GB1583052A - Ceramic heat exchangers - Google Patents

Description

(54) CERAMIC HEAT EXCHANGERS (71) We, ROSENTHAL TECHNIK AG, a German company, of Postfach 1508, 8672 Selb/Bayern, Germany do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to heat exchangers suitable for use in the fields of, for example, gas turbine construction, process engineering, chemical technology and internal combustion engines, where liquid and/ or gaseous heat exchange media are nowadays needed to work at temperatures of up to 1,400"C.

According to the present invention, a heat exchanger comprises a plurality of ceramic layers of parallel ducts, the layers being stacked with either the same orientation for the ducts in adjacent layers or with mutually perpendicular orientation for the ducts in adjacent layers, and the layers being secured to one another to form a composite body in which each of the layers is gas-tight, with respect to the layer or layers adjacent thereto, said layers being formed of silicon carbide or silicon nitride, which layers are rendered gas-tight by being interposed with layers formed of cordierite or by being impregnated with or covered by cordierite, tungsten. silicon, or a metallic silicide, the arrangement being such that in use heat exchange media flow through the ducts of adjacent layers but cannot intermix.

In one embodiment of the invention, adjacent layers are stacked with mutually perpendicular orientation, and each of the layers is formed as a plate having a plurality of the ducts extending therethrough. The ceramic plates are further preferably formed to be of identical shape regardless of whether they are to receive, in use, the relatively hot or the relatively cold heat exchange medium. An advantage of those heat exchangers is that, due to the crosswise position of the individual identical plates one upon the other, the effect of any weak portions which may exist in the ceramic material or materials is minimised. A significant disadvantage, however, is that their efficiency in terms of heat-transmission is not particularly good.

In a preferred embodiment of the invention, however, adjacent layers are stacked with the same orientation, and alternate layers have their ducts in communication with one another near their ends, which are blocked, whereby a heat exchange medium flowing through said alternate layers, in use, follows either al2-shaped or-shaped path with respect to a heat exchange medium flowing through the remaining layers. Although some, or even all, of the layers can again be constituted by respective plates each having a plurality of the ducts extending therethrough, manufacture and assembly of the stacked layers is substantially simplified by the use of prefabricated bars and sheets. Each of the layers then includes a plurality of bars, with the bars of each said alternate layers being covered by a pair of facing sheets.This preferred manner of construction, in association with the preferred counter-flow mode of operation. has been found to lead to an increase in the efficiency of around seven to eight percent.

Not only are heat exchangers according to the present invention capable of attaining the high working temperatures referred to above, but they may be corrosion-resistant, thermalshock resistant and have high heat stability. as well as being gas-tight at internal pressures of up to 5 atmospheres.

The following materials are used for forming the ceramic layers: silicon nitride - in contrast to most other ceramic materials of high strength and corrosion resistance, its thermal shock resistance is also extraordinarily high; cordierite - in the fired state this material has a relatively low co-efficient of expansion and a high thermal shock resistance; and, silicon carbide - a material of good strength and chemical stability at high temperatures, where there is only slight shrinkage.

Although only cordierite of the above-listed ceramic materials is gas-tight, both silicon nitride and silicon carbide can be rendered gas-tight by covering or impregnating with cordierite, tungsten, silicon or a metallic silicide. This is of importance where all the layers of the stack are formed primarily of silicon carbide or silicon nitride. Where the stack consists of alternate plates formed respectively of silicon nitride and cordierite, or silicon carbide and cordierite, however, there is no need to cover or impregnate the porous plates in the above-described manner, because one can rely upon the gas-tightness of the cordierite plates located alternately therewith.

Preferably, the manufacture of heat exchangers according to the present invention includes the steps of extruding the ceramic layers and of securing those layers to one another by sintering. Moreover, it should be noted that heat exchangers according to the present invention may be assembled in flow communication with one or more other heat exchangers of substantially identical construction.

