Patent Document

BACKGROUND OF THE INVENTION 
     The invention relates to a filtering device for highly viscous media. 
     Such filter devices are known from DE 102 54 022 A1 or DE 103 17 170 A1, for example. In particular, they have either two screen plugs each with a screen cavity or one screen plug having two screen cavities. As a result, it is possible to keep one of the screen cavities in production operation, while the other one is being cleaned. Such a filtering device therefore allows a continuous operation of an installation, in which a highly viscous medium, particularly a plastic melt, has to be cleaned. 
     Although the known filtering devices have proven useful, a partial material degradation can occur, particularly when filtering plastic melts, leading to the partial carbonization of the melt on the clean side of the screen, which causes defects precisely in the case of transparent plastics. These problems occur particularly if larger filter surface areas are required due to small installation throughputs, because the aim is to achieve a particularly fine filtration. 
     These degradation processes are explained by the fact that the medium to be filtered has to be kept hot in the filtering device, in order to keep the viscosity of the medium as low as possible, and thus be able to push the medium through the filter inserts at a relatively low pressure. 
     In the normal uninterrupted throughput at a defined flow rate, the influence of the temperature does not yet have a decomposing effect. However, the above-described defects can occur if, given certain rheological properties of the medium, flow zones form within the flow channels, in which zones the melt remains for a longer time and is exposed to the influence of the temperature. 
     The two known variants of continuous screen changers always require several and also larger flow canals which together have a relatively large volume, which in turn entails a lower flow rates or higher residence times of the medium. 
     Besides the considerations relating to flow technology, the manufacturing costs play a role. As a rule, continuously operating screen changers have two screen plugs each with one screen cavity. For this purpose, larger housings have to be provided, and two fittings between the housing bore and the screen plug have to be established. Moreover, two hydraulic units are required to shift the screen plug. 
     The problem of the present invention therefore is to reduce such residence zones, also referred to as dead water zones, in order to achieve the most consistent possible rinsing through of the entire volume area within the screen cavities, and to achieve a consistent residence time for all the current filaments of a flow through a screen plug. An additional problem is to allow a more cost effective construction of a continuously operated filtering device. 
     SUMMARY OF THE INVENTION 
     This problem is solved according to the present invention by a filtering device with the characteristics of claim  1 . 
     It is essential for the invention that the inlet zone of the outlet channel is arranged off center on the clean side of a screen element in the screen cavity, in particular in an edge region of the narrowing funnel section, which extends in particular entirely up to the periphery of the screen cavity. As a result, a direction of flow is forced on the filtered medium which is such that it prevents turbulence in the melt or other medium in a lateral edge region, and thus the melt or other medium has a longer residence time in that area, in comparison to the central current filaments. 
     “Funnel-shaped,” in the sense of the present invention, denotes not only the shape of a truncated cone, but rather it also includes other narrowing shapes that are suitable for channeling the fluid from a large cross-sectional surface area behind the filter element and lead it to the inlet zone of the channel. 
     Moreover, it is provided according to the invention that the outlet channel has an outlet zone which is designed as a groove that extends axially, and in particular:
         either only outside on the jacket of the particularly cylindrical screen plug   or only in the wall of the screen plug bore   or on both sides, so that partial grooves result, which complement each other in the production position of the screen plug to form a complete groove, and partially overlap in other positions.       

     Between the inlet zone, which is adjacent to the funnel section behind the screen cavity, and the groove-shaped outlet zone, a deflecting point is provided. The inlet and the outlet zones intersect in the area of a deflecting point, and enclose an angle between 30° and 90° there. Due to the forced sharp deflection, the mixing of the medium is further improved. 
     It is preferable to provide that the inlet zone of the screen plug outlet channel is triangular viewed in cross section. The triangular inlet zone is delimited by a first flank which is adjacent to the central tip of the funnel area and leads outward from there. This first flank extends preferably in the extension of the conical wall of the funnel area, so that, viewed in cross section, a seamless transition from the funnel area into the inlet zone of the screen plug outlet channel occurs. The other delimitation of the inlet zone is preferably formed by a second flank which is oriented in the throughflow direction and which is directly adjacent to the outer periphery of the screen element. This flank thus extends—again viewed in the cross section through the screen plug—nearly in the extension of the outer delimitation of the screen cavity. Here, only a slight offset is necessary in order to brace the screen element that has been inserted into the screen cavity. 
     The above descriptions relate to a cross section which is located in the plane of the center axis and at the same time also intersects the housing-side inlet and outlet bores. 
     In a cross-sectional plane that is transverse to the screen plugs and lengthwise through one of the screen cavities, the inlet zone has the appearance of a disk which is placed against the hollow conical funnel area of the screen cavity. 
