AU1226483A - Silmaskin - Google Patents

Description

SCREEN MACHINE

The present invention concerns a screen machine, preferably for cellulose pulp and recycled pulp including a screen roto which is rotatable within a housing having an inlet for un¬ screened suspension, an outlet for accepts and adischarge opening for rejects.

From U.S. Patent Specification 1,429,707 is known a pulp screen of this kind having a cylindrical screen drum and self clearing action. The pulp is fed into the drum in one end thereof and is forced to rotate with the drum by means of blades. By the action of centrifugal force fibers and water are forced radially outwards through the screen slots. The casing of the machine is elliptical, and, thus, at two points the casing is located closer to the drum than at other points. At these two points, therefore, the screen¬ ed pulp located outside the drum and also rotating there¬ with will be "compressed" and cause a back-flow radially inward through the screen slots. Thereby these are cleared fr too big fibers etc. which are brought out through a dis- charge at the other end of the drum.

The screen machine according to the present invention also has a self-clearing action which, however, has been accom¬ plished by the utilization of centrifugal force. The cha- racteristic features of the invention will appear from the annexed claims.

The basis conception of the invention, thus, is to clear screen holes in a screen rotor by means of centrifugal force immediately after each screen zone, within which screening takes place against the action of centrifugal force. Cylindrical as well aε conical screen drums can be contemplated, as well as disc-shaped screen rotors, in the latter case, of course, the screen holes will have to be inclined against the axis of rotation in order to obtain centrifugal pumping action.

An embodiment of the invention will now be described, reference being made to the accompanying drawing, wherein

Fig. 1 is an axial section through a machine according to the invention, and

Fig. 2 is a section along line II-II of Fig. 1.

In fig. 1 is shownamachine casing 10, in which a shaft 12 driveableby a pulley 11 is journalled in bearings 13, 14. In the other end of shaft 12 is mounted a screen rotor 15 for rotation with the shaft, said rotor being concentric¬ ally located within a substantially cylindrical screen housing 16.

In the example shown the shaft 12 is horizontal and the screen housing 16 is provided with an inlet 17 for un¬ screened stock located at the top of the periphery, a central outlet 18 for screened stock (accepts) coaxial with the screen rotor 15, and a selectively openable dis¬ charge opening 19 for non-screenable goods (rejects) . The screen rotor 15 has a-tight hub disc 20 attached to the shaft 12 and an envelope plate 21 arranged at the peri¬ phery of the hub disc, the envelope disc being provided with a plurality of screen openings 22. Straight in front of the hub disc 20 the screen rotor 15 is open towards the wall 23 of the screen housing,in which the outlet 18 is disposed. Between the wall 23 and the edge of the envelope plate 21 there is an interspace 24.

The arrangenment is such that the pulp suspension flows from the inlet 17, via the annular space 25 between the housing 16 and the rotor wall 21 through the screen openings, and to the outlet 18, whether the rotor is rotating or not.

OMPI

^ The pressure of the stock at the inlet 17 and within the annular space 25, thus, shall be so much greater than the pressure in the outlet 18, that a flow takes place radially inwards through the screen openings 22 in spite of rotation of the rotor. This flow is indicated in Fig. 2 by little arrows in positions about 12, 3, 6 and 9 o'clock.

Now, according to the invention, within the rotor is arrang¬ ed a number of fixed baffle plates 26, 27, 28, 29 shutting off one sector each of the interior of the rotor from the outlet 18. The baffle plates 26-29, thus, extend from the wall 23 of the screen housing into the rotor and terminate immediately adjacent the hub disc 20.

As mentioned above, there is an interspace 24 between the wall 23 and the open end of the rotor drum. Because of this interspace the pressure of the annular space 25 exists also within the sectors defined by the rotor envelope 21 and the baffle plates 26-29.

At rotation of the rotor in the direction of arrow A in Fig. 2, thus, screening takes place through the four portions of the screen plate 21 that are temporarily located between the baffle plates 29 and 26, 26 and 27, 27 and 28, and, 28 and 29. As in all screens clogging of the screen openings occurs - in this case on the outside of the screen plate 21 due to the screening radially inward.

