WO2000006825A1 - Air pumping belt - Google Patents

Air pumping belt Download PDF

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Publication number
WO2000006825A1
WO2000006825A1 PCT/US1999/014355 US9914355W WO0006825A1 WO 2000006825 A1 WO2000006825 A1 WO 2000006825A1 US 9914355 W US9914355 W US 9914355W WO 0006825 A1 WO0006825 A1 WO 0006825A1
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WO
WIPO (PCT)
Prior art keywords
press
sheet
nip
body portion
belt
Prior art date
Application number
PCT/US1999/014355
Other languages
French (fr)
Inventor
Leonard R. Lefkowitz
Original Assignee
Lefkowitz Leonard R
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Lefkowitz Leonard R filed Critical Lefkowitz Leonard R
Publication of WO2000006825A1 publication Critical patent/WO2000006825A1/en

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Classifications

    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21FPAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
    • D21F7/00Other details of machines for making continuous webs of paper
    • D21F7/08Felts
    • D21F7/086Substantially impermeable for transferring fibrous webs
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21FPAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
    • D21F3/00Press section of machines for making continuous webs of paper
    • D21F3/02Wet presses
    • DTEXTILES; PAPER
    • D21PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21FPAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
    • D21F7/00Other details of machines for making continuous webs of paper
    • D21F7/08Felts

Definitions

  • the present invention relates to an endless air pumping belt for the removal of water from a web or sheet in a press section of a paperma ing machine, and a press section which includes, and a method of using, such an endless air pumping belt.
  • web and sheet are terms used interchangeably in the papermaking industry and are used interchangeably herein.
  • the paper sheet is customarily pressed in the press section of a papermaking machine between two press rolls while it is being supported and conveyed on a porous press felt through the nip formed by such press rolls.
  • a porous press felt for use in the press section.
  • One known type is commonly referred to as a needled felt; that is, a construction which typically includes a woven base fabric to which a batt has been needled.
  • the body of such porous press felts absorbs water through the sheet contacting surface of the felt during the pressing operation and therefore must retain sufficient void space under nip pressure to accommodate water being expressed from the paper web.
  • felt flow resistance must be low to allow water to flow into the water absorbing press felt .
  • a known press felt is one which includes a textile base and a layer of flexible, polymeric resin particles which are fused to each other and to the textile base, leaving pores between the particles, such pores being provided to receive water removed from the paper web during the pressing operation.
  • An example of such a composite press felt is Dutt, U.S. patent no. 4,571,359 wherein it is stated that such composite press felts are an improvement over the prior art in regard to compression/recovery properties and uniform void size and distribution.
  • Such press felts absorb water like a sponge through the sheet contacting surface of the felt during the pressing operation, and like a needled felt must be porous to water, must retain sufficient void space under nip pressure to accommodate the water, and must have low felt flow resistance to allow water to flow therein.
  • the vacuum is present longer adjacent to the sheet surface, allowing a substantial part of the water just expressed from the sheet to reenter the sheet thereby causing an appreciable amount of sheet rewetting after the mid-nip and a decrease in sheet consistency as the sheet leaves the press nip.
  • one of the two press rolls is supplied v/ith a pressurized fluid such as air, and the cooperating press roll is supplied with a vacuum, the first roll serving as a source of air to displace water from the textile fabric, and the vacuum roll serving to remove the expressed liquid from the press roll system.
  • a pressurized fluid such as air
  • the cooperating press roll is supplied with a vacuum
  • the first roll serving as a source of air to displace water from the textile fabric
  • the vacuum roll serving to remove the expressed liquid from the press roll system.
  • the paper sheet does not have enough strength to support itself through pairs of press rolls v/ithout self destruction caused by forces in operation at the press nip.
  • the unsupported sheet would likely be extruded back out of the nip entryway, particularly when one of the press rolls is forcing air into the nip.
  • Press felts are known which have barrier layers which limit the flow of water therethrough or which provide patterned voids. Press felts are also known which are membrane-like. However, such press felts are permeable to liquid and act as a receptor of water expressed from a wet sheet or web.
  • One object of the present invention is to provide an air pumping belt, for use in a press section, which does not serve as a receptor for liquid expressed from a wet sheet being moved through a press nip in such press section yet allows passage of pressurized gas through the air pumping belt into the sheet.
  • Another object of the present invention is to provide such an air pumping belt which pumps air into a wet sheet as the sheet is being moved through a press nip in a press section.
  • a further object of the present invention is to mix air with water which is present in a sheet as the sheet is being moved through a press nip in a press section to form a momentary froth within the sheet so that less pure v/ater remains in the sheet.
  • Yet another object of the present invention is to expunge water which is present in a sheet and replace such expunged water
  • Another object of the present invention is to eliminate post mid-nip vacuum which occurs during conventional dewatering of a sheet as the sheet is moved through a press nip in a press section so that the water once expressed from the sheet does not reenter the sheet.
  • a further object of the present invention is to improve sheet consistency of a sheet exiting from a press nip.
  • Another object of the present invention is to provide a slight burst of air at the time that a sheet is under nip pressure, to displace at least some of the water that resides within the sheet pore spaces.
  • Yet another object of the present invention is to provide an air pumping belt for extended nip presses that will reduce or eliminate the vacuum next to a plain press roll surface that draws water back into the sheet from the felt by supplanting the vacuum with positive air pressure, thereby eliminating rewetting.
  • a further object of the present invention is to provide a press section of a papermaking machine which includes an air pumping belt of the present invention.
  • Another object of the present invention is to provide an improved method of dewatering a sheet in a press nip of a papermaking machine by pumping air from an air pumping belt into a sheet in a press nip.
  • This invention achieves these and other objectives by providing an endless air pumping belt for a press section, comprising a first surface, the first surface being permeable to pressurized gas and substantially impermeable to liquid, and an opposite second surface. A body portion is provided which extends from the first surface to the second surface.
  • the body portion consists essentially of means for exhausting a burst of air when the body portion is subjected to nip pressure, from the body portion through the first surface at a first segment of the body portion being subjected to nip pressure in a press nip, sufficient to effect at the first segment (a) displacement of water from a sheet contacting the first surface and (b) mixture of air with water in the sheet, and for intaking air into the body portion through the first surface and/or the second surface at a second segment of the body portion not being subjected to nip pressure in a press nip.
  • This invention further achieves the above and other objectives by providing a press section which comprises a first press roll and a second press roll forming a press nip.
  • a press felt extends through the press nip for carrying a sheet through the press nip, the press felt having one surface structured and arranged to engage a first side of the sheet during operation of the press section.
  • An endless air pumping belt extends through the press nip, the endless belt comprising a first surface structured and arranged to engage a second side of the sheet during operation of the press section.
  • the first surface is permeable to pressurized gas and substantially impermeable to liquid.
  • the endless air pumping belt also includes an opposite second surface, and a body portion which extends from the first surface to the second surface.
