CA1202806A - Operation of a high speed paper making machine - Google Patents
Operation of a high speed paper making machineInfo
- Publication number
- CA1202806A CA1202806A CA000413180A CA413180A CA1202806A CA 1202806 A CA1202806 A CA 1202806A CA 000413180 A CA000413180 A CA 000413180A CA 413180 A CA413180 A CA 413180A CA 1202806 A CA1202806 A CA 1202806A
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- CA
- Canada
- Prior art keywords
- couch
- box
- roll
- fabric
- forming section
- Prior art date
- Legal status (The legal status 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 status listed.)
- Expired
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Abstract
IMPROVEMENT IN THE OPERATION OF
A HIGH SPEED PAPER MAKING MACHINE
ABSTRACT
The disclosure teaches a method of improving the operation of a high speed paper forming machine using synthetic forming fabrics by replacement of the perforated vacuum couch roll with a solid power or drive roll preceded immediately upstream by a specially designed static slotted couch box, having one or more narrow slot openings with land areas of wear resistant material having low coefficient of friction with the forming fabric, and operating with a source of vacuum in the order of 15 to 30 inches of mercury.
A HIGH SPEED PAPER MAKING MACHINE
ABSTRACT
The disclosure teaches a method of improving the operation of a high speed paper forming machine using synthetic forming fabrics by replacement of the perforated vacuum couch roll with a solid power or drive roll preceded immediately upstream by a specially designed static slotted couch box, having one or more narrow slot openings with land areas of wear resistant material having low coefficient of friction with the forming fabric, and operating with a source of vacuum in the order of 15 to 30 inches of mercury.
Description
~Z~28~6 BACKGROUND ~F INVENTION
(a) Field of Invention This invention relates to a method of improving the operation of a high speed paper making machine and, more particularly, to an improved forming section of the machine wherein the conventional perforated vacuum couch roll is re~laced with a fabric support roll having a solid outer surface and a high vacuum static slotted couching box is disposed directly upstream of the said drive roll between a last one of a plurality of dewatering devices and the fabric support roll.
(b) Description of Prior Art As is well known, in a conventional paper making apparatus, pulp stock consisting of a mixture of fibers suspended in water in a proportion typically 1 part fiber to 200 parts water by weight is transformed into a sheet of paper. In the initial section of the apparatus a forming "wire" or "fabric" which is a woven belt of either metal or plastic strands with mesh openings of a size to allow water to drain through, but to retain the fibers on the surface thereof, runs around rollers including a breast roll at the upstream end of the section and a couch roll at the downstream end. The pulp stock is fed evenly onto the surface of the fabric at the upstream end and is carried by the fabric over several different types of conventional dewatering devices such as table rolls, foils and suction boxes which drain water from the pulp through the fabric.
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(a) Field of Invention This invention relates to a method of improving the operation of a high speed paper making machine and, more particularly, to an improved forming section of the machine wherein the conventional perforated vacuum couch roll is re~laced with a fabric support roll having a solid outer surface and a high vacuum static slotted couching box is disposed directly upstream of the said drive roll between a last one of a plurality of dewatering devices and the fabric support roll.
(b) Description of Prior Art As is well known, in a conventional paper making apparatus, pulp stock consisting of a mixture of fibers suspended in water in a proportion typically 1 part fiber to 200 parts water by weight is transformed into a sheet of paper. In the initial section of the apparatus a forming "wire" or "fabric" which is a woven belt of either metal or plastic strands with mesh openings of a size to allow water to drain through, but to retain the fibers on the surface thereof, runs around rollers including a breast roll at the upstream end of the section and a couch roll at the downstream end. The pulp stock is fed evenly onto the surface of the fabric at the upstream end and is carried by the fabric over several different types of conventional dewatering devices such as table rolls, foils and suction boxes which drain water from the pulp through the fabric.
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2 --~L20Z~1~6 At the end of the initial or forming section, the proportion of fibers in the pulp is typically of the order of 15 to 20%. The pulp, now in the form of a wet web, is drawn away or picked up from the forming belt and is transferred to the press section of the paper machine.
In the press section, the web is further dewatered by press rolls which act much like a wringer in an old fashioned washing machine. The web is then passed to a dryer section where the final removal of water takes place by a vaporization process.
As it costs rnuch less to drain water from a sheet than to press it out in the press section or to vaporize it in the dryer section, and as press felts are more effective and last longer if they have less water to remove, it is desirable to attain a more effective removal of water in the forming section. However, the amount of water which can be removed in the forming section is limited because of the need to avoid excessive loss of fibers through the mesh of the forming fabric, since not only water but also fibers may be drawn through the openings in the fabric if the dewatering action is too violent.
In conventiollal paper machines, in order to increase the effectiveness of water removal in the forming section, a vacuum couch roll is used. This vacuum couch roll comprises a large outer shell perforated with a plurality of holes. A suction chamber within the roll is connected through a stationary hollow shaft to a high , vacuum source. The vacuum is communicated from the chamber through the perforations and through the mesh of the wire ~Z0213Q6 or fabric to the web of the water-fiber mixture so that water is drawn off the mixture adjacent the chamber. To insure that an appreciable amount of water is drawn off at the couch roll, a relatively high vacuum is applied at the vacuum chamber within the roll. It will be appreciated that, with such an arrangement, the wet web must be picked-off the wire or fabric on the couch roll just after it passes adjacent the downstream lip of the vacuum chamber or, in the case where the machine is equipped with a vacuum assisted pick-up, on a downward sloping portion of the wire or fabric between the couch roll and a wire turning or forward drive roll as it is usually called.
