WO1991017807A1

WO1991017807A1 - Band screen

Info

Publication number: WO1991017807A1
Application number: PCT/GB1991/000833
Authority: WO
Inventors: Maurice James Reaney; Peter Ross Blake
Original assignee: Brackett Green Limited
Priority date: 1990-05-24
Filing date: 1991-05-24
Publication date: 1991-11-28
Also published as: AU7985291A; WO1991017808A1; GB9011682D0; JPH05506810A; AU652758B2; SG44483A1; EP0531388A1; AU7901191A; EP0531375A1

Abstract

A band screen (1) for installation across a channel (2) containing a flowing liquid (6) comprises an endless belt of screening panels (19), each provided with inwardly extending leading and trailing edge walls (26) and so attached to the belt as to be pivotable relative thereto. Cam plates (28) on the sides of the panels (19) slide along inner and outer guide tracks to control the panel orientations around the belt, causing the leading edge walls (26) to form upwardly facing steps on the upgoing side on the screen and to flatten subsequently to facilitate cleaning.

Description

Band Screen

This invention relates to a type of screening apparatus known as a travelling or band screen, which is used to remove unwanted solid material from liquids flowing in channels. The invention is especially applicable to band screens in which only very small apertures are provided, so as to achieve fine screening.

A band screen is a continuous travelling belt of screening panels to be installed vertically in, and above, a channel containing a flowing liquid. In contrast to revolving screens, band screens are compact and versatile, and yet still allow a larger screening area to be presented to the flow than would otherwise be possible with a stationary screen. In the case of a uniflow type band screen, for example, the panels will be arranged across the channel at right angles to the direction of flow, so that all the liquid flows through first the upstream side and then the downstream side of the belt of panels. Screening takes place on the upstream side, with debris being transported up out of the flow.

Screening panels are usually provided with deep buckets formed on their trailing edges, which buckets lift large or heavy debris out of the flow on the screening side of the belt. Such panels cannot be effectively cleaned with brushes or back-washed with spray jets. Further, in the case of a uniflow band screen, the two rows of panels cause an undesirable pressure drop or headloss to develop across the screen, which effect is worsened if the screen becomes clogged, for example, with fibrous material. Moreover, in recent years, increasingly stringent standards for water purity have increased the demand for screens with finer aperture sizes, and aperture sizes of 6mm diameter or less are now commonplace. "Fine screens" are, of course, more susceptible to "blinding up", and hence, the cleaning and headloss problems mentioned above are exacerbated in such screens.

It is an object of the present invention substantially to overcome or mitigate the above-mentioned problems of conventional band screens. It is a particular object of the present invention to provide a band screen that is able to cope with larger debris as well as conventional band screens, and yet is also able to be cleaned easily.

The present invention provides a band screen comprising an endless belt that includes a succession of screening panels and that is mounted for movement around a closed path, each screening panel being so mounted on the belt as to be pivotable relative to the belt about a transverse axis in the plane of the belt, the screen also comprising controlling means for pivoting the panels about their respective axes as they pass around the closed path.

Thus, in contrast to conventional band screens in which the panels are arranged in a substantially fixed, end-to-end relationship, the band screen of the present invention allows the characteristics of the belt to be altered around its path, for example, to pivot the panels into one specific orientation for cleaning and another orientation for removing large debris from the flow.

In a preferred arrangement on the upwardly moving side of the belt the controlling means causes the panels to pivot relative to one another such that the panels form upwardly facing steps. The steps are capable of removing large sized debris from the liquid, thereby ensuring that the apparatus does not become blocked and that pretreatment of the liquid upstream of the apparatus is unnecessary. The steps also intercept any smaller material falling from the inclined panel faces, so that such material does not reenter the liquid. Each panel may be provided with an inwardly extending, leading edge portion that forms the upwardly facing step, and that leading edge portion may conveniently be caused to move outwardly to form the step. Preferably, each panel is provided with a screening surface and an edge wall that extends inwardly from the leading edge of the screening surface and that forms the upwardly facing step, so that the screening surface is not required to form part of the step.

