Filter material construction and method

A preferred filter media is provided. The media includes a fine fiber web secured to the surface of a coarse fiber support. A preferred filter media, comprising multiple layers of fine fiber media separated by coarse fiber support, is provided. Advantageous filter constructions result and are provided. Also according to the disclosure, methods for using such arrangements to filter are provided.

 

What is claimed is:

1. An air filter construction comprising:

(a) a first filter media arrangement having a configuration of one of pleated and corrugated, the media arrangement comprising a multi-layer composite and including at least first, second, and third layers of fine fibers therein;

(i) said first layer of fine fibers comprising a most upstream layer of said first, second, and third layers of fine fibers;

(A) said first layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said first layer of fine fibers having a first efficiency for filtering, on its own, of no greater than 32.6%;

(ii) said second layer of fine fibers being downstream of said first layer of fine fibers;

(A) said second layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said second layer of fine fibers being spaced from said first layer of fine fibers by a distance no greater than about 254 microns;

(C) said second layer of fine fibers having a second efficiency for filtering, on its own, of no greater than 32.6%;

(iii) said third layer of fine fibers being downstream of both of said first layer of fine fibers and said second layer of fine fibers;

(A) said third layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said third layer of fine fibers being spaced from said second layer of fine fibers by a distance no greater than about 254 microns;

(C) said third layer of fine fibers having a third efficiency for filtering, on its own, that is greater than said second efficiency for filtering and that is no greater than 68.8%.

2. An air filter construction according to claim 2 wherein:

(a) said media arrangement comprises a pleated construction including more than two pleats per 2.54 cm.

3. An air filter construction according to claim 2 wherein:

(a) said pleated construction includes each pleat having a pleat depth of at least 0.6 cm.

4. An air filter construction according to claim 1 wherein:

(a) said third efficiency for filtering is greater than said first efficiency for filtering.

5. An air filter construction according to claim 1 wherein:

(a) said first efficiency for filtering is at least 8.8%;

(b) said second efficiency for filtering is at least 8.8%; and

(c) said third efficiency for filtering is at least 32.6%.

6. An air filter construction according to claim 5 wherein:

(a) said first efficiency for filtering is no greater than 25.5%;

(b) said second efficiency for filtering is no greater than 25.5%; and

(c) said third efficiency for filtering is no greater than 54.5%.

7. An air filter construction according to claim 6 wherein:

(a) said first efficiency for filtering is about 10%;

(b) said second efficiency for filtering is about 20%; and

(c) said third efficiency for filtering is about 40%.

8. An air filter construction according to claim 1 wherein:

(a) said second layer of fine fibers is spaced from said first layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said second layer of fine fibers; and

(b) said third layer of fine fibers is spaced from said second layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said third layer of fine fibers.

9. A system comprising:

(a) a gas turbine apparatus having a gas flow intake; and

(b) a gas filter construction oriented to filter gas flowing into said gas flow intake; said gas filter construction including:

(i) a first filter media arrangement having a configuration of one of pleated and corrugated; the first filter media arrangement comprising a multi-layer composite including at least first, second, and third layers of fine fibers therein;

(A) said first layer of fine fibers comprising a most upstream layer of said first, second, and third layers of fine fibers;

(1) said first layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(2) said first layer of fine fibers having an efficiency for filtering, on its own, of no greater than 32.6%;

(B) said second layer of fine fibers being downstream of said first layer of fine fibers;

(1) said second layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(2) said second layer of fine fibers being spaced from said first layer of fine fibers by a distance no greater than about 254 microns;

(3) said second layer of fine fibers having an efficiency for filtering, on its own, of no greater than 32.6%; and

(C) said third layer of fine fibers being downstream of both of said first layer of fine fibers and said second layer of fine fibers;

(1) said third layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(2) said third layer of fine fibers being spaced from said second layer of fine fibers by a distance no greater than about 254 microns;

(3) said third layer of fine fibers having an efficiency for filtering, on its own, that is greater than the efficiency for filtering of said second layer, and no greater than 68.8%.

10. A system according to claim 9 wherein:

(a) said first layer of fine fibers has an efficiency for filtering at least 8.8%;

(b) said second layer of fine fibers has an efficiency for filtering at least 8.8%; and

(c) said third layer of fine fibers has an efficiency for filtering at least 25.5%.

11. A system according to claim 9 wherein:

(a) said first layer of fine fibers has an efficiency for filtering that is no greater than 25.5%; and

(b) said second layer of fine fibers has an efficiency for filtering that is no greater than 25.5%.

12. A system according to claim 9 wherein:

(a) said third layer of fine fibers has an efficiency for filtering that is no greater than 54.5%.

13. A system according to claim 9 wherein:

(a) said second layer of fine fibers is spaced from said first layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said second layer of fine fibers; and

(b) said third layer of fine fibers is spaced from said second layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said third layer of fine fibers.

