EP0147350B1

EP0147350B1 - Converflo trailing element

Info

Publication number: EP0147350B1
Application number: EP84630167A
Authority: EP
Inventors: Jose Juan Antonio Rodal; James Leroy Ewald
Original assignee: Beloit Corp
Current assignee: Beloit Technologies Inc
Priority date: 1983-11-25
Filing date: 1984-11-09
Publication date: 1989-03-15
Also published as: FI81145B; CA1230251A; NO162476B; EP0147350A3; ZA848555B; ES8507641A1; AU3564284A; FI844059L; KR860001627B1; EP0147350A2; ES537930A0; IN162165B; MX161597A; JPS6146597B2; DE147350T1; BR8405925A; NO844431L; FI844059A0; FI81145C; JPS60134093A

Description

The invention relates to a headbox for delivering stock to a forming surface as defined in the pre-characterizing portion of claim 1.
Such a headbox is known from DE-A-2 142 533. The trailing elements of this headbox have corrugations to generate turbulence in the stock flowing toward and through the slice opening. Due to the corrugations the trailing elements have a structural stiffness greater in the cross-machine direction than in the machine direction. However, the material of which the trailing elements are formed is isotropic, i.e. it has the same properties in all directions.
The concept of providing a freely movable self-positionable trailing element in the slice chamber of a headbox was first disclosed in U.S. Patent 3 939 037, Hill. In U.S. Patent RE 28 269, Hill et al, trailing elements are disclosed extending pondside to pondside. These trailing elements are capable of generating or maintaining fine scale turbulence in the paper stock flowing toward and through the slice opening. The concepts of the foregoing patents may also be employed to utilize their advantage and to function in a machine for making multi-ply paper wherein stocks of different characteristics are fed to chambers on opposite sides of the trailing elements where the elements extend pondside to pondside.
A basic limitation in headbox design has been that the means for generating turbulence in fiber suspension in order to disperse the fibers have been only comparatively large-scale devices. With such devices, it is possible to develop small scale turbulence by increasing the intensity of turbulence generated. Thus, the turbulence energy is transferred naturally from large to small scales and the higher the intensity, the greater the rate of energy transfer and hence, the smaller the scales of turbulence sustained. However, a detrimental effect also ensued from this high intensity large-scale turbulence, namely, the large waves and free surface disturbance developed on the Fourdrinier table. Thus a general rule of headbox performance has been that the degree of dispersion and level of turbulence in the headbox discharge was closely correlated; the higher the turbulence, the better the dispersion.
In selecting a headbox design under this limiting condition then, one could choose at the extreme, either a design that produces a highly turbulent, well-dispersed discharge, or one that produces a low-turbulent, poorly dispersed discharge. Since either a very high level of turbulence or a very low level (and consequent poor dispersion) produces defects in sheet formation on the Fourdrinier machine, the art of the headbox design has consisted of making a suitable compromise between these two extremes. That is, a primary objective of the headbox design up to that time had been to generate a level of turbulence which was high enough for dispersion, but low enough to avoid free surface defects during the formation period. It will be appreciated that the best compromise would be different for different types of papermaking furnishes, consistencies, Fourdrinier table design, machine design, machine speed etc. Furthermore, because these compromises always sacrifice the best possible dispersion and/or the best possible flow pattern on the Fourdrinier wire, it is deemed that there is a great potential for improvement in headbox design today.
The unique and novel combination of elements of the aforementioned patents provide for delivery of the stock slurry to a forming surface of a papermaking machine having a high degree of fiber dispersion with a low level of turbulence in the discharge jet. Under these conditions, a fine scale dispersion of the fibers is produced which will not deteriorate to the extent that occurs in the turbulent dispersion which are produced by conventional headbox designs. It has been found that is the absence of large-scale turbulence which precludes the gross reflocculation of the fibers since flocculation is predominately a consequence of small scale turbulence decay and the persistence of the large scales. Sustaining the dispersion in the flow on the Fourdrinier wire then, leads directly to improved formation.
The method by which the above is accomplished, that is, to produce fine scale turbulence without large scale eddies, is to pass the fiber suspension through a system of parallel cross machine channels of uniform small size but large in percentage open area. Both of these conditions, uniform small channel size and large exit percentage open area, are necessary. Thus, the largest scales of turbulence developed in the channel flow have the same order of size as the depth of the individual channels by maintaining the individual channel depth small, the resulting scale of turbulence will be small. It is necessary to have a large exit percentage open area to prevent the development of large scales of turbulence in the zone of discharge. That is, large solid areas between the channel's exits, would result in large-scale turbulence in the wake of these areas.
In concept then, the flow channel must change from a large entrance to a small exit size. This change should occur over a substantial distance to allow time for the large-scale coarse flow disturbance generated in the wake of the entrance structure to be degraded to the small-scale turbulence desired. The area between channels approaches the small dimension that it must have at the exit end. This concept of simultaneous convergence is an important concept of design of this invention.
Under certain operating conditions, the trailing members which are employed to obtain the fine scale turbulence are not necessarily stable. Cross-machine transient pressures tend to bend the trailing element in the cross-machine direction and cause cross-machine uniformity variances in the paper. Resistance to deformation along the machine direction length of the trailing elements can cause slight digressions in the uniform velocity of the stock flowing off the surfaces at the trailing edge of the trailing element. Static or dynamic instability can occur at certain operating conditions and resonant frequencies can be reached dependent on the hydrodynamic forces. It has been discovered that the inertia and hydrodynamic couplings can be broken by suitable distribution of the mass and elasticity of the trailing structure with proper mass distribution and stiffness distribution being of importance.
It is accordingly an object of the invention to provide an improved trailing element design which avoids disadvantages that occur at certain operating conditions in structures heretofore available, and particularly a trailing element which offers resistance to a deflection in the cross-machine direction and which offers minimal resistance to deformation in the fluid flow stream so that pressure are balanced on opposite sides of the trailing edge of the trailing elements.

