CELLULOSE PULP IN A CELLULOSE PULP DRYER

Field of the Invention

The present invention relates to a gas blowing cellulose pulp dryer being operative for drying a web of cellulose pulp by means of gas supplied from gas outlets arranged to blow gas towards the web. The present invention further relates to a method for controlling tension in a web of cellulose pulp in a cellulose pulp dryer.

Background of the Invention

Cellulose pulp is often dried in a dryer having several superposed horizontal drying decks. Large dryers may have more than 50 drying decks and each drying deck may be about 60 meters in length and 10 meters in width. Turning rolls are arranged at the ends of the drying decks, such that the web may travel in a zigzag manner through the dryer. Cellulose pulp having about 50% water content is fed into the upper drying deck. A web of cellulose pulp is conveyed along the drying decks and the turning rolls convey the web to the next, lower, drying deck. Dry cellulose pulp, having about 10% water content, leaves the end of the lowest drying deck. An example of a complete cellulose pulp dryer is illustrated in WO 99/36615.

A cellulose pulp dryer of the kind illustrated in WO 99/36615 may be a convective type of dryer and operate in accordance with the air borne web principle. An example of such a dryer is described in more detail in

WO 2009/154549. Hot air is blown onto a web of cellulose pulp by means of upper blow boxes and lower blow boxes. The air blown by the blow boxes transfer heat to the web to dry it. In addition, the air blown by the lower blow boxes keeps the web floating above the lower blow boxes. Hot air is supplied to the blow boxes by means of a circulation air system comprising fans and steam radiators heating the drying air.

The drying capacity of a cellulose pulp dryer depends upon a number of parameters such as the size of the drying surface, evaporation ability, and total time necessary for maintenance work. There is a need to make the cellulose pulp dryers of today more efficient and to improve the drying capacity.

Summary of the Invention

According to a first aspect, the inventive concept relates to a gas blowing cellulose pulp dryer operative for drying a web of cellulose pulp, wherein gas is supplied from gas outlets arranged to blow gas towards the web for drying and stabilizing the web while the web travels through the dryer, wherein the cellulose pulp dryer comprises at least one turning roll making the web turn when travelling from a first web tension zone of the cellulose pulp dryer to a second web tension zone of the cellulose pulp dryer, and wherein the cellulose pulp dryer comprises at least a first web tension control device arranged at the turning roll for controlling the tension of the web in at least one of the web tension zones.

By "gas blowing cellulose pulp dryer" is meant a dryer for drying cellulose pulp using hot gas, wherein the hot gas is blown towards a web of cellulose pulp for drying the web and for stabilizing the web while the web travels through the dryer. In other words, in a gas blowing cellulose pulp dryer the gas blown towards the web is used both for drying the web and for stabilizing the web. Therefore the web in a gas blowing cellulose pulp dryer may not need to be supported by any wire or other support structure since the web is stabilized and/or supported by the gas blown from the gas outlets. The web is a gas stabilized web. Gas blowing cellulose pulp dryers are known in the art, for instance in WO 99/36615. WO 99/36615 discloses a gas blowing cellulose pulp dryer having horizontal drying sections which are also referred to as drying decks. Moreover, gas blowing cellulose pulp dryers comprise vertical dryers, such as the dryer shown in WO 96/22419. In a vertical gas blowing cellulose pulp dryer the gas blown towards the web is used both for drying and for stabilizing the web.

The web tension control device may be used for controlling the tension of the cellulose pulp web in one or several portions of the cellulose pulp dryer. Depending on the type of cellulose pulp dryer and the position of the web tension control device in the dryer the tension of the web may be decreased or increased to optimize the efficiency the dryer. Control of the tension of the cellulose pulp web may lower the risk of web breaks, in particular in the first portion of the dryer where the tensile strength of the web is relatively low due to relatively large moisture content, and where the web is also quite heavy due to such large moisture content. If the web tension is lowered in the first portion of the cellulose pulp dryer, web breaks may be avoided or at least become less frequent.

In addition, control of the tension of the cellulose pulp web, by increasing the web tension, may lower the risk of formation of folds in the web. In particular in the middle and last portion of the cellulose pulp dryer, where the web is relatively light and thus may tend to flutter, an increased web tension may minimize the risk of formation of web folds. Since the cellulose pulp web has a higher dry solids content in the middle and last portions of the dryer than in the first portion of the dryer the cellulose pulp web may withstand larger tensile forces in the middle and last portions of the dryer than in the first portion of the dryer.

In addition, the first web tension control device may comprise at least one of a motor and a brake. The first web tension control device is arranged at one of the turning rolls. If the web tension control device comprises a motor it may be used for driving the turning roll and thus increase the rotational speed of the turning roll. An increased rotational speed of a turning roll may increase the tension of the cellulose pulp web upstream of that turning roll. If the web tension control device comprises a brake it may be used for slowing down the turning roll and thus decrease the rotational speed of the turning roll. A decreased rotational speed of a turning roll may decrease the tension of the cellulose pulp web upstream of that turning roll. It may thus be preferred that the web tension control device comprises both a motor and a brake.

In one embodiment the tension of the web in the first web tension zone is controlled by the first web tension control device and the tension of the web in the second web tension zone is controlled by a second web tension control device. The tension of the web in the first web tension zone is controlled by the first web tension control device and the tension of the web in the second web tension zone is controlled by a second web tension control device. Two or several web tension zones enable adjustment of the web tension to different levels in different parts of the dryer. The second web tension control device may be arranged at the outlet of the dryer, for instance at a nip roll, or it may be arranged inside or outside of the dryer. The second web tension control device may be arranged at a different turning roll of the cellulose pulp dryer than that turning roll at which the first web tension control device is arranged.