There are several substantial disadvantages in using vacuum couch rolls. In general, water is drawn out of the web through the holes in the couch shell as they pass over the vacuum chamber. Since the holes constitute only about 22 to 60 percent of the total area of the couch shell, only a corresponding portion of the static pulp web area is subjected to this dewatering action. This is, of course, a very inefficient use of the vacuum in the vacuum chamber and the dewatering is not as adequate as is desirable.
Another disadvantage is that a great amount of power is wasted in continually evacuating air from the holes in the couch roll as they pass into the area over the vacuum chamber within the roll and before any water can be withdrawn from them. For example, in a conven-tional COUch roll operating on a 24 foot wide machine at 2000 feet per minute having a shell 2-5/8 inches thick and 50/O open area, the quantity of air that has to be ~LZ028Q~i continually evacuated from the holes, defined as the "wasted volume of the couch" amounts to 5250 cubic feet per minute. As machine speeds are increased towards 4000 feet per minute the energy required to evacuate this wasted volume becomes economically intolerable.
Another disadvantage is that some droplets of water will remain in the couch holes after they have passed over the downstream lip of the vacuum chamber, and, as there is now no force to draw the droplets inwards, they will be ejected outwards by centrifugal force. Air rushing into these holes from the interior of the roll, not influenced by the vacuum chamber, will aid in the expulsion of the droplets of water. In apparatus using a vacuum assisted pick-up, these droplets impinge on the underside of the wire or fabric and from there are re-introduced into the web thus increasing the proportion of the water in the web and rendering the system less effi-cient because of poor runability from a water-filled pick-up felt.
A i-urther disadvantage of the vacuunl couch roll is that part of the power input to rotate the fabric is used to overcome the friction between the seal strips at the upstream and downstream lips of the vacuum chamber within the couch roll shell. This power loss can be a considerable amcunt because of the high vacuum that must be maintained in the chamber and the high rotational speed of the roll.
In the present design of vacuum couch rolls for high speed paper machines a misting water shower is placed near the upstream seal strip of the vacuum chamber iZ(~28~6 to provide necessary lubrication and sealing. This shower requires constant attention or it tends to apply more water than necessary which, under centrifugal acti~,n, finds its way to the outside surface of the couch roll where it combines with the water already being ejected by the couch, forms a water film, and re-wets the sheet of pulp at the ingoing nip on the roll. The sheet becomes wetter than it was when it left the last conventional suction box. This results in a condition known as "pinholing" whereby the sheet which is too wet becomes perforated over the holes in the couch by the very high vacuum within. As a further complication the re-wetting causes short fibers of the pulp to be pulled vertically downward into the mesh of the fabric because of the sudden application of vacuum and promotes a fuzziness on the fabric side of the sheet. Then, as the sheet passes through the dryers, some of these fibers are broken off leading to excessive dust in the dryer section which can lead to fires or, if the dust adheres to the sheet of paper by static electricity, to difficulties when the paper is printed. Again, if the sudden impact of vacuum is severe enough, fibers can be carried through the fabric and into the holes and vacuum chamber of the couch roll where they get wedged between the shell of the couch and the seal strips resulting in a loss in vacuum. Also these fibers may stick to the sides of the holes, gradually blocking them off, until the machine has to be shut down to be cleaned. The re-wetting also causes another problem known as "wire marking" wherein the mesh of the fabric is impressed into the softened sheet of paper making OZ8~6 printiny very difficult. This also interferes with lifting the sheet off the forming fabric and causes some fibers to be left entwined in the mesh of the fabric where they interfere with drainage.
Another effect of extra water returning to the sheet is a condition called "shell marking". This occurs when the sheet is severely re-wetted and the high velocity of water removal at the perforations of the couch roll leaves an imprint of the couch shell in the paper.
A further disadvantage of the vacuum couch roll is that, as the perforations pass over the extremities of the vacuum chamber a siren effect is produced and a pierc-ing whistle is emitted to the detriment of the operators whose hearing can be affected by the noise pollution which may exceed 90 db. Although efforts have been made to eliminate or modify this sound, it still remains a danger to the hearing of the operators and a bar ~o effective verbal communication in the area of the paper making apparatus.
A further disadvantage of the drilled couch roll with its complicated vacuurn chamber is its extreme high original cost and costly maintenance requirement.
In the early days of paper making the machines were not equipped with a vacuum couch roll having a perforated shell. At that time a machine would usually run at a speed of 800 ft. per min. or less and, with conventional table rolls and suction boxes having drilled tops, there was ample time to drain sufficient water from the pulp on the forming table. Drilled suction box covers having a pattern of holes leading to the vacuum chamber l~Z8~)6 were used because, with the materials available at the time, slotted box covers which provide better sheet coverage, caused excessive drag. The slots also caused premature fracture of the seam of metal forming wires.
S With the introduction of improved dewatering sections, and with devices such as foils, it was possible to run the machines at increased speeds up to about 2500 ft. per minute. The innovation of the vacuum couch roll helped to maintain reasonable fiber consistency (fiber to water ratio) at the increased speed and although there were disadvantages, as outlined above, the system was economically viable.
