Field of the Invention

The invention relates to drying of a pulp web. Particularly, the present invention is directed to a suction roll for this purpose, a suction roll arrangement, and a method for drying a pulp web using such a suction roll arrangement.

State of the Art

Pulp for paper and board manufacture is produced in a separate process wherein the pulp web produced is dried and packaged for storage and shipping. The pulp web is formed on a wire, followed by dewatering at a wire section, then at a press section, and finally by applying heat to the pulp web at a drying section. At the press section, the aim is to sufficiently increase the relative dry matter content of the web to enable efficient and uninterrupted running thereof at the actual drying section.

Various solutions for the design of the press section are known. In one known solution, the web is carried to the press section on a wire running around a first roll. In the vicinity of the roll, there is a second roll with a felt running around it. The web is passed through the nip between the first and second rolls, between the wire and the felt, the compression by the nip resulting in removal of liquid from the web to the felt running around the second roll. In the vicinity of the first and second rolls, there may be a third roll forming a second nip with the second roll for further dewatering of the web. A felt also runs around this third roll. The felts are arranged to run along an endless path where they are dried prior to redirecting back to the rolls. Such an arrangement is often called a combination press.

For more efficient drying of the web, it is common to use a suction roll as the second roll, having a perforated outer surface, through which liquid is removed from the web by suction at a location where the felt passing around the roll is in contact with the web. A problem caused by the relatively high rotational speed of the suction roll is the spraying of the liquid remaining in the openings due to centrifugal forces. That is, part of the liquid drawn into the suction roll sprays in the surrounding space, thus wetting it. A particular disadvantage is the spraying of liquid onto the dried felt being redirected onto the suction roll, thus decreasing the ability thereof to absorb liquid from the web.

Attempts have been made to moderate the above problem by also applying suction to other areas of the suction roll, such as in areas in which the felt passing around the suction roll is just about to loose or has just lost contact with the surface thereof, that is, in the so-called cleaning zone. However, due to increasingly higher rotational speeds of the rolls, unreasonably complicated solutions are necessary for producing a sufficiently high vacuum.

Objects and summery of the Invention

The object of the invention is to provide a roll arrangement and a suitable suction roll therefor to prevent spraying of the liquid remaining in the flow channels of the suction roll to the surrounding space. Further, the object of the present invention is to provide an improved method for drying a web, wherein the problems associated with the state of the art are avoided.

The suction roll of the invention is characterized in that it comprises a substantially stationary pressurized area limited to a portion of the inner surface of the shell of the suction roll to generate a blowing current, directed away from the suction roll, through the flow channels located at that moment in the pressurized area on the inner surface of the shell of the suction roll.

It is characteristic of a roll arrangement according the invention, comprising a suction roll, that within the suction roll, there is a substantially stationary pressurized area for removal of the liquid remaining in the flow channels by a blowing current, said pressurized area being directed to an area on the inner surface of the suction roll shell that substantially corresponds to at least a portion of the outer surface of the suction roll shell from which the web has lost contact with the felt passing around the suction roll.

A method according to the invention characteristically comprises steps for removal of the liquid remaining in the flow channels of the suction roll by directing a blowing current through the flow channels of the suction roll by means of the pressurized area within the suction roll, on at least portion of the outer surface area of the shell where the felt passing around the suction roll is no longer in contact with the web.

Characteristic features of other embodiments of the invention will become obvious from the appended dependent claims.

Brief description of the Figures

In the following, the invention is explained in more detail with reference to the appended drawing, in which:

Figure 1 shows a cross-sectional presentation of a roll arrangement according to an embodiment of the invention.

Said drawing is presented in a simplified form to illustrate more clearly the operation of the invention.

Detailed description of the Invention

The roll arrangement of Figure 1 comprises a suction roll 1, and backing rolls 3 and 4. A felt la passes around the suction roll 1 in contact with a portion of the outer surface of the suction roll shell. Similarly, a wire 3a runs around the backing roll 3 in contact with a portion of the outer surface of the backing roll 3 shell, and respectively, a felt 4a runs around the backing roll 4 in contact with a portion of the outer surface of the backing roll 4 shell. A web 2 is conveyed on the wire 3a to a nip between the suction roll 1 and the backing roll 3 through which the web 2 is passed and compressed between the wire 3a and the felt la. The web 2 is further conveyed in contact with the felt la along the outer surface of the suction roll 1 shell to a nip between the suction roll 1 and backing roll 4, through which the web 2 is passed and compressed between the felts la and 4a.

