Reciprocating compressor with improved sealing for low-molecular-weight gases

Abstract

A compressor for compressing a low-molecular-weight gas, wherein between the central part (13) of a cylinder (10) and the central part (33) of a head portion (30) of a slidable member (20), slidably arranged within the cylinder (10), such as a pressure or plunger, of a compression unit (1,2) an annular gap (50) is defined as having a predetermined thickness (G), and wherein the central part (33) of the head portion (30) comprises an inlet opening (17) of a barrier-fluid (9) available at a predetermined barrier pressure higher than the delivery pressure of the compression unit. The cylinder-piston unit can be a single-acting unit (1,2,3) having a compression chamber (12) defined in the cylinder (10) by a compression face (36) at one end of the head portion (30) opposite to the rod portion (39), or a double-acting unit, in which the head portion (30) has a further compression face (32) at the other end of the head portion, defining in the cylinder (10) a second compression chamber (22) along and inside which the rod portion (39) is arranged longitudinally. In an advantageous embodiment, the central part (33) of the head portion (30) has a diameter (D1 ) which decreases going from the proximal part (31 ) towards the distal part (35) of the head portion (30). A second barrier chamber can also be defined about the rod portion (39), in particular, arranged to be supplied with the same barrier-fluid (9) as the head portion (30) of the slidable member (20).

Description

TITLERECIPROCATING COMPRESSOR WITH IMPROVED SEALING FOR LOW- MOLECULAR-WEIGHT GASESDESCRIPTIONScope of the invention

[0001] The present invention relates to a reciprocating compressor comprising at least one compression unit, such as a cylinder-piston unit or a plunger unit including a slidable member movably arranged in a cylinder to define a compression chamber, between which an improved sealing system is provided to prevent a low-molecular-weight and / or high-pressure gases, in particular hydrogen or hydrogen-containing gas mixtures, from leaking out of the compression chamber.Prior Art - Technical problems

[0002] As well known, for some decades now hydrogen has been receiving increasing attention as a possible energy vector to limit the widespread use of fossil fuels. In this perspective, it is necessary to set up hydrogen generation plants, as well as hydrogen distribution networks and hydrogen storage systems that, to be economically viable, must operate at high pressures, even in the order of hundreds of bars, which can be reached by the use of reciprocating compressors.

[0003] It is also known how difficult is to provide low-molecular-weight gastight sealing systems, in particular, for hydrogen-containing gases, due to the tendency for those gases to leak through conventional containment systems. The normal leak of small quantities of hydrogen from an equipment into the atmosphere is an unwelcome event. Apart from the value of the lost product, it must be taken into account the high cost of protection measures that have to be adopted to comply with the potentially explosive atmosphere regulations.

[0004] A possible solution to the problem of hydrogen leakage from a reciprocating machine is to provide the piston with a large number of piston rings as sealing elements about the piston end portion(s) facing a compression chamber(s) of the machine. However, this would require a longer piston-cylinderunit, thus increasing the compressor manufacturing costs. Similarly, a plunger compressor should be equipped with a plunger long enough to act as a seal portion and prevent the gas leakage.

[0005] A further issue with reciprocating compressors, in particular if the compression ratio is high, is the uneven wear of the connecting rod bearing. As well known, when the top dead centre is trespassed by the slidable member, i.e. when the thrust direction is reversed after a compression stroke, the upper half of the connecting rod bearing remains fully loaded, whereas the lower half remains unloaded, which causes the uneven wear of the bearing. Single-acting compressors, including plunger compressors, are particularly affected by this issue, as the thrust acting on the connecting rod due to the pressure of the gas in the only compression chamber of the compression is not counterbalanced by a counter-pressure due to the gas present in a second opposite compression chamber, as it is the case for double-acting compressors.

[0006] EP2796719 A1 , US 4,174,929 and US 2018 / 051684 A1 deal with particular compressor unit arrangements.Summary of the invention

[0007] It is therefore an object of the present invention to provide a reciprocating compressor providing an improved seal against leakage and outward loss of low-molecular-weight gases, in particular a hydrogen-containing gas, with respect to most commonly used reciprocating compressors, for a same type of gas and a given operating pressure.

[0008] It is also an object of the present invention to provide a high-pressure reciprocating compressor, i.e. one working at a delivery pressure in the order of 100 bar or more, which provides an improved seal against gas leakage and outward loss with respect to most commonly used reciprocating compressors, for a same type of gas.

[0009] It is a particular object of the invention to provide such a reciprocating compressor including at least one piston-cylinder unit, which allows a tightness level while including a relatively small number of piston rings arranged about the piston end portion(s) facing a compression chamber(s), or while having a relatively short plunger sealing area of an own plunger unit thereof, for a same type of gas and / or a given operating pressure.

[0010] It is another particular object of the invention to provide such a reciprocating compressor that overcomes the rod bearing differential wear issue due to the upper rod bearing half failing to become disengaged at the moment of the thrust inversion at top dead centre.

[0011] Moreover, it is an object of the invention to provide such a compressor that is able to achieve a better compression efficiency than conventional reciprocating compressors when the latter are used to treat a low- molecular-weight gas.

[0012] These and other objects are achieved by a compressor configured to compress a low-molecular-weight gas up to a predetermined delivery pressure, as described in claim 1 . Advantageous embodiments of that compressor are defined in the dependent claims.

