BESA's DW/144 (2016) circular and radiused rectangular ductwork bends are sized by the throat radius which measures to the inner radius. Whereby 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 for duct diameters greater 100 mm.

Short, medium or long radius bends are therefore a multiple of the duct diameter (D) for circular ducts or the duct width (W) for rectangular ducts. Where a short radius bend is 0.5D, a medium radius bend is 1D and a long radius bend is 1.5D. See further detail below.

A short radius bend or elbow would be used where space is a premium. Short (sharp or small) radius measured to centre line = 0.5x duct diameter.

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.

A medium radius bend or elbow is the standard or default bend referenced by BESA's DW/144 (2016) that would normally be used in a ventilation system.

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. 100mm Bend = 50mm throat radius.

Alternatively, a circular medium radius bend measures 1.0x the duct diameter to the duct centre line.

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.

A long radius bend or elbow will occupy the greatest space but with smoother change in air path the air resistance is lower. Long radius bends are therefore suited to systems in which air turbulence and pressure losses are kept to a minimum.

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.

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.