Abstract:
A backwashable filtering device for thermoplastic plastics material is disclosed. The device can filter thermoplastic plastics material through one or more nests of screens in the filtering position, while submitting some nests of screens to the backwashing process that cleans them. The filtering position and the backwashing position are selectable by displacing one or more carrying bodies with respect to a housing and/or by displacing one or more control bodies with respect to the carrying bodies. The backwashing can be performed over an entire nest of screens or over a portion of a nest of screens. The nests of screens can be straight or curved.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a backwashable filtering device for thermoplastic plastics material, comprising a housing in which at least one feed channel is provided for the material to be filtered and at least one delivery channel is provided for the filtered material, and comprising at least one carrying body which is arranged in the housing, carries at least two nests of screens and is displaceable in the housing between a filtering position and at least one backwashing position associated with a nest of screens, wherein, in the filtering position, distribution chambers arranged on the inflow side of the nests of screens are fluidically connected to at least one feed channel via inflow channels, and collecting chambers arranged on the outflow side of the nests of screens are fluidically connected to at least one delivery channel via outflow channels, and, in the backwashing position of the carrying body, filtered material passes from the collecting chamber of one nest of screens, through a backwashing channel arranged in the carrying body and into the collecting chamber of another nest of screens to be cleaned and, when the outflow channel of this other nest of screens is closed, is dischargeable together with the impurities from this nest of screens into a backwashing outlet channel via the distribution chamber of this nest of screens. This type of construction of a backwashable filtering device is known from EP 540 546 B1 and has proved its worth. 
   SUMMARY OF THE INVENTION 
   Nevertheless, it is an object of the present invention to improve this construction still further by shortening the backwashing paths and improving the conditions when switching between filtration and backwashing. The invention achieves this object in that at least one control body, which is displaceable in the housing relative to the carrying body, is associated with each nest of screens for the backwashing of this nest of screens portion by portion, each of these control bodies forming a discharge channel which, in the backwashing position, is fluidically connected to at least one backwashing outlet channel via at least one control opening. By suitably arranging this control body relative to the position of the backwashing outlet channel of the carrying body, the length of the backwashing outlet channel can be greatly shortened in comparison with the initially described, known construction, which is synonymous with a reduction in the risk of cracking of the plastics material in this outlet channel. At the same time, with suitable displacements of the carrying body and the control body, the advantage is achieved that the backwashing process does not begin when the carrying body is moved into the backwashing position, but only when the control opening of the control body, by appropriate displacement thereof, is brought into alignment with the backwashing outlet channel of the carrying body. This leads to a decrease in wear to the carrying body, which is synonymous with an increase in the service life of the carrying body and with it the entire device. 
   According to a preferred embodiment of the invention, at least two control bodies, which are displaceable independently of one another, are provided for each nest of screens, each control body being associated with a portion of the nest of screens for the backwashing of that portion. Although the nest of screens can also be backwashed portion by portion without this measure, this portion-wise backwashing process can be further improved by the last-mentioned measure and, with appropriate construction, takes place uniformly for all portions of the nest of screens. 
   According to a further development of the invention, each control body is formed by a slider which is displaceable in its longitudinal direction and/or rotatable about its longitudinal axis, wherein the discharge channel extends in the axial direction of the slider and each control opening pierces the wall of the discharge channel. In this case, at least one slider has at least two control openings which, for a displaceable slider, are spaced apart in the longitudinal direction of the slider and, for a rotatable slider, are spaced apart in the circumferential direction of the slider. 
   A particularly simple structure is produced when each control body is formed by a tube which is displaceably and/or rotatably guided in a bore of the housing. The input required for manufacturing the control body is thus reduced to the formation of the control openings, which are easily producible by boring, since commercially available components can be used for the tube of the control body. 
   Particular advantages are produced within the framework of the invention if each backwashing outlet channel is arranged substantially centrally in relation to its associated portion of the nest of screens because, in this way, the amount of cleaned plastics material required for the backwashing process can be reduced and uniform conditions are achievable for all portions of the nest of screens. 
