by John Woolever, Donaldson Torit Product Manager
Dust collection using
pleated media filter elements known as cartridges
has become a
ubiquitous solution for industrial ventilation requirements in recent decades.
Cartridge dust collection equipment can be grouped into two categories based on
cartridge orientation and the cabinet’s airflow design:
1) A downflow
collector design with cartridge filters positioned horizontally and the dirty
air inlet positioned above all filtration media and
2) Crossflow or upflow collector designs with cartridge
filters hung vertically and the dirty air inlet positioned below or at the side
of the filtration media.
Some in the industrial ventilation marketplace make claims
that a collector with vertically-hung cartridge is superior by citing anecdotal
examples without supporting data. However, credible facts support the
advantages of a downflow collector design, including the American Conference of
Governmental Industrial Hygienists (ACGIH®) Industrial Ventilation Manual:
Section 8.3.2 of Industrial Ventilation Manual states, EPA-Sponsored1
research has shown that superior performance results from the downward flow of
the dirty airstream. This downward flow reduces redisposition since it aids
gravity in moving dust particles toward the hopper.
Before reviewing other factors on this subject, a brief
review of the origins of cartridge collection is beneficial.
In the 1970’s, cartridge collectors were originally designed
to replicate baghouse dust collectors that contained vertically-hung cloth
filter bags with dirty air entering the collector below the filters. The
pleated media of a cartridge filter element provided superior filtration
efficiency and reduced emissions over a comparable bag fabric. However, the new
style dust collector still had many of the same limitations of its baghouse
• Dust discharged from cleaned cartridge filters still
needed to fight past incoming air before settling in the hopper/containment
vessel. This led to dust re-entrainment on the filter media surface, with
higher net operational pressure drop, and shorter filter life.
• Filter change-out activities remained time-consuming,
dirty, and generally unpleasant because servicing the filters still required an
operator to open the dirty air plenum via a large access door.
Design improvements to the vertically-hung collector design
were realized over the years with the inclusion of side inlets (where the
operator could enter the collector next to filtration media instead of below
it); this led to some reduction of dust re-entrainment. However, cleaned dust
particles were still impacted by incoming air, and cartridge change processes
still exposed operators and the dust collector surroundings to a large opening
to the dirty air plenum.
Finally in the 1980’s, a breakthrough was achieved by
rotating the filter cartridge orientation from vertical to horizontal and by
positioning the dirty air inlet above all filtration media. This change
resulted in an incoming air flow pattern aligned with the desired final
location of the dust discharged from the cleaned filter cartridges. Since
gravity naturally directs discharged dust particles down towards the
containment vessel located at the bottom of the collector, the incoming air
pattern of a downflow collector now assists the movement of particles to the
storage vessel at the base of the unit, rather than fight against it.
This improved airflow management design reduced dust
re-entrainment, which could be enhanced further by using filter cartridges
containing surfacing-loading nanofiber filtration media. Reductions in dust
re-entrainment can lower operational pressure drop, extend filter life, and
reduce compressed air consumption used for pulse cleaning filter elements - all
of which benefit the dust collector’s operational expense and improve the total
cost of ownership for the collector.
Advanced airflow analysis methods have allowed further
improvements in downflow collector designs. Optimizing the
cartridge-to-cartridge spacing and the cartridge-to-cabinet wall dimension
allows for greater air filtering capacity for a given collector housing size.
Even more gains are possible with new approaches to positioning dirty air entry
for low-loss pre-separation. The example analysis below compares the results of
sophisticated airflow management designs for a downflow and side entry
collector. The downflow collector on the left shows lower velocities in the
dirty air plenum, which allows dust particles to drop out of the airstream more
effectively. In contrast, the vertically-hung collector to the right shows
higher velocities as well as upward “sweeping” patterns in the hopper section,
which impacts how cleaned dust particles settle into the containment vessel
under the hopper.
Some collector manufacturers point to anecdotal images of horizontal
filters with dust build up and bridging across the top surfaces of filter
elements. In reality, properly designed, operated, and maintained downflow
collectors have pulse cleaning systems that limit this dust build-up on
filtration media surfaces. A knowledgeable cartridge collector supplier should
be able to provide comprehensive pulse signature data quantifying the cleaning
energy delivered to the filtration media.
When superior airflow management design, effective pulse
cleaning performance, and surface loading nanofiber filtration media are
brought together in a single package, a downflow dust collector can have
impressive air filtering capacity benefits. One way to evaluate a collector’s
airflow capacity is by comparing volumetric airflow rates against filtration
media area. This air-to-media ratio (AMR) or filtration velocity is
typically used to size equipment for given applications.
Beyond dust collector performance and capacity, scalability
and configurability are two additional advantages downflow collectors have over
vertically-hung collectors. A downflow collector’s horizontal filter
orientation allows cartridges to be positioned in a matrix that can increase in
height as well as width for greater flexibility and reductions in total
footprint on the shop floor. Vertically-hung collectors have only one layer of
filters, so increases in filtration media area require increases in width and
depth because height is fixed. This often results in a larger total footprint
for an increased air handling capacity.
Explosion venting is another area where downflow collectors
are different than vertical element collectors. Explosion relief vents can be
located on the roof or the side of a downflow collector with no change in
equipment footprint. Because explosion vents must be located on the side of
vertically-hung collectors, they typically require an extended footprint in
order to direct vents upward. Many facilities prefer upward or roof-mounted
explosion vents because the deflagration flame front and the materials
discharged during a deflagration event are both directed upward where there is
reduced exposure to occupied areas.
the filter change process in a downflow collector simplifies element
replacement and helps minimizes operator exposure to collected contaminates
compared to the older, vertically-hung collector design. The downflow collector
typically has covers that provide access to just a few cartridges at a time.
This can be a great advantage if the collector is located outdoors in
potentially windy locations. The old-style, vertically-hung collectors
typically require larger access doors that expose the operator and surrounding
area to the entire dirty air plenum. Downflow style collector’s horizontal
cartridge removal is also simpler and cleaner, requiring the operator to place
only his/her arm into the collector to reach cartridge filters. Vertically-hung
collectors either require special tools to reach into the cavernous dirty air
plenum, or actual physical entry (potentially confined space entry) by the
operator to access cartridge filters deeper in the cabinet structure. While
nearly all dust collector designs accommodate industrial
platforms for filter access, downflow collectors can also be accessed by a step
ladder to help reduce the impact on the shop floor area.
Because of the larger door and more difficult access to cartridge filters, as well as the typically larger/ heavier filters, the filter
replacement on vertically-hung collectors typically requires a platform, a
man-lift, or some other kind of elaborate access.
More than 40 years after its inception, cartridge collectors
continue to be a compelling and economical solution for facility operators in
many different industries. Downflow style collectors deliver reduced dust
re-entrainment, minimized footprint, greater configuration flexibility, and
superior filter change experiences. With these user benefits, the case for
downflow cartridge collection is clear.
1 Leith, D.; Gibson, D.D.; First, M.W.:
Performance of Top and Bottom Inlet Pulse-Jet Fabric Filters. Journal of Air
Pollution Control Association 24:1150 (1974).