METHOD AND APPARATUS FOR KEEPING A SCREEN OR FILTER SURFACE CLEAR

Abstract: The present invention relates to a method and apparatus for keeping a screen surface or a filter surface clear. The method and apparatus according to the invention is especially suit­ able in connection with thickeners, screens and filters in­ tended for treating the fiber suspensions in the pulp and paper industry. The greatest disadvantage of all above-mentioned equipment is the tendency of perforated screen/filter surfaces to become gradually clogged as fibers of pulp suspensions adhere to the openings in the screen/filter surface. In the method of the invention, pressure fluid is introduced from a conduit (5) to the secondary side (10) of the screen/­ filter drum (6), which fluid flows, during the underpress­ urized stage caused by the blades (9) of the rotor (7) rotating on the primary side of the screen/filter drum (6), through the screen/filter drum (6) from the secondary side to the primary side thereby rinsing the openings of the drum (6) and making them clear.

Description: The present invention relates to a method and apparatus for keeping a screen surface or filter surface clear. It is especially suitable for the perforated surfaces of the equip­ ment treating fiber suspensions in the pulp and paper in­ dustry, which surfaces are inclined to become clogged every now and then because of fibers penetrating the openings of the screen surface and adhering to them. Fiber material, especially cellulose and wood fiber pulp, is treated in the form of fiber suspension where the fibers are, depending on the consistency of the suspension, more or less loose. Such pulps are treated by, for example, pumps, thickeners and various types of screens and filters. Part of such means is intended for separation of air from the fiber suspension, another part for separation of liquid from the fiber suspension, still another part for screening the fiber suspension into various fractions and a further part merely for conveying the fiber suspension. All means performing said functions are provided with perforated surfaces, in other words, surfaces with holes or slots where the fibers are prone to penetrate, thus clogging the perforations. In previously known means, the screen/filter surface has mostly been kept clear by a rotor, which either mechanically wipes the perforated surface or rotates very close to the surface, thus directing, by means of its front edge, a slight pressure stroke towards the screen/filter surface and, in the area of its back edge, a slight pressure stroke in the opposite direction creating an underpressure zone, which is intended for loosening the fiber mat formed on the screen/­ filter surface, mainly by creating turbulence that breaks the fiber layer, thereby opening the perforations of the surface. In the long run, the openings however become clogged with fibers, which do not become loose by means of the under­ pressure caused by a rotor blade but on the contrary, the capacity of the screen/filter surface slowly and continuously lowers or the pressure difference allowed over the screen becomes restricted. On the other hand, a method of clearing filter surfaces by means of compressed air is known, for example, with disc filters. An underpressure prevails on the bottom sector inside the filter disc, by means of which underpressure the fiber suspension is drawn against the filter surface, whereby liquid becomes filtered through the filter surface into the disc. While the disc is rotating and the fiber cake adhered to said disc is rising up from the pulp vat, a compressed air flow is directed outwards from the inside of the filter disc, which flow forces the pulp cake off of the filter surfaces. This method, even if it were applied to, for example, drum filters or screen drums, is not capable of keeping the screen/filter surfaces as clear as they should be, but the surfaces accumulate a substantial fiber layer instead, which slows down the function of the apparatus and lowers its capacity. The object of the invention is, therefore, to provide a method and apparatus for introducing a continuous flow of fiber suspension onto the screen/filter surface, which fiber sus­ pension is, at no stage, allowed to form a fiber mat on said screen/filter surface to hamper the operation thereof, but said fiber suspension is allowed to flow along the surface and towards the outlet opening whilst being treated effi­ ciently all the time. The basis of the apparatus of the example may be a conventional apparatus equipped with a screen drum, in which apparatus, close to the drum surface there rotates a rotor, which may be a conventional blade type member or equivalent. The method of the invention is characterized in that the se­ condary side of the screen/filter surface is supplied with fluid, either liquid or gas, the pressure p1 of which is lower than the pressure p2 of both the suspension to be treated and the centrifugal force on said fluid suspension, to which pressure p2 the screen/filter surface is subject, but higher than the pressure p3 being created in the area between the screen/filter surface and a member, i.e. the rear edge of a rotor blade or equivalent, moving relative to the screen/­ filter surface in the suspension. This is best accomplished by regulating the inlet flow of cleaning gas and the only possible passage for the gas being through the screen plate. The surface which is being cleared follows the rotor blades and the area of that surface is dependent on both the pressure fluctuations on the screen/filter surface and the amount of gas used. The apparatus of the invention is characterized in that the outer shell of the pulp treating apparatus is provided with a conduit for the pressure fluid, by which conduit the space on the secondary side of the screen/filter surface can be pressurized. Advantages gained by the method and apparatus of the invention are, for example, such that maintaining the screen surface clear is secured because bigger pressure differences are available in comparison with earlier known means. As a con­ sequence of the screen/filter surface staying clear the pulp treating process is much more efficient than with the equip­ ment of the prior art, thus resulting in a considerably higher pulp treating capacity than before. The method an apparatus of the invention will be described in greater detail in the following with reference to the accom­ panying drawings, in which Fig. 1 is an illustration of a conventional pulp treating apparatus modified for the requirements of the invention, and Fig. 2 is a detailed illustration of the function of the screen. In accordance with Fig. 1, the pulp treating apparatus com­ prises an outer shell 1, an inlet conduit 2 for the pulp, a discharge conduit 3 for the pulp, a discharge conduit 4 for the accept or the filtrate, a conduit 5 for compressed air or equivalent, and a screen drum 6 inside the shell and a rotor 7 with a shaft 8 and drive equipment. Preferably the rotor 7 is disposed inside the screen drum so as to dispose the rotor blades 9 or equivalent close to the surface of the screen drum 6. The rotor blades 9 are preferably similar in shape to the blades of screening apparatuses, whereby the front edges of the rotor blades develop a slight pressure pulse towards the screen drum and the rear edges of the blades develop a slight pressure pulse directed from the screen drum towards the rotor. Deviating from conventional arrangements, there is a space 10 for accept or filtrate behind the screen drum 6, which space is separate from the rest of the pulp treating apparatus, said space being provided with protruding conduits 4 and 5 for the accept/filtrate and for compressed air respectively. The inlet conduit for compressed air or for corresponding gas is preferably provided with a pressure valve 11 for regulating the pressure prevailing in the filtrate space 10 if desired. The method of the invention functions (as shown in Fig. 2) so that there has been arranged in the accept/filtrate space or a so-called secondary side, by means of a valve, such a pressure p1 that the centrifugal force of both the pulp and the suspension, and the inner pressure, i.e. the pressure p2 of the so-called primary side against the screen surface prevent the gas from discharging itself to the opposite side of the screen/filter surface, but during the underpressurized stage (pressure p3), which is caused by the rotor blade, the gas discharges itself from the area of the rear edge of the blade through the screen/filter surface, thus securing that the openings of the screen/filter surface certainly become clear. Hence, p1 < p2 but p1 > p3. After the effect of the rotor blade has stopped, the centrifugal force and the pressure prevailing inside the screen together overcome the gas pressure and the fiber suspension/liquid is filtered through the screen/filter surface conventionally, whereby the gas seeks its way to the center of the rotor. Thus, every single blade with its rear edge causes a back-blow through the screen/filter surface, which blow clears the openings of the screen/filter surface. In other words, during one revolution of the rotor the screen/filter surface is blown clear as many times as the rotor has blades or equivalent. Considering that the rotor surface may be grooved in a manner corresponding to modern screen surfaces, it can be noticed that the number of clearing times per each revolution of the rotor may be even several dozens. Naturally, it is possible that the rotating part of the appar­ atus is the screen/filter drum itself, whereby the stationary blades or equivalent create the necessary pressure difference over the screen/filter surface. It is also possible that the gas or other equivalent fluid is fed from a stationary, press­ urized space disposed close to the blades on the opposite side of the screen/filter surface. At this stage, it is worth mentioning that the gas mixed with the pulp and introduced through the screen/filter surface need not cause any big problems because it can be discharged, for example, with a centrifugal pump provided with an air separ­ ation system, which pump is needed anyway for conveying the pulp. On the other hand, it must be noticed that in a de­ pressurized system the gas mostly discharges by itself from the apparatus. As can be seen from the above description, a completely new and simple method and apparatus has been developed to highly efficiently keep the screen/filter surfaces clear by pre­ venting the accumulation of a fiber mat onto the screen/filter surfaces. As it appears from the above, the method and apparatus according to the present invention is well appli­ cable to a wide variety of pulp treating equipment. By increasing the pressure of the secondary side, a back-flow can be effected in the whole area of the screen/filter drum, by means of which back-flow the screen/filter surface can be easily cleared at the same time if, for example, said surface has for some reason become clogged. In principle, the inven­ tion is applicable to all places where clogging of a per­ forated surface constitutes a problem and where compressed air or equivalent gas can be considered to be a means of clearing thereof. In some cases, the use of fluid for rinsing the screen/filter surfaces is also worth its while. Hence, what has been described above, is in no way intended to limit the invention, but it discloses only a few especially preferable embodiments of the invention, the protective scope of which is defined in the accompanying claims only.