At the present time, with machine speeds in the range of 3000 ft. per minute or more and particularly with twin wire machines that are capable of even higher speeds, the disadvantages of the conventional perforated vacuum couch roll have become extremely limiting to the efficient production of paper.
I have now found, as an entirely new concept related spec:Lfically to high speed paper machines using synthetic forming fabrics that the inefficient, noisy, expensive vacuum couch roll can be eliminated and replaced with a solid power roll if it is preceded, immediately upstream, by a static slotted couching box of a specific design as described herein.
Earl~er paper machines equipped only with table rolls, deflectors and seven or eight suction boxes and using metal forming wires could not be adapted to this new concept. Now, however, with the use of synthetic forming fabrics which have a relatively low drag factor ~2(~Z8.r~6 and much higher resistance to flexural fatigue and by means of foils a~d/or low vacuum suction boxes it is possible to manage water removal without undue fiber loss with half the number of regular suction boxes at increased machine speeds and also keep power usage under control. Further, the development of the technology of constructing suction box covers having continuous slots with no cross-member supports in the machine direction to cause interference and with narrow fabric supporting surfaces of fine grained ceramics having low coefficient of friction with the fabric, the introduction of the static slotted couch of the invention, operating under high vacuum, is feasible and is adaptable as this new concept to eliminate the disadvantages of the vacuum couch roll.
SUMMARY OF INVENTION
It is therefore a feature of the invention to overcome the disadvantages inherent in the use of the conventional vacuum couch roll on a high speed paper making machine using synthetic forming fabrics.
It is a further feature of the invention to modify the forming section of a paper making apparatus i~
improve the effectiveness of the dewatering process.
It is a still further feature of the invention to provide a static couching box suitable for use in the forming section of a paper making apparatus as modified.
In general, the above features are attained by eliminating the vacuum structure of the couch roll which has become a limiting factor, and providing instead a static slotted couching box adjacent the upper run of a _ g _ ~2028~6 forming section between the dewatering devices thereof and the couch roll thereof, directly adjacent to and upstream of the couch roll. The vacuum couch roll is replaced by a solid couch or turning roll preferably of the same diameter and preferably covered with a friction cover to prevent slippage of the fabric.
The invention also relates to a high vacuum, static couching box arrangement for use in the forming~
section of a paper making apparatus: the couching box being an elongated hollow, enclosed structure with a cover member comprising preferably from 1 to 6 narrow, slotted openings: a bottom wall opposite the cover member and side and end walls: the interior of the box being adapted to communicate with a source of high vacuum through an opening in one of the walls: characterized in that the width of the slotted openings in the cover is in the range of 1/8" to 5/8" and the fabric supporting surfaces between slots is 3/4 of an inch or less. The vacuum maintained in the couching box would be in the order of 15 to 30 inches of mercury. The cover member comprises members of a wear resistant material such as silicon carbide, aluminum oxide or any material having a low coefficient of frlction with the forming fabric and also low abra-siveness.
The invention also relates to an improved forming section and method of dewatering pulp in a paper making machine comprising an endless belt passing around and ovQr a breast roll at one end of the section and a solid unperforated couch roll at the other end of the section, conventional dewatering devices between the lZ028~6 upper and lower runs of the endless belt which lie adjacent and in contact with the upper run and a high vacuum static couching box disposed adjacent to and in contact with the upper run and located between the conventional dewatering devices and the couch roll:
the couching box being as above described.
The reason it is feasible to put a high vacuum couching box following the conventional dewatering devices is that generally, by the time the pulp stock on the forming belt reaches the suction boxes, the web is substantially formed and fiber loss is less likely to occur. Higher velocities of water removal may therefore be tolerated in the area of the suction boxes and parti-cularly downstream of the suction boxes.
According to a broad aspect of the present invention there is provided a high vacuum static couching box for use in a forming sectioan of a high speed paper making machine and positioned in an area between a couch roll having a solid outer surface and a last one of a plurality of dewatering devices in contact with a synthetic forming fabric which is rotated about the roll. The couching box is an elongated hollow box-like structure having a slotted upper surface of wear ~esistant material. Each slot in the box-like structure has a width in the range of from 0.12S to 0.75 of an inch. Means is also provided to connect a high vacuum source to the interior of the box-like structure.
According to a still further broad aspect of the present invention there is provided an improved methocl of dewatering pulp in a forming section of a lZ02~6 high speed paper making machine which uses a synthetic forming fabric belt for supporting the pulp. The forming secticn has a breast roll, a couch roll, and dewatering devices disposed between the rolls in contact with the forming fabric which is rotated about the rolls. The method comprises extracting water from the pulp by means of a high vacuum static slotted couch box supported in a fixed position behind the belt and in contact therewith in an area of the forming section between the last one of the dewatering devices and the couch roll. The couch box has a vacuum in the range of between 15 to 30 inches of mercury. The couch roll is a fabric support roll having a solid outer shell whereby the amount of water remaining in the pulp is substantially unaffected when the pulp passes over the fabric support roll.
BRIEF DESCRIPTION OE DRAWINGS
The invention will be better understood by an examination of the following description, together with the accompanying drawings illustrating an example of the preferred embodiment, in which:
FIGURE 1 is a schematic side elevation of a portion of a forming section of the prior art;
FIGURE 2 is a schematic side elevation of a portion of a forrning section of Figure 1 modified according to the invention, and FIGURE 3 is a perspective view of a static slotted couching box for use in the forming section as modified accordiny to the invention.
