Naturally, prior art knows several devices used for screening fiber suspension. Refer- ence is here made to solutions according to US patents 5,000,842,5,224,603 and 5,547,083 which are meant for screening fiber suspensions at a relatively high consis- tency, which in the field of screening means a consistency of about 2.5-5 %. Said con- sistency is so high, that in order to maintain the screenability of the pulp, special char- acteristics and construction are required from the pulsation member, i. e. the rotor, to prevent the pulp from forming excessively large and strong fiber accumulations in the screening area. E. g., in the above patents the rotor is essentially cylindrical and on the surface of the cylinder there are so-called bulges arranged according to a certain con- figuration for maintaining the turbulence level and pulsation of the suspension.

During the years, the screen comprising said rotor has proved to be a reliable and ad- vantageous apparatus, but in some situations the consistency of the suspension in the screening area between the screen drum and the rotor tends to rise so high that said bulges are not capable of increasing the turbulence level high enough to maintain opti- mum screening efficiency. One solution for the problem is to apply dilution of the sus- pension in the screening area.

Accordingly, prior art knows several various solutions for diluting the pulp between the cylindrical rotor and the also cylindrical screen drum. As an example of these solutions e. g. US patent 4,234,417 may be mentioned, in which the surface of the rotor cylinder is provided with blades extending to the whole height of the rotor. On the trailing side of these blades, when looking in the rotating direction of the rotor, there are outlet ports via

which dilution liquid is fed in the suspension. Said outlet ports lead inside the rotor, where there are three annular chambers arranged one upon the other so that dilution liq- uid is fed into each chamber from outside the screen.

Said construction has both advantages and disadvantages. The only advantage worth mentioning is that the dilution liquid is fed specifically via the rotor, whereby it would not be necessary to guide it near to the screening surface. Nevertheless, when perform- ing according to the patent, i. e. when bringing the dilution liquid via the blades of the rotor onto the back surface of the blades, to the area of intense suction, there is a great risk that the dilution liquid is passed onto the screening surface and therethrough quickly further to the accept side without actually diluting the pulp in the screening area. A sec- ond disadvantage worth mentioning is the complexity of the construction, as e. g. making the hole of Fig. 5 requires two opposite drillings in the blade and additionally one drill- ing in the surface of the rotor cylinder.

Said problems have been solved by the arrangement and rotor according to the present invention, the characteristic features of which are disclosed in the appended claims.

In the following, the invention is disclosed in more detail with reference to the appended figures, of which Fig. 1 illustrates an arrangement according to a preferred embodiment of the invention in side projection from the direction of the screen axis, and Fig. 2 illustrates an arrangement according to a second preferred embodiment of the in- vention in side projection from the direction of the screen axis.

The invention relates to a screen, preferably a pressure screen, comprising an essentially cylindrical outer casing with its end, a stationary essentially cylindrical screen drum (although in some circumstances a conical screen drum is also used) arranged inside it, which leaves between itself and said casing a so-called accept space. Inside the screen drum, there is a rotor arranged at the shaft led via the other end, which rotor is provided

with members for creating pressure impulses required for the screening in the annular so-called screening space between the screen drum and the rotor. The outer casing of the screen is further provided with at least three conduits. A feeding conduit communicates with the screening space and when the screen has been positioned to a vertical position said conduit is arranged at the upper end of the screen casing. An accept discharge con- duit is arranged on the outer casing of the screen and communicates with the accept space. And a third conduit is a reject conduit, which communicates with the screening space, from the direction of the feeding conduit with the opposite end thereof. In some cases there is further an apparatus for separating so-called heavy or coarse reject ar- ranged at the upper end of the screen above the screen drum.

The screen 10 according to Fig. 1 comprises in this embodiment primarily the compo- nents described above, of which e. g. the following may be mentioned here: an essen- tially cylindrical casing 12, inside of which there is arranged an essentially cylindrical screen drum 14 attached to the casing, inside of which drum there is a rotating essen- tially cylindrical rotor 16. From the screening space 18, more specifically from the lower end thereof, between the outer casing 12 and the screen drum 14, a so-called accept conduit 20 leads out of the screen. The outer surface of the rotor 16 is provided with pulsation members located at certain intervals, e. g. so-called bulges 22, which are de- scribed e. g. in US patent 5,000,842. According to a preferred embodiment of the inven- tion illustrated in this figure, there is/are at least one, preferably three annular chambers 24', 24"and 24"'arranged inside the rotor 16. Dilution liquid is fed into these by means of at least one, preferably three tubes 26', 26", 26"'from outside the screen 10.

