Heat exchange device of the perforated plate exchanger type with improved sealing

A heat exchange device is provided whose exchange zone is formed of a stack of perforated plates having perforations disposed so that superimposition of the perforations creates flow spaces for at least two fluids at different temperatures, said perforated plates being separated two by two by at least one seal disposed so that each perforation of the plates corresponding to a flow space through which fluid passes is separated from the perforations corresponding to the flow spaces through which a different fluid passes, said stack being kept at a clamping pressure of 2 to 50 bars by means of a plurality of tie-rods passing through it and said seal being formed by an expanded graphite manufactured under such conditions that it has a bulk density of about 200 to 500 kg.m.sup.-3.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to a heat exchange device of a perforated 
plate exchanger type with improved sealing. 
2. Description of the Prior Art 
For some time, so-called "compact" exchangers have been proposed formed of 
stacks of perforated plates, in which the perforations are disposed so 
that stacking thereof creates separate spaces in which the fluids 
(liquids, gases or smoke) taking part in the heat exchange may flow. 
The applicant has himself described devices of this type, in particular in 
the French Pat. No. 2455721 "Compact heat exchanger" and the French patent 
application No. 2500610 "Perforated plate heat exchanger". 
Depending on the arrangement of the perforations on the plates and the 
arrangement of the plates in the stack forming the exchanger, the fluid 
flow spaces may consist of channels whose direction is perpendicular to 
the plane of the plates or "flow networks" created by interconnection of 
perforations between the adjacent plates of the stack. 
In all cases, the flow spaces through which the fluids taking part in the 
heat exchange travel (assemblies of channels or networks of interconnected 
perforations) must be separated from the adjacent flow spaces through 
which flow another fluid so as to reduce as much as possible leaks between 
the different flow spaces through which different fluids pass. To obtain 
this result, it would be necessary to use perforated plates of very good 
inherent flatness having a good surface condition, very tightly clamped 
against each other. 
Obtaining improved flatness and surface condition for the perforated plates 
forms a restriction which may lead to considerable increase of the 
complexity and of the cost of their fabrication. Since the desired sealing 
further requires considerable clamping of the stack, it would be necessary 
to use means such as flanges of sufficient thickness on each side of the 
stack (which would make the assembly considerably much heavier), as well 
as tie-rods in sufficient number and quality for reducing the internal 
leaks to a value compatible with the desired operation of the device. 
There would also follow an increase in the complexity and cost of mounting 
exchangers of this type. 
Under these conditions, it was particularly important to efficiently 
overcome the problem raised and it has occured to turn to the use of 
joints, disposed alternately with all the plates of the stack. Now, the 
constructional characteristics of compact exchangers and particularly the 
pressure to be exerted for maintaining cohesion of the stack are such that 
it is particularly difficult to select a material adapted for the 
contemplated use. 
Now, it has been discovered recently that, among the very large number of 
materials which could be contemplated for this use, certain expanded 
graphites such as defined further on offer an assembly of properties which 
make them particularly well adapted to this use. 
The main object of this invention is to provide a plate heat exchanger 
which, in addition to the advantages of low cost, compactness, relative 
lightness and ease in distributing the fluids, does not have any leaks or 
has negligible leaks between the fluids between which the heat exchange is 
to take place. 
SUMMARY OF THE INVENTION 
The heat exchangers of the invention may be defined generally as comprising 
a stack of perforated plates with, between any two consecutive plates, at 
least one seal disposed so that each perforation of the plate 
corresponding to a flow space through which a fluid passes is separated 
from the perforations corresponding to the flow spaces through which a 
different fluid passes, cohesion of the plates stack being provided by a 
plurality of tie-rods passing through said stack perpendicularly to the 
planes of the plates distributed over the whole volume thereof and 
exerting a clamping pressure of about 2 to 50 bars, said seal being formed 
of an expanded graphite manufactured under conditions such that it has 
bulk density of about 200 to 500 kg.m.sup.-3. 
