EP0131502B1 - Heat exchanger moulded from refractory material - Google Patents

Description

The invention relates to heat exchangers molded from refractory material.

There are many areas of industry in which heat exchangers capable of working, at low or high temperatures, with corrosive and / or abrasive fluids are needed, it being understood that in general, the term "low temperature" »A temperature lower than approximately 700 ° C whereas by“ high temperature ”one targets those which go from 700 ° C to approximately 1400 ° C.

• - centrales thermiques au charbon ou au fuel lourd (réchauffeurs d'air travaillant sur des fumées riches en S0₂ et en cendres abrasives);
• - réchauffeurs d'air sur chaudières à soufre;
• - foyers d'incinération produisant des fumées riches en CI, HCI, S0₂, S0₄H₂ et N0₃H;
• - fours de grillage de minerai produisant des fumées riches en Cl, S0₂ et oxydes métalliques;
• - fours de verrerie produisant des fumées agressives;
• - fours de métallurgie (fours poussant, fours Pitts) produisant des fumées riches en oxyde de fer;
• - fours de briquetteries et de cimenteries produisant des fumées riches en cendres abrasives;
• - condenseurs de vapeurs agressives sur réacteurs de synthèse.

Non-limiting examples of such fields are as follows:

• - coal or heavy fuel oil power plants (air heaters working on fumes rich in S0 ₂ and abrasive ash);
• - air heaters on sulfur boilers;
• - burning incinerators producing smoke rich in CI, HCI, S0 ₂ , S0 ₄ H ₂ and N0 ₃ H;
• - ore roasting ovens producing fumes rich in Cl, S0 ₂ and metal oxides;
• - glass furnaces producing aggressive smoke;
• - metallurgical furnaces (push furnaces, Pitts furnaces) producing fumes rich in iron oxide;
• - briquetting and cement kilns producing smoke rich in abrasive ash;
• - aggressive vapor condensers on synthesis reactors.

Ceramic heat exchangers are already known which are suitable for use at high temperatures.

GB-A-766 668 describes the manufacture of relatively small size monolithic heat exchangers with straight rectilinear channels, by pressing and sintering alumina powder at high temperature. Several exchangers thus obtained can be assembled to form a larger exchanger, which will obviously be non-monolithic. The manufacturing process described uses solid inserts (rods) which can be removed for the formation of the exchanger channels.

US-A-4156625 describes a heat recuperator in refractory material consisting of a monolithic central block comprising rectilinear crossed channels, flanked by two end blocks in which are formed cavities ensuring the connection of some of the channels of the central block between them. The different constituent parts of the exchanger can be produced by casting in a mold. Some of the channels of the central block have a flared shape to facilitate the release of the central block.

The heat exchangers of refractory ceramic material of the prior art are exchangers whose efficiency from the point of view of heat exchanges is only average due to the cross configuration of the channels.

In addition, these exchangers are either small in size, or require the assembly of several elements together, which poses problems of tightness at the joints and, therefore, of reliability of the exchanger.

It would therefore be desirable to be able to have monolithic heat exchangers made of refractory ceramic material having improved heat exchange properties and which can be produced in large sizes.

The invention aims to provide monolithic heat exchangers whose channels are parallel to each other over a major part of their length and which can be produced in large sizes.

More particularly, the invention relates to a heat exchanger essentially consisting of a monolithic body, molded from refractory material based on at least one metal oxide, this body comprising a plurality of first tubular continuous channels for a first fluid to be heated and a plurality of second continuous tubular channels for a second fluid to be cooled, these channels being in mutual heat exchange relationship; the first channels each having a first end intended to be connected to an inlet of said first fluid and a second end intended to be connected to an outlet of said first fluid, the second channels each having a first end intended to be connected to an inlet of said second fluid and a second end intended to be connected to an outlet of said second fluid, characterized in that:

the first and second channels extend parallel to each other over a major part of their length, the channels of at least one of the two groups are bent, and the body is a body, made of a refractory material making taken at room temperature and exhibiting a shrinkage of less than 0.5%.

The invention lends itself particularly well to the manufacture of large exchangers whose body has a mass greater than 500 kg.

Any refractory composition having low shrinkage (less than 0.5%) and good flowability and giving, after setting or ceramization, a refractory material having good resistance to resistance properties, can be used for molding the exchanger. abrasion and chemical agents as well as low permeability, that is to say less than 5 nanoperms.