   Within the framework of the invention, there is the further possibility of reducing the size of the distribution chambers. According to the invention, this can be achieved by at least one nest of screens having two curved perforated plates, between which is arranged a screen which is circular when laid flat, wherein the two perforated plates are inserted into a receiving opening in the carrying body, the receiving opening having an oval cross-section corresponding to the curvature of the perforated plates when seen in the axial direction of the receiving opening, and wherein the perforated plates are arranged so that their convex side lies on the inflow side during the filtering process. This produces the aforementioned reduction in the size of the distribution chambers and, at the same time, an increase in the size of the collecting chambers, the latter circumstance having a favorable effect during the backwashing process since the cleaned melt, which is used during the backwashing process, can distribute itself more evenly over the entire surface of the filter portion to be backwashed. At the same time, the possibility of using commercially available, circular screens is retained and rotation of the perforated plates is prevented. 
   Within the framework of the invention, the surface of each nest of screens can be divided into any number of portions which are backwashable individually or in groups. Strict division of the screen surface into individual, separately backwashable portions can easily be achieved if, in the case of at least one nest of screens, the collecting chamber is divided into collecting-chamber portions by at least one transverse wall supporting the screen. These transverse walls simultaneously help to support the screen during the backwashing process, which has a highly beneficial effect, in particular in the case of the aforementioned construction with curved perforated plates. 
   Embodiments of the invention are schematically shown in the drawings. 

   
     SUMMARY OF THE INVENTION 
       FIG. 1  shows the device in the filtering position in section through the axis of the carrying body, although the latter is shown in side view, 
       FIG. 2  shows the device according to  FIG. 1  in section along the line II-II in  FIG. 1 , 
       FIGS. 3 and 4  are sections similar to  FIGS. 1 and 2 , but show the device in the backwashing position for the left-hand half of the nest of screens arranged at the bottom in  FIG. 4 , 
       FIGS. 5 and 6  are sections similar to  FIGS. 3 and 4 , but show the device in the backwashing position for the right-hand half of the nest of screens arranged at the top in  FIG. 6 , 
       FIG. 7  shows the device with a control body formed as a longitudinal slider in the filtering position in section through the axis of the control body, 
       FIG. 8  shows the device according to  FIG. 7  in a position in which the entire nest of screens is being backwashed, 
       FIG. 9  shows the device according to  FIGS. 7 and 8  in a position for backwashing the upper portion of the nest of screens in  FIG. 9 , 
       FIG. 10  shows the device according to  FIGS. 7 to 9  in a position for backwashing the lower portion of the nest of screens in  FIG. 10 , 
       FIG. 11  shows, in a section similar to  FIG. 1 , an embodiment in which the nests of screens have curved perforated plates, and 
       FIG. 12  shows a variant of  FIG. 11 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the embodiment according to  FIGS. 1 to 6 , the device comprises a housing  1  having a feed channel  2  for the thermoplastic plastics material to be filtered and a delivery channel  3  for the filtered material. On the way from the feed channel  2  to the delivery channel  3 , the material flows through a carrying body  4  in the form of a rotational cylinder which is guided in a cylindrical bore  5  of the housing  1  so as to be longitudinally displaceable along its axis  6 . A screen arrangement  7  is embedded in the carrying body  4  and has two nests of screens  8 ,  9 , each of which comprises two perforated plates  12 ,  13 , between which lies a filter insert  33  for filtering the plastics material. In the filtering position of the device ( FIGS. 1 ,  2 ), the material to be filtered is fed to the nests of screens  8 ,  9  by means of inflow channels  10 ,  11  which lead into distribution chambers  14 ,  15  of the carrying body  4 , the distribution chambers  14 ,  15  lying upstream of the nests of screens  8 ,  9 . Downstream of the nests of screens  8 ,  9 , seen in the flow direction of the melt during the filtering process (arrows  20 ), collecting chambers  16 ,  17  for the melt lie in the carrying body  4 , from which spaces the melt flows through outflow channels  18 ,  19  into the delivery channel  3 , which is common to both outflow channels  18 ,  19 . The inflow channels  10 ,  11  of the two nests of screens  8 ,  9  each open on both sides of a central wall  21  which stands perpendicularly against the two perforated plates  12 ,  13  of the respective nest of screens  8 ,  9  and extends from these perforated plates as far as the circumference of the carrying body  4 . Consequently, this wall  21  supports the associated screen against the wall of the bore  5  during backwashing. The two perforated plates  12 ,  13 , together with the filter insert  33  lying between them, are inserted into a receiving opening  32  in the carrying body  4 . The wall thus divides each associated distribution chamber  14 ,  15  into two compartments of substantially equal volume, thereby defining portions  14 ′,  14 ″ and  15 ′,  15 ″ of the respective nest of screens  8 ,  9 . 
