Patent Application
20210062424
In the manufacture of high-bulk tissue products, such as facial tissue, bath tissue, paper towels, and the like, it is common to use one or more through-air dryers for partially drying the web or to bring the tissue web to a final dryness or near-final dryness. Through-air dryers typically include a rotating cylinder having an upper deck that supports a drying fabric which, in-turn, supports the web being dried. Heated air is passed through the web in order to dry the web. For example, in one embodiment, heated air is provided by a hood, which is generally retractable, above the drying cylinder. Alternatively, heated air is provided to a center area of the drying cylinder and passed through to the hood.
When incorporated into a papermaking system, through-air dryers offer many and various benefits and advantages. For example, through-air dryers are capable of drying tissue webs without compressing the web. Thus, moisture is removed from the webs without the webs losing a substantial amount of bulk or caliber. In fact, through-air dryers, in some applications, may even serve to increase the bulk of the web. Through-air dryers are also known to contribute to various other important properties and characteristics of the webs.
Commonly through-air dryers are equipped with a vacuum system that may be operated to maintain a neutral air pressure along the gap formed between the retractable hood and the drying cylinder. If this pressure becomes negative, cold machine room air may be drawn through the hood/drying cylinder gap and drying may be negatively affected. It is difficult, however, to maintain a neutral pressure over the entire width of the through-air dyer because the air permeability of the sheet changes as it dries. Thus, in practice, it is common to operate the system with a positive air pressure.
Operating with positive air pressure however, increases the amount of heated air that escapes through the hood/drying cylinder gap, which may represent a significant loss of energy and may cause heating of the machine room and make it uncomfortable to work near the dryer.
Several sealing means have been proposed to address the problem of heated air escaping from an enclosed dryer. For example, GB Publication No. 773,908 suggests installing skirts extending downwardly from a hood and beyond the rotating dryer. In other instances, the skirts may extend short of the dryer and be provided with a piece of flexible material, such as felt, which may contact the dryer to form a seal. Forming a seal between the dryer surface and the hood in this manner however, is complicated by thermal expansion of the components during startup of the dryer.
In other instances, to prevent heated air from exiting the hood/drying cylinder gap a thin Teflon strip may be installed on the edge of the retractable hood. This approach however, is also complicated by thermal expansion of both the drying cylinder and the hood, making it difficult to properly size the strip and seal the gap.
In still other instances, such as that described in U.S. Pat. No. 3,432,936, the hood is sealed at its side edges adjacent the surface of the cylinder by providing a raised portion or strip extending around the outside surface of the cylinder. A seal strip made of a resilient material, such as rubber or plastic, is bolted to the lower end of an outer sidewall of the hood and arranged to ride in sealing contact with the strip on the surface of the cylinder. The '936 patent further describes introduction of a second airstream into an outer portion of the hood to seal the hood and prevent leakage. This sealing arrangement is also complicated by thermal expansion of the hood and dryer and involves operating a second air system, which adds costs and complicates operation.
Thus, there remains a need in the art for a sealing mechanism that effectively seals the hood/drying cylinder gap, prevents the loss of heated air when the dryer is operated at positive air pressure and is effective over a range of dyer temperatures.
It has now been discovered that a through-air drying system comprising a hood and a rotatable cylinder with a porous cylindrical deck may be operated at positive air pressure by providing the system with a seal and more particularly a seal along the lateral edges of the system. The seal generally prevents the exhaust of drying medium from the system through the gap between the hood and the rotatable cylinder. The seal may be formed by providing the hood with an axial hood flange, which extends at least partially along the bottom edge of the hood and overlaps an axial annular flange disposed on the cylinder when the hood is in a closed and sealed position.