  • the body portion consists essentially of means for exhausting a burst of air, from the body portion through the first surface at a first segment of the body portion being subjected to nip pressure in the press nip, sufficient to effect at the first segment (a) displacement of water from the sheet and/or (b) mixture of air with water in the sheet, and for intaking air into the body portion through the first surface and/or the second surface at a second segment of the body portion not being subjected to nip pressure in the press nip.
  • This invention further achieves the above and other objectives by providing a method of dewatering a sheet in a press section of a papermaking machine, the press section including an endless press felt and an endless air pumping belt which travel through a press nip formed by a first press roll and a second press roll of the press section, comprising the steps of intaking air into the endless air pumping belt at a segment of the endless air pumping belt not being subjected to nip pressure in the press nip; sandwiching a sheet between the press felt and the endless air pumping belt; moving the sandwiched sheet, press felt and endless air pumping belt through the press nip; exhausting a burst of air in the press nip, from the endless air pumping belt into the sheet, causing in the press nip (a) displacing of water from the sheet and (b) mixing of air with water in the sheet.
  • Fig. 1 is a view of a press section employing a schematically illustrated air pumping belt of the present invention
  • Fig. 2 is a view of a portion of a press section and one embodiment of an air pumping belt of the present invention
  • Fig. 3 is a cross section view of another embodiment of the air pumping belt of the present invention viewed in the machine direction;
  • Fig. 4 is a cross section view of yet another embodiment of the air pumping belt of the present invention viewed in the machine direction;
  • Fig. 5 is a cross section view of another embodiment of the air pumping belt of the present invention viewed in the machine direction;
  • Fig. 6 is a view of the embodiment of the 'present invention of Fig. 5 taken along line 6-6;
  • Fig. 7 is a view of the embodiment of the present invention of Fig. 5 taken along line 7-7.
  • Fig. 1 depicts a press section of a papermaking machine including a conventional first press roll 2 and a conventional plain second press roll 4 which cooperate to form a press nip 6.
  • Various guide rolls are provided such as guide rolls 8, 10 and 12. It will be apparent to those skilled in the art that more or less guide rolls may be provided if desired.
  • a conventional endless press felt 14 extends about press roll 2 and guide rolls 8, 10, and 12 and is caused to travel in the machine direction 16 in the press nip in a conventional manner.
  • Fig. 1 depicts a schematically illustrated air pumping belt 30 of the present invention extending through press nip 6.
  • Belt 30 includes a void containing collapsible body portion 32, a sheet contacting surface 34 that is permeable to gas and substantially impermeable to liquid, and a roll contacting surface 36.
  • air pumping belt 30 travels through press nip 6, and about guide rolls 38 and 40.
  • Fig. 2 is an enlarged view of a portion of a press section of a papermaking machine similar to Fig. 1 and includes details of one embodiment of an air pumping belt of the present invention.
  • an air pumping belt 42 enters and extends through a press nip 6 with a sheet contacting surface 44 coming into pressure contact with a sheet 18 while a felt 14 presses against the other side of the sheet.
  • the surface 44 is permeable to pressurized gas and substantially impermeable to liquid, as described in more detail hereinafter.
  • Belt 42 also includes an opposite roll contacting surface 46.
  • a body portion 48 extends from surface 44 to surface 46.
  • Body- portion 48 consists essentially of means for exhausting a burst of air when the body portion is subjected to nip pressure, from the body portion through the surface 44 at the segment of the body portion being subjected to nip pressure in the press nip 6.
  • Such burst of air is sufficient to effect at the segment of the body portion in the press nip 6 (a) displacement of water from the sheet 18 contacting the first surface 44 and (b) mixture of air with water in the sheet 18.
  • Such means is also provided for intaking air into the body portion 48 through surface 44 and/or surface 46 at the segment of the body portion 48 not being subjected to nip pressure in press nip 6.
  • a resilient, elastomeric material comprising void spaces such as, for example, discrete elastomeric particles 50 scattered throughout the body portion 48 and separated by void spaces 52.
  • void spaces such as, for example, discrete elastomeric particles 50 scattered throughout the body portion 48 and separated by void spaces 52.
  • Such particles may comprise interconnected surfaces and may be substantially equal or vary in size and composition.
  • particles 50 may be polyurethane particles having surfaces which are bonded together as for example, by fusion bonding or sintering and the like.
  • the void spaces within the body portion of the belt are capable of being collapsed under nip pressure so as to expel air, which is contained in the segment of the belt passing through the nip, into the sheet during passage through the press nip.
  • a major advantage of the air pumping belt of the present invention is the ability to affect the sheet 18 by delivery of positive air pressure for a brief instant within the press nip 6 and beyond the duration of the mechanical pressure pulse. It is believed that this momentary continuation of pneumatic pressure beyond the duration of mechanical pressure eliminates vacuum at the sheet-felt interface which otherwise would occur in the expanding press nip egress, acting in this way to prevent sheet rewetting.
  • the void spaces within the air pumping belt may be formed by uniformly depositing fusion bondable elastomeric particles on top of a support surface and then sintering or fusing the particles together under pressure at a temperature above the softening temperature but below the degradation temperature of the particles, to form a strong cohesive structure.
  • Strength may be added by using a base fabric as the support surface, the particles being fused together and to the base fabric.
  • the void spaces within the air pumping belt are preferably interconnected and vented to the sheet contacting surface through micro-porous passageways v/hich are permeable to pressurized air but substantially impermeable to water under the dynamic short term pressure pulse conditions caused by passage through the press nip.
  • the air pumping belt of the present invention cannot serve as a water receptor, because fluid flow passageways at the sheet contacting surface are too small to allow for any significant amount of water to flow into the belt during the brief period of mechanical pressure exposure in the press nip.
  • most of the void spaces within the air pumping belt will contain pressurized air and v/ill collapse during entry into the nip.
  • the elastomer material may be in the form of a discontinuous elastomeric resin treatment or in some other form, provided that in all cases there are void spaces within the body portion of the belt capable of being collapsed under nip pressure so as to expel air, which is contained in the segment of the belt passing through the nip, into the sheet during passage through the press nip and to recover from deformation so that the void spaces will fill with air before reentry of such segment into nip.
  • the void spaces may be interconnected to facilitate venting of the belt under nip pressure through the sheet contact surface layer of the belt.
  • an air pumping belt of the present invention may be provided having a body portion similar to body portion 48 and containing a woven reinforcement, and/or reinforcement fibers, for strength and stability.
  • an air pumping belt 54 of the present invention is provided which includes a woven fabric reinforcement 56 within the body portion 58 to facilitate belt stability in operation on the press.
  • resilient body portion 58 includes void spaces 60 and elastomeric resin material in the form of discrete particles 62 scattered throughout the body portion. Reinforcement fibers 64 are also provided in the body portion 58.
  • 3 may be made by needling successive layers of fibers into the base fabric 56, while adding fine elastomer particles between fiber applications. Particles may be dispersed more uniformly throughout the fiber matrix with the aid of vibratory means. After the requisite fiber applications have been completed, additional particles may be added and the entire structure may be consolidated and compacted by further needling, followed by heat and pressure treatment to induce sintering and fusion bonding of particles and the assembly.