   The two collecting chambers  16 ,  17  are separated from one another by a dividing wall  22  extending transversely to the axis of the carrying body  4 . This dividing wall is pierced by at least one central bore or opening, which bore connects the two collecting chambers  15 ,  16  to one another and forms a backwashing channel  23 . 
   The housing  1  also has, for each portion  14 ′,  14 ″,  15 ′,  15 ″ of the nests of screens  8 ,  9 , a control body  31  which, in this embodiment, is formed by a tube forming a slider  34  which is guided so as to be longitudinally displaceable in a bore  35  of the housing  1  in the longitudinal direction of the tube. The hollow space of this tube forms a discharge channel  36  for the impurities released from the backwashed screen during the backwashing process. This discharge channel  36  leads out of the housing  1  and into the open air or into a collecting chamber which collects these impurities. The wall of the tube of the control body  31  is pierced by at least one control opening  37  which, in the backwashing position, is brought into alignment with a backwashing outlet channel  38  of the housing  1 , but in the filtering position is covered by means of corresponding displacement of the control body  31  of the housing  1 . 
   The two outflow channels  18 ,  19  are spaced apart by an amount T ( FIG. 1 ) in the direction of the axis  6 . The distance T is measured so that, in the filtering position shown in  FIGS. 1 and 2 , the mouths of both outflow channels  18 ,  19  are overlapped by the inlet opening of the delivery channel  3  so that the filtered material can flow unimpeded out of the carrying body  4 . This is made easier by the delivery channel  3  having a funnel-shaped widening facing the two channels  18 ,  19 . The same spacing T is found on the inflow side, namely the mouths of the two inflow channels  10 ,  11  are likewise spaced apart in the direction of the axis  6 . In the filtering position ( FIGS. 1 ,  2 ), the mouths of these inflow channels  10 ,  11  in the wall of the carrying body  4  are entirely overlapped by the funnel-shaped widening of the mouth of the feed channel  2 . Therefore, in the filtering position, the melt to be filtered is fed uniformly to all four portions  14 ′,  14 ″,  15 ′,  15 ″ of the two nests of screens  8 ,  9 . This melt flows through the two filter inserts  33 , is thereby cleaned and the cleaned melt flows through the outflow channels  18 ,  19  into the delivery channel  3 . 
   By means of suitable measures, the carrying body  4  cannot be rotated about its axis  6 , but can be displaced along this axis  6  as desired into one of a plurality of backwashing positions. Two of these backwashing positions are shown in  FIGS. 3 ,  4  and  5 ,  6 . In the position shown in  FIGS. 3 and 4 , the left-hand portion  15 ′ of the nest of screens  9  is being cleaned. In the position shown in  FIGS. 5 and 6 , the right-hand portion  14 ″ of the nest of screens  8  is being backwashed. In the backwashing position shown in  FIGS. 3 and 4 , the carrying body  4  has been displaced downwards from the filtering position shown in  FIGS. 1 and 2  until on the one hand the outflow channel  19  and on the other hand the inflow channel  11  are closed by the inner wall  26  of the housing  1 , along which inner wall the carrying body  4  is longitudinally displaceably guided. The plastics melt to be cleaned, which is fed into the feed channel  2  in the direction of the arrow  27 , can therefore only flow from the feed channel  2  into the inflow channel  10 , where it flows in the direction of the arrows  20  into the two portions  14 ′,  14 ″ of the distribution chamber  14 . The melt therein passes through the filter insert  33  of the nest of screens  8 , in so doing is cleaned, and flows in part via the outflow channel  18  into the delivery channel  3  (arrows  39 ), from which it flows in the direction of the arrow  28  to the point of application, e.g. to a screw extruder with connected mold. The other part of the melt flows out of the collecting chamber  16  in the direction of the arrows  40 , through the backwashing channel  23  and into the collecting chamber  17  of the other nest of screens  9 . From this collecting chamber  17 , the melt can pass through the nest of screens  9  only into the left-hand portion  15 ′ of the distribution chamber  15 , since only from there is it possible for the melt to flow any further. For this portion  15 ′, the control opening  37  of the slider  34  forming the control body  31  is fluidically connected to the backwashing outlet channel  38  associated with this screen portion  15 ′. However, the control slider  34  associated with the portion  15 ″ of the nest of screens  9  is in a position in which its control opening  37  is not fluidically connected to the backwashing outlet channel  38  associated with this portion  15 ″. Therefore, only the left-hand portion of the nest of screens  9  is backwashed by the cleaned melt supplied from the backwashing channel  23 . The impurities which are firmly attached to the filter insert  33  in this portion  15 ′ are released