WO2007054952A2

WO2007054952A2 - Pleated filter with isokinetic spacers

Info

Publication number: WO2007054952A2
Application number: PCT/IN2005/000432
Authority: WO
Inventors: Hamish Chandru Shahani
Original assignee: Hamish Chandru Shahani
Priority date: 2005-11-09
Filing date: 2005-12-26
Publication date: 2007-05-18
Also published as: WO2007054952A3

Abstract

A pleated filter media suitable for intercepting and immobilizing submicronic airborne particulates is disclosed with distinguishing features that minimize pressure loss due to inserts and spacers necessary to hold the pleats in position without distortions interpleat collapse. The plurality of pleats in a filter media are spaced uniformly and separated by cylindrical ceramic or polytetrafluroethylene tube sections that are hollow and through which air can pass isokinetically and isobarically without the formation of eddies within the interpleat pathways. State-of-the-art spacers add to filter pressure loss and also act as collection sites for particulates that egress the media downstream of the pleated matrix thus vitiating the very purpose of the filter.

Description

FIELD OF THE INVENTION

The present invention relates to fibrous filters that are widely exploited to capture and immobilize airborne submicronfc particulates. These filters are usually constructed for use in clean-air devices within the scope of US Military Standards MIL-F-51068 : Filters, Particulate, High Efficiency, Fire Resistant and MIL-F-51477 :

Military specification for Filters, Particulate, High Efficiency, Fire Resistant, Biological Use, General Specification for HEPA Filters. Their derivative standards are IES-RP-CC001.3 HEPA and ULPA Filters and International Standard ISO 14644 -1 Cleanrooms and associated controlled environments.

Apart from their cost, the performance of these filters is characterized by three parameters : a) Collection Efficiency b) Pressure Drop or Flow Resistance ; and, c) Filter Life .

Filter life is finite because the resistance to flow eventually increases to an unacceptable level as the filters become clogged with deposited particles. For reasons of economy and in keeping with their efforts to optimize the utilization of media surface area, prior-art inventors have developed different methods and materials for maintaining the spacing of the filter pleats. That's because the performance of the filter is enhanced or diminished by the ability of the contaminated air to pass freely and completely through the extended media.

When obstruction-free airflow paths are maintained between the pleats, the filter normally operates at its optimum. If, on the other hand, these streamlines through the filter are in some way impeded or reduced, for instance when pleats bunch together, the filtering capacity of the matrix is diminishes. That's why maintaining the designed spacing between successive pleats in a filter element is extremely important for the overall performance of the filter.

The microfibreglass flat-sheet media is conventionally rated at a pressure drop of 25 mm d'eau for a traverse velocity of 0.05 meters per second. Industry practice is to rate brand-new filters at a starting differential pressure of 25 mm d'eau w^'rth its life-cycle ending when the pressure drop becomes double or in excess of 50 mm d'eau. At that pressure, particularly when the pleat density is high, say, about 30 to 35 pleats per 100 mm, the filter is prone to pleat-bunching from interpleat static pressure. Several methods for achieving pre-determined pleat-spacing have been devised and are extensively known in the art. These include threads coated with adhesrves or ribbons of glass filter media or hot-melt adhesive extrusions or embossments that are hot-pressed into the media and serve to align, space and support the pleats in the filter matrix. These passive prior-art separators restrict airflow by roughly 10% over the unimpeded insert-free fiat-sheet media. That correspondingly adds to the cost of the filter. In other words, to achieve rated volumetric flow requirements for uniform exit air velocities, such as, say, 0.4 to 0.6 meters per second, the interpleat flow volume needs to be as much as 10% higher. This aggravates air- stream turbulence at each point the spacer intervenes the air paths, and raises the cumulative pressure drop across the filter. These pleat-separating materials also suffer from the insufficiency that they cannot function at air temperatures above 200 deg Celsius.

