The invention relates generally to producing fiber webs. Particularly the invention relates to a production line for producing a fiber web according to the preamble part of the independent production line claim and to a method for producing a fiber web according to the preamble part of the independent method claim.

Background As known from the prior art in fiber web machines, especially in paper and board machines, the fiber web is produced and treated in an assembly formed by a number of apparatuses arranged consecutively in a process line. Atypical production line comprises a forming section comprising a headbox and a forming unit and a press section as well as a subsequent drying section and a reel-up. The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a size press, a calender, a coating section. The production and treatment line also comprises typically at least one winder for forming customer rolls as well as a roll packaging apparatus. In this description and the following claims by fiber webs are meant especially board webs. The invention is in particular suitable in connection with production of multi-ply fiber webs.

The task of the headbox is to supply fiber suspension for the fiber web production into the forming unit. In a multilayer headbox more than one fiber suspension flows are discharged from the headbox via flow channels for pulp suspension layers, each for forming one layer of a multi-ply fiber web.

The task of a forming unit is to remove water from fiber suspension fed by the headbox. When the web is manufactured of watery fiber stock, water in the stock is removed on the forming section through a forming wire or forming wires for starting the formation of the web. Fibers remain on the forming wire or between the forming wires moving together. Depending on the grade of the web being manufactured, different types of stocks are used. The volume for which water can be removed from different stocks for achieving a web of good quality is a function of many factors, such as e.g. a function of the desired basis weight of the web, the design speed of the machine, and the desired level of fines, fibers and fill materials in the finished product. Many types of devices are known on the forming unit such as foil strips, suction boxes, turning rolls, suction rolls, and rolls provided with an open surface, which are used in many different arrangements and arrays when trying to optimize the volume, time and location of water being removed when forming the web. The manufacturing a high-quality end-product of desired grade is a function of the volume of dewatering, the dewatering method, the duration of dewatering, and the location of dewatering.

A commonly used method of making a multi-ply fiber web is based on the use of several separate web forming units in which the different layers of the fiber web are caused to be drained in a layer by layer fashion either onto one another or onto separate wires, in which case they are combined with one another after partial dewatering. Typically in multi-ply / multilayer fiber web production, i.e. when producing a fiber web having more than one layers, term “multi-ply” is used when the layers are formed separately in the forming section and term “multilayer” is used when a multilayer headbox is used for feeding suspension layers to the forming section even though these terms multi-ply / multilayer are used very often synonymously and thus the difference can be defined by the context only. In this application by the term multi-ply is to be interpreted synonymous to the multilayer.

In production of fiber webs, for example in production of paper or board webs, sizing is used to alter the properties of a fiber web by adding sizing agents, for example starch or other sizing agents. Sizing can be divided to internal sizing and surface sizing. In internal sizing, sometimes called stock sizing, the sizing agent is added to pulp in the wet end of the fiber web machine before forming. In surface sizing the sizing agent is added onto the surface of the fiber web typically at the dry end of the fiber web machine. The surface sizing of the fiber web typically utilizes an application device - a sizer typically comprising two sizing rolls forming a sizing nip in which the sizing agent applied directly onto the fiber web or indirectly via the surface of the sizing roll is pressed onto the fiber web one- or two-sidedly. The sizer together with for example drying devices following the sizer form the sizing section of the fiber web production line. In connection with the sizers different kinds of application technology for application of the sizing agent on the fiber web are employed in prior art arrangements, for example curtain technology or blade application technology or film transfer technology or rod application technology or air brush application technology or spray application technology or pond application technology. Sizing is used in order to improve paper web properties, in particular water resistance, water absorption properties, strength, internal strength and bending stiffness. In addition, runnability as well as dusting tendency can be affected favorably. It is known, that in connection with production of multi-ply fiber webs sizing can also be provided in the forming section by applying sizing agent in between layers of the multi-ply fiber web by spray or foam application.

A foam application system comprises typically a mixing device, a pumping device, a foaming device, a piping system and an application unit. The foaming is based on powerful mixing for adding air into the liquid-based substance with a surfactant additive, resulting substantial increase in the specific volume and thus making the handling and application of the substance much easier at small dry substance weights. The foam application device typically comprises an application head with a slot-type nozzle for extruding the foam onto the web. Very shortly after the application, the foamy structure disintegrates, absorbs and spreads into the surface structure of the web, leaving a wetted substance layer.

An object of the invention is to create a production line for producing a fiber web and a method for producing a fiber web, in which the disadvantages and problems of prior art are eliminated or at least minimized.

A particular object of the invention is to provide an improved production line for producing a fiber web and an improved method for producing a fiber web, by which improved sizing result is achieved, especially in view of strength improvement, reduced drying need and energy consumption and optimal use of sizing agents. Summary

In order to achieve the abovementioned objects, the production line for producing a fiber web according to the invention is mainly characterized by the features of the characterizing clause of the independent production line claim and the method for producing a fiber web according to the invention is mainly characterized by the features of the characterizing clause of the independent method claim. Advantageous embodiments and features are disclosed in the dependent claims.

According to the invention the production line for producing a fiber web comprises a forming section comprising at least one headbox and a forming unit comprising at least one wire and at least one water removal means, and which production line comprises a sizing section comprising a sizer with a sizing nip formed between two sizing rolls, wherein the production line comprises at least one application device located in the forming unit and configured to apply sizing agent in form of foam for inner part sizing of the fiber web and the sizer located in the sizing section comprises at least one application device is configured to apply sizing agent for surface sizing of the fiber web.

According to an advantageous feature of the invention the production line is for producing a multi-ply fiber web, which has at least two layers, and the forming unit of the production line comprises at least one twin-wire forming part formed between a wire for the first layer of the multi-ply fiber web and a wire for the second layer of the multi-ply fiber web, in which twin-wire part in the layers for the multi-ply fiber web are joined and treated layers combined, which forming unit comprises at least one application device for applying sizing agent in form of foam in between at least two layers of the multi-ply fiber web.

According to an advantageous feature of the invention the production line is for producing a multi-ply fiber web, which has at least three layers, in which at least one application device for applying sizing agent in form of foam on top of the filler layer, which will be combined with top and back layers in the twin-wire part. According to an advantageous feature of the invention the application device located in the forming unit is a curtain type application device configured to apply the sizing agent in form of foam.

According to an advantageous feature of the invention the application device located in the sizing section is a curtain type application device located in connection with the sizing roll and configured to apply the sizing agent in form of foam or liquid indirectly via surface of the sizing roll to be applied onto the fiber web in the sizing nip.

