First of all a big thank you to all those who attended my webinar last Friday on 'The General Principles of Smoke Control'. If you missed it or would like to watch it again, a recorded version is available here. Many questions were asked after the presentation and I decided to post them here for everybody’s benefit.
- How does smoke clearance and smoke control differ from smoke management?
Smoke management is generally the American term used in NFPA, so within NFPA, smoke management has generally been used to cover exactly the same things as we mean in theUKby smoke control. Smoke clearance and smoke control are just variations on smoke management. They are doing exactly the same thing; they’re managing where the smoke goes and how long it stays in the buildings and what the temperatures are, so we are not looking at different things, we are looking at basically the same thing.
- In relation to mechanical smoke ventilation shafts, why does approved Document B make no reference to the height of shafts required above the roof or the effects of surrounding structures, which it does do in terms of natural shafts?
The reason that those requirements are there for natural shafts is that it is very important that the top of the shaft ventilator is not affected by adverse wind conditions. There is a requirement that the shaft should be at least half a metre above any adjacent roof and also that the top of the shaft should be at least 2.5 metres above the ceiling of the highest level ventilated by that shaft. These requirements are there to ensure that firstly we have a chimney effect above the top level and secondly to avoid any adverse wind effects.
As soon as we go for a mechanical shaft, then those applications don’t apply; we don’t have to worry about the chimney effect because we have a fan which is generating the suction pressure, so the 2.5 metre limit no longer applies. Because we have got a fan in there, the pressure developed by the fan is much higher than will ever be developed by a natural system and therefore wind effects have much less effect. The only thing we are concerned about in terms of the location of our exhaust is to try and make sure it is located so that we minimise the risk of any smoke being drawn back in through any inlets.
- Is there a maximum height that a mechanical system will work for?
No, as long as the shaft is big enough. If we look at the work BRE did on natural shafts, they looked at those up to 100m high and CFD showed that they worked. We have successfully done mechanical shafts in buildings of over 50 storeys.
With a mechanical shaft, there is no reason why we can’t do exactly the same thing. The shaft will differ in area, though, because we don’t want to end up in a situation where we have a very high pressure drop between the top and bottom of the shaft which would cause us to get a vast variation in the ventilation performance depending on which floor the fire was on, therefore if we have a very tall shaft, the shaft size will get larger.
Typically, for a small building, we would be looking at a shaft size of 0.6 m², and 1m² – 1.2 m² for a big building. If you can’t provide that sort of height, there is nothing to stop us looking at a system which will split into two. For example, a 100m high building could have two systems, each dealing with 50 metres of the building.
- If there is a fire inside a shopping mall, won’t the shops themselves be full of smoke?
Yes. The smoke control system is generally not there to protect people within the shop; it is there to protect the people within the mall. Assuming that the people in a shop itself are protected by the alarm system and the means of escape from the shop, they will get out of the shop quickly and won’t be in danger from the smoke. In general, smoke control systems are not there to protect shop occupants. They could be designed to do so but this is not required by the Regulations.
- What is the difference between approved Document B and a BRE shaft?
In approved Document B, we look at shafts for fire fighting (BRE) and residential systems, which are smaller than the BRE shaft. By contrast the BRE shaft was developed for fire fighting shafts in commercial buildings and has a 3 m² cross section of shaft.
The shaft shown in approved Document B is for shafts in residential buildings and is a 1.5 m² shaft. There are differences as well in terms of the shaft requirements; approved Document B gives more detail about the shaft than is found with the BRE Shaft.
Although both shaft designs were developed by BRE, the first one was the BRE shaft, and the set of requirements given for it are a bit cruder than approved Document B. I would advise, even if you are doing a BRE shaft, to take into account the extra requirements in Approved Document B, because they are there for a very good reason, to ensure the shaft works as well as possible.
- In car parks, which extract system is more efficient; horizontal or vertical?
It depends on what you are trying to achieve: if we are looking at a smoke clearance system, then it really does not matter what you use, as you have a very limited flow rate and are not going to clear the car park or any areas of the car park in the early stages.
In terms of a smoke control system, where you are trying to keep an area of the car park clear, then which is better will depend to a large extent on the car park geometry. Most car parks are quite shallow in terms of height and it is very difficult to get a decent clear area below a smoke layer, and therefore in car parks we almost invariably use horizontal systems.
If however, you are looking at something like a loading bay where you have a tall space to accommodate the entry of lorries, then you have more height to play with, therefore a system with vertical extraction may well be better, but all it comes down to geometry.
- Will the PowerPoint slides from this webinar be available online?
Yes, the slides and a link to watch this webinar again are available on our website at https://www.coltinfo.co.uk/general-principles-of-smoke-control-cpd.html.
Paul Compton is a Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.
