EP3593077B1

EP3593077B1 - Variable passes heat exchanger for organic rankine cycle systems

Info

Publication number: EP3593077B1
Application number: EP18719640.7A
Authority: EP
Inventors: Mario Gaia; Roberto Bini; Isaia CASALI; Andrea Buttani; Paolo Belotti
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2017-02-20
Filing date: 2018-02-17
Publication date: 2022-03-09
Anticipated expiration: 2038-02-17
Also published as: WO2018150380A1; EP3593077A1; IT201700018674A1

Description

Technical field of the invention

The present invention relates to a variable passes heat exchanger used, in particular, in organic Rankine cycle plants (ORC). The subject heat exchanger will therefore be controlled either by the heating and/or vaporization phase or by the condensation phase of an organic working fluid of the ORC cycle. The present invention, as will be seen, is in any case also applicable to heat exchangers having any other type of application.

Known art

As is known, a heat exchanger is a thermal device allowing the exchange of heat between two fluids, in particular from the cooling fluid with higher temperature to the fluid at lower temperature which then heats up. In an organic Rankine cycle plant, for example, in the heat exchanger heating and/or vaporization (in some cases even overheating) of the organic working fluid takes place thanks to the heat supplied by a higher temperature fluid. Said fluid can comprise either a high temperature thermal source in the gaseous or liquid phase (for example, biomass combustion products, geothermal source, etc.) and in such case we speak of a direct thermal exchange, or it can consist of a fluid (for example, a diathermic oil) circulating in an intermediate circuit between the high temperature source and the organic working fluid. The diathermic oil, heated by the high temperature source, in turn provides the heat to the organic working fluid. Furthermore, in a heat exchanger for an ORC plant, vapor condensation of the organic working fluid can occur, for example, by cooling water which when heated can be supplied to a thermal user.
Independently from the type of fluids used, a heat exchanger is commonly defined a "shell & tube " when one of the two fluids flows inside a tube bundle the tubes of which, in general, are arranged parallel to each other, whereas the second fluid is located outside the tube bundle and is confined in an outer casing, called a shell.
The first fluid can pass the tube bundle several times within the exchanger itself and such crossings are called passes. The number of passes, i.e. the number of passages in the pipes, of a heat exchanger is decided during the sizing process, based on different parameters.
Let us consider for example the evaporator of an ORC plant the thermal source of which is represented by geothermal water. In such case, water flows inside the tubes (which are more easily cleaned using brushes), whereas the organic working fluid evaporates within the shell.
The choice of the number of passes is a compromise between heat exchange performance and load losses due to the crossing of the tube bundle. As a matter of fact, once defined the flow rate of the fluid flowing through the tubes and the number of tubes of the tube bundle, by increasing the number of passes of the fluid it will distribute on a smaller number of tubes for each pass, and then its speed and the heat exchange coefficients will increase. Clearly, the increase in speed, however, leads to an increase of load losses, as well as possible erosion or vibration problems.
In a "kettle" type evaporator the organic liquid fluid is practically stationary in the shell, which has a cross section larger than the minimum size which would be necessary for accommodating the tube bundle. In such case, the criterion for choosing the number of passes on the pipe side is reduced according to what said above, i.e. to the optimization of the compromise between performance and load losses.
In cases in which the shell & tube exchanger has also (or just) the purpose of preheating the organic fluid in the liquid phase, it may be advantageous to use transverse baffles also on the shell side. When one or more transverse baffles are present, the organic working fluid is also moving and takes several passes; substantially, the liquid is forced by the baffles to flow into narrower cross sections of the overall cross section of the exchanger shell, with a deliberately tortuous path. The choice of the number of passes is therefore more complex: as a matter of fact, in addition to always considering the increase of the load losses, in such case the increase in the number of passes does not necessarily imply an increase in thermal performance, as in some places the organic working fluid is flowing in a direction globally equal to that of the geothermal water inside the tubes.
Furthermore, it must be considered that often the geothermal water flow rate of an ORC plant (the same obviously applies for the flow rate of cooling water in the condenser of the same ORC plant) is not constant, but can undergo even very substantial variations, both during the year and over the years . The change in the water flow rate therefore involves the removal from the optimal conditions sought by sizing the number of passes; furthermore, especially in geothermal plants, if the water speed is too low, there is a greater probability of having problems with fouling/encrustation of the tubes.
It is therefore important to be able to vary the number of passes on the tube side of a heat exchanger even during the operation of the ORC plant.
Figure 1 shows a shell & tube exchanger with straight and two-passes tubes according to the known art. According to a known convention, "X" indicates an arrow seen from the tail representing the flow, then entering with respect to the sheet, whereas "·" indicates an arrow seen from the tip, then representing the outgoing flow with respect to the sheet.
Let us consider a kettle type ORC evaporator in which the working organic fluid (fluid) is located on the shell 13 side, whereas the source fluid (a source, for example, relates to a diathermic oil or a geothermal water) enters the left-hand head 70, which is divided in two portions, marked as 1 and 3, and passes through the tube bundle 80. In particular, the source fluid passes in a suitable flanged nozzle and enters the portion 1 of the left-side head, then passes through a first portion 81 of the tube bundle 80 (first "pass", placed in the upper half of the heat exchanger) and then finally into the right-hand head 70', the only portion of which is marked as 2. From the right-hand head 70' then the source fluid flows into a second portion 82 of the tube bundle 80, arranged in the lower half of the heat exchanger (second pass) and then returns to the left-hand head, more precisely into the portion 3, from which it then exits through an outlet nozzle.
Figure 2 shows the same exchanger, but with four passes on the tube side. Substantially the source fluid flows through the tubes four times inside the exchanger, therefore the left-hand head 70 is divided into three portions 1, 3, 5 , whereas the right-hand one is divided into two portions 2, 4.
In particular, flow enters the portion 1, passes through a first tube bundle which carries it to portion 2; from 2 it returns to the head 70, in particular to portion 3, by passing through second tube bundle. Then the fluid passes through a third tube bundle which connects the lower part of portion 3 with portion 4 of head 70'; finally it returns to portion 5 of the head 70 by passing through a fourth tube bundle. Symbols "X" and "·" will help to understand the flow direction within the various passes according to the convention explained above.
As can be seen from figures 1 and 2, in order to convert the heat exchanger from a two-passes to a four-passes configuration, it would be sufficient to change the arrangement of the separating baffles which divide the two heads into different volumes and thus determine the fluid distribution into the tubes.
Separating baffles are usually sheets welded to the walls of the head itself and the tube bundle plate, therefore it is impossible to change their position during operation of the plant.
US patent 4,105,065 (A ) indicates how it is possible not to weld the separating baffles inside the housing of the heat exchanger, but to realize them as removable plates which are inserted into the housing by means of suitable guides. Also in such case however, in order to change the number of passes, it is necessary to stop the system and empty the heat exchanger. US5178102 A and CN103175347 A disclose relevant prior art.
There is therefore a need to define a heat exchanger in which it is possible to modify the number of passes during the operation of the same, without requiring the ORC system to stop and/or the heat exchanger to be emptied.

