Evaporative cooling is a very efficient form of heat rejection. Therefore it is typically cost effective overall to run the cooling tower fan at constant full speed in applications when another part of your process gains benefit from colder water on cooler days. Process benefits may include energy savings (e.g. in chillers or compressors) and/or increased product (e.g. power generation or petrochemical throughput).

In applications where the fan runs full speed the fan energy cost typically equals capital cost of the installed cooling tower every 2 years. However a physically larger tower rejects heat with less fan power. If a larger tower is selected the fan energy cost can be reduced by up to 50%. There is a choice of many sizes to suit your project criteria for capital cost versus operating cost.

See the cooling tower sizing and selection tool and use the Evaluation Parameters tab to input your criteria…or contact MFC, your SPX Cooling Technologies representative.
http://qtcapps.ct.spx.com/update/Login.aspx

In contrast, applications that do not take benefit of colder water on cooler days are good candidates for variable speed drive (VSD). [When a tower is running a VSD at less than full speed it is analogous to the physically larger tower with low fan power described above.] If a tower is running a VSD at full speed most of the time then consideration should be given to installing more cooling tower capacity…to make it the (always) low fan power tower described above.