Abstract:
A filter for made of randomly arranged filaments. The filter has an upstream side where fluid enters and a downstream side where fluid exits. The filaments are greatest in diameter near the upstream side wherein the filaments continuously and gradually decrease in thickness toward the downstream side. The pores or spaces between the filaments are largest near the upstream side and decrease gradually toward the downstream side. This allows various size particles to become entrained in the filter in an evenly distributed manner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 11/255,585, filed Oct. 21, 2005 and patent application Ser. No. 11/255,584, filed Oct. 21, 2005; both of which claim the benefit of and priority to U.S. provisional application Ser. No. 60/672,894, filed Apr. 19, 2005, the subject matter of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Filters composed of an isotropic material are subject to premature clogging due to particulate collecting on the upstream surface of the filter, where fluid enters. Ideally filtered particles would be evenly distributed throughout the thickness of a filter so that longer filter life could be realized. The best way to achieve such a particulate distribution is to have porosity continuously decrease throughout the thickness of the filter in the direction of fluid flow through the filter.  
         [0003]     One method of achieving a more uniform particulate distribution in a filter is disclosed in U.S. Pat. No. 6,926,828. The filter medium used in this invention is a flexible, isotropic, and porous material such as expanded foam, which is enclosed in a case body. The case body progressively compresses the filter medium along the fluid flow direction such that the pores in the filter material are progressively compressed tighter together, thereby capturing finer particles. This design requires an external structure such as a case body to support and compress the filter medium.  
         [0004]     Another method of achieving a more uniform particulate distribution is to “intercalate” foam on a porous substrate as disclosed in US Pat. Pub. 2003/0084788 A1. This invention puts a polymeric or other type of expanding foam onto a porous substrate, then allowing the foam to expand. The expansion of the foam through the substrate and outside of the substrate produces distinct regions with different porosity. However, this does not produce a material with continuously and gradually decreasing pore size.  
         [0005]     An attempt to produce a filter medium with varying pore sizes is disclosed in U.S. Pat. No. 6,387,141. This invention uses an isotropic nonwoven fiberous medium which is subjected to a liquid jetting. This compresses the fibers on the side facing the liquid jet, thereby reducing the porosity on the jetted side. Another embodiment of this invention is to mix fibers of different coarseness together to form at least two layers of different properties. The porosity is changed by changing the ratio of coarse fibers to fine fibers in the mixture forming each layer. The assembled layers are then liquid jetted on one side to produce intertwinements of the fibers that connect the layers.  
         [0006]     Thus there remains a need to produce a filter that structurally supports itself, has a pore size that continuously and gradually decreases through the filtration medium, without relying on compression of the medium or mixing of different fibers to achieve a porosity gradient.  
       SUMMARY OF THE INVENTION  
       [0007]     This invention is a filter for fluids. The filter has an upstream side where fluid enters and a downstream side where fluid exits. The filter is made of filaments that are the greatest thickness near the upstream side gradually and continuously decreasing in thickness toward the downstream side. This results in the spaces or pores between the filaments being largest near the upstream side. The pores gradually and continuously decrease in size toward the downstream size. This causes particles of different sizes to be evenly distributed through the filter.  
         [0008]     Accordingly, it is an object of this invention to provide a filter which is of economical construction and which is of efficient operation.  
         [0009]     Still another object of this invention is to provide a filter for fluids that provides for more even distribution of filtered particulate matter throughout the thickness of the filter.  
         [0010]     Other objects of the invention become apparent upon the reading of the following description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is shows the machine used to make the filter;  
         [0012]      FIG. 2  is a side view of a filter bag;  
         [0013]      FIG. 3  is a microscopic view of the filaments taken along line  3 - 3  in  FIG. 2 ;  
         [0014]      FIG. 4  is a side view of a filter bag; and  
         [0015]      FIG. 5  is a microscopic view of the filaments taken as shown in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0016]     The preferred embodiment herein described is not intended to be exhaustive or to limit the invention to precise form disclosed. It is chosen and described to explain the principles of the invention and its application and practical use to enable others skilled in the art to best utilize the invention.  
