EP4650516A1

EP4650516A1 - Drying section of a fiber web production line

Info

Publication number: EP4650516A1
Application number: EP25173528.8A
Authority: EP
Inventors: Jyrki Huovila; Kari Juppi; Petri Norri; Jere NÄVERI; Hans Sundqvist
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2024-05-13
Filing date: 2025-04-30
Publication date: 2025-11-19
Also published as: FI20245598A1; FI131771B1

Abstract

The invention relates to a drying section for drying a fiber web, which drying section (10) comprises at least one cylinder drying unit (11; 13) and at least one superheated steam drying (SSD) unit (12), and a steam system (20) comprising an exhaust steam recovery system (30). The exhaust steam recovery system (30) is configured to receive exhaust steam (Sout) from the SSD unit (12) and comprises a steam cleaning unit (24) configured to remove contaminants by cleaning and/or by filtration and/or by separation from the SSD unit (12) received exhaust steam (Sout). The exhaust steam recovery system (30) comprises a super heated steam generation (279 configured to heat at least with an electric heater a first part (S1) of the recovered steam (S1) cleaned in the steam cleaning unit (24) and at least one steam compressing unit configured to compress a second part (S2) of the recovered steam cleaned in the steam cleaning unit (24) and connected to the at least one cylinder drying unit (11; 13) configured to receive the pressurized second part of the recovered steam cleaned in the steam cleaning unit (24) to be used as a heating medium.

Description

Technical field

The invention relates generally to production of fiber webs and to drying of fiber webs. Especially, the invention relates to a drying section according to the preamble of the independent drying section claim.

Background

It is known that fiber webs, e.g., paper and board webs, are manufactured in devices and machines which together constitute a fiber web production line, which can be hundreds of meters long. As known from the prior art in fiber web production lines typically comprise an assembly formed by a number of the devices and the machines arranged consecutively in a process line. A typical production line for fiber web comprises a forming section comprising a head box and a wire section, a press section and a drying section and a reel-up. The production line can further comprise other sections and devices for treatment and/or finishing the fiber web, for example a sizer, a coating device, a calender. The production and treatment line also typically comprises at least one slitter-winder for forming customer rolls as well as a roll packaging apparatus.
As known from the prior art, drying sections may comprise cylinder drying, in which drying groups based on cylinder drying in a fiber web production line employ twin wire draw and/or single wire draw, i.e. twin or single tier drying groups. In twin wire draw, the groups of drying cylinders comprise two wires, which press the fiber web, one from above and the other one from below, against heated cylinder surfaces. In single wire draw, i.e. in single tier drying groups, each group of drying cylinders comprises only one drying wire on whose support the fiber web runs through the entire group so that the drying wire presses the fiber web against the heated cylinder surfaces of the drying cylinders and the fiber web remains at the side of the outside curve of the reversing cylinders or rolls situated between the drying cylinders. Thus, in single wire draw in single tier drying groups, the drying cylinders are arranged outside the drying wire loop, and the reversing cylinders or rolls are arranged inside the loop. In the following description, by the term 'reversing cylinder' is also meant a reversing roll alternatively placed in a corresponding position, and the terms 'reversing cylinder' and 'reversing roll' are used synonymously in this description. In the single tier drying groups of drying sections based on cylinder drying in fiber web production lines, a pocket space defined by a wire is formed between two adjacent drying cylinders and a reversing cylinder situated between them in a lower row.
As known from the prior art in drying of fiber webs in a fiber web production line superheated steam drying can also be used. In superheated steam drying (SSD) the fiber web to be dried is guided to an SSD unit, in which superheated steam is blown towards the surface of the fiber web as impingement drying. The SSD unit typically comprises a rotating cylinder, on surface of which the fiber web is run, and an impingement hood structure covering at least wrap area of the fiber web on the cylinder, inside of which impingement hood the superheated steam is provided to create drying effect on the passing fiber web. The superheated steam drying is, thus an impingement drying technology, which uses superheated steam as drying medium. In SSD systems typically temperature of superheated steam is greater than 200°C, preferably in temperature range of 300-350°C. In patent application publication EP1279764A2 is disclosed a method of producing a sheet material in a selected drying zone comprising the steps of: supporting an inner surface of the sheet material in a wet condition by an externally heated cylindrical rotor having a rotatable arc-shaped smooth surface mounted in a sealed hood, inside of which is maintained at a predetermined pressure and temperature; and clamping the outer surface of said sheet material by an endless heat-resistant fabric belt movable in synchronous with the rotation of said rotor and having gas permeability of at least 7,500 cm 3 /cm 2 /min, while said fabric belt applying a tension so as to be capable of restraining dry shrinkage of the sheet material by pressing on said rotor by means of a belt roll; and blowing heated gas from the outer surface of said rotor through the fabric belt in a selected drying zone according to the paper grade to thereby dry said sheet material mainly by external heating, wherein the inside of said hood is set to a gas temperature of at least 130°C and wherein said heated gas for blowing is super-heated gas of at least 150°C, preferably at least 250°C. During the drying in the SSD unit exhaust steam comprising also moisture evaporation from the fiber web is generated. This exhaust steam comprises, thus energy and evaporated moisture, which is wasted unless recovered.
An object of the present invention is to create a drying section of a fiber web production line comprising cylinder drying in at least one cylinder drying group and superheated steam drying in an SSD unit, in which the exhaust steam from the SSD unit is recovered in an energy and cost-efficient way.
An object of the invention is to provide an improved drying section of a fiber web production line comprising cylinder drying in at least one cylinder drying group and superheated steam drying in an SSD unit to eliminate and/or minimize the problems and disadvantages associated with the drying sections of this type known from prior art, in particular relating to problems and disadvantages relating to recovering of exhaust steam from an SSD unit.

