US20080264616A1

US20080264616A1 - Novel Heat Exchanger Corrugations and Applications Thereof

Info

Publication number: US20080264616A1
Application number: US12/097,621
Authority: US
Inventors: Sophie Deschodt; Francois Leclercq; Marc Wagner
Original assignee: Individual
Current assignee: LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2005-12-22
Filing date: 2006-12-20
Publication date: 2008-10-30
Also published as: FR2895493B1; EP1966560A1; WO2007074290A1; FR2895493A1; CN101341372A

Abstract

The invention relates to a strip of corrugations (2) for brazed plate and fin heat exchangers of the type with partial offset, comprising a series of zones for brazing on a first separation sheet (1), a series of zones for brazing on a second separation sheet (1) adjacent to the first separation sheet (1) and a series of wings (13, 14), said wings forming the fins, of which at least one of the wings (14) of a corrugation is inclined without being perpendicular to the separation sheets (1) and of which at least one of the wings (13) has at least one section perpendicular to the separation sheets (1) and a brazed plate or fin exchanger comprising said corrugations.

Description

The present invention relates to novel heat-exchange corrugations and to applications thereof.
The technology involving brazed plate and fin exchangers (BPFEs) is commonly used to offer a large heat-exchange area within a very compact component.
BPFEs are composed of a stack of embossed sheets, called the “corrugations”, separated by flat sheets, termed “separation sheets”, and are closed on their sides by bars. The assembly constitutes a fluid flow layer.
These exchangers thus consist of a plurality of superposed plates between which are inserted heats exchange corrugations or heat-exchange fins whose geometry is particularly varied. These constituent elements of the exchanger are generally made of aluminum. They are assembled by salt bath brazing or vacuum brazing.
Each fluid flows in the space contained between two adjacent separation sheets, this space being called a passage. The flows can be counter-current, co-current or cross-current flows.
The benefit provided by these exchangers is that they offer a large heat-exchange area within a relatively reduced volume. These are very compact exchangers. An exchanger body can consist of a large number (more than one hundred) of fixed layers. In the case of large-scale processes, it is frequently the case that the exchanger is composed of one or more parallel bodies mounted in banks.
The corrugations inserted between the plates have the function of increasing the heat-exchange area and of thus increasing the overall transfer performance: being secured to the primary area by brazing, they transfer heat flow by conduction. The area of the separation sheets in direct contact with the fluid is the primary area, and the secondary area, formed by the transfer corrugations, represents about 50% to 90% of the total heat-exchange area.
The corrugations perform a dual function in BPFEs. In addition to supplying the major part of the heat-exchange area, the brazed corrugations provide the exchanger as a whole with its mechanical integrity. The heat-exchange corrugations based on a folded, perforated, lanced or stamped flat product have given rise to a large number of variants.
The following corrugations may be mentioned: the straight corrugation, which is a simple embossed metal plate of generally crenelated shape; the perforated corrugation, which is a straight corrugation produced with perforated bands (plates); and the partially offset (serrated) corrugation. The partially offset corrugation is characterized by a shape of corrugations such that the legs of the corrugations are perpendicular to the separation sheets and by an offset of the strips of corrugations at regular intervals.
Choosing which type of corrugation is used in an exchanger depends on the heat exchange required and on the maximum admissible pressure drops. However, a corrugation which has good heat performance often generates high pressure drops; the best solution thus generally involves a trade-off between these two variables.
The most frequently used corrugation is the partially offset corrugation. Its manufacture is widespread and well controlled: it is produced by folding and stamping sheets, generally aluminum sheets, using suitable tools.
While keeping all the other parameters constant, it is found that increasing the frequency of the offsets improves heat performance and causes higher pressure drops at the same time.
Including the extreme case of the straight corrugation, the family of partially offset corrugations having the same density is a good example of the trade-off to be found between heat performance and pressure drops.
The pattern of the partially offset corrugation is represented below in the figures.
At each offset line, the thickness of the corrugations creates a restriction for the fluid flow. Moreover, the edge of the fins creates a stop point for the fluid. These two effects combined increase the pressure drop in the corrugations, increasingly so when the offsets are frequent.
FR-A-2807828 proposes an improvement in these corrugations that consists in removing material over a small length at the start of the partial offset: the pressure drop is then lower and the heat exchange is barely diminished.
However, it would be desirable to make available exchangers having brazed plates or fins with an improved heat-exchange coefficient with respect to the partially offset corrugations.
Moreover, EP-A1123763 describes partially offset corrugations in which the legs of the corrugations are inclined with respect to the separation sheets and the length of each connecting part in the longitudinal direction of each strip of corrugations is less than or equal to the thickness of the plate forming said strip, so as to minimize the length of connection between adjacent strips.
Now, following research conducted over a long period, the Applicant has discovered that modifying the inclination of the legs of the corrugations having conventional partially offset geometry results in corrugations which make it possible to involve the entire fluid in the heat exchange, without leaving any more weakly mixed regions and offering a gain in area in the case of low densities of corrugations.
Accordingly, the subject of the present application is a strip of corrugations for BPFEs of the partially offset type, comprising a series of regions intended to be brazed to a first separation sheet, a series of regions intended to be brazed to a second separation sheet, adjacent to the first separation sheet, and a series of legs, said legs forming fins, characterized in that at least one of the legs of at least one corrugation is inclined, without being perpendicular to the separation sheets, and in that at least one of the legs has at least one portion perpendicular to the separation sheets.
