DRYING UNIT AND DRYER SECTION THAT MAKES USE OF SUCH UNITS

Abstract: The invention concerns a drying unit in the dryer section of a paper or board machine, in which drying unit there is a loop of a drying wire (32; 32a, 32b, 32c, 32d). The drying unit comprises a large-diameter D1 impingement-drying and/or through-drying cylinder (31), which is fitted inside the drying-wire loop (32, 32a, 32b, 32c, 32d). On top of said cylinder (31) and/or in the vicinity of said cylinder, at both sides, smooth-faced heated contact-drying cylinders (30) are placed, whose diameter D2 < D1. The impingement-drying and/or through-drying cylinder (31) is placed in the spaces (KT) underneath the floor level (K1-K1) of the paper machine hall and provided with an openable and closable blow hood (35) so that the removal of broke out of connection with the hood (35) takes place substantially by the force of gravity. The central axes of the contact-drying cylinders (30) placed in the vicinity of said cylinder (31) are placed in the vicinity of, or above, the floor level (K1-K1) of the paper machine hall. The curve sector b of the paper web (W) on the outer face of the drying wire (32; 32a, 32b, 32c, 32d) over said cylinder (31) is b > 180 DEG .

Description: Drying unit and dryer section that makes use of such units The invention concerns a drying unit in the dryer section of a paper or board machine, in which drying unit there is a loop of a drying wire and which drying unit comprises a large-diameter impingement-drying and/or through-drying cylinder, which is fitted inside said drying-wire loop and on top of which cylinder / in the vicinity of which cylinder, at both sides, smooth-faced heated contact-drying cylinders are placed, whose diameter D 2 < Di . Also, the invention concerns a dryer section of a paper machine, which comprises one or several group(s) with single-wire draw that is/are open towards the bottom and in which group(s) there are contact-drying cylinders in the upper row and reversing suction cylinders or rolls in the lower row, which cylinders or rolls are preferably connected to a vacuum. Even though, above and in the following, paper and paper machines are spoken of, in the present invention this notion also includes board and board machines. 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 ( « 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. One parameter that illustrates the drying capacities of the prior-art multi-cylinder dryers is the amount of water evaporated in the dryer section per unit of length and width, i.e. per floor area covered by the web to be dried, within a unit of time. In the prior-art multi-cylinder dryers, this parameter is typically in the range of 50...80 kilograms of H 2 O per square metre per hour. It is known from the prior art to use various 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-57, 457 (equivalent to SE-C-7503134-4), FI-B-87,669, and FI-A-931263 (equivalent to EP- 0,620, 313-A1). 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. In respect of the utilization of space, it is a particular object to utilize the basement space underneath the paper machine hall more efficiently, which basement space exists or is otherwise in any case needed. The efficiency of utilization of this basement space has remained relatively low with the use of so-called normal single-wire groups open towards the bottom, in which the contact-drying cylinders are placed in the upper row and the reversing suction cylinders or rolls in the lower row in the different wire groups. Thus, it is an object of the present invention, in connection with increasing the speeds of paper machines and with modernizations, to permit the fitting of the new dryer section in the place of the old multi-cylinder dryer. In relation to this, it is a further object of the invention to provide a dryer section concept which makes it possible to construct dryer sections of shorter length as 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 disturbance 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 impingement drying and/or through-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 a drying method and drying equipment which are also applicable to modernizations of dryer sections. 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 drying method and a dryer section that applies said method 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. Of the prior art cited above, the dryer section concept described in the applicant's FI Patent Application 931263 (equivalent to EP-0,620,313 Al) is most closely related to the present invention. Thus, the object of the present invention is further development and modification of this dryer section concept, which is in many respects favourable and advanced. The method in accordance with the applicant's said FI Pat. Appl. 931263 is mainly characterized in that the method comprises a combination of the following steps (a), (b), (c), and (d): (a) the paper web is contact-dried by pressing it with the drying wire on the cylinder face, whose diameter is chosen as D 2 > 1.5 m, on a sector b, whose magnitude is chosen as b > 180°; (b) evaporation drying is carried out as impingement drying and/or as through- drying by means of high-velocity drying-gas jets applied to the web on said drying wire on the face of the following large-diameter D j > 2 m cylinder on a sector a > 180° while the web is at the side of the outside curve; (c) a step (a) substantially equal to that defined above is carried out; (d) before the step (a) and/or after the step (c), the web to be dried is passed over the sector c of the suction roll, which sector c is subjected to a vacuum, while the web is supported on the drying wire at the side of the outside curve, the magnitude of said sector being chosen as c > 160°, and the diameter D 3 of said suction roll being chosen as D 3 < D 2 . On the other hand, the drying module in accordance with said FI Pat. Appl. 931263 is mainly characterized in that the drying module comprises a large-diameter Di impingement-drying and/ or through-drying cylinder, whose diameter Di > 2 m and which cylinder is placed inside the drying- wire loop, that, in the vicinity of said impingement-drying and/or through-drying cylinder, at both sides of said cylinder, smooth-faced heated contact-drying cylinders are placed, whose diameter D 2 < Di and which contact-drying cylinders are placed outside the same drying- wire loop, that, in the running direction of the web, before and/ or after said contact-drying cylinder, inside the same drying-wire loop, a reversing suction roll or rolls is/are placed, whose diameter D 3 < D 2 , that said drying cylinders and reversing suction rolls are placed so in relation to one another that on them the contact sectors of the web and of the drying wire are a > 180°, b > 180°, and the outer mantle of said impingement-drying and/or through-drying cylinder is provided with grooves and/or is penetrable by drying gas, and a drying hood being provided on the contact sector a of said mantle, in the interior of which hood, in the vicinity of the outer face of the web to be dried, there is a nozzle field, through which a set of drying-gas jets can be applied at a high velocity against the free outer face of the web to be dried over a substantial area of said sector a. In the present invention, the drying modules described in said FI Patent Application 931263 are applied in a novel way and the constructions of said modules and the overall configuration of the paper machine composed of them are modified and developed further. In said FI patent application, the impingement-drying/through- drying units are placed alternatingly as upper and lower units, and even the lower units are not placed in the basement space, but they are placed so that they can be serviced