It is known that variations in the concentration of the pulp suspension are of decisive importance for the screening process. A reduction in the concentration implies an increase of the hydraulic load on the screen means, i. e. the flow rate through the orifices in the screen means increases. At concentrations below about 0,5 % the capacity becomes unacceptably low. An increase in the concentration implies an increase of the energy intensity required for breaking up the fiber network to individual fibers and causes it to become fluid, so-called fluidization, which is a prerequisite for the screening process. The concentration, therefore, sets a limit for an efficient utilization of the screen. As a result of too high a concentration, the flocks of the pulp will not be broken up, which implies that the screening process cannot continue.

At a conventional pressurized screen for pulp suspensions, the thickening along the length of the screening zone from the inlet for unscreened pulp to the outlet for the discharge of enriched impurities, is the physical problem limiting the effectiveness of the screen with regard both to capacity and efficiency. From a physical point of view, the thickening implies that the concentration of the fiber suspension increases from the inlet to the reject outlet along the surface of the screen basket. Increased concentration causes the strength of the fiber network to increase considerably.

Due to the fact that the rotating means of the screen rotate at equal speed along the entire length of the screening zone, the energy supply is substantially constant from the inject end to the reject end of the screen means. This implies that the screening must start at too low a concentration at the beginning of the screening zone, in order to prevent the pulp concentration from rapidly becoming so high that a large portion of the screening zone operates as a thickener. An energy intensity too high in relation to the pulp concentration implies that the fiber suspension is overfluidized at the beginning of the screening zone, which yields an unnecessarily high turbulence level and, as a result thereof, a deteriorated separation selectivity. Afte a short zone with ideal conditions, the pulp concentration will be too high, the energy is not any longer sufficient for breaking up the fiber network, and the final portion of the screening zone operates as a thickener. In other words, the thickening implies that the screen looses efficiency and capacity.

At certain modem pulp screens one has succeeded to increase the pulp concentration by providing inside the screen means a rotor with pulsation-creating wings, which yield an extended suction pulse producing a vacuum adjacent the screen means, in order in this way to recover a certain amount of the liquid lost by the thickening. At the same time, overpressure arises on the inside of the pulsation wings. Extended suction pulses by wide pulsation wings render it possible to increase the concentration in a screen, but at this high concentration the process according to the above reasoning becomes sensitive from an optimization aspect. Variations in the pulp concentration, dewatering properties or fiber length distribution affect the critical balance between network strength and energy supply. As a result thereof, one is forced to operate the screen at a number of revolutions higher than at optimum in order to manage the operability even at normal process variations. Especially at the end of the screening zone the effect of the suction pulses tends to diminish, with thickening problems resulting thereof.

According to the invention, the above problems can be reduced considerably by positioning a flow restriction at the reject outlet between the rotor and the end of the pulsation wings in order to restrict the flow of the fiber suspension past the end of the wings. As mentioned above, the pulsation wings have the object to produce suction pulses by the pressure difference between the front and rear side of the wings. The invention, thus, implies that this desired pressure difference can be maintained along the entire length of the wings in that a pressure balancing flow past the ends of the wings substantially is prevented.

The characterizing features of the invention are apparent from the attache claims.

The invention is described in greater detail in the following, with reference to the accompanying drawing illustrating an embodiment of the invention.

Fig. 1 is a schematic cross-section of a screening device according to the invention; Fig. 2 is a cross-section according to II-II in Fig. 1; The screening device shown comprises an air-tight casing 1 with inlet 2 for the pulp suspension (inject) and outlets 3 and 4, respectively, for accept and reject, respectively. A rotation-symmetric screen means 5 is located stationary, preferably with vertical symmetry axis, in the casing 1. In the screen means 5 a rotor 6 is located which extends along the entire screen means 5. The rotor 6 is concentric with the screen means, so that an overall screening zone 7 is formed between the rotor 6 and screen means 5. Alternatively, the screen means can be rotary at a speed relative to the rotor 6.

The inject inlet 2 for the pulp suspension is connecte to the casing 1 for the supply of the pulp from below to the inside of the rotor 6. The rotor 6 is formed as a drum, through which the supplie pulp suspension is intended to flow upward and through one or several openings 8 in the upper portion of the rotor 6 for transferring the pulp to the upper end of the screening zone 7. The rotor is provided on its outside with pulsation wings 9, which extend along the entire screening zone 7. Said wings 9 are located spaced from the rotor and are formed with a leading edge 20 located near the screen means 5 and a rear edge 21 located at a greater distance from the screen means. The wings 9 thereby produce a suction pulse when they move along the screen means 5, which keeps the screen means 5 open and promotes the separation of the accept. The wings 9 can be of the kind shown in SE-PS 464 473.

One prerequisite for the favorable pulsations is the pressure difference arising between the front side and rear side of the pulsation wings 9. For maintaining this pressure difference even at the end of the pulsation wings to the reject outlet, a stationary flow restriction 10 in the form of an overall plate is located between this end and the rotor 6. The distance between the restriction 10 and the end of the wings 9 shall be small, preferably 1-5 mm, in order to prevent overflow. The restriction can be provided on a wall element 11, which at the same time forms a gap seal to the rotor 6 for defining the screening zone 7 from the inside of the rotor.

Pulp to be screened is supplie via the inlet 2 to the inside of the rotor 6 and further through the openings 8 in the rotor to the screening zone 7. In the screening zone 7 the pulp is moved downward from one end to the other. The accept passes through the screen means 5 by the effect of the pulsation wings 9, which facilitates the fractionation and counteracts the thickening of the reject. Short-circuit of the space outside and inside the pulsation wings 9 below their lower end is prevented, so that the favorable suction pulses are maintained all the way to the end of the screening zone 7. By preventing in this way a secondary flow, an effective discharge of the reject along the lower portion of the screen means 5 is facilitated. The screen means 5 thereby can be utilized at maximum, so that the capacity of the screen increases.

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