Patent Application
20230295871
Generally speaking, paper products are formed by depositing an aqueous slurry of papermaking fibers onto a forming section of a papermaking machine, to form a wet, slushy, paper web, and then dewatering the wet web, including through a press section where water is forced from the web by hydraulic forces imposed by mechanical pressure or suction, before the wet paper web reaches a dryer section where final removal of water is accomplished by heat-induced evaporation, to form a “dried” paper product. A wide variety of papermaking machines and methods are used to form the nascent, wet paper web, and to subsequently dewater, and dry, the web. In papermaking processes, there are many ways to remove water, each with substantial variability. As a result, the great variety of known paper products have a great variety of properties. Commonly employed methods of dewatering the nascent paper web ahead of an evaporative-contact dryer section include mechanical pressing of the web in one or more heavily-loaded nips where the web is essentially crushed to mechanically drive water out of the web, passing the web through a vacuum/suction chamber, and blowing hot air on the web in a through-air dryer.
The typical use of mechanical pressure generally translates to a hydraulic pressure being exerted on the water contained in the web, whereby water is removed by hydraulic pressure gradients. A limitation of use of mechanical pressure in dewatering the web is that the thickness of the web, after exiting the press section, is typically reduced as a result of such pressing, compared to thickness of the web entering the press section.
The amount of water which can be removed by pressing is limited by the degree to which the thickness of the web can be reduced in the press nip, and wherein the remaining interstices in the web, and namely the web, itself, are fully saturated with water at the point of maximum compression. As the web exits the nip, the web experiences a rebound increased thickness as the pressure is released, whereupon the web is no longer saturated with water. During such rebound, a slight negative pressure, vacuum exists in the expanding web, which draws some of the water which has been removed from the web, back into the web. Such phenomenon is commonly known as “rewet” of the nascent paper web as the web exits the press section of the papermaking machine. Accordingly, the fraction of the water which can be removed by mechanical pressing has a practical limit, generally related to the intensity of the mechanical pressing and the rebound in thickness of the web during the time when the mechanical pressure on the web is being released.
As an attempt to remove additional water from the web before the web reaches the evaporative dryer, water can be removed, either in combination or in the alternative, by subjecting the web to suction/vacuum at one or more vacuum boxes, which can be separate from, or co-located in, the mechanical press section. If done in the press section, a vent roll can include vacuum/suction apparatus which applies vacuum/suction to the web simultaneously with the application of mechanical pressure at the press section. If done separately, the vacuum/suction section is typically downstream from the mechanical pressing section.
While suction/vacuum can increase the amount of water removed from the web by subjecting the water in the web to an air pressure differential, the magnitude of the air pressure differential is limited by e.g. currently existing atmospheric pressure of e.g. 14.7 pounds per square inch (psi).
Typically, the degree/amount of water removed from such fibrous webs is expressed as the solids content of the web after the web has passed through the water removal/press section of the papermaking machine. Typically, higher solids content, and relatively greater web thickness, are desired, in order to limit the amount of energy needed to evaporate some of the remaining water, in the dryer section of the papermaking machine.
The finished/dried paper product typically has a residual water content less than 10 percent by weight, typically in the range of about 5 percent by weight. Whatever amount of water is left in the web after passing through the dewatering/press section of the papermaking machine, namely water that needs to be removed to achieve the finished paper product, is removed in the dryer section of the papermaking machine, by evaporating the water from the web. Evaporating water from the web is the most energy demanding method of removing water from the web. Accordingly, the greater the amount of water that needs to be removed in the dryer section, the greater the energy demand on the dryer section, namely the greater the energy demand on the papermaking process. By corollary, the greater the solids content as the web enters the dryer section, the less the amount of water which needs to be removed in the dryer section, and the lower the energy demand on the dryer section, thus the lower the overall energy demand on the papermaking process. It is an object of this invention to increase the amount of water which is removed from the nascent paper web before the web reaches the dryer section of the papermaking machine.
Accordingly, there remains a need, in the papermaking industry, for additional improvements in apparatus for dewatering nascent paper webs formed in a papermaking machine.
There is also a need for improved methods for dewatering nascent paper webs in the papermaking process.
There is further a need for methods and apparatus for limiting the amount of energy required for manufacturing paper products.
There is still further a need for papermaking methods which provide paper webs having increased bulk, which typically is accompanied by increased water absorbency.
There is also a need for increasing solids content of a paper web before the web reaches the dryer section, in order to reduce the energy demand on the dryer section.
There is also a need for apparatus and methods which facilitate making multiple grades of paper on a given papermaking machine by making adjustments in the water removal section of the papermaking machine, while varying desired mechanical load intensity on the fibrous web in the press section.
There is also a need for apparatus and methods for making multiple grades of paper on a given papermaking machine by making adjustments in the water removal section of the papermaking machine, while varying the pressure intensity exerted on the fibrous web in the water removal section.
There is also a need for novel papermaking fabrics which facilitate removal of water ahead of the dryer section of a papermaking machine.
These and other objects and needs are alleviated, or at least attenuated, by the novel apparatus, methods, and products of the invention.
This invention relates to improvements in removal of water from a nascent paper web, sometimes known as a foraminous web at a water removal section of a papermaking machine. More particularly, this invention relates to improved efficiencies in removing water from such nascent fibrous web. Further, the invention relates to reducing water content of the nascent paper web at the water removal section, before the web reaches the dryer section. Yet further, the invention relates to increasing solids content of a wet paper web at the water removal section of the papermaking machine and/or providing a resulting paper product which can have a relatively greater resulting bulk/thickness, softness, and or greater dryness as a result of the web having traversed the water removal section of the invention. As seen following, in the invention, the producer of paper products has increased latitude in determining the intensity of any mechanical pressing to be used in removing water from the fibrous web before the web reaches the dryer section of the papermaking machine whereby referring to such portion of the papermaking machine as a “press” section, where displacement dewatering of the invention is used, can be misleading; and “water removal section” is a more representative expression for such apparatus.
Accordingly, the invention discloses novel apparatus, methods, and fabrics for removing water from a nascent paper web, carried in a web sandwich, in a water removal section of a papermaking machine. In the invention, water is removed from the nascent paper web by a combination of mechanical pressure and compressed air. The mechanical pressure and air pressure can be applied in the same nip, or in separate nips where the mechanical pressure is applied upstream, in the papermaking machine, from the air pressure application. The mechanical pressure can be lower than pressures commonly used in conventional wet press sections of known papermaking machines/processes. The intensity of the air pressure is no more than the pressure which can be contained in a seal section between the web sandwich and the structure supplying the compressed air. Fabrics supporting the nascent paper web through an air dewatering station of the invention have limited lateral air flow under compressed air conditions, whereby air flows generally perpendicularly through the web being dewatered.
Conventional dewatering of nascent paper webs includes passing the web through a nip press and/or across a vacuum box/roll, or drawing a vacuum through the web while the web is in such nip press, followed by applying thermal energy in the form of a hot drum or passing heated air through the web, or otherwise applying thermal energy to the web, to complete the drying of the web by evaporation of additional water from the web.
In the invention, after the web has been formed, one or more carrier fabrics are joined with the nascent paper web, thereby defining a web sandwich, and the web sandwich is passed through a dewatering station defined by a pressure apparatus and a pressure receptive vent apparatus. At some point, mechanical pressure is applied to the web sandwich to mechanically/hydraulically remove a first portion of water from the web sandwich. In addition, pressurized gas, such as compressed air, is passed into and through the nascent paper web optionally after the web exits an earlier mechanical pressing step. A fabric/layer between the nascent paper web and the pressure receptive vent apparatus, as part of the web sandwich, receives pressurized gas, as well as water which has been removed from the nascent paper web, and impedes return of a rewet portion of such water to the web as the web passes through, and out of, the air removal station.
In a first family of embodiments, the invention comprehends a method for removing water from a nascent paper web in a papermaking machine in the process of fabricating a finished paper product, the nascent paper web comprising papermaking fibers and water, and having first and second opposing web surfaces which extend between first and second opposing edges of such nascent paper web, the nascent paper web having a generally continuous length, and a width, a first carrier fabric being disposed on the first surface of the nascent paper web and a second carrier fabric being disposed on the second opposing surface of the nascent paper web, the first and second carrier fabrics and the nascent paper web collectively defining a web sandwich, the web sandwich having first and second opposing sandwich surfaces, and a web machine direction corresponding with a machine direction of the papermaking machine, and a web cross machine direction, the web sandwich moving in the machine direction of the papermaking machine, the method comprising applying mechanical pressure to the web sandwich and thereby driving water from the nascent paper web; providing a gas pressure source and a gas receptive vent, the gas pressure source having a length extending along the width of the nascent paper web; and providing a generally constant compressed gaseous pressure along the entire cross direction length of the gas pressure source, the gas pressure source thereby conveying compressed gas to the web sandwich and thereby developing a seal zone at the web sandwich.
In some embodiments, the method comprises a gas pressure chamber in gaseous communication with the gas pressure source providing the compressed gaseous pressure, the gas pressure chamber being adapted and configured to constantly provide a generally constant compressed gaseous pressure along the entire length of the gas pressure source.
In some embodiments, the gas pressure chamber comprising a pressure box disposed inside the gas pressure source.
In some embodiments, the gas pressure chamber comprises an enclosure having an edge extending along the length of the gas pressure source, the enclosure being open to a surface of the gas pressure source, further comprising a seal extending about the enclosure along the length of the gas pressure source and between the enclosure edge and the respective surface of the gas pressure source, such seal inhibiting leakage of gas between the gas pressure chamber and the gas pressure source, between the first and second edges of the nascent paper web.
In some embodiments, the gas pressure source comprises a pressure roll having an outer shell, the gas pressure chamber being disposed inside the pressure roll, the seal being disposed at an inner surface of the outer shell.
In some embodiments, the method comprises providing vacuum at the gas receptive vent to assist in receiving gas and water from the web sandwich, and conveying such received gas and water away from the web sandwich.
In some embodiments, the gas pressure source and the gas receptive vent define a nip, the nip comprising first and second loading devices which apply mechanical pressure to the web sandwich in the nip, the method further comprising providing a pressure device adapted and configured to apply the compressed gaseous pressure in the nip through one of the first and second loading devices.