In the present application and in the following, the angles indicated are those formed by a leg as seen in cross section with respect to the direction of flow intended for a fluid. By convention, the angle zero is the direction of the separation sheets as represented horizontally in the figures which follow. The nature of the modifications is indicated by taking as reference the conventional rectangular shape of a partially offset corrugation seen in cross section with respect to the direction of flow intended for a fluid, and the shapes, unless stated otherwise, are indicated by taking as point of view the direction of flow intended for a fluid and therefore by observing the section transversely to this direction of flow.
The term “corrugation”, given its wavy shape, refers to that part of a strip of corrugations which is equivalent to the notion of “wavelength” in physics.
In preferred conditions of implementing the invention, at least one of the legs of a corrugation is inclined at an angle of 43° to 80°, advantageously 45° to 75°, preferably 50° to 75°, particularly 60° to 75°, very particularly 60° to 70°. Thus, the conventional rectangular shape of a corrugation, and preferably of each corrugation, is modified to form a trapezoid having a right angle. This type of modification is called type 1.
Only part of the corrugations may have one of its legs which is inclined, but preferably at least 20%, particularly at least 40%, very particularly at least 60%, of the corrugations. Very advantageously, all the corrugations of a strip of corrugations have one of their legs which is inclined.
Preferably, only one of the legs of a corrugation is inclined, particularly always the one situated on the same side of the corrugation (left-hand side or right-hand side) of a strip of corrugations.
In other preferred conditions of implementing the invention, the inclined leg of a corrugation has an additional fold. This type of modification, represented below, is called type 2. Advantageously, this additional fold is not made parallel to the general direction of folding of the plates forming the corrugations, which is the general direction of flow intended for a fluid. A vortex is thus generated in the inclined part of the corrugation, which improves the heat transfer. This folding makes it possible to create a three-dimensional flow.
In other preferred conditions of implementing the invention, an outward fold is provided on a vertical leg so as to form an inclined portion as represented in the figures which follow. This type of modification is called type 3. Preferably, an inclined leg and a vertical leg having an inclined region are combined on one and the same corrugation.
In yet other preferred conditions of implementing the invention, folds are provided on a leg so as to form at least one step. This type of modification is called type 4. Preferably, an inclined leg and a folded leg are combined on one and the same corrugation so as to form a step.
The various folds above can advantageously be combined with one another on one and the same corrugation or on the corrugations of one and the same strip of corrugations.
The patterns proposed advantageously allow a junction between the successive partial offsets so as to form a corrugation mat obtained by folding a flat sheet and capable of being easily installed in an exchanger.
In yet other preferred conditions of implementing the invention, the regions of a corrugation which are intended to be brazed to a first separation sheet and the regions intended to be brazed to a second separation sheet are parallel to one another, particularly when they are brazed to their separation plates or sheets.
In yet other preferred conditions of implementing the invention, apertures are provided in the sides of the corrugations using the same principle as the conventional partially offset apertured corrugations.
These novel corrugations and strips of corrugations can be manufactured from flat products by folding, as in the case of the conventional partially offset corrugation, with the profiles of the customary tools being modified. The cut-outs and folds can be made in the same direction as with the tool for manufacturing conventional partially offset corrugations, the shape of the folding blades being different.
The corrugations and strips of corrugations forming the subject of the present invention have very advantageous properties and qualities.
It will be recalled that the partially offset corrugation is characterized particularly by an offset of the corrugations at regular intervals.
The inclinations of the legs vary periodically between the partial offsets in such a way as to periodically change the direction of the leading edge. The leading edges can be vertical, horizontal or inclined.
The solution proposed by the invention, taking account of the periodic change in direction of the leading edges at each strip of corrugations, makes it possible to involve the entire fluid in the heat exchange without leaving any more weakly mixed regions.
It also makes it possible to render the partial offsetting more effective in the case of a flow which has not had time to recombine between two partial offsets: here the flow is recombined (and turbulence is created) in another plane.
These novel corrugations offer a gain in area in the case of low densities. The brazing area is then reduced in relation to the partially offset corrugation. These corrugations are thus particularly advantageous in the case of medium-pressure or low-pressure flows in which it is sought to reduce the pressure drop (where use is conventionally made of high-density and low-density partially offset corrugations). The corrugations have a height of between 3 and 10 mm.
These properties are illustrated below in the experimental section. They justify the use of the corrugations and strips of corrugations described above in the manufacture of exchangers having brazed plates or fins (BPFEs).
They find applications in condensation or in distribution, where their characteristics of low pressure drops can be significant factors.
They find large utility in the case of two-phase fluids in evaporation, or condensation.
Accordingly, another subject of the present application is a device comprising at least two parallel separation plates or sheets between which are installed strips of corrugations having corrugations as defined above, in particular an exchanger having brazed plates or fins with corrugations as defined above.
Between two parallel separation sheets, when, for example, the conventional shape of each partial offset is deformed so as to form a trapezoid comprising a right angle, it is possible at each partial offset for the pattern to be inverted and offset. Thus, in the course of its movement the fluid comes up against leading edges which are successively inclined to the right and to the left.
The preferred conditions of implementing the above-described corrugations apply equally to the other subjects of the invention addressed above, particularly to the strips of corrugations, to the devices comprising at least two parallel plates between which strips of corrugations are installed, and to the exchangers having brazed plates or fins as defined above.

