GB2591485A

GB2591485A - Fan system for a road sweeping vehicle

Info

Publication number: GB2591485A
Application number: GB2001267.0A
Authority: GB
Inventors: James Knopp Gareth; Sammartano Vincenzo; Frederick Offley Clive
Original assignee: Bucher Municipal Ltd
Current assignee: Bucher Municipal Ltd
Priority date: 2020-01-30
Filing date: 2020-01-30
Publication date: 2021-08-04
Anticipated expiration: 2040-01-30
Also published as: GB202001267D0; GB2591485B

Abstract

A fan system suitable for a road cleaning machine comprising an impeller mounted in a fan case comprising a hub, a plurality of blades, an impeller outlet, a hub attachment means 36 for attaching the hub to a drive shaft, the hub attachment means being located within a hub cover 30. The hub cover having the form of a cone comprising a hollow body 37, an upper frustoconical section and a rounded cone tip 38 (e.g. torispherical, dished, hemispherical, semi-ellipsoidal), wherein shape and the height of the hub cover maintains laminar air flow entering the impeller and gradually accelerate the air flow and turn it into the impeller blades through the impeller outlet. A crown radius of the cone tip and the angle of slant of the frustoconical upper section of the cone body is selected to provide a smooth transition where the cone body and cone tip join. The body and cone tip may be formed integrally or separately. The cone tip apex may comprise a recess. The walls of the cone body may be convex or concave. The impeller may comprise backward curved blades. Road cleaning machine comprising the fan system, chassis, hopper and suction conduit.

Description

FAN SYSTEM FOR A ROAD SWEEPING VEHICLE

The present disclosure relates to a fan system suitable for use in, but not limited to, road cleaning vehicles of the suction-type.

Road cleaning vehicles (also known as sweepers) are commonly used to remove unwanted debris from streets. Many suction-type road cleaning vehicles utilise a centrifugal exhauster fan for their sweeping action. The fan is the means used for generating the air flow in a suction or vacuum-type machine. The fan generates a negative pressure within an airtight container mounted on the vehicle chassis. Debris from the road is initially collected by rotating brushes which are positioned adjacent the mouth of one or more suction conduits connected to the container. The negative pressure created by the fan in the container causes a high velocity air flow in the conduit, which induces the debris into the conduit mouth. The debris is sucked through suction conduit and deposited in the container. Once in the container, the debris is separated from the air by means of a separation system, and the air flow is exhausted by the fan to the atmosphere.

One such centrifugal fan is disclosed in GB-A-2225814. The centrifugal fan comprises an impeller having circular front and back plates and a plurality of blades there between. The blades are each joined at one end to a generally cylindrical hub. Means are provided, commonly in the form of a motor, for rotating the hub and thereby the impeller. The impeller is housed in a casing having a volute portion and an air outlet. The sides of each blade are welded to the back plate and front plate of the impeller. The front plate comprises an air inlet to allow air to enter the impeller.

However, there is always a desire to improve the efficiency of the fan system and to reduce the noise associated with the fan system.

The invention therefore provides a hub cover for covering the hub of an impeller in a fan system, said hub cover having the form of a rounded cone comprising a hollow body, at least an upper section of which is frustoconical, and a rounded cone tip attached to the upper section of the body.

Preferably the body and cone tip are formed integrally. Alternatively the body and cone tip are formed separately and the cone tip is removably attached to the body. -2 -

The cone tip preferably comprises a recess in its apex and an aperture located at the end of the recess.

Preferably the whole of the cone body is frustoconical.

The side walls of the cone body are preferably convex or they may be concave.

Preferably the cone tip is torispherical, dished, hemispherical or semi-ellipsoidal.

The diameter of a lower edge of the cone tip is preferably the same as the radius of a top surface of the cone body, and a crown radius of the cone tip and an angle of slant of the frustoconical upper section of the cone body are selected to provide a smooth transition where the cone body and cone tip are attached.

The invention further provides a fan system for a road cleaning machine comprising a rotatable impeller mounted in a fan case, said impeller comprising a hub; and a plurality of blades attached to the hub; hub attachment means for attaching the hub to a drive shaft for rotatably driving the impeller; and a hub cover as described above removably attached over the hub; wherein the means for attaching the hub to a drive shaft are located within the hub cover.