[0013] The compressor includes at least one compression unit comprising a cylinder and a slidable member arranged within the cylinder, the slidable member having a head portion and a rod portion that is integral with the head portion and is arranged to cause the head portion to perform a stroke within the cylinder, said cylinder having a cylinder proximal part, a cylinder central part and a cylinder distal part, with respect to the rod portion, the cylinder proximal part and the cylinder distal part having a first inner diameter of the cylinder, the head portion having a head portion proximal part, a head portion central part and a head portion distal part, with respect to the rod portion, the head portion proximal part and the head portion distal part having a first outer diameter of the head portion, wherein the first inner diameter of the cylinder and the first outer diameter of the head portion are equal to a common nominal diameter of the cylinder and head portion, apart from a predetermined coupling tolerance, said cylinder central part having a second inner diameter, said head portion central part having a second outer diameter, the distal part of the head portion comprising a compression face on the opposite side with respect to the central part of the head portion, the compression face defining a compression chamber within the cylinder, the compression chamber having a suction port and a delivery port for the low- molecular-weight gas,wherein, according to the invention, the second inner diameter of the cylinder central part is larger than the second outer diameter of the head portion central part by a predetermined amount, i.e., by a predetermined gap thickness, such that an annular gap of a thickness equal to said predetermined amount is defined between the head portion central part and the cylinder central part, and wherein the cylinder central part has at least one inlet opening of a barrierfluid, wherein the inlet opening is in a fluid communication with a source of said barrier-fluid configured to feed the barrier-fluid at a predetermined barrier pressure into the annular gap, so that the barrier-fluid occupies the annular gap, and the annular gap forms a barrier chamber between the compression chamber and a space opposite to the compression chamber with respect to the head portion of the slidable member.

[0014] In the case of a compressor equipped with a single-acting compression unit, such as a single-acting cylinder-piston unit or a plunger unit, a sealing chamber is present between the compression chamber and the outside of the compressor, and such compression chamber is configured to contain a barrier-fluid at a pressure that is always higher than or at least equal to the pressure in the compression chamber. This sealing chamber provides an additional seal effect between the compression chamber and the outside, i.e. the proximal-side space adjacent to the head portion proximal part. In the case of cylinder-piston unit 1 or 2, this additional seal effect is summed to the seal effect provided by piston rings 41 ,45 of cylinder-piston units 1 and 2. Instead, in the case of plunger unit 3, this additional effect is summed to the seal effect provided by respective mutually-contacting surfaces of proximal and distal parts 1 1 ,15 of cylinder 10, on the one hand, and of proximal and distal parts 31 ,35 of plunger 20, on the other hand.

[0015] In other words, the barrier chamber cooperates with the piston rings, in the case of a single-acting cylinder-piston unit, or with the sealing portion of a cylinder-plunger coupling, in the case of a plunger unit, to contain the gas being compressed within the compression chamber. Since the barrier-fluid is less prone to leakage than a low-molecular-weight gas, it is therefore possible to ensure a given sealing performance with a smaller number of piston rings, in the case of a cylinder-piston unit, or with a shorter sealing portion of the cylinderplunger coupling, in the case of a plunger unit. With a smaller number of pistonrings, or with a shorter length of the sealing portion, it becomes possible to reduce the length and, therefore, the overall dimensions of the compressor unit and of the compressor itself, which simplifies the compressor construction and allows containing the manufacturing costs.

[0016] Thanks to the invention, a space- and cost-saving compressor is provided that makes it possible to limit the loss of compressed gas to the environment and that, in the case of a double-acting compressor unit, as described below, makes it possible to limit the gas leakage between the opposite compression chambers, thus improving the compression efficiency. This way, the economic losses due to compressed gas leakage can be reduced. Moreover, in the case of such a flammable gas as hydrogen or of certain mixtures thereof, the risk of forming an explosive atmosphere around the compressor can be limited as well.

[0017] In this description, expressions such as "in a pneumatic communication" and "in a hydraulic communication" between two spaces indicate that a connection means is present between these spaces, such as ducts, piping elements, etc., through which a gas or a liquid, respectively, can flow. Moreover, these gas and liquid are not limited to air and water, respectively, contrary to a common meaning of “pneumatic” and “hydraulic”.

[0018] Preferably, the thickness of the annular gap between the cylinder central part and the head portion central part of the compression unit is at least 1 mm, in particular said thickness is at least 5 mm, more in particular said thickness is at least 10 mm.

[0019] In some embodiments of the invention, the second inner diameter of the cylinder central part is equal to the first inner diameter of the cylinder distal and proximal parts, and is therefore equal to the common nominal diameter; the second outer diameter of the slidable member head portion central part is smaller than the common nominal diameter by the above predetermined gap thickness, i.e. the difference between the common nominal diameter and the second outer diameter of the head portion central part is equal to that gap thickness; the head portion central part has a length at least equal to the stroke of the slidable member.

[0020] In other words, the annular gap, and so the sealing chamber, can be made by using a piston or a plunger whose intermediate portion has a diameter smaller than the nominal one, which can be obtained, for instance, by removing material only from a conventional piston or plunger having a same outer diameter throughout its length. These embodiments particularly relate to use the invention in existing compressors and compression units thereof, as a slidable member can be more easily machined than the inside of a cylinder.

[0021] In this case, the barrier chamber moves integrally with the slidable member while the latter performs its stroke within the cylinder.

[0022] However, in other embodiments of the invention, the annular gap, and so the sealing chamber, can be made by using a cylinder that has an intermediate portion whose diameter is larger than the nominal diameter, which can be obtained, for example, by removing material only from a conventional cylinder having a same inner diameter throughout its length. In other words, in this case: the second outer diameter of the slidable member head portion central part is equal to the first inner diameter of the cylinder distal and proximal parts, and is therefore equal to the common nominal diameter; the second inner diameter of the cylinder central part is larger than the common nominal diameter by the above predetermined gap thickness, i.e. the difference between the second inner diameter of the cylinder central part and the common nominal diameter is equal to the gap thickness; the central part of the cylinder has a length at most equal to a difference between the length of the head portion central part and the stroke of the slidable member.

