EP0354892B1 - Heat exchanger for use between a gas and a fluid, with increased thermal exchange properties - Google Patents

Description

The invention relates to a heat exchanger according to the preamble of claim 1. Such a heat exchanger is known according to FR-A-793 344.

The heat exchange between a gas and a fluid separated by a wall is difficult to achieve economically if the difference between the temperatures of the gas and the fluid is small.

The present invention effectively uses a well-known physical datum, namely that the metallic wire is the best heat convector between a gas and a solid, since it constitutes almost entirely a leading edge.

Currently, this difficult exchange of enthalpy between a gas and a solid is most often achieved using bundles of finned tubes or a series of parallel plates crimped on tubes in which the fluid circulates.

In general, devices using these methods require air velocities circulating between these plates or between these fins, sufficiently high to obtain a satisfactory exchange coefficient between gases and tubes.

This results in high energy consumption, and high temperature differences between gas and a fluid that is difficult to improve economically.

According to document FR-A-793344, it is known to use a metallic fabric which is obtained from filiform elements and conduits, these two elements respectively forming the weft and the warp of a metallic fabric.

This texture is very unfavorable to high heat exchanges, and is also the source of higher pressure drops; it is very different from the woven wire fabrics welded to tubes, according to the present invention. In addition, patent FR-A-793344 indicates only incidentally and very briefly the use of the two metallic fabrics attached to either side of the tubes.

The object of the present invention is to economically solve, by a simple technical arrangement, heat exchange problems which are difficult to solve by the use of existing techniques, thanks to the particularly efficient characteristics of exchangers with woven wire cloths.

The subject of the present invention is a heat exchanger between a gas and a fluid formed by one or more unitary exchange panels made up of fabrics woven from metal wires with high thermal conductivity, between which metal tubes are arranged at intervals. substantially parallel to each other, in which the fluid circulates, characterized in that the unitary panel comprises two sets of at least two fabrics, the gas preferably flowing perpendicular to the fabrics; the fabrics of the same assembly are spaced between the welding points in contact with the tubes.

In order to increase the contact surfaces between fluids, the number of fabrics woven into metal wires forming the assemblies may include two, three or more fabrics.

The unitary panel can thus comprise two sets of 2 or 3 or more fabrics, woven into metallic wires between which metallic tubes are welded at intervals.

The number of fabrics to be used per set will be limited to 2 or 3 fabrics and preferably two fabrics for reasons of construction and optimization of a methodical exchange.

In the case of the use of two sets of several metallic fabrics, and in order to multiply the ruptures of gas streams, the fabrics of the same assembly will be spaced between their welding points in contact with the tubes.

By way of example, the fabrics are spaced a few millimeters apart, for example 3 mm.

The chosen gas is generally air, which does not exclude the use of other gases or vapors. The fluid circulating in the tubes is a liquid or a gas, and can result from a change of state liquid / gas or gas / liquid, the latter fluids having a very good coefficient of heat exchange with the interior wall of the tube. The fluids used include in particular water, saline or other solutions, solvents, heat transfer liquids, etc.

The unitary panel comprises two sets of at least two fabrics of metal wires with high thermal conductivity, of any structure and nature, for example of copper. These wires have a diameter between 0.1 and 3 millimeters, and generally between 0.2 and 2 millimeters. In addition, the diameters of warp threads and weft threads can be different.

The size of the mesh of the canvas is the result of a compromise specific to each application, between a dense structure of the fabrics to multiply the leading edges of the wires, and a sufficient opening of the mesh, to reduce to an economic level the pressure drop during the passage of gas through the fabrics.

The metal wires of the fabrics perpendicular to the tubes are preferably of a larger diameter than the wires parallel to the tubes.

Indeed, small diameter wires are the best convectors; they are suitable for wires parallel to the tubes. On the other hand, the wires perpendicular to the tubes must have a sufficient diameter to ensure the transfer by thermal conductivity between these wires and the tubes, of the enthalpy received or emitted by the wires constituting the fabrics.

In the description of the invention, the wires perpendicular to the tubes are designated by warp yarns and the wires parallel to the tubes by weft yarns. It is obvious that a reverse arrangement could be adopted without departing from the scope of the present invention.

For example, take a 0.5 mm wire in warp, and a 0.25 mm wire in weft; but it is not excluded to use other reports.

According to a preferred embodiment, the contact surfaces between wires are increased by rolling the fabrics, so as to pass from a point contact to a contact of the meniscus type between the two wires. This enlarged contact promotes heat transfer between weft and warp threads, and makes it possible to obtain a higher overall heat exchange coefficient of the unitary panel.

This rolling is carried out under a load such that only the contact surfaces between wires are increased, and it does not in any way modify the profile of the wires between the contact surfaces.

