HEADBOX, SHEET FORMING UNIT WITH A HEADBOX AND METHOD FOR OPERATING A SHEET FORMING UNIT

Abstract

A headbox includes a feed device feeding the fibrous material suspension, a nozzle, and a turbulence-generating device. The fibrous material suspension is guided in the turbulence-generating device in sub-flows through turbulence-generating channels. Within the individual turbulence-generating channel of the turbulence-generating device, at least one region forming a fluidization region is provided, in which a pressure loss can be generated in the sub-flow of the fibrous material suspension guided through said region. The nozzle and the turbulence-generating device are designed and dimensioned such that they are suitable for setting a dwell time of ≦200 ms of the fibrous material suspension flowing through the same from a final fluidization region of an individual turbulence-generating channel before inlet into the nozzle as far as the outlet gap of the nozzle, and a pressure loss of ≧50 mbar in the final fluidization region upstream of the inlet into the nozzle.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application No. PCT/EP2010/056309, entitled “HEADBOX, SHEET-FORMING UNIT HAVING A HEADBOX AND METHOD FOR OPERATING A SHEET-FORMING UNIT “, filed May 10, 2010, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a headbox for a machine for producing fibrous webs, in particular paper, cardboard or tissue webs from at least one fibrous stock suspension, comprising at least one feed device feeding the at least one fibrous stock suspension, a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, and a turbulence generating device arranged directly upstream of the nozzle in flow direction; whereby during operation of the headbox the at least one fibrous stock suspension is guided in partial flows through a plurality of turbulence generating channels, whereby within the individual turbulence generating channel of the turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in the partial flow of the fibrous stock suspension being guided through said region.

The invention also relates to a sheet forming unit for a machine for producing fibrous webs, in particular paper, cardboard or tissue webs, comprising a headbox and a forming unit arranged downstream thereof, into which the fibrous stock suspension is supplied from the outlet gap of the headbox in the free jet onto at least one clothing; and a method for operating a sheet forming unit of this type.

2. Description of the Related Art

The production process of fibrous webs is substantially dependent on the stock consistency of the fibrous stock suspension being used. With increasing stock consistency of the used fibrous stock suspension a deteriorating formation of the fibrous web at the end of the process which can be described through the macroscopic and microscopic distribution of fibers and fillers can be observed. In order to achieve satisfactory results in regard to the quality of the fibrous web, fibrous stock suspensions having stock consistencies in the range of 0.8-1.2% are brought into the downstream forming units in the current conventional headboxes. If stock consistencies with higher values are used, a coarsely clouded formation inside the fibrous stock suspension is to be expected, already at the discharge of the jet from the headbox, due to heavy fiber flocculation. Measures are therefore to be taken to facilitate destruction of these flakes and timely fixing of the flow. In particular, an as flake-free fibrous stock suspension jet as possible is to be provided through the headbox at its outlet. Inside the turbulence generating device which is arranged before the nozzle, regions serving the de-flocculation and better fluidization for the fibrous stock suspension are therefore provided by different means in the turbulence generating channels of the turbulence generating device. Many times these are however not sufficient. The reason is the greatly reduced re-flocculation time with increased stock consistency. However, in order to achieve satisfactory formation parameters for the developing fibrous web, re-flocculation of the fibrous stock suspension is to be completely avoided if possible in the headbox after the most recent fluidization. This, however, assumes appropriately short construction of units, which again are adverse to other requirements, in particular rigidity and reduction of the vibration tendency, as well as avoidance of hydraulic disturbances.

The problem of flake formation and its effect upon the quality of the developing fibrous web is described in publication EP 1 313 912 B1. As a solution, one design of a headbox with a modified turbulence generating device is suggested, whereby inside the turbulence generating device a fluidization is undertaken only once in one step in each turbulence generating channel of the turbulence generating device, thereby causing an acceleration of the flow and short dwell time of the fibrous stock suspension in the headbox. The level of fluidization can be maintained through the special design of the lamellas of the nozzle. For the fluidization, graduated changes of the cross sectional area of the individual turbulence generating channel of the turbulence generating device and lengths of the individual partial regions of the flow channels of the turbulence generating device forming the fluidization region are suggested which result in a length of the turbulence generating device in a range of 400 mm.

To improve the formation and the tear length properties of the developing fibrous web, a multitude of additional measures are already known which are characterized through a modification of the nozzle or the turbulence generating device.

Publication DE 101 06 684 A1 discloses one embodiment of a headbox with a specially designed lamella end for avoiding instabilities in the flow inside the nozzle and thereby a stimulation of vibration, whereby said lamella end is slanted on the side facing the nozzle wall and which, on the side facing away from the nozzle wall is provided with a structure. To influence the formation it is also known from publication DE 199 02 621 A1 to design the nozzle with different geometric regions to produce different flow cross sections inside the nozzle.

Publication WO 2008/077585 A1 discloses the promotion of the development of symmetric properties in Z-direction over symmetrically designed headbox nozzles and the embodiment and dimensioning of same.

Measures for improving the transverse rigidity through alignment of the fibers in the region of the outlet from the nozzle are described in publication EP 1 022 378 A2. Design of the nozzle includes a region having a constant cross sectional reduction and an adjacent shorter region of constant cross sectional expansion.

In order to avoid bursting of the free jet during its discharge from the nozzle, documentation DE 297 13 433 U1 discloses one embodiment of a headbox having a nozzle formed by machine-wide limiting areas of contact whereby at least one of the limiting surfaces is characterized by at least three segments of different angles of convergence.

