DRYING A WEB OF CELLULOSE PULP

Field of the Invention

The present invention relates to a cellulose pulp drying box for drying a web of cellulose pulp, wherein the cellulose pulp drying box comprises blow boxes that are operative for blowing gas towards the web of cellulose pulp for drying the pulp.

The present invention further relates to a method of drying a web of cellulose pulp.

Background of the Invention

Cellulose pulp is often dried in a convective type of drying box operating in accordance with the airborne web principle. An example of such a drying box is described 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 blow boxes have blow openings through which air is blown towards the web. The air blown by the blow boxes transfer heat to the web to dry it, and also 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. A complete cellulose pulp drying box is illustrated in WO 99/36615.

With increasing demands for increased pulp production in pulp mills, there is a desire to increase the drying capacity of a pulp drying box without increasing its size, or increasing its size only slightly, without lowering the standard of functionality. Summary of the Invention

An object of the present invention is to provide an arrangement for drying a cellulose pulp web, the arrangement being more space efficient than the prior art arrangements without lowering the standard of functionality. This object is achieved by means of a cellulose pulp drying box for drying a web of cellulose pulp, comprising blow boxes that are operative for blowing gas towards the web of cellulose pulp for drying the pulp, wherein at least 10% of the total number of blow boxes of the drying box are provided, in their respective face facing the web during use of the drying box, with blow openings having a characteristic measure of 1.5 to 5.5 mm, through which blow openings gas is blown towards the web during use of the drying box, wherein each blow opening is defined by a outlet opening facing the web during use of the drying box, a inlet opening facing the interior of the blow box, and a lateral face arranged between the outlet opening and the inlet opening, wherein, for the purpose of sound attenuation, at least a portion of the lateral face of the respective blow opening is provided with a shaped contour.

The understanding of "lateral face" should be the side or envelope surface of the three-dimensional blow opening, as opposed the planar "base" of the three-dimensional blow opening. In other words, the "base" is the blow opening face, i.e. the periphery of the blow opening lying in the face of the blow box wall.

By "shaped contour" is in this context meant that a portion of the lateral face of the blow opening has been shaped to influence the sound attenuation of the noise which may arise due to gas flowing through the blow opening and hitting the web. In other words, the lateral surface is, at its way from the first periphery to the second periphery, provided with a shaped contour.

An advantage with having the lateral face of the blow opening provided with a shaped contour is that sound or noise arising as gas passes through the blow opening and/or as the gas hit the web is dampened. It may not be necessary that every blow opening in the blow box has a lateral face provided with a shaped contour. To achieve an acceptable level of noise it may be enough to provide the blow openings at some portions of the blow box with a shaped contour.

The material thickness of the face provided with the blow openings is preferably between 0.7 - 2.0 mm. Since the blow openings are through openings in the blow box face, the depth of the lateral face of the blow opening corresponds to the thickness of the blow box face. In one

embodiment, the shaped contour is provided at at least 0.3 mm of the depth of the lateral face. In another embodiment, the shaped contour is provided at a portion of the lateral face which has a depth equal to the material thickness of the face subtracted by 0.3 mm. In other words, if the material thickness is 2.0 mm, the shaped contour may be provided at a depth between 0.3 - 1 .7 mm of the depth of the lateral face. Accordingly, if the material thickness in 1 .5 mm the shaped contour may be provided at a depth between 0.3 - 1 .2 mm of the depth of the lateral face. In addition, if the material thickness in 1 .0 mm the shaped contour may be provided at a depth between 0.3 - 0.7 mm of the depth of the lateral face.

In one embodiment the material thickness of the face provided with the blow openings is between 0.7 - 2.0 mm and at least 25% of the lateral face is provided with a shaped contour. More preferably, 30 - 80% of the lateral face is provided with a shaped contour.

In one embodiment, at least a portion of the inlet opening of the respective blow opening is provided with a shaped contour. If a portion of the inlet opening is provided with a shaped contour it may be possible to have a smaller portion of the lateral face provided with a shaped contour than if the inlet opening is not provided with a shaped contour. Thus, the total intrusion at the blow opening may be less extensive if the inlet opening is provided with a contoured portion. Another advantage with having the inlet opening provided with a shaped contour is that the shaped contour may facilitate for air flowing through the inlet opening. Hence, it may be possible to make the blow openings smaller than if no shaped contour is present at the inlet opening, which may lead to increased heat transfer.

In one embodiment, at least a portion of the outlet opening of the respective blow opening is provided with a shaped contour. Having the outlet opening provided with a shaped contour may be preferred since it easier to provide at an existing blow box.

Preferably, the shaped contour comprises an inwardly protruding portion protruding inwardly into the blow box. An inwardly protruding portion may have a larger affect on the gas fluid properties, which may cause the noise. It is also possible that the shaped contour comprises an outwardly protruding portion.

In one embodiment, the shaped contour is uniform as seen along the periphery of the blow opening. If the shaped contour is provided during manufacturing of the blow box it may be easier to provide a shaped contour which is uniform along the entire periphery of the blow opening.

In one embodiment, the shaped contour is irregularly shaped. If the shaped contour is provided after the blow box is assembled it may be easier to provide a shaped contour which is irregularly shaped since the blow opening may not be as easily accessible.

In one embodiment, the shaped contour comprises a counter bore. A counter bore may be a relatively easy way of constructing the shaped contour. The counter bore may have straight or chamfered sides.

In one embodiment, the shaped contour comprises a punched counter bore. Punching may be a fast and effective way of establishing a counter bore, in particular if a large number of blow box openings are to be provided with a counter bore.

In addition, the above mentioned object is achieved by means of a method for manufacturing a sound attenuated blow box for a cellulose pulp drying box, wherein the blow box is provided, in a blow box face, with blow openings through which gas is blown towards a web of cellulose pulp during use of the drying box, wherein the blow openings has a characteristic measure of 1 .5 to 5.5 mm, the method comprising arranging a periphery modifying device at at least one blow opening position of the blow box face; forcing the periphery modifying device towards the blow box face and forming a contoured portion of the blow opening.

The method may further comprise arranging the periphery modifying device in the blow opening; and tilting the periphery modifying device in the blow opening and forming the contoured portion in a lateral face of the blow opening. An advantage with such tilting is that there is no need for any anvil or holding-up tool. It may not be necessary to tilt the entire tool, perhaps only the portion of the tool which is inserted in the blow opening needs to be tilted depending on what tool is used. It is possible that the tilting of the periphery modifying device in the blow opening comprises forming a contoured portion which comprises a first portion of the lateral face and an opposite second portion of the lateral face.

In one embodiment the first portion comprises an inwardly protruding portion protruding inwardly into the blow box.

It is possible that the step of forming a contoured portion is carried out after the blow opening has been formed in the blow box face. Thus, it is possible to provide blow openings in an existing blow box with contoured portions.

Alternatively, it is possible that the step of forming a contoured portion is carried out in conjunction to forming the blow opening in the blow box face. The step of forming a contoured portion may in addition be carried out prior to forming the blow opening in the blow box face. Thus, the contoured portions may be provided during manufacturing of the blow box.

A further object of the present invention is to provide a method of drying a web of cellulose pulp in an efficient manner.

This object is achieved by means of a method of drying a web of cellulose pulp in a pulp drying box comprising blow boxes blowing gas towards the web of cellulose pulp, wherein at least 10% of the total amount of gas blown towards the web of cellulose pulp is blown from blow openings having a characteristic measure of 1 .5 to 5.5 mm, wherein each such blow opening is defined by an outlet opening facing the web, an inlet opening facing the interior of the blow box, and a lateral face arranged between the outlet opening and the inlet opening, wherein at least a portion of the lateral face of the respective blow opening is provided with a shaped contour via which the gas is blown to attenuate sound.