14. A system according to claim 9 wherein:

(a) said first filter media arrangement comprises a pleated construction having more than two pleats per 2.54 cm.

15. A filter element comprising:

(a) first and second, opposite, end caps;

(b) a region of depth media extending between said first and second end caps;

(i) said region of depth media having first and second, opposite sides;

(ii) said region of depth media including a tangle of fibrous material of about 2-3% solidity;

(c) a media construction extending between said first and second end caps; said media construction being secured to said first side of said region of depth media;

said media construction having a configuration of one of pleated and corrugated and including at least first, second, and third layers of fine fibers therein;

(i) said first layer of fine fibers comprising a most upstream layer of said first, second, and third layers of fine fibers;

(A) said first layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said first layer of fine fibers having a first efficiency for filtering, on its own, of no greater than 32.6%;

(ii) said second layer of fine fibers being downstream of said first layer of fine fibers;

(A) said second layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said second layer of fine fibers being spaced from said first layer of fine fibers by a distance no greater than about 254 microns;

(C) said second layer of fine fibers having a second efficiency for filtering, on its own, of no greater than 32.6%;

(iii) said third layer of fine fibers being downstream of both of said first layer of fine fibers and said second layer of fine fibers;

(A) said third layer of fine fibers having an average fiber diameter of no greater than about 5 microns;

(B) said third layer of fine fibers being spaced from said second layer of fine fibers by a distance no greater than about 254 microns;

(C) said third layer of fine fibers having a third efficiency for filtering, on its own, that is greater than said second efficiency for filtering and that is no greater than 68.8%.

16. A filter element according to claim 15 wherein:

(a) said first efficiency for filtering is at least 8.8%;

(b) said second efficiency for filtering is at least 8.8%; and

(c) said third efficiency for filtering is at least 32.6%.

17. A filter element according to claim 16 wherein:

(a) said first efficiency for filtering is not greater than 25.5%;

(b) said second efficiency for filtering is not greater than 25.5%; and

(c) said third efficiency for filtering is not greater than 54.5%.

18. A filter element according to claim 17 wherein:

(a) said second layer of fine fibers is spaced from said first layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said second layer of fine fibers.

19. A filter element according to claim 15 wherein:

(a) said third layer of fine fibers is spaced from said second layer of fine fibers by a distance of no more than 5 times of the average fiber diameter of said third layer of fine fibers.

20. A filter element according to claim 15 further including:

(a) an inner liner extending between said first and second end caps; and

(b) an outer liner extending between said first and second end caps.

FIELD OF THE INVENTION

The present invention relates to filters, filter constructions, materials for use in filter constructions and methods of filtering. Applications of the invention particularly concern filtering of particles from fluid streams, for example from air streams. The techniques described herein particularly concern the utilization of arrangements having one or more layers of fine fibers in the filter media, to advantage.

BACKGROUND OF THE INVENTION

Fluid streams such as air and gas streams often carry particulate material therein. In many instances, it is desirable to remove some or all of the particulate material from the fluid stream. For example, air intake streams to the cabins of motorized vehicles, to engines for motorized vehicles, or to power generation equipment; gas streams directed to gas turbines; and, air streams to various combustion furnaces, often include particulate material therein. In the case of cabin air filters it is desirable to remove the particulate matter for comfort of the passengers and/or for aesthetics. With respect to air and gas intake streams to engines, gas turbines and combustion furnaces, it is desirable to remove the particulate material because it can cause substantial damage to the internal workings to the various mechanisms involved.

In other instances, production gases or off gases from industrial processes or engines may contain particulate material therein. Before such gases can be, or should be, discharged through various downstream equipment and/or to the atmosphere, it may be desirable to obtain a substantial removal of particulate material from those streams.

A variety of fluid filter arrangements have been developed for particulate removal. For reasons that will be apparent from the following descriptions, improvements have been desired for arrangements developed to serve this purpose.

A general understanding of some of the basic principles and problems of air filter design can be understood by consideration of the following types of media: surface loading media; and, depth media. Each of these types of media has been well studied, and each has been widely utilized. Certain principles relating to them are described, for example, in U.S. Pat. Nos. 5,082,476; 5,238,474; and 5,364,456. The complete disclosures of these three patents are incorporated herein by reference.

In general, for any given application, filter design has typically concerned a trade off of features designed for high filter efficiency and features designed to achieve high capacity (i.e. long filter lifetime). The "lifetime" of a filter is typically defined according to a selected limiting pressure drop across the filter. That is, for any given application, the filter will typically be considered to have reached its lifetime of reasonable use, when the pressure buildup across the filter has reached some defined level for that application or design. Since this buildup of pressure is a result of load, for systems of equal efficiency a longer life is typically directly associated with higher capacity.

