METHOD FOR MONITORING AND CONTROLLING THE OPERATION OF A FLOW GENERATOR

Abstract

The invention relates to a method for monitoring and controlling the operation of a liquid flow generator (1) configured for operation in a tank (18) housing in a liquid comprising solid matter. The flow generator (1) comprises a propeller (3) and a main body (7) having a drive unit (4), wherein a control unit (4) is operatively connected to the flow generator (1) in order to monitor and control the operation of the flow generator (1), the method comprises the steps of: a) driving the propeller (3) in a normal direction of rotation, wherein the liquid flow is directed from an upstream side of the propeller (3) towards a downstream side of the propeller (3), wherein the main body (7) is located at the upstream side of the propeller (3), b) performing a cleaning sequence in response to a main body cleaning signal, wherein the cleaning sequence comprises the steps of: i) stopping the propeller (3) from rotating in the normal direction of rotation, ii) driving the propeller (3) in a reverse direction of rotation, wherein the liquid flow is directed from the downstream side of the propeller (3) towards the upstream side of the propeller (3) and along the main body (7) in order to remove any solid matter accumulated on the main body (7), and iii) stopping the propeller (3) from rotating in the reverse direction of rotation, c) resume driving of the propeller (3) in the normal direction of rotation.

Claims

1. A method for monitoring and controlling the operation of a liquid flow generator configured for operation in a tank housing in a liquid comprising solid matter, the flow generator comprising a propeller and a main body having a drive unit, wherein the drive unit comprises an electric motor located in said main body and a drive shaft connected to the electric motor and extending from the main body, wherein the propeller comprises a hub and a plurality of blades (14) connected to said hub, wherein the hub is connected to the drive shaft and driven in rotation by said electric motor during operation of the flow generator, a control unit being operatively connected to the flow generator in order to monitor and control the operation of the flow generator, the method comprises the steps of: a) driving the propeller in a normal direction of rotation in order to generate a bulk flow of liquid in the tank, wherein the liquid flow is directed from an upstream side of the propeller towards a downstream side of the propeller, wherein the main body is located at the upstream side of the propeller, b) performing a cleaning sequence in response to a main body cleaning signal, wherein the cleaning sequence comprises the steps of: i) stopping the propeller from rotating in the normal direction of rotation, ii) driving the propeller in a reverse direction of rotation in order to generate a local flow of liquid from the downstream side of the propeller towards the upstream side of the propeller and along the main body in order to remove any solid matter accumulated on the main body, and iii) stopping the propeller from rotating in the reverse direction of rotation, c) resume driving of the propeller in the normal direction of rotation, wherein the duration of the reverse operation of the propeller during the cleaning sequence is equal to or more than 5 seconds and is equal to or less than 60 seconds, and wherein the rotational speed of the flow generator during the reverse operation of the propeller during the cleaning sequence is equal to or more than 25 % of the max rotational speed of the flow generator and is equal to or less than 100 % of the max rotational speed of the flow generator.

2. The method according to claim 1, wherein the control unit verifies that the propeller is standing still before initiating the reverse operation of the propeller during the cleaning sequence.

3. The method according to claim 1, wherein the control unit verifies that the propeller is standing still before resuming the normal operation of the propeller after the cleaning sequence.

4. The method according to claim 1, wherein the duration of the reverse operation of the propeller during the cleaning sequence is equal to or more than 15 seconds and is equal to or less than 30 seconds.

5. The method according to claim 1, wherein the rotational speed of the flow generator during the reverse operation of the propeller during the cleaning sequence is equal to or more than 60 % of the max rotational speed of the flow generator and is equal to or less than 80 % of the max rotational speed of the flow generator.

6. The method according to claim 1, wherein the main body cleaning signal is trigged on a time-based condition, the time-based condition comprising a time interval between two consecutive main body cleaning signals, wherein the time interval is equal to or more than 15 minutes and is equal to or less than 1440 minutes, preferably 30-360, most preferably about 60 minutes.

7. The method according to claim 1, the flow generator comprising a temperature sensor located in the main body, wherein the main body cleaning signal is trigged on a temperature-based condition, the temperature-based condition comprising a predetermined temperature threshold, wherein the main body cleaning signal is trigged in response to the temperature of the temperature sensor (24) exceeds said temperature threshold.

8. The method according to claim 7, wherein the temperature threshold is equal to or more than 50° C. and is equal to or less than 150° C.

9. The method according to claim 1, wherein the flow generator comprises a temperature sensor located in the main body and wherein step b) comprises: b) performing a cleaning sequence in response to a main body cleaning signal, wherein the cleaning sequence comprises the steps of: i) stopping the propeller from rotating in the normal direction of rotation, ii) driving the propeller in a reverse direction of rotation in order to generate a local flow of liquid from the downstream side of the propeller towards the upstream side of the propeller and along the main body in order to remove any solid matter accumulated on the main body, iii) stopping the propeller from rotating in the reverse direction of rotation, iv) in response to the temperature of the temperature sensor exceeds a temperature threshold during the step of iii) stopping the propeller from rotating in the reverse direction of rotation, driving the propeller in the normal direction a predetermined time period that is equal to or more than 10 minutes and is equal to or less than 20 minutes and thereafter returning to the step of i) stopping the propeller from rotating in the normal direction of rotation, otherwise proceeding to step c) resume driving of the propeller in the normal direction of rotation.

10. The method according to claim 9, wherein an alarm “cleaning failed” is trigged in response to five returning to step i) of the cleaning sequence are performed and the temperature of the temperature sensor (24) exceeds the temperature threshold during the step of iii) stopping the propeller from rotating in the reverse direction of rotation.

11. The method according to claim 1, wherein the flow generator comprises a temperature sensor (24) located in the main body, wherein the main body cleaning signal is trigged on a temperature-based condition, the temperature-based condition comprising a predetermined temperature derivative threshold, wherein the main body cleaning signal is trigged in response to the derivative of the temperature change of the temperature sensor (24) exceeds said temperature derivative threshold.

12. The method according to claim 11, wherein the temperature derivative threshold is equal to or more than 2° C. per 5 minutes and is equal to or less than 10° C. per 5 minutes.

13. The method according to claim 6, wherein the rotational speed of the flow generator during the reverse operation of the cleaning sequence is higher in response to temperature-based trigging than in response to time-based trigging.

14. The method according to claim 6, wherein the duration of the reverse operation of the cleaning sequence is greater in response to temperature-based trigging than in response to time-based trigging.

15. A computer-readable storage medium having computer-readable program code portions embedded therein, wherein the computer-readable program code portions when executed by a computer cause the computer to carry out the steps of the method according to claim 1 in order to perform a cleaning of the flow generator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] A more complete understanding of the abovementioned and other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

[0032] FIG. 1 is a schematic perspective view of an inventive flow generator,

[0033] FIG. 2 is a schematic cross sectional side view of the flow generator according to FIG. 1, and

[0034] FIG. 3 is a schematic side view of a tank comprising such a flow generator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0035] Reference is initially made to FIGS. 1 and 2. The present invention relates especially to a flow generator or mixer, generally designated 1, suitable for treatment/transportation of liquid comprising solid matter, such as wastewater/sewage/biomass slurry, and relates especially to a method for monitoring and controlling the operation of such a flow generator 1.

[0036] The flow generator 1 is a submersible mixer machine and comprises three major parts, a drive unit, generally designated 2, a rotatable impeller/propeller 3 and a control unit 4. The control unit 4 monitors and controls the operation of the flow generator 1. The drive unit 2 drives the propeller 3 in rotation and the propeller 3 propels the liquid, i.e. provides thrust to the liquid. The drive unit 2 and the propeller 3 are always part of the flow generator 1, and in the disclosed embodiment the control unit 4 is integrated into and constitutes a part of the flow generator 1. In an alternative embodiment the control unit 4 is constituted by a separate member and is operatively connected to the flow generator 1. The flow generator 1 is also commonly called mixer machine or mixer. In the disclosed embodiment the flow generator is a submersible flow generator, i.e. configured to be located entirely submerged. However, it shall be pointed out that a submersible flow generator 1 can be partly located above the liquid surface during operation. The flow generator 1 is cooled by the liquid surrounding the drive unit 2.

