Abstract:
Apparatus for removing fiber webs from fiber conveying supports are disclosed including a doctor beam, a doctor blade having a web contacting edge, a hose for fluids for applying a force between the doctor beam and the doctor blade when pressurized to move the web contacting edge towards the fiber conveying support, the fiber web having a basis weight over 1000 g/m 2 , the doctor blade and doctor beam being pivotably attached, and the hose arranged for applying the force at a distance form the pivotable attachment to move the web contacting edge by pivoting the doctor blade relative to the doctor beam.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates in general to a fibre dewatering press and in particular to arrangement for removing a fibre web from a wire in a wire press. 
       BACKGROUND 
       [0002]    Dewatering presses for dewatering of a fibre suspension and forming of a continuous web thereof are previously known. One example of a known dewatering press is a twin-wire press, see e.g. the published international patent application WO 2008/105706. Dewatering of the pulp is usually done from an inlet pulp concentration of 3-8% by weight to an output pulp concentration of 30-50% by weight. According to the state of the art, such twin-wire presses comprises two endless wires cooperating for dewatering fibre suspensions provided between them. The fibres form a continuous fibre web provided at an outlet from the twin-wire press. 
         [0003]    When the continuous fibre web leaves the twin-wire press, the wires are fed back into the press while the continuous fibre web is supposed to be released from the wires. In order to ensure that parts of the fibre web do not follow the wires back in to the twin-wire press, doctor blades are typically provided at the surface of the wires at the outlet from the twin-wire press. It is desired that the doctor blade follows the surface of the wire in close proximity when there is a potential risk for fragments of the fibre web to follow the wire. However, the roll driving the wire, which roll typically is made of a relatively soft rubber material, is easily worn. After some operation time, the roll presents a worn and uneven surface. Consequently, the wire that is formed according to the roll surface therefore also often exhibits an uneven surface, even if the wire itself is undamaged. A doctor blade originally mounted in close proximity to the original wire surface is thereby separated from at least parts of the wire surface. Also, if the doctor blade is pushed against such an uneven surface, the doctor blade may damage the raised parts of the wire. The operating conditions at this position are also relatively demanding. The temperature is often in the range of 70-90° C. and may vary with time and the environment typically comprises peroxides. In order to reduce the wear of the wire and at the same time withstand the demanding conditions, the doctor blade is typically made of a polymer material, typically a high-molecular polythene material. Furthermore, at these temperatures, plastic deformation of the material is not unusual as well as displacements resulting from changing temperatures in different parts of the press. 
         [0004]    A general problem in prior art is that the doctor blade arrangements cause too much wear on the wire and/or cannot compensate for worn wires, doctor blade deformation or shifting surrounding temperatures. 
       RELATED ART 
       [0005]    Doctor blades as such are also utilized e.g. in connection with paper machines, such as e.g. disclosed in the U.S. Pat. No. 1,566,358 or 2,914,788. However, the fibre web in a paper machine is considerably lighter than for pulp applications. In pulp applications, the basis weight is typically higher than 1000 g/m 2 . Furthermore, also other conditions differ considerable between pulp and paper applications. Fibre webs of different weights and at different conditions will behave very differently and doctor blade solutions found in paper manufacturing applications cannot without careful modifications be utilized for pulp manufacturing purposes. 
       SUMMARY 
       [0006]    An object of the present invention is therefore to provide a doctor blade arrangement presenting low wear on the wire, well adapted resilience behaviour and suitable scraping properties. 
         [0007]    The above objects are achieved by arrangements and methods according to the enclosed patent claims. In general words, in a first aspect, an arrangement for removal of a fibre web from a fibre conveying support comprises a doctor beam and a doctor blade having a web contacting edge. The fibre web has a basis weight over 1000 g/m 2 . The doctor blade and the doctor beam are attached to each other by a pivotable attachment. The arrangement further comprises a hose for fluids arranged for applying a force between the doctor beam and the doctor blade at a distance from the pivotable attachment when being pressurized to move the web contacting edge towards the fibre conveying support by pivoting the doctor blade relative to the doctor beam. 
         [0008]    In a second aspect, a twin-wire press for dewatering of a fibre suspension, comprising lower rolls, an endless lower wire, upper rolls and an endless upper wire, and an arrangement according to the first aspect arranged for removal of a fibre web from at least one of the endless upper wire and the endless lower wire. 