The expanded graphite used as the material forming the seals of the 
exchangers of the invention is advantageously in the form of flexible 
sheets of variable thickness obtained by compression moulding of expanded 
graphite particles, under temperature and pressure conditions such that 
they have the aboved-mentioned bulk density, as well as suitable 
compressibility characteristics so that crushing thereof, under the 
clamping pressures used, allows them to play their role, that is to say, 
to compensate for the flatness defects of the plates of the stack, these 
defects being generally of a few tenths of a millimeter, in particular 
when the plates are made from metal (for example sheets of steel or of 
different alloys). 
Some other physical and mechanical characteristics of the expanded graphite 
considered are given hereafter: 
Young's modulus: 0.7.10.sup.6 to 150.10.sup.6 N.m.sup.-2 
Heat conductivity: 
in a direction parallel to the plane of the graphite sheet: 15 to 400 
W.m.sup.-1..degree.C..sup.-1 
in the direction perpendicular to the plane of the graphite sheet: &lt;15 
W.m.sup.-1..degree.C..sup.-1. 
In the stacks forming the exchange zones of the devices of the invention, 
the thickness of the expanded graphite seals represents in general 2.5 to 
10 times the mean amplitude of the flatness defects which said seals must 
compensate for. In particular when the plates are made from metal and have 
flatness defects of a few tenths of a millimeter, for example from about 
0.05 to 0.5 mm, the thickness of the seals may be about 0.1 to 5 mm. It is 
very often between about 0.5 and 2.5 mm. The thickness of the plates is 
generally from 2 to 20 mm. 
Considering the compressibility characteristics of the graphite used, the 
clamping pressure applied, which may be from about 2 to 50 bars, causes 
crushing of the expanded graphite seals by 10 to 90% with respect to their 
initial thickness. In some cases, a clamping pressure of about 2 to 25 
bars may be sufficient. It is very often between about 10 and 25 bars. The 
crushing may then be from about 40 to 70% of the initial thickness.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The construction of the perforated plate heat exchangers of the invention, 
having expanded graphite seals, will be better understood from the 
following description of different preferred embodiments, in which the 
overall structure of the exchangers responds to structures already 
described in the French patent application No. 2500610, the disclosure of 
which is included in the present description by reference. 
In these particular embodiments, the stacked plates for forming the heat 
exchange zone comprise elongate perforations disposed in parallel rows. 
Other forms of perforations and other arrangements may be contemplated. 
In the particular embodiments described hereafter, the sealant may consist 
of a suitably perforated expanded graphite sheet or an assembly of 
expanded graphite strips, themselves suitably perforated, or else and 
assembly of suitably disposed unperforated expanded graphite strips. 
In a first particular embodiment, the exchange zone properly speaking is 
formed essentially of a stack forming a right prism, of polygonal plates 
having preferably at least a pair of sides parallel to each other (for 
example rectangular plates) and seals of the same shape but of a thickness 
not necessarily equal to the thickness of the plates, said plates and said 
seals being alternated in the stack so that, preferably, a single seal is 
inserted between successive perforated plates of the stack, said plates 
and said seals being provided with elongate perforations disposed in rows 
parallel with each other, said perforations being disposed and said plates 
and said seals being stacked so that the rows of perforations of one plate 
are superimposed on the rows of perforations of the seals which are 
adjacent thereto. 
Furthermore, if we consider the whole of the plates of the stack, for at 
least a part of the rows of perforations of any intermediate plate, each 
perforation is in communication with two perforations of the corresponding 
row of the plate which precedes it and with two perforations of the 
corresponding row of the plate which follows it. 
The intermediate seals may have, for the rows considered, perforations 
coinciding with those of the corresponding row of the plate which precedes 
each said seal. Or else the intermediate seals may have, for the rows 
considered, perforations coinciding with those of the corresponding row of 
the plate which follows each said seal. Or else again, the intermediate 
seals may have, for the rows considered, perforations coinciding 
alternately with those of the corresponding row of the preceding plate for 
one seal and with those of the corresponding row of the following plate 
for the following seal, this alternation of arrangement of the 
perforations being repeated over the whole stack. 
In practice, the alternated stack of plates and seals may be formed by 
alternately superimposing perforated plates and unperforated expanded 
graphite sheets, by cutting out the perforations of each expanded graphite 
sheet through the perforations of the plate which follows said expanded 
graphite sheet to be perforated, during stacking. 
