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

Definitions

• the invention concerns a method for evaporation drying of the paper web that comes from the press section of a paper machine from a dry solids content of kg ⁇ 35...55 % to a dry solids content of k j « 90...98 % .
• the invention concerns a dryer section of a paper machine for carrying out the method.
• twin- wire draw and/ or single-wire draw is/are employed.
• twin-wire draw the groups of drying cylinders comprise two wires, which press the web one from above and the other one from below against heated cylinder faces.
• the rows of drying cylinders which are usually horizontal rows
• twin- wire draw the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular so in the stages of the drying in which the web is still relatively moist and, therefore, of low strength.
• each group of drying cylinders includes just one drying wire, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas on the reversing cylinders or rolls between the drying cylinders the web remains at the side of the outside curve.
• the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside said loop.
• dryer sections are known that comprise so-called normal groups with single- wire draw only, in which the drying cylinders are placed in the upper row and the reversing cylinders or rolls are in the lower row.
• the prior-art multi-cylinder dryers would also become uneconomical, because the cost of investment of an excessively long paper machine hall would become unreasonably high. It can be estimated that the cost of a paper machine hall is at present typically about 1 million FIM per metre in the machine direction.
• One object of the invention is, in connection with increasing of paper machine speeds and with modernizations, to permit fitting of a new dryer section in the place of an existing multi-cylinder dryer.
• the main object of the invention is to provide a novel drying module for a paper web and dryer sections that make use of said module/modules, which are suitable for use at high web speeds of v > 25 metres per second, which speeds can be up to an order of v « 30...40 metres per second or even higher.
• said diameter ratio D/d of drying cylinder to reversing roll optimally taking into account the different evaporation curves that are carried into effect in different areas of the dryer section.
• said diameter ratio D/d that is used in the initial end of the dryer section, preferably in one group, is higher than average, compared with the middle area of the dryer section, for example in the second, third and fourth wire groups.
• the last mentioned wire groups are in the area where the main evaporation of water takes place from the web.
• Said higher diameter ratio D/d is also employed in the final end of the dryer section, in which a significant proportion of the evaporation takes place on the curve sectors of the wire and the web on the drying cylinders.
• An object of the present invention is further development of the evaporation drying and the dryer sections in paper machines so that the drying process in different parts of the dryer section, in different phases of the drying process, and the dryer section configuration can be optimized and the length of the dryer section shortened or kept unchanged while the speeds become higher.
• the nature of the drying procedure has been clarified further in the applicant's recent research and in dryer sections that are in operation and in test runs on a test device.
• the invention is partly based on the observation that in the dryer section of a paper machine the drying process can be divided into three process stages that are different from each other:
• An object of the present invention is further development of said prior art, elimin- ation of drawbacks of the prior art that were mentioned above and of those that will come out later, and implementation of other objectives of the invention.
• the method of the invention is mainly characterized in that the method consists of three successive stages I, II and III that are carried out in the direction of progress of the web in the sequence given as follows:
• the paper web coming from the press section of the paper machine is heated in a short section of the paper machine in the machine direction quickly to a temperature of 55...85 °C, preferably to a temperature of about 70°C, and in this section the web is passed so that web breaks of the relatively moist and, thus, weak web are minimized,
• this second stage II after the first stage I, in this second stage II the main evaporation drying of the web is carried out with such an evaporation efficiency and rate of increase in dry solids content per unit of length of the dryer section in the machine direction that said evaporation efficiency and rate of increase in dry solids content of the web are substantially higher than in the first stage or in the final stage III, and the web temperature does substantially not rise in the second stage while the drying proceeds,
• the drying is continued with a decreasing evaporation efficiency and with such an average rate of increase in the dry solids content of the web in the machine direction as is lower than in the preceding stage II but higher than in a conventional cylinder drying with single-wire draw so that the paper quality can be controlled at the same time.
• the dryer section in accordance with the invention is mainly characterized in that, after the press section of the paper machine, the dryer section comprises the following dryer units that are placed in the given sequence in the machine direction:
• the first unit is a drying wire unit in which the paper web runs past blow boxes and/ or radiation dryer units, by whose means the web is heated without a direct contact with heated faces,
• dryer units that comprise at least one single- wire group with single- wire draw that is open towards the bottom, in which the contact drying cylinders are in the upper row and the reversing suction cylinders are in the lower row so that removal of broke can take place downwards by the effect of gravity.
