The heat transfer and flow characteristics of the nanofluids in the horizontal spirally coiled tubes are investigated. The spirally coiled tubes are fabricated by bending a 8.50 mm inner diameter straight copper tube into a spiral-coil of five turns. The test section with three different curvature ratios of 0.035, 0.043, 0.06 are tested. Effects of curvature, nanofluids concentration and hot water temperature on the nanofluids heat transfer characteristics and pressure drop are considered. The results showed that the Nusselt number is about 21.29%, 29.02%, 34.07% for (0.01%, 0.025%, 0.05% by volume concentration, respectively) higher than the Nusselt number obtained for water as working fluid. However, the friction factor of nanofluids as working fluid increase slightly compared that of water as working fluid. Two correlations for predicted the Nusselt number and friction factor in the spirally coiled tube under constant wall temperature are proposed. (C) 2015 Elsevier Ltd. All rights reserved,