Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A METHOD AND A PLANT FOR THE MANUFACTURE OF AIR-CLEANING FILTERS AS WELL AS A FILTER MANUFACTURED IN ACCORDANCE WITH THE METHOD
Document Type and Number:
WIPO Patent Application WO/2001/039889
Kind Code:
A1
Abstract:
The invention relates to a method suited for the manufacture of air-cleaning filters, which includes the steps to feed out a rigid, although formable strip (8) in a lying state from a store (7), to advance a lying mat (24) of threads (3) transverse to the strip and connect one end of the thread mat to the strip (8), to give the strip (8) equipped with threads a generally wave-like profile shape, and to cut off an individual strip piece (8') from the continuous, thread-equipped strip in order to use this to finally form a thread-carrying filter body (2) in which the wave troughs and wave crests, respectively, of the profiled strip piece forms air-transmitting ducts in the body.

Inventors:
BJOERKEBAUM HANS (SE)
Application Number:
PCT/SE2000/002226
Publication Date:
June 07, 2001
Filing Date:
November 14, 2000
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
FRESHMAN FLIMMER AB (SE)
BJOERKEBAUM HANS (SE)
International Classes:
B01D46/28; B03C3/28; (IPC1-7): B03C3/30; B01D46/28
Domestic Patent References:
WO1995000248A11995-01-05
WO1995033567A11995-12-14
WO1995033569A11995-12-14
Attorney, Agent or Firm:
Johansson, Lars (Lars Johansson Patentbyrå AB Box 68 Gävle, SE)
Download PDF:
Claims:
Claims
1. A method for the manufacture of aircleaning filters of the type that comprises a body (2) having a plurality of diminu tive, airtransmitting ducts (5), which are delimited by a lat tice (6) of thin walls and extend axially between opposite ends of the body, as well as a set of fine threads (3) of an elec trostatically chargeable material which are connected to and carried by the body (2), c h a r a c t e r i z e d by the steps of: making a continuous work piece in the form of a rigid, although formable strip (8) and a set of threads (3) protruding at an angle therefrom, giving the strip (8) of said work piece a generally wavelike profile shape, and cutting off an individual strip piece (8') from the continuous, threadequipped strip (8) in order topossibly together with other threadequipped strip piecesuse the same for finally forming said threadcarrying body (2), wave troughs and wave crests (42,43), respectively, in the profiled strip piece (8') forming said ducts (5) in the body.
2. A method according to claim 1, c h a r a c t e r i z e d in that the manufacture of the work piece takes place by feeding out a strip (8) in a flat lying state in a first direction from a store (7) and by advancing a flat lying mat (24) of threads (3) in a second direction transverse to the firstmentioned one, and uniting one end of this thread mat with the strip.
3. A method according to claim 2, c h a r a c t e r i z e d in that a weldable plastic or composite material is used as mate rial in the strip (8) and the threads (3), respectively, and that the thread mat (24) is united to the strip by means of welding.
4. A method according to claim 1, c h a r a c t e r i z e d in that the strip of the work piece is provided by welding to gether end portions of threads in the thread mat in order to jointly form a continuous, integrated material strand.
5. A method according to any one of the preceding claims, c h a r a c t e r i z e d in that profiling of the thread equipped strip (8) is carried out by bringing the strip to pass between a pair of cooperating, rotatable wheels (29,29') hav ing peripheral rims of teeth (30), the shape of which decides the profile shape of the strip.
6. A method according to any one of the preceding claims, c h a r a c t e r i z e d in that the work piece after having been formed with the strip (8) as well as the threads (3) in a lying state is redirected to an upright state in which the strip is vertically oriented and the threads hang down verti cally therefrom.
7. A method according to any one of the preceding claims, c h a r a c t e r i z e d in that the strip (8) is advanced from the position for the manufacture of the work piece by means of a first pair of feed rollers (22), which cooperates with a second pair of feed rollers (28) located at a distance from the firstmentioned pair, the part of the strip (8) which extends between the two feed roller pairs (22,28) being kept in a slack state of standby.
8. A method according to any one of the preceding claims, c h a r a c t e r i z e d in that the threadequipped and pro filed strip (8) is cut into comparatively short pieces (8'), which are permanently united to other threadequipped, short strip pieces, more precisely by bringing together a plurality of strip pieces and uniting the same side by side during suc cessive construction of the threadcarrying body.
9. A method according to claim 8, c h a r a c t e r i z e d in that said strip pieces (8') are united by welding, e. g. spot welding.
10. A method according to claim 8 or 9, c h a r a c t e r i z e d in that the threadequipped and profiled strip is cut into pieces (8') having different lengths for successive con struction of threadcarrying bodies having a nonrectangular crosssection shape.