In one embodiment the shaped contour comprises a counter bore. In one embodiment the periphery modifying device comprises a punching device. The punching device may be forced towards the blow box face to form the contoured portion of the blow opening by punching the blow box face. Punching may be an effective way of establishing contour portions. Further, punching may be used for establishing contoured portions at several blow openings simultaneously. 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 drying box for drying a web of cellulose pulp.

Fig. 2 is a schematic side view, and illustrates the area II of Fig. 1.

Fig. 3 depicts schematic top and cross-sectional views, and illustrates a first lower blow box as seen in the direction of the arrows Ill-Ill of Fig. 2.

Fig. 4 is a schematic side view, and illustrates the area IV of Fig. 1 . Fig. 5 is a schematic top view, and illustrates a second lower blow box as seen in the direction of the arrows V-V of Fig. 4.

Figs. 6a-e are schematic side views illustrating a blow opening of a blow box in cross section.

Fig. 7a is a perspective view of a blow opening.

Fig. 7b is a perspective view of a blow opening provided with a shaped contour.

Fig. 8a is a side view illustrating a blow opening, in cross section, which is provided with a shaped contour according to one alternative embodiment.

Fig. 8b is a side view illustrating a blow opening, in cross section, which is provided with a shaped contour according to one alternative embodiment.

Fig. 8c is a side view illustrating a blow opening, in cross section, which is provided with a shaped contour according to one alternative embodiment.

Fig. 8d is a side view illustrating a blow opening, in cross section, which is provided with a shaped contour according to one alternative embodiment.

Fig. 9 is a perspective, and partly broken, view of a blow opening which is provided with a shaped contour according to one alternative embodiment.

Fig. 10 is a perspective view of a tool for providing a blow opening with a shaped contour. Fig. 1 1 is a schematic side view, and illustrates a drying box for drying a web of cellulose pulp according to another embodiment.

Fig. 12 is a schematic side view, and illustrates the area XII of Fig. 1 1 .

Fig. 13a is a perspective view and a side view showing a tool for providing blow openings with shaped contours, and a portion of a blow box having blow openings.

Fig. 13b is a side view showing a tool for providing blow openings with shaped contours, and a portion of a blow box having a blow opening provided with a shaped contour.

Description of preferred Embodiments

Fig. 1 illustrates a cellulose pulp drying box 1 for drying cellulose pulp in accordance with a first embodiment of the present invention. The drying box 1 comprises a housing 2. Inside the housing 2 a first drying zone 4, a second drying zone 6, and an optional cooling zone 8 may, in one exemplary embodiment, be arranged, with the first drying zone 4 arranged in the upper region of the housing 2, the cooling zone 8 arranged in the lower region of the housing 2, and the second drying zone 6 being arranged between the first drying zone 4 and the cooling zone 8.

At a first end 10 of the housing 2 a first column of turnings rolls 12 is arranged, and at a second end 14 of the housing 2 a second column of turning rolls 16 is arranged. A wet pulp web 18 enters the drying box 1 via an inlet 20 arranged in the housing 2. In the embodiment of Fig. 1 , the inlet 20 is arranged in the upper portion of the housing 2, but the inlet may, in an alternative embodiment, be arranged in the lower portion of the housing. The web 18 is forwarded horizontally, towards the right as illustrated in Fig. 1 , in the drying box 1 until the web 18 reaches a turning roll. In the drying box 1 illustrated in Fig. 1 , the web 18 will first reach a turning roll 16 of the second column of turning rolls. The web 18 is turned around the turning roll 16, and then travels horizontally towards the left, as illustrated in Fig. 1 , in the drying box 1 until the web 18 reaches a turning roll 12 of the first column of turning rolls, at which the web 18 is turned again. In this manner the web 18 travels, in a zigzag manner, from the top to the bottom of the drying box 1 , as illustrated by arrows P. The web 18 leaves the drying box 1 , after having been dried in the first and second drying zones 4, 6 and having been cooled in the cooling zone 8, via an outlet 22 arranged in the housing 2. In the embodiment of Fig. 1 , the outlet 22 is arranged in the lower portion of the housing 2, but the outlet may, in an alternative embodiment, be arranged in the upper portion of the housing.

Typically, a gas in the form of air of a temperature of 80 to 250°C is utilized for the drying process. The web 18 of cellulose pulp entering the drying box 1 , from an upstream web forming station, not shown in Fig. 1 , typically has a dry solids content of 40-60 % by weight, and the web 18 of cellulose pulp leaving the drying box 1 has a dry solids content of typically 85- 95 % by weight. The web 18 of cellulose pulp leaving the drying box 1 typically has a basis weight of 800 to 1500 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.

The first drying zone 4 comprises at least one first drying deck 24, and typically 3-15 first drying decks 24. In the embodiment of Fig. 1 , the first drying zone 4 comprises 8 first drying decks 24. Each such first drying deck 24 comprises a number of blow boxes, as will described in more detail hereinafter, and is operative for drying the web 18 while the web 18 travels horizontally from one turning roll 12, 16 to the next turning roll 16, 12. Each first drying deck 24 comprises a number of first lower blow boxes 26 and a number of first upper blow boxes 28 that are arranged for blowing a hot drying gas towards the cellulose pulp web 18. Typically, each first drying deck 24 comprises 20-300 first lower blow boxes 26 and the same number of first upper blow boxes 28, although in Fig. 1 in the interest of maintaining clarity of illustration only a few blow boxes are illustrated. The first lower blow boxes 26 are operative for keeping the web 18 in a "floating" and fixed condition, such that the web 18 becomes airborne at a distance from the first lower blow boxes 26 during the drying process, as will be described in more detail hereinafter.

The second drying zone 6 comprises at least one second drying deck 30, and typically 5-40 second drying decks 30. In the embodiment of Fig. 1 , the second drying zone 6 comprises 1 1 second drying decks 30. Each such second drying deck 30 comprises a number of blow boxes, as will described in more detail hereinafter, and is operative for drying the web 18 while the web 18 travels horizontally from one turning roll 12, 16 to the next turning roll 16, 12. Each second drying deck 30 comprises a number of second lower blow boxes 32 and a number of second upper blow boxes 34 that are arranged for blowing a hot drying gas towards the cellulose pulp web 18. Typically, each second drying deck 30 comprises 20-300 second lower blow boxes 32 and the same number of second upper blow boxes 34, although in Fig. 1 in the interest of maintaining clarity of illustration only a few blow boxes are illustrated. The second lower blow boxes 32 are operative for keeping the web 18 in a "floating" condition, such that the web 18 becomes airborne at a distance from the second lower blow boxes 32 during the drying process, as will be described in more detail hereinafter.

The first drying decks 24 of the first drying zone 4 have a different mechanical design than the second drying decks 30 of the second drying zone 6, as will be described in more detail hereinafter. Often the first lower blow boxes 26 of the first drying decks 24 would have a different mechanical design than the second lower blow boxes 32 of the second drying decks 30, as will be illustrated by means of an example hereinafter.

The cooling zone 8 comprises at least one cooling deck 36, in Fig. 2 two such cooling decks 36 are illustrated, each such deck 36 comprising a number of third lower blow boxes 38 and third upper blow boxes 40 that are arranged for blowing a cooling gas towards the cellulose pulp web 18. The lower blow boxes 38 are operative for keeping the web 18 in a "floating" condition, such that the web 18 becomes airborne during the cooling process. Typically, air of a temperature of 15 to 40°C is utilized as a cooling gas for the cooling process. An isolated wall 42 separates the second drying zone 6 from the cooling zone 8.