         [0009]    In a third aspect, a method for operating an arrangement for removal of a fibre web from a fibre conveying support comprises pressurizing of a hose for fluids arranged for applying a force between a doctor beam and a doctor blade at a distance from a pivotable attachment between the doctor beam and the doctor blade to move a web contacting edge of the doctor blade towards the fibre conveying support by pivoting the doctor blade relative to the doctor beam. The method further comprises controlling of the pressurizing dependent on an operating condition of a device providing the fibre web. 
         [0010]    One advantage with the present invention is that the wear on doctor blades and fibre conveying supports are significantly lowered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic illustration of a twin-wire press; 
           [0013]      FIG. 2  is a schematic illustration of an embodiment of a doctor arrangement according to the present invention; 
           [0014]      FIG. 3  is a schematic illustration of the embodiment of  FIG. 2  with pressurized hose; 
           [0015]      FIG. 4  is a block diagram of an embodiment of a twin-wire press arrangement according to the present invention; 
           [0016]      FIG. 5  is a flow diagram of steps of an embodiment of a method according to the present invention; and 
           [0017]      FIG. 6  is a schematic illustration of another embodiment of a doctor arrangement according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Throughout the drawings, the same reference numbers are used for similar or corresponding elements. 
         [0019]      FIG. 1  illustrates schematically a twin-wire press  1 . The illustration is very simplified in order to facilitate the understanding of the general principle. The twin-wire press  1  comprises an endless upper wire  6 A running in a path around upper rolls  8 A. The twin-wire press  1  also comprises an endless lower wire  6 B running in a path around lower rolls  8 B. A fibre suspension is entered into the space between the endless upper wire  6 A and the endless lower wire  6 B through a headbox  10  at an inlet  2  of the twin-wire press  1 . The flow path of fibre suspension/web is indicated by the arrows  5 . The fibre suspension is thus provided into a space  14  between the endless upper wire  6 A and the endless lower wire  6 B. The two wires  6 A,  6 B cooperate with each other through a dewatering section  20  of the path  5 , in which the wires  6 A,  6 B form a wedge-shaped dewatering space for the fibre suspension between each other. During displacement of the wires  6 A,  6 B through the dewatering section  20 , the wires  6 A,  6 B thus successively compress the fibre suspension in the wedge-shaped space, whereby the fibre suspension is initially pressed and dewatered and formed to a continuous fibre web between the wires  6 A,  6 B. At an outlet  3  from the twin-wire press  1 , the fibre web is released from the wires  6 A,  6 B and collected in a shredder  12 . Doctor arrangements  30  are provided at the surface of the wires  6 A,  6 B at a respective roll  8 A,  8 B. 
         [0020]    At the outlet from the twin-wire press  1 , the fibre web is peeled off from the wires. In most situations, the fibre web is uniting in itself and the fibre web is typically released from the wires without problems. However, at some occasions, e.g. when starting or stopping the twin-wire press or when disturbances normal operation conditions appear, there might be portions (or even the entire web) which continue to follow one of the wire surfaces instead of being released. If such fibre material portions are allowed to follow the endless wire into the interior of the twin-wire press again, this can cause serious problems. A doctor arrangement  30  is therefore typically provided in connection with the outlet  3  from the twin-wire press  1 . As described in the background section, prior art doctor arrangements have a number of drawbacks. The doctor arrangement  30  is provided at least one of the wires  6 A,  6 B, and typically at both. 
         [0021]    One embodiment of an arrangement for removal of a fibre web from a wire according to the present invention is illustrated in  FIG. 2 , i.e. a doctor arrangement  30 . A roll  8  drives a fibre conveying support  21 , in this embodiment an endless wire  6 , in a conveying direction  26 . In other embodiments, the fibre conveying support  21  could e.g. be the surface of a roll. A fibre web, in this embodiment a web of cellulose pulp, is intended to be conveyed on a surface  24  of the endless wire  6  and be released therefrom before the endless wire  6  returns into the interior of the twin-wire press. The doctor arrangement  30  comprises a doctor blade  32  attached to a doctor beam  34 . The doctor blade  32  is provided for prohibiting any parts of the fibre web to follow the fibre conveying support beyond the outlet  3 . The doctor blade  32  is mechanically supported by the doctor beam  34 , and the doctor beam  34  constitutes the attachment of the doctor blade  32  to the main fibre handling equipment, i.e. in this embodiment the twin wire press. In this embodiment, a portion of the doctor beam is formed as a clamping plate  36 , which clamps the doctor blade  32  against the doctor beam  34  with a pivoting point  35 . The clamping plate  36  is firmly attached to the main part of the doctor beam  34 , in this embodiment by bolts  38 . The clamping plate  36  is in the present embodiment bent in such a way that outside the pivoting point  35 , there is a distance  39  between the doctor blade  32  and the clamping plate  36 , which allows the doctor blade to pivot out from the doctor beam surface around the pivoting point  35 . The clamping plate  36  and the end of the doctor beam  34  thus together form a pivotable attachment  37  of the doctor blade  32 . 
