How to Find the Duty & Operating Points

Learn more about how to find the operating point using a system curve and a static fan curve.


Finding the Duty Point & Operating Point

In this post we will be looking how to determine the operating point using a system curve and a static fan curve. If you’ve not yet read up on how to read a performance fan curve and know how they are produced, we suggest before reading this post, that you familiarise yourself with fan curves here.

Why do we need to find the Operating Point?

The manufacturers of filters, heating and cooling coils etc are usually able to give a static pressure drop for their product at a stated airflow, and for ducting, standard charts are available to identify the resistance per metre of different duct sizes at different airflows.

Standard factors are available to calculate the pressure drop caused by bends and other restrictions in ducting, and the discharge dynamic pressure is easily calculated.

Using the second fan law, we can calculate the resistance of each element at the required duty point, or air volume, and the sum of these resistances at the duty point and a number of other volumes. Plotting these points on a volume/pressure graph we create a “System Curve”. This will be a square curve.

The System Curve

The system curve is the resistance created at a set number of air volumes.

The System Curve Axair Fans

What Is The Operating Point?

We can now superimpose on this graph the fan curve for the fan we propose to use, the point where the fan curve and system curve meet is called the Operating Point and represents the airflow and pressure we will achieve in that system with that fan.

The Operating Point Axair Fans

At Axair, our technical engineers have a wealth of experience in both fan selection and technical expertise. If you require assistance selecting an operating point  or duty point on a fan curve of system curve, contact us on 01782 349 430. For more information visit our technical information page