In addition, the first web tension zone may be equipped with a first type of blow boxes suitable for a cellulose pulp web with relatively high basis weight and relatively low tensile strength. Further, the second web tension zone may be equipped with a second type of blow boxes suitable for a cellulose pulp web with relatively low basis weight and relatively high tensile strength. The second type of blow boxes may have a different mechanical design than the first type of blow boxes. By "relatively low" and "relatively high" basis weight is meant that the basis weight of the cellulose pulp web is higher in the first zone than in the second zone. By "relatively low" and "relatively high" tensile strength is meant that the tensile strength of the cellulose pulp web is lower in the first zone than in the second zone. A high moisture content of the web gives a heavy web having low tensile strength, whereas a web with higher dry solids content has a lower weight and a higher tensile strength. Thus the first web tension zone will precede the second web tension zone, as seen in the travelling direction of the web. It becomes possible to control the tension of the web in the different web tension zones, and thereby use different types of blow boxes in the different web tension zones. Therefore the drying process becomes more efficient and the energy consumption may be reduced. The length of the dryer may also be reduced due to the more efficient drying process.

Preferably, the gas blowing cellulose pulp dryer comprises several turning rolls, wherein a separate web tension control device is arranged at at least 3 of the turning rolls. Adjustment of the web tension may be facilitated and/or the web tension may be controlled with better accuracy if several of the turning rolls comprise a web tension control device. In one embodiment each turning roll has a separate web tension control device. In such an embodiment it may be advantageous if at least some of the web tension control devices have a rotational frequency control unit. Each rotational frequency control unit may be used driving a turning roll which may for instance be advantageous during tail threading of the cellulose pulp dryer. Prior art cellulose pulp dryers usually has two tail treading motors for driving the turning rolls during tail treading. The tail treading motors in the prior art cellulose pulp dryers are usually located at the ends of the dryer and the driving force is forwarded to the turning rolls using a chain transmission. Thus, each turning roll in the prior art cellulose pulp dryers has an

overrunning clutch to allow the turning rolls to be freewheeling during operation of the dryer. If the rotational frequency control units are used for controlling the rotational frequency of the turning rolls during tail threading the chain transmission and overrunning clutch become unnecessary. Thus, maintenance jobs of the chain transmission and overrunning clutch is eliminated. Also, the level of noise in the cellulose pulp dryer may be lowered if the chain transmission is removed since the chain transmission is rather noisy.

In addition it is possible that the web tension control device comprises at least one torque control device for measuring the tension of the cellulose pulp web.

The gas blowing cellulose pulp dryer may be selected in the group of dryers comprising airborne web dryers and vertical dryers.

The gas blowing cellulose pulp dryer may comprise a first web tension zone, in which the tension of the cellulose pulp web is held at a first web tension level, and a second web tension zone, in which the tension of the cellulose pulp web is held at a second web tension level, wherein the second web tension level is different from the first web tension level. In accordance with one embodiment the first web tension zone is located upstream, as seen in the direction of travel of the cellulose pulp web, of the second web tension zone, wherein first web tension level is lower than the second web tension level to compensate for the web being more fragile in the first web tension zone than in the second web tension zone. It is also possible that the gas blowing cellulose pulp dryer comprises a third web tension zone, in which the tension of the cellulose pulp web is held at a third web tension level. The third web tension level may be higher than the second web tension level since the web may be stronger in the third web tension zone than in the first and second web tension zones. However, it is also possible that the third web tension level is lower than the second web tension level since the web strength may be decreased at the last portion of the dryer due to cooling water being sprayed to the web in the cooling zone near the outlet of the dryer

The first web tension zone may be arranged upstream of the second web tension zone, as seen in the direction of travel of the pulp web in the dryer. The second web tension zone may be arranged upstream of the third web tension zone, as seen in the direction of travel of the web in the dryer.

According to one embodiment, the web tension control device is arranged with a switch to allow the web tension control device to be controlled in either torque control mode or speed control mode. A web tension control device run at torque control mode allows the web tension to be controlled using a particular torque for driving the turning roll. A web tension control device run at speed control mode controls the tension of the web by controlling the rotational speed of the turning rolls. The switch allows to easily switching between speed control mode and torque control mode. For instance, speed control mode may be suitable during treading of the dryer, whereas torque control mode may be suitable during normal operating conditions.

According to a second aspect, the inventive concept relates to a method for controlling tension in a web of cellulose pulp in a gas blowing cellulose pulp dryer operative for drying the web by means of blowing hot gas towards the web. The cellulose pulp dryer comprises at least a first web tension zone and a second web tension zone. The method comprising feeding the web of cellulose pulp in the first web tension zone; turning the web of cellulose pulp by a turning roll allowing the web to turn when the web travels from the first web tension zone to the second web tension zone;

feeding the web of cellulose pulp in the second web tension zone; and controlling the tension of the web in at least one of the first and the second web tension zones by driving or braking the turning roll.

Preferably the cellulose pulp dryer comprises several turning rolls and the method may comprise controlling the tension of the cellulose pulp web in the first web tension zone by driving or braking at least one turning roll of the first web tension zone; and controlling the tension of the cellulose pulp web in the second web tension zone separately from the controlling of the tension of the cellulose pulp web in the first web tension zone.