Efficiency is the propensity of the media to trap, rather than pass, particulates. It should be apparent that typically the more efficient a filter media is at removing particulates from a gas flow stream, in general the more rapidly the filter media will approach the "lifetime" pressure differential (assuming other variables to be held constant).

Paper filter elements are widely used forms of surface loading media. In general, paper elements comprise dense mats of cellulose fibers oriented across a gas stream carrying particulate material. The paper is generally constructed to be permeable to the gas flow, and to also have a sufficiently fine pore size and appropriate porosity to inhibit the passage of particles greater than a selected size therethrough. As the gases (fluids) pass through the filter paper, the upstream side of the filter paper operates through diffusion and interception to capture and retain selected sized particles from the gas (fluid) stream. The particles are collected as a dust cake on the upstream side of the filter paper. In time, the dust cake also begins to operate as a filter, increasing efficiency. This is sometimes referred to as "seasoning," i.e., development of an efficiency greater than initial efficiency.

A simple filter design such as that described above is subject to at least two types of problems. First, a relatively simple flaw, i.e. rupture of the paper, results in failure of the system. Secondly, when particulate material rapidly builds up on the upstream side of the filter, as a thin dust cake or layer, it eventually substantially blinds off or occludes portions of the filter to the passage of fluid therethrough. Thus, while such filters are relatively efficient, they are not generally associated with long lifetimes of use, especially if utilized in an arrangement involving the passage of large amounts of fluid therethrough, with substantial amounts of particulate material at or above a "selected size" therein; "selected size" in this context meaning the size at or above which a particle is effectively stopped by, or collected within, the filter.

Various methods have been applied to increase the "lifetime" of surface-loaded filter systems, such as paper filters. One method is to provide the media in a pleated construction, so that the surface area of media encountered by the gas flow stream is increased relative to a flat, non-pleated construction. While this increases filter lifetime, it is still substantially limited. For this reason, surface-loaded media has primarily found use in applications wherein relatively low velocities through the filter media are involved, generally not higher than about 20-30 feet per minute and typically on the order of about 10 feet per minute or less. The term "velocity" in this context is the average velocity through the media (i.e., flow volume.div.media area).

In general, as air flow velocity is increased through a pleated paper media, filter life is decreased by a factor proportional to the square of the velocity. Thus, when a pleated paper, surface loaded, filter system is used as a particulate filter for a system that requires substantial flows of air, a relatively large surface area for the filter media is needed. For example, a typical cylindrical pleated paper filter element of an over-the-highway diesel truck will be about 9-15 inches in diameter and about 12-24 inches long, with pleats about 1-2 inches deep. Thus, the filtering surface area of media (one side) is typically 37 to 275 square feet.

In many applications, especially those involving relatively high flow rates, an alternative type of filter media, sometimes generally referred to as "depth" media, is used. A typical depth media comprises a relatively thick tangle of fibrous material. Depth media is generally defined in terms of its porosity, density or percent solids content. For example, a 2-3% solidity media would be a depth media mat of fibers arranged such that approximately 2-3% of the overall volume comprises fibrous materials (solids), the remainder being air or gas space.

Another useful parameter for defining depth media is fiber diameter. If percent solidity is held constant, but fiber diameter (size) is reduced, pore size is reduced; i.e. the filter becomes more efficient and will more effectively trap smaller particles.

A typical conventional depth media filter is a deep, relatively constant (or uniform) density, media, i.e. a system in which the solidity of the depth media remains substantially constant throughout its thickness. By "substantially constant" in this context, it is meant that only relatively minor fluctuations in density, if any, are found throughout the depth of the media. Such fluctuations, for example, may result from a slight compression of an outer engaged surface, by a container in which the filter media is positioned.

Gradient density depth media arrangements have been developed. Some such arrangements are described, for example, in U.S. Pat. Nos. 4,082,476; 5,238,474; and 5,364,456. In general, a depth media arrangement can be designed to provide "loading" of particulate materials substantially throughout its volume or depth. Thus, such arrangements can be designed to load with a higher amount of particulate material, relative to surface-loaded systems, when full filter lifetime is reached. However, in general the tradeoff for such arrangements has been efficiency, since, for substantial loading, a relatively low solids media is desired. Gradient density systems such as those in the patents referred to above, have been designed to provide for substantial efficiency and longer life. In some instances, surface-loading media is utilized as a "polish" filter in such arrangements.

SUMMARY OF THE INVENTION

According to certain aspects of the present invention, a filter media construction is provided. The filter media construction can be used as a filter media in preferred filter arrangements. It may, in some instances, be utilized as one layer of media in a multi-layer arrangement, for example. In some arrangements, layers of filter media according to the present invention can be stacked, to create a preferred construction.

