EP0074471B1 - Process for the production of heat exchangers from ceramic foils - Google Patents

Abstract

Process and apparatus for manufacturing heat exchangers from ceramic sheets, wherein different flow channels are stamped from or pressed into the sheets, and the formed sheets are joined together with a laminating agent. The stacking of the individual sheets is effected using apparatus in which the sheets are transported to the forming means, applicator means and laminating means by horizontally and vertically displaceable, rotatable and pivotable suction plates. The organic component of the ceramic sheets is expelled from the heat exchanger block obtained in two heating steps with an intermediate forming operation to bring the heat exchanger block to its final dimensions, and the block then fired between 1,200 DEG to 1,700 DEG C. The actual sintering temperature depends on the particular ceramic used, which may comprise Si3N4, SiC, cordierite and/or semiconductive barium titanate compounds.

Description

The invention relates to a method for producing ceramic heat exchangers by punching out and stacking pieces of film made from film strips cast using ceramic slips, the block-shaped heat exchanger thus produced being subjected to a lamination process using a plastic compound or an adhesive, and then the raw body being subjected to a sintering process becomes.

A process for the production of heat exchangers from ceramic foils is already known from DE-A No. 2841571 by stacking foils punched out between two base plates with spacers and additionally milling so-called windows into the top wall. Such a block-shaped heat exchanger is then subjected to a cold or hot lamination process. Although such a process entails higher manufacturing costs than the conventional extrusion of ceramic heat exchangers, very thin walls are obtained. On the other hand, with the extrusion technique, it is not possible to install so-called baffles transverse to the direction of the flow channels. Handling when assembling such heat exchangers from rods and thin-walled foils is also very difficult, and the manufacturing method is also very labor-intensive. Furthermore, it was found during lamination that not all of the foils adhere evenly to one another, and in particular the tools easily smear during the raw machining of the unsintered heat exchanger block due to the organic binder content of the foils. If you remove the entire binder from the ceramic, the body becomes very brittle, so that it is difficult to work with.

Furthermore, from GB-A No. 1418459 a method is known which provides for the production of heat exchangers from foils. The films of approximately 0.15 mm thickness are produced on a combustible carrier material by means of the doctor blade method, it having proven to be particularly disadvantageous that the spacers between the media partition walls are produced using very complex technology which is unsuitable for series production. The heat exchanger is constructed by alternately stacking silicon plastic films and spacers that are applied to cast films. The individual parts of the heat exchanger are joined using pressure and temperature as well as a solvent or adhesive. When burning, the paper must first be removed, then the binder and finally the nitriding process carried out. When the paper is burned, it must also be ensured that the fine silicon structure is not damaged, and the ash formed in the process is removed by ultrasound washing. Furthermore, partial nitridation of the heat exchanger block must be carried out before the paper is burned.

The defects that have occurred in the described method do not allow rational mass production, in particular if undercutting or curved channel guides are necessary. The heat exchangers often have an inhomogeneous structure after their completion. This is particularly evident in the case of heat exchangers made of silicon nitride, since the porous surface of this material means that no smooth flow channels are obtained and the flow behavior is therefore not optimal.

The invention is therefore based on the object of improving a method of the type mentioned at the outset such that engagement and curved duct guides are also possible, and furthermore when the individual foils are joined to form a heat exchanger, the spacers, the end pieces and the milling of the windows are eliminated, in particular, the lamination errors are eliminated and the processing of the laminated raw body is possible with appropriate tools.

According to the invention, this object is achieved by the method steps specified in the main claim. Further features with regard to the invention and the materials used result from the subclaims.

Conventional ceramic slips are used to produce the films. The slip consists of a ceramic powder, organic binders, dispersing or diluting agents and, if necessary, plasticizers and other auxiliaries in the form of oils. The starting point is mainly silicon slip, to which 3 to 10% by weight of cordierite is preferably added. Other ceramic powders consist of cordierite with the composition of 9 to 20 wt.% MgO, 30 to 50 wt.% Al ₂ O ₃ and 41 to 57 wt.% SiO ₂ . Silicon carbide is also suitable, the mixture consisting of 70 to 92% by weight of SiC and 8 to 30% by weight of C. Furthermore, semiconducting barium titanates can be used if the heat exchanger block is to be used as a heating element at the same time, by being supplied with electrical current. The organic binder per se is not subject to any particular restriction provided that a good bond to the ceramic powder is guaranteed and the required toughness and dimensional stability are present in the film, if necessary in combination with the plasticizer. Polyvinyl acetates and polyvinyl butyral have proven particularly successful. Water or organic solvents such as ethanol, toluene and trichlorethylene are used as dispersing and diluting agents. Frame formulations which are particularly suitable according to the invention for the production of the ceramic films are given below, the slip formulations being broken down by ceramic raw materials and binders or solvents:

The viscosity of the slip can be influenced in particular by the solvent content. It has also been found that the use of ultrasound in the treatment of the casting slip is particularly advantageous. This treatment gives a casting slurry with greater homogeneity, better casting properties and a maximum content of solids, which has a particular effect on the bulk density of the film. In this way, films with a higher packing density and improved mechanical properties can be obtained. Furthermore, it makes sense to provide a vibrating device on the casting belt that compresses the casting slurry again or enables a uniform film thickness over the entire range.

