Abstract:
A method is provided for filtering a fluid. The method includes the step of selecting a first surface filtration medium to include layers of a first mesh material (FMM). The method also includes the step of selecting a second surface filtration medium to include layers of a second mesh material (SMM). The layers of FMM are selected to include a first number of layers (FNL) for collectively providing a first absolute micron filter rating (FAMFR) and a first capacity for collecting a first type of particles. The layers of SMM are selected to include a second number of layers (SNL) different from the FNL. The SNL is selected for collectively providing a second absolute micron filter rating (SAMFR). The SAMFR is different from the FAMFR. The SNL is also selected for providing a second capacity for collecting a second type of particles. The second capacity is different from the first capacity.

Description:
BACKGROUND OF THE INVENTION 
     1. Statement of the Technical Field 
     The invention concerns a method for filtering particles from a fluid. More particularly, the method concerns a method for filtering particles of two or more size ranges, and having different relative concentrations. 
     2. Description of the Related Art 
     A filter typically consists of a cartridge or bag for use in a pressurized filter system. A pressurized filter system utilizing filter bags is typically comprised of a filter housing having a body and a cover. The body contains a filter basket for placing the filter bag therein. The filter housing is comprised of a mounting stand, an input orifice and an output orifice. A contaminated fluid is pumped into the filter housing through the input orifice. The contaminated fluid contains one or more types of particles to be filtered (PTBFs). Each type of PTBF has a different particle size. The contaminated fluid also contains different concentrations of each PTBF type. The PTBFs are filtered from the contaminated fluid as the fluid flows through the filter bag and then exists through the output orifice. Such a pressurized filter system is disclosed in U.S. Pat. No. 5,246,581. 
     Fluids to be filtered often contain various types of particles which are to be removed by the filtering operation. For example, a first type of particles may have a first range of particle sizes, whereas a second type of particulate may have a second range of particle sizes. Filtration apparatus such as filter bags or filter cartridges can be specially designed for these types of filtering operations. A filter bag or cartridge can often include a first type of filter material intended to filter the first type of particle, and a second type filter material intended to filter the second type of particle. For example, the first filter material can be designed to capture relatively large particles of the first type whereas the second filter material can be selected to capture relatively smaller particles of the second type. The first and second filter materials are commonly arranged in layers through which the fluid is passed. These arrangements can be quite effective for efficiently filtering the two different particle types. 
     Still, there are problems which tend to arise when attempting to filter various different types of particles contained in a fluid. For example, the concentration of the various different types of particles may be different. The concentration of the first type of particle within the fluid can be much higher as compared to the second type of particle. Stated differently, the two or more particle streams have different percentages of concentration and different weights. Alternatively the concentration of the second type of particle might be much higher as compared to the first type of particle. Over time, this difference in particle concentration will generally result in a larger quantity of one particle type or the other being collected in the filter. 
     Significantly, each filter material will have a limited capacity or capability to capture particulate matter. After this capacity is reached, the filter can become so heavily laden with filtered particulate matter that the flow rate of fluid through the filter material is substantially diminished. This condition is sometimes referred to as blinding. When a first type of particle is captured in a first filter material layer and a second type of particle is captured in a second filter material layer one filter material layer may become blinded before the other. For example, a concentration of a first particle type can be ten times higher as compared to a second particle type. In such cases, a first filter material for capturing the first particle type might become blinded at a rate that is ten times faster as compared to a second filter material for capturing a second particle type. Notably, the filter bag or cartridge must be replaced once any of the filter materials layers becomes blinded, even if other filter materials comprising the filter bag or cartridge still have additional capacity. This is due to the fact that the tilter material layers are arranged serially. If one layer is clogged with particles, the other layers will no longer function effectively. 
     The foregoing problem can result in a shortened life cycle for a filter bag or cartridge. A single layer of filter material, once clogged, can effectively require replacement of the entire filter bag. Accordingly, there is a need for an improved filter bag in which the life of filter mediums provided for particles having different types and concentrations is more effectively equalized. 
