WO2010109079A2

WO2010109079A2 - Method and apparatus for controlling the shrinkage of a fibre web

Info

Publication number: WO2010109079A2
Application number: PCT/FI2010/050239
Authority: WO
Inventors: Henri Chydenius; Heikki Niskanen; Pertti Vainio; Pertti Heikkilä
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2009-03-26
Filing date: 2010-03-25
Publication date: 2010-09-30
Anticipated expiration: 2011-09-26
Also published as: FI20095313A0; EP2411310A2; FI20095313L; CN102365221A; FI124075B; CN102365221B; WO2010109079A3

Abstract

The invention relates to a method and an apparatus for controlling the shrinkage of a fibre web. In the method the fibre web is transferred at a first transfer speed and at least one operation is performed on it. During the operation the fibre web is transferred at a second operational speed, which is lower than the first speed, or the fibre web is even completely halted for the duration of the operation. The fibre web is dried in a controlled manner before the operation is performed. The according to the invention comprises an operational unit, such as a slitter-winder and drying means arranged in the immediate vicinity of the fibre web for drying the fibre web. The drying means are arranged before the operational unit in the travel direction of the fibre web. The invention also relates to a method for rebuilding an existing slitter-winder or rereeler section in a paper machine.

Description

METHOD AND APPARATUS FOR CONTROLLING THE SHRINKAGE OF A FIBRE WEB

The invention relates to a method and an apparatus according to the preambles of the independent claims presented hereafter. The invention especially relates to a novel manner of controlling the shrinkage of a fibre web, such as a paper web or a paperboard web.

PRIOR ART In the dry end of a paper machine room the relative humidity of the air is often low. This is due to the fact that there are many heat sources in the dry end of the machine room, but barely any moisture sources. In the dry end of the paper machine room the water amount in the air is often almost the same as in the outside air. Especially in the winter the relative humidity of the air in the dry end is very low, even 20 %. To be in equilibrium with the fibre web to be cut, the air humidity should be about 50 %, depending on the residual moisture of the fibre web.

Air which is dryer than the equilibrium moisture content causes drying of the fibre web, and as a result thereof, shrinking especially in the cross direction of the fibre web. The problem is especially discernible when processing a fibre web coming from a roll. Inside the roll the fibre web stays moist, but when it is unwound, the fibre web begins to dry and simultaneously to shrink. The shrinking may cause tearing of the fibre web during the roll change or another idle time. The fibre web may for example tear at the slitter knives in connection with splicing or other deceleration or halting.

Among other things the moistening of the whole dry end of the paper machine room has been presented as a solution. Such a solution however usually requires a large amount of air and water and may thus consume a great deal of energy. It has been possible to solve the problem also by moistening only a limited space of air in the dry end of the paper machine room. Traditionally the moistening is continuous, even though for example the halts of the slitter-winder typically last only a few minutes per hour. Moistening, especially continuous moistening, consumes a great deal of energy.

In the solution presented in publication Fl 89952, the air humidity is increased locally by blowing moist air to the necessary location, for example to the area of the slitter knives.

In the solution presented in publication Fl 108633, moist air is blown underneath the slitter-winder to the area of the slitter knives.

OBJECT OF THE INVENTION

One object of the present invention is to reduce or even eliminate the above- mentioned problems appearing in prior art.

One object of the present invention is to provide a solution, with which the shrinkage of a fibre web, such as a paper web, is controlled in connection with operations.

One object of the present invention is to provide a solution, with the aid of which the shrinkage of the fibre web may be generally controlled.

Another object of the present invention is to provide a solution, which decreases tearing of the fibre web, such as the paper web, at the various sections of the slitter-winder, which are situated after the unwinder.

Still another object of the present invention is to provide a solution, which decreases the difference in the shrinkage between two fibre webs, which are joined together by splicing.

DESCRIPTION OF THE INVENTION In a typical method according to the invention for controlling the shrinkage of a fibre web, the fibre web is transferred at a first transfer speed, at least one operation is performed on the fibre web, during which operation the fibre web is transferred at a second operational speed, which is lower than the first speed, or the fibre web is even completely halted for the duration of the operation, and the fibre web is dried in a controlled manner before the operation is performed.

