Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...
Reexamination Certificate
1995-11-15
2002-04-09
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Apparatus
Running or indefinite length product forming and/or treating...
C162S359100, C162S360300
Reexamination Certificate
active
06368466
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a press section of a paper making machine in which water is removed from a paper web formed in a preceding forming section by the application of mechanical pressure to the paper web.
BACKGROUND OF THE INVENTION
It is known from the prior art to use one or more extended-nip presses in a press section that follows the forming section of a paper machine. In such an extended-nip press, the length of the pressing zone extending in the running direction of the paper web is substantially longer than that encountered in a roll press nip. Traditionally, extended-nip presses were commonly used only in paperboard machines and to make thicker grades of paper. However, more recently, extended-nip presses have also been introduced for use in the production of thinner paper grades, such as newspaper and fine papers.
Known press sections have various problems, especially when used in the high speed manufacture of relatively thin paper products. Relatively thin paper products are much more likely to be damaged during their manufacture than are thicker paper products, especially at relatively high web speeds of about 25 to about 35 m/sec. The relatively low number of intertwined fibers due to the very thinness of such thin paper products is one cause of such vulnerability. This latter factor also contributes to the difficulties encountered when attempting to separate such relatively thin paper stock from relatively smooth surfaces because considerable adhesion forces have to be overcome. Consequently, the incidence of tearing of and other damage to relatively thin paper webs is much higher even in such currently available press sections that employ extended nip presses than that encountered with thicker materials.
Examples of prior art that is considered to be most closely related to the present invention can be found in the following published patent documents: Finnish patent application No. 890530 (corresponding to U.S. Pat. No. 4,923,570 and published German patent document No. DE-OS 3808293.4), German utility model No. DE-GBM 8805966, Finnish patent application No. 913886 (corresponding to U.S. patent No. 5,178,732 and published German patent document No. DE-OS 4026021), Finnish patent No. 75,382, Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), and U.S. Pat. Nos. 4,257,844, 4,551,205, 4,704,192, and 5,120,399. Further, reference is made to the paper in the journal Wochenblatt für Papierfabrikation 19 (1993), pages 780 to 782 “Die Flexonip®-Pressen”, which describes some of the latest extended nip press constructions of J. M. Voith GmbH. The press sections in these prior art devices have various problems.
In Finnish patent application No. 890,530, especially with reference to
FIGS. 3 and 4
(and in the corresponding U.S. Pat. No. 4,923,570), the paper web is transferred from one extended nip press into a second and/or third extended nip press on the smooth surface of a gliding belt mantle of the extended nip press. Because the gliding belt mantle is smooth, water is pressed out of the paper web only through the surface of the paper web which faces away from the mantle because the smooth surface of the mantle has no route by which water can leave the paper web. Additionally, such a smooth surface mantle has pronounced adhesion properties and therefore poor paper web transfer properties. Moreover, the first nip is not an extended nip that removes water efficiently through both surfaces of the paper web, and the overall concept disclosed is not that of a compact press section of several nips.
In the
FIGS. 1 and 2
embodiments in German utility model No. DE-GMB 88 05 966, the press section consists solely of two separate extended-nip presses in which the high load pressing is carried out on the same face of the same pressing felt. Moreover, the paper web is transferred over the long distance between the two nips by confining the paper web between two wet felts and on the face of one felt, thus resulting in the rewetting of the web by water previously expelled from the paper web to the felt. Consequently, the water content of the paper web increases after it is pressed in this so-called rewetting process. While the geometries of the press sections that are shown in
FIGS. 3
,
5
and
6
are more compact than those of
FIGS. 1 and 2
, the same press felt runs through both of the extended nips, thereby reducing the water absorbing capacity of the felt in the second extended nip. The transfer of the paper web from one extended nip to the other relies fully on the differences in the surface characteristics of the pressing felts as there is no additional mechanism to ensure the transfer of the paper web.
In Finnish patent application No. 913886 (corresponding to U.S. Pat. No. 5,178,732),
FIGS. 1
,
1
a
,
2
,
3
,
3
a
and
3
b
, a transfer means, which has a dewatering capacity lower than that of a felt, runs through all the press nips. Since the largest amount of water is removed in the first nip, the dewatering in one direction (i.e. through one surface of the paper web) is limited by the transfer means contacting that surface. In
FIGS. 2
,
2
a
,
3
,
3
a
and
3
b
, the transfer means in the first nip consists of a gliding belt which is impervious to water and smooth so that dewatering through one surface of the paper web is completely precluded.
Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), Finnish patent No. 75,382, and U.S. Pat. No. 4,257,844 do not disclose or suggest the use of an extended nip press with a high dewatering capacity at the first nip or anywhere else, or a closed web transfer to the dryer section.
U.S. Pat. No. 5,120,399 merely discloses the use of a single felt extended nip at the first nip. In this case, water is removed from the paper web in one direction only. Also, the press sections comprise only two nips, and no compact multi-nip press section is suggested.
SUMMARY OF THE INVENTION
The present invention involves a press section of a paper making machine for dewatering a paper web formed in a preceding forming section. In the press section of the present invention, a pick-up felt is used to remove the paper web from the forming wire that transports the paper web through the forming section of the paper making machine. The pick-up felt is applied to the upper surface of the paper web and then a lower felt is applied to the lower surface of the paper web. The two felts are comprised of a fabric material which are preferably slightly heavier and thicker than normal so that the felts absorb a substantial amount of water from the paper web. The two felts may also or instead be water permeable. The paper web, sandwiched between the two felts, passes through a first press nip which is an extended nip in that the paper web is pressed along its width and, in particular, along a portion of the length of the paper web, i.e., in the direction of travel or running direction of the web, rather than along a line of contact as in a conventional press nip. In the first extended nip, water is pressed from the upper surface of the paper web and is absorbed by the pick-up felt, and water is pressed from the lower surface of the paper web and is absorbed by the lower felt. Thus, in the first extended nip, water is simultaneously and symmetrically removed from both the upper and lower surfaces of the paper web along an extended length of the paper web. Consequently, good symmetry of the structure of the paper web through its thickness is obtained also resulting in symmetry of the density and porosity of both surfaces of the paper web. In this first press nip of the press section, relatively high pressures (about 100 to about 1,400 kN/m) can be applied to the paper web because it is sandwiched between the two felts. As a result, a substantial amount of water can be removed from the paper web in the first extended nip, thereby increasing its dry solids content and its strength and reducing the possibility that the paper web will break or otherwise be damaged when
Cohen & Pontani, Lieberman & Pavane
Hastings Karen M.
Valmet Corporation
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