and cleaned from the floor level of the paper machine hall. Thus, the utilization of the basement space of the paper machine hall remain deficient. Also, the impingement-drying/through-drying units described in said FI patent application are difficult to service and to cleanse from paper broke in the event of a web break. Nor have the different requirements of the different stages in the drying of paper in respect of drying devices and drying techniques in view of achieving an optimal final result, utilization of space and an optimal dryer-section geometry been recognized in said FI patent application. In view of achieving the objectives stated above and those that will come out later as well as in view of avoiding the drawbacks mentioned above, the drying unit in accordance with the invention is mainly characterized in that said impingement- drying and/or through-drying cylinder is placed in the spaces substantially underneath the floor level of the paper machine hall and provided with an openable and closable blow hood so that the removal of broke out of connection with said hood takes place substantially by the force of gravity, that the central axes of the heated contact-drying cylinders placed at both sides in the vicinity of said impingement- drying and/or through-drying cylinder are placed in the vicinity of, or above, the floor level of the paper machine hall, and that the curve sector b of the paper web to be dried on the outer face of said drying wire over said impingement-drying and/or through-drying cylinder is b > 180°. On the other hand, in view of the objectives stated above, the dryer section in accordance with the invention is mainly characterized in that the dryer section comprises one or several drying unit(s) in accordance with the invention, which unit(s) is/are provided with a blow hood module which is placed in the space underneath the paper machine hall, and that said group(s) with single- wire draw is/are placed substantially above the floor level of the paper machine hall. In the present invention, impingement-drying/through-drying modules are applied, which are placed in connection with a large-diameter through-drying cylinder (in the following, large cylinder) and preferably in the basement space underneath the dryer section. Said impingement-drying/ through-drying modules are provided with hoods which can be opened quickly and simply for cleaning, such as removal of broke, and for servicing, and which can be closed likewise. For this purpose, said modules and their hoods are open or openable towards the bottom, so that removal of broke out of connection with the large cylinders can take place substantially by the effect of gravity without manual operations, at least without considerable or time-consuming manual operations. Said hoods are preferably divided into two parts which are substantially symmetric in relation to the vertical plane in the cross direction of the machine and which can be displaced by means of power units mechanically in the machine direction and in the horizontal direction in view of quick and easy opening and closing of the hoods. The diameter Di of said large cylinder is commonly chosen so that D l > 2 m, preferably Di « 2...4 m. A sufficiently large diameter of the large cylinder and a sufficiently large turning sector b ~ 220...280° of the drying wire and the web have the effect that the web has a sufficiently long impingement-drying/through-drying distance and time on said large cylinder even at high speeds. Further, said diameter of the large cylinder is chosen such that the large cylinder with its auxiliary equip- ment can be accommodated well in the basement space and that an adequate space still remains below the large cylinder for other devices, such as a broke conveyor and air ducts. The number of said impingement-drying/through-drying modules in the basement space underneath the dryer section is, as a rule, 1 to 5, preferably placed in the initial part of the dryer section. A modern high-speed dryer section can, as a rule, be accomplished by means of three impingement-drying/through-drying modules of said sort, together with the connected other drying devices. When said impingement- drying/through-drying modules are employed in renewals of dryer sections in connection with increasing the speed of a paper machine, as a rule, one such module is sufficient. Further, in the present invention, consideration has been given to the factor, decisive from the point of view of runnability of the dryer section, that, when the web is placed on the impingement-drying and/or through-drying cylinders (large cylinders) and on the reversing suction rolls, on support of the wire, at the side of the outside curve, it tends to be separated from the drying wire by the effect of centrifugal forces, while the separation force is proportional to the term 2 x v 2 / Di , wherein Di is said diameter of the large cylinder. In order to prevent this separation, on said impingement-drying and/or through-drying cylinders and reversing rolls, preferably a difference in pressure is arranged, which is measured high enough so that separation of the web is prevented in all cases and the runnability is retained also in this respect. Said difference in pressure can also be used in particular on the impinge- ment-drying and/or through-drying cylinders to promote the through-drying. In the present invention, as the drying gas, preferably either air or superheated steam is used. The state of the drying gas is chosen in each drying stage in consideration of the way in which the water is bound in the fibre mesh of the paper web in each particular drying stage. In this way, it is possible to accomplish drying stages which are optimal both in respect of the quality of the paper, in respect of the drying, and in respect of the construction of the dryer section. In a drying module in accordance with the invention, as an impingement-drying and/or through-drying cylinder and as a reversing suction roll, it is most advantageously possible to employ such drying cylinders and reversing suction rolls provided with grooved mantles with through perforations as are marketed by the applicant with the trade mark VAC™-roll and whose details come out from the applicant's FI Patent No. 83,680 (equivalent to US Pat. 5,022,163). As a through-drying cylinder it is possible to use a through-blow roll which has a higher vacuum and a larger open area. One such roll is, for example, the product marketed by the applicant with the trade mark "HONEYCOMB" roll. According to the invention, when the web is kept in stable contact with the drying wire substantially over the entire length of the dryer section, if necessary, by employing a difference in pressure on the curve sectors on which the web remains outside, cross-direction shrinkage of the web during drying is prevented, whereby cross-direction inhomogeneity of the web, arising from an uneven cross-direction shrinkage profile, is eliminated. In the present invention, the hood of the impingement-drying and/or through-drying cylinder is also understood as a pressurized hood or as a counterflow hood, and/or said large cylinder can be understood as a cylinder provided with a grooved mantle or with an equivalent wire sock mantle. In such a case, said difference in pressure, by whose means the web is kept on support of the drying wire, can be produced substantially by means of said pressurization of the hood, by means of which pressurization, if necessary, the flow of the drying gases through the web can also be produced. In respect of the details of the construction and the operation of a counterflow hood, reference is made to the FI Patent No. 83,679 of Messrs. Teollisuusmittaus Oy. In a drying module in accordance with the present invention or in a number of successive