by this backwashing melt and carried by the flowing melt out of the respective portion  15 ′ of the distribution chamber  15  in the arrow direction  40  and conveyed via the backwashing outlet channel  38  and the control opening  37  fluidically connected thereto into the discharge channel  36  of the control slider and from there into the open air or into the aforementioned collecting chamber. As is apparent, with this cleaning process restricted to the left-hand half of the nest of screens  9 , the melt supply to the system connected to the delivery channel  3  is maintained and there is only a slight drop in pressure in the delivery channel  3  relative to the normal filtering position ( FIGS. 1 ,  2 ). As only one-half of the nest of screens  9  is backwashed, the pure melt flow supplied via the backwashing channel  23  is concentrated on this left-hand screen half, with the result that there is an increased rate of flow of the melt through this screen half and with it an improved cleaning effect. After the backwashing time has elapsed, the slider  34  is displaced again so that its control opening  37  is closed, whereupon the backwashing process for this screen portion  15 ′ is terminated. If no other screen portion is to be backwashed, the carrying body  4  is moved back into the filtering position ( FIG. 1 ), whereupon filtering of the melt supplied via the feed channel  2  is once more fully implemented. 
   Conditions are analogous if the right-hand half of the nest of screens  9  is to be backwashed. The carrying body  4  is displaced into the backwashing position in which, as described in connection with  FIGS. 3 and 4 , the outflow channel  19  is closed off from the delivery channel  3 , and the inflow channel  11  is closed off from the feed channel  2 . The control slider  34  of the right-hand screen portion  15 ″ is then moved into the backwashing position, whereby its control opening  37  is fluidically connected to the associated backwashing outlet channel  38 . The backwashing material can therefore flow out of that part of the collecting chamber  15  which is associated with the portion  15 ″ of the nest of screens  9 . The backwashing process is terminated as soon as the control slider  34  is displaced so that its control opening  37  is closed. 
   The conditions for backwashing the nest of screens  8  arranged at the top in the drawings are analogous to the above-described backwashing positions according to  FIGS. 3 and 4 . The melt flow for backwashing the right-hand portion  14 ″ of the chamber  14  is indicated by the arrows  40  in  FIG. 6 . 
   As is apparent, the carrying body  4  has to be displaceable into different positions, but does not necessarily have to be displaceable in the longitudinal direction. Instead, it is also entirely possible to achieve the described opening and closing of the individual channels by rotating the carrying body  4  about its axis  6 . The same applies to the control bodies  31 . Instead of longitudinally displaceable sliders, they can also be formed by rotary sliders. A combination of the two variants (displacement and rotation) for the control sliders  31  is even conceivable, in particular when each nest of screens is divided into more than two portions. 
   As is apparent, the nests of screens  8 ,  9  are inserted into their receiving openings  32  in the carrying body  4  so that the screen surfaces are parallel to one another. This forms a clear arrangement which is easy to manufacture and facilitates assembly and disassembly. However, in order to obtain particular flow conditions, it would also be possible to embed the two nests of screens  8 ,  9  in the carrying body  4  in a substantially V-shaped arrangement, just as more than two nests of screens may also be provided. Furthermore, as already mentioned, each nest of screens can also be divided into more than two portions for backwashing, for which purpose only the arrangement of the backwashing outlet channels  38 , like that of the backwashing outlet sliders  34 , has to be selected accordingly. The use of equal-sized portions, into which the distribution chambers  14 ,  15  of the nests of screens are divided, is recommended for reasons of uniformity, in particular with respect to the throughput delivered via the delivery channel, but is not absolutely necessary. 
   The ratio of the melt flow, which flows from the respective collecting chamber into the respective active discharge channel  36  of the control slider  34  during backwashing, to the entire flow of plastics material flowing through the device during the filtering process is substantially determined by the channel cross-sections. This ratio can be influenced by the insertion of different flow restrictors into the channels which are active during the backwashing process, in particular the backwashing channel  23 . An exchangeable throttle element  30 , formed as a throttle nozzle and influencing the flow resistance, is schematically indicated in  FIG. 12 . Optionally, this type of throttle element can be adjustable with respect to its flow cross-section so as to be adaptable to different operating conditions. 