  • the sheet contact layer 68 may be further perfected by the application of a discontinuous resin treatment 66 which when cured, forms a tough surface that under nip pressure, allows for the escape of compressed air from voids within the belt while being substantially impermeable to the admittance of water from the sheet during the brief press nip mechanical pressure exposure.
  • the delayed elastic recovery properties of the elastomeric components in the belt cause the collapsed void spaces to recover from deformation, inducing a slight vacuum, so that these internal void spaces expand and fill with air prior to reentry into the press nip.
  • the collapsed segment of the air pumping belt 54 recovers from compression and fills with air through both the sheet contacting surface 68 and the roll contacting surface 70 prior to re- entering the nip.
  • This exposure to air from both sides of the belt may be a preferred embodiment, for example, where the press configuration limits the length of the belt that can be used, or where the sheet is particularly thick, or v/here for some other reason it is desired to have the air pumping belt contain more air than could enter the non-compressed segment of the belt within the time constraints of machine speed and press configuration if air entry into such segment prior to the reentry of the belt into the press nip was restricted to only the sheet contacting side 68 of the belt.
  • roll contacting surface 70 of the air pumping belt 54 is of the same material as the body portion 58.
  • the air pumping belt it is preferable for the air pumping belt to wrap around the surface of a plain press roll 4 sufficiently to create an effective seal, as shown in Fig. 2 regarding belt 42, for preventing the escape of air entrapped in the belt from being expelled through surface 70 and into the atmosphere as the belt enters the press nip 6.
  • a plain press roll 4 for preventing the escape of air entrapped in the belt from being expelled through surface 70 and into the atmosphere as the belt enters the press nip 6.
  • the embodiment depicted in Fig. 3 is a cross section viewed in the machine direction of the belt 54.
  • a cross section of belt 54 viewed in the cross-machine direction would be similar to Fig. 3.
  • an air pumping belt 72 which includes a reinforcement woven base fabric 74 embedded within a body portion 76.
  • body portion 76 comprises elastomeric particles such as urethane 78 bonded or interconnected together at points of contact by fusion bonding or sintering so as to form a strong durable matrix having void spaces 80 between particles.
  • void spaces are formed between particles that are joined at points of contact, void spaces vent for the most part either directly or indirectly through the sheet contact surface of the belt.
  • roll contacting surface 86 is rendered completely impermeable, for example, by melting a solid layer of fusible urethane elastomer into the roll contacting surface of the air pumping belt. Thereafter, sheet contacting surface 82 serves as the only path for entry of air into pore spaces 80 between particles 78.
  • sheet contacting surface 82 includes a matrix of closely joined elastomeric particles, such as polyurethane particles 88 embedded within a urethane resin treated layer 90, with micro porous flow passageways 92 between solid matter.
  • the porosity of the sheet contacting surface 82 can be adjusted as desired to provide the prerequisite flow properties such that under nip pressure in nip 6, fluid flow passageways 92 will allow only for the escape of compressed air from collapsing void spaces 80 in the body portion 76 of the air pumping belt, while preventing the flow of water from a sheet 18 through surface 82 and into the air pumping belt.
  • body portion 76 includes larger urethane particles near the roll contacting surface 86 and smaller particles near the sheet contacting surface 82.
  • the air pumping belt of Fig. 4 may be the preferred embodiment when, for example, the belt cannot wrap the top roll or when the escape of any contained compressed air to atmosphere other than through the paper sheet v/ould reduce operating efficiency.
  • Variables in construction of the air pumping belt of the present invention include, without limitation, elastomer hardness and type, pore size and flow resistance of the sheet contact surface, contained void volume, deformation response to nip pressure, and recovery properties after deformation. All of these variables can be specified and controlled by the design engineer .
  • Fig. 4 The embodiment depicted in Fig. 4 is a cross section viewed in the machine direction of belt 72.
  • a cross section of belt 72 viewed in the cross-machine direction v/ould be similar to Fig. 4.
  • the fluid flow passageways in the sheet contacting surface of an air pumping belt of the present invention are extremely flow restrictive, substantially permitting only the passage of air from cavities in the air pumping belt into the paper sheet during passage through the press nip, hile at the same time, substantially preventing the flow of water from the sheet into the belt.
  • water from the sheet is driven into the much larger void spaces of a felt 14.
  • felt void space may be augmented by additional void space provided by a press roll 2 which is grooved. It should be noted that passage of water into a felt 14 is not restricted by narrow entry flow paths that prevent liquid flow, as in the case with the sheet contacting surface of the air pumping belt.
  • Figs. 5 to 7 illustrate another embodiment of the present invention incorporating a nonv/oven substrate 94 of a type, for example, taught in United States patent no. 4,740,409.
  • an air pumping belt 94 is depicted which includes a roll contacting surface 96 formed by an outer portion 98 of a solid elastomeric matrix member 100.
  • the solid elastomeric matrix member 100 includes an inner portion 104 which comprises the body portion of the belt. Inner portion 104 faces towards the sheet contacting surface 106.
  • Such inner portion 104 includes a plurality of spaced wall segments 108 which are defor able, resilient and impervious.
  • the wall segments 108 extend towards the sheet contacting surface 106 and are arranged to form a plurality of open-ended cavities 110 which extend and open in the direction 112 towards the sheet contacting surface 106.
  • the sheet contacting surface 106 is permeable to pressurized gas and substantially impermeable to liquid.
  • cavities 110 vent air in the direction 112, which may be substantially perpendicular to surface 106, through the sheet contacting surface 106 and into a sheet 18.
  • the spaced wall segments 108 prevent lateral movement of the compressed air within the body portion of the belt.
  • machine direction strands 114 may be embedded in the elastomeric matrix member 100.
  • the cavities 110 may be at least partially filled v/ith elastomeric particles 116, if desired.
  • v/hen subjected to pressure in a nip 6 the elastomeric material of the inner portion 104 will collapse causing respective wall segments 108 to partly fill the voids within the cavities 110.
  • the elastomeric particles 116 within cavities 110 also deform during nip pressure exposure, so that the combined response is to substantially entirely eliminate all of the void spaces within the structure during passage through the press nip 6.
  • a count of 14 cavities 110 per inch in both the machine and cross machine direction of the belt may be used.
  • the solid elastomer matrix member 100 of Fig. 5 may be Shore A scale hardness 85
  • the particles 116 v/ithin cavities 110 may be Shore A scale 90, the harder particle material offsetting the more deformable nature of the particles which have only point to point contact with each other, there being void spaces between particles 116.
  • the particles 116 used to provide the filler v/ithin the cavities 110 may be polyurethane having an average particle size of more than 600 micrometers.
  • the sheet contact surface 106 may be composed of similar plastic material, but v/ith an average particle size of less that 600 micrometers which provides a sheet contacting surface having pores which are sufficiently fine to provide a sheet contacting surface which is permeable to pressurized gas and substantially impermeable to liquid as discussed herein.
  • the void volume of the sheet contact surface may be minimized as required to provide only minute crevices between densely packed particles as a result of time, temperature and pressure exposure during formation of the sheet contact surface.