Preferably, the method comprises controlling the tension of the cellulose pulp web in the second web tension zone by driving or braking a turning roll of the second web tension zone.

Preferably, the method comprises controlling the tension of the cellulose pulp web in at least three consecutive web tension zones, wherein the maximum web tension is higher in the third web tension zone than in the second web tension zone, and the maximum web tension is higher in the second web tension zone than in the first web tension zone. The three consecutive web tension zones may be arranged consecutively along the travel direction of the web, with the first zone arranged upstream of the second zone, and the second zone arranged upstream of the third zone.

Brief description of the Drawings

The invention will now be described in more detail with reference to the appended drawings in which:

Fig. 1 is a schematic side view, and illustrates a dryer for drying cellulose pulp according to one embodiment of the present invention;

Fig. 2 is a diagram showing cellulose pulp web tension along the length of the cellulose pulp dryer, according to another embodiment of the present invention;

Fig. 3 is a schematic side view, and illustrates the area III of Fig. 1 ; Fig. 4 is a schematic top view, and illustrates a second type of lower blow box as seen in the direction of the arrows IV-IV of Fig. 4;

Fig. 5 is a schematic side view, and illustrates a dryer for drying cellulose pulp according to another embodiment of the present invention; Fig. 6 is a schematic side view, and illustrates the area VI of Fig. 5; and Fig. 7 is a schematic side view, and illustrates a dryer for drying cellulose pulp according to an alternative embodiment of the present invention.

Description of preferred Embodiments

Fig. 1 illustrates a cellulose pulp dryer 1 for drying cellulose pulp in accordance with the air borne web principle where cellulose pulp is dried by means of hot air while travelling along horizontal drying decks 2. It is possible to use another gas than air for drying the web.

Typically, a dryer 1 would comprise 4 - 40 drying decks 2. Large dryers may even comprise 50 drying decks or more, but for clarity purposes a smaller number of drying decks 2 is illustrated in Fig. 1 . The dryer 1 illustrated in Fig. 1 comprises 23 superposed drying decks 2 arranged in a housing 3. Optionally, the dryer 1 may also comprise one or more cooling decks, not illustrated in Fig. 1 , that are operative for cooling the web after the drying thereof.

At a first end 4 of the housing 3 a first column of turnings rolls 6, 7 is arranged, and at a second end 8 of the housing 3 a second column of turning rolls 10, 1 1 is arranged. The turning rolls 6, 7, 10, 1 1 are rotatable and arranged at the ends of the drying decks 2, i.e. in the vicinity the first and second ends 4, 8, respectively, of the housing 3. Each drying deck 2 may typically be between 15 and 80 meters in length and between 1 and 15 meters in width, why the turning roll side walls 12 of the housing 3 normally constitute the short sides of the housing 3. For clarity purposes, only the end portions of the dryer 1 , i.e. the portions of the dryer 1 which are close to the turning roll side walls 12 are illustrated in Fig. 1 . The middle section of the dryer 1 is cut away, which is illustrated by vertical dotted lines in Fig. 1 .

A wet pulp web 14 enters the dryer 1 via an inlet 16 arranged in a turning roll side wall 12 of the housing 3. In the embodiment of Fig. 1 , the inlet 16 is arranged in the upper portion of a turning roll side wall 12, but the inlet may, in an alternative embodiment, be arranged in the lower portion of a turning roll side wall 12. The web 14 is forwarded horizontally, towards the right as illustrated in Fig. 1 , in the dryer 1 until the web 14 reaches a turning roll 10. In the dryer 1 illustrated in Fig. 1 , the web 14 will first reach a turning roll 10 of the second column of turning rolls. The web 14 is turned around the turning roll 10, and travels horizontally towards the left as illustrated in Fig. 1 , in the dryer 1 until the web 14 reaches a turning roll 6 of the first column of turning rolls, at which the web 14 is again turned. In this manner the web 14 is fed through the housing 3 from the inlet 16 and travels, in a zigzag manner, from the top to the bottom of the dryer 1 , as illustrated by arrows P. The web 14 leaves the dryer 1 via an outlet 18 arranged in one of the turning roll side walls 12 of the housing 3. In the embodiment of Fig. 1 , the outlet 18 is arranged in the lower portion of the turning roll side wall 12, but the outlet 18 may, in an alternative embodiment, be arranged in the upper portion of the turning roll side wall 12.

Of the totally 23 drying decks illustrated in Fig. 1 the first 10 drying decks 2, as seen in the direction of the travelling direction P of the web 14, belong to a first web tension zone 19. The next 1 1 drying decks 2 belong to a second web tension zone 20 and the last 2 drying decks 2 belong to a third web tension zone 22. The web tension zones 19, 20, 22 will be described in more detail below. It is possible, in another embodiment, to have any number of drying decks in a web tension zone, for instance one web tension zone may comprise one drying deck.

Blow boxes 21 , 23, 24, 26 are arranged in each of the drying decks 2. Each drying deck 2 is defined by a row of juxtaposed lower blow boxes 21 , 24, which at their upper side discharge heated air for drying the web 14. Each row of lower blow boxes 21 , 24 is associated with a row of juxtaposed upper blow boxes 23, 26, which at their underside discharge heated air for drying the web 14. The air is blown through air outlets 25, 27 formed in the respective blow boxes 21 , 23, 24, 26. The air outlets 25, 27 may have any suitable shape, such as circular perforations or so called "eyelid perforations", which may have a similar design as the openings referred to as "eyelid perforations" in WO 97/16594. The air outlets may for instance be designed to keep the web 14 in a floating manner above the lower blow boxes 21 , 24 in accordance with the air borne web principle. Typically, each drying deck 2 comprises 20-300 lower blow boxes 21 , 24 and the same number of upper blow boxes 23, 26, although in Fig. 1 only the portions of the drying decks 2 close to a turning roll side wall 12 are shown, why only 16 lower blow boxes 21 , 24 and 16 upper blow boxes 23, 26 are illustrated in each drying deck 2.

