This study presents an experimental investigation of the characteristics of convective heat transfer in horizontal shell and coil heat exchangers in addition to the friction factor for fully developed flow through their helically coiled tube (HCT). Five heat exchangers of counterflow configuration were constructed with different HCT-curvature ratios () and tested at different mass flow rates and inlet temperatures of γ-Al2O3/water nanofluid in the HCT. The tests were performed for γ-Al2O3 with average size of 40 nm and particles volume concentration () from 0% to 2% for . Totally, 750 test runs were performed from which the HCT-average Nusselt number () and fanning friction factor () were calculated. Results illustrated that and of nanofluids are higher than those of the pure water at same flow condition, and this increase goes up with the increase in . When increases from 0% to 2%, the average increase in is of 59.4–81% at lower and higher HCT-Reynolds number, respectively, and the average increase in is of 25.7% and 27.4% at lower and higher HCT-Reynolds number, respectively, when increases from 0% to 2% for . In addition, results showed that and increase by increasing coil curvature ratio. When increases from 0.0392 to 0.1194 for , the average increase in is of 130.2% and 87.2% at lower and higher HCT-Reynolds number, respectively, and a significant increase of 18.2–7.5% is obtained in the HCT-fanning friction factor at lower and higher HCT-Reynolds number, respectively. Correlations for and as a function of the investigated parameters are obtained.