Summary

To achieve the above-mentioned objects and those which come out later, the drying section according to the invention is mainly characterized by what is presented in the characterizing part of the independent drying section claim. Advantageous features and embodiments of the invention are defined in dependent claims.
According to the invention the drying section for drying a fiber web comprises at least one cylinder drying unit and at least one superheated steam drying (SSD) unit, and a steam system comprising an exhaust steam recovery system, wherein the exhaust steam recovery system is configured to receive exhaust steam (Sout) from the SSD unit and comprises a steam cleaning unit configured to remove contaminants by cleaning and/or by filtration and/or by separation from the SSD unit received exhaust steam (Sout), wherein the exhaust steam recovery system comprises a superheated steam generator configured to heat with at least an electric heater a first part of the recovered steam cleaned in the steam cleaning unit and at least one steam compressing unit configured to compress a second part of the recovered steam cleaned in the steam cleaning unit and connected to the at least one cylinder drying unit configured to receive the pressurized second part of the recovered steam cleaned in the steam cleaning unit to be used as a heating medium
According to an advantageous feature of the invention the superheated steam generator is configured to pressurize the first part of the recovered steam cleaned in the steam cleaning unit and the exhaust steam recovery system comprises at least one desuperheating unit configured to desuperheat the second part of the recovered steam cleaned in the steam cleaning unit and connected to the at least one cylinder drying unit configured to receive the pressurized and desuperheated second part of the recovered steam cleaned in the steam cleaning unit to be used as a heating medium.
By the cleaning unit different kinds of contaminants: impurities, for example fiber and other dry substance particles, and possible surplus gases, for example air, as well as condensated gases and other condensates, are cleaned and/or filtered and/or separated from the exhaust steam. The cleaning unit may comprise one or more cleaning units for removing the different kinds of contaminants. In particular, gas, especially air removal, is important in order to keep the system correctly operational. The removal of air is important as air might function as insulant or even create air locks in the system and as air might discharge the superheating during, when warming and expanding in the system and thus decrease the efficiency of the system.
According to an advantageous feature of the invention the exhaust steam recovery system comprises at least one desuperheating medium source configured to provide the desuperheating medium to the desuperheating unit.
According to an advantageous feature of the invention the desuperheating medium source is connected to the at least one cylinder drying unit and configured to receive condensated water from the at least one cylinder drying unit to be used as desuperheating medium.
Advantageously, at least part of the condensate that is generated by the drying cylinders using the steam from the SSD unit/s can be used in heat exchangers to heat process streams and/or as cooled as process water.
According to an advantageous feature of the invention the exhaust steam recovery system comprises a compressor in connection with the at least one desuperheating unit configured to control pressure of the second part of the in the steam cleaning unit cleaned recovered steam to be used as the heating medium in the at least one cylinder drying unit.
According to an advantageous feature of the invention the superheated steam generator is a two-stage generator configured to first to compress the steam in temperatures about 180 °C and there after heat the compressed steam by the electric heater in temperatures about 300 °C.
According to an advantageous feature of the invention the SSD dryer comprises a moving element, such as a rotating roll, metal belt loop or fabric, on surface of which the fiber web is run, and a steam impingement dryer structure covering at least part of the support / wrap area of the fiber web on the moving element, inside of which steam impingement dryer the superheated steam is provided to create drying effect on the passing fiber web. The steam impingement dryer comprises at least one steam impingement hood having distribution channels for superheated steam feed from an external superheater, blowing nozzles with blow openings to blow said steam directly against the surface of the moving fiber web and collecting means for the used steam and saturated steam evaporated from the fiber web. Metal belt or fabric is favorably impervious to fluids. Typically, several supporting rolls are provided under the belt or the fabric at the impingement area to minimize bending of the fabric or belt. Distance between two next to each other located supporting rolls tangent to tangent is 0,8-3 m, advantageously 1- 1,8 m. The web/fabric/belt wrap over the support rolls is 5-50 degrees under blowing surface area.
