Next time you disassemble these unit for cleaning, try to establish what is causing the fouling. Is it suspended solids in the water or is it crystalization, is it chemical or is organic growth. Once you determine the source of the fouling they you can determine whether a filter will help or...
A Brazed Plate Heat Exchanger could possibly provide a faster payback. Try API Heat Transfer, they can look at shell and tube, plate and frame and brazed plate heat exchangers.
Tom
Delta T Heat Exchangers
www.deltathx.com
BAC typically manufactured (realtively speaking) smaller cooling towers which would lend themselves to be roof mounted. Cooled loop cooling towers can be smaller and be self contained with an integral basin but as timbones suggested there would usually be a driver to use a closed loop cooling...