Technical Field

The present invention relates in general to gas filters. In particular, the present invention relates to an improved cleanable high efficiency filter for gasses and a system employing such filters. Background Art

In recent years it has typically been required to filter air or other gases to remove the particulate matter entrained therein due to industrial processes. One common filtering arrangement is a baghouse having a filter media through which the air stream passes, with the particulate material entrained in the air stream being collected upon the filter media. Gas jet nozzles are provided adjacent the downstream side of the filter media, such that periodic activation of the air jet nozzles will reverse the air flow through the filter media to clean by blowing the collected particles from the media. While this arrangement is advantageous for its cleaning ability, the rugged filter media required for such an application is typically not sufficient by itself to produce acceptable pollution emission rates.

To overcome this unacceptable emission rate, it has been attempted to employ high efficiency filter media set across large rectangular frames. While this has produce acceptable emission rates initially, it has been found that the high efficiency filter media quickly becomes blinded with particulate matter, reducing the gas flow rate and increasing the possibility of a catastrophic bursting which emits a concentrated cloud of pollutants. Additionally, due to the delicate nature of the high efficiency filter media, it has not been possible to provide reverse flow cleaning, such that the filters must be physically replaced at periodic intervals. This requires a shut down in the gas flow to the particular filter and is labor intensive. Additionally, where hazardous or toxic particulate material is involved there is a great danger to the personnel

involved, along with increased cost associated with disposal of the filter at a hazardous waste facility. Disclosure of Invention

An object of the present invention is to provide a high efficiency filter which may be readily cleaned without manual intervention.

Another object of the present invention is to provide a high efficiency filter which may be cleaned by back flow air pressure. Another object of the present invention is to provide a high efficiency filter which maybe readily incinerated upon the end of its life cycle.

Yet another object of the present invention is to provide a filter system which allows the use of high efficiency filters.

These and other objects are achieved by a high efficiency filter cartridge and system using same. The filter cartridge consists of a high efficiency HEPA filter media which is pleated in an accordion fold and formed into a sealed cylinder. For additional support the cylinder of pleated filter media may be placed between cylindrical inner and outer support cages. A first longitudinal end of the cylinder, and possibly the cages, is then sealed with an end cap, while the second longitudinal end is sealed with an annular flow cap. By this arrangement the high efficiency filter media is sufficiently supported and protected to allow for back flow cleaning, yet has a large area for excellent filter characteristics. A plurality of the cartridges may be placed within a main housing with the flow cap connected to a passage leading to a secondary housing. Appropriate conduits for the contaminated gas are connected to the main housing, such that the gas flow will pass into the main housing, radially inward through the filter cartridge, and into the secondary housing. An appropriate fan may also be provided to aid in gas flow. A baffle plate is located between the flow entering the main housing and the filter cartridge, to ensure an even accumulation of

particulate about the filter. Appropriate back flow air nozzles may be located in the secondary housing and directed though the passage into the interior of the filter cartridge. When cleaning of the filter cartridge is desired, the gas flow may be halted, and clean gas passed through the nozzles into the interior of the cartridge to dislodge the particulate matter from the filter media, with the particulate matter falling downward into collection bins. Brief Description of Drawings

The objects and features of the invention noted above are explained in more detail with reference to the drawings, in which like reference numerals denote like elements, and in which: Fig. 1 is a side view of a filter system according to the present invention;

Fig. 2 is a front view of the system of Fig. 1; Fig. 3 is a top view of the system of Fig. 1; and Fig. 4 is a perspective view with partial cut-away of a filter cartridge according to the present invention. Modes for Carrying Out the Invention

With reference to Fig. 4, a filter cartridge according to the present invention, is generally designated by reference numeral 10. The cartridge 10 includes a filter media 12 which is preferably formed of a glass micro fiber or other equivalent material meeting the standards of a HEPA filter media. Such media will typically have a capture efficiency of 99.97% of particles having a diameter of 0.3 (three tenths) microns or greater. This capture efficiency can provide an emission rate on the order of 0.000 000 4 grains of particulate matter per cubic foot of exhaust air. The filter media 12 is arranged in the form of a pleated cylinder. In particular, the filter media 12 is provided with accordion folds extending substantially parallel to the longitudinal axis of the cylinder, and is arranged with adjacent fold lines being substantially parallel and

circumferentially spaced.

