<p>Growing awareness of the harmful environmental impacts of fossil fuel-based systems, combined with technological advancements and increasing energy demand, has highlighted the need for more efficient energy systems. This has resulted in a greater use of shell and helically coiled tube heat exchanger (SHCTHE) owing to its superior performance. This study focuses on enhancing the thermal efficiency of SHCTHE by improving the heat transfer coefficient using green-synthesized silver (Ag) nanoparticles derived from Azadirachta indica and Melia composita Willd with varying concentrations (0.02%, 0.04%, and 0.06%) dispersed in deionized water. Both computational and experimental approaches were used to evaluate the impact of nanofluids on the efficiency of the SHCTHE. Simulations were performed with ANSYS Fluent under consistent conditions, varying the flow rates. The findings revealed that nanofluids synthesized from Melia composita exhibited a 21% intensification in the mean heat transfer rate and a 15% augmentation in the heat transfer coefficient compared to those derived from Azadirachta indica. Building on promising simulation results, the Ag-DI nanofluids were tested in a heat exchanger under laboratory conditions, where the experimental outcomes closely matched the simulations, revealing only minor discrepancies. The average discrepancy between the simulated and experimental heat transfer rate observed for Azadirachta indica and Melia composita Willd was 3.54% and 2%, correspondingly, at a cold FR of 1.5&#xa0;l/min. MC exhibited an elevated heat transfer coefficient spanning from 2238.611 to 3805.155 W/m²K, making it a promising green nanofluid for SHCTHE with substantial thermal performance enhancement.</p>