Working out the static pressure a fan has to overcome
Velocity, friction factor and pressure drop per 100 ft, and what that means for the fan duty to specify.
The engineering question
The fan selection needs a static pressure. Where does the number come from before the fan is chosen?
Worked example
The figures below were produced by running the Duct Friction & Pressure Drop Calculator with the same inputs — during the build, from its authoritative engine.
| Velocity | 1432 fpm |
|---|---|
| Friction factor | 0.0173 – |
| Pressure drop per 100 ft | 5.3275 in.w.g. |
| Total pressure drop | 5.3275 in.w.g. |
| Reynolds number | 201845 |
| Basis | Swamee–Jain friction coefficient for galvanized round duct (0.0003 ft roughness). Flexible duct needs a roughness ~0.003 ft and 1.25× diameter allowance for the equivalent length note. |
- Area from the diameter gives the velocity.
- Reynolds number and friction factor follow from roughness and diameter.
- Pressure drop per 100 ft scales with the real route length; fittings are added separately.
Sanity check
Friction factor should fall as diameter rises. A friction factor that increases with duct size indicates an error.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
- Duct Friction & Pressure Drop Calculator Calculate round duct velocity, Reynolds number, friction factor and pressure drop per 100 ft before selecting a fan.
- Duct Sizing Calculator Size round and rectangular ducts from airflow and a design velocity, returning the next standard size and the actual velocity.
References & standards
- ASHRAE Handbook — duct design and fitting losses
- SMACNA — engineering manual
Related problems
Choosing duct size from airflow and a velocity you can live with
Turning airflow and a velocity budget into a standard duct size, and the rectangular equivalent that fits the ceiling void.
Finding the duty point instead of guessing a pump size
Reading a pump curve as a system curve, and matching the two to find where the system will actually operate.