Typically, air of a temperature of 80 to 250°C is utilized for the drying process. The cellulose pulp entering the dryer 1 , from a wet forming station (not illustrated) typically has a dry solids content of 40-60% by weight, and the cellulose pulp web 14 leaving the dryer 1 has a dry solids content of typically 85-95% by weight. The cellulose pulp entering the dryer 1 typically has a basis weight of 1500 to 2500 g/m ² , when measured at a moisture content of 1 kg water per kg dry substance, and a thickness of 3 to 6 mm. The cellulose pulp web 14 leaving the dryer 1 typically has a basis weight of about 800 to 1400 g/m ² , when measured at a moisture content of 0.1 1 kg water per kg dry substance, and a thickness of 0.8 to 3 mm.

As described above, the web 14 is fed in a floating manner between the lower blow boxes 21 , 24 and the upper blow boxes 23, 26. The lower blow boxes 21 , 24 are provided with the air outlets 25 and the upper blow boxes 23, 26 are provided with the air outlets 27. The vertical web space of a drying deck 2, i.e. the vertical distance between the upper side of the lower blow boxes 21 , 24 and the lower side of the upper blow boxes 23, 26 of a drying deck 2, is relatively small, for instance between 5 and 50 millimetres, whereas the width and length of a drying deck 2 is relatively large, for instance between 1 to 15 meters in width and between 15 and 80 meters in length.

Web tension control devices 28, 30 are included in the dryer 1 and are schematically shown in Fig. 1. As is described above the characteristics of the cellulose pulp web 14 vary along the length of the dryer 1 ; the web 14 enters the dryer 1 at high basis weight, but a low tensile strength, and then gradually, as moisture is dried off from the web 14, the basis weight is reduced, and the tensile strength is increased. Therefore the web tension control devices shown in Fig. 1 comprise a first web tension control device 28 which controls the web tension in the first web tension zone 19, and a second web tension control device 30 which controls the web tension in the second and third web tension zones 20, 22. For instance, the first web tension control device 28 may be used for braking a first driven turning roll 7 and thereby lowering the web tension in the first web tension zone 19. In addition the web tension in the second web tension zone may be influenced by the first web tension control device 28, since a braking of the first driven turning roll 7 may increase the web tension in the drying decks 2 downstream of the first driven turning roll 7. The second web tension control device 30 may be used for braking, or increasing the speed of, a second driven turning roll 1 1 and thereby decreasing, or increasing, the web tension in the second web tension zone 20. Controlling the second driven turning roll 1 1 may, in addition, influence the web tension in the third web tension zone 22, which is arranged downstream of the driven turning roll 1 1 . It is also possible to have a third web tension control device (not shown) for additional control of the web tension in the third web tension zone 22, or in any other web tension zone 19, 20. For instance a nip roll 40 located on the exterior side of the housing outlet 18 may be used to decrease or increase the web tension in one or several of the web tension zones 19, 20, 22.

In the first web tension zone 19 of the embodiment shown in Fig. 1 the web 14 has relatively high moisture content and is therefore relatively heavy and has a low tensile strength. For this reason web breaks occurs more frequently in the first part of the prior art cellulose pulp dryers. If the web tension is kept lower in the first web tension zone 19 than in the rest of the dryer web breaks may be prevented. However, in the second web tension zone 20, which is located downstream, as seen in the direction P of forwarding the web 14, of the first web tension zone 19, the web 14 is relatively dry, light and strong and may thus withstand higher web tension. In the third web tension zone 22 the web 14 may be even stronger than in the second web tension zone 20 or it may be, which will be described later, weaker due to increased moisture.

A first type of blow boxes 21 , 23 may be used in the first web tension zone 19. For instance, the first type of blow boxes 21 , 23 may be so-called fixation blow boxes that utilize drying air being blown towards the web in an inclined manner, such that the web is kept, by means of Coanda-effect, at a fixed distance of typically 1 -3 mm above the blow boxes. An example of such blow boxes is described in WO 97/16594. A second type of blow boxes may be used in the second and third web tension zones 20, 22. The second type of blow boxes 24, 26 may be so called non-contact blow boxes, or air cushion types of blow boxes, that may require a controlled web tension for optimal use. The non-contact blow boxes give non contact web transport using static over pressure. Such blow boxes may be designed to give perpendicular impingement of drying air supplied mainly through round holes in the blow boxes. At a suitable web tension the non-contact blow boxes yield a controlled web position and a high heat transfer. The second type of blow boxes will be described in more detail with reference to Fig. 3 and Fig. 4 below.

It is also possible to have more than three web tension zones. For instance five web tension zones (not illustrated) may be used in the cellulose pulp dryer 1 . A first web tension zone may then be equipped with a first type of blow boxes suitable for a cellulose pulp web with high basis weight and low tensile strength for the interval where the dryness is low. A second, third and fourth web tension zone may be equipped with a second type of blow boxes suitable for a cellulose pulp web with medium basis weight and medium tensile strength for the middle portion of the cellulose pulp dryer. A fifth web tension zone may equipped with a third type of blow boxes suitable for a cellulose pulp web with low basis weight and high tensile strength for the interval where the dryness is higher. It is also possible to have several further web tension zones, for instance between four and 30 web tension zones. Still further, the web tension zones may have similar or different types of blow boxes. It is also possible to arrange one web tension zone with two or more different types of blow boxes.

