Abstract:
A filter candle for a precoated filter for filtering and/or stabilizing fluids, especially beer, the filter candle comprising a tubular filter body that is formed from a spirally wound wire, with gaps being provided between the wire windings as fluid passages. To improve the adhesion of the precoated filter in such a filter candle, the surface of the wound wire that rests against the exterior of the filter body is inclined by an angle α relative to the longitudinal axis of the filter candle.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a filter candle for a precoated filter for filtering and/or stabilizing fluids, especially beer, the filter candle comprising a tubular filter body formed from a spirally wound wire, with gaps being provided between the wire windings as fluid passages. 
   BACKGROUND OF THE INVENTION 
   Precoated filters are especially needed in beer brewing to filter out the turbid matter contained in the beer and/or to stabilize the beer. In a known precoated filter, filter candles are used, in which the filter body comprises a wound wire, with gaps between the wire windings serving as fluid passages. The wound wire is either self-supported, or it is held by a carrier communicating with the wound wire. The filter candles of the precoated filter are arranged in an unfiltrate chamber, then passing the filtrate onwards into a filtrate area. For filtration, filtering aids, such as kieselguhr (diatomaceous earth), cellulose, pearlite, activated carbon, plastic fibers, glass fibers, cellulose fibers, or the like, are added to the beer to be filtered. During filtration the filtering aids are precoated on the exterior of the filter body at the beginning of the filtering operation, resulting in the formation of a precoat layer which, in turn, serves as a filtering layer. 
   The adhesion of the precoat layer to the filter body surface is of great importance, especially to process reliability, because a slipping off may lead to a destruction of the filter cake structure and thus also to an inadequate filter quality. The known filter bodies of wound wire, however, do not always yield an optimum adhesion of the filtering aid to the filter body, so that there may be malfunctions. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of the present invention to provide a filter candle for filtering fluids, the filter candle permitting improved adhesion of the precoat layer. 
   According to the invention said object is achieved in that the surface of the wound wire that rests against the exterior of the filter body is inclined by an angle α relative to the longitudinal axis L of the filter candle. 
   Due to the inclination of the surface of the wound wire that rests against the exterior of the filter body, there is an offset of the surfaces of successive windings, so that the precoat layer can adhere in an improved manner. Despite said improved adhesion, there is also an improved back-washing possibility of the precoat layer because the risk of blocking is smaller thanks to the inventive arrangement of the wound wire. 
   The same advantages are also achieved during the separate or simultaneous stabilization of beer with stabilizing agents, such as polyvinylpolypyrrolidone (PVPP) or filtering aids, such as silica gels, because the adhesive power thereof is particularly critical. 
   The angle α is preferably within a range between 1° and 10°, preferably between 3° and 5°. In this range the inclined winding of the wire can be realized in a simple way, entailing a distinct improvement of the adhesion of the precoat layer. 
   Preferably, the wire has a substantially triangular, inwardly tapering cross-section, so that the baseline of the triangle rests on the exterior of the filter body. This yields a gap width that increases from the exterior of the filter body to the interior. 
   Advantageously, the upper edge and the lower edge of the inclined surface of the wound wire are rounded, which may already be due to manufacture. The rounded edges can reduce the risk of blocking even further during back-washing of the filter. 
   In a filter candle developed according to the invention, the lower edge of the inclined surface of a first winding relative to an upper edge of the inclined surface of a wire winding positioned thereunder is offset inwards, the radial offset thereinbetween being in the order of tenths of a mm. Preferably, all lower edges of the inclined surfaces are positioned along a straight line parallel to the longitudinal axis L of the filter body, and all upper edges of the inclined surface along a second straight line parallel to the longitudinal axis L of the filter body. This arrangement makes it possible that the precoat material, such as kieselguhr, can easily deposit between the mutually offset lower and upper edges of the successive inclined surfaces. 
   The present invention also relates to a precoated filter which comprises a filter vessel with an unfiltrate chamber, with filter candles being arranged in the unfiltrate chamber for filtering fluids. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention shall now be explained with reference to the following figures in more detail, in which: 
       FIG. 1  schematically shows a longitudinal section through an embodiment of the filter candle according to the invention; 
       FIG. 1A  shows a section along line A—A of  FIG. 1 ; 
       FIG. 2  shows an enlarged section Z of  FIG. 1 ; 
       FIG. 3  shows an enlarged section Y of  FIG. 2 ; and 
       FIG. 4  is a schematic section through a precoated filter comprising the filter candles according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As becomes apparent from  FIG. 4 , the filter candles according to the invention for filtering fluids, particularly beer, can be suspended in an unfiltrate chamber  15  of a filter vessel  13 . In this instance, the filter candles are secured to a partition wall  12  which subdivides the filter vessel  13  into a filtrate chamber  14  and the unfiltrate chamber  15 . The fluid to be filtered, such as beer, to which kieselguhr has been added as a filtering aid, is introduced via the inlet  30  into the unfiltrate chamber of the filter vessel  13 . From said place it passes through the precoat layer, which has formed before and during filtration by kieselguhr depositing on the exterior surface of the filter candles, and then passes through the fluid inlets  12  ( FIG. 2 ), which are formed by gaps between the wire windings  17 , into the interior of the filter candle and is finally passed into the filtrate chamber  14  from where it then exists via an outlet  16  out of the filter vessel. 
