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The power required by a centrifugal fan varies as

  • A.the cube of the speed
  • B.the square of the speed
  • C.the speed itself
  • D.the fourth power of the speed at which the fan is being driven

Answer: A

Correct answer: the cube of the speed

A centrifugal fan’s torque varies as the square of the speed, and power is torque times speed, so

\[ P \propto \omega^3 \]

This cube law is why variable speed control of fans and pumps saves so much energy: reducing the speed by twenty per cent halves the power. It is also why a fan takes so long to run up, since the accelerating torque available shrinks steeply as the speed rises.

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