[0037] An electric cable 5 extending from a power supply, for instance the power mains, provides power to the flow generator 1, the flow generator 1 comprising a liquid tight lead-through 6 receiving the electric cable 5. The electric cable 5 may also comprise signal wires for data communication between the flow generator and an external control unit (not shown). A main body 7 comprises, in the present embodiment, the control unit 4 and the drive unit 2.

[0038] In FIG. 2 some internal parts of the flow generator 1 are schematically disclosed. The drive unit 2 comprises an electric motor, generally designated 8, and a drive shaft 9 connected to and driven in rotation by said electric motor 8 during operation of the flow generator 1. The electric motor 8 comprises in a conventional way a stator 10 and a rotor 11. The main body 7 is located upstream the propeller 3 during normal operation of the flow generator 1.

[0039] In the disclosed embodiment the drive shaft 9 comprises a rear end portion connected to and co-rotational with the rotor 11 in a conventional way and a forward end portion connected to and co-rotational with the propeller 3 in a conventional way, wherein a mechanical transmission unit 12 is arranged between the rear end portion and the forward end portion. The transmission unit 12 may have a fixed gear ratio wherein the propeller 3 has a lower rotational speed than the rotor 11 of the electric motor 8, i.e. reduced gearing. The gear ratio is preferably equal to or less than 60:1, and preferably equal to or higher than 15:1. According to alternative embodiments the gear ratio is 1:1, i.e. no gearing, wherein the rear end portion and the forward end portion are constituted by the same shaft member. The drive unit also comprises necessary bearings and seals (not shown). The rotational speed of the impeller/propeller 3, i.e. the operational speed of the flow generator 1, during normal operation of the flow generator 1 is equal to or less than 1000 rpm, and equal to or higher than 70 rpm.

[0040] In the disclosed embodiment the rear end portion and the forward end portion of the drive shaft 9 both extends in an axial direction, and are preferably collinear. According to alternative embodiments the mechanical transmission unit 12 is angled, i.e. it is an angle between the rear end portion and the forward end portion of the drive shaft 9, for instance 90 degrees.

[0041] The electric motor 8 is located in the main body/housing 7 and in the disclosed embodiment the propeller 3 is located in direct contact with the main body 7, the main body 7 being a liquid tight housing. However, in alternative embodiments the propeller 3 is located at a distance from the main body 7, i.e. the drive shaft 9 is visible between the main body 7 and the propeller 3.

[0042] The propeller 3 comprises a hub 13 connected to the drive shaft 9 and a plurality of blades 14 connected to said hub 13, wherein the drive shaft 9 extends in an axial direction and each blade 14 extends in a radial direction seen from its base to its top, wherein each blade 14 is connected to the hub 13 at its base and wherein the top of the blade 14 is the outermost part of the propeller 3. In the disclosed embodiment both the leading edge 15 and the trailing edge 16 of the blade 14 are curved, the leading edge 15 is convex and the trailing edge 16 is concave. It shall be pointed out that the blades 14 naturally also have an extension in the axial direction, i.e. has a pitch, in order to generate thrust to the liquid.

[0043] The control unit 4 is operatively connected to the electric motor 8, the control unit 4 being configured for monitoring and controlling the operation of the flow generator 1. The electric motor 8 is configured to be driven in operation by the control unit 4. Thus, the control unit 4 is configured to control the rotational speed at which said electric motor 8 of the flow generator is to be driven, for instance by controlling the frequency of the current operating the electric motor 8. According to the disclosed embodiment, the control unit 4 comprises a Variable Frequency Drive (VFD) 17. Thus, the flow generator 1 is configured to be operated at variable operational speed (n). The control unit 4 is configured to perform the inventive method.