Aiming to overcome those deficiencies, it is the essence of the present invention to provide a unique air filter pack for use in air supply and exhaust filtration systems that is more efficacious and effectual than those using spacers made from the presently prevalent ribbon, hot-melt, embossment, and other diverse means that provide interpleat integrity to filters used for clean and particulate-controlled spaces and environments, and for depyrogenation of medical containers at above 350 deg Celsius at which temperatures the air weighs about 0.56 kilograms per cubic meter in relation to the sea-level air at 20 deg Celsius which has a density of 1.28 kilograms per cubic meter.

DESCRiPTION OF PRIORART

US Pat 5,071,555 discloses one form of pleated filter pack having continuous ribbons of hot-melt adhesive between layers of filtering media and also teaches methods of manufacturing pleated filter elements wherein adhesive between layers of the filter- media is extruded in a particular pattern to provide increased stiffness in the machine direction of the pleated filter-media.

US Pat 6,824,581 B1 proposes the pleat spacers as embossments, both continuous and intermittent pot-pressed into a non-standard fibrous media impregnated with thermosetting compounds.

US Pat 6,709,480 B2 propounds the insertion of sandwiched reinforcing strips to maintain pleat spacing and rigidity.

Under US Pat 6,165,241 Kyiing^Ju discloses inserts of wedge-shaped pliable strips held by adhesives on the pleat face to maintain interpleat distance and geometric integrity. The inventor achieves the same purpose in a sequelae US Pat 6,165,242 which proposes placement of daubs of hot-melt adhesive set into pre-scored cavities for holding the pleats apart.

Another fitter matrix that is very much exploited commercially has ribbons of fibrous media to maintain interpleat distance for uniform airflow pathways.

The patents exemplified above as also their research data and literature sources are incorporated in this patent by reference.

UMTATIONS OF PRIOR ART

All of the patents cited herein and in the referential bibliography [56] have in their own way and manner provided methods and means to sub-serve their objects with incomparable grasp of detail and function. Each of their disclosures and those in the referral list has singular distinguishing attributes, but none has addressed the implanted increase in pressure drop caused by the addition of passive inserts to hold the pleats apart Neither have they considered the impact of MAXWELL slip flow for boundary conditions for laminar viscous flow in-between the pleats. Actually that is finite and determined by diffusion at the media surface and air-streams around the eddies caused by passive inserts which is the major drawback of the state-of-the-art.

Neither have those patentees addressed the need for hot-air applications say for endotoxin inacth/ation, spore annihilation and depyrogenafon of medical glassware at about 320 deg Celsius.

More particularly, in the case of the popular ribbon-type filter elements, the inadequacies have been : a) The continuous ribbon separators, slit from fibrous fitter-media are prone to sporadic shedding of minute fibres from the sliced edges under normal usage conditions. Such shedding becomes a major defect where extremely tow particle counts are obligatory for ultra-clean processes; such as for semi-conductor and micro-electronics production ;

^■ b) Ribbon separators inserted for the purpose of spacing the pleats uniformly, block airflow upto 15% of the useful media surface and in effect reduces the effective utilization of the filter media ; c) Ribbons slit from microfiberglass filter-media, are prone to absorb moisture by capillary-action, through both edges of the ribbon. That moisture promotes colonization and proliferation of viable microflora trapped in the intra-crevical space between the ribbon and the filter-media during the life-cycle of the filter and vitiates the very purpose of using such filters for bioclean applications ; d) At differential pressure in excess of 50 mm d'eau, turbulence makes the nonadherent ribbons flutter which increases the shedding of both the microbiologically inactive and viable particles trapped between the non-adherent ribbon and filter- media.

SUMMARY OF THE OBJECTS OF THE INVENTION

The main object is to provide separators that are hollow kinetic inserts which allow internal in-line and unklirectional passage of air with minimal interpleat pressure loss due to 1) InerrJal Bernoulli How, 2) Viscous Laminar Poiseuille - JHagen _Fl.ow,_.χ - 4≥!ϋ!!SI*_L and 3) Maxwell SGp Row.

Another object is to prevent interpleat collapse attributable to passive or non- kinetic inserts that offer resistance to airflow and cause turbulence and eddies in- between the pleats.. A further object is to eliminate dead or high flow-resistant spots within the filter matrix on which particulates can agglomerate and act as nutrient sites for microbial proliferation and increase the probability of bio-entities growing through and past the media to downstream regions.

