Mechanical smoke extract system for small lobby adjacent to staircase in flats

[kurnal]

I have been asked if it is posible to source a smoke extract system with a smaller duct size than the Colt system- this is for a very small lobby adjacent to a staircase where there is insufficient space for even the 0.6m colt shaft. Not found anything on google.

The lobby is very small - only doors are the staircase, the corridor and two flats. It is a multi staircase existing building being converted to flats. No problem with natural ventilation generally, other than for one staircase.

Does anybody know if there is anything smaller on the market please?

[Midland Retty]

Im not aware of anything 'off the shelf', so to speak, thus you may have to look at a bespoke system.  What available size have you got to work with ?

[kurnal]

We could accommodate a duct of say 1200mm by 400mm. I read some guff some time ago from a London company together with a case study that mentioned a very small 600x600 duct but they seem to have dissapeared.

[Midland Retty]

Are you looking for fan to be inside the fire zone (inline with ducting) or could you get away with the fan being outside of the "fire zone" ?

You need to consider flow rates, and base your ducting size / fan selection on that. 

Vent Axia, Sodeca, Nu-aire to name a few sell a range of S.E. fans in different sizes, shapes, flowrates etc. Give them a try they may be able to advise if they have off the shelf complete kits which include fan, ducting, and accessories.

[wee brian]

if the duct is too small then you get high velocities - weird and unhelpful swirly things happen to the smoke

[colin todd]

"Swirly things". I think that the term ought to be included in AD B.  Best technical term I have heard since Peter Hinckley used to talk about "all the stuff" as a description of smoke and hot gases.

Wasnt it to avoid swirly things that they introduced the variable speed limits on the M25.

[kurnal]

I wonder if swirlygigs would matter in this scenario? I assume from my dim recollection of the BRE study into the ventilation of communal areas of flats that the purpose of the ventilated lobby is to ensure that the staircase remains clear of smoke. Any positive pressure between the flat on fire and the staircase can be vented to outside via the lobby vents and make up air can be sourced from the staircase (and of course from the flat on fire if the door is open). So long as there remains a slightly negative pressure differential between the staircase and lobby, provided this small differential doesnt impede persons wishing to open doors, the staircase will be protected from the ingress of fire and smoke.

I have always assumed that the reason the BRE calculations were based on doors being ajar by about 100mm was that this is due to the firefighters 90mm hose being used during the intervention phase, and passing through doorways holding them open. Is this a correct assumption?

[wee brian]

yes and yes, thats what I remember.

(yes- i think the swirly thing isnt a problem for this app) (yes that's where the 100mm came from -I think)

my worry with these things was how you get a pressure difference across the stair/lobby door without sucking smoke in from the fire room/flat.

passive solutions dont have the same risks. but need a bigger shaft.