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OPERATION
When the 8840/8839 (8840 or 8839) is operating, a sample of air is drawn
from the duct and passed through the sampling chamber by means of the
inlet sampling tube. The air sample passes through the smoke detector
mounted in the duct housing and is exhausted back into the duct through
the outlet sampling tube.
MOUNTING THE AIR DUCT HOUSING
Location on Duct System
Locate the air duct detector in the main supply duct, downstream from the
filters and positioned so as to operate reliably in case of smoke in any part
of the air stream. In instances where filters are capable of removing
smoke, install detectors both upstream and downstream from the filters.
The air duct detectors use sampling tubes which monitor the full width of
an air duct to overcome the limitations of spot-type smoke and heat detec-
tors in the duct. However, since stratification can occur in the air stream
after a long duct run, locate the detector after bends or inlets which create
turbulence, and hence, a more homogeneous mixture of air. The detector
should, when possible, be located a minimum of six duct-widths downstream
from the source of the turbulence (See Figure 1). A 12 inch by 12 inch ac-
cess hole should be cut in the duct adjacent to the detector to permit
checking and cleaning of the sampling tubes, if necessary.
OUTLET
SAMPLING TUBE
INLET
SAMPLING TUBE
AT LEAST
SIX DUCT
WIDTHS
Locate the air duct detector in the air handling system, as shown in Figure
2 and in conformance with NFPA Pamphlet No. 90A, Air Conditioning
and Ventilating System and with NFPA 72 Detectors. (Both publications
are available from the National Fire Protection Association, Batterymarch
firealarmresources.com
Figure 1
Typical Mounting of Duct
2
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