In some embodiments, the method comprises applying the mechanical pressure to the nascent paper web, through the first and second carrier fabrics, at a first dewatering station in the papermaking machine, and applying the compressed gas to the nascent paper web, through the first and second carrier fabrics, in a second dewatering station, separate and distinct from the first dewatering station and downstream in the papermaking machine from the first dewatering station.
In some embodiments, the method comprises, after applying the mechanical pressure to the nascent paper web at the first dewatering station, and before applying compressed gas to the nascent paper web at the second dewatering station, replacing at least one of the first and second carrier fabrics in the web sandwich with a third carrier fabric, different from the respective first or second carrier fabric.
In some embodiments, the method comprises, after applying the mechanical pressure to the nascent paper web at the first dewatering station, and before applying the compressed gas to the nascent paper web at the second dewatering station, replacing the first carrier fabric in the web sandwich with a third carrier fabric and optionally replacing the second carrier fabric in the web sandwich with a fourth carrier fabric.
In some embodiments, the method includes providing, as the first loading device, a roll comprising a shell, the shell having a length and extending about a circumference of the roll, apertures being arrayed about the circumference, and along the length of the shell, and extending through the shell, the roll being equipped with a pressure box supplying compressed gas, through the apertures and to the web sandwich.
In some embodiments, the method includes providing, as the first loading device, a vented sleeve or a vented belt.
In some embodiments, the method includes providing, as the second loading device, a loading device selected from the group consisting of a vented roll, a shoe press, a vented fabric, or a suction roll.
In some embodiments, the method includes providing the mechanical pressure at a first nip, at an average nip pressure of up to about 400 pounds per square inch, optionally up to about 600 pounds per square inch, optionally up to about 800 pounds per square inch, optionally up to about 1000 pounds per square inch of nip pressure provided by mechanical force between the first and second loading devices at the first dewatering station.
In some embodiments, the gas pressure source and the gas pressure receptive vent define a first nip, the method including providing, at the second dewatering station, a second nip providing an average mechanical loading of the web sandwich at a nominal amount of pressure and optionally an average pressure of up to about 2 pounds per square inch, optionally an average pressure of up to about 5 pounds per square inch, optionally an average pressure of up to about 100 pounds per square inch, optionally an average pressure of up to less than about 300 pounds per square inch, optionally an average pressure of up to 500 pounds per square inch, optionally an average pressure of up to less than about 800 pounds per square inch, and optionally a seal, optionally a self-loading seal, optionally a self-loading ganged seal, in the nip.
In some embodiments, the method includes providing, at the second dewatering station, a gas pressure shoe providing the compressed gas to the web sandwich.
In some embodiments, the method includes providing, at the second dewatering station, a porous sleeve or belt at a surface of the web sandwich opposite the pressure shoe, the porous sleeve or belt receiving and venting water and gas passing through the web sandwich.
In some embodiments, the first carrier fabric comprises a layer having at least one property selected from the group consisting of (i) a basis weight of up to about 1200 grams per square meter, (ii) no more than about 40 percent void space, (iii) a moisture ratio of no more than 0.4, optionally no more than 0.25, optionally no more than 0.15, all prior to applying the compressed gaseous pressure to the web sandwich.
In some embodiments, the method optionally includes providing the first carrier fabric on the web sandwich surface to which the gas is applied, an amount of water removed from the first carrier fabric, as the compressed gaseous pressure is applied to the web sandwich, being less than two times the amount of water removed from the nascent paper web.
In some embodiments, the method includes providing the second carrier fabric on the web sandwich surface which is away from the compressed gaseous pressure being applied to the web sandwich, the second carrier fabric being adapted and configured to readily convey flow of water, which is pushed from the nascent paper web by the compressed gas, and to inhibit movement of water back into the nascent paper web under relatively lower ambient or otherwise reduced gaseous pressure.
In some embodiments, the method includes providing at least one of the first and second carrier fabrics comprising a perforated membrane.
In some embodiments, the method includes providing a respective carrier fabric wherein the perforated membrane is a first layer or sub-layer, and the respective carrier fabric further comprises a second layer or sub-layer.
In some embodiments, the perforated membrane, along with associated sub-layers, facilitates development of relative uniformity of gas flow through the nascent paper web in the seal zone.
In some embodiments, the method includes providing, in the first carrier fabric, on the web sandwich surface to which the compressed gaseous pressure is applied, a layer or sub-layer having low voids, optionally compacted or filled, optionally thermoplastic yarns, optionally mono mesh fabrics and/or monofilament fabrics, such layer or sub-layer optionally having less than 40 percent void volume.
In some embodiments, the method includes providing a multifilament or batt structure or other structure in contact with the nascent paper web and thereby limiting in-plane gas leakage in the nascent paper web.
In some embodiments, the method includes providing the first carrier fabric on the web sandwich surface to which the compressed gas is being applied, the first carrier fabric inhibiting web machine direction flow of such compressed gas.
In some embodiments, the gas pressure source and the gas pressure receptive vent define a nip, the first carrier fabric being disposed on the web sandwich surface to which the compressed gaseous pressure is being applied, the nip comprising first and second loading devices which apply sufficient mechanical pressure to the web sandwich to cause a reduction in thickness of the web sandwich, to a minimum thickness, in the nip, as the compressed gas is being passed through the web sandwich, the web sandwich rebounding from the minimum thickness as the web sandwich exits the nip, the method comprising continuing to provide compressed gaseous pressure to at least the first carrier fabric and the nascent paper web after the web sandwich exits the nip.
In some embodiments, the method comprises providing a third carrier fabric as part of the web sandwich, the third carrier fabric being disposed on a surface of the second carrier fabric remote from the nascent paper web and remote from the compressed gaseous pressure source, the method comprising separating the third carrier fabric from the web sandwich when the web sandwich exits the nip while continuing to optionally provide compressed gaseous pressure to the first carrier fabric, the nascent paper web, and the second carrier fabric after the first carrier fabric, the second carrier fabric, and the nascent paper web have exited the nip.
In some embodiments, the third carrier fabric expands as the web sandwich moves toward a nip exit, and thereby draws water from the web sandwich and into expanding voids in the third carrier fabric.
In some embodiments, the method includes providing the first carrier fabric on the web sandwich surface to which the compressed gaseous pressure is being applied, the first carrier fabric being designed and configured to selectively inhibit lateral web machine direction flow of water in the first carrier fabric.
In some embodiments, the method includes providing the second carrier fabric on the web sandwich surface remote from the surface to which the compressed gaseous pressure is being applied, the second carrier fabric having a greater affinity for water removed from the nascent paper web than a web affinity for water extant in the nascent paper web.
In some embodiments, the method includes applying and adjusting intensity of compressed gaseous pressure, and optionally warmed, being applied to the nascent paper web, independent of intensity of any mechanical pressure being applied to the nascent paper web.
In some embodiments, the method comprises applying and adjusting intensity of compressed gaseous pressure, and optionally warmed, being applied to the nascent paper web, independent of intensity of any mechanical pressure being applied to the nascent fibrous web.
In some embodiments, the method comprises specifying compressed gaseous pressure and mechanical pressure according to properties to be provided in the finished paper product.
In some embodiments, the method further comprises extending a seal from a body of the pressure device to the web sandwich, the seal extending about the seal zone, the seal limiting lateral gas leakage in the web machine direction and the web cross machine direction from the seal zone.
In some embodiments, the method comprises providing the compressed gas at pressures, measured at the gas pressure chamber, of about 5 pounds per square inch to about 125 pounds per square inch, optionally about 5 pounds per square inch to about 75 pounds per square inch, optionally about 10 pounds per square inch to about 60 pounds per square inch.
In some embodiments, the method includes a gas pressure chamber in gaseous communication with the gas pressure source providing the compressed gaseous pressure, the gas pressure chamber comprising a first gas pressure chamber providing a first compressed gaseous pressure of a first magnitude to the web sandwich, the method further comprising providing a second gas pressure chamber downstream in the web machine direction from the first gas pressure chamber, and proximate the first gas pressure chamber, the second gas pressure chamber being in gaseous communication with the gas pressure source, the second gas pressure chamber providing a generally constant compressed gaseous pressure of a second magnitude, optionally less than the first magnitude of the compressed gaseous pressure provided by the first gas pressure chamber, the gas pressure source conveying the compressed gaseous pressure of the second gas pressure chamber to the web sandwich.
In some embodiments, the gas pressure source conveys the compressed gaseous pressure of the second gas pressure chamber to the web sandwich along the entire length of the gas pressure source.
In some embodiments, at least one of the first and second carrier fabrics having sufficiently low permeability to the pressure of the compressed gas that the respective carrier fabric provides functional mechanical loading to the web sandwich, thereby aiding in release of water from the nascent paper web.
In some embodiments, the method further comprises controlling and adjusting mechanical pressure applied to the web sandwich and air pressure, optionally air temperature, applied to the web sandwich and thereby adjusting and controlling properties of a paper product produced from the nascent paper web.
In some embodiments, the gas pressure source comprises a pressure roll having an outer shell, further comprising an air permeable layer disposed on an outer surface of the outer shell, the air permeable layer being in direct contact with the nascent paper web, the method further comprising diffusing compressed air through the air permeable layer.
In some embodiments, the first carrier fabric comprises an outer layer extending about a circumference of an outer surface of the shell of a pressure roll, the outer layer being adapted and configured to receive gas passing through apertures in the shell and to assist in diffusing such gas so as to provide for increased uniformity of gas flow across the width and length of the nascent paper web, whereby the first carrier fabric comprises a temporary element of the web sandwich.
In some embodiments, the first carrier fabric, on the outer surface of the shell, is made using materials selected from the group consisting of sintered polymer, sintered metal, a shrunken sleeve, and a nonwoven fabric.
In some embodiments, the method includes a papermaking machine employing a method of the invention.
