Heat exchanger tube bundle and related heat recovery steam generator

The present invention relates to a heat exchanger, such as an economi zer tube bundle for a heat recovery steam generator, preferably in hori zontal ( gas ) path design . Moreover, a heat recovery steam generator and a related combined cycle power plant are provided .

Heat exchanger tube bundles in heat recovery steam generators are used to warm up or heat an incoming fluid, condensate or feed water to almost saturated conditions . Di f ferent heat trans fer concepts result in di f ferent heating surface si zes , depending on the performance and ef fectiveness of the particular configuration . Moreover, cost and/or operational complexity of the steam generator system is af fected not only by the configuration of the heating surface of the related heat exchanger, but also by the complexity and ef fort required to connect each tube row or tube bundle to the next . At least for hori zontal path heat recovery steam generators - for which tube orientation is consequently vertical - it is important to maintain or achieve a proper optimum or minimum feed water velocity inside the tubes under the relevant operating conditions . For heat exchangers where the operating fluid is water, such as an economi zer, this is in order to avoid e . g . flow instability, like an irregular or even stagnant or reversed water flow . This may in turn lead to a signi ficant loss of performance , overheating of the related tube bundle and damage of the overall system .

Most common heat exchanger concepts for hori zontal heat recovery steam generators make use of a design with a series of similar racks . In order to maximi ze an ef fective surface area for heat exchange , several tube rows are usually connected to an inlet and to an outlet or related headers . During an operation of such heat exchangers , a fluid is guided through the tubes of the tube bundle . In case of heat exchangers operating with water, such as economi zers the fluid is particularly fed to the parallel tubes or tube rows of the bundle through the inlet header and e . g . collected in the outlet header . In order to generate or uphold a favorable velocity of water passing through the tube bundle , e . g . partition plates are usually welded inside the headers .

Such partition plates aim to create a number of subsequent circuits ( tube passes ) in which the feed water is forced to pass several times through the same rack in up and down flow direction . Subsequent racks or so-called "harps" are connected to each other by means of external piping elements , or so- called "crossovers" for instance . A main disadvantage of this principle is its low ef fectiveness in heat recovery or exchanging heat with an exhaust or flue gas , such as an exhaust gas of a gas turbine . This is due to the inherently unfavorable cross flow properties of the design, rendering the principle inef ficient as regards heat recovery . Particularly, not all of the exhaust gas can - according to this principle - be exposed to the cold water and therefore be cooled to the lowest possible temperature . Consequently, heat recovery ef ficiency or as the case may be the amount of exchanged heat is reduced as compared to a principal achieving a lower number of tube passes per row .

Another known concept provides single tube rows which are connected in a series to inlet and outlet headers having usually smaller dimensions . The latter concept provides the advantage of dispensing with crossovers and of a higher overall ef ficiency of heat recovery . However, a drawback of this concept relates to its constructability being much less compact than the above-mentioned one . In turn, this also leads to an increased overall cost . Another disadvantage lies in the impossibility to tune water or fluid velocity, for an acceptable pressure drop, towards a balance or stable state which can be facilitated by partition plates . It is an obj ect of the present invention to provide an optimi zed heat exchanger design, particularly a design of an improved heat exchanger tube bundle . Preferably, the presented novel configuration combines the advantages of known methods , whereas - at the same time - its drawbacks can be reduced or prevented at all .

The mentioned obj ect is achieved by the subj ect-matters of the independent claims . Advantageous embodiments are subj ectmatters of the dependent claims .

An aspect of the present invention relates to a heat exchanger tube bundle of hori zontal gas path design, the tube bundle comprising a sequence or series of bottom headers and corresponding top headers , wherein each (header ) unit of the sequence comprises a row of tubes for exchanging heat like a flue gas , wherein each bottom header is fluidly connected to a corresponding top header via at least two similar tube rows for passing a fluid, like e . g . feed water, in a first direction between the bottom header and the top header or through the similar tubes of the row, respectively . Each unit of the sequence further comprises at least one further tube row in fluid connection with ( only) one of said bottom and top header, wherein the further tube row is further fluidly connected to a header of a subsequent or approximate unit , and wherein the further row' s tubes are configured for passing the fluid in a second direction opposite to the first direction . The heat exchanger tube bundle is preferably configured for an application of a heat recovery steam generator . Particularly, the provided configuration of flow tubes inherently allows to tackle the above-mentioned challenges , and therewith to provide for a far improved heat exchanger design . In this way, the stringent demands of today' s energy transition and related challenges for ef ficient components is addressed .

