Pump and method

A pump having at least one inlet and one outlet for use in a liquid circulation system, the liquid having a dynamic viscosity, the circulation system in use having a back pressure at the pump outlet, the pump comprising:

• a rotary rotor and a stator providing first and second spaced-apart surfaces defining a generally annular passage therebetween, the passage having a central axis and a clearance height, the clearance height being a radial distance from the first surface to the second surface, the rotor in use adapted to rotate at a rotor speed,
• at least one thread mounted to the first surface and extending helically around the central axis at a thread angle relative to the central axis, the thread having a height above the first surface and a thread width, the thread height less than the clearance height, the thread width together with a thread length providing a thread surface area opposing the second surface,
• wherein the rotor, in use, rotates at a rotor speed relative to the stator which results in a viscous drag force opposing rotor rotation, said drag force caused by shearing in the liquid between the thread and first surface and the second surface, the viscous drag force having a corresponding viscous drag pressure,
• wherein the thread height, thread surface area and thread angle are adapted through their sizes and configurations to provide a viscous drag pressure substantially equal to the back pressure, and
• wherein the clearance height is sized to provide for a non-turbulent liquid flow between the first and second surfaces.