Compression equipment and filling stations equipped with such equipment

Abstract

To transmit a large amount of force and power from an actuating mechanism to a cold head, and to minimize input of heat into the cold head.SOLUTION: An apparatus comprises pistons 5, 8 and sleeves 13, 14 delimiting compression chambers 3, 4, and a shaft that is able to move in translation along a longitudinal axis A. The shaft is connected to the pistons or sleeves and able to move alternately in two opposite directions to ensure a phase of compressing a fluid and a phase of taking the fluid into the compression chamber by relatively moving the pistons and the sleeves. The shaft comprises a portion 151 of reduced cross section in the longitudinal direction, separating two adjacent parts 153, 154 of the shaft. A linking element 155 made of a material that is less thermally conductive than a constituent material of the shaft, in particular made of a composite material, has two ends connected respectively to the two adjacent parts of the shaft.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a compression device, in particular for cryogenic fluids, and to a filling station equipped with such a device.

[0002] The invention more particularly relates to a device for compressing a cryogenic fluid in at least one compression stage, said device comprising at least one piston and at least one sleeve defining at least one pressurization chamber, and a shaft capable of translational movement along a longitudinal axis, said shaft being connected to the piston(s) or to the sleeve(s) and capable of alternating movement in two opposite directions in order to ensure, by moving the at least one piston and the at least one sleeve relative to one another, stages of compression and of suction of the fluid into the at least one pressurization chamber. [Background technology]

[0003] Cryogenic fluids have a much higher density than gaseous fluids. As a result, cryogenic pumps (as opposed to gas compressors) have higher mass flows, a smaller installation area, consume less energy and require less maintenance. For these reasons, cryogenic pumps are used in many areas, such as units for separating air and gas, reformers, filling stations and the maritime sector.

[0004] The fluids in question typically include oxygen, nitrogen, natural gas, argon, helium, or hydrogen. These compressors (or pumps) function to pressurize the cryogenic fluid to a target flow rate.

[0005] In particular, the present invention relates to apparatus for compressing or pumping cryogenic gases and / or fluids.

[0006] In the following text, in particular, the terms "compression device" and "pump" may be used interchangeably, as well as the terms "pumping" and "compression". In particular, the device that is the subject of the present invention is a device for pumping and / or compressing fluids and / or gaseous and / or supercritical cryogenic fluids.

[0007] For example, the cryogenic piston pump may be located in a dedicated cryogenic tank (also known as a "sump") located directly in-line or adjacent to the outlet of the cryogenic source reservoir and fed directly by the main storage tank.

[0008] For various reasons, but especially for reasons of management and design convenience, cryogenic pumps generally exhibit alternating motion and are inserted into a tank so as to be submerged in the cryogenic fluid to be pumped.

[0009] Cryogenic pumps typically have an inlet pressure of 1 to 20 bar and an outlet pressure of 20 to 1000 bar, depending on the application. The pump may have one or more compression stages using reciprocating motion.

[0010] Pumps for high pressure and adequate flow rates frequently exhibit an alternating motion with two stages: one stage for intake (in one direction) and one stage for delivery (in the other direction). Non-return valves (generally two per compression stage: one for intake or intake, one for exhaust or delivery) allow to increase the pressure of a fixed volume of fluid by reducing its volume through the relative translational motion between the piston and the sleeve.

[0011] The two translational movements have different design requirements in terms of the mechanical power and forces required for this movement: the suction movement is typically at pressures of 1 to 20 bar, while the compression movement can range up to 1000 bar. These two movements are therefore subject to very different mechanical and pressure stresses.

[0012] The shaft connecting the cold head of the cryogenic pump to the actuating mechanism (such as a motor) is therefore designed in terms of cross section and material for the maximum stress it will be subjected to, i.e. the most demanding movement (high pressure compression, in the case exemplified here).

[0013] For cryogenic applications, the amount of heat conducted through the shaft should be minimized to limit the thermal impact on the main pressurized chamber. Limiting the conduction through the shaft also makes it possible to limit the heat input when the pump is stopped or in "standby" between ambient temperature and the cryogenic temperature of the liquefied gas. Reducing the conduction of the shaft makes it possible to reduce the heat input to the liquid reservoir whether the pump is running or not, and therefore reduces the evaporation of the liquefied gas.