modules, the hood of the impingement-drying and/or through-drying cylinder can be divided in the cross direction, by means of walls placed in the machine direction, into a number of blocks, into which drying gases of different temperature, humidity and/or pressure are passed, or in said blocks sets of drying- gas jets of different velocities are employed. In this way, the drying of the paper web can be regulated in the cross direction, and it is possible to achieve a favourable moisture profile of a certain shape, most commonly uniform, in the cross direction. In a dryer section in accordance with the invention, the pocket placed underneath the "large cylinder" is not supposed to be subjected to a vacuum by means of a suction device placed inside the fabric loop, which is the case in said US Patent 4,033,048. Said large cylinder and so also the smaller reversing suction rolls placed in the gaps between the drying cylinders, for example the applicant's VAC™ rolls, are each of them provided with a suction duct of their own fitted in the axle of the roll. In said US Patent, between the large suction rolls, "middle rolls", which employ the same support fabric, there is just one outer roll, which can be heated. When the drying method and the drying device in accordance with the invention are employed, the drying effect is applied to the paper web from the side of its bottom face preferably over the entire length of the dryer section. This results in the tendency of curling of the web mentioned above. In order to prevent this, it is possible to use various methods and devices developed by the applicant, in respect of which reference is made by way of example to the applicant's non-public FI Pat. Appl. 964830 (filed on Dec. 03, 1996), and to the rest of the prior art referred to in said patent application. 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 drawings, the invention being by no means strictly confined to the details of said embodiments. Figure 1 shows a first preferred overall concept of the invention, which consists of three drying units in accordance with the invention and of other dryer groups in themselves known. Figure 2 is a more detailed side view of a drying unit and an impingement-drying hood module in accordance with the invention. Figure 3 shows a preferred exemplifying embodiment of the system of circulation of drying gases in connection with an impingement-drying hood module in accordance with the present invention. Figure 4 shows a preferred exemplifying embodiment of a drying unit in accordance with the invention and of the preceding group and the following group. Figure 5 is an illustration corresponding to Fig. 4 of a second exemplifying embodiment of the draw across a group gap and of the wire circulation arrangement. Figure 6 is an illustration corresponding to Figs. 4 and 5 of a third exemplifying embodiment of the group-gap draw and of the wire circulation arrangement. Figure 7 illustrates a second overall concept of a dryer section in accordance with the invention. Figure 8 illustrates a third overall concept of a dryer section in accordance with the invention, which consists of three impingement-drying hood modules in accordance with the invention and of four normal groups with single- wire draw. Fig. 1 shows a particularly favourable overall concept of a dryer section in accord- ance with the invention. In Fig. 1, it has been necessary to illustrate the oblong dryer section as two parts, which have been placed one above the other and which parts have been cut off at the cross-direction vertical plane A—A to make parts illustrated one above the other. As is shown in Fig. 1, the paper web W is passed from the press section 10 of the paper machine at a dry solids content of IC Q =*-= 35...55 % and at a temperature of T 0 ~ 30...65°C on the bottom face of the press fabric 11 and supported by a PressRun™ box 11a onto the top face of the drying wire 12 over its guide roll 13. The first planar drying unit Ri 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 R j , an intensive drying energy impulse is applied to the web W, in which connection, after the unit R j , the temperature T j of the web W is Ti « 60...80°C. In the unit Ri , primarily heating of the web W and of the water contained in it take place, but no substantial evaporation of water as yet. The length L j of the unit Ri in the machine direction is typically of an order of L j « 3...10 m. In the unit R 1 ; the paper web runs on support of the upper run of the drying wire 12 along a linear path in the horizontal plane so that it has no major changes in the direction and that, thus, no high dynamic forces are applied to it which might produce a web break in the web, which is still relatively moist and, thus, of low strength. 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. Additionally or alternatively, it is possible to employ infrared heaters. Said blow devices and/or radiators in the unit R, can be arranged so that their output in the cross direction of the web 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 R 2 , onto whose drying wire 22 the web W is transferred as a closed draw in the area of the first reversing suction roll 21. The single- wire unit R 2 , and so also the subsequent single-wire units R 4 , R 6 , R 8 , R 9 and R 10 that are open towards the bottom comprise steam-heated contact-drying cylinders 20 fitted in the upper row and reversing suction rolls 21 fitted in the lower row, for example the applicant's said VAC-rolls™. Below the cylinders 20, there are doctors 24 and ventilation blow devices 25. The paper web W to be dried enters into direct contact with the faces of the steam-heated drying cylinders 20, and on the reversing suction rolls 21 the web W remains on the drying wire 22 at the side of the outside curve. In Fig. 1, after the group R 2 with single- wire draw, there follows a drying unit R 3 in accordance with the invention, which comprises two contact-drying cylinders 30 and a large-diameter Di 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-dry ing/through-drying hood module Mi of the drying unit R 3 is fitted in the basement space KT underneath the floor level Ki— Ki of the paper machine hall on support of the floor level K 2 — -K 2 of said space. The central axes of the contact- drying cylinders 30 in the unit R 3 and in the corresponding following drying units R 5 and R 7 in accordance with the present invention are placed substantially in the floor plane of the paper machine hall or in the vicinity of said plane Ki— Ki , preferably slightly above said plane. The paper web W to be dried is passed from the single-wire unit R 2 as a closed draw onto the first drying cylinder 30 in the drying unit R 3 , after which the web W is passed on the wire 32 of the unit R 3 over the large cylinder 31 of the first module M j on a remarkably large sector b ~ 220...280° on support of the drying wire 32 and further onto the second drying cylinder 30 in the unit R 3 . From this drying cylinder 30 the web W is transferred as a closed draw into the next normal unit R 4 with single-wire draw, which unit is substantially similar to the unit R 2 described above. After this, there follows the second drying unit R 5 in accordance with the invention, which unit is similar to the drying unit R 3 described above and whose large cylinder 31 is also placed in the basement space KT. After the drying unit R 5 the web W is passed as a closed draw into the next single- wire unit Rg, which is followed by the third drying unit R 7 in accordance with the invention, whose large cylinder 31 is likewise placed in the basement space KT. The unit R 7 is followed by three successive single- wire units R 8 , R 9 and R 10 , and the web W out is passed from the last one of said units to the reel-up or into a finishing unit (not shown). In the basement space, besides the modules M 1 ( M 2 and M 3 , 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 R , R 6 , R 8 , R and R 10 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 Mi , M 2 and M 3 are open or openable towards the bottom so that the paper broke falls out of connection with them, substantially by the effect of gravity, without major manual operations, onto the broke conveyor 41 placed underneath. Underneath the modules M j , M 2 and M 3 , above the floor level K 2 — K 2 of the basement space KT, there is still space KT 0 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 Mi , M 2 and M 3 . Said lower space KT 0 is defined from below by the floor level K 2 — K 2 of the basement space and from above by the partition wall 42 placed below the broke conveyor 41. On the drying units R 2 ...R 10 there is an air-conditioned hood 50 in itself known. Figure 2 is a more detailed illustration of the impingement-drying/through-drying hood module M in accordance with the invention. As is shown in Fig. 2, the wire 32a which runs around the large cylinder 31 is first passed around the last lower cylinder 21a in the preceding group R n _ι with single- wire draw onto the first contact-drying cylinder 30 in the unit R n , from it further as a short straight run over the sector b ~ 280° of the large cylinder 31 onto the second contact-drying cylinder 30 in the group R- j and over said cylinder on a sector of about 90°. After this the web W follows the face of the cylinder 10 and is transferred as a closed draw onto the drying wire 22 of the next group R n+ l . The hood of the large cylinder 31, which consists of two parts 35, covers the cylinder substantially over the entire curve sector b of the web W. On the sector b the web W remains on the wire 32a at the side of the outside curve, so that its outer face is free. The large cylinder 31 is mounted on its axle journals 36, through which a communication is arranged with vacuum devices (not shown), by whose intermediate a suitable vacuum is produced in the interior of the cylinder 31, which vacuum is of an order of p 0 ~ 1...3 kPa. This vacuum p 0 keeps the web W on the wire 32a when the web W is at the side of the outside curve, and, at the same time, the vacuum p 0 also promotes possible through-drying taking place through the web W and the wire 32a. The sector 360°-b that remains outside the sector b on the large cylinder is covered by a cover plate 34 placed in the gap between the drying cylinders 30, and so also the last cylinder 21a in the group R n , which can also be called the reversing cylinder of the group B^, is covered by an obstacle plate 29. As to its more detailed embodiment, the perforated and grooved outer mantle 31a of the large cylinder 31 is, for example, similar to that described in said FI Pat. Appl. 931263 and illustrated above all in Fig. 11 of said patent application, so that the construction will not be described again in this connection. The large cylinder 31 is mounted by means of its axle journals 36 on support of the frame construction 37. In this frame construction, both at the driving side and at the tending side, there are horizontal beams 37a, on whose top face, or on rails provided on said top face, the hood halves 35 are arranged to be movable on wheels 39, which hood halves are illustrated in the open position 35a, in which the module M can be serviced. The hood halves 35 are displaced into the open and closed positions by actuating cylinders 38. The module M and its hood 35 are open towards the bottom, so that broke can be removed in the direction of the arrows WA substan- tially by the effect of gravity onto the broke conveyor 41 placed underneath without substantial manual operations, also when the hoods 35 are in the closed position. The top face of the hood 35 has been shaped as smoothly downwards inclined so as to improve the removal of broke. Further, in the open position 35a of the hood 35, the module M can also be serviced and cleaned easily in other respects. The diameter Di of the large cylinder 31 is, as a rule, chosen in the range of D, > 2 m, as a rule in the range of Di « 2...8 m, preferably Di ==• 2...4 m. The diameter D 2 of the drying cylinders 30 in the group R n is, as a rule, chosen in the range of D 2 •= 1.5...2.5 m, preferably in the range of D 2 « 1.8...2.3 m. In the groups R^ j and R- j +i with single-wire draw, the diameter of the drying cylinders 20 is preferably -=* D 2 . The diameter D 3 of the reversing suction cylinders 21,21a is, as a rule, chosen in the range of D 3 « 0.6...1.8 m, preferably D 3 « 1.0...1.5 m. The top face of the hood 35 has been shaped as smoothly downwards inclined to improve the removal of broke. The wire 32a guide roll 33a placed above the latter drying cylinder 30 can be stationary or displaceable. Between the groups R^, R jj and R n+ i a little difference in speed is employed, which is, as a rule, about 0.1...0.2 %, so that, in particular in the initial end of the dryer section on the wires 22, 32a, 22, the speed becomes higher when the web W moves forwards. In the final end of the dryer section, the speed can also be reduced because of shrinkage of the web in the machine direction. Fig. 3 shows an exemplifying embodiment of the arrangement of circulation of the drying gas blown through the hood 35 in the module M. In Fig. 3, a hood half 35 is shown, whose nozzle face 60, which follows the curve form of the large cylinder 31, is provided with nozzle openings 61, through which the blowings P j are directed at the outer face of the web W through the narrow gap space V between the nozzle face 60 and the outer face of the web. The radial extension of this space V is of an order of 10...50 mm. The circulation arrangement of drying gas comprises a circulation air blower 66, which blows a flow A 4 of circulation air to the gas burner 65. Into the burner 65, a combustion air flow A 2 and a fuel gas flow Ai are supplied. From the burner 65, the blow air flow A 3 departs into the impingement- drying hood 35. Out of the nozzle openings 61 in the nozzle face 60, said blowings Bi are blown against the outer face of the web W, and the temperature of said blowings is in a range of 250...400°C, preferably about 300°C. The velocity of the blowings Bi is of an order of 60...140 metres per second, preferably about 100 mps. Out of the space V between the nozzle face 60 and the outer face of the web W, the moistened air flow A 5 is sucked back to circulation. Part of this moistened air is removed as a flow A 6 through a heat exchanger unit 67. Into the heat exchanger unit 67, a dry air flow A 8 is passed, and from said unit a dry heated air flow A 7 is taken, which flow is, together with a part A 51 of the exhaust flow A 5 , passed into the circulation air blower 66 to constitute the intake air flow A 4 of said blower. Fig. 3 schematically shows the partition walls 35k fitted in the hood 35 in the vertical direction and machine direction, by means of which partition walls both of the hood halves 35 can be divided into blocks mi ...m N . By into said blocks mi ...m N feeding drying gas flows A ml ...A mN whose condition can be regulated, the cross- direction profile of the web W to be dried can be regulated, e.g., on the basis of the signal of measurement of the cross-direction profile, for example moisture profile, given by the measurement frame placed in the dry end of the dryer section. Fig. 4 shows an alternative embodiment of the circulation of the wire 32b in the drying unit R^ and for carrying out the closed draw of the web between the preceding single-wire unit R n .