The steam impingement dryer of the SSD unit is advantageously located above a rotating roll with diameter of 2-3,6 m to blow superheated steam directly to the fiber web passing the steam impingement dryer on support of the rotating roll. The steam impingement dryer is divided to two dryers/hoods so that maximum blowing length can be over 180 degrees, advantageously 200-270 degrees.
The inlet and outlet for the fiber web run in the SSD unit and respectively out from the SSD unit are advantageously sealed with rotating members, like rolls, located against the fiber web and the impingement roll, which provides for transverse sealing. Curved side seals of the steam impingement dryer contour the impingement roll surface for side sealing. Said seals can be made of graphite that has very good thermal properties and can be even in contact to roll surface. Moreover, labyrinth structure of the seals can be used to reduce leakage and fluid sealing, like in liquid seal or sealing blows, can be used in the outer edge. Typically, one side seal type is enough, when pressure difference between the steam and the outside air is lower than 0,2 bar. Combination of at least two of said seal type solutions is advantageous for maximum separation of fluids (separation of the steam and the air) and/or in cases of higher pressure differences of 0,2-0,5 bar. In addition to seal a narrow box structure can be arranged just outside the steam impingement dryer hood seal structure, which sealing box structure prevents steam escaping from the hood, and steam can be recovered also by providing a ventilation outlet/-s. This steam recovery box is attached to the steam impingement dryer outer edge and sealed against the moving surface. These steam recovery boxes are attached at least on both sides of the steam impingement hood. The inlet and outlet seal rolls can have their own recovery boxes. Covering the seal roll surface free of the fiber web with the steam dryer impingement hood and the steam recovery box the roll surface remains over the condensing temperature and thus, heat is not wasted to cooler outer air.
For a web break situation, the steam impingement dryer on a roll is divided to at least two steam impingement hoods, which in the web break situation are opened for providing access between the steam impingement hood and the roll for cleaning the broke. On such occasion fast and efficient closing of the steam feed into the SSD unit and removal are needed for preventing the air entering the steam system.
In a start up, the steam impingement dryer is preheated over the condensing temperature with secondary preheating heating means. Then air is removed from the closed and sealed steam impingement hoods before superheated steam system is opened and steam enters. Efficient preheating system ensures that the impingement dryer system is free of condensate that can reduce the fiber web quality with droplets and even break it. Preheating system can use the same channels and nozzles as the superheated steam dryer. Advantageously, heated air (from an electric heater) or exhaust gases (from a gas burner) can be used in the preheating of the SSD unit. Further, compact superheated steam impingement dryers on a single roll or a metal belt make the steam system volume smaller than in cases the common hood containing several cylinders inside it, thus compact superheated steam impingement dryers provide that air removal is more efficient too.
According to an advantageous feature of the invention the at least one cylinder drying unit comprises at least one single tier drying group.
According to an advantageous feature of the invention the steam system comprises a superheated steam supply, from which initiating superheated steam is configured to be guided to the SSD unit as inlet steam for initiating superheated drying of the fiber web passing through the SSD unit.
According to an advantageous feature of the invention the superheated steam supply is connected via a three-way valve to the SSD unit to select start-up steam for the superheating or the superheated, cleaned steam from the exhaust recovery system.
It has been surprisingly noted that in the steam system of the drying section by heating and pressing the exhaust steam from the SSD unit, advantageously by desuperheating the exhaust steam from the SSD unit, to latent heat before applying the latent heat into drying cylinders in form of saturated steam, the exhaust steam can be directly applied into the drying cylinders as thus, the steam specifications of drying cylinders are directly met Advantageously the recovery steam from SSD unit is after the cleaning unit in the temperature of 110 - 140°C and thus, condensation can be avoided and maximum energy can be transferred to the fiber web. This also helps to reuse the second part of the steam in the drying cylinders, as the steam can be directly pressurized to the feed pressure of the inlet of the cylinder dryer section. Thereafter temperature can be adjusted with desuperheater using condensate from the cylinders. The pressurized and desuperheated exhaust steam for the drying cylinders is typically at least 110 °C, advantageously at least 140 °C. Thus, decree of superheating is decreased by the desuperheating to suitable level for the drying cylinders. The steam specifications for the SSD system are different from those of the drying cylinders and thus by first processing the exhaust steam by heating and pressurizing, advantageously by pressurizing and desuperheating, it can be directly applied into drying cylinders.
