US2928466A - Means for improving drainage on paper machines - Google Patents

Description

March 15, 1960 s BURKHARD ETAL 2,928,466

MEANS FOR IMPROVING DRAINAGE ON PAPER MACHINES Filed Dec. 27, 1955 2 Sheets-Sheet l PRIOR ART FOURDRINIER MACHINE March 15, 1960 e. BURKHARD ETAI- 2,923,466

MEANS FOR IMPROVING DRAINAGE 0N PAPER MACHINES Filed Dec. 27, 1955 2 Sheets-Sheet 2 FEE WP/ST' KTTOE/VEY United States PatentO MEANS FOR IMPROVING DRAINAGE ON PAPER MACHINES George Burkhard and Peter Ellis Wrist, Baie Comeau,

Quebec, Canada, assignors to The Ontario Paper Company Limited, Thorold, Ontario, Canada Application December 27, 1955, Serial No. 555,662

Claims. (Cl. 162-354) This invention relates to the paper making art and in particular to that portion of the art as applied to the operation of the wire part of a Fourdrinier paper machine.

In a Fourdrinier machine a suspension of fibres or stock is discharged through an orifice onto a moving, endless wire screen or wire. The moving wire travels around and between a breast roll in the general location of the slice and a couch roll adjacent to the press section of the machine. Following the breast roll it is common in the art to lead the wire over one or more forming boards followed by a plurality of table rolls each of which creates a suction on the web and effects a measure of drainage. Following the table rolls are inserted a plurality of suction boxes each connected to a vacuum source. Next the wire passes round the couch roll and returns to the breast roll. On the return run the wire is supported, tensioned and guided by suitable rolls. A greater part of the water associated with the stock as discharged from the slice is drained away through the wire leaving most of the fibres on the wire in the form of a continuous paper web. The formed paper web leaves the wire on or closely following contact with the couch roll. On a modern high-speed machine the breast roll, table rolls, suction boxes, and couch roll typically require a total machine directional operating length of between thirty-five and fifty-five feet. More than half of this distance is occupied by the table rolls.

The use of table rolls gives rise to several distinct disadvantages. First, on wide machines and at the high speeds in modern operation it is ditiicult to design table rolls which are both mechanically strong enough to withstand defiection and which are also dynamically balanced for rotation, without recourse to excessive diameter. Further, the intense suction created by each table roll at high speeds produced violent disturbances and kick-up of the stock on the wire which are detrimental to paper quality. Kick-up is the formation in the course of travel of the stock on the wire over the Fourdrinier table of upward splashings of columns of stock projecting above the mass of the stock on the wire. Kick-up becomes manifest and increasingly severe as the speed of the paper machines increases. These disturbances may also be ag gravated by the carry-over of water from one roll to the succeeding roll which thereupon throws it back onto the under side of the wire. These disadvantages have been reduced by the use of bafiles interposed between rolls to deflect the water from the wire and also by the use of scrolled or grooved table rolls which have a less efficient, and therefore less severe, drainage action. These measures have only been moderately successful. In addition the use of drainage means of decreased efiiciency such as grooved table rolls has necessitated the use of longer Fourdrinier tables.

to produce a sheet suitable for couching and it is a "ice further object of this invention to provide such a method as will effect the drainage with greatly reduced disturbing action on the forming web of paper.

An important additional object of our invention is to permit operation of Fourdrinier paper machines at higher head box consistencies than has heretofore been possible.

Our invention consists broadly in substituting for one or more of the table rolls in a conventional Fourdrinier machine a pluraliy of stationary drainage members extending transversely across the wire and in contact with the under side thereof, such members being so dimensioned and in such spaced relationship that a greater drainage is effected over the relevant section of wire than has hitherto been obtained and with less disturbance to the web.

