Note: Descriptions are shown in the official language in which they were submitted.
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HOLLOW PLATE HEAT EXCHANGERS
The invention relates principally to a heat exchanger, made up of a stack of
hollow plates, which has a very high level of performance, that is, very high
bulk
conductance combined with a small front surface area, low mechanical power
requirements for the propulsion of the fluids involved and the possibility of
handling
liquid and/or gaseous fluids at relatively high differential pressures and
temperatures.
The invention relates secondarily to heat exchangers similar to the above,
overall with a lower level of performance than it has, but likely to be better
suited to
certain specific applications.
Hollow plate heat exchangers have much higher performance levels than the
solid fin exchangers on the radiators for heat engines. In fact, for the same
bulk
conductance, in such a liquid/gas heat exchanger, the gap between adjacent
hollow
plates is much greater than the gap between solid fins. As a result, the
weight of the
fonner, their bulk, front surface area and power consumed (pumping of
liquid(s)
and/or blowing of gas) are significantly lower than those of the latter. And
yet solid
metal fin heat exchangers continue to be universally used in a number of
fields. Under
these circumstances, when heat engines are fitted with normal water/air
radiators, the
front surface area (main cross-section) of these radiators measures
approximately
0.3dm2 per kW to be discharged, whilst their operation consumes mechanical
power
(ventilation and pumping) equal to up to 10% of the thermal power to be
dissipated,
or even more if the temperature differences are small. This demonstrates the
advantage of hollow plate heat exchangers.
Heat exchangers made up of a one-piece stack of hollow plates made from
polymer, glass or metal, are described in European Patent EP 1 579 163 B 1,
held by
TET. The method of producing one of these exchangers consists of
manufacturing, by
thermoblowing of a polymer parison, an accordion-shaped blank provided with
biconvex bellows, with walls embossed with steep alternating bosses, and then
carrying out a controlled compression of this blank. Following this
compression, these
bellows take on the final form of a one-piece stack of rigid hollow plates,
with a
narrow internal channel, connected to two internal manifolds. Such one-piece
polymer
heat exchangers provide completely satisfactory results for numerous
applications, as
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long as the bulk conductance sought remains in the mid-range (20W/ C/dm3 at
most)
and the fluids handled are at moderate differential pressure (0.1 MPa at most)
and not
at a very high temperature (< 100 C). In fact, in a number of specific cases,
their
advantages in terms of weight, cost, bulk and power consumption (3 to 5% of
the
thermal power to be discharged) largely compensate for this limited
performance,
particularly when the initial temperature difference between the two fluids in
question
is relatively small (< 60 C).
This one-piece heat exchanger, made up of hollow plates with embossed
polymer walls, has multiple advantages. Its walls combine a certain stiffness
and a
certain thinness, which are mutually contradictory characteristics, such that
its weight,
cost and bulk are low. Despite the laminar flow of the cooling liquid, its
narrow
internal channel allows for good thermal conductance between the liquid and
the wall
of the hollow plate. On the other hand, its embossed walls generate relatively
significant turbulence of the flow of air between the plates, which allows for
the gap
between them to be increased greatly. This considerably reduces the energy
needed to
propel the air between the plates. In addition, this significant turbulence in
the air
circulating between the plates increases the apparent thermal conductivity of
the air
and therefore the overall thermal conductance of the exchanger.
However, experience has shown that this two-stage technique of
thermoblowing and then controlled compression of the biconvex bellows of a
polymer
blank comes up against limited results if one seeks to increase the desired
performance level and in particular the bulk conductance of the heat exchanger
thus
produced. Indeed, with this technique, it is impossible to completely control
the two-
stage manufacturing process of a one-piece stack of hollow plates, with regard
to the
thicknesses of the internal channel and of the walls of the plates, even
though these
thicknesses are decisive parameters for the bulk conductance value of the
exchanger.
In practice, for the internal channel of the hollow plates, this results in a
thickness
with an average value of around two millimetres, with a dispersion of at least
thirty
percent. With regard to the thickness of their walls, the average value is in
the region
of one millimetre and the dispersion is approximately fifty percent, this
dispersion
being mainly due to the uneven narrowing of the wall during the thermoblowing
of
the blank.
In addition to the limitation of performance attributable to these thickness
problems, it must be noted that the presence of the internal manifolds of the
stacked
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hollow plates adds another aspect to such limitation: the creation of a
central channel,
common to all of these hollow plates, which allows for the direct rapid flow
of the
liquid between these two manifolds. As a result, this relatively large central
channel
scarcely contributes to the desired heat exchange.
