Xtralis VESDA VLC-505, Manual

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VESDA by Xtralis VESDA Pipe Network Design Guide
www.xtralis.com 19
4.6 In-duct Sampling
In a fire event, ventilation duct systems can convey smoke; usually hot toxic gases and flames from one area
to another. Duct systems may also supply air to aid combustion in the fire location. Therefore, the effective
management of smoke control, such as dampers and shutters to contain smoke spread and fire growth is
essential for life safety.
VESDA VLC models VLC-500 and VLC-505 are recommended for duct applications. Xtralis has conducted
extensive testing in duct environments to determine the optimum installation parameters.
Key Design Considerations
Sampling tubes should be oriented to overcome thermal stratification due to buoyancy of the smoke in the
upper half of the duct. This condition occurs where duct velocities are low, buoyancy exceeds flow inertia, or
the detector is installed close to the fire compartment. A vertical orientation of sampling tubes overcomes the
effects of differential buoyancy.
Where a detector is installed on a duct serving a single fire compartment, where the buoyancy exceeds the
flow inertia of the air in the duct and the sampling tube cannot be oriented vertically, then the effects of thermal
stratification can be minimized by locating the detector sampling tube in the upper half of the duct.
Thermal stratification due to buoyancy of the smoke occurs where duct velocities are low, buoyancy exceeds
flow inertia, or the detector is installed close to the fire compartment. This condition occurs in the upper half of
the duct. The effects of differential buoyancy are overcome by vertical orientation of sampling tubes. In a
single fire compartment, the detector sampling tube is located in the upper half of the duct to minimize the
effects of thermal stratification.
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The inlet pipe must be inserted at a distance between six to ten duct widths (width is considered the
longest side for oblong ducts) or diameters from any disturbances to the flow generated by sharp bends,
plenums, nozzles, branch connections, etc.
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The inlet and exhaust pipes must have the same length. They must be sealed at the far end with an end-
cap.
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The holes on the inlet and exhaust pipes should be facing the airflow as shown in Figure 4-20. Holes with
the same orientation eliminate unwanted flow faults associated with cyclical operation, maintenance or
power failure of the duct system. However, in some industrial applications where the quality of air inside
the duct is poor, it is recommended to face all holes on the inlet and exhaust pipes downstream (i.e. 180°
to incoming airflow).
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The pipes should always be supported at the duct walls by using fittings such as a rubber grommet.
Appropriate sealant must be used to ensure an airtight seal.
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Make sure that the sampling hole at either end is at least 50 mm (2 in) from the duct walls.
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The exhaust pipe must have 4 x Ø10 mm (21/32 in) holes. The holes should be concentrated in the
middle of the duct’s width and spaced accordingly.
Small Duct: Width < 1 m (3 ft)
Figure 4-19 shows a side view of a duct section with the insertion positions for the inlet and exhaust pipes.
The inlet pipe is to be installed in the middle of the duct height (H) or diameter. The exhaust pipe should be
inserted approximately 0.5 m (1.64 ft) further downstream at a quarter of the height of duct.
Legend
A Exhaust Pipe
B Inlet Pipe
C Air flow
Figure 4-19: Small duct sampling - side view