The construction, mode of operation and method of manufacture of seven heat exchangers according to the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a single-module cross-flow heat exchanger; Figure 2 shows a multiple arrangement, based on the heat exchanger of Figure 1, for use with perpendicular incident flow; Figure 3 shows a multiple arrangement, based on the heat exchanger of Figure 1, for use with oblique incident flow; Figure 4 shows a counter-flow heat exchanger in perspective representation with its alternate layers being trapezoidal in overall outline; Figure 5 shows, in a similar manner to Figure 4, a heat exchanger whose alternate layers are constituted by a plurality of bars;; Figure 6 is an exploded perspective view illustrating a heat exchanger formed entirely of groups of bars separated by sheets; and, Figure 7 is a schematic perspective representation showing how twelve heat exchangers may be assembled by virtue of their modular construction.

A heat exchanger 1 of single-module cross-flow type is shown in Figure 1 to include a plurality of ceramic plates 2 and 3, having corresponding ducts, which are stacked crosswise one on top of the other as shown, and are secured together to form a gas-tight composite body. The ducts may be of rectangular cross-section and extend parallel to one another in the plane of each of the plates. Instead of being of rectangular cross-section, however, the ducts may be hexagonal, square or circular. Moreover, adjacent plates may comprise respectively ducts of different shape. and different wall thickness, depending upon the intended use, provided that the shape of each plate is preferably such that it may be extruded with the aid of an appropriate multiple-tool extrusion die.The plates should be allowed to dry, and then stacked crosswise on top of one another before finally being secured by sintering to form a gas-tight composite body.

The cube-shaped heat exchanger modules l may then be assembled to form larger units.

Two possibilities are shown in Figures 2 and 3.

Figure 2 shows an arrangement in which a total of four heat exchanger modules 1 are disposed in two parallel rows. A cold medium flows, in the direction indicated by reference CI, into a space Sa formed between two of the modules 1 and a wall 6a. After the cold medium has passed through the modules l in the upper half of the drawing. it is conveyed to the lower modules l by deflecting ducts 7. In the process. an equalisation of temperature takes place with a hot medium which enters the two rows of the modules 1 in directions HI and leaves at low temperature in directions HO. On the other hand, the initially cold medium leaves at increased temperature in direction CO.

In Figure 3. the individual modules l are arranged obliquely. The cold medium flow enters spaces 5b. as indicated by references CI. and comes out again at increased temperature in the directions CO. The hot medium is again passed through the modules as indicated by the directional arrows HI and HO. The individual inlet and outlet spaces or chambers are formed by the modules and walls 6b.

It has provcd to be particularly advantageous to obtain the above-described heat exchanger plates by extruding the above-listed ceramic materials with suitable binders, in addition to any of the above-listed impermeable materials which may be present as impregnation, the following detailcd Examples illustrating just a few of the very many composite ceramic structures which may be obtained.

Example 1: The materials silicon nitride and cordierite have the same co-efficient of thermal expansion and an adequate wettability so that sintering together is possible. This method of assembly from silicon nitride and cordierite heat exchanger plates has the advantage that, as a result of the plastic deformation of the cordierite plates during firing, no intermediate spaces occur between the individual heat exchanger plates during use and, consequently, a good passage of heat is ensured.

The extruded ceramic plates of silicon nitride and cordierite may have approximately the following dimensions: Distance from duct to plate face 0.2 to 5 mm Distance from duct to duct 0.4 to 5 mm Square duct size 0.7 to 2 mm In use of a cross-flow heat exchanger of this kind, cold gas flows through the gas-tight cordierite plates, and hot corrosive waste gas flows through the silicon nitride plates. The latter plates, it is true, are porous, but they possess a substantially higher resistance to the corrosive waste gases than cordierite. With this combination, use is made of the gas-tightness of the cordierite, its smaller working temperature range compared with silicon nitride being acceptable.Each plate of silicon nitride is sealed by the adjacent plates of cordierite to form a rigid high-temperature-resistant structure or framework, even overheating in the cordierite causing no damage to the heat exchanger as a whole.

Example 2: In this case, extruded silicon nitride plates are stacked crosswise, being separated by metal foils of tungsten or silicon, and are fired in an electric kiln at 1400"C under low pressure. In this way, the porous ceramic plates are sealed so that they are gas-tight in relation to one another.

