Abstract:
A filter element has an upstream-side filter layer and a downstream-side filter layer. The downstream-side filter layer is attached to the upstream-side filter layer. The upstream-side filter layer has a coarser mesh than the mesh of the downstream-side filter layer. The upstream-side filter layer has a plurality of through-holes formed therein and extending throughout the thickness of the upstream-side filter layer. The plurality of through-holes allows the filter element to continue to be used when the rest of the upstream-side filter layer is clogged and the downstream-side filter layer still possesses filtering capability.

Description:
[0001]     This application claims priority to Japanese patent application serial number 2004-358449, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to filtering elements for filters and in particular to filter elements that are used for holding dust and contaminates in the air and have a relatively coarse upstream-side layer and a relatively fine downstream-side layer.  
         [0004]     2. Description of the Related Art  
         [0005]     Japanese Laid-Open Patent Publication No. 2001-523562, corresponding to International Publication WO99/26719, teaches a known filter element. As shown in  FIG. 7 , a filter element  90  of this publication has an upstream-side, non-woven fabric layer  92 , and a downstream-side, filter paper layer  94 . In this filter element  90 , the filtration ability is configured such that the filtration ability increases in a direction toward the downstream-side Thus, the upstream-side non-fabric layer  92  is more coarse than the downstream-side filter paper layer  94 . Therefore, the non-fabric layer  92  can hold dust particles having a relatively large diameter, while the paper filter layer  94  can hold dust particles having a relatively small diameter. In the example of this publication, the weight per unit area of the non-fabric layer  92  is determined to be within a range between 15 g/m 2  and 150 g/m 2 . The weight per unit area of the filter paper layer  94  is determined to be within a range between 50 g/m 2  and 200 g/m 2 .  
         [0006]     In order to improve the dust-holding (i.e., filtering) ability of the upstream-side non-woven fabric layer  92 , the mesh of the upstream-side non-woven fabric  92  of the filter element  90  may be configured as a finer mesh. However, with this configuration, the non-fabric layer  92  may become clogged earlier than the downstream-side paper filter layer  94 . When this occurs, no more use of the filter is possible even though the paper filter layer  94  may still be sufficient enough for further use. Therefore, the total dust-holding ability of the filter may be reduced.  
         [0007]     If the mesh of the upstream-side no-woven fabric layer  92  is set to be coarser in order to avoid potential clogging, the amount of dust that can be held by the non-woven fabric layer  92  may be reduced. As a result, the total available holding amount of dust by the filter may be reduced.  
       SUMMARY OF THE INVENTION  
       [0008]     It is accordingly an object of the present invention to teach improved techniques for increasing the total available dust-holding amount of a filter element while effectively utilizing the dust-holding abilities of the upstream-side filter layer and the downstream-side filter layer of the filter element.  
         [0009]     According to one aspect of the present teachings, filter elements are taught for holding dust in the air. The filter elements may include an upstream-side filter layer and a downstream-side filter layer. The upstream-side filter layer may be bonded to the downstream-side filter layer and has a coarser mesh than the mesh of the downstream-side filter layer. The upstream-side filter layer has a plurality of through-holes formed therein and extending throughout the thickness of the upstream-side filter layer.  
         [0010]     With this arrangement, in addition to the air being filtered by the upstream-side filter layer, the portion of the air (e.g., ambient air) that flows through the through-holes of the upstream-side filter layer may reach the downstream-side filter layer. Therefore, even in the event that dust or other contaminants in the air has clogged the upstream-side filter layer during a long time period of use, the downstream-side filter layer may still filter the air flowing though the through-holes. As a result, this may reduce the occurrence of the problem of a filter element no longer able to be used even though the downstream-side filter element is still usable for filtration.  
         [0011]     In addition, it is not necessary to form an upstream-side filter layer having a mesh that is needlessly coarse in order to avoid the clogging of the upstream-side filter layer. Therefore, the upstream-side filter layer may have an improved dust-holding ability.  
         [0012]     Because the dust-holding abilities of both of the upstream-side filter layer and the downstream-side filter layer can be effectively utilized, the available dust-holding capacity (i.e., amount) of the entire filter element can be increased.  