Claims: 1. A method of keeping screen surfaces or filter surfaces clear during the treatment of suspensions of solid materials, characterized in that fluid is introduced to the secondary side of the screen/filter surface, the pressure p1 of which fluid is lower than the pressure p2, said p2 being the sum of the pressures of both the suspension treated and the centrifugal force affecting said suspension and said p2 being directed against the screen/filter surface, but higher than the pressure p3 being formed in the area between an element i.e. the rear edge of a rotor blade or equivalent moving relative to the screen/filter surface in the sus­ pension. 2. A method as claimed in claim 1, characterized in that the pressure p1 is adjustable. 3. An apparatus for keeping a screen/filter surface clear, which apparatus is arranged in connection with a pulp treating means, said pulp treating means comprising an outer shell (1), an inlet conduit (2) for the pulp, conduits (3, 4) for at least two fractions and a pulp treating drum (6) being dis­ posed inside the outer shell (1) and provided with a screen/­ filter surface and an element i.e. a rotor (7) or equivalent moving relative to the screen/filter surface and being dis­ posed in the vicinity of said surface, character­ ized in that the outer shell (1) is provided with a conduit (5) for a pressure fluid, by means of which conduit (5) the space (10) on the secondary side of the screen/filter surface is arranged to be pressurized.

Inventors: NISKANEN, TOIVO, VIKMAN, VESA
Applicants: A. AHLSTROM CORPORATION
Application year: 1988
Main classification: D21D5
IPC codes: D21D5, B01D29