Preferably the impeller is a centrifugal impeller having a plurality of backwards curved blades.

The cone tip preferably comprises a recess in its apex and an aperture located at the end of the recess for receiving the hub attachment means such that one end of the hub attachment means is located with the recess.

The invention further provides a road cleaning machine comprising a chassis, a hopper mounted on the chassis, at least one suction conduit connected to the hopper, the fan system as described above for generating negative pressure within the hopper, and means for driving the impeller, wherein the hub is attached to said drive means. -3 -

By way of example only, embodiments of a road cleaning vehicle are now described with reference to, and as show in, the accompanying drawings, in which: Figure 1 is a side elevation of a road cleaning machine incorporating the fan system

of the present disclosure;

Figure 2 is a perspective side elevation of the road cleaning machine of Figure 1, showing the air flow through the machine; Figure 3 is a front elevation of the fan system of the present disclosure; Figure 4 is a cross sectional view of the inlet ducting and fan case of the fan system of Figure 3; Figure 5 is an end elevation of the fan system of Figure 3, showing the fan case in section Figure 6 is a cross sectional elevation of the fan case of the fan system of Figure 3, showing an alternative dome cone; Figure 7a is a perspective view of the outlet grill in the inlet ducting shown in Figure Figure 7b is a perspective view of alternative outlet grill to that of Figure 7a; Figure 8 is a plan view of the fan case and a first section of the inlet ducting of the fan system of Figure 3; Figure 9 is a perspective elevation of the fan system of Figure 3; Figures 10 and 11 are different perspective views of a section of the inlet ducting of the fan system of Figure 3; Figure 12 is a perspective view of the fan case and outlet ducting of the fan system of Figure 3; Figures 13 and 14 are different perspective views of the outlet ducting of the fan system of Figure 3.

Referring to Figs. 1 and 2, a typical road cleaning machine 10 comprises a vehicle chassis 11 on which is mounted an engine (not shown) and a hopper 12, in the form of an airtight container. The engine is preferably a high performance electric motor, for example a 60kW electric motor powered by a suitable battery pack. Alternatively, the engine may be a diesel or hydraulic motor. Attached to the hopper 12 are a pair of suction conduits 13 (only one of which is shown), each having an inlet nozzle 14 at the free end thereof. Located adjacent the inlet nozzle 14 is a rotatable sweeping brush 15, (only one of which is shown) for positioning debris at the mouth of the nozzle 14. A long sweep brush (not shown) may be -4 -located under the road cleaning machine 10 which is able to direct debris to the right or left.

Positioned within the hopper 12 is a mesh 18, which separates debris entrained in the air passing through the hopper 12, a fan system 16 for creating negative pressure within the hopper 12, and consequently creating the air flow, and an air discharge outlet 17. The direction of the air flow (shown in Figure 2) is such that the air (and debris) is sucked up through the suction conduits 13 into the hopper 12 and passes through the mesh 18. The air passes through the fan system 16 before being discharged to atmosphere via the air discharge outlet 17 in a wall of the hopper 12.

The hopper 12 may be mounted on the chassis 11 by pivot means which allows it to tip about a pivot point from a working position, for the collection of debris, to an emptying position, to empty any debris collected therein.

The fan system 16 is comprises a fan case 21, inlet ducting 22 and outlet ducting 23 (see Figures 3 and 4). Located within the fan case 21 is a centrifugal impeller 25. The impeller 25 has circular front and back plates 27, 28 and a plurality of blades 26 therebetween. Suitable impellers 25 may be 760mm or 800mm in diameter. However, the diameter will vary according to required suction power of the fan system 16. The blades 26 are each joined at one end to a generally cylindrical hub. The sides of each blade 26 are welded to the front and back plates 27, 28. The front plate 27 comprises a fan eye 29 to allow air to enter the impeller 25.

The fan system 16 is driven by the engine via suitable means, such as a mechanical direct drive, hydraulic or electric motor transmission system. The engine may be coupled to a hydraulic pump, which in turn drives a hydraulic motor which is coupled to the impeller 25. The hydraulic pump may be of the variable displacement type and the motor may be of the fixed displacement type. Alternatively, a direct mechanical drive through a fluid coupling and gear box or an electric drive coupling may be used. The impeller hub is attached to a drive shaft, such as a fan axle of the transmission system which, during operation, cause the hub, and thereby the impeller 25, to rotate.

