It depends, too, on the type of cooling tower, type of fan, local design outdoor wet bulb temperature, entering and leaving condensor water temperatures, and whether the water is mixed with glycol.
I probably left out a couple of things.
The very easiest way is to pick up the phone and call a cooling tower manufacturer. Marley, for example, will do all that for you.
Best to you,
Goober Dave
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Waht are you talking about; fan HP or cooling water pump HP?? Must have discharge pressure (calculated through a static and friction pressure calculation) to get pump HP.