Documentation DE 102 34 559 A1 discloses one embodiment of a headbox in a sheet forming system, wherein the nozzle is characterized by a length of ≧400 mm, whereby the turbulence block which is formed by the turbulence generating device and which is located upstream from said nozzle preferably is also within this length range.

All already known measures are however not suitable for bringing the dwell time of the individual fibrous stock suspension below its re-flocculation time, in particular at a higher stock consistency.

It is therefore the aim of the current invention, and what is needed in the art is, to further develop a headbox of the type referred to at the beginning for a machine to produce fibrous webs, in particular paper, cardboard or tissue webs so that the aforesaid disadvantages are avoided. In particular, re-flocculation of the fibrous suspension is to be avoided after the last fluidization inside the turbulence generating device prior to the nozzle until the outlet from the nozzle, and if possible also after the nozzle.

SUMMARY OF THE INVENTION

A headbox for a machine for producing fibrous webs, in particular paper, cardboard or tissue webs from at least one fibrous stock suspension, includes at least one feed device feeding the at least one fibrous stock suspension, a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, and a turbulence generating device arranged directly upstream of the nozzle in flow direction; whereby during operation of the headbox the at least one fibrous stock suspension is guided in partial flows through a plurality of turbulence generating channels which are arranged preferably in series; whereby within the individual turbulence generating channel of the turbulence generating device at least one region forming a fluidization region is provided in which a pressure loss can be produced in the partial flow of the fibrous stock suspension being guided through said region. The headbox is characterized according to the present invention in that the nozzle and the turbulence generating device arranged directly upstream of the nozzle are designed and dimensioned such that they are suitable for setting a dwell time of ≦200 ms, preferably ≦175 ms, in particular ≦150 ms of the fibrous stock suspension flowing through the same from a final fluidization region of an individual turbulence generating channel of the turbulence generating device before inlet into the nozzle as far as the outlet gap of the nozzle, and a pressure loss of ≧50 mbar, preferably ≧75 mbar, in particular ≧100 mbar, most preferably ≧150 mbar in the final fluidization region before the inlet into the nozzle.

Stated another way, the invention relates to a headbox for a machine for producing fibrous material webs from at least one fibrous material suspension, comprising at least one feed device feeding the at least one fibrous material suspension, a nozzle having an outlet gap for dispensing the fibrous material suspension in a free jet, and a turbulence-generating device arranged directly upstream of the nozzle in the flow direction. During operation of the headbox, the at least one fibrous material suspension is guided in the turbulence-generating device in sub-flows through a plurality of turbulence-generating channels. Within the individual turbulence-generating channel of the turbulence-generating device, at least one region forming a fluidization region is provided, in which a pressure loss can be generated in the sub-flow of the fibrous material suspension guided through said region. The headbox according to the invention is characterized in that the nozzle and the turbulence-generating device arranged directly upstream of the nozzle are designed and dimensioned such that they are suitable for setting a dwell time of ≦200 ms of the fibrous material suspension flowing through the same from a final fluidization region of an individual turbulence-generating channel of the turbulence-generating device before inlet into the nozzle as far as the outlet gap of the nozzle, and a pressure loss of ≧50 mbar in the final fluidization region upstream of the inlet into the nozzle.

A fluidization region is to be understood to be a region where the fibrous stock suspension, in particular the respective partial flow is actively or passively influenced so that almost no fiber network is formed. The influence can hereby occur actively in regard to its effect through controllable elements, for example static mixing devices or passively through the geometric design of the flow path and the thereby determined generation of turbulences on the fibrous stock suspension resulting in disintegration of accumulations, in particular flakes. Viewed in flow direction, the region may be limited locally on a line in cross machine direction, or can be designed progressing in flow direction.

The inventive solution offers the advantage of an expansion of the range of application of headboxes to fibrous stock suspensions with increased stock consistencies (fibers and fillers), preferably of ≧1%, in particular in the range of ≧0.5% to ≦4%, preferably ≧1% to ≦3%, in particular ≧1% to ≦2.5% and at the same time optimized fiber and filler distribution or respectively formation at the discharge of them in a free jet into the forming unit by avoiding fiber and filler agglomeration. New development of flakes in flow direction as far as to the discharge from the nozzle which was safely disintegrated by the minimum pressure loss in the last fluidization region can be safely avoided. The mobility of the fibers and thereby the fluidization level is maintained in the free jet right through to the outlet because of the short dwell time.

The design of the individual turbulence generating channel of the turbulence generating device preferably provides that the dwell time of the fibrous stock suspension between the last fluidization region of the individual turbulence generating channel of the turbulence generating device and the outlet of the turbulence generating device is ≧10 ms to ≦100 ms.

An especially advantageous design of the headbox is characterized in that the nozzle has a length l_D in the range of 100 mm≦l_D≦500 mm, preferably 100 mm≦l_D≦400 mm, in particular 200 mm≦1_D≦400 mm and that the distance between the final fluidization region inside the individual turbulence generating channel of the turbulence generating device upstream from the nozzle and the outlet from the turbulence generating device is ≦180 mm, preferably ≦150 mm, especially ≦120 mm, more especially 100 mm. This combination of measures permits a short and compact design of a headbox, suitable for fibrous stock suspensions having a wide consistency range, as well as avoidance of re-flocculation due to the minimum dwell time based on the minimum distance from the final fluidization region and outlet from the nozzle and the acceleration resulting from the pressure loss.

