Bosch D296 - Stratification; Environmental Considerations; Outdoor

Before installing beam smoke detectors , consider the following:
– Is there a clear line of sight between the transmitter and receiver at all times?
– Might objects such as signs or boxes be placed in the beam’s path in the future?
– Will moving objects such as traveling cranes or forklift trucks enter the beam path?
– What type of fire might occur?
– Would air movement or obstacles prohibit smoke from reaching the detectors?
Stratification
Depending on a number of environmental conditions, smoke can stratify as it rises. The
smoke’s upward movement in the plume depends on the smoke’s buoyancy relative to the
surrounding air. Stratification occurs when the smoke or hot gases flowing from the fire fail to
ascend to the smoke detectors mounted at a particular level (usually the ceiling) above the
fire due to lost buoyancy.
See the following figure:
Figure 3.2: Stratification
1
Projected beam 3 Stratification
2 Hot fresh air
This phenomenon occurs because of the continuous entrapment of cooler air into the fire
plume as it rises, cooling the smoke and fire plume gases. The cooling of the plume reduces
buoyancy. Eventually the plume cools to a point where its temperature equals that of the
surrounding air and its buoyancy is reduced to zero. Once this point of equilibrium is reached,
the smoke stops flowing upward and forms a layer. It maintains its height above the fire,
regardless of the ceiling height, unless and until the fire provides sufficient additional thermal
energy to raise the layer resulting from its increased buoyancy. The maximum height the
smoke ascends, especially early in the development of a fire, depends on the fire’s convective
heat release rate and the compartment’s ambient temperature.
Environmental Considerations
Outdoor
Do not use the D296/D297 detectors for outdoor applications. Environmental conditions such
as temperature extremes, bright sunlight, rain, freezing rain, snow, sleet, fog and dew can
interfere with the proper operation of the detector. Outdoor conditions also make smoke
behavior impossible to predict.
3.2
3.3
3.3.1
6 en | Document Introduction Projected-beam Smoke Detectors
2013.02 | 02 | F.01U.500.951 Application Guide Bosch Security System, Inc.