High-performance cooling devices for various applications are described in
international application WO 2006/010822, filed by TET. In these devices, the
radiators are heat exchangers produced in accordance with the method in TET's
European patent. For one particular application (the cooling of the exhaust
gases from
a diesel engine with a view to recycling them), provision is made in the
application
for using a one-piece heat exchanger having hollow metal plates, capable of
withstanding much higher differential pressure and temperature than those to
which a
one-piece polymer exchanger can be subjected. To this end, the metal accordion-
shaped blank for the heat exchanger had to be manufactured by hydroforming.
This
known technique seems promising in the field of one-piece heat exchangers
having
hollow metal plates but, at the moment, it has not yet been possible to
implement it
correctly and, moreover, it is itself limited with regard to its theoretical
efficiency.
Indeed, as the thermal resistivity of the cooling liquids, water or oil, is
high, the
thermal resistance of the layer of liquid, flowing laminarly in such hollow
plates, is
inevitably high, given an average thickness of at least 2 mm. This removes a
large part
of the advantage of the low thermal resistance that would be provided by the
metal
walls envisaged.
Consequently, another way of producing metal heat exchangers had to be
developed for several specific applications, in particular for the application
initially
envisaged and, more generally, for any device involving the possibility of
having very
high-performance heat exchangers. To this end, these new metal heat exchangers
must
have weight, bulk, front surface area and mechanical power consumption that
are as
low as those of the one-piece exchangers described above. They must do this
whilst
having a much higher bulk conductance (at least 100 W/ C/dm3, for example)
and,
above all, the possibility of operating correctly at high differential
pressures and
temperatures, for example 1 MPa and 600 C. In addition, derived from these
first
metal exchangers, other lower-performance polymer or glass exchangers are also
possible, which relate to specific particular applications, notably those that
use
corrosive fluids.
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To this end, unlike the one-piece metal exchangers, initially envisaged for
the
cooling of the exhaust gases of diesel engines, the new heat exchangers,
particularly
envisaged for this specific use, must be fitted with hollow metal plates,
provided with
an internal channel as narrow and accurate as possible and walls that are both
stiff and
very thin. With regard to the general characteristics of such a heat
exchanger, they
will obviously be completely different from those of the previous heat
exchangers.
They will be borrowed from a heavy, bulky heat exchange device developed for
cooling electric transformers in power distribution systems, described in
patents US 3
153 447 of 1964 and US 3 849 851 of 1974. This device is made up of large
hollow
metal plates with embossed walls, connected by welding to two external
manifolds,
capable of being arranged vertically and cooled by air circulating by natural
convection.
The first subject of the invention is a high-performance heat exchanger, made
up of hollow plates with thin metal walls stiffened by appropriate embossing,
simultaneously having a low weight, bulk and surface area, low mechanical
power
consumption and high bulk conductance, whilst being suitable for reliable,
easy to
control industrial production and, moreover, capable of handling liquid and/or
gaseous fluids, at high temperatures and/or differential pressures.
The second subject of the invention relates to improved heat exchangers,
similar to the previous exchanger, with lower performance than it, but better
suited to
given particular applications, different from those of the previous exchanger,
comprising a stack of hollow plates with thin polymer or glass walls,
stiffened by
appropriate embossing.
The third subject of the invention is a compact radiator with a small front
surface area, made from these improved heat exchangers, having high thermal
conductivity and requiring very low pumping and ventilation power.
According to the invention, a heat exchanger with low weight and bulk and
very high bulk conductance, capable of handling fluids at high differential
pressures
and temperatures, in which:
- hollow metal plates, with a narrow internal channel, are stacked evenly
spaced and connected to external manifolds;
- these plates comprise a embossed central zone, located between two
connecting zones provided with narrow openings with an area
approximately equal to the area of a cross-section of the central zone;
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- the walls of these plates have been produced by stamping and cutting a
metal sheet;
- the lateral edges of the two walls of a hollow plate are welded;
is characterised in that:
5 - the walls of each hollow plate are both rigid and very thin, their
embossed central zone having one or more sets of alternating aligned
bosses, provided with steep strain hardened faces, creating a large
number of sharp edges, facing in directions oblique or perpendicular to
the alignment of the bosses;
- the gap between opposite faces is uniform, very small, exactly known
and practically constant, in the range of the envisaged differential
pressures;
- the gaps separating the plates are relatively narrow.