Yet another object is to prevent the sporadic dislodgεment of particulates that penetrate and collect on the passive inserts or embossments downstream of the fitter.

Still another object is to provide isokinetic spacers for filters that will sustain interpleat geometric integrity, despite the total binder-loss from the media due to temperatures in excess of 300 deg Celsius.

BRIEF DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from the following description when considered in conjunction with the accompanying drawings, in which :

Fig. 1 is cut-away segment showing the placement of the isokinetic or hollow spacers along the airflow pathways. Fig. 1a is a fragment of the Flg.1 segment clarifying the particulate disposition mechanisms relating to the isokinetic spacers. - Fig. 2 is a swath of a composite filtration media made-up of a primary microporous PTFE filtration layer sandwiched between woven polymer support scrims. Fig. 3 ^■ depicts a portion of the pleated filter matrix separated by ribbons that are cut from the fibrous filter media. Fig. 4 is a perspective view of a segment of the pleated fitter according to the prior art with a continuous ribbon of hot-melt adhesive. Fig. 5 is a partial cut-away view of a filter and frame where the pleats are spaced and held in position by embossments pressed into the media in either a continuous or intermittent fashion.

Fig. 6 is a partial segment of another embodiment of the filter matrix according to prior art.

Rg.7 shows passive wedge-shaped inserts glued to the filter media.

Fig.7a is another embodiment of Fig.7 with solid inserts that are cylindrical in shape.

Fig.8 shows daubs of hot-melt adhesive placed in pre-scored cavities pressed in the filter media.

Fig. 9 graphically gives a typical pressure drop performance curve for a filter matrix. C1 is for the media without spacers. C2 is for isokinetic spacers of this patent and C3 is for passive inserts in accordance with prior-art. The pressure drop threshold line A1 running parallel to the abscissa, indicates the datum differential pressure of 25 mm d'eau made possible by an Intervening pressure limiting orifice plate. DETAILED DESCRIPTICWi OF THE INVENTION

The attainment of the above objects is achieved through the physical embodiments depicted by the accompanying drawings.

To conveniently review the drawings, this explanatory text of the patent denotes each key component by an individual drawing figure identified by a single-digit numerical. The relevant features are denoted by a double-digit numerical, which carries the figure number as a prefix. . .

Fig. 1 depicts a cut-away section of the panel filter made according to this patent, wherein the media 11 is spaced by hollow isokinetic spacers 12 affixed to the pleat 13 to font) the matrix which is potted into a holding frame 14 onto which an elastomeric gasket 15 is fixed. Contaminated air 16 enters the matrix and emerges downstream as clean-air 17. This insert design for air velocities in the laminar regime serves to minimize the pressure drop across the filter when in use.

The paniculate removal efficiency of a given filter medium is dependent on several variables including particle size, flow velocity, and filter media thickness. For any given filter construction and flow conditions, there is a specific most-penetrating particle size at which the retention efficiency is at a minimum. The percentage penetration will be lower for particles which are larger or smaller than the most-penetrating particle size.

The effect of air-stream velocity on particle removal and pressure drop depends on the filtration mechanism. Filter pressure drop performance can be assessed and predicted in terms of different types of flow relating to the filter media or to the construction of the filter. These are :

1. Inertia! Flow or Bernoulli Flow : This flow is characterized by a simple exchange of potential energy to kinetic energy when the interpleat air velocity is increased as in the case with prior-art separators. 2. Viscous Laminar Flow or Streamline or Poiseulle - Hagen Row : This flow is characterized by a parabolic velocity profile over the cross section perpendicular to the flow direction. The forces required to produce flow are determined by the shear stress in the air as it traverses the pleats and varies during the life-cycle of the filter.

