EP0454550A1 - Heat exchanger, in particular for corrosive fluids - Google Patents

Abstract

The heat exchanger comprises a first group of ducts (2) for passage of a fluid. The ducts of the first group (2) open on two opposite faces of the exchanger which also comprises at least one other group of ducts (3) opening on at least one other face. The other group of ducts is equipped with means (4) of electric heating. Application in particular to heating and maintaining the temperature of corrosive fluids. <IMAGE>

Description

The invention relates to a heat exchanger intended in particular for corrosive fluids.

Corrosive fluids are used in the chemical, food and surface treatment industries which must be kept at relatively constant temperature. If these corrosive fluids are the site of endothermic or exothermic reactions, it is necessary to warm or cool them to keep them at working temperature.

To this end, the corrosive fluids are passed through heat exchangers most often outside a treatment tank or a reactor, the forced circulation of the fluids being effected by means of pipes connected to a circulation pump. These exchangers are generally made of metal using boilermaking techniques and are particularly sensitive to corrosion at the place of the welds or the work hardened parts subjected to stresses. In addition, these exchangers occupy an important place so that it is difficult to exceed with this known technique a heat exchange power greater than 1 MW per m³ of exchanger volume.

An object of the invention is to remedy the drawbacks of the aforementioned state of the art by creating an exchanger, in particular for corrosive fluids, with an improved heat exchange power of the order of 3 to 5 MW per m³ d exchanger, operating in a preferred operating temperature range between - 100 ° C and + 400 ° C, which allows a significant saving of space and a reduction of the occupied volume by more than half compared to the state of the art . As an indication, the volume of the exchangers according to the invention is, for common applications, between 10 and 500 dm³.

The exchanger according to the invention comprises a first group of channels for the passage of the fluid to be heated or cooled, said channels opening onto two opposite faces of the exchanger and is characterized in that it also comprises at least one other group of channels leading to at least one other face of the exchanger, said other group of channels being provided with electric heating means.

Hitherto, exchangers have been used for materials which are good conductors of heat, insensitive to corrosion and non-porous in contact with the fluids for which they are intended. These materials are, for example, copper alloys for liquids mainly comprising water, ferrous alloys for hydrocarbons, non-porous graphite or graphite impregnated with materials such as polytetrafluoroethylene or a phenolic resin, or carbon pure for strong acids. These materials, which have good thermal conductivity, also generally have good electrical conductivity, so that a person skilled in the art avoids furnishing these materials with electric heating means, taking into account the risks of short circuit and electrocution, particularly after a long service life in corrosive environments.

Advantageously, the exchanger is arranged so that the first group of channels is substantially orthogonal to the other group of channels.

This arrangement thus makes it possible to adjust the spacings of the groups of channels, so as to avoid the stresses due to thermal expansions and to prevent leaks from a first group of channels to another group of channels, as well as to release a third axis of work substantially orthogonal both to the axes of circulation of the corrosive fluid and to the axes of implantation of the electrical heating means, this third axis of work being able to be used to implant a third group of channels serving for the passage of a cooling fluid or preheating, in particular in the case of exothermic or endothermic reactions, respectively.
Preferably, the exchanger according to the invention comprises at least one distribution box or at least one cover with a seal.

The distribution boxes thus make it possible to pass the corrosive fluid in several successive passes in the exchanger so as to optimize the temperature rise profile and to increase the heat exchange efficiency. The covers isolate from sealingly the ends of the heating means projecting from the group of channels which they line.

Preferably, the exchanger according to the invention comprises at least one temperature sensor installed in a hole arranged in and in a heat exchange situation with the exchanger.

The temperature sensor thus transmits the temperature of the block which, in steady state, is substantially equal to the working temperature of the corrosive fluid, and also allows temperature regulation and protection of the entire exchanger against excessive temperatures. , incompatible with the mechanical resistance required during operation of the exchanger.

Advantageously, an exchanger according to the invention comprises at least one group of channels comprising plies of corrosion-resistant tubes and overmolded in a material which is a good conductor of heat.

This arrangement makes it possible, in the case of materials for tube plies resistant to corrosion that are too expensive, to overmold a set of plies in a material which is good conductor of heat at a lower price, which reduces the cost of the exchanger without affect the efficiency of heat transfer.

Advantageously, a group of channels is produced by drilling holes of predetermined diameters and spacings, passing through at least one block of non-porous material resistant to corrosion.

The realization of the hole by machining thus makes it possible to have a good precision, superior to the exchangers produced by boilermaking techniques, and to respect the dimensions and performance characteristics provided for the exchanger.