The diameter of the tubes must be chosen according to the characteristics of the fluid circulating in these tubes, its flow rate and the allowed pressure drops, while respecting a speed such as to provide a good heat exchange between the fluid and the wall of the metal tube. .

The tubes are of a material with high thermal conductivity, for example copper, and they are preferably arranged at regular intervals. They preferably have two flats parallel to each other on which the wire cloths are welded. By welding, we also understand any other means of fixing, for example brazing, bonding, etc.

The interval between the tubes depends on the diameter of the warp threads and the permissible temperature differences between gas and fluid. In general, the tubes are arranged substantially parallel to one another and spaced 1 to 10 cm apart, the diameter of the warp threads being chosen as a function of this deviation.

The tubes, generally circular, are partially flattened by any known method so as to present two parallel flats with one another, intended to ensure an effective connection between fabrics and tubes, both for a satisfactory solidity of the panel, and for a good transfer to the tubes of the enthalpy collected or emitted by the threads of the fabrics.

The unitary panels can have very variable dimensions and are generally of rectangular or square section.

The exchangers of the claimed type comprise a single panel or are formed by assembly of unitary panels the number of which can even exceed 10, depending on the magnitude of the temperature gradient to be ensured in a methodical exchange against the current between the gas and the circulating fluid. in the tubes.

In addition, the texture of the fabrics of the successive panels can be different to adapt to the evolution of the heat transfer to be ensured during the passage of the gas through the exchanger.

Spacers between the panels arranged perpendicular to the tubes ensure, with the tubes, the rigidity of all the panels and a regular spacing between two panels.

This spacing is generally constant, but it is not excluded to possibly vary it.

Collectors mounted at the ends of the tubes or connections between the ends of the tubes of successive panels, ensure the circulation of the fluid; these collectors are optionally provided with partitions to adapt the circulation of the fluid to the enthalpy level that the gas is capable of receiving or emitting. These separations allow a methodical exchange between gas and fluid whenever necessary and practicable.

The gas stream is preferably introduced perpendicularly to the panels by ducts of section generally identical to that of the exchanger; the same arrangement is adopted at the outlet of the exchanger.

The collectors or sheets in the case of connections between the ends of the tubes of successive panels, serve as ducts on two of the four sides of the exchanger; the collectors are in direct contact with the fabrics at the end of the tubes.

In the case of connections between the ends of the tubes of successive panels, a flexible spacer is mounted between the sheet and the metal fabrics so as to allow the differential expansion of the panels.

On the other two sides of the exchanger, sheets also mounted with a flexible spacer matching the edges of the wire mesh, form the two other sides of the sheath of the exchanger.

For reasons of protection against possible corrosion, a deposit or a surface treatment of any type may be applied to the constituent elements of the exchanger formed by unitary panels and collectors.

One can consider, for example, an electrolytic deposition of a metal or a protective coating based on a coating or a paint by electrodeposition or by electrophoresis.

According to a preferred embodiment in the case of air coolers, the metal mesh heat exchangers of the claimed type are grouped in a star. The panels of the exchangers are arranged vertically, and the exchangers are assembled in a star around a central air intake duct.

In fact, in industrial air coolers, the thermal powers to be evacuated can reach several tens of megawatts, which implies the use of large frontal surfaces.

The exchangers described above are grouped in pairs around a central duct which unites the veins of air collected between the two faces of the branches of the star.

In the case of air coolers, one or more fans installed in the duct above the star, create the vacuum necessary for the passage of the air current through metal fabric exchangers of the type claimed.

For this, the underside of the star is fully closed as well as the upper part of each branch located around the sheath. The speed of the fan (s) must be adjustable so as to limit the consumption of electricity according to weather conditions.

If the temperature of the fluid to be cooled is appreciably higher than that of the air, the addition of a chimney of sufficient height in the extension of the sheath is likely to create a natural draft capable of overcoming the pressure drop in passing air through appropriate wire mesh. This arrangement makes it possible to avoid the installation of fans, the consumption of electricity, and the noise pollution of fans.

The reverse arrangement with lower chimney allows the enthalpy of the air to be captured by a fluid to be heated.

The invention described which makes it possible to use the high convective power of metal wires has many advantages.

The most important advantage is certainly the maintenance of a high heat exchange coefficient even at low front speed of the gas perpendicular to the wire mesh.

This characteristic makes it possible to limit the speed of the gas to a level sufficient to ensure the flow of enthalpy given to the gas (in the case of air coolers) or extracted from the gas (in the case of gas coolers) during its passage through the fabric exchanger. metallic.

