Drying and gas or vapor contact with solids – Material treated by electromagnetic energy – Infrared energy
Patent
1997-12-03
1999-10-19
Bennett, Henry
Drying and gas or vapor contact with solids
Material treated by electromagnetic energy
Infrared energy
34269, 34119, 34120, D21F 500, F26B 1102
Patent
active
059668350
DESCRIPTION:
BRIEF SUMMARY
This application is a division of Ser. No. 08,462,755 filed Jun. 5, 1995 which is a PCT/U.S.96/0873 filed Jun. 5, 1996.
The present invention relates to heated rotary cylinders for treating webs of material. The invention in novel cylinders is described below as it applies to paper making apparatus, both because the novel cylinders have particular value in that context and because the novel cylinders form parts of novel paper making apparatus. However, in some respects the invention is applicable to other apparatus in which a web of material is heated by 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 moistureladen 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.degree. F. (187.degree. 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 heal, 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" (25mm. 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.)
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"Gas Infrared's Steady Success in Paper Drying" pp. A4-A7--Natural Gas Applications in Industry--Spring 1994 Issue.
Bennett Henry
Gravini Steve
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