I don't think a positive displacement pump is suitable for this application. They deliver PRESSURE at LOW FLOW whereas a cooling tower needs HIGH FLOW and just enough PRESSURE to lift the water to the top.
In all the refineries I've run the pumps are CENTRIFUGAL. Ther are cheaper and will give you the volume of water required.
As Reidh says, what are your required volumes and what is the height of the tower? With this information it is a simple task to source the right pump.