WO1998048108A1

WO1998048108A1 - Dryer section in a paper machine in which impingement and/or ventilation hoods are used

Info

Publication number: WO1998048108A1
Application number: PCT/FI1998/000352
Authority: WO
Inventors: Antti Kuhasalo; Hans Sundqvist
Original assignee: Valmet Corporation
Priority date: 1997-04-22
Filing date: 1998-04-21
Publication date: 1998-10-29
Also published as: FI971715A; FI971715A0; AU7046098A; DE19882376T1

Abstract

The invention concerns a dryer section of a paper machine, which consists of several groups with single-wire draw that include a drying wire (22, 32, 62, 72, 82, 92), inside whose loop there is a number of reversing cylinders (21, 61, 71, 81, 91). In the gaps between them there are contact drying cylinders (20, 60, 60A, 70, 70A, 80, 80A, 80B, 80C, 90, 90A) against whose heated faces the drying wire (22, 32, 62, 72, 82, 92) presses the paper web (W) into direct contact. In addition to normal contact drying cylinders (20, 60, 70, 80, 90), one or several contact drying cylinders (60a, 70A, 80A, 80B, 80C, 90A) with a larger diameter, the diameter being D4 > 2200 mm and, as a rule, in the range of D4 ≈ 2200... 3600 mm, is/are used in one or several groups (R6, R7, R8; Rn) with single-wire draw that are open towards the bottom. An impingement-drying/ventilation hood (66; 76; 86; 86A; 96) is fitted above said contact drying cylinders (60A, 70A, 80A, 80B, 80C, 90A) with larger diameter D4, which hood extends on a large sector above said contact drying cylinder and above the drying wire (22, 32, 62, 72, 82, 92) and the web (W) running over the cylinder. An impingement-drying/ventilation effect is applied to the drying wire (22, 32, 62, 72, 82, 92) through impingement-drying/ventilation hoods (66; 76; 86; 86A; 96) on said wide sector, by means of which effect the moist boundary layer in contact with the drying wire (22, 32, 62, 72, 82, 92) is disintegrated and drying is promoted from the side of the face of the web opposite to the web (W) face that is placed against the contact drying cylinders (60A, 70A, 80A, 80B, 80C, 90A).

Description

Dryer section in a paper machine in which impingement and/or ventilation hoods are used

The invention concerns a dryer section of a paper machine which consists of several groups with single-wire draw that include a drying wire, inside whose loop there is a number of reversing cylinders, and in the gaps between them there are contact drying cylinders, against whose heated faces said drying wire presses the paper web to be dried into direct contact on a curve sector of over 180°.

The highest web speeds in paper machines are to-day up to an order of 25 metres per second and slightly higher, but before long the speed range of 25...40 metres per second will be taken to common use. In such a case, a bottleneck for the runnability of a paper machine will be the dryer section, whose length with the prior-art multi- cylinder dryers would also become intolerably long. If it is imagined that a present- day multi-cylinder dryer were used in a newsprint machine at a web speed of 40 mps, it would include about 70 drying cylinders (φ ~ 1800 mm), and its length in the machine direction would be ~ 180 metres. In such a case, the dryer section would comprise about 15 separate wire groups and a corresponding number of draws over group gaps. It is probable that, in a speed range of 30...40 mps, the runnability of normal prior-art multi-cylinder dryers is no longer even nearly satisfactory, but web breaks would occur abundantly, lowering the efficiency of the paper machine.

In a speed range of 30...40 mps and at higher speeds, 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. As is known from the prior art, in multi-cylinder dryers of paper machines, twin- wire draw and/or single-wire draw is/are employed. In 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. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular so when the web is still relatively moist and, therefore, of low strength. This is why, in recent years, ever increasing use has been made of said single- wire draw, in which 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. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside said loop.

From experience it is known that, if paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried by means of normal groups with single-wire draw from the side of its bottom face and if such asymmetric drying is extended substantially over the entire length of the dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, seen from above.

It is known from the prior art to use various ventilation/impingement-drying/through- drying units for evaporation drying of a paper web, which units have been employed in particular in the drying of tissue paper. With respect to this prior art, reference is made, by way of example, to the following patent literature: US-A-3,301, 746, US- A-3, 418, 723, US-A-3, 447,247, US-A-3,541,697, US-A-3, 956,832, US-A-4,033,048, CA-A-2, 061, 976, DE-A-2, 212, 209, DE-A-2, 364, 346, EP-A2-0, 427, 218, FI-B-83, 679, FI-B-57,457 (equivalent to SE-C-7503134-4), FI-B-87,669, and FI-A-931263 (equivalent to EP-0,620,313-A1). Additionally with respect to the prior art related to the present invention, reference is made to the applicant's FI Patent Applications 913648, 940992, 941392 and 951746. Different dryer section concepts and geometries and impingement-drying units are described in the FI Patent Applications mentioned above.

Of the applicant's FI patent applications mentioned above, the FI Pat. Appl. 951746 is most closely related to the present invention. This FI patent application describes a dryer-section concept of a paper/board machine, wherein the dryer section comprises a number of drying cylinder groups, which comprise exclusively a single- wire draw, on whose support the web is guided so that it meanders as loop-shaped from a suction cylinder onto a drying cylinder and from the drying cylinder onto the second suction cylinder and from it further onto the second drying cylinder, and further in the group, and the web/wire is guided in connection with the drying cylinders so that the web is placed against the face of the drying cylinder and the wire is placed outside, and that in connection with the suction cylinders/suction rolls the web is guided so that, by means of suction, the web is kept in contact with the wire draw, and guided into connection with the second heated drying cylinder, and that, in the dryer-section concept, the web is passed from one drying cylinder group into the next dryer-section group and further. In said FI Pat. Appl. 951746 it has been considered novel in the context mentioned above that at least some of the drying cylinders include impingement-drying units or equivalent in connection with them, through which units a heated medium, preferably air or steam, is passed through the wire into connection with the web so as to produce a two-sided drying effect and to increase the drying capacity, and that the impingement units are fitted in the end of the dryer section in an area with such a dry solids content of the web that by means of impingement-drying it is possible to affect the curling of the we^"b and to prevent it. In this FI patent application said impingement-drying units are mounted above contact drying cylinders with normal diameter (</> usually about 1800 mm) and it is suggested that said impingement-drying units be placed both in the initial end, in the middle area and in the final end of the dryer section. Owing to the matters stated above, the drying concept in said FI patent application cannot achieve the objectives that the present invention aims to achieve, above all because the area of effect of the impingement units cannot be made wide enough if drying cylinders with normal diameters are used and the effect of the impingement-drying units cannot be concentrated on the process stage where their effect would be optimal in view of the objectives of the present invention.

