Drying and gas or vapor contact with solids – Apparatus – Rotary drums or receptacles
Reexamination Certificate
1999-09-02
2003-05-13
Weiss, John G. (Department: 3629)
Drying and gas or vapor contact with solids
Apparatus
Rotary drums or receptacles
C034S273000, C034S269000
Reexamination Certificate
active
06560893
ABSTRACT:
The invention is described below as it applies to paper-making apparatus, because of its particular value in that context. However, in some respects the invention is applicable to other uses in which material to be heat treated is carried into contact with a heated cylinder.
BACKGROUND OF THE INVENTION
The Fourdrinier process of paper making involves a succession of phases. Initially a slurry of cellulose fibers in water is distributed on a screen and some of the water is drained off. A web is formed which is then transported by a felt or a succession of felts to pass a number of nip rollers in a press section. The felt and the formed web are squeezed between the nip rollers to extract water mechanically. In current practice, the web leaving the press section contains from 35 to 45% solids. The web then passes through a dryer section consisting of heated cylinders, in which the water content of the web is reduced by evaporation to roughly that of the finished paper.
Size coaters often follow the dryer section, followed by afterdryers and calenders, ending with the reel. The dryers and afterdryer sections may contain 60 or more heated cylinders. A felt is used to hold the paper firmly against many of the heated dryer cylinders, for assuring contact of the web with the heated surface and thereby promoting drying efficiency. Drying the web is the result of evaporation, caused by conduction of heat from the cylinders into the fibrous moisture-laden web. The term moisture-laden refers to water in all forms carried by the web, as free water or as moisture bound to the web's fibers.
In the U.S., roughly half the production is paperboard, which is formed into substantially thicker and heavier sheets than paper and newsprint. Many paperboard machines do not use papermaker's felts in the final dryer sections, because they are not necessary.
When the cold web enters the dryer section, fibers may be picked out of the web, adhering to the hot dryer cylinders. To suppress that effect, the temperature of the first series of dryer cylinders is comparatively low. Each successive cylinder's temperature is progressively higher until the sheet has been warmed up sufficiently for the web to encounter a hot dryer cylinder without concern for “picking” of fibers.
The following series of dryer cylinders effect a constant rate of drying. In this region the cylinders' temperature may be uniform. The paper making machine includes a falling rate zone that follows after the constant rate zone. The temperature of the steam in the successive cylinders of the falling rate zone is increased to 370° F. (187° C.). This is the practical upper limit for cylinders heated by steam under pressure. In the falling rate zone, the rate of evaporation declines progressively, due to the relatively dry condition of the web; in that condition, the web is a poor heat conductor, so that the transfer of heat to the web declines.
The highest pressure steam is typically delivered to the final dryer section, and a cascade steam system delivers reduced temperature steam upstream, to each cylinder of the series of dryer cylinders. It is complicated and expensive to provide steam at a pressure such that a specified high temperature is maintained in each of the cylinders. This is especially true when temperature changes are to be made.
Steam-heated cylinders are massive, both because of their large size and substantial wall thickness, They are usually made of gray cast iron for economy, and their walls are quite thick; e.g., 1″ to 2″ (25 mm. to 51 mm.) or more, to withstand the high internal steam pressure. A web may be 25 ft. (7.6 m.) wide, requiring cylinders that are slightly longer. The web may travel at 3300 ft./min. (1000 m./min.), or roughly 37 miles/hr. (60 km./hr.). That speed is impressive. The dryer section typically includes 60 cylinders. By any standard, the capital investment in a paper making machine is huge, and a considerable amount of space is needed.
Various types of paper making apparatus differ from that outlined above. For example, the “Yankee” type is characterized by inclusion of one very large diameter dryer cylinder; e.g., a diameter of 12 ft. to 18 ft. (3.6 m. to 5.5 m.). There, the wall thickness is particularly great, to withstand the pressure of the contained high-temperature steam and to allow for periodic grinding to restore surface smoothness.
The highest temperature of any steam-heated cylinder is limited by the corresponding pressure of steam that can be safely contained within the cylinder. The maximum internal steam temperature of a dryer cylinder (see above) is approximately 370° F. (187° C.) because of concern for the high steam pressure. It has been widely recognized that higher regulated temperatures, if feasible, would accelerate the drying process and would reduce substantially the number of dryer cylinders required.
Paper machine drying sections, worldwide, are almost universally heated by steam under pressure. Accordingly, it is appropriate to consider such apparatus in further detail, as a basis for appraising the advance in the art represented by the present invention.
As noted above, the temperature of a drying cylinder in a paper making machine is not determined by that which would be desirable from the point of view of performance, but by the limitations of cylinders heated by steam to withstand high pressures safely. This is evidenced by the large numbers of drying cylinders required in high-speed paper making machines or by the limited machine speed with lower temperature cylinders performing the drying function. Cylinders heated by steam under pressure have other significant limitations.
The external surface of a steam-heated cylinder responds slowly to an adjustment in steam pressure. This slow response time is manifested, for example, by the many minutes needed to bring the paper making machine from a cold start to full-speed operation. It is also manifested by the delayed change of a cylinder's external temperature in response to an adjustment in steam pressure.
It is virtually impossible to regulate the temperature of a cylinder wall from point-to-point along its length, for developing a desired temperature profile across the width of a web being dried. It is well-known that steam cylinder dryers are hotter at the ends, where no moist paper is present to absorb thermal energy from the cylindrical shell and from the end walls of the cylinder. Complicated, cumbersome arrangements have been proposed in an effort to compensate for the otherwise excessive cylinder temperatures at the margins of the web. These have been intended to control edge curl caused by unrestrained and excessive drying at the edges of the sheet. However, no easy, pitaical way has been found for varying the cross-machine temperature profile of a cylinder heated by steam under pressure.
The cross-machine moisture profile of a web emerging from the main dryer in a machine for producing paper and paperboard tends to develop non-uniformity not only at the margins but also at other portions of its width. This results from cumulative effects in the forming, press, and dryer sections. A web with moisture streaks is poorly suited to being coated as with size; moisture variations of the web cause the coating to be non-uniform. Also, a web whose cross-machine moisture profile is non-uniform has a tendency to render the calendering non-uniform.
The foregoing and other characteristics of a machine for making paper or paperboard, having dryer cylinders heated by steam under pressure, are impaired by some of the traits of the cylinder wall. Transfer of heat from the steam to the outer surface of the cylinder which contacts the web is impeded by many factors, including:
a) The considerable thickness of the cylinder wall needed for containing steam under the high pressure corresponding to the steam temperature, noting that the actual wall thickness is greater by a safety factor of 2.8 times that theoretically required for withstanding the steam pressure;
b) The poor heat conductivity of gr
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