In a second family of embodiments, the invention comprehends apparatus for removing water from a nascent paper web in a papermaking machine in the process of fabricating a finished paper product, the nascent paper web comprising papermaking fibers and water, and having first and second opposing web surfaces which extend between first and second opposing edges of the nascent paper web, the nascent paper web having a generally continuous length, and a width, a first carrier fabric being disposed on the first surface of the nascent paper web and a second carrier fabric being disposed on the second opposing surface of the nascent paper web, a web sandwich comprising the first and second carrier fabrics and the nascent paper web, the web sandwich having first and second opposing sandwich surfaces, and a web machine direction corresponding with a machine direction of the papermaking machine, and a web cross machine direction, the web sandwich moving in the machine direction of the papermaking machine, the apparatus comprising apparatus applying mechanical pressure to the opposing web sandwich surfaces and thereby driving water from the nascent paper web; a gas pressure source and a gas receptive vent, the gas pressure source having a length extending along the width of the nascent paper web; and a gas pressure chamber providing a generally constant compressed gaseous pressure along the entire cross direction length of the gas pressure source, the gas pressure source thereby conveying compressed gas to the web sandwich and thereby developing a seal zone at the web sandwich.
In some embodiments, the gas pressure chamber is adapted and configured to constantly provide a generally constant compressed gaseous pressure along the entire length of the gas pressure source.
In some embodiments, the gas pressure chamber comprises a pressure box disposed inside the gas pressure source.
In some embodiments, the gas pressure chamber comprises an enclosure having an edge extending along the length of the gas pressure source, the enclosure being open to a surface of the gas pressure source, further comprising a seal extending about the enclosure along the length of the gas pressure source and between the enclosure edge and the respective surface of the gas pressure source, the seal inhibiting leakage of gas between the gas pressure chamber and the gas pressure source, between the first and second edges of the nascent paper web.
In some embodiments, the gas pressure source comprises a pressure roll having an outer shell, the gas pressure chamber being disposed inside the pressure roll, the seal being disposed at an inner surface of the outer shell.
In some embodiments, the apparatus comprises a vacuum source providing vacuum at the gas receptive vent to assist in receiving gas and water from the web sandwich, and conveying the received gas and water away from the web sandwich.
In some embodiments, the apparatus comprises first and second loading devices, loaded against each other and thereby defining a nip therebetween, the first and second loading devices applying mechanical pressure to the web sandwich in the nip, further comprising a pressure device adapted and configured to apply the compressed gaseous pressure in the nip through one of the first and second loading devices.
In some embodiments, the apparatus comprises a first dewatering station in the papermaking machine, the first dewatering station applying the mechanical pressure to the nascent paper web, through the first and second carrier fabrics, further comprising a second dewatering station, separate and distinct from the first dewatering station and downstream in the papermaking machine from the first dewatering station, the compressed gaseous pressure being applied to the nascent paper web, through the first and second carrier fabrics, at the second dewatering station.
In some embodiments, the apparatus comprises, after applying the mechanical pressure to the nascent paper web at the first dewatering station, and downstream of the first dewatering station and before applying the compressed gaseous pressure to the nascent paper web at the second dewatering station, the web sandwich comprising a third carrier fabric, different from the first and second carrier fabrics, on one of the first and second surfaces of the nascent paper web.
In some embodiments, the apparatus comprises, after applying the mechanical pressure to the nascent paper web at the first dewatering station, and downstream of the first dewatering station and before applying the compressed gaseous pressure to the nascent paper web at the second dewatering station, the web sandwich comprising third and fourth carrier fabrics, different from the first and second carrier fabrics, on the first and second surfaces of the nascent paper web; and in some embodiments, the third and fourth carrier fabrics have replaced the first and second carrier fabrics.
In some embodiments, the first loading device comprises a roll, the roll comprising a shell, the shell having a length and extending about a circumference of the roll, apertures being arrayed about the circumference and along the length of the shell, and extending through the shell, the roll being equipped with a pressure box supplying the compressed gaseous pressure, through the apertures and to the web sandwich.
In some embodiments, the first loading device comprises a vented sleeve or a vented belt.
In some embodiments, the second loading device being selected from the group consisting of a vented roll, a shoe press, a vented fabric, and a suction roll.
In some embodiments, average mechanical nip pressure between the first and second loading devices at the first dewatering station comprises up to about 400 pounds per square inch, optionally up to about 600 pounds per square inch, optionally up to about 800 pounds per square inch, optionally up to about 1000 pounds per square inch.
In some embodiments, the gas pressure source and the gas receptive vent define a second nip therebetween at the second dewatering station, and apply mechanical loading to the web sandwich at a nominal amount of pressure, optionally an average pressure up to about 2 pounds per square inch, optionally an average pressure up to about 5 pounds per square inch, optionally an average pressure up to about 100 pounds per square inch, optionally an average pressure of up to about 300 pounds per square inch, optionally an average pressure of up to about 500 pounds per square inch, optionally an average pressure of up to about 800 pounds per square inch, optionally including a seal, optionally a self-loading seal, optionally a self-loading ganged seal.
In some embodiments, the apparatus further comprises, at the second dewatering station, the gas pressure source comprises a gas pressure shoe providing the compressed gas to the web sandwich.
In some embodiments, the gas receptive vent, at the second dewatering station, comprises a porous sleeve or belt at a surface of the web sandwich opposite the pressure shoe, the porous sleeve or belt receiving and venting water and gas passing through the web sandwich.
In some embodiments, the first carrier fabric comprises a layer having at least one property selected from the group consisting of (i) a basis weight of up to about 1200 grams per square meter, (ii) up to about 40 percent void space by volume, (iii) a moisture ratio less than 0.4, optionally less than 0.25, optionally less than 0.15 prior to applying the compressed gas to the web sandwich.
In some embodiments, the first carrier fabric is disposed on the web sandwich surface to which the compressed gaseous pressure is applied, and wherein an amount of water removed from the first carrier fabric, as the compressed gaseous pressure is applied to the web sandwich, is less than two times the amount of water removed from the nascent paper web.
In some embodiments, the second carrier fabric is disposed on the web sandwich surface which is away from the gas pressure chamber, the second carrier fabric being adapted and configured to readily convey flow of water, which is pushed from the nascent paper web by the compressed gas, and to inhibit movement of water back into the nascent paper web under relatively lower ambient or reduced gaseous pressure.
In some embodiments, at least one of the first and second carrier fabrics comprises a perforated membrane.
In some embodiments, the perforated membrane is a first layer or sub-layer, and the respective carrier fabric further comprises a second layer or sub-layer.
In some embodiments, the perforated membrane and associated sub-layers facilitate development of relative uniformity of gas flow through the nascent paper web in the seal zone.
In some embodiments, the first carrier fabric is disposed on the web sandwich surface to which the compressed gas is applied, the first carrier fabric comprising a layer having low voids, optionally compacted or filled, optionally thermoplastic yarns, optionally mono mesh fabrics and/or monofilament fabrics, and optionally less than 40 percent void volume.
In some embodiments, a multifilament or batt structure or other structure is in contact with the nascent paper web and thereby limiting in-plane gas leakage in the nascent paper web.
In some embodiments, the first carrier fabric is disposed on the web sandwich surface to which the compressed gas is being applied, the first carrier fabric inhibiting web machine direction flow of compressed gas.
In some embodiments, the first carrier fabric is disposed on the web sandwich surface to which the compressed gaseous pressure is applied, first and second loading devices defining a nip therebetween, and applying sufficient mechanical pressure to the web sandwich in the nip to cause a reduction in thickness of the web sandwich, to a minimum thickness as the compressed gas is being passed through the web sandwich, the web sandwich rebounding from the minimum thickness as the web sandwich exits the nip, the gas pressure chamber continuing to provide compressed gaseous pressure to at least the first carrier fabric and the nascent paper web after the web sandwich exits the nip.
In some embodiments, the apparatus further comprises a third carrier fabric as part of the web sandwich, the third carrier fabric being disposed on a surface of the second carrier fabric remote from the nascent paper web and remote from the compressed gaseous pressure source, further comprising the third carrier fabric being separated from the web sandwich when the web sandwich has exited the nip while optionally continuing to provide compressed gaseous pressure to the first carrier fabric, the nascent paper web, and the second carrier fabric after the first carrier fabric, the second carrier fabric, and the nascent paper web have exited the nip.
In some embodiments, the third carrier fabric expands as the web sandwich moves toward a nip exit, and thereby draws water from the web sandwich and into expanding voids in the third carrier fabric.
In some embodiments, the first carrier fabric is disposed on the web sandwich surface to which the compressed gaseous pressure is being applied, the first carrier fabric being designed and configured to selectively inhibit lateral web machine direction flow of water in the first carrier fabric.
In some embodiments, the second carrier fabric is disposed on the web sandwich surface remote from the surface to which the compressed gaseous pressure is applied, the second carrier fabric having a greater affinity for water removed from the nascent paper web than a web affinity for water extant in the nascent paper web.
In some embodiments, the gas pressure source and the gas receptive vent collectively defining a nip therebetween, further comprising a gas pressure generator adapted and configured for applying and adjusting intensity of compressed gaseous pressure, and optionally heat, being applied to the nascent paper web in the nip, independent of intensity of any mechanical pressure being applied to the nascent paper web in the nip.
In some embodiments, the apparatus further comprises a gas pressure generator adapted and configured for applying and adjusting intensity of compressed gaseous pressure, and optionally heat, being applied to the nascent paper web independent of intensity of any mechanical pressure being applied to the nascent paper web in the nip.
In some embodiments, the apparatus comprises at least a first controller adapted and configured to specify compressed gaseous pressure, gas temperature, and mechanical pressure according to properties to be provided in the finished paper product.
In some embodiments, the apparatus further comprises a seal extending from a body of the pressure device to the web sandwich, the seal extending about the seal zone, the seal limiting lateral gas leakage from the seal zone, in the web machine direction and the web cross machine direction.
In some embodiments, the pressure chamber is adapted and configured to provide compressed gas at pressures, measured at the gas pressure chamber, of about 5 pounds per square inch to about 125 pounds per square inch, optionally about 10 pounds per square inch to about 75 pounds per square inch.
