Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE lNV~N-llON
A number of different requirements are required on the rolls
and roll faces in different parts of a paper machine, such as, for
example, properties of durability, adhesion, transfer and detaching.
For example, in the so called closed press sections, which
are commonly used in paper machines, in conn~ction with the center
roll, one, or as a rule, several, pressed nips are formed. An
example of such press section is the press section which is marketed
under the trademark "Sym-Press II", which is shown in U.S. Patents
Nos. 4,209,361 and 4,285,766.
The smooth faced center roll in the press section, which is
a roll of a larger diameter as compared with the other rolls, is
usually made of rock, as a rule of granite. Since granite is a
non-homogeneous natural material of low tensile strength, the use
thereof in machine construction leads to considerable problems. If
it is desired to heat a granite roll, its deformations dependent on
temperature are non-linear and consequently difficult to predict.
Granite has relatively good properties of adhesion, transfer
and detaching of the web, which are reA~onc for its popularity for
use as a press roll material. The properties of detaching, however,
require further improvement, particularly in the case of unbleached
paper qualities.
As as known in the prior art, the web is detached by pulling
it freely without support off the face of the center roll in the
press. This free draw is quite critical in view of the operation of
the paper machine. In the free draw, a difference in speed is used
which extends the web, and this results in certain drawbacks.
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Furthermore, the free draw forms a problematical point susceptible to
breaks in the paper machine. In earlier times, the detaching was
always carried out mech~nically by pulling, and the heating took
place in the roll body, resulting in the entire roll being hot. As a
consequence, the roll become worn and the intervals between grindings
became short. The free draw of the web has become an even more
important problem because of ever increasing running speeds of paper
machines, and because different paper qualities are often produced by
a paper machine, with different adhesion to the face of the rock
roll, resulting in variations in the necess~ry tension required for
detaching of the web.
In U.S. Patent 4,919,759, issued April 24, 1990,
corresponding to Finland Patent Application No. 870308, a system
of regulation is described wherein the detaching of the paper web
from the smooth face of the center roll in a press can be better
controlled. It is a particular object of the invention therein to
provide such a system of regulation of the detaching of the web
wherein the tension of detaching of the web can be set optimally
irrespective of the dry solids content of the paper web, of the
surface energy of the material, and of the running speed of the paper
machine. The method in accordance with this application is
characterized in that in the method the temperature of the face of
the smooth faced press roll is regulated, and by means of this
regulation, the adhesion between the roll face and the paper web to
be detached is affected and as a result the detaching angle and/or
the detaching tension of the paper web is/are set within an optimal
range.
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On the other hand, the device of the above application is
mainly characterized in that in co~ection with the smooth faced
press roll, heating devices are provided by means of which the
temperature of which the smooth face of the press roll and thereby
the detaching of the web from the roll face are affected.
Still further, the invention of the above application is
based on the observation that the t~ ?rature at the interface
between the paper web and the smooth roll face from the web to be
detached affects the dry solids content, of the web, the surface
energies of the materials in contact and of the viscosity of the
water, which parameters again affect the adhesion between the paper
web and the water cont~ine~ therein, on the one hand, and the smooth
roll face on the other hand.
By establi~hing the inter~ep~n~enc~ of these parameters, and
by controlling the same and further, on the basis of this
information, regulating the temperature of the roll face by means of
the regulating system of the invention, it is possible to adjust the
detaching tension of the paper web to a suitable level even within
highly varying operating conditions. Thus, in the described
invention, when r~nn~ng different paper qualities and with different
rllnning speed of the paper machine, it is possible to adjust the
temperature of the smooth face of the roll to a specific set value,
which provides optimal detaching of the web and optimal rllnning
quality with the web quality and machine speed that is used at any
particular time.
In the above described invention, the central roll of the
press, or any other correspo~ing smooth faced roll from which the
paper web is to be detached, is a substantially metal-mantle roll
coated with a metal, with a ceramic material, or with mixtures of the
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same, or a cast iron roll, or an uncoated metal roll which are
arranged to be heated by means of adjustable heating devices. The
heating may take place from inside and/or outside the roll, at least
partly by means of teçhniques known in the prior art.
The invention of the above patent application is not
restricted for use in the detaching of the web from the central roll
of closed press sections of paper machines alone, but the invention
is also suitable and intended for controlling the detaching of the
web from a smooth faced roll in a press in general, i.e. also from a
roll other than a central roll.
