Patent Publication Number: US-6036029-A

Title: Device to drain a fibrous pulp suspension

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 196 35 293.2 filed Aug. 30, 1996 and German Patent Application No. 196 35 217.7, filed Aug. 30, 1996, the disclosures of which are expressly incorporated by reference herein in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device to drain a fibrous pulp suspension. 
     2. Background of the Invention 
     A device for draining a fibrous pulp suspension is know which includes an arched surface as, for example, a roll. Moreover, it is known to have a portion of the circumference of the arched surface encircled by a screen belt. The screen belt and sleeve surface of the roll create an inlet slit for draining the fibrous pulp suspension. 
     Such a device is described in WO 96/08600. In this reference, the arched surface is a roll with openings that open into the roll interior. The roll is, moreover, placed under suction. A portion of the circumference of the roll surface is surrounded by a substantially impermeable belt. 
     This device is disadvantageous because the fibers adhere to the impermeable belt during operation. The fibers very quickly create fiber clumps and seriously interfere with the operation of the device. Further, to clean the device, it is necessary to stop the machine and disassemble the belt. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-noted defects and disadvantages of the prior art by improving the device to drain a fibrous pulp suspension. 
     The object of the present invention is to create a device for draining a fibrous pulp suspension which achieves a high draining capacity but which presents no or only minor cleaning problems. 
     According to one embodiment of the present invention, the device to drain a fibrous pulp suspension includes an arched surface, which is impermeable and which includes a plurality of recesses that are able to receive and transport a quantity of a fluid. The fluid received by the plurality of recesses is a fibrous pulp suspension. 
     The device further includes a permeable belt encircling the arched surface. Further, the permeable belt and the arched surface independently move, resulting in relative velocity between the permeable belt and the arched surface. 
     According to another aspect of the invention, the device may include a plurality of rails which create a plurality of recesses on the arched surface. Further, the device may include a cleaning device positioned near the arched surface and near the permeable belt for removing contaminants. The arched surface may be substantially smooth. Further, the arched surface may comprise a roll. 
     In accordance with another aspect of the invention, the permeable belt may have two ends, as opposed to being continuous. The permeable belt unwinds at a creeping speed from an unwinding roll and winds onto an uptake roll. The unwinding and winding are reversible and the roll is capable of being driven at a high speed relative to the speed of the permeable belt. 
     The permeable belt and the roll are driven at different speeds. The differential speed or velocity of the belt and roll may be as low as, e.g., approximately a few millimeters per second. However, the actual velocity of the belt and roll may be as low as, e.g., approximately a few millimeter or a few centimeters per second, or as high as, e.g., between approximately ten and fifty meters per second. 
     According to one embodiment of the invention, the permeable belt essentially comprises an elastic wire mesh. According to another aspect of the invention, the permeable belt may comprise a stiff, flexible material, such as, for example, an arched perforated metal sheet placed over the arched surface. 
     The plurality of recesses, which receive and transport the fluid, may be formed by a plurality of spherical dents. Further, the spherical dents may be spaced evenly across the arched surface. The plurality of recesses may be formed by a plurality of spherical humps, which may also be spaced evenly across the arched surface. The recesses may also comprise a plurality of grooves that extend substantially across the direction of the relative movement of the belt and the arched surface. The grooves, moreover, may be divided into longitudinal sections. 
     According to another embodiment of the invention, the device may include a plurality of rails which create the plurality of recesses. 
     Further, the device may include a plurality of bores that extend substantially perpendicular to the direction of motion from one face side of the arched surface to the other face side of the arched surface. The arched surface here is made of an elastic moldable material. The plurality of bores, moreover, may be inflatable and deflatable. 
     The device may include a support construction and a hose which is made of an elastic material and which encircles the support construction. The device may further include a plurality of support units or rails that are positioned between the hose and the support construction. The support units or rails form a plurality of gaps between adjacent support units. Each of the adjacent support units can be inflated or deflated. 
     According to another aspect of the invention, the arched surface may exhibit a plurality of different radii along the length of the arched surface. The different radii are measured to a center point of the arched surface. Further, the different radii may decrease in the direction of the belt rotation thereby increasing the draining pressure with a constant screen tension. 
