Alertness and Warning systems –fire Safety

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MIRROR: G  K Bhat
For the efficiency and effective function of any equipment, it has to be placed scientifically. Therefore we should know the basic functions and technique of such equipment in detail. There are few guide lines made for this , so that even a person who do not have technical knowledge of these equipment still if he follow the guidelines , one can install the system with accuracy. Let us understand the where the beam detectors are effective and why .
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Beam Smoke Detectors vs. Spot-Type Smoke Detectors : Even though beam and spot-type smoke detectors are governed by the same UL and NFPA standards, the requirements under these standards differ because the principle of their operation differs. It is important that designers understand and give full consideration to these differences when selecting and applying smoke detectors to fire alarm systems. Spot-type smoke detectors are considered to have a maximum coverage of 900 sq. ft. or 30×30’..
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The maximum length between detectors is 41 feet when the width of the area being protected does not exceed 10 feet, as in a hallway. Beam smoke detectors generally have a maximum range of 330 feet and a maximum distance between detectors of 60 feet. This gives the beam smoke detector theoretical coverage of 19,800 sq. ft. (Figure 2). Manufacturer’s recommendations and other factors, such as room geometry, may impose practical reductions of this maximum coverage. Even with these reductions beam smoke detectors can cover an area which would require a dozen or more spot-type detectors.
 
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Fewer devices mean lower installation and maintenance cost. A spot type smoke detector’s response time is generally increased as its distance from the fire/floor increases. When ceiling heights exceed 16 feet the designer should consider whether the spacing of spot-type detectors should be decreased. Coverage Ceiling Height This is not necessarily the case with beam smoke detectors, which are ideally suited for high ceiling applications. Some manufacturers allow increased coverage as the ceiling height increases. This is because of the anticipated behaviour of a plume of smoke. While not all fires start at the lower elevations of the hazard, at or near the floor level, this is a typical fire scenario. When this is the case the smoke produced by the fire will rise to, or near the ceiling. Typically the column of smoke begins to spread out as it travels from its point of origin, forming a smoke field in the shape of an inverted cone. The density of the smoke field can be affected by the rate of growth of the fire. Fast fires tend to produce more uniform density throughout the smoke field than slow burning fires where there may be dilution at the upper elevations of the smoke field. In some applications, especially where high ceilings are present, beam smoke detectors may be more responsive to slow or smouldering fires than spot-type detectors because they are looking across the entire smoke field intersecting the beam. Spot-type detectors can only sample smoke at their particular “spot”. The smoke which enters the chamber may be diluted below the alarm.
Note: During our discussion of the topic we will be coming across with the words like NFPA and UL standards. These are some standards accepted globally. We will discuss the same in our future topics separately
Will be continued…
G K Bhat
[email protected]

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