Note: Descriptions are shown in the official language in which they were submitted.
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1
FILTER
Field of the Invention
The following invention relates to a filter for use in an airflow passageway.
Background of the Invention
s A difficulty arises where air for human or animal inhalation or for any
other
purpose is contaminated with odours, gases, vapours containing undesirable or
harmful
chemicals and other pollutants. The area in which a human is to breathe may be
a
cabin of a tractor or other vehicle or it may be the space inside a gas mask.
It might
also be a room in a building. Alternatively a situation might exist where air
in a room
i o to be occupied by people or a chamber in which an animal is kept or a
machine is
operated might gradually become polluted so that air may need to be
recirculated
through a filter to keep it suitable for its intended purpose.
Various forms of air filter are known having various filter elements located
in
a housing with an air flow path defined through the filter elements within the
housing.
15 A filter according to PCT International Publication Number WO/92/20406 is
depicted in Figures 1 to 3 of the accompanying drawings. The filter includes a
rectangular filter housing 1 including walls 2, 3 extending along the length
of the filter
which provide a sealed passageway for air flow in the direction shown by
arrows 4.
Supported within the air passageway are two webs of expanded metal 5 and 6;
2o perforations 7 within such expanded webs permit air to flow therethrough.
The webs
of expanded metal are made rigid in the direction of air flow relative one to
the other
by a post 13. An adsorptive chamber 16 is defined between the two opposing
expanded
metal webs 5 and 6, side walls 2 and 3 and end walls.
When fully assembled as shown in Figure 3, the unit comprises three basic
Zs filtration layers. A mechanical filter layer 19, such as a fully
reticulated flexible
polyester polyurethane foam, is provided upstream to remove heavy dust and
large
particles which would otherwise clog the downstream layers. Downstream of the
mechanical filter is an electrostatic layer 21 which may or may not contain
electret
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activity 21, trapping particles down to 0.3 microns. Electrets act to convert
uncharged
particles into dipoles and the layer acts to filter out such dipoles. The
electrostatic
layer acts to filter out charged particles. An adsorptive medium layer 22,
such as
activated carbon or an activated alumina impregnated with potassium
permanganate, is
s provided within the adsorptive chamber 16 to provide the final downstream
filtration.
The adsorptive medium adsorbs a wide range of chemicals still present in the
air which
have passed through the mechanical and electrostatic layers.
The current inventor has found, however, that this arrangement does not have
the required efficiency for various applications and standards. A standard
test for
~o determining the efficiency of the filter is to provide ethylacetate at 1000
ppm in a flow
stream moving at 25 cubic feet per minute (0.7 cubic metres per minute)
through the
filter, and timing how long it takes for leakage to occur. Tests utilising the
filters
according to the prior art filter described above have resulted in leakage
typically
within approximately 6 to 10 minutes, thereby reducing the effective life of
the filter.
~ s The current inventor has found that such leakage is in part as a result of
air
flow bypassing the adsorptive medium and flowing around the periphery of the
adsorptive medium at the interface with the filter housing wall.
Object of the Invention
It is an object of the present invention to provide an improved air filter.
zo Summary of the Invention
In a broad form of the present invention there is provided an air filter for
filtering air passing through a flow path, the air filter including a housing
defining an
adsorptive chamber for receiving an adsorptive medium, the housing including a
baffle
or baffles extending into the adsorptive chamber from around a periphery
thereof for
is inhibiting air from passing between said adsorptive medium and said
housing.
The baffles) is/are typically in the form of a flange.
The baffles) may be angled against the direction of flow of air passing
through
the flow path.
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Alternatively , said baffles) comprises a sheet of perforated or otherwise
apertured material situated within the adsorptive chamber, the sheet being so
disposed
that air flow must pass through said perforations or apertures. The sheet will
typically
include predrilled holes of various diameter, and may be in the form of mesh.
s The air filter will typically include the following sequential filtering
layers in
the flow path: a mechanical filter, an electrostatic layer and said adsorptive
medium,
the layers being so arranged that air passing through the air filter passes
through each
of the filtering layers, said adsorptive medium being disposed in said
adsorptive
chamber such that said baffles) extending into said adsorptive medium.
i o It is envisaged, however, that the air filter may be supplied as a housing
without the filtration layers which can be fitted on site.
