Conveyor arrangement with two parallel conveying elements

Description

[0001] The present invention is based on a conveying arrangement for a medium, wherein the conveying arrangement has a conveying line via which the medium is conveyed from a source location to a destination location, wherein the conveying line has a conveying section between the source location and the destination location, wherein a conveying element is arranged in the conveying section, wherein the conveying element is controlled by a flow controller, wherein a flow setpoint and an actual flow value are supplied to the flow controller, wherein the flow controller calculates a manipulated variable for the pump based on the flow setpoint and the actual flow value using a control algorithm.

[0002] In many cases, a medium—especially a liquid—must be conveyed from a source to a destination using a conveying element. For example, oil or water must be transferred from a first tank to a second. For a liquid medium, the conveying element is a pump. For larger flow rates, the pump is usually a centrifugal pump. For a gaseous medium, the conveying element is usually a compressor, a blower, or a fan.

[0003] In some cases, uncontrolled flow of the medium is possible. However, often a very specific flow rate must be set, for example, 100 liters per second (although this figure is purely an example).

[0004] For controlled pumping, in the simplest design, the pumping element is operated at a constant speed and a continuously adjustable resistance, which is arranged upstream or downstream of the pumping element in the delivery line, is adjusted accordingly. In the case of a pump that pumps a liquid medium, this could be an adjustable valve, for example. Although this solution is simple, it is very inefficient in energy terms. In particular, the pumping element is permanently operated at full power so that it is able to deliver the maximum required flow rate. Furthermore, this approach has its limits at smaller flow rates, as excessive throttling can lead to turbulence in the flow and, moreover, cavitation can even occur in the valve gap when a valve is used. Cavitation leads to considerable wear.

[0005] An improved solution is to operate the conveying element at a variable speed. In this case, as mentioned above, the conveying element is controlled by a flow controller. A flow setpoint and an actual flow value are fed to the flow controller. The flow controller calculates a control variable for the conveying element based on the flow setpoint and the actual flow value using a control algorithm. In this case, the conveying element (more precisely: the drive of the conveying element) is controlled by the flow controller according to the control variable. The control variable is the speed of the conveying element or its drive.

[0006] The latter approach is very efficient, provided the flow rate only needs to be adjusted within relatively narrow limits. In this case, the pumping element and its drive can be optimized for the required flow rate, which, as a rule, only needs to be adjusted within relatively narrow limits. In this case, the ratio between the maximum flow rate and the minimum flow rate should be no more than 5:1, for example, with a liquid as the medium, and even better, 4:1 or less, such as 2:1.

[0007] If the ratio increases, for example, 5:1 or more, corresponding operation of the conveying element is still possible in principle – at least in some cases. However, such operation is energy-inefficient at relatively low flow rates, since both the conveying element itself and the drive of the conveying element are designed for a specific speed and exhibit significantly reduced efficiency at significantly lower speeds. Furthermore, the conveying element generally cannot be operated below a minimum flow rate.

[0008] JP 2001 175336 A discloses a method for controlling a valve which can control a flow rate over a wide range.

[0009] The object of the present invention is to create a possibility by means of which, even with a large ratio between a maximum required flow and a minimum required flow, operation is possible in principle and, furthermore, efficient operation of a correspondingly designed conveying arrangement is also possible.

[0010] The object is achieved by a conveyor arrangement having the features of claim 1. Advantageous embodiments of the conveyor arrangement according to the invention are the subject of dependent claims 2 to 4.

