Abstract:
A heat exchanging element for a heat exchanger is provided with a coating that prevents, or at least reduces, the amount of contaminating materials to be abrade from the heat exchanger and into the heat exchange media. A method for producing a heat exchanging element for a heat exchanger, a heat exchanger per se, and a method for retrofitting an existing heat exchanger, provide for the occurrence of impurities caused by abrasion in one or more heat exchanging media and/or corrosion to be prevented or at least reduced by providing the coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation, under 35 U.S.C. §120, of copending international application No. PCT/EP2011/059649, filed Jun. 10, 2011, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2010 030 780.7, filed Jun. 30, 2010; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a heat exchanging element for a heat exchanger, a method for manufacturing a heat exchanging element for a heat exchanger, a heat exchanger per se, specifically involving the use of the heat exchanging element according to the invention, as well as a retrofitting method for a heat exchanger. In particular, the present invention also relates to CVD-coated and impregnated graphite, which is used especially for a heat exchanging element or recuperator element, in particular in a core for a block heat exchanger. 
         [0003]    In many areas of chemical and/or physical process engineering, quantities of heat must be exchanged between at least two fluid media, whether it be liquids, gases, gels, pasty media or the like, for example in order to cool or heat a provided process medium. The heat exchangers or recuperators used here exhibit at least one heat exchanging element or recuperating element, the corresponding contact surfaces or contact areas of which receive the flow of actual process medium to be heated or cooled, and of at least one additional medium, which supplies or removes the quantity of heat, and is often referred to as the service medium. The quantity of heat is introduced in one of the contact areas or in one of the contact surfaces, transferred by way of a heat conducting mechanism of the heat exchanging element to another contact surface or another contact area, and then released by the latter to the other medium. 
         [0004]    In this context, use is often made of heat exchanging elements that consist essentially of a graphite material, which is impregnated with a resin material at the contact surface that comes into contact with a respective medium, for example to limit or even prevent the penetration of respective medium into the porous complex of the material that forms the basis of the heat exchanging element. 
         [0005]    During the inflow of the receptive medium, in particular the process medium, particles from the resin impregnation and/or the material comprising the basis of the heat exchanging element, e.g., the graphite material, frequently become physically and/or chemically detached, remain in the actual process medium and thereby contaminate the latter. This is often unacceptable. 
         [0006]    In addition, corrosion on the material of the heat exchanging element and/or a detachment of corroded material and contamination of the heat exchanging fluid(s) by corroded material of the heat exchanging element can in many instances not be tolerated. 
       SUMMARY OF THE INVENTION 
       [0007]    It is accordingly an object of the invention to provide a heat exchanging element for a heat exchanger, a manufacturing method for a heat exchanging element for a heat exchanger, a heat exchanger per se as well as a retrofitting method for a heat exchanger, which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which render it possible to easily and yet reliably reduce or even prevent contamination of the heat exchanging medium/media by material from the underlying heat exchanging element and corrosion to the material of the heat exchanging element(s) and contamination of the heat exchanging fluid(s) by corroded material. 
         [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a heat exchanging element for a heat exchanger, the heat exchanging element comprising: 
         [0009]    a body containing or essentially consisting of one or more materials selected from the group consisting of graphitic materials, graphites, and open-pored, unsintered silicon carbide materials; 
         [0010]    said body being formed with a first contact region and a second contact region for separate contact in terms of flow with a first heat exchanging medium forming a first process medium and with a second heat exchanging medium forming a second process medium, respectively, for effecting separate heat exchange; and 
         [0011]    a coating with one or more materials selected from the group of materials consisting of silicon carbide materials, carbide oxide materials, silicide materials, tungsten titanate materials, oxide materials, pyrocarbon, diamond, and derivatives and combinations thereof; 
         [0012]    said coating being disposed on at least one of said first and second contact regions and partially or completely coating the respective said contact region. 
         [0013]    The objects underlying the invention are achieved according to the invention by way of the claimed heat exchanging element, by way of the claimed method for manufacturing a heat exchanging element, by of the claimed heat exchanger, and the claimed method of retrofitting a heat exchanger. 
