Control of the flow generator in the storage tank

Abstract

This invention relates to a method for controlling a flow generator (1) in a tank (20) for containing a liquid comprising solid matter. The flow generator (1) includes an impeller and is located at a height (h - mixer) in the tank (20), the tank (20) having a predetermined maximum filling height (h - maximum). The flow generator (1) is configured to operate at a variable operating speed (n), and the operating speed (n - required) depends on the current liquid level in the tank (20) (h - current). The maximum operating speed of the flow generator (1) is... The operating speed (n-maximum) is the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-maximum). At least when [(h-mixer)+X]≤(h-current)≤(h-maximum), the current operating speed (n-current) is set to be equal to the required operating speed (n-required) of the flow generator (1), which is determined using formula (I), where a≥(1 / 4) and a≤1, X=radius of the impeller of the flow generator+1, and all heights and measurements are given in meters.

Description

Technical Field

[0001] This invention generally relates to the field of submersible devices for stirring, mixing, and / or moving liquids in a tank, and also to methods for controlling the operation of such devices. The invention particularly relates to the field of submersible flow generators or mixers for liquids containing solid substances, and also to methods for controlling the operation of such flow generators in tanks configured for temporarily storing liquids containing solid substances, and to tanks including such flow generators. This invention is particularly applicable to wastewater and sewage applications, such as treatment plants and racetracks. Background Technology

[0002] Some steps in the treatment process at the plant (e.g., tributary buffer tanks, runways, etc.) and other similar intermediate and / or processing tanks / storages, either inside or outside the plant, have varying input flow rates entering the tanks, resulting in different liquid level heights in a particular tank when the output flow rate from the tank is not the same as the input flow rate. The purpose of buffer tanks is to ensure a constant / uniform output flow rate to the next step in the treatment plant, regardless of the input flow rate. The input flow rate will vary throughout the day, week, year, and with weather conditions.

[0003] When wastewater / sewage / surface water arrives at the wastewater treatment plant, it undergoes initial screening to separate large objects from liquids / slurries. Then, the liquids / slurries reach buffers / tanks before entering the actual treatment steps.

[0004] The purpose of a buffer tank is to balance the inputs of the actual treatment steps to ensure uniform and stable treatment in subsequent steps. Therefore, the liquid level in the buffer tank will fluctuate significantly throughout the day and week, as well as due to factors such as rain. After initial screening, the liquid / slurry in the buffer tank still contains heavier solids, such as sand and stones, which will easily settle / deposit. It is important that these solids do not settle / deposit until after they leave the buffer tank and enter a specific sedimentation tank. Equally important is that lighter solids (typically biological material) are uniformly distributed throughout the liquid volume. Therefore, in such a tank, it is crucial that the liquid containing solids (e.g., sludge) has the most homogeneous and uniform mixture possible to optimize treatment in subsequent steps and to have a predictable waste load distribution.

[0005] When solid matter settles / deposits in the arrival / buffer tank, the tank needs to be cleaned by emptying the liquid / slurry before the sediment can be removed. Therefore, it should be understood that this cleaning is not possible without parallel arrival tanks, each of which is too large to receive all incoming wastewater / sewage when one tank is disconnected.

[0006] The need for mixing is high when the tank contains a large or maximum amount of liquid, and low when the tank contains a small amount of liquid. When the liquid volume is too small, output can be temporarily stopped, and mixing is not required. To prevent settling / deposition upon arrival at the tank, one or more mixers are provided, operating at a high and constant speed to ensure no solid matter settles / deposits when the buffer tank is full. Therefore, energy consumption is much higher than actually required most of the time because the liquid level in the tank is below or even far below the maximum fill level.

[0007] Therefore, despite these variations in mixing requirements, mixers / flow generators typically operate at a constant / maximum operating speed to prevent the settling of solids and to ensure that the output liquid is mixed as constantly / uniformly as possible, thus avoiding unnecessary high power consumption over a period of time.