Depending on the type and composition of the fibrous stock suspensions that is to be delivered, in particular depending on the level of the stock consistency, the length of the nozzle which is characterized by the distance between outlet of the upstream turbulence generating device and outlet gap is limited in an advantageous further development by the following predetermination which considers the damping effect of the fibers in order to ensure the stability of the fibrous stock suspension stream emerging from the outlet gap of the nozzle:

l_D×SK≦1000, preferably ≦800, especially ≦700,

whereby

l_D=length of nozzle, measured in mm; and

SK=stock consistency in %.

The nozzle chamber of the nozzle is limited by two outlet gap forming and in flow direction converging nozzle walls, an upper and a lower nozzle wall, whereby it is advantageous in regard to flow technological aspects if the angle of convergence between them, at least in the region of the outlet gap, is between 5° and 45°, preferably 10° and 20°.

To always avoid a segregation of fibers and fluid inside the final turbulence generating device before the nozzle an advantageous design provides that the length 1 of the turbulence generating device and therefore of the individual turbulence generating channel of the turbulence generating device is preferably in the range of 100 mm≦l_TE≦500 mm, preferably 100 mm≦l_TE≦400 mm, especially 150 mm≦l_TE≦300 mm.

In an especially advantageous further development the individual turbulence generating channel of the turbulence generating unit is designed and dimensioned so that in the final fluidization region prior to the inlet into the nozzle the pressure loss inside the partial flow guided in said region of ≧50 mbar, preferably ≧75 mbar, especially ≧100 mbar, most especially ≧150 mbar is produced. The size of the pressure loss offers the advantage of certain assurance of a high deflocculation level and high fiber mobility even at high consistencies which can be maintained over the aforementioned longitudinal regions in flow direction as far as the outlet from the nozzle and further.

Regarding realization of the pressure loss inside the final fluidization region upstream from the nozzle in flow direction a multitude of possibilities exist. Here, the final fluidization region viewed in flow direction may be strongly limited locally or may be designed over a partial region of the turbulence generating channel of the turbulence generating device, extending in flow direction. According to a first variation the pressure loss may be generated passively, in the most simple case as a function of the geometry and/or dimensioning of the flow path in the individual turbulence generating channel of the turbulence generating device, or actively through the provision of additional devices and/or possibilities for the energy supply into the fibrous stock suspension inside the turbulence generating channel of the turbulence generating device.

According to an especially preferred design of a first variation for generation of a pressure loss, the final fluidization region before the inlet into the nozzle is formed by a local graduated change of the cross sectional area of the individual turbulence generating channels of the turbulence generating device, viewed in flow direction. The cross sectional area of the individual turbulence generating channel of the turbulence generating device can be described through a geometric form and dimension. The graduated change offers the advantage of easier generation of higher pressure losses in a locally strictly limited area inside the flow path by generating a very strong turbulence to break up flakes, whereby overall the fluidization is improved. The thereby adjusted high fiber mobility is then maintained through the short dwell time according to the invention, as well as the short distance of the fluidization region from the outlet of the nozzle.

In an additional embodiment the final fluidization region before the inlet into the nozzle can be formed by a constant change of the cross sectional area of the individual turbulence generating channel of the turbulence generating device, viewed in flow direction.

The magnitude of the change of the cross sectional area - either the graduated or constant change from the minimum cross sectional area to the maximum cross sectional area—which can be described as the difference of the hydraulic diameters characterizing the cross sectional areas is selected suitable for generating the required minimum pressure loss. Depending upon the characteristics of the fibrous stock suspension which is to be used, the change of the cross sectional area in the fluidization region is selected and designed so that the change, in particular the level of progression characterizing the cross sectional change suits at least the medium fiber length of the fibrous stock suspension which is used. The fluidization level required for the short dwell time can hereby be ensured.

According to an additional embodiment the pressure loss can be brought about additionally or alternatively through at least one static mixing device provided in the fluidization region or by at least one means of furnishing energy by producing the desired pressure loss in the fibrous stock suspension. These options offer the advantage of an easily realizable free adjustability of the pressure loss, independent of the geometry in the turbulence generating channel of the turbulence generating device.

In an advantageous design the individual turbulence generating channel of the turbulence generating device is designed and dimensioned so that its maximum cross sectional area is characterized by a hydraulic diameter d_hydr in a range of 5 mm≦d_hydr≦25 mm, preferably 5 mm≦d_hydr≦20 mm, especially 10 mm≦d_hydr≦20 mm. Based on this, the adjusting flake size after fluidization can be kept to a minimum.

In order to avoid fiber wipe formations the hydraulic diameter d_hydr-8E is preferably in a range of 8 mm≦d_hydr-8E≦20 mm, preferably 10 mm≦d_hydr-8E≦20 mm, especially 10 mm≦d_hydr-8E≦15 mm at the inlet into the individual turbulence generating channel of the turbulence generating device.

Guidance of the respective partial flow of fibrous stock suspension from the final fluidization region before the inlet into the nozzle occurs in an advantageous design through an additional region with a constant cross sectional change in the range of 50 mm to 100 mm.