Fig. 2 is an enlarged side view of the area II of Fig. 1 and illustrates a first drying deck 24 of the first drying zone 4 illustrated in Fig. 1 . The first drying deck 24 comprises the first lower blow boxes 26 arranged below the web 18, and the first upper blow boxes 28 arranged above the web 18. The first lower blow boxes 26 blow hot drying air towards the web 18 both vertically upwards towards web 18, illustrated by arrows VU in Fig. 2, and in an inclined manner, at an angle of typically 5 to 60° to the horizontal plane, as illustrated by means of arrows IU in Fig. 2. The blowing of drying air at an inclination to the horizontal plane by the first lower blow boxes 26 yield both forces forcing the web 18 upwards away from the blow boxes 26, and forces forcing the web 18 downwards towards the blow boxes 26. The latter effect is sometimes referred to as the Coanda effect. This will result in the blow boxes 26 exerting a fixation force on the web 18, holding the web at a comparably well defined distance from the blow boxes 26. Typically, the average distance, or height H1 , between the lower side of the web 18 and the upper surface of the first lower blow boxes 26 is 3-6 mm during operation of the drying box 1 . If the web 18 would tend to move upwards, the fixation forces of the blow boxes 26 would drag the web 18 downwards, and if the web 18 would tend to move downwards, the air blown by the blow boxes 26 would force the web 18 upwards. Hence, the web 18 is transported horizontally along the first drying deck 24 in a relatively fixed manner, with little movement in the vertical direction, meaning that the web 18 is subjected to limited stretching forces. The first type of upper blow boxes 28 blow hot drying air towards the web 18 vertically downwards towards web 18, illustrated by arrows VD in Fig. 2.

Typically, the average distance, or height H2, between the upper side of the web 18 and the lower surface of the first upper blow boxes 28 is 10 to 80 mm. The hot drying air blown by the blow boxes 26, 28 is evacuated via gaps S formed between horizontally adjacent blow boxes 26, 28.

Fig. 3 is a schematic top view, and illustrates the first lower blow box

26 as seen in the direction of the arrows Ill-Ill of Fig. 2. An arrow P illustrates the intended path along which the web, not shown in Fig. 3, is to pass over an upper face 44 of the first lower blow box 26. The upper face 44 comprises centrally arranged first type of blow openings 46, which are "inclination type" blow openings of a type sometimes referred to as "groove perforations". By "inclination type" blow openings is meant that at least 25% of the air blown from those blow openings 46 is blown at an angle a of less than 60° to the upper face 44 of the first lower blow box 26, as is best illustrated in the cross- section B-B of Fig. 3. In the first lower blow box 26 at least 30%, often at least 40%, of the total flow of air supplied thereto is blown from blow openings of the "inclination type", for example via groove perforations 46. As best illustrated in the cross-section B-B included at the bottom of Fig. 3, the groove perforations 46 may be round holes, that are arranged in a groove 47 which is arranged centrally in the upper face 44 of the first lower blow box 26. An example of a blow box with a groove and having groove perforations arranged in the groove is illustrated in US 4,837,947. A portion of the flow of air blown via the groove perforations 46 may be blown at an angle which is larger than 60°. Of the total air flow supplied to the lower blow box 26, at least 25% may be blown at an angle a of less than 60° to the upper face 44 of the first lower blow box 26.

The groove perforations 46 provide the hot drying air blown

therethrough with an inclination, such that the inclined flows IU illustrated in Figs. 2 and 3 are generated. As can be seen from Fig. 3 of the present application, the perforations 46 are arranged in the groove 47 in an

alternating manner, such that every second flow IU will be directed to the left, as illustrated in Fig. 3, and every second flow IU will be directed to the right.

Continuing with the description of Fig. 3 of the present application, the upper face 44 is provided with a second type of blow openings 48 that are arranged between the groove 47 and the respective sides 50, 52 of the blow box 26. The second type of blow openings 48 are of a "non-inclined type" that are distributed over the upper face 44. By "non-inclined type" is meant that at least 80 % of the air blown from those blow openings 48 is blown at an angle to the upper surface 44 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 surface 44, from the blow openings 48 of the non-inclined type. The blow openings 48 may be round holes, with a characteristic measure in the form of a diameter of 1 .8 to 3.1 mm. According to one embodiment, the blow openings 48 have a diameter of 2.0 to 2.8 mm. According to a further embodiment, the blow openings 48 have a diameter of 2.2 to 2.7 mm. The second type of blow openings 48 blow the hot drying air upwards to form the flows VU, as best illustrated in the cross-section B-B of Fig. 3. As can be seen from the cross-section B-B of Fig. 3, the outer portions of the upper face 44 slope slightly downwards. This is done for the purpose of reducing the risk that the web 18 touches the blow box 26 adjacent to its sides 50, 52. Hence, those blow openings 48 that are located adjacent to the sides 50, 52 may blow most of the air supplied thereto at an angle of typically about 85° to the horizontal plane.

By varying the number and size of the first type of blow openings 46 and the number and size of the second type of blow openings 48 a suitable pressure-drop relation between first and second types of blow openings 46, 48 may be achieved, such that, for example, 65 % of the total flow of air blown to the first lower blow box 26 is ejected via the first type of blow openings 46, and 35 % of the total flow of air blown to the first lower blow box 26 is ejected via the second type of blow openings 48. A degree of perforation of a blow box 26 may be calculated by dividing the total open area of the blow openings 46, 48 of a representative portion of the upper face 44 by the horizontally projected area 49 of the representative portion of the upper face 44. By "representative portion" is meant a portion of the upper face 44 which is representative with respect to the blowing of air towards the web 18, i.e. disregarding for example the air inlet part of the blow box. The degree of perforation, may, for example, be 1 .5%. The degree of perforation can be varied to suit the weight, dryness, etc. of the web 18 to be dried. Often the degree of perforation of the first lower blow box 26 would be 0.5-3.0%. The second type of blow openings 48 being non-inclined type of blow openings and having a diameter of 1 .8 to 3.1 mm typically constitute at least 20% of the total degree of perforation of the first lower blow boxes 26, and typically 30-70 % of the total degree of perforation of the first lower blow boxes 26. The first type of blow openings 46 being inclination type of blow openings may typically constitute at least 30% of the total degree of perforation of the first lower blow boxes 26, and typically 40-80 % of the total degree of perforation of the first lower blow boxes 26.

For example, considering an area of the representative portion 49 of 5000 mm ² , and a degree of perforation of 2 %, the total area of the blow openings 46, 48, would be 100 mm ² . If the first type of blow openings 46 would constitute 50% of the degree of perforation that would correspond to 50 mm ² . This means that the second type of blow openings 48 would have a total open area corresponding to the remaining 50 mm ² , which, with blow openings 48 of a diameter of 2.5 mm, would correspond to about ten blow openings 48, each having an open area of about 4.9 mm ² .

Fig. 4 is an enlarged side view of the area IV of Fig. 1 and illustrates a second drying deck 30 of the second drying zone 6 illustrated in Fig. 1 . The second drying deck 30 comprises the second lower blow boxes 32 arranged below the web 18, and the second upper blow boxes 34 arranged above the web 18. The second lower blow boxes 32 blow hot drying air towards the web 18 vertically upwards towards web 18, illustrated by arrows VU in Fig. 4. The second lower blow boxes 32 of the second drying deck 30 exert a lower fixation force on the web 18 compared to the first lower blow boxes 26 of the first drying deck 24, illustrated in Figs. 2 and 3. The fixation force exerted on the web by the second lower blow boxes 32 is normally rather low, or even non-existing. Returning to Fig. 4, the hot drying air supplied from the second lower blow boxes 32 lifts the web to a height at which the weight of the web 18 is in balance with the lifting force of the hot drying air supplied by the second lower blow boxes 32. Typically, the average distance, or height H3, between the lower side of the web 18 and the upper surface of the second lower blow boxes 32 is 4 to 15 mm. Since there is a limited or even non- existing fixation force exerted by the second lower blow boxes 32 on the web 18, the vertical position of the web 18 will tend to fluctuate, during operation of the drying box 1 , somewhat more when passing the second drying decks 30, compared to when passing the first drying decks 24. Hence, the web 18 is transported horizontally along the second drying deck 30 in a relatively free manner, with some movement in the vertical direction, meaning that the web 18 is subjected to some stretching forces. The second type of upper blow boxes 34 blow hot drying air towards the web 18 vertically downwards towards web 18, illustrated by arrows VD in Fig. 4. Typically, the average distance, or height H4, between the upper side of the web 18 and the lower surface of the second upper blow boxes 34 is 5 to 80 mm. The hot drying air blown by the blow boxes 32, 34 is evacuated via gaps S formed between horizontally adjacent blow boxes 32, 34.