[0044] Reference is now also made to FIG. 3, wherein the flow generator 1 is located in a tank 18 that is configured for housing the liquid comprising solid matter. Thus, the control unit 4 that is configured for operating the flow generator 1 at variable operational speed (n), i.e. rpm of the impeller/propeller 3. The flow generator 1 is provided with a max operational/rotational speed which is the operational speed/thrust required when the liquid level in the tank 18 is equal to the maximum filling height (h-max), in order to obtain sufficient and appropriate mixing. It shall be pointed out that the specific flow generator 1 in a specific application may be dimensioned for operational/rotational speeds above said max operational speed, however when sizing the flow generator 1 one tries to have the smallest possible flow generator 1 capable of performing the necessary mixing in order to save costs. It shall be pointed out that the present liquid level in the tank 18 may be below the maximum filing height.

[0045] The tank 18 comprises an inlet 19 to admit liquid to enter the tank 18 and an outlet 20 through which the liquid is removed/discharged from the tank 18. The liquid may be pumped into the tank 18 and/or arranged to flow by means of gravity into the tank 18, and the liquid may be pumped from the tank 18 and/or arranged to flow by means of gravity from the tank 18.

[0046] The tank 18 has a predetermined maximum filling height (h-max), for instance defined by the vertical distance between an overflow outlet 21 and the floor of the tank 18. The maximum filling height (h-max) is usually equal to or more than 2 meters and equal to or less than 15 meters.

[0047] The tank 18 may comprise several flow generators 1, wherein one or more are controlled in accordance with the present invention. The flow generator 1 is located at a height in the tank 18 during operation, and the height of the flow generator 1 may be fixed or adjustable. In the disclosed embodiment the flow generator 1 is arranged to be guided along and by means of a guide bar 22 by being hoisted or lowered in a chain/wire 23. The guide bar 22, wire/chain 23 and a runner at the back of the flow generator 1, i.e. the hosting/lowering equipment, are also exposed for fouling and are cleaned by the inventive method.

[0048] The inventive method comprises the steps of: [0049] a) driving the propeller 3 in a normal direction of rotation in order to generate a bulk flow of liquid in the tank 18, wherein the liquid flow is directed from an upstream side of the propeller 3 towards a downstream side of the propeller 3, wherein the main body 7 is located at the upstream side of the propeller 3, [0050] b) performing a cleaning sequence in response to a main body cleaning signal, wherein the cleaning sequence comprises the steps of: [0051] i) stopping the propeller 3 from rotating in the normal direction of rotation, [0052] ii) driving the propeller 3 in a reverse direction of rotation in order to generate a local flow of liquid from the downstream side of the propeller 3 towards the upstream side of the propeller 3 and along the main body 7 in order to remove any solid matter accumulated on the main body 7, and [0053] iii) stopping the propeller 3 from rotating in the reverse direction of rotation, [0054] c) resume driving of the propeller 3 in the normal direction of rotation, [0055] wherein the duration of the reverse operation of the propeller 3 during the cleaning sequence is equal to or more than 5 seconds and is equal to or less than 60 seconds, and [0056] wherein the rotational speed of the flow generator 1 during the reverse operation of the propeller 3 during the cleaning sequence is equal to or more than 25 % of the max rotational speed of the flow generator 1 and is equal to or less than 100 % of the max rotational speed of the flow generator 1.

[0057] Thus, during the cleaning sequence there is a local flow of liquid at the flow generator 1, which does not extinguish the bulk flow of liquid in the tank 18, wherein the local flow of liquid will flush away the solid matter accumulated on the main body 7, the guide bar 22, the chain/wire 23, etc.

[0058] The duration of the reverse operation of the propeller 3 must not be too long, since a too long reverse operation will increase the risk that the bulk flow of liquid in the tank is negatively affected. Thereto, the duration of the reverser operation of the propeller 3 must not be too short, since a too short reverse operation will not provide sufficient flush away of accumulated solid matter. Similarly, the rotational speed during the reverse operation of the propeller 3 must not be too low, since a too low operational speed will not provide sufficient flush away of accumulated solid matter.

[0059] According to various embodiments, the rotational speed of the flow generator 1 during the reverse operation of the propeller 3 during the cleaning sequence is equal to or more than 50 % of the max rotational speed of the flow generator 1.

