Fan and method for regulating a fan

Abstract

A fan comprises two impellers that are arranged one behind the other in the direction of through-flow and have a common drive or each have a drive. One of the impellers has a smaller outside diameter than the other impeller. A method serves to regulate the fan.

Description

[0001] FAN AND METHOD FOR CONTROLLING A FAN

[0002] The present invention relates to a fan with two impellers. Reference is made to DE 10 2021 213 480 A1 merely as an example. Such fans are also known in practice. Furthermore, the invention relates to a method for controlling such a fan.

[0003] Fans of this type, especially those with two or more impellers arranged one behind the other, are problematic in terms of noise emissions during operation. These are excessively high for such fans. Furthermore, the efficiency of such fans decreases rapidly beyond the "design point," i.e., beyond the point of maximum efficiency, with increasing or decreasing throttle ratios, since typically only a single control variable is available: the fan speed. This is usually suitable for setting a required air flow rate, but does not serve to further optimize fan operation.

[0004] In fans with a single impeller, vibration-critical speeds can only be reduced or even eliminated by deviating from the required airflow, either upwards or downwards. The so-called long-throw behavior cannot be influenced during operation independently of the airflow. Accordingly, the uniformity of the air flow to a downstream machine, such as a heat exchanger or the like, cannot be influenced independently of the fan's airflow.

[0005] The present invention is based on the object of designing and developing the generic fan in such a way that it operates efficiently over a wide range of throttling conditions, is particularly powerful, and yet has the lowest possible noise levels. During operation, other parameters should be optimized for a given performance value, in accordance with the requirements of the specific application. Furthermore, the fan should differentiate itself from competitive products. The same applies to the process.

[0006] The above object is achieved by the features of claim 1, according to which the fan comprises two impellers arranged one behind the other in the flow direction. The two impellers can have a common drive or each a separate drive, which is usually an electric drive. One of the impellers has a smaller outer diameter than the other impeller. With regard to the method, the object is achieved by the independent claim 11.

[0007] Advantageously, the smaller impeller has an outer diameter that is at least 20% smaller than the larger impeller. This means that one of the impellers has a significantly smaller outer diameter than the other impeller.

[0008] A non-rotatable guide device can be provided between the two impellers. This can comprise guide vanes. The guide device is arranged in the flow area between the impellers and preferably has a supporting function with respect to the drive(s). This measure separates the aforementioned flow areas from each other.

[0009] In a further advantageous manner, the guide device comprises an outer flow region away from the axis and an inner flow region near the axis, wherein the flow regions are preferably separated from one another by an intermediate ring. It is further advantageous if the smaller impeller is assigned to the inner flow region closer to the axis.

[0010] Further advantageously, the larger impeller has a radial or diagonal design. The smaller impeller can be located in the area of ​​or within the suction inlet of the larger impeller, i.e., in the inflow area of ​​the larger impeller. It is advantageously mounted on a structure located upstream of the smaller impeller, for example, on an inflow grille. Furthermore, it is conceivable for the impellers to rotate in the same or opposite directions.

[0011] With regard to the method, the object is achieved by the features of claim 11, according to which the fan operation is optimized by adjusting / changing the speed of at least one of the impellers.

[0012] If independent drives for the impellers are provided, the speeds of both impellers can be adjusted or controlled independently of each other.

[0013] It is also conceivable that the speeds of the two impellers are coordinated with each other and preferably adjustable depending on the operating point.

[0014] By adjusting the speed of one of the impellers, preferably the second impeller with an EC drive, the fan operation can be optimized.

[0015] It is advantageous if the speeds of the impellers can be adjusted or controlled using a special controller, whereby this is done in particular using methods of artificial intelligence, machine learning and / or conventional control algorithms with regard to the desired air performance and, if necessary, with regard to at least one other target variable, for example the overall efficiency.

[0016] Other parameters that may be optimized include vibration values, axial thrust, efficiency, acoustics, psychoacoustic aspects, long-throw behavior, flow pattern in relation to a heat exchanger or the like, etc. Thus, the fan according to the invention forms a platform for the realization of a quasi-intelligent fan. Although such a fan per se represents a turbo-turbine, the invention can be applied to other turbo-turbine types.