According to an advantageous aspect of the invention in the drying section comprising cylinder drying in at least one cylinder drying group and superheated steam drying in an SSD unit, in the superheated steam drying the fiber web is run into the SSD unit and after necessary drying, the fiber web will be guided out from the SSD unit. During this superheated steam drying, exhaust steam will be released from the system comprising hot moisture from the fiber web and drying steam. This exhaust steam comprises different kinds of contaminants, which are at first separated from the exhaust steam before the exhaust steam is processed further. Part of the recovered steam will be recirculated back to the SSD unit, after it has been superheated again to keep up the process temperature of the SSD unit. The other part, i.e. the rest of the recovered steam, will be utilized as heating medium for at least some of the drying cylinders. Before the exhaust steam is utilized as the heating medium for the drying cylinder, the exhaust steam is pressurized and desuperheated by applying desuperheating medium, typically moisture into the exhaust steam, advantageously by applying water by water spray units. During reuse of typically saturated steam to be fed into the drying cylinders has under 10 °C superheating, which can be controlled by the desuperheating unit. During drying the fiber web the drying cylinders make condensated water, which condensated water is in the recovery system is recirculated to be used as the desuperheating medium, advantageously as the water for the water spray units. By the invention, the overall energy efficiency of the drying section is significantly increased as well as in the exhaust steam recovery system significant improvement is achieved, as the energy of the exhaust steam is reutilized, and the moisture created by the drying cylinders is also reutilized. Thus, in the drying section infeed of superheated steam is needed only for starting the superheated steam drying process.
According to an advantageous aspect of the invention the steam impingement dryer of the SSD unit is sealed such, that entering of gases (typically air) into the system is prevented. The sealing can be provided by non-contacting air-bearing sealing provided against the moving element supporting the fiber web passing in the SSD unit. The sealing of the inlet and outlet can be done by a smooth roll that presses the fiber web against the roll or by a roll supporting a metal or belt or fabric. The sealing can also be a contacting lubricated sealing against the moving element supporting the fiber web during its run in the SSD unit. The sealing preventing the air to enter the exhaust steam recovery is important as air might function as insulant or even create air locks in the system and as air might discharge the superheating during, when warming and expanding in the system and thus decrease the efficiency of the system.
According to an advantageous aspect of the invention air-bearing sealing is arranged at outside edges of the steam impingement hood. The air-bearing sealing forms a collar-like sealing at the outside edges of the steam impingement hood. An air-bearing sealing is also advantageous for the inlet and outlet ends of the steam impingement hood. For accurate distance control to the fiber web, the air-bearing sealing placed opposite a roll, in a metal belt SSD unit, at a lead roll of the metal belt. The air-bearing sealing is gentle for the fiber web, and it can also provide some heat to the fiber web, if hot air is used as sealing air. In an air-bearing sealing device there is advantageously a porous sealing surface that is pervious for the air and has pore diameter under 0,01-0,2 mm. This surface is advantageously only 0,5-5 mm thick preferably 0,5-2 mm. This thin, porous structure is enabled by a support structure with dense perforations. The air-bearing sealing is advantageously made by additive manufacturing from metal in one print comprising both the porous and support structures. With additive manufacturing of the sealing surface the air-bearing sealing can be made compact and efficient. The sealing effect can also be enhanced with fine perforation through part of the sealing surface. In the side air-bearing sealing the blown air is usually in the process temperature of the support surface for the web which is a metal belt in this example. Other types of sealing structures can be combined with the air-bearing sealing for example a simple labyrinth seal can be added to the steam impingement dryer. There is advantageously also a removal arrangement between the air-bearing sealing and the steam impingement hood for removal of leakage of superheated steam and of air from sealings or from outside.