Definition of Terms:

machine direction: flow direction
isotropic: having the same properties in all directions
anisotropic: not isotropic, that is exhibiting different properties when tested along axes in different directions

In accordance with the principles of the invention, the objectives are obtained by providing a trailing element in a headbox as defined in the pre-characterizing portion of claim 1, which trailing element is formed of a material having an anisotropic characteristic to provide for the greater structural stiffness in the cross-machine direction according to the characterizing portion of claim 1.
Preferably the trailing element is formed of a laminate with separate layers of the laminate providing the qualities of cross-machine stiffness and machine direction strength and flexibility by either material properties, direction, size or number. Alternates of woven or needled material with weave directions or materials, or size or numbers of filaments controlling directional stiffness. Preferably the trailing element has a structural stiffness in the cross-machine direction greater than in the machine direction at its downstream portion.
By utilizing an anisotropic material, design factors which are otherwise not always available can be included such as strength, stiffness, corrosion resistance, wear resistance, weight, fatigue life, thermal expansion or contraction, thermal insulation, thermal conductivity, acoustical insulation, damping of vibrations, buckling, low friction and optimal design in manufacture.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which:
Figures 1A, 1Bend 1Care side elevational views in section, shown somewhat schematic of a paper machine headbox embodying the principles of the present invention; and
Figure 2 is a perspective view partially in section of a trailing element of the headbox of Figure 1.
As illustrated in Figure 1, a headbox 10 has papermaking stock 11 delivered thereto to flow through the headbox toward a slice chamber. In a headbox, various arrangements are positioned upstream of the slice chamber to control the flow and turbulence of the stock. The stock flows forwardly through openings in a wall 14 at the entry to the slice chamber. Trailing elements 18 and 19, Figure 1A, extend downstream in the slice chamber pivoted at their upper ends and free along their lengths and at their lower ends to be positionable solely due to forces of the stock flowing toward the slice opening 16. As the stock is emitted from the slice opening 16, it is delivered onto a travelling forming surface. The trailing elements are pivotally mounted at their upstream ends, and the pivotal mounting is immediately followed by a bent or angular portion which permits a short portion of the trailing elements to extend at right angles to the wall 14 and because of the bend, the trailing elements immediately turn and extend in the direction of the slice chamber.
In Figure 1B, two outer trailing elements 18' extend substantially the length of the slice chamber, and an intermediate trailing element 19' is constructed of greater length to extend through and slightly beyond the slice opening.
In the arrangement of Figure 1C, the downstream ends of the trailing elements 18" and 19" are curved to substantially conform to the curvature of the slice chamber as shown in Figure 1C. The upper trailing element 18' terminates short of the slice opening 16, whereas the lower trailing element 19" extends beyond the slice opening a short distance.
In Figure 2, a form of trailing element 18'" is shown in detail. The trailing element 18"' has outer layers 18a and 18b and a central integrally sandwiched intermediate layer 18c therebetween. The upper end of the trailing element is pivotally supported in a wall 14' such as by an enlarged or bulbous ridge 24 at the upper end pivotally mounted in a slot 25 in the wall 14'. Directional lines are shown with a machine direction line shown at the 90° axis and the cross-machine direction shown at the 0° axis and the intermediate direction shown by the double arrowed line with the angle between the double arrowed line and the machine direction line shown as a.
Various forms of headboxes may be employed as will be recognized by those versed in the art, including such as shown schematically in the aforementioned patents, RE 28 269 and 3 939 037.
In structures heretofore available, the trailing elements were formed of metal or plastic or woven and were isotropic in nature in the sense that the trailing element stiffness (Young's modulus) was the same in the flow and cross-flow direction. In accordance with the present invention, the trailing elements which extend flat in a cross-flow direction either in separate strips or continuous from pondside to pondside, can be a single layer or multilayered, flat or curved, (in the flow direction) uniform thickness, or tapered, thin or thick. The material is anisotropic so as to have different strength and/or stiffness characteristics in different directions. The anisotropic trailing elements have a greater stiffness in the cross-machine direction than in the machine direction. This being more important at the downstream tip of the trailing element.