Each web tension control device 28, 30 comprise a rotational frequency control unit 32, 34 in the form of electrical motors 32, 34 having a drive mode and a brake mode and which are arranged at turning rolls 7, 1 1 . Thus, the web tension control devices 28, 30 control the rotational speed of a respective turning roll 7, 1 1 . Each web tension control device 28, 30 may be connected to one or several load cells 36 to convert the tension of the web 14 into an electrical signal. Alternatively, instead of using load cells, the motors 32, 34 may be controlled directly by torque or rotational speed, e.g. the motors may be run at a predefined torque or rotational speed, which is illustrated and described in more detail with reference to Fig. 7. The tension of the web 14 may be adjusted by adjusting the rotational speed of the

respective turning roll 7, 1 1 using the motors 32, 34 operating in drive mode or brake mode, depending on which regulation of the web tension that is required.

In the embodiment shown in Fig. 1 two web tension control devices 28, 30 are shown. Each web tension control device 28, 30 have a motor 32, 34 and use information from two load cells 36. The dotted lines in Fig. 1 illustrate that one web tension control device 28, 30 use information from two load cells 36. It is however possible to use several more web tension control devices and load cells. For instance one web tension control device having a motor may be arranged at each turning roll of the entire dryer 1 . Or, for example, every second or every fifth turning roll 6, 7, 10, 1 1 may have a web tension control device. It is also possible to use several more load cells than the four load cells illustrated here. One web tension control device may communicate with several load cells, or with all load cells, to optimize the web tension control in the dryer 1 .

It is possible to divide the drying decks 2 of the dryer 1 into several more web tension control zones than the three web tension control zones 19, 20, 22 illustrated in Fig. 1. It is also possible to have one of the web tension control devices located outside of the dryer 1 . As mentioned above a web tension control device (not shown) may for instance be arranged at a guiding roll 40 located at the outlet 18 of the dryer 1 .

Fig. 2 is a diagram illustrating a plot of the maximum web tension F _max (in Newton/metre) in a respective drying deck 2. In Fig. 2 F _max is plotted on the y-axis and the distance L (in metre) that the web 14 has travelled from the inlet 16 of the dryer 1 is on the x-axis. The dryer for which F _max is plotted in Fig. 2 has five web tension control zones. The web tension varies slightly along the individual drying deck, and is normally at its maximum at the end of the individual drying deck. In Fig. 2 the maximum web tension in the individual drying deck is plotted versus the distance L that the web has travelled when reaching the end of the drying deck 2 in question. The dotted line shows the maximum web tension in a respective drying deck for a prior art cellulose pulp dryer. Fig. 2 illustrates web tension for a dryer which has a design which is similar to that of the dryer 1 described in connection with Fig. 1 above.

However, the dryer 1 in the embodiment in Fig. 2 has five web tension control zones which are controlled by a number of motors arranged at different turning rolls.

The first web tension zone for the dryer embodiment in Fig. 2 is located between the points denoted a and b. Each turning roll of the first web tension zone, which may for instance comprise four turning rolls, is driven by a respective motor. Thus, the first web tension zone of the dryer embodiment in Fig. 2 may comprise four motors. The maximum web tension F _max for each drying deck in the first web tension zone, i.e. between the points a and b, is held essentially constant by means of the motors in the first web tension control zone.

The second web tension zone for the dryer embodiment in Fig. 2 is located between the points denoted c and d. The second web tension zone may comprise four turning rolls, of which two may be driven. The two driven turning rolls may be the first and the last turning roll of the second web tension zone, respectively, i.e. located at point c and point d. In the second web tension zone the maximum web tension F _max for each drying deck increases essentially linearly along the length of the dryer due to air friction, which increases along the length of the dryer.

Thereafter follows a third web tension zone between the points denoted e and f, a fourth web tension zone between the points denoted g and h, and a fifth web tension zone between the points denoted / and j. The third, fourth and fifth web tension zones may be arranged in the same manner as the second web tension zone, i.e. having two driven turning rolls in each web tension zone.

In other words the beginning and end of each zone in Fig. 2, respectively, is restrained by driven turning rolls and those driven turning rolls are shared with the preceding and subsequent zone, respectively. For instance, the second web tension zone is located between the first and third web tension zones, and thus share one turning roll with the first web tension zone and one turning roll with the third web tension zone. Thus, the turning roll located at point b is the same as the turning roll located at point c, which is a driven turning roll. In addition, the turning roll located at point d is the same as the turning roll located at point e, which is also a driven turning roll, etc.

The maximum web tension F _max for each drying deck is increased somewhat in each web tension zone due to air friction, except for the first web tension zone where the maximum web tension is held constant in all drying decks by means of the driven turning rolls. The maximum web tension F _max is substantially increased between the points b and c, i.e. the maximum web tension F _max is substantially higher in the second web tension zone than in the first web tension zone. The maximum web tension F _max is substantially increased between the points d and e, i.e. the maximum web tension F _max is substantially higher in the third web tension zone than in the second web tension zone. The maximum web tension F _max is substantially increased between the points and g, i.e. the maximum web tension F _max is substantially higher in the fourth web tension zone than in the third web tension zone. The reason for increasing the web tension along the length of the dryer has been discussed above and is based at the fact that the web is more tensile in the first portions of the dryer and stronger in the later portions of the dryer.