Brief description of the drawings

Aspects of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of some example embodiments when read in connection with the accompanying drawings and in the following the invention is described in more detail referring to the accompanying drawing, in which

In figure 1 is schematically shown an advantageous example of a drying section according to the invention,
In figure 2 is schematically shown an advantageous example of a steam impingement dryer,
In figures 3A-3B is schematically shown an advantageous example of a sealing arrangement of the exhaust steam recovery system and
In figure 4 is schematically shown an advantageous example of a metal belt superheated steam dryer.

Detailed description

In the following description same reference signs designate for similar components unless otherwise mentioned and it should be understood that the examples are susceptible of modification in order to adapt to different usages and conditions within the frames of the invention. Repetition of some reference signs may have been omitted in figures for clarity reasons.
In figures is schematically shown examples of a drying section 10 according to the invention. In the drying section a fiber web is dried and during its run through the drying section the fiber web is supported by drying fabrics 45, 45, typically drying wires. In the drying section 10 each drying unit /group comprises its own drying fabric 45, 46 and in between the drying units / groups the fiber web is transferred from a last drying cylinder of a preceding drying group to a drying fabric of the next drying group. The drying section 10 of the examples of figures comprises at least one superheated steam drying unit (SSD unit) 12. The SSD unit 12 is advantageously located after a press section of a fiber web production line before a first cylinder drying unit / group of the drying section, more advantageously outside a drying hood covering the cylinder drying unit/-s / group/-s. This provides good drying result and provides improved adjustability. The dry solids content of the fiber web is advantageously 65-75 %, when it enters the SSD unit 12. This way the sticking of the fiber web to the roll can be avoided. In other words the advantageous temperature of the fiber web is preferably at least 100 °C, which means that evaporation starts immediately, and the wet fiber web has a lot of water to be evaporated. The SSD unit 12 comprises a moving element such, as a rotating roll 61, metal belt or fabric, on surface of which the fiber web is run, and a steam impingement dryer hood structure 62, 63 covering at least part of the support / wrap area of the fiber web on the moving element 61, inside of which steam impingement dryer 62, 63 the superheated steam is provided to create drying effect on the passing fiber web. The SSD unit 12 also comprises as secondary preheating means 18, a preheater 18, by which the steam impingement dryer 62, 63 is preheated over the condensing temperature. Then air is removed from the closed and sealed steam impingement dryer hoods 62, 63 via an air removal channel 16 before the superheated steam system is opened and steam enters the SSD unit 12. The drying section 10 comprises also before and/or after the SSD unit/-s or in case there is more than one SSD unit, between the SSD units, located cylinder drying unit 11; 13, which comprises at least one twin or single tier drying group 11; 13. The SSD unit 12 can thus be located, as mentioned above, after a press section of a fiber web production line before a first cylinder drying unit / group of the drying section, more advantageously outside a drying hood covering the cylinder drying units / groups. Advantageously, the cylinder drying unit 11; 13 comprises at least one single tier drying group 11; 13, which comprises drying cylinders 41; 42 in the upper row and reversing cylinders 43 and in which the fiber web is dried against the heated drying cylinder surface. The drying section 10 also comprises a steam system 20 for providing steam and comprising a recovery system 30 for recovery of the exhaust steam, i.e. for reuse of the exhaust steam from the SSD unit 12 and for reuse of condensated water from cylinder drying groups 11; 13. The impingement roll or support rolls are favourably without heating means to help preventing the sticking.
In the example of the figure 1 the drying section 10 comprises at least one cylinder drying unit comprising at least one cylinder drying group 11; 13, advantageously a single tier drying group, in which the fiber web is dried against the heated surface of the drying cylinder 41; 42 and in which the fiber web is supported by the drying wire 45; 45, which runs as a loop and is guided by reversing cylinders 43 and guide rolls 44. The fiber web Win is guided to the SSD unit 12 from the preceding drying cylinder group 11. From the SSD unit 12 the fiber web Wout is guided to at least one cylinder drying unit 11; 13 comprising at least one cylinder drying group, advantageously a single tier drying group, in which the fiber web is dried.
The drying section 10 comprises the steam system 20, which comprises a superheated steam supply 21, from which initiating superheated steam is guided via a three-way valve 22, to the SSD unit 12 as inlet steam for initiating superheated drying of the fiber web passing through the SSD unit 12. During this superheated steam drying, exhaust steam Sout comprising hot moisture from the fiber web and used superheated drying steam is removed from the SSD unit 12 to the exhaust steam recovery system 30 and guided to be cleaned in a steam cleaning unit 24, advantageously in a steam filter 24, by cleaning and/or by filtration and/or by separation. In the steam filter 24 from the exhaust steam Sout different kinds of contaminants: impurities, for example fiber and other dry substance particles, condensate and possible surplus gases, for example air, are separated. After the filtration, the recovered steam is processed further. Part of the recovered steam S1, first part of the recovered steam S1, will be recirculated back to the SSD unit 12 via a blower 26 providing recirculation flow for the recovered steam from the SSD unit 12 and via a steam superheating generator 27 configured to heat with at least an electric heater a first part of the recovered steam. The steam superheating generator 27advantageously comprises a two-stage generator. From the cleaned recovered steam the superheated steam is superheated again to keep up the process temperature of the SSD