Duct Transition Loss Coefficient

Diffuser expansion & reducer contraction — Idelchik 5-2 & 5.23.

Skip ahead: size a transition in the free duct static pressure calculator and it computes the loss from the live coefficient tables — no interpolation by hand.

Expansion (diffuser) — Idelchik Diagram 5-2

When duct area enlarges, downstream velocity drops. The coefficient is referenced to the upstream (fast) velocity and depends on the included angle — steep transitions lose much more:

Area ratio	1.5	2.0	3.0	4.0	6.0	10
Gradual (~25°)	0.17	0.26	0.38	0.42	0.49	0.49
Abrupt (~60°)	0.22	0.33	0.55	0.68	0.76	0.80

Model limit: these are the calculator’s two angle presets, not an optimization of transition angle. The expansion table is derived from a conical-diffuser case with a specified inlet condition; rectangular or disturbed-inlet installations need separate review.

Contraction (reducer) — Idelchik Diagram 5.23

The contraction table below is referenced to the downstream (fast) velocity. The app converts it to an upstream reference by multiplying Co by (Ain/Aout)² before multiplying by upstream velocity pressure. Do not apply that conversion a second time.

Area ratio (in/out)	1.5	2.0	3.0	4.0	6.0
Gradual (~25°)	0.037	0.047	0.045	0.044	0.044
Abrupt (~60°)	0.054	0.068	0.066	0.068	0.073

Static pressure and irreversible loss

In a contraction, velocity and velocity pressure rise while static pressure falls. The favorable pressure gradient can limit separation, so a well-shaped contraction often has a small irreversible total-pressure loss. It does not recover static pressure. In an expansion, velocity pressure falls and some can become static pressure, but separation and friction reduce that recovery. A total-pressure loss coefficient describes the irreversible loss, not the full change in static pressure.

Pitfall: putting a large abrupt diffuser right after a fan is a classic source of unexpected static-pressure loss. Model it in the calculator and compare gradual vs. abrupt in the per-segment results.