The housing will typically include perforated walls extending across said flow
path defining upstream and downstream walls of said adsorptive chamber, the
perforations of said upstream wall being angularly offset from a direction of
flow of
~ s said flow path.
A further perforated wall may be located immediately upstream of said upstream
wall,
the perforations of said further wall being angled in a reverse direction to
said upstream
wall perforations.
In one embodiment, the housing is rectangular and includes a pair of opposed
zo side walls and a pair of opposed end walls and said baffles extend inwardly
from the
side walls and end walls.
The baffles may be integrally formed with the side walls and end walls.
Alternatively, the baffles are attached to the side walls and end walls.
In another embodiment the air filter is a cylindrical air filter with said
housing
Zs comprising a pair of end caps between which is defined a generally annular
adsorptive
chamber, one of said end caps being provided with a flow opening radially
interiorly of
said annular adsorptive chamber, said flow path being radially directed
through said
annular adsorptive chamber between said end caps, each said end cap being
provided
with a said baffle extending into said adsorptive chamber.
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Brief Description of the Drawings
Preferred forms of the present invention will now be described by way of
example with reference to the accompanying drawings, wherein:
Figure 1 is a cross sectional schematic view of a prior art filter unit
housing.
s Figure 2 is a cut-away view of the prior art housing of Figure 1.
Figure 3 is a cross sectional schematic view of the first embodiment of the
invention showing the filter layers,
Figure 4 is a cross sectional schematic view of a filter housing according to
an
embodiment of the present invention.
~ o Figure 5 is a schematic end elevational view of an extrusion used to form
the
side walls and end walls of an alternative filter housing.
Figure 6 is a schematic end elevational view of another extrusion used for
forming the side walls and end walls of another alternative housing.
Figure 7 is a cross sectional schematic view of a filter housing according to
i s another embodiment of the present invention.
Figure 8 is a schematic end elevational view of another filter housing, the
filter
housing including an internal diverting plate.
Figure 9 is a schematic plan view of the diverting plate shown in Figure 8.
Figure 10 is a cross sectional schematic view of a cylindrical filter housing.
Detailed Description of the Preferred Embodiments
An air filter according to a first embodiment of the present invention is
depicted in Figure 4. The air filter is of the same general as that of
WO/92/20406
depicted in Figures 1 to 3, including a housing 101 defining an adsorptive
chamber 116
for receiving an adsorptive medium. The housing 101 of the present invention,
25 however, includes baffles 100, 100' extending into the adsorptive chamber
116 from
around a periphery thereof for inhibiting air from passing between the
adsorptive
medium and the housing.
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The housing 101 is provided with side walls 102, 103 extending along the
length of the filter joining similar end walls to define the rectangular
housing. Two
perforated walls formed of webs of expanded metal 105, 106 span across the air
flow
path defined by the housing walls, and define the upstream and downstream
boundaries
s of the adsorptive chamber 116. The webs 105, 106 are here supported by C
channel
sections 111, 112 affixed to, or integrally formed with, the side walls 102,
103. The
webs 105, 106 contain perforations 107 which permit air to flow through the
filter in
the direction generally shown by arrows 104. Here the expanded metal webs are
formed of aluminium panels and angle the air flow so as to maximise contact
with the
i o adsorptive medium, increasing the effective length of the flow path
through the
adsorptive chamber and so that the residency time of air passing through the
adsorptive
medium chamber is increased.
Also shown in Fig. 4 is a support post 150 attached by rivets 160 or otherwise
attached to the webs 105, 106 within the adsorptive chamber. The post 150
serves the
~ 5 purpose of rigidly interconnecting the webs 105, 106 to maintain a
constant defined
space within the adsorptive chamber. There may be a number of posts 150 spaced
"into and out of the page of the drawing", depending on the length of the
chamber 1.
The posts are typically fabricated from extruded aluminium square or
rectangular
section, with separate lengths of the extrusion cut off to say 1 or 2 cm
lengths. These
20 lengths would be spaced "into the page" of Fig. 14.
The adsorptive chamber 116 is defined between the two opposing expanded
metal webs 115 and 116, side walls 112 and 113 and end walls. A means for
recharging the adsorptive chamber with a suitable adsorptive medium may be
provided
and thus an aperture through either one of the end walls or through a side
wall of the
z5 adsorptive chamber with a closure can be provided. A window in one of the
walls can
also be provided to enable visual inspection of the adsorptive medium and/or a
suitable
colour change indicator for monitoring when the adsorptive medium should be
replaced.
