‘Protecting Escape routes: Smoke shafts v pressurisation’

Pressurisation is one of the possible solutions to provide smoke ventilation in escape stairs, common lobbies and corridors in residential buildings as required by Approved Document B and is recommended for buildings taller than 30m (BS 9999 & BS 9991). 

Mechanical ventilation is mentioned in ADB, but only with reference to pressurisation. Nevertheless it has become commonly used as an alternative to natural smoke shafts, providing equivalent or improved performance. 

How do you decide which to use? The decision is influenced by legislation and standards, building configuration, budget and space requirements. There is no universal “right” choice, but there’s certainly a best choice for each individual building. 

1) Have you a view on dilution systems with push-pull fans and ducts?

We very often use push-pull systems in larger buildings. There is sometimes a query over the changes in pressurisation or de-pressurisation as the conditions change; but generally they work very well. They will generally clear a corridor of smoke within a couple of minutes from closing a door and they provide generally the best protection.

They are the most expensive mechanical shaft type system but they will give you the best protection.

2) Do we have any difficulty with excessive door opening forces with pressurisation systems used in conjunction with a storey smoke extraction system?

It’s not common to have problems but if we have a storey smoke extract system then generally that system will have its own source of air inlet, so that the extra de-pressurisation caused within the room will be relatively small.  If we know that there is a storey smoke extraction system installed then when we commission the pressurisation system, we would expect that system to be running and we would check the pressure differential is no more than 55 Pa when both systems are running together.

So, no it’s not a problem but we do need to know the systems are there so that we can make sure they are working in right condition.

3) Typically where does the air release get installed and what form does it take?

Generally if air release is possible through the perimeter of the building using automatically operated ventilators or windows then that is by far the best system. It doesn’t take any space and is relatively easy to install, and is easy to service and maintain in the future.

Where you can’t do that then you would use a shaft system. This is normally because a building is cellular. If it’s all open-plan then you’ve always got direct access from your pressurised spaces straight through to a window. If you haven’t got that due to corridors and cellular offices, then a shaft system from the space adjacent to the stair is the best solution. You have a free choice as to whether that’s mechanical or natural and it comes down to whether you want to pay the extra money for the fans or keep as cheap as possible and use a larger natural shaft.

4) There is a request for a reminder of the website for the free download of the Smoke Control Association guidance.

That’s www.feta.co.uk

5) What is the maximum length of a corridor for which you can use smoke shafts and are there any requirements on shaft locations?

If you are complying with Building Regulations in terms of the building layout then there is no particular requirement for the shaft location. It could be anywhere adjoining the lobby which is adjoining the stair.

If you have extended corridors then yes, there are requirements. In terms of the maximum corridor length, we can make the system work with any corridor length but what is more important is what sort of length are building control likely to accept as a single direction of travel with no fire doors etc in the way. Generally the guidance is that the absolute maximum that you’re likely to get approved is probably 30m. But generally people are comfortable with less than that. The majority of systems we see tend to be in the range of 18-25m. If you are going for that sort of system, it is very important that the inlet and the exhaust are separated as far as they possibly can be. So ideally you would have your inlet right at one end of the corridor and your extract right at the other end. In practice, that is not always practically possible but they do need to be as close to the ends of the corridors as they can be.

6) What is the current situation of BSEN 12101 part 6?

BSEN 12101 part 6 is the pressurisation standard. The standard is current within all of Europe. It actually requires CE marking of pressurisation systems on-site, but the way the standard is written it’s impossible to do that. So pressurisation systems at the moment are not CE-marked.

There is an issue there that is being addressed and the CEN Committee responsible for BSEN 12101 part 6 is currently re-writing it, with the intent of splitting the CE-marking requirements from the design requirements. Things will change but it’s probably 3-4 years before that’s finished and published.

7) What is the maximum building height in which a mechanical shaft system would work?

In theory there is no particular maximum. It may be that if you’ve got a particularly tall building then you may want to split it into 2-3 sections in terms of the mechanical system.

If you are doing a pressurisation system, then you may want to do exactly the same.

Where you are looking at natural shaft systems, BRE have proven them by CFD up to 100 storeys.

There is no reason why a mechanical system shouldn’t also work up to 100 storeys. You will end up with a larger shaft but practically, for 99% of buildings, there is no real limitation.

8) You mentioned you can use mechanical shafts for day-to-day ventilation; do you need separate fans for smoke and day-to-day ventilation?

That depends; you can use a separate fan as we do in some cases. In other cases we use the smoke fan but we must make sure they are attenuated and inverter driven to give us a generally lower flow rate and much lower noise level as noise is much more critical for day-to-day usage than for smoke control use.