         [0013]     In one embodiment, the through-holes have an average diameter of approximately 1 mm to 5 mm. A ratio of the total of the open areas of the through-holes to an effective filtration area of the downstream-side filter layer is set to be in the range of approximately 5% to 20%.  
         [0014]     Due to the setting of the open ratio to be smaller than 20%, a reduction in the available dust-holding capacity of the upstream-side filter element may be minimized. In addition, setting the open ratio to be greater than 5% allows the air to be smoothly delivered to the downstream-side filter layer even after the upstream-side filter layer has become clogged.  
         [0015]     In another embodiment, the upstream-side filter layer is formed by a non-woven fabric having a weight per unit area of 3 g/m 2  to 20 g/m 2 . The non-woven fabric is made of fibers having an average diameter of 5 μm to 10 μm.  
         [0016]     Therefore, the upstream-side filter layer may have a mesh that is proper for ensuring the dust-holding ability of the upstream-side filter layer.  
         [0017]     In a further embodiment, a filter paper or a non-woven fabric forms the downstream-side filter layer. For example, the filter paper may have filtration pores with an average diameter of 25 μm to 45 μm.  
         [0018]     In a still further embodiment, the through-holes are distributed equidistantly from each other within the plane of the upstream-side filter layer.  
         [0019]     In a still further embodiment, the upstream-side filter layer and the downstream-side filter layer are directly bonded to each other without any additional bonding agent. In the case that a non-woven fabric and a filter paper respectively form the upstream-side filter layer and the downstream-side filter layer, fibers may be formed into the non-woven fabric as the fibers are directly applied onto the filter paper. Preferably, the fibers fall in a semi-melted state onto the filter paper. The fibers contacting the filter paper are bonded to the filter paper as the fibers solidify. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a vertical cross-sectional view of a part of a filter element according to an embodiment of the present invention; and  
         [0021]      FIG. 2  is a plan view of a part of the filter element showing an arrangement of through holes of an upstream-side filter layer; and  
         [0022]      FIG. 3  is a sectional view similar to  FIG. 1  but showing the diffusion of air into a downstream-side filter layer; and  
         [0023]      FIG. 4  is a graph showing the relationship between an open ratio of the upstream-side filter layer and the dust-holding amount; and  
         [0024]      FIG. 5 (A) is a table showing samples of filter elements having differing open ratios and through-hole diameters; and  
         [0025]      FIG. 5 (B) is a graph showing the correlation between the open ratio and the through-hole diameter of the upstream-side filter layer and the dust-holding amount; and  
         [0026]      FIG. 6  is a schematic, side elevation view of an apparatus for manufacturing the upstream-side filter layer; and  
         [0027]      FIG. 7  is a vertical sectional view of a part of a known filter element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved filter elements and methods and apparatus for producing filter elements. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings.  
         [0029]     A representative embodiment of the present invention will now be described with reference to FIGS.  1  to  6 . Referring to  FIG. 1 , a representative filter element  10  is designed for use with an air filter and has an upstream-side filter layer  12  and a downstream-side filter layer  14 . The upstream-side filter layer  12  is made of non-woven fabric and is adapted to hold dust and other contaminants in the air that have relatively large diameters. For this purpose, the mesh of the upstream-side filter layer  12  is set to be coarser than the mesh of the downstream-side filter layer  14 .  
         [0030]     As shown in  FIG. 2 , a plurality of through-holes  12   h  are formed in the upstream-side filter layer  12  and are distributed to be approximately equidistantly spaced from each other. The diameter of each of the through-holes  12   h  may be within a range of between 1 mm and 5 mm, such that the ratio of the total open area of the through-holes  12   h  to an effective filtration area of the downstream-side filter element  14  (hereinafter called “open ratio”) is within a range of between 5% and 20%.  
         [0031]     The non-woven fabric of the upstream side filter layer  12  may be formed by thermoplastic resin fibers produced by a melt-blown process that will be explained later.  