Before a commentary is given on the advantage of these new arrangements, it
will be noted that in the US patents in question, the walls of the plates do
not need to
be thin and their rigidity is not a particular problem, in such a way that the
embossing
of the central zone of the walls is not a solution to a stiffness problem
which, in this
case, barely exists. A sufficient thickness of walls made from a normal metal
meets
this need without difficulty. The embossing is simply to increase the heat
exchange
area of the plates without increasing their dimensions. This is achieved by
longitudinal undulations that result from relatively small recesses, evenly
spaced in
the walls. The particular profile of these undulations is shown; it is
ordinary and can
hardly be characterised by any originality, as this aspect of things is of no
interest in
this type of exchanger. However, due to these undulations, the internal
channel of the
hollow plates has an undulating thickness that varies symmetrically around a
relatively high average value. Furthermore, the walls of the internal channel
do not
comprise opposing sloping faces.
According to the first arrangement of the invention, it firstly involves
plates
with very thin rigid walls (for example 0.15 mm for certain steels) that are
endowed
with a particularly high hardness and limit of elasticity by their strain
hardening,
obtained "as a bonus" at the time of standard (cold) stamping; each face of
these
hollows and bosses serves as a rigid strip and, moreover, each sharp edge
behaves as a
beam in which these strips are embedded. These strips can therefore only take
a very
limited deflection, under the action of the differential pressures applied.
Particularly
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when the overpressure is external, this deflection always remains considerably
less
than half of the internal thickness of the hollow plates, which thickness,
measured
between the faces of the bosses, is by design exactly known and particularly
small
(0.3 mm, for example). This prevents any contact between walls of opposite
faces so
that the heat exchange function between the two fluids is always correctly
performed.
Under these conditions, each embossed hollow plate according to the
invention owes its remarkable primary stiffness to the fact that the metal
constituting
its walls is strain hardened and, furthermore, that its alternating bosses
significantly
increase its moment of inertia. These doubly stiff very thin strips are thus
able to act
perfectly as an efficient heat exchanger between the two fluids circulating
along their
two surfaces, even if there is a high differential pressure between the
fluids. The
immediate characteristics of these stamped alternating bosses, which must
provide
this stiffness, define the basis of the invention. They take the form of steep
strain
hardened faces, generated by significant local elongations of the initial flat
sheet,
which thus create a number of very thin, very stiff strips all of the edges of
which are
embedded in beams formed by the sharp edges of the bosses.
The sharp edges of the dihedrons, which form between them these steep faces,
have a second known effect, that of increasing the apparent thermal
conductivity of
the air; the edges, which are orientated obliquely and/or perpendicularly to
the
direction of flow of the air, have the effect of generating significant
turbulence in the
generally fast airflow that passes through the relatively narrow gaps
separating the
plates. This arrangement would be meaningless in the case of the vertically
arranged
undulated plates in the US patents in question, as slow airflow passes through
the
gaps with unspecified dimensions separating them, circulating by natural
convection.
If we now, to conclude this argument, refer to TET's European patent, it can
be seen that all of the causes of the performance limitations set out above
are
eliminated in this new heat exchanger and replaced by their opposites: the
walls and
the internal channel have very thin, precise and well-known thicknesses, and,
as will
be set out in detail below, the central channel can disappear. Conversely, all
of the
positive characteristics, relating to the embossed walls of the hollow plates
of the one-
piece polymer heat exchanger described in this European patent, are retained.
These
characteristics are supplemented by those arising from the strain hardening of
the
sheets of metal used. Due to their combination with the advantages of the
exchanger
described in the US patents together with the use (a priori ill-advised, in
the context of
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the high differential pressures envisaged) of very thin walls and the creation
of a
particularly narrow internal channel, a new, non-obvious heat exchanger is
produced.
This new exchanger consequently has performance levels that greatly transcend
those,
already very efficient, of the one-piece polymer heat exchangers according to
TET's
European patent.
According to particular characteristics, supplementary to the main
characteristics above,
- each hollow plate comprises at least two rows of alternating bosses;
- two adjacent rows are separated by a narrow, straight partition, formed
by two stamped internal protrusions, assembled by welding;
- the height of these protrusions is equal to half of the value of the
internal
thickness of the plates, at the crests of their bosses.
