Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
1999-06-24
2001-03-13
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C362S263000, C362S266000, C362S469000, C362S469000, C362S469000, C073S037600, C073S159000, C700S128000, C700S129000
Reexamination Certificate
active
06200422
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of controlling a moving paper web during manufacture or subsequent processing. The invention further relates to an apparatus for controlling a moving paper web during manufacture or subsequent processing.
BACKGROUND OF THE INVENTION
The paper web deforms geometrically during manufacture, or during subsequent processing operations, especially operations involving wetting or drying of the paper web. This deformation includes shrinking as its water content falls below about 35% by mass, and stretching or elongation due to applied forces. Forces are applied especially in the direction of movement of the web, including a tension in the plane of the web. The tension is generally not uniform, since the shrinkage and elongation are not uniform, and these three properties are related to each other. However, shrinkage and elongation are properties of the paper web, while tension is a property of the process. Nonuniformity in the web tension leads to problems in operating the paper making or paper processing equipment, including web breaks which cause production interruptions. Nonuniformity in the elongation leads to a variety of problems, including sheet breaks, caused by the fact that the paper web is longer in some place than in others.
U.S. Pat. No. 5,694,448 discloses a solution for regulating the web tension profile. That method requires measurement of the caliper and grammage profiles in addition to the web tension profile. Further, it achieves its aim of controlling the web tension profile by manipulating the actuators which are also used to control the profiles of caliper and grammage. Thus, improvements in the web tension profile are generally achieved at the cost of effects in the caliper and grammage profiles, and a compromise solution is obtained. Further more, the solution enables the regulation of the web tension profile only, and does not consider the web elongation profile.
An article “Optimize or compromise? The art of former section tuning”. Odell M. 51 Appita Annual General Conference 1997 Proceedings, Volume 1 discloses controlling the web tension profile by using a steam box in the press section to adjust the shape of the moisture profile going into the dryers. By making the edges drier and thus changing the drying rate profile it is possible to reduce the tension profile error at the reel. The moisture profile at the reel is corrected using rewetting in the dryers. In this case too, only the web tension is adjusted, and does not consider the web elongation profile. In failing to consider the web elongation, control of the web tension can lead to an inferior solution with undesirable effects on the web elongation profile.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method and an apparatus, in which the above drawbacks can be avoided.
The method of the invention is characterized by comprising the steps of measuring a web elongation profile and a moisture profile and controlling the web elongation by at least two actuators which affect the cross machine direction (CD) moisture profile, said actuators being situated at different stages in the web manufacturing process, and means for measuring the web elongation profile and the moisture profile being situated after the CD moisture actuators.
Further, the apparatus of the invention is characterized by comprising means for measuring a web elongation profile and a moisture profile, at least two actuators, which affect the CD moisture profile and means for controlling the web elongation by said CD moisture actuators, said CD moisture actuators being situated at different stages in the web manufacturing process, and the means for measuring the web elongation profile and the moisture profile being situated after the CD moisture actuators.
The basic idea of the invention is that the web elongation is controlled by use of at least two actuators which affect the CD moisture profile, said actuators being situated at different stages in the web manufacturing process. Preferably, a first CD moisture actuator is situated in the wet press or elsewhere near the wet end of the paper machine, and a second CD moisture actuator is situated near the reel or elsewhere near the dry end of the paper machine. A first moisture profile measurement device and a first web elongation profile measurement device are situated after the second moisture profile actuator. The moisture and web elongation profile measurements need not be at exactly the same location, but preferably there are no wetting or drying processes between them.
The elongation profile can be measured directly for example by using a method disclosed by K. R. Wadhams et alii, “The measurement of differential CD shrinkage”, Paper Technology, January 1991. Alternatively, the elongation can be inferred tram a known tension profile and a known shrinkage profile. The tension profile can be measured for example by using the method and equipment presented in U.S. Pat. No. 5,052,233. The shrinkage profile can be measured for example by using the method disclosed by E. DiMauro et alii, “New on-line sensor for paper shrinkage measurement”, Proc Control Systems '94 (May 31-June 2, Stockholm, Sweden).
Measurements of shrinkage or elongation may be provided from laboratory analysis of samples taken from the paper, or may be provided using on-line measurements. Since tension is a property of the process, it must be provided from on-line measurements
Cellulose fibers swell when wetted, and shrink when water is removed. This dimensional change is less along the axis of the fiber than across the axis. Paper is formed from an aqueous suspension in the wet end and dewatered through pressing and drying to a moisture level of typically 5-7%. As a result, it shrinks by several percent. No shrinkage occurs in the initial dewatering in the forming section and presses, as the water content is too high and fiber bonding is weak. As soon as the sheet has sufficient fiber bonding strength to support itself, shrinkage will accompany further dewatering.
However, the paper web is dimensionally restrained for part of its journey, being supported and transported by fabrics which impose frictional constraints on shrinkage. It is completely supported through the forming and pressing section, but open draws occur thereafter. When subsequent dewatering occurs to these fabrics, the sheet receives an increment of strain, which can be relaxed only when it is in an open draw, where it is less constrained in dimension. If the sheet is sufficiently wet, the strain is generally plastic (no stress is induced), or viscous (stress depends on rate of deformation, rather than on amount). This is the case in the forming and pressing sections, and in the initial drying stages. Strain due to constrained shrinkage when the sheet is drier is generally elastic—it induces a stress or tension. The elastic strain will be maintained if suitable tension is applied, and will relax otherwise.
The sheet is under machine direction (MD) tension in open draws, to support the mass of the web, stabilize its path, provide forces needed to detach it from fabrics and cylinders, and to overcome aerodynamic drag. Local MD tension prevents relaxation of strain in the sheet in the MD. However, if the strain can relax in the CD, this leads to local Poisson or Poisson-like elongation in MD, and results in a lower local MD tension. There is no externally applied CD tension per se, but the sheet is dimensionally restrained in CD at each end of the draw, and the length of the draw is typically much shorter than the sheet width. As a result, there is a CD tension profile induced in the open draw as a result of the CD straining due to dewatering. This CD tension is much less than the MD tension, and falls essentially to zero at the sheet edges, which are less restrained than the sheet center. Thus, although the sheet stretches in the MD both plastically and elastically, more than countering the effect of shrinkage in MD, it shrinks overall in the CD, and the shrinkage is
Kniivila Juha
Shakespeare John
Alston & Bird LLP
Fortuna Jos'e A.
Neles Paper Automation Oy
Silverman Stanley S.
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