In some cases, for a part of the rows of perforations, each perforation of 
any intermediate plate may be in communication with a single perforation 
of the corresponding row of the preceding plate and with a single 
perforation of the corresponding row of the following plate. In this case, 
each intermediate seal has perforations which, for the rows considered, 
coincide substantially with the perforations of the corresponding rows of 
the plates, this arrangement of the perforations of the plates and the 
seals being kept over the whole stack. 
One embodiment of this type is illustrated in FIGS. 1, 1A, 2, 2A, 3 and 3A. 
FIG. 1 is an elevational view of a plate 1 with parallel rows 21 of 
elongate perforations 6, said perforations being of the same dimension, 
evenly spaced apart along said rows, the spacing between the closest ends 
of two adjacent perforations 6 in the same row is less than the length of 
the perforation 6, the ends of the perforations being in addition aligned 
with each other from one row to another, in a direction perpendicular to 
the direction of said rows. 
FIG. 1A shows a cross-sectional view of a plate 1 through the plane A.A of 
FIG. 1. 
FIG. 2 is an elevational view of a plate 2 with parallel rows 22 and 23 of 
perforations 9 and 10 respectively, these rows being at the same distance 
from each other as the rows 21 on the plates 1; the perforations 9 and 10 
having the same dimensions as perforations 6 on plates 1 and being, in the 
same row 22 or 23, evenly spaced apart in the same arrangement as the 
perforations 6 in the same row 21 of a plate 1 but, from a row 22 to a row 
23, the perforations 9 and 10 are offset in a staggered arrangement. 
FIG. 2A shows a cross-sectional view of a plate 2 through the plane A.A of 
FIG. 2. 
FIG. 3 is an elevational view of an expanded graphite seal 3 having the 
same shape as a plate 1 (it has perforations 11). 
FIG. 3A shows a cross-sectional view of the seal 3 through the plane A.A of 
FIG. 3. It shows, for the seal 3, a thickness different from the thickness 
of plates 1 and 2. 
FIG. 4 is an elevational view of an expanded graphite seal 4 having the 
same shape as a plate 2 (it has perforations 12 and 13). FIG. 4A shows a 
cross-sectional view of seal 4 through the plane A.A of FIG. 4. It shows, 
for seal 4, a thickness different from the thickness of plates 1 and 2. 
In a first variant of this embodiment the exchange zone is formed by the 
successive stacking of a plate 1, a seal 3, a plate 2, a seal 3 and so on. 
In a second variant, the exchange zone is formed by the successive 
stacking of a plate 1, a seal 4, a plate 2, a seal 4 and so on. Finally, 
in a third variant, the exchange zone is formed by the successive stacking 
of a plate 1, a seal 4, a plate 2, a seal 3 and so on. 
FIG. 5 is an elevational view of a stack 14, formed in accordance with the 
first above described variant. FIGS. 5A and 5B show respectively 
cross-sectional views of the stack 14 through the planes A.A and B.B of 
FIG. 5. 
FIGS. 5C and 5D are respectively cross-sectional views of the stack 14 
through the planes C.C and D.D of FIG. 5. 
The stacks corresponding to the second and third above-described variants 
have not been shown in the Figures. 
In a second particular embodiment, the construction of the stack of plates 
forming the exchange zone is similar to that described in the first 
embodiment above, but the seals inserted between the perforated plates are 
in the form of perforated strips whose thickness is the thickness of the 
seal, a perforated strip corresponding to one row of perforations out of 
two. 
This embodiment is illustrated by the FIGS. 1, 1A, 2, 2A, 6, 6A and 7 to 
7C. The plates are similar to plates 1 and 2 of FIGS. 1 and 1A, 2 and 2A 
respectively. 
FIG. 6 is an elevational view of a seal 5 in the form of a strip with 
perforations 15 corresponding to the perforations 6 of plates 1 or to the 
perforations 9 of plates 2. FIG. 6A shows a section of a seal 5 through 
the plane A.A of FIG. 6. The width 1 of strips 5 is for example from b+a/2 
to b+2a, if we designate by a the distance, measured on the plates, 
between the nearest edges of the perforations of two adjacent rows and by 
b the width of the perforations. This width of strips 1 is advantageously 
from b+a to b+2a. It is preferably b+2a. It is this preferred width which 
has been designated by 1 in FIGS. 1 and 2. 
