The present invention concerns a gas-gas shell and tube heat exchanger for performing heat exchange between a first and a second gas flow.

Heat exchangers of the shell and tube type are well known devices in the industry for performing heat exchange between fluids, in particular for liquid-liquid and gas-liquid heat exchange applications, due to their relatively simple design, robustness and the ability of easy cleaning. However, for heat exchange between two gas flows the typical shell and tube heat exchangers having a standard design exhibit a number of limitations, which results in either voluminous and heavy devices or in a high pressure drop across one or both sides (shell side and/or tube side).

The standard design of a gas-gas shell and tube heat exchanger is described, e.g. in patent documents US1814011 and US2817498. In these known heat exchangers one of the gas flows (shell side gas flow) passes multiple times in a cross flow configuration through a tube bundle comprising a number of parallel pipes, inside of which the other gas flow (tube side gas flow) passes. The shell side gas flow passes a segment of the tube bundle pipes equivalent to the spacing (so called baffle spacing) between two baffles. The baffles support the pipes of the tube bundle and direct the shell side gas flow in a direction cross to the tube bundle pipes. The forced shell side gas flow across the tubes creates a high turbulence, which turbulence improves the heat transfer capability (i.e. heat transfer coefficient).

However, at the same time it generates also a relatively high pressure drop for the shell side gas flow. The total pressure drop at the shell side adds up as result of the multiple times that the shell side gas flow crosses the tube bundle.

When a shell and tube bundle is scaled to higher gas flows, theoretically only more (not longer) tubes are required. However when the tube length and baffle spacing are kept the same and the tube bundle geometry is kept as a symmetrical lay-out for practical reasons, the pressure drop of the shell side gas flow will increase, due to the higher velocity and the increased number of tubes that the entire shell side gas flow needs to pass. In order to keep the pressure drop of the shell side gas flow the same the baffle spacing needs to increase significantly, resulting in a lower gas velocity (hence turbulence) compared to the original shell and tube design before upscaling. This lower velocity results in a lower heat transfer coefficient, which needs to be compensated with longer tubes. On its turn this compensation by longer tubes leads to a higher pressure drop at the tube side resulting in more parallel tubes to compensate this increase. In such a situation for achieving the same heat exchange (expressed as temperature difference DT) at the same pressure drop at the tube side and the shell side more heat transfer surface (pipe surface) is required, which again results in an additional pressure drop that needs to be compensated with increased baffle spacing. This so called negative feedback loop results in voluminous and heavy shell and tube exchangers for gas-gas applications. This effect increases with increasing volumetric flow rate of the gas flow at the shell side, since the entire shell side gas flow has to pass each segment for a standard tube bundle. The higher the flow rate at the shell side, the longer the tube length needs to be. This negative feedback loop is cut in half in US5477846. In this known device the tubes are arranged as one “hollow” cylinder shaped tube bundle. Air is introduced at the outside (periphery) of this tube bundle and passes inwardly through a segment of the tube bundle until it reaches the inside hollow core, which is free of tubes, of the hollow cylinder shaped tube bundle. A solid baffle in this hollow core forces the shell side gas flow outwardly through an adjacent segment of the tube bundle to the outside periphery of this tube bundle. In this case the pressure drop across one bundle segment is lower, because the shell side gas flow does not pass the tube bundle as a full cylinder shaped tube bundle, but only half of it. However, the tubes needs to be significantly longer when the volumetric flow rate should be increased. Thus the negative feedback loop applies for this design as well. Furthermore the tube bundle and associated baffles are assembled into one rigid tube bundle in this design. GB557671A has disclosed a tubular heat exchanger, the tubes of which are subdivided into groups.

GB2451848A discloses a heat exchanger having tube bundles, wherein the heat exchanger includes a plurality of plate-type baffles, which support the tube bundles and/or direct the flow of the shell side fluid over the tube bundles in a helical flow pattern.

The invention aims at breaking the negative feedback loop of longer tube length with increased shell side flow rate.

It is a further object of the invention to allow (linearly) up-scaling the number of tubes with increasing flow rate at a fixed tube length and a given temperature difference in a gas-gas shell and tube heat exchanger.

