U.S. patent application number 14/513502 was filed with the patent office on 2015-01-29 for layer heat exchanger for high temperatures.
The applicant listed for this patent is Behr GmbH & Co. KG. Invention is credited to Hans-Heinrich ANGERMANN.
Application Number | 20150027674 14/513502 |
Document ID | / |
Family ID | 42561187 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150027674 |
Kind Code |
A1 |
ANGERMANN; Hans-Heinrich |
January 29, 2015 |
LAYER HEAT EXCHANGER FOR HIGH TEMPERATURES
Abstract
A layer heat exchanger for high temperatures is provided that
includes a layer block having layer plates and cover plates and a
housing that accommodates the layer block. The housing has a high
heat resistance, combined with a high stiffness, and the layer
block has a core that is soft and tough relative to the
housing.
Inventors: |
ANGERMANN; Hans-Heinrich;
(Stuttgart, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Behr GmbH & Co. KG |
Stuttgart |
|
DE |
|
|
Family ID: |
42561187 |
Appl. No.: |
14/513502 |
Filed: |
October 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13305022 |
Nov 28, 2011 |
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14513502 |
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PCT/EP2010/057317 |
May 27, 2010 |
|
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13305022 |
|
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Current U.S.
Class: |
165/166 |
Current CPC
Class: |
F28D 9/0037 20130101;
F28F 2265/26 20130101; F28F 3/08 20130101; F28F 21/084 20130101;
F28F 21/083 20130101; F28F 9/00 20130101 |
Class at
Publication: |
165/166 |
International
Class: |
F28F 3/08 20060101
F28F003/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 28, 2009 |
DE |
10 2009 022 984.1 |
Claims
1. A layer heat exchanger for high temperatures, the heat exchanger
comprising: a layer block having layer plates and cover plates; and
a housing configured to accommodate the layer block, the housing
having a high heat resistance, associated with a high stiffness,
and the layer block having a soft and tough core relative to the
housing.
2. The layer heat exchanger according to claim 1, wherein the
housing is made of a material with a high heat resistance.
3. The layer heat exchanger according to claim 2, wherein the
material of the housing is a nickel alloy with the material No.
2.4856 according to DIN EN 10095 and the material designation
NiCr22Mo9Nb or a ferritic high-temperature stainless steel with the
material No. 1.4750.
4. The layer heat exchanger according to claim 2, wherein the
material of the housing is an austenitic high-temperature stainless
steel with the material No. 1.4876 and with the material
designation X10NiCrAlTi 32-20 or with the material No. 1.4835 and
with the material designation X9CrNiSiNCe 21-11-2.
5. The layer heat exchanger according to claim 2, wherein the cover
and layer plates are made of a material with a lower heat
resistance than the housing material.
6. The layer heat exchanger according to claim 3, wherein the
material for the cover and layer plates is a high-temperature
stainless steel with the material No. 1.4876 with the material
designation X10NiCrAlTi 32-20 or a Ni base material with the
material No. 2.4851 with the material designation NiCr23Fe.
7. The layer heat exchanger according to claim 3, wherein the
material for the cover and layer plates is a ferritic material with
the material No. 1.4725 according to DIN 17470 and the material
designation CrAl14 4 or the material No. 1.4767 according to DIN
17470 with the material designation CrAl20 5.
8. The layer heat exchanger according to claim 7, wherein the
material for the cover and layer plates contains Al with a content
of .gtoreq.2%, especially preferably of .gtoreq.3%.
9. The layer heat exchanger according to claim 1, wherein the wall
thickness of the housing is as great as possible in regard to the
wall thickness of the cover and layer plates.
10. The layer heat exchanger according to claim 9, wherein the wall
thickness of the housing is about 1.5 mm.
11. The layer heat exchanger according to claim 9, wherein the wall
thickness of the cover and layer plates is about 0.5 mm, preferably
0.4 mm, especially preferably 0.3 mm.
12. The layer heat exchanger according to claim 1, wherein the
housing material includes an especially high-temperature-resistant
material with low wall thickness, particularly 2.4856 with a
thickness of 1.0 mm or 0.5 mm, and the layer and cover plate
material includes an especially soft and thin material,
particularly a ferritic material having a thickness of 0.3 mm or
0.4 mm.
