Bottom or side outlet openings in horizontal or vertical supply ducts should be equipped with adjustable flow-equalizing devices. Figure 17-6 indicates the pronounced one-sided flow effect from an outlet opening. This is before the corrective effect of air-turning devices. A control grid is added in Fig. 17-7 to equalize flow in the takeoff collar. A Vectrol is added in Fig. 17-8 to turn air into the branch duct and provide volume control. Air-turning devices are recommended for installation at all outlet collars and branch duct connections.
Square unvaned elbows are also a source of poor duct distribution and high-pressure loss. Nonuniform flow in a main duct, occurring after an unvaned ell, severely limits the distribution of air into branch ducts in the vicinity of the ell. One side of the duct may be void, thus starving a branch duct. Conversely, all flow may be stacked up on one side. This requires dampers to be excessively closed, resulting in higher sound levels.
Sources :
1. HVAC Troubleshooting Guide
2. HVAC Fundamentals, Second Edition