[0023] In particular, the compression unit is a cylinder-piston unit in which the slidable member is a piston, wherein the head portion proximal and distal parts each comprise at least two elastic piston rings at a distance from one another and arranged to slide within the cylinder, and wherein the head portion central part is located between the elastic piston rings of the head portion proximal part and the elastic piston rings of the head portion distal part.

[0024] In an embodiment of the invention, the cylinder-piston unit is a double-acting cylinder-piston unit, i.e., the compression face of the head portion is a first compression face, the compression chamber is a first compression chamber, and the proximal part of the head portion comprises a second compression face at an opposite side of the first compression face with respect to the central part of the head portion, wherein the second compression face and the cylinder define a second compression chamber through the whole length of which the rod portion is arranged.

[0025] In this case, the barrier chamber remains defined between the two compression chambers, which are always operating at different pressures. The barrier chamber contains a fluid at a pressure constantly above or even at least equal to the maximum pressure reached in each compression chamber. Therefore, the barrier chamber contributes together with the piston rings to seal the compression chambers with respect to each other, also in this case with a limited number of piston rings and, therefore, with a limited length of the piston head portion.

[0026] This makes it possible, with a limited number of piston rings and thus a limited length of the piston head portion, to drastically limit the gas leakage between one compression chamber and the other, thus maintaining a high compression efficiency of the compressor.

[0027] Advantageously, the head portion central part of the of the slidable member has a shape whose diameter decreases from the proximal part towards the distal part of the head portion, in particular, this shape is a truncated-cone shape.

[0028] This way, the resultant of the pressure forces acting on the head portion central part due to the barrier-fluid contained in the barrier chamber has a component directed away from the only compression face, in the case of a single-acting unit, or directed away from the compression face opposite to the rod portion. This component contributes to cause the connecting rod bearing to be positioned in its seat in such a way that the lower portion of the bearing is compressed, i.e. the lower portion of the bearing surface thereof to be engaged, once the slidable member has trespassed the top dead centre, i.e. during the subsequent stroke of the slidable member. By “bottom dead centre” it is meant the extreme position of the slidable member corresponding to a minimumdimension of the compression chamber defined by the only compression face of the slidable member, or in any case, in a double-acting cylinder-piston unit, by the compression face that is opposite to the piston rod with respect to the piston head itself.

[0029] In some embodiments, the cylinder central part has, at a middle portion thereof, at least one barrier-fluid outlet opening. In this case, preferably, the inlet opening and the outlet opening are realised at the same cross-section of the cylinder, and the cylinder has, at said cross-section, a circumferential recess, i.e. a circumferential groove, communicating with the barrier-fluid inlet opening and with the barrier-fluid outlet opening. This allows a uniform distribution of the barrier-fluid within the barrier chamber.

[0030] In some embodiments of a compressor including a cylinder-piston unit, the annular gap providing the barrier chamber is a first annular gap providing a head portion annular chamber, and the rod portion is provided with proximal and distal seals between which a second annular gap is defined, arranged to be supplied by a rod portion barrier-fluid, so as to provide a rod portion barrier chamber. This is a further improvement of the sealing system of a double-acting cylinder-piston unit against the gas leakage to the environment, beside the improved seal provided by the head portion barrier chamber between the two compression chambers, and the related above-mentioned performance advantages.

[0031] In these cases, the rod portion barrier-fluid may be the same as the piston head portion barrier-fluid, and the head portion barrier chamber and the rod portion barrier chamber can be supplied by the same circuit.

[0032] In particular, the head portion barrier chamber and the rod portion barrier chamber, i.e. the first and second annular gaps, have a mutual connection selected between: a parallel mutual connection, wherein the first and second annular gaps are both arranged to receive the common barrier-fluid directly from the barrier-fluid source; a serial mutual connection, wherein the cylinder and the second annular gap also have respective barrier-fluid outlet openings, wherein the outlet opening of the cylinder hydraulically is connected to the inlet opening of the second annular gap or barrier chamber.

[0033] In a modification of the above embodiment, the compressor unit may comprise a closed barrier-fluid supply circuit including the barrier-fluid source.

[0034] In one embodiment, such a supply circuit comprises a compensation vessel within which a deformable diaphragm or membrane is arranged to define a first variable-volume chamber and a second variable-volume chamber that are in pneumatic communication with the compression chamber delivery port and with the cylinder inlet opening, respectively, and the second variable-volume chamber is configured to contain a predetermined quantity of the barrier-fluid, whereby the barrier-fluid can be fed into the annular gap at a pressure dependent on the pressure of the low-molecular-weight gas at the delivery port.

[0035] In this way, a closed circuit containing a predetermined amount of barrier-fluid is used for the barrier chamber, requiring only a minimal amount or no barrier fluid to be fed, while exploiting the compression power of the compressor itself to maintain the pressure in the barrier chamber. The complete separation of the sealing circuit from the process by the deformable membrane guarantees against gas leakage to the outside.

[0036] In an alternative embodiment, the compressor unit may comprise a cooling circuit configured to convey a cooling fluid at a predetermined cooling fluid pressure higher than the delivery pressure. In such a case, advantageously, the inlet opening of the cylinder is in a hydraulic communication with said cooling circuit, whereby the barrier-fluid comprises a portion of said cooling fluid. In other words, the source of the barrier-fluid is the cooling circuit of the compressor.

[0037] Also in this case, the sealing system can be made without introducing fluids and supply circuits other than those already 'naturally' present in the compressor, thus limiting the manufacturing and operation costs of the system.