Preferably there are regulation valves (not shown) arranged in connection with said tubes 26', 26", 26"', by means of which valves the pressure/amount of liquid flowing in the tubes may be regulated if desired. At the location of the chambers 24', 24", 24"', the surface of the rotor is perforated so that dilution liquid from the chambers is allowed to flow through the rotor casing. Said chambers 24', 24"and 24"'extend preferably to at least 50% of the whole length of the rotor, preferably to at least 70% of the length of the rotor. Preferably the lowest chamber 24"'is located in the vicinity of the lower edge

of the rotor so that liquid being fed from the chamber into the screening space dilutes the pulp in the space throughout the whole screen drum 14 and the lower edges of the rotor 16.

Fig. 2 illustrates a screen 100 according to another preferred embodiment of the inven- tion, which screen differs from that of figure 1 in that said screen 100 has two screening zones with two accept spaces 119 and 119'and two screen drums 114'and 114". Of course, it is possible to arrange the apparatus to operate with one screen drum only, whereby the type of perforation of the lower part of the screen drum 114"located at the second accept space is preferably different from that of the upper part of the drum 114'.

The embodiment of the figure has naturally also two accept discharge conduits, conduits 120'and 120", one from each accept space 119 and 119'. A further characteristic fea- ture of the invention of this embodiment is that the rotor 116 is further provided with still one dilution liquid chamber 124""located opposite the second accept space 119'.

The idea of the solution according to this embodiment is that among the reject i. e. the fiber suspension still present in the screening area after screening in the first screening stage, the upper one in the figure, possibly performed at a relatively high consistency, there still is acceptable fiber fraction that might be separated from the suspension in preferable conditions. These conditions may be created so that dilution liquid is fed am- ply from the lower dilution liquid chamber into the screening space, whereby the con- sistency of the fiber suspension in the screening space decreases very low, which in turn ensures that all acceptable fiber material is"washed"from the suspension. This way minimizing the reject amount may maximize the efficiency of the apparatus.

What makes the arrangement according to the present invention superior to prior art solutions is that the dilution liquid is led onto the cylindrical rotor as far away from the screen drum as possible, whereby the risk of the dilution liquid passing quickly into the accept is minimized. However, it is clear that the dilution liquid is efficiently mixed in the fiber suspension, as the continuously operating turbulence-generating elements in

the space between the rotor and the screen drum maintain continuous turbulence in the screening space.

In our experiments we have noticed that by bringing the dilution liquid to a distance of at least about 20 mm, though preferably at least 25 mm from the screening surface, the dilution of the accept may be minimized, while the consistency of the fiber suspension in the screening space may be maintained during the whole screening operation essen- tially the same as the consistency of the untreated fiber suspension being fed into the apparatus. Further our experiments attested our fear, i. e. that if the dilution liquid is brought nearer to the screening surface, the dilution of the accept initiates and the con- sistency of the suspension in the screening space increases, whereby part of the usable fiber material inevitably remains in the reject.

Additionally our experiments showed that the turbulence-generating elements on the surface of the rotor should preferably be, if not exactly similar to the ones described in US patent 5,000,842, at least relatively closely resembling them. That is, the turbulence- generating elements shall most preferably be angular and in many cases plow-like in or- der to both generate an intense turbulence in the fiber suspension and guide the move- ment of the suspension in the screening space to a desired direction.

As noticed from the above, a new way of treating pulp in connection with screening has been developed, in which way the consistency of the pulp in the screening space be- tween the rotor and the screen drum remains essentially the same during the whole screening operation. What has been presented in the above, is to be understood as just some preferable examples of the invention, from which the invention may differ in many relations within the scope of the appended claims. Thus, it is completely possible that inside the rotor there are not only one or three but e. g. two or four, or even more, dilution liquid chambers. The number of the chambers is completely dependent on the object of application of the rotor. Accordingly, it is totally possible and in some cases even recommendable that the walls between the chambers are not completely imperme-

able, but between them may be arranged some kind of e. g. throttled flow connection.

Further it is possible to lead dilution liquid into all chambers via one and the same inlet tube and to regulate the flow of the liquid into the screening space by changing the size of the flow openings in the longitudinal direction of the rotor. Further it is naturally clear that the screen may be positioned in another position than the vertical position pre- sented in the above description.