A preferred filter media construction according to the present invention includes a first layer of permeable coarse fibrous media having a first surface. A first layer of fine fiber media is secured to the first surface of the first layer of permeable coarse fibrous media. Preferably the first layer of permeable coarse fibrous material comprises fibers having an average diameter of at least 10 microns, typically and preferably about 12 (or 14) to 30 microns. Also preferably the first layer of permeable coarse fibrous material comprises a media having a basis weight of no greater than about 50 grams/meter.sup.2, preferably about 0.50 to 25 g/m.sup.2, and most preferably at least 8 g/m.sup.2. Preferably the first layer of permeable coarse fibrous media is at least 0.0005 inch (12 microns) thick, and typically and preferably is about 0.001 to 0.010 inch (25-254 microns) thick.

In preferred arrangements, the first layer of permeable coarse fibrous material comprises a material which, if evaluated separately from a remainder of the construction by the Frazier permeability test, would exhibit a permeability of at least 150 meters/min, and typically and preferably about 200-450 meters/min. Also preferably, it is a material which, if evaluated on its own, has an efficiency of no greater than 10% and preferably no greater than 5%. Typically, it will be a material having an efficiency of about 1% to 4%. Herein when reference is made to efficiency, unless otherwise specified, reference is meant to efficiency when measured according to ASTM #1215-89, with 0.78.mu. monodisperse polystyrene spherical particles, at 20 fpm (6.1 meters/min) as described herein.

Herein, when a layer of material utilized in arrangements according to the present invention is characterized with respect to properties it "has" or would exhibit "on its own" or when tested "separately from the remainder of the construction", it is meant that the layer of material is being characterized with respect to the source from which it is derived. That is, for example, if reference is made to the "coarse" layer of material, in a composite, the description when characterized as referenced above, is with respect to the material and its properties as it would have existed before being incorporated into the construction. Reference in this context is not necessarily being made to the specific numerical characteristics of, or performance of, the layer as it operates in the composite structure.

Preferably the layer of fine fiber material secured to the first surface of the layer of permeable coarse fibrous media, is a layer of fine fiber media wherein the fibers have average fiber diameters of no greater than about 10 microns, generally and preferably no greater than about 8 microns, and typically and preferably have fiber diameters smaller than 5 microns and within the range of about 0.1 to 3.0 microns. Also, preferably the first layer of fine fiber material secured to the first surface of the first layer of permeable coarse fibrous material has an overall thickness that is no greater than about 30 microns, more preferably no more than 20 microns, most preferably no greater than about 10 microns, and typically and preferably that is within a thickness of about 1-8 times (and more preferably no more than 5 times) the fine fiber average diameter of the layer.

Preferably, when the application is for air filter applications such as engine induction systems, gas turbines, cabin air filtration, and HVAC (heat, ventilation and air conditioning) systems, the preferred upper basis weights for the fine fiber layers are as follows: for a layer of glass fiber material average size 5.1 micron, about 35.8 g/m.sup.2 ; for glass materials average fiber size 0.4 micron, about 0.76 g/m.sup.2 ; and, for glass fibers average size 0.15 micron, about 0.14 g/m.sup.2 ; for polymeric fine fibers average size 5.1 micron, about 17.9 g/m.sup.2 ; for polymeric fibers average size 0.4 micron, about 0.3 g/m.sup.2 ; and, for polymeric fine fibers 0.15 micron average size, about 0.07 g/m.sup.2. In general, preferably the most upstream layers of fine fibers has a basis weight of no greater than about 1 g/m.sup.2, for such applications.

When the material is utilized for high efficiency applications, such as selected indoor air applications and liquid applications (such as lube oil, hydraulic fluid, fuel filter systems or mist collectors), in general the preferred upper limits of the basis weights for the fine fiber layers will be as follows: for glass fibers average size 2.0 micron, about 15.9 g/m.sup.2 ; for glass fibers average size 0.4 micron, about 1.55 g/m.sup.2 ; and, for glass fibers average size 0.15 micron, 0.14 g/m.sup.2 ; for polymeric fine fibers average size 2.0 micron, about 8.0 g/m.sup.2 ; for polymeric fibers average size 0.4 microns, about 0.78 g/m.sup.2 ; and, for polymeric fibers average size 0.15 microns, about 0.19 g/m.sup.2. In general, preferably the most upstream layer of fine fibers has a basis weight of no greater than about 1 g/m.sup.2, for such applications.

The upper limits given for the air filtration applications, such as air induction systems etc., were based upon fine fiber layer thicknesses of about 5 fiber diameters, and an LEFS efficiency of 50% for the layer. For the high efficiency applications, the assumption was based upon five fine fiber thicknesses and an LEFS efficiency of about 90% per layer.

