Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2001-01-02
2002-08-13
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S278000, C162S358100
Reexamination Certificate
active
06432273
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a former-press section of a paper machine in which the former section comprises a lower wet wire and an upper wet wire or an equivalent transfer wire, and in which the press section comprises one or more dewatering press nips, and between which former section and press section there is a pre-press zone which removes water from a web and through which the web is passed as a closed draw from the former section to the press section.
The invention also relates to a method of transferring a paper web from a former section of a paper machine to a press section thereof, which former section is provided with a lower wet wire and with an upper wet wire or with an equivalent transfer wire, and which press section is provided with one or more dewatering press nips, and between which former section and press section water is removed from the web in a pre-press zone through which the web is passed as a closed draw from the former section to the press section.
BACKGROUND OF THE INVENTION
Increased speeds of paper and board machines provide new problems to be solved, which problems are mostly related to the rumnnability of the machine. Currently, speeds of up to from about 1600 to about 1700 meters per minute are employed in paper machines. At these speeds, the so-called closed press sections, which comprise a compact combination of press rolls fitted around a smooth-faced centre roll for the most part still operate satisfactorily. The applicant's Sym-Press II™ and Sym-Press O™ press sections may be mentioned as examples of these press sections.
Dewatering taking place by pressing is more advantageous than dewatering by evaporation from the point of view of energy economy. For this reason, attempts should be made to remove a maximal amount of water from the web by pressing, in order that the proportion of water to be removed by evaporation could be made as low as possible. Increased speeds of paper and board machines, however, provide new, so far unsolved problems expressly for dewatering taking place by pressing because the press impulse cannot be increased sufficiently by the prior art means, above all because at high speeds the nip times remain insufficiently short and, on the other hand, the peak pressure of compression cannot be increased beyond a certain limit without destroying the structure of the web. In addition to the drawbacks mentioned above, the efficiency of the prior art suction rolls is lowered significantly at particularly high web speeds, because the suction does not have sufficient time to act upon the web in the intended manner through the relatively long perforations in the mantle of the suction roll and through non-porous and thick pick-up press felt. This both limits the efficiency of dewatering and makes the transfer of the web more difficult with the help of a vacuum in the suction roll.
With increasing speeds of paper machines, the problems of runnability of a paper machine are also manifested with higher emphasis, because a web with a high water content and low strength does not endure especially the dynamic forces produced by high web speeds and changes in the direction of the web, but rather web breaks and other operational malfunctions arise, causing downtime periods. In modern printing paper machines, the cost of downtime is today about FIM 50,000 per hour.
In the prior art press sections, the web is generally passed from the forming wire into the first press nip on a pick-up felt, which also operates as a press fabric that receives water in the first press nip, which is either a roll nip or an extended nip. In the first press nip, a relatively high compression pressure is employed and large quantities of water are dealt with, and it is one of the drawbacks arising from this that the outer face of the press felt tends to be contaminated and its porous fabric structure tends to be partially blocked. Attempts are made to prevent this by means of efficient felt conditioning devices, which are, however, quite expensive, spacious components which consume an abundance of energy.
Recently, even speeds as high as about 40 meters per second=2400 meters per minute have been contemplated as speeds of printing-paper machines. Applications at speeds as high as this, in particular in wide machines, provide ever more difficult problems to be solved, of which problems the most important ones are runnability and adequate dewatering capacity of the machine at a high web speed. Similarly, in board machines (basis weight of the web>100 grams per square meter) attempts are made to increase the present web speeds (8-15 meters per second) to the level of 15-25 meters per second.
Important drawbacks of the press felts used in the prior art press sections include the effect of rewetting the web and the tendency of contamination, because, in particular when said press felts run through a high-pressure nip or nips, particles of contaminants tend to be affixed and to adhere to the press fabrics, wherefore the operation of the press fabrics is disturbed and their cleaning requires efficient conditioning devices, which consume a considerable amount of energy.
Moreover, in high-pressure press nips, the prior art porous press felts are subjected to intensive wear and strain, so that the felts must be replaced rather frequently, which increases the costs to a considerable extent.
With respect to the prior art related the invention, reference is made to U.S. Pat. Nos. 4,197,158, 4,879,001, 5,308,450 and 5,736,011. The last-mentioned U.S. Pat. No. 5,736,011 discloses a wet end of a paper machine which comprises a headbox of special construction and a twin-wire former on whose upper wire a web is passed as a closed draw to a pick-up point. The US patent does not disclose a pre-press nip nor any pre-pressing stage integral to the present invention. A further difference is that the present invention does not have the special limitations and structural requirements described in the above-mentioned US patent in connection with the headbox. The geometry of the twin-wire former and the transfer of the web from the former to a press section disclosed in the US patent are also substantially different from those of the present invention.
With respect to the prior art most closely related to the invention, reference is made to the applicant's FI patent 98843 (corresponding WO 97/13030 and U.S. Pat. No. 5,792,320). This FI patent discloses a method for removing water from a paper or board web and for passing it as a closed draw from a forming wire or transfer wire of a web former section to a press section. In the method of the FI patent, the web running on the forming wire or on the transfer wire is caused to adhere in a transfer and pre-press zone to the outer face of a transfer belt which is substantially non-water-receiving, and after the pre-press zone, the web is separated substantially immediately from said wire and passed on support of the transfer belt loop onto the next press fabric and/or into the next press nip in the press section.
In addition, the above-mentioned FH patent discloses a press section in which the press section includes a pre-press zone or zones and a transfer belt loop which is substantially non-water-receiving and has an outer face capable of adhesion to the paper web. This transfer belt loop is passed through the pre-press zone, or if two zones are present, at least through the latter zone. In the pre-press zone, the paper web is caused to adhere to the outer face of the transfer belt loop, and after the zone, it is separated substantially immediately from the forming wire without substantial rewetting of the web. On the transfer belt, the web is passed as a closed and supported draw onto the next press fabric and/or through the next press zone in the press section.
In FI patent 98843, a reliable and closed draw of the web is accomplished from the former section to the dryer section without risk of rewetting of the web. Also, in connection with the forming wire or an equivalent transfer wire it is possible t
Honkalampi Petter
Laapotti Jorma
Metso Paper Inc.
Nguyen Dean T.
Steinberg & Raskin, P.C.
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