APPARATUS FOR MEASURING MASS FLOW

Abstract: In an apparatus for weighing continuously flowing flowable material, in which the mass flow is determined as a function of measurements of forces exerted on at least one plate (1) in contact with the flowing material, the main novel features are that the apparatus is adapted to perform at least two of said measurements in substantial dependence of the distribution of weight, such as between third points (A, B), on a basically rectilinear inclined plate, on which the material flows slidingly. With this arrangement, the results of the measurement may be used for creating an expression of the mass flow, the accuracy of which is independent of the flow velocity, density and friction of the material.

Description: APPARATUS FOR MEASURING MASS FLOW TECHNICAL FIELD The present invention relates to an apparatus * for weighing a continuous stream of granular or powdery material flowing freely under the influence of gravity, such as set forth in the preamble of claim 1. BACKGROUND ART It is well-known practice to measure mass flow by measuring the deflection force upon change of direction of the flow, and such "collision-plate" weighing apparatus are described in several patent publications, such as US patent specification No. 4,637,262, German patent specification No. 3,410,845 and German patent specification No. 3,541,872. As the deflection force as measured in Newtons equals the mass flow measured in kg/s multiplied by the velocity change in the direction of the force as measured in m/s, the mass flow is only proportional to the force as measured, provided that the change in velocity is independent of the mass flow, and that it can also be held constant under varying conditions with regard to the material's consistency, layer thickness and friction against the collision plate etc.. With the usual type of collision-plate weighing apparatus, attempts have been made at solving this problem by delivering the material in a freely-falling vertically downwardly directed stream and measuring the horizontal force exerted on a plate adapted to deflect the stream to flow in an oblique direction. In this case, it is a condition for ensuring a constant value of the horizontal velocity component of the deflected flow that the height of fall is at least approximately constant, but normally this condition will not be completely fulfilled. Attempts at maintaining a constant height of fall for all particles are described in the German published patent specifications No. 2,346,967 and 2,412,258, but even with a constant height of fall, the velocity of fall will vary with the size of the particles, and in addition to this, the velocity of the deflected flow, which is the parameter of interest in the present connection, will vary depending on the coefficient of friction between the material and the plate. It is commonly known that the friction against the collision plate has a deleterious effect on the accuracy of the measurement, and various attempts have been made to remedy this, e.g. as described in German published patent specification No. 1,802,213 and Swedish published patent specification with publication No. 418,420. None of these attempts have, however, neither alone nor in combination, been able to compensate for all those erratic conditions influencing the horizontal component of the outlet velocity, for which reason all these methods for determining mass flow suffer from a degree of uncertainty, being especially high in the case of a stream of material of varying consistency, such as e.g. grain (cereals) with different degrees of purity and moisture percentage, in which the coefficient of friction only may vary between 0.2 and 0. 7. Basically, a direct measurement of the outlet velocity would remove this uncertainty, but it is difficult to carry out in practice with the sufficient precision, and will usually have to be based on mechanical arrangements, such as e.g. disclosed in German published patent application No. 2,950,925. The same method of measuring the outlet velocity is disclosed in the international patent application WO 89/11082, but in this case in connection with an inclined plane plate, on which the material passes slidingly, while the weight of the amount of material present on the plate at each instant is registered by the force it exerts against the plate. Thus, in this case it is not the force of deflection upon change of direction of the flow, but the weight of material being present on a known length of plate, in connection with a measured flow velocity, being utilized to determine the mass flow. In addition to the requirement that this velocity measurement is to be carried out with the sufficient precision independently of the thickness of the layer of the flowing material (and the consequent differences in radius from the axis of the measuring drum), it is a prerequisite for the last-mentioned arrangement that the flow velocity on the whole plate is uniquely determined by the outlet velocity. This is normally not the case, as a coefficient of friction, that is smaller or greater than the tangent to the angle of inclination of the plate relative to the horizontal will cause an acceleration or a deceleration respectively of the material, so that the mean velocity over the plate will be smaller or greater respectively than the outlet velocity as measured. DISCLOSURE OF THE INVENTION It is the object of the present invention to provide an apparatus of the kind initially