Machine having a dryer section and method for drying a web of fibrous material

Abstract

The invention relates to a dryer section for drying a web (10) of fibrous material, particularly paper, cardboard, tissue / sanitary paper, having at least one drying unit (11) for reducing the moisture content of the web (10) by means of convection drying. The drying unit (11) has at least one nozzle (12) for applying a drying fluid to the web (10), at least one discharge device (13) for the moist air produced during convection drying, and transport means for moving the web (10) relative to the drying unit (11). The drying unit (11) has at least one ultrasonic generator (15), which is arranged, for excitating vibrations in the drying fluid, in the region of the nozzle (12) in such a manner that the reduction of the moisture content by ultrasound can be supported over the entire width of the web.

Description

[0001] The invention relates to a machine for producing a web of fibrous material, comprising a drying section for drying the web of fibrous material, with the features of the preamble of claim 1. Such a machine with a drying section is known, for example, from DE 199 44 266 A1. The invention further relates to a method for drying a web of fibrous material.

[0002] The dryer section of a paper machine is located between the press section, which mechanically dewaters the web, and a reel unit, which winds up the dried web. The dryer section's function is to thermally remove the moisture remaining in the web after mechanical dewatering until the desired final moisture content is reached. This is conventionally achieved using steam-heated cylinders over which the web is guided. In these dryer sections, which are based on the principle of contact drying, the heat required for moisture removal is introduced into the web by the drying cylinders. DE 199 44 266 further describes the use of drying or nozzle hoods arranged along the direction of travel of the web in the dryer section. Such hoods serve to expose the web to a hot fluid and operate on the principle of convection drying.

[0003] A significant cost factor in paper production is the energy required to dry the paper web. The dryness level typically ranges from 45% to 98%. This cost factor will become increasingly important in the foreseeable future, given that rising energy prices make energy conservation a key challenge for the paper industry. Furthermore, the dryer section of a paper machine requires the most space. Therefore, the dryer section is a limiting factor not only in terms of energy consumption but also in terms of space requirements.

[0004] US patent 3,999,302 discloses a device for drying a fiber web. In the device, the web is guided around a cylindrical roller or between two cylindrical rollers, whereby the rollers are excited to ultrasonic vibrations and the fiber web can also be heated dielectrically.

[0005] DE 2 053 284 describes a drying device for a material web, comprising sound generators arranged in a reflector arrangement, which reflects the sound energy from the sound generators practically perpendicular to a support surface of the material web and directs sound energy reflected from the material web back to the sound center in order to enhance the sound field on the web surface.

[0006] US Patent 3,694,926 describes a device for drying paper webs which has a sound-assisted drying section in which the paper webs are exposed to high-intensity noise to remove moisture from the web.

[0007] The invention is based on the objective of providing a machine with a drying section for drying a web of fibrous material, in particular paper, cardboard, tissue / hygiene paper, which is improved with regard to energy efficiency. The invention is further based on the objective of providing a corresponding method for drying a web of fibrous material.

[0008] According to the invention, this problem is solved by a machine for producing a web of fiber material with the features of claim 1. With regard to the method, the problem is solved by the subject matter of claim 12.

[0009] According to the invention, a machine for producing a web of fibrous material, in particular paper, cardboard, or tissue / hygiene paper, is proposed, comprising a device for mechanically dewatering the web, in particular a press section, and a drying section for drying the web. The drying section is located downstream of the mechanically dewatering device in the transport direction of the web and includes at least one drying unit for reducing the moisture content of the web by convection drying. The drying unit has at least one nozzle for applying a drying fluid to the web. The drying section further comprises a discharge device for the moist air generated during convection drying and a transport means for moving the web relative to the drying unit.The drying unit has at least one ultrasonic generator which is arranged in the area of ​​the nozzle to excite vibrations in the drying fluid, wherein the ultrasonic generator is integrated into the nozzle so that the drying fluid flowing through the nozzle passes by the ultrasonic generator and is set into ultrasonic vibrations, and wherein the nozzle is directed towards a surface of the web so that a vibrationally excited stream of drying fluid exiting the nozzle impinges on the web, thereby enabling the reduction of the moisture content for the entire web thickness to be supported by ultrasound.

