Abstract:
A filtering device for high-viscous fluids having a two pistons being movably disposed in bores is disclosed. Each piston has at least one cavity and at least one filter element. The device also includes a displacing piston for generating fluid pressure and reversing the flow direction at the fluid during a back-flushing mode in an outlet channel or in a reservoir connected therewith.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent App. No. EP 15165817.6, filed Apr. 29, 2015, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a screen changer or a filtering device for high-viscous fluids, having a production mode and a back-flush mode, with the features described in the preamble of claim  1 . Such filtering devices are used, in particular, to clean molten thermoplastic fluid from impurities and agglomerations which can disturb subsequent production stages in plastic processing. 
       BACKGROUND 
       [0003]    Filtering devices -called piston type screen changers are known for filtering high-viscosity media such as plastic melts. The filtering device has a housing and feed channels. Two pistons are arranged movably in the housing. Each of these pistons has at least one cavity. At least one filter element or screen is placed in each cavity. In the production mode of operation, the feed channels in the housing branch off towards the cavities in such a way that melt is guided through the respective filter elements. Provided behind the filter element, when viewed in the direction of flow, are partial channels through which the plastic melt is discharged. The partial channels are combined at some point in the housing. 
         [0004]    The filtering device with two pistons bearing the filter elements or screens and being movable in a direction basically perpendicular to the flow-through direction of the fluid has several advantages. One of these is that the piston is not only a filter carrier but closes and opens flow paths with its movement so that no additional valves are needed. By a suitable design of flow channels the piston can be moved from a production mode of operation into a back-flushing mode of operation. In the back-flushing mode of operation the fluid is led from the outlet side in a reverse flow direction to the screen, in order to detach impurities stuck to the screen and to flush them out of the housing. Furthermore the piston can be partially moved out of the housing in order to give access to a filter element when cleaning or replacement thereof has become necessary. The screen on the other piston is not adversely affected by the back-flushing process. Thus the filtering device remains operational with at least 50% of the screens. 
         [0005]    DE 10 2005 043 096 A1 discloses a filtering device having two piston. Each piston has two filter cavities. While the screen in one cavity is cleaned by back-flushing or is replaced, not only the screens on the other pistons but also the second screen on the same piston remains operational whereby 75% of the total screen area in the filtering device is still used during cleaning or replacement of one screen. 
         [0006]    EP 554 237 A1 discloses a filtering device with two pistons, each piston having at least one cavity. Each cavity having a filter element or screen which can be back-flushed by means of an external unit having a collective reservoir and a hydraulically driven single displacing piston disposed therein. By retracting the displacing piston fluid is collected in the reservoir. By pushing the piston forward the pressure in all downstream flow channels is increased in order to improve the cleaning effect during back-flushing. The drawback of this device is that the increased fluid pressure does not only effect the screens in the cavities of the pistons but also effects other equipment in the other direction of the flow channel such as an extrusion die which might be adversely affected by the increased fluid pressure. 
         [0007]    In EP 1778 379 B1 an improved filtering device is disclosed having an additional displacing piston for each screen cavity which is used for back-flushing. The displacing piston is inserted into a partial outlet channel which leads to one cavity. The displacing piston presses clean fluid, in reverse flow, from the backside through the screen. The effective pressure results from the force of the displacing piston thus being independent from the fluid pressure in the system. 
         [0008]    DE 10 2011 001 262 A1 shows a further improvement of such a filtering device. The displacing piston has an internal channel which, in the production mode, is part of the partial outlet channel extending from an inlet opening in the front face of the displacing piston to an outlet opening at the side wall. Once the displacing piston is retracted the flow path is interrupted and simultaneously located in front of the piston, a reservoir with a larger diameter, is filled with fluid coming through the screen. For back-flushing the displacing piston is moved forward in order to push out the fluid portion stored in the reservoir in a reverse flow direction through the screen. 