[0011] In the conveying arrangement for a medium according to the invention, that the conveying arrangement has a conveying line via which the medium is conveyed from a source location to a destination location, that the conveying line has a conveying section between the source location and the destination location, that two conveying elements designed as pumps, compressors, condensers or blowers are arranged in the conveying section, that the conveying elements are connected in parallel to one another, that the conveying elements are controlled by a respective flow controller, that both flow controllers are supplied with a uniform flow setpoint and a uniform flow actual value for both flow controllers, that the flow controllers calculate a respective manipulated variable for the respective conveying element based on the flow setpoint and the flow actual value using a respective control algorithm, that the conveying device has a switching device to which the flow setpoint is supplied,that the switching device is designed such that at any given time only one of the two conveying elements is controlled according to the control variable of the associated flow controller, while the other conveying element is not controlled, that the switching device is designed such that when the flow setpoint falls below a lower switching threshold, it suppresses the control of one conveying element by one flow controller and enables the control of the other conveying element by the other flow controller, so that the medium flows exclusively via the other conveying element as long as the flow setpoint remains above a minimum value and below an upper switching threshold, and that the switching device is designed such thatthat when the flow setpoint rises above the upper switching threshold, it suppresses the control of the other conveying element by the other flow controller and enables the control of one conveying element by the one flow controller, so that the medium flows exclusively via the one conveying element as long as the flow setpoint remains below a maximum value and above the lower switching threshold.

[0012] According to the invention, two conveying elements are connected in parallel. In the case of pumps, the conveying elements are generally designed as centrifugal pumps. In any case, however, the conveying elements are differently dimensioned. Since the two conveying elements are differently dimensioned, one of the two conveying elements can be referred to as the large conveying element and the other as the small conveying element, without loss of generality. Variable-speed drives are assigned to the conveying elements as drives. The drives can, in particular, be inverter-controlled electric drives. Each conveying element is controlled by its own flow controller, whereby the respective flow controller can be parameterized to be optimized for the control and operation of the respective conveying element. However, both flow controllers are supplied with the same flow setpoint and the same flow actual value.At any given time, however, only one of the two conveying elements is controlled according to the control value of the corresponding flow controller. The other conveying element is not controlled.

[0013] Assume that at a specific point in time the large conveying element is controlled. This state is maintained until the flow setpoint falls below the lower switching threshold. The switching device monitors whether the flow setpoint falls below the lower switching threshold. If the flow setpoint falls below the lower switching threshold, from that point onwards the large conveying element is no longer controlled according to the manipulated variable of its assigned flow controller, but rather the system switches to the small conveying element, so that the small conveying element is controlled by its assigned flow controller. This state is now maintained until the flow setpoint rises above the upper switching threshold. The switching device also monitors whether the upper switching threshold is exceeded.If the flow setpoint exceeds the upper switching threshold, from this point onwards the small conveying element is no longer controlled according to the control variable of the flow controller assigned to it, but it is switched back to the large conveying element so that the large conveying element is controlled according to the control variable of the flow controller assigned to it.

[0014] Of course, the lower switching threshold must not be higher than the upper switching threshold. However, the two switching thresholds can have the same value. Preferably, however, the upper switching threshold is even higher than the lower switching threshold. This creates a hysteresis behavior, ensuring stable operation of the conveyor system. In any case, the maximum value is higher than either of the two switching thresholds, and the minimum value is lower than either of the two switching thresholds.

[0015] When switching from one conveying element to the other, unstable conditions may occur briefly. To achieve the most stable and smooth switching possible, the flow controller whose conveying element is not controlled is constantly tracked. This means that a tracking value is supplied to the flow controller for which control of the associated conveying element is suppressed. The tracking value is calculated by multiplying the manipulated variable of the flow controller whose conveying element is controlled by it by a respective predetermined factor. The predetermined factor for one conveying element preferably corresponds to the quotient of the nominal flow rate of the other conveying element divided by the nominal flow rate of one conveying element.Similarly, the predetermined factor for the other conveying element preferably corresponds to the quotient of the nominal flow rate of one conveying element divided by the nominal flow rate of the other conveying element. In any case, however, both factors are reciprocal. Thus, if one factor is designated f1 and the other factor f2, then f1*f2 = 1.

