Duct Bend Radius: Geometry and Pressure Loss

Centreline, inside and outside radius

For a circular duct bend, centreline radius R is measured from the centre of curvature to the centre of the duct. The inside or throat radius is R minus half the duct diameter. The outside radius is R plus half the diameter, using the same dimensional basis.

For example, an idealised 200 mm diameter bend with a 300 mm centreline radius has a 200 mm inside radius and a 400 mm outside radius. Its R/D ratio is 1.5. These are geometric example dimensions, not a product specification or a required minimum.

For rectangular duct, state which dimension lies in the plane of the bend. Turning through the wide side and the narrow side produces different geometry even if the duct's area is unchanged.

Duct elbow radius and bend angle

The bend angle tells you how far the direction changes; the radius tells you how gradually it changes. A 45-degree bend and a 90-degree bend can share the same radius but occupy different space and have different losses.

For an ideal circular arc, centreline arc length is R × θ, with θ in radians. A 90-degree turn has θ = π/2. At R = 300 mm, the centreline arc length is about 471 mm. This is the curved centreline length, not the overall fitting length including sockets or straight ends.

Give the fabricator a dimensioned sketch rather than ordering from arc length alone. Include connection lengths, joint details and the intended orientation.

How radius affects pressure loss

A tighter turn can increase flow separation and loss, while a smoother, more gradual bend may reduce it. The actual result also depends on bend angle, cross-section, construction, roughness and the approach flow. Use the relevant fitting data rather than assuming a fixed saving from changing radius.

Where a loss coefficient K is supplied, the fitting loss is commonly expressed as Δp = K × ρv²/2. Confirm the velocity and geometry basis used by that coefficient. Do not take a value for a smooth round elbow and apply it unchanged to a segmented rectangular bend.

The duct sizing guide explains the wider pressure budget. Closely spaced bends and fan connections also need a system effect review.

Check the space around the bend

Allow for the outside envelope, connection sections, supports and installation access. A bend that fits on a centreline drawing can still clash with a wall or another service at its outer edge. Review the route in both plan and elevation.

For chemical extraction, consider how condensate or deposits will behave through the change in direction. The designer should set the required slope, drainage and access where applicable. Do not introduce low points unintentionally while adjusting the route to fit.

Thermal movement and support arrangements belong in the same review. The duct should not pull on the fan or cupboard because an offset was forced into position.

A clear PVC bend fabrication brief

Specify duct diameter or width and height, bend angle, radius reference, connection arrangement and material requirements. Add the airflow, chemical conditions, temperature and location. Mark any dimension that cannot change because of an existing connection or structural obstruction.

We fabricate and install PVC chemical ductwork in circular diameters from 150 to 400 mm. Send the route and required dimensions so we can discuss the bend and adjoining components. Our PVC duct fittings guide and ductwork design guide help organise the complete brief.