Our invention may be better understood from a consideration of the following detailed description taken in conjunction with the attached drawings in which Fig. 1 illustrates a side elevation of a conventional Fourdrinier section of a newsprint machine. Fig. 2 illustrates a side elevation of the Fourdrinier section of reduced length made possible through the use of our invention. Fig. 3 is a cross-sectional view corresponding to a side elevation of a portion of a drainage unit constructed so as to embody features of our invention and adapted for use at the breast roll end of the Fourdrinier section. Fig. 4 is a cross-sectional view corresponding to a side elevation of a portion of a drainage unit constructed so as to embody features of this invention and adapted for use in lieu of the table rolls near the suction box section of a conventional Fourdrinier machine.

Referring to Fig. l, l is a Fourdrinier wire which at the slice end of the machine (the left hand side as viewed in Fig. 1) passes around a breast roll 2. One or more forming boards, for example 3a and 3b, are provided immediately following the breast roll. These are followed by the table rolls 4a, 4b, 4c, etc. lnterposed between the table rolls may be baflies 5, of any type in current usage in Fourdrinier machines. Following the table roll section is a plurality of suction boxes 6a, 6b, etc. and the couch roll 7. The web is transferred at the couch into the press section (not shown). The Fourdrinier wire 1 is endless and following the couch may pass over a forward driving roll 8 and return rolls 9a, 9b, etc. The couch roll 7 and forward driving roll 8 are driven by shafts 7a and 8a respectively. This invention contemplates the replacement of the forming boards such as 3a and 3b, and one or more of the table rolls 4a, 4b, 40, etc. with any associated baifles such as 5 by one or more units consisting of a plurality of spacedapart stationary drainage elements adapted to support the under side of the Fourdrinier wire. Each of the drainage units comprises a plurality of elements constructed generally in accordance with either Figures 3 or 4 and having an acute angled leading edge 10, a fiat portion 11 in contact with the under side of the wire and a trailing portion 12 diverging from the wire. The acute angled leading edge 10 acts as a scraper to remove water adhering to the under side of the wire 1 and the flow of water downward over this edge from the wire prevents the entry of air thereover and into the diverging nip formed between the trailing portion 12 and the wire. The entry of such air would have the effect of reducing the suction effect produced by the passage of the wire and carried stock over the trailing portion 12. The flat portion 11 acts to support the wire and at the same time to provide a seal against air being drawn over the leading edge into the nip formed between the trailing portion and the wire.

'Where the rate of drainage is high so that the flow of The length of the trailing portion 12, and the magni tude of the angle of divergence between the wire and the said trailing portion are both critical. For a given length of trailing portion, a given machine speed, and a given stock condition, the volume of drainage is substantially proportional to the magnitude of the angle up to a maximum critical angle of approximately beyond which drainage ceases. For a given stock, increasing the length of the trailing portion increases the drainage up to a pomt where the drainage resistance of the stock becomes controlling and thereafter very little further drainage occurs with further increase of length of the trailing portion. For a given stock and given length of trailing portion drainage also increases as machine speed increases. The practical limitations of the speed-drainage relation are determined by the point at which the suction developed in the drainage zone is sufficient to cause cavitation and by the extent of kiclcup as hereinafter discussed. It will, of course, be recognized that the drainage capacity of our invention is influenced by the characteristics of the stock being drained. The operation is a filtration process and the properties of any particular stock which influence its filterability will similarly influence the application of our invention.

At high speeds modifications in the use of our invention are required to minimize kick-up. In general, the probability of hick-up increases as a function of the length of the trailing portion 12 the angle of divergence between the trailing portion and the wire and the wire speed. Kick-up is also influenced by the condition of the stock above the wire as it passes onto the drainage unit in accordance with our invention. Kick-up tends to be cumulative and any pre-existing disturbance will become accentuated under conditions conductive to kickup. It is recognized that extensive kick-up occurs in current operating practice over and following certain drainage elements in common use on high speed Fourdrinicr paper machines. On any given machine, therefore, some degree of kick-up can be tolerated and, in practice, is tolerated. Diminution of kick-up, however, is desirable and the precise details of our invention will be determined by the factors hercinbefore discussed having regard to the extent of kick-up which is operationally acceptable.