         [0032]     A manufacturing apparatus  20 , for manufacturing the non-woven fabric of the upstream-side filter layer  12 , is schematically shown by a side elevation view in  FIG. 6 . The manufacturing apparatus  20  includes a conveyor  24  that extends along a Y-direction. A belt-like base fabric  24   m  is laid horizontally on the conveyor  24 . Preferably, the base fabric  24   m  may be made of a meshed air-permeable fabric and is adapted to receive fibers F in a semi-melted or a semi-solid state. The fibers F are sprayed from a spinning nozzle  26 . Therefore, the fibers F may fall onto the base fabric  24   m  to be laid thereon to a predetermined thickness as the conveyer  24  moves in the Y-direction.  
         [0033]     The spinning nozzle  26  is positioned at a predetermined level above the base fabric  24   m  and is oriented downward. The spinning nozzle  26  is designed to produce the fibers F by a melt-blown process. More specifically, hot air may be blown out of the hot-air discharge holes  26   a  of the spinning nozzle  26 , against the melted resin that is ejected as the resin fibers F from a central resin ejection hole  26   b  of the spinning nozzle  26 . The resin fibers F are blown by the hot air and may fall onto the base fabric  24   m  while the resin fibers F are still in a semi-melted state. Each of the resin fibers F may contact with other resin fibers F and may be bonded thereto at the contact points. As a result, the bonded resin fibers F may form a non-woven fabric.  
         [0034]     The diameter of the resin fibers F constituting the upstream-side filter layer  12  may be determined as desired by adjusting the flow rate of the resin from the resin ejection hole  26   b , and/or by adjusting the flow rate of the hot air blown out of the hot-air discharge holes  26   a . In this representative embodiment, the diameter (i.e., average diameter) of the resin fibers F is set to be within a range of between 5 μm and 10 μm.  
         [0035]     The weight per unit area of the non-woven fabric of the upstream-side filter layer  12  may also be determined as desired by adjusting the transferring speed of the conveyor  24 . Thus, as the speed of the conveyor  24  decreases, the amount of fibers F laid on the base fabric  24   m  may increase, resulting in the increase of the weight per unit area of the non-woven fabric. On the contrary, as the speed of the conveyor  24  increases, the amount of fibers F laid on the base fabric  24   m  may decrease, resulting in the decrease of the weight per unit area of the non-woven fabric. In this representative embodiment, the weight per unit area of the non-woven fabric of the upstream-side filter layer  12  is set to be within a range of between 3 g/m 2  and 20 g/m 2 .  
         [0036]     After the non-woven fabric has been thus manufactured, the through-holes  12   h  of the upstream-side filter layer  12  may be formed by vertically sticking heated needles through the non-woven fabric, or cutting the non-woven fabric by circular blades or the like at a normal temperature.  
         [0037]     The downstream-side filter layer  14  may be formed by a filter paper and may serve to hold relatively small sized dust that has passed through the non-woven fabric of the upstream-side filter layer  12 , and to hold dust that has traveled through the through-holes  12   h . Therefore, the mesh of the filter paper of the downstream-side filter layer  14  is configured to be finer than the mesh of the upstream-side filter layer  12 . Preferably, the filter paper of the downstream-side filter layer  14  has filtration pores having an average diameter of between 25 μm and 45 μm.  
         [0038]     The non-woven fabric constituting the upstream-side filter layer  12  and the filter paper constituting the downstream-side filter layer  14  are joined together by an appropriate process, such as an embossing process and a lamination process. For example, clamping the non-woven fabric and the filter paper between the upper and lower members, one of which has small projections that are heated for partly fusing the non-woven fabric, may perform the embossing process. Thus, as the small projections stick into the non-woven fabric, the non-woven fabric may be bonded to the filter paper at the fused portions of the non-woven fabric. Placing an air-permeable hot melt sheet between the non-woven fabric and the filter paper may be done to perform a laminating process. By heating and pressing the non-woven fabric together with the filter paper, the non-woven fabric and the filter paper may be bonded together by the hot melt sheet.  