Fig. 5 is a schematic top view, and illustrates a second lower blow box 32 as seen in the direction of the arrows V-V of Fig. 4. An arrow P illustrates the intended path along which the web, not shown in Fig. 5, is to pass over an upper face 54 of the second lower blow box 32. The upper face 54 extends between the sides 56, 58 of the blow box 32 and comprises blow openings 60 of the "non-inclined type" that are distributed over the upper face 54. By "non- inclined type" is, in accordance with the previous definition, meant that at least 80 % of the air blown from those blow openings 60 is blown at an angle to the upper face 54 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 54, from the blow openings 60 of the non-inclined type. In the second lower blow box 32 at least 75% of the total flow of air supplied thereto is blown from blow openings of the non-inclined type. In the embodiment illustrated in Fig. 5, 100% of the total flow of air supplied thereto is blown from the blow openings 60 of the non-inclined type. The blow openings 60 may be evenly distributed over the face 54, but may also be distributed in an uneven manner. As can be seen from Fig. 5, the concentration of blow openings 60 (blow openings per square centimetre of upper face 54) is somewhat higher adjacent to the sides 56, 58.

When seen from above, as in Fig. 5, the blow openings 60 of the blow box 32 have a somewhat elliptic appearance. However, as is sees in the enlarged portion of Fig. 5, the shape of the blow openings 60 is not really elliptical, but rather has an irregular and partly broken shaped periphery.

Six blow openings 60 are shown in the enlarged portion of Fig. 5. Each blow opening 60 is provided with a shaped contour 62 in the form of an inwardly protruding portion 64, which will be described in connection to Figs 6c-e below. The inwardly protruding portion 64 shown in Fig. 5 has an irregular shape and may also be referred to as a broken periphery portion due to its cracked or broken appearance. The inwardly protruding portion 64 is arranged at about one half of the periphery 65 of the blow opening 60. The other half of the periphery 65 is flush with the outer face 54 of the blow box wall. The surprising effect of providing the blow openings 60 with shaped contours 62 is that noise arising when gas blown through the blow openings 60 and towards the web is lowered or even damped out. The gas is usually air and will hereinafter be referred to as air.

The characteristic measure of the blow openings 60 shown in Fig. 5 is between 1 .5 to 5.5 mm, preferably between 1 .8 to 3.1 mm. The characteristic measure of such a shape, i.e. a shape which is not a round opening, relates to the diameter of a round opening having the same open area as the opening in question. Hence, for example, a square opening having a side of 2.2 mm would have an open area of about 4.9 mm ² . A round hole with that same open area of 4.9 mm ² would have a diameter of 2.5 mm. Thus, the characteristic measure of the square opening having a side of 2.2 mm would in fact be 2.5 mm, since 2.5 mm is the diameter of a round hole having the same open area as the square opening in question. In other words, to arrive at the characteristic measure for the somewhat elliptical shaped blow openings 60 shown in Fig. 5 the opening area of the blow openings 60 should be measured and converted to a corresponding circular area, and thereafter the characteristic measure is given as the diameter of the circular opening.

According to one embodiment, the blow openings 60 have a

characteristic measure of 2.0 to 2.8 mm. According to a further embodiment, the blow openings 60 have a characteristic measure of 2.2 to 2.7 mm. The blow openings 60 blow the hot drying air vertically upwards to form the flows VU.

It has been found that the heat transfer of the second drying zone 6 is considerably higher than that of the first drying zone 4. The first drying zone 4, on the other hand, provides a more stable control of the forwarding of the web 18, resulting in less stretching forces being exerted on the web 18. The tensile strength of the web 18 tends to increase with decreasing moisture content. Hence, the web 18 is comparably weak adjacent to the inlet 20 of the drying box 1 , illustrated in Fig. 1 , and is comparably strong adjacent to the outlet 22 of the drying box 1 . In the first drying zone 4 the web 18 is, hence, dried under low stretching conditions, with a quite stable path of the web, until the web has been dried to, for example, a dry solids content of about 55-80%. Then, with the web 18 having obtained a higher tensile strength, the web 18 is dried in the second drying zone 6 at conditions of higher stretching, but also with a very high heat transfer, making the drying efficient.

It is possible to provide some portions of the first lower blow boxes 26 with blow openings similar to the blow openings in the second lower blow boxes 32 to provide an improved heat transfer also at in the first zone 4.

One way of accomplishing blow openings 60 with a shaped contour 62, as illustrated in Fig. 5, will now be described with reference to Figs 6a-e. Fig. 6a shows a circular blow opening 60 which is provided in a blow box face 54 of a second lower blow box 32. In other words, the blow opening 60 shown in Fig. 6a is one of the blow openings 60 shown in Fig. 5 prior to providing the blow opening 60 with a shaped contour 62. The blow opening shown in Fig. 6a is also referred to as non-modified circular blow opening 60.

The blow opening 60 comprises a lateral face 66 arranged between an outlet opening 68 and an inlet opening 70. The outlet opening 68, or outlet periphery of the blow opening 60, faces the web during use of the drying box. The inlet opening 70, or inlet periphery of the blow opening 60, faces the interior of the blow box 32. Gas inside the blow box 32 is blown towards the web during use of the drying box. The gas enters the blow opening 60 via the inlet opening 70 and is blown towards the web via the outlet opening 68.

Fig. 6b shows the blow opening in Fig. 6a which is about to be provided with a shaped contour by means of a periphery modifying device 72. The periphery modifying device 72 is a rod-shaped tool having a diameter matching the diameter of the non-modified circular blow opening 60. By matching is meant that the diameter of the rod-shaped periphery modifying device 72 is about the same as, or slightly smaller than, the diameter of the blow opening 60 before the blow opening 60 is provided with a shaped contour, thereby the periphery modifying device 72 fits in the blow opening 60 and abuts to the entire lateral face 66 or at least to a large portion of the lateral face 66 of the blow opening 60. The periphery modifying device 72 illustrated in Fig. 6b approaches the blow opening 60 from above. An axis 74 along the rod shaped periphery modifying device 72 is essentially perpendicular to a plane 76 of the blow box wall 54 as the periphery modifying device 72 is inserted in the blow opening 60.

After being inserted in the blow opening 60, the periphery modifying device 72 is tilted toward one side such that the axis 74 along the periphery modifying device 72 approaches the plane 76 of the blow box wall 54. Fig. 6c illustrates tilting of the periphery modifying device 72 towards the left. A first portion 78 of the lateral face 66, which is a portion at the left side of the periphery modifying device 72 in Fig. 6c, is thereby forced downwards, towards the interior of the blow box 32, by the periphery modifying device 72. In addition, a second portion 80 of the lateral face 66, which is a portion at the right side of the periphery modifying device 72 in Fig. 6c, is forced upwards, towards the outside of the blow box 32, by the periphery modifying device 72. In other words, Fig. 6c illustrates the periphery modifying device 72 inserted in the blow opening 60 and tilted to break the lateral face 66 of the blow opening 60.