In some embodiments, the gas pressure chamber comprises a first gas pressure chamber providing a first compressed gaseous pressure of a first magnitude to the web sandwich, further comprising a second gas pressure chamber downstream in the web machine direction from the first gas pressure chamber, and proximate the first gas pressure chamber, the second gas pressure chamber being in gaseous communication with the gas pressure source, the second gas pressure chamber providing a generally constant compressed gaseous pressure of a second magnitude, optionally less than the first magnitude of the compressed gaseous pressure provided by the first gas pressure chamber, the gas pressure source conveying the compressed gaseous pressure of the second gas pressure chamber to the web sandwich.
In some embodiments, the gas pressure source conveying the compressed gaseous pressure of the second gas pressure chamber to the web sandwich along the entire length of the gas pressure source.
In some embodiments, at least one of the first and second carrier fabrics having sufficiently low permeability to the compressed gas that the respective carrier fabric provides functional mechanical loading to the web sandwich, thereby aiding in release of water from the nascent paper web.
In some embodiments, the gas pressure source, the gas receptive vent, and the gas pressure chamber collectively defining a first dewatering station adapted and configured to pass compressed gas through the web sandwich in a first direction, further comprising a second dewatering station, separate and distinct from the first dewatering station, the second dewatering station comprising a second gas pressure source, a second gas receptive vent, and a second gas pressure chamber, collectively adapted and configured to pass compressed gas through the web sandwich in a second opposing direction.
In some embodiments, the apparatus further comprises a controller adapted and configured to control and adjust mechanical pressure applied to the web sandwich and air pressure and temperature applied to the web sandwich and thereby to adjust and control properties of a paper product produced from the nascent paper web.
In some embodiments, the gas pressure source comprises a pressure roll having an outer shell, further comprising an air permeable layer disposed on an outer surface of the outer shell, the air permeable layer diffusing compressed air passing therethrough, the and being in direct contact with the nascent paper web.
In some embodiments, the invention comprises a papermaking machine comprising apparatus of the invention.
In some embodiments, the gas pressure source, the gas receptive vent, and the gas pressure chamber collectively define a first dewatering station and pass compressed gas through the web sandwich in a first direction, further comprising a second gas pressure source, a second gas receptive vent, and a second gas pressure chamber defining a second dewatering station, separate and distinct from the first dewatering station, the second gas pressure source, the second gas receptive vent, and the second gas pressure chamber collectively passing compressed gas through the web sandwich in a second opposing direction.
In a third family of embodiments, the invention comprehends as papermaking fabric having lateral leakage, using the “DB Lateral Air Flow Test”, of no more than 8 cubic feet per minute, optionally no more than 6 cubic feet per minute, optionally no more than 3 cubic feet per minute, optionally no more than 2 cubic feet per minute.
In some embodiments, the papermaking fabric has a basis weight less than 1200 grams per square meter.
In some embodiments, the papermaking fabric has voids occupying less than 40 percent of the volume of the papermaking fabric.
In some embodiments, the papermaking fabric contains a perforated layer.
In some embodiments, the papermaking fabric is made using one or more materials selected from the group consisting of polyamide, polyester, polyolefin, and mixtures thereof.
The invention is not limited in its application to the details of construction, or to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various other ways. Also, it is to be understood that the terminology and phraseology employed herein is for purpose of description and illustration and should not be regarded as limiting. Like reference numerals are used to indicate like component.
A CWP papermaking machine, such as papermaking machine 100, can produce parent roll 190 at speeds of about three thousand feet per minute to in excess of five thousand feet per minute. Papermaking using CWP is a mature process that provides a papermaking machine having high runability and high uptime. As a result of the compaction used to dewater web 102 at press nip 130, the resulting paper product typically has low bulk with corresponding high fiber cost. While such parent roll can be converted into rolled paper products, such as paper towels or toilet paper, having high sheet count per roll, such paper products generally have low absorbency and can feel rough to the touch.
Speaking of papermaking in general, energy demand per pound of water removed is less in the mechanical dewatering section than in the dryer section. In some instances, the compressed air provided by pressure source 628 can be heated above adiabatic conditions, thereby incrementally increasing the amount of water removed by operation of the air press assembly. Accordingly, such adding of incremental heat in the dewatering section results in overall reduction in energy use in achieving a dried paper product, compared to removal of the same amount of water in the dryer section. Thus, adding such heat to the pressurized air reduces the overall energy load used in achieving an end product of a dried paper web.
The carrier fabrics accompanying web 102 are designed to enhance water removal from the web. For example, fabric 524 is designed and structured to uniformly distribute air from apertures 630, including through optional cross direction grooving in shell 622, to web 102. Additionally, fabric 524 is designed and preconditioned to release little water within pressurized seal zone 634. This can be attained by using fabrics which are dewatered to a condition at or below 0.4 moisture ratio (MR) prior to entry into seal zone 634 or by using fabrics with low voids, for example less than 40 percent voids, or by using low basis weight fabrics e.g., less than 1200 grams per square meter or a combination of these characteristics. Anti-rewet layer 526 is designed to prevent rewet by capturing and preferentially holding air, such that the air/water layer impedes movement of water back into web 102 as the web and fabrics exit the nip into a lower air pressure ambient environment.
Web sandwich 631, including web 102, fabric 524, and anti-rewet layer 526, is retained/held in the nip in a sealing zone 632 which extends from inside an entrance locus of the nip, namely where the web sandwich enters the nip, to and inside an exit locus where the web sandwich exits the nip. Seal zone 632 is defined as that length of travel through the nip wherein compression of the web sandwich is adequate to allow the pressure roll to seal to fabric 524 and apply air to the web 102 with little or no leakage. Thus, the seal zone has an entrance locus just beyond the leading edge of the nip, where the web sandwich is beginning to be held/controlled, and an exit locus just before the trailing edge of the nip, where the web sandwich is released from compression.
The web sandwich, namely web 102, fabric 524, anti-rewet layer 526, and optional fabric 528, is exposed to a substantially stationary seal zone 634 where air flows from pressure box 624 through outer shell 622, downstream (MD) of the entrance locus of the nip. Seal zone 634 is thus confined to an area inside mechanical pressure zone 632. Since web 102 expands while exiting seal zone 634, the web expansion can cause water to be absorbed into the web (referred to elsewhere herein as rewet). In this invention, air is preferably used to fill the expanding web 102 rather than allow the web to expand and absorb (rewet) water. Accordingly, the seal zone, and thus the area wherein air is passing through the web can be extended beyond where the web sandwich exits the nip normal minimal leakage seal zone 634 to reduce rewet. As discussed hereinafter, where air flow continues in the web sandwich after the web sandwich exits the nip, operating parameters may be adjusted to maintain continued runability of the web.
Machine direction position of pressure box 624 can be adjusted to insure best operation depending on machine conditions. Thus, the path of least resistance for air flow through apertures 630 to 102 is through all of the fabrics in the nip, e.g., in
Anti-rewet layer 526 can be carried as a belt supported by a plurality of rolls (not shown) in addition to roll 512. In addition, an air diffusion and/or venting and/or expanding fabric, such as a felt 528 can be interposed between roll 512 and anti-rewet layer 526. Fabric 528 enhances water removal by reducing rewet of web 102, namely by urging and thereby enhancing water flow away from web 102.
Choosing to not be bound by theory, the inventor contemplates that the additional gains in sheet solids may be due to providing supplemental voids and flow channels in fabric 528 which preferentially attract water which might otherwise be drawn back into web 102 as the web expands toward the exit end of the nip. As discussed hereinafter, web 528 can be separated from the web sandwich at the nip exit, driven by drive apparatus (not shown), further isolating the web sandwich from any water which may be present in web 528. A porous air diffusion sleeve (not shown), wrapped around/mounted to roll 512, can be used in place of fabric 528.
The importance of fabric 524 has not been previously realized as it relates to gas displacement pressing. Since water removal occurs in one direction, namely water flows in only away from pressure box 624, any water released by fabric 524 necessarily enters web 102. Accordingly, the less the water which can be released from fabric 524 the better. Using fabrics 524 of low basis weight e.g. 1200 grams per square meter or less, low void capacity e.g. 40 percent voids or less, and dewatering fabric 524 to 0.4 moisture ratio (MR) or less prior to use in mechanical pressure zone 632 are methods for limiting water load from fabric 524. Water removed from web 102 at nip 610 can be discharged in a variety of ways known to those skilled in the art.
Typically, the outer surface of roll 512 has a pattern of grooves which carry, away from the nip, water expressed from web 102. Alternatively, blind drilled or suction rolls, can be used as roll 512 to aid flow of air through the web and through roll 512, or into optional fabric 528 to aid in removal of water expelled from web 102 using apparatus and methods of the invention. Fabric 528 can also be used to aid removal of water from the sheet. Fabric 528 void structure and void capacity assist in holding water as web 102 exits the nip and limit water from being centrifugally released from vented roll 512 and reaching departing web 102, whereby web 102 can be removed from the vicinity of water which has been removed from the web.
Shoe 510 is stationary in sleeve 712. Anti-rewet layer 526 is driven through nip 640 by a drive mechanism (not shown). Grooved, tubular sleeve 712, travels about a support structure (not shown) and passes between shoe 510 and anti-rewet layer 526. A thin layer of oil or other lubricant is typically deposited between shoe 510 and sleeve 712, thereby to facilitate the movement of the sleeve through nip 640.
As in the embodiment of
As with the embodiment of
Water removed from fiber web 102 at nip 640 can be discharged from the respective grooved sleeve 712 in a variety of ways known to those skilled in the art. Typically, outer surfaces of sleeve 712 have a pattern of grooves, blind drilled apertures, cavities, or a combination of such structures, which carry, away from the nip, water expressed from web 102.
Displacement pressing with the aid of compressed air, as in this invention, provides the option for increasing levels of web bulk and/or web dryness.
Fabric 526 uses the air flowing through the fabric to develop an air layer which works to isolate the paper web 102 from being re-wet by water, namely water which has been driven from fabric 524 and web 102. Preventing water from re-wetting the paper web is a significant factor in maintaining dryness of web 102 as the web leaves air press assembly 514.
Successful displacement dewatering of the web by passing gas/air through the web has 2 major requirements.
