What is fouling in heat exchangers and why is it important?

Fouling is the accumulation of unwanted deposits on heat transfer surfaces, reducing heat transfer efficiency and increasing pressure drop. Types include: scaling (mineral deposits from hard water), biological fouling (algae, bacteria), chemical reaction fouling (polymerization, corrosion products), particulate fouling (suspended solids), and solidification fouling (wax, ice). Fouling factors are added to design calculations to account for performance degradation, typically increasing required surface area by 15-30%.