In the present research, laminar, steady state flow in helically coiled tubes at a constant wall temperature was studied numerically and experimentally. Pressure drop and the convective heat transfer behavior of nanofluid were investigated. In the experimental section, a heat exchanger was designed, capable of providing constant wall temperature for coils with different curvature and torsion ratio for the ease of assembly. Pressure drop measurement and average convective heat transfer coefficient calculation were carried out. In the numerical study, the three dimensional governing equations were solved by finite difference method with projection algorithm using FORTRAN programming language. Homogeneous model with constant effective properties was used. The difference between numerical and experimental results was significant. Dispersion model was employed to make the observed difference between numerical and experimental results negligible. Dispersion model was modified to be applicable for helical tubes. This modification resulted in negligible difference between the numerical and the experimental results. More enhanced heat transfer was observed for tubes with greater curvature ratio. Moreover, the performance evaluation of these enhanced heat transfer methods presented. Utilization of base fluid in helical tube with greater curvature compared to the use of nanofluid in straight tubes enhanced heat transfer more effectively. (C) 2012 Elsevier Ltd. All rights reserved.