  • the sheet contact surface of the belt may be compressed under heat and pressure to develop optimum resistance to flow such that under the nearly instantaneous dynamic conditions of high pressure exposure in passing through a press nip, flow passageways are restricted so as to only permit passage of very low viscosity air from voids within the air pumping belt into the sheet, while preventing the entry of water into the belt. Density of the sheet contact surface may be further increased by resin treatment.
  • any of the methods described herein of providing the sheet contacting surface may be considered for use with any embodiment of the present invention.
  • Other methods such as treatment with polymeric resins, elastomeric resins, partial sealing of the surface with lower melting temperature particles, combining resin treatment with fusible fibers in the matrix, as well as other methods may also be used.
  • nip pressure on a modern paper machine press is typically in the range of from one to three milliseconds. Even with an extended nip or shoe press, the pressure duration is still only about four to six milliseconds.
  • the air pumping belt of the present invention will be exposed to zero mechanical pressure for several hundred times the nip exposure time during the return run and before re-entry into the press nip. During this relatively long time interval, the resilient body portion of the belt will have ample time to recover from pressure deformation causing void spaces v/ithin the belt to fill with air prior to re-entry into the nip.
  • the air pumping belt of the present invention will be tailored to suit each particular position it is to be used on to provide optimum performance in improving sheet dewatering. It is believed that improvements in sheet consistency on the order of about tv/o to three consistency percentage points may be possible through the use of the air pumping belt of the present invention. In addition to an improvement in sheet dewatering, improvements in other sheet properties may also be achieved through use of the air pumping belt of the present invention. For example, sheet mark due to roll grooves below the felt may be significantly reduced because the air pumping action of the belt may eliminate wet streaks that would otherwise occur in the sheet above press roll grooves.

Abstract

An endless air pumping belt (42) for a paper machine press section includes a void containing collapsible resilient elastomeric body portion (48) and a sheet contacting surface layer (44) which is permeable to pressurized gas and substantially impermeable to liquid during passage through a press nip.

Description

AIR PUMPING BELT
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an endless air pumping belt for the removal of water from a web or sheet in a press section of a paperma ing machine, and a press section which includes, and a method of using, such an endless air pumping belt. The terms "web" and "sheet" are terms used interchangeably in the papermaking industry and are used interchangeably herein.
2. Description of the Prior Art
In a typical papermaking operation, the paper sheet is customarily pressed in the press section of a papermaking machine between two press rolls while it is being supported and conveyed on a porous press felt through the nip formed by such press rolls. Various types of press felts for use in the press section are known. One known type is commonly referred to as a needled felt; that is, a construction which typically includes a woven base fabric to which a batt has been needled. The body of such porous press felts absorbs water through the sheet contacting surface of the felt during the pressing operation and therefore must retain sufficient void space under nip pressure to accommodate water being expressed from the paper web. In addition, felt flow resistance must be low to allow water to flow into the water absorbing press felt .
Another example of a known press felt is one which includes a textile base and a layer of flexible, polymeric resin particles which are fused to each other and to the textile base, leaving pores between the particles, such pores being provided to receive water removed from the paper web during the pressing operation. An example of such a composite press felt is Dutt, U.S. patent no. 4,571,359 wherein it is stated that such composite press felts are an improvement over the prior art in regard to compression/recovery properties and uniform void size and distribution. Such press felts absorb water like a sponge through the sheet contacting surface of the felt during the pressing operation, and like a needled felt must be porous to water, must retain sufficient void space under nip pressure to accommodate the water, and must have low felt flow resistance to allow water to flow therein.
The present situation in press dewatering of a paper web or sheet is that while very significant advances have been made in almost every aspect of papermaking, mechanical pressing to remove water from a paper web or sheet has progressed only to a limited extent. At the present time, the removal of water by mechanical wet pressing produces a sheet with a consistency of about 50%, despite the ability to utilize very high press loadings. It is commonly believed that this limitation in the extent to which water can be extracted from the paper web or sheet by mechanical pressing is mainly due to the effect of sheet "rewetting" after the mid-nip. An understanding of press dewatering is useful in understanding sheet rewetting after the mid-nip.
In a typical papermaking operation, as the mechanical pressure at the press nip compresses the sheet and felt, water is expressed from the sheet into the pore spaces of the felt. Under maximum press load during mid-nip passage; that is, at the middle or mid-point of the press nip where the distance between the two press rolls is at a minimum, a large portion of the water formerly contained within the pore spaces of the sheet is squeezed or expressed from within the sheet and caused to reside within the interface between sheet and press felt, and within the void spaces of the press felt. During post mid-nip passage; that is, immediately upon passing beyond the mid-point of the press nip, the rolls begin to diverge, and in the case of present conditions in wet pressing of paper, a vacuum is created due to the increase in volume between facing pairs of press rolls effected by the expanding nip caused by the divergence of the rolls. As a result of this vacuum, and because the capillaries within the sheet are finer than those within conventional press felts, the pressure differential caused by the widening nip is filled by air entering from the felt side of the nip. The fact that the felt is a more open substrate relative to the sheet and therefore provides easy egress for air to enter the system also contributes to this condition. As a result of the vacuum being relieved from the felt side rather than from the sheet side of the press nip, the vacuum is present longer adjacent to the sheet surface, allowing a substantial part of the water just expressed from the sheet to reenter the sheet thereby causing an appreciable amount of sheet rewetting after the mid-nip and a decrease in sheet consistency as the sheet leaves the press nip.
Some researchers recognize the desirability of introducing air under modest pressure into the nip, just at the point where the nip starts to open up, to overcome the problem of rewetting after the mid-nip. In particular, it has been reported that 65% dryness in milliseconds at room temperature has been achieved using a press simulator. To this end, a "brief pulse of modestly pressurized air, applied at the mid nip while the sheet is compressed causes the free water to move out of the sheet and into the felt" (Recent Highlights in Paper Technology, Douglas Wahren, TAPPI Journal, March 1986). However, heretofore no practical method of introducing air under pressure into a press nip of a papermaking machine press section has been available.
In the processing of wet textile materials, a problem similar to the rewetting problem discussed above exists. Various attempts have been made to increase the dryness of such textile materials beyond that which can be achieved by conventional pressing using pairs of steel, rubber or urethane rolls. One such attempt is described in Masuda, United States patent no. 4,535,611, which relates to the utilization of a system of two cooperating press rolls comprising axially mounted molded fiber web nonwoven porous discs wherein the textile to be dried passes between such two press rolls under pressure. According to Masuda, one of the two press rolls is supplied v/ith a pressurized fluid such as air, and the cooperating press roll is supplied with a vacuum, the first roll serving as a source of air to displace water from the textile fabric, and the vacuum roll serving to remove the expressed liquid from the press roll system. Such a system is not believed to be adaptable to a papermaking operation for several reasons. First, the weak nature of the paper web would make it impossible for the sheet to survive passage into and through a press nip where compressed air was being introduced, without the sheet being destroyed. For example, in a press dewatering step in papermaking, unlike textiles, the paper sheet does not have enough strength to support itself through pairs of press rolls v/ithout self destruction caused by forces in operation at the press nip. The unsupported sheet would likely be extruded back out of the nip entryway, particularly when one of the press rolls is forcing air into the nip.
