Helical coil heat exchanger

Abstract

A helical coil heat exchanger (10) comprises a shell (12), at least one shell inlet (14) for inletting a first fluid into the shell (12), at least one shell outlet (16) for the outlet of the first fluid from the shell (12), and at least one helical coil tube bundle (18), which is at least partially enclosed by the shell (12). At least one tube inlet (20) for inletting a second fluid into the helical coil tube bundle (18) is provided, so that heat exchange occurs between the first fluid and the second fluid through the helical coil tube bundle (18), and so that during heat exchange the second fluid is composed of a part of second fluid in liquid phase (P1) and a part of second fluid in vapor phase (P2). At least one tube outlet (22) for the outlet of the second fluid from the helical coil tube bundle (18) is also provided. At least one start-up system (24) is at least partially contained within the shell (12). The start-up system (24) comprises at least one separator device (26), which is in fluid connection with the helical coil tube bundle (18) and which is designed to receive the second fluid when heat is exchanged with the first fluid and when at least part of the second fluid is in vapor phase (P2), so as to separate the vapor phase (P2) from the liquid phase (P1) of the second fluid, and at least one collecting vessel (28), which is in fluid connection with the separator device (26) and which is designed to collect at least a part of the second fluid in liquid phase (P1) after separation from the vapor phase (P2). At least one warm-up system too is at least partially contained within the shell (12) and is hydraulically connected with the helical coil tube bundle (18). The warm-up system comprises at least one heating device (30) for heating the second fluid after entering through the tube inlet (20) and being recirculated in the helical coil tube bundle (18) and in the separator device (26).

Description

Field of the invention

[0001] The present invention refers in general to a helical coil heat exchanger and, more specifically, to a helical coil once through heat exchanger.Background of the invention

[0002] Helical coil heat exchangers are well-known heat transfer devices. These devices are used for transferring heat from a first fluid to a second fluid. A helical coil heat exchanger mainly comprises a helical coil tube bundle through which flows a first fluid. The helical coil tube bundle is enclosed in a shell, which usually consists of a long round cylindrical tank within which a second fluid flows. When the first fluid flows through the helical coil tube bundle, the curvature of said helical coil tube bundle will cause the first fluid to generate a secondary flow, and turbulence will occur earlier, thereby enhancing its heat transfer effect with the second fluid, but the pressure loss will also increase.

[0003] In the specific field of application of steam production for industrial plants in general, the use of steam generators is known. For example, when an industrial plant produces green electric energy, and when green electric energy production is higher than electricity demand, energy is stored as sensible heat by heating a specific fluid, usually consisting of a molten salt, that is stored in a respective tank. Before storage, the molten salt flows in a steam generator, generating superheated steam.

[0004] Helical coil once through heat exchangers are therefore well-known steam generators for generating superheated steam: they have been developed decades ago for nuclear power stations and are still used today in various applications. Helical coil once through heat exchangers perform, in one single equipment, the functions performed by four separate devices (preheater, evaporator, superheater, drum) on drum type steam generators. Helical coil once through heat exchangers use water as a fluid to produce steam. Once through means that water is not recirculated, as in drum type steam generators, but passes only once in the heat exchanger. Water passes through the helical coil tube bundle, whereas another hot fluid, for example a molten salt, flows on shell side. The heat transferred to the water by this hot molten salt allows the generation of steam.

[0005] Molten salt freezes at temperatures below around 150°C. Consequently, heating devices are required to heat the molten salt before it enters the helical coil heat exchanger if this molten salt is not already at a high enough temperature to prevent it from freezing.

[0006] Additionally, during the start-up of the helical coil heat exchanger the steam production is very low. In helical coil heat exchangers of known type, therefore, arrangements are required aimed at obtaining an efficient and hydraulically stable heat exchange during the start-up. Consequently, helical coil heat exchangers of known type must be associated with separate steam generators, such as steam drums, to achieve the above functionality.