[0008] The same or related problem of unnecessarily high power consumption associated with the instantaneous liquid / slurry volume in the tank also exists in other tanks besides the buffer tank, because when the liquid / slurry level in a particular tank is low, a higher operating speed than required will be used.

[0009] Purpose of the invention

[0010] The purpose of this invention is to eliminate the aforementioned disadvantages and defects of previously known methods for controlling flow generators in tanks, and to provide an improved method, tank, and flow generator.

[0011] The primary objective of this invention is to provide an improved method for controlling a flow generator in a tank of a initially defined type, wherein the liquid volume / liquid level in the tank varies / changes over time. Another objective of this invention is to provide a method for controlling a flow generator in a tank to reduce power consumption while ensuring proper mixing of the liquid containing solids. Yet another objective of this invention is to provide a method for controlling a flow generator in a tank to increase the operational life of the flow generator by not operating it at high / maximum operating speeds and by not subjecting the flow generator to strenuous operation. Summary of the Invention

[0012] According to the invention, at least the primary objective is achieved by the initially defined method, tank, and flow generator having the features defined in the independent claim. Preferred embodiments of the invention are further defined in the dependent claims.

[0013] According to a first aspect of the invention, a method for controlling a flow generator in a tank, as initially defined, is provided, wherein the flow generator includes an impeller and is located at a height (h-mixer) in the tank, the tank having a predetermined maximum filling height (h-maximum), the flow generator being configured to operate at a variable operating speed (n), and requiring the operating speed (n-required) to depend on the current liquid level in the tank (h-current), the maximum operating speed (n-maximum) of the flow generator being the operating speed required when the liquid level in the tank is equal to the maximum filling height (h-maximum), and at least when [(h-mixer)+X]≤(h-current)≤(h-maximum), the current operating speed (n-current) is set to be equal to the required operating speed (n-required) of the flow generator, which is determined using the following formula:

[0014] (n-current) = (n-required) = (n-maximum) / [(h-maximum) / (h-current)]^a

[0015] Where a ≥ (1 / 4) and a < 1, X = radius of the impeller of the flow generator + 1, and all heights and measurements are given in meters.

[0016] According to a second aspect of the invention, a tank is provided, configured to contain a liquid comprising solid matter, the tank including a flow generator, a control unit, and a height detection device, the control unit being configured to operate the flow generator at a variable operating speed (n), the height detection device being configured to determine a current liquid level (h-current) in the tank, wherein a maximum operating speed (n-maximum) of the flow generator is the operating speed required when the liquid level in the tank is equal to a maximum filling height (h-maximum), the control unit being configured to set the current operating speed (n-current) of the flow generator to be equal to a required operating speed (n-required) at least when [(h-mixer)+X]≤(h-current)≤(h-maximum), the required operating speed (n-required) depending on the current liquid level (h-current):

[0017] (n-current) = (n-required) = (n-maximum) / [(h-maximum) / (h-current)]^a

[0018] Where a ≥ (1 / 4) and a < 1, X = radius of the impeller of the flow generator + 1, and all heights and measurements are given in meters.

[0019] According to a third aspect of the invention, a flow generator suitable for such a tank is provided, wherein the flow generator includes an impeller and a fixed body having a drive unit, the impeller being connected to and driven to rotate by the drive unit. The flow generator includes a control unit, wherein the flow generator is configured to operate at a variable operating speed (n) via the control unit, and the flow generator includes a pressure sensor operatively connected to a drive unit. The operating speed (n) of the flow generator depends on a current pressure (P-sensor) determined by the pressure sensor, the current pressure (P-sensor) being proportional to a current liquid level (h-current). The maximum operating speed (n-maximum) of the flow generator is the operating speed required when the liquid level in the tank is equal to the maximum filling height (h-maximum). The control unit is configured to set the current operating speed (n-current) of the flow generator to be equal to a required operating speed (n-required) at least when [(h-mixer)+X]≤(h-current)≤(h-maximum), the required operating speed (n-required) depending on the current liquid level (h-current):

[0020] (n-current) = (n-requirement) = (n-maximum) / [(h-maximum) / (h-current)]^a, where a ≥ (1 / 4) and a < 1.