Also preferably, the exchanger according to the invention comprises at least one electrical heating resistor.

The electric heating resistor quickly reaches a high temperature as a function of the intensity of the current flowing through it, which makes it possible to follow exactly the temperature profile to be respected by means of the current passing through the resistor.

The exchanger according to the invention advantageously comprises inside the channels a heat conducting powder between two closed ends of the channels.

The heat-conducting powder thus constitutes an effective means of thermal transfer between the channels of the exchanger in which the electrical heating means are housed and the rest of the exchanger, while absorbing thermal expansions without creating excessive internal stresses. .

Preferably, said powder immobilizes in place in a hole an electric heating wire so as to produce an electric heating resistance.

This arrangement thus makes it possible to produce a one-piece exchanger with integrated resistors having better heat transfer than the shielded resistors of known types inserted in the channels and in contact with the powder, preferably electrically insulating.

• la figure 1 représente une vue schématique en perspective éclatée d'un échangeur selon l'invention,
• les figures 2A, 2B,2C représentent des vues latérales de trois faces d'un bloc d'un échangeur selon les directions A,B,C de la figure 1.
• les figures 3, 4, 5 représentent des vues de dessus et en coupe d'un échangeur suivant l'invention,
• la figure 6 représente une vue schématique en perspective d'un autre mode de réalisation d'un échangeur selon l'invention,
• la figure 7 représente une vue en coupe schématique d'une résistance électrique de chauffage garnissant un canal d'un échangeur suivant l'invention.

The invention will now be described with reference to a particular embodiment taken by way of nonlimiting example with reference to the appended drawings in which:

• FIG. 1 represents a schematic exploded perspective view of an exchanger according to the invention,
• FIGS. 2A, 2B, 2C represent side views of three faces of a block of an exchanger in the directions A, B, C of FIG. 1.
• FIGS. 3, 4, 5 represent views from above and in section of an exchanger according to the invention,
• FIG. 6 represents a schematic perspective view of another embodiment of an exchanger according to the invention,
• Figure 7 shows a schematic sectional view of an electrical heating resistor lining a channel of an exchanger according to the invention.

With reference to FIG. 1, an exchanger according to the invention capable of operating up to a working pressure of 10 bar absolute, comprises a central block of compact material which is good conductor of heat and resistant to corrosion, such as graphite impregnated with polytetrafluoroethylene.

Block 1 is produced in the form of a straight prism comprising substantially planar faces. In the embodiment of FIG. 1, the block 1 is substantially cubic and comprises a first group 2 of channels substantially parallel to the direction A, intended for the passage of a corrosive fluid to be maintained at temperature. A first group of channels 2 is produced in the form of substantially cylindrical holes passing through the block and opening out on the two opposite faces of the cube orthogonal to direction A. Block 1 comprises another group of channels 3 substantially parallel to direction B and opening onto at least one face of the cube orthogonal to direction B, furnished with electric heating means 4 capable of heating the block 1 and the fluid flowing in the channels 2.

The block 1 also includes a group of channels 5 substantially parallel to the direction C and opening out on at least one face of the cube orthogonal to the direction C, allowing the passage of a cooling or temperature-maintaining fluid, corrosive or not.

The electric heating means 4 can be electric heating resistors, for example shielded heating elements whose external sheath is electrically insulated from the live conductors. The shape of the cross sections of the substantially cylindrical channels 2, 3, 5 is adapted according to the desired heat exchange surface. This generally circular section can also be oval, polygonal or star-shaped, preferably with rounded edges.

In the case where the exchanger is connected to corrosive fluid circulation pipes, it includes distribution boxes 6, 7 as well as seals 8 so as to insulate in a sealed manner with respect to the outside and between they circulate corrosive fluid and coolant. These joints can be cut into sheets from a material such as polytetrafluoroethylene or made from polytetrafluoroethylene foam or compressed expanded graphite. The electric heating means are also protected by a cover 9 isolating the energized ends of the heating means 4 electric. The cover 9 is a box in the shape of a parallelepiped open on one side, similar to that of the distribution boxes 6, 7.

The distribution boxes or covers are protected against corrosion by a suitable coating, for example polytetrafluoroethylene (called tefloning).

The channels 2 which are substantially parallel to each other are not intersecting either with the channels 3 or with the channels 5 which are also not intersecting with each other.

The sealing of the fluids with one another and with the electric heating means is therefore achieved in the absence of porosities of the material capable of causing the different channels 2, 3, 5 to communicate.

