BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a paper machine, and, more particularly, to a dewatering fabric used in a paper machine for the manufacturer of a fibrous web.
2. Description of the Related Art
A fabric is utilized to carry the fiber web during the formation of the web. After the web takes form it is usually subjected to a drying process. The same fabric used during formation of the web or another fabric may come in contact with the web, to move the web across a vacuum section for the remove of moisture from the web. The fabric may additionally absorb moisture from the web and the moisture so absorbed is subsequently removed from the fabric at a later point in the process.
Dewatering fabrics used in the paper industry are usually referred to as press fabrics. Press fabrics are characterized by a structure that carries a lot of water. Press fabrics are normally used to transport the wet web after forming through at least one pressing point to dewater the web. This normally occurs in two ways. First, a negative pressure is applied to the press fabric to remove excess water prior to pressing. During this process, the web passes some water into the press fabric, thus increasing the solid content of the web prior to pressing. Secondly, pressing removes water through mechanical compaction. This step can be detrimental to the properties of the final dried web, specifically, the caliper and absorbency.
The method disclosed in U.S. Pat. No. 5,598,643, entitled, “Capillary Dewatering Method and Apparatus” discloses a way of dewatering the web without compaction. Additionally, U.S. Pat. No. 5,437,107, entitled, “Limited Orifice Drying” discloses a limited flow membrane. However, these disclosed dewatering methods are very different from Applicants' invention.
- SUMMARY OF THE INVENTION
What is needed in the art is a fabric, which provides dewatering of a fiber web, is less complicated and less costly to install.
The present invention provides a dewatering fabric for use in a papermaking machine.
The invention comprises, in one form thereof, a dewatering fabric for use in a paper machine, the dewatering fabric including a woven permeable fabric and a polymeric layer having openings therethrough, the polymeric layer connected to the permeable fabric.
BRIEF DESCRIPTION OF THE DRAWINGS
An advantage of the present invention is that it allows airflow therethrough for the removal of water from a fibrous web.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a cross-sectional schematic diagram of an Advanced Dewatering System using at least one of the embodiments of the dewatering fabric of the present invention;
FIG. 2 is a cross-sectional schematic view of an embodiment of a dewatering fabric used in the apparatus of FIG. 1;
FIG. 3 is a perspective view directed toward a roll side of yet another embodiment of a dewatering fabric used in the apparatus of FIG. 1;
FIG. 4 is a sectioned perspective view directed toward a roll side of yet another embodiment of a dewatering fabric used in the apparatus of FIG. 1; and
FIG. 5 is a sectioned perspective view directed toward a roll side of still yet another embodiment of a dewatering fabric used in the apparatus of FIG. 1.
- DETAILED DESCRIPTION OF THE INVENTION
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
Referring now to the drawings, and more particularly to FIG. 1, there is shown an Advanced Dewatering System 10, which is a fluid displacement apparatus, for the processing of fibrous web 12. System 10 includes a fabric 14, a suction box 16, a vacuum roll 18, a dewatering fabric 20, a belt press assembly 22, a belt 24, a pick-up suction box 26, showers 30 and save alls 32. Fibrous web 12 is formed from a fibrous slurry and is then transported along fabric 14, which is a structured fabric 14, past suction box 16 towards vacuum roll 18. At suction box 16, sufficient moisture is removed from web 12 to achieve a solids level of between 15% and 25% on a typical 20 g/m2 (gsm) web suction box 16 runs at −0.2 to −0.8 bar vacuum, with a preferred operating level of −0.4 to −0.6 bar.
As fibrous web 12 proceeds in machine direction M it comes into contact with dewatering fabric 20. Web 12 then proceeds toward vacuum roll 18 between structured fabric 14 and dewatering fabric 20. Vacuum roll 18 is operated at a vacuum level of −0.2 to −0.8 bar with a preferred operating level of at least −0.4 bar. Dewatering fabric 20, web 12 and fabric 14 are pressed against vacuum roll 18 by belt press assembly 22. A vacuum present in vacuum zone Z pulls a drying fluid, such as air, through permeable belt 24, then through fabric 14, then through web 12 and then through dewatering fabric 20. Moisture collected in vacuum roll 18 is removed through a vacuum pump (not shown) and some is discharged to save alls 32.