13. The layer heat exchanger according to claim 1, wherein the
housing and cover plate material is the same
high-temperature-resistant material and the layer plates are of the
soft material.
14. The layer heat exchanger according to claim 1, wherein the
cover and layer plates of the layer block are connectable to one
another by bonding on the front side.
15. The layer heat exchanger according to claim 14, wherein the
cover and layer plates are soldered to one another on the front
side.
16. The layer heat exchanger according to claim 14, wherein the
layer plates have sealing edges and wherein the layer plates are
welded to one another in the area of their sealing edges.
17. The layer heat exchanger according to claim 1, wherein the
housing and the layer block are made of the same material.
18. A layer heat exchanger for high temperatures, the heat
exchanger comprising: a layer block having layer plates, a top
cover plate and a bottom plate; and a housing configured to
accommodate the layer block, the housing having a higher heat
resistance and a higher stiffness than the layer block such that
the layer block is softer than the housing, wherein the cover and
layer plates are made of a material with a lower heat resistance
than the housing material, and wherein the material for the cover
and layer plates contains Al with a content of .gtoreq.2%.
Description
[0001] This nonprovisional application is a continuation of U.S.
application Ser. No. 13/305,022, which was filed on Nov. 28, 2011
and which is a continuation of International Application No.
PCT/EP2010/057317, which was filed on May 27, 2010, and which
claims priority to German Patent Application No. DE 10 2009 022
984.21, which was filed in Germany on May 28, 2009, and which are
both herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a layer heat exchanger for high
temperatures.
[0004] 2. Description of the Background Art
[0005] Layer heat exchangers are known from the conventional art
and include a layer block, which is made up of stacked layer plates
and cover plates and is used for the heat exchange between two
media, and a housing, which accommodates and seals the layer block
and has connections for supplying and removing the media. Layer
heat exchangers of this type are notable for a high specific heat
transfer performance, based on their volume.
[0006] German Offenlegungsschrift No. DE 103 28 274 A1 of the
applicant disclosed this type of layer heat exchanger and a method
for the manufacture thereof. The layer block of the prior-art heat
exchanger has separating and cover plates, which are fully soldered
to one another, i.e., at their points of contact. This produces a
relatively stiff layer block, which is welded and/or soldered to
the housing. A similar layer heat exchanger was disclosed in German
Offenlegungsschrift No. DE 10 2007 006 615 A1. German
Offenlegungsschrift DE 10 2006 011 508 A1 of the applicant
disclosed a layer heat exchanger with a layer block of layer
plates, which have an edge region bent by 180.degree., which is
connected by bonding to a similar edge region of a neighboring
layer plate, particularly by soldering with use of solder adhesive
tape. This creates the advantage of a layer heat exchanger with a
relatively soft layer block with respect to its mechanical
properties, which nonetheless has a high tightness.
[0007] German Offenlegungsschrift No. DE 10 2007 008 341 A1 of the
applicant disclosed a layer heat exchanger for use as a
high-temperature heat exchanger, particularly in the periphery of a
high-temperature fuel cell (solid oxide fuel cell=SOFC).
High-temperature fuel cells of this type are used for providing
electric power in motor vehicles as a so-called ARU (auxiliary
power unit). Heat exchangers for SOFCs are used, for example, for
heating process air and are supplied with hot combustion gases
within a temperature range of about 950.degree. C. This application
of a greatly and rapidly changing temperature leads in the heat
exchanger structure to thermomechanical stress, which leads to
problems with respect to the tightness of the layer block in the
interior and tightness of the heat exchanger outwardly. To counter
this problem, it was proposed in the layer heat exchanger disclosed
in DE 10 2007 008 341 A1 to solder the layer block only on the
front side, so that any internal soldering of the sheet metal
layers is avoided. It is advantageous in a layer block soldered
only outwardly in such a way that the sheet metal layers can avoid
the stresses better by elastic or optionally plastic deformations.
This results in a reduction of damage but is not sufficient.
