Abstract:
A worm metering apparatus which includes a metering tube having an end, the end has an annular barrier element, made of sintered metal, that communicates with a negative-pressure source and an overpressure source gas source. A pressure meter is also provided, which measures the negative pressure that prevails in a line to the barrier element. To prevent the barrier element from becoming clogged with bulk material particles, the overpressure gas is conducted to the barrier element during the operation of the worm metering apparatus, in order to detach adhering particles of bulk material.

Description:
PRIOR ART 
     The invention relates to a metering apparatus for pourable bulk material, of the kind known for instance from German Patent Disclosure DE 39 15 144A1. The known metering apparatus, embodied as a worm metering apparatus, has an annular barrier element, preferably of sintered metal, on its metering tube end; via a line, the barrier element communicates with a negative-pressure source. Downstream of the metering end, the ring element is subjected to negative pressure, causing air located in the admission cross section of the ring element to be aspirated away. As a result, the metering tube in the region of the ring element becomes clogged with product material, so that product material is no longer dispensed from the metering tube any longer, for instance into a packaging container. The known metering apparatus has the disadvantage that over the course of successive metering operations, the pores of the gas-permeable ring element become plugged with product particles, thus impairing its function. 
     An object of the invention is therefore to refine the know metering apparatus for pourable bulk material in such a way that a function of the metering apparatus is always assured over a high number of metering operations. This object is attained with the characteristics of the body of claim  1 . 
     Further advantages and advantageous refinements of the metering apparatus of the invention for pourable bulk material will become apparent from the description set forth herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention is shown in the drawing and will be described in further detail below. 
     FIG. 1 shows part of a metering apparatus, partly in longitudinal section and partly schematically, and 
     FIGS. 2 a  through  2   e  are flow charts for various functions of the metering apparatus over time, in a simplified description. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, the metering tube end  10  of a worm metering apparatus  1  is shown, which in a known manner, by the rotation about a certain angular amount of a metering worm  12  disposed in the metering tube  11  dispenses pourable bulk material, such as coffee, flour, and so forth, into packaging containers placed in readiness below the metering tube end  10 . To prevent the bulk material from continuing to trickle after the end of the rotation of the metering worm  12 , an annular barrier element  13  below the metering worm  12  communicates with the substantially vertically disposed metering tube  11 . The barrier element  13  comprises a gas-permeable material, preferably sintered metal. on the side toward the metering worm  12 , the barrier element  13 , at its region of transition to the metering tube  11 , has an encompassing chamfer  14  on the inside. As a consequence, the inside diameter of the barrier element  13  is somewhat less than the inside diameter of the metering tube  11 . The barrier element  13  is secured to the metering tube end  10  by means of a bushlike housing  15 . The housing  15 , whose inside wall surrounds the metering tube end  10  and at least partly surrounds the barrier element  13  on its outer circumference, has a shoulder region  17  of narrowed inside diameter on its end opposite the metering tube end  10 . The region  17  has an inside diameter that corresponds approximately to the inside diameter of the barrier element  13 ; the face end of the barrier element  13  oriented toward the shoulder region  17  rests against the region  17 , so that the barrier element  13  is axially fixed by the region  17 . 
     In the middle portion of the housing  15 , an encompassing annular groove  18  is formed, forming a chamber  19  that communicates with a negative-pressure source  22  by means of a first line  21 . For subjecting the chamber  19  to negative pressure, a first barrier valve  23  is disposed in the first line  21 ; this valve can be triggered by the control device  25  of the worm metering apparatus  1 . A branch  26  is disposed in the first line  21 , upstream of the first barrier valve  23 , and a second line  27  originates at this branch and communicates with an overpressure source and/or protective gas source  28 . A second barrier valve  29 , which is likewise triggerable by the control device  25 , is connected between the branch  26  and the overpressure source and/or protective gas source  28 . A pressure meter  30  is also interposed in the first line  21  between the housing  15  and the branch  26 , and values of pressure meter are delivered as input values to the control device  25 . 
     To describe the mode of operation of the worm metering apparatus  1  of the invention, FIGS. 2 a  through  2   e  will now be described. 
     FIG. 2 a  shows the intermittent operation of the metering worm  12  over time; in each of the operating phases, a certain quantity of bulk material is dispensed. The first barrier valve  23  is opened (FIG. 2 b ) in-phase with the stopped phases of the metering worm  12 , so that the chamber  19  is subjected to negative pressure. In a known manner, the effect of this is that the air is aspirated out of the open region of the barrier element  13  for the bulk material, through the pores of the barrier element  13 , so that bulk material continuing to trickle collects along the inner wall of the barrier element  13  and no longer drops out of the worm metering apparatus  1 . At the end of each of the stopped phases of the metering worm  12 , or at the end of the phases in which the negative-pressure source  22  is connected through to the chamber  19 , the negative pressure prevailing in the first line  21  is detected by the pressure meter  30  and supplied (FIG. 2 c ) as an input variable to the control device  25 . A limit value for a negative pressure is stored in memory in the control device  25 ; this is the maximum value that can be allowed to be attained if the barrier element  13  is to function properly. Since depending on the type of bulk material, the pores of the barrier element  13  become more or less clogged on the inside remote from the chamber  19  during the stopped phases of the metering worm  12 , the result sooner or later is that the barrier element  13  becomes less and less gas-permeable, and the negative pressure can no longer act on bulk material that continues to trickle in. This clogging of the pores of the barrier element  13  has the effect that gradually, a greater negative pressure is measured in the first line  21 . FIG. 2 d  shows that during a measurement of the negative pressure, the limit value stored in memory in the control device  25  is reached. As a result, as shown in FIG. 2 e,  in the ensuing operating phase of the metering worm  12 , the second barrier valve  29  is opened by the control device  25 , so that overpressure is carried to the chamber  19  via the two lines  21 ,  27 . The effect of the overpressure is that particles of bulk material adhering inside the pores of the barrier element  13  are detached from the pores and dispensed together with the bulk material that has just been metered. The application of the positive pressure can be done after the next container to be filled is in place. Therefore, additional material will not be added to the filled container. 
     The mode of operation described above for the worm metering apparatus  1  can be modified in manifold ways. For instance, it is conceivable to have the cleaning of the barrier element  13  take place not only during a single metering phase but instead during several successive metering phases. To reduce the expense for equipment for the worm metering apparatus  1 , it is also conceivable to dispense with the pressure meter  30 . In that case, the surge of compressed air can take place either in each metering phase of the worm metering apparatus  1 , or after a certain number of metering phases that is dependent on the bulk material. 
     Cleaning the barrier element  13  by means of overpressure has the advantage that instead of compressed air, a pressurized protective gas can be employed. This variant is used particularly for bulk materials that have a tendency to spoil and are vulnerable to oxygen, such as coffee. 
     Instead of pneumatic cleaning of the barrier element  13 , mechanical cleaning is also conceivable. This then includes a vibrator device, coupled to the barrier element  13 , that is triggered in accordance with the above examples, either via the measured values detected by the pressure meter  30  or after a predetermined number of metering operations. The vibrator device also has the effect of loosening adhering bulk material particles in the pores of the barrier element  13 , thus assuring proper operation of the worm metering apparatus  1 . 
     In the above exemplary embodiments, the metering apparatus is embodied as a worm metering apparatus  1 . The problems of bulk material that continues to trickle in are also known, however, in other metering apparatuses, such as chamber metering apparatuses, so that the cleaning mechanism according to the invention can also be applied to metering apparatuses of the kind in which similar barrier elements are used. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.