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Flanges 108 and 109 will typically fit tightly against the outlet of an air-
conditioning unit so as to provide a sealing against the unit. This sealing
could be
enhanced by the use of a gasket of a suitably deformable material.
Baffles 100 extend into the absorptive chamber from around the outer
periphery thereof, and are here in the form of flanges which are angled
against the
direction of flow of air passing through the flow path. The baffles 100 will
present a
barrier to air flowing between any gap at the interface between the adsorptive
medium
in the adsorptive chamber 116 and the side walls 102, 103 of the housing,
redirecting
any such flow back in to the absorptive medium as indicated by the arrows 104'
in
o Figure 4 .
The outer edges of the posts 150 supporting the expanded webs 105, 106 also
present a leak path for air to escape around the inner periphery of the
adsorptive
medium, and hence further baffles 100' may be provided on the wall of the post
150.
As the posts 150 are usually only very short in depth, of the order of 10 mm,
it may
~ s not necessarily be required to provide such baffles 100' at the support
post 150. If such
baffles 100' are provided, however, they may either be fastened to the post
150 or
formed integrally with an extrusion taking in the cross sectional shape of the
post 150.
Whilst the baffles 100' depicted in Figure 4 can be seen to be fastened to the
side walls (or at least the C channels 111, 112) of the housing 101, a more
cost
Zo effective manner of forming the housing with baffles is to provide the
baffles and
support means for the expanded webs 105, 106 integrally with an extruded wall
component. An example of the cross sectional shape of such an extrusion which
can be
used to form end walls or side walls of a filter housing is depicted in Figure
5. The
extruded component 120 includes a base corresponding to side walls 102, 103 or
either
2s of the end walls. Extending from the base and to reside with the adsorptive
chamber is
a baffle 100" . The baffle 100" extends at an angle of about 30° from
the base and, in
use will again be directed against the air flow 104 extending into the
adsorptive
material so as to redirect air bypassing around the outer periphery of the
adsorptive
material back into the adsorptive material. The baffle 100" will be effective
at any
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angle, so long as it extends into the adsorptive chamber 116 to block air
passing
between the adsorptive medium and housing walls 102, 103
Similarly, in the embodiment of Figure 6, an extrusion 130 is provided.
Rather than having an integrally formed baffle 100, separately formed baffles
100a and
s 100b are provided. Baffle 100a typically extends from the base 102, 103 at
an angle of
about 72° whereas the baffle 100b typically extends from the base at an
angle of about
102°. Where one or both of baffles 100a and 100b are used, they are
singularly or both
intended to extend into the adsorptive material within the adsorptive chamber.
The
baffles can be affixed to the extruded base by screws or rivets for example.
The
~ o channels 131, 132 formed in the extrusion 130 by protruding flanges are
utilised to
support the expanded webs 105, 106.
The particular baffles depicted as examples in Figures 5 and 6 were developed
with the assistance of Wisdom Agricultural Limited, however other forms of
baffle will
also be suitable as will be appreciated by persons skilled in the art.
~ s An alternative embodiment, depicted in Figure 7 has an additional expanded
web lOSa formed immediately upstream of the first expanded web 105 which
defines
the upstream boundary of the adsorptive chamber 116. The perforations of the
further
web lOSa are angled in a reverse direction to that of the first web 105. With
this
configuration air flowing through the filter will first hit the web lOSa and
be forced
2o through a change in direction one way prior to immediately passing through
another
change of direction in the opposing way as it hits the web 105. This will slow
the air
down before it enters the adsorptive medium, increasing the residency time of
the air
passing through the adsorptive medium. In this embodiment the side walls 102,
103
are integrally formed as an aluminium extrusion incorporating the baffles 100
and the
Zs channels for supporting the expanded webs 105, lOSa and 106.
In the embodiments of Figures 4 through 7, the fully assembled unit will
typically be provided with a mechanical filter layer 119, electrostatic layer
121 and
adsorptive medium layer 122 in much the same manner as the prior art filter
depicted
particularly in Figure 3. The baffles of the present invention are, however,
equally
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applicable to other configurations of air filtration layers incorporating an
adsorptive
medium layer.