3. Slip Flow or Maxwell Slip Flow : This flow is in addition to the laminar flow and is caused by slip at the pleat path wall and around the spacers. A natural assumption for boundary conditions for laminar viscous flow is a velocity of zero at the pleat path wall. Actually the velocity at the wall is not zero but is finite and determined by diffusion at the wall. When the inserts or spacers intervene the flow path perpendicular to the mean free path of the air molecules, this diffusional slip velocity is small, as it is with the isokinetic spacers of this invention, in relation to the velocity at the centre of the flow channel and it can be ignored. With passive or dead obstructions the magnitude of slip flow affecting pressure loss is to be empirically assessed.

4. Turbulent Flow : This will not become a factor until the Reynolds number is greater than about 2000 and does not apply to the filters covered by this patent.

An increase in velocity will improve the capture of particles by inertia! impaction because the inerb^'al effects which deviate particles from the airflow streamlines are increased. The effect is to extend the regime of this mechanism to smaller particle sizes.

For particulates below 1.0 micron diffusional interception is the predominant capture mechanism, and an increase in velocity will decrease removal efficiency. As the residence time in the filter media is decreased, the probability of interception due to random particle motion is decreased. The net effect of increasing flow velocity through a filter media will be a decrease in the minimum efficiency and a decrease in the most- penetrating particle size which is a direct consequence of prior-art pleat-spacing means.

Fig. 1a clarifies the way the hollow isokinetic spacer 12 is attached to the media by adhesive 18 that has minimal bonding-contact radially while the axial interface is only a line-seam which minimally impedes the air-streams as they can enter the hollow inserts of this patent without being subjected to slip flow boundary conditions that would stem from a solid insert which would in addition cause eddies and vortices along its axis. One each isokinetic spacer per pleat of depth L2 which usually is 45 to 55 millimeters, is positioned and oriented along the vertical plane at distance L1 which is about 25 millimeters on the incident and the exit side of the media. The pleat fold line F is the crest that ensures that the opposed spacers coincide and abut, thereby bracing the media and providing adequate interpleat rigidity to prevent their collapse at dynamic air-streams differential pressure above 50 mm d'eau.

The pliable adhesive 18 is a three-component epoxy resin for the PTFE media with polymeric tubular inserts 12. For high temperature air filtration the hollow spacers made from heat-resistant material such as ceramic are bonded to the media with commercially available heat-resistant dimethyl potysitoxane which is custom-blended with fillers such as colloidal ceramic and fumed silica for compensating the tensile strength lost when the media binders are bumt-out at about 300 deg Celsius. The colloidal ceramic amalgamates with the microfibreglass web by coalescing within the binder-tree glass strands which are approximately 19 microns in diameter and readily absorb the cement-like bonding material that cures by gradual drying. This expedient is indispensable as the isokinetic inserts are the only means of structural support for the multipleat filter matrix.

The enlarged view of the separator shows the other particulate disposition mechanisms by the media from the air streamlines 121 in relation to the hollow inserts. Those are inertial impaction 122 , interception 123 and diffusion 124. With solid inserts the interception would lead to progressive accretion of particles that would sporadically and randomly dislodge from the downstream pleats and compromise the air quality of the filter.

These isokinetic inserts are about 2 millimeters in diameter and roughly 18 millimeters length for room temperature applications and 25 millimeter in length for plus 300 deg Celsius use. One such hollow insert per pleat is placed midways along the depth, which is between 45 to 50 millimeters for the filters functioning at room temperatures and 75 millimeters pleat depth for hot-air sterilization systems. The pleat fold crest is also indicated by the same solid line F. When the accordion style folds are compressed to form a matrix their density is usually nearabout 30 pleats per 100 millimeters. These novel and unique separators provide smooth, rounded surfaces in engagement with the media. They permit assembly of the filter by allowing the pleats to be formed around the spacers by positioning them against the media during formation of the pleats. Besides that advantage, the airflow between the pleats is shared by the hollow inserts which prolongs the life of the filter while the knife-edge contact increases the area of media exposed to airflow, thus enhancing capacity and efficiency. In effect, this minimizes the blockage of the filter media by providing only one adhesive line contact with the media, midways along the pleat fold area whereby eddies and vortices at the edges of the isokinetic spacers do not arise.. Fibrous filter media which can withstand air temperatures in excess of 300 deg

Celsius, generally has a depth of 300 to 400 microns for maximal retention capacity. During its life-cycle almost all airborne particulates are trapped within the incident surface upto no more than 200 microns depth, beyond which the pressure drop becomes energy-prohibitive and uneconomic.