[0014] Therefore, a key characteristic of a cryopump shaft having alternating motion is its ability to transfer a large amount of force and power from the actuation mechanism to the cold head, while minimizing heat input to the cold head.

[0015] These two constraints are difficult to meet.

[0016] Currently, motion transmission shafts are formed of long metal rods which increase axial space requirements, wear and guidance complexity.

[0017] SUMMARY OF THE PRESENT EMBODIMENT It is an object of the present invention to ameliorate some or all of the disadvantages of the prior art detailed above. Summary of the Invention

[0018] To this end, the compression device according to the invention, otherwise according to its generic definition given in the preamble above, is essentially characterized in that the shaft comprises a portion of reduced cross section in the longitudinal direction, said portion of reduced cross section separating two adjacent parts of the shaft, and that the shaft also comprises at least one connecting element made of a material, in particular a composite material, having a lower thermal conductivity than the material of which the shaft is made, said at least one connecting element having two ends respectively connected to the two adjacent parts of the shaft.

[0019] Thus, the invention makes it possible to limit the heat input through the activation shaft of the cryopump.

[0020] Furthermore, embodiments of the invention may include one or more of the following features: in the section of reduced cross section, the shaft has a cross section which is reduced by 30 to 85%, preferably by 65 to 80%, compared to the cross section of the remaining section of the shaft; at least one connecting element comprises a cable; the device comprises a plurality of cables arranged around a locally reduced cross-section, the reduced cross-section portion is dimensioned in such a way that it transmits a given compressive load between two adjacent portions when the shaft moves in a first direction, and at least one connecting element and possibly also the locally reduced cross-section portion is dimensioned in such a way that it transmits a given tensile load between two adjacent portions when the shaft moves in a second direction opposite to the first direction, the compressive load being less than the tensile load, the compressive load is between 1% and 50%, preferably between 2 and 25%, of the tensile load; - two adjacent parts of the shaft are separated from each other, - the device has a layer of insulation material inserted between two adjacent parts, the connecting element is made of at least one of the following materials: Kevlar fibre, glass fibre, epoxy resin with carbon fibre, the device is of the single-stage compression type and comprises a casing housing a single pressurization chamber, an intake system communicating with the pressurization chamber and configured to allow the fluid to be compressed to enter said pressurization chamber, and a movable piston to ensure the compression of the fluid in the pressurization chamber, the device also comprising an outlet configured to allow the discharge of the compressed fluid from the pressurization chamber, the device is of the two-stage compression type and comprises a first pressurization chamber, a second pressurization chamber, an intake system connected to the first pressurization chamber and configured to allow the fluid to be compressed to enter said first pressurization chamber, a transfer system connected to the first and second pressurization chambers and configured to allow the transfer of fluid from the first pressurization chamber to the second pressurization chamber, and a movable piston for ensuring the compression of the fluid in the first and second pressurization chambers, the device also comprising an outlet connected to the second pressurization chamber and configured to allow the compressed fluid to be discharged, the second pressurization chamber being defined by a part of the body of the piston and by a fixed wall of the device, the intake system being located at a first end of the device, the outlet being located at a second end of the device and the transfer system being located between the intake system and the outlet, The device is housed in a sealed enclosure containing a bath of cryogenic cooling fluid.

[0021] The invention also relates to a station for filling tanks with pressurized gas, comprising a source of liquefied gas, in particular liquefied hydrogen, and a withdrawal circuit having a first end connected to the source of liquefied gas and at least one second end intended to be connected to the tank to be filled, the withdrawal circuit comprising a pump device or a device for compressing a fluid according to any one of the above or below characteristics.

[0022] The invention may also relate to any alternative device or method comprising any combination of the features set out above or below within the context of the claims.