9) Natural shafts are only allowed in buildings of no more than 30m. Are there any circumstance where this can be extended to a greater height?

That is the requirement in BS9991 and BS9999, if you are using those standards. If you are not using those standards and are using BS5588 then there is no problem using natural shafts much beyond that.

If you are using BS9991 or BS9999 then it’s basically down to the designers and building control to decide what is acceptable. Certainly, technically there is no reason why you shouldn’t use natural shafts above 30m, it’s simply that the British Standard committee felt that pressurisation gives better protection and therefore they wanted it in taller buildings.

10) Can you comment on the suitability of a mechanical venting system in regard to protecting refuges in protected common corridors and lobbies? Can the system ensure tenable conditions in these spaces or is the design reliant on accommodating the refuge area within the stair?

If you are using a mechanical ventilation system then I would strongly recommend that the refuge should be within the stair rather than within the lobby or corridor. Even if you’ve got an enhanced mechanical system intended to clear the lobby very quickly or the corridor very quickly, the lobby or corridor will get smoke into it every time the door from the fire room is opened and therefore there will be periods when it will be relatively smoky. So if you’ve got a refuge and you want to use a mechanical shaft system then you should put that refuge within the stair.

11) With a mechanical shaft system, what size ventilator is required from the lobby to the shaft and at the top of the stair?

From the lobby into the shaft, the size will be fairly dependent on the choice of the type of ventilator you use and the manufacturer’s guidance. Generally we would be looking at something with a face area in the order of 0.6 or 0.8m² for a Colt system. For the ventilator at the head of the stair, we would normally recommend 1m² free area in compliance with ADB so that if the mechanical system does fail you’ve still got a compliant ventilator at the head of the stair. We don’t actually need it to be quite that big when the mechanical vent systems working but it’s just an extra safety feature.

12) Is it possible under the BSEN to look at lower over pressures based on the provision sprinklers, similar to the approach taken by NFPA92?

There is no provision within EN12101-6 for lower pressures depending on whether you have sprinklers or anything else. It’s just a fixed rate and in that respect we do differ quite considerably from the American approach. There is no reason why on an individual job you couldn’t take a fire engineered approach and by CFD or other methods get building control to accept a lower pressure as an acceptable design. But if you do that you are not actually EN12101-6 compliant.

13) As I understand it, shaft systems are designed using a 2-2.5MW ultra fast design fire. Can you please comment on the sensitivity to other design fire sizes, particularly in non-sprinklered buildings where a larger fire is realistic?

Yes, in most cases people do design on fires up to 2.5MW. This generally applies to residential buildings because we have relatively small compartments. This fire size can apply to sprinklered office accommodation and may apply to cellular un-sprinkled office accommodation. Certainly if you have open-plan office accommodation which is unsprinkled then we wouldn’t design for a 2.5MW fire. In fact for the Colt shaft system we have actually developed a version which is based upon on the 12MW fire which is recommended in BR368. So yes, if you have unsprinklered open-plan then a 2.5MW fire is not enough.

14) Isn’t it possible to get a reverse flow of ventilation in a natural shaft in cold climates?

Generally you shouldn’t. The shaft itself normally has a ventilator at the top which is closed and therefore it’s exposed to the outside for a very small area and it’s exposed to the interior through the shaft and through the vents for quite a large area, so the shaft itself should not get anywhere near the cold external air temperatures. Certainly, if it’s very cold outside when you start the system you will have a column of what is probably cooler air than inside. But of course, you’ve also got the buoyancy of the heat from the fire which will over come that.

15) Is there a maximum air velocity on smoke outlet openings in a smoke shaft system?

In a smoke shaft system there is no particular maximum air velocity. The shaft system itself is generally sized to give a reasonable balance between saving space and not creating too much fan power. So you tend to end up with a discharge velocity which is probably in the region of 5-10m per second. That’s the norm but there is no particular reason why you can’t go outside that if you’ve got a particular reason.

16) Can you comment on provision of the plant for a mechanical shaft at the base of the smoke exhaust shaft rather than the head?

That’s perfectly possible to do. When we look at it, the fan itself will have quite a high ability to overcome pressure losses and the buoyancy pressures of smoke are relatively low, at perhaps 10-20Pa. The fan is probably rated at 300-400 Pa.

So, yes it’s perfectly possible to do a design with the fan at the bottom or middle of the shaft if you wanted to. The only disadvantage there is that you are possibly discharging at ground level and you need to make sure your discharge location won’t affect people escaping from the building or lead to smoke being re-circulated back into the building.