unit: first by compressing the steam in temperatures about 180 °C and there after heating the steam by electric heating in temperatures about 300 °C to the superheated steam. The superheated steam formed of the recovered steam in the steam superheating unit 27 is then recirculated via the three-way valve 22 to the SSD unit. The other part, i.e. the rest of the recovered steam, the second part of the cleaned recovered steam S2 will be guided to be utilized as heating medium for the drying cylinders in at least one of the cylinder drying unit 11; 13. Before the recovered steam is utilized as the heating medium for the drying cylinders 41, 42 in at least one of the cylinder drying unit 11; 13 as indicated by the dashed lines in figure 1, the recovered steam is pressurized and desuperheated in at least one desuperheating unit 31; 32 by applying desuperheating medium, typically moisture, into the recovered steam from a desuperheating medium source 37, 38. To the desuperheating unit 31; 32 moisture, advantageously water is provided from a water spray unit 34; 35. During desuperheating the exhaust steam is also pressurized by a compressor 33 to pressure suitable for the heating medium of the drying cylinders of the cylinder drying unit 11; 13. During drying the fiber web the drying cylinders 41, 42 of the cylinder drying unit 11; 13 condensated water is created, which condensated water is in the recovery system 30 is recirculated to be used as the desuperheating medium, advantageously as the water, for the desuperheating sources, advantageously water spray units 37; 38. The exhaust steam comprises a mixture of superheated steam portion that has reduced temperature and saturated steam portion that is evaporated from the fiber web. Moisture from the drying cylinders of the cylinder unit 11; 13 is condensed from saturated steam and is also suitable for reutilization as liquid.
Thus, the overall energy efficiency of the drying section 10 is significantly increased as well as in the exhaust steam recovery system 30 of the steam system 20 significant improvement is achieved, as the energy of the exhaust steam is reutilized, and the moisture created by the drying cylinders is also reutilized. Thus, in the drying section 10 infeed of superheated steam from the superheated steam supply 21 is needed mainly only for starting the superheated steam drying process as the process has started and as long as the drying process continues the superheated steam is provided by the recovery steam system 30. The steam system 20 naturally comprises pipelines connecting the units as described above, as well as valves 23, 25, 34, 35 for controlling the steam flows in respective pipelines. The steam system 20 may also comprise other control means and/or measuring means.
In the example of figure 2 is in more detail shown the SSD unit 12. In the left side the steam impingement hood 62 as a cross section view with the steam impingement arrangement, whereas in the right side the steam impingement hood 63 is shown the outer view and seal sectors, too. The SSD unit 12 comprises, as shown in the example of figure 2, a moving element 61 such, as a rotating roll 61, metal belt or fabric, on surface of which the fiber web is run, and a steam impingement dryer hood structure 62, 63 covering at least part of the support / wrap area of the fiber web on the moving element 61, inside of which steam impingement hood 62, 63 the superheated steam is provided to create drying effect on the passing fiber web. In the steam impingement hood 62, 63 there are transverse steam nozzles 65 having openings for blowing steam against the fiber web and opposite to those at the bottom of the nozzle bigger openings 66 to feed said nozzles from steam distribution channels 68. Said channels 68 are perpendicular to nozzles 65, 66 being in web travelling direction. Between said steam blow nozzles 65, 66 there are exhaust openings 67 provided with blocking structure to prevent web parts from entering steam system. Bloking structure can be net/mesh like assembly between the nozzle surfaces. The steam impingement dryer 12 has only these nozzles 65, 66, exhaust openings 67, feed 68 and exhaust 69 channels. Whereas auxiliary feeding and exhaust systems and blowers are outside the steam impingement dryer. In the partial enlargement of figure 2 is schematically shown the distribution and exhaust channels 68, 69, nozzles 65, 66 and arrows indicating flow directions. In the distribution channel 68 the flow is indicated by arrows in the channel direction. The flow enters via the open space of the nozzles 65, 66 and next are located by dashed arrows the exhaust flows from between the nozzles and the distribution channels 68 through the exhaust openings 67. The dotted lines indicated the openings of the nozzles and between the nozzles the blocking structure with the exhaust openings are located.
The steam impingement dryer hood 62, 63 of the SSD unit is advantageously located at the moving element 61, in the example the rotating roll 61 with diameter of 2-3,6 m to blow superheated steam directly to the fiber web passing the steam impingement hood 62, 63 on support of the rotating roll and in the example of figure 4 the moving element 61 is a metal belt 61. The steam impingement dryer is divided to two halves so that maximum blowing length can be over 180 degrees, advantageously 200-270 degrees. The inlet and outlet of the fiber web into the SSD unit 12 and out from the SSD unit 12 are sealed with rotating members 51, 53, 55; 52, 54, 56, like rolls, located against the fiber web and the impingement roll 61, which provides for transverse sealing. In this example the transverse sealing is provided by belt loops 51; 52 running and supported by rolls 53, 55; 54, 56, which seal the inlet and the outlet of the SSD unit 12 being arranged against the surface of the rotating roll 61 of the SSD unit 12. Curved side seals 71 (figs. 3A-3B) of the steam impingement dryer contour 61, 62 the impingement roll surface 61. Said seals can be made of graphite that has very good thermal properties and can be even in contact to roll surface. Moreover, labyrinth structure of the seals can be used to reduce leakage and fluid sealing, like in liquid seal or sealing blows, can be used in the outer edge. Typically, one side seal type is enough, when pressure difference between the steam and outside air is lower than 0,2 bar. Combination of at least two of said seal solutions is advantageous for maximum separation of fluids (steam & air) or in cases of higher pressure differences of 0,2-0,5 bar. For a web break situation, the steam impingement dryer on the roll is divided to at least two steam impingement hoods 62, 63, that in the web break situation are opened for providing access between the steam impingement hood and the roll for cleaning the broke as indicated by arrows S62, S63.