A typical apparatus according to the invention for controlling the shrinkage of a fibre web comprises an operational unit and drying means for drying the fibre web arranged in the immediate vicinity of the fibre web, which drying means are arranged before the operational unit in the travel direction of the fibre web.

In a typical method according to the invention for rebuilding an existing slitter- winder or rereeler section in a paper machine, where the slitter-winder or rereeler section comprises a moistening system for decreasing the shrinkage of a fibre web by moistening, the moistening system is converted to operate as a drying system, with which air is blown towards the fibre web in order to dry it.

It has now been found out that by drying the fibre web in a controlled manner before operations are performed to the fibre web, the operations requiring a substantial decrease in speed or even halting of the fibre web, the shrinkage of the web may be controlled and minimised during the operation. With the aid of the invention the fibre web may be dried to such a moisture content that uncontrolled shrinking of the fibre web does not occur during the operation, whereby also possible disadvantages related to the shrinking of the web are avoided, such as for example the tearing of the web in the longitudinal direction. With the aid of the present invention, many operations and operational stages to be performed to the fibre web in the dry end of the paper machine may be made easier, such as for example the cutting of the fibre web at a slitter-winder, the splicing of the web or the patching of holes in the fibre web.

In this application the term fibre web means a continuous web containing fibres of plant origin, such as wood or cellulose fibres, which web is manufactured in a paper or board machine. The fibre web may for example be a paper web or a board web. The web may usually also contain other material commonly used in paper or board manufacturing, such as pigments and fillers.

In this application the term operation especially means the processing of a fibre web in the dry end of the paper machine in a finishing treatment, where the fibre web, which has been produced in the paper machine and is on a machine roll or otherwise on a roll, is either unwound, wound, cut in the running direction of the fibre web, spliced or holes in the fibre web are patched.

In one embodiment of the invention operation especially means operations occurring in connection with the cutting of a fibre web, wherein the transfer speed of the fibre web has to be decreased or the web has to be halted. The fibre web is cut in many different process stages. Strips, so-called trimmings, may be removed from the edges of the fibre web, in order to tidy the edges of the web. The cutting of the trimmings may be performed for example on a rereeler or in the slitter- winder. Due to splicing, machine or customer roll change or another process disruption, the transfer speed of the fibre web may have to be decreased or the fibre web may have to be completely halted. The fibre web may have to be cut also in the cross direction, for example in order to remove a defect therein. In that case the fibre web is halted, the defective part is removed from the fibre web by cutting, and the ends of the defect-free fibre webs are spliced together, whereafter the transfer of the fibre web is continued.

In another embodiment of the invention operation especially means patching or splicing of the fibre web, which takes place on a rereeler, where a fibre web on the machine roll is unwound in order to check its quality, whereafter it is reeled back into a roll. When observing a quality defect in the fibre web, for example a hole, it can be patched permanently or temporarily with a suitable hole patch label, or the defect may be marked for later removal. For the duration of the patching or marking, the transfer speed of the fibre web may have to be decreased or the fibre web may have to be completely halted. In this application slitter-winder means an apparatus meant for cutting the fibre web, the apparatus with which the fibre web may be cut substantially in its travel direction, with one or more knives, into two or more sections. Generally, in the slitter-winder the wide web on the machine roll is unwound and cut in a slitter section into several narrower partial webs, which are rewound in a reeling section into separate customer rolls. When the customer rolls are ready, the slitter-winder is halted and the customer rolls are removed from the winder. When the fibre web runs out at the machine roll, the slitter-winder is halted and a new machine roll is brought thereto, the end of which is spliced together with the end of the previous fibre web which ran out.

In this application machine direction or travel direction of the fibre web means the direction, in which the fibre web travels in a normal running situation. Cross direction or cross machine direction is on the other hand the direction, which is transverse in relation to the machine direction, parallel to the level of the fibre web.

According to one embodiment of the invention the fibre web is dried with drying means arranged in the immediate vicinity of the fibre web. The drying means may be any means, which are intended or suitable for drying a fibre web. The fibre web may especially be dried by blowing air or heated air against the fibre web with drying means arranged in the immediate vicinity of the fibre web, by bringing it into contact with a hot surface, such as a cylinder or a roll, and/or by directing onto it electromagnetic radiation, such as IR radiation or microwaves.