In general, the preferred basis weight for any given situation would depend upon such variables as: the application involved (for example coarse or fine particles, or both, to be trapped in operation, high efficiency or lower efficiency needs); the desired life; the fiber material selected; and, the fiber size used. In general, when relatively high single-layer efficiency is desired (for example on the order of 90% LEFS), generally the glass fibers will work well, and the system will involve higher basis weights (for example about 20 g/m.sup.2), at higher fiber diameters (for example 2-3 microns).

On the other hand, when relatively low single-layer efficiencies are desired, but relatively high lifetime until loaded (resulting from the use of a number of layers) relatively low efficiencies for any given layer will be used (for example on the order of 10% LEFS). This will involve relatively low basis weights and fairly small diameter fiber. Polymer fibers may be usable for this (although glass ones could also), and thus basis weights on the order of 0.005 g/m.sup.2, with a fiber size of about 0.2 microns will be usable. Herein, when the basis weights are given, for glass fibers the assumption is a density of 2.6 g/cc, and for polymer fibers the assumption is a density of 1.3 g/cc.

In general, then, if what is desired by the engineer is to provide longer life, generally more layers, each layer having relatively low efficiency, will be used. If the engineer desires a very high efficiency filter, and long life is not necessarily desired, in general fewer layers with higher LEFS efficiency per layer will be used.

Herein the term "first" or "second" in reference to a construction, for example surfaces of media, is not meant to refer to any particular location in the media. For example, the term "first surface" on its own is not intended to be indicative of whether the surface referred to is upstream or downstream of other surfaces, or positioned above or below other surfaces. Rather, the term is utilized to provide for clarity in reference and antecedent basis. The term "1-8 fine fiber average diameters" is meant to reference a depth or thickness of about 1 times to 8 times the average diameters of the fine fibers in the fine fiber layer referenced.

In typical preferred systems, the fine fibers of the first layer of fine fiber media comprise fibers with diameters of no greater than about 1/6th, preferably no greater than about 1/10th and in some instances preferably no greater than about 1/20th of the diameters of the fibers in the first layer of permeable coarse fibrous media.

For certain applications, preferably the first layer (most upstream in operation) of fine fiber material is constructed and arranged to provide the resulting composite (i.e. the combination of the first layer of permeable coarse media and the first layer of fine fiber media) with an overall LEFS efficiency of at least 8%, preferably at least 10%, typically within the range of 20 to 60%, and most preferably at least 30% and no greater than about 70%. Such composites can then be stacked to create very efficient, for example greater than 97%, filters. They may also be used for less efficiency but very long life filters, for example 50-97% efficient. Also, preferably, the first (most upstream in operation) layer of fine fiber media is constructed and arranged such that the resulting composite (i.e. the combination of the first layer of permeable fibrous media with the first layer of fine fiber media thereon) has an overall permeability of at least 20 meters/min, and typically and preferably about 30 to 350 meters/min. Herein the term "most upstream" or "outermost" in connection with a fine fiber layer refers to the layer of fine fiber material (average fiber diameter less than 8 microns) in the position to be most upstream, relative to other fine fiber layers, in use. There may be more upstream layers of media (not fine fiber) than the most upstream fine fiber layer.

The first layer of permeable coarse fibrous material may be fibers selected from a variety of materials, including for example polymeric fibers such as polypropylene, polyethylene, polyester, polyamide, or vinyl chloride fibers, and glass fibers.

According to certain aspects of the present invention, a filter construction is provided which includes more than one layer, and preferably at least 3 layers, of fine fiber material. Typically the arrangements will include three or more such layers. It is not a requirement that the fine fiber layers in such a multi-layered system be identical to one another. However, preferably each fine fiber layer is a layer within the general description provided above for the first layer of fine fiber media in the media construction as described. Preferably in such arrangements each layer of fine fiber material is separated from its next adjacent layer of fine fiber material, by a layer of permeable coarse fibrous material.

The layers of permeable coarse fibrous material need not be identical, but preferably each is within the general description above with respect to the filter media construction, for the first layer of permeable coarse fibrous media. In certain preferred arrangements, the overall composite media construction also has a layer of permeable coarse fibrous media, as described, on both the most upstream and most downstream surfaces.

The filter construction may comprise a pleated arrangement of the composite, if desired. For example, such an arrangement can have pleats that are 0.25 to 12 inches (0.6-30.5 cm) deep, with a pleat density of at least 1-15 pleats/inch (1-15 pleats/2.5 cm). When it is said that the pleat density is at least 1-15/inch, and the arrangement is configured in a cylindrical pattern, with the pleats extending longitudinally, reference is made to pleat spacing around the inner diameter or surface.