[0007] All the separate devices mentioned above result in reduced functionality, increased thermal inertia, increased losses, and increased overall dimensions of helical coil heat exchangers of known type.Summary of the invention

[0008] One object of the present invention is therefore to provide a helical coil heat exchanger, more specifically a helical coil once through heat exchanger, which is capable of resolving the drawbacks of the prior art in a simple, inexpensive and particularly functional manner.

[0009] In detail, one object of the present invention is to provide a helical coil heat exchanger which is capable of always receiving one of the two fluids, which is in particular but not exclusively a molten salt, at the most suitable temperature for heat exchange with the other fluid, which is in particular but not exclusively water intended for the generation of steam.

[0010] Another object of the present invention is to provide a helical coil heat exchanger which is capable of obtaining an efficient heat exchange between the two fluids right from the start-up of the heat exchanger itself.

[0011] A further object of the present invention is to provide a helical coil heat exchanger with integrated heater to improve start-up warming and compensate heat losses when the plant in which the heat exchanger is installed is not in operation.

[0012] Still another object of the present invention is to provide a helical coil heat exchanger which is capable of improving functionality, reducing thermal inertia, reducing losses, and reducing overall dimensions compared with helical coil heat exchangers of known type.

[0013] These and other objects are achieved according to the present invention by providing a helical coil heat exchanger as set forth in the attached claims.

[0014] Further features of the invention are underlined by the dependent claims, which are an integral part of the present description.

[0015] The helical coil heat exchanger according to the present invention comprises a shell, at least one shell inlet for inletting a first fluid into the shell and at least one shell outlet for the outlet of the first fluid from the shell. The helical coil heat exchanger also comprises at least one helical coil tube bundle, which is at least partially enclosed by the shell, and at least one tube inlet for inletting a second fluid into the helical coil tube bundle, so that heat exchange occurs between the first fluid and the second fluid through the helical coil tube bundle, and so that during heat exchange the second fluid is composed of a mixture of second fluid in liquid phase and in vapor phase. The helical coil heat exchanger further comprises at least one tube outlet for the outlet of the second fluid from the helical coil tube bundle.

[0016] At least one start-up system is at least partially contained within the shell. The start-up system comprises at least one separator device, which is in fluid connection with the helical coil tube bundle, and which is designed to receive the second fluid when heat is exchanged with the first fluid and when at least part of the second fluid is in vapor phase, so as to separate the vapor phase from the liquid phase of the second fluid. The start-up system also comprises at least one collecting vessel, which is in fluid connection with the separator device, and which is designed to collect at least a part of the second fluid in liquid phase after separation from the vapor phase.

[0017] At least one warm-up system is at least partially contained within the shell and is hydraulically connected with the helical coil tube bundle. The warm-up system comprises at least one heating device for heating the second fluid after entering through the tube inlet and being recirculated in the helical coil tube bundle and in the separator device.

[0018] Preferably, the start-up system is fully contained within the shell and is surrounded by the helical coil tube bundle. The start-up system may comprise at least one evaporator device and at least one superheater device.

[0019] Still preferably, the heating device is fully contained within the shell and is placed at the tube inlet. More preferably, the heating device is positioned below the collecting vessel and in proximity of the helical coil tube bundle.

[0020] At least one vapor chamber may be positioned at a top portion of the separator device. The vapor chamber is in fluid communication with the tube outlet. The helical coil tube bundle may be provided with a plurality of tube connections for the fluid connection with the vapor chamber, and the tube connections may be joined to the vapor chamber through a tangential nozzle-shell junction.

[0021] Preferably, the separator device is a cyclonic separator device. At least one vapor vortex breaker may be interposed between the vapor chamber and the tube outlet.

[0022] The shell may be closed, at an end thereof, by at least one tube-sheet, in such a way that the tube-sheet is crossed by the helical coil tube bundle in a toroidal arrangement, and / or in such a way that the tube connections are arranged to exit from the shell at the tube-sheet, so as to connect with the vapor chamber outside the shell, to improve maintenance and availability.

