Abstract:
A method of operating a heat exchanger assembly for combustion devices which operate by using condensing boiler technology and produce dust-containing flue gases, e.g. from the combustion of biomass, reduces the formation of dust deposits in flue gas channels of the heat exchanger. A heat exchanger assembly suitable for operation in the presence of dust-containing flue gases is also provided.

Description:
BACKGROUND OF THE INVENTION  
     FIELD OF THE INVENTION  
       [0001]     The invention relates to methods of operating block heat exchanger assemblies for combustion devices which operate by using the condensing boiler technology and produce dust-containing flue gases. The invention also relates to block heat exchanger assemblies suitable for operation in the presence of dust-containing flue gases.  
         [0002]     In condensing boiler technology, the flue gases to be taken off from a combustion device are cooled in a heat exchanger to below to the dew point, so that at least partial condensation of the water vapor present in them is brought about and the heat withdrawn in the process is utilized by transferring it to a low-temperature heat transfer medium, e.g. the return stream of a heating circuit, or to the feed water for a hot water source. Stainless steel heat exchangers are usually used for condensation.  
         [0003]     The condensate includes water and possibly further water-soluble-flue gas constituents, e.g. oxides of sulfur (SO 2 , SO 3 ), which form corrosive acids in water. The composition of the condensate and its corrosive action depend very greatly on the fuel being used. While the combustion of desulfurized natural gas is relatively unproblematical, high concentrations of SO 2  and SO 3  are formed in the flue gas in the combustion of heating oil or biomass. In the case of heating oil having a sulfur content of 1,000 ppm, considerable corrosion problems occur in metallic heat exchangers, according to information from the manufacturers.  
         [0004]     A further problem is the relatively high chlorine content of biomass fuels such as straw, cereal, wood chips, wood pellets or firewood. This, too, leads to corrosion of metallic condensing boilers. For this reason, the reduction of the flue gas temperature to below the dew point and the utilization of the heat of condensation is frequently not carried out in the case of these fuels.  
         [0005]     Corrosion-resistant heat exchangers are necessary, to be able to exploit the advantages of the condensing boiler technology even in the case of devices operated by using biomass or fuels having a high sulfur content. A heat exchanger block composed of plates of graphite is known from German Utility Model 296 04 521 U1. Channel systems for two media are disposed on different planes within the block. The channels for the gaseous medium which gives off heat are formed by grooves inserted into the flat sides of the plates, and ridges remain between them. The channels for the second medium to be heated (hereinafter referred to as the heat transfer medium) are formed by holes penetrating through the plates. The thickness of the plates is chosen in such a way that the two channel systems located on different planes are separated by a thin material barrier which offers little hindrance to heat transfer but has a thickness which is sufficient to separate the two channel systems from one another in a fluid-tight manner. The channels of the two groups can be disposed parallel or perpendicular to one another, depending on whether the media are to flow in countercurrent or cocurrent or in cross-current.  
         [0006]     A heat exchanger block includes at least two superposed plates, at least one of the adjoining flat sides of which is provided with channel-forming grooves. Since a plurality of such pairs of plates can be laid side by side and on top of one another, many combinations are possible. This modular construction allows specific matching of the heat exchanger to various requirements. The plates are joined in a fluid-tight manner through the use of adhesives or through the use of gaskets and tensioning or clamping anchors.  
         [0007]     In these heat exchangers, the channels for the gaseous medium are preferably configured in such a way that a high ratio of heat exchange surface to channel volume is achieved. However, it has to be noted that the free flow cross section has to be large enough to ensure outflow of the gases as a result of the chimney draft. One suitable structure includes channels in the form of grooves having a high ratio of depth to width.  
         [0008]     However, the effectiveness of those measures for improving heat transfer and flow conditions is impaired if solid deposits are formed on the walls of the gas channels. That problem occurs in particular in the case of biomass, e.g. combustion devices operated by using wood chips or pellets, since their offgases have a high dust and ash content. The fouling of the heat exchange surface leads to an increase in the heat transfer resistance and consequently to a reduction in the rate of heat transfer. At the same time, the deposition of dust inevitably leads to significant pressure drop, since the free cross section of the gas channels is reduced.  
