GB2222967A

GB2222967A - Appliance for screening fibrous suspensions

Info

Publication number: GB2222967A
Application number: GB8920948A
Authority: GB
Inventors: Reimund Rienecker
Original assignee: JM Voith GmbH
Current assignee: JM Voith GmbH
Priority date: 1988-09-19
Filing date: 1989-09-15
Publication date: 1990-03-28
Anticipated expiration: 2009-09-15
Also published as: AT396695B; GB8920948D0; ATA216989A; IT8967772A0; GB2222967B; IT1232285B; FR2636987B1; DE3831845C2; FR2636987A1; DE3831845A1

Description

2222967 Screening Appliance for Fibrous Suspensions

The invention relates to a screen of the type having a wire cage with a substantially rotationally symmetrical shape, which is open at both its axial ends (inlet and outlet end) and having a rotor bearing a screening vane or screening components, the axis of rotation of which is the same as the axis of rotational symmetry. A screen of this type for fibrous suspensions is known from U.S. Patent Specification 4,383,918. In this screen a rotor having screening components attached to radial arms is disposed inside the wire cage and said cage has radially extending bars at regular intervals so as to produce a vorticity of the flow', to be exact a microturbulence, which is intended to permit a better separation of the components of the fibrous suspension to be screened, namely the fibres an the one hand and the impurities on the other hand. In this case the entire screen, i.e. which naturally also includes all the wire chambers, is filled with the fibrous suspension, which is supplied at a certain pressure which is at least 0.8 to 1.5 bar above atmospheric pressure. A corresponding pressure is also maintained throughout the screen housing, i.e. these screens have been designated as pressurised screens. An attempt is made to achieve particularly high screening efficiency by using very fine perforations, ie narrow wire slits or small wire holes. However in such a case it is difficult to obtain adequately large throughputs.

The object of the invention is to provide a screen of the type discussed above which has a higher. throughput compared to the prior art.

According to the invention this object is achieved by the features set out in the characterising part of Claim 1.

In the invention it has been recognised that a higher throughput is maintained over the entire length of the wire cage (or height of said wire cage, as these wire cages normally constructed with a vertical axis for the rotor is n-eant its axis relative velocity between wire cage rotational respectively, by which s-,r,n,etry), by a f are or of the 2 screening components or screening vanes respectively and the fibrous suspension (which relates to its peripheral velocity components). German Offenlegungsschrift 37 01 400 has already disclosed a wire cage designed with a diameter which increases towards the outlet end. However this only applies to a wire cage in a combination machine for screening coarse reject with the continual addition of diluting medium, during which a consistency of at least 10 % is achieved in the end of the wire cage which has the greater diameter, and the only result is intended to be great drainage efficiency.

In this case it is important that coarse reject having as high consistency as possible is discharged at the outlet end of the wire cage. As far as possible this consistency should be at least app-roximately 20 %. In this case it is is no longer necessary to screen fibres on a la rge scale because here just the drainage efficiency is still of paramount importance. It is no longer possible to achieve satisfactory screening efficiency here.

The invention is explained below with Figures 1 to 4 by reference to four exemplified embodiments which are principally shown in cross section.

In said figures each screen consists of a wire cage, which is disposed inside a housing 1 and which in many cases is assembled from several parts; rotor components in the form of screening vanes, which can, however, also be made from different screening components supported on radial arms of the rotor; a rotor which is not constructed with arms and which supports the screening vanes, with a wire chamber being formed between the rotor and the wire cage or the parts of the wire cage respectively; and also perhaps baffle rings. Each housing 1 has an inlet E and an outlet f or accept G and for any overf low which does not pass through the perforations of the wire cage, and which is designated by D. In the diagrammatic sketches the position of the t rotor is merely indicated. The screening vanes or components are known from the aforementioned German Offenlegungsschrift 37 01 400 and also from the aforementioned U.S. Patent Specification or also from German Auslegeschrift 27 12 715.

There follows a list of the reference numbers used for the individul components:

Wire cage Screening vanes Rotor drum Wire chamber on the overflow side of the wire cage radial baffle ring inlet end outlet end of the wire cage -2; 12, 12'; 22, 22'; 32, 3V; -3; 13, 13'; 23, 23'; 33, 33'; -4, 14, 24, 34, -6; 16, 16'; 26, 26'; 36, 3P; -43, 53; -101, 103, 105, 107 -102, 104, 106, 108.

The minimum increase in diameter of the wire cage between its inlet end and its outlet end is 50%. As shown in Figure 1 a conical wire cage and also a conical rotor drum 4 are provided. The peripheral velocity of the screening vanes 3 also increases in conformity with the increase in diameter. By their rotation they produce an ever increasing peripheral velocity towards the outlet end of the wire cage,' while conveying the fibrous suspension. However the constant increase in the peripheral velocity of the screening vanes towards the outlet end always maintains a considerable difference of at least 5 rn/s between said velocity and the peripheral velocity of the fibrous suspension. As a result the throughput of pre-screened accept is not continually reduced to the same extent towards the outlet end of the wire cage as is the case with known wire caces.

- 4 The same ef f ect is also achieved by the sudden increase in diameter as shown in Figure 2.

Instead of stepped increases, baf f le components may also be provided in the form of rings 43 and 53 in Figures 3 and 4 respectively, which likewise reduce the peripheral velocity (i.e. generally the speed of flow of the fibrous suspension) in wire chambers 26, 26' and 36' by slowing down at the transition between the wire chambers 26 and 26' or 36 and 36' divided by them. In the case of Figure 4 there is the combination of a sudden increase in diameter and a radial baffle ring 53. Both measures supplement a reduction in the peripheral velocity of the fibrous suspension.

