GB2124669A - Positive pulse preconditioning blade - Google Patents

Abstract

This invention involves the use of a positive pulse blade to precondition a wet web of material carried on a wire mesh. The blade (22) is removably mounted in fixed position relative to a wet web of material (28) and mesh to be passed thereover. An upper wear surface (52) is on the blade for engagement with the mesh. Force is applied to entrain water from the web on the underside of the mesh. The blade has a leading edge (48) with a configuration tending to direct all of the water entrained on the underside of the mesh when it is moved toward the blade up through the web to apply a positive pulse to the wet web and precondition the web for facilitating later dewatering of the web. <IMAGE>

Description

SPECIFICATION Positive pulse preconditioning blade This invention relates to a positive pulse preconditioning blade for facilitating the removal of liquid from a wet web of material.

In dewatering machinery, particularly in the papermaking industry where dewatering processes are common on the wet end of a papermaking machine; development work is constantly being pursued to provide for more efficient and effective dewatering of materials.

An example of typical applicable machinery is a Fourdrinier paper machine utilizing a dewatering box with vacuum applied to draw water from a combination wire mesh and sheet of material in the form of a wet web. A difficulty that occurs in a process of this type is that the material being dewatered accumulates in the form of a slurry (wet web) on the wire mesh, and, as the combination progresses along the machinery, it has a tendency to compress or mat. This matting action makes it more difficult to remove water. This concern is enhanced when one considers that vacuum is applied at various points along the papermaking machinery particularly on the wet end portion. The constant application of vacuum results in a constant negative pulse to the wet web further tends to mat the slurry on the mesh. The result is a reduction in efficiency as the process continues.

Various attempts have been made to increase dewatering efficiency. For example, increased vacuum has been used, either as a constant greater suction force or as a progressively increasing suction force as matting increases during the dewatering process. A variety of different types of blades, foils and deflectors have been developed with a variety of different configurations to enhance the dewatering process by vacuum and by negative pulse. This includes reversing traditional blade shapes and forming stepped blades in various shapes.

A positive pulse concept has been explored to a certain degree particularly in regard to table rolls.

This involves bringing the mesh carrying the wet web into engagement with table rolls which through the mesh force the web upward and provide a possible positive pulse. However this action is uncontrollable and is limited to a direct speed/roll diameter relationship. It is not capable of being controlled to provide the more efficient dewatering procedure particularly when combined with blades and with vacuum. There is clearly room for an improved deliquefying ordewatering system which makes use of the advantages of blade configurations for negative pulsing and drawing water through the mesh from the wet web and also for applying a positive pulse through the mesh to the web to loosen the matted web and enhance the dewatering.

Naturally the designed system should be adaptable for application of appropriate negative and positive pulses along the length of the machine, particularly if it is the wet end of a papermaking machine.

Naturally a simple and inexpensive structure is desirable, for example an improved single blade configuration which can be removably mounted in a stationary position on existing machinery such as a Fourdrinier machine.

Attempts at combining positive pulsing with other treatment activity have produced problems in the success of ultimate dewatering of the web. For example, those system which dewater and then contain the water in a base reservoir or other structure until the water level in the reservoir causes upward pulsing of the mat is undesirable for several reasons. In some of these systems there exists no easy means of controlling the positive pulsing action. Also, in some a substantially increased system weight is effected. Others do not allow water contained in the slurry to be removed through the mesh and then reintroduced through the mesh into the slurry. In all of these systems an accumulation of dirt and ultimate clogging in the system is produced and the captured water in the reservoir interferes with the integrity of the entrained water on the bottom of the mesh.The water from the slurry entrained on the bottom of the mesh is desirable for increasing the efficiency of the pulsing action in that the moving entrained layer of water can be controlled and directed through the web with the most positive pulsing action obtainable.

In our British patent application publication No. 2 082 644 a positive pulse dewatering blade is described which simultaneously applies a positive pulse to the web and removes a substantial part of the water entrained on the underside of the mesh.

