1. Field of the Invention
This invention relates to a dryer apparatus for drying a web extending through a dryer section of a paper machine. More specifically, this invention relates to a TOTAL BELRUN dryer apparatus. TOTAL BELRUN is a registered Trademark of Beloit Corporation.
2. Information Disclosure Statement
In the paper drying art, one of the primary problems with the high speed operation of a paper dryer section is sheet flutter which occurs when an unsupported web extends between successive dryers in a dryer section.
By the introduction of the so-called "single felt" configuration, sheet flutter was minimized. Such "single felt" configuration included an upper and a lower tier of dryers with the web and dryer felt extending contiguously around upper and lower dryers of the respective tiers. Although the "single felt" configuration avoided the problems associated with an unsupported web, such "single felt" or "serpentine" configuration introduced a further problem in that during passage of the web and felt around the dryers of the lower tier, the dryer felt was disposed between the respective dryers and the web. Therefore, the drying capability of the lower tier of dryers was inhibited. Additionally, in the "single felt" arrangement, the web had a tendency to separate from the felt during travel of the web towards, around and away from the dryers of the lower tier. Furthermore, initial threading of the web through a "single felt" dryer section was relatively difficult.
The aforementioned problems of "single felt" dryer sections was overcome by the provision of the TOTAL BELRUN arrangement which is the subject of co-pending patent application 014,569 filed Feb. 13, 1987. In the TOTAL BELRUN configuration, the lower tier of dryers in the "single felt" arrangement are replaced by vacuum transfer rolls. The vacuum transfer rolls avoid the necessity for a redundant lower tier of dryers. Furthermore, due to the application of vacuum, the tendency of the web to separate from the dryer felt during movement around the transfer roll is inhibited. Also, the draw between the single tier of dryers and the transfer roll is reduced thereby enhancing the stability of the web relative to the dryer felt. Additionally, by the provision of such vacuum rolls, initial threading of the web is facilitated.
Recent installations of the TOTAL BELRUN concept have indicated that such concept can be extended to include a large number of dryers without the introduction of any adverse effect on the web runnability. Such runnability is attained because the vacuum rolls are capable of conveying the web along the felt-supported spans without the need for sheet tension or section draw points.
Nevertheless, one potential problem with the TOTAL BELRUN system is that there remains a short draw length between the dryers and the effective vacuum zones of the intermediate vacuum transfer rolls. Although the web will generally be conveyed through the short draws with little or no separation from the felt, it has been observed during machine upsets that the edges of the web may separate by as much as one inch from the felt. The aforementioned upsets are typically caused by short periods of time in which the basis weight or grammage of the web and the moisture content of the web are nonuniform. The release characteristics of the web edges from the dryer surfaces during the aforementioned periods occasionally cause the web to be pulled from intimate contact with the felt. The web is then subjected to the disturbing influences of local air currents and centrifugal forces. The result of such disturbances can be undesirable wrinkling of the sheet and in extreme cases web breakage.
In U.S. Pat. No. 3,868,780 to Soininen, the dryers are arranged in such a configuration as to define an enclosure from which air is evacuated. The resultant partial vacuum generates a partial vacuum within perforate transfer rolls thereby drawing the web towards the dryer felt during transit of the web and felt around such perforate rolls. However, the provision of the aforementioned evacuated enclosure involves various sealing problems relative to the edges of the web and complications relative to access to the various dryers of the dryer section.
Other attempts to reduce edge flutter of the web relative to conventional "single felt" arrangements are disclosed in U.S. Pat. No. 4,502,231 to Fissmann et al, dated Mar. 5, 1985, U.S. Pat. No. 4,359,828 to Thomas dated Nov. 23, 1982 and U.S. Pat. No. 4,553,340 to Petersson dated Nov. 19, 1985. However, the aforementioned patents which all relate to "single felt" arrangements require full-width air nozzles to induce vacuum adjacent to the felt on the opposite side relative to the web in the felt supported draws. The nozzles require large quantities of air and the associated power requirements for the fans can be relatively high. Such boxes further necessitate the provision of a large space between adjacent dryers in order to accommodate boxes of adequate structural strength. Such large spaces increase the machine direction length of the dryer section and increase the cost of the building required to house such a dryer section. Additionally, dust and paper debris can accumulate on the aforementioned boxes and interfere with the action of the air jets.
Also, the vacuum induced by the aforementioned boxes causes deflection of the felt towards the boxes and, in severe cases, the felt will contact the box and cause excessive felt wear. Because of the aforementioned wear condition, the vacuum levels must be maintained at a relatively low level.
Additionally, the vacuum induced by the aforementioned prior art boxes must prevent the web from separating from the fabric as the web approaches the felt roll converging nip and as the web wraps the roll. The aforementioned nip and wrapping locations are the most critical locations. However, in the prior art no vacuum is applied directly at these critical locations.
U.S. Pat. No. 4,441,263 to Vedenpaa includes a vacuum box in association with a grooved roll so that the vacuum can extend further into the aforementioned critical locations. However, the vacuum applied as disclosed in 4,441,263 is highest in the pocket area above the grooved roll and lowest in the aforementioned converging nip location and where the web wraps the grooved roll.
The aforementioned problems are solved by utilizing vacuum rolls as taught in the aforementioned U.S. Ser. No. 014,569 instead of using grooved rolls or dryers in the intermediate position. Such vacuum rolls include seals for sealing the area between the dryers above the associated vacuum roll. In the present invention, rather than drawing the vacuum from the pocket area as taught in U.S. Pat. No. 4,441,263, the vacuum is drawn from the inside of the perforate vacuum roll. In this manner, the maximum vacuum is produced in the critical locations, including the area where the web wraps the vacuum roll. Furthermore, a lesser vacuum is applied at the critical location where the web approaches the vacuum roll and where the web leaves the roll. Additionally, such vacuum generates a minimal vacuum level within the pocket for drawing the web into close conformity with the dryer felt during transit between the dryers and the transfer roll.
More particularly, the air which flows into the roll encounters a pressure drop across the perforate shell of the transfer roll so that the vacuum level in the enclosed pocket is less than the vacuum level applied at the aforementioned critical locations.
Additionally, by locating the vacuum roll close to the dryers, the felt-supported draw length is minimized. Such minimization of the draw length not only reduces the tendency for the web to separate from the fabric, but also reduces the amount of fabric deflection which will occur for a given vacuum level.
Also, by the provision of the present invention, the stationary internal center shaft and relatively complex sealing arrangements associated with the vacuum roll of the aforementioned Ser. No. 014,569 is avoided. Pocket seals are included in the present invention for restricting the vacuum roll from drawing in excessive volumes of air and to extend the vacuum to the felt-supported draw lengths.
The stationary internal roll components can additionally be replaced by various divider plates or orifice plates which assist in concentrating the vacuum drawn out of the vacuum roll to compartments disposed adjacent to the front and back of the vacuum roll where the web is most susceptible to separation from the felt.
Accordingly, it is a primary object of the present invention to provide a dryer apparatus which overcomes the aforementioned inadequacies of the prior art dryer sections and which provides a significant contribution to the paper web drying art.
Another object of the present invention is the provision of a dryer apparatus in which the transfer rolls can be constructed without any internal stationary center shafts.
Another object of the present invention is the provision of a dryer apparatus in which the dryer pockets can be enclosed with sealing plates or boxes.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings and from the appended claims.