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 mnnabilify 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 ( = 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 particu- lar 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, 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-Al).

Additionally with respect to the prior art related to the present invention, reference is made to the applicant's Fl 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 com- prises 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 web 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 moderniz- ations 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 D4 = 2200...3600 mm, is/are used in one or several groups with single-wire draw, that an impinge- ment-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 accom- panying 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 ko ~ 35...60 % and at a temperature of To - 30...65"C on the bottom face of the press fabric 11 and supported by a Press Run'" box 1 la onto the top face of the drying wire 12 over its guide roll 13.

The first planar drying unit R1 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 R1, an intensive drying energy impulse is applied to the web W, in which connection, after the unit R1, the temperature of the web W is T1 ~ 65...85°C. In the unit R1, 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 R1, 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 R1 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 R1 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 R8 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 D1 impinge- ment-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 M1 of the drying unit R3 is fitted in the basement space KT underneath the floor level K1-K1 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 K1-K1, 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 M1 on a remarkably large sector > 180O 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 trans- ferred 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 M1 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 R1 directly into the pulper 40a placed underneath.

The single-wire units R2, R4, R6, R7 and R8 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 M1 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 M1 and M2, above the floor level K2-K2 of the basement space KT, there is still space KTo 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 M1 and M2 and also into the hoods 66 of the cylinders 60A of the units R6, R7 and R8. Said lower space KTo 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 .... .R8 there is an air-conditioned hood 50 in itself known.

The more detailed construction of the impingement-drying hood modules M1 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 R1, the stage II by the units R2, R3, R4, 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 Rn and Rn+1 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 Rn from the

preceding group (not illustrated). There are five reversing suction cylinders 61 inside the wire loop 62 in the groups Rn and R,+1. The groups Rn and Rn+ 1 include five contact drying cylinders 60,60A. Two middle ones 60A 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 D4 z 2200...

3000 mm (in Fig. 2 D4 = 2350 mm and D5 = 1500 mm), whereas the diameter of the smaller cylinders 60 is D5 z 1300...1800 mm and the diameter of the reversing suction cylinders 61 is D6 Z 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 D4 > 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 60,60A 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 Rn and Rn+ 1 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 60A of large diameter D4, into which hoods ventila- tion/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 = 1800 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 Rn and Run+ 1 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 vg > 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 vg z 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 Fl Pat. Appl. No. 971713. On the other hand, if the hoods 66 operate as ventilation hoods, the moistened drying gases are allowed to be dis- charged 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 vg 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 Rn and Run+1 illustrated in Fig. 2 are preferably applied in the final part of the dryer section so that group Run+ 1 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 60A, 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 Rn 1 Rns Rn+19 which are special groups with single-wire draw and open towards the bottom, whose more detailed construction is described in the applicant's Fl Patent Application No. 971713. Fig.

3 shows grouping between heated drying cylinders 70A and 70 and cold cylinders 71A 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 Y1 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 Y1.

Together with the wire 72, the paper web W is brought onto the drying cylinder 70A on the level Y1 from the reversing cylinder 71A on the level Y'2. From the drying

cylinder 70A the web W is passed onto the reversing cylinder 71A 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 Y1, from where the web W is passed onto the reversing cylinder 71A on the level Y'2. 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 71A 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 70A 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 71A of the level Y'2 are placed symmetri- cally to the vertical centre plane and at the same distance from it.

In Fig. 3, in the group R11, 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 Rns but it can be extended to cover several, also all of the cylinders 70A in the group RnX The air intake pipe of the hood 76 is denoted with the reference numeral 77.

The two successive wire groups Rn and Run+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 z 2200...3600 mm, are placed on the highest level T1 and the small cylinders 80 with smaller diameter D8 are placed on a lower level T2, the difference in height between these levels T1-T2 being h2-h1. Reversing cylinders 81 have been placed between the drying cylinders 80, 80A on a level T3 substantially lower than said levels T1 and T2. The paper web W to be dried runs through the dryer groups R, Run+ 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 diam- eters of the large cylinders 80A, the small cylinders 80 and the reversing cylinders 81 are chosen so that D4 > D8 > Dg.

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,. 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 1800, 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 D4 > 2200 mm of the large cylinders 80A, a sufficiently wide impinge- ment-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 2200...2600.

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 Rn,Rn+i... and a large cylinder 80A (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 11, and the spacing of the reversing cylinders 81 placed at both sides of the large cylinders 80A is 12 and 11 > 12. The horizontal spacing between the large and small cylinders 80A,80 is uniform spacing s both within a group and in group gaps between groups in Rn+ 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 Rn with single-wire draw shown in Fig. 4A 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 Rn shown in Fig. 4A, placed one after the other, there is a pair of drying cylinders 80B with larger diameter D4, 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 Run with single-wire draw as shown in Fig. 4A 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 Rn with single-wire draw, a through-drying hood 86A 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 86A extends over the

adjacent cylinders 80B and 80C on a sector of about 1800, 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 86A, 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 hodd 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 Run 1, Rn and Run+ 1 A drying module M applying the invention has been fitted inside the latter normal group Run+1 which module com- prises 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,. The web W comes from the drying cylinder 90 of the normal group Run+ 1 as a short open draw W0 to the large cylinder 90A and moves from the large cylinder 90A correspondingly as a short open draw W0 to the drying wire 92 of the group Run+1 at the leading roll 93a and from there further to the drying cylinder 90b, continuing through the group Run+ 1 supported by the same drying wire 92. In addition to the drying module M fitted inside the normal group Run+1 it is possible to use a drying module similar to that shown in Fig. 5 in the group gaps between the normal groups Run 1 Rn and Run+1 In connection with a drying module M fitted inside a normal group Rn+ 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 inven- tion, inside the loop of the outer wire 97, which hood extends on the large cylinder 90A over a sector of about 1800 so that the area of effect of impingement-dryingi-

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 90A is chosen as D4 > 2200 mm, preferably D4 z 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;86A 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 K1-K1 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 (A1-A9, 65...67) and cross-direction profiling arrangements of the web W (m1...mN, Amp..

AmN, 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- <BR> <BR> <BR> <BR> drying - 100.. .2000C and in a ventilation hood - 20...200°C. The rate of blowing<BR> <BR> <BR> <BR> <BR> <BR> is chosen in impingement-drying in a range of - 60... 140 metres per second and in<BR> <BR> <BR> <BR> <BR> 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; 86A;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 compen- sation 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 pro- vided, 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.