As shown in figures the steam impingement dryer of the SSD unit 12 is sealed such, that entering of gases (typically air) into the system is prevented. The belt-roll systems 51, 53, 55; 52, 54, 56 that arranged for the transverse sealing of the SSD unit 12 can be seen in the fig. 2. Whereas the side seals 71, 72, 73, 74, 75 of the SSD unit 12 are shown in the fig 3. A further sealing example, an air-bearing sealing 85, 86, 78, is shown schematically in figure 4. The sealing arrangements can be combined and/or varied in connection. The steam impingement dryer 62, 63 of the SSD unit is sealed such, that entering of gases (typically air) into the system is prevented. The sealing can be provided by non-contacting air-bearing sealing 72 provided against the moving element 61 supporting the fiber web passing in the SSD unit 12. The sealing of the inlet and outlet can be done by a smooth roll 51, 52 or a belt-roll system 51, 53, 55; 52, 54, 56 with a smooth roll 51, 52 that presses the fiber web against the rotating roll 61. The inlet and outlet for the fiber web run in the SSD unit 12 and respectively out from the SSD unit 12 are advantageously sealed with rotating members, like rolls, located against the fiber web and the impingement moving surface 61, which provides for transverse sealing. Curved side seals 71-75 of the steam impingement dryer 62, 63 contour the impingement surface 61 for side sealing. Said seals can be made of graphite that has very good thermal properties and can be even in contact to roll surface. Moreover, labyrinth structure 74 of the seals can be used to reduce leakage and fluid sealing, like in liquid seal or sealing blows 75, can be used in the outer edge. Typically, one side seal type is enough, when pressure difference between the steam and the outside air is lower than 0,2 bar. Combination of at least two of said seal type solutions is advantageous for maximum separation of fluids (separation of the steam and the air) and/or in cases of higher pressure differences of 0,2-0,5 bar. In addition to seal a narrow box structure 76, 77 can be arranged just outside the steam impingement dryer hood seal structure, which sealing box structure 76, 77 prevents steam escaping from the hood, and steam can be recovered also by providing a ventilation outlet/-s 76. This steam recovery box 76,77 is attached to the steam impingement dryer outer edge and sealed 77 against the moving surface 61. These steam recovery boxes 76, 77 are attached at least on both sides of the steam impingement hood 62, 63. The inlet and outlet seal rolls can have their own recovery boxes.
In the example of the figure 4 is shown an example of a superheated steam impingement dryer that corresponds to those of the above examples but with a metal belt 61 as the moving element 61 supporting the fiber web during its run in the superheated steam impingement dryer with an SSD unit 12 and with an air-bearing sealing 8, 86, 87. The metal belt 61 is supported by lead rolls 81. The SSD unit 12 is advantageously located after a press section of a fiber web production line before a first cylinder drying unit / group of the drying section, more advantageously outside a drying hood covering the cylinder drying unit/-s / group/-s. This provides good drying result and provides improved adjustability. In this example an air-bearing sealing 85, indicated by small arrows, is provided. Air is provided for the air-bearing sealing 85 as feed air flow S85in and outlet flow is indicated by the arrow S85out. The feed flow S85in is advantageously located at a lead roll and the outlet flow S85out is located in between two lead rolls 81. The air-bearing 85 is formed like a collar at outside edges of the impingement hood 62, 63 in cross-direction of the fiber web. The air-bearing sealing 86, 87 is also advantageous for the inlet and outlet ends of the steam impingement hood 62, 63. The feed air flow of the inlet and outlet end air-bearing sealings 86, 87 is shown by arrows S86, S87. For accurate distance control to the fiber web the air-bearing sealing 86, 87 is placed opposite a lead roll 81. Between the impingement hood 62, 63 ends and the inlet and outlet air-bearing sealing 86, 87, respectively, a discharge S88, S89 can be arranged. The air-bearing sealing 85, 86, 87 is gentle for the fiber web and it can also provide some heat to the fiber web when hot air is used as the sealing air flow. In the blow device for the air-bearing sealing 85, 86, 78 there is a porous sealing surface that is pervious for the air and has pore diameter under 0,01-0,2 mm. This surface is advantageously only 0,5-5 mm thick, preferably 0,5-2 mm thick. Thin, porous structure is enabled by a support structure with dense perforations. The air-bearing sealing 85, 86, 87 is advantageously made by additive manufacturing from metal in one print comprising both the porous and support structures. With additive manufacturing of the sealing surface the air-bearing sealing 85, 86, 87 can be made compact and efficient. The sealing effect can also be enhanced with fine perforation through part of the sealing surface. In the side air-bearing sealing the blown air is usually in the process temperature of the support surface for the fiber web, which is a metal belt 61 in this example. Some simple labyrinth seal can be added to steam impingement dryer according to fig 2A. There is also the discharges S88, S89 as removal arrangement between the air-bearing sealing 86, 87 and the steam impingement hood 62, 63 for removing any leakage of superheated steam and of air from the air-bearing sealing 86, 87 or from outside.
In the description in the foregoing, although some functions have been described with reference to certain features, those function may be performable by other features whether described or not. Although features have been described with reference to certain embodiments or examples, those features may also be present in other embodiments or examples whether described or not. Above the invention has been described by referring to some advantageous examples only to which the invention is not to be narrowly limited. Many modifications and alterations are possible within the invention as defined in the following claims.