11. A plant for the manufacture of aircleaning filters of the type that comprises a body (2) having a plurality of diminu tive, airtransmitting ducts (5), which are delimited by a lat tice (6) of thin walls and extend axially between opposite ends of the body, as well as a set of fine threads (3) of an elec trostatically chargeable material which are connected to and carried by the body, c h a r a c t e r i z e d in that the same comprises an arrangement (25) to make a continuous work piece in the form of a rigid, although formable strip (8) and a set of threads (3) which protrude at an angle therefrom, a device (29,29') to give the strip (8) of the work piece equipped with threads a generally wavelike profile shape; a cutting device (14) to separate individual strip pieces (8') from the continu ous, threadequipped strip (8), as well as a final forming sta tion (13) in which one or more threadequipped, profiled strip pieces are machined to form said threadcarrying body (2), wave troughs and wave crests (42,43), respectively, in the profiled strip piece (8') forming said ducts (5) in the body (2).
12. A plant according to claim 11, c h a r a c t e r i z e d in that the same comprises a store (7) for a continuous strip (8) of a rigid, although formable character, which may be fed out from the store in a first direction in a flat, lying state on a first substratum (21) following after the store, which cooper ates with a second substratum (23) on which a flat, lying mat (24) of threads (3) may be advanced to the strip (8) in a di rection transverse thereto in order to connect an end portion of the thread mat (24) with the lying strip (8).
13. A plant according to claim 11 or 12, c h a r a c t e r i z e d in that downstream said arrangement (25) for the manu facture of said work piece there is, seen in the strip feeding direction, arranged a first pair of feed rollers (22) with the purpose of intermittently advancing the strip from said device, a second pair of feed rollers (28) being arranged at a distance from the first pair in order to enable feeding of the strip in a slack state between the two feed roller pairs (22,28).
14. A plant according to claim 12 or 13, c h a r a c t e r i z e d in that said arrangement for the manufacture of the work piece consists of a welding machine (25) which is located between the store (7) and the first feed roller pair (22), as well as in the main in line with the second substratum (23) on which the thread mat (24) is advanced to the strip.
15. A plant according to any one of claims 1114, c h a r a c t e r i z e d in that a pair of wheels (29,29') serving as profiling devices having peripheral rims of teeth (30) is ar ranged upstream the final forming station (13), between which wheels the threadequipped strip (8) may be fed in order to be given a profile shape corresponding to the shape of the teeth.
16. A plant according to any one of claims 1115, c h a r a c t e r i z e d in that the cutting device (14) is located in the final forming station (13), the final forming station in cluding, on one hand, a welding machine (15) and, on the other hand, a picking apparatus (16) with the purpose of taking care of and holding cut off, short strip pieces (8') and bringing these to positions in which the same may be welded together side by side by means of the welding machine.
17. An aircleaning filter, including a body (2) having a plu rality of diminutive airtransmitting ducts (5), which are de limited by a lattice (6) of thin walls and extend axially be tween opposite ends of the body, as well as a set of fine threads (3) of an electrostatically chargeable material which are connected to and carried by the body (2), c h a r a c t e r i z e d in that the body (2) includes one or more indi vidual pieces (8') of strip of a rigid, although formable mate rial, at least certain strip pieces (8') after profiling having a generally wavelike profile shape, and that said profiled strip piece (8') is equipped with a mat (24) consisting of fine threads (3), which extends along wave troughs and wave crests (43,43) of the profile so as to obtain an efficient length which is larger than the installation length of the proper strip piece, the wave troughs and wave crests, respectively, in the profiled strip pieces forming said ducts (5) in the body (2).
18. An aircleaning filter according to claim 17, c h a r a c t e r i z e d in that all strip pieces (8') in the body (2) are profiled and interconnected side by side, with the wave crests of a given strip piece united to opposite wave crests of adja cent strip pieces.
19. An aircleaning filter according to claim 17 or 18, c h a r a c t e r i z e d in that said strip pieces (8') are made of a weldable plastic or composite material and intercon nected by welding.
20. An aircleaning filter according to any one of claims 17 19, c h a r a c t e r i z e d in that the strip pieces (8') are differently long.
Description:
A METHOD AND A PLANT FOR THE MANUFACTURE OF AIR-CLEANING FILTERS AS WELL AS A FILTER MANUFACTURED IN ACCORDANCE WITH THE METHOD Technical Field of the Invention In a first aspect, this invention relates to a method for the manufacture of air-cleaning filters of the type that comprises a body having a plurality of diminutive, air-trans- mitting ducts, which are delimited by a lattice of thin walls and extend axially between opposite ends of the body, as well as a set of fine threads of an electrostatically chargeable material which are connected to and carried by the body.

