Air press for dewatering a wet web

Details Air press for dewatering a wet web Air press for dewatering a wet web

1999-04-23

2001-10-23

Hastings, Karen M. (Department: 1731)

Paper making and fiber liberation

Apparatus

Running or indefinite length product forming and/or treating...

C162S301000, C162S359100, C034S634000

Reexamination Certificate

active

06306257

ABSTRACT:

BACKGROUND OF THE INVENTION
There are may characteristics of tissue products such as bath and facial tissue that must be considered in producing a final product having desirable attributes that make it suitable and preferred for the product's intended purpose. Improved softness of the product has long been one major objective, and this has been a particularly significant factor for the success of premium products. In general, the major components of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness.
While enhanced softness is a desire for all types of tissue products, it has been especially challenging to achieve softness improvements in uncreped throughdried sheets. Throughdrying provides a relatively noncompressive method of removing water from a web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until dry. The resulting dried web is softer and bulkier than a conventionally-dried uncreped sheet because fewer bonds are formed and because the web is less compressed. Thus, there are benefits to eliminating the Yankee dryer and making an uncreped throughdried product. Uncreped throughdried sheets are typically quite harsh and rough to the touch, however, compared to their creped counterparts. This is partially due to the inherently high stiffness and strength of an uncreped sheet, but is also due in part to the coarseness of the throughdrying fabric onto which the wet web is conformed and dried.
Therefore, what is lacking and needed in the art is a method for manufacturing tissue products having improved softness, and in particular uncreped throughdried tissue products having improved softness, as well as an apparatus that permits the manufacture of such tissue products.
SUMMARY OF THE INVENTION
It has now been discovered that an improved uncreped throughdried web can be made by dewatering the web to greater than about 30 percent consistency prior to transferring the wet web from a forming fabric to one or more slower speed intermediate transfer fabrics before further transferring the web to a throughdrying fabric for final drying of the web. In particular, increasing the consistency of the uncreped throughdried web before the point of differential speed transfer has surprisingly been found to result in: (1) both higher machine direction and cross direction tensile properties, contributing to improved runnability of the web; and (2) reduced modulus, that is increased softness, when the tensile strength is adjusted to the normal value. This discovery allows for the manufacture of tissue products with lower modulus at given tensile strengths as compared even to tissue products produced by undergoing differential speed transfer at lower consistencies.
One aspect of the present invention concerns an air press for noncompressively dewatering the wet web. The air press is a particularly desirable apparatus for dewatering the uncreped throughdried web to about 30 percent consistency or greater prior to the differential speed transfer. While pressurized fluid jets in combination with a vacuum device have previously been discussed in the patent literature, such devices have not been widely used in tissue manufacturing. Principally, this appears to be due to the fact that it had not been previously recognized that dewatering the web to greater than about 30 percent consistency in advance of the differential speed transfer would result in the improved product properties identified herein. Moreover, the disincentive to using such equipment is also believed to be attributable to the difficulties of actual implementation, including disintegration of the tissue web, pressurized fluid leaks, seal and/or fabric wear, and the like. The air press disclosed herein overcomes these difficulties and provides a practical apparatus for dewatering a wet web to consistency levels not previously thought possible at industrially useful speeds without thermal dewatering.
Hence, in one embodiment, an air press for dewatering a wet web according to the present invention comprises: support fabrics adapted to sandwich the wet web therebetween and transport the wet web through the air press; a first dewatering device comprising a pair of cross-machine direction sealing members including sealing blades; a second dewatering device comprising a cross-matching direction sealing member formed of a deformable material, the first and second dewatering movable relative to one another and adapted to assume an operating position in which the first and second dewatering devices are operatively associated with one another and at least one sealing blade impinges upon the support fabrics and is opposed on the other side of the support fabrics by the sealing member formed of deformable material; and wherein one of the first and second dewatering devices comprises an air plenum operatively connected to a source of pressurized fluid and the other comprises a collection device operatively connected to a vacuum source.
In another embodiment, an air press for dewatering a wet web according to the present invention comprises: support fabrics adapted to sandwich the wet web therebetween and transport the wet web through the air press; an air plenum positioned on one side of the wet web and operatively connected to a source of pressurized fluid, the air plenum comprising a sealing assembly that is adapted to move between an operating position and a retracted position, the sealing assembly comprising a pair of machine direction sealing members and a pair of cross-machine direction sealing members that form an integral seal with the wet web when the sealing assembly is in the operating position; a collection device positioned on the opposite side of the wet web and operatively associated with the air plenum, the collection device defining therein a pair of sealing slots that extend across the width of the wet web and also defining therein a central passageway disposed between the sealing slots and adapted to receive pressurized fluid from the air plenum and water from the wet web, the collection device comprising deformable sealing members disposed within the sealing slots; means for moving the machine direction sealing members into and out of contact with one of the support fabrics, the machine direction sealing members positioned opposite and a forming a seal against the deformable sealing members when the sealing assembly is in the operating position, and means for moving the cross-machine direction sealing members into and out of contact with one of the support fabrics.
The air press able to dewater the wet web to very high consistencies due in large part to the high pressure differential established across the web and the resulting air flow through the web. In particular embodiments, for example, the air press can increase the consistency of the wet web by about 3 percent or greater, particularly about 5 percent or greater, such as from about 5 to about 20 percent, more particularly about 7 percent or greater, and more particularly still about 7 percent or greater, such as from about 7 to 20 percent. Thus, the consistency of the wet web upon exiting the air press may be about 25 percent or greater, about 26 percent or greater, about 27 percent or greater, about 28 percent or greater, about 29 percent or greater, and is desireably 30 percent or greater, particularly about 31 percent or greater, more particularly about 32 percent or greater, such as from about 32 to about 42 percent, more particularly about 33 percent or greater, even more particularly about 34 percent or greater, such as from about 34 to about 42 percent, and still more particularly about 35 percent or greater.
The air press is able to achieve these consistency levels while the machine is operating at industrially useful speeds. As used herein, “high-spee operation” or “industrially useful speed” for a tiss

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