At a predetermined angle τ between the longitudinal axis 74 of the periphery modifying device 72 and the plane 76 of the blow box wall 54, tilting of the periphery modifying device 72 is interrupted and the periphery modifying device 72 is removed from the blow opening 60. In Fig. 6c the predetermined angle τ is about 45°. The predetermined angle τ at which the periphery modifying tool 72 is tilted varies with for instance blow box wall 32 material and thickness. For instance, the tilt angle τ may be from about 30° to about 60°.

To reduce the risk of damaging the web, the outwardly protruding edge 80 is grinded off, which is illustrated in Fig. 6d. Thus, the second portion 80 of the lateral face, i.e. the outwardly protruding periphery edge 80 which is formed by the tilting of the periphery modifying device 72 shown in Fig. 6c, is removed by grinding the blow box face 54, which is the upper outside surface of the blow box 32. Preferably several or all blow openings 60 of one blow box 32 is modified using the periphery modifying device 72 according to the steps illustrated in Figs 6a-c, thereafter the grinding step illustrated in Fig. 6d is carried out for all or several blow openings. Thereafter the blow box may be varnished giving a durable surface. Fig. 6e shows a blow opening 60 which has been provided with a shaped contour 62 according to the steps in Figs 6a-d. The shaped contour 62 comprises an inwardly protruding portion 64, at one side of the blow opening 60, i.e. the left side of the blow opening 60 as illustrated in Fig. 6e, and a tapering portion 82 at the other side of the blow opening 60, i.e. the right side of the blow opening 60 as illustrated in Fig. 6e.

Alluding to the method for obtaining the blow openings 60 with a shaped contour 62 described with reference to Figs 6a-e, the blow openings 60 may also be referred to as broken blow openings, cracked blow openings or collapsed blow openings.

Moreover, two or several blow openings 60 of a blow box 32 may be provided with shaped contours simultaneously using a tool (not shown) having several periphery modifying devices arranged at the same tool. For instance a pair of pliers of the kind called external circlip pliers may be used for providing two adjacent blow openings with shaped contours.

Fig 7a shows the blow opening 60 in Fig. 6a, i.e. a blow opening 60 in a blow box face 54 of a second lower blow box 32 before the blow opening 60 is provided with a shaped contour. The blow opening 60 shown in Fig. 7a is circular. The periphery 65 of the blow opening 60 comprises the outlet opening 68, i.e. the periphery portion of the blow opening 60 facing the web 18 during use of the lower blow box 32 in a drying box (see Fig. 1 ). Further, the periphery 65 comprises the inlet opening 70, facing the interior of the blow box 32, and the lateral face 66 connecting the outlet opening 68 and the inlet opening 70.

Fig. 7b shows the blow opening 60 in Fig. 7a after it has been provided with a shaped contour 62 according to the steps described in connection to Fig. 6b and Fig. 6c. The outwardly protruding portion 80 shown in Fig. 7b has not yet been grinded off from the blow opening 60.

Figs 8a-d show four alternative embodiments of a blow opening 60 of a second lower blow box 32 provided with a shaped contour 62 for the purpose of dampening or reducing noise in the second lower blow box 32. The direction of the air flow (not shown in Figs 8a-d) is thus directed essentially vertically upwards through the blow opening 60. Apart from the shape of the contoured blow openings 60 shown in Figs 8a-d, the blow box 32 has the same properties and functions as the second lower blow box 32 illustrated and described with reference to Fig. 1 and Figs 4-5 above. The material thickness at the upper face, denoted t in Figs 8a-d, is about 1 mm for the embodiments illustrated in Figs 8a-d. A preferred material thickness t for a blow box 32 of the kind illustrated here is about 0.7 - 2.0 mm, more preferably about 0.8 - 1 .5 mm. The blow box material of the face is preferably homogeneous along the thickness t of the face. The blow box material of the face is preferably made in one piece. The diameter of the non- modified blow opening 60 is denoted d in Figs 8a-d and is about 1 .8 mm in the embodiments shown in Figs 8a-d.

Fig. 8a shows a blow opening 60 at which the inlet opening 70, i.e. the periphery portion facing the interior of the blow box 32 has been provided with a shaped contour 62. The shaped contour 62 includes the entire inlet opening 70 and about half of the lateral surface 66 between the inlet opening 70 and the outlet opening 68. The depth of the blow opening 60 at which the shaped contour 62 is provided is denoted f, and the depth of the blow opening 60 which is not provided with a shaped contour is denoted e. In other words, the total material thickness t of the face 54 is equal to e+f, as seen in Fig. 8a. As mentioned above the total material thickness t of the face 54 in Fig. 8a is 1 .0 mm and it is thus realized that the embodiment shown in Fig. 8a has f = 0.5 mm and e = 0.5 mm. The inclination ω of the shaped contour 62 is about 45° to a normal to the blow box wall 32.

In other words, in the embodiment shown in Fig. 8a, the shaped contour 62 has the shape of an inclined counter bore on the interior side of the blow opening 60 in the lower blow box 32.

Fig. 8b shows a blow opening 60 at which the outlet opening 68, i.e. the periphery portion facing the interior of the blow box 32 has been provided with a shaped contour 62. The shaped contour 62 includes the entire outlet opening 68 and about three quarters of the lateral surface 66 between the outlet opening 68 and the inlet opening 70. The depth of the blow opening 60 at which the shaped contour 62 is provided is denoted f, and the depth of the blow opening 60 which is not provided with a shaped contour is denoted e. Just as in Fig. 8a, the total material thickness t of the face 54 is equal to e+f, and as mentioned above is equal to 1 mm. Further, the embodiment shown in Fig. 8b has f = 0.75 mm and e = 0.25 mm. The inclination ω of the shaped contour 62 is about 45° to a normal to the blow box wall 32.

Thus, in the embodiment shown in Fig. 8b, the shaped contour 62 has the shape of a rather deep inclined counter bore on the exterior side of the blow opening 60 in the lower blow box 32.

Fig. 8c shows a blow opening 60 at which the inlet opening 70, i.e. the periphery portion facing the interior of the blow box 32, has been provided with a shaped contour 62 in the form of a rounded periphery. The shaped contour 62 thus includes the entire inlet opening 70. The radius r of the rounded inner periphery is about 0.7 mm. In other words the radius r is smaller than the material thickness t, however still the entire lateral face 66 is contoured due to material abatement during formation of the shaped contour 62. Alternatively it is possible to have a radius r which is equal to the material thickness t, or to have a radius r which is larger than the material thickness t.

Fig. 8d shows a blow opening 60 at which the inlet opening 70, i.e. the periphery portion facing the interior of the blow box 32 has been provided with a shaped contour 62. The shaped contour shown in Fig 8d comprises the inlet opening 70 and about three quarters of the lateral face 66. The last quarter of the lateral face 66, i.e. the portion of the lateral face which is next to at the outlet opening 68, is thus not provided with a shaped contour. The depth e of the portion of the blow opening 60 which is not provided with a shaped contour is thus about 0.25 mm in Fig. 8d. The shaped contour shown in Fig. 8d is not a radius but rather has an exponential increasing diameter, as seen from the outlet opening 70 towards the inlet opening 68. In the embodiments shown in Figs 8a-d the contoured opening is symmetrical around the centre of the blow opening 60. In addition, it is possible to provide the contoured opening non-symmetrical with respect to the centre of the opening.

Even though the embodiments illustrated in Figs 8a-d are described with reference to the second lower blow box 32 illustrated in Fig. 4 and Fig. 5 it is realised that the blow openings 60 in Fig. 5 that appear elliptical are circular for the embodiments illustrated in Figs 8a-d. Thus, if a lower blow box 32 having blow openings 60 with shaped contours 62 according to anyone of the embodiment in Figs 8a-d is seen from above, corresponding to the view in Fig. 5, the blow openings 60 will appear circular.