These latter arrangements, taken from a possibility envisaged in the US
patents in question to improve the rigidity of the plates when they have large
dimensions (m), give two particularly advantageous results for the heat
exchanger
according to the invention. Firstly, under the effect of a relatively high
internal
overpressure, applied to the hollow plates of such a heat exchanger, the
straight
internal partition maintains the internal thickness of the embossed central
zones at a
value that is practically independent of the differential pressure to which
the thin walls
of the plates are subjected. The result of this is that the hollow plates,
with very think
walls stiffened by appropriate embossing, are able to withstand a relatively
high
internal overpressure without damage. Without such welded internal
protrusions, the
adjacent rows of highly rigid alternating bosses would be separated by a
flexible zone
acting as a hinge. In response to such overpressure, this would lead to a
slight bulging
of the plates, causing a significant reduction in the heat exchanges in the
gaps between
them or even rapid deterioration of the plates. However, with such a partition
formed
by these two welded internal protrusions, it is not necessary to
systematically increase
the thickness of the very thin walls of the hollow plates to enable them to
withstand a
temporarily high internal overpressure. This means that lighter, less costly
heat
exchangers can be produced.
The second advantage of these welded internal protrnsions comes in the form
of greater efficiency of the desired heat exchange. The internal partition
formed in this
way between two adjacent rows of alternating bosses constitutes a barrier for
the flow
of liquid entering the hollow plate. The first effect of each barrier is to
prevent a
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significant direct flow between the two external manifolds, along a smooth
wall with
a small surface area and therefore inefficient for the desired heat exchange,
in that this
surface is not swept by a strong airflow as it is in the rear zone of the
upstream
manifold. On the other hand, the second effect of this barrier is to direct
the incoming
flow towards the two rows of alternating bosses, which have high heat exchange
efficiency, and thus maximise the heat exchanges performed.
It will be noted that these two advantages are of little interest for the heat
exchanger with large hollow plates with relatively thick walls described in
the US
patents in question. In this exchanger, the maximum differential pressure,
which
occurs at the foot of the large vertical plates, is the relatively low
hydrostatic
overpressure generated by the cooling oil. This does not concern the heat
exchanger
according to the invention, which can obviously be installed in any relevant
position
and above all can operate with very high differential pressures. Moreover, as
the oil
circulates from top to bottom by natural convection in hollow plates much
larger than
the external manifolds, the low upstream dynamic pressure, due to a low
circulation
speed, prevents it from being able to favour a rapid direct trajectory from
one
manifold to another.
According to characteristics complementary to the previous ones:
- the angles formed by the normals to two adjacent faces of the alternating
bosses measure at least 30 , so that the sharp edges of these faces can be
effective in the creation of turbulence and comparable to beams in which
the faces of these bosses are embedded;
- the maximum angle of the normals to two adjacent faces is limited by the
restrictions imposed on the conditions under which the metal sheet in
question is stamped.
According to a characteristic complementary to the previous ones, the
opposite faces of a plate have parallel walls and the gap separating these
walls is
constant and of the same order of magnitude as their thickness.
According to characteristics complementary to the previous ones:
- the alternating bosses have, of their own, two faces in the form of an
isosceles trapezium, having a common longitudinal edge and, shared, two
rhomboid faces;
- the long diagonal of the rhomboid faces can measure several tens of times
the thickness of the wall of the plates.
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According to characteristics alternative to the previous ones:
- the alternating bosses have, of their own, two triangular faces and, shared,
two hexagonal faces, having a common transverse edge;
- the distance between the transverse edges of the hexagonal faces can
measure several tens of times the thickness of the wall of the plates.
According to a characteristic complementary to the previous ones, the
embossed central zone of each hollow plate is connected to the external
manifolds by
two connecting zones provided with lateral edges having a significant slant
and
smooth walls comprising portions of truncated cones.
According to a characteristic complementary to the previous ones, the external
manifolds have an aerodynamic profile capable of minimising their drag.
According to a possible characteristic complementary to the previous ones,
symmetrical boss faces appear to be cut in a diamond pattern and comprise
several
secondary faces, provided with additional sharp edges.