• (i) 55-99% de particules d'un matériau réfractaire fondu et coulé contenant une phase vitreuse, ce matériaux étant constitué principalement par les oxydes de zircone-silice, de zircone-silice-alumine ou de zircone-silice-alumine-oxyde de chrome, ces particules ayant la distribution granulométrique suivante: 15-45% de grains d'une grosseur de 2 à 5 mm, 20-40% de grainette d'une grosseur de 0,5 à 2 mm, 15-30% de farine d'une grosseur de 40 micromètres à 0,5 mm et 0-40% de fines d'une grosseur inférieure à 40 micromètres;
• (ii) 1 - 5% d'un ciment hydraulique; et
• (iii) 1-15% d'une charge constituée de particules d'une grosseur de 0,01 à 5 micromètres, sensiblement sphériques, d'un oxyde de métal, la surface spécifique de ces particules étant supérieure à 5 m²/g; la proportion de chacun des constituants (i), (ii), et (iii) étant donnée par rapport au total des ingrédients (i), (ii) et (iii).

Among these refractory compositions and according to a preferred embodiment, the refractory material has the following composition, in% by weight:

• (i) 55-99% of particles of a molten and cast refractory material containing a vitreous phase, this material consisting mainly of the oxides of zirconia-silica, zirconia-silica-alumina or zirconia-silica-alumina-oxide of chromium, these particles having the following particle size distribution: 15-45% of grains with a thickness of 2 to 5 mm, 20-40% of grainette with a thickness of 0.5 to 2 mm, 15-30% of flour of a thickness of 40 micrometers to 0.5 mm and 0-40% fines less than 40 microns in size;
• (ii) 1 - 5% of a hydraulic cement; and
• (iii) 1-15% of a charge consisting of particles of a thickness of 0.01 to 5 micrometers, substantially spherical, of a metal oxide, the specific surface of these particles being greater than 5 m ² / g ; the proportion of each of the constituents (i), (ii), and (iii) being given relative to the total of the ingredients (i), (ii) and (iii).

The above-mentioned refractory material is described in detail in European patent 0021 936 of the applicant. Preferably, component (ii) is a superaluminous cement and component (iii) consists of vitreous silica.

This refractory material has the distinction of having a very low shrinkage (less than 0.1%) when taken. This property makes it possible to obtain complex structures with high geometric precision and to introduce into the mass networks of hollow channels of organic material without the appearance of cracks between these networks which would put the fluid channels to be heated into communication with the channels of fluid to cool.

This refractory material has a low permeability to gases and liquids even under pressure, which is less than 1 nanoperm and in general of the order of 0.3 nanoperm.

The preferred refractory material used to manufacture the heat exchangers of the invention is implemented like a concrete by mixing it intimately before use with an amount of water between 3 and 25%, preferably between 4 and 10% by weight, and with 0.01 to 1% of a surface-active dispersing agent relative to the total weight of the ingredients (i) to (iii).

According to a preferred embodiment, the channels of the first network and those of the second network open onto different faces of the body of the exchanger.

According to another particular embodiment, the refractory material further comprises reinforcing fibers, preferably made of short stainless steel. As an indication, it is possible to incorporate 0.5 to 3% by weight of such fibers in the refractory composition, preferably approximately 1.5% by weight. These fibers strengthen the mechanical properties of the body and improve the resistance to thermal variations of the refractory material.

• a) disposition dans un coffrage ou moule ayant la forme désirée pour le corps de l'échangeur, d'une pluralité d'inserts positionnés et maintenus aux endroits correspondant aux emplacements désirés des canaux du corps;
• b) coulée dans le coffrage ou moule de la matière réfractaire additionnée d'eau de mise en oeuvre avec application de moyens de compactage de la composition coulée;
• c) séchage du corps moulé, puis étuvage de ce corps à une température suffisante pour provoquer l'élimination des inserts, caractérisé en ce que lesdits inserts sont constitués de tubes et/ou profilés creux en matière plastique rigide et en ce qu'on fait passer à travers lesdits tubes et/ou profilés creux un gaz à une température suffisamment élevée pour provoquer l'élimination desdits tubes et/ou profilés en matière plastique noyés au sein du corps séché;
• d) éventuellement céramisation du corps par chauffage à une température élevée appropriée.

The invention also relates to a method of manufacturing an exchanger according to the invention, which comprises the following steps:

• a) arrangement in a formwork or mold having the desired shape for the body of the exchanger, of a plurality of inserts positioned and maintained at the locations corresponding to the desired locations of the channels of the body;
• b) pouring into the formwork or mold the refractory material added with processing water with the application of means for compacting the poured composition;
• c) drying of the molded body, then steaming of this body at a temperature sufficient to cause the elimination of the inserts, characterized in that said inserts consist of tubes and / or hollow profiles made of rigid plastic and in that passing through said hollow tubes and / or profiles a gas at a sufficiently high temperature to cause the elimination of said plastic tubes and / or profiles embedded in the dried body;
• d) optionally ceramization of the body by heating to an appropriate high temperature.