According to an advantageous feature of the invention the application device located in the forming unit is configured to apply the sizing agent in form of foam in solids content, preferably in 0,5 - 20% solids content and the application device located in the sizing section is configured to apply the sizing agent in high solids content, preferably in 10-60 % solids content, more preferably in 20 - 40 % solids content.

According to an advantageous feature of the invention the sizing rolls of the sizing section are hard rolls.

According to an advantageous feature of the invention the forming unit of the production line comprises a suction or an underpressure device on the opposite side of the fiber web and its support wire in relation to the application device.

According to an advantageous feature of the invention the fiber web production line comprises an air guide, preferably an air curtain and/or a reversing blade before the application device for redirecting boundary air flow off the running direction of the fiber web before curtain application of the sizing agent in form of foam.

According to an advantageous feature of the invention the production line comprises a metal-belt calender or a heat-treatment device located in or after the sizing section. According to the invention in the method for producing a fiber web, which has at least one layer, in a production line comprising a forming section comprising at least one headbox and a forming unit, in the forming unit the fiber web is supported by at least one wire and water is removed from the fiber web by at least one water removal means, and the fiber web is sized in a sizing section comprising a sizer with a sizing nip formed between two sizing rolls, wherein inner part of the fiber web is sized by at least one application device located in the forming section and configured to apply sizing agent in form of foam for the inner part sizing of the fiber web and the fiber web is surface sized in the sizer located in the sizing section by at least one application device is configured to apply sizing agent for surface sizing of the fiber web.

According to an advantageous feature of the invention in the method a multi ply fiber web, which has at least two layers, is sized by application of the sizing agent in form of foam for the inner part sizing of the fiber web and by application of the sizing agent by at least one application device located in the sizing section of the production line for surface sizing of the fiber web.

According to an advantageous feature of the invention in the method the sizing agent for the inner part sizing is applied as a curtain in form of foam.

According to an advantageous feature of the invention in the method the surface sizing of the fiber web is followed by heat treatment of the fiber web.

According to an advantageous feature of the invention the sizing agent for the surface sizing is applied in high solids content, preferably in 10-60 % solids content, more preferably in 20 - 40 % solids content.

According to an advantageous main aspect of the invention in production of a fiber web the sizing of the fiber web is conducted in at least two main sizing stages comprising inner part sizing of the fiber web and surface sizing of the fiber web. By the term inner part sizing of the fiber web is meant that sizing agent penetrates especially in the inner part of the fiber web in the web thickness direction and surface sizing means that main content of the sizing agent remains near the surface of the fiber web. In the inner part sizing of the fiber web internal strength is provided and in case the fiber web being a multi-ply fiber web also improved plybond between layers of the multi-ply fiber web is provided. Also by the inner part sizing of the fiber web by application of the sizing agent in form of foam internal sizing in the headbox can be replaced at least partly. The inner part sizing of the fiber web in the forming section by application of the sizing agent in form of foam improved penetration and sizing response of the sizing agent is achieved compared to applying the sizing agent by liquid dispersion. The application of the sizing agent in form of foam also provides for the possibility of using higher dosages of the sizing agent compared to internal sizing in the headbox. In the application of the sizing agent in form of foam very shortly after the application, the foamy structure disintegrates, absorbs and spreads into the surface structure of the web, which provides for the possibility of using large dosages of the sizing agent, for faster absorption of the sizing agent into the fiber web. The application of the sizing agent in form of foam for the inner part thus provides for the possibility of using smaller amount of sizing agent for the surface sizing. Thus, decreased drying capacity is needed after the surface sizing. The solution according to invention creates optimal z-direction strength to the fiber web, because inner part sizing improves internal strength and then in the surface sizing can be concentrated for creating stiff surface layer.

Surprisingly it has been found out that solution according to invention creates fiber web which has higher stiffness and strength. It has so called I-beam structure in which surfaces are stiff and by that way web bending stiffness is good. In addition, with hard nip sizing with high solids content in the surface sizing, need for drying capacity is decreased, fiber web surfaces come smoother, which reduces need for calendering and by that way fiber web bulk remains. In general invention improves board stiffness, plybond, internal strength and surface strength and has better pre-coating properties.

In the surface sizing stage strength properties and impermeability properties of the surface/-s of the fiber web are improved. Additionally hydrophobic sizing can be used to improve flexural rigidity and moisture resistance of the fiber web. Advantageously in the inner part sizing stage more sizing agent is applied than in the surface sizing stage.

Advantageously the surface sizing agent solids content is higher than inner part sizing agent solids content.

Due to the first conducted inner part sizing in the surface sizing high penetration is not needed and thus, sizing agent with high viscosity can be used to achieved effective strengthening of the surface/-s of the fiber web. The possibility of using high viscosity sizing agent also decreases the wet addition and thus, need of drying. Advantageously, the fiber web is sized with sizing agent having viscosity of 5 cP (centipoise) or over, preferably 5 cP - 1000 cP, more preferably 20 - 200 cP.

According to an advantageous aspect of the invention the inner part sizing of the fiber web is conducted in the forming section of the fiber web production line by applying the sizing agent onto the surface/-s of the fiber web in form of foam. Due to suction of water removal devices used in the forming section the sizing agent is absorbed into the inner part of the fiber web, which is further effected by pressure of press nips in the press section of the fiber web production line. Thus, improved sizing effect especially in the inner part of the fiber web is provided.

According to an advantageous aspect of the invention the inner part sizing of a multi-ply fiber web is conducted in the forming section of the fiber web production line by applying the sizing agent in form of foam onto a surface of at least one layer of the multi-ply fiber web and/or in between at least two layers of the multi-ply fiber web. This provides sizing effect especially in the inner part of the multi-ply fiber web and improves plybond between the layers of the multiply fiber web. Especially in at least three layer fiber webs this improves filler layer properties, like internal strength as well as plybond between filler layer and back and top layers.

The application device for the inner part sizing is advantageously an applicator which creates a curtain-like sizing film, for example a slot-type or a slide-type curtain application device. The slot-type curtain application device is advantageous because dwell time is shortest from the nozzle to the fiber web. Also, curtain jet impact angle can be controlled easier by adjusting the inclination angle of the applicator.

According to an advantageous aspect of the invention the surface sizing of the fiber web is conducted after the inner part sizing of the fiber web, advantageously at a later production section of the fiber web production line, i.e. in a section following the forming section in the production line of the fiber web. Advantageously, the surface sizing of the fiber web is conducted as in a sizer in a sizing section of the fiber web production line after the pre-drying section. The sizing agent is applied in the sizer and directly or indirectly utilizing curtain technology, spray technology or film transfer technology or pond technology. Preferably, the sizing agent is applied utilizing curtain technology. Advantageously, the fiber web is sized with high solids content sizing agent, preferably in solids content 10 - 60 %, more preferably 20 - 40 %. More preferably the fiber web is sized by curtain technology and with high solids content sizing agent. Advantageously, the sizing agent is starch.