[0023] Further particular features and advantages will become apparent from the following description, which is provided with reference to the drawings. [Brief description of the drawings]

[0024] [Figure 1] 1 shows a schematic partial view illustrating an example of a filling station in which the compression device can be used; [Diagram 2] 1 shows a schematic partial view illustrating the structure of a first exemplary embodiment of a compression device according to the invention in longitudinal vertical section; [Diagram 3] 2 shows a schematic partial view illustrating the structure of a second exemplary embodiment of a compression device according to the invention in longitudinal vertical section; [Figure 4] 1 shows a schematic partial view in longitudinal vertical section illustrating details of an example of the construction of the shaft of a compression device according to the invention; [Diagram 5] FIG. 5 shows a perspective view of a detail of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention may be applied to any apparatus for compressing a cryogenic fluid having at least one compression stage.

[0026] In the example of Fig. 2, the compression device 1 is of the single-stage compression type. The device 1 comprises a single compression chamber 3 and a suction system 2 connected to this compression chamber 3 and allowing the fluid to be compressed to enter said compression chamber 3.

[0027] The intake system 2 may for example comprise at least one of one or more non-return valves, one or more openings or holes, at least one disk-shaped valve, or any other device or valve allowing the fluid to be compressed to enter the pressurization chamber 3 during the intake phase and preventing the discharge of the fluid during the compression phase. In particular, this intake system 2 opens when there is a given pressure difference between its two ends. In addition, the pressurization chamber 3 may optionally be equipped with a safety valve or some other safety element configured to limit the pressure in the chamber below a given safety threshold.

[0028] The pressure chamber 3 may be defined by a part of the body of the piston 5 and a fixed wall of the device. The piston 5 is capable of translational movement in a longitudinal direction A.

[0029] At a first end, the piston 5 may have a tubular portion mounted around a fixed central guide 8 .

[0030] As illustrated, the pressurization chamber 3 may be formed within a fixed chamber or tubular cavity within the piston 5, which cavity or chamber is closed at a first end. Thus, the pressurization chamber 3 may be defined at its lower portion by the closed tubular lower end of the piston 5. The intake system 2 may be located at the front end of the piston 5.

[0031] The rear end of the piston 5 may be mounted to slide against a fixed cross plate carried by a longitudinal strut. The structure of the piston 5 is designed in this case to allow a part of the piston 5 (the rear part) to slide on said plate (or other support(s)).

[0032] For example, the lower part of the piston 5 is tubular (and forms the pressure chamber 3), while the opposite part (upper part) of the piston 5 is designed to be slidable relative to a support plate. For example, the upper part of the piston 5 has one or more openings to allow the plate or support to pass through. The piston 5 can be made of one or more pieces that are joined / fixed to each other.

[0033] A first end of the central guide 8 may form a fixed wall that defines a second end of the pressure chamber 3. The other part of the pressure chamber 3 is defined by a sealing system 10 (section or the like) formed between the central guide 8 and the piston 5.

[0034] In other words, the tubular portion of the piston 5 forms a housing surrounding the entire pressurized chamber 3. Thus, the pressurized chamber 4 may be entirely contained within the tubular portion of the piston 5. The piston 5 may thus constitute the casing of the pressurized chamber 3. This architecture allows the pressurized chamber 3 to be contained within the piston 5, the walls of which may be efficiently thermalized (i.e. kept cool) as will be explained below.

[0035] The device 1 also comprises an outlet 7 configured to allow the compressed fluid to be exhausted from the pressure chamber 3 .

[0036] The outlet 7 may be provided with an anti-backflow system which may be of the same type as that of the intake system 2 (for example closed as long as the pressure difference between the pressure chamber 3 and the outside is below a given threshold value).

[0037] The piston 5 is fixed to a shaft 15, for example a metal shaft, which can move translationally along a longitudinal axis A.

[0038] The shaft 15, and therefore the piston 5, can be moved alternately in two opposite directions to ensure compression phases (in this example by applying a tensile load to the shaft 15) and intake phases of fluid into the pressure chamber 3 (by applying a compressive load to the shaft 15).

[0039] According to an advantageous particular feature, the shaft 15 comprises a locally reduced cross-section portion 151 located longitudinally between two adjacent portions 153, 154 of the shaft 15, and at least one connecting element 155 made of a material with low thermal conductivity and having two ends respectively connected to the two adjacent portions 153, 154 of the shaft 15.

