From NO-PS No. 161 341 and U.S. Pat. No. 4 679 393, among others, a pressure exchanger of the above mentioned kind is known, in which the rotor ducts substantially extend along cylinder faces the longitudinal axis of which coincides with the longitudinal rotor axis, and the rotor is made to rotate by the aid of a motor or by the fact that the velocities of the fluids flowing in and out have different components in the circumferential direction, so that the fluid exerts a turning moment on the rotor. Furthermore, the fluid flow may be achieved by the aid of circulation pumps or by the rotating rotor. It is advantageous that the rotation of the rotor provides the flow, because pumps will render the structure more expensive and complicated, especially in case of low pressures and large volumes of passing flow. The above concept, however, has a limited applicability in this connection, since pressure exchangers functioning in this manner can only provide low feed pressures, while most processes in which pressure recovery may advantageously be used, e.g. processes comprising reverse osmosis, require high feed pressures on the high pressure side. Also, with this manner of operating the rotor, only low initial turning moments can be provided so that rotation of the rotor might easily be prevented by particles brought along by the flow. Pressure exchangers are also known, which operate with high volumes of passing through flows and low pressures, but these are complicated and expensive.