[0017] There are now various possibilities for advantageously embodying and developing the teaching of the present invention. Reference is made, on the one hand, to the claims subordinate to claim 1 relating to the fan, and to the claim subordinate to claim 11 relating to the method, as well as, on the other hand, to the following explanation of preferred embodiments of the invention with reference to the drawings. In conjunction with the explanation of the preferred embodiments of the invention with reference to the drawings, generally preferred embodiments and developments of the teaching are also explained. The drawings show:

[0018] Fig. 1 shows a perspective view from the downstream side of an embodiment of a fan according to the invention with a larger and a smaller impeller, wherein a supporting guide device is arranged between the two impellers,

[0019] Fig. 2 in a planar axial plan view from the downstream side, the fan with two impellers of different diameters from Fig. 1 ,

[0020] Fig. 3 in a side view and in section on a plane through the axis of the fan according to Fig. 1 and 2 with two impellers of different diameters, wherein in fan operation the flow flows essentially from left to right,

[0021] Fig. 4 in a side view and in section on a plane through the axis, similar to the view according to Fig. 3, a further embodiment of a fan with two impellers of different diameters, wherein an outer diffuser on the outer casing projects axially beyond the smaller impeller in the outflow direction, and

[0022] Fig. 5 shows, in a perspective view from the downstream side, an embodiment similar to that shown in Fig. 4, wherein the intermediate ring of the guide vane unit and the outer wall of the smaller impeller are manufactured integrally. Fig. 1 shows, in a perspective view from the downstream side, an embodiment of a fan 1 according to the invention of axial design with a larger impeller 19A and a smaller impeller 19B. A supporting, non-rotating guide device, which in particular has the supporting guide vanes 11, is arranged between the two impellers 19A and 19B.

[0023] The guide device is integrated in one piece into a guide housing 2, which, in addition to the supporting guide vanes 11, also has the outer housing contour with the sections inlet nozzle 9 (see also Fig. 3), running area 29A for the larger impeller A and a diffuser area 10. During fan operation, flow medium is sucked into the fan 1 through the inlet nozzle 9, flows within the housing 2 first through the running area 29A, then through the diffuser area 10, before exiting the fan 1 again, in the view according to Fig. 1 diagonally to the left and towards the viewer. As a result of the rotation of the two impellers 19A and 19B during operation, energy is transferred between the impellers 19A, 19B and the flow medium and the flow transport is maintained.

[0024] The guide device or the guide vane housing 2 is designed with an intermediate ring 5 manufactured integrally with the guide vane housing 2 and the outer strut vanes 3, which are also manufactured integrally with the guide vane housing 2. The guide vanes 11 are designed as inner guide vanes, which run from a hub ring 4, which is also manufactured integrally, in the radial, or spanwise direction, only as far as the intermediate ring 5, which extends in the circumferential direction and is arranged between the radially inner hub ring 4 and the outer housing contour, formed by the inlet nozzle 9, running area 29A and diffuser 10. The strut vanes 3, in turn, extend between the intermediate ring 5 and the outer housing contour. Overall, the strut vanes 3, the intermediate ring 5 and the guide vanes 11 thus assume the load-bearing connection of the hub ring 4 and the outer housing contour.

[0025] The two impellers 19A and 19B, with their respective drives 34A and 34B, are directly or indirectly attached to the hub ring 4. The guide vane casing 2, and thus the entire fan 1, can be attached to a higher-level ventilation system, typically in the area of ​​the outer casing contour, for example, in the area of ​​the inlet nozzle 9 with the upstream fastening devices 20 or in the area of ​​the diffuser 10 with the downstream fastening devices 21.

[0026] On the downstream side of the guide housing 2, fastening provisions 25 for a protective grid are advantageously arranged such that a suitable protective grid can be attached such that it does not protrude axially beyond the guide housing 2, namely by screwing the protective grid axially countersunk into the guide housing 2.

[0027] The intermediate ring 5 separates a radially inner flow area 7 from a radially outer flow area 6, both of which run within the outer housing contour, defined by the inlet nozzle 9, the running area 29A of the large impeller 19A and the diffuser wall 10. In a sense, the total flow rate flowing through the fan 1 and also through the area of ​​the large impeller 19A with its blades 22A is divided between the inner flow area 7 and the outer flow area 6. The smaller impeller 19B, however, only approximately covers the area of ​​the inner flow area 7 in the radial or spanwise direction and therefore has a significantly smaller diameter than the larger impeller 19A, for example a diameter less than 75% of the diameter of the larger impeller 19A.

