How to Read Fan Curves

Gain a basic knowledge of fan curves by understanding how to read fan performance curves.


How to Read Fan Performance Curves

Engineers and designers who select and specify fans should have a good basic knowledge of fan curves and  an understanding of how these are produced is vital for verifying the original fan selection, trouble shooting after the installation, and understanding future flexibility.

Fan curves are simply graphs showing fan performance, normally with air volume on the horizontal “x” axis, and pressure on the vertical “y” axis. To obtain a fan curve the fan is placed in a test rig in which air pressure and volume can be measured and the pressure can be varied by adjusting a damper or venturi of known characteristics. For a fan driven by an electric motor, the input voltage remains the same throughout the test.

Free Air & Cut Off in Fan Curves

The pressure is varied between zero, when the fan delivers its maximum volume, this point is known as “free air”, and by stages up to the point when the fan moves no air and develops maximum pressure. This is often referred to as “cut off” or “shut off”.

At each pressure the volume is noted and the “operating points” are plotted on a graph, these are then joined with a line and then become the “fan curve”.

The curve below is a static curve, some manufacturers show total pressure which includes velocity pressure at the fan discharge. On a normal graph the various curves would be shown alongside one another.

Static Fan CurvesFan Performance Tables

Manufacturers typically publish catalogs containing performance or rating tables for each specific fan size. These tables are printed in a compact format, showing only the minimum information necessary to select a fan to match a desired performance. Performance tables are very easy to use for making an initial selection.

Static Pressure

Signed as Pst or sometimes on fan curves as Pfa. This is the difference in  pressure either across an element in the system. For example a carbon filter or a heating coil, or between the inside of the system and outside atmosphere. Static pressure can be either positive or negative.

Dynamic Pressure

Usually signed as pD or PDy. As the name suggests, this is pressure created by the movement of air. It increases as velocity increases and is always positive.

Total Pressure

Usually signed as Pt ot pT. This is the sum of all the static elements plus the dynamic pressure at the discharge:

Pt = Pst + pD

Many people are confused over total pressure and only total pressure and only include the sum of the static elements, calling this “the total pressure drop” but when using total pressure curves you must always add the dynamic pressure due to the velocity at the discharge from the system. For example, at 10 metres per second discharge velocity Pd is 60 Pascals.

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 or system curve, contact us on 01782 349 430.

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