[0021] X = radius of the impeller of the flow generator + 1, and

[0022] All heights and measurements are given in meters.

[0023] Therefore, this invention is based on the insight that a smaller liquid volume in the tank requires less thrust to agitate / mix the liquid and keep solids in the liquid suspended, while an increased liquid volume in the tank requires more thrust to agitate / mix the liquid and keep solids in the liquid suspended. Thus, this invention is based on the inventors' understanding that there is a predictable relationship between thrust demand (i.e., the operating speed of the flow generator / impeller) and the liquid volume in the tank (i.e., the liquid level). By determining / monitoring the instantaneous thrust demand in a tank with varying liquid / slurry volumes and correspondingly controlling the operating speed of the flow generator, the goal of minimizing power consumption and extending the operational life of the flow generator is achieved. This invention is also based on the understanding that there is a relationship between the pressure level in the tank and the liquid volume in the tank (i.e., the liquid level).

[0024] According to various embodiments of the present invention, when (h-mixer) ≤ (h-current) ≤ (h-fixed), the current operating speed (n-current) is set to be equal to a fixed value (n-fixed) of the flow generator, which is the operating speed required when the liquid level in the tank is equal to a predetermined filling height (h-fixed), and is determined using the following formula:

[0025] (n-current) = (n-fixed) = (n-maximum) / [(h-maximum) / (h-fixed)]^a where, (h-fixed) ≤ [(h-mixer) + X].

[0026] This means that when the liquid level is too close to the impeller of the flow generator, it deviates from the normal operating strategy, and the flow generator operates at a fixed speed, meaning it does not change according to further reductions in the liquid level in the tank. Therefore, when the impeller blades approach or cut into the liquid surface, the mixing effect decreases drastically, but the flow generator continues to operate—the impeller rotates—so as not to be forced to start from rest when the liquid level rises again, and because the lower operating speed would not provide any useful liquid mixing, but merely consume energy. When the liquid does not cover the flow generator body, the liquid will not provide cooling, and the flow generator is preferably stopped.

[0027] According to various embodiments of the invention, a flow generator is associated with a pressure sensor for determining the pressure (P-sensor) at the height (h-mixer) of the flow generator in the tank, wherein the liquid height (h-above) above the flow generator depends on the current pressure (P-sensor), and the current liquid level height (h-current) in the tank is determined using the following formula: (h-current) = (h-mixer) + (h-above).

[0028] According to various embodiments of the invention, the liquid height (h-above) above the flow generator depends on the sum of the current pressure (P-sensor) minus the air pressure deviation (ΔP-air), wherein the air pressure deviation (ΔP-air) is determined based on the current air pressure relative to the air pressure (P-correction) present during pressure sensor calibration.

[0029] Other advantages and features of the invention will become clear from the other dependent claims and the following detailed description of preferred embodiments. Attached Figure Description

[0030] The above and other features and advantages of the present invention will be more fully understood through the following detailed description of preferred embodiments in conjunction with the accompanying drawings, in which:

[0031] Figure 1 This is a schematic perspective view of the flow generator of the present invention.

[0032] Figure 2 It is based on Figure 1 A schematic cross-sectional side view of the flow generator, and

[0033] Figure 3 This is a schematic side view of the tank including this flow generator. Detailed Implementation

[0034] First refer to Figure 1 and 2 The present invention particularly relates to a flow generator or mixer, generally designated 1, suitable for treating / transporting liquids containing solid substances, such as wastewater / sewage, and particularly to a method for controlling such a flow generator 1.

