Selection of a radial fan arrangement among a plurality of centrifugal fan arrangements

Abstract

The disclosure relates to an array of centrifugal fan arrangements (10), said array comprising at least a first and a second centrifugal fan arrangement (10a; 10b; 10c) differing in at least one mode of operation, each of said fan arrangements (10a; 10b; 10c) comprising a plurality of fan blades (15a; 15b; 15c), each having a predetermined height (h1, h2, h3), wherein said height (h1, h2, h3) of the fan blades (15a; 15b; 15c) is different in order to distinguish a first fan arrangement (10a) from at least a second fan arrangement (10b; 10c). Furthermore, each fan arrangement (10a; 10b; 10c) is configured for operating at said mode of operation with a predetermined air flow (Q1; Q2; Q3) which depends on said height (h1, h2, h3).

Description

[0001] TITLE

[0002] SELECTION OF A RADIAL FAN ARRANGEMENT AMONG A PLURALITY OF CENTRIFUGAL FAN ARRANGEMENTS

[0003] TECHNICAL FIELD

[0004] The invention relates to an array of radial fan arrangements, said array comprising at least a first and a second centrifugal fan arrangement differing in at least one mode of operation, each of said fan arrangements comprising a plurality of fan blades, each having a predetermined height, wherein said height of the fan blades is different in order to distinguish a first fan arrangement from at least a second fan arrangement.

[0005] BACKGROUND

[0006] In the field of air treatment systems, it is known to install so-called heating and ventilation air conditioning (HVAC) systems in buildings of different types. An HVAC system generally comprises an air handling unit (AHU) which is configured for intake of fresh supply air into a building and for discharge of air from the building. To this end, the AHU normally comprises an extract air channel and a supply air channel.

[0007] Furthermore, the AHU is often provided with a heat exchanger arrangement which is configured so as to provide an exchange of heat from the exhaust air to the intake air. Such an AHU can consequently be used for optimizing the energy consumption in an HVAC system.

[0008] Systems for demand controlled ventilation (DCV) are also often used in buildings and are configured with a plurality of sensor devices in order to provide relevant information for controlling the ventilation in the building based on demand and on information from the sensor devices.

[0009] Furthermore, an AHU normally comprises at least one fan which is configured for inducing a flow in the air ventilation ducting system. In general, when the AHU includes a supply air channel and an extract air channel, there is a fan provided in both the supply air channel and extract air channel. In this manner, supply air and extract air can be fed through these channels and through the ventilation system. Often, a fan which is used in an air handling unit is constituted by a centrifugal fan in which air is drawn in a direction which is parallel to a fan motor shaft and diffused in a radial direction, generally perpendicular to said motor shaft, i.e. generally perpendicular to the direction of the intake of air.

[0010] A centrifugal fan of a known type normally comprises a front shroud, a back plate and a plurality of fan blades which are located between the front shroud and the back plate. The front shroud, the back plate, and the fan blades can be manufactured as separate units which are assembled to form the complete fan.

[0011] A common type of HVAC system can be configured with one or more centrifugal fans. During a configuration of a ventilation system, for example in a building, it is common that such a system is dimensioned in accordance with a predetermined requirement for an air flow which should be generated by such a centrifugal fan. The magnitude of the required air flow is normally based on a specified need for ventilation of supply air and extract air in to and out of the building in question.

[0012] As an example, the ventilation system can be equipped with a centrifugal fan for supply air which is designed to provide the specified and required air flow. It should be noted that a centrifugal fan of conventional type has a mode of operation in which its efficiency varies with the air flow which is generated by the fan. This means that the efficiency of the fan can be maximized at a very specific air flow value.

[0013] In order to maximize the efficiency of the centrifugal fan - which for example is advantageous for maintaining a low energy consumption of the fan - there is consequently a need to use a centrifugal fan which has a maximum efficiency at the specified air flow value for which the entire ventilation system has been designed. However, the above-mentioned strategy for designing a ventilation system has a disadvantage in the event that the ventilation system in question must be reconfigured and adapted to a different air flow. Such a situation may arise for example if the building in question is renovated or re-structured internally. Also, the air flow may be changed in a system which is configured for demand controlled ventilation. In such cases, the installed centrifugal fan may have to be operated at a different air flow value, which means that the fan will no longer be operated at the specified air flow value which provides the maximum efficiency of the fan. This is obviously a disadvantage as regards the efficiency of the fan.

[0014] In summary, today's ventilation system are configured to be used with one or more centrifugal fans which are chosen to be operated at a specified air flow value. In most cases, however, the ventilation systems of today are operated at conditions in which the efficiency of the centrifugal fans is not maximized. This is a drawback as regards the energy consumption of the ventilation system and the flexibility as regards designing and operating the ventilation system.

[0015] Consequently, with regard to the technical field of ventilation systems involving operation of centrifugal fans, there is a need for further improvements. In particular, there is a need to provide improved efficiency for the centrifugal fans described above.

[0016] SUMMARY

[0017] Consequently, an object of the invention is to provide improved ventilation arrangements which solves the above-mentioned problems and which offers an improved quality and efficiency of an HVAC system.

[0018] The above-mentioned object is achieved by an array of centrifugal fan arrangements, said array comprising at least a first and a second centrifugal fan arrangement differing in at least one mode of operation, each of said fan arrangements comprising a plurality of fan blades, each having a predetermined height, wherein said height of the fan blades is different in order to distinguish a first fan arrangement from at least a second fan arrangement. Furthermore, each fan arrangement is configured for operating at said mode of operation with a predetermined air flow which depends on said height.

[0019] By means of the invention, certain advantages are obtained. Firstly, it should be noted that the solution involving an array of centrifugal fan arrangements according to this disclosure is based on the insight that a maximum coefficient of efficiency (q) can be maintained in each fan arrangement within the array, by choosing a suitable fan within the array. In other words, the maximum coefficient of efficiency (q) can be “changed” or “moved” between different values, as required when the desired air flow in the ventilation system is changed, by replacing one given fan arrangement (optimized for a certain air flow Qi) with another fan arrangement (optimized for another air flow Q2), wherein the two fan arrangement are designed with different fan blade heights.

[0020] In particular, the invention is advantageous in conditions and air handling units in which the difference between two (or more) air flow levels is relatively small, since the maximum efficiency may decrease if the diameter of the fan, and consequently also the air flow, is decreased.

[0021] It should be noted that for air handling systems in which specifications regarding a required air flow is changed, the diameter of the supply air channel and extract air channel (and also the overall size of the air handling system) is normally fixed, i.e. it cannot easily be changed. This means that any replacement fan must also be dimensioned with the same diameter as the original fan. Consequently, new requirements for the air flow in the air handling unit cannot be met by changing the diameter.

[0022] The invention is particularly useful within the field of air treatment systems, in particular heating and ventilation air conditioning (HVAC) systems for buildings of different types. The invention is particularly intended to be used for increasing flexibility by allowing different radial fan arrangements - having mutually different functional characteristics and modes of operation - to be used within an air handling unit (AHU).

[0023] According to an embodiment, said predetermined air flow corresponds to a maximized efficiency level (q) in said mode of operation.

[0024] According to an embodiment, the centrifugal fan arrangement comprises a fan being formed by a front shroud which configured to be arranged concentrically around an end portion of an inlet duct of said fan arrangement, a back plate and a plurality of fan blades which are arranged between said front shroud and said back plate.

[0025] According to an embodiment, each one of said fan blades has a leading edge and a trailing edge.

[0026] According to an embodiment, the diameter of each fan arrangement is fixed at generally the same value.

[0027] The above-mentioned object is also achieved by a method for configuring an air handling unit comprising the following steps: providing an array of centrifugal fan arrangements, said array comprising at least a first and a second centrifugal fan arrangement differing in at least one mode of operation, each of said fan arrangement comprising a plurality of fan blades, each having a predetermined height, selecting said height of the fan blades differently in order to distinguish a first fan arrangement from at least a second fan arrangement. The method comprises the following step: configuring each fan arrangement to operate at said mode of operation with a predetermined air flow which depends on said height. In the context of this disclosure, the term “mode of operation” is used for describing a functional characteristic of a centrifugal fan in which the efficiency of the fan varies with its supplied air flow. The efficiency of the fan can be regarded as the ratio between the power which is used in the form of an air flow and the power which is consumed by the fan. This means that the fan has a mode of operation in which a specified air flow value which corresponds to a maximum fan efficiency value. From this follows that different types of centrifugal fans - i.e. having different dimensions, design, power consumption etc. - may have mutually different modes of operation and different air flow values corresponding to the maximum fan efficiency.

[0028] In the context of this disclosure, the term “array” is used to describe a number of centrifugal fans or centrifugal fan arrangements which can be used within an air handling unit in a ventilation system. In other words, the array of centrifugal fan arrangements defines an “assortment”, “variety” or “selection” from which one specific fan arrangement can be selected to be used in an air handling unit, depending on the requirements regarding the air flow of the air handling unit.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention will now be described more in detail with reference to the accompanying drawings, in which:

[0031] Fig. 1 shows an air handling unit (AHU) of a type which is used in a heating and ventilation air conditioning system;

[0032] Fig. 2 shows a perspective view of a centrifugal fan of a type which can be used in accordance with the present disclosure and which comprises a front shroud, a back plate and a number of fan blades;

[0033] Fig. 3 shows a side view of said centrifugal fan shown in Fig. 2; Fig. 4 is a graph which shows how the coefficient of efficiency of three different centrifugal fans varies depending on the air flow induced by each on the fans; and

[0034] Fig. 5 shows an array of three different radial fan arrangements being configured for different modes of operation, in accordance with the principles of the present disclosure.

[0035] DETAILED DESCRIPTION

[0036] Different embodiments of the present invention will now be described with reference to the accompanying drawings. The arrangements described below and defined in the appended claims can be realized in different forms and should not be construed as being limited to the embodiments described below.

[0037] The invention is particularly intended for increasing the flexibility in a HVAC system by allowing a number of different radial fan arrangements, i.e. an array of centrifugal fan arrangements - having mutually different modes of operation - which are configured to be used within one specific air handling unit.

[0038] Referring to Figure 1 , there is shown an embodiment of an air handling unit (AHU) 1 in which the present invention can be used. In particular, Figure 1 discloses an AHU 1 which is connected to an air ventilation ducting system 2 comprising an extract air channel 3 and a supply air channel 4.

[0039] The extract air channel 3 comprises an extract air inlet 5 connected to the air ventilation ducting system 2 for exhausting air from a building via an extract air outlet 6 for discarding extract air to the environment. The supply air channel 4 comprises a supply air inlet 7 for inlet of fresh air from the external environment and a supply air outlet 8 for guiding fresh air to the air ventilation ducting system to be distributed via the air ducting system 2 to a building. The air handling unit 1 is configured for regulating and ventilating air as part of a heating and ventilation air conditioning (HVAC) system, which is previously known as such.

[0040] The extract air channel 3 and the supply air channel 4 are in a heat exchanging relation via a heat exchanger 9 in order to exchange heat between the extract air and the supply air. The extract air channel 3 is provided with an extract air fan 10 in order to induce a flow of extract air from a building via the air ventilation ducting system 2 and the supply air channel 4 is provided with a supply air fan 11 for inducing a flow of fresh air in the supply air channel 4 in order to distribute supply air to the building via the air ventilation ducting system. The AHU 1 is also connected to an electronic control unit (ECU) 12 for control of the fans 10,11 .

[0041] Figure 1 shows schematically how an AHU 1 comprising a pair of centrifugal fans 10, 11 according to the invention may be designed. The AHU 1 may include further devices such as dampers for controlling the flow, additional air treatment units, e.g. humidifiers, filters or additional heat regulating devices such as a heat pump or electrical heaters, as well as sensors for sensing relevant parameters concerning air quality and temperature of the air.

[0042] According to further embodiments, the invention can be used in other applications (which are not shown in the drawings), for example an air handling unit of the single direction type which only provides flow of air in one direction. For this reason, such an air handling unit comprises a supply air channel but no extract air channel. Fresh air is then guided through an inlet and further via an air outlet and to an air ventilating ducting system in order to distribute fresh air to a building. The air handling unit is furthermore provided with a fan, such as the above-mentioned centrifugal fan, in order to induce a flow of fresh air into the air ventilating ducting system.

[0043] A further example of an air handling unit (not shown in the drawings) is a unit which comprises an extract air channel but no supply air channel. The air handling unit is therefore designed to only provide a flow of extract air from a building. The extract air is guided from an air ventilating ducting system to the air handling unit via an extract air inlet guided through the AHU to a supply air outlet to the environment. The air handling unit is further provided with a fan, such as the above-mentioned centrifugal fan, to induce a flow of extract air in the air ventilating ducting system.

[0044] The above description gives a few examples of different kinds of air handling units which may suitably be used for the present concept, involving centrifugal fans 10, 11 of generally the same type as will be described in the following.

[0045] Figure 2 shows a perspective view of a centrifugal fan 10, i.e. a fan which can be used for example as the above-mentioned extract air fan in the arrangement shown in Figure 1. Sometimes centrifugal fans are referred to as radial flow type fans, or radial fans.

[0046] The fan 10 shown in Figure 2 is in principle based on a concept for centrifugal fan arrangements which is previously known, and which comprises a generally circular front shroud (also known as front disc) 13, a back plate 14 and a multitude of fan blades 15 which are interposed between the front shroud 13 and the back plate 14. Each fan blade 15 is formed with a leading edge 15a and a trailing edge 15b. The leading edge 15a is closer to the center axis of the fan 10 than the trailing edge 15b, so that the leading edge 15a will “meet” the flow of air entering through the hole 16 in the front shroud 13. The trailing edge 15b of each blade 15 is positioned further away from the centre axis of the fan 10, normally close to the vicinity of the periphery of the back plate 14.

[0047] The above-mentioned type of centrifugal fan is often referred to as a backward curved centrifugal fan, or a centrifugal fan with backward curved blades. It should be mentioned that according to further embodiments, the concept according to this disclosure is also applicable to centrifugal fans with forward curved blades. Also, fans which are used in ventilation systems are often also referred to as low- speed centrifugal fans, in order to distinguish them from fans within other technical fields.

[0048] According to an embodiment, the front shroud 13, the back plate 14 and the fan blades 15 are produced as separate units which are assembled together by attaching each one of the fan blades 15 to the front shroud 13 and back plate 14 by any suitable means, e.g. by welding or by screws, or by through going pins. Although not shown in detail in Fig. 2, the fan 10 is suitably provided with attachment means for attaching the fan blades 4 to the front shroud 2 and to the back plate 3. However, the fan 10 could also be moulded as a single piece.

[0049] In the centre of the front shroud 13, there is provided an opening or hole 16 through which incoming air may enter into the fan 10 in an axial direction. Air will enter through the hole 16 when the fan arrangement is operating and the fan 10 is rotating so that an air flow will be directed through the fan 10, in a radial direction along the periphery of the fan 10, in all radial directions. The blades 15 of the fan 10 are designed to produce a radial flow of air and together with the back plate 14 and the front shroud 13, which are functioning as guides, so as to redirect the axial air flow entering in the front shroud 13 to provide an induced radial flow which is discharged at the outer periphery.

[0050] Each fan blade 15 has an upper edge which is in contact with, and faces towards, the front shroud and a lower edge which is in contact with, and faces towards, the back plate. Also, each fan blade has a blade body with a first side and a second side.

[0051] The centrifugal fan 10 shown in Figure 2 is designed with a predetermined outer diameter d, which is chosen to fit into an opening with a corresponding diameter in an air handling unit 1 (see Figure 1), i.e. either in the extract air outlet 6 or in the supply air outlet 8. In the context of the present invention, the diameter d is defined as the outer diameter of the fan blades 15, or more precisely, the diameter of a circle which is formed by the position of the trailing edges 15b during rotation of the fan 10. This diameter d is indicated in Fig. 2. Preferably, the chord length and radius of the chord line are the same for the blades independent of the height of the fan. Also, the profile of the blade is preferably the same.

[0052] When the air handling unit 1 is designed and configured, a required air flow level (Q) is first defined and a suitable fan 10 is chosen and dimensioned for providing the required air flow (Q). A requirement for the fan 10 is obviously also that it has a diameter d which fits into the corresponding opening in the air handling unit 1 .

[0053] Figure 3 shows a side view of the centrifugal fan 10 shown in Figure 2. The diameter d of the fan 10 corresponds to the diameter of both the front shroud 13 and the back plate 14.

[0054] Furthermore, the fan 10 shown in Figure 3 is designed with a number of fan blades 15 (see also Figure 2) which are designed with a height h, i.e. which corresponds to the distance from the back plate 14 to the front shroud 13. The present concept relies on the insight that the value of the height h can be dimensioned in order to obtain a desired air flow for the fan 10.

[0055] A centrifugal fan arrangement is provided by means of the fan 10 shown in Figure 2 and Figure 3, which is arranged together with an inlet duct (not shown in detail in the drawings but normally arranged for example as shown in Figure 1 ) which is positioned upstream of the fan 10 in a HVAC system. The front shroud 13 is arranged generally concentrically around an end portion of such an inlet duct.

[0056] Also, the inlet duct normally has a “trumpet” shape, i.e. it forms a tapering shape with a variable cross-sectional area which is narrowed in a direction towards the front shroud 13.

[0057] Figure 4 shows a diagram corresponding to three different modes of operation of three different centrifugal fans, which are of the same type as described above with reference to Figures 1-3. As mentioned initially, a centrifugal fan is normally designed to provide a specific level of air flow Q which is required in an air handling system. In this regard, the centrifugal fan 10 as shown in Figure 2 and 3 is configured for a mode of operation where the efficiency (i.e. the coefficient of efficiency q) of the fan 10 varies with the air flow which is generated by the fan. Figure 4 shows three different graphs corresponding to three different fans, but suitably having the same diameter d in order to fit in the same air handling unit. Furthermore, each of the fans operates at a mode of operation with a maximum efficiency at three different flow values Qi, Q2, Q3.

[0058] Furthermore, and in order to design a certain fan 10 for a certain air flow Q, the height h of the fan blades 15 for a given fan type is chosen to a suitable value which corresponds to the required magnitude of air flow. This means that the efficiency of a specific fan can be maximized at a specified air flow Q value by choosing a corresponding height h.

[0059] With reference to Figure 4, the three graphs describe the mode of operation for a first fan with a relatively low blade height hi , corresponding to a relatively low air flow level Qi with maximum efficiency, and a second fan with a second blade height h2 (which is higher than the first blade height hi ), corresponding to a second flow level Q2 (which is higher than the first air flow level Qi) with maximum efficiency, and finally a third fan with a third blade height hs (which is higher than the second blade height h2), corresponding to a third air flow level Q3 (which is higher than that the second air flow level Q2) with maximum efficiency.

[0060] In most applications relating to the present disclosure, the maximum efficiency which is obtained by a centrifugal fan is normally in the magnitude of approximately 70-80%. Also, the volume air flow depends on the diameter, which is normally in the magnitude of approximately 0,2-0, 8 meters. In normal cases, the centrifugal fan provides an air flow Q which normally is in the magnitude of 2-6 m3 / s. In order to maximize the efficiency of the centrifugal fan 10 - which for example is advantageous for maintaining a low energy consumption of the fan - the present concept is based on the insight that a centrifugal fan should be used which has a maximum efficiency at the specified air flow value Q for which the air handling unit 1 and the ventilation system is designed.

[0061] In the event that the ventilation system in question is to be re-configured for a different air flow, the currently used centrifugal fan will have to be changed to another centrifugal fan within the array of fans. The new fan consequently needs to be configured with a different blade height h, which corresponds to the new required air flow level Q where efficiency is maximized. In other words, a different fan is then chosen which has a maximum efficiency at the new required air flow level. This new fan consequently has a different blade height than the first fan.

[0062] Figure 5 shows three different centrifugal fans 10a, 10b, 10c, where each fan is of the same type as described above with reference to Figure 2 and Figure 3, but which has a blade height hi , h2, hs which is different for each fan 10a, 10b, 10c and which is associated with a corresponding value of the air flow Qi, Q2, Q3 which leads to maximum efficiency for the fan. The three fans 10a, 10b, 10c consequently define an array which can be used depending on the requirements regarding air flow. Also, according to a preferred embodiment, the three fans 10a, 10b, 10c in the array have the same diameter d.

[0063] In particular, it should be noted that the diameter d of all the fans 10a, 10b, 10c is fixed, i.e. it is adapted to the dimensions of the air handling unit 1 in which one of the fans 10a, 10b, 10c should be mounted. This means that the diameter of the fans cannot be adjusted in order to provide a specific air flow at which the efficiency of the fan is maximized.

[0064] In summary, this disclosure relates to an array of centrifugal fan arrangements 10 in which the array comprises at least a first and a second centrifugal fan arrangement (also referred to as “centrifugal fan”) differing in at least one mode of operation, and wherein each of said fan arrangements comprises a plurality of fan blades 15a; 15b; 15c, each having a predetermined height hi; h2; hs. Also, the height of the fan blades 15a; 15b; 15c is different for different fan arrangements in order to distinguish a first fan arrangement 10a from at least a second fan arrangement 10b; 10c. Furthermore, each fan arrangement 10a; 10b; 10c is configured for operating at its mode of operation with a predetermined air flow Qi; Q2; Q3 which depends on said height (hi , h2, hs).

[0065] The invention is particularly intended for an array of fan arrangements in which the fan arrangements differ in at least one mode of operation in which the efficiency of the fan varies with the air flow (Qi; Q2; Q3) which is suppled by each fan arrangement. The term “mode of operation” is used for describing that when different fans are operated, different relationships between the efficiency of the fan and the air flow (Q) which is generated by a specific fan will be obtained. Due to such different “modes of operation”, different types of fans may present a maximum coefficient of efficiency at different values of the airflow. A consequence of this is that in situations where the requested air flow is changed, a fan which is currently used may not always operate at optimal efficiency (as also mentioned above). This is a problem within the field of air treatment systems which can be solved by means of the array of centrifugal arrangements according to this disclosure, by operating each fan at a mode of operation with an air flow (Q) which depends on the height (h) of the fan blades of each fan.

[0066] Furthermore, in many cases it should understood that the relationship between the coefficient of efficiency q and the air flow Q also depends on the pressure generated by a fan arrangement. It can generally be assumed that all the situations described in Fig. 4 (i.e. corresponding to the three different graphs) occur at generally the same pressure.

[0067] Also, the desired air flow in a ventilation system needs to be controlled by means of a suitable control unit. The array of fan arrangements according to the disclosure is based on a concept where different requirements for ventilation, for example in an office building, may occur over time. In contrast to known technology, the array is designed so as to provide a maximized efficiency level even in the event of such different requirements. According to an embodiment, each of the fan arrangements within the array is controllable by means of an electronic control unit, i.e. based on relevant operational parameters. Alternatively, one single control unit is used for all the fan arrangements.

[0068] According to a further embodiment, all the fan arrangements which together form an array can be provided with the same number of fan blades. This means that all fan arrangements will have generally the same type of fan operation, for example as regards the dependency between the efficiency and the air flow (and also generally the same noise level) and will also be easier to manufacture, compared to the case where different fan arrangements have a different number of fan blades. However, the general purpose of the array is obtained also if the number of fan blades is not the same for all the fan arrangements.

[0069] It should be noted that the number of centrifugal fans within the array is not limited to just three fans with three different heights, as described in the embodiment above. In fact, the concept of the invention can be applied to arrays having two or more centrifugal fans, i.e. consequently with two or more different heights of the fan blades.

[0070] It is a particularly advantageous feature of the array that the predetermined air flow Qi; Q2; Q3 is set to correspond to a maximized efficiency level (q) in said mode of operation.

[0071] Finally, the inventive concept is not limited to the embodiments above but can be varied within the scope of the appended claims

Claims

CLAIMS1 . An array of centrifugal fan arrangements (10), said array comprising at least a first and a second centrifugal fan arrangement (10a; 10b; 10c) differing in at least one mode of operation, each of said fan arrangements (10a; 10b; 10c) comprising a plurality of fan blades (15a; 15b; 15c), each having a predetermined height (hi, h2, hs), wherein said height (hi, h2, hs) of the fan blades (15a; 15b; 15c) is different in order to distinguish a first fan arrangement (10a) from at least a second fan arrangement (10b; 10c); characterized in that each fan arrangement (10a; 10b; 10c) is configured for operating at said mode of operation with a predetermined air flow (Qi ; Q2; Q3) which depends on said height (hi , h2, hs).

2. An array of centrifugal fan arrangements (10) according to claim 1 , wherein said predetermined air flow (Qi; Q2; Q3) corresponds to a maximized efficiency level (q) in said mode of operation.

3. An array of centrifugal fan arrangements (10) according to claim 1 or 2, wherein the centrifugal fan arrangement (10) comprises a fan (10a; 10b; 10c) being formed by a front shroud (13) which configured to be arranged concentrically around an end portion of an inlet duct of said fan arrangement (10), a back plate (14) and a plurality of fan blades (15) which are arranged between said front shroud (13) and said back plate (14).

4. An array of centrifugal fan arrangements (10) according to any one of claims 1- 3, wherein each one of said fan blades (4) has a leading edge (4a) and a trailing edge (4b).

5. An array of centrifugal fan arrangements (10) according to claim 1 or 2 , wherein the diameter (d) of each fan arrangement (10a; 10b; 10c) is fixed at generally the same value.

6. A method for configuring an air handling unit (1 ) comprising the following steps:- providing an array of centrifugal fan arrangements (10), said array comprising at least a first and a second centrifugal fan arrangement (10a; 10b; 10c) differing in at least one mode of operation, each of said fan arrangement (10a; 10b; 10c) comprising a plurality of fan blades (15a; 15b; 15c), each having a predetermined height (hi , h2, hs),- selecting said height (hi, h2, hs) of the fan blades (15a; 15b; 15c) differently in order to distinguish a first fan arrangement (10a) from at least a second fan arrangement (10b; 10c); characterized in that the method comprises the following step:- configuring each fan arrangement (10a; 10b; 10c) to operate at said mode of operation with a predetermined air flow (Qi; Q2; Q3) which depends on said height (hi , h2, hs).

7. A method according to claim 1 , said method further comprising:- selecting said predetermined air flow (Qi; Q2; Q3) so as to correspond to a maximized efficiency level (q) within said mode of operation.

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