The maximum web tension F _max is substantially decreased between the points h and / ^' , i.e. the maximum web tension F _max is substantially lower in the fifth web tension zone than in the fourth web tension zone. One reason for decreasing the web tension in the last portion of the dryer is that a cooling zone, where water is sprayed onto the web, may be arranged in the last portion of the dryer. Thus, the web strength may be decreased somewhat in the last portion of the dryer. An example of a dryer having a cooling zone at the end portion of the dryer is shown in WO 2009/154549.

Fig. 3 is an enlarged side view of the area III of Fig. 1 and illustrates a drying deck 2 of the second drying zone 20 illustrated in Fig. 1 . The second drying deck 2 comprises the second type of lower blow boxes 24 arranged below the web 14, and the second type of upper blow boxes 26 arranged above the web 14. The second type of lower blow boxes 24 blow hot drying air towards the web 14 vertically upwards towards web 14, illustrated by arrows VU in Fig. 3. The second lower blow boxes 24 of the second drying deck 2 exert a lower fixation force on the web 14 compared to the first type of lower blow boxes 21 of the first drying deck 2, illustrated in Fig. 1 . As described above the first type of blow boxes 21 may be so-called fixation blow boxes that utilize drying air being blown towards the web in an inclined manner. The fixation force exerted on the web 14 by the second type of lower blow boxes 24 is normally rather low, or even non-existing. Returning to Fig. 3, the hot drying air supplied from the second lower blow boxes 24 lifts the web 14 to a height at which the weight of the web 14 is in balance with the lifting force of the hot drying air supplied by the second type of lower blow boxes 24. Typically, the average distance, or height H1 , between the lower side of the web 14 and the upper surface of the second type of lower blow boxes 24 is 4 to 20 mm. Since there is a limited or even non-existing fixation force exerted by the second type of lower blow boxes 24 on the web 14, the vertical position of the web 14 will tend to fluctuate, during operation of the dryer 1 , somewhat more when passing the drying decks 2 of the second web tension zone 20, compared to when passing the drying decks 2 of the first web tension zone 19. Hence, the web 14 is transported horizontally along the second drying deck 2 in a relatively free manner, with some movement in the vertical direction, meaning that the web 14 is subjected to some stretching forces. The second type of upper blow boxes 26 blow hot drying air towards the web 14 vertically downwards towards web 14, illustrated by arrows VD in Fig. 3. Typically, the average distance, or height H2, between the upper side of the web 14 and the lower surface of the second type of upper blow boxes 26 is 5 to 80 mm. The hot drying air blown by the blow boxes 24, 26 is evacuated via gaps S formed between horizontally adjacent blow boxes 24, 26.

Fig. 4 is a schematic top view, and illustrates the second type of lower blow box 24 as seen in the direction of the arrows IV-IV of Fig. 3. An arrow P illustrates the intended path along which the web, not shown in Fig. 4, is to pass over an upper face 42 of the second type of lower blow box 24. The upper face 42 extends between the sides 44, 46 of the blow box 24 and comprises openings 48 of the "non-inclined type" that are distributed over the upper face 42. In other words, the air outlets 25 (seen in Fig. 1 ) are the openings 48. By "non-inclined type" is meant that at least 80 % of the air blown from those openings 48 is blown at an angle to the upper face 42 which is at least 70°. Typically, almost the entire flow of air would be blown almost vertically, i.e., at an angle of close to 90° to the upper face 42, from the openings 48 of the non-inclined type. In the second type of lower blow box 24 at least 75% of the total flow of air supplied thereto is blown from openings of the non-inclined type. In the embodiment illustrated in Fig. 4, 100% of the total flow of air supplied thereto is blown from the openings 48 of the non- inclined type. The openings 48 may be evenly distributed over the face 42, but may also be distributed in an uneven manner. As can be seen from Fig. 4, the concentration of openings 48 (openings per square centimetre of upper face 42) is somewhat higher adjacent to the sides 44, 46. The openings 48 of the blow box 24 may be round holes, with a characteristic measure in the form of a diameter of 1 .8 to 5.0 mm. According to one embodiment (not illustrated), the openings 48 have a diameter of 1 .8 to 3.1 mm, and according to another embodiment (not illustrated), the openings 48 have a diameter of 2.0 to 2.8 mm. The openings 48 blow the hot drying air vertically upwards to form the flows VU.

The degree of perforation, as defined hereinabove, may, for example, be 1 .5% in the second type of lower blow box 24. The degree of perforation can be varied to suit the weight, dryness, etc. of the web 14 to be dried. Often the degree of perforation of the second type of lower blow box 24 would be 0.5-3.0%. The openings 48 having a diameter of 1 .8 to 3.1 mm typically constitute at least 75% of the total degree of perforation of the second lower blow boxes 24, and typically 80-100 % of the total degree of perforation of the second type of lower blow boxes 24. The openings 48 having a diameter of 1 .8 to 3.1 mm constitute, for example, 100 % of the total degree of perforation in the exemplary lower blow box 24 illustrated in Fig. 4. The first type of upper blow boxes 23 and the second type of upper blow boxes 26, seen in Fig. 1 , may have the same general design as the second lower box 24 illustrated in Fig. 4.

The above mentioned average distances H1 , H2 refer to the shortest distance between the face of the respective blow box 21 , 23, 24, 26 and the web 14.