The front of the fan case 21 has an inlet opening located over the fan eye 29. The inlet ducting 22 is attached at a proximal end 42 to the inlet opening in the front of the fan case 21, so as to direct the air flow into the impeller 25 through the fan eye 29. The attachment -5 -may be made by any suitable means, such as a bolts. A distal end 45 of the inlet ducting 22 is attached to the hopper 12. As the hopper 12 is rotatable relative to the chassis 11, to which the fan case 21 is mounted, the inlet ducting 22 includes a "make or break" seal 46 along its length. When the hopper 12 is tipped to an emptying position, the seal 46 breaks and when the hopper 12 is tipped back to the working position, the seal 46 is remade.

Located in a side of the fan case 21 is an outlet opening 32, to which a proximal end 50 of the outlet ducting 23 is attached, to enable air to flow out of the impeller 25 into the outlet ducting 23. A distal end 51 of the outlet ducting 23 is attached to the hopper 12. A further "make or break" seal 52 is therefore used at the point of attachment of the first end 50 of the outlet ducting 23 and the outlet opening 32 of the fan case 21. When the hopper 12 is tipped to an emptying position, the seal 52 breaks and when the hopper 12 is tipped back to the working position, the seal 52 is remade. An outlet grill 33 is attached across the outlet opening 31 which prevents debris falling into the impeller 25, especially when the hopper 12 is in a tipped state, and to assist in noise attenuation. The outlet grill 33 is attached by suitable means, such as bolts.

There are two major problems with fan systems 16 of this type, namely efficiency and noise. Apart from motor and bearing noise, most of the acoustic power of a fan system 16 comes from the eddy swirls following the trailing edge of the blade 26 after it passes by the fan throat 35. There is also an outward pulse of air as the leading edge of each blade 26 pushes forward cutting the air.

As the impeller 25 rotates, the tips of the blades 26 interact with the throat 35 of the outlet ducting 23 causing a pulsation or clipping effect. The frequency at which this effect occurs can be calculated based on the formula: N t

BPF

Where: I3PF= Blade Pass Frequency (Hz) N = rotation velocity (rpm) t = number of blades in impeller The trailing eddies produce a broad spectrum of random noise, modulated by the fan blade frequency. The outward pulses occur at the blade frequency, with harmonics. Both are stronger near the tip of the blade 26, because the tip is moving faster. Faster splitting of the -6 -air means a sharper leading edge of the pressure pulse, so the higher frequency noise is especially concentrated near the tips of the blades 26.

The spectrum of fan noise consists of pure tones and a continuous spectrum. The frequencies of the pure tones are the fan blade passing frequency and its harmonics. N t n Fn =

Where: Fn = Frequency Harmonic N= rotation velocity (rpm) n = Harmonic Number (1,2,3,...,n) t = number of blades in impeller The sound power produced by centrifugal fans can be approximated by an equation: Lw = Kw + 10 logic Q + 20P + BFI + CN where: Lw= sound power level (dB) Kw = specific sound power level depending on the type of fan Q = volume flow rate (cfm) P= total pressure (inches of H20) BFI = Blade Frequency Increment = correction for pure tone produced by the BPF whose centre frequency is closest to the blade passing frequency.

BPF= blade passing frequency CN = efficiency correction (because fans that are operated off their optimum flow conditions get noisier) CN = 10 + 10 10g10 (1-n)/ This however can cause other resonant installation effects. Once a fan is installed, even though the fan is well designed acoustically, an unexpected noise problem can arise.

This has been described as installation effects, and two types are applicable to this type of fan system 16. A structure that affects the inlet flow of a fan causes installation effects. For -7 -example, in their publication "Vergleich verschiedener Gerauschmessnerfahren fur Ventilatoren. Forschungsbericht FLT 3/1/31/87, Forschungsvereinigung fur Luft-und Trocknungstechnik e. V., Frankfurt/Main, Germany", Hoppe & Neise showed that with and without a bell mouth nozzle at the inlet flange of 500mm fan can change the noise power by 50dB. This is also energy that is wasted in the formation of noise.

