems cause sufficient restriction or disruption to the mass flow to require the provision of more powerful extract fans?

It depends upon the attenuators that are provided.  There are three types of attenuators typically used. One is a circular attenuator, straight through with no pod. This has no effect on pressure loss. The second is a circular attenuator with a pod in the middle to provide additional attenuation and this has a small additional pressure loss. Finally if you require more attenuation than these can provide, rectangular splitter attenuators are available.

Splitter attenuators certainly will have a significant additional pressure drop, which will require a more powerful fan.  Typically splitter attenuators would be selected giving a pressure loss up to 100 Pascal for each attenuator, so with both inlet and discharge attenuators the fan will need to handle up to 200 Pascal additional pressure. Thus the fan will be more expensive and more powerful, needing a higher electrical input putting an extra load on your generator.

For mechanical extract in a residential block, with the extract in a lobby to protect a single stair: Do we need to ensure that the AOV ventilator at the head of the stair will not be affected by wind?

Generally, if there is mechanical extract system from the lobby, the ventilator in the stair will be used primarily as a source of air inlet, so therefore we are not particularly worried about whether it is in the vertical or the horizontal or about the wind pressure effects.  There is however a risk (no matter how small) that we may lose both the fans or both of the power supplies and therefore the mechanical system may not work as intended. In that case the smoke ventilator at the head of the stair will then be used to get rid of any smoke which may then get into the stair.  It’s a very small risk, but having said that, if you can provide the ventilator in a position where it will not be adversely effected by wind, then it’s beneficial, but it’s not a particular requirement. 

Can we employ a smoke control system that we use for dwellings in hotels to extend over 9m for one dead end?

That’s not something that is generally done. What we generally find is that the extended travel distance systems are used for residential buildings but if there is a particular requirement for a hotel then yes, it would be perfectly possible to look at fire engineering a mechanical shaft system to suit a particular application.  It really comes down to looking at the particular application and sorting out the design with the design team and obviously with Building Control and the Fire Service as well.         

With car park smoke ventilation systems, can a natural solution be used with openings on opposite sides of the car park in the slab above rather than in the walls?

Generally yes, but it does come down to satisfying Building Control that what is being done will be suitable.  The purpose of requiring ventilation on two opposing sides is to ensure that whatever the wind direction is there’s a sensible through flow. If the vents are on the roof of the car park, then in theory both or all openings could be in the same pressure regime and therefore you would not get the benefits of wind-driven through flow that you might expect.  CFD might be needed to satisfy Building Control but, yes, it is certainly possible and has been done.

Is it required to have a smoke control system for a single-storey industrial building measuring 50m x 12m?

Normally not, because the travel distances would normally be well within the normal limits and therefore there is no UK Regulatory requirement for smoke control in a building of that size.  The only reason you would have smoke control there would be if the client particularly wanted it to protect the building.

Can we use a supply fan as an exhaust fan in case of fire?

I presume there we are talking about using a supply fan on a normal day-to-day ventilation system and reversing it to extract in case of fire.  That would be perfectly possible but the fan would have to be selected in accordance with EN 12101-3 as a smoke ventilation fan, to be sure that the fan will work as required in case of fire.  You would also, of course, need to make sure that any other components of the ductwork system are suitable as well. You would need to know, for instance, if there is a filter or anything else in there that would have to be by-passed. So the answer to the question is yes, as long as it is rated as a smoke ventilator, but you do need to look at the entire system not just the fan.

During a recent project where mechanical ventilation was incorporated, we couldn’t find any prescriptive guidance on a minimum distance between the inlet and outlet vents, so is there a minimum distance prescribed in current guidance to prevent expelled smoke from an extract vent re-entering the building through an inlet vent?

There is no specific guidance in any of the smoke control documentation.  The general recommendations which are given are that, as far as possible, the inlet should be 5m apart from the exhaust in a horizontal plane. If both are on a wall, then the exhaust should be also 1m higher than the inlet. This is taken from guidance for day-to-day ventilation but it is the only specific guidance that we are aware of and it certainly doesn’t guarantee that there will be no re-circulation of air at all; it gives you a condition where you would not expect significant re-circulation.

I have been involved in a scheme using a mechanical shaft system in a common corridor to extend the travel distance to 29m. What is the maximum distance permitted from shafts to end of corridors? Can it be more than 5m, and who is responsible to do modelling if it is required?

This is for a residential building with an extended corridor and a 29m extended travel distance from the furthest flat door to the place of safety, which will either be the lobby or stair. But how far can the smoke shaft be from the end of the corridor?

There is no specific maximum distance but there is guidance from the Smoke Control Association in a free guide, which can be downloaded from www.FETA.co.uk. In the Smoke Control Association’s Section you can download several very useful Smoke Control Association Guides for Residential Buildings, CFD, Car Parks/Loading Bays etc.

This guidance recommends that extended travel distances shouldn’t exceed 30m.  For the distance from the shaft to the end of the corridor, or the furthest door, the SCA document recommends a maximum distance of 5m. The reason for that is that the area beyond the shaft will not be ventilated anywhere near as well, so what we don’t want to find is that we have got quite a long distance beyond the shaft which remains smoke logged for some significant time.

If it is CFD modelled then the likelihood is that you will see that the rest of the corridor is cleared of smoke very quickly, but that end section beyond the shaft will remain smoky for a relatively long time. Of course if we didn’t have the extended travel distance, for a normal system with an AOV in the wall, then we would expect the 7.5m travel distance to remain pretty smoky anyway, so we are not any worse than with a conventional compliant system. 

In common parts of a staircase to a block of flats where AOV’s are installed, linked to smoke detection, do you need a fireman’s switch control to be able to close the AOV?

The Standards in England and Wales generally recommend that the stairwell vent should have a manual switch at the bottom of the stair.  In Scotland the requirements are that you have a manual switch of the bottom of the stair and at the top of the stair.

In a residential block with one staircase, if we use a pressurisation system within the corridors and have a full sprinkler system throughout the block, what will be the longest travel distance we can achieve if the ceiling height’s just below 3m?

In a residential block, ceiling height is not important in terms of design.  If we are looking at pressurisation within the corridors there isn’t any guidance in terms of using pressurisation systems to provide extended travel distances. The reason for this is that for a pressurisation system we need to have accommodation air release so that when the door from the pressurised space is open, we can maintain a velocity through that door and therefore we don’t pressurise up to the apartment door; instead we pressurise the stair and then provide the accommodation air release from the lobby or corridor. This accommodation air release doesn’t normally justify an extended travel distance, so pressurisation is not used to allow an extended travel distance.  If you need to have extended travel distances then you need to go for the mechanical extract type system.

What considerations would you take into account for location of smoke exhaust to the outside?

I think that this is partly answered by one of my previous answers, that we want to be at least 5m from any air inlets.  The other requirement is that the design of the exhaust should be such that smoke is not blown onto the building itself or any adjacent building. Also of course, if there are windows near to the ventilation outlet, we would try to re-design so that the outlet is as far from them as possible.

Paul Compton Paul Compton is Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.

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Topics: Smoke Control, Webinar, CPD