Therefore, it will be seen that for a given stock condition there is a length for the trailing portion 12 which will give maximum drainage without the deleterious effects of kick-up. We have found that if the trailing portion 12 is reduced below this otherwise optimum length a plurality of identical drainage elements may be employed to form a drainage unit, these elements being so spaced from each other as to materially reduce those portions of the wire over which no drainage is discharged from the under side of the wire and thereby greatly to increase the total drainage for a given Wire section. Fig. 3 illustrates an example of drainage element, adapted for use as part of a drainage unit near the breast roll end of the machine in which the leading edge is at an angle 30'' to the direction of travel of the wire, the flat surface 11 is approximately /2" long in the machine direction, the trailing portion 12 is approximately 1%" long and the spacing between adjacent elements is not less than and preferably in the range Vs to di Fig. 4 similarly shows an example of elements adapted for us: in the section near the suction boxes in which the leading edge 10 makes an angle of 60 to the direction of travel of the wire, the fiat portion 11 being approximately VB" and the trailing portion 12 being approximately /8" and the spacings between adjacent elerrients being approximately /5". For optimum use of drainage units comprising elements of the type shown in Fig. 3 for use on paper machines running at speeds in excess of 1,500 feet per minute we have found that kick-up is minimized if the elements are combined in units or groups in which the angle made by the trailing portion 12 with the wire 1 (measured while the machine is statidm ary), progressively increases in the direction of travel of the wire. In a typical arrangement the first group may have an angle of 1, so measured, the second group 1 /2 and the third group 2. The number of elements in each group should be chosen according to the particular drainage characteristics of the stock. For a typical newsprint machine the number of elements per group may be live. Within a group of each of the first several elements provides substantially the same drainage. Then, the angle of divergence remaining constant, the drainage efficiency decreases for additional elements. The precise number of elements up to the point of appreciable loss of efficiency is determined by the stock characteristics and is roughly constant for similar stocks. In designing units the number of elements per group would normally be predetermined.

As the consistency of the stock on the wire increases the stage is reached in which the full length of the trailing portion 12 of elements of the type shown in Figure 3 is not fully effective in producing drainage. At this stage it is advantageous to employ elements of the type shown in Figure 4. Under these conditions two advantages follow. First, the machine directional length occupied by the ineffective part of the trailing portion 12 is eliminated and, secondly, it has been found in practice that the tendency of these elements to produce kickup at very high speeds at a location immediately preceding the suction boxes is even less than that of the elements of the type shown in Figure 3 at the same location. When the stock consistency on the wire reaches approximately 3% it becomes advantageous to employ suction boxes as the means of further drainage.

The embodiment shown in Fig. 3 is especially designed for use near the breast roll and the embodiment shown in Fig. 4 is especially designed for use near the suction box section and it is to be noted that the differences between these embodiments, namely, (1) an increasing angle of the leading edge, (2) a decreasing length of the flat portion and (3) a decreasing length of the trailing portion according to the location of the unit in the direction of travel of the wire disclose the changes in design which we have discovered are desirable as the consistency of stock over the element increases or as the drainage resistance of the stock increases.

Referring now to Figure 2 it will be noted that the unit comprising elements of the type illustrated in Figure 3 and denoted as A in Figure 2 replaces not only the first few table rolls 4a, 4b, etc. but also the forming boards such as 3a and 3b of Figure 1. We have found that the immediate drainage of water from the jet landing on the wire helps to eliminate entrapment of air between the wire and the said jet and also serves to reduce instability in the top surface of the jet caused by the impact of landing. Units comprising elements of the type shown in Fig. 4 and denoted as B in Figure 2 replace the latter table rolls. Three such units are shown. In this arrangement these units also perform part of the function of the first suction boxes and therefore the total number of suction boxes, 6a, 6b, etc., may be reduced.

