When the fitting is at the fan inle

When the fitting is at the fan inlet, for example an elbow or a fan inlet duct/box (Figure Z). the velocity profi les at the inlet to the fitting may be uniform and the fitting pressure loss as measured or estimated from standard handbooks may be valid. However, the flow patterns at the fan inlet (or fitting outlet) may be disturbed with the presence of a vortex, spin or vena-contracta. This less than optimum flow condition at fan inlet caused by the fitting will lead to a reorganization of the flow inside the impeller and therefore a deterioration of fan performance in relation to catalogue ratings. Not only the fan curve may be affected by this disturbing obstacle but also somet imes, but not always, the fan power curve. A companion document will be drafted at a later date to show the influence of the inlet obstacles on the fan power curve for the same configurations of fans and fittings as in this document.