What is a typical fouling factor for cooling water service, and how often should exchangers be cleaned?
This depends heavily on your cooling water quality, but as a starting point, TEMA gives fouling resistances in the range of 0.0002 to 0.0005 m²·K/W (0.001 to 0.003 hr·ft²·°F/BTU) for treated cooling tower water, going up toward 0.0005-0.001 for untreated or brackish water, and higher still for seawater or water with high suspended solids.
If you're doing a new design, don't just grab a number from a table blindly, you should ask your water treatment vendor for actual fouling data from similar service, because the TEMA values are conservative defaults from decades ago and can lead to serious over-design (bigger exchanger, higher pressure drop, higher cost) if applied everywhere without thought.
On cleaning frequency that should be less about a fixed calendar and more about tracking actual performance. Trend your exchanger's overall U value (back-calculated from operating data) over time. Once it drops to somewhere around 75-80% of clean/design U, or once your approach temperature starts creeping up and you can't hit your outlet target even with full water flow, that's your signal to clean. Typical intervals for cooling water service run anywhere from 6 months to 2 years depending on water treatment quality, but let the data decide, not the calendar.