When used in combination with baffles and scrapers the diameter of the table rolls, in order that they may be structurally stable and dynamically balanced over the width of the paper machine, together with the number required to give adequate drainage, requires that the table roll section of a typical newsprints Fourdrinier machine such as is illustrated in Fig. 1 and running at machine speeds in excess of about 1,000 feet per minute comprises a length of the general order of twenty to thirty feet in the machine direction. On a similar machine using units comprising elements in accordance with our invention the same result can be achieved in a length in the machine direction of approximately 6 feet with no loss in total drainage and in addition with considerably reduced distut-hence to the forming web. It will be appreciated,

therefore, that newsprint machines for operating at speeds in excess of about 1,500 feet per minute with a Fourdrinier table of length less than 25 feet between breast and couch roll centres becomes possible for the first time. The region between the slice and this point is thus the dilute end of the Fourdrinier table and when in the specification and claims we refer to the dilute end of the wire section of the Fourdrinier machine we mean and have reference to the zone between the breast roll and a line at which the consistency of stock on the wire becomes about 3%. It is obvious that any diminution possible in the length of the table roll section would automatically constitute a corresponding diminution in the length of the Fourdrinier wire substantially equal to twice the diminution of the length in the table roll section, thus accomplishing a material cost saving in machine operation.

In the art it has only been previously possible to decrease the length of the Fourdrinier table on high speed machines below about 35 feet in the manufacture of tissue grades of paper. Tissue paper is defined in the Dictionary of Paper, published in 1951 by the American Paper and Pulp Association as a general term indicating a class of papers of characteristic gauzy texture made in weights lighter than 18 pounds (24" x 36"-500)" and when we refer to tissue grades we have reference to this class of papers. In these grades there is no special requirement of surface characteristics for printing. In addition, the light weight of the sheet and the free draining properties of tissue stock enable very rapid drainage means to be used. It has, therefore, been possible in these grades to effect very rapid drainage by the use of pressure drainage, namely, drainage by means of the projection of the paper stock against the Fourdrinier wire in such a way that the momentum of the stock creates a pressure differential through the wire. This is different from the means used on conventional Fourdrinier machines where drainage is effected principally by the application of suction to the under side of the wire by such drainage means as table rolls and suction boxes. Pressure drainage as described above has only proved satisfactory for tissue grades of paper. For other types of paper the heavy loss of fines through the wire and the particular type of formation achieved by pressure drainage result in a finished product which does not possess qualities of smooth surface and good printability. In addition, pressure drainage is limited in its application to light weight grades of paper. It will be appreciated that a distinguishing feature of pressure drainage is that the major portion of the drainage is effected over a very short length of the wire of the order of six to ten inches. With a unit constructed according to our invention the rate of drainage is effected at an appreciably slower rate but is continued in a substantially continuous manner over the whole width of the unit. It will be understood that, on conventional Fourdrinier machines employing table rolls, drainage is effected by short pulses of suction at each table roll interspaced with intervals of negligible drainage.

It will be obvious that the replacement of table rolls by units comprising elements in accordance with our invention is beneficial even where no shortening of the table is effected since the drainage is performed with less disturbance on the forming sheet and a more uniform product derived. It will be realized that the two forms of elements described in Figures 3 and 4 are only examples and that for other furnishes than newsprint modifications of the dimensions described above will produce beneficial results.

When drainage units in accordance with our invention are emoloved increased drainage and minimized disturbance of stock on the wire as disclosed above permit operation of the Fourdrinier section of a paper machine in a novel manner not previously possible in operating practice with important additional benefits. The consistency of the stock as discharged from the slice onto the Fourdrinier wire is determined by the stock characteristics and the grade of paper being made. Thus, in the manufacture of newsprint at high speeds fibre consistencies in the range of 0.50-0.65 are commonly employed. Where non-fibrous fillers are used, the total solid content may exceed this figure by the non-fibrous filler content. In the making of newsprint we know of no instance where on machines operating at speeds in excess of 1,500 feet per minute, the consistency of the stock is greater than about 0.65%. It will be appreciated that in high-speed machine operation the uniformity of discharge and consistency of the stock from the slice of the stock inlet become increasingly important. This factor is still more important when extremely rapid means, such as provided by our invention, and the use of higher initial fibre consistencies are employed. If it is attempted to operate conventional machines at higher fibre consistencies than about 0.65% the degree of disturbance introduced by one or more of the drainage elements as heretofore used causes serious nonuniformity and hence poor machine operation and poor paper quality. This is especially so at higher machine speeds, i.e. at speeds in excess of 1,500 feet per minute. By employing our invention it becomes feasible to make newsprint at head box consistencies exceeding 0.75% fibrous content and even as high as 1.0% and possibly higher at speeds in excess of 2,000 feet per minute. To effect this it is necessary only to employ drainage units in accordance with our invention designed for application at the appropriate consistency of stock over the wire. The principles of such design have been previously discussed. It is recognized that, in practice, the limits of head box consistency and machine speed are set by a complex of many factors of stock quality and machine construction. The important advance in the art which our invention makes possible for the first time is that of substantially mitigating the previously most important limiting factor to the use of higher head box consistencies for newsprint manufacture at high machine speeds namely the provision of adequate drainage efficiency without undue and unacceptable disturbance on the wire.

As a result of the use of increased head box consistency very great reductions in the volume of stock processed in the head box system become possible. The use of 1.0% consistency of head box stock instead of 0.6% reduces the necessary rate of slice discharge by 40%. The increased economy of handling substantially smaller volumes of stock in the head box section will be obvious. This, however, is only a small part of the benefit because reduction in volume from the slice means a corresponding reduction in water which had to be removed on the wire to give a final equal dryness or concentration of fibres at the couch. The lower drainage capacity needed on a given machine therefore results in important economics in the number of drainage elements required and in the necessary length of the Fourdrinier wire. It will thus be apparent that our invention substantially modifies the whole complex of the Fourdrinier section promoting reduced size and/or improved product quality with additional operating economies and so constitutes an important advance in the art.

We have described our invention in the embodiments mentioned above. It is to be understood, however, that these are given by way of illustration only and are not to be construed as limitations of our invention. Such embodiments of our invention, therefore, as come within the scope and purview of the appended claims, are to be considered as part of our invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A drainage unit for the dilute end of the wire section of a Fourdrinier paper machine adjacent the breast roll, said unit comprising a plurality of stationary elements in spaced relationship to one another and in supporting relationship to the Fourdrinier wire and extending across the machine transversely of the direction of wire travel, each of said elements having an acute-angled leading edge, a flat portion in supporting engagement with the Fourdrinier wire and a trailing portion diverging from the said wire at an angle therefrom greater than and not greater than said angle being measured when the wire is not in motion, in which unit the angle of divergence of the trailing portion of the said elements increases relative to the corresponding angle of divergence of at least one such clement disposed nearer the breast roll.

2. A drainage unit according to claim 1 in which the angle of the acute-angled leading edge increases relative to the corresponding acute-angled leading edge of at least one such element disposed nearer the breast roll.

3. A drainage unit according to claim 1 in which the length of the flat portion in engagement with the Fourdrinier wire decreases relative to the corresponding flat portion of at least one such element disposed nearer the breast roll.

4. A drainage unit in accordance with claim 1 in which the length of the divergent trailing portion decreases relative to the corresponding divergent trailing portion of at least one such element disposed nearer the breast roll.

