WO1998049393A1

WO1998049393A1 - High consistency stock profiling

Info

Publication number: WO1998049393A1
Application number: PCT/US1998/004667
Authority: WO
Inventors: Charles L. Sanford
Original assignee: Beloit Technologies Inc
Current assignee: Beloit Technologies Inc
Priority date: 1997-04-29
Filing date: 1998-03-10
Publication date: 1998-11-05
Anticipated expiration: 1999-10-29
Also published as: AU6695798A

Abstract

The present invention is a method for injecting stock onto a forming wire to form a web of increased cross machine direction uniformity. The method consists of supplying stock containing a substantially higher concentration of paper fibers, the stock being then used to form the web to selective regions of the headbox to control web basis weight uniformity in a cross machine direction.

Description

HIGH CONSISTENCY STOCK PROFILING

FIELD OF THE INVENTION

The present invention relates to papermaking headboxes in general and in particular to headboxes employing constant volumetric flow tubes between the headbox manifold and the slice.

BACKGROUND OF THE INVENTION

In the formation of paper, wood fibers are dispersed in water to form a papermaking stock. The stock is usually at least 99 percent water and contains one-half to one percent paper fibers. The paper stock is injected through a tapered flow control channel known as a slice onto a fourdrinier moving wire screen or forming fabric to form the paper web. In some circumstances the stock is injected between two moving wire screens on a so-called twin wire machine. Water is drawn from the stock through the forming screens or wires leaving a web of paper fibers which is pressed and dried to form a web of paper.

Modern papermaking machines are between one hundred and four hundred inches wide and operate at speeds up to and in excess of 6,000 feet per minute. Thus, the headbox and the slice which supply the paper stock which is formed into the paper web must supply not only a large quantity of stock to meet the high forming speeds of modern papermaking processes, but also supply the stock extremely uniformly if the sheet of paper formed is to be of uniform thickness across the width of the web. To achieve the high flow rates and uniformity of stock injected through the slice, the stock is pumped by means of pumping equipment. An attenuator is disposed upstream relative to the headbox for damping pressure pulses caused by the stock pumping equipment. The arrangement is such that the rate of stock entering the headbox is relatively constant.

To achieve a uniform flow of stock onto the forming wire or wires, the headbox employs an inlet header or manifold which is of a tapered configuration. Between the inlet header and the slice are a plurality of distributor tubes which are arrayed in a tube bank. The tube bank is typically in the neighborhood of six tubes high by several hundred tubes long. The stock flows from the tapered tube inlets through each tube disposed within the tube bank. It is essential that the rate of flow of stock through each distributor tube be uniform in order that the stock exiting the lips of the slice be uniform from one edge of the forming wire to the other.

In order to achieve such a constant flow rate, the inlet header or manifold is tapered in the cross-machine direction. In other words, the width of the manifold in the machine direction decreases further away from the stock inlet. The cross-sectional area of the inlet header at its narrowest is equal to the cross-sectional area of the inlet header at the stock inlet less three times the total area of the tubes opening off the header. As the flow of stock moves down the tapered header, a portion of the main flow is diverted through the tubes. Therefore, the cross-sectional area of the header is reduced as it moves in the cross-machine direction so that its area remains substantially equivalent to three times the cross-sectional area of the tubes not yet reached by the header. Thus, the cross-sectional area of the header is decreased in order to compensate for the loss of fluid volume as paper stock flows from one side of the header to the other. This change in cross-sectional area maintains the same pressure in the header in the cross-machine direction which in turn maintains the same flow through the tubes in the cross-machine direction.

Consequently, the rate of flow of stock through all of the tubes in the cross-machine direction is maintained substantially constant. However, in practice the consistency has not been sufficiently uniform to prevent some variation in paper weight or thickness in the cross-machine direction. Thus, in some paper forming headboxes actuators on the lip of the slice have been used to deform the slice lip to change the width of the slice opening in an effort to maintain a uniform paper weight across the paper web. In one recently developed system, described in U.S. Patent 5, 1 96,091 to Richard E. Hergert and incorporated herein by reference, the injection of diluting water into the headbox header or manifold adjacent to the tube inlets has been used to control the dilution of the stock in the cross-machine direction. This dilution control in turn acts to control the paper web weight or thickness. This technique in fact has resulted in the production of paper webs of more uniform characteristics. Another system is described in U.S. Patent No. 5,560,807 to Hauser, and incorporated herein by reference, which utilizes supply conduits connected to a stock supply wall in the interior of the headbox to add emollients to the stock. Such emollient can be evenly or unevenly distributed in the cross machine direction, and/or selectively placed in the Z direction.