Summary of the invention

Subject of the present invention is therefore an innovative variable passes heat exchanger, the passes variation of which is made during the operation of the heat exchanger, either according to a predetermined frequency or according to a control method which continuously calculates the number of passes optimizes performances, as specified in the appended independent claim.
Dependent claims define particular and further advantageous aspects of the invention.

Brief description of the drawings

Different embodiments of the invention will now be described, by way of examples, with reference to the accompanying drawings in which:

Figure 1 shows a shell & tube heat exchanger with straight tubes and with two passes, according to the known art,
Figure 2 shows the same exchanger as in Fig. 1, but with four passes on the tube side, according to the known art,
Figures 3a and 3b schematically show a variable passes heat exchanger, in particular the input/output head of the source fluid, according to a first embodiment of the present invention,
Figure 4 shows the heat exchanger of Figs. 3a and 3b, according to a different operating mode,
Figure 5 shows the opposed head of the heat exchanger of Fig. 4,
Figure 6 schematically shows a variable passes heat exchanger according to a second example which is not part of the claimed invention,
Figure 7 shows in a simplified way a constructive solution of one of the movable separating baffles of the heat exchanger of Fig. 6,
Figure 8 is a detail of Fig. 7 and in particular a detail of the sealing area of the shaft according to the constructive solution of Fig. 7,
Figure 9 shows the same detail of Fig. 7 but showing a different example of the shaft sealing according to the constructive solution of Fig. 7,
Figure 10 shows the variable passes heat exchanger of Fig. 6, according to a different operating mode,
Figure 11 schematically shows a variable passes heat exchanger according to a third embodiment of the present invention,
Figure 12 schematically shows a variable passes heat exchanger according to a fourth example which is not part of the claimed invention,
Figure 13 schematically shows a variable passes heat exchanger comprising tubes made in the shape of a "U".

