Apparatus for drying a web

Abstract: A method of drying a web of paper (12) emerging from a press section (14) of a papermaking machine comprises the steps of: transferring the web (12) from the press section to a first dryer section (16); initiating the drying of a first side (18) of the web during passage of the web through the first dryer section (16); transferring the web (12) between the first dryer section (16) and a downstream second dryer section (22) such that the web is reversed so that drying of the second side (24) of the web is initiated during passage of the web through the second dryer section (22), and transferring the web (12) between subsequent dryer sections (118, 120, 122) such that the first and second sides (18, 24) of the web (12) are alternately exposed to the drying effect of the subsequent dryer sections (118, 120, 122) in sequence. Each dryer section (16, 22, 118, 120, 122) includes a plurality of dryers (32-42, 58-63, 94-95) arranged in a single tier, a plurality of transfer rolls (44-54, 64-70, 100-106) each of which is disposed between and in close proximity to adjacent dryers, and a felt (56, 72, 110) extending alternately past each dryer and transfer roll in serpentine configuration such that the web is held between the felt and each dryer and the felt is disposed between the web and each transfer roll. Vacuum rolls are used as said transfer rolls and a vacuum within the range of 0,249 - 2.49 kPa (1 - 10 inches WC) is applied in the vacuum rolls so as to restrain the web during drying.

Description: This invention relates to a method of drying a web of paper emerging from a press section of a papermaking machine, said method comprising the steps defined in the preamble of the claim. In an article published in WOCHENBLATT FÜR PAPIERFABRIKATION No. 16 of 1986, J. Linderot: "10 Jahre Erfahrung mit Geschlossener Bahnführung in der Trockenpartie", pages 623-628, reference is made, on page 628, to a drying apparatus having press-to-dryer transfer means and a substantially horizontally extending drying section which includes three successive, substantially horizontally extending BEL RUN single-tier dryer sections and a fourth, single-felted two-tier dryer section disposed downstream relative to the BEL RUN sections. Each BEL RUN section comprises a plurality of top-felted dryers for drying the bottom side of the web and a plurality of vacuum rolls each being disposed below and between and in spaced close proximity to adjacent dryers. The fourth dryer section has top and bottom dryers and a single felt which is sandwiched between the web and the bottom dryers so that only the top dryers can contact the web. A two-tier two-felt section for continuing the drying process by drying the web directly from alternate sides is disposed downstream relative to the fourth dryer section. The web is unsupported as it extends between the top and bottom dryers of the two-tier two-felt section. The method carried out by the afore-mentioned drying apparatus includes the steps of transferring the web from the press section to the first dryer section which consists of three successive BEL RUN single-tier dryer sections; initiating the drying of a first side of the web during passage of the web through the first dryer section; and transferring the web without reversal between the third BEL RUN section and a fourth dryer section which is a single-felted two-tier dryer section having top and bottom dryers. Such a dryer section is also known in the art as Uno-Run dryer section. The top dryers of the fourth dryer section continue to dry the first side of the web whereas the heat transfer from the bottom dryers to the second side of the web is reduced due to the fact that the felt is sandwiched between the web and the bottom dryers. Nevertheless drying of the second side may be initiated during passage of the web through the single-felted two-tier dryer section. The method further comprises the step of transferring the web between the single-felted two-tier dryer section and a subsequent two-tier two-felt section in which each side is brought in direct contact with the top and bottom dryers. Although the cited article does not indicate the vacuum level applied in the vacuum rolls of the first dryer section, it is known in the art, that the vacuum level applied in the vacuum rolls of a BEL RUN section is approximately 0.996 kPa (4 inches WC). The above article also indicates that, in the future, when operating speeds may be above 1300 m/min, single felting may be necessary in all dryer sections of a drying apparatus, and that for certain paper grades, this may necessitate installing single felting alternately above and below in order to avoid twosidedness. WO-A-88/04206 (not pre-published) of prior European patent application 88901398 discloses a method of drying a web. Alternate sides of the web are being dried during passage of the web through