To keep the inserts in place, the ends of these protruding inserts of the formwork or mold can be fixed through openings of corresponding shape provided in the walls of said formwork or mold, and / or hold them in place by a set of sieves, in particular by stainless steel wires connected to the formwork and having a mesh corresponding to the diameter of the tube. In the latter case, the various steel wire screens used remain in the mass of the refractory.

Preferably, polyvinyl chloride tubes or profiles are used (abbreviated as P.V.C.). Such tubes or profiles, as well as sleeves and elbows making it possible to produce any desired bends, are readily available commercially. After baking, such tubes or profiles leave a perfectly smooth imprint.

As means for compacting the cast composition, vibrations can be used. This can be obtained, for example, by sending compressed air at low frequency into a few suitably chosen tubes or profiles or by using a vibrating table or suitable vibrators of the pneumatic or electric vibrator or vibrating needle type.

Once the ceramization has been carried out and the body has cooled, the latter can be insulated and possibly protected by an envelope.

The exchangers of the invention have numerous advantages over conventional devices, such as great resistance to aggressive chemical agents, such as chlorine, sulfuric anhydride, strong acids, strong bases, silicates and oxides of metals, etc. Their high hardness also gives them excellent resistance to erosion by gases circulating at high speed and loaded with abrasive ash. This high hardness makes it possible to circulate the fluids at high speeds, at least twice higher than those admissible in conventional steel tube exchangers, which ensures a good coefficient of heat exchange between the fluids and the walls of the body. and advantageously compensates for the lower thermal conductivity of the ceramic material relative to the metal, so that the exchange surfaces to be provided for the same heat exchange power are equal or less.

It should also be noted that the possibility of operating with fluids circulating at great speed promotes self-cleaning of the channels, which avoids the use of an expensive sweeping installation.

The high refractoriness of the refractory material and the large thermal inertia of the body allow the use of the exchangers of the invention at gas temperatures which can reach 1500 ° C. under variable conditions without risk of cracking under the action of thermomechanical stresses. ques.

Finally, the cost of manufacturing an exchanger according to the invention is much lower (up to 4 times) than that of a conventional exchanger, mainly because of its simplicity of manufacture which requires a reduced number of hours of workforce.

If desired, the exchanger can be manufactured on the site of use. Also, it is possible to vary the composition of the refractory material during the casting operation so that the body has zones of different compositions best suited to the working conditions to which they will be exposed in service.

The description which follows, made with reference to the appended drawings, will make the invention better understood.

Figure 1 is a schematic perspective view illustrating the manufacture of a heat exchanger body according to the invention. FIG. 2 is a view in axial longitudinal section of a heat exchanger according to the invention intended to be used with an industrial waste incinerator.

Example 1

This example illustrates the production of a monolithic exchanger body with separate fluids according to the invention of dimensions 1 m × 1 m × 1 m.

In a removable wooden mold 1 with internal dimensions L = 1 meter, 1 = 1 meter and H = 1.20 m (Figure 1), there is, on the one hand, a network of 36 rectilinear tubes 2 in PVC of 6 cm in diameter intended to be traversed, for example, by hot smoke. These tubes are held in place by the perforated plate 3 located above the mold and the perforated plate 4 forming the bottom of the mold. On the other hand, there is a network of 49 tubes 5 bent at 90 ° PVC 2.5 cm in diameter intended, for example, to be traversed by air to be heated. The tubes 5 are held in place by the perforated plate 3 and by the perforated side plate 6. For reasons of simplicity of representation, only 8 tubes 2 and 4 tubes 5 have been shown in FIG. 1.

The upper part of the mold is flared and two passages 7 have been made there through which the refractory material will be poured into the mold.

The whole mold-networks of PVC tubes is placed on a vibrating table (not shown) and the refractory composition of the type described in European patent 0 021 is poured into the mold through the passages 7. 936 and sold commercially under the trademark ERSOL ^O by the Applicant. This refractory material comprises by weight, 91 parts of grains melted and poured from a refractory material composed of 50.6% of Al ₂ O _3, 32.5% of ZrO _z , 15.7% of Si0 ₂ , 1, 1% Na ₂ 0, 0.1% Fe ₂ 0 ₃ , and 0.1% Ti0 ₂ (product n ° 1 of table 1 of the European patent 0 021 936 mentioned above).