Certain preferred arrangements according to the present invention include media as generally defined, in an overall filter construction. Some preferred arrangements for such use comprise the media arranged in a cylindrical, pleated configuration with the pleats extending generally longitudinally, i.e. in the same direction as a longitudinal axis of the cylindrical pattern. For such arrangements, the media may be imbedded in end caps, as with conventional filters. Such arrangements may include upstream liners and downstream liners if desired, for typical conventional purposes. The constructions may be utilized in association with inner wraps or outer wraps of depth media, for example in accordance with the arrangements described in U.S. patent application No. 08/426,220, incorporated herein by reference.

It is foreseen that in some applications, media according to the present invention may be used in conjunction with other types of media, for example conventional media, to improve overall filtering performance or lifetime. For example, media according to the present invention may be laminated to conventional media, be utilized in stack arrangements; or be incorporated (an integral feature) into media structures including one or more regions of conventional media. It may be used upstream of such media, for good load; and/or, it may be used downstream from conventional media, as a high efficiency polishing filter. The many variations possible will be apparent, from the more detailed descriptions below.

Certain arrangements according to the present invention may also be utilized in liquid filter systems, i.e. wherein the particulate material to be filtered is carried in a liquid. Also, certain arrangements according to the present invention may be used in mist collectors, for example arrangements for filtering fine mists from air.

According to the present invention, methods are provided for filtering. The methods generally involve utilization of media as described to advantage, for filtering. As will be seen from the descriptions and examples below, media according to the present invention can be specifically configured and constructed to provide relatively long life in relatively efficient systems, to advantage.

As will be apparent from the above discussions, and the detailed description below, certain specifically preferred arrangements, especially preferred for air filter constructions, are provided. A form of these is characterized as filter media constructions. The preferred filter media constructions comprise a plurality of layers of fine fiber media, i.e. at least two layers, each of the layers of fine fiber media comprising fibers having diameters of no greater than about 8 microns. The plurality of layers of fine fiber media include an outermost layer. Again, by "outermost" in this context, it is meant that there is a layer of fine fibers in the media which, when the media is organized or oriented for use as a filter media, would be positioned more upstream than any other layer of fine fiber material. This does not mean that the first "outermost" layer of fine fiber material is the outermost layer of media in the construction. Rather, it is the "outermost" or end layer among the plurality of fine fiber layers. When this filter media construction is in use, then, this fine fiber layer will be the upstream fine fiber layer of media in the construction. Preferably this outermost layer of fiber fibers includes fibers having an average diameter of no greater than about 5 microns, and a thickness of no greater than about 5 times the fine fiber average diameters in that outermost layer. Thus, it would have a thickness of no greater than about 25 microns maximum, and in typical applications wherein smaller diameters than 5 microns are used, a substantially smaller thickness. Preferably this outermost layer of fine fibers is relatively permeable having, on its own, a permeability for air of at least 90 meter/min. Of course, if the permeability of this fine fiber layer is measured in association with a coarse supporting substrate, if the overall combination has a permeability of at least 90 meter/min, the fine fiber layer itself does.

Preferably in this construction there is a layer of permeable coarse fibrous media positioned between each layer of fine fiber media. Preferably each layer of permeable coarse fibrous media comprises fibers of at least 10 microns in diameter and preferably each layer has an efficiency, if evaluated separately from the construction, of no greater than 10%, for 0.78.mu. particles as defined.

Preferably this media construction includes at least three layers of fine fiber material, although the at least two layers downstream from the "outermost" layer need not necessarily have an average diameter smaller than 5 microns, but rather it would be preferred that they are at least smaller than 8 microns; and, they may be less permeable than the outermost layer of fine fiber material, preferably each having a permeability on its own of at least 45 meter/min.

Additionally, a preferred filter media construction according to the present invention may be defined as having a first layer of permeable coarse fibrous media comprising coarse fibers having an average diameter of at least 10 microns, an efficiency of no greater than about 5%, for 0.78.mu. particles, and a first surface on which is positioned a first layer of fine fiber media. Preferably the first layer of fine fiber material comprises fibers having an average diameter of no greater than about 5 microns, and a thickness of no greater than about 5 times the average diameter of the fine fibers in this first layer. Preferably this material has a permeability, on its own of at least about 90 meter/min. This media construction, of course, can be utilized in association with other layers of fine fiber and coarse fiber material, and may even be utilized in overall media constructions that use other types of media, for example in association with paper or glass media or other types of depth media. The media construction of this embodiment may also include a plurality of further layers of fine fiber material, each of which is spaced from the next adjacent one by a layer of coarse media.

An overall filter construction may be provided, using media according to the present invention, and as defined in either of the above two identified preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a cross section of a theoretical mono-layer fine fiber filter media.

FIG. 2 is a schematic representation of a cross section of a theoretical mono-layer coarse fiber filter media.

FIG. 3 is a schematic representation of a cross section of a theoretical mono-layer fine fiber filter media; FIG. 3 being of a different media than that shown in FIG. 1.

