Abstract:
A filter unit ( 10 ) and a filter device formed from several such units for filtering a fluid, especially a process fluid such as required for chip-removing machining processes. The filter units ( 10 ) each comprise a frame ( 11 ) and a filter medium ( 16 ). The frame ( 11 ) has support webs ( 12 ), which reinforce the frame ( 11 ) and support the filter medium ( 16 ). The support webs ( 12 ) are arranged on the downstream side ( 18 ) of the filter medium ( 16 ) to prevent clogging on the upstream side ( 17 ) and permit a simple removal of the filter cake. The filter units ( 10 ) are stacked on each other in alternating orientation such that the upstream sides ( 17 ) or the downstream sides ( 18 ) of two adjacent individual filter units ( 10 ) face each other. The frame ( 11 ) forms unfiltered fluid and filtered fluid flow nozzles ( 13 ), ( 14 ) through which the fluid to be filtered or the filtered fluid can flow.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of international patent application no. PCT/EP2006/060252, filed Feb. 24, 2006 designating the United States of America and published in German on Aug. 31, 2006 as WO 2006/089944, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on Federal Republic of Germany patent application no. DE 10 2005 008 924.0, filed Feb. 24, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to filter units and a filter device for filtering a fluid in accordance with the preamble of claims  1  and  9 . The invention further relates to a filtration process in accordance with the preamble of claim  11 .  
         [0003]     Gutman et al., U.S. Pat. No. 5,429,742 (=EP 508,645) discloses a filter composed of stackable filter units for filtering a fluid. The filter units are constructed as stackable disks which have a plastic frame and a filter medium which is molded into the plastic frame. By stacking together a plurality of filter units, an unfiltered channel for the fluid being filtered is formed, which is connected to the upstream sides of the filter units. The filter units furthermore form a filtered channel, which is connected to the downstream side of the filter units. To form a filter, the stacked filter units are provided with end plates, such that the first end plate has an aperture for the unfiltered fluid and the second end plate has an aperture for the filtered fluid. The disclosed filter uses dead-end filtration, that is to say 100% of the introduced unfiltered fluid must pass through the filter units and exit the filter through the filtered channel. Filtered material deposits on the filter elements and a filter cake forms on the filter units, which the fluid being filtered must penetrate. When the filter cake becomes thick enough, the filter becomes obstructed and flow through the filter becomes almost impossible.  
       SUMMARY OF THE INVENTION  
       [0004]     The object of the invention is to provide filter units, a filter device and a filtration process that can be used for reliable, simple and cost-effective filtration of a fluid.  
         [0005]     These and other objects are achieved in accordance with the present invention by providing a filter unit for filtering a fluid, the filter unit comprising a frame and a filter medium mounted on and sealingly connected to the frame, the frame having an outer contour region which extends on both the upstream side and the downstream side of the filter medium, and forming an unfiltered flow nozzle for discharging unfiltered fluid upstream of the filter medium and a filtered flow nozzle for collecting filtered fluid downstream of the filter medium, and the frame further comprising a plurality of support webs disposed on the downstream side of the filter medium for supporting the filter medium, with the filter medium being connected to the support webs.  
         [0006]     In accordance with a further aspect of the invention, the objects are achieved by providing a filter comprising a stack of at least two filter units as described above, the frames of the adjacent filter units sealingly adjoining each other, and the unfiltered fluid flow nozzles of adjacent filter units adjoining each other to form an unfiltered fluid flow channel and filtered fluid flow nozzles of the adjacent filter units adjoining each other to form a filtered fluid flow channel, and the support webs of the adjacent filter units facing and mutually supporting each other.  
         [0007]     In yet another aspect of the invention, the objects are achieved by providing a filter device for filtering a fluid comprising a tank, an unfiltered fluid connection, a filtered fluid connection and a concentrate connection, wherein at least one filter as described above is disposed inside the tank, and the unfiltered fluid flow channel is connected to the unfiltered fluid connection and the concentrate connection, and the filtered fluid flow channel is connected to the filtered fluid connection.  
