Note: Descriptions are shown in the official language in which they were submitted.
2~18660
A~varatus for treatina a material web
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an apparatus for treating a
material web, preferably for dewatering a paper web in the
pulse drying method, comprising a heating roll and a
counter member which together with the heating roll forms
a pressing gap, the heating roll comprising a stationary
axle, a roll case and at least one bearing element for
supporting the roll case on the axle and the roll case
including a foundation layer and a thin ceramic heatable
surface layer applied to the foundation layer.
2. Description of the Prior Art
In the treatment of a material web, in particular when
dewatering a paper web having a high water content, the
material web is led through an apparatus comprising at
least one pressing gap or nip which is formed by a roll and
a counter member arranged opposite the roll. The counter
member may be constructed both as a second roll or roller
and as a hydraulio shoe. In particular in the dewatering
of a paper web, the web still containing a higher
proportion of water passes through the pressing gap and the
water is mechanically expelled from the paper web between
the roll and the counter member. An improvement of the
dewatering is achieved in the prior art in that the roll is
constructed as heating roll. The objective of the
apparatus known in the art is to obtain the greatest
possible dewatering of the material web. It has been found
disadvantageous here that the still not very strong paper
web adheres to the pressing roll and as a result tearing of
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the paper web may occur. These production disturbances are
due in particular to an unsuitable surface constituency of
the roll case.
Consequently, from the prior art various working layers for
the surface of the pressing rolls are known. For example,
from DE-PS 103,677 the use of natural granite as material
for the pressing roll is disclosed. This material
admittedly has good detachment properties but has the
disadvantage that in the event of temperature differences
in the granite as arise by heating with steam boxes from
the outside said granite is destroyed by thermal stresses.
Therefore, in the course of time various working layers
have been developed and have been used as substitute for
granite rolls. These solutions known from the prior art
comprise as a rule a roll base body of cast iron or steel
to which layers are applied which are equivalent to the
detachment properties of granite. The detachment
properties are based essentially on surface caverns and the
mixture of water-repellant and water-attracting components.
For example, filling materials such as granite powder have
been worked into plastic coatings. Such a working layer is
known from EP-OS 0 337 934.
Furthermore, from DE-OS 3,546,343 a pressing roll is known
for an apparatus of the aforementioned type which has a
working layer of its cylinder case which is formed from a
mixture of metal powder and a powder of inorganic material.
This inorganic material is a ceramic material which can be
produced for example by thermal spraying on to the roll
body.
In addition, from EP-PS 02 07 921 a pressing roll for a
paper machine is made which includes in integrated manner
a metal core on which a metallic foundation layer and a
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ceramic working layer are applied, the foundation layer
having a coefficient of expansion which lies between that
of the metal core and the surface layer. The ceramic layer
has a permeability of 1 to 30%. The ceramic layer has a
thickness of 1 to 30 mm. With a layer thickness below 1 mm
the detachment behaviour was disturbed. Furthermore, it is
known from the prior art, that is EP-OS 0 481 321, to add
to the ceramic layer described above mica particles as
water-repellant component and impregnations on the basis of
synthetic resins.
Finally, it is known in the art to dewater a material web,
preferably a paper web, by means of pressing rolls which
are heated to very high temperatures. On contact of the
wet material web part of the water in the paper surface is
suddenly evaporated. The steam forming presses the
material web in positive manner away from the pressing roll
so that the detachment behaviour otherwise to be considered
no longer plays a decisive part. However, it has been
found that in particular with metallic surfaces of the
pressing rolls with very good thermal conduction properties
too much steam is formed and tears the interfibre bonding
of a paper web. For this reason it has been found
advantageous in this known method as well to provide the
metallic surfaces of the pressing rolls with an energy
brake, for example a thin ceramic layer. This avoids
excessive steam formation. A disadvantage of this known
apparatus is however that apart from the roll surface the
entire roll and the hydraulic oil of its hydrostatic or
hydrodynamic support is heated and said oil loses its
lubricating ability at temperatures of more than 150C. It
follows necessarily from this that the hydraulic oil must
be cooled and this results in a heat loss which reduces the
thermal efficiency of the pressing roll for the pulse
drying.
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Admittedly, the heat loss of the aforementioned pressing
roll towards the inside can be limited in that in the roll
bore between the roll wall and the bearing elements an
insulating layer is arranged, for example a ceramic layer;
however, introduction of this layer is very complicated and
involves high constructional expenditure.
Apart from this, in this manner only the heat dissipation
into the bore is restricted. It is not possible with this
step to avoid the entire metal roll body assuming a high
temperature. Since however the coefficient of expansion of
ceramic material compared with the metal roll body is
almost 50% less, the outer ceramic layer as well as the
inner insulating layer is subjected in operation to tensile
stresses due to the heating thereof. The ceramic layers
can however withstand only small tensile stresses.