An object of the present invention is to enhance and to further develop the above prior-art dryer sections in themselves known in view of achieving the objectives to be stated in the following.

The primary object of the present invention is to provide a novel dryer section concept based on evaporation drying, by whose means it is possible to utilize the space in the paper machine hall more efficiently. 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. In relation to this, it is a further object of the invention to provide a dryer section concept that permits ever shorter dryer sections compared with the prior-art dryer sections.

It is a particular object of the invention to provide a dryer section in which the removal of broke can take place primarily by the force of gravity and in which it is not at all necessary to use so-called inverted wire groups, in which the contact- drying cylinders are placed in the lower row and the reversing suction cylinders in the upper row and which are closed towards the bottom, so that, in the event of web breaks, the removal of broke from them must, as a rule, be carried out manually, which is time-consuming and which is also work that is difficult from the point of view of safety at work.

It is a further object of the present invention to provide a dryer section in which it is possible to achieve good runnability and a substantially closed draw of the web and threading of the leader end of the web even without systems of threading ropes. It is a further object of the invention to make it possible to provide a dryer section concept in which different evaporation devices and techniques can be applied optimally in the different stages of drying so that a short construction of the dryer section, a good quality of the paper and a runnability sufficiently free from disturb- ance are achieved.

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.

It is a further object of the present invention to increase the drying capacity by means of ventilation and/ or impingement-drying and in this way to make the length of the dryer section shorter, which contributes to an improvement of the runnability of the dryer section.

It is a further object of the invention to provide such a drying method and drying equipment by whose means, in said high speed range, the length of the dryer section in the machine direction can, nevertheless, become reasonable so that its length does not, at least not substantially, exceed the length of the cylinder dryers currently in operation. An achievement of this objective would permit renewals and modernizations of paper machines in existing paper machine halls up to, and even beyond, a web speed of v = 40 metres per second.

It is a further object of the invention to provide a dryer section wherein the web is reliably affixed to the drying wire over the entire length of the dryer section so that cross-direction shrinkage of the web can be substantially prevented and that, thus, cross-direction inhomogeneity in the web arising from an uneven cross-direction shrinkage profile can be avoided. It is a further object of the present invention to provide a dryer section that permits quick changes of paper grade, and in this way it is possible to improve the overall efficiency of operation of the machine.

It is a further object of the invention to provide a dryer section in which the removal of broke takes place primarily so that the times of breaks and standstills can be reduced and manual handling and disposal of the broke be practically eliminated.

It is a further object of the invention to provide a dryer section which permits profiling of the paper web that is being produced both in the machine direction and in the cross direction in view of producing a paper that is of a quality as uniform as possible and that complies with the different criteria of quality.

In view of achieving the above objectives and those that will come out later, the invention is mainly characterized in that, in addition to normal contact drying cylinders, one or several contact drying cylinders with a larger diameter, the diameter being D₄ > 2200 mm and, as a rule, in the range of D ~ 2200...3600 mm, is/are used in one or several groups with single-wire draw, that an impingement-drying/ventilation hood is fitted above said contact drying cylinder(s) with larger diameter, which hood extends on a considerably large sector above said contact drying cylinder and above the drying wire and the web running over the cylinder, and that an impingement-drying/ventilation effect is applied to the drying wire through said impingement-drying/ ventilation hoods on said wide sector, by means of which effect the moist boundary layer in contact with the drying wire is disintegrated and drying is promoted from the side of the face of the web opposite to the web face that is placed against the contact drying cylinders.

The invention applies impingement-drying or ventilation by using a ventilation hood, through a drying wire placed on the outer face of a paper web under contact drying on a cylinder with large diameter, which hood can extend over a sufficiently long sector in the machine direction because of the large diameter of the contact drying cylinder concerned. Thus, the large diameter of the contact drying cylinder con- cerned can be utilized efficiently by means of these impingement-drying/ventilation hoods, and by means of the jets of drying gases applied from the hoods or by means of air flows of a direction contrary to the running direction of the wire, it is possible to disintegrate the moist boundary layer on the outer face and in the loops of the wire efficiently, and thus it is possible to prevent a growth of the component pressure of water steam in the wire placed on the drying cylinder and thereby to improve the heat- transfer coefficient.

A hood arrangement in accordance with the invention is preferably applied in the final end of the dryer section, where the rate of evaporation tends to become lower and where also water present in the fibres is removed. In this connection the web temperature also rises quite quickly. The invention is preferably applied to the last stage of drying and especially when the three stage method is applied that is described in the FI Patent Application No 971714 to be filed on the same day with the present patent application.

The dryer section in accordance with the invention also operates preferably so that in the first stage of the drying, in which the web is mainly heated, the drying effect, preferably a drying effect free of contact, is applied by means of drying gas flows and/or infra radiation on a short section through the upper face of the web. In the following stage, i.e. the main evaporation stage, and/or in the area of decreasing rate of evaporation in the end of the dryer section, the drying effect is applied by means of contact drying cylinders, preferably with the aid of impingement-drying through the lower face of the web, and in the final stage, in which also water present in the fibres is removed and the temperature of the web is raised at the same time, the drying effect is applied through both sides of the web, from the lower face with contact drying cylinders and from the upper face with ventilation/ impingement- drying hoods in accordance with the invention.