referred to, with which it is possible to achieve a more accurate measurement of the mass flow independently of the various parameter variations reducing the accuracy of the previously known methods, e.g. as discussed above, and this object is achieved with an apparatus, which according to the present invention is characterized by the features set forth in the characterizing clause of claim 1. Thus, while the above-mentioned known weighing apparatus does not take into account the variations in the velocity of the material during the sliding movement on the plate, the weighing apparatus according to the present invention thus in fact exploits the variation in velocity from inlet to outlet caused by friction and inclination, as the invention is substantially based on measuring the distribution of the material on the plate in combination with a measurement of the frictional force on the latter. How exactly it is possible to use the results of this measurement to produce an expression for the mass flow, which is independent of the unknown values for flow velocity and frictional resistance, will be explained below with reference to Figure 1. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present specification, the present invention will be explained in more detail with reference to the drawings, in which, Figure 1 shows the basic principles of the present invention, and Figures 2-4 show four different exemplary embodiments of an apparatus according to the present invention as envisaged for use in measuring mass flow of grain (cereals). DESCRIPTION OF THE PREFERRED EMBODIMENT For a sliding movement with friction on an inclined plate 1 being restricted against longitudinal movement and transversally supported in its third points, it applies that the ratio A/B between the forces exerted in the upper and lower points of support 2 and 3 respectively equals the ratio V2/v between the inlet and outlet velocities of the material. This relation can be proved on the basis of the ordinary equations relating to movement, the only prerequisite being that the coefficient of friction can be considered as being constant over the full length of the inclined plate 1. Further, the passage time along a plate with length L (m) equal to 2 /(vι+V2), the latter multiplied with the mass flow M (kg/s) and the acceleration of gravity g (m/s^) giving the gravitational force exerted by the flowing material on the plate, this again being equal to (A + B)/cosα, where is the angle of inclination of the plate relative to the horizontal. With the frictional force F (=μ(A+B)) as measured, the acceleration will be: a = g* (sinα-F* coso./(A+B) ) , with leading to the equation: this being theoretically correct for all values of flow velocities and coefficients of friction. The two weighing points for the inclined plate 1 may be situated in other positions than the third points. If for example the lower transverse support is placed in the mid-point of the plate instead of in its lower third point, while the upper support is moved to the top edge of the plate, then the force exerted will be changed to A' = (A-B)/3 above and B' = (A+2B)2/3 below, which when inserted in the equation for determining M will give M = (A' + B' ) /(A'+B')*tanα-F 6*g*L*cos a *A' By thus replacing the difference between the two measurements of A and B by the single measurement of A', errors caused by even small discrepancies between the constants of the two weighing cells will be avoided in cases where the magnitude A-B is much smaller than each of the magnitudes of A and B. It is a necessary condition that the angle of inclination a always differ from the friction angle, as both numerator and denominator approach zero, when the coefficient of friction approaches tano, whereby the flow velocity ( and hence the thickness of the layer of material) becomes constant. Since the absolute magnitude of the forces in relation to a given mass flow decreases rapidly with an increase in the inclination of the plate, the above-mentioned necessary condition implies a limitation of the practical accuracy of this method in cases, where large variations in the friction between the flowing material and the plate can be expected, e.g. because of variations in the material's water content. In such cases, a substantial improvement of the accuracy may be achieved by adjusting the angle of inclination of the inclined plate depending on the material's coefficient of friction, so as to achieve an optimal increase in velocity from the inlet velocity v^ to the outlet velocity V2 together with the consequent distribution of weight on the plate. The exemplary embodiment of a weighing apparatus according to the invention shown in Figure 2 is well suited to such an angular adjustment, the inclined plate 1 in this case being supported vertically with the result that the reaction forces A' and B' on the plate in the equation above in this case are resolved into - vertical forces A" = A'/cosα and B" = B'/cosα, both meing measured directly through the load on — the weighing cells 4 and 5, and the components in the plane of the plate, i.e. A'tanα and B'tanα acting at an oblique angle upwardly in the plane of the plate. As the sum of these components of force exceeds the frictional force F, the resultant reaction force in the plane of the plate will be directed opposite to the direction of movement of the material and will be equal to F" = (A' + B' )tanα-F, the latter being measured through the weighing cell 6. Insertion in the expression above for the mass flow M will now produce an expression, which is independent of the angle α, while at the same time all difference values between measured forces are eliminated, viz.: M = (A" + B") F" 6gLA" The angle of the inclination a is adjusted by means of the adjustment screw 7, being adapted to hold the beam 8 in the desired position relative to its pivot point 9 on the fixed vertical column 10, the latter together with a linkage rod 11 ensuring that the inlet plate 12 is parallel to the beam 8 and the plate 1 parallel to this beam. For materials with a coefficient of friction μ in the interval 0.2 < μ < 0.5 a fixed angle a = approx. 