Detailed description

With reference to aforementioned figures, a tube bundle heat exchanger is disclosed performing a thermal exchange between a pair of fluids and therefore comprising a tube bundle inside which a first fluid flows, which crosses the tube bundle or portions of the same according to a variable number of passes; it also comprises a shell internally containing the tube bundle and the second fluid (the latter being external to the tube bundle). The shell is provided with at least one fluid IN inlet nozzle and at least of one fluid OUT outlet nozzle of said second fluid. The heat exchanger also comprises at least one distribution head of the first fluid inside the tubes of the tube bundle. The head is provided for this purpose of a variable number of separating baffles, which may be fixed or movable as necessary. Finally, the head is provided with at least one source IN input nozzle and at least of one source OUT outlet of said first fluid.
With reference to Figures 3 to 13, some implementation modes of the present invention will be shown, all of them sharing the possibility of making the variation of the passes of a heat exchanger, during the operation of the organic Rankine cycle plant without the need to stop the plant itself and/or empty the machine. It has to be understood that, even if the illustrated examples refer to heat exchangers used in ORC plants, the present invention can be applied to any kind of a "shell & tube" heat exchanger, i.e. to a tube bundle.
Following description shows the possibility of varying during operation the number of passes of a heat exchanger, from two to four passes and vice versa. It has to be understood that the variation from two to four passes is represented only by way of example and in a non-limiting way, as the invention in its implementation modes described below allows any variation of the number of passes with obvious modifications with respect to the presented examples.
Fig. 3 shows a first embodiment of the present invention. As shown in Figure 3a, for a generic heat exchanger 100 a first possibility is to rotate the movable separating baffles by hinging them nearly at the center of the inlet/outlet head 70 of the source fluid. The separating baffle C is fixed, whereas the separating baffles A and B are hinged to the center of the head and rotate from a first position P2, in which they are superimposed and define a two-passes heat exchanger, to a second position P4 in which they are arrange opposed along the vertical direction, in such a way to make a four-passes heat exchanger. As a matter of fact, what is shown in the left heads 70 of the exchangers in figures 1 and 2 is realized.
A first kinematics mode for rotating the movable separating baffles A and B is schematically shown figure 3b. The movable separating baffles A and B are hinged at respective coaxial shafts of the toothed wheels F and G, which mesh into a worm screw H. The rotation of the worm screw H causes the rotation of the toothed wheels F and G (nuts) in opposite directions. The system composed of F, G and H is external to the head 70 and constitutes a worm screw reducer with two counter-rotating output shafts.
Possibly the movable separating baffles A and B can be rotated by means of shafts independently moved from each other.
With reference to Fig. 4, a second mode for rotating the movable separating baffles A and B is to connect them with a kinematic with a worm screw H', which by rotating moves a sleeve M (nut), in turn connected to the baffle with a suitable arm K, hinged to the baffle in a suitable intermediate position. The movable separating baffles A and B can be moved through a single threaded rod, possibly with two left and right threading directions for the two sleeves (the one placed at the upper half and the other one at the lower half), or by two worm screws.
The entire kinematic can be made inside the head 70 and the only point of possible leakage of fluid towards the outside would occur by the threaded rod, which could be moved manually or with a suitable actuator.
The internal leakage between the different sections of the case can be limited by the use of suitable seals at the abutment points of the plates, at positions P2 and P4.
Figure 5 shows the opposed head 70' of the heat exchanger 100, whereas it is necessary on the contrary to shift from two to just one section. The left image shows the movable separating baffles A' and B' close to the first position P2 (two passes), whereas at the right one such baffles are close to position P4 (four passes).
It must be noted that in the 2-passes configuration (left image) the position of the two vertically superimposed baffles A' and B' in the upper portion of the head does not represent an obstacle to the flow which in this mode and with this head is substantially vertical (see figure 1, at 70').
If the shifting between the two-passes and four-passes configuration or vice versa is made with an ORC functioning, the opening or closing sequence of the movable separating baffles must be made following such a sequence, in which there is no interruption of the tube side.