Rg. 2 is a swath of an ULPA media 21 made-up of microporous PTFE membrane 22 sandwiched between an upstream polypropylene scrim 23 and a finer mesh downstream scrim 24 which can also be in polypropylene. This media yields a penetration of .0001% for 0.1 micron particulates with pressure drop of 25 mm d'eau at traverse air velocities through the media ranging from 2 to 3 meters per minute.

Fig.3 is a segment of a pleated filter where the zig-zag folds 33 are spaced by ribbons 31 that are about 3 mm wide and placed about 25 millimeters apart along the running length of the flat sheet media 32. These filters can be used for high temperature applications which none of the state-of-the-art inventions as disclosed herein have addressed.

The density of air at 350 deg. Celsius is about 0.56 kilograms per cubic meter about half the weight of standard air at sea-level. Thermodynamically, for heating up glass containers to inactivate endotoxin, annihilate spores and depyrogenate them for medical use it is axiomatic to increase air velocities to more than 1.2 metres per second to obtain adequate enthalpy for heat transfer.

Fig.4 shows a slice of an air filter comprising a sheet of filter media 42 folded in zig-zag configuration. Before it is folded deposits of a polymerizing substance 41 , for instance some thermosetting glue, synthetic resin or other suitable bonding material is applied in such a pattern that they act as spacer elements creating a wider space at the outlet part of the wedges than the one adjacent to the apex of the wedges. In its simplest form this can be achieved by applying a double layer of glue in an intermittent way on both sides of the pleat 43 at the wider end; and, on one side close to the inner end a single glue layer: Of course, other ways to apply glue or other deposits can be used. A thermosetting adhesive is preferably used since it will hold the folds of the filter together during fabrication and increase the strength of the filter when it is used or transported.

The adhesive can be kept in an uncured state until the folding has been done or it could be allowed to harden and then re-melted so that it gets tacky on the surface.

Fig. 5 is also a partial cut-away portion of another style of a panel filter element 53 having a pleated filter media with a plurality of embossments. The first set of embossments project from the clean side of the pleats and second set of embossments project from the dirty side of the pleats. These arrays of spaced embossments 51 and continuous embossments 52 are hot pressed directly into the media. That is made possible with media that is pre-impregnated with thermosetting binders during the media manufacturing process.

Fig. 6 is yet another fragment of a self-supporting pleated filter. Its construction relies on forming In the usual manner, a plurality of pleats 62 from the filter media that extends along a front face and a rear face. The pleats include a plurality of pleat tips and sloping sides supported by a reinforcing member 61 engaged in the direction of pleating and bonded to the rear face of the filter media prior to the step of forming the pleats. The reinforcing member is positioned along at least one of the sloping side surfaces of the pleats on the rear face in a direction perpendicular to the direction of pleating.

Fig. 7 shows a cut-away pleat swath on which wedge-like pliable strips 71 are stuck on at least one face of the media 72 so as to separate the pleats for passage of air.

Fig.7a is another embodiment of Fig. 7 with passive 73 inserts that are cylindrical in shape.

Fig. 8 shows a cut-away pleat proportion on which hot-melt daubs 81 are placed on at least one face of the media 82 so as to separate the pleats for passage of air.