Reference is also made as prior art to U.S. Patent
4,889,598, issued December 26, 1989, corresponding
to Finnish Patent Application No. 870309. According to this
application, the water present between the web and roll face is
heated, preferably vaporized, locally, within the area of the
detaching point, by means of heat applied, among other ways, to the
roll mantle, this to detach the web from the roll face. Since, in
accordance with this application, the work for the detaching of the
web is carried out by vaporization, it is not necefisAry to extend the
web to detach the web, which again permits a closed draw from the
center roll in the press to the drying section, e.g. onto the drying
wire.
It is accordingly an object of the present invention
to improve on the solutions achieved according to the Fi nni ~h Patent
Application Nos. 870308 and 870309.
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It is another object of the present invention to
provide better control of the detaching of the paper web
from the smooth face of the center roll in a press, and also
to provide a better system for the regulation of the
detaching of the web.
As still a further object, the present invention seeks to
provide suitable surface materials for use wherein a heating device
outside the roll is used.
With the above and other objects in view, the arrangement in
acGordance with the invention is mainly characterized in a
combination of a heating device and a roll coating such that the
heating radiation penetrates only through the paper or only directly
to the roll face, but does not heat the roll itself at a depth
greater than the roll face.
In accordance with the present invention, the combination of
the coating and the heating is effected in such a manner that the
dark surface of the surface material absorbs the light coming form
the heating device in an efficient manner, so that the only roll face
is heated and the inner part of the roll is not heated. Moreover,
the coating material is chosen so that it acts as a thermal
insulation, whereby the remainder of the roll i8 not heated and the
heating power is not wasted.
More specifically, the invention provides an arrange-
ment in a paper machine, comprising a roll having a roll
face with a coating thereon, the coating including an inner
layer comprising ZrO2 which functions as an insulating
layer, underneath an outer layer selected from the group
consisting of A1203, SiO2, Na20, Fe203, CaO, MgO, TiO2Y203,
Cr203, and mixtures thereof. An external heating device is
adapted to heat the roll face in order to dry or detach a
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paper web running over the roll face, that coating having a
sufficient thickness to provide thermal insulation to the
roll, such that heat from the external heating device heats
the roll face without heating the roll to a depth greater
than the roll face.
The external heating device is advantageously a short wave
infrared heater or a laser heater which is capable of heating the
coating. The use of an infrared heater provides the advantage that
the face can also be heated through the paper. Only a thin surface
layer of the roll need be heated while the remainder of the roll
remains at the temperature of the environment. Embodiments of
suitable heaters are described in the above Fi nni sh Patent
Applications Serial Nos. 870308 and 870309.
BRIEF DESCRIPTION OF THE DRAWINGS
The background of the invention and certain of the
exemplifying embodiments thereof will be described in greater detail
below with reference to the accompanying drawings, in which
Fig. 1 is a schematic view of an arrangement showing
infrared heating in accordance with the present invention;
Fig. 2 is a schematic view showing inductive heating in
accordance with the present invention.
DESCRIPTION OF ~K~KK~U EMBODIMENTS
As shown in Fig. 1, heat from an infrared heater 2, the
energy of which is supplied by an energy source 3, passes through a
paper web 4 and heats only the surface layer 7 of the roll 1. It is
also possible to directly heat the surface layer 7 without the paper
web over the same.
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As shown in Fig. 2, the same effect is achieved by inductive
heating. The heat from the magnetic coil 5 supplied through coil 6
pA~ses through the paper web 4 and heats only the surface layer 7 of
the roll 1.
The surface layer 7 of the roll 1 i8 a coating, the purpose
of which is to apply a thermal shock to the paper. The thermal shock
can be utilized, for example, for drying the paper or for detaching
the paper from the roll. Properties of this coating include dark
face, good capacity of thermal in~ tion~ good resistance to heat,
good resistance to thermal shock, low coefficient of thermal
e~pAncion, high heat capacity, good wear resistance and/or good
resistance to corrosion.
A suitable coating for this purpose is, for example
chromia-silica composite, prepared by means of thermal spraying. It
is an advantage of this material that it is dark, corrosion proof and
wear resistant, and in accordance with the invention, the same is
applied to various objects of use, for example, to a calendar.
Thus, as the roll coating, a dark readily heatable material
is used. The final specific coating material in detail is chosen in
accordance with the above described ultimate additional properties.
The coating may, for example may be a ceramic or a ceramic composite
or a mixture of a ceramic and a metal.