[0028] In order to achieve good efficiency values, the smaller impeller 19B with its blades 22B runs in an area defined radially or in the spanwise direction by an outer wall 8. The outer wall 8 is simply designed as an extension of the intermediate ring 5, integrally attached to the guide vane casing 2, up to a trailing edge 12. Since the outer wall 8 does not rotate, a gap is formed between the smaller impeller 19B and the outer wall 8 of the smaller impeller B. In this context, the outer ends of the blades 22B of the smaller impeller 19B can advantageously be provided with winglets 38B in order to minimize noise generation and maximize efficiency. While essentially the entire flow rate flows through the large impeller 19A or is subjected to power, the smaller impeller 19B is only flowed through by a partial flow rate or interacts by transferring power only with the partial flow rate.This is particularly evident in the smaller outer diameter of the smaller impeller 19B. In a radially inner region of the fan 1, the efficiency and pressure stability of the entire fan 1 can be significantly influenced by influencing the flow there, i.e., by power transmission. However, this can be achieved in radially inner regions with relatively low noise generation, i.e., for the smaller impeller 19B with its blades 22B, the noise generation, for example, due to rotor-stator interaction and / or rotor-rotor interaction and / or head gap vortex noise, is relatively low.

[0029] The non-rotating guide vanes 11 arranged between the two impellers 19A and 19B can provide an intermediate rectification (swirl reduction) of the flow, which can be advantageous, whereby the guide vanes 11 also have a supporting function.

[0030] In other embodiments, it is also conceivable that no intermediate ring is provided in the guide vane housing, and that guide vanes extend continuously from a hub ring to an outer housing contour. Advantageously, the smaller impeller then has a co-rotating cover ring, which then connects the vane tips of this smaller impeller to each other and separates the partial flow flowing through the smaller impeller, radially or in the spanwise direction, from the outer flow not flowing through it.

[0031] In addition, in the embodiment according to Fig. 1, the larger impeller 19A rotates in the direction of rotation 32A and the smaller impeller 19B in the direction of rotation 32B. The direction of rotation of the two impellers is opposite.

[0032] In the case where the direction of rotation of the two impellers is opposite, other embodiments can dispense with the aerodynamically effective guide vanes 11 located between the impellers 19A and 19B. Depending on the embodiment, it is also conceivable for both impellers to have the same direction of rotation. It has been shown that embodiments with the same direction of rotation, in combination with the use of aerodynamically effective guide vanes, are advantageous for low fan noise levels.

[0033] The larger impeller 19A is essentially formed from a hub ring 31A (Fig.3) and vanes 22A attached thereto and is attached to a drive 34A with its hub ring 31A.

[0034] In other embodiments, the larger impeller may additionally have a cover ring that connects the outer ends of its blades in the circumferential direction.

[0035] The impeller 19B is essentially formed from a hub ring 31B and vanes 22B attached thereto and is attached to a drive 34B with its hub ring 31B.

[0036] In other embodiments, the smaller impeller could also additionally have a cover ring that connects the outer ends of its blades in the circumferential direction.

[0037] Fig. 2 shows a plan axial plan view from the downstream side of the fan 1 with two impellers 19A, 19B of different diameters according to Fig. 1. In addition to the descriptions for Figure 1, the different directions of rotation 32A of the larger impeller 19A and 32B of the smaller impeller 19B can be clearly seen. It can also be seen that the outer strut blades 3, viewed in the axial direction, are set at a strong angle to the trailing edges 26A of the blades 22A of the larger impeller 19A, for example at an angle of > 45° to the trailing edges 26A of the impeller 19A in the projection onto the view according to Fig. 2. This is particularly advantageous for low rotational noise generation in the radially outer region with high rotational speeds. In the radially inner region, a relatively large number of inner guide vanes 11 are opposite the blades 22B of the smaller impeller 19B.The rotational tone is therefore generated at a higher frequency there, which is often advantageous, and less intense.

[0038] The trailing edges 26A of the blades 22A of the large impeller 19A are serrated to ensure low trailing edge noise.

[0039] The trailing edges 26B of the blades 22B of the smaller impeller 19B are not serrated, since the smaller impeller 19B is less critical for the overall sound power of the fan 1 than the larger impeller 19A. It is also conceivable—in other embodiments—to design the trailing edges of the smaller impeller wheels 19B or their blades 22B serrated.

[0040] Mounting openings 24 are provided on the hub 31B of the smaller impeller 19B, which facilitate the installation, in this case of the motor 34A (not visible here) with the support unit integrated into the guide vane housing 2. For fans with two impellers of different diameters, it may be advantageous to have such or similar mounting openings in one of the impellers, typically in the hub area. If necessary, these can also be closed after the fan has been installed, e.g., to avoid noise during operation.

