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Title:
A CELLULOSE PULP DRYING BOX HAVING BLOW BOXES
Document Type and Number:
WIPO Patent Application WO/2013/089629
Kind Code:
A1
Abstract:
A cellulose pulp drying box for drying a web (18) of cellulose pulp comprises a lower blow box (26) for blowing gas upwards towards the web (18) via blow openings, for drying the pulp in accordance with the airborne web principle. The lower blow box (26) comprises a central zone (58) comprising blow openings (70, 72) forming a first type of perforation, and a first lateral zone (52) comprising blow openings (70, 80) forming a second type of perforation, the second type of perforation being different from the first type of perforation. The blow openings (72) of the central zone (58) forma first degree of perforation, and the blow openings (80) of the first lateral zone (52) form a second degree of perforation which is larger than the first degree of perforation. As alternative to or in combination therewith the first lateral zone (252; 352)comprises blow openings (70; 370) arranged to form an air cushion above the lateral zone (252; 352) to lift the web (18) adjacent to a lateral edge (82) thereof.

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Inventors:
KAMPRIS ROLAND (SE)
OLSSON CHRISTER (SE)
PETERSSON ROLF (SE)
HOLMBERG PER (SE)
Application Number:
PCT/SE2012/051393
Publication Date:
June 20, 2013
Filing Date:
December 14, 2012
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ANDRITZ TECH & ASSET MAN GMBH (AT)
KAMPRIS ROLAND (SE)
OLSSON CHRISTER (SE)
PETERSSON ROLF (SE)
HOLMBERG PER (SE)
International Classes:
F26B13/10; D21C9/18; D21F5/18; F26B13/20
Foreign References:
US4719708A1988-01-19
EP2053663A12009-04-29
US5471766A1995-12-05
US4021931A1977-05-10
US6092304A2000-07-25
Attorney, Agent or Firm:
AWAPATENT AB (P.O. Box 99, Växjö, SE)
Download PDF:
Claims:
CLAIMS

A cellulose pulp drying box for drying a web (18) of cellulose pulp, wherein the drying box (1 ) comprises lower blow boxes (26; 32) that are configured for blowing gas upwards towards the web (18) of cellulose pulp via blow openings, for drying the pulp in accordance with the airborne web principle, c h a r a c t e r i s e d in the drying box (1 ) comprising at least one lower blow box (26; 32) comprising, as seen along a horizontal direction perpendicular to a direction (P) in which the web (18) is to travel over the lower blow box (26; 32);

i. a central zone (58) comprising blow openings (72) forming a first type of perforation, and

ii. at least a first lateral zone (52) comprising blow openings (80) forming a second type of perforation, the second type of perforation being different from the first type of perforation, wherein the blow openings (72) of the central zone (58) of the lower blow box (26; 32) form a first degree of perforation, and the blow openings (80) of the at least a first lateral zone (52) of the lower blow box (26; 32) form a second degree of perforation, the second degree of perforation being larger than the first degree of perforation, and/or the first lateral zone (252; 352) of the lower blow box (232; 332) comprises blow openings (70; 370) arranged to form an air cushion above the lateral zone (252; 352) to lift the web (18) of cellulose pulp adjacent to a lateral edge (82) thereof.

A drying box according to claim 1 , wherein the lower blow box (26; 32) further comprises a second lateral zone (54) comprising blow openings (80) forming a third type of perforation which is different from the first type of perforation of the central zone (58), the central zone (58) being arranged between the first and second lateral zones (52, 54).

3. A drying box according to any one of the preceding claims, wherein the blow openings (72) of the central zone (58) of the lower blow box (26; 32) form a first degree of perforation, and the blow openings (80) of the at least a first lateral zone (52) of the lower blow box (26; 32) form a second degree of perforation, the second degree of perforation being larger than the first degree of perforation, preferably a factor of 1 .1 -2.0 larger than the first degree of perforation.

A drying box according to any one of the preceding claims, wherein the drying box further comprises at least one upper blow box (28) comprising, as seen along a horizontal direction perpendicular to a direction (P) in which the web (18) is to travel below the upper blow box (28);

i. a central zone (66) comprising blow openings (88) forming a fourth degree of perforation, and

ii. at least a first lateral zone (60) having a fifth degree of

perforation, the fifth degree of perforation being smaller than the fourth degree of perforation.

A drying box according to claim 4, wherein the at least one upper blow box (28) further comprises a second lateral zone (62) having a sixth degree of perforation which is smaller than the fourth degree of perforation of the central zone (66), the central zone (66) of the at least one upper blow box (28) being arranged between the first and second lateral zones (60, 62).

A drying box according to any one of the preceding claims, wherein the first lateral zone (252; 352) of the lower blow box (232; 332) comprises blow openings (70; 370) arranged to form an air cushion above the lateral zone (252; 352) to lift the web (18) of cellulose pulp adjacent to a lateral edge (82) thereof.

A drying box according to claim 6, wherein the first lateral zone (252; 352) of the lower blow box (232; 332) further comprises two rows (273, 275) of inclined type of blow openings (70; 370) adapted to blow air flows (IU1 , IU2) toward each other.

8. A drying box according to any one of the preceding claims, wherein the first lateral zone (52; 152) of the lower blow box (26; 32) has a width (LZ1 ; LZ3), as seen in the horizontal direction perpendicular to the direction (P) in which the web (18) is to travel over the lower blow box (26; 32), of 100 to 950 mm.

9. A drying box according to any one of the preceding claims, wherein the first lateral zone (52; 152) of the lower blow box (26; 32) is arranged for extending in under the web (18) a distance (DZ1 ; DZ3) of about 50 to 600 mm from a lateral edge (82) of the web (18).

10. A drying box according to any one of the preceding claims, wherein the drying box comprises a first drying zone (4) comprising a first type of lower blow boxes (26), and a consecutive second drying zone (6) comprising a second type of lower blow boxes (32).

1 1 . A drying box according to any one of the preceding claims, wherein at least some of the blow openings (80) of the first lateral zone (52) have a different size than the blow openings (72) of the central zone (58).

12. A drying box according to any one of the preceding claims, wherein the first lateral zone (152) has a different number of blow openings (172) per m2 of blow box surface (168) area than the central zone (158). 13. A drying box according to any one of the preceding claims, wherein at least one of the at least one lower blow box (26; 232; 332) comprises a combination of inclined type blow openings (70; 370) and non-inclined type blow openings (72; 172; 372). 14. A drying box according to any one of the preceding claims, wherein at least one of the at least one lower blow box (32) comprises solely non- inclined type blow openings (172).

