Pumps and Power: Advanced Applications II – VFD Affinity-Law Applications & Integrated Problems
The affinity laws introduced in the System Optimization guide aren't just theory — they are the mathematics behind one of the most common capital-improvement decisions in water and wastewater utilities: installing a variable-frequency drive (VFD) to slow a pump down and cut its energy bill. This guide puts real dollar figures behind a VFD speed-reduction decision, then works through fully integrated, exam-style problems that combine flow, head, efficiency, and cost into a single multi-part calculation — the level of problem you should expect on a top-tier (Class A / Class 4) certification exam.
Why VFDs Save Disproportionate Energy
Recall the affinity laws from the System Optimization guide:
$$\frac{Q_2}{Q_1} = \frac{N_2}{N_1} \qquad \frac{H_2}{H_1} = \left(\frac{N_2}{N_1}\right)^2 \q