PLASMA-TREATED HEAT EXCHANGERS

ECHANGEUR THERMIQUE A TRAITEMENT PLASMIQUE

English Abstract

The invention relates to a temperature-controlled, rotating, rotational
symmetric heat exchanger, particularly for the pressing mechanism, drying
mechanism or smoothing mechanism of a machine for producing web-like products
such as paper webs or plastic films. The heat exchanger can be temperature-
controlled preferably with fluids or vaporous heat transfer media or with
heating involving electrical means. The aim of the invention is to improve a
heat exchanger of the aforementioned type as to achieve surface hardnesses of
far greater than 400 - 450 HV and to minimize the chemical/mechanical
corrosion even at heat transfer capacities > 80 kW/m. To this end, the heat
exchanger is plamatized.

French Abstract

L'invention concerne un échangeur thermique à symétrie de rotation, rotatif, pouvant être tempéré, notamment destiné à l'unité de compression, de séchage ou de lissage d'une machine de fabrication de produits en bande, notamment de bandes de papier ou de plastique. Ledit échangeur peut de préférence être tempéré au moyen de fluides ou de caloporteurs vaporeux ou d'un chauffage éléctrique. L'invention vise à améliorer un tel échangeur thermique de manière à atteindre des duretés de surface nettement supérieures à 400-450 HV, et à réduire la corrosion chimique/mécanique même pour des puissances de transfert thermique > 80 KW/m. A cet effet, l'échangeur thermique selon l'invention est à plasma.

Claims

CLAIMS
1. Temperable, rotatable, rotationally symmetric heat
exchanger, especially for the pressing, drying, or smoothing
unit of a machine for the production of web-like products such
as paper webs or plastic films, which heat exchanger can be
tempered preferably with fluid or gaseous heat-transfer media or
by an electrical heating system, characterized in that the heat
exchanger is plasma-treated.
2. Heat exchanger according to Claim 1, characterized in
that the surfaces of the necks of the rolls are partially or
completely plasma-treated.
3. Heat exchanger according to Claim 1 or Claim 2,
characterized in that the contact surface of the heat exchanger
with the web-like product is plasma-treated.
4. Heat exchanger according to one of Claims 1-3,
characterized in that the surface of the barrel of the heat
exchanger is plasma-treated.
5. Heat exchanger according to one of Claims 1-4,
characterized in that the surface of the roll is plasma-treated.
6. Heat exchanger according to Claim 1 or Claim 3,
characterized in that the surfaces in contact with the heat
transfer medium are completely or partially plasma-treated.
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7. Heat exchanger according to one of Claims 1-6,
characterized in that a bonding layer is formed at the surface.
8. Heat exchanger according to one of Claims 1-6,
characterized in that no bonding layer is formed at the surface.
9. Heat exchanger according to one of Claims 1-8,
characterized in that subsurface corrosion is prevented.
10. Heat exchanger according to one of Claims 1-9,
characterized in that the plasma-treated calender roll can be
used at process temperatures between -20°C and 500°C.
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Description

CA 02516951 2005-08-23
TRANSLATrON (T~!-651PCT .-- original)
wo 2004/04,571 A1
PCT/EP2004/001340
PLASMA-TREATED HEAT EXCHANGERS
The invention pexta,ins to a heat exchanger according to the
introductory clause of Claim 1. These types~of heat exchangers
axe used in the production or finishing of web-like products in
the industry, primarily in calenders.
The heat~transfer outputs and surface temperatures required
in the new mufti-nip calender rolls subject the conventional
calender roll materia~.s such as white cast iron, spheroidal
graphite iron, and chromium alloy cast~irorz to loads which.
exceed their capacity. Forged steel is currentJ.y the material
which is able to fulfill these requirements. Because adequate
forged steel can be hardened and tempered only up to 400-450 HV
without sacrificing the advantages of forged steel, the surfaces
of these rolls are coated either by electrocoating ox by a
thermal coating process or they are case-hardened by inductive
hardening or flame hardening.
Thermally applied surface coatings suffer not oxzly from
insufficient reliability but also from the disadvantage of the
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CA 02516951 2005-08-23
relatively high porosity of the finished layer. dear particles
are able to penetrate into these pores and into the softer
matrix regions and to wash them out. Case-hardened forged steel
rolls suffer not only fxom inadequate tempering properties and
the associated lack of sufficient hardness but also from an
unfavorable internal stress distribution, caused by the case-
hardening process. High tensile stresses in the area of the
zone affected by the hardening process destroy the advantages of
the good mechanical properties of forged steel.
New coating compounds and feed materials being used in
paperma~ing call for increased chemical and mechanical corrosion
resistance not only of the barrel of the roll but also of the
entire surface exposed to the environment and to the product
web. Because of the enormous heat-transfer outputs (> 80 kW/m)
and the continually increasing heat-transfer temperatures, the
contact surfaces exposed to the fluid heat transfer media are
subject to ever greater chemical/mechanical coxxosion.
The invention is based on the task of improving the heat
exchanger of the general type in question in such a way that it
no longex suffers from the disadvantages mentioned above,
especially so that surface hardnesses of considerably more than
400-450 HV axe reached, and so that chemical/mechanical
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CA 02516951 2005-08-23
corrosion is minimized eaen at heat-transfer outputs of % 80
kw/m.
According to the invention, the previously described
hardened and tempered forged steel rolls axe plasma-treated in
the desired areas. The term "plasma-treating" is used i,n the
following as a collective term for "plasma nitriding", "plasma
nitrocarburizing", "plasma oxidizing", "plasma carbonitriding",
and "plasma carborizing". This operation is usually carried out
in a vacuum chamber at treatment pressures of 0.~-10 mbars. The
treatment temperature is in the range of 404-600°C, depending on
the task to be accompl~,shed. Ammonia, nitrogen, methane, and
hydrogen are usually used as the process gases. pressure,
temperature, time, and gas type are the parameters used to vary
the nature of the layers and the desired surface hardnesses.
Hardnesses of up to 1200 HV can be reached on the desired
surfaces, and a hardening depth of up to 1 mm is realized. zn.
addition to this extreme hardness, these diffusion layers do npt
produce any i,ncxease in the porosity which could be damaging to
the wear process. The porosity of the substrate remains intact.
In addition, the resistance to chemical corrosion is enormously
improved.
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CA 02516951 2005-08-23
Subsurface corrosion, known from thermal spray coatings or
even hard chromium alloy coatings, is not possible when plasma-
treating is used. The bonding layer with the surface passivates
the suxface against oxidative attach. Microcracks, also known
from the processes mentioned above, do not develop either.
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