[0008] Another solution to the problem of the thermally induced
mechanical stress was proposed by the applicant in German
Offenlegungsschrift No. DE 10 2007 056 182 A1 for a layer heat
exchanger, whereby the layer block is supported elastically within
the housing by a decoupling device. Thermally induced expansions of
the layer block, which is stiff in itself, are compensated by the
decoupling device, e.g., in the form of mineral fiber mats and thus
kept away from the housing. The layer block is arranged virtually
floating within the housing and can be used advantageously for high
temperature differences up to 900.degree. C. The structure-related
high internal leak rate of the heat exchanger is disadvantageous
here, however.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of the present invention to design
and construct a layer heat exchanger in such a way that it
withstands high temperatures, particularly stress caused by
cyclical temperature changes to about 950.degree. C., so that a
sufficient internal tightness, however, especially an absolute
tightness of the heat exchanger toward the outside, is assured
during operation. The outer tightness is especially important, for
example, during use of the heat exchanger for SOFC systems.
[0010] According to the invention, the layer heat exchanger is
characterized by a housing with a high heat resistance and
stiffness, as well as a layer block with a soft and tough core
relative to the housing. Compared with the layer block, the housing
thereby forms a relatively stiff counter support, which based on
its novel design is capable, particularly at high temperatures, of
absorbing thermally induced expansion forces originating from the
layer block. Based on this firm counter support, the soft and tough
core of the layer block will deform elastically or also plastically
in part, which is taken into account. It turned out surprisingly
that the internal leakage of the layer heat exchanger of the
invention is much lower than in the aforementioned prior art layer
heat exchangers. The internal leakage arises during operation,
e.g., during use of the heat exchanger in the periphery of a
high-temperature fuel cell because gas that has a temperature of
900 to 950.degree. C. encounters the cold layer block during a cold
start. The thin layer plates heat more rapidly than the thicker
housing material, so that because of the thus predefined thermal
expansion c of the plates, according to Hook's law, a force
F.about.E.epsilon. (E=elasticity modulus) on the housing arises.
When the housing of the invention opposes the force exerted by the
plates with a sufficiently high heat resistance, the layer plates
in the interior (of the layer block) are more likely to deform
elastically or plastically and thereby to reduce the force. It is
accepted in this case that the layer plates in the interior may be
damaged with often repetition of the cold start; i.e., a relative
internal leakage is permitted according to the invention. It is
advantageous that the damage is limited locally, namely, to regions
that reach the highest temperatures. This limits the internal
leakage. The extent to which an internal leakage forms during
operation depends on the selection of the materials, particularly
the material of the layer plate. The external tightness is
absolutely assured, however; i.e., the escape to the outside of gas
that has a temperature of 950.degree. C. and may also contain
hydrogen is prevented in each case.
[0011] Materials and structural realizations for the housing and
the layer and cover plate, which satisfy the aforementioned
criteria, will be indicated hereafter.
[0012] According to an embodiment, the housing is made of a
material with a high heat resistance. A high heat resistance is
understood to be a high hot yield strength .sigma..sub.0.2. Nickel
alloys are preferred, particularly a readily available
high-temperature-resistant material with the material No. 2.4856
according to DIN EN 10095 and the material designation NiCr22Mo9Nb.
This material is notable for good mechanical behavior at
temperatures above 500.degree. C. Thus, the yield strength of this
material occurs at 900.degree. C. at a high 200 N/mm.sup.2.
[0013] According to another embodiment, austenitic high-temperature
stainless steel with the material No. 1.4876 and the material
designation X10NiCrAlTi 32-20 or with material No. 1.4835 can also
be used as materials for the housing. These materials are more
cost-effective than the aforementioned nickel alloy, but do not
have the very high heat resistance of the material with the
material No. 2.4856.
[0014] According to another embodiment, the cover and layer plates
are made of a material that has a lower heat resistance compared
with the material of the housing, particularly a low hot yield
strength .sigma..sub.0.2. By means of this pairing of materials
with a different heat resistance for the housing, on the one hand,
and the layer block, on the other, the aforementioned expansion
behavior of the layer block is achieved at high temperature
stresses; i.e., a sufficient internal tightness and a complete
external tightness are assured.