[0023] The collecting vessel may be provided with at least one collecting vessel outlet for the outlet of the second fluid in liquid phase from the collecting vessel, and a recirculation conduit with at least one recirculation pump may be provided between the collecting vessel outlet and the tube inlet. At least one blowdown tank, with at least one blowdown valve, may be provided along the recirculation conduit.

[0024] At least one liquid chamber for the second fluid in liquid phase may be positioned between the tube inlet and the helical coil tube bundle.

[0025] Preferably, the heating device is an electrical heater.

[0026] The first fluid may be a hot molten salt to be cooled, and / or the second fluid may be water.

[0027] The present invention further relates to a method for operating the helical coil heat exchanger described so far. The method comprises the following steps in sequence: before operating the helical coil heat exchanger, carrying out a start-up preparation comprising the sub-step of filling the helical coil tube bundle with the second fluid until a predefined tube level of the second fluid is reached; starting the warm-up procedure by heating the second fluid through the heating device and circulating the second fluid in the helical coil tube bundle; when a first predefined temperature value is reached, which is equivalent to the saturation temperature, increasing the pressure value inside the helical coil heat exchanger; when a second predefined temperature value is reached, introducing the first fluid into the shell; as soon as heat exchange between the first fluid and the second fluid is effective, steam is produced and exported, and the separation between the vapor phase of the second fluid and the liquid phase of the second fluid is done by the separator device, in such a way that the liquid phase of the second fluid is sent to the collecting vessel, while the vapor phase of the second fluid is routed to the tube outlet; controlling the flow of the second fluid in order to maintain a level between a predefined minimum level and a predefined maximum level. Brief description of the drawings

[0028] The features and advantages of a helical coil heat exchanger according to the present invention will be clearer from the following exemplifying and nonlimiting description, with reference to the enclosed schematic drawings, in which: Figure 1 is a schematic front sectional view of a preferred embodiment of a helical coil heat exchanger according to the present invention; Figure 2 is a schematic side sectional view of the helical coil heat exchanger of Figure 1; Figure 3 is a partial sectional view of one of the internal components (heating device) of the helical coil heat exchanger of Figure 1; Figure 4 is a schematic view of another internal component (start-up system) of the helical coil heat exchanger of Figure 1; Figure 5 shows the process flow diagram of the helical coil heat exchanger of Figure 1, where the Roman numeral "V" indicates a detail of the side view of bottom portion of the helical coil heat exchanger; Figure 6 is a schematic view of another internal component (separator device) of the helical coil heat exchanger of Figure 1; and Figure 7 is a sectional view, obtained along line VII-VII of Figure 6, of the component of Figure 6. Detailed description of the invention

[0029] With reference in particular to Figures 1 and 2, a helical coil heat exchanger 10 is shown. The helical coil heat exchanger 10 comprises, in a well-known manner per se, a shell 12, at least one shell inlet 14 for inletting a first fluid into the shell 12, and at least one shell outlet 16 for the outlet of the first fluid from the shell 12.

[0030] The helical coil heat exchanger 10 also comprises at least one helical coil tube bundle 18, which is at least partially enclosed by the shell 12, and at least one tube inlet 20 for inletting a second fluid into the helical coil tube bundle 18, so that heat exchange occurs between the first fluid and the second fluid through the helical coil tube bundle 18, and so that during heat exchange the second fluid is composed of a mixture of second fluid in liquid phase P1 and in vapor phase P2 (see for example Figure 6). The helical coil heat exchanger further comprises at least one tube outlet 22 for the outlet of the second fluid from the helical coil tube bundle 18.

[0031] For example, the first fluid can be a hot molten salt to be cooled by the second fluid. As shown in Figure 5, the hot molten salt can be contained in one or more hot molten salt tanks 48 and can be fed into the helical coil heat exchanger 10 through one or more hot molten salt conduits 50 which are in fluid communication with the shell inlet 14. One or more shell-side fluid control valves 52 can also be provided on the hot molten salt conduits 50, so as to control the amount of molten salt fed into the helical coil heat exchanger 10. Once cooled inside the helical coil heat exchanger 10, the molten salt can be collected in one or more cold molten salt tanks 54 which are in fluid communication with the shell outlet 16.

