1. FIELD OF THE INVENTION
This invention relates to a vacuum roll transfer apparatus for drying a web extending through a single tier dryer section of a papermachine. More specifically, this invention relates to a vacuum transfer roll for a Total BelRun dryer section. Total BelRun is a Registered Trademark of Beloit Corporation.
2. INFORMATION DISCLOSURE STATEMENT
In co-pending U.S. Pat. application Ser. No. 014,569 filed Feb. 13, 1987 assigned to Beloit Corporation, a dryer section is disclosed which includes a plurality of single tier dryer sections, each dryer section including at least two dryers with a vacuum transfer roll interposed between adjacent dryers.
The aforementioned Total BelRun arrangement includes a dryer felt for each dryer section with the felt extending contiguously with the web alternately around each dryer and transfer roll. The partial vacuum applied to the vacuum roll provides a positive restraint to the web as the web and felt extend around the transfer roll with the felt being disposed between the web and the transfer roll.
As disclosed in U.S. Ser. No. 014,569, the vacuum roll includes a perforate shell and an internal duct arrangement with a seal or gasket disposed between the duct and the rotatable shell so that the vacuum can be concentrated in the vicinity of the wrapped portion of the transfer roll for urging the web into close conformity with the dryer felt. The aforementioned restraint helps to minimize cross-directional shrinkage and to improve the sheet quality and uniformity. The level of sheet restraint, and hence sheet quality can be increased by increasing the vacuum that is applied to the sheet as it wraps around the vacuum roll.
The problem with increasing the vacuum is that the volume of air which is drawn into the roll and which must thereafter be exhausted from the roll is proportionately increased. Such increased flow rate coupled with increased vacuum level causes an undesirable increase in the exhaust fan power requirements and additionally increases the amount of makeup air which must be supplied to the dryer hood.
The present invention overcomes the aforementioned problem by providing at least two internal flow restrictions. Such restrictions are located inside the roll near the outer edges thereof to allow the vacuum to be maintained at a high level adjacent to the edges or ends of the shell where the increased vacuum is most needed. Additionally, the arrangement of the present invention further allows the vacuum to be maintained at a lower level in between the edge or end portions of the roll shell where the increased vacuum is not required. Such an arrangement greatly reduces the volume of air which must be exhausted from the roll and thereby minimizes the exhaust fan horsepower.
Therefore, it is a primary object of the present invention to provide a vacuum roll transfer apparatus that overcomes the aforementioned problems associated with the dryer section arrangement disclosed in U.S. Ser. No. 014,569 and which makes a considerable contribution to the paper drying art.
Other objects and advantages of the present invention will be apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention as described hereinafter taken in conjunction with the annexed drawings and as defined in the appended claims.