GB2072033A - Paper pulp screen - Google Patents

Abstract

A screen for paper pulp suspensions has an inlet 3, an outlet 6 for the cleaned suspension, a rejects outlet 7, a disc-like sieve 2 between the inlet and the outlet 6 and a revolving rotor 9. The rotor has sieve-cleaning propeller-like blades 10 having an angle of incidence which is such that a rotating low or high pressure area is created between the blades 10 and the surface of the sieve. The blades can be arranged upstream or downstream of the sieve and are twisted so that their angle with the sieve decreases radially outwards. <IMAGE>

Description

SPECIFICATION A disc sorter for cleaning fibre suspensions The invention relates to a disc sorter for cleaning fibre suspensions having an inlet, and an outlet for the cleaned suspension, a disc-like sieve between inlet and outlet, a revolving rotor with cleaning arms arranged co-axially to the sieve axis in front of or behind the sieve, the cleaning arms scraping over the surface of the sieve.

Sorters with disc-like sieves are used in the paper industry for cleaning, sorting and deflaking (e.g.

DE-AS 2237 521, US-PS 2 180 080).

The rotors of these sorters are provided either with star-shaped hubs having plate-like members attached to the ends of their arms, their front ends being arranged substantially vertically to the plane of the sieve. In their simplest form the rotors comprise of a multi-arm star which is flame cut out of sheet metal with the cut extending vertically to the plane of the sheet.

These rotors with their cleaning arms serve both to keep the sieve clean for the sorting operation as well as for comminuting, fibrillating or deflaking the larger components of the suspension.

However, these rotors have the disadvantage in that the cleaning arms exhibit a high degree of flow resistance and therefore high energy consumption.

Furthermore, the small distance of the cleaning arms from the sieve results in extensive wear of the sieve screen.

Another disadvantage with known disc sorters is the fact that the peripheral speeds along the cleaning arms vary. The smallest diameter of the perforated sieve surface determines the cleaning arm speed and thus the number of rotations necessary for keeping the sieve free. If the large diameter were used, the peripheral speed of the cleaning arms would then be unnecessarily high and would thus lead to increased energy consumption.

The present invention therefore seeks to provide a disc sorter of the aforementioned type in which the energy requirement can be reduced while still maintaining the present throughput capacity.

Accordingly the invention provides A disc sorter for cleaning fibre suspensions having an inlet, an outlet for the cleaned suspension, a disc-like sieve between inlet and outlet, a revolving rotor with cleaning arms arranged co-axially to the sieve axis in front of or behind the sieve, the cleaning arms scraping over the surface of the sieve, the cleaning arms being in the form of propeller-like blades, having an angle of incidence which is such that when arranged in front of the sieve this creates a low pressure area between the blades and the surface of the sieve and when arranged behind the sieve a high pressure zone is produced between the blades and the surface of the sieve.

Propeller-like blades have a diameter which exhibits good flow characteristics and therefore reduced flow resistance. This leads to a reduction in the energy requirements. Furthermore, less wear takes place at the sieve surface. The wear on the blades themselves is also reduced. For this reason it is not necessary to apply reinforcements to the front edges as is customary with conventional rotors.

Unlike the use of centrifugal speed which was hitherto used for keeping the sieve free, the propeller-like blades of this invention utilise the principle of suction and pressure produced by a propeller-like blade. It is merely necessary to select the blade arrangement with respect to the disc-like sieve as to whether the blades are arranged before or after the sieve. If they are in front of the sieve, i.e. on the flow-in side, the blades must produce a low pressure so that impurities which have become deposited on the surface of the sieve can be released and allowed to reach an outflow opening.

In reverse, when the rotor with its blades is arranged behind the sieve, i.e. on the discharge side, the angle of incidence of the blades has to be such as to produce a positive pressure area in the direction towards the sieve surface. This positive pressure then acts on the sieve in a penetrating manner, thus likewise removing any impurities.