         [0008]     In accordance with a still further aspect of the invention, the objects are achieved by providing a method of filtering a fluid comprising providing a filter device as described above; introducing unfiltered fluid into the filter unit stack through the unfiltered fluid connection and the unfiltered fluid flow channel into an unfiltered fluid zone arranged between facing upstream sides of the filter medium of two adjacent filter units; passing a portion of fluid through the filter medium of one filter unit to a filtered fluid zone arranged between facing downstream sides of the filter medium of the one filter unit and a third filter unit, whereby the fluid is filtered; discharging filtered fluid from the filtered fluid zone through the filtered fluid flow channel and the filtered fluid connection; and discharging a remaining portion of the unfiltered fluid as a concentrate from the unfiltered fluid zone through unfiltered fluid flow channel and the concentrate connection.  
         [0009]     The filter unit according to the invention comprises a frame and a filter medium, such that the filter medium is sealingly connected to the frame. The filter medium can be a membrane or a fabric, with the pore size or mesh size adapted to the particles to be filtered. Suitable pore sizes for the filtration of fluids can range from approximately 0.3 to 1 μm. Fabrics normally have coarser mesh sizes. A thermoplastic material is preferably used for the filter medium, but other media suitable for filtration, such as metal or ceramics, may also be used. The frame is formed of a thermoplastic material, which may be unreinforced or reinforced with carbon fibers, glass fibers, glass beads or other materials. Advantageous synthetic resin materials include, for example, polyamide, polyethylene or polypropylene. The selection of the frame material should be adapted to the fluid being filtered so that the frame is resistant to the medium being filtered and to the process parameters, such as temperature. The frame has an outer contour region extending on both the downstream side and the upstream side of the filter medium. As used herein, the terms upstream and downstream refer to successive positions along a path of flow and not necessarily to a consistent physical direction. The frame furthermore has an inner contour, which forms an unfiltered flow nozzle, a filtered flow nozzle and support rails or webs. The filter medium is sealingly fixed to the unfiltered flow nozzle on the one hand and to the filtered flow nozzle on the other. The support webs may extend in any direction, e.g., horizontally and/or vertically to the outer contour region of the frame. The number and arrangement of the support webs may be selected as desired. They reinforce the outer contour region of the frame on the one hand and support the filter medium in flow direction on the other, so that the filter medium is protected against damage from excessive pressure forces. To this end, the filter medium is non-detachably fixed to the support webs on the downstream side. The support webs make it possible to realize even large-area filter units of inexpensive materials. Support webs may also be arranged on the upstream side to support the filter medium during backwashing.  
         [0010]     The unfiltered flow nozzle is connected for communication with the upstream side of the filter medium, such that the height H unfiltered  of the unfiltered flow nozzle corresponds to at least the height H contour  of the outer contour region of the frame extending on the downstream side of the filter medium. With this geometric configuration, the fluid to be filtered can reach only the upstream side of the filter medium. The filtered flow nozzle is connected for communication with the downstream side of the filter medium, such that the height h filtered  of the filtered flow nozzle corresponds to at least the height h contour  of the outer contour region of the frame extending on the upstream side of the filter medium. Thus no unfiltered fluid can reach the downstream side of the filter medium.  
         [0011]     If a thermoplastic material is used for both the frame and the filter medium, the filter medium can be inserted into the frame mold and the frame can be injection molded around it. The filter medium then melts in the region of the frame and bonds to the material of the frame to produce a non-detachable connection between the filter medium and the frame.  
         [0012]     According to one advantageous embodiment of the invention, the support webs extend diagonally to the outer contour region of the frame. This gives greater rigidity to the frame and increases its resistance to pressure. Furthermore, flow channels are formed between the support webs, which rest crosswise on each other when a plurality of filter units is stacked. As a result, the filtered fluid must flow along these crossed flow channels. This ensures uniform support of the support webs of a filter unit against the adjacent filter unit.  
         [0013]     Advantageously, the frame in the area of its outer contour region has a sealing contour on its end faces. On the first end face this sealing contour is configured as a concave contour and on the second end face as a convex contour. When a plurality of filter units is stacked together, the convex contour of the one filter unit therefore engages the concave contour of the adjoining filter unit. This creates a fluid-tight connection between the adjoining filter units, so as to avoid the need for additional sealing members. Because of the fluid tight joining of the filter units, the outer contour of the frame further forms a pressure-tight housing, within which the fluid flows.  