[0015] According to another embodiment, a high-temperature
stainless steel with the material No. 1.4876 and the material
designation X10NiCrAlTi 32-20, which is matched particularly to the
aforementioned housing material with the No. 2.4856, is selected
for the material of the layer and cover plates. If the layer plate
material 1.4876 were to lead to unacceptably high internal damage,
a relatively cost-effective Ni base material such as 2.4851
(NiCr23Fe) can also be used as the layer plate material. 2.4851 has
a higher heat resistance compared with 1.4876 but a lower heat
resistance than 2.4856.
[0016] According to another embodiment, a ferritic material can
also be selected for the material of the layer and cover plates.
With a view to the use of the layer heat exchanger in the periphery
of the SOFCs, particularly an Al-containing material is selected as
the ferritic material, because this material has a high
high-temperature corrosion resistance and low Cr evaporation. For
example, a material with the material No. 1.4725 according to DIN
17470 and the material designation CrAl14 4 is suitable.
[0017] Alternatively, another ferritic material with the material
No. 1.4767 according to DIN 17470 and the material designation
CrAl20 5 can be selected. These ferritic materials are especially
advantageous in a pairing with the aforementioned austenitic
housing materials with the material number 1.4876 (material
designation: X10NiCrAlTi 32-20) or 1.4835. An advantage during use
of ferritic stainless steels for layer and cover plates is their
high elongation at break; i.e., the layer plates deform plastically
in fact, but because of their high heat ductility have only a low
tendency for developing leakage such as cracks. It should be
emphasized that the conventional reservations in regard to
different thermal expansion coefficients in the welding of ferrites
and austenites do not apply here, because the expansion
coefficients of the ferritic FeCrAl and austenitic Ni alloys
selected here have only minor differences.
[0018] According to another embodiment, during use of the
aforementioned ferritic materials for layer and cover plates, an
especially relatively high-temperature-resistant ferritic material
such as, for example, 1.4750 can also be selected advantageously
for the housing.
[0019] Alternatively or also cumulatively to the aforementioned
material pairing, different wall thicknesses can be selected for
the housing, on the one hand, and the layer and cover plates, on
the other, i.e., a high wall thickness for the housing material and
a considerably lower wall thickness for the plate material. A stiff
housing and a softer, elastically deformable core of the layer
block with the aforementioned advantages of an internal and
external tightness are achieved by means of this dimensioning of
the wall thicknesses.
[0020] According to an embodiment, the wall thickness of the
housing is about 1.5 mm and that of the layer plates about 0.3 mm.
The wall thickness of the housing material, in contrast to the wall
thickness of the layer and cover plate material, can be relatively
low, when the heat resistance of the housing material is high
relative to the heat resistances of the layer plate material. It is
especially preferable accordingly when the housing material
consists of a high-temperature-resistant material with a low
thickness such as, for example, 2.4856 with a 1.0 mm or 0.5 mm wall
thickness and the layer and/or cover plate material of a soft
material such as the aforementioned FeCrAl alloys. The especially
minor difference in mass between the housing, on the one hand, and
the layer and cover plates, on the other, leads to especially low
thermal stresses. An Al content of .gtoreq.2%, especially
preferably .gtoreq.3% is especially useful in this case.
[0021] According to another embodiment, it is provided
alternatively or cumulatively that the layer and cover plates are
connected to one another by bonding only on the front side at their
sealing edges, preferably by soldering or welding. Ribs or nubs,
which are located in the interior of the heat exchanger or lead
from the outside into the interior of the heat exchanger, should
thereby not be connected to one another by bonding or at least to
the lowest extent possible. The advantage is achieved thereby that
the layer block is securely sealed toward the outside and remains
soft and elastically deformable in the interior in its core.
[0022] In an alternative construction form, the housing and the
cover plates consist of the same highly temperature-resistant
material and only the layer plates consist of the soft
material.
[0023] Finally, according to another embodiment, the housing and
the layer and cover plates can be made of the same material. In
this regard, the soft core of the layer block can be achieved by a
lower plate wall thickness compared with the wall thickness of the
housing and/or by the soldering or welding of the layer block on
the front side.
[0024] The use of the layer heat exchanger of the invention proves
to be advantageous particularly in the periphery of a
high-temperature fuel cell, preferably in motor vehicles, to meet
the strict conditions applicable there in regard to an internal and
external tightness of the heat exchanger.
[0025] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus, are
not limitive of the present invention, and wherein:
[0027] FIG. 1 shows a layer heat exchanger in an exploded view;
and
[0028] FIG. 1 a shows a layer block in an exploded view.
DETAILED DESCRIPTION
[0029] FIG. 1 shows a layer heat exchanger 1 in an exploded view.
Such a layer heat exchanger 1 is known from the aforementioned
state of the art with respect to its structure. A partially shown
layer block 2, through which two media can flow in crossflow, is
arranged in the interior of layer heat exchanger 1. Layer block 2
is housed in four box-like housing sections 3, 4, 5, 6, which in
turn have connecting pieces 3a, 4a, 5a, 6a for supplying and
removing the media flowing through layer block 2. The four box-like
housing sections 3, 4, 5, 6 including the connecting pieces 3a, 4a,
5a, 6a are together also called a housing 7 below. Layer block 2
and housing 7 are connected together by twelve welding seams, of
which welding seams 8a, 8b, 8c are designated by way of
example.
[0030] In FIG. 1 a, layer block 2 according to FIG. 1 is shown
schematically by a bottom cover plate 9 and a top cover plate 10
and by two layer plates 11, 12. Layer plates 11, 12, also called
layer sheets 11, 12, are formed contoured; i.e., they have flow
channels and sealing edges 11a, 11b, 12a, 12b intersecting at
90.degree. (which are not provided with reference numbers). All
parts are connected together by bonding, preferably soldered and/or
welded, as emerges in particular from the aforementioned state of
the art, to which reference is made herewith.
[0031] According to the invention, housing 7 has a high heat
resistance in relation to layer block 2; i.e., cover plates 9, 10
and layer plates or sheets 11, 12 have a lower heat resistance. As
a result, for layer block 2 a softer and tougher core is achieved,
which is capable of deforming elastically or plastically with high
temperature-induced expansions in the internal region, for example,
to curve. Housing 7, in contrast, owing to its elevated heat
resistance and stiffness should preferably not deform, but absorb
the reaction forces resulting from the layer block.
[0032] According to an exemplary embodiment, layer heat exchanger 1
is used in the periphery of a high-temperature fuel cell (SOFC),
which is not shown, as it is employed for providing electric power
in motor vehicles as a so-called ARU (auxiliary power unit). Layer
heat exchanger 1 is used in particular here for the recovery of
exhaust gas heat of the fuel cell and for heating process air for
the SOFC. A fuel cell system of this type was disclosed in US
2003/0031904 A1. The exhaust gases of the SOFC with a temperature
of about 950.degree. C. encounter the structure of layer heat
exchanger 1, in which the process air for the cathode of the SOFC
is to be heated to about 750.degree. C. Particularly in the case of
a cold start, this would lead to great thermomechanical stresses in
the heat exchanger structure. Because of the tough and soft core of
layer heat exchanger 1 constructed as taught by the invention,
these stresses can be reduced, however, by the elastic or plastic
deformations of the core. Layer block 2 is supported thereby by the
highly heat-resistant solid housing 7, which absorbs the reaction
forces without significantly deforming in so doing. Layer heat
exchanger 1 therefore remains outwardly tight in each case, so that
no hot exhaust gases can escape from the system. It is possible
that the layer plates are deformed beyond their yield strength,
i.e., permanently. This can be accepted, however, because such
damage is limited locally to areas where the highest temperatures
are reached. As a result, the internal leakage of layer heat
exchanger 1 is limited.
[0033] According to an embodiment, materials with different heat
resistance values are selected for housing 7 and layer and cover
plates 9, 10, 11, 12, particularly with a focus on the hot yield
strength .sigma..sub.0.2. Preferably, a nickel alloy with the
material No. 2.4856 and the material designation NiCr22Mo9Nb
according to DIN EN 10095 is selected as the material for housing
7, therefore box-like housing sections 3, 4, 5, 6. In contrast, a
material with a lower heat resistance, namely, a heat-resistant
high-temperature stainless steel, e.g., with the material No.