The surface sizing in the sizer is advantageously conducted by a hard-nip sizer comprising a hard sizing nip formed between two hard sizing rolls. Advantageously as the hard roll is used a roll produced of hard material or provided with a hard coating or cover. Advantageously, as the hard roll is used a ceramic or metallic roll or advantageously, a roll with a hard-polymeric roll cover (rubber, polyurethane or composite) having surface hardness 60 - 100 shoreD, advantageously 80 - 95 shoreD. The hard nip enhances the fiber web strength increase. Advantageously, linear load in the sizing nip is 5 - 450 kN/m, more preferably the linear load in the sizing nip is 5 - 200 kN/m.

The surface sizing can be conducted by applying the sizing agent in form of liquid or in form of foam utilizing foam application technology.

The production line of the fiber web according to an advantageous aspect comprises heat-treatment of the fiber web in a metal-belt calender or in a heat- treatment zone of a heat-treatment device. When the heat-treatment is conducted in addition to surface sizing the heat-treatment enhances effect of the sizing agent. The heat-treatment zone is advantageously formed between a heated belt loop, in particularly metal belt loop, and a roll, advantageously a hard roll, or between two belt loops, in particularly metal belt loops, at least one of which is advantageously heated. Advantageously, the length of the heat-treatment zone is 0,2 - 15 m, advantageously 0,2 - 5 m. Advantageously the surface temperature in the heat-treatment zone is 100 - 250 °C. Advantageously, the pressure in the heat-treatment zone is 0,1 MPa - 30 MPa, advantageously 0,1 - 5 MPa. The pressure comprises at least a low basic pressure in the heat- treatment zone but in addition thereto the pressure during the heat-treatment zone can vary, for example at least one high pressure pulse created by at least one additional roll-nip, a heat-treatment nip, placed at a location in the heat- treatment zone can be used. Also, instead a loading sector formed by a loading element located in the heat-treatment zone can be used to variate the pressure in the heat-treatment zone. Advantageously, the dwell time of the fiber web in the heat-treatment zone is at least 20 ms, advantageously 100 - 10000 ms.

According to one embodiment of the invention the inner part sizing of the fiber web is conducted in the forming section by applying the sizing agent in form of foam and the surface sizing of the fiber web is conducted in a sizer, advantageously in a hard nip sizer.

According to another embodiment of the invention the inner part sizing of the fiber web is conducted in the forming section by applying the sizing agent in form of foam and the surface sizing of the fiber web is conducted in a sizer, advantageously in a hard nip sizer by utilizing curtain application of the sizing agent with high solids content.

According to another embodiment of the invention the inner part sizing of the fiber web is conducted in the forming section by applying the sizing agent in form of foam and the surface sizing of the fiber web is conducted in a sizer, advantageously in a hard nip sizer by utilizing curtain application of the sizing agent with high solids content, and as heat treatment in a heat-treatment zone of a heat-treatment device, advantageously by metal belt heat-treatment. Advantageously the fiber web production line comprises a final calendering stage in a final calender after the surface sizing stage.

The fiber web production line can also comprise a coating section with a coater and drying equipment.

According to an advantageous aspect of the invention the sizing agent applied in form of foam in the inner part sizing penetrates deep into the structure of the fiber web and advantageously the conveyance and the penetration of the sizing agent in form of foam is guided to desired direction and depth by a water- removal means and by controlling amount and viscosity of the foam, by conveying the sizing agent to both layers on each side of the boundary of the web layers to be joined the sizing in the boundary zone between the layers joined is efficient.

According to an advantageous aspect of the invention the multi-ply fiber web is cartonboard, for example folding box board (FBB), white lined chipboard (WLC), solid bleached board (SBS) or liquid packaging board (LPB). In addition, the multi-ply board is advantageously containerboard, for example krafliner (KL), testliner (TL) or white top liner (WTL).

Brief description of the drawings

In the following the invention is explained in detail with reference to the accompanying drawing to which the invention is not to be narrowly limited.

In figure 1 is shown schematically an advantageous example of a production line for producing a fiber web according to the invention.

In figure 2 is shown schematically another advantageous example of a production line for producing a fiber web according to the invention. In figure 3 is shown schematically yet another advantageous example of a production line for producing a fiber web according to the invention. In figure 4 is shown schematically a further advantageous example of a production line for producing a fiber web according to the invention.

Detailed description

During the course of the following description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention and its advantageous examples. In the figures some repetitive reference signs have been omitted for clarity reasons.

In figure 1 is shown an example of a production line 200 for producing a two layer fiber web W. The production line 200 comprises in this example a forming section 210 comprising two headboxes M10; M20 and a forming unit 220 and a press section 230 as well as a subsequent drying section 240 and a reel-up 280. The production line 200 in this example further comprises a sizing section 250 with a drying unit 260 and a calender 105.

In the example the forming section 210 for production of a multi-ply fiber web, in this example for a two-layer W10; W20 fiber web W. The forming section comprises a headbox M10; M20 for each layer W10, W20, from which the stock suspension is fed to the forming unit 220 beginning as one wire part comprising a wire 10 for first layer W10 of the multi-ply fiber web W and a wire 20 for second layer W20 of the multi-ply fiber web W, each wire 10, 20 comprising rolls 12, 22 for guiding, tensioning and/or driving the wire 10; 20 as an endless loop. The stock suspension M10; M20 is first fed onto the wire 10; 20 and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20 located water removal means 11 ; 21 ; 15; 25. Inside of the loop of wire 10; 20 at least one suction means 15; 25 are located. During the run on the one wire part the water removal from the web is substantially horizontal. A curtain application device C10; C20 is located at the run of the stock layer the one wire part for applying sizing agent in form of foam by a curtain type application device onto the pre-dewatered fiber web and to be in between at the layers of the multi-ply fiber web in forming section. The curtain application device C10; C20 is advantageously located such that the suction means 15; 25 are located on the opposite side of the multi-ply fiber web inside of the loop of the wires 10; 20. Also, an air guide 14, 24, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10, 20 at a location before the curtain application device C10, C20 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C10, C20.

Alternatively, only one application device C10; C20 for the foam could be used, which device C10 would advantageously be applicating the foam onto the fiber web layer on the lower wire 10. The application device C20 for the foam can also be only applicating onto the fiber web on the upper wire 20. It is also possible to use two application devices C10; C20 as shown in the example of the figure. Also, more than one application devices C10; C20 can be used consecutively for one fiber web layer.

