This study experimentally examines the hydrothermal performance attributes of a horizontal shell and semi-circular tubes (SCTs) heat exchanger of a counter-flow configuration. The tests are done by employing cold pure water or Multi-Walled Carbon Nanotube (MWCNT)/water nanofluid in the shell-side. The SCTs spacing ratio (0.157≤δ≤0.472), MWCNT volume concentration (0≤φ≤0.5%), and the operational conditions of the shell-side (3351≤Resh≤13461, 4.85≤Prsh≤7.54) are the key parameters throughout this study. Compared with the case of employing complete circular tubes and pure water in the shell-side, the results state that splitting the internal tubes and/or adding MWCNT increase both the shell-average Nusselt number (¯Nu_sh) and Fanning friction factor (fsh), by average increases of 44.4-106.2% and 7.1-29.6%, respectively, when δ increases from 0.157 to 0.472, and with average increases of 8.2-41.8% and 5.9-33.6%, respectively, when φ increases from 0.1% to 0.5%. The hydrothermal performance index (HTPI) is assessed to compare the performance of the heat exchanger with SCTs and with/without MWCNT to that of the case of circular tubes and pure water. The results record that incorporating both SCTs (δ=0.472) and MWCNT (φ=0.5%) provides the highest HTPI; 2.46, and it is reduced by their reduction. Finally, correlations for ¯Nu_sh, fsh, and HTPI are proposed.