According to one embodiment of the invention the fibre web is thus dried with air drying, i.e. by blowing air towards the fibre web. Air may be blown advantageously over the entire width of the web for example at 0.05-1 (m³/s)/machine width meter, typically 0.22-0.66 (m³/s)/machine width meter, more typically 0.1-0.3 (m³/s)/machine width meter. Air may also be blown in the range of 0.11-0.2 (m³/s)/machine width meter. When blowing drying air the air velocity in the nozzle may be for example 10-100 m/s, typically 20-70 m/s, more typically 25-55 m/s, usually 30-45 m/s. Air drying may be used at various fibre web speeds, for example at a normal transfer speed of a fibre web, at a crawling speed or when the fibre web is stationary. Air drying is a technique known as such, and all of its details are not described in this application.

In those embodiments of the invention, where the fibre web is dried by blowing air towards it, the blown air may be unheated or heated air. The temperature of the unheated air is typically below 50 ¹C, more typical Iy 20-35 "C. The air of the machine room may be used as the unheated air. The temperature of the heated air may be for example 30-200 O, 75-150 ¹C, 100-15 0 O or 77 O, 100 "C, 150 "C or 170 O. Heated air may be produced for exampl e with an air-conditioning device, which comprises for example a filter, a blower and a heating unit. The heating unit may comprise an electric resistor, a steam radiator, a water radiator, a gas burner or another corresponding heating device, with which the air meant for blowing is heated to a desired temperature. From the air-conditioning device the air is directed into the air dryer along pipes or channels.

The drying means may thus comprise an air dryer, which is arranged to blow air towards the fibre web, preferably over the entire width of the fibre web. In one embodiment the air dryer may comprise a gap having a width of the entire fibre web, from which gap air is blown towards the fibre web. Preferably the air dryer has nozzle members, such as a nozzle level, a nozzle or a nozzle group or line, whose length is mainly the same as the width of the entire fibre web and which are mainly parallel with the fibre web, the fibre web being arranged to travel past the nozzle members. The nozzle members may comprise several blow openings or gaps, from which air is arranged to be blown towards the fibre web. There may be several nozzle members one after the other in the travel direction of the fibre web. Exit openings for removing moist air from between the nozzle level and the fibre web may have been arranged between the nozzle members. Such nozzle members are reliable and dry the fibre web efficiently.

In one embodiment of the invention the air to be dried is blown to the fibre web over the entire width of the fibre web, with one or more nozzles. The nozzle members of the air dryer may for example be impulse nozzles known as such, suspension nozzles or gap nozzles. The nozzle members may be arranged close to the fibre web to be dried. Typically the distance of the nozzle member from the fibre web is 0.005-0.02 m or 0.01-0.05 m when the nozzle member is situated close to the web, or 0.1-0.5 m, or 0.5-1 m, or 1-2 m when the nozzle member is situated farther from the web. The distance depends on the used nozzle member type, and on the free space in the vicinity of the fibre web. For example channel nozzles may be at a distance of up to 2 meters from the fibre web, whereby they function as an air distribution device. When using air flotation nozzles, they may be arranged even very close to the fibre web in order to maximise the drying effect and the energy saving. The typical distance of an air flotation nozzle from the fibre web is 5-30 mm, more typically 5-15 mm. The typical distance of a direct impingement nozzle from the fibre web is 15-100 mm, more typically 20-50 mm.

According to one embodiment of the invention the blowing nozzles used in the air drying are situated on the edges of the fibre web, whereby they are arranged to blow air from the edges of the fibre web towards its centre, in a direction which differs from the travel direction of the fibre web, for example in the cross machine direction. The blowing nozzles may also be situated above the fibre web, whereby air is blown to the upper surface of the fibre web, or underneath it, whereby air is blown to the lower surface of the fibre web. There may be several nozzles and they may be situated in different locations in relation to the fibre web. Air can for example be blown at the same time only from one side of the fibre web or from both sides. The nozzle members situated in different locations in relation to the fibre web may be similar to or different from each other. For example the nozzle members on the first side of the fibre web may be different in terms of structure and/or blowing effect from the nozzle members on the second side of the fibre web. Air may be blown over the entire width of the fibre web or only over a part of the width of the fibre web. Air can be blown perpendicularly towards the surface of the fibre web. Air can be blown diagonally in the running direction of the web, substantially in the running direction of the web or in the opposite direction to the running direction of the web. The invention thus makes possible the optimisation of the drying of the fibre web, so that the shrinkage of the fibre web may be controlled as precisely as possible. In one embodiment of the invention an air moistening system already existing in the slitter-winder section may be changed for a dry-blowing system. Thus the air to be blown towards the fibre web may be brought into the winder section through the same pipes, through which moist air was previously brought. Previous already existing moistening air nozzles may be used as nozzles, or they may be replaced with blowing members, such as blowing nozzles. Such alteration work is quick and inexpensive to perform. When heated air is used for drying of the fibre web, the air moistening unit of the former moistening system is changed for an air heating unit.

