EP0406094B1 - Process for automatically driving a discontinuously working centrifuge - Google Patents

Description

The present invention relates to centrifugal dryers with a cylindrical basket operating in repetitive cycles during which the product to be spun is loaded into the basket, the loading being controlled by a device which interrupts the feeding when the layer of product to be spun reaches a predetermined thickness. , then, after a first spin, the product is washed by a fluid sprayed by means of a ramp pierced with orifices or provided with spray nozzles and placed inside the basket and undergoes a final spin and, finally, the product washed and spun is unloaded from the basket and evacuated. Such a wringer is used in particular in the sugar industry to wring out the sugar crystals of a cooked mass.

The cycle of such a wringer is defined by a certain number of parameters: acceleration, deceleration, speed step, time delay, etc. Currently, these parameters are entered, individually, in the automatic control of the wringer, either by the production manager or by the operator, and are possibly adjusted empirically as a function of the observed results of the wringing.

In current installations, certain parameters, such as the time available for the cycle and the volume of the washing fluid to be used, depend on the load of the basket. As it is difficult to measure this load, we try to keep it constant and equal to a predetermined optimal value. For this, a feeler is used placed inside the basket and which controls the closing of the product supply valve to be wrung when the thickness of the layer of product in the basket reaches a predetermined set value. For various reasons, in particular because the product to be spun does not distribute itself uniformly over the entire height of the basket during loading, the thickness measured by means of the probe continues to increase for a certain time after the valve has closed. The maximum thickness reached is greater than the set value, and the difference between the two is variable depending on the characteristics of the product. For the same setpoint, we can therefore have different basket loads from one cycle to another.

To overcome this drawback, it is proposed in accordance with the invention to determine, at the end of each cycle, a new set thickness layer value for closing the supply valve as a function of the difference between the measured value and a predetermined value of the maximum layer thickness reached during the cycle which has just ended. We could, for example, add to the old setpoint the algebraic difference between the measured and predetermined values of the maximum thickness.

To determine the new set value for the layer thickness, it is also possible to take account of the change, compared to the previous cycle, of the slope of a curve representative of the variations in the thickness of the product layer in the basket as a function of time.

• La figure 1 est le diagramme du cycle d'une essoreuse du type concerné par l'invention. Sur cette figure, on a également représenté la courbe des variations de l'épaisseur de la couche de produit dans le panier de l'essoreuse pendant le cycle, et
• La figure 2 est une représentation schématique, en coupe verticale, d'une essoreuse du type concerné.

The following description refers to the accompanying drawings, in which:

• Figure 1 is a diagram of the cycle of a wringer of the type concerned by the invention. This figure also shows the curve of the variations in the thickness of the product layer in the basket of the wringer during the cycle, and
• Figure 2 is a schematic representation, in vertical section, of a wringer of the type concerned.

On the diagram of FIG. 1, the time in seconds is plotted on the abscissa and the speed in revolutions per minute is plotted on the ordinate.

The cycle includes an acceleration phase AB, a loading phase BC at constant speed VC, an acceleration phase CE, a final spin phase EF, at constant speed VE, a deceleration phase FG and an unloading phase GH, at constant speed VD; before the final spin, the layer of product loaded in the basket is washed, starting at D.

avec

N

: nombre d'essoreuses disponibles

Ch

: charge du panier imposée (en m3)

Q

: débit du produit à essorer (en m3/h)

TIC

: marge de sécurité, de 2 à 30 secondes, que l'on ménage entre la fin d'un cycle et le début du cycle suivant.

At the end of each cycle, the time available for the next cycle is determined using the formula $$\text{TCD =}\frac{\text{3600 x N x Ch}}{\text{Q}}\text{- ICT}$$

with

NOT

: number of wringers available

Ch

: basket load imposed (in m3)

Q

: flow rate of the product to be wrung (in m3 / h)

TIC

: safety margin, from 2 to 30 seconds, which is taken care of between the end of a cycle and the start of the next cycle.

Q can, for example, come from the production management system upstream of spinning or from a level measurement in a tank or a mixer feeding the extractors.

If the calculated TCD value differs in more or less from the duration tH - tA of the previous cycle, we set for the spin stop EF of the following cycle a duration TE = tF - tE longer or shorter, respectively, of such so that the total cycle time is equal to TCD.