         [0022]    The doctor arrangement  30  is preferably originally mounted in such a way that a web contacting edge  33  of the doctor blade  32  is placed just at the surface  24  of the wire  6 , when the doctor blade  32  is positioned along the surface of the doctor beam  34 . When surrounding conditions are changed, e.g. due to wear of the roll  8  or wire  6 , different temperatures or plastic deformation of the doctor blade, a slit may be left between the web contacting edge  33  and the surface  24  of the wire. For such occasions, a hose  40  for fluids is provided in a recess  41  of the doctor blade  32 . The hose  40  can generally be pressurized with any fluid-gas or liquid and is therefore adapted for being connected to a hose pressurizing arrangement. In the present embodiment, the hose  40  is intended to be pressurized by air. 
         [0023]      FIG. 3  illustrates a situation when the hose  40  is pressurized by a (not shown) hose pressurizing arrangement. The hose  40  expands and protrudes outside the recess  41  and applies a force onto a support surface  42  of the doctor beam  34 . This support surface  42  can be a portion of the main doctor beam  34  itself or be a portion provided with a surface coating adapted for interaction with the expanded hose  40 . In general words, the hose  40  is arranged for applying a force between the doctor beam  34  and the doctor blade  32  at a distance from the pivoting point  35  when being pressurized. The rear end of the doctor blade  32  is thereby moved out from the surface of the doctor beam  34 , leaving a space  43  there between. This force acts to move the web contacting edge  33  towards the fibre conveying means  21 , in this embodiment the wire  6 . The web contacting edge  33  is thereby held against the surface  24  of the wire  6  with a force that is determined by the pressure applied in the hose  40 . The pivoting of the doctor blade  32  is typically limited by the distance  39  to the clamping plate  36 . The web contacting edge  33  follows the surface  24  of the wire  6  even if the roll or wire is unevenly worn. Also mechanical movements in the doctor arrangement  30  as a result of e.g. temperature differences are compensated by such an arrangement. Furthermore, if the doctor blade  32  itself, which typically is made of high-molecular polythene, undergoes plastic deformation, also this is compensated. The doctor blade  32  is preferably manufactured in a material that is relatively soft, such as e.g. high-molecular polythene, which allows the doctor blade  32  to adapt and/or be worn according to the surface profile of the wire and/or roll. 
         [0024]    Now returning to  FIG. 2 . When the rather thick fibre web comes into contact with the doctor blade  32  that is gently pressed against the wire  6 , the fibre web also creates a force F on the doctor blade  32 . This force F increases the pressure on the web contacting edge  33  against the surface  24  of the wire  6 . The doctor blade  32  wants to rotate around the pivot point  35 . A higher normal force N is thereby obtained, which increases the friction force R from the wire  6 . The higher the force F becomes the better and tighter the doctor blade  32  is pressed against the wire  6 , and the doctor blade  6  doctors away the fibre web from the wire. Because of this function, there is only need of a small pressure in the hose  40  at normal running to keep the doctor blade  32  close to the wire  6 . Such a lower pressure gives a lower friction and longer lifetime of the wire  6  and doctor blade  32 . 
         [0025]    Thus, during the majority of the operation time of a twin-wire press, the release of the fibre web, i.e. in this case the release of a pulp mat, functions without any need for assistance from any fluid loaded doctor arrangements. During such periods, it would therefore be beneficial if the doctor blade  32  is not actively pressed against the wire  6  surface. As mentioned above, this could reduce the wear both on the wire  6  and/or roll  8  and on the doctor blade  32 . There is thereby a need for being able to control the pressurizing of the hose  40  depending on the actual operating conditions of the twin-wire press. 
         [0026]    The upper doctor blade of a twin-wire press is typically in no contact with the pulp mat when the machine is running. The doctor blade only picks up or pulls out small amount of fibres that is stuck into the wire. The smaller fibre particles more or less follow the wire, even if the doctor blade has loosened the fibre from the wire. Spraying systems are typically provided to handle these particles later. 
