Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F5/00—Dryer section of machines for making continuous webs of paper
  • D21F5/02—Drying on cylinders
  • D21F5/04—Drying on cylinders on two or more drying cylinders
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21F—PAPER-MAKING MACHINES; METHODS OF PRODUCING PAPER THEREON
  • D21F5/00—Dryer section of machines for making continuous webs of paper
  • D21F5/02—Drying on cylinders
  • D21F5/04—Drying on cylinders on two or more drying cylinders
  • D21F5/042—Drying on cylinders on two or more drying cylinders in combination with suction or blowing devices
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21G—CALENDERS; ACCESSORIES FOR PAPER-MAKING MACHINES
  • D21G9/00—Other accessories for paper-making machines
  • D21G9/0063—Devices for threading a web tail through a paper-making machine

Definitions

• TITLE A Process For The Restrained Drying Of A Paper Web
• the present invention relates to a process for the restrained drying of a paper web in a dryer section of a paper machine. More particularly, the present invention relates to a process which includes restraining the web against cross-machine direction shrinkage so that edge curl of the web is inhibited.
• a typical dryer section includes a plurality of dryers arranged as a first and a second tier with the first tier disposed above the lower tier.
• the web is threaded around alternate upper and lower dryers in serpentine configuration so that alternate sides of the web are sequentially exposed to the external surfaces of the dryers.
• the upper tier of dryers is wrapped by an upper dryer felt and the lower tier of dryers is wrapped by a lower dryer felt.
• the arrangement is such that the upper and lower felts restrain the web against cross-machine direction shrinkage during passage of the web around the respective upper and lower dryers.
• the web is unsupported by either the upper or rhe lower felt during transit of the web between the upper and lower dryers. Such unrestrained movement of the web permits cross-machine direction shrinkage with the attendant resultant edge curl.
• dryer sections have been installed in which a single dryer felt extends contiguously with the web around respective upper and lower dryers so that no open draw or unsupported passage of the web between dryers is permitted. While the aforementioned single felted or Uno-run dryer sections assist in supporting the web during transit between upper and lower dryers such single felted arrangements suffer from at least the following disadvantages.
• the felt is disposed between the web and the lower dryer thereby rendering the lower dryers redundant.
• the web is disposed on the outside of the felt during passage around the lower dryers so that there exists a tendency for the web to flutter relative to the felt during passage around the lower dryers. Such fluttering of the web means that the. eb is unrestrained during passage around the lower dryer drum.
• the web should be restrained against cross-machine direction shrinkage during the entire passage of the web through the dryer section.
• Such.ideal situation is achieved by the employment of the TOTAL BEL RUN arrangement proposed in copending patent application 014,569 filed February 13, 1987 and assigned to Beloit Corporation.
• cross-machine direction shrinkage is not only inhibited during passage of the web around the dryer but also around the vacuum guide roll.
• the vacuum guide roll being of a diameter considerably less than that of the dryer results in a joint run of the web and felt between the dryers and the guide rolls which is minimal so that the web is restrained against cross-machine direction shrinkage throughout most of the passage through the dryer section.
• the aforementioned sheet restraint reduces edge curl and cockle and the graininess of the resultant sheet at the edges thereof. Furthermore, by the provision of such sheet restraint, the slice opening in the headbox is able to be more uniform and the cross direction fiber orientation profile is improved.
• nonuniform cross directional sheet shrinkage which occurs during conventional drying processes.
• the aforementioned nonuniform shrinkage is responsible for nonuniformities in headbox slice profiles, in fiber orientation, and in the sheet elongation and tensile energy absorption.
• TEA Tensile energy absorption hereinafter referred to as TEA is defined in "The Dictionary of Paper” Fourth Edition, published 1980, as the energy absorbed when a paper specimen is stressed to rupture under tension. It is expressed in energy units per unit area eg kg-cm/cm 2 . It is useful in evaluating packaging materials subject to rough handling.
• the continuous drying restraint which is applied by a total no-draw dryer section as exemplified in the TOTAL BEL RUN concept of Beloit Corporation, has a direct effect on the finished sheet properties by controlling the cross-directional elongation and TEA profiles. Additionally, such reduced shrinkage reduces the cockles and graininess of the sheet edges.
