Understanding Fan Velocity Pressure
Fan velocity pressure is the pressure linked to moving air. It is not the same as static pressure. Static pressure acts in all directions. Velocity pressure follows the air stream. Together, they help explain total pressure inside a duct or fan system.
Why It Matters
Designers use velocity pressure to judge air speed, duct balance, and fan performance. A high value can show fast air, small duct area, or excessive energy use. A low value can show slow air, poor capture, or a weak fan setting. The number also helps when comparing pitot tube readings with planned airflow.
Useful Inputs
The calculator accepts velocity directly, or it can derive velocity from airflow and area. Round duct, rectangular duct, and open area options are included. This saves time during field checks. It also reduces unit mistakes. Air density can be entered manually. It can also be estimated from dry air temperature and pressure.
Reading the Result
Velocity pressure rises with the square of velocity. Doubling velocity makes the pressure four times larger. This is why small duct changes can greatly affect fan energy. The result is shown in pascals, inches of water, and millimeters of water. These units cover common laboratory, HVAC, and industrial reports.
Good Practice
Measure airflow in a straight duct section when possible. Keep probes away from elbows, dampers, and fan outlets. Turbulence can distort velocity readings. Use several traverse points for large ducts. Average the readings before using the calculator. Check density when temperature, altitude, or process gas conditions are unusual.
Design Use
Engineers often use velocity pressure while selecting fans, louvers, filters, and exhaust hoods. The value supports pressure loss checks. It also shows whether air speed is within a chosen comfort or capture range. When the calculator derives velocity from airflow, the selected area must match the real free area, not only the outside duct size. This improves estimates for grilles, screens, and partially blocked openings during maintenance checks too.
Practical Notes
This tool gives engineering estimates. It does not replace a full system test. Use it with pressure gauges, pitot tubes, fan curves, and balancing records. Always keep units consistent. Review safety limits before changing fan speed or duct layout.