         [0009]    The known filtering devices with a separate displacing piston per cavity achieve very good cleaning results without adverse impact on the filtering process, which is still going on through the remaining cavities. However, having many displacing piston units results in high complexity of both mechanical as well as electrical design. 
         [0010]    The problem addressed by the present invention is to improve a filtering device of the aforementioned kind. 
       SUMMARY OF THE INVENTION 
       [0011]    This problem is solved by a filtering device having a displacing piston for boosting fluid pressure in the back-flushing mode in such a manner that a pressure boost is achieved for each screen or filter element to be cleaned. 
         [0012]    The filtering device of the invention requires only one displacing piston as a pressure generator instead of two in a two-cavity filtering device or four in a four cavity filtering device. In comparison with the prior art the filtering device of the present invention is much more compact and less complex. Relating to the four-cavity embodiment, not only are three hydraulic pistons saved but also the wiring, electrical control units, sensors, tubing and pressure pumps needed to drive the hydraulic pistons are saved. This results in a reduction of the weight of the filtering device by about 20% in comparison to a device built in accordance with DE 10 2011 001 262 A1. 
         [0013]    In the prior art, hydraulic pistons had to be arranged on the outside of the housing resulting in a large amount of space being used for positioning the filtering device in a plastics production line. In the present invention, the hydraulic piston is either integrated into a supplementary housing or requires only a minor enlargement of the basic housing for integration so there are no protruding units. 
         [0014]    In spite of the savings to be achieved in weight and space for the filtering device of the present invention as well as the reduced costs of manufacturing the performance of the filtering device is not derogated. 
         [0015]    Below, the invention is explained in greater detail with a view to an advantageous embodiment. The accompanying drawing shows the filtering device in the form of a sectional view, without any hashing but with hidden lines only in order to clearly present the complex arrangement of the inner flow channels arranged inside the housing and partially extending through different planes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The figures of the drawing show: 
           [0017]      FIG. 1  illustrates a filtering device in the production mode in a horizontal view; 
           [0018]      FIG. 2  illustrates the filtering device seen from the left side; 
           [0019]      FIG. 3  illustrates a housing only in a vertical sectional view; 
           [0020]      FIG. 4  illustrates the filtering device in the production mode in a vertical sectional view; 
           [0021]      FIG. 5  illustrates the filtering device in back-flushing mode, in a horizontal sectional view through the plane of the upper piston; 
           [0022]      FIG. 6  illustrates the filtering device in back-flushing mode in a vertical sectional view; 
           [0023]      FIG. 7  illustrates the filtering device in screen changing mode, in a horizontal sectional view; 
           [0024]      FIG. 8  illustrates the filtering device in screen changing mode, in a vertical sectional view; 
           [0025]      FIG. 9A-9C  illustrate the filtering device in different intermediate positions of the upper piston, each in a vertical sectional view; 
           [0026]      FIG. 10  illustrates an enlarged detail of  FIG. 6 ; and 
           [0027]      FIG. 11A, 11B  illustrate the supplementary housing with the displacing piston in different positions, each seen in a sectional view from the left side. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 1  shows a filtering device  100  comprising a housing  10  and two pistons of which a first piston  22  can be seen in the sectional view through the housing. In the view according to  FIG. 1  the second piston  21  is hidden under the piston  22 . In  FIG. 1  as well as in the following sectional views the piston itself is not shown in section. 
         [0029]    The flow direction through the filtering device  100  is illustrated by the arrows. Fluid enters the housing at an inlet opening  11 , which connects to different partial inlet channels  4 . The piston  22 , in the preferred embodiment, has two cavities  23 . 3  and  23 . 4 . However, the piston  22  may have one or more than two cavities. 
         [0030]    One pair of inlet channels  4  leads to each cavity  23 . 3 ,  23 . 4  in the piston  22 . On the inlet side at which the fluid enters into the filtering device additional back-flushing channels  8 . 1 ,  8 . 2  are installed. The back-flushing channels are separated from the inlet opening  11  and from the inlet channels  4  and do not have contact with the cavities  23 . 3 ,  23 . 4  during the production mode which is shown in  FIG. 1 . 