[0040] In particular, the portion with reduced cross-section 151 may be dimensioned to transmit a given compressive load between the two adjacent portions 153, 154 when the shaft 15 moves in a first direction, and the at least one connecting element 155 (and possibly also the portion with reduced cross-section 151 locally) for its part is dimensioned to transmit a given tensile load between the two adjacent portions 153, 154 when the shaft 15 moves in a second direction opposite to the first direction. The compressive load will be significantly less than the tensile load.

[0041] Preferably, the cross section of shaft 15 is reduced to a value equal to or greater than the minimum value such that the pressure to which shaft 15 is subjected does not exceed the maximum allowable pressure for the material from which it is made.

[0042] This allows the cross section of the shaft 15 to be at least locally smaller than normal in order to reduce the heat conduction in the axial direction. In particular, this reduced cross section and the structure of the connecting element 155 make it possible to reduce the heat conduction between the relatively cool and hot parts of the device 1.

[0043] The mechanical properties of the shaft 15 are not compromised by this reduction in cross section, thanks to the connecting element(s) 155 which at least partially take on the larger tensile load (tension on the shaft) during the compression phase. For example, the composite structure of the connecting element(s) 155 also helps to reduce the transfer of heat along the shaft 15. For example, at least one connecting element 155 is made of at least one of the following materials: Kevlar fiber, glass fiber, epoxy resin with carbon fiber, or any other material suitable for withstanding tensile loads and having a thermal conductivity lower than that of metals or alloys traditionally used to make shafts.

[0044] Thus, the upper and lower parts of the shaft 15 may be connected by cables 155, wires, composite braids and will only meet through the bond between the cables 155 and their metal parts. This creates additional insulation.

[0045] These materials have the advantage of a higher fracture strength and a lower thermal conductivity compared to metals. These composite materials have a very high tensile strength and a low compressive strength. However, depending on the arrangement of the invention, these connecting elements are essentially or exclusively loaded in tension. In the phase of compressive loading of the shaft, it is the locally reduced cross-section portion 151 that takes up all or most of the load or stress. This is because the structure of the rod or cable 155 does not allow the latter to take a compressive load (for example through slack or relative flexibility of the cable 155). Furthermore, it is conceivable that when under tension, the two adjacent portions 153, 154 of the shaft 15 move away from each other in tension and butt up against each other in compression (although this embodiment is not preferred).

[0046] For example, during a compressive movement of the shaft (e.g., upwards when the device is oriented vertically), the shaft 15 is pulled upwards by the upper part 153, which is connected to the motor 21. The connecting element 155 is pulled under tension and therefore transmits the force to the bottom part 154. During a downward movement, when only a small portion of the upward force is needed, the upper part 153 moves downwards and presses against the lower part 154. A small contact surface is sufficient to transmit the compressive load downwards.

[0047] In the portion 151 of reduced cross section, the shaft 15 may have a cross section that is reduced by 30 to 85%, preferably by 65 to 80%, compared to the cross section of the remaining portions of the shaft 15. In other words, compared to the usual dimensions of the shaft 15 of the prior art embodiments, the present invention allows a local reduction of a critical dimension, which reduces the heat transfer but does not jeopardize the mechanical integrity requirements for the passage of loads, in particular tensile loads.

[0048] The reduced cross-sectional portion 151 may be formed along a short length, for example up to 5% of the length of the shaft 15 .

[0049] For example, this reduction in cross section is suitable for a load during inhalation which may be between 1% and 50% of the tensile load, preferably between 2 and 25%.

[0050] The at least one connecting element 155 may comprise a number of cables 155 or rods having two ends respectively fixed to two adjacent portions 153, 154 of the shaft 15 (eg, the upper and lower portions, respectively).

[0051] As illustrated, the cable 155 may be disposed around the locally reduced cross-section portion 151 .

[0052] The two adjacent portions 153, 154 of the shaft 15 located on either side of the portion 151 of reduced cross section may be of one piece or of separate pieces (of the same or different compositions).

[0053] 4, at least one layer 156 of insulating material may be inserted between two adjacent portions 153, 154, for example in the locally reduced cross-section portion 151. This layer may comprise, for example, glass fiber, hard or chrome steel, a PTFE type polymer, etc.