According to this process, the ceramic films are brought to their final dimensions after lamination. If thick foils or very high flow channels are required, which exceed the foil thickness of 0.1 to 1.5 mm, the foils are connected to individual cards with a lamination aid in a pre-lamination process. Various flow channels are then punched out of these foils or cards or the foil is subjected to an embossing process. In the latter case, the ceramic foils are exposed in matrices at 20 to 120 ° C and pressures of 5 to 100 bar, which results in comb-like projections.

The punched or embossed cards are then built up by means of a device to form a heat exchanger block, with which the individual layers are laminated together with the aid of a laminating press.

In the lamination process, a press device is used at pressures of 0.1 to 15, preferably 1 bar and time intervals of 1 to 10 s. Normally work is done at room temperature, but temperatures up to 100 ° C can also be used. In individual cases, the pressure used depends on the organic content and the type of lamination aid. For the lamination process, either a paste is used, which preferably contains a ceramic filler, or a purely organic adhesive, which is applied by screen printing, spraying or rolling. The use of lamination aids has several advantages. On the one hand, low pressures are made possible during the lamination process, whereby deformation of the flow channels is avoided. Furthermore, the waviness of the foils is compensated and finally the lamination aid effectively reduces the lamination errors. The organic constituents are then heated from 40 to 60% of the plastic content, which results in an additional raw strength. This also ensures that the heat exchanger block is easy to machine without the tools smearing through the organic components of the ceramic film. The remaining organic components are then heated and the heat exchanger block sintered between 1200 and 1700 ° C. It may still be necessary to rework the inlet and outlet openings of the flow channels in order to maintain a good connection to the various supply and discharge media.

The method according to the invention also enables extensive automation, since in the previous production, due to the individual handling during punching, positioning and laminating, no continuous work process could be carried out. According to the method of the invention, heat exchangers are obtained which are very homogeneous and show very good contact between the individual layers after sintering. The improved method also results in better quality heat exchangers, and / or with little effort, so-called baffles can be installed transversely to the flow device of the channels. The baffles can be selected and are no longer dependent on the manufacturing process. Another possibility provides that curved flow channels can be produced. This means that asymmetrical and cylindrical heat exchangers can also be manufactured. Furthermore, heat exchangers can be obtained which optionally consist of layers of silicon nitride, silicon carbide and cordierite in the form of plates or foils in accordance with DE-A No. 2631092. This use of cordierite, in particular in the case of silicon nitride, gives smooth and therefore low-resistance flow channels.

• Fig. 1 den Verfahrensstammbaum des erfindungsgemässen Verfahrens,
• Fig. 2 eine Draufsicht auf Karten mit und ohne verschiedenen ausgestanzten Strömungskanälen, und
• Fig. 3 die perspektivische Ansicht einer Ausführungsform gemäss dem erfindungsgemässen Verfahren.

Further special features of the method according to the invention for producing heat exchangers from individual foils result from the description part with reference to the drawings. Show it:

• 1 the process family tree of the process according to the invention,
• Fig. 2 is a plan view of cards with and without different punched-out flow channels, and
• Fig. 3 is a perspective view of an embodiment according to the inventive method.