According to one embodiment of the invention the drying means of the apparatus may comprise a hot surface, such as a heatable cylinder or roll, via which the fibre web is arranged to travel. The fibre web is thus dried by bringing it into contact with the hot surface. The temperature of the hot surface to be arranged into contact with the fibre web may for example be 30-300 O, typically 100-150 O, more typically 50-100 O, in some cases 60-120 "C. The hot surface may for example be such a cylinder or roll, which is already a part of the paper or board machine and in connection with which surface heating means may be arranged.

The drying means of the apparatus may also comprise an electromagnetic radiation source, such as an infrared dryer or a microwave dryer, the radiation produced by which is arranged to be directed onto the fibre web. According to one embodiment of the invention the fibre web may thus be dried by directing onto it electromagnetic radiation, such as infrared radiation, i.e. IR radiation, or microwaves. By IR radiation is in this context meant radiation having a wavelength in the range from 700 nm to 1 mm. By microwaves is in this context meant radiation having a wavelength in the range from 1 mm to 1 meter.

The used drying means may already be a part of some finishing unit of the paper or board machine, or the drying means may be arranged in connection with one or more finishing unit, if needed. Different drying methods and means may be combined with each other, for example crawling may be combined with the blowing of heated air or IR radiation, or a single finishing unit may have both a radiation source and an air dryer. The drying means may be in operation during short periods only when needed, for example before the fibre web is halted and/or during splicing. The drying can also be continuous.

In one method according to one embodiment of the invention the drying of the fibre web with drying means is begun when transferring the fibre web at the first transfer speed. Thus the fibre web is dried with the drying means already before its speed is decreased. Thus the drying time may be adjusted to suit each situation. The drying means may be active for example for 0-100 s, 0-30 s, 10-40 s or 50-100 s before the running speed of the web is decreased to the operational speed or before the web is completely halted.

Occasionally the drying of the fibre web occurs by decreasing the speed of the fibre web to a crawling speed before the operation is performed, which crawling speed is lower than the transfer speed of the fibre web and as high as or higher than the operational speed of the fibre web. For example, the drying of the fibre web may thus be started at the normal transfer speed of the fibre web, then decreasing the speed to the crawling speed, for example before the halt. When drying a fibre web with drying means as it advances at the crawling speed, it is possible to ensure that the duration of the drying is sufficient to achieve a suitable moisture level in the web. In this application the normal transfer speed of the fibre web means the speed, at which the fibre web runs when the process functions normally. The normal transfer speed of the fibre web can for example in the slitter- winder be 2000-3500 m/min or 2500-3000 m/min. Generally the typical transfer speed of the fibre web is > 500 m/min, very typically 1500-2500 m/min.

According to one embodiment of the invention the drying of the fibre web is begun when transferring the fibre web at a speed which is slower than the normal running speed. Thus the active drying of the fibre web with drying means is not necessarily needed at all, but the fibre web may be dried by prolonging its contact time with the surrounding dry machine room air. The fibre web can thus be dried simply by transferring the fibre web at a lower speed than the normal running speed, at a so-called crawling speed. With a running speed which is lower than the normal running speed, a longer drying time may be achieved, whereby the fibre web has time to dry more even over a small distance. A typical crawling speed of the fibre web is < 100 m/min, typically < 50 m/min, typically 0.5-17 m/min, more typically 0.5-15 m/min, very typically < 10 m/min, for example 1-10 m/min, occasionally 2-5 m/min. In connection with splicing, the crawling speed may be for example 30-45 m/min, typically about 40 m/min. The fibre web may be transferred at the crawling speed while simultaneously drying it for example for 0- 200 s, 0-30 s, 10-40 s or 50-100 s. The fibre web may be dried with the drying means at the same time as the fibre web is transferred at the crawling speed.