An acoustic loading effect is also present in duct systems. The sound power generated by a fan system 16 is not only a function of the design of the impeller 25 and the speed and operating conditions, but also depends on the acoustic impedances of the duct systems connected to the inlet and outlet of the fan. Therefore, fan and duct system should be matched not only for aerodynamic noise reasons but also because of acoustic considerations.

The described vector motion of the blade tips is a sine wave. This sinusoid is a mathematical curve that describes a smooth repetitive oscillation. The most basic form as a function of time (t) is: where: * A, the amplitude, is the peak deviation of the function from zero * f, the ordinary frequency, is the number of oscillations (cycles) that occur each second of time * w = 2.rtf, the angular frequency, is the rate of change of the function argument in units of radians per second * cp, the phase, specifies (in radians) where in its cycle the oscillation is at t = 0.

A number of features are included in the fan system 16 of the present disclosure each of which helps to improve its efficiency and to reduce the noise output. These features may be used individually in a fan system 16 or together. Including one or more of these features enables the size of the impeller 25 to be reduced compared to fan systems without these features, whilst still maintaining the same performance. These features are as follows.

The impeller 25 is a multi-bladed, backwards curved impeller. The impeller 25 may be an asymmetrically spaced, multi-bladed, backwards curved impeller which has unequal -8 -spacing on its blades 26, i.e. they are axially asymmetric, as this assists in breaking up the blade pass frequency (BPF) of the system, shifting the peak sound generated away from the usually dominant frequency, and subsequent harmonics of this frequency. This is advantageous in that it helps to reduce the fan noise.

A removable hub cover 30 (see Figures 4, 5 and 6), at least the upper part of which has the form of a rounded cone, is located over the impeller hub, which reduces energy dissipation as it improves the change of direction of the airflow entering the impeller 25. The hub cover 30 provides a reduction in the flow separation at the fan eye 29, reduces vortex formation, and better the flow turning from axial to radial direction. The hub cover 30 is designed so that the flow is laminar, with the direction of the entering flow gradually changes from axial to radial. Specifically, the shape and the height of the hub cover 30 are such that the flow gradually accelerates and turns into the blades 26 through the impeller outlet 31.

The hub cover 30 has a hollow cone body 37, at least an upper section of which is frustoconical, having a smooth outer surface, and a rounded cone tip 38. In the embodiment shown in Figures 4 and 5, the whole of the cone body 37 is frustoconical In the embodiment shown in Figure 6, only the upper section of the cone body 37 is frustoconical and the lower section flares outwardly. The frustoconical section may be convex or concave. The rounded cone tip 38 may be torispherical, dished, hemispherical or semi-ellipsoidal. The diameter of the lower edge of the cone tip 38 is the same as the radius of the top surface of the cone body 37 and the crown radius of the cone tip 38 and the angle of slant of the frustoconical upper section of the cone body 37 are selected to provide a smooth transition where the cone body 37 and cone tip 38 join.

At least one hub attachment means 36, such as a bolt, passes through an aperture in the apex of the cone tip 38, an aperture in the hub and is attached to the axle of the transmission system. The top of the cone tip 38 is preferably recessed, so that the free end of the hub attachment means 36 lies within the cone tip 38 and does not protrude from the hub cover 30. The aperture, in this embodiment, lies at an end of the recess in the cone tip 38. As such the turbulence and vortex shredding which occurs in the known arrangements, which results in lost energy and pressure, is reduced. Preferably the cone tip 38 is formed integrally with the cone body 37. Alternatively, the cone body 37 and cone tip 38 may be separate elements. The hub attachment means 36 in this embodiment may attach the cone body 37 to the hub, and the cone tip 38 may be attached to the cone body 37, for example -9 -by means of a clip mechanism, to cover the end of the hub attachment means 30. The shape of the hub cover 30 improves the pressure coefficient and divergence efficiency to create linear variation with radius to maintain an almost constant flow to the impeller outlet 31 The configuration of the fan case 21, inlet ducting 22 and outlet ducting 23, when connected, has an overall spirally shaped contour. Air enters into the impeller 25 from the inlet ducting 22, which provides the transition from the hopper 12 to the fan case 21, following the axial direction. It is then forced to change direction moving inside the blades 26 along the radial direction before being expelled via the outlet ducting 23, which provides the transition from the fan case 21 to the environment. In airflow any change of direction requires energy, and, specifically in this case, a reduction of the fan performance. Indeed, any sudden change of direction of the flow direction turns into a flow separation; adverse pressure gradient and so vortexes that dissipates a fraction of the total energy of the airflow. To increase the fan efficiency, and so to better transform the kinetic energy imparted to the air by the impeller 25 into potential energy.