[0041] Fig. 3 shows a side view and a section through the axis of the fan 1 according to Figs. 1 and 2 with two impellers 19A and 19B of different diameters. During fan operation, the flow flows - in this view - essentially from left to right through the fan 1 or its housing 2 within the outer housing contour, first through the area inside the inlet nozzle 9, then through the cylindrical area 29A of the outer housing contour, which here also forms the area for the larger impeller 19A, and then through the area of ​​the outer diffuser wall 10. Approximately after passing through the larger impeller 19A, the flow splits into two flow areas, an inner flow area 7 and an outer flow area 6, which are separated from each other by a wall.

[0042] The wall that separates the inner flow area 7 and the outer flow area 6 from each other is a non-rotating intermediate wall integrated as one piece into the entire guide housing 2, which equally forms the intermediate ring 5 of the guide housing 2 or the guide device integrated there with the guide vanes 11 as well as the outer wall 8 of the smaller impeller 19B. After flowing through the larger impeller 19A, the flow flows over the upstream edge 23 of the intermediate ring 5 and is divided there between the two flow areas, the inner flow area 7 and the outer flow area 6.

[0043] Essentially, the portion of the flow flowing through the inner flow region 7 also flows through the smaller impeller 19B and, as a result of its rotation, is additionally subjected to power transfer. Power transfer is possible in both directions; in any case, power transfer from the smaller impeller 19B to its flow, which can be assigned to the inner flow region 7, is advantageous. The portion of the flow flowing through the outer flow region 6 essentially does not flow through the smaller impeller 19B, so that in this region, for example, no additional noise can be generated by interaction with rotating blades.

[0044] It is also conceivable for an inner and an outer flow area to be separated by a rotating ring integrated into the smaller impeller, connecting the outer blade ends, and for no non-rotating partition to be formed, for example, if the advantageously supporting guide vanes are designed to extend continuously from a hub ring to the outer housing contour. In any case, only a portion of the total flow through the fan 1 flows through an inner flow area 7 and thus the smaller impeller 19B, and an inner flow area 7 and an outer flow area 6 are separated by a partition. It is also conceivable for a smaller impeller to be arranged upstream of the larger impeller in the flow direction.In such embodiments, the total flow of the fan is divided into two flow areas upstream of the large impeller, so that after passing through the two flow areas, the flow can be combined and flow through the larger impeller. The partition between the inner flow area and the outer flow area can be either rotating and integrated into the smaller impeller, or non-rotating and integrated into a support structure, for example, comprising guide vanes.

[0045] In an advantageous embodiment, especially when the larger impeller is in the form of a radial or diagonal fan, the smaller impeller can be arranged in the area in front of or in the suction opening (in the inlet area) of the larger impeller. In this case, the smaller impeller can have the design, in particular, of an axial impeller or a more axially oriented diagonal impeller.

[0046] It is generally also conceivable that the separation into inner and outer flow areas 7 and 6 is continued in the area of ​​the large impeller, in particular in the form of a rotating partition wall integrated into the large impeller and connecting the blades in the span direction between the hub of the large impeller and the outer casing contour.

[0047] The large impeller may also have a circumferential shroud connecting the outer blade tips. In such cases, this outer shroud can help define the outer casing contour, which in this case is more accurately referred to as the outer flow contour.

[0048] The impeller 19A according to the embodiment according to Fig. 3 or its hub 31A is connected to the rotating part of the drive 34A at connecting devices 30A, advantageously screwed.

[0049] The impeller 19B or its hub 31B is connected to the rotating part of the drive 34B by connecting devices 30B, preferably by screwing. Fig. 3 clearly shows the respective drives, preferably electric drives, 34A and 34B of the large impeller 19A and the small impeller 19B, respectively. The impellers 19A and 19B or their hubs 31A and 31B are connected to the rotating interfaces of the drives 34A and 34B, respectively, so that the drives 31A, 31B hold the impellers 19A, 19B and drive them in the direction of rotation, whereby a power transfer takes place between the drives and the impellers.

[0050] The larger impeller 19A typically has a larger, more powerful, or more torque-enhancing drive 34A than the smaller impeller 19B, which requires only a smaller, less powerful, or less torque-enhancing drive 34B. Since the less powerful drive 34B requires only a small fraction of the manufacturing effort, expressed in terms of costs or CO2 equivalent, of the larger drive 34A (<50%), the manufacturing effort is only slightly higher than for a comparable state-of-the-art system without the small impeller and small drive. Despite this only slightly increased manufacturing effort, the power density, efficiency, application range, and adaptability can be significantly increased compared to the state-of-the-art fan.

