Presses – Concurrent pressing and conveying – Roll type
Reexamination Certificate
1999-09-02
2001-02-20
Vo, Peter (Department: 3721)
Presses
Concurrent pressing and conveying
Roll type
C100S099000, C100S15500G, C100S172000, C100S327000
Reexamination Certificate
active
06189442
ABSTRACT:
TECHNICAL FIELD
This invention relates to an apparatus for performing calendering of a fiber web, preferably using an enclosed shoe roll.
PRIOR ART AND PROBLEMS
Calendering of paper is performed in order to achieve a smooth surface of a fiber web, e.g. paper. Traditionally this is achieved by using two counter-acting rolls forming a nip within which a high pressure is applied to the paper surface in order to even out irregularities of the paper surface so as to form a smooth surface. A disadvantage by using the above mentioned method is that the high pressure acting on the web may cause excessive compaction of the web. As a result the thickness of the paper will be significantly reduced leading to relatively poor stiffness of the web after the calendering.
The above mentioned disadvantage can be reduced by using heat in combination with a relatively moderate pressure. The reason for this is that the fibers of the paper are plasticized if the temperature is sufficiently high (The temperature of plastification is normally about 170-210° C., i.e. depending on the moisture content and the properties of the fibers.) Accordingly if a sufficiently heated roll, e.g. 250° C., is used and sufficient heat transfer is achieved to the surface of the web passing the roll, a web may be produced having a smooth surface and relatively large thickness, which results in a much stiffer product than if a high pressure nip without heat would have been used.
For the above reason there are many applications where heat calendering is desired for the calendering process. A relatively recent problem in relation to heat calendering is the achievement of sufficient heat transfer, due to the trend towards higher and higher speed of the web. The faster the web moves through the nip the shorter time there will be for the transfer, i.e. shorter retention time. In U.S. Pat. No. 5,163,364 there is shown a method for solving the latter problem. U.S. Pat. No. 5,163,364 describes the use of an extended nip for obtaining sufficient retention time to ensure sufficient heating of the web surface during its travel through the nip. As shown in U.S. Pat. No. 5,163,364 the calendering zone is made up by a heated roll pressing from one side and an endless flexible belt which is pressed by means of a concave press shoe against the heated roll.
The endless flexible belt is preferably made of a material that comprises polymers, resulting in relatively poor heat resistance, i.e. if the heat exceeds a certain temperature, normally about 100° C., the flexible belt will be destroyed. Since the cost of such a belt is considerably high any over heating of the flexible belt must be avoided. This can be achieved by the paper web absorbing most of the heat in combination with a cooling of the flexible belt after having passed through the nip. However, if the paper web is broken, an arrangement as shown in U.S. Pat. No. 5,163,364 in combination with the use of a flexible belt comprising polymers (not mentioned in U.S. Pat. No. 5,163,364) would lead to the destruction of the flexible belt due to overheating, since the heated roll would then act directly on the flexible belt. This problem would be even worse if an enclosed shoe roll would have been used, since the cooling of an open flexible belt is easier to achieve than in a closed roll, i.e. an enclosed shoe roll. Furthermore, the use of an enclosed shoe roll requires that the flexible jacket is longer than the load shoe, i.e. it extends outside the nip on each side. Accordingly there will exist portions of the jacket which normally would not be covered by the web, since these portions do not participate in the calendering within the nip. This would result in a direct heat radiation from the heated roll to these portions, which might lead to over heating of the jacket and premature destruction.
Another related problem is the start-up process. Normally, the jacket of an enclosed shoe-roll is not driven by itself, but by means of friction once in contact with the fiber web. It is evident for the skilled person that the web will be negatively affected by such a starting-up process. Furthermore, such a start-up process also presents a possible risk of overheating of the belt at the moment of the start-up, since the belt does not move during the first contact with the web within the heated nip, i.e. an extreme heat transfer to the belt will occur.
Another related problem is how to avoid undesired wear of a flexible belt/jacket.
SOLUTION AND ADVANTAGES
The object of the invention is to provide a process and apparatus which eliminates or at least minimizes some of the disadvantages mentioned above. This is achieved according to one aspect of the invention.
A method for calendering a fiber web is provided, wherein the fiber web passes through an extended and heated nip, said nip being formed on one side by a cylindrical heated roll and on the other side by a flexible tubular jacket which is pressed against the heated roll by means of a concave load shoe, said tubular jacket surrounding a stationary support beam, which supports at least one actuator which may urge said load shoe and said flexible tubular jacket against said heated roll, and wherein the extension of the load shoe in the axial extension is less than the axial extension of the jacket and the heated roll such that when the load shoe is urged against the heated roll there will be formed tapered sections, at each side end of the jacket within the nip characterized in that said tapered section are substantially covered by said fiber web such that a small strip of the fiber web at each edge is not calendered in said extended nip.
By the solution according to the invention the jacket is protected from over heating also at the end portions by means of covering them with the web. This leads to big cost savings, because of prolonging the lifetime of the jacket. The uncalendered strips may be cut off at a later stage, but according to a preferred aspect said small strips are calendered in a preceding or in a subsequent nip, which results in unproblematic rolling up of the produced web and possibly also obtaining a larger amount of the fiber web as a final product, which increases the income for the producer.
According to further aspects related to the invention,
said flexible tubular jacket forms a part of an enclosed shoe roll such that the ends of said jacket has end walls mounted thereto, which end walls are rotatably mounted in relation to said support beam to form a sealed space together with said jacket, and in that at least one of said end walls is driven by means of a drive arrangement which drive arrangement may be activated to drive the end walls and thereby also the jacket independently of its position in relation to the fiber web or the heated roll.
said driving means is activated before the nip is closed in order to ensure a desired speed of the jacket at the moment of closure of the nip.
the speed of the web is measured and that the speed of the belt is synchronized with said speed of the web before it is brought into contact with it.
a detecting means which detects if the paper web is broken and a control system interconnected with said detecting means in such a manner that said driving means is activated if the web is broken and also at the same time that the jacket is moved away out of contact with the heated roll.
the speed of the web exceeds 600 m/min, preferably exceeds 800 m/min, and more preferably exceeds 1000 m/min.
the web being produced is paper whereby the speed of the web exceeds 1000 m/min, preferably exceeds 1500 m/min, and more preferably exceeds 1800 m/min.
the temperature of the surface of said heated roll exceeds 150 C, preferably exceeds 170 C, and more preferably is about 200-250 C.
the linear load within the nip is less than 500 kN/m, preferably less than 400 kN/m, and more preferably about 320-380 kN/m.
the linear maximum pressure within the nip is less than 15 MPa, preferably less than 13 MPa, and more preferably about 8-12 MPa.
the force transmission from the drive arrangement to at least one of sa
Berglind Thomas
Honkalampi Petter
Safman Nils-Erik
Torvi Timo
Alston & Bird LLP
Huynh Louis
Valmet Corporation
Vo Peter
LandOfFree
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