A typical screening device of this kind can comprise a pressurized screen housing with a chamber, in which the screening means is located. On a rotor shaft inside of the screening means a rotary rotor means is mounted, which suitably comprises a pulse transducing element of some kind, as for example wings or buckles.

The screening means divides in radial direction the interior of the screen housing into an accept chamber located outside the screening means and a screen chamber located inside the screening means.

The screening device further comprises an inlet means for the supply of fibrous suspension to the screening device, at least one reject outlet means for reject from the screening device and at least one accept outlet means for accept from the screening device. The pulp suspension to be screened is introduced via the inlet means to the screen chamber, where the accepted fraction, the accept, flows through the screening means into the accept chamber.

At rotation of the rotor means the pulse transmitting elements create pulses, which assist in moving the accept through the screening means.

At screening devices of this kind problems arise with the thickening of the pulp at one end of the screening means, the reject end. The pulp passing along the screening means is dewatered and thereby thickened at its passage along the screening means in the direction to the reject outlet.

The thickening causes a deteriorated functioning of the screening means, owing a. o. to an increase of the friction between the rotor and screening means. In the worst case the rotor can come to a complete standstill.

The thickening also causes wear. When abrasive components, for example sand, are included in the pulp suspension, a screening means and/or pulse transmitting means can be worn down very rapidly.

For solving these problems, it was tried a. o. to supply dilution water in order to dilute the pulp suspension in the area where the thickening reaches a troublesome magnitude.

It was also tried to develop different types of rotor wings intended to assist in decreasing the thickening.

The present invention proposes a device where the problem of thickening and wear is considerably reduced or entirely eliminated.

At screening means according to known art the diameter of the screening means is about 1 to 1,5 times greater than its height.

It was found at experiments, that the problems of thickening are substantially smaller when the dimensions of the screening means are changed so that the diameter of the screening means is made still greater than its height.

The characterizing features of the invention appear from the attached claims.

A preferred embodiment will be described in the following with reference to the accompanying drawing where: Figure 1 shows a screening device with a screening means according to the invention, Figure 2 shows a screening means according to the invention.

The screening device in Fig. 1 comprises a pressurized screen housing 1, in which a stationary screening means 4 is located. Inside of the screening means 4 a rotary rotor means 2 is mounted on a rotor shaft 3. The rotor means 2 comprises as pulse transmitting elements wings 14.

The screening means 4 divides in radial direction the interior of the screen housing into an accept chamber 5 located outside the screen means 4 and a screen chamber 6 located inside the screening means.

The screening device further comprises an inlet means 7 for the supply of fibrous suspension to the screening device, at least one reject outlet means 8 for reject from the screening device and at least one accept outlet means 9 for accept from the screening device.

The screening device also comprises a coarse reject outlet (a scrap trap) 10 for greater particles.

Through a dilution liquid inlet means 11 dilution liquid can be supplied to a dilution chamber 12. The need thereof, however, is reduced or entirely eliminated by the invention.

The screening means can be any kind of screening means, with screening apertures of suitable size for allowing passage of the desired portion of the pulp suspension.

The screening means, for example, can have slits with openings between 0,1 mm and 0,5 mm or holes with hole diameters between 0,1 and 12 mm.

The height A of the screening means 4 in Fig. 2 relates to effective screening height, i. e. to that portion of the screening means 4 where there are screening apertures. The screening means 4 suitably also comprises support rings 13.

The diameter B relates to the inner diameter of the screening means, if the screening means 4 is intended for screening from the inside out, and its outer diameter, if the screening means 4 is intended for screening from the outside in.

The fibrous suspension to be separated is introduced via the inlet means 7 to the screen chamber 6 where the accepted fraction, the accept, flows through the screening means 4 into the accept chamber 5. When the fibrous suspension flows along the screening means in direction to the reject outlet 8, the concentration of fibers in the screen chamber becomes higher and higher the closer to the reject outlet the fibrous suspension flows. This is due to the fact that the liquid (water) in the fibrous suspension separates to a higher degree and at the beginning of the screening path flows through the screening means 4 into the accept chamber 5. A certain thickening, thus, takes place.

At the embodiment shown the diameter B is about 3 times greater than the height A.

Due to the low height of the screening means 4, a problem with great thickening at the screening means 4 does not arise. The fibrous suspension has no time to thicken so much on the relatively short path.

The problem with thickening decreases already at a screening means where the diameter B is 1,8 times greater than the height A. The diameter B, however, advantageously can be 2 or even 2,5 times greater than the height A, and still more suitably 3 times or 4 times greater than the height A.

The diameter B, however, shall not be more than 10 times greater than the height A, or preferably 5 times greater than the height A, because the screening means 4 in this case becomes so low, that all fibers, which should flow through the screening means 4, do not have the time to do so. The screening device then will be given a low capacity.

Typical is a diameter B of the screening means 4 of about 800 mm to about 1500 mm, and an effective screening height A of about 300 mm to about 600 mm. The diameter B, however, can also be from about 150 mm to about 3600 mm, and the height A can be from about 50 mm to about 2000 mm. The diameter B, for example, can be 800 mm, and the height A 300 mm, i. e. the diameter is about 2,7 times greater than the height A.

A screening means designed according to the invention has also advantages from a strength point of view compared to screening means according to known art. The rods in a rod screening basket, for example, then are not so long, which from a strength aspect, of course, is an advantage.

A screening means according to the invention can also be obtained in that a screening means according to known art partially is blinded so that a lower effective screening height is obtained and, thus, the relation between the diameter of the screening means and the effective screening height increases. I. e. that the effective screening height A is obtained in that a higher screening means is partially blinded..

The screening means 4 is thereby partially blinded so that an effective screening height A is obtained which is lower than the original height of the screening means 4 so that the diameter B becomes at least 1,8 times greater than the effective screening height A.

The diameter B, however, advantageously can be 2 or even 2,5 times greater than the height A, and still more suitably 3 times or 4 times greater than the height A. The diameter B, however, shall not be more than 10 times greater than the height A, or preferably 5 times greater than the height A By partially blinding one in a screening device existing screening means it is possible, without rebuilding the screening device or investing in a new screening device, to get a screening device as works much better.

A screening device of the stated kind, of course, can comprise more than one screening step.

The screening device can also be designed so that the screening means is intended to be rotary. The screening device and the screening means can also be designed so that the fibrous suspension is intended to flow from the outside in through the screening means.

The accept outlet means, reject outlet means and inlet means, of course can be located in places other than indicated at the embodiment shown. The number of accept outlet means and reject outlet means and their location is determined by the design of the screening device and by the number of screening steps it contains.

The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the claims with reference to description and Figures.