ι and the following single-wire unit R n+ ι . The wire 22 of the preceding unit R n-j , guided by the wire guide roll 23b, is in contact with the first contact-drying cylinder 30 in the unit R- on a sector of about 45°, in the area of which cylinder 30 the web W is transferred onto the smooth face of the cylinder 30 and further to under the drying wire 32b of the unit R n while turning on the sector Ci onto the large cylinder. Similarly, on the second contact-drying cylinder 30 in the unit R n , the web W is transferred on the sector c 2 , being pressed by the wire 32b, onto the face of the cylinder 30 and on said face further onto the drying wire 22 of the latter group R n + ι and on its face further over the first reversing suction cylinder 21 in the unit R n + i- The wire 32b guide roll 33b placed above the latter drying cylinder 30 in the unit R jj can be either stationary or of adjustable position in view of achieving an optimal closed draw of the web W. Fig. 5 shows an alternative embodiment of the arrangement of the closed draw between the drying unit R„ in accordance with the invention and the preceding unit R n .ι and the following unit R n+ l and of the circulation of the wire 32c in the unit R n . The wire 32c runs over the reversing suction cylinder 21c, which is in the position of the last reversing cylinder in the preceding unit R n .ι , on a sector of about 180°, after which the wire 32c and the web W run over the first drying cylinder 30 in the unit R n on a sector of about 180° and further onto the large cylinder 31 in the module M. From the large cylinder 31 the web W is transferred on the drying wire 32c at the transfer point Si onto the latter drying cylinder 30c so that the wire 32c contacts the face of the cylinder 30c tangentially . The wire 22 of the latter unit R n+ 1 contacts the latter drying cylinder 30c on a sector of about 90°. In the other respects the embodiment of the hood module M is similar to that described above. Fig. 6 shows an alternative embodiment of the arrangement of the closed draw of the web W between two successive modules Mi and M 2 and of the circulation of the wire 32d in the unit R n . The drying wire 32d of the unit R n is arranged to run over two drying cylinders 20d and three reversing suction cylinders 2 Id so that the drying wire 32d forms a sort of a short group with single-wire draw and the drying wire of the latter module M 2 . From the last reversing suction cylinder 2 Id the drying wire 32d and the web W are passed over the first drying cylinder 30 and further over the large cylinder 31 onto the latter drying cylinder 30 and from it further onto the wire 22 of the latter group Rn+ j . Fig. 7 shows an overall concept of a dryer section in accordance with the invention, which is a modification of the concept shown in Fig. 1. As is shown in Fig. 7, the web W, which has been dried in the press section 10 to a dry solids content of kg ~ 35...55 % , is passed along a linear path through the planar drying- wire unit R j . The unit R j is, for example, similar to that described in relation to Fig. 1. After this, there follows the first group R 2 with single- wire draw open towards the bottom, and after that the first drying unit R 3 in accordance with the invention provided with a module M j . After this, there follows a group R with single-wire draw provided with three drying cylinders 20 and the second drying unit R5 in accordance with the invention provided with a module M 2 . After the drying unit R 5 , there follows again a single-wire group R 6 provided with three drying cylinders 20 and the third drying unit R 7 in accordance with the invention provided with a module M 3 , and the last group is a group R 8 with single- wire draw provided with four drying cylinders 20. Between the groups R 2 , R , R 6 and R 8 with single- wire draw and the drying units R 3 , R 5 and R 7 in accordance with the invention, the web W has closed draws, which are shown in Fig. 7 to be accomplished mainly in the way illustrated in more detail in Fig. 4, but in connection with the overall concept illustrated in Fig. 7, where applicable, it is also possible to employ the group-gap draws and wire circulation arrangements shown in Figs. 1,2,5 and 6. The modules M j ,M 2 and M 3 are similar to that described above and placed in the basement space KT. The removal of broke and the other arrangements are similar to those described above in relation to Fig. 1 , or equivalent. Fig. 8 shows an overall concept of a dryer section composed of drying units R j , R 3 , R 5 , R 7 in accordance with the invention and of their modules Mi , M 2 , M 3 , M and of groups R 2 , R , R 6 , R 8 with single- wire draw. The paper web W to be dried is passed on the lower fabric 10a of the press section 10 onto the bottom face of the drying wire 32 of the first group Ri on support of the PressRun™ boxes 11a. The first group Ri is a dryer group in accordance with the invention provided with a hood module Mi . The group R j is followed by a normal group R 2 with single-wire draw, in which there are three drying cylinders 20 and four reversing suction cylinders 21. The group R 2 is again followed by a second dryer group R 3 in accordance with the invention, which is provided with a second module M 2 placed in the basement space KT, after which there is further a fourth single- wire group R provided with three drying cylinders 20 and four reversing suction cylinders 21. This group is followed by a third group R 5 in accordance with the invention, which is provided with a hood module M 3 placed in the basement space KT. The group R 5 is followed by a single-wire group R 6 provided with three drying cylinders 20 and four reversing suction cylinders 21, after which there follows the last group R 7 in accordance with the invention, which is provided with a fourth hood module M placed in the basement space KT. The last group is a single- wire group R 8 provided with three drying cylinders and four reversing suction cylinders 21, from which group the web W out is passed to finishing or to a reel-up. In Fig. 8, the draws of the web W over the group gaps are closed, and they have been accomplished substantially in the way illustrated in Fig. 4, but in the overall concept shown in Fig. 8 it is also possible to use any of the group-gap draws and wire circulation arrangements shown in Figs. 1, 2, 3, 5 or 6, where applicable. The embodiments of the hood modules M j ...M and of the groups R j , R 3 , R 5 , R 7 are in the other respects similar to those described above, or equivalent. In a dryer section as illustrated in Figs. 1 and 7 the drying is carried out so that, in the first wire group R j , the drying energy is applied to the web from the side of its top face. After this, in the groups R 2 ...RJO (Fig- 1) or m t e groups R 2 ...R 8 (Fig. 7), the drying energy is applied to the web exclusively from the side of its bottom face. If tendencies of curling occur in this connection, they can be compensated for in the finishing treatment of the web or in some other ways in themselves known, as has been stated earlier. In Fig. 8, the drying of the web is carried out by applying drying energy to the web exclusively from the side of its bottom face, in which case a compensation for the tendency of curling of the web is often necessary. By means of the present invention and in particular by means of the overall concepts illustrated in Figs. 1 and 7, it is favourably possible to carry out the novel three- stage optimal drying method that is described in more detail in the applicant's FI Patent Application No. (971714) to be filed on the same day with the present application. In respect of the drying method and of preferred embodiments of the different drying parameters, reference is made to said FI patent application, and what is stated in said FI patent application is not considered necessary to repeat herein in this respect. 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 the details described above by way of example only.

