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Power number is conceptually the ratio of power draw to impeller parameters, speed, and density. It is defined as:

(3.3)

The main use of power number is to calculate power draw. It is a function of impeller type, Reynolds number, and various geometric factors. When using SI units, the power will be expressed in watts. If one chooses to use inches, rpm, Hp, and specific gravity, we can insert the conversion factor and rearrange for power:

(3.4)

Same caution as for Eq. (3.2)

Note that power draw is quite sensitive to both impeller diameter and shaft speed.