• graininess is defined in the aforementioned dictionary as small variations in the surface appearance of a paper or board, resulting from any of a variety of causes, such as impressions of wires or felts, irregular distribution of color, and uneven shrinkage in drying.
• the opening of the slice lip of the headbox may be maintained more uniform and an improved cross direction fiber orientation profile is obtained as stated more particularly hereinafter.
• the aforementioned TOTAL BEL RUN includes the transfer of the web between dryers with positive support and restraining the sheet with fabric pressure and roll vacuum.
• the combination of the aforementioned arrangement has improved sheet threading, machine runability, and sheet properties.
• the wet paper is dried by intermittent contact with cast-iron, steam heated dryers.
• the thermal contact between the paper and the dryer is maintained by tensioned dryer fabrics which apply a pressure to the paper as it wraps the dryer.
• Typical fabric tensions range from 8 to 12 pounds per linear inch (PLI) which depending on the dryer diameter, will apply a fabric pressure which is in the range of 0.25 to 0.35 PSI which is 6 to 10 inches water gauge (WG) .
• the aforementioned fabric pressure not only improves the drying contact, but also applies a restraint to the paper to prevent shrinkage from occurring. Such restraint, however, is repeatedly released as the sheet passes through the open draws between, conventional dryer cylinders as described hereinbefore.
• the fabric pressure continues to provide some restraint in the machine direction by maintaining a machine direction draw, but in the cross-machine direction, the paper is virtually unrestrained.
• the paper shrinks freely in the cross direction particularly at the edges and somewhat less so near the center of the web where the sheet is at least partly restrained by the outer portions.
• nonuniform cross-machine shrinkage gives rise to nonuniform cross-directional sheet properties such as stretch, TEA and tensile.
• Stretch is defined in the aforementioned dictionary as "the elongation corresponding to the point of rupture in a tensile strength measurement; it is usually expressed as a percentage of the original length.”
• the high cross-directional edge shrinkage also aggravates the susceptibility of the sheet to edge cockle, curl and graininess.
• Hygroexpansivity is defined in "The Dictionary of Paper” as "the change in dimension of paper that results from a change in the ambient relative humidity; it is commonly expressed as a percentage and is usually several times higher for the cross direction than for the machine direction. This property is of great importance in applications where the dimensions of paper sheets and cards or construction board (wallboard, acoustical tile, etc.) are critical.”
• the first phase of such study was directed at quantifying the nonuniformity on commercial paper machines and then determining the effect that the nonuniform shrinkage has on the machine operation and on the finished sheet properties.
• the cross-directional sheet shrinkage was determined by metering fine drops of ink onto the stock as it discharged from the slice lip of the headbox. The distance between marks at the wet end were then compared to the distances at the dry end to determine the cross-directional shrinkage profile.
• Tensile strength is defined in the aforementioned dictionary as "the maximum tensile stress developed in a specimen before rupture under prescribed conditions; it is usually expressed as force per unit width of the specimen.”
• the machine direction tensile was fairly uniform, again being affected in part by the machine direction draw which does not vary in the cross direction.
• the cross direction tensile profile is nonuniform and exhibits a slight hyperbolic configuration. The lowest tensile occurs near the sheet edges, again where the cross-direction shrinkage was the greatest.
• the tensile ratio is the ratio of the tensile in the cross direction to the tensile in the machine direction and will be discussed in detail hereinafter.
• the TEA profiles were also measured for the sample.
• the cross-direction profile reflected the nonuniformity in cross-direction stretch.
• the TEA profile does not exhibit quite as much variation as the CD stretch, because the loss in stretch near the machine center is greatly offset by the increase in tensile strength.
• the increased shrinkage which occurs near the edges also has an adverse effect on headbox performance.
• the slice opening In order to produce a level basis weight profile at the reel, the slice opening must be closed down near the edges. Such closing down near the edges of the slice opening reduces the basis weight at the edges to compensate for the higher shrinkage which occurs near the edges. Such reduction in basis weight causes the paper to go through the press section and earlier dryer sections with light edges which eventually heavy up as the edges shrink.