~2~28~6 DESCRIPTION OF PREFERRED EMsoDIMENT
Referring to Figure 1, the portion of the forming section of a paper making machine of the prior art, indi-cated generally at 10 includes an endless fabric forming belt 11 having an upper run 12 and a lower run 13~ The belt passes over suction boxes 14, over a sector of vacuum couch roll 15, which is rotated in the direction shown by arrow A, and then around a forward drive roll 16 before returning, over a series of return rolls, not shown, to a breast roll, also not shown. Pulp web 20 partially dewatered, passes on fabric 11 over the sector of the couch roll and is rernoved from the fabric after it leaves the couch roll by a vacuum pick-up roll 21 on pick-up fabric 22. The shell 17 of couch roll 15 is drilled with a plurality of holes 18 through which water is drawn from the pulp web through fabric 11 into vacuum chamber 19 which is located within the couch roll adjacent the sector of the shell over which the fabric 11 passes.
Chamber 19 is equipped with lips 23 and 24 which are in intimate contact with the inner surface of couch shell 17 and is connected to a source of vacuum through outlet 25. In the case of a typical high speed, large paper making machine the vacuum couch roll will be about 4 feet in diameter, the shell about 2-5/8" thick, the drilled holes about 1/4" in diameter and the open area provided by the array of holes from about 22 to 60 percent. The re-wetting problem, discussed above, wherein water is ejected from the holes in the couch shell by centrifugal force, is illustrated at B and C. Numeral 26 indicates a wipe apron which may be installed about 60 to 90 ~Z02~6 before top dead center of the couch roll in an attempt to reduce re-wetting.
Referring now to Figure 2 which shows the portion of the forming section of Figure 1 modified according to the invention, the vacuum couch roll is replaced by a couch roll 30 having an undrilled solid shell 31. A
static slotted couching box 32 is located, as shown, directly upstream of the couch roll and between it and the conventional suction boxes 14. The pulp web 20 may be picked off the fabric over the couch roll at C or prefer-ably before the power roll at D, since this arrangement gives the least angular change and therefore fewer run-nability problems.
It will be apparent that with the modified system re-wetting of the pulp web at the couch roll is eliminated.
Also the very expensive drilled vacuum couch roll can be eliminated. Further, there is no need to equip the machine with a forward drive roll since the warp of the fabric around the solid shelled couch or power roll provides adequate contact to drive the fabric particularly if covered with a friction material. As a result the controls to operate the paper machine become much less complicated.
Figure 3 shows, in simplified form, a perspec-tive view of a static slotted couching box which comprises an elongated, hollow, box-like structure 40, which extends across and beyond the width of the forming fabric, and a slotted cover member or upper surface, indicated generally at 41. The cross-section of the box structure is wide and deep relative to the dimension of the slotted cover member to provide rigidity in the machine direction and also in ~Z~
the direction normal to the plane of the fabric Wi th which it is in contact. The cover member comprises a plurality of elongated fabric supporting members 42 positioned parallel to and spaced from each other ~nd extending the entire length of the box structure~ Mernbers 42 are provided with fine grained ceramic fabric contact-ing surfaces 43. These members are held in recesses 44 in the ends of the box and are spaced by end blocks 45 to form slots 46. End blocks 45 also provide vacuum seals at the edges of the forming fabric. Recesses 44 may be large enough to accommodate up to 7 supporting members and, if less than 7 are employed, a spacer bar 47 is added to fill the excess space. Members 42 are supported and braced within the box in a known manner so that there are no cross-member supports in the machine direction and near the fabric contacting surfaces to cause interference with the effective removal of water from the pulp web on the fabric. The fabric supporting surfaces 43 may comprise plates segments, inserts or coatings of wear resistant cerarnic material having a low coefficient of friction with the forming fabric. These surfaces may be glued on with epoxy resin or attached b,~
any other known method. The slots 46 communicate with the interior of the box-like structure and an opening ~8 is provided in the bottom of the structure which is con-nected to a source of high vacuum which is not shown.
The width of the support members is in the range of 0.375 to 0.75 of an inch, preferably 0.6, and the width of the slots is in the range of 0.125 to 0.75 of an inch.
The number of slots in the static slotted couch will range ~Z(:~Z8~6 from 1 to preferably not more than 6, depending on machine operating conditions, and the vacuum will range from 15 to 30 inches of mercury.
It is possible to use a high vacuum in the couching box in accordance with the invention for two reasons: at the position at which the couching box is disposed, the web is substantially formed so that fibers are not likely to be drawn out of the sheet. In addition, with the use of box covers comprisins n~rrow fabric supporting surfaces of long wearing ceramics, known to have a low coefficient of friction with the fabric, there is virtually no danger of an excess of power being required by the system to overcome drag caused by friction. Further, because the slots of the box 1 are relatively very narrow, and because the high vacuum is applied at only this one place, there is little danger that the belt will be stalled by the action of the high vacuum upon it.
The invention is well adapted for use in the type of machine that is already equipped with a vacuum pick-up and has a forward drive roll located downstream and below the couch roll as shown in Figure 1. It is in this type of machine that rewetting of the web before it is picked-off, as previously described, always occurs.
The invention is also particularly well adapted to substitute for a vacuum couch roll on any twin wire or multi wire paper making machine. It may be used also to improve the efficiency on any standard Fourdrinier machine that is not equipped with a vacuum couch roll.