[0032] Still for example, the second fluid can be water. Water can be fed into the helical coil heat exchanger 10 through one or more water conduits 55 which are in fluid communication with the tube inlet 20. One or more feedwater control valves 56, as well as one or more feedwater pumps 58, can also be provided on the water conduits 55. Additionally, one or more deaerators 60 may be optionally provided on the water conduits 55.

[0033] After heat exchange with the molten salt, at least part of the water changes to the vapor phase P2 and steam is formed. Steam is expelled from the helical coil heat exchanger 10 through the tube outlet 22, which then operates as a steam outlet, and can be conveyed into a steam pipe 62 for reuse and / or vent. Steam outlet control valves 64 can be placed on the steam pipe 62, as well as various sensors 66, such as temperature, pressure and / or level sensors.

[0034] According to the invention, at least one start-up system 24 is at least partially contained within the shell 12 of the helical coil heat exchanger 10. More specifically, the start-up system 24 comprises: at least one separator device 26, which is in fluid connection with the helical coil tube bundle 18 and which is designed to receive the second fluid when heat is exchanged with the first fluid and when at least part of the second fluid is in vapor phase P2, so as to separate the vapor phase P2 from the liquid phase P1 of the second fluid; and at least one collecting vessel 28, which is in fluid connection with the separator device 26 and which is designed to collect at least a part of the second fluid in liquid phase P1 after separation from the vapor phase P2.

[0035] Additionally, at least one warm-up system is at least partially contained within the shell 12 and is hydraulically connected with the helical coil tube bundle 18. The warm-up system comprises at least one heating device 30 for heating the second fluid after entering through the tube inlet 20 and being recirculated in the helical coil tube bundle 18 and in the separator device 26.

[0036] Preferably, the start-up system 24 is fully contained within the shell 12 and is surrounded by the helical coil tube bundle 18. The start-up system 24 may also comprise at least one evaporator device 68 and at least one superheater device 70.

[0037] Still preferably, the heating device 30 is fully contained within the shell 12 and is placed at the tube inlet 20. The heating device 30 may also be positioned below the collecting vessel 28 and in proximity of the helical coil tube bundle 18.

[0038] The helical coil heat exchanger 10 according to the invention is thus provided with and integrated warm-up and start-up system. Moreover, the helical coil heat exchanger 10 incorporates, in one single equipment, functions performed by four different devices of steam generators (preheater, evaporator, superheater, drum). Although the helical coil heat exchanger 10 according to the invention is preferably designed to process hot molten salt on shell side, thus operating as a steam generator using water flowing on tube side, this helical coil heat exchanger 10 can also operate with any other fluid.

[0039] The warm-up system is schematically shown in Figure 3. Since molten salt freezes at a temperature below around 150°C, the helical coil heat exchanger 10 must be warmed before the molten salt enters through the shell inlet 14. The heating is carried out by the heating device 30, which is preferably an electrical heater.

[0040] The start-up system is schematically shown in Figure 5. When the helical coil heat exchanger 10, molten salt enters through the shell inlet 14 and steam is produced. Steam pushes water out from the helical coil heat exchanger 10 (swelling effect). Water is collected in the collecting vessel 28 of the start-up system 24, which is integrated in the helical coil heat exchanger 10. During start-up steam production is very low. In order to have efficient heat exchange and stable hydraulics, a circulation pump 44 may be used in the start-up system 24. In these conditions, the helical coil tube bundle 18 works as an assisted circulation drum type evaporator. Feedwater is controlled to compensate steam production by water level control in the collecting vessel 28 by means of the sensors 66. Since steam-water mixing is generated by the helical coil heat exchanger 10, steam is separated from water in the separator device 26 in order to produce dry steam.

[0041] Preferably, at least one vapor chamber 32 is positioned at a top portion of the separator device 26 and is in fluid communication with the tube outlet 22. The vapor chamber 32 is designed to collect steam after evaporation of the second fluid (water). The helical coil tube bundle 18 is provided with a plurality of tube connections 34 for the fluid connection with the vapor chamber 32. Preferably, the tube connections 34 are joined to the vapor chamber 32 through a tangential nozzle-shell junction, as schematically shown in Figure 7. A protection shield 77 may enclose the tube connections 34.