The invention can also be carried into effect so that the dryer section in accordance with the invention comprises one or several groups with single- wire draw, in which group/groups, there are contact drying cylinders with larger diameter and contact drying cylinders with smaller diameter alternating with each other, or two successive contact drying cylinders with larger diameter and one contact drying cylinder with smaller diameter alternating with each other. In such a case, the cylinders with smaller diameter are fitted on a lower level in the gaps between the cylinders or pairs of cylinders with larger diameter. Above the cylinders with larger diameter and possibly also above the cylinders with smaller diameter, inside the drying-wire loop, an impingement-drying/ventilation hood or hoods has/have been fitted.

In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments.

Figure 1 is a schematic side view of a highly favourable dryer section in accordance with the invention.

Figure 2 shows two groups with single-wire draw applicable in the invention and the closed draw over the group gap between said groups, in a scale larger than Fig. 1.

Figure 3 shows such a wire group applicable in the invention in which drying and reversing cylinders are used that are placed on four horizontal levels, one level above the other.

Figure 4 shows such a group with single-wire draw applicable in the invention in which contact drying cylinders of small diameter and of large diameter are used that are placed alternatingly in the gaps between said cylinders.

Figure 4A shows such a variation and modification of the group with single-wire draw as shown in Fig. 4 in which there are, fitted alternatingly as interlocking with each other, pairs of contact drying cylinders with large diameter as well as contact drying cylinders with small diameter. Figure 5 shows such an embodiment of the invention in which, in one cylinder group, a drying cylinder with large diameter is used together with a wire group that is provided with an impingement hood fitted in accordance with the invention.

Figure 1 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 IC_Q = 35...60 % and at a temperature of T₀ ~ 30...65 °C on the bottom face of the press fabric 11 and supported by a Press Run™ 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. In the unit Ri , an intensive drying energy impulse is applied to the web W, in which connection, after the unit R_j, the temperature of the web W is T_j ~ 65...85°C. In the unit R_j, primarily heating of the web W and of the water contained in it takes place, but no substantial evaporation of water as yet.

In the unit Ri , the paper web W 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 W, which is still relatively moist and, thus, of low strength. 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 W. Additionally or alternatively, it is possible to employ 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.

In Fig. 1, the unit R_j is followed by the first so-called normal (not inverted) single- wire unit R₂, 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 R₂, and so also the subsequent single- wire units R₄, R₆, R₇ and Rg that are open towards the bottom comprise steam-heated contact-drying cylinders 20;60,60A fitted in the upper row and reversing suction rolls 21;61 fitted in the lower row, for example the applicant's said VAC-rolls™. Below the cylinders 20, there are doctors 24; 64 and ventilation blow devices 25; 65. The paper web W to be dried enters into direct contact with the faces of the steam-heated drying cylinders 20;60;60A, and on the reversing suction rolls 21;61 the web W remains on the drying wire 22 at the side of the outside curve.

In Fig. 1 , after the group R₂ with single- wire draw, there follows a drying unit R₃ which comprises two contact-drying cylinders 30 and a large-diameter D_j impingement-drying/through-drying cylinder 31 with a perforated mantle, which cylinder will be called a large cylinder in the following. Around the contact-drying cylinders 30 and around the large cylinder 31, 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_γ of the drying unit R is fitted in the basement space KT underneath the floor level K_i—K_j of the paper machine hall on support of the floor level K₂-K₂ of said space. The central axes of the contact-drying cylinders 30 in the unit R₃ and in the corresponding second drying unit R₅ 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^— K_j, preferably slightly above said plane. The paper web W to be dried is passed from the single-wire unit R₂ as a closed draw onto the first drying cylinder 30 in the drying unit R₃, after which the web W is passed on the wire 32 of the unit R₃ over the large cylinder 31 of the first module M_j on a remarkably large sector > 180° on support of the drying wire 32 and further onto the second drying cylinder 30 in the unit R₃. From this drying cylinder 30 the web W is transferred as a closed draw into the next normal unit R₄ with single-wire draw, which unit is substantially similar to the unit R₂ described above. After this, there follows the second drying unit R₅, which is similar to the drying unit R₃ described above and whose large cylinder 31 is also placed in the basement space KT. In the basement space, besides the modules M_j and M₂, Fig. 1 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. In the event of a web break, the web W can be passed after the unit R_j directly into the pulper 40a placed underneath. The single- wire units R₂, R₄, R₆, R₇ and R_g 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. Also the modules M_j and M₂ 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.

Underneath the modules M_j and M₂, above the floor level K₂— K₂ of the basement space KT, there is still space KT₀ 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_j and M and also into the hoods 66 of the cylinders 60 A of the units R₆, R₇ and R₈. Said lower space KT₀ is defined from below by the floor level K₂— K₂ of the basement space and from above by the partition wall 42 placed below the broke conveyor 41. On the drying units R₂...R₈ there is an air-conditioned hood 50 in itself known.

The more detailed construction of the impingement-drying hood modules M_l and M₂, shown in Fig. 1 is described in the applicant's FI Patent Application No. 971713 that was filed on the same day with the present application. Additionally, the method in accordance with Fig. 1 preferably applies the three-stage drying method described in the applicant' s FI Patent Application No. 971714 which was filed on the same day with the present application. In this case the stage I of the latter FI patent application is carried out by the unit R_j, the stage II by the units R₂, R₃, R , R₅, R₆ and the stage III by the special units R₇, R₈ or by the last special unit R₈ only.

Figure 2 shows two successive groups with single-wire draw R_n and R_n+ ι to be applied in the invention and the closed draw S of the web W between said groups. The paper web W to be dried is brought as a closed draw to the group R^ from the preceding group (not illustrated). There are five reversing suction cylinders 61 inside the wire loop 62 in the groups R_n and _n+ι. The groups R.. and Rn+ i include five contact drying cylinders 60,60A. Two middle ones 60 A of these cylinders are contact drying cylinders with a diameter larger than that of the other cylinders 60, and their diameter is D₄ > 2200 mm and as a rule in the range of D = 2200... 3000 mm (in Fig. 2 D == 2350 mm and D₅ = 1500 mm), whereas the diameter of the smaller cylinders 60 is D₅ ~ 1300...1800 mm and the diameter of the reversing suction cylinders 61 is D₆ •=• 600...1500 mm. In order that a significant advantage could be achieved over the prior art solutions with an arrangement in accordance with the invention, the diameter D₄ must be D > 1,2 x D₅. In the gaps between the reversing suction cylinders 61 there are blowing devices 65 for ventilation of the spaces between the cylinders 60, 60 A and 61 and also for promoting the drying. The upper sector free from the web W and the wire 62 on the reversing suction cylinders 61 is covered with a blocking part 64 which promotes the maintenance of the vacuum produced inside the cylinders 61.