     The described preferred embodiment of the inlet zone with the inclined flank has the following advantages in terms of flow technology: 
     Due to the offset arrangement of the outlet channel, at least of the area of the outlet channel extending up to the deflecting point, not only does the throughflow through the screen cavity occur in the housing throughflow direction, but a clear lateral swirl is also generated within the screen cavity, which counteracts the development of turbulence. 
     Due to the inclined flank, the area of wall adhesion of the melt is also increased. Indeed, as long as the effect of the wall adhesion on the flowing melt is strong enough, the flow remains laminar. 
     The construction according to the invention not only leads to an advantageous filtering device each with two screen plugs and one screen cavity, but it allows in particular the construction of a filtering device with only one screen plug and two screen cavities. As a result, considerable savings in manufacturing costs can be achieved. The construction with only one plug is made possible, on the one hand, by the groove-shaped outlet zone. Consequently, independently of the axial position of the screen plug, an area of said plug is always in connection with the housing outlet channel, that is to say also when a screen cavity has been removed from the housing for cleaning. 
     In order to be able to produce large filter surface areas, it is advantageous if two screen plugs that can be moved mutually independently in the housing are provided each with two pairs of screen cavities. The screen cavities of a pair are here arranged on facing sides of the screen plugs. In this manner, a filtering device having a total of eight screen cavities is produced. 
     In the filtering device according to the invention, it is also advantageous to use oval and other elongate shapes of filter elements, and accordingly to configure the screen cavities so that they are elongate. The flow guidance according to the invention promotes flow in the longitudinal extension of the screen cavity in the outlet zone. In contrast to the situation in the case of a central outflow from the screen cavity into the housing outlet channels according to the state of the art, in the design according to the invention there is no risk of formation of dead water zones in edge regions, even in screen cavities that extend over a great distance. 
     The flow deflection according to the invention has not only an advantageous effect in the normal operating position, but also precisely in the screen change position, because, in contrast to the known devices which provide several screen plug outlet channels for each screen cavity, of which only one remains operational in the screen change position, and in which the melt remains motionless in the other channel, in the filtering device according to the invention, owing to the fact that the outlet zone is always located before the housing outlet channel, no substantial reduction of the outlet-side flow cross section occurs and there is no negative change in the flow on the clean side of the screen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail below in reference to the drawings. A detailed description of the figures follows: 
         FIG. 1  shows a filtering device according to a first embodiment in cross section in the production position; 
         FIG. 2  shows the filtering device according to  FIG. 1  in a cross-sectional plane offset by 90°; 
         FIG. 3  shows the filtering device according to  FIG. 1  in a screen change position from the top in cross section; 
         FIG. 4  shows the filtering device according to  FIGS. 1-3  in the ventilation position from the top in cross section; 
         FIG. 5  shows a filtering device according to a second embodiment in the production position from the top in cross section; 
         FIG. 6  shows the filtering device according to  FIG. 5  in the backwash position from the top in cross section; 
         FIG. 7 a    shows a filtering device according to a third embodiment in the production position in a side view; 
         FIG. 7 b    shows a filtering device according to the third embodiment in the screen change position in a side view, and 
         FIG. 8  shows the filtering device according to the third embodiment in cross section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a filtering device  100  which consists substantially of a housing  20  and a screen plug  10 . The particularly cylindrical screen plug  10  is movably mounted in a screen plug bore  27  of the housing  20 . 
     On the inlet side of the housing  20 , an inlet opening  23  is provided, which branches into two housing inlet channels  21 ,  22 . Each housing inlet channel  21 ,  22  is in connection with a screen cavity  13 ,  14  in the screen plug  10 , when the screen plug  10  is in the production position depicted in  FIG. 1 . 
     On the other side of the housing  20 , an outlet opening  26  is provided, at which the two housing outlet channels  24 ,  25  join. The housing outlet channels  24 ,  25  each are in connection with the clean side of the screen cavities  13 ,  14 . 
     Within the screen cavities  13 ,  14 , screen elements  13 . 1  are inserted. Behind the screen elements  13 . 1 , viewed in the throughflow direction, the screen cavity  11  in each case narrows to a funnel area  11 . 6 . Whereas in conventional filtering devices, a screen plug outlet channel usually starts directly from the tip of the funnel area  13 . 1 , in the filtering device  100  according to the invention a screen plug outlet channel  11  is provided with an inlet area  11 . 1  which is directly adjacent to the funnel area  11 . 6 . 
     The inlet zone  11 . 1  is delimited by a flank  11 . 4  which starts approximately from the tip of the conical funnel area  11 . 6  and from there extends over some distance toward a second flank  11 . 5 . The second flank  11 . 5  extends approximately in the extension of the lateral delimitation of the screen cavity  11  and thus approximately parallel to the center axis of the screen cavity  11 . 