15. A drying box according to any one of the preceding claims, wherein the drying box comprises at least one drying deck (24; 30) in which at least 90% of the total number of lower blow boxes (26; 32) of that drying deck comprises the central zone (58; 158) and the first lateral zone (52; 152).

16. A method of drying a web (18) of cellulose pulp by means of lower blow boxes (26, 32) configured for blowing gas upwards towards the web (18) of cellulose pulp via blow openings, for drying the pulp in accordance with the airborne web principle, c h a r a c t e r i s e d in

utilizing at least one lower blow box (26; 32) comprising, as seen along a horizontal direction perpendicular to a direction (P) in which the web (18) is to travel over the lower blow box (26; 32), a central zone (58) comprising blow openings (70, 72), and at least a first lateral zone (52) comprising blow openings (70, 80),

blowing a first relative gas flow in a first flow pattern towards the web (18) by blow openings (70, 72) of the central zone (58), and

blowing a second relative gas flow in a second flow pattern, which is different from the first flow pattern, towards the web (18) by blow openings (70, 80) of the first lateral zone (52),

wherein the second relative gas flow is larger than the first relative gas flow, and/or the second relative gas flow is blown in the second flow pattern towards the web (18) to form an air cushion lifting a lateral edge (82) of the web (18) of cellulose pulp.

17. A method according to claim 16, further comprising utilizing in the at least one lower blow box (26; 32) a second lateral zone (54) comprising blow openings (70; 80), the central zone (58) being arranged between the first and second lateral zones (52, 54), and blowing a third relative gas flow in a third flow pattern, which is different from the first flow pattern, towards the web (18) by blow openings (70, 80) of the second lateral zone (54). 18. A method according to any one of claims 16-17, wherein the second

relative gas flow is larger than the first relative gas flow, preferably the second relative gas flow is a factor of 1 .1 -2.0 larger than the first relative gas flow.

19. A method according to any one of claims 16-18, further comprising blowing the second relative gas flow in the second flow pattern towards the web to form an air cushion lifting a lateral edge (82) of the web (18) of cellulose pulp.

20. A method according to any one of claims 16-19, further comprising

utilizing at least one upper blow box (28) comprising, as seen along a horizontal direction perpendicular to a direction (P) in which the web (18) is to travel below the upper blow box (28), a central zone (66) comprising blow openings (88), and at least a first lateral zone (60), and blowing a fourth relative gas flow towards the web by blow openings (88) of the central zone (66), and blowing a fifth relative gas flow, which is smaller than the fourth relative gas flow, towards the web (18) by the first lateral zone (60).

21 . A cellulose pulp drying box for drying a web (18) of cellulose pulp,

wherein the drying box (1 ) comprises lower blow boxes (26; 32) that are configured for blowing gas upwards towards the web (18) of cellulose pulp via blow openings (70, 72), and at least one upper blow box (28) for blowing gas downwards towards the web (18) of cellulose pulp via blow openings (88), for drying the pulp in accordance with the airborne web principle, c h a r a c t e r i s e d in the at least one upper blow box (28) comprising, as seen along a horizontal direction perpendicular to a direction (P) in which the web (18) is to travel below the upper blow box (28);

i. a central zone (66) comprising blow openings (88) forming a fourth degree of perforation, and

ii. at least a first lateral zone (60) having a fifth degree of

perforation, the fifth degree of perforation being smaller than the fourth degree of perforation, such that the drying effect at the first lateral zone (60) is lower than the drying effect at the central zone (66) of the at least one upper blow box (28).

Description:
A CELLULOSE PULP DRYING BOX HAVING BLOW BOXES

Field of the Invention

The present invention relates to a cellulose pulp drying box for drying a web of cellulose pulp, wherein the drying box comprises blow boxes that are operative for blowing air towards the web of cellulose pulp for drying the pulp in accordance with the airborne web principle.

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 cellulose pulp 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 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 cellulose pulp drying box without increasing its size, or increasing its size only slightly.

Summary of the Invention

An object of the present invention is to provide a cellulose pulp drying box for drying a cellulose pulp web, the drying box being more efficient in drying the web than the prior art drying boxes.

This object is achieved by means of cellulose pulp drying box for drying a web of cellulose pulp, wherein the drying box comprises lower blow boxes that are configured for blowing gas upwards towards the web of cellulose pulp via blow openings, for drying the pulp in accordance with the airborne web principle. The drying box comprises at least one lower blow box comprising, as seen along a horizontal direction perpendicular to a direction in which the web is to travel over the lower blow box, a central zone comprising blow openings forming a first type of perforation, and at least a first lateral zone comprising blow openings forming a second type of perforation, the second type of perforation being different from the first type of perforation, wherein the blow openings of the central zone of the lower blow box form a first degree of perforation, and the blow openings of the at least a first lateral zone of the lower blow box form a second degree of perforation, the second degree of perforation being larger than the first degree of perforation, and/or the first lateral zone of the lower blow box comprises blow openings arranged to form an air cushion above the lateral zone to lift the web of cellulose pulp adjacent to a lateral edge thereof.

An advantage of this drying box is that it is more efficient in supporting a lateral edge of the web, resulting in the web travelling in a more horizontal manner as seen across its width, resulting in more efficient drying.

Furthermore, problems related to dust generated by a lateral edge of the web touching a lower blow box may be reduced. The advantageous effect may be achieved by the blow openings of the central zone forming a first degree of perforation, and the blow openings of the first lateral zone forming a second degree of perforation which is larger than the first degree of perforation. As alternative to or in combination therewith the first lateral zone comprises blow openings arranged to form an air cushion above the lateral zone to lift the web adjacent to a lateral edge thereof.

According to one embodiment the lower blow box further comprises a second lateral zone comprising blow openings forming a third type of perforation which is different from the first type of perforation of the central zone, the central zone being arranged between the first and second lateral zones. An advantage of this embodiment is that a web may be more efficiently dried, and better supported, at both of its lateral edges.

According to one embodiment the blow openings of the central zone of the lower blow box form a first degree of perforation, and the blow openings of the at least a first lateral zone of the lower blow box form a second degree of perforation, the second degree of perforation being larger than the first degree of perforation. An advantage of this embodiment is that the lifting force will increase in the lateral zone. According to a preferred embodiment the second degree of perforation is a factor of 1 .1 -2.0, preferably 1 .1 -1 .7, larger than the first degree of perforation.

According to one embodiment the drying box further comprises at least one upper blow box comprising, as seen along a horizontal direction perpendicular to a direction in which the web is to travel below the upper blow box, a central zone comprising blow openings forming a fourth degree of perforation, at least a first lateral zone having a fifth degree of perforation, the fifth degree of perforation being smaller than the fourth degree of perforation. An advantage of this embodiment is that a drying effect of the upper blow box at a lateral edge of the web is reduced to compensate, at least partly, for an increased drying effect of the first lateral zone of the lower blow box.