Simultaneous positive pulsing and dewatering action in combination detracts from the desired result. The dewatering action is less efficient in that only a portion of the water from the slurry is directed back through the wire to the web and consequently the agitation is less pronounced.

Another attempt concerning dewatering is one which separates the water being removed from the web so that a major portion is disposed of in a drainage slot and a smaller portion is directed as a film over the wear surface of the foil or blade. This is designed for producing a lubricated surface for the mesh as it passes over the stationary blade. In this type of system, there is little or no effect on the condition of the fibers on the web. It is not designed for agitation or passing fluid up through the mesh into the web. The only action is in the formation of a thin film for lubrication purposes.

Accordingly, there is a clear absence of any concept for separating dewatering and preconditioning of the web so that the web fibers can be worked prior to the dewatering vacuum action to provide an optimum and efficient removal of liquid orwater therefrom.

With the above background in mind the invention provides a blade adapted to be mounted on a dewatering machine such as a papermaking machine. The blade is designed with an upper wear surface over which a moving porous conveyor carrying a wet web of material is passed. By contact with a predetermined configuration of a leading edge of the blade substantially all of a predetermined amount of liquid entrained on the undersur face of the porous conveyor is directed up through the latter to precondition the web for later dewatering.

Later dewatering of the web can then occur with or without the assistance of the trailing edge portion of the blade or by introduction to later dewatering blades. At that time, a negative pulse produced by vacuum may be introduced to the wet web to aid the dewatering purposes.

A blade of the above type may be unitary in construction and adapted to be mounted on a conventional well known type of machine such as a Fourdrinier machine. The blade may be adapted to be mounted in sequence.as a row of blades having aligned wear surface for passage of the porous conveyor and sheet thereover in the form of a wet web. The blade may be adapted to be mounted so that a suitable force, such as vacuum or gravity will entrain a predetermined portion of water on the underside of the porous conveyor and when the latter is directed to the blade the water will be pulsed up through the porous conveyor into the wet web to precondition the web for later dewatering. The preconditioning occurs with substantially no drainage or removal of any water or liquid from the web porous conveyor system.

The blade may have a leading edge portion having a configuration including a tip which is spaced from the undersurface of the porous conveyor a distance equal to or greater than the thickness of the entrained liquid on the underside of the porous conveyor as it moves toward the blade. In this manner, the entrained liquid passes through the porous conveyor and the tip and is then directed by the blade up toward and through the conveyor into the wet web. The result is a positive pulsing action only for preconditioning the web.

It is contemplated that the blade portion between the tip and the upper wear surface can be at any acute or obtuse angle or any combination of acute or obtuse angles as well as containing arcuate as long as the blade will direct the entrained liquid up through the conveyor into the web. An example of an effective angle is one between 0 and 10 degrees.

An example of an effective distance for the tip of the leading edge from the underside of the conveyor relative to an entrained amount of liquid on the underside of the conveyor would be 6 millimeters with the thickness of the liquid being equal to or less than 6 millimeters and the tip being spaced a distance equal to or greater than 6 millimeters from the underside of the conveyor. In this manner, substantially all of the entrained water is directed up through the conveyor into the web and substantially none is drained from the system.

The invention also provides one alternative form of the blade of the present invention in which it has a configuration adjacent and subsequent to the upper wear surface designed to facilitate the application of a negative pulse. In one form, this would include the use of a step to provide a recess adjacent and subsequent to the upper wear surface and a drainage slot and vacuum means after the blade in the direction of travel of the conveyor. In lieu of reintroducing the entrained water a subsequent blade designed for dewatering can be employed.

The blade is designed to be suitable for preconditioning a wet web as the first blade in a series of blades on the wet end of a machine such as a papermaking machine. In this way, the web would be preconditioned before subjecting it to a series of dewatering blades. Alternatively, the positive pulsing blade for preconditioning the web can be one of a series of such blades and also can be one of a series of blades alternated with a corresponding series of dewatering blades so that the wet web is preconditioned with a positive pulse by one blade prior to introduction of the dewatering blade. In this manner, dewatering is facilitated by continuously preconditioning the web prior to dewatering. The blades would be spaced to provide appropriate drainage slots therebetween in a conventional manner.