         [0009]     In a published article in the journal “Sonne, Wind und Wärme” from the year 2002 (Number 03/2002, pp. 50-59), a necessary development time of 10 years was predicted for the utilization of condensing boiler technology for wood-fired boilers because of the difficulties associated with the high ash content. In a market review published in the same journal, the engineering parameters and operating properties of various wood-fired systems available on the market were compared and the necessity of regular cleaning of the flue gas heat exchanger to remove fly ash was emphasized. That has to be carried out manually 15-29 times per heating period for one third of the apparatuses tested, while other manufacturers offer an automatic cleaning facility for the heat exchanger, but that increases the price of the heating system.  
         [0010]     The publication “Erprobung der Brennwerttechnik in häuslichen Holzhackschnitzelfeuerungen mit Sekundärwärmetauscher”“ (issued by the Technologie-und Förderzentrum im Kompetenzzentrum für Nachwachsende Rohstoffe Straubing 2004, pp. 21-23) also addresses the problem of the dust content of the flue gas. An experimental device included a boiler fired with wood chips and a shell-and-tube heat exchanger having a plurality of horizontal bundles of tubes composed of a special ceramic having a high thermal conductivity, disposed next to and above one another. Flue gas and outflowing condensate were conveyed in cocurrent within the shell-and-tube heat exchanger.  
         [0011]     Studies were carried out to determine whether or not the dust content of the offgas and the deposition of dust on the heat exchanger tubes could be reduced by spraying of fresh water (quenching) into the flue gas stream immediately before it enters the heat exchanger. However, both objectives were not able to be achieved, and the dust emission in quench operation was actually higher. The configuration of the heat exchanger tubes above one another was intended to prevent the dust from depositing on the heat exchanger tubes and blocking the spaces between the tubes. However, it was found that it could not be achieved even through the use of quench operation in the case of the flue gases laden with a large amount of dust from the combustion of wood chips.  
         [0012]     Deposition of dust in shell-and-tube heat exchangers is less problematical than in the block heat exchangers known from German Utility Model 296 04 521 U1, since in those heat exchangers the milled channels become unavailable for flue gas transport and thus for heat transfer over their entire length in the case of blockage, so that the heat transfer area decreases drastically. On the other hand, the compact construction of the block heat exchangers made up of plates as described in German Utility Model 296 04 521 U1 is a great advantage over shell-and-tube heat exchangers.  
       SUMMARY OF INVENTION  
       [0013]     It is accordingly an object of the invention to provide a block heat exchanger assembly for a combustion device, which operates by using condensing boiler technology that produces dust-containing flue gases, and a method for operating the block heat exchanger assembly, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensure that walls of gas channels required for heat transfer and condensate formation are reliably kept free.  
         [0014]     With the foregoing and other objects in view there is provided, in accordance with the invention, a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises causing flue gas and condensate to flow in countercurrent in a heat exchanger block.  
         [0015]     With the objects of the invention in view, there is also provided a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises spraying condensate from a neutralization and collection vessel and/or fresh water, continuously or at intervals, into hot flue gas, at or immediately before entry into a heat exchanger block.  
         [0016]     With the objects of the invention in view, there is additionally provided a method of operating a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The method comprises causing flue gas and condensate to flow in cocurrent. Condensate from a neutralization and collection vessel and/or fresh water are/is sprayed into hot flue gas at or immediately before entry into a heat exchanger block, in an amount causing a flue gas stream to be saturated with water vapor.  
         [0017]     In accordance with another mode of the invention, the heat exchanger block is made of graphite, and clarified condensate is used without neutralization for moistening the flue gases.  
         [0018]     The method of the invention simultaneously increases the condensate formation per unit area and ensures that the maximum calorific value of the fuels is utilized to the greatest possible extent. The fuel usage is reduced due to the improved utilization of the maximum calorific value according to the invention.  