Claims

Drying section for drying a fiber web, which drying section (10) comprises at least one cylinder drying unit (11; 13) and at least one superheated steam drying (SSD) unit (12), and a steam system (20) comprising an exhaust steam recovery system (30), characterized in, that the exhaust steam recovery system (30) is configured to receive exhaust steam (Sout) from the SSD unit (12) and comprises a steam cleaning unit (24) configured to remove contaminants by cleaning and/or by filtration and/or by separation from the SSD unit (12) received exhaust steam (Sout), that the exhaust steam recovery system (30) comprises a superheated steam generator (27) configured to heat at least with an electric heater a first part (S1) of the recovered steam (S1) cleaned in the steam cleaning unit (24) and at least one steam compressing unit configured to compress a second part (S2) of the recovered steam cleaned in the steam cleaning unit (24) and connected to the at least one cylinder drying unit (11; 13) configured to receive the pressurized second part of the recovered steam cleaned in the steam cleaning unit (24) to be used as a heating medium.
Drying section according to claim 1, characterized in, that the superheated steam generator (27) is configured to pressurize the first part (S1) of the recovered steam (S1) cleaned in the steam cleaning unit (24) and that the exhaust steam recovery system (30) comprises at least one desuperheating unit (31; 32) configured to desuperheat the second part (S2) of the recovered steam cleaned in the steam cleaning unit (24) and connected to the at least one cylinder drying unit (11; 13) configured to receive the pressurized and desuperheated second part of the recovered steam cleaned in the steam cleaning unit (24) to be used as a heating medium.
Drying section according to claim 1 or 2, characterized in, that the exhaust steam recovery system (30) comprises at least one desuperheating medium source (37; 38) configured to provide the desuperheating medium to the desuperheating unit (31; 32).
Drying section according to claim 3, characterized in, that the desuperheating medium source (37; 38) is connected to the at least one cylinder drying unit (11; 13) and configured to receive condensated water from the at least one cylinder drying unit (11; 13) to be used as desuperheating medium.
Drying section according to any of claims claim 1-4, characterized in, that the exhaust steam recovery system (30) comprises a compressor (33) in connection with the at least one desuperheating unit (31; 32) configured to control pressure of the second part (S2) of the in the steam cleaning unit (24) cleaned recovered steam to be used as the heating medium in the at least one cylinder drying unit (11; 13).
Drying section according to any of claims 1-5, characterized in, that the superheated steam generator (27) comprises a two-stage generator configured to first to compress the steam in temperatures about 180 °C and there after heat the compressed steam by the electric heater in temperatures about 300 °C.
Drying section according to any of claims 1-6, characterized in, that the SSD dryer (12) comprises a moving element, on surface of which the fiber web is run, and a steam impingement dryer structure covering at least part of support / wrap area of the fiber web on the moving element, inside of which steam impingement dryer the superheated steam is provided to create drying effect on the passing fiber web.
Drying section according to any of claims 1-7, characterized in, that the at least one cylinder drying unit (11; 13) comprises at least one single tier drying group.
Drying section according to any of claims 1-8, characterized in, that the steam system (20) comprises a superheated steam supply (21), from which initiating superheated steam is configured to be guided to the SSD unit (12) as inlet steam for initiating superheated drying of the fiber web passing through the SSD unit (12).
Drying section according to claim 9, characterized in, that the superheated steam supply (21) is connected via a three-way valve (22) to the SSD unit (12) to select start-up steam or the superheated, cleaned steam from the exhaust recovery system.
Drying section according to any of claims 1-10, characterized in, that the SSD unit (12) is configured to be preheated by heated air, advantageously provided from an electric heater arranged as a secondary preheating means in connection with the SSD unit (12) and/or by exhaust gases from a gas burner.
Drying section according to any of claims 1-11, characterized in, that the SSD unit (12) comprises secondary preheating means (18), by which the steam impingement dryer is preheated over the condensing temperature.
Drying section according to any of claims 1-12, characterized in, that the SSD unit (12) comprises an air removal channel (16) configured to remove air from the steam impingement dryer hood before the superheated steam system is opened and steam enters the SSD unit.
Drying section according to any of claims 1-13, characterized in, that an air-bearing sealing (85) is arranged in the SSD unit (12) at outside edges of the steam impingement dryer hood.
Drying section according to any of claims 1-14, characterized in, that an air-bearing sealing (86, 87) is located at the inlet and outlet ends of the steam impingement dryer hood of the SSD unit (12).