Patent Publication Number: US-2019193012-A1

Title: Air filter systems and methods of using the same

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 61/943,036 filed on Feb. 21, 2014 titled AIR FILTER SYSTEMS AND METHODS OF USING THE SAME; U.S. Provisional Patent Application No. 61/789,385 filed on Mar. 15, 2013 titled OVATE TUBULAR FILTER CARTRIDGES AND FILTER SYSTEMS USING THE SAME; and U.S. Provisional Patent Application No. 61/772,198 filed on Mar. 4, 2013 titled DIVERGING NOZZLES AND FILTER ELEMENT CLEANING SYSTEMS USING DIVERGING NOZZLES—each of which is hereby incorporated by reference in its entirety. 
    
    
     Air filter systems with pulse generators, pulse collectors and related components, along with methods of using the same are described herein. 
     Many industries encounter particulate matter suspended in the atmosphere. In some industries, this particulate matter is a valuable product (for example, starch), and it would be beneficial if the suspended particulate matter could be recovered and reintroduced into the process. For other industries (for example, metal or wood working), it may be desirable to remove the particulate matter from the air in order to provide a clear working environment. 
     Systems for cleaning an air or other gas stream laden with particulate matter include air filter assemblies that have filter elements disposed in a housing. The filter element may be a bag, sock or cartridge including a suitable filter media, e.g., fabric, pleated paper, etc. The gas stream contaminated with particulate matter is typically passed through the housing so that the particulate matter is captured and retained by one or more filter elements. 
     In a standard design of air filter system, an air filter system has a clean air chamber and a dirty air chamber. The two chambers are separated by a structure that is commonly referred to as a tube sheet. The tube sheet has a number of openings so that air can pass between the clean and dirty air chambers. The filter elements are positioned over the openings so that particulate-laden air (dirty air) introduced into the dirty air chamber must pass through a filter element to move into the clean air chamber. The particulate matter in the dirty air collects on the filter elements as the air moves through the filter elements. From the clean air chamber, the cleaned air is exhausted into the environment, or recirculated for other uses. See, for example, U.S. Pat. No. 3,942,962 (Duyckinck), U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 4,424,070 (Robinson), U.S. Pat. No. 4,436,536 (Robinson), U.S. Pat. No. 4,443,237 (Ulvestad), U.S. Pat. No. 4,445,915 (Robinson), U.S. Pat. No. 4,661,131 (Howeth), U.S. Pat. No. 5,207,812 (Tronto et al.), U.S. Pat. No. 4,954,255 (Muller et al.), U.S. Pat. No. 5,222,488 (Forsgren), U.S. Pat. No. 5,211,846 (Kott et al.), U.S. Pat. No. 5,730,766 (Clements), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. No. 6,902,592 (Green et al.), and U.S. Pat. No. 7,641,708 (Kosmider et al.). 
     As the filter elements capture particulate matter, flow through the system is inhibited and periodic cleaning of the filter elements can be performed to increase air flow through the system. Cleaning can be accomplished by periodically pulsing a brief jet of pressurized air into the interior of the filter element to reverse the air flow through the filter element, causing the collected particulate matter to be driven off of the filter element. The pressurized air may be directed into pulse collectors as described in, e.g. U.S. Pat. No. 3,942,962 (Duyckinck), U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. Nos. 4,395,269, 6,902,592 (Green et al.), U.S. Pat. No. 7,641,708 (Kosmider et al.), and US Patent Application Publication US 2006/0112667 A1. 
     SUMMARY 
     The air filter systems described herein include one or more pulse collectors and pulse generators aligned along pulse axes. 
     In one or more embodiments, the pulse generators and filter elements attached to the pulse collectors are arranged in an air filter system as described herein along a pulse distance as measured from a pulse outlet to a filter element opening to improve efficiency of the pulse regeneration process. In particular, the pulse distances, when selected to fall within the parameters described herein may exhibit improve pulse cleaning performance. 
     In one or more embodiments, the pulse collectors and the filter elements used in an air filter system as described herein may have openings with a relationship between them (as described herein) that may improve efficiency of a pulse regeneration process as, e.g., a cleaning pulse moves from the pulse collector into the filter element and/or as clean air moves from the interior volume of the filter element into the pulse collector. 
     In one or more embodiments, the pulse collectors used in air filter systems described herein may have a relationship between their hydraulic diameter and their length that may improve efficiency of a pulse regeneration process as, e.g., a cleaning pulse moves from the pulse collector into the filter element and/or as clean air moves from the interior volume of the filter element into the pulse collector. 
     In one or more embodiments, the pulse collectors used in air filter systems as described herein may include a filter section and a pulse section that meet at a junction along a length of the pulse collector. In one or more embodiments, the portions of the passageways in the pulse collectors defined by the pulse sections of the pulse collectors have a hydraulic diameter that increases when moving from the junction to the tube sheet opening of the pulse section. In one or more embodiments, the portions of the passageways defined by the filter sections of the pulse collectors have a hydraulic diameter that remains constant when moving from the junction to the filter end opening of the filter section. Pulse collectors having a pulse section within an increasing hydraulic diameter and a filter section with a constant hydraulic diameter as described herein may, in one or more embodiments, improve efficiency of a pulse regeneration process. 
     In one or more embodiments, air filter systems using filter elements/cartridges having filter media shaped or formed into ovate cross-sections as described herein may exhibit improved particulate loading capacity because, e.g., more of the filter media faces downward than upward. The downward facing filter media may, in or more embodiments, be less susceptible to particulate loading during use than filter media facing upward. Although described as ovate or ovoidal in shape, the cross-sections of the tubular filter media in the ovate filter elements/cartridges described herein may, in one or more embodiments, have one or more flat edges, i.e., the ovate or ovoidal cross-sections may not be true ovoids including only curved lines. Rather, only portions of the cross-sectional shapes of one or more embodiments of the tubular filter elements/cartridges may be in the form of true ovoids. In one or more embodiments, the inner perimeters of the ovate cross-sections of the filter elements/cartridges used in air filter systems as described herein may be asymmetric, i.e., there may be no line about which the inner perimeters of the cross-sections of the tubular filter media in the filter elements/cartridges are symmetric. In one or more alternative embodiments, the inner perimeters of the ovate cross-sections of the tubular filter media in the filter elements/cartridges may have only one line of symmetry. That single line of symmetry may, in one or more embodiments, be described as extending through a top and a bottom of the tubular filter media. Improved particulate loading capacity in such filter elements/cartridges may offer the advantage of reduced pulse cleaning requirements in terms of, e.g., fewer pulses required, reduced pulse energy required per pulse, etc. 
     In one or more embodiments of the air filter systems described herein, the pulse generators may include diverging pulse guides having shapes that may, in one or more embodiments, provide improvements in the cleaning of filter elements using reverse pulses by increasing the average peak pressure as measured in the interior surfaces of the filter elements used in air filter systems as described herein. 
     Air filter systems that include one or more of the various features and components described herein may offer one or more advantages such as, e.g., improved energy efficiency, reduced noise generation, etc. by, in one or more embodiments, reducing pressure drops within the air filter systems both during primary flow operation and pulse cleaning of the filter elements (where primary flow operation occurs when the air filter system is removing particulate matter from a dirty air stream), reducing frictional losses in the air filter systems (both during primary flow operation and pulse cleaning of the filter elements, improving particulate loading characteristics (thus potentially requiring fewer cleaning pulses), etc. 
     Other potential advantages of one or more embodiments of air filter systems that include asymmetrically arranged support beams in the yokes used to support filter elements in the air filter systems may include, e.g., accurate and repeatable alignment of the filter elements in a selected rotational orientation relative to the pulse axis extending through the yoke during placement. The asymmetric yokes may also assist in retention of the rotational orientation of the filter elements during use—which, in the case of one or more embodiments of the ovate and/or asymmetric filter cartridges described herein, provide for improved use of the enhanced particulate loading capacity of the filter cartridges. 
     These advantages may, in one or more embodiments be synergistic, i.e., the energy efficiency, reduced noise, etc. may be improved by using two or more of the features and/or components together in the same air filter systems. 
     In a first aspect, one or more embodiments of the air filter systems described herein may include: a tube sheet configured to separate a housing into a dirty air chamber and a clean air chamber; a pulse collector defining a passageway that extends through the pulse collector from a filter end opening at a filter end of the pulse collector element to a tube sheet opening at a tube sheet end of the pulse collector; an aperture in the tube sheet, wherein the tube sheet end of the pulse collector is configured for attachment to the tube sheet such that the tube sheet opening of the pulse collector is aligned with the aperture such that air passing from the dirty air chamber into the clean air chamber through the aperture passes through the passageway of the pulse collector; a filter element attached to the filter end of the pulse collector such that air passing into the passageway of the pulse collector through the filter end opening of the pulse collector passes through an interior volume of the filter element before reaching the filter end opening, wherein the filter element comprises a filter element opening at a junction between the filter end of the pulse collector and the filter element; a pulse generator located in the clean air chamber and positioned to deliver pulses of air into the interior volume of the filter element, the pulses of air passing through the aperture and the passageway of the pulse collector before reaching the interior volume of the filter element, wherein the pulse generator is configured to deliver the pulses of air along a pulse axis that extends from the pulse generator through the aperture in the tube sheet, the tube sheet opening in the pulse collector, and the filter end opening in the pulse collector, wherein the pulse generator comprises a pulse outlet located on the pulse axis and through which the pulses of air are delivered along the pulse axis, the pulse outlet defined by opposing walls that do not diverge with respect to the pulse axis, and wherein the pulse outlet defines a pulse outlet hydraulic diameter; wherein a pulse distance measured along the pulse axis from the pulse outlet to the filter element opening is 30 or more times the pulse outlet hydraulic diameter. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse distance is 60 times or less the pulse outlet hydraulic diameter. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse distance is 35 or more times the pulse outlet hydraulic diameter. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse distance is 50 times or less the pulse outlet hydraulic diameter. 
     In one or more embodiments of the first aspect of the air filter systems described herein, a hydraulic diameter of the filter element opening is 112% or less of a hydraulic diameter of the filter end opening of the pulse collector. In one or more embodiments, the hydraulic diameter of the filter element opening is 90% or more of the hydraulic diameter of the filter end opening of the pulse collector. In one or more embodiments, the hydraulic diameter of the filter element opening is 108% or less of the hydraulic diameter of the filter end opening of the pulse collector. In one or more embodiments, the hydraulic diameter of the filter element opening is 95% or more of the hydraulic diameter of the filter end opening of the pulse collector. 
     In one or more embodiments of the first aspect of the air filter systems described herein, an absolute value of a difference between a hydraulic diameter of the filter element opening and a hydraulic diameter of the filter end opening of the pulse collector is within 2% or less of the hydraulic diameter of the filter element opening. 
     In one or more embodiments of the first aspect of the air filter systems described herein, an offset between an inner surface of the filter element opening and an inner surface of the filter end opening of the pulse collector is no more than 15 millimeters about a perimeter of the filter element opening. 
     In one or more embodiments of the first aspect of the air filter systems described herein, an offset between an inner surface of the filter element opening and an inner surface of the filter end opening of the pulse collector is no more than 10 millimeters about a perimeter of the filter element opening. 
     In one or more embodiments of the first aspect of the air filter systems described herein, an offset between an inner surface of the filter element opening and an inner surface of the filter end opening of the pulse collector is no more than 5 millimeters about a perimeter of the filter element opening. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse collector comprises a passageway length measured along the pulse axis that is equal to or greater than a hydraulic diameter of the filter end opening of the pulse collector. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse collector comprises a passageway length measured along the pulse axis that is no more than three times a hydraulic diameter of the filter end opening of the pulse collector. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse collector comprises a filter section and a pulse section, wherein the filter section and the pulse section meet at a junction located between the filter end and the tube sheet end of the pulse collector; wherein a portion of the passageway defined by the pulse section comprises a hydraulic diameter that increases when moving from the junction to the tube sheet opening; and wherein a portion of the passageway defined by the filter section comprises a hydraulic diameter that remains constant when moving from the junction to the filter end. In one or more embodiments, the filter section comprises a filter section length measured along the pulse axis from the filter end to the junction and the pulse section comprises a pulse section length measured along the pulse axis from the tube sheet end to the junction, wherein the filter section length is less than or equal to the pulse section length. In one or more embodiments, the filter section length and the pulse section length are both equal to or less than 1.5 times a hydraulic diameter of the filter end opening of the pulse collector. In one or more embodiments, the filter section length and the pulse section length are both equal to or less than a hydraulic diameter of the filter end opening of the pulse collector. 
     In one or more embodiments of the first aspect of the air filter systems described herein in which the pulse collector comprises a filter section and a pulse section that meet at a junction located between the filter end and the tube sheet end of the pulse collector and in which the portion of the passageway defined by the pulse section comprises a hydraulic diameter that increases when moving from the junction to the tube sheet opening, the pulse section comprises opposing walls defining the portion of the passageway in the pulse section that diverge from the pulse axis at an included angle that is greater than zero (0) degrees and less than or equal to ten (10) degrees. In one or more embodiments, the included angle is equal to or greater than three (3) degrees. In one or more embodiments, the included angle is less than or equal to eight (8) degrees. In one or more embodiments, the included angle is equal to or greater than five (5) degrees. In one or more embodiments, the included angle is less than or equal to seven (7) degrees. 
