Fan Power and Static Pressure Calculator

How is fan power calculated?

The power a fan delivers to the air is flow times pressure rise: P_air = Q × Δp. With flow in m³/s and pressure in Pa the result is in watts. Shaft power is this divided by fan efficiency: P_shaft = Q × Δp / η_f. For the power drawn from the supply, motor and drive efficiency are included as well.

Where does fan static pressure come from?

The fan has to overcome every resistance the air meets on its way through the system. They fall into two groups: the duct side (straight duct friction, elbows, branches, dampers, terminals) and the equipment side (filters, coils, silencers, heat recovery). The total is taken along the most resistant path and a safety margin is added.

Example

Air at 7200 cubic metres per hour (2 m³/s) against a total loss of 500 Pa. Air power is 2 × 500 = 1000 W. At 60 percent fan efficiency the shaft power is 1667 W, and at 90 percent motor and drive efficiency the input power is 1852 W. Specific fan power is 926 W/(m³/s).

Specific fan power (SFP)

Specific fan power is the electrical power used to move one cubic metre of air per second. It is used to compare the energy efficiency of ventilation systems. For limits and classes see the energy performance regulations and the standard that apply to your project.

How does power change with flow?

In the same system, raising the flow raises pressure loss with the square of flow and power roughly with its cube. A 10 percent increase in flow costs about 33 percent more power, so an oversized airflow is expensive.

Limits

This calculation gives power, it does not select a fan. Efficiency varies strongly with the duty point and must be read from the manufacturer's curve. Density correction, system effect and motor sizing margin are assessed separately.