Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21'794~fi.~
HEAT EXCHANGER WITH IMPROVED PLATES
DESCRIPTION
This invention relates to a plate-type heat
S exchanger made up of a stack of corrugated plates
delimiting channels of variable section.
Corrugations have the general purpose of
disturbing the flow of fluids to increase the heat
transfers through the plates, but they have the drawback
of making the pressure drops much larger than with flat
plates. An ear.Lier French patent (FR-A-2 648 220
described a particular form of corrugated plate which
made it possible to reduce the volume of dead or
recirculation zones in which the fluid stagnates
substantially, which is one of the main causes of losses
in heat exchange efficiency and the fouling of plates if
the fluid is charged with particles, because they then
deposit much more easily. The plates of that earlier
invention had facets of two alternately different lengths
and were assembled together such that the corrugations of
the consecutive plates formed angles of preference of
nearly 180°C, i.e. such that, in each channel, the long
facets were oriented substantially in the same direction
corresponding to the direction of fluid flow in the
channel: the short facets thus faced the fluid and, owing
to their steeper slope, deviated it strongly towards the
long facets of the other plate delimiting the channel.
The result was that the fluid stream licked the long
facets over a large part of their surface, and the
recirculation zones which formed behind the corrugations
which restricted the channel, namely in front of the long
facets, were reduced as a consequence. One thus obtained
better heat exchange performance as we:l1 as easier flow.
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The objectives assigned t:o that invention
were thus met, but certain drawbacks may be found to such
a configuration of plates. First of all, the corrugations
composed of a long facet and a short facet have, for the
same pitch (the pitch designating t:he width of the
corrugation), a smaller height than the corrugations of a
conventional corrugated plate composed of two similar
facets. In other words, the channels have a smaller
average section. In practice, it is however desirable not
to reduce this section, thus requiring the use of plates
of similar shape but with larger corrugations having a
larger pitch. The lines of corrugations then have a
larger spacing and the number of contact points between
the plates is smaller, thus reducing the mechanical
strength of the stack.
Another drawback stems from the fact that the
beneficial effect: is obtained only for one direction of
fluid flow in each channel, alternating in adjacent
channels, thus imposing countercurrent circulation of
fluids which is not always desired. Finally, if pressure
drops are lower than with conventional corrugated plates,
they remain significant.
The present invention may be regarded as an
improvement of the earlier invention, because it offers
substantially equivalent advantages as concerns the heat
exchange performance and the reduced fouling of the walls
of the plates, :out also substantially smaller pressure
drops by the reduction of vortex movements, while still
involving the same number of contact points between
plates as ordinary corrugated plates. In its best
embodiments, the invention also lends itself to similar
flows of fluids in both directions, thus allowing the
free choice of parallel current flows as well as
counterflows.
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The invention relates, in its most general
form, to a heat exchanger composed of corrugated plates
placed side by side to delimit channel;;, the plates being
similar, composed of facets joined ~by bottom and top
lines, the plates being joined to each other at contact
points; the plates are alternately turned over and joined
either through their top lines or through their bottom
lines; it is characterized in that the facets include
bosses near the top lines and hollows near the bottom
1 roes .
This thus provides an arrangement i.n which
the corrugations are flattened and squeeze the section of
the channels near the contact points of the corrugations,
thanks to this combination of hollows and bosses as
IS described and to the alternate turnover of the plates.
The United States patent US-A-4 014 385 describes an
arrangement in which the facets include, on the contrary,
hollows at the top and bosses at the bottom, so that they
are connected to each other to form right angles, which
allows the stiffening of the plates but has an effect
opposite to that of the invention on the flow of fluid.
Another design i.s presented in the UTlited States patent
US-A-3 661 203: the top lines of every other plate are
broken by depressions into which penetrate the bottom
lines of the other plates. This inter-penetration offers
good cohesion between the plates and great regularity of
channel width.
It is recommended that hollows and bosses
should be discontinuous along the facets, with the bosses
located near th~s contact points, thus enabling them to
play the channel volume reduction role only at the
locations in which dead or stagnant zones are most likely
to form. The bosses are
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The invention will now be described in
greater detail in conjunction with the following figures
appended by way of non-limitative illu~~tration:
~ Figure 1 is a general view of a plate-type heat
exchanger in which the plates area represented in an
exploded view for clarity;
~ Figure 2 is a partial sectional view of a plate
according to the invention;
~ Figure 3 is a top view of a plate of the invention,
showing the distribution of the bosses and hollows;
~ Figure 4 illustrates how the plates are superposed
and in particular the angle of their corrugations
and
~ Figure 5 illustrates the respective positions of
the hollows and bosses on the two superposed
plates, according to a plane section indicated by
the line V-V on a plate in Figure 3.
A plate-type heat exchanger of a current type
is shown in Figure 1. It is made up of a superposition of
rectangular plates 1, having four bores 2 in the corners,
a smooth peripheral groove 3 and corrugations 4 on the
rest of their surface. The plates may be produced by
various means, by stamping, machining or casting, and in
the real heat exchanger they bear on each other through
their corrugations 9. Gaskets, not represented, are then
compressed between the grooves 3 and provide
leaktightness. The stack is held by clamping.