[0016] To avoid overloads when starting up the conveying elements, a respective ramp sensor is preferably assigned to the flow controllers. Using the respective ramp sensor, the speed of the respective conveying element is increased according to a predetermined ramp, starting from the moment the respective flow controller releases control of the respective conveying element. This stabilizes the operating behavior of the conveying arrangement.

[0017] Preferably, the two flow controllers are coupled together so that they not only always receive the same flow setpoint and the same flow actual value, but also always operate in the same operating mode. Possible operating modes include manual and automatic mode. In manual mode, an operator sets a speed for one of the pumping elements. In automatic mode, the flow is controlled according to the flow setpoint and the actual flow value.

[0018] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of the embodiments, which are explained in more detail in conjunction with the drawings. Herein, in schematic representation: FIG 1 shows part of a technical system, FIG 2 shows a conveyor arrangement, FIGS 3 to 5 show time diagrams and FIG 6 shows another conveyor arrangement.

[0019] According to FIG 1 A medium 1 is to be transported from a source location 2 to a destination location 3. Medium 1 is usually a liquid medium, such as water or oil. However, in individual cases, it can also be a gaseous medium.

[0020] Source location 2 can, for example, be a tank from which medium 1 is withdrawn. Destination location 3 can, for example, be a reactor in which medium 1 is mixed with at least one other medium or otherwise processed. However, other configurations are also possible.

[0021] For conveying the medium 1 from the source location 2 to the destination location 3, a conveying arrangement has a conveying line 4. The conveying line 4 has a conveying section 5 between the source location 2 and the destination location 3. The conveying section 5 is in FIG 1bounded by vertical dashed lines and in FIG 2 presented in more detail.

[0022] According to the FIG 1 and 2 Two conveying elements 6, 7 are arranged in the conveying section 5. If the medium 1 is a liquid, the conveying elements 6, 7 are designed as pumps. In this case, the pumps are usually designed as centrifugal pumps. If the medium 1 is a gas, the conveying elements 6, 7 can be designed, for example, as compressors, condensers, or blowers. In each case, however, the conveying elements 6, 7 are dimensioned differently.

[0023] The conveying elements 6, 7 are connected in parallel. The conveying section 5 thus has a front junction 8, where the conveying line 4 forks into two branches 9, 10. One of the two conveying elements 6, 7 is located in each of the two branches 9, 10. Often, there is also a rear junction 11, where the two branches 9, 10 rejoin.

[0024] The conveyor elements 6, 7 are arranged according to FIG 2 controlled by a respective flow controller 12, 13. The two flow controllers 12, 13 are usually designed as PI controllers or PID controllers. The letters P, I, and D in this context, as is generally known, stand for P = proportional, I = integral, and D = differential.

[0025] A set flow rate F* is supplied to the flow controllers 12, 13. An actual flow rate F is also supplied to the flow controllers 12, 13. The actual flow rate F can be detected, for example, by a flow sensor 14 arranged in the delivery line 4, either upstream of the front junction 8 or—if present—downstream of the rear junction 11.

[0026] It is clear that the same flow setpoint F* and the same flow actual value F are supplied to the flow controllers 12, 13. The flow setpoint F* and the flow actual value F are therefore the same for both flow controllers 12, 13. Furthermore, a summation point 15 can be arranged upstream of the flow controllers 12, 13, in which the control difference δF is formed, i.e., the control difference between the flow setpoint F* and the flow actual value F. In this case, the control difference δF can be supplied to the flow controllers 12, 13. Alternatively, the control difference δF can also be formed within the flow controllers 12, 13.