• Basis weight is defined in the aforementioned dictionary as "the weight in pounds of a ream cut to a specified basis size. The number of sheets in a ream is usually 500.”
• the aforementioned nonuniform slice opening is known to cause a distortion of the fiber orientation by inducing cross flows.
• the fiber orientation was determined for the aforementioned sample by measuring the sonic modulus profile as discussed hereinafter.
• the fiber orientation is indicated as the angle of the primary axis of the modulus envelope from the machine direction. A positive angle indicates the fibers are oriented towards the back side of the web and a negative angle indicates the fibers are oriented towards the front side.
• the fibers are all oriented towards machine center line as would be expected because the slice opening is closed down near the edges to compensate for edge shrinkage.
• a common commercial arrangement for eliminating open draws is the single felt or serpentine dryer section. Although such serpentine arrangement does eliminate the open draws, it does not replace the open draws with positive restraint and it dries the sheet from one side only.
• the intermediate vacuum roll of the aforementioned single tier section acts much like the fabric vacuum box used in laboratory studies discussed in detail hereinafter.
• the vacuum maintains the restraint which is applied by the dryer fabric pressure as the sheet is transferred between dryers.
• a vacuum level of 6 to 8 inches water column in the vacuum rolls is essentially equal to the restraint which is applied by the dryer fabric. Such vacuum level is also the level which is required for positive sheet restraint as discussed hereinafter.
• the sheet restraint used in the various laboratory studies was applied continuously and in order to achieve the same property improvements on a commercial machine, the drying restraint was also applied continuously or at least in those sections where the sheet is shrinking the most. Very specific laboratory tests were carried out on pilot machine samples in order to determine the natural or unrestrained shrinkage characteristics.
• the machine direction and cross-direction shrinkage is very low as the sheet is dried from 40 to 60 percent dry. Once the sheet reaches 60 percent dry, the shrinkage increases and continues at a high rate until the sheet is essentially dry.
• Another object of the present invention is the provision of a process for restrained drying of a paper web in a dryer section which includes the steps of wrapping the web with a felt during passage of the web with the felt around the dryer of the dryer section and thereafter wrapping the web around a portion of a guiding device disposed immediately downstream relative to the dryer so the edge curl of the web is minimized.
• Another object of the present invention is the provision of a process which includes restraining the web against cross-machine direction shrinkage during passage of the web past the guiding device so that edge curl of the web is inhibited.
• Another object of the present invention is the provision of a process for the restrained drying of a paper web which reduces cockle and graininess of the sheet edges.
• Another object of the present invention is the provision of a process which allows the slice opening to be more uniform thereby improving the cross-direction fiber orientation profile.
• This invention relates to a process for the restrained drying of a paper web.
• the process includes the steps of wrapping the web with a felt during passage of the web with a felt around the dryer of the dryer section and thereafter wrapping the web around a portion of a guiding device disposed immediately downstream relative to the dryer so the edge curl of the web is minimized.
• the process includes the steps of restraining the web against cross-machine direction shrinkage during passage of the web around a portion of the external surface of a rotatably cylindrical dryer, passing the web from the dryer to a web guiding device disposed closely adjacent to and immediately downstream relative to the dryer, wrapping the web around a part of the external face of the guiding device and restraining the web against cross-machine direction shrinkage during passage of the web past the guiding device so the edge curl of the web is inhibited.
• the step of restraining the web during passage around the external surface includes wrapping the web with a felt such that the web is sandwiched between the felt and the external surface so that the felt restrains the web against cross-machine direction shrinkage.
• the step of passing the web from the dryer to the web guiding device includes passing a joint run of the web and felt from the dryer to the web guiding device such that the web is disposed between the felt and the dryer.
• the step of wrapping the web around part of the external face of the guiding device includes wrapping the web and felt disposed contiguously relative to the web around part of the external face of the guiding device such that the felt is sandwiched between the web and the part of the external face of the guiding device.