[0038] In one embodiment, the compressor includes multiple compression stages, i.e. multiple compression units that are arranged to operate at consecutive suction-delivery pressure ranges with respect to each other. In this case, the inlet opening of the barrier-chamber of a compression unit arranged to perform an earlier compression stage may be in pneumatic communication with the delivery port of the compression chamber of a compression unit arranged to perform a later compression stage, whereby the barrier-fluid comprises the low- molecular-weight gas itself. A pressure-reducing device may be provided along a duct provided to allow said pneumatic communication, in order to possiblyreduce the gas withdrawn from the delivery port of the unit performing the later, high-pressure stage, to a value low enough to properly work as the barrier-fluid in one or more of the earlier subsequent stages, without entering it. In this way, no fluid extraneous to the process is introduced into the barrier chamber, which prevent the risk of foreign-fluid contamination of the process itself.Brief description of the drawings

[0039] The invention will be illustrated below with a description of some embodiments, by way of example and not of limitation, with reference to the accompanying drawings, in which:Figs. 1 and 2 are diagrammatic partial longitudinal cross-section views of a single-acting cylinder-piston unit of a reciprocating compressor according to an embodiment of the invention, in which the piston is at the top dead centre and at the bottom dead centre, respectively;Fig. 3 is a diagrammatic side view of the piston head portion of the cylinderpiston unit in Figs. 1 and 2;Figs. 4 and 5 are diagrammatic partial longitudinal cross-section views of a single-acting cylinder-piston unit of a reciprocating compressor according to another embodiment of the invention, in which the piston is at the top dead centre and at the bottom dead centre, respectively;Fig. 6 is a diagrammatic longitudinal cross-section view of the cylinder of the cylinder-piston unit in Figs. 4 and 5;Figs. 7 and 8 are diagrammatic partial longitudinal cross-section views of a plunger unit of a reciprocating compressor according to a further embodiment of the invention, in which the plunger is at the top dead centre and at the bottom dead centre, respectively.Fig. 9 is a diagrammatic side view of the plunger in Figs. 7 and 8;Figs. 10 and 1 1 are diagrammatic partial longitudinal cross-sectional views of double-acting cylinder-piston units of a reciprocating compressor according to two alterative embodiments of the invention, in which the barrier chamber has a single barrier-fluid inlet-outlet opening, and is internally defined by a cylindrical surface and by a truncated-cone surface, respectively;Figs. 12 and 13 are diagrammatic partial longitudinal cross-section views of double-acting cylinder-piston units of a reciprocating compressoraccording to two further alterative embodiments of the invention, in which the barrier chamber has a barrier-fluid inlet opening and a barrier-fluid outlet opening distinct from the barrier-fluid inlet opening, and is internally defined by a cylindrical surface and by a truncated-cone surface, respectively;Fig. 14 is a diagrammatic longitudinal cross-section view showing the double-acting cylinder-piston unit of Fig. 12 in greater detail;Figs. 15 and 16 are longitudinally sectioned perspective views of the cylinder-piston unit and of the cylinder of Fig. 14, respectively;Fig. 17 is a partial longitudinal cross-section view of a cylinder-piston unit as in Fig. 2, in which a barrier chamber is also provided on the piston rod portion;Figs. 18 and 19 show feeding diagrams of the head portion barrier chamber and of the rod portion barrier chamber of Fig. 17, in which the two barrier chambers are arranged parallely and serially to each other, respectively, to be fed with a same barrier-fluid;Fig. 20 is a diagrammatic partial longitudinal cross-section view of a cylinder-piston unit as in Fig. 12, in which the head portion barrier chamber and the rod portion barrier chamber are arranged to receive a cooling fluid of the cylinder-piston unit as the barrier-fluid;Fig. 21 is a diagrammatic partial longitudinal cross-section view of a cylinder-piston unit as in Fig. 10, in which a barrier-fluid supply circuit including a compensation vessel is provided.Description of preferred embodiments

[0040] With reference to Figs. 1 -9, three compression units 1 , 2 and 3 of compressors configured to compress low-molecular-weight gases up to a predetermined delivery pressure are described. Compression units 1 , 2 and 3 comprise a cylinder 10 and a slidable member 20. Slidable member 20 includes a head portion 30 slidingly coupled within cylinder 10, in the direction of a common longitudinal axis 5 along which cylinder 10 and head portion 30 extend.

[0041] Cylinder 10 and head portion 30 comprise respective parts that are in mutual sliding contact with each other, and that have a common nominal diameter D and respective predetermined coupling tolerances, which can bedecided in a manner known to a skilled person, according to the operating conditions of the compression units.

[0042] In particular, compression units 1 and 2 consist of cylinder-piston units (Figs. 1 -3 and 4-6), in which slidable member 20 is a piston. On the other hand, compression unit 3 consists of a plunger unit (Figs. 7-9) in which slidable member 20 is a plunger.

[0043] Beside head portion 30, slidable member 20 includes a rod portion 39 integrally connected to head portion 30, and arranged to cause head portion 30 to perform a stroke of length C within cylinder 10. Rod portion 39 is typically a connecting rod that is linked, in a manner not shown, to a crankshaft that can be a common crankshaft of several compression units 1 , 2 or 3 and respective modifications thereof.

[0044] Rod portion 39 has a diameter D3 which, in cylinder-piston units 1 and 2 (Figs. 1 -6) is smaller than nominal diameter D of head portion 30, that is therefore a maximum diameter of rod portion 39. On the contrary, in plunger unit 3 (Figs. 7-9) rod portion diameter D3 is substantially equal to nominal diameter D of head portion 30, in other words rod portion 39 forms an extension member of the same nominal diameter D of head portion 39, said extension member oriented towards the transmission, not shown, of the compressor.