     In one or more embodiments of the first aspect of the air filter systems described herein in which the pulse collector comprises a filter section and a pulse section that meet at a junction located between the filter end and the tube sheet end of the pulse collector, the filter section and the pulse section comprise separate articles attached to each other at the junction. In one or more embodiments, the filter section and the pulse section are welded together at the junction. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the pulse generator comprises a diverging pulse guide attached to the pulse outlet. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the filter element is supported on a yoke extending away from the pulse collector along the pulse axis, wherein the yoke comprises two or more support beams aligned with the pulse axis, wherein the two or more support beams are arranged asymmetrically about the pulse axis. In one or more embodiments, the filter element opening comprises alignment features arranged to align with the two or more support beams when the filter element is in only one rotational orientation relative to the pulse axis. In one or more embodiments, the filter element comprises a distal end located away from the pulse collector, wherein a distal end opening is located at the distal end of the filter element, and wherein the distal end opening comprises alignment features arranged to align with the two or more support beams when the filter element is in only one rotational orientation relative to the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the filter element is supported on a yoke extending away from the pulse collector along the pulse axis, wherein a second filter element is supported on the yoke, and wherein the filter element is located between the pulse collector and the second filter element. In one or more embodiments, the yoke comprises two or more support beams aligned with the pulse axis, wherein the two or more support beams are arranged asymmetrically about the pulse axis. In one or more embodiments, the filter element opening comprises alignment features arranged to align with the two or more support beams when the filter element is in only one rotational orientation relative to the pulse axis, and wherein the second filter element comprises a second filter element opening that comprises alignment features arranged to align with the two or more support beams when the second filter element is in only one rotational orientation relative to the pulse axis. In one or more embodiments, the filter element comprises a distal end located away from the pulse collector, wherein a distal end opening is located at the distal end of the filter element, and wherein the distal end opening comprises alignment features arranged to align with the two or more support beams when the filter element is in only one rotational orientation relative to the pulse axis; and wherein the second filter element comprises a distal end located away from the pulse collector, wherein a distal end opening is located at the distal end of the second filter element, and wherein the distal end opening of the second filter element comprises alignment features arranged to align with the two or more support beams when the second filter element is in only one rotational orientation relative to the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems described herein, the filter element comprises: tubular filter media defining an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter; wherein the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (Wmax), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; and wherein the bottom axis intersection point does not bisect the maximum height of the cross-section as measured between the top and bottom points. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point, and wherein the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. In one or more embodiments, the bottom section height is greater than zero. In one or more embodiments, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. In one or more embodiments, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. In one or more embodiments, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the axis of maximum height does not lie on a line of symmetry of the inner perimeter of the cross-section. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the inner perimeter of the cross-section defines only one line of symmetry. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the inner perimeter of the cross-section defines only one line of symmetry, and wherein the axis of maximum height is coincident with the line of symmetry. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the tube axis is collinear with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media as described herein, the tubular filter media defines an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter having an ovate shape; wherein the ovate shape of the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (Wmax), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; and wherein the bottom axis intersection point does not bisect the maximum height of the cross-section as measured between the top and bottom points. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point, and wherein the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. In one or more embodiments, the bottom section height is greater than zero. In one or more embodiments, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. In one or more embodiments, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. In one or more embodiments, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element including tubular filter media defining a cross-section with an inner perimeter having an asymmetric ovate shape as described herein, the tube axis is collinear with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the filter element includes tubular filter media defining an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter; wherein the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (Wmax), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; wherein the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point; wherein the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the bottom section height is greater than zero. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the axis of maximum height does not lie on a line of symmetry of the inner perimeter of the cross-section. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the inner perimeter of the cross-section defines only one line of symmetry. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the inner perimeter of the cross-section defines only one line of symmetry, and wherein the axis of maximum height is coincident with the line of symmetry. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, the tube axis is collinear with the pulse axis. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, an inscribed circle located within the inner perimeter of the cross-section occupies less than all and 60% or more of an inner area defined by the inner perimeter. In one or more embodiments, the inscribed circle located within the inner perimeter of the cross-section occupies 70% or more of the inner area defined by the inner perimeter. In one or more embodiments, the inscribed circle located within the inner perimeter of the cross-section occupies 80% or more of the inner area defined by the inner perimeter. 
     In one or more embodiments of the first aspect of the air filter systems including a filter element as described herein, an inscribed circle located within the inner perimeter of the cross-section defines a maximum radial gap between the circle and the inner perimeter that is 0.5 or less of a diameter of the inscribed circle, wherein the maximum radial gap is measured along a radial line extending through a center of the inscribed circle. In one or more embodiments, the maximum radial gap is 0.25 or less of the diameter of the inscribed circle. 
     In a second aspect, one or more embodiments of an air filter system as described herein may include: a tube sheet configured to separate a housing into a dirty air chamber and a clean air chamber; a pulse collector defining a passageway that extends through the pulse collector from a filter end opening at a filter end of the pulse collector element to a tube sheet opening at a tube sheet end of the pulse collector; an aperture in the tube sheet, wherein the tube sheet end of the pulse collector is configured for attachment to the tube sheet such that the tube sheet opening of the pulse collector is aligned with the aperture such that air passing from the dirty air chamber into the clean air chamber through the aperture passes through the passageway of the pulse collector; a pulse generator located in the clean air chamber and positioned to deliver pulses of air along a pulse axis that extends from the pulse generator through the aperture in the tube sheet, the tube sheet opening in the pulse collector, and the filter end opening in the pulse collector; a filter element attached to the filter end of the pulse collector such that air passing into the passageway of the pulse collector through the filter end opening of the pulse collector passes through an interior volume of the filter element before reaching the filter end opening. In air filter systems of the second aspect, the filter element comprises: tubular filter media defining an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter; wherein the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (Wmax), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; and wherein the bottom axis intersection point does not bisect the maximum height of the cross-section as measured between the top and bottom points. 
     In one or more embodiments of air filter systems according to the second aspect as described herein, the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point, and wherein the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the bottom section height is greater than zero. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the axis of maximum height does not lie on a line of symmetry of the inner perimeter of the cross-section. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the inner perimeter of the cross-section defines only one line of symmetry. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the inner perimeter of the cross-section defines only one line of symmetry, and wherein the axis of maximum height is coincident with the line of symmetry. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second aspect as described herein, the tube axis is collinear with the pulse axis. 
     In a third aspect, one or more embodiments of an air filter system as described herein may include: a tube sheet configured to separate a housing into a dirty air chamber and a clean air chamber; a pulse collector defining a passageway that extends through the pulse collector from a filter end opening at a filter end of the pulse collector element to a tube sheet opening at a tube sheet end of the pulse collector; an aperture in the tube sheet, wherein the tube sheet end of the pulse collector is configured for attachment to the tube sheet such that the tube sheet opening of the pulse collector is aligned with the aperture such that air passing from the dirty air chamber into the clean air chamber through the aperture passes through the passageway of the pulse collector; a pulse generator located in the clean air chamber and positioned to deliver pulses of air along a pulse axis that extends from the pulse generator through the aperture in the tube sheet, the tube sheet opening in the pulse collector, and the filter end opening in the pulse collector; a filter element attached to the filter end of the pulse collector such that air passing into the passageway of the pulse collector through the filter end opening of the pulse collector passes through an interior volume of the filter element before reaching the filter end opening. The third aspect of air filter systems as described herein include a filter element comprising tubular filter media defining an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter having an ovate shape; wherein the ovate shape of the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (Wmax), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; and wherein the bottom axis intersection point does not bisect the maximum height of the cross-section as measured between the top and bottom points. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point, and wherein the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the bottom section height is greater than zero. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the third aspect as described herein, the tube axis is collinear with the pulse axis. 
     In a fourth aspect, one or more embodiments of an air filter system as described herein may include: a tube sheet configured to separate a housing into a dirty air chamber and a clean air chamber; a pulse collector defining a passageway that extends through the pulse collector from a filter end opening at a filter end of the pulse collector element to a tube sheet opening at a tube sheet end of the pulse collector; an aperture in the tube sheet, wherein the tube sheet end of the pulse collector is configured for attachment to the tube sheet such that the tube sheet opening of the pulse collector is aligned with the aperture such that air passing from the dirty air chamber into the clean air chamber through the aperture passes through the passageway of the pulse collector; a pulse generator located in the clean air chamber and positioned to deliver pulses of air along a pulse axis that extends from the pulse generator through the aperture in the tube sheet, the tube sheet opening in the pulse collector, and the filter end opening in the pulse collector; and a filter element attached to the filter end of the pulse collector such that air passing into the passageway of the pulse collector through the filter end opening of the pulse collector passes through an interior volume of the filter element before reaching the filter end opening. The fourth aspect of air filter systems as described herein include a filter element comprising tubular filter media defining an interior surface facing an interior volume of the filter element and an exterior surface facing away from the interior volume, wherein the tubular filter media defines a tubular filter media length measured along a tube axis extending from a first end to a second end of the tubular filter media; a filter element housing comprising a first end cap at the first end of the tubular filter media and a second end cap at the second end of the tubular filter media; wherein, in a cross-section taken transverse to the tube axis at any location along a majority of the tubular filter media length, the interior surface of the tubular filter media defines an inner perimeter; wherein the cross-section comprises a maximum height measured between a top point and a bottom point, wherein the top point and the bottom point are located on the inner perimeter and an axis of maximum height (Hmax) that extends across the cross-section at a location and in an orientation such that the top point and the bottom point are points on the inner perimeter that are furthest apart from each other along any straight line extending across the cross-section; wherein the cross-section comprises a maximum width measured between a first point and a second point located on the inner perimeter and on an axis of maximum width (W max), wherein the axis of maximum width is located along a straight line perpendicular to the axis of maximum height, and wherein the axis of maximum width intersects the axis of maximum height at a bottom axis intersection point where the first point and the second point are furthest from each other on any straight line perpendicular to the axis of maximum height; wherein the cross-section comprises a bottom section height measured along the axis of maximum height from the bottom point to the bottom axis intersection point; wherein the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point, wherein the bottom perimeter section comprises a bottom perimeter section length measured along the inner perimeter from the first point to the second point; wherein the inner perimeter of the cross-section comprises a top perimeter section containing the top point, wherein the top perimeter section extends from a first end to a second end, wherein the first end is located on the inner perimeter between the first point and the top point and the second end is located on the inner perimeter between the second point and the top point, wherein first end and the second end of the top perimeter section are the points at which a top perimeter section line intersects the inner perimeter, and wherein the top perimeter section comprises a top perimeter section length measured along the inner perimeter from the first end to the second end; wherein the top perimeter section line is a straight line that is perpendicular to the axis of maximum height and that intersects the axis of maximum height at a top axis intersection point, wherein the cross-section comprises a top section height measured along the axis of maximum height from the top axis intersection point to the top point on the inner perimeter; wherein the top section height is equal to the bottom section height; and wherein the bottom perimeter section length is greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the bottom perimeter section length is 1.2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the bottom perimeter section length is 2 or more times greater than the top perimeter section length. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the bottom section height is greater than zero. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the bottom section height is greater than or equal to 0.1 of the maximum height. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the bottom section height is less than or equal to 0.4 of the maximum height measured along the axis of maximum height from the top point to the bottom point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the inner perimeter of the cross-section comprises a bottom perimeter section containing the bottom point and extending from the first point to the second point, wherein the entire bottom perimeter section is continuously curved from the first point to the second point. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, no section of the inner perimeter between the first point and the second point lies on a straight line for a distance of more than 1 centimeter. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the axis of maximum height does not lie on a line of symmetry of the inner perimeter of the cross-section. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the inner perimeter of the cross-section defines only one line of symmetry. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the inner perimeter of the cross-section defines only one line of symmetry, and wherein the axis of maximum height is coincident with the line of symmetry. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the inner perimeter of the cross-section is asymmetric. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the tube axis is aligned with the pulse axis. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the fourth aspect as described herein, the tube axis is collinear with the pulse axis. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second, third, or fourth aspect as described herein, an inscribed circle located within the inner perimeter of the cross-section occupies less than all and 60% or more of an inner area defined by the inner perimeter. In one or more embodiments, the inscribed circle located within the inner perimeter of the cross-section occupies 70% or more of the inner area defined by the inner perimeter. In one or more embodiments, the inscribed circle located within the inner perimeter of the cross-section occupies 80% or more of the inner area defined by the inner perimeter. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the second, third, or fourth aspect as described herein, an inscribed circle located within the inner perimeter of the cross-section defines a maximum radial gap between the circle and the inner perimeter that is 0.5 or less of a diameter of the inscribed circle, wherein the maximum radial gap is measured along a radial line extending through a center of the inscribed circle. In one or more embodiments, the maximum radial gap is 0.25 or less of the diameter of the inscribed circle. 
     In one or more embodiments of air filter systems including a filter element having tubular filter media according to the first, second, third, or fourth aspect as described herein, the cross-section is taken transverse to the tube axis at any location along 10% or more, optionally 25% or more, and optionally 50% or more of the tubular filter media length. 