In one advantageous embodiment of the invention the operation performed on the fibre web is performed in connection with the slitter-winder, whereby the part of the fibre web, which remains in the vicinity of the slitter knives when the fibre web is halted or decelerated, is dried. The operational unit is thus the slitter-winder, and the drying means are arranged in the vicinity of the slitter knives. By drying the part of the fibre web, which remains at the slitter knives as the fibre web is halted or decelerated, the shrinkage of the fibre web can be minimised at the slitter knives. When the shrinkage is decreased at the slitter knives, the lateral movement which occurs due to shrinkage and a notch which possibly forms at the cutting location are decreased. Additionally the risk of tearing of the paper, caused by shrinkage, at the slitter knives is decreased. The fibre web may have to be halted for example for the duration of the hole patching or for the duration of splicing, for example when the machine roll or intermediate roll runs out.

According to one embodiment of the invention the drying of the fibre web is discontinued before the fibre web is halted for the duration of the operation. The drying of the fibre web may for example be discontinued before the fibre web is halted. Since the fibre web has thus been dried before halting, the shrinking of the fibre web is lower during the operation.

According to one embodiment of the invention the drying of the fibre web is discontinued when the fibre web is stationary. The drying of the fibre web may for example be begun at the normal transfer speed, whereafter the fibre web may be transferred as it is dried during some time at the crawling speed. The drying of the fibre web may be discontinued for example when the fibre web has halted for splicing. The fibre web may for example be dried so that the drying starts in the transfer direction of the fibre web before the possible operational stage of the finishing.

According to one embodiment of the invention the drying of the fibre web is discontinued when the fibre web has been accelerated to the normal transfer speed after a halt or slower running speed. In some embodiments the drying according to the invention is active mainly during the entire halt of the fibre web. In some embodiments the drying according to the invention is active during part of the time, of which the fibre web is halted. The fibre web may for example be dried at the splicing location and/or near the splicing location when the fibre web is halted. By drying the fibre web at the splicing location and/or near the splicing location, the difference in shrinkage between two fibre webs, which are to be joined together by splicing, may be decreased.

By drying the web for a short time before the operation, for example before the halting or before the splicing, savings in drying costs are achieved. Typically the fibre web is in the normal running situation during the majority of time, whereby the drying according to the invention is not needed.

According to one embodiment of the invention the operation to be performed on the fibre web comprises joining two fibre webs together by splicing, whereby the fibre web, which remains in the vicinity of the slitter knives during the splicing, or the fibre web, which travels past the slitter knives during the splicing, is dried before the splicing. The fibre web is joined by splicing to a second fibre web, for example when the machine roll or intermediate roll runs out. During the splicing the fibre web is halted, for example when making a butt joint splice, or the fibre web is run at a speed which is lower than the normal running speed, when making for example a flying splice. According to one embodiment of the invention, the operation performed on the fibre web comprises joining two fibre webs together by splicing, whereby at least one of the fibre webs is dried at the splicing location and/or near the splicing location before the splicing. Two fibre webs are joined together by splicing for example when a machine roll runs out. The moisture content of the fibre webs to be spliced may differ and the fibre webs may thus shrink at different rates during the splicing or after the splicing. By drying for example the first fibre web, which has been brought to the slitter-winder, at the splicing location and/or near the splicing location, the difference between its shrinkage and the shrinkage of the second fibre web, which comes from a new machine roll, may be decreased. The operational unit is thus a splicing apparatus, and the drying means for drying the fibre web are arranged near the splicing apparatus.

According to one embodiment of the invention the fibre web is dried before patching of tears or defects, such as holes, in the fibre web. Tears or holes in the fibre web may be patched for example by patching the fibre web with other pieces of the fibre web or by taping. For example by arranging the fibre web near the tear to be essentially as dry as the piece of another fibre web, arranged to be a patch for the fibre web, disadvantages caused by different shrinkage in different fibre webs may be minimised. At the same time the drying of the fibre web to be patched during the patching, and the formation of possible further tears, is also prevented.