         [0014]     In another advantageous embodiment of the invention, the frame has a centering contour on its end faces in the area of its outer contour region. This centering contour causes the adjoining filter units to be lined up exactly so as to prevent slippage and the formation of leakage points. This centering contour may be configured as a tongue-and-groove geometry extending linearly on the end face of the outer contour region. It is also conceivable, however, to arrange centering contours only over partial areas, which may be distributed around the circumference of the outer contour.  
         [0015]     In a particularly advantageous embodiment, the centering contour and the sealing contour are combined in a common contour. This simplifies the geometry of the frame and makes it more cost effective to manufacture.  
         [0016]     In a further development of the invention, the height H unfiltered  of the unfiltered flow nozzle is greater than the height H contour  of the outer contour region. This makes it possible for adjoining unfiltered flow nozzles to engage so as to produce a better joint between the unfiltered flow nozzles. In this case, the adjoining unfiltered flow nozzles can, in particular, be pressed together. In one advantageous embodiment, the unfiltered flow nozzle has a sealing shoulder disposed particularly in the region protruding over the height H contour . In another embodiment of the unfiltered flow nozzle a circumferential projection may be formed which engages the unfiltered flow nozzle of the adjoining filter unit. This ensures a reliable seal of the unfiltered flow nozzles in relation to each other. The projection may be disposed either on the female part or on the male part.  
         [0017]     The foregoing explanations regarding the configuration of the unfiltered flow nozzle apply analogously to the configuration of the filtered flow nozzle, such that the sealing geometry of the filtered flow nozzle corresponds to the sealing shoulder of the unfiltered flow nozzle.  
         [0018]     The stack of filter units according to the invention has at least two filter units as described above. The frames of the individual filter units are sealingly connected to each other so as to form a volume within the frame. The unfiltered flow nozzles of the adjoining filter units contact each other to form an unfiltered fluid channel. The same is true for the filtered flow nozzles, which form a filtered fluid channel. The unfiltered fluid channel has openings communicating with the upstream side of the filter medium. The fluid being filtered flows through these openings to the filter medium. The filtered fluid channel has openings connecting the downstream side of the filter medium to the filtered fluid channel. Thus the filtered fluid can pass through these openings and reach the filtered fluid channel. The filter units have support webs on the upstream side of the filter media. The support webs of adjacent filter units contact each other, such that the support webs of the filter units rest on each other, thereby improving the stability of the filter unit stack. If the support webs extend diagonally, the support webs of the adjacent filter units extend crosswise to each other, so that the support webs contact each other only at the crossing points. Since the crossing points are distributed over the entire area, high stability is achieved. A flow of the filtered fluid toward the filtered fluid channel is furthermore ensured.  
         [0019]     The unfiltered and filtered flow nozzles may each be integrally formed with the frame. The height of the nozzle is may be selected as desired, provided however, that the unfiltered flow nozzle of the first filter unit contacts the unfiltered flow nozzle of the second filter unit. To this end, the unfiltered flow nozzle may also be formed by an additional element that is detachably or non-detachably connected to the frame. An additional element may, for example, be bonded, welded or screwed on so as to produce a tight joint. What has been said about the unfiltered flow nozzle applies analogously to the filtered flow nozzle.  
         [0020]     The filter device according to the invention is used to filter a fluid contaminated with particles. Such fluids may be gases or liquids, particularly coolants used in chip-removing machining processes producing small chip sizes. Small chip sizes ranging from approximately 1-10 μm may for example be created in grinding, honing or lapping operations. The filter device has a tank with an unfiltered fluid connection, a filtered fluid connection and a concentrate connection. Filter unit stacks of at least two filter units are formed of the above-described filter units and are inserted into the tank of the filter device. The filter unit stack is connected with the connections of the tank, so that the fluid being filtered can flow into the filter unit stack through the unfiltered fluid connection. The filtered fluid exits through the filtered fluid connection. The unfiltered concentrated fluid flows out of the filter device through the concentrate connection.  
         [0021]     According to another further embodiment of the filter device, the filter unit stack is clamped with clamping members, particularly more than three, such that the clamping members are fixed in a distributor plate on the one hand and in a clamping plate on the other. This makes it possible to clamp the filter units into a filter unit stack. Clamping prevents leakage flows. Higher pressures can at the same time act within the filter unit stack to push the fluid through the filter medium or to suck the filtered fluid out of the filter unit stack.  