Typical emollients suggested by Hauser include long chain polymers which aid in the retention of the fiber on the forming fabric. Other materials suggested are softening agents used with certain grades of tissue paper. Additional additives which Hauser suggests are those which facilitate the bonding of fibers to one another, for example starch. What is needed is additional ways of controlling a paper web basis weight or thickness in the cross machine direction.

SUMMARY OF THE INVENTION

The present invention is a method for injecting stock onto a forming wire to form a web of increased cross machine direction uniformity. The method employs a housing which is connected to a pressurized source of stock. The housing defines a stock manifold or headbox which is tapered in the machine direction. A tube bank composed of a multiplicity of tubes allows stock to flow from the stock manifold to a slice for injecting stock onto a forming wire.

The tubes forming the tube bank are connected to the interior of the headbox manifold along a stock supply wall or surface. A plurality of supply conduits are connected to the plenum supply wall in a manner similar to the tubes for conducting stock to the slice. The supply conduits open between tube drain openings. The supply tubes supply stock containing a substantially higher concentration of paper fibers then the stock being supplied to the headbox manifold. The amount of basis weight control a given level of additive injected into the headbox gives depends on the difference in fiber content between the stock supplied to the headbox and the stock injected for basis weight control.

Paper is typically formed with stock having only about one-half of one percent fiber by weight. Flowable stocks containing considerably more than one percent are possible. Thus greater control of basis weight is possible with the injection of stock containing a higher fiber content than by injecting a stock containing lower fiber content such as clarified white water. This is particularly true where low fiber content stocks are being used to form a paper web. It is a feature of the present invention to provide method for forming a paper web which can provide basis weight control in a web formed from a stock with a low fiber content.

It is another feature of the present invention to provide a headbox which controls basis weight profile.

It is also a feature of the present invention to provide an apparatus and method for injecting stock containing substantially higher fiber content that the stock supplied to a headbox for forming a paper web.

Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the headbox apparatus of this invention.

FIG. 2 is an enlarged isometric view, partly cut away, of the headbox apparatus of FIG. 1 .

FIG. 3 is a cross-sectional view of the apparatus of FIG. 1 taken along section line 3-3.

FIG. 3A is an alternative embodiment of the headbox apparatus of this invention.

FIG. 4 is an enlarged isometric view of one of the tubes of the apparatus of FIG. 1 .

FIG. 5 is a diagrammatic representation of the tapered tubes taken along section line 5-5 of FIG. 6.

FIG. 6 is a cross-sectional view of the apparatus of FIG. 1 taken along section line 6-6. DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring more particularly to FIGS. 1 -6 wherein like numbers refer to similar parts, a headbox apparatus 10 is shown in FIG. 1 . As shown in FIG. 2, the headbox 10 has a housing 14 which is connected to a pressurized source 1 5 of stock. The housing 1 4 defines a tapered inlet of the stock supply manifold 1 6 through which stock is introduced to a tube bank 1 8. The tube bank 1 8 comprises an array of tubes 24 which are stacked alongside and one above the other. A means for selectively introducing stock containing a higher fiber content in the cross machine direction is provided by an arrangement of supply conduits as shown in FIG. 3 or alternatively as shown in FIG. 3A and as described more fully below.

Paper is formed from stock typically containing about one-half of one percent wood fibers by weight. However the range is from about 0.2 percent for tissue grades and around one percent for certain other grades. Paper formed from stock having a very low fiber content may be more readily controlled by adding stock containing higher fiber content— perhaps one to three percent fiber by weight. Because basis weight control is nearly proportional to the difference between the forming stock and the stock injected for profile control, a greater range of control is provided by using stock of higher fiber content as compared to stock of low or no fiber content.

Each tube 24 shown in FIG. 2 extends from the supply manifold 1 6 to the slice chamber 30. The tube bank thus has an upstream end 20 at the manifold 1 6, and a downstream end 22 at the slice chamber 30. The upstream end 20 of the tube bank 1 8 joins the interior of the headbox manifold 1 6 at a stock supply wall or surface 21 , shown in FIG. 2. Thus, the individual tubes 24 penetrate the stock supply wall 21 and thus communicate with the interior 23 of the headbox manifold 1 6 and are thus supplied with stock.

The tube bank 1 8 has an array of tubes 24. The array has a plurality of super-positioned rows 50 of tubes 24, generally five to seven rows, or the exemplary six rows shown in FIGS. 1 , 2, and 3. Each row 50 has up to several hundred tubes 24 and extends substantially the entire length of the housing 14. The length of the housing 1 4 is approximately equal to the width of the paper web formed by the stock flowing through the headbox 1 0.