It is another object of the invention to design a gas-gas shell and tube heat exchanger with relatively short tube length.

It is yet another object of the invention to standardize a tube bundle for a specific temperature trajectory and balanced flows at either side, like in air preheating and drying processes with air.

Still a further object of the invention is to provide a heat exchanger allowing to cool an acidic flue gas beyond the acid dew point without the risk of corroding.

The gas-gas shell and tube heat exchanger for heat exchange between a first gas flow and a second gas flow according to the invention is defined in claim 1. The gas-gas shell and tube heat exchanger for heat exchange between a first gas flow and a second gas flow according to the invention comprises a housing (or shell) that delimits a heat exchanger chamber (or shell space) and is provided with at least one inlet (also known as shell nozzle) for feeding the second gas flow and with at least one outlet (also known as shell nozzle) for discharging the second gas flow after heat exchange. Typically the inlet and outlet are arranged at the opposite longitudinal ends of the heat exchange chamber. The cross section of the heat exchange chamber may have any shape, such as circular, oval, rectangular or square. In the heat exchanger chamber a plurality of tube bundles are arranged in parallel. The tube bundles may be positioned according to a regular pattern, such as a symmetrical lay-out, at a fixed distance (bundle pitch) from one another. Externally the bundles leave free a gap between the bundles through which gap the second gas flow is passed. Thus the gap is part of the second flow path. In another embodiment the tube bundles may be arranged asymmetrically, e.g. the density of the tube bundles increases from the inlet of the second gas flow and/or in the direction of the outlet of the second gas flow, allowing for an increased flow of the second gas flow, because the gap between the bundles is enlarged. Each tube bundle comprises a set of multiple tubes. These tubes are arranged parallel to the longitudinal axis of the shell and at a distance from one another. Typically the set of multiple tubes in a tube bundle are arranged according to a regular pattern at a fixed distance (tube pitch), such as according to a triangular or square design. Generally the tube pitch is (much) smaller than the bundle pitch. Internally the tubes define a flow path (tube side) for the first gas flow and externally the tubes in combination with the housing define a flow path (shell side) for the second gas flow. Thus the space between the tube bundles within the housing and the space between the tubes in the tube bundles are in fluid communication with each other and form the flow path of the second gas flow. At an entry end of the housing a first plenum (or front header) for receiving the first gas flow and distributing thereof into the number of tubes of each tube bundle is arranged. The first plenum has an inlet for feeding the first gas flow. In the housing also a second plenum (typically in a single pass configuration a rear header at the opposite end of the housing) for receiving and collecting the first gas flow after heat exchange is arranged, that is provided with an outlet for discharging the first gas flow. The two plenums are in communication with each other via the individual tubes of the tube bundles. Respective partitions separate the heat exchanger chamber from the first plenum and the second plenum. The partitions have bundle openings for accommodating the plurality of tube bundles in a sealing manner. Thus gas is prevented from passing from a plenum into the heat exchanger chamber and vice versa.

According to the invention the heat exchanger chamber is provided with at least one separation plate at a separation position. The separation plate has openings, through which the plurality of tube bundles extend. This separation plate extends from the inner wall of the housing in the gaps between adjacent tube bundles. Thus the separation plate surrounds the periphery of the tube bundles and covers the cross section of the heat exchange chamber except for the tube bundles. The separation plate forces the second gas flow in a direction that deviates from the longitudinal direction. In particular the separation plate provides a cross vector component to the second gas flow resulting in a flow into the tube bundles and thus the separation plate blocks the direct flow of the second gas flow in the longitudinal direction outside the periphery of each tube bundle. In other words, the second gas flow is bended away from a longitudinal direction parallel to the tubes of the tube bundle. For sake of convenience hereinafter the changed direction is also called cross flow. As a result the second gas flow is forced to change directions from parallel (typically countercurrent) flow to cross flow with respect to the heat exchanger tubes in the tube bundles.

According to the invention also each tube bundle is provided with at least one baffle at a baffle position at a longitudinal position different from the position of the separation plate, having baffle openings through which the tubes of the tube bundle extend. The baffle plate forces the second gas flow in cross flow direction outwardly from the space in the tube bundle towards the gaps between adjacent tube bundles, thereby locally inhibiting flow of the second gas flow between the tubes of the respective tube bundle in the longitudinal direction of the housing within the entire periphery of that tube bundle at the position of its baffle. The heat exchanger according to the invention with one or more separation plates and one or more baffle plates is especially beneficial for heat transfer over longer temperature trajectories.