Claims: Claims 1. A drying unit in the dryer section of a paper or board machine, in which drying unit there is a loop of a drying wire (32;32a,32b,32c,32d) and which drying unit comprises a large-diameter Di impingement-drying and/or through-drying cylinder (31), which is fitted inside said drying-wire loop (32, 32a, 32b, 32c, 32d) and on top of which cylinder (31) / in the vicinity of which cylinder, at both sides, smoothfaced heated contact-drying cylinders (30) are placed, whose diameter D 2 < Di , characterized in that said impingement-drying and/ or through-drying cylinder (31) is placed in the spaces (KT) substantially underneath the floor level (Ki— K j ) of the paper machine hall and provided with an openable and closable blow hood (35) so that the removal of broke out of connection with said hood (35) takes place substantially by the force of gravity, that the central axes of the heated contact-drying cylinders (30) placed at both sides in the vicinity of said impingement-drying and/or through-drying cylinder (31) are placed in the vicinity of, or above, the floor level (K j — K j ) of the paper machine hall, and that the curve sector b of the paper web (W) to be dried on the outer face of said drying wire (32;32a,32b,32c,32d) over said impingement-drying and/or through-drying cylinder (31) is b > 180°. 2. A drying unit as claimed in claim 1, characterized in that the diameter Di of the impingement-drying and/or through-drying cylinder (31) has been chosen in the range of D j « 2...8 m, preferably in the range of Di ~ 2...4 m, and that the diameter D 2 of said contact-drying cylinders (30) has been chosen in the range of D « 1.5...2.5 m, preferably in the range of D 2 « 1.5...2.3 m, and that the turning sector b of the drying wire (32) and of the web to be blow-dried and placed on the outer face of said wire on the impingement-drying and/or through-drying cylinder (31) has been chosen in the range of b ~ 200...300°, preferably in the range of b « 220...280°. 3. A drying unit as claimed in claim 1 or 2, characterized in that the blow hood (35) of the drying unit is composed of two hood halves (35) placed one against the other, which hood halves can be opened into an open position (35a) in the machine direction substantially horizontally by means of power units (38,39), in which open position the removal of broke takes place by the force of gravity onto the broke conveyor (41) or equivalent placed underneath through the opening remaining between the hoods (35). 4. A drying unit as claimed in claim 3, characterized in that said blow hood (35) is composed of two hood halves (35) which are placed one against the other and which are symmetric in relation to the vertical plane placed through the central axis of the impingement-blowing and/or through-drying cylinder (31) in the cross direction of the paper machine. 5. A drying unit as claimed in claim 3 or 4, characterized in that in said hood halves (35) there is a nozzle face (60), which is provided with nozzle openings (61), which complies with the curve form of the impingement-drying and/or through- drying cylinders (31), and which is placed at a small distance from the outer face of the web (W) to be dried, and that the interior of said hood (35) and the narrow gap space between the nozzle face and the outer face of the web (W) to be dried communicate with devices for circulation of drying gas (65,66,67) (Fig. 3). 6. A drying unit as claimed in any of the claims 1 to 5, characterized in that said hood (35) has been divided, by means of machine-direction vertical partition walls (35k), into blocks (mi ...m N ), into which drying gas flows (A ml ... A mN ) of regulated different conditions can be passed so as to regulate the cross-direction profile, in particular the moisture profile, of the web (W) to be dried, preferably based on. a profile measurement signal. 7. A drying unit as claimed in any of the claims 1 to 6, characterized in that the sector of the impingement-drying and/or through-drying cylinder (31) placed outside the sector (b) of contact of the drying wire (32) with said cylinder is covered by means of a sealing element (34) or by means of an equivalent closing arrangement. 8. A drying unit as claimed in claim 7, characterized in that said sealing element (34) is a blow box. 9. A drying unit as claimed in any of the claims 1 to 8, characterized in that the upper faces of said hood half (35) have been made smoothly inclined in order to facilitate the removal of broke. 10. A drying unit as claimed in any of the claims 1 to 9, characterized in that the axle journals of the impingement-drying and/or through-drying cylinder (31) in the drying unit are mounted in connection with frame parts (37), which frame parts (37) comprise horizontal frames (37a), on whose support said hood halves (35) have been arranged to be shifted by means of power units (38), preferably hydraulic or pneumatic cylinders, to be shifted in the machine direction, and that said horizontal frames (37a) or equivalent frame constructions are supported in connection with the floor level (K 2 — K 2 ) of said basement space (KT) so that, between said floor level (K 2 — K 2 ) and said horizontal beams (37a), a space remains in which there is the broke conveyor (41) and, below said broke conveyor, separated by a partition wall (42), and above the floor level (K 2 — K 2 ) there is a space (KT 0 ) for the ducts for drying gas and/or for possible other devices, if any. 11. A dryer section of a paper machine, which comprises one or several group(s) (Rι ...R 8 ) with single-wire draw that is/ are open towards the bottom and in which group(s) there are contact-drying cylinders (20) in the upper row and reversing suction cylinders (21) or rolls in the lower row, which cylinders or rolls are prefer- ably connected to a vacuum, characterized in that the dryer section comprises one or several drying unit(s) as claimed in any of the claims 1 to 10, which unit(s) is/are provided with a blow hood module (M) which is placed in the space (KT) underneath the paper machine hall, and that said group(s) (Ri ...R ) with single- wire draw is/are placed substantially above the floor level (Ki— K t ) of the paper machine hall. 12. A dryer section as claimed in claim 11, characterized in that the first dryer group in the dryer section is a drying-wire unit (Ri , 12...15), which comprises a drying wire (12), which brings the paper web (W) from the press section (10) as a closed draw and in which unit (Ri , 12...15) there is a run of the drying wire (12) guided by guide rolls (13,14) or equivalent, on the top of which run there are devices (15) which apply a drying gas flow and/or radiation to the paper web (W) substantially free of contact. 13. A dryer section as claimed in claim 11 or 12, characterized in that the drying wire (32a) of a drying unit ( ^ as claimed in any of the claims 1 to 10 is passed around the reversing suction cylinder (21a) placed in the position of the last revers- ing suction cylinder (21) in the group (R n _ι) preceding said unit, after which the web (W) is passed, being pressed by said drying wire (32a), around the first contact- drying cylinder (30a) in the drying unit R^ and further over said impingement- drying and/or through-drying cylinder (31) and further over the latter drying cylinder (30) in the unit (R n ), from which drying cylinder (30) the web (W) is transferred onto the drying wire (22) of the following group (Rn+ i ) with single-wire draw as a closed draw (Fig. 2). 14. A dryer section as claimed in any of the claims 11 to 13, characterized in that the paper web (W) is passed onto the first contact-drying cylinder (30) in a drying unit (R n ) as claimed in any of the claims 1 to 10 on the drying wire (22) of the preceding group with single-wire draw by passing said drying wire (22) into contact with said contact-drying cylinder (30) on a certain sector (Fig. 4). 15. A dryer section as claimed in any of the claims 11 to 14, characterized in that the drying wire (22) of the group (R n+ 1 ) with single-wire draw following after a drying unit (R n ) as claimed in any of the claims 1 to 10 is passed over the latter contact-drying cylinder (30c) in said drying unit (R JJ ) on a certain sector, and that the drying wire (32c) of said unit (R-) is passed so that it reaches contact with said contact-drying cylinder (30c) tangentially (Si) or on a small sector (Fig. 5). 