The invention will be better understood with reference to the appended drawings, in which:

FIG. 1 represents a perspective view, partially in section and exploded, of a conventional brazed plate exchanger (BPFH);

FIG. 2 represents a sectional view of three successive patterns of conventional partially offset corrugations;

FIG. 3 represents the leading edges encountered in the direction of the flow of a fluid in an exchanger provided with conventional partially offset corrugations;

FIG. 4 represents a sectional view of six successive patterns of corrugations according to the present invention, in which the left-hand leg or the right-hand leg of each corrugation is inclined such that each corrugation has a right-angled trapezoid shape;

FIG. 5 is a view analogous to that shown in FIG. 3, but in the case of the strips of corrugations shown in FIG. 4;

FIG. 6 is a perspective view of the series of patterns of corrugations shown in FIG. 4;

FIG. 7 represents a variant of a corrugation shown in FIG. 4, 5 or 6, in which the inclined leg of the corrugation has an additional fold in order to create a vortex effect;

FIG. 8 represents a series of 9 strips of corrugations comprising trapezoidal corrugations with a right angle, and corrugations of this same type but additionally having a stepped profile; the same sequence is repeated after the eighth strip;

FIG. 9 represents a view analogous to those shown in FIGS. 3 and 5, but implementing the succession of patterns of corrugations shown in FIG. 8; and, finally,

FIG. 10 also represents a series of 7 patterns of corrugations, all according to the present invention.

FIG. 1 shows a whole series of parallel flat separation sheets 1 which define superposed levels for the flow of fluids. Between each pair of adjacent separation sheets 1 are situated strips of heat exchange corrugations 2 brazed to the separation sheets 1. In a general manner, a whole series of bars 3 installed around the periphery of the exchanger, and generally brazed to the sheets, are responsible for sealing the assembly. The system of pipes 4, comprising cylindrical distribution boxes, and the design of the assembly are embodied to allow at least two fluids to be able to flow such that each fluid flows at different levels.
FIG. 2 shows the shape of a conventional partially offset corrugation, together with the lateral offset of the successive patterns of corrugations. The upper surface 11 and lower surface 12 of a strip of corrugations are brazed to upper and lower separation sheets 1 (not shown). It is possible to observe the rectangular general shape in cross section of the conventional partially offset corrugation, and FIG. 3 shows the succession of leading edges encountered by a fluid as it flows. Each corrugation also has two lateral surfaces 13 and 14 perpendicular to the separation sheets 1. The legs 13 and 14 of the corrugations are perpendicular to the separation sheets 1 and the strips of corrugations are offset at regular intervals.
FIG. 4 represents the modified profile of the corrugations according to the invention, these being designated type 1 corrugations. They have lost their rectangular shape, which has been modified to become a trapezoidal shape with right angles. It can be seen on the strip represented at the upper part of the drawing that the right-hand leg 14 is inclined by an angle α (alpha) of 45° with respect to the surface of the separation sheets 1 (not shown). The angle of 45° corresponds to the opening angle of a leg with respect to the conventional rectangular shape of a conventional partially offset corrugation. The left-hand leg is entirely perpendicular to the separation sheets (angle α (alpha) equal to 90°). For successive strips of corrugations it will be observed that the left-hand leg and then the right-hand leg are alternately inclined so as to form, in cross section, a trapezoid.
FIG. 6 is a perspective view of such a chain, in which view the strip of corrugations represented in the first plane corresponds to that represented at the top of FIG. 4, and successively.
FIG. 5 reveals that when the fluid flows between two separating plates provided with such a succession of strips of corrugations, the leading edges are not only perpendicular, as in the case of strips of conventional partially offset corrugations, but also inclined in the model represented at 45°. An increase of almost 15% of the secondary area is thus obtained in relation to a serrated corrugation having the same height, thickness and density.
FIG. 7 reveals that the inclined leg 14 of a corrugation shown in FIGS. 4, 5 and 6 has an additional fold. Thus, the right-hand leg 14 of the corrugation represented in FIG. 7 is formed by two plates 15 and 16. Furthermore, the fold between the strips 15 and 16 is not made parallel to the fold formed between the strip 16 and the upper surface 11 or between the upper surface 11 and the leg 13. Consequently, a vortex effect is obtained during the flow of the fluid. This type of modification has been called type 2.
FIG. 8 reveals that the patterns of corrugations A, C, D, F and H are of type 1, while the patterns of corrugations B, E and G are patterns of corrugations of the same type but ones in which the leg 13 perpendicular to the separation sheets 1 has two folds which give this leg 13 a stepped profile thus having two portions perpendicular to the separation sheets 1.
The leading edges encountered by the fluid as it flows between two separation sheets 1 provided with such a succession of such patterns of corrugations is represented in FIG. 9.
FIG. 10 reveals that the strip of corrugations C comprises left-hand and right-hand lateral legs, each provided with an additional fold, such that each of the lateral legs comprises a part perpendicular to the separation sheets 1 and an inclined part. Such a leg is inclined without being perpendicular to the separation sheets and at the same time has a portion perpendicular to the separation sheets.
Finally, the dotted arrows in FIGS. 8 and 10 represent the lines along which legs of successive corrugations are interconnected, thus allowing them to be manufactured from a flat sheet.
In the present inventions and in a general manner, the profiles of the legs of corrugations, and their sequences, will advantageously be chosen such that forming the corrugation is possible starting from a flat sheet both in respect of the legs and in respect of the upper and lower surfaces of the strips of corrugations.

Claims

1-10. (canceled)

11: A strip of corrugations for brazed plate and fin exchangers of the partially offset type, comprising a series of regions intended to be brazed to a first separation sheet, a series of regions intended to be brazed to a second separation sheet, adjacent to the first separation sheet, and a series of legs, said legs forming fins, wherein at least one of the legs of at least one corrugation is inclined, without being perpendicular to the separation sheets, and in that at least one of the legs has at least one portion perpendicular to the separation sheets.

12: The strip of corrugations of claim 11, wherein at least one of the legs of a corrugation is inclined at an angle of 43° to 80°.

13: The strip of corrugations of claim 11, wherein at least 60% of the corrugations have at least one of their legs which is inclined.

14: The strip of corrugations of claim 11, wherein only one of the legs of a corrugation is inclined.

15: The strip of corrugations of claim 11, wherein the inclined leg of a corrugation has an additional fold.

16: The strip of corrugations of claim 15, wherein the corrugations are formed by folding plates, and in that the additional fold is not made parallel to the general direction of folding of the plates forming the corrugations.

17: The strip of corrugations of claim 11, wherein one and the same corrugation comprises an inclined leg and a vertical leg having an inclined region.

18: The strip of corrugations of claim 11, wherein folds are provided on a leg of at least one corrugation so as to form at least one step.

19: The strip of corrugations of claim 11, wherein various folds are combined with one another on one and the same corrugation or on the corrugations of one and the same strip of corrugations.

20: An exchanger having brazed plates or fins with corrugations as defined in claim 11.