In order that the objectives of the invention could be achieved by means of the special groups 1^ and R_{n+ j} shown in Fig. 2 and that a sufficiently high evaporation efficiency and an adequate increase in the web W temperature could be obtained, a drying effect is applied to the web W on the contact drying cylinders 60A of large diameter (D₄ > 2200 mm) also from the top face of the web W, i.e. from the side of the drying wire 62. For this purpose impingement-drying/ ventilation hoods 66 are mounted above the cylinders 60 A of large diameter D , into which hoods ventilation/drying gases are passed through the intake pipe 67. The humid ventilation air is discharged from the pressurized interiors of the ventilation hoods 66 into the hood 50 that surrounds the dryer section, from which hood the air is removed in a way known from the prior art. These drying gases are blown against the drying wire 62 on the sector d of the cylinders 60A, said sector being preferably d •=■ 180° or even larger. In this way evaporation of water through the upper face of the web W through the wire 62 is promoted. The ventilation hoods 66 are shown in their open position 66a and so also their air intake pipes are shown in their open position 67a. In this position 66a it is possible to clean and to service the ventilation hoods, and the web W threading can also be carried out most favourably then.

In the invention, by means of the special groups R_n and R_n+ ι and of other equival- ent special groups, to be described in relation to Figs. 3...5, it is possible to carry out double-sided application of drying energy and an intensified evaporation from the web W, especially in the final stage of the drying.

If the hoods 66 illustrated in Fig. 2 are, in accordance with the invention, impinge- ment-drying hoods, they have a perforated nozzle face corresponding to the curve form of the large cylinder 60A, from which nozzle face drying gases are blown with the velocity of v„ > 60 m/s, the gas temperature T being in the range of T -== 100... 300°C. The blow velocity is as a rule in the range of v_g ~ 60...140 m/s. When impingement-drying is used, the moistened drying gases are removed, for example, using the kind of a circulation arrangement that is illustrated in Fig. 3 of the applicant's said FI Pat. Appl. No. 971713. On the other hand, if the hoods 66 operate as ventilation hoods, the moistened drying gases are allowed to be discharged from the edges of the hood into the interior of the hood 50 of the dryer section, from where they are removed in a way in itself known. When a ventilation hood is used, the velocity of the drying gases is, as a rule, in the range of v„ « 40...120 m/s, and the temperature T of the drying gases is in the range of T « 20...200°C. The hoods 66 and the equivalent hoods 76,86,96 shown in Figs. 3...5 can be divided into segments by means of vertical walls placed in the machine direction, into which segments different drying gases can be passed for profiling the web W in the cross direction.

In respect of the hood constructions and of the heating and circulation equipment of the ventilation/ impingement-drying gases in connection with the hoods, reference is made to the applicant's said FI Patent Application 951746 and especially to the solutions of construction illustrated in Figs. 4A,4B,4C and 5 of said application, reproducing of said solutions being not considered necessary in this connection. The single- wire groups R_n and Rn+ _j illustrated in Fig. 2 are preferably applied in the final part of the dryer section so that group R^ i is, for example, the last group in the dryer section, in which the method stage that is described in the last stage III in the applicant's FI Patent Application No. 971714 filed on the same day with this application is applied, in which the web W is dried highly efficiently and double- sidedly; by means of the cylinders 60 and 60A as contact drying from the lower side, and from the upper side drying and evaporation efficiency are promoted by means of efficient impingement-drying/ventilation hoods 66 above the large cylinder 60 A, which hoods act on a sufficiently large sector d above the large cylinders 60A and apply a drying effect to the web W also through its upper face. In this way a double-sided, efficient drying is achieved in the last stage of drying, in which water is also removed from inside the fibres in the web W, which requires more than an average amount of drying energy. At the same time it is possible to reduce the curl caused by one-sided contact drying.

In respect of the different details of construction and operation of the hoods 66, reference is additionally made to the prior art disclosed in the applicant's FI Patent Application 951746 and in the FI Patent 83,679 of Teollisuusmittaus Oy.

Figure 3 shows three successive dryer groups R_n.ι , R^ R-n+ i, which are special groups with single- wire draw and open towards the bottom, whose more detailed construction is described in the applicant's FI Patent Application No. 971713. Fig. 3 shows grouping between heated drying cylinders 70A and 70 and cold cylinders 71 A and 71, i.e. reversing rolls. As is shown in Fig. 3, each dryer group is com- posed of a group of cylinders in which the heated drying cylinders 70A, 70 are placed on two levels, the levels Y_j and Y₂. The reversing cylinders 71A have been placed on the level Y'₂ and the reversing cylinders 71 on the level Y₃. The heated drying cylinders 70 placed on the middle level Y₂ are, moreover, of a smaller diameter than the drying cylinder 70A placed on the level Yi .

Together with the wire 72, the paper web W is brought onto the drying cylinder 70A on the level Y_j from the reversing cylinder 71 A on the level Y'₂. From the drying cylinder 70 A the web W is passed onto the reversing cylinder 71 A and from there further, together with the wire 72, to the middle heated drying cylinder 70. From the heated drying cylinder 70 the paper web W and the wire 72 are passed onto the second reversing cylinder 71 on the level Y₃. From the reversing cylinder 71 the paper web W is passed onto the second heated drying cylinder 70A with a larger diameter on the level Y_j, from where the web W is passed onto the reversing cylinder 71 A on the level Y _. In a corresponding way the wire 72 is passed from one cylinder onto another. The paper web W runs against the faces of the drying cylinders on the heated drying cylinders 70A and 70. On the reversing cylinders 71 A and 71, the paper web W runs on the outside and the wire 72 against the face of the cylinder. The reversing cylinders 71 are placed symmetrically at the same distance from the vertical plane drawn through the centre of the heated drying cylinder 70 of smaller diameter. The heated drying cylinders 70 A of larger diameter D₄ > 2200 mm are also placed symmetrically at the same distance from the vertical plane. In a corresponding way the reversing cylinders 71 A of the level Y'₂ are placed symmetrically to the vertical centre plane and at the same distance from it.