The preconditioning positive pulsing blade tends to first loosen the matted web, the water of which thereafter can be again entrained on the bottom of the conveyor by the same blade or dewatered when subjected to a following dewatering means.

The positive pulsing blade of the present invention may be used in conjunction with a vacuum source so that a controlled amount of vacuum is applied to the web to entrain a predetermined amount of liquid on the underside of the conveyor to be directed into the leading edge configuration of the blades for pulsing the liquid up through the conveyor into the web of material as a preconditioning action.

The blade structure may induce positive pulses in locations including the first blade on a Fourdrinier having a series of blades in which the following blades are dewatering blades, or as one of an alternate arrangement of preconditioning blades and dewatering blades. The preconditioning positive pulse blade of the present invention is designed for use with forming boards, wet boxes, vacuum augmented foils, twin wire, and in connection with flat box drainage systems. Also, the blade concept of the present invention can be extended to include the leading edge of any stationary device located beneath the forming media such as foils and/or deflectors.

The action of the blade of the present invention may be enhanced by the use of vacuum prior and adjacent to the leading edge said vacuum entraining the liquid on the underside of the conveyor also serves to hold the web down while the liquid is pulsed back up through the conveyor into the web to enhance the preconditioning action and also seais the system to further allow close control of the preconditioning for later efficient dewatering.

With an arrangement embodying the present invention it is possible to entrain and retain a predetermined amount of water on the underside of a moving porous conveyor to be directed back up through the web for conditioning the web for later dewatering. Virtually all of the liquid is reintroduced through the conveyor into the web and drainage and dewatering does not occur during the conditioning action. The result is an improved formation of the slurry on the conveyor and reduced filtration resistance of the web during later dewatering action.

Formation of the web is enhanced by the loosening and redistribution of fibres to a more effective arrangement. Additionally, the liquid or water is permitted to distribute because of the additional time on the web to normalize. This feature is provided by the fact that no dewatering occurs during the preconditioning action but only occurs later.

Close control of the preconditioning action is provided since the amount of vacuum applied in relation to the speed of movement of the conveyor and the web will enable close control of the amount of liquid entrained on the underside of the conveyor for contact with the blade and redirection of all of the liquid back up into the web. By using the water in the web and not collecting or containing water removed from the web in a reservoir reduces the possibility of dirt accumulation in the system and also reduces the weight load on the machinery. Also, there is no interference with the entrained liquid so that it is freely moving and controlled to move with the conveyor directly into contact with the blade and directly up through the conveyor into the web to get the most effective and positive results in the preconditioning action.

In summary, a system is provided for preconditioning a wet web of material to be later dewatered.

The system includes a positive pulse blade which is adapted to be removably mounted in fixed position relative to the conveyor carrying the wet web of material to be passed thereover. The blade has an upper wear surface for engagement with the conveyor carrying the wet web of material. The blade has a leading edge of such configuration as to direct substantially all of the liquid, entrained on the bottom of the conveyor by a downward force, up through the conveyor after it engages the leading edge in order to apply a positive pulse to the wet web and precondition the web for facilitating later deliquefying of the web.

Alternatively, it should be understood that the present invention may be oriented other than the horizontal arrangement depicted and described. For example, it may be employed in a vertical or inverted arrangement. However, the entrainment force could not be gravity in other than substantially the described position.

By way of example an embodiment of the invention will now be described with reference to the accompanying drawings, of which: Figure lisa fragmentary top plan view of the wet end of a papermaking machine with a succession of blades mounted thereon; Figure 2 is a fragmentary sectional elevational view thereof taken along the plane of line 2-2 of Figure 1; and Figure 3 is an enlarged fragmentary sectional elevation view thereof.