Prior Art Particle-separating filters of the kind generally mentioned above are previously known by, for instance, WO 95/00248, WO 95/33567 and WO 95/33569. In filters of this type, the fine threads have the purpose of attracting and capturing fine, usually microscopic or submicroscopic particles accompa- nying a passing air flow. Therefore, the threads are manufac- tured by a material, e. g. polypropene or another polymer mate- rial, which has the ability to produce static electricity when the threads are set in motion in relation to each other by means of passing air. In the operative state, the set of threads is suspended in and carried by the body in such a way that the threads hang vertically down from a lower end of the body. The proper body is mounted transverse to the tubular or duct-shaped conduit through which the flow of polluted air passes and is formed in such a way that the same intersects the cross-section of the conduit and entirely fills up the cross- section. In this way, all air is forced to pass through the diminutive ducts within the body without creeping flows arising between the filter and the surrounding, air-carrying conduit.

This means that the filter and the body thereof are to have a cross-section shape, which in all essentials corresponds with the cross-section shape of the air-carrying conduit. In this connection, it should be pointed out that, in practice, the filters usually also include a surrounding casing, open at opposite ends, which surrounds the body as well as the set of

threads, or alternatively a comparatively slender sealing ring which only surrounds the proper body, but not the set of threads. It should also be mentioned that the proper set of threads usually has a considerably larger axial extension than the body. Thus, in practice the body may be of a length of 10- 50 mm between opposite ends or end portions, while the length of the threads may be 2-20 times larger.

Air-cleaning filters of the type mentioned above are advantageous inasmuch as the particle capturing threads are continuously charged with static electricity simply by means of the motions of the threads in the passing air, and/or the fric- tion between the threads and the air. In other words, the need of supplying electric energy from outside to keep the threads electrostatically charged in the long run is eliminated. There- fore, by those skilled in the art, the filters in question fre- quently are denominated self-acting electrostatic filters. A disadvantage of the described filter technique is, however, that there are no methods for efficient and inexpensive serial production are missing, and no filter constructions which may be varied in different embodiments to provide for different requirements and needs. More specifically there is no technique which, on one hand, makes it possible to manufacture thread- carrying bodies, the design of which in respect of cross- section shape, pressure drop-generating resistance, air flow distribution, etc, may be optimised for different air-cleaning purposes and, on the other hand, be combined with particle cap- turing threads which are distributed evenly and dense in the air flow space downstream the body. To avoid creeping flows and bad efficiency in the set of threads downstream the body, it is generally required that the threads are distributed as even as possible in the set. This is something that means that the mu- tual distance between adjacent threads or bunches of threads should be minimised. In order to attain a good cleaning effect, usually there are also needs of maximising the number of threads per unit of cross-section area.

In the above-mentioned WO 95/33567, a filter is dis- closed which is based on the use of a work piece consisting of two semi-rigid, although pliable strips which are united and interspaced by means of a plurality of mutually spaced-apart

intermediate flanges, which define duct-like cavities or pipes.

In one of these two united strips, slits are cut between adja- cent intermediate flanges, whereby the second strip may be de- flected to be able to be wound to a helical shape. On one side turned from the intermediate flanges of this pliable strip, a thin plastic foil of an electrostatically chargeable material is glued which is cut into slits separating thread-like strips, which in the completed filter have the purpose of capturing and separating particles from a passing air flow. An aggravating disadvantage of this filter is, however, that the number of thread-like strips feasible to achieve is limited by the fact that they are extracted from a thin plastic foil, which is separated by the comparatively thick spiral coils in the body.