[0054] As illustrated in FIG. 3, this particular feature can also be applied to a compression device having two compression stages.

[0055] This non-limiting example of a compression device 1 comprises a first pressurization chamber 3, a second pressurization chamber 4 and an intake system 2 connected to the first pressurization chamber 3 and configured to allow the fluid to be compressed to enter said first pressurization chamber 3 (see above for possible realizations of this intake system 2). The device 1 also comprises a transfer system 6 connected to the first pressurization chamber 3 and the second pressurization chamber 4 and configured to allow the transfer of fluid from the first pressurization chamber 3 to the second pressurization chamber 4 (the transfer system 6 can be of the same type as that of the intake system described above).

[0056] The device also comprises a movable piston 5 for ensuring compression of the fluid in the first pressurized chamber 3 and in the second pressurized chamber 4. The device 1 also comprises an outlet 7 communicating with the second pressurized chamber 4 and configured to allow the compressed fluid to be discharged. This outlet system may comprise the same type of valve system as described above.

[0057] As illustrated, the second pressurized chamber 4 may be defined by a portion of the body of the piston 5 and a fixed wall of the device. In addition, the piston 5 may have a tubular portion mounted around a fixed central guide 8. The ends of the central guide 8 may form the fixed walls that define a portion of the second pressurized chamber 4. The device 1 comprises a sealing system 10 formed between the central guide 8 and the piston 5. The intake system 2 is located at a first end of the device 1, the outlet 7 is located at an opposite second end of the device, and the transfer system 6 is preferably located between the intake system 2 and the outlet 7.

[0058] More generally, the invention may be applied to any compression device comprising a shaft 15 driven by a drive member 21 and connected to a (piston / sleeve) mechanism for compressing a fluid, in particular a cryogenic fluid.

[0059] As illustrated, the compression system, and preferably the compression device 1, may be housed in a thermally insulated, sealed enclosure 13 that contains a reservoir 16 of cryogenic cooling fluid.

[0060] In particular, the pressurized chamber(s) 3, 4 may be submerged in a liquid phase. 30 The upper part may have a gas headspace to collect any leaks in the device 1.

[0061] Thus, the cold head of the apparatus 1 may be vertically submerged in a cryogenic bath (sometimes called a "sump").

[0062] In the case of two compression stages, the first compression stage collects fluid at low pressure and then compresses it to the second compression stage along the longitudinal axis A. High pressure fluid then leaves the second compression stage upwards (when axis A is not horizontal).

[0063] Thus, the present invention allows for the use of metal rods or shafts 15 with improved heat insulation. Preferably, composite or equivalent materials are used when under tension during high pressure travel, and metal sections are used when under compression.

[0064] The cross section of the shaft can be sized for the transmission of the force / power required during the intake movement, which is therefore much lower than if it were sized for the loads of the fluid compression movement.

[0065] This allows the length of the shaft 15 to be shortened compared to the prior art while improving thermal performance when the shaft 15 is subjected to large thermal gradients.

[0066] Compression devices 1 of this type (or multiples in series or parallel) may be used in any cryogenic installation requiring the pumping or compression of cryogenic fluids.

