The present invention relates to a straining means, intended to be used in strainers for the straining of pulps from cellulose and paper industries- Straining of suspensions of cellulose fibres is a very important work operation in the pulp and paper manufacturing, whereby impurities and shives, having detrimental effect on the quality of the paper or paper product produced therefrom, are removed. Straining is carried out in various phases of the production of the pulp or the paper by means of different strainers. Thereby, the straining means or strainer plate, if made of metal, is provided with round holes or slots, which form an essential part in strainers. The straining means may be flat, arched, or be the envelope surface of the frustum of a cone or of a cylinder. The separation of impurities from a fibre suspension is mainly carried out in open or closed centrifugal strainers. Hereby, the suspension is set in motion in relation to the plane of the straining means by driving the straining means or by causing the suspension to move in a stationary straining drum. This straining drum may be arranged either vertically or horizontally.

The relative motion between the fibre suspension and the plane of the strainer plate is of great importance for the separation mechanism itself. The flow direction of the fibre suspension may be divided into two vectors, one parallel with and one perpendicular to the straining means. Thereby, the ratio between these vectors, i.e. angle of incidence of the flow towards the straining means, is of great importance for the function of the strainer. The flow vector parallel with the straining means varies with the distance from the straining means. Due to this flow gradient fibres, oblong fibre clusters and shives or other unwanted particles in the suspension will orient them¬ selves in the flow direction.

The mechanism, that separates the impurities from the fibre suspension when these impurities have smaller cross-sections than the openings of the straining means, lies mainly in the above described flow, on approaching the straining means, being forced to change direction and velocity just when passing the straining means due to the circumstance that it approaches the straining means at an angle less than 90° and that the flow is accelerated on passage through the openings of the straining means.

Conventional straining means have even surfaces. The Swedish published patent application No. 7211272-5 discloses a straining means, which is not even. The surface is provided with recesses in order to produce openings arranged on an inclined surface, so that the downstream edge of the openings in the straining means is located lower than the upstream edge. In one application of this invention the surface of the straining means exhibits V-shaped grooves, the openings being located on the upstream side plane of the groove.

The purpose of the present invention is to improve the effectivity of new as well as already existing straining means, like cylindrical straining drums, in strainers used in cellulose and paper industries.

This purpose is attained by a straining means of the kind, stated in the preamble of claim 1, having the features, stated in the characterizing part of said claim. Preferred embodiments appear from the subclaims. The present invention gives rise to straining with less tendency for clogging of existing openings in the straining means.

The invention will be described in the form of examples with the assistance of the drawing, on which Fig. 1 shows a section through a flat straining means according to a preferred embodiment and Fig. 2-4 show other embodiments of the straining means in cross-section.

Fig. 1 shows an enlarged section through a straining means 1, on account of simplicity being shown flat but mostly being curved to a cylindrical straining drum. Such a straining drum is either manufactured from a strainer plate of the shown appearance or from an existing straining drum without grooves, which may have been in operation and later have been provided with the shown grooves. It is also possible to provide a straining drum, having grooves on one side, with grooves on the other side. The upper side 2 of the straining means 1 is its injection side and the lower side 3 is its acceptance side. The suspension to be strained is supposed to have a relative motion in relation to the straining means 1, in close proximity thereto, which may be divided into two flow vectors, one having the direction P, being parallel with the surface of the straining means and pointing to the right in the figures, and one having the direction towards the straining means.

The straining means 1 is provided with openings 4, being circular in cross-section. The opening consists of a cylindrical part 4' closest to the injection side 2 and a frusto-conical part 4", expanding from the cylindrical part 4' towards the acceptance side 3. The two parts 4' and 4" of the opening

4 have a centre line 5 in common. The smallest diameter of the part 4" is equal to the diameter of the cylindrical part 4'. The centre line 5 of the opening 4 forms an angle with the normal N of the straining means 1. It is not necessary for the projection of the centre line 5 of the opening 4 on the straining means 1 to be parallel with the flow vector P of the suspension, as shown in the figure, but the angle between said projection and flow vector P may assume all values of 0-360°. The angle, that the centre line 5 form with the normal, is preferably less than 25°, particularly less than 10°. The angle between the projection of the centre line 5 of the opening 4 on the straining means and the flow vector P of the suspension preferably assumes values of 155-180° or 0-25°.

The openings of the straining means 1 may, of course, have other forms. For example, these openings may be cylindrical or frusto-conical, whereby the base of the frustum of the cone is at the acceptance side. A preferred opening consists of two cylindrical parts with a centre line in common but with different sizes of diameters, whereby the part with the larger diameter is closest to the acceptance side. Even if openings with circular apertures in cross-section are preferred, it may be suitable in certain applications and in special embodiments, that the openings are slot shaped with a width, being constant or increasing continuously or step-by-step in the longitudinal direction of the opening from injection to acceptance side.