In Fig. 3, in the group R_JJ, a ventilation/impingement-drying hood 76 is fitted above the drying cylinders 70A of larger diameter, which hood extends over the cylinders 70A of large diameter D₄ > 2200 mm on a considerably large sector in order to apply the drying effect to the web W from the side of the drying wire 72, i.e. through the upper face of the web W while the web is on the cylinders 70A subject to a direct contact drying by the contact drying cylinders through its lower face. In Fig. 3 the hood 76 is shown to be placed above two middle cylinders 70A in the group R_JJ, but it can be extended to cover several, also all of the cylinders 70 A in the group R^ The air intake pipe of the hood 76 is denoted with the reference numeral 77.

The two successive wire groups R_n and R_{n+ 1} shown in Fig. 4 are in the other respects substantially similar to prior-art normal (non-inverted) dryer groups except that the dryer section comprises contact drying cylinders 80A and 80 of two different diameters D₄ and D₈, which cylinders are placed in a very compact arrangement alternatingly. Large cylinders 80A with larger diameter D₄ > 2200 mm, as a rule D₄ ~ 2200...3600 mm, are placed on the highest level T_j and the small cylinders

80 with smaller diameter D₈ are placed on a lower level T₂, the difference in height between these levels Ti— T₂ being h₂— hi . Reversing cylinders 81 have been placed between the drying cylinders 80, 80A on a level T₃ substantially lower than said levels T_j and T₂. The paper web W to be dried runs through the dryer groups R_JJ, R_{n+ 1} supported by the drying wires 82, guided by leading rolls 83, so that on the reversing cylinders 81 the web W remains at the outside curve and on the drying cylinders 80,80A the web W, pressed by the drying wire 82, enters into a direct and immediate contact with the heated face of the drying cylinders 80,80A. The diameters of the large cylinders 80A, the small cylinders 80 and the reversing cylinders

81 are chosen so that D₄ > D₈ > D₉.

Fig. 4 shows a wide impingement-drying/ventilation hood 86 to be fitted inside the drying- wire loop 82, which hood extends above all large cylinders 80A in the group R_n. Said hood 86 extends on the first large cylinder 80A onto its sector whose magnitude is about 90°, on the middle large cylinder 80A onto a sector whose magnitude is about 180°, and on the last large cylinder 80A onto a sector, whose magnitude is also about 90°. In this way, also taking into consideration the large diameter D > 2200 mm of the large cylinders 80A, a sufficiently wide impingement-drying/ventilation effect can be achieved through the wire 82 in order to enhance the drying taking place from the upper surface of the web W in the way described above and in accordance with the present invention.

In accordance with Fig. 4, it is possible to provide a particularly compact dryer section so that substantially more "hot coverage" than in prior art is obtained per metre of horizontal length of the dryer section in the machine direction. This can be achieved partly thereby that, by means of the drying cylinders 80,80A, cover sectors larger than previously can be obtained. Said cover sectors are typically in the range of 220°...260°. In the exemplifying embodiment shown in Fig. 4 there is a small cylinder 80 (smaller diameter D₈) as the first cylinder in each wire group R_n,R_{jj+ 1}... and a large cylinder 80 A (larger diameter D₄) as the last cylinder, on whose free sector the web W is transferred as a closed draw WC onto the wire 82 of the next group and further onto the first reversing cylinder 81. In Fig. 4 the horizontal spacing of the reversing cylinders 81 placed at both sides of the small cylinders 80 is l _χ, and the spacing of the reversing cylinders 81 placed at both sides of the large cylinders 80 A is 1₂ and l _j > 1₂. The horizontal spacing between the large and small cylinders 80 A, 80 is uniform spacing s both within a group and in group gaps between groups R_JJ, R_{JJ+ 1}. . . , in which a closed draw WC is preferably applied. In group gaps, said spacing can also be different from the above.

Fig. 4A shows a modification and variation of the group with single-wire draw described above in relation to Fig. 4. In the following, mainly just the features of the group R_JJ with single- wire draw shown in Fig. 4 A will be described that differ from those described above in relation to Fig. 4, because the other constructions, measures, and operational properties are similar to those described above in relation to Fig. 4. In the group R_JJ shown in Fig. 4 A, placed one after the other, there is a pair of drying cylinders 80B with larger diameter D , which pair is preceded and followed by a contact drying cylinder 80 with smaller diameter D₈. The latter cylinder 80 with smaller diameter is again followed by a pair of drying cylinders 80C with larger diameter. In the gaps between the cylinders 80,80B,80,80C, on the lowest level T₃, there are reversing suction cylinders 81 in the way described in relation to Fig. 4. The particular features and advantages of the cylinder geometry of a group R_JJ with single- wire draw as shown in Fig. 4 A are described in more detail in the applicant's FI Patent Application No. 971655.

As is shown in Fig. 4A, inside the wire loop 82 of the group R_JJ with single-wire draw, a through-drying hood 86 A for blowing through the wire has been fitted. This hood 86A extends and covers the latter cylinder in the pair of cylinders 80B and the former cylinder in the pair of cylinders 80C as well as the cylinder 80 with smaller diameter D₈ placed between said larger cylinders. The hood 86 A extends over the adjacent cylinders 80B and 80C on a sector of about 180°, and over the smaller cylinder 80 placed between said cylinders on a sector of about 90°. The latter covering sector on the cylinder 80 is defined by the lower walls 87 of the hood 86 A, which walls are placed in the intermediate spaces between the cylinders 80B, 80 and 80C. The cross-direction edges and the machine-direction end edges of the hood 86A are provided with sufficiently small gaps in relation to the drying wire 82 in order to minimize escaping of the drying air. By means of the hood 86A, through-drying of the web W by blowing through the drying wire 82 is carried out in the way described above.

