Any guide or standard gives general recommendations that are designed to provide an adequate level of safety in virtually all cases. Any particular building has its own set of circumstances that can demand slightly different solutions to those given in the standard.
In this building you might choose to determine what the detection is there to achieve. You might want to look at the fire loading in the space below the detectors. Any fire that gives out a decent amount of heat will be detected with only a small delay. Any fire that sits and smoulders with very little heat production will have the detection system at the mercy of the ambient air currents in the space.
In a hall like this, let us suppose that there is some fire loading but nothing that is likely to cause a rapid flaming fire. The most likely fire scenario will start with an incubation period of some (difficult to determine) length of time. The current point detectors are likely not to detect the incubating fire for some time but, if the fire becomes established, then the heat generated will carry the smoke to the detectors. If the fire does not become well established but continues to smoulder and if the ambient air is still then the stratification mentioned earlier may occur.
Now, if you think about what the detection is trying to achieve, you can make a judgement as to whether the current provision is adequate.
Would the undetected stratified smoke layer from the
smouldering fire threaten anyone's means of escape? Would it threaten a protected staircase? Unlikely. Would it threaten a route through the space from an inner room? Possible. (Are the occupants awake or might they be asleep?)
Would the delay in detecting the
fire that becomes established threaten a protected staircase or a route through the space? I'm sure you'd agree that it is very unrealistic to believe that a fire might come anywhere close to threatening a correctly constructed protected route before smoke reaches the detectors. Could this fire scenario threaten the route through the space from an inner room? It is likely that the delay in the detection actuating will be more than off-set by the benefits gained by the high ceiling. But there is the potential for concern here.
If the occupants are awake and if there are no inner rooms why would anyone not notice the fire before their means of escape was threatened.
If the system is for property protection then you have to think about whether you're protecting the contents or the structure or both and about the additional damage that is likely to be inflicted by the delay in alarm actuation.
There is a simple zone model that illustrates the sort of level that stratified smoke attains.
This is really most applicable for larger fires but if we plug in figures for a small fire and a large temperature gradient (say, a 50kW fire and a temperature gradient of 0.3K/m (i.e. approx 5 degrees hotter at detector level than at ground level)) we can get an idea of where the smoke might stratify. This gives about 23m for this fire. Reduce the fire to 25kW and the smoke will attain 19m. Even if the fire is only 10kW the smoke will reach 15m.
This model is not precise but it's a best guess without resorting to CFD.
An aspirating or beam system might be quicker but do the potential gains outstrip the definite costs.
Incidentally, how do you reach these detectors to service them? That might be a significant consideration when weighing up alternatives.
Stu
