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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Technical Advice => Topic started by: kurnal on May 01, 2007, 08:36:11 AM
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I was reading the BRE research report summary.
http://www.bre.co.uk/adb/section.jsp?id=555
The findings of the study stress the importance of inlet or make up air in conjunction with natural ventilation (ie AOVs) and for mechanical ventilation systems.
But neither Approved Document B or BS5588 part 1 refer to inlet air at all, whilst being very specific on the dimensions of AOV required.
Please can anyone cast any light on this?
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Thats what the vent at the head of the stairs is for
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I always understood that the vents in the stairs - which may be openable windows- were there for fire service use rather than as part of the arrangements to keep the corridors tenable? This was explicit in CP3 Chaper 4 part 1 and I think has continued as percieved wisdom since..
Theres a couple of schemes I am looking at at the moment- just out of critical interest really- one newly built and approved of 5 floors - has extended dead end travel distances of 18m and a single staircase, half way along the dead end corridor it has a 0.5 sq m fan assisted vent and the only inlet air is using the stair as a plenum. But the system will only have inlet air if the stair door is open and if the fire is in a flat within the 9m dead end beyond the smoke vent I cannot see how it is supposed to work.
Another based on 1.4 sq m AOVs into smoke shafts has a core stair with a 1sq m roof vent. Again inlet air will only be available if the door to the stairs is open and the BRE research appears to stress the importance of the make up air being at a low level. The concluding remarks of the report are that "an AOV can provide acceptable tenability ..........(b) if vents to the outside at the bottom parts of the end walls are installed".
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Wee Brian Usually when you require natural ventilation of an area you ask for high and low ventilation the high is outlet and the low inlet. If that is correct surely then the vent at the head of the staircase is an outlet not an inlet?
Kurnal I agree fully with your posting but we always required permanent vent at the head of a staircase if it was only for fire brigade use then we would have asked for control over these vents. It may have been naive but I think the concept was a vent at the head would keep the staircase clear of smoke for occupants and fire brigade.
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The guidance in the new ADB for smoke control in the common parts of flats is very different to older systems such as the ones given in CP3 and 5588 part 1 (neither of which work very well).
The new approach is designed to produce a pressure difference between the stair and the corridor. This prevents/reduces smoke movement. The vent at the head of the stair is an inlet (trust me on this) thats why there is a requirement for an interlock between the corridor vents and the stair vent.
The New AD does provide for a vent at the head of small blocks that acts as an outet. This is where there are no common lobbies or corridors. Its better than nothing but only just.
However - if you are looking at an alternative solution that is intended to clear the corridor then you will need low level make up air just like a little shopping centre.
I've come acoross these designs that rely on the stair door being open - this is rubbish.
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Thanks Wee B
Point taken on the old CP3 and 5588. I think the way I read the BRE research document it appeared to point me towards the traditional solutions, and being lazy I read the summary text rather than the sqiggles and diagrams in the report which appear to be presented without a clear explanation of what they are supposed to represent.
Do you know of any document that can be used as a source of reference explaining the new approach?
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Kurnal,
The detail is in the appendices. The work is an extension of the 2002 report by Harrison and Miles. It does not say who the BRE staff are who did this work - I know Roger Harrison left for 'down under' a year or two ago; Stuart Miles is probably involved on the computing side.
Which copy of ADB were you looking at? I haven't yet seen the new edition but would expect that the new one would give more information.
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There isnt really an "idiots guide" available. (not one that is any good anyway) It will take an awful lot of reading between lines to link the BRE report to the guidance in the AD.
The BRE report is a study that looks at all the options and how they work in different conditions. It would be a usefull reference when considering alternatives but it isnt a commentary on the ADB approach.
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wee brian
I assume you are refering to 2.27 in ADB..2007....but that para relates to mechanical ventilation, and as far as I can determine there is partically no change for natural ventilation from the old ADB to the new.
Conqueror.
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Conqueror - read it again s l o w l y
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wee....
I will indeed !!