a series of single-tier dryer sections. Transfer of the web between consecutive single-tier dryer sections is effected without open draw, positively and with an alternate reversal of the web. The single-tier dryer sections are arranged only over a part of the entire length of the drying section, said part including the wet end of the drying section. The vacuum level applied in the vacuum rolls of the single-tier dryer section is not indicated. The method provides a paper product having uniform surface characteristics, surface strength and smoothness and no tendency to curl. WO-A-82/02937 discloses a method of drying a web by means of a drying apparatus which has a two-tier configuration throughout its entire length. The web is supported by felts only until it has attained sufficient strength to be self-supporting. Each of the dryer groups in which the web is supported by felts, is served by a respective single felt. One of the single-felted two-tier dryer groups has the felt return run arranged above the top row of drying cylinders. Another dryer group, which is located immediately downstream of said one dryer group, has its felt return run arranged below the bottom row of drying cylinders. Therefore, the bottom side of the web is brought in direct contact with the drying cylinders of the top row of drying cylinders of said one dryer group, and its top side is brought in indirect contact with the drying cylinders of the bottom row of drying cylinders whereas, in the other dryer group, the bottom side of the web is brought in indirect contact with the drying cylinders of the top row of drying cylinders and the top side of the web is brought in direct contact with the drying cylinders of the bottom row of drying cylinders. Thus the web is transferred from said one dryer group to said other dryer group with a reversal of the web. Moreover the web is transferred without open draw and positively between said one single-felted two-tier dryer group and said other single-felted two-tier dryer group. Those top and bottom dryers at which the felt is sandwiched between the web and the dryers are grooved and a vacuum is applied in the grooves of each grooved dryer by means of a vacuum box which is disposed adjacent to the grooved dryer. The highest vacuum level applied in the vacuum box is 1.42 kPa. The web is allowed to shrink during drying so that the dried paper possesses stretchability of a higher degree than conventionally prepared papers that have been strained or stretched during manufacture. An article published in TAPPI JOURNAL, Vol. 70, No. 9, September 1987, Norcross, GA, USA, pages 65 to 69, "Advances in dryer section runability" by Gregory L. Wedel and Sam Palazzolo, suggests a future dryer section design which employs the single-tier concept through the entire dryer section, maintaining twosided drying by alternating the fabrics. Furthermore, all open draws will be removed, and the need for web tension to maintain web stability will be eliminated. Positive web support betweeen adjacent dryers will be provided by vacuum transfer rolls which apply a vacuum of 1 kPa to the web so as to hold the web to the fabric around the roll. In this way, any tendency for the web to flutter and wrinkle in the dryer section will be avoided. However, the article does not suggest a solution to the problem of non-uniform paper property development between the center and edges of the web. Nevertheless the article is considered to represent the nearest prior art, and the method carried out on the future dryer section design comprises the steps defined in the preamble of the claim. An object of the present invention is the provision of a method of drying a web of paper in which the quality of the sheet is improved. According to the invention, a method of drying a web of paper emerging from a press section of a papermaking machine, the method comprising the steps of: transferring the web from the press section to a first substantially horizontally extending single-tier dryer section means of a substantially horizontally extending drying section having a single-tier configuration throughout its entire length, passing the web through said drying section by passing it successively through said first, a second and subsequent single-tier dryer section means in close conformity with a respective one of a plurality of felts, each dryer section means comprising a plurality of dryers and a plurality of vacuum rolls each of which is disposed between and in spaced close proximity to adjacent dryers such that the web extends alternately past each vacuum roll and dryer in serpentine configuration, with only one side of the web contacting each dryer of the respective dryer section means, and positively transferring the web between adjacent dryer section means without open draw and with an alternate reversing of the web such that the first and second sides of the web are alternately