The stock forming the second layer W20 of the multi-ply fiber web W guided on the wire 20 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the first layer W10 and the wire 20 for the second layer W20 are united by a combining roll 23 to form a twin wire part and the webs for the first layer W10 and the second layer W20 are guided into a gap formed between the wires 10; 20 forming the twin-wire part of the forming unit. Web layers W10, W20 have been joined together by a combining roll 23 at the beginning of the twin-wire forming part. Combining roll 23 is situated in the second layer wire loop 20. Thus, the sizing agent applied in form of foam by the curtain application devices C10; C20 onto each layer W10, W20 of the web W is guided in between the layers. Advantageously, after applying the sizing agent in form of foam by the curtain application device C10 its conveyance and penetration into the web layer is assisted such that the sizing agent is forwarded deeper into the web layer on which the sizing agent is applied in form of foam by guiding the foam to desired direction. In the example of figure a suction means 15, 25 are provided inside the wire loops 10, 20, which suction means during and after the application of the sizing agent in form of foam by the curtain application device C10, C20 but before the web layers are joined by the combining roll 23 at the beginning of the twin-wire forming part assist the conveyance and penetration of the sizing agent into the web layer. It is also possible to use in one or more forming units two-sided dewatering before applying the sizing agent, this is done by using separate wire loop on top of the one-wire part.

After the joining of the layers W10, W20 the multi-ply fiber web W is guided on the wire 10 supporting the first side of the multi-ply fiber web W as a one wire part, during which run support foils 16 located inside the wire 10 loop. The support foils 16 do not deviate the run of the wire 10 only remove water from the first surface of the wire 10 and support the run of the wire 10 as the multi ply fiber web is guided via suction roll 13 towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric 40 of the press section 230. The press section also comprises a second press fabric 50 with a roll 51 . The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi-ply fiber web W is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 55. In this example the press section 230 comprises two sets of press fabrics 40, 50 with the rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop as well as the press nips formed between the first press rolls 45 and the second press roll 55. Absorption of the sizing agent into the inner part of the fiber web W is further effected by pressure of press nips between the press rolls 45, 55 in the press section 230. After the press section the fiber web is guided to a drying section 240.

The drying section 240 comprises means for drying the fiber web W. In this example is shown only beginning of the drying section, in which drying is effected by a one-wire drying cylinder group comprising a drying wire 60 with a pick-up roll 61 , rolls 62 for guiding, tensioning and/or driving the drying wire 60, reversing rolls 63 and drying cylinders 64. The drying section 240 typically comprises several drying groups with single- or double-wire draw. Also, other types of drying devices can be used. The drying section 240 is followed by the sizing section 250 with the drying unit 260.

In the sizing section of the example the fiber web is running in direction substantially vertically downwards from a guide roll 72. The fiber web W is sized by two curtain application devices C75, each located at a sizing roll 75, indirectly by first applying the sizing agent onto surface of the corresponding rotating sizing roll 75 and then the sizing agent is guided to a sizing nip formed between the sizing rolls 75. In rotating direction of the sizing roll 75 an air guide 74, preferably an air curtain and/or a reversing blade, is located for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent applied by the curtain application device C75. In the example the fiber web W is sized two-sidedly but also only one side of the fiber web W can be sized, in which case only one of the sizing rolls 75 is provided with the curtain application device C75 and the air guide 74. Advantageously, the sizing rolls 75 are hard rolls, i.e. a hard nip sizer is provided. Advantageously, as the hard roll is used a ceramic or metallic roll or advantageously, a roll with a hard-polymeric roll cover (rubber, polyurethane or composite) having surface hardness 60 - 100 shoreD, advantageously 80 - 95 shoreD. The hard nip enhances the strength increase. Advantageously, linear load in the sizing nip is 5 - 450 kN/m, more preferably the linear load in the sizing nip is 5 - 200 kN/m. Advantageously, the sizing agent is high solids content sizing agent, comprising advantageously fiber suspension, preferably 10-60 % solids content, more preferably 20 - 40 % solids content. Viscosity of the sizing agent is advantageously 5 cP or over, preferably 5 cP - 1000 cP, more preferably 20 cP - 200 cP. One of the hard sizing rolls 75 is a deflection compensated roll and thus a straight sizing nip is be formed. The curtain application device located in connection with each sizing roll 75 applies the sizing agent onto the surface of the sizing roll 75 and the fiber web W is sized by indirect sizing. The sizing agent is then via the surface of the sizing roll 75 guided onto the surfaces of the fiber web W such that in the sizing nip the sizing agent is pressed to the fiber web surfaces. After the sizing the fiber web W is guided via an air-borne drying device 87 of the drying unit 260, which air borne drying device 87 first dries the sizing agent preventing the sizing agent to adhere to the following drying cylinders 84 in the drying unit 260. The fiber web W is further dried in the drying unit 260, in this example comprising single wire drying and double wire drying. In the single wire drying the fiber web W runs supported by a drying wire 80 via drying cylinders 84 and reversing rolls 83. The drying wire 80 runs as a closed loop supported and rolls 82 are provided for guiding, tensioning and/or driving the drying wire 80. The drying unit 260 also comprises the double wire drying, in which the fiber web W is dried by drying cylinders 94U, 94L located in two rows. Each row has its own drying wire 90U, 90L and rolls 92U, 92L for guiding, tensioning and/or driving the corresponding drying wire 90U, 90L. Between the rows the fiber web W has free draws.

After the sizing section 250 with drying unit 260 the fiber web is guided to the calendering section 270, which in the example comprises one calendering nip formed between calendering rolls 105, 106. It is possible to have a coating section with a coater and drying equipment after the pre-calender and also final calender after coating section (not shown). Thereafter, the fiber web W is guided to the reel-up 280, in which the fiber web W is reeled to a parent roll 115 by means of a reeling cylinder 116.

In figure 2 is shown an example of a production line 200 for producing a two layer fiber web W. The production line 200 comprises in this example a forming section 210 comprising two headboxes M10; M20 and a forming unit 220 and a press section 230 as well as a subsequent drying section 240 and a reel-up 280. The production line 200 in this example further comprises a sizing section 250 with a drying unit 260 and a calender 105.