         [0019]     With the objects of the invention in view, there is also provided a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The assembly comprises a heat exchanger block having a flue gas inlet in a flue gas inlet region. A flue gas line has an entry into the flue gas inlet region of the heat exchanger block. A neutralization and collection vessel is provided for condensate with overflow. A nozzle is disposed at the flue gas inlet region of the heat exchanger block or at the flue gas line immediately before the entry into the heat exchanger block. A line connects the neutralization and collection vessel to the nozzle. A pump conveys the condensate from the neutralization and collection vessel through the line to the nozzle.  
         [0020]     With the objects of the invention in view, there is additionally provided a block heat exchanger assembly for a combustion device operating by using condensing boiler technology and producing dust-containing flue gases. The assembly comprises a heat exchanger block being made of graphite and having a flue gas inlet in a flue gas inlet region. A flue gas line has an entry into the flue gas inlet region of the heat exchanger block. A collection vessel is provided for condensate having an overflow connected to a neutralization and purification vessel. A nozzle is disposed at the flue gas inlet region of the heat exchanger block or at the flue gas line immediately before the entry into the heat exchanger block. A line connects the neutralization and collection vessel with the nozzle. A pump conveys the condensate from the collection vessel through the line to the nozzle.  
         [0021]     In accordance with another feature of the invention, the heat exchanger block includes flue gas channels having walls provided with a coating reducing adhesion of dust particles and having a thickness of from 30 to 500 μm. The coating includes a fluoropolymer or a dust-repellent paint or varnish.  
         [0022]     In accordance with a further feature of the invention, in the vicinity of the flue gas inlet, the heat exchanger block has flue gas channels formed by grooves defining ridges between the grooves being slanted in direction of the flue gas inlet, causing individual adjacent channels to open into a slit-shaped flue gas inlet opening extending over all of the flue gas channels at the flue gas inlet.  
         [0023]     In accordance with an added feature of the invention, the heat exchanger block has plates facing one another. The plates have gas channels, and the plates have ridges defining a gap between at least some of the ridges, for connecting adjacent gas channels to one another through the gap.  
         [0024]     In accordance with an additional feature of the invention, the heat exchanger block has gas channels and ridges, and the ridges have transverse channels milled into the ridges interconnecting adjacent gas channels.  
         [0025]     In accordance with yet another feature of the invention, the transverse channels are milled at an angle of between 30 and 75° relative to a direction of gas flow.  
         [0026]     In accordance with yet a further feature of the invention, the heat exchanger block has flue gas channels, and turbulence-inducing fixtures are disposed in the flue gas channels. The turbulence-inducing fixtures may be rod-shaped. The turbulence-inducing fixtures may also be wire matrices each containing a plurality of wire slings deposited in the flue gas channels.  
         [0027]     In accordance with a concomitant feature of the invention, there is provided a demister for separating condensate droplets from a flue gas stream leaving the heat exchanger block.  
         [0028]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0029]     Although the invention is illustrated and described herein as embodied in a block heat exchanger assembly for dust-containing flue gases and a method of operating the same, 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.  
         [0030]     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 DRAWINGS  
       [0031]      FIG. 1  is a diagram of a heat exchanger block assembly operating according to a first embodiment of the method according to the invention, with a countercurrent flow of flue gases and condensate;  
         [0032]      FIG. 2  is a diagram of a heat exchanger block assembly operating according to a second embodiment of the method according to the invention with a cocurrent flow of flue gases and condensate;  
         [0033]      FIG. 3  is a fragmentary, diagrammatic, perspective view of an advantageous embodiment of a heat exchanger block with ridges between gas channels slanted in direction of a flue gas inlet;  
         [0034]      FIG. 4  is an enlarged, fragmentary view of an advantageous embodiment of a heat exchanger block with gaps between the ridges of abutting plates;  
         [0035]      FIG. 5  is a view similar to  FIG. 1 , with a line connected between the vessel and a nozzle disposed at the flue gas outlet region, as well as a nozzle for fresh water injection;  
         [0036]      FIG. 6  is another view similar to  FIG. 1 , with a demister at the upper end of the heat exchanger block;  
         [0037]      FIG. 7A  is a view similar to  FIG. 4 , but with turbulence-inducing fixtures;  
         [0038]      FIG. 7B  is a sectional view taken along a line VIIB-VIIB of  FIG. 7A , in the direction of the arrows;  
         [0039]      FIG. 8  is a block diagram of an external neutralization vessel; and  
         [0040]      FIG. 9  is an enlarged, plan view of the heat exchanger block with transverse channels extending at an angle. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen an illustration of the basic principle of the method of the invention for operating a block heat exchanger assembly for dust-containing flue gases. According to this basic principle of the method, the dust is flushed away as soon as it touches the walls of the flue gas channels so as to prevent formation of dust deposits.  