[0010] The invention enables a significant increase in drying efficiency, i.e., the energy input per unit weight of water removed, compared to the prior art. The invention therefore possesses considerable economic potential, which increases accordingly with rising energy prices. In addition to the advantage of low energy consumption when using the drying section according to the invention, the typically high investment costs in the industry are reduced, since the invention requires less mechanical engineering effort for the production of the drying section or the paper machine with such a drying section.If the drying section of an existing system is retrofitted for ultrasound-assisted drying according to the invention, the small space requirement of the invention also offers the advantage that, for a given system size, an increase in the overall drying capacity is achieved, thereby increasing the machine output.

[0011] The improved drying performance is achieved by the ultrasonic generator provided according to the invention, which serves to vibrate the drying fluid exiting the nozzle of the drying unit. The acoustic vibrations of the drying fluid prevent or reduce the formation of an air cushion on the web to be dried, which would impede the convection flow. Thus, a larger quantity of fluid directly impacts the web. The drying section according to the invention is designed not only to dehumidify the web surface. Rather, the drying section according to the invention is designed so that the moisture content can be reduced to the desired final moisture content across the entire web thickness, with the reduction of the moisture content being supported by ultrasound. The drying section according to the invention therefore allows for through-drying of the entire web in the thickness direction with the support of ultrasound.

[0012] It is assumed – without committing to this specific principle – that applying ultrasound to the web produces a dual effect. Firstly, as already mentioned, the moisture-saturated air cushion above the web is disrupted by the acoustic excitation of the drying fluid, allowing unsaturated air or, more generally, unsaturated drying fluid to be drawn in. This allows more water to escape from the web overall. A second effect may also occur, based on the fact that the ultrasonic vibrations are transmitted into the web material and excite its fiber structure. It is assumed that this further mechanically drives out the water contained within the web material.

[0013] The inventive method is based on the fact that, for drying a web of fibrous material, in particular paper, cardboard, or tissue / hygiene paper, the web is moved through the drying section and at least partially dried convectively, thereby reducing the moisture content of the web. During the drying process, the web is subjected to ultrasonic waves such that the reduction of the moisture content across the entire web thickness is supported by ultrasound. In the inventive method, as in the inventive drying section, the convection current is acoustically excited, thereby disrupting a layer of saturated air forming on the web and allowing the convection current to reach the web essentially unimpeded. In the inventive method, the moisture content is also reduced across the entire web thickness, with this reduction being ultrasound-assisted.This process therefore involves not only surface drying, but also thorough drying of the entire web. In this context, reference is made to the explanations regarding the drying section according to the invention.

[0014] In a preferred embodiment of the drying section according to the invention, one or more ultrasound-assisted drying units and further drying units are combined. The further drying units can, for example, be at least one drying cylinder and / or at least one infrared drying device and / or at least one air hood. This generally means that the ultrasound-assisted drying units can be combined with units based on the principles of contact drying and / or convection drying and / or infrared drying. This results in an increase in the drying capacity of conventional drying units, which can be achieved both when retrofitting an existing system and during new installations.

[0015] Alternatively, it can be provided that only ultrasound-assisted drying units are used. These can be arranged on one side and / or both sides and / or alternately along the web to be dried, the web being stabilized by web stabilization or web guiding aids in a manner known per se. This means that, within the scope of this embodiment of the invention, it is also possible, in extreme cases, to construct the entire drying section according to the principle of ultrasound-assisted convection drying.

[0016] Advantageously, the transport devices have guides for the track, in particular guide rollers, with one or more ultrasonically assisted drying units arranged in free movement between the guides. The ultrasonically assisted drying units are therefore flexible in their application and do not necessarily have to be combined with other, albeit known, drying units. This expands the design possibilities of the individual assemblies of the drying section.

[0017] The transport means preferably have a continuously movable carrier for the web, with one or more ultrasonically assisted drying units arranged in the area of ​​the carrier. The carrier can be, for example, a conveyor belt, a screen belt, or a wire mesh. Other carriers are possible. The advantage of this embodiment is that the ultrasonically assisted drying units are largely positionally independent and can therefore be assigned to system elements, such as the continuously movable carrier for the web. The ultrasonically assisted drying units can be arranged on the carrier side and / or the free web side. The drying units can be arranged on one side and / or both sides and / or alternately.

[0018] Pre-drying and / or post-drying of the web before passing through drying units known per se can be achieved by arranging one or more ultrasound-assisted drying units in the free path before and / or after further units, in particular drying units, such as a drying cylinder and / or an infrared drying device and / or an air hood.