In one method according to one embodiment of the invention, the air near the part of the fibre web to be dried is kept at a temperature of over +25 O. The air near the part of the fibre web to be dried may be kept at a temperature of over +25 C all the time, in the normal running situation, or the warming of the part to be dried may be started at a suitable time for example before the halting or the splicing.

The air temperature may be for example 25-50 O, 35-40 ¹C, 40-50 O or 37 O, 40 O, 45 O or 50 O. This can be achieved for exa mple by arranging a hood around the operational site. For example the fibre web coming to the knives of the slitter-winder over a distance of 0.5-2 meters or the area around the splicing location over a distance of 0.5-2 meters from the splicing location may be kept at this temperature.

Many things affect the drying of the fibre web. The need for change in the moisture content of the fibre web depends for example on the moisture of the surrounding air. For example newsprint paper, which at a temperature of 40 O has a moisture content of 8 %, is by drying attempted to get to a moisture content of 3.7 %, when the surrounding air temperature is 25 O and the relative moisture is about 20 %. For example the quality of the fibre web, the temperature and the moisture content of the fibre web before the drying, the speed of the fibre web and the drying method affect the required drying time and the change in the moisture content of the fibre web. The duration of the drying according to the invention, i.e. the time during which an individual part of the fibre web is subject to the drying according to the invention, may for example be 1-100 s, 1-10 s, 10-60 s or 15 s, 20 s, 30 s or 60 s.

The embodiments and advantages mentioned in this text are in suitable parts applicable to both each apparatus and method according to the invention, even if this is not always specifically mentioned.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described in more detail below with reference to the enclosed schematic drawing, in which

Figure 1 shows a cross-section in the machine direction of an apparatus according to an embodiment of the invention,

Figure 2 shows a part of an apparatus according to an embodiment of the invention, and

Figure 3 shows a part of an apparatus according to an embodiment of the invention. DETAILED DESCRIPTION OF THE EXAMPLES OF THE FIGURES In order to increase the clarity, the same reference numbers are used in the figures for parts in different examples which at least in some way correspond to each other.

Figure 1 shows as a schematic view a cross-section of an apparatus 1 according to the invention. From an unwinder 2 a fibre web 3 is led to knives 5 of a slitter- winder 4. The normal rotational direction of the unwinder 2 and the normal travel direction of the paper web 3 are depicted with arrows A and B. In connection with the winder 4, below the paper web 3 has been arranged an air inlet channel 6a in the cross machine direction, which channel is connected to a blowing nozzle 7a, which is arranged in the cross machine direction and has a width of the entire paper web. The blowing nozzle 7a is directed so that is blows air towards the lower surface 8a of the paper web. The blown air may be heated air or unheated air from the machine room.

A second air inlet channel 6b in the cross machine direction has been arranged above the travel path of the paper web 3. The inlet channel 6b is a pipe, in which has been formed a gap nozzle having a length of the entire paper web, i.e. a blowing nozzle 7b. The blowing nozzle 7b is directed so that it blows air towards the upper surface 8b of the paper web. In the example of Figure 1 , air coming from both the blowing nozzle 7a and from the blowing nozzle 7b is blown towards the paper web 3 against the travel direction B of the paper web. The air blows coming from the blowing nozzles 7 are depicted schematically with arrows.

In Figure 1 is shown with a dotted line 3b the location of the paper web when the paper roll 9 on the unwinder 2 is in its full original size. As the unwinding progresses, the diameter of the paper roll 9 decreases. The location of the paper web in a situation, where the paper roll 9 begins to reach its end, is denoted with the number 3.