[0042] The separator device 26 may be a cyclonic separator device, that is, a separator device wherein rotational effects and gravity are used to separate the vapor phase P2 from the liquid phase P1 of the second fluid. At least one vapor vortex breaker 36 may be interposed between the vapor chamber 32 and the tube outlet 22.

[0043] Preferably, the shell 12 is closed, at an end thereof (for example the upper end), by at least one tube-sheet 38, in such a way that the tube-sheet 38 is crossed by the helical coil tube bundle 18 in a toroidal arrangement. Additionally, or alternatively, the tube connections 34 are arranged to exit from the shell 12 at the tube-sheet 38, so as to connect with said vapor chamber 32 outside the shell 12. This arrangement makes the start-up system 24 more compact and efficient, as well as facilitating maintenance of the helical coil tube bundle 18.

[0044] As shown in Figure 5, the collecting vessel 28 is preferably provided with at least one collecting vessel outlet 40 for the outlet of the second fluid in liquid phase P1 from the collecting vessel 28. A recirculation conduit 42, with at least one circulation pump 44, can also be provided between the collecting vessel outlet 40 and the tube inlet 20. Optionally, at least one blowdown tank 72, with at least one blowdown valve 74, may be provided along the recirculation conduit 42. Finally, at least one liquid chamber 46 for the second fluid in liquid phase P1 may be positioned between the tube inlet 20 and the helical coil tube bundle 18.

[0045] The start-up procedure of the helical coil heat exchanger 10 described so far is as follows. During start-up preparation, i.e., before operating the helical coil heat exchanger 10, the steam outlet control valves 64 and a vent control valve 78 are closed. Using the feedwater pump 58, the helical coil tube bundle 18 is filled with the second fluid (water) until a predefined tube level L1 of the second fluid is reached (see figure 5). The circulation pump 44 is in operation.

[0046] At this point the warm-up procedure can begin. Using the heating device 30, water is heated and circulated in the helical coil tube bundle 18 through the circulation pump 44. When a first predefined temperature value is reached, which is equivalent to the saturation temperature, pressure value inside the helical coil heat exchanger 10 is increased by opening and controlling the vent control valve 78. Water chemistry is controlled by the blowdown valve 74. When a second predefined temperature value is reached, which is equivalent to the required temperature for start-up, the first fluid is introduced into the shell 12 through the shell inlet and the first fluid flow is controlled by the shell-side fluid control valve 52.

[0047] As soon as heat exchange between the first fluid and the second fluid is effective and water evaporation starts, steam is produced, and water is pushed out of the helical coil tube bundle 18 (swelling effect). The collecting vessel 28 is sized in order to accommodate swelled flow within a predefined maximum level L2 of the second fluid. Circulation in the helical coil tube bundle 18 is controlled by the circulation pump 44.

[0048] As soon as heat absorption increases, the pressure of the helical coil tube bundle 18 also increases. Pressurization is controlled by the vent control valve 78. In these conditions the helical coil tube bundle 18 works as a drum type boiler, with level control through the feedwater control valve 56. Level measurement is done through level transmittal sensors 66.

[0049] Steam is dried by the separator device 26. Inclined tube connections 34 enter tangentially to the vapor chamber 32 (see Figure 7). Separated steam is routed to the tube outlet 22, whereas separated water is routed to the collecting vessel 28. Feedwater flow is controlled in order to maintain a level between a predefined minimum level L3 of the second fluid and the predefined maximum level L2 of the second fluid. The separator vortex (schematically shown in figure 6) may be broken by a water vortex breaker 76 placed between the separator device 26 and the collecting vessel 28. If saturated steam exiting the helical coil heat exchanger 10 is deemed useful for other applications, as soon as flow conditions (pressure, flow) are acceptable, the steam outlet control valves 64 are properly controlled to flow the saturated steam through the steam line 62 and send it to the desired application.