     At a deflecting point  11 . 3 , the screen plug outlet channel  11  is deflected by an angle of 90° or also less, and transitions into an outlet zone  11 . 2 . 
     In the outlet zone  11 . 2 , the screen plug outlet channel  11  then extends farther as a groove, which is preferably configured approximately in the shape of a keyway, and finally transitions into the housing outlet channel  24 . 
     The same holds for the other screen cavity  12  on the right side of the filtering device  100  with a screen plug outlet channel  12  shown in  FIG. 1 , which also has an inlet zone  12 . 1 , a deflecting point  12 . 3 , and an outlet zone  12 . 2 . 
     In the represented embodiment example, the outer flanks  11 . 5 ,  12 . 5  of the inlet zone  11 . 1 ,  12 . 1  extend in each case parallel to the direction of flow, and the inclined flanks  11 . 4 ,  12 . 4  extend from inside to outside in the represented embodiment example, wherein “inside” is defined as the center line which extends between the screen cavities  13 ,  14 , and thus also, in the production position shown, centrally through the inlet opening  23  and the outlet opening  26 . 
     Whereas in  FIG. 1  the inlet zones  11 . 1 ,  12 . 1  have a triangular appearance, in a transverse cross-sectional plane through the screen plug  10 , as represented in  FIG. 2 , they appear only as a thick disk which is adjacent to the funnel area  11 . 6 . 
     The operation of the filtering device according to the invention is explained below: 
     During normal production operation according to  FIG. 1 , the two screen cavities  13 ,  14  are located before the inlet channels  21 ,  22 . On the outlet side, the openings of the outlet zones  11 . 2 ,  12 . 2  are also located before the openings of the outlet channels  24 ,  25 . The melt is able to flow uniformly through the two screen cavities  13 ,  14 , and the flow distribution indicated by the arrows becomes established. 
     In the case of soiling of a screen cavity—as shown in  FIG. 3 —the screen plug  10  can be withdrawn from the housing  20  until one of the screen cavities  14  is in a position that is freely accessible outside of the housing  20 . There, the screen cavity  14  including the outlet zone  12 . 2  can be cleaned without effort. The screen element  12 . 7  is also freely accessible, so it can be taken out and replaced. 
     In the mean time, the melt, as before, is able to flow through the housing inlet channel  22  into the screen cavity  13 , and from there it can flow off again through the housing outlet channel  25 , so that the production operation does not have to be interrupted. 
     After the screen cavity  12  has been cleaned and optionally provided with a new screen insert, the screen plug  10  is again moved back into the housing  20 , but at first not yet into the production position according to  FIG. 1 , but rather into a position shown in  FIG. 4 , in which a gradual filling of the previously cleaned screen cavity  12  with the melt and simultaneous ventilation of the screen cavity  12  occur. 
     In this position, the screen cavity  14  is in connection with the housing inlet channel  22  only by a very small connection site  14 . 2 , so that only a greatly slowed melt flow into the screen cavity  14  is possible. Consequently, the melt can accumulate in the screen cavity  14  and displace the air contained therein. 
     In the upper area of the screen plug, a ventilation groove  14 . 3  is provided. In fact, said ventilation groove is not located, as indicated here, in the cross-sectional plane through the center axis, but above said plane, on the cylindrical jacket of the screen plug. Therefore, it is indicated in  FIG. 4  only by the dashdotted line. Rising air can flow out of the bore  14 . 4 , and it reaches the groove  14 . 3  which extends outside of the housing edge of the housing  20 , so that air can escape there. 
     After the preflooding of the screen cavity  12  and the ventilation, the screen plug  10  is then moved back into the production position shown in  FIG. 1 . The groove  14 . 3  is then located again entirely within the housing  20 , and it is thus sealed off in the housing bore  27 . 
     In  FIG. 5 , an additional embodiment of a filtering device  100 ′ is represented. The housing design of the housing  20  corresponds to the above-described embodiment. Additional backwash bores  28  are provided only beneath the screen plug  10 ′. 
     The screen plug  10 ′ is also largely similar to the screen plug  10  of the first embodiment. The design, which is essential for the invention, of the screen plug outlet channel with an inlet zone  11 . 1 ′ and an outlet zone  11 . 2 ′ corresponds to that of the first embodiment. 
     The difference is that, besides the screen cavities  13 ′,  14 ′, a backwash opening  15 ′ is provided in each case, which has to be brought to correspond with the backwash bores  28  in the housing  20 . 
     In the position shown in  FIG. 5 , a normal production operation occurs. The two screen cavities  13 ′,  14 ′ are supplied through the housing inlet channels  21 ,  22 . 