[0027] The two flow controllers 12, 13 operate according to a respective control algorithm C1, C2. The two control algorithms C1, C2 determine, for example, whether the respective flow controller 12, 13 operates as a P controller, as a PI controller, as a PID controller, etc., which size the proportional gain has, which reset time the integral component has, etc. The two control algorithms C1, C2 are individually and independently tuned to the optimal control of the respective conveying element 6, 7. Each of the two flow controllers 12, 13 uses its control algorithm C1, C2 to determine a respective manipulated variable S1, S2 for the conveying element 6, 7 it controls based on the flow setpoint F* and the flow actual value F (or the control difference δF). The respective manipulated variable S1, S2 can, in particular, be a respective setpoint speed. The flow controllers 12, 13 control the respective conveying element 6, 7 - more precisely: the drive 16, 17 (see FIG 1) of the respective conveyor element 6, 7 - according to their respective control variables S1, S2. The conveyor elements 6, 7 and their drives 16, 17 are thus speed-controllable. The drives 16, 17 can be designed, in particular, as electric drives. In this case, the drives 16, 17 can be powered, for example, via a respective converter (not shown in the figures).

[0028] Of the two conveying elements 6, 7, only one is active at a time. To activate and deactivate the conveying elements 6, 7, the conveying arrangement has a switching device 18. In particular, the flow setpoint F* is supplied to the switching device 18. The switching device 18 is designed such that - depending on the value of the flow setpoint F* - it enables the control of one of the conveying elements 6, 7 by the corresponding flow controller 12, 13 and suppresses the control of the other of the two conveying elements 7, 6 by the corresponding flow controller 13, 12. This will be explained below with additional reference to the FIGS 3 and 4explained in more detail. Enabling and disabling the control of the conveyor elements 6, 7 (or their drives 16, 17) can be achieved, for example, by the switching device 18 outputting a respective activation signal A1, A2 to the two conveyor elements 6, 7. At any given time, the corresponding activation signal A1, A2 is output to only one of the two conveyor elements 6, 7. When the respective activation signal A1, A2 is switched off, the respective conveyor element 6, 7 is completely switched off.

[0029] According to FIG 3the flow setpoint F* has a minimum value MIN at a time t1. After time t1, the flow setpoint F* gradually increases as a function of time t to a maximum value MAX. Specifically, the flow setpoint F* reaches an upper switching threshold SO at a time t2 and the maximum value MAX at a time t3. From a time t4, the flow setpoint F* drops back to the minimum value MIN. Specifically, the flow setpoint F* reaches a lower switching threshold SU at a time t5 and the minimum value MIN at a time t6.

[0030] At time t1, as shown in FIG 4 The activation signal A2 is output, so that the conveying element 7 is controlled according to the control signal S2 determined by the flow controller 13. Correspondingly, the activation signal A1 is not output.

[0031] This state is maintained until the flow setpoint F* reaches the upper switching threshold SO at time t2. When the upper switching threshold SO is exceeded, the activation signal A2 switches to the activation signal A1. This ensures that the conveying element 6 is now controlled by the flow controller 12 according to the control signal S1 determined by it, and the conveying element 7 is no longer controlled by the flow controller 13.

[0032] This state is maintained until the flow setpoint F* reaches the lower switching threshold SU at time t5. When the flow falls below the lower switching threshold SU, the system switches back from activation signal A1 to activation signal A2.

[0033] As a result, the medium 1 flows exclusively through the conveying element 6, 7 to which the corresponding activation signal A1, A2 is applied. In particular, the medium 1 flows exclusively through the conveying element 6 as long as the flow setpoint F* remains above the upper switching threshold SO and below the maximum value MAX. Likewise, the medium 1 flows exclusively through the conveying element 7 as long as the flow setpoint F* remains below the lower switching threshold SU and above the minimum value MIN.

[0034] To completely suppress any flow of medium 1 through the non-activated conveying element 6, 7, a blocking element 19, 20, for example a corresponding valve, can be arranged in each of the two branches 9, 10. In this case, the respective blocking element 19, 20 is permanently kept fully open by the switching device 18 as long as its activation signal A1, A2 is supplied to the conveying element 6, 7 arranged in the same branch 9, 10. If its respective activation signal A1, A2 is not supplied to the respective conveying element 6, 7, the respective blocking element 19, 20 is closed.