     In one or more embodiments of air filter systems according to the first, second, third, or fourth aspect as described herein, a diverging pulse guide is operably connected to the pulse generator, wherein the pulses of air from the pulse generator pass through the diverging pulse guide, and wherein the diverging pulse guide comprises: a tubular wall comprising a connector end connected to the pulse generator and an open end located distal from the connector end; an interior channel extending through the diverging pulse guide from the connector end to the open end, wherein the interior channel defines a channel length extending from the connector end to the open end and a channel width defined by opposing interior surfaces of the tubular wall, wherein the channel length extends along a longitudinal axis and wherein the channel width extends transverse to the longitudinal axis; wherein the interior channel comprises a first section proximate the connector end and a second section proximate the open end such that the first section is located between the second section and the connector end and the second section is located between the first section and the open end; wherein the opposing interior surfaces of the diverging pulse guide in the first section diverge from the longitudinal axis at a first angle, wherein the first angle is greater than zero (0) degrees; wherein the opposing interior surfaces of the diverging pulse guide in the second section diverge from the longitudinal axis at a second angle that is greater than the first angle. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second section of the interior channel comprises a second section length measured along the longitudinal axis that greater than the channel width at the connector end. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second section of the interior channel comprises a second section length measured along the longitudinal axis that is two (2) or more times the channel width at the connector end. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is 1.5 or more times as large as the first angle. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the first angle is three (3) degrees or less. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is three (3) degrees or more. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is four (4) degrees or more. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is five (5) degrees or more. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is nine (9) degrees or less. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is eight (8) degrees or less. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is seven (7) degrees or less. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the second angle is six (6) degrees. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, the interior channel comprises a circular cross-section taken transverse to the longitudinal axis at any point along the longitudinal axis. 
     In one or more embodiments of a diverging pulse guide used in an air filter system as described herein, at least a portion of an exterior surface of the tubular wall comprises threads extending from the connector end towards the open end. 
     In a fifth aspect, one or more embodiments of a method of cleaning one or more filter elements in an air filter system as described herein may include: collecting particulate matter on a filter element located in a dirty air chamber; and directing a pulse of air into the interior volume of the filter element from a pulse generator located in the clean air chamber at a selected time after collecting the particulate matter on the filter element. 
     As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or “the” component may include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements. 
     It is noted that the term “comprises” and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably herein. 
     The above summary is not intended to describe each embodiment or every implementation of the air filter systems and methods described herein. Rather, a more complete understanding of the invention will become apparent and appreciated by reference to the following Description of Illustrative Embodiments and claims in view of the accompanying figures of the drawing. 
    
    
     
       BRIEF DESCRIPTIONS OF THE VIEWS OF THE DRAWING 
         FIG. 1  is a perspective view of one illustrative embodiment of an air filter system as described herein. 
         FIG. 2  is a side view of the air filter system depicted in  FIG. 1 . 
         FIG. 3  is a top view of the air filter system depicted in  FIGS. 1 and 2 . 
         FIG. 4  is a cross-sectional view of the air filter system of  FIGS. 1-3  taken along line  4 - 4  in  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the air filter system of  FIGS. 1-3  taken along line  5 - 5  in  FIG. 3 . 
         FIG. 6  is partially exploded perspective view of the air filter system of  FIGS. 1-5 . 
         FIG. 7  is a perspective view of an alternative illustrative embodiment of an air filter system as described herein. 
         FIG. 8  is a schematic diagram of one illustrative embodiment of a relationship between a pulse generator and a filter element attached to a pulse collector in an air filter system as described herein. 
         FIG. 9  is a cross-sectional view of one illustrative embodiment of a relationship between a pulse collector and a filter element at a junction between the pulse collector and the filter element. 
         FIGS. 10A and 10B  depict illustrative embodiments of offsets between the inner surfaces of a pulse collector and a filter element in an air filter system as described herein. 
         FIG. 11  depicts one illustrative embodiment of a pulse collector including a pulse section and a filter section as described herein. 
         FIG. 12  is a cross-sectional view of the pulse collector of  FIG. 12  taken along line  12 - 12  in  FIG. 11 . 
         FIG. 13  is an enlarged cross-sectional view of one embodiment of a junction in the pulse collector depicted in  FIG. 12 . 
         FIG. 14  depicts an alternative arrangement of a pulse collector and a tube sheet that may be used in one or more embodiments of an air filter system as described herein. 
         FIG. 15  depicts one illustrative embodiment of an arrangement of a pulse collector attached to a tube sheet, a filter cartridge located on a yoke extending out word from the pulse collector, and a pulse generator aligned with the pulse collector and the filter cartridge. 
         FIG. 16  is a view of the arrangement depicted in  FIG. 15  taken along pulse axis  651  from right to left. 
         FIG. 17  depicts one illustrative embodiment of an end cap that may be used on a filter cartridge/element used in an air filter system as described herein. 
         FIG. 18  is a perspective view of one illustrative embodiment of an ovate filter element/cartridge as described herein. 
         FIG. 19  is a perspective view of the filter media in the filter element/cartridge of  FIG. 18 . 
         FIG. 20  is a cross-sectional view of the filter media of  FIG. 19  taken in plane  3  as depicted in  FIG. 19 . 
         FIG. 21  is another cross-sectional view of the filter media of  FIG. 19  taken in plane  3  with an inscribed circle located within the inner perimeter. 
         FIG. 22  is a cross-sectional view of the filter media of an alternative embodiment of an ovate filter element/cartridge as described herein. 
         FIG. 23  is a cross-sectional view of the filter media of another alternative embodiment of an ovate filter element/cartridge as described herein. 
         FIG. 24  is a side elevational view of one illustrative embodiment of a yoke and venturi on which a filter element/cartridge as described herein may be mounted within an air filter system. 
         FIG. 25  is a perspective view of one illustrative embodiment of a diverging pulse guide connected to a pulse generator in fluid communication with a manifold containing pressurized gas as described herein. 
         FIG. 26  is a side elevational view of one illustrative embodiment of a diverging pulse guide as described herein. 
         FIG. 27  is an end view of the diverging pulse guide of  FIG. 26 . 
         FIG. 28  is a cross-sectional view of the diverging pulse guide of  FIG. 26  taken along line  28 - 28  in  FIG. 26 . 
         FIG. 29  is a partial cross-sectional view depicting one illustrative embodiment of a connection between a diverging pulse guide and a pulse generator as described herein. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In the following description of illustrative embodiments, reference is made to the accompanying figures of the drawing which form a part hereof, and in which are shown, by way of illustration, specific embodiments. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     Referring to  FIGS. 1-4 , one illustrative embodiment of an air filter system is depicted generally at  10 . The air filter system depicted in  FIG. 1  is generally in the shape of a box and includes an upper wall panel  16 , and two pairs of opposite side wall panels  17  (one of which is depicted in  FIG. 1 ). The air filter system  10  includes a dirty air conduit  11  for receiving dirty or contaminated air (i.e., air with particulate matter therein) into the filter system  10 . A clean air conduit  13  (see, e.g.,  FIGS. 3 and 4 ) may be provided for venting clean or filtered air from the filter system  10 . The air filter system  10  includes access openings  12  for multiple filter elements (not shown in  FIG. 1 ) configured together in a side-by-side arrangement. In use, each of the access openings  12  is sealed by a cover (not shown) such that dirty air entering the air filter system  10  does not escape through the access openings  12 . 
     The air filter system may also include a hopper  18  to collect particulate matter separated from the dirty air stream as described herein. The hopper  18  may include sloped walls to facilitate collection of the particulate matter and may, in some embodiments, include a driven auger or other mechanism for removing the collected particulate matter. 
     The air filter system of  FIG. 1  is depicted in a side elevation in  FIG. 2  and a top plan view in  FIG. 3 . The air filter system  10 , as seen in  FIGS. 2 and 3 , includes pulse generators  50  as part of a pulse-jet cleaning system, with the pulse generators  50  configured to direct a pulse of air into the filter elements as described herein. 
       FIG. 4  is a cross-sectional view of the air filter system  10  taken along line  4 - 4  in  FIG. 3  and shows the interior of the air filter system  10 . The interior of the air filter system includes a tube sheet  22  that separates the interior of the housing into a clean air chamber  24  and a dirty air chamber  26 . As depicted in  FIGS. 3 and 4 , the air filter system  10  includes a clean air conduit  13  through which clean air exits from the clean air chamber during operation of the air filter system  10 . 
     The depicted air filter system  10  includes pulse collectors  30  and filter elements  40  in the dirty air chamber  26 . The pulse collectors  30  are attached to the tube sheet  22  over an aperture in the tube sheet  22  (not seen in  FIG. 4 ) such that a pulse of air from the pulse generators  50  passing through the pulse collector  30  enters an interior volume  41  of the filter elements  40 .  FIGS. 5 and 6  are, respectively, a cross-sectional view of the air filter system of  FIGS. 1-4  taken along line  5 - 5  in  FIG. 3  and a partially exploded perspective view of the air filter system  10  with some of the walls removed to reveal the pulse collectors  30  and filter elements  40  located therein. 
     The apertures  28  in the tube sheet  22  over which the pulse collectors  30  are positioned are seen in the cross-sectional view of  FIG. 5 . Also seen in  FIG. 5  are yokes  42  attached to the pulse collectors  30  and/or the tube sheet  22 . The yokes  42  are provided to assist in supporting the filter elements  40  within the housing of the air filter system  10 . The use of yokes  42  and similar structures for supporting filter elements in an air filter system may be described in, e.g., U.S. Pat. No. 3,942,962 (Duyckinck), U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 5,562,746 (Raether), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. No. 6,902,592 (Green et al.), and U.S. Pat. No. 7,641,708 (Kosmider et al.). 
     Although the filter elements  40  depicted in  FIGS. 4-6  are in the form of two-part cartridges, the air filter systems described herein can be adapted to use a variety of filter elements provided the filter elements can be used in conjunction with pulse collectors. In one or more embodiments, the filter elements may take the form of, e.g., bags, socks, cartridges, etc. In one or more embodiments of the air filter systems described herein, the filter elements may, for example, include only a single cartridge. In one or more embodiments of the air filter systems described herein that include a filter element with two or more components (e.g., cartridges, bags, socks, etc.), two or more of the components may be the same or different form, size, shape, etc. 
     The pulse generators  50  of the air filter system  10  are configured to direct air into the pulse collectors  30  through the apertures  28  in the tube sheet  22 . The air from each of the pulse generators  50  enters the pulse collector  30  aligned with the pulse generator  50  and passes into the interior volume  41  of the filter element  40  to remove particulate matter from the filter elements  40  in a manner similar to that described in, e.g., U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 5,562,746 (Raether), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. No. 6,902,592 (Green et al.), U.S. Pat. No. 7,641,708 (Kosmider et al.), U.S. Pat. No. 8,075,648 (Raether), and US Patent Application Publication No. US2013/0305926 A1 (Raether). 
     The pulse generators  50  may be provided as part of a pulse-jet cleaning system including one or more sources of pressurized gas (e.g., air), valves and a control system. Illustrative embodiments of potentially suitable pulse-jet cleaning systems may be found in, e.g., U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 5,562,746 (Raether), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. No. 6,902,592 (Green et al.), U.S. Pat. No. 7,641,708 (Kosmider et al.), and U.S. Pat. No. 8,075,648 (Raether). 
       FIG. 7  depicts an illustrative embodiment of another air filter system  110 . The air filter system  110  is also generally in the shape of a box, but differs from the air filter system depicted in  FIGS. 1-6  because of the orientation of the components located within the air filter system. In particular, the air filter system  110  includes a tube sheet  122  that may, in one or more embodiments, be oriented generally horizontally with a clean air chamber  124  located above the tube sheet  122  and a dirty air chamber  126  located below the tube sheet  122 . Although the air filter systems depicted in, e.g.,  FIGS. 6 and 7 , include filter elements/cartridges in two different orientations (i.e., horizontal and vertical), air filter systems as described herein may include filter elements/cartridges in any orientation and/or arrangement. 
     The air filter system  110  also includes filter elements  140  attached to the tube sheet  122  through pulse collectors  130 . Dirty air entering the dirty air chamber  126  passes through the filter elements  140  and the pulse collectors  130  before entering the clean air chamber  124  above the tube sheet  122 . The tube sheet  122  includes apertures  128  over which the pulse collectors  130  are attached such that air passing from the pulse collectors  130  passes through the apertures  128  in the tube sheet  122  when moving from the pulse collectors  130  into the clean air chamber  124 . 
     The air filter system  110  also includes pulse generators  150  located in the clean air chamber  124  and are configured to direct pulses into the pulse collectors  130  through the apertures  128  in the tube sheet  122 . The pulse from each of the pulse generators  150  enters the pulse collector  130  over which the pulse generator is aligned and passes into the interior volume of the filter element  140  to remove particulate matter from that filter element as described herein. 
     In one or more embodiments of the air filter systems described herein, the distance between the pulse generators and filter elements may be selected to improve the cleaning or removal of particulate matter from the filter elements during use of the air filter systems. Referring to, e.g.,  FIG. 8 , one illustrative embodiment of an arrangement between a pulse generator  250 , tube sheet  222 , pulse collector  230 , and filter element  240  is depicted in the form of a simplified structure to more clearly illustrate and describe this feature. 