It is preferred that the panels are shaped so that they may be pivoted to form a surface that may be easily cleaned, i.e., a surface without crevices or external projections. Thus, preferably the panels will have only inwardly, as opposed to outwardly, extending side or edge walls. In a preferred configuration, each screening panel comprises a generally flat, rectangular screening surface with leading and trailing edge walls extending inwardly therefrom; the edge walls of adjacent pairs of panels may then be arranged abutting one another on the upstream side of the screen, so as to prevent screenings from passing between the panels of the belt.

Preferably, the screening panels are moulded from plastics material and are provided with a plurality of holes also formed during the moulding process. By moulding the panels from plastics material, it is possible to produce panels having low friction surfaces, as well as with smooth-walled holes, in a variety of sizes and thicknesses. Polyurethane is an especially suitable material from which to mould the panels.

Preferably, the panels are provided with a plurality of tapered holes which reduce in diameter towards the exterior of the screen. Such holes provide a substantial improvement in the efficiency of the screen: whereas any fibres or particles entering a straight hole and having a width similar to the hole diameter would be likely to become lodged therein, with a tapered hole, such fibres or particles may be subsequently freed, either by the flow itself, or by the application of cleaning means. Thus, the extent of clogging or "blinding up" of the screen can be greatly reduced, which also assists in reducing the headloss that develops across the screen as a result of the presence of the two rows of panels.

In view of the difficulty of drilling tapered holes, such holes are conveniently formed by moulding, where a plastics panel is required. In that case, there is the added benefit that the tapered hole facilitates the removal of the hardened panel from the mould.

Conveniently, along an upper portion of the path of the panels, the screening surfaces of adjacent panels lie flush with one another to form a substantially uninterrupted surface, from which surface screenings may then be relatively easily removed. Along at least part of that portion of the belt, the screening surfaces of the panels advantageously face downwardly, so as to assist in the removal of the screenings. Usually, cleaning means will be provided to remove screenings from the upper portion of the belt. For example, a rotating brush may be arranged to brush the screening surfaces of the panels, and outwardly directed spray jets may also be provided for backwashing.

Preferably, on a lower portion of the downwardly moving side of the belt the screening panels are caused to be so orientated that there is a gap between adjacent panels. The gaps allow screened liquid to pass easily through the downstream side of the belt thereby reducing the headloss across the apparatus as compared with the headlosses that occur across conventional screens. Furthermore, fine solid material, such as fibrous material, that is able to pass through the screening panel apertures, does not build up on the internal surfaces of the belt causing "blinding up", and a concomitant loss of efficiency.

The controlling means may comprise inner and/or outer guide tracks positioned along portions of the path of the belt and spaced at varying distances therefrom. Preferably, the guide tracks are provided on both sides along the panel belt. The controlling means may comprise cam means provided on the sides of each of the panels and so arranged as to be slideable along guide tracks disposed around the path of the belt.

In a preferred arrangement, each panel is pivotally attached at or near its trailing end to the belt and the cam portions are located at or near the front of the panel. Although it would be expected that it would be easier to pull the panels, as opposed to pushing them, it has been found that less difficulties are experienced with the latter arrangement; in that arrangement, however, it is normally necessary for the cam means to have both inner and outer portions that are so arranged as to be slideable along the inner and outer guide tracks, respectively, in order to provide sufficient control of the panels around the belt. In particular, guide tracks are required where the panels approach the top of the upgoing side, as without assistance, the panels are liable to stick and jam the screen, rather than dropping under the action of gravity, as would be expected.