[0045] Head portion 30 comprises a proximal part 31 , a central part 33 and a distal part 35 (Figs. 1 and 3). Similarly, cylinder 10 has a proximal part 1 1 , a central part 13 and a distal part 15 (Fig. 2). The words “proximal” and “distal” indicates the position of head portion parts 31 ,35 and of cylinder parts 1 1 ,15 with respect to rod portion 39. Moreover, as anticipated, the diameter of proximal parts 1 1 ,31 , and of distal parts 15,35 of cylinder 10 and head portion 30, respectively, is equal to nominal diameter D, apart from respective coupling tolerances that allows proximal parts 11 ,31 , and distal parts 15,35 to be in a sliding contact with each other.

[0046] Opposite to head portion central part 33, distal part 35 of head portion 30 comprises a compression face 36 defining a compression chamber 12 in cylinder 10, provided with a suction port 14 for the gas to be compressed and with a delivery port 16 for the compressed gas.

[0047] In particular, in the cylinder-piston units of Figs. 1 -6, at least two piston rings 41 ,45 are arranged on proximal part 31 and on distal part 35 of head portion 30 of piston 20, in such a way to slide in contact with the inner surface of cylinder 10. On the other hand, in plunger unit 3 of Figs. 7-9, proximal and distal parts 31 -33 of head portion 30 are smooth sealing portions arranged to slide themselves in contact to the inner surface of cylinder 10.

[0048] As shown, cylinder central part 13 has a second inner diameter D2 (Figs. 1 , 2 and 6), while piston head portion central part 33 has a second outer diameter D1 (Figs. 3, 4 and 5). Second inner diameter D2 of cylinder central part 13 and second outer diameter D1 of head portion central part 33 the differ by a predetermined quantity G = D2-D1 , more precisely, cylinder second inner diameter D2 is larger than head portion second outer diameter D1 by said predetermined quantity G (Figs. 1 and 4).

[0049] Therefore, an annular gap or chamber 50 is defined between cylinder central part 13 and head portion central part 33. Annular gap 50 has a major diameter equal to cylinder second inner diameter D2 and a minor diameter equal to head portion second outer diameter D1 .

[0050] A bore 17 is drilled through the wall of cylinder central part 13 to provide an inlet opening for a fluid into annular gap 50. Bore 17 is preferably arranged to be at a first axial end 51 of annular gap 50 when slidable member 20 is at the top dead centre (Figs. 1 , 4 and 7), and to be at a second axial end 52 of annular gap 50, opposite first axial end 51 , when slidable member 20 is at the bottom dead centre (Figs. 2, 5 and 8). Inlet opening 17 is in fluidic communication with a source 8 of a barrier-fluid 9 (Figs. 7 and 8), and is configured to feed barrier-fluid 9 into annular gap 50 at a predetermined barrier pressure.

[0051] Thus, barrier-fluid 9 fills and occupies annular gap 50, which forms therefore a barrier chamber 50 between compression chamber 12 and a space 22 opposite to compression chamber 12 with respect to head portion 30 of slidable member 20. Barrier-fluid 9 and barrier chamber 50 provide an additional seal effect between compression chamber 12 and space 22, in addition to the seal effect provided by piston rings 41 ,45 of cylinder-piston units 1 and 2 and, in the case of plunger unit 3, by respective surfaces contacting each other ofproximal and distal parts 1 1 ,15 of cylinder 10 and of proximal and distal parts 31 ,35 of head portion 30 of plunger 20.

[0052] In the figures, for the sake of clarity, the difference between common nominal diameter D and second outer diameter D1 of head portion central part 33 and the difference between cylinder second inner diameter D2 and common nominal diameter D, nominal are exaggerated, in other words thickness G of annular gap 50 is represented regardless of the scale. Thickness G is preferably at least 1 mm, more in particular it is at least 5 mm, more in particular it is at least 10 mm, depending on the operating pressure of compression unit 1 -3 in a manner that can be easily understood by the skilled person.

[0053] Figs. 1 -3 and 7-9 refer to embodiments of the invention in which cylinder second inner diameter D2 is equal to the first inner diameter of proximal and distal parts 1 1 and 15 of cylinder 10, and is therefore equal to common nominal diameter D of cylinder 10 and head portion 30, and in which second outer diameter D1 of head portion central part 33 of slidable member 20 is smaller than the first outer diameter of proximal and distal parts 31 and 35, and is therefore smaller than common nominal diameter D by gap thickness G. Therefore, annular gap 50 is included in the profile of head portion 30.

[0054] In this case, annular gap 50 can be obtained, for instance, by removing a portion of material of a given length L1 from central part 33 of head portion 30 of slidable member 20. In particular, in the case of cylinder-piston unit1 of Figs. 1 -3, central part 33 is located between piston rings 41 and 45 of head portion 30 of piston 20 (Fig. 3), therefore length L1 of central part 33 having reduced (second) outer diameter D1 is smaller than the distance between the innermost piston rings 41 and 45 of head portion 30. Moreover, as Figs. 1 and2 show, during stroke C of slidable member 20, annular gap 50 moves together with head portion 30, and length L1 of head portion central part 33 is preferably equal to or at least not shorter than stroke C of piston 20. This way, inlet opening 17 is longitudinally located at first and second axial ends 51 ,52 of annular gap 50 when the slidable member 30 is at the top dead centre (Figs. 1 and 7) and at the bottom dead centre (Figs. 2 and 8) of its stroke C, respectively, and in any case is always longitudinally located within length L1 of annular gap 50.

