Bosch D296 Application Manual page 3

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2.0 Applications
2.1 Coverage
Generally, the D296/D297 Series provides smoke
detection in large areas with open or high ceilings.
Figure 1 shows the D296/D297 Series coverage range of
up to 350 ft (106 m). On a smooth flat ceiling, you can
place the beams up to 60 ft (18 m) apart, allowing one
D296/D297 Series to replace up to 24 spot smoke
detectors with 30 ft (9 m) spacing.
You can install beam detectors vertically or at any
angle needed to allow protection of the hazard
involved, such as vertical beams through the open shaft
Figure 1: Coverage Range
30 ft
(9 m)
30 ft
(9 m)
2.2 Stratification
Section B.4.6.1 of NFPA 72, National Fire Alarm
Code (2002 Edition) states: "The potential for the
stratification of smoke is another concern in
designing and analyzing the response of detectors.
This is of particular concern with the detection of
low-energy fires and fires in compartments with high
ceilings."
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.
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
(Figure 2). 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
Bosch Security Systems, Inc. | 01/07 | F01U500951-01
D296/D297 | Application Guide | 2.0 Applications
area of a stairwell where there is a clear vertical space
inside the handrails).
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?
350 ft
(106 m)
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.
Figure 2: Stratification
1
3
1 - Projected beam
2 - Hot fresh air
3 - Stratification
60 ft
(18 m)
2
3