[0060] There is a correlation between the operational speed and the duration of the reverse operation of the propeller 3. In situations/applications the rotational speed is 100 % of the max rotational speed, the duration can be in the lower part of the stipulated range of duration, and in situations/applications the rotational speed is 25 % of the max rotational speed, the duration can be in the upper part of the stipulated range of duration, for a specific/optimized combination of flow generator 1 and tank 18.

[0061] According to various embodiments the duration of the reverse operation of the propeller 3 during the cleaning sequence is equal to or more than 15 seconds and is equal to or less than 30 seconds. In this more limited duration range there is no need to know the rotational speed within the set range, i.e. the entire rotational speed range stipulated above is valid in this more limited duration range.

[0062] Thereto, the information about the more limited duration range entails that in situations/applications the rotational speed is 100 % of the max rotational speed, the duration is preferably in the range 5-30 seconds, and in situations/applications the rotational speed is 25 % of the max rotational speed, the duration is preferably in the range 15-60 seconds.

[0063] According to various embodiments the rotational speed of the flow generator 1 during the reverse operation of the propeller 3 during the cleaning sequence is equal to or more than 60 % of the max rotational speed of the flow generator 1 and is equal to or less than 80 % of the max rotational speed of the flow generator 1 within the set range. In this more limited rotational speed range there is no need to know the duration, i.e. the entire duration range stipulated above is valid in this more limited rotational speed range.

[0064] Thereto, the information about the more limited rotational speed range entails that in situations/applications the rotational speed is in the range 60-100 % of the max rotational speed, the duration is preferably in the range 5-30 seconds, and in situations/applications the rotational speed is in the range 25-60 % of the max rotational speed, the duration is preferably in the range 15-60 seconds.

[0065] The rotational speed of the flow generator 1 in the normal/forward direction of rotation during step a) and c) is determined for the specific application, i.e. size of flow generator 1, size/type of tank 18, type of liquid/slurry, etc. For instance the rotational speed in the normal/forward direction of rotation is in the range 70-80 % of the max rotational speed.

[0066] During the cleaning sequence, according to various embodiments, after the step of stopping the propeller 3 from rotating in the normal direction of rotation, the control unit 4 verifies that the propeller 3 is standing still before initiating the reverse operation of the propeller 3. One way of verifying stand still is that no current/power is used by the electric motor 8, or that the output frequency from the control unit 4 to the electric motor 8 is zero. Alternatively, the propeller 3 is allowed to freewheel after stopping the forward operation of the propeller 3, but in such conditions the propeller 3 will be slowly rotated in the normal direction of rotation by the bulk flow of liquid.

[0067] During the cleaning sequence, according to various embodiments, after the step of stopping the propeller 3 from rotating in the reverse direction of rotation, the control unit 4 verifies that the propeller 3 is standing still before resuming the normal operation of the propeller 3 after the cleaning sequence. One way of verifying stand still is that no current/power is used by the electric motor 8, or that the output frequency from the control unit 4 to the electric motor 8 is zero. Alternatively, the propeller 3 is allowed to freewheel after stopping the backward operation of the propeller 3, but in such conditions the propeller 3 will be slowly rotated in the normal direction of rotation by the bulk flow of liquid.

[0068] Thus, stopping the propeller 3 means that the rotational speed of the propeller 3 is decreased in a controlled manner by the control unit 4 and/or by disengaging the control unit 4 from the electric motor 8.

[0069] According to various embodiments, the main body cleaning signal is trigged on a time-based condition. The time-based condition comprising a time interval between two consecutive main body cleaning signals, wherein the time interval is equal to or more than 15 minutes and is equal to or less than 1440 minutes (24 hours). The time interval is preferably equal to or more than 30 minutes and equal to or less than 360, and most preferably about 60 minutes. The duration of the entire cleaning sequence is about 1 minute.

[0070] According to various embodiments, the flow generator 1 comprising a temperature sensor 24 located in the main body 7, wherein the main body cleaning signal is trigged on a temperature-based condition, the temperature-based condition comprising a predetermined temperature threshold, wherein the main body cleaning signal is trigged in response to the temperature of the temperature sensor 24 exceeds said temperature threshold.