Conveniently, each cam means forms part of a cam plate that is sealingiy disposed between the side of the panel and the adjacent edge of the screen and that is fixed to the panel. Such a design has a number of advantages. Firstly, the provision of a single member as opposed to a plurality of, for example, cam levers, reduces the complexity and cost of the screen. Secondly, one of the main problems with band screens is achieving adequate sealing while maintaining sufficient clearances for the moving parts. The provision of a cam plate on each side of the panel, is a simple but extremely effective way of controlling the panel, while retaining a seal on each side thereof; furthermore, the cam plate may be separately replaced, if necessary. Each cam plate and its respective panel may conveniently be coaxially, pivotally mounted on the belt, in addition to being secured together away from the axis.

The panels may be mounted on a pair of chains, each of which includes a plurality of rollers rotatably mounted thereon and moveable along tracks disposed around the screen, each panel being pivotally mounted about the same axis as a roller. Each chain roller and panel may be mounted on a common pivot pin. Preferably, the cam means and rollers travel along the same tracks or travel along adjacent tracks that are stepped in relation to one another.

One form of band screen, constructed in accordance with the invention, will now be described, by way of example only, with reference to the accompanying drawings, of which:

Fig.l is a longitudinal sectional view of the screen installed in a channel;

Fig.2a is a perspective view of a screening panel;

Fig.2b shows the arrangement of the holes on the screening panel; and,

Fig.2c is a side view of a cam plate;

Fig.3 is a schematic sectional view along the line III-III of Fig.l;

Fig.4 is a schematic sectional view of the top part of the screen of Fig.l, to a larger scale than Fig. 1;

Fig.5 is a schematic sectional view of the bottom part of the screen of Fig.l, to a larger scale than Fig.l; and,

Fig.6 is a schematic sectional view along the line VI-VI of Fig.4.

Referring to Fig. 1, the band screen is of the uniflow type and is shown installed vertically in and above a sewage channel, extending thereacross.

The screen 1 comprises a single structure that may be lowered into, or raised from, the channel 2 as a complete unit and stands on supporting feet 3 that, when the screen is installed, rest on a deck 4 on either side of the channel. Neoprene rubber flexible contact strips (not shown) are provided at the front of the screen between the screen structure and the channel walls 2 to prevent the passage of any solid material therethrough. The screen structure is made from robust steel sections 7 that are suitably braced to withstand the hydraulic forces that may be imposed by the sewage flow 6.

The panels together define a belt 5 which comprises a base section 8 submerged in the flow 6, an upstream, screening side 9 on which the panels pass upwardly thereby removing debris from the flow, a head section 10 at the top of the screen where the debris is removed and a downstream, return side 11.

The head section 10 of the screen, shown in greater detail in Fig.4, contains a single speed electric motor 12 coupled to a fully enclosed, shaft mounted double worm gearbox 13. Two sprocket wheels 14 are positioned at the front of the head section 10, one on either side of the panel belt 5, and are keyed to the main drive shaft 15. An endless movable chain 16 is attached to each side of the panel belt 5. Each chain 16 passes over a respective sprocket wheel 14 by which it is driven around inner and outer tracks 17 and 18, respectively, positioned on both sides of the panel belt 5 along its straight sections and around its head and base sections 10 and 8, respectively. Next to the sprocket wheels 14 a header pipe carrying a row of wash-water jet spray nozzles 20 is positioned horizontally above and across the inside of the return side 11 of the belt 5. A spiral rotating brush 21 is positioned horizontally across and against the outside of the panel belt 5, below the spray nozzles 20. The brush 21 is driven by a timing belt from the main drive shaft 15 and is provided with polypropylene bristles.

Referring now to Fig.l and also to Fig.5, in which the base section 8 of the screen 1 is shown in greater detail, the screening side 9 of the panel belt 5 is provided with a straight sealing brush 22 that extends horizontally across the bottom of the belt to prevent the passage of solid material under the belt. In addition, curved sealing brushes (not shown) are provided on frame members on each side of the screening panels 19 and extend up from the horizontal brush 22 to the point (60 in Fig.5) at which the curved base section of the belt merges into a straight section; above this point, nylon sealing angles, which extend up to the level of the deck 4, are provided for the same purpose.