~LZ~)Z8~
Although a single example of the preferred embodiment was described above, this was or the purpose of illustrating, but not limiting the invention. Various modifications which will come readily to the mind of one skilled in the art, are intended to be covered, provided such modifications fall within the scope of the appended claims.
In the press section, the web is further dewatered by press rolls which act much like a wringer in an old fashioned washing machine. The web is then passed to a dryer section where the final removal of water takes place by a vaporization process.
As it costs rnuch less to drain water from a sheet than to press it out in the press section or to vaporize it in the dryer section, and as press felts are more effective and last longer if they have less water to remove, it is desirable to attain a more effective removal of water in the forming section. However, the amount of water which can be removed in the forming section is limited because of the need to avoid excessive loss of fibers through the mesh of the forming fabric, since not only water but also fibers may be drawn through the openings in the fabric if the dewatering action is too violent.
In conventiollal paper machines, in order to increase the effectiveness of water removal in the forming section, a vacuum couch roll is used. This vacuum couch roll comprises a large outer shell perforated with a plurality of holes. A suction chamber within the roll is connected through a stationary hollow shaft to a high , vacuum source. The vacuum is communicated from the chamber through the perforations and through the mesh of the wire ~Z0213Q6 or fabric to the web of the water-fiber mixture so that water is drawn off the mixture adjacent the chamber. To insure that an appreciable amount of water is drawn off at the couch roll, a relatively high vacuum is applied at the vacuum chamber within the roll. It will be appreciated that, with such an arrangement, the wet web must be picked-off the wire or fabric on the couch roll just after it passes adjacent the downstream lip of the vacuum chamber or, in the case where the machine is equipped with a vacuum assisted pick-up, on a downward sloping portion of the wire or fabric between the couch roll and a wire turning or forward drive roll as it is usually called.
There are several substantial disadvantages in using vacuum couch rolls. In general, water is drawn out of the web through the holes in the couch shell as they pass over the vacuum chamber. Since the holes constitute only about 22 to 60 percent of the total area of the couch shell, only a corresponding portion of the static pulp web area is subjected to this dewatering action. This is, of course, a very inefficient use of the vacuum in the vacuum chamber and the dewatering is not as adequate as is desirable.
Another disadvantage is that a great amount of power is wasted in continually evacuating air from the holes in the couch roll as they pass into the area over the vacuum chamber within the roll and before any water can be withdrawn from them. For example, in a conven-tional COUch roll operating on a 24 foot wide machine at 2000 feet per minute having a shell 2-5/8 inches thick and 50/O open area, the quantity of air that has to be ~LZ028Q~i continually evacuated from the holes, defined as the "wasted volume of the couch" amounts to 5250 cubic feet per minute. As machine speeds are increased towards 4000 feet per minute the energy required to evacuate this wasted volume becomes economically intolerable.
Another disadvantage is that some droplets of water will remain in the couch holes after they have passed over the downstream lip of the vacuum chamber, and, as there is now no force to draw the droplets inwards, they will be ejected outwards by centrifugal force. Air rushing into these holes from the interior of the roll, not influenced by the vacuum chamber, will aid in the expulsion of the droplets of water. In apparatus using a vacuum assisted pick-up, these droplets impinge on the underside of the wire or fabric and from there are re-introduced into the web thus increasing the proportion of the water in the web and rendering the system less effi-cient because of poor runability from a water-filled pick-up felt.
A i-urther disadvantage of the vacuunl couch roll is that part of the power input to rotate the fabric is used to overcome the friction between the seal strips at the upstream and downstream lips of the vacuum chamber within the couch roll shell. This power loss can be a considerable amcunt because of the high vacuum that must be maintained in the chamber and the high rotational speed of the roll.
In the present design of vacuum couch rolls for high speed paper machines a misting water shower is placed near the upstream seal strip of the vacuum chamber iZ(~28~6 to provide necessary lubrication and sealing. This shower requires constant attention or it tends to apply more water than necessary which, under centrifugal acti~,n, finds its way to the outside surface of the couch roll where it combines with the water already being ejected by the couch, forms a water film, and re-wets the sheet of pulp at the ingoing nip on the roll. The sheet becomes wetter than it was when it left the last conventional suction box. This results in a condition known as "pinholing" whereby the sheet which is too wet becomes perforated over the holes in the couch by the very high vacuum within. As a further complication the re-wetting causes short fibers of the pulp to be pulled vertically downward into the mesh of the fabric because of the sudden application of vacuum and promotes a fuzziness on the fabric side of the sheet. Then, as the sheet passes through the dryers, some of these fibers are broken off leading to excessive dust in the dryer section which can lead to fires or, if the dust adheres to the sheet of paper by static electricity, to difficulties when the paper is printed. Again, if the sudden impact of vacuum is severe enough, fibers can be carried through the fabric and into the holes and vacuum chamber of the couch roll where they get wedged between the shell of the couch and the seal strips resulting in a loss in vacuum. Also these fibers may stick to the sides of the holes, gradually blocking them off, until the machine has to be shut down to be cleaned. The re-wetting also causes another problem known as "wire marking" wherein the mesh of the fabric is impressed into the softened sheet of paper making OZ8~6 printiny very difficult. This also interferes with lifting the sheet off the forming fabric and causes some fibers to be left entwined in the mesh of the fabric where they interfere with drainage.
