Paper making and fiber liberation – Processes and products – With measuring – inspecting and/or testing
Reexamination Certificate
1998-08-12
2001-08-28
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
With measuring, inspecting and/or testing
C162S199000, C162S252000, C162S262000, C162S263000, C034S242000, C034S405000
Reexamination Certificate
active
06280573
ABSTRACT:
BACKGROUND OF THE INVENTION
Many devices exist for performing an operation on a moving web wherein a gas or gas pressure differential is applied. For example, in the art of papermaking, a wet or embryonic paper web can be partially dried or dewatered by means of an applied gas pressure differential using nozzles, a compressive roll with holes or grooves for pressurized air, an “air press” or other devices wherein a gas pressure differential forces air into a web to displace liquid water and/or to remove water by evaporation. Many prior systems, particularly those having compressive rolls, impose high compressive force on the web and are not suitable for many low density materials such as tissue. Systems of nozzles are typically inadequate due to the low residence time provided for air penetration into the web. Nozzle systems also either suffer from high leakage if the nozzle is not in contact with the web or from high fabric wear if the fabric wraps the surfaces of the nozzle to provide some degree of sealing. A fabric wrapping a nozzle with a small effective radius of curvature is particularly likely to experience wear problems.
Flat pressurized boxes, such as steam boxes, while capable of good residence time in some cases, suffer from high leakage from the sides of the steam chamber. Steam boxes for heating paper webs in particular have tended to be flat boxes with a finite gap between the web and the sides of the box. Such gaps or clearances allow significant volumes of air to enter, in part due to the air boundary layer traveling with the web. Intentionally bleeding steam to oppose the boundary layer or using a steam curtain to prevent entry of the boundary layer is inherently inefficient.
Recognizing the difficulty of providing adequate seals in pressurized paper drying chambers, some have proposed the use of high-velocity heated air impingement that relies on the momentum of the air to push through the web for paper drying without attempting to use seals. This technique is intended to minimize lateral migration of the drying air along the surface of the web, thereby reducing the need for sealing. Even if this method reduces the lateral flow of air, the extent of treatment is limited by the brief contact time of the pressurized gas with the web due to the narrow jets employed. Without suitable seal elements, leakage still will not be prevented.
Rotary devices, such as cylindrical through dryers and suction rolls, can be operated to pass air through a fibrous sheet but are complex and costly devices. Further, the surface of the rotary device or other supporting surfaces in contact with the web have significant closed areas where gas flow is blocked, resulting in nonuniform penetration of the gas through the web.
An excellent system for effective gas treatment of a moving web is the dewatering system disclosed in commonly owned copending application Ser. No. 08/961,915 of Hada et al. filed Oct. 31, 1997 and incorporated herein by reference in its entirety. This application teaches an air press wherein a non-rotating upper plenum is used to apply pressurized air to a moist paper web while the web is sandwiched between two pervious fabrics. The pressurized plenum cooperates with a lower vacuum box on the opposite side of the sandwiched paper web such that the overall pressure differential across the web is greater than if the pressurized plenum were used alone at a predetermined pressure. An important issue in the operation of an air press is preventing unnecessary leakage of gas out of the plenum into the surrounding atmosphere. Hada et al. disclose a set of cross-direction seals (seals running in the cross-direction) for the leading and trailing edges of the plenum (the leading edge being closest to the headbox) and a pair of machine-direction seals running in the machine direction to seal the side edges of the plenum. Hada et al. also disclose a lever system for increasing sealing force on the seals responsive to measurement of air pressure in the plenum. The principle of operation is that excessive leakage will result in a reduction of pressure in the plenum, which can then be compensated by increasing the applied pressure to the systems of seals.
Though capable of opposing leakage and preventing large leaks, such a system may lead to excess fabric wear, particularly in wide machines, because sealing is generally performed across the entire width of the machine, rather than solely in the locations where leak reduction is needed. Further, use of internal pressure measurements to detect leak control may lead to some false readings of leakage when pressure fluctuates for other reasons, such as changes in web properties or compressor operation or air temperature. Further still, a single macroscopic measure of pressure cannot be used to locate specific leaks, only leakage in general. Therefore, what is lacking and needed is a method and apparatus for detecting the specific location of leaks and applying corrective remedies to prevent leakage only at the locations where leakage is occurring.
SUMMARY OF THE INVENTION
Pressurized or depressurized web treatment chambers for processing moving webs can operate more efficiently if fluid leaks are detected and controlled with localized leak detection means and with localized leak reduction means, wherein the leak reduction means is operatively responsive to the leak detection means such that local leaks are effectively sealed or reduced in severity.
In particular, a control method has been discovered for web treatment systems for moving webs, the treatment system involving at least one chamber at a pressure substantially different from the ambient pressure, wherein localized leaks are detected by one or more leak detectors and wherein the leaks are reduced by application of pressure reduction means responsive to the one or more leak detectors. The control method of the present invention can provide improved means for prevention of leaks in web treatment systems of all kinds wherein a moving web passes through a pressurized treatment chamber.
Further, the control system of the present invention can be used to prevent leakage of chemicals from a web treatment chamber or to prevent excess infusion of atmospheric air or oxygen into web treatment chamber by means of localized leak or infusion detectors operatively associated with localized leak reduction means to apply improved sealing in the localized regions where such action is needed. Such a system is desirable when the fluid in the web treatment chamber has a substantially different chemical composition than the ambient atmosphere and it is desired to prevent leakage of the treatment fluid into the atmosphere or to prevent leakage of air into the chamber. Again, it is desirable to maintain effective seals across substantial distances as a web passes through the seals through a combination of localized leak detection and reduction, rather than subjecting the entire apparatus or entire extent of the web and the seals to increased clamping pressures. As used herein, the term “leak” encompasses both the escape of fluid from within the chamber (e.g., the escape of pressurized air from an air press) and the infusion of fluid into the chamber (e.g., infusion of the atmosphere into a low pressure treatment chamber).
Cross-directional flexible seals with leakage control according to the present invention are desirable for the entrances and exits to enclosed pressurized web treatment chambers, such as steam heating chambers, while machine-direction edge seals and a corresponding control system are desirable for other web treatment devices.
Hence, in one aspect, the present invention resides in a control system for detecting and reducing fluid leaks along a seal between a moving web and a web treatment chamber, wherein the web treatment chamber applies a fluid at a pressure other than the ambient pressure to a surface of the moving web, the control system comprising:
a) a leak detector for indicating the presence and location of a fluid leak between the moving web and the seal; and
b) localized
Hada Frank Stephen
Hermans Michael Alan
Lindsay Jeffrey Dean
Charlier Patricia A.
Chin Peter
Gage Thomas M.
Kimberly--Clark Worldwide, Inc.
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