In the example the forming section 210 for production of a multi-ply fiber web, in this example for a two-layer W10; W20 fiber web W. The forming section comprises a headbox M10; M20 for each layer W10, W20, from which the stock suspension is fed to the forming unit 220 beginning as one wire part comprising a wire 10 for first layer W10 of the multi-ply fiber web W and a wire 20 for second layer W20 of the multi-ply fiber web W, each wire 10, 20 comprising rolls 12, 22 for guiding, tensioning and/or driving the wire 10; 20 as an endless loop. The stock suspension M10; M20 is first fed onto the wire 10; 20 and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20 located water removal means 11 ; 21 ; 15; 25. Inside of the loop of wire 10; 20 at least one suction means 15; 25 are located. During the run on the one wire part the water removal from the web is substantially horizontal. A curtain application device C10; C20 is located at the run of the one wire part for applying sizing agent in form of foam by a curtain type application device onto the pre-dewatered fiber web and to be in between at the layers of the multi-ply fiber web in forming section. The curtain application device C10; C20 is advantageously located such that the suction means 15; 25 are located on the opposite side of the multi-ply fiber web inside of the loop of the wires 10; 20. Also, an air guide 14, 24, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10, 20 at a location before the curtain application device C10, C20 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C10, C20.

The stock forming the second layer W20 of the multi-ply fiber web W guided on the wire 20 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the first layer W10 and the wire 20 for the second layer W20 are united by a combining roll 23 to form a twin wire part and the webs for the first layer W10 and the second layer W20 are guided into a gap formed between the wires 10; 20 forming the twin-wire part of the forming unit. Web layers W10, W20 have been joined together by a combining roll 23 at the beginning of the twin-wire forming part. Combining roll 23 is situated in the second layer wire loop 20. Thus, the sizing agent applied in form of foam by the curtain application devices C10; C20 onto each layer W10, W20 of the web W is guided in between the layers. Advantageously, after applying the sizing agent in form of foam by the curtain application device C10, C20 its conveyance and penetration into the web layer is assisted such that the sizing agent is forwarded deeper into the web layer on which the sizing agent is applied in form of foam by guiding the foam to desired direction. In the example of figure a suction means 15, 25 are provided inside the wire loops 10, 20, which suction means during and after the application of the sizing agent in form of foam by the curtain application device C10, C20 but before the web layers are joined by the combining roll 23 at the beginning of the twin-wire forming part assist the conveyance and penetration of the sizing agent into the web layer. It is also possible to use in one or more forming units two-sided dewatering before applying the sizing agent, this is done by using separate wire loop on top of the one-wire part.

After the joining of the layers W10, W20 the multi-ply fiber web W is guided on the wire 10 supporting the first side of the multi-ply fiber web W as a one wire part, during which run support foils 16 located inside the wire 10 loop. The support foils 16 do not deviate the run of the wire 10 only remove water from the first surface of the wire 10 and support the run of the wire 10 as the multi ply fiber web W is guided forwards on the wire 10. In connection with this run of the wire 10 a further curtain application device C50 is located for applying sizing agent on the surface of multi-ply fiber web W. The curtain application device C50 is advantageously located such that the suction means 56 are located on the opposite side of the multi-ply fiber web W inside of the loop of the wire 10. Also, an air guide 54, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10 at a location before the curtain application device C50 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C50.

Alternatively, only one application device C10; C20; C50 for the foam could be used, which device C10; C50 would advantageously be applicating the foam onto the fiber web layer on the lower wire 10. The application device C20 for the foam can also be only applicating onto the fiber web on the upper wire 20. It is also possible to use three application devices C10; C20; C50 as shown in the example of the figure. It is also possible to use two application devices C10; C20; C50. Also, more than one application devices C10; C20; C50 can be used consecutively for one fiber web layer.

After the forming section 210 the fiber web W is guided via suction roll 13 of the forming unit 220 towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric 40 of the press section 230. The press section also comprises a second press fabric 50 with a roll 51. The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi-ply fiber web W is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 55. In this example the press section 230 comprises two sets of press fabrics 40, 50 with the rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop as well as the press nips formed between the first press rolls 45 and the second press roll 55. Absorption of the sizing agent into the inner part of the fiber web W is further effected by pressure of press nips between the press rolls 45, 55 in the press section 230. After the press section the fiber web is guided to a drying section 240.

The drying section 240 comprises means for drying the fiber web W. In this example is shown only beginning of the drying section, in which drying is effected by a one-wire drying cylinder group comprising a drying wire 60 with a pick-up roll 61 , rolls 62 for guiding, tensioning and/or driving the drying wire 60, reversing rolls 63 and drying cylinders 64. The drying section 240 typically comprises several drying groups with single- or double-wire draw. Also, other types of drying devices can be used. The drying section 240 is followed by the sizing section 250 with the drying unit 260.

In the sizing section of the example the fiber web is running in direction substantially vertically upwards from a guide roll 72. The fiber web W is sized by two curtain application devices C75, each located at a sizing roll 75, indirectly by first applying the sizing agent onto surface of the corresponding rotating sizing roll 75 and then the sizing agent is guided to a sizing nip formed between the sizing rolls 75. In rotating direction of the sizing roll 75 an air guide 74, preferably an air curtain and/or a reversing blade, is located for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent applied by the curtain application device C75. In the example the fiber web W is sized two-sidedly but also only one side of the fiber web W can be sized, in which case only one of the sizing rolls 75 is provided with the curtain application device C75 and the air guide 74. Advantageously, the sizing rolls 75 are hard rolls, i.e. a hard nip sizer is provided. Advantageously, as the hard roll is used a ceramic or metallic roll or advantageously, a roll with a hard-polymeric roll cover (rubber, polyurethane or composite) having surface hardness 60 - 100 shoreD, advantageously 80 - 95 shoreD. The hard nip enhances the strength increase. Advantageously, linear load in the sizing nip is 5 - 450 kN/m, more preferably the linear load in the sizing nip is 5 - 200 kN/m. Advantageously, the sizing agent is high solids content sizing agent, comprising advantageously fiber suspension, preferably 10-60 % solids content, more preferably 20 - 40 % solids content. Viscosity of the sizing agent is advantageously 5 cP or over, preferably 5 cP - 1000 cP, more preferably 20 cP - 200 cP. One of the hard sizing rolls 75 is a deflection compensated roll and thus a straight sizing nip is be formed. The curtain application device located in connection with each sizing roll 75 applies the sizing agent onto the surface of the sizing roll 75 and the fiber web W is sized by indirect sizing. The sizing agent is then via the surface of the sizing roll 75 guided onto the surfaces of the fiber web W such that in the sizing nip the sizing agent is pressed to the fiber web surfaces. After the sizing the fiber web W is guided via an air-borne drying device 87 of the drying unit 260, which air borne drying device 87 first dries the sizing agent preventing the sizing agent to adhere to the following drying cylinders 84 in the drying unit 260. The fiber web W is further dried in the drying unit 260, in this example comprising one wire drying and twin-wire drying. In the single wire drying the fiber web W runs supported by a drying wire 80 via drying cylinders 84 and reversing rolls 83. The drying wire 80 runs as a closed loop supported and rolls 82 are provided for guiding, tensioning and/or driving the drying wire 80. The drying unit 260 also comprises the double wire drying, in which the fiber web W is dried by drying cylinders 94U, 94L located in two rows. Each row has its own drying wire 90U, 90L and rolls 92U, 92L for guiding, tensioning and/or driving the corresponding drying wire 90U, 90L. Between the rows the fiber web W has free draws. The drying unit 260

After the sizing section 250 with drying unit 260 the fiber web is guided to the calendering section 270, which in the example comprises one calendering nip formed between calendering rolls 105, 106. It is possible to have a coating section with a coater and drying equipment after the pre-calender and also final calender after coating section (not shown). Thereafter, the fiber web W is guided to the reel-up 280, in which the fiber web W is reeled to a parent roll 115 by means of a reeling cylinder 116.