Regarding the construction and design of the turbulence generating device there are basically several options for which however the above described conditions apply. The turbulence generating device can consist of a plurality of machine-wide turbulence generating channels which are arranged vertical to the flow direction above one another, or of a plurality of individually designed turbulence generating channels arranged in rows in cross machine direction and in columns arranged vertical to the cross machine direction. In an advantageous embodiment however, the number of rows of flow channels in the turbulence generating device is selected such that the flow speed of the partial flow traveling in the narrowest cross section of such a turbulence generating channel of the turbulence generating device is between 5 m/s and 20 m/s, preferably between 7 m/s and 15 m/s. This design offers the advantage together with the constructive characteristics of a sensitive and effective fluidization.

A headbox according to the present invention (as described above) is preferably utilized in a sheet forming unit for a machine for producing fibrous webs, in particular paper, cardboard or tissue webs, also comprising a downstream forming unit whereby the fibrous stock suspension is fed in the form of a free jet, defined as line of impingement from the outlet gap of the headbox into the forming unit, in particular onto a clothing or between two sections of clothing. The design and arrangement of headbox and forming unit occurs hereby so that the dwell time of the fibrous stock suspension from the final fluidization region to the line of impingement is ≧30 ms to ≦300 ms, preferably ≧50 ms to ≦200 ms, especially ≧80 ms to ≦200 ms. The inventive arrangement of an inventively designed headbox and of a forming unit offers the advantage that up to impingement onto the clothing an optimized fibrous stock suspension with a view to the targeted formation of the fibrous web and in regard to fiber distribution and orientation is provided.

The forming unit may be in the embodiment of a hybrid former, gap former comprising two wire belts which form an inlet gap for the fibrous stock suspension, or a Fourdrinier wire former, comprising a wire belt onto whose surface the fibrous stock suspension is delivered by means of the headbox.

The inventive method for operating a sheet forming unit (as described above), whereby the at least one fibrous stock suspension of the headbox is fed across the machine width, is directed or guided by forming of partial flows in a plurality of turbulence generating channels of the turbulence generating device and is fed to a nozzle from where the at least one fibrous stock suspension is delivered in the free jet into the forming unit, particularly onto the clothing of the forming unit under definition of line of impingement, whereby inside one individual turbulence generating channel of the turbulence generating device a pressure loss is adjusted in the fibrous stock suspension, is characterized in that in a final fluidization region of an individual turbulence generating channel of the turbulence generating device upstream from the inlet into the nozzle a pressure loss inside the fibrous stock suspension of ≧50 mbar, preferably ≧75 mbar, especially ≧100 mbar, most especially ≧150 mbar is produced and that the fibrous stock suspension is guided from this final fluidization region to the outlet gap from the nozzle so that its dwell time in the region described or extending from the final fluidization region as far as the outlet gap is ≦200 ms, preferably ≦175 ms, in particular ≦150 ms, and/or that the dwell time in the region extending from the final fluidization region to the line of impingement is ≧30 ms to ≦300 ms, preferably ≧50 ms to ≦200 ms, especially ≧80 ms to ≦200 ms.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a detail of an axial section of a sheet forming unit for a machine for producing a material web and the construction of an inventive headbox;

FIG. 2 illustrates with the assistance of a diagram the connection between stock consistency and formation of the flake structure in the free jet;

FIG. 3 again shows a detailed section from an inventive headbox according to FIG. 1;

FIGS. 4a1 and 4a2 show a first arrangement of the turbulence generating channels of the turbulence generating device;

FIGS. 4b1 and 4b2 show a second arrangement of the turbulence generating channels of the turbulence generating device; and

FIG. 5 shows a particularly advantageous design of a turbulence generating channel of the turbulence generating device.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 clarifies in a schematically simplified illustration of a diagram the influence of the level of stock consistency SK inside a fibrous stock suspension FS upon the formation. For this purpose the formation of the flake structure FL in the free jet in regard to the dimension of the forming flakes is plotted above stock consistency SK. From this the connection can be seen between high stock consistency SK and an uneven and coarsely clouded formation in regard to the arrangement of the fibers and fillers based on increased fiber flocculation, that is the tendency in conventional known headboxes toward larger flakes in the free jet F of fibrous stock suspension being delivered from the outlet gap of a headbox. It can also be seen that with fibrous stock suspensions with lower stock consistency—in this example below a stock consistency parameter SK_x of 1.2%—the flake formation is less, meaning that only smaller flakes are observed in the free jet at the discharge from the outlet gap of the headbox. FIG. 2 only illustrates the basic connection between consistency of a fibrous stock suspension FS and the susceptibility to flakes. This also depends on the fiber stock.

In order to reduce and if possible avoid reflocculation, that is re-occurrence of flakes within the fibrous stock suspension FS before or during discharge from headbox 1, an inventive headbox 1 according to the illustration in FIG. 1 is suggested for use in a sheet forming unit 3. Here, headbox 1 is located upstream from a forming unit 2 and together with this unit forms the sheet forming unit 3 for a machine for producing a material web, in particular a fibrous web in the form of a paper, cardboard or tissue web. Headbox 1 serves the machine-wide feeding of at least one fibrous stock suspension FS into forming unit 2. For clarification of the individual directions a coordination system is placed on sheet forming unit 3, whereby X-direction describes the longitudinal direction which is also referred to as machine direction MD and which coincides with the direction of travel of fibrous web F. Y-direction describes the direction transverse to the direction of travel of the fibrous web, in particular the width direction of the machine which is therefore also referred to as the cross machine direction CD, whereas Z-direction characterizes the height direction.