According to one embodiment the at least one upper blow box further comprises a second lateral zone having a sixth degree of perforation which is smaller than the fourth degree of perforation of the central zone, the central zone of the at least one upper blow box being arranged between the first and second lateral zones. An advantage of this embodiment is that over-drying of the web at the lateral edges thereof may be avoided, or at least reduced.

According to one embodiment the first lateral zone of the lower blow box comprises blow openings arranged to form an air cushion above the lateral zone to lift the web of cellulose pulp adjacent to a lateral edge thereof. An advantage of this embodiment is that the web is efficiently supported at its lateral edge.

According to one embodiment the first lateral zone of the lower blow box further comprises two rows of inclined type of blow openings adapted to blow air flows toward each other. An advantage of this embodiment is that a "collision" of air flows blown by two opposing inclined type blow openings generates a lifting force supporting a lateral edge of the web of cellulose pulp.

According to one embodiment the first lateral zone of the lower blow box has a width, as seen in the horizontal direction perpendicular to the direction in which the web is to travel over the lower blow box, of 100 to 950 mm. An advantage of this embodiment is that a suitable support of a lateral edge of the web is achieved.

According to one embodiment the first lateral zone of the lower blow box is arranged for extending in under the web a distance of about 50 to 600 mm from a lateral edge of the web. A first lateral zone extending less than 50 mm under the web may not provide sufficient support for the lateral edge. A first lateral zone extending more than 600 mm under the web may result in unnecessary over drying of the web, without further improving the manner in which the web travels through the dryer.

According to one embodiment the drying box comprises a first drying zone comprising a first type of lower blow boxes, and a consecutive second drying zone comprising a second type of lower blow boxes. An advantage of this embodiment is that the drying zones of the drying box can be adapted to the varying properties if the web, as regards basis weight, strength, etc., in various positions along the dryer.

According to one embodiment at least some of the blow openings of the first lateral zone have a different size than the blow openings of the central zone. An advantage of this embodiment is that the respective degrees of perforation of the first lateral zone and the central zone can be arranged in an efficient manner, optionally utilizing the same pattern of the openings, but with different sizes.

According to one embodiment the first lateral zone has a different number of blow openings per m 2 of blow box surface area than the central zone. An advantage of this embodiment is that the respective degrees of perforation of the first lateral zone and the central zone can be arranged in an efficient manner, optionally utilizing the same size of the openings, but arranged in different patterns.

According to one embodiment at least one of the at least one lower blow box comprises a combination of inclined type blow openings and non- inclined type blow openings. An advantage of this embodiment is that the web is supported and forwarded at a well-defined height above the lower blow box, which reduces the mechanical strain on the web. According to one embodiment at least one of the at least one lower blow box comprises solely non-inclined type blow openings. An advantage of this embodiment is that a high drying effect is achieved.

According to one embodiment the drying box comprises at least one drying deck in which at least 90% of the total number of lower blow boxes of that drying deck comprises the central zone and the first lateral zone. An advantage of this embodiment is that efficient drying and low risk of the web touching the lower blow boxes is achieved throughout the entire drying deck.

A further object of the present invention is to provide a method of drying a cellulose pulp web in a more efficient manner than the methods of the prior art.

This object is achieved by means of a method of drying a web of cellulose pulp by means of lower blow boxes configured for blowing gas upwards towards the web of cellulose pulp via blow openings, for drying the pulp in accordance with the airborne web principle, the method comprising: utilizing at least one lower blow box comprising, as seen along a horizontal direction perpendicular to a direction in which the web is to travel over the lower blow box, a central zone comprising blow openings, and at least a first lateral zone comprising blow openings,

blowing a first relative gas flow in a first flow pattern towards the web by blow openings of the central zone, and

blowing a second relative gas flow in a second flow pattern, which is different from the first flow pattern, towards the web by blow openings of the first lateral zone,

wherein the second relative gas flow is larger than the first relative gas flow, and/or the second relative gas flow is blown in the second flow pattern towards the web to form an air cushion lifting a lateral edge of the web of cellulose pulp.

An advantage of this method is that the web is efficiently dried with little generation of dust particles. The advantageous effect may be achieved by the second relative gas flow being larger than the first relative gas flow. As alternative to or in combination therewith the second relative gas flow is blown in the second flow pattern towards the web to form an air cushion lifting a lateral edge of the web of cellulose pulp.

According to one embodiment the method further comprises utilizing a second lateral zone comprising blow openings in the at least one lower blow box, the central zone being arranged between the first and second lateral zones, and blowing a third relative gas flow in a third flow pattern, which is different from the first flow pattern, towards the web by blow openings of the second lateral zone. An advantage of this embodiment is that the web is supported in an efficient manner at both of its lateral edges.

According to one embodiment the second relative gas flow is larger than the first relative gas flow. According to a preferred embodiment the second relative gas flow is a factor of 1 .1 -2.0, preferably 1 .1 -1 .7, larger than the first relative gas flow. An advantage of this embodiment is that a lateral edge of the web of cellulose pulp is hindered, at least partly, from touching the lower blow box.

According to one embodiment the method further comprises blowing the second relative gas flow in the second flow pattern towards the web to form an air cushion lifting a lateral edge of the web of cellulose pulp. An advantage of this embodiment is that a lateral edge of the web of cellulose pulp will be less prone to touch the lower blow box.

Further objects and features of the present invention will be apparent from the description and the claims.

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 cellulose pulp 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 is a front view of a first drying deck as seen in the direction of the arrow III of Fig. 2.

Fig. 4 is a top view of a first lower blow box.

Fig. 5 is a top view of a first upper blow box.

Fig. 6 is a top view of a second lower blow box. Fig. 7a is a top view of a third lower blow box.

Fig. 7b is an enlarged view of a lateral zone of the third lower blow box. Fig. 7c is a cross-section of the lateral zone of Fig. 7b.

Fig. 7d is an enlarged view of an alternative lateral zone.

Fig. 7e is a cross-section of the lateral zone of Fig. 7d.

