Drying and gas or vapor contact with solids – Apparatus – For diverse operations on treated material
Patent
1997-07-15
1998-07-14
Bennett, Henry A.
Drying and gas or vapor contact with solids
Apparatus
For diverse operations on treated material
34 95, 34116, F26B 1326
Patent
active
057785554
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an apparatus for drying and smoothing a fibre web, the apparatus comprising two endless bands that are impermeable to air and have a good thermal conductivity; first turning rolls, the first band being arranged to turn around said first turning rolls; and second turning rolls, the second band being arranged to turn around said second turning rolls; the first and second bands being arranged to run part of the way in parallel such that they define a drying zone between them, the first band being heated and the second band being cooled, and a fibre web and at least one felt or wire running between the bands such that the fibre web is in contact with the first, heated band, and the felt or wire is between the fibre web and the second, cooled band.
Drying of a fibre web between two parallel metal bands moving in the same direction such that the fibre web is in contact with the heated metal band and that there is a felt between the fibre web and the second, cooled metal band, whereby the steam separated from the fibre web by heating condenses to the felt by the effect of the cold metal band, is previously known from many patent publications, such as Finnish Patents 61,537 and 78,755. The basic idea is that two endless metal bands are arranged to run around turning rolls and that against the surface defined by the loops that the bands form are provided pressure chambers containing hot steam and water, respectively, such that the pressure produced presses the hot and cold bands against the fibre web and felt located between them. The bands, which are located between the pressure chambers, together with seals provide one side for the pressure chambers such that the steam and water can directly affect the bands. The operation of the apparatus is fully known per se and has been disclosed e.g. in the above patent publications, which are incorporated herein by reference.
In the solution of Finnish Patent 61,537, the temperature of the web is quite high, typically over 120.degree. C., and due to the pressurized steam and water, the mechanical z-pressure is also high, typically over 1 bar. Dried in this manner, webs that contain large quantities of lignin and hemicellulose are rended very strong, and a smooth surface is provided on the side of the web that faces the heated metal band. A drawback of the high compression force, however, is that the web density becomes very high in the direction of thickness, i.e. the web thickness collapses in a way, which often impairs the stiffness of the web. If this drawback were to be avoided in the solution of the publication, the temperature of the cooling water would have to be raised so close to the temperature of the heating steam that the drying rate would notably slow down and the apparatus would have to be made much longer.
Finnish Patent 61,537 also teaches that the cooling space of the apparatus can be divided into various separate spaces. This, however, is very complicated to implement, and it is equally complicated to arrange counter supports on the steam box side, which are needed because of different pressures prevailing in the cooling space.
Finnish Patent 78,755, in turn, teaches a solution in which there is essentially no external excess pressure outside the metal band, but the metal bands are subjected to approximately normal atmospheric pressure. The thermal energy needed for drying the web is obtained by pre-heating the metal band, whereby the thermal energy stored therein vaporizes the water present in the web, starting the drying process. The hot metal band enters the drying zone at a temperature of about 150.degree. to 200.degree. C., cooling correspondingly in the travel direction of the machine as heat transfers from the band to the web. The web is here subjected to a mechanical z-pressure of at most close to 100 kPa, while the temperature of the cooled metal band is maintained low, e.g. at about 20.degree. C., by the use of e.g. a cold water jet or some other cooling method. If the difference of temperature between the metal bands rem
REFERENCES:
patent: 4461095 (1984-07-01), Lehtinen
patent: 4719707 (1988-01-01), Klapp
patent: 4958444 (1990-09-01), Rautakorpi et al.
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patent: 5553392 (1996-09-01), Hanaya
patent: 5594997 (1997-01-01), Lehtinen
patent: 5619806 (1997-04-01), Warren
patent: 5706587 (1998-01-01), Lehtinen et al.
Juntunen Taisto
Lehtinen Jukka
Rautakorpi Paavo
Bennett Henry A.
Gravini Steve
Valmet Corporation
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