         [0031]    In each of the cavities  23 . 3 ,  23 . 4  at least one screen or filter element is placed in order to retain any contaminants contained in the fluid coming from the inlet side of the screen, which is also designated as the dirty side. The screens are fastened and secured against pressure in reverse flow direction by screen retainer elements. There are also grooves on the top of piston  22 , which are necessary for venting and refilling cavities after screen replacement. All these elements are not shown in  FIG. 1  as they are well known in the prior art. 
         [0032]    On the outlet side, also known as the clean side, the piston  22  has in total four outlet openings  25 . 3 ,  26 . 3 ,  25 . 4 ,  26 . 4 . Two of the outlet openings are behind each screen cavity  23 . 3 ,  23 . 4 . In the production mode, the outlet openings  25 . 3 ,  25 . 4 ,  26 . 3 ,  26 . 4  match with partial outlet channel openings  5 . 3 ,  5 . 4 ,  6 . 3 ,  6 . 4  in the housing  10 . 
         [0033]    As far as described above, the filtering device  100  still corresponds to the basic design of such filtering devices known from the prior art. 
         [0034]    The improvement of the invention is having the supplementary housing unit  13  incorporating a single pressure generator in form of a displacing piston  16  and by having a differentiation between two groups of outlet channels. The supplementary housing unit  13  can be made as an integral part of the housing  10  or as a separate unit detachably mounted to the housing  10 . 
         [0035]    One group of outlet channels is designated as direct outlet channels  3 . 3 ,  3 . 4 . In the production mode their openings  5 . 3 ,  5 . 4  are in flow communication with the direct outlet openings  25 . 3 ,  25 . 4  on the backside of the piston  22 . The direct outlet channels  3 . 3 ,  3 . 4  are directly connected with an outlet opening  12  on the supplementary housing  13  without any interruption. 
         [0036]    In the production mode, the second group of outlet channels is designated as indirect outlet channels  7 . 3 ,  7 . 4 . The openings  6 . 3 ,  6 . 4  of the indirect outlet channels  7 . 3 ,  7 . 4  are in flow or fluid communication with the indirect outlet openings  26 . 1 ,  26 . 2  on the backside of the piston  22 . 
         [0037]    The channel system of the invention is shown in  FIG. 2 . On the right side there is the inlet opening  11 . The partial inlet channels  4  connect the inlet opening  11  to the cavities  23 . 1 ,  23 . 3  inside the pistons  21 ,  22 . From both pistons  21 ,  22  the indirect outlet channels  7 . 1 ,  7 . 3  extend towards a collective reservoir  7 . The reservoir  7  is terminated on the lower side by a lid at the bottom of the supplementary housing  13  and on the upper side by the displacing piston  16 , which is disposed in the collective reservoir  7 . 
         [0038]    Features of the filtering device  100  include the distinction between different groups of outlet channels and the shape, inclination and position of the openings of the outlet channels in the housing and the corresponding openings on the pistons. 
         [0039]    With reference made to  FIG. 3  the shape, inclination and position of the openings is explained in further detail: 
         [0040]      FIG. 3  shows the housing  10  without any pistons in a vertical sectional view. The sectional plane runs through the middle axes of the bores  14 . 1 ,  14 . 2  for the pistons  21 ,  22 . The view is directed onto the inlet opening  11 , from which different inlet partial channels  4  are extending and are opening out into the bores  14 . 1 ,  14 . 2 . 
         [0041]    For each cavity in the pistons there are two inlet channels  4  provided because a certain axial extension of an inlet zone is needed in order to bring the fluid to the screen in a number of axial positions of the piston and to achieve a uniform flow over the whole area of the screen. Instead of two separated openings one elongated hole could be provided to achieve the same hydraulic effect but two separated partial inlet channels  4  leading to separate openings are easier to manufacture by simply drilling the channels from the inlet opening  11 . 