Rg. 9 depicts pressure drop performance curves for the media C1 without spacers, C2 with isokinetic spacers and C3 with passive inserts. Cl is hypothetical and extrapolated from the results based on measurement made on the filter media as a flat- sheet. Thafs why if s a straight One. All three curves show differential pressure plotted against the flow volume through the pleated 610 W x 610 H x 80 D panel filters, the numericals being millimeters. Curves C2 and C3 are based on measurements that were made on the same test-rig. Several styles of prior-art filters in accordance with the US Patents cited herein were tested by rotation and their average plotted. The datum line A1 is shown as running parallel to the abscissa and denotes the pressure differential threshold of 25 millimeter d'eau which is the constant static pressure derived downstream of the orifice plate. Ordinate datum line O1 is for the 800 cubic meters per hour air quantity passing through the filter at a velocity of 0.6 meters per second. Plotted against that is Cl with the air volume V1 estimated to be in the neighbouhood of 770 cubic meters per hour. C2 with the pleated matrix of this patent has an empirically tested volume just below 700 cubic meters per hour and C3 for the average the air volume roughly between 640 to 660 cubic meters per hour. This air volume was measured through four different commercially purchased HEPA fitters, each with pleated matrice of prior-art, as cited herein.

As filter pressure drop was the intent of the measurements, an orifice plate device similar to the one used by US Department of Energy Filter Test Facilities located at Oak Ridge, TN, Rocky Flats, Co, and Hartford, WA. The device is a flat plate with 3 mm diameter holes drilled symmetrically 25 mm apart horizontally and vertically. This device was calibrated to an accuracy of 1% for a flow rate of 1000 cubic meter per hour as shown as the datum line parallel to the abscissa in FIg. 9. The orifice plate exactly matches 610 m x 610 m filter projected face and was positioned in-line at a distance of about 30 centimeters upstream of the filter. The differential pressure acting upon the filter was @ 25 mm d'eau despite the system supply pressure being over 100 mm d'eau.

When the ambient pressure and temperature are factored, the consistency of the orifice device is as good as the reproducibility of the pressure, pressure drop and temperature measurements. It is relatively easy to get these measurements to accuracy of better than 1%. The standard used for the flow measurements was a viscous laminar airflow meter with an accuracy of ± 0.5% , and traceable to the US Bureau of Standards.

This test-rig ensuring differential pressure of 25 mm d'eau across the HEPA filters was used to test all the matrices described by Rg. 3, Fig.4, Fig.5 and Fig.6, that generated curves C2 and C3 shown in Fig.9.

Volume flow is simply the ratio of mass flow to the gas density. . The volume flow rate is also a measure of the gas velocity averaged over a cross section normal to the flow path. With viscous laminar flow, the volume flow is independent of the gas density and proportional to the ratio of the pressure drop to the gas viscosity. The equation for pressure drop for viscous laminar flow using volume flow is, ΔP_V = ICnQ where ;

Δ P_V = pressure drop from pure viscous laminar volume flow

K_r = constant relative to viscous laminar volume flow.

Q = air volume flow rate n = air viscosity

For the filters disclosed by this patent, the pressure drop is deemed to be directly proportional to the product of the volume flow and the gas viscosity. ADVANTAGES OF THE PATENT

In essence the objects of this patent are to overcome the inadequacies of prior- art methodologies. All the patents listed in the citation, conceptually and substantivefy, overlook the resistance to the flow of air caused by the passive spacers. These passive prior-art separators restrict airflow by roughly 10% over the unimpeded insert-free flat-sheet media, correspondingly adding to the cost of the filter.

State-of-the-art minipleat separation means cause turbulence resulting in eddies at each point the spacer intervenes the air-streams and consequently increase cumulative pressure drop across the filter. These pleat separating materials suffer from the insufficiency that they can only function at air temperatures below 200 deg. Celsius.

The advantages of the present invention is that the above drawbacks have been overcome by an unique method and device hitherto not envisaged by those skilled in the art.

SUMMATION

The present invent'on is directed at a novel, non-obvious and commercially feasible filter construction that enhances filter performance by reducing pressure drop, the primary parameter that impacts on the economics of filter-life. The comparable performances are shown by a graphic depiction of relative air volumes plotted against an estimate mathematically derived from flat-sheet traverse velocities. Fig.9 should therefore be taken here as a non-limiting specificity.

