System Sensor Pipe Installation - Manual (Page 18)

User Guide: Aspirating Smoke Detector Pipe Installation
18
Large Duct Sampling
For ducts with a width of 3 ft. to 7 ft. (1 m to 2 m), two branch
pipes are recommended for the inlets. Inlet pipes should enter
a quarter of the way from the top and bottom of the duct, as
shown in figure 24.
The exhaust pipe should be inserted approximately 18" (0.5 m)
from the inlet pipes and half way up the height of the duct.
To avoid dilution, sampling pipes should be located before
fresh air intakes and before exhaust air output.
Duct Width
Number
of Ports
Port Size
Nominal Pipe
Flow Rate (CFM)
3 ft. 4 in.
(1 m)
6
9/64 in.
(3.5 mm)
1.77 cfm
(50.2 L/min)
5 ft.
(1.5 m)
8
1/8 in.
(3 mm)
1.80 cfm
(50.9 L/min)
6 ft. 6 in.
(2 m)
10
1/8 in.
(3 mm)
2.10 cfm
(59.6 L/min)
Table 5: Port sizes for large ducts.
The information in table 4, on page 17, and table 5, above,
applies to a 15 ft. (4.6 m) inlet pipe and a 10 ft. (3 m) exhaust
pipe. Always check local codes and standards for port size
and spacing.
Port Orientation
Sampling response time can also be improved by avoiding high
and low velocity air flows. Ports on the inlet pipes should be
facing 20-45 degrees from the airflow. The angle of the ports
can be adjusted for complete flow independence from the AHU
by following some simple steps outlined in the Duct Application
white paper on the FAAST website. Ports on the exhaust pipe
should be facing away from the airflow.
For additional information relating to duct applications, refer to
the duct application white paper at systemsensor.com/faast.
Monitoring Voids
In these high air exchange environments, detection systems
should be installed in any void, unless the void is completely
empty and presents no fire risk. If the void contains cabling
and/or equipment that can initiate or contribute to a fire,
monitoring for combustion is highly recommended. The
sampling port spacing for these areas is the same as the
requirement for the rest of the area, in accordance with the
relevant local codes and standards.
When sampling pipes are installed in shallow voids having poor
ventilation, special care should be taken to position the piping
as close to the top of the void as possible. This gives the best
early warning due to the likelihood of the initial smoke layer
taking up only the top 10% of the void height.
A
B
C
20° - 45°
from direction
of Air Flow
Inlet Pipe
Inlet Pipe
Outlet Pipe
3H/4
H/4
H/2
Air Flow
Large Diameter Duct
Figure 25: Large duct sampling.
Figure 26: Port orientation.
A. Airflow Streamlines
B. Low velocity (high static pressure) area
C. High velocity (low static pressure) area