Conqueror
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Good man
If you still cant see any difference then I'll explain it.
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Hi Wee Brian
How would you feel about explaining hw the new approach actually works?
I believe what you say of course but I do like to understand it as well.
I accept the objective is to keep the stair clear by ensuring that the adjoining lobbies or corridors are at a slightly lower pressure so any flow is from stair to corridor.
I would have expected a vent at the head of the stair to be an outlet because of the stack effect- the air temperature at the top will be warmer than than the air outside so warm air will flow out of the the vent and cooler air from outside will enter at a lower level if it can get in. And if make up air cant get in then the outflow will be affected and reduced. So I cant see how a vent at the top can be an inlet?
The AOV in the corridor is there to ensure that any bouyant hot smoke vents to outside rather than building up a pressure and passing into the staircase, and as a bonus the smoke layer in the corridor will also be kept at a higher level making conditions a little more tenable than otherwise. I also accept the BRE report and parameters based on a 250kw fire and a flat door slightly ajar even though I am not convinced that this is the most likely scenario. In many cases I think the smoke will be cooler.
Whilst the objective is not to clear the lobby of smoke, only to reduce the pressure that would otherwise push smoke into the stairway, surely the AOV will only be partially efficient because without inlet air the best it can achieve is a neutral plane, there will be inertia, pulsing, and if the outflow is substantial it is bound to entrain air against the flow increasing the smoke volume and causing smokelogging further back in the lobby.
There is no flow from the stair into the lobby unless the door is open.
Now, please Brian where have I got this wrong?
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Kurnal I agree with all you say in the previous posting but you only need the ventilation during the evacuation phase of the fire floor. During this time the door will be open for considerable time changing the dynamics of the system. Because the ventilation duct is a minimum of 2.5m above the vent in the staircase enclosure then the flow of air should be from the vent into the stair, from the stair into the lobby, from the lobby up the duct to atmosphere keeping the staircase enclosure clear and reducing the density of the smoke in the lobby. When the evacuation is over and the door is closed the system will revert to your scenario removing any smoke that may have entered the stair case enclosure.
Go on load your ack ack guns and shoot me down.
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TW
Thanks for the explanation, it does sound feasible, I do have to keep pulling myself back to the concept of only protecting conditions in the staircase and not the corridor.
So it goes against the grain but if we accept that conditions in the corridor will only start to improve to any significant effect once the stairway door is open then yes it may work to some extent. I do accept Wee Bs point about the historic cross ventilation system not working, especally if the wind was too weak, too strong or in the wrong direction.
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I agree and await WB`s explaination of the new approach.
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If he is kind/patient enough to offer one!!
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As I know that Wee B is a very busy man I thought I’d add my two-penneth, as, in a previous life, I had something to do with the ‘new’ concept.
The AOV at the top of the stair only acts as an inlet when smoke from the lobby/corridor is ventilated to a smoke-shaft which is sealed at the base (either naturally or mechanically driven). This is due to the stack-effect (or pressure drop due to a mechanical fan) in the smoke shaft causing the lobby/corridor to be under reduced pressure. This will cause air to be drawn from the stair to the lobby, provided there is some form of inlet in the stair. There will be a flow from the stair to the lobby when the stair door is open, preventing smoke from entering the stair at all using the concept of depressurisation….and when the stair door is closed, there will still be a sufficient pressure difference across the stair door preventing smoke from entering the stair via any possible gaps around the door.
The bottom line is;
door to stair open = stairway clear of smoke
door to stair closed = stairway clear of smoke
Of course, if another source of inlet air is provided to the stair…say the Fire Service prop open the door at the base of the stair (which leads to external air), then the inlet air may well come from there instead, and the AOV at the top may be ‘neutral’, it is dependent on the location of the fire. The inlet air will just take the path of least resistance. The AOV at the top of the stair provides inlet air (similar to a U-tube shaft) prior to the stair being accessed by the Fire Service/or occupants so that the system works from detection.