In the case of sorters having a cylindrical sieve drum it is already known to arrange a rotor co-axially to the axis of the sieve basket which has blade-like cleaning arms fixed to it and which revolve on a circular path in front of the sieve basket. However, this does not relate to propeller-like blades. Furthermore, the problem was not of how to keep the sieve sufficiently free because the peripheral speed of the blades along the sieve basket was always the same.

Hitherto it has been the belief that with disc sorters, rotors should be used in which the cleaning arms form a sharp front edge which scrape as close as possible along the sieve surface. The belief was held that only with arrangements of this type was it possible to achieve sufficient sieve clearance, even at different peripheral speeds especially at lower peripheral speeds. The hitherto used sharp edged rotors were arranged in such a way that for keeping clear the sieve holes they utilised the high speed component directed centrifugally parallel to the sieve surface.

It was believed that the correct flow around an aerodynamically favourably shaped body which is directed parallel close to an adjacent surface, sieve surface in this case, is disturbed to such an extent that the suction and pressure forces necessary for keeping the sieve free are too much impaired.

However, it was surprisingly discovered that the forces which are produced by a blade shape so as to exhibit favourable flow characteristics when working in the vicinity of the wall are much stronger than had been anticipated. These forces are much more effective for keeping the sieve free than the conventional cleaning arms.

An additional advantage furthermore is the fact that the suction forces of the aerodynamically formed blades are still effective even when a substantial distance away from the sieve surface. This enables a larger gap to be provided between the blade and the sieve screen which facilitates fitting and which prevents impurities from becoming embedded and also reduces wear.

A further development of the invention provides that the angle of incidence of the blades becomes smaller from inside outwards and in particular in such a way that approximately the same suction or pressure effect exists everywhere in the radial direction along the sieve surface.

According to a preferred feature of the invention the angle of incidence is now changed. By selecting a higher angle of incidence in the area of the rotor hub where the peripheral speed is low, an adequate suction effect is achieved while the angle of incidence becomes progressively smaller from the inside towards the outside. In this way it is possible to even out the different peripheral speeds so as to achieve the same strong suction effect everywhere.

The propeller-like blades contain a sufficient twist.

This feature achieves additional savings in energy.

According to another preferred feature of the invention it is further proposed that the front edges of the blades be inclined towards the rear extending from the inside to the outside against the direction of rotation.

This feature ensures that no whirling impurities become lodged in the blades. Impurities which deposit themselves, as a result of the centrifugal forces, slide towards the rear and are deflected on the outer circumference.

According to another preferred feature of the invention it is proposed to trim the front edges of the blades up to the point closest to the sieve.

With the selected shape of the blades the point closest to the sieve is not located on the front edge of the blades, but is located a small distance behind it.

If the front edge of the blades is trimmed right up to the point closest to the sieve, it is thereby prevented that large impurities such as pieces of wood, branches, wires, sheet metal etc. become lodged between the sieve surface and the blades, or cause excessive wear and tear.

It is an advantage in this case when the trimmed front edge extends backwards at an angle in a way that the remaining front edge has an angle of less than 90s.

This feature reduces even more the danger of wear and pinching.

For energy saving it can also be an advantage if the blade width diminishes from the centre outwards.

Afurther advantage results when the blade ends are provided with a terminal disc for reducing the pressure euqualisation between the pressure and suction side.

The blades according to the invention can also be used for multiple rotors. For example, in a disc sorter with the two parallel disc-like sieves arranged opposite each other between which is arranged a rotor with two rows of cleaning arms, each row being arranged with respect to one sieve surface, the blades of one row being arranged in the circumferential direction respectively displaced with respect to the other row.

This measure prevents the mutual interaction of the blades.

A further very advantageous embodiment of the invention consists in the fact that the outer ends of the blades are provided with terminal pieces arranged approximately vertically to the longitudinal axis of the blade and tangentially to the blade end.

This measure serves to maintain the low pressure at the blade ends since this substantially reduces the flow from the positive to the negative pressure and this also prevents the formation of a vortex, and thus energy losses are prevented.

