The well-known equations for the local Nusselt numbers Nu(x) have been reinterpreted. By integrating them, simple equations were derived while consistently adhering to validity limits for the mean Nu numbers. With these it is possible to consider the temperature dependency of material data in a targeted way. The derived correction factors show the different influences of viscosity, thermal conductivity and specific heat capacity. The Prandtl number Pr, which is usually used as a corrective, does not stand up to theoretical considerations. In addition, depending on the operating conditions - such as counter- or cocurrent - the effects of a non-constant wall temperature were mathematically recorded.