Smoke ventilation of common areas of flats- BRE report

[kurnal]

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?

[wee brian]

Thats what the vent at the head of the stairs is for

[kurnal]

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".

[Tom Sutton]

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.

[wee brian]

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.

[kurnal]

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?

[John Webb]

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.

[wee brian]

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.

[Martin Burford]

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.

[wee brian]

Conqueror - read it again s l o w l y

[Martin Burford]

wee....
I will indeed !!
Conqueror

[wee brian]

Good man

If you still cant see any difference then I'll explain it.

[kurnal]

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?

[Tom Sutton]

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.

[kurnal]

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.