Abstract:
A device for granulating sludge has a hollow-cylindrical die into which sludge to be granulated is to be injected and in which a stripper with at least one knife is rotatingly arranged. A knife holder holds at least one knife radially movably on the rotating stripper.

Description:
BACKGROUND 
     1. Field of the Invention 
       [0001]    The invention relates to a device for granulating sludge having a hollow-cylindrical die into which sludge to be granulated is to be injected and in which a stripper with at least one knife is rotatingly arranged. The invention further relates to a method for operating a device for granulating sludge, in which method sludge to be granulated is injected into a hollow-cylindrical die and a stripper is rotated in the die. 
         [0002]    The at least one knife can also be referred to as a scraper or stripping disc, which, driven by the stripper, is moved or pushed over the inner surface of the hollow-cylindrical die and thereby, in combination with applied pressure, forces the sludge to be granulated from the inside to the outside through passage openings which are formed in the die. 
       2. Description of the Related Art 
       [0003]    A device and a method of the generic type are known from DE 202 02 626 U1 originating from the applicant. With the device described there, mechanically pre-dewatered sludge is if necessary processed into a free-flowing granulate, in order to be able to dry this granulate subsequently in a rapid as well as energy- and cost-effective manner. A granulation of the sludge before drying brings about an increase in efficiency during the subsequent drying, because the granulate has a relatively large surface to volume ratio. Thus, evaporation of water from the granulate is simplified and the residence time in the subsequent drying is considerably reduced. In addition, a dried granulate is easier to handle, in particular in respect of the packaging and/or the metering as fuel. 
         [0004]    The object on which the invention is based is to provide a device and a method for granulating sludge which, compared with conventional devices and methods, are even more efficient and productive. It is particularly relevant that the sludge to be granulated may have greatly varying compositions and consistencies and also contain foreign substances, such as hair. 
       SUMMARY 
       [0005]    This object is achieved according to the invention by a device for granulating sludge having a hollow-cylindrical die into which sludge to be granulated is to be injected and in which a stripper with at least one knife is rotatingly arranged. According to the invention, the at least one knife is held substantially radially movably on the rotating stripper by means of a knife holder. The knife held radially movably in the stripper according to the invention can, when sweeping over the die and its outlet openings, evade the sludge material which may accumulate at the outlet openings, as investigations according to the invention have shown. Owing to the fact that the knife according to the invention surmounts or sweeps over this banked-up sludge material, it does not block the rotation of the associated stripper and thus ensures the continuous operation thereof. The knife according to the invention is preferably arranged substantially radially directed, which means that its pawl or disc plane extends substantially radially. 
         [0006]    The knife according to the invention mounted evasively in its height position is moved particularly advantageously within the die in alternating circumferential direction, and therefore the associated stripper rotates alternatingly first clockwise and then anticlockwise. Thus, the aforementioned outlet openings in the die are swept over first clockwise and then subsequently anticlockwise. As a result of this, by the stripper according to the invention, with its knives, residues or banked-up sludge material which has accumulated on one side of the respective outlet opening is moved towards the other side into the centre of the outlet opening and then out of the die through the outlet opening. 
         [0007]    In the device according to the invention for granulating sludge, the at least one knife held substantially radially movably on the knife holder may be freely displaceable under its own weight. Such a design is particularly simple and therefore cost-effective and operationally reliable, because then no additional apparatus is required for the pressing of the knife according to the invention onto the inner surface of the die. Alternatively, the knife according to the invention can also be pressed against the inner surface of the hollow-cylindrical die by means of an elastic element, such as a helical spring or a leaf spring. 
         [0008]    The freely displaceable knife may be held on the knife holder in the circumferential direction of the hollow-cylindrical die with a clearance of between 1% to 4% of the inside diameter of the hollow-cylindrical die, preferably between 2% and 3% of the inside diameter of the hollow-cylindrical die. Such a clearance or free space between knife holder and knife allows on the one hand a sufficiently precise guidance of the knife and on the other hand a movement space for the knife which is of such a size that jamming or clogging during the operation of the device according to the invention is safely avoided. 
         [0009]    A plurality of individual knives may be arranged successively on the knife holder in the axial direction of the hollow-cylindrical die. The plural individual knives allow a pointwise evasion or yielding at individual portions of the axial extent of the hollow-cylindrical die. At other regions, however, the knives there remain placed against the die, so that they work there accordingly without residue. 
