System Sensor FAAST Comprehensive - Manual (Page 34)

hiGh Air eXChAnGe AreAS
Typically, high air exchange areas have some form of mechanical ventilation
to maintain constant or cyclical air flow for heating, cooling or maintaining
some other sort of special environment. Smoke tends to travel with the air
flow, so positioning sampling pipes near the return of an air handling unit or
heating/air conditioning unit ensures early detection of particulate in the area.
NFPA classifies air exchange area as: medium – 6 air exchanges per hour; high
– 12 air exchanges per hour.
Normal sampling methods for high air exchange areas are a combination of
return air and ceiling sampling. The return air sampling provides protection
when the air flow is present. The ceiling network provides protection when
the air flow is off. Local codes typically require smaller sample areas (closer
spacing of sample holes) as the air flow rate increases.
return Air Sampling
Return air sampling provides an effective means of very early warning in a
high air velocity environment, such as a mechanical air conditioned area or an
area equipped with an Air Handling Unit. Placing the pipe network sampling
holes directly in the air stream at a return air grill allows the system to monitor
air that has circulated throughout the protected area.
The following guidelines should be reviewed and followed to ensure proper
sampling by the detector system.
1. More than one sampling location may be required for large air grills.
NFPA 76 recommendations specify that each sampling hole can cover a
maximum of 4 sq. ft. (0.4 m2).
2. Sampling holes should be aligned at an angle of 20 to 45 degrees to the
direction of the maximum air flow.
3. Sampling pipes should be placed in the path of greatest air flow.
4. The number of bends in the pipe network should be kept to a minimum.
5. Pipe ends should be capped with an end cap. Depending on the pipe
design and PipeIQ recommendations, the end caps may or may not have
a sampling hole.
6. Socket unions should be used in locations where the pipe network requires
the removal of the pipes on a regular basis for maintenance purposes.
7. Use standoff fittings to keep the pipe network at least 2 to 8 in. (50 to 200
mm) in front of the grill for high velocity air flow locations. Installing the
network any closer to the input grill locates the sample point in an area
of negative air pressure.
8. Always keep in mind that the monitored environment should still ensure
coverage if the manufactured air flow gets disrupted.
Generally, the FAAST detector should not monitor more than one air handler.
The number of air handlers monitored is limited by the maximum length of
the pipe network runs. However, the degree of particulate dilution and air
movement that occurs with multiple air handlers can adversely affect sys-
tem response times. Final system testing should be done to confirm actual
response times.
in-DuCT SAmplinG
The FAAST detector is approved for in duct applications. National and local
safety standards and codes recognize the ability of air duct systems to transfer
smoke, toxic gases, and flame from area to area. Sometimes smoke can be of
such quantity as to be a serious hazard to life safety unless blowers are shut
down and dampers are actuated. The primary purpose of duct smoke detec-
tion is to prevent injury, panic, and property damage by reducing the spread
(recirculation) of smoke. Duct smoke detection also can serve to protect the
air conditioning system from fire and smoke damage, and can be used to assist
in equipment protection applications, for example, in the ventilation/ exhaust
duct work of mainframe computers and tape drives. For additional informa-
tion relating to duct applications refer to the System Sensor website.
Design Considerations for in-Duct Sampling
The following guidelines are necessary to obtain the best installation results.
1. Pipes should always be supported at both duct walls – rubber grommets
can be used. Silicon sealer must also be used to ensure an airtight seal in
the duct walls.
2. Inlet pipes must be inserted between six and ten duct widths or diam-
eters (for round ducts) from any disturbances to the flow generated by
sharp bends, plenums, nozzles or branch connections, etc.
3. Position the sampling hole at either end of the sampling pipe at least 2
in. (50 mm) from the duct walls.
FiGure 2. in-DuCT pipe orienTATion
4. Holes on the inlet pipe should face 20-45° into the air flow with the holes
concentrated at the center of the duct as shown in Figure 2.
5. The exhaust pipe must have 4, 3 /
8
in. (9.5 mm) holes. Holes should be
concentrated in the middle of the duct’s width and spaced evenly. Holes
on the exhaust pipe should be oriented such that they face away from the
air flow.
Small Duct Sampling
For ducts with a width less than 3 ft. (1 m), the inlet pipe should be in-
stalled at the midpoint of the duct height or diameter. Exhaust pipes should
be inserted 18 in. (0.5 m) downstream from the input pipe. The exhaust pipe
should be at one quarter of the duct height or diameter, as shown in Figure
3. To avoid dilution, sampling pipes should be located before fresh air intakes
and before the exhaust air output.
FiGure 3. SmAll DuCT SAmplinG pipe plACemenT
TABle 1. hole SiZeS For SmAll DuCTS
DuCT WiDTh no. oF
holeS hole SiZe nominAl pipe
FloW rATe (CFm)
12 in .
(300 mm)
2 1/4 in .
(6 .5 mm)
1 .84 cfm
(52 .0 L/min)
20 in .
(500 mm)
3 1/4 in .
(6 .5 mm)
1 .83 cfm
(51 .9 L/min)
28 in .
(700 mm)
4 11/64 in . (4 .5
mm)
1 .70 cfm
(48 .1 L/min)
36 in .
(900 mm)
5 5/32 in .
(4 mm)
1 .81 cfm
(51 .2 L/min)
large Duct Sampling
For ducts with a width of 3 ft. to 7 ft. (1 m to 2 m), two branch pipes are rec-
ommended for the inlets. Inlet pipes should enter a quarter of the way from
the top and bottom of the duct, as shown in Figure 4.
The exhaust pipe should be inserted approximately 18 inches (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 the exhaust air output.
Small Diameter Duct
Air Flow
Inlet Pipe
H/2
Outlet Pipe
H
H/4
ASP-36
ASP-64
Direction of
Air Flow
Air Supply to
FAAST Device
Exhaust Pipe from
FAAST Device
SS-400-007 35 E56-3621-003
Application and Design