The top surface of a portion of the wet end of a Fourdrinier papermaking machine 20 is depicted in Figure 1. Figure 2 shows a side elevation view of a portion of machine 20 with a series of spaced aligned blades removably mounted thereon in a conventional manner. Of the depicted blades, blade 22 is a positive pulse preconditioning blade designed in accordance with the present invention and is positioned between two conventional dewatering blades 24 and 25. A slot 26 is provided between blades 24 and 22 and a slot 27 is provided between blades 22 and 25. Downward forces which may be vacuum from a conventional vacuum source are separately applied to slots 26 and 27 and thus to the upper surface of the machine 20. Suitable end deckles 62 are provided to seal the ends so that the slots 26 and 27 are located in the central portion of the machine 20.The wet web of material 28 is in the form of a web of fibres 30 positioned on a porous conveyor, for example a wire mesh 32. The fibre web is initially in the form of a slurry which is compacted during travel when subjected to normal gravitational forces and additional vacuum force when applied during processing. The object of the machinery is to ultimately remove water or dewater or otherwise deliquefy the wet web 28.

Suitable controls may be provided from a standard vacuum source to individually control the vacuum applied to the slot 26 and applied to slot 27.

Normally they are of different value. The gravitational or vacuum force applied to slot 26 is not for dewatering purposes but for the purpose of entraining liquid on the underside of the mesh 32 and also for holding the mesh in contact with the upper wear surface 52 of blade 22. On the other hand, the vacuum applied through slot 27 is designed to assist in dewatering the web 28 and collect and remove the water from the machinery.

In connection with slot 27, vacuum is applied to entrained water from the wet web to be subsequently dewatered by the leading edge of blade 25. In contrast, the water that is drawn from the web 28 is entrained on the underside of the mesh 32 and travels with the mesh 32 toward the blade 22.

However, none of this entrained water is removed, it is retained with the moving mesh and web.

Blade 24 is a conventional step type blade with a leading horizontal wear surface portion 34 which terminates in a shoulder 36 communicating with a recess surface 38. The vertical space between surfaces 34 and 38 forms an area for generation of a negative pulse to the wet web through communication with slot 26. A predetermined amount of vacuum is applied to the web through slot 26 to entrain an amount of liquid 40 on the underside of the mesh 32 with a predetermined thickness "t".

Gravity and this negative pulsing action tends to compact and mat the web slurry on the mesh thus making deliquefying of the web more difficult. Blade 22 is utilized to precondition the matted web to make it more susceptible to efficient deliquefying. Commonly, the liquid being removed from the web is water and, in further describing the depicted embodiment, water will be identified as the liquid being removed from the web. This is a common procedure in papermaking machinery.

Blade 22 includes a downwardly extending projection 42 from its undersurface 44 for interlocking engagement with receiving surfaces on a conventional papermaking machine. Blade 22 with projection 42 is adapted to be mounted in any one of a number of positions on the papermaking machinery.

It can be the first in a line of blades with the blade 22 being a conditioning blade and the blades following being dewatering blades such as blade 25, or there can be a combination of both types. It is possible to alternate preconditioning blades such as blade 25 so that the web is preconditioning to enhance the dewatering action before it is introduced to the dewatering blade.

The amount of downward force used at slot 26 is sufficient to entrain water on the underside of the mesh. Alternatively to the use of a blade, such as blade 24, at the entrance to the machine, an ordinary support or roll can be used or any other conventional support means. The blade contacting the mesh prior to any dewatering within this system would be blade 22.

Blade 22 includes a leading edge 44 for initial communication with the moving mesh 32 and the water 40 entrained therewith. The lower portion 46 of leading edge 44 may be beveled down and toward the trailing end 45 of blade 22. The upper portion 48 of leading edge 44 is tapered upward and toward the trailing end of blade 22. Lower portion 46 and upper portion 48 meet at an apex or tip 50. The other end of lower portion 46 terminates at lower edge 44 of blade 22 and the other end of upper portion 48 terminates at an upper wear surface 52 on the blade which contacts the mesh 32 passing thereover and is sealed in that interengaging arrangement when subjected to vacuum through slot 26. Thus, the upper portion 48 forms a nip with the underside of the mesh 32.