Secondly, the extremely high speeds used in making paper make it impossible to process unsupported wet paper sheets into and through pairs of press rolls as described by Masuda. In particular, in Masuda a bonded fiber web axially compressed nonwoven vacuum roll is provided in direct contact with the textile, to absorb and remove the expressed water. But in the case of paper machines, operating at many times the speed of textile machinery, there would be insufficient time for such a vacuum roll to absorb and convey away the moisture from the press nip. Further, an arrangement similar to that described in Masuda would not prove satisfactory in present day high speed papermaking, because the roll surface would not have time to rid itself of sufficient water before the next revolution of the press roll. Finally, should a vacuum roll be used in combination v/ith the pressurized roll as taught by Masuda, the vacuum roll would cause the sheet to stick to the surface of the roll, and break down, rather than continue to pass through the press nip on to the next step in the papermaking process.
In an effort to reduce rewetting in the manufacture of paper, Lundstrom, US patent no. 4,588,475, describes a water impermeable resilient mat which is passed through a press nip next to one side of the sheet while a conventional press felt contacts the opposite side. This mat is stretched as it emerges from the nip to shorten the time that the felt remains in pressure contact with the sheet, thereby reducing the amount of water that can transfer back into the sheet during exit from the nip. This mat is subject to constant stretching and relaxing that may result in early failure. In addition, the effects of shortening the nip length after the mid-nip point may be very limited in terms of its effect on rewetting and sheet moisture content.
Press felts are known which have barrier layers which limit the flow of water therethrough or which provide patterned voids. Press felts are also known which are membrane-like. However, such press felts are permeable to liquid and act as a receptor of water expressed from a wet sheet or web.
One object of the present invention is to provide an air pumping belt, for use in a press section, which does not serve as a receptor for liquid expressed from a wet sheet being moved through a press nip in such press section yet allows passage of pressurized gas through the air pumping belt into the sheet.
Another object of the present invention is to provide such an air pumping belt which pumps air into a wet sheet as the sheet is being moved through a press nip in a press section.
A further object of the present invention is to mix air with water which is present in a sheet as the sheet is being moved through a press nip in a press section to form a momentary froth within the sheet so that less pure v/ater remains in the sheet.
Yet another object of the present invention is to expunge water which is present in a sheet and replace such expunged water
Another object of the present invention is to eliminate post mid-nip vacuum which occurs during conventional dewatering of a sheet as the sheet is moved through a press nip in a press section so that the water once expressed from the sheet does not reenter the sheet.
A further object of the present invention is to improve sheet consistency of a sheet exiting from a press nip.
Another object of the present invention is to provide a slight burst of air at the time that a sheet is under nip pressure, to displace at least some of the water that resides within the sheet pore spaces.
Yet another object of the present invention is to provide an air pumping belt for extended nip presses that will reduce or eliminate the vacuum next to a plain press roll surface that draws water back into the sheet from the felt by supplanting the vacuum with positive air pressure, thereby eliminating rewetting.
A further object of the present invention is to provide a press section of a papermaking machine which includes an air pumping belt of the present invention.
Another object of the present invention is to provide an improved method of dewatering a sheet in a press nip of a papermaking machine by pumping air from an air pumping belt into a sheet in a press nip.
SUMMARY OF THE INVENTION This invention achieves these and other objectives by providing an endless air pumping belt for a press section, comprising a first surface, the first surface being permeable to pressurized gas and substantially impermeable to liquid, and an opposite second surface. A body portion is provided which extends from the first surface to the second surface. The body portion consists essentially of means for exhausting a burst of air when the body portion is subjected to nip pressure, from the body portion through the first surface at a first segment of the body portion being subjected to nip pressure in a press nip, sufficient to effect at the first segment (a) displacement of water from a sheet contacting the first surface and (b) mixture of air with water in the sheet, and for intaking air into the body portion through the first surface and/or the second surface at a second segment of the body portion not being subjected to nip pressure in a press nip.
This invention further achieves the above and other objectives by providing a press section which comprises a first press roll and a second press roll forming a press nip. A press felt extends through the press nip for carrying a sheet through the press nip, the press felt having one surface structured and arranged to engage a first side of the sheet during operation of the press section. An endless air pumping belt extends through the press nip, the endless belt comprising a first surface structured and arranged to engage a second side of the sheet during operation of the press section. The first surface is permeable to pressurized gas and substantially impermeable to liquid. The endless air pumping belt also includes an opposite second surface, and a body portion which extends from the first surface to the second surface. The body portion consists essentially of means for exhausting a burst of air, from the body portion through the first surface at a first segment of the body portion being subjected to nip pressure in the press nip, sufficient to effect at the first segment (a) displacement of water from the sheet and/or (b) mixture of air with water in the sheet, and for intaking air into the body portion through the first surface and/or the second surface at a second segment of the body portion not being subjected to nip pressure in the press nip.
This invention further achieves the above and other objectives by providing a method of dewatering a sheet in a press section of a papermaking machine, the press section including an endless press felt and an endless air pumping belt which travel through a press nip formed by a first press roll and a second press roll of the press section, comprising the steps of intaking air into the endless air pumping belt at a segment of the endless air pumping belt not being subjected to nip pressure in the press nip; sandwiching a sheet between the press felt and the endless air pumping belt; moving the sandwiched sheet, press felt and endless air pumping belt through the press nip; exhausting a burst of air in the press nip, from the endless air pumping belt into the sheet, causing in the press nip (a) displacing of water from the sheet and (b) mixing of air with water in the sheet.
BRIEF DESCRIPTION OF THE DRAWINGS This invention may be clearly understood by reference to the attached drawings wherein like elements are designated by like reference numerals and in which:
Fig. 1 is a view of a press section employing a schematically illustrated air pumping belt of the present invention;
Fig. 2 is a view of a portion of a press section and one embodiment of an air pumping belt of the present invention;
Fig. 3 is a cross section view of another embodiment of the air pumping belt of the present invention viewed in the machine direction;
Fig. 4 is a cross section view of yet another embodiment of the air pumping belt of the present invention viewed in the machine direction;
Fig. 5 is a cross section view of another embodiment of the air pumping belt of the present invention viewed in the machine direction;
Fig. 6 is a view of the embodiment of the 'present invention of Fig. 5 taken along line 6-6; and
Fig. 7 is a view of the embodiment of the present invention of Fig. 5 taken along line 7-7. DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of this invention which is illustrated in Fig. 1 is particularly suited for achieving the objects of this invention. Fig. 1 depicts a press section of a papermaking machine including a conventional first press roll 2 and a conventional plain second press roll 4 which cooperate to form a press nip 6. Various guide rolls are provided such as guide rolls 8, 10 and 12. It will be apparent to those skilled in the art that more or less guide rolls may be provided if desired. A conventional endless press felt 14 extends about press roll 2 and guide rolls 8, 10, and 12 and is caused to travel in the machine direction 16 in the press nip in a conventional manner. A sheet 18 carried by a conventional upstream endless web- carrying medium 20 travelling in direction 22 around rolls which include, for example, roll 24, is transferred from the medium 20 to a surface 26 of the press felt 14 at a nip 28 provided by rolls 8 and 24 in a conventional manner. Movement of the press felt 14 causes the press felt to extend, and carry the sheet 18, through the press nip 6.