[0050] When steam flow and heat exchanger duty are suitable for steam superheating, the control system of the helical coil heat exchanger 10 is changed to once through mode, vent control valve 78 is closed, steam outlet control valve 64 is open and feedwater flow is controlled on the basis of the steam outlet temperature measured by the sensors 66. Being steam outlet temperature lower than setpoint, the feedwater control valve 56 is closed until the steam outlet temperature reaches required setpoint. This dries the collecting vessel 28. When the water level reaches the predefined minimum level L3, the circulation pump 44 is shutdown.

[0051] In case of failure of the helical coil tube bundle 18, the respective tubes can be plugged at helical coil tube bundle 18 inlet, from liquid chamber 46, and at the tube outlet 22 (tube connection to the vapor chamber 32). To allow access to the tube connections 34 to the vapor chamber 32, these tube connections 34 can be placed outside the shell 12 and the tubes of the helical coil tube bundle 18 passes across the tube-sheet 38.

[0052] It is thus seen that the helical coil heat exchanger according to the present invention achieve the previously outlined objects.

[0053] The helical coil heat exchanger of the present invention thus conceived is susceptible in any case of numerous modifications and variants, all falling within the same inventive concept; in addition, all the details can be substituted by technically equivalent elements. In practice, the materials used, as well as the shapes and size, can be of any type according to the technical requirements.

[0054] The scope of protection of the invention is therefore defined by the enclosed claims.List of references

[0055] 10: helical coil heat exchanger; 12: shell; 14: shell inlet; 16: shell outlet; 18: helical coil tube bundle; 20: tube inlet; 22: tube outlet; 24: start-up system; 26: separator device; 28: collecting vessel; 30: heating device; 32: vapor chamber; 34: tube connections; 36: vapor vortex breaker; 38: tube-sheet; 40: collecting vessel outlet; 42: recirculation conduit; 44: circulation pump; 46: liquid chamber for the second fluid; 48: hot molten salt tank; 50: hot molten salt conduit; 52: shell-side fluid control valve; 54: cold molten salt tank; 55: water conduit; 56: feedwater control valve; 58: feedwater pump; 60: deaerator; 62: steam pipe; 64: steam outlet control valve; 66: sensors; 68: evaporator device; 70: superheater device; 72: blowdown tank; 74: blowdown valve; 76: water vortex breaker; 77: protection shield; 78: vent control valve; L1: tube level of the second fluid; L2: maximum level of the second fluid; L3: minimum level of the second fluid.

Claims

1. A helical coil heat exchanger (10) comprising: - a shell (12); - at least one shell inlet (14) for inletting a first fluid into said shell (12); - at least one shell outlet (16) for the outlet of the first fluid from said shell (12); - at least one helical coil tube bundle (18), which is at least partially enclosed by said shell (12); - at least one tube inlet (20) for inletting a second fluid into said helical coil tube bundle (18), so that heat exchange occurs between the first fluid and the second fluid through said helical coil tube bundle (18), and so that during heat exchange the second fluid is composed of a mixture of second fluid in liquid phase (P1) and in vapor phase (P2); and - at least one tube outlet (22) for the outlet of the second fluid from said helical coil tube bundle (18), wherein at least one start-up system (24) is at least partially contained within said shell (12), wherein said start-up system (24) comprises: - at least one separator device (26), which is in fluid connection with said helical coil tube bundle (18) and which is designed to receive the second fluid when heat is exchanged with the first fluid and when at least part of the second fluid is in vapor phase (P2), so as to separate said vapor phase (P2) from the liquid phase (P1) of the second fluid; and - at least one collecting vessel (28), which is in fluid connection with said separator device (26) and which is designed to collect at least a part of the second fluid in liquid phase (P1) after separation from the vapor phase (P2), and wherein at least one warm-up system is at least partially contained within said shell (12) and is hydraulically connected with said helical coil tube bundle (18), wherein said warm-up system comprises at least one heating device (30) for heating the second fluid after entering through said tube inlet (20) and being recirculated in said helical coil tube bundle (18) and in said separator device (26).