As a result of these different arrangements, the bulk conductance of the heat
exchanger thus produced is particularly high. There are several reasons for
this: (1)
the plates have metal walls that have negligible thermal resistance, (2) the
thermal
resistance of the very thin layer of water or oil inside the plates is low,
despite the
laminar flow of the layer and the relatively high thermal resistivity of these
liquids
and (3) the turbulence and the apparent thermal conductivity of the air
circulating
between the plates increase with the height of the bosses and the total number
of sharp
edges they comprise. With at least two rows each comprising several
alternating
bosses, provided with faces sloped at approximately 45 , an efficient
compromise is
achieved between the different parameters involved. The stamping of the
bosses, the
slope of the faces of which is less than approximately 50 , is a standard
operation that
poses no production problems, and a minimum angle of 30 between the normals
to
two adjacent faces ensures satisfactory turbulence in the air flow and a
minimum
width for each of the rows of bosses in the central zone of the plates, when
the height
of these hollows and bosses is fixed. Furthermore, a minimum angle of 30
between
the normals to two adjacent faces gives the edge in question sufficient
stiffness for it
to be comparable to a beam, and the edges are then collectively comparable to
a
network of beams.
Moreover, with a heat exchanger formed by the stacking of a large number of
such identical plates, connected to two external manifolds, the pressure drop
of a
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liquid circulating within them at a constant flow rate and flowing laminarly,
which
can, if applicable, be relatively fast, can be reduced considerably. In any
event, such a
stack significantly reduces the power necessary to pump the liquid. In
addition to the
use of external manifolds with an aerodynamic profile, despite the relatively
large gap
5 separating the plates, their largest dimension, installed parallel to the
flow velocity of
the two intersecting fluids, leads to a significant reduction in the
aerodynamic drag of
the radiator and/or the power necessary to ventilate it.
With regard to the metals that can be used for the production of the walls of
the hollow plates according to the invention, it will be noted that these are
not
10 numerous but are well known to specialists in stamping and that ultimately
the choice
(aluminium or steel, for example) will mainly be determined by the mechanical
behaviour of these metals in the operating temperature range of the heat
exchangers
that will incorporate these plates.
As a result of these various arrangements, the industrial manufacture of the
very high-perforniance heat exchangers according to the invention comprises a
set of
completely controllable operations that are relatively easy to automate, which
results
in an advantageous cost price for the mass production of such exchangers.
These
operations are as follows:
1) stamping and cutting of identical plate walls from a thin metal sheet;
2) turning of one wall head to foot;
3) assembly of two adjacent walls, by welding of their lateral flanges and
the protrusions of their internal central partition;
4) mounting and fixing by welding of these hollow plates to their two
external manifolds.
According to the invention, a compact radiator with very high bulk
conductivity is characterised in that:
- it comprises two identical groups of thin metal hollow plate heat
exchangers, associated with two main upstream and downstream
manifolds, provided with flat rectangular trapezoid surfaces, slightly
separate from each other and arranged so that their square corners are
opposite each other;
- the individual upstream and downstream manifolds of the exchangers in
each group are connected respectively, at constant intervals slightly
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larger than the width of the central zone of the exchangers, to the two
surfaces of the two main upstream and downstream manifolds.
By means of these arrangements, a radiator can be constructed with very high
bulk conductivity and as small a main cross-section as possible (up to 0.10
dm2 per
kW to be discharged). A large number of heat exchangers themselves formed by a
large number of metal hollow plates stacked according to the invention can
easily be
assembled on either side of the two main flat manifolds. This compact radiator
also
requires particularly low pumping and ventilation power, around five times
lower than
the power required by solid fm radiators with the same thermal conductance.
The characteristics and advantages of the invention will become apparent in
more detail on reading the following description of a non-limitative
embodiment of
the invention, given in reference to the appended drawings, in which:
- Figure 1 is a top view of a first embossed wall of a hollow plate
according to the invention;
- Figure 2A is a top view of a second embossed wall of a hollow plate
according to the invention and figures 2B and 2C are views of particular
faces of this wall;
- Figure 3 is a longitudinal cross-section of the alternating bosses on this
first wall;
- Figure 4 is a longitudinal cross-section of one end of a hollow plate,
welded to a manifold;
- Figure 5 is an isometric perspective view of a heat exchanger with fifteen
hollow plates;
- Figure 6 is a top view of a radiator according to the invention,
constructed using these heat exchangers.
Figure 1 shows a first embodiment of a thin metal wall 10 of a hollow plate.
This wall has been stamped and then cut so as to have an embossed central zone
13,
arranged between two connecting zones. As an example, this wall is made from
aluminium and is 0.3 mm thick and its embossed central zone is 60 mm wide and
76
mm long. This central zone 13 is made up of two adjacent identical rows 12 and
14 of
alternating bosses, separated by a narrow straight zone 16, 4 mm wide. The two
connecting zones 18 and 20 have smooth walls. Each row comprises two identical
areas of alternating embossing, made up of bosses and hollows, that is, for
rows 12-14,
four bosses 221_2 and 241_2 on the one hand and four hollows 22'1_2 and 24'1_2
on the
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other, these latter being shown in grey. Each boss 221_2-241_2 or each hollow
22'1_2-
24'1_2 is shaped like a roof with four slopes having four very steep sharp
edges,
namely for each alternating boss in row 12: (1) of its own, two symmetrical
trapeziums 261_2 and 281_2 for the bosses and 26'1_2 and 28'1_2 for the
hollows, all with
a 19 mm large base, (2) shared with the adjacent boss in the same row, two
isosceles
triangles 301_2 and 321_2 for the bosses and 30'1_2 and 32'1_2 for the
hollows, all with a
28 mm base, (3) a longitudinal crest 341_2 for the bosses and 34'1_2 for the
hollows, all
5 mm long, and (4) the same height of 5 mm. It will be noted that the two
pairs of
isosceles triangles 302-30', and 30'2-32, in row 12 (and similarly in 14),
which belong
to two consecutive alternations of the alternating embossing, form two flat
rhombuses.
At the centre of the narrow straight zone 16, which splits in two the embossed
central zone 13 of the wall 10 shown, an internal protrusion 36 2 mm wide is
produced by stamping, with symmetrical sides as stiff as the stamping
technology
allows. Such a protrusion 36 has a height equal to half of the maximum gap
separating
the crests of the bosses on the two walls of the hollow plate produced (that
is, 0.2 mm,
as specified below). Two lines 38-40 separate the parallel external edges of
the two
rows 12-14 of alternating bosses on one hollow plate wall from the pair of
parallel
external flanges 42-44, which form part of the sealing surface of two plate
walls. The
lines 38-40 and the flanges 42-44 are 1 mm wide and form a small step 0.2 mm
high,
which determines half of the internal thickness of a plate at the crests of
its bosses.
These two flat lines 38-40 end in the two flat parts 46-48 of the two
connecting zones
18-20 of the wall 10 and these two parallel flanges 42-44 end with the two
pairs of
oblique external flanges 501-502 and 521-522 of these same connecting zones;
they
form the other part of the sealing surface of the plate walls. Each flange
501_2 or 521_2
forms an angle of 60 with the longitudinal line of symmetry of the wall 10.
The end
of each connecting zone 18-20 comprises an almost flat truncated cone portion
54-56,
with a half-cone angle of 87.5 . This tapered portion is delimited by two
pairs of arcs
581_2 and 601_2, the latter pair being 8 mm long. Their ends are connected to
each other
by two steps 1.5 mm high, such that the area of each of the upstream or
downstream
openings, thus made for a hollow plate, measures 24 mm2, i.e. approximately
the area
of the transverse cross-section of the internal space of the embossed central
zone 13 of
the plate.
Figure 2A shows a thin metal wall 11, stamped and then cut, which constitutes
a second embodiment of a hollow plate wall according to the invention. This
wall 11
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only differs from the previous wall 10 in its embossed central zone, which
only
comprises a single row of bosses 15, 26 mm wide, and in the shape of its
alternating
bosses. This single row comprises three bosses 22b1_3 and three hollows
22'bl_3, the
latter being shown in grey. Each boss 22b1_3 and each hollow 22'bl_3 is shaped
like a
roof with four steep slopes. For the three alternating bosses in row 15, this
gives for
each one: (1) of its own, pairs of symmetrical lateral triangles 25b1_2,
27bl_2, 29b,_2 for
the bosses, and similarly 25'bl_2, 27'bl_2, 29'b1_2 for the hollows, all with
a 14 mm
large base and (2) shared with the adjacent boss, central hexagons 311_5, all
with a
transverse crest 18 mm long, and the same height of 5 mm.
Figures 2B and 2C show, as variants to the faces of the bosses in Figure 1 and
2A, two of the main faces in Figure 2A having secondary faces. Figure 2B shows
a
triangular lateral face 25, provided with three secondary faces 371-3 forming
a
relatively flat trihedron with three sharp edges, with a diamond point 39,
located at the
centre of gravity of this triangle. Figure 2C shows a hexagonal longitudinal
face 31,
provided with six triangles with coplanar sides 411_6, with a central diamond
point 431_
6, similar to the point 39 in Figure 2B. The height of these points is
determined by the
limits of the metal sheet stamping technology.
Figures 1 and 2A show two of the possible forms that the bosses on the
embossed walls of the hollow plates according to the invention can take.