exposed to the drying effect of said first, second and subsequent dryer section means in sequence, characterised in that said web of paper to be dried is a web of fine paper and is restrained against cross-directional shrinkage while it is not held between each dryer and a respective one of the felts, said web being so restrained by application of a vacuum in said vacuum rolls within the range of 0.249-2.49 kPa (1-10 inches wc) , whereby the edges of the web are not permitted to shrink more than the center portion of the web. The method according to the present invention has the capability of affecting the sheet properties as follows. First, the tensile strength and stretch of the web in the machine direction is improved. Second, the tensile strength and stretch of the web in the cross machine direction is improved. Third, the tendency for the sheet to curl is reduced and fourth, the tendency to generate edge cockles is reduced. Once the sheet leaves the press section, the machine direction tensile and stretch are affected primarily by the press section draw and somewhat less by the draw between dryer sections. In conventional drying sections, these dryer draws are usually set as low as possible but the minimum draw is dictated by runability and sheet control. However, in the drying method according to the present invention, there is no minimum draw requirement for runnability. Nevertheless, the machine direction properties can still be affected in the usual way by pulling a draw between sections if such is deemed desirable. In a somewhat similar manner, the cross machine direction tensile and stretch are affected by the cross machine direction sheet restraint. In an open draw, there is no restraint, so the sheet freely shrinks (although this shrinkage is greatest at the edges). In operation of a typical Uno-Run type drying section, there is a degree of web restraint as evidenced by the increase in trim and the decrease in edge cockles. In the drying method according to the present invention, the sheet restraint is more positive than that provided by a typical Uno-Run system and this further reduces any tendency for edge cockles to develop. With the improved restraint provided by the present invention, a slight increase in cross machine direction tensile and a decrease in cross machine direction stretch is evident. The decrease in cross machine direction stretch is most pronounced at the edges where the sheet normally has the least restraint. Accordingly, the net effect according to the present invention, will be a sheet with more uniform properties in the cross machine direction. The propensity to curl is caused by three factors as follows. First, fiber orientation, second fines and filler (bonding) distribution, and third, residual fiber stress. The dryer section affects only the fiber stress. Curl is normally controlled by adjusting the drying from each side of the sheet by separate control of the top and bottom dryer steam pressures. Accordingly, in the drying method according to the present invention, such control is provided by adjusting the steam pressures in successive sections. Another particularly important object of the present invention is the provision of a method of drying fine paper grades where directionality is important. More particularly, restrained drying of the web is an important feature of the present invention in that it will not permit the edges of the sheet to shrink more than the center portion of the sheet. The key to restraining the sheet while it is not held between the felt and the dryer drum is the vacuum in the transfer, or turning roll, below adjacent drums. The sheet is restrained as stated hereinbefore by application of a vacuum within the range of 0.249-2.49 kPa (1-10 inches wc) and preferably approximately 0.996 kPa (4 inches wc). Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the following detailed description taken in conjunction with the annexed drawings. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a side elevational view of an apparatus for carrying out the method according to the present invention showing the press section, a first transfer means, a first dryer section means, a second dryer section means, and a first dryer transfer means; Figure 2 is an enlarged fragmentary view of Figure 1 showing the press section, and more particularly, the first transfer means for transferring the web from the press section to the first dryer section; Figure 3 is an enlarged fragmentary view of Figure 1 showing the first dryer section means, including the first dryer section and a second dryer section; Figure 4 is an enlarged fragmentary view of Figure 1 showing the second dryer section means; Figure 5 is an enlarged fragmentary view of a third dryer section means; Figure 6 is an enlarged fragmentary view of Figure 1 showing a fourth dryer section means; Figure 7 is an enlarged fragmentary view of