Where the filter media cylinder is formed from an initially flat piece of material, the ends of the material brought together to form the cylinder are sealed together along the entire longitudinal length by an adhesive 14. This arrangement will define an interior and exterior face for the filter media, with the longitudinally extending fold lines defining interior and exterior peripheries for the filter media cylinder. A first longitudinal end of the media cylinder 12 is received within an end cap 20 which includes a peripheral side wall 22. The exterior periphery of the media cylinder is received within the side wall 22, and this first longitudinal end of the cylinder is sunk within an appropriate potting compound, such as an epoxy resin. As the end cap is not permeable, and the first end of the cylinder is received within the potting compound, gas flow may only pass through the filter media 12, and not about the end cap 20. Similarly, the second longitudinal end of the cylinder 12 is received within a flow cap 24 having the form of an annulus with interior and exterior peripheral walls 26 and 28. As with the end cap 20, the exterior side wall 28 receives the exterior periphery of the cylinder, while the interior side wall 26 receives the interior periphery of the cylinder 12. Also as before, a potting compound 30 seals the media cylinder 12 to the flow cap 24, such that the flow of air from the exterior of the media cylinder 12 to the interior thereof must pass either through the media cylinder 12, or through a throat 32 of the cartridge defined by the interior side wall 26 of the flow cap. Finally, the flow cap may include means for mounting the cartridge, which may advantageously take the form of one or more threaded studs 33 extending outwardly from the flow cap in a direction substantially parallel to the longitudinal axis of the cartridge.

In this arrangement the accordion fold of the media produces a cylinder 12 having a radial depth. Additionally, the fixing of the ends of the cylinder in the potting compound fixes the spaced fold arrangement of the media, producing improved structural strength compared to a flat expanse of the media or a cylinder of the media without pleating. In particular, the cylinder is fairly rugged in compression and bending, although it readily twists in torsion. The structural strength of such a cartridge is of course dependent upon its size. Where the cartridge has a relatively short longitudinal length its compression strength is greater than a cartridge having a relatively long length, as may be readily envisioned.

As such, for cartridges which are relatively short, the above elements may be sufficient by themselves to comprise the cartridge. However, for cartridges having a preferred length on the order of 50cm (20 in.) with a diameter on the order of 15cm (6 in.), additional reinforcement of the media may be required not only for sufficient strength during handling, but to withstand the forces applied to the media during use.

A first reinforcing arrangement is to simply employ a media for the cylinder 12 which retains the HEPA filtering characteristics, but with improved structural strength. For example, the media may be a composite of a central HEPA filter media layer with outer top and bottom layers of foraminous polyester or other material bonded thereto. Such a material employing polyester exterior layers is commercially available from SF Filtration AG. of Switzerland. Simply employing such a composite media in the pleated cylinder may be sufficient to provide the necessary strength in a cartridge of the preferred size.

Another alternative, usable with the standard or the composite HEPA media, is the provision of additional support elements in the cartridge. Such support elements may take the form of interior and exterior support cages 16 and 18. Each of the support cages is in the form of a

cylinder which will closely contact the associated set of interior or exterior fold lines of the filter media cylinder. The support cages are formed of a foraminous material such as an expanded metal or plastic mesh which will allow good gas flow therethrough, yet which is fully self supporting such that the cages may lend support to the media cylinder 12. The support cages may be secured in place in the same manner as with the cylinder, i.e. by fixing their longitudinal ends within the potting compound. Where the cartridge 10 is to be employed with particulate emissions which are toxic or hazardous, it may be preferred that all elements comprising the cartridge be formed of a material which may be easily incinerated. As such, the end cap, flow cap and studs, and the support cages if employed, may be formed of a plastic or rubber.

While various steps could be performed to produce the cartridge 10, it has been found advantageous to begin with a sheet of the filter media which is repeatedly subject to alternating equidistant folding or pleating to form the accordion fold arrangement until the desired length is reached, at which point the media is cut. The free ends of the pleated media which are parallel to the fold lines are then brought together and sealed along their length with the adhesive 14. The cylinder thus formed is then supported in the cylindrical configuration by manual grasping, placing an elastic band about its exterior or by other means. While maintaining the proper pleated cylinder configuration, cylinder 12 is placed within one of the end cap 20 or flow cap 24 containing the uncured potting material, such that the end of the cylinder is immersed in the potting material. Once the potting material has sufficiently set, the other end of the partially formed cartridge is placed within the other of the end cap or flow cap with the potting material 30 therein. When this potting has sufficiently solidified, the cartridge is complete.