         [0017]     This invention may be best understood by the following descriptions and the workings of the equipment used to produce the filter, which is shown as a filter bag  10 . As illustrated in  FIG. 1 , a quantity of polymer material, preferably polypropylene or other thermoplastic materials capable of producing filaments  20  when molten and air dried, is introduced into an extruder  12  at hopper  14  and is fed through a nozzle  16 . A plurality of ring heaters  18  circumscribe the nozzle  16  and serve to produce heat sufficient to liquefy the polymer material as it flows through the nozzle  16 . The nozzle  16  terminates in a plurality of laterally spaced discharge outlets  22  through which the polymer material in its molten state is propelled to form filaments  20 . Air heated in manifolds  23 , then directed through ducts  17 , and blown across the filaments from above and below at an angle as shown in  FIG. 1 . The air produces a turbulent flow. The air from the manifolds  23  helps to propel and stretch the filaments  20  as they leave the discharge outlets  22 . The filaments  20  are propelled toward a mandrel  28 . The mandrel  28  may be formed from metal, wood or similar material and resembles in its outer configuration the intended shape of the filter bag  10  to be produced. Mandrel  28  is rotated about an axis  35  in the direction shown within the flow path of the filaments  20  from the discharge outlets  22 . The mandrel  28  is placed between 1 to 3 feet from outlets  22  and rotated at a constant speed such as between 30 to 80 rpm. The filaments  20  are sufficiently cooled from a molten state such that the filaments  20  adhere to each other to form a sidewall  32  of the filter bag  10 . Turbulence, as the filaments  20  are propelled from the discharge outlets  22  toward the mandrel  28 , causes the filaments  20  to overlap in a random pattern as they are deposited on the mandrel  28 .  
         [0018]     The random distribution of filaments  20  is shown in  FIGS. 3 and 5 . The filaments  20  define pores  24 . In the preferred embodiment of this invention, the diameters of filaments  20  continuously and gradually decrease as the thickness of the sidewall  32  increases. This is best illustrated in viewing  FIGS. 3 and 5  where a section of the sidewall  32  of the filter bag  10  has been magnified for illustrative purposes.  FIGS. 3 and 5  show the largest diameter filaments  20  being at the inside of the bag  10  and the smallest diameter of the filaments  20  being at the outside of the bag. The direction of fluid flow through the bag  10  being from the inside towards the outside of the bag  10  as shown by arrow A. Where the filaments  20  are largest, near the inside of the bag  10 , the pores or spaces between the filaments  20  are the largest, and as the filaments  20  decrease in size the pores or spaces  24  between the filaments  20  decrease in size. Thus, the larger particulate matter being filtered from the fluid will first become entrained within the filter bag  10  closer to the inside of the bag  10  and particulate matter of gradually decreasing size will be distributed throughout the thickness of the sidewall  32 , with the smallest particles captured near the outside of the bag  10 .  
         [0019]     The thickness of the filaments  20  of bag  10  may range from 50 to 200 microns towards the inside of the bag and continuously become smaller in thickness to 0.5 microns at the outside surface of the bag  10  with sidewall  32  of the bag  10  being approximately ¾ to 1 inch thick. The precise thickness of the filaments  20  and thickness of the bag  10  can vary depending upon the type of material intended to be filtered and the size of the filter bag  10 .  
         [0020]     In producing filter bag  10 , the thicker filaments  20  are first deposited upon the mandrel  28  and then as the bag&#39;s thickness progressively increases, the filaments  20  are decreased in size until the filaments  20  smallest in size at the outside surface of the bag  10 . This progressively decreasing filament  20  thickness is accomplished by varying three parameters which are: (1) airspeed of the air blown across the filaments, (2) temperature of the molten polymer, and (3) throughput of molten polymer leaving the nozzle  16 . Considering the parameter of airspeed alone, increasing the airspeed will decrease the thickness of the filaments  20  and increase their length. If the temperature of the molten polymer alone is changed, an increase in the temperature will decrease the filament  20  thickness, and decreasing the temperature will make the filaments  20  thicker. Changing throughput alone will thicken the filaments  20  when throughput is increased, and decrease the thickness when the throughput is reduced. During production more than one parameter may be changed in particular combinations such that a filter bag  10  with desired characteristics is produced.  
         [0021]     Also, in addition to varying the thickness of the filaments  20 , by varying the three parameters, the stiffness of the filaments  20  can be increased so that the inner wall of the filter bag becomes stiff or rigid. Generally filaments  20  of greater thickness will be more rigid. Since the filaments  20  near the inside of the bag  10  will be the largest in the filter these will be the most rigid filaments  20 . These filaments  20  nearest the inside will help the filter bag  10  maintain its shape. Additionally, bag sidewalls  32  may substantially collapse, due to pressure forcing the inside of the bag toward the outside of the bag during use, reducing the bag&#39;s  10  permeability and filtering capacity. By rigidifying the sidewall of the bag  10  at the side where the fluid first contacts, the bag  10  filaments remain intact and provide pores or spaces  24  between the filaments  20  to catch or entrain filtered particles.  
         [0022]     The invention is not to be limited to the details above given but may be modified within the scope of the claims.