By increasing the stiffness in the cross direction, deformations due to pressure variations are reduced'or eliminated. By having the trailing element flexible in the machine direction, effects or pressure differences upstream on the trailing element have a minimum effect on the position of the downstream edge of the trailing element so that it functions to maintain the velocities equal of the layers emerging off of the edge to minimize shear between the layers.
In a preferred arrangement, the difference between the stiffness in a cross-machine direction and a machine direction is a minimum of 5% and preferred to be 500% or more. Presently, the stiffness limit as designated by Young's modulus in the cross-machine direction is a maximum 689500 MPa (100 000 000 psi), and a minimum stiffness in the machine direction is 344,75 MPa (50 000 psi), due to existing materials properties.
The anisotropic trailing elements can be formed of a composite material, that is, a laminate wherein the different physical properties of the different layers can be taken advantage of. For example, if a three layered trailing element is provided, the outer layers can be formed with cross-direction fibers of a material such as graphite, with the inner layer containing a weaker stiffness material oriented in the machine direction, such as fiberglass. This would give greater stiffness in the cross direction, and less stiffness in the machine direction due to material stiffness, and material position within the matrix. The anisotropic trailing elements can be formed from composite materials such as graphite, kevlar, boron, glass, carbon, beryllium, steel, titanium, or aluminum fibers in matrices such as epoxy, polyamide, carbon, polyester, phenolic, silicone, alkyd, melamine, fluorocarbon, polycarbonate, acrylic, acetal, polypropylene, ABS copolymer, polysulfone, polyethylene, PEEK, polystyrene, PPS, nylon, thermoset, plastics, thermoplastics, glass, metal or other matrices. Different materials can be combined, not such as in alloying where the result is homogeneous, and isotropic. The advantage of a composite laminate is that it may attain the best qualities of the constituents and often qualities that neither alone possess. Tailoring of an anisotropic material yields not only the stiffness, strength, thermal expansion, thermal conductivity, acoustic insulation, fatigue and life required in a given direction, but functions in an improved manner during service of the headbox. The relative factors sought after are: strength, stiffness, thermal expansion, thermal conductivity and so forth. If an isotropic material were used, a compromise would have to be reached as to the material chosen. This compromise is not necessary in an anisotropic structure, wherein the desirable properties of different directions may be exploited. Outstanding mechanical properties can be combined with unique flexibility. Properties that can be improved by using an anisotropic design are strength, stiffness, corrosion resistance, wear resistance, weight, fatigue, life, thermal expansion or contraction, thermal insulation, thermal conductivity, acoustical insulation, damping of vibrations, buckling, low friction and optimum design and manufacture.
By design the inertia and hydrodynamic couplings can be broken by suitable distribution of the mass and elasticity of the structure with proper mass and stiffness distribution being of significant importance. An anisotropic design can attain stability with improved function of the trailing elements.
While the structure is shown with the trailing elements being pivotally mounted at their upstream end, this is a preferred arrangement and other forms of mounting may be employed which need not be pivotal. It is important, however, that the trailing element be self-positionable so that the position is controlled by the pressure of the stock flowing on opposite sides of the trailing element. The element is preferably free of attachment at the pondsides, but can be attached at the pondsides in some structures where movement due to hydraulic forces is small. While a trailing element formed of a single material may be used, a laminate may be employed such as illustrated in Figure 2 wherein different physical properties of different layers can be taken advantage of. Various thicknesses of the trailing edge of the elements may be employed, but 0.254 mm (10 mils) to 3.048 mm (120 mils) is a thickness that has been found to be satisfactory.
Thus, it will be seen that we have provided an improved headbox design which meets with the objectives and advantages above set forth and avoids problems existent under certain operating conditions heretofore present in the art.