To manufacture cube 1, a block of non-porous material resistant to corrosion and good conductor of heat is preferably machined by drilling parallel holes therein of predetermined diameters and spacings opening onto pairs of opposite faces of cube 1. The parallel holes 2 of the first group of channels are located in parallel planes substantially orthogonal to the drilling plane of another group of channels 3, also substantially orthogonal to the drilling plane of the group of channels 5.

With reference to FIGS. 2A, 2B, 2C, three faces of the cube 1 are shown: a face 10 for the entry of the corrosive fluid, a face 11 for installing the electrical heating means and a face 12 for the outlet of the coolant. as well as seals for the aforementioned fluids. Identical references to those in FIG. 1 designate identical elements in FIGS. 2A, 2B, 2C. It will be noted that in FIG. 2B, the channels 3 are not provided with electric heating means 4. Each channel being produced so as to have no intersection with any other channel, the heat transfer is necessarily carried out by conduction at through the block 1. The efficiency of the heat transfer therefore depends on the quality of the thermal contact at the wall of the channels and on the thermal conductivity of the material chosen to make the block 1. In particular, as the electrical resistors 4 heat the block 1 when they are in service, which block in turn transmits this heat to the fluids which pass through, the quality of the thermal contact between the electrical resistances 4 and the inner wall of the channels 3 is essential for the proper functioning of the exchanger. However, too tight an adjustment would lead to excessive stresses resulting from thermal expansions: a large number of thermal cycles would then lead to the formation of cracks in the block or to the deformation, jamming, or even seizure, of the electrical resistances 4 in the channels 3.

In the case of an exchanger communicating with a fluid pipe, the distribution boxes 6, 7 are assembled to the cube 1 with interposition of the seals 8 by known means such as tie rods, studs, threaded rods and associated nuts, or by collage.

Advantageously, one or more temperature sensors are placed in the channels 3 or in a specially arranged hole in the cube 1 and in a heat exchange situation with the material of the cube 1. This heat exchange is obtained by means of an adhesive or a conductive grease or the interposition of a silicone-based material charged with a metal such as copper, aluminum, silver or the like. In view of the fact that all the heat exchanges, either by heating or by cooling, are carried out via block 1, it can be seen that block 1 constitutes a thermal mass which tends to maintain the temperature of the fluids circulating therein at a temperature very close to that of the block. This temperature maintenance limits the volume of corrosive fluid within the exchanger and thus the total amount of corrosive fluid to be used. The temperature sensors or measurement probes in a heat exchange situation with the cube 1 are generally celled with control devices of known types for controlling the supply of electrical energy to heat the corrosive fluid and the cooling fluid or preheating.

Referring to Figures 3, 4, 5, a heat exchanger according to the invention similar to that shown in Figure 1 comprises a cube 21 produced in the form of elementary parallelepipeds 21 a , 21 b , 21 c , 21 d pierced with corresponding channels with corrosive fluid and cooling fluid, then assembled together in a leaktight manner by means of threaded rods 22 or other means such as gluing, consecutive bolting or chassis clamping. The desired seal is obtained by gluing, by interposing a paste or by interposing seals, not shown, similar to seal 8 in FIG. 1.

This arrangement in elementary blocks allows a modular construction and a production of the elementary blocks in series leading to a reduction in costs by economy of scale.

It can also be seen in FIG. 5 that the electrical protective cover 23 allows connection of the electric heating elements 4, for example a series connection as shown in FIG. 5. Likewise, the distribution boxes 24 a , 24 b 24 c and 25 a , 25 b can be arranged so as to circulate one of the fluids in several passes, for example 4 passes in the case of FIG. 5, so as to increase the heat transfer between the block and said fluid (we designates by "pass" in the terms of the trade a round trip of a fluid in an exchanger). This arrangement makes it possible to easily insulate the assembly by a heat-insulating coating 26 (limited by dashed lines) from which protrudes the cover 23 of electrical protection, which is located at a distance from the heating part of the heating elements 4 substantially equal to the thickness. insulation 26.

In the cover 23 remote from the block 1, which is at a high temperature, the connections of the electrical resistors 4 are thus arranged so as not to be damaged by the high temperature of the block 1. A temperature sensor 27, for measuring the temperature substantially in the central position of the block 21 and housed in an axis hole substantially parallel to the electric heating elements 4, also opens out inside the electric protective cover 23, the connections of the sensor 27 not being shown.

With reference to FIG. 6, the groups of channels, instead of being made by drilling in a block of material, consist of sheets of corrosion-resistant tubes 32, 33, 35, located in substantially orthogonal planes l 'to each other, so that these tubes are non-intersecting and regularly spaced between them. Once all the plies of tubes have been mounted at predetermined distances, a good material is used. heat conductor 30 such as cement, resin or aluminum alloy melt-cast to coat the three groups of channels. The molding thus produced of the above-mentioned assembly constitutes a block with pipes or passages reserved. Preferably, this overmolding is carried out under vacuum or under a pressure such as to avoid the appearance of porosities or casting defects detrimental to good heat transmission.

