The determination of the overall heat-transfer coefficient is necessary in order to
determine the heat transferred from the inner pipe to the outer pipe. This coefficient takes into
account all of the conductive and convective resistances (k and h, respectively) between fluids
separated by the inner pipe, and also takes into account thermal resistances caused by fouling
(rust, scaling, i.e.) on both sides of the inner pipe. For a double-pipe heat exchanger the overall
heat transfer coefficient, U, can be expressed as
.