[0035] The flow generator 1 is a submersible mixer and comprises three main parts: a stationary body generally designated 2, a rotatable impeller / propeller 3, and a control unit 4. The control unit 4 controls a drive unit located inside the stationary body 2, which drives the impeller 3 to rotate, and the impeller 3 propels the liquid, i.e., provides thrust to the liquid. The stationary body 2 and the impeller 3 are always part of the flow generator 1, and in the disclosed embodiment, the control unit 4 is integrated into the flow generator and constitutes part of it. In an alternative embodiment, the control unit 4 is a separate component and is operatively connected to the flow generator. The flow generator 1 is also referred to as a mixer or blender. In the disclosed embodiment, the flow generator is a submersible flow generator, i.e., configured to be fully submerged. However, it should be noted that the submersible flow generator 1 can be partially positioned above the liquid surface during operation. The flow generator 1 is cooled by the liquid surrounding the stationary body 2.

[0036] A cable 5 extending from a power source (e.g., a power line) supplies power to a flow generator 1, which includes a liquid-tight lead 6 for receiving the cable 5. The cable 5 may also include signal lines for data communication between the flow generator and an external control unit (not shown).

[0037] exist Figure 2The diagram schematically discloses some internal components of the flow generator 1. The drive unit includes an electric motor, generally designated 7, and a drive shaft assembly 8, which is connected to and driven by the electric motor 7 during operation of the flow generator 1. The electric motor 7 conventionally includes a stator 9 and a rotor 10. In the disclosed embodiment, the drive shaft assembly 8 includes a drive shaft 11 (i.e., the rear end portion) and a propeller shaft 12 (i.e., the front end portion), wherein a mechanical transmission unit 13 is arranged between the drive shaft 11 and the propeller shaft 12. The rotor 10 is connected to and rotates with the drive shaft 11 of the drive shaft assembly 8. An impeller 3 is conventionally connected to and rotates with the propeller shaft 12 of the drive shaft assembly 8. The transmission unit 13 has a fixed gear ratio, wherein the impeller 3 has a lower rotational speed than the rotor 10 of the electric motor 7, i.e., a reduction gear device. The gear ratio is preferably equal to or less than 100:1, more preferably equal to or less than 60:1, and more preferably equal to or greater than 2:1, even more preferably equal to or greater than 15:1. According to an alternative embodiment, the gear ratio is 1:1, i.e., there is no gear shifting, and the drive shaft 11 and the propeller shaft 12 are constructed from the same shaft components. The drive unit also includes necessary bearings and seals (not shown). During normal operation of the flow generator 1, the rotational speed of the impeller / propeller 3 (i.e., the operating speed of the flow generator 1) is equal to or less than 1000 rpm and equal to or greater than 200 rpm.

[0038] In the disclosed embodiment, both the drive shaft 11 and the propeller shaft 12 extend in an axial direction and are preferably collinear. According to an alternative embodiment, the mechanical transmission unit 13 is angled, i.e., there is an angle, for example, 90 degrees, between the drive shaft 11 and the propeller shaft 12. In the latter case, the propeller shaft 12 extends in an axial direction.

[0039] The electric motor 7 is located within the housing 14 of the fixed body 2. In the disclosed embodiment, the impeller 3 is positioned in direct contact with the housing 14, which is a liquid-tight housing. However, in an alternative embodiment, the impeller 3 is located at a distance from the housing 14, i.e., the propeller shaft 12 of the drive shaft assembly 8 is visible between the housing 14 and the impeller 3. According to the alternative embodiment, the electric motor 7 can be located in a dry environment. In most applications, the flow generator is a submersible flow generator, i.e., both the drive unit and the impeller 3 are located below the liquid surface during operation. In an alternative embodiment, when the impeller 3 is below the liquid surface, the housing 14 and the electric motor 7 are not simultaneously located in the liquid, i.e., a so-called top-entry or side-entry flow generator.

[0040] The impeller 3 includes a hub 15 connected to a propeller shaft 12 of a drive shaft assembly 8 and a plurality of blades 16 connected to the hub 15. The propeller shaft 12 extends axially, and each blade 16 extends radially from its base to its tip. The blade 16 is connected to the hub 15 at its base, and the tip of the blade 16 is the outermost portion of the impeller 3. In the disclosed embodiment, both the leading edge 17 and the trailing edge 18 of the blade 16 are curved, with the leading edge 17 being convex and the trailing edge 18 being concave. It should be noted that the blade 16 naturally also has an axially extending portion, i.e., a pitch, to generate thrust on the liquid. The impeller 3 has a radius measured from the central axis of the impeller 3 and the propeller shaft 12 to the outermost portion of the blade 16.

