There is disclosed a pressure vessel (1) provided with a first port (10) acting as a high pressure inlet of a first stream and a second port (11) acting as a high pressure outlet. A rotatable valve element (9) is located in the centre of the machine and includes a centre plate (19), which is utilized to separate high pressure streams. At each end of the valve element (9) are valves. The valves ensure that as the valve element (9) rotates the exchange ducts (3a and 3b) are either both isolated or that one is exposed to high pressure while the other is exposed to low pressure. In operation, a fluid stream is introduced to the machine at high pressure through port (10) and flows around the outside of the exchange duct (3b) towards the centre of the machine. The stream then flows downwardly to the valve element (9), where it then passes through the open ports of the valve element (9) and into flow distributor (6). The stream then passes into and upwardly in the exchange duct (3a), causing upward displacement of the duct piston (4a), resulting in the pressurization and flow of the second fluid above the duct piston (4a). The second fluid then flows into the upper flow distributor (5), into the valve element (9), and then downwardly and finally between the exchange duct (3a) and out through the high pressure port (11). At the same time a fluid stream is introduced to the machine at low pressure through port (16). This flows into the valve element (9) and then into the flow distributor (5). From the flow distributor (5) it flows and downwardly into the pressure exchange duct (3b), causing downward displacement of duct piston (4b) and resulting in flow of the first fluid below the duct piston (4b), which then flows into the lower flow distributor (6), into the valve element (9), and then out of the lower sealing plate (8) at port (15).