16. A dryer section as claimed in any of the claims 11 to 15, characterized in that the drying wire (32d) of a drying unit (R^) as claimed in any of the claims 1 to 10 is also passed so that it constitutes the drying wire of the preceding and/or the following group (R n _ι and/or R n+ ι) with single- wire draw so that it runs over one or several drying cylinders (20d) and reversing suction cylinders (2 Id) or rolls in said preceding and/or following group (Fig. 6). 17. A dryer section as claimed in any of the claims 11 to 16, characterized in that the dryer section is composed of one or several drying units (R n ) whose blow-hood modules (Mι ,M 2 ,M 3 and/or M ) are placed in the basement space (KT) and of groups (R 2 , R 4 , R 6 , R 8 ) with single-wire draw between said units (Fig. 7). 18. A dryer section as claimed in any of the claims 11 to 17, characterized in that the dryer section includes two or more, preferably three or four, blow-hood modules (M), which are placed in the basement space (KT), and between said modules (M) there are groups (R-) with single-wire draw, and that the last group in the dryer section is a group with single- wire draw. 19. A dryer section as claimed in any of the claims 11 to 18, characterized in that the first unit in the dryer section is a drying- wire group (R j , 12...16), in which an intensive blowing of drying gas and/or electromagnetic radiation is/are applied to the top face of the web (W) free of contact, that said first unit (R j ) is followed by a single-wire unit (R 2 ), after that by a blow-hood unit (R 3 ,M j ), which is followed by a second single- wire unit (R 3 ) and after that by a second blow-hood unit (R 5 ,M 2 ), after which there follows a third single-wire unit and further a third blow-hood unit (R 7 ,M ), after which there follow(s) one to three single-wire units. 20. A dryer section as claimed in claim 19, characterized in that all of said single- wire units are open towards the bottom, and said blow-hood units are open towards the bottom, so that removal of broke can be carried out by the force of gravity onto the broke conveyor (41) placed below said blow-hood units (M). 21. A dryer section as claimed in any of the claims 11 to 20, characterized in that the dryer section is an earlier multi-cylinder dryer which has been modernized in order to increase its drying capacity, one or several drying units provided with a module (M) as claimed in any of the claims 1 to 10 having been placed in the space (KT) underneath the floor level of said multi-cylinder dryer. 22. A dryer section as claimed in any of the claims 11 to 21, characterized in that below the frame parts of said modules (M), the broke conveyor (41) is fitted, and at least at one end of said conveyor, preferably in the final end of the dryer section, there is a pulper (40b), and that below said broke conveyor (41), above the floor level (K 2 — K 2 ) of the basement space, there is a space (KT 0 ) in which air ducts and/or equivalent of the modules (M) and/or other necessary devices can be fitted. 23. A dryer section as claimed in any of the claims 11 to 22, characterized in that over the entire length of the dryer section the drying of the paper web (W) is carried out from the side of its bottom face (Fig. 8). 24. A dryer section as claimed in any of the claims 11 to 22, characterized in that, in the first stage of the drying, preferably in the first wire group (Ri , 12...16), the drying of the paper web is carried out by applying the drying energy through, and from the side of, its top face, and that after the first stage the drying of the paper web (W) is carried out by applying the drying energy exclusively from the side of its bottom face (Figs. 1 and 7). AMENDED CLAIMS [received by the International Bureau on 31 August 1998 (31.08.98); original claim 1 amended; remaining claims unchanged (6 pages)] 1. A drying unit in the dryer section of a paper or board machine, in which drying unit there is a loop of a drying wire (32;32a,32b,32c,32d) and which drying unit comprises a large-diameter D, impingement-drying and/or through-drying cylinder (31), which is fitted inside said drying-wire loop (32,32a,32b,32c,32d) and on top of which cylinder (31 ) / in the vicinity of which cylinder, at both sides, smooth-faced heated contact-drying cylinders (30) are placed, whose diameter D 2 < D,, so that the curve sector b of the paper web (W) to be dried on the outer face of said drying wire (32;32a,32b,32c,32d) over said impingement-drying and/or through-drying cylinder (31) is b > 180°, characterized in that said impingement-drying and/or through- drying cylinder (31) is placed in the spaces (KT) substantially underneath the floor level (K j — K j ) of the paper machine hall and provided with an openable and closable blow hood (35) so that the removal of broke out of connection with said hood (35) takes place substantially by the force of gravity through an uncovered sector of the hood (35), that the central axes of the heated contact-drying cylinders (30) placed at both sides in the vicinity of said impingement-drying and/or through-drying cylinder (31) are placed in the vicinity of, or above, the floor level (K j — K,) of the paper machine hall. 2. A drying unit as claimed in claim 1 , characterized in that the diameter D, of the impingement-drying and/or through-drying cylinder (31) has been chosen in the range of D { ~ 2...8 m, preferably in the range of D j = 2...4 m, and that the diameter D 2 of said contact-drying cylinders (30) has been chosen in the range of D 2 ~ 1.5...2.5 m, preferably in the range of D 2 ~ 1.5...2.3 m, and that the turning sector b of the drying wire (32) and of the web to be blow-dried and placed on the outer face of said wire on the impingement-drying and/or through-drying cylinder (31) has been chosen in the range of b = 200...300°, preferably in the range of b = 220...280 0 . 3. A drying unit as claimed in claim 1 or 2, characterized in that the blow hood (35) of the drying unit is composed of two hood halves (35) placed one against the other, which hood halves can be opened into an open position (35a) in the machine direction substantially horizontally by means of power units (38,39), in which open position the removal of broke takes place by the force of gravity onto the broke conveyor (41) or equivalent placed underneath through the opening remaining between the hoods (35). 4. A drying unit as claimed in claim 3, characterized in that said blow hood (35) is composed of two hood halves (35) which are placed one against the other and which are symmetric in relation to the vertical plane placed through the central axis of the impingement-blowing and/or through-drying cylinder (31) in the cross direction of the paper machine. 5. A drying unit as claimed in claim 3 or 4, characterized in that in said hood halves (35) there is a nozzle face (60), which is provided with nozzle openings (61), which complies with the curve form of the impingement-drying and/or through-drying cylinders (31), and which is placed at a small distance from the outer face of the web (W) to be dried, and that the interior of said hood (35) and the narrow gap space between the nozzle face and the outer face of the web (W) to be dried communicate with devices for circulation of drying gas (65,66,67) (Fig. 3). 6. A drying unit as claimed in any of the claims 1 to 5, characterized in that said hood (35) has been divided, by means of machine-direction vertical partition walls (35k), into blocks (m^-.m N ), into which drying gas flows (A^.-.A,^) of regulated different conditions can be passed so as to regulate the cross-direction profile, in particular the moisture profile, of the web (W) to be dried, preferably based on a profile measurement signal. 7. A drying unit as claimed in any of the claims 1 to 6, characterized in that the sector of the impingement-drying and/or through-drying cylinder (31 ) placed outside the sector (b) of contact of the drying wire (32) with said cylinder is covered by means of a sealing element (34) or by means of an equivalent closing arrangement. 