Example 3: About 10% by weight of cordierite and suitable binders are mixed with silicon powder. The mass or composition is then drawn into plate-shaped hollow bodies and nitrided between 1000 and 1500"C. In this process, however, a vitreous phase is not yet formed through the presence of the cordierite. This is obtained only when the body is heated to about 1400"C in an oxygen atmosphere. The creation of a glazed surface layer is used to sinter the separate plates together to form a heat exchange module.

The above-discussed ceramic materials are also used, in a substantially similar manner, in the construction of the counter-flow heat exchangers to be described hereinafter with reference to the remaining Figures of the accompanying drawings.

Indeed, Figure 4 shows a heat exchanger unit which utilises the counter-flow mode of operation and consists of a plurality of plates each having a plurality of parallel ducts extending therethrough. The plates are shown stacked one above the other with the same orientation. that is so that all of the ducts extend parallel to one another. the plates having been secured to one another. for example by sintering. to form a gas-tight composite body.

Those plates which have been referenced 2' are intended to receive, in use. a high pressure and high temperature heat exchange medium flowing in the direction of the arrows HI and HO. In contrast thereto. the alternate plates reference 3' are intended to receive. in use, a low pressure and low temperature heat exchange medium flowing in the directions of the arrows CI and CO.

It is clear from Figure 4 that each of the plates 3' is trapezium-shaped so that heat exchange medium flowing therethrough follows aLJ-shaped path with respect to the heat exchange medium flowing through the plates 2'. Alternatively, however, the plates 3' could be of rhombic overall outline to permit flow therethrough in a t-shaped path. In both of these arrangements. it will be seen that the ducts of the plates 3' are in communication with one another near their ends. which are blocked by end webs 14. The opposed outer faces of the heat exchanger. one of which is indicated at 18. preferably form parts of the hot medium plates 2'. and preferably lie in horizontal planes rather than in vertical planes as shown.

While with the embodiment shown in Figure 4 one obtains a counter-flow heat exchanger with better efficiency. compared with the above-mentioned cross-flow heat exchangers. the efficiency is still reduced by the walls of double-thickness which separate the ducts of adjacent plates. In order to obtain a thinner wall between the high and low temperature ducts. guide bars 13 of different lengths are used instead of the plates 3'. the bars 13 being so arranged that. for example. aU-shaped flow path is produced as represented in Figure 5. The bars 13 and 14 thus serve the twin functions of supports and seal elements against the plates 2'.The reader will appreciate that for convenicnce like constituent parts illustrated in the various Figures will be given like reference numerals even though the heat exchangers embodying those constituent parts are different for each Figure.

A particularly simple method of production for a ceramic heat exchanger appears from Figure 6, in that both the high-pressure and the low-pressure layers of the stack are assembled from bars and sheets. In fact, the low temperature and high temperature layers include a plurality of bars 13 and 15 respectively, the bars of any two adjacent layers being separated by respective facing sheets 16, with a further pair of opposed end sheets 16 being provided as shown. One of a similar pair of cover walls 17, likewise formed as sheets, includes openings 26 for permitting flow of the cold heat exchange medium.

In a typical application, twelve substantially identical counter-flow heat exchanger modules are assembled together, as shown in Figure 7. The relatively cold medium enters in the direction of the arrow CI through an inlet passage 20 and is uniformly distributed to the alternate low-temperature layers of three heat exchanger units 21, 22 and 23. Then the cold medium flows in aL2-shaped path into a by-pass passage 7' from where it is conducted to a further three heat exchanger units in fluid communication therewith. The cold medium finally issues in a heated condition through an outlet passage 25 in the direction of the arrow CO. The cold medium is thus heated in a counter-flow mode of operation by a higher temperature medium flowing through the remaining layers of the heat exchanger units in the directions of the arrows HI and HO.It will be noted that the passages 20 and 25 extend perpendicularly to the planes of the layers forming the assembled heat exchanger. The other six modules function similarly.

The series connection of the heat exchanger units, with constant medium flow, leads to a better transmission of heat, the parallel connection between the heat exchange units (for example, units 21, 22 and 23) naturally leading to a higher medium throughput. Sealing between the individual heat exchanger units and the various inlet, by-pass and outlet passages, in order to compensate for the different expansions of their different materials when heated, may be effected by asbestos.An alternative seal material for use at lower temperatures is graphite/MoS2, further materials for use at higher temperatures being NiO/CaF2, NiO/SrF2 and CaFe/BaF2/AlPO4. Although the assembled heat exchanger units are located in a rigid frame there is still sufficient space to compensate for expansion under different temperature conditions.