Therefore, between radially adjacent strips rather wide, air- transmitting gaps are formed, which entails that the air-clean- ing ability of the set of strips becomes limited. In addition, said publication does not describe any manufacturing method which would allow the design of filters in different embodi- ments which may provide for varying requirements for the per- formance of the filters and which have the ability to be ad- justed to air-carrying conduits having different cross-section shapes.

Objects and Features of the Invention The present invention aims at obviating the above- mentioned shortcomings and disadvantages of prior art. There- fore, a primary object of the invention is, in a first aspect, to provide a manufacturing method which enables quick and effi- cient serial production of air-cleaning filters of the type initially mentioned, and which should permit conversion of the manufacture so as to make it possible to produce highly differ- ent embodiments in long as well as short series while providing for varying market demands and needs. In particular, filters with varying cross-section shape (circular as well as polygo- nal) should be possible to produce. An additional object is to provide a manufacturing method by means of which the closeness of threads in the completed filter may be varied in a simple way. Another object of the invention is to provide a manufac- turing method, which may be carried out mechanically, i. e.

without other manual measures than refilling in stores for strips and threads.

According to the invention, at least the primary ob- ject is attained by the features defined in the characterizing clause of claim 1. Preferred embodiments of the method accord- ing to the invention are furthermore defined in the dependent claims 2-10.

In a second aspect, the invention also relates to a plant by means of which the method according to the invention may be carried out. The features of the plant according to the invention are defined in claim 11. Preferred embodiments of this plant are furthermore defined in the dependent claims 12- 16.

In a third aspect, the invention also relates to an air-cleaning filter produced in accordance with the method according to the invention. The features of this new filter are defined in claim 17. Preferred embodiments of the filter according to the invention are furthermore defined in the dependent claims 18-20.

Brief Description of the Appended Drawinas In the drawings: Fig 1 is a partly cut, schematic perspective view of a ready-made filter having a rectangular cross-section shape produced according to the method of the inven- tion, Fig 2 is an enlarged planar view from above of the filter according to fig 1, Fig 3 is a partly cut perspective view of an alternative embodiment of a filter having a circular cross-section shape, Fig 4 is an enlarged planar view from above of the filter according to fig 3, Fig 5 is a schematic planar view from above of a plant, in which the method according to the invention is applied, Fig 6 is an enlarged, schematic planar view from above of a final forming station included in the plant,

Fig 7 is an enlarged perspective view schematically illus- trating a str : p profiling device included in the plant, Fig 8 is a partial perspective view illustrating one side of a strip piece equipped with threads in a certain manu- facturing state, Fig 9 is an analogous perspective view showing the opposite side of the same strip piece, and Fig 10 is an enlarged, schematic side view illustrating a joint between two strip parts.

Detailed Description of Preferred Embodiments of the Invention Before the proper manufacturing method and the plant for the realization thereof are described in detail, reference is made to figs 1-4, which schematically illustrate the basic structure of a filter according to the invention in two differ- ent versions. In fig 1 and 2, a filter 1 formed with a rectan- gular cross-section shape is shown. In a previously known way the filter includes a body or skeleton 2, as well as a set of threads or thread-like elements 3 which are connected to and carried by the body, more precisely in a vertically hanging state. On the outside of the body 2 and the set of threads 3, a casing 4 is arranged, e. g. a simple cardboard capsule that is open at opposite, upper and lower ends. In a way previously known by the initially mentioned publications, the threads 3 consist of an electrostatically chargeable material, e. g. poly- propene, or another suitable polymer material.

In the body 2, a plurality of diminutive ducts 5 are included, which extend axially between the upper and lower ends of the body so that polluted air may pass in the direction from above and downwards through the body and the subsequent set of threads. Said ducts are delimited by a number of thin walls in- cluded in a lattice generally designated 6. The structure of this lattice will be described in detail below.

The filter according to figs 3 and 4 differs from the filter according to figs 1 and 2 only in that it has a circular cross-section shape instead of a rectangular one.

In practical operation, the filters described above are installe in a surrounding, pipe-like or tubular conduit

through which air may pass, e. g. by being sucked (or pressed) through the conduit by means of a fan.

In order to provide terminological simplicity, the invention is accounted for in the description as well as the claims with reference to the purification of air only, since it is axiomatic that the invention may also be applied to separate particles from other arbitrary gases.