         [0014]    A first aspect of the present invention provides a novel heat exchanging element for a heat exchanger, which, in order to achieve a separate heat exchange in terms of flow between a first heat exchanging medium as the process medium and a second heat exchanging medium as the service medium, exhibits a first contact region and a second contact region for a separate contact in terms of flow with the first heat exchanging medium or the second heat exchanging medium, which essentially contains or consists of one or more materials from the group of materials that exhibits graphite materials, graphites and open-pored and unsintered SiC or silicon carbide materials, and in which at least one of the first and second contact regions is partially or completely coated with one or more materials from the group of materials as the coating, which exhibits SiC or silicon carbide materials, carbide oxide materials, silicide materials, tungsten titanate materials and their derivatives and combinations. The first and second heat exchanging media can here be fluids, e.g., liquids, gases, gels or pasty media, foams, slurries and their combinations and mixtures. 
         [0015]    Therefore, one core idea of the present invention involves reducing or preventing the likelihood that material comprising the basis of the heat exchanging element will detach and/or corrode, by giving the material comprising the basis of the heat exchanging element an especially resistant or abrasion-proof coating, at least on one of the first and second contact regions or contact surfaces of the heat exchanging element, precisely when the material comprising the basis of the heat exchanging element is made out of a graphite material or an open-pored and/or unsintered SiC or silicon carbide material. 
         [0016]    An impregnation can be composed of an impregnating material containing or consisting of one or more materials from the group exhibiting resin materials, phenol resin materials and their derivatives and combinations. In particular, impregnation with the impregnating material serves to prevent one of the heat exchanging media from advancing too deeply and in particular penetrating the material comprising the basis of the heat exchanging element, and remaining therein, so as to thereby reduce or prevent a material intermixing, even if this were only possible in the long term, and/or a contamination when exchanging media. 
         [0017]    The impregnation with the impregnating material can be formed entirely or partially on and/or in the coating and/or entirely or partially on and/or in the first contact region and second contact region. Since the coating at least partially, if not completely, seals the material comprising the basis of the heat exchanging element anyway so as to prevent contamination, e.g., through abrasion or the like, i.e., closes pores at that location, it is especially advantageous for an impregnation to have been or be formed on or in the coating to prevent contamination. This also offers procedural advantages, since thermal boundary conditions for the impregnating material need not be taken into account when processing the coating on the material comprising the basis of the heat exchanging element. For example, high-temperature stages can be run through in the manufacturing process, without having to expect that the impregnating material will become damaged or degraded, since the latter can then be applied or introduced after the fact, i.e., following the high-temperature stage. 
         [0018]    The coating with the coating material can conceivably have varying structures and methods involved in its manufacture. 
         [0019]    The coating may be configured as a CVD coating (CVD, chemical vapor deposition). 
         [0020]    Alternatively or additionally, the coating can be configured as a chemical and/or physical conversion area—in particular via a process involving the entire or partial chemical and/or physical conversion of the material comprising the first and/or second contact region. 
         [0021]    Further, the coating can alternatively or additionally be formed by method of plasma spraying and/or flame spraying. In addition, successful tests have already been performed on the formation of a solid layer through so-called liquid silicidation, both in an immersion process and evaporation process, as well as in a wicking process. 
         [0022]    Depending on the kind of material provided for coating purposes or the materials provided for coating purposes, use can be made of different coating mechanisms and corresponding manufacturing methods, but the coating formed as a chemical and/or physical conversion layer is especially elegant in particular when no or only a slight quantity of additional material components must be provided for coating purposes. 
         [0023]    Different manufacturing methods and structures can have been or be combined with each other while building up the coating. 
         [0024]    The heat exchanging element according to the invention can be designed as a heat exchanger plate or recuperator plate of a plate heat exchanger or plate recuperator. 
         [0025]    The heat exchanging element according to the invention can also be designed as a heat exchanger core or block or as a recuperator core or block of a block heat exchanger or block recuperator. 
         [0026]    Further, the heat exchanging element according to the invention can be designed as a heat exchanger tube or recuperator tube of a tubular heat exchanger or tubular recuperator. 
