The present invention relates to optimizing a fibrous web processing apparatus with a metal belt loop for different uses, the processing apparatus comprising a belt arranged to rotate around at least one guide means. In this application, the term λweb processing' refers to the various measures relating to the processing of a fibrous web in a paper/board machine, such as pressing, drying, calendering, coating, sizing and smoothing. The processing apparatus may also be a fibrous web finishing machine, such as a separate coating machine, printing apparatus or calender. A processing apparatus provided with a metal belt allows the use of an extremely broad pressure range and effective period (heat transfer time and/or processing time) in the processing zone, thus making it possible to use the same apparatus for processing numerous different coated and uncoated printing papers, boards and other paper grades, and it may be used, for example, as a precalender before coating and an final calender after the paper machine or coating, a breaker stack, a wet stack calender or drying apparatus, a coating apparatus, a sizing apparatus, a printing apparatus and/or a press. A processing apparatus with a metal belt can conceivably be used to replace, for example, a soft calender, a multi-nip calender, a machine calender, a shoe calender, the Yankee cylinder part of a dryer group, or a press.

There are numerous different paper and board grades and they can be divided into two categories by grammage: papers, which have one layer and a grammage of 25-300 g/m2 and boards made by multi-layer technique and having a grammage of 150-600 g/m2. As can be seen from this, the borderline between paper and board is a sliding one since boards having the lightest grammage are lighter than the heaviest of papers. Paper is generally used for printing and board for packaging. The following descriptions are examples of values currently used for fibrous webs and they may contain considerable variation from the disclosed values. The descriptions are based mainly on the source publication Papermaking Science and Technology, section Papermaking Part 3, edited by Jokio, M., published by Fapet Oy, Jyvaskyla 1999, 361 pages.

Mechanical pulp based, i.e. wood-containing printing papers include newsprint, uncoated magazine paper and coated magazine paper.

Newsprint is composed either completely of mechanical pulp or it may contain some bleached softwood pulp (0-15%) and/or recycled fibre to replace some of the mechanical pulp. As general values for newsprint may be regarded the following: grammage 40-48.8 g/m2, ash content (SCAN-P 5:63) 0-20%, PPS-slO roughness (SCAN-P 76-95) 3.0-4.5 μm, Bendtsen roughness (SCAN-P21:67) 100-200 ml/min, density 600-750 kg/m3, brightness (ISO 2470:1999) 57-63%, and opacity (ISO 2470:1998) 90-96%.

Uncoated magazine paper (SC = supercalendered) usually contains 50-70% mechanical pulp, 10-25% bleached softwood pulp, and 15-30% fillers. Typical values for calendered SC paper (including e.g. SC-C, SC-B, and SC- A/A+) are: grammage 40-60 g/m2, ash content (SCAN-P 5:63) 0-35%, Hunter gloss (ISO/DIS 8254/1) < 20-50%, PPS SlO roughness (SCAN-P 76:95) 1.0-2.5 μm, density 700-1250 kg/m3, brightness (ISO 2470:1999) 62- 70%, and opacity (ISO 2470:1998) 90-95%.

Coated magazine paper (LWC = light weight coated) contains 40-60% mechanical pulp, 25-40% bleached softwood pulp, and 20-35% fillers and coaters. As general values for LWC paper may be regarded the following: grammage 40-70 g/m2, Hunter gloss 50-65%, PPS SlO roughness 0.8-1.5 μm (offset) and 0.6-1.0 μm (roto), density 1100-1250 kg/m3, brightness 70- 75%, and opacity 89-94%. As general values for MFC paper (machine finished coated) may be regarded the following: grammage 50-70 g/m2, Hunter gloss 25-70%, PPS SlO roughness 2.2-2.8 μm, density 900-950 kg/m3, brightness 70-75%, and opacity 91-95%.

As general values for FCO paper (film coated offset) may be regarded the following: grammage 40-70 g/m2, Hunter gloss 45-55%, PPS SlO roughness 1.5-2.0 μm, density 1000-1050 kg/m3, brightness 70-75%, and opacity 91- 95%.

As general values for MWC paper (medium weight coated) may be regarded the following: grammage 70-90 g/m2, Hunter gloss 65-75%, PPS SlO roughness 0.6-1.0 μm, density 1150-1250 kg/m3, brightness 70-75%, and opacity 89-94%.