Reference is now made to figs 5 and 6, which in de- tail, although schematically, illustrate the manufacturing method according to the invention as well as the plant in which the method is practised. Through the plant according to the in- vention, a continuous path or manufacturing line extends along which different stations are placed. The most important of these stations (seen in a feeding direction) are: A store 7 for strips 8.

A station 9 for equipping the strip 8 with threads 3.

A station 11 for storing the strip in a stand-by or waiting state.

A station 12 for profiling of the strip 8.

A final forming station in its entirety designated 13 in which a strip cutting device 14, a welding machine 15 and a picking apparatus 16 are included.

The strip store 7 positioned up-stream in the manu- facturing line includes a shaft or bobbin 17 to which a roll 18 of strip may be applied in an operative state. Beside the op- erative strip roll 18, additional strip rolls 18'may be stored in order to be transferred to the bobbin 17, when so required, by a sensor 19, which detects the consumption of the strip roll 18 and may activate a transfer mechanism (not shown) in order to apply a new roll on the bobbin 17, when needed. Downstream the store 7, there is a strip splicing device 20 in which means are included to join or splice two consecutive strips. The splicing may advantageously take place in the way which is illustrated in fig 10, more precisely by letting two strip ends overlap and be punched by means of a tool which in the two strip ends brings about an oblique cut, as well as two short cuts perpendicularly to the strip. After removal of the surplus material, the strip ends shaped in this way may be located in a common plane and be welded in a splice 10 with the edge sur-

faces of the strip ends in contact with each other. Thus, by means of the splicing device 20 strips may continuously be fed out in the manufacturing line.

In this connection, it should be pointed out that the strip 8 most preferably consists of a rigid, although formable plastic or composite material of a weldable character. Advanta- geously, the strip may consist of the same material as is used or is included in the threads 3 with which the strip should be equipped, e. g. polypropene or another similar polymer material.

Polypropene constitutes a specially suitable material by virtue of the low melting point (approx. 160° C) thereof, which makes it possible to connect the threads with the strip by simple heat welding.

From the splicing device 20, the strip 8 is fed out on a table or another suitable substratum 21 in a flat lying state. Advantageously, the table 21 may be elongate and extend in the feeding direction of the strip. Feeding-out of the strip 8 takes place by means of a first pair of feed rollers 22, one of which may be stationary and drivable, while the second one is spring-activated and serves as a non-driven counter-roll.

Here it should be pointed out that a special feeder device (not shown) may be arranged in or up-stream the splicing device in order to enable feeding of strip from a new strip roll 18 to this device in connection with a splicing operation.

At an angle, more precisely at a right angle to the substratum 21, a second substratum 23 extends, which advanta- geously may consist of a conveyor, e. g. a strip conveyor. From a store (not shown), a number of thread braids may be received on said second substratum 23, which braids by means of suitable means (not shown) may be spread while forming a mat of threads 3. By choosing a suitable number of thread braids, the thick- ness and thereby the thread closeness or density in said thread mat 24 may be varied within wide limits. Already now, it should be pointed out that the thickness of this thread mat decides the thread closeness in the set of threads in the completed filter 1. After expansion on the substratum 23, the thread mat is advanced on the substratum 21 to be stopped when the front end of the mat is located on top of the strip 8. After this, the thread mat is united with the strip; something which in the

preferred embodiment example is carried out by welding by means of a welding machine 25. Advantageously, the welding of threads on the strip may take place by heat welding or ultrasonic weld- ing while forming a continuous weld 41. In the direct connec- tion with the welding operation, the thread mat is cut off in a section 26 by means of a cutting device placed in the area be- tween the two substrata 21,23. For space-saving purposes this device has not been shown in fig 5. During the welding and cut- ting, the strip 8 is immobile by the fact that the feed rollers 22 have been stopped. Thus, the feeding of the strip to the feed roller pair 22 takes place intermittently. In the station 9, sensors 27 are included by means of which it can be checked whether the thread mat advanced to the strip 8 has the prede- termined width (that the threads have a correct length) and closeness (= number of threads/width unit). These sensors may also check whether the threads in the thread mat have the pre- determined electrostatical charge after having passed a device for applying electric charge (not shown) placed along the sub- stratum 23. For space-saving purposes the sensors 27 are shown at a distance from the thread mat, although they in practice are positioned closer to the mat.