[0055] On the other hand, Figs. 4-6 refer to an embodiment of the invention in which second outer diameter D1 of head portion central part 33 is equal to thefirst outer diameter of the corresponding proximal and distal parts 31 and 35 of head portion 30, and is therefore equal to common nominal diameter D of cylinder 10 and head portion 30, and in which second inner diameter D2 of cylinder central part 13 is larger than the first inner diameter of the corresponding proximal and distal parts 1 1 and 15, and is therefore larger than common nominal diameter D by gap thickness G. Therefore, annular gap 50 is included in the profile of cylinder 10.

[0056] In this case, annular gap 50 can be obtained, for instance, by removing a portion of material of a given length L2 from central part 13 of cylinder 10. In any case, cylinder central part 13 having increased (second) inner diameter has a length L2 at most equal to a difference L-C between the length L of head portion central part 33 and stroke C of slidable member 20, where head portion central part 33 is understood as the region of central part 33 between the innermost piston rings 41 ,45. During stroke C of slidable member 20, fixed annular gap 50 is always between cylinder central part 13 and head portion and central part 33.

[0057] Similar considerations apply to a compression unit consisting of a plunger unit, in which the diameter of the plunger central part is larger than the nominal common diameter of the plunger and the cylinder.

[0058] Figs. 10-14 and 17-21 refer to double-acting piston-cylinder units 1 a- 1 i according to advantageous embodiments of piston-cylinder unit 1 of Figs. 1 and 2, which, on the other hand, can refer to a single-acting embodiment. In double-acting piston-cylinder units 1 a-1 i, proximal part 31 of head portion 30 comprises a second compression face 32 opposite to first compression face 36 with respect to head portion central part 33. Cylinder 10 and second compression face 32 defines a second compression chamber 22 through the whole length of which rod portion 39 is arranged. Second compression chamber 22 is provided with a suction port 24 for the gas to be compressed and with a delivery port 26 for the compressed gas.

[0059] In two embodiments shown in Figs. 10 and 12, head portion central part 33 of piston-cylinder units 1 a and 1 c has a cylindrical shape of diameter D1 . As an alternative, in two advantageous embodiments shown in Figs. 1 1 and 13, head portion central part 33 of piston-cylinder units 1 b and 1d has a truncated- cone shape or, more generally, a shape in which the diameter D1 decreasesfrom a maximum value D1 " next to head portion proximal part 31 , to a minimum value D1 ' next to head portion distal part 33.

[0060] Fig. 14 and, partially, Figs. 15 and 16, show piston-cylinder unit 1 c of Fig. 12 in greater detail, in particular in Fig. 16 cylinder 10 of piston-cylinder unit 1 c is represented without head portion 30 of piston 20, in order to show the inside of cylinder 10.

[0061] With reference to Figs. 12-16, cylinder central part 13 of pistoncylinder unit 1 c is provided with an outlet hole or opening 18 for barrier-fluid 9, in addition to inlet opening 17. Outlet opening 18 is arranged along cylinder 10 so as to be located at first axial end 51 of head portion central part 33 when piston 20 is at the top dead centre, as in Fig. 1 , and to be located at second axial end 52 of head portion central part 33, when piston 20 is at bottom dead centre, as in Fig. 2. This way, barrier-fluid 9 can pass through annular gap 50, beside pressurizing it. This feature is obviously extendable to the case of the singleacting cylinder-piston unit 20 of Figs. 1 -3, and to the case of the plunger unit 3 of Figs. 7-9.

[0062] In particular, as shown in Fig. 16, cylinder 10 has a recess or circumferential groove 19 at a cross-section, preferably at the cross-section at which input and output openings 17,18 are made, with which the circumferential groove 19 is in communication.

[0063] Figs. 17-19 refer to three double-acting cylinder-piston units 1 e, 1 f and 1 g according to respective modifications of an advantageous embodiment of the invention, in which rod portion 39 is provided with proximal and distal seals, not shown, between which a second annular gap is defined, providing a rod portion barrier chamber, i.e. a chamber arranged to be supplied by a rod portion barrier-fluid 9 or 9’. In cylinder-piston units 1 f and 1 g of Figs. 18 and 19, both annular gaps, i.e. head portion barrier chamber 50 and the rod portion barrier chamber, are arranged to receive a same barrier-fluid 9 from a common barrier-fluid source 8. In particular, the second annular gap of piston-cylinder unit 1 g (Fig. 19) has an inlet opening 67 in hydraulic communication with barrierfluid source 8, whereby both barrier chambers are arranged to receive common barrier-fluid 9 directly from barrier-fluid source 8, i.e., they are arranged in parallel with respect to barrier-fluid source 8. As an alternative, cylinder 10 and the second annular gap of piston-cylinder unit 1 f (Fig. 18) have respective outletopenings 18,68 for common barrier-fluid 9, wherein outlet opening 18 of head portion barrier chamber 50 is hydraulically connected with inlet opening 67 of the rod portion barrier chamber, in other words the two barrier chambers are serially arranged with respect to the common barrier-fluid source 8.

[0064] Even if Figs. 17-19 only show double-acting cylinder-piston units 1 e- g in which the barrier chambers are provided with distinct inlet openings 17,67 and outlet openings 18,68, the scope of the invention also encompasses single- or double-acting cylinder-piston compression units in which a rod portion barrier chamber is provided besides head portion barrier chamber 50, and each barrier chamber has one inlet / outlet opening, or distinct inlet and outlet openings, which can be obtained by modifying what is shown in Figs. 17-19 in a way that is obvious to the skilled person.