HWC (heavy weight coated) has a grammage of 100-135 g/m2 and it can be coated even more than twice.

Woodfree printing papers made of chemical pulp, or fine papers, include uncoated and coated chemical-pulp based printing papers, in which the proportion of mechanical pulp is less than 10%.

Uncoated chemical-pulp based printing papers (WFU) contain 55-80% bleached birchwood pulp, 0-30% bleached softwood pulp, and 10-30% fillers. With WFU, the values vary considerably: grammage 50-90 g/m2 (up to 240 g/m2), Bendtsen roughness 250-400 ml/min, brightness 86-92%, and opacity 83-98%.

In coated chemical-pulp based printing papers (WFC), the amounts of coating vary greatly in accordance with the requirements and intended use. The following are typical values for once and twice coated chemical-pulp based printing paper: once coated, grammage 90 g/m2, Hunter gloss 65- 80%, PPS SlO roughness 0.75-2.2 μm, brightness 80-88%, and opacity 91- 94%, and for twice coated grammage 130 g/m2, Hunter gloss 70-80%, PPS SlO roughness 0.65-0.95 μm, brightness 83-90%, and opacity 95-97%.

Release papers have a basis weight ranging from 25 to 150 g/m2.

Other papers include packing papers (Sackkraft), tissues, and wallpaper bases.

Boards constitute a fairly heterogeneous group which includes grades having a high grammage of up to 500 g/m2 and grades having a low grammage of about 120 g/m2, the grades ranging from ones based on virgin fibre to 100% recycled fibre based grades, and from uncoated to multiply coated. In the following, board grades are divided into coated and uncoated grades because coating has the greatest effect on the calendering method. In coated grades, both precalendering before the coating machine and the final calender after the coating machine are used. Uncoated grades are only subjected to final calendering. These two groups include several board grades as follows:

Coated board: - virgin fibre based folding boxboard (FBB), bleached pulp board (SBS = solid bleached board), liquid packaging board (LPB), coated white top liner, carrier board - recycled fibre based white-lined chipboard (WLC), coated recycled board.

Uncoated board: - virgin fibre based (kraft liner, white top liner, liquid packaging board) - recycled fibre based (test liner) In the following are described calendering concepts for different coated board grades such as folding boxboard, white-lined chipboard, solid bleached board and liquid packaging board. Coated board grades vary from one-layer to five-layer board. The most important qualities are large bulk, rigidity and smoothness. The board is often one-sided, but may also be two-sided (SBB boards).

Precalendering is applied before the coating machine to reduce roughness and porosity to a target level characteristic of the coating machine. The precalendering method is dependant on many variables, the most important being the following: - the structure of the board machine (Yankee cylinder) - raw materials (virgin fibre versus recycled fibre; European fibre versus fibre from the southern part of the USA).

Precalendering mainly serves the purpose of CD-direction calibre control when there is a Yankee cylinder in the board machine (typically a folding boxboard machine). The Yankee cylinder produces an extremely smooth surface with large bulk. Precalendering is usually done with one hard-nip calender, based on either thermal or hydraulic calibre control. Line loads are typically fairly low, 10-30 kN/m, and thermo roll temperatures 70-100°C.

Typical board grades made without a Yankee cylinder are solid bleached board, white top liner, coated recycled board and liquid packaging board.

Traditionally, precalendering was carried out by using a multi-roll hard-nip calender and the calendering effect was enhanced by adding water by means of water boxes (wet stack calenders). The number of rolls varies from 4 to 11 depending on the board grade; the more readily calendered European fibres do not require as many nips as fibres originating from the southern parts of the USA. Recycled fibres are likewise more readily calendered than virgin fibres.

The ability to raise the temperature of the thermo roll has shifted precalendering towards hot calendering. Today, the aim is to use hot hard nip calendering or soft calendering. Increased thermo roll temperatures (even exceeding 200 °C) result in bulk savings due to the temperature gradient. The runnability of a hot hard nip calender or soft calender is better than that of a multi-roll hard nip calender. Likewise the application of water on wet stack calenders is not easy to control.

Uncoated board grades often have only one or two layers. Multi-layered, uncoated boards, such as liquid packaging boards, can also be produced. As with coated boards, the calendering of uncoated boards should also save on bulk and rigidity.

