Paper making and fiber liberation – Apparatus – Web creping or crinkling type
Reexamination Certificate
1993-12-08
2001-05-15
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Apparatus
Web creping or crinkling type
C034S085000, C015S256510
Reexamination Certificate
active
06231724
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to papermaking structures which resist thermal distortion in general, and to doctor blade support structures in particular.
BACKGROUND OF THE INVENTION
Paper is manufactured in widths of up to four-hundred inches. The paper web must be uniform across its entire width. The width of the paper web, combined with the requirement for uniformity, means that the components of a papermaking machine must maintain dimensional stability across the width of a paper web.
Components of a papermaking machine where dimensional stability is particularly important include the head box, which must uniformly distribute paper fibers under the fourdrinier paper-forming screen, quarter heads, size press heads, the support beam for induction heaters, and the supporting framework for cross machine scanners, as well as cooled reel drums.
One area where dimensional stability is particularly important is in the support of doctor blades which are used in applying coatings to the web as well as for cleaning roll surfaces and, in the case of the Yankee dryer, removing the dried web from the dryer surface. To function properly, doctor blades must engage the surface of the roll or the surface of the web uniformly over the entire length of the web. A problem associated with thermal distortion of the doctor blade support makes retaining dimensional stability across the width of the papermaking machine difficult. Doctor blades are invariably utilized on heated rolls forming part of a dryer. Thus, the blade and its support structure are inevitably heated by radiative heat transfer from the roll or the overlying web to the exposed surfaces of the doctor support structure. Radiative heat transfer has an effect on only the exposed or illuminated portions of the support structure, thus inducing uneven heating within the structure. This leads to thermal distortion and lack of dimensional stability.
One solution to thermal distortion in the doctor blade support structure is the simple and expedient method of applying steam heating to the structure so that all the surfaces comprising the support structure are held at a uniform temperature by the condensation of steam thereon. However, this process is wasteful of energy and requires constantly supplying and controlling the steam pressure and quality supplied to the doctor blade support structure.
What is needed is a self-contained doctor support structure which prevents thermal gradients from forming within the support structure and is thus structurally stable when used in proximity to heated surfaces.
SUMMARY OF THE INVENTION
The doctor of this invention includes a doctor blade and a doctor structural support which spans the width of the roll used in paper manufacturing. The doctor structural support is positioned above a heated dryer roll. The doctor blade is mounted to the support and extends into engagement with the dryer roll surface for cleaning thereof. The doctor blade structural support has the form of a hollow three-walled structural member of roughly triangular cross-section, one side of the triangular cross-section being closely spaced from and parallel to a plane substantially tangent to the surface of the dryer roll. A quantity of water is disposed within the hollow structural member and resides on the inside surface of the structural member which faces the heated dryer roll surface.
The interior of the doctor support structure is sealed from the atmosphere and is evacuated so that the interior of the structure contains only water and water vapor. Heat radiating from the dryer roll surface to the doctor support structure causes water disposed on the inside surface adjacent to the dryer roll surface to evaporate, thus raising the internal vapor pressure in the sealed interior of the doctor support structure. The water vapor continuously condenses on the inside surfaces of the support structure which are not exposed to radiant heating from the dryer roll. This evaporation and condensation of water within the interior hollow support structure maintains relative temperature uniformity from top to bottom of the support structure. The temperature uniformity in turn leads to a minimizing of thermal distortion caused by the heat transfer between the doctor support structure and the dryer roll.
The apparatus disclosed herein can more generally be used wherever thermal distortion is a problem in a papermaking machine. In general, support structures subject to thermal distortion can be formed with hollow interiors which communicate with all portions of the structure subject to thermal distortion. A liquid can be retained on the surfaces of the support structure which receives heating, the interior of the support structure being evacuated so that only the liquid and its vapor are present in the interior void. The liquid is chosen for a particular application depending on the temperature extremes to which the support structure is exposed.
Examples of other papermaking equipment which could benefit from this apparatus for preventing thermal distortion include a paper forming head box, coater heads, size press heads, support beams for induction heaters, supporting frames for cross machine scanners, and cooled reel drums. The above list is not to be considered exhaustive, but the apparatus disclosed herein can be used anywhere in a papermaking machine where thermal uniformity needs to be maintained.
It is an object of the present invention to provide a doctor and doctor support structure which are less sensitive to thermal distortion.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3001296 (1961-09-01), Carvill
patent: 3387317 (1968-06-01), De Noyer et al.
patent: 3450098 (1969-06-01), Williams, Jr.
patent: 3526017 (1970-09-01), Kleimola
patent: 3611471 (1971-10-01), Meskanen et al.
patent: 3624860 (1971-12-01), Grimston et al.
patent: 1177467 (1959-07-01), None
patent: 2929620 (1979-07-01), None
Busker Leroy H.
Chance J. Larry
Pulkowski Jeffrey H.
Beloit Technologies Inc.
Lathrop & Clark LLP
Nguyen Dean T.
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