GB2045113A - Rotary screen for refuse sizing - Google Patents

Abstract

In a rotary screen (1) for refuse sizing longitudinal lands (4) defining the screen apertures (3) extend radially outward of the screen such that the land width, as measured over the radially outer surface of the lands, exceeds the circumferential width of the apertures. A ratio of effective land width to aperture width of at least 1.2:1 has been found to reduce the tendency for the apertures to become blocked by long flexible items wrapped around the lands. Conveniently the lands have smoothly arched outer surfaces. To assist in clearing any items which may still become wrapped around the land, stiff rubber plates (6) are provided to brush against the screen as it rotates. <IMAGE>

Description

SPECIFICATION Rotary screen for refuse sizing The present invention relates to rotary or trommel screens and in particular to rotary screens which are designed for use in the sieving of relatively coarsely-sized items from a mixed refuse containing long flexible items.

Rotary screens conventionally take the form of a cylindrical sieve which is caused to rotate by eg an electric motor. Material to be sieved is fed into the interior of the screen at one end thereof, and as the screen rotates is shifted longitudinally along the screen, for example by inclining the screen or by attaching helical vanes within the screen which then act as a mechanical screw on the material inside the screen. As the material passes along the screen undersize items fall through apertures in the screen and are collected beneath it. The screen may be provided with apertures of several different sizes arranged in zones along the screen so that items of progressively larger size ranges are sieved out as the material moves along the screen. At the downstream end of the screen residual material is discharged for further treatment.

Where the material is being separated in a rotary screen contains long flexible items, these tend to wrap themselves around the longitudinally extending lands of the screen as it rotates and in this way block up or "blind" the apertures in the screen. Not only does this cause a reduction in sieving rate but since it alters the sizes of the items which are caused to fall out of the screen by reducing the effective size of the apertures in the screen, it is necessary frequently to stop the screen and clear out the apertures. This causes lost separating time and is in itself a labour intensive task so that such blinding of the screen apertures may be seen to have a very detrimental effect on the efficiency of the sieving process.

The problem of blinding is particularly severe where the feed material to the screen is a mixed domestic refuse containing items of discarded clothing and other relatively long textile items. Although many of these items can be successfully removed using a rotary screen of the type described in UK patent application No 1533167 there is always a tendency for some of the apertures in the screen to become blocked over a period of use. It is an object of the present invention to provide a screen in which this tendency is, at least in part, overcome. Afurther object is to do this whilst maintaining or even improving the capacity of the rotary screen, in terms of rate of throughput of material to be sieved per unit screen size.

These objects are satisfied in part by the apparatus of the present invention, which comprises a rotary screen for the grading of material having a range of sizes, said screen being rotatable about a longitudinal axis and having a plurality of longitudinally-extending lands and a plurality of circumferentially-extending lands said lands defining therebetween a plurality of apertures each having a throat, said longitudinally extending lands being so shaped that the width thereof as measured over the radially outer surface thereof in a circumferential direction exceeds the circumferential width of the throats of the apertures circumferentially adjacent thereto.

The width of an aperture as measured in the circumferential direction may vary according to the radius at which it is measured, the radially innermost part of the aperture haing minimum circumferential width being referred to hereinafter as the aperture throat. The term "radially outer surface" is used herein as meaning that part of the surface of a longitudinally extending land which has on the side of the land radially outwards of the adjacent aperture throats.

The screen of the present invention embodies two novel principles concerning the design and operation of rotary screens particularly for sieving material containing long flexible items. Firstly it has been appreciated by the applicant that the tendency of such items to become wrapped around the longitudinal lands of the screen may be diminished by the provision of a land which has a width greater than the width of the respective apertures separated by the land.Preferably the land width, as measured over the surface of the lands, exceeds the circumferential width of the respective apertures by a factor of at least 1.2:1. Secondly it has been appreciated by the applicant that while maintaining the above proportions, the capacity of the screen can be increased, ie the number of apertures in a screen of given diameter and length increased, by causing the lands to project outwardly of the mean radius of the screen. In this way the land width as measured over the radially outward surface of the land may exceed the aperture width measured over the screen circumference even although the land width as measured over the screen circumference may in fact be less than the aperture width measured in the same manner.By adopting this arrangement therefore a rotary screen is obtained in which the apertures are relatively closely spaced whilst the intervening longitudinally-extending lands have a radially outer surface measured in the circumferential direction which exceeds the aperture width and so helps to prevent blocking of the apertures by long flexible items. In practice the narrowness of the lands measured on the circumference of the screen will be determined largely by the strength required for the lands, since these are major structural components of the screen.