Introduction of a thin foundation layer with a coefficient
of expansion between that of the ceramic layer and that of
the base body and intended to serve as bonding agent did
not result in an appreciable solution of the aforementioned
problems either. For the thin foundation layers described
in EP-PS 02 07 921 and EP-OS O 481 221 expand together with
the base body so that the ceramic working layer is again
subjected to high tensile stresses.
SUMMARY OF THE INVENTION
The invention therefore has as its object, proceeding from
the prior art described above, to provide an apparatus for
treating a material web, preferably for dewatering a paper
web by the pulse drying method, which avoids the
disadvantages of the prior art described above and in
particular haR an improved energy efficiency and improved
mechanical stability.
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The invention therefore proposes in an apparatus for
treating a material web, preferably for dewatering a paper
web in the pulse drying method, comprising a heating roll
and a counter member which together with the heating roll
forms a pressing gap, the heating roll comprising a
stationary axle, a roll case and at least one bearing
element for supporting the roll case on the axle and the
roll case including a foundation layer and a thin ceramic
heatable surface layer applied to the foundation layer, the
improvement wherein the ceramic working layer has a
thickness of less than 1 mm, the foundation layer consists
of ceramic material as thermal insulation and the
foundation layer has a thickness which is greater than the
thickness of the ceramic working layer.
An apparatus thus configured has the advantage that the
material web to be dewatered adheres to the roll surface to
a far lesser extent and at the same time the dewatering of
the material web, preferably the paper web, can be improved
by a high temperature and a high pressure. It is of
particular advantage here that the thin ceramic working
layer with external heating has a high temperature at its
surface but a low thermal conductivity so that the roll
case of metal is heated to a far lesser extent. At the
same time, the different expansion of the ceramic working
layer and the metallic roll case are adapted to each other
by the thicker foundation layer of ceramic material
compared with the working layer and consequently no tensile
stresses arise in the ceramic working layer.
A simple production of a heating roll for the apparatus
according to the invention is achieved in accordance with
a further feature thereof in that the thermally in~ulating
foundation layer is applied to the roll case by water-
stabilized plasma spraying. In the calculation it has been
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found particularly advantageous here to apply the
foundation layer to the roll case with a thickness of 5 to
30 mm. This avoids tensile stress in the working layer.
According to a further feature of the invention a
particularly efficient reduction of the tensile stress in
the working layer can be achieved in that the foundation
layer has a thickness of 8 to 10 mm, preferably about 9.5
mm. Such a foundation layer can be economically made by
using Al203.
It is particularly advantageous for the ceramic working
layer to consist of zirconium oxide and have a thickness of
0.5 mm. This ceramic material has particularly suitable
thermal conductivity values which permit an optimum
temperature effect even with a layer thickness of 0.5 mm.
In a further development of the apparatus according to the
invention it is advantageous for pulse drying that the roll
case of the heating roll consists of clear chilled casting
which in the outer region comprises white iron and in the
inner region gray iron. This is a cast iron which due to
its particular alloy and casting technique is formed in the
outer region as white iron with high hardness between 450
and 600 HV, a high E modulus of about 180,000 N-mm2and a
poor thermal conductivity of about 20 W/mK. A roll case
formed in this manner has the advantage that it provides a
good support effect and thus had good mechanical properties
and at the same time acts as a thermal barrier. The
thickness of the layer in which the iron forms as white
iron can be influenced in accordance with the alloy. It
has been found to be particularly advantageous for the
white iron to have a layer thickness of 20 to 80 mm. Here,
the low coefficient of expansion of the white iron has a
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favourable effect on the thermal stress distribution within
the roll case.
In the apparatus according to the invention the heating
roll is preferably heated from outside. This achieves a
rapid heating of the ceramic working layer and avoids large
heat losses. According to a further feature of the
invention it has been found advantageous to mix the ceramic
working layer with a metal powder so that said working
layer is electrically conductive. An apparatus formed with
this feature then has the advantage that according to a
further feature of the invention the ceramic working layer
can be heated by induction of eddy currents in the working
layer.
To avoid altering the surface of the ceramic working layer
mixed with the metal powder and thus to retain the
advantageous low adhesion tendency of the metal web to the
working layer, according to a further feature of the
invention it is proposed that an electrically conductive
layer be located directly beneath the working layer.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in detail hereinafter with
reference to an example of embodiment and with the aid of
the attached schematic drawings, wherein:
Fig. 1 is a side elevation of an apparatus for treating
! a material web;
Fig. 2 is a front view of the apparatus according to
Fig. 1;
Fig. 3 is a front view of a heating roll and
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' Fig. 4 is an enlarged frayment of the heating roll
corresponding to the circle A indicated in Fig.
i 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An apparatus as apparent from Fig. 1 and indicated by the
reference numeral 1 for treating a material web, pre~erably
for dewatering a paper web 2 by the pulse drying method,
comprises a heating roll 3 and a hydraulic shoe roll 4
which serves as counter member and together with the
heating roll 3 forms a pressing gap 5~ The counter roll 4
may also be replaced by a hydraulic shoe.