Claims

1. A headbox (10) for delivering stock (11) to a forming surface, the headbox (10) comprising a slice chamber and a slice opening (16); a trailing element (18, 19; 18', 19'; 18", 19"; 18"') positioned in the slice chamber for stock flow induced movement, said element (18, 19; 18', 19'; 18", 19"; 18"') extending. transversely of said headbox (10) and having a greater structural stiffness in the cross-machine direction than in the machine direction so that the element (18,19; 18', 19'; 18", 19"; 18"') resists deflection in the cross-machine direction by transient pressure variations and offers low resistance to deformation in the fluid flow stream for balancing pressure forces on opposite sides of the element (18, 19; 18', 19'; 18", 19"; 18"'); and means (14; 14', 24, 25) anchoring said element (18,19; 18',19'; 18",19"; 18"') in the slice chamber at an upstream portion with the downstream portion unattached and constructed to be self-positionable so as to be responsive to forces exerted thereon by the stock

(11) flowing over the surfaces of the element (18, 19; 18', 19'; 18", 19"; 18"'), characterized in that said element (18, 19; 18', 19'; 18", 19"; 18"') is formed of a material having an anisotropic characteristic to provide for the greater structural stiffness in the cross-machine direction.

2. A headbox as claimed in claim 1, wherein the element (18, 19; 18', 19'; 18", 19"; 18"') has a maximum stiffness of substantially 689 500 MPa in the cross-machine direction as designated by Young's modulus and a minimum stiffness of substantially 344.75 MPa in the machine direction as designated by Young's modulus.

3. A headbox as claimed in claim 1 or 2, wherein the element (18, 19; 18', 19'; 18", 19") is constructed of a graphite epoxy with unidirectional layers in the laminate material.

4. A headbox as claimed in any one of claims 1 to 3, wherein the element (18"') being constructed so that at least a proportion thereof has a structural stiffness in the cross-machine direction greater than in the machine direction.

5. A headbox as claimed in claim 4 wherein the element (18"') is formed in layers (18a, 18b, 18c) with one of the layers constituting said portion having a structural stiffness in the cross-machine direction greater than in the machine direction and another of the layers having uniform stiffness in each direction.

6. A headbox as claimed in claim 5, wherein the element (18"') has outer layers (18a, 18b) and an intermediate layer (18c), and the intermediate layer has a structural stiffness in the cross-machine direction greater than in the machine direction.

7. A headbox as claimed in claim 5, wherein the element (18"') has outer layers (18a, 18b) and an intermediate layer (18c) with at least one of the outer layers (18a, 18b) having a structural stiffness in the cross-machine direction greater than in the machine direction.

8. A headbox as claimed in any one of claims 4 to 7, wherein said portion is formed of an anisotropic material selected from the group of graphite, kevlar, boron, glass, carbon, beryllium, steel, titanium or aluminum fibers in matrices chosen from the group of epoxy, polyamide, carbon, polyester, phenolic, silicone, alkyd, melamine, fluorocarbon, polycarbonate, acrylic, acetal, polypropylene, ABS copolymer, polyethylene, polysulfone, polystyrene, nylon, plastics, thermoplastics, thermoset plastics, glass or metal.

9. A headbox as claimed in any one of the preceding claims, wherein only the downstream portion of said element (18,19; 18', 19'; 18", 19") has a structural stiffness in the cross-machine direction greater than in the machine direction.

10. A headbox as claimed in any one of the preceding claims, wherein the trailing edge of the element (18, 19; 18', 19'; 18", 19"; 18"') has a thickness in the range of 0.254 mm to 3.048 mm.

11. A headbox as claimed in any one of the preceding claims, wherein a plurality of trailing elements (18, 19; 18', 19'; 18", 19"; 18"') are provided in the slice chamber of substantially similar construction.