Filter bags are installed within filter housings, a pressure vessel that
directs the process flow through the filter media to remove unwanted particles.
Filter bags are designed to be thrown away after use, attempting to clean and
reuse them is not recommended. The “cost” of a filter bag begins with the media
itself but also involves the frequency of filter bag changes and the annual
disposal costs. It is the frequency of changing and perhaps reduced
quantity of filter bags required annually that often contributes most to
reduction in operational costs.
Filter Bag Designs
Needle-felt filter bags are generally the least expensive style and are
the most common with mesh styles being a little more expensive mostly due to the
materials compatible with the required manufacturing process. Melt-blown
and multi-layered filter bags are the most expensive but also provide higher
Filter Bag Sealing Rings
A snap-ring is a metal or plastic ring sewn within the opening of the
filter bag, which aligns with a groove within the filter bag housing.
Based upon the tolerance of the groove and ring, the filter bag is held firmly
in-place yet can be removed by hand for change-outs; filter bags have handles to
grab so they can be lifted from the vessel.
A crush-seal ring is a geometrical shaped plastic ring that also has an
outer diameter that “snaps” into the support basket groove. Sealing is
further enhanced by downwards pressure onto the top of the sealing ring, which
creates an outward force and improving the seal between the ring and sealing
grove. The downwards pressure is achieved by the filter vessel cover,
either directly or via a hold-down ring to transmit the downwards force of a
closed chamber cover to the top of the filter bag.
Filter Bag Efficiency
There is no industry standard relating “nominal” to a specific efficiency;
each manufacturer determines the acceptable range of efficiency and thus it
is necessary to compare filter bag performance and NOT rely on the stated
The efficiency for our nominally rated filter bags tends to be 65-70% and
I am told that some manufacturers reduce costs by accepting lower efficiencies,
so “you get what you pay for” applies to filter bags as most other things in
Although also an accurate description of the seal type and generically
referred to as “snap-ring” it is also a registered trademark owned by Eaton
Hydraulics, Filtration Division LLC. These represent their inexpensive
line of filter bags, offered in polyester, polypropylene, nylon, nomex and PTFE
felt materials. The rings sewn into the filter bag openings are zinc
plated steel, 316SS or polypropylene.
SNAP-RING® filter bags have a sewn construction. A web of
material is perforated with a needle board to create the holes determining
retention and one side is singed to reduce the change of fibers “migrating”
Materials are mass produced by mills and tested for their retention
efficiencies, then sold to companies which manufacture filter bags.
The snap-ring style filter bags are also available in woven materials such
as nylon, essentially weaving single or multi-filament polymeric threads into a
single layer grid-like pattern. The manufacturing process and materials are more
expensive than needle felt filter bags and they still have nominally rated
retention efficiencies because the spacing, angle of weave and stabilization of
the mesh grid are difficult to control.
Crush-Seal Ring Filter Bags
SENTINEL® filter bags have nominally rated retentions, however
their sealing ring design and having a welded seam instead of a sewn seam
increases their relative efficiency and strength when compared to snap-ring
style filter bags.
A version of this bag, referred to as the UNIBAG® is
made from recycled materials to support “green” products and they are less
expensive than snap-ring style filter bags.
As with the snap-ring style filter bags, the crush-ring style can be
applied to mesh design filter bags.
Premium Efficient Filer Bags
Carry-out (process) fluid loss
Reduced exposure to process fluid
Employing a melt-blown manufacturing process, the fibers used have a
smaller diameter and are less likely to “migrate” downstream. The melt-blown
process provides control over the material density and thickness.
Polypropylene, polyester and nylon can all be manufactured with a melt-blown
The well documented efficiencies results from the multi-layer approach;
the coarser materials remove larger particles and the increasingly finer layers
remove finer particles. Relying on multiple layers also ensures purity of
materials as no binders or additives are required for formulating a single
PROGAF® filter bags represent the most
complex design available, having efficiencies, which rival filter cartridges
while providing holding capacities and supporting flow rates that would require
a multiple cartridge housing design.
The premium efficiency stems from the number of layers and a modified
crush-seal ring which is welded to each layer to ensure a specific flow path.
The HAYFLOW™ design offers greater surface area; its needle-felt
material with crush-ring is “doubled-up” within the filter bag housing to
support increased particulate loading and higher flow rates.
Choosing the Right Filter Bag
Thankfully almost all filter bag housing manufacturers offer housings for
#1 - #4 size filter bags, the most common being #1 and #2 sizes (having a 7”
diameter opening and 16” or 32” length). The filter bag size will begin to limit
your options if its anything other than a #2 size.
Materials of Construction
Melt-blown materials are ideal for ½ to 25 micron retentions, nominal or
absolute efficiencies, provide increased depth filtration and thus typically are
not replaced as often.
Mesh materials are best for solid particles and can be gently cleaned in
some cases for re-use; these are typically used for larger size particle
retention, >100 microns (nominally rated).
Filter Bag Positioners are cage-like structures that limit the movement of
the filter bag and thus help prevent damage due to pressure spikes related to
fast acting valves within a piping system.
Using 9300 Gauss rare earth magnet assemblies within a bag positioner
increases the efficiency and retention capacity of magnetic particles.