With some grades, such as fluting and certain untreated test liner grades, calendering is not required. Traditionally, calendering was almost always carried out on hard nip calenders. The display and advertising function of fluted packages has recently become increasingly important. This has increased demands for a higher-quality, bleached surface layer.

Soft calenders are, therefore, used also for uncoated board grades when good printability is required. Steam-jet sprayers may be used before the calender to improve the outcome of calendering if the board is cooled.

The future trend, as with coated grades, is toward long nip calendering. This technology makes it possible to calender board, to higher formation scale roughness and bulk with lower micro roughness (PPS) and better printability. Earlier international published applications WO 03/064761, WO 03/064764 and WO 03/064762 by the applicant of the present invention describe in greater detail a fibrous web processing apparatus with a metal belt loop and its applications to different uses.

A general aim of the present application is to enhance the operation of the processing apparatus with a metal belt loop and to improve its usability at different points on the production line and/or finishing stages of the fibrous web. On the basis of trial runs, it has been found that the ratio between the length of the metal belt and the width of the metal belt is a significant factor from the functioning viewpoint in processing apparatuses with metal belt loops having a belt width within the range from about 1.5 m to about 12 m.

The aim of the invention is, therefore, to provide a fibrous web processing apparatus with a metal belt loop for different uses in a paper/board machine and/or finishing machine, the processing apparatus comprising a metal belt arranged to rotate around at least one guide means. To achieve this aim, the solution relating to the invention is characterised in that the ratio of the length of the metal belt to its width is greater than 0.55. The said ratio is preferably more than 0.8, and more preferably more than 1.

Further preferred embodiments of the invention are described in the dependent claims.

Outside the metal belt loop is preferably arranged at least one counter- element forming a contact surface with the belt, in such a way that between the belt and the counter-element is formed a web processing zone through which the web to be processed is led.

A processing apparatus with a metal belt loop according to the invention is preferably located at least at two different points on the paper/board production line, the at least two processing apparatuses having at least 40% of identical, interchangeable parts. This solution results in a fall in maintenance costs, a reduction in the need for spare parts and in lower production costs due to the possibility of serial production. For example a calender provided with a metal belt loop and a coating machine may have about 70% of interchangeable parts and a calender and press equally about 70%, whereas a press and a coating machine may have some 40% of common parts.

In connection with the metal belt loop are preferably arranged cleaning means for cleaning the processing surface of the metal belt, the said cleaning means being selected from a group including a felt cleaner, a brush cleaner, cleaning by spraying washing solution, steam or compressed air under pressure from a washing device, and cleaning based on ultrasonic vibration or other oscillation.

In conventional calendering, the insufficiency of heat transfer in a short nip constitutes a bottleneck. Especially off-line soft and multi-nip calendering benefit a great deal if the web is preheated immediately before calendering. Patent application FI 20035030 discloses one solution for modernisation.

Thus, one aim of the present invention is also an arrangement for calendering a fibrous web by means of a calender comprising a conventional nip, a long contact zone being incorporated in the calender for enhancing the heat transfer of the web. To achieve this aim, in one embodiment of the invention, a metal belt loop is incorporated outside a roll in a conventional calender, in such a way that a long preheating zone is formed immediately before the conventional nips of , the calender, whereby a high preheating effect is achieved. The belt contact is preferably added before the first nip. According to yet another embodiment of the invention, a metal belt loop is incorporated outside a conventional calender roll in such a way that a long final cooling zone is formed after the conventional calender nips. The web is cooled. Cooling preferably takes place against the final roll, most preferably against a polymer roll. Cooling makes it possible to use higher calendering temperatures without endangering rolling or the fibrous web drying excessively.

By means of calendering enhanced according to the invention are produced glossy, coated or uncoated printing papers and boards in such a way that the enhancement of heat transfer will promote the smoothing process. Examples include wood-containing uncoated printing paper (SC: Hunter 40-60%), wood-containing coated printing paper (LWC Hunter 50-70%, MWC Hunter 65-75%, HWC) and coated fine paper (e.g. WFC: Hunter 70-90%) and coated folding boxboard (more detailed values are available in the publication FAPCT: Papermaking, Part a: Finishing).