As mentioned above the shaped contours shown in Figs 8a-b has the shape of inclined counter bores on the exterior side, i.e. at the outlet opening 68, of the blow opening 60. In addition, the shaped contours 62 shown in Figs 8c-d are, in the context of this application, to be seen as counter bores. In other words, counter bores may be provided at the inlet opening 70 or at the outlet opening 68 of the blow box opening, or even at both the inlet opening 70 and the outlet opening 68 of a blow box opening 60. According to one embodiment the counter bore is essentially uniform along the entire inlet or outlet opening periphery.

Any suitable tool may be used to provide the shaped contours 62, i.e. the counter bores, of the blow openings 60 illustrated in Figs 8a-d. For instance, a punching tool may be used. Alternatively, a drilling tool or a grinding tool may be used to provide the shaped contour 62 shown in Figs 8a- d.

Fig. 9 shows yet another embodiment of a blow opening 60 which is provided with a shaped contour 62. Apart from the shape of the contoured blow opening 60 shown in Fig. 9, the blow box 32 has the same properties and functions as the second lower blow box 32 illustrated and described with reference to Fig. 1 and Figs 4-5 above.

A periphery modifying device 72 in the form of a screwdriver for so called Phillips© screws is shown in Fig. 9 and has been used for providing the shaped contour 62 of the blow opening 60 shown in Fig. 9. The outlet opening 68 shown in Fig. 9 has accordingly, been provided with four notches 84 corresponding to the shape of the screwdriver. The notches 84 are arranged in the lateral face 66 of the blow opening 60 and are extended from the outlet opening 68 to the inlet opening 70. Each notch 84 forms an inwardly protruding portion 84 at the inlet opening 70. The inwardly protruding portions 84 extend towards the interior of the blow box 32. The inwardly protruding portions 84 originate from the screwdriver being forced downwards towards the blow opening 60. Thus, the size of the screwdriver, i.e. the diameter of the screwdriver, is larger than the characteristic measure of the blow opening 60. The periphery modifying device 72 in the form of a screwdriver as shown in Fig. 9 is an example of a punching device for providing a shaped contour 62 at a blow box opening 60.

Fig. 10 illustrates yet another periphery modifying device 72 in the form of a punching device. The punching device comprises a central punching portion 86 having a sharp periphery edge 90 which may be used to punch a blow opening in a blow box 32. Four upwardly slanting punching portions 88 are arranged at the periphery edge 90 and may be used to provide notches, similar to the notches 84 shown in Fig. 9, to a blow opening of a blow box. It is possible to use the punching device in Fig. 10 to provide an existing blow opening with a shaped contour, or to punch a shaped contour in conjunction with punching the blow opening.

Alternatively, which is not illustrated, it is possible to provide a shaped contour in a blow box wall prior to providing the blow opening itself.

Thereafter, a blow opening may be provided at predetermined location in the blow box wall, which corresponds to the location of the shaped contour to arrive with a blow opening provided with a shaped contour.

The degree of perforation, as defined hereinabove, may, for example, be 1 .5% in the second lower blow box 32. The degree of perforation can be varied to suit the weight, dryness, etc. of the web 18 to be dried. Often the degree of perforation of the second lower blow box 32 would be 0.5-3.0%. The blow openings 60 having a characteristic measure of 1 .8 to 3.1 mm typically constitute at least 75% of the total degree of perforation of the second lower blow boxes 32, and typically 80-100 % of the total degree of perforation of the second lower blow boxes 32. The blow openings 60 having a characteristic measure of 1 .8 to 3.1 mm constitute, for example, 100 % of the total degree of perforation in the exemplary lower blow box 32 illustrated in Fig. 5.

The first upper blow boxes 28 of the first drying decks 24, illustrated in

Fig. 2, and the second upper blow boxes 34 of the second drying decks 30, illustrated in Fig. 4, may have the same general design as the second lower box 32 illustrated in Fig. 5, as indicated by dashed arrows in Fig. 5. Furthermore, the third lower blow boxes 38 and the third upper blow boxes 40 of the cooling zone 8 may also have a similar design as the second lower blow boxes 32 illustrated in Fig. 5, as illustrated by means of dashed arrows. In accordance with an alternative embodiment, the third lower blow boxes 38 may have a similar design as the first lower blow boxes 26 illustrated in Fig. 3, as illustrated by means of a dashed arrow.

Similarly, the first lower blow boxes 26, illustrated hereinbefore with reference to Fig. 3, may also be provided on its upper face 44 with blow openings 48 having a characteristic measure of for instance 2.5 mm and being provided with a shaped contour. Those blow openings 48 would behave in a similar manner as the blow openings 60, and provide an improved heat transfer over prior art blow boxes having blow openings of a diameter of, for example, 5 mm. The groove perforations 46 of the first lower blow box 26 have a somewhat different purpose, namely that of stabilizing the web 18, and the diameter of those blow openings 46 may thus be influenced by other parameters, possibly resulting in a different hole diameter than the blow openings 48.

The above mentioned average distances H1 , H2, H3, H4, all refer to the shortest distance between the face 44, 54 of the respective blow box 26, 28, 32, 34 and the web 18.

Fig. 1 1 illustrates a vertical cellulose pulp drying box 201 in which a wet pulp web 18 is dried by means of hot air while travelling along a number of drying sections 224, that may, in a vertical cellulose pulp drying box 201 , be referred to as drying windings 224. The cellulose pulp web 18 is dried in the vertical cellulose pulp drying box 201 while travelling vertically upwards and downwards along the drying windings 224 between upper turning rolls 212 and lower turning rolls 216.

The vertical drying box 201 may typically comprise 4-80 windings 224, for example 40 windings 224. For clarity purposes a smaller number of windings 224 are illustrated in Fig. 1 1 , and the middle section of the drying box 201 is cut away, which is illustrated by vertical dotted lines in Fig. 1 1 .

A wet pulp web 18 enters the drying box 201 via an inlet 220 arranged in a first side wall 210 of a housing 202. In the embodiment of Fig. 1 1 the inlet 220 is arranged in the central portion of the side wall 210, but the inlet 220 may, in an alternative embodiment, be arranged in another position along the height of the side wall 210. The web 18 is, after entering the housing 202 via the inlet 220, forwarded essentially vertically upwards, as illustrated with an arrow P in Fig. 9, in the drying box 201 until the web 18 reaches an upper turning roll 212. The web 18 is turned around the upper turning roll 212 and travels essentially vertically downwards in the drying box 201 until the web 18 reaches a lower turning roll 216 at which the web 18 is again turned. In this manner the web 18 is fed through the housing 202 and travels vertically upwards and downwards in an alternating manner from the inlet 220 at the first side wall 210 of the housing 202 to an outlet 222 arranged in a second side wall 214 of the housing 202. The dried web 18 leaves the drying box 201 via the outlet 222 which, in the embodiment of Fig. 1 1 , is arranged in the lower portion of the second side wall 214. The outlet 222 may, in an alternative embodiment, be arranged in another position along the height of the side wall 214.

The web 18 is dried by means of air blown from blow boxes 32 arranged to the left and to the right of each winding 224, as will be described in more detail hereinafter with reference to Fig. 12. As is seen in Fig. 1 1 the length of the windings 224 is not constant throughout the entire drying box 201 . Those windings 224 that are arranged adjacent to the inlet 220 have a shorter length than the windings 224 arranged in the other parts of the drying box 201 . As illustrated in Fig. 1 1 that winding 224 which is arranged immediately after the inlet 220 is the shortest one, and is followed by a stepwise increase in the length of the following four windings 224. The sixth winding 224 and the windings 224 following thereafter, have a full length. With a stepwise increase in the length of the windings 224, as seen in the direction of web travel, the risk of web break is reduced in that portion of the drying box 201 which is closest to the inlet 220, where the web 18 is relatively heavy, due to a large water content, and fragile. Thus, having shorter windings 224 adjacent to the inlet 220 decreases the risk of web breaks. It is, however, possible to have the same length of all windings 224 in the entire drying box 201 . The vertical length of each winding 224, i.e. the vertical distance between an upper turning roll 212 and a lower turning roll 216, may typically be 2-60 meters.