The first requirement of successful displacement dewatering is that the wet paper web be compressed, generally fixing in position, the web fibers, thus generally preventing the fibers in the web from moving relative to each other. The compression also reduces voids in the web and compresses/squeezes the web fibers, thus driving water out of interstices in the web paper fibers.
The second requirement of successful displacement dewatering is to apply compressed air or other gas to a surface of the web, generally perpendicular to the surface of the web, which provides a pressure gradient, driving the gas through the thickness of the web. Movement of the gas through the web displaces/pushes water out of the web.
In the invention, where the first-required mechanical compression and the second-required air pressure are exerted at the same nip, e.g. first and second rolls are forced together, compressing the web between themselves as the web passes through the nip, similar to what occurs in a conventional paper machine press section. However, in the invention, magnitude of the web compression, as by the e.g. first and second rolls, is controlled independent of the control of the magnitude of the air pressure. The independent control of mechanical press load and magnitude/pressure of the compressed air allow for increased independence in controlling web solids relative to web bulk. For a press loading in the invention which approaches, in magnitude, a conventional press loading in a conventional press section in a conventional papermaking machine, the addition of the flow of compressed air as in this invention yields paper webs which exceed dryness achieved by using the conventional press loading alone, while providing similar bulk to that provided by using the conventional press loading alone. In the invention, lower press load, and thus less compression of the web in the nip, is typically used. With the addition of air flow, additional water is removed, compared to use of conventional wet pressing alone, resulting in relatively greater solids in the web compared to conventional wet pressing. As a result of the use of compressed air/gas as in the invention, the paper maker can use the combination of nip compression and air flow to control sheet properties while limiting/reducing the amount of energy needed to be used in finishing the drying of the web.
All of the pressure curves in
In addition to mechanical compression, the second requirement of displacement dewatering using compressed gas, in a single station air displacement press, is that air, or other gas, be passed through the paper web while the web fibers are being restrained in the seal zone. The instant invention accomplishes such restraint by using a roll 516 similar to a suction roll but with 2 distinct differences.
First, instead of the respective roll communicating with a suction source, roll 516 is in communication with a gas pressure generator 628, namely a source of compressed gas/air. Since roll 516 is in gaseous communication with a compressed gas generator, rather than being a suction roll, roll 516 is a pressure roll.
The air seal zone experienced by wet paper web 102 is generally co-located consistent with the location of pressure box 624 inside pressure roll 516. Pressure box 624 is adapted, configured, and located to apply air/gas pressure within the boundaries of nip 610 where the web is being pressed, and denoted by areas where nip curve “PT” is high enough to avoid disruption of the fibers in the wet paper web by the applied gaseous pressure.
Pressure roll shell 622 is similar to a suction roll shell in that the shell is drilled with a series of through apertures, from the inner surface of the shell to the outer surface of the shell, allowing flow of air from pressure box 624, through the shell in a generally radial direction. The radially extending apertures communicate with optional voids, such as grooves extending in the cross machine direction, optionally in the machine direction, in the surface of roll shell 622 so that air, and correspondingly air press load, are distributed over the surface of web 102 in a manner that provides generally uniform mechanical loading, and generally uniform air flow through the web. Pressure box 624 is stationary in pressure roll 516, within the rotating roll shell. Pressure box 624 has a fixed location relative to the press nip. One or more seals 762 are disposed between the distal edge of the pressure box, adjacent shell 622, and the inner surface of shell 622. Such seals 762 maintain the compressed gas in the pressure box, whereby the only exits from the pressure box are through apertures 630 in shell 622, whereby the compressed gas flows, by way of the apertures, through the shell at that arc segment of the shell which is in communication with the pressure box at any given point in time, and thence into and through the web sandwich.
Opposite pressure roll 516 is gas receptive vent roll 512 which has optional flow channels, such as perforations, cavities, knurling, a sintered shell, or grooves, such that the channels communicate with a lower pressure sink such as atmospheric pressure or vacuum source. Roll 512 can be vented e.g. by grooves extending in the machine direction, or by using a suction roll with or without vacuum or a combination of grooves and vacuum/suction.
Referring to
Restated, the invention uses press rolls or other structures which develop a controlled level of forces which mechanically compress web sandwich 631, including compressing web 102. The press rolls or other structures are used to develop an e.g. vertical hydraulic pressure gradient across the web sandwich in seal zone 634. Typically, air is used to create the seal zone but other compressible gases can be used as the pressure source for applying a gas pressure gradient at the nip.
In conventional wet pressing, web dewatering occurs when the web is compressed to saturation. When this level of compression is surpassed, hydraulic pressure is created in the water which is in the web whereupon web dewatering begins and water flows from areas of relatively higher hydraulic pressure to areas of relatively lower pressure. In
By contrast, in the invention where gaseous pressure can be simultaneously applied by pressure box 624, positive gaseous pressure is maintained throughout the nip, and optionally for a period after the web sandwich exits the nip. The magnitude of the gaseous pressure can be controlled independently from the magnitude of the mechanical pressure. By using gaseous pressure, dewatering time can be increased and water flow rates out of the web can be increased since gaseous pressure is controlled independently, and not by mechanical web compression. Accordingly, displacement dewatering using air/gas can lead to greater dryness than seen in conventional pressing. Further, dewatering of the web can be continued while the nip is expanding after passing the locus of maximum compression so long as the pressure box maintains a positive pressure gradient into and through the web.
An important advantage of displacement dewatering is the ability to reduce web rewet. This ability to reduce web rewet is a result of air having been moved into the web and the lower carrier fabrics, downstream in the air flow from web 102. Such air in the web and in the carrier fabrics creates an at least partial air barrier that inhibits water flow re-entering the web after the web leaves the locus of maximum compression in the nip. The stability of these barriers within the web and the lower carrier fabric(s) can produce a high solids web and maintain those high solids after the web exits the nip.
In displacement dewatering, it is necessary to control the size, shape, and uniformity of the seal zone. The size, shape and uniformity of the seal zone depends in part on uniformity of the radial permeability of the pressure roll at shell 622. The smaller the drill size and the closer packed the drill pattern, the more uniform the seal zone.
In order to smooth out the pressure and flow variations that occur from a given aperture 630 in shell 622 to the surrounding land area at the roll surface, the fabric in contact with the pressure roll shell desirably has sufficient MD/CD in-plane lateral permeability, along with its vertical permeability, to provide a laterally relatively consistent flow of air surrounding the aperture.
In the invention, the in-plane permeability of e.g. fabric 524 between web 102 and shell 622 allows air to flow laterally from a drilled aperture area of roll 516, within the respective fabric, to surrounding areas of the wet web to be dewatered. Restated, air flow from an aperture or void in the pressure roll spreads laterally as the air travels vertically through a fabric between web 102 and shell 622 so the air flow from a given aperture in shell 622 desirably overlaps air flow from adjacent apertures and at least partially masks land areas in the shell where no vertical air flow can occur. Wet web 102 thus experiences an averaging effect, whereby areas of the web under land areas of shell 622 are relatively more uniformly dewatered than if the air flow could not move laterally after exiting shell apertures 630. However, the air flow does not spread out beyond the nip entrance or nip exit or beyond edges of the web, namely not outside the seal zone, because flow outside the seal zone could cause disruption of web fibers, e.g. destruction, of respective portions of the web.
Structure of bottom fabric 526, namely a fabric between web 102 and vent roll 512, can be optimized to take advantage of the flowing water and air to minimize re-wet of web 102 when the web sandwich expands when experiencing a decreasing or negative mechanical pressure “PH” as the web approaches the exit of the nip.
One major advantage of displacement pressing is that high bulk paper can be made, with relatively higher dryness than is now available using conventional manufacturing apparatus, before nascent web 102 reaches the dryer section of the papermaking machine. One way to achieve a web 102 with higher bulk is to reduce the peak mechanical pressure in the press nip to levels not generally seen on papermaking machines. Compared to conventional wet press papermaking, peak mechanical pressure “PT” (
Displacement pressing requires passing air (or other gas) through a restrained web, such as at a nip. A relatively longer nip provides a correspondingly longer period of time during which displacement pressing can effect removal of water from the web. Elastic roll covers, compressible fabrics, thicker or a greater number of carrier fabrics, increased roll diameters, and increased mechanical press loads all help to increase nip length, thus a longer (machine direction) displacement pressing/seal/dewatering zone, and thus dewatering time and area/distance. But even where such parameters are used, there are limits to how (MD) long the nip can be when cylindrical rolls are used. Typically, the (MD) length of web restraint in the nip, using cylindrical rolls, is about 2.5 inches.
One way to increase nip length beyond what is possible in a nip defined by two rolls as in e.g.
Conventional dewatering presses which use a shoe in the nip typically have a rotating elastic sleeve which is attached to bearings at the end of the sleeve. Internally, the shoe is stationary, pressing into the sleeve through a hydraulic film on which the sleeve floats.
To this point, the invention has been described in terms of apparatus and methods for operating a single-stage displacement dewatering process, where the web sandwich is mechanically compressed in a nip, and compressed gas is forced through the web sandwich in that same nip. In the single-stage embodiments, the mechanical compression drives water out of the web sandwich by creating hydraulic pressure in the water which is in wet web 102, such that the water in the web moves toward reduced pressure in an underlying vent roll, vacuum roll, vent fabric, or other venting device, resulting in a substantial reduction in the hydraulic pressure in the web sandwich. At or proximate the locus of maximum compression in the nip, the web sandwich has reached the minimum thickness which will be obtained while passing through the nip. At that thickness, the web sandwich is fully occupied by the fibers in the web sandwich and water which saturates substantially all of the voids/interstices in that portion of the web sandwich which has been reduced to its minimum thickness. No further reduction in water content of the web is available as a result of the mechanical compression of the web sandwich in the nip. As compressed gas is simultaneously being pushed through the web sandwich at pressures up to about 100 pounds per square inch gauge, and while the web sandwich is experiencing the mechanical compression, that compressed gas displaces some of the water in the voids/interstices in the web sandwich. Because the compressed air can move a substantial amount of water, the magnitude of pressing in the nip can be less than is used in conventional wet pressing, thus achieving relatively higher sheet bulk, while still achieving improved web solids. The lesser degree of mechanical compression provides opportunity for achieving a resultant thicker and/or softer finished paper product. Thus, mechanical compression can be as little as about 20 pounds per square inch (psi) and up to as much as about 1000 psi, and all 1-psi increments in between those two pressures. Gas/Air pressure can be nominal, or between about 2 psi gauge and up to about 100 psi gauge, and all 1-psi increments in between those two pressures as is consistent with good dewatering efficiency. While in some instances, vacuum can be used e.g. in the gas receptive vent roll, compressed air pressure is used for its ability to sustain higher pressure differentials than can be achieved by the limited pressure differentials available with vacuum, as well as the lower cost to generate compressed air for the needed air flow, and the corresponding inherent ability to present relatively higher dewatering forces to the web sandwich.