[0065] Fig. 20 shows a cylinder-piston unit 1 h according to an embodiment of the invention, in which a cooling circuit 70 is configured to convey a cooling fluid 7 available at a cooling fluid pressure higher than the delivery pressure of the compressor including cylinder-piston unit 1 h. Moreover, both rod and head portion barrier chambers are hydraulically connected with the cooling circuit 70 in parallel with each other, as shown, or, in an unshown alternative modification of this embodiment, they can be serially connected to each other. Therefore, a portion of cooling fluid 7 contained in cooling circuit 70 supplies barrier-fluid 9 to both rod and head portion barrier chambers, and inlet opening 17 of barrier chamber 50 is in a hydraulic communication with cooling circuit 70.

[0066] Even if Fig. 20 only shows a double-acting cylinder-piston unit 1 h that includes both the head portion barrier chamber and the rod portion barrier chamber, and in which the barrier chambers are provided with both inlet openings 17,67 and outlet openings 18,68, the scope of the invention also encompasses compressors including a single-acting cylinder-piston unit or a double-acting cylinder-piston unit provided with a head portion barrier chamber only, as well as compressors including a single-acting cylinder-piston unit or a double-acting cylinder-piston unit including a single- or double-acting cylinderpiston unit with a head portion barrier chamber and possibly with a rod portion barrier chamber, the chamber(s) having (each) a single inlet / opening 17 or distinct inlet and outlet openings 17,18 and an outlet opening, wherein the chamber or chambers-barriers are in hydraulic communication with a coolingcircuit of the cylinder-piston unit, with modifications to Fig. 20 obvious to the person skilled in the art.

[0067] With reference to Fig. 21 , in a piston-cylinder unit 1 i of a compressor according to an embodiment of the invention, supply circuit 4 comprises a compensator tank 40 within which a deformable membrane 45 defines a first chamber 41 and a second chamber 42 with variable volume. The first chamber 41 of the compensator tank 40 is in pneumatic communication with the delivery ports 16,26 of the compression chambers 12,22, whereby, during operation of compressor unit 1 i, it contains a variable volume of the low-molecular-weight gas treated by compressor unit 1 i, at the delivery pressure of compressor unit 1 i. The second chamber 42 is in communication with the suction ports 17,67 of the annular gaps or the barrier chambers of head portion 30 and piston rod portion 39 of piston 20, and contains a predetermined quantity of barrier-fluid 9, at a pressure which depends on the pressure in the first chamber 41 , i.e. the delivery pressure of compressor unit 1 i.

[0068] Even if Fig. 21 only shows a double-acting cylinder-piston unit 1 h that includes both the head portion barrier chamber and the rod portion barrier chamber, the scope of the invention also encompasses compressors including a single-acting cylinder-piston unit or a double-acting cylinder-piston unit provided with a head portion barrier chamber only, as well as compressors including a single-acting cylinder-piston unit or a double-acting cylinder-piston unit including a single- or double-acting cylinder-piston unit with a head portion barrier chamber and possibly with a rod portion barrier chamber, wherein the first chamber of the compensation vessel is in pneumatic communication with the delivery port of the one or more compression chambers, and the second chamber is in communication with the suction port(s) of the barrier chamber(s), with modifications to Fig. 21 obvious to the person skilled in the art.

[0069] In an embodiment not shown, the compressor has multiple compression stages, i.e. it comprises a plurality of serially-arranged compression units 1 or 1 a-g or 2 or 3 as shown in Figs. 1 -21 , these compression units operating within consecutive suction-delivery pressure intervals. In this case, inlet opening 17 of barrier chamber 50 of a upstream compressor unit arranged to perform a lower-pressure compression stage may be in pneumatic communication with the delivery port 16,26 of the compression chamber of adownstream compressor unit arranged to perform a higher-pressure compression stage, whereby barrier chamber 50 of the upstream, lower- pressure compression unit is supplied with a barrier-fluid 9 that includes the low- molecular-weight gas processed by the downstream, higher-pressure compression unit.

[0070] The aforementioned description of embodiments of the invention is capable of showing the invention from a conceptual point of view in such a way that others, using the known technique, will be able to modify and / or adapt in various applications such specific embodiments without further research and without departing from the inventive concept, and, therefore, it is understood that such adaptations and modifications will be considered equivalent to the modifications and specific embodiments. The means and materials for realising the various functions described may be of various kinds without departing from the scope of the invention. It is understood that the expressions or terminology used are purely descriptive and, therefore, not limiting.

Claims

CLAIMS1. A compressor configured to compress a low-molecular-weight gas up to a predetermined delivery pressure, wherein at least one compression unit (1 ,1 a-i,2,3) is provided comprising a cylinder (10) and a slidable member (20) arranged within said cylinder (10), said slidable member (20) having a head portion (30) and a rod portion (39) integral with said head portion(30) and arranged to cause said head portion (30) to perform a stroke (C) within said cylinder (10), said cylinder (10) having a proximal part (1 1 ), a central part (13) and a distal part (15), with respect to said rod portion (39), said proximal part (1 1 ) and said distal part (15) of said cylinder (10) having a first inner diameter, said head portion (30) having a proximal part (31 ), a central part (33) and a distal part (35), with respect to said rod portion (39), said proximal part(31 ) and said distal part (35) of said head portion (30) having a first outer diameter, wherein said first inner diameter of said cylinder (10) and said first outer diameter of said head portion (30) are equal to a common nominal diameter (D) of said cylinder (10) and said head portion (30), apart from a predetermined coupling tolerance, said central part (13) of said cylinder (10) having a second inner diameter (D2), said central part (33) of said head portion (30) having a second outer diameter (D1 ), said distal part (35) of said head portion (30) comprising a compression face (36) on the opposite side with respect to said central part (33) of said head portion (30), said compression face (36) defining a compression chamber (12) within said cylinder (10), said compression chamber having a suction port (14) and a delivery port (16) for said low-molecular-weight gas, characterised in that said second inner diameter (D2) is larger than said second outer diameter (D1 ) by a predetermined amount (G), in such a way that an annular gap (50) having a thickness (G) equal to said predetermined amount is defined between said central part (33) of said head portion (30) and said central part (13) of said cylinder (10),and in that said central part (13) of said cylinder (10) has at least one inlet opening (17) of a barrier-fluid (9), wherein said inlet opening (17) is in a fluid communication with a source (8) of said barrier-fluid (9) configured to feed said barrier-fluid (9) at a predetermined barrier pressure into said annular gap (50), so that said barrier-fluid occupies said annular gap (50), and said annular gap (50) forms a barrier chamber between said compression chamber (12) and a space (22) opposite to said compression chamber (12) with respect to said head portion (30) of said slidable member (20).