By providing one or more separation plates and spaced apart tube bundles the detrimental feedback loop can be prevented, allowing to design a dense pattern of tubes having a relatively short length without the limitations of an unpractical pressure drop at the shell side gas. Thus the invention allows a compact design. In particular, the design of a gas-gas heat exchanger according to the invention allows to scale up an existing facility without requiring essentially higher or longer space and thus longer tubes. Thus the volumetric footprint of the gas-gas shell and tube heat exchanger is relatively reduced in relation to the heat transfer capacity.

In the heat exchanger according to the invention due to the at least one separation plate and the at least one baffle in each tube bundle, the heat transfer is improved at essentially the same pressure drop of the second gas flow allowing to upscale the heat exchanger for higher heat exchanging capacity at higher volumetric flow rates compared to a heat exchanger without separation plate and baffle, if any. Furthermore the heat exchanger according to the invention has a number of benefits with respect to replacing existing heat exchangers in existing constructions, and/or refurbishing thereof. The total weight (or weight per m ² ) and height of the heat exchanger according to the invention is less compared to a standard design of a shell and tube heat exchanger. As a result additional heat exchanging surface area may be fitted in an existing construction, which could also be a plastic cold end part for cooling down a flue gas below its acid dew point, as will be explained below in more detail.

Preferably a first separation plate and a second separation plate are provided that are arranged on either side of the at least one baffle at a longitudinal distance from the at least one baffle. More preferably, the separation plates and baffles are arranged in an alternating order, with the proviso that the alternating order starts and ends with a separation plate. As the second gas flow is introduced from the outside of the shell, and typically in the outer peripheral space between the shell and the plurality of tube bundles the flow resistance is low, the partial flow of the second gas through this space is also guided by the most upstream separation plate, as seen in the direction of the second gas flow, into the tube bundles, thereby preventing the second gas flow from bypassing the tube bundles. The same applies to the most downstream separation plate in relation to the outlet of the second gas flow.

Here it is noted that a heat exchanger design having a single tube bundle, having alternating donut and disc shaped baffles is known. In this known design the donut disc shaped baffle extend from the shell interior wall into the tube bundle, while the disc shaped baffle does not extend up to the periphery of the tube bundle.

The shape of the shell is not limited. It may have a circular or oval cross section like a round cylinder shell, or a polygonal cross section, like triangular, rectangular such as square or pentagonal, hexagonal or decagonal.

In an advantageous embodiment the gas-gas shell and tube heat exchanger according to the invention is a single pass heat exchanger, wherein the first and second plenum are arranged at opposite ends of the heat exchanger.

In order to distribute the second gas flow over the shell space at the entry side the heat exchanger chamber may have multiple inlets, such as two inlets, for feeding the second gas flow. For similar reasons, at the exit side the heat exchanger chamber may be provided with multiple outlets. Advantageously the number of inlets is equal to the number of outlets.

In a further or alternative embodiment the density of the tube bundles increases from the inlet for feeding the second gas flow over at least a part of the flow path of the second gas flow. Typically the second gas flow inlet will be in fluid communication with a feed section of the heat exchanger chamber, and the outlet of the second gas flow with a discharge section of the heat exchanger chamber. Due to the larger space (gap) between the tube bundles in the feed section the second gas flow rate may be increased without jeopardizing the pressure drop. In an embodiment thereof wherein the inlet and outlet of the second gas flow are at the same side the density of the tube bundles will also decrease in the discharge section, further contributing to the possibility of increased flow rate of the second gas flow.

In an advantageous countercurrent embodiment of the gas-gas shell and tube heat exchanger according to the invention the at least one outlet of the second gas flow is arranged at the end of the heat exchanger chamber, adjacent the first plenum. Advantageously the at least one inlet of the second gas flow is arranged at the end of the heat exchanger chamber, adjacent the second plenum.

The at least one separation plate is preferably fixedly mounted to the inner wall of the housing, more preferably in a sealing manner, although some leakage is allowable as the main stream of the second gas flow is directed inwards into the plurality of tube bundles.

In a preferred embodiment the assembly of a tube bundle and its at least one baffle is releasably and retractably arranged in the heat exchange chamber, allowing easy inspection and maintenance including mechanical or chemical cleaning of the outer surfaces of the individual tubes, and/or repair and/or replacement.

In a further preferred embodiment thereof the ends of the individual tubes of a tube bundle are arranged in tubesheets, wherein one tubesheet has dimensions larger than the accommodating opening in a partition. In a vertical arrangement of the gas-gas shell and tube heat exchanger according to the invention this tubesheet rests in a sealing manner on the upper partition, typically in the first plenum. This tubesheet may also be fixedly mounted on this partition. In order to allow release and retraction the tubesheet at the other end has dimensions smaller than the opening in the at least one separation plate. Appropriate sealing of the tubesheets in the openings of the partitions may be accomplished using a flexible seal, e.g. an O-ring that is partly accommodated in a recess in the wall of the partition, which wall delimits the opening.

Advantageously the baffle in a tube bundle is connected to the number of tubes in the tube bundle in a fluidly tight manner. However, some “leakage” through the baffle is allowable, e.g. when using so called orifice baffles.

In a preferred embodiment, in particular in view of cooling acidic flue gasses, preferably an overall countercurrent embodiment, a tube bundle comprises a first tube bundle part and a second tube bundle part, wherein the tubes, e.g. made of metal, of the first tube bundle part are at one end in fluid communication with the first plenum and at the opposite end in fluid communication with a first end of the tubes of the second tube bundle part, which tubes, made of a corrosion resistant material (e.g. plastic, glass, or (metal) tubes internally coated with a corrosion resistant coating like glass or (fluor containing) polymer), of the second tube bundle part at the second end are in fluid communication with the second plenum, wherein at the connecting position of the first tube bundle part to the second bundle part connecting means configured to allow the fluid communication of the tubes of the first tube bundle part with the tubes of the second bundle part and configured for guiding the second gas flow outwardly from the second (corrosion resistant) tube bundle part to the first tube bundle part. In this “cold end" embodiment the second bundle part comprising corrosion resistant tubes forms a cold end, allowing cooling an acidic flue gas beyond its acid dew point, without the risk of corrosion of the second tube bundle part due to condensed acid.

In an embodiment the connecting means comprise an O-ring chamber engaging a first part tubesheet of the first tube bundle part and a second part tubesheet of the second tube bundle part. The first part tubesheet and the second part tubesheet block the second gas flow in the longitudinal direction, while the O-ring chamber connects the respective tubesheets of the first tube bundle part to the tubesheets of the second tube bundle part. Thus the tube ends of the tubes of the first tube bundle part are in fluid communication with the tube ends of the tubes of the second tube bundle part through the O-ring chamber. Typically the O-ring chambers would be mounted to strips having openings for the tube bundle parts. The O-ring chambers also allow expansion of the tube bundles of the first and second bundle parts. In an alternative embodiment the connecting means comprise a single tubesheet that engages the respective tube ends of the first tube bundle part on one side and the tube ends of the second tube bundle part at the opposite side thereof. In yet another embodiment the connector means comprise a direct connection, such as a weld or adhesive connection between the tube ends of the first and second tube bundle parts, as well as strips or plates having openings for the connected tubes.

The invention also relates to a method of performing heat exchange between a first gas flow and a second gas flow using the gas-gas shell and tube heat exchanger according to the invention, wherein the first gas flow is fed to the at least one gas inlet of the first plenum, a second gas flow is fed to the gas inlet of the heat exchange chamber, the first gas flow is allowed to pass through the tubes of the plurality of tube bundles to the second plenum and is subsequently discharged, while the second gas flow is allowed to pass through the heat exchange chamber and in between the tubes of the tube bundles towards the gas outlet in an alternating pattern of parallel flow and cross flow to the plurality of tube bundles.

From viewpoint of heat transfer, generally the second gas flow fed to the chamber has a lower temperature than the first gas flow that is fed to the first plenum.