For example, let us consider a geothermal water evaporator according to figures 1 to 5. Let us suppose that at the beginning the heat exchanger is working with two passes. In order to shift to four passes, first the bulkheads of the left head 70 move until reaching the position 4P. In this way, the left head is in position 4P (as in Fig. 2), whereas the right head 70' is in position 2P (as in Fig. 1); during this transitional phase, the source fluid enters the left head at portion 1 (Fig. 2), passes through the tubes and reaches the right head 70', still in the configuration of Fig. 1, then returns to the left head but only at portion 5 (Fig. 2), and then comes out from the heat exchanger; as a matter of fact, the water cannot flow through the exchanger portion corresponding to the portion 3 of the left head (Fig. 2). Therefore the geothermal water passes only through one-half of the exchanger. Once completed the positioning of the movable separating baffles of the left head, one proceeds with the right one. If, on the other hand one should intervene on the right head (70') and then the on the left one (70), the geothermal water would stop at the upper half of the exchanger, without reaching the outlet nozzle, so blocking the operation of the plant.
In a second example which is not part of the claimed invention, instead of moving the movable separating baffles with a kinematic connected to a worm screw, it is possible to open or close passages among movable separating baffles in the heads, by rotating them as if they were throttle valves. For example, in Figure 6 an example which is not part of the claimed invention is shown for the left head 70 of the exchanger 110, completely similar to that shown in Figs. 1 and 2. The head 70 has a fixed separating baffle C (horizontally placed at the right half) and three movable separating baffles D, E, and F. In order to shift from two to four passes the three movable separating baffles must be rotated from the horizontal first position P2 to the vertical second position P4.
Figure 7 shows in a simplified way a possible constructive solution of one of the movable separating baffles D, E, F as shown in Fig. 6. Figure 8 shows a detail of the sealing zone of the shaft.
Figure 7 shows a partial section of a straight-tube heat exchanger 110. Tubes 14, through which the source fluid flows, that is the one at the highest temperature, are fixed to the tube plate 12, whereas the organic fluid is confined in the shell 13. The head 11 is closed by a cover 10 constrained to the head itself by threaded connections 26.
The change in the number of passes is carried out by rotating a shaft 19, to which a rectangular plate 17 is fixed, that is a movable separating baffle as shown in Fig. 6. The sealing between the movable separating baffles 17 and the fixed separating baffles 27 is carried out through suitable sealing means 16 (for example of the flexible blade type), which preferably act on a projection 15 of the fixed separating baffle 27.
The shaft can rotate being inserted in a hole of cover 10 on one side, and in a sleeve 18 on the other one, rigidly constrained to the tube plate. During assembly, the shaft is inserted into the element 18, then the cover 10 is mounted in such a way that shaft 19 fits into the respective hole of cover. The shaft has a certain axial clearance given by the space remaining between its end and the base of element 18; in order to avoid excessive axial movements however, a widening 25 of the shaft section is provided, such as to possibly abut on the cover (which therefore acts as a limit switch.
In the example of Fig. 7, the shaft is moved by a manual hand wheel 20, but can also be rotated with a pneumatic, electric or hydraulic actuator, normally with the interposition of a speed reducer, for example of the worm screw/nut type.
Figure 8 shows a detail of a possible sealing system towards the external environment. It should be kept in mind, however, that small losses can be accepted, both if the fluid passing through the tubes and the head is for example a geothermal water, and if such small losses are still collected and conveyed in suitable collection volumes.
In particular, a gland sealing system is shown. The stuffing box 23 is pressed into a suitable seat by the externally threaded gland 22, which is screwed to the internally threaded element 21 and welded to the cover 10. It is possible to install different sealing systems, such as O-Rings (for example the O-Ring 24 in Fig. 8) or a gland with compression spring system 28, as in Figure 9, or a combination thereof.
The same sealing systems can also be applied to the worm screw of the first embodiment of the invention (the one shown in Figs. 3b , 4 and 5).
The movable separating baffles inside the case can be rotated as previously shown, or transferred as shown in Figure 10. The movable separating baffle 61 rests on guides 60, 62 formed in the fixed separating baffle 63. The displacement of the movable separating baffle 61 is generated by the rotation of the threaded shaft 29, which is inserted into a threaded hole made of a protuberance 61' of the movable separating baffle.