[0067] For example, a station for filling tanks with pressurized gas (e.g. hydrogen) may comprise a source 17 of liquefied gas and a withdrawal circuit 18 having a first end connected to the source and at least one second end intended to be connected to a tank 19 to be filled, the withdrawal circuit 18 comprising such a pump device 1. The pumped fluid is pumped through a downstream exchanger (not shown) and optionally stored in one or more pressurized buffer tanks 20. The following is a summary of the claims as originally filed: [1] A device (1) for compressing a cryogenic fluid in at least one compression stage, comprising at least one piston (5,8) and at least one sleeve (13,14) defining at least one pressurization chamber (3,4), and a shaft (15) capable of translational movement along a longitudinal axis (A), said shaft (15) being connected to one or more of said pistons or to one or more of said sleeves and capable of alternating movement in two opposite directions to ensure, by moving said at least one piston (5) and said at least one sleeve (13,14) relative to one another, a phase of compression of a fluid and a phase of intake into said at least one pressurization chamber (3,4 ... 1. A device comprising a portion (151) of reduced cross-section in a longitudinal direction (A), said portion (151) of reduced cross-section separating two adjacent portions (153, 154) of a shaft (15), said shaft (15) also comprising at least one connecting element (155) made of a material having a lower thermal conductivity than the material of which said shaft (15) is made, said at least one connecting element (155) having two ends respectively connected to said two adjacent portions (153, 154) of said shaft (15), characterized in that in said portion (151) of reduced cross-section, said shaft (15) has a cross-section which is reduced by 30 to 85%, preferably by 65 to 80%, compared to the cross-section of the remaining portions of said shaft (15). [2] The device described in [1], characterized in that the at least one connecting element (155) comprises a cable. [3] The device described in [2], characterized in that it comprises a plurality of cables (155) arranged around the portion (151) with a locally reduced cross-section. [4] The device according to any one of [1] to [3], characterized in that the portion (151) with reduced cross-section is dimensioned so as to transmit a given compressive load between the two adjacent portions (153, 154) when the shaft (15) moves in a first direction, and that the at least one connecting element (155) and possibly also the portion (151) with locally reduced cross-section are dimensioned so as to transmit a given tensile load between the two adjacent portions (153, 154) when the shaft (15) moves in a second direction opposite to the first direction, the compressive load being less than the tensile load. [5] The device according to [4], characterized in that the compressive load is between 1% and 50%, preferably between 2 and 25%, of the tensile load. [6] The device according to any one of [1] to [5], characterized in that the two adjacent parts (153, 154) of the shaft (15) are separated from each other. [7] The device according to [6], characterized in that it has a layer (156) of insulating material inserted between the two adjacent parts (153, 154). [8] The device according to any one of [1] to [7], characterized in that the connecting element (155) is made of at least one of the following materials: Kevlar fiber, glass fiber, epoxy resin with carbon fiber. [9] The device according to any one of [1] to [8], characterized in that it is of the single-stage compression type, comprising a casing housing a single pressurization chamber (3), an intake system (2) connected to the pressurization chamber (3) and configured to allow the fluid to be compressed to flow into the pressurization chamber (3), and a movable piston (5) for ensuring the compression of the fluid in the pressurization chamber (3), the device (1) also comprising an outlet (7) configured to allow the compressed fluid to be discharged from the pressurization chamber (3).

[10] A two-stage compression type, characterized in that the device (1) comprises a first pressurization chamber (3), a second pressurization chamber (4), an intake system (2) connected to the first pressurization chamber (3) and configured to allow the fluid to be compressed to flow into the first pressurization chamber (3), a transfer system (6) connected to the first pressurization chamber (3) and the second pressurization chamber (4) and configured to allow the transfer of fluid from the first pressurization chamber (3) to the second pressurization chamber (4), and a transfer system (7) for ensuring the compression of the fluid in the first pressurization chamber (3) and the second pressurization chamber (4). and a movable piston (5) connected to the second pressurized chamber (4) and configured to allow for the discharge of compressed fluid, the second pressurized chamber (4) being defined by a part of a body of the piston (5) and a fixed wall of the device, the suction system (2) being located at a first end of the device (1), the discharge port (7) being located at a second end of the device, and the transfer system (6) being located between the suction system (2) and the discharge port (7).

[11] The device described in any one of [1] to [9], characterized in that it is contained in a sealed housing (16) containing a liquid tank (13) of a cryogenic cooling fluid.

[12] A station for filling tanks with pressurized gas, comprising a source (17) of liquefied gas, in particular liquefied hydrogen, and a withdrawal circuit (18) having a first end connected to the source (17) of liquefied gas and at least one second end intended to be connected to a tank (19) to be filled, said withdrawal circuit (18) comprising a pump device (1) or a device for compressing a fluid (1) according to any one of [1] to

[11] .

