GB2110804A

GB2110804A - Drying paper webs

Info

Publication number: GB2110804A
Application number: GB08229878A
Authority: GB
Inventors: Edgar J Justus
Original assignee: Beloit Corp
Current assignee: Beloit Corp
Priority date: 1981-10-21
Filing date: 1982-10-19
Publication date: 1983-06-22
Also published as: IN157463B; IT8223804A0; JPS5870794A; ES8400522A1; ES516678A0; KR840002050A; IT1152725B; GB2110804B

Abstract

In a tiered paper making machine dryer section, a vacuum pocket 12 is provided in the space between a lower tier roll 11 and free running spans of a pervious belt 8 carrying a freshly felted paper web 7 extending between a pair of upper tier rolls 10 and the lower tier roll 11, and a vacuum is drawn in the pocket for retaining the web in contact with the belt 8 against any tendency of the web to pull away from the belt in the vicinity of the pocket. The vacuum may be drawn through one of the walls 13 serving as end dams at opposite ends of the pocket, or may be drawn through a top closure 14 for the pocket. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the drying of paper webs This invention relates to improvements in the drying of paper webs and is more particularly concerned with a new and improved paper making machine dryer section and method.

As production speeds in paper making have been increased, particularly during the manufacture of lightweight newsprint and lightweight directory grade paper sheet, stability problems have been experienced in the first dryer which receives the wet paper web immediately after the web has been felted and while it is still fragile. Production speeds may be up to 3000 feet per minute (914 metres per minute) or greater. Because of web fragility, the wet web is supported by porous co-running pervious belt means, sometimes referred to as a felt or as a screen. In the dryer section, the web in contact with the belt means, travels at high speed in a rollwrapping serpentine path over dryer rolls.

Certain dryer sections employ a double webcarrying belt arrangement, such as represented in U.S. Patent Specification No. 3,956,832, and foraminous dryer rolls, hot air being drawn through the belt-carried web wrapping the foraminous rolls. This ensures that the fragile web will be thoroughly supported between the belts, but it is a rather complex and high cost system.

Another arrangement is shown in U.S. Patent Specification No. 4,033,049, wherein multi-stage drying is effected by running the web over and about a pair of spaced heated solid or foraminous surfaced rolls and a larger hollow perforated perimeter drum, heated air being directed through the web and carrying belt into the drum. A suction chamber is designed to draw moisture-laden air from the drum and from the wrapped perimeters of the rolls and the arrangement is stated as maintaining leader of the web in adherence to the outer side of the screen which carries the web into direct engagement with the perimeters of the heated rolls and provides a spacer between the web and the perimeter of the perforated drum.The suction chamber has partitions which extend into the area between the belt and the heated rolls and the space between the partitions opens toward the perimeter of the perforated drum to facilitate drawing of the heated moisture laden air from the drum. This is also a complex and high cost system.

For the sake of economy and operational simplicity, it is advantageous to use all solid heated dryer rolls arranged in tiers with a single paper-webcarrying belt or felt. Such a system is disclosed in U.S. Patent Specification No.4,202,113. In this arrangement, a vacuum is pulled with respect to a lower tier of grooved dryer rolls which is alleged to maintain the paper web against the fabric carrying belt as the belt wraps the lower tier of rolls.

However, at high operating speed, there is a tendency for the wet paper web to separate from the pervious belt as the belt and web leave the upper tier of rolls where the paper web is in direct wrapping engagement with the roll perimeters.

An important object of the present invention is to overcome the disadvantages, drawbacks, inefficiencies, shortcomings and problems inherent in prior arrangements, and to provide a new and improved paper making dryer section which will provide the advantages of a single carrying belt arrangement efficiently and in a highly stable manner.

According to the invention there is provided a paper making machine dryer section comprising vertically spaced horizontal first and second tiers of heated dryer rolls, wherein a lower tier roll is located opposite a space between a pair of upper tier rolls, a paper web to be dried being adapted to travel at high speed in a roll-wrapping serpentine path in contact with co-running pervious belt means in such relation that the web is in direct contact with said pair of first tier rolls and the belt means intervenes between the web and said second tier roll, a top closure extending between said pair of upper tier rolls which, together with said lower tier roll and spans of said belt means running freely between said lower tier roll and said pair of upper tier rolls, defines a vacuum pocket, said web travelling on said belt means outside of said pocket, and means for drawing a vacuum in said pocket for retaining said web in contact with said belt means against any tendency of said web to pull away from said belt means in the vicinity of said pocket.

