Fan Affinity Laws
What a fan's airflow, static pressure, and power do when you change its speed.
Example
You enter
- Baseline RPM 1750
- Baseline CFM 5000
- Baseline SP (in wc) 1
- Baseline kW 5
- Target RPM
- Target value 1500
You get
- Ratio 0.857
- CFM 4285.7
- SP 0.735 in wc
- kW 3.149
Details, formula, and sources
Q ~ N, P ~ N^2, kW ~ N^3 across RPM, CFM, SP, and kW changes.
Q2/Q1 = (N2/N1); H2/H1 = (N2/N1)²; P2/P1 = (N2/N1)³. Laws apply for geometrically similar fans / pumps at the same point of operation.
Hydraulic Institute / AMCA by name; classical pump and fan theory.
Principles free in published engineering texts.
Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
Field names used by the API: baseline_RPM, baseline_CFM, baseline_SP_in_wc, baseline_kW, target_kind, target_value, ratio, CFM, SP_in_wc, kW
- Operating point constant (no system-curve change)affinity-law assumption