Equipment Apparatus № 08
VFD Affinity Curves
The most important law in HVAC energy: fan and pump power
scales as the cube of speed. Slow a fan to 80% flow and you
use only 51% of the power.
But real systems don't always follow the textbook. Throttling valves
add static pressure that flattens the curve. Constant
static-pressure control with VFDs gets you closest to the theoretical
cube law.
P ∝ Q³
In Practice
The ABB ACH580 drives at your properties run this math
continuously. If a VFD is pinned near 100%, it's defeating its
own purpose — the curve flattens to a horizontal line and you
pay full power for partial flow.