Figures 2B
and 2C show the possible variants of the main faces of these bosses, in order
to
improve their ability to produce turbulence in the airflow between plates.
Figure 3 shows an enlargement of a longitudinal cross-section along an axis
AA' (see fig. 1) of one of the ends of part of a hollow plate before it is
connected to a
manifold. This plate is the result of the welding of the two walls l0a and
lOb, the wall
10b being the wall 10a turned head to foot, around the transverse axis of
symmetry
BB' (see fig. 1). This cross-section AA' is made along the crests 352 and 35'2
of the
alternating embossing formed by the boss 242 and the hollow 24'2 in row 14 and
it
passes through the connecting zone 18 of the wall 10a of this plate. Figure 4
shows an
enlargement of a cross-section of the same plate end, made along the
longitudinal line
of symmetry CC' (see fig. 1) of rows 12 and 14 of alternating bosses and the
connecting zones 18 and 20 of the wall 10a.
In Figure 3, the bosses and hollows of the first embodiment of the bottom wall
lOb and the top wall l0a of a plate are reversed, so that parts 242 and 24'2
of the top
wall 10a, seen in profile in Figure 3, appear respectively as a hollow and a
boss. The
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14
boss 24', and the hollow 24, on the wall 10b defmed above are nested in the
hollow
and the boss respectively. The thickness of the part 62 of the internal
channel of a
hollow plate, located between the nested crests 341-352 or 34'1-352 of the
embossed
zone of the plate is 0.4 mm, and the thickness of the part 64 of the internal
channel,
located between the 45 slopes of the ascending or descending sides of the
bosses is
0.28 mm. The thickness of the internal channel 66, between the flat parts of
the
connecting zones 18 and 20, is 0.4 mm.
According to Figure 3, the right-hand part of the cross-section along the line
AA' shows (1) the start 68 of the gradual separation of the walls from the two
facing
conical sections 54-56 of the walls l0a-lOb, which end these two connecting
zones,
(2) the two symmetrical steps of these walls that start with the circles 582
and 58, and
(3) the two symmetrical external flanges 522 and 50, which define the sealing
surface
of the walls 10a and l Ob.
According to Figure 4, the cross-section shown is made along the longitudinal
axis of symmetry CC' of one hollow plate end engaged and welded with a bead
70, in
the edges and the ends of a slot 72, in the form of a 120 arc, made in the
connecting
shell 74 of an external manifold 75, formed by two elongated shells welded to
each
other. The cross-section represented shows two parallel sections 16a and 16b
of the
narrow central zones of the walls l0a and 10b, separated by a 0.4 mm gap 66,
and two
other divergent sections 54 and 56 corresponding to the facing conical
sections of the
connecting zones of the two walls l0a and 10b of the hollow plate. The gap
between
the extreme edges of these two divergent sections is 3 mm and the length of
the 120
arcs 602 and 601 (see fig. 1) is 8 mm. As a result, the area of the straight
cross-sections
of the internal channel with embossed walls and that of the openings in the
ends of the
fm are approximately equal.
Figure 5 shows an elementary heat exchanger 76 comprising fifteen thin
hollow metal fms 781_15 with embossed walls. The ends of these hollow plates
are
engaged and welded as shown above in slots with circular edges, 3.5 mm wide
and
spaced 8 mm apart, made in the walls of the external manifolds 80-82, with an
aerodynamic profile. To enable the easy production of such welds, the
manifolds 80-
82 are made up of two elongated shells, with a U-shaped transverse cross-
section,
welded to each other along a line 83. They are made from metal strips cut out
of
sheets identical to those used for the production of the stamped plate walls.
Slots with
appropriate width, length and spacing are made in half of these strips, then
the two
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types of strip prepared in this way are turned into front closure and
connecting shells
75, by means of two matching templates, with protruding and hollow profiles.
Then,
the openings in the various hollow plates are welded to the slots in the
connecting
shells. Then, two front closing shells are in turn welded to the previous two
and one of
5 their ends is sealed off, to form both the two streamlined external
manifolds and the
exchanger itself.
Figure 6 shows a top view of a compact radiator 81. Six identical heat
exchangers 761_6 can be mounted in parallel on either side of two flat main
manifolds
84-86, in the form of rectangular trapeziums arranged head to foot, in order
to form a
10 compact radiator with appropriate overall thermal conductance. These flat
manifolds
84-86 have parallel sides 881_2 and 901_2 and a thickness approximately equal
to the
maximum dimension of the straight cross-sections of the external manifolds
801_2.
