Xtralis VESDA VLC-505, Manual

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VESDA by Xtralis VESDA Pipe Network Installation Guide
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5.2.3 Duct Sampling and Condensation
Duct sampling allows you to monitor the return and supply air within the protected enclosure. The sampling
pipe is connected to a sampling probe to sample the air from within the duct. To avoid the potential effects of
pressure differentials the air is exhausted back into the duct through an exhaust probe. The exhaust probe
holes must also face the same direction as the intake probe holes.
The detector must be installed in the inverted position when sampling air from ducts. This is done because
condensation may occur when the dew point temperature of the duct air is at or above the ambient
temperature of the detector. This usually occurs when the temperature of the humid duct air is higher than the
ambient temperature of the detector.
In this situation, longer pipe runs outside the duct should be used. Initial regular inspections must be carried
out to determine whether there is condensation forming inside the pipes, especially during the winter season.
Smoke tests should be conducted regularly in the first two months of installation.
Duct Installation Considerations
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In air-return ducts, the detector must be installed where the static pressure is not below -500Pa and the
air duct velocity is not above 20 m/sec (4,000 fpm). Static pressure is the pressure generated by the
fan to deliver a specific flow-rate in the duct. Static pressure is a function of the flow-rate and
impedance of the duct system. Convert to volumetric flow rate.
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Always vent the exhaust pipe back into the duct even when the duct pressure is sometimes very small
(e.g. less then 20Pa). This is because the duct system operation may change in the future.
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In air-return (negative pressure) ducts, the detector must be installed away from any fans and closer to
air vents; this reduces large negative pressures.
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For air-return (negative pressure) ducts, the detector must be installed before humidifiers, heating coils,
and filters.
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In air-return (negative pressure) ducts, the detector must be installed before fresh air intakes.
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Do not sample from multiple ducts (i.e. use a different detector for each duct).
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Do not sample duct and ambient environments together.
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For a detector mounted on the duct, ensure no tangible vibrations exist when the duct system is in full
operation. If vibrations exist, insert visco-elastic foam between the detector’s mounting bracket and
the external surface of the duct.
Probe Installation
The differential pressure across the inlet and exhaust pipe needs to be kept within ±10Pa in the presence of
airflow, regardless of duct air velocity.
Differential pressure is created by the orientation of the holes on both pipes in relation to the duct flow. This
differential pressure across the inlet and exhaust pipes will either aid or impede the flow through the detector.
A very low differential pressure ensures the normal operation of the detector. To achieve this, perform the
following steps:
1. Ensure the duct system is operating and airflow is present in the duct.
2. Drill holes in the inlet and exhaust pipes as required, draw a line along the holes to the end of the pipe.
3. Insert the pipes in the duct, and rotate the pipe until the line indicates that the holes are facing into the
airflow
4. Using the Manometer testing (Figure 5-11) or flexible U testing (Figure 5-12) methods to minimize the
pressure differential.
5. When you are happy with the pipe orientation, mark the position of the pipes’ orientation on the duct
and pipes.
6. Secure the pipe installation and run pipe to the detector.
Face the holes of both pipes to the airflow and slightly rotate the pipes so the pressure reading is within
±10Pa. Mark the position of the pipes’ orientation on the pipe and the outside of the duct. Connect the detector
after securing the pipe network.