2. The compressor according to claim 1 , wherein said thickness (G) of said annular gap (50) between said central part (13) of said cylinder (10) and said central part (33) of said head portion (30) of said compressor unit (1 ,1 a-i,2,3) is at least 1 mm, in particular is at least 5 mm, more in particular is at least 10 mm.

3. The compressor according to claim 1 , wherein: said second inner diameter (D2) of said central part (13) of said cylinder (10) of said compression unit (1 ,1a-i,2) is equal to said common nominal diameter (D); said second outer diameter (D1 ) of said central part (33) of said head portion (30) is smaller than said common nominal diameter (D) by said predetermined quantity (G); said central part (33) of said head portion (30) has a length (L1 ) at least equal to said stroke (C).

4. The compressor according to claim 1 , wherein: said second outer diameter (D1 ) of said central part (33) of said head portion (30) of said compression unit (3) is equal to said common nominal diameter (D); said second inner diameter (D2) of said central part (13) of said cylinder (10) is larger than said common nominal diameter (D) by said predetermined quantity (G); said central part (13) of said cylinder (10) has a length (L2) at most equal to a difference between said length (L1 ) of said central part (33) of said head portion (30) and said stroke (C).

5. The compressor according to claim 1 , wherein said compression unit (1 a- i) is a cylinder-piston unit and said slidable member is a piston (20), wherein said proximal part (31 ) and said distal part (35) of said head portion (30) each comprise at least two elastic piston rings (41 ,45) at a distance from one another, said elastic piston rings (41 ,45) arranged to slide within said cylinder (10), and wherein said central part (33) of said head portion (30) is located between said elastic piston rings (41 ) of said proximal part (31 ) of said head portion (30) and said elastic piston rings (45) of said distal part (35) of said head portion (30).

6. The compressor according to claim 5, wherein said cylinder-piston unit (1 a- i) is a double-acting cylinder-piston unit, wherein said compression face (36) of said head portion (30) is a first compression face, said compression chamber (12) is a first compression chamber, and said proximal part (31 ) of said head portion (30) comprises a second compression face (32) at an opposite side of said first compression face with respect to said central part (33) of said head portion (30), wherein said second compression face and said cylinder (10) define a second compression chamber (22) through the whole length of which said rod portion (39) is arranged.

7. The compressor according to claim 1 , wherein said second diameter (D1 ) of said central part (33) of said head portion (30) of said compression unit (1 b,1 d) decreases from said proximal part (31 ) towards said distal part (35) of said head portion (30).

8. The compressor according to claim 1 , wherein said central part (13) of said cylinder (10) of said slidable member (20) of said compressor unit (1 c- h,2,3) has at least one outlet opening (18) of said barrier-fluid (9).

9. The compressor according to claim 8, wherein said inlet opening (17) and said outlet opening (18) are made at a same cross-section of said cylinder (10), and said cylinder (10) has a circumferential groove (19) at said crosssection, said circumferential groove communicating with said inlet opening (17) and with said outlet opening (18).

10. The compressor according to claim 6, wherein said annular gap (50) is a first annular gap, and said rod portion (39) of said slidable member (20) of said compressor unit (1 e-i) is provided with proximal and distal sealsbetween which a second annular gap of said rod portion (39) is defined, said second annular gap having at least one inlet opening (67) arranged to be supplied by a rod portion barrier-fluid (9,9').

11. The compressor according to claim 10, wherein said first annular gap (50) and said second annular gap of said compressor unit (1 f-i) are arranged to receive a common barrier-fluid (9) and have a mutual connection selected between: a parallel mutual connection, wherein said first and second annular gaps are arranged parallely to each other, i.e. they are both arranged to receive said common barrier-fluid (9) directly from said source (8); a serial mutual connection, wherein said cylinder (10) and said second annular gap also have respective outlet openings (18,68) for said common barrier-fluid (9), an outlet opening (18) of said cylinder hydraulically connected to said inlet opening (67) of said second annular gap.

12. The compressor according to claim 1 , wherein said compressor unit (1 h) comprises a cooling circuit (70) of said cylinder (10) and said slidable member (20), said cooling circuit configured to convey a cooling fluid (7) at a predetermined cooling fluid pressure above said delivery pressure, wherein said inlet opening (17) of said cylinder (10) is in a hydraulic communication with said cooling circuit (70).

13. The compressor according to claim 10, wherein said compression unit (1 i) comprises a supply circuit (4) in which a compensation vessel (40) is provided within which a deformable membrane is arranged to define first and second variable-volume chambers (41 ,42), wherein said first and second variable-volume chambers (41 ,42) are in a fluid communication with said delivery port (16) of said compression chamber (12) and with said inlet opening (17) of said cylinder (10), respectively, wherein said second variable-volume chamber (42) is configured to contain a predetermined quantity of said barrier-fluid (9), whereby said barrier-fluid can be fed into said annular gap (50) at a pressure dependent on a pressure of said low-molecular-weight gas at said delivery port (16).

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