Drying and gas or vapor contact with solids – Apparatus – Rotary drums or receptacles
Reexamination Certificate
1999-11-23
2001-04-24
Gravini, Stephen (Department: 3749)
Drying and gas or vapor contact with solids
Apparatus
Rotary drums or receptacles
Reexamination Certificate
active
06219934
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to drying sections of paper making machines and, more particularly, to substitutes for and/or improvements of vacuum rolls and/or felt rolls used in drying sections.
As is well known, in a single tier dryer section the drying cylinders are arranged in a line following one another. In contrast, in a double-tier dryer section these cylinders are stacked in two tiers—two lines—, the paper web meandering alternatingly between the two tiers, in a zigzag pattern.
Although single tier dryer sections were commercialized in a meaningful way only relatively recently in the early eighties, they have been described in the literature for at least a half a century. As to double tier dryers, those have been in actual use for well over a century. Accordingly, to avoid needless description of well known technology, reference is made to the following United State patents which mostly describe single tier drying sections: 1,656,853; 2,537,129; 3,448,529; 3,868,780; 4,359,827; 4,427,736; 4,483,083; 4,677,762; 4,807,371; 4,850,121; 4,876,803; 4,882,854; 4,972,608; 4,974,340; 4,980,979; 4,982,513; 5,101,577; 5,105,501; 5,135,614; 5,144,758; 5,146,696; 5,269,074; and 5,279,049. The contents of each of the above patents is incorporated by reference herein.
Common to both single and double tier drying sections, the drying cylinders are divided into groups, each group having an associated felt or fabric, in well known manner. Of relevance here is the fact that in a double tier dryer section the felt is guided from one drying cylinder the next by so-called felt rolls. Similarly, in modern single tier drying sections, the same guiding function is carried out by so-called vacuum rolls, which also go by other names, e.g. suction rolls, suction felt rolls, etc.
With the passage of time, the operational speeds and the widths of paper machines have steadily increased. For example, nowadays finished paper measuring about 400 inches in width spews out of these paper machines at speeds well in excess of 4,000 feet per minute (about 45 miles per hour) and often even faster. In other words, the machine produces in one minute a sheet of paper that is 400 inches wide and almost a mile long. To produce 400 inch wide paper, the drying cylinders of these machines have to have axial lengths of well over 400 inches. In fact, these drying cylinders are huge, not only in width but also in diameter which typically can be on the order of 6 to 7 feet. These devices weigh many, many tons. Nonetheless, these cylinders are well worth their weight and cost because they provide very large heated surfaces over which the paper web travels as it is produced, and are the very reason which has enabled the realization of very fast and efficient paper making machines.
In contrast, there is no intrinsic overpowering reason for felt or vacuum rolls to be of gigantic size. These latter devices function primarily to transfer the felt, and in the case of vacuum rolls the felt and the web, from one drying cylinder to the next. Still, these devices have to be as long axially as the drying cylinders so as to span the width of the paper machine. Moreover, their diameters must still be quite large to ensure that they will not sag or bow at their centers, which would be extremely detrimental to the reliability and runability of the paper machine. In any event, current thinking in the art is that the ever increasing machine speeds encountered today make it useful to support the paper web substantially throughout its journey through the drying section, certainly during the first few dryer sections of the drying section and preferably throughout. In this regard, paper makers speak of the need to operate their machines without any “open draw,” i.e. with the paper web firmly supported on the dryers, or on the felts when the web travels between dryers, typically through the use of intermediate vacuum rolls. Therefore, many drying section designers are compelled to use expensive, massive vacuum rolls and/or very long and large felt rolls which substantially increases machine costs and machine operation expenditures.
SUMMARY OF THE INVENTION
Accordingly, a key object of the invention is to provide a drying section which obviates the need to fabricate and operate expensive vacuum rolls and/or felt rolls.
Another object of the invention is to provide a drying section which simplifies the construction of vacuum rolls and improves their operation in machine designs which insist on the use of vacuum and/or felt rolls.
A still further object of the invention is to provide a drying section which provides the advantages of a single tier drying section in a conventional two tier arrangement.
It is also an object of the present invention to provide a drying section design which substantially improves the drying cylinder heat utilization, enabling far higher paper machine speeds.
The present invention also envisions and provides a flexible dryer to dryer web transfer mechanism that will avoid spurious paper web breakages, occasionally experienced by single tier drying sections.
The foregoing and numerous other objects of the invention are realized by a drying section which still employs the traditional single or double tier configurations, but which introduces a radical departure from conventional notions concerning how the paper web or the felt should be guided between drying cylinders. The invention discloses the use of a novel structure which the present inventor has dubbed the “vacuum roller bridge.” The vacuum roller bridge (VRB) of the present invention is unlike any known drying section guiding roll. The bridge itself is stationary. It does not rotate around a fixed center of rotation which determines the path of the paper web, as with a conventional guiding roll. In the main, the bridge comprises a pair of spaced walls which extend in the cross direction of the paper machine and define a vacuum chamber therebetween. In the machine direction, each vacuum bridge extends in a somewhat arcuate path from one drying cylinder to the next.
The upper wall of the bridge which faces the pocket between the dryers is air tight. In contrast, the lower wall is perforated to allow the vacuum within to act on any felt or web/felt joint run that is guided over the lower wall. Further, the lower wall supports a plurality of rollers which are comparatively short, small diameter cylindrical bodies which protrude a very small distance beyond the lower wall, for example, an inch or two or so. These rollers are distributed in spaced relation to each other over the lower wall, lengthwise and widthwise of the drying section.
In operation of the novel drying section of the present invention, the joint run of felt and web, which conventionally is guided over a vacuum roll between adjacent dryers, is instead guided over the vacuum roller bridge. In doing so, the felt engages and travels over the rollers without touching the lower wall of the bridge while, at the same time, permitting the vacuum exerted through the lower wall to act on the paper web by drawing it and supporting it against the felt in a manner that provides without open draw guiding of web between drying cylinders. In other words, the invention guides the web/felt over the non-linear path extending from one dryer cylinder to the other with the novel vacuum roller bridges, instead of with conventional vacuum roller.
Another embodiment of the invention uses a pair of vacuum rolls between adjacent dryers. However, unlike all known vacuum rolls, which span the entire width of the machine and which are rotatably supported solely at their distal ends, the novel vacuum rolls of the present invention are substantially smaller in diameter and are also rotatably supported at one or more points intermediate their distal ends, in a manner and for a purpose that is described in more detail further on.
REFERENCES:
patent: 4483083 (1984-11-01), Chance
patent: 4710271 (1987-12-01), Miller
patent: 5832625 (1998-11-01), Skaugen et al.
patent: 5921000 (1999-07-01), Wedel et al.
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