   The filtering position is also shown in  FIG. 7 , in a different view. It can be seen that the carrying body  4  and the control body  31  are each in a position such that two backwashing outlet channels  38  of the housing  1 , which are associated with the nest of screens  9 , are closed by the wall of the slider  34 , i.e. so that the control openings  37  piercing this wall are not in alignment with the backwashing outlet channels  38 . As is apparent, this is irrespective of whether or not a wall  21  is arranged in the distribution chamber  15 . The corresponding displacement of the carrying body and the control body  31  is carried out by means of double-acting hydraulic or pneumatic cylinders  41 ,  42  which are fixed to a frame  43  of the device and the piston rods  44  and  45  of which are connected to the carrying body  4  and the control body  31  respectively, these components thus being displaceable in the direction of the double arrow  46 . 
   In the backwashing position shown in  FIG. 8 , the arrangement is such that the entire surface of the nest of screens  9  is backwashed. For this purpose, the carrying body  4  and the control body  31  are displaced by the cylinders  41  and  42  so that the control openings  37  of the control body  31  are fluidically connected to the backwashing outlet channels  38 , the latter lying so that they are arranged as centrally as possible in relation to the area of the screen to be backwashed. The flow of the melt used during the backwashing process is indicated by the arrows  40 . 
   For backwashing a portion of the nest of screens  9 , there are two possibilities: either the carrying body  4  is displaced so that the backwashing outlet channel  38  to be rendered inactive is covered by the housing  1 , while the other backwashing outlet channel  38  is fluidically connected to one of the two control openings  37 , or, as a second possibility, the control slider  34  is displaced so that only one of the two control openings  37  is fluidically connected to the respective backwashing outlet channel  38  to be rendered active. 
   Whereas  FIGS. 7 and 8  show embodiments in which the control slider  34  has two control openings  37  which are axially spaced from one another,  FIGS. 9 and 10  show an embodiment in which the control slider  34  has only one control opening  37 . This control opening  37  can be fluidically connected to one of the two backwashing outlet channels  38 . In  FIG. 9 , this is shown for backwashing the upper portion of the nest of screens  9 , and in  FIG. 10  for backwashing the lower portion of the nest of screens  9 . 
   Although there is no wall  21  shown here which divides into two portions the distribution chamber  15  of the nest of screens  9  to be backwashed, the nest of screens  9  is nevertheless substantially backwashed in portions. This is because, with a suitable arrangement of the backwashing outlet channel  38  in relation to the screen portion to be backwashed, flow conditions which favor backwashing of the screen portion adjacent to the backwashing outlet channel  38  are produced in the distribution chamber  15 , as indicated by the arrows  40  in  FIGS. 9 and 10 , i.e. for the upper portion of the nest of screens  9  in  FIG. 9  and for the lower portion of the same in  FIG. 10 . 
     FIG. 11  shows an embodiment in which the nests of screens have a curved or bowed form. For this purpose, the two perforated plates  12 ,  13  of the nest of screens are curved and, when inserted into the receiving opening  32 , lie with their convex side facing outwards, i.e. on the inflow side in the filtering state. The filter insert  33  lies between the two perforated plates  12 ,  13  and has a circular circumference when laid flat, with the result that commercially available filter inserts can be used. In order to make this possible, the receiving opening  32  and the two perforated plates  12 ,  13  have an oval circumferential shape corresponding to the selected curvature of the perforated plates  12 ,  13 . The perforated plates  12 ,  13  can be supported by walls  21  against the wall  47  of the bore  5 , in which the carrying body  4  is displaceably guided. As a result, on the one hand the perforated plates  12 ,  13  are supported during the backwashing process, and on the other hand the respective distribution chamber  14 ,  15  is divided into the two portions  14 ′,  14 ″ and  15 ′,  15 ″, as already mentioned. The curved formation of the perforated plates  12 ,  13  enlarges the collecting chamber  16 ,  17  supplying the backwashing melt during the backwashing process and correspondingly reduces the size of the distribution chamber  14 ,  15  lying downstream of the perforated plates  12 ,  13  (seen in the flow direction of the melt) during the backwashing process. On the one hand this promotes the intensity of the backwashing process, and on the other hand reduces the melt volume in the respective distribution chamber  14 ,  15  at the start of the backwashing process. In addition, the oval circumferential shape of the perforated plates secures them against rotation. 
