Swirl control
According to Euler's equation for turbomachinery, both the increase in energy in a centrifugal pump'simpeller and the pump's characteristic head and input power curves can be determined by changes in the fluid's swirl between the impeller's inlet and outlet. As the impeller's geometry cannot be changed during pump operation (with the exception of variable pitch propeller pump blades), a centrifugal pump's characteristic curve depends solely on the rotational speed if, as is generally the case, the approach flow to the impeller is swirlfree.
By adjusting (controlling) the flow's swirl upstream of the impeller, it is possible to influence the characteristic curves during operation even with unchanged impeller geometry and at constant rotational speed. A swirl rotating in the opposite direction of the impeller leads to an increase in pump head and pump input power, while a swirl rotating in the same direction as the impeller entails a lower head and lower pump input power. The effect of swirl control increases with the pump's specific speed (ns) and is therefore very significant for axial flow pumps. See Fig. 10 Cooling water pumps
Swirl control equipment is installed upstream of the impeller inlet to control the flow's angular momentum. This comprises mechanically adjustable swirl control devices which may induce a swirl both in the direction of rotation of the impeller and the opposite direction.