     SUMMARY OF THE INVENTION 
     A method is provided for filtering a fluid. The method includes the step of selecting a first surface filtration medium to include layers of a first mesh material (FMM). The layers of FMM are arranged in a stack. The method also includes the step of selecting a second surface filtration medium to include layers of a second mesh material (SMM). The layers of SMM are also arranged in the stack. The layers of FMM are selected to include a first number of layers for collectively providing a first absolute micron filter rating and a first capacity for collecting a first type of particles. The layers of SMM are selected to include a second number of layers for collectively providing a second absolute micron filter rating and a second capacity for collecting a second type of particles. The second number of layers is different front the first number of layers. Likewise, the second absolute micron filter rating is different from the first absolute micron filter rating. Similarly, the second capacity is different from the first capacity. 
     According to an aspect of the invention, a ratio is determined using a concentration value of the first type of particles and a concentration value the second type of particles. She second number of layers is selected using the determined ratio. 
     According to another aspect of the invention, the number of layers of the first and second mesh materials are selected to provide a useful life expectancy of the first surface filtration medium approximately equal to a useful life expectancy of the second surface filtration medium when filtering a known fluid having substantially unequal concentrations of the first and second type of particles. 
     According to another aspect of the invention, the number of layers of the second mesh material is selected using a known linear or non-linear relationship between particle concentrations and life expectancies of multilayer surface filtration mediums. Alternatively, the number of layers of the second mesh material is selected using known life expectancies of multilayer surface filtration mediums. 
     According to another aspect of the invention, the first number of layers is selected using a mathematical equation AMR=0.72 n-1 ·p, where AMR is an absolute micron rating, p is a predetermined pore size, and n is a number of layers. The first mesh material is selected to have a predetermined pore size larger than a highest limit value of a size range of the first type of particles. The first predetermined pore size is selected to be to be larger than the first absolute micron filter rating. The first absolute micron filter rating is selected to have a value less than or equal to a lowest limit value of a size range of the first type of particles. 
     According to yet another aspect of the invention, the second mesh material is selected to have a predetermined pore size larger than a highest limit value of a size range of the second type of particles. The second predetermined pore size is selected to be larger than the second absolute micron filter rating. The second absolute micron filter rating is selected to have a value less than or equal to a lowest limit value of a size range of the second type of particles. If the predetermined pore size of the first mesh material is larger than a predetermined pore size of the second mesh material, then the first surface filtration medium may be placed downstream from the second surface filtration medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which: 
         FIG. 1  is a flow diagram of a method for filtering a fluid including particles that is useful for understanding the present invention. 
         FIG. 2  is an enlarged cross-sectional view of a first surface filtration media that is useful for understanding the invention. 
         FIG. 3  is an enlarged cross-sectional view of a second surface filtration media that is useful for understanding the invention. 
         FIG. 4  is an enlarged cross-sectional view of a capacity based filter media that is useful for understanding the invention. 
         FIG. 5  is a perspective view of filter bag that is useful for understanding the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention concerns a method for designing a filter. The invention also concerns a method for filtering a fluid using a capacity based filter media. The capacity based filter media is designed in accordance with particle profiles. Each particle profile includes a particle type and a particle size range. The capacity based filter media is also designed in accordance with particle concentrations. The phrase “particle concentration” as used herein refers to the amount of a particle type existing within a fluid to be filtered. The capacity based filter media includes two or more stacks of surface filtration mediums. The phrase “surface filtration medium” as used herein refers to a medium that relies on the direct interception to trap particles and achieves filtration on a single plane. Each stack is provided to filter a particular particle type from the fluid. The stacks of surface filtration mediums are selectively designed to have the same or substantially similar life expectancies. Stated differently the stacks experience blinding at approximately the same point in time regardless of different concentrations of particle types in the fluid. The term “blinding” as used herein refers to the clogging of a filtration medium to an extent that a fluid flow rate through the filtration medium is substantially diminished as compared to its original flow rate. 