In the particular embodiment described, the exchange zone is formed by the 
successive stacking of a plate 1 of a suitable number of strips 5, a plate 
2, again strips 5 and so on. 
FIG. 7 is an elevational view of such a stack 16. 
FIG. 7A shows a cross-sectional view of stack 16 through the plane A.A of 
FIG. 7. FIGS. 7B and 7C are respectively cross-sectional views of the 
stack 16 through the planes B.B and C.C of FIG. 7. 
In a third embodiment, the construction of the stack of plates forming the 
exchange zone is similar to that described in the above embodiment, but 
the seals inserted between the perforated plates are in the form of strips 
whose thickness is that of the seal, a strip corresponding to the 
separation gap disposed between two adjacent rows of perforations. 
This embodiment is illustrated in FIGS. 1, 1A, 2, 2A, 8, 8A and 9. The 
plates are similar to plates 1 and 2 of FIGS. 1 and 1A, 2 and 2A. 
FIG. 8 is an elevational view of a seal 7 in the form of an unperforated 
strip. The width d of a strip 7 is preferably equal to the distance, 
measured on the plates, between the closest edges of the perforations of 
two adjacent rows, that is to say to the width of the separation gaps 8. 
The preferred width d has been shown in FIGS. 1 and 2. However, the width 
d may be less than the above indicated value so that, if a is the distance 
between the closest edges of the perforations of two adjacent rows, d may 
be generally between a/10 and a, and advantageously between a/2 and a. 
In the particular embodiment described, the exchange zone is formed by the 
successive stack of a plate 1, a suitable number of strips 7, a plate 2, 
again a set of strips 7 and so on. 
FIG. 9 is a perspective view of such a stack 17. 
In the heat exchangers of the invention, clamping of the stack formed of 
perforated plates and seals is provided by means of metal tie-rods 2 which 
pass through said stack perpendicularly to the planes of the plates, said 
tie-rods being advantageously introduced into a part of the ducts 11 
formed by superimposition of a part of the perforations 1 of said plates 
and said seals. The clamping properly speaking, the detail of which is not 
supplied in the present description, may be provided by conventional 
means, such as threaded rods terminating the tie-rods 20 and nuts 21 
bearing during tightening thereof on the endmost plates of the stack or on 
flanges disposed on each side of the stack so as to transmit the clamping 
force while distributing it over the whole surface of the plates. Clamping 
may also be applied to the stack by inserting between the nuts or other 
clamping means and the endplates 22 or endflanges of the stack 
spring-washers or another resilient device so as to allow the variations 
of height of the stack related to the variations of the temperature 
thereof, while maintaining sufficient and not excessive clamping on the 
stack so as to ensure the internal sealing of the exchanger during its 
operation. The fraction of the so-called "straight" ducts, formed by the 
superimposition of a part of the perforations of the plates and the seals, 
occupied by the tie-rods is limited so as to leave free passage for the 
fluid in a sufficient number of said straight ducts, but this fraction of 
straight ducts occupied by the tie-rods must also be sufficient for the 
clamping force made possible by the number and tensile strength of the 
tie-rods in extreme operating conditions to reach the clamping pressure 
required for the internal sealing of the exchanger. The seals considered 
in the invention, made from expanded graphite, allow such internal sealing 
to be obtained under as low a pressure as possible compatible with a 
sufficient mechanical maintenance of the stack. A complementary advantage 
is obtained by the fact that the seal used is a good heat conductor so 
that it participates in the transfer of heat from the relatively hot fluid 
towards the relatively cold fluid. 
In the heat exchangers of the invention, the perforated plates may be made 
from metal. They may be also formed of other materials, such for example 
as synthetic thermoplastic or heat hardenable materials, ceramic material 
or else high density graphite. 
These heat exchangers are more particularly used for exchanges between two 
fluids, particularly for recovering heat from furnace or boiler smoke 
(first fluid), the recovered heat serving for heating for example, air 
(second fluid).