Background of the Invention 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 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.

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; 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 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

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 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 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.

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.

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 inhibiting air from passing between said adsorptive medium and said housing.

The baffle (s) is/are typically in the form of a flange.

The baffle (s) may be angled against the direction of flow of air passing through the flow path.

Alternatively said baffle (s) 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.

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 baffle (s) extending into said adsorptive medium.

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 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 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 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.

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.

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.

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 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, 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.

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 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 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 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 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 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.

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 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 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 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 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

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 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 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.

An alternative embodiment, depicted in Figure 7 has an additional expanded web 105a 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 105a 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 105a and be forced 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 channels for supporting the expanded webs 105,105a 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

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 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 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 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 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 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

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 carbon is granulated to a size with 90% of granules being between 4.76 to 2.38 mm, with an apparent density of 580 kg/in3.

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 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 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 CH3CHO Aceticacid CH3COOH Acetylene HCCH Acrolein CH2CHCHO Acrylonitrile CH2CHCN 1,3-Butadiene CH2CHCHCH2 Butyricacid CH3CH2CH3000H Carbon disulfide CS2 Chlorinedioxide C102 Cresol HOC6H4CH3 Diethylamine (C2H5) 2NH Dimethylamine (CH3) 2NH Ethanol CoHOH Ethylamine C2HsNH2 Formaldehyde HCHO Formicacid HCOOH Freon 11 CCI3F Hydrazine H2NNH2 Hydrogen chloride HCI Hydrogencyanide HCN Hydrogen sulfide H2S Indole C8H7N Isoprene CH2CCH3CHCH2 Isoproponal CH3CHOHCH3 Methanol CH30H Methylacrylate CH2CHCOOCH3 Methyl disulphide CH3SSCH3 Methyl ethyl ketone CH3COCH2CH3 Methyl mercaptan CH3SH Methyl sulphide (CH3) 2S Methyl vinyl ketone CH3COCHCH2 Methylamine CH3NH2 Nitric oxide NO Nitrogen dioxide NO2 Phenol C6H5OH Ozone 03 Styrene C6H5CHCH2 Sulfurdioxide S02 Sulfurtrioxide S03 Trichloroethylene CCI2CHCI Triethylamine (C2H5) 3N Trimethylamine (CH3) 3N Vinylchloride 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 increased through use of the baffles.

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 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 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 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 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, 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

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 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 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 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 from the end walls 202a, 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 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.

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.