     The invention will now be described more fully hereinafter with reference to accompanying drawings, in which illustrative embodiments of the invention are shown. This invention, may however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
     Referring now to  FIG. 1A , there is provided a flow diagram of a method  100  for filtering a fluid including contaminant particles. As shown in  FIG. 1 , the method  100  begins at step  102  and continues with step  104 . In step  104 , two or more types of particles to be filtered (PTBF) from a fluid are selected. Each particle type has a different range of particle sizes. For example, a first particle type can have a particle size equal to one hundred fifty (150) microns+/−40 microns. A second particle type can have a particle size of thirty (30) microns+/−10 microns. Still, the invention is not limited in this regard. For example, the types of particles can be selected in accordance with an International Standard Code (ISO), such as ISO 4406. 
     After step  104 , the method  100  continues with step  106 . In step  106 , the size range for each type of PTBF is determined. In step  108 , the concentration of the PTBFs is determined. As should be understood, a fluid may contain different concentrations of the particle types. A concentration is determined for each particle type in step  108 . Steps  106 - 108  are performed to obtain size and concentration values for use in designing a capacity based filter media (described below in relation to steps  110 - 116  of  FIG. 1A , steps  118 - 124  of  FIG. 1B  and  FIGS. 2-4 ). 
     Subsequent to completing step  108 , the method  100  continues with step  110 . In step  110 , a first mesh material is selected. The first mesh material has a pore size and micron rating greater than a size of a first PTBF. Mesh materials are well known to persons skilled in the art, and therefore will not be described in detail herein. However, it should be appreciated that the first mesh material is a highly porous woven or non-woven mesh material. The highly porous woven or non-woven mesh material can be formed of a non-fiber shedding material. The first mesh material can be made from any suitable material that is temperature and fluid compatible with the filtering application to be carried out. For example, the first mesh material is made of a thermoplastic having polypropylene for low temperature filtering applications. Alternatively, the first mesh material is made of a thermoplastic having nylon for high temperature filtering applications. Still, the invention is not limited in this regard. 
     Referring again to  FIG. 1A , the method  100  continues with step  112 . In step  112 , a first number of layers of the first mesh material is determined using the micron rating of the first mesh material selected in step  110 . The first number of layers is selected to be sufficient in combination for providing a first surface filtration medium having a desired absolute micron rating (AMR). The first surface filtration medium will be described below in relation to step  114  of  FIG. 1A  and  FIG. 2 . 
     The desired AMR is selected to have a value less than or equal to the lower limit value of the size range of the first type of PTBF. However, the desired AMR can advantageously be achieved by using multiple layers of mesh with pores or apertures having sizes larger than the highest limit value of the size range of the first type of PTBF. The phrase “absolute micron rating” as used herein means that nearly one hundred percent (100%) of all particles larger than a stated micron size will be removed from a fluid being filtered. 
     Absolute micron ratings for various multilayer surface filtration mediums as described herein are listed in Table 1. 
                                                                                                                             TABLE 1                   Micron Rating           Of Mesh   Number of Mesh Material Layers            Material   2   3   4   5   6   7   8   9   10   11   12                    100   72.0   51.8   37.3   26.9   19.3   13.9   10.0   7.2   5.2   3.7   2.7       150   108.0   77.7   56.0   40.3   29.0   20.9   15.0   10.8   7.8   5.6   4.0       200   144.0   103.7   74.6   53.7   38.7   27.9   20.1   14.5   10.4   7.5   5.4       400   288.0   207.4   149.3   107.5   77.4   55.7   40.1   28.9   20.8   15.0   10.8       600   432.0   311.0   223.9   161.2   116.1   83.6   60.2   43.3   31.2   22.5   16.2                    
As illustrated in Table 1, the absolute micron rating achieved using the multi mesh layer as described is defined by: (a) a micron rating of a mesh material used to construct a surface filtration medium; and (b) the number of layers of mesh material used to form the surface filtration medium. For example, if a surface filtration medium is comprised of four (4) layers of a mesh material having a micron rating of four hundred (400), then an approximate value of the absolute micron rating of the surface filtration medium is computed using the following mathematical equation (1).
 