Referring to Fig.2a, in which one of the screening panels 19 is shown, each panel may be made from perforated, rectangular stainless .steel plate or moulded from plastics material. The perforations 24 are perferably circular holes (to ensure uniformity of screening) of a relatively fine aperture size of, for example, 2 to 6mm diameter. Two end walls 26 extend from the two longer edges 25 of the panel a short distance perpendicularly away from the rectangular face 27. In the case of a stainless steel panel, the perforated plate may be folded over to form the two end walls 26, so that all three surfaces are perforated. In the case of a moulded plastics panel, holes may be moulded in the rectangular face and the two end walls may be solid. Stainless steel side plates 28 are welded across the left and right edges of the panel face 27 between the two end walls 26 so that each panel 19 is shaped in the form of a tray.

A cam plate 29, made from hard wearing plastics material such as high density, high molecular weight polyethylene, is fixedly attached to each side plate 28 of the panel 19, so that there is no relative movement between the cam 29 and panel. The front half of the cam 29 has a rounded projection 30 that extends outwardly relative to the plane of the panel belt 5, as shown in Fig.4, and an angular projection 31 that extends inwardly relative thereto. It is important that the cam plate has the particular shape and angles shown in Fig.2c, in order to achieve the desired pivoting of the panels around the path of the belt, the projections 30, 31 allowing the position of the panel 19 to be controlled in a manner to be explained below.

The screen 1 consists of fifty such screening panels 19. These panels 19 are so mounted on the two chains 16 that their end walls 26 are in abutting relationship and, together with the side plates 28, extend inwardly relative to the panel belt 5 so that each panel 19 outwardly presents the flat rectangular face 27. Fig.3 shows how each panel 19 is attached to a chain 16. Each chain comprises a plurality of nylon rollers 32, each of which is rotatably mounted on a stainless steel pivot pin 33, and two pairs of steel side bars, an inner pair 34 and an outer pair 35, which are also pivotally mounted on the pivot pin 33 either side of the roller 32, one pair being linked to the roller in front and the other pair to the roller behind; the side bars may be made from stainless steel or from zinc plated carbon steel. The panels 19 are attached to the chain 16 by means only of the pivot pin 33, which passes through holes 36, 37 provided near the back ends of the cam and side plate, respectively, as seen in Figs. 2A and 2C. Thus, as each panel 19 is only attached to each chain 16 by the pivot pin 33, the front end of each panel is freely rotatable about the axis of the pin, thereby pivoting relative to the plane of the belt 5, as may be seen in Fig.l. The head of the pin lies against the inside of the side plate 28 and the roller 32 and side bars "4, 35 are secured in position with a self-locking nut 38. It will be appreciated that such an arrangement allows the panels 19 to be easily interchanged or replaced.

As the panel belt 5 moves along its own length, the relative positions of the panels are altered as a result of the action of the cams 29; for example, where the chain roller 32 is running along an inner track, the inner cam projection 31 may run along that track as well, or alternatively, may run along an adjacent track stepped in relation thereto; where the roller 32 is running along an outer track, the outer cam projection 30 may act in a similar fashion.

The details of the cam action and the arrangement of the tracks around different parts of the panel belt 5 will now be described. The tracks themselves comprise steel frame members along which the cam projections 30, 31 usually slide. The outer portions of the steel tracks 39 are fitted with replaceable, polypropylene wearing strips 40 along which the rollers 32 travel; occasionally wider strips are provided on which the cam projections 30, 31 can also slide. Fig.3 shows a panel in the lower region of the screening side 9 of the belt. In this region the chain roller 32 and inner cam projection 31 are both running along a polypropylene wearing strip 40. The position of the cam 29 causes the leading end of each panel 19 to be pushed outwardly from the plane of the belt 5 so that the panels form a stepped arrangement in which the leading end wall 26 forms an exposed and approximately level surface. A guide angle 41 is secured to an outer frame member 42 and prevents the cam 29 from being displaced too far from the inner track if its motion is interrupted suddenly. A nylon sealing angle 43, as described earlier, is also secured to the outer frame member 42 and has a distal portion 44 that lies flat against the outside surface of the cam 29 to prevent the ingress of any solid material through the panel belt 5.