The heat exchanger and heat exchanging method according to the invention can be employed for any heat exchange between two gases. A typical application would be (pre) heating or cooling a process gas, such as preheating air that is to be used in a combustion process. Another example is drying processes with air. A further example is cooling of flue gas as the first gas flow, preferably using the cold end embodiment as explained above, wherein in the first tube bundle part the first gas flow being flue gas is cooled to a temperature above the acid dew point of the flue gas and in the second tube bundle part the flue gas is further cooled to a temperature below the acid dew point of the flue gas.

The invention is further illustrated by means of the attached drawings, wherein: Fig. 1 diagrammatically shows a first embodiment of a gas-gas heat exchanger according to the invention;

Fig. 2 shows a cross section of the embodiment of Fig. 1 ;

Fig. 3 is an embodiment of a tube bundle of the first embodiment of a gas-gas heat exchanger according to the invention;

Fig. 4 shows an embodiment of a baffle of a gas-gas heat exchanger according to the invention;

Fig. 5 is an embodiment of a separation plate of a gas-gas heat exchanger according to the invention;

Fig. 6 is a second embodiment of a gas-gas heat exchanger according to the invention;

Fig. 7 is a cross section of a third embodiment of a gas-gas heat exchanger according to the invention;

Fig. 8 is a fourth embodiment having a corrosion resistant cold end section of a gas-gas heat exchanger according to the invention; and

Fig. 9 shows an embodiment of a connection plate of the fourth embodiment.

In the drawing the same or similar components of the device are indicated by the same reference numeral.

In Fig. 1 a diagram of a first embodiment of a heat exchanger for heat exchange between a first and second gas flow according to the invention is indicated in its entirety by reference numeral 10. The heat exchanger 10 comprises a housing or shell 12, that delimits a heat exchanger chamber 14. At the top end of the heat exchanger chamber 14 a first (entry) plenum 16 is provided, which has an inlet 18 for feeding a first gas flow (indicated by arrow A). At the bottom end of the heat exchange chamber 14 a second (exit) plenum 20 is provided, which has an outlet 22 for discharging the first gas flow after heat exchange from the heat exchanger. The plenums 16 and 20 are separated from the heat exchanger chamber 14 by partitions 24 and 26 respectively. At the lower end of the heat exchanger chamber 14 and adjacent to the second plenum 20 a gas inlet 28 for introducing a second gas flow (indicated by arrow B) into a feed compartment 29, delimited by partition 26 and lower separation plate 40 is provided. A gas outlet 30 for discharging the second gas flow from a discharge compartment 31 delimited by partition 24 and upper separation plate 40 of the heat exchanger 14 is provided at the upper end of the heat exchanger chamber 14 adjacent the first plenum 16. A plurality of tube bundles 32 are arranged vertically and parallel to one another in the heat exchange chamber 14. The broken lines indicate the periphery of a tube bundle 32. A tube bundle 32 comprises a number of parallel, individual heat exchange tubes 34 as shown in the embodiment of Fig. 3 in more detail. The first plenum 16 is in fluid communication with the second plenum 20 by means of the individual tubes 34 of the plurality of tube bundles 32. These tube bundles 32 are arranged at a distance from one another leaving free open space in the heat exchanger chamber 14, also indicated as shell space or gap, which in its entirety bears reference numeral 36. The interior of the individual tubes 34 is the flow path of the first gas flow. The exterior (that is to say space or gap 36) of the individual tubes 34 defines the flow path of the second gas flow within the housing 12. In this embodiment each tube bundle 32 has two baffles 38. In the gap 36 of heat exchanger chamber 14 at least one separation plate 40 is mounted to the housing 12, in the embodiment shown three plates 40. The configuration of alternating separation plates 40 and baffles 38 causes the second gas flow in a combined pattern of parallel flow and cross flow. See also Fig. 3 wherein the arrows indicate this flow pattern.

A baffle 38 is a plate like component that is provided with openings 42 (see Fig. 4) for accommodating the individual tubes 34. The baffles 38 are configured such that their dimensions essentially corresponds with the periphery of the cross-section of the respective tube bundle 32. In the embodiment shown the baffle spacing of the two baffles 38, in this case about one third of the tube bundle length in the heat exchanger chamber 14, is such that a tube bundle 32 in the heat exchange chamber 14 is divided in three sections 44 (see Fig.

