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Filtration
Most laser fume purification systems
have 3 stages of filtration. A pre filter to protect the
higher cost HEPA (High Efficiency Particulate Air) for the
finer particulates and finally a gas/chemical filter.
Pre Filter
The efficiency of the pre filter is directly related to
the life of the HEPA in that the more particulates captured
in it, the less can pass through and prematurely block the
HEPA. Most pre filters have an efficiency grading of F5
- F7. When plastics are being lasered, the particulate size
generated is typically 0.2microns. At this level of filtration
the media has an efficiency of 35% in F5 to 65% in F7. When
metals are being lasered the particulate size being generated
is typically below 1micron. This equates to an efficiency
of 65% in F5 to 80% in F7.
The Bofa Advantage range of laser
fume purification systems has an F8 grade media which gives
an efficiency of 80% at 0.2 microns and 95% at 1micron.
Pre filters most commonly come in
2 styles:-
- Panels
- Bags
Panels
Most panel filters have a nominal depth of 50mm. They have
few pleats to obtain some increase in surface area. In short
we would not recommend the use of this type of filter in
any laser marking or cutting exercise as the surface areas
are far too small for the amounts and the size of particulate
generated.
Bags
Bag filter are commonly used in laser fume elimination for
their volumetric containment properties. These bags have
a variety of shapes including multi pocket styles to increase
surface area and reduce pressure drops.
The BOFA 2 Stage Filter (Deep
Mini Pleat Cartridge)
As the contaminated air enters the inside of the cartridge
chamber, the velocity drops slowing down the airflow causing
in the larger and heavier particulates to fall within the
cartridge sump. The sump also allows larger particulates
that build up during operation to drop away from the filter
through the true nature of reverse airflow design. See fig
1.
Surface area and depth of media
Filter life is directly related to surface area. As previously
mentioned the larger dry particulates build up and fall
away during operation. Sticky particulates on the other
hand will hit the media surface and dry on contact irrespective
of the progressive depth of the media. The end result of
this is skinning or masking off to the surface media. Therefore
the greater the surface area the longer the filter will
last.
Electrically Charged Media
It should be worth mentioning at this point that electrostatic
charged fibres have in recent years been applied to
a range of panel and bag filtration products. Extensive
field trials have shown that the charged fibres are adversely
affected and the charge can to a large extent be Œnegated‚
by humidity or by coming into contact with carbonaceous
contaminants. This can result in a severe decrease in efficiency
after a few weeks operation. The particles are held within
a much more open matrix, in the event of vibration caused
by air movement through the filter, particles can become
dislodged and carried into the 'clean' airstream.
All pre filters used in the BOFA
Advantage range have a very high degree of particulate separation
(F8, which is 95% @ 1 micron). The Advantage 180 and 400
having 9m2 of surface area and the 1500 and 4000 being as
high as 20m2. The whole filter is designed as a cartridge
with a sump capacity of 10Kgs for the 180 and 400 series
and 25Kgs for the 1500 and 4000 series. Where bag filters
are used in conjunction with a reverse airflow system the
whole bag acts as the filter. This results in larger particulates
falling back within the filter media itself. If that media
is pleated, the surface area for fall back contaminants
can lay in up to 50% of the construction of that filter.
See fig 2.
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