Accordingly, in one embodiment the present invention provides a through-air drying apparatus having longitudinal, transverse and axial directions comprising: a rotatable cylinder having a pair of spaced apart headers; an annular flange attached to each of the pair of spaced apart headers and extending axially therefrom; a hood having a pair of spaced apart sidewalls terminating at bottom sidewall edges, the hood movable between an open and closed position for interacting with and covering at least a portion of the rotatable cylinder; a hood flange having a substantially axially extending portion attached to the sidewall and a substantially longitudinally extending portion; wherein the annular flange and the hood flange cooperate with one another when the hood is in a closed position to form a substantially sealed enclosure containing the rotatable cylinder.
In another embodiment the present invention provides a through-air drying apparatus having longitudinal, transverse and axial directions comprising: a rotatable cylinder having a porous cylindrical deck and a pair of spaced apart headers; an annular flange attached to each of the pair of spaced apart headers and extending axially therefrom; a hood having a pair of spaced apart sidewalls terminating at bottom sidewall edges and being operable for passing drying air between an interior of the hood and an interior of the rotatable cylinder, the hood movable between an open and closed position for interacting with and covering at least a portion of the rotatable cylinder; a hood flange having a substantially axially extending portion attached to the sidewall and a substantially longitudinally extending portion; wherein the annular flange and the hood flange cooperate with one another when the hood is in a closed position to form a substantially sealed enclosure containing the rotatable cylinder.
In still another embodiment the present invention provides a system for enclosing a through-air dryer having a rotatable cylinder with a porous cylindrical deck and having at least one continuous drying fabric wrapped about a portion of the circumference thereof adapted to transport a wet paper web thereabout, the system comprising: an annular flange extending axially from the cylinder; a hood for covering the portion of the cylinder about which the fabric and the web are wrapped, the hood being operable for passing drying air between the interior of the hood and the interior of the cylinder; and a hood flange having an axially extending portion and a longitudinally extending portion, wherein the longitudinally extending portion of the hood flange overlaps at least a portion of the annular flange.
In yet other embodiments the present invention provides a method of through-air drying a tissue web comprising the steps of transferring a partially dewatered fibrous web to a through-air drying fabric, transporting the through-air drying fabric and partially dewatered fibrous web over a drying cylinder having an annular flange extending axially therefrom, providing a hood that engages with and at least partially encloses the drying cylinder, the hood having a longitudinally extending portion that overlaps the annular flange to form a seal therebetween, and pressurizing the hood with heated air. In certain instances, the seal formed by overlapping the longitudinally extending portion of the hood and the axially extending dryer flange enables the through-air drying system to be operated at a positive pressure, such as a pressure from about 3,000 to about 7,000 pascals.
The sealed through-air drying system of the present invention is well suited for drying of a wide variety of web materials, particularly fibrous webs and more particularly fibrous tissue webs that may be converted into tissue products such as such as facial tissue, bath tissue, paper towels, and the like. In certain instances, a partially dewatered web, such as a fibrous tissue web, is transferred to a through-air drying fabric and carried around a cylindrical deck of a pressurized through-air dryer system generally made in accordance with the present invention. The through-air dryer system generally includes a retractable hood and a drying cylinder. A drying medium such as a heated gas, and more particularly heated air, is introduced to the system and used to dry the web as it is transported along the drying cylinder.
In certain instances, a burner may be used to heat ambient air, which may then be forced by a fan into the hood. The hood, in-turn, directs the heated air through the web carried on the through-air drying fabric. The heated air is drawn through the web, drying the web, and through the surface of the drying cylinder. In certain embodiments at least a portion of the hot air is re-circulated back to the burner using a fan. In one embodiment, in order to avoid the build-up of moisture in the system, a portion of the spent heated air is vented, while a proportionate amount of fresh make-up air is fed to the burner.
While in certain embodiments heated air travels from the hood through the tissue web and then through the drying cylinder, the invention is not so limited. In other embodiments, the heated air may be fed through the drying cylinder and then forced into the hood. Regardless of the direction of air flow in the through-air dyer, heated air is passed through the tissue web as it is supported by a through-air drying fabric to dry the web.