According to a third embodiment of the invention, it is possible to shift from a two-passes to a four-passes configuration by using tubes and valves external to the heat exchanger, thus avoiding the movement of the separating baffles. Therefore, the heat exchanger heads will be provided only with fixed separating baffles. By proceeding with the description by way of example of passes variation from two to four, it is evident that the left head can be divided into three or four portions whereas the right head will be divided into two portions. Depending on the position of the external valves, some portions of the external circuit will or not always be accessible to the source fluid, also determining in this case its operation with two or four passes.
Due to this embodiment of the invention, its advantage is represented by the absence of possible leakage points due to the need to handle internal mechanisms to the heat exchanger, whereas its disadvantage consist in cost and overall encumbrance of the machine due to tubes (also provided with large diameters therefore requiring wide curves), valves and also nozzles.
Figure 11 shows a first example of a third embodiment of the invention. Heat exchanger 120 is provided with a left head 70 divided into four portions and with a pair of inlet nozzles 71 of the source fluid and with a pair of outlet nozzles 72 for the same fluid. A first valve 30 is fluid-dynamically connected to the two inlet nozzles 71; a second valve 31 fluid-dynamically connected to an inlet nozzle and an outlet nozzle, which are angularly adjacent one another; a third valve 32 makes instead a fluid-dynamic connection between the two outlet nozzles 72. Starting from the left-hand head, a tube bundle 80 extends to the right-hand head 70', which is divided in two portions being in fluid-dynamic connection according to the opening or closing position of a fourth valve 33.
During a two-passes operation ( Fig. 11 - configuration 2P), the valve 31 is closed whereas the valves 30, 32 and 33 are open (in the figures, the open valves are indicated with ON, whereas the closed ones are indicated with OFF). In this way the source enters the two upper nozzles 71 of the left head 70, passes through a first upper portion 81 of the tube bundle 80, enters the upper portion of the right end head 70', passes through the duct controlled by the valve 33, and returns to the bottom of the lower head portion; finally it crosses a second lower portion 82 of the tube bundle 80 and returns to the left head 70, where it exits through the lower nozzles 72.
During a four-passes operation (Fig. 11 - configuration 4P), the valve 31 instead is open, whereas the valves 30, 32 and 33 are closed. The source fluid will then follow this path: it enters the portion 1 of the left head 70, crosses a first portion of the tube bundle 80 and arrives to the portion 2 of the right head 70'. Starting from here it crosses a second portion of the tube bundle and opens into the portion 3 of the left head 70. Being the valve 31 open, the fluid can then flow into the portion 4 of the left head 70 and from there crosses a third portion of the tube bundle and opens into the portion 5 of the right head 70'. Finally, it crosses a fourth portion of the tube bundle and arrives to the portion 6 of the left head, from which it emerges through an outlet nozzle.
According to an example which is not part of the claimed invention, the solution of figure 12 is very similar to the previous one, from which it differs in that the heat exchanger 130 does not need any valve in the right head 70' and has a smaller number of nozzles.
In particular, the heat exchanger 130 is provided with a left head 70 and is divided in three portions with a pair of inlet nozzles 71 of the source fluid and an outlet nozzle 72 of the same fluid. A first valve 40 is fluid-dynamically connected to the two inlet nozzles 71; a second valve 41 and a third valve 42 are in fluid-dynamic connection with each other and with an inlet nozzle and an outlet nozzle, mutually angularly adjacent to one another. The tube bundle 80 departs from the left head and reaches the right head 70', which is divided into two portions separated from each other by a fixed separating baffle.
In the configuration with two passes (2P) of Figure 12, the source fluid enters sections 1 from both upper nozzles of the left head (valve 40 open, 41 closed), flows in the tubes and reaches the right head, where it distributes in the lower tube bundles, then exiting in section 3 of the left head (valve 42 open).
In the four-passes configuration (4P) of Figure 12, the source fluid enters portion 1 in only one of the upper nozzles (valve 40 closed), flows through the tubes, reaches portion 2 along lower tubes and reaches portion 3. Being valve 42 closed, source fluid continues flowing through the other lower tube bundle reaching portion 4 of the opposed head; at 4 source goes back towards last upper tube bundle and reaches portion 5 from which it then leaves the heat exchanger 130 (valve 41 open).