     In particular, the pulse collector  230  includes a filter end opening  231  at the end of the pulse collector element to which the filter  240  is attached. The filter element  240  includes a filter element opening  245  at the interface between the filter end opening  231  of the pulse collector  230  and the filter element  240 . At the opposite end of the pulse collector  230 , a tube sheet opening  232  is, in one or more embodiments, aligned with an aperture  228  in the tube sheet  222 . 
     The illustrative embodiment of pulse generator  250  depicted in  FIG. 8  (which, for the sake of clarity, is not depicted in scale with the other components seen in  FIG. 8 ) includes a pulse outlet  254  defined at the end of a delivery tube  252 . The pulse generator  250  is configured to deliver pulses of air along a pulse axis  251  that extends from the pulse generator  250  through the aperture  228  in the tube sheet  222 , the tube sheet opening  232  and the filter end opening  231  in the pulse collector  230 . The pulse generator  250  includes a pulse outlet  254  located on the pulse axis  251  and through which the pulses of air are delivered along the pulse axis  251 . 
     Although the pulse axis  251  in one or more embodiments of air filter systems described herein may be oriented and located such that the pulse axis  251  passes through a center of all of the pulse outlet  254 , the aperture  228  in the tube sheet  222 , the tube sheet opening  232  and the filter end opening  231  in the pulse collector  230 , the filter element opening  245 , and the interior volume  241  of the filter element  240 , the pulse axis  251  may, in one or more embodiments, be positioned such that the pulse axis  251  does not pass through the center of one or more of those features/openings. 
     In one or more embodiments such as the illustrative embodiment depicted in  FIG. 8 , a diverging pulse guide  290  is attached to the pulse generator  250  such that air leaving the pulse outlet  254  is at least partially contained within the diverging pulse guide  290  before exiting the diverging pulse guide  290  at its open end  293 . The diverging pulse guide  290  depicted in  FIG. 8  is only one example of a diverging pulse guide which may be used in connection with the air filter systems described herein. Other diverging pulse guides may be used such as, e.g., those described herein, as well as those described in, e.g., U.S. Provisional Patent Application No. 61/772,198, titled DIVERGING NOZZLES AND FILTER ELEMENT CLEANING SYSTEMS USING DIVERGING NOZZLES. 
     The pulse outlet  254  of the pulse generators described herein is the opening through which pulses pass that is defined by opposing walls in the pulse generator  250  that do not diverge. In the illustrative embodiment depicted in  FIG. 8 , the pulse outlet  254  is defined by the walls of delivery tube  252  which may be parallel to each other. In one or more alternative embodiments, however, the walls of delivery tube  252  leading to the pulse outlet  254  may be converging. The pulse outlet  254  is not, however, defined by walls that are diverging as are the walls defining the diverging pulse guide  290  attached to the pulse generator  250 . The converging or diverging orientation of the opposing walls defining the pulse outlets in pulse generators described herein are determined with respect to the pulse axes passing through the pulse generators, i.e., when not parallel to each other, the converging or diverging nature of the opposing walls is determined with when moving along the pulse axis in a direction towards the filter elements. 
     The relationship between the pulse generator and filter element in air filter systems as described herein is, in one or more embodiments, related to the pulse distance (pd as seen in  FIG. 8 ) and the pulse outlet hydraulic diameter (dpo as seen in  FIG. 8 ). 
     The pulse distance (pd) is the distance measured along the pulse axis  251  from the pulse outlet  254  to the filter element opening  245 , where the filter element opening  245  is the location along the pulse axis  251  at which the hydraulic diameter (dfe) of the filter element opening is determined as discussed herein in connection with  FIG. 9 . The pulse axis  251  extends from the pulse outlet  254  through the aperture  228 , pulse collector  230  and into the interior volume  241  of the filter element  240 . In one or more embodiments in which the delivery tube  252  defines the pulse outlet  254  with walls that are parallel to each other, the pulse axis  251  may be aligned with those parallel walls. 
     The hydraulic diameter (dpo) of the pulse outlet  254  can be determined by measuring the cross-sectional area of the pulse outlet  254 , multiplying that area by four, and then dividing the resultant by the length of the perimeter of the pulse outlet  254 . Calculation of the hydraulic diameter of a pulse outlet is represented by the following equation. 
         dpo =4*(area of pulse outlet)/perimeter of pulse outlet 
     In one or more embodiments of air filter systems described herein, the hydraulic diameter (dpo) of the pulse outlets may be as small as, e.g., 8 millimeters and as large as, e.g., 150 millimeters. The sizing of the pulse outlets will vary depending on many different factors such as, e.g., the size of the filter elements, flow rates through the system, etc. 
     In one or more embodiments of the air filter systems described herein, the lower end of the range for the pulse distance (pd) may be 30 or more times the pulse outlet hydraulic diameter (dpo). In one or more alternative embodiments of the air filter systems described herein, the lower end of the range for the pulse distance (pd) may be 35 or more times the pulse outlet hydraulic diameter (dpo). In one or more embodiments of the air filter systems described herein, the upper end of the range for the pulse distance (pd) may be 60 times or less the pulse outlet hydraulic diameter (dpo). In one or more embodiments of the air filter systems described herein, the upper end of the range for the pulse distance (pd) may be 50 times or less the pulse outlet hydraulic diameter (dpo). 
     One or more embodiments of the air filter systems described herein may also be characterized in terms of a relationship between hydraulic diameters of the filter element openings and the filter and openings of the pulse collectors to which the filter elements are attached. A simplified schematic diagram of the junction between a pulse collector  330  and a filter element  340  that are located along a pulse axis  351  is depicted in  FIG. 9  and will be used to describe the relationship between those hydraulic diameters. 
     As depicted in  FIG. 9 , the pulse collector  330  includes an inner surface  333  that defines the filter end opening  331  of the pulse collector  330 . In one or more embodiments, the pulse collector  330  may include a flange  335  that can be used as a surface against which a filter element can be sealed during use of the air filter systems described herein. 
     The filter element  340  depicted in  FIG. 9  includes filter media  347  to which an end cap  380  is connected. In one or more embodiments, the end cap  380  may be configured to receive the filter media  347  such that an air-tight connection is provided between the filter media  347  and the end cap  380 . In the depicted illustrative embodiment, sealant  387  in the form of, e.g., potting material may be used to provide an air-tight connection between the end cap  380  and the filter media  347  (although many other air-tight connections could be used to secure an end cap to filter media). 
     A gasket  383  is, in the depicted illustrative embodiment, located between the flange  335  of the pulse collector  330  and the end cap  380  to form a seal between the pulse collector  330  and the filter element  340 . In the air filter systems described herein, one or more gaskets or other sealing structures may be used to seal the connection between a filter element and a pulse collector. 
     In one or more embodiments of the air filter systems described herein, the hydraulic diameter of the filter element opening (dfe) may be related to the hydraulic diameter of the filter end opening of the pulse collector (dpc). 
     The hydraulic diameter (dpc in  FIG. 9 ) of the filter end opening of the pulse collectors described herein can be determined in a plane that is transverse to the pulse axis  351  at a location within 25 millimeters or less of the filter end opening  331  of the pulse collector  330  along the pulse axis  351  where the cross-sectional area of the passageway through the pulse collector  330  is smallest. With reference to  FIG. 9 , it is the distance DI that is 25 millimeters or less. As a result, minor changes in the cross-sectional area of the passageway through the pulse collector  330  near the junction of the pulse collector and the filter element  340  (such as, e.g., curvature of the pulse collector  330  at its filter end opening where the pulse collector  330  widens due to, e.g., manufacturing requirements) will not affect an accurate determination of the hydraulic diameter dpc of the pulse collector  330  as described herein. The hydraulic diameter dpc of the filter end opening of the pulse collector  330  is calculated according to the equation described above in connection with the hydraulic diameter of the pulse outlet, i.e., the hydraulic diameter is four times the cross-sectional area of the pulse collector at the selected location divided by its perimeter at that location. 
     The hydraulic diameter of the filter element opening (dfe in  FIG. 9 ) is, likewise, determined in a plane that is transverse to the pulse axis  351 . In particular, as used herein, the hydraulic diameter of the filter element opening (dfe) is determined at a location where the interior of filter media  347  of the filter element  340  is exposed to the interior volume  341  of the filter element  340  such that air can pass through the filter media  347  into and out of the interior volume  341  around a perimeter of the interior volume of the filter element  340 . In one or more embodiments in which an end cap  380  is used, that location will be found at an interior edge  388  of the end cap  380 . The hydraulic diameter of the filter element opening  345  is also calculated according to the equations described above, i.e., the hydraulic diameter of the filter element opening dfe is four times the cross-sectional area of the filter element opening at the selected location divided by its perimeter at that location. In the case of, e.g., pleated filter media, the cross-sectional are is defined by the locations of the inner edges of the folds making up the pleats in the filter media. 
     Although not depicted in the schematic diagram of  FIG. 9 ., in one or more embodiments of the filter elements as described herein, an inner liner may be provided over the inner surface of the filter media  347  to offer e.g., protection, support, etc. to the filter media. Examples of some liners that may be used in connection with the filter elements described herein may be found in, e.g., U.S. Pat. No. 6,488,746 (Kosmider et al.), U.S. Pat. No. 8,128,724 (Mills et al.), etc. In such an arrangement, the hydraulic diameter of the filter element opening dfe is determined using the inner surface of the inner liner. 
     In one or more embodiments of the air filter systems described herein, the hydraulic diameter of the filter element opening (dfe) is 112% or less of the hydraulic diameter of the filter end opening of the pulse collector (dpc). In one or more alternative embodiments of the air filter systems described herein, the hydraulic diameter of the filter element opening (dfe) is 108% or less of the hydraulic diameter of the filter end opening of the pulse collector (dpc). 
     In one or more embodiments of the air filter systems described herein, the hydraulic diameter of the filter element opening (dfe) is 90% or more of the hydraulic diameter of the filter end opening of the pulse collector (dpc). In one or more alternative embodiments of the air filter systems described herein, the hydraulic diameter of the filter element opening (dfe) is 95% or more of the hydraulic diameter of the filter end opening of the pulse collector (dpc). 
     In one or more alternative embodiments of the air filter systems described herein, the absolute value of a difference between the hydraulic diameter of the filter element opening (dfe) and the hydraulic diameter of the filter end opening of the pulse collector (dpc) is within 2% or less of the hydraulic diameter of the filter element opening. 
     Another manner in which the air filter systems described herein may be characterized can be described in connection with  FIGS. 10A and 10B , which depict cross-sectional views of enlarged portions of the interface between the filter end opening  431  of a pulse collector  430  and a filter element  440 . The filter element  440  defines an inner surface  446  while the pulse collector  430  defines an inner surface  433 . In one or more embodiments, the inner surface  433  of the pulse collector  430  is in alignment with the inner surface  446  of the filter element  440  at the filter end opening  445  of the filter element  440 . In one or more embodiments, that alignment may be measure at the locations used to determine the hydraulic diameters of the filter end opening of the pulse collector and the filter element (dpc and dfe as described above in connection with  FIG. 9 ). 
     In some instances, however, there may be an offset between the inner surface  433  of the filter end opening  431  of the pulse collector  430  and the inner surface  446  of the filter element opening  445  of the filter element  440 . In particular, that offset (do in  FIGS. 10A and 10B ) may result in an arrangement in which the inner surface  433  and  446  do not align with each other around the perimeter of the junction between the filter end opening  431  and the filter element opening  445 .  FIG. 10A  depicts an example in which the inner surface  433  of the filter end opening  431  of the pulse collector  430  is located inwardly from the inner surface  446  of the filter element  440  at the filter end opening  445  at an offset distance (do) as seen in  FIG. 10A .  FIG. 10B  depicts an example in which the inner surface  446  of the filter element  440  at the filter end opening  445  is located inwardly from the inner surface  433  of the filter end opening  431  of the pulse collector  430  at an offset distance (do) as seen in  FIG. 10B . 
     In one or more embodiments, the offset (do) between the inner surface  446  of the filter element opening  445  and the inner surface  433  of the filter end opening  431  of the pulse collector  430  is no more than 15 millimeters at any location about a perimeter of the filter element opening  445 . In one or more alternative embodiments, the offset (do) between the inner surface  446  of the filter element opening  445  and the inner surface  433  of the filter end opening  431  of the pulse collector  430  is no more than 10 millimeters at any location about a perimeter of the filter element opening  445 . In one or more alternative embodiments, the offset (do) between the inner surface  446  of the filter element opening  445  and the inner surface  433  of the filter end opening  431  of the pulse collector  430  is no more than 5 millimeters at any location about a perimeter of the filter element opening  445 . 
     The air filter systems described herein include, in one or more embodiments, a pulse collector located between the tube sheet and the filter element on the dirty air chamber side of the tube sheet. In one or more embodiments, the pulse collector may be in the form of a venturi element including a throat that constricts the passageway through the pulse collector at a location between its ends as described in, e.g., one or more of the following: U.S. Pat. No. 3,942,962 (Duyckinck), U.S. Pat. No. 4,218,227 (Frey), U.S. Pat. No. 6,090,173 (Johnson et al.), U.S. Pat. No. 6,902,592 (Green et al.), U.S. Pat. No. 7,641,708 (Kosmider et al.), and US Patent Application Publication No. US2013/0305667 A1. 
