The present invention relates to a screen for treating fibrous suspensions, such as pulps, of the wood processing industry. Especially it relates to the construction of a rotor element for the screen.
Pressure screens are essential devices in the production of pulp and paper. They remove from the pulp suspension mainly impurities, over-sized pieces of wood and fiber bundles as well as other undesired substances. The screen can also fractionate fibers according to their length for improving the properties of the pulp. The precise function of the screen is dependent on the location in the process where it is used. In the screening process the water suspension of the pulp fibers is typically pumped into a cylindrical chamber, wherein the suspension is brought to contact with the screen surface and a rotor moving at high velocity. The rotational velocity of the rotor pushes the fibrous material into movement, whereby part of it is passed as accept through apertures in the screen surface. The high-speed rotor applies positive and negative impact pulses to the suspension. The positive impact pulses push the fibers through the apertures in the screen and may fractionate the fibers. The negative impact pulses provide for a regular flush-back of the apertures in the screen surface so that the fibers do not plug the apertures.
The pulp suspension consists of millions of elastic fibers that easily attach to each other forming so-called fiber flocks. Even at a low consistency such as 0.01% the fibers form unstable flocs. In a typical screening consistency, 1-3% the fibers form stable flocks and fiber networks hamper the screening. The fibers and undesired solid matter are periodically removed from the net in order to enable the screening the remaining fibers from the flocks and fiber networks into reject and accept fibers. When the pulp consistency increases, the force required for decomposing the fiber network increases intensively and finally a process limit is reached, where the apertures in the screen surface or the reject line is clogged. A large number of various rotor solutions has been developed with the aim of ensuring a continuous screening operation.
In principle, the rotors can be divided into two basic groups, open and closed rotors. Both are being used and their purpose is, as known, to keep the screening surface clean, i.e. to prevent the formation of a fiber mat on the screening surface. The first group is characterized in that the interior of the screen drum is provided with a rotary shaft or a rotor, whereto blades are attached by means of arms. An example of this kind is the rotor solution according to U.S. Pat. No. 4,193,865, where the rotor is arranged rotatably inside a cylindrical stationary screen drum, said rotor comprising blades located in the vicinity of the screen drum surface, which blades in the construction according to said patent form an angle with the drum axis i.e. the blades extend obliquely from one end of the screen drum to another. When moving, the blades impact pressure pulses on the screen surface, which pulses open the surface apertures. There are also solutions, in which the blades have been located on both sides of the screen drum. In that case, the suspension to be treated is fed to the inside or to the outside of the drum and the accept is, respectively, discharged from the outside or inside of the drum.
In stationary rotors the rotor is an essentially closed cylindrical piece, the surface of which is provided with pulsation members, for instance almost hemispherical protrusions, so-called bulges. In this kind of an apparatus the pulp is fed into a treatment space located between the rotor cylinder and the screen drum outside thereof, whereby the purpose of the rotor protrusions, e.g., the bulges, is both to press the pulp against the screen drum and by means of its trailing edge to withdraw the fiber mat off the screen drum apertures. The bulges can be replaced by other kinds of protrusions.
A solution widely used in the market is a represented by a method according to FI patent 77279 (U.S. Pat. No. 5,000,842) and the solution developed for the implementation thereof. The method according to said patent is characterized in that the fiber suspension is subjected to axial forces with varying intensity and effective direction, the direction and intensity of which are determined based on the mutual axial positioning of the point of application and the countersurface of the screen drum and by means of which the axial velocity profile of the fiber suspension is changed while maintaining the flow direction continuously towards the discharge end. Preferably the surface of the rotor is divided into four zones: feed, feed and mixing, mixing, and efficient mixing. The rotor surface is typically provided with 10-40 protrusions, the shape of which varies according to the zone i.e. the axial part of the rotor that they are located on. The protrusions on the housing surface of the rotor are mainly formed of front surfaces facing the flow, preferably surfaces parallel to the housing surface and back surfaces that descend towards the housing surface of the rotor. The housing surface of the rotor is provided with protrusions of several different forms, which have been arranged onto the rotor housing so that two or more circumferential zones are formed separated from each other in the axial direction of the rotor, such as e.g. 4 zones. At least part of the front surfaces of the protrusions forms an angle with the axial direction. The front surface of the protrusions can be divided into two parts that form with the axial direction angles of different size. The variation interval of the angles is −45°-+45° compared to the axial direction. However, the functioning principle of the protrusions is the same as in other corresponding devices. The abrupt front surface imparts a strong pressure shock to the fiber mat on the screen drum, whereby the accept is pressed through the apertures of the drum. The sloping back surface of the protrusion withdraws some water back to the screening zone and thus releases from the grooves and apertures major particles and fiber flocks thus cleaning the screen drum.
U.S. Pat. No. 5,192,438 describes a rotor which provides high intensity axial shear stress in addition to high positive pulses and negative pulses. The rotor has a contoured surface including a plurality of protrusions. A protrusion has a front plane, an upper plane, an inclined plane and edge surfaces, which may converge. The trailing surface of the protrusion is abrupt.
So, in prior known solutions the functional prerequisite of pressure screens starts from the presumption that the rotor element is to develop an adequate pressure impulse on the interface to make the fiber particles flow through the screening surface and that the rotor element is to create by its trailing edge a negative pressure impulse to generate a turbulence that cleans the apertures clogged by the previous positive impulse. It has also been generally presented in the field that a negative impulse withdraws liquid back towards the feeding space preventing excess thickening of the fiber suspension in the feeding space and in its part cleaning the apertures of the screening surface. For enabling to create these conditions, the rotor must have an adequate rotational speed, which is, however, limited by energy consumption and mechanical durability of the screen, a typical speed for a rotor described in FI-patent 77279 (U.S. Pat. No. 5,000,842) is 24 m/s.
In the present industrially used pressure screen applications the rotor solutions have enabled to reach the maximum feed consistency level of pulp. The consistency level is almost the same for different rotor types, for instance for softwood (SW)-pulp approximately 2-3%. Thus, there is a need in the field to develop a screen rotor that will allow higher feed consistencies.