Where the support cages are employed the process is similar, although the cages may be employed to maintain

the cylinder in its proper configuration. In particular the cylinder 12 may be placed within the exterior support cage 18 and subjected to any final positioning to provide equidistant peripheral spacing to the longitudinal fold lines. The interior support cage 16 is then placed within the filter media cylinder 12, and this resulting structure secured to the end and flow caps as described above.

As may be readily envisioned from the above description of manufacture, the cartridge according to the present invention requires sealing only along the single joined end of the sheet of filter media and at the top and bottom of the cartridge. This reduces the manufacturing costs, as does the reduced amount of frame structure required for the present cartridge. Additionally, as the filter media is formed as a pleated cylinder, a large amount of filter media area is available for use while requiring very little floor space to house the cartridge. This also helps to reduce the operating costs of the present cartridge. The pleated cylindrical shape additionally improves the operation of the present gas filter. Specifically, during operation the contaminated gas will flow from the exterior of the cartridge radially inward, passing through the filter media cylinder 12, and exiting through the throat 32. At periodic intervals the contaminated gas flow will be halted and a reverse, clean gas flow will be directed into the throat 32 and radially outward of the filter media cylinder 12 to dislodge the particulate matter from the filter media and thus clean the filter cartridge. During, either of these gas flow conditions, it may be seen that the generally cylindrical shape of the filter media 12 helps to distribute stresses, providing improved operational life compared to a planar filter arrangements and permitting the use of the high efficiency filter media.

Additionally, the pleated arrangement of the filter media cylinder provides further stress reducing

features. Specifically, the pleated cylinder forms a plurality of longitudinally extending planar surfaces between each fold line, yet these planar surfaces are of a sufficiently small dimension (at least in the direction between the fold lines) that the fragile filter media may withstand the pressures caused by the gas flow. This reduced stress upon the filter media 12 allows the use of a high efficiency filter media, which will typically be subjected to greater stresses than a low efficiency filter media due to the increased amount of particulate matter trapped upon and within the high efficiency filter media, causing a greater pressure differential during gas flow.

The pleated radial configuration of the filter media cylinder 12 also optimizes the ability of the filter element to capture a large amount of particulate matter, and release the particulate during the cleaning cycle. Specifically, the radial pleat design results in the pleat spacing (the circumferential distance between fold lines) to be wider on the exterior or "dirty" side of the filter media where the particulate matter is captured, compared to the pleat spacing on the interior or radially inner side of the filter media. This provides a larger and more easily accessed filter surface on the exterior of the cylinder 12, where the particulate matter will impact upon the cartridge. Additionally, during the cleaning mode the increased spacing on the exterior side allows the captured particulate to be more easily removed, as the media does not interfere with, or touch, the particles as they are blown outward from the cylinder 12. While the pleated configuration provides reduction in the stresses during the gas flow, the use of a high efficiency HEPA filter media may still produce sufficient stresses upon the cylinder such that the interior and exterior support cages 16 and 18 are required to maintain the media in place. This is most likely to be the case in large size cartridges. While the use of support cages increases the material costs for producing the filter, the

support provided by the cages allows normal handling of the cartridges without damage to the filter media.

With reference to Figs. 1-3, a system employing the cartridge 10 according to the present invention is generally designated by reference numeral 34. The system 34 includes an entrance conduit 36 which conveys the contaminated gases. While not shown in the figures, the conduit 36 will be connected to industrial machinery of various applications. The entrance conduit 36 is operatively connected to a main housing 38, preferably with an appropriate shut off valve 40, such as a butterfly valve, interposed between the entrance conduit 36 and the main housing 38. While the main housing may take other configurations, it is preferably formed as a generally rectangular enclosure which defines a main cavity therein. The entrance conduit 36 communicates with the main cavity 42 by way of a front wall 44 of the main housing, with the generally rectangular configuration of the main housing resulting in the main housing also including a rear wall 46 and right and left side walls 48 and 50. One or more of the side walls and rear wall may include one or more sealed doors to provide access to the main cavity 42.

While the main housing could include a bottom wall extending between the front, rear and side walls, it is preferred that the bottom of the main housing include one or more transition zones 52 having a generally inward taper and connected to an equal number of removable dust collection pans 54. Where this is the case, the main housing may include a plurality of support legs 56 to cause the dust collection pans 54 to be spaced from the ground to allow them to be removed for a purpose described below.

