Air speed is the velocity of air movement around an occupant, typically measured in feet per second (ft/s) or meters per second (m/s). Air speed influences convective heat transfer from the body and affects thermal comfort perception. Excessive or insufficient air movement creates discomfort and draft complaints in HVAC systems.
Technical Details
ASHRAE Standard 55 limits mean air speed to 0.3 m/s (59 ft/min) for sedentary activities and 0.5 m/s (98 ft/min) for light activity in comfort zones. Air speeds above 0.15 m/s may cause draft discomfort, while speeds below 0.1 m/s feel stagnant. Turbulence intensity and temperature fluctuations compound air speed effects. Anemometers and hot-wire probes measure localized air velocities during commissioning and troubleshooting.
Applications in HVAC Systems
Diffuser design and placement control air speed distribution. Displacement ventilation systems maintain low air speeds at occupied zones while exhausting from upper areas. VAV (variable air volume) systems modulate supply air speed based on load.
Practical Significance
Proper air distribution prevents draft complaints and dead zones, improving occupant satisfaction and system efficiency.