Duct Velocity Pressure
Duct velocity pressure is the kinetic-energy component of airflow inside a duct, representing pressure from air speed rather than wall pressure. It is expressed in inches of water gauge or pascals. In HVAC and industrial ventilation, total pressure equals static pressure plus velocity pressure, with velocity pressure derived by subtracting static pressure from total pressure.
On a shop floor, duct velocity pressure is used to verify that exhaust hoods and dust-collection ducts achieve intended capture velocities, diagnose fan or duct issues like blockage or leakage, and size or balance ventilation systems to control contaminants. It is measured indirectly by subtracting static pressure from total pressure using probes. Continuous monitoring of airflow via duct static pressure or air speed, with alarms for low flow, is emphasized in Canadian safety contexts like WorkSafeBC guidance for laboratory ventilation.
- Confusing static pressure with velocity pressure, leading to wrong airflow calculations and false confidence in ventilation performance.
- Measuring at poor locations or with wrong probe orientation, corrupting total/static pressure readings and derived velocity pressure.
- Assuming one duct velocity pressure value guarantees adequate capture at the hood, ignoring hood entry losses, duct layout, and fan performance.
Is velocity pressure the same as dynamic pressure?
In HVAC and industrial ventilation usage, yes: velocity pressure is commonly treated as the pressure associated with air motion, i.e., dynamic pressure.
Can velocity pressure be negative?
In standard duct-flow practice, it is treated as always positive because it is based on the square of velocity and represents kinetic energy in the flow direction.
How do you measure it directly in a duct?
You measure total pressure with the probe facing the flow and static pressure with a static port or probe oriented to exclude velocity component; the difference is velocity pressure.