The radially extending lands preferably become narrower towards their radially outward extremity so that the apertures between the lands are narrowest at their radially innermost extent. This helps to prevent items of refuse becoming jammed in the apertures. In particular it is preferred that the lands have smoothly arched outer surfaces and the term "arched land" is used hereinafter for convenience though it will be appreciated that in general the lands may take any shape provided only that they extend radially of the screen.

The beneficial effect secured by the design of rotary screens according to this invention is maintained whatever the shape of the apertures, though for simplicity of construction these are often of square or other rectangular shape. However it should be appreciated that only the longitudinally-extending lands need extend radially since it is only around these lands that long flexible items tend to become wrapped as the screen rotates: it is not necessary to radially extend the circumferentially-extending lands running between longitudinally-adjacent apertures.

As regards the construction of the rotary screen of this invention this may, for example, be of cylindrically-formed sheet material with the apertures punched out, or of distinct members forming the longitudinal lands held in a cylindrical outline by a series of annular members which themselves form the circumferentially-extending lands. In the former case the arched surface of the land may be provided by a distinct element which may be bolted onto the cylindrical screen over the longitudinal lands, whilst in the latter case the separate members forming the lands may themselves be shaped to provide a convex outer surface. The construction of the arched lands in a screen may be a combination of these two basic arrangements.

Operation of the screen of the present invention to reduce or eliminate binding by long flexible items may be improved further by the provision, adjacent the apertures in the screen, of resiliently deflectable members such as a plate or group of fingers or a resilient material which extend towards the screen perpendicularly of its longitudinal axis and are of sufficient length to project through the apertures in the screen when these are aligned with the plates or fingers. As the screen rotates therefore, the plates or fingers alternately bend as their end portions wipe across the arched land surface and flick straight as the end of the plate or finger comes off the edge of the land and into the succeeding aperture. As the plate or finger strikes the edge of the next following land it is bent back again to repeat the whole process as each aperture passes by.In this way the fingers or plates help to clear the lands of any items with which, despite the arching of the lands, they may have become entangled. It is advantageous to place the fingers at a point just above the bottom of the screen on its upward-moving side so that any material which might start to wrap itself around the lands as the screen rotates will be immediately subjected to the wiping action of the fingers or plates. By virtue of the fact that the fingers or plates flex as they pass over the lands and then "flick" quickly into their straight condition any material which is "caught" by the fingers or plates will tend to be projected through the next following aperture and back into the screen. The fingers or plates are preferably formed of a relatively stiff rubber material.

The invention will now be more particular described with reference to the accompanying drawings in which: Figure 1 is a perspective view of an embodiment of a rotary screen according to the present invention; and Figures 2 and 3 are views in transvrse section of two different embodiments of screen according to the invention.

In Figure 1, a rotary screen is shown generally at 1 and is arranged to be driven by externally-mounted wheels (not shown) bearing against circumferential tracks 2,2'. The screen has a regular array of square apertures 3 separated by longitudinally extending lands 4 and by circumferentially extending lands 5. The longitudinal lands 4 have convex outer surfaces. A series of stiff rubber plates 6 is fixed to a bar 7 running parallel to the rotary axis of the screen and spaced from it by a short distance. The plates 6 are sufficiently long to extend through the apertures 3 when the latter are in alignment therewith and sufficiently flexible to be bent out of the way by an approaching land as the screen rotates.The plates are preferably located at a position around the screen which is just forward (in the intended direction of rotation of the screen) of the lowermost point thereof, but in Figure 1, for the sake of clarity, they are shown at a position near the top of the screen.

The rotary axis of the screen is inclined somewhat to the horizontal and in operation is fed at the upper end with a material containing items of different size ranges. The screen is rotated by external drive means and as the material tumbles round within the screen undersized items are sieved out through the apertures to fall onto a collecting means, eg a conveyor belt running beneath the screen. The externally arched shape of the lands 4 results in there being little tendency for any long flexible items to become hooked around the lands.

