The current study focuses on numerical and experimental investigation of the laminar, steady state flow in helical tubes with constant wall heat flux boundary condition. Pressure drop and convection heat transfer behavior of water-silver nanofluid in helical coils were evaluated and compared to the pure water. In the numerical part of the study, incompressible Navier-Stokes equations, derived from an orthogonal helical coordinate system were solved by finite difference method with projection algorithm using FORTRAN programming language. Homogeneous model with constant effective properties was used for nanofluid. The Local Nusselt number in the entrance region of the helical tubes and the effects of curvature and torsion ratios and Reynolds number have been discussed. In the experimental part, six helical heat exchangers with different curvature and torsion ratios were designed, capable of providing a constant wall heat flux. Pressure drop measurement and fully developed heat transfer coefficient and Nusselt number calculations were carried out and compared with the numerical results. The results show that for laminar flow with constant wall heat-flux boundary condition, the helical tubes with larger curvature ratio lead to higher heat transfer enhancement and higher pressure drop increase, when employing the nanofluid instead of the base fluid. Moreover, compared to using metal Ag nanofluid in straight tubes, utilization of base fluid in helical coils with greater curvature increases heat transfer more effectively. Finally, we conclude that since using Ag nanofluid increases the heat transfer of helical heat exchangers about 3.5-3.8%, utilizing this nanofluid could enhance the thermal performance of heat exchangers, which can benefit the industry. (C) 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.