Heat exchange – Casing or tank enclosed conduit assembly
Reexamination Certificate
1999-04-01
2001-09-18
Flanigan, Allen (Department: 3743)
Heat exchange
Casing or tank enclosed conduit assembly
C165S166000, C165S162000, C165S170000, C029S890042
Reexamination Certificate
active
06289977
ABSTRACT:
The present invention relates to a heat exchange bundle intended to constitute the thermally active part of a heat exchanger.
The present invention also relates to a method of welding together two sheets by means of weld beads.
The present invention also relates to a method of producing an elementary two-plate module for a heat exchange bundle.
The present invention also relates to a heat exchanger incorporating the heat exchange bundle.
It is possible to make a basic distinction between two categories of industrial exchangers:
tube and trough exchangers,
plate exchangers.
Tube and trough exchangers are the most widely used today. They consist of a bundle of parallel tubes in which a first exchange fluid flows, and of means of making the second exchange fluid flow several times in succession across regions of the bundle which are successively closer to the inlet of the first fluid. These exchangers are universally known, and their use in industry for more than a century has demonstrated their reliability. The manufacturing methods are also known, and accessible to innumerable companies throughout the world. They are relatively easy to clean, which allows them to be used with clogging fluids.
On the other hand, they are heavy and bulky, which increases their installation cost.
From the heat exchange point of view, the profile defined by the interior of the tubes is the profile exhibiting the best thermal and hydrodynamic performance. In fact, the exchange coefficient/head loss ratio is better than with any other profile for high flow speeds and fluids having low viscosity (the general case of gasses).
Unfortunately, this remarkable performance is counterbalanced by mediocre efficiency with regard to the fluid flowing outside the tubes. The latter must make numerous changes of direction throughout its passage in the exchanger in order to convert its circuit in the transverse direction with respect to the direction of flow in the tubes into a “pseudo” counter-flow. The said changes of direction result in high head losses without improving the heat exchange, and create dead zones in which the flow of fluid is virtually zero, which greatly diminishes the area effectively used for heat exchange.
Because of this, the thermal and hydraulic performances of tube and trough exchangers are relatively modest in overall terms.
Plate exchangers appeared more recently on the exchanger market (after the second world war). They made an important inroad, particularly in the food industry sector, in the form of an assembly of plates, rendered fluid-tight by flexible seals, which allows easy dismantling and rapid cleaning. This property in fact makes it possible to solve the problems of bacterial contamination which is very much present in the food industry.
Whether in the form of plates and seals, or in the form of welded plates, they are lighter than tube and trough exchangers and are particularly compact. This compactness generally derives from the small space between two consecutive plates, which results in a hydraulic diameter which is also very small. In fact it is known that, for equal performance, the length of an exchanger is proportional to its hydraulic diameter: that smaller the latter is, the shorter the exchanger will be.
For mainly mechanical reasons, the plates most often have undulations in a zig-zag pattern (“herring bones”)consisting of straight sections a few centimetres long, followed by a change of direction, as shown in G-A-798 535.
This profile has a particularly good performance when the flow speeds are low and the viscosity of the fluids is relatively high (the case of liquids).
On the other hand, this profile is less well adapted thermally in the case of gasses.
The ideal would be to be able to produce an exchanger simultaneously combining the properties of compactness and lightness of plate exchangers with the good thermal and hydraulic performances of the internal tubular profile.
U.S. Pat. No. 4,029,146 proposes heat exchangers consisting of plates with straight longitudinal undulations assembled in pairs in order to form individual modules defining, inside of them, longitudinal ducts in which a first heat exchange fluid flows. These modules are grouped to form a bundle. The second fluid flows in the intervals formed by spacers between the modules. The path of the second fluid is not very appropriate, either from the point of view of heat exchanges or from that of head load.
JP-A-58 128 236 describes a welding and hydroforming method for manufacturing a heat module. For this purpose two metal sheets are welded to each other along longitudinal lines which are parallel with one another and by a peripheral weld bead interrupted at two opposite corners of the module for the inlet and outlet of the fluid. There is then introduced between the two sheets a fluid under pressure which deforms the structure by inflation between the weld beads in such a way as to define on the one hand longitudinal channels between the longitudinal beads and, on the other hand, end collectors along the transverse sides, at each end of the longitudinal channels. In order to control the inflation of the module during the hydroforming phase, the module is placed between two fixed surfaces which limit the expansion of the module in the direction of the thickness. Such a module raises production problems and operational efficiency problems. One of the observations which is the reason for the present invention is that hydroforming gives industrially acceptable results only if the shapes sought correspond to mutually compatible deformation modes from one region of the part to another. In particular, the part cannot inflate without its overall width diminishing. Now the transverse parts of the peripheral weld of JP-A-58 128 236 constitute a reinforcement which opposes such deformation, at least in the zones close to the ends of the longitudinal channels. Similarly, the transverse collectors assume, during hydroforming, a generally tubular profile which also behaves like a powerful transverse stiffener. Furthermore, the fluid inlets and outlets through the corners of the module result in an unequitable distribution of fluid in the various longitudinal ducts.
The production of longitudinal weld beads raises problems because the sheets tend to deform and detach from one another during the welding under the effect of thermal stresses. This is true even with the laser welding method, even though a lower energy density is used.
Certainly, FR-A-2 685 462 proposes limiting the welding of the module to points of contact distributed over the surface. But the structure thus obtained is inefficient from the hydrodynamic and thermal points of view. The method mentioned for arranging the ends of the bundle as a plate, by the formation of bent rims on each sheet, is complex. The bending of non-flat zones of the sheet raises production problems.
The purpose of the present invention is thus to propose an exchange bundle, a heat exchanger and production methods which make it possible to produce a heat exchanger which is optimized in terms of fluid exchanges and thermal exchanges without causing particular difficulties in its industrial production.
According to a first aspect of the invention, the heat exchange bundle comprising:
modules each comprising two sheets laser welded along longitudinal lines which between them define first passages, of substantially tubular shape, for a first exchange fluid;
positioning means, for positioning the modules with respect to each other in such a way as to define, between the modules, second passages for a second exchange fluid;
is characterized by distributer means for distributing, in a substantially equitable and/or symmetrical manner, the flow of at least one of the first and second fluids between all of the corresponding passages.
Advantageously, the distributer means comprise, at least at one of the ends of each module, a transition zone which defines for the first fluid a first transition passage having, across the width of the module, a dimension progressively narrowing from
Claudel Michel
Fauconnier Jean-Claude
Guidat Roland
Flanigan Allen
Greer Burns & Crain Ltd.
Ziepack
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