Abstract:
A silo has more than one discharge passage, and each passage has the ability to increase or decrease its respective flow rates. The silo and each portion of the silo that feeds into each individual discharge passage can measure a feature of the solid being dispensed, such as its moisture content or its temperature. A computer controller is then used to take this feedback and adjust the rate of flow from each discharge passage so that the flows from each passage are kept the same despite the variation in moisture, temperature or any other characteristic of the flowing solid. A method to utilize this device is also taught.

Description:
TECHNICAL FIELD 
     The present invention concerns a method of discharging a solid from a silo with a polygonal or round discharge cross-section, wherein the solid flows continuously through the silo. 
     The invention further concerns a discharge apparatus for a solids silo having a polygonal or round discharge cross-section, as well as a solids silo, wherein the silo is designed for a solid to flow therethrough. 
     STATE OF THE ART 
     In regard to the metered withdrawal from silos with a rectangular or square discharge opening, it is known that, when using a screw conveyor in which the screw is of a constant core and outside diameter and has a constant screw pitch, the bulk material is withdrawn only at the rearward end of the silo while a dead zone is formed in the forward region of the silo. By virtue of adaptation of the screw geometry, for example by a reduction in the core diameter in the conveyor direction and an increase in the outside diameter or the screw pitch, the screw can pick up bulk material along the entire discharge cross-section, SCHULZE, Dietmar. Grundlagen und Möglichkeiten der Schüttguttechnik. Schüttgut—Informationen für die Schüttgutindustrie (Agrichema GmbH). 
     DE 3717748 (ZIPPE GMBH U. CO, 6980 WERTHEIM) May 26, 1987 discloses a plate heat exchanger for preheating bulk materials, in which the problem of an irregular withdrawal of solid material at the lower end of the heat exchanger is avoided by symmetrically arranged outlet shafts with flange-mounted, non-controllable shaker conveyors of equal conveyor output. 
     In the case of bulk materials which flow very poorly, those known measures nonetheless frequently still result in a non-homogeneous mass flow of the solid material over the cross-section of the apparatus. If the solid material in the silo is at the same time heated or cooled or if a reaction takes place during the flow of solid material therethrough, then the unequal mass flow can result for example in locally different temperatures and thus different product properties. 
     DE 3214472 (EIRICH, HUBERT ET AL) Apr. 20, 1982 discloses a controllable discharge apparatus for an apparatus for heating electrically conductive bulk materials, in which the discharge speed and the electrical heating power are matched to each other in order to achieve a temperature which is as constant as possible in the discharged product. 
     In apparatuses for heating electrically conductive bulk materials by means of resistance heating by way of oppositely disposed electrodes, the power input at the electrodes is dependent on the resistance of the bulk material disposed therebetween. As the current which is passed through the bulk material has a tendency to flow along the path of least resistance, when dealing with an irregular mass flow across the cross-section of the silo-form apparatus, that results in temperature differences between regions which are flowing more quickly and more slowly. Particularly in a situation involving changing flow properties in respect of the intake substances, due for example to changing intake temperature, material moisture content or particle size distribution, there is hitherto no possible way of influencing the locally different discharge speed, which arises as a result thereof, from the solids silo. 
     DISCLOSURE OF THE INVENTION 
     The problem of the present invention is to provide a method and a discharge apparatus for a solids silo as well as a solids silo which can be equipped with such a discharge apparatus, which permit a controllable solids discharge which is regular over the cross-section of the silo, and thus permit the production of bulk materials which are treated physically or chemically when flowing through the silo, being in particular heated or cooled, with properties which are as homogeneous as possible, in particular with slight temperature differences. In addition in its preferred configuration the invention permits automatic adaptation to changing flow properties in respect of the intake substances used. 
     The silo discharge according to the invention divides the withdrawal cross-section or discharge cross-section into a plurality of preferably mutually equal partial cross-sections, to each of which a respective continuous controllable discharge member is flange-mounted. The solids flow issuing from the controllable discharge members can be collected together for example by means of a continuous conveyor device disposed therebeneath and removed. 
