Danfoss VLT FC 102 Series Design Manual page 32

Introduction to VLT® HVAC D... Design Guide
2.7.17 Cooling Tower Fan
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Cooling tower fans are used to 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, onto the cooling tower's "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.
2.7.18 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 HVAC dedicated adjustable frequency drive, the HVAC adjustable frequency drive can be
utilized to improve the performance of a cooling tower fans application. 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, program the adjustable frequency drive 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.
30 Danfoss A/S © Rev. 06/2014 All rights reserved. MG11BC22