Abstract: "Theoretical equations have been derived for calculating heat transfer coefficients for fluids flowing through concentric annuli for the following two cases: (A) constant and equal heat fluxes from both walls, and (B) constant, but unequal, heat fluxes from the walls, with equal wall temperatures at a given axial position along annular channel. In the derivations, the conditions of fully-established flow, and independence of physical properties with temperature variation across the flow channel, were assumed. The only geometrical parameter in this general case is the radius ratio r2/r1, and in the study it was varied from 1.0 to 10.0."