         [0042]     This can be achieved most simply by conveying the flue gases in the heat exchanger block  1  in countercurrent to the condensate outflow, i.e. the flue gas flows through the heat exchanger block from the bottom upward and cools in this way. Water vapor condenses and wets the walls of the gas channels, i.e. the heat transfer surfaces. The further the stream of flue gas travels in an upward direction, the more it is cooled and the more condensate is formed. Since the flue gas has been cooled to the greatest extent at the flue gas outlet of the heat exchanger block, continual moistening of the gas channels with condensate is ensured. The downward-flowing condensate flushes the dust from the walls of the channels.  
         [0043]     In addition, as is seen in  FIG. 1 , metered spraying of condensate from a neutralization and collection vessel  2  into the hot flue gas, at or immediately before the entry into the heat exchanger block  1 , can increase the water vapor content of the flue gas, so that more condensate is formed in the heat exchanger block  1  and the cleaning action is increased. In this mode of operation, it is particularly advantageous to use a heat exchanger block  1  made of graphite since neutralization of the condensate is not absolutely necessary because of the corrosion resistance of graphite. However, the condensate should be clarified to avoid blockages in the spray nozzles, and appropriate facilities for this purpose should be provided. It is only the condensate which flows out at the condensate overflow and is no longer utilized for moistening the flue gases, that has to be neutralized and purified.  
         [0044]      FIG. 5  illustrates a heat exchanger block assembly similar to that shown in  FIG. 1 , except that a line is provided connecting the collection vessel  2  (or neutralization and collection vessel, if the heat exchanger block is not made from graphite) with a nozzle disposed at the flue gas outlet region of the heat exchanger block  1  and/or a nozzle  10  is provided for injection of fresh water. Therefore, additional condensate and/or fresh water can be sprayed into the heat exchanger block from above, resulting in more effective flushing away of deposited dust.  
         [0045]      FIG. 6 , which also illustrates a heat exchanger block assembly similar to that shown in  FIG. 1 , includes a demister  11  provided at the upper end of the heat exchanger block  1 . Although only shown for countercurrent operation in  FIG. 6 , the demister can also be used in cocurrent operation, as is shown in  FIG. 2 . This is advantageous for the following reasons. The demister  11  separates condensate droplets from the flue gas stream leaving the heat exchanger block. Since those droplets usually form with small dust particles acting as condensation nuclei, those fine dust particles, together with the droplets, are separated from the flue gas outlet stream when passing the demister.  
         [0046]     For example, by providing a demister constructed for separation of droplets with a diameter of 10 μm, dust particles with a diameter below 10 μm can be separated from the flue gas. Thus, removal of dust from the flue gas is further increased. Preferably, the demister is disposed in the assembly in such a way that it can be flushed with water from the nozzle  10  and/or condensate from the nozzle  4 . Demisters, for example demisters equipped with a wire matrix, are known in the art.  
         [0047]     In countercurrent operation, the heat exchanger block has to be constructed in such a way that the flow velocity of the flue gases is not so high that the condensate is blown out from the flue gas channels.  
         [0048]     In heat exchangers in which flue gas and condensate flow in cocurrent in the direction of gravity, as is seen in  FIG. 2 , dust deposits can likewise be avoided by condensate being sprayed into the hot flue gas at or immediately before the entry into the heat exchanger block  1 . The amount of condensate sprayed in is preferably such that the dew point is reached, i.e. the flue gas becomes saturated with water vapor. The condensate formed then develops its cleaning action immediately after the flue gas comes into contact with the walls of the gas channels which act as cooling surfaces.  