Claims

1. A device (1) for compressing a cryogenic fluid in at least one compression stage, comprising at least one piston (5, 8) and at least one sleeve (13, 14) defining at least one pressurization chamber (3, 4), and a shaft (15) capable of translational movement along a longitudinal axis (A), said shaft (15) being connected to one or more of said pistons or to one or more of said sleeves and capable of alternating movement in two opposite directions in order to ensure, by moving said at least one piston (5) and said at least one sleeve (13, 14) relative to one another, stages of compression and of suction of a fluid into said at least one pressurization chamber (3, 4), said shaft 1. A device comprising: a shaft (15) having a portion (151) of reduced cross section in said longitudinal direction (A), said portion (151) of reduced cross section separating two adjacent portions (153, 154) of said shaft (15), said shaft (15) also comprising at least one connecting element (155) made of a material having a lower thermal conductivity than the constituent material of said shaft (15), said at least one connecting element (155) having two ends respectively connected to said two adjacent portions (153, 154) of said shaft (15), characterized in that in said portion (151) of reduced cross section said shaft (15) has a cross section which is reduced by 30 to 85% compared to said cross section of the remaining portions of said shaft (15).

2. The device of claim 1, wherein the at least one connecting element (155) comprises a cable.

3. 3. A device according to claim 2, characterized in that it comprises a number of cables (155) arranged around said portion (151) of locally reduced cross section.

4. 4. The device according to claim 1, characterized in that the portion (151) with reduced cross section is dimensioned so as to transmit a given compressive load between the two adjacent portions (153, 154) when the shaft (15) moves in a first direction, and that the at least one connecting element (155) and possibly also the portion (151) with locally reduced cross section are dimensioned so as to transmit a given tensile load between the two adjacent portions (153, 154) when the shaft (15) moves in a second direction opposite to the first direction, the compressive load being less than the tensile load.

5. 5. The apparatus of claim 4, wherein the compressive load is between 1% and 50% of the tensile load.

6. 6. Device according to any one of the preceding claims, characterized in that the two adjacent parts (153, 154) of the shaft (15) are separated from each other.

7. 7. Device according to claim 6, characterized in that it comprises a layer (156) of insulating material interposed between said two adjacent parts (153, 154).

8. 8. The device according to claim 1, wherein the connecting element (155) is made of at least one of the following materials: Kevlar fibre, glass fibre, epoxy resin with carbon fibre.

9. An apparatus as described in any one of claims 1 to 8, characterized in that the apparatus is of a single-stage compression type and comprises a casing accommodating a single pressurized chamber (3), an intake system (2) connected to the pressurized chamber (3) and configured to allow the fluid to be compressed to flow into the pressurized chamber (3), and a movable piston (5) for ensuring compression of the fluid in the pressurized chamber (3), the apparatus (1) also comprising an outlet (7) configured to allow the compressed fluid to be discharged from the pressurized chamber (3).

10. The device is of a two-stage compression type, and the device (1) comprises a first pressurized chamber (3), a second pressurized chamber (4), an intake system (2) connected to the first pressurized chamber (3) and configured to allow a fluid to be compressed to flow into the first pressurized chamber (3), a transfer system (6) connected to the first pressurized chamber (3) and the second pressurized chamber (4) and configured to allow a fluid to be transferred from the first pressurized chamber (3) to the second pressurized chamber (4), and a movable piston (8) for ensuring the compression of the fluid in the first pressurized chamber (3) and the second pressurized chamber (4).

9. The device according to claim 1, further comprising a piston (5) and a discharge port (7) connected to the second pressurization chamber (4) and configured to allow discharging compressed fluid, the second pressurization chamber (4) being defined by a part of the body of the piston (5) and by a fixed wall of the device, the suction system (2) being located at a first end of the device (1), the discharge port (7) being located at a second end of the device, and the transfer system (6) being located between the suction system (2) and the discharge port (7).

11. An apparatus as described in any one of claims 1 to 9, characterized in that the apparatus is contained in a sealed housing (130) including a liquid tank (16) of low-temperature cooling fluid.

12. 12. A station for filling tanks with pressurized gas, comprising a source (17) of liquefied gas and a withdrawal circuit (18) having a first end connected to said source (17) of liquefied gas and at least one second end intended to be connected to a tank (19) to be filled, said withdrawal circuit (18) comprising a pump device (1) or a device (1) for compressing a fluid according to any one of claims 1 to 11.

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