By means of calendering enhanced according to the invention are produced low-gloss, coated or uncoated printing paper and board products such as newsprint (Hunter 20-30%) and uncoated fine paper (copying paper, WFU: Hunter < 30%).

By means of calendering enhanced according to the invention can be produced so-called matt grades. This means especially WFC matt grades (Hunter < 35%). In a matt run, it is most preferable to run only through one or two of the "first" nips, thus passing the gloss-producing nips. The surface of the support surface (= a metal belt or a thermo roll placed against it) of the first or second "nip contact" is patterned or rough (Ra > 0.3 μm) or it is coated with a coating producing a matt surface (e.g. ceramic surface, porous metal surface). If necessary, the roll used in the matt run can be loaded separately so that the entire set of rolls will not have to be used in a partial nip run.

By means of calendering enhanced according to the invention may also be calendered low- or high-gloss boards. In this case, the metal belt loop is preferably incorporated in conjunction with a soft or shoe calender.

By means of calendering enhanced according to the invention may also be calendered release paper. It is particularly advantageous to connect the belt loop to a multi-nip calender.

The belt loop according to the invention, which forms a long preheating or final cooling zone to enhance calendering may thus be connected either to a multi-nip calender (super or OptiLoad or TwinLine) or to a soft/shoe calender. The calendering described may be either precalendering or final calendering.

The invention is described in greater detail in the following, with reference to the accompanying drawings, in which:

Figure 1 shows diagrammatically some alternatives for positioning the processing apparatus with a metal belt loop on an LWC paper production line,

Figure 2 shows a diagrammatic side view of an embodiment of the processing apparatus with a metal belt loop,

Figure 3 shows diagrammatically the felt cleaner of the metal belt loop,

Figure 4 shows a cross-section of the structure of the felt of the felt cleaner, Figure 5 shows the positioning of the felt cleaner with respect to the metal belt to be cleaned,

Figure 6 shows the washing arrangement of the felt of the felt cleaner,

Figure 7 shows a steel brush cleaner,

Figure 8 shows a cleaning beam solution, where the mechanical cleaning means is movable with respect to the beam towards the surface to be cleaned and away from it,

Figures 8A and 8B show a cleaning beam solution, where a mechanical cleaning means and suction are used,

Figure 8C shows a modification of Figures 8A and 8B,

Figure 9 shows a metal belt cleaner arrangement,

Figure 10 and 11 show figures in principle of incorporating the metal belt loop in a multi-nip calender,

Figure 12 shows diagrammatically a Yankee cylinder, and

Figures 13 and 14 show different embodiments of the invention as an apparatus replacing the Yankee cylinder.

Figure 1 shows an embodiment of a LWC paper production line illustrating the parts of the line from the press section I onwards. The press section is followed by a dryer section II, the end part of which is marked with reference marking III. The dryer section is followed by a precalendering stage IV and then the coating stage V, which is divided into coating head Va and dryer section Vb. The coating head is followed by the final calendering stage VI and last by the final processing stages VII including rolling and slitting. The processing apparatus with the metal belt could conceivably be located, for example, at the points marked with references a, b, c and/or d on an on-line production line for LWC paper. In addition to, or instead of, these positioning locations, the processing apparatus according to the invention could conceivably be located, for example, in place of the press of the press section I, the end part III of dryer section and/or the precalender IV and/or the final calender VI.

Figure 2 shows diagrammatically a processing apparatus with a metal belt loop comprising a calendering belt 2 made of metal which rotates around guide rolls 3, of which guide rolls at least a part are movable for adjusting the belt 2 tension and/or the overlap angle as desired. The calendering belt 2 travels around a roll 5 arranged outside the belt loop, thus forming a calendering zone between the belt 2 and the roll 5. The material web W being calendered travels through the calendering zone, whereby it is subjected to the desired pressure impulse and thermal effect as a function of time. Figure 1 shows in dotted line 9 the shape of the pressure impulse when inside the calendering belt 2 is arranged a nip roll 4 acting as a pressing means, the nip roll pressing the belt against the roll 5, thus forming a higher pressure within the calendering zone of the processing zone. By broken line 8 is, on the other hand, illustrated the shape of the pressure impulse when the contact pressure prevailing in the calendering zone is created by belt 2 tension alone, the roll 4 being out of pressing contact with the belt 2 (or when no roll 4 at all is fitted inside the belt 2). The roll 4 may in addition be arranged to be movable for changing the length of the processing zone and/or belt tension. There may also be more than one press roll 4 inside the metal belt loop. The metal belt may also be coated. In the embodiment shown in Figure 1, the nip roll is a shoe roll. Reference numeral 6 denotes heating means, such as an induction heater, an infrared radiator, a gas burner or a capacitive heater.

