Double-pipe heat exchangers are typically employed in a variety of applications, including refineries and waste heat recovery systems. They are generally used for light-duty applications. They have limitations for heavy-duty applications. Heat transfer must be intensified to overcome this problem. Twisted tapes with diamond cuts of different sizes are inserted into the heat exchanger tubes to improve heat transfer. This creates added areas that disrupt the flow and help increase turbulence, thus promoting better mixing of the fluid. The diamond-cut twisted inserts have a constant twist ratio of 3 and varying scale ratios of 25, 50, and 75. The outcomes suggest that the Nusselt number has increased by 81.43% and the friction factor has increased by a factor of 1.01 compared with a plain tube. The maximum thermal performance factor is found to be 1.3 for a
scale ratio of 50. The diamond-cut twisted tapes produced strong swirls, periodic boundary layer disruption, and controlled flow separation and reattachment. These changes increased turbulence, thereby intensifying heat transfer. Generalized correlations were developed for the Nusselt numbers and friction factors.