The apparatus in Figure 1 may function for example in the following way. From the unwinder 2 the fibre web 3 is fed at a normal running speed to the slitter-winder 4 and further in the direction shown by the arrow B. When the halt of the fibre web is approaching, the speed of the fibre web 3 under transfer is slowed down from the normal running speed to a crawling speed. At the same time the drying of the fibre web 3 is begun in the area between the unwinder 2 and the slitter-winder 4 by blowing air from the blowing nozzles 7a, 7b towards the lower surface 8a and upper surface 8b of the fibre web. After a certain time the fibre web 3 is halted for splicing. In this example the splicing speed is thus 0 m/s. The fibre web 3 is spliced with the splicing device in a manner known as such. The halting and splicing is performed when the fibre web coming for example from the machine roll or intermediate roll is running out. The drying, i.e. the blowing of air from the blowing nozzles 7a, 7b is discontinued for example just before the halting. The fibre web can further be dried at the splicing location and/or near the splicing location before the splicing. Due to the drying, the fibre web 3 shrinks in the cross machine direction already before the halting. Thus essentially no further shrinking of the fibre web 3 occurs during the halt, whereby forces directed to the web and tearing it, are minimised. After the splicing, the unwinder 2 is started again and the speed of the unwinder 2 and the fibre web 3 is again accelerated up to the normal running speed.

It can be seen from Figure 1 that by properly selecting the location of the fixed blowing nozzles 7a, 7b in relation to the fibre web, the blowing may in all situations be performed relatively close to the fibre web 3 and 3b. For example in the solution in Figure 1 , the upper nozzle 7b is close to the paper web when the roll 9 is full and the lower nozzle 7b is close to the paper web when the roll 9 is empty. Thus the paper web 3 may be efficiently dried at least on one side.

It is possible that the inlet channels 6 and the blowing nozzles 7a, 7b have previously formed a part of the air moistening system of the winder section, for example so that moist air has been brought to the apparatus 1 with the inlet channel 6a, which air has been led towards the paper web 3 from the nozzle 7a. According to one embodiment of the invention such a moistening system may have been replaced with a drying apparatus 1 according to the invention, so that a device, which produces dry, possibly heated air, has been connected to the inlet channel 6a. Dry, possibly heated air may be produced for example with an air- conditioning device, which comprises for example a filter, a blower and a heating unit. The heating unit may comprise an electric resistor, a steam radiator, a water radiator, a gas burner or another corresponding heating device, with the aid of which the air meant for blowing is heated to a desired temperature. The blowing nozzles are connected to the air-conditioning device with a channel system.

Figure 2 shows a part of a drying apparatus according to one embodiment of the invention. The directions of the air streams are depicted with arrows. In Figure 2 several air inlet channels 6 have been arranged in the cross machine direction above the paper web 3. A nozzle surface 10, which has several small blow openings 7, has been arranged over the entire width of the paper web on the lower inlet channel surface intended to be towards the paper web. The nozzle surface 10 is arranged to be mainly parallel with the fibre web to be dried. The distance between the fibre web 3 and the nozzle surface 10 may during drying be for example 20 mm. The air passes from the inlet channel 6 to several individual openings 7, through which the air stream is directed towards the surface of the fibre web 3, whereby the air stream dries the fibre web. Outlet channels 11 have been arranged between the inlet channels 6, into which outlet channels the moistened air flows and through which the air travels out of the vicinity of the fibre web 3. The inlet channels and the outlet channels between them may for example be surrounded with a hood or another casing, whereby the moist air may be removed from the apparatus in a controlled manner.

Figure 3 shows a blowing nozzle, a so-called air flotation nozzle 12, of a drying device according to one embodiment of the invention. The air flotation nozzle 12 may be arranged for example below the fibre web 3, whereby an air cushion floating the fibre web may be created with it between the nozzle 12 and the fibre web 3. The air flotation nozzle 12 shown in Figure 3 consists of an air inlet channel 6 in the cross machine direction and of parallel blowing channels 13 on both of its sides. The air travels from the inlet channel 6 into the blowing channels through openings 14 in the wall between them. The upper surface of the blowing channel 13, which is meant to be towards the fibre web 3, forms a nozzle surface 10, which is meant to be mainly parallel with the fibre web to be dried. Several small blow openings 7 have been formed in the nozzle surface 10. The air stream from the blow openings 7 is directed towards the surface of the fibre web 3, whereby the air stream dries the fibre web. A part of the blow openings 13 may be arranged to blow air from both blowing channels 13 towards each other in order to optimise the flotation effect. In the example in Figure 3, the blow openings 15 in the edges of the blowing channels 13, which are closest to each other, have been directed towards each other. There may be several air flotation nozzles 12 subsequently in the travel direction of the web. The removal of moist air may be arranged for example between adjacent air flotation nozzles 12. It is also for example possible that the moistened air flows away from the vicinity of the fibre web 3 via the apparatus edges in the machine direction. The air flotation nozzles 12 may for example be surrounded with a hood or another casing, whereby the moist air may be removed from the apparatus in a controlled manner.