         [0027]    Therefore, the concept according to the invention can basically be utilized in all heat exchangers or recuperators in which one or more heat exchanger elements or recuperator elements are used that follow the principle outlined in greater detail above, specifically that it receives a flow of heat exchanging medium, in particular a process medium, at least on one contact region or on one contact surface, thereby coming into mechanical contact with the latter, as a result of which the physical and/or chemical interaction conceivably leads to an abrasion on a surface of the contact area or the contact surface of the heat exchanging element. 
         [0028]    Another aspect of the present invention provides a method for manufacturing a heat exchanging element for a heat exchanger, in which the heat exchanging element for a separate heat exchange in terms of flow between a first heat exchanging medium as the process medium and a second heat exchanging medium as the service medium is designed with a first contact region and with a second contact region for separately contacting the first heat exchanging medium or second heat exchanging medium in terms of flow, in which the heat exchanging element essentially contains or consists of one or more materials from the group of materials that exhibits graphite materials, graphites and open-pored and unsintered SiC or silicon carbide materials, and in which at least one of the first and second contact regions is entirely or partially coated with one or more materials from the group of materials as the coating, which exhibits SiC or silicon carbide materials, pyrocarbon, oxide ceramics, e.g., chromium oxides, diamond, carbide oxide materials, silicide materials, tungsten titanate materials and their derivatives and combinations. 
         [0029]    An impregnation can be composed of an impregnating material containing or consisting of one or several materials from the group exhibiting resin materials, phenol resin materials and their derivatives and combinations. 
         [0030]    The impregnation with the impregnating material can be formed entirely or partially on and/or in the coating and/or entirely or partially on and/or in the first contact region and second contact region. 
         [0031]    The coating may be configured as a CVD coating (CVD, chemical vapor deposition). 
         [0032]    The coating may also be configured as a CVI coating (CVI, chemical vapor infiltration). 
         [0033]    Alternatively or additionally, the coating can be designed as a chemical and/or physical conversion area—in particular via a process involving the entire or partial chemical and/or physical conversion of the material comprising the first and/or second contact region. 
         [0034]    Further, the coating can alternatively or additionally be formed by method of plasma spraying and/or flame spraying. 
         [0035]    The heat exchanging element can be designed as a heat exchanger plate or recuperator plate of a plate heat exchanger or plate recuperator. 
         [0036]    The heat exchanging element can also be designed as a heat exchanging core or block or as a recuperator core or block of a block heat exchanger or block recuperator. 
         [0037]    The heat exchanging element according to the invention can also be designed as a heat exchanger tube or recuperator tube of a tubular heat exchanger or tubular recuperator. 
         [0038]    Another aspect of the present invention also provides a heat exchanger, in which one or more heat exchanging elements according to the invention have been or will be formed. 
         [0039]    In addition, another aspect of the present invention also indicates a method for retrofitting an already existing heat exchanger, in which one or more present and in particular conventional heat exchanging elements are replaced by one or more corresponding heat exchanging elements according to the invention and/or in which one or more present and in particular conventional heat exchanging elements are converted into heat exchanging elements according to the invention, wherein in particular the method according to the invention is used. 
         [0040]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0041]    Although the invention is illustrated and described herein as embodied in a heat exchanging element for a heat exchanger, a method for manufacturing a heat exchanging element for a heat exchanger, a heat exchanger, and a retrofitting method for a heat exchanger, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0042]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0043]      FIG. 1  is a schematic and perspective exploded view of an embodiment of a heat exchanger according to the invention taking the form of a plate recuperator using an embodiment of the heat exchanging element according to the invention in the form of a recuperator plate. 
           [0044]      FIG. 2  is a schematic and perspective side view of a single heat exchanging element from the assembly on  FIG. 1 . 
           [0045]      FIGS. 3A and 3B  show schematic sectional side views of two intermediate states attained in a manufacturing process according to the invention for a heat exchanging element according to the invention. 
           [0046]      FIG. 4  is a schematic and perspective side view of another embodiment of a heat exchanger, specifically taking the form of a block recuperator. 
           [0047]      FIGS. 5A and 5B  show schematic sectional side views of two intermediate states that are attained in another embodiment of a manufacturing process according to the invention for a heat exchanging element according to the invention. 
           [0048]      FIGS. 6A and 6B  show schematic sectional side views of two intermediate states that are attained in another embodiment of a manufacturing process according to the invention for a heat exchanging element according to the invention. 