     The device further may include a covering screen to cover the recesses of the arched surface. The covering screen may surround the arched surface. 
     Another way to describe the present invention is a device to drain a fibrous pulp suspension comprising an arched surface wherein the arched surface is impermeable and includes a plurality of recesses that are capable of receiving a quantity of fluid. The arched surface further includes a grid structure for creating the plurality of recesses. The device further includes a permeable belt that is moved relative to the arched surface resulting in a relative velocity between the permeable belt and the arched surface. 
     According to another aspect of the invention, the plurality of recesses are able to receive a quantity of the fibrous pulp suspension. Further, the grid structure may be in the shape of a honeycomb. 
     The arched surface may be a circumference area or a sleeve of a roll. The device, moreover, may include a screen to cover the plurality of recesses. The screen may surround the arched surface. 
     According to another aspect of the present invention, the arched surface and the permeable belt form an inlet slit to receive the fluid, which can comprise a fibrous pulp suspension. Further, the arched surface may comprise a roll. 
     According to another embodiment of the invention, the roll may include a support construction which comprises either a hollow cylindrical core or a solid cylindrical core. In addition to the cylindrical core, the support construction may also include a plurality of rails that form a plurality of gaps. The rails extend radially from the surface of the cylindrical core. Further, the roll includes a flexible hose that is water impermeable and that is capable of shrinking onto the plurality of rails, forming a wave profile over which the permeable belt travels. 
     According to the present invention, because the belt is essentially permeable and is continuously rinsed during operation, the device presents almost no cleaning problems with respect to the cleanliness of the belt surrounding the roll, because the belt is essentially permeable and is continuously rinsed during operation. In addition, the belt can, if required, be cleaned outside the working zone through suitable conventional means. Another advantage of the present invention is exemplified by the very high dry contents that may be achieved with the device, depending on established operating parameters such as the type of pulp of the fibrous pulp suspension, the screen tension, the circulating velocity of the roll, and so on. The pulp, which is substantially dry, is extracted from the fibrous pulp suspension, is present in the form of a sheet. In some cases, the pulp is in the form of small fibrous pulp rolls, whose axes extend parallel to the roll axes during their creation, and which can have a diameter of less than one millimeter to several millimeters. 
     The present invention also applies the known principle that a relative velocity exists between the sleeve surface of the roll and the screen. The screen or the roll can remain completely stationary during the operation, i.e. perform no translational motion. They can, however, move at variable speeds, including creeping speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by referring to the following description with reference to the drawings. The drawings illustrate, by way of non-limiting examples, embodiments of the invention, with like reference numbers representing similar parts throughout the several views, and wherein: 
     FIG. 1 is a schematic diagram of one embodiment of a device for draining a fibrous pulp suspension; 
     FIG. 1A is a schematic diagram of one aspect of one embodiment of a device for draining a fibrous pulp suspension; 
     FIG. 1B is a schematic diagram of the device for draining fibrous showing troughs for collecting the drained fluid; 
     FIG. 2 is a schematic diagram of a second embodiment of the present invention; 
     FIG. 2A is a schematic diagram of one aspect of the second embodiment of the present invention; 
     FIG. 3 is a schematic diagram of a third embodiment of the present invention; 
     FIG. 3A shows a cross-sectional view of the sleeve showing a depression formed in the surface; 
     FIG. 3B shows a cross-sectional view of the sleeve showing a raised portion formed in the surface; 
     FIG. 3C shows the embodiment of FIG. 3 including a screen covering for the roll; 
     FIG. 4 is a schematic diagram of a fourth embodiment of the present invention; 
     FIG. 5 is a schematic diagram of another aspect of the device for draining a fibrous pulp suspension; 
     FIG. 6 a schematic diagram of another embodiment of the present invention; and 
     FIG. 7 is a schematic diagram of another embodiment of the device for draining a fibrous pulp suspension. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for the fundamental understanding of the invention, the description taken with the drawing figure making apparent to those skilled in the art how the invention may be embodied in practice. 
     As shown in FIG. 1, the device, in accordance with the invention, includes an arched surface and a screen belt 2 which encircles the arched surface on part of the circumference of the arched surface. In the present embodiment, the arched surface is the top surface of a roll 1. The roll 1 exhibits a uniform curvature of radius R across the entire circumferential area of the roll 1. However, it is possible to have an arched surface that includes a changing radius. 