Another effect of extra water returning to the sheet is a condition called "shell marking". This occurs when the sheet is severely re-wetted and the high velocity of water removal at the perforations of the couch roll leaves an imprint of the couch shell in the paper.
A further disadvantage of the vacuum couch roll is that, as the perforations pass over the extremities of the vacuum chamber a siren effect is produced and a pierc-ing whistle is emitted to the detriment of the operators whose hearing can be affected by the noise pollution which may exceed 90 db. Although efforts have been made to eliminate or modify this sound, it still remains a danger to the hearing of the operators and a bar ~o effective verbal communication in the area of the paper making apparatus.
A further disadvantage of the drilled couch roll with its complicated vacuurn chamber is its extreme high original cost and costly maintenance requirement.
In the early days of paper making the machines were not equipped with a vacuum couch roll having a perforated shell. At that time a machine would usually run at a speed of 800 ft. per min. or less and, with conventional table rolls and suction boxes having drilled tops, there was ample time to drain sufficient water from the pulp on the forming table. Drilled suction box covers having a pattern of holes leading to the vacuum chamber l~Z8~)6 were used because, with the materials available at the time, slotted box covers which provide better sheet coverage, caused excessive drag. The slots also caused premature fracture of the seam of metal forming wires.
S With the introduction of improved dewatering sections, and with devices such as foils, it was possible to run the machines at increased speeds up to about 2500 ft. per minute. The innovation of the vacuum couch roll helped to maintain reasonable fiber consistency (fiber to water ratio) at the increased speed and although there were disadvantages, as outlined above, the system was economically viable.
At the present time, with machine speeds in the range of 3000 ft. per minute or more and particularly with twin wire machines that are capable of even higher speeds, the disadvantages of the conventional perforated vacuum couch roll have become extremely limiting to the efficient production of paper.
I have now found, as an entirely new concept related spec:Lfically to high speed paper machines using synthetic forming fabrics that the inefficient, noisy, expensive vacuum couch roll can be eliminated and replaced with a solid power roll if it is preceded, immediately upstream, by a static slotted couching box of a specific design as described herein.
Earl~er paper machines equipped only with table rolls, deflectors and seven or eight suction boxes and using metal forming wires could not be adapted to this new concept. Now, however, with the use of synthetic forming fabrics which have a relatively low drag factor ~2(~Z8.r~6 and much higher resistance to flexural fatigue and by means of foils a~d/or low vacuum suction boxes it is possible to manage water removal without undue fiber loss with half the number of regular suction boxes at increased machine speeds and also keep power usage under control. Further, the development of the technology of constructing suction box covers having continuous slots with no cross-member supports in the machine direction to cause interference and with narrow fabric supporting surfaces of fine grained ceramics having low coefficient of friction with the fabric, the introduction of the static slotted couch of the invention, operating under high vacuum, is feasible and is adaptable as this new concept to eliminate the disadvantages of the vacuum couch roll.
SUMMARY OF INVENTION
It is therefore a feature of the invention to overcome the disadvantages inherent in the use of the conventional vacuum couch roll on a high speed paper making machine using synthetic forming fabrics.
It is a further feature of the invention to modify the forming section of a paper making apparatus i~
improve the effectiveness of the dewatering process.
It is a still further feature of the invention to provide a static couching box suitable for use in the forming section of a paper making apparatus as modified.
In general, the above features are attained by eliminating the vacuum structure of the couch roll which has become a limiting factor, and providing instead a static slotted couching box adjacent the upper run of a _ g _ ~2028~6 forming section between the dewatering devices thereof and the couch roll thereof, directly adjacent to and upstream of the couch roll. The vacuum couch roll is replaced by a solid couch or turning roll preferably of the same diameter and preferably covered with a friction cover to prevent slippage of the fabric.
The invention also relates to a high vacuum, static couching box arrangement for use in the forming~
section of a paper making apparatus: the couching box being an elongated hollow, enclosed structure with a cover member comprising preferably from 1 to 6 narrow, slotted openings: a bottom wall opposite the cover member and side and end walls: the interior of the box being adapted to communicate with a source of high vacuum through an opening in one of the walls: characterized in that the width of the slotted openings in the cover is in the range of 1/8" to 5/8" and the fabric supporting surfaces between slots is 3/4 of an inch or less. The vacuum maintained in the couching box would be in the order of 15 to 30 inches of mercury. The cover member comprises members of a wear resistant material such as silicon carbide, aluminum oxide or any material having a low coefficient of frlction with the forming fabric and also low abra-siveness.
The invention also relates to an improved forming section and method of dewatering pulp in a paper making machine comprising an endless belt passing around and ovQr a breast roll at one end of the section and a solid unperforated couch roll at the other end of the section, conventional dewatering devices between the lZ028~6 upper and lower runs of the endless belt which lie adjacent and in contact with the upper run and a high vacuum static couching box disposed adjacent to and in contact with the upper run and located between the conventional dewatering devices and the couch roll:
the couching box being as above described.
The reason it is feasible to put a high vacuum couching box following the conventional dewatering devices is that generally, by the time the pulp stock on the forming belt reaches the suction boxes, the web is substantially formed and fiber loss is less likely to occur. Higher velocities of water removal may therefore be tolerated in the area of the suction boxes and parti-cularly downstream of the suction boxes.
