GB2241905A

GB2241905A - Rotary screen

Info

Publication number: GB2241905A
Application number: GB9104976A
Authority: GB
Inventors: Kenneth Albert Eden
Original assignee: Tomkinson & Co Ltd E
Current assignee: Tomkinson & Co Ltd E
Priority date: 1990-03-16
Filing date: 1991-03-08
Publication date: 1991-09-18
Anticipated expiration: 2011-03-08
Also published as: GB9104976D0; GB2241905B; GB9006026D0

Abstract

A rotary screen, for removing solids from sewage or river water, comprises a perforate cylinder 16 which receives the liquid from duct 12 at one end and an imperforate cylinder 17 attached to cylinder 16 at one end and closed by wall 18 at the other. Cylinder 17 has internal vanes 21, inclined to the radii, which scoop up the heavy solids, while cylinder 16 retains the lighter solids as liquid flows through it into chamber 13. A chute 25 carries a scraper or rake which engages cylinder 16 and receives the solids from cylinders 16 and 17. Cylinder 16 may comprise axially spaced rings of wedge-section, with wide edges inside or outside. Instead of the scraper or rake, material can be washed from cylinder 16 by an external water jet. <IMAGE>

Description

SCREENS This invention relates to rotatable screens for use primarily, but not exclusively, in separating solid elements such as rags and the like from the liquid phase, primarily water, of the inlet flow into a sewage treatment plant.

A previously proposed rotatable screen for use in screening the flow entering a sewage treatment plant is known as a cup screen and comprises a cylindrical member of circular cross-section, mounted for rotation about its longitudinal axis, which axis is positioned horizontally.

One axial end of the cylinder is closed by an end wall, and the opposite, open end of the cylinder is presented to the inlet flow, the cylinder being mounted such that the inlet flow all enters the cylinder. The cylindrical wall of the cylinder is perforated with the intent that the liquid phase of the flow will pass through the perforations leaving rags and like solid matter held against the inner surface of the cylinder by differential water pressure. As the cylinder slowly rotates the expectation is that rags and solid material adhering to the inner surface of the screen will drain and ultimately, after about one half of a revolution of the screen, will drop under gravity into a collection chute.

It is recognised that inevitably the liquid flowing to the screen will contain solid elements which are too dense to adhere to the inner surface of the screen and others which will float in the liquid and thus will not attach themselves to the screen. In order to Separate such dense, or floating elements the inner cylindrical surface of the screen is provided with a plurality of equi-angularly spaced, axially extending, radially inwardly projecting walls or paddles which physically collect the heavy, or floating objects and carry them upwardly as the screen rotates. Unfortunately, the provision of the paddles separates the interior of the screen into a plurality of circumferentially spaced compartments thus precluding the use of a simple scraper or the like engaging the inner surface of the screen, as the screen rotates, to dislodge materials adhering to the screen.In order therefore to clean the screen it is usual to provide continuously operating water jets or sprays directed against the exterior of the screen, and through the perforations, to wash adherent material from the interior of the screen into the collection chute.

Thus in addition to the disadvantage of having to provide water jets or sprays there is the attendant disadvantage that materials entering the chute are extremely wet and a proportion of the jetted or sprayed water also flows into the chute, so that collected materials may have to be subjected to an additional dewatering process before they can be disposed of.

It is an object of the present invention to provide a rotatable screen wherein the aforementioned disadvantages are minimised or obviated.

In accordance with the present invention there is provided a screen comprising a hollow cylindrical body of circular cross-section including a first axially extending region having a perforated cylindrical wall and being open at one end to receive the liquid flow to be screened, and a second axially extending region positioned co-axially with, and of larger diameter than, said first axially extending region, said second region communicating at one axial end with the downstream axial end of said first region and being closed at its opposite axial end, said second region being divided internally by a plurality of circumferentially spaced generally axially extending walls to define a plurality of circumferentially extending compartments or buckets, means for mounting said hollow cylindrical body for rotation about its longitudinal axis, chute means for receiving screened material from said first and second axial regions of said body, and rake means for dislodging material adherent to the inner surface of said first axial region.