• the first stage I in the method in accordance with the invention such a construc- tion of the dryer section is used as also has optimal runnability properties so that in this stage, when the web is still moist and relatively weak, web breaks can be minimized.
• the final stage III of the method of the invention is carried out with such solutions of equipment as also permit control of quality properties of paper, such as brightness, curl, etc.
• the web is preferably dried sot that in the first stage I the drying energy is at least mainly applied from the side of and through the upper surface of the web, in the second stage II the drying energy is applied to the web from the side of and through its lower surface, and in the third stage III the drying energy is applied to the web from and through its both surfaces.
• Figure 1A is a schematic side view of a dryer section in accordance with the invention in which the method in accordance with the invention can be applied favourably.
• Figure IB shows a preferred contact-drying/impingement-drying unit used in a dryer section in accordance with the invention, of which units there are three in the dryer section shown in Fig. 1, separated from one another by single- wire groups.
• Figure 1 C shows the last wire group of the dryer section in a scale larger than Fig. 1A, in which group the stage III of the method in accordance with the invention is carried out.
• Figure 2 is a graphic illustration of the different stages of the method in accordance with the invention in a system of coordinates of dry solids content of the web.- length of the dryer section in the machine direction, compared with a prior-art multi cylinder dryer.
• Figure 3 is a graphic illustration similar to Fig.2 of the drying method in accordance with the invention and of a prior-art drying method in a system of coordinates of evaporation capacity - length of the dryer section in the machine direction.
• Figure 4 is an illustration similar to Figs. 2 and 3 of the distribution of paper web temperature in the machine direction of the dryer section.
• Figure 5 illustrates the evaporation capacity of stage III in accordance with the invention as a function of the dry solids content percentage of the web in the method in accordance with the invention and in a prior-art dryer section.
• FIG. 1A shows a particularly favourable overall concept of a dryer section in accordance with the invention.
• the paper web W is passed from the press section 10 of the paper machine at a dry solids content of - g ⁇ 35...55 % and at a temperature of T 0 ⁇ 30...60°C on the bottom face of the transfer fabric 11 and supported by a PressRunTM box 11a onto the top face of the drying wire 12 over its guide roll 13.
• the first planar drying unit R j comprises a blow hood 15, under which the web W to be dried runs on the horizontal run of the wire 12, which is supported by the rolls 14.
• Said horizontal run of the wire 12 forms a plane consisting of grooved rolls and/or of suction boxes or blow boxes to support the web W.
• T 1 60...85°C.
• the length L j of the unit R j in the machine direction is typically of an order of L j « 3.. J0 m.
• the paper web runs on support of the upper run of the drying wire 12 along a linear path in the horizontal plane so that it has no major changes in the direction and that, thus, no high dynamic forces are applied to it which might produce a web break in the web, which is still relatively moist and, thus, of low strength.
• the blow hood 15 In the interior of the blow hood 15, there is a nozzle arrangement, by whose means hot drying gases, such as air or steam, are blown against the top face of the web.
• infrared heaters Said blow devices and/or radiators in the unit R j can be arranged so that their output in the cross direction of the web W is adjustable so as to provide profiling of the web W in the cross direction.
• the unit R j is followed by the first so-called normal (not inverted) single-wire unit R 2 , onto whose drying wire 22 the web W is transferred as a closed draw in the area of the first reversing suction roll 21.
• the single- wire unit R 2 , and so also the subsequent single- wire units R 4 , R 6 and R ⁇ that are open towards the bottom comprise steam-heated contact-drying cylinders 20 fitted in the upper row and reversing suction rolls 21 fitted in the lower row, for example the applicant's said VAC-rollsTM. Below the cylinders 20, there are doctors and ventilation blow devices 25.
• the paper web W to be dried enters into direct contact with the faces of the steam-heated drying cylinders 20, and on the reversing suction rolls 21 the web W remains on the drying wire 22 at the side of the outside curve.
• a drying unit R 3 in accordance with the invention which, in accordance with Fig. IB, comprises two contact-drying cylinders 30 and a large-diameter T> ⁇ impingement-dry ing/through- drying cylinder 31 with a perforated mantle, which cylinder will be called a large cylinder in the following.
• a drying wire 32 is fitted to run, which wire is guided by the guide rolls 33.