1 shows the manufacturing process of a gas / liquid heat exchanger made of silicon nitride. To produce the ceramic casting slip, 100% by weight of silicon powder with 24% by weight of ethanol, 10% by weight of toluene, 1.5% by weight of Menhaden oil, 8% by weight of polyvinyl butyral and 5% by weight as plasticizer. -% Palatinol and / or Ucon-Oil added. This mixture is milled in a drum mill with Al ₂ O ₃ balls for 20 h and the slip is then evacuated. The bad The warping of the slip for film production takes place on a steel belt. The slip is added via a casting shoe, the film thickness being determined by an adjustable gap height of 0.2 to 1.5 mm on the casting shoe. The film is then pulled off the steel strip and separated. It has proven to be expedient to construct so-called prelaminates from two to three foils. The individual foils are connected to one another by spraying or by applying a laminating aid. In the latter case, a paste is used which consists, for example, of 65% by weight of silicon and / or cordierite or of mixtures thereof. The paste also contains 20 to 40% by weight of unsaturated alcohols and 3 to 10% by weight of binder which contain plasticizers and polyvinyl butyral. In this case, the paste is printed using the screen printing process. At the same time, the unevenness of the surface is compensated for by the solid content of the paste. The paste also dissolves the surface, which later leads to homogeneous connections of the individual layers. When using silicon foils, it makes sense to print the pre-laminates completely, especially if the paste contains a cordierite component which, with the later silicon nitride, leads to the exudation of a glass phase in the sintering process, which causes smooth and dense surfaces of the flow channels. Otherwise only those places are printed which are necessary for the connection of the foils. This ensures that the punched-out parts can be returned and placed in the casting slip.

2 shows the rectangular foils or cards with a thickness of 0.9 mm for the construction of a gas heating heat exchanger. These cards 1 have dimensions of 120 x 400 mm and are provided with an additional edge 2, which is removed during subsequent post-processing. In the punched-out cards with a thickness of 1.8 mm, the flue gas ducts 3 have a width of 50 mm and the walls 4 have a width of 3 to 7 mm. While a width of 100 mm has been chosen for the punched-out water pockets 5 and provided with baffles 6 perpendicular to the direction of flow, the card thickness here is 2.7 mm. The baffles are intended, in particular, to ensure that the temperature distribution is uniform in the flow channels.

The stacked heat exchanger block, after having been removed from the laminating press, is subjected to a temperature treatment at temperatures between 100 and 200 ° C. The organic components, especially the plasticizer and the lamination aid, evaporate. The baking process takes 1 to 2 days, with 40 to 60% of the organic components being expelled from the heat exchanger block. The heat exchanger block can then be machined by milling or sawing so that it obtains its final mass. The remaining content of organic constituents is then baked out at temperatures between 200 and 300 ° C. over a period of about 2 to 3 days. This measure eliminates the usual pre-sintering or pre-nitriding at 1100 to 1300 ° C, in particular in the case of silicon foils. The nitriding is then carried out in a known manner between 1300 and 1400 ° C. As already mentioned, the tightness of the finished silicon heat exchanger can be increased further by expediently 3 to 10% by weight of silicon is replaced by cordierite in the lamination aid. This measure can also take place with the pouring slurry. Then, however, post-sintering at temperatures between 1300 and 1400 ° C. is necessary, in the presence of oxygen, as can be seen from DE-A No. 2544437. The result of this process is a homogeneous one-piece heat exchanger, which is characterized by a uniform strength.

Claims (6)

1. Method of manufacturing ceramic heat exchangers using foil bands moulded from ceramic slurrys in which dimensionally correct pieces of foil are punched out and stacked, whereby the block-shaped heat exchanger thus produced is subjected to a laminating process using a plastics composition or an adhesive and the semi-finished body is subjected to a sintering process, characterised in that differently shaped flow channels including baffle plates and openings for the media connections are directly stamped or impressed into the foils, whereby a predetermined layer thickness is obtained by prelamination of individual foils, the individual foils are individually laminated together with a pressure of 0.1 to 15 bar and at time intervals of 1 to 6 s, whereby each individual layer is provided with a laminating aid, preferably a paste with a high proportion of solid material, to compensate for the roughness of the foils; in that the organic components up to 40 to 60% of the plastics proportion are baked out in order to make the semi-finished body processable, the heat exchanger block is post-treated, and subsequently the remaining organic components are removed at between 200 to 300°C, and in that an optional post-treating of the inlet and outlet openings of the flow channels occurs after the firing at between 1,200 to 1,700° C.

2. Method as claimed in Claim 1, characterised in that the silicon slurry preferably contains 3 to 10% by weight of cordierite.

3. Method as claimed in Claim 1, characterised in that the ceramic proportion in the cordierite slurry comprises 9 to 10% by weight of MgO, 30 to 50% by weight of AI₂0₃, and 41 to 57% by weight of Si02.

4. Method as claimed in Claim 1, characterised in that the ceramic proportion in the silicon carbide slurry comprises 70 to 92% by weight of SiC and 8 to 30% by weight of C.

5. Method as claimed in Claim 1, characterised in that the moulding slurry contains semi-conductive barium titanate compounds.

6. Method as claimed in any of Claims 1 to 5, characterised in that the flow channels are produced by stamping whilst the ceramic foils are subjected in matrices to 20 to 120° C and pressures of 5 to 15 bar.

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