Drying and gas or vapor contact with solids – Process – Diverse types of drying operations
Reexamination Certificate
1999-09-10
2001-05-29
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Diverse types of drying operations
C034S425000, C034S453000, C034S456000, C034S464000, C034S117000, C034S122000, C034S123000
Reexamination Certificate
active
06237248
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention claims the priority under 35 U.S.C. §119 of German Patent Application No. 198 41 767.5 filed on Sep. 11, 1998, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a dry end of a machine for manufacturing a material web, in particular a paper or cardboard web. It also relates to the convection drier particularly suited for use in such a dry end.
2. Discussion of Background Information
In previously known multi-cylinder dry ends, in order to be dried, the paper web is guided by steam-heated cylinders or of an arrangement of steam-heated cylinders and wire suction rolls. Particularly at the beginning of the drying, however, problems frequently arise in the web guidance which can in particular be traced back to the fact that the still-moist web remains stuck to the smooth contact surfaces of the cylinders or rolls that are absolutely required for a sufficient heat transmission. This frequently leads to web tears and to an overstretching of the web edges. Therefore, as a rule, the assurance must be made that the drying occurs more slowly, which means that the necessary dry end becomes longer as a whole. The above-mentioned problems occur to a more pronounced degree at higher web speeds.
Thus, an object of the present invention is to produce a dry end of the type mentioned above which simultaneously assures a drying rate which is as optimal as possible and thereby an overall length of the dry end which is as short as possible, as well as a reliable web guidance. Furthermore, a convection drier should be produced which is particularly suited for a dry end of this kind.
SUMMARY OF THE INVENTION
With regard to the dry end, this object is attained according to the present invention in that the dry end includes at least one convection drier which has at least one drying section by means of which the material web is guided between two air-permeable wire belts and in which the material web can be acted on with hot air and/or hot vapor on one or both sides through the wire belts.
Based on this embodiment, it is particularly possible to assure a sufficient heat transmission onto the still relatively moist material web at the beginning of the dry end without the necessity of the material web being guided by smooth contact surfaces for his purpose. Instead, in a respective drying section, the material web is guided between two air-permeable wire belts having surfaces oriented toward the material web which are open, i.e., not smooth. Since the smooth contact faces are avoided at the beginning of the drying, the danger of web tears and an overstretching of the web edges is practically eliminated. In particular, higher drying rates are also now possible, such that the overall length of the dry end is considerably shortened.
The drying capacity of the drying process is only limited by a possible influence of the paper quality at an excessive drying speed and no longer by the web guidance of the moist paper web. As long as it is sufficiently moist, the paper is not heated significantly above the cooling limit temperature. Furthermore, with the convection drier that can be more flexibly and rapidly regulated, the paper quality can be influenced more favorably. Thus, a correction of moisture cross direction profiles is also possible at the beginning of the drying phase, particularly with a deliberate heating and drying. Since pressing against heated surfaces is no longer required, the shrinkage prevention at the beginning of the drying is also very slight, which can have a positive effect on the propensity to wrinkle or pucker. The lower heat transmission coefficient in the convection, as compared to a heat transmission by means of smooth contact surfaces, is balanced out, or even overcompensated for, by means of a greater temperature difference and the two-sided drying.
In a preferred practical embodiment of the dry end according to the invention, the convection drier includes at least one drying section that is at least essentially straight, or slightly curved, in terms of the web travel direction, such that a reliable and exact guidance is always assured.
It is advantageous if at least two drying sections are disposed generally one above the other in the convection drier. Such a multi-tier disposition can considerably reduce the structural length such that, particularly in connection with the double-sided drying, an undesirable uneven drying on opposite sides of the web can be prevented.
In one embodiment of the present invention, at least one drying section of the convection drier is formed between two canopy hoods or nozzle hoods. At least one such canopy hood is preferably simultaneously provided with a vapor or air exhaust, which further reduces the required structural length of the dry end. Therefore, an exchange between the heating and evaporation is no longer necessary, which means that the additional evaporation surface is eliminated.
The material web can also be at least partially laterally enclosed by the convection drier at least partially. Since this prevents the exhaust air from mixing with colder ambient air, the exhaust air can be used further at a higher temperature.
If the convection drier has a plurality of drying sections, then the drying sections can also be at least partially controlled separately, particularly in terms of their heating capacity. Thus, a generally more selective and sensitive influence on the material web is possible.
For moisture profiling, the convection drier can suitably have at least one drying section that is divided into zones lateral to the web travel direction. Alternatively or additionally, it is also conceivable to divide the respective canopy hoods at least partially in the web travel direction.
According to a preferred embodiment of the present invention, a closed web guidance is provided throughout the entire convection drier. A closed web guidance of this kind particularly benefits the above-mentioned multi-tier disposition of a plurality of drying sections disposed generally above one another.
A web deflection after a respective drying section is preferably carried out by means of a respective wire suction roll, from which the respective outer wire belt, which is guided through the preceding drying section, is removed so that a shearing of the material web is prevented.
Suitably, the material web is acted on in at least one drying section of the convection drier with hot air and/or hot vapor, having a temperature in a range from approximately 150 to approximately 450° C. and particularly from approximately 200 to approximately 400° C.
According to another aspect of the present invention, the material web is acted on, in at least one drying section of the convection drier, by hot air and/or hot vapor having a flow speed in a range from approximately 60 to approximately 120 m/s, particularly from approximately 90 to 110 m/s, and preferably in the vicinity of 100 m/s.
In a preferred embodiment of the dry end according to the present invention, at least one convection drier is arranged in the web travel direction before a single or double-row cylinder drying group such that, upon reaching the first drying cylinder of this drying group, the material web has preferably already been dried to a dry matter content in the range from approximately 55 to approximately 65%.
The convection drier can thus be provided directly at the beginning or in a beginning region of the dry end, after one or a number of cylinders. Additionally or alternatively, however, it is also possible to provide at least one such convection drier in a main evaporation zone and/or in the end region of the dry end. As a result, an even higher drying rate is achieved, which further shortens the dry end as a whole.
According to another aspect of the invention, a dry end of a machine is provided for manufacturing a material web, in particular a paper or cardboard web. The dry end includes at least one convection dr
Greenblum & Bernstein P.L.C.
Voith Sulzer Papiertechnik Patent GmbH
Wilson Pamela
LandOfFree
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