PRESSURE SCREEN FOR SCREENING PULP The invention relates to a pressure screen for screening pulp, the pressure screen comprising a housing and a rotor rotatabfy mounted within the housing, at least two axially successive screen cylinders, mounted coaxially with the rotor, the pulp being introduced from one end of the pressure screen into the screen cylinder, and the reject that remains after screening being dis- charged from the other end of the pressure screen, and foil blades coupled to the rotor for wiping the inner surface of the screen cylinders.

Pressure screens are used to screen pulp received in defibration so as to obtain an optimally suitable mass for the production of paper and paper- board. This is done during screening by removing various impurities, such as slivers, possible fibre bundles, and other impurities hampering production and impairing the quality of the finished web. This is accomplished by screens comprising one or more screen cylinders which are provided with openings whose shape and size are dimensioned so as to allow the accepted pulp frac- tion to pass through the openings of the screen cylinders as easify as possible.

In contrast, dirt and fibres that are too large cannot penetrate the openings.

Since in modern paper production it is very important that the capacity of a screen be as high as possible, the openings are usually made larger than would actually be necessary. As a result, part of the rejected materai pene- trates the screen and hence mufti-stage screening is employed to ensure the required quaiity. Similarly, owing to the operation of the device, it is obvious that some usable fibres are discarded together with the rejected material.

Hence it is economical to screen the reject again in order to recover this fibre material.

To increase the efficiency of screening and simplify the equipment, various multi-stage screens have been designed. In these screens the pulp is screened in several successive stages identical to reject screening. Finnish Published Specification No. 309/67 discloses a device comprising two opera- tionally successive screening stages. The pulp to be screened is introduced into a first screen, in which the pulp having penetrated the screen cylinder pro- ceeds to a discharge conduit. The rejected pulp fraction, i. e. the reject, is di- luted with water and introduced into a second screening stage in which the pulp fraction that has passed the screen drum, i. e. the accept, is again led to the discharge conduit, i. e. the reject is separately led out.

Finnish Published Specification No. 47789 discloses a device also

comprising two successive screening stages, connected in series. In this solu- tion the pulp to be screened is fed to a first screening stage and the rejected pulp fraction is discharged from the device. The pulp having penetrated the screen cylinder is then led to a second screening stage, from which the ac- cepted pulp fraction that has passed the screen cylinder, i. e. the accept, is carried further. The rejected pulp fraction, i. e. the reject, in turn, is discharged.

The problem in this solution is that accepted fibres are discharged with reject removal, and consequently the solution calls for separate reject screening.

Finnish Published Specification No. 51221 discloses a solution in which a second screen cylinder is arranged between the reject conduit and the pulp to be screened. From this cylinder, liquid and fibres are able to return from the reject to the pulp to be screened. In practice, this solution corre- sponds to a solution with two screens, in which the rejected pulp is separately screened and the accept obtained from it is recycled to the pulp entering the first screening stage. The operation of this device is, however, very unreliable as the movement of the liquid and fibres from the reject conduit to the pulp to be fed is subject to the pressure in the reject conduit being higher, which is contrary to the actual pressure ratio of the device.

Finnish Published Specification No. 93979 discloses a solution with three separate screening steps that can be linked by screening the pulp in two successive screening steps and recycling the accept to the second screening step. In this solution the reject is returned to screening via a conduit in the ro- tor axle, whereby pump impellers are secured to the rotor to provide the extra pressure needed, the impellers producing the required pressure rise.

It is the object of the present invention to provide a pressure screen with which pulp can be screened more effectively and simply than with known solutions. The invention is characterized in that a ring-shaped exhaust cham- ber is arranged outside each screen cylinder, said exhaust chambers being separate from other exhaust chambers, that a ring-shaped conduit is arranged between two axially successive screen cylinders, the conduit being in contact with the space inside the screen cylinders so as to allow dilution water to be fed from it to the pulp flowing by in order to be able to dilute the reject that failed to permeate the previous screen cylinder in the flow direction of the pulp for the screening performed by the following screen cylinder.

It is an essential idea of the invention that a pressure screen com- prises a plurality of successive coaxial screen cylinders and inside them a ro-

tating rotor, puip being introduced into one end of the screen cylinders and flowing in the axial direction of the screen cylinders and being simultaneously screened so as to finally discharge the last reject at the other end of the screen cylinder. The invention is further characterized in that a ring-shaped conduit is arranged between two successive screen cylinders, allowing dilution water to be fed from said channel to the reject that failed to permeate the pre- vious screen cylinder for its next screening stage.

It is an advantage of the pressure screen of the invention that it is simple and easy to build as it only requires one rotor to move the blades needed by the screen cylinder. A further advantage of the invention is that it is possible to use either one-piece foil blades wiping the inner surfaces of all screen cylinders, or separate foil blades, possibly having different characteris- tics, for each screen cylinder so as to allow the screening to be carried out as efficiently as possible. It is stili another advantage of the invention that no pipe systems or pumping is needed between the separate screening stages.