Headbox 1 comprises a feeding device 4 through which the at least one fibrous stock suspension FS can be distributed across the entire width of headbox 1. In the most simple case this is in the embodiment of an element representing a distribution channel, in particular a distribution pipe extending in cross machine direction CD and which tapers in flow-through direction in cross machine direction. In the illustrated example fibrous stock suspension FS comes from feed device 4 for example into a first turbulence generating device 5, comprising a plurality of turbulence generating elements. Turbulence generating device 5 may be of varying construction and in the simplest scenario is in the embodiment of flow channels, in particular turbulence generating channels 6 describing and through-flow opening comprising orifice plate or bundle of pipes. Viewed in flow-through direction a space 13 is located adjacent to the first turbulence generating device 5 which is followed by an additional second turbulence generating device 7, comprising turbulence generating elements forming turbulence generating channels 8. Following second turbulence generating device 7, located at its outlet 7A is a nozzle 9, whereby a nozzle chamber 10 is formed which is capable of substantially accelerating the flow of fibrous stock suspension FS during operation and whereby fibrous stock suspension FS is delivered for example by means of an aperture 11 and through outlet gap 12 to forming unit 2 for the machine for producing a material web, whereby nozzle chamber 10 is limited by nozzle wall 16.1, 16.2 in a directional plane vertical to machine direction MD and cross machine direction CD. Inside individual turbulence generating devices 5 and 7 the fibrous stock suspension FS is apportioned according to a pre-defined separation and travels on, distributed into partial flows. Turbulence generating devices 5 or respectively 7 comprise a plurality of turbulence generating channels 6, 8 extending in longitudinal direction of the machine, that is in machine direction MD and which are either machine-wide or are arranged parallel to each other in cross machine direction CD in rows and in vertical direction in columns, in other words vertical to a plane which can be described by the flow-through direction and cross machine direction CD.

Inside individual turbulence generating channel 8 of turbulence generating device 7 at least one region representing a fluidization region 15 is provided where a pressure loss can be produced in the individual partial flow of fibrous stock suspension FS being guided in this region.

According to the invention the second turbulence generating device 7 which is located upstream from nozzle 9 viewed in direction of flow of fibrous stock suspension FS and nozzle 9 are designed and dimensioned so that the dwell time T_v of fibrous stock suspension FS when running through the second turbulence generating device 7 as far as to the outlet from outlet gap 12 of nozzle 9 is ≦200 ms, preferably ≦175 ms, in particular ≦150 ms. This is achieved in an especially advantageous design example through appropriate matching of the geometry of second turbulence generating device 7 that is the element of the headbox which is located immediately prior to nozzle 9, and the design of nozzle 9. Second turbulence generating device 7 is designed, arranged and dimensioned so that by means of it at least a pressure loss of ≧50 mbar, preferably ≧75 mbar, especially ≧100 mbar, most especially ≧150 mbar is produced in the last fluidization region 15 before nozzle 9 within the partial flow guided in this region. Several options are conceivable here, whereby one differentiates between active and passive measures, in other words between a permanent adjustment of the achievable pressure loss or an open adjustability. As further explained below, the pressure loss can be achieved through the geometric design of the individual turbulence generating channel 6, 8, in particular through local change of the cross sectional areas and/or arrangement of additional devices such as static mixing devices or an additional energy supply into the individual partial flow.

The length of final turbulence generating device 7 upstream from nozzle 9, viewed in machine direction MD is indicated as l_TE and is characterized by a length in the range of ≧100 mm to ≦500 mm, preferably ≧100 mm to ≦400 mm, especially ≧150 mm to ≦300 mm. Length l_D of nozzle 9, measured from outlet 7A from turbulence generating device 7 to outlet gap 12 in machine direction MD is ≧100 mm to ≦500 mm, preferably ≧100 mm to ≦400 mm, especially ≧200 mm to ≦400 mm. The stability of the jet can hereby only be maintained if the damping effect of the fibers increases and length lD of nozzle 9 meets condition

l_D×SK≦1000, preferably ≦800, especially ≦700,

whereby l_D is consistent with the length of the nozzle in mm, and SK with the stock consistency in %.

An additional substantial geometric characteristic is length l₁ which describes the distance between final fluidization region 15 in the turbulence generating device 7 located immediately before nozzle 9 and outlet 7A from turbulence generating device 7 which coincides with an inlet 14 into nozzle 9 and which is ≦180 mm, preferably 150 mm, especially ≦120 mm, more especially ≦100 mm.

Angle of convergence a is provided in the area of outlet gap 12 between individual nozzle walls 16.1, 16.2 which define nozzle chamber 10 and describes the angle between these in the area of the outlet gap 12 and is selected within a range of 5° and 45°, preferably between 10° and 20°. With this geometric design of the combination of the characteristics, whereby essentially the length of nozzle l_D and distance l₁ are decisive the dwell time T_v can be adjusted to a duration within a predetermined range and in particular to below the reflocculation time of fibrous stock suspension FS with higher stock consistency SK.

FIG. 3 illustrates again in a detailed section of headbox 1 the components essential to the invention for producing the necessary geometric conditions on headbox 1.