         [0042]    In the rear, one can see the indirect outlet channels  7 . 1 ,  7 . 2 ,  7 . 3 ,  7 . 4 , which lead to the collective reservoir  7 . These channels  7 . 1  . . .  7 . 4  open into the bores  14 . 1 ,  14 . 2  at the outlet openings  6 . 1  . . .  6 . 4 , of which one is related to each cavity. 
         [0043]    As it can be seen in  FIG. 3 , the elongated openings  6 . 1  . . .  6 . 4  are neither extending in longitudinal direction of the bores  14 . 1 ,  14 . 2  nor perpendicular thereto but they are arranged in an oblique position with view to the middle axes of the respective bores  14 . 1 ,  14 . 2 . The openings  6 . 1 ,  6 . 3  on the left side are arranged inversely to the openings on the right side  6 . 2 ,  6 . 4  such that they build a V-type arrangement. The reason for this arrangement is described in further detail with reference to  FIG. 6  but first it has to be noted that the openings  5 . 1 ,  5 . 2 ,  5 . 3 ,  5 . 4  from which the direct outlet channels extend and are arranged in the slightly oblique matter. 
         [0044]    Referring now to  FIG. 4 , the filtering device  100  is shown in the same sectional view in a vertical sectional plane as in  FIG. 3  but here with both pistons  21 ,  22  inserted. For the sake of clarity hashing are omitted. 
         [0045]    The axial position of the pistons  21 ,  22  is such that all openings  6 . 1  . . .  6 . 4 ,  5 . 1  . . .  5 . 4  of the outlet channels match to the respective openings  25 . 1  . . .  25 . 4  and  26 . 1  . . .  26 . 4  on the pistons  21 ,  22 . In the production mode, as illustrated in  FIG. 4 , each cavity is connected with two openings of partial inlet channels  4  and with one opening of a direct outlet channel as well as with one indirect outlet channel. The shape, position and inclination of the openings  25 . 1  . . .  25 . 4  and  26 . 1  . . .  26 . 4  on the piston is the same as of the openings of the channels  6 . 1  . . .  6 . 4 ,  7 . 1 ,  7 . 2  in the housing  10 . 
         [0046]    During the production mode, fluid enters into the housing at the inlet opening  11  and is split into the eight inlet partial channels  4 , two for each cavity. Fluid flows into the cavity and through the screen positioned therein, which is not shown. Having passed the screen in the cavity, a part of the fluid runs to both the direct outlet channels  25 . 1  . . .  25 . 4  and to the indirect outlet channels  26 . 1  . . .  26 . 4 . The portion of the fluid which runs through the direct outlet channel  25  . . .  25 . 4  is lead directly to the outlet opening  12 . The portion of the fluid which leaves the cavity through the indirect outlet openings  26 . 1  . . .  26 . 4  runs through the indirect outlet channels  7 . 1  . . .  7 . 4  into the collective reservoir  7  in the lower part of the supplementary housing  13 . 
         [0047]    The displacing piston  16  is shown in enlarged form in  FIGS. 11A and 11B . In the basic position according to  FIG. 11A , the inner channel  17  opens into the reservoir  7  and matches with an outlet channel  7 . 5  leading directly to the outlet opening  12 . In the production mode, fluid from the reservoir  7  flows through the displacing piston  16  to the outlet opening  12 . 
         [0048]    In the back-flushing mode, the displacing piston  16  moves forward in the reservoir  7 , as illustrated in  FIG. 11B , whereby two functions are achieved. First, the displacing piston  16  raises the pressure of the fluid in the reservoir  7  and the connected direct outlet channels  7 . 1  . . .  7 . 3  and pumps fluid towards the screen which must be cleaned. Second the displacing piston  16  serves as a valve and interrupts the connection between the reservoir  7  and the outlet opening  12 . 
         [0049]    The back flushing mode of the filtering device is shown in  FIGS. 5 and 6  wherein the screen in the upper left cavity is to be cleaned. 
