Elbow types range from mitered to long radius. Long radiused elbows are most efficient. An elbow with a centerline radius (r/D or r/W) of 1.5 is very efficient (C=0.19) and should be used in cases where duct air velocity is 13 m/s (2,500 fpm) or higher. A standard radiused elbow (r/D of 1.0) is more economical and only slightly less efficient (C=0.23); it is generally preferred. Sometimes only a mitered elbow (C=1.15) will fit into the space provided. In this case, a turning vane will reduce pressure losses if properly installed. However, a turning vane should not be used in a transitional (drop check) elbow or an elbow with an angle other than 90°. Accurate turning vane installation is critical to performance, so factory-made or carefully fabricated shop units are necessary. Improperly installed turning vanes can be counterproductive. A double thickness turning vane is more costly and its performance is much poorer than that of a single vane; however, a double thickness vane is required when spanning 0.9 m (3 ft) or more because of structural needs. Turning vanes have been known to become dislodged and have substantially blocked airflow because of seismic disturbance. There are almost always alternatives to the mitered elbow and vane combination. A very short radiused elbow with a single fully radiused vane (C=0.43) is often a good compromise. [Rozell, 1974]