The invention also provides a method of operating a paper making machine dryer section having vertically spaced horizontal first and second tiers of heated dryer rolls, wherein a lower tier roll is located opposite a space between a pair of upper tier rolls, a paper web to be dried being adapted to travel at high speed in a roll-wrapping serpentine path in contact with co-running pervious belt means in such relation that the web is in direct contact with said pair of first tier rolls and the belt means intervenes between the web and said second tier rolls, the method comprising: drawing a vacuum into a vacuum pocket defined between said lower tier roll, spans of said belt means running freely between said lower tier roll and said pair of upper tier rolls, and a top closure extending between said pair of upper tier rolls, said web travelling on said belt means outside of said pocket, and thereby retaining said web in contact with said belt means against tendency of said web to pull away from said belt means.

The following is a detailed description of embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a schematic illustration of a paper making machine dryer section embodying the invention, Figure2 is a sectional view taken substantially along the line ll-ll of Figure 1, and Figure 3 shows a modification.

Referring to Figure 1, a first dryer section 5 is adapted to receive a newly felted wet paper web 7 from forming apparatus (not shown), and is adapted to dry the web sufficiently to be further handled in a self-sustaining condition, the thus dried web passing on to further processing apparatus as may be desirable or customary with respect to the particular type of paper web being handled. For example, the paper web may be lightweight newsprint, lightweight directory grade sheet, or the like. Production of such lightweight grades of paper is desirably effected at speeds of 3000 feet per minute (914 metres per minute) or greater.

At the entry end of the dryer section 5, the wet paper web 7 joins in co-running relation with a pervious carrying belt 8, which may be of the kind commonly referred to as a screen, wire or felt. As is customary, the belt 8 is trained over suitable guide rollers outside of the press section 5, and runs over a guide roller 9 at substantially the point of initial contact with the paper web 7 at the entry end of the dryer section. Then the co-running belt 8 and paper web 7 travel on at the high operating speed in a roll-wrapping serpentine path over a first tier of upper heated dryer rolls 10 which are in horizontally spaced alignment, and a lower second tier of heated dryer rolls 11, which are in horizontally spaced relation to one another and respectively vertically spaced below the spaces between respective pairs of the dryer rolls 10.Any preferred construction of solid dryer rolls may be employed in respect of the rolls 10 and 11, and any suitable means of driving the two tiers of rolls in unison may be employed. Although either or both of the tiers of rolls 10 and 11 may be solid grooved rolls, herein only the lower rolls 11 are shown as being of peripherally grooved construction. Solid heated dryer rolls and the means for driving the same are well known in the art and will thus not be described in more detail.

In the dryer section 5, the co-running, intimately contacting web 7 and belt 8 run in such relation that the web 7 is in direct wrapping contact with the first tier rolls 10, and the belt 8 intervenes between the web 7 wrapping the second tier rolls 11, that is the belt 8 is in direct wrapping contact with the rolls 11. It will be understood that means, not shown, housing the dryer section 5, will provide a moisture laden air evacuating system as the moisture is driven from the web 7 by the heated dryer rolls as the web progresses through the dryer.

Because of the high moisture content and low tensile strength of the web 7 as it is being treated in the dryer 5, the web is dangerously susceptible to sheet breaks. For example, as the web 7 and the pervious belt 8 leave the first tier rolls 10, the web may tend to separate and bulge away from the belt 8 due to surface tension at the perimeter of the upper tier rolls. Such a tendency to separate may also develop at other areas of the free running spans of the belt and web extending between the upper tier and lower tier rolls.

To avoid any such tendency of the web 7 to separate from the belt 8, means are provided to define a vacuum pocket within the area defined by the lower tier roll 11 and the spans of the belt 8 running freely between the lower tier rolls and the associated upper tier rolls. For enclosing the pocket, respective opposite end walls 13 are associated with the opposite are associated with the opposite ends of the rolls 10 and 11 and a closure 14 extends between the top ends of the walls 13. In a preferred construction, the end walls 13 have edges which cooperate closely with the respective opposite margins of the belt 8 and may be provided with dynamic sealing strip means 15 of a suitable low friction and abrasion resistant plastics material.At their lower edges, the walls 13 desirably conform substantially to the radius of curvature of and cooperate as closely as practicable to the perimeter of the opposite end portions of the associated lower tier roll 11. Suitable low friction and abrasion resistant dynamic sealing strips 17 may be carried by the lower edges of the walls.

Dynamic sealing means in the form of generally channel shaped bars 18 extend along the edges of the top closure 14 and along the associated upper tier rolls 10 desirably adjacent where the downrunning span of the belt 8 leaves the left hand upper tier roll 10 and the uprunning span meets the right hand upper tier roll 10, as viewed in Figure 1. Both of the upper and lower edges of the seal bars 18 are preferably provided with dynamic sealing means, such as sealing strips 19 of a suitable abrasion resistant and low friction plastics material, such as nylon, polytetrafluoroethylene or the like. In order to avoid excessive wear, the various edges and seals of the end walls 13 and the seal bars 18 approach the adjacent surfaces to be sealed as closely as practicable, but without intentional direct contact. This affords ample sealing for the present purpose.The double seals 19 extending along the opposite edges of the seal bars 18 afford excellent sealing against vacuum loss and ensures the provision of a vacuum effect for retaining the web 7 on the belt 8 in the tangency angle areas 20 and 21 between the web 7 carried by the face of the belt 8 outside of the vacuum chamber 12 and the respective upper tier rolls 10. This effect isbspecially desirable at the tangency area 20 to counteract any tendency of the web 7 to pull away from the belt 8, but to retain the web firmly on and in co-running relation with the belt 8.