     The uniform discharge of solid material at the continuously operating discharge members is in that case controlled in dependence on measurement signals from a plurality of similar sensors which detect the locally prevailing mass flow or another measurement parameter in the corresponding partial portions of the silo, by way of the conveyor delivery of the discharge member associated with the respective sensor. 
     To detect the local mass flow, for example the electrical power input at an electrically heated sensor can be used to maintain a preset temperature at the sensor tip, GERL, Stefan et al, Sensor auf Transistorbasis zur In-line-Restfeuchtemessung in ruhenden Haufwerken, Technisches Messen. 1997, Vol 64, No 7/8, pages 268-275, or, in the case of electrically conductive bulk materials, the current strength at oppositely disposed electrodes. 
     The local energy input of heat exchangers through which fluid or vapour flows can also be detected and utilised as a signal for the local mass flow. 
     Furthermore the solids mass flow can be ascertained directly in each discharge member associated with a partial portion of the silo by means of weighing in respect of each discharge member in conjunction with the respective discharge speed of the discharge member. 
     In addition, in the case of silos through which the material flows continuously, the mass flow is adapted to the feed mass flow of the feed member, by way of the rotary speed of the discharge members, in such a way that the filling level within the silo remains constant during the flow therethrough. 
     The uniform controllable removal of material permits for example uniform heating/cooling of the product over the entire cross-section of the silo without locally different product temperatures. At the same time the capacity of the heat transfer arrangement can also be fully utilised. 
     As an alternative thereto control of the discharge speeds of the individual discharge members can be effected by way of measurement of the temperature of the solid. With a uniform heating power in all regions of the silo the solid is heated more greatly in those regions in which it remains for longer. In the case of heating power which is irregularly distributed over the cross-section, some regions are heated more greatly and other regions less greatly, at the same height. If now the temperature in the silo or in the region of the discharge members is measured the conveyor speeds of the discharge members can be so adapted that the solid material from all regions is at the same temperature upon being removed from the silo. In other words, the discharge speed is slowed down in a region if the temperature of the solid as measured there is below a predeterminable first reference value and speeded up if the temperature of the solid as measured there is above a second reference value. That overall ensures a uniform discharge temperature for the solid, which is between the first and second reference values (which can also be the same) in all partial cross-sections of the discharge cross-section. In that respect it is possible to use various control procedures which are known in the state of the art such as for example PID control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is diagrammatically illustrated by way of example in the drawings in which: 
         FIG. 1  is a diagrammatic isometric view of a silo which is subdivided into four partial portions, with mass flow sensors, a signal evaluation and control unit and controllable discharge screws, 
         FIG. 2  is a plan view of a rectangular discharge floor of a silo along section X-X′ in  FIG. 1  with four withdrawal screws with a progressive screw pitch, 
         FIG. 3  is a diagrammatic side view of a weighed-out silo through which solids continuously flow, with mass flow sensors, a signal evaluation and control unit and controllable progressive withdrawal screws, 
         FIG. 4  is a diagrammatic isometric view of a weighed-out silo which is subdivided into four partial portions, with controllable discharge screws, electrodes for heating electrically conductive solids and a signal evaluation and control unit, 
         FIG. 5  is a diagrammatic isometric view of a silo which is subdivided into four partial portions, with a filling level sensor, controllable discharge screws, heat exchanger elements, mass flow sensors and a signal evaluation and control unit, 
         FIG. 6  is a diagrammatic isometric view of a silo which is subdivided into four partial portions, with controllable discharge screws, heat exchanger elements operating as mass flow sensors and a signal evaluation and control unit, 
         FIG. 7  is a diagrammatic side view of a silo with controllable cell wheel lock devices, 
         FIG. 8  is a diagrammatic side view of a silo with controllable conveyor screws with oppositely disposed discharge openings, 