The invention also contemplates apparatus and methods for a two-stage displacement dewatering process, using a modified conventional wet press (CWP) papermaking machine. In the two stage displacement dewatering process, the first stage is provided by a conventional wet press which mechanically presses water from web 102 without the use of air. In such conventional wet press, web compression collapses web fibers and releases water internal to the fibers into the area outside the fibers due to the forced collapse of the fibers during compression. In addition, a portion of the water which was originally outside the fibers, as the web entered the web press, is driven out of the web according to hydraulic pressure being experienced by the water as a result of the mechanical pressure being applied during conventional wet pressing. One or more felts support web 102 in the web sandwich, and receive at least some of the water released from the web in the nip, whereby the freed water is held external to the fibers by e.g. surface tension in the interstices of web 102.
The second stage in the two-stage displacement dewatering process is located downstream in the papermaking machine from the first stage wet press. The second stage in the 2-stage displacement dewatering process uses an air press to apply an air pressure gradient across the web to remove additional water that was not removed by the conventional wet press in the first stage. The two-stage process benefits from the fact that the first stage mechanical press collapses the paper fibers, releasing water from within the fibers, whereby release of water internal to the paper fibers is not needed at the second stage air press station. Since the second stage air press no longer has to compress the web, the second stage can optionally be less robust than in single-stage displacement dewatering because the second stage air press does not need to accommodate the higher press loads needed for web compression in the single stage displacement dewatering process. Accordingly, existing commercially operating paper machines can use existing press sections along with a second stage air press and still get the benefits of displacement dewatering using compressed gas, of the invention. Because little mechanical compaction is required in the second stage air press, the second stage air press can use a pressurized sleeve air press instead of a loaded pressurized roll shell air press to apply air pressure to the web. The two stage configuration allows use of existing wet presses along with the addition of a less expensive sleeve type air press to gain the advantages of displacement dewatering using compressed gas, namely the advantages of higher bulk and/or solids. For two stage displacement pressing, the air press 649 should follow shortly after the mechanical press lest the web fibers complete their potential for re-swelling and re-absorbing water into the internal fiber structure before the air press can act on the web.
Air press assembly 649 shown in
Layer 752 (e.g.
The overall compressive force received by web 102 in air press assembly 514 applies some of the mechanical force required to establish, and stabilize, the seal zone, thereby preventing such lateral movement as would disrupt the lay and orientation of fibers in web 102. Namely, pressure shoe 754, itself, does not substantially compress web 102.
Namely, in
Web sandwich 631 loading can be generated by increasing pressure differential across the e.g. sleeve or belt 752. Such pressure differential provides mechanical pressure which is equal to the difference in pressure times the seal zone area. In this embodiment, sleeve 752 acts like a piston to compress the web to the extent needed to establish the seal zone.
Such vented sleeve or belt 752 can generate a pressure drop, namely mechanical pressure on web 102, in a number of ways, namely any structure that limits air flow, and any structure which creates a pressure differential.
Sufficient tension in the sleeve or belt generates pounds per linear inch/arc radius (PLI/R) tension forces against the counter/vented roll. Sleeve or belt tension can be controlled by rollers (not shown) which are loaded to expand or reduce the circumference of the sleeve or belt.
Those skilled in the art will now see that a wide variety of pressing fabrics, counter roll constructions, sleeve stiffness, sleeve tension, and sleeve constructions can be optimized for displacement dewatering processes of the invention.
Self-loading and/or auxiliary loading methods can be used to load seal 756 into the sleeve or belt. Such sealing forces can be adjusted in combination with tensioning of the sleeve to maintain desired operating parameters. Placement of the sleeve tensioning rollers near the press nip can enhance PLI/R nip counter roll pressure issues.
In some embodiments, as in
Where a pressure box is used inside a pressure roll, the stationary pressure box has a seal 762 which engages the rotating perforated shell. Since seal 762 is stationary and the roll shell is rotating, there is some seal wear. The seal has low friction, low sealing pressure, low air leakage, and long life. Suitable material for such seal is a long-wearing material such as nylon, or polyolefins, for example nylon filled composite, and without limitation, carbon, or carbon fiber accompanied by polytetrafluoroethylene (PTFE) fillers.
Seal 762 is directed against e.g. shell 622 of pressure roll 516 with limited pressure. The pressure can be so limited by making the seal into a hollow piston that slides within pressure box walls.
The portions of the roll shell shown in
In
Rather than use an O-ring seal between the seal piston and the stationary box, an exemplary seal 762 has a piston cup 770. Piston cup 770 is attached to pressure shoe 754 and slides on the wall of pressure box 624. Air pressure and the natural shape of piston cup 770 cause the cup to seal to the stationary pressure box yet slide up or down as needed with little friction.
Other loading mechanisms can be used such as, for example and without limitation, air cylinders (not shown), in which case the seal can be attached to a frame and wherein the loading cylinders press on the frame to load the seal into the roll shell.
To affect displacement pressing, air is forced through the web. If the web is weak, air flow could disrupt the fibers in the web in which case the web must be restrained before air flow occurs. To avoid disrupting a weak web, the restraint must be sufficiently robust. Web restraint can occur within the nip due to nip loading, and/or by fabric tension as the fabric wraps the pressure roll since fabric wrap tension can compress the web against the roll over the radial angle by which the fabric wraps the roll with compression. To restrain the sheet, using web tension, fabrics of the web sandwich 631 apply restraining force to web 102 forcing the web into the roll. Depending on the tension used, wrap angle and web type, tensioning fabric 524 can apply enough pressure to restrain the web even before the web experiences any additional restraint from interaction with the air press. Once the web sandwich makes contact with sleeve or belt 752, the web sandwich can apply restraining force though tension on the web sandwich.
Web restraint is a condition where fibers are not disrupted and thus web quality is not adversely affected when air is forced through the web. Mechanical compression of the web is one effective way of restraining the web, but the web has internal integrity, and thus the web may require no mechanical compression in order for the web to withstand the flow of air necessary to displacement dewatering of the web. Seal zone boundaries where air is applied to the web vary widely and are determined by the on-machine conditions. Location and size of mechanical pressure zone 632 are adjusted depending on grade, speed, and/or furnish used in the paper web, and press load magnitude, press load sequence, machine clothing, operating temperature, and air pressure used in the papermaking machine, all of which effect the disrupting force imposed on the web and the amount of restraint required to resist such disruption.
In some cases, it is helpful for the web to wrap the pressurized side of the air press, especially on the exit side of the nip, such as in
Web disruption can occur if in plane flow occurs, allowing a fluid pressure field to occur that overcomes web restraint. To avoid in-plane flow, fabrics are constructed to favor vertical (through the fabric) flow of both the air and the water.
In general, when web bulk is preferred, low mechanical pressure is preferred in the nip in order to limit compaction of the web. In this case, peak mechanical pressures are lowered relative to conventional wet pressing, which can be accomplished by using soft rolls and high basis weight felts which also increases the machine direction nip length as well as reducing compaction of the web. Increased nip machine direction length provides a relatively longer seal zone, which can accommodate a relatively longer time for air to be passed through a given area of the web. In addition, soft nips compact the fabrics less so void capacity at the, middle of the length of the nip is also increased when peak pressures are lowered. So, a soft nip is desirable when sheet bulk is important.
Dewatering speed is a significant factor in determining the speed of the web through the papermaking machine. A relatively soft nip at the dewatering section of the papermaking machine provides increased permeability of the web sandwich in the nip, at least partially a result of the web experiencing low compression and thus higher thickness. Removal of water from the web can progress at a relatively faster pace because of the greater void space through which the water can travel. Use of air hotter than ambient can affect increased temperature of web 102, which hotter air further increases the amount of water removed from the web in the dewatering section.
Efficient removal of the water requires substantially uniform air flow throughout web 102, namely about the area of the seal zone. If there were areas of seal zone 634 where there might be little or no air flow through the web wet areas/spots in the web would be the result. Since air originates from apertures in either roll shell 622 or sleeve or belt 752 or the like, the air must be spread out uniformly as applied to web 102 if wet spots are to be avoided. To spread air flow emanating from e.g. apertures/holes 630 of shell 622, cross direction flow is preferred along with limited machine direction flow since machine direction flow can cause machine direction lateral leakage under seal 756. One way to get the desired through-web air flow with limited machine direction flow is to create fabric 524 with relatively cross direction orientated impermeable structures, namely structures which impede at least some of the machine direction flow of the air. Namely, the fabric structure may provide for greater cross direction flow of air than machine direction flow of air yet have enough machine direction flow so that a relatively uniform pressure field is ultimately applied to the web. Relative favoring of cross direction lateral air flow can also be provided by use of perforated, otherwise impermeable, structures within or as a layer or sub-layer in the fabric. Such favored cross direction air flow can be achieved, for example and without limitation, by providing e.g. roll channels 630 elongate in the cross machine direction.
The structure of any fabric layer or fabric sub-layer can have one or more perforated layers which act to restrict overall flow of air as well as to preferentially impede machine direction flow of air. For example, a fabric structure having an array of uniformly-spaced perforations, having open areas of common size, and wherein the perforations are uniformly spaced about the area of the fabric, can have flow resistance high enough that flow channels through that perforated structure dominate, or greatly influence, the e.g. vertical permeability of the fabric while inhibiting in-plane/lateral flow of the air. In such embodiment, the so-perforated fabric acts to present a uniform flow of air across the entire area of the web in the seal zone due to the resistance at the lands between the perforations and the uniformity of the areas of the flow channel. In addition, the flow channels can present the air to an air distribution layer/fabric which can spread the air laterally before the air reaches web 102.
