In the present paper, heat transfer characteristics of Al2O3 nanofluid flow inside shell and helical tube heat exchangers are numerically investigated. The numerical computations are performed by using SST turbulence model as well as by implementing conjugate heat transfer from hot fluid in the tube to cold fluid in the shell. The thermo-physical properties of the fluids were considered to be temperature dependent. The coil-side Reynolds number, the shell-side Reynolds number and the Prandtl number ranged between 9000 and 36,000, 600 and 2600 and 2.2 and 8.3, respectively. The effects of nanoparticle volume concentration on the temperature of coil- and shell-side flows, heat transfer rate and convective heat transfer coefficients are presented. The results indicate that the presence of 0.2% and 0.3% nanoparticle volume concentration increases the mean heat transfer rate by approximately 14% and 18%, respectively. The results also show that with the increase in particle volume concentration the coil-side, shell-side and overall heat transfer coefficients enhance. It is indicated that for the same mass flow rate, the heat transfer rate of nanofluid enhances noticeably compared to water and it increases marginally with the further increase in the nanoparticle volume concentration. The performance of the heat exchanger was evaluated and the results reveal that the effectiveness enhances by decreasing the mass flow rate and increasing the particle volume concentration, tube diameter and coil diameter. Moreover, the performance index enhances by increasing the tube diameter and reducing the coil diameter. (C) 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.