[0041] Control unit 4 is operatively connected to electric motor 7 and is configured to monitor and control the operation of flow generator 1. Electric motor 7 is configured to be driven by control unit 4 during operation. Therefore, control unit 4 is used to control the rotational speed (at which electric motor 7 of the mixer is driven), for example by controlling the frequency of the current operating the electric motor 7. According to a disclosed embodiment, control unit 4 includes a variable frequency drive (VFD) 19. Therefore, flow generator 1 is configured to operate at a variable operating speed (n). Control unit 4 is configured to perform the method of the invention. Therefore, tank 20 includes control unit 4, which is configured to operate flow generator 1 at a variable operating speed (n) (i.e., RPM of impeller / propeller 3). Flow generator 1 is provided with a maximum operating speed (n-maximum), which is the operating speed, i.e., thrust, required when the liquid level in tank 20 is equal to the maximum filling height (h-maximum) in order to obtain sufficient and suitable mixing. It should be noted that the size of a particular flow generator 1 in a particular application can be set for an operating speed higher than the maximum operating speed (n-maximum). However, when determining the size of the flow generator 1, one tries to make the smallest possible flow generator 1 capable of performing the necessary mixing in order to save costs.

[0042] The following also refers to Figure 3 The flow generator 1 is located in a storage tank 20, which is configured to contain a liquid containing solid matter. The storage tank 20 includes an inlet 21 and an outlet 22, the inlet 21 allowing liquid to enter the storage tank 20 and the outlet 22 removing / draining the liquid from the storage tank 20. Liquid can be pumped into the storage tank 20 and / or arranged to flow into the storage tank 20 by gravity, and liquid can be pumped from the storage tank 20 and / or arranged to flow out of the storage tank 20 by gravity.

[0043] The tank / basin / storage container 20 is constructed, for example, as a buffer tank in a wastewater treatment plant. The tank 20 has a predetermined maximum filling height (h-maximum), determined, for example, by the vertical distance between the overflow outlet 23 and the bottom plate of the tank 20. The maximum filling height (h-maximum) is typically equal to or greater than 4 meters and equal to or less than 15 meters.

[0044] The tank 20 may include a plurality of flow generators 1, wherein one or more flow generators 1 are controlled according to the present invention. The flow generators 1 are located at a certain height (h-mixer) within the tank 20, and the height of the flow generators 1 may be fixed or adjustable. In a disclosed embodiment, the flow generators 1 are arranged to be guided along guide rod 24 by being raised or lowered in chain / line 25. The height of the flow generator 1 (h-mixer) is defined as the height of the central axis of the impeller / propeller 3 of the flow generator 1 relative to the bottom plate of the tank 20. The maximum filling height (h-maximum) or maximum liquid level is known and set by the operator / technician. The height of the flow generator (h-mixer) is known and set by the operator / technician and / or automatically determined during the raising / lowering of the flow generator 1. For example, when the flow generator 1 is at its maximum filling height (h-maximum), the operator can set a value equal to the maximum filling height (h-maximum). Then, the control unit 4 determines how far the flow generator 1 descends along the guide rod 24, where the difference gives the height of the flow generator (h-mixer). In various embodiments, the flow generator 1 is located at a fixed height, the height of the flow generator (h-mixer) is known, and is set by the operator / technician. Additionally, the chain / line 25 may have markings to help the operator / technician set the correct height of the flow generator.