         [0049]     In addition, the moistening, according to the invention, of the flue gas in heat exchangers operated in cocurrent leads to an improvement in efficiency, since condensation takes place immediately at the flue gas inlet because of the saturation with water vapor. As a result of the condensation, the heat transfer coefficient a increases appreciably (from 5-120 W/m 2 *K in the case of gas cooling alone to 4,000-12,000 W/m 2 *K in the case of film condensation, and in the case of drop condensation even to 35,000-45,000 W/m 2 *K).  
         [0050]     On the other hand, without flue gas moistening, condensation commences only in the middle part of the heat exchanger, so that the conditions which favor heat transfer occur only in this part of the heat exchanger. As a result, only a small total quantity of heat can be transferred per unit area.  
         [0051]     The condensate is conveyed through the use of a pump  3  from the collection vessel  2  through a connecting line to a nozzle  4  and is sprayed there into the flue gas line (as shown by dotted lines) or the flue gas inlet region of the heat exchanger block  1 .  
         [0052]     Fresh water can also be sprayed-in, in place of or in addition to condensate. A nozzle or water dispersion device  10  operated only at intervals is possible, particularly in the case of moistening with fresh water.  
         [0053]      FIG. 8  shows an external neutralization vessel  13  having a condensate inlet  14  receiving condensate from the condensate overflow shown in  FIG. 2 . The neutralization vessel  13  also has a neutralization agent  15 , a neutralized condensate outlet  16  and a desludge outlet  17 .  
         [0054]     In principle, the method of the invention is not tied to heat exchanger blocks made of graphite. However, they are preferred because of their corrosion resistance. In addition, no heavy metal ions can be formed in the condensate when graphite heat exchangers are used.  
         [0055]     The key component of this heat exchanger is a block  1  which is composed of graphite plates and through which channels run in a plurality of planes. The flue gas channels run through the block in a longitudinal direction and the channels for the heat transfer medium (cooling channels) pass through it in a transverse direction, so that these two media are conveyed in cross-current relative to one another. As an alternative, the channels can be disposed in parallel as is known from German Utility Model 296 04 521 U1, so that flue gases and heat transfer medium can be conveyed in cocurrent or countercurrent.  
         [0056]     At least one flat side of each plate is provided with a plurality of parallel grooves which are separated from one another by ridges. The plates are stacked on top of one another so that the flat sides provided with grooves in each case abut, with the grooves in the two plate surfaces being added together and thus forming the channels which are bounded by the abutting ridges of the two plates. If only one flat side of each plate is provided with grooves, the flat side having no grooves is usually combined with the flat side having no grooves of the next plate, so that flat sides having grooves and flat sides of the plates having no grooves are always alternately adjacent one another. The outward-facing flat sides of the first plate and the last plate of a stack are not provided with grooves.  
         [0057]     In order to obtain a channel cross section which is at the same time favorable in terms of flow dynamics and offers a large heat transfer area, grooves having a large depth compared to their width are preferred. The ratio of groove width to depth can be up to about 1:50, with a ratio of from about 1:1 to 1:10 being particularly advantageous for graphite apparatuses for production and process engineering reasons. The plate thickness is selected in such a way that a layer of material which is sufficient to ensure mechanical stability and fluid-tightness remains between the bottoms of the grooves and the holes.  
         [0058]     The flue gas stream is distributed among the longitudinal channels opening on the narrow side of the block and flows through them. The heat transfer medium to be heated, e.g. water, flows successively through individual groups of transverse channels (cooling channels), with the medium flowing firstly through transverse channels at the end of the block opposite to the flue gas inlet and last through transverse channels in the vicinity of the flue gas inlet (cross-countercurrent). The openings of the transverse channels are closed by caps (lids) made of steel. The cap over the inlet openings of the group of transverse channels through which the medium flows first is connected to an inlet pipe for the heat transfer medium and thus forms an inlet and distribution chamber, while the cap over the outlet openings of the group of transverse channels through which the medium flows last is connected to the outlet pipe for the heated heat transfer medium and thus forms a collection and outlet chamber. The openings of the transverse channels located in between are covered by caps which form chambers that change the flow direction. Each of these caps extends over the openings of a group of transverse channels through which the medium flows first and a group of transverse channels through which the medium flows subsequently. As a result, the transverse channels through which the medium flows successively are connected to one another and the flow direction of the heat transfer medium is changed.  
