Wednesday 23 November 2016

AFFINITY LAWS OF PUMP

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
  1. change in speed of wheel - revolutions per minute (rpm)
  2. 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:

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