In usual manner the heating roll 3 is in the form of a tube
rotatably mounted about its longitudinal axis. The heating
roll 3 may be supported both hydrostatically and
hydrodynamically on the continuous stationary shaft 6. The
counter roll 4 is a shoe roll and likewise supported
hydrostatically or hydrodynamically (14).
In the vicinity of the surface of the heating roll 3 an
external heating 8 is provided which for example is in the
form of an induction heating which couples electromagnetic
energy into the material of the roll case 7 of the heating
roll 3 and thus produces a temperature increase, in
particular in the working surface.
In Figures 3 and 4 the constructional makeup of the heating
roll 3 is illustrated in detail. As can be seen from Fig.
3, a coating is applied to the roll case 7. This coating
consists of an insulating foundation layer 9, a ceramic
working layer 10 and an electrically conductive layer 11
which is arranged between the foundation layer 9 and the
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working layer lo and consists of a metal powder. In simple
manner the layer 11 makes it possible to heat up the
working layer 10 by inducing eddy currents in the layer 11.
In the example of embodiment illustrated the ceramic
working layer 10 has a thickness of about 0.5 mm and
consists of zirconium oxide. The foundation layer 9
consisting of Al2O3 has a layer thickness of about 9.5 mm
and is applied by water-stabilized plasma spraying to the
roll case 7. The layer 11 of metal powder arranged between
the foundation layer 9 and the working layer 10 has a
thickness which is less than the thickness of the working
layer 10 and is about 0.4 mm. It is also possible to
integrate the layer 11 into the working layer 10.
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The roll case 7 of the heating roll 3 consists of clear
chilled casting. This is a cast iron which due to a
particular alloy and casting technique forms in the outer
region 12 as so-called white iron and in the inner region
13 as so-called gray iron. The white iron arranged in the
outer region 12 has a high hardness between 450 and 600 HV,
a high E modulus of about 180,000 M/mm2 and a poor thermal
conductivity of about 20 W/nK. In addition, a roll case of
this structure has a high mechanical strength and acts as
thermal barrier. The layer thickness of the white iron in
the outer region 12 is about 30 mm and the layer thickness
of the gray iron disposed in the inner region 13 is about
110 mm so that the diameter of the roll case 7 without the
foundation layer 9 and the working layer 10 is about 140 mm
and with the foundation layer 9 and the working layer 10
about 150 mm.
Below the expansion average values of the various layers of
a roll case 7 with external heating according to the prior
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art and according to the invention are compared, all the
particulars relating to mean values of the respective
layers:
Layer Thick~ess Expansion Prior Art C Invention
Coeff. C dl~mm/m) dl(mm/m)
Working 0.5 6.5x1 o-6/K 330 2.0 330 .2.0
lay~r
Foundation 9.5 6.5x10 ~/K -- -- 276 1.6
layer
White 30.0 8.5x1 0-~/K -- -- 207 1.59
iron
Gray 110.0 10.5x10-9/K 257 2.7 176 1.64
iron
It can be seen that a roll case 7 according to the prior
art expands thermally more than the working layer and this
results in tensile stresses which oxide ceramic for example
can hardly withstand.
In the construction according to the invention the thermal
expansion of all the sublayers bearing he working layer 10,
i.e. the foundation layer 9 and the regions 12, 13 of the
roll case 7, is of the same order of magnitude and moreover
less than that of the working layer 10. The working layer
10 is thus under compressive stress during operation and
this has a favourable effect on the life of the working
layer. Between the foundation layer 9 and the working
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11
layer 10 on the one hand and the white iron in the region
12 on the other hand no appreciable relative stresses are
present. On a comparison of the energy loss of a heating
roll of 1350 mm outer diameter and 150 mm wall thickness of
gray iron comprising a 0.5 mm thick layer of zirconium
oxide and heated externally to 300 to 400C, it is found
that the heat flow with a heating roll 3 constructed
according to the invention is reduced from 2Z0 kW/m tube
length to about 71 kW/m tube length. As a result, the
thermal efficiency of 55% in the prior art is improved to
79% with an apparatus according to the invention and the
energy losses are correspondingly reduced.
This also results in the advantage that the cooling means
for the hydraulic oil for the hydrostatic or hydrodynamic
support can be made substantially smaller and thus more
economical.
