Smoke ventilation of common areas of flats- BRE report

[Tom Sutton]

I agree and await WB`s explaination of the new approach.

[kurnal]

If he is kind/patient  enough to offer one!!

[Crusader]

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.

[kurnal]

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?

[jokar]

Just a small question then, what happens is external weather patterns disrupt the air flow?

[Crusader]

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.

[kurnal]

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.

[Crusader]

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.