AMR=0.72 n-1   ·p   (1)
 
where AMR is an absolute micron rating, p is the pore size of each mesh layer, and n is the number of layers. Still, the invention is not limited in this regard.
 
     Referring once again to step  112 , a user can begin by selecting a preferred number of layers for the first surface filtration medium. Once the preferred number of layers is selected, the mathematical equation (1) or Table 1 can be used to determine the micron rating of a mesh material required in order to achieve a desired AMR for the first surface filtration medium. For example, if it is desired that the first surface filtration medium shall be comprised of four (4) layers, and an AMR of seventy-five (75) microns is desired, then Table 1 indicates that a mesh material having a micron rating of two hundred (200) should be chosen to form the first surface filtration medium. Such a selection will provide an AMR of seventy-four point six (74.6) microns. 
     Referring again to  FIG. 1A , the method  100  continues with step  114 . In step  114 , a first surface filtration medium is formed by arranging the first number of layers of first mesh material in a stack. A schematic illustration of a first surface filtration medium  200  is provided in  FIG. 2 . In this example, the first number of layers is equal to two (2). 
     As shown in  FIG. 2 , the surface filtration medium  200  is formed of layers of a highly porous mesh material  202 ,  204 . Although the surface filtration medium  200  of  FIG. 2  includes two (2) layers of a mesh material, the invention is not limited in this regard. For example, the layers  202 ,  204  can include (a) two or more layers of a woven-mesh material, (b) two or more layers of the non-woven mesh material, or (c) two or more layers of a woven mesh material and a non-woven mesh material. As noted above, the exact number of layers is determined in step  112 . 
     The multiple layers of mesh material  202 ,  204  advantageously provide a filtration material that has sufficient mechanical stability and strength so as to withstand the mechanical and hydraulic stresses to which the filtration material will be exposed when used in a high pressure filtration process. The multiple layers of mesh material  202 ,  204  also provide a surface filtration medium  200  with an increased surface area for collection of contaminants as compared to a filtration medium formed of a single layer of mesh material. It should be appreciated that the increased surface area is a three dimension (3D) surface area including a plurality of passages  206 . In effect, the surface filtration medium also has: (a) a relatively high particle capacity as compared to conventional filtration mediums; (b) a relatively high fluid throughput as compared to conventional filtration mediums; and (c) an increased life expectancy as compared to conventional filtration mediums. It should be noted that the features (a) and (b) results from a direct interception of particles within the tortuous paths created by the misaligned pores of mesh material. The features (a) and (b) also result from a bridging of particles on the surface filtration medium. The term “bridging” as used herein refers to the collection of particles in a manner so as to form a bridge over a passage of a mesh material. Bridging is well known to persons skilled in the art, and therefore will not be described in great detail herein. 
     As shown in  FIG. 2 , the layers of a mesh material  202 ,  204  are not aligned with cacti other. Each layer of the mesh material  202 ,  204  has the same pore structure  206 . This pore configuration advantageously provides a multilayer redundant architecture which achieves a desired AMR (described above in relation to step  112  of  FIG. 1A ) using a mesh material having a pore size that is substantially larger than the AMR to prevent blinding. Still, the invention is not limited in this regard. For example, the surface filtration medium can include layers of a mesh material that are aligned with each other. 
     Referring again to  FIG. 1A , the method  100  continues with step  116 . In step  16 , a ratio is determined using, a first and second concentration value obtained in step  108 . The first concentration value denotes the amount of a first type of PTBF in a fluid to be filtered. The second concentration value denotes the amount of a second type of PTBF in the fluid to be filtered. The term “ratio” as used herein refers to a quantity that denotes the magnitude of a concentration of a first type of PTBF relative to a concentration of a second type of PTBF. The ratio can be calculated by dividing the first concentration value by the second concentration value. For example, if the first concentration value equals four (4) and the second concentration value equals eight (8), then the ratio is one to two (1:2). The ratio (1:2) indicates the there are twice as many particles of the second type than the first type in a fluid to be filtered. 
     Referring again to  FIG. 1A , the method  100  continues with step  118  of  FIG. 1B . In step  118 , a number of layers of a second mesh material is determined. The number of layers is selected to be sufficient in combination for providing a second surface filtration medium having a life expectancy approximately equal to a life expectancy the first surface filtration medium (described above in relation to step  114  of  FIG. 1A  and  FIG. 2 ). Stated differently, the first and second surface filtration mediums will experience blinding at approximately the same point in time regardless of different concentrations of the first and second PTBF types in the fluid. As a result, the second surface filtration medium has a non-blinding time frame approximately equal to a non-blinding time frame of the first surface filtration medium. The phrase “non-blinding time frame” as used herein refers to the period of time during which blinding of a surface filtration media does not occur. 
     The number of layers of mesh material can be determined (a) using a known linear relationship between particle concentrations and life expectancies of multilayer surface filtration mediums, (b) using a known non-linear relationship between particle concentrations and life expectancies of multilayer surface filtration mediums, or (e) using known life expectancies of multilayer surface filtration mediums configured for filtering different types of PTBF and different concentrations of PTBF. The known life expectancies can be determined heuristically. 
     In many filtering applications, a linear relationship exists between particle concentrations and life expectancies of surface filtration mediums. In such a scenario, the number of layers of mesh material can be determined using the ratio computed in step  116  of  FIG. 1A . For example, if the ratio is one to two (1:2), then the number of layers of a second mesh material is twice the number of layers of the first mesh material used to form the first surface filtration medium. In such a scenario, it should be appreciated that the capacity of each stack can be defined by the mathematical equation (2).
 
