Velocity begins with area
Velocity (FPM) = airflow (CFM) ÷ duct area (ft²)
A 400 CFM flow through 0.50 ft² has 800 FPM velocity. Through 1.00 ft², the same 400 CFM has 400 FPM. The airflow did not change; the cross-sectional area did.
What changes when velocity changes
| Higher velocity can | Lower velocity can |
|---|---|
| Use less duct area where routing space is limited. | Require more physical duct area. |
| Increase straight-duct and fitting pressure losses. | Reduce velocity-related pressure loss for the same airflow. |
| Increase sound and discharge concerns if the system is not designed for it. | Change cost, space, and distribution feasibility. |
| Demand more available fan pressure. | Still require fitting, terminal, and fan checks. |
Why a single FPM target is not enough
Main ducts, branches, returns, exhaust paths, terminals, and noise-sensitive rooms can have different constraints. The selected value must work with the complete pressure path: straight duct, fittings, filters, coils, grilles, dampers, and actual fan performance.
Practical review sequence
- Confirm assigned CFM for the run.
- Calculate area and actual velocity for the available duct geometry.
- Check friction and fitting losses against the pressure budget.
- Check noise, balancing, distribution, and terminal performance.
- Verify the fan can deliver the required flow at the calculated system resistance.
FAQ
Can this guide select a design velocity?
No. A design velocity must be selected from the system’s airflow, route, pressure budget, acoustic goals, equipment, and applicable requirements.
Does lower velocity always solve a duct problem?
No. It may require impractically large ducts and does not remove losses from fittings, terminals, filters, or equipment.