The pouring is stopped when the material level reaches a few centimeters above the desired level (1 meter in the example) and continues to vibrate until densification of the product. We unmold after setting. The body is then subjected to a heat treatment comprising a drying step at a temperature in the range of 100-150 ° C, a steaming step used to remove the PVC tubes (generally by progressive heating up to 400 ° C approximately) and finally a high temperature ceramization step (generally in the range of 800-1200 ° C approximately). Finally, it is allowed to cool to room temperature.

The same molding operation is repeated with a similar refractory material except that 1.5 parts by weight of stainless steel fibers of the registered brand DRAMIXZP, quality 30/40 sold by the Belgian company BEKAERT, are incorporated therein. These fibers are in the form of U-shaped staples with a diameter of 0.3 mm and a length of 40 mm. They exist in AISI 302 steel for applications at temperatures not exceeding 1000 ° C or in AISI 314 steel for applications at temperatures above 1000 ° C. Also, 4.7 parts of water are used instead of 4.5 parts.

After cooking at approximately 1000 ° C, the bodies obtained, with or without the presence of steel fibers, are compact.

Example 2

This example describes the production on the site of use of a heat exchanger, according to the invention, for an industrial waste incinerator, in which it is a question of recovering approximately 1,000,000 Kcal / hour by heating the incoming air to 28 ° C approximately to approximately 650 ° C by means of hot fumes arriving at approximately 950 ° C and leaving at approximately 250 ° C.

As shown in FIG. 2, the body 21 of the exchanger comprises 360 channels 22 intended to be traversed by the flue gases and 360 channels 23 intended to be traversed by the air, all with a diameter of 2.5 cm. The channels 22 are rectilinear and extend from the base to the top of the body, while the channels 23 are bent at 90 °, in opposite directions, at each of their ends so as to extend parallel to the channels 22 on the most of their length but leading to the perimeter of the body, at 24 and 25 as illustrated in Figure 2. The exchange surface is around 198 _M ^2.

The body, which has a diameter of 1.1 m and a height of 7 meters, is molded in the space of a few hours on site by casting about 15 tonnes of the material described in Example 1 (with fibers) in formwork of appropriate shape. After formwork removal, apply to the body a layer 26 of insulating cellular concrete with a thickness of approximately 100 mm, a metal casing 27 made of sheet steel 10 mm thick, and finally a mattress 28 of rock wool with a thickness of 20 mm. Metal flanges, such as 29, are provided around the areas where the channels open in order to facilitate the connection of the fluid inlets and outlets. Obviously, only one layer of insulation can be used, either in the form of concrete or in the form of fibers.

To make this apparatus, we used the solution which consists in positioning the networks of tubes 22 and 23 in the meshes of a set of stainless steel sieves of approximate 25 mm opening (mesh sieve "1 inch") fixed to a frame.

The refractory mixture is poured in sections of 850 mm in height using removable chutes which facilitate the operation. The formwork made up of two semi-cylindrical shells is placed section after section by sliding it inside the support frame.

Due to the size of the part, the effect of vibrators external to the formwork is combined with the effect of vibrators acting in the mass of the refractory.

The heat treatment for removing PVC tubes and ceramization is carried out using hot fumes available on site or burners.

As an indication, the labor required to set up the formwork and the positioning of the tubes on the site is around 60 hours.

For gas speeds of 15 Nm / second, the heat exchange coefficient is 45 Kcal / h · m ^{2 ·} ° C.

By way of comparison, the equivalent solution of steel tubes weighs 20 tonnes and consists of an exchanger comprising 121 tubes with a diameter of 8 cm and has an exchange surface of 214 m ² . Its exchange coefficient is 20 Kcal / h · m ^2. ° C for gas speeds of 2 Nm / s. In addition, the pressure drops of fluid to be heated are twice as great. Such an exchanger requires approximately 400 hours of welding and assembly.

The invention therefore applies universally to all types of low and high temperature exchangers and makes it possible to solve here times the problems of tightness between the channels, refractoriness, good heat exchange, resistance to erosion and corrosion by various aggressive fluids or loaded with aggressive agents.

Example 3

This example describes the production on the site of use of a heat exchanger operating at high temperature for a pushing furnace from the steel industry, in which it is a question of reheating the incoming air at about 27 ° C to 670 °. C approximately by means of hot fumes arriving at approximately 800 ° C and leaving at approximately 400 ° C.