The main housing 38 is completed by a main barrier wall 56 which forms the top of the main housing, and includes a plurality of cartridge passages 58 extending therethrough. The cartridge passages 58 have a configuration and size roughly corresponding to that of the

throat 32 of the cartridge 10, and the main barrier wall 56 includes appropriate bolt holes (not shown) for passage of the mounting studs 33 of the cartridge. As may be readily envisioned, one cartridge 10 is associated with each of the cartridge passages 58, with the longitudinal axis of cartridge 10 extending substantially vertically downward into the main cavity 42 of the main housing.

The main barrier wall 56 which forms the top of the main housing 38 also forms the bottom of a secondary housing 60 mounted upon the main housing 38. As with the main housing, the secondary housing may take many configurations but is preferably formed as a generally rectangular enclosure which serves to define a secondary cavity 62 therein. Supported upon either or both of the main and secondary housings is an air supply tank 64 which serves to store a supply of cleaning air or other gas for a purpose made clear below. To ensure that the gas within the tank 64 is maintained at a proper pressure, the air supply tank may include an appropriate relief valve 66 and regulating valve 68.

As is best shown in Figs. 1 and 3, one or more tank conduits 70 extend from the air supply tank 64 into the secondary cavity 62 of the secondary housing, and extend to a position above, and in proximity to, the cartridge passages 58. Each of the tank conduits 70 includes at least one appropriate nozzle 72 which is directed into the cartridge passage 58. Finally, there is provided a supply valve 74 for each of the conduits 70, such that the supply valve will control the flow of gas from the supply tank 64 through the conduit 70 and out of the nozzle 72.

While the secondary housing may include an appropriate outlet opening serving as an exhaust for the system, it is preferred that the secondary housing include a secondary barrier wall 76 having one or more cartridge passages 78 similar to those in main barrier walls 56. This will allow one or more additional cartridges 10 to be

located within the secondary housing to provide further filtration for gas passing through the system. The cartridge passages 78 open into an exhaust chamber 80 having an exhaust duct 82 connected to a fan 84 which is arranged to draw air from the exhaust chamber 80 and expel it from its exhaust outlet 86.

As should be apparent from the above description, contaminated gas passing through the entrance conduit 36 will enter into the main cavity 42 of the main housing. The gas will pass radially through the filter media cylinders 12 of the cartridges 10 to remove the particulate matter entrained within the gas flow, with the cleaned gas flowing from the interior of the cartridges 10 through the cartridge passages 58 into the secondary cavity 62 of secondary housing 60. While the cleaned gas may simply be exhausted at this point, it is preferred that the gas pass through the additional set of cartridges 10, with the yet further cleaned gas passing through the cartridge passages 78 into the exhaust chamber 80, through the exhaust duct 82 and fan 84 to be finally expelled through the exhaust outlet 86. Appropriate piping may be provided from the exhaust outlet to an exterior portion of a building within which the system is housed.

This arrangement will serve to trap the particulate matter within the gas upon the filter media cylinders 12, thus cleaning the gas. At periodic intervals the filter media cylinders 12 must, however, be cleaned. To effect this the shut off valve 40 may be moved from an open to a closed position to reduce the pressure differential across the filter media 12, and the supply valve 74 moved from the normally closed to an open position. This will allow the cleaning gas housed within the supply tank 64 to pass through the tank conduit 70, through the nozzle 72 and into the interior of the cartridge 10. Such reverse flow cleaning, when employed in a baghouse environment has typically taken the form of either a high pressure, low volume burst of cleaning gas or a low

pressure high volume flow of cleaning gas. In the present invention the high efficiency filter media is rather delicate, yet traps a great percentage of the particulates. Due to these two characteristics, it is believed that neither of the these typical cleaning extremes is advantageous, but that a medium pressure, medium volume blast would provide the best cleaning without damaging the filter media. While the volume of cleaning gas employed will depend in part upon the size of the cartridge, it is currently preferred that the nozzles eject the cleaning gas at approximately 48E3 N/m ² (7 psi.)