In the presently preferred embodiments, the mechanical filter layer 119 could
be a fully
reticulated flexible polyester polyurethane foam, such as sold under the trade
name
s Meracell, and the grade sold as Me 080 (having a density of 28 Kg/in3 and 80
cells per
25 mm) is found to give an excellent result. The open pore structure allows
for good
flow-through of air without leading to too great a pressure drop.
Alternatively the mechanical layer 119 could comprise of two layers. A first
coarse layer may comprise a filter pad manufactured from gradual synthetic
fibres made
io up in graduated layers, bonded and subsequently baked. A second mechanical
filter
layer is downstream of the said coarse filter layer and can be made of a
similar material
with a finer pore size.
Downstream of the mechanical filter 119 is the electrostatic layer 121 which
may or may not contain electret activity. Electrets act to convert uncharged
particles
~ s into dipoles and the layer acts to filter out such dipoles. The
electrostatic layer acts to
filter out charged particles. A suitable medium for this layer is sold under
the name of
FILTRETE G (Trade Mark) by the 3M Company. The advantage of such a layer is
its
compactness and airflow properties to draw out electrically particles of small
size
without the need to have bulky charge inducing means, and then means to
attract such
2o charged particles.
Whilst a layer having both activities is shown, it may be desired to have the
two activities separated, this later alternative however has the disadvantage
of extra
thickness.
The adsorptive medium layer 122 is provided within the adsorptive chamber
25 116. In one embodiment such adsorptive material is provided by an activated
alumina
impregnated with potassium permanganate. The potassium permanganate content is
5
by volume and contains no more than 5 mg per cubic metre respirable dust.
Other adsorptive media may also be used, and thus activated carbon can be
used as an adsorptive media. The adsorptive chamber with non~granulated
activated
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carbon will typically require a lining with a smaller pore size so that the
activated
carbon is not lost through the expanded metal webs.
In one form the activated carbon could be a granulated coconut shell based
steam activated carbon impregnated with 7.5% potassium bromide. This activated
s carbon is granulated to a size with 90 % of granules being between 4.76 to
2.38 mm,
with an apparent density of 580 kglin3.
Alternatively it may be desired to use other adsorptive media, especially
where
a specific adsorptive requirement exists. The choice of adsorptive medium is
however,
limited to some extent by the requirement to keep the reduction in airflow to
a
i o minimum.
In use dust and large particles are removed by the mechanical filter layer
119.
Particles down to 0.3 microns are trapped by the electrostatic medium and
electrostatic
layer 121. These layers effectively remove particles of size unable to absorb
to the
adsorptive medium 122 and such particles as are likely to clog up the
adsorptive
~ s medium.
Activated alumina impregnated with potassium permanganate useful as the
adsorptive medium is able to absorb a large range of chemicals and the
following are
examples.
Acetaldehyde CH CHO
Acetic acid CH COOH
Acetylene HCCH
Acrolein CH2CHCH0
Acrylonitrile CH2CHCN
1,3-Butadiene CHZCHCHCH2
Butyric acid CHgCH2CH3000H
Carbon disulfide CSZ
Chlorine dioxide C 102
Cresol HOC6H4CH3
Diethylamine (C2H )2NH
Dimethylamine (CH ) NH
Ethanol C~HSOH
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Ethylamine C2HgNH2
Formaldehyde HCHO
Formic acid HCOOH
Freon 11 CCI3F
Hydrazine H NNH2
Hydrogen chloride HCI
Hydrogen cyanide HCN
Hydrogen sulfide HzS
Indole CgH7N
Isoprene CH2CCH3CHCH2
Isoproponal CH CHOHCH3
Methanol CHgOH
Methyl acrylate CH2CHCOOCH3
Methyl disulphide CH3SSCH3
Methyl ethyl ketone CH COCH2CH~
Methyl mercaptan CHgSH
Methyl sulphide (CH3)2S
Methyl vinyl ketone CH COCHCH2
Methylamine CH NHZ
Nitric oxide NO
Nitrogen dioxide N02
Phenol C6HgOH
Ozone 03
Styrene C6HSCHCH2
Sulfur dioxide S02
Sulfur trioxide SOg
Trichloroethylene CCI2CHCI
Triethylamine (CzH )3N
Trimethylamine (CH )gN
Vinyl chloride CH2CHCI
Tests conducted as per that discussed above in relation to the prior art air
filter
of WO/92/20406 indicate that use of the baffles described above with the
described
filtration layers may increase the time before leakage of the test ethyl
acetate from 6 to
10 minutes to in excess of 33.5 minutes. The useful life of the filter is thus
markedly
s increased through use of the baffles.