Figure 1 showing a fifth dryer section means; Figure 8 is an enlarged fragmentary view of Figure 1 showing two of the vacuum rolls; and Figure 9 is a side elevational view showing air nozzle means for assisting guidance of the tail of the web from a dryer to a further dryer. Figure 1 is a side elevational view showing the apparatus generally designated 10 for drying a web 12 of paper emerging from a press section, generally designated 14 of a paper making machine. The apparatus 10 includes a first dryer section means, generally designated 16 for initiating the drying of a first side 18 of the web 12. A first transfer means generally designated 20 transfers the web 12 from the press section 14 to the first dryer section means 16. A second dryer section means generally designated 22 is disposed downstream relative to the first dryer section means 16. This second dryer section means 22 initiates the drying of a second side 24 of the web 12. The second side 24 of the web 12 being opposite to the first side 18 thereof. A first dryer transfer means generally designated 25 transfers the web 12 without open draw between the first and second dryer section means 16 and 22 respectively. The first dryer transfer means 25 permits both threading of the web 12 without the assistance of threading ropes and the drying of both sides 18 and 24 of the web 12. Figure 2 shows in more detail the first transfer means 20 and will be described in more detail hereinafter. Figure 3 shows in detail the first dryer section means 16. This first dryer section means 16 includes a first dryer section generally designated 26 for initiating the drying of the first side 18 of the web 12. A second dryer section generally designated 28 is disposed downstream realtive to the first dryer section 26 for continuing the drying of the first side 18 of the web 12. A second dryer transfer means generally designated 30 transfers the web 12 without open draw between the first and the second dryer sections 26 and 28 respectively. More particularly, with reference to Figure 3, the first dryer section also includes a first plurality of dryers 32, 34, 36, 38, 40 and 42 respectively. The first dryer section 26 also includes a first plurality of vacuum rolls 44, 46, 48, 50, 52 and 54 respectively. Each vacuum roll of the first plurality of vacuum rolls 44 to 54 is disposed adjacent to a corresponding dryer of the first plurality of dryers 32 to 42 such that the web 12 extends alternately past each vacuum roll 44 to 54 and dryer 32 to 42 in serpentine configuration. A first felt 56 extends around the first plurality of dryers 32 to 42 and the first plurality of vacuum rolls 44 to 54 in close conformity with the web 12. The second dryer section 28 also includes a second plurality of dryers 58, 59, 60, 61, 62 and 63. The second dryer section 28 also includes a second plurality of vacuum rolls 64, 65, 66, 67, 68, 69 and 70. Each of the vacuum rolls 64 to 70 is disposed adjacent to a corresponding dryer of the second plurality of dryers 58 to 63 such that the web 12 extends alternately past each vacuum roll 64 to 70 and dryer 58 to 63 in serpentine configuration. A second felt 72 extends around the second plurality of dryers 58 to 63 and the vacuum rolls 64 to 70 respectively such that the second felt 72 is disposed in close conformity with the web 12. The second felt 72 and an unfelted portion 74 of the downstream dryer 42 of said first plurality of dryers 32 to 42 defines a first pick-up section generally designated 76 for transferring the web 12 from the unfelted portion 74 onto the second felt 72 so that the web 12 is transferred without open draw from the first dryer section 26 to the second dryer section 28. Each of the vacuum rolls of the first and the second dryer sections 26 and 28 is disposed in spaced close proximity to its adjacent corresponding dryer such that the felt draw between each of the vacuum rolls and its corresponding dryer is minimal, thereby inhibiting any tendency of the web to flutter relative to the supporting felts 56 and 72 respectively. As shown in Figure 3 the apparatus 10 also includes a base frame 78 for rotatably supporting both the first and the second plurality of dryers such that the axes of the first and second plurality of dryers are disposed in a first plane 80 as shown in Figure 3. Additionally, the frame 78 rotatably supports the first and second plurality of vacuum rolls such that the axes of the first and the second plurality of vacuum rolls are disposed in a second plane 82 shown in Figure 3. The first plane 80 is disposed above the second plane 82 as shown in Figure 3. As shown in Figure 3 the apparatus 10 includes an upstream vacuum roll 64 of the second plurality of vacuum rolls and this vacuum roll 64 is disposed in spaced close proximity to the unfelted portion 74 of the downstream dryer 42 of the first dryer section 26. A first felt roll 84 is rotatably