Fire alarm spacing and siting of fire detectors

When designing a fire alarm, the heat and smoke detectors installed, depend on convection to transport hot gases and smoke from the fire to the detector.
When designing fire alarms, spacing and siting of these detectors needs to be based on the need to restrict the time taken for this
movement and to ensure that the products of combustion reach the fire detection in adequate concentration. In a building, the hottest gas and the greatest concentration of smoke will generally form at the highest parts of the enclosed areas, and it is here, therefore, that the fire detection need to be sited.
As the smoke and hot gases from a fire rise, they become diluted with clean, cool air, which is drawn into the plume. Hence, the size of fire required in order to operate heat or smoke detectors increases rapidly as the height of the ceiling above the fire increases. To some extent, this effect can be countered by the use of more sensitive detectors.
fire alarm system, incorporating optical beam detectors are less sensitive to the effects of ceiling height than are point-type detectors, since the increased size of plume will involve a greater proportion of the path length of the optical beam and help to alleviate the effects of reduced smoke density. In addition, the entrainment of air cools the gases. If the ceiling is high and the ambient temperature in the
uppermost areas within the protected space is high, the plume of smoke and hot gases may reach ambient temperature before reaching the ceiling. If the temperature of the surrounding air increases with height, (e.g. as a result of solar gain), it is possible for the air at the uppermost levels to be at a higher temperature than that of the plume. The plume will then spread out to form a smoke layer before it reaches the ceiling, as though there were an “invisible ceiling” at a specific height within the protected space. This is known as
stratification, and, at this stage of the fire growth, the smoke and hot gases will not operate ceiling mounted detectors, regardless of their sensitivity.