The OZA-VКO inlet header is used to correctly form a uniform velocity diagram when the flow enters the blades of an axial fan impeller when operating without a network at the inlet. OZA-VKO must also be installed on the suction duct during subsequent installation of the fan, since with a flanged flow inlet into an axial fan or duct, there is a significant reduction in the flow rate and pressure generated by the fan due to losses at the flange edge.