According to a broad aspect of the present invention there is provided a high vacuum static couching box for use in a forming sectioan of a high speed paper making machine and positioned in an area between a couch roll having a solid outer surface and a last one of a plurality of dewatering devices in contact with a synthetic forming fabric which is rotated about the roll. The couching box is an elongated hollow box-like structure having a slotted upper surface of wear ~esistant material. Each slot in the box-like structure has a width in the range of from 0.12S to 0.75 of an inch. Means is also provided to connect a high vacuum source to the interior of the box-like structure.
According to a still further broad aspect of the present invention there is provided an improved methocl of dewatering pulp in a forming section of a lZ02~6 high speed paper making machine which uses a synthetic forming fabric belt for supporting the pulp. The forming secticn has a breast roll, a couch roll, and dewatering devices disposed between the rolls in contact with the forming fabric which is rotated about the rolls. The method comprises extracting water from the pulp by means of a high vacuum static slotted couch box supported in a fixed position behind the belt and in contact therewith in an area of the forming section between the last one of the dewatering devices and the couch roll. The couch box has a vacuum in the range of between 15 to 30 inches of mercury. The couch roll is a fabric support roll having a solid outer shell whereby the amount of water remaining in the pulp is substantially unaffected when the pulp passes over the fabric support roll.
BRIEF DESCRIPTION OE DRAWINGS
The invention will be better understood by an examination of the following description, together with the accompanying drawings illustrating an example of the preferred embodiment, in which:
FIGURE 1 is a schematic side elevation of a portion of a forming section of the prior art;
FIGURE 2 is a schematic side elevation of a portion of a forrning section of Figure 1 modified according to the invention, and FIGURE 3 is a perspective view of a static slotted couching box for use in the forming section as modified accordiny to the invention.
~2~28~6 DESCRIPTION OF PREFERRED EMsoDIMENT
Referring to Figure 1, the portion of the forming section of a paper making machine of the prior art, indi-cated generally at 10 includes an endless fabric forming belt 11 having an upper run 12 and a lower run 13~ The belt passes over suction boxes 14, over a sector of vacuum couch roll 15, which is rotated in the direction shown by arrow A, and then around a forward drive roll 16 before returning, over a series of return rolls, not shown, to a breast roll, also not shown. Pulp web 20 partially dewatered, passes on fabric 11 over the sector of the couch roll and is rernoved from the fabric after it leaves the couch roll by a vacuum pick-up roll 21 on pick-up fabric 22. The shell 17 of couch roll 15 is drilled with a plurality of holes 18 through which water is drawn from the pulp web through fabric 11 into vacuum chamber 19 which is located within the couch roll adjacent the sector of the shell over which the fabric 11 passes.
Chamber 19 is equipped with lips 23 and 24 which are in intimate contact with the inner surface of couch shell 17 and is connected to a source of vacuum through outlet 25. In the case of a typical high speed, large paper making machine the vacuum couch roll will be about 4 feet in diameter, the shell about 2-5/8" thick, the drilled holes about 1/4" in diameter and the open area provided by the array of holes from about 22 to 60 percent. The re-wetting problem, discussed above, wherein water is ejected from the holes in the couch shell by centrifugal force, is illustrated at B and C. Numeral 26 indicates a wipe apron which may be installed about 60 to 90 ~Z02~6 before top dead center of the couch roll in an attempt to reduce re-wetting.
Referring now to Figure 2 which shows the portion of the forming section of Figure 1 modified according to the invention, the vacuum couch roll is replaced by a couch roll 30 having an undrilled solid shell 31. A
static slotted couching box 32 is located, as shown, directly upstream of the couch roll and between it and the conventional suction boxes 14. The pulp web 20 may be picked off the fabric over the couch roll at C or prefer-ably before the power roll at D, since this arrangement gives the least angular change and therefore fewer run-nability problems.
It will be apparent that with the modified system re-wetting of the pulp web at the couch roll is eliminated.
Also the very expensive drilled vacuum couch roll can be eliminated. Further, there is no need to equip the machine with a forward drive roll since the warp of the fabric around the solid shelled couch or power roll provides adequate contact to drive the fabric particularly if covered with a friction material. As a result the controls to operate the paper machine become much less complicated.
Figure 3 shows, in simplified form, a perspec-tive view of a static slotted couching box which comprises an elongated, hollow, box-like structure 40, which extends across and beyond the width of the forming fabric, and a slotted cover member or upper surface, indicated generally at 41. The cross-section of the box structure is wide and deep relative to the dimension of the slotted cover member to provide rigidity in the machine direction and also in ~Z~
the direction normal to the plane of the fabric Wi th which it is in contact. The cover member comprises a plurality of elongated fabric supporting members 42 positioned parallel to and spaced from each other ~nd extending the entire length of the box structure~ Mernbers 42 are provided with fine grained ceramic fabric contact-ing surfaces 43. These members are held in recesses 44 in the ends of the box and are spaced by end blocks 45 to form slots 46. End blocks 45 also provide vacuum seals at the edges of the forming fabric. Recesses 44 may be large enough to accommodate up to 7 supporting members and, if less than 7 are employed, a spacer bar 47 is added to fill the excess space. Members 42 are supported and braced within the box in a known manner so that there are no cross-member supports in the machine direction and near the fabric contacting surfaces to cause interference with the effective removal of water from the pulp web on the fabric. The fabric supporting surfaces 43 may comprise plates segments, inserts or coatings of wear resistant cerarnic material having a low coefficient of friction with the forming fabric. These surfaces may be glued on with epoxy resin or attached b,~
any other known method. The slots 46 communicate with the interior of the box-like structure and an opening ~8 is provided in the bottom of the structure which is con-nected to a source of high vacuum which is not shown.