Fig. 5 shows an embodiment of the invention in which there is closed draw between the successive normal groups R_JJ.J , R_JJ and R_{n+ 1}. A drying module M applying the invention has been fitted inside the latter normal group R_JJ+ J , which module comprises a large cylinder 90A against whose heated face 90' the face of the web W is placed that is opposite to the face placed against the drying cylinders 90 in normal groups R_JJ. The web W comes from the drying cylinder 90 of the normal group R-_i-_i- l as a short open draw W₀ to the large cylinder 90 A and moves from the large cylinder 90A correspondingly as a short open draw W₀ to the drying wire 92 of the group R_n+ ι at the leading roll 93 a and from there further to the drying cylinder 90b, continuing through the group R_{n+ 1} supported by the same drying wire 92. In addition to the drying module M fitted inside the normal group R_JJ+ J it is possible to use a drying module similar to that shown in Fig. 5 in the group gaps between the normal groups R_n.j, R_jj and R_{n+ 1}. In connection with a drying module M fitted inside a normal group R_n+ι it is, naturally, also possible to use a closed draw of the web W without an open gap.

A wire 97 that is placed outside the module M is fitted in connection with the large cylinder 90A, which wire is guided by the leading rolls 98 and 93. Said outer wire 97 presses the web W to be dried against the heated face of the large cylinder 90A. An impingement-drying/ventilation hood 96 is fitted, in accordance with the invention, inside the loop of the outer wire 97, which hood extends on the large cylinder 90 A over a sector of about 180° so that the area of effect of impingement-drying/- ventilation blowing can be made large enough in view of the objectives of the invention. This is achieved partly thereby that the diameter D₄ of the large cylinder 90 A is chosen as D₄ > 2200 mm, preferably D₄ ~ 2200...3600 mm.

In the module M the effect of the impingement-drying/ventilation hood 96 is applied to the web W through its lower face in connection with the wire 97 and through the wire, contrary to the hoods 66; 76; 86; 86 A shown in Figs. 1...4 and 4A. On the other hand, the large cylinder 90 dries the web W as contact drying through its upper face. In Fig. 5 the module M and its hood 96 are placed in the basement space KT underneath the floor level Kι— K_j of the paper machine hall, above the floor level K₂— K₂ of said space KT. There are no broke removal problems in a drying module M in accordance with Fig. 5, because the outer wire 97 prevents access of shreds of paper broke to the nozzle face (not illustrated) in the hood 96.

In respect of the construction of the impingement-drying/ventilation hoods 66; 76; 86;86A;96 and of the related air circulation arrangements, reference is made, by way of example, to said FI Patent Application 971713, to be filed on the same day with the present application, and especially to its Fig. 3. The hood (35), its nozzle face (60), its nozzle perforations (61), and the air circulation arrangements (A A₉, 65...67) and cross-direction profiling arrangements of the web W (m _x ... m_N , A_{JJJ x} ... - A_jj,_N, 35k), illustrated in said Fig. 3, can be applied, as modified if necessary, also with the present invention, taking into account the preferred dimensioning and operating parameters and arrangements of equipment suggested in the following.

The temperature of the blowing air in the hoods 66; 76; 86; 96 is in impingement- drying ~ 100...200°C and in a ventilation hood ~ 20...200°C. The rate of blowing is chosen in impingement-drying in a range of ~ 60...140 metres per second and in the ventilation hood in the range ~ 40...120 metres per second. The distance from the nozzle face to the web W or wire 62; 72; 82; 92 is dimensioned in the range of 10...50 mm, preferably in the range 20...30 mm. The diameter of the nozzle holes is 3...10 mm, preferably 4...6 mm. The proportion of open area of the blow nozzles in the nozzle face is dimensioned in the range 0,5...5 %, preferably in the range 0,5...2 %.

The air system of the hoods 66;76;86;86A;96 can be one of the following systems (1), (2) or (3) depending on the temperature level applied, on the amount of blowing air, or on the location in the dryer section:

(1) A separate air circulation arrangement, in which recirculated air is sucked from the impingement-drying/ ventilation hoods 66; 76; 86; 86 A; 96, from which air the water evaporated from the area of the hood 66;76;86;86A;96 is removed by means of a certain exhaust air flow, and to which air compensation air and/or combustion air, depending on the method of heating, is added in order to maintain the air balance. In this arrangement the hoods 66;76;86;86A;96 comprise a return air space connected with the return air side of the circulation air system, into which return air space the blowing air used in drying and the water evaporated in drying are sucked.

(2) An air circulation arrangement, where the air needed for blowing is mainly taken from the closed hood 50 covering the dryer section. Compensation air, which can be pre-heated, can be mixed with this air in order to lower the humidity of the air. Heating of either blowing air or of compensation air is used to reach the correct temperature level of the blowing air. In this case the evaporated water is discharged along with the used drying air into the closed hood 50 and is carried out along with the exhaust air flow. One alternative is a solution in which the blowing air taken from the closed hood 50 is sucked from a point that is close to the impingement-drying/ventilation hoods 66;76;86;86A;96, and, thus, a certain air circulation arrangement is provided, even though there is no return air space proper in the hoods, and in this way a better energy efficiency can be achieved.

(3) Direct blowing-air arrangement, in which the drying air is taken from outside the dryer section and heated. The air devices can be placed outside the hoods proper or be partly or totally integrated with the impingement-drying/ventilation hoods 66;76;86;86A;96 proper. Depending on the temperature level applied, on the amount of blowing air, on the location in the dryer section, or on the energy cost level of the mill and on the availability of different fuels, the heating of the blow air for the hoods 66; 76; 86; 86A;96 can be carried out by one of the following methods or by combinations of the methods: by means of a gas burner either directly in the drying air flow or in a heat exchanger system; by means of an oil burner either directly in the drying air flow or in a heat exchanger system; by means of steam radiators or electric radi- ators.