         [0022]     The method for filtering a fluid according to the invention is carried out with one of the above-described filter devices. The fluid being filtered is introduced into the filter unit stack through the unfiltered fluid connection. The fluid flows through the unfiltered flow nozzle to the unfiltered side, which is disposed between the upstream sides of the filter medium. The fluid flows through the filter medium and thus reaches the filtered side disposed between the downstream sides of the filter units. The filtered fluid flows through the filtered flow nozzle to the filtered fluid connection and then exits the filter device. The concentrate flows through the unfiltered flow nozzles until it exits the filter device through the concentrate nozzle. The concentrate may be fed back into the unfiltered fluid via a recirculation line, such that an additional filter device may optionally be provided, so that the concentrate flows in a loop through the filter device. As an alternative, the concentrate may circulate through the filter device in a loop until no further filtration is possible. The heavily contaminated concentrate is then disposed of before new fluid is fed into the filter device for filtration.  
         [0023]     According to another advantageous embodiment of the method for filtering a fluid, the filter units may be cleaned through a backwashing process. The filter cake which forms on the upstream side of the filter medium is thereby separated and removed from the filter unit stack, so that the capacity of the filter units is restored. A portion of the filtered fluid or some other rinsing fluid may be used for this backwashing process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The invention will be described in further detail hereinafter with reference to illustrative preferred embodiments shown in the accompanying drawing figures, in which:  
         [0025]      FIG. 1  shows a filter unit according to the invention;  
         [0026]      FIG. 2  is a sectional view of a filter device comprised of a stack of filter units according to the invention;  
         [0027]      FIG. 3  is a view of a twin filter unit;  
         [0028]      FIG. 4  is a sectional view of the twin filter unit of  FIG. 3  taken along line A-A;  
         [0029]      FIG. 5  is a sectional view of the twin filter unit of  FIG. 3  taken along line B-B;  
         [0030]      FIG. 6  is a sectional view through a stack of filter units taken along line A-A;  
         [0031]      FIG. 7  is a sectional view through a stack of filter units taken along line B-B;  
         [0032]      FIG. 8  is a perspective view of a filter unit according to the invention;  
         [0033]      FIG. 9  is a detail view Z of a filter unit according to  FIG. 8 ;  
         [0034]      FIG. 10  is a perspective view of a filter unit module;  
         [0035]      FIG. 11  is a schematic diagram of a filtration system;  
         [0036]      FIG. 12  is a flow diagram showing flow through a filter according to the invention, and  
         [0037]      FIG. 13  is an alternative flow diagram. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]      FIG. 1  is a schematic representation of a filter unit  10 . The filter unit  10  comprises a frame  11  with a circumferential outer contour region  15  and support rails or webs  12 . The support webs  12  extend diagonally within the outer contour region  15 . The support webs  12  are spaced apart from each other such that the largest possible flow area is available for the fluid being filtered while sufficient stability is produced for the outer contour region  15 . The frame  11  further has an unfiltered flow nozzle  13  and a filtered flow nozzle  14 . A flat filter medium  16 , which is shown in part, is disposed within the outer contour region  15 . The filter medium  16  is molded into the outer contour region  15 . The filter medium  16  is furthermore joined to the support webs  12  and the unfiltered flow nozzle and the filtered flow nozzle  13 ,  14 . Within the unfiltered flow nozzle and the filtered flow nozzle  13 ,  14 , the filter medium  16  has a cut out opening.  