In an embodiment , the heat exchanger tube bundle comprises only two similar tube rows and only one further tube row . This advantageously allows for an easy, robust and simple construction, and at the same time to tailor and/or optimize flow, particularly downflow, stability and to avoid any related feed water flow reversion and/or disturbance.

In an embodiment, the heat exchanger tube bundle is configured for only one fluid pass per tube, or as the case may be, per tube row.

In an embodiment, the similar row's tubes are configured for an upflow fluid pass, and the further row's tubes are configured for a downflow fluid pass.

In an embodiment, the similar row's tubes and the further row's tubes are all of similar dimension.

In an embodiment, the similar row's tubes and the further row's tubes are all of equal or similar cross section.

According to these embodiments, upflow and downflow dynamics are already (or can be) optimized by the presented tube setup, e.g. featuring a favorable balance of up- and downflow behavior. Similarly, flow instabilities can advantageously be avoided, or a stable operating range extended.

In an embodiment, the similar row's tubes are of equal dimension and a cross section of the further row's tubes differs from that one of the similar row's tubes. Likewise, this embodiment advantageously enables to maintain stable upflow dynamics, and to reduce flow instabilities.

In an embodiment the cross section of the further row' s tubes is smaller than that one of the similar row's tubes. This can be achieved e.g. by selecting a smaller tube diameter or fewer tubes per row of the downflow tubes.

These embodiments further enable an optimization of flow dynamics through the tubes during an operation of the heat ex- changer. At the same time, heat transfer efficiency can advantageously be optimized.

In an embodiment, the heat exchanger tube bundle comprises more than two similar tube rows per (header) unit.

In an embodiment, the heat exchanger tube bundle comprises more than two further tube rows per unit.

In an embodiment the heat exchanger tube bundle is free of partition plates, crossovers and/or external piping. This embodiment advantageously eases construction, particularly saves material and additional welding effort, and also allows to achieve an improved heat transfer efficiency (see above) . A further aspect of the present invention relates to a heat exchanger comprising the tube bundle as described herein.

A further aspect of the present invention relates to a heat recovery steam generator of horizontal gas path design comprising the heat exchanger tube bundle, wherein the heat exchanger tube bundle is functionally set up for transferring (waste) heat from a flue gas, such as exhausted by a gas turbine, to a feed water when passing it through the tube bundle .

A further aspect of the present invention relates to a combined cycle power plant, wherein the heat exchanger tube bundle is applied in an economizer and/or an evaporator of the power plant. Expediently, the power plant may to this effect comprise a steam turbine and a gas turbine or be functionally coupled to said turbines.

A still further aspect of the present invention relates to a method of heat recovery or heat transfer from an exhaust gas in the heat exchanger, such as within an intended operation of the heat exchanger or heat recovery steam generator. To this effect, the exhaust gas may be provided by a gas turbine engine or similar entity. In fact any source of (waste) heat, even when provided otherwise than through a gas turbine exhaust , can be contemplated . Moreover, the generated steam may be used as a working fluid in a steam turbine and/or as process steam for any other industrial application .

Advantages and embodiments relating to the described heat exchanger ( tube bundle ) and/or the described heat recovery steam generator may as well pertain or be valid with regard to the power plant and vice versa .

Further, features and advantageous embodiments become apparent from the following description of the exemplary embodiment in connection with the Figures .

Figure 1 shows a simpli fied perspective view on a heat exchanger design as economi zer of the prior art .

Figure 2 shows a simpli fied side view of another heat exchanger design as economi zer of the prior art .

Figure 3 indicates - similarly to Figure 2 - a simpli fied side view of a heat exchanger tube bundle of the present invention .

Figure 4 indicates in a simpli fied sketch a combined cycle power plant , along with an inventive heat exchanger as shown in Figure 3 being functionally integrated to the power plant process .

Like elements , elements of the same kind and identically acting elements may be provided with the same reference numerals in the Figures . The Figures are not necessarily depicted true to scale and may be scaled up or down to allow for a better understanding of the illustrated principles . Rather, the described Figures are to be construed in a broad sense and as a qualitative base which allows a person skilled in the art to apply the presented teaching in a versatile way . The term „and/or" as used herein shall mean that each of the listed elements may be taken alone or in conj unction with two or more of further listed elements .

Figure 1 shows a known principle of a heat exchanger, particularly an economi zer 10 for a heat recovery steam generator (HRSG) application in a hori zontal gas path design . The heat exchanger as shown preferably relates to a so-called "harp" design . The component 10 comprises a plurality of such harps or racks , wherein two racks are exemplarily depicted in Figure 1 in a series connection, which are connected to each other by means of external piping elements 6 , also called "crossovers" . In order to maximi ze a heat trans fer-ef fective surface for each rack, several rows of tubes 3 are connected to inlet and outlet collecting headers 2a, 2b, respectively .

