Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
1998-12-08
2001-04-03
Gravini, Stephen (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S454000
Reexamination Certificate
active
06209224
ABSTRACT:
BACKGROUND OF THE INVENTION
In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels and the like, the use of a throughdrying process is well known. In general, the throughdrying process involves passing hot air through a dewatered tissue web while the web is supported by a throughdrying fabric. More specifically, the fabric-supported web is dried while it is maintained in contact with a rotating throughdrying drum, which supplies the hot drying air. Depending upon the particular process, the dried web can be subsequently creped or left uncreped.
However, the current throughdrying methods are dependent upon throughdrying fabrics to provide the desired texture and structure to the resulting tissue product. The designs of such fabrics are limited by the inherent nature of the weaving process and the properties of the filaments used to weave the fabrics. A particular problem associated with many filament materials is their inability to withstand high temperatures. Of course, as efforts continue to increase the productivity of throughdrying tissue making machines, there is an incentive to be able to increase dryer temperatures and drying energy efficiency and shorten drying times. However, the throughdrying fabric is a limiting factor due to several reasons such as temperature degradation, loss of latent heat in each revolution of the dryer, etc.
Therefore there is a need for a method of improving the drying efficiency of throughdrying tissue machines.
SUMMARY OF THE INVENTION
It has now been discovered that the throughdrying fabric can be eliminated from the tissue machine by properly designing the throughdryer. This not only eliminates the costs of purchasing throughdrying fabrics and the down time costs associated with fabric changes, but the thermal efficiency of the drying process can also be improved.
Hence, in one aspect the invention resides in a method of making a throughdried tissue sheet comprising:(a) forming a tissue web; (b) dewatering the tissue web; (c) transferring the dewatered web from a supporting fabric to the surface of a throughdrying drum having a plurality of holes through which hot air passes to dry the web; (d) drying the web while on the surface of the throughdrying drum; and (e) removing the dried web from the surface of the throughdrying drum. Transfer of the web onto the surface of the throughdrying drum can be assisted by a blow box which blows compressed air through the supporting fabric towards the surface of the throughdrying drum. In addition, the transfer can be further assisted by the presence of a vacuum box within the throughdrying drum at the point of transfer. Removal of the dried web from the surface of the throughdrying drum can be assisted by the presence of a blow box within the throughdrying drum which blows air outwardly toward the dried web at the point of removal. Alternatively or in addition, a doctor blade can be used to skim the dried web from the surface of the throughdrying drum. In some cases, if the strength of the web is adequate, the dried web can be simply peeled off.
In another aspect the invention resides in a method of making a throughdried tissue sheet comprising:(a) forming a tissue web; (b) dewatering the tissue web; (c) transferring the dewatered web to the surface of a throughdrying drum without being supported by a throughdrying fabric, said surface of the throughdrying drum having a plurality of circumferential ridges; (d) drying the web while on the surface of the throughdrying drum; and (e) removing the dried web from the surface of the throughdrying drum. In this embodiment, the presence of the circumferential ridges impart significant bulk, cross-machine directional stretch and flexibility to the resulting dried web.
In another aspect, the invention resides in a throughdrying apparatus for drying a web, said apparatus comprising a perforated throughdrying drum having a plurality of circumferential ridges, a hot air hood for supplying hot air to the surface of the throughdrying drum, a vacuum zone within the throughdrying drum for drawing hot air from the hot air hood inwardly through the surface of the throughdrying drum to dry the web, and means for removing the dried web from the surface of the throughdrying drum. The means for removing the dried web from the surface of the throughdrying drum can include a ribbed doctor blade which is positioned against the surface of the throughdrying drum which skims the web from the surface and/or a blow box within the throughdrying drum which utilizes pressurized air to blow the web off of the surface of the throughdrying drum.
As used herein, a dried “tissue” web is a low density sheet of papermaking fibers suitable for use as facial tissue, bath tissue or paper towels, as distinguished from other more dense grades of paper such as writing papers.
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Croft Gregory E.
Gravini Stephen
Kimberly--Clark Worldwide, Inc.
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