Illustrated is nozzle 9 and the last region actively influencing the fibrous stock suspension FS which is located upstream, viewed in flow direction and which is formed by a turbulence generating device 7 and includes a fluidization region 15. Also illustrated are again the basic geometric dimensions l_D in form of the length of nozzle, l₁ as distance of the final fluidization region 15 within turbulence generating device 7 prior to inlet 14 into nozzle 9. The distance is hereby measured at the end of fluidization region 15. Fluidization region 15 may be planar, extending over a partial region of the flow path or may be linear in cross machine direction CD, that is locally strictly limited. Also illustrated is the angle of convergence a of nozzle 9 in the region of outlet gap 12 and length l_TE of turbulence generating device 7, as well as length l₁ for identification of the distance between fluidization region 15 and inlet 14 into nozzle 9 in flow direction. The dwell time in l₁ is hereby between 10 ms and 100 ms.

FIGS. 4a1, 4a2 and 4b1, 4b2 show a schematically greatly simplified illustration of advantageous designs of turbulence generating devices 7. Turbulence generating device 7 which is utilized for fluidization of fibrous stock suspension FS may take different forms. According to FIGS. 4a1, 4a2 it can consist of a plurality of channels 8 in the embodiment of individual channels which are arranged in rows in cross machine direction CD and in columns in height direction. Individual channels 8 of turbulence generating device 7, in this example 8.11 to 8.nn can be as already known in the form of pipes, square or rectangular profiles, etc. Moreover, integration of them into orifice plates is conceivable. FIG. 4a2 illustrates the arrangement in rows without offset relative to each other in cross machine direction CD. It is understood that also alternating offset of individual channels 8 of turbulence generating device 7 between two rows relative to each other arranged vertically on top of one another is possible.

According to FIG. 4b2 it is moreover conceivable to design flow channels 8 as channels 8.1 to 8.5 of turbulence generating device 7 extending over the width in cross machine direction CD which are arranged on top of one another in height direction. These channels of turbulence generating device 7 are identified here for example with 8.1 to 8.n and are illustrated in two views in FIGS. 4b1, 4b2. The coordinate system according to FIG. 1 was transferred for the purpose of directional allocation.

All embodiments have the design of the channel geometry in common which provides an area characterized by a graduated cross sectional change 17, in particular by progression. An example of such a turbulence generating channel 8 of turbulence generating device 7 is illustrated in FIG. 5. This view shows the extension in longitudinal direction that is in flow-through direction when installed in a machine for producing material webs. FIG. 5 clarifies the design of individual turbulence generating channel 8 of turbulence generating device 7 in schematically greatly simplified depiction. In this example turbulence generating channel 8 of turbulence generating device 7 is separated into a plurality of different partial regions 18.1 to 18.4. Inlet side 8E of turbulence generating channel 8 of turbulence generating device 7 describes together with additional such channels of turbulence generating device 7 inlet 7E into turbulence generating device 7. Outlet 8A corresponds to inlet 14 into nozzle 9. Between these, several partial regions 18.1 to 18.4 having different cross sectional areas Q1 to Q3 are arranged. The region of the final fluidization prior to the outlet into nozzle 9 is hereby realized through a graduated cross sectional change 17, in particular through a progression between two cross sectional areas Q1 and Q2. Here, turbulence generating channel 8 of turbulence generating device 7 has a first partial region 18.1 which is characterized by a constant cross sectional area Q1 over its extension range in flow-through direction, which is described by a hydraulic diameter d_hydr, in the illustrated example by a circular cross section through a diameter D1. Second partial region 18.2 which is located adjacent in flow-through direction between inlet 8E to outlet 8A is also characterized over the extension of the partial region 18.2 in flow direction, through a constant cross section which can be described by a diameter D2. The second partial region is followed by a transition area 18.3 which permits a constant, that is continuous transition to a third partial region 18.4 which is characterized by a cross sectional area Q3 which can be described by a diameter D3.

The design of the progression that is the cross sectional change 17 between cross sectional areas Q1 to Q2 which is characterized advantageously by a diameter change D2/D1 of the geometry describing the partial regions of turbulence generating channel 8 of turbulence generating device 7 occurs so that a pressure loss between the first partial region 18.1 and the second partial region 18.2 of greater than 50 mbar is produced. It is decisive however, that length l₁ of second partial region 18.2 and third partial region 18.4 under consideration of transitional region 18.3 which characterizes the distance from fluidization region 15 formed by progression region 18.3 to outlet 8A from turbulence generating channel 8 or respectively from turbulence generating device 7 must be at least ≦180 mm, preferably ≦150 mm, especially ≦120 mm, more especially 100 mm. Length l_TE of individual turbulence generating channel 8 is in the range between 100 mm and 500 mm, preferably 100 mm to 500 mm, especially between 150 mm and 300 mm.

If cross sectional areas Q1, Q2 and Q3 cannot be described by a diameter D1, D2 and D3—in other words, in the case of other cross sectional geometries—then the hydraulic diameter D_hydr=4·Q/U, with Q=cross sectional area and U=circumference is used.

According to a particularly advantageous design the final progression which is necessary for fluidization and which is located before nozzle 9 should be at least in the range of the medium fiber length of the used fibrous stock suspension FS, that is (D2-D1)/2≧l_Fmittel, whereby here the diameter with a circular cross section is formulated, otherwise the respective hydraulic diameter d_hydr.

Since after fluidization, that is after the last progression viewed in flow direction the formed flake size inside fibrous stock suspension FS depends on the available space or in other words on the cross sectional area Q, the largest hydraulic diameter d_hydr-8 inside turbulence generating channel 8 of turbulence generating device 7 should be in the range of 5 mm≦d_hydr≦25 mm, preferably 5 mm≦d_hydr≦20 mm, especially 10 mm≦d_hydr≦20 mm, and because of the fiber wipe formation the hydraulic diameter d_hydr-8E in the area of inlet 8E on turbulence generating channel 8 of turbulence generating device 7 should be selected in the range of 8 mm≦d_hydr-8E≦20 mm, preferably 10 mm≦d_hydr-8E≦20 mm, especially 10 mm≦d_hydr-8E≦15 mm.