This reverse flow of gas will serve to dislodge the particulate matter trapped upon the filter media cylinders 12, allowing the particulate material to fall from the cartridges through the transition zone 52 to the collection pan 54. As should be apparent, the vertical orientation of the cartridges, and in particular the fold lines, helps to ensure that the particles will not simply fall from one section of the cartridge to a lower section. After a sufficient amount of reverse flow has occurred to provide a cleaning action, the supply valves 74 are again moved to the closed position and the shut off valves 40 opened to resume the normal cleaning of the gas passing through conduit 36. The operation of the shut off valve 40 and supply valve 74 may be effected manually, may be automatically effected at intervals controlled by a timer, or by other arrangements. In particular, it is preferred that various sensors be employed, such as pressure sensors 88 to sense the pressure within the main cavity 42 and secondary cavity

62. The difference in pressure sensed may provide an indication of the gas flow, and thus the amount of particulate material trapped upon the filter media cylinders 12. When a sufficient drop in pressure is observed, a control means 96 receiving signals from the sensors 88 may cause the valves 40 and 74 to be automatically activated to perform the cleaning cycle described above.

To further improve the operation of the system 34, there may be provided one or more main dividing walls 90 which serve to substantially divide the main cavity 42 into plural sections, each having one or more cartridges. The entrance conduit 36 would in this case be divided such that the entrance conduit communicates with each of these sections, with a shut off valve 40 provided with each section. With this arrangement a first of the sections may be closed off to the supply of contaminated gas by activation of its associated shut off valve 40, thus allowing cleaning of the cartridges 10 within that section, while the contaminated gas within conduit 36 may continue to flow to the remaining section or sections of the main cavity. This will help to provide a continuous cleaning process without costly interruptions.

Yet further improvement may be achieved by providing an inlet transition 92 between the entrance conduit 36 and the main housing 38, with the inlet transition increasing the cross sectional area of the conduit 36 to thus provide a decrease in the velocity of the contaminated gas supplied to the main housing, assuming a constant volumetric flow rate. Additionally, there may be provided a baffle plate 94 interposed between the entrance of the entrance conduit 36 and the cartridges 10. The baffle plate 94 will preferably extend completely across the main housing (or section of the main housing) , yet terminate at a position above the transition zone 52 leading to the dust collection pans 54.

The baffle plate is preferably perforated, possibly having approximately .5cm (3/16 in.) diameter holes on an approximately ,6cm (1/4 in.) stagger. This baffle plate will further reduce the velocity of the contaminated gas entering the main housing, and will serve to deflect a portion of the gas downward and towards the rear of the housing. This velocity reduction, in combination with that produced by the inlet transition 92, will serve to reduce the size of particles which may remain entrained within the

contaminated gas. As such, a certain percentage of the largest particles will fall from the contaminated gas flow through the transition zone 52 and into the dust collection pans 54 without ever contacting the filter cartridges 10. This will prolong the cartridge life and reduce the frequency of the cleaning cycles.

Finally, the reduced velocity of the contaminated gas due to the presence of the baffle plate will reduce the stresses upon the filter media cylinders 12, helping to allow the use of the high efficiency filter media.

The deflection of the gas stream caused by the baffle plate will additionally serve to provide a better distribution of the contaminated gas, such that there is not a build up of particles upon those cartridges which are closest to the entrance conduit, and those portions of cartridges which are closest to the entrance conduit. This will result in a more uniform accumulation of particulate matter upon the filter media cylinders 12, reducing the required time before effecting a reverse flow cleaning cycle.

As may be envisioned, the particulate material which falls from the contaminated gas flow due to reduced gas velocity, and which falls from the cartridges 10 during the reverse flow cleaning cycle, will fall downward to be collected in the collection pans 54. At desired intervals, the shut off valve 40 may be moved to the closed position and the pans 54 removed for emptying. Additionally, the access doors in the main housing may, when necessary, be opened to provide access for changing the cartridges 10. It will be apparent to those skilled in the art that various modification may be made without departing from the scope of the invention. For example, the pleated media need not be arranged in the preferred circular cross section, but the cross section may be elliptical, rectangular, triangular, etc. In each of these alternatives, however, the pleated media will have the general form of a tube, which term is intended to encompass

various cross sectional forms. In each case the pleating will still be formed by fold lines substantially parallel to the longitudinal axis of the tube, and thus be spaced peripherally about the tube. While the terms "radially inner" and "radially outer" are employed to identify those fold lines defining the inner and outer peripheries of such tubes, the use of "radial" should not be taken as a limitation of a circular cross section, but may apply to other cross sectional forms as well. From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative, and not in a limiting sense.