When given a performance requirement selection of a fan is fairly straightforward.

For example if a requirement is given for 700m³/hr @ 450 Pa then this duty would fit the fan performance graph perfectly.

However, it is rare that a required duty fits a fan perfectly.

What if the required duty was 1000m³/hr @ 200 Pa.

What would the fan actually give?

Would the fan operate at 1000m³/hr @ 325 Pa?

Or would it operate at 1350m³/hr @ 200 Pa?

The answer is neither. The system the fan is applied to has a system resistance. You will recall that the system resistance follows a square law. Therefore with the given duty of 1000m³/hr @ 200 Pa you can draw a system resistance line as shown below.

Where the system resistance line crosses the fan curve will be the actual duty point, so in this case the fan will produce approximately 1150m³/hr @ 275 Pa.

System resistance accuracy
You will achieve the airflow shown on the previous section and pressure development in the application when using the fan described if the original specified requirements were correct.

Very often specified volume flow and pressure development are estimates so in the application there could be a different resistance curve and therefore the fan will produce a different airflow and pressure.

This is a very important point to consider and if a customer complains the fan is not doing the required duty in the catalogue, how accurate are they with the system resistance calculation.

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