Abstract: 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).