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A cooling tower fan on a variable-speed drive is controlled from

  • A.the water temperature leaving the tower
  • B.the speed of the fan measured by a tachogenerator
  • C.the current drawn by the fan motor
  • D.the ambient air temperature alone

Answer: A

Correct answer: the water temperature leaving the tower

The tower exists to cool water to a required temperature, so that is the controlled variable and the fan speed is the means. Controlling on fan speed or motor current would control the actuator rather than the process, and ambient temperature is a disturbance to be rejected rather than a setpoint.

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