The inlet ducting 22 of the present disclosure is designed to ensure that the transition of airflow reduces the amount of energy taken to drive the system by improving the change of direction (through 90 degrees) of the entering air flow. An amount of pressure drop occurs by virtue of the shape of the transitions in direction along the flow path.

The inlet ducting 22 (see Figures 3, 9, 10 and 11) comprises a first section 40 and a second section 41. A first end 42 of the first section 40 (i.e. the proximal end of the inlet ducting 22) is attached to the inlet opening of the fan case 21 and a second end 43 of the first section 40 is attached to a first end 44 of the second section 41. The attachment is made at the seal 46, which may be a flange seal, located at the second end 43 of the first section 40 and at the first end 44 of the second section 41. A mouth 49, which provides an air inlet, at the second end 45 of the second section 41 (i.e. the distal end of the inlet ducting 22) is positioned in the hopper 12, preferably at the top of the hopper 12.

The cross sectional area of the inlet ducting 22 gradually reduces along the majority of its length, from the mouth 49 to the first end 42 of the first section 40. By gradually decreasing the cross sectional area the speed and pressure of the air increases. Furthermore, the air flow is aligned in the most efficient manner to avoid the formation of vortices and/or -10 -pressure fluctuations. By making the flow as uniform as possible, this minimises vortex shredding when transifioning from one surface to another during a change in direction, or where there is a bend in the inlet ducting 22. A proportion of the first section 40 may have a substantially constant cross section towards the first end 42 where it abuts the fan case 21. The proportion of the inlet ducting 22 along which the cross section area reduces, the reducing portion, is at least 50% of the overall length of the inlet ducting 22 (measured along the central cord length), and is more preferably at least 85%. Most preferably it is in the range of 60% to 90%. In order to optimise the airflow within the inlet ducting 22 and reduce the pressure drop, the rate of change of cross sectional area measured along the central chord length of the inlet ducting 22 from the distal end in the reducing portion is in the range of 0.0249:1 -0.0361:1.

The second section 41 has an inner wall 47, which is closest to the fan case 21, and an opposing outer wall 48. The inner and outer walls 47, 48 are connected by side walls. At least one of the inner and outer walls 47, 48 or the side walls, preferably the inner wall 47, has a smooth concave inner surface to improve the flow characteristics. The opposing wall may be straight, as shown in the illustrated embodiment, or it may also have a smooth concave inner surface. The curvature of the concave walls 47, 48 helps to prevent flow separation and eddy currents, thereby improving the airflow.

Typically, inlet ducting 22 is made from multiple flat sheets, preferably of mild or stainless steel, which are folded over and welded and have an angled or convex configuration designed to fit around the engine. Preferably the inlet ducting 22 of the present disclosure is made from a single sheet which is rolled and folded over for welding. The smooth concave shape is beneficial in overcoming the disadvantages described above and to improve the performance of the fan system 16.

The outlet ducting 23 (see Figures 12, 13 and 14), forms the transition from the impeller 25 to the environment. The purpose of the outlet ducting 23 is to gradually reduce the velocity of the air moving through it to where it is finally discharged, which results in an increase in pressure. The outlet ducting 23 disclosed herein has been designed to ensure that transition of airflow reduces the amount of energy taken to drive the fan system 16. The design of the outlet ducting 23 has been optimised to recover static pressure, by converting fan velocity pressure into fan static pressure. This pressure is then available to the rest of the fan system 16, thereby providing an efficiency saving.