The screening panels move up the screening side 9 in stepwise formation until the inner track curves to form a horizontal section, as shown in Fig. 4. A further guide angle 45 is provided at this point. The part of the polypropylene track 40 next to the panel 19, along which the cam 29 is running, then slants downwardly at a point 46 before terminating so that the inner cam projection 31 passes onto the steel track 39 below, causing the front of the panel to drop so that the panel belt 5 adopts a flatter configuration. The panels 19 then pass along the tracks in the lowered position until the sprocket wheel 14 is reached, where the tracks terminate. The teeth of the wheel 14 then lift the chain links and carry them around the head of the belt 5, a further, curved guide angle 47 being provided around the sprocket wheel 14. On the return side 11, the chain 16 passes off the wheel 14 onto an outer track section 18 where the chain rollers 32 and outer cam projections 30 again travel along an upper polypropylene strip 48 and lower steel track 49, respec¬ tively, as shown in section in Fig. 6. The position of the cam 29 now causes the panels 19 to lie completely flat so that the panel belt 5 forms a continuous flat surface as it passes the spray nozzles and rotating brush for cleaning. The panels 19 then pass around a concave portion, an inner guide angle 50 being provided inside the panel belt 5 at this point, and down a straight section leading to the base section 8, as seen in Fig. 1. At the curved portion, a short section of wider polyprop¬ ylene strip causes the outer cam projection 30 to move inwardly relative to the belt 5 so that the fronts of the panels 19 lift slightly passing round the curve. Once on the straight section of the return side 11 of the belt the rollers 32 run along the outer polypropylene track, and an outer guide angle 51 runs along the belt 5 at a distance spaced sufficiently therefrom to allow the panels to hang freely vertically without the outer cam projections 30 contacting the guide angle 51. The presence of the guide angle, however, serves to confine the cam projections and thus prevents the panels from pivoting too far from the plane of the belt 5.

Referring now to Fig. 5, at the base of the down¬ stream side 11, the outer cam projection 30 is forced inwards by the outer guide track 51 curving inwardly. Inner and outer tracks 17 and 18, respectively, then cause the chain rollers 32 and cams 29 to follow a semi¬ circular path around the base section 8, the chain rollers being confined within a pair of tracks 52 that are disposed between the pair of cam tracks 53, as seen in Fig.5. Once the panels 19 have travelled around the curved base section 8, the outer tracks 18 terminate and the inner cam track 53 rejoins the inner roller track 52 (at point 60) so that the fronts of the panels 19 lift to form a stepped arrangement once more, as described previously.

The screen 1 operates in the following manner. On the upstream, screening side 9, the panels 19 entrain solid material from the flow 6 and carry it upwardly out of the water. Large sized debris, which conventional screens usually remove with scoops provided on the outsides of the panels, is lifted out of the flow on the rising platform "steps" formed by the front walls 26 of the panels 19 due to the cam action. Thus the screen will not become blocked on its upstream side 9, either by large sized debris collecting on the channel floor against the belt or debris floating on the surface of the flowing sewage 6. The screen may therefore be inserted directly at the inlet to a sewage works without any pretreatment of the flow being required. Furthermore, any fibrous material falling from the vertical panel faces 27 collects on the "steps" 26 rather than reenter- ing the flow .