The strip 8 equipped with threads leaves the substra- tum 21 in a lying state, but is then re-directed or deflected in such a way that the plane of the strip is directed verti- cally before the strip enters into the pinch between the pair of feed rollers 22, which are rotatable around vertical geomet- ric axes. This deflection may take place simply by the fact that the feed rollers are placed at a sufficiently large dis- tance from the substratum 21 in order to give the strip time to turn 90°. It is also feasible to install a special re-directing device between the substratum 21 and the feed rollers. In all events, the threads 3 will hang down vertically from the strip 8 directed vertically when the feed rollers are passed.

At a distance from the first pair of feed rollers 22, a second pair of feed rollers 28 is arranged. The space between the pair of feed rollers forms the station 11 in which the strip may be temporarily stored in a state of stand-by. Thus, the part of the strip which extends between the feed roller pairs 22,28 may be held in a slack, stress-free state in which

this strip part is independent of, on one hand, the feeding out from the strip store 7 and, on the other hand, the further feeding to the final forming station 13. Advantageously, the strip part in question may in a suitable way be held in a coil, as is outlined in fig 5.

In the profiling station 12 (see also fig 7), a pro- filing device is included in the form of a pair of wheels 29, 29'which are rotatable around vertical shafts and which have peripheral rims of teeth or cogs 30. The shape of these teeth decides the profile shape of the strip 8 when this is brought to pass between the wheels. The two wheels may be made in the same way as conventional cogwheels, e. g. by being cut or cast, but it is also feasible to equip the periphery of the wheels with individual, replaceable teeth having different shapes. In this connection, it should be mentioned that consecutive teeth may be asymmetrically made, i. e. have varying shapes and/or be placed at different mutual distance in order to enable increase and decrease, respectively, of the fall of air pressure in dif- ferent parts of the body and the cross-section surface of the completed filter. The profile shape of the teeth and the pro- filed strip, respectively, may vary within wide limits. In the drawings, a trapezoid-like profile shape is illustrated throughout, but it is also feasible to give the strip another basic shape, e. g. square, rectangular, triangular, partly cir- cular, arch-like, sinusoidal, etc. The only essential thing with the profile shaping is that the strip after the profiling has wave crests and wave troughs. Of the two profiling wheels 29,29', at least one is driven, while the second one may be inactive and spring-activated, the feeding of the strip from the feed rollers 28 and through the pinch between the wheels being guaranteed by the driven wheel. In order to facilitate the profile shaping of the strip, this should be heated to a suitable temperature, e. g. by radiant heat, heated air or con- tact heat via the wheels.

It should be mentioned that the profiling station 12 may include a plurality of tooth wheel pairs which may be ar- ranged in a revolving arrangement and be used individually for the manufacture of different types of strip pieces. Preferably it should be possible to separate the tooth wheels so as to al-

low the straight flat strip 8 to pass without being profile- shaped, if required. This makes it possible to finalize the manufacture of a filter with a strip or strip portion which is flat and may be brought to surround individual profiled strip pieces which are put together in the final forming station 13, the surrounding strip forming a sealing ring around the body.

In the final forming station 13 positioned downstream the profiling station 12, the cutting device 14 is included, which includes means to carry out cutting of the strip and the appurtenant threads, e. g. by means of ultrasound, laser, high- pressure water jets or heating. The cutting has to comprise the proper strip as well the threads, since the thread mat in an advantageous embodiment includes not only a plurality of axial, parallel threads, but also transverse threads or fibres, which together gives the mat a net-like structure.

The cutting device 14 co-operates with an adjustable anvil 31 by means of which the advanced strip 8 may be cut off into different, desired lengths. Also the cutting device 14 may be adjustable in different positions along the strip in order to guarantee that the cut off strip piece is centred in rela- tion to a central line through the final forming station.

In figs 5 and 6, an individual cut-off strip piece is designated 8'. The picking apparatus designated 16, which is included in the station 13, includes first and second holding devices 33,34 which are individually shown in first and second settings with continuous and dash-dotted lines, respectively.

Each one of these holding devices includes transverse guides 35 which extend between longitudinal guides 36 and are movable to and fro along the same. In each holding device, two gripping devices 37 and 38, respectively, are also included. In the po- sition shown with dash dotted lines, the holding device 33 has captured a cut-off strip piece 8'and is about to bring this forward to the welding machine 15, which is axially movable along a transverse guide 39, which in turn is vertically ad- justable along vertical guides 40. By this mobility in two planes, the welding machine may be raised and lowered in order to allow passage of the advanced strip piece, as well as enable welding of spot welds at different levels.