• the impingement-drying/through-drying hood module M j of the drying unit R 3 is fitted in the basement space KT underneath the floor level K j — K j of the paper machine hall on support of the floor level K 2 — K 2 of said space.
• the central axes of the contact-drying cylinders 30 in the unit R 3 and in the corresponding following drying units R 5 and R 7 in accordance with the present invention are placed substantially in the floor plane of the paper machine hall or in the vicinity of said plane K j — K 1 ⁇ preferably slightly above said plane.
• the paper web W to be dried is passed from the single-wire unit R 2 as a closed draw onto the first drying cylinder 30 in the drying unit R 3 (R n ), after which the web W is passed on the wire 32 of the unit R 3 over the large cylinder 31 of the first module M j on a remarkably large sector b » 220...280° on support of the drying wire 32 and further onto the second drying cylinder 30 in the unit R 3 (R n ). From this drying cylinder 30 the web W is transferred as a closed draw into the next normal unit R 4 with single-wire draw, which unit is substantially similar to the unit R 2 described above.
• the second drying unit R 5 ( ⁇ , which unit is similar to the drying unit R 3 described above and whose large cylinder 31 is also placed in the basement space KT.
• the drying unit R 5 the web W is passed as a closed draw into the next single- wire unit R 6 , which is followed by the third drying unit R 7 (1 ⁇ ), whose large cylinder 31 is likewise placed in the basement space KT.
• the unit R 7 is followed by a particular single- wire unit R 8 , from which the web W out is passed to the reel-up or into a finishing unit (not shown).
• the construction and operation of the particular unit R 8 will be described in more detail later with reference to Fig. IC.
• Fig. 1A also shows the pulpers 40a and 40b, between which there is the broke conveyor 41, which carries the paper broke into the pulper 40a and/or 40b.
• the web W can be passed after the unit R j directly into the pulper 40a placed underneath.
• the single- wire units R 4 , R , and R 8 are open towards the bottom, and therefore the paper broke falls from them by the effect of gravity onto the broke conveyor 41 placed underneath or directly into the pulpers 40a, 40b.
• the modules M 1 ; M 2 and M 3 are open or openable towards the bottom so that the paper broke falls out of connection with them, substantially by the effect of gravity, without major manual operations, onto the broke conveyor 41 placed underneath.
• the modules M j , M 2 and M 3 above the floor level K 2 — K 2 of the basement space KT, there is still space KT Q for various devices, such as ducts through which the heating medium, such as heated air or steam, is passed into the interior of the hoods 35 of the modules M l , M 2 and M 3 .
• Said lower space KT 0 is defined from below by the floor level K 2 -K 2 of the basement space and from above by the partition wall 42 placed below the broke conveyor 41.
• On the drying units R 2 ...R 8 there is an air-conditioned hood 50 in itself known.
• Figure IB is a more detailed illustration of the impingement-drying/through-drying hood module M in accordance with the invention.
• the wire 32a which runs around the large cylinder 31 is first passed around the last lower cylinder 21a in the preceding group R ⁇ with single- wire draw onto the first contact-drying cylinder 30 in the unit R ⁇ from it further as a short straight run over the sector b ⁇ 220 ... 280° of the large cylinder 31 onto the second contact-drying cylinder 30 in the group R n and over said cylinder on a sector of about 90°.
• the large cylinder 31 is mounted on its axle journals 36, through which a communication is arranged with vacuum devices (not shown), by whose intermediate a suitable vacuum is produced in the interior of the cylinder 31, which vacuum is of an order of p 0 ⁇ 1...3 kPa.
• This vacuum p 0 keeps the web W on the wire 32a when the web W is at the side of the outside curve, and, at the same time, the vacuum p 0 also promotes possible through-drying taking place through the web W and the wire 32a.
• the sector 360°-b that remains outside the sector b on the large cylinder 31 is covered by a cover plate 34 placed in the gap between the drying cylinders 30, and so also the last cylinder 21a in the group R n , which can also be called the reversing cylinder of the group R ⁇ , is covered by an obstacle plate 29.
• the perforated and grooved outer mantle 31a of the large cylinder 31 is, for example, similar to that described in said FI Pat. Appl. 931263 and illustrated above all in Fig. 11 of said patent application, so that the construction will not be described again in this connection.
• the large cylinder 31 is mounted by means of its axle journals 36 on support of the frame construction 37.
• the hood halves 35 are arranged to be movable on wheels 39, which hood halves are illustrated in the open position 35a, in which the module M can be serviced.