Ceramic heat exchangers with-shaped oaths for the relatively cold medium are especially suitable for installation in gas turbines. Naturally, however, heat exchanger units having Q-shaped flow paths may also be connected in series, the resultant assembly again being long and thin, but in practice extending vertically rather than horizontally.

Regarded as a whole. considerable advantages result from our counter-flow heat exchangers in comparison with our cross-flow heat exchangers. In addition to the increased efficiency already discussed, importance is attached to the compact style of construction of the individual heat exchanger units. whereby the absolute amounts of expansion are correspondingly kept small. By use of the above-listed ceramic materials, namely silicon carbide, silicon nitride and/or cordierite, one can construct heat exchangers for use at temperatures of up to 1400"C. Moreover, the above-described modular construction allows better operational reliability and easier possibility of repair in that each heat exchanger unit is relatively simply and individually replaceable.

The present invention utilises the properties of ceramic materials, whereby both high heat stability and corrosion resistance to hot media. as well as a sufficient impermeability to gas, are achieved in each module.

The advantages of the present heat exchangers lie in that brazing and complicated shaping, as in known heat exchangers. are not necessary. Moreover. the present heat exchangers have manifold uses. have a good heat transfer capacity and can be used for corrosive media. In particular. such heat exchangers can be produced at comparatively modest cost.

WHAT WE CLAIM IS: 1. A heat exchanger comprising a plurality of ceramic layers of parallel ducts, the layers being stacked with either the same orientation for the ducts in adjacent layers or with mutually perpendicular orientation for the ducts in adjacent layers, and the layers being secured to one another to form a composite body in which each of the layers is gas-tight, with respect to the layer or layers adjacent thereto. said layers being formed of silicon carbide or silicon nitride, which layers are rendered gas-tight by being interposed with layers formed of cordierite or by being impregnated with or covered by cordierite. tungsten. silicon or a metallic silicidc. the arrangement being such that in use heat exchange media flow through the ducts of adjacent layers but cannot intermix.

2. A heat exchanger according to claim 1. in which adjacent layers are stacked with mutually perpendicular orientation. and each of the layers is formed as a plate having a plurality of the ducts extending therethrough.

3. A heat exchanger according to claim 1. in which adjacent layers are stacked with the same orientation. and alternate layers have their ducts in communication with one another

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   those constituent parts are different for each Figure.

   A particularly simple method of production for a ceramic heat exchanger appears from Figure 6, in that both the high-pressure and the low-pressure layers of the stack are assembled from bars and sheets. In fact, the low temperature and high temperature layers include a plurality of bars 13 and 15 respectively, the bars of any two adjacent layers being separated by respective facing sheets 16, with a further pair of opposed end sheets 16 being provided as shown. One of a similar pair of cover walls 17, likewise formed as sheets, includes openings 26 for permitting flow of the cold heat exchange medium.

   In a typical application, twelve substantially identical counter-flow heat exchanger modules are assembled together, as shown in Figure 7. The relatively cold medium enters in the direction of the arrow CI through an inlet passage 20 and is uniformly distributed to the alternate low-temperature layers of three heat exchanger units 21, 22 and 23. Then the cold medium flows in aL2-shaped path into a by-pass passage 7' from where it is conducted to a further three heat exchanger units in fluid communication therewith. The cold medium finally issues in a heated condition through an outlet passage 25 in the direction of the arrow CO. The cold medium is thus heated in a counter-flow mode of operation by a higher temperature medium flowing through the remaining layers of the heat exchanger units in the directions of the arrows HI and HO.It will be noted that the passages 20 and 25 extend perpendicularly to the planes of the layers forming the assembled heat exchanger. The other six modules function similarly.