Fabric 14 is a structured fabric having a three dimensional structure that is reflected in web 12. part of web 12 is embedded into structured fabric 14, these areas are usually referred to as ‘pillows’. The pillows are protected during pressing as they are within the body of structured fabric 14. As such the pressing imparted by belt press assembly 22 upon web 12 does not negatively impact web quality, while it increases the dewatering rate of vacuum roll 18. In a No Press/Low Press papermaking machine the pressure is transmitted through dewatering fabric 20, also known as a press fabric. In such a case web 12 is not protected inside structured fabric 14. This is still advantageous, because the press nip is much longer than a conventional press, which results in a lower specific pressure and less compaction of web 12.
In the No Press/Low Press TissueFlex process dewatering fabric 20 replaces the standard press fabric, often felt, in a Crescent Former. The improvement in this type of replacement raises web solids to a level that is high enough to either eliminate or greatly reduce the need for Yankee pressing, thereby increasing the quality of web 12.
Belt press assembly 22 fits over vacuum roll 18 and it has a single fabric 24 capable of applying pressure to the non-sheet contacting side of structured fabric 14 that carries web 12 around vacuum roll 18. A hot air hood may be fit over vacuum roll 18 inside belt press assembly 22 to improve dewatering. The circumferential length of vacuum zone Z can be from 200 mm to 2,500 mm, with a preferred range of from 300 mm to 1,200 mm and a more preferred range of 400 mm to 800 mm. The solids content of web 12 leaving suction zone Z is 25% to 55%, preferably greater than 30%, more preferably greater than 35% and even more preferably greater than 40%, depending on installed options.
FIG. 2 is a side illustration of a preferred embodiment of the present invention, included is a woven single layer base fabric 50. Base fabric 50 includes machine direction yarns 54 and cross direction yarns 56. Yarn 54 is a 3 ply multifilament twisted yarn. Yarn 56 is a monofilament yarn. Yarn 54 can also be a monofilament yarn and the construction can be of a typical multilayer design. In either case, base fabric 50 is needled with fine batt fiber 58 having a weight of less than or equal to 700 gsm, preferably less than or equal to 150 gsm and more preferably less than or equal to 135 gsm. The batt fiber encapsulated the base structure giving it sufficient stability. The needling process can be such that straight through channels are created. The sheet contacting surface is heated to improve its surface smoothness.
Now, additionally referring to FIG. 3 there is illustrated another embodiment of dewatering fabric 20. In this embodiment, base fabric 50 has attached thereto a lattice grid 74 made of a polymer, such as polyurethane, that is put on top of base fabric 50. The side of dewatering fabric 20 that runs against the vacuum roll is illustrated in FIG. 3. The opposite side of dewatering fabric 20 (not shown), which is an opposite side of base fabric 50, is the side that contacts web 12. Grid 74 may be put on base fabric 50 by utilizing various known procedures, such as, for example, an extrusion technique or a screen-printing technique. As shown in FIG. 3, lattice 74 is put on base fabric 50 with an angular orientation relative to machine direction yarns 54 and cross direction yarns 56. Although this orientation is such that no part of lattice 74 is aligned with machine direction yarns 54 as shown in FIG. 3, other orientations such as that shown in FIG. 4 can also be utilized. Although lattice 74 is shown as a rather uniform grid pattern, this pattern can actually be discontinuous in part. Further, the material between the interconnections of the lattice structure may take a circuitous path rather than being substantially straight, as that shown in FIG. 3. Lattice grid 74 is made of a synthetic, such as a polymer or specifically a polyurethane, which attaches itself to base fabric 50 by its natural adhesion properties.
Lattice grid 74 being a polyurethane has good frictional properties, such that it seats well against the vacuum roll. This then forces vertical airflow and eliminates any x, y plane leakage. The velocity of the air is sufficient to prevent any rewetting once the water makes it through lattice 74
Additionally, grid 74 may be a thin perforated hydrophobic film 74 having an air permeability of 35 cfm or less, preferably 25 cfm or less having pores therein of approximately 15 microns. Here too we have vertical airflow at high velocity to prevent rewet.