Bends in the outlet ducting 23 provide are a prolific source of unnecessary pressure loss and unwanted noise. However, whilst straight outlet ducting 23 would minimise the pressure loss, this is not feasible in a road cleaning machine 10 as, due to design constraints, the outlet ducting 23 generally has to bend in two directions to move air from the fan case outlet opening 32 to the final discharge point in the air discharge outlet 17. The design of the outlet ducting 23 of the present disclosure manages the losses in an efficient manner and recovers a similar level of pressure to a straight design.

The design of the outlet ducting 23 of the present disclosure comprises a first end 50 which is attached to the outlet opening 32 of the fan case 21. The attachment is made at the seal 52, which may be a flange seal, located at the first end 50 and the outlet opening 32. An outlet hood 53 is provided at a second end 51 of the outlet ducting 23, in which is located the air discharge outlet 17. The outlet ducting 23 has an inner wall 54 and an opposing outer wall 55, the inner and outer walls 54, 55 being connected by side walls 56, 57. At least one of the walls 54, 55, 56, 57 has a section which has a smooth concave inner surface, such as the inner wall 54, which is beneficial in improving the air flow through the outlet ducting 23, which in turn improves the performance of the fan system 16. The wall having the smooth concave surface may also have a section which has a smooth convex inner surface, such as the inner wall 54. This enables the inlet ducting to bend in two directions. More preferably each of the walls 54, 55, 56, 57 have a section which has a smooth concave inner surface, and even more preferably at least two of the walls 54, 55, 56, 57 have a smooth concave inner surface.

The cross sectional area of the outlet ducting 23 gradually increases along a portion of its length, from the first end 50 to the second end 51. A proportion of the outlet ducting 23, where the outlet hood 53 is located may have a substantially constant cross section. The proportion of the outlet ducting 23 along which the cross section area increases, the expanding portion, is at least 50% of the overall length of the outlet ducting 23 (measured along the central cord length), and is more preferably at least 85%. Most preferably it is in the range of 60% to 90%. In order to optimise the airflow within the outlet ducting 23 and reduce the pressure losses, the rate of change of cross sectional area measured along the central chord length of the outlet ducting 23 from the distal end is in the range of 0.0807:1 and 0.1421:1.

-12 -Preferably the outlet ducting 23 disclosed herein is also made from a single sheet. preferably of mild or stainless steel. which is rolled and folded over for welding.

The volute towards the outlet opening 32 of the fan case 21 is shaped so that there is a smooth transition between the fan case 21 and the outlet ducting 23. This also helps to improve the air flow.

The outlet grill 33 (see Figures 4, 7a and 7b), which is attached across the outlet opening 31 of the fan case 21, has also been modified to help improve the efficiency of the fan system 16. Prior art outlet grills are generally made from a series of flat plates in a grid configuration which act as a flow straightener to help promote a laminar flow. The flat plates create a series of channels which extend along the length of the outlet grill. However, as the air flow from the impeller 25 is variable in this type of application, the currently used outlet grills actually reduce the flow efficiency. This is due to the fact that, as each impeller blade 26 passes the outlet grill 33, the air flow changes direction significantly; it is rarely in a direction which aligns with the flow straighteners and can be in a direction in which the flow straighteners block the air flow, and thus induces turbulence. Due to the proximity of the outlet grill 33 to the fan blades 26, the flow is not fully formed when it reaches the outlet grill 33 and is experiencing transient pulses and fluctuations in the direction of the flow. Both effects are stronger nearer the blade tip, because the blade tip is moving faster. Fast splitting of the air means a sharper leading edge of the resulting pressure pulse. As the fan rotates the tips of the blades interact with the throat of the outlet ducting causing a pulsation or clipping effect with an oscillation in flow vector direction in relation to the BPF. There is also an outward pulse of air as the leading edge of each blade pushes forward cutting the air: -BPF = N t /60 where: BPF = Blade Pass Frequency (Hz) N = rotation velocity (rpm) t = number of blades in impeller This reduces the efficiency of the fan system 16.