The number of rows, in other words the number of flow channels 8 within one column should be selected so that the flow speed in the narrowest cross section is between 5 m/s and 20 m/s, preferably between 7 m/s and 15 m/s.

A headbox 1 of this type can be further modified as desired. There may be headboxes equipped with lamellas and/or with dilution fiber technology, meaning with at least one metering device for adding a fluid into flow channels 8.

The inventive headbox can moreover be used in combination with randomly designed forming units 2, in particular Fourdrinier wire, Hybrid Former and Twin Wire Former. The example illustrated in FIG. 1 represents an advantageous design in combination with a Gap Former whereby the free jet F is directed into a gap 19 between clothing 20.1, 20.2 which is supported by two rollers. It is however not limited to this.

While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

COMPONENT IDENTIFICATION LIST

• 1 Headbox
• 2 Forming unit
• 3 Sheet forming unit
• 4 Feed device
• 5 Turbulence generating device
• 6 Turbulence generating channel
• 7 Turbulence generating device
• 7E Inlet into turbulence generating device
• 7A Outlet from turbulence generating device
• 8 Turbulence generating channel
• 8.1-8.2, 8.11-8.n Turbulence generating channel
• 8E Inlet into turbulence generating channel
• 8A Outlet from turbulence generating channel
• 9 Nozzle
• 10 Nozzle chamber
• 11 Aperture
• 12 Outlet gap
• 13 Space
• 14 Inlet
• 15 Region
• 16.1 Nozzle wall
• 16.2 Nozzle wall
• 17 Cross sectional change
• 18.1 First partial region
• 18.2 Second partial region
• 18.3 Transitional region
• 18.4 Third partial region
• 19 Gap
• 20.1, 20.2 Wire belt
• 21 Line of impingement
• CD Cross machine direction
• D1 Diameter of first partial region
• D2 Diameter of second partial region
• D3 Diameter of third partial region
• d_hydr Hydraulic diameter
• d_hydr-8 Hydraulic diameter of turbulence generating channel
• d_hydr-8E Hydraulic diameter at inlet into turbulence generating channel
• 8E Inlet
• 8A Outlet
• FL Flake structure
• FS Fibrous stock suspension
• l₁ length of distance between progression and inlet into nozzle
• l_D Length of nozzle
• l_Fmittel Medium fiber length
• l_TE Length of turbulence generating device
• MD Machine direction
• Q1 Cross sectional area of first partial region
• Q2 Cross sectional area of second partial region
• Q3 Cross sectional area of third partial region
• SK, SK_x Stock consistency
• T_v Dwell time
• α Nozzle angle of convergence
• Δp Pressure loss

Claims

1. A headbox for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said headbox comprising:

at least one feed device feeding the at least one fibrous stock suspension;

a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and

a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle.

2. The headbox according to claim 1, wherein said dwell time is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar.

3. The headbox according to claim 1, wherein said nozzle has a length (lD) in a range of 100 mm≦lD≦500 mm, and a distance between said final fluidization region inside said individual one of said plurality of turbulence generating channels of said turbulence generating device and said first inlet into said nozzle is ≦180 mm.

4. The headbox according to claim 3, wherein said length (lD) is one of 100 mm≦lD≦400 mm and 200 mm≦lD≦400 mm, and said distance is one of ≦150 mm, ≦120 mm, and 100 mm.

5. The headbox according to claim 1, wherein said nozzle has a length which, under consideration of a stock consistency of the at least one fibrous stock suspension which is guided through said nozzle, meets the following requirements: lD×SK one of ≦1000, ≦800, and ≦700, wherein lD=said length of said nozzle, measured in mm, and SK=said stock consistency in %.

6. The headbox according to claim 1, wherein said nozzle includes a nozzle chamber, a first nozzle wall, a second nozzle wall, and a region of said outlet gap, said first nozzle wall and said second nozzle wall converging relative to one another in said flow direction and forming said outlet gap, said nozzle chamber being limited by said first nozzle wall and said second nozzle wall, an angle of convergence between said first nozzle wall and said second nozzle wall in said region of said outlet gap being one of between 5° and 45° and between preferably 10° and 20°.

7. The headbox according to claim 1, wherein said turbulence generating device has a length (lTE) viewed in a flow-through direction in a range of one of 100 mm≦lTE≦500 mm, 100 mm≦lTE≦400 mm, and 150 mm≦lTE≦300 mm.

8. The headbox according to claim 1, wherein said final fluidization region before said first inlet into said nozzle is formed by a local graduated change of a cross sectional area of said individual one of said plurality of turbulence generating channels of said turbulence generating device, viewed in said flow direction.

9. The headbox according to claim 1, wherein said final fluidization region before said first inlet into said nozzle is formed by a constant change of a cross sectional area of said individual one of said plurality of turbulence generating channels of said turbulence generating device, viewed in said flow direction.

10. The headbox according to claim 9, wherein said constant change of said cross sectional area in said final fluidization region suits at least a medium fiber length of the at least one fibrous stock suspension which is used.

11. The headbox according to claim 10, wherein a level of progression characterizing a cross sectional change in said final fluidization region suits at least said medium fiber length of the at least one fibrous stock suspension which is used.