In figure 3 is shown an example of a production line 200 for producing a one layer fiber web W with single layer headbox or a multilayer fiber web W with multilayer headbox. The production line 200 comprises in this example a forming section 210 comprising one headbox M10 and a forming unit 220 and a press section 230 as well as a subsequent drying section 240 and a reel-up 280. The production line 200 in this example further comprises a sizing section 250 with a drying unit 260 and a calender 105. In the example the forming section 210 for production of a fiber web W, which can contain one or more fiber web layers. The forming section 210 comprises a headbox M10, from which the stock suspension is fed to the forming unit 220 beginning as one wire part comprising a wire 10 for one wire runs and a wire 20 for two wire runs for forming the fiber web W. In case in which single layer fiber web is manufactured, to the headbox M10 only one stock suspension is fed, but in case in which multilayer fiber web is manufactured, to the headbox M10 is fed two or more equivalent or different kind of stock suspensions. Stock suspensions can differ from each other based on raw material, fiber length, fiber freeness or additives difference. Each wire 10, 20 comprises rolls 12, 22 for guiding, tensioning and/or driving the wire 10; 20 as an endless loop. The stock suspension M10 is first fed onto the wire 10 and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20 located water removal means 17, 11. The first water removal means 17 is advantageously a forming shoe 17, which is advantageously under-pressured and may have a configured surface, such that the forming shoe 17 does not cause substantial pressure pulses i.e. the forming shoe 17 can be a so called non-pulsating forming shoe 17. Non-pulsating forming shoe 17 can be formed by cross machine direction lists which are arranged after each other with small distance in machine direction and forming shoe cover is straight. During the run on the wire 10 the fiber web water removal is substantially horizontal. The first one-wire run is followed by a twin-wire run, during which the fiber web is running between two wires 10, 20. During this twin-wire run water is removed by water removal means 18, 19 and 16. The first water removal means 18 in the example is a suction box 18 with pulsating water removal having optional adjustable loading blades P in the opposite wire loop 10, the second water removal means in the example is a suction box 19, as third water removal means 16 a suction box can be used. The suction box 16 ensures fiber web separation from wire 20 and functioning also as support means for supporting the run of the fiber web W. The twin-wire run is followed by another one wire run, on which run a curtain application device C50 is located for applying sizing agent on the surface of fiber web W in form of foam. The curtain application device C50 is advantageously located such that the suction means 56 are located on the opposite side of the fiber web W inside of the loop of the wire 10. Also, an air guide 54, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10 at a location before the curtain application device C50 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C50.

After the forming section 210 the fiber web W is guided via suction roll 13 of the forming unit 220 towards a pick-up roll 41 for transferring the fiber web to a first press fabric 40 of the press section 230. The press section also comprises a second press fabric 50 with a roll 51 . The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the fiber web W is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 55. In this example the press section 230 comprises two sets of press fabrics 40, 50 with the rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop as well as the press nips formed between the first press rolls 45 and the second press roll 55. Absorption of the sizing agent into the inner part of the fiber web W is further effected by pressure of press nips between the press rolls 45, 55 in the press section 230. After the press section the fiber web is guided to a drying section 240.

The drying section 240 comprises means for drying the fiber web W. In this example is shown only beginning of the drying section, in which drying is effected by a single-wire drying cylinder group comprising a drying wire 60 with a pick-up roll 61 , rolls 62 for guiding, tensioning and/or driving the drying wire 60, reversing rolls 63 and drying cylinders 64. The drying section 240 typically comprises several drying groups with single- or double-wire draw. Also, other types of drying devices can be used. The drying section 240 is followed by the sizing section 250 with the drying unit 260.

In the sizing section of the example the fiber web is running in direction substantially vertically upwards from a guide roll 72. The fiber web W is sized by two curtain application devices C75, each located at a sizing roll 75, indirectly by first applying the sizing agent onto surface of the corresponding rotating sizing roll 75 and then the sizing agent is guided to a sizing nip formed between the sizing rolls 75. In rotating direction of the sizing roll 75 an air guide 74, preferably an air curtain and/or a reversing blade, is located for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent applied by the curtain application device C75. In the example the fiber web W is sized two-sidedly but also only one side of the fiber web W can be sized, in which case only one of the sizing rolls 75 is provided with the curtain application device C75 and the air guide 74. Advantageously, the sizing rolls 75 are hard rolls, i.e. a hard nip sizer is provided. Advantageously, as the hard roll is used a ceramic or metallic roll or advantageously, a roll with a hard-polymeric roll cover (rubber, polyurethane or composite) having surface hardness 60 - 100 shoreD, advantageously 80 - 95 shoreD. The hard nip enhances the strength increase. Advantageously, linear load in the sizing nip is 5 - 450 kN/m, more preferably the linear load in the sizing nip is 5 - 200 kN/m. Advantageously, the sizing agent is high solids content sizing agent, comprising advantageously fiber suspension, preferably 10-60 % solids content, more preferably 20 - 40 % solids content. Viscosity of the sizing agent is advantageously 5 cP or over, preferably 5 cP - 1000 cP, more preferably 20 cP - 200 cP. One of the hard sizing rolls 75 is a deflection compensated roll and thus a straight sizing nip is be formed. The curtain application device located in connection with each sizing roll 75 applies the sizing agent onto the surface of the sizing roll 75 and the fiber web W is sized by indirect sizing. The sizing agent is then via the surface of the sizing roll 75 guided onto the surfaces of the fiber web W such that in the sizing nip the sizing agent is pressed to the fiber web surfaces. After the sizing the fiber web W is guided via an air-borne drying device 87 of the drying unit 260, which air borne drying device 87 first dries the sizing agent preventing the sizing agent to adhere to the following drying cylinders 84 in the drying unit 260. The fiber web W is further dried in the drying unit 260, in this example comprising single wire drying and double wire drying. In the single wire drying the fiber web W runs supported by a drying wire 80 via drying cylinders 84 and reversing rolls 83. The drying wire 80 runs as a closed loop supported and rolls 82 are provided for guiding, tensioning and/or driving the drying wire 80. The drying unit 260 also comprises the double wire drying, in which the fiber web W is dried by drying cylinders 94U, 94L located in two rows. Each row has its own drying wire 90U, 90L and rolls 92U, 92L for guiding, tensioning and/or driving the corresponding drying wire 90U, 90L. Between the rows the fiber web W has free draws. After the sizing section 250 with drying unit 260 the fiber web is guided to the calendering section 270, which in the example comprises one calendering nip formed between calendering rolls 105, 106. It is possible to have a coating section with a coater and drying equipment after the pre-calender and also final calender after coating section (not shown). Thereafter, the fiber web W is guided to the reel-up 280, in which the fiber web W is reeled to a parent roll 115 by means of a reeling cylinder 116. In figure 4 an example of a production line 200 for producing a three layer fiber web W. The production line 200 comprises in this example a forming section 210 comprising three headboxes M10; M20; M30 and a forming unit 220 and a press section 230 as well as a subsequent drying section 240 and a reel-up 280. The production line 200 in this example further comprises a sizing section 250 with a drying unit 260 and a calender 105.

