How are fouling factors incorporated in heat exchanger design and what values are typical?

Fouling factors (R_f) add thermal resistance to the clean overall coefficient: 1/U_dirty = 1/U_clean + R_f,i + R_f,o. TEMA standards provide typical values: cooling tower water (0.00035 m²-K/W), seawater (0.00009), clean steam (0.00009), fuel oil (0.0009), river water (0.00035-0.0005). Design must balance over-sizing (cost, weight) against maintenance frequency. Fouling rate depends on fluid velocity (higher velocity reduces deposition), wall temperature (scaling increases with temperature), fluid chemistry, and material compatibility. Excess area of 10-25% is common. Modern approaches use fouling monitors and predictive maintenance rather than fixed factors.