[0071] It is preferred that the temperature sensor 24 is located in the control unit 4 portion of the flow generator 1, but it is also conceivable that the temperature sensor 24 is located in direct vicinity to the electric motor 8, i.e. in the motor compartment of the main body 7. It is also conceivable to have both types of temperature sensors.

[0072] According to various embodiments, the temperature threshold is equal to or more than 50° C. and is equal to or less than 150° C. In applications the temperature sensor 24 is located in the control unit 4 portion, the temperature threshold is in the range 50-100° C., preferably equal to or more than 60° C. and equal to or less than 80° C. In applications the temperature sensor 24 is located at or in the electric motor 8, the temperature threshold is in the range 100-150° C. Each degree above a normal/reference temperature level will have negative effect on the expected life of the flow generator 1 and the time interval between service will decrease. Thus, the temperature threshold is dependent on the normal/reference temperature of the specific flow generator 1 and application. Thanks to the present invention, i.e. secure proper cooling of the flow generator by having no accumulation of solid matter on the main body 7, the temperature of the liquid/slurry surrounding the flow generator 1 is allowed to be higher than before.

[0073] According to various embodiments, the main body cleaning signal is trigged on a temperature-based condition, the temperature-based condition comprising a predetermined temperature derivative threshold, wherein the main body cleaning signal is trigged in response to the derivative of the temperature change of the temperature sensor 24 exceeds said temperature derivative threshold. Thereby a sudden increase of the temperature will give an early indication that a large piece of solid matter obstructs the flow of liquid along the main body 7 during normal operation of the flow generator 1, and thereby the cooling is negatively affected. In response to a trigging of the main body cleaning signal based on temperature derivative threshold, it is preferred to perform two or three cleaning sequences with a time interval of 10-15 minutes and normal operation therebetween since the rapid increase of temperature indicates a large/single piece of solid matter obstructing the flow of liquid along the main body and it is important to rapidly remove such solid matter.

[0074] The temperature derivative threshold is equal to or more than 2° C. per 5 minutes and is equal to or less than 10° C. per 5 minutes. Preferably the temperature derivative threshold is equal to or more than 3° C. per 5 minutes and is equal to or less than 5° C. per 5 minutes.

[0075] It shall be pointed out that different trigging of the main body cleaning signal may be used in combination, i.e. time-based as well as temperature-based. Another trigging of the main body cleaning signal is a manual trigging, on site or remotely.

[0076] Trigging of the main body cleaning signal based on temperature derivative threshold will take place also when the temperature is well below the temperature threshold. It is preferred that the rotational speed of the flow generator 1 during the reverse operation of the cleaning sequence is higher in response to a temperature-based trigging than in response to a time-based trigging. Thus, a temperature-based trigging of the main body cleaning signal is directly associated with accumulation of solid matter obstruction the flow of liquid/slurry along the main body 7 during normal operation of the flow generator 1, and the time-based trigging is more of a safety measure should the temperature sensor 24 malfunction. For instance the rotational speed during reverse operation in response to temperature-based trigging is in the range 20-100 % higher than the rotational speed during reverse operation in response to time-based trigging, preferably about 50-60 % higher.

[0077] For the same reason, it is also preferred that the duration of the reverse operation of the cleaning sequence is greater in response to a temperature-based trigging than in response to a time-based trigging. For instance the duration of the reverse operation in response to temperature-based trigging is in the range 20-100 % longer than the duration of the reverse operation in response to time-based trigging, preferably about 50-60 % longer.

