What is the overall heat transfer coefficient and how is it calculated?

The overall heat transfer coefficient (U) combines all thermal resistances in a heat transfer system into a single value. For a plane wall with convection on both sides: 1/UA = 1/(h₁A₁) + L/(kA) + 1/(h₂A₂). For heat exchangers, U relates to heat transfer rate as Q = UA×ΔT_mean. Typical values: water-to-water (1000-2500 W/m²-K), water-to-air (30-60 W/m²-K), steam condensers (1500-4000 W/m²-K). U decreases over time due to fouling, requiring fouling factors in design calculations.