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, 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. The third lower blow boxes 38 may, for example, have a design which is similar to that of the first or second lower blow boxes 26, 32, that are described in more detail hereinafter. The third upper blow boxes 40 may have a design which is similar to that of the first or second upper blow boxes 28, 34, that are described in more detail hereinafter. 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 18 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 1 -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 the 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 front view of the first drying deck 24 as seen in the direction of the arrow III of Fig. 2. Drying air is supplied from a fan, not shown for reasons of maintaining clarity of illustration, and enters interior 44 of the first lower blow box 26 at a first end 46 of lower blow box 26, as illustrated by means of an arrow DA1. The air supplied to interior 44 of blow box 26 is blown towards the web 18 as air streams VU and IU via openings, described hereinafter with reference to Fig. 4. Returning to Fig. 3, drying air is also supplied from the fan, not shown, and enters interior 48 of the first upper blow box 28 at a first end 50 of upper blow box 28, as illustrated by means of an arrow DA2. The air DA2 supplied to interior 48 of blow box 28 is blown towards the web 18 as air streams VD via openings, described hereinafter with reference to Fig. 5.

To obtain efficient drying and minimum runability problems, the vertical distances between the web 18 and the respective lower and upper blow boxes 26, 28 should preferably be rather constant at the heights H1 and H2, depicted in Fig. 2, across the width WW of the web 18. It has been found that in the prior art, the web 18 might in some cases, in particular at its lateral ends, deviate from the desired heights H1 , H2, as is illustrated in Fig. 3 with broken lines denoted PA. Such deviation means that the web 18 is not dried in an optimum manner.

The first lower blow box 26 has a total width WB which may typically be 2-16% larger than the width WW of the web 18. Typically, the width WW of the web 18 is 1 -15 meters, more typically 2-12 meters. The first lower blow box 26 is divided into zones along its width WB, i.e., along a horizontal direction being perpendicular to the direction P in which the web 18 travels over the blow box 26. The first lower blow box 26 has a first lateral zone 52 arranged adjacent to the first end 46 of the blow box 26, a second lateral zone 54 arranged at a second end 56 of the blow box 26, and a central zone 58 arranged between the first and second lateral zones 52, 54.

The first upper blow box 28 has a first lateral zone 60 arranged adjacent to the first end 50 of the blow box 28, a second lateral zone 62 arranged at a second end 64 of the blow box 28, and a central zone 66 arranged between the first and second lateral zones 60, 62.

As will be described in more detail hereinafter with reference to Figs. 4 and 5, the arrangement of the lateral zones 52, 54, and of the lateral zones 60, 62 results in the web 18 being carried in an almost horizontal manner over the entire width WW of the web 18, as illustrated with an unbroken line in Fig. 3. Hence, the large deviations from the desired heights H1 , H2 of the prior art, as illustrated with the broken lines PA, may be avoided, or at least

substantially reduced. It is also acceptable if the web 18 travels at a

somewhat higher height H1 above the lower blow box 26 at the lateral zones 52, 54 than at the central zone 58, since such does not tend to influence the drying operation in a negative manner.

Fig. 4 is a top view of the first lower blow box 26 as seen in the direction of arrows IV-IV of Fig. 3. The first lower blow box 26 has an upper surface 68 adapted to face the web 18 and is, as described hereinbefore with reference to Fig. 3, divided into the first lateral zone 52 arranged adjacent to the first end 46, the second lateral zone 54 arranged adjacent to the second end 56 of the blow box 26, and the central zone 58 arranged between the first and second lateral zones 52, 54. Hence, the first and second lateral zones 52, 54 are arranged on opposite sides of the central zone 58.

The central zone 58 is provided with centrally arranged first type of air blow openings 70, which are "inclined type" openings which may, for example, be of a type sometimes referred to as "eyelid perforations" and which are arranged in the upper surface 68. By "inclined type" openings is meant that at least 25% of the air blown from those openings 70 is blown at an angle of less than 60° to the horizontal plane. The eyelid perforations 70, which may have a similar design as the openings referred to as "eyelid perforations 6" in WO 97/16594, and which are described with reference to Figs. 2 and 3 of WO 97/16594, provide the hot drying air blown therethrough with an inclination, resulting in the air streams IU illustrated in Fig. 2 of the present application. In the first lower blow box 26 at least 30 %, often at least 40%, of the total flow of air supplied thereto is blown via eyelid perforations 70.

Continuing with the description of Fig. 4 of the present application, the central zone 58 is provided with a second type of air blow openings 72 that are arranged close to sides 74, 76 of the blow box 26. The second type of openings 72 are of a "non-inclined type" that are distributed over the upper surface 68 within central zone 58. By "non-inclined type" is meant that at least 80 % of the air blown from those openings 72 is blown at an angle to the horizontal plane which is at least 70°. The second type of openings 72 may, for example, be round, square or triangular holes. The second type of openings 72 may, when round, typically have a diameter of 1 to 10 mm. In one example, the second type of openings 72 are round holes with a diameter of 2.5 mm. The second type of openings 72 blow the hot drying air upwards to form the air streams VU, illustrated in Fig. 2.

Returning to Fig. 4, a first degree of perforation of the central zone 58 may be calculated by dividing the total open area of the openings 70, 72 of a representative portion of the central zone 58 by the horizontally projected area 78 of the representative portion of the upper surface 68 within the central zone 58. By "representative portion" is meant a portion of the upper surface 68 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. Typically, the degree of perforation of the central zone 58 would be 0.5-3.0%. In one example the degree of perforation of the central zone 58 is 1 .5%. The degree of perforation can be varied to suit the weight, dryness, etc. of the web 18 to be dried. The second type of openings 72 being non-inclined type of openings 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 openings 70 being inclined type of 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 78 of

4000 mm 2 , and a degree of perforation of 1 .5 %, the total area of the openings 70, 72, would be 60 mm 2 . If the first type of openings 70 would constitute 50% of the degree of perforation that would correspond to 30 mm 2 . This means that the second type of openings 72 would have a total open area corresponding to the remaining 30 mm 2 , which, with openings 72 of a diameter of 2.5 mm, would correspond to about six openings 72, each having an open area of about 4.9 mm 2 .

The first lateral zone 52 is provided with centrally arranged first type of air blow openings 70, being of the eyelid perforation type, and being of a similar design as the first type of openings 70 of the central zone 58.

Furthermore, the first lateral zone 52 comprises a third type of air blow openings 80 that are arranged close to sides 74, 76 of the blow box 26. The third type of openings 80 are of the "non-inclined type" that are distributed over the upper surface 68 within first lateral zone 52. The third type of openings 80 may be round, square or triangular holes, and may, when round, typically have a diameter of 1 to 10 mm. In one example, the third type of openings 80 are round holes with a diameter of 3.1 mm that blow hot drying air upwards to form the air streams VU, illustrated in Fig. 2. Hence, the third type of blow openings 80 of the first lateral zone 52 has a different size than the second type of blow openings 72 of the central zone 58. Thus, the openings 70, 72 of the central zone 58 form a first type of perforation, which is different from a second type of perforation formed by the openings 70, 80 of the lateral zone 52. The third type of blow openings 80 of the first lateral zone 52 are arranged in a pattern which is similar to the pattern of the second type of blow openings 72 of the central zone 58.