         [0010]    The plurality of individual knives may be offset in the circumferential direction of the hollow-cylindrical die. With the offset, in the circumferential direction, of the knives arranged axially one behind the other, the stability or rigidity of the associated stripper can be increased. The individual knives do not all sweep over associated passage openings simultaneously, if these openings are advantageously arranged in an axial row, in particular at the downward-facing region of the hollow-cylindrical die. 
         [0011]    The at least one knife that is held substantially radially movably, may have a knife height that is between 95% to 98% of the inside diameter of the hollow-cylindrical die, preferably between 96% and 97% of the inside diameter of the hollow-cylindrical die. With such a design the knives according to the invention can yield, but at the same time circulate substantially the entire sludge material in the hollow-cylindrical die upon rotation of the associated stripper. 
         [0012]    The knife holder may be formed by a tube extending axially in the hollow-cylindrical die. The tube of this kind is cost-effective to produce and can at the same time transmit a high torsional moment, with the result that it is then possible to drive the stripper, rotating according to the invention, at only one of its axial ends. 
         [0013]    The tube may have at least one through opening that extends transverse to the alignment of the at least one substantially radially movably held knife. The at least one through opening is thus situated on the tube in a portion that is comparatively far from the holder of the knives. This is advantageous both with regard to a low-resistance flow through the tube and with regard to a high rigidity of the tube. In order to further improve the flow through the stripper according to the invention, it is furthermore advantageous also to design the at least one knife with through openings in its disc plane. 
         [0014]    The at least one knife held substantially radially movably may be held at its two opposite end regions by the knife holder. The knife is thus held, in relation to the hollow-cylindrical die, therefore at diametrically opposite end regions or driven with respect to its rotational movement. With this support, a high torque can be transmitted to the knife without the risk of tilting or even deformation of the knife. 
         [0015]    The at least one substantially radially movably held knife may be V-shaped in cross-section at least at one end facing the die. Owing to the V-shape the knife advantageously slides over the die and also over banked-up material therein. At the same time, the V-shape forms a kind of cutting edge, by means of which banked-up material can be pushed away particularly strongly. 
         [0016]    The invention further is directed to a method for operating a device for granulating sludge, in particular of the above-mentioned kind, in which method sludge to be granulated is injected into a hollow-cylindrical die, a stripper is rotated in the die, and the stripper is rotated alternatingly with respect to its direction of rotation. Thus, as explained above, the die is swept over first clockwise and then subsequently swept over counterclockwise. As a result of this, by the stripping according to the invention, residues or banked-up sludge material can be pushed away in both circumferential directions. An accumulation of material on only one side of associated outlet openings of the die is avoided reliably. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a perspective view of an exemplary embodiment of a device for granulating sludge according to the invention. 
           [0018]      FIG. 2  is a section taken along line II-II in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In  FIGS. 1 and 2  there is illustrated a device  10  for granulating sludge  12 , in which the sludge  12  is distributed by means of a main supply line  14 , by a plurality of valves  16  connected thereto, into respectively associated individual supply lines  18 . The individual supply lines  18  respectively lead to a place on the longitudinal extent of a supporting tube  20  at which they are fluid-conductingly coupled there by means of respectively one line connection  24 . The supporting tube  20  is mounted stationarily by means of a tube holder  22 . In the tube holder  22  there are embedded, inter alia, viewing windows  26  that allow an operator of the device  10  to look under the supporting tube  20 . 
         [0020]    Situated in the supporting tube  20  is a hollow-cylindrical die  28 , the outside diameter of which is only slightly smaller than the inside diameter of the supporting tube  20 . The hollow-cylindrical die  28  extends substantially over the entire length of the supporting tube  20  and has, in its upper portion in relation to  FIG. 2 , respective inlet openings  30  through which sludge  12  to be granulated can then be led from the individual supply lines  18  and the associated line connections  24  into the interior of the hollow-cylindrical die  28 . Furthermore, at the lower portion of the die  28  in relation to  FIG. 2 , there is situated in the longitudinal direction thereof a row of outlet openings  32 . The sludge  12  can then be led out of the die  28  again through these outlet openings  32 , congruent openings also being situated in the supporting tube  20 , although they are not shown in the present case. 