3). In the embodiment shown the three separation plates 40 are arranged at a position midway the length of a section 44. The lower separation plate 40 is positioned near the position of the gas inlet 28 and the upper separation plate is positioned near the gas outlet 30. A separation plate 40 is a plate like element that is provided with openings 46 (see Fig. 5) for the plurality of tube bundles 32. A separation plate 40 is fixed to the inner wall of the shell 12 and covers essentially the whole cross section of the heat exchange chamber 14 except at the positions of the plurality of tube bundles 32.

Fig. 2 shows a cross section according to line I - I of the embodiment of a gas-gas heat exchanger of Fig. 1 in a diagrammatic way. As shown within the rectangular shell 12 a plurality of tube bundles 32 having a circular cross section are arranged. Fig. 2 also shows the separation plate 40. In Fig. 2 the individual tubes and baffles of a tube bundle 32 are not drawn.

Fig. 3 is a diagrammatic view of an embodiment of a tube bundle 32 of the gas-gas heat exchanger 10 according to the first embodiment of the gas-gas heat exchanger according to the invention. In this embodiment a tube bundle 32 comprises a number of individual heat exchange tubes 34, that are aligned parallel to one another. The upper ends of the tubes 34 are accommodated in a tubesheet 50 in a common manner and are in communication with the upper plenum (not shown in Fig. 3). The tubesheet 50 having cross dimensions larger than the co-operating partition opening 51 in upper partition 24 is releasably mounted to the upper partition 24. The lower ends of the tubes 34 are also accommodated in a tubesheet 52 and are in communication with the lower plenum (also not shown). The first gas flow passes through the individual tubes 34 from the first plenum to the second plenum. The baffles 38 separate the sections 44 of the tube bundle 32. At the outer periphery of the tube bundle 32 the separation plates 40 each covering the gap 36 in the heat exchanger chamber 14 and extending from the interior wall of the shell (not shown in Fig. 3) to the outer tubes 34 of the tube bundles 32 are positioned. At the openings 46 (see Fig. 5) in the separation plates 40 for the plurality of tube bundles 32 flexible seals 56 are positioned that hold the outer tubes 34 of a tube bundle 32. In a practical embodiment a flexible seal 56 is a ring having an inner circumference matching the contour of the outer tubes of a tube bundle as a whole. Such a ring may leave a tiny gap between its body and the opening 46 in the separation plate 40, wherein the tube bundle 32 is accommodated. The separation plates 40 are positioned such that a baffle 38 has an upstream separation plate and a downstream separation plate as seen in the direction of the flow of the second gas, which is indicated by the curved arrows in Fig.

3. The second gas flow entered to the gas inlet of the heat exchanger chamber is forced between the lower partition 26 that accommodates the tubesheet 52 by means of O-ring 58 in partition opening 60, and the lower separation plate 40 into the tube bundle 32. In the tube bundle 32 the second gas flow is allowed to pass the separation plate 40 in the longitudinal direction and then guided outwardly by means of the lower baffle 38. This changing of the direction from the second gas flow is repeated at each separation plate and baffle that are arranged further downstream. Thus the direction of the second gas flow is composed of an alternating cross and countercurrent flow with respect to the first gas flow in the heat exchanger tubes.

A tube bundle 32 including its baffles 38 and its tubesheets 50 and 52 is retractable as a single unit from the top end of the heat exchanger chamber.

Fig. 4 shows an embodiment of a baffle 38. The baffle 38 is a plate like component having openings 42 through which the tubes 34 extend. Generally a baffle 38 is fixed to the outer periphery of the tubes 34, such that an assembly of tube bundle 32 and associated baffles 38 can be retracted as a single unit or module from the heat exchanger.

Fig. 5 shows an embodiment of a separation plate 40. The separation plate 44 is a plate like element, that is provided with large openings 46, one for each tube bundle 32. At the circumference of the openings a flexible seal 56, e.g. O-ring seated in an appropriate circumferential recess, is mounted.