         [0039]      FIG. 2  is a sectional view of stacked filter units  10  depicted in  FIG. 1 . Parts corresponding to those of  FIG. 1  are identified by the same reference numerals. The filter medium  16  has an upstream side  17  and a downstream side  18 . The upstream side  17  admits the fluid to be filtered and is therefore in communication with the unfiltered flow nozzle  13 . After the fluid has passed through the filter medium  16 , the fluid flows from the downstream side  18  to the filtered flow nozzle  14 . The individual filter units  10  contact each other so as to form a seal. The filter units  10  are arranged in such a way that the unfiltered flow nozzles and the filtered flow nozzles  13 ,  14  contact each other respectively to form a seal, such that the upstream sides  17  of two adjacent filter units  10  or the downstream sides  18  of the adjacent filter units  10  lie opposite each other. The unfiltered flow nozzle  13  has a height H unfiltered , which corresponds to the height H contour  of the outer contour region  15 . The height H contour  extends from the filter medium  16  in the same direction as the height H unfiltered  of the unfiltered flow nozzle  13 . Similarly, the filtered flow nozzle  14  has a height h filtered , which equals the height h contour  of the outer contour region  15 . The sum of the heights H contour  and h contour  together with the thickness of the filter medium equal the total height of the outer contour region  15 . The filter units  10  form a filter unit stack  19 , which can have any number of filter units  10 . The filter unit stack  19  is terminated by a distributor plate  20 . This distributor plate  20  has a concentrate outlet  21  and a filtered fluid outlet  22 . To connect the filtered spaces  26  between the support webs  12  to the filtered fluid outlet  22 , the support webs  12  can have apertures (not shown). As an alternative, indentations may be formed in the distributor plate  20 . The concentrate outlet  21  is in communication with the unfiltered flow nozzle(s)  13 , so that the concentrated fluid can be removed from the filter unit stack  19 . The filtered fluid connection  22  is connected to the filtered flow nozzle(s)  14 , so that the filtered fluid can flow out of the filter unit stack  19 . Opposite the distributor plate  20  is a connection plate  23 , which has an unfiltered fluid inlet  24 . The unfiltered fluid inlet  24  is connected to the unfiltered flow nozzle(s)  13 . To connect the filtered spaces  26  to the filtered fluid outlet  22 , apertures may be provided in the support webs or indentations in the connection plate  23 , as described above.  
         [0040]     The fluid being filtered flows in the direction of arrow A through the unfiltered fluid inlet  24  into the unfiltered flow nozzle  13  and thus into the filter unit stack  19 . Between the upstream sides  17  facing each other of the filter medium  16 , an unfiltered space  25  is formed into which the fluid being filtered flows. The unfiltered flow nozzles  13  distribute the fluid being filtered in all the unfiltered spaces  25  of the filter unit stack  19 . The fluid flows into contact with the filter medium  16 , such that the fluid to be filtered passes through the filter medium  16  in the direction of arrow B and collects in the filtered space  26  between the downstream sides  18  of the filter media  16  and then flows along the support webs  12  to the filtered flow nozzle  14 . The filtered fluid, which may be as little as approximately 20% or less of the fluid introduced, exits the filter unit stack  19  through the filtered fluid outlet  22  in the direction of arrow C. The concentrated fluid, which may be as much as approximately 80% or more of the introduced fluid, flows in the direction of arrow D out of the filter unit stack  19  through the concentrate outlet  21 .  
         [0041]      FIG. 3  shows a twin filter unit  10 ′. Once again, parts corresponding to those of  FIG. 1  are identified by the same reference numerals. The twin filter unit  10 ′ comprises two separate filter units  10  connected by webs  27 . The interconnected filter units  10  are precisely positioned in relation to each other by the webs  27 . Furthermore, a single mold can be used to produce twice the number of filter units  10 . Another advantage of this embodiment is that the outer contour regions  15  have matching end faces, which fit precisely on top of each other during assembly. The geometry of the end faces of the outer contour regions  15  will be described in greater detail with reference to  FIGS. 4 and 5 . In the area of the unfiltered flow nozzles  13  the outer contour region  15  is funnel-shaped. This embodiment prevents contaminants from accumulating in the corner areas, which receive less flow. In the area of the filtered flow nozzle  14  the outer contour region  15  does not need to be funnel-shaped since there is little contaminant loading in this area.  