Reference is now made to figs 8 and 9, which in per- spective show a cut-off strip piece 8'with an appurtenant set of threads 3. As is seen in the figures, the threads are at- tached to one side of the profiled strip piece by means of the weld 41 which has been applied to the strip already in the welding machine 25, i. e. in the state when the strip is still flat. During the profiling in the station 12, the strip is formed with wave crests and wave troughs 42 and 43, respec- tively. Said wave crests and wave troughs entail that the proper length of the strip piece becomes larger than the in- stallation length (= the distance between opposite ends of the strip piece) of the strip piece. Because the mat of threads follows the profile shape of the strip piece, the thread mat gets a length that is larger than the installation length of the strip piece. Therefore, in comparison with a flat strip piece, the profiled strip piece according to the invention may receive a larger number of threads per length unit, i. e. an increased closeness of threads. Furthermore, threads are dis- tributed or displaced laterally in relation to each other, whereby an improved distribution of the threads is obtained in the completed filter.

The gripping devices 37, which capture the strip piece 8'from above, are by the mounting thereof on the above- mentioned guides 35,36 movable in a system of co-ordinates which means that they may be manoeuvred to arbitrary points in the horizontal plane within the limits determined by the dis- tance between the guides 36, as well as the distance between the front and rear end positions of the guide 35. The transport of the gripping devices may, in practice, take place by means of linear motors, screws or the like, the function of which may be controlled by suitably placed position indicators. From the position shown with dash dotted lines, the holding device 33 is transported to the position shown with continuous lines, in which the captured strip element 8'is turned over to the sec- ond holding device 34. More precisely, the gripping devices 38 take over the strip piece 8', and then the holding device 33 may return to the starting position. The gripping devices 37 as well as the gripping devices 38 also serve as fixture elements with the help of which the strip piece is correctly positioned

laterally in relation to the aforementioned system of co-ordi- nates. A number of dollies 44 also co-operate with the holding device 34. These dollies are, like the holding devices and their gripping devices, movable between different exact posi- tions in the system of co-ordinates. Advantageously, the number of dollies 44 may correspond to the desired number of spot welds which are provided by means of the welding machine 15 (in this connection, it should be pointed out that more than one welding machine 15 may be movable along the guide 39 so as to simultaneously weld the strip piece in several spaced-apart spots). By the vertical mobility thereof, the welding machine 15 may be used to apply spot welds at different levels; some- thing which is essential if the strip piece has a large width (actually, a large height).

When an individual strip piece is welded on a preced- ing strip piece, the completed part of the filter is moved a distance (= the profile thickness) forwards by means of the holding device 34, whereas the holding device 33 after picking up a new strip piece at the cutting arrangement conveys the new strip piece forwards for welding.

The above operations are repeated until a body 2 to- gether with the appurtenant threads is completed. During the construction of the filter, sensors 32 (see fig 5) monitor that all motions and working moments are carried out correctly and that the filter is built up according to a predetermined, de- sired pattern.

As is outlined in fig 5, a flat strip 45 may be twined round the body 2. This strip 45 may, like the profiled strip on one side thereof, be provided with a thread mat which surrounds the welded profiled strip pieces so as to seal against air leakage. Furthermore, the strip in question has a form-stabilizing and uniting effect. It is also feasible to apply a surrounding strip without interior threads or fibres.

On the outside of the filter formed by the set of threads 3 and the body 2 (possibly provided with a surrounding strip 45) a casing 4 may also be applied which houses the body as well as the set of threads.

After or in connection with assembling and before de- livery, the filter should be blown through with compressed air.

Such a through-blowing serves three purposes, viz. to distrib- ute the threads evenly across the filter cross-section, to in- crease the electric charge in the filter, and to remove loose particles, which would otherwise whirl out in the air passing through the filter.