         [0021]    In the interior of the hollow-cylindrical die  28  there is situated coaxially thereto a stripper  34  which rotates in the die  28  and thereby sweeps over the outlet openings  32  in such a manner that the sludge  12  forced out through these outlet openings  32  is portioned or granulated. 
         [0022]    For this purpose, the stripper  34  is provided with a plurality of plate-shaped scrapers or knives  36  that are held, successively in the longitudinal or axial direction of the stripper  34  and offset relative to one another in the circumferential direction, on a knife holder  38  so as to be movable substantially in the radial direction. The knife holder  38  is formed by a tube  40  that extends likewise coaxially relative to the hollow-cylindrical die  28  and has an outside diameter of about ½ to ⅔ of the inside diameter  44  of the die  28 . Formed in the tube  40  are respectively mutually diametrically opposite longitudinal slots  42 , in which respectively one of the knives  36  is held with a clearance  46  in the circumferential direction of about ½ of the knife thickness or in the present case of between 2% and 3% of the inside diameter of the hollow-cylindrical die  28 . The respective knife  36  is otherwise freely movable in the two associated longitudinal slots  42  of its knife holder  38  and is thus forced downwards in relation to  FIG. 2  by its own weight and in the direction of the outlet openings  32  during the rotation of the stripper  34 . The knives  36 , when sweeping over the associated outlet openings  32 , therefore press radially downwards due to their own weight. The knives  36  accordingly lie either directly on the inner surface of the hollow-cylindrical die  28  at the lower region thereof or they sweep over any sludge material which may have accumulated there at the edges of the outlet openings  32 . The individual knives  36  have respectively a knife height  48  that is less than the inside diameter  44  of the hollow-cylindrical die  28 , namely in the present case between 96% and 97% of the inside diameter  44  of the hollow-cylindrical die  28 . The knives  36  therefore have, with respect to the inside diameter  44  of the hollow-cylindrical die  28 , inside the latter likewise some play or freedom of movement, in order to be able to surmount sludge material that has banked up to a certain extent and not block the rotational movement of the stripper  34 . 
         [0023]    For the rotational or rotary movement of the stripper  34 , a drive  50  is provided at one end of the supporting tube  20 , by means of which drive the stripper  34  can be rotated alternatingly with respect to its direction of rotation. The stripper  34  can therefore be rotated both clockwise and anticlockwise. With this alternating rotation, it is possible to change the stripping direction of the knives  36  over the outlet openings  32 . The outlet openings  32  are therefore, in relation to the sectional view according to  FIG. 2 , swept over first clockwise and then anticlockwise. In this way it is possible for sludge residues that have accumulated on one side of one of the outlet openings  32  to be removed on the return sweep of the associated knife  36 . 
         [0024]    It should further be noted that the tube  40  as well as the knives  36  have through openings  52  and  54 , respectively. The through openings  52  pass through the wall of the tube  40  in its centre transversely to the alignment of the respectively adjacent knife  36 . The through openings  54  also pass through the respective knife  36  centrally transversely to its disc plane  56 . 
         [0025]    The knives  36 , held in such a way with play at their opposite end regions  58  and  60  by means of the tube  40  and its longitudinal slots  42 , are moreover of V-shaped form in cross-section at these end regions  58  and  60 , the two flanks or legs of the V-shape each facing the inner surface of the hollow-cylindrical die  28 . 
         [0026]    In conclusion, it should be noted that all the features that have been mentioned in the application documents and in particular in the dependent claims, despite their formal dependence on one or more specific claims, should also be accorded independent protection individually or in an any arbitrary combination. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  device 
           12  sludge 
           14  main supply line 
           16  valve 
           18  individual supply line 
           20  supporting tube 
           22  tube holder 
           24  line connection 
           26  viewing window 
           28  die 
           30  inlet opening in the die 
           32  outlet opening in the die 
           34  stripper 
           36  knife 
           38  knife holder 
           40  tube 
           42  longitudinal slot 
           44  inside diameter 
           46  clearance 
           48  knife height 
           50  drive 
           52  through opening of the tube 
           54  through opening of the knife 
           56  disc plane of the knife 
           58  end region 
           60  end region