• the process also includes in the step of wrapping the web and felt around part of the external face of the guiding device, rotating the guiding device by frictional contact with the felt.
• the step of restraining the web during passage past the guiding device also includes sandwiching the felt between the web and the external face of the guiding device which is a rotatable guide roll so that the web is restrained against cross-machine direction shrinkage during passage of the web around the rotatable guide roll.
• the step of restraining the web during passage past the guide roll includes directing a flow of air in a direction from the web towards the guide roll such that the web is pressed into close conformity with the felt during passage of the web and felt past the guide roll so that the web is restrained against cross-machine direction shrinkage.
• Figure 1 is a side-elevational view of a typical prior art double-felted dryer section.
• Figure 2 is a graph showing the percentage of shrinkage from the front to the back edge of the sheet.
• Figure 3 is a graph comparing sheet elongation profiles in a machine direction and a cross-machine direction.
• Figure 4 is a graph comparing the sheet tensile strength profiles in a machine direction and cross-machine direction.
• Figure 5 is a graph showing the sheet tensile ratio profile from the front to the back edge of the sheet.
• Figure 6 is a graph showing the sheet tensile energy absorption profiles from the front to the back edge of the sheet for the machine direction and the cross-machine direction respectively.
• Figure 7 is a graph showing the dry weight of a sheet from the front to the back edge thereof.
• Figure 8 shows a slice profile of a headbox with the opening profile configured such that a dry weight is obtained as shown in figure 7.
• Figure 9 is a graph showing sheet fiber orientation profile from the front to the back edge of the sheet.
• Figure 10 is a graph comparing machine direction to cross-machine direction shrinkage and the effects thereon of sheet vacuum restraint on such sheet shrinkage.
• Figure 11 is a graph similar to that shown in figure 9 but showing the effect of sheet vacuum restraint on sample stretch.
• Figure 12 is. a graph showing the effect of sheet vacuum restraint on sample tensile strength.
• Figure 13 is a graph showing the effect of sheet vacuum restraint on sample TEA.
• Figure 14 is a side-elevational view of a single felt dryer section or serpentine run or Uno-run dryer section.
• FIG 15 is a side-elevational view of a TOTAL BEL RUN single tier dryer section as described in copending patent application 014,569.
• Figure 16 is a graph showing sample shrinkage characteristics in a machine direction and in a cross-machine direction respectively.
• Figure 17 is a graph showing the effect of restrained compared to nonrestrained on hygroexpansivity.
• Figure 18 is a copy of a photomicrograph showing the surface of a freely dried sheet and figure 19 is a photomicrograph showing the surface of a restraint dried sheet .
• FIG. 1 is a side-elevational view of a typical double-felted dryer section generally designated 10 including dryers 11 and 12 of an upper tier generally designated 13.
• the dryer section 10 also includes lower dryers 14 and 15 of a lower tier generally designated 16.
• the web W extends in sinusoidal configuration past dryers 14, 11, 15 and 12 respectively so that alternate sides of the web are dried as they come into contact with the respective external surfaces 17, 18, 19 and 20 of dryers 14, 11, 15 and 12.
• An upper felt 21 extends around a guide roll 22 and then around dryer 11. The upper felt 21 then extends around a further guide 23 and the upper dryer 12.
• a lower felt 24 after extending around dryer 14 extends around a lower guide roll 25 and dryer 15 and then around a further lower guide roll 26.
• Figure 2 is a graph showing the results for a fine paper machine wherein the shrinkage was found to be highly nonuniform with the graph being almost parabolic. As expected, the highest shrinkage was found to occur at the edges where the sheet has the least cross-directional restraint and the sheet shrinkage was the lowest near the center where the paper was at least partly restrained by the outer portion.
• the x axis includes readings taken from the front edge to the back edge of the sample web and the amount of shrinkage is shown as a percentage of the initial width.
• Figure 3 is a graph showing a cross-directional paper sample tested in the laboratory to determine the variations in sheet properties. As shown in figure 3, the machine direction and cross-machine direction sheet strength profiles are demonstrated. The machine direction stretch is very uniform in the cross direction because it is controlled by the machine direction draws. However, the cross-machine direction stretch is very nonuniform as shown by the graph. From a comparison of the graph 28 with the graph 29 of the machine direction, it appears there is a direct reflection of the cross-direction shrinkage, that is the highest stretch occurs at the edges where the sheet has experienced the greatest shrinkage.