         FIG. 9  is a diagrammatic side view of a silo with controllable conveyor screws with a central discharge opening and orthogonally arranged conveyor device, and 
         FIG. 10  is a diagrammatic side view of a weighed-out, negatively conical silo through which solids continuously flow, with a signal evaluation and control unit and controllable progressive screw carriages. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a rectangular silo  1  with a solids fill  2 , the silo  1  being divided in the bottom region into four uniform portions  3 ,  4 ,  5  and  6 . Each of the portions  3 ,  4 ,  5  and  6  has its own continuous controllable discharge device or member  7 ,  8 ,  9  and  10 , for example a discharge screw, which can continuously remove the solid material  2  from the respective portion. Arranged above each portion  3 ,  4 ,  5  and  6  is at least one respective mass flow sensor  11 ,  12 ,  13  and  14  associated with the respective portion. Each of the similar sensors  11 ,  12 ,  13  and  14  detects the local flow of the solid material fill  2  in the portion in which the measurement field of each sensor is disposed. The signals  11   a ,  12   a ,  13   a  and  14   a  from the respective sensors  11 ,  12 ,  13  and  14  are passed to a signal evaluation and control unit  15 . The signal evaluation and control unit  15  produces setting signals  7   a ,  8   a ,  9   a  and  10   a  for the controllable discharge devices  7 ,  8 ,  9  and  10  in such a way that the signals which occur at the sensors  11   a ,  12   a ,  13   a  and  14   a  and which are proportional to the solid mass flow are of the same magnitude and thus the solid mass flow in each portion is equal. 
       FIG. 2  shows a plan view of the discharge bottom of a silo according to the invention along section X-X′ in  FIG. 1 . Over the discharge cross-section of the silo  16  two respective screws  17 ,  18  are arranged in mutually juxtaposed relationship and two respective screws  17 ,  19  and  18 ,  20  are arranged in mutually superposed relationship. The screws can be provided for example with a progressive pitch. In the discharge region  21  into which all screw outlets open, the solid which is withdrawn from the silo drops in the direction of the force of gravity into downstream-disposed installation portions (not shown). To provide for stepless adjustability of the discharge speed of each screw, it is provided with a motor  22  with a frequency converter  23  or an adjusting transmission (not shown). The discharge speed in each portion or partial cross-section of the discharge bottom  3 ,  4 ,  5  and  6  of the silo can thus be individually set. 
       FIG. 3  shows a silo  1  with the discharge bottom according to the invention, solids  2  flowing continuously through the silo. 
     The silo  1  is charged at the upper end with solids  25  which are pourable, by way of a metering member  24 , for example a variable-speed conveyor belt, and the solid is continuously drawn off in the bottom region. In order to be able to maintain a defined degree of filling within the silo and to prevent overfilling, the degree of filling is detected for example by way of a weighing device by means of weighing cells  26 . 
     The measurement signals of the sensors  11  and  13  which are of the same design configuration and which detect the solid mass flow in each portion  3 ,  5  of the withdrawal region of the silo are detected by means of a signal evaluation and control unit  15 , and the filling level within the silo is detected by way of the weighing cells  26 . The signal evaluation and control unit  15  controls the speed of the discharge members  18 ,  20  on the basis of the input signals  11   a ,  13   a  and  26   a , by way of the controllable drive units  18   a ,  20   a , in such a way that the filling level within the silo remains constant and all solid mass flow sensors  11 ,  13  register the same level in respect of the measurement signal  11   a ,  13   a.    
     In a further variant a plurality of discharge members, for example 17+18 and 19+20 or 18+20 and 17+19 can be combined together in terms of control procedures. 
     In addition, instead of the filling level within the silo, the solids flow  25  which is supplied by way of the metering member  24  and which is determined by measuring procedures can be utilised for controlling the discharge speed of the discharge members  18 ,  20 . 
       FIG. 4  shows a rectangular silo  1  with a solids fill  2 , which is divided in the bottom region into portions  3 ,  4 ,  5  and  6 . Each of the portions  3 ,  4 ,  5  and  6  has a continuous controllable discharge device  7 ,  8 ,  9  and  10 , for example a discharge screw, which can continuously withdraw the solid  2  from the respective portion. The entire silo  1  is supported on weighing cells  26  in order to ensure a constant degree of filling. Alternatively it is also possible to use filling level sensors  31  ( FIG. 5 ). 
