When sizing ventilation ductwork, the maximum air velocity should be limited to avoid noise whilst minimum velocities must be maintained for effective particle suspension in fume extract and local exhaust ventilation (LEV) systems.
Maximum Velocity
Maximum air velocity limits are applied to control noise levels or avoid nuisance draughts. Velocity limits are also used to minimise pressure losses through equipment and fittings. Operational limits are applied to equipment such as cooling coils to prevent moisture carryover or air diffusers to ensure optimum performance. Typically, in a general office [NR 35-40] maximum velocities are 6.0 m·s⁻¹ in the branch duct, 3.5 m·s⁻¹ in the run-out ductwork and 2.5 m·s⁻¹ at a supply diffuser.
Maximum recommended air velocities should comply with CIBSE Guide B (2016) Table 2.16 where noise generation is the controlling factor. Maximum velocities in risers and ceilings and supply and return openings are listed in Table 2.18 and Table 2.19 respectively.
Minimum Velocity
Minimum air velocities are applied to ensure contaminants are safely and effectively removed from working areas and kept suspended in the air stream whilst being transported a safe exhaust location. Equipment may require a minimum operating air velocity to ensure it functions satisfactory. Typically, kitchen extract air velocity should be 6-9 m·s⁻¹ in main ductwork and 5-7 m·s⁻¹ at canopy spigots. In fume extract systems the air velocity will vary by contaminant type ranging from 5 m·s⁻¹ for gases and vapours to 25 m·s⁻¹ for large particles.
Minimum air velocities by contaminant type for local exhaust ventilation (LEV) can be found in HSG258 (2017) Table 12 and BESA’s DW/172 (2018) Table 11 for kitchen ventilation systems.
Pressure Loss
Increasing air velocity raises the velocity pressure which is a measure of the pressure exerted by the air due to its motion. Pressure drop in duct fittings will rise with an increase in velocity pressure. Therefore, maintaining the lowest possible air velocities throughout a ventilation system is essential for achieving minimal total pressure loss to limit fan energy consumption.
Comments are closed.