In a processing apparatus with a metal belt loop, it is advantageous to use guide rolls 3 as internal guide means of the belt loop 2, the said rolls being heatable and having a large diameter, e.g. within the range from 1000 to 2000 mm. The large diameter eliminates metal belt 2 fatigue because the radial bending radius does not allow a fatiguing load with the metal belt thicknesses used. On the other hand, a large guide roll diameter provides the guide roll with high axial rigidity and thus significantly reduces the deflection of the guide roll due to metal belt tension. It is often desirable to heat the metal belt to achieve two-sided calendering during one process stage. The drying cylinders of paper and board machines meet the foregoing demands well. Furthermore, some of the drying cylinders remain unused when machines are modernized and thus re-use of cylinders as belt calender guide rolls is possible. The durability of the cylinder surface can be improved by coating. Therefore, according to the present invention, a drying cylinder of a paper/board machine acts as the at least one guide means internally of the metal belt loop, the diameter of the cylinder being about 1000 mm to about 2000 mm.

In conjunction with the metal belt loop are preferably arranged cleaning means for cleaning the processing surface of the metal belt, the cleaning means being selected from a group comprising a felt cleaner, a brush cleaner, cleaning by spraying a washing solution, steam or compressed air under pressure from a washing device, and cleaning based on ultrasonic vibration or other oscillation of the belt. The cleaning devices can be made essentially shorter than the width of the belt and arranged to move in an oscillating manner along a beam or other support structure extending transversely to the belt. The cleaning methods may be divided into three groups: contacting, non- contacting and semi-contacting methods.

Contacting methods

The felt cleaner 30 shown in Figure 3 is based on a felt 31 travelling around rolls 33, 34 which felt rubs the belt being cleaned when the felt rotates against the belt 2 of the metal belt. If necessary, an additional load shoe 35 may be used to increase compression and contact between the belt and the cleaning felt. The apparatus may be continuously operating or it may be started up when necessary (intermittent). The felt 31 of the felt cleaner may be composed of one layer of felt material or be multi-layered incorporating, for example, a plastic wire or support structure 31b of other material (Figure 4) to support the felt material 31a. The felt material may be soft felt (e.g. material corresponding to the bottom felt) or abrasive material. An advantage of abrasive material is that it also evens out the belt loop's roughness.

In principle, the belt cleaning system may be freely located in the belt loop of the calender, but a primary location would be after the nip outlet, as marked with reference numeral 10 in Figure 2, to ensure that the dirt removed will not enter the nip pressing zone. Locating the belt cleaner in an open draw will also dampen the vibration/oscillation of the belt. With respect to the metal belt 2, the cleaner 30 may be located either horizontally against the belt loop or at a certain angle so that the outer edge of the cleaner will extend beyond the edges of the belt, as shown diagrammatically in Figure 5.

The felt may be humidified by means of nozzles referred to by reference numeral 32 in Figure 3 either outside or inside the felt loop. To the humidifier unit of the felt loop may also be incorporated a washing function, for example, a combined compressed air/water wash, steaming (steam wash/heating of the felt) or solvent wash. The liquid used as detergent may be either volatile or non-volatile, a solvent or other liquid. Alternatively, the washing/humidifying of the felt may be carried out as pool washing, for example, as shown in Figure 6. The washing device or its part is wetted either by directly inclining or turning into a washing trough 37 filled with washing liquid (volatile/non-volatile solvent). The felt may also be washed by means of vibrating methods (e.g. ultrasound). When the felt washing device is in operation, dirt adheres to the washing felt and/or the removed liquid/solid matter is collected into a collection trough 36 under the washing device.

