EP2660390A1 - Head box for a paper machine - Google Patents

Abstract

A turbulence generator (8) consists of a fixed part (9) containing fixed sections (10) of an array of parallel ducts, and a movable part (14) with, for each duct, a movable section (15) with a diameter smaller than the minimal diameter of the adjacent fixed section (10). If the overall rate of the stock flow is large a maximal flow velocity sufficient for producing a required degree of turbulence can be reached with the fixed part (9) alone and the movable part (14) is removed from the flow path. Where the flow rate is too small for that the movable part (14) is placed in the flow path and the minimal overall cross section area reduced. This results in an increase of the maximal velocity of the flow which is then again sufficient to engender turbulence of the desired degree and intensity. In this way two different ranges of paper which require different ranges of flow rates can be produced.

Description

Field of the invention
• The invention concerns a head box for a paper machine. Such head boxes are used for distributing paper stock onto a conveyor of the machine.
Prior art
• A head box of the generic type which has a fixed turbulence generator is known from DE 26 58 844 A1 . The head box serves, among other purposes, to impart an appropriate degree of turbulence to the flow of the stock which is an important factor for the quality of the resulting paper. It depends, apart from the viscosity of the stock, upon the maximal velocity the stock flow assumes in the head box. The maximal velocity depends on the overall flow rate of the stock and the minimum overall cross section of the turbulence generator. As the flow rate varies with the type of paper which is to be produced by the paper machine, only a small range of papers can be made with satisfactory quality using the known head box because the desired quality and degree of turbulence can only be achieved if the maximal velocity is contained in a small interval which in turn limits possible variations of the overall flow rate.
Summary of the invention
• It is the object of the invention to provide a head box of the generic type which can easily be adjusted to the production of at least two different ranges of papers. This object is achieved by the features in the characterising portion of claim 1.
• The head box according to the invention can easily be switched between two or, depending on the particulars of the embodiment, more states each of which is appropriate for the production of a certain range of papers, i.e., for a range of flow rates of the paper stock which is required for producing them while at the same time the maximal velocity of the stock flow is kept within an interval where an appropriate amount of turbulence is ensured.
Brief description of the drawings
• The invention is explained more fully in the following with reference to figures which represent merely an exemplary embodiment, and wherein:

Fig. 1

shows schematically a vertical cross section of a head box according to the invention in a first state, and

Fig. 2

shows the head box of Fig. 1 in a second state.

Description of the preferred embodiments
• The head box serves to distribute paper stock from a central distributor onto a conveyor of a paper machine. The direction of flow of the stock is indicated by arrows. The head box comprises at an upstream end an inflow chamber 1 surrounded by a housing 2 with a plurality of connectors 3 arranged side by side. At each of the connectors 3 a flexible feed pipe is attached which extends from the central distributor. The feed pipes have all the same length. At an opposite downstream end the head box has an outflow chamber 4 which is confined between a bottom plate 5 and a cover plate 6 and which has a constant cross section in a lateral direction perpendicular to the flow direction. The clearance of the outflow chamber 4 decreases in the flow direction such that the chamber ends in a horizontal outflow slot 7 which is arranged just above the conveyor.
• Between the inflow chamber 1 and the outflow chamber 4 a turbulence generator 8 is arranged. It comprises a fixed part 9 which contains a plurality of parallel fixed sections 10 of ducts arranged according to a pattern, usually forming, as represented, a rectangular array consisting of several rows which are arranged one above the other, each row extending in the lateral direction from one side of the head box to the other. The fixed sections 10 are preferably equal, each of them beginning, at its upstream end, with a first subsection 11 of constant circular cross section, with a first diameter d_f, the minimal diameter of the fixed section 10, immediately followed by a second subsection 12, equally of constant circular cross section, with a larger second diameter, and a third subsection 13 where the cross section changes continuously from circular to quadratic, the cross section area increasing at the same time, reaching a maximum its downstream end. The fixed part 9 can consist of stainless steel or high density polyethylene.
• The turbulence generator 8 further comprises a movable part 14 which is arranged immediately upstream of the fixed part 9. It has an active zone in the shape of a rectangular plate of a constant thickness of between 30mm and 70mm, preferably between 35mm and 60mm. It contains movable sections 15 for the ducts which sections are distributed according to a pattern which corresponds to the pattern defining the positions of the fixed sections 10 in the fixed part 9, forming a congruent array of parallel rows which extend in the lateral direction.
• Each of the movable sections 15 has a constant circular cross section and a diameter d_m which is smaller than the diameter d_f of the first subsection 11 of the fixed section 10 whose position in the array corresponds to that of the movable section 15 in question.
• If the ducts are equal the overall flow rate Q is $$\mathrm{Q}={\mathrm{<figure-callout\; id="2"\; label="N\pi d"\; filenames="imgf0001.png"\; state="\{\{state\}\}">N\pi d</figure-callout>}}^{\mathrm{2}}\cdot \mathrm{v}$$