           [0049]      FIGS. 7A and 7B  show schematic sectional top views of two intermediate states that are attained in another embodiment of a manufacturing method according to the invention for a heat exchanging element according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    A variety of embodiments and implementations of the present invention will be described below. It will be understood that the various embodiments of the invention along with their technical features and properties can be individually isolated or optionally combined with each other in any way desired and without limitation. 
         [0051]    Structurally and/or functionally identical, similar or equivalent features or elements will be identified with common reference numerals throughout the figures. A detailed description of these features or elements will not be repeated in each instance. 
         [0052]    Referring now to the figures of the drawing in general, the present invention relates to a heat exchanging element  10 ,  20  for a heat exchanger  100 ,  200 , a method for manufacturing a heat exchanging element  10 ,  20  for a heat exchanger  100 ,  200 , a heat exchanger  100 ,  200  per se, as well as a method for retrofitting an existing heat exchanger, in which contaminants generated via abrasion and/or corrosion in one or more heat exchanging media M 1 , M 2  and/or corrosion have been or will be prevented or at least reduced by providing a coating  30 . 
         [0053]    In addition to the general principle described above, the present invention also relates to CVD-coated and impregnated graphite, and in particular to its use for configuring heat exchanging or recuperator elements  10 ,  20 . 
         [0054]    In particular in the fine chemical and pharmaceutical industry, heat exchangers  100 ,  200  and recuperators  100 ,  200  made out of graphite impregnated with synthetic resin are frequently used for cooling or heating media M 1 , M 2 . In the context, it happens that graphite particles or particles stemming from impregnation, i.e., resin particles, are detached and/or corroded from the surfaces  20   v  or walls  20   v  of the product boreholes  22  by the process medium M 1  flowing through the heat exchanger  100 ,  200 . The latter may, in particular, take the form of a block recuperator  200 . These particles must be understood as foreign particles, because they contaminate the end product, so that the entire production lot must be discarded in the worst-case scenario. 
         [0055]    In order to avoid or circumvent this problem, for example, consideration can be given to using recuperators  100 ,  200 , whose heat exchanging elements  10 , or recuperator elements  10 ,  20  are made completely out of silicon carbide (SiC). The advantage to silicon carbide by comparison to recuperator elements or heat exchanging elements consisting of graphite is that the abrasion and/or corrosion resistance is clearly higher, so that practically no silicon carbide particles are encountered in the heat exchanging medium M 1 , M 2 , in particular in the product medium M 1  or product solution M 1 . 
         [0056]    The disadvantage to a heat exchanging element or recuperator element made completely out of silicon carbide is that the material and manufacturing costs are several times higher by comparison to pure graphite, so that this option can as a rule be used only in exceptional cases. In addition, the extremely brittle behavior of SiC ceramics is often disadvantageous owing to the application. 
         [0057]    The present invention also makes use of the understanding that in particular graphite surfaces can be coated with silicon carbide or SiC in a CVD process, especially one carried out at temperatures exceeding 1000° C., so as to thereby raise the abrasion and/or corrosion resistance of the material  10 ′,  20 ′ comprising the basis of a heat exchanging element  10 ,  20 , meaning in particular of the underlying graphite material. This also holds true for substrates, e.g., CSiC material. The diminished corrosion resistance of the free carbon is problematical in this context, and poses an obstacle to an application of the kind described above. 
         [0058]    The advantage here is that the use of silicon carbide, for example, as the coating material  30 ′ yields a higher abrasion resistance on the one hand, while not inordinately increasing the manufacturing costs on the other, since the underlying material  10 ′,  20 ′ can still continue to be a low-cost material, in particular a graphite material, which then is quasi refined on its surface via the coating  30 . 
         [0059]    In particular in the case of so-called block recuperators  200 , it might also be possible for the recuperator block  20 , especially in terms of the product boreholes  22 , to have situated upstream from it a block element made completely out of an abrasion-resistant material, e.g., silicon carbide, but containing no service boreholes  24  for the second heat exchanging medium M 2 . This ensures that an abrasion-resistant component will handle first contact with the process medium M 1 . 