Optionally, the drying box 201 could be provided with a first drying zone 204, comprising the first five windings 224, and a second drying zone 206, comprising the remaining windings 224. The two drying zones 204, 206 could be provided with blow boxes of different mechanical design, and/or could be supplied with drying air of different temperatures, and/or could be supplied with different relative amounts of drying air, and/or could have different lengths of the windings 224, to achieve low risk of web breaks and optimum drying both in the first drying zone 204, in which the web 18 is relatively heavy and has a high water content, and in the second drying zone 206, in which the web 18 is relatively dry, and has a lower weight.

Fig. 12 is an enlarged side view of the area XII of Fig. 1 1 and illustrates a portion of a winding 224 in which the web 18 travels vertically downwards. Blow boxes 32 are arranged to the left and to the right of the web 18 and discharge hot air onto the web 18 from the left, illustrated by arrows VL, and from the right, illustrated by arrows VR. The distance D between the web 18 and the blow boxes 32 may typically be 4 to 50 mm, preferably 5 to 30 mm, and most preferably 5 to 20 mm. The hot drying air blown by the blow boxes 32 is evacuated via gaps S formed between vertically adjacent blow boxes 32. The blow boxes 32 are of the type which is illustrated in Fig. 5, although the blow boxes 32 are arranged in the drying box 201 for blowing drying air from the side, in a horizontal direction, instead of upwards as in the drying box 1 , and comprises blow openings 60 provided with shaped contours that are distributed over the face 54, which is adapted to face the web 18, of the respective blow box 32. The blow openings 60 distributed over the face 54 of the blow box 32 may have a characteristic measure of 1.8 to 3.1 mm.

According to one embodiment, the blow openings 60 have a characteristic measure of 2.0 to 2.8 mm. According to a further embodiment, the blow openings 60 have a characteristic measure of 2.2 to 2.7 mm.

Figs 13a-b show an example of a blow box opening 60 provided with a shaped contour 62 and an example of a periphery modifying device. Fig. 13a shows two views of a portion 132 of a blow box 32 and a periphery modifying device, i.e. a punching tool 172, for providing blow openings 60 of the blow box 32 with shaped contours. Fig. 13a illustrates the blow box portion 132 prior to the openings 60 being provided with shaped contours.

The left view of Fig. 13a shows the blow box portion 132 and the punching tool 172 in perspective, and the right view of Fig. 13a shows the blow box portion 132 and the punching tool 172 from the side. In addition, the right view of Fig. 13a shows an anvil 300 arranged below the blow box portion 132. The blow box 32 shown in Fig. 13a may be the same kind of blow box as the second lower blow box 32 described in connection to Fig. 5 above, and the blow box 32 illustrated in Fig. 13a may be arranged in a cellulose pulp drying box of the same kind as the cellulose pulp drying box 1 described in connection to Fig. 1 above. Thus, the upper face 54 of the blow box portion 132 shown in Fig. 13a comprises blow openings 60 of the "non-inclined type" that are distributed over the upper face 54. The blow box portion 132 shown in Fig. 13a is illustrated with four blow box openings 60. The blow openings 60 may be provided in the blow box 32 according to any suitable known technique.

An arrow A1 in the right view of Fig. 13a illustrates that the punching tool 172 is approaching the blow box portion 132, i.e. the punching tool 172 has not yet punched the openings 60.

The punching tool 172 is designed to allow several blow box openings 60 to be provided with shaped contours simultaneously. For this purpose the punching tool 172 comprises a base portion 302 provided, on its side 303 facing the blow box 32 during punching, with four circular truncated cones 304. Each truncated cone 304 comprises a circular end portion 306 and a lateral area 308. As with all circular cones, the circumference of the lateral area 308 is increasing towards the base of the truncated cone 304, hence the circumference of the lateral area 308 decreases towards the circular end portion 306 of each truncated cone 304. The blow box openings 60 shown in Fig. 13a are circular and have a characteristic measure, i.e. a diameter, of 1 .5 to 5.0 mm, preferably 2.0 to 3.0 mm, more preferably 2.2 to 2.7 mm. The end portions 306 of each one of the truncated cones 304 are designed to fit into the respective blow box opening 60. Thus, the diameter of the end portions 306 of the truncated cones 304 is slightly smaller than the diameter of the blow box openings 60. The circular end portions 306 may for instance have a diameter of 1 .0 to 4.5 mm, preferably 1 .3 to 2.7 mm, more preferably 1.5 to 2.0 mm. The blow box portion 132 illustrated in the left view of Fig. 13a is chosen such that one of the truncated cones 304, i.e. the truncated cone 304 at the extreme right in the left view of Fig. 13a, is shown cut through and so is one of the blow box openings 60 shown in the left view of Fig. 13a.

Fig. 13b illustrates the same blow box opening 60 as shown in the right view of Fig. 13a, however in Fig. 13b the punching tool 172 has been forced towards the opening 60 to provide the opening 60 with a shaped contour 62 in the form of an inclined counter bore. The arrow A2 in Fig. 13b illustrates that the punching tool 172 is lifted from the blow box opening 60 after the punching action has taken place. The anvil 300 makes sure that the underside of the blow box portion 132, i.e. the side of the blow box portion 132 that is not being punched, is kept unaffected or nearly unaffected.

The height of the truncated cones 304 is denoted h in Fig. 13b and measured perpendicular to the side 303 of the base portion 302, from the side 303 to the end portion 306. The height h may for instance be 0.4 to 4.5 mm, preferably 0.5 to 2.5 mm, more preferably 0.6 to 1 .2 mm. For instance, if the thickness of the blow box wall, denoted b in Fig. 13b, is about 1.0 mm it may be suitable that the height h of the truncated cone 304 is about 0.9 mm.

Naturally, the truncated cones 304 should be arranged on the base portion 302 of the punching tool 172 in a pattern that corresponds to the pattern of the blow box openings 60.

The punching tool 172 illustrated in Figs 13a-b may be used for punching a blow box 32 from the outside of the blow box or from the inside of the blow box 32. Further, the punching tool 172 illustrated in Figs 13a-b may be used for punching the blow openings of a lower blow box as well as the blow openings of an upper blow box, and for punching blow openings of blow boxes in a vertical dryer such as the dryer shown in Fig. 1 1 above. It will be appreciated that numerous variants of the above described embodiments are possible within the scope of the appended claims.

Several embodiments of blow openings provided with a shaped contoured are described above. In a situation where two or more

embodiments are equally suitable with regard to the situation, i.e. blow box dimension and material, or whether or not the blow box is assembled, it is possible to choose the embodiment which gives favourable heat transfer properties.

Hereinbefore it has been described that the blow openings 60 in the second drying zone 6 are provided with a shaped contour 62 to dampen noise. It may not be necessary to provide all blow openings 60 in the second drying zone 6 with a shaped contour 62. For instance, noise may tend to arise in the stagnation portion of the drying box 32. Thus, in one embodiment only the blow openings at the stagnation end of a blow box are provided with a shaped contour.

Blow openings 60 with a shaped contour 62 have been described refereeing to a blow box 32 of the second drying zone 6. However it is possible to provide the so called non-inclination blow openings 48 in the blow boxes 26 of the first drying zone 4, or the cooling zone 8, with a shaped contour.

Hereinbefore it has been described that the blow openings 48, 60, prior to forming all or some of the blow openings 48, 60 with a shaped contour 62, are round holes. It will be appreciated that other shapes than round holes are also possible for use as blow openings. For example, the blow openings 48, 60 could be given the shape of a square, a rectangle, a triangle, an oval, a pentagon, a hexagon, etc. The characteristic measure of such an alternative shape always relates to the diameter of a round blow opening having the same open area as the blow opening in question. Such non-round blow openings may also be provided with a shaped contour for the purpose of sound attenuation, i.e. to dampen noise.

