As known from the prior art, in a dryer section of a paper machine, single-wire draw or twin-wire draw or various combinations of the same are employed. In single-wire draw, so-called "normal groups with single-wire draw" are used, in which drying cylinders are arranged in an upper row and reversing suction cylinders or rolls are arranged in a lower row below the upper row of drying cylinders. In this case, the dryer section is open toward the bottom, for example, to enable easy removal of broke. In so-called "inverted groups with single-wire draw", the drying cylinders are arranged in a lower row and the reversing suction cylinders or rolls are arranged in an upper row above the lower row of reversing suction cylinders or rolls. In this case, it is a recognized drawback that the inverted groups are closed toward the bottom and, for example, there are problems associated with the removal of broke because it cannot be arranged to take place by the force of gravity (which is how it is done in "normal groups with single-wire draw").
In the past, in the dryer section of a paper machine, so-called breaker stacks (intermediate calenders) were employed quite commonly, which stacks are usually arranged between groups with twin-wire draw (which will be discussed below). These breaker stacks have been largely abandoned, partly because of runnability problems, and partly because their us causes an increase in the length of the dryer section in the machine direction since, between a breaker stack and the wire groups preceding and following it, it was usually necessary to provide relatively long unsupported draws of the web. Such draws are susceptible to fluttering and web breaks and are also problematic in view of the threading of the web.
In recent years, some breaker stacks arranged inside dryer sections have been suggested, which stacks are formed between heated drying cylinders and particular calender rolls. With respect to this prior art, reference is made, as an example, to the U.S. Pat. No. 5,127,168, to published German Patent Application No. DE 4,407,405 A1 (which corresponds to English-language Canadian Patent Application No. 2,143,912), and to a press section marketed by Black Clawson--Kennedy Inc. under the trademark "HYDRA NIP".TM.. It is not known if a patent or patent application in respect of the latter press section has been filed or issued. With respect to the prior art, reference is made further to Japanese Patent Application No. 56040/1992 (published application No. 222691/1993), in the name of Mitsubishi Heavy Industries Ltd.
In FIG. 5 in U.S. Pat. No. 5,127,168, an arrangement is illustrated in which a first nip of intermediate calendering is formed in connection with the first cylinder in the first group with twin-wire draw in a multi-cylinder dryer, and a water-receiving felt of the press section of the paper machine is passed through this first nip of intermediate calendering. A second nip of intermediate calendering is formed in connection with the first lower cylinder in the second group with twin-wire draw. It is a drawback of the arrangement of intermediate calendering known from this U.S. patent that in the group gap in which an intermediate calendering nip is used, the web will have a very long unsupported draw, and further, it is a disadvantage that the group gap makes the dryer section remarkably longer in the machine direction. The U.S. patent also does not suggest any solutions for carrying out intermediate calendering in modern dryer sections in which groups with single-wire draw and closed draw of the web are applied.
In FIG. 1 of the German Patent Application No. DE 4,407,405, an arrangement of intermediate calendering is illustrated in which an intermediate calendering nip is arranged in connection with the last drying cylinder in an inverted group with single-wire draw. The arrangement of intermediate calendering in accordance with the German patent application involves the drawback that the group gaps in which intermediate calendering is applied become relatively long and spacious in the machine direction and thereby unduly increase the length of the dryer section in the machine direction. Moreover, in the German patent application, an arrangement of intermediate calendering is described as being applied in connection with an inverted group with single-wire draw and in group gaps between such an inverted group and a normal group with single-wire draw only. As such, the German patent application does not suggest any solution for intermediate calendering in dryer sections in which exclusively groups with single-wire draw or various combinations of such groups and groups with twin-wire draw or combinations of such groups and so-called hybrid groups are employed.
In particular in connection with modernizations of paper machines, for example in order to increase their running speed, it is also necessary to increase the drying capacity of the dryer section. The prior art arrangements of intermediate calendering mentioned above, with the exception of that described in U.S. Pat. No. 5,127,168, are not suitable for modernizations of paper machines, because they increase the length of the dryer section and/or reduce the drying capacity.