In a similar manner as described hereinbefore with respect to the central zone 58, it is possible to calculate a second degree of perforation for the first lateral zone 52. Typically, the degree of perforation of the first lateral zone 52 would be 0.8-3.5%. In one example, with third type of openings 80 having a diameter of 3.1 mm, the degree of perforation of the first lateral zone 52 is 1 .9 %. Thus, the degree of perforation of the first lateral zone 52 is larger than the degree of perforation of the central zone 58. In accordance with one embodiment, the degree of perforation of the first lateral zone 52 is a factor of 1 .1 -2.0, preferably 1 .1 -1 .7, larger than the degree of perforation of the central zone 58. Hence, if, for example, the degree of perforation of the central zone 58 is 1.5% then the degree of perforation of the first lateral zone 52 would be within the range of 1 .65 to 3.0 %.

The second lateral zone 54 may have a similar design as the first lateral zone 52. Hence, in one example the second lateral zone 54 comprises first type of openings 70 of the eyelid perforation type, and third type of openings 80 that are round and have a diameter of 3.1 mm, resulting in a third degree of perforation of the second lateral zone 54 of 1 .9 %. Thus, the degree of perforation of the second lateral zone 54 is larger than the degree of perforation of the central zone 58.

During operation of the drying box 1 drying air DA1 is supplied to the first lower blow box 26. Since the openings 70, 72 of the central zone 58 form a first type of perforation, which is different from a second type of perforation formed by the openings 70, 80 of the lateral zone 52, air will be blown in a first flow pattern from the openings 70, 72 of the central zone 58, which is different from a second flow pattern in which the air is blown from the openings 70, 80 of the lateral zone 52. Furthermore, air will be blown in a third flow pattern from the openings 70, 80 of the second lateral zone 54, which is different from the first flow pattern in which the air is blown from the openings 70, 72 of the central zone 58. Due to the degree of perforation of the first lateral zone 52 being larger than the degree of perforation of the central zone 58 a first relative air flow, in, for example, the unit: m 3 of air per hour and mm 2 of upper surface 68 area, i.e., m 3 /(h, mm 2 ), blown towards the web 18 by openings 70, 72 of central zone 58 will be smaller than a second relative air flow, in m 3 of air per hour and mm 2 of upper surface 68 area, blown towards the web 18 by openings 70, 80 of first lateral zone 52.

Similarly, a third relative air flow, in m 3 of air per hour and mm 2 of upper surface 68 area, i.e., m 3 /(h, mm 2 ), blown towards the web 18 by openings 70, 80 of second lateral zone 54 will be higher than the first relative air flow of the central zone 58. This will cause an improved lifting force on the web 18 at the lateral zones 52, 54, i.e., an improved lifting force adjacent to lateral edges 82, 84 of the web 18, such that the web 18 "floats" in a desired manner, as illustrated in Fig. 3, above the first lower blow box 26. Furthermore, the drying effect will be increased at the lateral edges 82, 84 due to the larger flow of air at the lateral zones 52, 54 compared to the central zone 58. In accordance with one embodiment, the second and third relative air flows blown by the first and second lateral zones 52, 54, respectively, is a factor of 1 .1 -2.0, preferably 1 .1 -1 .7, larger than the first relative air flow blown by the central zone 58.

Typically, the first lateral zone 52 may extend under the web 18 a distance DZ1 of about 50 to 600 mm from the lateral edge 82. Similarly, the second lateral zone 54 may extend under the web 18 a distance DZ2 of about 50 to 600 mm from the lateral edge 84. The blow box 26 has, as described hereinbefore, a width WB which is larger than the width WW of the web 18. The first lateral zone 52 preferably has a total width LZ1 , as seen from the first end 46, of 100 to 950 mm, and the second lateral zone 54 has a total width LZ2, as seen from the second end 56, of 100 to 950 mm. Preferably, the central zone 58 is at least as wide as each of the lateral zones 52, 54. Hence, if, for example, the lateral zones 52, 54 have widths LZ1 , LZ2, respectively, of 400 mm, then the central zone 58 should have a width of at least 400 mm.

In one example, the blow box 26 has a width WB of 5000 mm, the web 18 has a width WW of 4600 mm, each of the first and second lateral zones 52, 54 has a width LZ1 , LZ2, respectively, of 500 mm, and the central zone 58 has a width LC of 4000 mm, resulting in the first lateral zone 52 extending under the web 18 a distance DZ1 of about 300 mm from the lateral edge 82, and the second lateral zone 54 extending under the web 18 a distance DZ2 of about 300 mm from the lateral edge 84.

Fig. 5 is a top view of the first upper blow box 28 as seen in the direction of arrows V-V of Fig. 3. The first upper blow box 28 has a lower surface 86 adapted to face the web 18 and is, as described hereinbefore with reference to Fig. 3, divided into the first lateral zone 60 arranged adjacent to the first end 50, the second lateral zone 62 arranged at the second end 64 of the blow box 28, and the central zone 66 arranged between the first and second lateral zones 60, 62.

The central zone 66 is provided with a fourth type of air blow openings

88 that are distributed over lower surface 86 of the blow box 28 within central zone 66. The fourth type of openings 88 are of the "non-inclined type" and may be round, square or triangular holes, and may, when round, typically have a diameter of 1 to 10 mm. In one example, the fourth type of openings 88 are round holes with a diameter of 5 mm that blow hot drying air

downwards to form the air streams VD, illustrated in Fig. 2.

In a similar manner as described hereinbefore with respect to the central zone 58, it is possible to calculate a fourth degree of perforation for the central zone 66. Typically, the degree of perforation of the central zone 66 would be 0.5-3.0%. In one example, with fourth type of openings 88 having a diameter of 5 mm, the fourth degree of perforation of the central zone 66 is 1 .5 %. The first lateral zone 60 of the first upper blow box 28 has no openings at all. A fifth degree of perforation of the first lateral zone 60 is, thus, 0 %. Hence, the fifth degree of perforation of the first lateral zone 60 is smaller than the fourth degree of perforation of the central zone 66. Similarly, the second lateral zone 62 of the first upper blow box 28 has no openings at all. Hence, a sixth degree of perforation of the second lateral zone 62 is smaller than the fourth degree of perforation of the central zone 66. In accordance with an alternative embodiment, the first and/or second lateral zones 60, 62 may be provided with openings, for example round openings, of such a size and number that the first and/or second lateral zones 60, 62 would still have a lower degree of perforation than the central zone 66.