     For example, as shown in FIG. 1A, the radius may decrease in the direction away from the wedge-shaped inlet slit 3, which is formed by the arched surface or by the sleeve of the roll 1 and the screen belt 2. The fibrous pulp suspension is introduced into the inlet slit 3. In FIG. 1A, the screen belt 2 winds on the roll 1 where the roll 1 has the smaller curvature, with radius R 1 , and unwinds from the roll 1 where the roll 1 has the larger curvature, with radius R 2 . 
     Devices of the type described in FIG. 1 can be easily cleaned by rotating the body surrounded by the arched surfaces by 180° around the center M. The dirty side, thus, is easily accessible for cleaning while ensuring only a brief interruption of operation. 
     According to one aspect of the invention, as shown in FIG. 1A, the watery fiber suspension, coming from the inlet slit 3, is first only somewhat drained. With the decreasing radius of the arched surface and a constant tension of the screen belt 2, the draining pressure on the suspension, located between screen belt 2 and the arched surface or the sleeve surface of the roll 1, increases. Thus, high draining capacities can be achieved with already partially drained fiber suspensions. A nozzle 4, which extends over the width of the machine, is directed against the wedge-shaped inlet slit 3. The nozzle 4 supplies the fibrous pulp suspension to the slit 3. As shown in FIG. 1A, the screen belt 2 may be a continuous screen belt. 
     In accordance with another aspect of the present invention, the sleeve surface of the roll is substantially smooth. However, the sleeve surface has elements that serve to transport the fibrous pulp suspension and the screen belt 2 in the rotation direction of roll 1. The following figures illustrate the transport elements in more detail. 
     The sleeve surface may also include a rough top surface. It is crucial in the present invention that the arched surface or the sleeve surface of the roll and the roll do not exhibit any openings. Thus, the draining of the fibrous pulp does not occur into the roll interior, as in WO 96/08600. Rather, the draining occurs outside the roll thereby assuring that the screen belt 2 is kept free from contamination due to the constant flow-through. 
     The fluid drained from the fibrous pulp suspension may be collected in one or more troughs or similar collection devices. An exemplary arrangement is illustrated in FIG. 1B in which drained fluid, indicated by arrows 12, pass through screen 2 to be collected in troughs 8 and 8&#39;. 
     As shown in FIG. 1, any contaminants deposited on the arched surface or sleeve surface of roll 1 can be removed without stopping the machine. For example, the contaminants may be rinsed from the circulating roll or the circulating arched surface with the aid of spray nozzle 20. 
     The speed of the roll 1 and the screen belt 2 are selected such that a difference exists between the speed of the sleeve surface of the roll 1 and the screen belt 2. Thus, a relative velocity between the sleeve surface of the roll 1 and the screen belt 2 exists. 
     Thus, each of the elements, i.e., the arched surface (of roll 1) and screen belt 2, is driven at a different speed. However, it is also contemplated that only one of the elements is driven and the other element remains substantially stationary. The differential speed or velocity between the elements may be maintained to be low, e.g., approximately a few centimeters or even a few millimeters per second, and may be maintained to be as high as, e.g., between approximately ten and fifty meters per second. Further, the individual elements may be driven at actual velocities of as low as, e.g., approximately a few millimeters or a few centimeters per second, and as high as, e.g., between approximately ten and fifty meters per second. 
     In a first example, roll 1 (in FIG. 1) may be driven at a speed of, e.g., approximately 10 m/sec, and screen 2 may be driven at a speed of, e.g., approximately 10.1 m/sec. Thus, a differential velocity or speed of, approximately 0.1 m/sec may be established between the elements in the draining device. In a second example, roll 1 may be driven at a speed of, e.g., approximately 10 m/sec and screen 2 may be driven at a speed of, e.g., approximately 0.1 m/sec. In a third example, roll 1 (in FIG. 1A) may stand still and only screen 2 may be driven at a speed of e.g., approximately 10 m/sec. 