[0019] In another preferred embodiment, the ultrasonically assisted drying units are arranged on one side and / or both sides, in opposition and / or alternating on both sides, relative to the web. The one-sided arrangement may be sufficient to ensure complete drying of the web across its entire thickness when using high-performance ultrasonically assisted drying units. The two-sided arrangement of the ultrasonically assisted drying units, in opposition and / or alternating on both sides, allows the web to be exposed to ultrasonically excited drying fluid from both sides, resulting in particularly uniform drying of the web from both sides. In this configuration, the use of comparatively less powerful drying units is possible, but not mandatory.

[0020] Another example of the positionally flexible replaceability of the drying units is that one or more ultrasonically assisted drying units are arranged along the entire or part of the web's wrap-around area of ​​at least one roller and / or at least one cylinder. Advantageously, the cylinder comprises a drying cylinder with an air hood that at least partially, and in particular completely, surrounds the wrap-around area of ​​the drying cylinder.

[0021] The drying section can have a control system set such that the final moisture content of the web is a maximum of 15%, preferably a maximum of 10%. The term "through-drying" refers to the final moisture content that is established over the entire web thickness.

[0022] The drying section is part of a machine for producing a web from fibrous material, comprising, among other things, a device for mechanically dewatering the web, in particular a press section, wherein the drying section is located downstream of the device for mechanical dewatering in the transport direction of the web. A sizing press and / or a coating unit and / or a calender and / or a device for winding the web may be located downstream of the drying section.

[0023] In the process according to the invention, it is preferably provided that the fiber material comprises a wet-strength agent, in particular a melamine resin, a urea-formaldehyde resin, or an epichlorohydrin resin. Such materials require superdrying, i.e., extensive removal of water, for the purpose of condensation of the wet-strength resin. For this, a sufficiently high web temperature is required during drying. The process according to the invention, in which the moisture content is reduced across the entire web thickness by ultrasound-assisted convection drying, is therefore particularly attractive in connection with the use of wet-strength agents because reducing the water content of the web below the level of the finished paper web requires a particularly high energy input, especially when removing residual water at low moisture contents.The method according to the invention is not limited to the drying of webs with a fiber material that has a wet-strength agent, but merely offers particular advantages in this regard.

[0024] The invention is explained in more detail below with reference to the accompanying schematic drawings. These show: Fig. 1 a side view of a drying section according to an embodiment of the invention; Fig. 2 a side view of a drying cylinder combined with ultrasonically assisted drying units; Fig. 3 a side view of a drying cylinder with an air hood in which several ultrasonically assisted drying units are integrated; Fig. 4 a side view of a drying cylinder with an air hood in which additional ultrasonically assisted drying units are arranged in the free passage in front of the air hood; Fig. 5 the drying cylinder according to Fig. 4 , which is additionally equipped with infrared drying devices, wherein the infrared drying devices are arranged on one side; Fig. 6 the drying cylinder according to Fig. 5 , wherein the infrared drying devices as well as the ultrasound-assisted drying units are arranged on both sides, and Fig. 7 shows a simplified cross-section through an ultrasound-assisted drying unit.

[0025] Fig. 1 Figure 1 shows a drying section 20 according to an example according to the invention, which is part of a paper machine. The drying section 20 can also be part of a machine for the production of tissue or hygiene paper webs or part of a machine for the production of cardboard webs.

[0026] The dryer section 20 is arranged between a press section and a reel unit. The paper machine can be equipped with or without a calender(s) and / or with or without a size or film press and / or an online coating unit. The aforementioned assemblies can be located downstream of the dryer section in the direction of web travel. The dryer section 20 features ultrasonically assisted drying units 11, which, together with the other drying units of the dryer section, reduce the moisture content of the web to the desired final moisture content.

[0027] The task of the drying section 20 is to bring the paper web transferred from the press section to the desired final moisture content. The final moisture content can be in a range of approximately 2 to 15%, particularly in a range of approximately 2 to 10%, particularly approximately 2 to 8%, particularly approximately 2 to 6%, and particularly approximately 2 to 4%. Generally, contact drying, convection drying, and infrared drying are suitable for drying, with drying section 20 employing one of these methods according to [specification / specification]. Fig. 1 Convection drying is intensified by exciting the drying fluid with an ultrasonic frequency. In other words, the drying fluid, for example hot air or steam, is sprayed at an ultrasonic frequency onto the wet paper web exiting the press section.