During operation of the drying box 1 drying air DA2 is supplied to the first upper blow box 28. Due to the degree of perforation of the first and second lateral zones 60, 62 being lower than the degree of perforation of the central zone 66 a fourth relative air flow, in the unit: m 3 of air per hour and mm 2 of lower surface 86 area, blown towards the web 18 by openings 88 of central zone 66 will be higher than a fifth relative air flow of the lateral zones 60, 62. This will cause a reduced drying effect of the lateral zones 60, 62 at the edges 82, 84 of the web 18, which will compensate, at least partly, for the increased drying effect caused by the lateral zones 52, 54 of the first lower blow box 26, as described hereinbefore. Such compensation, with reduced drying effect in the lateral zones 60, 62 of the upper blow box 28, reduces the risk that the web 18 is, adjacent to its lateral edges 82, 84, over-dried, which could result in quality problems. Furthermore, the forces pressing the web 18 downwards towards the first lower blow box 26 will be somewhat reduced at the edges 82, 84, since no air streams are blown downwards from the lateral zones 60, 62, thereby further improving the "floating" of the web 18.

Typically, the lateral zones 60, 62 may extend under the web 18 a respective distance DZ1 , DZ2 of about 50 to 600 mm from the respective lateral edge 82, 84. The lateral zones 60, 62 preferably have respective widths LZ1 , LZ2, as seen from respective ends 50, 64, of 100 to 950 mm. The central zone 66 has a width LC which depends from the width WB of the blow box 28, and the widths of the lateral zones 60, 62, between which the central zone 66 is located.

In accordance with an alternative embodiment, a first upper blow box could be arranged which has a single zone, having openings through which drying air is blown towards the web 18. Such an alternative first upper blow box would not involve compensation for the increased drying effect of the lateral zones 52, 54 of the lower blow box 26.

The second upper blow boxes 34 of the second drying decks 30 of the second drying zone 6 illustrated in Fig. 1 may have a similar design as the first upper blow box 28 illustrated in Fig. 5.

Fig. 6 is a top view of the second lower blow box 32 as seen in a similar perspective as the first lower blow box 26 of Fig. 4. The second lower blow box 32 has an upper surface 168 adapted to face the web 18 and is divided into a first lateral zone 152 arranged adjacent to a first end 146 of the blow box 32, a second lateral zone 154 arranged adjacent to a second end 156 of the blow box 32, and a central zone 158 arranged between the first and second lateral zones 152, 154. Hence, the first and second lateral zones 152, 154 are arranged on opposite sides of the central zone 158.

The central zone 158 is provided with fourth type of air blow openings 172 that are distributed over the upper surface 168 of the lower blow box 32 within the central zone 158. The fourth type of openings 172 are of a "non- inclined type" and may, for example, be round, square or triangular holes. The fourth type of openings 172 may, when round, typically have a diameter of 1 to 10 mm. In one example, the fourth type of openings 172 are round holes with a diameter of 2.5 mm.

Typically, the degree of perforation of the central zone 158 would be 0.5-3.0%. In one example the degree of perforation of the central zone 158 is 1 .5%. For example, considering an area of 4000 mm 2 of a representative portion 178 of the upper surface 168 within central zone 158, and a degree of perforation of 1 .5 %, the total area of the openings 172 would be 60 mm 2 . A total area of 60 mm 2 and a diameter of each opening 172 of 2.5 mm would correspond to about twelve openings 172, each having an open area of about 4.9 mm 2 . The first lateral zone 152 is provided with fourth type of air blow openings 172 that are distributed over the upper surface 168 of the lower blow box 32 within first lateral zone 152. The fourth type of openings 172 of first lateral zone 152 may be similar in size and shape to the openings 172 of the central zone 158. Hence, the fourth type of openings 172 of the first lateral zone 152 may, for example, be round holes with a diameter of 2.5 mm.

Typically, the degree of perforation of the first lateral zone 152 would be 0.8-3.5%. In one example, the degree of perforation of the first lateral zone 152 is 2.0 %. Thus, the degree of perforation of the first lateral zone 152 is larger than the degree of perforation of the central zone 158. In accordance with one embodiment, the degree of perforation of the first lateral zone 152 is a factor of 1 .1 -2.0, preferably 1.1 -1 .7, larger than the degree of perforation of the central zone 158. For example, considering an area of 4000 mm 2 of a representative portion 179 of the upper surface 168 within the first lateral zone 152, and a degree of perforation of 2.0 %, the total area of the openings 172 would be 80 mm 2 . A total area of 80 mm 2 and a diameter of each opening 172 of 2.5 mm would correspond to about sixteen openings 172, each having an open area of about 4.9 mm 2 . Hence, the first lateral zone 152 has a different number of blow openings 172 per m 2 of blow box upper surface 168 area than the central zone 158. In this embodiment the first lateral zone 152 has more blow openings 172 per m 2 of blow box upper surface 168 area than the central zone 158. The blow openings 172 of the first lateral zone 152 are arranged in a pattern which is different from the pattern in which the blow openings 172 of the central zone 158 are arranged. Hence, the openings 172 of the central zone 158 form a first type of perforation, which is different from a second type of perforation formed by the openings 172 of the lateral zone 152.

The second lateral zone 154 may have a similar design as the first lateral zone 152. Hence, in one example the second lateral zone 154 comprises fourth type of openings 172 that are round and have diameter of 2.5 mm, resulting in a degree of perforation of the second lateral zone 154 of 2.0 %. Thus, the degree of perforation of the second lateral zone 154 is larger than the degree of perforation of the central zone 158. In the second lower blow box 32 the openings 172 have the same size and shape in all zones, but the concentration of openings 172, i.e. the number of openings per surface 168 area, is higher in the lateral zones 152, 154 than in the central zone 158, resulting in a larger degree of perforation in the lateral zones 152, 154.

During operation of the drying box 1 drying air DA3 is supplied to the second lower blow box 32. Typically, the average distance, or height H1 , between the lower side of the web 18 and the upper surface 168 of the second lower blow box 32 is 2-8 mm during operation. Since the openings 172 of the central zone 158 form a first type of perforation, which is different from a second type of perforation formed by the openings 172 of the lateral zone 152, air will be blown in a first flow pattern from the openings 172 of the central zone 158, which is different from a second flow pattern in which the air is blown from the openings 172 of the lateral zone 152. Due to the degree of perforation of the first and second lateral zones 152, 154 being larger than the degree of perforation of the central zone 158 a first relative air flow, in the unit: m 3 of air per hour and mm 2 of upper surface 168 area, blown towards the web 18 by openings 172 of central zone 158 will be smaller than a second relative air flow, in m 3 of air per hour and mm 2 of upper surface 168 area, blown towards the web 18 by openings 172 of lateral zones 152, 154, causing an improved lifting force on the web 18 at the lateral zones 152, 154, i.e., an improved lifting force adjacent to lateral edges 82, 84 of the web 18, such that the web 18 "floats" in a desired manner.