         [0042]      FIG. 4  shows a section of the twin filter unit  10 ′ of  FIG. 3  taken along line A-A. Again, parts corresponding to those of  FIG. 3  are identified by the same reference numerals. The outer contour region  15  has end-face sealing contours  28 . In the upper filter unit  10 ′, the sealing contour  28  is convex on the side facing the support webs  12 . In the lower filter unit  10 ′ the side facing the support webs  12  is concave. The opposite sealing contour  28 , which is disposed on the upstream side  17 , has a negative contour to the sealing contour  28  associated with the support webs  12 . Thus a concave sealing contour  28  is arranged opposite an end face that has a convex sealing contour  28 . The upper filter unit  10 ′ has the oppositely oriented sealing contour  28  relative to the lower filter unit  10 ′. The twin filter units  10 ′ can therefore be stacked on each other in alternate direction such that the upstream sides  17  and the downstream sides  18  are facing each other respectively. Thus a single mold is sufficient to produce identical twin filter units  10 ′, which can be assembled into a filter unit stack  19  as shown in  FIG. 2 . The filtered flow nozzle  14  is configured in such a way that a nozzle area  29  is provided which protrudes above the height of the frame  11 . The filtered flow nozzle  14  further has a seating area  30  in which the nozzle area  29  of the adjacent filter unit  10  engages to produce a tight connection (see  FIG. 6 ).  
         [0043]      FIG. 5  is a section of the twin filter unit  10 ′ of  FIG. 3  taken along line B-B. Again, parts corresponding to those of  FIGS. 3 and 4  are identified by the same reference numerals. Since the sealing contour  28  extends uniformly on an end face, the explanations given above with reference to  FIG. 4  apply likewise to  FIG. 5 . The unfiltered flow nozzle  13  has geometries for sealing purposes similar to those of the filtered flow nozzle  14 .  
         [0044]      FIG. 6  is a partial section of stacked filter units  10  of  FIG. 3  taken along line A-A. As already explained with reference to  FIG. 5 , the nozzle areas  29  engage in the seating areas  30 , thereby producing a filtered fluid channel  31 , which is sealingly separated from the unfiltered fluid space  25 . The filtered fluid space  26  is formed by clearances between the support webs  12  and is connected to the filtered fluid channel  31 . The sealing contours  28  of the frame  11  engage each other, such that the concave and the convex regions, respectively, are configured so that the flat regions of the frame  11  are first superimposed on each other and the curved regions are initially only used for centering. This has the advantage that the flat regions are first pressed together to form a seal before the curved regions contact each other in a planar manner. As a result smaller clamping forces are sufficient to assure a tight connection between stacked members.  
         [0045]      FIG. 7  is a partial sectional view of the stacked filter units  10  shown in  FIG. 3  taken along line B-B. Parts corresponding to those of  FIG. 5  are again identified by the same reference numerals. As the filter units  10  are stacked, the unfiltered flow nozzles  13  form the unfiltered fluid channel  32 , which is connected to the unfiltered fluid space  25 . The filtered fluid space  26  is separated from the unfiltered fluid space  25  by the filter medium  16 .  
         [0046]      FIG. 8  shows a filter unit  10 ″. Parts corresponding to those of  FIG. 3  are again identified by the same reference numerals. The filter unit  10 ″ comprises a frame  11  with an outer contour region  15  along which centering contours  33  are arranged on the end-face side. The centering contours  33  are formed as holes  34  and pins  35 , as depicted in detail Z shown in  FIG. 9 . The holes  34  and the pins  35  are distributed around the end face, with the pins  35  registering with and engaging in the holes  34  of the adjacent filter unit  10 ″ to position the filter units on each other. The number and distribution of the centering contours  33  depends on the size and geometry of the filter unit  10 ″. The filter unit  10 ″ further comprises support rails or webs  36 , which are arranged on both sides of the filter medium  16 . As a result the filter medium  16  is also supported on the upstream side, so that the filter unit  10 ″ can be backwashed without excessively stressing the filter medium  16 .  
         [0047]      FIG. 10  shows a filter unit module  37 . The filter unit module  37  has four filter unit stacks  19 . In the drawing, one filter unit stack  19  at the right hand side is indicated only by a single filter unit  10 . The filter unit stacks  19  are sealingly mounted between a distributor plate  38  and clamping plates  39 . The individual filter units  10  are tightly clamped by clamping members  40 , which are secured to the distributor plate  38  on the one hand and to the clamping plates  39  on the other. The clamping plates  39  are configured separately for each filter unit stack  19 , so that tolerances in the filter units  10  can be compensated, and each filter unit stack  19  can be separately clamped with a predetermined clamping force.  