Tip 50 is spaced a predetermined distance "d" beneath the undersurface of the mesh 32. This distance is determined to be equal to or greater than the thickness "t" of the entrained water 40 on the undersurface of the mesh 32 when vacuum is applied to slot 26. The surface 48 extends forwardly for an distance at least equal but preferably greater than the thickness "t" of entrained water. Thus, all of the entrained water 40 passes between tip 50 and the undersurface of mesh 32 and is directed upward and through the mesh 32 as a positive pulse. No water is drained through slot 26. Pulsing the water upward through the web loosens the matted slurry on the mesh so that water can be withdrawn from the wet web more easily and efficiently thereafter. This action is defined as application of a positive pulse.In addition to an improved formation of slurry, positive pulsing in this manner also reduces filtration resistance of the web and provides for an improved redistribution of water and fibres through the web before actual dewatering takes place at a later time and location.

The web then passes beyond wear surface 52 of blade 22 at which time it is exposed to the dewatering vacuum in slot 27. The preconditioned web is then dewatered and the water is collected through slot 27 for disposal. As previously stated, the number of preconditioning blades 22 and dewatering blades 25 is a matter of choice and sequence.

Several preconditioning blades can be placed in sequence followed by several dewatering blades or they could be alternated.

Each of the blades is supplied with a notch 54 in its trailing edge for facilitating mounting of end deckle 62 to seal the ends of the papermaking machinery in a conventional manner so that the vacuum is concentrated in the open central portion of the slots.

The blades are interchangeable and removable so that they can be replaced or interchanged at any desired time. Additionally, they can be formed of a conventional plastic or nylon material in a conventional well known manner.

In papermaking machinery, it has been found effective for dewatering to form a thickness "t" of up to no greater than 6 millimeters for the entrained water and a distance "d" of the tip 50 from the mesh 32 at least equal to or greater than 6 millimeters. In this manner, all of the entrained water is directed by leading edge portion 48 back up through the mesh 32 for conditioning purposes.

In the depicted embodiment, an acute angle is shown between the upper portion 48 of leading edge 44 and the mesh 32. The angle is a matter of choice and can be acute, for example up to 10 degrees as shown, or a right angle or obtuse angle at the point of introduction to the mesh or a series of stepped sections of different angles, or one or more arcuate surfaces. The only requirement is that the water entrained on the undersurface of the mesh be directed up through the web for conditioning purposes.

In operation, as shown, the mesh 32 travels from left to right driven by suitable drive means of a conventional type on the machine. It is directed over wear surface 34 of blade 24 after which it becomes exposed to the slot 26 where the predetermined vacuum applied to that slot will cause a predetermined thickness of water to become entrained on the undersurface of mesh 32 as it moves. None of this water is drained through slot 26. The web and entrained water then is directed to leading edge 44.

All of the water will contact the leading edge 44 above tip 50 and will be directed upward through the web by engagement with upper surface portion 48 of leading edge 44. This causes a positive pulsing of the web and conditions the web for later dewatering.

The conditioned web is then introduced to the vacuum of slot 27 where the water is entrained on the underside of the mesh for subsequent dewatering by the leading edge of blade 25 prior to disposal.

The conditioned web 28 enhances the dewatering action. To facilitate collection of the water, the leading edge 56 of blade 25 is beveled downward and rearward toward the trailing edge of blade 25 to aid in directing the air and water mixture for drainage through the slot 27.

The web 28 is then passed to further conditioning and/or dewatering blades and slots for further treatment in the same manner. The dewatering action may occur after a negative pulse applied to the web as opposed to a positive pulse which is applied during the conditioning action.