A refractory material such as that of Example 1 (with steel fibers) is poured onto the site into a formwork of 1.3 x 1.3 x 10 m furnished with a network of 625 tubes (25 x 25) with an outside diameter of 6cm in order to obtain an exchange surface of the order of 1000 m ² . 313 of these tubes are straight and are intended to form the smoke channels, while the other 312 tubes, intended to form the air channels, are bent at 90 °, in opposite directions, at each of their ends so as to extend parallel to the first 313 tubes over most of their length, but lead to the periphery of the body, in a similar manner to what was described in example 3 with reference to FIG. 2. During casting, we vibrate either by injecting compressed air into the tubes, or by using vibrators as is commonly practiced on concrete sites. It is removed from the mold after 24 hours and the molded body is left to age for 8 days. Then, the heat exchanger body is thermally insulated using a layer of insulating concrete or a mattress of insulating fibers and a metal envelope is put in place keeping everything. The isolated body is then subjected to a heat treatment similar to that described in Example 1, using the hot fumes available in the factory which are passed through part or all of the channels of the body, as required. .

Claims (12)

1. Heat exchanger consisting essentially of an as-molded monolithic body made of a refractory material based on at least one metal oxide, said body comprising a plurality of first tubular continuous channels for a first fluid to be heated and a plurality of second tubular continuous channels for a second fluid to be cooled, said channels being in a mutual heat-exchange relationship; each of the first channels having a first end for connection to an inlet of said first fluid and a second end for connection to an outlet of said first fluid, each of the second channels having a first end for connection to an inlet of said second fluid and a second end for connection to an outlet of said second fluid, characterized in that: the first and second channels being mutually parallel along a major part of their length, the channels of at least one of said group are bent, and the body is a body made of a refractory material setting at ambient temperature and exhibiting a shrinkage lower than 0.5%.

2. Heat-exchanger as claimed in claim 1, characterized in that refractory material contains grains of fused cast metal oxides of one of the following systems: ZrO_z-SiO_z, Zr0₂-S'0₂-AI₂0₃ and ZrO₂-SiO₂-Al₂O₃-CrO₃.

3. Heat exchanger as claimed in claim 1 or 2, characterized in that the refractory material has the following composition in % by weight:

(i) 55-99% of particles of a fused cast refractory material containing a vitreous phase based on zirconia-silica, zirconia-silica-alumina or zirconia-silica-alumina-chromium oxide, these particles having the following size distribution: 15-45% of grains with a size of 2 to 5 mm, 20-40% of small grains with a size of 0.5 to 2 mm, 15-30% of dust with a size of 40 micrometers to 0.5 mm and 0-40% of fines with a size of less than 40 micrometers;

(ii) 1-5% of a hydraulic cement; and

(iii) 1-15% of a filler consisting of approximately spherical particles of a metal oxide with a size of 0.01 to 5 micrometers, the specific surface area of these particles being greater than 5 m²/g, the proportion of each of the constituents (i), (ii) and (iii) being given relative to their total.

4. Heat exchanger as claimed in claim 3, characterized in that the constituent (ii) is a superalu- minous cement and the constituent (iii) is vitreous silica.

5. Heat exchanger as claimed in any of claims 1 to 4, characterized in that reinforcing fibers are incorporated into the refractory material.

6. Heat exchanger as claimed in claim 5, characterized in that the reinforcing fibers are stainless steel fibers present in a proportion of 0.5 to 3% by weight, relatively to the refractory material.

7. Heat exchanger as claimed in any of claims 1 to 6, characterized in that it has a weight of more than 500 kilograms.

8. Heat exchanger as claimed in any of claims 1 to 7, characterized in that the body is thermally- insulated.

9. Heat exchanger as claimed in any of claims 1 to 8, characterized in that it comprises metal clamp provided around the regions where the channels emerge so as to facilitate the connection of the fluid inlets and outlets.

10. A process for the manufacture of an exchanger as claimed in claim 1, which comprises the following steps: a) arranging in a shuttering or mold having the shape desired for the body of the exchanger, a plurality of inserts positioned and held at the points corresponding to the desired locations of the channels in the body;

b) casting, into the shuttering or mold, the refractory material to which working water has been added, with the application of means for compacting the cast compositions; and

c) drying the molded body, then stoving said body at a sufficiently high temperature to cause the removal of said inserts, characterized in that said inserts are tubes and/or hollow profiles made of rigid plastic and in that the interior of said tubes and/or hollow profiles is contacted with a gas at a temperature high enough for causing the removal of said tubes and/or profiles made of plastic and embedded within the dried body.

11. The process as claimed in claim 10, characterized in that the molded body is, in additon, cer- amized by heating to an appropriate high temperature.

12. The process as claimed in claim 10 or 11, characterized in that the tubes or profiles are made of polyvinyl chloride.

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