Fig. 1 depicts a schematically illustrated air pumping belt 30 of the present invention extending through press nip 6. Belt 30 includes a void containing collapsible body portion 32, a sheet contacting surface 34 that is permeable to gas and substantially impermeable to liquid, and a roll contacting surface 36. In Fig. 1, air pumping belt 30 travels through press nip 6, and about guide rolls 38 and 40.
Fig. 2 is an enlarged view of a portion of a press section of a papermaking machine similar to Fig. 1 and includes details of one embodiment of an air pumping belt of the present invention. In particular, an air pumping belt 42 enters and extends through a press nip 6 with a sheet contacting surface 44 coming into pressure contact with a sheet 18 while a felt 14 presses against the other side of the sheet. The surface 44 is permeable to pressurized gas and substantially impermeable to liquid, as described in more detail hereinafter. Belt 42 also includes an opposite roll contacting surface 46. A body portion 48 extends from surface 44 to surface 46. Body- portion 48 consists essentially of means for exhausting a burst of air when the body portion is subjected to nip pressure, from the body portion through the surface 44 at the segment of the body portion being subjected to nip pressure in the press nip 6. Such burst of air is sufficient to effect at the segment of the body portion in the press nip 6 (a) displacement of water from the sheet 18 contacting the first surface 44 and (b) mixture of air with water in the sheet 18. Such means is also provided for intaking air into the body portion 48 through surface 44 and/or surface 46 at the segment of the body portion 48 not being subjected to nip pressure in press nip 6. The exhausting and intaking means of the embodiment of Fig. 2 may include a resilient, elastomeric material comprising void spaces such as, for example, discrete elastomeric particles 50 scattered throughout the body portion 48 and separated by void spaces 52. Such particles may comprise interconnected surfaces and may be substantially equal or vary in size and composition. For example, particles 50 may be polyurethane particles having surfaces which are bonded together as for example, by fusion bonding or sintering and the like. The void spaces within the body portion of the belt are capable of being collapsed under nip pressure so as to expel air, which is contained in the segment of the belt passing through the nip, into the sheet during passage through the press nip. Once the compressed segment of the air pumping belt emerges from the pressure in the nip the resileint body portion v/ill recover from deformation and the void spaces v/ill fill v/ith air before reentry of such segment into the nip.
Under ideal conditions, air pressurized within collapsed air pumping belt void spaces v/ill continue to escape from the air pumping belt for a brief instant beyond the application of mechanical pressure because of the compressible nature of gases and the delayed elastic recovery properties of elastomeric materials. Thus, a major advantage of the air pumping belt of the present invention is the ability to affect the sheet 18 by delivery of positive air pressure for a brief instant within the press nip 6 and beyond the duration of the mechanical pressure pulse. It is believed that this momentary continuation of pneumatic pressure beyond the duration of mechanical pressure eliminates vacuum at the sheet-felt interface which otherwise would occur in the expanding press nip egress, acting in this way to prevent sheet rewetting. After exiting the press nip, and all during the return run, resilience of the elastomeric materials within the air pumping belt causes the collapsed void spaces to once again recover from deformation and fill with air prior to re-entry into the press nip.
In fabricating an air pumping belt of the present invention, the void spaces within the air pumping belt may be formed by uniformly depositing fusion bondable elastomeric particles on top of a support surface and then sintering or fusing the particles together under pressure at a temperature above the softening temperature but below the degradation temperature of the particles, to form a strong cohesive structure. Strength may be added by using a base fabric as the support surface, the particles being fused together and to the base fabric.
The void spaces within the air pumping belt are preferably interconnected and vented to the sheet contacting surface through micro-porous passageways v/hich are permeable to pressurized air but substantially impermeable to water under the dynamic short term pressure pulse conditions caused by passage through the press nip. The air pumping belt of the present invention cannot serve as a water receptor, because fluid flow passageways at the sheet contacting surface are too small to allow for any significant amount of water to flow into the belt during the brief period of mechanical pressure exposure in the press nip. Furthermore, most of the void spaces within the air pumping belt will contain pressurized air and v/ill collapse during entry into the nip.
In an alternative embodiment , the elastomer material may be in the form of a discontinuous elastomeric resin treatment or in some other form, provided that in all cases there are void spaces within the body portion of the belt capable of being collapsed under nip pressure so as to expel air, which is contained in the segment of the belt passing through the nip, into the sheet during passage through the press nip and to recover from deformation so that the void spaces will fill with air before reentry of such segment into nip. The void spaces may be interconnected to facilitate venting of the belt under nip pressure through the sheet contact surface layer of the belt.
In an alternative embodiment, an air pumping belt of the present invention may be provided having a body portion similar to body portion 48 and containing a woven reinforcement, and/or reinforcement fibers, for strength and stability. For example, in the embodiment depicted in Fig. 3, an air pumping belt 54 of the present invention is provided which includes a woven fabric reinforcement 56 within the body portion 58 to facilitate belt stability in operation on the press. In Fig. 3, resilient body portion 58 includes void spaces 60 and elastomeric resin material in the form of discrete particles 62 scattered throughout the body portion. Reinforcement fibers 64 are also provided in the body portion 58. The embodiment depicted in Fig. 3 may be made by needling successive layers of fibers into the base fabric 56, while adding fine elastomer particles between fiber applications. Particles may be dispersed more uniformly throughout the fiber matrix with the aid of vibratory means. After the requisite fiber applications have been completed, additional particles may be added and the entire structure may be consolidated and compacted by further needling, followed by heat and pressure treatment to induce sintering and fusion bonding of particles and the assembly.
In some cases, the sheet contact layer 68 may be further perfected by the application of a discontinuous resin treatment 66 which when cured, forms a tough surface that under nip pressure, allows for the escape of compressed air from voids within the belt while being substantially impermeable to the admittance of water from the sheet during the brief press nip mechanical pressure exposure.
It should be understood that whereas mechanical pressure in the press nip persists only while the press rolls are mechanically in contact with felt, sheet, and air pumping belt, the pneumatic pressure induced within the partly collapsed pore spaces of the air pumping belt does not drop instantaneously to zero upon exit from the press nip. Pneumatic pressure persists for a brief instant beyond mechanical pressure, while entrapped compressed air within the belt continues to escape from internal pore spaces into the sheet surface.
In a like manner, during the return run of the air pumping belt back to the press nip, the delayed elastic recovery properties of the elastomeric components in the belt cause the collapsed void spaces to recover from deformation, inducing a slight vacuum, so that these internal void spaces expand and fill with air prior to reentry into the press nip.