     In one or more alternative embodiments, the pulse collectors used in the air filter systems described herein may be in the form of straight to this without any constriction or divergence between the tube sheet and the filter element. One example of such a pulse collector is depicted in, e.g.,  FIG. 8 . 
     In still other embodiments, the pulse collectors used in the air filter systems described herein may include a pulse section and a filter section that meet at a junction located between the filter end and the tube sheet end of the pulse collector. One illustrative embodiment of such a pulse collector  530  is depicted in  FIGS. 11-13 . The pulse collector  530  includes a pulse section  536  and a filter section  537  that meet at a junction  538  at a location between the filter end  531  and the tube sheet end  532  of the pulse collector  530 . As with the other embodiments of pulse collectors as described herein, the pulse axis  551  extends through the pulse collector  530 . 
     In one or more embodiments, the pulse collectors having both a pulse section and a filter section as described herein may have a pulse section  536  in which the portion of the passageway through the pulse collector  530  defined by the pulse section  536  has a hydraulic diameter (see, e.g., d 1  in  FIG. 12 ) that increases when moving from the junction  538  towards the tube sheet end  532  of the pulse collector  530 . The hydraulic diameter of the pulse section  536  is determined according to the principles described herein, i.e., the hydraulic diameter of the pulse section  536  at any point along the pulse axis  551  is the product of four times the cross-sectional area of the pulse section  536  divided by the perimeter at that location. 
     In one or more embodiments, the pulse collectors having both a pulse section and a filter section as described herein may have a filter section  537  in which the portion of the passageway through the pulse collector  530  defined by the filter section  537  has a hydraulic diameter (see, e.g., d 2  in  FIG. 12 ) that remains constant when moving from the junction  538  towards the filter end  531  of the pulse collector  530 . The hydraulic diameter of the filter section  537  is determined according to the principles described herein, i.e., the hydraulic diameter of the filter section  537  at any point along the pulse axis  551  is the product of four times the cross-sectional area of the filter section  537  divided by the perimeter at that location. It should be understood that the filter section  537  may have a hydraulic diameter that increases slightly at the filter end  531  due to manufacturing limits in the forming of the materials used to manufacture the filter section  537 . The hydraulic diameter of the filter section  537  may, however, be constant over substantially its entire length with the exception of that small transition area which, in one or more embodiments, constitutes less than 10% of the overall length of the filter section  537 . 
     In one or more embodiments of the pulse collectors described herein that include a pulse section  536  and a filter section  537 , the pulse section  536  and the filter section  537  may be in the form of separate articles attached to each other at the junction  538 . In one or more embodiments, the pulse section  536  and the filter section  537  may overlap each other within or near the junction  538  as seen in, e.g., the enlarged cross-sectional view of  FIG. 13 . It should be noted that the precise location of the junction  538  is, in the illustrative embodiment depicted in  FIGS. 11-13 , selected as the location at which the pulse collector  530  begins to diverge such that the hydraulic diameter increases when moving towards the tube sheet end  532 . 
     The connection made near the junction  538  of the pulse collector  530  may be constructed using a variety of techniques and/or components. For example, the pulse section  536  and filter section  537  may be connected to each other using adhesives, clamps, mechanical fasteners, etc. In one or more embodiments, the pulse section  536  and the filter section  537  may be welded together. 
     In one or more embodiments of the pulse collectors described herein, the pulse collector  530  may be described as having a passageway length (see, e.g., lp in  FIG. 12 ) measured along the pulse axis  551  that is equal to or greater than a hydraulic diameter of the filter end opening  533  at the filter end  531  of the pulse collector  530 . Further, in one or more embodiments of the pulse collectors described herein, the pulse collector  530  may be described as having a passageway length measured along the pulse axis  551  that is no more than three times the hydraulic diameter of the filter end opening  533  at the filter end  531  of the pulse collector  530 . These relationships between the passageway length and the hydraulic diameter of the filter end opening  533  at the filter end  531  of the pulse collector  530  apply regardless of whether or not the pulse collector has the specific construction of pulse collector  530 . In other words, the relationship between the passageway length and the hydraulic diameter at the filter end opening of a pulse collector used in air filter systems described herein may, in one or more embodiments, be applied to any pulse collector including those that include a throat and/or those that have a constant hydraulic diameter along their entire length (e.g., are in the form of a simple straight wall tube). 
     In one or more embodiments of the pulse collectors described herein that include a pulse section  536  and a filter section  537 , the filter section  537  may have a filter section length (see, e.g., l 1  in  FIG. 12 ) measured along the pulse axis  551  from the filter end  531  to the junction  538  and the pulse section  536  has a pulse section length (see, e.g., l 2  in  FIG. 12 ) measured along the pulse axis  551  from the tube sheet end  532  to the junction  538 . In one or more embodiments of the pulse collectors described herein, the filter section length (l 1 ) is less than or equal to the pulse section length (l 2 ). 
     In one or more embodiments of the pulse collectors described herein that include a pulse section  536  and a filter section  537 , the filter section length (l 1 ) and the pulse section length (l 2 ) may have one or more selected relationships with the hydraulic diameter of the filter end opening  533  (d 2 ) at the filter end  531  of the pulse collector  530 . For example, in one or more embodiments the filter section length (l 1 ) and the pulse section length (l 2 ) are both equal to or less than 1.5 times the hydraulic diameter of the filter end opening  533  (d 2 ) at the filter end  531  of the pulse collector  530 . In one or more alternative embodiments, the filter section length (l 1 ) and the pulse section length (l 2 ) are both equal to or less than the hydraulic diameter of the filter end opening  533  (d 2 ) at the filter end  531  of the pulse collector  530 . 
     As discussed in connection with the pulse section  536  of the pulse collector  530 , in one or more embodiments of pulse collectors that may be used in air filter systems as described herein, the pulse section  536  may have a hydraulic diameter (d 1 ) that increases when moving from the junction  538  to the tube sheet end  532  of the pulse collector  530 . In one or more embodiments, that increasing hydraulic diameter is a function of an included angle formed by the opposing walls defining the portion of the passageway in the pulse section  536 , with the opposing walls diverging from the pulse axis  551  at an included angle (see, e.g., angle θ (theta) in  FIG. 12 ). 
     In one or more embodiments, that included angle may be described as being greater than 0° and less than or equal to 10°. In one or more alternative embodiments, that included angle may be described as being greater than 3° or, in one or more alternative embodiments, greater than 5°. In one or more alternative embodiments that included angle may be described as being less than or equal to 8°, and in still other embodiments, the included angle may be described as being less than or equal to 7°. Any combination of these upper and lower limits for the included angle may be used to characterize the divergence of opposing walls of a pulse section of a pulse collector as described herein. 
     Although the illustrative embodiments of air filter systems depict arrangements in which the pulse collectors are located on the dirty air chamber side of the tube sheet with the tube sheet end of the pulse collector located on dirty air chamber side of the tube sheet, in one or more embodiments the tube sheet end of the pulse collectors may be located on the clean air chamber side of the tube sheet. One illustrative embodiment of such an arrangement is depicted in  FIG. 14  including a pulse collector  530 ′ and a tube sheet  522 ′ that separates a clean air chamber  524 ′ from a dirty air chamber  526 ′. In the depicted illustrative embodiment, the pulse collector  530 ′ is positioned relative to the tube sheet  522 ′ such that the tube sheet end  532 ′ is located in the clean air chamber  524 ′ while the filter end  531 ′ of the pulse collector  530 ′ remains in the dirty air chamber  526 ′. 
     The pulse collector  530 ′ can be described as having a passageway length lp measured along a pulse axis  551 ′ as discussed herein in connection with other illustrative embodiments. In one or more embodiments, the portion of the passageway length lp located in the clean air chamber  524 ′ (i.e., on the clean air chamber side of the tube sheet  522 ′) may be limited to 50% or less of the total passage way length lp. 
     Additional features that may be provided in one or more embodiments of the air filter systems described herein are depicted in connection with  FIGS. 15-17 . In particular,  FIG. 15  depicts an arrangement that includes a tube sheet  622  having a pulse collector  630  attached thereto. A filter cartridge  644  is depicted as loaded onto a yoke  642  that extends from the pulse collector  630 , with space on the yoke  642  for a second filter cartridge  644  to form a filter element attached to the pulse collector  630  as described in connection with the air filter systems described herein. The arrangement depicted in  FIG. 15  further includes a pulse generator  650  aligned along a pulse axis  651  to provide for cleaning of the filter element as described herein. 
     The filter cartridge/element  644  depicted in  FIG. 15  includes end caps  680  that may include features such as gaskets, etc. that allow the cartridge  644  to form a seal with a second cartridge that may be located on the yoke  642 , as well as the pulse collector  630 . 
     An end view of the components depicted in  FIG. 15  is provided in  FIG. 16 , with the view taken along the pulse axis  651 . Among the features depicted in the view of  FIG. 15  is the noncircular shape of the end cap  680  and the associated filter element/cartridge  644 . 
     The filter elements used in one or more embodiments of the air filter systems described herein may be supported on a yoke that extends away from the pulse collector along the pulse axis. In the illustrative embodiment of such an arrangement as seen in, e.g.,  FIGS. 15 and 16 , the yoke  642  includes support beams  672 ,  674  and  676  that are aligned with the pulse axis  651 . The support beams are connected to each other along the length of the yoke  642  using struts  670 . In one or more embodiments, the yokes used in air filter systems as described herein may be constructed of any material or combination of materials that defines support beams that mate with and support filter elements along the yoke. For example, although the yoke  642  is constructed of rod-shaped material, in one or more alternative embodiments, the yokes used to support filter elements in air filter systems as described herein may be constructed of, e.g., sheet metal or any other suitable material. 
     Although the illustrative embodiment of yoke  642  includes three support beams, in one or more alternative embodiments the yoke  642  may include as few as two support beams. Unlike conventional yokes used in air filter systems, the support beams used in one or more embodiments of yokes as described herein may be arranged asymmetrically about the pulse axis  651  and extending through the yoke. That asymmetry of the support beams of yoke  642  is seen in both  FIGS. 15 and 16 . 
     The asymmetry of the support beams in the yokes used to support filter elements in air filter systems as described herein may, in one or more embodiments, be used to align the filter elements in a selected rotational orientation relative to the pulse axis extending through the yoke during placement and to assist in retention of their rotational orientation during use. Such alignment requirements may be helpful where, for example, the filter elements have orientations that may or may not properly sealed with other features such as, e.g., the filter end of a pulse collector. In particular, the location and placement of support beams  672 ,  674 , and  676  of yoke  642  limit the placement of a filter element  644  having a shape such as that seen in, e.g.,  FIGS. 16 and 17  to only one selected rotational orientation relative to the pulse axis  651 . For example, the end cap  680  may include alignment features  682 ,  684 , and  686  that are configured to receive the corresponding support beams  672 ,  674 , and  676  of the yoke  642 . The alignment features aligned with their corresponding support beams only when the end cap  680  is in one selected rotational orientation relative to the pulse axis  651 . In one or more embodiments, the end caps  680  may include a visual alignment aid  688  indicating, e.g., an upward direction for the end cap  680  and, therefore, for its corresponding filter element. 
     Ovate Filter Elements/Cartridges 
     In one or more embodiments, the filter elements used in connection with the air filter systems described herein may have one or more ovate cross-sectional shapes as described in, e.g., U.S. Provisional Patent Application No. 61/789,385, titled OVATE TUBULAR FILTER CARTRIDGES AND FILTER SYSTEMS USING THE SAME. 
     Examples of some illustrative embodiments of ovate tubular filter elements/cartridges that may be used to provide filter elements in air filter systems as described herein are depicted and described in connection with  FIGS. 18-24 . Although described below as filter elements, two or more of the filter elements may be combined in the air filter systems as described herein to form a single composite filter element (in which case each filter element described below may sometimes be referred to as a cartridge). 
     One illustrative embodiment of an ovate filter element that may be used in one or more embodiments of the air filter systems as described herein is depicted in the perspective views of  FIGS. 18 and 19 . The filter element includes filter media  1110  having end caps  1120  located on each of the first end  1112  and the second end  1114  of the filter media  1110 . 
     The end cap  1120  on first end  1112  of the filter media  1110  may, in one or more embodiments, have an opening that allows access to the interior volume of filter element. The end cap  1120  on the opposite end of the filter media  1110  may, in one or more embodiments, be closed so that it prevents access to the interior volume of the filter element and so that gas (e.g., air) entering the interior volume of the filter element through the end cap  1120  on the first end  1112  of the filter media  1110  must exit through the filter media in the filter element. In one or more alternative embodiments, both end caps  20  may be open to allow access to the interior volume of the filter element. 
     In one or more embodiments, a gasket  1122  may be provided on the end cap  1120  to seal the filter element over an opening in, e.g., a tube sheet, a venturi, or other structure through which gas (e.g., air) is delivered into the interior volume of the filter element. 