     FIG. 11  shows the backwashing process for that part of the nest of screens  9  which is associated with the right-hand portion  15 ″ of the distribution chamber  15 , wherein only the control opening  37  of the control body  31  lying bottom right is fluidically connected to the backwashing outlet channel  38  associated with the control opening  37 . The control openings of the other control bodies  31  are arranged so that there is no fluidic connection to the respective backwashing outlet channel. 
   The embodiment according to  FIG. 12  differs from that according to  FIG. 11  only in that the wall  21  has been omitted. In order to create a sufficiently large distribution chamber  15  in this case, but still securely hold the perforated plates  12 ,  13  and the filter  33  lying therebetween, the edge of the perforated plate  13  adjacent to the wall of the bore  5  has projections  48  or correspondingly formed surfaces which are supported against this wall. This additional support can also be beneficial when the wall  21  is used. 
   In addition to the difference in relation to  FIG. 11 ,  FIG. 12  shows the backwashing process for the left-hand portion  15 ′ of the distribution chamber  15 . The flow conditions here are similar to those described in connection with  FIGS. 9 and 10 . 
   In all embodiments, the backwashing outlet channels  38  are arranged as centrally as possible in relation to the respective associated portion of the nest of screens. In this way, equally long drainage paths are provided for the dirt particles, together with uniform backwashing of the respective portion of the nest of screens. Furthermore, the amount of backwashing material required is reduced. 
   It is advantageous if the dividing wall  22 , which lies between the two collecting chambers  16 ,  17 , is formed as thinly as possible, in particular in the central portion of the dividing wall  22 . This produces a corresponding enlargement of the cross-sections of the outflow channels  18 ,  19  and, as a result, particularly favorable flow conditions for the plastics melt. Furthermore, the length of the backwashing channel  23  is thereby restricted to a minimum. This backwashing channel  23  therefore only needs to be as long as is necessary to hold the throttle element  30  securely. The short length of the backwashing channel  23  also makes it easier to install the throttle element  30  and gain access to it for cleaning and replacement purposes. Moreover, the short length of the backwashing channel  23  has a favorable effect on the prevention of cracking of the plastics material in the backwashing channel  23 , particularly as melt only flows through this channel during backwashing. 
   The control body  31  does not have to be a slider which is displaceable in its longitudinal direction; it can also be formed as a rotary slider. In this case, a plurality of control openings  37  are distributed over the circumference of the tube of the rotary slider. The principles of a longitudinally displaceable slider and a rotary slider can also be combined; i.e. the tubular slider is both displaceable in its longitudinal direction and rotatable about its axis. This embodiment is suitable in particular when the surface of the nests of screens  8 ,  9  is divided not only in the axial direction of the carrying body  4 , as shown in  FIGS. 1 to 12 , but also transversely thereto, e.g. by (additional) walls  21  lying parallel to the plane of the drawing. However, it must always be ensured that the walls  21  do not significantly impede the inflow, from the inflow channels  10 ,  11 , of the melt to be filtered. 
   Furthermore, the configuration according to the invention is also applicable to devices having two or more supporting bodies  4 . In a device with two supporting bodies each having two nests of screens, and in a construction in which each nest of screens is divided into two portions for backwashing, only an eighth of the screen area remains in the backwashing flow during the backwashing process; i.e. the amount of material used during the backwashing process is concentrated on a small screen area which is backwashed with a higher rate of flow so that good cleaning is achieved. The configuration according to the invention is also applicable to devices in which a single carrying body  4  carries two or more screen-nest pairs, i.e. four or more nests of screens. 
   For the sake of simplicity, the embodiments shown in the drawings are each shown with a single feed channel  2  and a single delivery channel  3 . In practice, it is usually preferable to provide more than one feed channel  2  and delivery channel  3 , in which case it is simply necessary to preserve the aforementioned connection relationships. 
   In the construction according to the invention, the release of dirt particles from the nests of screens can easily be improved by the discharge channels  36  being alternately opened and closed by a periodic reciprocating movement of the control openings  37  in the respective control body  31 , thereby producing an intermittent backwashing process.