Abstract: A positive-displacement unitary pump and turbine is operable as a fluid energy exchanger using a charging fluid as motive force and acting upon a separate feed fluid that exits the turbine at an elevated energy state. The rotor casing defines a rotor chamber having a contoured wall that forms a plurality of lobes, typically in an even number. Each lobe has an inlet port and an outlet port defined by the contoured wall, and the rotor has a plurality of vanes that follow the contoured wall as the rotor spins. The rotor is driven by the charging fluid entering first and second lobes, located generally opposite one another, and exiting the lobes at a lower energy state. The driven rotor is operable to elevate the energy level of a feed fluid in third and fourth lobes, located generally opposite one another.