  

  
  with N the number of ducts, d the minimal diameter of the duct and v the maximal velocity of the stock flow. Nπd² is the minimal overall cross section area of the turbulence generator 8. It follows from (1) that, for a given overall flow rate Q, the maximal velocity equals $$\mathrm{v}=\mathrm{Q}/{\mathrm{<figure-callout\; id="2"\; label="N\pi d"\; filenames="imgf0001.png"\; state="\{\{state\}\}">N\pi d</figure-callout>}}^{\mathrm{2}}\mathrm{.}$$

  
• The maximal velocity v is the most important of the parameters determining the quality and intensity of turbulence in the stock flow which is decisive for the quality of the resulting paper. What is desired is micro-turbulence with an intensity restricted to a rather small range. As the said intensity is a function of v² this in turn requires that the maximal velocity v be contained in a rather small interval.
• If the maximal velocity v is too low, macro-turbulence which results in defects like the formation of flocks and an unequal distribution and orientation of the fibres in the finished paper is produced instead of the desired micro-turbulence. If, on the other hand, the maximal velocity v is higher than would be required for creating the required amount of micro-turbulence, the flow resistance is unnecessarily high and it may be impossible to achieve the required overall flow rate.
• The required overall flow rate Q depends essentially upon the type of paper which is produced. With a given minimal diameter of each duct and the consequent overall cross section area the velocity is a consequence of the flow rate as is apparent from (2).
• In the first state of the head box (Fig. 1) the movable part 14 of the turbulence generator 8 is in a first position where it is completely removed from the flow path such that only the fixed part 9 of the turbulence generator 8 has any effect on the flow of the stock. The ducts consist only of their fixed sections 10. The minimal cross section area of the turbulence generator 8 is therefore Nnd_f ², as in each case the diameter d_f of the first subsection 11 of the fixed section 10 of the duct, i.e., its diameter at the upstream end, is at the same time the minimal diameter of the duct, defining the minimal overall cross section area. The flow rate is $${\mathrm{<figure-callout\; id="1"\; label="Q"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Q</figure-callout>}}_{\mathrm{1}}={{\mathrm{<figure-callout\; id="2"\; label="N\pi d\; f"\; filenames="imgf0001.png"\; state="\{\{state\}\}">N\pi d</figure-callout>}}_{\mathrm{f}}}^{\mathrm{2}}\cdot {\mathrm{<figure-callout\; id="1"\; label="v"\; filenames="imgf0001.png"\; state="\{\{state\}\}">v</figure-callout>}}_{\mathrm{1}}$$

  

  
  where v₁ is the maximal velocity, i.e., the velocity the stock flow assumes in the first subsections 11 and $${\mathrm{<figure-callout\; id="1"\; label="v"\; filenames="imgf0001.png"\; state="\{\{state\}\}">v</figure-callout>}}_{\mathrm{1}}={\mathrm{<figure-callout\; id="1"\; label="Q"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Q</figure-callout>}}_{\mathrm{1}}/{{\mathrm{<figure-callout\; id="2"\; label="N\pi d\; f"\; filenames="imgf0001.png"\; state="\{\{state\}\}">N\pi d</figure-callout>}}_{\mathrm{f}}}^{\mathrm{2}}$$

  
• With an adequate choice of the diameter d_f the maximal velocity v₁ will produce just the desired quality and degree of turbulence.
• If a different type of paper is produced which requires a flow rate Q₂ which is smaller than Q₁ the corresponding maximal velocity v₂ may be too small to create the desired micro-turbulence. In this case the movable part 14 of the turbulence generator 8 is lowered to a second position such that its active zone is in the flow path (Fig. 2). The turbulence generator 8 is now in a second state where each duct consists of one of the fixed sections 10 and, immediately upstream of the same, one of the movable sections 15. The diameter d_m of the latter is now the minimal diameter defining the minimal overall cross section area. The resulting maximal velocity v₂ of the flow is therefore the velocity in the movable section 15 where the diameter of the duct is minimal, i.e., $${\mathrm{<figure-callout\; id="2"\; label="v"\; filenames="imgf0001.png"\; state="\{\{state\}\}">v</figure-callout>}}_2={\mathrm{<figure-callout\; id="2"\; label="Q"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Q</figure-callout>}}_2/{{\mathrm{<figure-callout\; id="2"\; label="N\pi d\; m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">N\pi d</figure-callout>}}_{\mathrm{m}}}^{\mathrm{2}}$$