Two adjacent exchangers are mounted so that the lateral edges of their plates
are
practically abutting or slightly interleaved. In the first case, the feet of
the upstream
15 801_6 and downstream 821_6 external manifolds are engaged to the same depth
in
appropriate circular openings, 941_6 and 961_6, made at constant intervals
along the
long sides 92-93 of the faces of the main manifolds, and they are then welded.
In the
second case, the depths of insertion of the manifolds are different for the
exchangers
in the odd and even rows. The length of the longest 882 - 902 of the parallel
sides of
the two main manifolds 84-86 is determined by the number of heat exchangers 76
to
be mounted. The short sides of the two main manifolds 84-86 have lengths
determined by the spacing of the external manifolds 80-82 and by the gap 100
(typically 5 mm) that separates their oblique sides.
Such an assembly of heat exchangers formed by stacks of thin hollow metal
plates, with very thin walls stiffened by embossing, allows for the formation
of a
compact radiator that is particularly advantageous for the cooling of high
power
thermal engines (> 100 kW). They have a very small main cross-section, very
high
thermal conductance, low pumping and ventilation power consumption and limited
bulk and weight. It is also suitable for the processing of diesel engine
exhaust gases,
used cooled to improve their operation at low speeds. More generally, any heat
exchange between two fluids, particularly between a liquid and a gas, having a
high
temperature and/or differential pressure (up to around 600 C and 1 MPa) can be
carried out efficiently by means of such a compact metal assembly.
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16
The invention is not limited to the examples described. The length and width
of the hollow plates can be significantly greater than those shown in Figure 1
and
measure several decimeters. The same can be said of the number of alternating
bosses
in each row and the number of rows on each plate. The maximum dimensions of a
plate are in practice determined by those of the table on the stamping press
available.
With regard to the number of hollow plates in a heat exchanger, this can rise
to
several tens. The same can be said of the total number of exchangers assembled
in a
compact radiator.
It will also be noted that it is possible to produce a hollow plate according
to
the invention using two appropriate embossed walls that are similar but not
identical
due to their different lateral edges. Instead of two identical walls, with
lateral flanges
comprising a small step defining the half-thickness of the internal channel of
the
central zone, it would be possible to have one wall with flanges having a step
twice as
high as the previous step and another wall without any step. This would
require the
use of two different pairs of stamping moulds but would have little economic
impact
when the production rate is high.
The figures above show hollow plates for a liquid/gas heat exchanger. The
liquid circulates in these metal plates with a very narrow internal channel
(0.3 mm). In
the case of a gas/gas heat exchanger, the thickness of this internal channel
is
obviously much greater (typically > 1 mm) and the gap between plates is
generally
smaller than that on the exchanger shown. This is so that the mass flows and
speeds of
the two gases are comparable on either side of the walls of the hollow plates.
Moreover, for particular applications, notably in chemistry and any other
field
in which corrosive fluids are used, it is often desirable and sometimes
necessary to
have access to high-performance glass heat exchangers perfectly suited to
their
conditions of use. To this end, these glass heat exchangers will be provided
with high
bulk conductance, but half way between the conductances given above for hollow
plate exchangers made either from a one-piece polymer or from metal of the
type
according to the invention (20 or 100 W/ C/dm3). With regard to the maximum
temperatures and differential pressures that can be applied to these glass
heat
exchangers, they will be lower than those withstood by the metal exchangers
according to the present invention and higher than those relating to the one-
piece
polymer exchangers according to TET's European patent. For this same type of
application, it can also be advantageous to have access to polymer heat
exchangers
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17
with a bulk conductance approximately 50% higher than that of these one-piece
exchangers, whilst retaining their differential pressure and temperature
ranges.
To this end, the new technology for metal heat exchangers according to the
invention can be adopted and adapted and, instead of a metal sheet, a sheet of
glass or
polymer can simply be used and processed by hot stamping or thermoforming. The
manufacturing methods used in these two sheet forming techniques are similar
to each
other: the first uses mechanical pressure and two matching moulds comprising
hollows and/or protrusions, and the second uses pneumatic pressure and a
single
mould with hollows and/or protrusions; both use appropriate heating. However,
no
strain hardening is produced.
The thicknesses of the walls and internal channels of such a glass or polymer
hollow plate heat exchanger with embossed walls and external manifolds will
inevitably be increased in accordance with the specific mechanical properties
of the
type of glass or polymer used. Their performance will be derived directly from
them,
as explained above.
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