1.4876, is provided as the material for the cover and layer plates
9, 10, 11, 12. The housing material with the material No. 2.4856
has a yield strength of .sigma..sub.0.2=345 MPa at 760.degree. C.,
whereas the corresponding hot yield strength in the case of the
plate material with the material No. 1.4876 is only 90 MPa (1 MPA=1
N/mm.sup.2). When the thermomechanical stresses occurring during
operation in the plates exceed their yield strength, these stresses
in the plates are reduced by plastic deformation, whereas the
housing is at most slightly elastically deformed; i.e., no
permanent damage occurs in the housing and the layer heat exchanger
remains outwardly securely tight. The Ni base material 2.4851 can
be used for higher thermomechanical requirements for the layer
plate material.
[0034] According to another embodiment, ferritic materials,
particularly Al-containing ferritic materials, can also be used
advantageously for the cover and layer plates 9, 10, 11, 12 of
layer block 2, e.g., with the tool numbers 1.4725 or 1.4767, which
according to DIN 17470 correspond to the material designations
CrAl14 4 or CrAl20 5. For these plate materials, i.e., a ferritic
core of the layer block, a more cost-effective housing material
would also be advantageous, namely, an austenitic high-temperature
stainless steel, e.g., with the material numbers 1.4876 or 1.4835
or a ferritic stainless steel such as the 1.4750.
[0035] According to another embodiment of the invention, the
aforementioned measures of different materials for the housing and
plates can be supported by structural measures, particularly by the
selection of wall thicknesses. In an alternative, the wall
thickness of housing 7 or the box-like housing sections 3, 4, 5, 6
are selected as high as possible and the wall thickness of cover
and layer plates 9, 10, 11, 12 as low as possible. In a preferred
exemplary embodiment, a wall thickness of about 1.5 mm is provided
for housing 7 and a wall thickness of about 0.3 mm for the plates.
Such a selection of different wall thicknesses would support the
aforementioned selection of different materials or strengthen the
effect of the invention.
[0036] In another embodiment, the wall thicknesses of the housing,
i.e., of material for the box-like sections and cover plates, are
selected as low as possible, for example, 1.0 mm or even 0.5 mm, in
comparison with 0.3 mm- or 0.4 mm-thick layer plate material. The
advantage here is that with rapid heating of the block formed from
the layer plates the temperature difference relative to the housing
is low and thereby the thermomechanical stresses are low. The
relative stiffness of the housing must then be achieved with the
heat resistances of the materials used for the housing. An
exemplary combination of materials for the housing (plates for the
box-like sections and covers) would be a high-temperature-resistant
Ni base material, such as, for example, 2.4856 or 2.4851 and for
the layer plates (folded sheets) of the block, a
low-temperature-resistant but ductile ferritic stainless steel.
Especially preferred in this case are Al-containing ferritic
stainless steels because of their good high-temperature corrosion
resistance and the low Cr evaporation.
[0037] According to another embodiment of the invention, the
aforementioned measures of the invention can be supplemented and
supported by a suitable joining technology for layer block 2. Thus,
according to a first exemplary embodiment, it is provided that
layer block 2 is soldered only on the front side, which is known
per se, namely, from DE 10 2007 008 341 A1 of the applicant. The
object of this publication is incorporated in its entirety in the
disclosed content of the present application. A soft, i.e., movable
core arises as an advantage of this front-side soldering, i.e., an
elimination of a complete soldering of the layer plates, because
the individual layer plates in the core region can slide one on top
of another. Thus, discrete flow channels are no longer present, as
could emerge from FIG. 1a. An elastic yielding to the thermal
stresses arising during operation is thus supported. Instead of
soldering, there is also the possibility of welding layer block 2
only at sealing edges 11a, 11b, 12a, 12b (cf. FIG. 1a) and avoiding
soldering completely. A still greater mobility of layer block 2 or
its layer plates 11, 12 is achieved thereby.
[0038] According to another aspect of the invention, it is possible
to select the same material for housing 7 and cover and layer
plates 9, 10, 11, 12, when it is assured simultaneously that
housing 7 has a sufficiently greater heat resistance than layer
block 2 or cover and layer plates 9, 10, 11, 12. This can be
achieved, as stated above, by appropriate selection of wall
thicknesses and/or of suitable joining technique.
[0039] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are to be included within the scope of the following
claims.
* * * * *