supported by the base frame 78 for guiding the second felt 72 past and in conformity with the unfelted portion 74 of the downstream dryer 42 and thereafter around the upstream vacuum roll 64 of the second dryer section 28 such that the web 12 is transferred from the unfelted portion 74 to the second felt 72 without open draw. As shown in Figure 2 referred to hereinbefore the apparatus 10 includes a first transfer means 20 for transferring the web 12 from the press section 14 to the first dryer section means 16. This first transfer means 20 further includes a lead in roll 86 which is disposed in spaced close proximity relative to the press section 14. The first felt 56 extends around this lead in roll 86 for transferring the web 12 from the press section 14 to the first dryer section means 16. A guide roll 88 is disposed between the lead in roll 86 and the first dryer section means 16 for assisting the transfer of the web 12 from the press section 14 towards the first dryer section means 16. A transfer felt 90 extends around the guide roll 88 such that the transfer felt 90 and the first felt 56 define therebetween a transfer section 92 for transferring the web 12 from the press section 14 toward the first dryer section means 16. With further reference to Figure 2, the first transfer means 20 further includes an upstream vacuum roll 44 of said first dryer section means 16. The upstream vacuum roll 44 cooperates with the first felt 56 and the transfer felt 90 such that the transfer section 92 extends from the guide roll 88 to the upstream vacuum roll 44 so that the web 12 emerging from the transfer section 92 is guided around the upstream vacuum roll 44 into the first dryer section means 16. With reference to Figure 4 the second dryer section means 22 also includes a third plurality of dryers 94, 95, 96, 97, 98 and 99. The third plurality of dryers is disposed downstream relative to the first dryer section means 16. Each vacuum roll of a third plurality of vacuum rolls 100, 101, 102, 103, 104, 105 and 106 is disposed in spaced close proximity relative to a corresponding dryer of the third plurality of dryers such that the web 12 extends alternately past each vacuum roll and dryer of the second dryer section means 22 in serpentine configuration. As shown in Figure 4 the base frame 78 rotatably supports each of the dryers of the third plurality of dryers such that the axis of the dryers are disposed in the third plane 107. The base frame 78 also rotatably supports each of the vacuum rolls such that the axis of each of the vacuum rolls of the third plurality of vacuum rolls is disposed in a fourth plane 108 with the fourth plane being disposed above the third plane. A third felt 110 extends past the third plurality of dryers and vacuum rolls such that the third felt supports the web through the second dryer section means 22 with the second side of the web being urged by the third felt 110 into close conformity with each dryer of the third plurality of dryers. As shown in Figure 4 the first dryer transfer means includes the downstream vacuum roll 70 of the first dryer section means 16 and a downstream felt roll 112 of the first dryer section means 16. The second felt 72 of the first dryer section means 16 extends between the downstream vacuum roll 70 and the downstream felt roll 112. The second felt 72 supports the web 12 such that the web is conveyed and disposed between the second felt 72 and the second dryer section means 22. The first dryer transfer means also includes the upstream vacuum roll 100 and an upstream felt roll 114. The third felt 110 extends between the upstream felt roll 114 and the upstream vacuum roll 100 of the second dryer section means 22 such that the third felt 110 and the second felt 72 define therebetween a first dryer transfer means section 116 for transferring the web without open draw from the second to the third felts 72 and 110 respectively. The third felt 110 presses against the web such that the second side of the web is pressed into close conformity with each dryer of the third plurality of dryers such that the second side of the web is dried. Figures 5, 6, and 7 respectively show third, fourth, and fifth dryer section means 118, 120 and 122 and second, third and fourth dryer transfer means 124, 126 and 128 for transferring and reversing the web as the web progresses through the drying apparatus. The first, second, third and fourth dryer transfer means 25, 124, 126 and 128 permit the transfer of the web between the respective dryer section means, 16, 22, 118, 120 and 122 without open draw and with an alternate reversing of the web such that the first and second sides of the web are alternately dried as the web extends through the apparatus and past consecutive dryer section means. Figure 8 shows the details of two of the vacuum rolls 46, 48 in which pressure seals 130 may be moved from the position shown with reference to the roll 46 to that shown relative to roll 48 for counteracting the tendency of the web to part from the felt. The first dryer transfer means 25 includes a joint run of the felt 72 and the further felt 110 such that the web is disposed between the felt 72 and the further felt 110 during passage through the joint run. Also, the first dryer transfer means 25 further includes vacuum means disposed downstream relative to the joint run for positively maintaining the web in close conformity with the further felt 110 when the felt 72 and further felt 110 diverge relative to each other downstream relative to the joint run. In a specific embodiment of the present invention, the vacuum means 100 is the vacuum roll 100. Preferably, the single tier drying section 16 extends from the press section 14 to a calender section 130 or to a size press (not shown) or throughout the entire dryer section. The single tier drying section 16 includes a multiplicity of single tier subsections 16,22,118,120 and 122 and a plurality of dryer transfer means 25,124,126 and 128 each dryer transfer means 25,124,126 and 128 being disposed between adjacent subsections such that as the web progresses through consecutive subsections 16,22,118,120 and 122 alternate sides of the web are dried. Additionally, the subsections 16,22,118,120 and 122 are disposed at different heights relative to each other and preferably every other subsection 16,118 and 122 and every other subsection 22 and 120 are disposed at the same height relative to each other. As shown particularly in figure 9, the first dryer transfer means 25 also includes air nozzle means 132 for assisting guidance of the web from the dryer 63 to the further dryer 94. In operation of the apparatus, the web is transferred from the press section to the first dryer section means 16 of the apparatus. Drying of the first side 18 of the web 12 is initiated during passage of the web through the first dryer section means 16. The web is transferred without open draw between the first dryer section means 16 and the downstream second dryer section means 22 with the web transfer being such that the web is reversed so that drying of the second side 24 of the web 12 is initiated during passage of the web through the second dryer section means 22. In operation of the apparatus the web is also transferred without open draw between subsequent dryer section means such that the first and second sides of the web are alternately exposed to the drying effect of the subsequent. dryer section means in sequence. The apparatus is capable of operating at extremely high speeds as no open draws exist between the various sections thereof. Furthermore, the apparatus permits threading without the use of threading ropes.

Claims: A method of drying a web (12) of paper emerging from a press section (14) of a papermaking machine, the method comprising the steps of: transferring the web (12) from the press section (14) to a first substantially horizontally extending single-tier dryer section means (16) of a substantially horizontally extending drying section having a single-tier configuration throughout its entire length, passing the web (12) through said drying section by passing it successively through said first (16), a second (22) and subsequent single-tier dryer section means (118, 120, 122) in close conformity with a respective one of a plurality of felts (56, 72, 110), each dryer section means comprising a plurality of dryers (32, 34, 36, 38, 40, 42, 58, 59, 60, 61, 62, 63; 94, 95, 96, 97, 98, 99) and a plurality of vacuum rolls (46, 48, 50, 52, 54, 64, 65, 66, 67, 68, 69; 101, 102, 103, 104, 105) each of which is disposed between and in spaced close proximity to adjacent dryers such that the web (12) extends alternately past each vacuum roll and dryer in serpentine configuration, with only one side (18; 24) of the web (12) contacting each dryer of the respective dryer section means, and positively transferring the web (12) between adjacent dryer section means (16, 22, 118, 120, 122) without open draw and with an alternate reversing of the web (12) such that the first and second sides (18, 24) of the web (12) are alternately exposed to the drying effect of said first, second and subsequent dryer section means (16, 22, 118, 122) in sequence, characterised in that said web of paper (12) to be dried is a web of fine paper and is restrained against cross-directional shrinkage while it is not held between each dryer and a respective one of the felts, said web (12) being so restrained by application of a vacuum in said vacuum rolls within the range of 0.249-2.49 kPa (1-10 inches wc), whereby the edges of the web are not permitted to shrink more than the center portion of the web.

Inventors: WEDEL GREGORY L, WEDEL, GREGORY L.
Applicants: BELOIT TECHNOLOGIES INC, BELOIT TECHNOLOGIES, INC.
Application year: 1988
Main classification: D21F5
IPC codes: D21G9, D21F5