     As shown in FIG. 1, at the end of the winding zone of the screen belt 2 around the sleeve surface of the roll 1, a substantially dry accumulation of fibers 6 is created which can be in the form of a sheet or, in some cases, may be in the form of small fiber rolls. The drained fibers are removed with suitable conventional known methods as, for example, being transferred to an uptake or removing roll 5 and scraped off with a scraper 7. 
     In accordance with a second embodiment of the invention, as shown in FIG. 2, the screen belt 2 is not an continuous belt. Rather, the screen belt 2 has two ends. The roll 1 rotates, while the screen belt moves very slowly in the direction of rotation or against the direction of rotation of the roll 1, as indicated by arrow 50 of FIG. 2. The screen belt 2 is thereby unwound from a roll 2.1 and wound onto a roll 2.1. The unwinding and winding processes may occur over an extended period of time. 
     The roll is shown with rails or cross-pieces 1.10 that are parallel to the axes. These cross-pieces 1.10 may extend over the entire width of the roll 1. However, the cross-pieces 1.10 may also be divided in their longitudinal direction, and the partial cross-pieces may be offset from each other in the rotational direction of the roll 1. The cross-pieces 1.10 serve to transport the fibrous pulp suspension that again is injected in the slit 3 by the pulp head box nozzle 4. The rails or cross-pieces 1.10 can be relatively small in the radial direction of the roll 1. FIG. 2 shows the cross-pieces 1.10 in an exaggerated fashion. The cross-pieces 1.10 may exhibit a radial width of several 1/10 millimeters up to several millimeters. The opposing distance of the rails or cross-pieces 1.10 can measure a few centimeters, or even a few decimeters. It is important that the cross-pieces 1.10 are able to transport the fibrous pulp suspension. 
     A certain tensile force, which is adjustable, is exerted on the screen belt 2. This adjustable force may also be present and effective in other embodiments of the present invention. 
     The screen belt 2 is moved at a creeping speed from roll 2.1 to roll 2.2 to prevent potential screen wear by not allowing the cross-pieces 1.10 to make contact at the same locations. Thus, there is uniform wear across the length of the screen belt 2. 
     As shown in FIG. 2A, in accordance with another aspect of the invention, a stiff yet flexible metal sheet 42 is used instead of a relatively flexible or elastic screen belt 2. The metal sheet 42 is clamped in position like the screen belt 2 by its ends 40, 41. Preferably, the metal sheet 42 remains stationary. Because the draining occurs through the metal sheet to the exterior, as shown by the arrows in FIG. 2A, a perforated metal sheet is particularly well suited. 
     In a particularly preferred embodiment, the metal sheet creates a tapered slit away from the inlet slit 3 in the rotational direction of the roll. This results in a higher draining pressure which improves draining. 
     FIG. 3 shows yet another embodiment of the present invention. In accordance with one aspect of the invention, the sleeve 1.1 of the roll 1 comprises a material of a certain elasticity and compressibility. The sleeve 1.1 may consist of synthetic material, for example of polyurethane. The arched surface or, in the present example, the sleeve surface 1.2 exhibits recesses 1.3 and bulges 1.4. As shown in FIG. 3, the recesses 1.3 and bulges 1.4 unfold in the form of a wave. 
     As shown in FIG. 3A, the recesses 1.3 may further be formed by a depression 1.3&#39;. Depression 1.3&#39; may be in the form of, e.g., a spherical dent, and the arched surface may include a plurality of these spherical dents being evenly spaced over the surface of the roll. Alternatively, the depression 1.3&#39; may also be formed, e.g., by grooves extending transverse to the direction of relative movement of the belt and the arched surface. Further, the grooves may be divided into longitudinal sections. As shown in FIG. 3B, the bulges 1.4 may be formed by raised portions 1.4&#39;. The raised portions may be in the form of, e.g., spherical humps, and the arched surface may include a plurality of these spherical humps being evenly spaced over the surface of the roll. 
     Further, as shown in FIG. 3C, the device may further include a covering screen 1.6 to cover the recesses formed on the arched surface of roll 1. 
     The sleeve surface 1.2 is enclosed by screen belt 2. The recesses 1.3 represent a transport volume by which the pulp suspension is transported in the rotational direction of the roll. During the transport, a draining of the fibrous pulp suspension occurs radially outwardly through the screen belt 2, as shown by the arrows in FIG. 3 that extend in the radial direction. 