[0028] An example of an ultrasound-assisted drying unit 11 used for this purpose is shown in Fig. 7 The drying unit 11 is shown schematically. It has a nozzle 12 directed towards the surface of the paper web 10, so that the drying fluid flow exiting the nozzle 12 impinges on the web 10. The nozzle 12 extends transversely to the web's longitudinal direction. The width of the nozzle 12 corresponds essentially to the width of the web to be dried, so that the desired final moisture content can be adjusted across the entire web width. The nozzle 12 can be designed as a slot nozzle, with the slot extending transversely to the web's longitudinal direction. Alternatively, the nozzle 12 can be designed as a perforated plate with one or more rows of holes, each extending transversely to the web's longitudinal direction. The nozzle 12 is connected to a fluid supply 21 through which the drying fluid can be supplied to the nozzle 12. The fluid flow is in Fig. 7 labelled with the arrows F.

[0029] The drying unit 11 has a discharge device 13 through which the moist air generated during drying is discharged. The airflow is indicated by the arrows L. In the simplest case, the discharge device 13 is designed as a perforated plate or a slotted plate, the openings of which extend transversely to the longitudinal direction of the web. A separate arrangement of the discharge device from the drying unit 11 is possible.

[0030] A special feature of the drying unit 11 is the ultrasonic generator 15, which serves to subject the drying fluid exiting the nozzle 12 to an ultrasonic frequency. In the embodiment according to Fig. 7 The nozzle walls are designed as an ultrasonic generator 15. In other words, the ultrasonic generator 15 is integrated into the nozzle 12, so that the drying fluid flowing through the nozzle 12 passes by the ultrasonic generator 15 and is set into ultrasonic vibrations.

[0031] The drying capacity of the drying unit 11 is adapted so that a paper web of typical thickness can be dehumidified across its entire thickness, allowing the desired final moisture content to be set. It is therefore not only a matter of ultrasonically drying the top side of the web 10 facing the nozzle 12. Rather, the drying unit 11 is designed to reduce moisture across the entire web thickness, thus achieving the advantage of reduced energy consumption.

[0032] It is assumed that an amplitude of the ultrasonic waves effective at the surface of the web 10 is in the range of approximately 120 dB to approximately 190 dB, particularly in the range of approximately 140 dB to approximately 190 dB, particularly in the range of approximately 160 dB to approximately 190 dB, and particularly in the range of 160 dB to approximately 185 dB. The required distance between the nozzle opening 12 and the surface of the web 10 is set by a person skilled in the art. The distance between the nozzle opening 22 and the web surface on the nozzle side can also be determined based on the wavelength, where the distance corresponds approximately to (λ) (n / 4). λwavelength of the ultrasonic oscillation and n± 0.5 of an odd integer The value n represents a range, for example, 0.5 to 1.5, 2.5 to 3.5, 4.5 to 5.5, etc. Preferably, n is an odd integer (1, 3, 5, 7, etc.). This ensures that the amplitude of the ultrasonic vibration reaches its maximum approximately at the surface of the web, in order to disrupt the buffer layer forming above the web as effectively as possible.

[0033] To adjust the distance between the outlet opening 22 and the web surface, an adjustment surface can be provided, which is arranged opposite the outlet opening 22. The adjustment surface (not shown) guides the web to be dried at the desired distance from the outlet opening 22. The adjustment surface can, for example, be a flat plate, with the web being guided over the surface by means of a support belt. Alternatively, the adjustment surface can be formed by one or more rollers on which the web is guided directly, or by a support belt that supports the web (without an adjustment plate). Other embodiments of the adjustment surface are possible. Additional or alternative adjustment mechanisms can be provided to adjust the distance between the outlet opening 22 and the web surface, for example, an adjusting device for changing the position of the outlet opening 22 relative to the web surface.

[0034] The following will refer to the Fig. 1 bis 6 Various possibilities are described for how the ultrasonically assisted drying units 11 can be positioned in relation to conventional drying units. In general, the ultrasonically assisted drying units can be used in combination with conventional drying units based on the principles of contact drying, convection drying, and / or infrared drying. The ultrasonically assisted drying units 11 can be arranged in place of, in front of, behind, or between drying units that are common or possible in the drying section. The aforementioned positioning specifications refer to the direction of travel of the web. The drying units combined with the ultrasonically assisted drying units 11 can be cylinders heated with different heat transfer media (e.g., water, steam, thermal oil), infrared dryers, or air hoods.