FIG. 4 is a schematic representation of a cross section of a theoretical mono-layer coarse fiber media arrangement having the same percent solidity as the arrangement shown in FIG. 3.

FIG. 5 is a schematic fragmentary plane view of a surface of a media construction according to the present invention.

FIG. 6 is a schematic cross sectional view of a media according to FIG. 5.

FIG. 7 is a schematic fragmentary cross sectional view of a multi-layer media construction according to the present invention.

FIG. 8A is a fragmentary schematic perspective view of a pleated media arrangement including a media construction according to the present invention.

FIG. 8B is an enlarged fragmentary schematic cross-sectional view of a portion of the arrangement shown in FIG. 8A.

FIG. 9 is a schematic representation of a media according to the present invention threaded on a mechanical support structure.

FIG. 10 is a side elevational view of a filter arrangement incorporating a filter media construction according to the present invention therein.

FIG. 11 is an enlarged fragmentary schematic cross sectional view taken generally along line 11--11 of FIG. 10.

FIG. 12 is a scanning electron micrograph of a conventional air-laid polymeric fiber media.

FIG. 13 is a scanning electron micrograph of a conventional air-laid glass fiber media.

FIG. 14 is a scanning electron micrograph of a conventional two-phase media.

FIG. 15 is a scanning electron micrograph of the same conventional two-phase wet-laid glass media as shown in FIG. 14; FIG. 15 being taken of an opposite side of the media from that shown in FIG. 14.

FIG. 16 is a scanning electron micrograph of a media according to a first embodiment of the present invention.

FIG. 17 is a scanning electron micrograph of a media according to a second embodiment of the present invention.

FIG. 18 is a scanning electron micrograph of a media according to a third embodiment of the present invention.

FIG. 19 is a scanning electron micrograph of a media according to a fourth embodiment of the present invention.

FIG. 20 is a scanning electron micrograph of a media according to a fifth embodiment of the present invention.

FIG. 21 is a scanning electron micrograph of the media of FIG. 19, after NaCl loading according to a description herein.

FIG. 22 is a plot of data from Experiment 5.

FIG. 23 is a plot of certain data from Experiment 6.

FIG. 23 A is another plot of data from Experiment 6.

FIG. 24 is a scanning electron micrograph of a media according to the present invention shown after NaCl loading.

FIG. 25 is a schematic of a custom salt bench used in certain experiments.

DETAILED DESCRIPTION

A. Filtration Advantages of Fine Fibers

In general, in filter media constructions, some filtration advantages are theoretically provided by utilizing relatively fine fibers instead of coarse fibers, for the media. Consider for example FIGS. 1 and 2. FIG. 2 is a schematic illustrating a "single" or "mono-" layer of fine fiber media, with a fixed interfiber distance, D.sub.x, representing the distance between the surfaces of adjacent fibers. FIG. 2 is a schematic representation depicting a single layer with the same D.sub.x but wherein the fiber diameter is about 12 times larger than the fiber diameter in FIG. 1.

Comparing FIGS. 1 and 2, it is apparent that, for an area of fixed media perimeter (i.e. area) the total amount of air space or void space between the fibers in the arrangement of FIG. 2 is substantially smaller than the void space in the arrangement of FIG. 1. Thus, in the arrangement of FIG. 2, there is significantly less volume available for loading of particulate material trapped by the system. In addition, air flow is more disrupted by the arrangement of FIG. 2, than it is in the arrangement of FIG. 1, since a smaller percent of the surface is open for undisrupted air flow therethrough.

From a comparison of FIGS. 1 and 2 it is apparent that if average interfiber distance (D.sub.x) is maintained constant, but average fiber size is reduced, typically a greater space available for loading results and higher permeability to air flow results.

Now consider the arrangements of FIGS. 3 and 4. FIGS. 3 and 4 are intended to schematically represent a single layer of fibers in two depth media systems in which fibers of different sizes are used, but percent solidity or density is held constant. From a review of the figures, it should be apparent that the arrangement with the larger fibers, i.e. the arrangement of FIG. 4, has potentially such large open areas that the filter efficiency is relatively low (but permeability is very high), by comparison to an arrangement with smaller diameter fibers but the same percent solidity, i.e. the arrangement of FIG. 3.

Theoretical considerations of the effects of utilizing smaller fiber diameters have been studied and have been represented quantitatively by the Stokes Number and Interception Parameter.

The dimensionless Stokes Number is represented by the following formula:

STOKES NUMBER=d.sub.p.sup.2 .rho..sub.p v/9 d.sub.f .mu.

wherein: d.sub.f =fiber size (diameter), d.sub.p =particle size (diameter), .rho..sub.p =particle density; v=approach velocity and .mu.=fluid viscosity.

From the formula it will be apparent that (at least theoretically) as d.sub.f (fiber size) is decreased, Stokes Number is increased (assuming no change in the other variables).

