This page gives examples of dimensioning for small, medium and long radiused ventilation ductwork bends. This article only covers swept radius bends and does not consider square bends or bends with turning vanes.

Standard radiused rectangular and circular ductwork bends are detailed in BESA's DW/144 (2016). Short, medium and long radiused bends are detailed for rectangular ductwork and only medium and long radiused for circular ductwork. Each bend is sized by the throat radius which measures to the inner radius. The default [standard] circular bend detailed in Figure 93 shows the throat radius dimension is equal to D/2 or 0.5x D.

BS EN 1506 (2007) Figure 2 radius measurement is to the centre line of a duct for circular pressed bends with the dimension bend radius r_m equal to 100 mm for ducts less than or equal to 100 mm diameter and r_m equal to the duct diameter (r/d = 1) for duct diameters greater 100 mm. BS EN 1506 does not identify bends as either short, medium or long or anything other than r/d = 1.

Medium or long radius bends a favoured over short radius bends as pressures losses are greatest in short radius bends. Air pressure losses can be found using the pressure loss factor ζ, where approximate ζ, refer to details on Pressure Losses.

A medium (normal or standard) radius bend or elbow is the default bend referenced by BESA's DW/144 (2016) that would normally be used in a ventilation system. Medium radius bends in DW/144 have a throat radius equal to D⁄2 (half the diameter) which is equal to 1.0x duct diameter measured to the centre line of the duct.

Medium radius bends have a lower air resistance than short bends but marginally higher than long radius bends. Medium radius bends are therefore the standard default bend choice for most systems suitable for velocities up to 7.5 m/s. For higher velocities and dust extract systems long radius bends should be used.

BESA's DW/144 (2016) circular duct Figure 93 using the throat radius D/2; DW144 references this as standard. The DW/144 is also half the duct diameter (0.5x D) and sometimes identified as "half radius" e.g., 100 mm diameter bend = 50 mm throat radius.

Alternatively, a circular medium radius bend measures 1.0x the duct diameter to the duct centre line as with BS EN 1506 (2007) for duct diameters greater than 100 mm.

BESA's DW/144 (2016) rectangular duct Figure 41 the measurement is to throat radius equal to W/2, that is, half the duct width. Equally, a rectangular ductwork medium radius bend measured to the duct centre line equals 1.0x the duct width.

Long (slow or large) radius bends will occupy the greatest space but with a smoother change in air path the air resistance is lower so are suited to systems in which air turbulence and pressure losses are kept to a minimum. The long radius bend measured to the duct centre line = 1.5x duct diameter (or r/d = 1.5) and is regarded as standard bend to use in a ventilation system by ASHRAE.

A long radius bend or elbow is the preferred choice for local exhaust ventilation (LEV) systems (see Further Information on LEV systems) which operate at high velocity (usually greater than 8 m/s). The smoother airflow that long radius bends provide ensures that higher velocities can be maintained for effective particle suspension which is critical in local exhaust ventilation (LEV) systems.

BESA's DW/144 (2016) circular duct Figure 93 note refers to the long radius throat radius equal to D the duct diameter. Alternatively, a circular long radius bend is 1.5x the duct diameter measured to the duct centre line.

BESA's DW/144 (2016) rectangular duct Figure 41 note refers to the long radius throat radius equal to W, that is equal to the duct width. Equally, a long radius bend equals 1.5x the duct width measured to the duct centre line.

Bends in local exhaust ventilation (LEV) systems

Long radius bends should be used in fume extract or local exhaust ventilation (LEV) systems. The requirement for a long radius bend is defined in the Health and Safety Executive's HSG 258 Controlling airborne contaminants at work: A guide to local exhaust ventilation (LEV). Chapter 7 of HSG 258 details ductwork design requirements to include Figure 33 which details bends with a long radius stating, "R not less than 1½ times D". Section 168 states that sharp bends should be avoided, and references Figure 33.

The British Standard for Fume Cupboards BS EN 14175-2 (2003 with 2014 amendments) provides similar guidance on ductwork design given in HSG 258. The BS does not specify a preferred or minimum bend radius but section NA.2.5.2.3 does state that bends should have the largest radii practicable.

A short (sharp or small) radius bend or elbow would be used where space is a premium. Typically, short radius bends measured to the centre line equal 0.5x duct diameter, but this dimension can vary.

A short radius bend or elbow will occupy the least space but with a tight change in air path the air resistance is higher than medium and long bends. Short radius bends are therefore suited to low velocity systems (<5 m/s) installed in congested spaces. Ideally, the quantity of short radius bends should be kept to a minimum.

BESA's DW/144 (2016) does not identify measurements for a short circular duct bend. Manufacturer's such as Lindab (here ) supply pressed short radius bends sized at 0.6x the duct diameter (centreline radius) in a limited number of sizes.

In BESA's DW/144 (2016) the rectangular duct Figure 40 refers to the short bend throat radius, which measures to the inner radius, as a minimum 100 mm for ducts up to 400 mm wide.

Change in the air direction at ductwork elbows will cause pressure losses. The minor or dynamic loss (ΔP) due to duct system components in a ventilation system is found using the CIBSE Guide C (2007) Equation 4.11 with appropriate values of the pressure loss factor ζ (Zeta) combined with the velocity pressure or dynamic pressure equation. The velocity pressure (Pv) or dynamic pressure is the kinetic energy per unit volume of a fluid.

The pressure loss (ΔP) through a fitting will increase with rising velocity or a higher pressure loss factor value. Changes in circular duct size and the height to width ratio in rectangular ducts will have a bearing on the ζ value. Approximate ζ values used for comparison of a ∅250 mm ducts are 0.9 for short bends, 0.33 for medium (standard) bends and 0.24 for long radius bends. In this example with a 0.3 m³/s air volume at 6.1 m/s air velocity the pressure losses would be 20.2 Pa for a short bend, 7.4 Pa for a medium and 5.4 Pa for a long bend. Smoother airflow in long radius bends results in lower air resistance and pressure drop which is crucial for maintaining efficient airflow.

Component pressure loss equation