To reduce air pressure losses associated with air turbulence provide a straight air duct at the fan inlet and outlet. The minimum straight segment length must be at least 1 air duct diameter at inlet and 3 air duct diameters at outlet. No filters or other similar equipment is allowed inside the air ducts.
For rectangular ducts the respective air duct diameter is calculated as follows:
D = air duct diameter
H = air duct height
B = air duct width