Chapter 2: Installation and configuration
LaserSense Nano Aspirating Smoke Detector Installers Handbook 13
Return air duct sampling method
Duct sampling generally is the most cost-effective method of air sampling
because the pipe runs are minimal and a single detector may be used to cover a
larger area. The speed of response of the detector to smoke is given by the
exchange rate in the rooms ventilated by the duct ventilation system. This tends
to be rapid, giving early warning of any smoke present. This type of sampling is
particularly suited to aspirated smoke detection, since the smoke content in the
air will tend to be diluted to a level below that of point type detectors. Also, the
relatively high airflow in the duct reduces the effectiveness of point-detection
devices.
The duct sampling method does have one major disadvantage. If the ventilation
becomes inoperative, the air-flow through the duct system ceases and the
smoke-detection system becomes ineffective.
Figure 6: Return air duct sampling
(1) 1/4 to 1/3 of duct width
(2) 45 degree
(3) 11.81 in. (300 mm) minimum
(4) 45 degree
(5) 2/3 to 3/4 of duct width
(6) Direction of air flow
(7) Intake pipe to detector
(8) Exhaust pipe from detector
Figure 6 above shows a typical sampling pipe arrangement for an air duct. The
right pipe is the sampling pipe and the holes on it are drilled 4 inches apart and
face into the oncoming air stream. The left pipe exhausts air from the detector.
The detector is UL 268A and CAN/ULC-S529 approved for duct applications with
an operating air velocity range of 300 to 4000 ft./min (1.52 to 20.32 m/sec).
The following guidelines apply:
• Only one duct can be monitored per detector.
• If the air sampling pipe system and aspirating detector is used as the primary
smoke detection system, methods should be employed to notify stoppage of
airflow in the ducts.
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