[0078] According to various embodiments, wherein the flow generator 1 comprises a temperatures sensor 24 the step of b) performing a cleaning sequence in response to a main body cleaning signal of the inventive method comprises more substeps concerning a temperature test after cleaning. More precisely the cleaning sequence comprises the steps of: [0079] i) stopping the propeller 3 from rotating in the normal direction of rotation, [0080] ii) driving the propeller 3 in a reverse direction of rotation in order to generate a local flow of liquid from the downstream side of the propeller 3 towards the upstream side of the propeller 3 and along the main body 7 in order to remove any solid matter accumulated on the main body 7, [0081] iii) stopping the propeller 3 from rotating in the reverse direction of rotation, [0082] iv) in response to the temperature of the temperature sensor 24 exceeds a temperature threshold during the step of iii) stopping the propeller 3 from rotating in the reverse direction of rotation, driving the propeller 3 in the normal direction a predetermined time period that is equal to or more than 10 minutes and is equal to or less than 20 minutes and thereafter returning to the step of i) stopping the propeller 3 from rotating in the normal direction of rotation, otherwise proceeding to step c) resume driving of the propeller 3 in the normal direction of rotation.

[0083] Thus, these embodiments of the inventive method comprise a cleaning sequence loop in response to the main body cleaning signal is temperature-based trigged and the temperature does not decrease under the threshold during the stopping of the propeller 3 from rotating in the reverse direction. The cleaning sequence loop continues until the threshold is no longer exceeded.

[0084] According to various embodiments comprising the cleaning sequence loop, an alarm “cleaning failed” is trigged in response to five returning to step i) of the cleaning sequence are performed and the temperature of the temperature sensor 24 exceeds the temperature threshold during the step of iii) stopping the propeller 24 from rotating in the reverse direction of rotation. In response to the trigging of the alarm “cleaning failed” the flow generator 1 preferably enters a safety mode, i.e. operation having a decreased operational speed and operation in the normal/forward direction until service is performed.

[0085] As an alternative, after a cleaning sequence and the step of c) resume driving of the propeller 3 in the normal direction of rotation has been initiated, the temperature test after cleaning is performed. In response to the temperature of the temperature sensor 24 exceeds a temperature threshold during step c) after a cleaning sequence, driving the propeller 3 in the normal direction a predetermined time period that is equal to or more than 10 minutes and is equal to or less than 20 minutes and thereafter returning to step b) performing a cleaning sequence. Said predetermined time period shall be shorter than the normal time interval between two consecutive main body cleaning signals for time-based trigging of the main body cleaning signal.

[0086] When entering a cleaning sequence loop, the duration and/or rotational speed of the flow generator 1 increases each new cleaning sequence. For instance the duration will increase by 5 seconds each cleaning sequence and/or the rotational speed of the flow generator 1 will increase by 5 % of the max rotational speed.

[0087] According to various embodiments, the default value of the duration of the cleaning sequence is about 20 seconds. According to various embodiments, the default value of the rotational speed is about 50 % of the max rotational speed.

[0088] Following a cleaning sequence loop, the default values of duration and rotational speed for a cleaning sequence may be updated

[0089] The flow generator 1 comprises means adapted to execute the steps of the above method. Many of the steps of the above method are preferably performed/controlled by the control unit 4, and thus the term “the flow generator 1 comprises means...” does not necessarily imply that said means has to be located within the main body 7. Thus the term also includes means accessible/available/operatively connected to the mixer machine.

[0090] A computer program product/package comprising instructions to cause the flow generator 1 to execute the steps of the above method, is accessible/available/operatively connected to the flow generator 1. Said computer program product is preferably located/run in the control unit 4. Thus, the computer-readable storage medium has computer-readable program code portions embedded therein, wherein the computer-readable program code portions when executed by a computer cause the computer to carry out the steps of the described method.

Feasible Modifications of the Invention

[0091] The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is intended to cover all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined by the wording of the appended claims and the equivalents thereof. Thus, the equipment may be modified in all kinds of ways within the scope of the appended claims.

[0092] It shall also be pointed out that all information about/concerning terms such as above, under, upper, lower, etc., shall be interpreted/read having the equipment oriented according to the figures, having the drawings oriented such that the references can be properly read. Thus, such terms only indicate mutual relations in the shown embodiments, which relations may be changed if the inventive equipment is provided with another structure/design.

[0093] It shall also be pointed out that even thus it is not explicitly stated that features from a specific embodiment may be combined with features from another embodiment, the combination shall be considered obvious, if the combination is possible.

[0094] Throughout this specification and the claims which follows, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or steps or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.