What Is Air Velocity? Understanding Airspeed, Airflow, and Ventilation
Air velocity describes how fast air moves at a point. Airflow describes how much air passes through an area over time. Both matter in HVAC commissioning, duct measurements, ventilation diagnostics and room air distribution.
Air Velocity, Flow Rate and ACH
Velocity is usually measured in metres per second (m/s) or feet per minute (fpm). Volumetric flow rate is expressed in m³/s, m³/h or cfm. For approximately uniform flow, Q = v × A, where Q is volumetric flow rate, v is average velocity and A is cross-sectional area. Air changes per hour (ACH) relates a volumetric flow rate to room volume: ACH = flow (m³/h) ÷ volume (m³).
Why Average Velocity Is Important
Velocity is often not uniform across a duct because walls, elbows, dampers and fans disturb the profile. Multiplying a single centerline velocity reading by duct area can misrepresent the total flow. A properly designed traverse measures several locations so that an appropriate average can be estimated.
In Ducts vs. Occupied Rooms
Duct velocity helps determine airflow and system operation. In occupied rooms, local air speed affects draught sensation and thermal comfort; high velocity is not automatically better ventilation. Fresh-air delivery also depends on outdoor-air supply, distribution effectiveness, mixing and contaminant removal.
Measurement Technologies
Thermal anemometers infer air velocity from the cooling of a heated sensing element. Pitot arrangements use pressure differences and require the correct airflow-density assumptions and installation geometry. Vane anemometers are useful for suitable grilles and openings. Every technology has operating ranges, orientation requirements and limitations at very low speeds.
Worked Example
For a duct with a cross-sectional area of 0.20 m² and a representative average velocity of 3 m/s, airflow is Q = 0.20 × 3 = 0.60 m³/s, or 2,160 m³/h. This is an illustrative calculation, not an installation reading. Real duct measurements require adequate straight runs or a validated measurement approach.
Product Selection
Explore AirAlign’s Air Velocity Transmitters directory for installed devices. Confirm whether the sensing technology, installation depth, duct size, output protocol and accuracy suit the location.
References and Further Reading
AirAlign publishes independent educational information. Consult applicable standards, manufacturers and qualified professionals when specifying safety-critical systems.