2. The helical coil heat exchanger (10) according to claim 1, wherein said start-up system (24) is fully contained within said shell (12) and is surrounded by said helical coil tube bundle (18).

3. The helical coil heat exchanger (10) according to claim 1 or 2, wherein said start-up system (24) comprises at least one evaporator device (68) and at least one superheater device (70).

4. The helical coil heat exchanger (10) according to any one of claims 1 to 3, wherein said heating device (30) is fully contained within said shell (12) and is placed at said tube inlet (20).

5. The helical coil heat exchanger (10) according to claim 4, wherein said heating device (30) is positioned below said collecting vessel (28) and in proximity of said helical coil tube bundle (18).

6. The helical coil heat exchanger (10) according to any one of claims 1 to 5, wherein at least one vapor chamber (32) is positioned at a top portion of said separator device (26) and is in fluid communication with said tube outlet (22), wherein said helical coil tube bundle (18) is provided with a plurality of tube connections (34) for the fluid connection with said vapor chamber (32), and wherein said tube connections (34) are joined to said vapor chamber (32) through a tangential nozzle-shell junction.

7. The helical coil heat exchanger (10) according to claim 6, wherein said separator device (26) is a cyclonic separator device.

8. The helical coil heat exchanger (10) according to claim 7, wherein at least one vapor vortex breaker (36) is interposed between said vapor chamber (32) and said tube outlet (22).

9. The helical coil heat exchanger (10) according to any one of claims 6 to 8, wherein said shell (12) is closed, at an end thereof, by at least one tube-sheet (38), in such a way that said tube-sheet (38) is crossed by said helical coil tube bundle (18) in a toroidal arrangement, and / or in such a way that said tube connections (34) are arranged to exit from said shell (12) at said tube-sheet (38), so as to connect with said vapor chamber (32) outside the shell (12).

10. The helical coil heat exchanger (10) according to any one of claims 1 to 9, wherein said collecting vessel (28) is provided with at least one collecting vessel outlet (40) for the outlet of the second fluid in liquid phase (P1) from said collecting vessel (28), wherein a recirculation conduit (42) with at least one circulation pump (44) are provided between said collecting vessel outlet (40) and said tube inlet (20).

11. The helical coil heat exchanger (10) according to claim 10, wherein at least one blowdown tank (72), with at least one blowdown valve (74), are provided along said recirculation conduit (42).

12. The helical coil heat exchanger (10) according to any one of claims 1 to 11, wherein at least one liquid chamber (46) for the second fluid in liquid phase (P1) is positioned between said tube inlet (20) and said helical coil tube bundle (18).

13. The helical coil heat exchanger (10) according to any one of claims 1 to 12, wherein said heating device (30) is an electrical heater.

14. The helical coil heat exchanger (10) according to any one of claims 1 to 13, wherein the first fluid is a hot molten salt to be cooled by the second fluid, and / or the second fluid is water.

15. A method for operating a helical coil heat exchanger (10) according to any one of claims 1 to 14, the method comprising the following steps in sequence: - before operating the helical coil heat exchanger (10), carrying out a start-up preparation comprising the sub-step of filling the helical coil tube bundle (18) with the second fluid until a predefined tube level (L1) of the second fluid is reached; - starting the warm-up procedure by heating the second fluid through the heating device (30) and circulating the second fluid in the helical coil tube bundle (18); - when a first predefined temperature value is reached, increasing the pressure value inside the helical coil heat exchanger (10); - when a second predefined temperature value is reached, introducing the first fluid into the shell (12); - as soon as heat exchange between the first fluid and the second fluid is effective, steam is produced and exported, and the separation between the vapor phase (P2) of the second fluid and the liquid phase (P1) of the second fluid is done by the separator device (26), in such a way that the liquid phase (P1) of the second fluid is sent to the collecting vessel (28), while the vapor phase (P2) of the second fluid is routed to the tube outlet (22); - controlling the flow of the second fluid in order to maintain a level between a predefined minimum level (L3) and a predefined maximum level (L2).

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