For the sake of simplicity, tubes 3 are only depicted schematically, wherein the dashed lines shall indicate an interspace and/or separation between di f ferent tubes in a row .

Reference numeral 4 as used herein shall particularly denote a so-called upflow tube , whereas reference numeral 5 shall indicate a downflow tube , all of which characteri zing the hori zontal path design of heat recovery steam generators for the related heat exchanger section .

During an operation of the economi zer, incoming water (not explicitly indicated) is fed to the parallel tubes 3 through the respective inlet header and collected in the corresponding outlet header ( cf . as well arrows in the Figures indicating flow direction of the feed water during an intended operation .

In order to ensure a favorable feed water velocity partition plates 9 ( only partially indicated in the Figures ) are welded inside the headers 2 , which ef fectuate creation of a number of subsequent or separated water circuits - also called "passes per row" or "tube passes" . According to this embodi- ment of the economizer, the partition plates 9 force the feed water to pass several times, e.g. twice, through the same rack in up and down flow direction. Thus, for each rack, a number of tubes are grouped in the design of the heat exchanger as shown in Figure 1.

Furthermore, numeral 7 indicates a general feed water inlet, whereas numeral 8 indicates a related water outlet in the depicted tube bundle 10. Depending on the intended fluid flow direction, an inlet may principally relate to an outlet, and vice versa.

A main technical drawback of this configuration is its quite complex design and relatively low effectiveness in exchanging heat with the exhaust gas, the course of which is indicated by the larger arrow and numeral G. In fact, since not all exhaust gas G can be exposed to the coldest, i.e. upstream, feed water, also not the whole share of heat of gas G can be extracted by the heat exchanger 10 and transferred to the feed water. In other words, the gas can in this way not be cooled to the lowest possible temperature; or as the case may be, the water heated to the maximum possible temperature. Consequently, heat exchange is reduced due to this less favorable crossflow geometry, as compared to the inventive design as shown in Figures 3 onwards, for instance.

Figure 2 shows an alternative (known) heat exchanger concept that may pertain to a so-called "harmonica"-type of economizer used for horizontal path HRSG applications. Therein, single, parallel tube or tube rows 3 are arranged in a series and each of which (or each row) connected to a top header 2a and an bottom header 2b. Each bottom header 2b is preferably fluidly connected via an upflow tube 4 with the corresponding top header 2a. Such a sequential unit 1 of the whole component 10 is encircled by a dashed square. In total, six sequential units or tube elements are shown connected in series. The larger square (not explicitly indicated) between the headers 2a, 2b indicates an exhaust or flue gas channel , the economi zer is , preferably subj ected to in its intended operation, such as an operation of a related steam generator . Again, the arrow G indicates a possible direction of exhaust gas for the expedient heat trans fer .

As compared to the "harp-design" of Figure 1 , this concept stands out for a higher heat trans fer ef ficiency due to a lack of cross flow loops . However, a main disadvantage relates to the constructability and/or a less compact design which increases the overall costs . Another disadvantage compared to the harp design is the impossibility to tune feed water velocity and a related flow stability, for an acceptable pressure drop, due to a lack of partition plates or similar measures .

Figure 3 illustrates a simpli fied sketch of a heat exchanger design featuring a tube bundle according to the present invention . Said heat exchanger 10 may as well be set up for heat recovery steam generators ( cf . Figure 4 below) . This tube bundle 10 comprises a sequence 1 of bottom headers 2b and corresponding top headers 2a, wherein each unit la of the sequence comprises a row of tubes , wherein each bottom header 2b is fluidly connected to a corresponding top header 2a via at least two similar tube rows 3 for passing a fluid in a first direction between the bottom header 2b and the top header 2a, respectively . The presented heat exchanger 10 is further characteri zed in that each unit of the sequence further comprises at least one further tube row 3 in fluid connection with one of said bottom and top header, wherein the further tube row is further fluidly connected to a header of a subsequent unit lb . The further row' s tubes 3 are configured for passing the fluid preferably in a second direction opposite to the first direction ( cf . again arrows indicating the flow directions of a fluid guided through the tube rows in an operation of the heat exchanger 10 ) . As shown in Figure 1, this configuration of heat exchanger tube bundle comprises a number of only four connected serial header units.

In contrast to the known bundle configuration as described by way of Figure 2, particularly two upflow tubes 4 (each of which shall indicate a whole row of tubes connecting the bottom header and the top header in Figure 3) are arranged in fluid communication for a fluid or feed water flow from a given bottom header 2b to the top header 2a, respectively.

