Fitting Loss Coefficients: Method

Loss Coefficients and Equivalent Length

Air turning a corner, changing area or splitting at a branch loses total pressure in proportion to its velocity pressure: Δp = C ρV²/2. The coefficient C is a property of the fitting's shape, nearly independent of velocity at duct Reynolds numbers, and is tabulated by ASHRAE (the Duct Fitting Database, several hundred geometries) and SMACNA. The catalog on this page holds representative values for the common fittings; a long-radius five-gore round elbow loses about a tenth of a velocity pressure, a mitered elbow without vanes more than a full one, and turning vanes bring the mitered elbow back to a tenth or a third depending on the vane type.

The velocity used is that in the duct where the fitting sits, or for a transition the smaller duct, and for a branch the branch velocity. Branch coefficients depend on the flow split; the values here are for a branch taking half the common flow.

The equivalent length is the straight run of the same duct with the same loss, L_e = C D / f. It lets fitting losses be added to the duct lengths in the equal-friction method; because f varies with size and velocity, the equivalent length is computed for each row's own duct. The total of the run can also be entered in the Duct Sizing tool as each segment's ΣC.

Not included: system effect at the fan, the dependence of branch coefficients on the flow ratio, and the interaction of closely spaced fittings (two elbows within a few diameters lose more than their sum).