• the graph shown in figure 4 includes a graph of the sheet tensile strength profiles for the cross-machine direction 30 and the graph for the machine direction 31.
• the machine direction tensile as shown in figure 4 is fairly uniform again being affected in part by the machine direction draw which does not vary in the cross-direction.
• the cross-direction tensile profile is nonuniform. It exhibits a slight "frown" or hyperbolic shape. The lowest tensile occurs near the sheet edges again where the cross-machine direction shrinkage was the greatest.
• Figure 6 shows two graphs 33 and 34.
• Graph 33 demonstrates the sheet TEA profile in a machine direction where graph 34 shows the sheet TEA profile for the cross-machine direction.
• the TEA profiles also measured for the same sample.
• the cross-machine direction profile shown in figure 6 reflects the nonuniformity in the cross-machine direction stretch.
• the TEA profile does not exhibit quite as much variation as the cross-machine direction stretch because the loss in stretch near the machine center is partly offset by the increase in the tensile strength.
• the increased shrinkage which occurs near the edges can also have an adverse effect on the headbox performance.
• the slice opening In order to produce a level basis weight profile at the reel, the slice opening must be closed down near the edges such reduction of the basis weight at the edges to compensate for the higher shrinkage which occurs near the edges. This cause the paper to go through the press section and early dryer section with light edges which eventually heavy up as the edges shrink.
• Figure 7 is a graph 35 showing the dry weight of a sample sheet from the front to the back edge thereof.
• Figure 8 is a graph showing the slice profile required in order to obtain the result shown in figure 7. As shown in figure 8 the slice openings are reduced at the respective edges in order to obtain a relatively uniform resultant web after shrinkage.
• the fiber orientation was determined for the sample by measuring the sonic modulus profile.
• the profile is shown in figure 8 which is a graph from the front to the back of the sheet.
• the graph indicates actual readings whereas the graph 36 shows the average orientation.
• the fiber orientation is indicated as the angle of the primary axis of the modulus envelope from the machine direction. A positive angle indicates that the fibers are oriented towards the back side, and a negative angle indicates that the fibers are oriented towards the front side.
• the fibers were all oriented towards the machine center line, as expected, because the slice opening was closed down near the edges to compensate for edge shrinkage.
• the samples were manufactured on pilot two-wire machines at commercial speeds. These sheets were then freely dried on a dryer fabric which was supported by a vacuum box. Separate sheets were dried with different levels of vacuum in the box to provide different levels of sheet shrinkage restraint.
• figure 10 shows the effect of sheet vacuum restraint on sheet shrinkage for machine direction as shown by graph 37 and for cross-machine direction as shown by graph 38.
• Figure 11 shows the effect of sheet vacuum restraint on sample stretch and shows graph 39 for the machine direction and graph 40 for the cross-machine direction.
• Figure 12 shows the effect of sheet vacuum restraint on sample tensile strength with the machine direction graph 41 and the cross-machine direction 42.
• Figure 13 shows the effect of sheet vacuum restraint on sample TEA with graph 43 indicating machine direction and graph 44 showing cross-machine direction.
• a common commercial arrangement for eliminating open draws is the single felt or serpentine dryer section shown in figure 14.
• dryers 100, 101 and 102 constitute a upper tier generally designated 103 whereas dryers 104 and 105 constitute a lower tier 106.
• a joint run of the web WA and felt F extends in serpentine configuration respectively around the dryers 100, 104, 101, 105 and 102.
• blow boxes 107 and 108 draw the web towards the felt during transit of the web between dryers, such vacuum is insufficient to cause any appreciable restraint of the web.
• this arrangement does eliminate the open draws, it does not replace the open draws with positive restraint and it dries the sheet from one side only.