The downstream end 22 of the tube bank 1 8 is connected to the inlet or upstream end 32 of the slice chamber 30. The stock supplied to the slice chamber 30 passes through the slice chamber 30 and is ejected from the downstream end or lip 34 of the slice chamber 30 onto a forming wire 1 2, shown in FIG. 1 . The rows 50 of the tube bank 1 8 define the width of the paper web formed on the wire 1 2 and each of the rows defines a portion of the through thickness or z-direction of the web. As shown in FIG. 2, trailing elements 64 are long, thin hinged members disposed between rows 50 of the tube bank 1 8, which keep the flow from the individual rows 50 separated from one another. The trailing elements 64 terminate adjacent to the lip 34 of the slice 30.

As shown in FIG. 3, individual rows 50 of tubes 24 provide a nearly continuous sheet of stock to the slice 30. The rows 50 of tubes 24 are super-positioned with the uppermost row 51 corresponding to the uppermost layer of fibers in the paper web formed. The lowermost row 53 corresponds to the paper fibers at the bottom of the sheet in the z-direction which are formed against the moving wire 1 2. As shown in FIG. 5, six rows of individual tubes 24 are vertically arrayed and extend from the supply wall 21 . The tubes 24 thus are positioned to receive stock from the stock manifold 1 6. Each tube 24 in a vertical array is from a different super-positioned row 50 of the tube bank 1 8. A plurality of supply conduits 36 discharge stock having a fiber content substantially greater than the stock passing through the manifold 1 6. A supply conduit 36 injects high fiber content stock into the manifold 1 6 through the stock supply wall 21 .

Although injection openings 39 may be positioned at different levels within the manifold as shown in FIG. 3A, an exemplary supply conduit 36 is shown in FIG. 5 injecting stock between two rows 50 of tubes 24. As shown in FIG. 3, a plurality of supply conduits 36 connect a source of high fiber content stock 38 to a multiplicity of stock injection points or openings 39 between individual tubes 24 in a row of tubes 50.

The headbox 10 is designed to produce a uniform orientation and consistency of fibers laid down in the cross-machine direction on the wire 1 2. This uniformity starts with an attenuator (not shown) disposed upstream relative to the headbox for damping pressure pulses caused by the stock pumping equipment. The stock then flows into the manifold 1 6. The manifold is tapered in a cross-machine direction, either linearly or parabolically so that the pressure within the manifold remains constant in the cross-machine direction.

The job of the manifold and the tubes 24, is to change the direction of the stock flow from the cross-machine direction to the machine direction. Each tube 24, as shown in FIG. 4, has an upstream section 54 which is generally cylindrical and which receives stock from the manifold 1 6. The upstream section 54 is joined at an expansion joint 61 to a flattened downstream section 60 which discharges stock onto the wire 1 2. The length of the upstream section 54 of the tube 24 is selected so the flow becomes completely symmetrical and aligned in the machine direction. The flow then undergoes a sudden expansion at the juncture 61 with the downstream section 60. The sudden expansion creates shear for improved fiber dispersion and also creates head loss for cross-machine uniformity. Because flow through a pipe is dependent on the entire pressure drop caused by the pipe, a large pressure drop at the expansion joint 61 in the tube 24 dominates and thus reduces the effect of upstream pressure variations. This mechanism increases the uniformity of the flow through all of the tubes 24 in the tube bank 1 8.

The transition between the circular first section 54 and the circular second section 60 produces uniform and stable profiles within a short distance downstream of the expansion joint 61 . The flow then smoothly transitions to a generally rectangular shaped outlet 62. The perimeter of the tube is kept constant, allowing the cross-sectional area to be decreased. The result is a tube section in which the flow accelerates, enhancing both flow stability and uniformity.

The critical parameter is the length of the downstream section 60 after the expansion joint 61 . Proper length prevents a water rich, low consistency layer from building up near the tube walls.

The pressure drop in the tubes 24 combined with the uniform pressure profile within the manifold 1 6 means that the injection points 39 of the supply conduit 36 have minimal or no effect on the volumetric flows through the individual tubes 24. Flow stability is enhanced in the slice chamber 30 by utilizing trailing elements 64 which have thicker base dimensions which limit the expansion of the flow as it enters the nozzle formed by the slice 30. For grades that are sensitive to paper fiber orientation, it is desirable to align the flow path so that it is in line from the manifold 1 6 through the tube bank 1 8 and the slice 30.

