Danfoss VLT HVAC Basic Drive FC 101 Design Manual page 22

Product Overview Design Guide
2.6.16 Cooling Tower Fan
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Cooling tower fans cool condenser water in water-cooled chiller systems. Water-cooled chillers provide the most efficient
means of creating chilled water. They are as much as 20% more efficient than air-cooled chillers. Depending on climate,
cooling towers are often the most energy efficient method of cooling the condenser water from chillers.
They cool the condenser water by evaporation.
The condenser water is sprayed into the cooling tower until the cooling towers "fill" to increase its surface area. The tower
fan blows air through the fill and sprayed water to aid in the evaporation. Evaporation removes energy from the water, thus
dropping its temperature. The cooled water collects in the cooling towers basin, where it is pumped back into the chiller's
condenser, and the cycle is then repeated.
2.6.17 The VLT Solution
With an adjustable frequency drive, the cooling towers fans can be set to the speed required to maintain the condenser
water temperature. The adjustable frequency drives can also be used to turn the fan on and off as needed.
Several features of the Danfoss dedicated adjustable frequency drive, the HVAC adjustable frequency drive can be utilized to
improve the performance of cooling tower fans applications. As the cooling tower fans drop below a certain speed, the
effect the fan has on cooling the water becomes insignificant. Also, when utilizing a gear box to frequency control the
tower fan, a minimum speed of 40–50% may be required.
The customer programmable minimum frequency setting is available to maintain this minimum frequency even as the
feedback or speed reference calls for lower speeds.
Also as a standard feature, the adjustable frequency drive can be programmed to enter a "sleep" mode and stop the fan
until a higher speed is required. Additionally, some cooling tower fans have undesirable frequencies that may cause
vibrations. These frequencies can easily be avoided by programming the bypass frequency ranges in the adjustable
frequency drive.
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