     A tube axis  1111  extends through the tubular filter element between the first end  1112  and the second end  1114 . The filter media  1110  has a length L between its first end  1112  and its second end  1114  as depicted in  FIG. 19 . The filter media  1110  in the filter elements described herein defines an exterior surface  1116  and interior surface  1118  located around the tube axis  1111 . The interior surface  1118  faces an interior volume of the filter element  1110  and the exterior surface  1116  faces away from that interior volume. 
     In one or more embodiments in which the ovate filter elements are used in air filter systems as described herein, the tube axis  1111  may be aligned with a pulse axis defined by a pulse generator in the air filter system. In one or more alternative embodiments in which the ovate filter elements are used in air filter systems as described herein, the tube axis  1111  may be collinear with a pulse axis defined by a pulse generator in the air filter system. 
     Although not depicted in the illustrative embodiment of filter element, in one or more embodiments of the filter elements as described herein, an outer liner may be provided over the exterior surface of the filter media and/or an inner liner may be provided over the inner surface of the filter media to offer e.g., protection, support, etc. to the filter media. Examples of some liners that may be used in connection with the filter elements described herein may be found in, e.g., U.S. Pat. No. 6,488,746 (Kosmider et al.), U.S. Pat. No. 8,128,724 (Mills et al.), etc. One or both the liners may, in one or more embodiments, be flexible enough to adopt the ovate cross-sectional shape of the tubular filter elements as described herein. In one or more alternative embodiments, one or both of the liners may be formed into the ovate cross-sectional shapes described herein and retain those shapes in the absence of any external force acting on the liner. 
       FIGS. 20 and 21  depict one illustrative embodiment of an ovate cross-section formed by tubular filter media  1110  in the filter element, with the cross-section being taken transverse to the tube axis  1111 . For example, the cross-section seen in  FIGS. 19 and 20  may be taken in the plane  3  depicted in  FIG. 19 , where plane  3  is oriented orthogonal to the tube axis  1111 . The cross-sections of the tubular filter media described herein may, in one or more embodiments, be taken at any location along the length L of the filter element containing the filter media  1110 . In one or more alternative embodiments, the cross-sections of the tubular filter media  1110  as described herein may be found at any location along 10% or more of the length L of the filter media. In other words, there may be portions of the length L of the tubular filter media that do not exhibit the characteristics described herein in a cross-section thereof. In one or more alternative embodiments, the cross-sections of the tubular filter media as described herein may be found at any location along 25% or more of the length L of the filter media. In one or more alternative embodiments, the cross-sections of the tubular filter media as described herein may be found at any location along 50% or more of the length L of the filter media. 
     In one or more embodiments, the tubular filter media  1110  may have the same shape along the entire length L, although that is not required in all embodiments (i.e., in one or more embodiments, the cross-sectional shape of the tubular filter media  30  may change over the length L). 
     As seen in the ovate cross-section depicted in  FIGS. 20 and 21 , the tubular filter media  1110  defines an inner perimeter that corresponds to the interior surface  1118  of the filter media  1110 . Because the inner perimeter of the cross-section is essentially coincident with the interior surface  1118  of the filter media  1110 , reference number  1118  may also be used herein to refer to the inner perimeter of the cross-section. The filter media provided in the filter elements may take a variety of different forms, but in one or more embodiments, the filter media  1110  may include pleats  1119  having internal folds located along the inner perimeter as represented by the interior surface  1118  and external folds located along the outer perimeter of the filter media  1110 . In one or more embodiments, the folds in the pleats  1119  will typically be located along the surface of an inner liner that follows and/or defines the shape of the inner perimeter and the interior surface  1118  of the filter elements described herein. 
     Although the cross-sections of the tubular filter media in the filter elements described herein are discussed using terms such as up, down, top, bottom, etc., those terms are used only to provide a frame of reference for describing the shapes and/or features of the cross-sections. In particular, it should be understood that the filter elements described herein may be used in a filter system in any orientation. For example, in one or more embodiments, a surface identified as a “bottom” of the filter media or filter element may be found on a top surface of the filter element (relative to the direction of gravity) when the filter elements installed within a filter system. 
     In one or more embodiments, the ovate cross-section of the filter media  1110  has a maximum height  1133  (Hmax) that is measured between a top point  1131  and a bottom point  1132  along an axis of maximum height  1130 . The top point  1131  and the bottom point  1132  are located on the inner perimeter  1118  of the cross-section of the filter media  1110  and are, in one or more embodiments, the points that are furthest apart from each other along any straight line extending across the inner perimeter  1118  of the cross-section. In some instances, the inner perimeter  1118  may have two or more axes of maximum height, each of which intersects the inner perimeter  1118  at two points that are equidistant apart from each other along two or more different straight lines extending across the inner perimeter  1118  of the cross-section. In such a case, any one of the axes of maximum height may be used to characterize the ovate cross-section as described herein. 
     The inner perimeter  1118  of the ovate cross-section of the filter media  1110  as described herein also has a maximum width  1143  (Wmax) measured between a first point  1141  and a second point  1142  on the inner perimeter  1118 . The first point  1141  and the second point  1142  are located on an axis of maximum width  1140  that is located along a straight line perpendicular to the axis of maximum height  1130 . The axis of maximum width  1140  intersects the axis of maximum height  1130  at a bottom axis intersection point  1144  where the first point  1141  and the second point  1142  at which the axis of maximum width  1140  intersects the inner perimeter  1118  are located furthest apart from each other on any straight line perpendicular to the axis of maximum height  1130 . 
     Because of the ovate or ovoid shape of the cross-section of the filter media  1110 , the bottom axis intersection point  1144  does not, in one or more embodiments, bisect the maximum height of the cross-section as measured between the top point  1131  and the bottom point  1132  along the axis of maximum height  1130 . 
     In one or more embodiments of the tubular filter media described herein, the ovate cross-section, as depicted in, e.g.,  FIG. 20 , may define a bottom section height  1134  (Hwmax) measured along the axis of maximum height  1130  from the bottom point  1132  to the bottom axis intersection point  1144 . In one or more embodiments, the bottom section height  1134  may be less than or equal to 0.4 of the maximum height as measured along the axis of maximum height  1130  from the top point  1131  to the bottom point  1132 . In one or more embodiments, the bottom section height  1134  is greater than zero. In one or more embodiments, the bottom section height  1134  is greater than or equal to 0.1 of the maximum height as measured along the axis of maximum height  1130  from the top point  1131  to the bottom point  1132 . 
     Another manner in which the ovate cross-sections of tubular filter media of filter elements as described herein may be characterized is in terms of the length of the inner perimeter at both the top and bottom of the filter media of the filter element. For example, the inner perimeter  1118  of the cross-section of filter media of filter elements as described herein may define a bottom perimeter section containing the bottom point  1132  and extending from the first point  1141  to the second point  1142  at which the axis of maximum width  1140  intersects the inner perimeter  1118 . The bottom perimeter section, i.e., the portion of the inner perimeter  1118  from the first point  1141  to the second point  1142  (and including bottom point  1132 ) has a bottom perimeter section length measured along the inner perimeter  1118  from the first point  1141  to the second point  1142 . 
     The inner perimeter  1118  of the ovate cross-section of filter media  1110  of filter elements as described herein may also define a top perimeter section containing the top point  1131  at which the axis of maximum height  1130  intersects the inner perimeter  1118 . The top perimeter section extends from a first end  1146  to a second end  1147  on the inner perimeter  1118 , the first end  1146  being located on the inner perimeter  1118  between the first point  1141  and the top point  1131  and the second end  1147  being located on the inner perimeter  1118  between the second point  1142  and the top point  1131 . The first end  1146  and the second end  1147  of the top perimeter section are the points at which a top perimeter section line  1145  intersects the inner perimeter  1118 . The top perimeter section line  1145  is a straight line that is perpendicular to the axis of maximum height  1130  and intersects the axis of maximum height  1130  at a top axis intersection point  1149 . The top axis intersection point  1149  is located within the inner perimeter  1118  between first point  1131  and second point  1132  at which the axis of maximum height  1130  intersects the inner perimeter  1118 . The top axis intersection point  1149  defines a top section height  1135  measured along the axis of maximum height  1130  from the top axis intersection point  1149  to the top point  1131  on the inner perimeter  1118 . 
     In one or more embodiments, the top section height  1135 , e.g., the distance from the top axis intersection point  1149  to the top point  1131  in the illustrative embodiment depicted in  FIG. 20 , is equal to the bottom section height  1134  in the ovate cross-sections of filter media in filter elements as described herein. In one or more embodiments, the bottom perimeter section length as measured along the inner perimeter  1118  between points  1141  and  1142  (and including bottom point  1132 ) is greater than the top perimeter section length as measured along the inner perimeter  1118  between first end  1146  and second end  1147  (and including top point  1131 ). In one or more embodiments, the bottom perimeter section length may be 1.2 or more times greater than the top perimeter section length. In one or more alternative embodiments, the bottom perimeter section length may be two or more times greater than the top perimeter section length. 
     In one or more embodiments of the filter media  1110  in filter elements as described herein, the bottom perimeter section of the inner perimeter  1118  located between the first point  1141  and the second point  1142  may be continuously curved from the first point  1141  to the second point  1142 . As used herein, “continuously curved” means that the inner perimeter  1118  includes no straight portions between first point  1141  and second point  1142 , although the curvature of the inner perimeter  1118  may not be uniform along the entire length of the bottom perimeter section. In one or more alternative embodiments, the bottom perimeter section of the inner perimeter  1118  may include one or more limited portions that form a straight line, however, no portion of the bottom perimeter section of the inner perimeter  1118  lies on a straight line for a distance of more than 1 centimeter. 
     One or more embodiments of the filter media  1110  in filter elements as described herein may also include a line of symmetry defined by the inner perimeter  1118  of the cross-section as depicted in, e.g.,  FIGS. 20 and 21 . In particular, filter elements having the ovate shapes described herein may, in one or more embodiments, define only a single line of symmetry. In the illustrative embodiment depicted in  FIGS. 20 and 21 , the inner perimeter  1118  of the cross-section of filter media  1110  defines a single line of symmetry that is coincident with the axis of maximum height  1130 . Such a relationship between a line of symmetry and an axis of maximum height may not, however, necessarily be required in all embodiments described herein. 
     In one or more embodiments of the filter elements described herein, it may be possible to provide an inscribed circle  1150  located within the inner perimeter  1118  of the ovate cross-section of the filter media  1110 , with the inscribed circles discussed herein being the largest inscribed circles that may be located within the inner perimeter  1118  of the cross-section of the filter media  1110 . Because the inner perimeter  1118  is not circular in shape, the inscribed circle  1150  occupies less than all of the area within the inner perimeter  1118 . In the view as seen in  FIG. 21 , the inscribed circle  1150  does not occupy areas  1152 ,  1154 , and  1156  within the inner perimeter  1118  of the filter media  1110 . 
     In one or more embodiments, the inscribed circle  1150  located within the inner perimeter  1118  may occupy 60% or more of the inner area defined by the inner perimeter  1118 . In one or more alternative embodiments, the inscribed circle  1150  may occupy 70% or more of the inner area defined by the inner perimeter  1118 . In one or more additional alternative embodiments, the inscribed circle  1150  may occupy 80% or more of the inner area defined by the inner perimeter  1118 . In the illustrative example depicted in  FIG. 21 , the inscribed circle  1150  occupies more than 80% of the inner area defined by the inner perimeter  1118 . 
     The use of inscribed circles may also provide another way in which the inner perimeters of the cross-sections of tubular filter media in filter elements as described herein may be characterized. In connection with the illustrative embodiment depicted in, e.g.,  FIG. 21 , the inscribed circle  1150  located within the inner perimeter  1118  can be described as defining a maximum radial gap between the inscribed circle  1150  and the inner perimeter  1118 . As depicted in  FIG. 21 , the maximum radial gap may be measured between points  1158  and  1159  located along axis  1157  that passes through the center  1151  of the inscribed circle  1150 . In one or more embodiments, the maximum radial gap as measured between points  1158  and  1159  may be 0.5 or less of the diameter of the inscribed circle  1150 . In one or more alternative embodiments, the maximum radial gap between an inscribed circle and the inner perimeter of a cross-section of filter media in which the inscribed circle is located may be 0.25 or less of the diameter of that inscribed circle. Limiting the maximum radial gap between an inscribed circle and the inner perimeter may, in one or more embodiments, provide improvements in pulse cleaning of a filter element having such characteristics. Further, although this characteristic is not described with respect to the other alternative illustrative embodiments described below with respect to  FIGS. 21 and 22 , this characteristic may be determined with respect to any tubular filter media used in filter elements as described herein and may, in one or more embodiments, be controlled to the ratios described above. 
     Another illustrative embodiment of an ovate cross-section of filter media  1210  that may be used in a tubular filter element as described herein is depicted in  FIG. 22 . Unlike the cross-section of filter media  1110  as depicted in  FIGS. 19 and 20 , the ovate cross-section of filter media  1210  depicted in  FIG. 22  has an inner perimeter  1218  that defines no lines of symmetry, i.e., the inner perimeter  1218  of the filter media  1210  is asymmetric. 