         [0027]    Also the lower doctor blade is in no contact to the pulp mat when the machine is running properly. The pulp mat structure and its own weight is pulling the pulp mat out from the wire so that the doctor blade goes free without contact to the pulp mat. 
         [0028]    This typical phenomenon occurs in particular with pulp mats that has a sufficient strength and weight, and it is presently believed that it is a requirement to have a basis weight that is over 1000 g/m 2 , and preferably over 1200 g/m 2 , to achieve this phenomenon. In start up processes and shut down processes of the machine, often both the upper and lower doctor blades are in contact with the pulp mat. Then there is a need to have a strong doctor blade, but made of a gently material for reducing the damage on the wire. This is because the high contact forces that this thick pulp mat generates. The minimum basis weight for pulp mats produced by twin-wire presses is 1200 g/m 2 , which makes the present doctor blade arrangements according to the present invention particularly suitable. 
         [0029]    A further advantageous function for the lower doctor blade arrangement is to provide for a contact between the pulp mat and the clamping plate. This stabilizes the pulp mat on its way to the shredder screw. Without this contact, the pulp mat is more easily broken because of the action of the forces that are applied at the shredder screw and from the weight itself. When a break occurs, the pulp mat looses the force from its own weight that pulls out the pulp mat and there is typically a doctor blade contact against this very thick pulp mat that gives more fibre losses and wearing on the doctor blade. Typically, the speed of the wire is in the range of 10-40 m/min at normal operation. 
         [0030]    A fibre handling system according to an embodiment of the present invention is illustrated in  FIG. 4 . A twin-wire press  1  for dewatering of a fibre suspension has doctor arrangements  30 , e.g. according to the embodiment of  FIGS. 2 and 3 , mounted at the outlet from the twin-wire press  1 . A hose pressurizing arrangement  50  is connected to a hose of the doctor arrangements  30 . A controller  52  is arranged for controlling the hose pressurizing arrangement  50 . The controller  52  is connected for receiving information about the operation conditions of the twin-wire press  1 . Based on this information, the controller  52  can pressurize the hoses of the doctor arrangements  30  when needed, e.g. during starting or stopping of the twin-wire press  1 , or when other disturbances in the operation is detected. During normal operation of the twin-wire press  1 , the controller  52  can instead allow a release of the pressure and thereby allow the doctor blade to pivot out from the wire surface. 
         [0031]      FIG. 5  illustrates a flow diagram of steps of an embodiment of a method according to the present invention. A method for operating an arrangement for removal of a fibre web from a wire starts in step  200 . In step  210 , a hose for fluids is pressurized. The hose is arranged for applying a force between a doctor beam and a doctor blade at a distance from a pivotable attachment between the doctor beam and the doctor blade to move a web contacting edge of the doctor blade towards the wire. The pressurizing is in step  212  controlled dependent on an operating condition of a device providing the fibre web. The procedure ends in step  299 . 
         [0032]    The embodiment illustrated in  FIGS. 2 and 3  has the hose provided in a recess in the doctor blade and arranged to actuate on the doctor beam. Furthermore, the hose applies the force on the doctor blade at a side opposite to the web contacting edge relative to the pivotable attachment  37  and pivoting point  35 . However, there are also alternative embodiments. One alternative embodiment is illustrated in  FIG. 6 . Here, the hose  40  is provided in a recess  41  in the doctor beam  34  instead. The hose  40  is thereby arranged to actuate on a support surface  42  at the doctor blade  32 . In another alternative embodiment the hose  40  can be provided such that the hose applies the force on the doctor blade at a same side as the web contacting edge relative to the pivotable attachment. 
         [0033]    The clamping of the doctor blade  32  to the doctor beam  34  at limited areas gives rise to a simple embodiment of a pivotable attachment. Anyone skilled in the art realizes that also other types of pivotable attachments can be utilized, such as different kinds of hinges. 
         [0034]    In the embodiments above, the fibre conveying support is exemplified by a wire. However, the present ideas also operate well with also other types of fibre conveying support, such as e.g. different kinds of rolls. 
         [0035]    The conditions at the outlet from a fibre web handling arrangement, such as a twin-wire press, are relatively special. The environment is hazardous, typically comprising peroxides. At the same time, the temperatures are often in the range of 70-90° C., but may also vary considerably, in particular at starting and stopping the fibre web handling arrangement. The presented solutions are well adapted to withstand such environments at the same time as they provide for simple and cost-efficient operation. 
         [0036]    The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.