  

  
  such that, with an appropriately chosen d_m, v₂ is again in a range where the stock flow through the turbulence generator 8 produces micro-turbulence of the desired intensity. In this way, two ranges of paper qualities can be covered with the head box, with only a minor adjustment of the turbulence generator 8 which can be carried out easily in a moment.
• The movable part 14 is preferably a vertically movable slide whose position can be changed by an electric motor via a rack-and-pinion mechanism, threaded spindles engaging with threaded bores or by other means. It can have a plurality of active zones, each with an array of movable sections of ducts with different diameters of the movable sections pertaining to different active zones. It is also possible to provide several movable parts, which are arranged following one upon the other in the flow direction and have congruent arrays of movable sections of ducts, preferably with constant circular cross sections and equal diameters which grow smaller in the upstream direction. It is also possible to do without a fixed part, i.e., to provide a turbulence generator which consists of a movable part only which has at least two active zones but the division into a fixed part and at least one movable part is, in general, much preferable as the moveable part or parts can usually be comparatively small and therefore easier to handle.
List of reference symbols

1

inflow chamber

2

housing

3

connector

4

outflow chamber

5

bottom plate

6

cover plate

7

outflow slot

8

turbulence generator

9

fixed part

10

fixed section

11

first subsection

12

second subsection

13

third subsection

14

movable part

15

movable section

Claims (12)

1. Head box for a paper machine, comprising:

   - at an upstream end an inflow chamber (1) in a housing (2) with at least one connector (3) for a feed pipe,

   - at an opposite downstream end an outflow chamber (4) ending in a horizontal outflow slot,

   - between the inflow chamber (1) and the outflow chamber (4) a turbulence generator (8) comprising an array of parallel ducts,
   and characterised in that
   the turbulence generator (8) comprises a movable part (14) which can, in a plane perpendicular to a flow path connecting the inflow chamber (1) and the outflow chamber (4), assume at least a first position and a second position different from the first position, such that in the second position of the movable part (14) the minimal cross section area of each duct is smaller than in the first position of the movable part (14).
2. Head box according to one of claim 1, characterised in that the movable part (14) is a slide translatable in a direction essentially perpendicular to the flow path.
3. Head box according to claim 1 or 2, characterised in that the turbulence generator (8) comprises a fixed part (9) containing at least a fixed section (10) of each duct whereas the movable part (14) is arranged upstream of the fixed part (9), with at least one active zone containing a movable section (15) for each duct which, in the second position of the movable part (14), immediately precedes the fixed section (10) of the duct.
4. Head box according to claim 3, characterised in that the length of the movable part (15) in the direction of the flow path is between 30mm and 70mm.
5. Head box according to claim 3 or 4, characterised in that the movable part (14) is, in its first position, completely removed from the flow path.
6. Head box according to one of claims 3 to 5, characterised in that the at least one active zone of the movable part (14) is a plate with the movable sections (15) for the ducts arranged in a pattern congruent with the pattern of the fixed sections (10) of the ducts.
7. Head box according to claim 6, characterised in that the movable sections (15) for the ducts have each a constant cross section.
8. Head box according to claim 6 or 7, characterised in that for each duct the cross section area of the movable section (15) at a downstream end of the same is smaller than the cross section area of the fixed section (10) at an upstream end of the same.
9. Head box according to one of claims 3 to 8, characterised in that the cross section of the movable section (15) is circular in each case.
10. Head box according to one of claims 3 to 9, characterised in that the cross section area of the fixed section (10) is larger at its downstream end than it is at its upstream end in each case.
11. Head box according to one of claims 3 to 10, characterised in that the cross section of the fixed section (10) is circular at its upstream end in each case.
12. Head box according to one of claims 1 to 11, characterised in that the turbulence generator (8) consists of stainless steel or of high-density polyethylene.

Images