     In one or more embodiments, the ovate cross-section of the filter media  1210  has a maximum height  1233  (Hmax) that is measured between a top point  1231  and a bottom point  1232  along an axis of maximum height  1230 . The top point  1231  and the bottom point  1232  are located on the inner perimeter  1218  of the cross-section of the filter media  1210  and are, in one or more embodiments, the points that are furthest apart from each other along any straight line extending across the inner perimeter  1218  of the cross-section. 
     The inner perimeter  1218  of the ovate cross-section of the filter media  1210  as described herein also has a maximum width  1243  (Wmax) measured between a first point  1241  and a second point  1242  on the inner perimeter  1218 . The first point  1241  and the second point  1242  are located on an axis of maximum width  1240  that is located along a straight line perpendicular to the axis of maximum height  1230 . The axis of maximum width  1240  intersects the axis of maximum height  1230  at a bottom axis intersection point  1244  where the first point  1241  and the second point  1242  at which the axis of maximum width  1240  intersects the inner perimeter  1218  are located furthest apart from each other on any straight line perpendicular to the axis of maximum height  1230  between top point  1231  and bottom point  1232 . 
     Because of the ovate or ovoid shape of the cross-section of the filter media  1210 , the bottom axis intersection point  1244  does not, in one or more embodiments, bisect the maximum height of the cross-section as measured between the top point  1231  and the bottom point  1232  along the axis of maximum height  1230 . 
     In one or more embodiments of the tubular filter media described herein, the ovate cross-section, as depicted in, e.g.,  FIG. 22 , may define a bottom section height  1234  (Hwmax) measured along the axis of maximum height  1230  from the bottom point  1232  to the bottom axis intersection point  1244 . In one or more embodiments, the bottom section height  1234  may be less than or equal to 0.4 of the maximum height as measured along the axis of maximum height  1230  from the top point  1231  to the bottom point  1232 . In one or more embodiments, the bottom section height  1234  is greater than zero. In one or more embodiments, the bottom section height  1234  is greater than or equal to 0.1 of the maximum height as measured along the axis of maximum height  1230  from the top point  1231  to the bottom point  1232 . 
     Another manner in which the cross-sections of tubular filter media of filter elements as described herein may be characterized is in terms of the length of the inner perimeter at both the top and bottom of the filter media of the filter element. For example, the inner perimeter  1218  of the cross-section of filter media  1210  as described herein may define a bottom perimeter section containing the bottom point  1232  and extending from the first point  1241  to the second point  1242  at which the axis of maximum width  1240  intersects the inner perimeter  1218 . The bottom perimeter section, i.e., the portion of the inner perimeter  1218  from the first point  1241  to the second point  1242  (and including bottom point  1232 ) has a bottom perimeter section length measured along the inner perimeter  1218  from the first point  1241  to the second point  1242 . 
     The inner perimeter  1218  of the cross-section of filter media  1210  of one or more embodiments of filter elements as described herein may also define a top perimeter section containing the top point  1231  at which the axis of maximum height  1230  intersects the inner perimeter  1218 . The top perimeter section extends from a first end  1246  to a second end  1247  on the inner perimeter  1218 , the first end  1246  being located on the inner perimeter  1218  between the first point  1241  and the top point  1231  and the second end  1247  being located on the inner perimeter  1218  between the second point  1242  and the top point  1231 . The first end  1246  and the second end  1247  of the top perimeter section are the points at which a top perimeter section line  1245  intersects the inner perimeter  1218  on opposite sides of the axis of maximum height  1230 . The top perimeter section line  1245  is a straight line that is perpendicular to the axis of maximum height  1230  and intersects the axis of maximum height  1230  at a top axis intersection point  1249 . The top axis intersection point  1249  is located within the inner perimeter  1218  between first point  1231  and second point  1232  at which the axis of maximum height  1230  intersects the inner perimeter  1218 . The top axis intersection point  1249  defines a top section height  1235  measured along the axis of maximum height  1230  from the top axis intersection point  1249  to the top point  1231  on the inner perimeter  1218 . 
     In one or more embodiments, the top section height  1235 , e.g., the distance from the top axis intersection point  1249  to the top point  1231  in the illustrative embodiment depicted in  FIG. 22 , is equal to the bottom section height  1234  in the cross-sections of filter media in one or more embodiments of filter elements as described herein. In one or more embodiments, the bottom perimeter section length as measured along the inner perimeter  1218  between points  1241  and  1242  (and including bottom point  1232 ) is greater than the top perimeter section length as measured along the inner perimeter  1218  between first end  1246  and second end  1247  (and including top point  1231 ). In one or more embodiments, the bottom perimeter section length may be 1.2 or more times greater than the top perimeter section length. In one or more alternative embodiments, the bottom perimeter section length may be two or more times greater than the top perimeter section length. 
     In one or more embodiments of the filter media  1210  in filter elements as described herein, the bottom perimeter section of the inner perimeter  1218  located between the first point  1241  and the second point  1242  may be continuously curved from the first point  1241  to the second point  1242 . In one or more alternative embodiments, the bottom perimeter section of the inner perimeter  1218  may include one or more limited portions that form a straight line, however, no portion of the bottom perimeter section of the inner perimeter  1218  lies on a straight line for a distance of more than 1 centimeter. 
     Still another illustrative embodiment of tubular filter media  1310  having an ovate cross-section that may be used in a tubular filter element as described herein is depicted in  FIG. 23 . Unlike the cross-section of filter media  1110  as depicted in  FIGS. 20 and 21  or the cross-section of filter media  1210  depicted in  FIG. 22 , the cross-section of filter media  1310  depicted in  FIG. 23  has an inner perimeter  1318  that includes flat or straight sections. For the purposes of the filter elements described herein, however, the cross-section formed by filter media  1310  is ovate because it has a base wider than a top. 
     In one or more embodiments, the cross-section of the filter media  1310  has a maximum height  1333  (Hmax) that is measured between a top point  1331  and a bottom point  1332  along an axis of maximum height  1330 . The top point  1331  and the bottom point  1332  are located on the inner perimeter  1318  of the cross-section of the filter media  1310  and are, in one or more embodiments, the points that are furthest apart from each other along any straight line extending across the inner perimeter  1318  of the cross-section. 
     The inner perimeter  1318  of the cross-section of the filter media  1310  as described herein also has a maximum width  1343  (Wmax) measured between a first point  1341  and a second point  1342  on the inner perimeter  1318 . The first point  1341  and the second point  1342  are located on an axis of maximum width  1340  that is located along a straight line perpendicular to the axis of maximum height  1330 . The axis of maximum width  1340  intersects the axis of maximum height  1330  at a bottom axis intersection point  1344  where the first point  1341  and the second point  1342  at which the axis of maximum width  1340  intersects the inner perimeter  1318  are located furthest apart from each other on any straight line perpendicular to the axis of maximum height  1330  between top point  1331  and bottom point  1332 . 
     Because of the ovate or ovoid shape of the cross-section of the filter media  1310 , the bottom axis intersection point  1344  does not, in one or more embodiments, bisect the maximum height of the cross-section as measured between the top point  1331  and the bottom point  1332  along the axis of maximum height  1330 . 
     In one or more embodiments of the tubular filter media in filter elements described herein, the cross-section, as depicted in, e.g.,  FIG. 22 , may define a bottom section height  1334  (Hwmax) measured along the axis of maximum height  1330  from the bottom point  1332  to the bottom axis intersection point  1344 . In one or more embodiments, the bottom section height  1334  may be less than or equal to 0.4 of the maximum height as measured along the axis of maximum height  1330  from the top point  1331  to the bottom point  1332 . In one or more embodiments, the bottom section height  1334  is greater than zero. In one or more embodiments, the bottom section height  1334  is greater than or equal to 0.1 of the maximum height as measured along the axis of maximum height  1330  from the top point  1331  to the bottom point  1332 . 
     Another manner in which the cross-sections of tubular filter media of filter elements as described herein may be characterized is in terms of the length of the inner perimeter at both the top and bottom of the filter media of the filter element. For example, the inner perimeter  1318  of the cross-section of filter media  1310  as described herein may define a bottom perimeter section containing the bottom point  1332  and extending from the first point  1341  to the second point  1342  at which the axis of maximum width  1340  intersects the inner perimeter  1318 . The bottom perimeter section, i.e., the portion of the inner perimeter  1318  from the first point  1341  to the second point  1342  (and including bottom point  1332 ) has a bottom perimeter section length measured along the inner perimeter  1318  from the first point  1341  to the second point  1342 . 
     The inner perimeter  1318  of the cross-section of filter media  1310  of one or more embodiments of filter elements as described herein may also define a top perimeter section containing the top point  1331  at which the axis of maximum height  1330  intersects the inner perimeter  1318 . The top perimeter section extends from a first end  1346  to a second end  1347  on the inner perimeter  1318 , the first end  1346  being located on the inner perimeter  1318  between the first point  1341  and the top point  1331  and the second end  1347  being located on the inner perimeter  1318  between the second point  1342  and the top point  1331 . The first end  1346  and the second end  1347  of the top perimeter section are the points at which a top perimeter section line  1345  intersects the inner perimeter  1318  on opposite sides of the axis of maximum height  1330 . The top perimeter section line  1345  is a straight line that is perpendicular to the axis of maximum height  1330  and intersects the axis of maximum height  1330  at a top axis intersection point  1349 . The top axis intersection point  1349  is located within the inner perimeter  1318  between first point  1331  and second point  1332  at which the axis of maximum height  1330  intersects the inner perimeter  1318 . The top axis intersection point  1349  defines a top section height  1335  measured along the axis of maximum height  1330  from the top axis intersection point  1349  to the top point  1331  on the inner perimeter  1318 . 
     In one or more embodiments, the top section height  1335 , e.g., the distance from the top axis intersection point  1349  to the top point  1331  in the illustrative embodiment depicted in  FIG. 23 , is equal to the bottom section height  1334  in the cross-sections of filter media in filter elements as described herein. In one or more embodiments, the bottom perimeter section length as measured along the inner perimeter  1318  between points  1341  and  1342  (and including bottom point  1332 ) is greater than the top perimeter section length as measured along the inner perimeter  1318  between first end  1346  and second end  1347  (and including top point  1331 ). In one or more embodiments, the bottom perimeter section length may be 1.2 or more times greater than the top perimeter section length. In one or more alternative embodiments, the bottom perimeter section length may be two or more times greater than the top perimeter section length. This relationship between the bottom perimeter section length and the top perimeter section length may be one way of describing that more of the filter media  1310  faces downward than upward. 
     In one or more embodiments of the filter media  1310  in filter elements as described herein, the bottom perimeter section of the inner perimeter  1318  located between the first point  1341  and the second point  1342  may be continuously curved from the first point  1341  to the second point  1342 . In one or more alternative embodiments, the bottom perimeter section of the inner perimeter  1318  may include one or more limited portions that form a straight line, however, no portion of the bottom perimeter section of the inner perimeter  1318  lies on a straight line for a distance of more than 1 centimeter. 
     The filter media  1310  is another illustrative example of a cross-section having an inner perimeter  1318  that has a line of symmetry. In particular, the inner perimeter  1318  defines, at most, only a single line of symmetry. In the illustrative embodiment depicted in  FIG. 22 , the single line of symmetry is coincident with the axis of maximum height  1330 . Such a relationship may not, however, necessarily be required in all embodiments described herein. 
     Although only three different ovate cross-section shapes for filter media  1110 ,  1210 , and  1310  in filter elements as described herein are discussed in connection with  FIGS. 18 to 23 , the descriptions of the various characteristics of those ovate cross-sectional shapes can be applied to an infinite number of different ovate shapes that may be used to form filter media used in tubular filter elements in air filter systems as described herein. Accordingly, the specific embodiments disclosed herein should be considered to be illustrative in nature only. 
     Referring to  FIGS. 18 and 24 , in one or more embodiments of the filter elements described herein, the end caps  1120  on the filter elements may include an alignment mechanism in the form of, e.g., optional tabs  1124  in which notches  1126  are located. The notches  1126  may be sized to receive upper and lower members  1152  and  1154  of a yoke  1150  over which the filter element may be mounted in one or more embodiments of an air filter system as described herein. Each of the notches  1126  may be described as having, in one or more embodiments, an opening that faces the interior volume of the filter elements, with the notch  1126  extending towards the inner perimeter  1128  of the end cap  1120 . Although each notch  1126  is formed in a single tab  1124  in the depicted embodiment, in one or more alternative embodiments, a notch  1126  may be formed between two members that protrude from the inner perimeter  1128  of the end cap  1120  where the two members forming the notch  26  are not the same structural member. 
     One illustrative embodiment of a yoke  1150  having upper and lower support beams  1152  and  1154  is depicted in  FIG. 24 . The upper and lower support beams  1152  and  1154  are structurally connected to each other by strut  1156  which, in the illustrative embodiment of  FIG. 24 , maybe a continuous member that provides structural support to the upper and lower support beams  1152  and  1154  and increases the rigidity of the yoke  1150 . Also depicted in  FIG. 24  is a venturi/pulse collector  1160  that may be used to move gas into and out of the interior volume of a filter element located on yoke  1150 . The venturi/pulse collector  1160  may have a filter end  1162  against which the end cap (e.g., end cap  1120 ) of a filter element may be positioned and a tube sheet end  1164  configured to be attached over an aperture in a tube sheet of an air filter system as described herein. 