In the example shown in Fig. 3, after passage through nip 6, the collapsed segment of the air pumping belt 54 recovers from compression and fills with air through both the sheet contacting surface 68 and the roll contacting surface 70 prior to re- entering the nip. This exposure to air from both sides of the belt may be a preferred embodiment, for example, where the press configuration limits the length of the belt that can be used, or where the sheet is particularly thick, or v/here for some other reason it is desired to have the air pumping belt contain more air than could enter the non-compressed segment of the belt within the time constraints of machine speed and press configuration if air entry into such segment prior to the reentry of the belt into the press nip was restricted to only the sheet contacting side 68 of the belt. In the example shown in Fig. 3, roll contacting surface 70 of the air pumping belt 54 is of the same material as the body portion 58. In this case, where the sheet contacting surface 68 and the roll contacting surface 70 of the air pumping belt 54 are permeable to air, it is preferable for the air pumping belt to wrap around the surface of a plain press roll 4 sufficiently to create an effective seal, as shown in Fig. 2 regarding belt 42, for preventing the escape of air entrapped in the belt from being expelled through surface 70 and into the atmosphere as the belt enters the press nip 6. Upon nip entry, air prevented from escaping from the air pumping belt at surface 70 by roll 4 is forced under pressure in nip 6 to escape only through sheet contacting surface 68. The embodiment depicted in Fig. 3 is a cross section viewed in the machine direction of the belt 54. A cross section of belt 54 viewed in the cross-machine direction would be similar to Fig. 3.
In an alternative embodiment of the present invention depicted in Fig. 4, an air pumping belt 72 is provided which includes a reinforcement woven base fabric 74 embedded within a body portion 76. In this embodiment, body portion 76 comprises elastomeric particles such as urethane 78 bonded or interconnected together at points of contact by fusion bonding or sintering so as to form a strong durable matrix having void spaces 80 between particles. When such void spaces are formed between particles that are joined at points of contact, void spaces vent for the most part either directly or indirectly through the sheet contact surface of the belt. Some of such points of contact are illustrated at 84.
In the embodiment of Fig. 4, roll contacting surface 86 is rendered completely impermeable, for example, by melting a solid layer of fusible urethane elastomer into the roll contacting surface of the air pumping belt. Thereafter, sheet contacting surface 82 serves as the only path for entry of air into pore spaces 80 between particles 78. In the example of Fig. 4, sheet contacting surface 82 includes a matrix of closely joined elastomeric particles, such as polyurethane particles 88 embedded within a urethane resin treated layer 90, with micro porous flow passageways 92 between solid matter. The porosity of the sheet contacting surface 82 can be adjusted as desired to provide the prerequisite flow properties such that under nip pressure in nip 6, fluid flow passageways 92 will allow only for the escape of compressed air from collapsing void spaces 80 in the body portion 76 of the air pumping belt, while preventing the flow of water from a sheet 18 through surface 82 and into the air pumping belt. In the embodiment of Fig. 4, to provide good void volume as well as uniform pressure against the sheet, body portion 76 includes larger urethane particles near the roll contacting surface 86 and smaller particles near the sheet contacting surface 82.
The air pumping belt of Fig. 4 may be the preferred embodiment when, for example, the belt cannot wrap the top roll or when the escape of any contained compressed air to atmosphere other than through the paper sheet v/ould reduce operating efficiency.
Variables in construction of the air pumping belt of the present invention include, without limitation, elastomer hardness and type, pore size and flow resistance of the sheet contact surface, contained void volume, deformation response to nip pressure, and recovery properties after deformation. All of these variables can be specified and controlled by the design engineer .
The embodiment depicted in Fig. 4 is a cross section viewed in the machine direction of belt 72. A cross section of belt 72 viewed in the cross-machine direction v/ould be similar to Fig. 4.
In use, the fluid flow passageways in the sheet contacting surface of an air pumping belt of the present invention are extremely flow restrictive, substantially permitting only the passage of air from cavities in the air pumping belt into the paper sheet during passage through the press nip, hile at the same time, substantially preventing the flow of water from the sheet into the belt. Under nip pressure, water from the sheet is driven into the much larger void spaces of a felt 14. In many cases, felt void space may be augmented by additional void space provided by a press roll 2 which is grooved. It should be noted that passage of water into a felt 14 is not restricted by narrow entry flow paths that prevent liquid flow, as in the case with the sheet contacting surface of the air pumping belt.
Figs. 5 to 7 illustrate another embodiment of the present invention incorporating a nonv/oven substrate 94 of a type, for example, taught in United States patent no. 4,740,409. In particular, an air pumping belt 94 is depicted which includes a roll contacting surface 96 formed by an outer portion 98 of a solid elastomeric matrix member 100. The solid elastomeric matrix member 100 includes an inner portion 104 which comprises the body portion of the belt. Inner portion 104 faces towards the sheet contacting surface 106. Such inner portion 104 includes a plurality of spaced wall segments 108 which are defor able, resilient and impervious. The wall segments 108 extend towards the sheet contacting surface 106 and are arranged to form a plurality of open-ended cavities 110 which extend and open in the direction 112 towards the sheet contacting surface 106. The sheet contacting surface 106 is permeable to pressurized gas and substantially impermeable to liquid. In use, when compressed in a nip 6, cavities 110 vent air in the direction 112, which may be substantially perpendicular to surface 106, through the sheet contacting surface 106 and into a sheet 18. The spaced wall segments 108 prevent lateral movement of the compressed air within the body portion of the belt. In a preferred embodiment, machine direction strands 114 may be embedded in the elastomeric matrix member 100. The cavities 110 may be at least partially filled v/ith elastomeric particles 116, if desired. In such an embodiment, v/hen subjected to pressure in a nip 6, the elastomeric material of the inner portion 104 will collapse causing respective wall segments 108 to partly fill the voids within the cavities 110. The elastomeric particles 116 within cavities 110 also deform during nip pressure exposure, so that the combined response is to substantially entirely eliminate all of the void spaces within the structure during passage through the press nip 6. As a result, most if not all of the air contained within the belt cavities 110 will be expelled under pressure through the sheet contact layer 106, thereby producing a momentary burst of air pressure into sheet 18 at the surface of the sheet adjacent the air pumping belt. This positive pressure burst drives water from the sheet into the felt 14, mixes air with the water in the sheet, prevents sheet rewetting upon exiting the nip 6, and provides improved sheet dewatering in the press section. In the preferred embodiment particles 116 in each cavity 110 will be interconnected as described herein.