Schematically, the fabric against the pressure roll is illustrated as wear layer 774 in
In
The second sub-layer of fabric 524 is perforated sub-layer 776 which has orifices 778 which are typically, but not necessarily, uniform in size, shape, and distribution about the area of sub-layer 776, and when so uniform in size, shape, and distribution, sub-layer 776 is uniform in permeability and air flow. Lower diffusion sub-layer 780 allows, typically, a limited amount of in-plane air flow whereby the air is, again, urged to spread out and present uniform pressure, and uniform rate of flow, to web 102. The nature of sub-layer 780 to spread the air flow is denoted by diagonally down-directed arrows in sub-layer 780 which represent spreading of air flow from any given orifice in all down directions. Air flow spreading from an individual orifice preferably so overlaps air flowing from next adjacent orifices that a relatively consistent distribution of flow of air enters the entrance surface of web 102 and exits the exit surface of web 102 whereby, if a given orifice is plugged, or partially plugged, air flowing from adjoining orifices helps provide water removal at and adjacent the respective plugged or partially plugged orifice. In some embodiments, a perforated shrink screen roll cover 782 is provided in direct contact with the outer surface of roll 516 to facilitate the interaction between fabric 524 and the outer surface of roll 516.
Schematically, anti-rewet layer 526 is disposed against vent roll 512 as illustrated in
The smaller a given perforation 783, the greater the pressure required to dislodge air within the perforation, so small perforations full of air are an effective anti-rewet tool/structure. At the same time, it is air is allowed to readily flow through web 102 in order to avoid wet areas in the web.
Perforated sub-layer 786 of anti-rewet layer 526 has the function of accelerating flow of water through layer 526 under the influence of the compressed air from apertures 630, thus ejecting water from the area of web 102. Because vertical flow of air and water necks down in the perforated area, water flow away from the web is accelerated, whereby the momentum of the exiting velocity of water flowing away from web 102 helps remove the water from the area of the web, thus helping the water maintain separation from the web area, and thereby preventing return of that water back into layer 102. Lower sub-layer 788 of anti-rewet layer 526 is typically more porous than upper sub-layer 784. Lower sub-layer 788 is sufficiently porous to transport air and water uniformly from perforated sub-layer 786 to grooves or other vents in vent roll 512. The vent roll can be a suction roll, a grooved roll, or other surface characterization of the roll, or a vent fabric which provides flow paths for air and water to move away from the nip. Where the vent roll is rotating at any substantial velocity, surface tension of the removed water and air is accelerated by the circumferential rotational speed of the roll surface whereby the removed air and water is physically urged in a machine direction away from the vent roll by the surface speed of the vent roll.
In the invention, where an air pressure nip is used to perform displacement dewatering, mechanical web compression and fluid gaseous e.g. air flow are independently controllable. Web compression is controlled by the applied mechanical nip loading, if any, while hydraulic pressure on the water contained in the web is controlled by air pressure being applied from pressure box 624.′ Namely, a more compacted web (e.g. less bulk and more strength) can be obtained by increasing the mechanical PLI load at the nip. As desired, and independent of the mechanical load, air pressure applied at the nip can be increased, to offset the reduced permeability of the more compressed web. Respectively, where mechanical loading is less, more space unoccupied by fiber is available in web 102 whereby movement of air through web 102 is facilitated, whereby the air pressure required for removing a given amount of the entrained water is lessened. The ability to adjust air flow independently of mechanical press loading allows more accurate control of web properties such as bulk and solids.
A further advantage of the displacement dewatering nip of the invention is that a wide range of fabric thicknesses for e.g. fabrics 524, 526, and any others desired, can be used.
In the invention, the air seal, defining the pressure zone, occurs at the pressure roll, or the pressure box, as applies. Where a pressure roll is used, there is no issue with fabric edge seals since air is applied to the surface of the web sandwich while the web sandwich is restrained in the nip, and there is a positive seal between a respective web sandwich fabric and the press roll shell. The seals within the roll shell are in a controlled environment which is not affected by fabric wear.
Referring first to
Another known conventional process aims to add bulk to tissue by means of a texturized belt. The texturized belt process typically uses vacuum dewatering ahead of the belt to dewater the web, typically to the 20-25 percent solids range and then presses the web against the texturized belt where web solids remain low when the web is transferred to the Yankee. A drawing of the texturized belt process is shown in
In
The texturized belt process can be modified with a displacement press by replacing vacuum dewatering roll 792 with a displacement dewatering station. The new configuration is illustrated in
Compared to the configuration of
As illustrated in
When air is applied to the web through pressure box 624, the hydraulic pressure curve becomes that of curve “HPZ”. In curve “HPZ”, hydraulic pressure increases and remains constant throughout the press nip since a constant supply of compressed air is provided to the nip. The pressurized air causes dewatering over substantially the fully length of the pressure zone. With the application of air pressure, as the web expands, the web absorbs air instead of water.
Choosing to not be bound by theory, the inventor contemplates that, in nip 640, the compressed air arriving through apertures 630 first drives water from overlying distribution fabric 524 into web 102, thence drives the water collectively residing in layer 102 into anti-rewet layer 526, and thence from layer 526 to and through any other underlying fabric/layer and out of the web sandwich. Water leaving the web sandwich is generally presented to, and carried away by, the underlying pressure receptive vent roll or belt or sleeve.
In displacement dewatering, air passing through the web flows in a single direction, from a region of relatively higher pressure, originating at the pressure roll or pressure box, toward a region of relatively lower pressure, which is toward the vented or suction side of the nip, thence to ambient pressure discharge. Such unidirectional flow of air is illustrated in
Since air distribution layer/fabric 524 is located on the higher pressure side of web 102, any free water in air distribution layer 524 flows out of fabric 524 and into web 102, thereby adding to the water load to be removed from web 102. In addition, the water from fabric 524 must travel through the entire thickness of web 102 before exiting the web being dewatered and allowing such web to reach higher solids. Given that water being driven from the air distribution fabric, in the dewatering of web 102, must traverse both fabric 524 and web 102, the water being driven from the air distribution layer takes longer to be removed than water which was initially present in web 102, because of the distance that must be travelled, both from fabric 524 and through web 102, which slows down the water removal process.
The inventor surprisingly discovered that, under some conditions, web 102 became wetter after displacement dewatering using compressed gas, and observed that using wet felts as an air distribution layer resulted in enough water being released from the felt that sheet dryness was reduced, actually making a web 102 which was wetter after the dewatering process than before entering the dewatering process.
The inventor ran a series of experiments, in a hand displacement press, designed to reveal the amount of water in a vacuum dewatered air distribution fabric, followed by the amount of water which was removed from the air distribution fabric during displacement dewatering using both mechanical pressure and compressed air. Results of the experiments are shown in the following Table 1.
The top 2 fabrics were used as air distribution layers. As indicated in the above table, the wet felts gave up 300-755 grams of water per square meter, which inherently passed into the underlying web 102 and, accordingly, had to be driven from the web being dewatered, in addition to the water which was originally in the web being dewatered. Table 2 shows how the amount of water given up by the air distribution fabric compares to the water to be removed from the web being dewatered at various sheet basis weights and for various changes in sheet solids.
Table 2 shows that, for example, a 20 percent solids sheet, 50 grams per square meter (gsm), such as copy paper, needs to have 150 gsm of water removed in order to reach 50% solids. Table 1 shows that, if a wet felt is used as the air distribution fabric, the felt gives up 300-755 gsm of water into the web being displacement dewatered, namely at least twice the amount of water originally resident in the web being dewatered.
Thus, in order to successfully dewater the 150 gsm web to 50 percent solids, the amount of water which must be driven from that web is at least three times the amount of water, driven from the web, which was originally resident in that web. By contrast, the perforated anti-rewet fabric, and the forming fabric, gave up negligible amounts of water under the same test conditions whereby a shorter exposure to compressed air, or a lower air pressure, were effective in achieving the same level of web solids in the web being dewatered. The experiments summarized in Tables 1 and 2 thus show that selection of material for the air distribution fabric 524 is critical to efficient displacement dewatering.
Because water given up by the air distribution layer is so important, water in the distribution fabric is kept low. Sufficiently low water content can be achieved with fabrics which have low voids. Namely, such fabrics can be e.g. compacted or filled fabric structures. Such fabrics typically are less than 40 percent void. Such void levels can be achieved with fabrics which have been compacted with optional heat and pressure, or fabrics which use thermoplastic yarns along with heat and pressure to fix the fabric structure in a compacted condition.
In addition to, or in place of, fabric selection, another step which can be taken to limit the amount of water which must be removed from the air distribution layer during displacement dewatering is to displace water in the air distribution fabric ahead of the displacement dewatering station. Such removal of water can be achieved, for example and without limitation, by conditioning the fabric by e.g. subjecting the fabric to vacuum dewatering at a vacuum station, or by subjecting the fabric to compressed air dewatering at a compressed air station similar to a displacement station of the invention.
Still referring to the air distribution layer, fabric pore structure also has an influence on fabric water holding capacity. The larger the pores the less water the fabric will hold. Fabrics using primarily monofilament construction instead of multifilament construction have relatively larger pores and thus hold less water. Monofilament fabrics also dewater readily so monofilament construction is preferred for the air distribution layer along with optional use of multifilament or batt structure to effect sealing to the sheet being dewatered in order to avoid in-plane leakage at the surface of fabric 524. Basis weight is also an effective parameter in that lower basis weight is generally accompanied by less water holding capacity. Basis weight of the air distribution layer must be less than 1200 grams per square meter, preferably less than 800 grams per square meter and optionally less than 500 grams per square meter. Conventionally available papermaking fabrics do not limit in-plane flow, namely do not channel flow vertically, between opposing surfaces of the fabric as required in this invention.