It will be recognised that since an internal rake can be utilized to dislodge materials adherent to the inner surface of the first axial region then water jets and sprays are not needed thus effecting a saving in both manufacturing and running costs. Moreover since the collected materials are much drier then no additional dewatering stage is needed and the materials can be processed by a screw compactor for subsequent disposal.

In the conventional cup screen the perforated cylinder is a cylindrical surface pierced with circular or rectangular holes. It is found that fibrous material in the flow being screened tends to become entangled in and around the holes, restricting the flow through the screen and proving difficult to remove, and it is an object of preferred embodiments of the invention to provide a rotatable screen wherein this problem is minimised.

Thus preferably said perforated cylindrical wall of said first axial region of said body is defined by a plurality of axially spaced hoops, the perforations being the gaps between the hoops.

Preferably said hoops are of wedge-shaped cross-section.

Desirably the hoops are arranged with their narrowest regions radially outermost so that the wedge-shaped gaps defined between adjacent hoops diverge from the inner surface of the first axial region.

Alternatively the hoops are arranged with their narrowest regions radially innermost.

Preferably said walls dividing said second axial region into circumferentially extending compartments are disposed at an angle to radii of the body passing through the respective point of contact of a wall with the cylindrical surface of the second axial region of the body.

Desirably the chute means is an inlet chute of a screw compactor.

One example of the invention is illustrated in the accompanying drawings wherein Figure 1 is a diagrammatic perspective view of a rotating screen, Figure 2 is a diagrammatic side-elevational view, partly in section, of the screen illustrated in Figure 1, Figure 3 is a diagrammatic transverse sectional view of part of the screen shown in Figure 1, and Figure 4 is an enlarged sectional view on the line 4-4 in Figure 1.

Referring to the drawings, the inlet flow of liquid, basically water, with entrained solid material including rags and the like flows into the sewage treatment plant along a channel 12 in a cast concrete screen base 11.

The channel 12 terminates, within the base, in a chamber 13, the chamber 13 being of greater depth and width than the channel 12. The channel 12 is of part circular cross-section and thus defines a part circular step 14 at its junction with the chamber 13.

The rotatable screen is supported partially within the chamber 13 by a structure (not shown) anchored to the base 11. The rotatable screen is of cylindrical form, and is supported for rotation about its longitudinal axis with its longitudinal axis positioned horizontally and substantially parallel to the axis of the channel 12.

The rotatable screen includes a rotatable cylindrical body 15 having first and second co-axial cylindrical regions 16, 17. The first cylindrical region 16 comprised a perforated cylindrical wall and the body 15 is so positioned that an open axial end of the region 16 is adjacent the step 14 such that the inlet flow passing along the channel 12 all enters the region 16. The region 17 is of larger diameter than the region 16, and communicates with the region 16 at its axial end remote from the step 14. Te opposite axial end of the region 17 is closed by an axial end wall 18, and the difference in diameter between the regions 16 and 17 is closed by an annular wall 19 parallel to the wall 18.

The parallel walls 18 and 19, and the cylindrical wall of the region 17 are imperforate.

It will be recognised that internally the region 17 defines an annular chamber lying spaced axially from, and extending radially outwardly from the inner surface of the region 16. The annular chamber is divided by a plurality of equi-angularly spaced axially extending walls or paddles 21 to define a circumferentially extending row of compartments or buckets 22. The walls 21 may be positioned radially with respect to the region 17, but preferably, as shown in Figure 3, the walls 21 are disposed at an angle to respective radii of the regions 17 passing through the points of contact between the walls 21 and the cylindrical wall of the region 17.

The inclination of the walls 21 is such that their radially innermost ends are disposed forwardly of their radially outermost ends in the direction of rotation of the rotatable screen, this inclination of the walls providing a "scoop-like" action.

Extending transversely in front of the open end of the region 16 is the barrel 24 of a screw compactor 23.

Extending from the barrel 24 into the rotatable body 15 is an arcuate tapering chute 25 which extends axially to the end wall 1# of the region 17, and the arcuate extent of which is approximately 900 from the top point of the rotatable body 15 in a direction opposite the rotation of the body 15. The inclination of the walls 21 within the region 17 is such that during rotation of the body they do not reach a horizontal orientation until after they have passed the forward edge of the chute 25.