For pulling a vacuum in the vacuum pocket 12, a vacuum port 22 is desirably provided in at least one of the end walls 13 and is connected by means of a duct 23 with a vacuum generating source 24.

Alternatively, a vacuum port 22' may be provided in the top closure 14 and the vacuum pocket 12 thereby placed in communication by means of the duct 23 with the vacuum generating source 24, as shown in Figure 3.

At a web and belt velocity of about 3000 fpm (914 metres per minute) a vacuum of the order of 0.5 inches (1.27 cm) of water is indicated for thin, e.g.

tissue, webs. Higher orders of vacuum may be employed for heavier grades of web.

Although for some purposes only the first threesome set of rolls, that is one pair of upper tier rolls 10 and an associated lower tier roll 11, may be equipped with the vacuum pocket 12. For other purposes, each threesome set of pairs of upper tier rolls and associated lower tier rolls may be so equipped, substantially as shown in Figure 1.

Beyond the last of the dryer rolls 11 in the lower tier series, the belt 8 may be guided toward its return run by means of a guide roller 25, and the at least substantially dry web 7 may be led off by means of a guide roller 27.

Claims

1. A paper making machine dryer section comprising vertically spaced horizontal first and second tiers of heated dryer rolls, wherein a lower tier roll is located opposite a space between a pair of upper tier rolls, a paper web to be dried being adapted to travel at high speed in a roll-wrapping serpentine path in contact with co-running pervious belt means in such relation that the web is in direct contact with said pair first tier rolls and the belt means intervenes between the web and said second tier roll, a top closure extending between said pair of upper tier rolls which, together with said lower tier roll and spans of said belt means running freely between said lower tier roll and said pair of upper tier rolls, defines a vacuum pocket, said web travelling on said belt means outside of said pocket, and means for drawing a vacuum in said pocket for retaining said web in contact with said belt means against any tendency of said web to pull away from said belt means in the vicinity of said pocket.

2. A paper making machine dryer section according to claim 1, wherein said vacuum pocket is also defined by respective walls associated with opposite ends of said rolls, said closure extending between the top ends of said walls, and dynamic sealing means being provided along edges of said top closure extending along said pair of upper tier rolls.

3. A paper making machine dryer section according to claim 2, wherein said vacuum drawing means include a duct connected to and communicating with said pocket through one of said walls.

4. A paper making machine dryer section according to claim 1 or claim 2, said vacuum wherein drawing means include a duct connected to and communicating with said pocket through said top closure.

5. A paper making machine dryer section according to any of claims 1 to 4, inciuding a respective vacuum pocket associated with each of a plurality of pairs of said first tier dryer rolls and respective lower tier dryer rolls below the spaces between said pairs of upper tier rolls, and said means for drawing vacuum being effective in all of said vacuum pockets.

6. A method of operating a paper making machine dryer section having vertically spaced horizontal first and second tiers of heated dryer rolls, wherein a lower tier roll is located opposite a space between a pair of upper tier rolls, a paper web to be dried being adapted to travel at high speed in a roll-wrapping serpentine path in contact with corunning pervious belt means in such relation that the web is in direct contact with said pair of first tier rolls and the belt means intervenes between the web and said second tier rolls, the method comprising: drawing a vacuum into a vacuum pocket defined between said lower tier roll, spans of said belt means running freely between said lower tier roll and said pair of upper tier rolls, and a top closure extending between said pair of upper tier rolls, said web travelling on said belt means outside of said pocket, and thereby retaining said web in contact with said belt means against tendency of said web to pull away from said belt means.

7. A method according to claim 6, comprising providing respective walls associated with opposite ends of said rolls, said top closure extending between top ends of said walls, and effecting dynamic sealing along edges of said top closure extending along said pair of upper tier rolls.

8. A method according to claim 7, which comprises drawing said vacuum through means communicating through one of said walls.

9. A method according to claim 6 or claim 7, which comprises drawing said vacuum through means communicating through said top closure.

10. A method according to any of claims 6 to 9, defining a vacuum pocket associated with each of a plurality of pairs of said first tier rolls and lower tier rolls below the spaces between the upper tier rolls, and drawing a vacuum effective in all of said pockets.

11. A paper making machine dryer section substantially as hereinbefore described with reference to the accompanying drawings.

12. A method of operating a paper making machine dryer section substantially as hereinbefore described with reference to the accompanying drawings.