     In a particularly advantageous configuration of the invention, arranged within the silo  1  in the upper region are one or more, preferably identical electrodes  27  (+pole), over the entire silo cross-section, while arranged in the lower region are one or more, preferably identical electrodes  28   a ,  28   b ,  28   c  and  28   d  (−pole), above each withdrawal cross-section  3 ,  4 ,  5  and  6 . The reverse polarity of the electrodes  27  and  28   a ,  28   b ,  28   c  and  28   d  is equally possible. A current  29  flows between the electrodes and the electrically conductive solids fill  22 , the strength of the current  29  being dependent on the resistance and thus the temperature of the solid disposed therebetween. The current strength  27 ′ measured in the input power is divided to the corresponding number of electrodes  28   a ,  28   b ,  28   c  and  28   d  in the withdrawal region, wherein the measured current strengths  28   a ′,  28   b ′,  28   c ′ and  28   d ′ of each electrode  28   a ,  28   b ,  28   c  and  28   d  varies in dependence on the resistance of the solid material in each withdrawal portion  3 ,  4 ,  5 ,  6 . 
     The measured current strengths  28   a ′,  28   b ′,  28   c ′ and  28   d ′ of the respective electrodes  28   a ,  28   b ,  28   c  and  28   d  are passed to a signal evaluation and control unit  15 . The current strength  27 ′ at the upper electrode  27  as well as the weight of the silo from the weighing cells  26  together with the measured temperature of the solid material  30  at the discharge region  21  are also fed into the signal evaluation and control unit  15 . The signal evaluation and control unit  15  produces setting signals  7   a ,  8   a ,  9   a  and  10   a  for the controllable discharge devices  7 ,  8 ,  9  and  10  in such a way that the current strength  28   a ′,  28   b ′,  28   c ′ and  28   d ′ at the electrodes  28   a ,  28   b ,  28   c  and  28   d  are of equal magnitude and thus the solid mass flow in each portion is of the same magnitude and in addition the filling level within the silo  1  remains the same. 
     In addition the signal evaluation and control unit  15  detects the temperature  30  of all the discharged solid and controls the inputted power at the electrodes  27 ,  28   a ,  28   b ,  28   c  and  28   d  in such a way that the desired final temperature of the product is achieved at the discharge. 
     When using a plurality of electrodes within a withdrawal portion the measured current strengths are suitably combined together to form an evaluatable measurement signal. 
       FIG. 5  shows a variant of  FIG. 4  and  FIG. 1 , in which heating or cooling of the solid within the silo  1  is effected by way of example by way of heat exchanger elements  32  through which pass vapour, thermal oil or cooling fluid and which in a further variant could also be electrically heated. The solid mass flow in each portion  3 ,  4 ,  5  and  6  is detected as shown in  FIG. 1  by way of a plurality of mass flow sensors  11 ,  12 ,  13 ,  14  and the signals  11   a ,  12   a ,  13   a  and  14   a  are fed to a signal evaluation and control unit  15  which generates therefrom corresponding setting signals for the discharge devices  7 ,  8 ,  9  and  10  as set forth in the description relating to  FIG. 1 . The power input  33  at the heating or cooling elements  32  within the silo, controllable for example by way of the through-flow of the heating or cooling medium, is effected in dependence on the measured final temperature  30  at the discharge of the withdrawal screws. 
       FIG. 6  shows a further variant of  FIG. 5 , in which the heat exchanger elements  32   a ,  32   b ,  32   c  and  32   d  through which a heating or cooling medium flows are used at the same time as mass flow sensors insofar as a heat exchanger element  32   a ,  32   b ,  32   c  and  32   d , through each of which a respective heating or cooling medium flows, is allocated to each withdrawal portion  3 ,  4 ,  5  and  6 . Setting signals for the discharge devices  7 ,  8 ,  9  and  10  can be produced, in accordance with the description relating to  FIG. 1 , by way of the cooling medium mass or volume flow  36   a ,  36   b ,  36   c  and  36   d  which is detected individually for each portion, and the energy input which is ascertained by way of the respective temperature difference between the intake  34   a ,  34   b ,  34   c  and  34   d  and the outlet  35   a ,  35   b ,  35   c  and  35   d , by the signal evaluation and control unit  15 . 
       FIG. 7  shows a variant of  FIG. 3 , in which the discharge of the solid in the partial portions  37 ,  38  and  39  is effected by way of a plurality of controllable cell wheel lock devices which deliver the discharged solid on to a continuously operating conveyor device  40  which is disposed therebeneath and which combines the individual solid mass flows together and conveys them to a predefined delivery point  41 . Control of the discharge speed of the cell wheel lock devices is effected in a similar manner to the foregoing description by way of the mass flow sensors (not shown). 
       FIG. 8  shows a further variant of  FIG. 3  in which discharge is effected by way of screws  42 ,  43  which deliver the solid which has been withdrawn from the partial portions  3  and  5 , by way of oppositely disposed discharge openings  44  and  45 , on to a continuously operating conveyor device  46  which is disposed therebeneath and which combines the individual solid mass flows together and delivers them at a predefined point. In this case also control of the discharge speed of the screws  42 ,  43  is effected by way of the mass flow sensors (not shown) similarly to the foregoing description. 
       FIG. 9  shows a further variant of  FIG. 8 , in which a plurality of withdrawal screws  47 ,  49  convey the solid which has been withdrawn from the partial portions  3  and  5  respectively towards the middle of the silo  1  and the total solid flow is combined together by an orthogonally arranged continuous conveyor device  48  and transported away to a predefined point. 
       FIG. 10  shows a variant of  FIG. 3  with a negatively conical silo  1  through which solids  2  continuously flow. The silo  1  is charged with pourable solids  25  at the upper end by way of a metering member  24 , for example a variable-speed conveyor belt, and the solid is continuously withdrawn in the bottom region. In order to be able to maintain a defined degree of filling within the silo and to prevent overfilling, the degree of filling is detected for example by way of the weight of the silo, by means of weighing cells  26 . Discharge is effected by way of a plurality of screw weighing arrangements  50  and  51 . 
     The negatively conical structural configuration of the silo  1  provides that compacting of the solid  2  in lower layers is counteracted by the weight of the solid material itself. The fill density and thus for example also the electrical resistance of the material fill remain constant over the height involved. 
     The weights of the conveyor screws  50   b  and  51   b  in each portion  3 ,  5  of the withdrawal region of the silo are detected by means of a signal evaluation and control unit  15  and the solid mass flow of each screw is calculated by way of the speed of the respective screw. In addition, the filling level within the silo is detected by way of the weighing cells  26 . The signal evaluation and control unit  15  controls the speed of the discharge members  50 ,  51 , on the basis of the input signals  50   c ,  51   c  and  26   a , by way of the controllable drive units  50   a ,  51   a , in such a way that the filling level within the silo remains constant and all solid mass flows which are calculated from the weight  50   c ,  51   c  and the rotary speeds of the screws  50  and  51  are of the same magnitude. Alternatively to the screw weighing arrangement it is also possible to use a belt weighing arrangement or a weighed-out oscillating or shaker conveyor. 
     In principle the invention is not limited to the discharge devices set forth but can be carried into effect with any continuously operating and controllable discharge member. The same applies for the continuous conveyor device which is disposed beneath the discharge members and which brings together the solid material flow issuing from the discharge devices and transports it away. Instead of a continuous conveyor device the solid issuing from the discharge members can also be fed directly to an item of equipment connected at a downstream location. The discharge cross-section of the silo is not restricted to a polygonal shape, preferably rectangular or square, but can also be round. 
     For the purposes of original disclosure it is pointed out that all features which are to be deduced by a man skilled in the art from the present description, the drawings and the claims, even if they were described in specific terms only in connection with given further features, can be combined both individually and also in any combinations with others of the features or groups of features disclosed herein, unless that has been expressly excluded or technical factors make such combinations impossible or meaningless. A comprehensive explicit representation of all conceivable combinations of features is dispensed with here only for the sake of brevity and readability of the description.