Typically, each lateral zone 152, 154 may extend under the web 18 respective distances DZ3, DZ4 of about 50 to 600 mm from the respective lateral edge 82, 84. The blow box 32 has a width WB which is larger than the width WW of the web 18. The first and second lateral zones 152, 154 preferably has respective total widths LZ3, LZ4 as seen from the respective first and second ends 146, 156, of 100 to 950 mm.

The respective distances DZ3, DZ4 of the second lower blow box 32 may be the same or different compared to the respective distances DZ1 , DZ2 of the first lower blow box 26.

Fig. 7a is a top view of an alternative second lower blow box 232 as seen in a similar perspective as the second lower blow box 32 of Fig. 6. The alternative second lower blow box 232 has an upper surface 268 adapted to face the web 18 and is divided into a first lateral zone 252 arranged adjacent to a first end 246 of the blow box 232, a second lateral zone 254 arranged adjacent to a second end 256 of the blow box 232, and a central zone 258 arranged between the first and second lateral zones 252, 254.

The central zone 258 is provided with fourth type of air blow openings 172 that are distributed over the upper surface 268 of the lower blow box 232 within the central zone 258. The fourth type of openings 172 are of a "non- inclined type" and may, for example, be round, square or triangular holes, and may, when round, typically have a diameter of 1 to 10 mm. In one example, the fourth type of openings 172 are round holes with a diameter of 3.0 mm.

Typically, the degree of perforation of the central zone 258 would be 0.5-3.0%. In one example the degree of perforation of the central zone 258 is 1 .5%.

The first lateral zone 252 is provided with first type of air blow openings

70, which are "inclined type" openings which may, for example, be of the "eyelid perforation" type and which are arranged in the upper surface 268. The eyelid perforations 70, which may have a similar design as the openings referred to as "eyelid perforations 6" in WO 97/16594, and which are described with reference to Figs. 2 and 3 of WO 97/16594, provide the hot drying air blown therethrough with an inclination.

Typically, the degree of perforation of the first lateral zone 252 would be 0.5-3.0%. In one example, the degree of perforation of the first lateral zone 252 is 1 .5 %. Thus, the degree of perforation of the first lateral zone 252 is the same as the degree of perforation of the central zone 258.

For example, considering an area of 4000 mm 2 of a representative portion 278 of the upper surface 268 within the central zone 258, and an area of 4000 mm 2 of representative portion 279 of the upper surface 268 within the first lateral zone 252, and a degree of perforation of 1 .5 %, the total area of the openings 172, 70, respectively, would be 60 mm 2 in each representative portion 278, 279. In accordance with alternative embodiments, the degree of perforation of the first lateral zone 252 could be either larger or smaller than the degree of perforation of the central zone 258.

The second lateral zone 254 may have a similar design as the first lateral zone 252. In the alternative second lower blow box 232 the lateral zones 252, 254 have the same degree of perforation as the central zone 258, but a different pattern of the perforation, with non-inclined type of openings 172 in the central zone 258, and inclined type of openings 70 in the lateral zones 252, 254.

Typically, each lateral zone 252, 254 may extend under the web 18 respective distances DZ3, DZ4 that are within similar ranges as the distances DZ3, DZ4 described hereinbefore with regard to the blow box 32. Similarly, the first and second lateral zones 252, 254 may have respective total widths LZ3, LZ4 that are within similar ranges as the total widths LZ3, LZ4 described hereinbefore with regard to the blow box 32.

During operation drying air DA4 is supplied to the second lower blow box 232. Since the openings 172 of the central zone 258 form a first type of perforation, which is different from a second type of perforation formed by the openings 70 of the lateral zone 252, air will be blown in a first flow pattern from the openings 172 of the central zone 258, which is different from a second flow pattern in which the air is blown from the openings 70 of the lateral zone 252. Due to the arrangement of the openings 70 of the lateral zones 252, 254 air cushions are formed generating improved lifting forces on the web 18 at the lateral zones 252, 254, i.e., an improved lifting force acts on the web 18 adjacent to lateral edges 82, 84 thereof, such that the web 18 "floats" in a desired manner.

Figs. 7b and 7c illustrate the arrangement of the openings 70 of the first lateral zone 252 of Fig. 7a in more detail, with Fig. 7b being an enlarged top-view, and Fig. 7c being a cross-section taken along the line Vllc-Vllc of Fig. 7b. The second lateral zone 254 may have a similar design as the first lateral zone 252. As can be seen from Fig. 7b the openings 70 are of the inclined type, for example the above mentioned eyelid perforation type, and are arranged in a first row 273 adjacent to a first side 274 of the blow box 232, and in a second row 275 adjacent to a second side 276 of the blow box 232. Within the lateral zone 252 the openings 70 are arranged in such manner that each opening 70 of the first row 273 opposes an opening 70 of the second row 275. Hence, air blown in inclined manner, illustrated by arrows IU1 , from openings 70 of the first row 273 will "collide" with air blown in an inclined manner, illustrated by arrows IU2, from opposing openings 70 of the second row 275. As best illustrated in Fig. 7c, such collision between air flows IU1 , IU2 above the centre of upper surface 268 of the blow box 232 generates a static pressure P s tat that acts to lift the web 18 of Fig. 7a at the lateral zones 252, 254, such that the web 18 "floats" in a desired manner.

Figs. 7d and 7e illustrate an alternative first lateral zone 352 and a portion of a central zone 358 of a further alternative lower blow box 332, with Fig. 7d being an enlarged top-view, and Fig. 7e being a cross-section taken along the line Vlle-Vlle of Fig. 7d. It will be appreciated that the lower blow box 332 may be provided with two lateral zones arranged on opposite sides of the central zone 358, although not illustrated in the drawings.