     The yoke  1150  is depicted as being partially inserted into the filter element in  FIG. 18 . Although depicted only on the nearest end cap  1120  in  FIG. 18 , in one or more embodiments, the end caps  1120  on both ends of the filter element of  FIG. 18  may include tabs  1124  having notches  1126  formed therein. The use of two tabs  1124  in combination with a yoke  1150  having two support beams  1152  and  1154  may be, in one or more embodiments, be beneficial to prevent, or at least limit, rotation of a filter element about its tube axis  1111  when installed on the yoke  1150  in an air filter system as described herein. 
     Although one or more embodiments of the tubular filter media provided in the filter elements described herein may be in the form of pleated filter media, in one or more alternative embodiments, the tubular filter media may or may not be pleated. Further, although the filter media used in the filter elements described herein may be used to filter particulate matter from a gas/air stream, in one or more embodiments, the filter media may be further capable of removing other materials from a gas/air stream such as, e.g., chemical contaminants, etc. 
     Pulse Generators with Diverging Pulse Guides 
     In one or more embodiments, the pulse generators used in connection with the air filter systems described herein may have a diverging pulse guide having a shape as described in, e.g., U.S. Provisional Patent Application No. 61/772,198, titled DIVERGING NOZZLES AND FILTER ELEMENT CLEANING SYSTEMS USING DIVERGING NOZZLES. 
     Examples of some illustrative embodiments of diverging pulse guides that may be used in connection with the pulse generators in air filter systems as described herein are depicted and described in connection with  FIGS. 25-29 . The diverging pulse guide  1490  is attached to a pulse generator  1450  which is, in turn, attached to a manifold  1458  that supplies pressurized air to the pulse generator  1450  in an air filter system as described herein. 
     In particular, the diverging pulse guide  1490  is depicted as being connected to the pulse generator  1450  with a collar  1456 , although many other different connection mechanisms could be used to attach to attach a diverging pulse guide  1490  to a pulse generator  1450 . Although the diverging pulse guide  1490  is depicted as being connected directly to the pulse generator  1450 , in one or more alternative embodiments, the diverging pulse guide  1490  may be connected to the pulse generator  1450  through one or more intermediate conduits as needed. Even though one or more intermediate conduits may be provided between the pulse generator  1450  and the diverging pulse guide  1490 , the diverging pulse guide  1490  is still, for the purposes of the systems and methods described herein, still connected to the pulse generator  1450  because pressurized gas delivered by the pulse generator  1450  will eventually pass through the diverging pulse guide  1490 . 
     Referring now to  FIGS. 26-28 , the illustrative embodiment of diverging pulse guide  1490  is treated in more detail. The diverging pulse guide  1490  includes a connector end  1492  and an open end  1493  with a longitudinal axis  1491  extending between the connector end  1492  and the open end  1493 . A tubular wall  1494  extends between the connector end  1492  and the open end  1493  and defines an interior channel  1495  through which pressurized gas is delivered from the pulse generator  1450 . In particular, pressurized gas enters the interior channel  1495  of the diverging pulse guide  1490  through the connector end  1492  and exits the interior channel  1495  through the open end  1493 . 
     In one or more embodiments, the interior channel  1495  may have a circular cross-section taken transverse to the longitudinal axis  1491  (in which case, the channel width may be defined as the diameter of the channel  1495 ). In one or more embodiments, the longitudinal axis  1491  may be the same as (i.e., collinear) with the pulse axes described in connection with the air filter systems described herein. Although, in the depicted embodiments, the interior channel  1495  has a circular cross-section, variations in a circular cross-section may be allowable in diverging pulse guides as described herein. For example, in one or more embodiments, the cross-section of the interior channel  1495  may be in a shape that approximates a circle, such as, e.g., a hexagon, octagon, etc. 
     The interior channel  1495  of the diverging pulse guide  1490  defines a channel length LT that extends from the connector end  1492  to the open end  1493  of the diverging pulse guide  1490 . The diverging pulse guide  1490  also includes a channel width that is defined by opposing interior surfaces  1496  of the tubular wall  1494  (it being understood that the opposing interior surfaces  1496  may, in one or more embodiments, be parts of the same surface that are simply located on opposite sides of the channel  1495  relative to the longitudinal axis  1491 ). In general, the channel length LT can be described as extending along the longitudinal axis  1491  while the channel width can be described as extending transverse to the longitudinal axis  1491 . 
     In one or more embodiments, the interior channel can be provided in parts. In particular the interior channel  1495  may include a first section  1497  proximate the connector end  1492  and a second section  1498  proximate the open end  1493  of the diverging pulse guide. As a result, the first section  1497  is located between the second section  1498  and the connector end  1492  while the second section  1498  is located between the first section  1497  and the open end  1493  of the diverging pulse guide  1490 . In one or more embodiments, the first section  1497  may begin at the connector end  1492 , while in one or more alternative embodiments, the first section  1497  may begin at some location between the connector end  1492  and the second section  1498 . 
     In one or more embodiments, the channel width of the interior channel  1495  of the diverging pulse guide  1490  may begin increasing while the opposing interior surfaces begin diverging at or near the connector end  1492 . In the embodiment depicted in, e.g.,  FIG. 28 , the opposing interior surfaces  1496  of the diverging pulse guide  1490  in the first section  1497  diverge at an angle α (alpha). In one or more embodiments, the angle α (alpha) at which the opposing interior surfaces  1496  of the first section  1497  diverge is greater than zero. In one or more embodiments, the angle α (alpha) may be 3° or less. 
     In one or more embodiments, the channel width of the interior channel  1495  of the diverging pulse guide  1490  continues increasing as the opposing interior surfaces  1496  continue diverging in the second section  1498 . In the embodiment depicted in  FIG. 28 , the divergence of the opposing interior surfaces  1496  of the interior channel  1495  within second section  1498  is represented by angle β (beta) as seen in  FIG. 28 . In one or more embodiments of the diverging pulse guides as described herein, the opposing interior surfaces  1496  in the second section  1498  of the interior channel  1495  of the diverging pulse guide  1490  may diverge at an angle β (beta) that is greater than the angle α (alpha) at which the first section  1497  diverges. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 1.5 or more times as large as the angle α (alpha) at which the opposing interior surfaces  1496  in the first section  1497  diverge. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 3° or more. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 4° or more. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 5° or more. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 9° or less. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 8° or less. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 7° or less. In one or more embodiments, the angle β (beta) at which the opposing interior surfaces  1496  in the second section  1498  diverge may be 6°. 
     Although the illustrative embodiment of diverging pulse guide  1490  depicted in  FIGS. 26-28  includes a first section  1497  and a second section  1498  in which the opposing interior surfaces  1496  diverge at different angles, in one or more alternative embodiments, the diverging pulse guide  1490  may include an interior channel  1495  with opposing interior surfaces  1496  that diverge at the same angle along its entire length or that includes a first section  1497  in which opposing interior surfaces  1496  defining the channel width do not diverge (i.e., the opposing interior surfaces  1496  of the interior channel  1495  are substantially parallel to each other such that the interior channel width is constant along the length of the first section  1497  (with allowances for manufacturing tolerances). 
     In either embodiment, i.e., whether the opposing interior surfaces  1496  of the interior channel  1495  diverge at one angle along the entire length of the interior channel  1495  (in which case the second section  1498  may be described as having a length L 2  equal to the total length LT of the diverging pulse guide  1490 ), or the opposing interior surfaces  1496  of the interior channel  1495  diverge only within a second section  1498  that occupies less than the total length LT of the diverging pulse guide  1490  (while the opposing interior surfaces  1496  in a first section  1497  do not diverge), the opposing interior surfaces  1496  defining the channel width of the channel  1495  may diverge in the diverging section from the longitudinal axis  1491  at an angle that is, at a lower end, 2° or more (where the diverging section occupies all or less than the entire length LT of the channel  1495 ). At the upper end of both of the two embodiments, the divergence of the opposing interior surfaces  1496  of the diverging section of the diverging pulse guide may be at an angle of 7° or less. At the upper end of both of the two embodiments, the divergence of the opposing interior surfaces  1496  of the diverging section of the diverging pulse guide may, in one or more alternative embodiments, be at an angle of 3° or more. In still other embodiments, the divergence of the opposing interior surfaces  1496  of the diverging section of the diverging pulse guide of both embodiments may be at an angle of 4° or more. In still other embodiments, the divergence of the opposing interior surfaces  1496  of the diverging section of the diverging pulse guide of both embodiments may be at an angle of 5° or more. In both of the two embodiments, the divergence of the opposing interior surfaces  1496  of the diverging section of the diverging pulse guide may be at an angle of 6°. 
     In one or more embodiments of the diverging pulse guides as described herein, the length L 2  of the diverging second section  1498  of the interior channel  1495  (see  FIG. 28 ) as measured along the longitudinal axis  1491  may be related to the channel width of the interior channel  1495  at the connector end  1492  of the diverging pulse guide  1490  (where the diameter of the interior channel  1495  at the connector end  1492  is measured transverse to the longitudinal axis  1491 ). The relationship between the channel width of the interior channel  1495  and the length L 2  of the diverging second section  1498  holds for those embodiments in which the diverging second section  1498  occupies all or less than the total length LT of the channel  1495 . For example, in one or more embodiments, the length L 2  of the diverging second section  1498  of the interior channel  1495  may be at least as long as the channel width of the interior channel  1495  at the connector end  1492 . In one or more other embodiments, the length L 2  of the diverging second section  1498  of the interior channel  1495  may be two or more times the channel width of the interior channel  1495  at the connector end  1492 . In still other embodiments, the length L 2  of the diverging second section  1498  of the interior channel  1495  may be three or more times the channel width of the interior channel  1495  at the connector end  1492 . In yet other embodiments, the length L 2  of the diverging second section  1498  of the interior channel  1495  may be four or more times the channel width of the interior channel  1495  at the connector end  1492 . 
     Another feature that may be provided in one or more embodiments of the diverging pulse guides as described herein are threads  1499  located at the connector end  1492  of the diverging pulse guide  1490  and extend from the connector end  1492  towards the open end  1493 . The threads  1499  may be used to connect the diverging pulse guide  1490  to a pulse generator or to an intermediate conduit interposed between the diverging pulse guide  1490  and a pulse generator. Although the diverging pulse guide  1490  includes threads to assist in connecting the diverging pulse guide  1490  to a pulse generator, many other fluid connection structures may be used to connect the diverging pulse guide  1490  to a pulse generator, such as, e.g., quick connect couplings, etc. In the depicted embodiment, the threads  1499  are located on the exterior surface of the diverging pulse guide  1490 . In one or more alternative embodiments, however, the threads or other connection mechanism may be located on the interior surface  1496  of the diverging pulse guide  1490 . 
     Referring to  FIG. 29 , one illustrative embodiment of a connection of a diverging pulse guide  1490  to a delivery tube  1452  of a pulse generator  1450  is depicted. The diverging pulse guide  1490  includes threads  1499  on its exterior surface at the connector end  1492  of the diverging pulse guide  1490 . Those threads  1499  mate with a set of internal threads  1459  on the interior surface of the delivery tube of the pulse generator  1450  to retain the diverging pulse guide  1490  in fluid communication with the pulse generator  1450 . 
     In one or more embodiments, the interior diameter of the delivery tube  1452  of the pulse generator  1450  as defined by the interior wall  1453  may be substantially equal to the interior diameter of the diverging pulse guide  1490  at the connector end  1492  such that gas flowing through the pulse generator  1450  into the diverging pulse guide  1490  sees little or no discontinuity when moving from the delivery tube  1452  of the pulse generator  1450  to the diverging pulse guide  1490 . Such a smooth transition between the pulse generator  1450  and the diverging pulse guide  1490  may be useful in limiting pressure losses, reducing noise, etc. 
     For those embodiments of the diverging pulse guides described herein in which the opposing interior surfaces  1496  diverge beginning at the free end  1492  of the pulse guide  1490  and in which the opposing interior walls  1453  of the pulse generator  150  (e.g., the delivery tube  1452 ) to which the pulse guide  1490  is attached are either parallel to each other or converging as described herein, the pulse generator  1450  can be described as having a pulse outlet  1454  that is located essentially at the connector end  1492  of the pulse guide  1490 . 
     For those embodiments of the diverging pulse guides described herein that include a first section  1497  in which the opposing interior surfaces  1496  do not diverge, the pulse guide  1490  may effectively move the pulse outlet  1454  (see, e.g.,  FIG. 29 ) to a location between the connector end  1492  and the open end  1493  of the pulse guide  1490  at which the opposing interior surfaces/walls  1496  of the pulse guide  1490  do begin to diverge. 
     The complete disclosure of the patents, patent documents, and publications identified herein are incorporated by reference in their entirety as if each were individually incorporated. To the extent there is a conflict or discrepancy between this document and the disclosure in any such incorporated document, this document will control. 
     Illustrative embodiments of the air filter systems and components thereof, as well as methods of using the same, are discussed herein some possible variations have been described. These and other variations and modifications in the invention will be apparent to those skilled in the art without departing from the scope of the invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein. Accordingly, the invention is to be limited only by the claims provided below and equivalents thereof. It should also be understood that this invention also may be suitably practiced in the absence of any element not specifically disclosed as necessary herein.