In general, the Stokes Number is reflective of inertial impaction. This can be understood by considering the likelihood that as an airstream is distorted or curved around a fiber, a particle within the airstream and directed toward the fiber will leave the airflow (rather than curve with the air flow) and impact the fiber. The variables reflected in the formula above for the Stokes Number logically reflect that, in general, an increase in momentum of the particle (from increasing density and/or velocity) is associated with a greater likelihood that the particle will not flow around the fiber with the airflow stream, but rather that it will leave the airflow stream and directly impact the fiber. The formula also indicates that this likelihood is greater when the fiber diameter is smaller, due, at least in part, to the fact that when the fiber diameter is smaller, the fiber will disrupt the airflow stream to a lesser extent. This brings the effected flow field of the airstream, as it curves around the fiber, into closer proximity to the surface of the fiber and increases the likelihood that a lower momentum particle will still leave the air stream sufficiently to encounter (impact) the fiber.

Another consideration relating to why certain fine fiber systems are theoretically generally more efficient as filters than coarse fiber systems, is particle interception, reflected by the Interception Parameter. Interception Parameter (R) can be represented by the following formula:

R=d.sub.p /d.sub.f

wherein d.sub.p and d.sub.f are defined as above.

In general, Interception Parameter is velocity and momentum independent, and relates to the size of the particle and the size of the fiber. In general, it relates to the likelihood that a particle (which tends to curve with the airstream, as the airstream is distorted around the surface of the fiber), will nevertheless encounter the fiber and become trapped. Thus, it does not directly relate to the likelihood that the momentum of the particle will carry it out of the airstream and into the fiber, but rather whether, while within the airflow stream, the particle will nevertheless encounter the fiber. In general, since smaller fibers disrupt the airflow to a lesser extent, and the distortion in the air flow (from linear) occurs closer to the surface of the fiber, smaller fibers are associated with higher efficiencies and a higher rate of interception impactions than larger fibers.

In general, the advantages associated with the use of fine fibers in a media are more pronounced with relatively small particles. Thus the advantages of fine fibers may be of particular interest when the filter application will require filtering of small particles, especially those 10 microns or less in size (diameter).

B. Some Problems and Limits Associated with Utilization of Relatively Fine Fibers in Filter Media

In the previous section, theoretical advantages available from selection of small diameter fibers in a filter media, relative to coarser fibers, were provided. Problems would result, however, if coarse fibers, i.e. on the order of about 10 or 12 microns (diameter) on up, were simply replaced in depth media by very fine fibers, i.e. on the order of about 8 microns and below, typically 5 microns and below, especially on the order of about 0.2-3.0 microns. For example, constructions made from fibers on the order of about 0.2-5 microns in size would be more difficult to handle (than constructions of coarser fibers) and would tend to collapse in use, creating a very low permeability. That is, it is relatively difficult to maintain a substantially open structure for high loading and high flow therethrough, with a construction merely comprising fibers of 5 microns or below in diameter, since such media typically possesses insufficient mechanical strength (or "body") to resist collapse. When the media collapses, the spaces between the fibers become relatively small, and the construction, while perhaps quite efficient as a filter, loads fairly rapidly and is not very permeable. Indeed, such a system will begin to approximate a surface-loading system in behavior, since a relatively low porosity and shallow depth is, in effect, what results.

One can conceive of a construction in which extremely fine fibers are intimately mixed with (i.e. are intangled with) coarse fibers. However, construction of effective filter arrangements, especially using conventional techniques for creating depth media of mixed fiber diameters, is not readily achieved when the diameters of the fibers are greatly different. For example, consider a theoretical system in which the fine fibers are 1/20th of the diameter of the coarse fibers. If the filter media, that the air being filtered passes through, comprises 50% by weight of the coarse fiber and 50% by weight of the fine fiber, the system is one in which there is a very high number of fine fibers relative to coarse fibers (or fine fiber length relative to coarse fiber length). This would be a system with a relatively low interfiber spacing or porosity. It might be relatively efficient, but it would still load fairly rapidly. In general, if the weight of the coarse fibers relative to the fine fibers is reduced, the problem is exacerbated. If the weight of the coarse fibers relative to the fine fibers is increased, the advantages associated with fine fibers and related to interception and inertial impaction would be compromised.

C. Some Conventional Uses of Fine Fibers in Media

There has been some conventional use of fine fibers in media. In particular, Donaldson Company Inc. of Bloomington Minn., the Assignee of the present invention, has utilized fine fiber technology in its Ultra-Web.RTM. products. These products have generally comprised surface loading cellulose media, which has a web or net of polymeric microfibers, of less than 1 micron in diameter, on an upstream surface.

Such media has typically found use in pulse cleaned dust collectors. In operation, and without the fine fibers, the coarse, surface