• the hood halves 35 are displaced into the open and closed positions by actuating cylinders 38.
• the module M and its hood 35 are open towards the bottom, so that broke can be removed in the direction of the arrows WA substantially by the effect of gravity onto the broke conveyor 41 placed underneath without substantial manual operations.
• the top face of the hood 35 has been shaped as smoothly downwards inclined so as to improve the removal of broke.
• the diameter D j of the large cylinder 31 is, as a rule, chosen in the range of D j > 2 m, as a rule in the range of D j ⁇ 2...8 m, preferably D j ⁇ 2...4 m.
• the diameter D 2 of the drying cylinders 30 in the group R JJ is, as a rule, chosen in the range of D 2 ⁇ 1.5...2.5 m, preferably in the range of D 2 « 1.8...2.2 m.
• the diameter of the drying cylinders 20 is preferably ⁇ D 2 .
• the top face of the hood 35 has been shaped as smoothly downwards inclined to improve the removal of broke.
• the wire 32a guide roll 33a placed above the latter drying cylinder 30 can be stationary or displaceable. Between the groups R ⁇ i , Rn and R n+ 1 a little difference in speed can be employed, which is, typically, about 0J...0.2 %, so that, on the wires 22,32a,22, the speed becomes higher when the web W moves forwards. In the final end of the dryer section, the difference in speed can also be reversed.
• the more detailed construction of the hood 35 of the module M and the circulation arrangements of the drying gases that are blown through it are described in detail in the FI Patent Application No. (971713) to be filed on the same day with the present application by the applicant, especially in its Fig. 3 and the related specification part, to which reference is made in this connection.
• Fig. IC shows, in a larger scale than Fig. 1A, the last group R 8 with single-wire draw in the dryer section in accordance with the invention, in which group the third stage of the invention is carried out.
• the paper web W to be dried is brought into the group R 8 from the last contact-drying cylinder 30 of the module M 3 shown in Fig. 1A as a closed draw onto the first reversing suction roll 61 of the group R 8 .
• the group R 8 includes five contact drying cylinders 60,60A.
• Two middle ones 60A of these cylinders are contact drying cylinders whose diameter, which is larger than that of the other cylinders 60, is D 4 « 1.8...2.5 m, whereas the diameter of the smaller cylinders 60 is D 5 « 1.0...1.8 m, and the diameter of the reversing suction cylinders 61 is D 6 « 1.0... 5 m.
• a drying effect is applied to the web W by means of contact drying cylinders 60A with large diameter also from the top face of the web W, i.e. from the drying wire 62.
• ventilation hoods 66 are provided above the cylinders 60 A, into which hoods sufficiently hot and dry drying air gases are passed through the intake pipe 67.
• the humidified ventilation air is discharged into the hood 50 around the dryer section, from where it is removed in a way known from the prior art.
• These drying gases are blown against the drying wire 62 in the sector d of the cylinders 60 A, said sector being preferably d ⁇ 180° or even larger.
• evaporation of water is promoted through the upper face of the web W through the wire 62.
• the ventilation hoods 66 are shown in their open position 66a, as well as their air intake pipes are shown in their open position 67a. In this position 66a it is possible to clean and service the ventilation hoods, and the web W threading is also carried out most favourably then.
• the ventilation hoods 66 can be similar to those that are described in more detail in the FI Patent Application (971713) to be filed on the same day with the present applica- tion.
• Fig. 2 shows the development of the dry solids content KA of the paper over the length L of the dryer section in the machine direction as a function.
• the curve K represents an optimized method in accordance with the invention
• the curve K PA represents the development of the dry solids content with a method and a dryer section of prior art.
• the curves K and K pA have been obtained by means of computer simulation using the applicant's dryer section process model.
• the basis for the curve K pA is the applicant's prior-art SymRunTM dryer section concept, which consists of N pcs. of successive groups with single-wire draw that are open towards the bottom, and the curve K is based on a dryer section concept in accordance with Fig. 1.
• the method in accordance with the invention is divided into three different stages I, II and III.
• the rate of increase in dry solids content KA of the web W becomes higher from the initial value Kg more steeply in accordance with the curve K, in comparison with the curve K pA , because the initial temperature of the web W is higher, which becomes clear from a comparison of the temperature curves T and T PA of the stage I in the figure.
• the first stage I as is shown in Fig.