If the hardness and wear-resistance is desired for the
material, advantageously materials based on alumina (A1203) or
chromia (Cr203) are used. On the other hand, if heat resistance,
impact strength and/or capacity of thermal insulation is/are desired
for the material, advantageously materials are based on zirconia
(ZrO2) are chosen. In surface finish applications of high
requirements, advantageously materials based on titania (Tio2) are
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used. Coating materials can be used alone or in layers dep~n~ing on
the specific of use, e.g. to provide a suitable resistance to thermal
shocks or a capacity of thermal insulation.
one ob;ect of use in accordance with the invention is in a
calendar roll, wherein the heating effect of the heating device
penetrates through the paper, whereby the hot roll face modifies the
surface properties of the paper. Suitable coating materials for this
purpose are e.g. Cr203, A1203, Tio2. If desired, zirconia
or equivalent may be used as a thermal insulation underneath a thin
ceramic mantle.
The invention may also be employed to detach the paper from
the web by heating (by affecting the viscosity of the medium) or by
vaporizing the interface, e.g. by vaporizing the water, in the case
of a press roll, or the coating agent, in the case of a guide roll in
a coating device. In other words, the arrangement can be used in a
guide roll in a separate sizing cylinder to facilitate the scraping
by means of a doctor, in which case the hot roll face vaporizes some
of the size layer and detaches the paper from the roll. Coating
materials suitable for this purpose include, e.g. Cr203.
An object of application may also be a pick-up roll wherein
there is a felt between the paper and the roll.
The thickn~sc of the coating may be, e.g. 10 ~m to 10 mm,
most appropriately about 50 ~m to 150 ~m.
In accordance with the invention, when it is desirable to
obtain good adherence of the coating to the roll, increased
to~ghness, and good wear-resistance of the coating at the same time
in the coating material, it is possible to use a metal as an alloying
agent, the ratio being, for example, 1 to 10-30 (metal to ceramic).
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One example of a coating might be a surface layer of WC 86 Co 20 Cr4
isolated from the roll body by a layer of Zr203.
To manufacture a roll coating, it is possible to employ
prior art processes, e.g. powder-metallurgic processes.
By means of the invention, a number of advantages are
obtained over the prior art. It is thus possible, in accordance with
the invention, to provide a wear-resistant face and at the same time,
the detaching and other properties of the face can be modified very
precisely in the desired way by means of the system of heating of the
roll and by means of a suitable choice of materials.
Thus, the various objects of the invention are briefly
detaching of paper from a roll by heating (by acting upon viscosity)
or by vaporizing the interface, heating of the paper, and/or
modifying the surface properties of the paper by heating.
For these applications, short-wave radiation is suitable,
the wavelength being e.g. 1 ~m when the radiation is supposed to
penetrate through the web (infrared heater); when the roll face is
heated directly, a radiation of longer wavelength is suitable
(infrared heater 1-5 ~m, C02-laser 10.6 ~m). The requirement of
power depends on the web length, web thic~ess, web moisture, and on
the filler material.
The following examples are given of materials suitab-le for
use in accordance with the present invention. The scope of the
invention is not, however, meant to be limited to the specific
details of the examples.
_g_
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Examples of surface material compositions based on different oxides.
Con- Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6. Ex. 7 Ex. 8
stit- (White (Gray (Al~ 'nA (Zirco- (Yttria (Alumina (Tita- (Chro-
uents Alumina) Al 'nA) Titania) nia) stabi- Zirconia) nia) mia)
lizing
Zirconia)
A12~3 99.64 96.0658.29 0.45 73.82 0.20 1.83
SiO2 0.10 0.600.13 0.40 0.12 0.10 0.12 6.23
Na20 0.15 0.15 0.02
Fe203 0.03 0.400.04 0.13 0.03 0.06 0.40 0.30
CaO 0.04 0~120.02 5.51 0.03 0.45
MgO 0.04 0.120.01 1.43 1.40
TiO2 2.7041.36 0.10 0.10 2.32 99.20
Zr~2 91.64 89.78 25.68
Y203 9.97
Cr203 89.50
Therm_l
expans .
co~ff. 7.4 7.6 7.8 9.5 5.0 5.0
X 10 - 16/oC
(20...1500~C)
Thermal
conduc-
tivity 2.36 0,99 2.98
R Cal/
m hr ~C (550...................... (550........................ (600~C)
1100~C) 1100~C)
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While the invention ha~ been described in particular ~ith
~ respect to certain specific embodiments, it is apparent that
variations and modifications of the invention can be made without
departing from the spirit or scope thereof.
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