A CENTRIFUGAL FAN ARRANGEMENT

Abstract

A centrifugal fan arrangement (7) comprises an inlet duct (6) for an incoming flow of air, and a fan (1; 1a; 1b) formed by a front shroud (2) which is arranged concentrically around an end portion (4) of the inlet duct (6). A back plate (3) is provided and a plurality of fan blades (4) are arranged between the front shroud (2) and the back plate (3). Each one of the fan blades (4) has a leading edge (4a) and trailing edge (4b), and the diameter (d1) of the inlet duct (6) is smaller than the inlet diameter (d2) of the front shroud (2), so that a gap (9) is formed between the inside of the front shroud (2) and the outside of the inlet duct (6), the gap (9) extending around the circumference of the inlet duct (6). The centrifugal fan arrangement (7) is configured so that the diameter (d3) of the gap (9), at an exit portion (6a) of the inlet duct (6), is greater than the inlet diameter (d2) of the front shroud (2); and the diameter (d3) of the gap (9), at the exit portion (6a) of the inlet duct (6), is also greater than or equal to a diameter (d4) which is defined by the position of each leading edge (4a) of the fan blades (4).

Description

TECHNICAL FIELD

The invention relates to a centrifugal fan arrangement comprising an inlet duct for an incoming flow of air and a fan being formed by a front shroud, which is arranged concentrically around an end portion of said inlet duct, a back plate and a plurality of fan blades which are arranged between said front shroud and said back plate; wherein each one of said fan blades has a leading edge and trailing edge, and wherein the diameter (d₁) of the inlet duct is smaller than the inlet diameter (d₂) of the front shroud, so that a gap is formed between the inside of said front shroud and the outside of said inlet duct; and wherein said gap extends around the circumference of the inlet duct.

BACKGROUND

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 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. 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.

Also, 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.

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. Often, a fan in an AHU 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.

A centrifugal fan of known type normally comprises a front shroud (also called front disc), 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. The separate units can, for example, be manufactured by moulding a polymer material, but other materials are also possible, such as steel or aluminium.

According to known technology, the front shroud is designed with a central hole where air enters. The fan blades are arranged to provide a radial flow or mixed flow of air from the fan. Also, each fan blade is arranged to have its leading edge closer to the centre axis of the fan than its trailing edge, such that the leading edge will meet the flow of air entering through the hole in the front shroud. The trailing edge of each fan blade is positioned further away from the centre axis of the fan, normally in the vicinity of the periphery of the fan, where the air leaves the fan. Each fan blade 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. Normally, the fan blades are aerodynamically designed and shaped as aerofoils in order to adapt its high pressure side and low pressure side, respectively, to the flow of air along the blades through the fan.

Furthermore, the outer diameter of the inlet duct is slightly smaller than the inner diameter of the opening in the front shroud. This allows the fan with the front shroud to rotate in relation to the stationary inlet duct.

An important criterion for evaluating the comfort level of an indoor environment is its sound quality. In this regard, it should be noted that internal noise from HVAC systems can be noticeable and also difficult to isolate. Furthermore, it is previously known that low-speed centrifugal fans which are installed within a HVAC system can be a dominant noise contributor. More precisely, it is previously known that low speed centrifugal fans may generate unwanted, audible noise. In particular, noise in the form of a tone having a predetermined frequency can be generated by such centrifugal fans. This is obviously a disadvantage.

A solution to the problem of noise which is generated by centrifugal fans is to place silencers in ducts, but the silencers may introduce additional skin friction and decrease the cross-sectional area of the flow in the system. Also, the silencers are mainly effective in absorbing broadband noise rather than the tonal noise which was described above. They can be tuned to dampen the tonal noise at certain frequencies, while the tuning is not valid for a wider range of frequencies.

Furthermore, the patent document US 2021/0270286 teaches an air handling arrangement with a radial offset between an air intake and an inlet shroud. This offset causes turbulence, which in turn causes unwanted noise. In order to solve this, said patent document teaches an arrangement in which the radial offset is eliminated at the interface between the air intake and an inlet shroud. Also, the inlet shroud and the air intake cooperate in a manner so as to define a smooth flow path for the airflow.

Although there exist known solutions which aim at solving the above-mentioned problem with unwanted noise in centrifugal fans, there is still a need for further improvements within this field of technology. In particular, there is a need to provide an improved centrifugal fan arrangement by means of which the tonal noise can be attenuated in a more optimal manner as compared with the prior art.

SUMMARY

Consequently, an object of the invention is to provide an improved centrifugal fan arrangement which solves the above-mentioned problems associated with unwanted noise, in particular tonal noise, and which offers lower noise and an improved sound quality of a HVAC system.

The above-mentioned object is achieved by a centrifugal fan arrangement comprising an inlet duct for an incoming flow of air and a fan being formed by a front shroud, which is arranged concentrically around an end portion of said inlet duct, a back plate and a plurality of fan blades which are arranged between said front shroud and said back plate; wherein each one of said fan blades has a leading edge and trailing edge, and wherein the diameter (d₁) of the inlet duct is smaller than the inlet diameter (d₂) of the front shroud, so that a gap is formed between the inside of said front shroud and the outside of said inlet duct; and wherein said gap extends around the circumference of the inlet duct. Furthermore, the centrifugal fan arrangement is configured so that the diameter (d₃) of the gap, at an exit portion of the inlet duct, is greater than the inlet diameter (d₂) of the front shroud; and that the diameter (d₃) of the gap, at the exit portion of the inlet duct, is also greater than or equal to a diameter (d₄) which is defined by the position of each leading edge of said fan blades.

In a centrifugal fan which is suitable for an HVAC system, there is normally a gap (also termed clearance) between the rotating fan shroud and the stationary inlet duct. The pressure difference between the fan's inner and outer sides drives air to pass through the gap.

The present disclosure is based on the insight that a predominant source of the unwanted noise in prior art fan arrangements is turbulence stemming from the gap between the front shroud and the inlet duct. This is not known from prior art arrangements. In fact, it has been discovered that the turbulence evolves along with the front shroud and is swept downstream so as to interact with the top side of the leading edges of the fan blades. The interaction accounts for uneven surface pressure distribution on the blades. Moreover, the pressure is significantly unsteady near the shroud.

It has also been discovered that the primary source of the unwanted tonal noise in prior art arrangements, which is a tone having a particular frequency, is the interaction between the gap turbulence and the blades. In fact, the frequency (Hz) is equal to the so-called blade passing frequency (BPF) which is calculated with the following formula:

$BPF=\frac{n·z}{60}$

• • where n is the rotation velocity (rpm) of the fan and z is the number of blades. The gap turbulence exists near the shroud wall upstream of the blades.

The arrangement according to this disclosure focuses on the noise which is generated by the gap turbulent flow. By means of the invention, certain advantages are obtained. Firstly, it should be noted that the unwanted tonal noise caused by turbulence can be greatly reduced by designing the fan arrangement so as to eliminate the gap turbulence.

According to an embodiment, the centrifugal fan arrangement is arranged so that the diameter d₃ of the gap, at the exit portion of the inlet duct, is greater than the diameter d₄ which is defined by the position of each leading edge of said fan blades. In this manner, the turbulent flow of air which exits the gap at a downstream position will not be guided into the leading edges of the fan blades. Instead, a turbulent flow of air will be formed in a position which is on the rear side of (i.e. after) the position of each of the leading edges. In other words, a turbulent flow of air will be formed at the exit of the gap but the actual position where it will be formed will not be in front of said leading edges but rather after the leading edges.

According to an embodiment, the centrifugal fan arrangement is arranged so that the diameter d₃ of the gap, at the exit portion of the inlet duct, is equal to the diameter d₄ which is defined by the position of each leading edge of said fan blades. By using such an arrangement, it can also be avoided that the turbulent flow of air which exits the gap is guided into the leading edges of the fan blades, as discussed above.

According to an embodiment, the centrifugal fan arrangement is arranged so that the diameter d₃ of the gap, at the exit portion of the inlet duct, is greater than or equal to a diameter d₄ which is defined by the position of each leading edge of said fan blades, and that a diameter d₃ of the gap, at the exit portion of the inlet duct, is less than a diameter de which is defined by the position of each trailing edge of said fan blades.

According to an embodiment, the centrifugal fan arrangement is arranged so that the diameter d₃ of the gap, at the exit portion of the inlet duct, is greater than or equal to a diameter d₄ which is defined by the position of each leading edge of said fan blades, and that said diameter d₃ is less than a value defined by (d₆−d₄)/2, where de is a diameter which is defined by the position of each trailing edge of said fan blades.

According to an embodiment, the centrifugal fan arrangement is arranged so that the diameter (d₃) of the gap, at the exit portion of the inlet duct, is greater than a diameter (d₄) which is defined by the position of each leading edge of said fan blades, and that said diameter (d₃) is less than a value defined by: (d₆−d₄)/2, where de is a diameter which is defined by the position of each trailing edge of said fan blades.

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.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1a shows an air handling unit (AHU) of a first type;

FIG. 1b shows an air handling unit (AHU) of a second type;

FIG. 1c shows an air handling unit (AHU) of a third type;

FIG. 2 shows a perspective view of a centrifugal fan in accordance with the present disclosure and comprising a front shroud, a back plate and fan blades;

FIG. 3 shows a side view of a centrifugal fan arrangement comprising the centrifugal fan shown in FIG. 2;

FIG. 4A shows an enlarged view of a particular section of the fan arrangement shown in FIG. 3;

FIG. 4B is a schematical top view of certain dimensions which are shown in the fan arrangement of FIGS. 3 and 4A; and

FIG. 5 shows a perspective view of a part of the fan arrangement which is shown in FIGS. 2, 3, 4A and 4B.

DETAILED DESCRIPTION

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.

The invention is particularly intended to be used for reducing unwanted noise which is generated within air handling units. With initial reference to FIGS. 1a-1c, different embodiments of air handling units (AHU's) 100 in which the present invention can be used are shown. FIG. 1a discloses an AHU 100 which is connected to an air ventilation ducting system 101 comprising an extract air channel 102 and a supply air channel 103. The extract air channel 102 comprises an extract air inlet 102a connected to the air ventilation ducting system 101 for exhausting air from a building via an extract air outlet 102b for discarding extract air to the environment. The supply air channel 103 comprises a supply air inlet 103a for inlet of fresh air from the environment and a supply air outlet 103b for guiding fresh air to the air ventilation ducting system to be distributed via the air ducting system 101 to a building.

The extract air channel 102 and the supply air channel 103 are in a heat exchanging relation via a heat exchanger 104 in order to exchange heat between the extract air and the supply air. The extract air channel 102 is provided with an extract air fan 1a in order to induce a flow of extract air from a building via the air ventilation ducting system 101 and the supply air channel 103 is provided with a supply air fan 1b for inducing a flow of fresh air in the supply air channel 103 in order to distribute supply air to the building via the air ventilation ducting system. The AHU 100 is also connected to an electronic control unit (ECU) 105 for control of the fans 1a, 1b.

The design of the AHU 100 in FIG. 1a discloses schematically how an AHU comprising a pair of fans 1a, 1b according to the invention may be designed. The AHU 100 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.

FIG. 1b discloses another example of an AHU 100a in which the present invention can be used and which is of the single direction (SD) type. An AHU of the SD type only provides flow of air in one direction and for this reason, the AHU 100a shown in FIG. 1b comprises a supply air channel 103 but no extract air channel. The AHU 100a is furthermore designed to only provide a flow of fresh air entering through a supply air inlet 103a to the AHU 100a and to be further guided via a supply air outlet 103b to an air ventilating ducting system 101 in order to distribute fresh air to a building. The AHU is further provided with a fan 1 in order to induce a flow of fresh air in the air ventilating ducting system. The AHU 100a is also provided with a filter 104a in order to clean the supply air and a temperature conditioning unit 104b in order to adjust the temperature of the supply air stream.

FIG. 1c discloses still another example of an AHU 100b in which the present invention suitably can be used. This AHU 100b is of the SD kind and comprises an extract air channel 102 but no supply air channel. The AHU 100b is thus designed to only provide a flow of extract air from a building. The extract air is guided from the air ventilating ducting system 101 to the AHU 100b via an extract air inlet 102a guided through the AHU 100 to a supply air outlet 102b to the environment. The AHU 100b is further provided with a fan 1 in order to induce a flow of extract air in the air ventilating ducting system 101.

The AHU 100b in FIG. 1c does not include any air treatment units but could of course be provided with additional devices, e.g. some kind of heat recovery arrangement in order to regain heat from the air exhausted. For example, in case there is a single direction (SD) AHU present in a building for guiding extract air out of a building to the environment, as disclosed in FIG. 1c, there is also often present a supply air single direction AHU as disclosed in FIG. 1b. Hence, the AHU 100a in FIG. 1b and the AHU 100b in FIG. 1c may both comprise heat pumps connected between them such that heat is transferred between the supply air stream of the AHU 100a in FIG. 1b and the extract air stream of the AHU 100b in FIG. 1c.

Hence, the above description, with reference to FIGS. 1a-1c, serves as a few examples of a multitude of different kinds of AHUs which may suitably be used for a fan 1, 1a, 1b of generally the same type as will be described below.

FIG. 2 shows a perspective view of a centrifugal fan 1, i.e. a fan which provides a radial flow as described above. The fan 1 is in principle based on a concept for centrifugal fan arrangements which is previously known, but has been modified and improved as compared with prior art solutions, as will be described in greater detail in the following. The fan 1 as shown in FIG. 2 comprises a generally circular front shroud (also known as front disc) 2, a back plate 3 and a multitude of fan blades 4 which are interposed between the front shroud 2 and the back plate 3. Each fan blade 4 is formed with a leading edge 4a and a trailing edge 4b. According to an embodiment, the front shroud 2, the back plate 3 and the fan blades 4 are produced as separate units which are assembled together by attaching each one of the fan blades 4 to the front shroud 2 and back plate 3 by any suitable means, e.g. by welding or by through going pins. However, the fan 1 could also be moulded as a single piece.

In the centre of the front shroud 2, there is provided an opening or hole 5 through which air may enter into the fan 1. Although not shown in detail in FIG. 2, the fan 1 is suitably provided with attachment means for attaching the fan blades 4 to the front shroud 2 and to the back plate 3.

Furthermore, FIG. 3 teaches a centrifugal fan arrangement 7 in which the fan 1 shown in FIG. 2 is arranged together with an inlet duct 6 which is positioned upstream of the fan 1 in a HVAC system (which is not shown in further detail but which can be designed for example as shown in FIGS. 1a-c). The front shroud 2 is arranged generally concentrically around an end portion 6a of the inlet duct 6. Also, the inlet duct 6 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 2. More precisely, the cross-sectional area of the inlet duct 6 is narrowed to a minimum value area near the inlet of the fan 1, i.e. as it reaches the front shroud 2, and downstream of that position, the cross-sectional area of the inlet duct 6 increases slightly.

Furthermore, the outer diameter of the inlet duct 6 is slightly smaller than the fan inlet diameter—i.e. as regarded along the inside of the inlet portion of the front shroud 2—as indicated by means of reference numeral 2a in FIG. 3. In this manner, a gap 8 (i.e. a clearance) is formed between the inlet duct 6 (i.e. which is stationary) and the front shroud 2 (which is rotating). This gap 8 will be described in more detail below. The fan 1 and the inlet duct 6 are suitably placed within a downstream duct of the HVAC system and the inlet duct 6 is connected to an upstream duct.

As described above, air will enter through the hole 5 when the fan arrangement is operating and the fan 1 is rotating so that an air flow through the fan 1 will be directed in radial direction along the periphery of the fan 1 in all radial directions. The blades 4 of the fan 1 are thus designed to produce a radial flow of air and together with the back plate 3 and front shroud 2, which are functioning as guides, redirect the axial air flow entering in the front disc 2 to provide an induced radial flow when the fan 1 is operating.

Also, each fan blade 4 is arranged to have its leading edge 4a closer to the centre axis of the fan 1 than its trailing edge 4b, such that the leading edge 4a will meet the flow of air entering through the hole 5 in the front shroud 2. The trailing edge 4b of each fan blade 4 is positioned further away from the centre axis of the fan, normally in the vicinity of the periphery of the fan, where the air leaves the fan.

As mentioned above, the fan arrangement according to the disclosure is configured so as to decrease, or at least eliminate, the noise which is generated in previously known fan arrangements. This disclosure is in particular based on the insight that such noise is generated (in previously known arrangements) by a turbulent flow of air in a gap between an inlet duct and a front shroud. For this reason, the fan arrangement according to the embodiment in FIGS. 2 and 3 has been improved as compared with prior art arrangement. This will now be described in detail.

In summary, FIGS. 2 and 3 teach a centrifugal fan arrangement 7 with the inlet duct 6, the front shroud 2 and the fan 1. The fan 1 is formed by the front shroud 2, the back plate 3 and the fan blades 4 which are arranged between said front shroud 2 and the back plate 3, wherein each one of said fan blades 4 has a leading edge 4a and trailing edge 4b. Also, with reference to FIG. 4A, which shows an enlarged section from FIG. 3 (as indicated by means of the box 9 with broken lines in FIG. 3), the inlet duct 6 has an outer diameter d₁ which varies along the curvature of the inlet duct 6, and the front shroud 2 has an inner diameter d₂ which varies along the curvature of the front shroud 2. At any given position along the front shroud 2 and inlet duct 6, as shown in FIG. 4A, the diameter d₁ of the inlet duct 6 is smaller than the inlet diameter d₂ of the front shroud 2, so that the gap 8 is formed between the inside of the front shroud 2 and the outside of the inlet duct 6. Also, this gap 8 extends around the circumference of the inlet duct 6.

In contrast to prior art fan arrangements, which may cause unwanted tonal noise as described above, the present invention relies on a specific geometrical arrangement of the fan 1 as shown in FIGS. 2 and 3. More precisely, the centrifugal fan arrangement 7 is configured so that a diameter d₃ of the gap 8, as regarded at an exit portion 6a of the inlet duct 6, is greater than the above-mentioned inlet diameter d₂ of the front shroud 2. Also, said diameter d₃ of the gap 8 as regarded at the exit portion 6a of the inlet duct 6, is greater than or equal to a further diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4. It should be noted that the term “exit portion” is used to define a position at which a flow of air which is guided along the gap 8 (from the uppermost part of the front shroud 2 and in a direction which is indicated by means of an arrow in FIG. 4A) actually leaves said gap 8. Such a position is generally located at the lower end of the inlet duct 6, as also shown in FIG. 4A.

By means of the fan arrangement 7 which is configured with the geometry as described above, the leading edge 4a of each fan blade 4 will not cross any turbulent flow of air located downstream of the exit portion 6a of the inlet duct 6. The result of this arrangement is that virtually no tonal noise will be generated in the manner as described above with reference to previously known fan arrangements.

There will be a certain amount of turbulence in the fan arrangement 7 according to the invention, but it will be generated at a position which is beyond the position of the leading edge 4a of the fan blade 4, more precisely just outside the exit portion 6a. The fact that the gap 8 follows the curve shape of the front shroud 2 and also extends between the inlet duct 6 and front shroud 2 all the way beyond the leading edge 4a (i.e. so that the exit portion 6a is positioned outside the position of the leading edge 4a), means that the leading edge 4a will not hit the turbulent flow. This means that the tonal noise can be virtually eliminated.

According to a particular embodiment, the fan arrangement 7 is arranged so that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is greater than the diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4.

According to a further embodiment, the fan arrangement 7 is arranged so that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is equal to the diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4.

According to a further embodiment, the fan arrangement 7 is arranged so that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is greater than or equal to a diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4, and also that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is less than a diameter de which is defined by the position of each trailing edge 4b of said fan blades 4.

According to a further embodiment, the fan arrangement 7 is arranged so that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is greater than or equal to a diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4, and that said diameter d₃ is less than a value defined by (d₆−d₄)/2, where de is a diameter which is defined by the position of each trailing edge 4b of said fan blades 4.

According to a further embodiment, the fan arrangement 7 is arranged so that the diameter d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is greater than a diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4, and that said diameter d₃ is less than a value defined by (d₆−d₄)/2, where do is a diameter which is defined by the position of each trailing edge 4b of said fan blades 4.

FIG. 4B is a schematical top view generally corresponding to the embodiment in FIG. 4A and indicating the three above-mentioned diameters d₃, d₄ and de. It should be noted that all the above-mentioned embodiments involving these diameters are possible within the scope of this enclosure, i.e. not only the particular embodiment shown in FIG. 4B in which da is greater than d₄, and de is greater than both d₃ and d₄.

FIG. 5 shows a further perspective view of a further part of the fan arrangement 7 according to FIG. 3, in particular as indicated by means of the box 10 with broken lines in FIG. 3). FIG. 5 shows in particular how the leading edge 4a of the fan blade 4 is positioned in relation to the gap 8.

As an additional effect of the above described arrangement, the coefficient of efficiency of the fan arrangement 7 will be improved. This is due to the fact that turbulence in the flow of air in the fan is generated at a position which is further downstream, as compared with known arrangements. This means that the separation at the trailing edge 4b (which otherwise would affect the efficiency in a negative way) can be delayed.

Furthermore, it should be noted that the width of the gap 8 (i.e. d₂−d₁) can be varied, as long as the d₃ of the gap 8, at the exit portion 6a of the inlet duct 6, is greater than the diameter d₄ which is defined by the position of each leading edge 4a of said fan blades 4. Also, the coefficient of efficiency of the fan arrangement will generally be increased as the gap 8 is decreased.

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

Claims

1-7. (canceled)

8. A centrifugal fan arrangement comprising:

an inlet duct for an incoming flow of air; and

a fan formed by a front shroud, which is arranged concentrically around an end portion of said inlet duct, a back plate and a plurality of fan blades arranged between said front shroud and said back plate;

wherein each of said fan blades has a leading edge and trailing edge, and

wherein a diameter (d1) of the inlet duct is less than an inlet diameter (d2) of the front shroud, so that a gap is formed between an inside surface of said front shroud and an outside surface of said inlet duct; and wherein said gap extends circumferentially around the inlet duct;

wherein a diameter (d3) of the gap, at an exit portion of the inlet duct, is greater than the inlet diameter (d2) of the front shroud; and

wherein the diameter (d3) of the gap at the exit portion of the inlet duct, is also greater than or equal to a diameter (d4) defined by a position of each leading edge of said fan blades.

9. The centrifugal fan arrangement according to claim 8, wherein:

the diameter (d3) of the gap at the exit portion of the inlet duct is greater than the diameter (d4) defined by the position of each leading edge of said fan blades.

10. The centrifugal fan arrangement according to claim 8, wherein:

the diameter (d3) of the gap at the exit portion of the inlet duct, is equal to the diameter (d4) defined by the position of each leading edge of said fan blades.

11. The centrifugal fan arrangement according to claim 8, wherein:

the diameter (d3) of the gap at the exit portion of the inlet duct, is greater than or equal to the diameter (d4) defined by the position of each leading edge of said fan blades, and wherein the diameter (d3) of the gap at the exit portion of the inlet duct, is less than a diameter (d6) defined by a position of each trailing edge of said fan blades.

12. The centrifugal fan arrangement according to claim 8, wherein: ( d 6 - d 4 ) / 2

the diameter (d3) of the gap at the exit portion of the inlet duct, is greater than or equal to the diameter (d4) defined by the position of each leading edge of said fan blades, and

wherein said diameter (d3) is less than a value defined by:

where d6 is a diameter which is defined by a position of each trailing edge of said fan blades.

13. The centrifugal fan arrangement according to claim 8, wherein: ( d 6 - d 4 ) / 2

the diameter (d3) of the gap at the exit portion of the inlet duct, is greater than the diameter (d4) defined by the position of each leading edge of said fan blades, and

wherein said diameter (d3) is less than a value defined by:

where d6 is a diameter which is defined by a position of each trailing edge of said fan blades.

14. An air handling unit having an extract air channel comprising at least one extract air inlet for guiding extract air from a building to outside the building through at least one of an extract air outlet and a supply air channel comprising at least one supply air inlet for guiding supply air from outdoor the building into the building through at least one supply air outlet adapted to be connected to an air ventilation ducting system;

wherein the air handling unit comprises the centrifugal fan arrangement according to claim 8, for inducing an air flow through at least one of said extract air channel and said supply air channel.