Estimates the energy and cost savings of controlling a centrifugal pump or fan's flow with a VFD (reducing motor speed) instead of a throttling valve/damper (running the motor at full speed against added flow resistance), using the standard affinity laws for centrifugal loads: power scales with the cube of speed, and speed reduction tracks flow reduction for a quadratic system curve.
Standards followed
Standard centrifugal-load affinity laws (textbook fluid-machinery physics), the same relationship underlying DOE/utility VFD retrofit guidance
What it can do
- VFD power at a reduced flow: rated power × (flow fraction)³.
- Comparison against a throttled/undamped baseline power (adjustable — defaults to 100% of rated power, a conservative assumption since throttling barely reduces a centrifugal load's motor power).
- Annual energy and cost savings from the operating hours and electricity rate.
Example problem
75kW pump motor, reducing flow by 30% (to 70% of full flow), 100% baseline power assumption, 6,000 operating hours/year at $0.12/kWh.
- Flow fraction = 1 − 0.30 = 0.70.
- VFD power = 75 × 0.70³ = 75 × 0.343 = 25.73kW.
- Baseline (throttled) power = 75 × 100% = 75kW.
- Power savings = 75 − 25.73 = 49.28kW → annual energy = 49.28 × 6,000 = 295,650kWh → cost savings = 295,650 × $0.12 = $35,478.
≈$35,478/year in estimated savings. Feed this into the Life-Cycle Cost calculator as the 'annual operating cost' difference for a full VFD payback analysis.
Want the full step-by-step version?
The 100% throttled-baseline assumption is deliberately conservative/illustrative — a real valve/damper-throttled system's power at reduced flow depends on the specific pump/fan curve and system curve shape, and is often somewhat lower than 100% of rated power. If you have actual trended power data or a pump/fan curve for the throttled condition, override the baseline percentage for a more accurate estimate. This tool assumes a quadratic (friction-dominated) system curve, typical of most pump/fan applications but not universal (e.g. systems with a large static head component behave differently).