[0045] Furthermore, the tank 20 includes a height detection device for determining the current liquid level (h - current) in the tank 20. The height detection device can be configured as a liquid level sensor located within the tank 20, and can be a submerged liquid level sensor or a dry-mounted liquid level sensor suspended above the liquid level (e.g., using ultrasound, radar, etc.). In a preferred embodiment, the liquid level sensor is configured as a pressure sensor 26 located in the submerged position within the tank 20; most preferably, the pressure sensor 26 is part of or connected to the flow generator 1. The pressure sensor 26 is operatively connected to the control unit 4 and the drive unit, wherein the operating speed (n) of the impeller 3 of the flow generator 1 depends on the current pressure (P - sensor) determined by the pressure sensor 26.

[0046] Therefore, pressure sensor 26 and control unit 4 are used to determine the pressure (P-sensor) at the height (h-mixer) of flow generator 1 in tank 20, wherein the liquid height (h-above) above the flow generator depends on the current pressure (P-sensor), wherein the current liquid level height (h-current) in the tank is determined using the following formula: (h-current) = (h-mixer) + (h-above).

[0047] In various embodiments, the pressure sensor 26 is not located at the same height as the flow generator 1 (h-mixer). The current pressure (P-sensor) is added to / subtracted from the height difference between the position of the pressure sensor 26 and the height of the flow generator (h-mixer), plus the height of the flow generator (h-mixer), to determine the current liquid level (h-current).

[0048] Therefore, the current liquid level (h-current) is proportional to the current pressure (P-sensor) of pressure sensor 26.

[0049] The control unit 4 is configured to set the current operating speed (n-current) of the flow generator 1, which is equal to the required operating speed (n-required), which depends on the current liquid level (h-current) in the tank 20.

[0050] According to the method of the present invention, the required operating speed (n-required) of the flow generator 1 is determined using the following formula at least when [(h-mixer)+X]≤(h-current)≤(h-maximum):

[0051] (n-current) = (n-required) = (n-maximum) / [(h-maximum) / (h-current)]^a

[0052] Where a ≥ (1 / 4) and a < 1, X = radius of impeller 3 of the flow generator + 1, and all heights and measurements are given in meters.

[0053] According to various embodiments, a ≥ (1 / 3) and / or a ≤ 2 / 3, with a = 1 / 2 being the most preferred. Theoretically, there is a linear relationship between the liquid volume (i.e., the liquid depth in the tank) and the thrust of the flow generator 1 required to achieve the desired mixing, and a quadratic relationship between the thrust of the flow generator 1 and the operating speed of the flow generator 1 (i.e., the RPM of the impeller 3). Therefore, theoretically, the variable "a" should be equal to 1 / 2; however, in specific applications, the value of the variable "a" can be set / adjusted by the operator / technician. In particular, when the tank 20 has a non-uniform shape / area along the vertical direction, i.e., different areas at different depths and / or different areas based on the properties of the specific liquid (i.e., sludge) containing solid matter in the tank 20, when the solution of the present invention is to reduce the operating speed linearly with respect to the reduced liquid level height, i.e., when the variable "a" equals 1, this will necessarily result in too low an operating speed to perform the required mixing task, i.e., too small a thrust. Of course, this becomes increasingly applicable when the variable "a" is greater than 1. When the variable "a" is less than 1 / 4, the operation speed will be too high and consume too much energy to perform the requested hybrid task, i.e., too much thrust.

[0054] When the current liquid level (h-current) decreases too close to the flow generator 1 and / or the flow generator 1 increases too close to the current liquid level (h-current), the method preferably includes the following steps: when (h-mixer) ≤ (h-current) ≤ (h-fixed), the current operating speed (n-current) is set to a fixed value (n-fixed) equal to that of the flow generator 1, which is the operating speed required when the liquid level in the tank 20 is equal to a predetermined filling height (h-fixed), and is determined using the following formula:

[0055] (n-current) = (n-fixed) = (n-maximum) / [(h-maximum) / (h-fixed)]^a where, (h-fixed) ≤ [(h-mixer) + X].

[0056] Therefore, operating speeds (n) lower than the fixed operating speed (n-fixed) are inefficient and should be avoided (in cases where the blades of impeller 3 cut the liquid surface). Alternatively, the flow generator 1 can be shut off when the current liquid level (h-current) is equal to or less than the predetermined filling height (h-fixed). According to a preferred embodiment, the predetermined filling height (h-fixed) = [(h-mixer) + X].

