Mechanical Smoke Extract Systems to Protect a Fire Fighting Stairwell

[DVM]

Hello,

I am an Electrical Engineer and require to carry out a Fire Engineering module and need your help.

One of the tasks in my assessment is to use Branzfire to analyse a typical high rise building with a firefighting stairwell with a lobby on each floor and a smoke extract system off of the lobby area. I have carried out various tests unsing natural vent, which is ok but the confusion for me is when using a mechanical extract system. The fire is 5MW and I am assuming that the earlier simulatin tests where 0.282kg /s for the mass as shown in the earlier results is what the extract vent should be sized to / or should it be the plume mass flow?

Also Colt has a website where they claim to be able to reduce the standard 3m2 shaft to 20% of this value?

Any help would be greatly appreciated as my knowledge in this field is very limited .

[John Webb]

I regret that as I have been retired for a few years I'm not totally up to speed on current standards or on what Branzfire is. But I think you need to look at the relevent part of BS5588 or possibly the BS on Fire Safety Engineering. As I recall, it is not the total products of combustion of a 5MW fire that need to be extracted from the lobby but only possible leakage? And there is the alternative of positive pressurisation of the staircase/lobby to consider as well as extraction. This latter method has positive benefits for the firefighter.
(I'm ex-Fire Research Station - used to work with Dr H Morgan on sprinklers and smoke extraction.)

[jimi902]

I have completed a similar coursework this year. As far as I can rememer I used an arbritary 20m3/s.

For further information:

Smoke Shafts protecting Fire-Fighting Shafts:

their performance and design, (Harrison And Miles) 2002.


http://projects.bre.co.uk/frsdiv/smokeshafts/

[wee brian]

The smoke shafts that H&M looked at were naturally ventilated stacks. They work by generating a pressure differential between the stair and the lobby so that smoke does not pass into the stair.

Assuming your mechanical system is intended to provide the same performance then the fire size is irrelevant.

As John W says, if you want a mechnical system then pressurisation is better.