Any such items which nevertheless do so, will tend to be detached therefrom by the plates as the respective lands pass under the plates and fall back into the screen. Such items and all other oversize material is discharged eventually from the lower end of the screen.

Figure 2 shows, in transverse section, a cylindrical screen 11, in which the lands comprise portions 12 of the body of the screen between the apertures 13. Bolted to the portions 12 are arched strips 14 each extending either the length of a single aperture or along a part or the whole of the length of the screen as is convenient. Plates or sets of fingers 16 project towards the screen at the position shown for this screen which is designed for counter-clockwise rotation, ie just forward (in the intended direction of rotation of the screen) of the lowermost point of the screen.

In the arrangement shown in Figure 3, the lands 24 are unitary members forming the longitudinal framework of the screen. The lands 24 are fixed to peripheral members, the relative spacing of the two sets of land determining the size of the apertures 23. Plates or sets of fingers 26 project towards the screen at a position which has been found to be preferred for a clockwise-rotating screen, ie again just forward of the lowermost part of the screen.

A rotary screen according to the present invention was constructed having two sets of apertures, a first set of 40mm diameter round apertures in that half of the screen between the input end and the centre thereof and a second set of 225mm square apertures in the other half of the screen. The total area of the screen was 48m2 and the area of the first and second set of apertures was respectively 11 m2 and 10.2m2 representing, respectively, 47% and 43% of the total area of each half of the screen. The 225 mm square apertures were arranged in rows around the screen circumference and adjacent apertures within each row were separated by an arched land having a width (measured over the land surface) of 275 mm, ie 1.2 times the aperture width. The screen was horizontal, the material being carried along it from the input end to the output end by virtue of a helical vane projecting with the screen.

In a test run a mixed domestic refuse was fed onto the rotary screen at the input end at a rate of 4.7 ton/hr whilst the screen was rotating at 10 rpm, this being 35% of the critical speed at which the screen begins to behave as a centrifuge.

Under these conditions the output of refuse material from the screen was found to be as follows: Material size Weight Volume range (mm) % Below 40 49 20 40 to 225 47 63 Above 225 4 17 100 100 The importance of keeping the 225 mm apertures clear will be appreciated from these figures which show that more than 60% by volume of the refuse passes through these apertures.

Claims (9)

1. A rotary screen for the grading of material having a range of sizes, said screens being rotatable about a longitudinal axis and having a plurality of longitudinally-extending lands, and a plurality of circumferentiallyextending lands said lands defining therebetween a plurality of apertures each having a throat, said longitudinally-extending lands being so shaped that the width thereof as measured overthe radially outer surface thereof in a circumferential direction exceeds the circumferential width of the throats of the apertures circumferentially adjacent thereto.

2. A rotary screen according to claim 1 wherein the land width as measured over the radially outer surface of the lands exceeds the circumferential width of the throats of the circumferentially adjacent apertures by a factor of at least 1.2:1.

3. A rotary screen according to claim 1 or claim 2 wherein the said radially outer surfaces of the longitudinally extending lands are smoothly arched.

4. A rotary screen according to any one of claims 1 to 3 including resiliently deflectable members mounted adjacent the screen and of sufficient length alternately to project into the apertures between the longitudinal lands of the said screen and to be resiliently deflected over the surface of the longitudinally extending lands as the screen rotates about its axis.

5. A rotary screen according to claim 4 where the said resiliently deflectable members are formed of a stiff rubber material.

6. A rotary screen according to any one preceding claim wherein at least in part the longitudinallyextending lands and the circumferentially-extending lands are formed integrally from sheet material.

7. A rotary screen according to claim 6 wherein the radially outer surface of the said longitudinally extending lands is provided by distinct elements attached thereto.

8. A rotary screen according to any one of claims 1 to 5 formed from a plurality of distinct members forming the said longitudinal land mounted on a plurality of longitudinally spaced annular members forming the circumferentially extending lands.

9. A rotary screen substantially as hereinbefore described with reference to the accompanying drawings.