[0035] In order to enable the smoothest possible switching from one conveying element 6 to the other conveying element 7 and vice versa, a tracking value N1 is supplied to the flow controller 12 whenever the conveying element 6 controlled by it is not being controlled. In a similar manner, a tracking value N2 is supplied to the flow controller 13 whenever the conveying element 7 controlled by it is not being controlled. The tracking values ​​N1, N2 determine the value for the integral component with which the respective flow controller 12, 13 is initialized at the time at which it is activated (i.e., the output of the manipulated variable S1, S2 determined by it to the corresponding conveying element 6, 7 is released). In the example according to the FIGS 3 and 4For the flow controller 12, this is time t2. For the flow controller 13, this is time t5. In particular, the integral component of the respective flow controller 12, 13 is determined such that the manipulated variable S1, S2 of the respective flow controller 12, 13 is equal to the respective tracking value N1, N2 at the time t2 or t5 of the switching.

[0036] The tracking values ​​N1, N2 are calculated according to FIG 2by the switching device 18. However, they could also be determined by another device. The determination is carried out by multiplying the respective tracking value N1 or N2 by the manipulated variable S2 or S1 of the other flow controller 13, 12 by a respective predetermined factor f1, f2. Therefore, N1 = f1*S2 and N2 = f2*S1. When determining the tracking values ​​N1 or N2, the manipulated variable limitations of the flow controllers 12, 13 are also taken into account. The two factors f1, f2 are reciprocal to each other. Therefore, f1*f2 = 1 or f2 = 1 / f1.

[0037] The two factors f1 and f2 can be determined as needed. However, they are preferably determined based on the nominal flow rates of the corresponding conveying elements 6 and 7. If the nominal flow rate of conveying element 6 is designated FN1 and the nominal flow rate of conveying element 7 is designated FN2, then preferably f1 = FN2 / FN1 and, conversely, f2 = FN1 / FN2.

[0038] After switching the conveyor elements 6, 7, the newly activated conveyor element 6, 7 requires a certain amount of time until it has reached the desired speed (as determined by the respective control variable S1, S2 or the corresponding tracking value N1, N2). Since the newly activated conveyor element 6, 7 runs up against resistance, it can be overloaded if the actually desired speed is reached too quickly. For this reason, the conveyor elements 6, 7 are assigned a certain time as shown in FIG 2a respective ramp encoder 21, 22 is assigned. The ramp encoders 21, 22 can, for example, be arranged in the inverters if the conveying elements 6, 7 are driven by converter-fed drives. By means of the respective ramp encoder 21, 22, starting from the activation of the respective conveying element 6, 7, a speed of the respective conveying element 6, 7 is increased according to a predetermined respective ramp. The increase takes place up to a limit speed. The limit speed can, for example, be determined by the tracking value N1, N2 at the time of activation of the corresponding conveying element 6, 7. After completing the respective ramp, the respective ramp encoder 21, 22 is switched inactive again. From this point on, the normal control of the respective conveying element 6, 7 takes place via the respective flow controller 12, 13. This is described below in connection with FIG 5for the conveying element 6 and its flow regulator 12 are explained in more detail.

[0039] FIG 5 shows an example of a possible time course of the flow setpoint F*. According to FIG 5 the flow setpoint F* rises over time t at a time t11 above the upper threshold value SO. At time t11, the system switches from the conveying element 7 to the conveying element 6. From time t11 onwards, the control of the conveying element 7 is suppressed and the conveying element 6 is controlled instead. The upper threshold value SO in the stable state corresponds to a specific speed nS1. Starting at time t11, the ramp generator 21 outputs a speed setpoint n1 according to its ramp. The speed setpoint n1 rises as shown in FIG 5from time t11, the ramp increases linearly until the end of the ramp is reached at time t12. Only at this point is the output of the "normal" manipulated variable S1 to conveyor element 6 enabled.