This results in the possibility of large reductions in the frontal air speed, which can bring very high savings in the energy consumed by the fans. Indeed, the energy consumption for a given circuit is proportional to the cube of the gas speed. If the structure of the fabrics gives the same pressure drop as exchangers with finned tubes or plates, the possibility of reducing the gas speed for example from 3 m / sec to 1 m / sec leads to a reduction in the consumption of energy in the ratio 1³ / 3³ = 1/27.

In addition, if a small difference between the temperatures of the gas and of the fluid circulating in the tubes is required, as for example in the drying processes which are the subject of Belgian patent n ° 902120 and French patent n ° 8604883, the exchangers with metallic fabrics make it possible to economically obtain this small difference, because even at low gas speed, the difference between the temperature of the gas and the temperature of the fluid can be very small.

Due to their design, metal mesh heat exchangers are significantly more compact and lighter than the finned tube heat exchangers usually used in industry, and the star arrangement makes it possible to greatly reduce the floor space occupied.

In particular, in certain processes of the chemical industry involving the condensation of a gas, it is possible, thanks to the reduced difference between the temperature of the air and that of the cooled fluid, to dissociate the elimination of enthalpy in the atmosphere by an air cooler with metal sheets of the claimed type, and the condensation of the gas with possible cooling of the liquid obtained, using a heat transfer liquid intermediate between the air cooler and the condenser / cooler of the chemical. This liquid can be possibly glycol water for protection against freezing.

The present invention will be better understood using FIGS. 1 and 2.

FIG. 1 represents a unitary panel formed by 2 sets of 2 metallic fabrics between which are welded at intervals, metallic tubes arranged substantially parallel to each other, in which the fluid circulates.

Figure 2 shows a plan view of a set of 24 exchangers arranged in pairs in the 12 branches of a star.

According to FIG. 1, the unitary panel 1 comprises the assemblies 2 and 3 each formed of 2 metallic fabrics 4 and 5, 6 and 7, which are welded respectively at points 8 and 9, 10 and 11, to the tubes 12 and 13.

The two assemblies 2 and 3 are welded at regular intervals to the neighboring tubes 12 and 13.

The metallic fabrics used are obtained by weaving metallic threads, for example copper, with a diameter of 0.3 mm in weft and 0.6 mm in warp.

The tubes with a diameter of 5 mm are arranged parallel to each other and spaced apart, for example 3 cm.

The metal tubes 12 and 13 have flats at the welding points 8 and 9, 10 and 11.

The flats were obtained by any known means such as a partial flattening of the tube, or directly in manufacture from a profiled die.

The flats of the tubes used in the example presented in FIG. 1 have a width of approximately 3 mm.

The fabrics are separated from each other by a space of a few millimeters.

According to FIG. 2, a set 14 of 24 exchangers arranged vertically and grouped in pairs 15 and 16, in 12 branches similar to branch 17, form the star 18 around and below the circular sheath 19.

The vertical axis fan 20 centered in the sheath 19 creates in the star 18 the vacuum necessary for the suction of air through the exchangers.

The air circuit for exchanger 15 has been indicated by arrows 21 and 22.

The exchangers shown diagrammatically in FIG. 2 can be of any type. For example, they can be made up of unitary panels 1 of FIG. 1, which are assembled so as to create a staggered arrangement of the metal tubes of the successive panels. The unit heat exchanger used can include from 1 to more than 10 unit panels.

Such embodiments make it possible to obtain high heat exchange coefficients.

Claims (7)

1. Heat exchanger between a gas and a fluid made with one or more exchange unit panels formed by wire woven grids of high heat conductivity between which, at intervalls and in a substantially parallel mutual relationship, metal tubes are welded in which a fluid is circulated, characterized in that the unit panel comprises two assemblies of at least two grids and the grids of a same assembly are spaced apart between the welding points in contact with the tubes.
2. Heat exchanger between a gas and a fluid according to claim 1, characterized in that the unit panel comprises two assemblies of 2 or 3 woven grids.
3. Heat exchanger between a gas and a fluid according to claims 1 and 2, characterized in that the metal wires of high heat conductivity of the woven grids are made of copper with a diameter between 0.1 and 3 mm.
4. Heat exchanger between a gas and a fluid according to claims 1 to 3, characterized in that the metal wires of the grids perpendicular to the tubes have a higher diameter than the wires parallel to the tubes.
5. Heat exchanger between a gas and a fluid according to claims 1 to 4, characterized in that the contact surfaces between wires are increased by rolling the grids.
6. Heat exchanger between a gas and a fluid according to claims 1 to 5, characterized in that the metal tubes are located at regular intervalls and have two mutually parallel flat parts to which the wire grids are welded.
7. Heat exchange assembly between a gas and a fluid according to claims 1 to 6, characterized in that the heat exchangers used as air-coolers are located vertically in a star-shaped disposition around and under an air exhausting shaft.

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