The through-air drying system of the present invention is provided with a seal and more particularly a seal along the lateral edges of the system. The seal generally prevents the exhaust of drying medium from the system through the gap between the hood and the rotatable cylinder, which gap generally extends longitudinally along the lateral edge of the system. The seal may be formed by providing the hood with an axially extending hood flange, which extends at least partially along the bottom edge of the hood and overlaps an axially extending annular flange disposed on the cylinder when the hood is in a closed and sealed position.
In certain instances, the seal is configured to reduce the gap width, the transverse distance between the cylinder flange and the hood flange, and increase the gap length, the axial length between seal and the cylinder head. By reducing the gap width and increasing the gap length the area that heated air may exit the system may be reduced while also reducing the discharge coefficient and velocity of the exiting air. For example, the sealing mechanism may reduce the gap width to less than about 1.00 cm, such as less than about 0.50 cm, and more preferably less than about 0.125 cm, such as from about 0.125 to about 1.00 cm. In other instances, the sealing mechanism may increase the gap length to greater than about 4.0 cm, such as greater than about 5.0 cm. In this manner the seal may decrease the gap area and decrease the discharge coefficient of heated air to improve operation of the system under positive air pressures, such as pressures from about 3,000 to about 7,000 pascals.
With reference to
The drying cylinder may comprise a stationary support shaft, or journal, that is concentrically positioned with respect to the cylindrical deck. The shaft may extend from a first side or end of the drying cylinder to a second and opposite side. The cylindrical deck is intended to rotate about the shaft's central axis 125.
In certain instances, a pair of journals may extend axially outwardly from either end of the cylindrical dryer and be carried by a pair of journal bearings. In certain instances, the journal may extend beyond the bearing and carry a seal bearing on its outermost end. Generally, the journal, and any extending portion, are hollow and allow the passage of fluids from the interior of the cylindrical shell therethrough. A duct member may be attached to the seal bearing and connected to a vacuum source. In this manner, air passing through the web carried on the surface of the dryer is withdrawn from the interior of the deck through the passage in the journal and out through the duct.
With continued reference to
In other embodiments the drying cylinder may include various other internal components that assist in supporting the cylindrical deck. For instance, the drying cylinder may include a tube disposed over the hub, internal support members, or a deck support ring, that all rotate with the cylindrical deck. The internal support members may be attached to the rotating tube on one end and to the deck support ring on an opposite end. In this manner, the deck support ring may support the cylindrical deck at a mid region between each end of the cylindrical deck. The internal support members can be in the shape of plates and can assist in directing air flow through the dryer. The internal support members may be of a single piece construction or may be of a multi-piece construction as desired.
The cylindrical deck may be made from a single piece of welded steel having a honeycombed structure providing high structural strength while having a high amount of open area to permit air flow therethrough, or it may comprise a plurality of individual plates. In those embodiments where the deck comprises a plurality of plates, the plates may be connected to the deck support ring in a manner that allows thermal expansion. For instance, in one embodiment, each plate may include an indentation into which the deck support ring is received. In this manner, the plates may move relative to the deck support ring while remaining supported by the deck support ring.
With reference to
An annular flange 150, which forms a portion of the inventive sealing mechanism, is attached to the header 160 near the header outer peripheral edge 147. The annular flange 150 extends axially outwardly from the header 160 and forms a portion of the dryer first end 122. The annular flange 150 generally comprises an axially extending portion 151 and a longitudinally extending portion 152. The distance that the axially extending portion extends from the header may be adjusted to achieve the desired reduction in the discharge coefficient and velocity of heated air exiting the system.
In certain embodiments, such as that illustrated in
While annular flanges having a closed D-shape may be preferred, the invention is not so limited and one skilled in the art will appreciate that other cross-section shapes, including both closed loop and open, may be used. For example, in other embodiments the annular flange may comprise an open U-shaped structure.