By way of example, a specific embodiment of the invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through an embodiment of a disc sorter according to the invention; Figure 2 is a plan view of a rotor with propeller-like blades; Figure 3 is an enlarged cross-section through a blade; Figure 4 is a side elevation of a blade in the direction of the arrow in Figure 2; and Figure 5 is a fragmentory sectional view on line V-V of Figure 2.

The disc sorter has a housing 1 in which is arranged a stationary disc-shaped sieve 2. Through an inlet 3 the suspension to be cleaned reaches the cleaning chamber 4. Behind the sieve 2 there is provided a collector chamber 5 from which the cleaned suspension is removed from the housing 1 through outlet 6.

The cleaning chamber 4 is furthermore provided with an outlet 7 for removing the separated impurities. Through a drilling in the housing 1 or a housing lid, a shaft 8 enters the interior of the housing 1.

Shaft 8 is rigidly connected with rotor 9 which has propeller-like blades 10. The blades are twisted in such a way that the angle of incidence in the area of rotor 9 is largerthan in the area of the outer circumference. This becomes clear from the drawing in Figure 1.

As can furthermore be seen from Figure 2, the front edges 11 or 11 of the blades 10 are inclined from the inside towards the outside against the direction of rotation. Similarly, the blade width diminishes from the centre outwards.

In Figure 3 there is shown an enlarged blade 10 in cross-section. As can be seen, the front edge 11' of the blade 10 is trimmed off up to the point 12 nearest to the sieve. The trimmed front edge extends backwards at an angle. The broken line depicts the shape of a blade 10 which is not trimmed.

At their ends the blades 10 are provided with terminal members 13 which are arranged approximately vertically to the blades longitudinal axis and at a tangent with the same. The terminal members are roughly triangular in shape, one side being arranged parallel with the sieve and at a small distance from the same. The arrangement and construction of the terminal members 13 is illustrated particularly in Figures 4 and 5.

For a twin disc sorter with a common cleaning chamber 4 it is necessary merely to attach a second sieve opposite to and in parallel with the disc-shaped sieve 2. In this case the rotor 9 carries a second row of blades 10 which rotate in front of the second sieve.

The propeller-like blades 10 essentially utilise the properties of a propeller, namely intense suction forces vertically to the propeller plane and low resistance in the individual blades.

The invention is not restricted to the details of the foregoing embodiments.

Claims (9)

1. A disc sorter for cleaning fibre suspensions having an inlet, an outlet forthe cleaned suspension, a disc-like sieve between inlet and outlet, a revolving rotor with cleaning arms arranged co-axially to the sieve axis in front of or behind the sieve, the cleaning arms scraping over the surface of the sieve, the cleaning arms being in the form of propeller-like blades, having an angle of incidence which is such that when arranged in front of the sieve this creates a low pressure area between the blades and the surface of the sieve and when arranged behind the sieve a high pressure zone is produced between the blades and the surface of the sieve.

2. A disc sorter according to Claim 1, in which the angles of incidence of the blades decrease from the centre outwards in such a way that approximately an even suction or pressure effect is achieved in the radial direction along the surface of the sieve.

3. A disc sorter according to Claim 1 or 2, in which the front edges of the blades are inclined backwards from the centre outwards against the direction of rotation.

4. A disc sorter according to any one of Claims 1 to 3, in which the front edges of the blades are trimmed up to the point lying closest to the sieve.

5. A disc sorter according to Claim 4, in which the trimmed front edge extends backwards at an angle in such a way that the remaining front edge makes an angle smaller than 90 .

6. A disc sorter according to any one of the Claims 1 to 5, in which the blade width decreases from the centre outwards,

7. A disc sorter according to any one of Claims 1 to 6, having at least two parallel and oppositely arranged disc-shaped sieves between which rotates the rotor having at least two rows of cleaning arms, each row being provided for one sieve surface, the blades of each row being arranged in the peripheral direction respectively displaced with respect to the other row.

8. A disc sorter according to any one of Claims 1 to 7, in which the outer ends of the blades are provided with terminal members arranged approximately vertically to the longitudinal axis of the blades and at a tangent with the end of the blade.

9. A disc sorter constructed and arranged substantially as herewith described with reference to the accompanying drawings.