The invention is not meant to be limited to the embodiments shown as examples above, but the aim is to interpret it extensively within the scope of protection defined in the claims presented below.

Claims

1. A method for controlling the shrinkage of a fibre web (3), which method comprises

- transferring the fibre web at a first transfer speed,

- performing at least one operation on the fibre web, wherein the fibre web is processed in the dry end of the paper machine or board machine in a finishing treatment, whereby - in the operation the fibre web, which has been produced in the paper machine or board machine, which fibre web is on a machine roll or otherwise on a roll, is unwound, and

- the fibre web is cut in the running direction, spliced, patched and/or wound, - transferring the fibre web during the performed operation at a second operational speed, which is lower than the first transfer speed, or the fibre web is even completely halted for the duration of the performed operation, characterised in drying the fibre web in a controlled manner before the operation is performed.

2. The method according to claim 1 , characterised in drying the fibre web (3) with drying means arranged in the immediate vicinity of the fibre web

- by blowing air or heated air towards the fibre web,

- by bringing the fibre web into contact with a hot surface, such as a cylinder or roll, and/or

- by directing onto the fibre web electromagnetic radiation, such as IR radiation or microwaves.

3. The method according to claim 2, characterised in beginning the drying with the drying means when transferring the fibre web (3) at the first transfer speed.

4. The method according to claim 1 , characterised in that the drying of the fibre web (3) occurs by decreasing the speed of the fibre web to a crawling speed before said operation is performed, which crawling speed is lower than the transfer speed of the fibre web and as high as or higher than the operational speed of the fibre web.

5. The method according to any of the preceding claims, characterised in performing the operation performed on the fibre web in connection with a slitter- winder, whereby the part of the fibre web, which remains in the vicinity of the slitter knives when the fibre web (3) is halted or decelerated, is dried.

6. The method according to any of the preceding claims, characterised in that in the operation performed on the fibre web, two fibre webs (3) are joined by splicing, whereby at least one of the fibre webs is dried at the splicing location and/or near the splicing location before splicing.

7. The method according to any of the preceding claims, characterised in drying the fibre web (3) before tears and/or defects in the fibre web are patched.

8. An apparatus for controlling the shrinkage of a fibre web (3), which apparatus comprises a operational unit, such as a slitter-winder or a reeler, characterised in that the apparatus further comprises drying means arranged in the immediate vicinity of the fibre web for drying the fibre web, which drying means are arranged before the operational unit, such as the slitter-winder or the reeler, in the travel direction of the fibre web.

9. The apparatus according to claim 8, characterised in that the operational unit is a slitter-winder, and that the drying means are arranged in the vicinity of the slitter knives.

10. The apparatus according to claim 8, characterised in that the operational unit is a splicing device, and that the drying means for drying the fibre web (3) are arranged in the vicinity of the splicing device.

11. The apparatus according to any of the preceding claims 8-10, characterised in that the drying means comprise an air dryer, which is arranged to blow air towards the fibre web (3), preferably over the entire width of the fibre web.

12. The apparatus according to claim 11 , characterised in that the air dryer has a nozzle member, such as a nozzle level, a nozzle or a nozzle group or line, whose length is mainly the same as the width of the entire fibre web (3) and which are mainly parallel with the fibre web, past which nozzle member the fibre web is arranged to travel, and which nozzle member (10) has several blow openings or gaps (7), from which air is arranged to be blown toward the fibre web.

13. The apparatus according to any of the preceding claims 8-12, characterised in that the drying means for drying the fibre web comprises a hot surface, such as a heatable cylinder or roll, via which the fibre web (3) is arranged to travel.

14. A method for rebuilding an existing slitter-winder or rereeler section in a paper machine, where the slitter-winder or rereeler section comprises a moistening system for decreasing the shrinkage of a fibre web by moistening, characterised in that the moistening system is converted to operate as a drying system, with which air is blown towards the fibre web in order to dry it.

15. The method according to claim 14, where the nozzles of the moistening system already existing in the slitter-winder section are replaced by blowing members, such as blowing nozzles.