Heat exchange – Movable heating or cooling surface – Rotary drum
Patent
1996-04-01
1997-08-12
Rivell, John
Heat exchange
Movable heating or cooling surface
Rotary drum
165 89, 492 46, 432246, F28D 1102
Patent
active
056555962
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a steam-heated roll of the kind as described in the preamble of claim 1.
BACKGROUND OF THE INVENTION
For the treatment of web-like media, such as e.g. paper, by pressing, drying and smoothing, rolls are used which have large diameters and are heated. In this context "large" is appreciated to be a diameter of at least 1,000 mm.
These involve e.g. the so-called "center rolls" of a press section of a paper machine, the usual drying cylinders in the drying section and particularly the heated rolls in smoothing sections, the so-called "gloss cylinders" and "soft calenders". The so-called "Yankee cylinder" represents a special case, a combination of drying cylinder and smoothing roll.
Heating such rolls is done preferably from within by means of heat transfer fluids, preferably water, steam or heat transfer oil. The increase in the surface temperature of the rolls achieved thereby permits either their function, by affecting in the case of drying cylinders the evaporation of the moisture in the paper web, or promotes their function, because the water in the press can be more easily squeezed out from the paper web at elevated temperature and reduced viscosity or because the plastification ability of the paper fibers is enhanced at elevated temperature so that they can be smoothed more easily.
When heating is done by means of steam, the rolls are normally of the cylindrical kind. The steam is usally introduced into the interior of the roll cylinder where it condenses on the inside surfaces. The condensate is drawn off from the roll via a syphon.
The drawback in this arrangement is that in such a configuration the complete interior space is subjected to steam pressure. This results in restrictions mainly stemming from safety considerations. These relate to the level of the permissible steam pressure and thus to the achievable surface temperature which, in turn, restricts the effectiveness of the process. Restrictions also exist--depending on where employed--as regards the application of roll materials. Since, for instance in U.S.A. the so-called chilled cast iron, as is usually employed for a smoothing roll, is not standardized, it is out of the question for such applications. Large rolls for gloss calenders thus need to be manufactured from gray cast iron or nodular cast iron which as regards the wearing properties of the roll surface is unfavorable.
Due to the mechanical stresses the wall of the roll cannot be designed as thin as would be desirable for a good conduction of the thermal energy to the heat transfer medium.
When certain rotative speeds are attained, a stable ring of condensate is formed on the inside of the roll due to the centrigugal effect and dependent on the roll diameter. Then, the steam no longer condenses on the wall of the roll, but in the condensate ring. This results in a deterioration, on the one hand, of the heat transfer from the steam to the roll, but also, on the other, of the temperature profile at the roll surface.
Some of the cited drawbacks, such as e.g. the poor temperature profile, were overcome in the case of cylindrical rolls by changing to hot water for the heating. For this purpose a cylindrical displacement body was secured in the central bore of the roll so that a narrow, ring-shaped gap remained between the displacement body and the inner bore. The hot water was then forced through this gap at velocities exceeding 1 m/s.
The drawbacks of the large wall thickness, required for mechanical reasons, and the restriction due to the internal pressures remained, however, and apart from this such rolls are of a heavy-weight structure.
The aforementioned drawbacks were overcome in part by the so-called peripherally drilled rolls. In this case the heat transfer fluid--water or heat transfer oil--flows through axially parallel drilled passages located just beneath the surface of the roll. This enabled the distance for conducting the heat from the heat transfer medium to the surface of the roll to be decisively reduced.
Unfortunately,
REFERENCES:
patent: 4658486 (1987-04-01), Schonemann
patent: 4955433 (1990-09-01), Zaoralek
patent: 4964202 (1990-10-01), Pau et al.
patent: 5072497 (1991-12-01), Zaoralek et al.
patent: 5152333 (1992-10-01), Barbe et al.
International Search Report.
Atkinson Christopher
Rivell John
Schwabische Huttenwerke GambH
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