     From the production position according to  FIG. 5 , the screen plug  10 ′ can be moved sideways out of the housing  20 , so that one of the screen cavities, in this case the right screen cavity  14 ′, is completely cut off from the housing inlet channels  21 ,  22 . The melt can now flow through the left screen cavity  13  and the screen plug outlet channel  11  thereof into the inlet channel  24 , and then again through the outlet channel  25  into the right screen cavity  14 ′, because there is no longer any pressure applied to the soiled side of the screen cavity  14 ′. 
     Instead, in this position, there is a connection to the backwash bore  15 ′ on the soiled side of the screen cavity  14 ′. The melt can flow through said connection to the backwash bore  28  in the housing, and from there it reaches the outer side of the housing  20 . Coarse soiling particles that adhere to the soiled side of the screen element  13 . 1  can be detached by backwashing in a manner which in itself is known. 
     In the second embodiment of a filtering device  100 ′, the screen plug  10 ′ can be moved even farther out of the housing  20 , so that a screen replacement analogous to the position in  FIG. 3  is possible. Similarly, a ventilation and preflooding of the clean screen cavity is possible analogously to the procedure described in  FIG. 4 . 
     A third embodiment of a filtering device  200  is represented in  FIGS. 7 a , 7 b    and  8 . It comprises two identical screen plugs  210 ,  210 ′, which can be shifted independently of each other via the hydraulic cylinders  231 ,  232  in a housing  220 . 
     Each screen plug  210 ,  210 ′ has, as can be seen in  FIGS. 7 a  and 7 b   , two screen cavities  213 ,  214 ,  213 ′,  214 ′ next to each other and, moreover, as can be seen again in the cross section according to  FIG. 8 , again the same number of screen cavities on the rear side, so that the filtering device  200  has a total of eight screen cavities. 
     In  FIG. 7 a   , the production position is shown in a side view on the filtering device  200 , in which the two screen plugs  210 ,  210 ′ are positioned in such a manner that there is flow through all the screen cavities  213 ,  214 ,  213 ′,  214 ′. 
     The fluid flows through an inlet opening  223  at the top in the housing into housing inlet channels  221 ,  222  that branch off from said housing, into the screen cavities  213 ,  214 ,  213 ′.  214 ′, each of which has at least one filter element that is not shown here. 
     In the flow direction behind the filter elements, in each case—as also in the embodiments according to  FIGS. 1-6 —a funnel area is present, in which the fluid collects, and is led to an inlet zone  211 . 1 ,  212 . 1  of a screen plug outlet channel  211 ,  212  which starts in an edge region of the given screen cavity  213 ,  214 ,  213 ′.  214 ′. Adjacent to this there is in each case, in the screen plug outlet channel  211 ,  212 , a deflecting point  211 . 3 ,  212 . 3 , wherein a transition into the outlet zone  211 . 2 ,  212 . 2  occurs. The latter extends parallel to the center axis of the screen plugs  210 ,  210 ′, until they open into the V-shaped mutually separated housing outlet channels  224 ,  225 . The housing outlet channels  224 ,  225  lead to a common outlet opening  226  at the bottom on the housing  220 . 
       FIG. 7 b    shows a situation in which the upper screen plug  210  is in a so-called screen replacement position. Here, the screen cavity  213  which is visible on the left outside and the associated screen cavity of the same pair are positioned on the rearward side of the screen plug  210  outside of the housing  220 . The screen cavities  213  can be cleaned, and the filter elements contained therein can be removed. The right screen cavity  214 , on the other hand, has a connection to the left housing outlet channel  224  and can thus continue to be fully used. 
       FIG. 8  shows a cross section along the dashdotted line in  FIG. 7 a   , which extends through the inlet and the outlet openings  223 ,  226 . 
     The screen cavities  214 ,  214 ′ which are visible in  FIGS. 7 a  and 7 b    from the side have, on the other side of the screen plugs  210 ,  210 ′, counterpieces in the form of screen cavities  216 ,  216 ′. 
     The broken lines indicate the course of the housing inlet channels  221 ,  221 ′ and the housing outlet channels  225 . In order to be able to supply a total of eight screen cavities of the filtering device  200 , four vertical housing inlet channels  221 ,  221 ′ are provided, which first branch away from the inlet opening  223 , and then lead downward on both sides of the screen plugs  210 ,  210 ′, wherein they tangentially intersect the screen cavities  214 ,  216 ,  214 ′,  216 ′. At the same time, four housing outlet channels  225  are provided, which branch off in the longitudinal direction from an outlet opening  226  (see  FIG. 7 a , 7 b   ) or open into said opening, and which also strut apart height-wise (see  FIG. 8 ) in such a manner that they cut the upper screen plugs  210  at the bottom and the bottom screen plugs  210 ′ at the top. 
     In this representation, one can clearly see the groove-shaped outlet zones  212 . 2 ,  212 . 2 ′, which are in connection with the housing outlet channels  225 .

Technology Category: 7