 C   stack   =C   L   ·N   (2)
 
where C stack  represents the capacity of a stack of mesh material, C L  represents the capacity of a layer of mesh material, and N is the number of layers of mesh material in the stack. Still, the invention is not limited in this regard.
 
     Referring again to  FIG. 1B , the method  100  continues with step  120 . In step  120 , a second mesh material is selected for providing a second surface filtration medium having a desired absolute micron rating (AMR). The AMR of the second surface filtration medium is defined by the following mathematical equation (3).
 
AMR 2 =0.72 n-1   ·p   (3)
 
where AMR 2  is an absolute micron rating, p is the pore size of each mesh layer, and n is the number of layers. The second mesh material can be selected using Table 1 (described above in relation to step  110  of  FIG. 1A ) and the number of layers determined in step  118  of  FIG. 1B . The second mesh material is also selected to have a pore size and micron rating greater than the highest limit value of the size range of the second type of PTBF. The second mesh material is further selected to be a highly porous woven or non-woven mesh material. The highly porous woven or non-woven mesh material can be formed of a non-fiber shedding material. The second mesh material can be made from any suitable material that is temperature and fluid compatible with the filtering application to be carried out.
 
     After step  120 , the method  100  continues with step  122 . In step  122 , a second surface filtration medium is formed by arranging the number of layers of second mesh material in a stack. A schematic illustration of a second surface filtration medium  300  is provided in  FIG. 3 . 
     As shown in  FIG. 3 , the second surface filtration medium  300  is formed of layers of a highly porous mesh material  302 ,  304 ,  306 ,  308 . Although the second surface filtration medium  300  of  FIG. 3  includes four (4) layers of a mesh material, the invention is not limited in this retard. For example, the layers  302 ,  304 ,  306 ,  308  can include (a) two or more layers of a woven-mesh material, (b) two or more layers of the non-woven mesh material, or (c) two or more layers of a woven mesh material and a non-woven mesh material. 
     As shown in  FIG. 3 , the layers of a mesh material  302 ,  304 ,  306 ,  308  are not aligned with each other. Each layer of the mesh material  302 ,  304 ,  306 ,  308  has the same pore structure  312 . This pore configuration advantageously provides a multilayer redundant architecture which achieves a desired AMR using mesh material having a pore size that is substantially larger than the AMR to prevent blinding. Still, the invention is not limited in this regard. For example, the surface filtration medium can include layers of a mesh material that are aligned with each other. 
     Referring again to  FIG. 1B , the method  100  continues with step  124 . In step  124 , a capacity based filter media is formed by joining the first and second surface filtration mediums together using any suitable technique known in the art. Such techniques include, but are not limited to, an ultrasonic welding technique. Notably, the AMR of such a capacity based filter media is defined by the following mathematical equation (4).
 