In the starting and heating phase of the hood system, in the event of a web break, during threading and in a maintenance and cleaning situation, the hoods 66;76;86; 86A;96 are preferably arranged to be moved to a larger distance from the web W or wire 62;72;82;92, which makes working and surveillance of the web W or its leader end easier and reduces the heat load caused by the hot nozzle face on the bare wire or cylinder faces.

In the starting and heating phase of the hood system, in the event of a web break and during threading, the air circulation arrangement based on use of gas or oil burners operates with a so-called short circulation. In the short circulation, blowing of hot blowing air directly onto bare wire or cylinder faces is prevented by directing the circulation air through a bypass duct straight into the return air duct. In such a case the burner does not have to be turned off, for example, in a break situation, which would require a new, rather long starting sequence.

In a way in itself known, impingement-drying/ventilation hoods 66;76;86;86A;96 arranged in accordance with the invention can also be used to regulate the cross- direction moisture profile of the web W by dividing the hoods in cross direction into different blocks, so that it is possible to pass a drying gas of adjustable state into each block. In the following, the patent claims will be given, and the various details of the invention can show variation within the scope of the inventive idea defined in said claims and differ from what has been described above by way of example only.

Claims

1. A dryer section of a paper machine which consists of several groups with single- wire draw that include a drying wire (22,32,62,72,82,92), inside whose loop there is a number of reversing cylinders (21,61,71,81,91), and in the gaps between them there are contact drying cylinders (20,60,60A,70,70A,80,80A,90,90A), against whose heated faces said drying wire (22,32,62,72,82,92) presses the paper web (W) to be dried into direct contact on a curve sector of over 180┬░, characterized in that in addition to normal contact drying cylinders (20,60,70,80,90), one or several contact drying cylinders (60 A, 70 A, 80 A, 90 A) with a larger diameter, the diameter being D₄ > 2200 mm and, as a rule, in the range of D₄ == 2200...3600 mm, is/are used in one or several groups (R₆,R₇,R₈;R_jj) with single-wire draw, that an impingement-drying/ventilation hood (66; 76; 86; 96) is fitted above said contact drying cylinder(s) (60 A, 70 A, 80 A, 90 A) with larger diameter D₄, which hood extends on a considerably large sector d above said contact drying cylinder and above the drying wire (22,32, 62,72,82,92) and the web (W) running over the cylinder, and that an impingement-drying/ventilation effect is applied to the drying wire (22,32,62,72, 82,92) through said impingement-drying/ventilation hoods (66;76;86;96) on said wide sector, by means of which effect the moist boundary layer in contact with the drying wire (22,32,62,72,82,92) is disintegrated and drying is promoted from the side of the face of the web opposite to the web (W) face that is placed against the contact drying cylinders (60A,70A,80A,80A).

2. A dryer section as claimed in claim 1, characterized in that the group or groups (R₆,R₇,R₈;R_n,R_{jj+ 1}) with single-wire draw provided with said impingement-drying/ventilation hoods (66; 76; 86; 96) is/are fitted in the final end of the dryer section mainly in the area in which the last stage of the evaporation drying of the web (W) is carried out so that also the water present in the fibres in the web (W) is evaporated while the drying efficiency is lowered and the web (W) temperature rises as the drying proceeds and approaches the end.

3. A dryer section as claimed in claim 1 or 2, characterized in that the coverage d of said impingement-drying/ventilation hoods (66; 76; 86; 96), at least on some cylinders (60 A, 70 A, 80 A, 90 A) with large diameter D₄, is d ┬½ 180┬░ or d > 180┬░.

4. A dryer section as claimed in any of the claims 1 to 3, characterized in that the dryer section comprises one or several groups (R₆,R₇,R₈) with single-wire draw which is/are open towards the bottom, that, in addition to a normal drying cylinder (60), said group or groups (R₆,R₇,R₈) comprise(s), at least two contact drying cylinders (60A) with large diameter (D₄), on whose top there is said impingement- drying/ventilation hood (66), and that on both sides of said contact drying cylinders (60A) with larger diameter (D₄) there are contact drying cylinders (60) with smaller diameter (D₅), and below said contact drying cylinders (60, 60 A) and at the gaps between them there are reversing suction cylinders (61) (Fig. 2).

5. A dryer section as claimed in any of the claims 1 to 4, characterized in that when an impingement-drying hood (66,76,86,96) is employed, the velocity v of the drying gases applied from its perforated nozzle face is in the range v_g > 60 m/s, preferably in the range v_g ΓÇó=ΓÇó 60...140 m/s, and the temperature of the impingement- drying gases is in the range T ~ 100...300┬░C, and that, when a ventilation hood (66; 76; 86; 96) is employed, the velocity of the ventilation gases is in the range v ~ 40...120 metres per second and the temperature in the range T ~ 20...200┬░C.

6. A dryer section as claimed in any of the claims 1 to 5, characterized in that in the beginning of the dryer section there is a particular dryer group (R_j) in which drying gases and/or drying radiation is applied to the upper face of the web (W) free of contact in a short section (L_╧ç) in the machine direction, that this is followed by groups (R₂,R₄) with single-wire draw that are open towards bottom and/or impingement-drying hood modules (M╬╣ ,M₂), after which there is/are one or several groups (R₆,R₇,R₈) with single-wire draw in which groups, in addition to normal drying cylinders (60), there are contact drying cylinders (60A) with a larger diameter (D₄) and provided with impingement-drying/ventilation hoods (66), the number of these last mentioned groups (R₆,R₇,R₈) being preferably two or three (Fig. 1).

7. A dryer section as claimed in claim 5, characterized in that the dryer section comprises one or more impingement-drying hood modules (M_|,M₂) that are open or openable towards the bottom and placed in the basement space (KT) below the dryer section so that removal of broke can take place by the force of gravity down onto the broke conveyor (41) placed in the lower part of the basement space (KT) (Fig. 1).