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As an alternative to baffles taking the form as shown in Figs. 4 to 7, a
single
diverting plate or multiple diverting plates can be provided. In Fig. 8, a
housing 101'
is depicted within which there is situated a mechanical prefilter 119 and an
electrostatic
layer 121 therebelow. The adsorptive chamber defined by webs 105, 106 is
divided
s into two subchambers by a single diverting plate 154. There may be one or
more than
one diverting plate 154 provided one above another with adsorptive medium 122
situated therebetween. The plate or plates 154 extend to the walls of the
filter housing.
As depicted in Figure 9, the diverting plate 154 includes a number of
apertures 151
which are predrilled therethrough and have differing sizes. The diverting
plate is
o intended to ensure that all air passing through the filter must pass through
the
adsorptive medium. The plate 154 might be gauze, mesh or a predrilled plate as
shown
or any other plate having apertures therethrough. The plate may be located
with
respect to the webs 105, 106 by means of support posts. The support posts
might
include baffles of the type shown in Fig. 4 for example. Another modification
might be
i s in the substitution of a plastics material for aluminium or steel. That
is, the disclosed
structures can be formed from moulded plastics material or plastics material
having
been fabricated by other means.
The use of baffles according to the present invention is not restricted to the
particular form of air filter described above, but can be utilised to restrict
the bypassing
zo of air around- the adsorptive medium located in the adsorptive chamber of
any of
various forms of air filter. Similar baffles to those described above could
readily be
incorporated in to circular respirator mask-type filters as an example.
Another form of filter that the present invention can also be applied to is a
cylindrical air filter, as depicted in cross section in Figure 10. In this
form of air filter,
zs the housing 201 is defined by two circular end caps with inner and outer
expanded
webs 205, 206 each formed into a cylinder and mounted between the end caps so
as to
define an annular adsorptive chamber 216. In such an air filter, the air flow
path 204 is
generally radially from outside of the filter, through an annular mechanical
filter
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disposed about the adsorptive chamber 216 and an electrostatic activity 221
between the
mechanical filter layer 219 and the adsorptive chamber 216, adjacent the outer
perforated web 205. The air flow path 204 extends radially from outside of the
air
filter through the mechanical filter layer 219 and the electrostatic layer
221, through the
s adsorptive medium 222 in the adsorptive chamber 216 and in to the hollow
interior 224
of the filter. The air flow path then exits through the large central aperture
of the end
cap 203. The end cap 203 will then be sealed against an opening of the wall of
whatever space is to be supplied with the filtered air as required.
In this form of air filter, a possible leak path exists at the interface
between
~o each of the end caps 202, 203 and the adsorptive medium 222 within the
adsorptive
chamber 216, and thus accordingly baffles 200, 200' are provided which extend
into
the adsorptive medium 222 in the adsorptive chamber to direct air bypassing
the
adsorptive medium back in to the adsorptive medium. Here the baffle 200 has a
portion angled back towards the end cap 202 so as direct air back through the
i s adsorptive medium toward the end cap 202, whilst the baffle 200' is angled
away from
the end cap 203 so as to merely deflect the air flow through approximately 45
° in to the
adsorptive medium, preventing it from immediately escaping into the hollow
interior
224 of the filter.
Both baffles 200, 200' are generally formed as annular flanges which extend
zo from the end walls 202x, 203a of the end caps 202, 203 adjacent the
respective
perforated web 205, 206. Here the baffles 200, 200' are actually secured to
the ends of
the perforated webs 205, 206 and abut the end cap end walls 202a, 203a when
the
perforated webs 205, 206 are fitted to the end caps 202, 203. Alternatively
the baffles
200, 200' could be integrally formed with the end cap and extend therefrom
into the
is adsorptive chamber 216. The baffles 200, 200' could also be formed with the
angled
portions formed into separate radially projecting segments rather than a
continuous
annular flange, the segments being twisted so as to further disrupt the air
flow and
direct it into the adsorptive medium.
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The filters of the present invention may be used in various applications,
including air conditioning systems, respirator masks, or the filtration of
cabin air in
trucks, mining equipment and the like.
