The Affinity Laws of pump
- The Affinity Laws of centrifugal pumps or fans indicates the influence on volume capacity, head (pressure) and/or power consumption of a pump or fan due to
- change in speed of wheel - revolutions per minute (rpm)
- geometrically similarity - change in impeller diameter
- Hence, the Affinity Laws are mathematical expressions that define changes in pump capacity,head, and BHP when a change is made to pump speed, impeller diameter, or both.
- Affinity laws are useful when an existing pump must be modified ti give a higher or lower head or different capacity
- The Affinity Laws are valid only under conditions of constant efficiency.
According to Affinity Laws:Capacity, Q: changes in direct proportion to impeller diameter D ratio, or to speed N ratio:
Head, H: changes in direct proportion to the square of impeller diameter D ratio, or the square of speed N ratio:
BHP: changes in direct proportion to the cube of impeller diameter ratio, or the cube of speed ratio:
- Where the subscript: 1 refers to initial condition, 2 refer to new condition
- If changes are made to both impeller diameter and pump speed the equations can be combined to:
