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Variable Speed Cooling Tower Fan

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  rev. 2008-11-25        

Existing Conditions

There are xxx xxxhp cooling tower fan motors that cycle between fixed speed settings to maintain a setpoint condenser water temperature.  Installing variable frequency drives on these fans would reduce the fan motor consumption.

Retrofit Conditions

We recommend installing variable frequency drives and inverter duty motors on the cooling tower fans, and then controlling the speed to maintain a desired condenser water temperature, rather than on/off or high/low fan cycling.

Further Benefits

VFDs extend motor life due to reduced speed and torque, gentler starting, and gradual acceleration.

Application Details

Install variable-frequency drives and high efficiency Class F insulated motors (inverter duty).

Specify with manual bypass capability where the service is considered critical.

Issues and Concerns







Energy savings from VFDs come from the basic laws of fan or pump operation.  Fan power input is related to the cube of the flow rate (load), according to the equation:

W2 = W1 x (Q2/Q1)3

Q1 = first flow rate
Q2 = second flow rate
W1 = first fan power input
W2 = second fan power input

For example, if the flow rate is halved (Q1=1, Q2=1/2), the fan power input W2 will be one-eighth of W1.

In practice, the drive itself dissipates some of the power input as heat, so the savings are not quite as great as this.

When calculating the savings, project the number of hours operating annually at each of the current operating points, then smooth the curve for the same gross annual airflow and determine the new power consumption.

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