AMR CBFM =AMR 2   (4)
 
where AMR CBFM  is an absolute micron rating of the capacity based filter media and AMR 2  is the absolute micron rating of the second surface filtration medium. However, it should be understood that the AMR of a capacity based filter media can generally be defined by the following mathematical equation (5).
 
AMR CBFM =AMR N   (5)
 
where AMR CBFM  is an absolute micron rating of a capacity based filter media and AMR N  is the absolute micron rating of the outermost (or most downstream) surface filtration medium. As should be understood, the outermost (or most downstream) surface filtration medium has a pore size smaller than the pore sizes of the other surface filtration mediums used to form the capacity based filter media. A greatly enlarged side view of a capacity based filter media  400  is provided in  FIG. 4 .
 
     Referring now to  FIG. 4 , the capacity based surface filtration media  400  is configured to filter a first and second type of PTBF from a fluid to be filtered. The fluid to be filtered has a larger concentration of the second type of PTBF than the first type of PTBF. Accordingly, the first surface filtration medium  200  has a first capacity for capturing particles of the first type. The second filtration medium  300  has a second capacity for capturing particles of the second type. The first and second capacities are unequal, thereby providing a capacity based surface filtration media  400  having surface filtration mediums  200 ,  300  with equal useful life expectancies. 
     It should be noted that the first type of PTBF has a size range larger than a size range of the second type of PTBF. Accordingly, the capacity based filter media  400  is formed by placing the second surface filtration medium  300  downstream from the first surface filtration medium  200 . In effect, the capacity based filter media  400  has a graded or a tapered pore structure. The graded or tapered pore structure provides a capacity based filter media  400  whereby the pore size decreases in the direction of fluid flow. Still, the invention is not limited in this regard. 
     Referring again to  FIG. 1B , the method  100  continues with step  126 . In step  126 , a contaminated fluid is pumped (or passed) transversely through the surfaces defined by the various layers. Thereafter, step  128  is performed where the method  100  ends. 
     It should be noted that the above described method  100  can be implemented in a filter bag. An exemplary embodiment of a filter bag is provided in  FIG. 5 . As shown in  FIG. 5 , a filter bag  500  is comprised of a bag member  502 , a ring member  506  and an input orifice  504 . A contaminated fluid is pumped into the filter bag  500  through the input orifice  504 . The fluid flows through the filter bag  500  to produce a filtered fluid. Such fluids include, but are not limited to, water, beverages, alcohols, pharmaceutical liquids, oils, cosmetic liquids and bio-fuels. 
     The bag member  502  is composed of a capacity based filter media. The capacity based filter media is the same as the capacity based filter media formed in step  124  of  FIG. 1B . In this regard, it should be appreciated that the capacity based filter media is comprised of layers of a mesh material having a number of pores to control fluid flow. The layers of mesh material can have an identical pore structure or a graded (or tapered) pore structure. The graded or tapered pore structure provides a capacity based filter media whereby a pore size can decrease in the direction of fluid flow. i.e. from an input orifice  504  through the filter bag  500 . The ring member  506  is comprised of one or more handles  508 - 1 ,  508 - 2 . The ring member  506  is made of a semi-rigid material, such as a thermoplastic. 
     All of the apparatus, methods and algorithms disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the invention has been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the apparatus, methods and sequence of steps of the method without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain components may be added to, combined with, or substituted for the components described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined.