IMPELLER AND FAN

The present invention relates to an improved impeller for a fan, in particular to an impeller for a ventilation fan or blower, more particularly for a fan for use in domestic and/or commercial applications, for use mainly in a duct or airway or in a wall of a building. The present invention further relates to a fan comprising such an impeller.

It is known to provide ventilation fans where the air flow through the fan is centrifugal. In such fans, there is high turbulence at the centre caused by the rotation of the blades, leading to high losses and hence inefficiency. This is due to the central vortices effectively reducing the available inlet area into the fan such that there is high air velocity at the outer edges of the inlet in order to accommodate the pressure gradient caused by the fan. In view of the flow profile entering the fan, it is a further disadvantage of existing centrifugal fans that the minimum clearance for a sufficient volume of air to enter the inlet is typically one half of the diameter of the fan.

The high pressure gradients and other operational characteristics of conventional centrifugal fans do not pose a problem for general operation since the inlet opening can be designed to accommodate the required flow. However the efficiency of conventional centrifugal fans is significantly degraded when placed in narrow inlet and discharge airways.

It is a further disadvantage of conventional fans that the inner vortex which can develop during use of an in-line centrifugal fan, affects the pressure characteristics and thus further reduces fan efficiency. It is therefore necessary to provide a straightener or baffle in the duct, in order to prevent the formation of such a vortex. In the case of boxed centrifugal fans a straightener is also required to reduce dump losses.

It has been proposed to provide fans which have a ‘mixed flow’; the flow of air through the fan comprising an axial and a centrifugal component. However, in existing ‘mixed flow’ fans, the axial element of the air flow is very low, of the order of 5% of the total air flow, the remainder of the air flow being centrifugal. The existing mixed flow fans therefore exhibit the same disadvantages as conventional centrifugal fans.

Furthermore conventional mixed flow fans comprise a backward curved centrifugal with a dished back plate such that the impeller imparts a spiral or vortex flow characteristic on the fluid leaving the fan. Thus efficiency is lost due to the generation of an unwanted tangential, rather than radial, flow downstream of the fan.

It is an object of the present invention to provide a fan in which the above disadvantages are reduced or substantially obviated.

It is a further object of the present invention to provide a fan which can be mounted in an airway with a relatively narrow inlet and discharge area, compared to conventional ventilating fans, which fan is relatively shallow in depth, compared to conventional fans.

It is a still further object of the present invention to provide a fan which passes relatively high volumes of air through narrow airways.

The present invention provides an impeller for a fan, which impeller comprises a hub and one or more blades, wherein at least one blade has: a radially inner portion which is aerofoil in section; and, a centrifugal accelerator portion which extends radially outwardly from the aerofoil portion, wherein the aerofoil portion has a greater angle of attack than the accelerator portion.

The angle of attack defines the angle between the front of the blade and the direction of motion of the blade.

The impeller according to the present invention has part aerofoil and part centrifugal fan characteristics, thus drawing air towards the centre of the fan to a greater extent than conventional impellers. This provides a more even flow profile of air entering the fan, avoiding turbulence towards the centre and the resulting high velocity regions towards the perimeter of the fan inlet. The aerofoil portion operates in the manner of an axial impeller, whilst the accelerator section forces the air outward in a radial direction such that the axial and radial elements are both included in a ‘combined’ flow impeller according to the present invention.

In one embodiment, the radially inner, or aerofoil portion is helicoidal in shape. The helicoidal axial section at the root of the impeller imparts a substantially radial force upon the air entering the fan, forcing the air radially outward. Thus the air is forced outward to the long path lateral accelerator portion.

Preferably the curvature of the blade increases towards the axis of rotation or centre of the impeller. Typically the angle of the blade to the axis of rotation of the impeller is greatest at the central portion. Thus the angle of attack of the blade is greatest at the accelerator portion.

In a preferred embodiment the angle of attack of the blade is reduced in the aerofoil portion and typically the angle of attack is smallest at the radially innermost edge of the blade. Thus the orientation of the blade tends towards a outermost edge which is substantially parallel to the axis of rotation of the blade. However the outermost edge does not achieve a straight line but retains a slight curvature.

Preferably the curvature of the centrifugal accelerator portion is substantially constant such that the blade is substantially symmetrical at its outermost edge.

The orientation of the blade towards the outermost edge is particularly advantageous since the centrifugal accelerator portion takes a scoop or paddle-like shape so as to generate substantially radial, as opposed to tangential, flow from the impeller. In this regard the pressure drop over the accelerator portion is greater than the pressure drop over the aerofoil portion of the blade. Thus the aerofoil portion produces little drag, drawing air into the fan centre, whereas the elongated accelerator portion pushes air radially outward.

The long path accelerator portion is particularly advantageous since it obviates air turbulence and imparts energy to the air in order to generate a pressure gradient in a novel manner.

In addition the angle of attack is minimal at the centre, where the speed of the blade is minimal, and increases towards the outermost edge, where the speed is greatest. This promotes an even inlet flow profile over the whole of the impeller inlet.

According to a preferred embodiment, the leading edge of the blade is substantially straight. Preferably the leading edge is substantially tangential to a hub portion of the impeller.

The impeller preferably comprises 5 or 7 blades, each of which has an aerofoil helicoidal central portion which extends laterally into a centrifugal accelerator portion.

The present invention further provides a fan comprising an impeller and a housing in which the impeller is mounted for rotation, which impeller comprises one or more blades, wherein the blade or at least one of the blades has an aerofoil helicoidal central portion which extends laterally into a centrifugal accelerator portion.

In a preferred embodiment of a fan according to the present invention, the impeller is mounted within a volute formed within the housing.

It will be understood that the angle of attack refers to the angle between the blade and the direction of motion of the blade. The angle of attack for a particular blade section may be measured as the angle between the blade centerline and the direction of motion at the leading edge of the blade. For a fan according to the present invention, the direction of motion is generally approximately perpendicular to the axis of rotation.

Preferred embodiments of an impeller and a fan according to the present invention will now be described in further detail with reference to the accompanying drawings, of which:

FIG. 1 is a side view of a conventional centrifugal fan, showing the inlet velocity pattern;

FIG. 2 is a plan view of the fan of FIG. 1, showing the air discharge pattern;

FIG. 3 is an isometric view of an embodiment of an impeller according to the present invention;

FIG. 4 is a plan view from above of the impeller of FIG. 3;

FIG. 5 is a side view of a fan according to FIG. 3;

FIG. 6 is a side view of a generic fan according to the present invention;

FIG. 7 is a plan view of the fan of FIG. 6.

As can be seen from FIGS. 1 and 2, a conventional centrifugal fan shown generally at 10 comprises an impeller 2 having a diameter D¹and housed in a housing 4. The impeller 2 is driven by a motor 6. The inlet diameter D²of the impeller 2 is less than the diameter D¹of the impeller 2. Typical values for the diameters are that D¹=300 mm and D²=240 mm. For these diameters, the maximum air volume is 400 litres/second which equates to an average inlet velocity of 8.8 m/s. As can be seen from FIG. 1, the area of the housing 4 on the inlet side of the impeller includes a central low velocity and low turbulence area 8 surrounded by a high velocity area 12. For the dimensions and inlet velocity specified above, the velocity in the low velocity area 8 is of the order of 6.5 m/s and in the high velocity area 12, the velocity is of the order of 11 m/s.

As can be seen from FIG. 2, the impeller 2 is driven by a motor 6 which is located at the axis of rotation of the impeller 2. A curved internal wall 14 is located within the housing 4 and defines a volute 16 in which the impeller 2 is mounted for rotation. The curved internal wall 14 forms at one end a close throat plate 18.

Arrows A¹and A²show the air discharge pattern schematically. As can be seen from the arrows A¹and A², the air discharge pattern is curved as a result of the swirl which is imparted to the flow by the impeller blades. The discharge has a high inertia, of the order of 15-20 m/s for the fan dimensions described above.

An alternative design of fan having a high aspect ration and combined flow pattern is shown in FIGS. 3 to 7.

As can be seen from FIG. 3, an impeller shown generally at 20 comprises a plurality of blades, 22, 24, 26, 28, 30, 32, 34 spaced equiangularly about a hub portion 36. The hub 36 has a curved circumferential surface 35 which terminates at peripheral rim 37. The direction of rotation of the impeller is indicated by the arrow 38. Each of the blades 22, 24, 26, 28, 30, 32, 34 comprises a leading edge 40 which extends along an aerofoil portion 39 and a lateral air accelerator portion 42.

The outermost edge 41 is substantially perpendicular to the leading edge 40 and trailing edge 40′.

It can be seen that the blade is twisted such that the outermost portion of each blade tends towards an orientation which is substantially parallel to the axis of rotation of the impeller. The angle of curvature of the blade is also reduced along the length of the blade with distance from the axis of rotation. Thus the innermost section of the leading edge defines the aerofoil portion 39 with the air accelerator portion 42 extending radially therefrom.

FIG. 4 shows the impeller 20 of FIG. 3 from above. It can be seen that the depth of each blade in a circumferential direction is greatest adjacent the hub and diminishes towards the outermost edge 41. That is to say that the thickness of the blade reduces with distance from the hub 36 when viewed from above. This tapering of the blade is due to the blade twisting such that a greater surface area of the blade is presented to the airflow towards the hub of the impeller than towards the periphery, at which the blade is angled away from the airflow.

Also in FIG. 4, it can be seen that the leading edge 40 of each blade forms a substantially straight line, extending tangentially from the hub. The trailing edge 40′ of the blade in the accelerator portion is radially aligned with the axis of rotation 45.

Turning now to FIG. 5, it can be seen that the impeller blades extend radially outwardly from the curved outer circumferential surface 35. The depth of the blades in the axial direction is smallest at the innermost point of the blade and increases with distance from the axis of rotation by virtue of the curved shape of hub surface 35. In this regard each blade follows the curvature of the hub.

However, unlike conventional fans, the blades extend outwardly of the outermost rim of the hub portion. The axial depth of the blade in the acceleration portion 42, between the outer rim of the hub and the outermost edge 41 of the blade, is substantially constant, although each blade is preferably slightly tapered towards the axis of rotation.

The combination of the aerofoil inner blade section and the accelerator section extending radially outwardly therefrom has the effect of turning the air through substantially 90° as it passes through the fan.

In FIG. 6, the impeller 20 has diameter D³and is located within a housing 44 having an inlet ring 47. The impeller 20 is driven by a motor 46. The inlet diameter D⁴of the impeller 20 is less than the diameter D³of the impeller 20. Typical values for the diameters are that D³=300 mm and D⁴=280 mm. For these diameters, the maximum inlet volume is 400 litres/second which equates to 6.49 m/s. The inlet velocity is substantially the same across the whole area of D⁴on the inlet side of the impeller.

In the example shown, the height of the impeller H¹is approximately 50 mm, the height of the fan, including inlet ring, H²is approximately 80 mm and the total height of the cavity or ducting in which the fan is located, H³is approximately 150 mm. Thus the fan can operate effectively with a clearance of only 70 mm to provide an airflow of approximately 500 l/s. This is in contrast to a conventional fan which would require a cavity of typically 250 mm height in order to achieve a similar flow rate.

The air is drawn into the centre of the impeller by the aerofoil elements 40 of the impeller blades 22, 24, 26, 28, 30, 32 and 34. This prevents the generation of a flow profile as shown in FIG. 1 and causes air to be drawn substantially evenly over the inlet diameter D². However it will be appreciated that the exact curvature of the blade and the geometry of the aerofoil portion can be altered to achieve the desired flow profile for optimal performance.

The lines L¹and L²are parallel to the axis of rotation of the impeller. The angle made between the blade 24 and the lines L¹and L²are shown at α and β respectively.

The angle α represents the angle made between the leading edge of the aerofoil portion 39 and the axis of rotation, whilst the angle β is between the leading edge of the acceleration portion and the axis of rotation. The angle α may be between 45° and 90°, whilst the angle β is between 0° and 30°.

The angle of attack defines the acute angle between the blade and its direction of movement. The lines L¹and L²are perpendicular to the direction of rotation of the blades. Thus the angle of attack of the blade at the aerofoil portion can be defined as 90°−α and the angle of attack at the accelerator portion can be defined as 90°−β. Therefore the angle of attack at the aerofoil section may be between 0 and 45°, whereas the angle of attack for the accelerator portion may be between 60 and 90°.

Thus the angle of attack varies with distance along the leading edge from the hub. The angle of attack varies constantly along the length of the blade by virtue of the twisting of the blade about its leading edge. Thus the angle of attack will be minimal at the innermost point of the blade and maximal at the outermost edge 41. The angle of attack may vary from 0 to 90° over the length of the blade.

In addition, it can be seen that the skew of the blade varies along its length. The curvature of the blade is greatest in the vicinity of the leading edge within the aerofoil portion and reduces towards the trailing edge. However the curvature of the blade in the acceleration portion is substantially constant between the leading and trailing edges. At the outermost edge, the blade is substantially symmetrical about the mid point of the blade, although the blade may be curved slightly forward as shown in FIG. 4. In contrast the aerofoil section of the blade is highly asymmetrical.

As can be seen from FIG. 7, the impeller 20 is driven by motor 46 which is located at the axis of rotation 45 of the impeller 20. The motor is of external rotor type and is disposed within the hub portion so as to provide a compact design. A curved internal wall 48 is located within the housing 44 and defines a volute 50 in which the impeller 20 is mounted for rotation. It can be seen that throat 49 is retracted when compared with the throat plate 18 of FIG. 1 so as to define an open passageway for air leaving the housing 44.

Arrows B¹and B²show the air discharge pattern schematically. As can be seen from the arrows B¹and B², the air discharge pattern is straight. The discharge has a low inertia, of the order of 11 m/s maximum for the fan dimensions described above.

The principle of operation of the fan of FIGS. 3 to 7 is as follows. Conventional centrifugal fan inlet characteristics produce high velocity patterns at the outer edges and high turbulence at the centre as is shown in FIG. 1. The design of the impeller 20 of FIGS. 3 to 5 has an aerofoil helicoidal central portion running laterally into a centrifugal accelerator portion, so that the air velocity is even across the entire inlet area providing a low inertia air entry. This also enables a larger inlet to impeller diameter ratio than is possible with the conventional impeller of FIGS. 1 and 2, thus substantially reducing inlet losses and noise and allowing a much closer than normal inlet clearance.

The volute design is also modified relative to the volute of a conventional fan to allow, due to the impeller design, lower air discharge velocity than with a conventional fan. The impeller uses a long path accelerator to impart kinetic energy to the air providing increased lateral flow and obviating the need for a close throat plate on the discharge. All of the foregoing enable a very much narrower fan than would usually be expected to achieve the airflows, static pressures and noise levels attained. In particular, the lower velocity of the air leaving the blades with reduced swirl allows a fan of reduced depth according to the present invention to match or exceed the flow rate of a conventional fan since a more even flow into and from the fan can be achieved over wider ducting. This is in spite of the reduced depth of the ducting.

As stated above, the fan shown in FIGS. 3 to 7 has an aerofoil section to bring the air into the centre part of the impeller, effectively creating an axial portion. Since the fan has combined flow, the long path accelerator accelerates the air flow.

The motor for the fan is preferably a four pole motor running at 50 Hz, so that the fan is rotating at approximately 1500 rpm. Alternatively, two pole motors may be preferred for smaller diameter impellers.

It is a further advantage of the fan according to the invention that whereas a conventional fan needs a “straightener” or baffle in the outlet duct to prevent the formation of an inner vortex or, in other arrangements, a dump loss, the fan according to the invention does not need baffles or straighteners.

As with conventional fans, the fan according to the invention must have an odd number of blades, for example, 5 or 7. An even number of blades leads to noise problems due to the blade passage frequencies which would be generated.

IMPELLER AND FAN

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