An expansion device for an at least partially gaseous fluid, comprising: - a tubular inlet portion that is arranged to receive said at least partially gaseous fluid at a predetermined inlet pressure (P 1), said tubular inlet portion having an inlet port, an inlet surface consisting of the internal surface (42,72,92) of said tubular inlet portion, a longitudinal axis and a generally decreasing cross sectional area, starting from said inlet port; - a tubular outlet portion for said at least partially gaseous fluid; - a tubular throat portion between said tubular inlet portion and said tubular outlet portion, such that said tubular portions (31,65,66) form a passageway for said at least partially gaseous fluid (2,3); wherein in said inlet portion a central element having a central surface is arranged to define an annular passage, so as to direct said at least partially gaseous fluid according to a flow direction that is generally at an angle with respect to said longitudinal axis, in order to reduce the friction of said at least partially gaseous fluid while flowing through said device, in particular through said throat portion, wherein downstream of said central element a closing element is provided in said throat portion, wherein said closing element is a substantially cylindrical hollow body coaxially connected to one end (44,84) of said central element (40,80) opposite to said inlet port of said inlet portion, said cylindrical hollow body having a plurality of holes between an outer cylindrical surface and an inner cylindrical surface, said closing element adapted to cause an expansion with a pressure drop down to a predetermined discharge pressure, a cooling and a partial liquefaction of said at least partially gaseous mixture (2,3), said closing element adapted to gradually impart to said fluid a substantially swirling direction that is maintained within an inner recess of the closing element and/or within the outlet portion of the expansion device; characterised in that channels are arranged along said internal surface (42,72,92) of said inlet portion, said channels defined by a plurality of baffles (41,71,91) arranged according to said flow direction along said annular passage, wherein said channels have a helical profile so as to impart a swirling movement in said inlet portion to said at least partially gaseous fluid, such that a centrifugal force acts on said at least partially gaseous fluid and a transformation occurs of a pressure energy into a kinetic energy that is associated with said swirling movement in said inlet portion and such that said centrifugal force assists a separation between a gas phase and a progressively forming liquid phase, and in that at least one part of said holes is arranged proximate to said channels of said inlet portion, such that a portion of said at least partially gaseous fluid that leaves swirling one of said channels enters and flows through a respective hole of said closing element, and gradually achieves said substantially swirling direction that is maintained within said inner recess of the closing element and/or within said outlet portion of the expansion device. - a tubular inlet portion that is arranged to receive said at least partially gaseous fluid at a predetermined inlet pressure (P 1), said tubular inlet portion having an inlet port, an inlet surface consisting of the internal surface (42,72,92) of said tubular inlet portion, a longitudinal axis and a generally decreasing cross sectional area, starting from said inlet port; - a tubular outlet portion for said at least partially gaseous fluid; - a tubular throat portion between said tubular inlet portion and said tubular outlet portion, such that said tubular portions (31,65,66) form a passageway for said at least partially gaseous fluid (2,3);