As shown in FIG. 3, valves 88 may control the addition of high fiber content stock in the cross-machine direction from the high fiber weight stock source 38. This allows control of the basis weight of the web being formed. The valves 88 can be adjusted in response to downstream measurements of the basis weight thereby providing control of basis weight in real time.

FIG. 3A discloses an alternative arrangement of the injection points 39. The injection points 39 are offset in the z direction along the cross machine direction. This arraignment may limit stock flows in the slice 30 by evenly distributing the injection points 39 over the supply wall 21 .

The injection of high fiber content stock could also be combined with injecting white water to control the sheet consistency in the cross-machine direction. By injecting both lower consistency white water and higher consistency stock, maximum control of basis weight is produced. As shown in FIG. 2, a control means 40 may be installed between a source of stock 38 and the supply conduits 36. One typical control means may be a system of valves 88 and splitters 89 which can supply a precisely controlled quantity at a controlled flow rate of stock containing a high fiber content to the supply conduits 36 which inject through the injection points 39 into the manifold 1 6.

It should be understood that the high turbulence expansion joints 61 may facilitate the uniform mixing of higher fiber content stock with the stock flowing through the tubes 24. It should be understood that the use of flow splitters 89 which allow the use of larger valves facilitates the injection of stock containing high fiber content because larger valves are less subject to clogging with paper fibers. U.S. Patent Application No. 08/761 , 1 83 discloses an improved flow splitter which is incorporated herein by reference. It should be noted that "fiber content," as defined herein, is the fraction of the dry weight of the paper fiber to the total weight of the stock.

It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1 . A method of controlling basis weight in a paper web during web formation, comprising the steps of: injecting a supply of pressurized stock of a first fiber content into the manifold of a headbox, wherein the manifold is tapered in the cross-machine direction; simultaneously flowing the stock from the manifold through a tube bank comprised of at least two super-positioned rows of tubes, the rows of tubes extending in the cross-machine direction and corresponding to the width of the web formed and the individual super-positioned rows corresponding to portions of the web formed in the z-direction; and simultaneously with the injection of stock into the tubes, injecting along the width of the headbox at selected locations stock containing a higher fiber content then the first fiber content, wherein the higher fiber content stock is injected from points closely spaced to the upstream end of the tubes of the tube bank and wherein the injection points are positioned to supply the higher fiber content stock to a selected region of the paper web formed so that basis weight of the paper is affected by the higher fiber content stock injection.

2. The method of Claim 1 wherein the higher fiber content stock injected at selected points contains between one-half of one percent and three percent paper fiber by weight.

3. The method of Claim 1 wherein the higher fiber content stock which is injected at selected points contains at least twice the fiber content of the stock of a first fiber content which is injected into the manifold.

4. The method of Claim 1 further comprising the step of simultaneously with the injection of stock into the tubes, injecting along the width of the headbox at selected locations low fiber content stock, wherein the low fiber content stock is injected from points closely spaced to the upstream end of the tubes of the tube bank and wherein the injection points are positioned to supply the low fiber content stock to a selected region of the paper web formed so that basis weight of the paper is affected by the low and high fiber stock injection points.

5. A method of controlling basis weight in a paper web during web formation, comprising the steps of: injecting a supply of pressurized stock of a first selected fiber content into the manifold of a headbox; simultaneously flowing the stock from the manifold through a tube bank comprised of at least two rows of tubes wherein one of said rows is position above the other said row, the rows of tubes extending in the cross-machine direction and corresponding to the width of the web formed; and simultaneously with the injection of stock into tubes, injecting along the width of the headbox at selected locations stock containing a second selected fiber content higher than the first selected fiber content, wherein the higher fiber content stock is injected from points closely spaced to the upstream end of the tubes of the tube bank and wherein the injection points are positioned to supply the high fiber content stock to a selected region of the paper web formed so that basis weight of the paper is affected by the stock injection.

6. The method of Claim 5 wherein the first selected fiber content stock contains less than 0.5 percent fiber by weight and the second selected fiber content stock contains between one-half of one percent and three percent paper fiber by weight.

7. The method of Claim 5 wherein the stock containing a second selected fiber content stock contains at least twice the fiber content of the first selected fiber content stock.

8. The method of Claim 6 further comprising the step of simultaneously with the injection of stock into the tubes, injecting along the width of the headbox at selected locations low fiber content stock, wherein the low fiber content stock is injected from points closely spaced to the upstream end of the tubes of the tube bank and wherein the injection points are positioned to supply the low fiber content stock to a selected region of the paper web formed so that basis weight of the paper is affected by the low and high fiber stock injection points.