         [0050]      FIG. 5  shows the filtering device in a horizontal sectional view again. In comparison to the production position in  FIG. 1  to  FIG. 4 , the piston  22  has been moved to the left side. By this movement, the connection of the cavity  23 . 3  to the inlet channel  4  is interrupted as well as the flow through the opening  26 . 3  of the left cavity  23 . 3 . However, there is a connection from the rear side between the indirect outlet opening  6 . 3  and the opening  25 . 3  on the piston  21 . In addition, on the clean side of the cavity  23 . 3  there is a connection with the back flushing channel  8 . 3 . In order to clean the screen, which is positioned in cavity  23 . 3 , fluid is pumped from the reservoir  7  by the displacing piston  16  through the indirect outlet channel,  7 . 3  in a reverse flow direction through the outlet opening  6 . 3  into the outlet opening  25 . 3  and finally through the back flushing channel  8 . 3  to the outside of the housing  10 . The flow path of the fluid opening used to clean the filter element or screen in the cavity  23 . 3  is depicted by the dotted line in  FIG. 5 . 
         [0051]    Simultaneously, the filter element or screen in the cavity  23 . 4  remains in the production mode. Fluid is flowing through the filtering device  100  in the normal flow direction as illustrated by the large arrows in  FIG. 5 . The fluid enters the cavity  23 . 4  through two of the partial inlet channels  4  and flows through the screen and through the outlet openings  25 . 4  and  26 . 4 . The fluid then enters into two different direct outlet openings  5 . 3 ,  5 . 4 . However, there is no connection to the indirect outlet opening  6 . 4   
         [0052]    In the filtering device  100  only one displacing piston  16  is needed as a pressure generator for back-flushing any of the four screens, the pressure generated inside the reservoir  7  expands to all branches of the indirect outlet channels  7 . 1 , . . . , 7 . 4 . Thus, pressure generated in the collective reservoir  7  is effective to the right side, too, but the opening  6 . 4  is closed by the piston  22  and the flow can only run through the left cavity  23 . 3  positioned in the back flushing mode. 
         [0053]    As it can be seen in the side view in  FIG. 2  again, there is a connection of the indirect outlet channel system to both levels in which each one of the pistons  21 ,  22  is positioned. When a screen in the upper piston  22  is to be back-flushed, the flow through the indirect outlet channels running through the other piston  21  has to be interrupted because at this time the flow through the indirect outlet channel  7 . 1 ,  7 . 3  goes in reverse flow direction. 
         [0054]    In order not to adversely affect the production flow, which runs in the other direction, the lower piston  21  is moved slightly to the right side according to  FIG. 5 , thereby relocating the indirect outlet openings  26 . 1 ,  26 . 2  on the piston  21  apart from the indirect outlet openings  6 . 1 ,  6 . 2  in the housing  10 . Regardless of what is happening in the system of the indirect outlet channels  7 . 1  . . .  7 . 4  at this stage there will be no adverse effect on the production neither in the second cavity  23 . 4  in the upper piston  22  nor in any of the cavities  23 . 1 ,  23 . 2  in the lower piston  21 . 
         [0055]    In  FIG. 6  it can be seen that the openings  5 . 1 ,  5 . 2  relating to direct outlet channels  7 . 1  . . .  7 . 4  and to the openings  25 . 1 ,  25 . 2  on the piston  22  are partially overlapping. Also, a connection is formed between each cavity  23 . 1 ,  23 . 2  and at least one of the partial inlet channels  4  so that the flow in the production mode direction can still continue in three of the four cavities while one of the filter element or screen in the cavities is being cleaned. 
         [0056]    The interruption of the flow through the indirect outlet channel openings  6 . 1 ,  6 . 2  and the corresponding openings  26 . 1 ,  26 . 2  on the piston is achieved by arranging these openings parallel to each other in their longitudinal extension and by positioning both of them in an oblique direction with view to the middle axis of the piston. So the position according to  FIG. 6  represents a blocking mode which interrupts all connections to the collective reservoir that are not needed for the cleaning process on the respective other piston. 