Fig. 6 shows a second embodiment of a gas-gas heat exchanger according to the invention, which is similar to the embodiment of Fig. 1 except that two gas inlets 28 and two outlets 30 for the second gas flow are provided.

Fig. 7 shows a cross section of a third embodiment of a gas-gas heat exchanger according to the invention, wherein the density of the tube bundles 32 increases from the inlet 28 of the second gas flow (arrow B), such that there is a larger space (gap) between the tube bundles 32 at the second gas flow inlet side of the heat exchange chamber. As a result the second gas flow rate may be increased without jeopardizing the pressure drop. Using this lay-out In the embodiment of a heat exchanger as shown e.g. in Figs. 1 and 3, the density of the tube bundles will decrease in the upper part also in the direction of the outlet and further helps to increase the flow rate of the second gas flow.

Fig. 8 shows a fourth embodiment having a corrosion resistant cold end section of a gas-gas heat exchanger according to the invention, in particular for cooling of flue gas that comprises acidic compounds e.g. sulphur and/or phosphor containing acids. The heat exchanger 10 comprises a housing or shell 12, that delimits a heat exchanger chamber 14. At the top end of the heat exchanger chamber 14 a first (entry) plenum 16 is provided, which has an inlet 18 for feeding a first gas flow (indicated by arrow A). At the bottom end of the heat exchange chamber 14 a second (exit) plenum 20 is provided, which has an outlet 22 for discharging the first gas flow after heat exchange from the heat exchanger. The plenums 16 and 20 are separated from the heat exchanger chamber 14 by partitions 24 and 26 respectively. At the lower end of the heat exchanger chamber 14 and adjacent to the second plenum 20 a gas inlet 28 for introducing a second gas flow (indicated by arrow B) into the feed compartment 29 of heat exchanger chamber 14 is provided. A gas outlet 30 for discharging the second gas flow from the discharge compartment 31 the heat exchanger 14 is provided at the upper end of the heat exchanger chamber 14 adjacent the first plenum 16. A plurality of tube bundles 32A are arranged vertically and parallel to one another in the heat exchange chamber 14A. The broken lines indicate the periphery of a tube bundle 32. In this embodiment a tube bundle 32 comprises an upper tube bundle part 32a comprising tubes (not individually shown) e.g. made of metal, and a lower tube bundle part 32b comprising corrosion resistant (e.g. plastic) tubes (also not individually shown). The upper bundle part 32a can be inserted in the chamber 14 from the top and is fixed with its tubesheet 50 to the partition 24, e.g. similar to the situation in Fig. 3. The lower bundle part 32b can be inserted in the chamber from the bottom and is fixed with its tubesheet 50 to the partition 26. The ends of the tube bundle of part 32a and 32b that face each other are mounted in tubesheets 52a and 52b respectively, which are connected by means of suitable connectors, in this embodiment O-ring chambers 70. An O-ring chamber comprises O-rings 72 seated in connector bodies 74, that are mounted to connector strips 76. Fig. 9 shows an example thereof. The exemplified connector strips 76 extend from one interior wall of the housing 12 to the opposite wall thereof and mounted thereto. The strip 76 is provided with openings 80 for the tube bundle 32A and 32B. Between a side wall and adjacent strip as well as between adjacent strips passages 82 for the second gas flow are present. The first plenum 16 is in fluid communication with the second plenum 20 by means of the individual tubes of the plurality of tube bundles 32 comprising tube bundle parts 32a and 32b. These tube bundles 32 are arranged at a distance from one another leaving free open space 36 in the heat exchanger chamber 14. The interior of the individual tubes is the flow path of the first gas flow. The space or gap 36 between the tube bundles and between the individual tubes in a tube bundle defines the flow path of the second gas flow within the housing 12. In this embodiment each tube bundle part 32a has two baffles 38 and in the upper gap 36a of heat exchanger chamber 14 three separation plates 40. In the corrosion resistant cold section provided with the tube bundle parts 32b comprising corrosion resistant tubes a single separation plate 40 is provided in space 36b.

Instead of the passages 82 an external connection of space 36b to space 36a for the second gas flow requiring an additional intermediate outlet of space 36b and intermediate inlet to space 36a could be provided. Then the flow restrictor would only have openings for the connected tube ends.