         [0039]     The operation of the representative filter element  10  will now be described with reference to  FIG. 3 . Ambient air may first flow through the upstream-side filter layer  12 , and then may further flow though the downstream-side filter layer  14 . The ambient air may thereafter be brought to a predetermined location. Before filtration by the filter element  10 , the ambient air may contain dust having diameters of less than a few hundred μm. The upstream-side filter layer  12  may hold the dust having relatively large diameters. In addition, the downstream-side filter layer  14  may hold the remaining dust having relatively small diameters.  
         [0040]     In addition, a part of the ambient air may directly reach the downstream-side filter layer  14  through the through-holes  12   h  of the upstream-side filter layer  12 . The downstream-side filter layer  14  may then filter this part of the ambient air before the filtered air is brought to a predetermined site.  
         [0041]     If the non-woven fabric of the upstream-side filter layer  12  becomes clogged as a result of the filter element being used for a long time period, the ambient air may flow through the through-holes  12   h  of the upstream-side filter layer  12 . The downstream-side filter layer  14  may then filter the ambient air, as long as the downstream-side filter layer  14  still has some filtration ability. The air reaching the downstream-side filter layer  14  through the through-holes  12   h  may enter directly opposing portions of the downstream-side filter element  14 , where the through-holes  12   h  are directly opposed to the downstream-side filter element  14 . The air also may diffuse into the surrounding portions that surround the directly opposing portions of the downstream-side filter layer  14  and are positioned on the backside of the non-woven fabric, as indicated by the narrow cross-hatchings in  FIG. 3 . Thus, the air may be filtered by flowing linearly through the directly opposing portions and also by diffusing into the surrounding portions.  
         [0042]     The relationship between the open ratio H and the amount of dust held by the filter element  10  is shown by the graph in  FIG. 4 . Here the open ratio H is a ratio of the total amount of the open areas of the through holes  12   h  to the effective filtration area of the downstream-side filter layer  14 . The “S” line indicates a reference level dust-holding amount. As will be seen from  FIG. 4 , the dust-holding amount becomes smaller than the reference amount S as the open ratio H becomes smaller than 5%. Presumably, if the open ration H is smaller than 5%, the through-holes  12   h  may not effectively deliver the ambient air to the downstream-side filter layer  14  in the event that the upstream-side filter layer  12  has become clogged. Therefore, the dust-holding ability of the downstream-side filter layer  14  may not be effectively utilized. The dust-holding amount also becomes smaller than the reference amount S as the open ratio H becomes greater than 20%. Presumably, this may be caused because the dust-holding ability of the upstream-side filter layer  12  is lowered by the presence of the through-holes  12   h.    
         [0043]     Therefore, in this representative embodiment, the open ratio H of the filter element  10  is set to be within a range of between 5% and 20% (5%&lt;H&lt;20%).  
         [0044]     The correlation between the open ratio H and the average diameter of the through-holes  12   h , and the dust-holding amount of the filter element  10 , is shown by a graph in  FIG. 5 (B). This correlation has been obtained from Sample Nos.  1  to  6  of the filter element  10  as shown by the table in  FIG. 5 (A).  
         [0045]     Sample No.  1  has an open ratio H of 0% (i.e., having no through-holes  12   h ) as shown in  FIG. 5 (A). This results in a dust-holding amount T 1  (T 1 &lt;S) as indicated by ♦ No.  1  in  FIG. 5 (B).  
         [0046]     Sample No.  2  has an open ratio H of 7% with through-holes  12   h  having an average diameter of 1.5 mm, as shown in  FIG. 5 (A). Consequently, sample No.  2  has a dust-holding amount T 2  (T 2 &gt;S) as indicated by ♦ No.  2  in  FIG. 5 (B).  
         [0047]     Sample No.  3  has an open ratio H of 11% with through-holes  12   h  having an average diameter of 1.8 mm, as shown in  FIG. 5 (A). As a result, sample No.  3  has a dust-holding amount T 3  (T 3 &gt;T 2 &gt;S) as indicated by ♦ No.  3  in  FIG. 5 (B).  