• the evaporation efficiency PE is, in accordance with the curve PE j , substantially higher than in the prior-art method, curve PE pA of stage I (Fig. 3).
• the first phase I is carried out on a horizontal dryer unit R j where the web W temperature T w is raised to about 55...85°C, preferably to about 70 °C, as comes out from Fig. 4.
• this raising of the temperature can be carried out very quickly, because in the unit R j a highly energy-intensive impingement stage and/or infra radiation can be used, because heating of the web W takes place free of contact so that there is no risk of sticking.
• Stage II shown in Figs. 1...4, is the main evaporation area where, in accordance with Fig. 2, the dry solids content KA of the web increases more steeply than in stage I as the drying proceeds.
• Fig. 3 shows the three successive evaporation peaks PE j , PE- and PE 3 of stage II, at which the maximal evaporation efficiency PE is of an order of PE « 60 kg/m 2 /h (kilograms per square meter in an hour). These evaporation peaks are achieved by the hood modules Ml, M2 and M3 in the dryer section shown in Fig. 1. Depending on the mode of operation of the modules Ml, M2 and M3 or equivalent, the maximal evaporation efficiency can be even higher.
• the evaporation efficiency PE is of an order 20 kg/m 2 /h, i.e. of the same order of magnitude as the evaporation efficiency in accordance with the curve PE PA in Fig. 3 on the average.
• the web temperature T w stays substantially invariable in the stage II in accordance with the curves T j and T pA in a range of about 60...70°C.
• the stage II is the main evaporation area where the water is evaporated from between the fibres in the web W and from the fibre surfaces.
• the steepness of the increase in the dry solids content decreases in comparison with stage II.
• the evaporation effi- ciency also decreases in accordance with Fig. 3, whereas the web W temperature T w starts rising from about 70°C to 100.. J10°C.
• the evaporation efficiency still remains invariable, in accordance with the curve PE PA in Fig. 3, and so also the temperature in accordance with the curve T pA in Fig. 4.
• the stage III is carried out in the last cylinder group R 8 , where the evaporation is made more intensive by means of the hoods 66 that are placed above the cylinders 60 A with large diameter, in which hoods sufficiently powerful and hot drying gases are applied to the web W placed under the drying wire 62 and to the environment of the wire 62, so that the web W temperature T w can be raised very steeply in the stage III, in accordance with Fig. 4, in which connection also the water present inside the fibres in the paper web W can be efficiently evaporated on a sufficiently short length L of the dryer section in the machine direction.
• Fig. 5 illustrates the evaporation efficiency PE in the stage III of the invention, i.e.
• the curve PR represents the method in accordance with the invention, and the curve PE PA a corresponding curve carried out by means of the prior-art SymRunTM concept.
• Fig. 5 shows that in the beginning of the stage III, in accordance with the curve PE j , in the dry solids content area 80...82 the evaporation efficiency is substantially higher than in the prior-art concept and somewhat higher than in the dry solids content area 84...91 and in the dry solids content area 93...98.
• This improvement has mainly been carried out in the particular group R 8 by means of the drying cylinders 60A with large diameter and by means of the blowings from their ventilation hoods 66.
• the method stage I in accordance with the invention is carried out by applying drying energy mainly through the upper face of the web W.
• drying energy is applied to the web mainly through the lower face of the web only by means of the wire groups R 3 , R , R 5 , R 6 , R 7 and R 8 and by means of the hood modules M j , M 2 and M 3 , whereas in the group R 8 (Fig.

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Citations (3)

• 1997
  • 1997-04-22 FI FI971714A patent/FI971714A/fi unknown
• 1998
  • 1998-04-21 JP JP54509598A patent/JP3604704B2/ja not_active Expired - Fee Related
  • 1998-04-21 EP EP98917149A patent/EP1012386B1/de not_active Expired - Lifetime
  • 1998-04-21 AU AU70459/98A patent/AU7045998A/en not_active Abandoned
  • 1998-04-21 DE DE69833107T patent/DE69833107T2/de not_active Expired - Lifetime
  • 1998-04-21 AT AT98917149T patent/ATE315123T1/de not_active IP Right Cessation
  • 1998-04-21 WO PCT/FI1998/000351 patent/WO1998048107A1/en active IP Right Grant
  • 1998-04-21 CA CA002287355A patent/CA2287355C/en not_active Expired - Fee Related

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