Paper making and fiber liberation – Apparatus – Running or indefinite length product forming and/or treating...
Reexamination Certificate
1999-09-22
2001-10-30
Hastings, Karen M. (Department: 1731)
Paper making and fiber liberation
Apparatus
Running or indefinite length product forming and/or treating...
C162S358300, C162S361000, C100S172000, C492S046000
Reexamination Certificate
active
06309512
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to pressing devices for removing water from a web of fibrous material in a paper-making or board-making machine. The invention relates more particularly to impulse-pressing devices in which the web is passed through a nip and the web in the nip is rapidly heated such that high pressure and high temperature are simultaneously produced in the web.
BACKGROUND OF THE INVENTION
Pressing a wet paper web in a press section at elevated temperature has been shown to result in a number of beneficial effects. The technology, known as impulse technology, impulse drying or impulse pressing, was first described in SE-7803672-0, corresponding to U.S. Pat. No. 4,324,613, which discloses a roll press. The top pressure utilized in the process described therein lies within the range of 3-8 MPa and the surface temperatures between 150° C. and 350° C. for a cylinder in a press nip of conventional type. The dwell time during which a given region of the web is within the nip in such a roll press, however, is only a few milliseconds, which is too short a time to be able to take full advantage of the favorable effects of pressing at high temperature. It has therefore been proposed subsequently to utilize impulse drying in a heated shoe press in which the nip is extended so that the dwell time for the heat treatment is greatly extended.
In impulse drying, high temperature and high pressure are combined for a brief period of time in order to give extremely high heat flows to the web. The envelope surface of a steel or cast iron roll has maximum heat flow in the range of 2-8 MW/m
2
, which results in extremely high dewatering speeds. The mechanism forming the basis of these high dewatering speeds, with consequent high dry solids content, has not yet been fully discerned. One theory presented is that, through its expansion, the steam which is developed in the vicinity of the hot surface of the heat-transfer element nearest the fibrous web helps to displace water remaining in the fibrous web into the felt that is in contact with the fibrous web. Another theory is that high dry solids content is achieved by virtue of the reduced viscosity of the water caused by the high temperature, in combination with water at high temperature (>100° C.) being quickly vaporized when the web leaves the nip and the pressure thus drops to atmospheric pressure.
According to the '613 patent, a burner supplies heat to one roll in a press with two rotating rolls. The heat is supplied to the envelope surface of the roll immediately before the press nip. The patent specification states that the roll may have a surface layer with low thermal conductivity, thereby enabling it to maintain a high temperature.
U.S. Pat. No. 4,738,752 describes an extended, heated press nip where the fibrous web encounters a hot surface formed by a rotatable press roll or a metal belt passing in a loop around a plurality of guide rolls. The press roll or belt is heated by a heat source. The press roll may have a first coaxial layer and a second coaxial layer extending around the first layer and having a coefficient of thermal conductivity that is greater than that of the first layer. The first layer may consist of ceramic, whereas the second, outer layer consists of metal and has a thickness of 0.0127-1.27 cm. The layers are in intimate contact with each other and, together, form a unitary body.
When a roll of the described known types is heated from the outside, the outer layer will be hotter than the layer or layers disposed inside the outer layer which is rigidly joined to the next inner layer. The outer layer will therefore expand more than the inner layer, thereby causing stresses to arise between the two layers. Even if the roll is homogenous, the same differences will arise in expansion and stresses between the outer and inner parts. To reduce the risk of damage in the roll as a result of such stress differences, the initial heating of the roll must take place slowly. Another problem is that it is difficult to maintain the geometric shape of the roll across the machine direction, because of difficulties in maintaining the same temperature along the envelope surface of the roll and at the end walls. Considerable stresses also arise between these construction elements since they cannot expand freely independently of each other, and the envelope surface can become bowed outwardly or inwardly across the machine direction. Since the inner layer or part of the roll will also absorb some of the thermal energy supplied, the heating costs will be high.
One drawback with the use of a metal belt as a heat-transfer means is that it must be arranged in a loop with at least two rolls, thus constituting a space-consuming configuration. To allow the surface of the belt to be cleaned with a doctor, a counter roll must be arranged inside the loop of the belt in front of the doctor. Another drawback is that the belt cannot usually be coated with layers to achieve certain release properties and certain thermal conductivity properties.
A number of the problems discussed above exist also in roll presses and calendars that supply heat to the web.
SUMMARY OF THE INVENTION
The present invention seeks to overcome at least some of the drawbacks noted above, by providing a pressing device and associated method in which a heat transfer element that heats and presses the web is allowed to freely expand in all directions upon heating so that deleterious stresses tend not to develop in the heat transfer element. In accordance with a preferred embodiment of the invention, a pressing device comprises first and second press members defining a nip therebetween through which the web is passed. The first press member includes a rotating component defining a rotating surface for exerting pressure on the web. The second press member comprises a generally cylindrical press body that rotates about a central longitudinal axis thereof, and a generally tubular sleeve that surrounds the press body. The press body urges the sleeve toward the first press member. In preferred embodiments of the invention, the press body causes the sleeve to rotate with the press body, although alternatively the sleeve can be rotated by a separate driving mechanism. The sleeve is movably connected to the press body such that the sleeve is free to expand in all directions relative to the press body. The pressing device further includes a heat source arranged to continuously heat the sleeve such that heat is transferred from the sleeve to the web.
Advantageously, the sleeve's inner surface is radially spaced from the press body's outer surface over the majority of the circumference of the sleeve, the two surfaces coming into contact in the vicinity of the press nip. The sleeve may be substantially rigid or flexible. The second press member preferably includes one or more stop elements arranged at one end thereof for axially restraining a corresponding end of the sleeve, while still allowing the sleeve to expand axially as well as radially and circumferentially. Various types of stop elements are disclosed. As an alternative to the stop elements, the press body can be cambered over its length (i.e., bowed outwardly such that its outer surface is convex in the cross-machine direction), and the inner surface of the sleeve can be correspondingly cambered (i.e., concave in the cross-machine direction). The cambered surfaces tend to restrain the sleeve to reside in its desired location, while still permitting the sleeve to freely exand in all directions.
The sleeve advantageously can have a plurality of circumferential grooves formed in its inner surface facing the press body, the grooves being spaced axially along the length of the sleeve. The grooves reduce the contact area between the sleeve and the press body, thereby reducing heat transfer from the sleeve to the press body.
The sleeve can be driven to rotate by frictional contact between the press body and the inner surface of the sleeve. Alternatively, the press body and sleeve can have element
Bengtsson Jan Martin
Brox Erik
Helgesson Leif Erik
Jansson Kjell Sune Evald
Kilian Anna Malin Katarina
Alston & Bird LLP
Hastings Karen M.
Valmet Karlstad AB
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