As a variant, it is also possible to use the layers of heating resistors 33 to melt a mass of powder of material which is a good conductor of heat and thus coat the three groups of channels mentioned above.

Usually, resistive, shielded, insulated, substantially cylindrical elements with a metallic exterior surface are used to fill the holes 3 in FIG. 1. However, in the case of certain fragile materials used to make a block 1, the metal cylinder exterior of the resistor can create expansions, excessive stresses that can damage the block, causing cracks or shrinkage. It is possible, in particular in this case, to mount the insulated electrical resistors with cylindrical outer surface by means of a powder compacted inside the hole 3 around the heating element 4. This powder is preferably conductive of heat and comprises for example silica, magnesia, or a set of ceramic beads interposed in the powder. To avoid leakage or loss of powder, the ends of the hole are preferably sealed with a resin.

With reference to FIG. 7, it is also possible to directly produce an insulated electrical resistance inside the holes or channels 3 provided for this purpose in the block 1. For this purpose, a hole 3 arranged in the block 1 is furnished with wires. electric heaters 40 extended by current supply ends 41. The electric heating wire 40 is immobilized in place in the powder 42, preferably electrically insulating.

This powder is compacted by mechanical means or agglomerated by vibration inside the channel 3. The ends 43 and 44 of the channel 3 are generally closed by a resin, for example a so-called "high temperature" resin based on silicone, or by another heat-resistant sealing means. This arrangement thus makes it possible not to create thermal stresses on the block because none of the components is rigid and cannot exert significant stresses when expanding.

One can also advantageously use a set of ceramic beads or pierced rings, threaded on the electric heating wire 40 so as to reinforce the electrical insulation of the wire 40 relative to the hole 3, in particular in the case where the material of the block 1 has a notable electrical conductivity.

It is possible to use as sealing device for the ends of the channel 3, shutters 43, 44, made of polytetrafluoroethylene or other natural or synthetic material which does not conduct electricity.

Of course, the invention is not limited to the embodiment described above, and many variations can be made without departing from the scope of the invention.

Thus, the shape of the block can be arbitrary, insofar as it is capable of producing a two-dimensional or three-dimensional network of distinct channels, distributed in several groups of parallel and non-intersecting channels. A rectangular parallelepiped shape is preferably used in the case of parts machined along three orthogonal axes. The number of channels can be arbitrary, as well as the number of successive passes which one or the other fluid traverses in the exchanger. Insofar as almost the entire volume of the central block is used, the channels may or may not unclog on two opposite faces of the exchanger. Finally, the number of temperature sensors, their arrangement as well as that of their electrical connections can be arbitrary and take place as desired inside or outside the electrical connection box of the heating means.

Claims (10)

Heat exchanger, in particular for corrosive fluids, comprising a first group of channels for the passage of said fluid, said channels opening on two opposite faces of the exchanger, characterized in that it also comprises at least one other group of channels (3) opening onto at least one other face (11) of the exchanger, said other group of channels being provided with means (4) of electric heating. Exchanger according to claim 1, characterized in that the first group of channels (2) is substantially orthogonal to the other group of channels (3). Exchanger according to one of claims 1 or 2, characterized in that it comprises at least one distribution box (6,7) or at least one cover (9) with at least one seal (8). Exchanger according to one of claims 1 to 3, characterized in that it comprises at least one temperature sensor (27), installed in a hole provided in and in a heat exchange situation with the exchanger. Exchanger according to one of claims 1 to 4, characterized in that at least one group of channels comprises sheets of tubes (32,33) resistant to corrosion, overmolded in a material (30) good conductor of heat . Exchanger according to one of claims 1 to 5, characterized in that at least one group (2,3,5) of channels is produced by drilling holes of predetermined diameters and spacings, passing through at least one block ( 1.21) of non-porous corrosion-resistant material. Exchanger according to any one of the preceding claims, characterized in that it comprises at least one electrical heating resistor (4). Exchanger according to any one of the preceding claims, characterized in that it comprises inside the channels a powder (42) which conducts heat. Exchanger according to Claim 8, characterized in that the said powder (42) is electrically insulating and preferably comprises silica or magnesia. Exchanger according to claim 9, characterized in that said powder (42) immobilizes in place in a hole (3) an electric heating wire (40) by electrically insulating it from the wall of the hole (3).

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