At the fan discharge, due to centrifugal forces, the air velocity at the outer extremity of the casing (furthest from centreline) is higher than that at the other end of the discharge (nearest to centreline). This is even more so when the fan casing is fitted with a shield or tongue piece.
Bends fitted directly to the fan outlet flange therefore receive a distorted air velocity profile. It is always good practice to have at least 4.5 equivalent diameters of straight duct on the fan outlet to allow for good diffusion. Where this cannot be accommodated, the bend loss will be greater than normal. The approximate effect is given in Figure 3.69. More comprehensive information is given in Chapter 5, Section 5.6 and AMCA Publication 201.
Fans wtth ahiatd In discharge
Loaa 2 la i ■ normal
Air votocWy gvnoraay Mgh at *4a away trom MM Fans of any typa
Figure 3.69 Discharge bend losses
Posted in Fans Ventilation A Practical Guide