Regardless of the shape of the annular flange it is generally preferred that it be attached to the header and more preferably attached near the outer peripheral edge of the header. One means of attaching the annular flange to the header is illustrated in detail in
With reference now to
The hood 210 may comprise a sidewall 220, preferably a unitary piece of longitudinally extending metal as illustrated in
The extent to which the hood covers the drying cylinder when in a closed, sealed position may vary. In certain instances, the hood only partially covers the cylinder when the hood is in a closed position. More particularly the arc length of the drying cylinder covered by the hood may range from about 30 to about 60 percent of the perimeter length of the cylinder, such as from about 45 to about 55 percent.
In certain embodiments the hood may contain interior partitions to control the flow and the distribution of the drying medium. Further, depending on the configuration used, the hood may be operably arranged to provide the drying medium to the drying cylinders or to exhaust the drying medium therefrom. Accordingly, where the hood supplies the drying medium, it is configured such that the drying medium is directed through the web and the fabric about the cylinders.
To prevent the exhaust of drying medium from the portion of the through-air dryer system where the hood covers the drying cylinder, an edge seal according to the present invention is provided. The edge seal 215 comprises a hood flange 230 attached to and extending outwardly from the hood sidewall 220 in an axial direction and the annular flange 150, which is fastened to the header 160. When the hood 210 is in a closed position the hood flange 230 and annular flange 150 cooperate with one another to seal the through-air drying system and prevent the drying medium from exiting along its lateral edges.
With reference now to
The hood flange 230 is generally attached to the outer surface 222 of the hood sidewall 220 near its low edge 224 (shown in detail in
In certain preferred embodiments the hood flange 230 is generally L-shaped and comprises an axially extending portion 231 attached to, and extending axially outward from, the hood sidewall 220. The hood flange 230 also comprises a longitudinally extending portion 233 that extends generally downwardly from the axial portion 231 and terminates at a flange bottom edge 235.
In the embodiment illustrated in
In certain instances, the length of the longitudinally extending portion of the hood flange is substantially uniform along its entire length. In other instances, the length of the longitudinally extending portion of the hood flange varies along its entire length. For example, the length of the longitudinally extending portion of the hood flange may be greater at a first end of the flange compared to near the midpoint of the flange. In certain instances, it may be necessary for both the first and second ends of the hood flange to have longer longitudinally extending portions, compared to a point near the middle of the flange, to account for deflection of the hood sidewall in operation and form an operable seal.
Regardless of the degree of overlap between the hood flange 230 and the annular flange 150, it is generally preferred that a seal 215 is formed out of plane with the dyer surface plane 255. For example, as illustrated in
Not only is it desirable to form the seal out-of-plane with the dryer surface 130, it is generally preferred that the seal 215 be formed outboard, positioned axially outward, of the header 160 to seal the gap 240 between the hood sidewall 220 and the dryer surface 130. In this manner, as illustrated in
In certain instances, a slight longitudinally orientated gap, such as less than about 1.00 cm and more preferably less than about 0.5 cm, such as from about 0.10 to about 1.00 cm, is formed along the overlapped portions of the hood flange 230 and the annular flange 150. Despite this longitudinally orientated gap, the system may be sealed as a result of the gap being moved axially outward and reduced in size relative to the gap between the hood sidewall and the deck of the drying cylinder.
In certain instances, to avoid contact between the hood flange 230 and the annular flange 150, a wear resistant material 256 may be attached inside the face 254 of the hood flange 230. Suitable wear resistant materials include, for example, non-metallic materials such as polyetheretherketone (PEEK), polyetherketone (PEK), polyetherketoneetherketoneketone (PEKEKK), and polyetheretherketone-ketone (PEEKK), and generally a polyaryletheretherketone. Further other polyketones can be used as well as other thermoplastics. Other suitable wear resistant materials include, for example, silicone rubber or Teflon.
In operation, the hood 210 is lowered over the cylindrical dryer 100 and the hood flange 230 overlaps the annular flange 150 along a portion of the dyer's circumference to form an edge seal 215. A drying medium is introduced to the hood 210 at a pressure in excess of atmospheric pressure and the cylindrical dryer 100 is rotated. The hood 210 and hood flange 230 remain in a fixed position, while the annular flange 150 rotates with the dryer 100. The edge seal 215 prevents the pressurized drying medium from exiting the dryer along its lateral edge.