Either vacuum, gravity or pressure or a combination thereof may be used to provide the force to direct water through the mesh for entrainment thereon. The device would be determined by, among other things, the orientation of the web and mesh.

Thus the several aforenoted objects and advan tages are most effectively attained. Although several somewhat preferred embodiments have been disclosed and described in detail herein, it should be understood that the scope of this invention is to be determined with reference to the appended claims.

Claims (31)

1. A method of preconditioning a wet web of material being carried on a porous conveyor comprising: arranging a positive pulse blade and the wet web of material on the conveyor in position so that the conveyor carrying the wet web engages and passes over an upper wear surface on the blade, subjecting the wet web of material and conveyor to a predetermined force before it reaches the blade in the direction of travel to cause water from the web to be entrained on the underside of the conveyor and to be brought into engagement with the blade, and providing a leading edge on the blade having a configuration tending to direct substantially all of the entrained water up through the conveyor into the web to apply a positive pulse to the wet web and precondition the web for facilitating later dewatering of the web.

2. A method as claimed in claim 1 wherein the means for entraining water from the wet web on the underside of the conveyor includes at least one of the application of vacuum, gravity, and pressure to the web conveyor.

3. A method as claimed in claim 1 or 2 wherein the leading edge of the blade has a tip spaced a predetermined distance below the underside of the conveyor carrying the web and the predetermined distance being greater than the thickness of the liquid entrained on the underside of the conveyor.

4. A method as claimed in claim 3 wherein the tip of the leading edge of the blade is spaced from the underside of the conveyor a distance equal to or greater than 6 millimeters.

5. A method as claimed in claim 3 or4 in which the portion of the leading edge of the blade between the tip and the underside of the conveyor forms a predetermined acute angle with the underside of the conveyor to facilitate direction of the liquid entrained on the underside of the wire up through the conveyor into the web.

6. A method as claimed in any preceding claim wherein the blade has a trailing portion following the upper wear surface adapted to be positioned prior to a draining space and the portion having a configuration tending to entrain liquid on the bottom of the conveyor and apply a negative pulse to facilitate removal of liquid from the web to be collected through the drainage space after the web has been previously subjected to a preconditioning positive pulse.

7. A method as claimed in claim 6 wherein the negative pulse applied to the drainage space is provided by at least one of the application of vacuum, gravity and pressure.

8. A method as claimed in claim 6 or 7 wherein the trailing portion is adjacent the draining space.

9. A method as claimed in any preceding claim wherein the blade is mounted on the wet end of papermaking machinery for preconditioning a wet web for later dewatering.

10. A method as claimed in any preceding claim wherein the blade is mounted as one of a sequence of blades in the direction of travel of the wet web with the blade providing a positive pulse to precondition the wet web and at least one of the other blades providing a negative pulse with alternating pulses being applied to the wet web to provide efficient dewatering ofthe web.

11. A method as claimed in any preceding claim wherein 100 percent of the liquid entrained on the underside of the conveyor is pulsed back up through the conveyor by the blade.

12. A method as claimed in any preceding claim wherein the liquid entrained on the underside of the conveyor travels to the blade unsupported from below and without containment means until contacted by the leading edge of the blade.

13. A method as claimed in any preceding claim wherein a dewatering blade is positioned for engagement with the conveyor downstream of the preconditioning blade for dewatering of the preconditioned web and conveyor.

14. A method of preconditioning a wet web of material being carried on a porous conveyor comprising passing the wet web on the conveyor with liquid entrained on the underside of the conveyor over a wear surface of a preconditioning blade having a leading edge configured to direct substantially all of the entrained liquid up through the conveyor into the web to apply a positive pulse to the wet web and precondition the web for facilitating later removal of the liquid from the web.

15. A method of preconditioning a wet web of material being carried on a porous conveyor, the method being substantially as herein described with reference to and as illustrated by the accompanying drawings.

16. A method of treating a wet web of material, the method including carrying out a method as claimed in any preceding claim and subsequently removing liquid from the web.