The invention will be described in more detail in the attached draw- ings, in which Figure 1 schematically shows a side view of an embodiment of the pressure screen of the invention in partial section, Figure 2 schematically shows another embodiment of the pressure screen shown in Figure 1 in partial section, and Figure 3 schematically shows a functional diagram of the pressure screen of the invention.

Figure 1 shows an embodiment of the pressure screen of the in- vention, mainly as a diagram. Like numerals are use to designate like parts in the description of Figure 1 and also in association with Figures 2 and 3, and they are therefore not described in greater detail in connection with said Fig- ures.

The pressure screen comprises a body containing a housing 1 with a cover 2. A rotor 3 is coaxially arranged inside the housing 1 and rotates in a manner known per se with a motor (not shown). A conical inner housing 4 is secured to the rotor 3 with rods 5. Disciform blades 6 have been secured to the rotor for making the pulp rotate inside the pressure screen.

The pressure screen shows three successive screen cylinders 7a to 7c, and a ring-shaped exhaust chamber 8a to 8c outside each screen cylinder 7a to 7c, each exhaust chamber embodying an exhaust conduit 9a to 9c. Foil

blades 11 a to 11c are secured to the rotor 3 housing by rods 10 by each screen cylinder 7a to 7c, the blades rotating along the inner surfaces of the screen cylinders 7a to 7c as the rotor 3 rotates in a manner known per se and detaches the material pressed against the inner surfaces of the screen cylin- ders 7a to 7c by means of pulses. Between two successive screen cylinders 7a and 7b, and 7b and 7c, respectively, inlet conduits 12a and 12b for dilution water are arranged, the conduits being ring-shaped, preferably continuous slits surrounding the inner space of the pressure screen. Dilution water for diluting the pulp flowing past the corresponding conduit can be introduced into both ring-shaped conduits via dilution water units 13a and 13b. The pulp is intro- duced into the upper. part of the pressure screen via an inlet conduit 14 in its cover 2, and the pulp flows in the case presented in the Figure downwards along the downward narrowing ring-shaped space formed by the screen cylin- ders 7a to 7c and the inner housing 4. The pulp is screened by the screen cylinder 7a, from where the pulp that has penetrated the screen cylinder en- ters the exhaust chamber 8a and is then discharged via the exhaust conduit 9a. The pulp that does not permeate the screen cylinder 7a flows downward and is diluted before arriving at the second screen cylinder 7b by water intro- duced via the ring-shaped conduit 12a. Here the diluted reject is again screened and some of the pulp in it permeates the screen cylinder 7b to the exhaust chamber 8b and is further discharged via the exhaust conduit 9b. The pulp that fails to permeate the screen cylinder 7b is again diluted while by- passing the second dilution conduit 12b by the water supplied by it and will be again screened in the third screen cylinder 7c. Having permeated said screen cylinder, the pulp enters the exhaust chamber 8c and is further discharged via the exhaust conduit 9c. After this screening, the remaining reject is discharged via the ring-shaped reject chamber 15 and the reject conduit 16 associated therewith and located below the third screen cylinder 7c.

Figure 2 shows a pressure screen corresponding to that of Figure 1 except that a common foil blade has been used for all screen cylinders instead of several separate foil blades. This solution is easier to implement in practice, although in the solution shown in Figure 1, separate foil blades can be dimen- sioned for the different screen cylinders to achieve the desired effect.

Besides three-stage, the pressure screens shown both in Figure 1 and 2 can be built multi-stage simply by adding a desired number of screen cylinders and dilution water conduits between them. The number of screen

cylinders required is dependent on both the screening capacity required by the device and the quality required by the screening. Figure 3 schematically shows the operation of the pressure screens shown in Figures 1 and 2. As is evident from the Figure, the first screen cylinder 7a provides a first screening stage, into which pulp is introduced and from which the accept is discharged along the conduit 9a. The reject from the first screening stage is discharged to a second screening stage, provided by the screen cylinder 7b. In this case the reject from the first stage is diluted by the dilution water before its arrival at the second screening stage. The accept from the second screening stage is dis- charged again along the exhaust conduit 9b, and the reject is led to a third screening stage provided by the screen cylinder 7c. The reject is further di- luted by dilution water in this stage before it arrives at the screening stage, and the accept is discharged via the exhaust conduit 9c in the same way as the reject is discharged via the reject conduit 16. As the Figure shows, it is also in practice relatively simple to add such successive screening stages.

The Figure also shows by dashed lines a return conduit 17 with a pump 18 connected to it. A pump can be added to a pressure screen when the it is desirable to use the most diluted accept, i. e. that received from the last screening stage, instead of only water in the dilution stage of pulp from earlier screening stages. Usually this is not at all necessary as to the operation of the pressure screen.

The invention has been described above and in the drawings by way of example only, and is not in any way restricted to it. The diameters of the successive screen cylinders can be equal or unequal. The openings of the screen cylinders can be round, slit-like or of any other suitable shape in a manner known per se, and can be of the same or a different size in the differ- ent screen cylinders.