8. A dryer section as claimed in any of the claims 1 to 7, characterized in that the dryer section comprises a group or groups (R_n,R_{n+ 1}) with single-wire draw in which there are contact drying cylinders (80,80A;80,80B,80C) with a larger diameter (D₄) and with a smaller diameter (D₈) placed one after the other and between them, on a lower level (T₃), reversing suction cylinders (81), of which drying cylinders the cylinders (80) with a smaller diameter are placed on a lower level (T₂) than the cylinders (80A;80B,80C) with a larger diameter (D₄), and that above said cylinders (80A;80B,80C) with larger diameter and/or above said cylinders (80) with smaller diameter, an impingement-drying/ventilation hood (86;86A) or hoods has/have been fitted inside the drying wire loop (82) (Figs. 4 and 4A).

9. A dryer section as claimed in claim 8, characterized in that the dryer section comprises a group (R_JJ) or groups with single-wire draw, in which group/groups there are contact drying cylinders (80, 80 A) with larger diameter (D ) and with smaller diameter (D₈) alternating with each other (Fig. 4).

10. A dryer section as claimed in claim 8, characterized in that the dryer section comprises a group (R^) or groups with single-wire draw, in which group/groups there are pairs of contact drying cylinders (80B,80C) with larger diameter (D ) and, in the gap or gaps between said pairs, a contact drying cylinder (80) with smaller diameter (D₈), and that above said cylinders (80B,80C) with larger diameter (D₄) and preferably also above said cylinders (80) with smaller diameter (D₈) placed between said pairs of cylinders, a through-drying hood (80 A) or hoods for blowing through the wire has/have been fitted (Fig. 4A).

11. A dryer section as claimed in any of the claims 1 to 6, characterized in that the dryer section comprises a row of contact drying cylinders (70A) with larger diameter (D₄) placed on the highest level (Y_j), reversing suction cylinders (71 A) fitted at every second gap between said cylinders (70A) with larger diameter (D₄) and placed on the following, lower, level (Y'₂), and contact drying cylinders (70) with smaller diameter fitted between every other free gap between said cylinders (70A) with larger diameter (D₄) and placed on the following lower level (Y₂), and reversing cylinders (71) fitted on both sides of said contact drying cylinders (70) with smaller diameter and placed on the lowest level (Y ), and that above said contact drying cylinders (70 A) with larger diameter (D₄) placed on the highest level (Y_j), an impingement-drying/ventilation hood (76) or hoods have been fitted, which hoods extend over a considerably large sector (Fig. 3).

12. A dryer section as claimed in any of the claims 1 to 6, characterized in that inside a group (R_{n+ 1}) with single-wire draw or in a gap between two successive groups with single- wire draw, a contact drying cylinder (90 A) with a large diameter (D ) has been fitted instead of a reversing suction cylinder, and that the paper web (W) is passed around said contact drying cylinder (90A), the upper face of the web being arranged to be pressed from outside against the smooth face (90') of said contact drying cylinder (90 A) by means of the wire loop (97) placed below, inside which loop an impingement-drying/ventilation hood (96) has been fitted on a considerably large sector, preferably extending over 180┬░ (Fig. 5).

13. A dryer section as claimed in claim 12, characterized in that said lower contact drying cylinder (90A) with large diameter and the lower wire (97) running around it form a hood module (M) which is placed mainly in the basement space (KT) below the floor level (K_jΓÇö K ) of the paper machine hall (Fig. 5).

14. A dryer section as claimed in any of the claims 1 to 13, characterized in that, in order to carry out the last stage, the final end of the dryer section comprises one or several groups (R₆,R₇,R₈) with single- wire draw, in which, above one or more contact drying cylinders (60A), a hood structure (66) is placed, which enhances the evaporation through the wire (62).

15. A dryer section as claimed in any of the claims 1 to 14, characterized in that the dryer section comprises two or three groups (R₂,R₄,R₆) with single-wire draw that are open towards the bottom, between and/or after which groups there is/are an impingement-drying group or groups (R₃,R₅), provided with a drying wire, which groups comprise an impingement-drying/through-drying cylinder (31) with a large diameter (Di) and a hood (35) surrounding the cylinder and contact drying cylinders (30) above said cylinder (31) at both sides, into which groups the web (W) is brought from the preceding group (R₂,R₄) with single- wire draw and from the latter one of which groups (R₅) the web (W) is passed into the following group (R₆) with single- wire draw as a closed draw (Fig. 1).

16. A dryer section as claimed in claim 15, characterized in that said impingement- drying/through-drying cylinders (31) are placed in the basement space (KT) below the floor level (K_jΓÇö K_╧ç) of the paper machine hall while the groups (R₂,R₄,R₆, R₇,R₈) with single-wire draw are placed above said cylinders (31) substantially above the floor level (K_jΓÇö K_j) of the paper machine hall (Fig. 1).

17. A dryer section as claimed in any of the claims 14 to 16, characterized in that in respect of said groups (R₂,R₄,R₆,R₇,R ) with single-wire draw and while the hoods (35) of the impingement-drying/through-drying cylinders (31) are opened, removal of broke can take place by the force of gravity onto the broke conveyor (41) or equivalent placed below.

18. A dryer section as claimed in any of the claims 1 to 17, characterized in that, after its first unit (R_j), the dryer section comprises a unit (R₂) with single- wire draw, after that an impingement-drying hood unit (R₃,M₁), after that a unit (R ) with single- wire draw, after that a second impingement-drying hood unit (R₅,M₂), after that a third group with single-wire draw, after that a third impingement-drying hood unit and, in order to carry out the final stage of the drying, one or several single-wire units (R₆,R₇,R₈) in which there are reversing suction rolls (61) in the lower row inside a drying wire loop (62) and contact drying cylinders (60, 60 A) in the upper row, at least two of which cylinders (60A) have a larger diameter (D₄) than the other cylinders (60) have, which larger cylinders are provided with a ventilation/impingement-drying hood (66).

19. A dryer section as claimed in any of the claims 13 to 18, characterized in that the impingement-drying/ventilation hoods (66;76;86;86A;96) are divided into blocks in the cross direction of the machine, into which blocks it is possible to pass drying gases of different states in order to regulate the cross-direction profile of the web, especially the moisture profile.