How do you select appropriate fouling factors for heat exchanger design?

Fouling factors depend on fluid type, velocity, temperature, and operating history. Reference sources include TEMA standards and company experience. Typical values: clean water 0.0001 m2K/W, cooling tower water 0.0002-0.0004, crude oil 0.0003-0.0009, heavy hydrocarbon 0.0005-0.001. Higher factors provide more margin but increase surface area and cost. Consider: actual operating data from similar services, cleaning frequency planned, process criticality, and whether excess area causes operational issues (low velocity increasing fouling).