8. A drying unit as claimed in claim 7, characterized in that said sealing element (34) is a blow box. 9. A drying unit as claimed in any of the claims 1 to 8, characterized in that the upper faces of said hood half (35) have been made smoothly inclined in order to facilitate the removal of broke. 10. A drying unit as claimed in any of the claims 1 to 9, characterized in that the axle journals of the impingement-drying and/or through-drying cylinder (31) in the drying unit are mounted in connection with frame parts (37), which frame parts (37) comprise horizontal frames (37a), on whose support said hood halves (35) have been arranged to be shifted by means of power units (38), preferably hydraulic or pneumatic cylinders, to be shifted in the machine direction, and that said horizontal frames (37a) or equivalent frame constructions are supported in connection with the floor level (K 2 — K 2 ) of said basement space (KT) so that, between said floor level (K 2 — K 2 ) and said horizontal beams (37a), a space remains in which there is the broke conveyor (41) and, below said broke conveyor, separated by a partition wall (42), and above the floor level (K 2 — K 2 ) there is a space (KT 0 ) for the ducts for drying gas and/or for possible other devices, if any. 1 1. A dryer section of a paper machine, which comprises one or several group(s) (R,...R 8 ) with single-wire draw that is/are open towards the bottom and in which group(s) there are contact-drying cylinders (20) in the upper row and reversing suction cylinders (21) or rolls in the lower row, which cylinders or rolls are preferably connected to a vacuum, characterized in that the dryer section comprises one or several drying unit(s) as claimed in any of the claims 1 to 10, which unit(s) is/are provided with a blow hood module (M) which is placed in the space (KT) underneath the paper machine hall, and that said group(s) (R j ...R 8 ) with single-wire draw is/are placed substantially above the floor level (K j — K,) of the paper machine hall. 12. A dryer section as claimed in claim 1 1, characterized in that the first dryer group in the dryer section is a drying-wire unit (R ] 5 12...15), which comprises a drying wire (12), which brings the paper web (W) from the press section (10) as a closed draw and in which unit (R j , 12...15) there is a run of the drying wire (12) guided by guide rolls (13,14) or equivalent, on the top of which run there are devices (15) which apply a drying gas flow and/or radiation to the paper web (W) substantially free of contact. 13. A dryer section as claimed in claim 11 or 12, characterized in that the drying wire (32a) of a drying unit (R n ) as claimed in any of the claims 1 to 10 is passed around the reversing suction cylinder (21a) placed in the position of the last reversing suction cylinder (21) in the group (R n-1 ) preceding said unit, after which the web (W) is passed, being pressed by said drying wire (32a), around the first contact-drying cylinder (30a) in the drying unit (R n ) and further over said impingement-drying and/or through-drying cylinder (31 ) and further over the latter drying cylinder (30) in the unit (R n ), from which drying cylinder (30) the web (W) is transferred onto the drying wire (22) of the following group (R n+1 ) with single-wire draw as a closed draw (Fig. 2). 14. A dryer section as claimed in any of the claims 11 to 13, characterized in that the paper web (W) is passed onto the first contact-drying cylinder (30) in a drying unit (R n ) as claimed in any of the claims 1 to 10 on the drying wire (22) of the preceding group (R n-j ) with single- wire draw by passing said drying wire (22) into contact with said contact-drying cylinder (30) on a certain sector (Fig. 4). 15. A dryer section as claimed in any of the claims 11 to 14, characterized in that the drying wire (22) of the group (R n+1 ) with single-wire draw following after a drying unit (R n ) as claimed in any of the claims 1 to 10 is passed over the latter contact- drying cylinder (30c) in said drying unit (R n ) on a certain sector, and that the drying wire (32c) of said unit (R n ) is passed so that it reaches contact with said contact- drying cylinder (30c) tangentially (S,) or on a small sector (Fig. 5). 16. A dryer section as claimed in any of the claims 11 to 15, characterized in that the drying wire (32d) of a drying unit (R n ) as claimed in any of the claims 1 to 10 is also passed so that it constitutes the drying wire of the preceding and/or the following group (R n _ j and/or R n+1 ) with single- wire draw so that it runs over one or several drying cylinders (20d) and reversing suction cylinders (2 Id) or rolls in said preceding and/or following group (Fig. 6). 17. A dryer section as claimed in any of the claims 11 to 16, characterized in that the dryer section is composed of one or several drying units (R n ) whose blow-hood modules (M j ,M 2 ,M 3 and/or M 4 ) are placed in the basement space (KT) and of groups (R 2 , R 4 , R 6 , R 8 ) with single-wire draw between said units (Fig. 7). 18. A dryer section as claimed in any of the claims 1 1 to 17, characterized in that the dryer section includes two or more, preferably three or four, blow-hood modules (M), which are placed in the basement space (KT), and between said modules (M) there are groups (R,) with single-wire draw, and that the last group in the dryer section is a group with single-wire draw. 19. A dryer section as claimed in any of the claims 11 to 18, characterized in that the first unit in the dryer section is a drying-wire group (R ] 5 12...16), in which an intensive blowing of drying gas and/or electromagnetic radiation is/are applied to the top face of the web (W) free of contact, that said first unit (R,) is followed by a single-wire unit (R 2 ), after that by a blow-hood unit (R 3 ,M,), which is followed by a second single-wire unit (R 3 ) and after that by a second blow-hood unit (R 5 ,M 2 ), after which there follows a third single-wire unit and further a third blow-hood unit (R 7 ,M 3 ), after which there follow(s) one to three single-wire units. 20. A dryer section as claimed in claim 19, characterized in that all of said single- wire units are open towards the bottom, and said blow-hood units are open towards the bottom, so that removal of broke can be carried out by the force of gravity onto the broke conveyor (41) placed below said blow-hood units (M). 21. A dryer section as claimed in any of the claims 11 to 20, characterized in that the dryer section is an earlier multi-cylinder dryer which has been modernized in order to increase its drying capacity, one or several drying units provided with a module (M) as claimed in any of the claims 1 to 10 having been placed in the space (KT) underneath the floor level of said multi-cylinder dryer. 22. A dryer section as claimed in any of the claims 11 to 21 , characterized in that below the frame parts of said modules (M), the broke conveyor (41) is fitted, and at least at one end of said conveyor, preferably in the final end of the dryer section, there is a pulper (40b), and that below said broke conveyor (41), above the floor level (K 2 — K 2 ) of the basement space, there is a space (KT 0 ) in which air ducts and/or equivalent of the modules (M) and/or other necessary devices can be fitted. 23. A dryer section as claimed in any of the claims 1 1 to 22, characterized in that over the entire length of the dryer section the drying of the paper web (W) is carried out from the side of its bottom face (Fig. 8). 24. A dryer section as claimed in any of the claims 11 to 22, characterized in that, in the first stage of the drying, preferably in the first wire group (R,, 12...16), the drying of the paper web is carried out by applying the drying energy through, and from the side of, its top face, and that after the first stage the drying of the paper web (W) is carried out by applying the drying energy exclusively from the side of its bottom face (Figs. 1 and 7).

Inventors: KAIHORVITA JUHA, KOMULAINEN ANTTI, LIPPONEN JUHA, SALMINEN MARTTI, KAIHORVITA, JUHA, KOMULAINEN, ANTTI, LIPPONEN, JUHA, SALMINEN, MARTTI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1998
Main classification: D21F5
IPC codes: D21F5