Mechanical Extraction from Car Parks

[GB]

ADB 12.7.d states that extract points for a mechanical extract within a car park should be arranged so that 50% of the outlets are at high level and 50% low level.

What is the theory behind this? I would have thought that the extraction system would try and best use the buoyancy of the fire effluent therefore multipe extraction points at high level only?

I would appreciate if anyone can explain this logic a bit further for me.

[Midland Retty]

I stand corrected but I think in theory buoyant gas will be extracted by the high level vents, the less bouyant, denser gas by the lower level vents.

So in short low level vents are employed to extract less buoyant / denser gases.

[GB]

Many thanks Midland

I understand the theory of that.

Trying to visualise that however, would the velocity of the gases and entrainment of the air keep the gas up in the higher level - picturing a car on fire within an enclosed space - i have only really seen quite a high neutral plane (unless there were excessive downstands to force the stream downwards)

[John Webb]

Are the low-level extracts to remove fuel fumes if a vehicle starts to leak petrol or diesel from it's fuel tank or engine, rather than for fire gases?

[CivvyFSO]

Remember that you are essentially looking at a clearance system GB, often operated at the discretion of the fire service. Neutral planes, flowing layers etc will have very little to do with it. You will more than likely have a room completely full of smoke, from high to low level.

It is something that is often ignored nowadays, since impulse fans clearly only seem to move the smoke at high level, but their efficiency to move the whole volume has been proven via real tests, and CFD.

Also, remember that part F needs to be complied with, and this will involve the removal of CO etc. The two things are quite interlinked.

[GB]

I was of the opinion that the impulse fans effectively replace extracting ducting by moving the smoke around the compartment but essentially the extraction fans are still required? Is this not the case?

[CivvyFSO]

Yes, you still need extract with impulse fans. But they are rarely (read: never, so far, in my experience) in the form of high level & low level. The systems will quite often revolve around one very large set of extract fans.

[wee brian]

save youself some grief and read the current version of ADB

[CivvyFSO]

Well spotted WB, would seem to be 11.6 in ADB 2006. Same requirements though.

[GB]

Apologies Wee Brian - should have said on Isle of Man looking at ADB 2000 so it is the correct reference but as said by last post, same requirement in ADB Volume 2 item 11.6.d