         [0059]     Further information and details regarding suitable graphite materials, the production and joining of the plates, the run of the channels and the flow of the media can be found in German Utility Model 296 04 521 U1.  
         [0060]     The construction of the heat exchanger from individual plates makes it possible to match the size to the particular application, so that heat exchangers having a capacity suited to the desired use can be made available.  
         [0061]     This match can, firstly, be effected by relatively small units being disposed on top of and/or next to one another and thus being combined to form larger devices. However, for production reasons, it is more advantageous firstly to prefabricate large plates having a number of individual sets of channels and then to cut them up to form units of the desired size. The individual sets of channels on the plates are constructed in such a way that each set has the number of channels necessary for the smallest construction size.  
         [0062]     If necessary, a fan is provided behind the heat exchanger to accelerate the flow of the flue gases by producing a pressure gradient (suction).  
         [0063]     In a particular embodiment of the heat exchanger block  1  for the method of the invention which is shown in  FIG. 3 , ridges  5  between grooves forming flue gas channels  6  on plates  8 ,  8 ′ are slanted in the direction of the flue gas inlet in the vicinity of the inlet region for the flue gases. This means that the individual adjacent channels open into a slit-shaped flue gas inlet opening  7  which extends over all of the flue gas channels directly at the flue gas inlet.  
         [0064]     This embodiment has a number of advantages. In contrast to the construction without slanted ridges, where the end faces of the ridges are exposed directly to the hot flue gas stream, the hot flue gas stream does not impinge directly on the ridges at the entrance of the heat exchanger having slanted ridges, so that overheating of the ridges is avoided.  
         [0065]     As a result of the gradual rise of the milled-down ridges  5 , the cross section through which the flue gas flows decreases continuously until the ridges have reached their full height and only the cross sections of the individual flue gas channels are now available. This reduction in the cross section through which flow occurs can compensate the decrease in volume of the flue gases associated with cooling, so that a uniform flow velocity of the flue gases over the entire length of the heat exchange is obtained. In addition, an inlet region having widened flue gas channels has the advantage of easier cleaning with water or condensate, since it acts as a funnel.  
         [0066]     Coating of the walls of the gas channels with a fluoropolymer which prevents adhesion of dust, for example PFA or PTFE, has been found to be particularly advantageous for cleaning of the heat exchanger. Coatings including other dust-repellent materials or surface coatings, e.g. coatings including a nanoparticle-containing surface coating or a powder of nanoparticles which produce the known lotus effect, are also suitable. The advantages are that, firstly, the dusts adhere less readily to the surface and, secondly, cleaning by water or condensate is simplified since the water easily runs off the hydrophobicized surface. For this purpose, a thin coating which has a thickness of from 30 to not more than 500 μm and does not hinder heat transfer is satisfactory. It is also not necessary for the coating to be impermeable. In addition, a hydrophobic, i.e. not readily wettable, surface is a prerequisite for the condensation preferably occurring as droplet condensation. Droplet condensation is, as mentioned above, advantageous compared to film condensation because of the higher heat transfer coefficient.  
         [0067]     Coating of the walls of the gas channels is also suitable for conventional heat exchangers made of metallic materials. In this case, an impermeable, full-area-covering coating can simultaneously effect protection of the surfaces which are in contact with flue gas against corrosion.  