45° will often give acceptable results, and the exemplary embodiment of the apparatus shown in Figure 3, adapted to measure a horizontal reaction force D = F"coso: in combination with the vertical reaction force C = B"-F"coso., will in many cases be more useful, because in practice, measuring the force F" in the plane of a plate may be difficult to achieve. The vertical reaction force A" in the upper edge of the plate will in this case remain unaltered, and the mass flow is determined by the equation: M = (A"+C+Dtanα) 6gL COSQ.A" Another exemplary embodiment of the weighing apparatus according to the invention, in which the weighings with the inclined plate are combined with measurements of the deflection force on a basically vertical collision plate is shown in Figure 4, in which the material leaving the inclined plate 1 impacts against the rear wall of a vertical exit duct 13, the latter being supported by vertical linkage rods 14, so that solely the horizontal deflection force P acts upon the weighing cell 15. The plate 1 is supported transversely to its longitudinal direction in the points 2 and 3, in which the forces A and B are measured by means of weighing cells in exactly the same manner as in the diagrammatic Figure 1, while in this case it is not necessary to measure the longitudinally exerted force acting upon the rod 16. On the basis of the two equations for the mass floww M B (A+B)2 V7 and M = P 2gLcosαA V2Cθsα multiplication and extraction of roots will give as the unknown velocity V2 disappears. Thus, in this embodiment, the measurement of the frictional force F in the embodiments described above is replaced by measurement of the deflection force P, but by in this case additionally measuring the frictional force F, a double assurance for the accuracy of the result may be achieved, as a combined calculation based on measuring all four parameters A, B, F and P will produce a highly reliable result. In the embodiment according to Figure 4 also, it is possible to support the inclined plate 2 in other positions than the third points, as these have solely been chosen to simplify the explanation. At times it may be practically advantageous as shown in Figure 5 to place the inclined plate 1 downstream of the deflection plate 17, the latter in this case preferably being curved and arranged with a vertical inlet and an outlet in the direction of the inclined plate 1. As in this case, the inlet velocity vi un the inclined plate 1 will be used in the calculation instead of the outlet velocity V2, the mass flow M may be determined using the above equation by interchanging A and B. LIST OF PARTS inclined plate upper point of support upper point of support weighing cell weighing cell weighing cell adjustment screw beam pivot point column linkage point inlet plate exit duct linkage rod weighing cell rod deflection plate

Claims: C L A I M S 1. Apparatus for weighing continuously flowing flowable material, in which the mass flow is determined as a function of measurements of forces exerted on at least one plate in contact with the flowing material, characterized by being adapted to perform at least two of said measurements in substantial dependence on the distribution of weight on a basically rectilinear inclined plate, on which the material flows slidingly. 2. Apparatus according to claim 1, characterized by being adapted to measure transversal reaction forces A and B in the third points of the plate as well as the frictional reaction force F in the longitudinal direction of the plate, and to determine the mass flow as M = (A + B ) (A + B) *tanα-F 2*g*L*cos a *(A-B) in which L is the length of the plate, a is the angle of inclination of the plate relative to the horizontal, and g is the acceleration of gravity. 3. Apparatus according to claim 1, characterized by being further adapted to measure a horizontal deflection force on a rectilinear or curved plate with vertical inlet or outlet respectively and situated upstream or downstream respectively of the rectilinear plate, and to determine the mass flow as M = respectively, in which P is the horizontal force of deflection, and the remaining symbols have the same meaning as in claim 2 above. 4. Apparatus according to any one or any of the claims 1, 2 and 3, characterized by being adapted to measure the reaction forces A and B transversely to the plate and F longitudinally to the plate in other points and/or directions than set forth in claim 2, the equation used for determining the mass flow M being accordingly altered by substitution of the static equilibrium expressions of the measured forces A, B and F.

Inventors: OLESEN HENRY TOFTDAHL, OLESEN, HENRY, TOFTDAHL
Applicants: CIMBRIA UNIGRAIN LTD, CIMBRIA UNIGRAIN LTD. A/S
Application year: 1992
Main classification: G01F1
IPC codes: G01F1, G01G11