[0051] The impeller arranged first on the inflow side in the flow direction, here the larger impeller 19A, is equipped in the area of ​​its hub 31A or its inflow side with an aerodynamically designed, curved hub cap 37.

[0052] If the smaller of the impellers is arranged on the upstream side, it can advantageously be equipped with an aerodynamically designed hub cap.

[0053] As drives 34A, 34B, continuously variable speed electric motors are advantageously used, furthermore permanent magnet EC motors, furthermore advantageously with an external rotor design. The motors, including their respective control electronics, can be manufactured completely independently of each other, or a common control electronics can be used. In any case, the control electronics is or are arranged close to the motor, for example in the area of ​​a support unit, such as, in the illustrated embodiment, in the area within the hub ring 4 of the supporting guide vane unit of the guide vane housing 2.

[0054] Depending on the embodiment, one or more electronics housings 13 can be formed in the area of ​​the drives 34A, 34B, in which the control electronics are accommodated.

[0055] It is particularly advantageous if the speeds of the two impellers 19A and 19B, or their drives 34A and 34B, can be adjusted completely independently of each other. Thus, the speed ratio of the two impeller speeds can be optimally adjusted depending on requirements.

[0056] Particularly advantageously, the two speeds are controlled by a special controller. This controller must somehow ensure the required flow rate, but also has a further degree of freedom (because two speeds are available as control variables), for example, to maximize efficiency, minimize power consumption, minimize noise, minimize vibration values, optimize long-throw performance, or optimize the flow through a heat exchanger near the fan. In particular, vibration values ​​can also be minimized, which automatically prevents critical speeds of one or the other impeller.

[0057] Advantageously, the controller used to adjust the speed of the two impellers can be based on artificial intelligence or machine learning methods, but simpler, more deterministic control methods can also be very effective.

[0058] The drives 34A and 34B or the control electronics(s) must be electrically and / or electronically connected to the higher-level system; in particular, for transmitting electrical power, using power cables, but possibly also using control cables. Various devices can be provided for this purpose, for example, cable connections 53 on the control electronics(s) and / or cable penetrations, openings, or the like on the hub cup in the area of ​​the hub 4 of the suspension and / or cable holders 35 on guide devices or support struts and / or cable penetrations 36 in the area of ​​the outer housing (see Fig. 5).

[0059] Fig. 4 shows a side view and a section through the axis of a fan 1 with two impellers 19A and 19B of different diameters, similar to the view in Fig. 3. In contrast to the otherwise similarly constructed fan according to Fig. 1 to Fig. 3, the radially outer flow restriction has an additional extension 16 of the diffuser wall after the diffuser area 10 integrated into the housing 2. On the inner contour, this extension 16 continuously adjoins the integrated diffuser wall 10. Such a diffuser-like extension of the radially outer flow restriction of the fan 1, within which the flow cross-section continuously expands, can be advantageous for particularly high efficiency values.

[0060] The outer flow restriction in the form of the extension 16 of the diffuser wall projects beyond the smaller impeller 19B and its outer wall 8, viewed in the axial direction toward the downstream flow direction, which is located on the right in the illustration shown. This provides additional advantages in addition to the high efficiency. A flat contact guard can now be designed at the downstream end of the diffuser attachment 16 as a contact guard without causing collision with the smaller impeller 19B or its outer wall 8, or its drive 34B. This also makes it easier to stack multiple fans 1 on top of each other for transport purposes.

[0061] An axially long outer diffuser extending over the smaller impeller 19B or its outer wall 8 or its drive 34B can also be manufactured integrally with the housing 2.

[0062] If an attachment diffuser 16 is not manufactured integrally, it can be made of sheet metal, solid plastic or foamed plastic or the like.

[0063] In the exemplary embodiment, the outer wall 8 of the smaller impeller 19B is initially manufactured as a separate, annular part and is attached, advantageously glued, to the intermediate ring 5 of the guide vane housing 2 or its guide device. In advantageous embodiments, it is conceivable to retrofit the smaller impeller 19B, in particular with its drive 34B and, if necessary, with its outer wall 8, to an existing fan 1 according to the state of the art, for example, to achieve higher efficiency, lower specific sound power, higher air flow, greater operational flexibility, or other functional optimizations.