A commonly known measure of papermaking fabric moisture is the “Moisture Ratio”. Moisture Ratio is mass of water/mass of fabric. Preferred air distribution fabrics enter the displacement dewatering nip with moisture ratios of no more than 0.4 MR, preferably no more than 0.25 MR, optionally as low as possible below 0.25 MR, such as 0.15 MR. In preferred embodiments, the amount of water given up by air distribution fabric 524 in the displacement dewatering press is less than 2 times the amount of water removed from the nascent paper web. The amount of water removed from air distribution fabric 524 during displacement dewatering can be measured and compared with the amount of water removed from the web being dewatered using microwave moisture meters or nuclear gages designed to measure basis weight.
The first and second approaches use a novel combination of fabrics in the web sandwich to either inhibit movement of water back into web 102, or to compete with web 102 for any water which may move as the web sandwich expands while exiting the nip.
The third approach continues to apply compressed air to the web sandwich after the web sandwich has exited the nip.
The first approach to opposing web rewet is to break the hydraulic path which otherwise can allow available rewet water to flow back into web 102. The hydraulic path can be so broken by providing, optionally as part of the web sandwich, downstream of the air which is progressing through web 102, a fabric, illustrated as layer 526 in
The second approach to opposing web rewet is to provide, again downstream of the air which is progressing through web 102, and optionally as part of the web sandwich, a fabric, illustrated as fabric 528 in
The third approach to opposing web rewet begins with the understanding that there is a delay effect between the time web 102 leaves the nip and the time wherein water no longer has a tendency to move back into web 102 from fabrics which are downstream, in terms of air flow, from the web. Restated, as the web sandwich leaves the mechanical pressing force in the nip, there is a time delay where the mechanical force has been completely released but the restoration of the thickness of the web sandwich, specifically web 102, has not been completed. So for a short time after the web has exited nip 640, web 102 continues expanding and resolving internal forces among the fibers. As those forces work themselves out, the movements of the fibers can bring rewet water back into the web. The magnitude of those forces, however, tends to be relatively small. Accordingly, those forces can be countered by continuing to push compressed air through the web sandwich, namely through web 102, after the web sandwich has left nip 640. In
The above first and second fabric-based approaches to reducing rewet of web 102 are illustrated in
In general, displacement dewatering can remove a higher fraction of the water from the web than can be removed using a conventional dewatering press station, resulting in higher solids content in the paper web after the web has passed through the dewatering station. In the invention, the dewatering section/process necessarily passes the web through a nip. The nip has so far been described in terms of an air pressure roll and a pressure receptive vent apparatus, which has been illustrated as either a grooved vent roll, a suction roll, a blind drilled roll, or a shoe accompanied by a sleeve or belt which passes over the shoe in the nip. One of the advantages of the invention is that a wide variety of nip structures can be used. While an air pressure roll has been described and illustrated for applying the air pressure, a wide variety of structures known to those skilled in the art, which can supply the air pressure, and can be used to provide one side of the nip, and are accordingly included in this invention. Further, while the pressure receptive vent has been described as either a grooved vent roll, suction roll, blind drilled roll, or a shoe accompanied by a grooved sleeve or belt, any other vented structure, for example a vacuum roll, which can receive the released water, and convey that water away from the nip, is included in this invention. Accordingly, the invention includes any combination of structures which can apply enough mechanical nip pressure to web 102 to develop a functional seal zone if needed to prevent the web from disruption, in combination with driving air through the web at the same location where seal zone pressure is being applied. In some embodiments, mechanical pressing of the web, which displaces water from the fibers, can be displaced upstream in the process from the location where air is being driven through the web.
One such structure, contemplated as being within the scope of the invention includes a porous pressure belt and a grooved vent belt. The pressure belt is driven about a supporting frame, and cooperates with the grooved vent belt, in applying mechanical pressure in a nip, at a stationary pressure box which is at a fixed location along the path of the pressure belt. A web 102, supported by one or more carrier fabrics is drawn through the nip. The grooved vent belt both supports the mechanical pressure of the pressure belt, and receives water and air which is driven through web 102 and the supporting fabrics by air from the pressure box. Accordingly, neither the pressure belt nor the vent belt needs to take on the form of a roll per se. Yet the combination of pressure belt and vent belt, meeting at the nip, and where the stationary pressure box drives air or other gaseous material through the web to be dewatered, is effective to dewater the web so long as nip pressure is adequate to provide any restraint needed for the web/fabrics and the air pressure is adequate to move more of the water out of the web than can be removed by mechanical pressing, alone, of the web in the nip. Those skilled in the art will know other structures which can be combined to provide the illustrated e.g. mechanical pressure, if any, in the nip, in combination with the illustrated air pressure applied in the restraint zone of the nip. The floating seal, as at e.g.
In some embodiments, at least first and second fabrics are used to support web 102 in the dewatering process. A first fabric, which is between web 102 and pressure box 624, conveys a pushing flow of compressed air or other gas, from the pressure box, generally uniformly distributed about the seal zone, and generally perpendicular to the surface of the fabric, thus generally perpendicular to the closest surface of the web, and through the web, in air pressure assembly 514. The respective functions of the first fabric can be achieved by providing respective ones of the requirements in multiple layers as in
Similarly, the second fabric, namely the anti-rewet layer, on the opposing side of web 102, is designed to receive water being displaced from web 102, and to expeditiously transport water away from the web as well as to prevent back flow of water into the web, especially backflow of water into the web as the web begins to re-expand while approaching the exit locus, and after exiting the nip. The respective functions of the second fabric can be achieved by providing respective ones of the requirements in multiple layers as in
Such structures as are recited for fabrics 524, 526 can be incorporated in whole or in part within the structure of the pressure roll or in the pressure receptive vent roll, or both, to arrive at the overall desired result of achieving gaseous flow through web 102, the air flow being generally uniformly distributed about the seal zone, while the web is in the nip. For example, the perforated shell of pressure roll 516 can have a second layer 524A, shown on the roll in
This invention relies on flow of e.g. air generally perpendicular to that surface of web 102 which is proximate the pressure delivery structure, such as shell 622, seal 756, or the like. Correspondingly, lateral flow of air through the web sandwich, and especially through any fabric upstream, in the air flow, of web 102 diminishes the effectiveness of the invention in removing water from web 102. Accordingly, fabric 524, and any other fabric located between web 102 and structure delivering the pressurized air to the web sandwich, such as apertures 630, is designed and configured to limit lateral air flow in the fabric.
A seal ring 820 has an outer diameter of 3.3 inches and an inner diameter of 2.67 inches, and uniform wall thickness therebetween corresponding to the respective inner and outer diameters, and the inner and outer surfaces, of the wall of the loading head, and underlying the entire circumference of the wall of the loading head. Accordingly, the wall of the seal ring fully underlies the cylindrical wall of the loading head. Base seal 822 and seal ring 820 are neoprene seals, 0.063 inch thick, having Shore Hardness 60-70 on the A scale.
A papermaking fabric, such as a fabric 524 is tested as follows. Fabric 524 is placed between base seal 822 and seal ring 820, and extends across the entire areas overlain by loading head 810, seal ring 820, and base seal 822. An air supply line 826 supplies pressurized air to the open chamber inside the loading head. A downwardly directed air load of 50 pounds per square inch of seal area 820 is maintained at the seal through the loading head, thereby capturing and pressing downwardly on fabric 524 and sealing fabric 524 between seal ring 820 and base seal 822. With the fabric so held, pressurized air at 25 pounds per square inch is introduced, through air supply line 826, into the open chamber in the loading head. The pressurized air flows downwardly onto fabric 524 whereby flow of air downwardly through fabric 524 is blocked by base seal 822. Air pressure is generally uniform and generally constant throughout the volume of the air chamber. While air pressure is being maintained at 25 psi and the downward mechanical pressure on the ring seal is maintained at 50 psi, leakage of air between seal ring 820 and base seal 822, namely lateral leakage of air through the fabric being tested, is measured.
Using the above test, lateral leakage of air out of the open chamber, through a fabric acceptable for use in this invention between apertures supplying compressed air to the web sandwich and the web being dewatered, is no more than 8 cubic feet per minute, optionally no more than 6 cubic feet per minute, preferably no more than 3 cubic feet per minute.
For this invention, the seal zone is understood to mean the pressurized area in the nip where flow from the pressure box occurs primarily vertically through the nascent paper web toward the vent receptive structure. Flow outside this primarily vertical flow path, e.g., laterally, reduces efficiency of the process and may disrupt the sheet or machine operation. Fabric 524 is typically designed for sealing to pressure sources consistent with permeability requirements for adequate dewatering. For webs that are less permeable, higher gaseous pressure is required which in turn can lead to more difficulty developing a seal zone to direct gaseous flow through the sheet. In such difficult sealing cases, the MD seal zone boundaries can be moved inward toward nip center where higher mechanical pressure is impressed on the sealing surfaces before gas is applied to the web. Higher mechanical pressure helps to further compress the web sandwich, which inhibits lateral flow of air and also helps to make surface contacts more intimate thereby inhibiting lateral leakage. Because of the variety of furnishes used by the paper industry and the permeabilities such varieties of furnishes, the variety of fabric designs that can be used for displacement dewatering service, and variations in machine operating conditions, the sealing zone location can vary depending on the specific situation. In the cross machine direction, the sealing zone can extend to areas of the web sandwich which are substantially impermeable and which are designed to seal with the pressure elements 516, 752 or the like known to those skilled in the art. Additionally the cross direction dimension of the seal zone can be limited by the cross direction width of pressure box 624 and interaction of the seal zone with the web sandwich as at 758 as air flow passes to the vent receptive structure and downstream components of the displacement press.
In some embodiments, one or more fabric, typically a vent fabric, can be sufficiently thick, and sufficiently porous, to provide all venting of water removed from web 102.
Those skilled in the art will now see that certain modifications can be made to the apparatus, products, and methods herein disclosed with respect to the illustrated embodiments, without departing from the spirit of the instant invention. And while the invention has been described above with respect to the preferred embodiments, it will be understood that the invention is adapted to numerous rearrangements, modifications, and alterations, and all such arrangements, modifications, and alterations are intended to be within the scope of the appended claims. To the extent the following claims use means plus function language, it is not meant to include there, or in the instant specification, anything not structurally equivalent to what is shown in the embodiments disclosed in the specification.
| Number | Date | Country | |
|---|---|---|---|
| 63372095 | Feb 2022 | US |