         [0068]      FIG. 4  illustrates a further improvement in the heat transfer block  1  for the method of the invention, which includes providing a gap  9  between the ridges  5  of the plates  8 ,  8 ′ which face one another. The distance between the ridges  5  of the two plates, i.e. the width of the gap  9 , is preferably from half a groove width to a full groove width. However, the gap width is at least equal to the layer thickness of the above-mentioned adhesion-reducing coating. The adjacent flue gas channels  6 ,  6 ′ are connected to one another through this gap  9  between the ridges  5  of the two plates  8 ,  8 ′. Thus, the flue gas can be diverted into free channels in the event of temporary or long-term blockage of individual channels, while further downstream the flue gas stream once again flows through the previously blocked channels  6  after passing the blockage.  
         [0069]     The gap  9  can be formed in various ways. For example, the plates  8 ,  8 ′ which face one another can be kept apart through the use of a frame or a gasket or the applied adhesion-reducing coating so as to produce a gap between the ridges  5  which face one another.  
         [0070]     As an alternative, the height of the ridges  5  can be reduced to a level below the peripheral region of the plates by milling, so that the ridges no longer touch one another when the plates  8 ,  8 ′ are stacked on top of one another.  
         [0071]     Gaps  9  do not have to be provided between all of the ridges  5 . In one variant, individual ridges are configured in such a way that they come into contact with the respective opposite ridge. However, both of the ridges adjacent those ridges have a gap, so that each gas channel is connected with at least one of its neighboring gas channels. In this way, individual groups of gas channels connected to one another are formed.  
         [0072]     Connections between the flue gas channels running next to one another, which provide a bypass for the flue gas stream if individual channels become blocked, can also be produced by transverse channels milled into the ridges. For this purpose, a plurality of transverse channels are provided on the ridges at regular or irregular intervals over the length of the heat exchanger block.  
         [0073]     The emission of dust can be further reduced when the transverse channels are milled perpendicular to the ridges  5  under a defined angle.  FIG. 9  shows transverse channels  18  extending at an angle between 30 and 75° relative to the direction of the gas flow in the flue gas channels  6  within the block  8 , which is most advantageous. Ridges  5  can be seen between the flue gas channels. As a result of this, the laminar layer of the flow close to the channel wall is disturbed, resulting in impinging of the condensed droplets and dust particles onto the cooled walls of the gas channels. Thus, the dust particles can be flushed away by the downward flowing condensate.  
         [0074]     A further beneficial effect can be obtained by providing fixtures in the flue gas channels, which induce or intensify turbulences, for instance rod-like elements against which the flue gas flows perpendicularly. It can be shown by calculations of the fluidics of such a device that the heat transfer is increased by approximately 50% due to the turbulence-inducing rods. Since transfer of heat and transfer of mass follow analogous physical laws, the condensation will be increased, too. The increased gas flow at the turbulence-inducing fixtures leads to an increased deposition of dust due to the inertia of the dust particles, which can be flushed away by the downward-flowing condensate. Other turbulence-inducing fixtures (turbulators) such as wire matrices (i.e. a plurality of wire slings) provided in the flue gas channels or turbulators provided on the ridges, will have a similar beneficial effect.  
         [0075]      FIG. 7  shows such turbulence-inducing fixtures, namely rod-like fixtures  11  and fixtures  12  formed of a wire matrix. In contrast to the embodiment of  FIG. 4 , in  FIG. 7  there are no gaps  9  between the ridges  5 . However, it should be appreciated that the turbulence-inducing fixtures could also be provided in the channels  6  when there are gaps  9  between the ridges  5 .  
         [0076]     The heat exchanger is virtually maintenance-free, due to the corrosion-resistant graphite material and the operating method of the invention which prevents the formation of dust deposits. A further advantage is that its compact construction also allows it to be retrofitted into existing heating devices. Such retrofits are not only advantageous because of the fuel consumption which is reduced through the use of the method of the invention but in many cases are also absolutely necessary because of the coming-into-force of stricter regulations for the operation of heating devices, e.g. the German federal antipollution regulations which came into force on Nov. 1, 2004 and the energy saving regulations which come into force on Dec. 31, 2006.  
         [0077]     This application claims the priority, under 35 U.S.C. § 119, of German Patent Application 10 2005 009 202.0, filed Feb. 25, 2005; the prior application is herewith incorporated by reference in its entirety.