Once the panels 19 have travelled around the drive spocket 14, the cam action arranges the screening panels, as described above, to form a continuous flat surface. The panels 19 move past the row of spray nozzles 20 which backwash each screening panel using a high pressure fan shaped jet. The nozzles 20 are so arranged as to provide overlapping coverage so that the whole panel belt 5 is very effectively cleaned. The panels 19 are then vigorously brushed by the spiral rotating brush 21 to remove any remaining fibrous material, a build up of which can often give rise to problems with travelling band screens. It will be appreciated that the cam action allows the panels to be very efficiently cleaned, the flattened panels ensuring that all solid material becomes exposed and is thus easily removed. Screenings displaced by the cleaning treatment are discharged directly into a receptacle before being transported away for disposal.

Descending the return side 11, the panels hang vertically. As may be seen from Fig. 1, there is a considerable gap between adjacent panels 19. This enables the screened liquid to pass relatively unhindered through the downstream side 11 of the belt: there are two advantages associated with such arrangement. Firstly, there will be no internal build up of solid material between the upstream and downstream sides 9 and 11, respectively, of the belt, such "blinding up" being a common problem associated with screens having fine apertures. Secondly, the headloss across the screen is substantially reduced compared to that across a conven¬ tional "fine screen".

The screen 1 may run continuously. Preferably, however, ultrasonic level detection equipment is provided which activates control means that starts the motor and thus the screening panels and the cleaning brush, whenever a preset differential headloss is detected across the screen. The screen may also be activated if the equipment detects that the level upstream exceeds a certain height, irrespective of headloss. Ideally, the screen will be activated by the control means for a short preset period once every hour in order to prevent the screening panels drying out or solids accumulating upstream of the screen. Backwashing of the screening panels, which is separately controlled, may also be arranged to commence whenever the motor starts up and ideally, should continue for a short, predetermined period after the motor has stopped.

As an example of suitable dimensions, a screen for use in a sewage channel, for example, a channel of 2m width and 2m depth, may have a width of 1.75m and may have 50 panels each with a pitch of 200mm, the screen having a minimum immersed depth in normal operation of 1.8m. The straight, upstream and downstream sections of the belt may be inclined at an angle of 65° to the horizontal. The screen may be provided with 2mm thick, perforated stainless steel plate panels having 6mm diameter apertures provided in the rectangular screening face (and the two end walls), which apertures occupy about 50% of the plate area. The screen may have a belt speed of 8m/min and have a design capacity of 1.33m³/sec.

It will be appreciated that modifications may be made to the travelling band screen described above. For example, instead of forming the screening panels from 2mm stainless steel perforated plate, the panels may be moulded from plastics material, with the apertures also being formed during the moulding process. For many applications, such a panel may infact be preferred over the stainless steel panel. An example of one such panel will now be described:

The panel is moulded from polyurethane and comprises a rectangular screening face having a length of about 80cm, a width of about 20cm, and two edge walls of about ^•4cm width extending perpendicularly from the screening face. The panel has a thickness of 5mm.

The edge faces are solid but the screening face is provided with a plurality of apertures formed therein during the moulding process. Each aperture is of constant circular cross-section and of 5mm in diameter, each aperture having a length of about 5mm. The apertures are arranged, as shown in Fig.2b, over the whole face, in rows spaced 7mm apart (centre to centre spacing) , with the apertures in each row being separated by about 12mm (centre to centre spacing) and apertures in alternate rows being offset from one another by 6mm. The apertures occupy about 50% of the total area of the screening face.

In contrast to the stainless steel panel, the thickness of this panel is such that the length of each aperture is equal to its diameter. Such a ratio enables the problem of "hairpinning" of fibrous screenings in the apertures to be substantially overcome, and hence, there is less "blinding up" of the screen and consequently, a smaller headloss is likely to develop across the screen during use.