Generally, a wet paper web, such as a tissue web, is transported on a through-air drying fabric, which may be a continuous belt of porous construction forming a loop around the cylindrical dryer. The drying cylinder and fabric may be arranged such that the fabric is wrapped about a major portion of the circumference of the dryer. In addition to the cylindrical dryer, the through-air drying fabric may loop one or more fabric supports that may include, for example, a vacuum box, a rotatable roll, or the like. As the wet paper web is transported across the drying cylinder by the through-air drying fabric, the drying medium, which may be a heated gas such as air, or a hot vapor such as steam, passes through the web using applied differential pressure. Water is then removed from the web by the drying medium principally by the mechanism of forced convection.
In view of the foregoing description, it will be apparent to one of ordinary skill in the art that the following embodiments are within the scope of the present invention:
In a first embodiment the invention provides a through-air drying apparatus having longitudinal, transverse and axial directions comprising a rotatable cylinder having a porous cylindrical deck and a pair of spaced apart headers; an annular flange attached to each of the pair of spaced apart headers and extending axially therefrom; a hood having a pair of spaced apart sidewalls terminating at bottom sidewall edges and being operable for passing drying air between an interior of the hood and an interior of the cylinder, the hood movable between an open and closed position for interacting with and covering at least a portion of the cylinder; a hood flange having a substantially axially extending portion attached to the sidewall and a substantially longitudinally extending portion; wherein the annular flange and the hood flange cooperate with one another when the hood is in a closed position to form a seal.
In a second embodiment the invention provides the invention of the first embodiment wherein the longitudinally extending portion of the hood flange extends beyond the sidewall bottom edge.
In a third embodiment the invention provides the invention of either of the first or the second embodiments wherein the annular flange has a bottom edge portion lying in a first transverse plane and the vertical portion of the hood flange has a bottom edge lying in a second transverse plane when the hood is in a closed position and wherein the first and second transverse planes lie substantially in the same plane.
In a fourth embodiment the invention provides the invention of any one of the first through third embodiments wherein the annular flange has a bottom edge portion lying in a first transverse plane and the vertical portion of the hood flange has a bottom edge lying in a second transverse plane when the hood is in a closed position and wherein the first transverse plane lies above the second transverse plane.
In a fifth embodiment the invention provides the invention any one of the first through fourth embodiments wherein the annular flange has a closed D-shape and the hood flange has an open L-shape.
In a sixth embodiment the invention provides the invention of any one of the first through fifth embodiments further comprising a wear resistant nonmetallic material disposed on the vertical portion of the hood flange. In certain embodiments the non-metallic material may be selected from the group consisting of polyetheretherketone (PEEK), polyetherketone (PEK), polyetherketoneetherketoneketone (PEKEKK), and polyetheretherketoneketone (PEEKK), and generally a polyaryletheretherketone. Further other polyketones can be used as well as other thermoplastics. Other suitable wear resistant materials include, for example, silicone rubber and Teflon
In a seventh embodiment the invention provides the invention of any one of the first through sixth embodiments wherein the hood only partially covers the cylinder when the hood is in a closed position.
In an eighth embodiment the invention provides the invention of any one of the first through seventh embodiments wherein the arc length of the cylinder covered by the hood ranges from about 30 to about 60 percent of the perimeter length of the cylinder.
In a ninth embodiment the invention provides the invention of any one of the first through eighth embodiments wherein the length of the hood flange is equal to the arc length of the cylinder covered by the hood.
In a tenth embodiment the invention provides the invention of any one of the first through ninth embodiments wherein the length of the longitudinally extending portion of the hood flange is substantially uniform along the entire length of the hood flange.
In an eleventh embodiment the invention provides the invention of any one of the first through tenth embodiments wherein the length of the longitudinally extending portion of the hood flange varies along the length of the hood flange.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US19/48685 | 8/29/2019 | WO | 00 |