[0040] Analogous designs apply to the conveying element 7 and its flow controller 13, i.e., for the inverse switching. However, the run-up duration and the specific speed nS2 for the speed setpoint n2 according to the ramp of the ramp encoder 22 may differ from the corresponding values ​​of the ramp encoder 21.

[0041] The control-technical integration of the control-technical implementation of the conveying arrangement, i.e., the two flow controllers 12, 13, into a more comprehensive process control system is readily possible. In particular, it is possible, within the process control system, to interconnect the two controller instances by means of which the flow controllers 12, 13 are implemented, so that they are always supplied with the same flow setpoint F* and the same flow actual value F, or always with the same control difference δF. Furthermore, the two flow controllers 12, 13 can also be easily coupled to one another in the process control system, so that they not only always receive the same flow setpoint F* and the same flow actual value F, but are also always operated in the same operating mode (i.e., automatic or manual).Within the framework of an operating and monitoring system, it is also possible to display only a single flow controller for the conveying section 5, which (apparently) controls only a single conveying element. As a rule, this will be the flow controller 12, 13 for the conveying element 6, 7 with the larger nominal flow rate. However, it is equally possible to display the conveying section 5 as it is (i.e. with the two branches 9, 10 and the conveying elements 6, 7 arranged therein). In this case, however, only a single flow controller is displayed for both conveying elements 6, 7. The latter display can alternatively be permanent or, for example, be output instead of the former display in the event of a malfunction of one of the two conveying elements 6, 7.

[0042] It is also easily possible to expand the conveyor arrangement to three, four, etc., conveyor elements connected in parallel. This is briefly outlined below for an expansion to three conveyor elements.

[0043] So let us assume that we have, as shown in FIG 6 a configuration of a conveying arrangement in which the conveying section 5, in addition to the branches 9, 10 with the conveying elements 6, 7, has a further branch 23 in which a further conveying element 24 is arranged. Without limiting the generality, it can be assumed that the conveying element 7 is the conveying element with the smaller nominal flow and that the further conveying element 24 has a nominal flow that is smaller than the nominal flow of the smaller conveying element 7. In this case, switching between the conveying elements 6, 7, as described above in the FIGS 3 to 5A further switching between the smaller conveyor element 7 and the further conveyor element 24 can be carried out in a completely analogous manner to the explanations for the FIGS 3 to 5 However, as far as the interaction of the conveying element 7 and the further conveying element 24 is concerned, the following replacements must be made: The maximum value for the interaction of conveying element 7 and the additional conveying element 24 is equal to the upper or lower switching threshold SO, SU for the interaction of conveying elements 6 and 7. The upper or lower switching threshold for the interaction of conveying element 7 and the additional conveying element 24 is equal to the minimum value MIN. The minimum value for the additional conveying element 24 must be selected to be (sufficiently) small.

[0044] In a completely analogous manner, further extensions can be made to include even more conveyor elements.

[0045] In summary, the present invention relates to the following: A medium 1 is conveyed from a source location 2 to a destination location 3 via a conveying line 4. Between the source location 2 and the destination location 3, the conveying line 4 has a conveying section 5 in which two conveying elements 6, 7 are arranged. The conveying elements 6, 7 are connected in parallel and are controlled by a respective flow controller 12, 13. Both flow controllers 12, 13 are supplied with the same setpoint flow value F* and the same actual flow value F. From this, they determine a respective manipulated variable S1, S2 for the respective conveying element 6, 7.A switching device 18 of the conveying arrangement is designed such that, when the flow setpoint F* drops below a lower switching threshold SU, it suppresses the control of one conveying element 6 by one flow controller 12 and enables the control of the other conveying element 7 by the other flow controller 13, so that the medium 1 flows exclusively via the other conveying element 7 as long as the flow setpoint F* remains above a minimum value MIN and below an upper switching threshold SO.In an analogous manner, if the flow setpoint F* rises above the upper switching threshold SO, the control of the other conveying element 7 by the other flow controller 13 is suppressed and the control of one conveying element 6 by the one flow controller 12 is released, so that the medium 1 flows exclusively via the one conveying element 6 as long as the flow setpoint F* remains below a maximum value MAX and above the lower switching threshold SU.