The solution of Figure 12 is very similar to the previous one, with the advantage that the left head has one less nozzle and the right one does not need external valves/circuits. On the contrary, the solution of Figure 11 has another advantage: considering for example that by the tube side a hot source of an evaporator or a preheater is present, the hottest tubes (first pass in the 2P configuration, first and second pass in the 4P configuration) are always in the upper portion of the exchanger, whereas in Figure 12 in 4P configuration the upper portion of the shell is occupied by the tube bundles relating to the first and fourth pass. According to this configuration, it is therefore not possible to organize the path of the other flow which must be heated and which flows within the shell in counter-flow with respect to the flow passing through in the tubes.
The configurations shown in the present patent application can also be applied to heat exchangers with "U"-shaped tubes.
According to an example which is not part of the claimed invention, in Figure 13 a diagram is shown of a heat exchanger 140 or, in particular, a capacitor with "U"-shaped tubes with a variable number of passes. The variation of the number of passes is useful for optimizing the operation as a consequence of the variation of the cooling water flow rate to the capacitor; this variation can be due to the fact that at certain times of the year the plant is operated with a cogeneration function (the water heated in the capacitor is used for thermal purposes) and in other times of the year it is operated just for producing electric energy. By decreasing the number of passes, at the same speed inside the tubes and with the same thermal power transferred by the capacitor, a double temperature difference on the water will be available.
With reference to Figure 13, the heat exchanger 140 is provided with a left 70 divided into three portions and with a pair of inlet nozzles 71 of the source fluid source and an outlet nozzle 72 of the same fluid. A first valve 40 is fluid-dynamically connected to the two inlet nozzles 71; a second valve 41 is in fluid-dynamic connection with one of the two inlet nozzles and with the with a circuit of a thermal user; a third valve 42 is in fluid-dynamic connection with an outlet nozzle and with a feed pump 51. From the left head the tube bundle 80' is departing with a plurality of "U"-shaped tubes.
The two-passes configuration (2P) corresponds to the operation just for the production of electric energy, with a large water flow rate. The cooling water is circulated by means of at least one feed pump 51 and is cooled in the air heat exchanger 52, being valve 47 open and valves 44 and 46 closed. Cooling water is then distributed to the two upper portions of the head 70" of heat exchanger 140 being valve 40 open and valve 41 closed. Water then flows through the tube bundle 80' and returns to the lower portion of the head 70" exiting heat exchanger 140, valve 42 being open. The thermal user 50 may not require thermal power and in such case either valve 45 is closed (off) and pump 53 is off, or else heat power is required at a temperature such that it cannot use the one of the ORC capacitor; in such case valve 45 is open, pump 53 switched on and the thermal user is supplied with water heated by other heat sources (54) .
In the four-passes configuration (4P) with lower water flow rate, water after having cooled down by the thermal user 50, passes through valves 44 and 46 entering portion 1 of the exchanger (valve 40 closed). Then water passes through the tubes of the tube bundle 80' departing from portion 1 and arriving to underlying portion 2 (valve 42 closed), from where it then enters the tubes which occupy the right side of the exchanger and exits at portion 3, with valve 41 open. Finally, water having taken off heat from the condensing steam, exits heat exchanger 140 and reaches the thermal user 50, valve 45 being closed and pump 53 in operation. In four-passes configuration, water can either bypass the air exchanger 52, flowing through valve 46 or continue to pass through air heat exchanger 52, but with fans off.
Although at least one exemplary embodiment has been presented in the summary and the detailed description, it must be understood that there exists a large number of variations falling within the scope of the invention, for example with more than two portions connected to each other as in the proposed diagram. Furthermore, it must be understood that the embodiment or the embodiments presented are only examples which do not intend to limit in any way the scope of protection of the invention or its application or its configurations. On the contrary, the summary and detailed description provide the expert in the field with a convenient guide in order to implement at least one exemplary embodiment, being clear that numerous variations may be made in the function and assembly of the elements described therein, without departing from the scope of protection of the invention as established by the attached claims.