The embodiments described with reference to Figs 1 -4 refer to cellulose pulp dryers having horizontally extending drying decks. In addition the web tension control device 28 may be utilized in vertical cellulose pulp dryers. Fig. 5 and Fig. 6 illustrate a web tension control device 128 in a vertical cellulose pulp dryer 101 . Several features, such as the motors and the control functions, are the same as described above with reference to Figs 1 -4.

Fig. 5 illustrates a vertical cellulose pulp dryer 101 . Cellulose pulp web 1 14 is dried while travelling along vertical windings 150 between upper turning rolls 1 10, 1 1 1 and lower turning rolls 106. A vertical dryer 101 may comprise a high number of windings 150, for instance 40 windings. For clarity purposes a smaller number of windings 150 are illustrated in Fig. 5, and the middle section of the dryer 101 is cut away, which is illustrated by vertical dotted lines in Fig. 5. Thus, the dryer 101 illustrated in Fig. 5 comprises 14 windings 150 arranged in a housing 103. It is also possible to have a lower number of windings than the 14 windings illustrated in Fig. 5.

Turning rolls 106, 1 10, 1 1 1 are arranged at the top of the dryer 101 and at the bottom of the dryer in Fig. 5. The turning rolls 106, 1 10, 1 1 1 are rotatable and arranged at the ends of the windings 150. The web 1 14 thus travels up and down in a zigzag manner between the turning rolls 106, 1 10, 1 1 1 .

A wet pulp web 1 14 enters the dryer 101 via an inlet 1 16 arranged in a turning roll side wall 1 12 of the housing 103. In the embodiment of Fig. 5 the inlet 1 16 is arranged in the middle portion of the left side wall 1 12, but the inlet may, in an alternative embodiment, be arranged in another portion of the side wall 1 12. The web 1 14 is forwarded essentially vertically, upwards as illustrated with an arrow P in Fig. 5, in the dryer 101 until the web 1 14 reaches a turning roll 1 10. The web 1 14 is turned around an upper turning roll 1 10 and travels essentially vertically downwards as illustrated in Fig. 5, in the dryer 101 until the web 1 14 reaches a lower turning roll 106 at which the web 1 14 is again turned. In this manner the web 1 14 is fed through the housing 103 from the inlet 1 16 and travels, in a zigzag manner, from one side of the dryer to the other side of the dryer 101 . The web 1 14 leaves the dryer 101 via an outlet 1 18 arranged in another side wall 1 12 of the housing 103. In the embodiment of Fig. 5, the outlet 1 18 is arranged in the lower portion of the right side wall 1 12, but the outlet 1 18 may, in an alternative embodiment, be arranged in another portion of the side wall 1 12.

The first five windings 150 of the web 1 14, as seen in the direction of the travelling direction P of the web 1 14, belong to a first web tension zone 1 19. The latter windings 150 belong to a second web tension zone 120.

The web 1 14 is dried by air from blow boxes 124 arranged to the left and to the right of each winding 150 of the web 1 14. Thus, two columns 152, 154 of blow boxes 124 are arranged at each web winding 150. The blow boxes 124 of the left column 152 of blow boxes discharge air on their left side and the blow boxes 124 of the right column 154 of blow boxes discharge air on their right side.

The horizontal distance (D in Fig. 6) between two corresponding blow boxes 124, i.e. between a blow box 124 from a left column 152 of blow boxes to the closest blow box 124 from a right column 154 of blow boxes on the other side of the web 1 14, is relatively small, for instance between 4 and 80 millimetres, whereas the width and length of a winding 150 is relatively large, for instance between 1 to 15 meters in width and between 2 and 60 meters in height. Length is here referred to as the vertical distance between the upper turning rolls 10, 1 1 and the lower turning rolls 6. As is seen in Fig. 5 the length of the windings 150 is not constant in the entire dryer 101 . In the first web tension zone 1 19 the lower turning rolls 6 are located at a higher vertical level than the lower turning rolls 6 of the second web tension control zone 120 whereas all upper turning rolls 1 10, 1 1 1 are located at the same vertical level. Thus, the length of the windings 150 is shorter in the first web tension control zone 1 19 than in the second web tension control zone 120. Moreover, the length of the windings 150 in the first web tension zone 1 19 is increasing stepwise, the first winding being shortest and the second and third windings being longer than the first winding, and the fourth and fifth windings being still longer.

Having stepwise increasing length of the vertical windings 150, as is illustrated in the first web tension zone 1 19 of Fig. 5, gives a lower web tension in the first web tension zone 1 19 since gravity gives rise to a large portion of the web tension. It is possible to have variable length of the windings 150 also in the second web tension zone 120, or in any suitable number of web tension zones. It is also possible to have the same length of all vertical windings 150 in the entire dryer. In particular if one or several web tension control device 128, which will be described below, are used it may be possible to have only a few shortened windings 150, or even to allow all windings 150 to have the same length. In another embodiment it is possible that one web tension zone 1 19, 120 comprises for instance only one or two vertical windings, or it may comprise several more windings 150 than the windings illustrated here.

A web tension control device 128 is included in the dryer 101 of Fig. 5 and is illustrated schematically. As is described above the characteristics of the cellulose pulp web 1 14 vary along the length of the dryer 101. The web tension control device 128 controls the tension of the web 1 14 in the first web tension zone 1 19.