         [0060]    While this makes it possible to lower the extent of contamination, providing such a preliminary block made completely out of abrasion-resistant material in this way is also cost-intensive, and associated with the technical drawback that a dead volume is introduced prior to the actual recuperator process, thereby diminishing the overall effectiveness of such an installation. 
         [0061]    By contrast, the invention proposes a simplified approach, namely coating one or more heat exchanging elements  10 ,  20  of a heat exchanger  100 ,  200  with an abrasion and/or corrosion-resistant material  30 ′, specifically at least in the areas or partial areas where contact takes place with the process medium M 1 . 
         [0062]    As a result, the invention creates a variant that is cost-effective and potentially mechanically more tolerant to brittle fractures by comparison to recuperator elements  10 ,  20 , e.g., block recuperators, which are made completely out of an abrasion and/or corrosion-resistant material. Therefore, a cost-effective and previously conventional material  10 ′,  20 ′ can be used for the heat exchanging elements  10 ,  20 , in which all surfaces that come into contact with the abrasive and/or corrosive medium M 1 , M 2 , e.g., the two end surfaces  20   e  and the product boreholes  22  in the block recuperator  200 , are then protected by a layer  30  of abrasion and/or corrosion-resistant material  30 ′, e.g., silicon carbide, and their pores are then completely impregnated with a synthetic resin  40 ′, so as to ensure the tightness of the heat exchanging element  10 ,  20 , in particular of the recuperator block  20 . 
         [0063]    An impregnation  40  based on synthetic resin  40 ′ or some other type of impregnation  40  is often necessary, since it can frequently not be ensured that each surface of the heat exchanging element  10 ,  20 , in particular of the block recuperator  200 , coming into contact with the process medium M 1  is completely sealed by the used abrasion and/or corrosion-resistant material  30 ′, in particular by the silicon carbide. 
         [0064]    An impregnation  40 , in particular with synthetic resin  40 ′, must take place after the process of coating with the abrasion and/or corrosion-resistant material, since temperatures in excess of 1000° C. during the coating process can destroy the material  40 ′ for the impregnation  40 . 
         [0065]    An embodiment of a manufacturing method according to the invention for a heat exchanging element  10 ,  20 , in particular for a recuperator block  200  or the like, can have the following structure: 
         [0066]    A finished block  20 , e.g., consisting of graphite  20 ′ or the like, is coated with a silicon carbide coating  30  based on a CVD process, wherein, for example, the lateral surfaces  20   m  of the block can be covered with a graphite film, for example, so that no coating takes place there. 
         [0067]    Alternatively, the service boreholes  24  can be introduced into the block  20  after coating with the silicon carbide material  30 ′ is complete. 
         [0068]    The block  20  coated with silicon carbide  30 ′ is then given an impregnation  40  analogously to the manufacture of recuperator blocks, e.g., impregnated with a synthetic resin  40 ′. Prior to impregnation  40 , the two end surfaces  20   e  of the block  20  can be covered with two correspondingly large metal disks, wherein a seal between each block end face  20   e  and the metal disk prevents contact between the impregnating resin  40 ′ and the block end surfaces  20   e  and product boreholes  22 . The resin  40 ′ for impregnation  40  can penetrate into the block  20  via the lateral surfaces  20   m  and the service boreholes  24 . For example, the jacket disks are fixed in place and made taut by several tension rods, which are guided through the product boreholes  22 . After impregnation  40 , the block  20  with the braced metal plates is placed in an annealing oven. The synthetic resin  40 ′ is hardened in a standard procedure. Obtained as the end product is a block  20  coated with silicon carbide on the product side, and free of resin film in the product boreholes  22 . 
         [0069]    These and similar types of manufacturing methods according to the invention and heat exchanging elements  10 ,  20  according to the invention have a variety of advantages relative to prior art procedures: 
         [0070]    The heat exchanger  100 ,  200  or recuperator  100 ,  200 , in particular block recuperator  200 , manufactured according to the invention is resistant to abrasive and/or corrosive media, and just like a conventional heat exchanger, facilitates a complete heat exchange between a process medium M 1  and service medium M 2 , i.e., without the formation of dead volumes. 