[0057] According to various embodiments, the liquid height (h - above) above the flow generator depends on the sum of the current pressure (P - sensor) and the air pressure deviation (ΔP - air), where the air pressure deviation (ΔP - air) is determined based on the current air pressure relative to the air pressure present during the calibration process of the pressure sensor (P - calibration). The current air pressure is the air pressure acting on the liquid surface and can be measured on-site or obtained from weather forecasts.

[0058] According to various embodiments, the liquid height (h - above) above the flow generator depends on the sum of the current pressure (P - sensor) and the current air pressure. The current air pressure is the air pressure acting on the liquid surface and can be measured on-site or obtained from weather forecasts.

[0059] A computer program product / package, including instructions for causing the flow generator 1 to perform the steps of the above-described method, is accessible / available / operably connected to the flow generator 1. The computer program product is preferably located in / operates in the control unit 4.

[0060] Possible variations of the invention

[0061] This invention is not limited to the embodiments described above and shown in the accompanying drawings, which are primarily illustrative and exemplary. This patent application will cover all modifications and variations of the preferred embodiments described herein, and therefore the invention is determined by the wording of the appended claims and their equivalents. Thus, the device can be modified in various ways within the scope of the appended claims.

[0062] This invention is not limited to buffer tanks, but rather covers all tanks inside or outside wastewater treatment plants, tanks for temporary storage of liquids, and / or tanks for treating liquids.

[0063] It should also be noted that all information regarding terms such as above, below, upper, lower, etc., should be interpreted / read as referring to a device oriented according to the drawings, with the drawings oriented so that they can be properly read. Therefore, these terms merely indicate the relationships in the illustrated embodiments, and these relationships may change when the device of the present invention is provided with another structure / design.

[0064] It should also be noted that although it is not explicitly stated that features from a particular embodiment can be combined with features from another embodiment, such combination should be considered obvious when feasible.

[0065] Throughout the specification and the following claims, unless the context otherwise requires, the word “comprising” and its variations (e.g., “including”) shall be understood to imply inclusion of the said integer or step or group of integers or steps, but not to exclude any other integer or step or group of integers or steps.

Claims

1. A method for controlling a submersible mixer (1) in a tank (20) arranged for containing a liquid comprising solid matter, the submersible mixer (1) comprising an impeller (3) and being located at a height (h-mixer) in the tank (20), the tank (20) having a predetermined maximum filling height (h-max), the submersible mixer (1) being arranged to be operated at a variable operating speed (n) and a required operating speed (n-required) being dependent on a current liquid level (h-current) in the tank (20), wherein, The maximum operating speed (n-max) of the submersible mixer (1) is the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-max), and the current operating speed (n-current) is set equal to the required operating speed (n-required) of the submersible mixer (1) at least when [(h-mixer) + X] < (h-current) < (h-max), the required operating speed (n-required) being determined using the following formula: (n-current) = (n-required) = (n-max) / [(h-max) / (h-current)]^a where a > (1 / 4) and a < 2 / 3, X = radius of the impeller of the submersible mixer + 1, and all heights are given in meters.

2. The method of claim 1, wherein: a≥（1 / 3）。 3. The method of claim 1 or 2, wherein: The current operating speed (n-current) is set equal to a fixed value (n-fixed) of the submersible mixer (1) when (h-mixer) < (h-current) < (h-fixed), the fixed value (n-fixed) being the operating speed required when the liquid level in the tank (20) is equal to a predetermined filling height (h-fixed), and being determined using the following formula: (n-current) = (n-fixed) = (n-max) / [(h-max) / (h-fixed)]^a where (h-fixed) < [(h-mixer) + X].