[0035] As in Fig. 1 As shown, the ultrasonically assisted drying units of the cylinder assembly can be assigned to a double-row drying section 20'. It is also possible, as further shown in Fig. 1 The figure shows how to integrate the ultrasonically assisted drying units into an assembly of a single-row drying section with vacuum rollers arranged at the top (see reference numeral 20") or at the bottom (see reference numeral 20‴). ​​The drying units 11 can be assigned to the vacuum rollers and / or the drying cylinders. By combining the ultrasonically assisted drying units 11 with the vacuum rollers 23, a particularly efficient utilization of the installation space of the drying section is achieved.

[0036] The following positions of the ultrasound-assisted drying units 11 in the drying section 20 are possible, without the invention being limited thereto.

[0037] As in the Fig. 1 and 4 As shown, the drying units 11 can be arranged in free movement between two web guiding aids, such as paper guide rollers, or generally in free movement between guides 14a. The drying units 11 can be arranged on one side and / or both sides and / or alternately, as shown in Fig. 4 The drying units are shown, each arranged in relation to track 10. Another possible arrangement of the drying units is as shown in the Fig. 1 and 2 The invention discloses the installation of drying units 11 within the encirclement area 19 of cylinders, such as drying cylinders 16, vacuum cylinders 23, or paper guide rollers. The drying units can be arranged over the entire encirclement area 19 or partially over the encirclement area 19 (see also Fig. 4, 5 The drying units 11 can extend continuously along the cylinder circumference or be distributed around the cylinder circumference.

[0038] It is possible to guide a carrier web, such as a drying screen or a drying felt, between the drying units 11 and the web 10 without significantly impairing the operation of the ultrasound-assisted drying units 11. Fig. 1 The drying sieve is designated with the reference number 14b (generally carrier). As in Fig. 3 As shown, the drying cylinder 16 can additionally be surrounded by a high-performance hood or, more generally, an air hood 18 (see also Fig. 4, 5 and 6 The drying units 11 are integrated into the air hood. Another combination option is available in the Fig. 5 , 6 shown. Accordingly, the drying units 11 can be combined with infrared drying devices 17, wherein the devices 17 are one-sided ( Fig. 5 ) or on both sides (Fig. 17). Similarly, the ultrasound-assisted drying units 11 can also be arranged on one side only ( Fig. 5 ) or bilateral ( Fig. 6 ) be arranged. It is understood that the drying units 11 can be arranged on the same side as the equipment 17 ( Fig. 5 ). In both examples according to Fig. 5 , 6 The drying units 11 are arranged in the free passage before and / or after the infrared drying devices 17.

[0039] Another example of the arrangement of the drying units 11 is to mount them in the area where the web 10 is guided on a support 14a, 14b. The support can be, for example, a conveyor belt, a conveyor belt, a screen belt, a wire mesh, paper guide rollers, vacuum rollers, etc.

[0040] In summary, at least one drying unit, and in particular several drying units, can be combined with heated drying cylinders 16 and / or with infrared dryers 17 and / or with air hoods 18. It is also possible to provide exclusively ultrasonically assisted drying units 11, i.e., without additional conventional drying units.

[0041] The absolute number of drying units 11 required in the drying section depends on the amount of water to be removed per unit of time and is a function of basis weight, web speed, web width, web moisture content at the entrance to the drying section, and final web moisture content. Accordingly, a person skilled in the art selects the required number of drying units based on the specific circumstances.

[0042] The aforementioned arrangement options for the drying units according to the Fig. 1 bis 6 can be combined with each other.

[0043] The features of the various embodiments of the drying section described above are also disclosed in connection with the drying process that can be carried out using the respective embodiments. Within the framework of the process, ultrasound-assisted convection drying is performed in such a way that a reduction in moisture content occurs across the entire web thickness. The ultrasound-induced convection current can be applied to the web 10 on one side, both sides, and / or alternately. In cases where superdrying of the web, i.e., a reduction of the web's moisture content below that of the finished paper web, is required, the energy-saving advantage becomes particularly apparent. The removal of residual water at low moisture contents takes place in the rear section of the drying section. Superdrying is used, for example, for the purpose of condensing a wet-strength resin. Bezugszeichenliste

