What is the overall heat transfer coefficient (U) and what factors affect it?

The overall heat transfer coefficient (U) represents the total thermal resistance between two fluids separated by a solid wall, expressed in W/m2K. It combines individual resistances: 1/U = 1/hi + Rfi + Rw + Rfo + 1/ho, where hi and ho are inside and outside film coefficients, Rf values are fouling resistances, and Rw is wall resistance. Higher U means better heat transfer. Typical values range from 25-50 W/m2K for gases to 1000-3000 W/m2K for condensing steam.