Nip arrangement for a paper machine or the like
The present invention relates to a nip arrangement in a paper or paperboard machine or in a finishing machine for paper or paperboard, said nip arrangement comprising a first nip in which a belt loop travels between a first roll and a second roll.
Publication EP 0770727 discloses an arrangement in which two belt loops travel through a nip, and a paper web moves forward between said belt loops. The paper web is guided by means of suction affecting through the belt, which suction can be arranged by means of a suction roll.
The drawbacks of known solutions include for example the number of belt loops and the fact that when the web is conveyed between belts, an uneven surface complying with the surface structure of the belt is produced on the web.
By means of a nip arrangement according to the invention, it is pos- sible to avoid the above-mentioned problems. The nip arrangement according to the invention is characterized in that the nip arrangement comprises a second nip in which the same belt loop as in the first nip travels between the second and a third roll, and the web to be treated is in contact with the surface of the second roll between the first and the second nip.
In the nip arrangement according to the invention it is possible to reduce the number of belts. The surface of the web to be treated can be wet calendered so that it becomes smooth. Reliable closed sheet run of the web is also attained.
The nip arrangement comprises a first nip and a second nip. The same belt loop that conveys the web to be treated also travels via the first and the second nip. The first nip is formed between the first roll and the second roll, and the web to be treated is in contact with the surface of the second roll. The second nip is formed between the second roll and the third roll, i.e. both in the first nip and in the second nip the second
roll constitutes one counter surface forming the nip. The second nip is not necessarily subjected to a load, but the third roll is in contact with the belt, the belt is in contact with the web to be treated and the web is in contact with the second roll. Water is removed in the first nip in the press section and in the second nip the web is transferred from the smooth roll with the belt.
When the web to be treated travels through the first nip, the surface that is in contact with the surface of the web becomes smooth, i.e. the surface is wet calendered. The third roll may be equipped with a drive, wherein a local speed difference can be produced in the second nip to stretch the web and to release it from the surface of the second roll. The third roll can be a suction roll, wherein the suction effective through the belt loop moves the web easily on the surface of the belt after the second nip, and the web to be treated does not continue its travel on the surface of the second roll. Thus, a reliable closed sheet run of the web is attained, wherein the web does not break easily. The releasing of the web from the second roll after the nip can be enhanced by means of blowings directed in a suitable manner or by means of a doctor installed at a suitable location.
The nip arrangement according to the invention is advantageous in the press section of a paper or paperboard machine, where the strength level of paper or paperboard is still low. It is especially advantageous to utilize the invention when producing printing papers of various types, for example in connection with the manufacture of SC paper. The principle of the invention can also be applied for example in the drying of paper in the drying section as well as in calendering.
Because of the above-described operating conditions, the concept of a belt refers in this context to felts, wires, belts of a belt calender or other corresponding materials that can be utilized to form a part (belt loop) moving forward in a paper machine. In the press section the belt is typically a press felt.
The first and the second roll can be heated or unheated rolls. They can be hard rolls or resilient rolls. Possible roll types include for example
conventional rolls or shoe rolls. In case of a press section, the first roll is typically a shoe roll, or another conventional press roll that removes water by means of pressing, for example a roll with a grooved surface. The second roll is typically a roll with a smooth surface, for example a ceramic roll, a smooth roll coated with synthetic material or a granite roll. The third roll is advantageously a suction roll.
In the following, the invention will be described with reference to the appended drawings, in which
Figs 1 to 4 show some embodiments of the invention.
Fig. 1 shows a basic concept of the nip arrangement according to the invention. The first roll 1 is a shoe roll and the second roll 2 is a ce- ramie roll. A belt loop is formed of the belt 5 by means of auxiliary rolls. The belt 5 travels from the first nip N1 between the first and the second roll in such a manner that it touches the surface of the first roll 1 and the web to be treated touches the surface of the second roll 2. A second nip N2 is formed between the second roll 2 and the third roll 3, the belt 5 and the web moving through said nip. The third roll 3 can be a suction roll that promotes the transfer of the web from the surface of the second roll 2 on the surface of the belt 5. The third roll 3 can be equipped with a drive, wherein a local speed difference can be produced between the surfaces of the belt 5, said speed difference stretching the web. The web is wet calendered smooth against the second roll 2. To move the web away from the surface of the belt 5 and further with the next belt loop 11, the next belt loop can be provided with a suction roll 4.
Figs 2 to 4 show nip arrangements similar to the one shown in Fig. 1. The web is brought to the first nip on the surface of the roll 2, on which it is transferred from the surface of a transfer roll 9. The web travels further to the surface of a transfer belt 11 via the belt 5, and thereafter to the surface of a transfer belt 12, except in Fig. 4 in which the paper web is brought into the nip N1 directly on the surface of the transfer belt 5. The rolls 6, 7 and 8 are arranged in such a manner that when the web has travelled through the nips therebetween, the web is
treated on both sides according to the principle of the invention. The converter 10 of the moisture content of the transfer belt can be for example a suction box.
The invention is not restricted to the description above, but it may vary within the scope of the claims.