This concept worked for ‘cool’ smoke as well as ‘hot’ smoke, as the magnitude of stack effect required (for naturally ventilated shafts) was much less to prevent smoke ingress to the stair for ‘cool’ smoke. Of course, mechanically ventilated shafts will deal with most temperatures of smoke (in line with the temp rating of the fan), provided that door opening forces are not exceeded.
The main aim of this approach is to keep the stair clear of smoke. Conditions in the lobby/corridor are generally untenable when the door to the fire room is open (when the stair door is either open or closed), and conditions are not always much better for smaller quantities of cooler smoke. Because of this, the ‘new’ approach focussed on protecting the stairwell.
If you want to provide protection to the lobby/corridor as well, you really need to provide some form of dedicated smoke control system for that area (this is where high level vents and low level inlets are required).
When smoke is ventilated from the lobby/corridor via an external window, the pressure difference between the lobby/corridor and the stair is not large enough to prevent smoke from passing into the stair when the door to the stair is open (there is no stack effect). So the vent at the top of the stair WILL act as an outlet in this case.
Well……that’s probably as clear as mud….but I did try.
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Thanks Crusader that really is useful.
I now have a glimmering of understanding (or do I delude myself?) and I guess to add my two penn'orth it remains important that the smoke shaft serving the AOVs to the lobbies is higher than the head of the stairs (As per the 2 m clear of the roof in ADB) otherwise as we get higher in the building then the stack effect will diminish and we will find a neutral plane for a given smoke temperature and from then upwards the AOV will start behaving similar to a window.
Take as a hypothetical case a staircase with a large glasshouse style feature at the head of the tower staircase serving say 5 or 6 floors in which the temperature at the head of the stairs on a sunny day will get fairly high (unless for environmental control the vents are made automatic linked to temperature as well as to the smoke detection system.)
I perceive it likely that, especially for a fire in the upper floors, on operation of the fire detection system and the opening of the vent the initial stack effect in the staircase will cause a surge of air upwards in the staircase, this may exceed the stack effect of the AOV and it is the smoke shafts that may act as the inlet?
The ADB mentions that the either the vents should open simultaneously or that staircase vent may open just before the smoke shaft AOV which common sense would suggest may help to avoid the reverse movement of smoke.
Or do I still have it all round my neck?
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Just a small question then, what happens is external weather patterns disrupt the air flow?
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Kurnal
You are correct, if you want to keep the stairwell clear on all storeys, the extra 2 m shaft height is provided to generate sufficient stack effect to achieve this. If the shaft does not extend clear of the roof, then conditions are similar to those achieved when venting via and external window, which does not always keep the stair entirely clear of smoke. Ventilation to a smoke shaft generally provides improved conditions in the stair compared to external wall ventilation.
As for a possible reverse flow due to solar gain, you are correct again, probably the most important recommendation in the original (Fire-Fighting shaft) work was that the AOV serving the shaft must be as high as practically possible in the lobby/corridor to generate the stack effect in the shaft early, the bottom of the AOV MUST be at least be a high as the top of the door serving the stair, otherwise the stack effect may be generated in the stair first. The original work examined conditions in the stair when subjected to environmental effects by simulating 'adverse' pressure differentials tending to draw smoke from the lobby/corridor into the stair (either by external wind pressures or solar gain). The recommendations given in BRE report 79204, took into account these adverse conditions so that the system performed effectively. I assume this was also done for the common corridor work too...but I'd have to check. I suppose the advantage of a mechanically driven shaft is that adverse environmental effects are mininised.
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Very ocasionally there are some really valuable contributions made to the forum, when someone takes the time and trouble to set out in a straightforward and concise way something that would otherwise be shrouded in mystery and conjecture. Thankyou Crusader and we hope you stay around.
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Well, I'm glad it was of some use...but saying that, I've just spotted a 'deliberate mistake' in my last posting, it really should say that the top (not the bottom) of the AOV serving the shaft must be at least as high as the top of the door serving the stair. It was found that 'door' type (tall and narrow) vents worked just as well as 'letterbox' (wide and shallow) type vents.