17. An apparatus for preconditioning a wet web of material being carried by a porous conveyor comprising: mounting means for removably mounting a positive pulse preconditioning blade in fixed position relative to the conveyor and wet web of material passing thereover with liquid entrained on the underside of the conveyor, an upper wear surface on the blade for engagement with the conveyor carrying the wet web of material, a leading edge on the blade having a tip spaced from the underside of the conveyor a distance equal to or greater than 6 millimeters to direct up through the conveyor substantially all of the liquid entrained on the underside of the conveyor as it is moved toward the blade to thereby apply a positive pulse to the wet web and precondition the web for facilitating later removal of liquid from the web.

18. An apparatus as claimed in claim 17 wherein means for entraining liquid from the wet web on the underside of the conveyor are provided, the means including at least one of the application of vacuum, gravity, and pressure to the web and conveyor.

19. An apparatus as claimed in claim 17 or 18 wherein the portion of the leading edge of the blade between the tip and the underside of the conveyor forms a predetermined acute angle with the underside of the conveyor to facilitate direction of the liquid entrained on the underside of the conveyor up through the conveyor into the web.

20. An apparatus as claimed in any of claims 17 to 19 wherein the blade has a trailing portion following the upper wear surface adapted to be positioned prior to a drainage space and the portion having a configuration tending to entrain liquid on the bottom of the conveyor and apply a negative pulse to facilitate removal of liquid from the web to be collected through the drainage space after the web has been previously subjected to preconditioning positive pulse.

21. An apparatus as claimed in claim 20 wherein the negative pulse applied to the drainage space is provided by at least one of the application of vacuum, gravity, and pressure.

22. An apparatus as claimed in claim 20 or 21 wherein the trailing portion is adjacent the drainage space.

23. An apparatus as claimed in any of claims 17 to 22 wherein the blade is mounted on the wet end of papermaking machinery for preconditioning a wet web for later dewatering.

24. An apparatus as claimed in any of claims 17 to 23 wherein the blade is mounted as one of a sequence of blades in the direction of travel of the wet web with the blade providing a positive pulse to precondition the wet web and at least one of the other blades providing a negative pulse with alternating pulses being applied to the wet web to provide efficient removal of liquid from the web.

25. An apparatus as claimed in any of claims 17 to 24 wherein 100% of the liquid entrained on the underside of the conveyor is pulsed back up through the conveyor by the blade.

26. An apparatus as claimed in any of claims 17 to 25 wherein the liquid entrained on the underside of the conveyor travels to the blade unsupported from below and without containment means until contacted by the leading edge of the blade.

27. An apparatus as claimed in any of claims 17 to 26 wherein a dewatering blade is positioned for engagement with the conveyor downstream of the preconditioning blade for dewatering of the precondition web and conveyor.

28. A positive pulse blade for use in preconditioning a wet web of material being carried by a porous conveyor comprising: mounting means for removably mounting the blade in fixed position relative to the conveyor and wet web of material passing thereover, means for entraining water from the wet web on the underside of the conveyor, an upper wear surface on the blade for engagement with the wire carrying the wet web of material, a leading edge on the blade having a configuration tending to direct up through the conveyor substantially all of the liquid entrained on the underside of the conveyor as it is moved toward the blade to thereby apply a positive pulse to the wet web and precondition the web for facilitating later dewatering of the web.

29. A positive pulse blade for use in preconditioning a wet web of material being carried by a porous conveyor, the blade including mounting means for removably mounting the blade in fixed position relative to the conveyor and wet web of material passing thereover and an upper wear surface for engagement with the conveyor carrying the wet web of material wherein the blade has a leading edge having a tip spaced from the plane of the upper wear surface by a distance equal to or greater than 6 millimeters.

30. An apparatus for preconditioning a wet web of material being carried buy a porous conveyor, the apparatus being substantially as herein described with reference to and as illustrated by the accompanying drawings.

31. A positive pulse blade substantially as herein described with reference to and as illustrated by the accompanying drawings.