[0044] 10 Track 11 Drying unit 12 Nozzle 13 Discharge device 14a Guides 14b Carrier 15 Ultrasonic generator 16 Drying cylinder 17 Infrared drying device 18 Air hood 19 Enclosure area 20 Drying section 20', 20", 20' Drying section components 21 Fluid supply 22 Outlet opening 23 Vacuum rollers F Fluid L Air

Claims

1. A machine for producing a web (10) of fibrous material, particularly of paper, cardboard, tissue / sanitary paper, having a device for mechanically dewatering the web (10), especially a press section and a dryer section for drying the web (10) of fibrous material, wherein the drying section is arranged downstream of the device for mechanically dewatering in the transport direction of the web (10) and comprises: - at least one drying unit (11) for reducing the moisture content of the web (10) by means of convection drying, wherein the drying unit (11) has at least one nozzle (12) for applying a drying fluid to the web (10), - at least one discharge device (13) for the moist air being produced during the convection drying and - transport means to move the web (10) relative to the drying unit (11), characterised in that the drying unit (11) has at least one ultrasound generator (15), which is arranged to excite vibrations of the drying fluid in the region of the nozzle (12), wherein the ultrasound generator (15) is integrated into the nozzle (12) such that the drying fluid flowing through the nozzle (12) passes the ultrasound generator (15) and is made to carry out ultrasound vibrations, and wherein the nozzle is directed to a surface of the web (10) such that a vibration-excited drying fluid stream exiting the nozzle (12) impinges on the web (10), thereby causing that the reduction in the moisture content over the entire web thickness can be assisted by ultrasound.

2. A machine according to claim 1, characterized in that in the dryer section one or more ultrasound-assisted drying units (11) and further drying units (16, 17, 18), in particular at least one drying cylinder (16) and / or at least one device for infrared drying (17) and / or at least one air hood (18), are combined.

3. A machine according to claim 1, characterized in that in the dryer section exclusively ultrasound-assisted drying units (11) are provided.

4. A machine according to one of the claims 1 through 3, characterized in that the transport means has guides (14) for the web, particularly guide rollers, wherein one or more ultrasound-assisted drying units (11) are arranged in the open draw between the guides (14).

5. A machine according to one of the claims 1 through 4, characterized in that the transport means has a continuously movable carrier (14b) for the web (10), particularly a conveyor belt, a screen belt or a wire mesh, wherein one or more ultrasound-assisted drying units (11) are arranged in the region of the carrier (14b).

6. A machine according to one of the claim 1, 2, 4 and 5, characterized in that in the dryer section one or more ultrasound-assisted drying units (11) are arranged in the open draw before and / or after further units, particularly drying units such as a drying cylinder (16) and / or a device for infrared drying (17) and / or an air hood (18).

7. A machine according to one of the claims 1 through 6, characterized in that the ultrasound-assisted drying units (11) are arranged on one side and / or both sides in an opposing manner and / or alternating on both sides in relation to the web (10).

8. A machine according to one of the claim 1 through 7, characterized in that the dryer section comprises at least one roller and / or at least one cylinder, which each form a wrap region (19) of the web (10), wherein one or more ultrasound-assisted drying units (11) are arranged along the entire wrap region or a part thereof (19).

9. A machine according to claim 8, characterized in that the at least one cylinder comprises a drying cylinder (16) with an air hood (18), which at least partially, particularly completely, surrounds the wrap region (19) of the drying cylinder (16).

10. A machine according to one of the claims 1 through 9, characterized in that a control, which is adjusted in such a way that the final moisture content of the web (10) is a maximum of 15%, preferably a maximum of 10%.

11. A machine according to one of the claims 1 through 10, characterized in that a size press and / or a coating unit and / or a smoothing unit and / or a device for rolling the web are arranged downstream of the dryer section.

12. A method for drying a web (10) of fibrous material, particularly of paper, cardboard, tissue / sanitary paper, in a machine according to one of the preceding claims, wherein the web (10) is moved through the dryer section and is dried convectively, wherein the moisture content of the web (10) is reduced, characterized in that the nozzle (12) is directed to a surface of the web (10) such that a vibration-excited drying fluid stream exiting the nozzle (12) impinges on the web (10) during the drying process to thereby assist the reduction in the moisture content over the entire web thickness by ultrasound.

13. A method according to claim 12, characterized in that the fibrous material has a wet strengthening agent, particularly a melamine resin or a urea formaldehyde resin or an epichlorohydrin resin.

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