In the same manner as was described in connection to Fig. 1 , the web tension control device 128 comprises a rotational frequency control unit 132. The rotational frequency control unit 132 is an electrical motor having a drive mode and a brake mode and which is arranged at a driven turning roll 1 1 1 . The web tension control device 128 is connected to a load cell 136 to convert the tension of the web 1 14 into an electrical signal.

It is possible to include a second web tension control device, as described in connection to Fig. 1 , to control the tension of the web in the second web tension zone 20. It is also possible to use the web tension control device 128 to control the tension of the web in the second web tension zone. For instance, braking the driven turning roll 1 1 1 may decrease the tension of the web 14 in the first web tension zone 1 19 and increase the tension of the web 14 in the second web tension zone 120. It is also possible to include several more web tension control devices, which is described in connection to Fig. 1 above.

Fig. 6 is an enlarged side view of the area VI of Fig. 5 and illustrates a portion of a winding 150. Blow boxes 124 are arranged to the left and to the right of the web 1 14 and discharge hot air onto the web 1 14 from the left, illustrated by arrows VL, and from the right, illustrated by arrows VR. As mentioned above, the average distance D between the web 1 14 and the blowing surface of the blow boxes 124 may be 4 to 80 mm. The hot drying air blown by the blow boxes 124 is evacuated via gaps S formed between vertically adjacent blow boxes 124. The air outlets of the blow boxes 124 of the dryer 101 in Fig. 5 may be of the same kind as the openings 48 illustrated in Fig. 4.

Fig. 7 shows the same kind of dryer 1 as is described in connection to Fig. 1 above. Therefore, the same reference numerals are used in Fig. 7 for illustrating the horizontal drying decks 2, the housing 3 and its ends 4, 8, side walls 12, inlet 16 and outlet 18. Moreover the turning rolls 6, 10, the web 14, the blow boxes 21 , 23, 24, 26, and the travelling direction P of the web 14 are the same in Fig. 7 as in Fig. 1 above. However, in Fig. 7 the tension of the web 14 is controlled by means of a respective web tension control device 50 arranged at every single turning roll 6, 10. Thereby the web tension control devices 50 shown in Fig. 7 allow the tension in the web 14 to be controlled individually at each drying deck 2.

Each web tension control device 50 is arranged with a switch 52 to allow the web tension control devices 50 to be controlled in either torque control mode (TC) 54 or speed control mode (SC) 56. In Fig. 7 all switches 52 are arranged in torque control mode 54. A web tension control device 50 run at torque control mode 54 allows the web tension to be controlled using a particular torque for driving the turning roll 6, 10. Motors 32, 34 (not shown in Fig. 7) of the kind mentioned with reference to Fig. 1 above may be used for driving the turning rolls 6, 10. The torque may be a positive value i.e. the turning rolls 6, 10 may be used for pulling the web 14, or a negative value i.e. the turning rolls 6, 10 may exert a braking force on the web 14. It is also possible to keep the torque at a neutral value, i.e. neither pulling nor slowing down the web 14. Since each drying deck 2 may be controlled individually it is possible to have different torque values at different drying decks 2, depending on the strength of the web 14 at a particular location in the dryer 1 .

A web tension control device 50 run at speed control mode 56 controls the tension of the web 14 by controlling the rotational speed of the turning rolls 6, 10. It is possible to keep all turning rolls 6, 10 at the same rotational speed, or to regulate the rotational speed of each turning roll 6, 10

individually. In the same manner as during torque control mode, motors of the kind described with reference to Fig. 1 may be used for driving the turning rolls 6, 10.

One example of an operating routine for the web tension control devices 50 shown in Fig. 7 may be to use speed control mode 56 during tail threading of the dryer 1 , and to switch to torque control mode 54 after threading is completed or at a particular width of the web 14. The web tension control devices 50 may be used for driving all or most of turning rolls 6, 10 at the same speed in order to reduce the risk of overloading the web 14 during tail threading. When a particular width of the web 14 is reached in the tail threading process, for instance at half web width, the switches 52 may be changed to torque control mode 54. Torque control mode 54 may be used for achieving a particular web tension in a particular portion of the dryer 1 , as described in connection to Figs. 1 -6 above.

In the illustrated example all switches 52 are arranged in torque control mode 54. It is possible to have some turning rolls 6,10 driven in torque control mode 54 and simultaneously have some other turning rolls 6, 10 driven in speed control mode 56, or to have all switches 52 in speed control mode 56.

It is possible to have the web tension control devices 50 arranged at fewer turning rolls 6, 10 than what is shown in Fig. 7. For instance every second turning roll may be equipped with a web tension control device 50, or all turning rolls 6, 10 in some portions of the dryer 1 may be equipped with a web tension control device 50 whereas other portions of the dryer 1 may have turning rolls 6, 10 that are not driven, i.e. some turning rolls lacking a web tension control device 50. It is possible to have the web tension control devices 50 arranged at a vertical dryer, such as the dryer 101 shown in Figs 5-6.

The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

For instance, it is possible to use any suitable number of web tension zones and to adjust the web tension in each zone in any suitable manner. As mentioned above the different drying zones may have different types of nozzles on the blow boxes. There may be several more kinds of drying boxes than the ones mentioned inhere which are suitable for use with the web tension control system.

Furthermore, one motor 28 may drive several turning rolls 6, 7, 10, 1 1 for example 2-4 turning rolls, via chain drive. It is also possible to drive some turning rolls 6, 7, 10, 1 1 with web tension control motors, and some turning rolls 6, 7, 10, 1 1 with chain drive, for instance during tail threading of the dryer.