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1. Low Velocity Duct Systems: Low-velocity ducts are characterized by air velocities in the range of 400 to 2000 feet per minute (fpm).
2. Medium Velocity Duct Systems: Medium-velocity (MV) duct systems are characterized by air velocities in the range of 2000 to 2500fpm.
3. High Velocity Duct Systems: High-velocity (HV) duct systems are characterized by air velocities in the range of 2500 to 3500 fpm.
low velocity frictional loss used 0.08 to 0.15 in w./100 ft
high velocity 2500 fpm to 4000 fpm with high frictional loss of 0.6 in w./100 ft
according to ASHRAE
Best way to calculate air velocity in the duct is from velocity pressure. Manometers with pitot tubes are used. These manometers are calibrated in inches of water. Then air velocity is calculated using formula. One can also use air velocity flow chart directly to calculate min and max air velocity. A lot of commercial gauges are available in market for the need. Eg Dawer 460.
