Calculating the change in fan performance due to small diameter changes, speed variations and density fluctuations is simply a matter of multiplying by ratios of the target parameter to the initial parameter (raised to some power). The table below shows the relationships and powers to be used in these calculations.

Typically, the performance of an axial flow fan blade can be displayed as a plot of Static Pressure versus Flow (cfm) (Example). Performance data must be presented for a given fan speed, fan diameter and air density. If any of these parameters change, the performance data changes.

Performance data may be calculated for changes in diameter, speed and density according to the table shown above. The following link shows the application of these laws to a change in speed, diameter and both:

Application of Fan Laws to Axial Fan Blades