   The series connection of the heat exchanger units, with constant medium flow, leads to a better transmission of heat, the parallel connection between the heat exchange units (for example, units 21, 22 and 23) naturally leading to a higher medium throughput. Sealing between the individual heat exchanger units and the various inlet, by-pass and outlet passages, in order to compensate for the different expansions of their different materials when heated, may be effected by asbestos.An alternative seal material for use at lower temperatures is graphite/MoS2, further materials for use at higher temperatures being NiO/CaF2, NiO/SrF2 and CaFe/BaF2/AlPO4. Although the assembled heat exchanger units are located in a rigid frame there is still sufficient space to compensate for expansion under different temperature conditions.

   Ceramic heat exchangers with-shaped oaths for the relatively cold medium are especially suitable for installation in gas turbines. Naturally, however, heat exchanger units having Q-shaped flow paths may also be connected in series, the resultant assembly again being long and thin, but in practice extending vertically rather than horizontally.

   Regarded as a whole. considerable advantages result from our counter-flow heat exchangers in comparison with our cross-flow heat exchangers. In addition to the increased efficiency already discussed, importance is attached to the compact style of construction of the individual heat exchanger units. whereby the absolute amounts of expansion are correspondingly kept small. By use of the above-listed ceramic materials, namely silicon carbide, silicon nitride and/or cordierite, one can construct heat exchangers for use at temperatures of up to 1400"C. Moreover, the above-described modular construction allows better operational reliability and easier possibility of repair in that each heat exchanger unit is relatively simply and individually replaceable.

   The present invention utilises the properties of ceramic materials, whereby both high heat stability and corrosion resistance to hot media. as well as a sufficient impermeability to gas, are achieved in each module.

   The advantages of the present heat exchangers lie in that brazing and complicated shaping, as in known heat exchangers. are not necessary. Moreover. the present heat exchangers have manifold uses. have a good heat transfer capacity and can be used for corrosive media. In particular. such heat exchangers can be produced at comparatively modest cost.

   WHAT WE CLAIM IS: 1. A heat exchanger comprising a plurality of ceramic layers of parallel ducts, the layers being stacked with either the same orientation for the ducts in adjacent layers or with mutually perpendicular orientation for the ducts in adjacent layers, and the layers being secured to one another to form a composite body in which each of the layers is gas-tight, with respect to the layer or layers adjacent thereto. said layers being formed of silicon carbide or silicon nitride, which layers are rendered gas-tight by being interposed with layers formed of cordierite or by being impregnated with or covered by cordierite. tungsten. silicon or a metallic silicidc. the arrangement being such that in use heat exchange media flow through the ducts of adjacent layers but cannot intermix.
2. 2. A heat exchanger according to claim 1. in which adjacent layers are stacked with mutually perpendicular orientation. and each of the layers is formed as a plate having a plurality of the ducts extending therethrough.
3. 3. A heat exchanger according to claim 1. in which adjacent layers are stacked with the same orientation. and alternate layers have their ducts in communication with one another

   near their ends, which are blocked, whereby a heat exchange medium flowing through said alternate layers, in use, follows either a-shaped or shape path with respect to a heat exchange medium flowing through the remaining layers.
4. 4. A heat exchanger according to claim 3, in which each of said alternate layers is constituted by a plurality of bars defining the ducts therebetween, whereas each of said remaining layers is constituted by a plate having a plurality of the ducts extending therethrough.
5. 5. A heat exchanger according to claim 3, in which each of the layers includes a plurality of bars, with the bars of each of said alternate laters being covered by a pair of facing sheets.
6. 6. A heat exchanger according to any one of claims 3 to 5, in which the ducts of each of said alternate layers present a trapezoidal or rhombic outline when viewed in combination.
7. 7. A heat exchanger according to any one of claims 3 to 6, in which a pair of passages are provided for directing flow of a heat exchange medium, in use, respectively to and from the ducts of all of said alternate layers simultaneously, both of the passages extending in a direction which is perpendicular to the layers forming the composite body.
8. 8. A heat exchanger according to claim 1 and substantially as hereinbefore described with reference as necessary to any one of the accompanying drawings.
9. 9. A heat exchanger according to any preceding claim whose manufacture included the steps of extruding the ceramic layers and of securing those layers to one another by sintering.
10. 10. A heat exchanger according to any preceding claim assembled in flow communication with one or more other heat exchangers of substantially identical construction.