12. The headbox according to claim 1, wherein said individual one of said plurality of turbulence generating channels of said turbulence generating device is designed and dimensioned so that a maximum diameter (dhydr) describing a cross sectional area is in a range of one of 5 mm≦dhydr≦25 mm, 5 mm≦dhydr≦20 mm, and 10 mm≦dhydr≦20 mm.

13. The headbox according to claim 1, wherein said individual one of said plurality of turbulence generating channels includes a second inlet, a hydraulic diameter (dhydr-8E) describing a cross sectional area at said second inlet of said individual one of said plurality of turbulence generating channels of said turbulence generating device being in a range of one of 8 mm≦dhydr-8E≦20 mm, 10 mm≦dhydr-8E≦20 mm, and 10 mm≦dhydr-8E≦15 mm.

14. The headbox according to claim 1, wherein a number of rows of said plurality of turbulence generating channels of said turbulence generating device is such that a flow speed in a narrowest cross section of said individual one of said plurality of turbulence generating channels of said turbulence generating device is one of between 5 m/s and 20 m/s and between 7 m/s and 15 m/s.

15. A sheet forming unit for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said sheet forming unit comprising:

a headbox including: at least one feed device feeding the at least one fibrous stock suspension; a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle; and

a forming unit which is arranged downstream of said headbox and to which the at least one fibrous stock suspension is supplied from said outlet gap of said headbox in said free jet.

16. The sheet forming unit according to claim 15, wherein said dwell time is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar.

17. The sheet forming unit according to claim 15, wherein said forming unit includes a clothing, the at least one fibrous stock suspension being deposited in said free jet onto said clothing of said forming unit creating a line of impingement, said headbox and said forming unit being designed and arranged so that a dwell time of the at least one fibrous stock suspension from said final fluidization region to said line of impingement on said clothing is one of ≧30 ms to ≦300 ms, ≧50 ms to ≦200 ms, and ≧80 ms to ≦200 ms.

18. A method to operate a sheet forming unit for a machine for producing a web of fibrous material from at least one fibrous stock suspension, the web being one of a paper web, a cardboard web, and a tissue web, said method comprising the steps of:

providing that the sheet forming unit includes: a headbox including: at least one feed device feeding the at least one fibrous stock suspension; a nozzle having an outlet gap for dispensing the fibrous stock suspension in a free jet, said nozzle having a first inlet; and a turbulence generating device arranged directly upstream of said nozzle in a flow direction, said turbulence generating device including a plurality of turbulence generating channels which are configured for guiding therethrough, during an operation of the headbox, the at least one fibrous stock suspension in a plurality of partial flows, within an individual one of said plurality of turbulence generating channels of said turbulence generating device at least one region representing a fluidization region is provided in which a pressure loss can be produced in a respective one of said plurality of partial flows of the at least one fibrous stock suspension being guided through said fluidization region, said nozzle and said turbulence generating device arranged directly upstream of said nozzle being designed and dimensioned such that said nozzle and said turbulence generating device are suitable for setting a dwell time of ≦200 ms of the at least one fibrous stock suspension flowing through said nozzle and said turbulence generating device from a final said fluidization region of said individual one of said plurality of turbulence generating channels of said turbulence generating device before said first inlet into said nozzle as far as said outlet gap of said nozzle, and for setting said pressure loss of ≧50 mbar in said final fluidization region before said first inlet into said nozzle; and a forming unit which is arranged downstream of said headbox and to which the at least one fibrous stock suspension is supplied from said outlet gap of said headbox in said free jet;

feeding the at least one fibrous stock suspension of said headbox across a machine width;

guiding the at least one fibrous stock suspension of said headbox by forming said plurality of partial flows in said plurality of turbulence generating channels of said turbulence generating device;

feeding the at least one fibrous stock suspension of said headbox to said nozzle from where the at least one fibrous stock suspension is delivered in said free jet into said forming unit;

adjusting, inside said individual one of said plurality of turbulence generating channels of said turbulence generating device, said pressure loss in the at least one fibrous stock suspension;

producing, in said final fluidization region of said individual one of said turbulence generating channels of said turbulence generating device upstream from said inlet into said nozzle, said pressure loss inside the at least one fibrous stock suspension of ≧50 mbar; and

guiding the at least one fibrous stock suspension from said final fluidization region to said outlet gap of said nozzle so that at least one of: (a) said dwell time of the at least one fibrous stock suspension in a region of the sheet forming unit extending from said final fluidization region as far as said outlet gap is ≦200 ms, and (b) a dwell time in a region of the sheet forming unit extending from said final fluidization region to a line of impingement is ≧30 ms to ≦300 ms, wherein the at least one fibrous stock suspension of said headbox is fed to said nozzle from where the at least one fibrous stock suspension is delivered in said free jet into said forming unit onto a clothing of said forming unit under definition of said line of impingement.

19. The method according to claim 18, wherein said dwell time of the at least one fibrous stock suspension in a region of the sheet forming unit extending from said final fluidization region as far as said outlet gap is one of ≦175 ms and ≦150 ms, and said pressure loss is one of ≧75 mbar, ≧100 mbar, and ≧150 mbar.

20. The method according to claim 18, wherein said dwell time in a region of the sheet forming unit extending from said final fluidization region to a line of impingement is one of ≧50 ms to ≦200 ms and ≧80 ms to ≦200 ms.