         [0048]      FIG. 11  is a schematic diagram of a filtration system using filter units according to the invention. The filtration system comprises a tank  41 , in which a filter unit module  37  with four filter unit stacks  19  as depicted in  FIG. 10  is disposed. The filter unit module  37  has an unfiltered inlet  42 , which can be opened and closed by a valve  43 . The unfiltered inlet  42  is connected to the unfiltered fluid channel  32  of each filter unit stack  19 . The filtered fluid channels  31  of the filter unit stack  19  are connected to a filtered fluid line  44 , such that each filter unit stack  19  can be opened and closed by a separate valve  45 . The filtered fluid line  44  opens into a backwash reservoir  46  on the one hand and into a filtered fluid tank  47  on the other. The filtered fluid tank  47  can be closed off from the filtered fluid line  44  by a valve  48 .  
         [0049]     The unfiltered inlet  42  is connected to an unfiltered fluid tank  49 . A pump  50  is provided to force the fluid to be filtered into the filter unit module  37 . The unfiltered inlet  42  is furthermore connected to a concentrate line  51 , which opens into a container  52 . The concentrate line  51  can be opened and closed by a valve  53 .  
         [0050]     The tank  41  has a discharge line  54 , which can be opened and closed by a valve  55 . The discharge line  54  is connected to the unfiltered fluid tank  49 , so that the fluid collected in the tank  41  can be returned to the unfiltered fluid tank  49 .  
         [0051]     The fluid to be filtered is located in the unfiltered fluid tank  49 . From there it is pumped into the filter unit module  37  through the unfiltered inlet  42  when the valve  43  is open. In this state, the valve  53  is closed. At least one of the valves  45  is open, so that the filtered fluid can exit the filter unit module  37  through the filtered fluid line  44 . The filtered fluid flows into the backwash reservoir  46  or, if valve  48  is open, into the filtered fluid tank  47  from where it can be supplied for further use. If one or more of the filter unit stacks  19  is dirty, the valve  48  is closed and the respective valve  45  is opened so that the filter unit stack  19  can be backwashed. For this purpose, valve  43  is also closed and valve  53  in the concentrate line  51  is opened. The dirty backwash fluid is collected in the container  52 . When the fluid contained in the filter unit module  37  cannot be filtered any further, the fluid is removed from the filter unit module  37  via the concentrate line  51 . When the container  52  is full, it is replaced or emptied. The concentrate is then disposed of. Since the filter units  10  (according to the figures described above) are only clamped together, minor leaks may occur. Any leaked fluid is then collected in the tank  41  and is removed from the tank  41  through the discharge line  54 .  
         [0052]     The filter unit stacks  19  may selectively receive flow and be backwashed by operation of the appropriate valves  45 .  
         [0053]     The flow path through the filter device is schematically illustrated in the exploded view of a filter unit module  37  of  FIG. 12 . The unfiltered fluid inlet  24  is disposed in the clamping plate  39 . The filtered fluid outlet  22  and the concentrate outlet  21  are disposed in the distributor plate  38 . The filter units  10  are depicted schematically and correspond to the filter units  10  described with reference to the preceding figures. The fluid to be filtered flows through the unfiltered fluid inlet  24  into the filter unit module  37  (thick arrow). Between the clamping plate  39  and the first filter unit  10 , the fluid flows downwardly to the unfiltered flow nozzle  13 . In the region of the filter medium  16 , previously filtered fluid passes through and flows through the filtered flow nozzle  14  in the second filter unit  10  toward the filtered fluid outlet  22  (thin arrow). The introduced unfiltered fluid flows through the unfiltered flow nozzle  13  into the next unfiltered space (thick arrow). The fluid flows through the individual unfiltered spaces until the fluid is discharged as a concentrate from the filter unit module  37  through the concentrate outlet  21 . Thus, the concentrate stream is guided across all the filter units  10 . The filtered fluid flows (along the thin arrows) through the filtered spaces to the filtered flow nozzles  14  and then exits the filter unit module  37  through the filtered fluid outlet  22 .  
         [0054]      FIG. 13  shows an alternative flow path to the flow path depicted in  FIG. 12 . Parts corresponding to those shown in  FIG. 12  are identified by the same reference numerals. The difference compared to the flow path described in  FIG. 12  is that each filter unit  10  has two unfiltered flow nozzles  13 . As a result, the concentrate stream no longer needs to be guided across all filter units  10 , which is advantageous particularly for the backwashing of the filter units.  
         [0055]     The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations within the scope of the appended claims and equivalents thereof.