4. The method of claim 3, wherein: (h-fixed) = [(h-mixer) + X].

5. The method of claim 1 or 2, wherein: The submersible mixer (1) is associated with a pressure sensor (26) for determining the pressure (P-sensor) at the height (h-mixer) of the submersible mixer (1) in the tank (20), wherein the liquid height (h-above) above the submersible mixer depends on the current pressure (P-sensor), and the current liquid level (h-current) in the tank (20) is determined using the following formula: (h-current) = (h-mixer) + (h-above).

6. The method of claim 5, wherein: The liquid height (h-above) above the submersible mixer depends on the sum of the current pressure (P-sensor) minus an air pressure deviation (AP-air), wherein the air pressure deviation (AP-air) is determined based on the current air pressure relative to an air pressure (P-calibration) present during a calibration process of the pressure sensor.

7. Tank (20) for containing a liquid comprising solid matter, said tank (20) comprising a submersible mixer (1) having an impeller (3) and located at a height (h-mixer) in the tank (20), and said tank (20) having a predetermined maximum filling height (h-max), characterized in that, The tank (20) further comprises a control unit (4) arranged for operating the submersible mixer (1) at a variable operating speed (n), and a height detection device for determining the current liquid level (h-current) in the tank (20), wherein the maximum operating speed (n-max) of the submersible mixer (1) is the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-max), wherein the control unit (4) is arranged for setting the current operating speed (n-current) of the submersible mixer (1) equal to a required operating speed (n-required) depending on the current liquid level (h-current) at least when [(h-mixer) + X] < (h-current) < (h-max), using the following formula: (n-current) = (n-required) = (n-max) / [(h-max) / (h-current)]^a where a > (1 / 4) and a < 2 / 3, X = radius of the impeller of the submersible mixer + 1, and All heights are given in meters.

8. The tank (20) of claim 7, wherein: The required operating speed (n-required) depends on the predetermined maximum filling height (h-max) and the maximum operating speed (n-max) of the submersible mixer (1), which is the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-max).

9. The tank (20) according to claim 7 or 8, wherein: The height detection device consists of a pressure sensor (26) for determining the pressure (P-sensor) at the height (h-mixer) of the submersible mixer (1) in the tank (20), the liquid height (h-above) above the submersible mixer depending on the current pressure (P-sensor), the current liquid level (h-current) in the tank (20) being determined using the following formula: (h-current) = (h-mixer) + (h-above).

10. Submerged mixer (1) suitable for use in a tank (20) according to any one of claims 7-9, wherein The submersible mixer (1) comprises an impeller (3) and a stationary body (2) with a drive unit, the impeller (3) being connected to and driven in rotation by the drive unit, characterized in that the submersible mixer (1) comprises a control unit (4), the submersible mixer (1) being arranged for operating with a variable operating speed (n) by means of the control unit (4), and the submersible mixer (1) comprising a pressure sensor (26) operatively connected to the drive unit, the operating speed (n) of the submersible mixer (1) depending on the current pressure (P-sensor) determined by the pressure sensor (26), the current pressure (P-sensor) being proportional to the current liquid level (h-current), the maximum operating speed (n-max) of the submersible mixer (1) being the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-max), the control unit (4) being arranged for setting the current operating speed (n-current) of the submersible mixer (1) equal to the required operating speed (n-required) depending on the current liquid level (h-current) using the following formula at least when [(h-mixer) + X] < (h-current) < (h-max): (n-current) = (n-required) = (n-max) / [(h-max) / (h-current)]^a where a > (1 / 4) and a < 2 / 3, X = radius of the impeller of the submersible mixer + 1, and All heights are given in meters. The required operating speed (n-required) depends on the predetermined maximum filling height (h-max) and the maximum operating speed (n-max) of the submersible mixer (1), which is the operating speed required when the liquid level in the tank (20) is equal to the maximum filling height (h-max). The height detection device consists of a pressure sensor (26) for determining the pressure (P-sensor) at the height (h-mixer) of the submersible mixer (1) in the tank (20), the liquid height (h-above) above the submersible mixer depending on the current pressure (P-sensor), the current liquid level (h-current) in the tank (20) being determined using the following formula: (h-current) = (h-mixer) + (h-above).

Images