Hereinbefore it has been described that the drying box 1 comprises a first drying zone 4 being provided with the first lower blow boxes 26, or 126, and a second drying zone 6 are provided with the second lower blow boxes 32. It will be appreciated that the drying box may have any number of drying zones, with or without a cooling zone. Furthermore, the drying box may have a single drying zone. Thus, for example, the drying box could be provided with solely first lower blow boxes 26, 126, of the types illustrated in Fig. 3. Furthermore, the drying box could be provided with solely second lower blow boxes 32 of the type illustrated in Fig. 5.

Hereinbefore it has been described, with reference to Fig. 1 , that the drying box 1 comprises a first drying zone 4, a second drying zone 6, and a cooling zone 8. It will be appreciated that many alternative embodiments are possible. For example, it is also possible to design a drying box having a first drying zone 4, and a second drying zone 6, but no cooling zone, in the event that cooling is not required.

As described hereinbefore, the third lower blow boxes 38 of the cooling zone 8 may have the same general design as the first lower blow boxes 26, 126 illustrated in Fig. 3, or the same general design as the second lower blow boxes 32 illustrated in Fig. 5.

Utilizing third lower blow boxes 38 having the same general design as the second lower blow boxes 32 as illustrated in Fig. 5 has the advantage that the heat transfer will be high, similar to the heat transfer illustrated for the second lower blow box 32. Hence, the cooling in the cooling zone 8 becomes very efficient.

Utilizing third lower blow boxes 38 having the same general design as the first lower blow boxes 26 or 126, as illustrated in Fig. 3, has the

advantage that the web 18 leaving the drying box 1 via the outlet 22 is stabilized, with little vertical movement. This may be an advantage to downstream equipment, such as a web position control unit, a web cutter etc. that handle the dried web 18 leaving the drying box 1 .

Hence, if heat transfer has the highest priority in the cooling zone 8, then it would be suitable to utilize as the third lower blow boxes 38 a design of the general type disclosed in Fig. 5. If, on the other hand, web stability has the highest priority in the cooling zone 8, then it would be suitable to utilize as the third blow boxes 38 a design of the general type disclosed in Fig. 3. A further option is to arrange a cooling zone 8 which has one or more cooling decks 36 having lower blow boxes 38 of the design illustrated in Fig. 5 to obtain efficient cooling, with such a cooling zone 8 having a last cooling deck 36, just upstream of the outlet 22 of the drying box 1 , which is provided with third lower blow boxes 38 of a design of the general type disclosed in Fig. 3 to obtain good web stability just before the web 18 leaves the drying box 1 . If web stability has the highest priority, but the drying box has no cooling zone, then a third drying zone could be arranged downstream of the second drying zone. Such a third drying zone would typically have drying decks that would resemble the first drying decks 24 of the first drying zone 4, and have first lower blow boxes 26 or 126 that would yield high web stability. Such a third drying zone would typically have just one to four drying decks.

Hereinbefore it has been described that the drying box 1 has totally 19 drying decks. Of these drying decks totally 8 decks (42 % of the total number of drying decks) belong to the first drying zone 4, and totally 1 1 decks (58 % of the total number of drying decks) belong to the second drying zone 6. In a drying box having two drying zones 4, 6 typically 10-70 % of the total number of drying decks would belong to the first drying zone 4 and be provided with first lower blow boxes 26 or 126 of the type illustrated in Fig. 3, and, correspondingly, typically 30-90 % of the total number of drying decks would belong to the second drying zone 6 and be provided with second lower blow boxes 32 of the type illustrated in Fig. 5. Normally, the first drying zone 4 would only have that many drying decks that are required for the web 18 to obtain a tensile strength being sufficient for the second drying zone 6. In case there is a third, and even fourth drying zone, those would normally reduce the number of drying decks of the second drying zone. Typically the first drying zone 4 would comprise at least two first drying decks 24.

Hereinbefore, it has been described that the first lower blow boxes 26 would be provided with inclination type blow openings 46 of the "groove perforation" type as disclosed in US 4,837,947, or inclination type blow openings 146 of the "eyelid perforation" type disclosed in WO 97/16594. It will be appreciated that the inclination type blow openings 46 may also have an alternative design. An example of such an alternative design is disclosed in US 5,471 ,766. In Fig. 6 of US 5,471 ,766 a blow box is disclosed which has a central V-shaped groove, which is similar to that of US 4,837,947, but which has a slightly lower depth.

Hereinbefore it has been described that the gas supplied to the blow boxes 26, 28, 32, 34, 40, 126, is air. It will be appreciated that in some cases the gas supplied to the blow box may be another type of gas, for example air mixed with combustion gases.

It will be appreciated that different types of fixation type of blow boxes could be utilized in the drying box. Hence, a first drying zone could be provided with first lower blow boxes 26, 126 of the type illustrated in Fig. 3. Hence, in the first drying zone a comparably large fixation force would be at hand. A second drying zone could be provided with first lower blow boxes being similar to the type illustrated in Fig. 3 but having a lower fixation force. Such lower fixation force could be achieved, for example, by increasing the number of second type of blow openings 48, 148, such that less drying air passes through the inclination type perforations 46, 146. This would yield a lower fixation force, which may still be acceptable, since the web has already gained an increased tensile strength in the first drying zone. Then a third drying zone commences, such third drying zone having drying decks and second lower blow boxes of the type illustrated in Figs. 4 and 5. Hence, the different types of blow boxes can be arranged in various ways to obtain suitable conditions with regard to the fixation force and the heat transfer for the particular web 18 that is to be dried in the drying box 1 . Thus, a drying box could be provided with two or more drying zones, typically 2 to 10 drying zones.

In Fig. 4 it has been illustrated that each upper blow box 34 is arranged vertically above a respective lower blow box 32. It will be appreciated that other arrangements of upper and lower blow boxes could also be utilized. One example of such an alternative arrangement is a so-called staggered arrangement in which each upper blow box 34 is centred above the gap S between two adjacent lower blow boxes 32.

Hereinbefore it has been described that the first drying zone 4 comprises first lower blow boxes 26, 126, and that the second drying zone 6 comprises second lower blow boxes 32. It will be appreciated that mixing of blow boxes in the respective drying zone is possible. Hence, the first drying zone 4 could, for example, comprise up to 25 % second lower blow boxes 32, and the second drying zone 6 could comprise up to 25 % first lower blow boxes 26, 126. Also other types of lower blow boxes could be comprised in the first and second drying zones. Preferably, in the first drying zone 4, at least 75% of the lower blow boxes should be first lower blow boxes 26, and in the second drying zone 6, at least 75% of the lower blow boxes should be second lower blow boxes 32.

To summarize, the cellulose pulp drying box for drying a web 18 of cellulose pulp comprises blow boxes 26, 32, 126 that are operative for blowing gas towards the web 18 of cellulose pulp for drying the pulp. At least 10% of the total number of blow boxes 26, 32, 126 of the drying box 1 are provided, in their respective face 44, 54, 144 facing the web 18 during use of the drying box 1 , with blow openings 60 having a characteristic measure of 1 .5 to 5.5 mm through which blow openings 60 gas is blown towards the web 18 during use of the drying box 1 . Each blow opening 60 is defined by an outlet opening 68 facing the web 18 during use of the drying box 1 , an inlet opening 70 facing the interior of the blow box 26, 32, 126, and a lateral face 66 arranged between the outlet opening 68 and the inlet opening 70. At least a portion of the lateral face 66 of the respective blow opening 60 is provided with a shaped contour 62 for the purpose of sound attenuation.