Improved manner of reducing vibration in fan walls

Abstract

A method for fan wall (10) comprising a plurality of fan modules (11), wherein the plurality of fan modules (11) is grouped into at least two fan groups (FG), each fan group comprising one or more than one fan (11), and wherein the method comprises receiving a control signal (CS), operating at least one selected fan group (FG-1) above an upper limit of a vibration range and operating the other fan group(s) (FG-2, FG-3) below a lower limit of the vibration range, whereby the least one selected fan group (FG-1) is operated to compensate for the other fan group(s) (FG-2, FG-3).

Description

[0001] IMPROVED MANNER OF REDUCING VIBRATION IN FAN WALLS TECHNICAL FIELD 5 The teachings herein relate to an improved fan wall and a method for improved manner of reducing vibration in a fan wall. BACKGROUND 10 As is well-known a fan comprises many moving parts which may cause vibration in the fan. Especially in fan walls – being large constructs – these vibrations can become a problem. Prior art solutions have tried to strengthen the 15 geometry, such as the housing, of the fan wall in order to push any resonance frequency out of the workable operating range for the fan wall. However, this requires that the fan housing is made very thick which is impractical as the fan wall then becomes very heavy. 20 There is thus a need for an improved manner of preventing or at least reducing vibration in a fan wall. SUMMARY The inventors have realized that by controlling the fans 25 individually or in groups and to control some fans to compensate for others it is possible to avoid any operating ranges where vibrations are prominent, and especially realized that when controlling fans in groups it is possible to control some groups of fans to compensate for other groups to avoid 30 any operating ranges where vibrations are prominent. The teachings herein thus provide fan wall comprising a plurality of fans and a controller, wherein the plurality of fans is grouped into at least two fan groups (FG), each fan group comprising one or more than one fan, and wherein the controller is configured to receive a control signal (CS), which control signal indicates a fan speed for a desired fan 5 effect (FE) for the fan wall, determine if the control signal (CS) indicates a fan speed that is inside a vibration range, and if so select at least one of the fan groups (FG), operate the at least one selected fan group (FG-1) at a fan speed above an upper limit of the vibration range and operate the 10 other fan group(s) (FG-2, FG-3) at a fan speed below a lower limit of the vibration range, whereby the least one selected fan group (FG-1) is operated to compensate for the other fan group(s) (FG-2, FG-3). For the context of the teachings herein, a fan effect is, 15 of course, the volume of air pushed by the fan in a given time, such as cubic meters of air pushed through per second (m3 / s). The fan effect can thus also be referred to as airflow or flow of air. In embodiments where the fan blades are fixed (to the hub, i.e. the angle of the blade is fixed) the flow of 20 air will be proportionate to the speed of fan and the flow of air is increased by increasing the speed (which is done by increasing the signal) and decreased by decreasing the speed (which is done by decreasing the signal). Also, for the context of the teachings herein, the 25 control of a fan group is achieved by sending a single control signal to (all) the fan(s) in the fan group. This provides a system where a minimum of control signals is used, or at least where a reduction in control signals needed is achieved. This allows for a simpler component architecture and also for the 30 use of simpler or less advanced components, compared to a system where each fan is individually controlled. Further embodiments and benefits are discussed in the detailed description. The teachings herein also provide a method for a fan wall comprising a plurality of fans, wherein the plurality of fans 5 is grouped into at least two fan groups (FG), each fan group comprising one or more than one fan, and wherein the method comprises receiving a control signal (CS), determining if the control signal (CS) is inside a vibration range, and if so selecting at least one of the fan groups (FG), operating the 10 at least one selected fan group (FG-1) above an upper limit of the vibration range and operating the other fan group(s) (FG- 2, FG-3) below a lower limit of the vibration range, whereby the least one selected fan group (FG-1) is operated to compensate for the other fan group(s) (FG-2, FG-3). 15 Unlike in the prior art, the solution(s) herein makes it possible to avoid vibrations even when it is not possible to modify the fan wall geometry. The solution(s) herein thus provide a fan wall that can be designed so there are never any harmful vibrations in the 20 active fan speed range. This solution can beneficially be used also when (other) mechanical solutions are not possible, such as when the resonance is inherent from the fan design, and it is not possible to strengthen the fan wall enough to push the vibration peak out of the working area. 25 Further embodiments and benefits of the present teachings will be apparent from the following description. BRIEF DESCRIPTION OF THE FIGURES The solution will be disclosed with simultaneous 30 reference to the appended figures, wherein Figure 1 shows a schematic view of a fan wall according to the teachings herein; Figure 2 shows a schematic view of an operating range for a fan wall and indicates an range where vibrations are prominent; Figures 3A, 3B, 3C and 3D each shows a schematic view of 5 various embodiments of how individual fans in a fan wall may be grouped according to the teachings herein; Figures 4A, 4B, 4C, and 4D each shows a schematic view of various embodiments of operating ranges for groups of fans in a fan wall according to the teachings herein; 10 Figure 4E shows a schematic view of various embodiments of combined operating ranges for groups of fans in a fan wall according to the teachings herein; Figure 4F shows a schematic view of an operating range for a fan wall and indicates a range where vibrations are 15 prominent as in Figure 2; Figure 4G shows a schematic view of various embodiments of a resulting operating range for groups of fans in a fan wall according to the teachings herein; Figure 5 shows a schematic view of various embodiments of 20 alternative operating ranges for groups of fans in a fan wall according to the teachings herein; and Figure 6 shows a flow chart for a general method for various embodiments of the teachings herein. 25 DETAILED DESCRIPTION The teachings herein will be disclosed with simultaneous reference to the appended figures. Figure 1 shows a schematic view of a fan wall 10 30 according to the teachings herein. The fan wall comprises a plurality of fans 11, possibly arranged in fans, where a fan can comprise one or more fans 11. As a skilled person would understand the fan(s) 11 are usually arranged in a regular pattern, depending on the size (dimensions) of the fan wall 11 and the number of fan(s) 11. In figure 1 there are 9 fan(s) 11-1 – 11-9 arranged in a 3x3 pattern. As a skilled person 5 would understand this is only one example and other numbers of fan(s) 11 and arrangements are possible, for example 4 fans arranged 2x2, 6 fans arranged 2x3, 8 fans arranged 2x4, 12 fans arranged 3x4, 16 fans arranged 4x4, to mention a few examples. 10 Figure 2 shows a schematic view of an operating range for a fan wall 10 and indicates a (fan speed) range where vibrations are prominent, i.e. a vibration range. Such a vibration range is also referred to herein as a blocking range, as the vibration range is to be avoided as per the 15 present invention. This range thus indicates a range where fan operation should be avoided due to prominent vibrations, i.e. a blocking range. Prominent vibrations may be defined as vibrations above a vibration threshold level and / or causing sound above a sound level. The graph shows how for a given 20 input signal, such as a control signal CS, a desired or expected effect FE for the fan wall 10 is provided, such as a desired operating speed, indicated as the revolutions per minute, RPM. The range where vibrations become prominent (the vibration range), as in (undesirably) noticeable (by human 25 observation or by instrument measuring), is marked by an upper limit UL and a lower limit LL. As a skilled person would understand, the exact values of this range is dependent on the interaction of many mechanical factors of the fan and the fan wall. 30 Returning to figure 1, the fan wall 10 also comprises a control unit 12. The control unit 12 is in some embodiments an electronic control unit such as a Programmable Logic Circuit or a processor. The control unit 12 is in some embodiments an electric circuit arranged to perform a wanted function given an input signal. The control unit 12 is connected to the fan(s) 11 for controlling the fans, and more particularly 5 controlling the speed of the fans 11. The connection may be individual for one, some or all fan(s) 11. The connection may also or alternatively be for groups of fans for some of the fan(s) 11. This enables the controller 12 to control the speed of each fan 11 individually or in groups of fans. 10 The control unit 12 is configured to receive a control signal CS, which control signal indicates a desired fan effect for the fan wall 10. As the fan effect is (nearly directly) proportional to the speed of the fans 11 (given that the external pressure drop in the system where the fan wall is 15 installed is sufficiently small), the control signal indicates a desired speed of the fans 11. In any subsequent discussion, it is assumed that the pressure drop of the system where the fan wall is installed is sufficiently moderate that the additions of different fan effects is linear within a 20 sufficient margin. It is also assumed that the lowest speed at which a fan is run is high enough that no back-draft through the fan will occur. In some embodiments the fan wall 10 also comprises a vibration sensor(s) 13 arranged to sense vibrations in some or 25 all of the fans 11. Alternatively, the vibration sensor(s) is external and used to measure or detect the vibrations. Of course, there can be both internal and external vibration sensors. This enables the vibrations to be measured, which enables the controller 12 to detect which control signals 30 provide vibrations that are undesirable. In some embodiments the controller 12 is therefore configured to receive a vibration sensing signal from the vibration sensor 13, and to detect for which control signals that vibrations exceeding a vibration threshold occurs. The controller 12 in such embodiments is thus further configured to determine the vibration range where such prominent vibrations occur. The 5 range may be determined as an upper limit and a lower limit for the range. In figure 2, this range is plotted against the speed of fans 11 of the fan wall 10, but can also be plotted to the control signal CS. In some embodiments, the blocking range is determined 10 during design or commissioning of the fan wall, possibly using a standalone measuring instrument that may or may not be part of the fan wall 10. Any time the fan wall 10 is operated in this range, there will be prominent vibrations. As will be discussed, the 15 teachings herein provide for a manner of operating the fan wall 10, where the fan(s) 11 are controlled individually and / or in groups, where some fan(s) is controlled to compensate for other fan(s), by being controlled to operate at a higher speed than the other fan(s), thereby providing a 20 resulting fan effect of the fan wall 10 that corresponds to the desired effect, even when the control signal (indicates a fan speed that) is inside the vibration range but without the prominent vibrations occurring. As one or some fans are controlled to compensate for other fans, the fan(s) 11 can be 25 controlled so that none of them actually operates in the vibration range where the prominent vibrations occur. Returning to the grouping of fan(s) 11 as mentioned above, the fans may be grouped in fan groups. This allows for fewer control lines to be used. Some fan walls 11 and some 30 controllers 12 only come with a limited number of control lines which prevents all fan(s) 11 to be individually controlled. Figures 3A, 3B, 3C and 3D each shows a schematic view of various embodiments of how individual fan(s) 11 in a fan wall 10 may grouped according to the teachings herein. As a skilled person would understand, many groupings are possible and the 5 shown examples are only some of many possibilities, both as regards the number of groups and the number of fans in each group, as well as the distribution of the fans in a given fan group. Figure 3A shows one possible fan grouping where a first 10 fan group FG-1 comprises a single (and middle) fan 11-5, a second fan group FG-2 comprises two side fan(s) 11-2 and 11-8, and a third fan group FG-3 comprises the top and bottom row of fan(s) 11-1, 11-4, 11-7, 11-3, 11-6, 11-9. The grouping in this example is this FG-1:1, FG-2:2 and FG-3:6. 15 Figure 3B shows one possible fan grouping where a first fan group FG-1 comprises a single (and middle) fan 11-5, a second fan group FG-2 comprises one side fan 11-2 and the top row of fan(s) 11-1, 11-4, 11-7, and a third fan group FG-3 comprises one side fan 11-8 and the bottom row of fan(s) 11-3,20 11-6, 11-9. The grouping in this example is this FG-1:1, FG- 2:4 and FG-3:4. Figure 3C shows one possible fan grouping where a first fan group FG-1 comprises a single (and middle) fan 11-5, a second fan group FG-2 comprises two diagonally arranged fan(s) 25 11-1, 11-9, a third fan group FG-3 comprises the other two diagonally arranged fan(s) 11-3, 11-7, and a fourth fan group FG-4 comprises the bottom and bottom middle fan(s) 11-2, 11-4, 11-6, 11-8. The grouping in this example is this FG-1:1, FG- 2:4, FG-3:2, and FG-4:4. 30 Figure 3D shows one possible fan grouping where a first fan group FG-1 comprises two (side and middle) fan(s) 112, 11- 8, a second fan group FG-2 comprises two side top row fan(s) 11-1 and 11-7 and the middle fan 11-5, and a third fan group FG-3 comprises the top middle fan 11-4 and the bottom row of fan(s) 11-3, 11-6, 11-9. The grouping in this example is this FG-1:2, FG-2:3 and FG-3:4. 5 As the inventors have realized, by having fan groups of different sizes, the resulting effect as the fans are combined can be increased incrementally in different steps. In some embodiments at least two of the fan groups thus comprise a different number of fan(s) 11. This will lead to that some fan 10 group(s) will have fewer fan(s) 11 than other fan group(s). Even if the first fan group in the examples given herein is always shown as having the smallest number of fan(s) 11, this is not necessary and is only a matter of labeling. In some embodiments at least one fan group has a single 15 fan 11. This allows for making small adjustments of the resulting effect. As the inventors have also realized, a suitable goal when determining the sizes of the fan groups, is which configuration will give the lowest maximum signal M, i.e. the 20 level of speed as the signal indicates the speed, as described below. This is because a large difference between the fan speeds of the fans in the fan wall may cause a larger error because of various effects such as backdraft or increased turbulence between the fans. The largest difference in speeds 25 M in vibration range can be denoted the spread (of speed levels). As the inventors have also realized, by having fan groups substantially centered around a center of the fan wall 10, the resulting effect as the fans are combined is more balanced, 30 than if a fan group had all fans on one side. In some embodiments the fan groups are thus distributed around a center of the fan wall 10. As the inventors have realized, the resulting effect of the fan wall 10 can be calculated as the sum of the contribution of the individual fan(s) 11 divided by the number of fans. One formula that can be used to calculate the resulting fan wall effect RFE is: (1) RFE =∑ிீ^∗ே^ே , where FGi is the fan effect (or speed) that a fan group i is run at, Ni is the number of fans in fan group i, and N is the number of fans in the fan wall. The controller 12 is thus configured to operate the fans individually or in groups so that the resulting effect RFE of the fan wall 10 is close to the intended (or original) effect FW, by controlling at least one fan to operate above the vibration range compensating for at least one other fan 11 which remains operating below the vibration range. The controller 12 is thus configured to control the fan(s) 11 so that RFE = FE + / - e, while all FGiare outside the vibration range, where e is an accepted error margin. In some embodiments the controller 12 is further configured to control the fan(s) 11 so that the resulting efficiency of the fan wall 10 is kept below a power level or at a minimum power given the criteria that RFE = FE+ / -e, while all FGi are outside the vibration range. As the inventors have realized, the power consumed by a fan is correlated to the speed of the fan 11. However, this relationship is not linear, and a higher speed for one fan to provide a given fan effect (i.e. airflow) gives a higher power consumption than two (or more) fans providing the same effect, but at lower speeds. To accomplish this the inventors are proposing that the controller 12 is configured to run all (or at least a group of) fans in unison (as normally) until a lower limit LL of a vibration range is reached, and then increase or jump the 5 speed of a first fan group to the upper limit of the vibration range while maintaining one or several fan groups below or at the lower limit. As the control signal increases (indicated a higher desired fan effect referenced FW), the speed of the first fan group is increased, but at a higher rate to 10 compensate for the fan group maintained at the lower limit. Alternatively or additionally, the increase in control signal can cause an increase in other fan groups that are operating below the lower limit, but only up to the lower limit, or possible up to above the upper limit, also to compensate for 15 the fan group maintained at the lower limit. In some embodiments the controller 12 is configured to increase the speed of the first fan group at a change rate not exceeding a threshold value in order to avoid oscillations as the fans are changing their speeds. 20 In some embodiments the controller 12 keeps increasing the first fan group as the control signal increases until a maximum (allowed) speed of the first fan group FG-1 is reached. Thereafter, the first fan group is not able to alone compensate for the other fan group(s) and a second fan group 25 is selected to compensate for the other fan group(s). The second fan group may be operated to compensate for the other fan group(s) in combination with the first fan group as a supplement, or alone. In embodiments where the second fan group is operated to 30 compensate for the other fan group(s) in combination with the first fan group the controller 12 is configured to increase the speed of the fan(s) 11 in the second fan group FG-2 to the upper limit UL and to decrease the speed of the fan(s) 11 in the first fan group FG-1 back down to the upper limit UL, while maintaining the other fan group(s) (for example the third fan group FG-3) at the lower limit LL. 5 Alternatively or additionally, the controller 12 is further configured to increase the speed of the fan(s) 11 in fan groups operating below the lower limit up to the lower limit. This increase may be done alone or as a supplement to increasing the speed of the fans operating above the upper 10 limit. By increasing the signal to (and thereby the speed of) the lower group(s) (fan groups operating below the lower limit) as an alternative to or as a supplement to increasing the speed of the upper group(s) (fan group(s) operating above the upper limit) enables for a small difference in fan speeds 15 and thus provides a more even flow of air, and also a more harmonized sound, compared to running fan groups at greatly differing fan speeds. In some embodiments, the controller 12 is further configured to increase the speed of the fan(s) 11 in the 20 second fan group FG-2 along with the speed of the fan(s) 11 in the first fan group FG-1 and / or along with the fan(s) operating below the lower limit as an increased control signal is received. I.e. all fan speeds (or rather fan group speeds when all fans in a group are set to operate at a same speed) 25 are increased at the same rate – except when there is a jump for moving one fan group to above the vibration range and / or possibly for moving one fan group to below the limit – both of which can happen at the same time. In some such embodiments the speed of the fan(s) 11 in 30 the first fan group FG-1 is increased at a same rate as the speed of the fan(s) 11 in the fan group FG-2. In some such embodiments the speed of the fan(s) 11 in the first fan group FG-1 is increased at a same rate as the speed of the fan(s) 11 in the fan groups operating below the lower limit. In some such embodiments the speed of the fan(s) 11 in the lower fan groups is increased at a higher rate than the 5 normal increase of fan speed, i.e. the rate of increase during normal operation (operation outside the vibration range). In some embodiments the increase is linear. In some embodiments the increase is non-linear. This allows for the first and second fan group to be increased intermittently, 10 where the first fan group may be repeatedly increased to a level, whereby the second group is increased to catch up, and the first level is again increased. In some alternative embodiments, the controller 12 is further configured to only increase the speed of the fan(s) 11 15 in the first fan group FG-1 as an increased control signal is received and maintain the speed of the fan(s) 11 in the second fan group FG-2 at the upper limit UL , or lower limit depending on which fan group is operating above the vibration range and which is operating below the vibration range. In some such 20 embodiments, one or more of the fan groups above the vibration range may be increased at a same rate, while the one or more fan groups below the vibration range are held at the lower limit of the vibration range. Similarly, one or more of the fan groups below the vibration range may be increased at a 25 same rate, while the one or more fan groups above the vibration range are held at the upper limit of the vibration range. The inventors are thus proposing to operate the fan group(s) individually and at control signals (and thus speeds) 30 outside the vibration range, where one or some fan groups are set to operate above or at an upper limit and one or some fan groups are set to operate below or at a lower limit, wherein the control signals for the fans are set to provide a resulting combined airflow without operating any fan in the vibration range, where the provided resulting airflow corresponds to a common airflow that would be provided by a 5 control signal in the vibration range if all fans were operated in unison in the vibration range. Which alternative is chosen depends on the design of the fan wall 10, the desired fan effect FE and the size of the vibration area. 10 Returning to which fan group is selected, the controller 12 is in some embodiments to simply select a next larger fan group. However, in some instances, the next fan group may have too many fans which when run at the upper limit will provide for a resulting effect that is too high. 15 The controller 12 is thus in some embodiments configured to select a next fan group to be run at (or higher than) the upper limit so that the resulting fan effect is not too high, i.e so that RFE = FE(+ / - e). Before showing an example, it could be noted that the 20 maximum allowed speed of a fan (i.e the maximum level M) is in some embodiments dependent on the design of the fan 11. In some embodiments it is the maximum speed that allows the fan to operate at an acceptable power consumption. In some embodiments the maximum speed is the speed needed to reach an 25 FE where a next group operating at the upper limit at the earliest provides RFE = FE. It could also be pointed out that the size of the smallest fan group depends on the number N of fans in the fan wall 10, the desired fan effect FE, and the distance between 30 the upper limit and the lower limit (i.e. how much one fan group has to compensate for the other fan group(s)) and the maximum (allowed) an speed (as this may not be enough to fully compensate – or to compensate in a power efficient manner. In some embodiments, the fan groups are controlled individually, but the fans inside the fan groups are not 5 controlled individually but all fans in the group are controlled in the same manner. This is achieved, in some embodiments, by providing the same control signal to all fans in a selected fan group thereby setting the same speeds for all fans in the group. This provides for a fan wall requiring 10 fewer control signals, where only the number of fan groups are needed as the number of control signals, as opposed to the number of fans. In some such embodiments, the fan groups are set during installation as opposed to being set during operation. In some alternative such embodiments, the fan 15 groups are set during manufacturing or assembly as opposed to being set during operation. In some such embodiments, the groups are thus hardcoded by how the control signals are connected to the fans, and cannot be changed – at least not easily – during operation. It is thus important in such 20 embodiments to design the fan groups (i.e. select which fans go in which fan group) prior to operation. As discussed above, the selection of which fan to go in which fan group may be based on achieving a smallest difference between fan speeds. As it is possible to determine how a controlling algorithm 25 will operate around the vibration range, it is also possible to determine which fans should be included in which fan groups prior to operation. In order to do this, all possible combinations of fans and groups are determined. In some embodiments the number m of control signals available (or 30 chosen) determines the number of fan groups. For n fans and m fan groups the number of combinations is equivalent to the number of surjections (onto 15 functions) from a set of ^^ objects to ^^ groups. The formula for this is given by: ^^!⋅^^(^^,^^), where ^^(^^,^^) is a Stirling number of the second kind, representing the number of ways to partition ^^ objects into ^^ non-empty subsets. If a fan group is allowed to be empty, the number of combinations becomes mn. As all combinations have been determined, and the vibration range has been determined, the control algorithm (as disclosed herein) is applied virtually to all combinations of fans and fan groups for a common control signal going over the vibration range, whereby the group control signals for the fan groups are determined as the common control signal goes through the vibration range. It is then determined which combinations give the smallest differences in the group control signals (a group control signal giving the fan speed in the fan group). In some embodiments the difference is determined as the largest difference at any point during operation, i.e. max(highesti- lowesti) for all time instances i. In some embodiments the difference is determined as the largest average difference, possible for a limited time period, during operation, i.e. max(average(highj,i-lowj,i))for all time instances i and (selected) time periods j. Figure 4A shows a schematic view of various embodiments of operating ranges for groups of fans in a fan wall according to the teachings herein. In figure 4A three fan groups FG-1, FG-2 and FG-3 are run in unison as long as the incoming control signal CS results in the operating range being outside the vibration range, which is indicated by that the fan speeds are below a lower limit of the vibration range. This is indicated in the figure as stage 1, S-1. As the control signal increases, as is shown in figure 4B, the controller selects one fan group, in this example FG- 1, to compensate for the other two fan groups. The fan speed of the selected fan group FG-1 is thus increased up to the upper limit, while the speed of the other fan groups (in this example FG-2 and Fg-3 is maintained at the lower limit. As the control signal increases, the fan speed of the selected fan 5 group FG-1 is increased, but at a higher rate to compensate for the other fan group(s). The fan speed of the selected fan group FG-1 is increased until a maximum allowed speed is reached. This is indicated in the figure as stage 2, S-2. As the control signal further increases, as is shown in 10 figure 4C, the controller selects another or one more fan group, in this example FG-2, to compensate. In this example the second fan group FG-2 is selected in addition to the first fan group FG-1. However, as discussed above, the second fan group FG-2 could be selected instead of the first fan group 15 FG-1. As is shown in figure 4C, the fan speed of the first fan group FG-1 is decreased back down to the upper limit and then increased again as the control signal increases. In this example, the first fan group is thus simply supplemented (or 20 backed up) by the second fan group. However, the second fan group can also be controlled so that the fan speed of the fans in that group is increased as well as the fan speed of the fans in first group. An example of how this can be done is shown in figure 4D. 25 The speed of the fans 11 in the selected fan group(s) is thus increased again (as the control signal increases) until a maximum level is reached. This is stage 3, S-3. As is seen in figure 4D, there may be more than one maximum level (or the maximum level may be different depending 30 on the stage). For the first fan group FG-1, the maximum level M1 for stage 2 is higher than the maximum level M2 for stage 3. As the first fan group is now increasing along with the second fan group a lower maximum is possible. Also, the second fan group may have another maximum than the first fan group. 5 Figure 4E shows a schematic view of various embodiments of combined operating ranges for groups of fans in a fan wall according to the teachings herein. In Figure 4E all individual fan group speeds are overlaid to show their dependencies on one another. 10 Figure 4F is the same as Figure 2 and is only repeated here for easier comparison of the desired fan effect FE shown in Figure 4F / 2 with the resulting fan effect RFE shown in Figure 4G which resulting fan effect RFE is provided as the weighted sum (1) discussed above. 15 Figure 5 shows a schematic view of various embodiments of alternative operating ranges for groups of fans in a fan wall according to the teachings herein. In figure 5, the second fan group is selected together with the first fan group already in stage 2 thereby reducing the rate at which the first fan group 20 need to be increased, which may save on power consumption as the power needed for a speed increase is not linear over the speed range. As noted above, the selection of which fan group to compensate may be done by simply choosing from a next in a 25 list (which may be based on previously made calculations), or to do the calculations discussed above dynamically. The examples above have discussed jumping or increasing the fan speed of a selected fan group so that it is at the upper limit. However, it should be clear that depending on the 30 number of fans in the selected fan group(s) and the upper limit, it may be necessary to increase the fan speed straight away to a level above the upper limit. Alternatively, it may be needed to decrease the speed of the other fan group(s) and thus not maintain them at the lower limit, but below the lower limit. The key is that no fan (group) is operating between the lower limit and the upper limit, only above the upper limit or 5 below the lower limit, compensating for one another. In some instances it may be needed to deselect a fan group, i.e to bring the speed of that fan group down below the lower limit (or lower), in order to select another fan group (having more fans) while still allowing the resulting fan 10 effect to not differ significantly from the desired fan effect, i.e. RFE = FE+ / -e. As discussed above, the vibration range is related to the speed of the fan(s). However, as the vibration range is determined (measured or calculated), the corresponding control 15 signals providing the lower limit and the upper limit are also determined (measured / noted or calculated), and the controller can thus determine that it is (or rather would be) in the vibration range simply by determining what the control signal is. If the control signal is above a lower level resulting in 20 the fan speed being above the lower limit, but below an upper level resulting in the fan speed being below the upper limit, the controller can determine that the fan wall would be in the vibration range. The two terminologies of whether the vibration range relates to the speed or to the control signal 25 are thus interchangeable as would be clear to a skilled person. Figure 6 shows a flow chart for a general method for various embodiments of the teachings herein. The method is for fan wall 10 as discussed herein and the method comprises 30 receiving 710 a control signal CS, and determining 720 if the control signal CS is inside a vibration range. If the control signal CS is inside the vibration range, at least one of the fan groups (FG) is selected 730 to be a compensating fan group and is operated 740 at a speed above an upper limit of the vibration range. The remaining or other fan group(s) (FG-2, FG-3) are operated 750 below a lower limit of the vibration 5 range, whereby the least one selected fan group (FG-1) is operated to compensate 760 for the other fan group(s) (FG-2, FG-3). The vibration range is, as discussed in the above – given both as a control signal range, and as a speed range, and the 10 two are interchangeable. The vibration range may thus be seen as a control signal range which provides a fan speed range, where a lower limit of the control signal range corresponds to a lower limit of the fan speed range and an upper limit of the control signal range corresponds to an upper limit of the fan 15 speed range. In the above a simple control formula is provided which is based on the contributory flow of each fan. However, the inventors have also realized that more complex control algorithms may be used, as below, but these are all based on 20 the same general principle and can thus be seen as a subspecies of the simple formula discussed above. As would be understood, there is a difference in selecting which fan group to activate or control, and which fans go in which fan group. The selection of which fan goes 25 into which fan group may be made before operation (i.e. such as during manufacturing, assembly and / or installation), whereas the selection of which fan group to control is made during operation. In the following a general control algorithm employing 30 many of the embodiments disclosed above will be detailed. The more complex algorithm is based on dividing the range of the control signal in sections and determining what fan groups will be active and at which speed in each section. The algorithm starts by ordering the fan groups in 5incrementing (or rather non-decrementing) order ^^^, ^^ଶ…^^^ basedon size of fan groups. Let ^^^be the number of fans which are active in section ^^, Let ^^^be the number of fans which are operating above the 10 blocking range in section ^^ Let ^^^be the number of fans which are operating below the blocking range in section ^^ Let ^^^^^^௪^^be lower limit of the blocking range and ^^^^௨^^^^and be upper limit of the blocking range. 15 The aim of the algorithm is to output a resulting signal which is linear while no fan is operating in the blocking range. When comparing different fan groupings as per the previous fan group, the main objective is to minimize the largest signal spread, which will also be considered in the 20 algorithm. In order to keep the resulting signal linear, the fans which are active needs to compensate for the fans which are stationary. In some embodiments all active fans should have the same rate of change in order to reduce the signal spred, 25 since otherwise the signal spread at that point would be higher than necessary. Therefore the rate of change in section ^^ should be 30 It is in some cases beneficial if as many fans as possible – or at least many fans - are increasing as the change rate ^^^will be lower and therefore the spread will be lower. Since the fans may not operate in the blocked range (the vibration range), the algorithm will be divided in different sections and the fans will need to pivot around the 5 blocking range – some fans will always need the operate above the blocking range and some fans below. Since the groups are ordered in incrementing order, the pivoting – opposing fans – will appear in a binary pattern. In order to minimize the range between the lowest and 10 highest control signal for the fan groups, there are two case situations: i) the active fans can increment until they reach ^^^^^^௪^^or ii) they can increment starting from ^^^^௨^^^^(until another configuration that will give a lower signal spread is possible) 15 In the first case i), since the active fans will be equal to ^^^^^^௪^^at the end of section, the end of section ^^ will be In the second case ii), the end of section ^^ can be found 20 by observing the equation for start of the next section There are two exceptions when defining the sections: 25 ^ The end point of the last segment is always 100% ^ There will be a middle segment which could either belong to the special cases discussed below with regards to partitions. However, the ideal solutions is to divide the middle segment into two equal segments where the first belongs 30 to (2) and the second to (3). This will minimize the spread. The above statements allow us to create a list of sections and their characteristics. The general case as described above is however only valid without modifications when: 51. ^^^ ≠ ^^^ା^ (all groups have different size) (all sums of the combinations of different groups (except the last) are unique) 3. (the sum of the groups (except10 the last) is less than the last) Examples of divisions that fulfils this are for instance [1,2,6], [1,2,4,9]. Examples of divisions that do not fulfill this are [1,1,7], [1,2,3,10], [2,3,4] (Violating 1,2 and 3 respectively). 15 In all these cases 1-3 above, a working algorithm can be obtained by simply removing sections that are invalid. Regarding case 1 and 2), the best workflow is to calculate which sums are distinct already before defining the algorithm and only use those. This could be done by generating 20 all binary combinations of the groups and once a sum is calculated, discard all future equal sums and their associated binary combination. If the sum of all groups except the last one are larger than the last one itself, i.e., if25 then some segments, as defined by the preceding conditions for ei, will in essence overlap. For the algorithm, the solution is to just remove any segment where ^^^ > ^^^ା^ andpossibly also adjust the starting point of the adjacent 30 section. Similarly, if ^ି^ ^^^^ = ^^^^ୀ^ then some sections will have exactly the same endpoint and those section can just be removed.(Since for instance, the “mid” section endpoint and the previous endpoint will by 5 definition be the same) Table 1 gives an example view of the general algorithm for n sections. Referring back to the selection of fan modules for the fan groups, in some embodiments, the selection of which fan 10 groups to be used can be done utilizing a recursive algorithm for fan wall having n fan modules 11 that will be partitioned into m fan groups: 15 having special cases (partitions(n≠0,m=0,start,subset)= ∅ (partitions(n=0,m=0,start,subset)= subset To determine the divisions of n fans into m groups, the 20 partitions should be determined, i.e. determine ^^^^^^^^^^^^^^^^^^^^(^^, ^^, 1, ∅).The algorithm can be understood as building configurations by attempting to removing groups of fans of all “possible” sizes (i.e., pulling fans from the pool of 25 “unassigned fans” into a specific group). If the final groups does not add up to the number of fans (i.e., if the number of “remaining fans” are not zero), the group configuration is invalid and discarded. To determine which fan grouping is the best, the algorithm above (and in table 1) can be applied and 30 the maximal signal spread of each division can be calculated. Sections herein are ranges within which one or more fan groups are controlled without making any jump, such as shown in figures 4A-4G and figure 5. 5

[0002] Section Group 1 Group 2 Group i Group n Change Section end e ^^^^^^^^^^^^^^^ g g g g Table 1

Claims

CLAIMS 1. A fan wall (10) comprising a plurality of fans (11) and a 5 controller (12), wherein the plurality of fans (11) is grouped into at least two fan groups (FG), each fan group comprising one or more than one fan (11), and wherein the controller (12) is configured to receive a control signal (CS), which control signal indicates a fan speed for a desired fan effect (FE) for the fan wall (10), determine if the control signal (CS) indicates a fan speed that is inside a vibration range, and if so select at least one of the fan groups (FG), operate the at least one selected fan group (FG-1) at a fan speed above an upper limit of the vibration range and operate the other fan group(s) (FG-2, FG-3) at a fan speed below a lower limit of the vibration range, whereby the least one selected fan group (FG-1) is operated to compensate for the other fan group(s) (FG-2, FG-3).

2. The fan wall (10) according to claim 1, wherein the controller (12) is further configured to detect that the received control signal (CS) is increasing and in response thereto increase the speed of the fan groups at a first rate when the control signal indicates a fan speed that is outside the vibration range, or increase the speed of the selected at least one fan group (FG-1) at a second rate when the control signal indicates a fan speed that is inside the vibration range,wherein the second rate is higher than the first rate.

3. The fan wall (10) according to claim 2, wherein the 5 controller (12) is further configured to increase the speed of at least one of the selected at least one fan group (FG-1) at the second rate and maintain the speed of the other (FG-2) of the selected at least one fan group (FG-1, FG-2) when the control signal indicates a fan speed that is inside the vibration range.

4. The fan wall (10) according to claim 2 or 3, wherein the controller (12) is further configured to maintain the speed of at least one of the other fan groups (FG-2, FG-3) at a speed below the lower limit when the control signal indicates a fan speed that is inside the vibration range.

5. The fan wall (10) according to any preceding claim, wherein the controller (12) is further configured to detect that the received control signal (CS) is increasing and in response thereto increase the speed of the fan groups at a first rate when the control signal indicates a fan speed that is outside the vibration range, or increase the speed of the at least one of the other fan groups (FG-2, FG-3) to a speed below or at the lower limit at a third rate when the control signal indicates a fan speed that is inside the vibration range, wherein the third rate is higher than the first rate.

6. The fan wall (10) according to any preceding claim, wherein the controller (12) is further configured to determine that the control signal (CS) indicates a fan speed that is at a lower limit of the vibration range, and in response thereto5 set the speed of the selected at least one fan group (FG- 1, FG-2) to an upper limit or above and to set the speed of the other fan groups (FG-2, FG-3) to a lower limit or below.

7. The fan wall (10) according to any preceding claim, wherein the controller (12) is further configured to determine that a maximum level (M) of at least one (FG-1) of the selected at least one fan groups (FG-1, FG-2) is reached, and in response thereto select at least one next fan group (FG-2, FG-1) and increase the speed of the next fan group (FG-2, FG-1) to the upper limit or above.

8. The fan wall (10) according to claim 7, wherein the controller (12) is further configured to deselect at least one of the selected fan group (FG-2, FG-1) and set the speed of the deselected at least one fan group to the lower limit or below.

9. The fan wall (10) according to claim 7 or 8, wherein the controller (12) is further configured to set the speed of at least one of the selected at least one fan group to the upper limit or above.

10. The fan wall (10) according to any preceding claim, wherein the controller (12) is further configured to determinethat a maximum level (M) of at least one (FG-1) of the selected at least one fan groups (FG-1, FG-2) is reached, and in response thereto select at least one fan group (FG-2, FG-3) of the fan 5 group(s) operating below the lower limit and increase the speed of the next fan group (FG-2, FG-3) to a speed below or at the lower limit.

11. The fan wall (10) according to claim 10, wherein the controller (12) is further configured to keep increase the speed of the next fan group (FG-2, FG-3) to the lower limit and then keep increasing the speed of the next fan group (FG- 2, FG-3) to the speed above or at the upper limit.

12. The fan wall (10) according to any preceding claim further comprising a vibration sensor (13), wherein the controller (12) is further configured to receive sensor input from the vibration sensor (13), the sensor input indicating a vibration level, determine a lower control signal for which the vibration level is above a vibration threshold, determine a fan speed at the lower control signal, set the fan speed at the lower control signal as the lower limit, determine a higher control signal for which the vibration level is above a vibration threshold, determine a fan speed at the higher control signal, and set the fan speed at the higher control signal as the upper limit.

13. The fan wall (10) according to any preceding claim, wherein the controller (12) is further configured to select the at least one fan group and a speed for that at least one fan group so that a resulting fan effect corresponds 5 to a desired fan effect, wherein the desired fan effect (FE) is the fan effect achieved if all fans (11) were run at the same speed, and the resulting fan effect (RFE) is the fan effect achieved when at least some of the fans (11) are run at the different speeds.

14. The fan wall (10) according to claim 12, wherein the controller (12) is further configured to determine that the resulting fan effect (RFE) corresponds to the desired fan effect (FE) when RFE = FE + / - e, where e is an acceptable error.

15. The fan wall (10) according to claim 13 or 14, wherein the controller (12) is further configured to determine that the resulting fan effect corresponds to the desired fan effect when a weighted sum of the effect provided by the fan groups corresponds to the desired effect, wherein the weighted sum is weighted based on number (N) of fans (11) in a fan group (FG).

16. The fan wall (10) according to claim 15, wherein the controller (12) is further configured to determine weighted sum asFGiis the effect that a fan group i is run at, Ni is the number of fans in fan group i, and N is the number of fans in the fan wall (10).

17. The fan wall (10) according to any preceding claim, wherein vibration range is a control signal range which provides a fan speed range, where a lower limit of the control signal range corresponds to a lower limit of the fan speed 5 range and an upper limit of the control signal range corresponds to an upper limit of the fan speed range, a lower limit of the vibration range thus relating to the lower limit of the control signal range and to the lower limit of the fan speed range and an upper limit of the vibration range thus relating to the upper limit of the control signal range and to the upper limit of the fan speed range.

18. A method for fan wall (10) comprising a plurality of fans (11), wherein the plurality of fans (11) is grouped into at least two fan groups (FG), each fan group comprising one or more than one fan (11), and wherein the method comprises receiving a control signal (CS), determining if the control signal (CS) is inside a vibration range, and if so selecting at least one of the fan groups (FG), operating the at least one selected fan group (FG-1) above an upper limit of the vibration range and operating the other fan group(s) (FG-2, FG-3) below a lower limit of the vibration range, whereby the least one selected fan group (FG-1) is operated to compensate for the other fan group(s) (FG-2, FG-3).

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