         [0048]     Sample No.  4  has an open ratio H of 18% with through-holes  12   h  having an average diameter of 2.0 mm, as shown in  FIG. 5 (A). This results in a dust-holding amount T 4  (T 2 &gt;T 4 &gt;S) as indicated by ♦ No.  4  in  FIG. 5 (B).  
         [0049]     Sample No.  5  has an open ratio H of 28% with through-holes  12   h  having an average diameter of 2.5 mm, as shown in  FIG. 5 (A). Sample No.  5  has a dust-holding amount T 5  (T 5 &lt;S) as indicated by ♦ No.  5  in  FIG. 5 (B).  
         [0050]     Sample No.  6  has an open ratio H of 40% with through-holes  12   h  having an average diameter of 2.5 mm, as shown in  FIG. 5 (A). Sample No.  6  has a dust-holding amount T 6  (T 6 &lt;T 5 &lt;S) as indicated by ♦ No.  6  in  FIG. 5 (B).  
         [0051]     As a result, it has been found that a dust collecting amount greater than the reference amount S may be achieved if the open ratio H is greater than 5% but smaller than 20% (5%&lt;H&lt;20%) with the condition that the average diameter of the through-holes  12   h  is between 1 mm and 5 mm (e.g., preferably between 1.5 mm and 2.0 mm).  
         [0052]     As described above, according to the representative filter element  10 , not only may the downstream-side filter layer  14  receive not the flow of the air filtered by the upstream-side filter layer  12 , but also receives the direct flow of the ambient air through the through-holes  12   h . Therefore, even in the event that the upstream-side filter layer  12  has been clogged by dust, the downstream-side filter layer  14  may filtrate ambient air. As a result, and in contrast to the known filter elements, if the downstream-side filter layer  14  still has filtration ability, the filter element  10  does not result in the problem that the filter element  10  can no longer be used.  
         [0053]     In addition, it is not necessary to form the upstream-side filter layer  12  to have a mesh that is needlessly coarse in order to avoid clogging of the upstream-side filter layer  12 . Therefore, the upstream-side filter layer  12  may have an improved dust-holding ability.  
         [0054]     Because the dust-holding abilities of both of the upstream-side filter layer  12  and the downstream-side filter layer  14  can be effectively utilized, the available dust-holding amount of the entire filter element  10  can be increased.  
         [0055]     Further, according to the representative embodiment, the average diameter of the through-holes  12   h  is set within a range of between 1 mm and 5 mm. The open ratio H is set within a range of between 5% and 20%. Therefore, the reduction in the available dust-holding amount by the upstream-side filter element  12  may be minimized, due at least in part to the configuration of the open ratio H of smaller than 20%. In addition, the ambient air may be smoothly delivered to the downstream-side filter layer  14  even in the event that the upstream-side filter layer  12  has clogged, due at least in part to the configuration of the open ratio H of greater than 5%.  
         [0056]     Furthermore, according to the representative embodiment, the upstream-side filter layer  12  is made of a non-woven fabric. The non-woven fabric may have a weight per unit area of 3 g/m 2  to 20 g/m 2  and is formed by fibers having an average diameter of 5 μm to 10 μm. Therefore, the upstream-side filter layer  12  may have a mesh that is proper for ensuring the dust-holding ability of the upstream-side filter layer  12 .  
         [0057]     Still furthermore, the air permeability of the filter element  10  may be improved because the upstream-side filter layer  12 . has a plurality of through-holes  12   h .  
         [0058]     Although the upstream-side filter layer  12  and the downstream-side filter layer  14  of the filter element  10  of the above representative embodiment are bonded together by an embossing process or a laminating process, any other type of bonding techniques can be used. For example, the upstream-side filter layer  12  may be directly bonded to the downstream-side filter layer  14  by having the downstream-side filter layer  14  directly receive the fibers F from the spinning nozzle  26 . This results in the non-woven fabric being directly formed on the downstream-side filter layer  14 .  
         [0059]     In addition, although filter paper is used as the downstream-side filter layer  14  in the above representative embodiment, filter paper may be replaced with a non-woven fabric having a fine mesh, e.g., with filtration pores having an average diameter of 20 μm to 90 μm.