The operation of the rotating screen is as follows. The inlet flow to the sewage treatment plant, which is basically water carrying solid materials including rags, dense objects which sink and are carried by the flow, and less dense floating objects, passes along the channel 12, and flows over the step 14 into the rotating body 15.

The open axial end of the region 16 is radially outward from the step 14 by a small distance, and is axially close to the step 14. Thus the flow in the channel 12 all passes into the body 15, and the only route by which liquid can pass into the chamber 13 is through the perforated cylindrical wall of the region 16. Thus solid material in the flow is retained by the body 15, rags and like material which can adhere to the perforated wall of the region 16 will do so by virtue of differential hydraulic pressure between the interior and exterior of the region 16, while objects which are too dense to adhere to the screen will roll into the lowermost compartments or buckets 22.Since the end walls 18, 19 and the cylindrical wall of the region 17 are imperforate then the compartments or buckets 22 will contain water and any objects floating in the water will be collected by the walls 21 as the body 15 rotates. Any objects collected within a compartment or bucket 22 will remain therein during rotation of the body and will lodge against a wall 21 until such time as that wall 21 passes through a horizontal orientation. Thereafter of course the objects will fall from the wall 21 and will be caught by the chute 25. Similarly, rags and similar material adhering to the inner surface of the region 16 will have drained, and therefore will have been substantially dewatered by the time they align with the chute 25.At this point some of the material adhering to the inner surface of the region 16 will fall under gravity into the chute 25, but rake means in the form of a rake or scraper blade 26 is fixed relative to the chute 25, adjacent the trailing end of the chute 25 and co-acts with the inner surface of the region 16 to dislodge any remaining adherent material clinging to the inner surface of the region 16.

It will be recognised that by comparison with the conventional cup screen described above the volume of liquid delivered into the chute is minimal, since the compartments 22 will substantially have drained before discharging into the chute 25. Moreover, efficient cleaning of the perforated component of the screen can be achieved by the use of the rake or scraper blade 26, it being understood that a similar rake or scraper blade could not be utilised in the aforementioned conventional cub screen owing to the presence of the axially extending walls.

Figure 2 illustrates diagrammatically the provision of supporting bearings 27 whereby the body 15 is mounted for rotation about its horizontally disposed longitudinal axis through to the intermediary of a support structure anchored to the base 11. Also shown diagrammatically in Figure 2 is a drive arrangement in which a power source, conveniently an electric motor 28 drives the body 15 either continuously, or in a series of steps, through the intermediary of a reduction gear drive transmission 29.

It will be recognised that the perforated region 16 of the body 15 can take a number of forms. For example, the region 16 could be formed from a strip of stainless steel which has been punched to provide therein a plurality of circular apertures, and which is then bent to the appropriate cylindrical form. As an alternative, the perforated region 16 could be formed from synthetic resin material. However, Figures 1 and 4 illustrate a preferred construction of the perforated region 16 in which the region 16 is constructed from a plurality of axially spaced circular hoops 31 of stainless steel, each hoop 31 being of wedge-shaped cross-section with its narrowest point radially outermost and its widest point radially innermost.The hoops are positioned spaced apart from one another with their axis co-extensive, and are secured together by being welded or otherwise affixed to a plurality of equi-angularly spaced external, axially extending tie bars 32. It will be understood that the widest surfaces of hoops 31 define the interior of the region 16, and thus the gaps between the hoops taper from a minimum width at the inner surface of the region 16 to a maximum width at the external surface of the region 16.

This tapering gap between the hoops defining the perforated region 16 is found to exhibit a self-cleaning action in that any article which can pass outwardly between the hoops will not wedge between the hoops and of course any item which cannot pass through the gap between hoops will be collected in the region 17 or will be subsequently removed from the interior of the region 16 by the rake or scraper 26.

An advantage of the arrangement described above is that solid material separated by the screen is deposited in the chute 26 with a relatively low water content by comparison with conventional cup screens. The delivery of solid material with a relatively low water content permits the use of a screw compactor for compacting the screenings without the need for an intermediate dewatering apparatus that would be needed to enable a screw compactor to operate efficiently upon the screenings from a conventional form of cup screen.