-13 -The outlet grill 33 of the present disclosure comprises a frame 60, which is shaped to fit within the outlet opening 32 of the fan case 21. Generally, this is a rectangular shape having a pair of opposing side members 61 joined at each end by an end member 62. A plurality of evenly spaced cross bars 63 extend transversely between the opposing side members 61. Whilst, the lower the number of bars 63, the less disruption there is to the air flow, for safety reasons, the spacing of the cross bars 63 is such that a human hand cannot pass between the bars 61. In an alternative embodiment, shown in Figure 7), the outlet grill 33 also has a plurality of longitudinal bars 64, which extend between the opposing end members 62. The cross bars 63 and, if present, the longitudinal bars 64 have a leading edge (i.e. the part of the bar 63, 64 which first contacts the air exiting the fan case 21) which is rounded. The bars 63, 64 may be round, elliptical or airfoil-shaped. The bars 63, 64 are preferably solid.

Using bars 63, 64 that have a rounded leading edge means that the barrier or impediment to air flow is axially symmetric, with the axis perpendicular to most prevalent possible direction of fluctuations. This has been found advantageously to reduce the drag force, thereby creating less disruption and interference to the air flow than the traditional flow straighteners, thereby providing a better energy transformation at the impeller outlet 31. The outlet grill 33 of the present disclosure surprisingly generates a straighter air flow path when the air flows towards the outlet ducting 23.

A further improvement can be achieved by the location of the outlet grill 33 relative to the periphery of the impeller 25. One method of determining the optimal distance of the outlet grill 33 relative to the periphery of the impeller 25is using the following equation: -I = N tk trr (D -d) * 105 where: I = distance of grill from periphery of impeller (m) N = rotation velocity (rpm) t = number of blades in impeller D = diameter of extremity of fan (m) d = diameter of fan eye(m) k = turbulent Energy -14 -The maximum distance of the outlet grill 33 relative to the periphery of the impeller 25 is 400m, which is the point at which the air flow has become substantially uniform. More preferably the distance lies in the range of 200mm to 400mm, with the preferred distance being 200mm.

Claims

-15 - CLAIMS:- 1. A hub cover for covering the hub of an impeller in a fan system, said hub cover having the form of a rounded cone comprising a hollow body, at least an upper section of which is frustoconical, and a rounded cone tip attached to the upper section of the body.
2. A hub cover as claimed in claim 1, wherein the body and cone tip are formed integrally.
3. A hub cover as claimed in claim 1, wherein the body and cone tip are formed separately and the cone tip is removably attached to the body.
4. A hub cover as claimed in any one of the preceding claims, wherein the cone tip comprises a recess in its apex and an aperture located at the end of the recess.
5. A hub cover as claimed in any one of the preceding claims, wherein the whole of the cone body is frustoconical.
6. A hub cover as claimed in any one of the preceding claims, wherein side walls of the cone body are convex.
7. A hub cover as claimed in any one of claims 1 to 5, wherein side walls of the cone body are concave.
8. A hub cover as claimed in any one of the preceding claims, wherein the cone tip is torispherical, dished, hemispherical or semi-ellipsoidal.
9. A hub cover as claimed in any one of the preceding claim, wherein the diameter of a lower edge of the cone tip is the same as the radius of a top surface of the cone body, and a crown radius of the cone tip and an angle of slant of the frustoconical upper section of the cone body are selected to provide a smooth transition where the cone body and cone tip are attached.
10. A fan system for a road cleaning machine comprising a rotatable impeller mounted in a fan case, said impeller comprising: -16 -a hub; and a plurality of blades attached to the hub; hub attachment means for attaching the hub to a drive shaft for rotatably driving the impeller; and a hub cover as claimed in any one of the preceding claims removably attached over the hub; wherein the means for attaching the hub to a drive shaft are located within the hub cover.
11. A fan system as claimed in claim 10, wherein the impeller is a centrifugal impeller having a plurality of backwards curved blades.
12. A fan system as claimed in claim 10 or claim 11, wherein the cone tip comprises a recess in its apex and an aperture located at the end of the recess for receiving the hub attachment means such that one end of the hub attachment means is located with the recess.
13. A road cleaning machine comprising a chassis, a hopper mounted on the chassis, at least one suction conduit connected to the hopper, the fan system as claimed in any one of claims 10 to 12 for generating negative pressure within the hopper, and means for driving the impeller, wherein the hub is attached to said drive means.