Preferably, as indicated in Figure 3, the tube bundle 10 comprises only two upflow tubes 3, 4 and only one downflow tube 3, 5 per unit. It is further shown that the tube bundle 10 is configured such that it allows for only one fluid or water pass per tube or tube row.

Of course, it is contemplated by the present invention, that the fluid flow direction could as well be established vice versa, i.e. such that there are two (similar) downflow rows 3, 5 per unit and e.g. only one (further) row set up for an upwardly guided fluid.

In a preferred embodiment of the present invention, the similar row's tubes 3, 4 and the further row's tubes 3, 4 are all of similar dimension, particularly of similar or equal cross section .

Alternatively and deviating from the indication of Figure 3, it is contemplated that only the similar tube rows 3, 4 (pointing upwards) are of equal dimension and e.g. a cross section of the further tubes 5 (pointing downwards) differs from that one of the similar tubes. More particularly, it may be advantageous - for optimization of the fluid flow and the avoidance of flow instability in any type of exchanger application - that e.g. the downflow tube's cross section (s) is/are smaller than that one of the upflow tubes 4. This advantageously allows to keep the downflow velocity at an opti- mum and/or a related downflow throughput ( through the tubes ) at a minimum, like in case of heat exchangers applied in an industrial economi zer and/or evaporator . In turn, thereby instabilities in the feed water flow are reliably avoided .

Preferably, the inventive heat exchanger is free of any partition plates , crossovers and/or external piping, thereby maintaining a compact layout .

As well , the design of the inventive tube bundle may be configured such that it comprises more than two similar up- or downflow tubes 3 or related tube rows connecting a given bottom header 2b and the corresponding top header 2a .

Conditionally, the inventive advantages may also mani fest in a design with a plurality of further up- or downflow tubes (or related rows ) per header unit . This would, however, most probably imply as well a plurality, particularly a larger number, of the respectively other type of tube rows .

The inventive design stands further out for an optimi zed number of top headers 2a and bottom headers 2b . As shown by way of example only, the depicted heat exchange tube bundle 10 comprises a number of four bottom headers 2b and/or a number of four, top headers 2a . It is emphasi zed that the technical advantages are preferably intrinsic to the described configuration, e . g . number and si ze , of headers , upflow and downflow tube rows with which fluid flow properties may be tailored better than with any known heat exchanger design to the respective application .

The inventive heat exchanger may relate to a so-called "Accordion" economi zer design, advantageously combining the design compactness and constructability of the harp design and the ef fectiveness of a harmonica design . Moreover, it allows to ef fectively avoid downflow instability by allowing a speci fic design of the downflow row' s tube si ze , i . e . independently from the other tube rows . This results in a more compact , robust and easily constructed bundle with a far greater potential of heat recovery in the operation of related steam generators or a higher-ranking power plant ( cf . Figure 4 below) . Particularly, as compared to the design shown in Figure 1 , the inventive concept excels in a better heating surface ef fectiveness and - at the same time - in a simpler construction due to a lack of crossover connections . Both advantages , in turn, result also in a cost advantage .

As compared to the "harmonica" design as illustrated in Figure 2 , furthermore , a bundle compactness is increased due to a lower number of headers , and to a shorter module depth . Consequently, the steam generator' s overall dimension can also advantageously be reduced and shop constructability improved .

Figure 4 shows a simpli fied sketch of at least parts of a combined cycle power plant 100 illustrating the basic principle of its assembly, e . g . comprising a heat exchanger 10 , such as the inventive heat exchanger configured for an application of heat recovery steam generator 40 . Also an evaporator assembly 20 as well as a superheater 30 are shown . All of these components are preferably functionally coupled in an expedient way . Moreover, the plant 100 may comprise or be connected to the flue gas path of a gas turbine GT via which an exhaust gas G is exposed to the superheater 40 ( or vice versa ) . Then the superheater 30 is connected or coupled to a hot steam path - fed in by a steam drum 21 of the evaporator section 20 . The steam ST is then expediently provided to a steam turbine of the plant 100 , or to another facilty . Downstream of the superheater assembly 30 - also comprising dedicated tube bundles for steam generation and/or heat trans fer - the exhaust gas G enters the tube bundle of the evaporator 20 . Later, in a rather low temperature section, the gas G finally arrives at the economi zer tube bundle 10 for ef ficient heat trans fer operation of the plant 100 . However, the assembly could also be di f ferent , such as using the heat exchanger 10 in another setup, like in a configuration for a heat trans fer from any kind of waste heat and the generation of related process steam (without any superheater or turbines involved) . Thus , besides the above-described (plant ) applications , the improved heat exchanger design imparts the outlined technical advantages as well to other application, such as downscaled and/or domestic application of heat exchangers .