The frictional characteristics of the plastics panel over a metal panel are markedly improved. The provision of the moulded apertures with their smooth inner walls and openings, also results in less screenings being trapped than would be the case with drilled or punched •holes of similar dimensions. In fact, the improved performance of the moulded plastics panel over the metal panel means that it may be possible to dispense entirely with the backwashing spray jets (which contribute substantially to the running cost of the screens), or at least, to reduce backwashing to short periods every hour, for example. The rotating brush may also not be required. In some applications, it may be worthwhile for all the structural members to be made from stainless steel. Furthermore, instead of attaching cam plates to the sides of the screening panels the cam projections may be integrally provided as part of the screening panels. As an alternative to a single speed motor, the screen may be powered by a two speed motor with, for example, the belt moving at either 8m/min or 16m/min, which speeds may be either manually or automatically selected.

Where it is desired to install a screen in a channel of greater width than the screen, one or both sides of the screen may be provided with a weir handstop, comprising a slidably mounted vertical plate which may be lifted manually to allow sewage to bypass the panel belt whenever necessary.

Claims

1. A band screen comprising an endless belt that includes a succession of screening panels and that is mounted for movement around a closed path, each screening panel being so mounted on the belt as to be pivotable relative to the belt about a transverse axis in the plane of the belt, the screen also comprising controlling means for pivoting the panels about their respective axes as they pass around the closed path.

2. A band screen as claimed in claim 1, wherein on the upwardly moving side of the belt the controlling means causes the panels to pivot relative to one another such that the panels form upwardly facing steps.

3. A band screen as claimed in claim 2, wherein each panel is provided with an inwardly extending, leading edge portion that forms the upwardly facing step.

4. A band screen as claimed in any one of claims 1 to

3, wherein each screening panel comprises a generally flat, rectangular screening surface with leading and trailing edge walls extending inwardly therefrom.

5. A band screen as claimed in any one of claims 1 to

4, wherein the screening panels are moulded from plastics material and are provided with a plurality of holes also formed during the moulding process.

6. A band screen as claimed in any one of claims 1 to

5, wherein the screening panels are provided with a plurality of tapered holes, which reduce in diameter towards the exterior of the screen.

7. A band screen as claimed in any one of claims 1 to 6, wherein along an upper portion of the path of the panels the screening surfaces of adjacent panels lie flush with one another to form a substantially uninterrupted surface.

8. A band screen as claimed in claim 7, wherein the screening surfaces of the panels face downwardly along at least part of the upper portion of the path where they lie flush with one another.

9. A band screen as claimed in claim 7 or claim 8, wherein cleaning means is provided to remove screenings from the upper portion of the belt.

10. A band screen as claimed in claim 9, wherein the cleaning means comprises a rotating brush that is arranged to brush the screening surfaces of the panels.

11. A band screen as claimed in any one of claims 1 to

10, wherein on a lower portion of the downwardly moving side of the belt the screening panels are caused to be so orientated that there is a gap between adjacent panels.

12. A band screen as claimed in any one of claims 1 to

11, wherein the controlling means comprises cam means provided on the sides of each of the panels and so arranged as to be slideable along guide tracks disposed around the path of the panels.

13. A band screen as claimed in claim 12, wherein each panel is pivotally attached at or near its trailing end to the belt and the cam means are located at or near the front of the panel.

14. A band screen as claimed in claim 12 or claim 13, wherein the cam means has both inner and outer portions that are so arranged as to be slideable along inner and outer guide tracks, respectively.

15. A band screen as claimed in any one of claims 12 to 14, wherein the cam means comprises a cam plate that is sealingiy disposed between the side of the panel and the adjacent edge of the screen and that is fixed to the panel.

16. A band screen as claimed in claim 15, wherein each cam plate and its respective panel are pivotally mounted about the same axis.

17. A band screen as claimed in any one of claims 1 to 16, wherein the panels are mounted on a pair of chains, each of which includes a plurality of rollers rotatably mounted thereon and moveable along tracks disposed around the screen, each panel being pivotally mounted about the same axis as a roller.

18. A band screen as claimed in claim 17, wherein each roller and panel are mounted on a common pivot pin.

19. A band screen as claimed in claim 17 or claim 18, wherein the cam means and rollers travel along the same tracks or travel along adjacent tracks that are stepped in relation to one another.