         [0057]    The enlarged detail of  FIG. 6  in  FIG. 10  shows that not only the opening  26 . 1  on the piston  21  and the corresponding indirect outlet channel opening  6 . 1  are arranged obliquely relative to the middle axis  21 . 1  but that also the opening  25 . 1  and the corresponding opening of the direct outlet channel  5 . 1  are arranged obliquely relative to the middle axis  21 . 1 . 
         [0058]    By arranging the other openings  5 . 1 ,  25 . 1  obliquely in a small angle of less than 30° relative to the middle axis  21 . 1  rather than letting them extend parallel along the middle axis  21 . 1  the opening  5 . 1  is turned away from the opening  26 . 1  in the blocking mode. So the opening  5 . 1  can extend longer than in an arrangement parallel to the axis  21 . 1  which is simulated in  FIG. 10  by the dotted line. 
         [0059]    By this arrangement of the openings  5 . 1 ,  6 . 1  in the housing and the opening  26 . 1  on the piston a movement of the piston  21  for a distance Δx causes a sufficient offset Δd of the edges of the opening  26 . 1  to both adjacent openings  6 . 1 ,  5 . 1  which is required for a reliable interruption of the flow of the fluid. 
         [0060]    Regarding the back-flushing mode only the separation of the cavities  23 . 1 ,  23 . 2  from the indirect flow channels  7 . 1  . . .  7 . 4  could also be achieved by round openings or by elongated holes being arranged perpendicular to the middle axis. The reason to arrange these openings in an oblique direction is given below in the explanation of the filtering device in the screen changing mode. 
         [0061]    The filtering device  100  can also be used in a screen changing mode if a screen cannot be cleaned further by back-flushing so it has to be replaced.  FIGS. 7 and 8  show the filtering device  100  in a screen changing mode, which is entered from the production mode according to  FIGS. 1 to 4  by moving only the upper piston  22  on which a screen has to be replaced. The other piston  21  remains in its original position as shown in  FIGS. 1 to 4  without any relative movement of matching openings  25 . 3 ,  25 . 4  and  26 . 3 ,  26 . 4  on the piston  21  and corresponding openings  5 . 3 ,  5 . 4 ,  6 . 3 ,  6 . 4  in the housing  10 . 
         [0062]    In the screen changing mode no pressure is generated in the indirect outlet channel system. Accordingly, it is not necessary to move the other piston  21  to the blocking position as shown in  FIG. 6  where both cavities  23 . 1 ,  23 . 2  are separated from the indirect outlet channels  7 . 1 ,  7 . 2  in the housing  10 . In comparison with the position in the production mode, as shown in  FIGS. 1 to 4 , the piston  22  in  FIG. 7  has been moved left so that the cavity  23 . 3  which has the screen which must be replaced, is moved out of the housing  10  and is freely accessible for an operator. At the other cavity  23 . 4  in the same piston  22  there is a connection both to the inlet channel  4  and to one outlet channel opening  6 . 3  which is part of the indirect outlet channel system. Consequently, production can be carried on through cavity  23 . 4  while maintenance work is performed on the screen in cavity  23 . 3 . 
         [0063]      FIG. 8  shows again in a vertical sectional view both pistons  21 ,  22 . At the piston  21  all outlet channel openings  25 . 1 ,  25 . 2 ,  26 . 1 ,  26 . 2  completely cover their related openings of the direct and indirect outlet channels in the housing. Also two partial inlet channels  4  open out into each cavity  23 . 1 ,  23 . 2 . The right cavity  23 . 4  on the upper piston  22  is still connected to the most left partial inlet channel  4 . 