In the example the forming section 210 for production of a multi-ply fiber web, in this example for a three-layer W10; W20; W30 fiber web W. The forming section comprises a headbox M10; M20; W30 for each layer W10, W20, W30 from which the stock suspension is fed to the forming unit 220 beginning as one wire part comprising a wire 10 for first layer W10 of the multi-ply fiber web W and a wire 20 for second layer W20 of the multi-ply fiber web W and a wire 30 for the third layer of the multi-ply fiber web, each wire 10, 20, 30 comprising rolls 12, 22, 32 for guiding, tensioning and/or driving the wire 10; 20; 30 as an endless loop. The first layer W10 can be called as back layer of the multi-ply fiber web W, the second layer W20 can be called as top layer of the multi-ply fiber web W and the third layer W30 can be called as filler layer of the multi ply fiber web W. The stock suspension M10; M20; M30 is first fed onto the wire 10; 20;30 and thereafter the stock on the wire is guided past inside the loop of the wire 10; 20; 30 located water removal means 11 ; 21 ; 31 ; 15; 25; 35. Inside of the loop of wire 10; 20; 35 at least one suction means 15; 25; 35 are located . During the run on the one wire part the water removal from fiber web is substantially horizontal. A curtain application device C10; C20; C30 is located at the run of the stock layer the one wire part for applying sizing agent in form of foam by a curtain type application device onto the stock layer and to be in between at the layers of the multi-ply fiber web in forming section. The curtain application device C10; C20; C30 is advantageously located such that the suction means 15; 25; 35 are located on the opposite side of the multi-ply fiber web inside of the loop of the wires 10; 20; 30. Also, an air guide 14, 24, 34, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10, 20; 30 at a location before the curtain application device C10, C20, C30 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C10, C20, C30.

The stocks forming the second and third layer W20, W30 of the multi-ply fiber web W guided on the wires 20, 30 is after the one-wire part guided downwards towards the first wire 10 and the runs of the wire 10 for the first layer W10 and the wires 20, 30 for the second and third layers W20, W30 are united by a combining roll 23 to form a twin wire part and the webs for the first layer W10, the second layer W20 and the third layer W30 are guided into a gap formed between the wires 10; 20 forming the twin-wire part of the forming unit. Web layers W10, W20, W30 have been joined together by a combining roll 23 at the beginning of the twin-wire forming part. Combining roll 23 is situated in the second layer wire loop 20. Thus, the sizing agent applied in form of foam by the curtain application devices C10; C20; C30 onto each layer W10, W20, W30 of the web W is guided in between the layers. Advantageously, after applying the sizing agent in form of foam by the curtain application device C10; C20; C30 its conveyance and penetration into the web layer is assisted such that the sizing agent is forwarded deeper into the web layer on which the sizing agent is applied in form of foam by guiding the foam to desired direction. In the example of figure a suction means 15, 25, 35 are provided inside the wire loops 10, 20, 30, which suction means during and after the application of the sizing agent in form of foam by the curtain application device C10, C20, C30 but before the web layers are joined by the combining roll 23 at the beginning of the twin-wire forming part assist the conveyance and penetration of the sizing agent into the web layer. It is also possible to use in one or more forming units two-sided dewatering before applying the sizing agent, this is done by using separate wire loop on top of the one-wire part. Especially with layers which basis weight is higher this is beneficial, like filler layers which typically has higher basis weight than back and top layers. Alternatively, only one application device C10; C20; C30; C50 for the foam could be used, which device C10; C50 would advantageously be applicating the foam onto the fiber web layer on the lower wire 10. The application device C20; C30 for the foam can also be only applicating onto the fiber web on one of the upper wires 20, 30. Advantageously the application device C30 applies sizing agent in form of foam on top of the filler layer, which will be combined with top and back layers in the twin-wire part. It is also possible to use four application devices C10; C20; C30; C50 as shown in the example of the figure. It is also possible to use two or three application devices C10; C20; C30; C50. Also, more than one application devices C10; C20; C30; C50 can be used consecutively for one fiber web layer.

After the joining of the layers W10, W20, W30 the multi-ply fiber web W is guided on the wire 10 supporting the first side of the multi-ply fiber web W as a one wire part, during which run support foils 16 located inside the wire 10 loop. The support foils 16 do not deviate the run of the wire 10 only remove water from the first surface of the wire 10 and support the run of the wire 10 as the multi-ply fiber web W is guided forwards on the wire 10. In connection with this run of the wire 10 a further curtain application device C50 is located for applying sizing agent on the surface of multi-ply fiber web W. The curtain application device C50 is advantageously located such that the suction means 56 are located on the opposite side of the multi-ply fiber web W inside of the loop of the wire 10. Also, an air guide 54, preferably an air curtain and/or a reversing blade, is located above the corresponding wire 10 at a location before the curtain application device C50 for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent in form of foam applied by the curtain application device C50.