The filter shown in figs 1 and 2 has a rectangular cross-section shape. This shape has been achieved by the fact that all strip pieces 8', which are included in the body 2, have been given one and the same length. In the filter having a circular cross-section shape shown in figs 3 and 4, the body 2 has, however been composed of strip pieces having different, successively increasing and decreasing length. A first strip piece 8', which is shown at the bottom in fig 4, has thus been given a minimal length. Next, the strip 8 arriving to the final forming station has been cut in successively longer and longer strip pieces, which have been welded together in way described above, viz. up to a stage when a longest strip piece has re- ceived a length corresponding to the diameter of the circular cross-section. After that, the length of the subsequent strip pieces has been successively decreased, the assembly being com- pleted by an upper strip piece having the same minimal length as the lower strip piece.

Thus, by varying the length of the individual strip pieces, filter bodies having a most varying cross-section shape may be built up, not only generally quadrangular and circular, respectively, but also other arbitrary cross-section shapes.

Avantages of the Invention A substantial advantage of the invention is that the same enables an efficient serial production of air-cleaning filters in many different embodiments in order to meet most varying needs and requirements. Thus, filters may be manufac- tured which not only have a varying cross-section shape so as to suit air flow ducts or conduits having different cross-sec- tion shapes, but may also be given a thread closeness which in a simple way may be varied to meet different requirements re- garding particle-separation ability, fall of pressure, service life, etc.

Feasible Modifications of the Invention The invention is not solely restricted to the embodi- ments described above and shown in the drawings. In the pre- ferred embodiments, which are shown in figs 1-4, the body 2 is composed of only profiled strip pieces, which have been united side by side with opposite wave crests in contact with each other. However, it is also feasible to form the thread-carrying body in another way. The profiled strip pieces may, for in- stance, be combined with flat strip pieces, e. g. in such a way that every second strip piece is flat and every second one pro- filed. Between flat strip pieces, triangular or zigzag-shaped strip pieces may be applied. As is pointed out in the introduc- tion, the profile shape of the profiled strip pieces may vary most considerably, provided that the same is generally waved with marked wave crests and wave troughs. It should also be pointed out that the two opposite edges of the strip not neces- sarily have to be straight and parallel as has been shown in the drawings. The lower as well as the upper edge on the strip pieces in the completed filter may, for instance, have toothed or wavelike shape. Furthermore, it is feasible to obliquely bevel, or in another way specially design the edges of the strip, e. g. directly after splicing, but before welding of the threads. The strip does neither necessarily need to be single with the threads attached to one side of the strip. Thus, the threads could be clamped up or in another way be fixed between two strip flanges that are folded to a U-shape in the strip equipping station. It is also possible to apply thread mats on opposite sides of a single strip. Furthermore, it should be pointed out that the strip not necessarily has to be of uniform thickness. Thus, the strip may have a slightly wedge-shaped cross-section and then be somewhat thicker at the top than at the bottom in the completed filter. Instead of composing the thread-carrying body of individual strip pieces side by side, it is also feasible to form, in the final forming station, one single continuous strip piece to a coil laid in a plurality of layers, the shape of which coil is adjusted to the cross- section shape of the air flow duct. Instead of uniting the arriving thread mat to the strip by welding, as is illustrated in fig 5, it is feasible to bring about fixing of the threads

in another way, e. g. by agglutination. It should also be men- tioned that the profiling of the strip may be realized in an- other way than by the fact that the strip is continuously fed between rotatable profiling wheels. Thus, it is feasible to use a conventional, intermittently activated press tool with suit- able profiling members to give the strip the desired profile shape step-by-step.

Throughout in the description above and the accompa- nying drawings, a method and a plant have been accounted for which are based on the intention to unite a separate strip to a separate thread mat, more precisely in the welding arrangement that is designated 25. However, within the scope of the inven- tion, as this is defined in the independent claims, it is also feasible to make a work piece of only threads, i. e. without the supply of any separate strip from a store. This alternative manufacturing method is specially applicable when the threads, as mentioned above, consist of polypropene or another polymer material having a low melting point. Thus, in order to provide such work pieces, the end portions of the threads in a wide- spread thread mat may be permanently inter-united while forming a continuous strip or strip-like material strand, threads being melted together by supply of heat. If the thread mat is formed with a sufficient thickness, it is possible to provide a suffi- ciently thick and rigid strip already by means of one single layer of threads. However, it is also feasible to fold the end portions of the threads overlapping in two or more layers which then are melt together while forming a continuous strip. Fur- thermore, it is also feasible to supply granule-shaped or par- ticle-shaped, meltable material in a strand along one end por- tion of the thread mat and melt or weld together this addi- tional material with the threads so as to form the desired work piece.