The straining means 1 according to Fig. 1 is on its injection side 2 provided with grooves 18 of V-shaped cross-section, the grooves being parallel with each other and forming an angle with the flow vector P of the suspension, whereby the angle may assume values of 50-130°. Thus, these grooves are perpendicular to the vector P or deviates from this direction not more than 40°. Between these grooves 18 the straining means 1 may exhibit the same curvature on its injection side as the straining means. Preferably, two adjacent grooves, 18 and 18' are located very close to each other, i.e. the distance between two adjacent grooves approaches zero, as in Fig. 1. The two legs 18a and 18b of the groove 18 may be of equal or different length. Preferably, the ratio of the lengths of the legs is 1:20 to 20:1, particularly 1:10 to 10:1. Suitably, the openings 4 are arranged in the longer leg if they are of different length and in one of them 18a if the legs 18a and 18b are of equal length. The depth of the grooves 18 depends i.a. on the overall thickness of the straining means and is usually less than 2 mm, particularly less than 1,5 mm.

The straining means 1 according to Fig. 1 is provided with grooves 28 of V-shaped cross-section also on the acceptance side 3. These grooves 28 form an angle of 50-130° with the flow vector P of the suspension on the injection side like the grooves 18 on the injection side do. Thereby, the grooves 28 on the acceptance side 3 form an angle of 0-80° with the grooves 18 on the injection side. According to a preferred embodiment the grooves 28 on the acceptance side 3 are parallel with the grooves 18 on the injection side 2. Preferably, two adjacent grooves on the acceptance side lie against each other with a boundary line in common. The ratio between the lengths of the two legs 28a and 28b, i.e. the width of the two sides of the groove, is 1:20 to 20:1, preferably 1:10 to 10:1. The depth of the grooves is not more than 2 mm, preferably not more than 1,5 mm. The sides 28a, 28b of the groove 28 form an essentially right or obtuse angle with each other. In Fig. 1 the boundary line of the whole opening 4 on the acceptance side 3 lies entirely within one side 28b.

The straining means 11 according to Fig. 2 resembles the one in Fig. 1, the form of the openings 14 having the same appearance as the openings in Fig. 1, but their centre line forming another angle with the flow vector P of the suspension. On the acceptance side 38 the V-shaped grooves are designed in such a way, that the leg 38a is longer than the leg 38b. Further, the boundary line of the acceptance side of the opening 14 touches the line in common, which joins the two sides 38a and 38b of the groove 38. In one embodiment the generatrix of the conical part 14" of the opening 14 is tangential to the side 38b.

Fig. 3 shows a straining means 21 with grooves of U-shaped cross- section on the injection side and the acceptance side 48 and 58 respectively. The openings 24 have a cylindrical form and their centre line is normal to the straining means. Of the two legs 48a, 48b of the groove 48 the leg 48a forms an angle of about 20° with the flow vector P but this angle may, of course, assume higher and lower values and the leg 48b is perpendicular to the flow vector P. The angle of the two legs 48a, 48b with the flow vector may assume the same values as the angle of the legs 18a, 18b of the V-shaped groove 18 in Fig. 1. The waist 48c of the U-shaped groove 48 is parallel with the flow vector P or the plane of the straining means. It is also possible for this plane 48c of the groove 48 to form an angle, greater than zero, with the flow vector P or the plane of the straining means. The longitudinal direction of the grooves 48 forms the same angle with the flow vector P as the grooves 18.

The groo.es 58 on the acceptance side ha\e two legs 58a and 58b, which consitute the two side planes of the groove. The leg 58a is perpendi¬ cular to the plane of the straining means 21. The leg 58b forms an angle of about 30° with the plane of the straining means. The \ alues of the angles, which the side planes 58a and 58b form with the plane of the straining means, \ ary within wide limits. The plane 58c between the side planes 58a and 58b is parallel with the plane of the straining means but may also form an angle greater than zero with said plane. The boundary line of the openings 24 on the acceptance side is entirely within the plane 58c. Hereby, the diameter of the opening may be as large as the length of the waist 58c (the width of the plane 58c) or be smaller than said length, as shown. This boundary line may also intersect the plane 58b alone or both 58b and 58c.

Fig. 4 shows a straining means 31 with cylindrical openings, the centre line of which forms an angle greater than zero with the normal of the straining means. The groo . es 68 and 78 on the injection and acceptance sides, respectiv ely, are U-shaped in cross-section. The upper boundary line of the opening 34 intersects plane 68c only, which is parallel with the plane of the straining means. In the figure the diameter of the opening is as large as the width of the plane 68c. The boundary line of the opening may intersect only one of planes 68c and 68a or both. On the acceptance side the boundary line of the opening intersects the side side plane 78b only but may also intersect the plane 78c.

The openings in the straining means may be manufactured after having prov ided the blank of the straining means with groov es. Alternativ ely, the openings may 3 drilled first and the groov es produced afterwards. The latter is the case when starting from a finished straining drum, which may hav e been in operation, and prov iding it with grooves to improv e the straining properties.

It has been possible under fav ourable conditions to use straining means, which hav e been prov ided with grooves according to the invention, in a pressure strainer without hav ing had a relativ e motion of the drum in relation to wings arranged therein.

The straining means may be composed of a metal or a metal alloy, e.g. of steel, or some other material like plastic.