It is contemplated that a variant of the cup screen described above may be used in an unorthodox arrangement where the screen is subjected to a sparge system of cleaning (utilizing an external high pressure water jet or jets) to wash material adhering to the screen, inwardly into the chute 25. The screen construction differs from that described above in that to facilitate sparge cleaning the hoops 31 are of reversed crosssection such that the gaps between hoops taper from a minimum width at the outer surface of the region 16 to a maximum width at the inner surface of the region 16.

Naturally the additional water entering the chute 25 from the sparge jet or jets may necessitate the provision of dewatering equipment in association with the compactor 23.

Although the rotatable screens described above is primarily intended for use in sewage treatment, it may, with minor modifications if necessary, find use in other liquid treatment plants, for example an industrial waste separation plant, or at the inlet of a water treatment plant where, for example, river water is to be screened before being passed for purification for industrial or domestic use.

Claims

1 A screen comprising a hollow cylindrical body of circular cross-section including a first axially extending region having a perforated cylindrical wall and being open at one end to receive the liquid flow to be screened, and a second axially extending region positioned co-axially with, and of larger diameter than, said first axially extending region, said second region communicating at one axial end with the downstream axial end of said first region and being closed at its opposite axial end, said second region being divided internally by a plurality of circumferentially spaced generally axially extending walls to define a plurality of circumferentially extending compartments or buckets, means for mounting sai hollow cylindrical body for rotation about its longitudinal axis, chute means for receiving screened material from said first and second axial regions of said body, and rake means for dislodging material adherent to the inner surface of said first axial region.

2 A screen as claimed in Claim 1 wherein said perforated cylindrical wall of said first axial region of said body is defined by a plurality of axially spaced hoops, the perforations being the gaps between the hoops.

3 A screen as claimed in Claim 2 wherein said hoops are of wedge-shaped cross-section and are arranged with their narrowest regions radially outermost so that the wedge-shaped gaps defined between adjacent hoops diverge from the inner surface of the first axial region.

4 A screen as claimed in Claim 2 wherein said hoops are of wedge-shaped cross-section and are arranged with their narrowest regions radially innermost so that the wedge-shaped gaps defined between adjacent hoops diverge from the outer surface of the first axial region.

5 A screen as claimed in any one of Claims 2 to 4 wherein said hoops are secured together by a plurality of axially extending tie bars provided on the external surfaces of the hoops.

6 A screen as claimed in any one of the preceding claims wherein said walls dividing said second axial region into circumferentially extending compartments are disposed at an eagle to radii of the body passing through the respective point of contact of a wall with the cylindrical surface of the second axial region of the body.

7 A screen as claimed in any one of the preceding claims wherein said walls dividing said second axial region are equi-angularly spaced.

8 A screen as claimed in any one of the preceding claims wherein the chute means is an inlet chute of a screw compactor.

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

Amendments to the claims have been filed as follows 1 A screen comprising a hollow cylindrical body of circular cross-section including a first axially extending region having a perforated cylindrical wall and being open at one end to receive the liquid flow to be screened, and a second axially extending region positioned co-axially with, and of larger diameter than, said first axially extending region, said second region communicating at one axial end with the downstream axial end of said first region and being closed at its opposite axial end, said second region being divided internally by a plurality of circumferentially spaced generally axially extending walls to define a plurality of circumferentially extending compartments or buckets, means for mounting said hollow cylindrical body for rotation about its longitudinal axis, chute means for receiving screened material from said first and second axial regions of said body, and means for dislodging material adherent to the inner surface of said first axial region.

6 A screen as claimed in any one of the preceding claims wherein said walls dividing said second axial region into circumferentially extending compartments are disposed at an angle to radii of the body passing through the respective point of contact of a wall with the cylindrical surface of the second axial region of the body.

9 A screen as claimed in any one of the preceding claims wherein said means for dislodging adherent material comprises rake means.

10 A screen as claimed in Claim 4 or any one of Claims 5 to 8 where dependent upon Claim 4 and wherein said means for dislodging adherent material comprises a sparge system for washing said adherent material from the screen.

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