Heat transfer of nanoparticle suspensions in turbulent flow is studied theoretically. The main idea on which this work is based is that nanofluids behave more like single-phase fluids than conventional solid-liquid mixtures. This hypothesis implies that all convective heat transfer correlations available in the literature for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing there are the effective properties of the nanofluids calculated at the reference temperature. . In this regard, two empirical equations, based on a wide variety of experimental data reported in the literature, are used to evaluate the effective thermal conductivity and dynamic viscosity of the nanofluid. Conversely, the other effective properties are calculated by traditional mixture theory. The novelty of the present study is that the merits of nanofluids compared to the corresponding base liquid are evaluated in terms of overall energy performance, and not simply from the common perspective of heat transfer enhancement. The two cases of constant pumping power and constant heat transfer rate are studied for different operating conditions, nanoparticle diameters and solid-liquid combinations. The fundamental result obtained is the existence of an optimal particle loading for maximum heat transfer at constant motive power or minimum operating cost at constant heat transfer rate. In particular, for any assigned combination of solid and liquid phases, it was found that the optimal concentration of suspended nanoparticles increases as the overall temperature of the nanofluid increases, the Reynolds number of the base fluid increases, and the ratio pipe length/diameter increases. is reduced, although it is practically independent of the diameter of the nanoparticles. The usual design requirements for modern heat transfer equipment are small size and high thermal performance. In this regard, during the last decades, considerable research effort has been devoted to the development of advanced methods for improving heat transfer, such as those based on new geometries and configurations, and those based on the use of extended surfaces and/or turbulators. On the other hand, according to a number of studies carried out recently, another important contribution could come from the replacement of traditional heat transfer fluids, such as water, ethylene glycol and mineral oils, with nanofluids, c that is to say colloidal suspensions of solids of nanometric size. particles, whose effective thermal conductivity has been demonstrated to be higher than that of the corresponding pure base liquid. The main results of previous work on flow in pipes, which is arguably one of the most studied topics in the field of convection in nanofluids, clearly show that nanoparticle suspensions provide better thermal performance than base liquids at the same Reynolds number, and that heat transfer increases with increasing nanoparticle