ASHRAE Journal - August 2024 - 20

COLUMN HVAC FUNDAMENTALS
case of an explosion. Figures 2 and 3 provide detailed
illustrations of how these devices work.
In Figure 2, the dirty air would come into the dust
collector from the bottom right. It would pass through
the spark detection and then through the inlet isolation
valve before entering the collector. If an explosion did
occur in the collector, the flames and pressure wave
would propagate back out through the inlet and outlet,
which would be very dangerous. This is the job of the
inlet isolation valve and the abort gate. The abort gate
is a passive member-it is forced closed by the pressure
wave and then directs the flames and pressure outside
and prevents them from reentering the building. The
inlet isolation valve is also passive (Figure 3) and would
close and prevent the flames from reentering the
building through the inlet duct.
For normal collector operation, an inlet isolation valve
allows dirty air to flow through it and to the collector
(Frame 1, Figure 3). However, in Frame 2 an explosion
happens downstream in the collector, and the pressure
wave closes the inlet flap. This also blocks flames and
smoke from traveling back upstream (Frames 3 and 4).
Double-check with the NFPA and your local authority
having jurisdiction to verify the requirements for all
needed safety devices.
Motor Requirements
Fan motors can be the source of ignition in a collection
system; therefore, it is prudent to specify a fan that helps
limit the ignition sources. An antistatic, shaft-grounded,
explosion-proof fan is able to reduce many of these
possibly dangerous conditions. Antistatic electricity fans
add positive and negative ions to the airstream, which
actually prevents the buildup of static electricity within
the airstream. A shaft-grounded fan has its main shaft
electrically grounded. While this is normally done to
protect the motor's bearings from damage, it also serves
to aid in preventing any spark transferring from the
motor to the airstream. An explosion-proof fan contains
any explosion originating from inside the motor housing
along with preventing any spark transferring to the
airstream or being created in the airstream by the fan
blades scraping the side of the fan wall.
Additional Considerations
Makeup air is one more thing to consider if you are
placing your collector outdoors. Obviously, if you are
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ASHRAE JOURNAL ashrae.org A U G UST 2024
FIGURE 3 Inlet isolation valve diagram.
1
2
3
4
exhausting thousands of cfm in operation, this will
cause your space to become extremely negatively
pressurized. A common solution is to return the clean
air out of the dust collector back into the building.
This will ensure that the space remains at the correct
pressurization while the collector is in use. However,
more safety accessories will be needed if you choose to
recirculate the air.
If you choose to not recirculate the air, then you must
provide a means for makeup air. This could be handled
through a dedicated outdoor air unit serving the space.
Or, if the design permits, gravity ventilators can also
provide outdoor air to maintain proper pressurization.
Another important consideration is the inclusion of
blast gates at all dust collecting branches. Blast gates
allow the branch to be manually closed off when not in
use. This aids in maintaining proper air speed within
the ductwork.
Also, some projects benefit from a dust hazard
analysis. These tests provide details about the dust
being collected and the facilities creating the dust.
These tests identify locations where explosible dust
cloud atmospheres could be present. Also, they identify
potential ignition sources that could be present
under normal and foreseeable abnormal conditions.
Depending on the circumstances and the intent of the
design, a dust hazard analysis could be required.
Relevant Codes
If you need further information regarding this subject,
the relevant codes that most directly deal with this
subject are IMC 511, NFPA 68, 69 and 664. Please refer to
these codes and your local authority having jurisdiction
for any more queries.
CAMFIL APC
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ASHRAE Journal - August 2024

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