The steel brush cleaner 40 shown in Figure 7 rotates either continuously or intermittently while cleaning the metal belt. The cleaner may rotate either independently or with the metal belt loop. Its cleaning effect is based on the abrasive impact of the steel bristles as they rotate. When rotating, the steel brush also grinds possible impurities and/or damage (e.g. grooves) on the metal belt. The steel brush may be located, as the felt washing device, freely on the belt loop of the calender, but the principal location would, however, be after the nip, in accordance with point 10 in Figure 2. Similarly, the steel brush cleaner may be located horizontally or obliquely with respect to the metal belt loop (Figure 5). In conjunction with the steel brush cleaner 40 (e.g. combined with the collection trough 41) may also be placed a magnet, which collects metal dust removed from the belt.

Figure 8 shows yet another contacting cleaning device 80 comprising a support beam 81, to which are joined two profiles 83, 84 within each other and moving with respect to each other, with an expanding means 85, for example a pneumatic hose, between them. The inner profile 83 moves with respect to the outer profile 84 towards the surface 86 to be cleaned. The cleaning means 82 is attached to the inner profile 83. The cleaning means may be, for example, a rubber moulding, a cord packing, etc. The profiles 83, 84 are preferably of extruded aluminium due to which the need for machining is minimal and manufacturing costs are low compared with, for example, welded structures or structures bent from sheet metal. Aluminium profile can be coated in different ways, thus making possible the use of a cleaning device also in hot, dirty and humid conditions. The surface treatment may be, for example, anodic coating, pulverisation, different types of painting, etc. The small number of parts and simple structure of the device make possible a functional and reliable implementation.

The cleaning device can be made relatively small and light by using aluminium profiles. The cleaning pressure can be distributed evenly on account of the pneumatic or hydraulic hose, regardless of defects in the form, such as deflection, of the support beam or surface to be cleaned. The cleaning means can be replaced, for example, by pulling out the profile inside and pushing a new one back after the removal of the locking means at the end, such as a locking pin. Due to the light and compact structure, the device can be made an oscillating one by installing, for example, a linear guide between the cleaning device and the support beam.

Figures 8A and 8B describe a cleaning device comprising a beam 120 implemented as a suction box, to which is connected a tube-loaded doctor blade part 121. The beam is turned by means of hydraulic cylinders 124 between a cleaning position, in which the doctor blade settles against the metal belt 2, and a free position in which the doctor blade is at a distance from the surface of the metal belt. The interior of the beam is connected to an external suction source (not shown) through a suction inlet 123 for collecting fine dust. On the upper part of the beam are arranged bristles 125 which prevent the belt from contacting the beam and also improve the suction effect. On the inside of the beam is preferably arranged a magnet 126 for collecting metal particles. The bottom part of the beam has been made to open by means of a hatch 122, through which larger impurities that are not removed by the suction air can be removed. Figure 8C shows a variation of Figures 8A and 8B, where a rotating brush 127, which may be oscillating, has been incorporated inside the beam. This type of brush enhances the cleaning of dents in the belt.

Non-contactinα methods

In non-contacting washing devices, the belt loop of the metal belt calender is cleaned by spraying washing solution, steam or compressed air 54 under pressure from a washing device 50 (Figure 9). The solution may be a solvent (volatile or non-volatile), for example water or other washing solution, or for example, dry ice, the cleaning effect of which is based on the thermal shock phenomenon. From the washing device may also be sprayed steam which will heat the metal belt and at the same time detach dirt particles. Compressed air may be used in the same manner. In conjunction with the non-contacting washing device may also be incorporated a collection trough 53 which will collect excess humidity and/or a hood 52, the purpose of which is to prevent liquid/drops/steam from spreading into other parts of the apparatus. To the hood may also be connected a suction device for collecting drops/steam. In non-contacting methods may also be included a doctor blade 51 which will dry the surface of the belt (e.g. a plastic blade). The position and manner of location are the same as in the contacting methods. Non-contacting washing methods may be used either intermittently or continuously. In a non-contacting method the substance being sprayed/injected may also be a solid substance, for example, washing granules that clean the belt.

Semi-contacting methods

In semi-contacting methods, the cleaning of the belt is based on, for example, ultrasonic vibration or other strong oscillation of the belt. During oscillation, the impurities on the belt are detached and can be collected into a collection trough.