This patent discloses a pleat-spacing method for achieving and maintaining pre-determined spatial relationships between adjacent folds in a filter element pleated from microfibreglass media for HEPA High Efficiency Particulate Air Filters @ 0.1% penetration for 0.3 micron particles and in a filter element pleated from rrdcroporous polytetrafluroethylene membrane for ULPA Ultra Low Penetration Air @ 0.001% penetration for 0.1 micron particles. The hollow isokinetic inserts that hold the pleats apart are made from PTFE or any extrudable polymeric material for room temperature use and from heat resistant material such as ceramic for hot-air applications above 300 deg Celsius. The filter-executions encompass other applications for pleated filters. Their required performance parameters are exceeded by innovative and ingenious isokinetic spacers placed originatively within the pleats to hold them rigidly apart with minimal obscuration of media surface and air-stream interference.

The same heat resistant isokinetic spacers when made, say, from ceramic are effective for hot-air spore inactivation of pharmaceutical glass containers as they can be used at air temperatures exceeding 350 degree Celsius without compromising filter integrity or particulates retention efficiency.

Claims

CLAIMS What is claimed is :

1. A pleated filter matrix made from a plurality of V-foIded flat-sheet media for submicronic particulates filtration from airstreams traversing the pleats wherein ; the interpleat pathways are uniformly spaced by means of isokinetic inserts that are interposed to hold the pleats apart and subjoined to the media by pliable adhesive. the inserts are hollow for air to flow isobaricaliy without vortices through their interior surface, the inserts are cylindrical and present a knife-edge contact to the flat-sheet media for minimal pore obscuration of flat-sheet surface and air-stream interference.

2. A pleated filter assembly with isokinetic inserts according to claim 1 wherein , the said inserts have internal pathway for passage of air-streams without deflections or obstructions ; the said hollow isokinetic inserts are disposed in arrays that precisely align and abut against each other on both the incident and leaving side of the media in a nesting relationship that holds the pleats rigidly parallel and perpendicular ; the said abutting isokinetic inserts equispace the pleats throughout the pleat depth.

3- A filter assembly according to claim 1 , comprising V-folded pleats wherein ; said pleats have inlet and outlet air passages ; said pleats are separated by isokinetic inserts as spacers that overlie each other to provide equidistant pathways for airstreams to traverse without turbulence, eddies or vortices ; the said inserts offer reduced resistance, to air-streams whereby they are dynamically isobaric with the interpleat differential pressure ; said pleats have a nesting density of 20 to 30 pleats per 100 millimeters, said pleats have a traverse velocity through the media of 2 to 3 meters per minutes, said pleats have a pressure drop of 20 to 25 mm d'βau for filter projected filter face velocities at exit of 0.4 to 0.6 meters per second.

4. A pleated HEPA filter assembly in accord with claim 1, that can withstand the passage of hot-air in excess of 350 deg Celsius wherein ; the hollow isokinetic inserts separating microtlbreglass media are made from heat-resistant material ; the said inserts are conjoined to the media by a pliable compound of colloidal silica and ceramic dispersed in a solvent ; the said colloidal bonding compound has the property to penetrate the binder-free media and strengthen the surface where the said inserts are conjoined.

5. A pleated filter assembly having isokinetic inserts whereby ; interpleat slip and transition airflow effects are attenuated by the passage of air through the hollow separating spacers ; interpleat perturbations from obstructions by passive inserts are eliminated ; interpleat airflow impeding effects by obstructing separator means are mitigated.

6. A pleated filter assembly whereby hot air upto 350 deg Celsius can be filtered for spore inacth/ation of glass containers comprising :

• isokinetic spacers made from heat-resistant material that separate and support the biπderiess microfibreglass media ;

• colloidal mixture of silica and ceramic in a carrier fluid that bonds the hollow inserts by amalgamation with the binderiess microfibreglass fibres media.

7. A pleated ULPA filter comprising a composite matrix wherein ; the media is a membrane of micropbrous porytetrafluroethylene sandwiched between two supporting layers of woven mesh of high molecular weight polymer ; the spacers are hollow polymeric isokinetic inserts that separate the said media and define a plurality of equispaced pleats ; the pressure drop is less than 25 mm d'eau for a traverse air velocity of 3 meters per minute across the composite media ; the percentage penetration is 0.001 % for 0.1 micron particulates.