The terms Q(flow) and RPM(speed of the fan or pump) in the above expressions, by virtue of Equation 1, are interchangeable. It is important to note that the above affinity laws as stated are only valid in theory where the flow Q1 has simply been changed to Q2 at the design stage. In this situation, the designer can determine the corresponding revised W2 required delivering the revised flow Q2. The designer can then resize the pump or fan accordingly.
Many engineers recognize the fact that the affinity laws are for theoretical ideal situations; in order to correct for this shortcoming in dealing with a real situation, every engineer brings his own adjustment factor to modify the affinity laws. The above cubic exponent in Equation 3 is modified and substituted by an exponent 2.x, where 0 < x < 9, to be conservative in their energy calculation estimates. There lies the dilemma to the owner, who has commissioned the evaluation of installing a VSD on a pump, for example. There is no consensus on a fixed value of 2.x, as 2.x is characterized by each engineer as "based on experience." This characterization leaves plenty of room for interpretation, as the amount of experience of each engineer is different.