Correspondingly, a new value for the volume of washing liquid to be used during the next cycle is fixed. In practice, the supply rate of the washing ramps 22 (FIG. 2) being constant, a new value for the duration of washing TL will be set.

To maintain the load of the basket 10 at the imposed value Ch, a layer sensor 12 is used placed inside the basket.

As soon as the basket loading has started, the probe is applied to the product layer 14 to measure its thickness continuously. This increases rapidly until reaching an EPD setpoint value - trip thickness. At this time, an automaton 24 closes the valve 16 placed on the feed chute 18 of the wringer. After closing the valve, the thickness of the layer 14 continues to increase, in particular due to the rise of the product from the bottom of the basket in the case of a vertical basket wringer such as that shown in FIG. 2 .

The maximum thickness of the EPM layer is therefore greater than EPD, and the difference between the two is variable depending on the characteristics of the product to be wrung, essentially its viscosity.

In accordance with the invention, a new EPD value is determined at the end of each cycle by comparing the maximum value of the layer measured by means of the probe 12 with the theoretical value EPC corresponding to the load on the imposed basket Ch. We calculate the difference E = EPC - EPM then the new value of EPD.

An improvement consists in taking into account the evolution of the slope of the curve giving the variations in the thickness of the layer of the product in the basket. This curve, which is shown in dashed lines in FIG. 1, is drawn from information from the probe 12 or from a gammadensimeter 20 whose radiation passes through the layer of products 14. This slope P is calculated at the start of loading and compared with the slope of the curve of the previous cycle. This calculation can be done, for example, from a measurement of the DTE time it takes to go from a thickness E1 to a thickness E2 during loading.

All the calculations are carried out automatically by a computer 26 which receives the necessary information from the operator and from appropriate sensors, in particular from the probe 12 or from the gammadensimeter 20. The calculated setpoint values are applied to the inputs of the controller 24 which controls the different parts of the wringer: basket drive motor, feed valve, feeler, etc.

   K étant un coefficient choisi par l'opérateur à partir de résultats d'essais;
   K pourra, par exemple, être pris égal à 0,8.When the spin dryers are started, the EPC setpoint, corresponding to the imposed basket load Ch, is introduced into the computer 26 by the operator. For the first cycle, the computer determines the EPD setpoint using the formula: $$\text{EPD (1) = K EPC}$$

K being a coefficient chosen by the operator from test results;
K could, for example, be taken equal to 0.8.

For the following cycles, the computer uses the formula below to determine the EPD setpoint

In this formula K1 and K2 are coefficients introduced by the operator in the memory of the computer. K1 could, for example, be chosen in a range from 0 to 2 and K2 in a range from 0 to 3. EPD (n) is the set value used in cycle n and EPD (n + 1) is the calculated value for cycle n + 1 (next cycle).

et à tenir compte de l'évolution de la pente de la courbe d'épaisseur de la façon suivante :A simplified solution consists in calculating EPD by the formula:

and to take into account the evolution of the slope of the thickness curve as follows:

If the trend is positive - steeper slope - and if the difference EPC - EPM (n) is also positive - loading during the previous cycle too low - we do not modify EPD for the current cycle EPD (n + 1 ) = EPD (n). Likewise, if the evolution of the slope of the load curve is negative - lower slope - and if the difference EPC - EPM (n) is also negative - loading too large during the previous cycle - we do not change EPD.

In other cases, EPD is calculated by the above formula.

Claims (2)

1. Process of automated control of a cylindrical basket and cyclical operation centrifugal according to which process, at each cycle, the feed valve (16) is made to close when the thickness of the layer of product in the basket reaches a predetermined value characterized in that the feed valve (16) is made to close when the thickness of the layer of the product loaded in the basket reaches the value of a first set point (EPD) lower than the thickness corresponding to a prescribed value (Ch) of the basket load, then the maximum thickness of the layer (EPM) is measured and, at each cycle, this measured maximum thickness is compared with the value of a second set point (EPC) corresponding to the said prescribed value of the basket load, and a new value for the first set point (EPD) is determined according to the difference between the measured maximum thickness (EPM) and the value of the second set point (EPC).
2. Process according to claim 1, characterized in that the new value of the first set point (EPD) is determined taking account of the evolution, in comparison with the previous cycle, of the slope (P) of the curve representative of the time dependence of the thickness variations of the layer of product in the basket.

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