         [0064]    Most important to be mentioned in this view are the position and the extension of the indirect outlet opening  26 . 4  on the upper piston&#39;s right cavity  23 . 4  compared to the shape, inclination and position of the indirect outlet opening  5 . 3  in the housing  10 . The opening  26 . 4  and the opening  5 . 3  are each extending longitudinally wherein the axis is arranged obliquely with view to the middle axis. The respective angles of inclination of the opening  26 . 4  and the opening  5 . 3  are arranged inversely thus resulting in a V-Type or X-Type arrangement when they are overlapping. Rather than being arranged parallel and spaced apart like in the blocking mode of piston  21  in  FIG. 6  the overlapping areas are achieved by this arrangement over quite a long axial distance thus maintaining the production mode for the right cavity  23 . 4  during screen changing works to be carried out on the other cavity. In the preferred embodiment of the invention, the flow through the remaining cavity is never interrupted while the piston is moved in order to carry out replacement operations of a screen in one cavity. Accordingly, 75% of the filter area always remains operational during replacement of the screen in one cavity. Thus, like in back-flushing mode, three of four screens are still used in screen changing mode. 
         [0065]    The oblique arrangement of elongated openings on both the pistons  21 ,  22  and in the housing  10  is an feature of the filtering device  100  in the preferred embodiment of the invention comprising two pistons  21 ,  22  which carry two screens each because this arrangement makes it possible to maintain a flow through one cavity on the piston during the whole movement from production mode position to screen changing position. This is further illustrated by  FIGS. 9A to 9C , which each show an intermediate position in which the upper piston  22  has been already moved for a certain distance to the left. 
         [0066]    The openings  6 . 1 , . . . , 6 . 4  of the indirect outlet channels  7 . 1  . . .  7 . 4  and the related openings  26 . 1  . . .  26 . 4  on the pistons  21 ,  22  are arranged obliquely with an angle of more than 45° relative to the middle axis  21 . 1 ,  22 . 2  of the pistons  21 ,  22 . The openings  5 . 1 , . . . , 5 . 4  of the direct outlet channels  5 . 1  . . .  5 . 4  and the related openings  25 . 1  . . .  25 . 4  from the cavities  23 . 1 ,  23 . 2 ,  23 . 3 ,  23 . 4  on the pistons  21 ,  22  are arranged obliquely with an angle of less than 45° in the same orientation as the openings  6 . 1 , . . . , 6 . 4  of the indirect outlet channels  7 . 1  . . .  7 . 4  and the related openings  26 . 1  . . .  26 . 4 . 
         [0067]    In  FIG. 9A , the left cavity  23 . 3  has already been separated from all openings in the housing  10  related to the production mode. The right cavity  23 . 4  is still connected to one inlet channel  4  and to a direct outlet channel opening  5 . 3  via opening  25 . 4 . 
         [0068]    In  FIG. 9B , the piston  21  has been moved further but has not completely been moved out of the housing  10 . On the inlet side the right cavity  23 . 4  is connected to two inlet channels  4  and on the outlet side there are overlapping areas both of opening  26 . 4  and housing opening  5 . 3  and of opening  25 . 4  and opening  6 . 3 . There is also an overlapping area at opening  25 . 4  and opening  5 . 3 . 
         [0069]    By further movement of the piston  22  to the left, a further intermediate position is entered, which is shown in  FIG. 9C . In this position, opening  25 . 4  has no more connection to opening  6 . 3  but still there are overlapping areas at both outlet openings  25 . 4 ,  26 . 4  with housing openings  5 . 3 ,  6 . 3 . There is no position on the way to the screen changing position, in which the flow through right cavity  23 . 4  is totally interrupted. Pressure surges in the filtering device  100  are thereby avoided. 
         [0070]    Of course, it is well understood that due to the symmetrical arrangement of all openings at all cavities  23 . 1 , . . . ,  23 . 4  on both pistons  21 ,  22  as well as of the openings in the housing the back-flushing and screen changing operations described above can also be carried out on the respective right cavity  23 . 4  by a right movement of the piston  22  as well as on both cavities  23 . 1 ,  23 . 2  on the lower piston  21 . The arrangement of all openings and openings on the left is repeated as a mirror image on the right and the arrangement of all openings and openings on the lower piston is repeated as a mirror image on the upper piston. Only the rims needed for venting and filling the cavities have to be arranged on the upper side of each piston.