[0046] The present invention offers the following advantages: The actual flow rate F of the medium 1 can be precisely controlled in an energy-efficient manner. Due to the use of dedicated flow controllers 12, 13, a high level of control quality is achieved. By tracking the inactive flow controller 12, 13, integrator windup of the flow controllers 12, 13 can be avoided, and an at least virtually smooth transition when changing the conveying elements 6, 7 can be achieved. For the system operator, however, there are hardly any changes.

[0047] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

1. Conveyor assembly for a medium (1), - wherein the conveyor assembly has a conveyor line (4) via which the medium (1) is conveyed from a source location (2) to a destination (3), - wherein the conveyor line (4) has a conveyor section (5) between the source location (2) and the destination (3), - wherein two conveyor elements (6, 7), which are embodied as pumps, compressors, condensers or fans, are arranged in the conveyor section (5), - wherein the conveyor elements (6, 7) are connected in parallel with one another, - wherein the conveyor elements (6, 7) are controlled by a respective flow controller (12, 13), - wherein a target flow value (F*) which is uniform for both flow controllers (12, 13) and an actual flow value (F) which is uniform for both flow controllers (12, 13) are supplied to both flow controllers (12, 13), - wherein the flow controllers (12, 13) calculate a respective manipulated variable (S1, S2) for the respective conveyor element (6, 7) on the basis of the target flow value (F*) and the actual flow value (F) using a respective control algorithm (C1, C2), - wherein the conveying device has a changeover device (18) to which the target flow value (F*) is supplied, - wherein the changeover device (18) is embodied such that only one of the two conveyor elements (6, 7) is controlled according to the manipulated variable (S1, S2) of the associated flow controller (12, 13) at each time, while the other conveyor element (6, 7) is not actuated, - wherein the changeover device (18) is embodied such that, if the target flow value (F*) falls below a lower changeover threshold (SU), it suppresses the actuation of the one conveyor element (6) by the one flow controller (12) and releases the actuation of the other conveyor element (7) by the other flow controller (13) so that the medium (1) flows solely via the other conveyor element (7) as long as the target flow value (F*) remains above a minimum value (MIN) and below an upper changeover threshold (SO), and - wherein the changeover device (18) is embodied such that, if the target flow value (F*) exceeds the upper changeover threshold (SO), it suppresses the actuation of the other conveyor element (7) by the other flow controller (13) and releases the actuation of the one conveyor element (6) by the one flow controller (12), so that the medium (1) flows solely via the one conveyor element (6) as long as the target flow value (F*) remains below a maximum value (MAX) and above the lower changeover threshold (SU).

2. Conveyor assembly according to claim 1, characterised in that - a tracking value (N1, N2) is supplied to the flow controller (12, 13) for which the actuation of the associated conveyor element (6, 7) is suppressed, - the tracking value (N1, N2) is obtained by multiplying the manipulated variable (S2, S1) of the flow controller (13, 12) which actuates its conveyor element (7, 6) by a respective predetermined factor (f2, f1), and - the two factors (f1, f2) are reciprocal to one another.

3. Conveyor assembly according to claim 1 or 2, characterised in that the flow controllers (12, 13) are assigned a respective ramp generator (21, 22) by means of which, calculated from the release of the actuation of the respective conveyor element (6, 7) by the respective flow controller (12, 13), a speed of the respective conveyor element (6, 7) is ramped up in accordance with a predetermined respective ramp.

4. Conveyor assembly according to claim 1, 2 or 3, characterised in that the two flow controllers (12, 13) are coupled to one another so that they not only always receive the same target flow value (F*) and the same actual flow value (F), but are also always operated in the same operating mode.