# How does fan airflow relate to its RPM setting?

A fan can run at 600 - 2000 RPM, at 2000 RPM it has 50 m3/h airflow.

0% = 0 RPM

100% = 2000 RPM

At 50% = 1000 RPM is the airflow halved to 25 m3/h?

Yes it does. Check this link.

The First Fan laws: Volume of Air

The first law of fans is a useful tool when working out the volumetric flow rate supplied by a fan under speed control or conversely working out what the RPM would be to deliver a required volume of air and hence what frequency to set a variable speed drive (VSD) to.

Volumetric flow rate (V, m³/hr) varies directly proportional to the ratio of the rotational speed (RPM) of the impeller.

$$V_2 = (\frac{U_2}{U_1})V_1$$

Where:

: Volume 1, m³/hr – Original volume of air

: Volume 2, m³/hr – New Volume of air

: RPM 1 u/min – Original Speed

: RPM 2, u/min – New Speed

• It might be worth pointing out that the energy requirement will be far from linear. I seem to remember a cube law in action. May 9, 2020 at 7:09
• Don’t fans, like propellors eventually stall? May 9, 2020 at 10:48
• @Transistor The link I gave tells about that too(The Third Fan Law: Power); and yup its a cube law. May 9, 2020 at 13:36