         [0071]    In addition, the abrasion-resistant layer  30  or coating  30 , in particular the SiC layer, prevents both the adsorption of media M 1 , M 2  and their subsequent desorption when changing out a product or the service medium M 2 . In addition, the abrasion and/or corrosion-resistant layer  30  or the SiC layer  30  can prevent any graphite particles or resin particles from abrading or accumulating in the process medium M 1 , and hence in the product solution, and/or corrosion. 
         [0072]    The substrate material  10 ′,  20 ′—e.g., the graphite  10 ′,  20 ′—and coating material  30 ′ can advantageously be coordinated in terms of their thermal expansion coefficients. 
         [0073]    This is because, in another aspect of the present invention, the ratio between the thermal expansion coefficients for the substrate material  10 ′,  20 ′, in particular the graphite substrate  10 ′,  20 ′, and the coating material  30 ′ and/or impregnating material  40 ′—in particular the CVD-SiC—can be selected and set in such a way, for example, as to exhibit values which, if at all possible, range between about 1.2 and about 0.8, preferably between about 1.1 and about 0.9, and especially preferably between about 1.05 and 0.95, in particular at the highest process temperature. Ideally, the thermal expansion coefficients of the material pairings are identical. 
         [0074]    It has surprisingly been discovered that layer thicknesses of below 5 μm are already abrasion and corrosion-proof. Particles are successfully retained, substrate corrosion is prevented, and an extreme increase is achieved in surface hardness. Therefore, layer thicknesses of between 5 and 1000 μm, preferably of between 20 and 400 μm, and especially preferably of between 50 and 200 μm are ideally applied. 
         [0075]    Preferred process temperatures here range in particular between about 1200° C. and 2400° C., depending on the used coating process, in particular the CVD procedure. 
         [0076]    Amazingly, absolutely tear-free coatings can be obtained by carefully selecting material pairings in this way in terms of their thermal expansion, thereby potentially eliminating the need for a seal, for instance with resins. 
         [0077]    Aside from a high wear resistance, surfaces fabricated in this way exhibit a high corrosion resistance, which in particular matches that of a SiC, alpha-SiC or a-SiC. 
         [0078]    Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a schematic and perspective exploded view of an embodiment for a heat exchanger  100  according to the invention in the form of a so-called plate heat exchanger  100 ′ or plate recuperator  100 ′. The latter is formed of an assembly  110  resembling a stack comprised of a plurality of heat exchanger elements  10  or recuperator elements  10  configured as heat exchanger plates  10  or recuperator plates  10 . 
         [0079]    The arrows denote the inflows and outflows of the first and second heat exchanging media M 1  and M 2 , which alternately flow in the gaps R 1 , . . . , Rn as flow spaces, wherein corresponding sealing devices are provided between the sequential heat exchanger elements  10  (not explicitly shown here), so as to prevent the first and second heat exchanging media M 1  and M 2  from mixing together. 
         [0080]      FIG. 2  presents a schematic and perspective side view of a single heat exchanging element  10  in the form of a heat exchanger plate  10  from the arrangement on  FIG. 1 . This heat exchanging element  10  in the form of plates essentially consists of a base material  10 ′, e.g., a graphite material, and exhibits an upper side  10   o  or front side  10   o  and a rear side  10   u  or lower side  10   u . The front side  10   o  and rear side  10   u  can be made out of corresponding flow channels in the surface of the underlying material  10 ′ of the plate  10 , so as to intensify the mechanical contact, and hence exchange of heat, between the two sides  10   o  and  10   u  of the plate  10 . These flow or streaming channels are not explicitly shown here, and form a kind of relief on the upper side  10   o  or lower side  10   u  of the plate  10 . 
         [0081]      FIGS. 3A and 3B  present various manufacturing stages for the heat exchanging element  10  depicted on  FIG. 2  in plate form. 
         [0082]      FIG. 3A  practically denotes the blank for the heat exchanging element  10  in plate form. This means that the plate  10  essentially consists of a conventional material  10 ′, e.g., a graphite material, as the plate substrate  10 ′. Also denoted are the upper side  10   o  and lower side  10   u  of the plate  10 . 
         [0083]    In the transition to the depiction on  FIG. 3B , a coating  30  consisting of an abrasion-resistant material  30 ′ is formed at least on the upper side  10   o  and lower side  10   u.    
         [0084]    It is often sufficient for the respective side—i.e., either the upper side  10   o  or the lower side  10   u —with the abrasion-resistant material  30 ′ to be designed as the coating  30  that comes into contact with the actual process medium, e.g., the heat exchanging medium M 1 , which as the product must not be contaminated. Whether the service medium, meaning the second heat exchanging medium M 2 , for example, becomes contaminated or not is frequently of secondary importance. Therefore, the side—the lower side  10   u  on FIGS.  3 A and  3 B—must often only be provided with the coating  30  as an option, as denoted on  FIG. 3B  with a dashed line. 
         [0085]      FIG. 4  presents a schematic and perspective side view of another embodiment of a heat exchanger  200  or recuperator  200  according to the invention, specifically in the form of a perfectly cylindrical block recuperator  200 ′ with a heat exchanger core or recuperator core  20  consisting of a material  20 ′ that exhibits first, perpendicular or vertical boreholes  22  or process boreholes  22  for the first heat exchanging medium M 1  or process medium M 1  parallel to the symmetry axis, i.e., in the Z direction, as well as second or horizontal boreholes  24  or service boreholes  24  perpendicular thereto for the second heat exchanging medium M 2  or service medium M 2 . 
         [0086]    The boreholes  22  and  24  do not communicate with each other, so that the first and second heat exchanging media M 1  and M 2  cannot become mixed together. A guiding disk frame  50 ,  60  with an array of several guiding disks  50  is provided to laterally and vertically limit and control the flows of the first and second heat exchanging media M 1  and M 2 . The guiding disks  50  are clamped in corresponding brackets  60 . Additionally depicted are the lateral surface  20   m  and end surfaces  20   e  of the heat exchanging element  20  designed as a block, as well as the surfaces  20   v ,  20   h  or inner surfaces  20   v ,  20   h  of the vertical or horizontal channels of boreholes  22 , or  24 . 
         [0087]    In the present invention, a block-shaped heat exchanging element  20  according to  FIG. 4  can have not just end surfaces  20   e  and a lateral surface  20   m , but also precisely the inner surfaces  20   v  and  20   h , of the first or second boreholes  22  or  24  for the process medium M 1  or service medium M 2  that consist of a corresponding coating  30  made of an abrasion-resistant coating material  30 ′. 
         [0088]    This is illustrated once again on  FIG. 5A to 7B  within the framework of two sequential procedural steps in a schematic and cut side view or a schematic top view. 
         [0089]      FIGS. 5A and 5B  present a section of the arrangement from  FIG. 4  for a block-shaped heat exchanging element  20 , depicting exclusively those vertical boreholes  22  parallel to the Z-direction that serve to transport the process medium M 1  or first heat exchanging medium M 1  and exhibit inner surfaces  20   v , for example. 
         [0090]    The base material  20 ′ of this heat exchanging element  20  can be a conventional material  20 ′. In the transition to the intermediate state shown on  FIG. 5B , the end surfaces  20   e  of the block-shaped heat exchanging element  20  and the inner surfaces  20   v  or inner sides  20   v  of the vertical boreholes  22  or vertical flow channels  22  are then provided with a coating  30  that contains or consists of the coating material  30 ′. If necessary, a corresponding coating  30  also arises on the end surface  20   e.    
         [0091]    The cross section of the vertical boreholes  22  is possibly slightly restricted, in which case the representation on  FIG. 5A to 7B  is not to scale; the actual reduction in the clear width of the boreholes  22  and  24  with the inner surfaces  20   v  and  20   h  is only minimally curtailed. 
         [0092]    The same holds true for a coating that relates to the lateral surface  20   m  and inner surfaces  20   h  or inner sides  20   h  of the horizontal boreholes  24 , as illustrated on  FIGS. 6A and 6B  analogously to  FIGS. 5A and 5B . 
         [0093]      FIGS. 7A and 7B  present a top view depicting the arrangement of the block recuperator  200 ′ on  FIGS. 4 to 6B  opposite the Z-direction, i.e., directly on the upper end surface  20   e  of the underlying cylinder. 
         [0094]    In the case of tubular heat exchangers not graphically depicted here, such a coating  30  on the inside and/or outside of a respective heat exchanger tube is also conceivable.