Figure 1 further shows that the suction roll 1 is provided with flow channels lb extending from the outer surface to the inner surface of the suction roll 1 shell. A vacuum area lc and a pressurized area Id are arranged within the suction roll 1. A suction roll 1 according to an embodiment of the invention comprises a cylindrical hollow body provided with several flow channels lb extending between the outer and inner surfaces of the suction roll 1 shell. The flow channels lb are preferably radial perforations. The perforation for the flow channels lb may, for instance, be provided by boring holes through the suction roll 1 shell. The suction roll 1 is rotationally supported at ends thereof and preferably provided with driving means for rotating the suction roll at a desired speed. There is a vacuum area lc arranged within the suction roll 1 for applying vacuum to a limited area of the inner surface of the suction roll 1 shell. That is, the vacuum area 1 is restricted to a limited area of the inner surface of the suction roll 1 shell. Further, there is a substantially stationary pressurized area Id arranged within the suction roll 1 for applying pressure to another limited area of the inner surface of the suction roll 1 shell. Similarly, said pressurized area Id is restricted to another limited area of the inner surface of the suction roll 1 shell. A pressure difference between the vacuum area lc and the surrounding space results in a suction current towards the inside of the suction roll through those flow channels lb that are at that moment in the vacuum area lc on the inner surface of the suction roll 1 shell. Correspondingly, a pressure difference between the pressurized area Id and the surrounding space results in a blowing current away from the suction roll 1 through those flow channels lb that are at that moment in the pressurized area Id on the inner surface of the suction roll 1 shell. As the suction roll 1 rotates, its flow channels are alternately connected to the various areas lc, Id, and the surrounding space.

Preferably, the vacuum area lc and pressurized area Id substantially extend over the whole length of the roll. In special cases, also other dimensions are possible. Each pressure in the vacuum and pressurized areas lc, Id may be independently adjusted to best correspond to each particular situation. In the vacuum area lc, negative pressures similar to those used in conventional suction rolls may for instance be applied, the negative pressures typically ranging from 35 to 65 kPa, being preferably about 50 kPa. In the pressurized area Id, for instance pressures exceeding the ambient pressure for instance by about 15 to 25 kPa, preferably by about 20 kPa, are preferable for the operation.

According to an embodiment of the invention, the roll arrangement comprises a rotating suction roll 1 having a felt la running around the roll, said felt being in contact with at least a part of the outer surface of the suction roll 1 shell. Preferably, the felt la runs around the roll as part of a more extensive felt system further comprising an apparatus (not shown) for drying the felt la. Moreover, the suction roll 1 comprises several flow channels lb between the inner and outer surface of the shell thereof. There is a substantially stationary vacuum area lc arranged within the suction roll 1. The vacuum area lc is arranged for applying vacuum to an area on the inner surface of the suction roll 1 shell, corresponding to at least part of the area on the outer surface of the suction roll 1 shell in contact with the felt la. Preferably, the vacuum area lc is restricted to an area on the inner surface of the suction roll 1 shell corresponding to an area on the outer surface of the shell, in which the web 2 is in contact with the felt la. Further, there is a substantially stationary pressurized area Id within the suction roll 1. As for the pressurized area Id, it is arranged to apply pressure to an area on the inner surface of the suction roll 1 shell, corresponding to at least a part of the area on the outer surface of the suction roll 1 shell, in which the web 2 is no longer in contact with the felt la running around the suction roll 1. Preferably, the pressurized area Id is directed to an area on the inner surface of the suction roll 1 shell corresponding to an area on the outer surface of the shell where the contact with the felt la has just been lost.