Claims

Heat exchanger (100, 120) having a tube bundle configured to achieve a heat exchange between a couple of fluids, the heat exchanger comprising:
- the tube bundle (80, 80') inside which a first fluid of the couple of fluids flows, said first fluid passing through the tube bundle or through its portions according to a variable number of passes,

- a shell (13) inside which the tube bundle (80, 80') and the second fluid of the couple of fluids are contained, the second fluid flowing outside of the tube bundle (80, 80'), said shell provided with at least an inlet nozzle and at least one outlet nozzle of said second fluid,

- at least a head (70, 70', 70") for the distribution of the first fluid inside the tubes of the tube bundle according to a variable number of passes, said head comprising for this purpose a variable number of separating baffles, fixed or movable, and at least one inlet nozzle and at least one outlet nozzle of said first fluid,
wherein said heat exchanger is configured so as to be able to vary the number of passes of the first fluid without having to open the at least one head and/or remove one of the two fluids from the heat exchanger, wherein the variation of the passes number is obtained by means of the movement of at least a movable separating baffle (A, B, A', B', D, E, F, 17, 61) inside of the heads (70, 70', 70") or by varying the opening and closing of a plurality of valves external to the heat exchanger, the heat exchanger (100, 120) being characterized in that:
when the variation of the passes number is obtained by means of the movement of at least a movable separating baffle (A, B, A', B', D, E, F, 17, 61) inside of the heads (70, 70', 70"),
- the movable separating baffles (A, B) are hinged near the head (70) center and rotate from a first position (P2), in which said separating baffles are overlapping and defining a two passes heat exchanger, to a second position (P4) rotated of 90° in which said separating baffles are opposite arranged along a straight line, so as to realize a four passes heat exchanger, or when the variation of the number of passes is obtained by varying the opening and closing of a plurality of valves external to the heat exchanger, the heat exchanger (120) comprising :

- a left head (70), divided into four portions, having a couple of inlet nozzles (71) for the first fluid and a couple of output nozzles (72) for the same fluid, and provided with a first valve (30) in fluid connection with the two inlet nozzles (71), a second valve (31) in fluid connection with an inlet nozzle and an outlet nozzle, angularly adjacent to each other and a third valve (32) in fluid connection with the two outlet nozzles (72),

- a right head (70'), divided into two portions, provided with a fourth valve (33) in fluid connection with said two portions of the right head (70').
Heat exchanger (100) according to claim 1, characterized in that the movable separating baffles (A, B) are hinged in corresponding coaxial shafts with toothed wheels (F, G), which engaged in a worm screw (H).
Heat exchanger (100) according to claim 1, characterized in that the movable separating baffles (A, B) can be rotated by means of shafts which can be moved independently between them.
Heat exchanger (100) according to claim 1 characterized in that the movable separating baffles (A, B, A', B') are connected by a kinematic mechanism to at least a worm screw (H'), that by rotating moves corresponding sleeves (M) in turn connected to the movable separating baffles (A, B, A', B') by an appropriate arm (K).
Heat exchanger (100) according to claim 1, characterized in that at least one of the two heads is provided with a movable separating baffle (D, E, F) that rotates from a first position (P2), in which it defines a two passes heat exchanger, to a second position (P4) in which it defines a four passes heat exchanger, being its rotational axis at the center of the baffle itself.
Heat exchanger (100) according to any of the preceding claims, characterized in that the variation of the number of passes is carried out by rotating a shaft (19, 29) to which the movable separating baffle is fixed (17), whose sealing against a fixed separating baffle (27) is realized by means of flexible sealing means (16), acting on a protrusion(15) of the fixed separating baffle (27).
Heat exchanger (100) according to claim 6, characterized in that the shaft (19, 29) is constrained to a sleeve (18) on one side and on the other side passes through a hole of a cover (10).
Heat exchanger (100) according to claim 6 or 7, characterized in that the shaft (19, 29) or the worm screw (H, H') are moved by a hand wheel (20) or by means of a pneumatic, electric or hydraulic actuator.
Heat exchanger (100) according to any of claims from 6 to 8, characterized in that the shaft (19, 29) sealing means are chosen between a stuffing box (23), O-rings (24), a stuffing box (23) having a compression spring system (28) or a combination thereof.
Heat exchanger (100) according to claim 1, characterized in that at least one movable separating baffle (61) rests on guides (60, 62) formed in the fixed separating baffle (63) and is traversed by means of the rotation of a threaded shaft (29), which fits into a threaded hole formed in a protuberance (61') in one piece with the movable separating baffle (61).
Heat exchanger (120) according to claim 1, characterized in that during two passes operations the second valve (31) is closed while the remaining valves (30, 32, 33) are open and during four passes operations the second valve (31) is open while the remaining valves (30, 32, 33) are closed.
Organic Rankine cycle system comprising a heat exchanger (100, 120) according to any of claims 1 to 11.
Organic Rankine cycle system according to claim 12, wherein said heat exchanger (100, 120) is designed to operate as an evaporator or preheater of an organic working fluid and characterized in that the heat source is the first fluid flowing inside the tube bundle.
Organic Rankine cycle system according to claim 12 or 13, wherein said heat exchanger (100, 120) is designed to operate as a condenser of the organic working fluid and characterized in that the cooling water is the first fluid flowing inside the tube bundle.