After the forming section 210 the fiber web W is guided via suction roll 13 of the forming unit 220 towards a pick-up roll 41 for transferring the multi-ply fiber web to a first press fabric 40 of the press section 230. The press section also comprises a second press fabric 50 with a roll 51 . The press fabrics 40, 50 comprise rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop. In the press section the multi-ply fiber web W is guided between the first press fabric 40 and the second press fabric 50 to a press nip formed between a first press roll 45 and a second press roll 55. In this example the press section 230 comprises two sets of press fabrics 40, 50 with the rolls 42, 52 for guiding, tensioning and/or driving the fabrics 40; 50 as an endless loop as well as the press nips formed between the first press rolls 45 and the second press roll 55. Absorption of the sizing agent into the inner part of the fiber web W is further effected by pressure of press nips between the press rolls 45, 55 in the press section 230. After the press section the fiber web is guided to a drying section 240.

The drying section 240 comprises means for drying the fiber web W. In this example is shown only beginning of the drying section, in which drying is effected by a one-wire drying cylinder group comprising a drying wire 60 with a pick-up roll 61 , rolls 62 for guiding, tensioning and/or driving the drying wire 60, reversing rolls 63 and drying cylinders 64. The drying section 240 typically comprises several drying groups with one- or two-wire draw. Also, other types of drying devices can be used. The drying section 240 is followed by the sizing section 250 with the drying unit 260.

In the sizing section of the example the fiber web is running in direction substantially vertically downwards from a guide roll 72. The fiber web W is sized by two curtain application devices C75, each located at a sizing roll 75, indirectly by first applying the sizing agent onto surface of the corresponding rotating sizing roll 75 and then the sizing agent is guided to a sizing nip formed between the sizing rolls 75. In rotating direction of the sizing roll 75 an air guide 74, preferably an air curtain and/or a reversing blade, is located for redirecting the boundary air layer and thus, preventing the boundary air layer to disturb the curtain of the sizing agent applied by the curtain application device C75. In the example the fiber web W is sized two-sidedly but also only one side of the fiber web W can be sized, in which case only one of the sizing rolls 75 is provided with the curtain application device C75 and the air guide 74. Advantageously, the sizing rolls 75 are hard rolls, i.e. a hard nip sizer is provided. Advantageously, as the hard roll is used a ceramic or metallic roll or advantageously, a roll with a hard-polymeric roll cover (rubber, polyurethane or composite) having surface hardness 60 - 100 shoreD, advantageously 80 - 95 shoreD. The hard nip enhances the strength increase. Advantageously, linear load in the sizing nip is 5 - 450 kN/m, more preferably the linear load in the sizing nip is 5 - 200 kN/m. Advantageously, the sizing agent is high solids content sizing agent, comprising advantageously fiber suspension, preferably 10-60 % solids content, more preferably 20 - 40 % solids content. Viscosity of the sizing agent is advantageously 5 cP or over, preferably 5 cP - 1000 cP, more preferably 20 cP - 200 cP. One of the hard sizing rolls 75 is a deflection compensated roll and thus a straight sizing nip is be formed. The curtain application device located in connection with each sizing roll 75 applies the sizing agent onto the surface of the sizing roll 75 and the fiber web W is sized by indirect sizing. The sizing agent is then via the surface of the sizing roll 75 guided onto the surfaces of the fiber web W such that in the sizing nip the sizing agent is pressed to the fiber web surfaces. After the sizing the fiber web W is guided via an air-borne drying device 87 of the drying unit 260, which air borne drying device 87 first dries the sizing agent preventing the sizing agent to adhere to the following drying cylinders 84 in the drying unit 260. The fiber web W is further dried in the drying unit 260, in this example comprising single wire drying and double-wire drying. In the single-wire drying the fiber web W runs supported by a drying wire 80 via drying cylinders 84 and reversing rolls 83. The drying wire 80 runs as a closed loop supported and rolls 82 are provided for guiding, tensioning and/or driving the drying wire 80. The drying unit 260 also comprises the double-wire drying, in which the fiber web W is dried by drying cylinders 94U, 94L located in two rows. Each row has its own drying wire 90U, 90L and rolls 92U, 92L for guiding, tensioning and/or driving the corresponding drying wire 90U, 90L. Between the rows the fiber web W has free draws. The drying unit 260

After the sizing section 250 with drying unit 260 the fiber web is guided to the calendering section 270, which in the example comprises one calendering nip formed between calendering rolls 105, 106. It is possible to have a coating section with a coater and drying equipment after the pre-calender and also final calender after coating section (not shown). Thereafter, the fiber web W is guided to the reel-up 280, in which the fiber web W is reeled to a parent roll 115 by means of a reeling cylinder 116.

As can be understood from the examples of the figures 1-4 in production of a fiber web W the sizing of the fiber web is conducted in at least two main sizing stages comprising inner part sizing of the fiber web W in the forming section 210 and surface sizing of the fiber web W, advantageously in the sizing section 250.

The inner part sizing of the fiber web W is conducted in the forming section 210 of the fiber web production line 200 by applying the sizing agent onto the surface/-s of the fiber web W and/or in between at least two layers W10; W20; W30 of the multi-ply fiber web W, advantageously in form of foam. Due to suction of water removal devices 11 , 15, 17, 21 , 25, 21 , 35, 56 used in the forming section 210 and subsequent press dewatering between press rolls 45, 55 the sizing agent is absorbed into the inner part of the fiber web W, which is further effected by pressure of the press nips formed between the press rolls 45, 55 in the press section 230 of the fiber web production line 200. The application device C10; C20; C30; C50 for the inner part sizing is advantageously a slot-type or a slide-type curtain application device. The slot- type curtain application device is advantageous because dwell time is shortest from the nozzle to the fiber web.

The surface sizing of the fiber web W is conducted after the inner part sizing of the fiber web W, advantageously at a later production section of the fiber web production line, i.e. in a section following the forming section in the production line of the fiber web, most advantageously in the sizing section 250 of the fiber web production line 200. The sizing agent is applied in the sizer and directly or indirectly utilizing curtain technology, spray technology or film transfer technology or bond technology. Preferably, the sizing agent is applied utilizing curtain technology by the curtain application device/-s C75. Advantageously, the fiber web W is sized with high solids content sizing agent, preferably in solids content 10 - 60 %, more preferably 20 - 40 %. More preferably the fiber web W is sized by curtain technology by the curtain application device/-s C75 and with high solids content sizing agent. Advantageously, the sizing agent is starch. The surface sizing in the sizer is advantageously conducted by a hard-nip sizer comprising a hard sizing nip formed between the two hard sizing rolls 75. The surface sizing can be conducted by applying the sizing agent in form of liquid or foam utilizing foam application technology. Advantageously, the fiber web production line 200 comprises a final calendering stage in the calender section 270 after the sizing section 250. Advantageously, the fiber web production line 200 comprises a pre-calender after the sizing section 250 drying unit 260 and after pre-calender coating section with a coater and drying equipment and final calendaring stage after that.

In the description in the foregoing, although some functions have been described with reference to certain features, those functions 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.