A centrifugal pump and centrifugal fan operate using one principle. They move fluid by creating differential pressure between their inlet and outlet with an impeller. Fluid is drawn into the center (eye) of the impeller and pushed toward the outlet by the differential pressure created when the impeller spins.
Affinity laws define how a centrifugal pump or centrifugal fan perform from 0-100% speed. They calculate Flow, Pressure and Horsepower at various points on a centrifugal pump curve and a centrifugal fan curve. It’s tough trying to understand a centrifugal pump or a centrifugal fan curve, but Affinity Laws are simple.
If you were looking to learn more about pumps go to Intro to Pumps
Affinity Laws apply to every centrifugal pump and every centrifugal fan. They are simple, but difficult to visualize. They are as follows. When a fixed impeller is spinning at 0-100% Motor Speed, Flow is proportional to Speed, Pressure is the Square of the Speed, and Power is the Cube of the Speed. Below is a table defining them using formulas
|MOTOR SPEED (X)||VARIABLE (Y)||FUNCTION TYPE||FUNCTION FORMULA|
It’s difficult to visualize formulas so let’s look at the curves generated by them. When you see all three curves together the laws become clear and you see something unexpected. When motor speed is reduced, power is reduced faster than flow, so it takes less power per unit flow.
The reduced power per unit of flow becomes energy savings when a Variable Frequency Drive (VFD) is used to reduce motor speed. This becomes significant as the motor size increases.
Affinity Laws Example
The chart belows shows at 80% motor speed, the flow is 80%, but the power is 52%.
- DESIGN – When sizing pumps and fans Engineers typically oversize them by 20% to insure adequate capacity.
- REQUIRED – Users typically need 80% of designed pump and fan flow capacity
- SAVINGS – Save / reduce energy 35% (1-.52/.80) by reducing motor speed / flow to 80%
Variable Frequency Drives
When a Variable Frequency Drive is being selected for a specific motor application the first question is “Is it a Variable Torque or a Constant Torque application ?” The short answer is
- VARIABLE TORQUE = All Centrifugal Pump and Fan motor applications
- CONSTANT TORQUE = All other motor applications