Figure 10 shows a figure in principle of incorporating a metal belt loop Hl and H2 to the start and end of a multi-nip calender. Belt loop Hl forms a lengthened preheating zone and H2 a lengthened final cooling zone. Wl and W2 illustrate alternative web draws to the calender, and W a route suitable for a matt run.

Figure 11 shows an example of a Twin-Line calender with added preheating and cooling metal belt loop H.

Figure 12 shows diagrammatically a Yankee cylinder 100, around which the fibrous web W travels. The impression cylinder 101 causes the web to remain in contact with the cylinder 100 surface.

Figure 13 shows the use of the processing apparatus with a metal belt loop 102 as an apparatus replacing the Yankee cylinder, preferably as a smoothing apparatus. The metal belt 102 rotates around guide rolls 105, 106. Rolls 103 and 104 form a press nip for bringing the fibrous web W into contact with the metal belt 102. 103a is a press roll which forms a second press nip with the guide roll 106. The apparatus may include a wire loop (not shown) which presses the web W against the metal belt 102.

In Figure 14, the metal belt loop is arranged to travel around the last drying cylinder 110 in the dryer section and the guide roll 112. The press rolls that press the fibrous web against the metal belt loop 102 are marked with reference numeral 111. The apparatus may include a wire loop (not shown) which presses the web W against the metal belt 102. Reference numeral 113 denotes a pulper. The processing apparatus with a metal belt can replace current precalendering and/or final calendering solutions for all board grades. The board may have the following properties: - it may be coated or uncoated - coated grades may have one or more coating layers - grammage may be 100...1000 g/m2, typically 120...500 g/m2 - it may be a single-layer or multi-layer product - the fibre raw material may be recycled or virgin fibre.

In the case of a coated board grade, the coating method may be any one of, for example, the following: blade coating, sizer, curtain coating, spray, or any combination of these in multi-layer coated grades.

Various combinations of a metal belt calender and modern technology are possible: - metal belt precalendering can be combined with metal belt final calendering - metal belt precalendering can be combined with a currently used final calendering method - a current precalendering method can be combined with metal belt final calendering - metal belt calendering and lighter than current final calendering by a modern method or without final calendering.

Metal belt precalendering is preferably carried out in such a way that the board web is led between the thermo roll and the metal belt. To control unequal sidedness, the metal belt may also be against a soft roll. Calendering preferably takes place in one stage. Both the metal belt and the thermo roll may be heatable. The metal belt calender can be used as a final calender after the dryer section and/or on the dryer section.

Typical process conditions on a metal belt calender are as follows: - contact time with the metal belt 10-1000 ms, preferably 20-40 ms - load on an optional, at least one additional load roll behind the metal belt 0-400 kN/m, preferably 15-100 kN/m - additional load provided by hard or soft roll - metal belt temperature 20-4000C, preferably 150-2000C - thermo roll temperature 20-4000C, preferably 150-200°C - board moistude 1-50%, preferably 8-15% - possibility of also using an on-line humidifier before the metal belt calender

The metal belt final calendering is preferably carried out so that the board web is led between a soft-surface roll and a metal belt. It is also possible to finally calender the board between the thermo roll and the coated metal belt. To obtain even gloss and printing ink absorption, a soft surface must be used on the nip, which adapts to variation in the formation scale. Calendering preferably takes place in two stages. However, if necessary, the metal belt final calendering may also be carried out on a metal-surface thermo roll and a metal-surface belt.

Typical process conditions on a metal belt calender include: - one or more processing zones, preferably two - contact time with metal belt 10-1000 ms, preferably 60-200 ms - load on an optional, at least one additional load roll behind the metal belt 0-400 kN/m, preferably 15-100 kN/m - metal belt temperature 20-4000C, preferably 150-2000C - thermo roll temperature 20-400°C, preferably 150-2000C - board moisture 1-50%, preferably 8-15% - matt grades can be made with appropriately patterned belt or thermo roll.

By means of the metal belt calender is provided an efficient nip, where it is possible to process both sides of the web on one nip. Furthermore, effective control of unequal sidedness by using heat or humidification. By means of a metal belt calender it is in addition possible to replace a part of the dryer section or to increase the speed of the board machine. Strength properties are improved in metal belt calendering compared with conventional calendering methods and good large-scale smoothness is obtained compared with a machine calender or soft calender (low Bendtsen roughness).