5. A drainage unit for the dilute end of the wire section of a Fourdrinier paper machine adjacent the breast roll, said unit comprising a plurality of stationary elements in spaced relationship to one another and in supporting relationship to the Fourdrinier wire and extending across the machine transversely the direction of wire travel, each of the said elements having an acute-angled leading edge, a fiat portion in supporting engagement with the Fourdrinier wire and a trailing portion diverging from the said wire at an angle therefrom greater than 0" and not greater than 5, said angle being measured when the wire is not in motion, in which unit groups of the said elements, as the distance of their location from the breast roll increases, are constructed with an increase in the angle of divergence of the trailing portion relative to the corresponding angle of divergence in the preceding group of said elements disposed nearer the breast roll.

6. A drainage unit according to claim 5 in which the angle of the acute-angled leading edge in each group of elements increases relative to the corresponding angle in the preceding group of said elements disposed nearer the breast roll.

7. A drainage unit according to claim 5 in which the length of the flat portion in each group of elements in engagement with the Fourdrinier wire decreases relative to the corresponding fiat portion of said elments in the preceding group of elements disposed nearer the breast roll.

8. A drainage unit according to claim 5 in which the length of the divergent trailing portion in each group of elements decreases relative to the corresponding divergent trailing portion of the preceding group of said elements disposed nearer the breast roll.

9. A drainage unit for the dilute end of the wire section of a Fourdrinier paper machine adjacent the breast roll, said unit comprising a plurality of stationary elements in spaced relationship to one another and in supporting relationship to the Fourdrinear wire and extending across the machine tranversely of the direction of wire travel, each of said elements having an acute-angled leading edge, a fiat portion in supporting engagement with the Fourdrinier wire, and a trailing portion diverging from the said wire at an angle therefrom (measured when the wire is not in motion) greater than 0" and not greater 5 than 5, in which unit the length of the diverging trailing portion of the said elements decreases relative to the corresponding diverging trailing portion of at least one such element disposed nearer the breast roll, and in which the angle of divergence of the trailing portion increases rela- 10 tive to the corresponding angle of divergence of at least one such element disposed nearer the breast roll.

10. A drainage unit for the dilute end of the wire section of a Fourdrinier paper machine adjacent the breast roll, said unit comprising a plurality of stationary elements in spaced relationship to one another and in supporting engagement with the Fourdrinier wire and extending across the machine transversely of the direction of wire travel, each of the said elements having an acuteangled leading edge, a fiat portion in supporting engage 2o ment with the Fourdrinier wire and a trailing portion diverging from the said wire at an angle therefrom (measured when the wire is not in motion) greater than 0 and not greater than 5", in which unit groups of the said elements, as the distance of their location from the breast roll increases, are constructed with a divergent trailing portion having a smaller length relative to the divergent trailing portion of the elements in the preceding group disposed nearer the breast roll, and in which the angle of divergence of the trailing portions in each group of elements is increased relative to the corresponding angle of divergence of the elements in the preceding group of elements disposed nearer the breast roll.

References Cited in the file of this patent UNITED STATES PATENTS 541,336 Savery June 18, 1895 1,638,975 Antoine Aug. 16, 1927 1,826,735 Cofrin Oct. 13, 1931 1,832,844 Cofrin Nov. 24, 1931 1,917,098 Cofrin July 4, 1933 2,170,109 Berry Aug. 22, 1939 2,274,641 Abbott et a1 Mar. 3, 1942 2,694,345 Hornbostel Nov. 16, 1954 2,744,454 Wagenknecht May 8, 1956 2,756,649 Lee July 31, 1956 FOREIGN PATENTS 399,739 Great Britain Oct. 12, 1933 577,195 Germany May 29, 1933 487,200 Canada Oct. 14, 1952 OTHER REFERENCES Witharn: Modern Pulp and Paper Making, page 396 (1942), second edition. published by Reinhold Publishing Co., New York, N.Y.

Casey: Pulp and Paper, volume 1, (1952), page 460, published by Interscience Publishers, Inc. New York,

Clapperton et al.: Modern Paper-Making," third edition (1947), page 272, published by Basil Blackwell, Oxford, England.

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