Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2002-02-14
2004-06-08
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S111000, C162S358300, C162S358500
Reexamination Certificate
active
06746574
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a machine for the manufacture of a fiber material web, in particular a tissue paper or hygienic paper web, having at least one pressing gap (nip) which is formed between a shoe pressing unit and a drying or tissue cylinder, and water absorbent carrier band, a water-impermeable pressing band and the fiber material web guided through the at least one gap. It also relates to a method for the manufacture of a fiber material web, e.g., a tissue or hygienic paper web, in which the fiber material web to be dewatered is passed together with a carrier band through a pressing gap (nip).
2. Discussion of Background Information
A plurality of embodiments of a machine of the initially named kind are described in DE-A-42 24 730. In this document, at least two pressing gaps are in each case provided in all embodiments. The main press, which lies to the rear when viewed in the direction of travel of the web, includes in each case a drying cylinder and an associated pressing element. A suction pressing roller or a shoe pressing roller can be provided as a pressing element of this kind.
Two pressing gaps are again also provided in a machine of the initially named kind which is known from DE-A-196 54 197. The rearwardly lying main press is formed by a shoe pressing unit and a drying cylinder.
Thus, in the known machines two or more pressing gaps are always provided. This is considered to be imperative in order to achieve on the one hand a careful dewatering without a squashing, which is demanded in particular in the manufacture of a tissue paper or hygienic paper web, and on the other hand an improved production performance through an increased dry content after the press. In this a careful dewatering of the web without a squashing is achieved with the complete or partial replacing of the roller gap presses by shoe presses as a result of the corresponding gap lengthening or the increase in the pressing time respectively.
A substantial disadvantage of known embodiments includes the relatively high investment and operating costs in comparison with the improvements achieved. In view of the previously usual limiting of the maximum line force to a value in the region of 90 kN e.g. for Yankee cylinders it was always necessary in the known press designs to provide at least two pressing gaps.
SUMMARY OF THE INVENTION
The present invention provides a machine and a method as generally discussed above in which as high a dry content and/or specific volume can be achieved while avoiding the above named disadvantages and while largely retaining the quality features which are demanded of the produced web.
In a dewatering press of a machine which serves for the manufacture of a paper and/or cardboard web and which is described in EP-A2-0 852 273 the dwell time of the web in a prolonged pressing gap amounts to less than 12 ms.
In accordance with the invention, the machine includes at least one pressing gap (nip) which is formed between a shoe pressing unit and the drying or tissue cylinder respectively. The length of this pressing gap when viewed in the direction of travel of the web is less than or equal to a value of approximately 60 mm and the resulting pressure profile over the pressing gap length has a maximum pressing pressure which is greater than or equal to a value of approximately 3.3 MPa.
A completely unexpected significant increase both in the dry content and also in the specific volume of the fiber material web is achieved with a development of this kind. This is all the more surprising as the proposed solution departs from the course which was previously taken in the press development, the goal of which was, with the replacement of the roller gap presses by shoe pressing with a prolonged pressing gap, in effecting a careful, slow dewatering as a prerequisite for ideal results. As a result of the development in accordance with the invention many tissue paper machines can now be realized with only one single shoe pressing gap.
In a preferred practical embodiment of the machine in accordance with the invention the pressing gap length is less than or equal to a value of approximately 50 mm and the maximum pressing pressure is greater than or equal to a value of approximately 4.3 MPa. The pressing gap length is defined as he contacted length between the drying cylinder and the associated pressing element and the new carrier band which is passing through.
In this the specific volume is likewise slightly increased. An increase of this kind can for example amount to approximately 5%. A considerable increase is arrived at with respect to the dry content, which holds in particular in comparison with a pressing gap which is formed by a suction pressing roller or by a shoe press with a gap length of for example 120 mm. In this case for example an increase in the dry content by approximately 2.5 to 3% is possible if a constant line force of approximately 90 kN/m is assumed. A value of this kind must not be exceeded in the previously usual Yankee cylinders in view of a maximum permissible mechanical stressing. The invention can thus be used with particular advantage in particular in conversions of existing presses. Thus dry content increases and constant or even higher values of the specific volume (bulk) can be achieved in comparison with the conventional presses in spite of a respective line force limiting.
In a further expedient embodiment the pressing gap length lies in a region of approximately 37 mm and the maximum pressing pressure in a region of approximately 4.8 MPa.
With short shoes it turned out that the angle between the tangent which is applied at the end of the pressing gap to the drying or tissue cylinder respectively and the carrier band which emerges from the pressing gap has a significant influence on the dry content of the tissue web. An advantageous embodiment is distinguished in that in particular for the manufacture of a tissue web this angle between the tangent which is applied at the end of the pressing gap to the drying or tissue cylinder respectively and the carrier band which emerges from the pressing gap is ≧10°, in particular is ≧18° and is preferably ≧20°. With this there results a dry content increase of approximately 1 to 3% in comparison with applications with suction pressing rollers. In this the pressing band is preferably grooved and/or blind bored.
In accordance with an alternative embodiment of the machine in accordance with the invention, at least one pressing gap is formed between a shoe pressing unit and the drying or tissue cylinder respectively. The length of the at least one pressing gap when viewed in the direction of travel of the web is greater than a value of approximately 80 mm and preferably less than 200 mm, in particular at most 150 mm. The pressure profile which results over the pressing gap length has a maximum pressing pressure which is less than or equal to a value of approximately 2 MPa. In this account is also taken in particular of the fact that in longer shoes the dry content which can be achieved, contrary to expectation, decreases.
In particular in this case it is advantageous when the dwell time of the fiber material web in the pressing gap is greater than or equal to a value of for example approximately 3.5 ms and is in particular greater than or equal to 4 ms. In this the dwell time can in particular be defined by the ratio of the gap or shoe length respectively to the web speed.
The maximum line force which is produced by the pressing gap can lie for example in a range from approximately 90 to approximately 120 kN/m.
In an advantageous practical embodiment of the machine in accordance with the invention the shoe pressing unit includes a pressing shoe which can be pressed against the drying cylinder via a plurality of pressing elements which are arranged adjacently to one another and transversely (cross-wise) to the direction of travel of the web. With this a respective desired pressing force profile can be set in particular at the web edges for the uniformiza-
Scherb Thomas Thoroe
Schmidt-Hebbel Harald
LandOfFree
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