Which fittings should be used and which ones should be avoided? Criteria for evaluation include: (1) pressure drop, which directly affects energy consumption, as expressed by C-coefficient (see Section 18.104.22.168) and (2) cost of manufacturing and installation. For example, consider the pressure loss in a smooth 90° elbow. If a splitting vane is added in a smooth elbow (see ASHRAE Duct Fitting Database 1994, page 66, fittings CR3-3), the pressure loss is 25 times lower than in a mitered elbow (see ASHRAE Duct Fitting Database 1994, page 66, fitting CR3-6) [Nillson, 1993].
Substantial savings can be achieved by designing around simple fittings and tap-ins instead of elaborate nested fittings. Selection of trunk, branch, and take-off fittings for connection to terminal boxes is very important to avoid air-flow imbalance. Air-flow imbalance results in more pressure than needed at terminal boxes located closer to the fan, which increases the potential for excessive air flow, instability, high air velocity through dampers, and noise. Trunk fittings should be selected to meet the pressure needs of each downstream trunk section and each terminal box.