In one example of the belt depicted in Fig. 5, a count of 14 cavities 110 per inch in both the machine and cross machine direction of the belt may be used. In order to produce even pressure against the sheet 18, it is useful to vary the hardness of the several belt components. For example, the solid elastomer matrix member 100 of Fig. 5 may be Shore A scale hardness 85, while the particles 116 v/ithin cavities 110 may be Shore A scale 90, the harder particle material offsetting the more deformable nature of the particles which have only point to point contact with each other, there being void spaces between particles 116. The particles 116 used to provide the filler v/ithin the cavities 110 may be polyurethane having an average particle size of more than 600 micrometers. The sheet contact surface 106 may be composed of similar plastic material, but v/ith an average particle size of less that 600 micrometers which provides a sheet contacting surface having pores which are sufficiently fine to provide a sheet contacting surface which is permeable to pressurized gas and substantially impermeable to liquid as discussed herein. The void volume of the sheet contact surface may be minimized as required to provide only minute crevices between densely packed particles as a result of time, temperature and pressure exposure during formation of the sheet contact surface. For example, the sheet contact surface of the belt may be compressed under heat and pressure to develop optimum resistance to flow such that under the nearly instantaneous dynamic conditions of high pressure exposure in passing through a press nip, flow passageways are restricted so as to only permit passage of very low viscosity air from voids within the air pumping belt into the sheet, while preventing the entry of water into the belt. Density of the sheet contact surface may be further increased by resin treatment.
Any of the methods described herein of providing the sheet contacting surface may be considered for use with any embodiment of the present invention. Other methods such as treatment with polymeric resins, elastomeric resins, partial sealing of the surface with lower melting temperature particles, combining resin treatment with fusible fibers in the matrix, as well as other methods may also be used.
The duration of nip pressure on a modern paper machine press is typically in the range of from one to three milliseconds. Even with an extended nip or shoe press, the pressure duration is still only about four to six milliseconds. In contrast to the very brief residence time in the press nip, the air pumping belt of the present invention will be exposed to zero mechanical pressure for several hundred times the nip exposure time during the return run and before re-entry into the press nip. During this relatively long time interval, the resilient body portion of the belt will have ample time to recover from pressure deformation causing void spaces v/ithin the belt to fill with air prior to re-entry into the nip. The air pumping belt of the present invention will be tailored to suit each particular position it is to be used on to provide optimum performance in improving sheet dewatering. It is believed that improvements in sheet consistency on the order of about tv/o to three consistency percentage points may be possible through the use of the air pumping belt of the present invention. In addition to an improvement in sheet dewatering, improvements in other sheet properties may also be achieved through use of the air pumping belt of the present invention. For example, sheet mark due to roll grooves below the felt may be significantly reduced because the air pumping action of the belt may eliminate wet streaks that would otherwise occur in the sheet above press roll grooves.
The embodiments which have been described herein are but some of several which utilize this invention and are set forth here by way of illustration but not of limitation. It is apparent that many other embodiments which v/ill be readily apparent to those skilled in the art may be made without departing materially from the spirit and scope of this invention.

Claims

I c la im :
1. An endless belt for a press section, comprising a first surface, said first surface being permeable to pressurized gas and substantially impermeable to liquid, an opposite second surface, and a body portion extending from said first surface to said second surface, said body portion consisting essentially of means for exhausting a burst of air when said body portion is subjected to nip pressure, from said body portion through said first surface at a first segment of said body portion being subjected to nip pressure in a press nip, sufficient to effect at said first segment (a) displacement of water from a sheet contacting said first surface and (b) mixture of air with water in said sheet, and for intaking air into said body portion through at least one of said first surface and said second surface at a second segment of said body portion not being subjected to nip pressure in a press nip.
2. The endless belt of claim 1 wherein said second surface is impermeable.
3. The endless belt of claim 1, wherein said second surface is permeable.
4. The endless belt of claim 1, wherein said exhausting and intaking means includes an elastomeric material comprising interconnected void spaces.
5. The endless belt of claim 4, wherein said elastomeric material includes polyurethane particles.
6. The endless belt of claim 4, wherein said polyurethane particles comprise interconnected surfaces.
7. The endless belt of claim 4 wherein said polyurethane particles comprise surfaces interconnected by sinter bonds.
8. The endless belt of claim 1, wherein said body portion includes at least one of a plurality of machine direction reinforcement strands and a plurality of cross-machine direction reinforcement strands.
9. The endless belt of claim 1, wherein said body portion includes a plurality of reinforcing fibers.
10. The endless belt of claim 4, wherein said body portion includes at least one of a plurality of machine direction reinforcement strands and a plurality of cross-machine direction reinforcement strands.
11. The endless belt of claim 10, wherein said body portion includes a plurality of reinforcing fibers.
12. The endless belt of claim 11, wherein said elastomeric material includes polyurethane particles comprising interconnected surfaces.
13. The endless belt of claim 12 wherein said second surface is impermeable.
14. The endless belt of claim 12, v/herein said second surface is permeable.
15. The endless belt of claim 2 further including an outer portion which comprises said second surface, and an inner portion which comprises said body portion, said inner portion facing towards said first surface and consisting essentially of a plurality of spaced wall segments v/hich are deformable, resilient and impervious, and which extend towards said first surface, said plurality of spaced wall segments arranged to form a plurality of open-ended cavities which extend and open towards said first surface.
16. The endless belt of claim 15 v/herein each cavity of said plurality of cavities is at least partially filled with elastomeric material comprising interconnected void' spaces.
17. The endless belt of claim 16 wherein said elastomeric material comprises particles.
18. The endless belt of claim 16 wherein said elastomeric material comprises particles having interconnected surfaces.
19. The endless belt of claim 15 wherein each spaced wall segment of a plurality of said plurality of spaced wall segments contains at least one machine direction reinforcement strand.
20. A press section, comprising: a first press roll and a second press roll forming a press nip; a press felt extending through said press nip for carrying a sheet through said press nip, said press felt having one surface structured and arranged to engage a first side of said sheet during operation of said press section; and an endless belt extending through said press nip, said endless belt comprising a first surface, structured and arranged to engage a second side of said sheet during operation of said press section, said first surface being permeable to pressurized gas and substantially impermeable to liquid, an opposite second surface, and a body portion extending from said first surface to said second surface, said body portion consisting essentially of means for exhausting a burst of air when said body portion is subjected to nip pressure, from said body portion through said first surface at a first segment of said body portion being subjected to nip pressure in said press nip, sufficient to effect at said first segment (a) displacement of water from said sheet and (b) mixture of air v/ith water in said sheet, and for intaking air into said body portion through at least one of said first surface and said second surface at a second segment of said body portion not being subjected to nip pressure in a press nip.
21. A method of dewatering a sheet in a press section of a papermaking machine, the press section including' an endless press felt and an endless air pumping belt which travel through a press nip formed by a first press roll and a second press roll of the press section, comprising the steps of: intaking air into said endless air pumping belt at a segment of said endless air pumping belt not being subjected to nip pressure in said press nip; sandwiching a sheet between said press felt and said endless air pumping belt; moving the sandwiched sheet, press felt and endless air pumping belt through said press nip; and exhausting a burst of air in said press nip, from said endless air pumping belt into said sheet, causing in said press nip (1) displacement of water from said sheet and (b) mixing of air with water in said sheet.
PCT/US1999/014355 1998-07-27 1999-06-24 Air pumping belt WO2000006825A1 (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008089118A2 (en) * 2007-01-18 2008-07-24 Lefkowitz Leonard R Anti-rewet transfer belt
WO2008089118A3 (en) * 2007-01-18 2008-12-11 Leonard R Lefkowitz Anti-rewet transfer belt

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