Self-cleaning filter particularly for high-viscosity pasty fluids
BACKGROUND OF THE INVENTION 
     The subject of the present invention is a self-cleaning filter particularly for high-viscosity pasty fluids. 
     High-viscosity pasty fluids, such as for example polymers at their liquefaction temperature, are subjected to filtration during their processing in order to remove impurities or foreign substances contained therein. 
     The presence of a filter in a duct in which a pasty fluid flows usually causes a drop in pressure the value whereof is not constant in time and therefore an amount of product available per unit time is not constant: the occlusion of the openings for the passage of the product, which progressively increases during filtration, in fact alters the flow, and so does the replacement of a dirty filter with a clean one. 
     If the fluid is processed by a machine after filtration, the pressure and flow rate variation in the flow of the fluid almost always causes a non-uniform characteristic of the finished product. 
     SUMMARY OF THE INVENTION 
     The technical aim of the present invention is to obviate the above described disadvantages, i.e. to provide a self-cleaning filter particularly for high viscosity pasty fluids by means of which the filtration conditions remain constant in the course of time. 
     Within the scope of this technical aim, an object of the present invention is to achieve said aim with a simple structure which is relatively easy to execute in practice, safe to use and effective in operation as well as relatively modest in cost. 
     This aim and these objects are achieved by the present self-cleaning filter particularly for high viscosity pasty fluids, characterized in that it comprises a fixed body which has a substantially cylindrical elongated chamber in which a filtration drum is supported, said drum defining a central region, at one end of which there is an inlet for the fluid to be filtered, and an outer annular region, at the other end whereof there is a duct for the removal of the filtered fluid, a shaft being axially supported and rotationally actuated in said chamber, said shaft having a helical crest the profile whereof is suitable for grazing the inner surface of the drum to remove therefrom the impurities which are present and is suitable for pushing the fluid toward a duct for the discharge of the impurities which is arranged at the other end of the chamber, the constant cleaning of the drum being suitable for keeping constant the pressure drop across said filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further peculiarities will become apparent and evident from the detailed description of a preferred but not exclusive embodiment of a filter according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: 
     FIG. 1 is a sectional side view, taken along a diametrical plane, of a self-cleaning filter for pasty fluids according to the invention; 
     FIG. 2 is a partial view of a detail of the filtration drum of the filter according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With particular reference to the above figures, the reference numeral 1 generally indicates the self cleaning filter for high-viscosity pasty fluids according to the invention. 
     The filter 1 is constituted by a fixed body 2 provided with a related closure 3; said body and said closure are mutually coupled by appropriate coupling means, which may be advantageously constituted by longitudinal bolts. 
     A cylindrical chamber 5 having an axis or chamber axis A is defined in the body 2, and at least three peripheral radial spacers 6 center and lock therein a series of rings 7; said rings 7 have a substantially trapezoidal cross section, the top side whereof extends with a portion 7a having a squared cross section which is suitable for fitting snugly in corresponding notches 8 of the spacers: the relative distances between the bases of the rings define narrow passage ports of a filtration drum generally indicated by the reference numeral 9. 
     The drum 9 divides the chamber 5 into a central region 10 extending along said chamber axis and being delimited by said rings and into an annular peripheral region 11 defined between said cylindrical chamber 5 between said rings 7 and said fixed body 2: a truncated-cone extension 12 of the region 10 is defined in the closure 3, is axially provided with a cylindrical seat for a bush 13 and is radially provided with an inlet 14 for feeding the pasty product to be filtered (the light colored arrows indicate the product, the black ones indicate the impurities); the input 14 is advantageously blended into 12 by means of the curved surface 15. 
     A bottom plate 16 is mounted by appropriate mounting means on the bottom of the chamber 5 and is centrally provided with an axial truncated-cone opening 17 which diverges toward the filtration drum 9 and converges toward a reduced-diameter axial extension 18 of the chamber 5; the bottom plate 16 is peripherally provided with a radial dead hole 19 which is aligned with the radial duct 20 defined in the body 2 for the removal of the filtered fluid. The radial hole 19 affects the bottom plate 16 up to the region aligned with the annular region 11 and blends with such profile (the blending being indicated by a broken line). 
     The external extension 18 is connected to a radial impurity discharge duct 21 which can have, at its output, an outer cooling jacket 22 fed with water. 
     A shaft 23, rotationally actuated by means not illustrated in the figure, is supported in the chamber axially with respect to A by means Of the bush 13 and of a sealed thrust bearing 24 not illustrated in detail in the figure. 
     A helical crest or auger 25 extends around and along the shaft 23, and a portion of its profile extends to a point sufficiently close to the inner surface of the filtering drum 9 and the inner surface of the truncated-cone 17 and the inner surface of the axial extension 18; at 17 and 18, to remove therefrom impurities which are present. The pitch of the crest 25 reduces in order to exert a greater thrusting action in the direction of the black arrows. 
     The operation of the filter according to the invention is as follows: by feeding the product through 14 with a preset pressure, said product is filtered through the ports defined between the rings 7 and is guided through the region 11 toward the filtered product output 20. 
     The impurities present in the product stop on the surface of the drum 9 and are progressively removed by the action of the profile of the crest 25; the impurities thus removed are conveyed and pushed toward 21 by the crest profiles which operate inside 17 and inside 18. 
     By cooling the fluid in output with 22 it is possible to slow this outflow of fluid to a greater or smaller extent according to the requirements. 
     It should be noted that by operating with the device according to the invention the filtration surface is always constantly clean from impurities and therefore no variations in the extrusion pressure due to the jamming conditions of the filter arise; the pressure loss due to the filter remains therefore substantially constant. 
     It is evident that the filtration characteristics of the filter vary by varying the distances of the rings 7 and therefore the dimensions of the openings which are defined between the various rings. 
     It has thus been observed that the invention achieves the proposed aim and objects, and in particular that since the filter is constantly kept clean no pressure variations arise due to impurities present thereon. 
     The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept. 
     All the details may furthermore be replaced with other technically equivalent ones. 
     In practice, the materials employed, as well as the shapes and dimensions, may be any according to the requirements without thereby abandoning the scope of the protection of the following claims.