Influence of Bends in Inlet Ducts on the Performance of Induced-Draft Fans

Abstract

Abstract This paper shows the great loss in capacity and efficiency of a fan that results from the location of elbows or bends close to the inlet boxes. The loss is shown to be a function of the type of bend, of its “aspect ratio” or ratio of depth to width, and of the direction in which the air is turned in the bend. By the use of guide vanes in the bend the loss can be almost entirely prevented. When scroll inlet boxes provided with guide vanes are used, this loss does not occur, but the capacity with such boxes is low, although the efficiency is maintained. The cause of the loss is not the friction in the bend, but is the poor distribution of the air leaving the bend and entering the inlet box. The use of a short length of straight duct between the bend and the inlet box will reduce the loss appreciably. Measurements by pitot tube of the distribution of velocity and static pressures at the entrance to the inlet boxes will indicate the existence of this loss and the desirability of inserting guide vanes in the bends.