Convection Film Resistance

A moving fluid does not touch a wall at the wall's temperature; it drags a thin, nearly stationary layer along with it, and all of the temperature drop happens inside that film. Treating it as a resistance, R = 1/(hA), lets you drop convection straight into the same series network as the conduction terms. A 2 m² panel in still air at h = 25 W/(m²·K) has a film resistance of 1/50 = 0.02 K/W: larger than 100 mm of concrete behind it, which is the whole reason surface films appear in every building-envelope calculation as fixed air-film allowances.

The trap is assuming the film is small enough to ignore because the coefficient is a big number. Compare resistances, not coefficients: on an air-cooled condenser the air film usually holds 80–95% of the total, so polishing the refrigerant side is wasted money while the fan speed is not. The other trap is area: in a finned or tubular geometry h and A must be quoted on the same surface, or the resistance you compute belongs to a different exchanger than the one on the drawing.