Fittings

Elbows, tees, transitions, and how their losses are calculated.

How Fitting Losses Work

Every fitting in a duct system causes a pressure drop due to turbulence and flow separation. DuctStatic calculates these using the dynamic loss coefficient method:

ΔP = Co × Pv

Where Co is a dimensionless loss coefficient and Pv is the velocity pressure at the reference cross-section (usually the inlet of the fitting). The model uses equations, interpolated tables, or fixed defaults depending on fitting type. Match the geometry, flow direction, and velocity reference to the source.

Fitting Categories

Elbows

Smooth radius (round and rectangular). Co depends on centerline radius-to-diameter ratio, bend angle, and Reynolds number. Available in R/D = 1.0, 1.5, and 2.0.
Mitered. Higher loss. Co based on deflection angle. Available with or without turning vanes.
Offset. Two-bend offset with a fixed loss coefficient.

Transitions

Gradual (~25°) or sharp (~60°). Used for both expansions and contractions. The same fitting types apply whether the duct is getting larger or smaller. Contractions are generally lower loss than expansions at the same angle.
Round-to-rectangular and rectangular-to-round. Shape-change transitions.

Tees and Wyes

Converging tee (return/exhaust side). Flow from two branches merging into one.
Diverging tee (supply side). Main flow splitting into branch and continuing trunk.
45° wye (converging and diverging). Lower loss than 90° tee when space allows.
90° Boot (45° entry). Common at branch takeoffs to floor or ceiling diffusers.

Tee note: Converging vs. diverging is set automatically based on system type (supply = diverging, return/exhaust = converging). You select the branch geometry only.

Loss Method Options

Calculated Co (default)

Calculated Co updates with the geometry and flow inputs used by that fitting model. It is an estimate within the model’s limits, not a measured value for the installed fitting. Check the applicability and source table, especially for approximations and out-of-range inputs.

Manual Co Override

If you have a Co value from a manufacturer's data sheet or another source, you can enter it directly. This overrides the calculated value for that fitting instance. Confirm the source uses the same reference velocity pressure as the calculation; a common-duct coefficient is not interchangeable with a branch-referenced coefficient.

Published Pressure Drop

For manufactured fittings (terminal units, VAV boxes, coils, dampers) where the manufacturer provides a pressure drop directly, you can enter that value in in. w.g. instead of using a Co. DuctStatic adds it as a fixed loss regardless of velocity. Replace defaults with data for the actual device at its design airflow and density. Check whether the published rating is static or total pressure and which terminal/exit effects it includes; see pressure boundaries and ratings.

Custom Fitting Library

You can save frequently used fittings (with a specific Co or published drop) to your personal fitting library. Saved fittings appear in the fitting picker for quick reuse across projects. Account required.