Baffle ring 53 is also constructed conically so as to achieve the increase in diameter.

It is also possible to dispose the inlet side of the wire cage radially outside said wire cage and not inside - as shown here. Obviously the rotor and its screening vanes or elements would then be disposed on this side.

It has been ascertained that if conventional, upright wire cages are used, i.e. they are not conical, a respective increase in both the length and diameter of the wire cage of 20 % only produces an increase in throughput of 20 %, and not of 40%, as was to be expected. With the wire cage specified by the invention, there is achieved a minimum increase in throughput of 70 % when compared with said increase in throughput achieved with the normal, enlarged wire cage without increasing the length, but merely by a corresponding increase in diameter.

The overall increase in diameter from the inlet end of the wire cage to the outlet end is at- most 100 % of the diameter at the inlet end, in particular at most 65 %, relative to a length of the wire cage which is equal to the initial 1 diameter thereof. As a result with a wire cage constructed with a uniform increase in diameter, at the production end of the conical shell there is produced an angle of inclination (a) of between a = 20 0 and a = 35 0, which can be described around the wire cage.

At least four screening components or screening vanes respectively or rows of screening vanes or screening components respectively, which are parallel to the rotor axis or are inclined by less than 200 towards the radial planes parallel to the rotor axis, are preferably distributed uniformly over the periphery of the rotor. This is the "normal" design of such screens required to achieve high screening efficiency. However this directly results in the strong rotational movement of the fluid to be screened, more particularly suspension in paper manufacture, inter alia the suspension made from waste paper.

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Claims

1. A screen having a wire cage with a substantially rotationally symmetricalshape, which is open at both its axial ends (inlet and outlet end) and having a rotor bearing a screening vane or screening components, the axis of rotation of which is the same as the axis of rotational symmetry,' characterised in that in the wire chamber (16; 26, 261; 36, 36'; 46, 46') there is maintained a differential velocity of at least 5 m/s between the screening vanes (3; 13, 131; 23, 23'; 33, 33') or screening components (99) and the peripheral velocity components of the suspension to be screened over the length of the wire cage (2; 12, 121; 22, 221; 32, 321) extending between the inlet and outlet end, which is achieved continuously or in stages by structural/ mechanical measures or means.

2. A screen as specified in Claim 1, characterised by a continuous or correspondingly sudden increase in the diameter of the wire cage between its inlet end (101) and its outlet end (102; 103, 104; 105, 106; 107, 108).

3. A screen as specified in Claim 2, characterised in that the total increase in diameter is at least 35 % of the diameter at the inlet end.

4. A screen as specified in Claim 2 or 3, characterised in that the total increase in diameter is at most 100 % of the diameter at the inlet end.

5. A screen as specified in Claim 4, characterised in that th.e i--c-ease in diameter is 80 % of the diameter at the inlet end.

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6. A screen as specified in one of Claims 3 to 5, characterised in that the maximum increase in diameter is 65 % of the diameter at the inlet end for a wire cage length corresponding to the initial diameter.

7. A screen as specified in one of Claims 3 to 6, characterised in that the angle of inclination (a) of the production end of the conical shell of the wire cage towards 0 0 its central axis is between 20 and 35

8. A screen as specified in one of Claims 1 to 7, characterised in that annular baffle components (43,53) which extend radially from the periphery of the wire cage into the wire chamber are provided at the stepped increases or instead of the steps.

9. A screen as specified in Claim 8, characterised in that when there are stepped increases in the region of the baffle components (53) there is a continuous increase in the diameter of the baffle components (53) towards the outlet end of the wire cage (107, 108).

10. A screen as specified in one of Claims 1 to 9, characterised in that the screening vanes (3; 13, jV; 23, 231; 33, 331) are disposed inside the wire cage (2; 12, 12',' 22, 22'; 32, 32').

11. A screen as specified in one of Claims 1 to 10, characterised in that the screening vanes (3; 13, 13': 23, 23'; 33, 331) are attached to a drumlike rotor (4, 14, 24, 34) so that the maximum radial extension of the wire chamber between the shell of the rotor drum and the wire cage is 1.8 times the radial extension of the screening vanes.

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12. A screen as specified in one of Claims 1 to 11, characterised in that- the rotor, which has screening vanes or components, is disposed on the overflow side of the wire cage and the diameter of said rotor increases between the 5 inlet and the outlet end of the wire cage.

13. A screen as specified in one of Claims 1 to 12, characterised in that there is at least one sudden increase in the diameter of the wire cage.

14. A screen as specif ied in one of Claims 1 to 13, characterised by a maximum consistency of 8 % on the oT.-er.'L2low side.

1 D- - A screen as specified in one of Claims 1 to.14, characterised by more than three paraxial rows of screening vanes (3; 13, 131; 23 e-tc) or screening co.mponents on the periphery of the rotor.

16. A screen constructed, arranged and adapted for use substantially as herein before described with reference to, and as shown in, figure 1, 2, 3 or 4 of the accompanying drawing.

Published 1990 a' The PatentOffice, State House, 66 71 High Holborn, LondonWCIR4TP.Ptirther copies maybe obtained from The PatentOffice. Sales Branch. St Mary Cray, Orpington. Kent BR5 3RD. Printed by Multiplex techniques Ad. St Mary Cray. Kent. Con. 1/87