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 [0001]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[0002]1. Field of the Invention[0003]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 ch...

AI Technical Summary

Benefits of technology

[0038]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.

[0039]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.

Problems solved by technology

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.

Many times these are however not sufficient.

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.

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.

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