• FIG 15 shows the TOTAL BEL RUN arrangement disclosed in copending patent application 014,569 including dryers 200, 201 and 202 arranged as a single tier generally designated 203. Interposed between the dryers 200 and 201 is a vacuum guide roll 204. Furthermore, another guide roll 205 is disposed between the dryers 201 and 202. In this design the bottom ineffective dryers of the serpentine section shown in figure 14 have been eliminated and replaced with vacuum rolls 204 and 205. Two-sided drying is maintained in this arrangement by alternating between top-felted and bottom-felted single tier sections as shown in copending application 014,569.
• the intermediate vacuum rolls 204 and 205 of the aforementioned single tier section 203 act much like the fabric vacuum box used in the aforementioned laboratory studies. This vacuum maintains the restraint which is applied by the dryer fabric pressure as the sheet is transferred between dryers.
• the vacuum which is induced by conventional serpentine blow boxes is typically only 0.1 to 0.2 inches water column and is clearly inadequate to provide significant shrinkage restraint as shown from figure 9. Additionally, this low level vacuum does not extend around the entire bottom dryer. With the long sheet length between top dryers, the sheet is left unrestrained for a significant portion of the drying cycle in the conventional serpentine- dryer section.
• a vacuum level of 6 to 8 inches WC in the vacuum rolls is essentially equal to the restraint which is applied to the dried fabric. It is also the vacuum level which is required for positive sheet restrain as indicated in figure 9.
• the serpentine and single tier dryer without vacuum guide rolls have been applied to the wet end of the dryer section. This has been done in order to improve runability.
• the single tier dryer section should be applied near the dry end of the machine.
• the single tier dryer section configuration should be applied to the entire dryer section.
• a sheet which is dried under a restraint is significantly different from one which is dried freely.
• FIG. 17 shows the effect of restraint on hygroexpansivity, the upper graphs 400, 401 and 402 representing freely dried sheets and the graphs 403, 404 and 405 representing sheets dried under restraint.
• the photomicrographs shown in figures 18 and 19 compare the fiber surface characteristics of a sheet taken from the center of the machine that is under partial cross-machine direction restraint to a sheet taken from the edges with unrestrained cross-machine direction. These micrographs show the same reduction in fiber kinks and caliper as seen in' laboratory dried samples.
• the cross-directional sheet shrinkage which occurs during the drying process is highly nonuniform.
• This nonuniform shrinkage directly affects the cross-machine direction stretch, tensile, modulus and TEA profiles.
• the greatest shrinkage occurs near the edges.
• the headbox slice opening In order to achieve a level basis weight profile at the reel, the headbox slice opening must be reduced near the edges to recompensate for the edge shrinkage.
• the nonuniform shrinkage thereby indirectly affects fiber orientation and a single tier dryer section with intermediate vacuum rolls can be used to control the cross-machine direction shrinkage. Vacuum levels in the intermediate rolls or guide rolls in the range 6 to 8 inches WC will continue the restraint applied by the dryer fabric pressure and substantially reduce the edge shrinkage.

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Citations (3)

• 1989
  • 1989-05-23 EP EP89907445A patent/EP0418317B2/en not_active Expired - Lifetime
  • 1989-05-23 DE DE68913738T patent/DE68913738T3/de not_active Expired - Lifetime
  • 1989-05-23 JP JP1506827A patent/JP2639736B2/ja not_active Expired - Fee Related
  • 1989-05-23 BR BR898907449A patent/BR8907449A/pt not_active IP Right Cessation
  • 1989-05-23 AU AU37733/89A patent/AU639414B2/en not_active Expired
  • 1989-05-23 WO PCT/US1989/002230 patent/WO1989012138A1/en active IP Right Grant
  • 1989-05-23 DE DE198989907445T patent/DE418317T1/de active Pending
  • 1989-05-31 CA CA000601289A patent/CA1335473C/en not_active Expired - Fee Related
  • 1989-06-01 ES ES8901890A patent/ES2015693A6/es not_active Expired - Fee Related
  • 1989-06-02 CN CN89103753A patent/CN1019406B/zh not_active Expired
• 1990
  • 1990-11-30 FI FI905932A patent/FI107548B/fi not_active IP Right Cessation

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