By means of the baffle plates 26-29 according to the inven- tion, however, after each screening zone is caused an auto¬ matic claring of the screen openings. This clearing is achieved by the action of the centrifugal force on the water columns that are in screen openings within the effective area of the baffle plates. Within the screening zones the centrifugal force is not capable of neutralizing the pressure difference between the outside of the rotor and its inside; this, however, is possible inside the baffle plates, where the outside pressure also prevails on the inside of the baffle plates. Aε soon as the rotor has rotated to a sector defined by a baffle plate, thus, the water column present in the respective screen hole is thruεted out, and with it the fibres, staples and otherthings, that may have clogged a screen opening. Replacement of the water or the suspen- εion thuεly centrifugally pumped out of the εpace between the baffle plates and the drum wall 21 takes place by flow through the interspace 24 as is indicated by little arrows in positions about 11, 8, 5 and 2 o'clock in Fig. 2.

In order to restrict the arc along which the centrifugal pumping takes place, plates 30-33 adapted to the curvature of the drum 21 are inεtalled along the latter part of the effective εector of the reεpective baffle plate. Hereby is achieved that particles that have just been thrusted out do not immediately turn inwards, but are given the oppor¬ tunity to undisturbedly leave the immediate vicinity of the screen openings.

Advantageously the edges of the plates 30-33 turned against the direction of rotation of the rotor are inclined towards the interspace 24, so that flow into the rotor is counter- -acted and some scraping action is obtained. In Fig. 1 the baffle plate 28 is shown-broken up so that the plate 32 with its inclined edge 32* is visible.

Advantageously scrapers can be arranged on the outside of the screen rotor at suitable locations along the clearing zones. An example of such a εcraper 34 is shown in Fig. 2 within the clearing zone of the baffle plate 26.

Summing up, the machine according to the invention has the following features:

- the suεpension to be screened is supplied on one side of the rotor such that screening takes place radially inward^; _S^SSE

OMP - screening flow through the screen rotor exclusively takes place due to the difference in suspension presεure at the inlet and outlet of the machine;

- intentional rotation of the suspension does not take place, neither inside nor outside the rotor;

the centrifugal force, that everywhere acts on the sus¬ pension which temporarily is present in the screen openings and, thus, there rotates with the rotor, is allowed to act only within those zones where the baffle plates separate the rotor from the pressure of the out¬ let which is low relative to the pressure of the inlet.

Particles that are cleared off the rotor are collected at the discharge opening 19 and may at need be discharged by means of e.g. a screw 36.

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Claims (7)

1. A screen machine, particularly for cellulose pulp and recycled pulp, including a screen means (15) rotatable about a rotational axiε and mounted in a housing (16) having an inlet (17) for pulp suεpension, an outlet (18) for accepts, and, a discharge opening (19) for rejects, c h a r a c t e r i z e d in that the screen means (15) is εo arranged between the inlet (17) and the outlet (18) that flow therethrough from its inlet side to its outlet side has a component directed towards the rotational axiε, that at leaεt one εtationary baffle means (26) is arranged on the outlet side of the screen means (15) which baffle means εeparateε a sector of the screen means from the out¬ let (18) , and that along at least a part of this sector there is communi¬ cation between the inlet (17) and the space between the rotor (15) and the baffle means (26), whereby the flow, taking place against the action of the centrifugal force at rotating screen means, is discontinued -within the sec¬ tions mentioned and is replaced by a counter-directed flow dependent on the centrifugal force.

2. A screen machine according to claim 1, c h a r a c t ¬ e r i z e d in that the-screen means is a drum (21) and that the communication mentioned takeε place around an edge of the drum.

3. A screen machine according to claim 2, c h a r a c t ¬ e r i z e d in that the communication takes place through an interspace (24) between a wall (23) of the housing (16) and the drum edge mentioned.

4. A screen machine according to claim 2 or claim 3, c h a r a c t e r i z e d in that the drum (15) is open at one end towards the wall mentioned (23) , and that the baffle means (26) , is attached to this wall and extends in the direction of the drum axis to the other end (20) of the drum.

5. A screen machine according to claim 3 or claim 4, c h a r a c t e r i z e d in that the outlet (18) is coaxial to the drum (15) and located in the wall (23) mentioned, and that a plurality of evenly distributed baffle means (26-29) is arranged within the drum (15).

6. A screen machine according to any of the preceding claims, c h a r a c t e r i z e d in that between each baffle means (26-29) and the screen means (21) is arrang¬ ed a fixed obstructing means (30-33) preventing flow through the screen means.

7. A screen machine according to any of the preceding claimε, c h a r a c t e r i z e d in that on the inlet side of the screen means (15) is arranged at least one scraper (34) .