   As becomes apparent from  FIG. 1 , the filter candle  1  in this embodiment comprises a filter body  2  which is formed from a spirally wound wire  17 . The filter body is connected via spacers  4  to an inner body, here an inner tube  3 , which serves to reduce the volume of the interior of the filter body  2 . The wound wire  17  is thus held and supported by the inner tube with the spacers  4 . The inner tube  3 , the spacers  4  and the wound wire  17  form filtrate channels  6  ( FIG. 1   a ) through which the filtrate can flow through the filter candle upwards to the recesses  9  and from there via the head portion  7  into the filtrate area  14 . The filter body  2  is closed by a head portion  7  and an end portion  5 . The invention, however, is not limited to said embodiment; of course, the wire wound according to the invention may also be self-supporting, e.g. due to welded support wires in axial direction, or it may comprise a different carrier construction. 
     FIG. 2  shows, on an enlarged scale, area Z of  FIG. 1 . As follows from said  FIG. 2 , the spirally wound wire  17 , which is here welded to the spacer element  4 , which extends in the form of a rod in the longitudinal direction of the filter body  2 , has a substantially triangular cross-sectional shape. The baseline g of the acute triangle is positioned on the exterior of the filter body  2 . The baseline g has preferably a length between 0.5 to 3 mm. Height h of the acute triangle is from 1 to 5 mm. The surface  20  of the wound wire  17 , which forms the exterior of the filter body  2  (baseline g), is inclined at an angle α relative to the longitudinal axis L of the filter candle. In this embodiment, said angle α is about 3°. The inclined surface  20  may either be achieved by an inclined winding of the wire  17  or by choosing the cross-section of the wire  17  such that the cross-section of the triangle is not an acute isosceles triangle, but an acute non-isosceles triangle, resulting also in the inclined position of the surface. The lower edge  21  of the inclined surface  20  of a first wire winding is offset inwards relative to the upper edge  22  of the inclined surface  20  of a wire winding positioned thereunder. The offset r ( FIG. 3 ) is identical with the distance between a first plane imagined along the inclined surface  20  of a first winding, to a second plane extending along the inclined surface  20  of a successive winding. The radial offset r is between about 0.5 to about 2 mm. Thanks to the offset of the lower edge  21  relative to the upper edge  22 , the upper edge  22  projects to the outside relative to the lower edge positioned thereabove, so that filtering aids can here very easily deposit as a precoat layer and can get fixed and secured against slipping downwards. All of the lower edges  21  of the inclined surface  20  are positioned along a straight line parallel to the longitudinal axis L of the filter body, and all of the upper edges  22  of the inclined surfaces  20  are along a second straight line parallel to the longitudinal axis L. Hence, the filter body  2  has a constant diameter over its whole length L. 
   As follows from  FIG. 3 , the lower edge  21  and the upper edge  2  are rounded. This is of advantage because the risk of blocking is thereby reduced further. 
   The fluid passages are formed by the gaps  11  between the respective wire windings. The gap width between the lower edge  21  of a first wire winding and the surface  23  of the wire of the neighboring winding b 1  facing the lower edge  21  is preferably between 5–8/100 mm when a beer filter is used, the perpendicular of the base g of a winding to the upper surface  23  of the neighboring winding being regarded as distance b 1 , as follows from  FIG. 3 . The gap width increases towards the interior of the filter body  2  due to the triangular cross-section of the wire. 
   In the illustrated embodiment, the upper edge of the wound wire is positioned relative to the filter body axis further to the outside than the lower edge of the wire. The present invention, however, should not be limited thereto because it is also possible, though not explicitly shown, that the lower edge of the wound wire is positioned further to the outside relative to the longitudinal axis L of the filter body  2  in comparison with the upper edge, i.e. the inclination is opposite to the inclination shown in the figures. In this instance, too, the irregular surface permits an improved deposition of the precoat layer, resulting, in addition, in a simplified back-washing.