Now, additionally referring to FIG. 4, which illustrates the vacuum roll contacting side of dewatering fabric 20. This is yet another embodiment of dewatering fabric 20 that includes permeable base fabric 50 having machine direction multifilament yarns 54 and cross-direction monofilament yams 56, that are adhered to grid 76, also known as an anti-rewet layer 76. Grid 76 is made of a composite material, which may be an elastomeric material the may be the same as that used in lattice grid 74. Grid 76 includes machine direction yarns 78 and a composite material 80 formed therearound. Grid 76 is a composite structure formed of elastomeric material 80, and machine direction yarn 78. Machine direction yarn 78 may be pre-coated with elastomeric material 80 before being placed in rows that are substantially parallel in a mold that is used to reheat elastomeric material 80 causing it to re-flow into the pattern shown as grid 76 in FIG. 4. Additional elastomeric material 80 may be put into the mold as well. Grid structure 76, also known as composite layer 76, is then connected to base fabric 50 by one of many techniques including laminating grid 76 to permeable fabric 50, melting elastomeric coated yarn 78 as it is held in position against permeable fabric 50 or by re-melting grid 76 onto base fabric 50. Additionally, an adhesive may be utilized to attach grid 76 to permeable fabric 50. Composite layer 76 seals well against the vacuum roll preventing x, y plane leakage and allowing vertical airflow to prevent rewet.
Now, additionally referring to FIG. 5, which illustrates the roll side of dewatering fabric 20. This structure includes the elements that are shown in FIG. 4 with the addition of batt fiber 82. Batt fiber 82 is needled into the structure shown in FIG. 4 to mechanically bind the two layers together, thereby forming a dewatering fabric 20 having a smooth needled batt fiber surface. Batt material 82 is porous by its nature, additionally the needling process not only connects the layers together, it also creates numerous small porous cavities extending into or completely through the structure of dewatering fabric 20.
Dewatering fabric 20 has an air permeability of from 5 to 100 cubic feet/minute preferably 19 cubic feet/minute or higher and more preferably 35 cubic feet/minute or higher. Mean pore diameters, as measured using a Coulter method, are from 5 to 75 microns, preferably 25 microns or higher and more preferably 35 microns or higher. Either surface of dewatering fabric 20 can be treated with a material to bake it hydrophobic. Lattice composite layer 76 may be made of a synthetic polymeric material or a polyamide that is laminated to fabric 50.
Batt fiber can range from 0.5 d-tex to 22 d-tex and may contain an adhesive to supplement fiber to fiber bonding. The bonding may result from the use of, for example, a low temperature meltable fiber, particles and/or resin. Dewatering fabric 20 is a very thin fabric, which reduces the amount of water carried therein. This improves the dewatering efficiency and reduces or eliminates a rewetting phenomena seen with prior art structures. The total thickness of dewatering fabric 20 is preferably less than 1.50 millimeters, and more preferably less than 1.25 millimeters and even more preferably less than 1.0 millimeter thick.
Machine direction yarns 54, shown in FIGS. 3, 4 and 5, also known as weft yarns 54 in an endless weaving process, are made of a multi-filament yam, normally twisted/plied or can be a solid monolithic strand usually of less than 0.40 millimeter diameter, with a preferable diameter of 0.20 millimeter or as low as 0.10 millimeter. Cross direction yarns 56, shown in FIGS. 3, 4 and 5, also known as warp yarns 56 when woven in an endless weaving process are made of a monofilament yarn, of a diameter greater than or equal to 0.2 mm, preferably 0.38 mm. The multifilament yarns are formed in a single strand, twisted cabled or joined side by side to form a flat shaped fabric 50. Woven permeable fabric 50 may have straight through channels needled through fabric 50, thereby causing a straight through drainage channel through dewatering fabric 20. Additionally, a hydrophobic layer may be applied to at least one surface.
As to the uses of dewatering fabric 20 in dewatering system 10, pressure is applied by belt press 22 against web 12 as a mechanical force that creates a hydraulic pressure in the moisture contained in web 12. The squeezing action is coupled with a vacuum at zone Z of vacuum roll 18, to drive moisture from web 12 and through dewatering permeable membrane 20. Advantageously, moisture is removed through the combination of the pressure applied by the extended nip press contact of belt 24 and the introduction of air through belt 24 and fabrics 14 and 20 enhance the dewatering capability of the present invention.
Dewatering fabric 20 has an air permeability of less than 130 cfm, preferably less than 100 cfm, more preferably less than 65 cfm and even more preferably less than 35 cfm.
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.