The further alternative lower blow box 332 comprises a central groove 377 arranged in an upper surface 368 of the blow box 332. The groove 377 is provided with a first side wall 373 and a second side wall 375. Inclined type blow openings 370 are arranged in the side walls 373, 375 of the groove 377. In the central zone 358 the openings 370 are arranged in the first and second side walls 373, 375 in an alternating manner. Additionally, the central zone 358 is provided with non-inclined type openings 372 arranged in a horizontal portion of the upper surface 368. The lateral zone 352 has no such non- inclined type openings. In the lateral zone 352 openings 370 arranged in the first side wall 373 of the groove 377 are arranged to oppose openings 370 arranged in the second side wall 375 of the groove 377. As best illustrated in Fig 7e, air blown in an inclined manner, illustrated by arrows IU1 , from openings 370 of the first side wall 373 within lateral zone 352 will "collide" with air blown in an inclined manner, illustrated by arrows IU2, from opposing openings 370 of the second side wall 375. Such collision between air flows IU1 , IU2 above the groove 377 of the blow box 332 generates a static pressure P s tat within lateral zone 352 which acts as an air cushion to lift the web 18 at the lateral zone 352, such that the web 18 "floats" in a desired manner.

Typically, the degree of perforation of the central zone 358 would be 0.5-3.0%. In one example the degree of perforation of the central zone 358 is 1 .5%. Typically, the degree of perforation of the lateral zone 352 is 0.4-2.7%. Since the lateral zone 352 is not provided, in this embodiment, with any non- inclined type openings 372, the degree of perforation of the lateral zone 352 is lower than the degree of perforation of the central zone 358. In one example, the degree of perforation of the central zone 358 is 1 .5 %, and the degree of perforation of the lateral zone 352 is 1 .3 %. In the alternative second lower blow box 332 the lateral zone 352 has a lower degree of perforation than the central zone 358, and a different pattern of the

perforation. Hence, a first relative air flow blown by openings 370, 372 of the central zone 358 is larger than a second relative air flow blown by openings 370 of the first lateral zone 352.

It will be appreciated that numerous variants of the above described embodiments are possible within the scope of the appended claims.

Hereinbefore it has been described that a drying box 1 comprises a first drying zone 4 and a second drying zone 6 that comprises different types of lower blow boxes 26, 32, respectively. It will be appreciated that a drying box may also comprise more than two consecutive drying zones, for example three to ten consecutive drying zones, each of which may have the same or different types of lower blow boxes. Furthermore, a drying box could also be designed with a single drying zone, comprising a single type of lower blow boxes. According to one such alternative embodiment, a drying box could be designed with a single drying zone, and comprising only lower blow boxes 26 of the type described hereinbefore with reference to Fig. 4. According to a further alternative embodiment, a drying box could be designed with a single drying zone, and comprising only lower blow boxes 32 of the type described hereinbefore with reference to Fig. 6.

Hereinbefore, it has been described that the type of upper blow boxes 28 described hereinbefore with reference to Fig. 5 may be utilized in both the first and the second drying zone 4, 6. It will be appreciated that other types of upper blow boxes 28 may also be utilized in one or more of these drying zones 4, 6. Furthermore, it is also possible to utilize, as the upper blow boxes in one or more of the drying zones, an upper blow box which is not divided into zones along its length. With such a non-zone divided upper blow box, a compensation for the higher drying effect in the lateral zones 52, 54, 152, 154 of the lower blow boxes 26, 32 would not be achieved, but this may be acceptable in some cases.

Hereinbefore it has been described that the first drying zone 4 comprises solely lower blow boxes 26 of the type described with reference to Fig. 4, and upper blow boxes 28 of the type described with reference to Fig. 5, and that the second drying zone 6 comprises solely lower blow boxes 32 of the type described with reference to Fig. 6, and upper blow boxes 34 being similar to the upper blow boxes 28 described with reference to Fig. 5. It will be appreciated that other combinations are also possible, including, for example, utilizing a combination of first lower blow boxes 26 and second lower blow boxes 32 in one and the same drying deck of a drying zone. For example, a drying deck could be arranged with every second lower blow box being a first lower blow box 26, and every second lower blow box being a second lower blow box 32.

Hereinbefore it has been described that the size of openings of the central zone 58 and of the lateral zones 52, 54 could be different, as described with reference to Fig. 4, or that the concentration of openings per m 2 of surface area of the central zone 158 and of the lateral zones 152, 154 could be different, as described with reference to Fig. 6. It will be appreciated that it is also possible to utilize in a lateral zone both a different size and a different concentration of openings, compared to a central zone. Still further, it is also possible to utilize in a lateral zone openings of a different shape than in the central zone. For example, round openings could be utilized in the central zone, and square openings could be utilized in the lateral zone.

Hereinbefore it has been described that the respective upper and lower blow boxes 26, 28, 32 extend under the entire width WW of the web 18, with drying air being supplied from one end 46, 50, 146 located adjacent to a first lateral edge 82 of the web 18. It will be appreciated that it would also be possible to arrange, as alternative to one such full length blow box, two blow boxes each having about half the width of the web 18 and extending from each of the two lateral edges 82, 84 of the web 18 and abutting each other at the centre of the web, with air being supplied at openings adjacent to each of the edges 82, 84. Each such half-length blow box would have a central zone, and a first lateral zone arranged under the respective edge 82, 84 of the web 18.

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

Hereinbefore it has been described that in the lateral zones 252, 352 of the blow boxes 232, 332 an air cushion effect is achieved by air flows blown by opposing blow openings 70, 370 "colliding" with each other, generating a static pressure P s tat- It will be appreciated that an air cushion effect could also be achieved by other arrangements of openings.

Hereinbefore it has been described that a second relative air flow blown by openings 70; 370 of the respective lateral zone 252, 352 of blow boxes 232, 332 is equal to or smaller than a first relative air flow blown by openings 172, 370, 372 of the respective central zone 258, 358. It will be appreciated that it may often be possible to achieve the desired air cushion effect also when supplying a second relative air flow supplied by the lateral zone 252, 352 which is higher than the first relative air flow supplied by the central zone 258, 358.

To summarize, a cellulose pulp drying box for drying a web 18 of cellulose pulp comprises a lower blow box 26 for blowing gas upwards towards the web 18 via blow openings, for drying the pulp in accordance with the airborne web principle. The lower blow box 26 comprises a central zone 58 comprising blow openings 70, 72 forming a first type of perforation, and a first lateral zone 52 comprising blow openings 70, 80 forming a second type of perforation, the second type of perforation being different from the first type of perforation. The blow openings 72 of the central zone 58 form a first degree of perforation, and the blow openings 80 of the first lateral zone 52 form a second degree of perforation which is larger than the first degree of perforation. As alternative to or in combination therewith the first lateral zone 252; 352 comprises blow openings 70; 370 arranged to form an air cushion above the lateral zone 252; 352 to lift the web 18 adjacent to a lateral edge 82 thereof.