[0064] Fig. 5 shows, in a perspective view from the downstream side, an embodiment of a fan 1 with two differently sized impellers 19A, 19B, similar to that shown in Fig. 4, wherein the intermediate ring 5 of the guide vane unit of the housing 2 and the outer wall 8 of the smaller impeller 19B are manufactured integrally. In this embodiment, an attachment diffuser 16 is provided, which has a more angular outer contour at the outlet. This enables a compact arrangement of several fans 1 next to one another, for example, on an air-conditioning device. Heat exchangers connected on the downstream side and with a more angular shape can be flowed through more homogeneously.

[0065] The speed ratio of the impellers 19A and 19B can also be controlled so that the flow through the heat exchangers connected on the pressure side is as homogeneous as possible.

[0066] Due to the effect of the smaller impeller 19B of fan 1, the heat exchangers connected to the pressure side, located opposite the central area of ​​fan 1, are flowed through much more homogeneously. Backflow in these areas is easily avoided.

[0067] The speed ratio of the impellers 19A and 19B can also be controlled so that the long-throw behavior is optimized, ie that the outflow of the fan into a room connected on the downstream side, for example, acts as far into the depth as possible or that the conveyed air penetrates as deeply as possible into the connected room.

[0068] In a particularly advantageous embodiment, the drive can also be designed in the form of a “double drive”, i.e. a drive with common components such as a common drive and / or electronics housing, but with two different drive speeds, whereby the two impellers can be connected to different rotors and can be driven with rotational speeds that are essentially independent of one another.

[0069] In other advantageous embodiments that require less manufacturing effort, the speeds of the two impellers can be coupled, for example, at a fixed ratio. This can be achieved with transmissions, such as gear drives or magnetic transmissions.

[0070] In other advantageous embodiments that require less manufacturing effort, the speeds of the two impellers can be the same, preferably with the same direction of rotation. The impellers can then be mechanically attached to the same shaft train.

[0071] Depending on the design and operating condition, the smaller impeller can also transfer power to its electric drive, which in this case becomes a generator, in a generator-like manner.

[0072] With regard to further advantageous embodiments of the teaching according to the invention, reference is made to the general part of the description and to the appended claims in order to avoid repetition.

[0073] Finally, it should be expressly pointed out that the embodiments described above serve only to explain the claimed teaching, but do not limit it to the embodiments.

[0074] List of reference symbols

Claims

Claims 1. Fan with two impellers arranged one behind the other in the direction of flow with a common drive or one drive each, one of the impellers having a smaller outer diameter than the other impeller.

2. Fan according to claim 1, characterized in that the smaller impeller has an outer diameter at least 20% smaller than the larger impeller.

3. Fan according to claim 1 or 2, characterized in that in the flow area between the two impellers a non-rotatable guide device is provided, which preferably carries the impellers together with the drive(s), and which may comprise guide vanes.

4. Fan according to claim 3, characterized in that the guide device has an outer, axially remote flow area and an inner, axially close flow area, wherein the flow areas are preferably separated from one another by an intermediate ring, and wherein the smaller impeller is assigned to the inner, axially closer flow area.

5. Fan according to one of claims 1 to 4, characterized in that the larger impeller has a radial or diagonal design and the smaller impeller is located in the region of or within the suction mouth of the larger impeller, ie in the inflow region of the larger impeller and is advantageously mounted on a structure which is arranged on the upstream side of the smaller impeller, for example on an inflow grille.

6. Fan according to one of claims 1 to 5, characterized in that the impellers rotate in the same or opposite direction.

7. Fan according to one of claims 1 to 6, characterized in that, when the impellers are driven independently of one another, the speeds of the two impellers can be adjusted or regulated independently of one another.

8. Fan according to one of claims 1 to 7, characterized in that the speeds of the two impellers are coordinated with one another and can advantageously be regulated depending on the operating point.

9. Fan according to one of claims 1 to 7, characterized in that the fan operation is optimized by adjusting the speed of one of the impellers or both impellers, preferably at least the smaller impeller with an EC drive.

10. Fan according to one of claims 1 to 9, characterized in that the speeds of the impellers can be optimally adjusted or regulated with the aid of a special controller, in particular using methods of artificial intelligence, machine learning and / or conventional control algorithms with regard to the desired air performance and optionally at least one further target variable, for example the overall efficiency.

11. A method for controlling a fan with two impellers according to one of claims 1 to 10, wherein the fan operation is optimized by adjusting / changing the speed of at least one of the impellers.

12. Method according to claim 11, characterized in that a requested power or air power of the fan is ensured while maximizing the overall efficiency and / or minimizing the overall noise emission and / or minimizing the overall vibration values ​​and / or the vibration values.

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