In the represented embodiment, there is a
countercurrent fluid flow, but it could be otherwise. The
corrugations 4 are of herringbone :Form but could be
straight. The fluids are generally liquids in existing
embodiments, but this is not obligatory, and there may
also be changes of state. The invention is applicable to
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all these categories of exchangers and even to different
kinds of exchangers.
According to the invention (Figures 2 to 5),
the corrugations 4 of the plates 1 may be broken down
into top lines 10 alternating with boi~tom lines 11, the
lines 10 and 11 all being parallel to each other and
separating contiguous facets 12. The facets 12 have a
rough surface, i.e. they are not straight over the
essential part of their length as in conventional
corrugated plates, but exhibit bosses 13 and hollows 19.
For the requirements of the description, the observation
reference is located in the channel 6 over the plate 1,
and the top lines 10 are over the boi=tom lines 11; the
bosses 13 are convex reliefs and the hollows 14 are
concave reliefs in this channel 6.
Hollows and bosses 14 and 13 are produced
without difficulty with the corrugations 4, for example
by stamping with special dies, without any special
operation. The general representation in Figure 3 shows
that the hollows 14 and the bosses 13 do not extend over
the entire length of the corrugations 9 but are, on the
contrary, discontinuous and that the bosses 13 extend
near the top lines 10, approximately over half of the
length of the corrugations 4, and to the locations near
the contact points 15 of the adjacent plate 1; more
precisely, the b~~sses 13 of the adjacent facets 12 extend
on either side of the middle top line 10 so as to
surround in pairs the contact points 15. The hollows 14
are adjacent to the bottom lines 11 and each extends
between two consecutive bosses 13 o:E the facet 12 to
which they belong (hollow 14 and bosses 13 thus alternate
along each of the facets 12), so that they form a roughly
uninterrupted series along each of the bottom lines 11,
alternately on the two facets 12 which border it. In this
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embodiment, the average direction of th.e flow of fluidin
the channel 6 is vertical (ac:cording to the
representation of this Figure 3) and t:he angle a of the
corrugations 4 with this direction is 60. The ot her
plate 1 delimiting the channel 6 wi:Ll be similar but
placed after having been tuned over, so that the plate s
1
will join through their top lines 10 (Figures 9 and 5),
their corrugations 4 being crossed and forming angles of
60 . The same will be true for each couple of plates 1,
which will be joined either through their respective top
lines 10 or through their bottom lines 11. Whether the
direction of f:Low is ascending or descending, the
characteristics of the flow are identical because the
form of the channels 6 is symmetrical. It is noted in
particular that the bosses 13 form zones in which the
section of the channel 6 is very small around the cont act
points 15, in which the fluid would have a tendency to
stagnate, but that the bosses 13 do not contribute to
hindering the flow at the other locations of the channels
6.
Similarly, the contact points 16 with the
other adjacent plate 1, located at the center of the
diamonds formed by four adjacent contact points 15, will
be surrounded by two pairs of hollows 14 of the two
plates 1 concerned, but which are seE~n as bosses 13 in
the adjacent channel 6 delimited by these two plates 1.
It is thus seen that all the channels will have the same
form.
The hollows 14 have hardly any influence on
the flow in the channels 6.
To allow the proper positioning of the plates
in relation to each other, hollows 17 may be provided on
the top lines 10 at the contact points 15, between the
bosses 13. These hollows make it po:>sible, on the one
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hand, to position the plates precisely and, on the other,
to better streamline the flow around the corrugation.
These hollows 17 have a depth of about 0.5 mm (between
0.3 mm and 1 mm) and their form allows. engagement of the
contact points 15 of the upper plate. They are made in
the same way as the rest of the plate, with no additional
cost. The hollows 17 are established on every other plate
1; the top lines 10 of the other plates 1 remain
straight.
Bosses could also be provided at the bottom
lines 11, at the location of the conl~act points 16, on
certain plates 1 to facilitate also the engagement of the
plates 1 by these bottom lines 11.
Other embodiments are possible depending in
1S particular on the angles formed by the corrugations in
relation to the average direction of f:Low.
An embodiment actually test=ed included plates
with corrugations 4 in which the angle a, in relation to
the flow was equal to 60°, the plates 1 having a pitch p
(Figures 2 and 3) of 13 mm and a height a (Figure 2) of
3.9 mm, for maximum heights and depths of 0.8 mm and
diameters from 3 to 4 mm for the bosses 13 and the
hollows 14. The channels 6 were 0.4 m long and 0.14 m
wide. The flow rate was 6 to 40 m3 per hour in each
2S channel 6. The plates 1 in accordance with the invention
produced a head loss (or pressure drop) from 30 to 50~
smaller than a conventional corrugated plate, i.e.
without bosses 13 and hollows 14. The heat exchange
coefficients were similar, with d.i.fferences of less than
5$. The plates c~~nsisted of stainless-steel stampings 0.6
mm thick.
The invention can be applied to all fields of
activity in which exchangers of this type are used, and
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in particular the chemical, para-chemical, petroleum,
climatic, agro-food, energy production and metallurgical
industries.
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