     Other than as described above, the sleeve surface 1.2 of the sleeve 1.1 is substantially smooth. 
     In accordance with another aspect of the invention, the recesses 1.3 and bulges 1.4 may be created in different ways. One may, for example, undertake parallel bores 1.5 at even distances to the roll axes. This results in recesses 1.3 being formed during operation. 
     Another possibility to create recesses is to insert objects into the sleeve material which are spaced across the circumference of the sleeve 1.1. The objects may, for example, consist of rods extending over the entire width of the sleeve 1.1. If these objects exhibit a different compressibility than the remaining sleeve material, a wave form of the sleeve surface 1.2 results. 
     Another possibility to create recesses is to embed strands made of multi-filaments into the sleeve material such that the strands extend in a direction parallel to the axes. The multi-filaments have a different compressibility than the sleeve material. 
     Finally, the bores 1.5, or hoses that are inserted into the bores 1.5, may be connected to pressure sources or vacuum sources. Thus, the bores 1.5 may be inflated and deflated. In this manner, the size of the recesses 1.3 can be adjusted during operation of the device. 
     In accordance with another embodiment of the invention, as shown in FIG. 4, the arched surface, here, the roll 1, includes a support construction. In the present embodiment, the support construction is a cylindrical core 1.6. The cylindrical core 1.6 may either be hollow or solid. Support units or rails 1.8 are located on the sleeve surface of the core 1.6. The support units or rails 1.8 stand radially on the sleeve surface and may be mounted by conventional methods, such as welding. 
     The support construction or the core 1.6 with the rails 1.8 is surrounded by a hose 1.7. The hose 1.7 consists of an elastic or flexible material. The hose 1.7 exhibits a substantially smooth outer surface and is water impermeable. The hose 1.7 can be shrunk onto the radial rails 1.8 so that a wave profile is achieved during the shrinking process. The gaps 1.9 between adjacent rails 1.8 can, however, also be put under a vacuum or over-pressure, thereby influencing or even changing the wave profile during operation. 
     As shown in FIG. 5, in accordance with another aspect of the invention, the roll sleeve 1.1 may be constructed so that along the roll surface any desired shape may be formed or defined, e.g., the surface may include a grid defining a honeycomb pattern 5.1. In this regard, the grid defining honeycomb pattern 5.1 may be formed, e.g., by the above-discuees elements or filaments having a different compression characteristic than the interior portions of the grid. Alternatively, the interior surfaces of the honeycomb pattern 5.1. may be formed, e.g., by recesses. Further, the grid may be formed, e.g., by raised portions or humps in the roll surface, or by objects or bores located within the sleeve. Still further, the grid may be formed by any similar type manner utilized in the prior art. 
     FIG. 6 is a schematic diagram of a side view of another embodiment of the present invention. In FIG. 6, as in FIG. 1 or FIG. 2, the roll sleeve 1.1 comprises a grid. The roll 1 rotates at a relatively high speed. The belt 2 is a finite belt, i.e. it has two ends, one of which may be unwound from one roll 2.1 and the other of which may be wound onto another roll 2.2. While the device is operating, the belt 2 only moves at a creeping speed to prevent wear from occurring at the same location on the belt 2. 
     In accordance with another aspect of the present invention, as shown in FIG. 7, a stiff yet flexible metal sheet 42 is used in place of a flexible or elastic screen belt 2. The metal sheet 42 is clamped in position like the screen belt 2 with ends 40, 41. Preferably, however, the metal sheet 42 remains stationary. Because the draining is to occur through the metal sheet to the exterior, as shown by the arrows extending radially from the roll 1, a perforated metal sheet 42 is particularly well suited. 
     Preferably, the metal sheet 42 creates a tapered slit away from the inlet slit 3 in the rotational direction of the roll. This results in a higher draining pressure, and therefore, an improved draining. 
     The embodiments shown and described are for illustrative purposes only and are not intended to limit the scope of the invention as defined by the claims. While the preferred embodiments of the invention have been illustrated and described, the present invention is not limited by the preferred embodiments as described and illustrated above. Various changes can be made therein without departing from the spirit and scope of the invention.