The width of the support members is in the range of 0.375 to 0.75 of an inch, preferably 0.6, and the width of the slots is in the range of 0.125 to 0.75 of an inch.
The number of slots in the static slotted couch will range ~Z(:~Z8~6 from 1 to preferably not more than 6, depending on machine operating conditions, and the vacuum will range from 15 to 30 inches of mercury.
It is possible to use a high vacuum in the couching box in accordance with the invention for two reasons: at the position at which the couching box is disposed, the web is substantially formed so that fibers are not likely to be drawn out of the sheet. In addition, with the use of box covers comprisins n~rrow fabric supporting surfaces of long wearing ceramics, known to have a low coefficient of friction with the fabric, there is virtually no danger of an excess of power being required by the system to overcome drag caused by friction. Further, because the slots of the box 1 are relatively very narrow, and because the high vacuum is applied at only this one place, there is little danger that the belt will be stalled by the action of the high vacuum upon it.
The invention is well adapted for use in the type of machine that is already equipped with a vacuum pick-up and has a forward drive roll located downstream and below the couch roll as shown in Figure 1. It is in this type of machine that rewetting of the web before it is picked-off, as previously described, always occurs.
The invention is also particularly well adapted to substitute for a vacuum couch roll on any twin wire or multi wire paper making machine. It may be used also to improve the efficiency on any standard Fourdrinier machine that is not equipped with a vacuum couch roll.
~LZ~)Z8~
Although a single example of the preferred embodiment was described above, this was or the purpose of illustrating, but not limiting the invention. Various modifications which will come readily to the mind of one skilled in the art, are intended to be covered, provided such modifications fall within the scope of the appended claims.
Claims (7)
1. An improved forming section for a high speed paper-making machine which uses a synthetic forming fabric belt for supporting pulp; said forming section having a breast roll, a couch roll, and dewatering devices disposed between said rolls in contact with said forming fabric which is rotated about said rolls, the improvement compris-ing a high vacuum static couch box having a slotted surface supported in a fixed position below said fabric belt and in contact therewith in an area of said forming section between a last one of said dewatering devices and said couch roll, said couch box having a vacuum in the range of between 15 to 30 inches of mercury, said couch roll having a solid outer shell whereby the amount of water remaining in said pulp is substantially unaffected when said pulp passes over said couch roll.
2. A forming section as claimed in claim 1 wherein said slotted couch box extends across the width of said belt and is formed as an elongated hollow box-like structure having an upper surface of wear resistant material with at least one elongated slot therein, said slot being narrow slots having a width in the range of from 0.125 to 0.75 of an inch, and a high vacuum source connected to the interior of said hollow box-like structure.
3. A forming section as claimed in claim 2 wherein said upper surface is provided with one to six elongated slots, each said slot being separated by a distance of 0.75 of an inch or less.
4. A forming section as claimed in claim 3 wherein said upper surface is constructed as a cover member formed by a plurality of elongated fabric supporting members each having a wear resistant outer surface, said members being positioned parallel to and spaced from each other and extending the entire length of said box-like structure, end blocks secured at opposed ends of said members and disposed in tight fit within said slots to seal said cover member and provide openings through said slots only.
5. A forming section as claimed in claim 4 wherein said members have a width in the range of from 0.375 to 0.75 of an inch and are held at their ends in recesses in opposed ends of said box-like structure, and a spacer bar disposed adjacent an end one of said members to fill any remaining space dependent on the number of members forming said cover member.
6. A forming section as claimed in claim 4 wherein said wear resistant outer surface is a fine grain ceramic surface having a low coefficient of friction with said fabric belt.
7. An improved method of dewatering pulp in a forming section of a high speed paper making machine which uses a synthetic forming fabric belt for supporting said pulp, said forming section having a breast roll, a couch roll, a dewatering devices disposed between said rolls in contact with said forming fabric which is rotated about said rolls, said method comprising extracting water from said pulp by means of a high vacuum static slotted couch box supported in a fixed position below said belt and in contact therewith in an area of said forming section between a last one of said dewatering devices and said couch roll, said couch box having a vacuum in the range of between 15 to 30 inches of mercury, said couch roll having a solid outer shell whereby the amount of water remaining in said pulp is substantially unaffected when said pulp passes over said couch roll.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US31881881A | 1981-11-06 | 1981-11-06 | |
| US06/318,818 | 1981-11-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1202806A true CA1202806A (en) | 1986-04-08 |
Family
ID=23239693
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000413180A Expired CA1202806A (en) | 1981-11-06 | 1982-10-08 | Operation of a high speed paper making machine |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1202806A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108201744A (en) * | 2016-12-20 | 2018-06-26 | 上海东冠纸业有限公司 | A kind of conduit assembly for facilitating paper injection vacuum cleaning |
-
1982
- 1982-10-08 CA CA000413180A patent/CA1202806A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN108201744A (en) * | 2016-12-20 | 2018-06-26 | 上海东冠纸业有限公司 | A kind of conduit assembly for facilitating paper injection vacuum cleaning |
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