The roll arrangement according to said embodiment of the invention further comprises at least one backing roll 3 forming a nip with the suction roll 1. The web 2 is passed through the nip pressed between the backing roll 3, and the suction roll 1 with the felt la running around it. Preferably, the web 2 is introduced to the nip on a wire 3 a running around the backing roll 3. In this case, the web 2 is pressed in the nip of the suction roll 1 and backing roll 3, between the felt la and the wire 3a. Pressing of the web 2 results in release of liquid therefrom, the liquid being then absorbed into the felt la. The liquid transferred from the web 2 to the felt la is drawn in by the suction roll 1 by passing a suction current through the flow channels lb. The suction current is provided by the vacuum area lc within the suction roll, the vacuum area la being connected to a water separation apparatus, and to an apparatus for producing vacuum. The liquid withdrawn by the suction roll 1 is further passed away from the inside of the suction roll 1. However, part of the liquid withdrawn from the felt la remains in the flow channels lb of the suction roll 1, and sprays in the surrounding space due to rotation of the suction roll 1. To prevent this, the liquid remaining in the flow channels lb is removed in a controlled manner by blowing with a blowing current provided by the pressurized area Id. Preferably, the removal of the liquid accumulated in the flow channels lb by a blowing current may be performed by absorbing the liquid blown out into the felt la.

A roll arrangement according to another embodiment of the invention further comprises a second backing roll 4 having a felt 4a passing around it, in contact with at least a portion of the outer surface of the second backing roll 4 shell. The second backing roll 4 is located to form a nip with the suction roll 1 having a felt la passing around it, the web 2 being arranged to pass through the nip. Preferably, the nip between the second backing roll 4 and the suction roll 1 is located in the effective range of the suction current provided by the vacuum area lc. Particularly preferably, the nip between the second backing roll 4 and the suction roll 1 is located, in the running direction of the web 2, at the end of the effective range of the suction current provided by the vacuum area lc. Both backing rolls 3, 4 may be provided with perforations on the respective shell outer surfaces. Preferably, the perforations of the backing rolls 3, 4 do not extend through the respective shells.

According to still another embodiment of the invention, in addition to the suction roll, the roll arrangement further comprises a separate suction apparatus for collection, and removal of the liquid blown from the flow channels lb. In order to achieve this, the suction , apparatus is located next to the area where the liquid accumulated in the flow channels lb is blown out. That is, the suction apparatus is located in the vicinity of the shell's outer surface of the suction roll 1 in the effective range of the blowing current provided by the pressurized area Id. Preferably, the suction apparatus is further in flow communication with the apparatus for water separation and for producing a vacuum.

In the method for drying a web according to an embodiment of the invention, the web 2 is passed to the nip between the suction roll 1, with the felt la passing around it, and at, least one backing roll 3· Due to the pressure in the nip, liquid is absorbed from the web 2 to the felt la. With the vacuum area lc within the suction roll 1, suction current through the flow channels lb is created at least on a portion of that outer surface area of the suction roll 1 shell, with which the felt la is in contact. That is, a suction current is directed to the felt la to absorb liquid therefrom. Preferably, the suction current is passed through the flow channels lb at least on a portion of that outer surface area of the suction roll 1 shell where the web 2 is in contact with the felt la. Liquid remains in the flow channels lb after the effective range of the suction current created by the vacuum area lc. For controlled removal of the liquid remaining in the flow channels lb, a blowing current is passed through the flow channels lb by the pressurized area Id within the suction roll 1.

Preferably, the blowing current is passed through the flow channels lb in an area of the outer shell of the suction roll 1 where the web 2 no longer contacts the felt la. More preferably, the blowing current is passed through the flow channels lb in an area where the felt la has substantially just lost contact with the outer surface of the suction roll 1. The liquid blown from the flow channels lb into the felt la may be removed from the felt la with a drying apparatus belonging to its respective felt circulation system.

In the method according to another embodiment of the invention, the web 2 is further passed through the second nip between the suction roll 1, with the felt la passing around it, and a second backing roll 4, preferably within the effective range of the suction current created by the vacuum area lc. Particularly preferably, the web 2 is passed through the second nip in its running direction in the final portion of the effective range of the suction flow. A, felt 4a may also pass around the second backing roll 4.

In the method according to still another embodiment of the invention, the liquid blown out of the flow channels lb is collected by the suction apparatus for further removal.

For those skilled in the art, it is obvious that the embodiments presented are not mutually exclusive, and thus the invention may be carried out by combining the embodiments presented above. For those skilled in the art, it is further obvious that in the solutions according to the invention, either one or both of the backing rolls 3, 4 may be designed to be structurally similar to the suction roll 1.